Dissecting the pathways that destabilize mutant p53: the proteasome or autophagy?

One fundamental feature of mutant forms of p53 consists in their accumulation at high levels in tumors. At least in the case of neomorphic p53 mutations, which acquire oncogenic activity, stabilization is a driving force for tumor progression. It is well documented that p53 mutants are resistant to proteasome-dependent degradation compared with wild-type p53, but the exact identity of the pathways that affect mutant p53 stability is still debated. We have recently shown that macroautophagy (autophagy) provides a route for p53 mutant degradation during restriction of glucose. Here we further show that in basal conditions of growth, inhibition of autophagy with chemical inhibitors or by downregulation of the essential autophagic genes ATG1/Ulk1, Beclin-1 or ATG5, results in p53 mutant stabilization. Conversely, overexpression of Beclin-1 or ATG1/Ulk1 leads to p53 mutant depletion. Furthermore, we found that in many cell lines, prolonged inhibition of the proteasome does not stabilize mutant p53 but leads to its autophagic-mediated degradation. Therefore, we conclude that autophagy is a key mechanism for regulating the stability of several p53 mutants. We discuss plausible mechanisms involved in this newly identified degradation pathway as well as the possible role played by autophagy during tumor evolution driven by mutant p53.