Abstract LB-487: Pim protein kinase inhibitors induce the unfolded protein response and sensitize prostate cancer cells to killing by ABT-737-mediated apoptosis

Abstract

Abstract Pim serine/threonine kinases contribute to prostate tumorigenesis and therapeutic resistance, yet anticancer efficacy of Pim kinase inhibitors on prostate cancer is unknown. We demonstrate for the first time that a genetically engineered decrease in Pim kinase levels or the addition of Pim kinase small molecule inhibitors (e.g. SMI-4a) to human prostate cancer cells, including LNCaP and PC-3, causes ER stress and activates the unfolded protein response (UPR) stimulating increases in eIF-2α phosphorylation, ATF-4, CHOP proteins, and cleavage of XBP-1. Because (1) Bcl-2 family proteins reside on the ER lumen and play an important role in regulation of XBP-1 splicing, (2) Bcl-2 family members are over expressed in prostate cancer to enhance Pim kinase anti-cancer activity, and (3) Pim kinase inhibitors decrease Mcl-1 and increase Noxa protein, we treated prostate cancer cell lines with SMI-4a plus the Bcl-2 family antagonist ABT-737, a small molecule antagonist of Bcl-2 family members. Strikingly, the addition of ABT-737 to Pim inhibitors triggered a robust apoptosis of prostate cancer cells in vitro and in vivo. Pim inhibitors decreased levels of the Bcl-2 family member Mcl-1, both by blocking 5′-cap dependent translation and decreasing protein half life. Additionally, Pim inhibition transcriptionally increased levels of the BH3 protein Noxa by activating the UPR, lead to eIF2α phosphorylation and increased expression of CHOP. Increased levels of Noxa also inactivated the remaining levels of Mcl-1 protein activity. Notably, these specific protein changes were essential to the apoptotic process because ABT-737 did not inhibit Mcl-1 protein activity and Mcl-1 overexpression blocked the apoptotic activity of ABT-737. We find that the Pim kinase inhibitors and ABT-737 are highly synergistic in killing prostate cancer both in vitro and when used together in a subcutaneous animal model of prostate cancer therapy. Our results therefore suggest that this combination treatment could be developed as a potential therapy for human prostate cancer where overexpression of Pim kinases and antiapoptotic Bcl-2 family members drives tumor cell resistance to current anticancer therapies. 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-487. doi:1538-7445.AM2012-LB-487