PPAR-alpha and -gamma but not -delta agonists inhibit airway inflammation in a murine model of asthma: in vitro evidence for an NF-kappaB-independent effect

1. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that have been proposed to regulate inflammation by antagonising the nuclear factor-kappaB (NF-kappaB) signalling pathway. We investigated the role of PPARs using synthetic agonists in murine models of airway inflammation, and addressed the possible effect on NF-kappaB signalling in vitro using a human epithelial cell line, A549. 2. Sensitised BALB/c mice exposed to an aerosol solution of ovalbumin had an increased number of airway eosinophils, neutrophils and lymphocytes. When given intranasally an hour before the aerosol challenge, a PPAR-alpha (GW 9578) and PPAR-gamma (GI 262570) selective agonist as well as a dual PPAR-alpha/gamma (GW 2331) agonist selectively inhibited allergen-induced bronchoalveolar lavage eosinophil and lymphocyte but not neutrophil influx. In contrast, a PPAR-delta agonist (GW 501516) was inactive. 3. When given intranasally an hour before challenge, PPAR-alpha and PPAR-gamma selective agonists as well as a dual PPAR-alpha/gamma agonist did not inhibit lipopolysaccharide-induced bronchoalveolar lavage neutrophil influx or tumour necrosis factor-alpha (TNF-alpha) and KC production. 4. In A549 cells, selective agonists for PPAR-alpha, -gamma and -delta did not inhibit intracellular adhesion molecule-1 expression following stimulation with proinflammatory cytokines. In addition, IL-8 release and the activation of an NF-kappaB-responsive reporter gene construct were inhibited only at micromolar concentrations, suggesting that these effects were not PPAR-mediated. 5. Our in vivo data show that agonists of PPAR-alpha and -gamma, but not -delta, inhibit allergen-induced bronchoalveolar lavage eosinophil and lymphocyte influx. In vitro data suggest that this effect might not be mediated by antagonism of the NF-kappaB pathway.