The small GTPase Rac1 is involved in the activation of the reduced NAD phosphate oxidase complex resulting in superoxide production. We recently showed that Bcl-2 overexpression inhibited apoptosis in leukemia cells by creating a pro-oxidant intracellular milieu, and that inhibiting intracellular superoxide production sensitized Bcl-2-overexpressing cells to apoptotic stimuli. We report here that silencing and functional inhibition of Rac1 block Bcl-2-mediated increase in intracellular superox-
The critical role of mitochondria in cell fate decisions has been well documented over the years. These observations have highlighted the way mitochondrial physiology controls cell survival and growth in the normal settings, the critical role of mitochondrial outer membrane permeabilization and altered mitoenergetics in cell death execution, and most importantly the association of altered mitochondrial metabolism with pathological states, in particular cancer. Reprogramming of cell metabolism, an invariable finding in cancer cells, is tightly linked to mitoenergetics as is evidenced by up-regulation of nutrient uptake and a pro-oxidant tilt in the intracellular milieu. The latter has also been demonstrated in oncogene-induced carcinogenesis models, notably as a functional outcome of Bcl-2 overexpression. Interestingly, even in that model, mitochondria appear to be the target as well. Thus the association of metabolic re-circuiting and altered mitoenergetics with the process of transformation has resulted in a paradigm shift in the way cancer development and progression is viewed today, which has tremendous implications for the development of novel and strategic therapeutic modalities.
We have previously reported that Bcl-2 expression resulted in an increase in intracellular superoxide anion and that a dominant negative mutant of the small GTPase Rac1 sensitized Bcl-2 expressing cells to apoptosis. Here we report that silencing and functional inhibition of Rac1 blocks Bcl-2 mediated increases in intracellular and mitochondrial superoxide levels in tumor cells. We provide evidence that this effect is mediated via specific interaction between the two proteins using co-immunoprecipitation, confocal and electron microscopy, as well as GST-fusion proteins. Analysis of the sub-cellular localization of these proteins revealed increased association of Bcl-2 and mitochondrial Rac1 in Bcl-2 overexpressing cells. This interaction can be blocked in vitro and in vivo by BH3 mimetics such as HA14-I, BH3-I as well as synthetic Bcl-2 BH3 domain peptides. That this interaction is functionally relevant is supported by the ability of the Bcl-2 BH3 peptide to inhibit intracellular superoxide production as well as overcome drug resistance in Bcl-2 overexpressing cells. Lastly, using patient-derived primary tissues, we observed the interaction only in cancerous tissues with marked overexpression of Bcl-2 and not in peripheral blood leukocytes or samples from non-cancerous tissue. These data provide a novel facet in the biology of Bcl-2 with potential implications for targeted anti-cancer drug design. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1025.
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