Type 1 diabetes mellitus (T1DM) is characterized by the deficiencies of insulin and C-peptide. Mounting evidences have proved the beneficial effects of C-peptide on the renal function in T1DM. However, it is still controversial about the roles of C-peptide in T2DM nephropathy since the level of C-peptide fluctuates greatly at different stages of T2DM. In the present study, we found that the serum C-peptide concentration was much lower in GK rats with diabetic nephropathy than that in normal counterparts. A sustained supplementation of C-peptide at a physiological level could ameliorate urinary albumin, independent of blood glucose control. C-peptide treatment improved glomerulosclerosis and podocyte morphology and reduced the thickness of glomerular basement membrane as compared with saline treatment control. Moreover, it decreased fibronectin synthesis in diabetic glomeruli and in cultured rat mesangial cells accompanied by a down-regulation of RAGE and an up-regulation of PICA. Interestingly, H-89, a PICA inhibitor, could reverse the inhibition effect of C-peptide on fibronectin production in cultured mesangial cells. These findings suggest that C-peptide level is low in T2DM rats with nephropathy and a treatment with a physiological dose of C-peptide can prevent renal injury in diabetic GK rats. (C) 2015 Elsevier Inc. All rights reserved.