Hyperglycemia increases the production of reactive oxygen species (ROS) from the mitochondrial electron transport chain in bovine endothelial cells. Because several studies have postulated a role for prostaglandins (PGs) in the glomerular hyperfiltration seen in early diabetes, we evaluated the effect of mitochondrial ROS on expression of the inducible isoform of cyclooxygenase (COX-2) in cultured human mesangial cells (HMCs). We first confirmed that incubation of HMC with 30 mmol/l glucose significantly increased COX-2 mRNA but not COX-1 mRNA, compared with 5.6 mmol/l glucose. Similarly, incubation of HMCs with 30 mmol/l glucose significantly increased mitochondrial membrane potential, intracellular ROS production, COX-2 protein expression, and PGE2 synthesis, and these events were completely suppressed by thenoyltrifluoroacetone or carbonyl cyanide m-chlorophenylhydrazone, inhibitors of mitochondrial metabolism, or by overexpression of uncoupling protein-1 or manganese superoxide dismutase. Furthermore, increased expression of COX-2 mRNA and protein was confirmed in glomeruli of streptozotocin-induced diabetic mice. In addition, hyperglycemia induced activation of the COX-2 gene promoter, which was completely abrogated by mutation of two nuclear factor kappaB (NF-kappaB) binding sites in the promoter region. Our results suggest that hyperglycemia increases mitochondrial ROS production, resulting in NF-kappaB activation, COX-2 mRNA induction, COX-2 protein production, and PGE2 synthesis. This chain of events might contribute to the pathogenesis of diabetic nephropathy.