High glucose-induced cytosolic phospholipase A2 activation responsible for eicosanoid production in rat mesangial cells.

The stimulation of prostaglandin E2 (PGE2) production in mesangial cells exposed to a high glucose level was studied from the viewpoint of its implication in the glomerular hyperfiltration in diabetic nephropathy. The basal PGE2 synthesis apparently increased in the cells on incubation with a high glucose level (20 mM) for 3-6 h. Under these conditions, secretory phospholipase A2 activity was not detected in the incubation medium, but cytosolic phospholipase A2 (cPLA2) activity in the cells increased time-dependently up to 6 h, compared with that with a normal glucose level (5 mM). However, no difference in the cPLA2 protein content between the two glucose levels was observed on immunoblot analysis, suggesting that the increased cPLA2 activity under high glucose conditions is not due to stimulation of de novo synthesis. Stimulation with a calcium ionophore markedly enhanced arachidonic acid liberation and PGE2 production by cells exposed to the high glucose level. Furthermore, mitogen-activated protein kinase (MAPK) activity increased time-dependently under high glucose conditions, the rate of increase being consistent with those in cPLA2 activity and PGE2 production under the same conditions. These data suggest that glucose-induced cPLA2 activation through MAPK activation is responsible for the enhancement of PGE2 production in mesangial cells.