Insulin Secretory Responses and Phospholipid Composition of Pancreatic Islets from Mice That Do Not Express Group VIA Phospholipase A2 and Effects of Metabolic Stress on Glucose Homeostasis

Abstract

Studies involving pharmacologic or molecular biologic manipulation of Group VIA phospholipase A 2 (iPLA 2 ␤) activity in pancreatic islets and insulinoma cells suggest that iPLA 2 ␤ participates in insulin secretion. It has also been suggested that iPLA 2 ␤ is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels and arachidonate incorporation into phosphatidylcholine (PC). We have generated iPLA 2 ␤-null mice by homologous recombination and have reported that they exhibit reduced male fertility and defective motility of spermatozoa. Here we report that pancreatic islets from iPLA 2 ␤-null mice have impaired insulin secretory responses to D-glucose and forskolin. Electrospray ionization mass spectrometric analyses indicate that the abundance of arachidonate-containing PC species of islets, brain, and other tissues from iPLA 2 ␤-null mice is virtually identical to that of wild-type mice, and no iPLA 2 ␤ mRNA was observed in any tissue from iPLA 2 ␤-null mice at any age. Despite the insulin secretory abnormalities of isolated islets, fasting and fed blood glucose concentrations of iPLA 2 ␤-null and wild-type mice are essentially identical under normal circumstances, but iPLA 2 ␤-null mice develop more severe hyperglycemia than wild-type mice after administration of multiple low doses of the ␤-cell toxin streptozotocin, suggesting an impaired islet secretory reserve. A high fat diet also induces more severe glucose intolerance in iPLA 2 ␤-null mice than in wild-type mice, but PLA 2 ␤-null mice have greater responsiveness to exogenous insulin than do wild-type mice fed a high fat diet. These and previous findings thus indicate that iPLA 2 ␤-null mice exhibit phenotypic abnormalities in pancreatic islets in addition to testes and macrophages.

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

“…2C). These effects of arachidonic acid are not use-dependent because Kv currents elicited after a 5-min incubation of cells with 5 M arachidonic acid when held at Ϫ80 mV show effects similar those observed (20) with cells subjected to depolarizing steps during the 5 min period of incubation with arachidonate ( Fig. 2B).…”

“…Because phospholipase C is also activated by Cch, these experiments utilized intracellular calcium buffering to prevent phosoholipase C activation. Carbachol and glucose were applied to control and iPLA 2 ␤ Ϫ/Ϫ mouse pancreatic islet ␤-cells, and Kv currents were then recorded (20). The control ␤-cell Kv currents exhibit a significant left shift in their time to peak response to 20 mM glucose, and this response is maintained upon addition of carbachol (500 M; Fig.…”

“…The progressive rise in insulin resistance during the evolution of type II diabetes requires development of a compensatory hypersecretion of insulin to maintain glucose homeostasis, and a rise in AA levels might stimulate such a response. The deterioration of glucose tolerance induced in mice fed a high fat diet is exacerbated in iPLA 2 ␤-null mice, and pancreatic islets isolated from these mice also fail to develop the exaggerated insulin secretory responses observed with islets isolated from wild-type mice (20). This suggests that iPLA 2 ␤ participates in ␤-cell compensatory responses to increasing insulin resistance, and our results suggest that modulation of Kv2.1 channel activity by fatty acids could be one of the steps involved in such ␤-cell compensation.…”

“…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)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). Pancreatic islet ␤-cells contain high levels of arachidonic acid compared with other tissues (7, 9 -12 24, 25), and about two-thirds of islet ␤-cell glycerophospholipids contain arachidonic acid as the sn-2 substituent (23)(24)(25).…”

Diminished Acyl-CoA Synthetase Isoform 4 Activity in INS 832/13 Cells Reduces Cellular Epoxyeicosatrienoic Acid Levels and Results in Impaired Glucose-stimulated Insulin Secretion

“…Based on studies that show that glucose stimulates the release and metabolism of AA, it has been hypothesized that AA and/or its metabolites play a significant role in insulin secretion (10 -12). The initial hydrolysis of AA from glycerophospholipids seems to be critical for insulin secretion, because knockdown or knock-out of phospholipase A 2 in INS-1 cells (13) or mouse islets (14) diminishes GSIS. Free AA is metabolized by one of several different enzyme systems that yield potent bioactive compounds with critical signaling roles.…”