The ubiquitin ligase SIAH-1 mediates overexpression of nucleic-acid binding protein FBP-3 and promotes HCC cell proliferation

Abstract

Background/Aims: Far upstream element (FUSE) binding protein (FBP)-1 and FBP-2 are potent regulators of hepatocellular carcinoma (HCC) cell proliferation and cell motility, respectively. In this study we aimed to analyze the expression, functional relevance, and regulation of FBP-3, another FBP family member in human hepatocarcinogenesis. Methods: Expression of FBP-3 was analyzed in normal livers, dysplastic nodules and HCCs. Functional consequences after RNAi-mediated reduction of FBP-3 on HCC cell proliferation, viability, and migration were analyzed. In order to identify potential upstream regulators of FBP-3, explorative statistical analyses using HCC-protein arrays were performed. Results: A strong nuclear overexpression of FBP-3 was observed in 74% of all analyzed human HCCs which significantly correlated with tumor dedifferentiation, tumor cell proliferation (p<0.001), and poor cumulative survival (p<0.05). FBP-3 predominantly supported proliferation but not cell migration in different HCC cell lines. Based on explorative analyses, nuclear expression of the E3-ubiquitin ligase seven in absentia homologue (SIAH)-1 significantly correlated with FBP-3 expression in dedifferentiated HCCs (p<0.01). Indeed, the knock-down of nuclear SIAH-1 as well as treatment with proteasome inhibitors drastically reduced FBP-3 transcript and protein levels. Because inhibition of SIAH-1 also reduced HCC cell viability, we tested whether exogenous FBP-3 might rescue from SIAH-1-mediated effects. Indeed, vector-based overexpression of FBP-3 partially rescued HCC cells from diminished cell viability after SIAH-1 knock-down. Conclusion: FBP-3 is strongly induced in the majority of human HCCs and represents a potent inducer of HCC cell proliferation. Its accumulation is regulated by proteasome-dependent effector mechanisms comprising nuclear activity of the ubiquitin ligase SIAH-1.