The inhibitor of apoptosis (IAP) proteins are major regulators of apoptosis that bind to and inhibit caspases 3, 7, and/or 9. Overexpression of IAP proteins has been demonstrated to confer protection against a variety of pro apoptotic stimuli, including chemotherapies, and is a marker for poor prognosis in a variety of solid tumors and hematologic malignancies. All IAP proteins contain one to three copies of the baculoviral IAP repeat (BIR) domain, zinc binding domains of about 80 amino acids, that are necessary for their interactions with a number of cytosolic target proteins, including activated caspases 3, 7, and/or 9. Antagonism of the IAP protein mediated inhibition of these caspases is required for caspase dependent cell death, and can be achieved by the mitochondrial protein second mitochondria derived activator of caspases/direct IAP binding protein with low pI (Smac/DIABLO), which is released into the cytoplasm in response to pro apoptotic stimuli. The pro apoptotic function of Smac/DIABLO is dependent on a conserved four residue IAP protein interaction motif (A-V/I-P/A-I/F/Y) found at the N-terminus of the mature, post translationally processed, protein. Structural studies have shown that such N terminal peptides bind to a surface groove on the IAP protein BIR domains. The Smac-binding groove of XIAP BIR3 also makes critical contacts with an IAP protein interacting motif located at the N terminus of the small subunit of caspase-9. The use of structure-based design to identify small molecules that potently antagonize IAP protein interactions will be discussed. These small-molecule IAP protein antagonists induce apoptosis in select cancer cell lines and have proven efficacious at inhibiting tumor growth in mouse xenograft models of breast cancer, NSCLC, pancreatic cancer, melanoma, and colon cancer. In vitro studies have established that IAP protein antagonists induce cell death that is dependent on de novo protein biosynthesis and TNF signaling. Remarkably, binding of small-molecule IAP protein antagonists to the BIR3 domains of c-IAP1 or 2 results in induction of auto-ubiquitination activity and rapid proteasomal degradation of these proteins. IAP protein antagonists also result in activation of the NF-kappaB signaling pathways and induction of TNFalpha expression. Given that c-IAP1 and 2 are recruited to the proximal TNFR1-signaling complexes where they cooperate with TRAF2 to block caspase-8 activation, their proteasomal degradation following IAP protein antagonist-induced auto-ubiquitination likely perturbs TNF signaling to favor an apoptotic outcome. By negating the activities of three antiapoptotic proteins, c-IAP1 and 2 through induced degradation, and XIAP through direct antagonism of caspase inhibition, IAP protein antagonists establish an intracellular environment that allows caspase activation, and thus advance an apoptotic fate when TNFalpha is produced in response to induction of NF-kappaB activity.
AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics-- Oct 22-26, 2007; San Francisco, CA