Role of PKC and MAPK in cytosolic PLA2 phosphorylation and arachadonic acid release in primary murine astrocytes

Although Group IV cytosolic phospholipase A(2) (cPLA(2) ) in astrocytes has been implicated in a number of neurodegenerative diseases, mechanisms leading to its activation and release of arachidonic acid (AA) have not been clearly elucidated. In primary murine astrocytes, phorbol myristate acetate (PMA) and ATP stimulated phosphorylation of ERK1/2 and cPLA(2) as well as evoked AA release. However, complete inhibition of phospho-ERK by U0126, an inhibitor of mitogen-activated protein kinase kinase (MEK), did not completely inhibit PMA-stimulated cPLA(2) and AA release. Epidermal growth factor (EGF) also stimulated phosphorylation of ERK1/2 and cPLA(2) [largely through a protein kinase C (PKC)-independent pathway], but EGF did not evoke AA release. These results suggest that phosphorylation of cPLA(2) due to phospho-ERK is not sufficient to evoke AA release. However, complete inhibition of ATP-induced cPLA(2) phosphorylation and AA release was observed when astrocytes were treated with GF109203x, a general PKC inhibitor, together with U0126, indicating the important role for both PKC and ERK in mediating the ATP-induced AA response. There is evidence that PMA and ATP stimulated AA release through different PKC isoforms in astrocytes. In agreement with the sensitivity of PMA-induced responses to PKC down-regulation, prolonged treatment with PMA resulted in down-regulation of PKCalpha and epsilon in these cells. Furthermore, PMA but not ATP stimulated rapid translocation of PKCalpha from cytosol to membranes. Together, our results provided evidence for an important role of PKC in mediating cPLA(2) phosphorylation and AA release in astrocytes through both ERK1/2-dependent and ERK1/2-independent pathways.