Regulation and actions of insulin-like growth factor binding proteins in vascular smooth muscle cells

Abstract: Insulin-like growth factor-I (IGF-I) has multiple actions on vascular smooth muscle cell (VSMCs) function. In the circulation and in extracellular fluids, IGF-I is complexed to high affinity IGF-binding proteins (IGFBPs), which modulate IGF-I bioactivity. The present study focused on expression and regulation of IGFBPs in VSMCs. and how they modulate IGF-1 effects in these cells.Rar VSMCs expressed mRNAs for IGFBP-2, -4 and -6, while human VSMCs expressed mRNAs for IGFBP-2, -3, -4, -5 and -6 as determined by solution hybridization. IGF-1 (10-8 M) increased and angiotensin TI (AII, 10-6 M) decreased IGFBP-2 and IGFBP-4 mRNA in rat VSMCs. Furthermore, AII and All and 1GF-I in combination increased IGF-I receptor mRNA and All decreased IGF-1 mRNA. We detected endogenous 1GFBP-2 and IGFBP-4 by Western immuno blot in concentrated conditioned medium of rat VSMCs. IGF-I did not affect steadystate levels of IGFBP-2 and -4, but increased the amount of fragments of these IGFBPs. Exogenously added IGFBP-2 and -4 were minimally degraded in serum free conditioned medium for up to 72h in the presence of VSMCs, but immunoreactive bands of the intact IGFBPs disappeared in the presence of IGF-I. We detected IGFBP-2, -4, -5 and -6 in concentrated conditioned medium of human VSMCs.IGFBP-1 and IGFBP-4 inhibited 1GF-1 (1 nM) induced DNA synthesis in rat VSMCs with IC50 values of 1.6 and 6.2 nM respectively, and they also inhibited IGF-I induced protein synthesis. IGFBP-2, if preincubated with IGF-I, also acted inhibitory on IGF-I action. IGFBP-1, -3 and- 4 (1.4-2.0 nM) inhibited IGF-I (I nM) induced DNA synthesis in human VSMCs, while IGFBP-2, -5 and -6 had no effects in these concentrations.IGF-I and AII had additive effects on DNA- and protein synthesis in rat VSMCs and these effects occurred at doses, at which the peptides added alone initiated DNA synthesis. Losartan blocked the synergistic effects and IGFBP-1, which inhibited IGF-I action, was not able to inhibit the action of AII.In conclusion, rat and human VSMCs express IGFBP-2, -4 and -6, but not IGFBP-1. The results on IGFBP-3 and IGFBP-5 are inconsistent. IGFBP-1, -3 and -4 act at physiological concentrations inhibitory on IGF-I stimulated effects. IGFBP-2 and IGFBP-4 are metabolized by proteases from rat and human VSMCs and the degradation of these IGFBPs is enhanced by IGF-I. There is an interaction of AII and the IGF-system in VSMCs and IGF-I and All have additive effects on DNA- and protein synthesis in these cells.