SIRT6 Protects Against Liver Fibrosis by Negatively Regulating YAP/TAZ.

Hepatic fibrosis often occurs after chronic liver injury, and it is largely attributable to the activation of hepatic stellate cells (HSCs). It is a determining step in the pathophysiology of non-alcoholic steatohepatitis (NASH), an advanced form of fatty liver disease. The Hippo pathway and its effectors Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), have been implicated in hepatic fibrogenesis. However, how YAP and TAZ are regulated in HSCs remains incompletely understood. Here in this work, we have identified Sirtuin 6 (SIRT6), a NAD+ dependent deacetylase, as a regulator of YAP and TAZ in HSCs. Using primary HSCs deficient in SIRT6 from mice fed a moderately-high-fat-cholesterol-cholate diet (HFCC) for one week as well as in vitro models using transforming growth factor-β (TGF-β) treated LX-2 cells, we have revealed that YAP and TAZ were acetylated during the HSC activation. Moreover, expression of YAP and TAZ target genes including connective tissue growth factor (Ctgf), cysteine-rich angiogenic inducer 61 (Cyr61) and ankyrin repeat domain 1 (Ankrd1) was increased in those HSCs. Immunofluorescence microscopy analysis of liver sections obtained from Sirt6 HSC-specific knockout mice fed HFCC diet for six weeks showed elevated YAP nuclear localization in the HSCs, suggesting an activation of YAP during fibrosis. Further in vitroanalysis of YAP/TAZ activity upon gain- and loss-of-function of SIRT6 in TGF-β treated LX-2 cells showed a decrease or increase of Ctgf, Cyr61 and Col3a1gene expression,respectively. To understand how SIRT6 regulates YAP and TAZ, we performed immunoprecipitation and lysine acetylation analysis, and our data showed that SIRT6 indeed interacted with YAP and TAZ and deacetylated them. Taken together, our study has identified a key regulatory axis in which SIRT6 interacts with YAP/TAZ and deacetylates them at several lysine residues, thereby suppressing their transcriptional co-activator activity during fibrosis.