Downregulation of high-density lipoprotein receptor in human fibroblasts by insulin and IGF-I

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Abstract
High-density lipoprotein (HDL3) particles bind to a cell surface receptor, thereby promoting the efflux of cholesterol from extrahepatic nonsteroidogenic cells. This receptor appears to be upregulated by increased cell cholesterol content and also may be responsive to the growth state of cells. Because insulin can be mitogenic, the effect of insulin on HDL-receptor function was tested. HDL-receptor activity of cholesterol-loaded fibroblasts was inhibited by insulin treatment. Insulin decreased HDL binding in a log-dose fashion (−25% at 67 nM insulin) in association with increases in [3H]thymidine incorporation into DNA. HDL-mediated cholesterol efflux from cholesterol-loaded cells was diminished by insulin treatment of cells in parallel with decreased HDL binding. Insulin induced reciprocal changes in HDL- and low-density lipoprotein (LDL)-receptor activity. In cells in which these receptors were upregulated by varying cell cholesterol content, insulin increased LDL binding (+ 88%) and decreased HDL binding (−24%). Insulinlike growth factor I (IGF-I, 100 ng/ml) also significantly decreased HDL binding and HDL-mediated cholesterol efflux to a comparable degree. Pooled human serum similarly induced a reduction in HDL binding to its receptor. These results are consistent with the hypothesis that growth factors in general, and insulin and IGF-I in particular, decrease HDL-receptor activity, possibly to promote retention of cholesterol needed for new membrane synthesis during cell proliferation. Such a mechanism could be partly responsible for accumulation of cholesteryl esters in arterial wall cells during atherogenesis in diabetes mellitus.