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Senescence in chronic liver disease: Is the future in aging?

  • Aloysious D. Aravinthan
    Affiliations
    Department of Medicine, University of Toronto, Toronto, Canada

    National Institute for Health Research (NIHR) Nottingham Digestive Diseases Biomedical Research Unit, University of Nottingham and Nottingham University Hospitals NHS Trust, Nottingham, UK
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  • Graeme J.M. Alexander
    Correspondence
    Corresponding author. Address: UCL Institute for Liver and Digestive Health, The Royal Free Trust, London, UK, or Department of Medicine, Box 210, Cambridge University Hospitals, Hills Road, Cambridge CB2 0QQ, UK. Tel.: +44 1223 217456.
    Affiliations
    UCL Institute for Liver and Digestive Health, The Royal Free Trust, London, UK

    Department of Medicine, University of Cambridge, Cambridge, UK
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      Summary

      Cellular senescence is a fundamental, complex mechanism with an important protective role present from embryogenesis to late life across all species. It limits the proliferative potential of damaged cells thus protecting against malignant change, but at the expense of substantial alterations to the microenvironment and tissue homeostasis, driving inflammation, fibrosis and paradoxically, malignant disease if the process is sustained. Cellular senescence has attracted considerable recent interest with recognition of pathways linking aging, malignancy and insulin resistance and the current focus on therapeutic interventions to extend health-span. There are major implications for hepatology in the field of fibrosis and cancer, where cellular senescence of hepatocytes, cholangiocytes, stellate cells and immune cells has been implicated in chronic liver disease progression. This review focuses on cellular senescence in chronic liver disease and explores therapeutic opportunities.

      Abbreviations:

      mTORC1 (mammalian target of rapamycin complex 1), SADF (senescence-associated DNA damage foci), SAHF (senescence-associated heterochromatic foci), SASP (senescence-associated secretory phenotype), SA-β-GAL (senescence-associated β-galactosidase)

      Keywords

      Cellular senescence

      What is cellular senescence?

      Cellular damage, if not repaired, leads to apoptosis or senescence, the fundamental cellular mechanisms of cancer prevention with a long evolutionary history. Apoptosis, a process of programmed cell death, eliminates damaged cells in a rapid but regulated manner with minimal interference to the microenvironment. On the other hand, senescence, which limits the proliferative potential of damaged cells through the induction of stable cell cycle arrest [
      • Campisi J.
      • d’Adda di Fagagna F.
      Cellular senescence: when bad things happen to good cells.
      ,
      • Serrano M.
      • Blasco M.A.
      Putting the stress on senescence.
      ,
      • Collado M.
      • Blasco M.A.
      • Serrano M.
      Cellular senescence in cancer and aging.
      ,
      • Ben-Porath I.
      • Weinberg R.A.
      The signals and pathways activating cellular senescence.
      ], results in substantial alterations to the microenvironment and tissue homeostasis. It is unclear why an injured cell that is incapable of repair should trigger a pathway to either apoptosis or instead to cellular senescence (and is beyond the scope of this review); however, one suggestion is that the greater the severity of DNA damage the more likely it is that a cell will undergo apoptosis [
      • Childs B.G.
      • et al.
      Senescence and apoptosis: dueling or complementary cell fates?.
      ]. Nevertheless, both pathways should be considered as normal, healthy responses to injury that protect both the organ and the organism (Fig. 1).
      Figure thumbnail gr1
      Fig. 1Consequences of cellular injury. The potential outcomes of DNA damage and cellular injury. In the setting of ‘controlled’ injury, the outcome depends on whether repair can be undertaken. The cell will attain pre-injury, normal status if damage is repaired completely. When the damage is irreparable, it leads to either apoptosis or cellular senescence. ‘Uncontrolled’ (extreme) cellular injury leads to death by lysis or necrosis, as often seen in acute liver failure.
      Cellular senescence was first described over half-a-century ago as a consequence of replicative exhaustion (replicative senescence), the concept that each individual cell has a finite potential for cell division [
      • Hayflick L.
      • Moorhead P.S.
      The serial cultivation of human diploid cell strains.
      ]. Cellular senescence also occurs in a much more rapid manner as part of an immediate response to cellular stresses such as oxidative stress, DNA damage, oncogene activation and disruptions to the epigenome (stress-induced premature senescence), approaches which have been exploited in laboratory studies [
      • Campisi J.
      • d’Adda di Fagagna F.
      Cellular senescence: when bad things happen to good cells.
      ,
      • Ben-Porath I.
      • Weinberg R.A.
      The signals and pathways activating cellular senescence.
      ] (Fig. 2).
      Figure thumbnail gr2
      Fig. 2Common causes and cellular changes in senescence. Cellular senescence is activated in response to various forms of cellular stresses, categorised broadly into replicative senescence and stress-induced (genotoxic, oxidative and oncogenic stresses) premature senescence. Cells, once senescent, acquire major morphological changes, become permanently cell cycle arrested, express senescence-associated β-galactosidase (SA-β-GAL), accumulate senescence-associated heterochromatic foci (SAHF) and senescence-associated DNA damage foci (SADF), attain the senescence-associated secretory phenotype (SASP), increase ROS production and activate autophagy. All play a crucial role in the induction and maintenance of cellular senescence and are known collectively as the ‘cellular senescence signature’.
      Although cellular senescence plays a crucial role in aging of both individual organs as well as the entire organism, as might be imagined from the nomenclature, it is important to appreciate that cellular senescence is also a normal part of wound healing and plays a critical part in normal embryonic development in animals and humans [
      • Storer M.
      • et al.
      Senescence is a developmental mechanism that contributes to embryonic growth and patterning.
      ,
      • Munoz-Espin D.
      • et al.
      Programmed cell senescence during mammalian embryonic development.
      ,
      • Rajagopalan S.
      • Long E.O.
      Cellular senescence induced by CD158d reprograms natural killer cells to promote vascular remodeling.
      ]. Cellular senescence under these circumstances can be considered an acute, time-limited event, which is part of the normal physiological response protecting the organ and in which senescent cells are removed subsequently, thus indicating that the process of cellular senescence can be terminated or reversed. If, however senescent cells persist, i.e., when the senescence process is disordered or chronic, this represents a breach of the normal healing process and the organ is then susceptible to injury mediated by the senescent cell per se as well as the initial cause of the DNA damage – the yin and yang of cellular senescence.
      Once cellular senescence is established, cells become resistant to apoptosis [
      • Marcotte R.
      • Lacelle C.
      • Wang E.
      Senescent fibroblasts resist apoptosis by downregulating caspase-3.
      ,
      • Hampel B.
      • et al.
      Apoptosis resistance of senescent human fibroblasts is correlated with the absence of nuclear IGFBP-3.
      ]. As a consequence, senescent cells then accumulate in organs and the decline in organ function with aging is thought to be related, at least in part, to the accumulation of senescent cells with altered physiology [
      • Herbig U.
      • et al.
      Cellular senescence in aging primates.
      ,
      • Jeyapalan J.C.
      • et al.
      Accumulation of senescent cells in mitotic tissue of aging primates.
      ,
      • Campisi J.
      Senescent cells, tumor suppression, and organismal aging: good citizens, bad neighbors.
      ]. Evidence from skin naevi suggests that senescent cells may persist in tissue for years [
      • Michaloglou C.
      • et al.
      BRAFE600-associated senescence-like cell cycle arrest of human naevi.
      ].

      Markers of cellular senescence

      Cells undergo significant morphological changes during senescence. Senescent cells become enlarged and flattened and contain enlarged nuclei. They express senescence-associated β-galactosidase (SA-β-GAL) detectable at pH 6, which differentiates senescent cells from pre-senescent and terminally differentiated cells [
      • Dimri G.P.
      • et al.
      A biomarker that identifies senescent human cells in culture and in aging skin in vivo.
      ].
      Senescent cells are classically cell cycle arrested at G1/S transition, thus typically display a DNA content characteristic of the G1 phase and express cell cycle inhibitors such as p21, p16 and p53 [
      • Campisi J.
      Replicative senescence: an old lives’ tale?.
      ]. Nuclear accumulation of senescence-associated heterochromatic foci (SAHF) and senescence-associated DNA damage foci (SADF) further discriminate senescent cells. Proteins involved in the formation of SAHFs induce senescence when over-expressed and are required for stable suppression of proliferation-associated genes [
      • Narita M.
      • et al.
      Rb-mediated heterochromatin formation and silencing of E2F target genes during cellular senescence.
      ,
      • Narita M.
      • et al.
      A novel role for high-mobility group a proteins in cellular senescence and heterochromatin formation.
      ], whilst SADFs contain proteins crucial for DNA damage response and senescence [
      • d’Adda di Fagagna F.
      • et al.
      A DNA damage checkpoint response in telomere-initiated senescence.
      ].
      Further, senescent cells secrete a wide variety of factors (Table 1), a phenomenon known as the senescence-associated secretory phenotype (SASP) [
      • Coppe J.P.
      • et al.
      Senescence-associated secretory phenotypes reveal cell-nonautonomous functions of oncogenic RAS and the p53 tumor suppressor.
      ,
      • Kuilman T.
      • Peeper D.S.
      Senescence-messaging secretome: SMS-ing cellular stress.
      ,
      • Davalos A.R.
      • et al.
      Senescent cells as a source of inflammatory factors for tumor progression.
      ,
      • Coppe J.P.
      • et al.
      The senescence-associated secretory phenotype: the dark side of tumor suppression.
      ]. Autophagy, which allows cells under conditions of stress to digest internal constituents to generate energy and metabolic precursors, is closely associated with senescence [
      • Hoare M.
      • Young A.R.
      • Narita M.
      Autophagy in cancer: having your cake and eating it.
      ]. This catabolic pathway is spatially and temporally coupled to the mammalian target of rapamycin (mTOR)-associated anabolic pathway, allowing protein degradation to feed raw materials directly into protein synthesis for the SASP [
      • Narita M.
      • et al.
      Spatial coupling of mTOR and autophagy augments secretory phenotypes.
      ].
      Table 1Common senescence-associated secretary phenotype (SASP) factors secreted by senescent cells. Adopted with modification from reference
      • Davalos A.R.
      • et al.
      Senescent cells as a source of inflammatory factors for tumor progression.
      .
      Please note that this table does not include all known SASP factors.
      IL, interleukin; GRO, growth-regulated oncogene; MCP, monocyte chemoattractant protein; MIP, macrophage inflammatory protein; GM-CSF, granulocyte-macrophage colony-stimulating factor; MIF, macrophage migration inhibitory factor; EGF, epidermal growth factor; FGF, fibroblast growth factor; HGF, hepatocyte growth factor; KGF, keratinocyte growth factor; VEGF, vascular endothelial growth factor; SDF1, stromal cell-derived factor 1; IGFBP, insulin-like growth factor-binding protein; PGE2, prostaglandin E2; MMP, matrix metalloproteinase; TIMP, tissue inhibitor of metalloproteinase; PAI-1, plasminogen activator inhibitor-1; tPA, tissue plasminogen activator; ICAM, intercellular adhesion molecule 1; sTNFRI, soluble tumour necrosis factor receptor I; TRAIL-R, TNF-related apoptosis-inducing ligand receptor.
      It is important to stress that there is no single marker of cellular senescence. The presence of permanent cell cycle arrest, the acquisition of major morphological change, expression of SA-β-GAL, accumulation of SAHFs and SADFs, acquisition of SASPs, increased reactive oxygen species (ROS) production and autophagy are known collectively as the ‘cellular senescence signature’ and is used to define cellular senescence [
      • Aravinthan A.
      Cellular senescence: a hitchhiker’s guide.
      ,
      • Sikora E.
      • et al.
      Impact of cellular senescence signature on ageing research.
      ,
      • Sharpless N.E.
      • Sherr C.J.
      Forging a signature of in vivo senescence.
      ] (Fig. 2).

      Senescent-associated secretary phenotype

      Senescent cells have considerable influence on their microenvironment and exert both beneficial and detrimental effects through SASP factors. These induce senescence in neighbouring cells in a paracrine fashion [
      • Nelson G.
      • et al.
      A senescent cell bystander effect: senescence-induced senescence.
      ,
      • Hoare M.
      • Narita M.
      Transmitting senescence to the cell neighbourhood.
      ], and promote inflammation, which aids clearance of senescent cells and cell debris [
      • Krizhanovsky V.
      • et al.
      Senescence of activated stellate cells limits liver fibrosis.
      ], encouraging tissue repair and remodelling. In addition, SASP helps removal of both pre-malignant and malignant cells, thus representing an important aspect of tumour surveillance [
      • Xue W.
      • et al.
      Senescence and tumour clearance is triggered by p53 restoration in murine liver carcinomas.
      ,
      • Kang T.W.
      • et al.
      Senescence surveillance of pre-malignant hepatocytes limits liver cancer development.
      ]. SASPs are also involved in refining embryogenesis. On the other hand, the SASP may also promote tumourigenesis [
      • Davalos A.R.
      • et al.
      Senescent cells as a source of inflammatory factors for tumor progression.
      ,
      • Coppe J.P.
      • et al.
      The senescence-associated secretory phenotype: the dark side of tumor suppression.
      ,
      • Yoshimoto S.
      • et al.
      Obesity-induced gut microbial metabolite promotes liver cancer through senescence secretome.
      ], which is occasionally referred to as the tumour-associated secretary phenotype. Such deleterious effects of the SASP are seen characteristically when senescent cells accumulate and become ‘chronic’ as seen with aging and in chronic diseases (Fig. 3).
      Figure thumbnail gr3
      Fig. 3‘Acute’ and ‘chronic’ senescence. An acute cellular injury leads to cellular senescence, which plays a pivotal role in tumour prevention, wound healing and regulation of embryonic development. These senescent cells are removed subsequently by the innate immune system. In contrast, on-going cellular injury, as seen in chronic liver disease, leads to continuous generation of senescent cells, which is accentuated by immune senescence leading to failure of immune-mediated clearance. This leads to a state of ‘chronic’ senescence, which leads eventually to tissue dysfunction and tumour promotion. Thus cellular senescence is a classic example of antagonistic pleiotropy.

      Cellular senescence in chronic liver disease

      Fibrosis progression, decompensation and the development of liver cancer are common to all chronic liver disorders, independent of aetiology. Hepatocyte injury per se, as opposed to aetiology, is therefore likely to play a crucial role in chronic liver disease progression. Cellular senescence of hepatocytes, cholangiocytes, stellate cells and immune cells has been demonstrated in a wide spectrum of chronic liver disorders using in vitro and in vivo experiments in cell lines, animal models and humans over many years. The involvement of senescence in liver disease has attracted increased attention recently. With clinical hepatologists and hepatobiologists in mind, this review examines the evidence and clinical implications of cellular senescence in chronic liver disease.

      Cellular senescence in epithelial cells (hepatocytes and cholangiocytes)

      Senescence of hepatocytes has been demonstrated indisputably in a range of chronic parenchymal liver disorders, including chronic viral hepatitis B and C [
      • Marshall A.
      • et al.
      Relation between hepatocyte G1 arrest, impaired hepatic regeneration, and fibrosis in chronic hepatitis C virus infection.
      ,
      • Sekoguchi S.
      • et al.
      Role of cell-cycle turnover and oxidative stress in telomere shortening and cellular senescence in patients with chronic hepatitis C.
      ,
      • Paradis V.
      • et al.
      Replicative senescence in normal liver, chronic hepatitis C, and hepatocellular carcinomas.
      ,
      • Tachtatzis P.M.
      • et al.
      Chronic Hepatitis B Virus Infection: The Relation between Hepatitis B Antigen Expression, Telomere Length, Senescence, Inflammation and Fibrosis.
      ], alcohol-related liver disease [
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ], non-alcohol-related fatty liver disease [
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ,
      • Richardson M.M.
      • et al.
      Progressive fibrosis in nonalcoholic steatohepatitis: association with altered regeneration and a ductular reaction.
      ,
      • Nakajima T.
      • et al.
      Nuclear size measurement is a simple method for the assessment of hepatocellular aging in non-alcoholic fatty liver disease: Comparison with telomere-specific quantitative FISH and p21 immunohistochemistry.
      ] and genetic haemochromatosis [
      • Wood M.J.
      • et al.
      Ductular reaction in hereditary hemochromatosis: the link between hepatocyte senescence and fibrosis progression.
      ]. Hepatocyte senescence appears an almost universal phenomenon in chronic liver disease, irrespective of aetiology [
      • Wiemann S.U.
      • et al.
      Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.
      ]. Evidence of hepatocyte senescence has also emerged from tolerant human liver allografts with normal hepatic function and histology [
      • Aini W.
      • et al.
      Accelerated telomere reduction and hepatocyte senescence in tolerated human liver allografts.
      ]. Hepatocyte senescence has also been demonstrated in animal models of liver disease [
      • Yang S.
      • et al.
      Oval cells compensate for damage and replicative senescence of mature hepatocytes in mice with fatty liver disease.
      ].
      As with hepatocytes in chronic parenchymal liver disease, senescence of cholangiocytes was detected readily within damaged small bile ducts and bile ductules in chronic biliary liver disorders such as primary biliary cholangitis (previously primary biliary cirrhosis) [
      • Sasaki M.
      • et al.
      Decreased expression of Bmi1 is closely associated with cellular senescence in small bile ducts in primary biliary cirrhosis.
      ,
      • Sasaki M.
      • et al.
      Telomere shortening in the damaged small bile ducts in primary biliary cirrhosis reflects ongoing cellular senescence.
      ] and primary sclerosing cholangitis [
      • Tabibian J.H.
      • et al.
      Cholangiocyte senescence by way of N-ras activation is a characteristic of primary sclerosing cholangitis.
      ]. Cholangiocyte senescence has also been demonstrated in advanced stages of chronic parenchymal liver diseases such as chronic viral hepatitis and non-alcohol-related fatty liver disease [
      • Sasaki M.
      • et al.
      Bile ductular cells undergoing cellular senescence increase in chronic liver diseases along with fibrous progression.
      ]. The mechanism for involvement of neighbouring cells has not yet been dissected, but is likely to reflect paracrine effects driven by senescent hepatocytes; these observations may have clinical significance if resting bile duct cells retain stem cell function [
      • Rodrigo-Torres D.
      • et al.
      The biliary epithelium gives rise to liver progenitor cells.
      ,
      • Lu W.Y.
      • et al.
      Hepatic progenitor cells of biliary origin with liver repopulation capacity.
      ], although this is contentious.
      In contrast to other organs/tissues, aging per se does not seem to induce senescence of hepatocytes and cholangiocytes in normal liver [
      • Verma S.
      • et al.
      Sustained telomere length in hepatocytes and cholangiocytes with increasing age in normal liver.
      ]. This finding may be of great value in the field of liver transplantation, as more and more liver grafts are being used from older donors who are otherwise healthy. It is conceivable that senescence markers might in the future be used to select suitable livers from older donors.

      Cellular senescence in non-epithelial cells (hepatic stellate cells and immune cells)

      Despite a potential and important role in fibrosis regression (see below), senescence of hepatic stellate cells has been studied only rarely in chronic liver disease. A study comparing hepatocytes with stellate cells in chronic liver disease demonstrated significantly longer telomeres in hepatic stellate cells [
      • Wiemann S.U.
      • et al.
      Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.
      ]. On the other hand, senescence of hepatic stellate cells has been demonstrated in animal models of obesity [
      • Yoshimoto S.
      • et al.
      Obesity-induced gut microbial metabolite promotes liver cancer through senescence secretome.
      ], which has not yet been shown in humans.
      Immune senescence has also been examined in patients with chronic liver disease. Accelerated telomere shortening in peripheral lymphocytes and has been demonstrated in chronic viral hepatitis [
      • Hoare M.
      • et al.
      CD4+ T-lymphocyte telomere length is related to fibrosis stage, clinical outcome and treatment response in chronic hepatitis C virus infection.
      ], cirrhosis [
      • Calado R.T.
      • et al.
      Constitutional telomerase mutations are genetic risk factors for cirrhosis.
      ], and liver transplant recipients with established grafts, independent of the aetiology of liver disease [
      • Gelson W.
      • et al.
      Features of immune senescence in liver transplant recipients with established grafts.
      ,
      • Uziel O.
      • et al.
      Telomere shortening in liver transplant recipients is not influenced by underlying disease or metabolic derangements.
      ]. Despite emerging evidence, the cause of immune senescence in chronic liver disease remains unknown. Peripheral monocyte telomere shortening has been demonstrated in type-2 diabetes mellitus, a disease which has strong associations with non-alcohol-related fatty liver disease [
      • Sampson M.J.
      • et al.
      Monocyte telomere shortening and oxidative DNA damage in type 2 diabetes.
      ]. Immune senescence in circulating mononuclear blood cells has been demonstrated in patients with non-alcohol-related fatty liver disease (Shah M and Alexander GJ, personal communication). No studies have yet examined senescence in intrahepatic immune cells.

      Mechanisms of cellular senescence in chronic liver disease

      Despite the convincing evidence for senescence of hepatocytes and cholangiocytes, the mechanisms that drive the evolution of senescence in chronic liver disease remain debatable. Telomere attrition from repeated cell proliferation causing replicative exhaustion (replicative senescence) has been considered the basis for hepatocyte senescence in chronic liver disease [
      • Sekoguchi S.
      • et al.
      Role of cell-cycle turnover and oxidative stress in telomere shortening and cellular senescence in patients with chronic hepatitis C.
      ,
      • Paradis V.
      • et al.
      Replicative senescence in normal liver, chronic hepatitis C, and hepatocellular carcinomas.
      ,
      • Wiemann S.U.
      • et al.
      Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.
      ]. However, this hypothesis is not supported by more recent studies, which place greater emphasis on stress-induced hepatocyte senescence [
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ,
      • Wood M.J.
      • et al.
      Ductular reaction in hereditary hemochromatosis: the link between hepatocyte senescence and fibrosis progression.
      ,
      • Nakajima T.
      • et al.
      Premature telomere shortening and impaired regenerative response in hepatocytes of individuals with NAFLD.
      ]. In particular the absence of hepatocyte proliferation in liver biopsies from patients with chronic liver disease, even in the early stages and the demonstration of a failure of cycle progression of hepatocytes in chronic liver disease make it unlikely that increased cell turnover underpins hepatocyte senescence [
      • Marshall A.
      • et al.
      Relation between hepatocyte G1 arrest, impaired hepatic regeneration, and fibrosis in chronic hepatitis C virus infection.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ]. The disparity in many studies in the literature regarding hepatocyte proliferation may be explained by a common misunderstanding of markers used to measure cell proliferation. Mcm, PCNA and Ki67, proteins commonly studied as markers of cell proliferation in these studies, are detectable throughout all or most phases of the cell cycle, at varying concentrations but not in the resting phase. Therefore, their presence cannot be interpreted exclusively as an indication of cell proliferation since cells that have entered the cell cycle but are in stable cell cycle arrest also express the same proteins.
      It has been argued that oxidative stress, a common cellular stress, leads to both telomeric and non-telomeric DNA damage and thereby causes telomere-dependent and telomere-independent cellular senescence [
      • Aravinthan A.
      Cellular senescence: a hitchhiker’s guide.
      ]. Similarly, stress-induced senescence has also been proposed for cholangiocyte senescence in chronic biliary liver disease [
      • Sasaki M.
      • et al.
      Decreased expression of Bmi1 is closely associated with cellular senescence in small bile ducts in primary biliary cirrhosis.
      ,
      • Sasaki M.
      • et al.
      Telomere shortening in the damaged small bile ducts in primary biliary cirrhosis reflects ongoing cellular senescence.
      ,
      • Tabibian J.H.
      • et al.
      Cholangiocyte senescence by way of N-ras activation is a characteristic of primary sclerosing cholangitis.
      ]. Nevertheless, it is unlikely that a single mechanism is solely responsible; both stress-induced and replicative senescence is likely to contribute to cellular senescence in chronic liver disease.
      Moreover, genetic mutations of components of the telomerase enzyme complex (telomerase reverse transcriptase and telomerase RNA component), which maintain telomere length, have been demonstrated in cirrhosis with a wide range of aetiologies [
      • Calado R.T.
      • et al.
      Constitutional telomerase mutations are genetic risk factors for cirrhosis.
      ,
      • Hartmann D.
      • et al.
      Telomerase gene mutations are associated with cirrhosis formation.
      ]. These studies highlight the genetic involvement of hepatocyte senescence in liver disease progression and show telomerase mutations as a risk factor for the development of cirrhosis [
      • Chaiteerakij R.
      • Roberts L.R.
      Telomerase mutation: a genetic risk factor for cirrhosis.
      ].
      Senescence of hepatocytes, cholangiocytes, stellate cells and immune cells is a key feature of chronic liver disease independent of aetiology.

      Accumulation of senescent cells in chronic liver disease

      Senescent hepatocytes have been shown to accumulate with progression of chronic liver disease [
      • Marshall A.
      • et al.
      Relation between hepatocyte G1 arrest, impaired hepatic regeneration, and fibrosis in chronic hepatitis C virus infection.
      ,
      • Sekoguchi S.
      • et al.
      Role of cell-cycle turnover and oxidative stress in telomere shortening and cellular senescence in patients with chronic hepatitis C.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ,
      • Richardson M.M.
      • et al.
      Progressive fibrosis in nonalcoholic steatohepatitis: association with altered regeneration and a ductular reaction.
      ]. An on-going liver insult, whether in the form of continuous alcohol ingestion, fat accumulation, viral or immune-related damage is likely to generate senescent hepatocytes continuously, leading eventually to accumulation. Lack of immune-mediated clearance of senescent hepatocytes due to immune senescence in patients with chronic liver disease is also likely to contribute towards further accumulation of senescent hepatocytes. Induction of senescence in hepatocytes in the immediate vicinity of an already senescent hepatocyte, a phenomenon known as senescence-induced senescence, could also contribute to the accumulation and explain the clustering of senescent hepatocytes [
      • Nelson G.
      • et al.
      A senescent cell bystander effect: senescence-induced senescence.
      ]. Although the lifespan of senescent hepatocytes in humans is not yet known, given the anti-apoptotic nature of senescent cells and immune senescence in chronic liver disease, senescent hepatocytes are likely to persist. In mice, senescent hepatocytes have been shown to persist up to 7 weeks after a single brief exposure to a DNA-damaging agent [
      • Panda S.
      • Isbatan A.
      • Adami G.R.
      Modification of the ATM/ATR directed DNA damage response state with aging and long after hepatocyte senescence induction in vivo.
      ]. In this study, the use of DNA-damaging agent may also have impaired immune responses, thus allowing senescent cells to persist longer (chronic senescence), mimicking the clinical scenario in chronic liver disease, as opposed to the prompt clearance of senescent hepatocytes reported by Kang et al. (acute senescence) [
      • Kang T.W.
      • et al.
      Senescence surveillance of pre-malignant hepatocytes limits liver cancer development.
      ], in which immune responses remained healthy, since senescence was induced specifically and exclusively in murine hepatocytes via oncogene activation.
      Senescent cholangiocytes have also shown to increase in number with progression of chronic liver disease [
      • Sasaki M.
      • et al.
      Bile ductular cells undergoing cellular senescence increase in chronic liver diseases along with fibrous progression.
      ]; evidence for a similar increase in number is lacking for other intrahepatic cell lineages.

      Consequences of cellular senescence in chronic liver disease – physiology turned pathology

      Cellular senescence and liver fibrosis

      Cellular senescence plays an important role in the progression of chronic liver disease.
      Accumulation of senescent cells is likely to influence disease progression through an impact on the microenvironment and tissue homeostasis. The diseased liver generates an enormous burden of senescence since as many as 80% of hepatocytes are senescent in advanced disease [
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ] and the evidence indicates that these have a marked effect on disease progression. A number of studies in chronic parenchymal liver disease demonstrated an increase in the proportion of senescent hepatocytes with inflammatory activity and fibrosis stage [
      • Marshall A.
      • et al.
      Relation between hepatocyte G1 arrest, impaired hepatic regeneration, and fibrosis in chronic hepatitis C virus infection.
      ,
      • Sekoguchi S.
      • et al.
      Role of cell-cycle turnover and oxidative stress in telomere shortening and cellular senescence in patients with chronic hepatitis C.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ,
      • Richardson M.M.
      • et al.
      Progressive fibrosis in nonalcoholic steatohepatitis: association with altered regeneration and a ductular reaction.
      ]. Similar associations were also demonstrated between hepatocyte nuclear area and fibrosis in alcohol-related liver disease, a finding that predates the concept of cellular senescence in the field of hepatology [
      • Gonzalez-Reimers C.E.
      • et al.
      Hepatocyte and nuclear areas in alcoholic liver cirrhosis: their relationship with the size of the nodules and the degree of fibrosis.
      ,
      • Gonzalez-Reimers E.
      • et al.
      Hepatocyte and nuclear areas and fatty infiltration of the liver in chronic alcoholic liver disease.
      ]. A rise or fall in the proportion of senescent hepatocytes with progression or regression of fibrosis respectively, has also been shown in chronic liver disease [
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ]. Moreover, a close geographical association is evident between hepatocyte senescence and activation of hepatic stellate cells in chronic liver disease [
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ]. Furthermore, the rate of hepatocyte telomere shortening has been shown to correlate with the rate of fibrosis progression [
      • Sekoguchi S.
      • et al.
      Role of cell-cycle turnover and oxidative stress in telomere shortening and cellular senescence in patients with chronic hepatitis C.
      ]. These data indicate a likely role for hepatocyte senescence in stellate cell activation and fibrosis progression. This is further corroborated in a study of p53-decifient mice with nutrition-directed steatohepatitis, which demonstrated reduced hepatocyte p21 expression and reduced hepatic stellate cell activation (reduced α-SMA and collagen expression) compared to the wild-type counterpart [
      • Tomita K.
      • et al.
      P53/p66Shc-mediated signaling contributes to the progression of non-alcoholic steatohepatitis in humans and mice.
      ]. However, experimental evidence for such direct causal link between hepatocyte senescence and stellate cell activation (and therefore fibrosis) is lacking in man.
      Cellular senescence plays a crucial role in wound healing. Induction of fibrosis by senescent hepatocytes is a way of limiting tissue injury as part of the normal wound healing process. Recruitment of immune cells for the clearance of cell debris and senescent cells also contributes towards wound healing. However, the continuing nature of insult seen in chronic liver disease allows continued senescence of hepatocytes, tipping the balance towards accumulation. Immune senescence of chronic liver disease may also contribute to such accumulation through impaired clearance of senescent cells. Simultaneously, accumulation of senescent hepatocytes leads to continuous activation of stellate cells and thereby fibrosis progression, making a physiological phenomenon pathological (Fig. 4).
      Figure thumbnail gr4
      Fig. 4Consequence of hepatocyte senescence and the role of SASP factors in chronic liver disease. Senescent hepatocytes are pro-inflammatory and SASP factors are involved in the activation of innate immune responses. SASP factors reinforce the senescent state in an autocrine manner and induce senescence in neighbouring normal hepatocyte, a phenomenon known as senescence-induced senescence. On the other hand, SASP factors may also promote tumourigenesis in neighbouring cells. Tumours may also develop from senescent hepatocytes if they acquire mutations, which inactivate or help bypass senescence. Senescent hepatocytes, through SASP factors, attract and activate hepatic stellate cells leading to fibrosis. Hepatocytes, once senescent, undergo major metabolic changes such as alteration in handling of bilirubin and acquisition of insulin resistance.
      An increase in the proportion of senescent cholangiocytes with fibrosis progression has also been demonstrated in both chronic parenchymal and biliary liver disease, although this was more marked in the latter [
      • Tabibian J.H.
      • et al.
      Cholangiocyte senescence by way of N-ras activation is a characteristic of primary sclerosing cholangitis.
      ,
      • Sasaki M.
      • et al.
      Bile ductular cells undergoing cellular senescence increase in chronic liver diseases along with fibrous progression.
      ]. Whether senescent cholangiocytes induce stellate cell activation or whether both cholangiocyte senescence and stellate cell activation are parallel consequences of biliary injury is not yet known.
      Whilst senescence of hepatocytes and cholangiocytes is associated with fibrosis progression, senescence of hepatic stellate cells caused fibrosis regression in a murine model [
      • Krizhanovsky V.
      • et al.
      Senescence of activated stellate cells limits liver fibrosis.
      ]. Stellate cell senescence, which has been attributed to replicative exhaustion, has been shown to limit fibrosis progression and lead to resolution of fibrosis upon withdrawal of the damaging agent [
      • Krizhanovsky V.
      • et al.
      Senescence of activated stellate cells limits liver fibrosis.
      ]. In these circumstances, senescence in the hepatic stellate cell can be regarded as a normal part of the repair process, in which the senescent stellate cells would be cleared at a later date.
      Unfortunately, none of the above studies have looked at senescence of hepatocytes, cholangiocytes and stellate cells contemporaneously in order to understand the influence of contrasting populations upon liver fibrosis in chronic liver disease. Such studies are essential.

      Cellular senescence and impaired hepatic function

      Hepatocyte senescence has strong links with liver dysfunction in chronic liver disease. The strongest evidence of such an association comes from a time when cellular senescence was not known to be associated with chronic liver disease. A study in 1988 demonstrated very elegantly a strong correlation between hepatocyte nuclear area and the development of jaundice, ascites, and encephalopathy in alcohol-related liver disease [
      • Gonzalez-Reimers E.
      • et al.
      Hepatocyte and nuclear areas and fatty infiltration of the liver in chronic alcoholic liver disease.
      ]. These findings were corroborated in a more recent study, which showed an association between an increased proportion of senescent hepatocytes and a decline in liver function in alcohol-related liver disease [
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ]. The decline in hepatocellular function is likely to be due to an alteration in the metabolic function of senescent hepatocytes. Differential expression of genes involved in the major metabolic pathways has been demonstrated in hepatocyte senescence [
      • Aravinthan A.
      • et al.
      The senescent hepatocyte gene signature in chronic liver disease.
      ]. Such changes in cellular function become more pronounced and detrimental when the balance is skewed towards senescent hepatocyte accumulation, leading to a decline in hepatic function and decompensation.
      In vivo and in vitro experiments demonstrated an alteration in the expression of bilirubin transporters (MRP2 and MRP3) in senescent hepatocytes [
      • Aravinthan A.D.
      • Alexander G.J.
      Hepatocyte senescence explains conjugated bilirubinaemia in chronic liver failure.
      ]. MRP2, which is restricted to the canalicular (apical) membrane was downregulated whilst MRP3, which is found only in the sinusoidal (basolateral) membrane of hepatocytes, was upregulated in senescent hepatocytes [
      • Aravinthan A.D.
      • Alexander G.J.
      Hepatocyte senescence explains conjugated bilirubinaemia in chronic liver failure.
      ] (Fig. 4). Thus, accumulation of senescent hepatocytes in advanced chronic liver disease leads to increased transport of conjugated bilirubin into the hepatic sinusoids rather than the bile canaliculi and therefore explains the development of jaundice (conjugated bilirubinaemia) in advanced liver disease [
      • Aravinthan A.D.
      • Alexander G.J.
      Hepatocyte senescence explains conjugated bilirubinaemia in chronic liver failure.
      ]. Similar in vitro experiments also show reduced synthetic function of senescent hepatocytes [
      • Aravinthan A.
      • et al.
      The senescent hepatocyte gene signature in chronic liver disease.
      ], which at least in part explains the decline in liver synthetic function in advanced liver disease.
      The metabolic alterations of senescent hepatocytes in conjunction with the deterioration of hepatic function in advanced liver disease is corroborated by the independent association demonstrated between the proportion of senescent hepatocytes and a subsequent adverse outcome in patients with chronic liver disease [
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ]. Progressive accumulation of senescent hepatocytes in chronic liver disease, which do not function like normal hepatocytes, is likely to disturb the metabolic function of the liver when sufficient cells are affected and thereby lead to decompensation and then an adverse outcome. Consistent with this is that the proportion of senescent hepatocytes was a stronger predictor of an adverse outcome than the clinical parameters used currently [
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ].
      A number of studies have demonstrated an association between hepatocyte senescence and insulin resistance or type-2 diabetes mellitus [
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ,
      • Richardson M.M.
      • et al.
      Progressive fibrosis in nonalcoholic steatohepatitis: association with altered regeneration and a ductular reaction.
      ,
      • Nakajima T.
      • et al.
      Premature telomere shortening and impaired regenerative response in hepatocytes of individuals with NAFLD.
      ]. Induction of senescence in hepatocytes has been shown cause selective insulin resistance through dissociation of Akt downstream pathways [
      • Aravinthan A.
      • et al.
      Selective insulin resistance in hepatocyte senescence.
      ] (Fig. 4). This in vitro finding, which has not been confirmed in vivo, might perhaps explain the development of insulin resistance in advanced liver disease, a phenomenon known as ‘hepatogenous diabetes’. Furthermore, altered expression of glucose transporters in hepatocyte senescence has been demonstrated in humans and is also related to clinical outcome in patients with chronic liver disease and could be used in both stratification and prognostication [
      • Aravinthan A.
      • et al.
      Selective insulin resistance in hepatocyte senescence.
      ].

      Cellular senescence and liver cancer

      Although the primary role of cellular senescence is cancer prevention, senescence is also involved in tumourigenesis [
      • Aravinthan A.
      Cellular senescence: a hitchhiker’s guide.
      ], and hepatocyte senescence is no exception to this. A study using a murine model illustrated elegantly the development of hepatocellular carcinoma from senescent hepatocytes in the absence of immune clearance of senescent cells [
      • Kang T.W.
      • et al.
      Senescence surveillance of pre-malignant hepatocytes limits liver cancer development.
      ]. This model is highly relevant to the development of hepatocellular carcinoma in advanced chronic liver disease in humans (Fig. 4), where both accumulation of senescent hepatocytes and immune senescence (thus impaired immune clearance of senescent hepatocytes) coexist. This may also explain the rarity of hepatocellular carcinoma outside the context of cirrhosis.
      Escape of hepatocytes from the senescent state is considered to be the primary mechanism involved in the development of hepatocellular carcinoma. Non-malignant hepatocytes of chronic liver disease demonstrated DNA damage foci, shorter telomeres and cell cycle inhibitors indicating hepatocyte senescence. Contrastingly, hepatocellular carcinoma from the same liver only demonstrated DNA damage foci and shorter telomeres but not cell cycle inhibitors [
      • Plentz R.R.
      • et al.
      Telomere shortening and inactivation of cell cycle checkpoints characterize human hepatocarcinogenesis.
      ], indicating escape from a senescent state. Gene expression studies of cirrhosis and hepatocellular carcinoma also demonstrated involvement of hepatocyte escape form senescence in the development of hepatocellular carcinoma [
      • Yildiz G.
      • et al.
      Genome-wide transcriptional reorganization associated with senescence-to-immortality switch during human hepatocellular carcinogenesis.
      ]. Whether on-going insult leads to further DNA damage in already senescent hepatocytes, which help these cells overcome senescence barriers, is not yet known.
      Tumours may also develop from neighbouring non-senescent hepatocytes (Fig. 4), which are in the vicinity of senescent hepatocytes. Senescent cells, through the SASP, are involved in angiogenesis and recruitment/proliferation of newer cells to replenish damaged cells; an essential role of senescent cells towards tissue integrity. This physiological phenomenon, however, is suspected to be involved in tumourigenesis [
      • Krtolica A.
      • et al.
      Senescent fibroblasts promote epithelial cell growth and tumorigenesis: a link between cancer and aging.
      ,
      • Parrinello S.
      • et al.
      Stromal-epithelial interactions in aging and cancer: senescent fibroblasts alter epithelial cell differentiation.
      ]. Accumulation of senescent hepatocytes in large proportions, as in advanced chronic liver disease, results in a SASP-rich microenvironment, which in turn may promote tumourigenesis in neighbouring DNA-damaged, pre-senescent hepatocytes.
      Although, based on the above association between hepatocyte senescence and development of hepatocellular carcinoma, it is tempting to postulate an association between cholangiocyte senescence and cholangiocarcinoma in chronic biliary disease, no studies have yet investigated this possible link.

      Cellular senescence and viral kinetics

      Finally, in contrast to chronic hepatitis C virus infection, the replication of hepatitis B virus seems to decline with disease progression. This association appears to be related to hepatocyte senescence. In a recent study, the markers of hepatitis B viral replication were confined to biologically younger hepatocytes, as opposed to older or senescent hepatocytes [
      • Tachtatzis P.M.
      • et al.
      Chronic Hepatitis B Virus Infection: The Relation between Hepatitis B Antigen Expression, Telomere Length, Senescence, Inflammation and Fibrosis.
      ]. This may explain the low levels of hepatitis B viral DNA and hepatitis B surface antigen usually seen in cirrhotic patients who have high proportions of senescent hepatocytes.

      The future – prognostic and therapeutic potential of cellular senescence

      The crucial role of cellular senescence in fibrosis and liver decompensation, both critical landmarks in the course of progressive liver disease, make it an ideal potential biomarker for disease stratification and prognostication. In fact, specific markers of hepatocyte senescence have already been demonstrated to have a role in predicting the outcome of chronic liver disease [
      • Aravinthan A.
      • et al.
      Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
      ,
      • Aravinthan A.
      • et al.
      Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
      ]. However, the requirement for liver tissue (in the form of liver biopsy) for the analysis of hepatocyte senescence is a clear disadvantage. Liver biopsy is invasive, associated, on occasions, with serious complications and as ever, the biopsy sample may not represent the state of hepatocyte senescence of the entire liver. It would be advantageous to identify and validate serum markers of hepatocyte senescence, which could improve the landscape for the evaluation of chronic liver disease, with the added value in some cases to anticipate fibrosis as an alternative to measuring established fibrosis in serum or within the liver.
      Secondly, accumulation of senescent hepatocytes is considered as a driver of chronic liver disease progression. Thus, treatment targeted towards eliminating senescent hepatocytes should, in theory, help alleviate senescence-related pathology. Consistent with this approach, clearance of senescent cells was shown to retard age-associated organ dysfunction, delay tumorigenesis and extend lifespan in murine models [
      • Baker D.J.
      • et al.
      Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders.
      ,
      • Baker D.J.
      • et al.
      Naturally occurring p16-positive cells shorten healthy lifespan.
      ]. A new class of drugs known as “senolytics”, which selectively target senescent cells, are under investigation in animal models, but so far with little effect on general applicability [
      • Zhu Y.
      • et al.
      The Achilles’ heel of senescent cells: from transcriptome to senolytic drugs.
      ]. An exciting, recent study, using a specific inhibitor of the anti-apoptotic proteins BCL-2 and BCL-xL, demonstrated selective elimination of senescent cells in culture in a cell type- and species-independent manner [
      • Chang J.
      • et al.
      Clearance of senescent cells by ABT263 rejuvenates aged hematopoietic stem cells in mice.
      ]. The same agent mitigated premature aging of the haemopoietic system through removing senescent bone marrow hematopoietic stem cells in a murine model [
      • Chang J.
      • et al.
      Clearance of senescent cells by ABT263 rejuvenates aged hematopoietic stem cells in mice.
      ]. This study indicates that pharmacological elimination of senescent cells with restoration of tissue function is now a real possibility.
      In an alternative approach in an experimental animal model, restoration of telomere function through telomerase gene delivery has been shown to alleviate cirrhosis and improve liver function [
      • Rudolph K.L.
      • et al.
      Inhibition of experimental liver cirrhosis in mice by telomerase gene delivery.
      ], thus corroborating the concept that progressive chronic liver disease could be halted by elimination or prevention of cellular senescence.
      Senescent cells affect neighbouring cells and microenvironment through a broad variety of SASP factors, which induce and reinforce senescence, activate immune responses, induce fibrosis and harbour pro-tumourigenic properties [
      • Aravinthan A.
      Cellular senescence: a hitchhiker’s guide.
      ]. The mammalian target of rapamycin complex 1 (mTORC1) is involved in the SASP production and inhibition of mTORC1 has been to shown to attenuate SASP production [
      • Herranz N.
      • et al.
      MTOR regulates MAPKAPK2 translation to control the senescence-associated secretory phenotype.
      ,
      • Laberge R.M.
      • et al.
      MTOR regulates the pro-tumorigenic senescence-associated secretory phenotype by promoting IL1A translation.
      ] and improve organismal longevity [
      • Harrison D.E.
      • et al.
      Rapamycin fed late in life extends lifespan in genetically heterogeneous mice.
      ,
      • Anisimov V.N.
      • et al.
      Rapamycin increases lifespan and inhibits spontaneous tumorigenesis in inbred female mice.
      ]. In an alternative approach, reprogramming of the SASP through inhibition of the JAK2/STAT3 pathway enhanced immune surveillance and tumour clearance [
      • Toso A.
      • et al.
      Enhancing chemotherapy efficacy in Pten-deficient prostate tumors by activating the senescence-associated antitumor immunity.
      ]. Similar such opportunities could, therefore, be optimally exploited to intervene in the process of hepatocyte senescence of chronic liver disease for therapeutic advantages.
      Understanding cellular senescence may extend therapeutic options in preventing or even reversing chronic liver disease progression.
      Paradoxically, there is a potential role for pro-senescent therapy for certain complications of chronic liver disease. Treatment of hepatocellular carcinoma with TGFβ, a potent hepatocyte cell cycle inhibitor, has been shown to induce senescence in tumour cells and inhibit tumour growth in a mouse model [
      • Senturk S.
      • et al.
      Transforming growth factor-beta induces senescence in hepatocellular carcinoma cells and inhibits tumor growth.
      ]. Reactivation of p53 in a mouse model of hepatocellular carcinoma using RNA interference has also been shown to cause tumour senescence and tumour regression [
      • Xue W.
      • et al.
      Senescence and tumour clearance is triggered by p53 restoration in murine liver carcinomas.
      ]. A targeted pro-senescent therapy approach, therefore, has immense clinical potential. Similarly, senescence-inducing therapy targeting the hepatic stellate cells based on murine work could slow or even prevent fibrosis progression in chronic liver disease. Although such pro-senescent therapies are currently being tried in cancer treatment, cell-specific pro-senescent therapies are not yet available for clinical or research purposes.

      Conclusions

      Senescence is a fundamental cellular process with colossal diagnostic, prognostic and therapeutic potential in liver disease. However, the role of cellular senescence is poorly understood and little studied in hepatology. Strong collaboration between basic and clinical scientists could rejuvenate the liver.

      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.

      Acknowledgements

      This article is based on Dr. A. D. Aravinthan’s Sheila Sherlock Award lecture at the British Association for the Study of the Liver (BASL).

      Supplementary data

      References

        • Campisi J.
        • d’Adda di Fagagna F.
        Cellular senescence: when bad things happen to good cells.
        Nat Rev Mol Cell Biol. 2007; 8: 729-740
        • Serrano M.
        • Blasco M.A.
        Putting the stress on senescence.
        Curr Opin Cell Biol. 2001; 13: 748-753
        • Collado M.
        • Blasco M.A.
        • Serrano M.
        Cellular senescence in cancer and aging.
        Cell. 2007; 130: 223-233
        • Ben-Porath I.
        • Weinberg R.A.
        The signals and pathways activating cellular senescence.
        Int J Biochem Cell Biol. 2005; 37: 961-976
        • Childs B.G.
        • et al.
        Senescence and apoptosis: dueling or complementary cell fates?.
        EMBO Rep. 2014; 15: 1139-1153
        • Hayflick L.
        • Moorhead P.S.
        The serial cultivation of human diploid cell strains.
        Exp Cell Res. 1961; 25: 585-621
        • Storer M.
        • et al.
        Senescence is a developmental mechanism that contributes to embryonic growth and patterning.
        Cell. 2013; 155: 1119-1130
        • Munoz-Espin D.
        • et al.
        Programmed cell senescence during mammalian embryonic development.
        Cell. 2013; 155: 1104-1118
        • Rajagopalan S.
        • Long E.O.
        Cellular senescence induced by CD158d reprograms natural killer cells to promote vascular remodeling.
        Proc Natl Acad Sci U S A. 2012; 109: 20596-20601
        • Marcotte R.
        • Lacelle C.
        • Wang E.
        Senescent fibroblasts resist apoptosis by downregulating caspase-3.
        Mech Ageing Dev. 2004; 125: 777-783
        • Hampel B.
        • et al.
        Apoptosis resistance of senescent human fibroblasts is correlated with the absence of nuclear IGFBP-3.
        Aging Cell. 2005; 4: 325-330
        • Herbig U.
        • et al.
        Cellular senescence in aging primates.
        Science. 2006; 311: 1257
        • Jeyapalan J.C.
        • et al.
        Accumulation of senescent cells in mitotic tissue of aging primates.
        Mech Ageing Dev. 2007; 128: 36-44
        • Campisi J.
        Senescent cells, tumor suppression, and organismal aging: good citizens, bad neighbors.
        Cell. 2005; 120: 513-522
        • Michaloglou C.
        • et al.
        BRAFE600-associated senescence-like cell cycle arrest of human naevi.
        Nature. 2005; 436: 720-724
        • Dimri G.P.
        • et al.
        A biomarker that identifies senescent human cells in culture and in aging skin in vivo.
        Proc Natl Acad Sci U S A. 1995; 92: 9363-9367
        • Campisi J.
        Replicative senescence: an old lives’ tale?.
        Cell. 1996; 84: 497-500
        • Narita M.
        • et al.
        Rb-mediated heterochromatin formation and silencing of E2F target genes during cellular senescence.
        Cell. 2003; 113: 703-716
        • Narita M.
        • et al.
        A novel role for high-mobility group a proteins in cellular senescence and heterochromatin formation.
        Cell. 2006; 126: 503-514
        • d’Adda di Fagagna F.
        • et al.
        A DNA damage checkpoint response in telomere-initiated senescence.
        Nature. 2003; 426: 194-198
        • Coppe J.P.
        • et al.
        Senescence-associated secretory phenotypes reveal cell-nonautonomous functions of oncogenic RAS and the p53 tumor suppressor.
        PLoS Biol. 2008; 6: 2853-2868
        • Kuilman T.
        • Peeper D.S.
        Senescence-messaging secretome: SMS-ing cellular stress.
        Nat Rev Cancer. 2009; 9: 81-94
        • Davalos A.R.
        • et al.
        Senescent cells as a source of inflammatory factors for tumor progression.
        Cancer Metastasis Rev. 2010; 29: 273-283
        • Coppe J.P.
        • et al.
        The senescence-associated secretory phenotype: the dark side of tumor suppression.
        Annu Rev Pathol. 2010; 5: 99-118
        • Hoare M.
        • Young A.R.
        • Narita M.
        Autophagy in cancer: having your cake and eating it.
        Semin Cancer Biol. 2011; 21: 397-404
        • Narita M.
        • et al.
        Spatial coupling of mTOR and autophagy augments secretory phenotypes.
        Science. 2011; 332: 966-970
        • Aravinthan A.
        Cellular senescence: a hitchhiker’s guide.
        Hum Cell. 2015; 28: 51-64
        • Sikora E.
        • et al.
        Impact of cellular senescence signature on ageing research.
        Ageing Res Rev. 2011; 10: 146-152
        • Sharpless N.E.
        • Sherr C.J.
        Forging a signature of in vivo senescence.
        Nat Rev Cancer. 2015; 15: 397-408
        • Nelson G.
        • et al.
        A senescent cell bystander effect: senescence-induced senescence.
        Aging Cell. 2012; 11: 345-349
        • Hoare M.
        • Narita M.
        Transmitting senescence to the cell neighbourhood.
        Nat Cell Biol. 2013; 15: 887-889
        • Krizhanovsky V.
        • et al.
        Senescence of activated stellate cells limits liver fibrosis.
        Cell. 2008; 134: 657-667
        • Xue W.
        • et al.
        Senescence and tumour clearance is triggered by p53 restoration in murine liver carcinomas.
        Nature. 2007; 445: 656-660
        • Kang T.W.
        • et al.
        Senescence surveillance of pre-malignant hepatocytes limits liver cancer development.
        Nature. 2011; 479: 547-551
        • Yoshimoto S.
        • et al.
        Obesity-induced gut microbial metabolite promotes liver cancer through senescence secretome.
        Nature. 2013; 499: 97-101
        • Marshall A.
        • et al.
        Relation between hepatocyte G1 arrest, impaired hepatic regeneration, and fibrosis in chronic hepatitis C virus infection.
        Gastroenterology. 2005; 128: 33-42
        • Sekoguchi S.
        • et al.
        Role of cell-cycle turnover and oxidative stress in telomere shortening and cellular senescence in patients with chronic hepatitis C.
        J Gastroenterol Hepatol. 2007; 22: 182-190
        • Paradis V.
        • et al.
        Replicative senescence in normal liver, chronic hepatitis C, and hepatocellular carcinomas.
        Hum Pathol. 2001; 32: 327-332
        • Tachtatzis P.M.
        • et al.
        Chronic Hepatitis B Virus Infection: The Relation between Hepatitis B Antigen Expression, Telomere Length, Senescence, Inflammation and Fibrosis.
        PLoS One. 2015; 10e0127511
        • Aravinthan A.
        • et al.
        Hepatocyte expression of the senescence marker p21 is linked to fibrosis and an adverse liver-related outcome in alcohol-related liver disease.
        PLoS One. 2013; 8e72904
        • Aravinthan A.
        • et al.
        Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease.
        J Hepatol. 2013; 58: 549-556
        • Richardson M.M.
        • et al.
        Progressive fibrosis in nonalcoholic steatohepatitis: association with altered regeneration and a ductular reaction.
        Gastroenterology. 2007; 133: 80-90
        • Nakajima T.
        • et al.
        Nuclear size measurement is a simple method for the assessment of hepatocellular aging in non-alcoholic fatty liver disease: Comparison with telomere-specific quantitative FISH and p21 immunohistochemistry.
        Pathol Int. 2010; 60: 175-183
        • Wood M.J.
        • et al.
        Ductular reaction in hereditary hemochromatosis: the link between hepatocyte senescence and fibrosis progression.
        Hepatology. 2014; 59: 848-857
        • Wiemann S.U.
        • et al.
        Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.
        FASEB J. 2002; 16: 935-942
        • Aini W.
        • et al.
        Accelerated telomere reduction and hepatocyte senescence in tolerated human liver allografts.
        Transpl Immunol. 2014; 31: 55-59
        • Yang S.
        • et al.
        Oval cells compensate for damage and replicative senescence of mature hepatocytes in mice with fatty liver disease.
        Hepatology. 2004; 39: 403-411
        • Sasaki M.
        • et al.
        Decreased expression of Bmi1 is closely associated with cellular senescence in small bile ducts in primary biliary cirrhosis.
        Am J Pathol. 2006; 169: 831-845
        • Sasaki M.
        • et al.
        Telomere shortening in the damaged small bile ducts in primary biliary cirrhosis reflects ongoing cellular senescence.
        Hepatology. 2008; 48: 186-195
        • Tabibian J.H.
        • et al.
        Cholangiocyte senescence by way of N-ras activation is a characteristic of primary sclerosing cholangitis.
        Hepatology. 2014; 59: 2263-2275
        • Sasaki M.
        • et al.
        Bile ductular cells undergoing cellular senescence increase in chronic liver diseases along with fibrous progression.
        Am J Clin Pathol. 2010; 133: 212-223
        • Rodrigo-Torres D.
        • et al.
        The biliary epithelium gives rise to liver progenitor cells.
        Hepatology. 2014; 60: 1367-1377
        • Lu W.Y.
        • et al.
        Hepatic progenitor cells of biliary origin with liver repopulation capacity.
        Nat Cell Biol. 2015; 17: 971-983
        • Verma S.
        • et al.
        Sustained telomere length in hepatocytes and cholangiocytes with increasing age in normal liver.
        Hepatology. 2012; 56: 1510-1520
        • Hoare M.
        • et al.
        CD4+ T-lymphocyte telomere length is related to fibrosis stage, clinical outcome and treatment response in chronic hepatitis C virus infection.
        J Hepatol. 2010; 53: 252-260
        • Calado R.T.
        • et al.
        Constitutional telomerase mutations are genetic risk factors for cirrhosis.
        Hepatology. 2011; 53: 1600-1607
        • Gelson W.
        • et al.
        Features of immune senescence in liver transplant recipients with established grafts.
        Liver transpl. 2010; 16: 577-587
        • Uziel O.
        • et al.
        Telomere shortening in liver transplant recipients is not influenced by underlying disease or metabolic derangements.
        Ann Transplant. 2013; 18: 567-575
        • Sampson M.J.
        • et al.
        Monocyte telomere shortening and oxidative DNA damage in type 2 diabetes.
        Diabetes Care. 2006; 29: 283-289
        • Nakajima T.
        • et al.
        Premature telomere shortening and impaired regenerative response in hepatocytes of individuals with NAFLD.
        Liver Int. 2006; 26: 23-31
        • Hartmann D.
        • et al.
        Telomerase gene mutations are associated with cirrhosis formation.
        Hepatology. 2011; 53: 1608-1617
        • Chaiteerakij R.
        • Roberts L.R.
        Telomerase mutation: a genetic risk factor for cirrhosis.
        Hepatology. 2011; 53: 1430-1432
        • Panda S.
        • Isbatan A.
        • Adami G.R.
        Modification of the ATM/ATR directed DNA damage response state with aging and long after hepatocyte senescence induction in vivo.
        Mech Ageing Dev. 2008; 129: 332-340
        • Gonzalez-Reimers C.E.
        • et al.
        Hepatocyte and nuclear areas in alcoholic liver cirrhosis: their relationship with the size of the nodules and the degree of fibrosis.
        Drug Alcohol Depend. 1987; 19: 357-362
        • Gonzalez-Reimers E.
        • et al.
        Hepatocyte and nuclear areas and fatty infiltration of the liver in chronic alcoholic liver disease.
        Drug Alcohol Depend. 1988; 22: 195-203
        • Tomita K.
        • et al.
        P53/p66Shc-mediated signaling contributes to the progression of non-alcoholic steatohepatitis in humans and mice.
        J Hepatol. 2012; 57: 837-843
        • Aravinthan A.
        • et al.
        The senescent hepatocyte gene signature in chronic liver disease.
        Exp Gerontol. 2014; 60: 37-45
        • Aravinthan A.D.
        • Alexander G.J.
        Hepatocyte senescence explains conjugated bilirubinaemia in chronic liver failure.
        J Hepatol. 2015; 63: 532-533
        • Aravinthan A.
        • et al.
        Selective insulin resistance in hepatocyte senescence.
        Exp Cell Res. 2015; 331: 38-45
        • Plentz R.R.
        • et al.
        Telomere shortening and inactivation of cell cycle checkpoints characterize human hepatocarcinogenesis.
        Hepatology. 2007; 45: 968-976
        • Yildiz G.
        • et al.
        Genome-wide transcriptional reorganization associated with senescence-to-immortality switch during human hepatocellular carcinogenesis.
        PLoS One. 2013; 8e64016
        • Krtolica A.
        • et al.
        Senescent fibroblasts promote epithelial cell growth and tumorigenesis: a link between cancer and aging.
        Proc Natl Acad Sci U S A. 2001; 98: 12072-12077
        • Parrinello S.
        • et al.
        Stromal-epithelial interactions in aging and cancer: senescent fibroblasts alter epithelial cell differentiation.
        J Cell Sci. 2005; 118: 485-496
        • Baker D.J.
        • et al.
        Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders.
        Nature. 2011; 479: 232-236
        • Baker D.J.
        • et al.
        Naturally occurring p16-positive cells shorten healthy lifespan.
        Nature. 2016; 530: 184-189
        • Zhu Y.
        • et al.
        The Achilles’ heel of senescent cells: from transcriptome to senolytic drugs.
        Aging Cell. 2015; 14: 644-658
        • Chang J.
        • et al.
        Clearance of senescent cells by ABT263 rejuvenates aged hematopoietic stem cells in mice.
        Nat Med. 2016; 22: 78-83
        • Rudolph K.L.
        • et al.
        Inhibition of experimental liver cirrhosis in mice by telomerase gene delivery.
        Science. 2000; 287: 1253-1258
        • Herranz N.
        • et al.
        MTOR regulates MAPKAPK2 translation to control the senescence-associated secretory phenotype.
        Nat Cell Biol. 2015; 17: 1205-1217
        • Laberge R.M.
        • et al.
        MTOR regulates the pro-tumorigenic senescence-associated secretory phenotype by promoting IL1A translation.
        Nat Cell Biol. 2015; 17: 1049-1061
        • Harrison D.E.
        • et al.
        Rapamycin fed late in life extends lifespan in genetically heterogeneous mice.
        Nature. 2009; 460: 392-395
        • Anisimov V.N.
        • et al.
        Rapamycin increases lifespan and inhibits spontaneous tumorigenesis in inbred female mice.
        Cell Cycle. 2011; 10: 4230-4236
        • Toso A.
        • et al.
        Enhancing chemotherapy efficacy in Pten-deficient prostate tumors by activating the senescence-associated antitumor immunity.
        Cell Rep. 2014; 9: 75-89
        • Senturk S.
        • et al.
        Transforming growth factor-beta induces senescence in hepatocellular carcinoma cells and inhibits tumor growth.
        Hepatology. 2010; 52: 966-974