Platelet-activating factor and retinoic acid synergistically activate the inducible prostaglandin synthase gene.

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Abstract
Platelet-activating factor (PAF), a potent lipid mediator generated in cell injury and in the inflammatory and immune responses, promotes transcriptional activation of several primary response genes. TIS10/PGS-2 is a primary response gene encoding the inducible form of prostaglandin synthase. The inductive effects of PAF and retinoic acid (RA), alone and in combination, were studied with the regulatory region of TIS10/PGS-2 transfected into an exponentially growing glioblastoma-neuroblastoma NG108-15 hybrid in the human SH-SY5Y neuroblastoma or in the NIH 3T3 cell. RA alone exhibited only a small inductive effect. However, in the presence of RA (100 nM), a PAF-dependent (1-50 nM) synergistic activation of luciferase reporter constructs driven by regulatory regions of the TIS10/PGS-2 gene was found. The hetrazepine BN-50730, an antagonist selective for intracellular PAF binding sites, inhibited PAF and RA induction of luciferase from the TIS10/PGS-2 promoter. Thus, the intracellular PAF binding site is involved in TIS10/PGS-2 expression. Induction is rapid, suggesting that the combination of PAF and RA activates a preexisting latent transcription factor(s). Deletion studies restrict the major PAF and RA cis-acting response element of the TIS10/PGS-2 gene to a 70-nucleotide sequence as an intracellular inducer of TIS10/PGS-2 expression. The synergistic effect of RA and PAF represents an unusual convergence of nuclear signaling pathways by which, through the modulation of preexisting transcription factors, specific gene expression can be upregulated. PAF-dependent induction of TIS10/PGS-2 expression may play a role in cell injury, differentiation, inflammation, and immune responses.