Background: Sorafenib, a multi-target kinase inhibitor, has been shown to downregulate the prosurvival Bcl-2 family protein, Mcl-1, by an as yet unclear mechanism. Signal transducer and activator of transcription 3 (Stat3) is constitutively active in human tumors and has been shown to regulate Mcl-1. Therefore, we determined if sorafenib can inhibit Stat3 signaling to downregulate Mcl-1. We also studied whether Mcl-1 suppression can potentiate apoptosis induction by TRAIL (TNF-related apoptosis-inducing ligand), a proapoptotic cytokine whose anti-tumor effects are negatively regulated by Mcl-1. Experimental Design: Human pancreatic cancer cell lines (PANC-1 and BxPC-3) were pre-incubated with sorafenib (Nexavar®) alone or followed by TRAIL and apoptosis was determined by annexin V labeling. Caspase-8,-9,-3 and poly (ADP-ribose) polymerase (PARP) cleavage, as well as Bax/Bak activation, were analyzed by immunoblotting. The effects of sorafenib on Stat3 phosphorylation (Tyr705) and downstream Mcl-1 and Bcl-xL protein expression were determined. Lentiviral shRNA was used to knockdown BRAF, Stat3, and Mcl-1 expression. Results: Sorafenib treatment inhibited Stat3 phosphorylation (Tyr705) and downregulated Mcl-1 and Bcl-xL proteins in a dose-dependent manner in both cell lines. Stat3 knockdown was shown to suppress Mcl-1 and Bcl-xL. Furthermore, knockdown of BRAF, a Raf kinase and target of sorafenib, inhibited Stat3 phosphorylation (Tyr705) and similarly inhibited Mcl-1 and Bcl-xL protein expression. Mcl-1 knockdown was shown to potentiate apoptosis induced by TRAIL, and pharmacologic suppression of Mcl-1 by sorafenib similarly enhanced TRAIL-mediated apoptosis that was associated with caspase-8,-9,-3 and PARP cleavage, as well as Bak activation. Conclusions: Sorafenib inhibits Stat3 phosphorylation to downregulate prosurvival Mcl-1 and Bcl-xL protein expression, resulting in augmentation of TRAIL-mediated apoptosis in human pancreatic cancer cells.
Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3693.
100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO