A tracer superfusion method to measure rates of entry, exit, metabolism, and synthesis of steroids in cells

Androgen metabolism in the human epididymis. Effect of in vivo estrogen administration

Androgen metabolism in human epididymis was studied by incubating tissue fragments with isotopically labeled testosterone (T) and androstenedione (A) under batch and superfusion conditions. Epididymides were obtained from 16 patients with prostatic cancer, 5 of them treated with diethylstilbestrol (2.5 mg/d) for several months prior to castration. Results from batch incubations with [3H]T (100 nM) for 2 h at 25 degrees C indicated a markedly lower 5 alpha-reductase activity in tissues from estrogen-treated patients, as evaluated by measuring the amounts of radioactive 5 alpha-dihydrotestosterone, 5 alpha-androstanediols and 5 alpha-androstanedione present in tissue and medium at the end of the incubation period. Superfusion experiments confirmed this estrogen effect and also showed a shift of the interconversion between A and T towards the reductive direction and a diminished tissue retention of DHT after estrogen treatment. These effects may contribute to the marked regression of the epididymal epithelium that was noted in the estrogen-treated patients, which is thought to be mainly the result of the inhibition of androgen biosynthesis caused by chemical hypophysectomy.

Estradiol entry into endometrial cells in suspension

Cells from a human endometrial adenocarcinoma cell line (HEC-50) were superfused with mixtures of [3H]E2 and [14C]E1 in order to estimate rates of entry and exit of E1 and E2 into and out of cells according to previously published procedures (J. steroid Biochem., 13 (1980) 1379). Proportionality between rates of entry and concentrations of E2 outside the cells, indicative of passive diffusion, was found at levels of E2 ranging from 1 to 100 ng/ml. Effects of albumin and of pure human sex steroid binding protein (SBP) on the rate of entry of E2 were also evaluated in parallel superfusions. In other single tracer experiments, [3H]E2 was used at concentrations as low as 100 pg/ml and the effects of plasma proteins on entry were evaluated by measuring steady-state concentrations of E2 and E1 in cells and superfusate. Results from these experiments indicate that albumin, and to a larger extent SBP, reduced the entry of E2 into HEC-50 cells. Similar results were obtained when CG-5 cells, a variant of the human breast cancer cell line MCF-7, were superfused with [3H]E2. Further experiments are needed, however, to determine the physiologic role of plasma estrogen binding proteins on the entry and metabolism of E1 and E2 into target cells.

In vitro metabolism of C19 steroids in human endometrium

In order to quantitate the extent of intracellular metabolic conversions of C19 steroids in human endometrium, specimens of proliferative and secretory tissue were superfused at a constant rate with several pairs of labeled compounds at low concentrations. About 16% of dehydroepiandrosterone sulfate interacting with endometrial cells was converted to dehydroepiandrosterone and about 3% of this compound was converted to androstenedione. Androstenedione was reversibly reduced to testosterone and the extent of this conversion was shown to be several-fold higher in secretory than in proliferative tissue. About 1% of testosterone entering the cells was reduced to 5 alpha-dihydrotestosterone. These results demonstrate that the conversion of the main circulating C19 steroids in women, i.e. dehydroepiandrosterone sulfate and androstenedione, to 5 alpha-dihydrotestosterone, the compound considered to be the true intracellular androgen, is very small. In contrast, formation of testosterone from androstenedione is extensive and increases during the luteal phase under the influence of progesterone, a hormone known to stimulate the activity of 17 beta-hydroxysteroid dehydrogenase in human endometrium.

Enzymatic modulation of hormonal action at the target tissue

The relevance of metabolism of steroids in target tissues to hormonal action is discussed. Two mechanisms of metabolic regulation at the cellular level are considered: formation of active steroids from steroidal prehormones, and controlled conversion of the active compound to inactive metabolites. Factors regulating the direction in which interconversions between active hormones and inactive metabolites preferentially proceed are mentioned; methods of estimating the preferred direction in vitro and in vivo are described and examples are given. The estradiol-estrone system in human endometrium is used to describe a case in which a hormone (progesterone) induces an enzymatic activity (17 beta-hydroxysteroid dehydrogenase) and lowers the intracellular/extracellular ratio of concentrations of the active hormone (estradiol) by increasing its conversion to an inactive, or less active, metabolite (estrone). That hormone metabolism can effectively determine the level of unbound hormone available for association to receptors is shown with published data on estradiol in human endometrium, using a kinetic analysis in which binding and Michaelis-Menten equations are combined.