Sinularin induces autophagy-dependent cell death by activating ULK1 and enhancing FOXO3-ATG4A axis in prostate cancer cells

Sinularin is a natural product extracted from soft coral and is shown to exhibit antitumor effects against multiple human cancers. We previously showed that Sinularin induces apoptotic cell death via stabilizing the FOXO3 protein in prostate cancer cells. In this study, we demonstrated that Sinularin triggers autophagy via two different mechanisms in prostate cancer cells. First, Sinularin reduced the S757 phosphorylation of ULK1 protein, which was mediated by mTOR, leading to ULK1 activation and autophagy initiation. Second, Sinularin enhanced the expression of autophagic protein ATG4A, which is the key regulator in the formation of autophagosome, through a FOXO3-dependent transcriptional mechanism. Next, we identified that ATG4A is a new target gene of the transcription factor FOXO3. Additionally, we also found that Sinularin-induced autophagy promoted survivin degradation and led to cell apoptosis. Taken together, these findings suggest that Sinularin induces prostate cancer cell autophagy by promoting autophagy initiation through activation of ULK1 and formation of autophagosome through the FOXO3-ATG4A pathway.