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Sinusoidal communication in liver fibrosis and regeneration

  • Giusi Marrone
    Affiliations
    Regenerative Medicine & Fibrosis Group, Institute for Liver & Digestive Health, University College London, Royal Free, London, UK
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  • Vijay H. Shah
    Affiliations
    Division of Gastroenterology and Hepatology, Mayo Clinic, Rochester, MN 55901, United States
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  • Jordi Gracia-Sancho
    Correspondence
    Corresponding author. Address: Barcelona Hepatic Hemodynamic Lab, IDIBAPS Biomedical Research Institute, C/Rosselló 149, 4th Floor, Room 4.5, 08036 Barcelona, Spain. Tel.: +34 932275707x4306.
    Affiliations
    Barcelona Hepatic Hemodynamic Lab, Institut d’Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS) – Centro de Investigación en Red en Enfermedades Hepáticas y Digestivas (CIBEREHD), Barcelona, Spain
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      Summary

      Cellular crosstalk is a process through which a message is transmitted within an individual cell (intracellular crosstalk) or between different cells (intercellular crosstalk). Intercellular crosstalk within the liver microenvironment is critical for the maintenance of normal hepatic functions and for cells survival. Hepatic cells are closely connected to each other, work in synergy, and produce molecules that modulate their differentiation and activity. This review summarises the current knowledge regarding paracrine communication networks in parenchymal and non-parenchymal cells in liver fibrosis due to chronic injury, and regeneration after partial hepatectomy.

      Abbreviations:

      ECM (extracellular matrix), LSEC (liver sinusoidal endothelial cells), HSC (hepatic stellate cells), KC (Kupffer cells), eNOS (endothelial nitric oxide synthase), VEGF (vascular endothelial growth factor), α-SMA (alpha-smooth muscle actin), KLF2 (kruppel-like factor 2), PMF (portal myofibroblast), BM SPC (bone marrow-derived endothelial sinusoidal progenitor cells)

      Keywords

      Introduction: Liver sinusoidal cells and cellular crosstalk

      The hepatic sinusoid represents a well-organized vascular matrix that provides the structural and biochemical environment in which non-parenchymal liver cells live and communicate. The space of Disse separates sinusoidal cells from parenchymal cells and contains extracellular matrix (ECM) components. Thus, the liver microenvironment can be described as a multidirectional interaction complex (cell-matrix-cell) organized to manage the delivery of molecular signals, where every piece has a crucial role. It is due to this particular structure that hepatic cells, composed mainly of hepatocytes, liver sinusoidal endothelial cells (LSEC), hepatic stellate cells (HSC), and Kupffer cells (KC), precisely “talk” to each other.
      Liver cells, mainly LSEC, KC, HSC and Hepatocytes readily interact each other, thus conforming a highly efficient signalling network that maintains sinusoidal homeostasis.
      The word crosstalk was firstly used in electronics and technology communication to explain any phenomena by which a signal transmitted on one circuit creates an undesired effect in another circuit. In biomedical science we do not view this phenomenon as undesired but consider it a mechanism cells have to transmit “instructions” both in physiological and pathophysiological situations. In the latter scenario instructions are considered as transmitted errors, and are therefore an event or multi-events that could be considered as biological targets.
      LSEC, the hepatic cell population that interfaces blood components and forms the barrier of sinusoids, are pivotal regulators of the liver microcirculation and have a key role in sinusoidal crosstalk. Their peculiarities (small cells with non-diaphragmed fenestrae and without a basement membrane) make them a vital link in the complex network of hepatic cellular interactions both in health and disease. Fenestrae, arranged in sieve plates, contribute to LSEC permeability and functions [
      • Braet F.
      • Wisse E.
      Structural and functional aspects of liver sinusoidal endothelial cell fenestrae: a review.
      ], facilitating oxygenation of hepatocytes and enhancing hepatocytes exposure to macromolecules from the portal circulation. They are capable of contracting or dilating, can change in size, porosity (number of fenestrae per μm2), and frequency, acting like a filter for the transit of substances. Modifications in fenestrae properties, a process known as pseudo-capillarization, are associated with aging and hypoxia [
      • Cogger V.C.
      • Warren A.
      • Fraser R.
      • Ngu M.
      • McLean A.J.
      • le Couteur D.G.
      Hepatic sinusoidal pseudocapillarization with aging in the non-human primate.
      ,
      • McLean A.J.
      • Cogger V.C.
      • Chong G.C.
      • Warren A.
      • Markus A.M.
      • Dahlstrom J.E.
      • et al.
      Age-related pseudocapillarization of the human liver.
      ], while capillarization (complete loss of fenestrae) [
      • Schaffner F.
      • Poper H.
      Capillarization of hepatic sinusoids in man.
      ] seems to precede the development of most chronic liver diseases [
      • Urashima S.
      • Tsutsumi M.
      • Nakase K.
      • Wang J.S.
      • Takada A.
      Studies on capillarization of the hepatic sinusoids in alcoholic liver disease.
      ,
      • Xu B.
      • Broome U.
      • Uzunel M.
      • Nava S.
      • Ge X.
      • Kumagai-Braesch M.
      • et al.
      Capillarization of hepatic sinusoid by liver endothelial cell-reactive autoantibodies in patients with cirrhosis and chronic hepatitis.
      ]. Note that the crude term “capillarization” in this specific context means that the unique and highly specialized phenotype of LSEC is lost and cells become ordinary non-specialized endothelial cells, or endothelial cells of an ordinary capillary.
      LSEC are the main source of the endothelium-derived nitric oxide (NO), an important modulator of vascular tone, where it is produced by the endothelial nitric oxide synthase (eNOS). Moreover, LSEC express singular sets of adhesion molecules that correlate with the micro-environmental characteristics of the sinusoidal wall [
      • Geraud C.
      • Evdokimov K.
      • Straub B.K.
      • Peitsch W.K.
      • Demory A.
      • Dorflinger Y.
      • et al.
      Unique cell type-specific junctional complexes in vascular endothelium of human and rat liver sinusoids.
      ]. These adhesion molecules include ICAM-1 (Intercellular Adhesion Molecule 1), VCAM-1 (vascular cell adhesion molecule 1), and selectins, are regulated by inflammatory cytokines, and influence cell-to-cell interactions [
      • Essani N.A.
      • McGuire G.M.
      • Manning A.M.
      • Jaeschke H.
      Differential induction of mRNA for ICAM-1 and selectins in hepatocytes, Kupffer cells and endothelial cells during endotoxemia.
      ]. Interestingly, LSEC phenotype is maintained, at least in part, by paracrine secretion of vascular endothelial growth factor (VEGF) by hepatocytes and HSC [
      • DeLeve L.D.
      • Wang X.
      • Hu L.
      • McCuskey M.K.
      • McCuskey R.S.
      Rat liver sinusoidal endothelial cell phenotype is maintained by paracrine and autocrine regulation.
      ].
      HSC, localized in the space of Disse, are the main collagen-synthesizers of the liver [
      • Friedman S.L.
      • Roll F.J.
      • Boyles J.
      • Bissell D.M.
      Hepatic lipocytes: the principal collagen-producing cells of normal rat liver.
      ], and contribute to its architecture and functions by interacting with neighbouring cells [
      • Sato M.
      • Suzuki S.
      • Senoo H.
      Hepatic stellate cells: unique characteristics in cell biology and phenotype.
      ]. They provide retinoid storage and homeostasis, ECM metabolism, and sinusoidal lumen diameter [
      • Pinzani M.
      • Gentilini P.
      Biology of hepatic stellate cells and their possible relevance in the pathogenesis of portal hypertension in cirrhosis.
      ]. A single stellate cell can wrap up to 4 sinusoids and regulate sinusoidal blood flow by contraction. Injuries to the hepatic microvasculature activate the trans-differentiation of HSC conferring them a proliferative and fibrogenic myofibroblast-like phenotype. HSC activities mainly depend on their interactions with ECM components, endothelial cells and hepatocytes [
      • Sato M.
      • Suzuki S.
      • Senoo H.
      Hepatic stellate cells: unique characteristics in cell biology and phenotype.
      ,
      • Wake K.
      Structure of the sinusoidal wall in the liver.
      ].
      KC are hepatic macrophages that have an essential role in the liver immune system [
      • Bilzer M.
      • Roggel F.
      • Gerbes A.L.
      Role of Kupffer cells in host defense and liver disease.
      ] and inflammation [
      • Liaskou E.
      • Wilson D.V.
      • Oo Y.H.
      Innate immune cells in liver inflammation.
      ]. They are attached to the sinusoidal endothelial layer, where they uniquely capture signals from the blood and contribute to hepatic blood flow regulation [
      • McCuskey R.S.
      Morphological mechanisms for regulating blood flow through hepatic sinusoids.
      ]. They are mainly the source rather than the target of soluble mediators [
      • Bilzer M.
      • Roggel F.
      • Gerbes A.L.
      Role of Kupffer cells in host defense and liver disease.
      ], eliciting a physiological response to all other liver cells.
      Hepatocytes represent the most abundant cell type within the liver and, although they are the major functional hepatic cells, their interactions with sinusoidal cells are totally necessary for their multiple activities. While consequences of endothelial capillarization and HSC phenotype dysregulation on liver function are well characterised, the exact process of hepatocyte dysfunction in chronic liver disease is not completely known. Indeed, the liver can function normally with less than half of its hepatocytes, although they have the unique ability for continuous turnover and replenishment [
      • Malhi H.
      • Guicciardi M.E.
      • Gores G.J.
      Hepatocyte death: a clear and present danger.
      ].
      As a consequence of parenchymal and non-parenchymal interactions, cells grow, proliferate, migrate and differentiate, preserving their normal cellular phenotype. In this interactive network, healthy or injured cells become the positive or negative regulators of the closest neighbouring cells (via juxtacrine signalling and receptor-ligand complexes or through indirect contact), or of the hypothetically most distant cells (via a variety of soluble paracrine and endocrine factors, such as cytokines, growth factors, second messengers and hormones) (Table 1). In this regard, ECM is considered a depository for growth factors, cytokines and other proteins that can be released when required to be used by proximal cells, contributing to cellular programming [
      • Wells R.G.
      The role of matrix stiffness in regulating cell behavior.
      ,
      • March S.
      • Hui E.E.
      • Underhill G.H.
      • Khetani S.
      • Bhatia S.N.
      Microenvironmental regulation of the sinusoidal endothelial cell phenotype in vitro.
      ]. Understanding the intercellular crosstalk within the sinusoids is critical for a better knowledge of the progression, aggravation, and resolution of liver disease, and modifications in the coordinated interactions may lead to disease development/improvement.
      Table 1Paracrine crosstalk agents.
      LGF, liver growth factor; VEGF, vascular endothelial growth factor; FGF, fibroblast growth factor; IGF, insulin growth factor; HB-EGF, heparin-binding EGF-like growth factor; PDGF, platelet derived growth factor; HGF, hepatocyte growth factor; TGF-β, transforming growth factor beta; CTGF, connective tissue growth factor; SDF-1α, Stromal cell-derived factor 1; MCP-1, Monocyte Chemoattractant Protein-1; ET-1, endothelin-1; NO, nitric oxide; cGMP, cyclic guanosine monophosphate.

      Sinusoidal crosstalk in fibrosis, cirrhosis and portal hypertension

      Fibrosis is characterised by intra-hepatic accumulation of ECM, mainly in the perisinusoidal space of Disse and portal tracts [
      • Friedman S.L.
      Mechanisms of disease: Mechanisms of hepatic fibrosis and therapeutic implications.
      ]. The formation of abnormal nodules with consequent distortion of the liver architecture, inflammation, vascular occlusion, and intra-hepatic angiogenesis aggravate the fibrotic state leading to the development of cirrhosis [
      • Pinzani M.
      • Rosselli M.
      • Zuckermann M.
      Liver cirrhosis.
      ]. Alterations in the normal crosstalk cause progressive microvascular dysfunction in the cirrhotic liver, increase in hepatic vascular resistance and development of the main complication of cirrhosis, portal hypertension [
      • Garcia-Pagan J.C.
      • Gracia-Sancho J.
      • Bosch J.
      Functional aspects on the pathophysiology of portal hypertension in cirrhosis.
      ]. All sinusoidal cells take part in this process: they communicate and acquire a vasoconstrictor phenotype that is further exacerbated in response to biomechanical, pathogenic and inflammatory stimuli [
      • Gracia-Sancho J.
      • Lavina B.
      • Rodriguez-Vilarrupla A.
      • Garcia-Caldero H.
      • Bosch J.
      • Garcia-Pagan J.C.
      Enhanced vasoconstrictor prostanoid production by sinusoidal endothelial cells increases portal perfusion pressure in cirrhotic rat livers.
      ,
      • Steib C.J.
      • Gerbes A.L.
      • Bystron M.
      • op den Winkel M.
      • Hartl J.
      • Roggel F.
      • et al.
      Kupffer cell activation in normal and fibrotic livers increases portal pressure via thromboxane A(2).
      ]. However, our understanding of the mechanisms underlying the changes in phenotype of sinusoidal cells and the divergent cellular communication during cirrhosis progression is far from complete. Unfortunately, the only treatment for end-stage cirrhosis is transplantation [
      • Said A.
      • Lucey M.R.
      Liver transplantation: an update 2008.
      ], and the need to develop anti-fibrotic compounds is a must.

      Liver fibrosis is initiated by crosstalk from LSEC and Hepatocytes

      The fibrogenic reaction is initiated by two major intercellular crosstalk pathways that are ultimately connected. In the presence of hepatic injury, LSEC become rapidly dysregulated and start de-differentiation towards a capillarized phenotype. This is accompanied by the production and release of soluble factors that rapidly travel to neighbouring cells affecting their phenotype [
      • DeLeve L.D.
      The hepatic sinusoidal endothelial cell: morphology, function, and pathobiology.
      ]. As an example, it has been demonstrated that LSEC-derived fibronectin affects HSC phenotype, promoting their activation [
      • Jarnagin W.R.
      • Rockey D.C.
      • Koteliansky V.E.
      • Wang S.S.
      • Bissell D.M.
      Expression of variant fibronectins in wound healing: cellular source and biological activity of the EIIIA segment in rat hepatic fibrogenesis.
      ]. In parallel, exogenous hepatic injury significantly modifies hepatocytes transcriptional programs promoting their proliferation and death. Hepatocyte apoptosis results in the formation of apoptotic bodies that once captured by the non-parenchymal cells (HSC and KC) contribute to their activation [
      • Malhi H.
      • Guicciardi M.E.
      • Gores G.J.
      Hepatocyte death: a clear and present danger.
      ,
      • Canbay A.
      • Feldstein A.E.
      • Higuchi H.
      • Werneburg N.
      • Grambihler A.
      • Bronk S.F.
      • et al.
      Kupffer cell engulfment of apoptotic bodies stimulates death ligand and cytokine expression.
      ,
      • Jiang J.X.
      • Mikami K.
      • Venugopal S.
      • Li Y.
      • Torok N.J.
      Apoptotic body engulfment by hepatic stellate cells promotes their survival by the JAK/STAT and Akt/NF-kappaB-dependent pathways.
      ]. In turn, HSC begin to proliferate, contract and deposit elevated amount of collagen fibers and extracellular matrix molecules in the hepatic parenchyma, contributing to the stiffening of the organ and perturbing all cellular functions. Interestingly, collagen accumulation in the space of Disse may contribute to the loss of endothelial fenestrations, aggravating hepatic fibrosis [
      • McGuire R.F.
      • Bissell D.M.
      • Boyles J.
      • Roll F.J.
      Role of extracellular matrix in regulating fenestrations of sinusoidal endothelial cells isolated from normal rat liver.
      ]. Thus, a self-perpetuating cycle between collagen-producing activated HSC and capillarized LSEC stimulate each other, further contributing to liver fibrosis (Fig. 1).
      Figure thumbnail gr1
      Fig. 1Sinusoidal crosstalk during chronic liver injury. Initial dysregulation in functional hepatocytes (fHep) and liver sinusoidal endothelial cells (LSEC) due to liver damage lead to complex paracrine interactions (red arrows) with quiescent hepatic stellate cells (qHSC) and Kupffer cells (KC), ultimately creating a dysfunctional sinusoidal microenvironment composed by dysfunctional LSEC (dxLSEC), activated HSC (aHSC), activated KC (aKC) and dysfunctional and necroptotic hepatocytes (dxHep). Phenotypic characteristics of each cell type are progressively lost during the progression of the liver disease, and new pathologic properties appear.
      Hepatocytes and LSEC also communicate with each other during fibrogenesis via the VEGF-A/VEGFR2 pathway [
      • Yamane A.
      • Seetharam L.
      • Yamaguchi S.
      • Gotoh N.
      • Takahashi T.
      • Neufeld G.
      • et al.
      A new communication system between hepatocytes and sinusoidal endothelial cells in liver through vascular endothelial growth factor and Flt tyrosine kinase receptor family (Flt-1 and KDR/Flk-1).
      ]. It has been recently observed that CD147, a transmembrane glycoprotein linked to liver fibrosis [
      • Zhang D.W.
      • Zhao Y.X.
      • Wei D.
      • Li Y.L.
      • Zhang Y.
      • Wu J.
      • et al.
      HAb18G/CD147 promotes activation of hepatic stellate cells and is a target for antibody therapy of liver fibrosis.
      ], promotes the expression and secretion of VEGF-A via the PI3K/Akt signalling pathway in hepatocytes, and upregulates VEGFR2 expression in LSEC, augmenting their proliferation and migration. Interestingly, anti-CD147 antibodies have been found to inhibit VEGF-A/VEGFR-2-mediated angiogenesis, which consequently attenuates liver fibrosis progression. The synchronistic expression of CD147 in both hepatocytes and LSEC suggest a regulatory role for CD147 in hepatocyte–LSEC interactions [
      • Yan Z.
      • Qu K.
      • Zhang J.
      • Huang Q.
      • Qu P.
      • Xu X.
      • et al.
      CD147 promotes liver fibrosis progression via VEGF-A/VEGFR2 signalling-mediated cross-talk between hepatocytes and sinusoidal endothelial cells.
      ]. Therapeutic trials blocking CD147-induced angiogenesis may be promising for the treatment of liver fibrosis.
      Considering the cellular communications initiating the fibrogenesis reaction, protection of LSEC and/or hepatocytes may be an effective strategy to avoid fibrosis progression. Potentially, drugs specifically designed to ameliorate LSEC phenotype may also prevent the progression of fibrosis, and its derived complications. Although this idea has been demonstrated in experimental models of cirrhosis regression [
      • Di Pascoli M.
      • Divi M.
      • Rodriguez-Vilarrupla A.
      • Rosado E.
      • Gracia-Sancho J.
      • Vilaseca M.
      • et al.
      Resveratrol improves intrahepatic endothelial dysfunction and reduces hepatic fibrosis and portal pressure in cirrhotic rats.
      ,
      • Guillaume M.
      • Rodriguez-Vilarrupla A.
      • Gracia-Sancho J.
      • Rosado E.
      • Mancini A.
      • Bosch J.
      • et al.
      Recombinant human manganese superoxide dismutase reduces liver fibrosis and portal pressure in CCl4-cirrhotic rats.
      ], it has not been validated in situations of liver fibrosis progression. Regarding hepatocytes, apoptosis occurs in physiological conditions with no release of pro-inflammatory cytokines resulting in a minimal immune response. It would be reasonable to extrapolate that under pathophysiological situations, the mechanisms that would otherwise protect neighbouring cells fail. Inhibiting hepatocyte apoptosis following injury does not convey benefit [
      • Bannert K.
      • Kuhla A.
      • Abshagen K.
      • Vollmar B.
      Anti-apoptotic therapeutic approaches in liver diseases: do they really make sense?.
      ], and an alternative strategy could be aimed at either blocking the capturing of the apoptotic bodies by non-parenchymal cells, or building a ‘screen’ between hepatocytes and HSC/KC. To test these hypotheses however, we first need to understand when cell dysfunction initially occurs, and to which cell.

      Communications between hepatic cells and ECM

      As stated, LSEC have a major role in hepatic fibrogenesis: they are contributors of extracellular matrix (ECM) deposition [
      • Wells R.G.
      Cellular sources of extracellular matrix in hepatic fibrosis.
      ] and regulators of ECM metabolism [
      • Myers P.R.
      • Tanner M.A.
      Vascular endothelial cell regulation of extracellular matrix collagen: role of nitric oxide.
      ], however, LSEC phenotype is strongly modulated by ECM itself. Ford and colleagues demonstrated how the phenotype of LSEC changes in response to matrix stiffness. Using collagen hydrogels with two different values for elastic modulus they reproduced healthy (6 kPa) and fibrotic (36 kPa) tissue matrix. LSEC seeded on 6 kPa substrates exhibited well-defined fenestrae arranged in sieve plate structures, and underwent pseudocapillarization after 96 h. However, as a result of increasing elastic modulus, LSEC seeded on 36 kPa substrates completely lost fenestrae and expressed CD31 at the surface just after 24 h [
      • Ford A.J.
      • Jain G.
      • Rajagopalan P.
      Designing a fibrotic microenvironment to investigate changes in human liver sinusoidal endothelial cell function.
      ], thus confirming the importance of HSC-derived ECM on LSEC phenotype [
      • Wells R.G.
      The role of matrix stiffness in regulating cell behavior.
      ].
      Liver ECM also markedly influences the behaviour of other non-parenchymal cells, especially portal myofibroblasts (PMF) and HSC. When cultured on stiff rather than soft collagen lattices PMF increase their expression of fibrillin-1 [
      • Li Z.
      • Dranoff J.A.
      • Chan E.P.
      • Uemura M.
      • Sevigny J.
      • Wells R.G.
      Transforming growth factor-beta and substrate stiffness regulate portal fibroblast activation in culture.
      ], and HSC become highly activated loosing their lipid droplets and expressing high levels of α-SMA [
      • Olsen A.L.
      • Bloomer S.A.
      • Chan E.P.
      • Gaca M.D.
      • Georges P.C.
      • Sackey B.
      • et al.
      Hepatic stellate cells require a stiff environment for myofibroblastic differentiation.
      ], demonstrating that the stiffness of the matrix has significant functional relevance for the contractile non-parenchymal cells. Interestingly, a recent paper further depicts the underlying mechanisms partly responsible for such activation revealing a negative regulation between RhoA and the cytosolic tyrosine kinase c-SCR [
      • Gortzen J.
      • Schierwagen R.
      • Bierwolf J.
      • Klein S.
      • Uschner F.E.
      • van der Ven P.F.
      • et al.
      Interplay of matrix stiffness and c-SRC in hepatic fibrosis.
      ].
      In addition to ECM stiffness, its chemical components can also actively modulate cell status. Hyaluronic acid (HA) is one of the main components of the ECM and influences cell proliferation and migration. It has been proposed as a marker of liver fibrosis since high HA serum levels correlate with fibrotic stages [
      • Halfon P.
      • Bourliere M.
      • Penaranda G.
      • Deydier R.
      • Renou C.
      • Botta-Fridlund D.
      • et al.
      Accuracy of hyaluronic acid level for predicting liver fibrosis stages in patients with hepatitis C virus.
      ]. Within the liver, HA is mostly synthesized by HSC and degraded by LSEC [
      • Tangkijvanich P.
      • Kongtawelert P.
      • Pothacharoen P.
      • Mahachai V.
      • Suwangool P.
      • Poovorawan Y.
      Serum hyaluronan: a marker of liver fibrosis in patients with chronic liver disease.
      ]. This efficient homeostasis is facilitated by the activity of KC, as their depletion does not allow HA uptake by LSEC [
      • Deaciuc I.V.
      • Bagby G.J.
      • Lang C.H.
      • Spitzer J.J.
      Hyaluronic acid uptake by the isolated, perfused rat liver: an index of hepatic sinusoidal endothelial cell function.
      ]. Therefore, maintenance of KC in a competent state could control ECM metabolism and contribute to fibrosis resolution. Conversely, KC inactivation using gadolinium chloride prevents the development of cirrhosis in experimental models of fibrosis [
      • Muriel P.
      • Escobar Y.
      Kupffer cells are responsible for liver cirrhosis induced by carbon tetrachloride.
      ].
      KC also play a key role modulating the phenotype of neighbouring non-parenchymal cells. This relationship has been characterised by Nieto and colleagues: in a seminal paper they described how KC negatively affect neighbouring HSC, promoting their activation (defined as high proliferation, high expressions of α-SMA and collagen-I, and reduced capability to degrade extracellular collagen) [
      • Nieto N.
      Oxidative-stress and IL-6 mediate the fibrogenic effects of [corrected] Kupffer cells on stellate cells.
      ]. Furthermore, the paper described an H2O2-IL-6-dependent mechanism responsible for such intercellular communication. Subsequent studies validated the KC-HSC crosstalk when M2 phenotype-like KC were co-cultured with HSC, thus confirming such communications in an injured microenvironment [
      • Cubero F.J.
      • Nieto N.
      Ethanol and arachidonic acid synergize to activate Kupffer cells and modulate the fibrogenic response via tumor necrosis factor alpha, reduced glutathione, and transforming growth factor beta-dependent mechanisms.
      ].
      In the clinic, liver stiffness can be measured with transient elastography and provide a measure of fibrosis [
      • Castera L.
      • Pinzani M.
      • Bosch J.
      Non invasive evaluation of portal hypertension using transient elastography.
      ], thus a hypothetical degree of cell-matrix-cell crosstalk could potentially be estimated at the bedside due to its quantifiable association with liver stiffness.

      LSEC and HSC communications in fibrosis and cirrhosis

      The interaction between LSEC and HSC in fibrosis/cirrhosis has been extensively investigated. In 2004 DeLeve and colleagues showed that the phenotype of rat LSEC was maintained by autocrine and paracrine regulation exerted by either healthy hepatocytes or stellate cells, and that there was no added effect of co-culturing both hepatocytes and stellate cells together with LSEC [
      • DeLeve L.D.
      • Wang X.
      • Hu L.
      • McCuskey M.K.
      • McCuskey R.S.
      Rat liver sinusoidal endothelial cell phenotype is maintained by paracrine and autocrine regulation.
      ]. Their experiments suggested the requirement of an autocrine production of NO by LSEC, since the addition of the inhibitor of eNOS L-NAME (L-NG-Nitroarginine Methyl Ester) was associated with a low number of surface CD31 positive cells, a marker of LSEC de-differentiation. Subsequently, they demonstrated that the maintenance of differentiated LSEC phenotype (with regular fenestrae) was due to both a VEGF-stimulated-NO-independent pathway and a VEGF-stimulated-NO-dependent pathway. Restoration of LSEC differentiation in vivo promoted HSC quiescence, prevention of fibrosis progression, and regression of mild fibrosis [
      • DeLeve L.D.
      • Wang X.
      • Guo Y.
      Sinusoidal endothelial cells prevent rat stellate cell activation and promote reversion to quiescence.
      ,
      • Xie G.
      • Wang X.
      • Wang L.
      • Wang L.
      • Atkinson R.D.
      • Kanel G.C.
      • et al.
      Role of differentiation of liver sinusoidal endothelial cells in progression and regression of hepatic fibrosis in rats.
      ]. Studies from our group further investigated the HSC/LSEC paracrine interactions, especially in pathological conditions, and demonstrated that co-culture of differentiated LSEC with HSC in which eNOS was inhibited by L-NAME blocked the ability of LSEC to maintain HSC quiescence [
      • Marrone G.
      • Russo L.
      • Rosado E.
      • Hide D.
      • Garcia-Cardena G.
      • Garcia-Pagan J.C.
      • et al.
      The transcription factor KLF2 mediates hepatic endothelial protection and paracrine endothelial-stellate cell deactivation induced by statins.
      ]. We proposed that the transcription factor Kruppel-like factor 2 (KLF2) is crucial in safeguarding the hepatic sinusoid via a bidirectional protective cell-to-cell communication in which growth factors and second messengers play a key role. Using in vitro and in vivo experimental models of liver cirrhosis, we demonstrated a paracrine effect whereby KLF2-mediated improvement in HSC phenotype ameliorated the dysfunctional phenotype of LSEC and vice versa. This led to a significant regression of liver cirrhosis and a marked reduction in portal pressure [
      • Marrone G.
      • Russo L.
      • Rosado E.
      • Hide D.
      • Garcia-Cardena G.
      • Garcia-Pagan J.C.
      • et al.
      The transcription factor KLF2 mediates hepatic endothelial protection and paracrine endothelial-stellate cell deactivation induced by statins.
      ,
      • Marrone G.
      • Maeso-Díaz R.
      • Garcia-Cardena G.
      • Garcia-Pagan J.C.
      • Bosch J.
      • Gracia-Sancho J.
      KLF2 exerts anti-fibrotic and vasoprotective effects in cirrhotic rat livers: behind the molecular mechanisms of statins.
      ].
      Sinusoidal paracrine interactions are rapidly dysregulated upon liver injury, actively contributing to fibrogenesis and hepatic microvascular dysfunction. Statins represent the most promising therapy to improve the sinusoidal milieu in chronically injured livers.
      Following the research initiated 20 years ago by Jaime Bosch [
      • Zafra C.
      • Abraldes J.G.
      • Turnes J.
      • Berzigotti A.
      • Fernandez M.
      • Garca-Pagan J.C.
      • et al.
      Simvastatin enhances hepatic nitric oxide production and decreases the hepatic vascular tone in patients with cirrhosis.
      ] and conducted subsequently by our group and others [
      • Abraldes J.G.
      • Rodriguez-Vilarrupla A.
      • Graupera M.
      • Zafra C.
      • Garcia-Caldero H.
      • Garcia-Pagan J.C.
      • et al.
      Simvastatin treatment improves liver sinusoidal endothelial dysfunction in CCl(4) cirrhotic rats.
      ,
      • Trebicka J.
      • Hennenberg M.
      • Laleman W.
      • Shelest N.
      • Biecker E.
      • Schepke M.
      • et al.
      Atorvastatin lowers portal pressure in cirrhotic rats by inhibition of RhoA/Rho-kinase and activation of endothelial nitric oxide synthase.
      ,
      • Trebicka J.
      • Hennenberg M.
      • Odenthal M.
      • Shir K.
      • Klein S.
      • Granzow M.
      • et al.
      Atorvastatin attenuates hepatic fibrosis in rats after bile duct ligation via decreased turnover of hepatic stellate cells.
      ,
      • Gracia-Sancho J.
      • Russo L.
      • Garcia-Caldero H.
      • Garcia-Pagan J.C.
      • Garcia-Cardena G.
      • Bosch J.
      Endothelial expression of transcription factor Kruppel-like factor 2 and its vasoprotective target genes in the normal and cirrhotic rat liver.
      ,
      • Klein S.
      • Klosel J.
      • Schierwagen R.
      • Korner C.
      • Granzow M.
      • Huss S.
      • et al.
      Atorvastatin inhibits proliferation and apoptosis, but induces senescence in hepatic myofibroblasts and thereby attenuates hepatic fibrosis in rats.
      ,
      • Kumar S.
      • Grace N.D.
      • Qamar A.A.
      Statin use in patients with cirrhosis: a retrospective cohort study.
      ,
      • Souk K.
      • Al-Badri M.
      • Azar S.T.
      The safety and benefit of statins in liver cirrhosis: a review.
      ], some of the mechanisms through which statins have beneficial effects on dysfunctional sinusoidal cells have been described. We observed that simvastatin is the optimal statin [
      • Marrone G.
      • Russo L.
      • Rosado E.
      • Hide D.
      • Garcia-Cardena G.
      • Garcia-Pagan J.C.
      • et al.
      The transcription factor KLF2 mediates hepatic endothelial protection and paracrine endothelial-stellate cell deactivation induced by statins.
      ] used to increase NO bioavailability selectively in the liver [
      • Abraldes J.G.
      • Rodriguez-Vilarrupla A.
      • Graupera M.
      • Zafra C.
      • Garcia-Caldero H.
      • Garcia-Pagan J.C.
      • et al.
      Simvastatin treatment improves liver sinusoidal endothelial dysfunction in CCl(4) cirrhotic rats.
      ]. Its anti-oxidant, anti-inflammatory, vasodilatory and anti-fibrotic effects are due to KLF2 activity [
      • Marrone G.
      • Maeso-Díaz R.
      • Garcia-Cardena G.
      • Garcia-Pagan J.C.
      • Bosch J.
      • Gracia-Sancho J.
      KLF2 exerts anti-fibrotic and vasoprotective effects in cirrhotic rat livers: behind the molecular mechanisms of statins.
      ,
      • Bosch J.
      • Forns X.
      Therapy. Statins and liver disease: from concern to ’wonder’ drugs?.
      ]. Interestingly, results from a recently published placebo-controlled randomized clinical trial demonstrate that simvastatin reduces mortality in patients with cirrhosis who have survived an episode of variceal haemorrhage [
      • Abraldes J.G.
      • Villanueva C.
      • Aracil C.
      • Turnes J.
      • Hernandez-Guerra M.
      • Genesca J.
      • et al.
      Addition of simvastatin to standard therapy for the prevention of variceal rebleeding does not reduce rebleeding but increases survival in patients with cirrhosis.
      ]. Other NO donors have undesired systemic effects that prevent their use at the bedside [
      • Bellis L.
      • Berzigotti A.
      • Abraldes J.G.
      • Moitinho E.
      • Garcia-Pagan J.C.
      • Bosch J.
      • et al.
      Low doses of isosorbide mononitrate attenuate the postprandial increase in portal pressure in patients with cirrhosis.
      ,
      • Fiorucci S.
      • Antonelli E.
      • Brancaleone V.
      • Sanpaolo L.
      • Orlandi S.
      • Distrutti E.
      • et al.
      NCX-1000, a nitric oxide-releasing derivative of ursodeoxycholic acid, ameliorates portal hypertension and lowers norepinephrine-induced intrahepatic resistance in the isolated and perfused rat liver.
      ,
      • Bosch J.
      Decreasing hepatic vascular tone by liver-specific NO donors: wishful thinking or a promising reality?.
      ,
      • Berzigotti A.
      • Bellot P.
      • DeGottardi A.
      • Garcia-Pagan J.C.
      • Gagnon C.
      • Spenard J.
      • et al.
      NCX-1000, a nitric oxide-releasing derivative of UDCA, does not decrease portal pressure in patients with cirrhosis: results of a randomized, double-blind, dose-escalating study.
      ], but simvastatin, an already established medication, appears to be a good anti-fibrotic candidate due to its simultaneous effects on different hepatic cells (Fig. 2). This potentially occurs through the ability of simvastatin to activate the appropriate molecules, such as KLF2, needed for cell communication [
      • Ray K.
      Liver: Sussing out statins in cirrhosis–KLF2 is the key.
      ,
      • Hide D.
      • Ortega-Ribera M.
      • Garcia-Pagan J.C.
      • Peralta C.
      • Bosch J.
      • Gracia-Sancho J.
      Effects of warm ischemia and reperfusion on the liver microcirculatory phenotype of rats: underlying mechanisms and pharmacological therapy.
      ].
      Figure thumbnail gr2
      Fig. 2Effects of statins on sinusoidal cells. Summary of cellular and molecular mechanisms underlying the beneficial effects of statins in liver cells (in blue, modifications due to statins administration). Please note that to simplify the figure, paracrine interactions between statin-improved cells are intentionally omitted. Complete explanation can be found in the text. α-SMA, smooth muscle actin alpha; Arg-1, arginase-1; CAM, cell adhesion molecules; cGMP, cyclic guanosine monophosphate; COX-1, cyclooxygenase-1; eNOS, endothelial nitric oxide synthase; HNF4a, hepatocyte nuclear factor 4 alpha; KLF2, Kruppel-like factor 2; MLCP, myosin light chain phosphatase; Mrp3, ATP-binding cassette sub-family C member 3; NFkB, nuclear factor kappa-light-chain-enhancer of activated B cells; NO, nitric oxide; NOX, NADPH oxidase; Nrf2, nuclear factor (erythroid-derived 2)-like 2; ROS, radical oxygen species; TM, thrombomodulin; TXA2, thromboxane A2.
      Additionally, recent data support the concept that sinusoidal communication not only occurs via the release of soluble molecules but also through the liberation of cellular microvesicles. Two papers describe the importance of this type of paracrine interactions in cirrhosis and portal hypertension [
      • Lemoinne S.
      • Cadoret A.
      • Rautou P.E.
      • El M.H.
      • Ratziu V.
      • Corpechot C.
      • et al.
      Portal myofibroblasts promote vascular remodeling underlying cirrhosis formation through the release of microparticles.
      ,
      • Wang R.
      • Ding Q.
      • Yaqoob U.
      • de Assuncao T.M.
      • Verma V.K.
      • Hirsova P.
      • et al.
      Exosome adherence and internalization by hepatic stellate cells triggers sphingosine 1-phosphate-dependent migration.
      ]. In the first study, using in vitro co-culture models of PMF and endothelial cells, authors describe the release of microparticles rich in VEGF by PMF, which in turn promote an increase in the pro-angiogenic activity of endothelial cells. The second study further supports the exosomal sinusoidal paracrine communication, demonstrating that secretion of exosome-packaged Sphingosine kinase-1 by endothelial cells paracrinally act on HSC, promoting their activation (in terms of elevated migration). These papers add key information to our understanding of sinusoidal communications, opening new avenues of research.
      In summary, findings confirm that LSEC act as “protective gatekeepers” of the microvascular milieu, and keeping LSEC in a healthy condition is an acute strategy for the development of new hepatoprotective drugs. Additionally, therapies aimed at controlling KC or HSC activation may also prevent the development of hepatic cirrhosis, probably in part due to the re-activation of proper hepatic crosstalk. To date, various drugs with anti-inflammatory or anti-oxidant properties have shown promising results on fibrosis at the benchside, but limited outcomes in clinical trials [
      • Nelson D.R.
      • Tu Z.
      • Soldevila-Pico C.
      • Abdelmalek M.
      • Zhu H.
      • Xu Y.L.
      • et al.
      Long-term interleukin 10 therapy in chronic hepatitis C patients has a proviral and anti-inflammatory effect.
      ,
      • Lebrec D.
      • Thabut D.
      • Oberti F.
      • Perarnau J.M.
      • Condat B.
      • Barraud H.
      • et al.
      Pentoxifylline does not decrease short-term mortality but does reduce complications in patients with advanced cirrhosis.
      ,
      • Van Wagner L.B.
      • Koppe S.W.
      • Brunt E.M.
      • Gottstein J.
      • Gardikiotes K.
      • Green R.M.
      • et al.
      Pentoxifylline for the treatment of non-alcoholic steatohepatitis: a randomized controlled trial.
      ,
      • Scorletti E.
      • Bhatia L.
      • McCormick K.G.
      • Clough G.F.
      • Nash K.
      • Hodson L.
      • et al.
      Effects of purified eicosapentaenoic and docosahexaenoic acids in nonalcoholic fatty liver disease: results from the Welcome∗ study.
      ,
      • Reverter E.
      • Mesonero F.
      • Seijo S.
      • Martinez J.
      • Abraldes J.G.
      • Penas B.
      • et al.
      Effects of sapropterin on portal and systemic hemodynamics in patients with cirrhosis and portal hypertension: a bicentric double-blind placebo-controlled study.
      ], likely because of unknown effects on cellular communication. Future studies are needed to discover new mechanisms of cellular crosstalk to determine at which stage of cirrhosis progression these interactions occur. The mechanism of microvesicle transport could potentially be productive in this regard.

      Sinusoidal crosstalk in liver regeneration and transplantation

      Parenchymal and non-parenchymal cells tightly regulate their proliferation. Sinusoidal cells are a key source of cytokines and growth factors needed by the hepatocytes, and sinusoidal cells growth is modulated by hepatocytes and other non-parenchymal cells [
      • Ding B.S.
      • Nolan D.J.
      • Butler J.M.
      • James D.
      • Babazadeh A.O.
      • Rosenwaks Z.
      • et al.
      Inductive angiocrine signals from sinusoidal endothelium are required for liver regeneration.
      ,
      • Ding B.S.
      • Cao Z.
      • Lis R.
      • Nolan D.J.
      • Guo P.
      • Simons M.
      • et al.
      Divergent angiocrine signals from vascular niche balance liver regeneration and fibrosis.
      ]. The maintenance of these intercellular relationships is essential to promote the replacement of missing hepatocytes during liver regeneration, a process that occurs more frequently after ischaemic or toxic injury. When the liver is damaged beyond its ability to regenerate, liver transplantation is the treatment of choice. Living-donor transplantation, bioengineered liver support systems or patient-specific hepatic cell transplantation are current alternatives to whole liver transplantation. Understanding the effects of sinusoidal cells on hepatocytes phenotype will help clarify sequalae associated with liver injury due to the ischemia/reperfusion occurring during hepatic resection and liver transplantation.

      Liver regeneration – classic and new views

      Most of the data regarding liver regeneration derives from experimental partial hepatectomy, which is the accepted animal model to mimic human major hepatic resection. However, the positive results obtained in the laboratory are difficult to successfully translate to clinical therapies probably because the regenerative program in humans differs greatly following damage by drug overdose, viral infection, or excessive alcohol consumption [
      • Mao S.A.
      • Glorioso J.M.
      • Nyberg S.L.
      Liver regeneration.
      ].
      In response to hepatectomy, liver sinusoidal and bone marrow-derived stem cells orchestrate a precise response to promote hepatocyte regeneration. VEGF plays a key role in such process.
      The initiation of liver regeneration after conventional two-thirds partial hepatectomy appears macrophage driven (Fig. 3). Peak macrophage proliferative response is at 48–72 h [
      • Widmann J.J.
      • Fahimi H.D.
      Proliferation of mononuclear phagocytes (Kupffer cells) and endothelial cells in regenerating rat liver. A light and electron microscopic cytochemical study.
      ], at which point they begin to communicate with hepatocytes mostly through the production of IL-6 [
      • Abshagen K.
      • Eipel C.
      • Kalff J.C.
      • Menger M.D.
      • Vollmar B.
      Loss of NF-kappaB activation in Kupffer cell-depleted mice impairs liver regeneration after partial hepatectomy6.
      ,
      • Xu C.S.
      • Jiang Y.
      • Zhang L.X.
      • Chang C.F.
      • Wang G.P.
      • Shi R.J.
      • et al.
      The role of Kupffer cells in rat liver regeneration revealed by cell-specific microarray analysis.
      ]. LSEC and HSC divide approximately 96 h after partial hepatectomy, primarily due to hepatocyte-derived VEGF/angiopoietin stimulation, and proteins relying on phosphoinositide 3-kinase for their mitogenic effect, respectively [
      • Taniguchi E.
      • Sakisaka S.
      • Matsuo K.
      • Tanikawa K.
      • Sata M.
      Expression and role of vascular endothelial growth factor in liver regeneration after partial hepatectomy in rats.
      ,
      • Sato T.
      • El-Assal O.N.
      • Ono T.
      • Yamanoi A.
      • Dhar D.K.
      • Nagasue N.
      Sinusoidal endothelial cell proliferation and expression of angiopoietin/Tie family in regenerating rat liver.
      ,
      • Mabuchi A.
      • Mullaney I.
      • Sheard P.W.
      • Hessian P.A.
      • Mallard B.L.
      • Tawadrous M.N.
      • et al.
      Role of hepatic stellate cell/hepatocyte interaction and activation of hepatic stellate cells in the early phase of liver regeneration in the rat.
      ]. There is an evident cooperation among non-parenchymal and parenchymal cells during regeneration [
      • Roskams T.
      Relationships among stellate cell activation, progenitor cells, and hepatic regeneration.
      ], however, the idea that bone marrow-derived endothelial sinusoidal progenitor cells (BM SPC), rich in hepatocyte growth factor (HGF), are also recruited to the site of the injury is now emerging [
      • Wang L.
      • Wang X.
      • Wang L.
      • Chiu J.D.
      • Van de Ven G.
      • Gaarde W.A.
      • et al.
      Hepatic vascular endothelial growth factor regulates recruitment of rat liver sinusoidal endothelial cell progenitor cells.
      ,
      • DeLeve L.D.
      Liver sinusoidal endothelial cells and liver regeneration.
      ]. Recently, Katagiri and colleagues observed that a small representation (1–2%) of bone marrow mesenchymal stem cells, named Muse, differentiate into liver-lineage cells and repair tissue. After their infusion in a model of partial hepatectomy, they integrated into regenerating areas and expressed liver progenitor markers during the early phase of regeneration to then differentiate into hepatocytes, KC and LSEC [
      • Katagiri H.
      • Kushida Y.
      • Nojima M.
      • Kuroda Y.
      • Wakao S.
      • Ishida K.
      • et al.
      A distinct subpopulation of bone marrow mesenchymal stem cells, muse cells, directly commit to the replacement of liver components.
      ]. Muse cells can be obtained easily from patients and donors, and could be used for clinical applications. Their differentiation into a specific hepatic cell may depend on cell-matrix-cell interactions; reinforcing the concept that liver matrix is much more than a simple scaffold.
      Figure thumbnail gr3
      Fig. 3Sinusoidal crosstalk during liver regeneration. Upon hepatectomy, initial signals (mainly IL-6) from Kupffer cells (KC), macrophages and other non-myeloid cells lead to the first burst of hepatocytes proliferation, which in turn will produce vascular endothelial growth factor (VEGF) to recruit bone marrow-derived sinusoidal progenitor cells (BM SPC) as a key source of hepatocyte growth factor (HGF). Latterly, hepatic stellate cells (HSC) and liver sinusoidal endothelial cells (LSEC) contribute to the regeneration process.

      Role of LSEC in liver repopulation

      Improved understanding of the cellular crosstalk occurring during liver regeneration, both in space and time, may provide new strategies to promote liver repopulation. In the experimental field of hepatocyte transplantation, Gupta and colleagues observed that cell engraftment in the liver involves the disruption of endothelial structures, entry of transplanted cells into the liver plates, reconstitution of plasma membrane structures with restoration of cell polarity, and integration of cells in the liver parenchyma [
      • Gupta S.
      • Rajvanshi P.
      • Sokhi R.
      • Slehria S.
      • Yam A.
      • Kerr A.
      • et al.
      Entry and integration of transplanted hepatocytes in rat liver plates occur by disruption of hepatic sinusoidal endothelium.
      ]. Interestingly, transplanted and host hepatocytes produce VEGF before the disruption of the endothelial layer, reaching a peak around 8 h post hepatocyte injection, but totally disappearing after 24 h. These observations correlate with more recent work in which loss of LSEC viability occurs after 24 h of liver injury, but before hepatocyte necrosis, suggesting that a VEGF-regulated engraftment of BM SPC, an alternative source of HGF, may latterly stimulate hepatocyte proliferation and liver regeneration [
      • Wang L.
      • Wang X.
      • Xie G.
      • Wang L.
      • Hill C.K.
      • DeLeve L.D.
      Liver sinusoidal endothelial cell progenitor cells promote liver regeneration in rats.
      ]. Certainly, VEGF knockdown significantly decreases partial hepatectomy-induced BM SPC proliferation and engraftment to the liver [
      • Wang L.
      • Wang X.
      • Wang L.
      • Chiu J.D.
      • Van de Ven G.
      • Gaarde W.A.
      • et al.
      Hepatic vascular endothelial growth factor regulates recruitment of rat liver sinusoidal endothelial cell progenitor cells.
      ]. These studies suggest that loss of LSEC functionality seems to be the input signalling in the bone marrow-liver crosstalk, which will promote the release of BM SPC, regeneration of the liver, and at a very final step reconstitution of LSEC structures will occur.
      The role of VEGF in the healthy and diseased liver is ambiguous, however, its relevance in liver regeneration was hypothesized two decades ago [
      • Mochida S.
      • Ishikawa K.
      • Inao M.
      • Shibuya M.
      • Fujiwara K.
      Increased expressions of vascular endothelial growth factor and its receptors, flt-1 and KDR/flk-1, in regenerating rat liver.
      ], and refined more recently when it was demonstrated that VEGF promotes proliferation of hepatocytes through the reconstruction of the liver sinusoids by proliferation of sinusoidal endothelial cells [
      • Taniguchi E.
      • Sakisaka S.
      • Matsuo K.
      • Tanikawa K.
      • Sata M.
      Expression and role of vascular endothelial growth factor in liver regeneration after partial hepatectomy in rats.
      ]. A recent paper by Hu and colleagues highlighted the underlying mechanisms involved in the space-temporal regulation of cellular crosstalk during liver regeneration [
      • Hu J.
      • Srivastava K.
      • Wieland M.
      • Runge A.
      • Mogler C.
      • Besemfelder E.
      • et al.
      Endothelial cell-derived angiopoietin-2 controls liver regeneration as a spatiotemporal rheostat.
      ]. They demonstrated how LSEC-angiopoetin 2 expression is dynamically regulated during the regeneration process to promote hepatocytes proliferation during the early phase, and to stimulate angiogenesis during the late phase. In this new vision of the regeneration puzzle, the exact role of the previously described BM SPC remains to be defined. Nevertheless, VEGF has to be considered as the signalling molecule par excellence involved in the hepatic intercellular crosstalk occurring during liver regeneration.

      Other non-parenchymal cells in liver regeneration

      Gupta et al. investigated whether transplanted hepatocytes could additionally interact with HSC, aware that cell-to-cell interactions could modulate their engraftment in the liver [
      • Benten D.
      • Kumaran V.
      • Joseph B.
      • Schattenberg J.
      • Popov Y.
      • Schuppan D.
      • et al.
      Hepatocyte transplantation activates hepatic stellate cells with beneficial modulation of cell engraftment in the rat.
      ]. This study demonstrated that HSC (as well as KC) become activated after only 24 h following hepatocyte transplantation, reaching a peak after 3 days. Interestingly, activated cells were in the immediate proximity of transplanted hepatocytes, suggesting they could interact with each other. Additionally, but perhaps counterintuitively, HSC and KC contributed to correct cell engraftment. Future works aimed to elucidate the exact role of every other non-parenchymal cell, in space and time, during the communication process that occurs in liver regeneration and transplantation will be of significant interest.

      Conclusions and future research

      Here, we have summarised the current knowledge about the sinusoidal crosstalk occurring in two pathological situations with similar cellular and molecular mechanisms, but with a significant difference: liver regeneration after hepatectomy progresses without signs of fibrosis, but liver repair in response to chronic liver injury is associated with fibrosis. Although research and knowledge has increased in recent years, our understating of these processes is still far from complete. Further investigation is required, particularly in cirrhosis, where we still do not know the exact spatial-temporal sinusoidal communications during the progression and [importantly] regression of the disease, especially considering the probable aetiology dependent differences. Improved knowledge of these essential processes would help develop efficient strategies to promote fibrosis resolution. The most recent studies in hepatic regeneration biology proposed new, but possibly incompatible, explanations of crosstalk and proliferation of hepatic cells in response to hepatectomy [
      • Hu J.
      • Srivastava K.
      • Wieland M.
      • Runge A.
      • Mogler C.
      • Besemfelder E.
      • et al.
      Endothelial cell-derived angiopoietin-2 controls liver regeneration as a spatiotemporal rheostat.
      ,
      • Fernandez-Mena C.
      • Almagro J.
      • Puerro M.
      • Quintana A.
      • Hidalgo J.
      • Bañares R.
      • et al.
      Effects of myeloid cell-selective deficiency of IL-6 for liver regeneration after partial hepatectomy (PH) in mice.
      ], and therefore further validation is required.
      In addition to disease-specific limitations, we should consider two important points that need to be addressed. Firstly, the possible experimental discrepancies between research groups (i.e., experimental models of hepatectomy or fibrosis induction), which may lead to divergent conclusions, and therefore inadvertently protract the development of proficient therapeutic strategies. Secondly, most data are obtained using in vitro methods of cell culturing, which may differ greatly to actual pathological mechanisms. For instance, sinusoidal cells rapidly de-differentiate after isolation, require specific matrices, and the endothelial layer needs biomechanical stimulation. New methods are needed to better explore the mechanisms underlying physiological and pathophysiological intercellular crosstalk. In this sense, the newest “liver on a chip” platforms [
      • Illa X.
      • Vila S.
      • Yeste J.
      • Peralta C.
      • Gracia-Sancho J.
      • Villa R.
      A novel modular bioreactor to in vitro study the hepatic sinusoid.
      ,
      • Prodanov L.
      • Jindal R.
      • Bale S.S.
      • Hegde M.
      • McCarty W.J.
      • Golberg I.
      • et al.
      Long-term maintenance of a microfluidic 3D human liver sinusoid.
      ] may have the potential to reproduce the exact liver-like micro network, taking into account the importance of hemodynamic and mechanical forces, and the liver architecture.
      In conclusion, acquiring more data inherent to intercellular crosstalk will certainly improve knowledge of the progression of liver disease, allowing the creation of in vitro models for the development of therapeutic strategies.

      Financial support

      This work was supported by grants FIS PI14/00029 and Explora BIO2014-61377 from the Instituto de Salud Carlos III , Funds FEDER “ una manera de hacer Europa ”, and Ministerio de Economía y Competitividad , Spain (Gracia-Sancho), R01 AA021171 & R01 DK59615 from the National Institutes of Health , USA (Shah), and the Sheila Sherlock Postdoctoral Fellowship from the European Association for the Study of the Liver (Marrone).

      Conflict of interest

      The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.

      Authors’ contributions

      G.M. literature search, writing and revision of the manuscript, and figure illustrations. V.H.S. conception of the paper and critically revision of the manuscript. J.G.-S. conception of the paper, and writing and revision of the manuscript.

      Acknowledgements

      Authors acknowledge Tom Shepherd (UCL) for the throughout revision of the manuscript.

      References

        • Braet F.
        • Wisse E.
        Structural and functional aspects of liver sinusoidal endothelial cell fenestrae: a review.
        Comp Hepatol. 2002; 1: 1
        • Cogger V.C.
        • Warren A.
        • Fraser R.
        • Ngu M.
        • McLean A.J.
        • le Couteur D.G.
        Hepatic sinusoidal pseudocapillarization with aging in the non-human primate.
        Exp Gerontol. 2003; 38: 1101-1107
        • McLean A.J.
        • Cogger V.C.
        • Chong G.C.
        • Warren A.
        • Markus A.M.
        • Dahlstrom J.E.
        • et al.
        Age-related pseudocapillarization of the human liver.
        J Pathol. 2003; 200: 112-117
        • Schaffner F.
        • Poper H.
        Capillarization of hepatic sinusoids in man.
        Gastroenterology. 1963; 44: 239-242
        • Urashima S.
        • Tsutsumi M.
        • Nakase K.
        • Wang J.S.
        • Takada A.
        Studies on capillarization of the hepatic sinusoids in alcoholic liver disease.
        Alcohol Alcohol Suppl. 1993; 1B: 77-84
        • Xu B.
        • Broome U.
        • Uzunel M.
        • Nava S.
        • Ge X.
        • Kumagai-Braesch M.
        • et al.
        Capillarization of hepatic sinusoid by liver endothelial cell-reactive autoantibodies in patients with cirrhosis and chronic hepatitis.
        Am J Pathol. 2003; 163: 1275-1289
        • Geraud C.
        • Evdokimov K.
        • Straub B.K.
        • Peitsch W.K.
        • Demory A.
        • Dorflinger Y.
        • et al.
        Unique cell type-specific junctional complexes in vascular endothelium of human and rat liver sinusoids.
        PLoS One. 2012; 7e34206
        • Essani N.A.
        • McGuire G.M.
        • Manning A.M.
        • Jaeschke H.
        Differential induction of mRNA for ICAM-1 and selectins in hepatocytes, Kupffer cells and endothelial cells during endotoxemia.
        Biochem Biophys Res Commun. 1995; 211: 74-82
        • DeLeve L.D.
        • Wang X.
        • Hu L.
        • McCuskey M.K.
        • McCuskey R.S.
        Rat liver sinusoidal endothelial cell phenotype is maintained by paracrine and autocrine regulation.
        Am J Physiol Gastrointest Liver Physiol. 2004; 287: G757-G763
        • Friedman S.L.
        • Roll F.J.
        • Boyles J.
        • Bissell D.M.
        Hepatic lipocytes: the principal collagen-producing cells of normal rat liver.
        Proc Natl Acad Sci U S A. 1985; 82: 8681-8685
        • Sato M.
        • Suzuki S.
        • Senoo H.
        Hepatic stellate cells: unique characteristics in cell biology and phenotype.
        Cell Struct Funct. 2003; 28: 105-112
        • Pinzani M.
        • Gentilini P.
        Biology of hepatic stellate cells and their possible relevance in the pathogenesis of portal hypertension in cirrhosis.
        Semin Liver Dis. 1999; 19: 397-410
        • Wake K.
        Structure of the sinusoidal wall in the liver.
        in: Wisse E. Knook D.L. Wake K. Cells of the hepatic sinusoid. The Kupffer Cell Foundation, Leiden1995: 241-246
        • Bilzer M.
        • Roggel F.
        • Gerbes A.L.
        Role of Kupffer cells in host defense and liver disease.
        Liver Int. 2006; 26: 1175-1186
        • Liaskou E.
        • Wilson D.V.
        • Oo Y.H.
        Innate immune cells in liver inflammation.
        Mediators Inflamm. 2012; 2012: 949157
        • McCuskey R.S.
        Morphological mechanisms for regulating blood flow through hepatic sinusoids.
        Liver. 2000; 20: 3-7
        • Malhi H.
        • Guicciardi M.E.
        • Gores G.J.
        Hepatocyte death: a clear and present danger.
        Physiol Rev. 2010; 90: 1165-1194
        • Wells R.G.
        The role of matrix stiffness in regulating cell behavior.
        Hepatology. 2008; 47: 1394-1400
        • March S.
        • Hui E.E.
        • Underhill G.H.
        • Khetani S.
        • Bhatia S.N.
        Microenvironmental regulation of the sinusoidal endothelial cell phenotype in vitro.
        Hepatology. 2009; 50: 920-928
        • Friedman S.L.
        Mechanisms of disease: Mechanisms of hepatic fibrosis and therapeutic implications.
        Nat Clin Pract Gastroenterol Hepatol. 2004; 1: 98-105
        • Pinzani M.
        • Rosselli M.
        • Zuckermann M.
        Liver cirrhosis.
        Best Pract Res Clin Gastroenterol. 2011; 25: 281-290
        • Garcia-Pagan J.C.
        • Gracia-Sancho J.
        • Bosch J.
        Functional aspects on the pathophysiology of portal hypertension in cirrhosis.
        J Hepatol. 2012; 57: 458-461
        • Gracia-Sancho J.
        • Lavina B.
        • Rodriguez-Vilarrupla A.
        • Garcia-Caldero H.
        • Bosch J.
        • Garcia-Pagan J.C.
        Enhanced vasoconstrictor prostanoid production by sinusoidal endothelial cells increases portal perfusion pressure in cirrhotic rat livers.
        J Hepatol. 2007; 47: 220-227
        • Steib C.J.
        • Gerbes A.L.
        • Bystron M.
        • op den Winkel M.
        • Hartl J.
        • Roggel F.
        • et al.
        Kupffer cell activation in normal and fibrotic livers increases portal pressure via thromboxane A(2).
        J Hepatol. 2007; 47: 228-238
        • Said A.
        • Lucey M.R.
        Liver transplantation: an update 2008.
        Curr Opin Gastroenterol. 2008; 24: 339-345
        • DeLeve L.D.
        The hepatic sinusoidal endothelial cell: morphology, function, and pathobiology.
        in: Arias I.M. The liver: biology and pathobiology. 5th ed. Wiley & Sons, Hokoben2009: 371-388
        • Jarnagin W.R.
        • Rockey D.C.
        • Koteliansky V.E.
        • Wang S.S.
        • Bissell D.M.
        Expression of variant fibronectins in wound healing: cellular source and biological activity of the EIIIA segment in rat hepatic fibrogenesis.
        J Cell Biol. 1994; 127: 2037-2048
        • Canbay A.
        • Feldstein A.E.
        • Higuchi H.
        • Werneburg N.
        • Grambihler A.
        • Bronk S.F.
        • et al.
        Kupffer cell engulfment of apoptotic bodies stimulates death ligand and cytokine expression.
        Hepatology. 2003; 38: 1188-1198
        • Jiang J.X.
        • Mikami K.
        • Venugopal S.
        • Li Y.
        • Torok N.J.
        Apoptotic body engulfment by hepatic stellate cells promotes their survival by the JAK/STAT and Akt/NF-kappaB-dependent pathways.
        J Hepatol. 2009; 51: 139-148
        • McGuire R.F.
        • Bissell D.M.
        • Boyles J.
        • Roll F.J.
        Role of extracellular matrix in regulating fenestrations of sinusoidal endothelial cells isolated from normal rat liver.
        Hepatology. 1992; 15: 989-997
        • Yamane A.
        • Seetharam L.
        • Yamaguchi S.
        • Gotoh N.
        • Takahashi T.
        • Neufeld G.
        • et al.
        A new communication system between hepatocytes and sinusoidal endothelial cells in liver through vascular endothelial growth factor and Flt tyrosine kinase receptor family (Flt-1 and KDR/Flk-1).
        Oncogene. 1994; 9: 2683-2690
        • Zhang D.W.
        • Zhao Y.X.
        • Wei D.
        • Li Y.L.
        • Zhang Y.
        • Wu J.
        • et al.
        HAb18G/CD147 promotes activation of hepatic stellate cells and is a target for antibody therapy of liver fibrosis.
        J Hepatol. 2012; 57: 1283-1291
        • Yan Z.
        • Qu K.
        • Zhang J.
        • Huang Q.
        • Qu P.
        • Xu X.
        • et al.
        CD147 promotes liver fibrosis progression via VEGF-A/VEGFR2 signalling-mediated cross-talk between hepatocytes and sinusoidal endothelial cells.
        Clin Sci (Lond). 2015; 129: 699-710
        • Di Pascoli M.
        • Divi M.
        • Rodriguez-Vilarrupla A.
        • Rosado E.
        • Gracia-Sancho J.
        • Vilaseca M.
        • et al.
        Resveratrol improves intrahepatic endothelial dysfunction and reduces hepatic fibrosis and portal pressure in cirrhotic rats.
        J Hepatol. 2013; 58: 904-910
        • Guillaume M.
        • Rodriguez-Vilarrupla A.
        • Gracia-Sancho J.
        • Rosado E.
        • Mancini A.
        • Bosch J.
        • et al.
        Recombinant human manganese superoxide dismutase reduces liver fibrosis and portal pressure in CCl4-cirrhotic rats.
        J Hepatol. 2013; 58: 240-246
        • Bannert K.
        • Kuhla A.
        • Abshagen K.
        • Vollmar B.
        Anti-apoptotic therapeutic approaches in liver diseases: do they really make sense?.
        Apoptosis. 2014; 19: 1243-1253
        • Wells R.G.
        Cellular sources of extracellular matrix in hepatic fibrosis.
        Clin Liver Dis. 2008; 12 (viii): 759-768
        • Myers P.R.
        • Tanner M.A.
        Vascular endothelial cell regulation of extracellular matrix collagen: role of nitric oxide.
        Arterioscler Thromb Vasc Biol. 1998; 18: 717-722
        • Ford A.J.
        • Jain G.
        • Rajagopalan P.
        Designing a fibrotic microenvironment to investigate changes in human liver sinusoidal endothelial cell function.
        Acta Biomater. 2015; 24: 220-227
        • Li Z.
        • Dranoff J.A.
        • Chan E.P.
        • Uemura M.
        • Sevigny J.
        • Wells R.G.
        Transforming growth factor-beta and substrate stiffness regulate portal fibroblast activation in culture.
        Hepatology. 2007; 46: 1246-1256
        • Olsen A.L.
        • Bloomer S.A.
        • Chan E.P.
        • Gaca M.D.
        • Georges P.C.
        • Sackey B.
        • et al.
        Hepatic stellate cells require a stiff environment for myofibroblastic differentiation.
        Am J Physiol Gastrointest Liver Physiol. 2011; 301: G110-G118
        • Gortzen J.
        • Schierwagen R.
        • Bierwolf J.
        • Klein S.
        • Uschner F.E.
        • van der Ven P.F.
        • et al.
        Interplay of matrix stiffness and c-SRC in hepatic fibrosis.
        Front Physiol. 2015; 6: 359
        • Halfon P.
        • Bourliere M.
        • Penaranda G.
        • Deydier R.
        • Renou C.
        • Botta-Fridlund D.
        • et al.
        Accuracy of hyaluronic acid level for predicting liver fibrosis stages in patients with hepatitis C virus.
        Comp Hepatol. 2005; 4: 6
        • Tangkijvanich P.
        • Kongtawelert P.
        • Pothacharoen P.
        • Mahachai V.
        • Suwangool P.
        • Poovorawan Y.
        Serum hyaluronan: a marker of liver fibrosis in patients with chronic liver disease.
        Asian Pac J Allergy Immunol. 2003; 21: 115-120
        • Deaciuc I.V.
        • Bagby G.J.
        • Lang C.H.
        • Spitzer J.J.
        Hyaluronic acid uptake by the isolated, perfused rat liver: an index of hepatic sinusoidal endothelial cell function.
        Hepatology. 1993; 17: 266-272
        • Muriel P.
        • Escobar Y.
        Kupffer cells are responsible for liver cirrhosis induced by carbon tetrachloride.
        J Appl Toxicol. 2003; 23: 103-108
        • Nieto N.
        Oxidative-stress and IL-6 mediate the fibrogenic effects of [corrected] Kupffer cells on stellate cells.
        Hepatology. 2006; 44: 1487-1501
        • Cubero F.J.
        • Nieto N.
        Ethanol and arachidonic acid synergize to activate Kupffer cells and modulate the fibrogenic response via tumor necrosis factor alpha, reduced glutathione, and transforming growth factor beta-dependent mechanisms.
        Hepatology. 2008; 48: 2027-2039
        • Castera L.
        • Pinzani M.
        • Bosch J.
        Non invasive evaluation of portal hypertension using transient elastography.
        J Hepatol. 2012; 56: 696-703
        • DeLeve L.D.
        • Wang X.
        • Guo Y.
        Sinusoidal endothelial cells prevent rat stellate cell activation and promote reversion to quiescence.
        Hepatology. 2008; 48: 920-930
        • Xie G.
        • Wang X.
        • Wang L.
        • Wang L.
        • Atkinson R.D.
        • Kanel G.C.
        • et al.
        Role of differentiation of liver sinusoidal endothelial cells in progression and regression of hepatic fibrosis in rats.
        Gastroenterology. 2012; 142: 918-927
        • Marrone G.
        • Russo L.
        • Rosado E.
        • Hide D.
        • Garcia-Cardena G.
        • Garcia-Pagan J.C.
        • et al.
        The transcription factor KLF2 mediates hepatic endothelial protection and paracrine endothelial-stellate cell deactivation induced by statins.
        J Hepatol. 2013; 58: 98-103
        • Marrone G.
        • Maeso-Díaz R.
        • Garcia-Cardena G.
        • Garcia-Pagan J.C.
        • Bosch J.
        • Gracia-Sancho J.
        KLF2 exerts anti-fibrotic and vasoprotective effects in cirrhotic rat livers: behind the molecular mechanisms of statins.
        Gut. 2015; 64: 1434-1443
        • Zafra C.
        • Abraldes J.G.
        • Turnes J.
        • Berzigotti A.
        • Fernandez M.
        • Garca-Pagan J.C.
        • et al.
        Simvastatin enhances hepatic nitric oxide production and decreases the hepatic vascular tone in patients with cirrhosis.
        Gastroenterology. 2004; 126: 749-755
        • Abraldes J.G.
        • Rodriguez-Vilarrupla A.
        • Graupera M.
        • Zafra C.
        • Garcia-Caldero H.
        • Garcia-Pagan J.C.
        • et al.
        Simvastatin treatment improves liver sinusoidal endothelial dysfunction in CCl(4) cirrhotic rats.
        J Hepatol. 2007; 46: 1040-1046
        • Trebicka J.
        • Hennenberg M.
        • Laleman W.
        • Shelest N.
        • Biecker E.
        • Schepke M.
        • et al.
        Atorvastatin lowers portal pressure in cirrhotic rats by inhibition of RhoA/Rho-kinase and activation of endothelial nitric oxide synthase.
        Hepatology. 2007; 46: 242-253
        • Trebicka J.
        • Hennenberg M.
        • Odenthal M.
        • Shir K.
        • Klein S.
        • Granzow M.
        • et al.
        Atorvastatin attenuates hepatic fibrosis in rats after bile duct ligation via decreased turnover of hepatic stellate cells.
        J Hepatol. 2010; 53: 702-712
        • Gracia-Sancho J.
        • Russo L.
        • Garcia-Caldero H.
        • Garcia-Pagan J.C.
        • Garcia-Cardena G.
        • Bosch J.
        Endothelial expression of transcription factor Kruppel-like factor 2 and its vasoprotective target genes in the normal and cirrhotic rat liver.
        Gut. 2011; 60: 517-524
        • Klein S.
        • Klosel J.
        • Schierwagen R.
        • Korner C.
        • Granzow M.
        • Huss S.
        • et al.
        Atorvastatin inhibits proliferation and apoptosis, but induces senescence in hepatic myofibroblasts and thereby attenuates hepatic fibrosis in rats.
        Lab Invest. 2012; 92: 1440-1450
        • Kumar S.
        • Grace N.D.
        • Qamar A.A.
        Statin use in patients with cirrhosis: a retrospective cohort study.
        Dig Dis Sci. 2014; 59: 1958-1965
        • Souk K.
        • Al-Badri M.
        • Azar S.T.
        The safety and benefit of statins in liver cirrhosis: a review.
        Exp Clin Endocrinol Diabetes. 2015; 123: 577-580
        • Bosch J.
        • Forns X.
        Therapy. Statins and liver disease: from concern to ’wonder’ drugs?.
        Nat Rev Gastroenterol Hepatol. 2015; 12: 320-321
        • Abraldes J.G.
        • Villanueva C.
        • Aracil C.
        • Turnes J.
        • Hernandez-Guerra M.
        • Genesca J.
        • et al.
        Addition of simvastatin to standard therapy for the prevention of variceal rebleeding does not reduce rebleeding but increases survival in patients with cirrhosis.
        Gastroenterology. 2016; 150: 1160-1170
        • Bellis L.
        • Berzigotti A.
        • Abraldes J.G.
        • Moitinho E.
        • Garcia-Pagan J.C.
        • Bosch J.
        • et al.
        Low doses of isosorbide mononitrate attenuate the postprandial increase in portal pressure in patients with cirrhosis.
        Hepatology. 2003; 37: 378-384
        • Fiorucci S.
        • Antonelli E.
        • Brancaleone V.
        • Sanpaolo L.
        • Orlandi S.
        • Distrutti E.
        • et al.
        NCX-1000, a nitric oxide-releasing derivative of ursodeoxycholic acid, ameliorates portal hypertension and lowers norepinephrine-induced intrahepatic resistance in the isolated and perfused rat liver.
        J Hepatol. 2003; 39: 932-939
        • Bosch J.
        Decreasing hepatic vascular tone by liver-specific NO donors: wishful thinking or a promising reality?.
        J Hepatol. 2003; 39: 1072-1075
        • Berzigotti A.
        • Bellot P.
        • DeGottardi A.
        • Garcia-Pagan J.C.
        • Gagnon C.
        • Spenard J.
        • et al.
        NCX-1000, a nitric oxide-releasing derivative of UDCA, does not decrease portal pressure in patients with cirrhosis: results of a randomized, double-blind, dose-escalating study.
        Am J Gastroenterol. 2009;
        • Ray K.
        Liver: Sussing out statins in cirrhosis–KLF2 is the key.
        Nat Rev Gastroenterol Hepatol. 2015; 12: 64
        • Hide D.
        • Ortega-Ribera M.
        • Garcia-Pagan J.C.
        • Peralta C.
        • Bosch J.
        • Gracia-Sancho J.
        Effects of warm ischemia and reperfusion on the liver microcirculatory phenotype of rats: underlying mechanisms and pharmacological therapy.
        Sci Rep. 2016; 6: 22107
        • Lemoinne S.
        • Cadoret A.
        • Rautou P.E.
        • El M.H.
        • Ratziu V.
        • Corpechot C.
        • et al.
        Portal myofibroblasts promote vascular remodeling underlying cirrhosis formation through the release of microparticles.
        Hepatology. 2015; 61: 1041-1055
        • Wang R.
        • Ding Q.
        • Yaqoob U.
        • de Assuncao T.M.
        • Verma V.K.
        • Hirsova P.
        • et al.
        Exosome adherence and internalization by hepatic stellate cells triggers sphingosine 1-phosphate-dependent migration.
        J Biol Chem. 2015; 290: 30684-30696
        • Nelson D.R.
        • Tu Z.
        • Soldevila-Pico C.
        • Abdelmalek M.
        • Zhu H.
        • Xu Y.L.
        • et al.
        Long-term interleukin 10 therapy in chronic hepatitis C patients has a proviral and anti-inflammatory effect.
        Hepatology. 2003; 38: 859-868
        • Lebrec D.
        • Thabut D.
        • Oberti F.
        • Perarnau J.M.
        • Condat B.
        • Barraud H.
        • et al.
        Pentoxifylline does not decrease short-term mortality but does reduce complications in patients with advanced cirrhosis.
        Gastroenterology. 2010; 138: 1755-1762
        • Van Wagner L.B.
        • Koppe S.W.
        • Brunt E.M.
        • Gottstein J.
        • Gardikiotes K.
        • Green R.M.
        • et al.
        Pentoxifylline for the treatment of non-alcoholic steatohepatitis: a randomized controlled trial.
        Ann Hepatol. 2011; 10: 277-286
        • Scorletti E.
        • Bhatia L.
        • McCormick K.G.
        • Clough G.F.
        • Nash K.
        • Hodson L.
        • et al.
        Effects of purified eicosapentaenoic and docosahexaenoic acids in nonalcoholic fatty liver disease: results from the Welcome∗ study.
        Hepatology. 2014; 60: 1211-1221
        • Reverter E.
        • Mesonero F.
        • Seijo S.
        • Martinez J.
        • Abraldes J.G.
        • Penas B.
        • et al.
        Effects of sapropterin on portal and systemic hemodynamics in patients with cirrhosis and portal hypertension: a bicentric double-blind placebo-controlled study.
        Am J Gastroenterol. 2015; 110: 985-992
        • Ding B.S.
        • Nolan D.J.
        • Butler J.M.
        • James D.
        • Babazadeh A.O.
        • Rosenwaks Z.
        • et al.
        Inductive angiocrine signals from sinusoidal endothelium are required for liver regeneration.
        Nature. 2010; 468: 310-315
        • Ding B.S.
        • Cao Z.
        • Lis R.
        • Nolan D.J.
        • Guo P.
        • Simons M.
        • et al.
        Divergent angiocrine signals from vascular niche balance liver regeneration and fibrosis.
        Nature. 2014; 505: 97-102
        • Mao S.A.
        • Glorioso J.M.
        • Nyberg S.L.
        Liver regeneration.
        Transl Res. 2014; 163: 352-362
        • Widmann J.J.
        • Fahimi H.D.
        Proliferation of mononuclear phagocytes (Kupffer cells) and endothelial cells in regenerating rat liver. A light and electron microscopic cytochemical study.
        Am J Pathol. 1975; 80: 349-366
        • Abshagen K.
        • Eipel C.
        • Kalff J.C.
        • Menger M.D.
        • Vollmar B.
        Loss of NF-kappaB activation in Kupffer cell-depleted mice impairs liver regeneration after partial hepatectomy6.
        Am J Physiol Gastrointest Liver Physiol. 2007; 292: G1570-G1577
        • Xu C.S.
        • Jiang Y.
        • Zhang L.X.
        • Chang C.F.
        • Wang G.P.
        • Shi R.J.
        • et al.
        The role of Kupffer cells in rat liver regeneration revealed by cell-specific microarray analysis.
        J Cell Biochem. 2012; 113: 229-237
        • Taniguchi E.
        • Sakisaka S.
        • Matsuo K.
        • Tanikawa K.
        • Sata M.
        Expression and role of vascular endothelial growth factor in liver regeneration after partial hepatectomy in rats.
        J Histochem Cytochem. 2001; 49: 121-130
        • Sato T.
        • El-Assal O.N.
        • Ono T.
        • Yamanoi A.
        • Dhar D.K.
        • Nagasue N.
        Sinusoidal endothelial cell proliferation and expression of angiopoietin/Tie family in regenerating rat liver.
        J Hepatol. 2001; 34: 690-698
        • Mabuchi A.
        • Mullaney I.
        • Sheard P.W.
        • Hessian P.A.
        • Mallard B.L.
        • Tawadrous M.N.
        • et al.
        Role of hepatic stellate cell/hepatocyte interaction and activation of hepatic stellate cells in the early phase of liver regeneration in the rat.
        J Hepatol. 2004; 40: 910-916
        • Roskams T.
        Relationships among stellate cell activation, progenitor cells, and hepatic regeneration.
        Clin Liver Dis. 2008; 12 (ix): 853-860
        • Wang L.
        • Wang X.
        • Wang L.
        • Chiu J.D.
        • Van de Ven G.
        • Gaarde W.A.
        • et al.
        Hepatic vascular endothelial growth factor regulates recruitment of rat liver sinusoidal endothelial cell progenitor cells.
        Gastroenterology. 2012; 143: 1555-1563
        • DeLeve L.D.
        Liver sinusoidal endothelial cells and liver regeneration.
        J Clin Invest. 2013; 123: 1861-1866
        • Katagiri H.
        • Kushida Y.
        • Nojima M.
        • Kuroda Y.
        • Wakao S.
        • Ishida K.
        • et al.
        A distinct subpopulation of bone marrow mesenchymal stem cells, muse cells, directly commit to the replacement of liver components.
        Am J Transplant. 2015; 10
        • Gupta S.
        • Rajvanshi P.
        • Sokhi R.
        • Slehria S.
        • Yam A.
        • Kerr A.
        • et al.
        Entry and integration of transplanted hepatocytes in rat liver plates occur by disruption of hepatic sinusoidal endothelium.
        Hepatology. 1999; 29: 509-519
        • Wang L.
        • Wang X.
        • Xie G.
        • Wang L.
        • Hill C.K.
        • DeLeve L.D.
        Liver sinusoidal endothelial cell progenitor cells promote liver regeneration in rats.
        J Clin Invest. 2012; 122: 1567-1573
        • Mochida S.
        • Ishikawa K.
        • Inao M.
        • Shibuya M.
        • Fujiwara K.
        Increased expressions of vascular endothelial growth factor and its receptors, flt-1 and KDR/flk-1, in regenerating rat liver.
        Biochem Biophys Res Commun. 1996; 226: 176-179
        • Hu J.
        • Srivastava K.
        • Wieland M.
        • Runge A.
        • Mogler C.
        • Besemfelder E.
        • et al.
        Endothelial cell-derived angiopoietin-2 controls liver regeneration as a spatiotemporal rheostat.
        Science. 2014; 343: 416-419
        • Benten D.
        • Kumaran V.
        • Joseph B.
        • Schattenberg J.
        • Popov Y.
        • Schuppan D.
        • et al.
        Hepatocyte transplantation activates hepatic stellate cells with beneficial modulation of cell engraftment in the rat.
        Hepatology. 2005; 42: 1072-1081
        • Fernandez-Mena C.
        • Almagro J.
        • Puerro M.
        • Quintana A.
        • Hidalgo J.
        • Bañares R.
        • et al.
        Effects of myeloid cell-selective deficiency of IL-6 for liver regeneration after partial hepatectomy (PH) in mice.
        Hepatology. 2016; 62: 848A
        • Illa X.
        • Vila S.
        • Yeste J.
        • Peralta C.
        • Gracia-Sancho J.
        • Villa R.
        A novel modular bioreactor to in vitro study the hepatic sinusoid.
        PLoS ONE. 2014; https://doi.org/10.1371/journal.pone.0111864
        • Prodanov L.
        • Jindal R.
        • Bale S.S.
        • Hegde M.
        • McCarty W.J.
        • Golberg I.
        • et al.
        Long-term maintenance of a microfluidic 3D human liver sinusoid.
        Biotechnol Bioeng. 2016; 113: 241-246