Acute nutrient regulation of the unfolded protein response and integrated stress response in cultured rat pancreatic islets.
OBJECTIVE
Inadequate chaperone function relative to client protein load in the endoplasmic reticulum triggers an adaptive unfolded protein response (UPR), including the integrated stress response (ISR), the latter being also activated by other types of stresses. It is well established that pancreatic beta cells, which synthesise and secrete insulin upon nutrient stimulation, are markedly affected by pathological disruption or excessive activation of the UPR. However, whether and how physiological nutrient stimulation affects the beta cell UPR has been little investigated.


METHODS
We compared the effects of increasing glucose concentrations and of endoplasmic reticulum Ca(2+) emptying with thapsigargin on the UPR (X-box binding protein [Xbp1] mRNA splicing and XBP1/activating transcription factor [ATF] 6-target gene expression) and ISR (eukaryotic translation initiation factor 2A phosphorylation, ATF4 protein levels and target gene expression) in isolated rat islets.


RESULTS
Thapsigargin strongly increased both UPR and ISR. In comparison, glucose moderately increased the UPR between 5 and 30 mmol/l, but exerted complex effects on the ISR as follows: (1) marked reduction between 2 and 10 mmol/l; (2) moderate increase parallel to the UPR between 10 and 30 mmol/l. These glucose effects occurred within 2 h, were mimicked by other metabolic substrates, but were independent of changes in Ca(2+) influx or insulin secretion. Remarkably, attenuating the glucose stimulation of protein synthesis with a low concentration of cycloheximide prevented UPR activation but not ISR reduction by high glucose.


CONCLUSIONS
Nutrient stimulation acutely activates rat islet UPR in a manner dependent on protein synthesis, while exerting complex effects on the ISR. These effects may contribute to nutrient-induced maintenance of the beta cell phenotype.
Elouil
H
H
Unit of Endocrinology and Metabolism, Faculty of Medicine, Université Catholique de Louvain, Avenue Hippocrate, 55 (UCL 55.30), 1200, Brussels, Belgium.
Bensellam
M
M
Guiot
Y
Y
Vander Mierde
D
D
Pascal
S M A
SM
Schuit
F C
FC
Jonas
J C
JC
D017398:Alternative Splicing; D000818:Animals; D002118:Calcium / Q000378:metabolism; D003470:Culture Media / Q000378:metabolism; D003513:Cycloheximide / Q000494:pharmacology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D007700:Kinetics; D018832:Molecular Chaperones / Q000378:metabolism; D010766:Phosphorylation; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D051381:Rats; D019284:Thapsigargin / Q000494:pharmacology
D003470:Culture Media; D007328:Insulin; D018832:Molecular Chaperones; D011500:Protein Synthesis Inhibitors; D019284:Thapsigargin; D003513:Cycloheximide; D005947:Glucose; D002118:Calcium