Studies of the Role of Group VI Phospholipase A2 in Fatty Acid Incorporation, Phospholipid Remodeling, Lysophosphatidylcholine Generation, and Secretagogue-induced Arachidonic Acid Release in Pancreatic Islets and Insulinoma Cells

Abstract

H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA 2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA 2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA 2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.

A Requirement for Calcium-independent Phospholipase A2 in Thrombin-induced Arachidonic Acid Release and Growth in Vascular Smooth Muscle Cells

“…Besides cPLA 2 , the other PLA 2 that is present in many cell types and involved in agonist-induced arachidonic acid release is the most recently characterized one, iPLA 2 (21-23). One of the major functions attributed to iPLA 2 is its role in lipid remodeling (39,40). In fact, using its selective inhibitors such as BEL, it was shown that arachidonic acid release in response to some agonists is mediated primarily by iPLA 2 (23).…”

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

“…These results suggest that muscarinic receptor occupancy increases ␤-cell Kv current inactivation by iPLA 2 ␤-regulated mechanism(s), and this results in a shorter time to peak Kv current response. Phospholipase A 2 Regulates Delayed Rectifier Current Activity-iPLA 2 ␤ is expressed by islet ␤-cells from rats (29), humans (47), and mice (48), and by several insulinoma cell lines (31,38,39). Upon activation, iPLA 2 ␤ hydrolyzes the sn-2 substituent from phospholipids substrates, such as phosphatidylcholine, to yield a free fatty acid and a lysophospholipid, such as LPC (44,54,55).…”

“…The phospholipid content of esterified AA is higher in pancreatic islets than in other tissues (23,25,26,29), and arachidonate is hydrolyzed from phospholipids upon stimulation of islets with secretagogues (13,14,27,31,36,44) and accumulates to micromolar concentrations (7,27,28) that are sufficient to modulate delayed rectifier Kv channel FIGURE 5. Effects of the muscarinic agonist carbachol on INS-1 cell-delayed rectifier currents.…”

“…The iPLA 2 ␤ enzyme is expressed by islets from rats, mice, and humans (20, 24 -26) and by several insulinoma cell lines (31,52,53), and iPLA 2 ␤ catalyzes the hydrolysis of the sn-2 ester bond of phospholipids to yield a free fatty acid, such as arachidonic acid, and a 2-lysophospholipid, such as LPC (54,55). Secretagogue-stimulated hydrolysis of AA from ␤-cell phospholipids is suppressed by the iPLA 2 ␤ inhibitor BEL (18,52,53), and overexpression of iPLA 2 ␤ in insulinoma cells causes a rise in PLA 2 product levels and is associated with amplification of insulin secretory responses (21,56).…”

“…The products of iPLA 2 ␤ action on its phospholipid substrates include a nonesterified fatty acid, such as AA, and a 2-lysphophosplipid, such as lysophosphatidylcholine (LPC), and ␤-cell levels of both products correlate with iPLA 2 ␤ expression level (21,22,31,32). Modulation of iPLA 2 ␤ activity and the rate of secretagogue-induced hydrolysis of arachidonic acid from ␤-cell membrane phospholipids might thus represent a tunable signal to amplify or attenuate islet insulin secretion in various (patho)physiologic settings (19,24).…”