Dysregulated mitochondrial metabolism has been linked with insulin resistance and diabetes in humans. SIRT3, a member of the sirtuin family of NAD+-dependent deacylases, has been shown to regulate a range of mitochondrial proteins, suggesting a key role for this enzyme in energy metabolism. Studies using in vitro models, SIRT3 knockout mice, and high fat diet (HFD) fed mice have suggested a link between deleted, or low SIRT3 protein and insulin resistance and metabolic inflexibility in skeletal muscle1. Given the detrimental effects of SIRT3 deletion on metabolic homeostasis, we have acutely overexpressed SIRT3 in rat skeletal muscle to investigate the effect in vivo on muscle glucose disposal and insulin sensitivity.
Overexpression of SIRT3 in male Wistar rats was achieved via intramuscular injection of AAV2/9.tMCK.SIRT3-FLAG in the right tibialis cranialis muscle, with the contralateral leg acting as a control. Rats were then fed a chow or HFD for 4 weeks.
Western blot analysis showed SIRT3 protein was overexpressed 9-fold in the right tibialis compared to the left. HFD significantly increased intramuscular triglyceride compared to chow (4.8±0.4 vs. 3.4±0.08 respectively, p=0.002), but SIRT3 overexpression in the tibialis had no effect on triglyceride content or glycogen accumulation in muscle.
Under hyperinsulinemic-euglycemic clamp conditions, glucose uptake into muscle was significantly decreased in muscles of high-fat fed rats (19.51±0.3 vs. 13.8±0.4 umol/100g/min, -29%, p<0.001), however overexpression of SIRT3 had no effect on glucose uptake into muscle or glucose incorporation into glycogen.
Despite high overexpression of SIRT3 in rat skeletal muscle, similar impairments in peripheral insulin sensitivity and triglyceride accumulation were seen in both limbs in response to HFD. These results suggest that acute overexpression of SIRT3 in rodent muscle in vivo does not protect against lipid accumulation and insulin resistance induced by HFD.