Insulin-like growth factor I receptors are more abundant than insulin receptors in human micro- and macrovascular endothelial cells

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Abstract
Micro- and macroangiopathy are major causes of morbidity and mortality in patients with diabetes. Our aim was to characterize IGF-I receptor (IGF-IR) and insulin receptor (IR) in human micro- and macrovascular endothelial cells. Cultured human dermal microvascular endothelial cells (HMVEC) and human aortic endothelial cells (HAEC) were used. Gene expression was measured by quantitative real-time RT-PCR and receptor protein by ligand-binding assay. Phosphorylation of IGF-IR β-subunit was analyzed by immunoprecipitation and Western blot. Glucose metabolism and DNA synthesis was assessed using [3H]glucose and [3H]thymidine incorporation, respectively. We detected gene expression of IGF-IR and IR in HAEC and HMVEC. IGF-IR gene expression was severalfold higher than that of IR. The specific binding of 125I-IGF-I was higher than that of 125I-insulin in HAEC and HMVEC. Insulin and the new, long-acting insulin analog glargine interacted with the IGF-IR with thousand- and hundred-fold less potency than IGF-I itself. Phosphorylation of the IGF-IR β-subunit was shown in HAEC for IGF-I (10−8 M) and insulin (10−6 M) and in HMVEC for IGF-I and glargine (10−8 M, 10−6 M). IGF-I 10−7 M stimulated incorporation of [3H]thymidine into DNA, and 10−9–10−7 M also the incorporation of [3H]glucose in HMVEC, whereas glargine and insulin had no significant effects at 10−9–10−7 M. Human micro- and macrovascular endothelial cells express more IGF-IR than IR. IGF-I and high concentrations of glargine and insulin activates the IGF-IR. Glargine has a higher affinity than insulin for the IGF-IR but probably has no effect on DNA synthesis at concentrations reached in vivo.