Protection against Fas-induced fulminant hepatic failure in liver specific integrin linked kinase knockout mice

Background

Programmed cell death or apoptosis is an essential process for tissue homeostasis. Hepatocyte apoptosis is a common mechanism to many forms of liver disease. This study was undertaken to test the role of ILK in hepatocyte survival and response to injury using a Jo-2-induced apoptosis model.

Methods

For survival experiments, ILK KO and WT mice received a single intraperitoneal injection of the agonistic anti-Fas monoclonal antibody Jo-2 at the lethal dose (0.4 μg/g body weight) or sublethal dose (0.16 μg/g body weight). For further mechanistic studies sublethal dose of Fas monoclonal antibody was chosen.

Results

There was 100% mortality in the WT mice as compared to 50% in the KO mice. We also found that hepatocyte specific ILK KO mice (integrin linked kinase) died much later than WT mice after challenge with a lethal dose of Fas agonist Jo-2. At sublethal dose of Jo-2, there was 20% mortality in KO mice with minimal apoptosis whereas WT mice developed extensive apoptosis and liver injury leading to 70% mortality due to liver failure at 12 h. Proteins known to be associated with cell survival/death were differentially expressed in the 2 groups. In ILK KO mice there was downregulation of proapoptotic genes and upregulation of antiapoptotic genes.

Conclusions

Mechanistic insights revealed that pro-survival pathways such as Akt, ERK1/2, and NFkB signaling were upregulated in the ILK KO mice. Inhibition of only NFkB and ERK1/2 signaling led to an increase in the susceptibility of ILK KO hepatocytes to Jo-2-induced apoptosis. These studies suggest that ILK elimination from hepatocytes protects against Jo-2 induced apoptosis by upregulating survival pathways. FAK decrease may also play a role in this process. The results presented show that the signaling effects of ILK related to these functions are mediated in part mediated through NFkB and ERK1/2 signaling.

Genes inducing iPS phenotype play a role in hepatocyte survival and proliferation in vitro and liver regeneration in vivo

Reprogramming factors have been used to induce pluripotent stem cells as an alternative to somatic cell nuclear transfer technology in studies targeting disease models and regenerative medicine. The neuronal repressor RE-1 silencing transcription factor (REST) maintains self-renewal and pluripotency in mouse embryonic stem cells by maintaining the expression of Oct3/4, Nanog, and cMyc. We report that primary hepatocytes express REST and most of the reprogramming factors in culture. Their expression is up-regulated by hepatocyte growth factor (HGF) and epidermal growth factor (EGF). REST inhibition results in down-regulation of reprogramming factor expression, increased apoptosis, decreased proliferation, and cell death. The reprogramming factors are also up-regulated after 70% partial hepatectomy in vivo.

CONCLUSION

These findings show that genes inducing the iPS phenotype, even though expressed at lower levels than embryonic stem cells, nonetheless are associated with control of apoptosis and cell proliferation in hepatocytes in culture and may play a role in such processes during liver regeneration.

Hepatocyte proliferation and hepatomegaly induced by phenobarbital and 1,4-bis [2-(3,5-dichloropyridyloxy)] benzene is suppressed in hepatocyte-targeted glypican 3 transgenic mice

Glypican 3 (GPC3) is a family of glycosylphosphatidylinositol-anchored, cell-surface heparan sulfate proteoglycans. Loss-of-function mutations of GPC3 cause Simpson-Golabi-Behmel syndrome characterized by overgrowth of multiple organs, including liver. Our previous study showed that in GPC3 transgenic (TG) mice, hepatocyte-targeted overexpression of GPC3 suppresses hepatocyte proliferation and liver regeneration after partial hepatectomy and alters gene expression profiles and potential cell cycle-related proteins. This study investigates the role of GPC3 in hepatocyte proliferation and hepatomegaly induced by the xenobiotic mitogens phenobarbital (PB) and TCPOBOP (1, 4-bis [2-(3, 5-dichloropyridyloxy)] benzene). Wildtype (WT) and GPC3 TG mice were given 0.1% PB in drinking water for 10 days or a single dose of TCPOBOP (3 mg/kg) by oral gavage. At day 5 the WT mice showed a 2.2- and 3.0-fold increase in liver weight, whereas the GPC3 TG mice showed a 1.3- and 1.6-fold increase in liver weight after PB and TCPOBOP administration, respectively. There was a significant suppression of proliferative response in the GPC3 TG mice, as assessed by percent of Ki67-positive hepatocyte nuclei. Moreover, gene array analysis showed a panel of changes in the gene expression profile of TG mice, both before and after administration of the xenobiotic mitogens. Expression of cell cycle-related genes in the TG mice was also decreased compared to the WT mice.

CONCLUSION

Our results indicate that in GPC3 TG mice, hepatocyte-targeted overexpression of GPC3 plays an important role for regulation of liver size and termination of hepatocyte proliferation induced by the xenobiotic mitogens PB and TCPOBOP, comparable to the effects seen in the GPC3 TG mice during liver regeneration after partial hepatectomy.

Long term cultures of primary human hepatocytes as an alternative to drug testing in animals

Due to species differences, primary human hepatocytes are still the in vitro system of choice to analyse liver specific processes and functions. Human hepatocytes were cultured for several weeks in a serum-free two-dimensional culture system, which was used to study the effects of acetaminophen (APAP) on hepatocellular functions and vitality. Non-invasive determinations of albumin, urea and lactate dehydrogenase concentrations in cell culture supernatants allowed continuous monitoring for at least two weeks. APAP was applied every 4 days for 24 h. Each application reduced urea production by 25% and albumin synthesis by approximately 70% without any effects on cellular viability. After removal of the substance, hepatocellular functions returned to control levels within one (urea) to three (albumin) days. The repetitive analyses of APAP-mediated effects on cellular metabolism led to identical results for up to five cycles. The drug also caused reversible and repetitive ultrastructural modifications, in particular an almost complete replacement of rough endoplasmic reticulum by smooth endoplasmic reticulum and a massive degradation of glycogen stores. The data demonstrate the suitability of the culture system to serve as a model for repetitive testing of drug-mediated changes on hepatocellular functions, thereby reducing animal studies during drug development.

Excessive hepatomegaly of mice with hepatocyte-targeted elimination of integrin linked kinase following treatment with 1,4-bis [2-(3,5-dichaloropyridyloxy)] benzene

TCBOPOP (1,4-bis [2-(3,5-dichaloropyridyloxy)] benzene) an agonist of the constitutive androstane receptor (CAR), produces rapid hepatocyte hyperplasia and hepatomegaly in the absence of hepatic injury. In this study we demonstrate that integrin-linked kinase (ILK), which is involved in transmission of the extracellular matrix (ECM) signaling by way of integrin receptors, plays an important role in regulating TCPOBOP-induced proliferation of hepatocytes and hepatomegaly. Hepatocyte-specific ILK knockout mice (ILK/liver-/- mice) and wildtype mice (WT) were given a single dose of TCPOBOP (3 mg/kg) by oral gavage. Mice were sacrificed at days 1, 2, 5, and 7 after TCPOBOP administration. WT mice showed maximum proliferation on days 1 and 2, which came back to baseline levels by days 5 and 7 after TCPOBOP administration. The ILK/liver-/- mice, on the other hand, showed a prolonged and a sustained proliferative response as evident by an increased number of proliferative cell nuclear antigen assay (PCNA)-positive cells even at days 5 and 7 after TCPOBOP administration. At day 7 the WT mice showed close to a 2.5-fold increase in liver weight, whereas the ILK/liver-/- mice showed a 3.7-fold increase in liver weight. The prolonged proliferative response in the ILK/liver-/- mice seems to be due to sustained induction of CAR leading to sustained induction of c-Myc, which is known to be a key mediator of TCPOPOP-CAR induced direct liver hyperplasia.

CONCLUSION

The data indicate that ECM-mediated signaling by way of ILK is essential for adjustment of final liver size and proper termination of TCPOBOP-induced proliferation of hepatocytes.

Scaffolds containing growth factors and extracellular matrix induce hepatocyte proliferation and cell migration in normal and regenerating rat liver

BACKGROUND & AIMS

Intrahepatic drug delivery from implantable scaffolds is being developed as a strategy to modulate growth and enhance regeneration at the time of liver resection. In this study we examine the effects of scaffolds containing hepatocyte growth factor, epidermal growth factor, fibroblast growth factor 1, fibroblast growth factor 2, and liver-derived extracellular matrix (L-ECM) when implanted into normal and partially hepatectomized rat livers.

METHODS

Scaffolds loaded with combinations of growth factors and L-ECM were implanted into normal livers (controls=L-ECM, polymer or sham) and livers following partial hepatectomy (controls=partial hepatectomy or sham). The primary end points were hepatocyte DNA synthesis and liver tissue penetration into scaffolds. Secondary end points included non-parenchymal cell DNA synthesis, liver weight analysis, liver function, and histological characterisation of the peri-implant parenchyma.

RESULTS

Four days after implantation in normal livers, there was significantly more hepatocyte proliferation around growth factor scaffolds than controls. Seven days after implantation, there was significantly more tissue penetration into growth factor scaffolds than control scaffolds. ED-1 and desmin positive cells were present in the pores of scaffolds. Two days after partial hepatectomy, there was significantly more hepatocyte proliferation around scaffold implanted livers than after partial hepatectomy alone.

CONCLUSIONS

Growth factors and L-ECM accelerated non-parenchymal cell migration into scaffolds and increased hepatocyte and non-parenchymal cell proliferation around them. These results demonstrate the potential for intrahepatic implantation of scaffolds containing growth factors and L-ECM to modulate growth in the normal and regenerating liver.

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.

Suppression of liver regeneration and hepatocyte proliferation in hepatocyte-targeted glypican 3 transgenic mice

Glypican 3 (GPC3) belongs to a family of glycosylphosphatidylinositol-anchored, cell-surface heparan sulfate proteoglycans. GPC3 is overexpressed in hepatocellular carcinoma. Loss-of-function mutations of GPC3 result in Simpson-Golabi-Behmel syndrome, an X-linked disorder characterized by overgrowth of multiple organs, including the liver. Our previous study showed that GPC3 plays a negative regulatory role in hepatocyte proliferation, and this effect may involve CD81, a cell membrane tetraspanin. To further investigate GPC3 in vivo, we engineered transgenic (TG) mice overexpressing GPC3 in the liver under the control of the albumin promoter. GPC3 TG mice with hepatocyte-targeted, overexpressed GPC3 developed normally in comparison with their nontransgenic littermates but had a suppressed rate of hepatocyte proliferation and liver regeneration after partial hepatectomy. Moreover, gene array analysis revealed a series of changes in the gene expression profiles in TG mice (both in normal mice and during liver regeneration). In unoperated GPC3 TG mice, there was overexpression of runt related transcription factor 3 (7.6-fold), CCAAT/enhancer binding protein alpha (2.5-fold), GABA A receptor (2.9-fold), and wingless-related MMTV integration site 7B (2.8-fold). There was down-regulation of insulin-like growth factor binding protein 1 (8.4-fold), Rab2 (5.6-fold), beta-catenin (1.7-fold), transforming growth factor beta type I (3.1-fold), nodal (1.8-fold), and yes-associated protein (1.4-fold). Changes after hepatectomy included decreased expression in several cell cycle-related genes.

CONCLUSION

Our results indicate that in GPC3 TG mice, hepatocyte overexpression of GPC3 suppresses hepatocyte proliferation and liver regeneration and alters gene expression profiles, and potential cell cycle-related proteins and multiple other pathways are involved and affected.

Cytosolic phospholipase A2alpha and peroxisome proliferator-activated receptor gamma signaling pathway counteracts transforming growth factor beta-mediated inhibition of primary and transformed hepatocyte growth

Hepatocellular carcinoma often develops in the setting of abnormal hepatocyte growth associated with chronic hepatitis and liver cirrhosis. Transforming growth factor beta (TGF-beta) is a multifunctional cytokine pivotal in the regulation of hepatic cell growth, differentiation, migration, extracellular matrix production, stem cell homeostasis, and hepatocarcinogenesis. However, the mechanisms by which TGF-beta influences hepatic cell functions remain incompletely defined. We report herein that TGF-beta regulates the growth of primary and transformed hepatocytes through concurrent activation of Smad and phosphorylation of cytosolic phospholipase A(2)alpha (cPLA(2)alpha), a rate-limiting key enzyme that releases arachidonic acid for the production of bioactive eicosanoids. The interplays between TGF-beta and cPLA(2)alpha signaling pathways were examined in rat primary hepatocytes, human hepatocellular carcinoma cells, and hepatocytes isolated from newly developed cPLA(2)alpha transgenic mice.

CONCLUSION

Our data show that cPLA(2)alpha activates peroxisome proliferator-activated receptor gamma (PPAR-gamma) and thus counteracts Smad2/3-mediated inhibition of cell growth. Therefore, regulation of TGF-beta signaling by cPLA(2)alpha and PPAR-gamma may represent an important mechanism for control of hepatic cell growth and hepatocarcinogenesis.

Integrin alpha 7 interacts with high temperature requirement A2 (HtrA2) to induce prostate cancer cell death

Integrins are a family of receptors for extracellular matrix proteins that have critical roles in human tissue development. Previous studies identified down-regulation and/or mutations of integrin alpha7 (ITGA7) in prostate cancer, liver cancer, soft tissue leiomyosarcoma, and glioblastoma multiforme. Here we report that expression of ITGA7 induced apoptosis in the human prostate cancer cell lines PC3 and DU145. Yeast two-hybrid analysis revealed that the C-terminus of ITGA7 interacts with high temperature requirement A2 (HtrA2), a serine protease with a critical role in apoptosis. Expression of ITGA7 increases the protease activity of HtrA2 both in vitro and in vivo. Deletion of the HtrA2 interaction domain abrogates the cell death activity of ITGA7, whereas down-regulation of HtrA2 dramatically reduced cell death mediated by ITGA7. In addition, site-directed protease-null mutant HtrA2S306A expression blocked apoptosis induced by ITGA7. Interestingly, interaction between ITGA7 and its ligand laminin 2 appears to protect against cell death, since depleting laminin beta2 with a small-interfering RNA significantly exacerbated apoptosis induced by ITGA7 expression. This report provides a novel insight into the mechanism by which ITGA7 acts as a tumor suppressor.

Bid agonist regulates murine hepatocyte proliferation by controlling endoplasmic reticulum calcium homeostasis

BH3-interacting domain death agonist (Bid), a BH3-only B cell lymphoma 2 family molecule, is generally known for its importance in activating the mitochondrial apoptosis pathway after death receptor engagement, particularly in hepatocytes. However, Bid also promotes hepatocyte proliferation during liver regeneration and carcinogenesis. This study was designed to examine the hypothesis that Bid regulates endoplasmic reticulum calcium concentration ([Ca(2+)](ER)) homeostasis to affect hepatocyte proliferation. We found that serum-stimulated hepatocyte proliferation was dependent on calcium, and the depletion of calcium with thapsigargin or ethylene glycol tetraacetic acid (EGTA) inhibited the proliferation. Subcellular fractionation showed that a portion of Bid was inserted into the endoplasmic reticulum (ER)-enriched membranes, and single-cell calcium imaging indicated that Bid was important for maintaining the [Ca(2+)](ER) level. Bid-deficient hepatocytes manifested delayed and reduced serum-stimulated proliferation, which was corrected by ionomycin or reconstitution of Bid, particularly an ER-targeted Bid. Finally, B cell lymphoma 2-associated X protein (Bax) could also be found in the ER-enriched membranes, and Bax deficiency caused the same proliferation defect. However, Bid/Bax double deletion in hepatocytes did not further augment the defect, which suggested that Bid and Bax worked by the same regulatory mechanism in [Ca(2+)](ER) control.

CONCLUSION

Bid regulates hepatocyte proliferation by positively affecting [Ca(2+)](ER) homeostasis, and this could be important for liver regeneration and carcinogenesis.

An autophagy-enhancing drug promotes degradation of mutant alpha1-antitrypsin Z and reduces hepatic fibrosis

In the classical form of alpha1-antitrypsin (AT) deficiency, a point mutation in AT alters the folding of a liver-derived secretory glycoprotein and renders it aggregation-prone. In addition to decreased serum concentrations of AT, the disorder is characterized by accumulation of the mutant alpha1-antitrypsin Z (ATZ) variant inside cells, causing hepatic fibrosis and/or carcinogenesis by a gain-of-toxic function mechanism. The proteasomal and autophagic pathways are known to mediate degradation of ATZ. Here we show that the autophagy-enhancing drug carbamazepine (CBZ) decreased the hepatic load of ATZ and hepatic fibrosis in a mouse model of AT deficiency-associated liver disease. These results provide a basis for testing CBZ, which has an extensive clinical safety profile, in patients with AT deficiency and also provide a proof of principle for therapeutic use of autophagy enhancers.

Interaction of integrin-linked kinase and miniature chromosome maintenance 7-mediating integrin {alpha}7 induced cell growth suppression

Mutation of integrin alpha7 (ITGA7) was previously identified in multiple human malignancies. Restoration of ITGA7 expression in prostate cancer and leiomyosarcoma cell lines suppressed tumor growth and cell motility both in vitro and in vivo. In this study, we showed that integrin-linked kinase (ILK) binds with miniature chromosome maintenance 7 (MCM7), a DNA replication licensing protein. A 58-amino acid ILK binding motif was identified in the NH(2)-terminus of MCM7. The expression of ITGA7 induced the phosphorylation of MCM7. Knocking down of ILK abrogated ITGA7-induced MCM7 phosphorylation. ANK, the dominant-negative mutant of ILK, also blocked the phosphorylation of MCM7 induced by ITGA7. The phosphorylation of MCM7 reduced MCM7 chromatin association and inhibited cell growth. A MCM7 mutant that does not bind with ILK did not respond to ITGA7 stimulation, and behaved similarly to a dominant MCM7-negative mutant and neutralized the effect of ITGA7. We conclude that ILK interaction with MCM7 and MCM7 phosphorylation may be a critical event in ITGA7 signaling pathway, leading to tumor suppression.

RNA interference against hepatic epidermal growth factor receptor has suppressive effects on liver regeneration in rats

Liver regeneration after a two-thirds partial hepatectomy (PHx) is a complex process requiring interaction and cooperation of many growth factors and cytokines and cross talk between multiple pathways. Along with hepatocyte growth factor and its receptor MET (HGF-MET), the epidermal growth factor receptor (EGFR) signaling pathway is activated within 60 minutes after PHx. To investigate the role of EGFR in liver regeneration, we used two EGFR-specific short hairpin silencing RNAs to inhibit EGFR expression in regenerating normal rat liver. Suppression of EGFR mRNA and protein was evident in treated rats. There was also a demonstrable decrease but not complete elimination of bromo-deoxyuridine incorporation and mitoses at 24 hours after PHx. In addition, we observed up-regulation of MET and Src as well as activation of the ErbB-3-ErbB-2-PI3K-Akt pathway and down-regulation of STAT 3, cyclin D1, cyclin E1, p21, and C/EBP beta. The decrease in the ratio of C/EBP alpha to C/EBP beta known to occur after PHx was offset in shEGFR-treated rats. Despite suppression of hepatocyte proliferation lasting into day 3 after PHx, liver weight restoration occurred. Interestingly, hepatocytes in shEGFR-treated rats were considerably larger when compared with ScrRNA-treated controls. The data indicate that although the MET and EGFR pathways are similar, the contributions made by MET and EGFR are unique and are not compensated by each other or other cytokines.

Liver regeneration after partial hepatectomy: critical analysis of mechanistic dilemmas

Liver regeneration after partial hepatectomy is one of the most studied models of cell, organ, and tissue regeneration. The complexity of the signaling pathways initiating and terminating this process have provided paradigms for regenerative medicine. Many aspects of the signaling mechanisms involved in hepatic regeneration are under active investigation. The purpose of this review is to focus on the areas still not well understood. The review also aims to provide insights into the ways by which current concepts of liver regeneration can provide understanding regarding malfunction of the regenerative process in liver diseases, such as acute liver failure.

Liver-specific ablation of integrin-linked kinase in mice results in enhanced and prolonged cell proliferation and hepatomegaly after phenobarbital administration

We have recently demonstrated that disruption of extracellular matrix (ECM)/integrin signaling via elimination of integrin-linked kinase (ILK) in hepatocytes interferes with signals leading to termination of liver regeneration. This study investigates the role of ILK in liver enlargement induced by phenobarbital (PB). Wild-type (WT) and ILK:liver-/- mice were given PB (0.1% in drinking water) for 10 days. Livers were harvested on 2, 5, and 10 days during PB administration. In the hepatocyte-specific ILK/liver-/- mice, the liver:body weight ratio was more than double as compared to 0 h at day 2 (2.5 times), while at days 5 and 10, it was enlarged three times. In the WT mice, the increase was as expected from previous literature (1.8 times) and seems to have leveled off after day 2. There were slightly increased proliferating cell nuclear antigen-positive cells in the ILK/liver-/- animals at day 2 as compared to WT after PB administration. In the WT animals, the proliferative response had come back to normal by days 5 and 10. Hepatocytes of the ILK/liver-/- mice continued to proliferate up until day 10. ILK/liver-/- mice also showed increased expression of key genes involved in hepatocyte proliferation at different time points during PB administration. In summary, ECM proteins communicate with the signaling machinery of dividing cells via ILK to regulate hepatocyte proliferation and termination of the proliferative response. Lack of ILK in the hepatocytes imparts prolonged proliferative response not only to stimuli related to liver regeneration but also to xenobiotic chemical mitogens, such as PB.

Hepatocyte growth factor (HGF) modulates GABAergic inhibition and seizure susceptibility

Disrupted ontogeny of forebrain inhibitory interneurons leads to neurological disorders, including epilepsy. Adult mice lacking the urokinase plasminogen activator receptor (Plaur) have decreased numbers of neocortical GABAergic interneurons and spontaneous seizures, attributed to a reduction of hepatocyte growth factor/scatter factor (HGF/SF). We report that by increasing endogenous HGF/SF concentration in the postnatal Plaur null mouse brain maintains the interneuron populations in the adult, reverses the seizure behavior and stabilizes the spontaneous electroencephalogram activity. The perinatal intervention provides a pathway to reverse potential birth defects and ameliorate seizures in the adult.

Liver regeneration: alternative epithelial pathways

Loss of hepatic tissue triggers a regenerative response in the whole organ. Under typical normal conditions, all hepatic cells (epithelial: hepatocytes and biliary epithelial cells; non-epithelial: stellate cells, macrophages and endothelial cells) undergo one to three rounds of replication to establish the original number of cells and restore organ size. The review summarizes the literature of regenerative patterns in situations in which proliferation of either hepatocytes or biliary epithelial cells is inhibited. The evidence strongly suggests that under these circumstances, hepatocytes or biliary epithelial cells can function as facultative stem cells for each other and replenish the inhibited cellular compartment by a process of transdifferentiation, involving complex signaling pathways. These pathways are activated under experimental conditions in rodents and in fulminant hepatitis associated with liver failure in humans. Mechanistic analysis of these pathways has implications for liver biology and for potential therapeutic modalities in human liver disease.

Liver regeneration: alternative epithelial pathways

Loss of hepatic tissue triggers a regenerative response in the whole organ. Under typical normal conditions, all hepatic cells (epithelial: hepatocytes and biliary epithelial cells; non-epithelial: stellate cells, macrophages and endothelial cells) undergo one to three rounds of replication to establish the original number of cells and restore organ size. The review summarizes the literature of regenerative patterns in situations in which proliferation of either hepatocytes or biliary epithelial cells is inhibited. The evidence strongly suggests that under these circumstances, hepatocytes or biliary epithelial cells can function as facultative stem cells for each other and replenish the inhibited cellular compartment by a process of transdifferentiation, involving complex signaling pathways. These pathways are activated under experimental conditions in rodents and in fulminant hepatitis associated with liver failure in humans. Mechanistic analysis of these pathways has implications for liver biology and for potential therapeutic modalities in human liver disease.

Enhanced liver regeneration following changes induced by hepatocyte-specific genetic ablation of integrin-linked kinase

Following liver regeneration after partial hepatectomy, liver grows back precisely to its original mass and does not exceed it. The mechanism regulating this "hepatostat" is not clear and no exceptions have been found to date. Although pathways initiating liver regeneration have been well studied, mechanisms involved in the termination of liver regeneration are unclear. Here, we report that integrin-linked kinase (ILK) (involved in transmission of the extracellular matrix [ECM] signaling by way of integrin receptors) and/or hepatic adaptations that ensue following ILK hepatocyte-targeted removal are critical for proper termination of liver regeneration. Following partial hepatectomy (PHx), mice with a liver-specific ILK ablation (ILK-KO-Liver) demonstrate a termination defect resulting in 58% larger liver than their original pre-PHx mass. This increase in post-PHx liver mass is due to sustained cell proliferation driven in part by increased signaling through hepatocyte growth factor (HGF), and the beta-catenin pathway and Hippo kinase pathways.

CONCLUSION

The data indicate that ECM-mediated signaling by way of ILK is essential in proper termination of liver regeneration. This is the first evidence of a defect leading to impaired termination of regeneration and excessive accumulation of liver weight following partial hepatectomy.

Investigation of the role of glypican 3 in liver regeneration and hepatocyte proliferation

Glypicans are heparan sulfate proteoglycans that are bound to the cell surface by glycosylphosphatidylinositol. While six members of the glypican family are known in mammals, our study focused on glypican 3 (GPC3). Loss-of-function mutations of GPC3 result in the Simpson-Golabi-Behmel syndrome, an X-linked disorder characterized by pre- and postnatal liver and other organ overgrowth. GPC3 is overexpressed in human hepatocellular carcinoma; however, its role in normal liver regeneration and hepatocyte proliferation is unknown. Here we investigated the role of GPC3 in hepatocyte proliferation. GPC3 mRNA and protein levels begin to increase 2 days after hepatectomy with peak expression levels by day 5. In hepatocyte cultures, GPC3 reaches a plateau when hepatocyte proliferation decreases. In vitro studies using Morpholino oligonucleotides showed that blocking GPC3 expression promoted hepatocyte growth. Yeast two-hybrid assays revealed that GPC3 interacts with CD81, a member of the tetraspanin family that is reported to be involved in hepatitis C virus infection and cell proliferation. We found that CD81 levels also increased 2 days after partial hepatectomy and toward the end of regeneration. Immunofluorescence showed that CD81 and GPC3 colocalize by 2 and 6 days after hepatectomy. Co-immunoprecipitation validated the interaction of GPC3 and CD81. Our results indicate that GPC3 may be a negative regulator of liver regeneration and hepatocyte proliferation, and that this regulation may involve CD81.

Investigating Multi-cancer Biomarkers and Their Cross-predictability in the Expression Profiles of Multiple Cancer Types

Microarray technology has been widely applied to the analysis of many malignancies, however, integrative analyses across multiple studies are rarely investigated. In this study we performed a meta-analysis on the expression profiles of four published studies analyzing organ donor, benign tissues adjacent to tumor and tumor tissues from liver, prostate, lung and bladder samples. We identified 99 distinct multi-cancer biomarkers in the comparison of all three tissues in liver and prostate and 44 in the comparison of normal versus tumor in liver, prostate and lung. The bladder samples appeared to have a different list of biomarkers from the other three cancer types. The identified multi-cancer biomarkers achieved high accuracy similar to using whole genome in the within-cancer-type prediction. They also performed superior than the one using whole genome in inter-cancer-type prediction. To test the validity of the multi-cancer biomarkers, 23 independent prostate cancer samples were evaluated and 96% accuracy was achieved in inter-study prediction from the original prostate, liver and lung cancer data sets respectively. The result suggests that the compact lists of multi-cancer biomarkers are important in cancer development and represent the common signatures of malignancies of multiple cancer types. Pathway analysis revealed important tumorogenesis functional categories.

Liver-specific ablation of integrin-linked kinase in mice results in abnormal histology, enhanced cell proliferation, and hepatomegaly

Hepatocyte differentiation and proliferation are greatly affected by extracellular matrix (ECM). Primary hepatocytes cultured without matrix dedifferentiate over time, but matrix overlay quickly restores differentiation. ECM also is critical in liver regeneration where ECM degradation and reconstitution are steps in the regenerative process. Integrin-linked kinase (ILK) is a cell-ECM-adhesion component implicated in cell-ECM signaling by means of integrins. We investigated the role of ILK in whole liver by using the LoxP/Cre model system. ILK was eliminated from the liver by mating homozygous ILK-floxed animals with mice expressing Cre-recombinase under control of the alpha fetoprotein enhancer and albumin promoter. After ablation of ILK, animals are born normal. Soon after birth, however, they develop histologic abnormalities characterized by disorderly hepatic plates, increased proliferation of hepatocytes and biliary cells, and increased deposition of extracellular matrix. Cell proliferation is accompanied by increased cytoplasmic and nuclear stabilization of beta-catenin. After this transient proliferation of all epithelial components, proliferation subsides and final liver to body weight ratio in livers with ILK deficient hepatocytes is two times that of wild type. Microarray analysis of gene expression during the stage of cell proliferation shows up-regulation of integrin and matrix-related genes and a concurrent down-regulation of differentiation-related genes. After the proliferative stage, however, the previous trends are reversed resulting in a super-differentiated phenotype in the ILK-deficient livers.

CONCLUSION

Our results show for the first time in vivo the significance of ILK and hepatic ECM-signaling for regulation of hepatocyte proliferation and differentiation.

Expression of specific hepatocyte and cholangiocyte transcription factors in human liver disease and embryonic development

Transcription factors are major determinants of cell-specific gene expression in all cell types. Studies in rodent liver have shown that alterations in transcription factor expression determine lineage specification during fetal liver development and signify transdifferentiation of cells of the biliary compartment into 'oval' cells and eventually hepatocytes in adult liver. We examined the cellular localization of hepatocyte- or BEC-associated transcription factors in human fetal and adult liver and in diseases in which transdifferentiation between hepatocytes and biliary cells may play a role. In the normal adult human liver, hepatocyte nuclear factor (HNF)4 alpha and HNF6 appeared exclusively in hepatocytes; HNF1beta, HNF3alpha, and HNF3beta were observed only in BEC. During fetal development both BEC and hepatocytes expressed HNF3alpha, HNF3beta, and HNF6. HNF1alpha was expressed only in fetal hepatocytes. We further examined expression of transcription factors in massive hepatic necrosis and in specific types of chronic liver disease. Hepatocyte-associated transcription factors HNF4 alpha and HNF6 also appeared in BEC in massive hepatic necrosis and chronic hepatitis C virus infection. Similarly, HNF3beta that is expressed only in BEC in normal adult liver was also observed in hepatocytes in primary biliary cirrhosis and chronic biliary obstruction. These data mimic previous findings in rodents in which hepatocyte-associated transcription factors appear in biliary cells prior to emergence of oval cells, which function as progenitor cells for hepatocytes when the regenerative capacity of the latter is compromised.