Hepatocyte-specific deletion of SIRT1 alters fatty acid metabolism and results in hepatic steatosis and inflammation

Abstract

Hepatic metabolic derangements are key components in the development of fatty liver, insulin resistance, and atherosclerosis. SIRT1, a NAD+-dependent protein deacetylase, is an important regulator of energy homeostasis in response to nutrient availability. Here we demonstrate that hepatic SIRT1 regulates lipid homeostasis by positively regulating peroxisome proliferators-activated receptor alpha (PPARalpha), a nuclear receptor that mediates the adaptive response to fasting and starvation. Hepatocyte-specific deletion of SIRT1 impairs PPARalpha signaling and decreases fatty acid beta-oxidation, whereas overexpression of SIRT1 induces the expression of PPARalpha targets. SIRT1 interacts with PPARalpha and is required to activate PPARalpha coactivator PGC-1alpha. When challenged with a high-fat diet, liver-specific SIRT1 knockout mice develop hepatic steatosis, hepatic inflammation, and endoplasmic reticulum stress. Taken together, our data indicate that SIRT1 plays a vital role in the regulation of hepatic lipid homeostasis and that pharmacological activation of SIRT1 may be important for the prevention of obesity-associated metabolic diseases.

Figure 1. Hepatic deletion of SIRT1 alters PPARα signaling

(A) Microarray analysis of PPARα signaling pathway from control and SIRT1 LKO mice (n=6). The log ratios of SIRT1 LKO/control were presented by heat map. (B) Venn-diagram representation of the subset of PPARα-regulated hepatic fatty acid metabolism genes that were significantly decreased in the liver of SIRT1 LKO mice (n=6, p<0.05). (C and D) Quantitative real-time PCR (qPCR) analysis of PPARα target genes involved in fatty acid oxidation in the liver of control (black bars) and SIRT1 LKO mice (white bars) on the chow diet, sacrificed at 4pm (C) or after a 16 h fast (D). In this and other figures, error bars represent mean ± SEM. (n=6, *p<0.05).

Figure 2. Loss of SIRT1 reduces the induction of PPARα targets and fatty acid oxidation in primary hepatocytes

(A) SIRT1 deficiency in primary hepatocytes reduces the induction of fatty acid oxidation gene expression by PPARα agonist WY14643. Primary hepatocytes from control (black bars) or SIRT1 LKO mice (white bars) were treated as described in the Experimental procedures, and mRNA were analyzed by qPCR (n=3, *p<0.05). (B) SIRT1 deficiency in primary hepatocytes reduces the rate of fatty acid oxidation. The oxidation rate of [3H]-palmitic acid in primary hepatocytes from control (black bars) and SIRT1 LKO (white bars) mice were measured as described in the Experimental Procedures (n=3, *p<0.05). (C) SIRT1 deficient primary hepatocytes show reduced induction of other PPARα targets by WY14643. (D) Overexpression of SIRT1 in primary hepatocytes induces the expression of PPARα targets. Primary hepatocytes from control and SIRT1 LKO mice were infected with lentiviruses expressing GFP (white bars) or SIRT1 (black bars) and mRNA were analyzed by qPCR (*p<0.05).

Figure 3. SIRT1 regulates ligand-dependent PPARα transactivation

(A) Reduction of SIRT1 activity decreases ligand dependent PPARα transactivation. (Left panel) pSuper (black bars) or pSuper-SIRT1 RNAi (white bars) HEK293T cells were cotransfected with luciferase reporters and a construct encoding Gal4 DBD-PPARα LBD fusion protein, then treated with DMSO or PPARα agonists and analyzed for luciferase activity as described in Experimental Procedures. (Right panel) pSuper (black bars) and pSuper-SIRT1 RNAi (white bars) HEK293T cells were transfected with 3xPPRE-luciferase construct or a control construct lacking the PPRE, together with constructs expressing murine PPARα and RXRα. Cells were then treated with increasing doses of WY14643 and analyzed for luciferase activity as described in the Experimental Procedures. (*p<0.05) (B) Loss of SIRT1 in primary hepatocytes reduces ligand dependent PPARα transactivation. (*p<0.05) (C) Overexpression of SIRT1 stimulates ligand-dependent PPARα transactivation in mouse H2.35 hepatoma cells. (*p<0.05)

Figure 4. SIRT1 interacts with PPARα

(A) SIRT1 interacts with PPARα in HEK293T cells. Cell lysates from HEK293T cells transfected with empty vector or HA-PPARα were treated with DMSO or WY14643, were then immunoprecipitated (IP) with anti-HA antibodies. (B) SIRT1 interacts with PPARα in the liver. Liver nuclear extracts from mice treated with vehicle or WY were immunoprecipitated with anti-PPARα antibodies. (C) SIRT1 interacts with PPARα in vitro. GST-SIRT1 fusion protein was incubated with HA-PPARα (left), or GST-PPARα fusion protein was incubated with myc-SIRT1 (right) in the presence of 15 μM WY. Top panels, immunoblots of HA-PPARα or myc-SIRT1. Bottom panels, ponceau S staining of GST fusion proteins. (D) GST-PPARα interacts with the catalytic core domain of SIRT1. GST-PPARα fusion protein was incubated with indicated myc-tagged SIRT1 fragments in the presence of 15 μM WY. (E) The catalytic core domain of SIRT1 interacts with the both DNA-binding domain (DBD) and ligand-binding domain (LBD) of PPARα. GST-SIRT1 (aa 194–497) fusion protein was incubated with HA-tagged full-length (F), DBD, or LBD of PPARα. (F) SIRT1 localizes to the PPRE of PPARα targets. Primary hepatocytes treated with DMSO or WY for 4 h were subjected to ChIP with IgG control (white bars) or anti-SIRT1 (black bars) antibodies and analyzed by qPCR with primers flanking PPRE in the promoters of FGF21, Acot1 and Ehhadh. (G) SIRT1 is recruited to the PPRE of Cyp4A10 upon fasting. ChIP assays were performed with chromatin extracts from livers of control mice that were fed, fasted for 24 h, or fasted for 24 h then refed for 24 h.

Figure 5. SIRT1 is required to activate PGC-1α

(A) PGC-1α accumulates on the PPRE of Acot1 in SIRT1 LKO primary hepatocytes. Control and SIRT1 LKO primary hepatocytes treated with DMSO or WY for 4 h were subjected to ChIP with IgG control or anti-PGC-1α antibodies and analyzed by qPCR with primers flanking PPRE in the promoter of Acot1. (B) Accumulation of PGC-1α and BAF60a on the PPRE of PPARα targets after fasting in the SIRT1 LKO liver. ChIP assays were performed with chromatin extracts from livers of control (black bars) and SIRT1 LKO (white bars) mice fasted for 16 h. (C) PGC-1α protein levels are induced by fasting and repressed by refeeding in both control and SIRT1 LKO mice. Liver extracts from control and SIRT1 LKO mice that were fed, fasted for 24 h, or fasted for 24 h then refed for 24 h, were immunoblotted with indicated antibodies. (D) Accumulation of PGC-1α on the PPRE regardless of feeding status. ChIP assays were performed with chromatin extracts from livers of control and SIRT1 LKO mice that were fed, fasted for 24 h, or fasted for 24 h then refed for 24 h. (E) SIRT1 deficiency in hepatocytes increases the acetylation of PGC-1α. FLAG-PGC-1α from control (Con) and SIRT1 LKO (LKO) primary hepatocytes were treated and immuno-purified as described in the Experimental Procedures, and analyzed with anti-acetyl-lysine (acetyl-K) antibodies. (*p<0.05). (F) Decreased PGC-1α co-activation activity on the expression of PPARα target genes in SIRT1 deficient primary hepatocytes. Primary hepatocytes from control and SIRT1 LKO mice were infected with adenoviruses expressing GFP or PGC-1α, and treated with DMSO or WY as described in the Experimental Procedures (*p<0.05). (G) SIRT1-mediated induction of FGF21 requires PPARα and PGC-1α. Primary hepatocytes from control mice were electroporated with a negative control siRNA or siRNAs against PPARα or PGC-1α. Cells were then infected with lentiviruses expressing GFP or SIRT1, and treated as described in the Experimental Procedures (*p<0.05).

Figure 6. Hepatic loss of SIRT1 function impairs lipid homeostasis upon high-fat diet feeding

(A) Body weight gain and food intake curves of control and SIRT1 LKO mice (n=10–11) under western diet. Arrow indicated the beginning of western diet feeding. (B–E) SIRT1 LKO mice accumulate more lipids in the liver after fasting as indicated by (B) Hematoxylin and eosin staining of liver sections from control and SIRT1 LKO mice; and total liver triglycerides (C), free fatty acids (D), and cholesterol (E) levels. (F–I) SIRT1 deficiency in the liver increases serum free fatty acids but reduces serum β-hydroxybutyrate after fasting. Serum triglycerides (F), free fatty acids (G), cholesterol (H), and β-hydroxybutyrate (I) were analyzed as described. (J) Relative expression of genes encoding key factors in hepatic fatty acid oxidation, oxidative phosphorylation (OXPHOS), fatty acid esterification, bile acid (BA) metabolism, and cholesterol uptake. (n=4–5, *p<0.05).

Figure 7. Hepatic deletion of SIRT1 causes hepatic inflammation and ER stress upon high-fat diet feeding

(A) SIRT1 deficiency in liver increases ER stress. ER stress markers were analyzed by immunoblotting as described. (B) SIRT1 LKO mice display increased hepatic inflammation. Expression of macrophage markers and pro-inflammatory genes in the livers of control (black bars) and SIRT1 LKO mice (white bars) was analyzed by qPCR. (C) SIRT1 LKO mice display increased hepatic levels of pro-inflammatory cytokines. Liver extracts from control and SIRT1 LKO mice were analyzed for TNFα and IL-1β by ELISA. (D) Normal deposition of collagen in the SIRT1 LKO livers. The liver sections from control and SIRT1 LKO mice were stained with Sirius red for collagen. Mice in (A to D) were fed a western diet and scarified after 16 h fasting. (n=4–5, *p<0.05). (E) Serum insulin and leptin from control (black bars) and SIRT1 LKO (white bars) mice. (F) SIRT1 LKO mice display signs of hepatic insulin resistance. (G) SIRT1 deficiency in the liver increases hepatic lipogenesis. Mice in (E to G) were fed a western diet and scarified after 4 h fasting. (n=4–5, *p<0.05).