p53-dependent regulation of autophagy protein LC3 supports cancer cell survival under prolonged starvation.

The p53 tumor suppressor is mutated in a high percentage of human tumors. However, many other tumors retain wild-type (wt) p53 expression, raising the intriguing possibility that they actually benefit from it. Recent studies imply a role for p53 in regulation of autophagy, a catabolic pathway by which eukaryotic cells degrade and recycle macromolecules and organelles, particularly under conditions of nutrient deprivation. Here, we show that, in many cell types, p53 confers increased survival in the face of chronic starvation. We implicate regulation of autophagy in this effect. In HCT116 human colorectal cancer cells exposed to prolonged nutrient deprivation, the endogenous wt p53 posttranscriptionally down-regulates LC3, a pivotal component of the autophagic machinery. This enables reduced, yet sustainable autophagic flux. Loss of p53 impairs autophagic flux and causes excessive LC3 accumulation upon starvation, culminating in apoptosis. Thus, p53 increases cell fitness by maintaining better autophagic homeostasis, adjusting the rate of autophagy to changing circumstances. We propose that some cancer cells retain wt p53 to benefit from the resultant increased fitness under limited nutrient supply.