The effect of hypocholesterolemic drug treatment on adrenocortical cell function

Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis

The steroidogenic tissues have a special requirement for cholesterol, which is used as a substrate for steroid hormone biosynthesis. In many species this cholesterol is obtained from plasma lipoproteins by a unique pathway in which circulating lipoproteins bind to the surface of the steroidogenic cells and contribute their cholesteryl esters to the cells by a 'selective' process in which the whole lipoprotein particle does not enter the cell. This review describes the lipoprotein selective cholesteryl ester uptake process and its specific partnership with the HDL receptor, scavenger receptor class BI (SR-BI). It describes the characteristics of the selective pathway, and the molecular properties, localization, regulation, anchoring sites and potential mechanisms of action of SR-BI in facilitating cholesteryl ester uptake by steroidogenic cells.

Distinctive properties of adrenal cortex mitochondria

The mitochondria in cells that synthesize steroid hormones not only have enzymes not present in mitochondria of non-steroidogenic cells but also have unique mechanisms for regulating the steroid substrate availability for certain of these enzymes. We have considered in detail the cytochrome P-450scc system that is located in the inner mitochondrial membrane and that catalyzes the initial and rate-determining step in the steroid hormone biosynthetic pathway. The flux through this pathway is regulated both by the levels of these catalysts themselves and by the availability of the substrate cholesterol for conversion to pregnenolone. These two levels of regulation occur in different time frames but are both controlled externally by the action of tissue-specific peptide hormone. We have used the adrenal cortex fasciculata cells as our paradigmatic cell type. The overall picture seems closely similar for mitochondria in other such steroidogenic cells when analogous data are available. Thus, in adrenal cortex fasciculata cells ACTH triggers several long-term (trophic) and short-term (acute) effects upon and within mitochondria that influence the initial and rate-determining step in the steroid hormone biosynthetic pathway. The only second messenger for both effects characterized thus far is cAMP. An increase in membrane-associated cAMP rapidly activates cAMP-dependent protein kinase, which in turn phosphorylates several cellular proteins, e.g., cholesterol ester hydrolase (vide supra). The trophic action, i.e., that produced by exposure of the cells to increased levels of ACTH or cAMP for a prolonged period (minutes to hours), increases the amounts of the steroid hormone synthesizing proteins in the mitochondria by increasing the transcription of the relevant nuclear genes. This latter process is not needed for the acute increase in the rate of steroid hormone biosynthesis. Whether induction of steroidogenic enzymes requires activation of a kinase has not been determined. However, the postulated SHIP proteins provide a mechanism by which cAMP levels and protein synthesis itself may regulate this induction. Mitochondria in steroidogenic tissues exert control over this process by their ability to recognize, import and process correctly the nuclear encoded precursors of the steroidogenic enzymes. Whether control at this level is ultimately dictated by nuclear or mitochondrial gene products or by an interplay between them is still unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of endogenous and exogenous cholesterol on the ultrastructure and steroid secretion of undifferentiated rat adrenocortical cells in primary culture

The present study was undertaken to define the effects of lipoprotein-derived cholesterol and endogenous, de novo synthesized cholesterol on the ultrastructure and function of undifferentiated rat adrenocortical cells [lipoprotein (HDL3 and LDL) receptor-negative, zona glomerulosa-like adrenocortical cells] in primary culture. For this purpose human plasma high density lipoprotein (HDL3) or low density lipoprotein (LDL) was added to culture medium devoid of cholesterol. Steroid secretion remained at the low basal level even after addition of lipoproteins, and the amount of intracellular lipid droplets did not increase. When mevinolin (0.96 microgram/ml), an inhibitor of cholesterol synthesis, was added to the culture medium, a low secretion of corticosterone was measured both in serum-free and serum-containing media. Ultrastructurally, lipid droplets disappeared after treatment with mevinolin in both media used. At this concentration of mevinolin cell proliferation was similar to that in the controls, but at higher concentrations (4.8 or 9.6 micrograms/ml) proliferation was inhibited to 42% and 26% in serum-free medium, and 20% and 12% in serum-supplemented medium, respectively. This study demonstrates that cell proliferation and synthesis of corticosterone by undifferentiated rat adrenocortical cells is identical in the absence or presence of exogenous lipoprotein cholesterol. Inhibition of de novo cholesterol synthesis by mevinolin over a period of 7 days does not inhibit corticosterone secretion or proliferation of cells but decreases the amount of intracellular lipid droplets, thus suggesting utilization of intracellular cholesterol esters. However, higher concentrations of mevinolin inhibit proliferation of cells both in serum-free and serum-containing media.

Regulation of steroid hydroxylase gene expression is multifactorial in nature

In summary, regulation of steroid hydroxylase gene expression is complex and multifactorial, involving cAMP-dependent and -independent mechanisms required for maintenance of optimal steroidogenic capacity, tissue-specific mechanisms which lead to different steroidogenic pathways in different tissues, and developmental mechanisms which lead to fetal imprinting of steroid hydroxylase expression and which probably overlap with both maintenance and tissue-specific mechanisms. Future studies will involve identification of the trans-acting factors associated with each of these aspects of the multifactorial regulation and characterization of the cis-regulatory elements to which they bind. Such studies will inevitably lead to the identification of genes encoding these trans-acting factors and investigation of their regulation. In this way, it will be possible to work outward from the steroid hydroxylase genes toward the cell surface receptors in order to elucidate the series of events which lead to cAMP-dependent and -independent regulation of steroid hydroxylase gene expression.

Sex differences in adrenocortical structure and function. XXII. Light- and electron-microscopic morphometric studies on the effects of gonadectomy and gonadal hormone replacement on the rat adrenal cortex

The cytological aspects of sexual dimorphism in the rat adrenal cortex and its relationship to the gonads have been investigated. The adrenal glands of mature female rats were heavier than those of males, and morphometry showed that this was almost exclusively due to conspicuous differences in the volume of cells of the zona fasciculata (ZF) and zona reticularis (ZR). Stereology demonstrated that the volume of the mitochondrial and lipid droplet compartments, as well as the surface area per cell of mitochondrial cristae and smooth endoplasmic reticulum tubules, were markedly higher in the ZF and ZR cells of female animals. Orchiectomy increased and ovariectomy decreased the adrenal weight, by eliciting hypertrophy and atrophy, respectively, of ZF and ZR cells; these effects of gonadectomy were reversed by the appropriate gonadal hormone replacement. It is suggested that the sexual dimorphism of the rat adrenal cortex may depend upon the inhibitory action of testosterone and the stimulatory effect of estradiol on the hypothalamo-hypophyseal-adrenal axis.

Morphological evidence of lysosomal uptake of high-density lipoproteins by rat adrenocortical cells in vitro

The high-density lipoprotein (HDL) pathway in rat adrenocortical cells was studied at the electron microscopic level in vitro via colloidal gold labelling. Steroid hormone assays were performed to confirm that the cells remained intact, viable, responsive to ACTH under the applied conditions, and to reveal the steroidogenic effect of HDL. The gold-labelled HDL particles (HDL-Au) were observed on the surface of the parenchymal cells, often attached to the membranes of the microvilli, but rarely in coated pits and coated vesicles. HDL-Au was accumulated by non-coated vesicles, multivesicular bodies and lysosomes. The lysosomes were identified by means of a non-specific esterase reaction. It is concluded that HDL particles are internalized by both zona glomerulosa and zona fasciculata cells. HDL is required for the enhanced functional activity of these cells in long-term incubation, and the lysosomes are involved in the process.

Control of steroid synthesis in adrenal fasciculata cells

The control of steroid synthesis in adrenal fasciculata cells is considered in terms of two types of control by ACTH: control of cholesterol availability to inner mitochondrial cytochrome P-450scc. This process controls total steroid synthesis and is rapidly activated by ACTH. The several steps in cholesterol transfer are examined. partitioning of metabolism by means of competition between enzymes for limiting amounts of steroid intermediates. Changes in such competition determine the ratio of steroid products from the adrenal cells. Such changes typically are a slower response to ACTH. A critical aspect of such competition is the modulation of multiple activity P-450 cytochromes: P-450(17 alpha) (17 alpha-hydroxylation and 17,20 lyase) and P-450(11 beta) (11 beta- and 18-oxidases). Factors such as substrate binding, electron transfer steps and lipid environment are considered in addition to new enzyme synthesis. The ACTH stimulation of steroid synthesis in bovine adrenal cell primary cultures is examined as a model for both types of regulation.

The role of lysine-rich proteins in the acute steroidogenic response of rat adrenal cells to ACTH

The possibility that a lysine-rich protein was involved in the acute stimulation of steroidogenesis by ACTH was investigated using [3H] labelled lysine and isolated adrenal cells. The results demonstrated that cycloheximide inhibited steroidogenesis in a dose-dependent, rapid fashion and inhibited the incorporation of radioactive lysine into protein. However cells incubated in a lysine-free medium showed the same response to ACTH as cells incubated in a lysine-containing medium. It was also demonstrated that ACTH had no effect on the incorporation of tritiated lysine into the protein or small peptide fractions. These observations suggest that a rapidly synthesised, lysine-rich protein is not involved in the acute response to ACTH.

Correlation of lipid droplet content and steroidogenic capacity in zona glomerulosa and fasciculata cells from lipoprotein-deficient rats

Lipoprotein deficiency was maintained for 48 h by applying 4-aminopyrazolo(3,4-d)pyrimidine in male albino rats and the effect on the structure and function of isolated adrenocortical cells was investigated. Neither the morphology nor the steroidogenic response of zona glomerulosa cells changed; however, in the zona fasciculata cells a marked reduction in the quantity of lipid droplets was associated with a decreased steroidogenic response to ACTH. The results indicate that despite lipoprotein deficiency, zona glomerulosa cells contain a reserve of intracellular cholesterol, thus retaining their responsiveness to stimulation. The fact that the lipoprotein deficiency does not affect lipid droplets located in the glomerulosa cells and in the fasciculata cells in a similar manner is probably due to the differences in their physical state and chemical composition.