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Up-regulation of inducible cyclooxygenase gene expression by platelet-activating factor in activated rat alveolar
M Thivierge1, M Rola-Pleszczynski
1Department of Pediatrics, Faculty of Medicine, University of Sherbrooke, Quebec, Canada.
Abstract:
Platelet-activating factor (PAF) can stimulate alveolar macrophages (AM) to produce IL-6 through a PG-dependent process. In this study, the modulation by PAF of the expression of COX-2, the inducible isoform of PG synthase, was investigated. Expression of COX-2 mRNA was increased in rat AM within 2 h after treatment with either of the bacterial products LPS or muramyl dipeptide (MDP) alone. Although PAF had no effect by itself, stimulation of AM with a combination of PAF (10(-10) to 10(-8) M) and LPS or MDP resulted in a synergistic, three- to fivefold increase in levels of COX-2 mRNA. No significant change was observed in the mRNA expression of the constitutive isoform, COX-1. The antagonist WEB 2170 blocked the action of PAF, whereas lyso-PAF was inactive on the COX-2 gene expression. The effect of PAF was rapid, being evident by 30 to 60 min of stimulation and was accompanied by enhanced production of PGE2. Two relatively selective inhibitors of COX-2 abolished the PAF-dependent increase in PGE2 production. Moreover, inhibition of transcription with actinomycin D completely abrogated the effect of PAF on both COX-2 mRNA and PGE2 production. These results suggest that COX-2 expression can be regulated at the transcriptional level by PAF, in synergy with bacterial products. PAF-dependent up-regulation of COX-2 expression may constitute a novel element in the interrelationship between PAF and prostanoids in the context of allergic, inflammatory, and immune processes.
Insights
Platelet-activating factor (PAF) synergizes with bacterial products to increase cyclooxygenase-2 (COX-2) mRNA expression in alveolar macrophages. This PAF-mediated upregulation of COX-2, a key enzyme in inflammation, occurs transcriptionally and enhances prostaglandin E2 production.
Area of Science:
- Immunology and Inflammation Research
- Molecular Biology of Macrophages
- Prostanoid Synthesis Pathways
Background:
- Platelet-activating factor (PAF) is known to stimulate alveolar macrophages (AM) to produce IL-6 via prostaglandin (PG)-dependent mechanisms.
- Cyclooxygenase-2 (COX-2) is an inducible isoform of PG synthase, crucial in inflammatory responses.
- Bacterial products like LPS and MDP can induce COX-2 expression in macrophages.
Purpose of the Study:
- To investigate the modulation of COX-2 expression by PAF in alveolar macrophages.
- To determine the synergistic effect of PAF with bacterial products (LPS, MDP) on COX-2 mRNA levels.
- To elucidate the mechanism underlying PAF-induced COX-2 expression and prostaglandin E2 (PGE2) production.
Main Methods:
- Rat alveolar macrophages were treated with PAF, LPS, MDP, or combinations thereof.
- Quantitative analysis of COX-2 and COX-1 mRNA expression using techniques like Northern blotting or RT-PCR.
- Assessment of PGE2 production and the effect of COX-2 inhibitors and transcription inhibitors (actinomycin D).
Main Results:
- LPS or MDP alone increased COX-2 mRNA in AM; PAF alone had no effect.
- Combinations of PAF (10(-10) to 10(-8) M) with LPS or MDP synergistically increased COX-2 mRNA levels by three- to fivefold.
- PAF rapidly enhanced PGE2 production, an effect blocked by COX-2 inhibitors and transcription inhibition, indicating transcriptional regulation.
Conclusions:
- PAF, in synergy with bacterial products, transcriptionally up-regulates COX-2 expression in alveolar macrophages.
- This PAF-dependent COX-2 induction contributes to enhanced PGE2 production.
- The findings reveal a novel mechanism in the interplay between PAF, COX-2, and prostanoids in inflammatory and immune processes.