Wg. Zhou et al., IMPAIRED SURFACE EXPRESSION OF PAF RECEPTORS ON HUMAN NEUTROPHILS IS DEPENDENT UPON CELL ACTIVATION, Archives of biochemistry and biophysics, 308(2), 1994, pp. 439-445
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.