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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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Tsc1−/− mice are resistant to diet-induced steatosis in a rapamycin-dependent manner.Six week-old Tsc1+/+ and Tsc1−/− littermates were randomly assigned to normal chow diet (NCD), high fat diet (HFD) or HFD with rapamycin (2 mg/kg IP, 3 times weekly during the last 4 weeks). At the end of 6 weeks on the assigned diets, mice were sacrifice following an overnight fast. A) H&E histology and Oil Red “O” (ORO) staining of representative liver sections from each of the 6 groups. Magnification 400X. B) Biochemical measurements of liver and plasma triglyceride (TG), plasma insulin and glucose levels are shown for each of the groups. Values represent mean ±SEM. * p<0.05. C) Western blot analyses of representative liver lysates from each group showing the effects on Akt/mTORC1 signaling. Blots from long exposure (LE) and short exposure (SE) are shown for p-Akt(Ser473). Actin, loading control.
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pone-0018075-g008: Tsc1−/− mice are resistant to diet-induced steatosis in a rapamycin-dependent manner.Six week-old Tsc1+/+ and Tsc1−/− littermates were randomly assigned to normal chow diet (NCD), high fat diet (HFD) or HFD with rapamycin (2 mg/kg IP, 3 times weekly during the last 4 weeks). At the end of 6 weeks on the assigned diets, mice were sacrifice following an overnight fast. A) H&E histology and Oil Red “O” (ORO) staining of representative liver sections from each of the 6 groups. Magnification 400X. B) Biochemical measurements of liver and plasma triglyceride (TG), plasma insulin and glucose levels are shown for each of the groups. Values represent mean ±SEM. * p<0.05. C) Western blot analyses of representative liver lysates from each group showing the effects on Akt/mTORC1 signaling. Blots from long exposure (LE) and short exposure (SE) are shown for p-Akt(Ser473). Actin, loading control.

Mentions: The correlation between hepatic Akt activity and steatosis led us to predict that mTORC1-mediated suppression of Akt may protect the Tsc1−/− liver from TG accumulation. To test this hypothesis, 6-week old Tsc1−/− and Tsc1+/+ littermates were placed on a high-fat diet or normal chow diet for 6 weeks. Those assigned to the HFD were further randomized to receive rapamycin (2 mg/kg IP, MWF) or vehicle control (DMSO) during the last 4 weeks of the experiment. Over this period, weight gains on the HFD were marginally less in the mutant animals compared to wild-type littermates (p>0.10), and the liver:body weight ratio remained higher in the Tsc1−/− mice as it was under the normal diet condition (data not shown). In response to HFD, fasting plasma glucose and insulin levels rose, and rapamycin treatment further increased the values (Figure 8B). These trends suggest that glucose intolerance worsened with HFD + rapamycin in both Tsc1+/+ and Tsc1−/− mice without significant differences between the two genotypes. Plasma TG increased with HFD in the wild-type, but not mutant animals, whereas rapamycin significantly elevated plasma TG levels in the mutant but not wild-type mice (Figure 8B). On histologic analysis, the livers from the wild-type and mutant mice in the NCD group were indistinguishable, but under HFD condition, the Tsc1+/+ mice developed significant steatosis while the Tsc1−/− livers showed minimal change based on H&E and Oil Red “O” staining (Figure 8A). Direct measurements of liver TG revealed significant increase in lipid accumulation following HFD in the wild-type animals but not in the Tsc1−/− mice (Figure 8B). We confirmed these findings using an independent cohort of animals under the same scheme. After 6 weeks, the results of the hepatic TG levels were nearly identical with the exception that the Tsc1−/− mice fed HFD had significantly lower hepatic TG levels than those fed NCD (6.5 vs. 8.3 mg/g, p<0.01). Together, the histologic and biochemical evidence suggest that the loss of Tsc1 protects the liver from TG accumulation.


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

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

Tsc1−/− mice are resistant to diet-induced steatosis in a rapamycin-dependent manner.Six week-old Tsc1+/+ and Tsc1−/− littermates were randomly assigned to normal chow diet (NCD), high fat diet (HFD) or HFD with rapamycin (2 mg/kg IP, 3 times weekly during the last 4 weeks). At the end of 6 weeks on the assigned diets, mice were sacrifice following an overnight fast. A) H&E histology and Oil Red “O” (ORO) staining of representative liver sections from each of the 6 groups. Magnification 400X. B) Biochemical measurements of liver and plasma triglyceride (TG), plasma insulin and glucose levels are shown for each of the groups. Values represent mean ±SEM. * p<0.05. C) Western blot analyses of representative liver lysates from each group showing the effects on Akt/mTORC1 signaling. Blots from long exposure (LE) and short exposure (SE) are shown for p-Akt(Ser473). Actin, loading control.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3066210&req=5

pone-0018075-g008: Tsc1−/− mice are resistant to diet-induced steatosis in a rapamycin-dependent manner.Six week-old Tsc1+/+ and Tsc1−/− littermates were randomly assigned to normal chow diet (NCD), high fat diet (HFD) or HFD with rapamycin (2 mg/kg IP, 3 times weekly during the last 4 weeks). At the end of 6 weeks on the assigned diets, mice were sacrifice following an overnight fast. A) H&E histology and Oil Red “O” (ORO) staining of representative liver sections from each of the 6 groups. Magnification 400X. B) Biochemical measurements of liver and plasma triglyceride (TG), plasma insulin and glucose levels are shown for each of the groups. Values represent mean ±SEM. * p<0.05. C) Western blot analyses of representative liver lysates from each group showing the effects on Akt/mTORC1 signaling. Blots from long exposure (LE) and short exposure (SE) are shown for p-Akt(Ser473). Actin, loading control.
Mentions: The correlation between hepatic Akt activity and steatosis led us to predict that mTORC1-mediated suppression of Akt may protect the Tsc1−/− liver from TG accumulation. To test this hypothesis, 6-week old Tsc1−/− and Tsc1+/+ littermates were placed on a high-fat diet or normal chow diet for 6 weeks. Those assigned to the HFD were further randomized to receive rapamycin (2 mg/kg IP, MWF) or vehicle control (DMSO) during the last 4 weeks of the experiment. Over this period, weight gains on the HFD were marginally less in the mutant animals compared to wild-type littermates (p>0.10), and the liver:body weight ratio remained higher in the Tsc1−/− mice as it was under the normal diet condition (data not shown). In response to HFD, fasting plasma glucose and insulin levels rose, and rapamycin treatment further increased the values (Figure 8B). These trends suggest that glucose intolerance worsened with HFD + rapamycin in both Tsc1+/+ and Tsc1−/− mice without significant differences between the two genotypes. Plasma TG increased with HFD in the wild-type, but not mutant animals, whereas rapamycin significantly elevated plasma TG levels in the mutant but not wild-type mice (Figure 8B). On histologic analysis, the livers from the wild-type and mutant mice in the NCD group were indistinguishable, but under HFD condition, the Tsc1+/+ mice developed significant steatosis while the Tsc1−/− livers showed minimal change based on H&E and Oil Red “O” staining (Figure 8A). Direct measurements of liver TG revealed significant increase in lipid accumulation following HFD in the wild-type animals but not in the Tsc1−/− mice (Figure 8B). We confirmed these findings using an independent cohort of animals under the same scheme. After 6 weeks, the results of the hepatic TG levels were nearly identical with the exception that the Tsc1−/− mice fed HFD had significantly lower hepatic TG levels than those fed NCD (6.5 vs. 8.3 mg/g, p<0.01). Together, the histologic and biochemical evidence suggest that the loss of Tsc1 protects the liver from TG accumulation.

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