BACKGROUND: We evaluated the role of oxidative stress in diabetic nephropathy by measuring intracellular reactive oxygen species (ROS) and redox-sensitive transcription factors in isolated peripheral mononuclear cells (PBMC). METHODS: From 66 diabetic patients with or without diabetic nephropathy (Group III and II, respectively) and 49 normal control subjects (Group I), spontaneous and stimulated ROS levels, activities of nuclear factor-kappa B (NF-kappaB), activator protein-1 (AP-1), and specificity protein1 (Sp1) in PBMC, urinary and PBMC TGF-beta1 (transforming growth factor-beta1), and 24-hour urinary albumin excretion (UAE) were measured. RESULTS: Spontaneous ROS was significantly higher in group III and II than group I (60.7 +/- 3.3 vs. 60.0 +/- 3.0 vs. 41.1 +/- 2.4%, respectively), and stimulated ROS were significantly higher in Group III compared to Group II (Increment of H2O2-induced ROS production: 21.8 +/- 2.2 vs. 11.1 +/- 2.0%, respectively; increment of PMA-induced ROS production 23.5 +/- 4.5 vs. 21.6 +/- 2.2%, respectively). The activities of NF-kappaB and AP-1, but not of Sp1, were significantly higher in Group III than in Group II (2.53 vs. 2.0 vs. 1.43-fold, respectively). Both PBMC- and urinary TGF-beta1 levels were higher in Group III than Group II (3.23 +/- 0.39 vs. 1.99 +/- 0.68 ng/mg in PBMCs, 16.88 +/- 6.84 vs. 5.61 +/- 1.57 ng/mL in urine, both respectively), and they were significantly correlated with activities of NF-kappaB and AP-1 and 24-hour UAE. CONCLUSIONS: Increased intracellular ROS generation in PBMCs of diabetic patients is involved in the pathogenesis of diabetic nephropathy through activation of NF-kappaB and AP-1, but not Sp1, and increased expression of TGF-beta1.