Introduction Insulin resistance predisposes to cardiovascular disease by inducing endothelial dysfunction. Our laboratory has shown that insulin resistance impairs the capacity for endothelial repair. Additionally, we have discovered that a circulating protein, insulin-like growth factor binding protein-1 (IGFBP-1), is potentially protective in the vasculature by stimulating nitric oxide production and enhancing insulin sensitivity. In cross-sectional studies, low IGFBP-1 levels are associated with diabetes and cardiovascular disease. In this project, we investigated whether IGFBP-1 can enhance vascular endothelial repair in insulin resistant mice in vivo and examined potential mechanisms in human endothelial cells and endothelial progenitor cells in vitro.
Methods To assess endothelial regeneration, insulin receptor (IR)+/− mice with or without transgenic over-expression of IGFBP-1 (IGFBP1tg) underwent femoral artery endothelial denuding wire-injury. After five days, the area of regenerated endothelium was quantified in en face sections of injured artery. Endothelial regeneration was also assessed ex-vivo in human tissues by seeding segments of endothelium-denuded human saphenous vein with a sub-confluent density of human coronary artery endothelial cells, which were pre-incubated with or without IGFBP-1. Adherent cells were quantified using con-focal microscopy. The effects of IGFBP-1 on the functional properties of endothelial cells in vitro were examined using cell migration assays.
Results Endothelial regeneration following wire injury was blunted in IR+/− mice, however regeneration was significantly improved in this model of insulin resistance by IGFBP-1 overexpression (n=5–10 per group). (Figure 1a) In human umbilical vein endothelial cells (HUVEC), insulin resistance was mimicked by the pro-inflammatory cytokine tumour necrosis factor-alpha, which significantly inhibited migration in a scratch wound assay (n=9). (Figure. 1b) Co-incubation with IGFBP-1 restored the migratory capacity of HUVEC to control levels. There was a trend to enhanced migration of Human Coronary Artery Endothelial Cells (HCAEC) in response to IGFBP-1 in a Boyden chamber assay (P=0.07). IGFBP-1 significantly enhanced endothelial cell adhesion on a human saphenous vein matrix (n=5, p<0.001). (Figure. 2)
Conclusions IGFBP-1 over-expression enhances endothelial repair in the insulin resistant setting. In endothelial cells, IGFBP-1 restores cell migration in a pro-inflammatory setting and improves cell adhesion. Ongoing studies are in progress to examine the underlying mechanisms. Collectively, these data raise the possibility that manipulating IGFBP-1 could be a strategy to enhance endothelial repair in patients with insulin resistance.