Sirt6 Alleviated Liver Fibrosis by Deacetylating Conserved Lysine 54 on Smad2 in Hepatic Stellate Cells

Backgrounds and Aims Activation of hepatic stellate cells (HSCs) is a central driver of fibrosis. This study aimed to elucidate the role of the deacetylase sirtuin 6 (Sirt6) in HSC activation and liver fibrosis. Approach and Results Gain-of-function and loss-of-function models were used to study the function of Sirt6 in HSC activation. Mass spectrometry was used to determine the specific acetylation site. The lecithin retinol acyltransferase-driven cyclization recombination recombinase construct (CreERT2) mouse line was created to generate HSC-specific conditional Sirt6-knockout mice (Sirt6(oHSC)). We found that Sirt6 is most abundantly expressed in HSCs as compared with other liver cell types. The expression of Sirt6 was decreased in activated HSCs and fibrotic livers of mice and humans. Sirt6 knockdown and Sirt6 overexpression increased and decreased fibrogenic gene expression, respectively, in HSCs. Mechanistically, Sirt6 inhibited the phosphorylation and nuclear localization of mothers against decapentaplegic homolog (Smad) 2. Further study demonstrated that Sirt6 could directly interact with Smad2, deacetylate Smad2, and decrease the transcription of transforming growth factor beta/Smad2 signaling. Mass spectrometry revealed that Sirt6 deacetylated conserved lysine 54 on Smad2. Mutation of lysine 54 to Arginine in Smad2 abolished the regulatory effect of Sirt6. In vivo, specific ablation of Sirt6 in HSCs exacerbated hepatocyte injury and cholestasis-induced liver fibrosis in mice. With targeted delivery of the Sirt6 agonist MDL-800, its concentration was 9.28-fold higher in HSCs as compared with other liver cells and alleviated hepatic fibrosis. Conclusions Sirt6 plays a key role in HSC activation and liver fibrosis by deacetylating the profibrogenic transcription factor Smad2. Sirt6 may be a potential therapeutic target for liver fibrosis.