The Cross-talk Between Apoptosis And Autophagy:Inhibitor Of Apoptosis Protein XIAP Inhibits Autophagy To Promote Tumorigenesis

The body depends on "self-killing"(apoptosis) to repress aberrant cell growth, and cells rely on "self-eating"(autophagy) to recycle obsolete components. However, more and more studies suggest the existence of an intricate cross-talk between apoptosis and autophagy. These two pathways may be regulated by same factors, and sometimes they share the common constituents. What’s more interesting is that increasing evidence favors the idea that inhibition of either apoptosis or autophagy can promote tumorigenesis. But which one plays a major role arid the detailed molecular mechanism behind that is still unknown.In the first part of this dissertation, we report the identification of X-linked inhibitor of apoptosis protein (XIAP) as an intrinsic inhibitor of autophagy. Among the mammalian IAPs, XIAP is most extensively studied and well characterized, and its biological function has been largely attributed to its effect on the apoptotic pathway, which is believed to be primarily related to inhibiting of caspases (the apoptotic proteases that are responsible for the initiation and execution of apoptosis). XIAP is well documented for its anti-apoptotic function. We find that XIAP suppresses autophagy by exerting a previously unidentified ubiquitin E3ligase activity towards Mdm2, and thereby inhibiting Mdm2-mediated p53degradation in the cytoplasm. The steady-state level of cytoplasmic p53suppresses autophagic process. The effect of XIAP on autophagy appears to be phosphorylation-dependent. Upon induction of autophagy by serum starvation, XIAP undergoes dephosphorylation due to inhibition of PI3K/AKT pathway. This dephosphorylation in turn promotes the dissociation of XIAP from Mdm2, which accelerates Mdm2-mediated p53degradation and facilitates serum starvation-induced autophagy. In mouse models, phosphorylation of XIAP is shown to be essential in regulating tumorigenecity of tumor cell. Additionally, this XIAP-mediated autophagy inhibition was also detected in clinical tumor samples with a relatively high proportion. In summary, these findings reveal a novel XIAP-Mdm2-p53pathway that mediates the inhibition of autophagy, by which XIAP contributes to tumorigenesis.During the subsequent second part of research, we would like to address whether the anti-autophagic function of XIAP could fully account for its tumor-promoting effect, because of XIAP knockout mice exhibiting no obvious apoptosis-related phenotype. In xenograft mouse model, we find that, independent of caspase-binding activity, XIAP mutant could still promote tumorigenesis, with dramatical inhibition of autophagy, not apoptosis. These findings strongly suggest that XIAP inhibiting autophagy, rather than apoptosis, may play a vital role in tumor formation. We employ Mass Spectrometry method to find that p62, a protein which serves as the transporter linking ubiquitinated substrates to autophagic machinery, could possibly be involved in XIAP-mediated autophagy. Subsequent studies show that p62is another substrate of XIAP as E3ligase, and physiologically regulated via ubiquitin-proteasome degradation by XIAP. Through p62, XIAP inhibits autophagy to promote cell proliferation and colony formation. Consistent with this, XIAP deficiency impedes tumorigenesis, and this defect is rescued by knockdown of p62. Thus, XIAP promotes tumorigenesis via inhibiting p62-mediated autophagy. Furthermore, by analyzing clinical specimens with different stages, we find the unexpected connection between XIAP, p62, LC3expression and tumor progression. Our data is well consistent with the current understanding of the role of autophagy in tumorigenesis, where autophagy is believed to inhibit tumor initiation but promote tumor metastasis. And XIAP-p62axis may be an attractive target to cancer.Overall, we reveal that anti-apoptotic protein XIAP as an inhibitor of autophagy to regulate tumorigenesis, and our findings have broad implications for understanding the importance of apoptosis and autophagy in cancer. Studies on this field will help idnetify valuable therapeutic targets for cancer therapy in the future.