Little is known about the regulatorymechanisms of endothelial cell (EC) proliferation by retinal pericytes and vice versa. In a model of coculture with bovine retinal pericytes lasting for 24 h, rat brain ECs showed an increase in arachidonic acid (AA) release, whereas Western blot and RT-PCR analyses revealed that ECs activated the protein expression of cytosolic phospholipase A2 (cPLA2) and its phosphorylated form and calcium-independent intracellular phospholipase A2 (iPLA2). No activation of the same enzymes was seen in companion pericytes. In ECs, the protein level of phosphorylated extracellular signal-regulated kinase (ERK) 1/2 was also enhanced significantly, a finding not observed in cocultured pericytes. The expression of protein kinase C-a (PKCa) and its phosphorylated form was also enhanced in ECs. Wortmannin, LY294002, and PD98059, used as inhibitors of upstream kinases (the PI3-kinase/Akt/PDK1 or MEK-1 pathway) in cultures, markedly attenuated AA release and the expression of phosphorylated forms of endothelial cPLA2, PKCa, and ERK1/2. By confocal microscopy, activation of PKCa in perinuclear regions of ECs grown in coculture as well as strong activation of cPLA2 in ECs taken from a model of mixed culture were clearly observed. However, no increased expression of both enzymes was found in cocultured pericytes. Our findings indicate that a sequential activation of PKCa contributes to endothelial ERK1/2 and cPLA2 phosphorylation induced by either soluble factors or direct cell-tocell contact, and that the PKCa-cPLA2 pathway appears to play a key role in the early phase of EC-pericyte interactions regulating blood retina or blood-brain barrier maturation.
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