Mechanistic Elucidation of the Function of Sirtuin 6 in the Regulation of Liver Fibrosis

Abstract

Hepatic fibrosis is a cellular repair mechanism that is initiated upon prolonged damage to the liver, resulting in an accumulation of excess extracellular matrix. This eventually leads to the formation of scar tissue, which disrupts the hepatic architecture and causes liver dysfunction. Hepatic stellate cells (HSCs) play a major role in hepatic fibrosis. However, the molecular mechanisms remain incompletely understood. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ or WWTR1), key players of the Hippo pathway, have been implicated in the liver fibrosis, but the HSC-specific functions of YAP and TAZ are largely unclear. Here we have identified Sirtuin 6 (SIRT6), an NAD+ dependent deacetylase, as a key epigenetic regulator in the protection against hepatic fibrosis by suppressing the YAP/TAZ activity. SIRT6 has been previously implicated in the regulation of the canonical transforming growth factor β (TGFβ)-SMAD3 pathway. This study has revealed the significant contribution of the non-canonical pathways including the Hippo pathway to the development of hepatic fibrosis. HSC-specific Sirt6 deficient mice developed severe fibrosis when fed a high-fat-cholesterol-cholate diet compared to their wild-type counterparts. YAP became more active in the SIRT6-deficient HSCs. Expression of the YAP/TAZ downstream genes like CTGF, CYR61 and ANKRD1 were elevated in the SIRT6-deficient HSCs. Biochemical and mutagenic analyses have revealed that SIRT6 deacetylates YAP and TAZ at key lysine residues and reprograms the composition of the TEA domain transcription factor complex to suppress the YAP/TAZ function in the hepatic fibrogenesis.