Background Insulin-like growth factor binding proteins (IGFBP) are key regulators of insulin-like growth factor (IGF) type 1 bioavailability at the cellular level and may also exert IGF-independent effects on target cells. In cross-sectional studies, circulating levels of IGFBP-1 inversely correlate with insulin resistance and vascular disease. In a previous study of transgenic mice overexpressing IGFBP-1 (but with intact fast/feed regulation of this axis), we showed a favourable effect on endothelial function and blood pressure, mediated by increased vascular nitric oxide. We thus sought to explore the hypothesis that IGFBP-1 favourably modulates metabolic homeostasis and vascular endothelial function in the setting of dietary obesity.
Methods Male C57Bl/6 mice with transgenic IGFBP-1 overexpression and littermate controls were fed an obesogenic diet, with subsequent metabolic and non-invasive haemodynamic testing in conscious mice. Endothelial function was assessed ex vivo in the organ bath by testing insulin-mediated blunting of vasoconstriction in excised aorta. These studies were complemented by Western blot assay of aortic endothelial nitric oxide synthase (eNOS) phosphorylation after in-vivo intraperitoneal insulin bolus (0.75 IU/kg). Separate in-vitro studies in the organ bath examined the vasomotor effect of exogenous IGFBP-1 (500 ng/ml) in healthy murine aorta. Western blot was used to study the effects of IGFBP-1 (500 ng/ml) on signalling pathways in cultured human coronary artery endothelial cells.
Results Weight-gain in response to a high-fat diet was unaltered by IGFBP-1 overexpression. However, transgenic obese mice were significantly more insulin-sensitive, glucose-tolerant, and less hypertensive (table). Insulin-mediated inhibition of vasoconstriction in the organ bath is lost in obese wild-type aorta, but relatively preserved in transgenic aorta. Preliminary study of signalling pathways in aorta after insulin challenge suggest preserved eNOS phosphorylation in obese transgenic compared with obese wild-type mice. Exogenous IGFBP-1 abrogates vasoconstriction in healthy lean aorta (fig 1; maximal constriction 0.46 ± 0.05 g with IGFBP-1, 0.67 ± 0.05 g without IGFBP-1, p = 0.02, n = 5), an effect that was completely abolished by the eNOS inhibitor l-NMMA. Moreover, ser1177eNOS phosphorylation in cultured endothelial cells was significantly increased by IGFBP-1 (fig 2; normalised ser1177eNOS level compared with control 202 ± 32%, p = 0.03, n = 5).
Implications These data demonstrate novel actions of IGFBP-1, with rescue of endothelial function in a model of insulin-resistance through preserved insulin-sensitivity and increased nitric oxide production. Attenuated IGFBP-1 levels in insulin-resistant states are thus implicated in endothelial dysfunction and pro-atherogenesis.