Neutrophil CR3 expression and specific granule exocytosis are controlled by different signal transduction pathways.

Abstract

Neutrophils express receptors (CR3) for the complement fragment C3bi. CR3 expression can be increased by exposure of the cells to chemotactic factors such as FMLP or to the calcium ionophore A23187. It has been suggested that CR3 moieties are stored in the membrane bounding either the secondary or the tertiary (gelatinase containing) granules. To help define the mechanisms mediating CR3 up-regulation, the effects of several inhibitors upon CR3 expression and secondary granule exocytosis were investigated. Pertussis toxin inhibited FMLP-induced (but not A23187-induced) CR3 expression and exocytosis, indicating that an early step in FMLP-induced CR3 expression is activation of a pertussis toxin-sensitive G protein. However, CR3 expression and exocytosis appeared to be controlled by separate mechanisms distal to G protein activation because 1) DBcAMP and the protein kinase inhibitor H-7 inhibited or stimulated exocytosis, respectively, without affecting CR3 expression; 2) the calmodulin (chlorpromazine and trifluoperazine) and myosin L chain kinase (ML9) inhibitors had greater effects on exocytosis than on CR3 expression; and 3) the kinetics of CR3 expression and exocytosis differed markedly. Thus, although G protein activation is a common early step in both processes, there is a bifurcation of the two processes distally. The mechanisms mediating CR3 up-regulation and tertiary granule exocytosis were also investigated. Extracellular Ca2+ was essential for tertiary granule exocytosis, but not for CR3 up-regulation. We conclude that because the mechanisms controlling CR3 up-regulation and exocytosis diverge soon after the binding of a chemotactic ligand to its receptor, that at least the bulk of increased CR3 expression is not simply a by-product of secondary and tertiary granule exocytosis but is the result of the mobilization of CR3 moieties from an intracellular pool of uncertain identity.