Degradative autophagy selectively regulates CCND1 (cyclin D1) and MIR224, two oncogenic factors involved in hepatocellular carcinoma tumorigenesis.

Overexpressed CCND1 (cyclin D1) is associated with hepatocellular carcinoma (HCC) and we used 147 tumor tissue samples from HCC patients and 3 murine models to reveal an inverse correlation between low autophagic activity and high CCND1 expression. These 2 phenomena in combination correlated with poor overall survival in HCC patients. Mechanistic analysis showed that activated autophagy triggered CCND1 ubiquitination followed by SQSTM1 (sequestosome 1)-mediated selective phagophore recruitment, autophagosome formation, fusion with a lysosome, and degradation. Functional studies revealed that autophagy-selective degradation of CCND1 suppresses DNA synthesis, cell proliferation, and colony, and liver tumor formation by arresting the cell cycle at the G phase. Most importantly, diverse pharmacological inducers (rapamycin and amiodarone) effectively suppress tumor growth in orthotopic liver tumor and subcutaneous tumor xenograft models. In conclusion, we have demonstrated a link between degradative autophagy and the cell cycle regulator CCND1, and have discovered the underlying mechanism by which the autophagic degradation machinery regulates the turnover of the cell-cycle regulator CCND1, which in turn affects HCC tumorigenesis. Abbreviations: CCDN1: cyclin D1; HBV: hepatitis B virus; HCC: hepatocellular carcinoma; HCV: hepatitis C virus; SQSTM1: sequestosome 1.