Glucose Decreases Na+,K+-ATPase Activity in Pancreatic β-Cells

Glucose stimulates both insulin secretion and hydrolysis of arachidonic acid (AA) esterified in membrane phospholipids of pancreatic islet ␤-cells, and these processes are amplified by muscarinic agonists. Here we demonstrate that nonesterified AA regulates the biophysical activity of the pancreatic islet ␤-cell-delayed rectifier channel, Kv2.1. Recordings of Kv2.1 currents from INS-1 insulinoma cells incubated with AA (5 M) and subjected to graded degrees of depolarization exhibit a significantly shorter time-to-peak current interval than do control cells. AA causes a rapid decay and reduced peak conductance of delayed rectifier currents from INS-1 cells and from primary ␤-cells isolated from mouse, rat, and human pancreatic islets. Stimulating mouse islets with AA results in a significant increase in the frequency of glucoseinduced [Ca 2؉ ] oscillations, which is an expected effect of Kv2.1 channel blockade. Stimulation with concentrations of glucose and carbachol that accelerate hydrolysis of endogenous AA from islet phosphoplipids also results in accelerated Kv2.1 inactivation and a shorter time-to-peak current interval. Group VIA phospholipase A 2 (iPLA 2 ␤) hydrolyzes ␤-cell membrane phospholipids to release nonesterified fatty acids, including AA, and inhibiting iPLA 2 ␤ prevents the muscarinic agonist-induced accelerated Kv2.1 inactivation. Furthermore, glucose and carbachol do not significantly affect Kv2.1 inactivation in ␤-cells from iPLA 2 ␤ ؊/؊ mice. Stably transfected INS-1 cells that overexpress iPLA 2 ␤ hydrolyze phospholipids more rapidly than control INS-1 cells and also exhibit an increase in the inactivation rate of the delayed rectifier currents. These results suggest that Kv2.1 currents could be dynamically modulated in the pancreatic islet ␤-cell by phospholipase-catalyzed hydrolysis of membrane phospholipids to yield non-esterified fatty acids, such as AA, that facilitate Ca 2؉ entry and insulin secretion.Glucose metabolism within the pancreatic islet ␤-cell generates a multitude of signals that regulate insulin secretion. Changes in the relative concentrations of the metabolites ATP and ADP cause ␤-cell depolarization via closure of ATP-sensitive potassium channels (K ATP ), 3 which results in activation of voltage-operated Ca 2ϩ channels, Ca 2ϩ influx, and insulin secretion (1). The extent of ␤-cell depolarization and insulin release are regulated in part by the activation of repolarizing ion channels, including the voltage-gated potassium channel, Kv2.1 (2-4). One mechanism employed by pancreatic ␤-cells to regulate the biophysical activity of the ion channels involved in insulin release involves hydrolysis of membrane phospholipids to yield mediators that include inositol triphosphates and free fatty acids (5-8).Pharmacologic, biochemical, and genetic evidence suggests that glucose-stimulated hydrolysis of esterified arachidonic acid from ␤-cell membrane phospholipids is required for physiological insulin secretion (7-25). Pancreatic islet ␤-cells contain high levels of arachidonic ac...

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Modulation of the Pancreatic Islet β-Cell-delayed Rectifier Potassium Channel Kv2.1 by the Polyunsaturated Fatty Acid Arachidonate

Jacobson

Weber

Bao

et al. 2007

“…Proposed functions include signaling in secretion (47,48,(52)(53)(54)(55)(56)(57), and BEL attenuates glucose-induced insulin secretion, arachidonate release, and rises in cytosolic [Ca 2ϩ ] in pancreatic islet ␤-cells and insulinoma cells (48,(52)(53)(54)(55)(56)(57).…”

mentioning

confidence: 99%

Effects of Stable Suppression of Group VIA Phospholipase A2 Expression on Phospholipid Content and Composition, Insulin Secretion, and Proliferation of INS-1 Insulinoma Cells

Bao

Bohrer

Ramanadham

et al. 2006

“…Although it has been shown in studies on tumor ␤-cell lines that a surprisingly small percentage of released radioactivity consists of metabolites (48), a recent study suggested a role for lipooxygenase-12 metabolites in ␤-cell function (57). Further studies need to be conducted to discriminate between AA and its metabolites produced by GIP stimulation of the ␤-cell.…”

Section: ؉mentioning

confidence: 99%

A New Pathway for Glucose-dependent Insulinotropic Polypeptide (GIP) Receptor Signaling

Ehses¹,

Lee

Pederson

et al. 2001

The hormone glucose-dependent insulinotropic polypeptide (GIP) is an important regulator of insulin secretion. GIP has been shown to increase adenylyl cyclase activity, elevate intracellular Ca 2؉ levels, and stimulate a mitogen-activated protein kinase pathway in the pancreatic ␤-cell. In the current study we demonstrate a role for arachidonic acid in GIP-mediated signal transduction. Static incubations revealed that both GIP (100 nM) and ATP (5 M) significantly increased [ 3 H]AA release. Our data suggest that cAMP is the proximal signaling intermediate responsible for GIP-stimulated AA release. Finally, stimulation of GIP-mediated AA production was shown to be mediated via a Ca 2؉ -independent phospholipase A 2 . Arachidonic acid is therefore a new component of GIP-mediated signal transduction in the ␤-cell.Glucose-dependent insulinotropic polypeptide (GIP, or gastric inhibitory polypeptide) 1 is a 42-amino acid polypeptide hormone synthesized by mucosal K cells of the duodenum and jejunum and released into the circulation in response to nutrient ingestion (1-4). GIP and glucagon-like peptide-1 (GLP-1) are thought to be the major hormones (incretins) that constitute the endocrine component of the enteroinsular axis in humans and are responsible for at least 50% of postprandial insulin secretion (5). In non-insulin-dependent diabetes mellitus (type 2 diabetes mellitus), the incretin effect following oral glucose administration is reduced or absent (6, 7), and the ability of intravenous GIP, but not GLP-1, to stimulate insulin secretion is severely blunted (7,8). This implies that a defective GIP signal transduction system and/or a reduced number of functional GIP receptors may contribute to the pathophysiology of type 2 diabetes. A greater understanding of the signal transduction systems activated by GIP should assist in determining whether reduced responsiveness involves changes at this level.The receptor for GIP (9 -11) is a member of the class II G protein-coupled receptor superfamily, which includes receptors for glucagon, GLP-1, secretin, and vasoactive intestinal polypeptide (12). Stimulation of the GIP receptor has been shown to stimulate adenylyl cyclase and elevate intracellular cAMP levels in pancreatic islets (13), islet tumor cell lines (14), and various cell lines transfected with the GIP receptor (10,15,16). In addition, GIP has been shown to increase uptake of Ca 2ϩ into isolated islets (17) and increase intracellular Ca 2ϩ levels in HIT-T15 (18), RINm5F (9), and COS cells (10). We have shown that the GIP receptor probably couples to various Ca 2ϩ channels (10), but there is no evidence for GIP-stimulated IP 3 production (18). There is, however, evidence that GIP stimulates insulin secretion (19) and activation of mitogen-activated protein kinase (20) via a wortmannin-sensitive pathway, implying a role for phosphatidylinositol 3-kinase. It is therefore clear that GIP action on the pancreatic ␤-cell involves several interacting signal transduction pathways.Phospholipase A 2 (PLA 2 ) catalyzes the...