Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor superfamily that induces apoptosis in many tumor cells. Previous studies suggested that TRAIL treatment might also cause release of lysosomal cathepsin proteases to the cytosol, thus further promoting apoptosis. L-leucyl-L-leucine methyl ester (LeuLeuOMe) is a lysosome-destabilizing agent that may cause release of cathepsins into the cytosol and ensuing apoptosis. We hypothesized that a combination of TRAIL and LeuLeuOMe may synergistically promote apoptosis in lung cancer cells. The human epidermoid lung carcinoma cell line Calu-1 (TRAIL-resistant) and human large cell lung carcinoma cell line NCI-H460 (TRAIL-sensitive) were assayed for sensitivity to TRAIL and LeuLeuOMe, given alone or in different combination doses. Each agent alone induced a dose-dependent cytotoxicity, with substantially different efficacies of the two agents for the two cell types. When both agents were combined, synergistic cytotoxicity was achieved even in the TRAIL-resistant cells. TRAIL-induced cytotoxicity was completely inhibited by pan-caspase inhibitor z-VAD-fmk, but not by cysteine protease inhibitor E-64d. Conversely, E-64d totally blocked LeuLeuOMe-induced cytotoxicity. TRAIL caused mitochondrial damage, while enlarged lysosomes and lysosomal rupture were observed in LeuLeuOMe-treated cells. Our data suggest that, while TRAIL and LeuLeuOMe cause apoptosis through pathways that differ in their involvement of lysosomal cysteine proteases, mitochondrial and lysosomal desta-bilization have converging pro-apoptotic effects. Thus, the synergy of TRAIL and LeuLeuOMe may be used therapeutically to promote apoptosis in lung cancers, even those with intrinsic or acquired resistance to TRAIL.
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