SIRT3 deficiency exacerbates fatty liver by attenuating the HIF1α-LIPIN 1 pathway and increasing CD36 through Nrf2

Background: Deficiency of mitochondrial sirtuin 3 (SIRT3), a NAD(+)-dependent protein deacetylase that maintains redox status and lipid homeostasis, contributes to hepatic steatosis. In this study, we investigated additional mechanisms that might play a role in aggravating hepatic steatosis in Sirt3-deficient mice fed a high-fat diet (HFD). Methods: Studies were conducted in wild-type (WT) and Sirt3(-/-) mice fed a standard diet or a HFD and in SIRT3-knockdown human Huh-7 hepatoma cells. Results: Sirt3(-/-) mice fed a HFD presented exacerbated hepatic steatosis that was accompanied by decreased expression and DNA-binding activity of peroxisome proliferator-activated receptor (PPAR) alpha and of several of its target genes involved in fatty acid oxidation, compared to WT mice fed the HFD. Interestingly, Sirt3 deficiency in liver and its knockdown in Huh-7 cells resulted in upregulation of the nuclear levels of LIPIN1, a PPAR alpha co-activator, and of the protein that controls its levels and localization, hypoxia-inducible factor 1 alpha (HIF-1 alpha). These changes were prevented by lipid exposure through a mechanism that might involve a decrease in succinate levels. Finally, Sirt3(-/-) mice fed the HFD showed increased levels of some proteins involved in lipid uptake, such as CD36 and the VLDL receptor. The upregulation in CD36 was confirmed in Huh-7 cells treated with a SIRT3 inhibitor or transfected with SIRT3 siRNA and incubated with palmitate, an effect that was prevented by the Nrf2 inhibitor ML385. Conclusion: These findings demonstrate new mechanisms by which Sirt3 deficiency contributes to hepatic steatosis.