EFFECT OF LIPOPOLYSACCHARIDE ON MITOGEN-ACTIVATED PROTEIN-KINASES ANDCYTOSOLIC PHOSPHOLIPASE A(2)

The addition of platelet-activating factor (PAF) to human neutrophils increases phosphorylation on tyrosine residues and stimulates the acti vity of p42(erk2)mitogen-activated protein kinase (MAP kinase). This a ction is rapid and transient. In contrast, p42(erk2), p44(erk1) and th e p40(hera) MAP kinase isoforms are all not tyrosine phosphorylated or activated in human neutrophils stimulated with low concentrations of lipopolysaccharide (LPS) in combination with serum. In spite of this, the PAF-induced tyrosine phosphorylation and activation of the p42(erk 2) MAP kinase are greatly potentiated in cells pretreated with LPS. Mo re interestingly, although low concentrations of LPS do not affect MAP kinase isoforms in these cells, they cause the phosphorylation of cyt osolic phospholipase A(2) (cPLA(2)), as evidenced by a decrease in the electrophoretic mobility of the enzyme. In addition, this stimulus-in duced upward shift in the mobility of the enzyme is not inhibited by t he tyrosine kinase inhibitor, genistein. Furthermore, LPS increases th e release of arachidonic acid in control and PAF-stimulated human neut rophils. These observations clearly show that cPLA(2) can be phosphory lated and activated by kinases other than the currently known MAP kina ses. It is proposed that there are MAP kinase-dependent and -independe nt mechanisms for the phosphorylation of cPLA(2).