Abstract 183: Loss of Bad phosphorylation and Mcl-1 expression is necessary for rapid apoptosisinduced by combination of TGFα-PE and ZSTK474 in prostate cancer cells

Abstract

Advanced androgen-independent prostate cancer is notoriously resistant to conventional systemic therapies, and at the moment there is no effective protocol for hormone-refractory advanced prostate cancer affected patients. One of possible mechanisms of such resistance is activation of the anti-apoptotic signaling PI3K/Akt pathway that is constitutively active in the 60% of advanced prostate cancers. However, multiple inhibitors of PI3K are just well studied and some have just gone on to clinical trials, but unfortunately they showed limited efficacy against prostate cancer alone. Our recent experiments has shown remarkable synergy in inducing rapid and massive apoptosis when PI3K inhibitor ZSTK474 is used in combination with Pseudomonas aeruginosa exotoxin A fragment fused with Transforming Growth Factor Alpha (TGFα-PE38). Time lapse video microscopy of prostate cancer C4-2 cell line has shown the substantial cell death within 4-6 hours with combined administration, compared to limited cell death in single agents-treated cells. Analysis of PARP and cleaved-caspase 3 fragment by Western blotting confirmed that cell death occurs via apoptosis. Quantitation of caspase 3 and 7 activity with luminescent AFC-DEVD substrate confirmed synergy in apoptosis activation by combination of TGFα-PE38 and ZSTK474, while single agents even in higher concentration did not induce substantial apoptosis within 6 hours time period. Analysis of the mechanisms underlying this synergy has shown that PI3K inhibitor ZSTK474 triggers Bad dephosphorylation, while TGFα-PE38 reduces expression levels of Mcl-1. A slight increase of Bim protein was detected and related with ZSTK474 administration. No major variations were detected in the expression levels of pro- and anti-apoptotic proteins Bax, Bcl-2 and Bcl-XL. To address the role of Bad phosphorylation and Mcl-1 expression in apoptosis induction by PI3K inhibitor ZSTK474 and TGFα-PE38, we examined apoptosis in C4-2 cells that express phosphorylation-deficient mutant Bad (BAD2SA). Results indicated that the expression of BAD2SA sensitized C4-2 cells to apoptosis induced by TGFα-PE38. Experiments that address the role of Mcl-1 loss in apoptosis induced by TGFα-PE38 are ongoing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 183. doi:10.1158/1538-7445.AM2011-183