Regulation of platelet-activating-factor receptors and the desensitization response in polymorphonuclear neutrophils

Abstract

Platelet-activating factor (PAF) desensitizes as well as stimulates its various target cells, We find that human polymorphonuclear neutrophils (PMN) exposed to PAF became maximally unresponsive to a second PAF challenge within 15-90 s in assays of Ca2+ mobilization and degranulation. The cells regained full PAF-sensitivity over the ensuing 20-40 min. These effects correlated with changes in PAF receptor availability. PMN treated with PAF, washed in regular buffer and assayed for PAF binding exhibited falls (maximal in 15 s), followed by rises (reaching control levels by 60 min), in the number of high-affinity PAF receptors. However, tracking studies showed that [3H]PAF accumulated on the cell surface for approximately 2 min before being internalized. Regular-buffer washes did not remove this superficial PAF, whereas a washing regimen using excess albumin to adsorb PAF removed 99% of the surface compound. PMN washed by the latter regimen after PAF exposure lost PAF receptors relatively slowly (maximal at approximately 5 min), but the ultimate extent of this loss and the rate at which receptor expression normalized were similar to those of cells washed in regular buffer. Neither cycloheximide nor actinomycin D influenced the course of the receptor changes, but two protein kinase C (PKC) blockers, staurosporine and 1-(5-isoquinolinesulphonyl)piperazine, inhibited the receptor-receptor-depleting actions of PAF. Indeed, a phorbol diester activator of PKC also caused PMN to decrease high-affinity PAF receptor numbers, and the two PKC blockers antagonized this action at concentrations that inhibited PAF-induced PAF receptor losses. We conclude that: (a) PAF induces PMN to down-regulate and then to re-express PAF receptors independently of protein synthesis; (b) these changes are likely to underlie the later stages and reversal of desensitization; (c) the onset (t < or = 2 min) of desensitization, however, precedes receptor down-regulation and must be due to receptor uncoupling from transductional elements; and (d) down-regulation of receptors for PAF appears to be mediated by PKC and/or elements inhibited by PKC blockers.

Activation Via Multiple Signaling Pathways Induces Down-Regulation of Platelet-Activating Factor Receptors on Human B Lymphocytes

“…Our finding that phorbol ester down-regulates PAFR is in agreement with several previous observations from human monocytes (23), neutrophils (29), and human neurohybrid cell lines (48,49). Nguer et al (11) reported that PMA increased PAFR mRNA and surface expression in human tonsillar B cells.…”

“…The combination of calcium ionophore and phorbol ester strongly activates B cells, and both agents have been reported to modulate the PAFR (23,28,29). In the present study, we found that both PDB and ION caused down-regulation of PAF binding sites on LA350 cells.…”

“…Changes in PAFR numbers have been shown to correlate well with changes in the biological response to PAF (29,42). To determine whether there was a functional decrease in PAFR responsiveness (Fig.…”

Regulation of platelet-activating factor receptor gene expression in vivo by endotoxin, platelet-activating factor and endogenous tumour necrosis factor

“…Shirasaki et al [22] showed that PAF rapidly doubled its receptor mRNA in human monocytes, whereas others showed that PAF markedly reduced its receptor mRNA in U937 cells [23], in keeping with earlier studies in PMNs [24] and Kupffer cells [25], which demonstrated that PAF caused a down-regulation of cell surface PAF-binding sites. Quantitative study of tissue PAF-R mRNA, especially of its change over time during inflammation, has so far been limited.…”

“…Few studies investigating tissue PAF-Rs in i o have been reported, and only relative amounts of PAF-R mRNA have been assessed [21]. Previous studies in itro have demonstrated various effects of PAF on PAF-R binding and PAF-R gene transcription, but the results have been contradictory [22][23][24][25], and may be irrelevant to inflammation in i o. Using a highly sensitive Q-PCR method to quantify accurately PAF-R transcripts, we report here high expression of PAF-R in the small intestine, especially the ileum, during both normal and inflamed states.…”