Journal of Hepatology
Volume 39, Issue 3 , Pages 431-434, September 2003

Progenitor cell involvement in cirrhotic human liver diseases: from controversy to consensus

Department of Morphology and Molecular Pathology, University Hospitals Leuven, Minderbroederstraat 12, 3000 Leuven, Belgium

See Article, pages 357–364

Article Outline

 

Support for the liver progenitor cell hypothesis has come mainly from rodent models of chemical hepatocarcinogenesis [1], [2], [3], [4], [5], from liver cell regeneration after chemical injury [6], [7], [8] and from cell culture data (for review see ref. [9]). In these models, a periportal population of small ‘primitive’ epithelial cells proliferates in association with or before hepatocyte multiplication. These cells were called oval cells because of their shape [1], [2]. Oval cells are related to terminal biliary ductules and the so-called canals of Hering [7], [10], [11], [12], [13] and they express phenotypic markers of both (immature) hepatocytes (like alpha-fetoprotein and albumin) and bile duct cells (like bile duct type cytokeratins) [14], [15], [16], [17]. They constitute a heterogeneous cell population [14], [15], [17], [18], [19], [20], but at least a subset of oval cells is pluripotent and has the capacity to differentiate towards hepatocytes, bile ductular cells and intestinal epithelium and can give rise to hepatocellular carcinoma and cholangiocellular carcinoma [5], [10], [21], [22], [23], [24], [25], [26], [27].

Although controversial for a long time, it becomes now generally accepted that also in human liver, progenitor cells exist and are activated in different liver diseases [28], [29], [30], [31], [32]. Human liver progenitor cells have mainly been studied in regeneration after severe hepatocellular necrosis [30], [32], [33], [34], [35], [36], but recent studies show that this cell compartment is also activated in chronic viral hepatitis [37], [38], [39], alcoholic liver disease [40], [41] and non-alcoholic fatty liver disease [41], the most important carcinogenic conditions in the Western world. The study of Falkowski et al. [42] in this issue of the Journal describing the presence of ductular reaction-associated progenitor cells in different chronic liver diseases, further supports these studies. Activation of progenitor cells in chronic liver diseases implies that they form a potential target cell population for hepatocarcinogens [43], [44], [45], [46]. This could be an important drawback for possible future treatment of chronic liver diseases with progenitor cells, as these cells would be introduced in a carcinogenic microenvironment. Understanding the mechanisms of activation and differentiation of human hepatic progenitor cells in different liver diseases and the possible role of these cells in hepatocarcinogenesis [47], [48] is mandatory in order to fully assess the therapeutic potential of progenitor cells [49].

Activation of hepatic progenitor cells (oval cells in rodents) is a term which is used for an increase in the number of progenitor cells and their differentiation towards the hepatocytic and/or biliary lineage [17], [50], [51]. In human liver, differentiation towards the biliary lineage leads to formation of reactive ductules, while differentiation towards the hepatocytic lineage occurs via intermediate hepatocytes. Reactive ductules are anastomosing strands of immature biliary cells with an oval nucleus and a small rim of cytoplasm, located at the mesenchymal (portal/septal)-parenchymal interface [30], [31], [52], [53], [54]. Intermediate hepatocytes are polygonal cells with a size and phenotype intermediate between progenitor cells and hepatocytes [30], [31], [55], [56]. Different subtypes of progenitor cells have also been recognized by electron microscopy: the most immature progenitor cell type besides cells already featuring some biliary or hepatocytic features [20], [39], [57], [58]. This spectrum of cells ranging from the most immature phenotype to ductules and intermediate hepatocytes, forms a cell compartment with a specific phenotype, and is often referred to as the ‘progenitor cell compartment’. The term ‘ductular reaction’ refers to proliferation of pre-existing ductules, progenitor cell activation and intermediate hepatocytes [54].

From animal models we have learned that progenitor cells (oval cells) are activated when damage or loss of hepatocytes and/or cholangiocytes is combined with impaired regeneration of the mature cell types involved [51]. Virtually every acute and chronic human liver disease is associated with damage to and loss of hepatocytes and/or cholangiocytes, the two major epithelial cell compartments of the liver. Injury can be caused by numerous triggers including viruses, alcohol, toxic substances, metabolic errors and unknown factors. Hence, it is not surprising that hepatic progenitor cell activation has been described in a variety of human liver diseases [28].

After submassive liver cell necrosis, reactive ductules, in continuity with intermediate hepatocytes, are seen at the periphery of the necrotic areas [31], [33], [56]. In patients studied with sequential liver biopsies, intermediate hepatocytes become more numerous with time and extend further into the liver lobule [30], [59]. This sequence of changes suggests gradual differentiation of human putative progenitor cells into intermediate hepatocytes, in analogy with what is seen in rat models of chemical injury associated with impaired hepatocyte replication [13], [29], [30], [60].

In chronic cholestatic human liver diseases, progenitor cell activation has also been described, based on immunohistochemical and ultrastructural investigations [30], [52], [57], [61], [62]. On ultrastructure, cells intermediate between progenitor cells and bile duct cells are much more numerous than intermediate hepatocytes, suggesting that in biliary diseases, progenitor cells mainly differentiate towards the biliary lineage [30], [53]. Intermediate hepatocytes are also present in the periportal zone in chronic cholestatic diseases, but less numerous than in regenerating liver after submassive necrosis [30]. These cells form small periportal clusters and rosettes, seemingly in continuity with reactive ductules [30], [52], [54], [63]. This continuity is now proven by the three-dimensional reconstruction studies by Falkowski in this issue. The presence of intermediate hepatocytes can however still be interpreted in two ways. Either these intermediate cells are the result of differentiation of progenitor cells into hepatocytes, triggered by hepatocyte damage due to cholestasis. Another possibility, not yet ruled out is ‘metaplasia’ of hepatocytes towards a more biliary phenotype, as a protection mechanism against accumulation of toxic bile salts [64]. Periportal hepatocytes are known to protect themselves against chronic cholestasis by upregulating bile salt transporters [65], pumping bile salts black into the blood. By doing this they become less cholestatic. Similarly, hepatocytes which undergo gradual change into a more biliary phenotype, will be able to survive, because this phenotype is more resistent to toxic bile salts. Falkowski et al. indeed show that intermediate hepatocytes, in continuity with reactive ductules/progenitor cells are rarely cholestatic; what is in fact expected and proofs that ductular metaplasia would be an effective protection mechanism. However, the authors interpret this absence of cholestasis as a proof against the existence of ductular metaplasia in chronic cholestatic diseases, which is an over interpretation of the data. They define cholestasis as bilirubinostasis and cholestatic Mallory bodies. These are severe signs of cholestasis, in fact signs of terminally damage … One would expect terminally damaged cholestatic hepatocytes to be isolated, not being able to link to the biliary tree, which is indeed what the authors of the article in this issue of J Hepatol show: isolated clusters of cholestatic hepatocytes are more numerous in cholestatic diseases than in chronic hepatitis. In addition, the authors use the high replicative rate of reactive ductules as an argument against the existence of ductular metaplasia of hepatoytes. However, they only studied a case of hepatitis C (in which reactive ductules are of course of the regenerative/proliferative type), and no cases of chronic cholestatic disease, the only situation in which the possibility of ductular metaplasia has ever been considered [63], [64]. So overall, Falkowski et al. very nicely show continuity between reactive ductules and intermediate hepatocytes (not being severely cholestatic), and in addition they show that small cirrhotic nodules are often associated with ductules and intermediate hepatocytes. But, based on their data, they are not able to rule out ductular metaplasia of hepatocytes as part of the protection mechanisms of hepatocytes in chronic cholestatic diseases.

In chronic viral hepatitis, progenitor cells are activated, even in mild degrees of inflammation [37], [39]. The number of progenitor cells (the degree of activation) correlates with the degree of inflammatory activity [37], [39]. Intermediate hepatocytes are only seen when a certain level of inflammation is reached, suggesting that progenitor cells only differentiate towards hepatocytes when a certain threshold of damage is reached [37]. Similarly, in haemochromatosis, in alcoholic liver disease, in non-alcoholic fatty liver disease as well as in chronic biliary diseases, the degree of activation of progenitor cells and the number of intermediate hepatocytes correlate with the stage of the disease, which is a measure for the degree of damage [30], [35], [41], [61], [66]. Therefore, progenitor cell activation seems to be a more generalized response to liver damage, irrespective of the underlying disease. What could be the common activating stimulus for progenitor cells? Animal data indicate that oval cells are activated when oxidative stress inhibits the regenerative capacity of more mature hepatocytes [41], [67], [68], [69], [70]. Inhibition of replicative activity of mature hepatocytes is also shown in human liver diseases like alcoholic hepatitis and viral hepatitis [71]. Recently, Wiemann et al. illustrated that hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis induced by a wide variety of etiologies [72]. The study of Falkowski et al. in this issue very nicely confirms inhibition of hepatocyte replication in the cirrhotic stage of chronic viral hepatitis and that this is accompanied by an increased replication rate of the progenitor cell compartment. This increase in progenitor cell activation has also been shown in (cirhhotic) alcoholic and non-alcoholic fatty liver disease, conditions in which replication of hepatocytes is also inhibited [41], [71]. Overall, oxidant-induced replicative senescence, as well as the progenitor cell response, seems to be stereotypical, irrespective of the type of underlying liver disease [30], [35], [37], [57]. Progenitor cell activation is particularly pronounced in the cirrhotic stage of a variety of chronic liver diseases, the stage in which most carcinomas arise. This should be kept in mind when progenitor cells are considered as a therapeutical option in chronic liver diseases.

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PII: S0168-8278(03)00333-7

doi:10.1016/S0168-8278(03)00333-7

Journal of Hepatology
Volume 39, Issue 3 , Pages 431-434, September 2003

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