Apposite Insulin-like Growth Factor (igf) Receptor Glycosylation Is Critical to the Maintanence of Vascular Smooth Muscle Phenotype in the Presence of Factors Promoting Osteogenic Differentiation and Mineralisation

Vascular calcification is strongly linked with increased morbidity and mortality from cardiovascular disease. Vascular calcification is an active, cell-mediated process that involves the differentiation of vascular smooth muscle cells (VSMCs) to an osteoblast-like phenotype. Several inhibitors of this process have been identified, including Insulin-like Growth Factor-I (IGF-1). These studies examined the role of the IGF receptor and the importance of IGF receptor glycosylation in the maintenance of the VSMC phenotype in the face of factors known to promote osteogenic conversion. IGF-I (25 ng/ml) significantly protects VSMCs from glycerophosphate-induced osteogenic differentiation (p<0.005) and mineral deposition (p<0.01), Mevalonic acid depletion (induced by cerivastatin, 100 nM) significantly inhibits these IGFprotective effects (p<0.01). Mevalonic acid depletion impairs IGFR processing, decreases the expression of mature IGF receptors at the cell surface, and inhibits the down-stream activation of Akt and MAPK. Inhibitors of N-linked glycosylation (tunicamycin, deoxymannojirimycin and deoxynojirimycin) also markedly attenuates the inhibitory effect of IGF-1 on -glycerophosphate-induced mineralisation (p<0.05), and activation of Akt and MAPK. These results demonstrate that alterations in the glycosylation of the IGFR disrupt the ability of IGF-I to protect against the osteogenic differentiation and mineralisation of VSMCs by several interrelated mechanisms: decreased IGFR processing, reduced IGFR cell-surface expression and reduced downstream signalling via the Akt and MAPK pathways. IGF-I thus occupies a critical position in the maintenance of normal vascular smooth muscle cell phenotype and protection from factors known to stimulate vascular calcification.