Abstract
X-linked inhibitor of apoptosis-associated factor 1 (XAF1) is a stress-inducible pro-apoptotic protein that is commonly inactivated in multiple human cancers. Nevertheless, the molecular basis for its tumor suppression function remains largely uncharacterized. Here we report that XAF1 antagonizes the oncogenic activity of tripartite motif containing 28 (TRIM28) ubiquitin E3 ligase through zinc finger protein 313 (ZNF313)-induced ubiquitination and proteasomal degradation. XAF1 exerts apoptosis-promoting effect more strongly in TRIM28+/+ versus XAF1-/- tumor cells and suppresses tumor cell growth, migration, invasion, and epithelial-to-mesenchymal transition and xenograft tumor growth in a highly TRIM28-dependent fashion. Mechanistically, XAF1 interacts directly with the RING domains of TRIM28 and ZNF313 through the ZF6 and ZF7 domain, respectively, thereby facilitating ZNF313 interaction with and ubiquitination of TRIM28. A mutant XAF1 lacking either ZF6 or ZF7 domain exhibits no activity to promote TRIM28 ubiquitination. By destabilizing TRIM28, XAF1 blocks TRIM28-driven ubiquitination of p53 and RLIM, p53-HDAC1 interaction, and TWIST1 stabilization. Intriguingly, TRIM28 destabilizes XAF1 through K48-linked polyubiquitination and proteasomal degradation to protect tumor cells from apoptotic stress, indicating its role as an intrinsic antagonist against XAF1 and the antagonistic interplay of XAF1 and TRIM28. XAF1 expression is inversely correlated with TRIM28 expression in cancer cell lines and tumor tissues and more tightly associated with the survival of TRIM28-high versus TRIM28-low patients. Together, this study uncovers a novel mechanism by which XAF1 suppresses tumor malignancy and an important role for XAF1-TRIM28 interplay in governing stress response, illuminating the mechanistic consequence of its alteration during tumorigenic process.