Glucose Activates a Protein Phosphatase-1-Mediated Signaling Pathway to Enhance Overall Translation in Pancreatic β-Cells

Abstract

Both the rate of overall translation and the specific acceleration of proinsulin synthesis are known to be glucose-regulated processes in the beta-cell. In this study, we propose that glucose-induced stimulation of overall translation in beta-cells depends on a protein phosphatase-1-mediated decrease in serine-51 phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha), a pivotal translation initiation factor. The decrease was rapid and detectable within 15 min and proportional to the range of glucose concentrations that also stimulate translation. Lowered net eIF2alpha phosphorylation was not associated with a detectable decrease in activity of any eIF2alpha kinase. Moreover, okadaic acid blocked glucose-induced eIF2alpha dephosphorylation, suggesting that the net effect was mediated by a protein phosphatase. Experiments with salubrinal on intact cells and nuclear inhibitor of protein phosphatase-1 (PP1) on cell extracts suggested that this phosphatase was PP1. The net effect contained, however, a component of glucose-induced folding load in the endoplasmic reticulum because coincubation with cycloheximide further amplified the effect of glucose on eIF2alpha dephosphorylation. Thus, the steady-state level of eIF2alpha phosphorylation in beta-cells is the result of a balance between folding-load-induced phosphorylation and PP1-dependent dephosphorylation. Because defects in the pancreatic endoplasmic reticulum kinase-eIF2alpha signaling system lead to beta-cell failure and diabetes, deregulation of the PP1 system could likewise lead to cellular dysfunction and disease.

Insulin demand regulates β cell number via the unfolded protein response

“…In low glucose, some hallmarks of the integrated stress response (ISR) were induced, including CHOP and cell death, while ATF6 was not increased (Figure 4, A, B, L, and Supplemental Figure 4C). Our findings are consistent with prior observations that ISR is activated in rat islets or the murine min6 pancreatic β cell line at 2 mM and 5 mM glucose, whereas UPR is activated in high glucose (58,59 F1804). Blots were developed using ECL, ECL prime (GE Healthcare) or SuperSignal West Femto Maximum Sensitivity Substrate (Thermo Scientific).…”

Cluster analysis of rat pancreatic islet gene mRNA levels after culture in low-, intermediate- and high-glucose concentrations

“…Please also note that changes in mRNA levels may result from altered transcription or mRNA stability and that the higher level of complexity due to alternative mRNA splicing was not addressed. Finally, changes in mRNA levels may not necessarily lead to parallel changes in protein levels, as glucose influences general translation as well as translation of specific mRNAs by various mechanisms [39,40].…”

Translational repression during chronic hypoxia is dependent on glucose levels

“…Since it has been shown that glucose starvation can have a suppressive effect on translation under normoxic conditions in certain cell lines (Gomez et al 2004;Vander Mierde et al 2007) we wanted to determine if the translational repression that we observe under hypoxic conditions is merely a consequence of glucose depletion. Our data clearly show that short-term (20 h) glucose depletion under normoxic conditions has little impact on protein synthesis while dramatically inhibiting translation under hypoxic conditions.…”