Cholesterol Availability and Adrenal Steroidogenesis

Steroid hormones are crucial for the regulation of many aspects of metabolism, homeostasis, immune function, reproduction, and development. Cholesterol is the precursor for the synthesis of steroid hormones by a succession of enzyme-catalyzed reactions. While studies of the regulation of steroidogenesis often focus on these enzymes, availability of cholesterol is also crucial for steroid synthesis. Its optimal intracellular levels for steroidogenesis and other essential functions are maintained through acquisition of cholesteryl esters from circulating lipoproteins, de novo synthesis, and retrieval of stored cholesteryl esters in lipid droplets, balanced by its removal by use, including for steroid synthesis, esterification for storage, or extrusion from the cell (1, 2). A recent publication addresses the importance of the adenosine triphosphate (ATP)-binding cassette transporter G1, which transfers intracellular cholesterol to circulating high-density lipoprotein (HDL), in decreasing its availability for steroidogenesis (3).

The initial step in steroid biosynthesis requires transport of cholesterol in the cytosol from the outer mitochondrial membrane into the inner mitochondrial membrane by the steroidogenesis acute regulator protein (StAR) where the CYP11A1 enzyme transforms cholesterol to pregnenolone, the starting steroid for all the steroidogenic cascades (4); however, regulation of the availability and presentation of cholesterol for StAR action in mitochondria is similarly important (1). While cholesterol synthesis within the cell by the rate-limiting enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase can provide the cholesterol needed for steroidogenesis, most of the adrenal cholesterol normally comes from circulating lipoproteins. There are 2 paths for circulating cholesterol uptake in the adrenal steroidogenic cells, low-density lipoprotein (LDL) receptor–mediated endocytosis and “selective” uptake by the HDL scavenger receptor, class B type I (SR-B1). Internalized LDL containing the esterified cholesterol is degraded in lysosomes, releasing cholesteryl esters that are hydrolyzed by the lysosomal acid lipase to produce free cholesterol for steroid synthesis. Cholesterol sequestered by SR-B1 remains in the plasma membrane and is mobilized for steroidogenesis on stimulation. In humans, the LDL receptor plays a greater role in providing cholesterol to adrenal steroidogenic cells; SR-B1 is more prevalent in rodents (1). Disruption of LDL function, or human carriers of functional mutations in the SR-B1, result in subtle abnormalities in cortisol response to adrenocorticotropin (ACTH) stimulation, which is more prominent in the case of SR-B1–knockout (KO) mice as predicted (2).

Unused, free cholesterol within the cell is reesterified by the Acyl CoA:cholesterol acyltransferse and stored in lipid droplets (1). On stimulation by ACTH, the cholesterol esters in the lipid droplets are hydrolyzed by the endosomal hormone-sensitive lipase close to the mitochondria where the StAR protein resides (5).

Lipoproteins appear to have roles in the regulation of adrenal steroidogenesis in addition to providing cholesterol (6, 7). In cell culture studies, HDL and very LDL stimulate aldosterone biosynthesis through a signaling cascade similar to that of angiotensin II by promoting the phosphorylation of the regulatory transcription factor cyclic adenosine monophosphate response element-binding protein (CREB), resulting in mobilization of cytosolic calcium levels, stimulation of phospholipase D, and expression of StAR protein and aldosterone synthase (CYP11B2) (6, 7). The mechanism(s) for this effect is not clear. Endogenous HDL is a carrier of many different proteins, one or several of which may mediate the stimulatory effect, as synthetic HDL does not have a similar effect (8). Whether these lipoproteins have a signaling role in regulating steroidogenesis in addition to being the main source of cholesterol esters to the adrenal in vivo is yet unknown.

Intracellular free cholesterol not used for the synthesis of steroids is reesterified and deposited into lipid droplets, or extruded from the cell through ATP-binding cassettes G1 and A1 and locally produced apolipoprotein E (2, 3). The roles of the latter proteins appear to be more complex than simply mediating cholesterol efflux from the cell. The family of ATP-binding cassette proteins uses ATP hydrolysis to transport a variety of substrates to multiple acceptors including LDL, HDL, and liposomes (2). Fasting stress in normal mice induces a significant increase in the messenger RNA expression of the SR-B1, a decrease in Ldlr, Apoe, and the ATP-binding cassette genes Abcg1 and Abca1 required for cholesterol transport, with no change in the Hmgcr required for cholesterol production (2). The impaired efflux of cholesterol in Apoe-KO mice is associated with increased secretion of glucocorticoids induced by stress, while mice with a global KO of the Abcg1 cassette showed an increase in corticosterone in response to fasting stress or ACTH challenge that was significantly less than in wild-type mice (2). The Abcg1-KO mice had no changes in adrenal weight or the relative messenger RNA expression of the various steroidogenic enzymes, a decrease in neutral lipid accumulation, a nonsignificant decrease in the adrenal levels of esterified cholesterol, and an increase in Apoe and Abca1 expression that also mediate the efflux of cholesterol from the adrenal (2). In contrast, the 40% reduction of Abcg1 specifically in the adrenal cortex did not alter levels of Abca1 expression, but paradoxically increased the expression of genes that promote cholesterol uptake, Ldlr, and synthesis, Hmgcr and Sqle. Insig1, which encodes the insulin-induced gene, was also increased, suggesting that cholesterol metabolism was dysregulated. Intracellular cholesterol pools, the expression of steroidogenic enzymes in the adrenal, glucocorticoid-responsive genes in the liver, and plasma ACTH were unchanged; however, there was a significant increase in corticosterone (3). Inactivation of the adrenal Abca1 gene in the adrenal cortex did not alter plasma ACTH or total adrenal steroid synthesis, nor was it reported to have similar effects on cholesterol transport that the Abcg1 partial inactivation did (3).

The efficacy of mechanisms mediating and regulating the efflux of cholesterol from adrenal steroidogenic cells is unclear. Adrenocortical cells of patients with congenital lipoid hyperplasia due to loss-of-function mutations of the StAR protein or StAR-KO mice have a significant increase in lipid droplets, suggesting that the efflux mechanism cannot compensate when use of cholesterol in the early phase of steroid synthesis is abnormal (4). However, the seemingly contradictory results suggest that Abcg1 affects adrenal steroidogenesis by mechanisms in addition to mediating cholesterol efflux from the adrenal cortical cell.

Abbreviations

ACTH

adrenocorticotropin

ATP, HDL

high-density lipoprotein

KO

knockout

LDL

low-density lipoprotein

SR-B1

scavenger receptor, class B type I

StAR

steroidogenesis acute regulator protein

Funding

C.E.G.S. is supported by the National Heart, Lung and Blood Institute (grant No. R01 HL144847), the National Institutes of General Medical Sciences (grant No. U54 GM115428), and the US Department of Veteran Affairs (grant No. BX00468).

Disclosures

C.E.G.S. and E.P.G.S. are editorial board members of Endocrinology.