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

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.

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.

Enzymatic regulation of estradiol-17 beta concentrations in human breast cancer cells

Estradiol-17 beta is known to be involved in both the etiology and maintenance of growth of breast cancer. However, blood levels of the hormone do not reflect those found within the cells due to a number of transformations catalysed by enzymes which may be under metabolite and/or hormonal regulation. Recognition of the importance of the hormone microenvironment within the cell focuses attention on these enzymes and provides the subject for this review. An interplay between the sex hormones, estrogen and progestin, can control estradiol-17 beta concentrations in breast cancer cells at the level of key transforming enzymes. In addition, some enzymes catalyse production of biologically inert derivatives which are rapidly eliminated from the cell. Other enzymes catalyse the formation of derivatives which are exclusively intracellular and can act as reserve forms of the hormone. Yet others lead to estradiol-17 beta metabolites which are cytotoxic. An improved understanding of the enzymes and the role of the related metabolites can provide the opportunity for the development of new therapeutic agents.

Metabolism of lipoidal derivatives of estradiol-17-beta in human mammary cancer tissue and cell lines

Estradiol-17 beta (E2) is converted exclusively to intracellular metabolites, termed lipoidal estrogens [long chain fatty acid 17 beta-esters (E2-L)], by human mammary cancer tissue and cell lines. In order to further evaluate the biological role of lipoidal estrogens, rates of saturation of the estrogen receptor (ER) along with formation of [3H]E2-L have been measured in human mammary cancer cells exposed to 5 nM [3H]E2. Extensive specific binding of E2 to ER in MCF-7 cells (approximately 37%) and ZR-75-1 cells (approximately 62%) occurred before appreciable synthesis of E2-L was evident and the maximum level of E2-L attained was only 3-9% of the E2 specifically bound to ER. In these ER positive cell lines, and in the ER negative cell line MDA-MB-231, an initial rise in the rate of E2-L formation was followed by a decrease at approximately 6 min and re-establishment of a new rate, indicating turnover of the E2-L fraction by esterification-de-esterification reactions. This data does not support the concept that E2-L acts in the transport of E2 to nuclear receptors, but rather than liberation of E2 from E2-L could serve to maintain occupancy of ER necessary for initiation of DNA synthesis. The esterase, as studied in pooled human mammary cancer tissue, was found to hydrolyse E2-17 beta-long chain fatty acid esters at different rates--the enzyme being less active towards E2-17 beta-stearate compared to E2-17 beta-oleate, -linoleate and -linolenate. Esterase activity was significantly higher in MDA-MB-231 cells compared to MCF-7 cells. Treatment of MCF-7 cells with E2 did not alter the specific activity of the esterase towards E2-17 beta-oleate as substrate. Similarly, addition of dibutyryl c-AMP to ZR-75-1 cell cultures was without effect on E2-L, both during the time when E2-L was accumulating, or during a subsequent phase when E2-L was decreasing following transfer to medium lacking E2. Calcitonin, which increases endogenous c-AMP in MCF-7 cells, had no effect on E2-L in this latter phase using this cell line. Thus, no evidence could be provided that the esterase was under E2 control, or control by polypeptide hormones which utilize c-AMP as a second messenger.

Estrogenic action of estriol fatty acid esters

Recent studies suggest that, estriol, like estradiol, is biosynthetically esterified with fatty acids. We have synthesized the stearate estriol, at C-16 alpha, C-17 beta and the diester, C-16 alpha,17 beta and tested these D-ring esters for their estrogenic action both in vivo and in vitro, comparing them to estradiol, estriol and estradiol-17-stearate. None of the estriol esters bind to the estrogen receptor. They are only weakly estrogenic in a microtiter plate estrogen bioassay: stimulation of alkaline phosphatase in the Ishikawa endometrial cells. However, both estriol monoesters are extremely potent estrogens when injected subcutaneously (in aqueous alcohol) into ovariectomized mice. Compared to the free steroids, they produced a dramatically increased uterine weight with a greatly prolonged duration of stimulation. The 16 alpha,17 beta-diester also induced a protracted uterotrophic response, but the stimulation of uterine weight was comparatively low. Since the esters of estradiol and estriol do not bind to the estrogen receptor, their estrogenic signal must be generated through the action of esterase enzymes. These naturally occurring esters have the potential of being extremely useful pharmacological agents for long-lived estrogenic stimulation.

Multiple steroid metabolic pathways in ZR-75-1 human breast cancer cells

In order to characterize the main enzymatic systems involved in androgen and estrogen formation as well as metabolism in ZR-75-1 human breast cancer cells, incubation of intact cells was performed for 12 or 24 h at 37 degrees C with tritiated estradiol (E2), estrone (E1), androst-5-ene-3 beta, 17 beta-diol (5-ene-diol), dehydroepiandrosterone (DHEA), testosterone (T), androstenedione (4-ene-dione), dihydrotestosterone (DHT) or androsterone (ADT). The extra- and intracellular steroids were extracted, separated into free steroids, sulfates and non-polar derivatives (FAE) and identified by HPLC coupled to a Berthold radioactivity monitor. Following incubation with E2, 5-ene-diol or T, E1, DHEA and 4-ene-dione were the main products, respectively, thus indicating high levels of 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD). When 4-ene-dione was used, on the other hand, a high level of transformation into 5 alpha-androstane-3,17-dione (A-dione), Epi-ADT and ADT was found, thus indicating the presence of high levels of 5 alpha-reductase as well as 3 alpha- and 3 beta-hydroxysteroid dehydrogenase. Moreover, some T was formed, due to oxidation by 17 beta-HSD. No estrogen was detected with the androgen precursors T or 4-ene-dione, thus indicating the absence of significant aromatase activity. Moreover, significant amounts of sulfates and non-polar derivatives were found with all the above-mentioned substrates. The present study shows that ZR-75-1 human breast cancer cells possess most of the enzymatic systems involved in androgen and estrogen formation and metabolism, thus offering an excellent model for studies of the control of sex steroid formation and action in breast cancer tissue.

Estradiol esters can replace 17 beta-estradiol in the stimulation of DNA and esterase synthesis by MCF-7 cells: a possible role for the estrogen-sensitive MCF-7 cell esterase

In this communication we extend our earlier observations on estrogen-sensitive carboxyl esterases in MCF-7 human breast cancer cells able to hydrolyze esters of estradiol. Using either estradiol acetate or p-nitrophenyl hexanoate as substrates, esterase activity was found to increase 2-3-fold in MCF-7 cells maintained in the presence of 10(-8) M estradiol. Following sucrose density centrifugation, over 85% of total esterase activity was found in the cytoplasmic fraction. No esterase activity was found in spent media from growing cells. By size exclusion chromatography, estradiol acetate esterase activity exhibited a mol. wt of 45-50 kDa. Attempts to demonstrate incorporation of [3H]estradiol into estradiol fatty acid esters by the above MCF-7 cell line (203P) were unsuccessful, although, such incorporation could be demonstrated in two other MCF-7 cell sublines. Incubation of the 203P cells with 10 nM [3H]estradiol in the presence of 0.5 mM radioinert estradiol acetate resulted in the incorporation of 35 +/- 12% of the label into the estradiol acetate in 10 min. In the absence of radioinert estradiol acetate, no incorporation was observed. When MCF-7 cells were incubated with [3H]estradiol in the presence of a large excess of radioinert estradiol valerate, label was found only in estradiol valerate. Similarly, when the incubation was carried out in the presence of a mixture of radioinert estradiol acetate and valerate, label was incorporated into both esters. We conclude that the apparent formation of radiolabeled estradiol esters by MCF-7 cells incubated under the above conditions, results at least in part, from an esterase-catalyzed exchange reaction. Under conditions where no ester hydrolysis could be detected in the absence of cells, valerate and stearate esters of estradiol were found to be as effective as unesterified estradiol in stimulating esterase synthesis and the incorporation of [3H]thymidine into DNA. These results are consistent with a model in which an intracellular esterase in MCF-7 cells can generate estradiol from an exogenous lipoidal steroid and elicit an estrogen response.

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.

Wide spectrum of steroids serving as substrates for the formation of lipoidal derivatives in ZR-75-1 human breast cancer cells

Recently, several natural steroids have been found to be esterified to long-chain fatty acids (FAE) in various mammalian tissues. The purpose of the present study was to determine the ability of a series of 3H-labeled steroids to serve as substrates for the formation and accumulation of such non-polar derivatives in intact cells, using the hormone-responsive ZR-75-1 human breast cancer cell line as model. All 14 steroids tested were found to be converted, directly or following further metabolism, to lipoidal ester derivatives. The percentage of intracellular steroids recovered as FAE derivatives was usually substantial (14-90%), especially in the case of C-19 steroids (75-90%). The composition of the lipoidal steroid fractions recovered from the labeled cell extracts was characterized by chromatographic comparison with synthetic steroid FAEs and by saponification of the steroid FAEs and identification of the released steroidal moieties. Following metabolism, most steroid substrates were converted into multiple lipoidal esters. Furthermore, 5 alpha-androstane-3 alpha, 17 beta-diol, 5 alpha-androstane-3 beta, 17 beta-diol, as well as androst-5-ene-3 beta, 17 beta-diol formed lipoidal diesters in addition to the monoester form. The high level of intracellular steroid FAE accumulation reported in this study suggests that these yet poorly known steroid derivatives may play important functions in the regulation of steroid hormone metabolism and action.

Long-acting estrogenic responses of estradiol fatty acid esters

Estradiol esters at C-17 and C-3 with palmitic, stearic and oleic acids were chemically synthesized and then evaluated for their long-acting estrogenic responses in ovariectomized rats. The duration of the biological effects was measured after a single subcutaneous dose of 0.1 mumol of each ester and compared with those observed with 17 beta-estradiol, estradiol 3-benzoate and estradiol 17-enanthate. Vaginal citology, uterophyc action, serum gonadotropins inhibition and 17 beta-estradiol levels were measured 0, 5, 10, 20, 30 and 60 days after injection. The results disclosed that most of the estradiol derivatives evaluated exhibited a long-acting estrogenic action. However, the monoesters at C-17 showed longer effects that monoesters at C-3, while the estradiol diesters exhibited the shortest effects. In addition as shown by its low serum levels, all estradiol esters with unsaturated fatty acids show a decreased E2 absorption. The overall results indicated that esterification of E2 with long chain fatty acids provided long-acting properties to it, being higher with C-17 esters. Whether some of these compounds could be employed in substitutive endocrine therapy remains to be established.

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.

Presence of fatty acid esters of pregnenolone in follicular fluid from women undergoing follicle stimulation

In the following investigation the presence of lipoidal pregnenolone derivatives in the preovulatory follicular fluid obtained from women undergoing in vitro fertilization was established. Concentrations of lipoidal pregnenolone proved to be at least twofold greater than those of the unconjugated counterpart. Indirect identification of these lipoidal pregnenolone derivatives was achieved by comparing the C-18 column, thin-layer silica gel (TLC), high performance liquid chromatography (HPLC), and gas chromatography/mass spectrometry (GC/MS) chromatographic properties of the endogenous lipoidal pregnenolone derivatives in follicular fluid with those of synthetic acyl pregnenolone esters. Lipoidal pregnenolone derivatives recovered after HPLC subfractionation were treated with alkali to hydrolyze the acyl group thus liberating nonconjugated pregnenolone. Concentrations of this steroid were then measured using radioimmunoassay upon which analysis of HPCL and gas chromatograms permitted the calculation of the individual pregnenolone ester contributions within the samples. Five lipoidal pregnenolone derivatives constituted more than 90% of the total lipoidal pregnenolone concentration observed, these derivatives being: pregnenolone oleate (30.7%), linoleate (20.7%), palmitate (20.1%), linolenate (14.8%), and palmitoleate (7.1%).

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.

Formation of lipoidal steroids in follicular fluid

The presence of high levels of lipoidal pregnenolone in follicular fluid has recently been established although no evidence has been presented concerning its possible origin. The following investigation focuses on the enzymatic conversion of non-conjugated steroids into their lipoidal derivatives in preovulatory follicular fluid obtained from women undergoing in vitro fertilization. Our observations indicated that pregnenolone, an important precursor steroid, was acylated at a similar rate as cholesterol in follicular fluid. Similar studies were subsequently conducted with serum obtained from a pool of normal women and women undergoing follicular stimulation which showed little difference to the results obtained in follicular fluid. Further studies using dehydroepiandrosterone, androst-5-ene-3 beta,17 beta-diol, estradiol and dihydrotestosterone were were also performed to monitor their respective lipoidal conversion percentages in follicular fluid which revealed a marked difference of conversion rates between steroids. The indirect identification of the lipoidal pregnenolone derivatives formed in follicular fluid was also conducted by incubating radiolabelled pregnenolone in follicular fluid. The fatty acid components of the resulting lipoidal pregnenolone derivatives showed a marked resemblance to those of cholesteryl esters formed in plasma by the enzymatic activity of lecithin:cholesterol acyltransferase. The pregnenolone derivatives were comprised predominantly of unsaturated fatty acids such as linoleate, palmitoleate, oleate, linolenate and arachidonate while saturated fatty acids, namely palmitate, constituted 20% of the total lipoidal pregnenolone.

Long-chain fatty acid esters of 5-androstene-3 beta, 17 beta-diol: composition and turnover in human mammary cancer cells in culture

Long-chain fatty acid esters of the adrenal-derived estrogen 5-androstene-3 beta, 17 beta-diol (ADIOL) were found to accumulate in four human mammary cancer cell lines (MCF-7, ZR-75-1, MDA-MB-231 and MDA-MB-330) when explosed to 10-30 nM ADIOL for variable time periods. At each time point examined, the monoester fraction, which represented the major component of the total lipoidal fraction, contained fatty acids linked to either the 3 beta- or 17 beta-positions. However, there was considerable variation in the ratio of 3 beta- to 17 beta-monoesters in the four cell lines. By means of reverse phase HPLC and referral to authentic synthesized compounds, each monoester fraction was found to contain a number of long-chain fatty acid components whose composition resembled that previously determined for the fatty acid esters formed from 17 beta-estradiol. A specific and measurable turnover of a subfraction of ADIOL-17 beta-monoesters composed of essential fatty acids (22:6, 20:4, 18:3) occurred in MCF-7 cells, and to a lesser extent in ZR-75-1 cells. No changes were observed with time in any of the components of the 3 beta- or 17 beta-monoester fractions in MDA-MB-231 and MDA-MB-330 cells. These results, coupled with other studies, now suggest that a very rapid turnover of some components of these lipoidal derivatives may be occurring. If so, it is possible that the system of acylation-deacylation may be involved in a transport mechanism for estrogens and perhaps other steroid hormones.

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.

Properties of fatty acyl-coenzyme A: estradiol-17 beta acyltransferase in bovine placenta microsomes

The properties of the enzyme catalyzing the formation of non-polar derivatives of estradiol-17 beta (E2) esterified to long-chain fatty acids have been investigated in microsomal preparations from bovine placenta cotyledons. A rapid enzyme assay has been developed which involves simple solvent partitioning. The membrane-bound enzyme showed a pH optimum of 5.0 and addition of fatty acyl-coenzymes A (CoAs), such as oleoyl-CoA, palmitoyl-CoA and palmitoleoyl-CoA, increased [3H]E2-fatty acyl ester formation from [3H]E2 by some 7-fold. Linoleoyl-CoA, linolenoyl-CoA and arachidonoyl-CoA were much less effective as acyl donors. Only 17 beta-fatty acyl monoesters were synthesized in each instance. Similar results were obtained with microsomes or mitochondria from bovine endometrium. The apparent Km for E2 employing placenta microsomes was 8.0 +/- 2.2 (SD) microM. Steroids such as testosterone, dehydroepiandrosterone and 5-androstene-3 beta, 17 beta-diol acted as competitive inhibitors (Ki values 79, 46 and 39 microM, respectively). These, and other data to be reported separately, which showed that these steroids were substrates for the enzyme, demonstrate that the latter is not specific for E2. The [3H]E2-fatty acyl ester fractions biosynthesized from [3H]E2 and bovine placental or endometrial tissue were analyzed by high pressure liquid chromatography (HPLC) and were found to have similar compositions characterized by a high percentage of unsaturated fatty acids.

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.

Metabolic fate of estradiol in human mammary cancer cells in culture: estrogen sulfate formation and cooperativity exhibited by estrogen sulfotransferase

The metabolism of 17 beta-estradiol in both estrogen receptor positive and negative human breast cancer cell lines has been compared. Initial experiments in which confluent cells were exposed to 1 nM [3H]17 beta-estradiol for 24 h, revealed that the main metabolites formed by estrogen receptor positive MCF-7 and ZR-75-1 cells were 17 beta-estradiol-3-sulfate (together with lesser amounts of estrone sulfate) and estrone. In estrogen receptor negative cell lines, production of estrogen sulfates was either significantly lower (MDA-MB-231 cells) than receptor positive cells, or failed to be produced at all (MDA-MB-330 cells). In both these receptor negative cell lines, production of estrone was significantly higher than in receptor positive cells. Accumulation of estrogen sulfates resulted from attainment of a steady state between synthesis catalysed by estrogen sulfotransferase and degradation catalysed by estrogen sulfatase. The former was present in the cytosol and showed a very high affinity for 17 beta-estradiol and estrone (low nM range). Complex initial velocity versus estrogen substrate curves were obtained with enzyme purified 106-fold by affinity chromatography. Such curves were consistent with a rate equation of degree 3 or 4 and suggest the presence of cooperatively linked dependent binding sites.

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.

Selective turnover of the essential fatty acid ester components of estradiol-17 beta lipoidal derivatives formed by human mammary cancer cells in culture

The properties of fatty acyl coenzyme A: estradiol-17 beta acyl transferase in microsomes derived from pooled human mammary cancer tissue have been examined. A pH optimum of 5.5 was found and addition of long-chained fatty acyl CoAs increased estradiol-17 beta (E2) 17-monoacyl ester synthesis; the apparent Km for E2 being 8 microM when oleoyl CoA, linolenoyl CoA or palmitoyl CoA were employed. Testosterone, dehydroepiandrosterone, and 5-androsterone-3 beta, 17 beta-diol acted as competitive inhibitors with Ki values of 36, 36 and 46 microM, respectively. The composition of E2 fatty acyl esters (E2-L) formed by incubation of [3H]E2 with human mammary cancer tissue and human mammary cancer cell lines has been determined by HPLC. Although the composition of E2-L in estrogen receptor negative cell lines (MDA-MB-231 and MDA-MB-330) was generally similar to that found for MCF-7 cells (estrogen receptor positive) and pooled human mammary cancer tissue, the former cell lines contained a 3-fold higher relative concentration of E2-17 beta stearate. MCF-7 cells were exposed to 30 nM [3H]E2 and the composition of the isolated [3H]E2-L fraction studied at various time intervals. At 0.5 h, the intracellular concentration of E2-L was 1.8 +/- 0.4 (SEM) pmol/mg DNA which increased to values of 3.6 +/- 0.6 and 4.3 +/- 0.5 at 4 h and 16 h, respectively. In the subsequent 3 h following transfer to medium lacking [3H]E2, the concentration of E2-L declined to 3.7 +/- 0.3 pmol/mg DNA. The subfraction of E2-L composed of E2-17 beta arachidonate, linolenate and docosahexaenoate, was seen to decline in relative abundance after 0.5 h and to reach significantly lower relative levels at 16 h, and again in the 3 h period following estrogen withdrawal. The data suggests that these components, derived from essential fatty acids, are more metabolically active. This may then provide a new lead to link these novel estrogen derivatives with the established relationship between unsaturated fatty acids and an increased mammary cancer incidence.

Estradiol metabolism in Ishikawa endometrial cancer cells

Estrogen-responsive human cells derived from a specimen of well differentiated endometrial adenocarcinoma (Ishikawa line) were incubated with [3H]estradiol (E2) at various concentrations and the medium was sampled at 3, 6 and 24 h to evaluate the kinetics of removal of the hormone and the formation of unconjugated or sulfated metabolites. The detectable products of metabolism were estrone and the conjugate estradiol-3-sulfate. The latter was identified by high pressure chromatography, before and after acetylation, oxidation, and hydrolysis. The disappearance of [3H]E2 from the medium was found to follow first order kinetics between 3 and 24 h, with half-lives increasing from 4.7 to 53 h as the initial concentrations of the hormone were raised from 10(-8) to 10(-6)M. At the lowest concentration, practically all of the [3H]E2 added to the cultures was converted to estradiol-3-sulfate in 24 h, whereas at 10(-6)M oxidation to estrone was quantitatively more important than sulfation. These results indicate the presence in Ishikawa cells of an estrogen sulfotransferase of low Michaelis constant for E2, and 17 beta-oxidoreductase activity that significantly contributes to the metabolism of E2 only at higher concentrations of substrate.

Solubilisation and reconstitution of acylcoenzyme A:estradiol-17 beta acyltransferase

Acylcoenzyme A:estradiol-17 beta acyltransferase in microsomes of bovine placenta cotyledons was strongly membrane bound. The enzyme was solubilised from microsomes by sodium cholate and was reconstituted into phospholipid vesicles. The apparent Km for estradiol-17 beta was 11 microM which was close to the value of 8 microM previously found with the membrane-bound enzyme. Testosterone was also a substrate for the reconstituted enzyme (apparent Km 62 microM) and was a competitive inhibitor (Ki 74 microM) of the acylation of estradiol-17 beta. Although various long-chained fatty acyl CoAs acted as acyl donors, these proved to have widely differing apparent Km values with palmitoleoyl CoA having the highest affinity (Km 24 microM) and arachidonoyl CoA the lowest affinity (Km 330 microM).