Nonalcoholic steatohepatitis (NASH) is a chronic liver disease that is manifested clinically by an elevation in hepatic triglycerides, inflammation, and fibrosis. The pathogenesis of NASH remains incompletely understood. Sirtuin 6 (Sirt6), an NAD-dependent deacetylase, has been implicated in the fatty liver disease; however, the underlying molecular mechanisms in the NASH pathogenesis are elusive. The aims of this study were to elucidate the role of hepatic Sirt6 in NASH. Wild-type (WT), liver-specific Sirt6 knockout (KO), hepatic stellate cell (HSC)-specific Sirt6 knockout (HSC-KO), and Sirt6 transgenic (Tg) mice were subjected to a Western diet for 4 weeks. Hepatic phenotypes were characterized and underlying mechanisms were investigated. Remarkably, both the liver-KO and HSC-KO mice developed much worse NASH than the WT mice did whereas the Tg mice were protected from the diet-induced NASH. Our cell signaling analysis revealed that Sirt6 negatively regulates the TGFβ-Smad3 pathway. Biochemical analysis showed a physical interaction between Sirt6 and Smad3 in hepatic stellate cells. Moreover, our molecular data further demonstrated that Sirt6 deacetylates Smad3 at key lysine residues K333 and K378 and attenuates its transcriptional activity induced by TGFβ in the hepatic stellate cells. Our data suggest that SIRT6 plays a critical role in the protection against the NASH development and it may serve as a potential therapeutic target for NASH.

Forkhead O transcription factors (FOXOs) have been implicated in glucose and lipid homeostasis; however, the role of FOXOs in the development of nonalcoholic fatty liver disease (NAFLD) is not well understood. In this study, we designed experiments to examine the effects of two different diets—very high fat diet (HFD) and moderately high fat plus cholesterol diet (HFC)—on wildtype (WT) and liver-specific Foxo1/3/4 triple knockout mice (LTKO). Both diets induced severe hepatic steatosis in the LTKO mice as compared to WT controls. However, the HFC diet led to more severe liver injury and fibrosis compared to the HFD diet. At the molecular levels, hepatic Foxo1/3/4 deficiency triggered a significant increase in the expression of inflammatory and fibrotic genes including Emr1, Ccl2, Col1a1, Tgfb, Pdgfrb, and Timp1. Thus, our data suggest that FOXO transcription factors play a salutary role in the protection against the diet-induced fatty liver disease.

Chronic exposure of pancreatic β-cells to abnormally elevated levels of free fatty acids can lead to β-cell dysfunction and even apoptosis, contributing to type 2 diabetes pathogenesis. In pancreatic β-cells, sirtuin 6 (SIRT6) has been shown to regulate insulin secretion in response to glucose stimulation. However, the roles played by SIRT6 in β-cells in response to lipotoxicity remain poorly understood. Our data indicated that SIRT6 protein and mRNA levels were reduced in islets from diabetic and aged mice. High concentrations of palmitate (PA) also led to a decrease in SIRT6 expression in MIN6 β-cells and resulted in cell dysfunction and apoptosis. Knockdown of Sirt6 caused an increase in cell apoptosis and impairment in insulin secretion in response to glucose in MIN6 cells even in the absence of PA exposure. Furthermore, overexpression of SIRT6 alleviated the palmitate-induced lipotoxicity with improved cell viability and increased glucose-stimulated insulin secretion. In summary, our data suggest that SIRT6 can protect against palmitate-induced β-cell dysfunction and apoptosis.