ABSTRACT
Certain classes of tumor cells respond favorably to TRAIL due to the presence of cell surface death receptors DR4 and DR5. Despite this preferential sensitivity, resistance to TRAIL remains a clinical problem and therefore the heightened interest in identifying compounds to revert tumor sensitivity to TRAIL. We recently demonstrated that the phosphatidylinositide-3-kinase (PI3K) inhibitor, LY294002, and its inactive analog LY303511, sensitized tumor cells to vincristine-induced apoptosis, independent of PI3K/Akt pathway. Intrigued by these findings, we investigated the effect of LY303511 on TRAIL-induced apoptosis in HeLa cells. Preincubation of cells with LY30 significantly amplified TRAIL signaling as evidenced by enhanced DNA fragmentation, caspases 2, 3, 8, and 9 activation, and reduction in the tumor colony formation. This increase in TRAIL sensitivity involved mitochondrial membrane permeabilization resulting in the egress of cytochrome c and second mitochondrial activator of caspase/direct IAP-binding protein with low PI, cleavage of X-linked inhibitor of apoptosis protein, and activation of caspase 9. We link this execution signal to the ability of LY30 to downregulate cFLIP(S) and oligomerize DR5, thus facilitating the signaling of the death initiating signaling complex. The subsequent exposure to TRAIL resulted in processing/activation of caspase 8 and cleavage of its substrate, the BH3 protein Bid. These data provide a novel mechanism of action of this small molecule with the potential for use in TRAIL-resistant tumors.
