Abstract 283: SGK1-Dependent Sirt3 Phosphorylation Regulates Mitochondrial Dynamics

Mitochondrial dynamics (i.e. fusion and fission) is impaired in models of obesity and can result in target organ dysfunction. However, the mechanisms that regulate mitochondrial dynamics in the setting of obesity are not completely understood. The objectives of this study are to examine a role for and determine the molecular mechanisms of serum and glucocorticoid-inducible kinase 1 (SGK1) in obesity-related mitochondrial dynamics in the vasculature. We recently reported that aortic expression of SGK1 is elevated in a model of diet-induced obesity (DIO) in vivo and by resistin; a fat-derived adipokine, in human aortic smooth muscle cells (SMC) in vitro . To directly examine the effects of SMC-derived SGK1 on mitochondrial dynamics, wildtype and SMC-specific SGK1 knockout mice were subjected to DIO for eight weeks. Our results indicate that SMC-specific deletion of SGK1 induced a fused, elongated mitochondrial phenotype in aortic SMC in vivo and attenuated obesity-mediated arterial mitochondrial fragmentation suggesting a role for SGK1 in stimulation of mitochondrial fission. To determine the molecular mechanism for this effect, we performed a proteomic screen for novel SGK1 substrates and identified the mitochondrial deacetylase SIRT3 as a novel SGK1 target. Mass spectrometry indicates SGK1 phosphorylates SIRT3 on serine103. Increasing doses of resistin augmented SIRT3-S103 phosphorylation and caused a concomitant decrease in total SIRT3 in rat aortic SMC in vitro . To examine whether SGK1-dependent SIRT3 phosphorylation regulates the mitochondrial fission protein machinery; we evaluated total and activated levels of Drp1, the mitochondrial fission regulator, in response to ectopic expression of SIRT3 wildtype, phospho-memetic (S103D) and phospho-deficient (S103A) mutants. SIRT3-S103D increased total Drp1 and activated Drp1 protein levels an effect inhibited by SIRT3-S103A. These findings implicate elevated resistin observed during obesity in stimulation of SGK1 and subsequent phosphorylation of SIRT3 leading to activation of Drp1 and stimulation of arterial mitochondrial fragmentation.