Abstract 3955: Akt Impairs Myocardial Mitochondrial Function by FOXO1 Mediated Regulation of OXPHOS Gene Promoters
Short-term activation of Akt is cardioprotective, but sustained Akt activation induces cardiac hypertrophy, which ultimately leads to heart failure. We tested the hypothesis that Akt-mediated cardiac hypertrophy is not accompanied by increased mitochondrial metabolic capacity. In mice with constitutive or inducible transgenic activation of Akt (caAkt), mitochondrial O2 consumption and ATP production rates with palmitoyl-carnitine and activity of the beta-oxidation enzyme hydroxyacyl CoA dehydrogenase or the TCA cycle enzyme citrate synthase were significantly decreased as early as 2-weeks after Akt transgene induction. To elucidate mechanisms for Akt-induced repression of mitochondrial function, microarray and proteomic analyses were conducted in LV mRNA and mitochondria respectively. PGC-1α expression was repressed by 71% and of 51 significantly regulated OXPHOS genes, 45 (88%) were repressed (19 by 30 –50% and 26 by 15–30%). Proteomic analysis confirmed that 60% of identified OXPHOS proteins were also repressed. Bioinformatic promoter alignment revealed significant enrichment of forkhead transcription factor (FOXO1/FOXO3) binding sites on promoters of PGC-1α and 33 of 45 repressed OXPHOS genes. We therefore performed chromatin immunoprecipitation with FOXO1 antibody on PGC-1α and three regulated candidate OXPHOS promoters: succinate CoA-ligase, GDP forming beta subunit (Suclg2), ATP synthase mitochondrial F0 complex subunit C (ATP5g1) and cytochrome C (Cytc). Invariant genes on the array that lacked FOXO1 binding sites served as controls. In caAkt hearts, FOXO1 promoter occupancy was reduced by 40%, 38% and 36% in Suclg2, ATP5g1 and Cytc promoters respectively (p<0.05), and similar to 33% and 39% reduction, respectively in promoter occupancy of the PGC-1α and FOXO1 genes. This study identifies a critical role for FOXO transcription factors in the regulation of mitochondrial OXPHOS gene expression in the heart via FOXO-dependent transactivation of the promoters of PGC-1α and many OXPHOS genes. Thus Akt-induced mitochondrial dysfunction is mediated by nuclear exclusion of FOXO1/3, leading to Akt-mediated repression of OXPHOS gene expression, which is amplified by a parallel repression of the co-activator PGC-1α.
This research has received full or partial funding support from the American Heart Association, Western States Affiliate (California, Nevada & Utah).