Liver zonation occurs through a beta-catenin-dependent, c-Myc-independent mechanism

Spontaneous repopulation of β-catenin null livers with β-catenin-positive hepatocytes after chronic murine liver injury

Prolonged exposure of mice to diet containing 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) results in hepatobiliary injury, atypical ductular proliferation, oval cell appearance, and limited fibrosis. Previously, we reported that short-term ingestion of DDC diet by hepatocyte-specific β-catenin conditional knockout (KO) mice led to fewer A6-positive oval cells than wildtype (WT) littermates. To examine the role of β-catenin in chronic hepatic injury and repair, we exposed WT and KO mice to DDC for 80 and 150 days. Paradoxically, long-term DDC exposure led to significantly more A6-positive cells, indicating greater atypical ductular proliferation in KO, which coincided with increased fibrosis and cholestasis. Surprisingly, at 80 and 150 days in KO we observed a significant amelioration of hepatocyte injury. This coincided with extensive repopulation of β-catenin null livers with β-catenin-positive hepatocytes at 150 days, which was preceded by appearance of β-catenin-positive hepatocyte clusters at 80 days and a few β-catenin-positive hepatocytes at earlier times. Intriguingly, occasional β-catenin-positive hepatocytes that were negative for progenitor markers were also observed at baseline in the KO livers, suggesting spontaneous escape from cre-mediated recombination. These cells with hepatocyte morphology expressed mature hepatocyte markers but lacked markers of hepatic progenitors. The gradual repopulation of KO livers with β-catenin-positive hepatocytes occurred only following DDC injury and coincided with a progressive loss of hepatic cre-recombinase expression. A few β-catenin-positive cholangiocytes were observed albeit only after long-term DDC exposure and trailed the appearance of β-catenin-positive hepatocytes.

CONCLUSION

In a chronic liver injury model, β-catenin-positive hepatocytes exhibit growth and survival advantages and repopulate KO livers, eventually limiting hepatic injury and dysfunction despite increased fibrosis and intrahepatic cholestasis.

The transforming growth factor-α and cyclin D1 genes are direct targets of β-catenin signaling in hepatocyte proliferation

BACKGROUND & AIMS

β-Catenin is an oncogene frequently mutated in hepatocellular carcinoma. In this study, we investigated target genes of β-catenin signaling in hepatocyte proliferation.

METHODS

We studied transgenic mice displaying either inactivation or activation of the β-catenin pathway, focusing on analysis of liver proliferation due to aberrant β-catenin activation, and on the regeneration process during which β-catenin signaling is transiently activated. We localized in situ the various partners involved in proliferation or identified as targets of β-catenin in these transgenic and regenerating livers. We also performed comparative transcriptome analyses, using microarrays. Finally, we extracted, from deep-sequencing data, both the DNA regulatory elements bound to the β-catenin/Tcf nuclear complex and the expression levels of critical targets identified in microarrays.

RESULTS

β-Catenin activation during liver regeneration occurred during G1/S cell cycle progression and allowed zonal extension of the normal territory of active β-catenin and panlobular proliferation. We found that β-catenin controlled both cell-autonomous and non-cell-autonomous hepatocyte proliferation, through direct transcriptional and complex control of cyclin D1 gene expression and of the expression of a new target gene, Tgfα.

CONCLUSIONS

We propose that β-catenin controls panlobular hepatocyte proliferation partly by controlling, together with its Tcf4 nuclear partner, expression of the pro-proliferation cyclin D1 and Tgfα genes. This study constitutes a first step toward understanding the oncogenic properties of this prominent signaling pathway in the liver.

Otto Warburg's contributions to current concepts of cancer metabolism

Otto Warburg pioneered quantitative investigations of cancer cell metabolism, as well as photosynthesis and respiration. Warburg and co-workers showed in the 1920s that, under aerobic conditions, tumour tissues metabolize approximately tenfold more glucose to lactate in a given time than normal tissues, a phenomenon known as the Warburg effect. However, this increase in aerobic glycolysis in cancer cells is often erroneously thought to occur instead of mitochondrial respiration and has been misinterpreted as evidence for damage to respiration instead of damage to the regulation of glycolysis. In fact, many cancers exhibit the Warburg effect while retaining mitochondrial respiration. We re-examine Warburg's observations in relation to the current concepts of cancer metabolism as being intimately linked to alterations of mitochondrial DNA, oncogenes and tumour suppressors, and thus readily exploitable for cancer therapy.

Therapeutic targeting of cancer cell metabolism

In 1927, Otto Warburg and coworkers reported the increased uptake of glucose and production of lactate by tumors in vivo as compared with normal tissues. This phenomenon, now known as the Warburg effect, was recapitulated in vitro with cancer tissue slices exhibiting excessive lactate production even with adequate oxygen. Warburg's in vivo studies of tumors further suggest that the dependency of tumors in vivo on glucose could be exploited for therapy, because reduction of arterial glucose by half resulted in a four-fold reduction in tumor fermentation. Recent work in cancer metabolism indicates that the Warburg effect or aerobic glycolysis contributes to redox balance and lipid synthesis, but glycolysis is insufficient to sustain a growing and dividing cancer cell. In this regard, glutamine, which contributes its carbons to the tricarboxylic acid (TCA) cycle, has been re-discovered as an essential bioenergetic and anabolic substrate for many cancer cell types. Could alterations in cancer metabolism be exploited for therapy? Here, we address this question by reviewing current concepts of normal metabolism and altered metabolism in cancer cells with specific emphasis on molecular targets involved directly in glycolysis or glutamine metabolism.

The Wnt/β-catenin signaling pathway in liver biology and disease

The Wnt signaling pathway is an evolutionarily conserved, highly complex signaling pathway that is critical for development, differentiation and cellular homeostasis. The protein β-catenin is the central player in one major arm of the Wnt pathway called the canonical Wnt pathway. As in other organs, the Wnt/β-catenin pathway is critical for liver development. However, recent research suggests that the pathway is also important in liver regeneration, liver metabolism and maintenance of normal function in the adult liver. Aberrant activation of β-catenin has also been implicated in the pathogenesis of hepatobiliary neoplasia, ranging from benign lesions to liver cancer. The explosion of research into the many roles of the Wnt/β-catenin pathway promises to change our fundamental understanding of normal liver biology and the aberrations that lead to disease and cancer.

Advances in hepatobiliary pathology: update for 2010

Recent publications on hepatology and hepatic pathology provide a wealth of new information on wideranging topics. Morphologic aspects of liver disease associated with hepatitis B and C viruses, autoimmune hepatitis, and HIV infection were addressed, as was the prevalent problem of nonalcoholic fatty liver disease. Advances in diagnosis and pathogenesis of primary biliary cirrhosis, primary sclerosing cholangitis, and the increasingly complex spectrum of IgG4 hepatobiliary diseases were also reported. The histologic and immunohistochemical features of the rare "calcifying nested stromal-epithelial tumor" of the liver were described in a 9-case series. For benign and malignant liver tumors, immunohistochemistry plays a major diagnostic role, and several recent studies demonstrate the value of immunostains in distinguishing between liver-cell adenoma and focal nodular hyperplasia.

Glucocorticoid-dependent transdifferentiation of pancreatic progenitor cells into hepatocytes is dependent on transient suppression of WNT signalling

Developmentally, the pancreas and liver are closely related and pathological conditions – including elevated glucocorticoid levels – result in the appearance of hepatocytes in the pancreas. The role of the WNT signalling pathway in this process has been examined in the model transdifferentiating pancreatic acinar AR42J-B-13 (B-13) cell. Glucocorticoid treatment resulted in a transient loss of constitutive WNT3a expression, phosphorylation and depletion of β-catenin, loss of β-catenin nuclear localisation, and significant reductions in T-cell factor/lymphoid enhancer factor (Tcf/Lef) transcriptional activity before overt changes in phenotype into hepatocyte-like (B-13/H) cells. A return to higher Tcf/Lef transcriptional activity correlated with the re-expression of WNT3a in B-13/H cells. β-catenin knock down alone substituted for and enhanced glucocorticoid-dependent transdifferentiation. Overexpression of a mutant β-catenin (pt-Xβ-cat) protein that blocked glucocorticoid-dependent suppression of Tcf/Lef activity resulted in inhibition of transdifferentiation. A small-molecule activator of Tcf/Lef transcription factors blocked glucocorticoid-dependent effects, as observed with pt-Xβ-cat expression. Quercetin – a Tcf/Lef inhibitor – did not promote transdifferentiation into B-13/H cells, but did potentiate glucocorticoid-mediated transdifferentiation. These data demonstrate that the transdifferentiation of B-13 cells into hepatocyte-like cells in response to glucocorticoid was dependent on the repression of constitutively active WNT signalling.

Computational experiments reveal plausible mechanisms for changing patterns of hepatic zonation of xenobiotic clearance and hepatotoxicity

No concrete, causal, mechanistic theory is available to explain how different hepatic zonation patterns of P450 isozyme levels and hepatotoxicity emerge following dosing with different compounds. We used the synthetic method of modeling and simulation to discover, explore, and experimentally challenge concrete mechanisms that show how and why biomimetic zonation patterns can emerge and change within agent-based analogues, expecting that those mechanisms may have counterparts in rats. Mobile objects map to compounds. One analogue represents a cross-section through a lobule. It is comprised of 460 identical, quasi-autonomous functional units called sinusoidal segments (SSs). SSs detect and respond to compound-generated response signals and the local level of an endogenous gradient. Each SS adapts by using those signals to adjust (or not) the probability that it will clear a detected compound during the next simulation cycle. The adjustment decision is based on the value of a biomimetic algorithm that is based on an assumed, evolution imposed, genetic mandate that normal hepatocytes resist increasing the cost of their actions. The algorithm estimates the long-term, discounted cost to a given SS of continuing to use its current clearance effort. Upon compound exposure, lobular analogues developed a variety of clearance and hepatotoxicity patterns that were strikingly similar to those reported in the literature. A degree of quantitative validation was achieved against data on hepatic zonation of CYP1A2 mRNA expression caused by three different doses of TCDD (2,3,7,8-tetracholorodibenzo-p-dioxone).

Organogenesis and development of the liver

Embryonic development of the liver has been studied intensely, yielding insights that impact diverse areas of developmental and cell biology. Understanding the fundamental mechanisms that control hepatogenesis has also laid the basis for the rational differentiation of stem cells into cells that display many hepatic functions. Here, we review the basic molecular mechanisms that control the formation of the liver as an organ.

Genetic dissection of differential signaling threshold requirements for the Wnt/beta-catenin pathway in vivo

Contributions of null and hypomorphic alleles of Apc in mice produce both developmental and pathophysiological phenotypes. To ascribe the resulting genotype-to-phenotype relationship unambiguously to the Wnt/β-catenin pathway, we challenged the allele combinations by genetically restricting intracellular β-catenin expression in the corresponding compound mutant mice. Subsequent evaluation of the extent of resulting Tcf4-reporter activity in mouse embryo fibroblasts enabled genetic measurement of Wnt/β-catenin signaling in the form of an allelic series of mouse mutants. Different permissive Wnt signaling thresholds appear to be required for the embryonic development of head structures, adult intestinal polyposis, hepatocellular carcinomas, liver zonation, and the development of natural killer cells. Furthermore, we identify a homozygous Apc allele combination with Wnt/β-catenin signaling capacity similar to that in the germline of the Apc^min mice, where somatic Apc loss-of-heterozygosity triggers intestinal polyposis, to distinguish whether co-morbidities in Apc^min mice arise independently of intestinal tumorigenesis. Together, the present genotype–phenotype analysis suggests tissue-specific response levels for the Wnt/β-catenin pathway that regulate both physiological and pathophysiological conditions.

Germline or somatic mutations in genes are the underlying cause of many human diseases, most notably cancer. Interestingly though, even in situations where every cell of every tissue of an organism carries the same mutation (as is the case for germline mutations), some tissues are more susceptible to the development of disease over time than others. For example, in familial adenomatous polyposis (FAP), affected persons carry different germline mutations in the APC gene and are prone to developing cancers of the colon and the rectum—and, less frequently, cancers in other tissues such as stomach, liver, and bones. Here we utilize a panel of mutant mice with truncating or hypomorphic mutations in the Apc gene, resulting in different levels of activation of the Wnt/β-catenin pathway. Our results reveal that different pathophysiological outcomes depend on different permissive signaling thresholds in embryonic, intestinal, and liver tissues. Importantly, we demonstrate that reducing Wnt pathway activation by 50% is enough to prevent the manifestation of embryonic abnormalities and disease in the adult mouse. This raises the possibility of developing therapeutic strategies that modulate the activation levels of this pathway rather than trying to “repair” the mutation in the gene itself.

Canonical Wnt signaling is antagonized by noncanonical Wnt5a in hepatocellular carcinoma cells

BACKGROUND

beta-catenin mutations that constitutively activate the canonical Wnt signaling have been observed in a subset of hepatocellular carcinomas (HCCs). These mutations are associated with chromosomal stability, low histological grade, low tumor invasion and better patient survival. We hypothesized that canonical Wnt signaling is selectively activated in well-differentiated, but repressed in poorly differentiated HCCs. To this aim, we characterized differentiation status of HCC cell lines and compared their expression status of Wnt pathway genes, and explored their activity of canonical Wnt signaling.

RESULTS

We classified human HCC cell lines into "well-differentiated" and "poorly differentiated" subtypes, based on the expression of hepatocyte lineage, epithelial and mesenchymal markers. Poorly differentiated cell lines lost epithelial and hepatocyte lineage markers, and overexpressed mesenchymal markers. Also, they were highly motile and invasive. We compared the expression of 45 Wnt pathway genes between two subtypes. TCF1 and TCF4 factors, and LRP5 and LRP6 co-receptors were ubiquitously expressed. Likewise, six Frizzled receptors, and canonical Wnt3 ligand were expressed in both subtypes. In contrast, canonical ligand Wnt8b and noncanonical ligands Wnt4, Wnt5a, Wnt5b and Wnt7b were expressed selectively in well- and poorly differentiated cell lines, respectively. Canonical Wnt signaling activity, as tested by a TCF reporter assay was detected in 80% of well-differentiated, contrary to 14% of poorly differentiated cell lines. TCF activity generated by ectopic mutant beta-catenin was weak in poorly differentiated SNU449 cell line, suggesting a repressive mechanism. We tested Wnt5a as a candidate antagonist. It strongly inhibited canonical Wnt signaling that is activated by mutant beta-catenin in HCC cell lines.

CONCLUSION

Differential expression of Wnt ligands in HCC cells is associated with selective activation of canonical Wnt signaling in well-differentiated, and its repression in poorly differentiated cell lines. One potential mechanism of repression involved Wnt5a, acting as an antagonist of canonical Wnt signaling. Our observations support the hypothesis that Wnt pathway is selectively activated or repressed depending on differentiation status of HCC cells. We propose that canonical and noncanonical Wnt pathways have complementary roles in HCC, where the canonical signaling contributes to tumor initiation, and noncanonical signaling to tumor progression.

Role of Wnt/β-catenin signaling in liver metabolism and cancer

Wnt/β-catenin signaling is known for its role in embryogenesis as well as carcinogenesis. In the liver, it plays many critical roles during hepatic development and regeneration, and its dysregulation is evident in aberrant hepatic growth during various liver tumors. Its chief cellular roles in the liver include regulation of processes of cell proliferation, apoptosis, oxidative stress and differentiation, which in turn contributes to hepatic growth, zonation, xenobiotic metabolism and other metabolic processes inherent to the liver. Most of these functions of the Wnt/β-catenin signaling are dictated through the highly temporal and tissue-specific or non-specific transcriptional targets of the pathway. In addition, some of the critical functions such as cell-cell adhesion and perhaps maintenance of various junctions that are critical from an epithelial cell biology perspective are also a function of β-catenin, which is the central component of the canonical Wnt pathway. Various animal models and clinical studies have demonstrated the spectra of Wnt/β-catenin signaling in liver health and disease. Thus therapeutic modulation of this pathway for improved hepatic health is inevitable in the future. The current review discusses the advances in our understanding of the Wnt/β-catenin signaling in liver physiology and pathology especially in hepatic metabolism and various tumors in adult liver and goes on to extrapolate the pre-clinical significance and possible translational implications of such findings.

Role of Wnt/β-catenin signaling in liver metabolism and cancer

Wnt/β-catenin signaling is known for its role in embryogenesis as well as carcinogenesis. In the liver, it plays many critical roles during hepatic development and regeneration, and its dysregulation is evident in aberrant hepatic growth during various liver tumors. Its chief cellular roles in the liver include regulation of processes of cell proliferation, apoptosis, oxidative stress and differentiation, which in turn contributes to hepatic growth, zonation, xenobiotic metabolism and other metabolic processes inherent to the liver. Most of these functions of the Wnt/β-catenin signaling are dictated through the highly temporal and tissue-specific or non-specific transcriptional targets of the pathway. In addition, some of the critical functions such as cell-cell adhesion and perhaps maintenance of various junctions that are critical from an epithelial cell biology perspective are also a function of β-catenin, which is the central component of the canonical Wnt pathway. Various animal models and clinical studies have demonstrated the spectra of Wnt/β-catenin signaling in liver health and disease. Thus therapeutic modulation of this pathway for improved hepatic health is inevitable in the future. The current review discusses the advances in our understanding of the Wnt/β-catenin signaling in liver physiology and pathology especially in hepatic metabolism and various tumors in adult liver and goes on to extrapolate the pre-clinical significance and possible translational implications of such findings.