Insulin excess counteracts the effects of HDL on intracellular sterol accumulation in cultured human skin fibroblasts

Both Type 1 (insulin-dependent) and Type 2 (non-insulin-dependent) diabetic individuals are at increased risk of developing ischaemic heart disease. Insulin excess, present in both diabetic groups, may play an important pathophysiologic role in accelerating the atherogenic process. In this study, cultured human skin fibroblasts were incubated with varying concentrations of insulin to test the role of insulin on cell cholesterol homeostasis and on HDL3-mediated removal of excess cholesterol from cells. Insulin excess (1-100 nmol/l) resulted in a significant dose-dependent reduction in HDL3-mediated cholesterol efflux from the intracellular unesterified cholesterol pool of cultured human skin fibroblasts. Similar insulin concentrations resulted in impaired HDL-mediated cholesterol efflux from the cell membrane but had no effect on non-HDL-mediated efflux. The effect of insulin on cholesterol esterification and biosynthesis was assessed by 14C-oleate labelling. The addition of HDL3 (50 micrograms) resulted in a significant decrease in 14C-labelled cholesterol ester, reflecting a decrease in intracellular unesterified cholesterol, which was partially reversed by the addition of insulin. Insulin had no effect on the incorporation of 14C-oleate into unesterified cholesterol. During simultaneous incubation of fibroblasts with LDL and HDL, insulin resulted in an increase in cholesterol esterification and inhibited ability of HDL to promote the decrease in esterification. Thus, we have shown that insulin excess counteracts the beneficial effects of HDL that involve removal of cellular cholesterol and may in part promote atherogenesis by this mechanism.