Naturally occurring lipoidal derivatives of 3 beta-hydroxy-5-pregnen-20-one; 3 beta,17 alpha-dihydroxy-5-pregnen-20-one and 3 beta-hydroxy-5-androsten-17-one

C19‐5‐ene Steroids in Nature

Dehydroepiandrosterone (DHEA), produced from cholesterol in the adrenals, is the most abundant steroid in our circulation. It is present almost entirely as the sulfate ester, but the free steroid is the form that serves as a precursor of estrogens and androgens, as well as 7- and 16-oxygenated derivatives. Mammalian tissues reduce the 17-keto Group of DHEA to produce androstenediol-a weak estrogen and full-fledged androgen. Its androgen activity is not inhibited by the anti-androgens commonly used to treat prostate cancer. It is probably responsible for the growth of therapy-resistant prostate cancer. DHEA is hydroxylated at the 7 alpha position, and this derivative is oxidized by 11 beta-hydroxysteroid dehydrogenase to form 7-keto DHEA. The latter is reduced by the same dehydrogenase to form 7 beta-hydroxy DHEA. When fed to rats, each of the latter three steroids induce the formation of two thermogenic enzymes in the liver. The late-term human fetus produces relatively large amounts of 16 alphahydroxy DHEA, which serves the mother as a precursor of estriol.

Long-lived testosterone esters in the rat

Over the past decade it has become increasingly clear that steroid hormones are enzymatically esterified with fatty acids. These steroidal esters are the natural analogs of synthetic esters that are used therapeutically. One such family of pharmacological steroids is the synthetic alkyl esters of testosterone, androgens with great hormonal potency. We have investigated whether testosterone esters exist naturally by using the rat as a model. Most tissues of male rats, including blood, have very little if any ester (quantified by immunoassay as a nonpolar saponifiable metabolite), but fat and testes have sizable quantities, approximately 3 ng of testosterone equivalents per g of tissue. Testosterone in fat averages 9 ng/g. The fat from female rats and long-term (> 2 weeks) castrated males has no detectable testosterone ester. The presence of testosterone esters was confirmed by GC/MS, which clearly showed the presence of testosterone in the hydrolyzed ester fraction of fat from intact males but not long-term castrates. Upon castration, testosterone levels in the fat completely disappear within 6 hr. To the contrary, it is not until 48 hr after castration that a measurable fall in the testosterone ester fraction was observed; even after 10 days a small amount of ester is still present in the fat. These experiments demonstrate the existence of a previously unknown androgen with a potentially important physiological impact; testosterone esters, natural analogs of potent therapeutic agents, occur in the fat where they can serve as a reservoir of preformed androgen to stimulate neighboring target tissues.

Aromatase and testosterone fatty acid esters: the search for a cryptic biosynthetic pathway to estradiol esters

The estradiol fatty acid esters (lipoidal derivatives, LE2) are extremely potent estrogens that accumulate in fat, including fat of menopausal women. These steroidal esters are protected from metabolism and are converted to the free, biologically active steroid through the action of esterases. Previous studies have shown that biosynthetic pathways in the adrenal gland exist in which steroid fatty acid esters are substrates. This led us to determine whether a cryptic aromatase pathway exists in which testosterone esters could be converted directly into LE2. We tested a representative fatty acid ester, testosterone stearate, both as an inhibitor and as a substrate for the aromatase enzyme from human placental microsomes. This ester had neither activity. In addition, we tested [1 beta-3H]testosterone acetate as a substrate for this enzyme complex, measuring the production of 3H2O as evidence of aromatization. Although the rate of reaction was considerably slower than that of testosterone, 3H2O was produced. However, when [2, 4, 6, 7-3H]testosterone acetate was incubated and the steroidal products isolated, we found that hydrolysis of the substrate had occurred. Both [3H]-labeled testosterone and estradiol were found, and very little if any [3H]estradiol acetate was formed. Thus, we conclude that an aromatase pathway involving testosterone esters does not exist and that the sole source of LE2 is through direct esterification of estradiol.

Steroidal fatty acid esters

Several years ago we discovered an unexpected family of steroidal metabolites, steroidal fatty acid esters. We found that fatty acid esters of 5-ene-3 beta-hydroxysteroids, pregnenolone and dehydroisoandrosterone are present in the adrenal. Subsequently, others have shown the existence of these non-polar 5-ene-3 beta-hydroxysteroidal esters in blood, brain and ovaries. Currently, almost every family of steroid hormone is known to occur in esterified form. We have studied the esters of the estrogens and glucocorticoids in some detail, and have found that these two steroidal families are esterified by separate enzymes. In a biosynthetic experiment performed simultaneously with estradiol and corticosterone, we established that the fatty acid composition of the steroidal esters is quite different. The corticoid is composed predominantly of one fatty acid, oleate, while the estradiol esters are extremely heterogeneous. Our studies have demonstrated that the estrogens are extremely long-lived hormones, that they are protected by the fatty acid from metabolism. They are extremely potent estrogens, with prolonged activity. Esterification appears to be the only form of metabolism that does not deactivate the biological effects of estradiol. We have demonstrated the biosynthesis of fatty acid esters of estriol, monoesters at both C-16 alpha and C-17 beta. They too are very potent estrogens. These fatty acid esters of the estrogens are the endogenous analogs of estrogen esters, like benzoate, cypionate, etc., which have been used for decades, pharmacologically because of their prolonged therapeutic potency. We have found that the estradiol esters are located predominantly in hydrophobic tissues, such as fat. Sequestered in these tissues, they are an obvious reservoir of estrogenic reserve, requiring only an esterase for activation. To the contrary the biological activity of the fatty acid esters of the glucocorticoid, corticosterone, is not different from that of its free parent steroid. We have shown that the rapid kinetics of its induction of gluconeogenic responses is caused by its labile C-21 ester which is rapidly hydrolyzed by esterase enzymes. While it appears that the physiological role of the estrogen esters may be related to their long-lived hormonal activity, the role of the other families of steroidal esters is not yet apparent. They, and perhaps the estrogen esters as well, must serve other purposes. Indeed they may serve important biological functions beyond those which we ordinarily associate with steroid hormones.

Estrogen regulation of methyl p-hydroxyphenyllactate hydrolysis: correlation with estrogen stimulation of rat uterine growth

We have recently demonstrated that methyl p-hydroxyphenyllactate (MeHPLA) is the endogenous ligand for nuclear type II binding sites in the rat uterus and other estrogen target and non-target tissues. MeHPLA binds to nuclear type II binding sites with a very high binding affinity (Kd approximately 4-5 nM), blocks uterine growth in vivo, and inhibits MCF-7 human breast cancer cell growth in vitro. Conversely, the free acid (p-hydroxyphenyllactic acid, HPLA) interacts with type II binding sites with a much lower affinity (Kd approximately 200 nM) and does not inhibit estrogen-induced uterine growth in vivo or MCF-7 cell growth in vitro. On the basis of these observations, we suggested that one way that estrogen may override MeHPLA inhibition of rat uterine growth may be to stimulate esterase hydrolysis of MeHPLA to HPLA. The present studies demonstrate that the rat uterus does contain an esterase (mol. wt approximately 50,000) which cleaves MeHPLA to HPLA, and that this enzyme is under estrogen regulation. This conclusion is supported by the observations that MeHPLA esterase activity is increased 2-3-fold above controls within 2-4 h following a single injection of estradiol, and is maintained at high levels for 16-24 h following hormone administration. This sustained elevation of MeHPLA esterase activity correlates with estradiol stimulation of true uterine growth and DNA synthesis.

Esterification of dehydroepiandrosterone by human plasma HDL

Evidence for metabolic esterification of dehydroepiandrosterone (DHEA) in human blood plasma, identification of the active lipoprotein (LP) subclass involved, namely HDL3, as well as positive identification of the long-chain fatty acid esters of DHEA formed as incubation products is presented. The esterification reaction of DHEA and subsequent transfer and transport of DHEA esters in human plasma appears to proceed in a manner similar to that of cholesterol. The experiments presented serve as a model predicting similar metabolic transformations during HDL3 interactions with other steroid hormones that have the delta 5-3 beta-hydroxy steroid ring structure and exhibit nonequilibrium associations with HDL. These observations imply that significant quantities of DHEA, particularly in the conjugated ester form, can enter cells via the membrane receptor-mediated pathways of LP internalization.

Biosynthesis of estradiol-17 beta fatty acyl esters by microsomes derived from bovine liver and adrenals

A fatty acyl coenzyme A:estradiol-17 beta acyl transferase activity has been detected in bovine hepatic and adrenocortical microsomes. It is thoroughly increased when adenosine triphosphate (5 mM) and coenzyme A (1 mM) are added to incubation buffer. Using a substrate concentration of 185 microM, the hepatic and adrenocortical microsomal activities have been found to be to 2.4 +/- 0.1 and 5.5 +/- 0.2 nmol/h/mg prot., respectively. Five major estradiol-17-esters have been isolated by reverse phase high performance liquid chromatography from both microsomal incubations, the fatty acid moieties being: arachidonate, linoleate, oleate, palmitate and stearate. However, the distribution of hepatic metabolites is quite different from that obtained with adrenocortical membranes, this is well explained by the corresponding differences between the endogenous contents of free fatty acids. With any of the two types of microsomal membranes used, the results show that estradiol is more susceptible to be esterified to polyunsaturated fatty acids than saturated ones. The possible physiological implications of such an activity in liver and adrenals are discussed.

The nonspecific esterases of human mononuclear leukocytes metabolize arylamine carcinogens and steroids esters

The presence of non-specific esterases in various leukocyte subfractions of whole blood is well established, but no endogenous substrates or function for these esterases have been identified. Here we report on the metabolism of N-acetoxy-2-acetylaminofluorene (NA-AAF) and beclomethasone-17-21-dipropinate (BDP) in viable human mononuclear leukocytes (HML). Conversion of NA-AAF to DNA binding intermediates and BDP to beclomethasone-17-monopropionate by a common esterase was demonstrated and then further characterized by a broad spectrum of effectors including well-established inhibitors and substrates for the nonspecific esterases. Two esters, beta estradiol-17-propionate and alpha naphtyl propionate, competitively inhibited this esterase activity. Together, these data identify at least one isozyme of A- or B-classes of HML nonspecific esterases as being responsible for the metabolism of NA-AAF and BDP. That HML nonspecific esterases may be functionally involved in arylamine carcinogenes (i.e. as it may relate to immune function) and in the endogenous production of steroids from their naturally occurring esters emphasizes the importance of continuing their characterization.

Hormone-sensitive lipase is involved in the hydrolysis of lipoidal derivatives of estrogens and other steroid hormones

Long-chain fatty acid esters of 17 beta-estradiol and other steroid hormones, which are formed in hormone-sensitive tissues, can be regenerated to the free hormone by the action of an esterase present in the cytosol. This esterase has now been examined in bovine placenta cotyledons. Activity towards steroid fatty acid esters was accompanied by activity towards a diacylglycerol analogue and cholesteryl oleate. During purification procedures, the ratio of activities towards the diacylglycerol analogue and estradiol 17 beta-oleate remained approximately constant. Activity towards these two substrates was inhibited by increasing concentrations of HgCl2 and phenylmethanesulfonyl fluoride in a parallel manner. Upon treatment with [3H]diisopropyl fluorophosphate, a major labelled species of Mr approx. 84,000 was formed. Activation by ATP and the catalytic subunit of cAMP-dependent protein kinase occurred. These properties were very similar to those of the hormone-sensitive lipase of bovine adipose tissue previously reported and run in parallel in this study. A highly purified preparation of this latter enzyme was found to hydrolyse steroid fatty acid esters and relative activities towards such substrates, diacylglycerol analogue and cholesteryl oleate, were similar to the placenta esterase. When the two esterases were phosphorylated with [gamma-32P]ATP, a labelled species of Mr 84,000 was isolated in both cases by use of an antibody raised against purified hormone-sensitive lipase of bovine adipose tissue. It is concluded that hormone-sensitive lipase is very likely the enzyme responsible for hydrolysis of steroid fatty acid esters in bovine placenta and possibly steroid hormone target tissues in general.

Capillary gas chromatography/mass spectrometry of intact fatty acyl esters of pregnane steroids

Synthetic long-chain fatty acyl esters of pregnenolone and 3 beta-hydroxy-5 alpha-pregnan-20-one (5 alpha-pregnanolone) were submitted to gas chromatography (GC) and GC/mass spectrometry (GC/MS). Analyses were readily performed using short (8-12 m) flexible fused-silica capillary columns coated with thin films of immobilized apolar stationary phase (e.g. OV-1 type). The electron impact (EI) mass spectra were found to be of limited value for structure investigations, as molecular ions (M+.) were either weak or absent. EI spectra also lacked characteristic fatty acyl fragment ions. In contrast, negative ion ammonia chemical ionization produced characteristic [M-H]- ions and diagnostic fragment ions. The application of the above methods to biochemical investigations is exemplified by GC/MS analysis of the steroidal fatty acyl esters in extracts of bovine adrenal tissue, and the metabolites produced by incubating starfish (Asterias rubens) testicular tissue with exogenous progesterone.

Formation and turnover of long-chain fatty acid esters of 5-androstene-3 beta, 17 beta -diol in estrogen receptor positive and negative human mammary cancer cell lines in culture

Microsomal preparations derived from bovine placenta cotyledons, previously investigated as a convenient source of fatty acyl coenzyme A: estradiol-17 beta-acyl transferase, have been shown to acylate other steroids bearing 3 beta- or 17 beta-hydroxyl groups. In the presence of 0.1 mM oleoyl CoA, the apparent Km values for dehydroepiandrosterone, testosterone, and 5-androstene-3 beta,17 beta-diol (delta 5-DIOL) were 45, 67, and 20 microM, respectively. Acylation of delta 5-DIOL occurred at either the 3 beta- or 17 beta-positions to give monoesters. Testosterone, estradiol-17 beta, and delta 5-DIOL acted as competitive inhibitors for the acylation of the 3 beta-hydroxyl group of dehydroepiandrosterone (Ki values 71, 75, and 41 microM, respectively). Such data indicate that a single enzyme of wide substrate specificity may be involved in these acylation reactions. When estrogen receptor (ER) positive and negative human mammary cancer cell lines were incubated with 10 nM [3H]delta 5-DIOL, intracellular accumulation of delta 5-DIOL long-chain fatty acid esters occurred; rates being higher (p less than 0.001) in ER negative cells (MDA-MB-231 and MDA-MB-330) compared to MCF-7 cells (ER positive), and higher (P less than 0.005) in MDA-MB-231 cells compared to ZR-75-1 cells (ER positive). After exposure to 10 nM [3H]delta 5-DIOL for 16 h, the total labeled steroid fatty acid fraction was composed predominantly of delta 5-DIOL-3 beta- and 17 beta-monoesters (approximately 85%), the remainder containing approximately equal amounts of delta 5-DIOL-diesters and dehydroepiandrosterone-3 beta-esters. Subsequent transfer to medium lacking delta 5-DIOL was accompanied by a breakdown of the labeled esters, which was more rapid in the ER positive cell lines. During this period, intracellular free delta 5-DIOL levels rapidly declined in MDA-MB-330 cells but were maintained in MCF-7 cells, presumably by binding to ER. This behavior parallels that of estradiol-17 beta previously observed in these cell lines and further emphasizes the potential importance of the adrenal-derived estrogen delta 5-DIOL in consideration of a hormone-based etiology of human breast cancer.

An estrogen-dependent esterase activity in MCF-7 cells

The presence of steroidal esters in hormonally sensitive tissues lends importance to the esterases which convert the biologically inactive adducts to the parent potent forms. Accordingly, esterase-activities were studied in a human breast cancer model--the MCF-7 cell line. Tritiated estradiol esters- estradiol-17-acetate (EA), estradiol-17-valerate (EV) and estradiol-17-stearate (ES) were tested systematically, but 3 beta-ol esters of androgens, and phorbol diesters were also investigated. All compounds tested, except the phorbol diesters were hydrolyzed either when added to growing cultures or to the 28,000 g supernate of homogenized MCF-7 cells. Among the estrogens, the relative rates of hydrolysis were EA greater than EV greater than ES. The esterase for EA was different as it was not inhibited by saturating concentrations of EV or ES, and unlike the others its activity was stimulated by the addition of estradiol to the culture medium. The antiestrogen keoxifene,[(6-Hydroxy-2-(4-hydroxyphenyl)benzo less than b greater than thien-3-yl greater than less than 4- less than 2-(1-piperidinyl)ethoxy greater than phenyl greater than methanone], negated the stimulatory effect. Other major classes of steroids did not influence EA esterase activity. Results of inhibition experiments indicated that the esterases are of the serine active-site types. The significance of the estrogen-dependent esterase activity can be assessed when the natural substrate(s) for the enzyme is elucidated.

Esterification-deesterification of estradiol by human mammary cancer cells in culture

Formation of lipoidal derivatives of estradiol-17 beta (E2) esterified to long-chained fatty acids has been reported to occur in estrogen target tissues. Employing human breast cancer cells in culture, we have detected the rapid synthesis of such compounds upon exposure of the cells to concentrations of [3H]E2 as low as 1 nM. When exposed to 10 nM [3H]E2 in the culture medium, synthesis of E2-lipoidal derivative (E2-L) reached 270 fmol/mg DNA in 2 h in the estrogen receptor positive MCF-7 human mammary cancer cell line. Higher rates (approximately 900 fmol/mg DNA in 2 h) were reached in 2 estrogen receptor negative human mammary cancer cell lines; MDA-MB-231 and MDA-MB-330. E2-L was the major form of estrogen in the latter cells at this time interval (E2-L/E2 approximately 3.0). Far higher concentrations of E2 were found in MCF-7 cells compared to 231 and 330 cells, and, in contrast to the latter, this was mostly specifically bound. Upon subsequent withdrawal of E2 from the medium, intracellular concentrations of E2-L decreased very rapidly in the first 5 h period, then declined more slowly to approximately 50 fmol/mg DNA at 24 h. Intracellular concentrations of E2 were maintained over this time period. E2-L was not present in the medium. Thus, accumulation of E2-L in cells upon continuous exposure to E2 represents the net result of esterification and deesterification reactions. These hydrophobic E2-derivatives may then be involved in the "capture" of E2 for transport through membranes and subsequent regeneration of E2 to maintain occupancy of the nuclear receptor.

Solid-phase reagents for the isolation and protection of carbonyl compounds

Resins which can serve as solid-phase Girard reagents have been prepared. These consist of porous cross-linked polystyrene-divinylbenzene matrices functionalized either with acid hydrazide groups, 1 or with O-alkylhydroxylamine groups, 2. Both resins react rapidly, under acid catalysis, with steroidal ketones and with glucose to form resin bound carbonyl derivatives. The carbonyl compound can be recovered by exchange with acetone under mildly acidic conditions. Resin 2 was used to extract steroidal ketones from an extract of bovine adrenals. By protecting the C-20 ketone group of allopregnanolone, resin 2 was used to prepare the counterpart of the steroid bearing tritium in the 3 alpha-position.

Glucosiduronidation and esterification of androsterone by human breast tumors in vitro

The metabolism of 3H-androsterone was studied in homogenates (fortified with uridine 5'-diphosphoglucuronic acid and adenosine 3'-phosphate 5'-phosphosulfate) of eighteen breast tumors, one muscle underlying the primary breast carcinoma and metastatic axillary lymph nodes from a patient with suspected primary breast cancer. The major metabolites identified were less polar than androsterone. On saponification these lipoidal derivatives afforded androsterone as the only product (3 to 48%). Unmetabolized androsterone and lesser quantities of epiandrosterone, 5 alpha-androstane- alpha, 17 beta-diol and 5 alpha-androstane-3,17-dione comprised the free steroid fraction. Androsterone glucosiduronate was isolated (0.17-4.1%) from weight breast tumor homogenates and from the node tissue incubation (17%). There was no apparent correlation between glucuronyltransferase activity and histopathology or estrogen receptor content.