ABSTRACT
Platelet-activating factor (PAF) is a potent inflammatory phospholipid mediator that is known to play a role in early-phase responses in asthma and other diseases. Through its high affinity receptor, PAFR, PAF is known to activate multiple signalling pathways contributing to its proinflammatory effects. Of these pathways, the mitogen-activated protein kinase (MAPK) cascade is initiated upon PAF stimulation, leading to the activation of the conventional MAPKs ERK1/2, p38 and JNK. Since dual-specificity phosphatases (DUSP) downregulated MAPK activity, we postulated that PAF could also enhance DUSP expression and thus induced an autoregulatory loop. In this report, we studied the effect of PAF on DUSP mRNA expression in human monocytes. Our results demonstrate that PAF induces DUSP1 and DUSP5 gene expression in a time- and concentration-dependent manner, with maximal effects at PAF 100 nM and at 20 - 30 min of stimulation. In contrast, DUSP2 and DUSP6 gene expression was not enhanced by PAF. Moreover, leukotriene D4, another lipid mediator of inflammation, was unable to modulate DUSP expression. PAF-induced DUSP expression was prevented by the PAFR antagonist WEB2170 and by pretreatment with the transcriptional inhibitor Actinomycin D. Moreover, enhanced DUSP5, but not DUSP1 expression was prevented by pretreatment with the ERK inhibitor PD98059 or the PI3K inhibitor Wortmannin. Taken together, our results indicate that PAF selectively enhances DUSP1 and DUSP5 gene expressions through PAFR activation, and suggest that PAF may have an active role in the resolution of inflammation by its ability to upregulate the two DUSPs and thus provide a negative auto-regulatory signalling mechanism.