Small G-Proteins Ras, Rac and Rho in the Regulation of the Neutrophil Respiratory Burst Induced by Formyl Peptide

Formylated peptides are specific ligands activating many of the neutrophil functions, their action is mediated by formyl peptide receptors (FPRs). FPRs belong to the family of receptors having seven transmembrane domains and coupled to G-proteins (GPCR). About a dozen of highly homologous genes of FPRs were found to be localized in mouse chromosome 17. By binding with labeled N-formyl-Met-Leu-Phe (fMLF), FPRs are classified as receptors with high (FPR1) and low (FPR2 and FPR3/FPRL1) affinity to formylpeptide. Binding of formylpeptide with FPRs triggers complex signaling pathways, of which the most studied are: phospholipase C (PLC) cascade with subsequent calcium signaling; pathway of mitogen-activated protein kinase (MAPK), and activation of phosphoinositol-3-kinase (PI3K) cascades. As we have shown previously, the priming of mouse neutrophil respiratory burst occurs under the cell activation by fMLF in high doses only, i.e., it is necessary to activate both high- and low-affinity FPRs. Besides, the usage of the specific MEK and p38MAPK inhibitors induced significant suppression of the response to 1 mu M fMLM, while the response to 50 mu M fMLF increased in the presence of the inhibitors. We have suggested that upon activation of high- and low-affinity fMLF receptors, there is a signal divergence, and a small G-protein-dependent signaling pathways could be the alternative signaling pathways in NADPH oxidase activation. Here we have demonstrated that Ras proteins participate in the respiratory burst activation, especially in activation through the high-affinity fMLF receptors. Activation of the Rho and Rac proteins induced the down-regulation of the respiratory burst under the stimulation of the high-affinity FPRs. The inhibition of the Rho proteins almost completely suppressed the respiratory burst activated through the high- and low-affinity receptors, probably due to inability to assemble the cytoskeleton proteins and NADPH oxidase components.