Enhanced hepatocarcinogenesis in mouse models and human hepatocellular carcinoma by coordinate KLF6 depletion and increased messenger RNA splicing

KLF6-SV1 (SV1), the major splice variant of KLF6, antagonizes the KLF6 tumor suppressor by an unknown mechanism. Decreased KLF6 expression in human hepatocellular carcinoma (HCC) correlates with increased mortality, but the contribution of increased SV1 is unknown. We sought to define the impact of SV1 on human outcomes and experimental murine hepatocarcinogenesis and to elucidate its mechanism of action. In hepatitis C virus (HCV)-related HCC, an increased ratio of SV1/KLF6 within the tumor was associated with features of more advanced disease. Six months after a single injection of diethylnitrosamine (DEN), SV1 hepatocyte transgenic mice developed more histologically advanced tumors, whereas Klf6-depleted mice developed bigger tumors compared to the Klf6fl(+/+) control mice. Nine months after DEN, SV1 transgenic mice with Klf6 depletion had the greatest tumor burden. Primary mouse hepatocytes from both the SV1 transgenic animals and those with hepatocyte-specific Klf6 depletion displayed increased DNA synthesis, with an additive effect in hepatocytes harboring both SV1 overexpression and Klf6 depletion. Parallel results were obtained by viral SV1 transduction and depletion of Klf6 through adenovirus-Cre infection of primary Klf6fl(+/+) hepatocytes. Increased DNA synthesis was due to both enhanced cell proliferation and increased ploidy. Coimmunoprecipitation studies in 293T cells uncovered a direct interaction of transfected SV1 with KLF6. Accelerated KLF6 degradation in the presence of SV1 was abrogated by the proteasome inhibitor MG132.

CONCLUSION

An increased SV1/KLF6 ratio correlates with more aggressive HCC. In mice, an increased SV1/KLF6 ratio, generated either by increasing SV1, decreasing KLF6, or both, accelerates hepatic carcinogenesis. Moreover, SV1 binds directly to KLF6 and accelerates its degradation. These findings represent a novel mechanism underlying the antagonism of tumor suppressor gene function by a splice variant of the same gene.

Hepatocyte growth factor enhances alternative splicing of the Kruppel-like factor 6 (KLF6) tumor suppressor to promote growth through SRSF1

Alternative splicing of the Krüppel-like factor 6 (KLF6) tumor suppressor into an antagonistic splice variant 1 (SV1) is a pathogenic event in several cancers including hepatocellular carcinoma (HCC) because elevated SV1 is associated with increased tumor metastasis and mortality. Ras activation is one factor that can enhance KLF6 splicing in cancer cells, however pathways driving KLF6 splicing are unknown. Splice site selection is regulated by splice factors that include serine/arginine-rich (SR) proteins such as SRSF1 (ASF-SF2), which in turn is controlled by phosphoinositide 3-kinase (PI3K)/Akt and the mitogen-activated protein kinase (MAPK) signaling pathway. Because signaling pathways downstream of the liver mitogen hepatocyte growth factor (HGF) include Akt, we explored whether HGF induces KLF6 alternative splicing. In HepG2 cells, HGF (25 ng/mL) significantly increases the ratio of SV1/KLF6 full by 40% through phosphorylation of Akt and subsequent downregulation of two splicing regulators, SRSF3 (SRp20) and SRSF1. Decreased SRSF3 levels regulate SRSF1 levels by alternative splicing associated with the nonsense-mediated mRNA decay pathway (AS-NMD), which stimulates cell growth by decreasing p21 levels. Enhanced cell replication through increased KLF6 alternative splicing is a novel growth-promoting pathway of HGF that could contribute to the molecule's mitogenic activity in physiologic liver growth and hepatocellular carcinoma.

Carcinogen-induced hepatic tumors in KLF6+/- mice recapitulate aggressive human hepatocellular carcinoma associated with p53 pathway deregulation

Inactivation of KLF6 is common in hepatocellular carcinoma (HCC) associated with hepatitis C virus (HCV) infection, thereby abrogating its normal antiproliferative activity in liver cells. The aim of the study was to evaluate the impact of KLF6 depletion on human HCC and experimental hepatocarcinogenesis in vivo. In patients with surgically resected HCC, reduced tumor expression of KLF6 was associated with decreased survival. Consistent with its role as a tumor suppressor, KLF6+/- mice developed significantly more tumors in response to the chemical carcinogen diethyl nitrosamine (DEN) than wild-type animals. Gene expression signatures in both surrounding tissue and tumors of KLF6+/- mice closely recapitulated those associated with aggressive human HCCs. Expression microarray profiling also revealed an increase in Mdm2 mRNA in tumors from KLF6+/- compared with KLF6+/+ mice, which was validated by way of quantitative real-time polymerase chain reaction and western blot analysis in both human HCC and DEN-induced murine tumors. Moreover, chromatin immunoprecipitation and cotransfection assays established the P2 intronic promoter of Mdm2 as a bona fide transcriptional target repressed by KLF6. Whereas KLF6 overexpression in HCC cell lines and primary hepatocytes led to reduced MDM2 levels and increased p53 protein and transcriptional activity, reduction in KLF6 by small interfering RNA led to increased MDM2 and reduced p53.

CONCLUSION

Our findings indicate that KLF6 deficiency contributes significantly to the carcinogenic milieu in human and murine HCC and uncover a novel tumor suppressor activity of KLF6 in HCC by linking its transcriptional repression of Mdm2 to stabilizing p53.

Bile acids induce hepatic stellate cell proliferation via activation of the epidermal growth factor receptor

BACKGROUND & AIMS

Hepatic stellate cell (HSC) proliferation is a key event in the development of liver fibrosis. In many liver diseases, HSCs are exposed to inflammatory cytokines, reactive oxygen species, and bile acids. Although inflammatory cytokines and reactive oxygen species are known to promote proliferation of HSCs, nothing is known about the effects of bile acids on HSC proliferation or apoptosis. The aim of this study was to investigate the effects of bile acids on HSC proliferation.

METHODS

HSCs were exposed to bile acids with different hydrophobicity (5-200 micromol/L). HSC proliferation and cell cycle-related events were assessed by bromodeoxyuridine incorporation, cell counting and proliferating cell nuclear antigen and cyclin E expression, apoptosis by caspase-3 activity assay, immunocytochemistry for active caspase-3 and acridine orange staining, and activation of signal transduction pathways by Western blot using phospho-specific antibodies. Uptake of bile acids was investigated using fluorescent bile acids.

RESULTS

All bile acids, at concentrations >25 micromol/L, induce a 2.5- to 3-fold increase in HSC proliferation via activation of the epidermal growth factor receptor. Bile acid-induced proliferation is mediated by activation of a protein kinase C/extracellular signal-regulated kinase/p70S6K-dependent pathway. Bile acids did not induce apoptosis in HSCs. HSCs do not take up fluorescent bile acids and do not express the bile acid importer ntcp.

CONCLUSIONS

Bile acids at levels reached in cholestatic conditions are an independent profibrogenic factor. Bile acids induce HSC proliferation via the activation of the epidermal growth factor receptor, whereas HSCs are protected against bile acid-induced apoptosis by excluding bile acids.