β-TrCP-mediated ubiquitination and degradation of Dlg5 regulates hepatocellular carcinoma cell proliferation

BACKGROUND: Discs large homolog 5 (Dlg5) is a member of the membrane-associated guanylate kinase (MAGUK) adaptor family of proteins and its deregulation has been implicated in the malignancy of several cancer types. Dlg5 was down-regulated in hepatocellular carcinoma (HCC) and lower Dlg5 expression was associated with poor survival of HCC patients. However, how to regulate Dlg5 remains largely unknown. METHODS: The co-immunoprecipitation assay was used to determine the interaction between Dlg5 and β-TrCP. The in vivo ubiquitination assay was performed to determine the regulation of Dlg5 by β-TrCP. CCK-8 and colony formation assay were implemented to detect the biological effect of Dlg5 on the growth of HCC cells in vitro. The effect of Dlg5 on HCC tumor growth in vivo was studied in a tumor xenograft model in mice. RESULTS: Here we report that Dlg5 is regulated by the ubiquitin proteasome system and depletion of either Cullin 1 or β-TrCP led to increased levels of Dlg5. β-TrCP regulated Dlg5 protein stability by targeting it for ubiquitination and subsequent destruction in a phosphorylation-dependent manner. We further demonstrated a crucial role of Ser730 in the non-canonical phosphodegron of Dlg5 in governing β-TrCP-mediated Dlg5 degradation. Importantly, failure to degrade Dlg5 significantly inhibited HCC cells proliferation both in vitro and in vivo. CONCLUSION: Collectively, our finding provides a novel molecular mechanism for the negative regulation of Dlg5 by β-TRCP in HCC cells. It further suggests that preventing Dlg5 degradation could be a possible novel strategy for clinical treatment of HCC.