Human aortic smooth muscle cells are insulin resistant at the receptor level but sensitive to IGF1 and IGF2

Whether insulin, at physiological concentrations, has direct effects on vascular smooth muscle cells (VSMCs) remains controversial. Our aim was to characterize the mechanism for insulin resistance in VSMCs. For comparison, the effects of IGF1 and IGF2 were also studied. Cultured human aortic smooth muscle cells (HASMC) were used. Receptor mRNA was analyzed by quantitative reverse transcription PCR and receptor protein by ELISA and western blot. Biological effects were studied by thymidine incorporation and glucose accumulation. In HASMC, both mRNA and protein expression of IGF1 receptors (IGF1 R) were fivefold higher compared to insulin receptor (IR). IR isoform A mRNA was 13-fold more expressed than IR isoform B. IR and IGF1R co-precipitated, indicating the presence of hybrid IR/IGF1R. Phosphorylation of the IGF1R beta-subunit was obtained by IGF1 10(-9)-10(-8) mol/l and IGF2 10(-8) mol/l. IR beta-subunit was phosphorylated by IGF1 10(-8) mol/l but not by insulin. IGF1 stimulated IR substrate-1 and AKT at 10(-8) mol/l and extracellular signal-regulated kinases 1 and 2 at 10(-9)-10(-8) mol/l respectively. IGF1 and 2 at a concentration of 10(-8)-10(-7) mol/l significantly stimulated H-3-thymidine incorporation, whereas insulin did not. C-14-Glucose accumulation was stimulated by IGF1 or IGF2 10(-8)-10(-7) mol/l, and also by insulin 10(-7) mol/l. Our results suggest that IGF1R and hybrid IR/IGF1R are activated by physiological concentrations of IGF1 and 2 in HASMC and this propagates downstream signaling and biological effects, while insulin has no effect on its receptor or downstream signaling probably due to a preponderance of IGF1R and incorporation of IR into hybrid IR/IGF1R.