Peroxisome proliferator-activated receptors (PPARs) are nuclear transcription factors that play central roles in metabolism and inflammation. Although a variety of compounds have been shown to activate PPARs, identification of physiologically relevant ligands has proven difficult. In silico studies of lipid derivatives reported here identify specific 5-lipoxygenase products as candidate physiologically relevant PPAR-α activators. Subsequent studies show both in vitro and in a murine model of inflammation that 5-lipoxygenase stimulation induces PPAR-α signaling and that this results specifically from production of the inflammatory mediator and chemoattractant leukotriene B4 (LTB4). Activation of PPAR-α is a direct effect of intracellularly generated LTB4 binding to the nuclear receptor and not of secreted LTB4 acting via its cell-surface receptors. Activation of PPAR-α reduces secretion of LTB4 by stimulating degradation of this fatty acid derivative. We also show that the LTB4 precursors leukotriene A4 (LTA4) and 5-hydroperoxyeicosatetrenoic acid (5-HPETE) activate PPAR-α but have no significant endogenous effect independent of conversion to LTB4. We conclude that LTB4 is a physiologically relevant PPAR-α activator in cells of the immune system. This, together with previous findings, demonstrates that different types of lipids serve as endogenous PPAR-α ligands, with the relevant ligand varying between functionally different cell types. Our results also support the suggestion that regulation of inflammation may involve balancing proinflammatory effects of LTB4, exerted through cell-surface receptors, and anti-inflammatory effects exerted through PPAR-α activation.
