Alpha-fetoprotein antagonizes X-linked inhibitor of apoptosis protein anticaspase activity and disrupts XIAP-caspase interaction.

Previous results have shown that the human oncoembryonic protein alpha-fetoprotein (AFP) induces dose-dependent targeting apoptosis in tumor cells, accompanied by cytochrome c release and caspase 3 activation. AFP positively regulates cytochrome c/dATP-mediated apoptosome complex formation in a cell-free system, stimulates release of the active caspases 9 and 3 and displaces cIAP-2 from the apoptosome and from its complex with recombinant caspases 3 and 9 [Semenkova et al. (2003) Eur. J. Biochem. 270, 276-282]. We suggested that AFP might affect the X-linked inhibitor of apoptosis protein (XIAP)-caspase interaction by blocking binding and activating the apoptotic machinery via abrogation of inhibitory signaling. We show here that AFP cancels XIAP-mediated inhibition of endogenous active caspases in cytosolic lysates of tumor cells, as well as XIAP-induced blockage of active recombinant caspase 3 in a reconstituted cell-free system. A direct protein-protein interaction assay showed that AFP physically interacts with XIAP molecule, abolishes XIAP-caspase binding and rescues caspase 3 from inhibition. The data suggest that AFP is directly involved in targeting positive regulation of the apoptotic pathway dysfunction in cancer cells inhibiting the apoptosis protein function inhibitor, leading to triggering of apoptosis machinery.