Overaccumulation of p53-mediated autophagy protects against betulinic acid-induced apoptotic cell death in colorectal cancer cells

Betulinic acid (BA) exhibits cytotoxic activity against some cancer cells. However, the molecular mechanism of BA against CRC cells was little reported. Here, we proved that BA elicited CRC cells' growth inhibition and apoptosis in a dose-dependent manner. In addition, BA treatment induced autophagy via inhibiting the AKT-MTOR signaling pathway. Inhibition of autophagy by either administration of autophagic inhibitor chloroquine or siRNA-mediated knockdown of ATG5 could augment BA-induced apoptotic cell death as well as inhibition of cell proliferation. Moreover, we found that p53 was firstly activated by short exposure to BA and then was rapidly degraded via the ubiquitin-mediated degradation pathway in both wtp53 and mutp53 CRC cells. Notably, more preferential cytotoxicity of BA was obtained in mutp53 cells (IC50 values: HT29, 125 mu M; SW480, 58 mu M) rather than wtp53 cells (IC50 values: HCT116, 178 mu M). Further experiments demonstrated that siRNA-mediated p53 knockdown attenuated BA-induced autophagy, and forced overexpression of p53 augmented BA-induced autophagy, indicating that p53-enhanced BA-induced autophagy. Moreover, BA enhanced the sensitivity of mutp53 cells to chemotherapy drugs such as 5-FU and ADR by degradation of mutp53. Overall, our study proved that BA could induce CRC cell death by inducing apoptosis and reduce the overaccumulation of BA-induced protective autophagy by degrading wtp53 and mutp53 dependent on the ubiquitin-mediated degradation pathway to achieve killer effect, suggesting that BA might serve as a novel desirable drug for mutp53 cancer therapy.