Engagement of L-selectin impairs the actin polymerizing capacity of β2 integrins on neutrophils

A sequential activation of L-selectin and β2-integrins on neutrophils is crucial for the rolling, adherence and subsequent migration of these cells on the endothelium. However, little is known about a possible interplay between these adhesion receptors in the final regulation of cell motility. The results presented here show that sulfatides themselves (here used as tools to activate L-selectins), have no major effect on the cellular content of filamentous actin (F-actin), but cause a time-related decrease in the β2-integrin-induced formation of F-actin. This effect of sulfatides was abolished in cells lacking L-selectin as a result of pretreatment with chymotrypsin. A similar sulfatide-induced activation of Lselectin also caused a pronounced and time-related decrease of a subsequent chemotactic peptide-induced F-actin response. The effect of sulfatides on both β2-integrin- and chemotactic peptide-induced F-actin were abolished if Lselectin were blocked by preincubating the cells with specific antibodies to L-selectin. These effects of L-selectin engagement on cellular F-actin content were neither abolished by blocking the cytosolic free Ca2+ signal with bis(2-amino-5-methylphenoxy)ethane-N,N,N’,N’-tetraacetic-acid tetraacetoxymethyly ester (MAPT/AM) nor by blocking a cAMP-induced activation of protein kinase A by pretreating the cells with adenosine-3’,5’-cyclic monophosphorothioate (Rp-cAMPS). Instead we found that L-selectin engagement impaired an early β2-integrin-induced tyrosine kinase activation, an event shown to be necessary for a normal β2-integrin-mediated F-actin response. The present demonstration of a negative feed-back function of L-selectin on β2-integrin-induced modulations of the actin cytoskeleton, suggests that the relative distribution and/or density of the respective L-selectin and β2-integrin ligands on endothelial cells might be important factors in determining the final site of firm adhesion and extravasation of neutrophils.