Abstract 2286: Regulation of GSK3β axis by combination treatment with TRAIL and Troglitazone in cancer cells

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Prostate cancer is estimated to account for 29% of all new cancers and is the second leading cause of cancer-related death in men in the United States. Hormonal therapy is the only treatment for advanced forms, but androgen-independence eventually develops in most patients. Developing new treatment strategies are urgently needed, which needs a deeper molecular understanding of the events that lead to tumor progression. TNF-related apoptosis inducing ligand (TRAIL) has gained much importance recently due to its ability to preferentially induce cell death in malignant and transformed cells. However, since many tumor cells develop resistance to TRAIL, recent approaches are focused on developing combinatorial therapeutic regimens that can enhance TRAIL sensitivity. There are multiple reasons behind TRAIL resistance, and recent studies (including ours) indicate that, Glycogen Synthase Kinase 3β (GSK3β) might be a key player in mediating this. Thus pathways that can antagonize GSK3β axis are important targets for cancer drug development. GSK3β is a serine/threonine kinase, initially identified as a critical mediator of glycogen metabolism and insulin signaling and is now well accepted to regulate various cellular processes including cell survival. In our earlier studies, combinatorial treatment with Troglitazone (TZD), a synthetic ligand for peroxisome proliferator-activator receptor gamma (PPARγ) and TRAIL induced significant apoptosis in TRAIL-resistant cancer cells. Utilizing this combination, we also observed an increase in GSK3βSer9 phosphorylation that preceded the onset of apoptosis. At a later time, however, TRAIL-TZD combination produced a dramatic reduction of total GSK3β levels, suggesting that GSK3β is being targeted by multiple pathways following this treatment. Similar results were observed in various prostate as well as pancreatic cancer cells, indicating this to be a generalized event. Interestingly, pretreatment of Bx-PC3 pancreatic cancer cells (with a shorter half-life of GSK3β) with protein synthesis inhibitor Cycloheximide (CHX) significantly reduced TRAIL-TZD-induced inhibition of total GSK3β expression, suggesting mostly a transcriptional regulation. Luciferase assays carried out in both pancreatic and prostate cancer cells indicated dramatic reduction of GSK3β promoter activity with TRAIL-TZD. Knockdown of endogenous PPARγ expression by PPARγ- siRNA resulted in a reduction of PPARγ levels with siRNA transfection, which was unable to antagonize TRAIL-TZD-induced reduction of GSK3β transcription or protein expression. These suggested a potential PPARγ-independent regulation of GSK3β in this pathway. Since the mechanisms that regulate GSK3β expression in cancer cells are largely unknown, these studies indicate a novel strategy of targeting GSK3β, elucidation of which might provide newer insights to improve our understanding of TRAIL resistance. Citation Format: Sunipa Majumdar, Sreevidya Santha, Ajay Rana, Basabi Rana. Regulation of GSK3β axis by combination treatment with TRAIL and Troglitazone in cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2286. doi:10.1158/1538-7445.AM2014-2286