Differential Activation Mechanisms of Erk-1/2 and p70^S6K by Glucose in Pancreatic β-Cells

Abstract

Glucose can activate the mitogen-activated kinases, Erk-1/2, and the ribosomal-S6 kinase, p70^S6K, in β-cells, contributing to an increase in mitogenesis. However, the signaling mechanism by which glucose induces Erk-1/2 and p70^S6K phosphorylation activation is undefined. Increased glucose metabolism increases [Ca²⁺]_i and [cAMP], and it was investigated if these secondary signals were linked to glucose-induced Erk-1/2 and p70^S6K activation in pancreatic β-cells. Blocking Ca²⁺ influx with verapamil, or inhibiting protein kinase A (PKA) with H89, prevented glucose-induced Erk-1/2 phosphorylation. Increasing cAMP levels by GLP-1 potentiated glucose-induced Erk-1/2 phosphorylation via PKA activation. Elevation of [Ca²⁺]_i by glyburide potentiated Erk-1/2 phosphorylation, which was also inhibited by H89, suggesting increased [Ca²⁺]_i preceded PKA for glucose-induced Erk-1/2 activation. Adenoviral-mediated expression of dominant negative Ras in INS-1 cells decreased IGF-1-induced Erk-1/2 phosphorylation but had no effect on that by glucose. Collectively, our study indicates that a glucose-induced rise in [Ca²⁺]_i leads to cAMP-induced activation of PKA that acts downstream of Ras and upstream of the MAP/Erk kinase, MEK, to mediate Erk-1/2 phosphorylation via phosphorylation activation of Raf-1. In contrast, glucose-induced p70^S6K activation, in the same β-cells, was mediated by a distinct signaling pathway independent of Ca²⁺/cAMP, most likely via mTOR-kinase acting as an “ATP-sensor.”