IGF-I but not insulin inhibits apoptosis at a low concentration in vascular smooth muscle cells

• Apoptosis of vascular smooth muscle cells (VSMCs) is of importance in the development of diabetic angiopathy. Our aim was to evaluate the effect of insulin and IGF-I on apoptosis in VSMCs. Rat aortic VSMCs were used and apoptosis was induced by serum starvation. As apoptotic markers we measured caspase-3 activity, histone-associated DNA fragments by ELISA and nuclear morphology by DAPI (4',6-diamidino-2-phenylindole) staining. Phosphorylation of IGF-I receptors was evaluated by Western blot. Serum starvation had increased caspase-3 activity even after 3 h. The highest activity was found after 3-12 h. IGF-I 10-9 M inhibited serum starvation-induced caspase-3 activity with a maximal effect after 12 h. When studied after starvation for 12 h, significant inhibitory effects on caspase-3 were found at IGF-I concentrations of 10-8-10-7 M (P<0.01) and at an insulin concentration of 10-6 M (P<0.01). DNA fragmentation was detected by ELISA after 24 h and chromatin condensation and nuclear fragmentation by DAPI staining after 24 and 48 h respectively. IGF-I dose-dependently reduced apoptosis evaluated by ELISA, reaching a maximal effect at 10-9 M. Insulin reduced apoptosis but the effect was weaker and a higher concentration was needed. IGF-I (10-8 M) and insulin at a very high concentration (10-6 M) phosphorylated IGF-I receptors. Taken together, IGF-I and insulin have anti-apoptotic effects on VSMCs but the effect of insulin is only found at high unphysiological concentration.

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