Cytohesin-1 regulates fMLF-mediated activation and functions of the beta 2 integrin Mac-1 in human neutrophils

The nucleotide exchange factor cytohesin-1 was previously reported to interact with the cytoplasmic domains of the integrin beta -chain common to all beta 2 integrins such as LFA-1 and Mac-1. We show here that cytohesin-1, which contributes to fMLF-induced functional responses in PMNs through activation of Arf6, restrains the activation of the beta 2 integrin Mac-1 ( alpha M beta 2) in PMNs or dcAMP-differentiated PLB-985 cells. We found that the cytohesin-1 inhibitor SecinH3 or siRNA increased cell adhesion to immobilized fibrinogen and fMLF-mediated conformational changes of Mac-1, monitored using mAb CBRM1/5, specific for the activation epitope of the alpha M subunit. In contrast, PLB-985 cells overexpressing cytohesin-1 showed little adhesion to fibrinogen. The use of SecinH3 and siRNA also revealed that interference with cytohesin-1 signaling also enhanced phagocytosis of zymosan particles and chemotaxis toward fMLF in transwell migration assays. These increments of phagocytosis and chemotaxis in cells treated with SecinH3 and cytohesin-1 siRNA were reversed by a blocking mAb to the integrin- alpha M subunit. We provide evidence for increased polymerized cortical actin in cells treated with SecinH3 and that altered signaling through cytohesin-1 increased cell surface expression of FPRL-1 and impairs the late calcium mobilization response elicited by fMLF. The data provide evidence that stimulation with fMLF initiates a signaling cascade that restrains Mac-1 activation in PMNs. Such crosstalk between FPRL-1 and Mac-1 involves cytohesin-1. We suggest that cytohesin-1 may coordinate activation of the beta 2 integrins to regulate PMN adhesion, phagocytosis, and chemotaxis.