Increased activation of the Wnt/β-catenin pathway in spontaneous hepatocellular carcinoma observed in farnesoid X receptor knockout mice

Small heterodimer partner overexpression partially protects against liver tumor development in farnesoid X receptor knockout mice

Farnesoid X receptor (FXR, Nr1h4) and small heterodimer partner (SHP, Nr0b2) are nuclear receptors that are critical to liver homeostasis. Induction of SHP serves as a major mechanism of FXR in suppressing gene expression. Both FXR(-/-) and SHP(-/-) mice develop spontaneous hepatocellular carcinoma (HCC). SHP is one of the most strongly induced genes by FXR in the liver and is a tumor suppressor, therefore, we hypothesized that deficiency of SHP contributes to HCC development in the livers of FXR(-/-) mice and therefore, increased SHP expression in FXR(-/-) mice reduces liver tumorigenesis. To test this hypothesis, we generated FXR(-/-) mice with overexpression of SHP in hepatocytes (FXR(-/-)/SHP(Tg)) and determined the contribution of SHP in HCC development in FXR(-/-) mice. Hepatocyte-specific SHP overexpression did not affect liver tumor incidence or size in FXR(-/-) mice. However, SHP overexpression led to a lower grade of dysplasia, reduced indicator cell proliferation and increased apoptosis. All tumor-bearing mice had increased serum bile acid levels and IL-6 levels, which was associated with activation of hepatic STAT3. In conclusion, SHP partially protects FXR(-/-) mice from HCC formation by reducing tumor malignancy. However, disrupted bile acid homeostasis by FXR deficiency leads to inflammation and injury, which ultimately results in uncontrolled cell proliferation and tumorigenesis in the liver.

Dichlorodiphenyltrichloroethane technical mixture regulates cell cycle and apoptosis genes through the activation of CAR and ERα in mouse livers

Dichlorodiphenyltrichloroethane (DDT) is a widely used organochlorine pesticide and a xenoestrogen that promotes rodent hepatomegaly and tumours. A recent study has shown significant correlation between DDT serum concentration and liver cancer incidence in humans, but the underlying mechanisms remain elusive. We hypothesised that a mixture of DDT isomers could exert effects on the liver through pathways instead of classical ERs. The acute effects of a DDT mixture containing the two major isomers p,p'-DDT (85%) and o,p'-DDT (15%) on CAR and ERα receptors and their cell cycle and apoptosis target genes were studied in mouse livers. ChIP results demonstrated increased CAR and ERα recruitment to their specific target gene binding sites in response to the DDT mixture. The results of real-time RT-PCR were consistent with the ChIP data and demonstrated that the DDT was able to activate both CAR and ERα in mouse livers, leading to target gene transcriptional increases including Cyp2b10, Gadd45β, cMyc, Mdm2, Ccnd1, cFos and E2f1. Western blot analysis demonstrated increases in cell cycle progression proteins cMyc, Cyclin D1, CDK4 and E2f1 and anti-apoptosis proteins Mdm2 and Gadd45β. In addition, DDT exposure led to Rb phosphorylation. Increases in cell cycle progression and anti-apoptosis proteins were accompanied by a decrease in p53 content and its transcriptional activity. However, the DDT was unable to stimulate the β-catenin signalling pathway, which can play an important role in hepatocyte proliferation. Thus, our results indicate that DDT treatment may result in cell cycle progression and apoptosis inhibition through CAR- and ERα-mediated gene activation in mouse livers. These findings suggest that the proliferative and anti-apoptotic conditions induced by CAR and ERα activation may be important contributors to the early stages of hepatocarcinogenesis as produced by DDT in rodent livers.

Bile acid transporters and regulatory nuclear receptors in the liver and beyond

Bile acid (BA) transporters are critical for maintenance of the enterohepatic BA circulation where BAs exert their multiple physiological functions including stimulation of bile flow, intestinal absorption of lipophilic nutrients, solubilization and excretion of cholesterol, as well as antimicrobial and metabolic effects. Tight regulation of BA transporters via nuclear receptors is necessary to maintain proper BA homeostasis. Hereditary and acquired defects of BA transporters are involved in the pathogenesis of several hepatobiliary disorders including cholestasis, gallstones, fatty liver disease and liver cancer, but also play a role in intestinal and metabolic disorders beyond the liver. Thus, pharmacological modification of BA transporters and their regulatory nuclear receptors opens novel treatment strategies for a wide range of disorders.

Bile Acid Receptors and Liver Cancer

Liver cancer, particularly hepatocellular carcinoma (HCC), is the third leading cause of cancer death in the world. Bile acids (BAs) are liver-produced amphipathic molecules that are required to facilitate the absorption of cholesterol, fat-soluble vitamins, and lipids in the intestine. However, BAs are also known to act as potential carcinogens and deregulation of BA homeostasis has been linked to HCC formation. Two key BA receptors, farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (TGR5), were recently identified, which provides great insights into BAs' normal physiological functions as well as their carcinogenic effects. In this review, we focus on the potential links among BAs, two BA receptors, and HCC. FXR and TGR5 not only play key roles in regulating BA homeostasis but also are essential in suppressing BAs' carcinogenic effects on liver cancer.

Hepatocyte-specific deletion of farnesoid X receptor delays but does not inhibit liver regeneration after partial hepatectomy in mice

Farnesoid X receptor (FXR), the primary bile acid-sensing nuclear receptor, also plays a role in the stimulation of liver regeneration. Whole body deletion of FXR results in significant inhibition of liver regeneration after partial hepatectomy (PHX). FXR is expressed in the liver and intestines, and recent reports indicate that FXR regulates a distinct set of genes in a tissue-specific manner. These data raise a question about the relative contribution of hepatic and intestinal FXR in the regulation of liver regeneration. We studied liver regeneration after PHX in hepatocyte-specific FXR knockout (hepFXR-KO) mice over a time course of 0-14 days. Whereas the overall kinetics of liver regrowth in hepFXR-KO mice was unaffected, a delay in peak hepatocyte proliferation from day 2 to day 3 after PHX was observed in hepFXR-KO mice compared with Cre(-) control mice. Real-time polymerase chain reaction, western blot and co-immunoprecipitation studies revealed decreased cyclin D1 expression and decreased association of cyclin D1 with CDK4 in hepFXR-KO mice after PHX, correlating with decreased phosphorylation of the Rb protein and delayed cell proliferation in the hepFXR-KO livers. The hepFXR-KO mice also exhibited delay in acute hepatic fat accumulation following PHX, which is associated with regulation of cell cycle. Further, a significant delay in hepatocyte growth factor-initiated signaling, including the AKT, c-myc, and extracellular signal-regulated kinase 1/2 pathways, was observed in hepFXR-KO mice. Ultraperformance liquid chromatography/mass spectroscopy analysis of hepatic bile acids indicated no difference in levels of bile acids in hepFXR-KO and control mice.

CONCLUSION

Deletion of hepatic FXR did not completely inhibit but delays liver regeneration after PHX secondary to delayed cyclin D1 activation.

FXR controls the tumor suppressor NDRG2 and FXR agonists reduce liver tumor growth and metastasis in an orthotopic mouse xenograft model

The farnesoid X receptor (FXR) is expressed predominantly in tissues exposed to high levels of bile acids and controls bile acid and lipid homeostasis. FXR^−/− mice develop hepatocellular carcinoma (HCC) and show an increased prevalence for intestinal malignancies, suggesting a role of FXR as a tumor suppressor in enterohepatic tissues. The N-myc downstream-regulated gene 2 (NDRG2) has been recognized as a tumor suppressor gene, which is downregulated in human hepatocellular carcinoma, colorectal carcinoma and many other malignancies.

We show reduced NDRG2 mRNA in livers of FXR^−/− mice compared to wild type mice and both, FXR and NDRG2 mRNAs, are reduced in human HCC compared to normal liver. Gene reporter assays and Chromatin Immunoprecipitation data support that FXR directly controls NDRG2 transcription via IR1-type element(s) identified in the first introns of the human, mouse and rat NDRG2 genes. NDRG2 mRNA was induced by non-steroidal FXR agonists in livers of mice and the magnitude of induction of NDRG2 mRNA in three different human hepatoma cell lines was increased when ectopically expressing human FXR. Growth and metastasis of SK-Hep-1 cells was strongly reduced by non-steroidal FXR agonists in an orthotopic liver xenograft tumor model. Ectopic expression of FXR in SK-Hep1 cells reduced tumor growth and metastasis potential of corresponding cells and increased the anti-tumor efficacy of FXR agonists, which may be partly mediated via increased NDRG2 expression. FXR agonists may show a potential in the prevention and/or treatment of human hepatocellular carcinoma, a devastating malignancy with increasing prevalence and limited therapeutic options.

Progress in understanding the role of FXR in promoting liver regeneration

The regenerative capacity of the liver is well known, and it can regenerate itself by a compensatory regrowth in response to partial hepatectomy or injury. Farnesoid X receptor (FXR) is a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. Bile acids are FXR physiological ligands. As a metabolic regulator, FXR plays key roles in regulating metabolism of bile acids, lipids and glucose. Recently, activation of intercellular signal transduction has been shown to play an important role in liver regeneration by binding of bile acids to their receptor FXR. Bile acid/FXR signaling pathway is required for normal liver regeneration. Furthermore, FXR promotes liver repair after injury, and activation of FXR is able to alleviate age-related defective liver regeneration. These novel findings suggest that FXR-mediated bile acid signaling is an important component of normal liver regeneration and highlight the potential use of FXR ligands to promote liver regeneration after segmental liver transplantation or resection of liver tumors. This review summarizes the recent progress in understanding the role of FXR in promoting liver regeneration.

Anti-inflammatory and metabolic actions of FXR: insights into molecular mechanisms

The farnesoid X receptor (FXR) is a ligand-activated transcription factor belonging to the nuclear receptor (NR) superfamily. FXR plays an important role in positively regulating genes (transactivation) involved in bile acid homeostasis, fat and glucose metabolism. Recently, it has become clear that an additional important role for FXR consists of downregulating genes involved in inflammation. Because of this broad spectrum of regulated genes, therapeutically targeting FXR with full agonists will likely result in adverse side effects, in line with what is described for other NRs. It may therefore be necessary to develop selective FXR modulators. However, the molecular mechanisms that distinguish between FXR-mediated transactivation and transrepression are currently unknown. For other NRs, post-translational modifications such as SUMOylation and phosphorylation have been reported to be unique to either transactivation or transrepression. Here, we review current knowledge on post-translational regulation of FXR with respect to transactivation and transrepression. Ultimately, increased understanding of the different mechanisms of transactivation and transrepression of nuclear receptors will aid in the development of NR drugs with fewer side effects.

Bile acids: from digestion to cancers

Bile acids (BAs) are cholesterol metabolites that have been extensively studied these last decades. BAs have been classified in two groups. Primary BAs are synthesized in liver, when secondary BAs are produced by intestinal bacteria. Recently, next to their ancestral roles in digestion and fat solubilization, BAs have been described as signaling molecules involved in many physiological functions, such as glucose and energy metabolisms. These signaling pathways involve the activation of the nuclear receptor FXRα or of the G-protein-coupled receptor TGR5. These two receptors have selective affinity to different types of BAs and show different expression patterns, leading to different described roles of BAs. It has been suggested for long that BAs could be molecules linked to tumor processes. Indeed, as many other molecules, regarding analyzed tissues, BAs could have either protective or pro-carcinogen activities. However, the molecular mechanisms responsible for these effects have not been characterized yet. It involves either chemical properties or their capacities to activate their specific receptors FXRα or TGR5. This review highlights and discusses the potential links between BAs and cancer diseases and the perspectives of using BAs as potential therapeutic targets in several pathologies.

Synergistic growth inhibition of human hepatocellular carcinoma cells by acyclic retinoid and GW4064, a farnesoid X receptor ligand

Abnormalities in the expression and function of retinoid X receptor (RXR), a master regulator of the nuclear receptor superfamily, are associated with the development of hepatocellular carcinoma (HCC). Dysfunction of farnesoid X receptor (FXR), one of the nuclear receptors that forms a heterodimer with RXR, also plays a role in liver carcinogenesis. In the present study, we examined the effects of acyclic retinoid (ACR), a synthetic retinoid targeting RXRα, plus GW4064, a ligand for FXR, on the growth of human HCC cells. We found that ACR and GW4064 preferentially inhibited the growth of HLE, HLF, and Huh7 human HCC cells in comparison with Hc normal hepatocytes. The combination of 1μM ACR plus 1μM GW4064 synergistically inhibited the growth of HLE cells by inducing apoptosis. The combined treatment with these agents acted cooperatively to induce cell cycle arrest in the G(0)/G(1) phase and inhibit the phosphorylation of RXRα, which is regarded as a critical factor for liver carcinogenesis, through inhibition of ERK and Stat3 phosphorylation. This combination also increased the expression levels of p21(CIP1) and SHP mRNA, while decreasing the levels of c-myc and cyclin D1 mRNA in HLE cells. In addition, a reporter assay indicated that the FXRE promoter activity was significantly increased by treatment with ACR plus GW4064. Our results suggest that ACR and GW4064 cooperatively inhibit RXRα phosphorylation, modulate the expression of FXR-regulated genes, thus resulting in the induction of apoptosis and the inhibition of growth in HCC cells. This combination might therefore be effective for the chemoprevention and chemotherapy of HCC.

Liver-specific loss of Atg5 causes persistent activation of Nrf2 and protects against acetaminophen-induced liver injury

Autophagy is an evolutionarily conserved biological process that degrades intracellular proteins and organelles including damaged mitochondria through the formation of autophagosome. We have previously demonstrated that pharmacological induction of autophagy by rapamycin protects against acetaminophen (APAP)-induced liver injury in mice. In contrast, in the present study, we found that mice with the liver-specific loss of Atg5, an essential autophagy gene, were resistant to APAP-induced liver injury. Hepatocyte-specific deletion of Atg5 resulted in mild liver injury characterized by increased apoptosis and compensatory hepatocyte proliferation. The lack of autophagy in the Atg5-deficient mouse livers was confirmed by increased p62 protein levels and the absence of LC3-lipidation as well as autophagosome formation. Analysis of histological and clinical chemistry parameters indicated that the Atg5 liver-specific knockout mice are resistant to APAP overdose (500 mg/kg). Further investigations revealed that the bioactivation of APAP is normal in Atg5 liver-specific knockout mice although they had lower CYP2E1 expression. There was an increased basal hepatic glutathione (GSH) content and a faster recovery of GSH after APAP treatment due to persistent activation of Nrf2, a transcriptional factor regulating drug detoxification and GSH synthesis gene expression. In addition, we found significantly higher hepatocyte proliferation in the livers of Atg5 liver-specific knockout mice. Taken together, our data suggest that persistent activation of Nrf2 and increased hepatocyte proliferation protect against APAP-induced liver injury in Atg5 liver-specific knockout mice.

Hepatocarcinogenesis in FXR-/- mice mimics human HCC progression that operates through HNF1α regulation of FXR expression

Farnesoid X receptor (FXR) (nuclear receptor subfamily 1, group H, member 4) is a member of nuclear hormone receptor superfamily, which plays essential roles in metabolism of bile acids, lipid, and glucose. We previously showed spontaneously hepatocarcinogenesis in aged FXR(-/-) mice, but its relevance to human hepatocellular carcinoma (HCC) is unclear. Here, we report a systematical analysis of hepatocarcinogenesis in FXR(-/-) mice and FXR expression in human liver cancer. In this study, liver tissues obtained from FXR(-/-) and wild-type mice at different ages were compared by microarray gene profiling, histological staining, chemical analysis, and quantitative real-time PCR. Primary hepatic stellate cells and primary hepatocytes isolated from FXR(-/-) and wild-type mice were also analyzed and compared. The results showed that the altered genes in FXR(-/-) livers were mainly related to metabolism, inflammation, and fibrosis, which suggest that hepatocarcinogenesis in FXR(-/-) mice recapitulated the progression of human liver cancer. Indeed, FXR expression in human HCC was down-regulated compared with normal liver tissues. Furthermore, the proinflammatory cytokines, which were up-regulated in human HCC microenvironment, decreased FXR expression by inhibiting the transactivity of hepatic nuclear factor 1α on FXR gene promoter. Our study thereby demonstrates that the down-regulation of FXR has an important role in human hepatocarcinogenesis and FXR(-/-) mice provide a unique animal model for HCC study.

Loss of FXR protects against diet-induced obesity and accelerates liver carcinogenesis in ob/ob mice

Farnesoid X receptor (FXR) is known to play important regulatory roles in bile acid, lipid, and carbohydrate metabolism. Aged (>12 months old) Fxr(-/-) mice also develop spontaneous liver carcinomas. In this report, we used three mouse models to investigate the role of FXR deficiency in obesity. As compared with low-density lipoprotein receptor (Ldlr) knockout (Ldlr(-/-)) mice, the Ldlr(-/-)Fxr(-/-) double-knockout mice were highly resistant to diet-induced obesity, which was associated with increased expression of genes involved in energy metabolism in the skeletal muscle and brown adipose tissue. Such a striking effect of FXR deficiency on obesity on an Ldlr(-/-) background led us to investigate whether FXR deficiency alone is sufficient to affect obesity. As compared with wild-type mice, Fxr(-/-) mice showed resistance to diet-induced weight gain. Interestingly, only female Fxr(-/-) mice showed significant resistance to diet-induced obesity, which was accompanied by increased energy expenditure in these mice. Finally, we determined the effect of FXR deficiency on obesity in a genetically obese and diabetic mouse model. We generated ob(-/-)Fxr(-/-) mice that were deficient in both Leptin and Fxr. On a chow diet, ob(-/-)Fxr(-/-) mice gained less body weight and had reduced body fat mass as compared with ob/ob mice. In addition, we observed liver carcinomas in 43% of young (<11 months old) Ob(-/-)Fxr(-/-) mice. Together these data indicate that loss of FXR prevents diet-induced or genetic obesity and accelerates liver carcinogenesis under diabetic conditions.