Human umbilical vein endothelial cells (HUVEC) express functional receptors to leptin, the product of the ob gene. As human obesity is associated with atherosclerosis and hyperleptinemia, we investigated whether leptin, in addition to its angiogenic properties, exerts atherogenic effects through the generation of oxidative stress in endothelial cells. In HUVEC leptin increased the accumulation of reactive oxygen species (ROS), as assessed by the oxidation of 2', 7'- dichlorodihydrofluorescein, in a time- and concentration-dependent manner. In addition, leptin activated the NH2-terminal c-Jun kinase/stress-activated protein kinase pathway as demonstrated by enhanced JNK activity and AP-1 DNA binding. Both effects were sensitive to antioxidant treatment with N-acetylcysteine. NF-kappaB, another redox-sensitive transcription factor, was also activated by leptin stimulation in an oxidant-dependent manner. Finally, activation of both AP-1 and NF-kappaB was associated with an enhanced expression of the monocyte chemoattractant protein-1 in HUVEC. These findings demonstrate that ROS are second messengers involved in leptin-induced signaling in endothelial cells. Thus, chronic oxidative stress in endothelial cells under hyperleptinemia may activate atherogenic processes and contribute to the development of vascular pathology.