We examined the mechanisms responsible for the production of fibronectin induced by oxidized low-density lipoprotein (oxLDL) in rat mesangial cells. oxLDL accelerated the production of fibronectin with the preceding generation of reactive oxygen species (ROS). Pretreatment with N-acetylcysteine suppressed the oxLDL-induced fibronectin production as well as ROS generation. oxLDL also elicited the activation of SP-1, nuclear factor-kappaB, and cAMP response element-binding protein, but not activator protein-1. Among these activated transcription factors, N-acetylcysteine inhibited the activation of SP-1 only. 7-Ketocholesterol, an oxidized lipid in oxLDL particles, induced the production of fibronectin and the activation of SP-1, those which were suppressed by N-acetylcysteine. Furthermore, mithramycin A, an inhibitor of SP-1, also suppressed the oxLDL- and 7-ketocholesterol-stimulated production of fibronectin. These results suggest that oxLDL stimulates fibronectin production, at least in part, through the ROS-dependent activation of SP-1 in rat mesangial cells, and further that the ROS-dependent cellular responses may be elicited by 7-ketocholesterol.