Inhibition of Signal Transducer and Activator of Transcription 3 (STAT3) by the Multi-Kinase Inhibitor Sorafenib Overcomes TRAIL-Resistance in Malignant Glioma Cells.
Markus D Siegelin. Columbia University College of Physicians & Surgeons, New York, NY
Background: Glioblastoma (GBM) is the most common primary malignant brain tumor with a dismal prognosis. Therefore, new therapeutic strategies are highly warranted. Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anti-cancer reagent that induces apoptosis specifically in cancer cells. However, many tumor cells, including GBM cells, are primary resistant to its pro-apoptotic effects. Sorafenib is an oral kinase inhibitor that has been shown to cross the blood-brain barrier and to inhibit tumor growth in vitro and in vivo by interfering with STAT3-signalling, being constitutively active in GBM. In this study, we analyzed whether sorafenib can overcome TRAIL-resistance in glioblastoma cells and the underlying molecular mechanisms.
Design: Glioma cell lines were treated with sorafenib, TRAIL or the combination of both. Cell viability was assessed by MTT-Assay. Apoptosis and caspase-activity were analysed by Annexin V/PI staining and immunoblotting. Analysis of anti-apoptotic proteins was assessed by immunoblotting. Expression levels of STAT3 were modulated by siRNA or plasmid-based over-expression of constitutively active STAT3.
Results: Single treatment with either suboptimal amounts of sorafenib or TRAIL did not result in efficient induction of apoptosis (programmed cell death). However, the combination treatment consisting of clinically achievable suboptimal doses of sorafenib and TRAIL led to rapid apoptosis in glioma cells. The combination treatment of sorafenib and TRAIL significantly activated both the initiator-caspases-8/-9 and effector-caspases-3/-7. Mechanistically, sorafenib inhibited signal transducers and activators of transcription 3 (STAT3) phosphorylation (Tyr 705) known to be constitutively active in both glioblastoma specimens and cell lines. Forced expression of constitutively active STAT3 (STAT3-CA) attenuated TRAIL-sorafenib mediated apoptosis. Conversely, acute ablation of STAT3 by siRNA sensitized glioma cells to TRAIL mediated cell death.
Conclusions: Targeting STAT3 by sorafenib in GBMs might be a powerful tool to overcome the highly apoptotic resistant phenotype of gliomas and to sensitize them to the proapoptotic effects of TRAIL.
Tuesday, March 1, 2011 1:00 PM
Poster Session IV # 233, Tuesday Afternoon