p21Ras downstream effectors are increased in activity or expression in mouse liver tumors but do not differ between ras-mutated and ras-wild-type lesions

The Ras/MAPK pathway and hepatocarcinoma: pathogenesis and therapeutic implications

BACKGROUND

Hepatocellular carcinoma (HCC) is still a major health problem, often diagnosed at an advanced stage. The multikinase inhibitor sorafenib is to date the sole approved systemic therapy. Several signalling pathways are implicated in tumour development and progression. Among these pathways, the Ras/MAPK pathway is activated in 50-100% of human HCCs and is correlated with a poor prognosis. The aim of this work was to review the main intracellular mechanisms leading to aberrant Ras pathway activation in HCC and the potential therapeutic implications.

MATERIALS AND METHODS

This review is based on the material found on PubMed up to December 2014. 'Ras signaling, Ras dysregulation, Ras inhibition, MAPK pathway, cancer, hepatocarcinoma and liver cancer' alone or in combination were the main terms used for online research.

RESULTS

Multiple mechanisms lead to the deregulation of the Ras pathway in liver cancer. Ras and Raf gene mutations are rare events in human hepatocarcinogenesis in contrast to experimental models in rodents. Downregulation of several Ras/MAPK pathway inhibitors such as GAPs, RASSF proteins, DUSP1, Sprouty and Spred proteins is largely implicated in the aberrant activation of this pathway in the context of wild-type Ras and Raf genes. Epigenetic or post-transcriptional mechanisms lead to the downregulation of these tumour suppressor genes.

CONCLUSION

Ras/MAPK pathway effectors may be considered as potential therapeutic targets in the field of HCC. In particular after the arrival of sorafenib, more Ras/MAPK inhibitors have emerged and are still in preclinical or clinical investigation for HCC therapy.

Hepatitis B virus regulation of Raf1 promoter activity through activation of transcription factor AP-2α

The X protein of hepatitis B virus (HBx) is one of the important factors in the development of hepatocellular carcinoma. Raf1 kinase is a central component of many signaling pathways that are involved in normal cell growth and oncogenic transformation. We previously demonstrated that hepatitis B virus regulates Raf1 expression in HepG2.2.15 cells by enhancing its promoter activity and that HBx and HBs might play an important role in this process. However, the underlying molecular mechanisms remain unclear. In this study, we show that nucleotides -209 to -133 of the Raf1 promoter sequence constitute the core region where hepatitis B virus is regulated. This regulation was found to require the involvement of cis-regulatory element AP-2α. We further demonstrated that AP-2α expression was higher in HepG2.2.15 cells (HBV-expressing cells) than in HepG2 cells in vitro. Silencing AP-2α expression by siRNA significantly inhibited the Raf1 promoter activity in HepG2.2.15 cells. These findings indicated that HBV regulates Raf1 promoter activity, possibly through AP-2α.

Transforming growth factor-β-stimulated clone-22 is a negative-feedback regulator of Ras / Raf signaling: Implications for tumorigenesis

Transforming growth factor-β (TGF-β)-stimulated clone-22 (TSC-22), also called TSC22D1-2, is a putative tumor suppressor. We previously identified TSC-22 downstream of an active mutant of fms-like tyrosine kinase-3 (Flt3). Here, we show that TSC-22 works as a tumor suppressor through inhibiting Ras/Raf signaling. Notably, TSC-22 was upregulated by Ras/Raf activation, whereas its upregulation was inhibited by concurrent STAT5 activation. Although TSC-22 was normally retained in the cytoplasm by its nuclear export signal (NES), Ras/Raf activation caused nuclear translocation of TSC-22, but not TSC22D1-1. Unlike glucocorticoid-induced leucine zipper (GILZ/TSC22D3-2) previously characterized as a negative regulator of Ras/Raf signaling, TSC-22 failed to interact physically with Ras/Raf. Importantly, transduction with TSC-22, but not TSC22D1-1, suppressed the growth, transformation and tumorigenesis of NIH3T3 cells expressing oncogenic H-Ras: this suppression was enhanced by transduction with a TSC-22 mutant lacking NES that had accumulated in the nucleus. Collectively, upregulation and nuclear translocation of TSC-22 played an important role in the feedback suppression of Ras/Raf signaling. Consistently, TSC22D1-deficient mice were susceptible to tumorigenesis in a mouse model of chemically-induced liver tumors bearing active mutations of Ras/Raf. Thus, TSC-22 negatively regulated Ras/Raf signaling through a mechanism different from GILZ, implicating TSC-22 as a novel suppressor of oncogenic Ras/Raf-induced tumors.

Transgenic overexpression of insulin receptor substrate 1 in hepatocytes enhances hepatocellular proliferation in young mice only

AIM

The insulin receptor substrate-1 (IRS-1) is a multisite docking protein which plays a central role in the signal transduction of growth factors such as insulin and insulin-like growth factors (IGF-1 and IGF-2). It is found to be frequently overexpressed in human hepatocellular carcinoma (HCC).

METHODS

To study IRS-1 overexpression in hepatocytes in vivo, transgenic mice overexpressing IRS-1 exclusively in hepatocytes were created, showing enhanced hepatocyte proliferation in young animals. In the present study, the phenotype of IRS-1 transgenic animals was characterized over a period of two years. The livers of transgenic and control mice were analyzed for IRS-1 expression and phosphorylation, activation of the downstream mitogen-activated protein kinase (MAPK) cascade and phosphatidylinositol 3' kinase (PI3'K) and macroscopical and histological abnormalities.

RESULTS

The enhanced hepatocyte proliferation observed in young IRS-1 transgenic animals was no longer detectable in adult mice. Despite constitutive overexpression and phosphorylation of IRS-1, MAPK- and IRS-1-associated PI3'K activity were significantly reduced in older transgenic mice. Furthermore, no premalignant lesions or HCC were detected in IRS-1 transgenic animals up to the age of 24 months.

CONCLUSIONS

Therefore, additional mechanisms such as enhanced growth factor expression or impaired negative feedback control mechanisms may augment IRS-1 overexpression in human hepatocarcinogenesis.

Differential selection for B-raf and Ha-ras mutated liver tumors in mice with high and low susceptibility to hepatocarcinogenesis

Activation of the Ras/Raf/MEK/ERK pathway is frequently observed in animal and human tumors. In our study, we analyzed B-raf codon 637 (formerly 624) and Ha-ras codon 61 mutations in liver tumors from C3H, B6C3F1 and C56BL mice which differ considerably with regard to their susceptibility to hepatocarcinogenesis. In total, 73% (102/140) of tumors induced by a single application of N-nitrosodiethylamine or 7,12-dimethylbenz[a]anthracene contained either B-raf or Ha-ras mutations and only <3% (4/140) were mutated in both genes. In addition, B-raf mutations were present in 76% (19/25) of early precancerous liver lesions. The prevalence of Ha-ras mutated tumors was significantly higher in the susceptible C3H and B6C3F1 mouse strains (39-50%) than in the comparatively resistant C57BL mouse (7%). B-raf mutated tumors, by contrast, were more frequent in C57BL mice (68%) than in the other two strains (17-45%). Taken together, our findings indicate that alterations affecting the Ras/Raf/MEK/ERK signalling pathway are a hallmark of carcinogen-induced liver tumors in mice. Moreover, our results show that mutational activation of B-raf in liver tumors of different mouse strains is, by contrast to Ha-ras, inversely related to their susceptibility to hepatocarcinogenesis. Although activated Ras and Raf proteins are assumed to have similar biological effects because they feed into the same signalling pathway, there seem to be subtle strain-specific differences in selection processes favouring the preferential outgrowth of either B-raf or Ha-ras mutated tumor populations in mouse liver.

Raf-1 and protein kinase B regulate cell survival through the activation of NF-kappaB in hepatitis B virus X-expressing cells

We previously demonstrated that activation of NF-kappaB by the hepatitis B virus X (HBx) gene plays an important role in cell survival. In the present study, we explored the upstream mediators of NF-kappaB activation and their correlations with cell survival. XTT assays and colony generation assays revealed that inhibition of NF-kappaB activation indeed increased cell death in HBx-expressing cells. Utilizing inactivating mutants of signal transducers, we showed that dominant negative mutants of stress-activated protein kinase/extracellular signal-regulated kinase (SEK1) or PKCalpha significantly diminished the HBx-mediated NF-kappaB activation. However, neither of these mutants significantly affected the cell survival in colony generation assays. In contrast, inactivating mutants of Raf-1 or PKB (protein kinase B)/Akt abrogated the HBx-mediated NF-kappaB activation and also suppressed the cell survival. Our results suggest that the Raf-1 or PKB-mediated NF-kappaB activation promotes cell survival in HBx-expressing cells.

Genetic alterations in cancer knowledge system: analysis of gene mutations in mouse and human liver and lung tumors

Mutational incidence and spectra for genes examined in both human and mouse lung and liver tumors were analyzed using the National Institute of Environmental Health Sciences (NIEHS) Genetic Alterations in Cancer (GAC) knowledge system. GAC is a publicly available, web-based system for evaluating data obtained from peer-reviewed studies of genetic changes in tumors associated with exposure to chemical, physical, or biological agents, as well as spontaneous tumors. In mice, mutations in Kras2 and Hras-1 were the most common events reported for lung and liver tumors, respectively, whether chemically induced or spontaneous. There was a significant difference in Kras2 mutation incidence for spontaneous versus induced mouse lung tumors and in Hras-1 mutation incidence and spectrum for spontaneous versus induced mouse liver tumors. The major gene changes reported for human lung and liver tumors were in KRAS2 (lung only) and TP53. The KRAS2 mutation incidence was similar for spontaneous and asbestos-induced human lung tumors, while the TP53 mutation incidence differed significantly. Aflatoxin B1, hepatitis B virus, hepatitis C virus, and vinyl chloride all caused TP53 mutations in human liver tumors, but the mutation spectrum for each agent differed. The incidence of KRAS2 mutations in human compared to mouse lung tumors differed significantly, as did the incidence of Hras and p53 gene mutations in human compared to mouse liver tumors. Differences observed in the mutation spectra for agent-induced compared to spontaneous tumors and similarities in spectra for structurally similar agents support the concept that mutation spectra can serve as a "fingerprint" of exposure based on chemical structure.

B-Raf and Ha-ras mutations in chemically induced mouse liver tumors

The mitogen-activated protein kinase signalling pathway is a central regulator of tumor growth, which is constitutively activated in chemically induced mouse liver tumors. In about 30-50% of cases this effect can be related to activation of the Ha-ras gene by point mutations, whereas in the remaining cases mutations may occur in other members within this pathway, such as Raf kinases. Recently, B-raf has been shown to be frequently mutated in human melanomas and certain other cancers, with a V599E amino-acid change representing the most predominant mutation type. We now screened 82 N-nitrosodiethylamine-induced liver tumors from C3H/He mice for mutations within the hotspot positions in the Ha-ras and B-raf genes. About 50% (39/82) of tumors showed Ha-ras codon 61 mutations and 16 tumors ( approximately 20%) harbored mutations at codon 624 of the B-raf gene, which corresponds to codon 599 in human B-raf. None of the tumors was mutated in both Ha-ras and B-raf. The high prevalence of Ha-ras and B-raf mutations in mouse liver tumors is in striking contrast to human hepatocellular cancers which very infrequently harbor mutations in the two genes. These fundamental differences between the biology of liver tumors in mice and man may be of toxicological relevance.

Contribution of dichloroacetate and trichloroacetate to liver tumor induction in mice by trichloroethylene

Determining the key events in the induction of liver cancer in mice by trichloroethylene (TRI) is important in the determination of how risks from this chemical should be treated at low doses. At least two metabolites can contribute to liver cancer in mice, dichloroacetate (DCA) and trichloroacetate (TCA). TCA is produced from metabolism of TRI at systemic concentrations that can clearly contribute to this response. As a peroxisome proliferator and a species-specific carcinogen, TCA may not be important in the induction of liver cancer in humans at the low doses of TRI encountered in the environment. Because DCA is metabolized much more rapidly than TCA, it has not been possible to directly determine whether it is produced at carcinogenic levels. Unlike TCA, DCA is active as a carcinogen in both mice and rats. Its low-dose effects are not associated with peroxisome proliferation. The present study examines whether biomarkers for DCA and TCA can be used to determine if the liver tumor response to TRI seen in mice is completely attributable to TCA or if other metabolites, such as DCA, are involved. Previous work had shown that DCA produces tumors in mice that display a diffuse immunoreactivity to a c-Jun antibody (Santa Cruz Biotechnology, SC-45), whereas TCA-induced tumors do not stain with this antibody. In the present study, we compared the c-Jun phenotype of tumors induced by DCA or TCA alone to those induced when they are given together in various combinations and to those induced by TRI given in an aqueous vehicle. When given in various combinations, DCA and TCA produced a few tumors that were c-Jun+, many that were c-Jun-, but a number with a mixed phenotype that increased with the relative dose of DCA. Sixteen TRI-induced tumors were c-Jun+, 13 were c-Jun-, and 9 had a mixed phenotype. Mutations of the H-ras protooncogene were also examined in DCA-, TCA-, and TRI-induced tumors. The mutation frequency detected in tumors induced by TCA was significantly different from that observed in TRI-induced tumors (0.44 vs 0.21, p < 0.05), whereas that observed in DCA-induced tumors (0.33) was intermediate between values obtained with TCA and TRI, but not significantly different from TRI. No significant differences were found in the mutation spectra of tumors produced by the three compounds. The presence of mutations in H-ras codon 61 appeared to be a late event, but ras-dependent signaling pathways were activated in all tumors. These data are not consistent with the hypothesis that all liver tumors induced by TRI were produced by TCA.

Searching new targets for anticancer drug design: the families of Ras and Rho GTPases and their effectors

The Ras superfamily of low-molecular-weight GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell growth and development, apoptosis, lipid metabolism, cytoarchitecture, membrane trafficking, and transcriptional regulation. More than 100 genes of this superfamily grouped in six subfamilies have been described so far, pointing to the complexities and specificities of their cellular functions. Dysregulation of members of at least two of these families (the Ras and the Rho families) is involved in the events that lead to the uncontrolled proliferation and invasiveness of human tumors. In recent years, the cloning and characterization of downstream effectors for Ras and Rho proteins have given crucial clues to the specific pathways that lead to aberrant cellular growth and ultimately to tumorigenesis. A direct link between the functions of some of these effectors with the appearance of transformed cells and their ability to proliferate and invade surrounding tissues has been made. Accordingly, drugs that specifically alter their functions display antineoplasic properties, and some of these drugs are already under clinical trials. In this review, we survey the progress made in understanding the underlying molecular connections between carcinogenesis and the specific cellular functions elicited by some of these effectors. We also discuss new drugs with antineoplastic or antimetastatic activity that are targeted to specific effectors for Ras or Rho proteins.

Sustained activation of the Raf/MEK/Erk pathway in response to EGF in stable cell lines expressing the Hepatitis C Virus (HCV) core protein

Chronic hepatitis C virus (HCV) infection is a leading cause of liver cirrhosis and hepatocellular carcinoma (HCC) worldwide. The HCV capside core is a multifunctional protein with regulatory functions that affects transcription and cell growth in vitro and in vivo. Here, we show that both HCV genotype 1a and 3 core proteins activate MEK1 and Erk1/2 MAP kinases and that the costitutive expression of the HCV core results in a high basal activity of Raf1 and MAP/kinase/kinase, as determined by endogenous Raf1 in vitro kinase assay and immunodetection of hyperphosphorylated Erk1 and Erk2 even after a serum starvation. Moreover, the activation of both Erk1/2 and the downstream transcription factor Elk-1 in response to the mitogenic stimulus EGF is significantly prolonged. The sustained response to EGF in cells expressing the HCV core occurs despite a normal induction of the MAP phosphatases MKP regulatory feedback and is likely due to the costitutive activation of Raf-1 activity. The ability of HCV core proteins to directly activate the MAP kinase cascade and to prolong its activity in response to mitogenic stimuli may contribute to the neoplastic transformation of HCV infected liver cells.

Role of the IkappaB kinase complex in oncogenic Ras- and Raf-mediated transformation of rat liver epithelial cells

NF-kappaB/Rel factors have been implicated in the regulation of liver cell death during development, after partial hepatectomy, and in hepatocytes in culture. Rat liver epithelial cells (RLEs) display many biochemical and ultrastructural characteristics of oval cells, which are multipotent cells that can differentiate into mature hepatocytes. While untransformed RLEs undergo growth arrest and apoptosis in response to transforming growth factor beta1 (TGF-beta1) treatment, oncogenic Ras- or Raf-transformed RLEs are insensitive to TGF-beta1-mediated growth arrest. Here we have tested the hypothesis that Ras- or Raf-transformed RLEs have altered NF-kappaB regulation, leading to this resistance to TGF-beta1. We show that classical NF-kappaB is aberrantly activated in Ras- or Raf-transformed RLEs, due to increased phosphorylation and degradation of IkappaB-alpha protein. Inhibition of NF-kappaB activity with a dominant negative form of IkappaB-alpha restored TGF-beta1-mediated cell killing of transformed RLEs. IKK activity mediates this hyperphosphorylation of IkappaB-alpha protein. As judged by kinase assays and transfection of dominant negative IKK-1 and IKK-2 expression vectors, NF-kappaB activation by Ras appeared to be mediated by both IKK-1 and IKK-2, while Raf-induced NF-kappaB activation was mediated by IKK-2. NF-kappaB activation in the Ras-transformed cells was mediated by both the Raf and phosphatidylinositol 3-kinase pathways, while in the Raf-transformed cells, NF-kappaB induction was mediated by the mitogen-activated protein kinase cascade. Last, inhibition of either IKK-1 or IKK-2 reduced focus-forming activity in Ras-transformed RLEs. Overall, these studies elucidate a mechanism that contributes to the process of transformation of liver cells by oncogene Ras and Raf through the IkappaB kinase complex leading to constitutive activation of NF-kappaB.

Ah receptor ligands and tumor promotion: survival of neoplastic cells

A number of agonists of the aryl hydrocarbon or dioxin receptor (AhR) are potent tumor promoters in rodent liver. The prototype compound is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Tumor promotion by TCDD is likely to be AhR-mediated. Tumor promoters may affect the rate of division, terminal differentiation or death (apoptosis) of tumor precursor cells. The present paper reviews some of the effects of TCDD on liver cell homeostasis that have been observed under diverse experimental settings and discusses some of the possible underlying mechanisms.

Hepatitis C virus core protein interacts with 14-3-3 protein and activates the kinase Raf-1

Persistent hepatitis C virus (HCV) infection is a major cause of chronic liver dysfunction in humans and is epidemiologically closely associated with the development of human hepatocellular carcinoma. Among HCV components, core protein has been reported to be implicated in cell growth regulation both in vitro and in vivo, although mechanisms explaining those effects are still unclear. In the present study, we identified that members of the 14-3-3 protein family associate with HCV core protein. 14-3-3 protein bound to HCV core protein in a phosphoserine-dependent manner. Introduction of HCV core protein caused a substantial increase in Raf-1 kinase activity in HepG2 cells and in a yeast genetic assay. Furthermore, the HCV core-14-3-3 interaction was essential for Raf-1 kinase activation by HCV core protein. These results suggest that HCV core protein may represent a novel type of Raf-1 kinase-activating protein through its interaction with 14-3-3 protein and may contribute to hepatocyte growth regulation.