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

An 85-kDa Group VI phospholipase A, enzyme (iPLA(2)) that does not require Ca2+ 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, (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 Ca2+ 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, Expression of mRNA species encoding two active iPLA(2) isoforms with distinguishable catalytic properties in two different types of human cells demonstrated here may have regulatory or functional implications about the roles of products of the iPLA(2) gene in cell biologic processes.