Human Pancreatic Islets Express mRNA Species Encoding Two Distinct Catalytically Active Isoforms of Group VI Phospholipase A2 (iPLA2) That Arise from an Exon-skipping Mechanism of Alternative Splicing of the Transcript from the iPLA2 Gene on Chromosome 22q13.1

Abstract

An 85-kDa Group VI phospholipase A 2 enzyme (iPLA 2 ) that does not require Ca 2؉ for catalysis has recently been cloned from three rodent species. A homologous 88-kDa enzyme has been cloned from human B-lymphocyte lines that contains a 54-amino acid insert not present in the rodent enzymes, but human cells have not previously been observed to express catalytically active iPLA 2 isoforms other than the 88-kDa protein. We have cloned cDNA species that encode two distinct iPLA 2 isoforms from human pancreatic islet RNA and a human insulinoma cDNA library. One isoform is an 85-kDa protein (short isoform of human iPLA 2 (SH-iPLA 2 )) and the other an 88-kDa protein (long isoform of human iPLA 2 (LH-iPLA 2 )). Transcripts encoding both isoforms are also observed in human promonocytic U937 cells. Recombinant SH-iPLA 2 and LH-iPLA 2 are both catalytically active in the absence of Ca 2؉ and inhibited by a bromoenol lactone suicide substrate, but LH-iPLA 2 is activated by ATP, whereas SH-iPLA 2 is not. The human iPLA 2 gene has been found to reside on chromosome 22 in region q13.1 and to contain 16 exons represented in the LH-iPLA 2 transcript. Exon 8 is not represented in the SH-iPLA 2 transcript, indicating that it arises by an exon-skipping mechanism of alternative splicing. The amino acid sequence encoded by exon 8 of the human iPLA 2 gene is proline-rich and shares a consensus motif of PX 5 PX 8 HHPX 12 NX 4 Q with the proline-rich middle linker domains of the Smad proteins DAF-3 and Smad4.

The Ca2+-sensing Receptor Activates Cytosolic Phospholipase A2 via a Gqα-dependent ERK-independent Pathway

“…iPLA 2 also appears to be ubiquitously expressed and is activated by receptors for ATP, ␣ 2 agonists, vasopressin, and Fas (15)(16)(17). The proposed functions of iPLA 2 include cellular lipid remodeling, generation of substrates for leukotriene biosynthesis, and generation of second messengers that regulate ion channel activity (11,18). iPLA 2 activity is inhibited by both AACOCF 3 and BEL (12).…”

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

“…Islet phospholipase A 2 (PLA 2 ) enzyme(s) activated by stimulation with glucose and/or by G-protein signaling mechanisms catalyze hydrolysis of arachidonic acid from ␤-cell membrane phospholipids and cause levels of free arachidonic acid to rise by up to 3-fold to the mid-micromolar range (7,13,14,27,28). Such concentrations of arachidonic acid amplify both the rise in ␤-cell [Ca 2ϩ ] and insulin secretion induced by glucose (7, 15-17, 23, 25, 27, 28).Pharmacologic inhibition or molecular biologic suppression of expression of the pancreatic islet ␤-cell Group VIA phospholipase A 2 (iPLA 2 ␤) enzyme (24,29,30) results in attenuation of insulin secretory responses induced by glucose and other secretagogues (18 -22, 24). The products of iPLA 2 ␤ action on its *…”

“…Pharmacologic inhibition or molecular biologic suppression of expression of the pancreatic islet ␤-cell Group VIA phospholipase A 2 (iPLA 2 ␤) enzyme (24,29,30) results in attenuation of insulin secretory responses induced by glucose and other secretagogues (18 -22, 24). The products of iPLA 2 ␤ action on its *…”

“…Pharmacologic inhibition or molecular biologic suppression of expression of the pancreatic islet ␤-cell Group VIA phospholipase A 2 (iPLA 2 ␤) enzyme (24,29,30) results in attenuation of insulin secretory responses induced by glucose and other secretagogues (18 -22, 24). 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).…”

Inactivation of Endoplasmic Reticulum Bound Ca2+-Independent Phospholipase A2 in Renal Cells during Oxidative Stress

“…Within the last 4 yr, a number of novel Ca 2ϩ -independent PLA 2 (iPLA 2 ) isoforms have been identified. Ma et al (4,5) described a cytosolic 85-kD iPLA 2 (Group VIA PLA 2 using the newer classification) that exists as two splice variants from the same gene (Group VIA-1 and Group VIA-2 PLA 2 ). A human microsomal-bound 85-kD iPLA 2 (Group VIB PLA 2 ) has also been expressed in insect and bacterial cells (6) and is hypothesized to protect against oxidant-induced cell death in renal cells (7).…”