Impor tantly, XIAP inhibits caspases at both the initiation phase and the execution phase of apoptosis. sellckchem In light of these activities, XIAP inhibition through small molecules or antisense has received considerable pharmaceutical industry focus, and multiple agents have progressed to clinical trials. A hallmark of apoptotic cell death is the presence of proteolytically cleaved, catalytically active caspases. Viable cells of many well studied cancer cell lines have been reported to exhibit high steady state levels of acti vated caspases in the absence of other markers of cell death. The resistance of these cells to apoptosis is thought to be mediated, at least in part, by XIAP. If XIAP function is essential for survival of these cancer cells, then its inhibition by pharmacological or genetic targeting should increase the rate of apoptosis, without the requirement of additional exogenous signals.

XIAP loss of function has been studied extensively using var ious genetic tools including germ line deletion, somatic cell deletion, and both transient and stable mRNA knockdown. The results have varied widely. in some reports XIAP knockdown alone resulted in decreased viability, while other studies demonstrated no effect. Mice harboring XIAP null alleles are viable and do not exhibit any overt defects in developmental or homeostatic apoptosis, aside from a slight delay in mammary alveolar development. These same XIAP null mice crossed to the TRAMP mouse model of prostate cancer did not result in an alteration in tumor progression, suggesting that XIAP is not a critical regu lator of tumor apoptosis in this context.

However, loss of XIAP function can sensitize some cell lines in vitro to apoptosis mediated by activation of either the extrinsic, caspase 8 dependent pathway, using exogenous ligands such as TRAIL and TNFa, or che motherapeutic agents, which largely activate the intrin sic, caspase 9 dependent pathway Some of the different outcomes in XIAP depleted cells may be attributable to varying functional dependence on XIAP. On the other hand, there are conflicting reports even in the same cell line. In MCF 7 cells, Hu et. al, reported that siRNA mediated knockdown of XIAP had no effect on cell viability in the absence of an exo genous apoptotic stimulus. GSK-3 By contrast, Zhang et. al. reported a 70% decrease in MCF 7 viability within 60 hr after transient siRNA mediated loss of XIAP. Also, Lima et. al. reported an approximately 50% decrease in viability in MCF 7 cells, 96 hr post transfec tion with XIAP targeted siRNA. In another example, the effect of XIAP depletion in NCI H460 cells ranged from approximately 20% to 55% reduced viability.