Itch/AIP4-independent proteasomal degradation of cFLIP induced by the histone deacetylase inhibitor SAHA sensitizes breast tumour cells to TRAIL

The histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA, vorinostat) is undergoing clinical trials as an antitumor drug and has received regulatory approval for cancer treatment. Here, we show that pre-treatment of human breast cancer cells with SAHA makes them susceptible to apoptosis induced by TRAIL (tumour necrosis factor-related apoptosis-inducing ligand). The apoptosis of breast tumour cells induced by TRAIL is blocked at the level of apical activation of caspase-8 and SAHA enhances the TRAIL-induced processing of procaspase-8. Consequently, a TRAIL associated pathway of apoptosis operated via mitochondria is activated in cells treated with SAHA. Interestingly, degradation of cellular FLICE-inhibitory proteins (cFLIP(L) and cFLIP(S)) by an ubiquitin/proteasome-dependent Itch/AIP4-independent mechanism is observed upon exposure to SAHA. Targeting cFLIP(L) directly with siRNA oligonucleotides also sensitizes human breast tumour cells to TRAIL-induced apoptosis. Furthermore, cFLIP(L) over-expression significantly inhibits the apoptosis elicited through the combined effects of SAHA and TRAIL. Together, these results indicate that SAHA sensitizes breast cancer cells to TRAIL-induced apoptosis by facilitating the activation of early events in the apoptotic TRAIL pathway. Therefore, the combination of TRAIL and SAHA may represent a therapeutic tool to combat breast tumours.