Potent inhibition of estrogen sulfotransferase by hydroxylated PCB metabolites: a novel pathway explaining the estrogenic activity of PCBs

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants which exert a variety of toxic effects in animals, including disturbances of sexual development and reproductive function. The estrogenic effects of PCBs may be mediated in part by hydroxylated PCB metabolites (PCB-OHs), but the mechanisms by which they are brought about are not understood. PCBs as well as PCB-Hs show low affinities for both alpha and beta estrogen receptor isoforms. In the present study we demonstrate that various environmentally relevant PCB-OHs are extremely potent inhibitors of human estrogen sulfotransferase, strongly suggesting that they indirectly induce estrogenic activity by increasing estradiol bioavailability in target tissues.

Cellular distribution of estrogen receptor beta in neonatal rat bone

Estrogens affect bone metabolism, and ovariectomy of rats results in marked bone loss caused by stimulation of osteoclastic bone resorption. Estrogen receptors (ER) have been demonstrated in osteoblasts and bone marrow stromal cells, but their presence in osteoclasts is controversial. Until recently, only one type of ER (now renamed ERalpha) had been identified. After the discovery of a novel ER subtype (ERbeta), it became necessary to re-investigate the ER expression in human and rodent bone. In the present study we examined the expression of ER mRNA in neonatal rat bone. Expression of ER alpha and beta mRNA (RT-PCR) was evident in femurs of 3-week-old male and female rats. In situ hybridization histochemistry of femural bones with digoxigenin labelled riboprobes, as well as radioactively labeled riboprobes, revealed that ERbeta mRNA was predominantly expressed in osteoblasts covering the metaphyseal bone trabecular surface. The presence of ERbeta mRNA in osteoblasts of rat bone suggests that ERbeta is involved in the mechanism of action of estrogens in bone.

Distinct effects on the conformation of estrogen receptor alpha and beta by both the antiestrogens ICI 164,384 and ICI 182,780 leading to opposite effects on receptor stability

Tissue-specific effects of 17beta-estradiol (E(2)) and synthetic estrogen receptor (ER) ligands on target gene regulation might, at least partly, be explained by a selective ligand-induced conformational change of their receptors (ERalpha and ERbeta). In this study, the effects of E(2) and the synthetic ER ligands tamoxifen (TAM), ICI 164,384, and ICI 182,780 on the conformation of ERalpha and ERbeta were examined using limited proteolytic digestion analysis. We found that E(2) induced a conformational change of ERalpha resulting in the protection of a 30-kDa product, whereas TAM protected a 28-kDa fragment. Strikingly, the ERalpha conformational change induced by both ICI 164,384 and ICI 182,780 did not result in protection but rather seems to induce a ligand concentration-dependent increase in proteolytic degradation of the 30- and 28-kDa products. Incubation of ERbeta with E(2) resulted in an increased protection of a 30-kDa fragment, whereas with TAM protection of a 29-kDa fragment was observed. In contrast to the situation with ERalpha, ICI 164,384 and ICI 182,780 incubation induced the protection in a manner similar to 30-kDa fragment E(2). In addition, the ICI compounds also induced in a dose-dependent manner the preservation of a 32-kDa fragment. Our observations demonstrate that ICI 164,384 and ICI 182,780 have distinct effects on the conformation of ERalpha and ERbeta, resulting in receptor subtype-selective opposite effects on receptor stability in vitro.

Expression of estrogen receptor alpha and beta during mouse embryogenesis

In adult mammals numerous target tissues and organs for estrogens exist. Little is known about possible target organs during embryogenesis other than the reproductive tract and the gonads. This is the first report on the expression of estrogen receptor beta (ERbeta) in comparison with ERalpha mRNA during mouse embryogenesis. We found expression of estrogen receptor mRNA in the reproductive tract, but also in the atrial wall, brain, kidney, urethra, bladder neck, mammary gland primordium, midgut, cartilage primordia and perichondria.

Ontogeny of estrogen receptor-beta expression in rat testis

The recently discovered estrogen receptor-beta (ERbeta) is expressed in rodent and human testes. To obtain insight in the physiological role of ERbeta we have investigated the cell type-specific expression pattern of ERbeta messenger RNA (mRNA) and protein in the testis of rats of various ages by in situ hybridization and immunohistochemistry. In fetal testes of rats 16 days postcoitum and testes of 4-day-old animals, fetal germ cells (gonocytes) reveal the ERbeta mRNA in their cytoplasm and the ERbeta protein in their nucleus. In testes of 11- and 15-day-old rats, ERbeta mRNA and protein were detected in Sertoli cells and type A spermatogonia. No signal was found in other types of germ cells. In the adult testes, expression of ERbeta mRNA as well as ERbeta protein was found in pachytene spermatocytes from epithelial stages VII-XIV and in round spermatids from stages I-VIII. Low ERbeta expression was observed in all type A spermatogonia, including undifferentiated A spermatogonia, whereas no expression was found in In and type B spermatogonia and early spermatocytes. At all ages, Sertoli cells showed a weak hybridization signal as well as weak immunoreactivity for ERbeta. In adult testes, no ERbeta mRNA or protein was detected in the interstitial tissue, indicating that Leydig cells and peritubular myoid cells do not express ERbeta. The expression of ERbeta in fetal and late male germ cells as well as in Sertoli cells suggests that estrogens directly affect germ cells during testicular development and spermatogenesis.

Interaction of estrogenic chemicals and phytoestrogens with estrogen receptor beta

The rat, mouse and human estrogen receptor (ER) exists as two subtypes, ER alpha and ER beta, which differ in the C-terminal ligand-binding domain and in the N-terminal transactivation domain. In this study, we investigated the estrogenic activity of environmental chemicals and phytoestrogens in competition binding assays with ER alpha or ER beta protein, and in a transient gene expression assay using cells in which an acute estrogenic response is created by cotransfecting cultures with recombinant human ER alpha or ER beta complementary DNA (cDNA) in the presence of an estrogen-dependent reporter plasmid. Saturation ligand-binding analysis of human ER alpha and ER beta protein revealed a single binding component for [3H]-17beta-estradiol (E2) with high affinity [dissociation constant (Kd) = 0.05 - 0.1 nM]. All environmental estrogenic chemicals [polychlorinated hydroxybiphenyls, dichlorodiphenyltrichloroethane (DDT) and derivatives, alkylphenols, bisphenol A, methoxychlor and chlordecone] compete with E2 for binding to both ER subtypes with a similar preference and degree. In most instances the relative binding affinities (RBA) are at least 1000-fold lower than that of E2. Some phytoestrogens such as coumestrol, genistein, apigenin, naringenin, and kaempferol compete stronger with E2 for binding to ER beta than to ER alpha. Estrogenic chemicals, as for instance nonylphenol, bisphenol A, o, p'-DDT and 2',4',6'-trichloro-4-biphenylol stimulate the transcriptional activity of ER alpha and ER beta at concentrations of 100-1000 nM. Phytoestrogens, including genistein, coumestrol and zearalenone stimulate the transcriptional activity of both ER subtypes at concentrations of 1-10 nM. The ranking of the estrogenic potency of phytoestrogens for both ER subtypes in the transactivation assay is different; that is, E2 >> zearalenone = coumestrol > genistein > daidzein > apigenin = phloretin > biochanin A = kaempferol = naringenin > formononetin = ipriflavone = quercetin = chrysin for ER alpha and E2 >> genistein = coumestrol > zearalenone > daidzein > biochanin A = apigenin = kaempferol = naringenin > phloretin = quercetin = ipriflavone = formononetin = chrysin for ER beta. Antiestrogenic activity of the phytoestrogens could not be detected, except for zearalenone which is a full agonist for ER alpha and a mixed agonist-antagonist for ER beta. In summary, while the estrogenic potency of industrial-derived estrogenic chemicals is very limited, the estrogenic potency of phytoestrogens is significant, especially for ER beta, and they may trigger many of the biological responses that are evoked by the physiological estrogens.

Interaction of estrogenic chemicals and phytoestrogens with estrogen receptor beta

The rat, mouse and human estrogen receptor (ER) exists as two subtypes, ER alpha and ER beta, which differ in the C-terminal ligand-binding domain and in the N-terminal transactivation domain. In this study, we investigated the estrogenic activity of environmental chemicals and phytoestrogens in competition binding assays with ER alpha or ER beta protein, and in a transient gene expression assay using cells in which an acute estrogenic response is created by cotransfecting cultures with recombinant human ER alpha or ER beta complementary DNA (cDNA) in the presence of an estrogen-dependent reporter plasmid. Saturation ligand-binding analysis of human ER alpha and ER beta protein revealed a single binding component for [3H]-17beta-estradiol (E2) with high affinity [dissociation constant (Kd) = 0.05 - 0.1 nM]. All environmental estrogenic chemicals [polychlorinated hydroxybiphenyls, dichlorodiphenyltrichloroethane (DDT) and derivatives, alkylphenols, bisphenol A, methoxychlor and chlordecone] compete with E2 for binding to both ER subtypes with a similar preference and degree. In most instances the relative binding affinities (RBA) are at least 1000-fold lower than that of E2. Some phytoestrogens such as coumestrol, genistein, apigenin, naringenin, and kaempferol compete stronger with E2 for binding to ER beta than to ER alpha. Estrogenic chemicals, as for instance nonylphenol, bisphenol A, o, p'-DDT and 2',4',6'-trichloro-4-biphenylol stimulate the transcriptional activity of ER alpha and ER beta at concentrations of 100-1000 nM. Phytoestrogens, including genistein, coumestrol and zearalenone stimulate the transcriptional activity of both ER subtypes at concentrations of 1-10 nM. The ranking of the estrogenic potency of phytoestrogens for both ER subtypes in the transactivation assay is different; that is, E2 >> zearalenone = coumestrol > genistein > daidzein > apigenin = phloretin > biochanin A = kaempferol = naringenin > formononetin = ipriflavone = quercetin = chrysin for ER alpha and E2 >> genistein = coumestrol > zearalenone > daidzein > biochanin A = apigenin = kaempferol = naringenin > phloretin = quercetin = ipriflavone = formononetin = chrysin for ER beta. Antiestrogenic activity of the phytoestrogens could not be detected, except for zearalenone which is a full agonist for ER alpha and a mixed agonist-antagonist for ER beta. In summary, while the estrogenic potency of industrial-derived estrogenic chemicals is very limited, the estrogenic potency of phytoestrogens is significant, especially for ER beta, and they may trigger many of the biological responses that are evoked by the physiological estrogens.

The estrogen receptor beta subtype: a novel mediator of estrogen action in neuroendocrine systems

The recent discovery that an additional estrogen receptor (ERbeta) subtype is present in many rat, mouse, and human tissues has advanced our understanding of the mechanisms underlying estrogen signalling. Ligand-binding experiments have shown specific binding of 17beta-estradiol by ERbeta with an affinity similar to that of ERalpha. The rat tissue distribution and/or the relative level of ERalpha and ERbeta expression seems to be quite different, i.e., moderate to high expression in uterus, testis, pituitary, ovary, kidney, epididymis, and adrenal for ERalpha and prostate, ovary, lung, bladder, brain, bone, uterus, and testis for ERbeta. Within the same organ it often appears that the ER subtypes are expressed in different cell types, supporting the hypothesis that the ER's may have different biological functions. The cell type-specific expression of ERalpha and ERbeta in rat prostate, testis, uterus, ovary, and brain and the distribution of ERbeta mRNA in the ERalpha knock-out mouse brain are discussed. The discovery of ERbeta suggests the existence of two previously unrecognized pathways of estrogen signalling; via the ERbeta subtype in tissues exclusively expressing this subtype and via the formation of heterodimers in tissues expressing both ER subtypes. The existence of two ER subtypes, their differential expression pattern, and different actions on certain response elements could provide explanations for the striking species-, cell-, and promoter-specific actions of estrogens and antiestrogens. The challenge for the future is to unravel the detailed physiological role of each subtype and to use this knowledge to develop the next generation of ER-targeted drugs with improved therapeutic profiles in the treatment or prevention of osteoporosis, cardiovascular system disorders, Alzheimer's disease, breast cancer, and disorders of the urogenital tract.

Increased expression of estrogen receptor-beta mRNA in male blood vessels after vascular injury

Estrogen exerts direct effects on vascular endothelial and smooth muscle cells that are important for vascular protection. Estrogen receptor-alpha (ERalpha) is expressed in vascular cells from males and females and may mediate some of the effects of estrogen on vascular tissue. However, we recently found that estrogen is able to protect against vascular injury in ovariectomized female ERalpha knockout mice. These mice express the newly described estrogen receptor-beta (ERbeta) in their aortas, suggesting that ERbeta may also mediate some of the direct effects of estrogen on the vasculature. In this study, the level of expression of ERalpha and ERbeta mRNA in male rat aortas was examined before and after vascular injury using en face (Häutchen) preparations and in situ hybridization. Little or no change in ERalpha expression was observed after vascular injury in either vascular endothelial or smooth muscle cells at any time point. In contrast, ERbeta mRNA was found to be expressed markedly after balloon injury. In endothelial cells, ERbeta was increased by 2 days after injury, and high levels of expression were maintained at 8 and 14 days. Furthermore, ERbeta expression was high in luminal smooth muscle cells at 8 and 14 days after injury and had decreased to low levels by 28 days after injury. These data demonstrate the presence of ERbeta in male vascular tissues and the induction of ERbeta mRNA expression after vascular injury, supporting a role for ERbeta in the direct vascular effects of estrogen.

Differential distribution and regulation of estrogen receptor-alpha and -beta mRNA within the female rat brain

In the present study, estrogen receptor (ER)alpha and ER beta genes were found to be differentially expressed in discrete subregions of the rat amygdaloid complex. The amygdala nuclei showing predominant ER alpha mRNA expression included the posterolateral cortical nucleus, amygdala hippocampal area, and lateral dorsolateral nucleus, whereas the amygdala areas with predominant ER beta mRNA expression were the medial anterodorsal and central nuclei. Both ER alpha and ER beta mRNAs were highly expressed in the medial posterodorsal nucleus. In addition to the discrete anatomical expression patterns, there appeared to be a differential regulation by estradiol of the ER alpha and ER beta mRNAs. Two weeks of estradiol (170 microgram total) treatment decreased ER alpha mRNA expression levels in the arcuate, ventromedial hypothalamus, and posterolateral cortical amygdala nucleus, but increased ER beta mRNA in the arcuate. In the medial amygdala nuclei, only ER beta mRNA levels were altered (reduced) by estradiol treatment. These results suggest that estrogen can modulate behaviors and functions mediated by the amygdala and hypothalamus via differentially regulated ER subtypes.

Differential expression of estrogen receptors alpha and beta mRNA during differentiation of human osteoblast SV-HFO cells

Estrogens have been shown to be essential for maintaining a sufficiently high bone mineral density and ER alpha expression has been demonstrated in bone cells. Recently, a novel estrogen receptor, estrogen receptor beta (ERbeta) has been identified. Here we demonstrate that also ERbeta is expressed in human osteoblasts, and that ER alpha and ERbeta are differentially expressed during human osteoblast differentiation. ERbeta mRNA expression increased gradually during osteoblast culture, resulting in an average increase of 9.9+/-5.3 fold (mean+/-S.D., n=3) at day 21 (mineralization phase) as compared to day 6 (proliferation phase). In contrast, ER alpha mRNA expression levels increased only slightly until day 10 (2.3+/-1.7 fold) and then remained constant. The observed differential regulation of ER alpha and beta is suggestive for an additional functional role of ERbeta to ER alpha in bone metabolism.

Differential ligand activation of estrogen receptors ERalpha and ERbeta at AP1 sites

The transactivation properties of the two estrogen receptors, ERalpha and ERbeta, were examined with different ligands in the context of an estrogen response element and an AP1 element. ERalpha and ERbeta were shown to signal in opposite ways when complexed with the natural hormone estradiol from an AP1 site: with ERalpha, 17beta-estradiol activated transcription, whereas with ERbeta, 17beta-estradiol inhibited transcription. Moreover, the antiestrogens tamoxifen, raloxifene, and Imperial Chemical Industries 164384 were potent transcriptional activators with ERbeta at an AP1 site. Thus, the two ERs signal in different ways depending on ligand and response element. This suggests that ERalpha and ERbeta may play different roles in gene regulation.

Mouse estrogen receptor beta forms estrogen response element-binding heterodimers with estrogen receptor alpha

The recent discovery that an additional estrogen receptor subtype is present in various rat tissues has advanced our understanding of the mechanisms underlying estrogen signaling. Here we report on the cloning of the cDNA encoding the mouse homolog of estrogen receptor-beta (ER beta) and the functional characterization of mouse ER beta protein. ER beta is shown to have overlapping DNA-binding specificity with that of the estrogen receptor-alpha (ER alpha) and activates transcription of reporter gene constructs containing estrogen-response elements in transient transfections in response to estradiol. Using a mammalian two-hybrid system, the formation of heterodimers of the ER beta and ER alpha subtypes was demonstrated. Furthermore, ER beta and ER alpha form heterodimeric complexes with retained DNA-binding ability and specificity in vitro. In addition, DNA binding by the ER beta/ER alpha heterodimer appears to be dependent on both subtype proteins. Taken together these results suggest the existence of two previously unrecognized pathways of estrogen signaling; I, via ER beta in cells exclusively expressing this subtype, and II, via the formation of heterodimers in cells expressing both receptor subtypes.

The novel estrogen receptor-beta subtype: potential role in the cell- and promoter-specific actions of estrogens and anti-estrogens

The recent discovery that an additional estrogen receptor (ER) subtype is present in various rat, mouse and human tissues has advanced our understanding of the mechanisms underlying estrogen signalling. The discovery of a second ER subtype (ERbeta) suggests the existence of two previously unrecognised pathways of estrogen signalling: via the ERbeta subtype in tissues exclusively expressing this subtype and via the formation of heterodimers in tissues expressing both ER subtypes. Various models have been suggested as explanations for the striking cell- and promoter-specific effects of estrogens and anti-estrogens, all on the basis of the assumption that only a single ER gene exists. This minireview describes several of these models and focuses on the potential role which the novel ERbeta subtype might have in this regard.

Comparison of the ligand binding specificity and transcript tissue distribution of estrogen receptors alpha and beta

The rat estrogen receptor (ER) exists as two subtypes, ER alpha and ER beta, which differ in the C-terminal ligand binding domain and in the N-terminal transactivation domain. In this study we investigated the messenger RNA expression of both ER subtypes in rat tissues by RT-PCR and compared the ligand binding specificity of the ER subtypes. Saturation ligand binding analysis of in vitro synthesized human ER alpha and rat ER beta protein revealed a single binding component for 16 alpha-iodo-17 beta-estradiol with high affinity [dissociation constant (Kd) = 0.1 nM for ER alpha protein and 0.4 nM for ER beta protein]. Most estrogenic substances or estrogenic antagonists compete with 16 alpha-[125I]iodo-17 beta-estradiol for binding to both ER subtypes in a very similar preference and degree; that is, diethylstilbestrol > hexestrol > dienestrol > 4-OH-tamoxifen > 17 beta-estradiol > coumestrol, ICI-164384 > estrone, 17 alpha-estradiol > nafoxidine, moxestrol > clomifene > estriol, 4-OH-estradiol > tamoxifen, 2-OH-estradiol, 5-androstene-3 beta, 17 beta-diol, genistein for the ER alpha protein and dienestrol > 4-OH-tamoxifen > diethylstilbestrol > hexestrol > coumestrol, ICI-164384 > 17 beta-estradiol > estrone, genistein > estriol > nafoxidine, 5-androstene-3 beta, 17 beta-diol > 17 alpha-estradiol, clomifene, 2-OH-estradiol > 4-OH-estradiol, tamoxifen, moxestrol for the ER beta protein. The rat tissue distribution and/or the relative level of ER alpha and ER beta expression seems to be quite different, i.e. moderate to high expression in uterus, testis, pituitary, ovary, kidney, epididymis, and adrenal for ER alpha and prostate, ovary, lung, bladder, brain, uterus, and testis for ER beta. The described differences between the ER subtypes in relative ligand binding affinity and tissue distribution could contribute to the selective action of ER agonists and antagonists in different tissues.

Estrogen receptor-beta mRNA expression in rat ovary: down-regulation by gonadotropins

We have examined the expression and regulation of the two estrogen receptor (ER alpha and ER beta) genes in the rat ovary, using Northern blotting, RT-PCR, and in situ hybridization histochemistry. Northern blotting results show that the ovary expresses both ER alpha and ER beta genes as single (approximately 6.5-kb) and multiple (ranging from approximately 1.0-kb to approximately 10.0-kb) transcripts, respectively. ER alpha mRNA is expressed at a level lower than ER beta mRNA in immature rat ovaries. This relationship appears unchanged between sexually mature adult rats and immature rats. In sexually mature adult rats undergoing endogenous hormonal changes, whole ovarian content of ER beta mRNA, as determined by RT-PCR, remained more or less constant with the exception of the evening of proestrus when ER beta mRNA levels were decreased. Examination of ER beta mRNA expression at the cellular level, by in situ hybridization, showed that ER beta mRNA is expressed preferentially in granulosa cells of small, growing, and preovulatory follicles, although weak expression of ER beta mRNA was observed in a subset of corpora lutea, and that the decrease in ER beta mRNA during proestrous evening is attributable, at least in part, to down-regulation of ER beta mRNA in the preovulatory follicles. This type of expression and regulation was not typical for ER alpha mRNA in the ovary. Although whole ovarian content of ER alpha mRNA was clearly detected by RT-PCR, no apparent modulation of ER alpha mRNA levels was observed during the estrous cycle. Examination of ER alpha mRNA expression at the cellular level, by in situ hybridization, showed that ER alpha mRNA is expressed at a low level throughout the ovary with no particular cellular localization. To further examine the potential role of the preovulatory pituitary gonadotropins in regulating ER beta mRNA expression in the ovary, we used immature rats treated with gonadotropins. In rats undergoing exogenous hormonal challenges, whole ovarian content of ER beta mRNA, as determined by RT-PCR, remained more or less unchanged after an injection of PMSG. In contrast, a subsequent injection of human CG (hCG) resulted in a substantial decrease in whole ovarian content of ER beta mRNA. In situ hybridization for ER beta mRNA shows that small, growing, and preovulatory follicles express ER beta mRNA in the granulosa cells. The preovulatory follicles contain ER beta mRNA at a level lower than that observed for small and growing follicles. In addition, there is an abrupt decrease in ER beta mRNA expression in the preovulatory follicles after hCG injection. The inhibitory effect of hCG on ER beta mRNA expression was also observed in cultured granulosa cells. Moreover, agents stimulating LH/CG receptor-associated intracellular signaling pathways (forskolin and a phorbol ester) readily mimicked the effect of hCG in down-regulating ER beta mRNA in cultured granulosa cells. Taken together, our results demonstrate that 1) the ovary expresses both ER alpha and ER beta genes, although ER beta is the predominant form of estrogen receptor in the ovary, 2) ER beta mRNA is localized predominantly to the granulosa cells of small, growing, and preovulatory follicles, and 3) the preovulatory LH surge down-regulates ER beta mRNA. These results clearly implicate the physiological importance of ER beta in female reproductive functions.

Cloning of a novel receptor expressed in rat prostate and ovary

We have cloned a novel member of the nuclear receptor superfamily. The cDNA of clone 29 was isolated from a rat prostate cDNA library and it encodes a protein of 485 amino acid residues with a calculated molecular weight of 54.2 kDa. Clone 29 protein is unique in that it is highly homologous to the rat estrogen receptor (ER) protein, particularly in the DNA-binding domain (95%) and in the C-terminal ligand-binding domain (55%). Expression of clone 29 in rat tissues was investigated by in situ hybridization and prominent expression was found in prostate and ovary. In the prostate clone 29 is expressed in the epithelial cells of the secretory alveoli, whereas in the ovary the granuloma cells in primary, secondary, and mature follicles showed expression of clone 29. Saturation ligand-binding analysis of in vitro synthesized clone 29 protein revealed a single binding component for 17beta-estradiol (E2) with high affinity (Kd= 0.6 nM). In ligand-competition experiments the binding affinity decreased in the order E2 > diethylstilbestrol > estriol > estrone > 5alpha-androstane-3beta,17beta-diol >> testosterone = progesterone = corticosterone = 5alpha-androstane-3alpha,17beta-diol. In cotransfection experiments of Chinese hamster ovary cells with a clone 29 expression vector and an estrogen-regulated reporter gene, maximal stimulation (about 3-fold) of reporter gene activity was found during incubation with 10 nM of E2. Neither progesterone, testosterone, dexamethasone, thyroid hormone, all-trans-retinoic acid, nor 5alpha-androstane-3alpha,I7beta-diol could stimulate reporter gene activity, whereas estrone and 5alpha-androstane-3beta,17beta-diol did. We conclude that clone 29 cDNA encodes a novel rat ER, which we suggest be named rat ERbeta to distinguish it from the previously cloned ER (ERalpha) from rat uterus.

Phosphotryptic peptide analysis of the human androgen receptor: detection of a hormone-induced phosphopeptide

Phosphorylation of the androgen receptor (AR) in human prostate tumor cells (LNCaP) is increased by androgens. The AR is expressed as two isoforms with apparent molecular masses of 110 and 112 kDa. Metabolic labeling experiments with [32P]orthophosphate revealed that only the 112 kDa isoform is radioactively labeled. Phosphoamino acid analysis revealed only phosphorylation on serine residues. Phosphotryptic peptide analysis of human AR protein by two-dimensional peptide mapping and by reverse-phase HPLC showed phosphorylation at multiple sites. Comparison of phosphopeptide maps of AR protein from cells incubated in the absence or presence of the synthetic androgen R1881 indicated that the ligand-stimulated phosphorylation is probably due to induction of phosphorylation at a new site rather than increased phosphorylation at an existing site. This result suggests that hormone-dependent AR phosphorylation might play a role in the signal transduction pathway of androgens.

Changes in the abundance of androgen receptor isotypes: effects of ligand treatment, glutamine-stretch variation, and mutation of putative phosphorylation sites

The SDS-polyacrylamide gel electrophoresis (SDS-PAGE) migration pattern of wild-type and mutated human androgen receptors (ARs) expressed in COS-1 cells was analyzed. In the absence of hormone, the wild-type AR migrated as a closely spaced 110-112 kDa doublet. Alkaline phosphatase treatment resulted in a single 110 kDa band showing that the 112 kDa upshift reflects receptors phosphorylation. Deletion of the N-terminal amino acids 46-101 or 100-142 resulted in mutant ARs migrating as single protein bands. Three consensus phosphorylation sites in this region were substituted, and the resulting mutated proteins were analyzed. Two Ser-Pro-directed kinase consensus sites at positions Ser-80 and Ser-93 were both necessary for the AR 112 kDa upshift. Substitution of the putative casein kinase II Ser-118 site had no effect on the AR migration pattern. Surprisingly, deletion of the glutamine repeat, located directly N-terminal of the Ser-Pro sites, resulted also in an AR single form. Lengthening of the glutamine repeat caused an increase in the spacing between the two isotypes of the doublet, showing that the number of glutamine residues determines the extent of the upshift. Hormone treatment induced an extra isotype with an apparent molecular mass of 114 kDa, resulting in a 110-112-114 kDa AR triplet. The hormone-induced upshift was dependent on the Ser-80 consensus phosphorylation site. Mutations in the DNA binding domain caused a different distribution of receptor protein over the three AR isotypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Steroid hormone receptor phosphorylation: is there a physiological role?

All members of the steroid hormone receptor family are phosphoproteins. Additional phosphorylation occurs in the presence of hormone. This hormone-induced phosphorylation, which is 2- to 7-fold more than the basal phosphorylation, is a rapid process. All steroid receptors are phosphorylated at more than one single site. Most phosphorylation sites are located in the N-terminal domain, and phosphorylation occurs mainly on serine residues. Phosphorylation on threonine residues occurs in only a few cases. Phosphorylation on tyrosine residues has been found only for the estrogen receptor. Six different protein kinases are possibly involved in steroid receptor phosphorylation (estrogen receptor kinase; protein kinase A; protein kinase C; casein kinase II; DNA-dependent kinase; Ser-Pro kinases). Steroid receptor phosphorylation has been directly implicated in: activation of hormone binding, nuclear import of steroid receptors, modulation of binding to hormone response elements, and consequently in transcription activation.

In vitro translation of androgen receptor cRNA results in an activated androgen receptor protein

Translation of androgen receptor (AR) cRNA in a reticulocyte lysate and subsequent analysis of the translation products by SDS/PAGE showed a protein with an apparent molecular mass of 108 kDa. Scatchard-plot analysis revealed a single binding component with high affinity for R1881 (Kd = 0.3 nM). All AR molecules synthesized specifically bound steroid. No evidence for AR phosphorylation during in vitro synthesis was found. When AR was labelled with [3H]R1881 and analysed on sucrose-density gradients, a complex of approx. 6 S was observed. The complex was shifted to a higher sedimentation coefficient after incubation with a monoclonal AR antibody directed against an epitope in the DNA-binding domain. In the presence as well as the absence of hormone, AR molecules were able to bind to DNA-cellulose without an activation step. Gel retardation assays revealed that the AR forms complexes with a DNA element containing glucocorticoid-responsive element/androgen-responsive element sequences. Receptor-DNA interactions were stabilized by different polyclonal antibodies directed against either the N- or C-terminal part of the AR and were abolished by an antibody directed against the DNA-binding domain of the receptor. In conclusion, translation of AR cRNA in vitro yields an activated AR protein which binds steroid with high affinity. It is proposed that AR antibodies enhance AR-DNA binding by stabilizing AR dimers when bound to DNA.

Localization and hormonal stimulation of phosphorylation sites in the LNCaP-cell androgen receptor

Phosphorylation of the androgen receptor in human prostate tumour cells (LNCaP) is increased by addition of androgens to intact cells. Double-label studies, using [35S]methionine incorporation into receptor protein, and [32P]P(i) to label metabolically receptor phosphorylation sites, have enabled us to determine the phosphate content, relative to receptor protein, of both nontransformed and transformed and androgen receptors generated in intact LNCaP cells. No net change in the phosphorylation of the intact 110 kDa steroid-binding component of the androgen-receptor complex was found upon transformation to the tight nuclear binding form in the intact cell. Partial proteolysis of androgen receptor protein metabolically labelled with [32P]P(i) and photolabelled with [3H]R1881 (methyltrienolone) revealed that phosphorylation occurs mainly in the N-terminal trans-activation domain, whereas no phosphorylation was detected in the steroid- and DNA-binding domains. The location of most (> 90%) of the hormonally regulated phosphorylation sites in the N-terminal trans-activation domain suggests a role of phosphorylation of the androgen receptor in transcription regulation.

Androgen receptor heterogeneity in LNCaP cells is caused by a hormone independent phosphorylation step

Androgen receptor synthesis and modification were studied in the human LNCaP cell line. Immunoblotting showed that the androgen receptor migrated as a closely spaced 110-112 kDa doublet on SDS-PAGE gels. Most of the receptor protein is present in the higher molecular mass form. Labelling experiments with [35S]methionine showed that the androgen receptor is synthesized as a single 110 kDa protein which is rapidly converted to a 112 kDa protein. Upon alkaline phosphatase treatment a gradual elimination of the 112 kDa isoform with a concomitant increase of the 110 kDa isoform was seen, indicating that the observed 110 to 112 kDa upshift reflects androgen receptor phosphorylation. Furthermore, it is shown that both isoforms can bind hormone and undergo a hormone dependent transformation to a tight nuclear binding form, indicating that the 110 to 112 kDa conversion is not an obligatory step for hormone binding or receptor transformation.

The human androgen receptor: structure/function relationship in normal and pathological situations

Discrete functions have been attributed to precise regions of the human androgen receptor (hAR) by expression of deletion mutants in COS and HeLa cells. A large C-terminal domain constitutes the hormone-binding region and a central basis, cysteine-rich domain is responsible for DNA binding. In addition, separate domains responsible for transactivation and nuclear translocation have been identified. In LNCaP cells (a prostate tumor cell line) the hAR is a heterogeneous protein which is synthesized as a single 110 kDa protein, but becomes rapidly phosphorylated to a 112 kDa protein. Metabolic labeling experiments using radioactive orthophosphate also indicated that the hAR is a phosphoprotein. Structural analysis of the AR gene in LNCaP cells and in 46, XY-individuals displaying androgen insensitivity (AIS) has revealed several different point mutations. In LNCaP cells the mutation affects both binding specificity and transactivation by different steroids. In a person with complete AIS a point mutation was identified in the splice donor site of intron 4, which prevents normal splicing and activates a cryptic splice donor site in exon 4. The consequence is a functionally inactive AR protein due to an in-frame deletion in the steroid-binding domain. In two unrelated individuals with complete AIS, two different single nucleotide alterations in codon 686 (Asp) were found. Both mutations resulted in functionally inactive ARs due to rapidly dissociating hormone-AR complexes. It is concluded that the hAR is a heterogeneous phosphoprotein in which functional errors have a dramatic impact on phenotype and fertility of 46, XY-individuals.

The androgen receptor in LNCaP cells contains a mutation in the ligand binding domain which affects steroid binding characteristics and response to antiandrogens

The human prostate tumor cell line LNCaP contains an abnormal androgen receptor system with broad steroid binding specificity. Progestagens, estradiol and several antiandrogens compete with androgens for binding to the androgen receptor in the cells to a higher extent than in other androgen sensitive systems. Optimal growth of LNCaP cells is observed after addition of the synthetic androgen R1881 (0.1 nM). In addition, estrogens, progestagens and several antiandrogens do not inhibit androgen responsive growth, but have striking growth stimulatory effects and increase EGF receptor level and acid phosphatase secretion. We have found that the androgen receptor in the LNCaP cells contains a single point mutation changing the sense of codon 868 (Thr to Ala) in the ligand binding domain. Expression vectors containing the normal or mutated androgen receptor sequence were transfected into COS or HeLa cells. Androgens, progestagens, estrogens and several antiandrogens bind the mutated androgen receptor protein and activate the expression of an androgen-regulated reporter gene (GRE-tk-CAT), indicating that the mutation directly affects both binding specificity and the induction of gene expression. Interestingly, the antiandrogen casodex showed antiandrogenic properties in growth studies of LNCaP cells and did not induce reporter gene activity in Hela cells transfected with the mutant receptor. The mutated androgen receptor of LNCaP cells is therefore a useful tool in the elucidation of different levels of action of steroids and antisteroids.

Substitution of aspartic acid-686 by histidine or asparagine in the human androgen receptor leads to a functionally inactive protein with altered hormone-binding characteristics

We have identified two different single nucleotide alterations in codon 686 (GAC; aspartic acid) in exon 4 of the human androgen receptor gene in three unrelated families with the complete form of androgen insensitivity. One mutation (G----C) results in an aspartic acid----histidine substitution (with 15-20% of wild-type androgen-binding capacity), whereas the other mutation (G----A) leads to an aspartic acid----asparagine substitution (with normal androgen-binding capacity, but a rapidly dissociating ligand-receptor complex). The mutations eliminate a Hinfl restriction site. Screening for the loss of the Hinfl site in both families with the Asp----Asn mutation resulted in the recognition of heterozygous carriers in successive generations of each. Both mutant androgen receptors were generated in vitro and transiently expressed in COS and HeLa cells. The receptor proteins produced had the same altered binding characteristics as those measured in fibroblasts from the affected subjects. R1881-activated transcription of a GRE-tk-CAT reporter gene construct was strongly diminished by both mutant receptors and was only partially restored using a 100-fold higher concentration of ligand compared with wild-type receptor. Thus, aspartic acid-686 appears essential for normal androgen receptor function. Substitution of this amino acid residue, by either histidine or asparagine, results in androgen insensitivity and lack of androgen-dependent male sexual differentiation.

Synthesis and post-translational modification of the androgen receptor in LNCaP cells

Androgen receptor synthesis and modification were studied in the human LNCaP cell line. Immunoblotting with a specific polyclonal antibody showed that the androgen receptor migrated as a closely spaced 110-112 kDa doublet on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels. Most of the receptor protein is present in the higher molecular mass form. Pulse labelling experiments with [35S]methionine showed that the androgen receptor is synthesized as a single 110 kDa protein which is rapidly converted to a 112 kDa protein. Alkaline phosphatase treatment of cytosols from [35S]methionine pulse labelled cells caused a gradual elimination of the 112 kDa isoform with a concomitant increase of the 110 kDa isoform. This indicates that the observed 110 to 112 kDa upshift of the newly synthesized androgen receptor reflects receptor phosphorylation. Both isoforms can bind hormone and can undergo a hormone dependent transformation to a tight nuclear binding form, indicating that the 110 to 112 kDa conversion is not an obligatory step for hormone binding or receptor transformation.

Androgen receptor abnormalities

The human androgen receptor is a member of the superfamily of steroid hormone receptors. Proper functioning of this protein is a prerequisite for normal male sexual differentiation and development. The cloning of the human androgen receptor cDNA and the elucidation of the genomic organization of the corresponding gene has enabled us to study androgen receptors in subjects with the clinical manifestation of androgen insensitivity and in a human prostate carcinoma cell line (LNCaP). Using PCR amplification, subcloning and sequencing of exons 2-8, we identified a G----T mutation in the androgen receptor gene of a subject with the complete form of androgen insensitivity, which inactivates the splice donor site at the exon 4/intron 4 boundary. This mutation causes the activation of a cryptic splice donor site in exon 4, which results in the deletion of 41 amino acids from the steroid binding domain. In two other independently arising cases we identified two different nucleotide alterations in codon 686 (GAC; aspartic acid) located in exon 4. One mutation (G----C) results in an aspartic acid----histidine substitution (with negligible androgen binding), whereas the other mutation (G----A) leads to an aspartic acid----asparagine substitution (normal androgen binding, but a rapidly dissociating androgen receptor complex). Sequence analysis of the androgen receptor in human LNCaP-cells (lymph node carcinoma of the prostate) revealed a point mutation (A----G) in codon 868 in exon 8 resulting in the substitution of threonine by alanine. This mutation is the cause of the altered steroid binding specificity of the LNCaP-cell androgen receptor. The functional consequences of the observed mutations with respect to protein expression, specific ligand binding and transcriptional activation, were established after transient expression of the mutant receptors in COS and HeLa cells. These findings illustrate that functional errors in the human androgen receptor have an enormous impact on phenotype and fertility.

The androgen receptor: functional structure and expression in transplanted human prostate tumors and prostate tumor cell lines

The growth of the majority of prostate tumors is androgen-dependent, for which the presence of a functional androgen receptor is a prerequisite. Tumor growth can be inhibited by blockade of androgen receptor action. However, this inhibition is transient. To study the role of the androgen receptor in androgen-dependent and androgen-independent prostate tumor cell growth, androgen receptor mRNA expression was monitored in six different human prostate tumor cell lines and tumors, which were grown either in vitro or by transplantation on (male) nude mice. Androgen receptor mRNA was clearly detectable in three androgen-dependent (sensitive) tumors and absent or low in three androgen-independent tumors. Growth of the LNCaP prostate tumor cell line can be stimulated both by androgens and by fetal calf serum. In the former situation androgen receptor mRNA expression is downregulated, whereas in the latter no effect on androgen receptor mRNA levels can be demonstrated. Sequence analysis showed that the androgen receptor gene from LNCaP cells contains a point mutation in the region encoding the steroid-binding domain, which confers an ACT codon encoding a threonine residue to GCT, encoding alanine.

A mutation in the ligand binding domain of the androgen receptor of human LNCaP cells affects steroid binding characteristics and response to anti-androgens

LNCaP prostate tumor cells contain an abnormal androgen receptor system. Progestagens, estradiol and anti-androgens can compete with androgens for binding to the androgen receptor and can stimulate both cell growth and excretion of prostate specific acid phosphatase. We have discovered in the LNCaP androgen receptor a single point mutation changing the sense of codon 868 (Thr to Ala) in the ligand binding domain. Expression vectors containing the normal or mutated androgen receptor sequence were transfected into COS or Hela cells. Androgens, progestagens, estrogens and anti-androgens bind the mutated androgen receptor protein and activate the expression of an androgen-regulated reporter gene construct (GRE-tk-CAT). The mutation therefore influences both binding and the induction of gene expression by different steroids and antisteroids.

Aberrant splicing of androgen receptor mRNA results in synthesis of a nonfunctional receptor protein in a patient with androgen insensitivity

Androgen insensitivity is a disorder in which the correct androgen response in an androgen target cell is impaired. The clinical symptoms of this X chromosome-linked syndrome are presumed to be caused by mutations in the androgen receptor gene. We report a G----T mutation in the splice donor site of intron 4 of the androgen receptor gene of a 46,XY subject lacking detectable androgen binding to the receptor and with the complete form of androgen insensitivity. This point mutation completely abolishes normal RNA splicing at the exon 4/intron 4 boundary and results in the activation of a cryptic splice donor site in exon 4, which leads to the deletion of 123 nucleotides from the mRNA. Translation of the mutant mRNA results in an androgen receptor protein approximately 5 kDa smaller than the wild type. This mutated androgen receptor protein was unable to bind androgens and unable to activate transcription of an androgen-regulated reporter gene construct. This mutation in the human androgen receptor gene demonstrates the importance of an intact steroid-binding domain for proper androgen receptor functioning in vivo.

Structural organization of the human androgen receptor gene

The complete coding region of the human androgen receptor gene has been isolated from a genomic library. The information for the androgen receptor was found to be divided over eight exons and the total length of the gene exceeded 90 kb. The sequence encoding the N-terminal region is present in one large exon. The two putative DNA-binding fingers are encoded separately by two small exons. The information for the hormone-binding domain is split over five exons. Positions of introns are identical to those reported for the chicken progesterone receptor and the human oestrogen receptor genes. Southern blot analysis of genomic DNA with various specific probes reveal that the human androgen receptor is encoded by a single-copy gene.

Structure and function of the androgen receptor

The androgen receptor in several species (human, rat, calf) is a monomeric protein with a molecular mass of 100-110 kDa. The steroid binding domain is confined to a region of 30 kDa, while the DNA-binding domain has the size of approx. 10 kDa. A 40 kDa fragment containing both the DNA and steroid binding domain displayed a higher DNA binding activity than did the intact 100 kDa molecule. cDNA encoding the major part of the human androgen receptor was isolated. The cDNA contains an open reading frame of 2,277 bp but still lacks part of the 5'-coding sequence. Homology with the progesterone and glucocorticoid receptor was about 80% in the DNA binding domain and 50% in the steroid binding domain. The present data provide evidence that the androgen receptor belongs to the superfamily of ligand responsive transcriptional regulators and consists of three distinct domains each with a specialized function.

The N-terminal domain of the human androgen receptor is encoded by one, large exon

Using specific cDNA hybridization probes, the first coding exon of the human androgen receptor gene was isolated from a genomic library. The exon contained an open reading frame of 1586 bp, encoding an androgen receptor amino-terminal region of 529 amino acids. The deduced amino acid sequence was characterized by the presence of several poly-amino acid stretches of which the long poly-glycine stretch (16 residues) and the poly-glutamine stretch (20 residues) were most prominent. Androgen receptor cDNAs from different sources contained information for poly-glycine stretches of variable size (23 and 27 residues, respectively). The androgen receptor amino-terminal domain was found to be hydrophilic and have a net negative charge. Combined with the previously described, partially overlapping cDNA clone 7A2M27 (Trapman et al. (1988) Biochem. Biophys. Res. Commun. 153, 241-248), the complete human androgen receptor was deduced to have a size of 910 amino acids.

Mechanism of androgen action: recent observations on the domain structure of androgen receptors and the induction of EGF-receptors by androgens in prostate tumor cells

In this paper two different aspects of androgen action are reviewed. Polyacrylamide gel electrophoresis of androgen receptors, photoaffinity labeled with R1881 showed that receptors isolated from both human prostate cells and calf uterine cytosol cells are proteins with a molecular mass of approx 110 kD. Purification to homogeneity of this form of the receptor from calf uterus also yielded a 110 kD protein. A molecular model for the DNA-binding form of the receptor is presented in which one polypeptide comprises three active domains: one for ligand binding, one for interaction with nuclear acceptor sites, and a third domain which modulates nuclear interaction. Mild digestion with chymotrypsin or a protease from rat prostates removes the modulating domain and leaves the ligand binding and nuclear interaction domain intact. Trypsin treatment yields a fragment of lower molecular mass containing the ligand binding domain with some affinity for RNA, but not DNA. In vitro studies with a human prostate tumor cell line (LNCaP), suggest that androgens not only directly effect cell growth, but also act indirectly. Both epidermal growth factor (EGF) and androgens stimulate cell growth. In addition androgens stimulate synthesis of receptors for EGF. Thus androgens effect tumor cell growth by autocrine or paracrine mechanisms by making the cells more sensitive for growth factor mediated stimuli.

The human androgen receptor: domain structure, genomic organization and regulation of expression

The domain structure and the genomic organization of the human androgen receptor (hAR) has been studied after molecular cloning and characterization of cDNA and genomic DNA encoding the hAR. The cDNA sequence reveals an open reading frame of 2751 nucleotides encoding a protein of 917 amino acids with a calculated molecular mass of 98,845 D. The N-terminal region of the hAR is characterized by a high content of acidic amino acid residues and by several homopolymeric amino acid stretches. The DNA-binding domain showed a high homology with the DNA-binding domain of the human glucocorticoid receptor (hGR) and the human progesterone receptor (hPR). The predominantly hydrophobic steroid binding domain of the hAR is 50-55% homologous with the ligand binding domains of the hGR and hPR. Transient expression of recombinant AR cDNA in COS-cells resulted in the production of a 110 kDa protein with the expected binding specificity of androgen receptors. Co-transfection with a reporter-gene construct [CAT(chloramphenicol acetyl transferase) under direction of the androgen regulated MMTV-promoter] showed that the protein is functionally active with respect to transcription regulation. In the LNCaP prostate carcinoma cell line two major (11 and 8 kb) and one minor (4.7 kb) mRNA species can be found which can be down-regulated by androgens. The hAR protein coding region was shown to be divided over eight exons with an organization similar to that of the progesterone and oestrogen receptor. The sequence encoding the N-terminal domain was found in one large exon. The two DNA-binding fingers were encoded by two small exons; the information for the androgen-binding domain was found to be distributed over five exons. Southern blot analysis of genomic DNA revealed that the hAR is encoded by one single gene, which is situated on the X-chromosome.

Cloning, structure and expression of a cDNA encoding the human androgen receptor

A cDNA clone has been isolated from a library prepared of mRNA of human breast cancer T47D cells with an oligonucleotide probe homologous to part of the region encoding the DNA-binding domain of steroid receptors. The clone has a size of 1505 bp and sequence analysis revealed an open reading frame of 1356 bp. The deduced amino acid sequence displays two highly conserved regions identified as the putative DNA-binding and hormone binding domains respectively of steroid receptors. Expression of this cDNA clone in COS cells produces a nuclear protein with all the binding characteristics of the human androgen receptor (hAR). The gene encoding the cDNA is assigned to the human X-chromosome. High levels of three hybridizing mRNA species of 11, 8.5 and 4.7 kb respectively are found in the human prostate cancer cell line (LNCaP), which contains elevated levels of hAR. The present data provide evidence that we have isolated a cDNA that encodes a major part of the human androgen receptor.

In situ photolabelling of the human androgen receptor

In situ photoaffinity labelling of the human androgen receptor has been performed in the LNCaP (Lymph Node Carcinoma of the Prostate) cell line. The covalently labelled receptors were identified by SDS-PAGE. Intact LNCaP cells, incubated with [3H]-R1881 and subsequently irradiated with u.v. light and directly solubilized in SDS-buffer, revealed two photolabelled protein bands at 110 and 50 kDa. Irradiation of intact cells and subsequent isolation of nuclei followed by extraction with 0.5 M NaCl resulted in one major photolabelled protein band at 110 kDa. The labelling of this band could be completely suppressed by a 100-fold molar excess of non-radioactive R1881. Photolabelling of androgen receptors in a cytosolic preparation of LNCaP cells after anion exchange chromatography resulted in a much lower labelling efficiency compared with the in situ labelling procedure, although the androgen receptor was purified 100-fold. The steroid binding domain of the human androgen receptor has been partially mapped with chymotrypsin and S. aureus V8 protease digestion. Proteolytic digestion with chymotrypsin of purified photoaffinity-labelled 110 kDa human androgen receptor resulted in the generation of a 15 kDa peptide which still contains the covalently linked hormone. It is concluded that the in situ photoaffinity labelling technique can be applied successfully for characterization of the steroid binding domain of androgen receptors in prostate cancer cells and in other androgen target cells. Furthermore, it was demonstrated that the human androgen receptor is a monomer with a molecular mass of 110 kDa, of which the steroid binding site is confined to a 15 kDa domain.

Analysis of steroid- and DNA-binding domains of the calf uterine androgen receptor by limited proteolysis

The DNA-binding form of the calf uterine androgen receptor (AR) was subjected to limited protease digestion using chymotrypsin, trypsin and a rat prostate cytosol protease. The properties of the generated polypeptide fragments were identified and compared with those of the intact AR. Physicochemical characterization was achieved through sedimentation analysis, gel filtration chromatography and DEAE anion exchange chromatography. Intactness of functional binding domains was evaluated by measuring the retention of steroid- and DNA-binding capacity. Under non-denaturing conditions the intact AR is a highly asymmetrical molecule with a Stokes radius (RS) of 45A, a sedimentation coefficient of 4.3S and a relative molecular mass of 80,000 daltons. This form of AR has an intrinsic binding affinity for DNA and was eluted from DNA-cellulose with 9 mM MgCl2. Chymotrypsin produced a more globular polypeptide (RS: 31A; 3.1S; 41,000 daltons) with a decreased net negative charge. This fragment also displayed DNA-binding affinity but required a higher concentration of MgCl2 (14 mM) for DNA-cellulose elution, indicating an increased affinity for DNA. The observed reduction in molecular size upon chymotrypsin treatment was confirmed when analysed by SDS-polyacrylamide gel electrophoresis after covalently labelling of the AR with [3H]R1881. Rat prostate cytosol contains a protease which is very active in generating an AR polypeptide with an increased affinity for DNA, without changing the AR net negative charge (RS: 33A; 3.7S; 51,000 daltons). The specificity of this protease remained unknown since none of a large number of inhibitors was able to inactivate this enzyme. The fragment generated is different from that obtained with chymotrypsin since significant differences in size as well as in charge were measured. Trypsin treatment generated a much smaller polypeptide (RS: 25A; 2.9S; 30,000 daltons) which had lost its DNA-binding capacity, but not its steroid binding site. This form probably represents the so-called meroreceptor. When intact AR was treated sequentially with prostate cytosol and trypsin, a polypeptide fragment with identical properties was obtained, indicating the spatial separation of two of the proteolytic cleavage sites. These studies provide evidence for the distinct nature of the molecular domains for androgen and DNA interaction on the calf uterine AR.

Characterization of androgen receptors in a transplantable human prostatic adenocarcinoma (PC-82)

The transplantable human prostatic adenocarcinoma, PC-82, has been shown to be a suitable model for the study of several aspects of androgen-regulated tumor growth. This tumor contains an androgen receptor, and the purpose of the present investigation was to characterize this androgen receptor with respect to hormone specificity, sedimentation coefficient, dissociation constant, Stokes radius, ionic properties, and molecular mass. Cytosol was prepared from tumor tissues grown in athymic nude mice, which were castrated 10 days before harvesting the tumor. Scatchard plot analysis revealed a binding protein with a Kd of 0.1 nM for R1881 (methyltrienolone) and binding capacity of 120 fmol/mg protein. The receptor showed a high affinity for R1881, testosterone, and 5 alpha-dihydrotestosterone, respectively, whereas no or little affinity was found for progesterone and estradiol. In the presence of 10 mM molybdate the androgen receptor in PC-82 cytosol eluted from an FPLC anion exchange column (Mono Q) at 0.32 M NaCl, which is identical to what has been found for androgen receptors from rat prostate and calf uterine cytosol. Photoaffinity labeling of the [3H]R1881-androgen receptor complex and subsequent analysis on SDS-polyacrylamide gels resulted in a covalently labeled protein with a molecular mass of approximately 50 kD. The androgen receptor of the PC-82 tumor had a sedimentation coefficient of 4S and a Stokes radius of 3.3 nm at high ionic strength (0.4 M NaCl). It is concluded that the PC-82 tumor contains a binding protein with the properties described for androgen receptors present in prostate tissue.

Characterization of androgen receptors after photoaffinity labelling with [3H]methyltrienolone (R1881)

The synthetic androgen 17 beta-hydroxy-17 alpha-[3H]methyl-4,9,11-estratrien-3-one (R1881) has been used as photoaffinity label to characterize androgen receptors in rat prostate, in a human transplantable prostatic adenocarcinoma (PC-82) and in calf uterus. Androgen receptors preparations were partially purified either via differential chromatography on 2',5'-ADP-Sepharose (rat prostate), via anion exchange fast protein liquid chromatography (rat prostate and PC-82) or via DNA-cellulose chromatography (calf uterus). Purification factors obtained with the three different methods were: 245, 75 and 40 respectively. Photolabelling of receptor preparations was performed via irradiation with a high pressure mercury lamp either before or after partial purification. Polyacrylamide gel electrophoresis under denaturing conditions showed that the DNA-binding form of the androgen receptor in calf uterus cytosol is a protein with a molecular mass of approx 95 kD. The covalent attachment of [3H]R1881 to the 95 kD protein could be completely suppressed by a 200-fold molar excess of dihydrotestosterone. In rat prostate cytosol an androgen receptor with a molecular mass of approx 50 kD could be photoaffinity labelled with R1881. A similar size was found for the androgen receptor in the human prostatic adenocarcinoma. Our results show that photoaffinity labelling of androgen receptors with [3H]R1881 as ligand can be applied for characterization of partial purified androgen receptor preparations.

Photoaffinity labelling of androgen receptors with 17 beta-hydroxy-17 alpha-[3H]methyl-4,9,11-estratrien-3-one

The synthetic androgen 17 beta-hydroxy-17 alpha-[3H]methyl-4,9,11-estratrien-3-one (R1881) has been used as photoaffinity label to characterize androgen receptors in calf uterus and rat prostate. Polyacrylamide gel electrophoresis under denaturing conditions showed that the DNA-binding form of the androgen receptor in calf uterus cytosol is a protein with a molecular mass of 98 kD. In rat prostate cytosol an androgen receptor with a molecular mass of 46 kD could be photoaffinity labelled with R1881. The photoaffinity labelling procedure described here provides a method for studying the hormone binding domain of androgen receptors in partial purified preparations.