Differential Activation Mechanisms of Erk-1/2 and p70S6K by Glucose in Pancreatic β-Cells
- From the Pacific Northwest Research Institute and Department of Pharmacology, University of Washington, Seattle, Washington
Glucose can activate the mitogen-activated kinases, Erk-1/2, and the ribosomal-S6 kinase, p70S6K, in β-cells, contributing to an increase in mitogenesis. However, the signaling mechanism by which glucose induces Erk-1/2 and p70S6K 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 p70S6K 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 p70S6K 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.”
Address correspondence and reprint requests to Christopher J. Rhodes, PhD, Pacific Northwest Research Institute, 720 Broadway, Seattle, WA 98122. E-mail:.
Received for publication 25 July 2002 and accepted in revised form 23 December 2002.
C.E.W. is currently affiliated with Medizinische Klinik I, Universitat Regensburg, F.S. Staub-Allee II, 93042 Regensburg, Germany.
EIA, enzyme immunoassay; GH, growth hormone; GLP-1, glucagon-like peptide-1; Grb2, growth factor receptor-bound protein-2; GSK3, glycogen synthase kinase-3; IRS, insulin receptor substrate; MAPK, mitogen-activated protein kinase; PDK1, phosphoinositide-dependent kinase-1; PI3K, phosphatidylinositol 3-kinase; PKA, protein kinase A; PKB, protein kinase B; MEK, MAP/Erk kinase; mSOS, mammalian Son of Sevenless; mTOR, mammalian target of rapamycin.