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Glucose amplifies fatty acid-induced endoplasmic reticulum stress in pancreatic beta-cells via activation of mTORC1.

Bachar E, Ariav Y, Ketzinel-Gilad M, Cerasi E, Kaiser N, Leibowitz G - PLoS ONE (2009)

Bottom Line: We found that glucose amplifies palmitate-induced ER stress by increasing IRE1alpha protein levels and activating the JNK pathway, leading to increased beta-cell apoptosis.Finally, we found that JNK inhibition decreased beta-cell apoptosis under conditions of glucolipotoxicity.Moreover, in stressed beta-cells mTORC1 inhibition decreases IRE1alpha protein expression and JNK activity without affecting ER protein load, suggesting that mTORC1 regulates the beta-cell stress response to glucose and fatty acids by modulating the synthesis and activity of specific proteins involved in the execution of the ER stress response.

View Article: PubMed Central - PubMed

Affiliation: Endocrinology and Metabolism Service, Department of Medicine, Hadassah--Hebrew University Medical Center, Jerusalem, Israel.

ABSTRACT

Background: Palmitate is a potent inducer of endoplasmic reticulum (ER) stress in beta-cells. In type 2 diabetes, glucose amplifies fatty-acid toxicity for pancreatic beta-cells, leading to beta-cell dysfunction and death. Why glucose exacerbates beta-cell lipotoxicity is largely unknown. Glucose stimulates mTORC1, an important nutrient sensor involved in the regulation of cellular stress. Our study tested the hypothesis that glucose augments lipotoxicity by stimulating mTORC1 leading to increased beta-cell ER stress.

Principal findings: We found that glucose amplifies palmitate-induced ER stress by increasing IRE1alpha protein levels and activating the JNK pathway, leading to increased beta-cell apoptosis. Moreover, glucose increased mTORC1 activity and its inhibition by rapamycin decreased beta-cell apoptosis under conditions of glucolipotoxicity. Inhibition of mTORC1 by rapamycin did not affect proinsulin and total protein synthesis in beta-cells incubated at high glucose with palmitate. However, it decreased IRE1alpha expression and signaling and inhibited JNK pathway activation. In TSC2-deficient mouse embryonic fibroblasts, in which mTORC1 is constitutively active, mTORC1 regulated the stimulation of JNK by ER stressors, but not in response to anisomycin, which activates JNK independent of ER stress. Finally, we found that JNK inhibition decreased beta-cell apoptosis under conditions of glucolipotoxicity.

Conclusions/significance: Collectively, our findings suggest that mTORC1 mediates glucose amplification of lipotoxicity, acting through activation of ER stress and JNK. Thus, mTORC1 is an important transducer of ER stress in beta-cell glucolipotoxicity. Moreover, in stressed beta-cells mTORC1 inhibition decreases IRE1alpha protein expression and JNK activity without affecting ER protein load, suggesting that mTORC1 regulates the beta-cell stress response to glucose and fatty acids by modulating the synthesis and activity of specific proteins involved in the execution of the ER stress response. This novel paradigm may have important implications for understanding beta-cell failure in type 2 diabetes.

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Effects of glucose, palmitate and rapamycin on mTORC1 signaling in β-cells.INS-1E cells were incubated overnight in RPMI medium containing 3.3 mmol/l glucose and 0.5% BSA without serum and then at 3.3 and 22.2 mmol/l glucose with and without 0.5 mmol palmitate and 50 nmol/l rapamycin for 4 h. S6(Ser235/236) and 4EBP1 phosphorylation were analyzed by Western blot. (A) A representative gel showing total and phosphorylated S6 and 4EBP1 is presented. (B) Quantification of S6 and 4EBP1 phosphorylation. Results are expressed as means±SE of 6 individual experiments. * p<0.05, ∧ p<0.001 for the difference between the indicated groups.
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pone-0004954-g002: Effects of glucose, palmitate and rapamycin on mTORC1 signaling in β-cells.INS-1E cells were incubated overnight in RPMI medium containing 3.3 mmol/l glucose and 0.5% BSA without serum and then at 3.3 and 22.2 mmol/l glucose with and without 0.5 mmol palmitate and 50 nmol/l rapamycin for 4 h. S6(Ser235/236) and 4EBP1 phosphorylation were analyzed by Western blot. (A) A representative gel showing total and phosphorylated S6 and 4EBP1 is presented. (B) Quantification of S6 and 4EBP1 phosphorylation. Results are expressed as means±SE of 6 individual experiments. * p<0.05, ∧ p<0.001 for the difference between the indicated groups.

Mentions: Since mTORC1 may function as an important modulator of glucolipotoxicity-induced β-cell apoptosis, we studied the effects of high glucose and palmitate on its signaling in β-cells. Glucose increased S6 and 4EBP1 phosphorylation (Figure 2), showing as expected, that glucose activates mTORC1 signaling. Palmitate partially reduced mTORC1 activation by high glucose, as seen by reduced S6 and 4EBP1 phosphorylation levels. However, mTORC1 activity was higher in β-cells treated with palmitate at 22.2 mmol/l than at 3.3 mmol/l glucose. Rapamycin completely abolished S6 phosphorylation and partially decreased 4EBP1 phosphorylation under all conditions, indicating that rapamycin efficiently inhibited mTORC1 in INS-1E cells exposed to high glucose and palmitate.


Glucose amplifies fatty acid-induced endoplasmic reticulum stress in pancreatic beta-cells via activation of mTORC1.

Bachar E, Ariav Y, Ketzinel-Gilad M, Cerasi E, Kaiser N, Leibowitz G - PLoS ONE (2009)

Effects of glucose, palmitate and rapamycin on mTORC1 signaling in β-cells.INS-1E cells were incubated overnight in RPMI medium containing 3.3 mmol/l glucose and 0.5% BSA without serum and then at 3.3 and 22.2 mmol/l glucose with and without 0.5 mmol palmitate and 50 nmol/l rapamycin for 4 h. S6(Ser235/236) and 4EBP1 phosphorylation were analyzed by Western blot. (A) A representative gel showing total and phosphorylated S6 and 4EBP1 is presented. (B) Quantification of S6 and 4EBP1 phosphorylation. Results are expressed as means±SE of 6 individual experiments. * p<0.05, ∧ p<0.001 for the difference between the indicated groups.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2654723&req=5

pone-0004954-g002: Effects of glucose, palmitate and rapamycin on mTORC1 signaling in β-cells.INS-1E cells were incubated overnight in RPMI medium containing 3.3 mmol/l glucose and 0.5% BSA without serum and then at 3.3 and 22.2 mmol/l glucose with and without 0.5 mmol palmitate and 50 nmol/l rapamycin for 4 h. S6(Ser235/236) and 4EBP1 phosphorylation were analyzed by Western blot. (A) A representative gel showing total and phosphorylated S6 and 4EBP1 is presented. (B) Quantification of S6 and 4EBP1 phosphorylation. Results are expressed as means±SE of 6 individual experiments. * p<0.05, ∧ p<0.001 for the difference between the indicated groups.
Mentions: Since mTORC1 may function as an important modulator of glucolipotoxicity-induced β-cell apoptosis, we studied the effects of high glucose and palmitate on its signaling in β-cells. Glucose increased S6 and 4EBP1 phosphorylation (Figure 2), showing as expected, that glucose activates mTORC1 signaling. Palmitate partially reduced mTORC1 activation by high glucose, as seen by reduced S6 and 4EBP1 phosphorylation levels. However, mTORC1 activity was higher in β-cells treated with palmitate at 22.2 mmol/l than at 3.3 mmol/l glucose. Rapamycin completely abolished S6 phosphorylation and partially decreased 4EBP1 phosphorylation under all conditions, indicating that rapamycin efficiently inhibited mTORC1 in INS-1E cells exposed to high glucose and palmitate.

Bottom Line: We found that glucose amplifies palmitate-induced ER stress by increasing IRE1alpha protein levels and activating the JNK pathway, leading to increased beta-cell apoptosis.Finally, we found that JNK inhibition decreased beta-cell apoptosis under conditions of glucolipotoxicity.Moreover, in stressed beta-cells mTORC1 inhibition decreases IRE1alpha protein expression and JNK activity without affecting ER protein load, suggesting that mTORC1 regulates the beta-cell stress response to glucose and fatty acids by modulating the synthesis and activity of specific proteins involved in the execution of the ER stress response.

View Article: PubMed Central - PubMed

Affiliation: Endocrinology and Metabolism Service, Department of Medicine, Hadassah--Hebrew University Medical Center, Jerusalem, Israel.

ABSTRACT

Background: Palmitate is a potent inducer of endoplasmic reticulum (ER) stress in beta-cells. In type 2 diabetes, glucose amplifies fatty-acid toxicity for pancreatic beta-cells, leading to beta-cell dysfunction and death. Why glucose exacerbates beta-cell lipotoxicity is largely unknown. Glucose stimulates mTORC1, an important nutrient sensor involved in the regulation of cellular stress. Our study tested the hypothesis that glucose augments lipotoxicity by stimulating mTORC1 leading to increased beta-cell ER stress.

Principal findings: We found that glucose amplifies palmitate-induced ER stress by increasing IRE1alpha protein levels and activating the JNK pathway, leading to increased beta-cell apoptosis. Moreover, glucose increased mTORC1 activity and its inhibition by rapamycin decreased beta-cell apoptosis under conditions of glucolipotoxicity. Inhibition of mTORC1 by rapamycin did not affect proinsulin and total protein synthesis in beta-cells incubated at high glucose with palmitate. However, it decreased IRE1alpha expression and signaling and inhibited JNK pathway activation. In TSC2-deficient mouse embryonic fibroblasts, in which mTORC1 is constitutively active, mTORC1 regulated the stimulation of JNK by ER stressors, but not in response to anisomycin, which activates JNK independent of ER stress. Finally, we found that JNK inhibition decreased beta-cell apoptosis under conditions of glucolipotoxicity.

Conclusions/significance: Collectively, our findings suggest that mTORC1 mediates glucose amplification of lipotoxicity, acting through activation of ER stress and JNK. Thus, mTORC1 is an important transducer of ER stress in beta-cell glucolipotoxicity. Moreover, in stressed beta-cells mTORC1 inhibition decreases IRE1alpha protein expression and JNK activity without affecting ER protein load, suggesting that mTORC1 regulates the beta-cell stress response to glucose and fatty acids by modulating the synthesis and activity of specific proteins involved in the execution of the ER stress response. This novel paradigm may have important implications for understanding beta-cell failure in type 2 diabetes.

Show MeSH
Related in: MedlinePlus