[Show abstract][Hide abstract] ABSTRACT: Citation: Dollé L and Gilgenkrantz H (2015) The Puzzle of Liver Homeostasis: The Centrilobular Hepatocyte, A Novel Master Piece on The Chessboard? Int J Stem Cell Res 1(2): http://dx.doi. As the largest internal organ, the liver has numerous essential functions, such as glucose and fatty acid metabolism or detoxification of endogenous and exogenous substances/drugs. Such functions are mainly attributed to the hepatocytes, which count for eighty percent of the liver cells population. As a result, loss of liver function results in organ failure and subsequent death within days. From a metabolic perspective, the liver of mammals is organized into functional units, called hepatic lobules, limited on one side by a central vein and on the other by a portal vein . The hepatocytes specialize as a function based on their position along this porto-central axis of the liver lobule, for example being responsible for gluconeogenesis in a periportal position and for glycolysis around the central vein [2-5]. Cellular complexity and multitasking of the liver maybe explain why this organ is gifted with remarkable regenerative abilities [1,6]. Most organs preserve homeostasis via two main non-mutually exclusive ways: cellular replication and differentiation from stem or progenitor cells. It is well established that differentiated hepatocytes that are in a quiescent state in the normal liver, are able to re-enter the cell-cycle and proliferate after a partial hepatectomy reconstituting the lost hepatic mass in record time. To elucidate the mechanisms that contribute to the liver repair in pathological conditions, the scientists have developed technical tricks to artificially induce liver damage in rodents by the " 2-hits " strategies (first hit inhibits the proliferation of mature hepatocytes and the second hit hurts the parenchyma).The general admitted conclusion is that in both acute and chronic diseases, the liver activates terminally differentiated epithelium to proliferate and repair the organ. When this capacity fails, the liver activates a population of liver progenitor cells, located around the portal vein, and which are able to proliferate, migrate and differentiate in order to restore both hepatic architecture and liver function. Nevertheless, while countless studies illustrate the ability of different cell types to replenish the liver parenchyma with different degrees of contribution, less fruitful is the discovery of the true nature of this stem cell compartment [1,7, 8]. Basically, until now, two theories were in conflict: the existence of a stem cell pool supplying the liver by maturation into functional hepatocytes by their offspring along the porto-central axis of the hepatocyte (" streaming liver " hypothesis), and the concept that the regeneration results solely from the adult hepatocyte dedifferentiation and/or division, without the intervention of a residual stem cell. In light with the recent work of Kaneko  it is clear that assuming that the liver stem cells are individual entities carrying specific markers, detaching from the biliary ducts to gradually spread out freely from the periportal toward the pericentral regions for liver repair is rather an outdated fact. The latest publications favor the hypothesis that virtually all hepatocytes hold a virtuoso performance in liver reconstitution, dedifferentiating to better proliferate or to change their fate whenever required [1,6-8,10]. The recently published study in the journal Nature, proposing a " counter-streaming " hypothesis in which hepatocytes located around the central vein reconstitute at least 1/3 of the liver lobule in one year, following a centro-portal axis sounds thus almost provocative! Whereas, animal models remain the primary if not the only lens through which scientists evaluate stem cell functions, very few of these models reproduce the etiology, natural history and progression of human liver diseases . In addition, remarkable heterogeneity displayed by liver stem cells in rodent models of stem cell-mediated liver regeneration has been demonstrated, part of it depending of the used artificial models of injury . To avoid biases, the scientists actually returned to an old concept: the use of unmanipulated mice that have a regular homeostasis. Less appreciated and understood is indeed the liver's ability to maintain itself day-by-day, replacing uninjured cells that die off naturally. In such a situation, the cell renewal rate is so low that it is supposed to be the result of an intrinsic proliferation phenomenon of mature hepatocytes. But do all hepatocytes play this role or are some hepatocytes more competent than others to fulfill this function? The group of Nusse tackles these questions and distinguishes a certain category of hepatocytes, located close to the central vein that ensures hepatic homeostasis . The use of a molecular lineage tracing strategy allowing the final labeling with GFP (Green Fluorescent Protein) of cells, which expressed the Axin-2 at the start of the analysis, in fact reveals the presence of a narrow ring of fluorescent hepatocytes along the central vein . These cells are distinguished from most other hepatocytes by expression of the transcription factor TBX3, important in the maintenance of pluripotency, and their diploid character, often associated with a proliferative capacity greater than that of other hepatocytes, which are mostly polyploid. During homeostatic renewal, the progeny of such labeled cells are hepatocytes lacking Axin-2 and TBX3, evolving towards polyploidy and replacing the hepatocytes along the portal centro-axis of the hepatic plate. In one year, these cells replenish up to 30% of the hepatocyte mass . Notably, these Axin-2 + hepatocytes are never replaced by Axin-2-hepatocytes suggesting that these cells self-renew and never give rise to biliary cells. By various approaches, the authors show that these Axin-2 + hepatocytes proliferate twice as fast as the others; thus, the authors called these hepatocytes stem cells because they self-renew and although they are unipotent. They unexpectedly replenish hepatocyte plate " against the stream ". What is the mechanistic underlying the existence of this centrilobular
[Show abstract][Hide abstract] ABSTRACT: Growth hormone (GH) pathway has been shown to play a major role in liver regeneration through the control of EGFR activation. This pathway is downregulated in non-alcoholic fatty liver disease (NAFLD). Since regeneration is known to be impaired in fatty livers, we wondered whether a deregulation of the growth hormone/EGFR pathway could explain this deficiency. Hepatic EGFR expression and triglyceride levels were quantified in liver biopsies of thirty-two obese patients with different degrees of steatosis. We showed a significant inverse correlation between liver EGFR expression and the level of hepatic steatosis. GH/EGFR downregulation was also demonstrated in two steatosis mouse models, a genetic (ob/ob) and a methionine and choline deficient (MCD) diet mouse model, in correlation with liver regeneration defect. Ob/ob mice exhibited a more severe liver regeneration defect after partial hepatectomy (PH) than MCD diet fed mice, a difference that could be explained by a decrease in STAT3 phosphorylation 32h after PH. Having checked that GH deficiency accounted for the GH signaling pathway downregulation in the liver of ob/ob mice, we showed that GH administration in these mice led to a partial rescue in hepatocyte proliferation after partial hepatectomy associated with a concomitant restoration of liver EGFR expression and STAT3 activation. In conclusion, we propose that the GH/EGFR pathway downregulation is a general mechanism responsible for liver regeneration deficiency associated with steatosis, which could be partially rescued by GH administration.
[Show abstract][Hide abstract] ABSTRACT: Since many years, Wnt canonical pathway was known to be involved in proliferation and cell fate. More recently, Hippo pathway has been recognized as a major actor in the control of organ size homeostasis. Both pathways are induced in the activation of stem cells, modulated during carcinogenesis and both use a second messenger, a cascade of phosphorylations and the same ubiquitin ligase degradation complex. Enough for their roads to cross! This review highlights the recent advances in the understanding of the complex crosstalks between Wnt/β-catenin and Hippo/YAP pathways, focusing on two tissues, liver and intestine. In the future, we hope that the identification of the molecular mechanisms underlining these entangled relationships will open towards novel therapeutic strategies for digestive carcinogenesis.
Medecine sciences: M/S 10/2013; 29(10):868-74. DOI:10.1051/medsci/20132910014 · 0.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Unraveling the molecular clues of liver proliferation has become conceivable thanks to the model of two-third hepatectomy. The synchronicity and the well-scheduled aspect of this process allow scientists to slowly decipher this mystery. During this phenomenon, quiescent hepatocytes of the remnant lobes are able to reenter into the cell cycle initiating the G1-S progression synchronously before completing the cell cycle. The major role played by this step of the cell cycle has been emphasized by loss-of-function studies showing a delay or a lack of coordination in the hepatocytes G1-S progression. Two growth factor receptors, c-Met and EGFR, tightly drive this transition. Due to the level of complexity surrounding EGFR signaling, involving numerous ligands, highly controlled regulations and multiple downstream pathways, we chose to focus on the EGFR pathway for this paper. We will first describe the EGFR pathway in its integrity and then address its essential role in the G1/S phase transition for hepatocyte proliferation. Recently, other levels of control have been discovered to monitor this pathway, which will lead us to discuss regulations of the EGFR pathway and highlight the potential effect of misregulations in pathologies.
[Show abstract][Hide abstract] ABSTRACT: Intrahepatic malignant tumours include hepatocellular carcinomas (HCC), cholangiocarcinomas (CC) and combined hepatocholangiocarcinomas (cHCC-CC), a group of rare and poorly characterized tumours that exhibit both biliary and hepatocytic differentiation. The aim of the study was to characterize the molecular pathways specifically associated with cHCC-CC pathogenesis. We performed a genome-wide transcriptional analysis of 20 histologically defined cHCC-CC and compared them with a series of typical HCC and of CC. Data were analysed by gene set enrichment and integrative genomics and results were further validated in situ by tissue microarray using an independent series of 152 tumours. We report that cHCC-CC exhibit stem/progenitor features, a down-regulation of the hepatocyte differentiation program and a commitment to the biliary lineage. TGFβ and Wnt/β-catenin were identified as the two major signalling pathways activated in cHCC-CC. A β-catenin signature distinct from that observed in well-differentiated HCC with mutant β-catenin was found in cHCC-CC. This signature was associated with microenvironment remodelling and TGFβ activation. Furthermore, integrative genomics revealed that cHCC-CC share characteristics of poorly differentiated HCC with stem cell traits and poor prognosis. The common traits displayed by CC, cHCC-CC and some HCC suggest that these tumours could originate from stem/progenitor cell(s) and raised the hypothesis of a potential continuum between intrahepatic CC, cHCC-CC and poorly differentiated HCC.
[Show abstract][Hide abstract] ABSTRACT: GH is a pleiotropic hormone that plays a major role in proliferation, differentiation, and metabolism via its specific receptor. It has been previously suggested that GH signaling pathways are required for normal liver regeneration but the molecular mechanisms involved have yet to be determined. The aim of this study was to identify the mechanisms by which GH controls liver regeneration. We performed two thirds partial hepatectomies in GH receptor (GHR)-deficient mice and wild-type littermates and showed a blunted progression in the G(1)/S transition phase of the mutant hepatocytes. This impaired liver regeneration was not corrected by reestablishing IGF-1 expression. Although the initial response to partial hepatectomy at the priming phase appeared to be similar between mutant and wild-type mice, cell cycle progression was significantly blunted in mutant mice. The main defect in GHR-deficient mice was the deficiency of the epidermal growth factor receptor activation during the process of liver regeneration. Finally, among the pathways activated downstream of GHR during G(1) phase progression, namely Erk1/2, Akt, and signal transducer and activator of transcription 3, we only found a reduced Erk1/2 phosphorylation in mutant mice. In conclusion, our results demonstrate that GH signaling plays a major role in liver regeneration and strongly suggest that it acts through the activation of both epidermal growth factor receptor and Erk1/2 pathways.
[Show abstract][Hide abstract] ABSTRACT: Even if the Greeks probably anticipated rather than discovered the extraordinary regenerative capacity of the liver with the Prometheus myth, this phenomenon still fascinates scientists nowadays with the same enthusiasm. There are good reasons to decipher this process other than to find an answer to our fantasy of immortality: it could indeed help patients needing large liver resections or living-donor liver transplantation, it could increase our understanding of liver pathology and finally it could enable novel cell-therapy approaches. For decades, most of our knowledge about the mechanisms involved in liver regeneration came from the classic two-thirds partial hepatectomy (PH) model. In this scenario, hepatocytes play the leading role, which raises the question of the simple existence of a stem cell population. Recently however, hepatic progenitor cells come again under the limelight, seeming to play a role in liver physiology and in various liver diseases such as steatosis or cirrhosis. Excellent reviews have recently addressed liver regeneration. Our goal is therefore to focus on recent improvements in the field, highlighting data mostly published in the last two years in order to draw a putative picture of what the future research axes on liver regeneration might look like.
[Show abstract][Hide abstract] ABSTRACT: It has been suggested that increasing age is correlated with an acceleration of the progression of liver fibrosis induced by various agents, such as hepatitis C virus or chronic alcohol consumption. However, the cellular and molecular changes underlying this predisposition are not entirely understood. In the context of an aging population, it becomes challenging to decipher the mechanisms responsible for this higher susceptibility of older individuals to this acquired liver disorder. To address this issue, we induced liver fibrosis by carbon tetrachloride (CCl(4)) chronic administration to 8-week- and 15-month-old mice. We confirmed that susceptibility to fibrosis development increased with age and showed that aging did not affect fibrosis resolution capacity. We then focused on the impairment of hepatocyte proliferation, oxidative stress, and inflammation as potential mechanisms accelerating the development of fibrosis in the elderly. We detected no inhibition of hepatocyte proliferation after CCl(4) injury in 15-month-old mice, whereas it was inhibited after a partial hepatectomy. Finally, we observed that, in a context in which liver oxidative stress was not differentially increased in both experimental groups, there was a higher recruitment of inflammatory cells, including mostly macrophages and lymphocytes, oriented toward a T helper 2 (T(H)2) response in older mice. Our data show that in conditions of equivalent levels of oxidative stress and maintained hepatocyte proliferative capacity, an increased inflammatory reaction mainly composed of CD4(+) lymphocytes and macrophages expressing T(H)2 cytokines is the main factor involved in the higher susceptibility to fibrosis with increasing age.
Rejuvenation Research 05/2011; 14(4):353-63. DOI:10.1089/rej.2010.1146 · 3.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The role of hepatocyte apoptosis in the physiopathology of obstructive cholestasis is still controversial. Although some data have strongly suggested that hepatocellular cholestatic injury is due to Fas-mediated hepatocyte apoptosis, some others concluded that necrosis, rather than apoptosis, represents the main type of hepatocyte death in chronic cholestasis. Moreover, it has also been suggested that the reduced liver injury observed in the absence of Fas receptor after bile duct ligation was not due to lower hepatocyte apoptosis but to the indirect role of this receptor in non-hepatocytic cells such as cholangiocytes and inflammatory cells. The aim of this work was therefore to determine whether a protection against cell death limited to hepatocytes could be sufficient to reduce liver injury and delay cholestatic fibrosis. With this purpose, we performed bile duct ligation in transgenic mice overexpressing Bcl-2 in hepatocytes and in wild-type littermates. We found that, compared with necrosis, apoptosis was negligible in this model. Our results also showed that hepatocyte Bcl-2 expression protected hepatocytes against liver injury only in the early steps of the disease. This protection was correlated with reduced mitochondrial dysfunction and lipid peroxidation. However, in contrast to Fas receptor-deficient lpr mice, fibrosis progression was not hampered and liver inflammatory response was not reduced by Bcl-2 overexpression. These results therefore comfort the hypothesis that Fas-mediated apoptotic hepatocyte pathway is not a significant contributing factor to the clinical features observed in cholestasis. Moreover, in the absence of a blunted inflammatory response in transgenic mice, Bcl-2 protection against hepatocyte mitochondrial dysfunction and lipid peroxidation was not sufficient to block fibrosis progression.
[Show abstract][Hide abstract] ABSTRACT: To develop and validate a transient micro-elastography device to measure liver stiffness (LS) in mice.
A novel transient micro-elastography (TME) device, dedicated to LS measurements in mice with a range of measurement from 1-170 kPa, was developed using an optimized vibration frequency of 300 Hz and a 2 mm piston. The novel probe was validated in a classical fibrosis model (CCl(4)) and in a transgenic murine model of systemic amyloidosis.
TME could be successfully performed in control mice below the xiphoid cartilage, with a mean LS of 4.4 ± 1.3 kPa, a mean success rate of 88%, and an excellent intra-observer agreement (0.98). Treatment with CCl(4) over seven weeks drastically increased LS as compared to controls (18.2 ± 3.7 kPa vs 3.6 ± 1.2 kPa). Moreover, fibrosis stage was highly correlated with LS (Spearman coefficient = 0.88, P < 0.01). In the amyloidosis model, much higher LS values were obtained, reaching maximum values of > 150 kPa. LS significantly correlated with the amyloidosis index (0.93, P < 0.0001) and the plasma concentration of mutant hapoA-II (0.62, P < 0.005).
Here, we have established the first non-invasive approach to measure LS in mice, and have successfully validated it in two murine models of high LS.
World Journal of Gastroenterology 02/2011; 17(8):968-75. DOI:10.3748/wjg.v17.i8.968 · 2.37 Impact Factor