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Platelet-activating factor production in stimulated macrophages is down-regulated by concurrently produced
1Department of Pathophysiological Biochemistry, Faculty of Pharmaceutical Sciences, Tohoku University, Sendai, Japan.
Abstract:
When rat peritoneal macrophages were incubated in medium containing 12-O-tetradecanoylphorbol 13-acetate (TPA), a protein kinase C activator, production of cell-associated platelet-activating factor (PAF) and extracellular prostaglandin E2 (PGE2) increased. In the presence of the cyclooxygenase inhibitor indomethacin, TPA-induced PAF production was further enhanced dose-dependently in accordance with decrease of PGE2 levels. In addition, indomethacin further enhanced PAF production that was stimulated by the protein kinase C activators, aplysiatoxin and teleocidin. Other cyclooxygenase inhibitors such as naproxen and ibuprofen also enhanced TPA-stimulated PAF production in accordance with inhibition of PGE2 production. Cyclooxygenase inhibitor-induced enhancement of PAF production was markedly prevented by exogenous PGE2. Exogenous arachidonic acid also inhibited TPA-induced PAF production in parallel with increase in PGE2 levels. Inhibition of PAF production by exogenous arachidonic acid was abolished by indomethacin. Furthermore, PAF production stimulated by the endomembrane Ca+2-ATPase inhibitors thapsigargin or thapsigargicin, or by the Ca+2 ionophore A23187, was also enhanced by indomethacin in compensation for the decrease in PGE2 production. In addition, the adenylate cyclase activator forskolin, or the cyclic adenosine monophosphate (cAMP) analogues 8-bromo cAMP and dibutyryl cAMP inhibited thapsigargin-induced PAF production. TPA-induced accumulation of intracellular cAMP was inhibited by indomethacin, and indomethacin-induced decrease of cAMP level was reversed by exogenous PGE2. These results suggested that concurrently produced PGE2 in stimulated macrophages down-regulates PAF production via adenylate cyclase and cAMP pathway.
Insights
Prostaglandin E2 (PGE2) produced by macrophages down-regulates platelet-activating factor (PAF) production. Inhibiting PGE2 production enhances PAF, suggesting a cAMP-mediated pathway for PAF regulation.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Macrophages play a critical role in immune responses.
- Platelet-activating factor (PAF) and prostaglandin E2 (PGE2) are key inflammatory mediators.
- Protein kinase C (PKC) activation influences macrophage mediator production.
Purpose of the Study:
- To investigate the relationship between PAF and PGE2 production in activated macrophages.
- To elucidate the role of cyclooxygenase (COX) and cyclic adenosine monophosphate (cAMP) in regulating PAF production.
Main Methods:
- Rat peritoneal macrophages were stimulated with phorbol ester (TPA), a PKC activator.
- The effects of COX inhibitors (indomethacin, naproxen, ibuprofen) on PAF and PGE2 production were assessed.
- The influence of exogenous PGE2, arachidonic acid, calcium ionophores, and cAMP modulators on PAF production was examined.
Main Results:
- TPA increased both PAF and PGE2 production.
- COX inhibition enhanced TPA-induced PAF production while decreasing PGE2 levels.
- Exogenous PGE2 reversed the enhancement of PAF production by COX inhibitors.
- Modulators of intracellular calcium and cAMP levels affected PAF production, with PGE2 negatively correlating with cAMP.
Conclusions:
- Concurrently produced PGE2 in stimulated macrophages down-regulates PAF production.
- This down-regulation appears to occur via the adenylate cyclase and cAMP signaling pathway.
- These findings reveal a novel regulatory mechanism for inflammatory mediator release in macrophages.