Sequential Expression of High and Low Density Lipoprotein Receptors in Differentiating Fetal Rat Adrenocortical Cells in Primary Culture*

Abstract

Binding of fluorescent human high density lipoprotein-3 (DIL-HDL3) and fluorescent human low density lipoprotein (DIL-LDL) to differentiating fetal rat adrenocortical cells was studied in primary culture, a system in which addition of ACTH induces differentiation of zona glomerulosa cortical cells into zona fasciculata-type cells. Undifferentiated adrenocortical cells did not bind DIL-HDL3 or DIL-LDL. Differentiation induced by ACTH was accompanied by intense binding of DIL-HDL3 and DIL-LDL to plasma membranes of adrenocortical cells. Binding of DIL-HDL3 and DIL-LDL was saturable and specific, and resulted in enhanced secretion of corticosterone and 18-hydroxydeoxycorticosterone, indicating that the binding sites for these lipoproteins represented functional receptors. Binding of DIL-HDL3 to the surface of adrenocortical cells was detected 6 h after the start of ACTH stimulation, whereas binding of DIL-LDL could not be detected until 12 h after ACTH stimulation was started. Thus, in the ACTH-induced differentiation of adrenocortical cells from zona glomerulosa-type into zona-fasciculata type, exogenous lipoprotein cholesterol is needed for enhanced growth and steroidogenesis, and an initial event in the differentiation of these cells is the sequential expression of specific receptors for HDL3 and LDL.