Usenamine A inhibits the growth of human hepatocellular carcinoma cells by activating autophagy through down-regulation of PLK1

Usenamine A (UA) is a dibenzofuran compound isolated from Usnea longissima Arch. Previous studies have demonstrated that UA exhibited significant in vitro and in vivo anti-hepatocellular carcinoma activity, inducing both apoptosis and autophagy in human hepatocellular carcinoma cells. This study aims to further elucidate the molecular mechanism underlying its anti-hepatocellular carcinoma effect. Based on data obtained from gene chip assay, real-time quantitative PCR, and Western blot analysis, we found that UA effectively inhibited the expression of polo-like kinase 1 (PLK1) in human hepatocellular carcinoma HepG2 and SK-HEP-1 cells. Furthermore, the administration of BI 6727, a PLK1 inhibitor, significantly diminished the inhibitory effect of UA on the viability of human hepatocellular carcinoma cells. Additionally, the knockdown of PLK1 expression via RNA interference markedly inhibited the proliferation of human hepatocellular carcinoma cells, and the inhibitory effect of UA on cell viability was attenuated upon PLK1 knockdown. The knockdown of PLK1 expression significantly upregulated the apoptosis rate of human hepatocellular carcinoma cells and notably diminished the apoptosis-inducing effect of UA on these cells. Additionally, the inhibition of autophagy using the autophagy inhibitor 3-MA reduced the proliferation-inhibitory effect of UA on human hepatocellular carcinoma cells. Utilizing the PLK1 inhibitor BI 6727 or RNA interference, we further demonstrated that PLK1 negatively regulated autophagy in human hepatocellular carcinoma cells. Consequently, the inhibition of PLK1 attenuated the autophagy induction by UA on these cells. Thus, PLK1 plays a crucial role in the inhibition of human hepatocellular carcinoma cell proliferation and the induction of apoptosis by UA. Moreover, UA activates autophagy through the inhibition of PLK1, which subsequently exerts an inhibitory effect on the growth of human hepatocellular carcinoma cells.