Loss of ATG5 expression in a subset of human prostate cancers promotes tumor growth through accumulation of p62

Loss-of-function mutations in autophagy-related (ATG) genes are rare in cancer. However, we report herein that ATG5 is fully deleted in [~]14% of prostate cancers (PCa), rivaling that of well-established tumor suppressor genes. ATG5 expression was downregulated at both mRNA and protein levels and was associated with poor patient survival. The DU145 PCa cell line, isolated from a brain metastasis, is entirely deficient in ATG5; and while ATG5 reintroduction restored autophagy, it dramatically inhibited tumor growth in vivo and led to near complete consumption of the multifunctional autophagy receptor/signaling protein, p62. Deletion of SQSTM1 confirmed that p62 was essential for tumor growth; and Reverse Phase Protein Array analysis revealed that p62 protein was significantly increased in prostate tumors, despite a reduction in mRNA expression. Thus, ATG5 appears to function as a novel tumor suppressor in a subset of prostate tumors and does so, at least in part, through autophagic degradation of p62.