Abstract 5274: Mitochondrial Transcription Factors, Tfam and Tfb2m Regulate the SERCA2 Gene Transcription -A Novel Mechanism of the Coordinate Regulation of Energy Production and Expenditure-
Background: The heart consumes large amount of ATP, which is mainly produced in mitochondria. Mitochondrial enzymes are produced under the control of mitochondrial specific transcription factor, Tfam and its transcriptional enhancer, Tfb2m. SERCA2 plays a central role in cardiac function by using high energy of ATP. Therefore, we tested a hypothesis if Tfam and Tfb2m activate SERCA2 gene transcription as well as mitochondrial gene and they serve as coordinate regulators of energy production and expenditure in the normal and diseased heart.
Methods and Results: Neonatal rat cardiac myocytes were used in this study. Localization of Tfam and Tfb2m in the nucleus as well as in mitochondria was demonstrated by immunofluorescence study. ChIP assay revealed that Tfam and Tfb2m directly bound to the −479 to +1 nt region of the rat SERCA2 gene promoter. Fluorescence correlation spectroscopy further revealed that Tfam and Tfb2m bound to the −122 to −114 nt and the −122 to −117 nt region, respectively. Mutation of these region abolished bindings of Tfam and Tfb2m to the SERCA2 gene promoter. Overexpression of Tfam and Tfb2m increased the SERCA2 gene transcription by 96% and the ablation of Tfam and Tfb2m by thier specific siRNAs diminished the transcription by 65 %. We next determined the pathophysiological role of Tfam and Tfb2m in the SERCA2 gene regulation. In the rat myocardial infarction model, SERCA2 mRNA expression level was significantly correlated with Tfam (r=0.52, p<0.001) and Tfb2m (r=0.72, p<0.001) mRNA expression. In the neonatal rat cultured cardiac myocytes, mutation of either Tfam or Tfb2m binding sites in the SERCA2 gene promoter decreased basal SERCA2 gene transcription level. Norepinephrine, an inducer of heart failure, decreased SERCA2 gene transcription and its effect was further enhanced when Tfam and Tfb2m binding sites were mutated.
Conclusion: While Tfam and Tfb2m have been recognized as mitochondrial DNA specific transcription factors, we disclosed that they also play a significant role in the regulation of nuclear DNA, the SERCA2 gene, in normal and pathological conditions. This is a novel mehcanism for the coordinate regulation of molecules for ATP production and expenditure and may serve as a therapuetic target for heart failure.