IMPORTANCE OF MEK IN NEUTROPHIL MICROBICIDAL RESPONSIVENESS

Exposure of neutrophils to inflammatory stimuli such as the chemoattra ctant FMLP leads to activation of responses including cell motility, t he oxidative burst, and secretion of proteolytic enzymes, A signaling cascade involving sequential activation of Raf-1, mitogen-activated pr otein kinase (MEK), and extracellular signal regulated kinase (ERK) is also rapidly activated after agonist exposure, The temporal relations hip between these events suggests that the kinases may be involved in triggering the effector functions, but direct evidence of a causal rel ationship is lacking, To assess the role of the MEK/ERK pathway in the activation of neutrophil responses, we studied the effects of PD09805 9, a potent and selective inhibitor of MEK, Preincubation of human neu trophils with 50 mu M PD098059 almost completely (>90%) inhibited the FMLP-induced activation of MEK-1 and MEK-2, the isoforms expressed by neutrophils. This dose of PD098059 virtually abrogated chemoattractant -induced tyrosine phosphorylation and activation of ERK-1 and ERK-2, i mplying that MEKs are the predominant upstream activators of these mit ogen-activated protein kinases, Pretreatment of neutrophils with the M EK antagonist inhibited the oxidative burst substantially and phagocyt osis only moderately, In addition, PD098059 antagonized the delay of a poptosis induced by exposure to granulocyte-macrophage CSF, However, t he effects of PD098059 were selective, as it failed to inhibit other r esponses, including chemoattractant-induced exocytosis of primary and secondary granules, polymerization of F-actin, chemotaxis, or activati on of phospholipase A(2). We conclude that MEK and ERK contribute to t he activation of the oxidative burst and phagocytosis, and participate in cytokine regulation of apoptosis.