Gambogenic acid induces proteasomal degradation of CIP2A and sensitizes hepatocellular carcinoma to anticancer agents.

Cancerous inhibitor of protein phosphatase 2A (CIP2A) is an oncoprotein that is overexpressed in many human malignancies. It regulates phosphorylated AKT and stabilizes c­Myc in cell proliferation and tumor formation, suggesting that CIP2A plays an essential role in the development of cancer. In the present study, we report that a natural compound, gambogenic acid (GEA), induced the degradation of CIP2A via the ubiquitin­proteasome pathway. Interestingly, the combination of GEA and proteasome inhibitors potentiated the accumulation of ubiquitinated CIP2A and aggresome formation. In addition, GEA exhibited an inhibitory effect on cell proliferation and CIP2A­downstream signaling molecules (c­Myc and pAKT). Furthermore, GEA and CIP2A silencing enhanced the chemosensitivity of hepatocellular carcinoma cells to anticancer agents, suggesting that a combination of a CIP2A inhibitor and anticancer agents could be a valuable clinical therapeutic strategy. These results indicate that GEA is a CIP2A inhibitor that interferes with the ubiquitination and destabilization of CIP2A, providing a promising strategy to enhance the combinational therapy for hepatocellular carcinoma.