IMPAIRED SURFACE EXPRESSION OF PAF RECEPTORS ON HUMAN NEUTROPHILS IS DEPENDENT UPON CELL ACTIVATION

The capacity of human neutrophils to bind PAF was rapidly diminished u pon cell stimulation with both physiological agonists (N-formylmethion ylleucylphenylalanine (FMLP), leukotriene B-4 (LTB(4))) and pharmacolo gic agonists (phorbol 12-myristate 13-acetate (PMA), A23187). As a con sequence, PAF responses in neutrophils were blunted, as monitored by a n inhibition of intracellular Ca2+ mobilization. Downregulation of the PAF receptor in neutrophils by diverse agonists was temperature-sensi tive and required intact cells. Scatchard analysis of binding data rev ealed that PAF binding sites were lost without an appreciable change i n the affinity of the ligand for the receptor. The binding of the PAF receptor antagonist WEB2086 to neutrophils decreased in parallel with PAF binding. PMA-induced PAF receptor downregulation was stauro-sporin e-sensitive while PAF receptor downregulation by A23187, FMLP, or LTB( 4) was staurosporine-resistant. Both neutrophil aggregation (a form of intercellular adhesion) and PAF receptor downregulation occurred only at high concentrations of agonists while other signaling processes su ch as the increase in [Ca2+](i), PKC activation, and PAF synthesis wer e stimulated at low concentrations of agonists. Furthermore, agonist-i nduced PAF receptor downregulation was observed only under conditions in which the activated neutrophils were stirred (or shaken) and were a llowed to aggregate. Additionally, chelation of extracellular Ca2+ wit h EGTA minimized cell aggregation and also inhibited PAF receptor down regulation. While the nature of the biochemical signal or the physical changes in the plasma membrane associated with aggregation or that fo llow aggregation remain to be elucidated, it is clear that full expres sion of cell activation (i.e., neutrophil aggregation) is required for PAF receptor downregulation. (C) 1994 Academic Press, Inc.