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Tuberous sclerosis complex-1 deficiency attenuates diet-induced hepatic lipid accumulation.

Kenerson HL, Yeh MM, Yeung RS - PLoS ONE (2011)

Bottom Line: These observations suggest that mTORC1 is neither necessary nor sufficient for steatosis.Instead, Akt and mTORC1 have opposing effects on hepatic lipid accumulation such that mTORC1 protects against diet-induced steatosis.These findings provide novel insights into the role of mTORC1 in hepatic lipid metabolism.

View Article: PubMed Central - PubMed

Affiliation: Department of Surgery, University of Washington, Seattle, Washington, United States of America.

ABSTRACT
Non-alcoholic fatty liver disease (NAFLD) is causally linked to type 2 diabetes, insulin resistance and dyslipidemia. In a normal liver, insulin suppresses gluconeogenesis and promotes lipogenesis. In type 2 diabetes, the liver exhibits selective insulin resistance by failing to inhibit hepatic glucose production while maintaining triglyceride synthesis. Evidence suggests that the insulin pathway bifurcates downstream of Akt to regulate these two processes. Specifically, mTORC1 has been implicated in lipogenesis, but its role on hepatic steatosis has not been examined. Here, we generated mice with hepatocyte-specific deletion of Tsc1 to study the effects of constitutive mTORC1 activation in the liver. These mice developed normally but displayed mild hepatomegaly and insulin resistance without obesity. Unexpectedly, the Tsc1- livers showed minimal signs of steatosis even under high-fat diet condition. This 'resistant' phenotype was reversed by rapamycin and could be overcome by the expression of Myr-Akt. Moreover, rapamycin failed to reduce hepatic triglyceride levels in models of steatosis secondary to Pten ablation in hepatocytes or high-fat diet in wild-type mice. These observations suggest that mTORC1 is neither necessary nor sufficient for steatosis. Instead, Akt and mTORC1 have opposing effects on hepatic lipid accumulation such that mTORC1 protects against diet-induced steatosis. Specifically, mTORC1 activity induces a metabolic shift towards fat utilization and glucose production in the liver. These findings provide novel insights into the role of mTORC1 in hepatic lipid metabolism.

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Hepatic mRNA expression of metabolic genes.Relative expression of genes involved in hepatic lipogenesis (SREBP1, ACLY, FASN), lipolysis (ATGL), gluconeogenesis (PEPCK), glycolysis (GK), mitochondrial respiration (PGC1α) and triglyceride secretion (ApoB, Mttp) were determined by RT-PCR analyses of RNA extracted from liver samples derived from experiments described in Figures 4, 6 and 8. All values represent mean ±SEM. A, B) Comparison of Tsc1+/+ and Tsc1−/− mice fed normal chow (NCD) and high-fat diet (HFD) with and without rapamycin (rapa). * p<0.05 (not all significant differences are highlighted). C) Pten−/− mice treated with rapamycin or vehicle (dmso) compared to wild-type littermates, * p<0.05 compared to Pten+/+. D) Gene expression in livers of wild-type mice fed NCD or HFD with or without rapamycin treatment. *p<0.05 compared to NCD, E) Reduced FoxO1 phosphorylation in Tsc1−/− livers. Tissue lysates from Tsc1+/+ and Tsc1−/− livers were analyzed for the expression of the indicated proteins by immunoblot analyses. Levels of FoxO1(Ser256) phosphorylation were quantified relative to total FoxO1 expression based on densitometric analyses (Image J).
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pone-0018075-g009: Hepatic mRNA expression of metabolic genes.Relative expression of genes involved in hepatic lipogenesis (SREBP1, ACLY, FASN), lipolysis (ATGL), gluconeogenesis (PEPCK), glycolysis (GK), mitochondrial respiration (PGC1α) and triglyceride secretion (ApoB, Mttp) were determined by RT-PCR analyses of RNA extracted from liver samples derived from experiments described in Figures 4, 6 and 8. All values represent mean ±SEM. A, B) Comparison of Tsc1+/+ and Tsc1−/− mice fed normal chow (NCD) and high-fat diet (HFD) with and without rapamycin (rapa). * p<0.05 (not all significant differences are highlighted). C) Pten−/− mice treated with rapamycin or vehicle (dmso) compared to wild-type littermates, * p<0.05 compared to Pten+/+. D) Gene expression in livers of wild-type mice fed NCD or HFD with or without rapamycin treatment. *p<0.05 compared to NCD, E) Reduced FoxO1 phosphorylation in Tsc1−/− livers. Tissue lysates from Tsc1+/+ and Tsc1−/− livers were analyzed for the expression of the indicated proteins by immunoblot analyses. Levels of FoxO1(Ser256) phosphorylation were quantified relative to total FoxO1 expression based on densitometric analyses (Image J).

Mentions: To explore the mechanism for the steatosis-resistant phenotype in the Tsc1−/− liver, we analyzed the hepatic expression of genes involved in lipid and glucose metabolism by RT-PCR analyses. Figures 9A and 9B show the results from the experiments described in Figure 8 highlighting the effects of Tsc1, HFD and rapamycin on hepatic metabolic gene expression relative to NCD-fed wild-type livers. In the Tsc1+/+ mice, HFD induced SREBP1c and glucose kinase (GK) expression and suppressed ATGL and PEPCK expression indicative of a metabolic shift towards fat synthesis and glucose utilization leading to steatosis. The loss of Tsc1 resulted in an opposite response to HFD with a significantly blunted increase in SREBP1c and GK expression and an exaggerated up-regulation of ATGL and PEPCK (Figure 9A). Further, these changes in the Tsc1−/− livers were reversed with rapamycin treatment such that the effects of mTORC1 inhibition resembled the response of the normal liver to HFD. These observations suggest that the protection from HFD-induced steatosis in the Tsc1−/− liver stems from a mTORC1-dependent switch in hepatic metabolism from fat synthesis to fat utilization and from glucose utilization to glucose production. Moreover, hepatic PGC1α expression was also rapamycin-sensitive and was significantly elevated in the Tsc1−/− livers suggestive of an increase in mitochondrial oxidation. While other factors such as TG export may influence hepatic lipid accumulation, we did not find a significant difference in the expression of hepatic microsomal triglyceride transfer protein (Mttp) between the groups although ApoB expression in the wild-type livers was significantly reduced when challenged with the HFD, a response not seen in the Tsc1−/− mice (Figure 9B).


Tuberous sclerosis complex-1 deficiency attenuates diet-induced hepatic lipid accumulation.

Kenerson HL, Yeh MM, Yeung RS - PLoS ONE (2011)

Hepatic mRNA expression of metabolic genes.Relative expression of genes involved in hepatic lipogenesis (SREBP1, ACLY, FASN), lipolysis (ATGL), gluconeogenesis (PEPCK), glycolysis (GK), mitochondrial respiration (PGC1α) and triglyceride secretion (ApoB, Mttp) were determined by RT-PCR analyses of RNA extracted from liver samples derived from experiments described in Figures 4, 6 and 8. All values represent mean ±SEM. A, B) Comparison of Tsc1+/+ and Tsc1−/− mice fed normal chow (NCD) and high-fat diet (HFD) with and without rapamycin (rapa). * p<0.05 (not all significant differences are highlighted). C) Pten−/− mice treated with rapamycin or vehicle (dmso) compared to wild-type littermates, * p<0.05 compared to Pten+/+. D) Gene expression in livers of wild-type mice fed NCD or HFD with or without rapamycin treatment. *p<0.05 compared to NCD, E) Reduced FoxO1 phosphorylation in Tsc1−/− livers. Tissue lysates from Tsc1+/+ and Tsc1−/− livers were analyzed for the expression of the indicated proteins by immunoblot analyses. Levels of FoxO1(Ser256) phosphorylation were quantified relative to total FoxO1 expression based on densitometric analyses (Image J).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3066210&req=5

pone-0018075-g009: Hepatic mRNA expression of metabolic genes.Relative expression of genes involved in hepatic lipogenesis (SREBP1, ACLY, FASN), lipolysis (ATGL), gluconeogenesis (PEPCK), glycolysis (GK), mitochondrial respiration (PGC1α) and triglyceride secretion (ApoB, Mttp) were determined by RT-PCR analyses of RNA extracted from liver samples derived from experiments described in Figures 4, 6 and 8. All values represent mean ±SEM. A, B) Comparison of Tsc1+/+ and Tsc1−/− mice fed normal chow (NCD) and high-fat diet (HFD) with and without rapamycin (rapa). * p<0.05 (not all significant differences are highlighted). C) Pten−/− mice treated with rapamycin or vehicle (dmso) compared to wild-type littermates, * p<0.05 compared to Pten+/+. D) Gene expression in livers of wild-type mice fed NCD or HFD with or without rapamycin treatment. *p<0.05 compared to NCD, E) Reduced FoxO1 phosphorylation in Tsc1−/− livers. Tissue lysates from Tsc1+/+ and Tsc1−/− livers were analyzed for the expression of the indicated proteins by immunoblot analyses. Levels of FoxO1(Ser256) phosphorylation were quantified relative to total FoxO1 expression based on densitometric analyses (Image J).
Mentions: To explore the mechanism for the steatosis-resistant phenotype in the Tsc1−/− liver, we analyzed the hepatic expression of genes involved in lipid and glucose metabolism by RT-PCR analyses. Figures 9A and 9B show the results from the experiments described in Figure 8 highlighting the effects of Tsc1, HFD and rapamycin on hepatic metabolic gene expression relative to NCD-fed wild-type livers. In the Tsc1+/+ mice, HFD induced SREBP1c and glucose kinase (GK) expression and suppressed ATGL and PEPCK expression indicative of a metabolic shift towards fat synthesis and glucose utilization leading to steatosis. The loss of Tsc1 resulted in an opposite response to HFD with a significantly blunted increase in SREBP1c and GK expression and an exaggerated up-regulation of ATGL and PEPCK (Figure 9A). Further, these changes in the Tsc1−/− livers were reversed with rapamycin treatment such that the effects of mTORC1 inhibition resembled the response of the normal liver to HFD. These observations suggest that the protection from HFD-induced steatosis in the Tsc1−/− liver stems from a mTORC1-dependent switch in hepatic metabolism from fat synthesis to fat utilization and from glucose utilization to glucose production. Moreover, hepatic PGC1α expression was also rapamycin-sensitive and was significantly elevated in the Tsc1−/− livers suggestive of an increase in mitochondrial oxidation. While other factors such as TG export may influence hepatic lipid accumulation, we did not find a significant difference in the expression of hepatic microsomal triglyceride transfer protein (Mttp) between the groups although ApoB expression in the wild-type livers was significantly reduced when challenged with the HFD, a response not seen in the Tsc1−/− mice (Figure 9B).

Bottom Line: These observations suggest that mTORC1 is neither necessary nor sufficient for steatosis.Instead, Akt and mTORC1 have opposing effects on hepatic lipid accumulation such that mTORC1 protects against diet-induced steatosis.These findings provide novel insights into the role of mTORC1 in hepatic lipid metabolism.

View Article: PubMed Central - PubMed

Affiliation: Department of Surgery, University of Washington, Seattle, Washington, United States of America.

ABSTRACT
Non-alcoholic fatty liver disease (NAFLD) is causally linked to type 2 diabetes, insulin resistance and dyslipidemia. In a normal liver, insulin suppresses gluconeogenesis and promotes lipogenesis. In type 2 diabetes, the liver exhibits selective insulin resistance by failing to inhibit hepatic glucose production while maintaining triglyceride synthesis. Evidence suggests that the insulin pathway bifurcates downstream of Akt to regulate these two processes. Specifically, mTORC1 has been implicated in lipogenesis, but its role on hepatic steatosis has not been examined. Here, we generated mice with hepatocyte-specific deletion of Tsc1 to study the effects of constitutive mTORC1 activation in the liver. These mice developed normally but displayed mild hepatomegaly and insulin resistance without obesity. Unexpectedly, the Tsc1- livers showed minimal signs of steatosis even under high-fat diet condition. This 'resistant' phenotype was reversed by rapamycin and could be overcome by the expression of Myr-Akt. Moreover, rapamycin failed to reduce hepatic triglyceride levels in models of steatosis secondary to Pten ablation in hepatocytes or high-fat diet in wild-type mice. These observations suggest that mTORC1 is neither necessary nor sufficient for steatosis. Instead, Akt and mTORC1 have opposing effects on hepatic lipid accumulation such that mTORC1 protects against diet-induced steatosis. Specifically, mTORC1 activity induces a metabolic shift towards fat utilization and glucose production in the liver. These findings provide novel insights into the role of mTORC1 in hepatic lipid metabolism.

Show MeSH
Related in: MedlinePlus