Role of the extracellular signal-regulated protein kinase cascade in humanneutrophil killing of Staphylococcus aureus and Candida albicans and in migration

Killing of Staphylococcus aureus and Candida albicans by neutrophils involv es adherence of the microorganisms, phagocytosis, and a collaborative actio n of oxygen reactive species and components of the granules. While a number of intracellular signalling pathways have been proposed to regulate neutro phil responses, the extent to which each pathway contributes to the killing of S. aureus and C. albicans has not been clearly defined. We have therefo re examined the effect of blocking one such pathway, the extracellular sign al-regulated protein kinase (ERK) cascade, using the specific inhibitor of the mitogen-activated protein kinase/ERK kinase, PD98059, on the ability of human neutrophils to kill S. aureus and C. albicans. Our data demonstrate the presence of ERK2 and a 43-kDa form of ERK but not ERK1 in human neutrop hils, Upon stimulation with formyl methionyl leucyl phenylalanine (fMLP), t he activities of both ERK2 and the 43-kDa form were stimulated. Despite abr ogating the activity of both ERK forms, PD98059 only slightly reduced the a bility of neutrophils to kill S. aureus or C. albicans. This is consistent with our finding that PD98059 had no effect on neutrophil adherence or degr anulation, although pretreatment of neutrophils with PD98059 inhibited fMLP -stimulated superoxide production by 50%, suggesting that a change in super oxide production per se is not strictly correlated with microbicidal activi ty. However, fMLP-stimulated chemokinesis was markedly inhibited, while ran dom migration and fMLP-stimulated chemotaxis mere partially inhibited, by P D98059. These data demonstrate, for the first time, that the ERK cascade pl ays only a minor role in the microbicidal activity of neutrophils and that the ERK cascade is involved primarily in regulating neutrophil migration in response to fMLP.