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. Treatment with 16 nM PMA for 5 min initiated a rapid reduction (50-75%) in [3H]WEB 2086 binding, which was maximal between 5 and 15 min. Scatchard analysis revealed that PMA treatment reduced the number of binding sites to 50% of control cells without an appreciable change in their affinity. In parallel cultures, flow cytometry analysis using anti-PAF-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 expression had declined by 80 to 90%. This PMA-induced down-regulation of surface PAF-R expression was preceded by a rapid down-regulation of PAF-R mRNA expression. PMA-mediated down-regulation could be blocked by the protein kinase C (PKC) inhibitors H-7 and calphostin C. Activation of PKC by the diacylglycerol analogue 1-oleoyl-2-acetylglycerol also resulted in down-regulation of PAF-R mRNA accumulation, whereas the inactive phorbol diester 4alpha-PMA was ineffective. The rapid disappearance of the PAF-R transcripts was associated with decreased stability 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 expression in human leukocytes can be regulated through a PKC-dependent pathway and involves post-transcriptional destabilization of receptor mRNA.