Reduction of the antiapoptotic protein cFLIP enhances the susceptibility of human renal cancer cells to TRAIL apoptosis

Human renal carcinoma cells (RCCs) were sensitized to the apoptotic effects of tumor necrosis factor (TNF)–related apoptosis-inducing ligand (TRAIL), by treatment with cycloheximide (CHX). In contrast to a previous study, a rapid and dramatic decrease in levels of cellular FLICE (Fas-associated death domain–like IL-1β–converting enzyme) inhibitory protein (cFLIP) following cycloheximide treatment was observed in all RCCs studied. The unambiguous detection of this decrease in cFLIP was dependent on the quality of the particular antibody preparation used to detect cFLIP. Cycloheximide treatment caused no major change in levels of pro-caspase-8 or cell surface expression of TRAIL receptors. Therefore, cycloheximide treatment resulted in an increase in the pro-caspase-8 to cFLIP ratio, which correlated with sensitization to TRAIL-mediated apoptosis. Furthermore, treatment of human RCCs with small interfering oligoribonucleotides (siRNA) for cFLIP caused a reduction of cFLIP protein and sensitized cells to TRAIL-mediated apoptosis. We concluded that in the presence of an intact TRAIL signaling pathway, a significant reduction of cFLIP alone is sufficient to sensitize human RCCs to TRAIL apoptosis.