Functional screening of genes suppressing TRAIL-induced apoptosis:: distinct inhibitory activities of Bcl-XL and Bcl-2

TNF-related apoptosis-inducing ligand (TRAIL) is known to selectively induce apoptosis in various tumour cells. However, downstream-signalling of TRAIL-receptor is not well defined. A functional genetic screening was performed to isolate genes interfering with TRAIL-induced apoptosis using cDNA retroviral library. Bcl-X-L and FLIP were identified after DNA sequencing analysis of cDNA rescued from TRAIL-resistant clones. We found that increased expression of Bcl-X-L, but not Bcl-2, suppressed TRAIL-induced apoptosis in tumour cells, Western blot and immunohistochemical analyses showed that expression of Bcl-X-L, but not Bcl-2, was highly increased in human breast cancer tissues, Exposure of MDA-MB-231 breast tumour cells to TRAIL induced apoptosis accompanied by dissipation of mitochondrial membrane potential and enzymatic activation of caspase-3, -8, and -9. However, SK-BR-3 breast tumour cells exhibiting increased expression level of Bcl-X-L were resistant to TRAIL, though upon exposure to TRAIL, caspase-8 and Bid were activated. Forced expression of Bcl-X-L, but not Bcl-2, desensitised TRAIL-sensitive MDA-MB-231 cells to TRAIL. Similar inhibitory effects were also observed in other tumour cells such as HeLa and Jurkat cells stably expressing Bcl-X-L, but not Bcl-2. These results are indicative of the crucial and distinct function of BCI-X-L and Bd-2 in the modulation of TRAIL-induced apoptosis. (C) 2003 Cancer Research UK.