Insulin-like growth factors (IGF) act in concert with insulin in glucose counter-regulation. Recent studies reveal that both hormones also have important vascular actions. A family of binding proteins (IGFBP) modulate IGF bioavailability at the cellular level. IGFBP-2 is abundant in serum and is expressed in the vascular wall; however, a potential role in vascular homeostasis remains unexplored. We overexpressed IGFBP-2 in transgenic mice in order to determine the role of the protein in vascular physiology.
All studies were conducted in accordance with Home Office regulations for animal experimentation. Transgenic mice were generated on a FVB/N background using a human IGFBP-2 cosmid clone. Transgenic mice overexpressing IGFBBP-2 and their wild-type littermates were studied between 30 and 40 weeks of age. Glucose and insulin levels were measured in the fasting state and after feeding with standard laboratory diet. Systolic blood pressure was recorded in conscious, restrained mice by tail cuff plethysmography on three occasions. Animals were then humanely killed and thoracic aortic rings were studied in vitro for (i) constriction to phenylephrine (PE, 1 nM-10 µM), (ii) relaxation to acetylcholine (ACh, 1 nM-10 µM) and sodium nitroprusside (SNP, 0.1 nM-10 µM), and (iii) maximal constriction to the NO synthase inhibitor L-NMMA (0.1 mM, 30 min). Data were analysed using analysis of variance repeated measures and are expressed as means ± S.E.M. P < 0.05 was considered significant. Results are summarised in Table 1.
Total IGFBP-2 levels were approximately twofold higher in transgenic mice than wild-type as assessed by Western blotting of serum proteins (data not shown). Fasting glucose and insulin levels were similar in transgenic and wild-type mice. Glucose was significantly lower in IGFBP-2 mice after feeding, despite similar insulin levels. Systolic blood pressure was significantly lower in IGFBP-2 mice than in controls. In aortic rings, dose-response curves for constriction to PE and relaxation to ACh and SNP did not differ between groups (maximal responses are shown in Table 1). The contractile response to L-NMMA was similar in IGFBP-2 and wild-type mice.
Thus over-expression of IGFBP-2 in mice is associated with lower blood pressure and blood glucose, despite normal insulin levels. These data support a potential role for IGFBP-2 in enhancing insulin sensitivity and lowering blood pressure. Further studies are warranted to explore the mechanisms involved.
This work was funded by the British Heart Foundation.