Permissive effect of insulin on the adenosine 3',5'-monophosphate-dependent up-regulation of low density lipoprotein receptors and the stimulation of steroid release in bovine adrenal cortical cells.

Abstract

Bovine adrenal cortical cells growth on extracellular matrix-coated dishes in the presence of F-12 medium supplemented with high density lipoprotein (30 micrograms protein/ml), insulin (50 ng/ml), transferrin (1 microgram/ml), and fibroblast growth factor (100 ng/ml) expressed high affinity low density lipoprotein (LDL) receptors (Kd = 2.0 X 10(-8) M) present at a density of 3 X 10(4) receptors/cell. The density of LDL receptors per cell could be increased 3-fold (9 X 10(4) receptors/cell) by incubating the cells for 24 h with cholera toxin (CT; 10 ng/ml), but not with insulin (100 ng/ml). This correlated with increased LDL internalization (10-fold) and degradation (5-fold). 11-Deoxycortisol (11 DOC) release was increased by 2.7-fold. The addition of insulin (100 ng/ml) together with CT (10 ng/ml) markedly potentiated the effects of CT on LDL binding, internalization, and degradation as well as on steroid release. Preincubation (24 h) of the cells with insulin (100 ng/ml) together with CT resulted in a 12.7-fold increase in high affinity LDL receptor density (3.6 X 10(5) receptors/cell), while LDL internalization and degradation were increased by 45- and 22-fold, respectively. This correlated with a 9.6-fold increase in the 11 DOC released into the medium. The effects of insulin on the induction of high affinity LDL receptors as well as increased internalization and degradation of [125I]LDL were both time and concentration dependent in cultures exposed to CT. Secretion of 11 DOC paralleled in a time-dependent manner the amount of internalized [125I]LDL. Exposure of the cells to chloroquine (100 microM) resulted in a 95% inhibition of [125I]LDL degradation correlating with an 80% decrease in 11 DOC released in cells preexposed to CT and insulin. The results of these studies suggest that in bovine adrenal cortex cells grown in chemically defined medium, insulin modulates steroidogenic cAMP-induced response by increasing both the LDL receptor pathway activity and 11 DOC release.