Conditional expression of the dual-specificity phosphatase PYST1/MKP-3 inhibits phosphorylation of cytosolic phospholipase A2 in Chinese hamster ovary cells

Inhibition of the MEK1/ERK pathway reduces arachidonic acid release independently of cPLA2 phosphorylation and translocation

BACKGROUND

The 85-kDa cytosolic phospholipase A2 (cPLA2) mediates arachidonic acid (AA) release in MDCK cells. Although calcium and mitogen-activated protein kinases regulate cPLA2, the correlation of cPLA2 translocation and phosphorylation with MAPK activation and AA release is unclear.

RESULTS

MEK1 inhibition by U0126 inhibited AA release in response to ATP and ionomycin. This directly correlated with inhibition of ERK activation but not with phosphorylation of cPLA2 on Ser505, which was only partially inhibited by ERK inhibition. Inhibition of AA release by U0126 was still observed when stoichiometric phosphorylation of cPLA2 on Ser505 was maintained by activating p38 with anisomycin. Translocation kinetics of wild-type cPLA2 and cPLA2 containing S505A or S727A mutations to Golgi were similar in response to ATP and ionomycin and were not affected by U0126.

CONCLUSIONS

These results suggest that the ability of cPLA2 to hydrolyze membrane phospholipid is reduced by inhibition of the MEK1/ERK pathway and that the reduction in activity is independent of cPLA2 phosphorylation and translocation to membrane. The results also demonstrate that cPLA2 mutated at the phosphorylation sites Ser505 and Ser727 translocated with similar kinetic as wild-type cPLA2.

Roles of cytosolic phospholipase A(2) and platelet-activating factor receptor in the Ca-induced biosynthesis of PAF

Casein-elicited peritoneal exudate cells (PEC), mainly consisted of neutrophils, were collected from platelet-activating factor receptor-knock-out (PAFR-KO), cytosolic phospholipase A(2) knock-out (cPLA(2)-KO), and wild-type (WT) mice. After stimulation of PEC with calcium ionophore A 23187, PAF levels were measured by radio-ligand binding assay using receptor-rich membrane fraction prepared from the PAF receptor transgenic mice. We found that the level of PAF production by PEC was not different between WT and PAFR-KO mice. On the other hand, cPLA(2)-KO mice were deficient in the PAF production. These results provide the direct evidence while cPLA(2) is essential in the production of PAF, PAF receptor deficiency has little effect on the PAF production.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.