Purpose: Flavopiridol is known to modulate the transcription of genes. We investigated the effect of flavopiridol pretreatment on TRAIL cytotoxicity and on the expression of FLIP(L) in different TRAIL-resistant cell lines, because FLIP expression is known to confer TRAIL-resistance.
Methods: Apoptosis was assessed by PI staining and protein expression by Western blotting. RT-PCR was used for mRNA quantitation. siRNA gene silencing was used to knock down FLIP(L).
Results: Flavopiridol pretreatment synergized TRAIL-induced apoptosis in human myeloma and breast cancer cells. Flavopiridol treatment repressed the transcription of FLIP(L) and downregulated its expression in both myeloma and breast cancer cells. Silencing of FLIP(L) gene by siRNA sensitized myeloma cells to TRAIL. Flavopiridol treatment downregulated the expression of the proapoptotic members of the Bcl-2 family proteins (Bak, Bax and PUMA-alpha). The expression of the antiapoptotic Bcl-2 members (Bcl-2 and Bcl-X(L)) was not altered by flavopiridol treatment in myeloma cells.
Conclusion: Our data indicate that flavopiridol synergizes TRAIL cytotoxicity by downregulation of FLIP(L) and this synergistic effect is Bcl-2 family independent.