Sodium nitroprusside enhances TRAIL-induced apoptosis via a mitochondria-dependent pathway in human colorectal carcinoma CX-1 cells

The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL, Apo-2L) is a recently characterized member of the family of programmed cell death- inducing ligands that includes TNF-alpha and CD95L (FasL), It is well known that TRAIL binds to the death signaling receptors, DR4 and DR5, and initia tes the TRAIL death pathway, Activation of this pathway, mediated through a caspase cascade, causes apoptosis, In this study, we hypothesized that oxi dative stress facilitates TRAIL-induced apoptosis by promoting caspase acti vity through cytochrome c release from mitochondria, Human colorectal carci noma CX-1 cells were treated with various concentrations of TRAIL (12.5-200 ng/ml) and/or sodium nitroprusside (SNP; 0.03-1 mM) for 12 h, SNP, a nitri c oxide donor, which had little toxic effect by itself, enhanced TRAIL-indu ced cytotoxicity, For example, TRAIL-induced apoptosis (200 ng/ml) was incr eased by a factor of 2.5-fold in the presence of 1 mM SNP. The combined tre atment also caused an increase in cytochrome c release, caspase-3 activity, and PARR cleavage, Overexpression of Bcl-2 completely blocked the SNP-prom oting effects, but only moderately inhibited TRAIL-induced apoptosis, Simil ar results were observed in the presence of hydrogen peroxide or peroxynitr ite. Taken together, the present studies suggest that SNP enhances TRAIL-in duced cytotoxicity by facilitating the mitochondria-mediated caspase signal transduction pathway.