Inhibitors of apoptosis (IAPs) as therapeutic targets in melanomas and other malignancies

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A family of anti-apoptotic regulators known as inhibitors of apoptosis (IAPs) were initially identified and functionally described in baculoviruses, and IAP homologues are now known in invertebrates and vertebrates. The ability of IAPs to physically interact with a variety of inducers and effectors of apoptosis, and to block apoptosis induced by diverse stimuli places IAPs in a central position as sensors and inhibitors of death signals that proceed through a number of different pathways. Melanoma inhibitor of apoptosis (ML-IAP) is a potent anti-apoptotic protein that is upregulated in a number of melanomas but not expressed in most normal adult tissues. Overexpression of IAPs, such as ML-IAP or XIAP, in human cancers has been shown to suppress apoptosis induced by a variety of stimuli. Peptides based on the processed N-terminus of Smac/DIABLO can bind to the single baculovirus IAP repeat (BIR) of ML-IAP and BIRs of XIAP, and negate their ability to suppress apoptosis. Based on this, we have developed SMAC-like peptides that sensitize melanomas and other cancer cell lines. Additionally, we have employed siRNA technology to demonstrate validity of knocking down the levels of ML-IAP in sensitizing melanomas. These findings emphasize the importance of IAP overexpression as a mechanism of resistance to apoptotic stimuli such as chemotherapeutic agents. Using phage display and X-ray crystallographic structural analysis we explored peptide-binding properties of ML-IAP and defined regions of the peptides that could be modified to provide increased binding affinity and/or selectivity relative to other IAPs. Employing similar structure/function analysis we investigated the mechanism of inhibition of apoptosis by ML-IAP and determined several key amino acid residues that are essential for binding and inhibition of caspases and SMAC-like molecules. Understanding differences between XIAP and ML-IAP regarding their role in inhibition of apoptosis will help in design of potent IAP-specific compound that will be useful in treatment of cancers, in particular melanomas, where ML-IAP expression contributes to resistance against conventional chemotherapeutic agents.