XAF1 directs apoptotic switch of p53 signaling through activation of HIPK2 and ZNF313

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a tumor suppressor that is frequently inactivated in many human cancers. However, the molecular mechanism underlying its growth-inhibitory function remains largely unknown. Here, we report that XAF1 forms a positive feedback loop with p53 and acts as a molecular switch in p53-mediated cell-fate decisions favoring apoptosis over cell-cycle arrest. XAF1 binds directly to the N-terminal proline-rich domain of p53 and thus interferes with E3 ubiquitin ligase MDM2 binding and ubiquitination of p53. XAF1 stimulates homeodomain-interacting protein kinase 2 (HIPK2)-mediated Ser-46 phosphorylation of p53 by blocking E3 ubiquitin ligase Siah2 interaction with and ubiquitination of HIPK2. XAF1 also steps up the termination of p53-mediated cell-cycle arrest by activating zinc finger protein 313 (ZNF313), a p21 ᵂᴬF¹-targeting ubiquitin E3 ligase. XAF1 interacts with p53, Siah2, and ZNF313 through the zinc finger domains 5, 6, and 7, respectively, and truncated XAF1 isoforms preferentially expressed in cancer cells fail to form a feedback loop with p53. Together, this study uncovers a novel role for XAF1 in p53 stress response, adding a new layer of complexity to the mechanisms by which p53 determines cell-fate decisions. Significance Epigenetic inactivation of X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is frequently observed in multiple human malignancies. However, the mechanisms underlying its tumor-suppression function remain largely undefined. Here, we identify XAF1 as a positive feedback regulator of p53, which directs the apoptotic switch of p53 signaling. As a unique transcription target of p53 in signaling apoptosis, XAF1 acts as a competitor of E3 ubiquitin ligase MDM2 in binding to p53 and thus disrupts the p53–MDM2 regulatory loop. Moreover, XAF1 appears to promote homeodomain-interacting protein kinase 2 (HIPK2)-mediated p53 phosphorylation by interrupting HIPK2-targeting function of E3 ubiquitin ligase Siah2 and promotes zinc finger protein 313 (ZNF313)-induced p21 ᵂᴬF¹ ubiquitination. XAF1 thus represents one critical regulator of p53’s cell-fate decision function, suggesting that restoring the p53–XAF1 feedback loop could be an attractive avenue for the therapeutic intervention of malignant tumor progression.