oa The role of Insulin-like Growth Factor-1 in Calcification of Human Aortic Valves

Insulin-like Growth Factor-1 (IGF-1) is a multi-functional protein that plays a role in survival, growth, proliferation and differentiation of various cell types. Particularly in the bone, IGF-1 is involved in tissue formation and growth, and the lack of expression of the gene leads to striking features such as short bone phenotype and low bone mineral density. In atherosclerosis, IGF system has been shown to stimulate vascular smooth muscle cell (SMC) proliferation, migration and extracellular matrix (ECM) synthesis contributing to maintenance of plaque stability. Consequently, a reduction of IGF-1 in atherosclerotic plaques has been suggested to increase SMC apoptosis and reduce ECM synthesis leading to weakening of the plaque. Despite extensive studies on its role in various organs and disease models, there is very little known about the effects of IGF-1 in human aortic valve (hAV) tissue that can suffer damaging effects such as cell death, differentiation and tissue mineralization, resulting into Aortic Valve Stenosis (AVS). The objective of the present study is therefore to investigate potential function of IGF-1 in this disease model. Human aortic valve samples were collected from patients undergoing aortic valve replacement or heart transplantation. The tissue expression of IGF-1 was analysed at mRNA level by quantitative Realtime PCR in calcified hAV (n=10-14) and non-calcified hAV (n=3-6), using the comparative Ct method. In vitro, the expression of endogenous IGF-1 was investigated on human valve interstitial cells (hVICs) upon administration of calcifying medium with or without transforming growth factor-beta1 (TGF-b1), a known pro-mineralization molecule (n=3). In addition to the expression of IGF-1, alkaline phosphatase (ALPL), TGF-b1 and osteopontin (OPN) were also examined at RNA level. IGF-1 expression was significantly up-regulated in the diseased valves compared to the controls (4.10 +/- 1.20 calcified vs. 1.10 +/- 0.76 control, p=0.0127). In vitro, there was an up-regulation of IGF-1 (p=0.06) and TGF-b1 expression (p=0.02) upon TGF-b1 stimulation. OPN expression was also highly up-regulated in calcifying media treated cultures, independent of TGF-b1 treatment. ALPL expression in contrast, was higher in normally growing cells compared to TGF-b1 and/or calcifying media treated cells (p=0.05). The significant elevation of IGF-1 in calcified valves in this small patient cohort strongly suggests that IGF-1 might be a key player in AVS disease. Whether the elevated IGF-1 expression is an effect or the cause of calcification is currently under investigation. Our preliminary data on TGF-b1 treatment of hVICs suggests that IGF-1 expression might be an effect of cells to counter apoptotic death conferred by TGF-b1 treatment. Further work on the detailed molecular mechanisms involved in IGF-1 action and its downstream signalling pathways is needed.