We recently showed that Bcl-2 overexpression induced a pro-oxidant intracellular milieu resulting in an inhibition of apoptotic execution and that reversing the pro-oxidant milieu by inhibiting intracellular superoxide production sensitized Bcl-2-overexpressing cells to apoptotic stimuli. We provide evidence that the small GTPase Rac1 is functionally involved since gene silencing and functional inhibition of Rac1 blocked Bcl-2-mediated increase in intracellular superoxide production in tumor cells. A co-localization and physical interaction between the two proteins were observed, which could be blocked by the BH3 mimetics as well as by synthetic Bcl-2 BH3 domain peptides both in vitro and in vivo. The Bcl-2 BH3 peptides as well as silencing and functional inhibition of Rac1 reversed intracellular superoxide levels and overcame Bcl-2-mediated drug resistance in human leukemia and cervical cancer cells, demonstrating that this interaction is functionally relevant. Notably, the interaction was observed in primary cells from patients diagnosed with B-cell lymphoma with Bcl-2 overexpression but not in noncancerous tissue. Furthermore, computer simulation driven virtual predictive experiments based on the protein pathway dynamic network created by Cellworks Group Inc. were carried out to study the functional implication of this interaction. An HCT116 human colorectal cancer cell line with KRAS over-activation, PI3K overexpression, CDKN2A deletion, β-catenin overexpression and Bcl-2 overexpression was created as a base line. Phenotypic indices of angiogenesis, proliferation, viability, metastasis as well as tumor volume were all found to be amplified in a variant cell line when Rac1 was overexpressed based on the above computer modeled HCT116. Strikingly, the expression levels of both STAT3 and β-catenin were significantly upregulated in the variant cell line; however upon Rac1 or Bcl-2 knockdown in both the base and the variant cell lines, apoptotic markers such as Bax, caspase 3 and cleaved PARP1 were amplified while STAT3 and β-catenin were significantly down-regulated. Intrigued by these predictions, we set out to verify the correlation between STAT3 signaling and Bcl-2-Rac1 interaction. Interestingly, there is indeed a strong correlation between the activation status of STAT3 (pY705) and Bcl-2 expression as well as Rac1 activation status. Furthermore, STAT3 was found to be localized to mitochondria and our preliminary results suggest an association with Bcl-2.

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 2003. doi:1538-7445.AM2012-2003