Abstract 4883: Insulin-like Growth Factor Binding Protein-3 (IGFBP3) Modulates Vascular Tone via Scavenging Receptor-B1 (SRB1)-Dependent Nitric Oxide Release

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Purpose: We recently reported the vascular protective effect of IGFBP3 in experimental models of ischemic vascular injury involving increased homing of circulating endothelial progenitor cells and activation of nitric oxide synthase (eNOS). The vascular protective potential of a molecule is determined in part by its ability to facilitate perfusion to the ischemic areas by dilation of the resident vasculature. Therefore, we hypothesized that IGFBP3 regulates vascular tone by causing vasodilation and thereby, enhances the perfusion of vascular bed. Methods: We evaluated the effect of intraluminal IGFBP3 on the pressure- and agonist-induced constrictions in rat posterior cerebral arteries (PCAs) and compared with that of HDL, a known activator of SRB1. NO release and endothelial intracellular Ca2+ ([Ca2+]i) were evaluated by DAF-FM and Fura-2 fluorescence microscopy, respectively, in intact arteries and human microvascular endothelial cells (HMVECs). Signaling pathways mediating the effects of IGFBP3 were delineated in intact arteries and HMVECs. Activation of eNOS by IGFBP3 was evaluated by Western blotting of ser1177 phosphorylated eNOS in HMVECs. IGF-independent effect was confirmed by using a non-IGF-binding mutant of IGFBP3. Results: Intraluminal IGFBP3 (100ng/ml) or HDL (1mg/ml) caused significant decrease in pressure- (40, 70 and 100 mmHg) and serotonin-induced constrictions of PCAs compared to the vehicle-control. These effects were blocked by L-NAME or SRB1-function blocking antibody (SRB1-ab). NO release was stimulated in PCAs by intraluminal IGFBP3 and HDL with no change in the [Ca2+]i. NO release by IGFBP3, with no change in the [Ca2+]i, in HMVECs was blocked by SRB1-ab, PP2 (cSrc inhibitor), LY294002 (PI3-kinase inhibitor) and triciribin (akt inhibitor) not by KN-92 (CamKII inhibitor). Non-IGF-binding mutant and wild type IGFBP3 showed similar effect in stimulating NO release in HMVECs. Phosphorylation of eNOS at ser1177 was increased in a time-dependent manner by IGFBP3 that was blocked by SRB1-ab. Conclusions: These results suggest that IGFBP3 modulates vascular tone by NO release via SRB1-dependent cSrc/PI3-kinase/akt pathway and this response may significantly contribute to the vascular protective effect of IGFBP3.