MODULATION OF HUMAN PLATELET-ACTIVATING-FACTOR RECEPTOR GENE-EXPRESSION BY PROTEIN-KINASE-C ACTIVATION

The effect of PMA on early and late regulation of platelet-activating factor receptor (PAF-R) expression was examined in human monocytes, Tr eatment with 16 nM PMA for 5 min initiated a rapid reduction (50-75%) in [H-3]WEB 2086 binding, which was maximal between 5 and 15 min, Scat chard analysis revealed that PMA treatment reduced the number of bindi ng sites to 50% of control cells without an appreciable change in thei r affinity, In parallel cultures, flow cytometry analysis using anti-P AF-R Abs failed to reveal any significant decrease in the level of PAF -R expression until after 4 h of treatment with PMA, By 24 h, PAF-R ex pression had declined by 80 to 90%, This PMA-induced down-regulation o f surface PAF-R expression was preceded by a rapid down-regulation of PAF-R mRNA expression, PMA-mediated down-regulation could be blocked b y the protein kinase C (PKC) inhibitors H-7 and calphostin C. Activati on of PKC by the diacylglycerol analogue 1-oleoyl-2-acetylglycerol als o resulted in down-regulation of PAF-R mRNA accumulation, whereas the inactive phorbol diester 4 alpha-PMA was ineffective. The rapid disapp earance of the PAF-R transcripts was associated with decreased stabili ty of receptor mRNA and not with a change in the nuclear transcription rate of the PAF-R gene, These findings indicate that PAF-R gene expre ssion in human leukocytes can be regulated through a PKC-dependent pat hway and involves post-transcriptional destabilization of receptor mRN A.