Abstract LB-79: Synergistic anticancer effects of the 9.2.27PE immunotoxin and ABT-737 in melanoma involves silencing of the anti-apoptotic protein Mcl-1

Abstract

Abstract In cancer, combinations of drugs targeting different cellular functions is well accepted to improve tumor control. We studied the effects of a Pseudomonas exotoxin A (PE)-based immunotoxin, the 9.2.27PE, and the Bcl-2 family inhibitor ABT-737 in a panel of melanoma cell lines. ABT-737 inhibits the Bcl-2, Bcl-xl and the Bcl-w, but not Mcl-1 and A1 of the Bcl-2 family. The 9.2.27PE + ABT-737 drug combination resulted in synergistic cytotoxicity in the melanoma cells independent of BRAF mutation status. The cell death observed was associated with apoptosis, as activation of caspase-3, inactivation of Poly (ADP-ribose) polymerase (PARP) and increased DNA fragmentation could be prevented by pre-treatment with caspase and cathepsin inhibitors. In addition, ABT-737 caused endoplasmic reticulum (ER) stress with increased GRP78, phosphorylated eIF2α protein levels and increased intracellular calcium levels. The increased intracellular calcium levels were enhanced by silencing of Mcl-1 by short hairpin RNA or by treatment with the 9.2.27PE immunotoxin. These results suggests that Mcl-1 has an inhibitory effect on calcium release from the ER, and that strongly decreased Mcl-1 protein levels after treatment with 9.2.27PE or shRNA may be responsible for the increased intracellular calcium levels in ABT-737 treated melanoma cells. Calcium release has been shown to take place through the inositol 1,4,5-triphosphate receptor in the ER membrane, and the Bcl-2 family proteins Bcl-2, Bcl-xl and Mcl-1 have been shown to interact with the inositol 1,4,5-triphosphate receptor, supporting our results.1 Notably, the combination of 9.2.27PE and ABT-737 caused growth delay in a human melanoma xenograft mice model. 1. Eckenrode EF et. Al., J Biol Chem. 2010 Apr 30;285(18):13678-84 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-79. doi:1538-7445.AM2012-LB-79