Lipoprotein Requirements for Secretion of Ultraviolet-Absorbing Corticosteroids by Guinea Pig Adrenocortical Cellsin Vitro: InnerVersusOuter Cortices; Zona GlomerulosaVersusZona Fasciculata*

Abstract

Most studies of lipoprotein requirements for steroid secretion by the adrenal have examined the mixed cell population of the whole gland; none have examined lipoprotein requirements of guinea pig adrenocortical cells. In this study the effect of exogenous lipoprotein on the ability of cells from each of the different regions of the guinea pid adrenal cortex to synthesize and secrete steroids has been analyzed in vitro, under baseline and ACTH-stimulated conditions. Most studies have assessed the effects of lipoprotein on one or a few selected steroids. In this study the effects of lipoprotein and ACTH were examined both by an assay for fluorogenic steroids and by HPLC analysis of the spectrum of UV-absorbing steroids. Guinea pig outer adrenocortices, containing zona glomerulosa and zona fasciculata, maintained in vitro as fragments or as isolated cells, secreted at least 5 times more steroid than the inner cortex, predominantly zona reticularis, and were dependent upon lipoproteins for their secretion. Corticosterone (B) and cortisol (F) were the predominant products of both zones. Aldosterone (Aldo), 18-hydroxycorticosterone (18-OH B), deoxycorticosterone (DOC), 11.beta.-hydroxyandrostenedione (11.beta.-OH And), androstenedione (And), and deoxycortisol were less abundant products of the outer cortex, while the inner cortex secreted only very small amounts of these steroids. Each of the outer cortical cell types secreted a distinct spectrum of steroids. Aldo, 18-OH B, and DOC were characteristic of glomerulosa cells, but B was most prominent. Fasciculata cells secreted primarily F, with 11.beta.-OH And as their next most prominent product. Low density lipoprotein (LDL) enhanced steroid secretion by glomerulosa cells to a greater extent than that by fasciculata cells, but the stimulation of LDL utilization by ACTH was greastest for fasciculata cells. LDL and ACTH also influenced the pattern of steroids by each cell type. Addition of LDL to glomerulosa cells enhanced secretion of DOC and B, but not that of Aldo or 18-OH B. In fasciculata cell cultures, LDL enhanced secretion of both F and 11.beta.-OH And. ACTH, particularly in the presence of LDL, stimulated secretion by glomerulosa cells of Aldo and 18-OH B, as well as that of F, And, and 11.beta.-OH And. The combined presence of ACTH and LDL in fasciculata cell cultures preferentially stimulated secretion of F and B. When the cultures were tested with mevinolin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme in cholesterol biosynthesis, inhibition of secretion was greater for zona glomerulose than for zona fasciculata cells. This suggests that in zona glomerulosa cells, although newly synthesized cholesterol contributes to steroid production, exogenous lipoprotein is essential for maximum secretion. Zona fasciculata cells, on the other hand, must depend more heavily on intracellular cholesterol stores to maintain appropriate levels of secretion in the face of fluctuating LDL levels.