ABSTRACT
Glucose sensing is essential for the ability of pancreatic beta-cells to produce insulin in sufficient quantities to maintain blood glucose within the normal range. Stress causes the release of adrenergic hormones that increase circulating glucose by promoting glucose production and inhibiting insulin release. We have shown that extracellular signal-regulated kinases 1 and 2 (ERK1/2) are responsive to glucose in pancreatic beta-cells and that glucose activates ERK1/2 by mechanisms independent of insulin. Here we show that glucose-induced activation of ERK1/2 is inhibited by epinephrine through the alpha2-adrenergic receptor. Epinephrine and the selective alpha2-adrenergic agonist UK14304 reduced insulin secretion and glucose-stimulated ERK1/2 activation in a pertussis toxin-sensitive manner, implicating the alpha subunit of a Gi family member. Alpha2-adrenergic agonists also reduced stimulation of ERK1/2 by glucagon-like peptide 1 and KCl, but not by phorbol ester or nerve growth factor. Our findings suggest that alpha2-adrenergic agonists act via a Gi family member on early steps in ERK1/2 activation, supporting the idea that ERK1/2 are regulated in a manner that reflects insulin demand.
Subject(s)
Epinephrine/pharmacology , Glucose/pharmacology , Islets of Langerhans/enzymology , Mitogen-Activated Protein Kinase 3/metabolism , Animals , Cell Line , Cyclic AMP/pharmacology , Enzyme Activation , Islets of Langerhans/drug effects , Kinetics , Pertussis Toxin/pharmacology , RatsABSTRACT
We discuss our work examining regulation and functions of mitogen-activated protein kinases, particularly ERK1 and ERK2, in pancreatic beta-cells. These enzymes are activated by glucose, other nutrients, and insulinogenic hormones. Their activation by these agents is calcium-dependent. A number of other stimuli also activate ERK1/2, but by mechanisms distinct from those involved in nutrient sensing. Inhibition of ERK1/2 has no apparent effect on insulin secretion measured after 2 h. On the other hand, ERK1/2 activity is required for maximal glucose-dependent activation of the insulin gene promoter. The primary effort has focused on INS-1 cell lines, with supporting and confirmatory studies in intact islets and other beta-cell lines, indicating the generality of our findings in beta-cell function. Thus ERK1/2 participate in transmitting glucose-sensing information to beta-cell functions. These kinases most likely act directly and indirectly on multiple pathways that regulate beta-cell function and, in particular, to transduce an elevated glucose signal into insulin gene transcription.
Subject(s)
Insulin-Secreting Cells/metabolism , Insulin/metabolism , MAP Kinase Signaling System , Animals , Calcium/metabolism , Cyclic AMP/metabolism , Enzyme Inhibitors/pharmacology , Humans , Mitogen-Activated Protein Kinase 1/metabolism , Mitogen-Activated Protein Kinase 3/metabolism , Models, Biological , Phorbol Esters/pharmacology , Rats , Time Factors , Transcription, GeneticABSTRACT
We showed previously that ERK1/2 were activated by glucose and amino acids in pancreatic beta cells. Here we examine and compare signaling events that are necessary for ERK1/2 activation by glucose and other stimuli in beta cells. We find that agents that interrupt Ca2+ signaling by a variety of mechanisms interfere with glucose- and glucagon-like peptide (GLP-1)-stimulated ERK1/2 activity. In particular, calmodulin antagonists, FK506, and cyclosporin, immunosuppressants that inhibit the calcium-dependent phosphatase calcineurin, suppress ERK1/2 activation by both glucose and GLP-1. Ca2+ signaling from intracellular stores is also essential for ERK1/2 activation, because thapsigargin blocks ERK1/2 activation by glucose or GLP-1. The glucose-sensitive mechanism is distinct from that used by phorbol ester or insulin to stimulate ERK1/2 but shares common features with that used by GLP-1.