Isolation and characterization of a putative liver progenitor population after treatment of fetal rat hepatocytes with TGF-beta

Transforming Growth Factor-β-Induced Cell Plasticity in Liver Fibrosis and Hepatocarcinogenesis

The Transforming Growth Factor-beta (TGF-β) family plays relevant roles in the regulation of different cellular processes that are essential for tissue and organ homeostasis. In the case of the liver, TGF-β signaling participates in different stages of disease progression, from initial liver injury toward fibrosis, cirrhosis and cancer. When a chronic injury takes place, mobilization of lymphocytes and other inflammatory cells occur, thus setting the stage for persistence of an inflammatory response. Macrophages produce profibrotic mediators, among them, TGF-β, which is responsible for activation -transdifferentiation- of quiescent hepatic stellate cells (HSC) to a myofibroblast (MFB) phenotype. MFBs are the principal source of extracellular matrix protein (ECM) accumulation and prominent mediators of fibrogenesis. TGF-β also mediates an epithelial-mesenchymal transition (EMT) process in hepatocytes that may contribute, directly or indirectly, to increase the MFB population. In hepatocarcinogenesis, TGF-β plays a dual role, behaving as a suppressor factor at early stages, but contributing to later tumor progression, once cells escape from its cytostatic effects. As part of its potential pro-tumorigenic actions, TGF-β induces EMT in liver tumor cells, which increases its pro-migratory and invasive potential. In parallel, TGF-β also induces changes in tumor cell plasticity, conferring properties of a migratory tumor initiating cell (TIC). The main aim of this review is to shed light about the pleiotropic actions of TGF-β that explain its effects on the different liver cell populations. The cross-talk with other signaling pathways that contribute to TGF-β effects, in particular the Epidermal Growth Factor Receptor (EGFR), will be presented. Finally, we will discuss the rationale for targeting the TGF-β pathway in liver pathologies.

TGF-β Family Signaling in Ductal Differentiation and Branching Morphogenesis

Epithelial cells contribute to the development of various vital organs by generating tubular and/or glandular architectures. The fully developed forms of ductal organs depend on processes of branching morphogenesis, whereby frequency, total number, and complexity of the branching tissue define the final architecture in the organ. Some ductal tissues, like the mammary gland during pregnancy and lactation, disintegrate and regenerate through periodic cycles. Differentiation of branched epithelia is driven by antagonistic actions of parallel growth factor systems that mediate epithelial-mesenchymal communication. Transforming growth factor-β (TGF-β) family members and their extracellular antagonists are prominently involved in both normal and disease-associated (e.g., malignant or fibrotic) ductal tissue patterning. Here, we discuss collective knowledge that permeates the roles of TGF-β family members in the control of the ductal tissues in the vertebrate body.

Expression kinetics of hepatic progenitor markers in cellular models of human liver development recapitulating hepatocyte and biliary cell fate commitment

Due to the limitations of research using human embryos and the lack of a biological model of human liver development, the roles of the various markers associated with liver stem or progenitor cell potential in humans are largely speculative, and based on studies utilizing animal models and certain patient tissues. Human pluripotent stem cell-based in vitro multistage hepatic differentiation systems may serve as good surrogate models for mimicking normal human liver development, pathogenesis and injury/regeneration studies. Here, we describe the implications of various liver stem or progenitor cell markers and their bipotency (i.e. hepatocytic- and biliary-epithelial cell differentiation), based on the pluripotent stem cell-derived model of human liver development. Future studies using the human cellular model(s) of liver and biliary development will provide more human relevant biological and/or pathological roles of distinct markers expressed in heterogeneous liver stem/progenitor cell populations.

New Insights into the Crossroads between EMT and Stemness in the Context of Cancer

The epithelial-mesenchymal transition (EMT) is an example of cellular plasticity, where an epithelial cell acquires a mesenchymal-like phenotype that increases its migratory and invasive properties. Stemness is the ability of stem cells to proliferate in an asymmetric way that allows them to maintain the reservoir of undifferentiated cells with stem cell identity, but also to produce new differentiated cells. Initial works revealed that activation of the EMT program in epithelial cells induces the acquisition of stem cell properties, which in the context of cancer may contribute to the appearance of tumor initiating cells (TIC). However, a number of groups have recently reported that mesenchymal-epithelial transition (MET) is required for efficient metastatic colonization and that EMT may be not necessarily associated with stemness. In this review, we summarize recent findings that extend our knowledge about the crossroads between EMT and stemness and their relevance under physiological or pathological conditions.

Role of the tissue microenvironment as a therapeutic target in hepatocellular carcinoma

Hepatocellular carcinoma is difficult to treat, primarily because the underlying molecular mechanisms driving clinical outcome are still poorly understood. Growing evidence suggests that the tissue microenvironment has a role in the biological behavior of the tumor. The main clinical issue is to identify the best target for therapeutic approaches. Here, we discuss the hypothesis that the entire tissue microenvironment might be considered as a biological target. However, the tissue microenvironment consists of several cellular and biochemical components, each of which displays a distinct biological activity. We discuss the major components of this environment and consider how they may interact to promote tumor/host crosstalk.

Mouse hepatic oval cells require Met-dependent PI3K to impair TGF-β-induced oxidative stress and apoptosis

We have previously shown that oval cells harboring a genetically inactivated Met tyrosine kinase (Met^−/− oval cells) are more sensitive to TGF-β-induced apoptosis than cells expressing a functional Met (Met^flx/flx), demonstrating that the HGF/Met axis plays a pivotal role in oval cell survival. Here, we have examined the mechanism behind this effect and have found that TGF-β induced a mitochondria-dependent apoptotic cell death in Met^flx/flx and Met^−/− oval cells, associated with a marked increase in levels of the BH3-only proteins Bim and Bmf. Bmf plays a key role during TGF-β-mediated apoptosis since knocking down of BMF significantly diminished the apoptotic response in Met^−/− oval cells. TGF-β also induced oxidative stress accompanied by NADPH oxidase 4 (Nox4) mRNA up-regulation and decreased protein levels of antioxidant enzymes. Antioxidants inhibit both TGF-β-induced caspase 3 activity and Bmf up-regulation, revealing an oxidative stress-dependent Bmf regulation by TGF-β. Notably, oxidative stress-related events were strongly amplified in Met^−/− oval cells, emphasizing the critical role of Met in promoting survival. Pharmacological inhibition of PI3K did impair HGF-driven protection from TGF-β-induced apoptosis and increased sensitivity of Met^flx/flx oval cells to TGF-ß by enhancing oxidative stress, reaching apoptotic indices similar to those obtained in Met^−/− oval cells. Interestingly, both PI3K inhibition and/or knockdown itself resulted in caspase-3 activation and loss of viability in Met^flx/flx oval cells, whereas no effect was observed in Met^−/− oval cells. Altogether, results presented here provide solid evidences that both paracrine and autocrine HGF/Met signaling requires PI3K to promote mouse hepatic oval cell survival against TGF-β-induced oxidative stress and apoptosis.

Culture of porcine hepatocytes or bile duct epithelial cells by inductive serum-free media

A serum-free, feeder cell-dependent, selective culture system for the long-term culture of porcine hepatocytes or cholangiocytes was developed. Liver cells were isolated from 1-wk-old pigs or young adult pigs (25 and 63 kg live weight) and were placed in primary culture on feeder cell layers of mitotically blocked mouse fibroblasts. In serum-free medium containing 1% DMSO and 1 μM dexamethasone, confluent monolayers of hepatocytes formed and could be maintained for several wk. Light and electron microscopic analysis showed hepatocytes with in vivo-like morphology, and many hepatocytes were sandwiched between the feeder cells. When isolated liver cells were cultured in medium without dexamethasone but with 0.5% DMSO, monolayers of cholangioctyes formed that subsequently self-organized into networks of multicellular ductal structures, and whose cells had monocilia projecting into the lumen of the duct. Gamma-glutamyl transpeptidase (GGT) was expressed by the cholangiocytes at their apical membranes, i.e., at the inner surface of the ducts. Cellular GGT activity increased concomitantly with the development of ductal structures. Cytochrome P-450 was determined in microsomes following addition of metyrapone to the cultures. In vivo-like levels of P-450s were found in hepatocyte monolayers while levels of P-450 were markedly reduced in cholangiocyte monolayers. Serum protein secretion in conditioned media was analyzed by Western blot and indicated that albumin, transferrin, and haptoglobin levels were maintained in hepatocytes while albumin and haptoglobin declined over time in cholangiocytes. Quantitative RT-PCR analysis showed that serum protein mRNA levels were significantly elevated in the hepatocytes monolayers in comparison to the bile ductule-containing monolayers. Further, mRNAs specific to cholangiocyte differentiation and function were significantly elevated in bile ductule monolayers in comparison to hepatocyte monolayers. The results demonstrate an in vitro model for the study of either porcine hepatocytes or cholangiocytes with in vivo-like morphology and function.

Epidermal growth factor induces cytokeratin 19 expression accompanied by increased growth abilities in human hepatocellular carcinoma

Cytokeratin (CK) 19-positive hepatocellular carcinoma (HCC) has been reported to have a poor prognosis. The mechanism of the development of CK19-positive HCC remains to be studied. To clarify this, in vitro experiments were performed using human HCC cell lines (PLC-5, HepG2), and the phenotypic changes after stimulation with several growth factors were examined using quantitative reverse transcriptase PCR, western blotting, and immunofluorescence staining. In vivo experiments using human HCC specimens obtained from a total of 78 patients and clinicopathological analysis were also performed. Among the growth factors tested, epidermal growth factor (EGF) had prominent effects on inducing CK19 expression in PLC-5 and HepG2, which was accompanied by the reduced expression of α-fetoprotein in PLC-5. The induction of CK19 expression after EGF stimulation was accompanied by the phosphorylation of c-Jun-N-terminal kinase (JNK)/stress-activated protein kinase, which was blocked by the addition of JNK inhibitors. EGF also increased proliferative abilities and invasive properties of the HCC cell lines. In vivo, 9 (12%) of 78 HCC cases showed positive immunohistochemical staining of CK19. The extent of positive immunohistochemical signals of EGF, EGF receptor (EGFR), and JNK expression was significantly intense in CK-19-positive HCC than those of CK19-negative HCC. Clinicopathological analysis showed that CK19-positive HCC had a high incidence of portal vein invasion, extrahepatic metastasis and an early relapse, which was associated with the worsened 2-year disease free survival. These results indicate that the activation of the EGF-EGFR signaling pathway is associated with the development of CK19-positive HCC, and the EGF-induced increase in growth abilities of HCC may account for the poor prognosis of the patients.

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.

Expression of hepatocytic- and biliary-specific transcription factors in regenerating bile ducts during hepatocyte-to-biliary epithelial cell transdifferentiation

Background

Under compromised biliary regeneration, transdifferentiation of hepatocytes into biliary epithelial cells (BEC) has been previously observed in rats, upon exposure to BEC-specific toxicant methylene dianiline (DAPM) followed by bile duct ligation (BDL), and in patients with chronic biliary liver disease. However, mechanisms promoting such transdifferentiation are not fully understood. In the present study, acquisition of biliary specific transcription factors by hepatocytes leading to reprogramming of BEC-specific cellular profile was investigated as a potential mechanism of transdifferentiation in two different models of compromised biliary regeneration in rats.

Results

In addition to previously examined DAPM + BDL model, an experimental model resembling chronic biliary damage was established by repeated administration of DAPM. Hepatocyte to BEC transdifferentiation was tracked using dipetidyl dipeptidase IV (DDPIV) chimeric rats that normally carry DPPIV only in hepatocytes. Following DAPM treatment, ~20% BEC population turned DPPIV-positive, indicating that they are derived from DPPIV-positive hepatocytes. New ductules emerging after DAPM + BDL and repeated DAPM exposure expressed hepatocyte-associated transcription factor hepatocyte nuclear factor (HNF) 4α and biliary specific transcription factor HNF1β. In addition, periportal hepatocytes expressed biliary marker CK19 suggesting periportal hepatocytes as a potential source of transdifferentiating cells. Although TGFβ1 was induced, there was no considerable reduction in periportal HNF6 expression, as observed during embryonic biliary development.

Conclusions

Taken together, these findings indicate that gradual loss of HNF4α and acquisition of HNF1β by hepatocytes, as well as increase in TGFβ1 expression in periportal region, appear to be the underlying mechanisms of hepatocyte-to-BEC transdifferentiation.

Growth factor- and cytokine-driven pathways governing liver stemness and differentiation

Liver is unique in its capacity to regenerate in response to injury or tissue loss. Hepatocytes and other liver cells are able to proliferate and repopulate the liver. However, when this response is impaired, the contribution of hepatic progenitors becomes very relevant. Here, we present an update of recent studies on growth factors and cytokine-driven intracellular pathways that govern liver stem/progenitor cell expansion and differentiation, and the relevance of these signals in liver development, regeneration and carcinogenesis. Tyrosine kinase receptor signaling, in particular, c-Met, epidermal growth factor receptors or fibroblast growth factor receptors, contribute to proliferation, survival and differentiation of liver stem/progenitor cells. Different evidence suggests a dual role for the transforming growth factor (TGF)-β signaling pathway in liver stemness and differentiation. On the one hand, TGF-β mediates progression of differentiation from a progenitor stage, but on the other hand, it contributes to the expansion of liver stem cells. Hedgehog family ligands are necessary to promote hepatoblast proliferation but need to be shut off to permit subsequent hepatoblast differentiation. In the same line, the Wnt family and β-catenin/T-cell factor pathway is clearly involved in the maintenance of liver stemness phenotype, and its repression is necessary for liver differentiation during development. Collectively, data indicate that liver stem/progenitor cells follow their own rules and regulations. The same signals that are essential for their activation, expansion and differentiation are good candidates to contribute, under adequate conditions, to the paradigm of transformation from a pro-regenerative to a pro-tumorigenic role. From a clinical perspective, this is a fundamental issue for liver stem/progenitor cell-based therapies.

Hepatic cancer stem cells and drug resistance: Relevance in targeted therapies for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of most common malignancies in the world. Systemic treatments for HCC, particularly for advanced stages, are limited by the drug resistance phenomenon which ultimately leads to therapy failure. Recent studies have indicated an association between drug resistance and the existence of the cancer stem cells (CSCs) as tumor initiating cells. The CSCs are resistant to conventional chemotherapies and might be related to the mechanisms of the ATP Binding Cassette (ABC) transporters and alterations in the CSCs signaling pathways. Therefore, to contribute to the development of new HCC treatments, further information on the characterization of CSCs, the modulation of the ABC transporters expression and function and the signaling pathway involved in the self renewal, initiation and maintenance of the cancer are required. The combination of transporters modulators/inhibitors with molecular targeted therapies may be a potent strategy to block the tumoral progression. This review summarizes the association of CSCs, drug resistance, ABC transporters activities and changes in signaling pathways as a guide for future molecular therapy for HCC.

Role of CXCR4/SDF-1 alpha in the migratory phenotype of hepatoma cells that have undergone epithelial-mesenchymal transition in response to the transforming growth factor-beta

Treatment of FaO rat hepatoma cells with TGF-beta selects cells that survive to its apoptotic effect and undergo epithelial-mesenchymal transitions (EMT). We have established a cell line (T beta T-FaO, from TGF-beta-treated FaO) that shows a mesenchymal, de-differentiated, phenotype in the presence of TGF-beta and is refractory to its suppressor effects. In the absence of this cytokine, cells revert to an epithelial phenotype in 3-4 weeks and recover the response to TGF-beta. T beta T-FaO show higher capacity to migrate than that observed in the parental FaO cells. We found that FaO cells express low levels of CXCR4 and do not respond to SDF-1 alpha. However, TGF-beta up-regulates CXCR4, through a NF kappaB-dependent mechanism, and T beta T-FaO cells show elevated levels of CXCR4, which is located in the presumptive migration front. A specific CXCR4 antagonist (AMD3100) attenuates the migratory capacity of T beta T-FaO cells on collagen gels. Extracellular SDF-1 alpha activates the ERKs pathway in T beta T-FaO, but not in FaO cells, increasing cell scattering and protecting cells from apoptosis induced by serum deprivation. Targeted knock-down of CXCR4 with specific siRNA blocks the T beta T-FaO response to SDF-1 alpha. Thus, the SDF-1/CXCR4 axis might play an important role in mediating cell migration and survival after a TGF-beta-induced EMT in hepatoma cells.

Expression of CD52 mRNA in the rat embryo

CD52 is a leukocyte differentiation antigen first discovered in humans as expressed on the surface of lymphocytes, monocytes and eosinophils. The human CD52 is found on chromosome 1, and two alleles are both known to be reasonably common. A closely homologous gene has been identified in the cynomologous monkey and related genes have been found in mouse, rat and dog. The role of CD52 in lymphocyte is still unclear but the anti-CD52 antibodies named CAMPATH-1 antibodies are largely used for therapy where depletion of lymphocytes is required. In the past expression of the antigen on progenitors of leukocytes in bone marrow had been excluded, but recent work indicates CD52 is highly expressed on cells with colony-forming and NOD/SCID (non-obese diabetic-severe combined immunodeficiency)-engrafting capacities, both at the mRNA and membrane protein level. We have investigated CD52 expression during development in rat embryos by in situ hybridization. We report here that the antigen is highly expressed in the liver that is the major organ where multipotent hematopietic stem cells differentiate but also in the splancnopleuric mesoderm, at early stages of embryo differentiation, where hematopietic stem cells are suggested to arise. CD52+ cells were found in areas active in vasculogenesis at early embryo stages and in the walls of the vessels in the liver at mid gestation. CD52+ cells were also found to emerge among c-Kit positive cells.

Pathobiology of biliary epithelia and cholangiocarcinoma: proceedings of the Henry M. and Lillian Stratton Basic Research Single-Topic Conference

In June 2008, the American Association for the Study of Liver Diseases (AASLD) sponsored the Henry M. and Lillian Stratton Basic Research Single-Topic Conference on the Pathobiology of Biliary Epithelia and Cholangiocarcinoma, which was held in Atlanta, GA. Attendees from 12 different countries participated in this conference, making it a truly international scientific event. Both oral and poster presentations were given by multidisciplinary experts, who highlighted important areas of current basic and translational research on biliary epithelial cell biology and pathophysiology, and on the etiology, cellular and molecular pathogenesis, and target-based therapy of cholangiocarcinoma. The specific goals and objectives of the conference were: (1) to advance knowledge of basic and molecular mechanisms underlying developmental and proliferative disorders of the biliary tract; (2) to foster a better and more comprehensive understanding of mechanisms regulating biliary epithelial (cholangiocyte) growth and transport, signaling, cell survival, and abnormalities that result in disease; and (3) to understand basic mechanisms of cholangiocarcinoma development and progression, with the added goal of identifying and exploiting potentially critical molecular pathways that may be targeted therapeutically. A number of interrelated themes emerged from the oral and poster sessions that affected current understandings of the complex organization of transcriptional and signaling mechanisms that regulate bile duct development, hepatic progenitor cell expansion, cholangiocyte secretory functions and proliferation, and mechanisms of cholangiocarcinogenesis and malignant cholangiocyte progression. Most notable were the critical questions raised as to how best to exploit aberrant signaling pathways associated with biliary disease as potential targets for therapy.