Human neutrophils utilize a Rac/Cdc42-dependent MAPK pathway to direct intracellular granule mobilization toward ingested microbial pathogens.

Elevated levels of mitogen-activated protein kinase/extracellular regulatory kinase (MAPK/ERK) activity are frequently found in some cancer cells. In efforts to reduce tumor growth, attempts have been made to develop cancer therapeutic agents targeting the MAPK. Here, by use of biologic, biochemical, and gene manipulation methods in human polymorphonuclear neutrophils (PMNs), we have identified a key pathway important in normal cell function involving MAPK/ERK in PMNs for growth inhibition of Candida albicans. Contact with C albicans triggered MAPK/ERK activation in PMNs within 5 minutes, and blocking of MAPK/ERK activation, either by the pharmacologic reagent PD098059 or by dominant-negative MAPK kinase (MEK) expression via vaccinia viral delivery, suppressed antimicrobial activity. Rac and Cdc42, but not Ras or Rho, were responsible for this MAPK/ERK activation. Expression of dominant-negative Rac (N17Rac) or Cdc42 (N17Cdc42) eliminated not only C albicans- mediated ERK phosphorylation but also phagocytosis and granule migration toward the ingested microbes, whereas dominant-negative Ras (N17Ras) and Rho (N19Rho) did not. PAK1 (p21-activated kinase 1) activation is induced by C albicans, suggesting that PAK1 may also be involved in the Rac1 activation of MAPK/ERK. We conclude from these data that Rac/Cdc42-dependent activation of MAPK/ERK is a critical event in the immediate phagocytic response of PMNs to microbial challenge. Therefore, use of MAPK pharmacologic inhibitors for the treatment of cancer may result in the interruption of normal neutrophil function. A balance between therapeutic outcome and undesirable side effects must be attained to achieve successful and safe anticancer therapy.