Abstract LB-158: STAT5 regulates mitochondrial gene expression through direct binding to mitochondrial DNA
Authors: Chueh Fu-Yu , Sun Haijing , Yu Chao-Lan 
Published: August 2012

Abstract
Cancer cells exhibit a distinct metabolic shift from oxidative phosphorylation in the mitochondrion to aerobic glycolysis in the cytoplasm, known as the Warburg effect. The mitochondrion generates ATP through electron transport chain (ETC) by consuming oxygen. ETC components are encoded by both nuclear and mitochondrial genomes suggesting the importance of coordinated nuclear and mitochondrial gene expression in cellular metabolism. However, it is not clear whether and how altered mitochondrial gene expression may contribute to this characteristic cancer metabolism. We reported previously that signal transducer and activator of transcription factor 5 (STAT5) translocated to mitochondria in both cytokine-stimulated cells and in leukemic T cells expressing oncogenic Lck kinase (BBRC 402:778, 2010). In this study, we further examined the role of Lck oncogene and STAT5 in regulating mitochondrial gene expression. Interleukin-3 (IL-3) is known to induce aerobic glycolysis in IL-3-dependent BaF3 cells. Consistent with this metabolic shift, we showed reduced mitochondrial gene expression in IL-3-stimulated BaF3 cells. Expression of oncogenic Lck in BaF3 cells resulted in similar metabolic shift and reduced mitochondrial gene expression. More importantly, we confirmed in vivo binding of STAT5 to the transcriptional control D-loop region of mitochondrial DNA (mtDNA). On the other hand, expression of oncogenic Lck kinase in HEK 293 cells that lack endogenous STAT5 did not alter mitochondrial gene expression. These data strongly support the role of mitochondrial STAT5 in regulating mtDNA transcription. To directly test this hypothesis, we used a mitochondrial-targeting construct to drive mitochondrial localization of exogenous STAT5a in HEK 293 cells. Consistent with our hypothesis, exogenous STAT5a bound to mtDNA in vivo and suppressed mitochondrial gene expression. To further determine the effects of tyrosine phosphorylation and DNA binding of STAT5a on mtDNA transcription, we examined a constitutively active STAT5a mutant, STAT5a 1*6, and the mutant defective in DNA binding, STAT5a 1*6VVV. Targeting of STAT5a 1*6 into the mitochondrion of HEK 293 cells caused a larger reduction of mitochondrial gene expression as compared to wild-type STAT5a. In contrast, the inhibitory effect on mtDNA transcription was reversed when STAT5a 1*6VVV mutant was targeted into the mitochondrion. These results suggest that tyrosine phosphorylation of STAT5 is not only important for its translocation into the mitochondrion, but also plays a key role in repressing mitochondrial gene expression through direct binding to mtDNA. All together, our novel findings reveal an important link between STAT5 mitochondrial translocation and reduced mitochondrial gene expression in the context of cytokine response and oncogenesis. It may represent a unique mechanism in redirecting cellular metabolism toward aerobic glycolysis.
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-158. doi:1538-7445.AM2012-LB-158