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


RESULTS
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
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.
Bachar
Etti
E
Endocrinology and Metabolism Service, Department of Medicine, Hadassah--Hebrew University Medical Center, Jerusalem, Israel.
Ariav
Yafa
Y
Ketzinel-Gilad
Mali
M
Cerasi
Erol
E
Kaiser
Nurit
N
Leibowitz
Gil
G
D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects; D002460:Cell Line; D003513:Cycloheximide / Q000494:pharmacology; D003924:Diabetes Mellitus, Type 2 / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000187:drug effects* / Q000378:metabolism; D004789:Enzyme Activation; D005849:Gerbillinae; D005947:Glucose / Q000378:metabolism*; D050417:Insulin-Secreting Cells / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism; D008297:Male; D000076222:Mechanistic Target of Rapamycin Complex 1; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D046912:Multiprotein Complexes; D018384:Oxidative Stress / Q000187:drug effects* / Q000502:physiology; D010168:Palmitates / Q000378:metabolism / Q000494:pharmacology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011506:Proteins; D015398:Signal Transduction / Q000502:physiology; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
D000903:Antibiotics, Antineoplastic; C519510:Crtc1 protein, rat; D008565:Membrane Proteins; D046912:Multiprotein Complexes; D010168:Palmitates; D011500:Protein Synthesis Inhibitors; D011506:Proteins; D014157:Transcription Factors; D003513:Cycloheximide; C486556:Ern2 protein, mouse; D000076222:Mechanistic Target of Rapamycin Complex 1; D017346:Protein Serine-Threonine Kinases; D058570:TOR Serine-Threonine Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D005947:Glucose; D020123:Sirolimus