Zonal hierarchy of differentiation markers and nestin expression during oval cell mediated rat liver regeneration

Restore the brake on tumor progression

Sonic hedgehog (Shh) signaling plays a key role in regulation of normal development. The negative feedback mechanism mediated by the transcriptional factor, Gli3, acts to finely tune Shh signaling, providing tight control of normal developmental processes. Hyperactivation of Shh signaling often leads to many human malignancies, including basal cell carcinoma and medulloblastoma (MB). However, how tumor cells sustain the aberrant activation of Shh signaling is still not completely understood. We recently revealed that during MB formation, tumor cells express Nestin, a type VI intermediate filament protein, which maintains uncontrolled Shh signaling by abolishing negative feedback by Gli3. Therefore, Nestin expression is a necessary step for MB formation. These findings highlight the novel function of Nestin in regulating Shh signaling, as well as the important role of a disrupted negative feedback mechanism in MB tumorigenesis. Further, restoration of the intrinsic negative feedback by repressing Nestin expression represents a promising approach to treat MB as well as other Shh signaling associated malignancies.

Hepatocellular carcinoma may display elevated nestin expression in endothelial cells: experimental study

CONTEXT AND OBJECTIVE

Nestin, a class VI intermediate filament protein, is highly expressed in the portal mesenchyme and sinusoidal endothelium of the human fetal liver, but scarcely expressed in adult portal vessel endothelium. During experimental liver regeneration, an increased number of nestin-positive parenchymal cells have been observed in the zone adjacent to the Hering canals. These parenchymal cells are regarded as hepatic stem cells or hepatoblasts, which may be involved in hepatocellular carcinogenesis. In the light of recent reports describing nestin-positive parenchymal cells in hepatocellular carcinoma, we aimed to use this tumor type as a positive control for immunohistochemical detection of nestin.

DESIGN AND SETTING

Experimental study conducted at a university hospital.

METHODS

Hepatocellular carcinoma sections from one case were analyzed for nestin expression by immunohistochemistry using confocal microscopy.

RESULTS

Surprisingly, a conspicuous pattern resembling liver sinusoid-like cytoarchitecture was observed upon nestin staining of endothelial cells.

CONCLUSIONS

This pattern has not been previously described. The preliminary results shown here suggest that nestin-positive endothelial cells are located in niches of immature or proliferative cells. Moreover, nestin expression in endothelial cells of hepatocellular carcinoma enhances the role of angiogenesis in this tumor type, although the prevalence of this immunohistopathological pattern remains to be determined. Finally, hepatocellular carcinoma is an effective positive control for nestin staining in fluorescent immunohistochemistry.

Evidence for a common progenitor of epithelial and mesenchymal components of the liver

Tissues of the adult organism maintain the homeostasis and respond to injury by means of progenitor/stem cell compartments capable to give rise to appropriate progeny. In organs composed by histotypes of different embryological origins (e.g. the liver), the tissue turnover may in theory involve different stem/precursor cells able to respond coordinately to physiological or pathological stimuli. In the liver, a progenitor cell compartment, giving rise to hepatocytes and cholangiocytes, can be activated by chronic injury inhibiting hepatocyte proliferation. The precursor compartment guaranteeing turnover of hepatic stellate cells (HSCs) (perisinusoidal cells implicated with the origin of the liver fibrosis) in adult organ is yet unveiled. We show here that epithelial and mesenchymal liver cells (hepatocytes and HSCs) may arise from a common progenitor. Sca+ murine progenitor cells were found to coexpress markers of epithelial and mesenchymal lineages and to give rise, within few generations, to cells that segregate the lineage-specific markers into two distinct subpopulations. Notably, these progenitor cells, clonally derived, when transplanted in healthy livers, were found to generate epithelial and mesenchymal liver-specific derivatives (i.e. hepatocytes and HSCs) properly integrated in the liver architecture. These evidences suggest the existence of a 'bona fide' organ-specific meso-endodermal precursor cell, thus profoundly modifying current models of adult progenitor commitment believed, so far, to be lineage-restricted. Heterotopic transplantations, which confirm the dual differentiation potentiality of those cells, indicates as tissue local cues are necessary to drive a full hepatic differentiation. These data provide first evidences for an adult stem/precursor cell capable to differentiate in both parenchymal and non-parenchymal organ-specific components and candidate the liver as the instructive site for the reservoir compartment of HSC precursors as yet non-localized in the adult.

Hepatic stem cell niches

Stem cell niches are special microenvironments that maintain stem cells and control their behavior to ensure tissue homeostasis and regeneration throughout life. The liver has a high regenerative capacity that involves stem/progenitor cells when the proliferation of hepatocytes is impaired. In recent years progress has been made in the identification of potential hepatic stem cell niches. There is evidence that hepatic progenitor cells can originate from niches in the canals of Hering; in addition, the space of Disse may also serve as a stem cell niche during fetal hematopoiesis and constitute a niche for stellate cells in adults.

Cultured hepatocytes adopt progenitor characteristics and display bipotent capacity to repopulate the liver

Clinical studies have proved the therapeutic potential of hepatocyte transplantation as a promising alternative to whole organ liver transplantation in the treatment of hereditary or end-stage liver disease. However, donor shortage seriously restricts cell availability, and the lack of appropriate cell culture protocols for the storage and maintenance of donor cells constitutes a significant obstacle. The aim of this study was to stimulate mature hepatocytes in culture to multiply in vitro and track their fate on transplantation. Rat hepatocytes isolated nonenzymatically were cultured serum free for up to 10 days. They were stimulated into proliferation in the presence of growth factors and conditioned media from nonparenchymal and hepatocyte culture supernatants, as well as 10 mM lithium chloride (LiCl). Cell proliferation was assessed by determining DNA content. Additionally, the extent of cell differentiation was estimated using immunofluorescence staining of hepatic, biliary, progenitor, and mesenchymal markers and gene expression analyses. Transplantation studies were performed on the Fischer CD26-mutant rat following pretreatment with retrorsine and partial hepatectomy. Proliferating hepatocytes increasingly adopted precursor characteristics, expressing progenitor (OV6, CD133), hepatic lineage (CK18), biliary (CD49f, CK7, CK19), and mesenchymal (vimentin) markers. The supplement of LiCl further enhanced the proliferative capacity by 30%. Transplantation studies revealed extensive repopulation by large donor hepatocyte clusters. Furthermore, bile duct-like structures deriving from donor cells proved to be immunoreactive to ductular markers and formed in close proximity to endogenous bile ducts. Mature hepatocytes reveal their potential to "switch" between phenotypes, adopting progenitor characteristics during proliferation in vitro. Following transplantation, these "retrodifferentiated" cells further expanded in vivo, thereby generating bipotentially differentiated progenies (hepatocytes and bile duct-like structures). This apparent plasticity of mature hepatocytes may open new approaches for cell-based strategies to treat liver disease.

Upregulation of nestin in proximal tubules may participate in cell migration during renal repair

The characteristics of renal tubular progenitor/precursor cells and the role of renal tubule regeneration in the repair of remnant kidneys (RKs) after nephrectomy are not well known. In the present study of a murine model of subtotal nephrectomy, we used immunofluorescence (IF), immunoblot analysis, and in situ hybridization methods to demonstrate that nestin expression was transiently upregulated in tubule cells near the incision edges of RKs. The nestin-positive tubules were immature proximal tubules that colabeled with lotus tetragonolobus agglutinin but not with markers of mature tubules (aquaporin-1, Tamm-Horsfall protein, and aquaporin-2). In addition, many of the nestin-expressing tubule cells were actively proliferative cells, as indicated by colabeling with bromodeoxyuridine. Double-label IF and immunoblot analysis also showed that the upregulation of tubular nestin was associated with enhanced transforming growth factor-β1 (TGF-β1) expression in the incision edge of RKs but not α-smooth muscle actin, which is a marker of fibrosis. In cultured human kidney proximal tubule cells (HKC), immunoblot analysis indicated that TGF-β1 induced nestin expression and loss of E-cadherin expression, suggesting an association of nestin expression and cellular dedifferentiation. Knockdown of nestin expression by a short hairpin RNA-containing plasmid led to decreased migration of HKC cells that were induced by TGF-β1. Taken together, our results suggest that the tubule repair that occurs during the recovery process following nephrectomy may involve TGF-β1-induced nestin expression in immature renal proximal tubule cells and the promotion of renal cell migration.

Participation of liver progenitor cells in liver regeneration: lack of evidence in the AAF/PH rat model

When hepatocyte proliferation is impaired, liver progenitor cells (LPC) are activated to participate in liver regeneration. We used the 2-acetaminofluorene/partial hepatectomy (AAF/PH) model to evaluate the contribution of LPC to liver cell replacement and function restoration. Fischer rats subjected to AAF/PH (or PH alone) were investigated 7, 10 and 14 days post-hepatectomy. Liver mass recovery (LMR) was estimated, and the liver mass to body weight ratio calculated. We used serum albumin and bilirubin levels, and liver albumin mRNA levels to assess the liver function. LPC expansion was analyzed by cytokeratin 19 (CK19), glutathione S-transferase protein (GSTp) immunohistochemistry and by CK19, CD133, transforming growth factor-β1 and hepatocyte growth factor mRNA expression in livers. Cell proliferation was evaluated by Ki67 and BrdU immunostaining. Compared with PH alone where LMR was ∼100% 14 days post-PH, LMR was defective in AAF/PH rats (64.1±15.5%, P=0.0004). LPC expansion was scarce in PH livers (0.5±0.4% of CK19(+) area), but significant in AAF/PH livers (8.5±7.2% of CK19(+)), and inversely correlated to LMR (r(2)=0.63, P<0.0001). A quarter of AAF/PH animals presented liver failure (low serum albumin and high serum bilirubin) 14 days post-PH. Compared with animals with preserved function, this was associated with a lower LMR (50±6.8 vs 74.6±9.4%, P=0.0005), a decreased liver to body weight ratio (2±0.3 vs 3.5±0.6%, P=0.001), and a larger LPC expansion such as proliferating Ki67(+) LPC covered 17.4±4.2% of the liver parenchyma vs 3.1±1.5%, (P<0.0001). Amongst those, rare LPC with an intermediate hepatocyte-like phenotype were seen. Also, less than 2% of hepatocytes were engaged into the cell cycle (Ki67(+)), while more numerous (∼25% of hepatocytes) in the livers with preserved function. These observations suggest that, in this model, the efficient recovery of the liver function was ensured rather by the proliferation of mature hepatocytes than by the LPC expansion and differentiation into hepatocytes.

Hedgehog signaling in the liver

Reactivation of Hedgehog (Hh), a morphogenic signaling pathway that controls progenitor cell fate and tissue construction during embryogenesis occurs during many types of liver injury in adult. The net effects of activating the Hedgehog pathway include expansion of liver progenitor populations to promote liver regeneration, but also hepatic accumulation of inflammatory cells, liver fibrogenesis, and vascular remodeling. All of these latter responses are known to be involved in the pathogenesis of cirrhosis. In addition, Hh signaling may play a role in primary liver cancers, such as cholangiocarcinoma and hepatocellular carcinoma. Study of Hedgehog signaling in liver cells is in its infancy. Additional research in this area is justified given growing experimental and clinical data supporting a role for the pathway in regulating outcomes of liver injury.

Disparate cellular basis of improved liver repair in beta-catenin-overexpressing mice after long-term exposure to 3,5-diethoxycarbonyl-1,4-dihydrocollidine

Administration of a hepatotoxic diet containing 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) induces biliary damage followed by hepatocyte injury, which is repaired through atypical ductular proliferation and oval cells and their subsequent differentiation to bile duct cells and hepatocytes. In this study, we examine whether excess β-catenin in transgenic (TG) mice would provide any reparative advantage in response to DDC. No differences in appearance or numbers of total A6-positive oval cells were observed after DDC administration. However, an increase in A6-positive "atypical hepatocytes" in the TG livers was observed after 14 and 28 days, coinciding with an increase in proliferating cell nuclear antigen-positive hepatocytes. Intriguingly, after chronic DDC administration for 150 days, a further increase in atypical hepatocytes was evident in TG mice, with higher numbers of proliferating cell nuclear antigen-positive hepatocytes exhibiting cytoplasmic/nuclear β-catenin and α-fetoprotein but not CK19, HNF1β, or Trop-2. Coincidently, we observed an improvement in intrahepatic cholestasis as seen by decreases in both serum bilirubin and alkaline phosphatase levels in TG mice, indicating an overall improvement in hepatic repair. TG mice exposed to DDC for 4 weeks followed by 2 days of normal chow showed decreases in alkaline phosphatase, atypical ductular proliferation, and periportal inflammation compared with wild-type animals, verifying improved biliary repair in TG livers. Thus, we report a potential role of β-catenin in liver repair, especially in enhancing the resolution of intrahepatic cholestasis after DDC injury.

Culture of newborn monkey liver epithelial progenitor cells in chemical defined serum-free medium

Studies with hepatic progenitor cells from non-human primates would allow better understanding of their human counterparts. In this study, rhesus monkey liver epithelial progenitor cells (mLEPCs) were derived from a small piece of newborn livers in chemical defined serum-free medium. Digested hepatic cells were treated in Ca(2+)-containing medium to form cell aggregates. Two types of cell aggregates were generated: elongated spindle cells and polygonal epithelial cells. Elongated spindle cells were expressed as vimentin and brachyury, and they were disappeared within 5 d in our cultures. The remaining type consisted of small polygonal epithelial cells that expressed cytokeratin 7 (CK7), CK8, CK18, nestin, CD49f, and E-cad, the markers of hepatic stem cells, but were negative for alpha-fetoprotein, albumin, and CK19. They can proliferate and be passaged, if on laminin or rat tail collagen gel, to initiate colonies. When cultured with dexamethasone and oncostatin M, the expression of mature hepatocyte markers, such as alpha-1-antitrypsin, intracytoplasmic glycogen storage, indocyanine green uptake, and lipid droplet generation, were induced in differentiated cells. If transferred onto mouse embryonic fibroblasts feeders, they gave rise to CK19-positive cholangiocytes with formation of doughnut-like structure. Thus, mLEPCs with bipotency were derived from newborn monkey liver and may serve as a preclinical model for assessment of cell therapy in humans.

The quest for liver progenitor cells: a practical point of view

Many chronic liver diseases can lead to hepatic dysfunction with organ failure. At present, orthotopic liver transplantation represents the benchmark therapy of terminal liver disease. However this practice is limited by shortage of donor grafts, the need for lifelong immunosuppression and very demanding state-of-the-art surgery. For this reason, new therapies have been developed to restore liver function, primarily in the form of hepatocyte transplantation and artificial liver support devices. While already offered in very specialized centers, both of these modalities still remain experimental. Recently, liver progenitor cells have shown great promise for cell therapy, and consequently they have attracted a lot of attention as an alternative or supportive tool for liver transplantation. These liver progenitor cells are quiescent in the healthy liver and become activated in certain liver diseases in which the regenerative capacity of mature hepatocytes and/or cholangiocytes is impaired. Although reports describing liver progenitor cells are numerous, they have not led to a consensus on the identity of the liver progenitor cell. In this review, we will discuss some of the characteristics of these cells and the different ways that have been used to obtain these from rodents. We will also highlight the challenges that researchers are facing in their quest to identify and use liver progenitor cells.

The role of stem cells in liver repair and fibrosis

In response to liver injury or loss of liver mass, proliferation of mature liver cells is the first-line defense to restore liver homeostasis. In the setting of chronic liver disease, however, the ability of hepatocytes and cholangiocytes to proliferate is blocked and small bipotential progenitor cells are activated. Recent studies have established the role of these facultative progenitor cells in injury repair and fibrosis in patients with chronic liver disease and in experimental models. Several signaling pathways linking progenitor cell activation and fibrosis have been identified, and there is increasing evidence that cross-talk (both physical and via soluble factors) between progenitor cells and myofibroblasts is essential for both fibrosis and parenchymal regeneration. Even more exciting are new data examining the cellular components of the progenitor cell niche, demonstrating that both resident liver cells and circulating cells from the bone marrow can function as stem cells, suggesting that there is a surprising degree of phenotypic plasticity such that progenitor cells can contribute to the myofibroblast population and vice versa. We highlight here recent findings from the literature demonstrating the cellular and functional complexity of the progenitor cell niche, and emphasize some of the important questions that remain to drive future research.

Overexpression of Dickkopf 3 in hepatoblastomas and hepatocellular carcinomas

Dickkopf 3 (Dkk3) is a protein expressed at a very early stage of hepatogenesis. In this study, we examined whether Dkk3 was related to a premature or dedifferentiated nature in hepatoblastomas (HBLs) and hepatocellular carcinomas (HCCs). It was demonstrated that Dkk3 was overexpressed in HBLs and HCCs and that its expression was more frequent in the former than in the latter, being consistent with the fact that most HBLs show an embryonal or fetal hepatic histology, whereas there was no distinct relationship between Dkk3 expression and clinical data or histology. All of the HBLs expressed Dkk3, alpha-fetoprotein (AFP), or both proteins, suggesting that, similar to AFP, Dkk3 is another potentially useful biomarker detecting a wide range of HBLs. Furthermore, Dkk3 and AFP were expressed reciprocally in the tumors. These results suggest that Dkk3 may be related to the premature or dedifferentiated nature of HBLs and HCCs, whereas AFP may be related to a more differentiated nature. Thus, assessment of Dkk3 and AFP may be useful in the diagnosis of hepatic tumors.

Relation between liver progenitor cell expansion and extracellular matrix deposition in a CDE-induced murine model of chronic liver injury

In chronic liver injury, liver progenitor cells (LPCs) proliferate in the periportal area, migrate inside the lobule, and undergo further differentiation. This process is associated with extracellular matrix (ECM) remodeling. We analyzed LPC expansion and matrix accumulation in a choline-deficient, ethionine-supplemented (CDE) model of LPC proliferation. After day 3, CDE induced collagen deposits in the periportal area. Expansion of LPCs as assessed by increased number of cytokeratin 19 (CK19)-positive cells was first observed at day 7, while ECM accumulated 10 times more than in controls. Thereafter, LPCs and ECM increased in parallel. Furthermore, ECM not only accumulates prior to the increase in number of LPCs, but is also found in front of LPCs along the porto-venous gradient of lobular invasion. Double immunostaining revealed that LPCs are embedded in ECM at all times. Moreover, LPCs infiltrating the liver parenchyma are chaperoned by alpha-smooth muscle actin (alpha-SMA)-positive cells. Gene expression analyses confirmed these observations. The expression of CK19, alpha-fetoprotein, E-cadherin, and CD49f messenger RNA (mRNA), largely overexpressed by LPCs, significantly increased between day 7 and day 10. By contrast, at day 3 there was a rapid burst in the expression of components of the ECM, collagen I and laminin, as well as in alpha-SMA and connective tissue growth factor expression.

CONCLUSION

Our data demonstrate that, in a CDE model, ECM deposition and activation of matrix-producing cells occurred as an initial phase, prior to LPC expansion, and in front of LPCs along the porto-venous gradient of lobular invasion. Those observations may reveal a fundamental role for the established hepatic microenvironment or niche during the process of activation and differentiation of liver progenitor cells.

Recent progress on tissue-resident adult stem cell biology and their therapeutic implications

Recent progress in the field of the stem cell research has given new hopes to treat and even cure diverse degenerative disorders and incurable diseases in human. Particularly, the identification of a rare population of adult stem cells in the most tissues/organs in human has emerged as an attractive source of multipotent stem/progenitor cells for cell replacement-based therapies and tissue engineering in regenerative medicine. The tissue-resident adult stem/progenitor cells offer the possibility to stimulate their in vivo differentiation or to use their ex vivo expanded progenies for cell replacement-based therapies with multiple applications in human. Among the human diseases that could be treated by the stem cell-based therapies, there are hematopoietic and immune disorders, multiple degenerative disorders, such as Parkinson's and Alzheimer's diseases, type 1 or 2 diabetes mellitus as well as eye, liver, lung, skin and cardiovascular disorders and aggressive and metastatic cancers. In addition, the genetically-modified adult stem/progenitor cells could also be used as delivery system for expressing the therapeutic molecules in specific damaged areas of different tissues. Recent advances in cancer stem/progenitor cell research also offer the possibility to targeting these undifferentiated and malignant cells that provide critical functions in cancer initiation and progression and disease relapse for treating the patients diagnosed with the advanced and metastatic cancers which remain incurable in the clinics with the current therapies.

Fate-mapping evidence that hepatic stellate cells are epithelial progenitors in adult mouse livers

Liver injury activates quiescent hepatic stellate cells (Q-HSC) to proliferative myofibroblasts. Accumulation of myofibroblastic hepatic stellate cells (MF-HSC) sometimes causes cirrhosis and liver failure. However, MF-HSC also promote liver regeneration by producing growth factors for oval cells, bipotent progenitors of hepatocytes and cholangiocytes. Genes that are expressed by primary hepatic stellate cell (HSC) isolates overlap those expressed by oval cells, and hepatocytic and ductular cells emerge when HSC are cultured under certain conditions. We evaluated the hypothesis that HSC are a type of oval cell and, thus, capable of generating hepatocytes to regenerate injured livers. Because Q-HSC express glial fibrillary acidic protein (GFAP), we crossed mice in which GFAP promoter elements regulated Cre-recombinase with ROSA-loxP-stop-loxP-green fluorescent protein (GFP) mice to generate GFAP-Cre/GFP double-transgenic mice. These mice were fed methionine choline-deficient, ethionine-supplemented diets to activate and expand HSC and oval cell populations. GFP(+) progeny of GFAP-expressing precursors were characterized by immunohistochemistry. Basal expression of mesenchymal markers was negligible in GFAP(+)Q-HSC. When activated by liver injury or culture, HSC downregulated expression of GFAP but remained GFP(+); they became highly proliferative and began to coexpress markers of mesenchyme and oval cells. These transitional cells disappeared as GFP-expressing hepatocytes emerged, began to express albumin, and eventually repopulated large areas of the hepatic parenchyma. Ductular cells also expressed GFAP and GFP, but their proliferative activity did not increase in this model. These findings suggest that HSC are a type of oval cell that transitions through a mesenchymal phase before differentiating into hepatocytes during liver regeneration. Disclosure of potential conflicts of interest is found at the end of this article.

Accumulation of hedgehog-responsive progenitors parallels alcoholic liver disease severity in mice and humans

BACKGROUND & AIMS

Improving outcomes in alcoholic liver disease (ALD) necessitates better understanding of how habitual ethanol (EtOH) consumption alters normal regenerative mechanisms within the liver. Hedgehog (Hh) pathway activation promotes expansion of progenitor populations in other tissues. We evaluated the hypothesis that chronic EtOH exposure activates Hh signaling in liver.

METHODS

Hh signaling, liver progenitors, transforming growth factor (TGF)-beta induction, and liver damage were compared in mice fed chow, high-fat diets (HF), or HF + EtOH for 4 weeks. Susceptibility to TGF-beta-mediated apoptosis was compared in Hh-responsive liver cells (eg, immature cholangiocytes and oval cells) and mature hepatocytes (which are unresponsive to Hh). Hepatic accumulation of Hh-responsive cells were compared in controls and ALD patients and correlated with a discriminant function (DF) that predicts subacute mortality.

RESULTS

Hh signaling and numbers of Hh-responsive cells were increased in HF mice and greatest in HF+EtOH mice. In both, progenitor and stromal cell populations harbored Hh-responsive cells. More ductular-type progenitors and fibrosis markers were noted in HF+EtOH mice than in HF mice. The former also expressed more TGF-beta-1. TGF-beta-1 treatment selectively promoted the viability of Hh-responsive immature liver cells and caused mature hepatocytes that survived to produce Hh ligands. Hh-responsive cells were increased in ALD patients. Lobular accumulation of Hh-responsive immature ductular cells was greater in those with a DF >32 than those with a DF <32.

CONCLUSIONS

Hh signaling is increased in ALD and may influence ALD outcomes by promoting hepatic accumulation of immature ductular cells.

Hepatic stellate cells: protean, multifunctional, and enigmatic cells of the liver

The hepatic stellate cell has surprised and engaged physiologists, pathologists, and hepatologists for over 130 years, yet clear evidence of its role in hepatic injury and fibrosis only emerged following the refinement of methods for its isolation and characterization. The paradigm in liver injury of activation of quiescent vitamin A-rich stellate cells into proliferative, contractile, and fibrogenic myofibroblasts has launched an era of astonishing progress in understanding the mechanistic basis of hepatic fibrosis progression and regression. But this simple paradigm has now yielded to a remarkably broad appreciation of the cell's functions not only in liver injury, but also in hepatic development, regeneration, xenobiotic responses, intermediary metabolism, and immunoregulation. Among the most exciting prospects is that stellate cells are essential for hepatic progenitor cell amplification and differentiation. Equally intriguing is the remarkable plasticity of stellate cells, not only in their variable intermediate filament phenotype, but also in their functions. Stellate cells can be viewed as the nexus in a complex sinusoidal milieu that requires tightly regulated autocrine and paracrine cross-talk, rapid responses to evolving extracellular matrix content, and exquisite responsiveness to the metabolic needs imposed by liver growth and repair. Moreover, roles vital to systemic homeostasis include their storage and mobilization of retinoids, their emerging capacity for antigen presentation and induction of tolerance, as well as their emerging relationship to bone marrow-derived cells. As interest in this cell type intensifies, more surprises and mysteries are sure to unfold that will ultimately benefit our understanding of liver physiology and the diagnosis and treatment of liver disease.

Recent progress in histochemistry

The progress in discerning the structure and function of cells and tissues in health and disease has been achieved to a large extent by the continued development of new reagents for histochemistry, the improvement of existing techniques and new imaging techniques. This review will highlight some advancements made in these fields.

E-cadherin as a reliable cell surface marker for the identification of liver specific stem cells

Oval cells are liver-specific bipotent stem cells which accumulate in injured liver when proliferation of mature hepatocytes and/or cholangiocytes is impaired. They represent an intermediary cell type with phenotypical characteristics of both, hepatocytes and cholangiocytes. Oval cells express specific cell surface proteins allowing their identification in situ. Most of these cell surface proteins, however, are recognized by antibodies in mouse liver tissue that are not commercially available or work only on frozen sections. We show herein the unequivocal identification of oval cells in paraffin-embedded mouse liver samples based on strong E-cadherin expression different from that of hepatocytes and bile duct cells. By comparing the pattern of E-cadherin expression with that of both, A6-antigen and CD44, we suggest a tight control of E-cadherin expression depending on the differentiation stage of the progenitor cells. In human cirrhotic liver samples E-cadherin expression was found as a common feature of both, typical and atypical reactions, and, thus, can also serve as an indication of the progenitor cell compartment activation.

Remarkable heterogeneity displayed by oval cells in rat and mouse models of stem cell-mediated liver regeneration

The experimental protocols used in the investigation of stem cell-mediated liver regeneration in rodents are characterized by activation of the hepatic stem cell compartment in the canals of Hering followed by transit amplification of oval cells and their subsequent differentiation along hepatic lineages. Although the protocols are numerous and often used interchangeably across species, a thorough comparative phenotypic analysis of oval cells in rats and mice using well-established and generally acknowledged molecular markers has not been provided. In the present study, we evaluated and compared the molecular phenotypes of oval cells in several of the most commonly used protocols of stem cell-mediated liver regeneration-namely, treatment with 2-acetylaminofluorene and partial (70%) hepatectomy (AAF/PHx); a choline-deficient, ethionine-supplemented (CDE) diet; a 3,5-diethoxycarbonyl-1,4-dihydro-collidin (DDC) diet; and N-acetyl-paraaminophen (APAP). Reproducibly, oval cells showing reactivity for cytokeratins (CKs), muscle pyruvate kinase (MPK), the adenosine triphosphate-binding cassette transporter ABCG2/BCRP1 (ABCG2), alpha-fetoprotein (AFP), and delta-like protein 1/preadipocyte factor 1 (Dlk/Pref-1) were induced in rat liver treated according to the AAF/PHx and CDE but not the DDC protocol. In mouse liver, the CDE, DDC, and APAP protocols all induced CKs and ABCG2-positive oval cells. However, AFP and Dlk/Pref-1 expression was rarely detected in oval cells.

CONCLUSION

Our results delineate remarkable phenotypic discrepancies exhibited by oval cells in stem cell-mediated liver regeneration between rats and mice and underline the importance of careful extrapolation between individual species.

Hedgehog-mediated mesenchymal-epithelial interactions modulate hepatic response to bile duct ligation

In bile duct-ligated (BDL) rodents, as in humans with chronic cholangiopathies, biliary obstruction triggers proliferation of bile ductular cells that are surrounded by fibrosis produced by adjacent myofibroblastic cells in the hepatic mesenchyme. The proximity of the myofibroblasts and cholangiocytes suggests that mesenchymal-epithelial crosstalk promotes the fibroproliferative response to cholestatic liver injury. Studying BDL mice, we found that bile duct obstruction induces activity of the Hedgehog (Hh) pathway, a system that regulates the viability and differentiation of various progenitors during embryogenesis. After BDL, many bile ductular cells and fibroblastic-appearing cells in the portal stroma express Hh ligands, receptor and/or target genes. Transwell cocultures of an immature cholangiocyte line that expresses the Hh receptor, Patched (Ptc), with liver myofibroblastic cells demonstrated that both cell types produced Hh ligands that enhanced each other's viability and proliferation. Further support for the concept that Hh signaling modulates the response to BDL was generated by studying PtcLacZ mice, which have an impaired ability to constrain Hh signaling due to a heterozygous deficiency of Ptc. After BDL, PtcLacZ mice upregulated fibrosis gene expression earlier than wild-type controls and manifested an unusually intense ductular reaction, more expanded fibrotic portal areas, and a greater number of lobular necrotic foci. Our findings reveal that adult livers resurrect developmental signaling systems, such as the Hh pathway, to guide remodeling of the biliary epithelia and stroma after cholestatic injury.

The histochemistry and cell biology vade mecum: a review of 2005–2006

The procurement of new knowledge and understanding in the ever expanding discipline of cell biology continues to advance at a breakneck pace. The progress in discerning the physiology of cells and tissues in health and disease has been driven to a large extent by the continued development of new probes and imaging techniques. The recent introduction of semi-conductor quantum dots as stable, specific markers for both fluorescence light microscopy and electron microscopy, as well as a virtual treasure-trove of new fluorescent proteins, has in conjunction with newly introduced spectral imaging systems, opened vistas into the seemingly unlimited possibilities for experimental design. Although it oftentimes proves difficult to predict what the future will hold with respect to advances in disciplines such as cell biology and histochemistry, it is facile to look back on what has already occurred. In this spirit, this review will highlight some advancements made in these areas in the past 2 years.