Age-related differences in insulin-like growth factor-1 receptor signaling regulates Akt/FOXO3a and ERK/Fos pathways in vascular smooth muscle cells

Advanced age is a major risk factor for atherosclerosis, but how aging per se influences pathogenesis is not clear. Insulin-like growth factor-I receptor (IGF-IR) promotes aortic vascular smooth muscle cell (VSMC) growth, migration, and extracellular matrix formation, but how IGF-IR signaling changes with age in VSMC is not known. We previously found age-related differences in the activation of Akt/FOXO3a and ERK 1/2 pathways in VSMC, but the upstream signaling remains unclear. Using explanted VSMC from Fischer 344/Brown Norway FI hybrid rats shown to display age-related vascular pathology similar to humans, we compared IGF-IR expression in early passages of VSMC and found a constitutive activation of IGF-IR in VSMC from old compared to young rats, including IGF-IR expression and its tyrosine kinase activity. The link between IGF-IR activation and the Akt/FOXO3a and ERK pathways was confirmed through the induction of IGF-I R with IGF-I in young cells and attenuation of IGF-IR with an inhibitor in old cells. The effects of three kinase inhibitors: AG 1024, LY294002, and TCN, were compared in VSMC from old rats to differentiate IGF-IR from other upstream signaling that could also regulate the Akt/FOXO and ERK pathways. Genes for p27kip-1, catalase and MnSOD, which play important roles in the control of cell cycle arrest and stress resistance, were found to be FOXO3a-targets based on FOXO3a-siRNA treatment. Furthermore, IGF-IR signaling modulated these genes through activation of the Akt/FOXO3a pathway. Therefore, activation of IGF-IR signaling influences VSMC function in old rats and may contribute to the increased risk for atherosclerosis.