Glucose activates a protein phosphatase-1-mediated signaling pathway to enhance overall translation in pancreatic beta-cells

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 eIF2alpha (eukaryotic translation initiation factor 2 alpha), 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, OA (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 NIPP1 (Nuclear inhibitor of protein phosphatase-1) on cell extracts suggested that this phosphatase was PP1 (protein phosphatase-1). The net effect contained, however, a component of glucose-induced folding load in the ER (endoplasmic reticulum) as coincubation with Chx (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. As defects in the PERK (PKR-like endoplasmic reticulum kinase) -eIF2alpha signaling system lead to beta cell failure and diabetes, deregulation of PP1 system could likewise lead to cellular dysfunction and disease.status: publishe