| Description |
Platelet activating factor (PAF) is a synthesized inflammatory phospholipid that signals through a seven-transmembrane G-protein receptor. Oxidation of phospholipids with sn-2 unsaturated fatty acyl residues can fragment the esterified chain forming numerous phospholipids; some are toxic and others are PAF-like lipids which can signal through the PAF receptor. Group VII phospholipases (PAF acetylhydrolases, PAF-AH) are Ca++-independent esterases that specifically hydrolyze short sn-2 residues, including the acetyl residue of PAF. We have observed that megakaryocytes derived from human umbilical cord blood CD34+ stem cells have increased PAF-AH activity over differentiation. Inhibition of PAF-AH activity in megakaryocytes with Pefabloc SC led to increased formation of PAF-like lipids that signaled through endogenous PAF receptors. Transient morphology changes were observed in megakaryocytes treated with Pefabloc or carbamyl-PAF (cPAF), a stable form of PAR These data suggest that PAF-AHs are important for controlling aberrant PAF or PAF-like lipid formation in megakaryocytes, and that dysregulation could lead to altered pro-platelet formation or function. The fission yeast Schizosaccharomyces pombe genome contains a putative gene, SPBC106.11c, homologous to group VII hydrolases that retains a lipase motif and residues that form the esterase catalytic triad. An active PAF-AH enzyme in an organism that cannot generate PAF may instead protect against membrane oxidation and death. We cloned this SPBC106.11c locus and expressed it in distantly related Saccharomyces cerevisiae, which lacks genes related by sequence, to find it encodes a functional phospholipase we named Plg7p. Ala substitution of the conserved Ser257 in the GXSXG lipase motif and two serine-directed inhibitors of plasma PAF acetylhydrolase, Pefabloc and methyl arachidonyl fluorophosphonate, suppressed Plg7p activity. S. pombe ?plg7 had no overt phenotype, but revealed a second Ca++-independent phospholipase activity not inhibited by the Ser-directed reagents. S. cerevisiae supplemented with polyunsaturated fatty acids and exposed to environmental Cu+ resulted in oxidized membrane lipids, and reduced viability. Expression of human type II PAF acetylhydrolase or Plg7p enhanced viability in this model of oxidative death. We conclude an organism distantly related to mammals expresses a phospholipase that is able to reduce oxidative death, suggesting this is the original role of the group VII phospholipase family. |
