CHOP Down-Regulates cFLIP(L) Expression by Promoting Ubiquitin/Proteasome-Mediated cFLIP(L) Degradation

Abstract

The transcription factor CHOP/GADD153 is induced during the unfolded protein response and is related to the induction of ER stress-mediated apoptosis. However, how CHOP is organized between the pro-survival and pro-apoptotic roles of ER stress remains largely undefined. In this study, we identified the apoptosis regulating protein suppressed by CHOP. We found that treatment of Caki cells with CHOP-inducing drugs including withaferin A, thapsigargin, brefeldin A, and silybin led to a strong reduction in cFLIPL protein levels together with a concomitant increase in the CHOP protein. Interestingly, Wit A down-regulated cFLIPL expression via both suppressing mRNA transcription and increasing cFLIPL protein instability. We also found that forced expression of CHOP dose-dependently led to a decrease of cFLIPL protein expression but did not alter cFLIPL mRNA levels. Additionally, we observed that siRNA-mediated CHOP silencing recovered the cFLIPL expression decreased by CHOP-inducing agents in Caki cells. Finally, we showed that CHOP facilitates ubiquitin/proteasome-mediated cFLIPL degradation, leading to down-regulation of cFLIPL. Finally, cFLIPL over-expression reduced cell death induced by treatment with brefeldin A, thapsigargin, and silybin. Taken together, our results provide novel evidence that cFLIPL is a CHOP control target and that CHOP-induced down-regulation of cFLIPL is due to activation of the ubiquitin/proteasome pathways.