Studies on the Relationship between Androgen Receptors and the Transport of Androgens in Rat Prostate

Relative effectiveness of alternative androgen withdrawal therapies in initiating regression of rat prostate

From a large number of potentially effective androgen withdrawal regimens including bilateral orchiectomy, estrogens, antiandrogens and LHRH agonists alone or in combinations, we compared the ability of 12 different treatment options to mimic the acute results of surgical castration on the rat prostate. Agents were administered s.c. in clinical doses to groups of male rats daily for three days. On day 4 the prostatic tissue was removed and analyzed by conventional methods for whole-tissue and nuclear concentrations of dihydrotestosterone, nuclear androgen receptor and cytoplasmic androgen receptor. Castration-like changes were most pronounced with the synergistic combinations of cyproterone acetate + low-dose diethylstilbestrol, and megestrol acetate + low-dose diethylstilbestrol. Comparing the effectiveness of single agents, low-dose diethylstilbestrol was superior to cyproterone acetate, megestrol acetate, flutamide, leuprolide and RU23908. Leuprolide combined with flutamide was superior to leuprolide + cyproterone acetate, leuprolide + cyproterone acetate + low-dose diethylstilbestrol or leuprolide + RU23908 after three days of administration; however, this advantage disappeared when the treatments were extended to seven days. The observations indicate that the most potent androgen withdrawal therapies such as cyproterone acetate + low-dose diethylstilbestrol and megestrol acetate + low-dose diethylstilbestrol at best approximate but do not surpass the early effects of surgical castration. During the same time course, other regimens are characterized by a slower onset of action and a lesser degree of suppression of androgenic mechanisms within the cell.

Androgen interactions with intact nuclear envelopes from the rat ventral prostate

Intact nuclear envelopes containing nuclear pore complexes have been prepared from the rat ventral prostate. The polypeptide profile of the nuclear envelopes from the rat prostate resembled that of nuclear envelopes prepared from the male rat liver. Isolation of the nuclear envelopes after incubation of purified nuclei with radioactive dihydrotestosterone results in labelling of the membrane. More dihydrotestosterone is bound after incubations at 22 degrees C for 18 h than at 2 degrees C for 18 h or 22 degrees C for 2 h. Scatchard analysis revealed a class of binding sites with an apparent Kd of 46 nM. Dihydrotestosterone, testosterone, cyproterone acetate and methyltrienolone were effective as competitors of labelled dihydrotestosterone binding to the nuclear envelopes, while estradiol did not compete. Castration of the rats 24, 48 and 96 h prior to preparation of nuclei resulted in loss of androgen binding to the membranes. Extraction with 0.6 M NaCl resulted in the loss of 72% of the androgen binding.

Factors influencing association of glucocorticoid receptor-steroid complexes with nuclei, chromatin, and DNA: interpretation of binding data

Attempts to reconstruct, in a test tube, the steroid-hormone system of a responsive cell are fraught with enumerable difficulties. In this chapter I have attempted to point out some of the factors that affect receptor-steroid complexes and their interactions with acceptors. In most cases there is a quantitative influence of these factors on the level of steroid complex binding to acceptors. In some cases, selected experimental designs that neglect these factors and methods of presenting the observed data may lead to artifactual conclusions. Several of these problems should disappear when the prospect of pure receptor-steroid complexes [127, 147, 150, 181, 247, 248] becomes a common occurrence. Nevertheless much has already been learned about the interactions of complexes with acceptors, which in turn have been used to help formulate models of steroid-hormone action.

The nuclear uptake of androgen by human benign prostate in vitro: action of antiandrogens

An in vitro test system suitable to assess the potency of putative antiandrogens has been developed, using the human benign prostatic tissue obtained at operation. The system circumvents some problems associated with using human tissue, such as the presence of endogenous steroid and contamination with plasma proteins (particularly sex hormone binding globulin). Slices of tissue were incubated in the presence of 3H-testosterone and the uptake into nuclei was determined. The nature of the nuclear radioactivity and the steroid specificity indicates a mechanism similar to the established in the rat ventral prostate. The action of antiandrogens (cyproterone acetate, diethylstilbestrol, flutamide, hydroxylated flutamide and gestonorone capronate) has been studied at various concentrations.

Ammonium sulfate precipitation as a tool for the study of androgen receptor proteins in rat prostate and mouse kidney

Ammonium sulfate precipitation has been used for the separation of bound and free steroids in rat prostate and mouse kidney cytosol equilibrated with tritiated androgens. A high affinity, low capacity binding protein has been identified in the 35% saturation precipitate. Biochemical and physiological data indicate that this protein is identical with the previously described 8-10 S androgen receptor. It has been demonstrated that this receptor protein binds 17 beta - hydroxy-5alpha-androstan-3-one (DHT) and testosterone in both tissues. The apparent dissociation constant (Kd) of the prostatic receptor for DHT and of the renal receptor for testosterone is 1-2 nM. The number of binding sites equals 57 and 23 fmoles/mg protein in prostate and kidney respectively. Dterminations of apparent inhibition constants (Ki) for 26 steroidal and non-steroidal compounds suggest that the binding sites in these tissues is similar or identical.

Androgen-binding proteins in rat epididymis: properties of a cytoplasmic receptor for androgen similar to the androgen receptor in ventral prostate and different from androgen-binding protein (ABP)

The cytoplasmic recptor (CR) in rat epididymal 105,000 g supernatant was separated from the androgen-binding protein (ABP) by gel electrophoresis following labeling with [1,2,6,7-3H]-testosterone in vivo. ABP disappeared from epididymal supernatants after castration of hypophysectomy, while CR remained unchanged. CR was evenly distributed between caput and cauda, while much more ABP was present in caput. Properties of CR in epididymis and prostate were similar and distinctly different from ABP. Binding to CR was destroyed by charcoal treatment (1 mg/mg protein) of supernatant for 0 degrees C for 6 h, heating at 50 degrees C for 30 min, or exposure to the sulfhydryl blocking reagent, p-chloromercuriphenylsulfonate (1mM) at 25 degrees C for 30 min, while binding to ABP was unaffected. The isoelectric pH of CR (5.8) was higher than that of ABP (4.6). Dissociation of radioactive 5alpha-dihydrotestosterone (DHT) from CR and nuclear receptors was extremely slow (half-time at 0 degrees C is greater than 2 days), while dissociation from ABP was rapid (half-time at 0 degrees C is similar to 6 min). Cyproterone acetate (250 mg/100 g body weight) inhibited binding to CR both in epididymis and ventral prostate but did not affect binding to ABP. Nuclear uptake was inhibited by cyproterone to the same extent as binding to CR, indicating that nuclear uptake and binding are dependent on CR and independent of ABP. The time-course of uptake and binding in epididymal supernatant and nuclear fractions was essentially the same 1 day after bilateral castration when both CR and ABP were present or 8 days after castration when CR alone was present. It is concluded that the cytoplasmic receptor for androgen in rat epididymis has properties very similar to the androgen receptor in ventral prostate but different from ABP.

Studies on the regulation of the concentration of androgens and androgen receptors in nuclei of prostatic cells

Experiments were performed to assess the effect of intracellular androgen metabolism and the availability of cytoplasmic receptors on the concentration of androgens and androgen receptors in nuclei of prostatic cells. It was found that androgens are incorporated into the nucleus by a regulated, selective process which appears to limit the type and amount of androgen transported across the nuclear membrane. The metabolic conversion of testosterone to dihydrotestosterone which takes place in cytoplasm does not reduce transport and, very likely, affects only the ratio of testosterone and dihydrotestosterone transferred into the nucleus. In vivo, when the intranuclear concentration of androgens approaches 250 nM (8 pmol per mg DNA), an apparent concentration ceiling is reached even in the presence of a downward concentration gradient that would be expected to promote further transport across the nuclear membrane. This finding strongly suggests that in vivo the nuclear membrane acts as a barrier to the passage of androgens and, therefore, mitigates against the possibility that passive diffusion is an important mechanism of afferent transport of androgens into the nucleus. The ability of the nucleus to concentrate testosterone and dihydrotestosterone was clearly demonstrated in vivo when cytoplasmic concentrations of androgens of approximately 20 nM were accompanied by intranuclear concentrations in the vicinity of 250 nM. Since the measured concentration of testosterone and dihydrotestosterone in prostate of several species fall within the 5-20 nM range, it is evident that androgen concentrations in the nucleus as high as 250 nM may be typical of the physiological steady state. At the latter concentration the nucleus contains 60 000 androgen molecules: in approximate terms one third of this total is bound to a large molecular weight component of the nucleus, one third is bound to a 3.3 S receptor and one third is free or loosely bound. Since 60 000 androgen molecules and 20 000 receptor molecules appear in the nucleus before transport stops, it seems that the quantity of 4.4 S cytoplasmic receptor estimated at 174 plus or minus 24 pmol per mg protein (equivalent to about 8000 molecules per cell) is insufficient to account for the total influx of androgens and androgen receptors into the nucleus. Thus, although these results support the view that cytoplasmic receptors and the capacity to transport androgens are closely linked phenotypic markers of intracellular steroid hormone action, they suggest that the control of androgen concentration in the nucleus is achieved in a more intricate fashion than simply through a dependence on the presumed translocation of 4.4 S androgen-receptor complex into the nucleus.

Androgen receptor in nuclei of rat testis

Testis nuclei of hypophysectomized rats selectively accumulate labeled testosterone and 5alpha-dihydrotestosterone following the injection of tritiated testosterone in vivo. Testosterone and 5alpha-dihydrotestosterone are bound to macromolecules in nuclei and can be extracted with 0.5 M KCl. Accumulation of protein bound radioactive androgens in nuclei of isolated seminiferous tubules is similar to that of whole testis. The relative amounts of testosterone and dihydrotestosterone in purified nuclei were similar to the relative amounts bound to cytoplasmic receptors, suggesting that cytoplasmic androgen-receptor complexes may be transported into the nuclei. Binding of labeled androgen is saturable and inhibited by prior injection of unlabeled testosterone or cyproterone acetate. Nuclear binding sites are destroyed by the proteolytic enzyme pronase, but not by DNase. Like the cytoplasmic androgen-receptor complexes in rat testis, nuclear androgen-protein complexes are heat labile and dissociate slowly at 0 degrees C. androgens fail to accumulate in testis nuclei of the Stanley-Gumbreck androgen insensitive rat, a species lacking cytoplasmic androgen receptors in testis and other androgen target tissues.

Androgen receptors: relationship to growth response and to intracellular androgen transport in nine variant lines of the Shionogi mouse mammary carcinoma

Aspects of the biological significance of androgen receptors have been studied in nine variant lines of the Shionogi carcinoma, two of which are androgen dependent and seven of which are autonomous. The dependent lines, and two of the seven autonomous lines, contain androgen receptors; this finding demonstrates that the presence of receptors is not an accurate marker of hormonal dependence in vivo. Since the ability to transport androgens into the nucleus, as judged from the relative maximal rates of transport, is virtually restricted to dependent and autonomous lines which possess cytoplasmic receptors, it is clear that such receptors may play a role in regulating the intranuclear concentration of androgens. The absence of cytoplasmic receptors and the comparative lack of perceptible transfer of androgens across the nuclear membrane are features peculiar to the autonomous condition.