Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that directly control numerous genes of lipid metabolism by binding to response elements in the promoter. It has recently been proposed that PPAR gamma may also regulate genes for proinflammatory proteins, not through PPRE binding but by interaction with transcription factors AP-1, STAT, and NF-kappa B. Recent studies with cultured human monocytes, however, have failed to observe an inhibitory effect of PPAR gamma agonists on induced expression of TNF alpha and IL-6, genes known to be controlled by AP-1, STAT, and NF-kappa B. In a similar fashion, we show here that PPAR alpha (fenofibrate) or PPAR gamma (rosiglitazone) agonists failed to modulate LPS-induced secretion of IL-8 in THP-1 cells. When we made parallel observations on another gene, matrix metalloproteinase 9 (MMP-9), we were surprised to find profound downregulation of LPS-induced secretion by both PPAR alpha or PPAR gamma agonists, These findings suggest that PPAR may regulate only a subset of the proinflammatory genes controlled by AP-1, STAT, and NF-kappa B. Effects of PPARs on MMP-9 may account for the beneficial effect of PPAR agonists in animal models of atherosclerosis.