P.A. Beranek

17β‐HYDROXYSTEROID DEHYDROGENASE AND AROMATASE ACTIVITY IN BREAST FAT FROM WOMEN WITH BENIGN AND MALIGNANT BREAST TUMOURS

The activity of 17β‐hydroxysteroid dehydrogenase and aromatase was studied in adipose tissue taken from women aged between 22 and 83 years with benign or malignant breast lesions. The benign and malignant groups showed no significant differences in the mean activities of either of the enzymes studied. Under the experimental conditions used, the rate of conversion of oestrone to oestradiol varied markedly between subjects (6‐169 ng oestradiol/mg protein/h), and there was a positive correlation between oestrone reduction and the total body weight of the tissue donor. In contrast, although the apparent Km for oestradiol (0·1–2·6 μmol/1) was lower than that for oestrone (9–14 μmol/1) the maximum velocity for oestrone production was very low (10–50 pmol/mg protein/h), and there was no obvious correlation with the age or body weight of the tissue donor. Aromatase activity in breast fat was at the lower end of the normal range of activity previously reported for abdominal adipose tissue, and there was no correlation between oestrone production and age or body weight.

The role of tissue steroids in regulating aromatase and oestradiol 17β-hydroxysteroid dehydrogenase activities in breast and endometrial cancer

We have observed that oestradiol concentrations in breast and endometrial tumours are relatively higher than oestrone, in contrast to peripheral tissues. Infusion of radiolabelled oestrogen also suggested there was a difference in metabolism between normal and tumour tissue. We have therefore looked for factors which could modulate tissue steroid metabolism and conclude that progesterone may influence aromatase, and that the adrenal androgens can inhibit oestradiol dehydrogenase activity. The latter mechanism, in particular, may be important in increasing tissue exposure to oestradiol.

The effect of epidermal growth factor, transforming growth factor and breast tumour homogenates on the activity of oestradiol 17β hydroxysteroid dehydrogenase in cultured adipose tissue

The effect of epidermal growth factor (EGF), transforming growth factor (TGF alpha) and breast tumour homogenates on oestradiol 17 beta hydroxysteroid dehydrogenase (E2DH) activity has been examined using cultured human breast adipose tissue. EGF (100-1000 ng/ml) inhibited E2DH activity (E1----E2) in a dose dependent manner. TGF alpha (250 and 500 ng/ml) stimulated E2DH activity, with conversion of E1----E2 increasing to a greater degree than E2----E1 activity. Breast tumour homogenates (2-10% w/v) also influenced E2DH activity. It is concluded that growth factors, produced by breast tumours, may modulate E2DH activity in tissues surrounding the tumour and thereby influence tumour growth.

Metabolism of [3H]oestradiol in vivo by normal breast and tumour tissue in postmenopausal women☆

Tumour and normal breast tissue was obtained from postmenopausal breast cancer patients following [3H]oestradiol infusion (50 mu Ci over a 12 h period). The fraction of radioactivity present as oestradiol or oestrone was measured and the results expressed both as the ratio of oestradiol-oestrone and as the percentage oestrogen present as oestrone, and the findings compared with in vitro measurements of 17 beta-hydroxysteroid dehydrogenase activity. Concentrations of 5-androstene-3 beta, 17 beta-diol, dehydroepiandrosterone and its sulphate and testosterone were measured and related to oestradiol metabolism. The study demonstrated that tumour tissue is less able to metabolise oestradiol to oestrone than is normal breast tissue and indicated that the ability of the tissue to detoxify oestradiol may be dependent on cofactor availability. The results also supported the possibility that increased tissue concentrations of adrenal androgens inhibit oestradiol and thus increase tissue exposure to oestradiol.

Free fatty acids: A possible regulator of the available oestradiol fractions in plasma

Consumption of dietary fats has been linked to the high incidence of breast cancer found in Western women. In vitro studies we have carried out show that unsaturated free fatty acids can increase the biologically available oestradiol fractions in plasma. It is possible therefore that the increased risk for breast cancer associated with a diet high in fats may be related to an elevation in the biologically available oestradiol fractions in plasma.

The regulation of the biologically available fractions of oestradiol and testosterone in plasma

The albumin bound fractions of oestradiol and testosterone have been measured in samples of plasma obtained over a 24 h period from women with breast cancer or polycystic ovarian disease and from pre- and postmenopausal control subjects and related to plasma levels of free fatty acids. For most subjects changes in the fraction of oestradiol bound to albumin were related to changes in plasma levels of free fatty acids. A significant decrease in the albumin bound testosterone fraction during the night was associated with increased plasma levels of cortisol.

Metabolism of adrenal androgens by human endometrium and adrenal cortex

The enzyme 17 beta-hydroxysteroid dehydrogenase (17OHSD) was studied in human endometrium and adrenal cortex with respect to the metabolism of 5-androstene-3 beta,17 beta-diol (androstenediol) and dehydroepiandrosterone (DHA). The aim was to provide further information concerning the origin and biological significance of these androgens in endometrium, particularly the increased concentrations of the secretory phase and to compare the characteristics of the enzyme in the two tissues. In both endometrium and adrenal cortex the metabolism of androstenediol to DHA was linear with time and increasing enzyme concentration. The preferred cofactor was NAD and the apparent Km values were 3.4 +/- 0.2 (SD) microM (n = 3) for endometrium and 30.5 +/- 6.1 microM (n = 3) for adrenal cortex. In endometrium DHA was not metabolised to androstenediol in the presence of either NADH or NADPH whereas in the adrenal cortex both cofactors were utilised. However, the concentration of NADH required to achieve maximum enzyme activity was 10-fold higher (1 mM) than for NADPH (0.1 mM) and maximum activity with NADH was only 30% of that using NADPH. The apparent Km was 125 microM DHA (n = 2). The study indicates that androstenediol in endometrium does not arise from DHA metabolism but that its presence could be due to a binding protein particularly during the secretory phase. Our findings also suggest that the enzyme of endometrium differs from that of the adrenal cortex and that the kinetic properties may be related to the physiological requirements of the two tissues.

Factors influencing estrogen production and metabolism in postmenopausal women with endocrine cancer

The role which steroid hormones play in the development and maintenance of human cancer is a problem which has attracted the attention of research investigators for many years. Oestrogens have come under particular scrutiny, since there is a good deal of indirect evidence that altered oestrogen production may be protective in some situations (e.g. the diminished risk of female breast cancer conferred by earlier castration) and disadvantageous in others (e,g, the enhanced risk of endometrial cancer attributable to increased oestrogen exposure in women bearing oestrogen-secreting tumours). Furthermore, in the last decade or so, studies of cellular receptors have clearly delineated the importance of oestrogens amongst other hormones in the maintenance of tumour growth. It is therefore entirely logical to investigate oestrogen metabolism in detail in the hope and expectation that some metabolic abnormality exists in women who are predisposed to these types of endocrine cancer, which may be revealed by careful biochemical examination. As a group, postmenopausal women are particularly important in this context, not only because of the increased incidence of breast and endometrial cancer at and around the menopause, but because it is at this time that major changes occur in the endocrine milieu which may relate directly to the risk of developing cancer.

Oestrogen production and metabolism in peri-menopausal women.

The metabolic clearance rates and production rates of oestrone and oestradiol have been measured in a group of peri-menopausal women either with breast or endometrial cancer or having an increased risk of developing one of these diseases. The results were compared with values for normal post-menopausal women in whom the menopause was established. The transfer constants for the conversion of oestrone to oestradiol and of oestradiol to oestrone were also measured. Metabolic clearance rates for oestrone (1946 +/- 406 1/24 h) and oestradiol (1296 +/- 261 1/24 h) for peri-menopausal women, and production rates of oestrone (90 +/- 38 micrograms/24 h) and oestradiol (45 +/- 33 micrograms/24 h) were significantly higher than for normal post-menopausal women. Transfer constants for the conversion of oestrone to oestradiol and of oestradiol to oestrone were similar in the peri- and post-menopausal women. Plasma progesterone concentrations were less than 0.4 ng/ml in both groups of women. It is possible that the higher oestrogen production and clearance rates of peri-menopausal women, at a time when progesterone production is greatly reduced, may in part account for the higher risk that post-menopausal women have for developing breast or endometrial cancer.

Androgen and estrogen metabolism in breast cancer patients.

Much indirect evidence, reviewed elsewhere,’ has implicated estrogens in the pathogenesis of certain endocrine cancers, and particularly breast cancer. However, the failure to find any consistent abnormality in the urinary excretion or plasma levels of estrogens in patients with breast cancer2 has led to a search for other mechanisms whereby tissue exposure to estrogens might be increased. Because breast cancer occurs most commonly in postmenopausal women, our own studies have examined possible factors that may regulate production of estrogens in this group of women. In postmenopausal women, estrogens are derived almost exclusively from the peripheral aromatization of androstenedione.” Adipose tissue is considered to be an important site in which aromatization occurs, for the transfer constant for the conversion of androstenedione to estrone shows a significant correlation with excess body weight.-’ Studies carried out in vivo in postmenopausal women with breast cancer, however, have not revealed any increase in the extent to which androstenedione is converted to estrone.4 The aromatase enzyme complex is also present in both normal breast tissue and in breast tumors,5-’ and it is therefore possible that high concentrations of estrogen could be produced in these tissues, resulting in increased exposure to estrogen and thus influencing tumor development. Since little is known about the regulation of androstenedione conversion to estrone, or the conversion of estrone to the biologically active estrogen estradiol, we are examining in vivo the factors that influence the activities of the enzymes responsible for these conversions. Tissue concentrations of certain steroids have also been measured and related to the activity of the aromatase enzyme complex and also to estradiol 17P-hydroxysteroid dehydrogenase (HSD), the enzyme responsible for the interconversion of estrone and estradiol. Because the regulation of the biologically active fraction of estradiol in plasma may influence tissue concentrations, we are investigating the possibility that dietary fats may alter the binding of estradiol to plasma proteins.