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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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mTORC1 activity is not sufficient for steatosis.Normal chow-fed, 20-week old male Tsc1+/+ and Tsc1−/− male mice were fasted overnight and sacrificed. Liver tissues were processed for histologic and biochemical analyses. A) Liver histology (H&E) and Oil Red “O” staining showing hepatic morphology and lipid content. Magnification 400X. B) Quantification of liver triglyceride content using TG assay kit (Roche Diagnostics, see Methods). C) Hepatocyte cell size was deduced based on the average number of hepatocytes per high-power field from 10 randomly selected fields. *, p<0.01 compared to Tsc1+/+. D) Expression of genes involved in lipogenesis (SREBP1), adipogenesis (PPARg), lipolysis (ATGL) and gluconeogenesis (PEPCK) were determined by quantitative RT-PCR. *, p<0.05 compared to Tsc1+/+. For all graphs, values represent mean ±SEM.
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pone-0018075-g003: mTORC1 activity is not sufficient for steatosis.Normal chow-fed, 20-week old male Tsc1+/+ and Tsc1−/− male mice were fasted overnight and sacrificed. Liver tissues were processed for histologic and biochemical analyses. A) Liver histology (H&E) and Oil Red “O” staining showing hepatic morphology and lipid content. Magnification 400X. B) Quantification of liver triglyceride content using TG assay kit (Roche Diagnostics, see Methods). C) Hepatocyte cell size was deduced based on the average number of hepatocytes per high-power field from 10 randomly selected fields. *, p<0.01 compared to Tsc1+/+. D) Expression of genes involved in lipogenesis (SREBP1), adipogenesis (PPARg), lipolysis (ATGL) and gluconeogenesis (PEPCK) were determined by quantitative RT-PCR. *, p<0.05 compared to Tsc1+/+. For all graphs, values represent mean ±SEM.

Mentions: To further examine the response to insulin in the liver, WAT and muscle tissues, fasted mice were given 0.5 U/kg of insulin 10 minutes before sacrifice. As expected, insulin led to a significant increase in Akt phosphorylation in all three tissues in the wild-type animals (Figure 2A,B). However, the Akt response to insulin was dramatically blunted in the tissues derived from the Tsc1−/− mice. In the case of the Tsc1−/− hepatocytes, hyperactivity of mTORC1 is known to inhibit Akt secondary to feedback inhibition on IRS1 [32]. Consequently, both baseline and insulin-stimulated Akt activities were suppressed in the Tsc1−/− liver (Figure 2A). In contrast, basal Akt phosphorylation in WAT and muscle tissues were elevated in the mutant mice relative to wild-type controls while insulin-stimulated response was significantly reduced (Figure 2B). This finding cannot be explained by the ‘feedback’ mechanism but we speculate that the chronic systemic exposure to post-prandial hyper-insulinemia (or other insulin-like growth factors) in the mutant animals may be responsible. Figure 3C highlights the relative changes in Akt(Ser473) phosphorylation between the saline- and insulin-treated wild-type and mutant animals. These results indicate that the loss of hepatic Tsc1 leads to hepatic and systemic insulin resistance.


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

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

mTORC1 activity is not sufficient for steatosis.Normal chow-fed, 20-week old male Tsc1+/+ and Tsc1−/− male mice were fasted overnight and sacrificed. Liver tissues were processed for histologic and biochemical analyses. A) Liver histology (H&E) and Oil Red “O” staining showing hepatic morphology and lipid content. Magnification 400X. B) Quantification of liver triglyceride content using TG assay kit (Roche Diagnostics, see Methods). C) Hepatocyte cell size was deduced based on the average number of hepatocytes per high-power field from 10 randomly selected fields. *, p<0.01 compared to Tsc1+/+. D) Expression of genes involved in lipogenesis (SREBP1), adipogenesis (PPARg), lipolysis (ATGL) and gluconeogenesis (PEPCK) were determined by quantitative RT-PCR. *, p<0.05 compared to Tsc1+/+. For all graphs, values represent mean ±SEM.
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getmorefigures.php?uid=PMC3066210&req=5

pone-0018075-g003: mTORC1 activity is not sufficient for steatosis.Normal chow-fed, 20-week old male Tsc1+/+ and Tsc1−/− male mice were fasted overnight and sacrificed. Liver tissues were processed for histologic and biochemical analyses. A) Liver histology (H&E) and Oil Red “O” staining showing hepatic morphology and lipid content. Magnification 400X. B) Quantification of liver triglyceride content using TG assay kit (Roche Diagnostics, see Methods). C) Hepatocyte cell size was deduced based on the average number of hepatocytes per high-power field from 10 randomly selected fields. *, p<0.01 compared to Tsc1+/+. D) Expression of genes involved in lipogenesis (SREBP1), adipogenesis (PPARg), lipolysis (ATGL) and gluconeogenesis (PEPCK) were determined by quantitative RT-PCR. *, p<0.05 compared to Tsc1+/+. For all graphs, values represent mean ±SEM.
Mentions: To further examine the response to insulin in the liver, WAT and muscle tissues, fasted mice were given 0.5 U/kg of insulin 10 minutes before sacrifice. As expected, insulin led to a significant increase in Akt phosphorylation in all three tissues in the wild-type animals (Figure 2A,B). However, the Akt response to insulin was dramatically blunted in the tissues derived from the Tsc1−/− mice. In the case of the Tsc1−/− hepatocytes, hyperactivity of mTORC1 is known to inhibit Akt secondary to feedback inhibition on IRS1 [32]. Consequently, both baseline and insulin-stimulated Akt activities were suppressed in the Tsc1−/− liver (Figure 2A). In contrast, basal Akt phosphorylation in WAT and muscle tissues were elevated in the mutant mice relative to wild-type controls while insulin-stimulated response was significantly reduced (Figure 2B). This finding cannot be explained by the ‘feedback’ mechanism but we speculate that the chronic systemic exposure to post-prandial hyper-insulinemia (or other insulin-like growth factors) in the mutant animals may be responsible. Figure 3C highlights the relative changes in Akt(Ser473) phosphorylation between the saline- and insulin-treated wild-type and mutant animals. These results indicate that the loss of hepatic Tsc1 leads to hepatic and systemic insulin resistance.

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