MAPPING OF THE C5A RECEPTOR SIGNAL-TRANSDUCTION NETWORK IN HUMAN NEUTROPHILS

Human neutrophils respond to chemoattractants, resulting in their accu mulation at an inflammatory site. Chemoattractants such as the C5a pep tide, derived from the C5 complement factor, bind to inhibitory guanin e nucleotide binding protein (G(i))-coupled seven membrane-spanning re ceptors expressed in neutrophils. C5a receptor activation results in t he G(i)-dependent activation of the mitogen-activated protein (MAP) ki nase pathway in human neutrophils. C5a receptor ligation activates bot h B-Raf and Raf-1, with B-Raf activation overlapping but temporally di stinct from that of Raf-1. B-Raf and Raf-1 both efficiently phosphoryl ate MAP kinase kinase (MEK-1). C5a also stimulates guanine nucleotide exchange and activation of Ras. Ras and Raf activation in response to C5a involves protein kinase C-dependent and -independent pathways. Act ivation of both Raf-1 and B-Raf was inhibited by protein kinase A stim ulation, consistent with the inhibitory effects of increased cAMP leve ls on neutrophil function. The findings define a functional signal tra nsduction pathway linking the neutrophil C5a chemoattractant receptor to the regulation of Ras, B-Raf, Raf-1, and MAP kinase.