Hormonal regulation of low-density lipoprotein (LDL) receptor activity in human hepatoma Hep G2 cells. Insulin increases LDL receptor activity and diminishes its suppression by exogenous LDL

Abstract

Low-density lipoprotein (LDL) receptors of approximate Mr 130,000 on non-reduced gels have been identified in Hep G2 cells by immuno- and ligand-blotting of cell extracts. Measurement of LDL receptor protein by scanning ligand blots was correlated with the specific binding, uptake and degradation of 125I-labelled LDL by intact cells, confirming that this is mediated by the LDL receptor. Cells incubated in medium with serum expressed significant LDL receptor activity. This increased when cells were transferred to medium containing lipoprotein-deficient serum (LPDS) but was not maximal because a further increase occurred when compactin was included in the medium. Inclusion of 17 alpha-ethinyl estradiol or 17 beta-estradiol in the medium at concentrations up to 500 ng/ml had no effect on LDL receptor activity in the cells as assayed by ligand blotting. Inclusion of insulin (100 mU/ml) in the preincubation medium containing LPDS resulted in a twofold increase in LDL-receptor protein and of LDL binding and degradation by intact cells. Insulin also diminished the suppressive effect of LDL on LDL receptor activity. If insulin exerts this effect in vivo it may partly explain why the liver expresses LDL receptors despite high levels of LDL in plasma and interstitial fluid.