In this study we have verified the existence of a cytosolic phospholipase A(2) (cPLA(2)) in rat-liver macrophages. Stimulation of these cells with phorbol 12-myristate 13-acetate (PMA), zymosan and lipopolysaccharide (LPS), but not with the Ca2+-ionophore A23187, leads to phosphorylation of cPLA(2) and activation of mitogen-activated protein (MAP) kinase, supporting the hypothesis that MAP kinase is involved in cPLA(2) phosphorylation. We show furthermore, that the tyrosine kinase inhibitor genistein prevents the LPS- but not the PMA- or zymosan-induced phosphorylation of cPLA(2) and activation of MAP kinase, indicating that tyrosine kinases participate in LPS-but not in PMA- and zymosan-induced cPLA(2) phosphorylation and MAP kinase activation. Phosphorylation of cPLA(2) does not strongly correlate with stimulation of the arachidonic acid (AA) cascade: (1) A23187, a potent stimulator of AA release, fails to induce cPLA(2) phosphorylation; (2) withdrawal of extracellular Ca2+, which inhibits PMA-stimulated AA release (Dieter, Schulze-Specking and Decker (1988) fur. J. Biochem. 177, 61-67), has no effect on PMA-induced phosphorylation of cPLA(2); (3) LPS induces cPLA(2) phosphorylation within minutes, whereas increased AA release upon treatment with LPS is detectable for the first time after 4 h; and (4) genistein, which prevents LPS-induced cPLA(2) phosphorylation, does not inhibit AA release in response to LPS. From these data we suggest that a rise in intracellular Ca2+, but not phosphorylation of cPLA(2), is essential for activation of the AA cascade in rat-liver macrophages.
