Abstract 4375: Hrk mediates 2 methoxyestradiol induced mitochondrial apoptotic signaling in prostate cancer cells.
Prostate cancer is one of the most prevalent cancers in males and ranks the second most common cause of cancer related deaths. 2 methoxyestradiol (2 ME) an endogenous estrogen metabolite, is a promising anticancer agent for various types of cancers. Although 2 ME has been shown to activate Jun N terminal kinase (JNK) and mitochondrial dependent apoptotic signaling pathways, the underlying mechanisms including downstream effectors remain unclear. Here, we report that the human Bcl 2 homology 3 (BH3) only protein harakiri (Hrk) is a critical effector of 2 ME induced JNK dependent apoptosis in prostate cancer cells. Hrk mRNA and protein are preferentially up regulated by 2 ME, and Hrk induction is dependent on JNK activation of c Jun. Hrk knockdown prevents 2 ME mediated apoptosis by attenuating the decrease in mitochondrial membrane potential, subsequent cytochrome c (cyt c) release and caspase activation. Conversely, Hrk over expression increases caspase dependent prostate cancer cell apoptosis. Involvement of the pro apoptotic protein Bak in this process suggested the possible interaction between Hrk and Bak. Thus, Hrk activation by 2 ME or its over expression displaced Bak from the complex with anti apoptotic protein Bcl xL, while deletion of the Hrk BH3 domain abolished its interaction with Bcl xL, reducing the pro apoptotic function of Hrk. Finally, Hrk is also involved in the 2 ME mediated reduction of X linked inhibitor of apoptosis (XIAP) through Bak activation in prostate cancer cells. Together, our findings suggest that induction of the BH3 only protein Hrk is a critical step in 2 ME activation of the JNK induced apoptotic pathway targeting mitochondria by liberating pro apoptotic protein Bak. Citation Format: Inik Chang, Shahana Majid, Sharanjot Saini, Mohd S. Zaman, Soichiro Yamamura, Takeshi Chiyomaru, Varahram Shahryari, Shinichiro Fukuhara, Guoren Deng, Rajvir Dahiya, Yuichiro Tanaka. Hrk mediates 2 methoxyestradiol induced mitochondrial apoptotic signaling in prostate cancer cells. [abstract] In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6 10; Washington, DC. Philadelphia (PA) AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4375. doi:10.1158/1538 7445.AM2013 4375