A comparison of the esterification of steroids by rat lecithin:cholesterol acyltransferase and acyl coenzyme A:cholesterol acyltransferase

17β-estradiol (E2) in membranes: Orientation and dynamic properties

Non-genomic membrane effects of estrogens are of great interest because of the diverse biological activities they may elicit. To further our understanding of the molecular features of the interaction between estrogenic hormones and membrane bilayers, we have determined the preferred orientation, location, and dynamic properties of 17β-estradiol (E2) in two different phospholipid membrane environments using (2)H-NMR and 2D (1)H-(13)C HSQC in conjunction with molecular dynamics simulations. Unequivocal spectral assignments to specific (2)H labels were made possible by synthesizing six selectively deuterated E2 molecules. The data allow us to conclude that the E2 molecule adopts a nearly "horizontal" orientation in the membrane bilayer with its long axis essentially perpendicular to the lipid acyl-chains. All four rings of the E2 molecule are located near the membrane interface, allowing both the E2 3-OH and the 17β-OH groups to engage in hydrogen bonding and electrostatic interactions with polar phospholipid groups. The findings augment our knowledge of the molecular interactions between E2 and membrane bilayer and highlight the asymmetric nature of the dynamic motions of the rigid E2 molecule in a membrane environment.

The orientation and dynamics of estradiol and estradiol oleate in lipid membranes and HDL disc models

Estradiol (E2) and E2 oleate associate with high-density lipoproteins (HDLs). Their orientation in HDLs is unknown. We studied the orientation of E2 and E2 oleate in membranes and reconstituted HDLs, finding that E2 and E2 oleate are membrane-associated and highly mobile. Our combination of NMR measurements, molecular dynamics simulation, and analytic theory identifies three major conformations where the long axis of E2 assumes a parallel, perpendicular, or antiparallel orientation relative to the membrane's z-direction. The perpendicular orientation is preferred, and furthermore, in this orientation, E2 strongly favors a particular roll angle, facing the membrane with carbons 6, 7, 15, and 16, whereas carbons 1, 2, 11, and 12 point toward the aqueous phase. In contrast, the long axis of E2 oleate is almost exclusively oriented at an angle of ∼60° to the z-direction. In such an orientation, the oleoyl chain is firmly inserted into the membrane. Thus, both E2 and E2 oleate have a preference for interface localization in the membrane. These orientations were also found in HDL discs, suggesting that only lipid-E2 interactions determine the localization of the molecule. The structural mapping of E2 and E2 oleate may provide a design platform for specific E2-HDL-targeted pharmacological therapies.

Acyl-CoA:cholesterol acyltransferases (ACATs/SOATs): Enzymes with multiple sterols as substrates and as activators

Cholesterol is essential to the growth and viability of cells. The metabolites of cholesterol include: steroids, oxysterols, and bile acids, all of which play important physiological functions. Cholesterol and its metabolites have been implicated in the pathogenesis of multiple human diseases, including: atherosclerosis, cancer, neurodegenerative diseases, and diabetes. Thus, understanding how cells maintain the homeostasis of cholesterol and its metabolites is an important area of study. Acyl-coenzyme A:cholesterol acyltransferases (ACATs, also abbreviated as SOATs) converts cholesterol to cholesteryl esters and play key roles in the regulation of cellular cholesterol homeostasis. ACATs are most unusual enzymes because (i) they metabolize diverse substrates including both sterols and certain steroids; (ii) they contain two different binding sites for steroidal molecules. In mammals, there are two ACAT genes that encode two different enzymes, ACAT1 and ACAT2. Both are allosteric enzymes that can be activated by a variety of sterols. In addition to cholesterol, other sterols that possess the 3-beta OH at C-3, including PREG, oxysterols (such as 24(S)-hydroxycholesterol and 27-hydroxycholesterol, etc.), and various plant sterols, could all be ACAT substrates. All sterols that possess the iso-octyl side chain including cholesterol, oxysterols, various plant sterols could all be activators of ACAT. PREG can only be an ACAT substrate because it lacks the iso-octyl side chain required to be an ACAT activator. The unnatural cholesterol analogs epi-cholesterol (with 3-alpha OH in steroid ring B) and ent-cholesterol (the mirror image of cholesterol) contain the iso-octyl side chain but do not have the 3-beta OH at C-3. Thus, they can only serve as activators and cannot serve as substrates. Thus, within the ACAT holoenzyme, there are site(s) that bind sterol as substrate and site(s) that bind sterol as activator; these sites are distinct from each other. These features form the basis to further pursue ACAT structure-function analysis, and can be explored to develop novel allosteric ACAT inhibitors for therapeutic purposes. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

17β-Estradiol and estradiol fatty acyl esters and estrogen-converting enzyme expression in adipose tissue in obese men and women

CONTEXT

Obesity is associated with increased circulating 17β-estradiol (E₂), but less is known about E₂ concentrations in adipose tissue. In addition to E₂, adipose tissue synthesizes E₂ fatty acyl esters (E₂-FAE).

OBJECTIVE

The aim was to compare estrogen concentrations and expression of estrogen-converting enzymes in adipose tissue between severely obese men and women.

DESIGN AND SETTING

Tissue samples were obtained during elective surgery in University Central Hospital in the years 2008 through 2011.

PATIENTS

We studied 14 men and 22 premenopausal women undergoing bariatric surgery and 10 control women operated for nonmalignant reasons.

INTERVENTIONS

Paired samples were taken from abdominal sc and visceral adipose tissue and serum and analyzed for E₂ and E₂-FAE by fluoroimmunoassay and liquid chromatography-tandem mass spectrometry. mRNA expression of genes was analyzed by quantitative PCR.

RESULTS

Compared with men, E₂ levels in sc adipose tissue in obese women were higher, along with higher relative mRNA expression of steroid sulfatase and 17β-hydroxysteroid dehydrogenases 1, 7, and 12. In men, E₂-FAE concentrations in adipose tissue were similar to E₂ but in women significantly lower compared with E₂. Adipose tissue E₂-FAE and serum E₂-FAE levels correlated positively in obese subjects. Serum E₂ did not significantly correlate with E₂ concentration or mRNA expression of genes in adipose tissue in obese men or women.

CONCLUSIONS

The production of E₂ by the large adipose mass was not reflected by increased circulating E₂ concentrations in severely obese men or women. However, adipose tissue may contribute to concentrations of serum E₂-FAE.

Fatty acyl esterification and deesterification of 17β-estradiol in human breast subcutaneous adipose tissue

CONTEXT

Adipose tissue has an important role in peripheral estrogen synthesis. One of the metabolic pathways of estradiol (E(2)) is its conversion to lipophilic fatty acyl esters.

OBJECTIVE

The aim was to study the metabolism of E(2) fatty acyl esters in adipose tissue and, specifically, the role of hormone-sensitive lipase (HSL) in steroid ester hydrolysis.

DESIGN AND SETTING

Tissue samples were obtained during elective surgery in University Central Hospital in the years 2008-2011.

PATIENTS

Women undergoing reduction mammoplasty (n = 27) or surgery for breast cancer (n = 16) participated in the study.

INTERVENTIONS

Two sc adipose tissue samples were taken from different quadrants of the breast. Radiolabeled steroids were incubated with tissue homogenate (esterase assay) or microsomal fraction (acyl transferase assay). E(2) and E(2) fatty acyl ester concentrations were determined by fluoroimmunoassay or liquid chromatography-tandem mass spectrometry.

MAIN OUTCOME MEASURES

We evaluated the hydrolysis rate of E(2) fatty acyl esters as well as the esterification rate of E(2); we also related tissue concentrations of E(2) and E(2) esters to serum estrogen concentrations.

RESULTS

Compared to esters of dehydroepiandrosterone and cholesterol, the hydrolysis of E(2) esters was much slower, whereas the esterification rate of E(2) was higher. The hydrolysis of E(2) esters in adipose tissue was reduced by 33-51% by inhibition of HSL. Estrogen concentration in sc adipose tissue was higher than in serum in both pre- and postmenopausal women.

CONCLUSIONS

E(2) fatty acyl esters in adipose tissue surrounding the mammary gland may act as a reservoir for conversion back to biologically active E(2). This is partly dependent on HSL activity.

Role of lysosomal acid lipase in the intracellular metabolism of LDL-transported dehydroepiandrosterone-fatty acyl esters

Dehydroepiandrosterone-fatty acyl esters (DHEA-FAE) belong to a unique family of naturally occurring hydrophobic steroid hormone derivatives that are transported in circulating lipoproteins and may act as a source of dehydroepiendrosterone (DHEA) and other biologically active steroid hormones in cells. Here, we studied the metabolic fate of low-density lipoprotein-associated [(3)H]DHEA-FAE ([(3)H]DHEA-FAE-LDL) and the possible role of lysosomal acid lipase (LAL) in the hydrolysis of DHEA-FAE in cultured human cells. When HeLa cells were incubated with [(3)H]DHEA-FAE-LDL, the accumulation of label in the cellular fraction increased with incubation time and could be inhibited by excess unlabeled LDL, suggesting LDL receptor or LDL receptor-related receptor-dependent uptake. During 48 h of chase, decreasing amounts of [(3)H]DHEA-FAE were found in the cellular fraction, while in the medium increasing amounts of unesterified [(3)H]DHEA and its two metabolites, [(3)H]-5alpha-androstanedione (5alpha-adione) and [(3)H]androstenedione (4-adione), appeared. As LDL-cholesteryl ester hydrolysis is dependent on LAL activity, we depleted LAL from HeLa cells using small interfering RNAs and compared the hydrolysis of [(3)H]DHEA-FAE-LDL and [(3)H]cholesteryl-FAE-LDL. The results demonstrated a more modest but significant reducing effect on the hydrolysis of [(3)H]DHEA-FAE compared with [(3)H]cholesteryl-FAE. Moreover, experiments in LAL-deficient human fibroblasts (Wolman disease patient cells) showed that [(3)H]DHEA-FAE hydrolysis was not completely dependent on LAL activity. In summary, LDL-transported [(3)H]DHEA-FAE entered cells via LDL receptor or LDL receptor-related receptor-mediated uptake, followed by intracellular hydrolysis and further metabolism into 5alpha-adione and 4-adione that were excreted from cells. Although LAL contributed to the deesterification of DHEA-FAE, it was not solely responsible for the hydrolysis.

Estradiol fatty acid esters in adipose tissue and serum of pregnant and pre- and postmenopausal women

CONTEXT

The 17beta-estradiol fatty acid esters are hormone derivatives with long-lasting estrogenic effect. They are transported in serum lipoproteins and thought to be sequestered in adipose tissue.

OBJECTIVE

Our objective was to determine the 17beta-estradiol fatty acid ester concentrations in serum and adipose tissue in women of various hormonal states.

DESIGN

After several chromatographic steps separating esterified from free estradiol, time-resolved fluoroimmunoassay was used as a quantifying tool.

PARTICIPANTS

Samples were obtained from pregnant women undergoing cesarean section (n = 13), or premenopausal (n = 8) and postmenopausal women (n = 6) during gynecological surgery.

MAIN OUTCOME MEASURES

17beta-Estradiol and 17beta-estradiol fatty acid ester concentrations in serum, and visceral and sc adipose tissue were examined.

RESULTS

The ratio of esterified to free estradiol in plasma increased with decreasing estradiol level from 0.5% in pregnant, to 15% in premenopausal and 110% in postmenopausal women. Estradiol esters constituted about 10% of the free estradiol present in adipose tissue in pregnancy. In nonpregnant women, most of the adipose tissue estradiol was in esterified form, the median ester to free ratio being elevated to 150-490%. After menopause, the overwhelming majority of estradiol in both free and esterified form was present in adipose tissue.

CONCLUSIONS

The overall higher ester to free estradiol ratio in adipose tissue than in serum indicates active esterification capacity in adipose tissue. The predominance of esterified and free estradiol in postmenopausal adipose tissue compared with serum suggests in situ production and storage. Whether the estradiol esters have an independent physiological role in adipose tissue remains to be clarified.

Postmenopausal estrogen therapy and serum estradiol fatty acid esters in women with and without previous intrahepatic cholestasis of pregnancy

BACKGROUND

Fatty acid esters of 17beta-estradiol (E2) are estrogen metabolites associated with lipoproteins in blood.

AIM

To study the effects of estrogen therapy on concentrations of serum E2 fatty acid esters in postmenopausal women with a history of an estrogen-related liver disorder, intrahepatic cholestasis of pregnancy (ICP), and in healthy women in a double-blind, crossover fashion.

METHOD

ICP (n = 10) and control women (n = 10) received increasing doses of E2 valerate orally 2-4 mg/day, or transdermal E2 50-100 microg/day for 6 weeks. After a 4-week wash-out period, the subjects crossed over to the alternate E2 treatment. Concentrations of serum E2 fatty acid esters were quantified after saponification by fluoroimmunoassay.

RESULTS

Oral E2 administration increased median serum E2 fatty acid ester concentrations from 57 to 73 pmol/L in the ICP and from 56 to 74 pmol/L in the control group, in association with elevations in serum E2, estrone and sex hormone-binding globulin levels. Transdermal E2 treatment did not increase serum E2 ester levels.

CONCLUSIONS

The increase in serum E2 fatty acid esters during oral E2 administration may be attributed, at least partly, to the higher estrogen dose during oral compared with transdermal therapy. A history of ICP did not affect esterification of E2 during estrogen therapy.

Antioxidant protection of LDL by physiological concentrations of 17 beta-estradiol. Requirement for estradiol modification

BACKGROUND

Exposure to estrogens reduces the risk for coronary artery disease and associated clinical events; however, the mechanisms responsible for these observations are not clear. Supraphysiological levels of estrogens act as antioxidants in vitro, limiting oxidation of low-density lipoprotein (LDL), an event implicated in atherogenesis. We investigated the conditions under which physiological concentrations of 17 beta-estradiol (E2) inhibit oxidative modification of LDL.

METHODS AND RESULTS

Plasma incubated with E2 (0.1 to 100 nmol/L) for 4 hours yielded LDL that demonstrated a dose-related increase in resistance to oxidation by Cu2+ as measured by conjugated diene formation. This effect was dependent on plasma, because incubation of isolated LDL with E2 at these concentrations in buffered saline produced no effect on Cu(2+)-mediated oxidation. Incubation of plasma with E2 had no effect on LDL alpha-tocopherol content or cholesteryl ester hydroperoxide formation during the 4-hour incubation. Plasma incubation with [3H]E2 was associated with dose-dependent association of 3H with LDL. High-performance liquid chromatographic analysis of LDL derived from plasma incubated with [3H]E2 indicated that the majority of the associated species were not detectable as authentic E2 but as nonpolar forms of E2 that were susceptible to base hydrolysis consistent with fatty acid esterification of E2. Plasma-mediated association of E2 and subsequent antioxidant protection was inhibited by 5,5'-dithiobis(2-nitrobenzoic acid), an inhibitor of plasma acyltransferase activity.

CONCLUSIONS

Exposure of LDL to physiological levels of E2 in a plasma milieu is associated with enhanced resistance to Cu(2+)-mediated oxidation and incorporation of E2 derivatives into LDL. This antioxidant capacity may be another means by which E2 limits coronary artery disease in women.