Suppression of glucosylceramide synthase restores p53-dependent apoptosis in mutant p53 cancer cells

Tumor suppressor p53 plays an essential role in protecting cells from malignant transformation by inducing cell-cycle arrest and apoptosis. Mutant p53 that is detected in more than 50% of cases of cancers loses its role in suppression of tumors but gains in oncogenic function. Strategies to convert mutant p53 into wild-type p53 have been suggested for cancer prevention and treatment, but they face a variety of challenges. Here, we report an alternative approach that involves suppression of glucosylceramide synthase (GCS), an enzyme that glycosylates ceramide and blunts its proapoptotic activity in cancer cells. Human ovarian cancer cells expressing mutant p53 displayed resistance to apoptosis induced by DNA damage. We found that GCS silencing sensitized these mutant p53 cells to doxorubicin but did not affect the sensitivity of cells with wild-type p53. GCS silencing increased the levels of phosphorylated p53 and p53-responsive genes, including p21(Waf1/Cip1), Bax, and Puma, consistent with a redirection of the mutant p53 cells to apoptosis. Reactivated p53-dependent apoptosis was similarly verified in p53-mutant tumors where GCS was silenced. Inhibition of ceramide synthase with fumonisin B1 prevented p53 reactivation induced by GCS silencing, whereas addition of exogenous C6-ceramide reactivated p53 function in p53-mutant cells. Our findings indicate that restoring active ceramide to cells can resuscitate wild-type p53 function in p53-mutant cells, offering preclinical support for a novel type of mechanism-based therapy in the many human cancers harboring p53 mutations.