Clinical studies have suggested that loss of insulin’s action and excessive IGF-1 levels in the glomeruli elevate the risk for diabetic kidney disease (DKD). To define the potential pathogenic role of IGF-1 and insulin signaling in DKD, we generated mice with selective loss of insulin receptor (IR) or IGF-1 receptor (IGF-1R) in the mesangial cells (MCs) by cross breeding IRflox/flox mice with Sm22α-Cre mice (SMIRKO) or IGF-1Rflox/flox mice with Myh11-Cre mice (SMIGF1RKO), and examined urinary albumin excretion (ACR) and mesangial expansion with 6 month duration of streptozotocin (STZ)-induced diabetes. IGF-1 levels were significantly increased by 1.5 fold in the renal cortex, but not in the plasma of all three types of diabetic mice. Diabetes induced elevation of ACR (2.0 fold), glomerular size and mesangial expansion (2.1 fold), in parallel with increased glomerular expressions of fibrosis-related (Tgfβ1, fibronectin and Col3a) and matrix (Has2) genes by 2-3 fold, significantly, in Wt and SMIRKO mice, but were prevented in diabetic SMIGF1RKO mice. In cultured MCs, IGF-1, at physiological levels, and insulin, only at supra-physiological levels, induced the expressions of Has2 (3.8 fold) and Tgfβ1 (1.7 fold). Deletion of IGF-1R, but not IR, in MCs inhibited both IGF-1 and insulin-induced expressions of Has2. Inhibitor of MAP kinase pathway, but not inhibitor of PI3 kinase reduced insulin or IGF-1-induced expressions of Has2 in MCs. IGF-1-induced phosphorylations of IGF-1R (39 fold), Akt (34 fold) and Erk/MAPK (8 fold) were significantly inhibited in both IR or IGF-1R-deleted MCs. Insulin-induced phosphorylations of IR-β (11 fold) and Akt (27 fold) were also decreased in IR-deleted MCs, but surprisingly increased in IGF-1R-deleted MCs. Thus, MAPK activation by elevated glomerular IGF-1 levels via IGF-1R, but not IR, on MCs, may contribute to the pathogenesis of DKD through induction of Has2 and fibrotic genes.
A. Ishikado: Employee; Self; Sunstar Group. Employee; Spouse/Partner; Sunstar Group. T. Shinjo: None. H. Yokomizo: None. Y. Maeda: None. K. Park: None. W. Qi: None. R. St-Louis: None. Q. Li: None. G.L. King: Research Support; Self; Janssen Pharmaceuticals, Inc.