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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 mTORC1 on ER stress-dependent and -independent JNK activation.TSC2-deficient and wild-type mouse embryonic fibroblasts were treated with 300 nmol/l thapsigargin for 24 h (A) or with 200 nmol/l anisomycin for 30 min (B) with and without 50 nmol/l rapamycin, as described under Materials and Methods. JNK and c-Jun phosphorylation were analyzed by Western blot. Representative gels of 3 individual experiments showing CHOP, phoshpo-c-Jun, total and phospho-JNK are presented.
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pone-0004954-g007: Effects of mTORC1 on ER stress-dependent and -independent JNK activation.TSC2-deficient and wild-type mouse embryonic fibroblasts were treated with 300 nmol/l thapsigargin for 24 h (A) or with 200 nmol/l anisomycin for 30 min (B) with and without 50 nmol/l rapamycin, as described under Materials and Methods. JNK and c-Jun phosphorylation were analyzed by Western blot. Representative gels of 3 individual experiments showing CHOP, phoshpo-c-Jun, total and phospho-JNK are presented.

Mentions: Figure 7A), indicating activation of the ER stress response. Basal and thapsigargin-induced JNK and c-Jun phosphorylation was higher in TSC2−/− cells than in wild-type cells. Moreover, rapamycin decreased CHOP, JNK and c-Jun phosphorylation in the TSC2−/− MEFs (Figure 7A). Collectively, these findings indicate that dysregulated mTORC1 activity sensitizes cells to JNK activation by ER stress.


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 mTORC1 on ER stress-dependent and -independent JNK activation.TSC2-deficient and wild-type mouse embryonic fibroblasts were treated with 300 nmol/l thapsigargin for 24 h (A) or with 200 nmol/l anisomycin for 30 min (B) with and without 50 nmol/l rapamycin, as described under Materials and Methods. JNK and c-Jun phosphorylation were analyzed by Western blot. Representative gels of 3 individual experiments showing CHOP, phoshpo-c-Jun, total and phospho-JNK are presented.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0004954-g007: Effects of mTORC1 on ER stress-dependent and -independent JNK activation.TSC2-deficient and wild-type mouse embryonic fibroblasts were treated with 300 nmol/l thapsigargin for 24 h (A) or with 200 nmol/l anisomycin for 30 min (B) with and without 50 nmol/l rapamycin, as described under Materials and Methods. JNK and c-Jun phosphorylation were analyzed by Western blot. Representative gels of 3 individual experiments showing CHOP, phoshpo-c-Jun, total and phospho-JNK are presented.
Mentions: Figure 7A), indicating activation of the ER stress response. Basal and thapsigargin-induced JNK and c-Jun phosphorylation was higher in TSC2−/− cells than in wild-type cells. Moreover, rapamycin decreased CHOP, JNK and c-Jun phosphorylation in the TSC2−/− MEFs (Figure 7A). Collectively, these findings indicate that dysregulated mTORC1 activity sensitizes cells to JNK activation by ER stress.

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