Steroid hormone receptors: biochemistry, genetics, and molecular biology

Beyond U0126. Dianion chemistry leading to the rapid synthesis of a series of potent MEK inhibitors

Employing phenylmalonitrile dianion chemistry, a large number of analogues of MEK inhibitor lead SH053 (IC(50)=140 nM) were rapidly synthesized leading to single digit nM inhibitors, displaying submicromolar AP-1 transcription inhibition in COS-7 cells. Compound 41, exhibiting a MEK IC(50)=12 nM showed ip activity in a TPA-induced ear edema model with an ED(50)=5 mg/kg.

MEK inhibitors: the chemistry and biological activity of U0126, its analogs, and cyclization products

In search of antiinflammatory drugs with a new mechanism of action, U0126 was found to functionally antagonize AP-1 transcriptional activity via noncompetitive inhibition of the dual specificity kinase MEK with an IC50 of 0.07 microM for MEK 1 and 0.06 microM for MEK 2. U0126 can undergo isomerization and cyclization reactions to form a variety of products, both chemically and in vivo, all of which exhibit less affinity for MEK and lower inhibition of AP-1 activity than parent, U0126.

Identification of a novel inhibitor of mitogen-activated protein kinase kinase

The compound U0126 (1,4-diamino-2,3-dicyano-1, 4-bis[2-aminophenylthio]butadiene) was identified as an inhibitor of AP-1 transactivation in a cell-based reporter assay. U0126 was also shown to inhibit endogenous promoters containing AP-1 response elements but did not affect genes lacking an AP-1 response element in their promoters. These effects of U0126 result from direct inhibition of the mitogen-activated protein kinase kinase family members, MEK-1 and MEK-2. Inhibition is selective for MEK-1 and -2, as U0126 shows little, if any, effect on the kinase activities of protein kinase C, Abl, Raf, MEKK, ERK, JNK, MKK-3, MKK-4/SEK, MKK-6, Cdk2, or Cdk4. Comparative kinetic analysis of U0126 and the MEK inhibitor PD098059 (Dudley, D. T., Pang, L., Decker, S. J., Bridges, A. J., and Saltiel, A. R. (1995) Proc. Natl. Acad. Sci U. S. A. 92, 7686-7689) demonstrates that U0126 and PD098059 are noncompetitive inhibitors with respect to both MEK substrates, ATP and ERK. We further demonstrate that the two compounds bind to deltaN3-S218E/S222D MEK in a mutually exclusive fashion, suggesting that they may share a common or overlapping binding site(s). Quantitative evaluation of the steady state kinetics of MEK inhibition by these compounds reveals that U0126 has approximately 100-fold higher affinity for deltaN3-S218E/S222D MEK than does PD098059. We further tested the effects of these compounds on the activity of wild type MEK isolated after activation from stimulated cells. Surprisingly, we observe a significant diminution in affinity of both compounds for wild type MEK as compared with the deltaN3-S218E/S222D mutant enzyme. These results suggest that the affinity of both compounds is mediated by subtle conformational differences between the two activated MEK forms. The MEK affinity of U0126, its selectivity for MEK over other kinases, and its cellular efficacy suggest that this compound will serve as a powerful tool for in vitro and cellular investigations of mitogen-activated protein kinase-mediated signal transduction.

Differential functional activities of rainbow trout and human estrogen receptors expressed in the yeast Saccharomyces cerevisiae

The cDNA of rainbow trout estrogen receptor (rtER), highly and stably expressed in yeast, Saccharomyces cerevisiae, was used to analyse the biological activity of the receptor. The rtER mRNA encoded a 65-kDa protein which was immunorevealed by a specific antibody and migrated with the authentic rtER major protein form detected in trout liver. Yeast rtER bound estradiol with high affinity and the dissociation constant (Kd = 1.35 nM) was very similar to the value measured from trout liver extracts but 3-5-fold higher than the Kd found for human estrogen receptor (hER). This indicates therefore that the rtER has a lower estradiol affinity compared to the human receptor. While the hER Kd remained unchanged at both 4 degrees C or 22 degrees C, it was slightly modified at 30 degrees C. The Kd measured for rtER at 22 degrees C and 30 degrees C were about 2-fold, and 12-fold higher, respectively, than the Kd obtained at 4 degrees C suggesting an alteration of the rtER affinity for its ligand at elevated temperature. To examine the estrogen-receptor-mediated activation of transcription in yeast, reporter plasmids integrated or not in the yeast genome were used. The reporter genes consist of one, two, or three copies of estrogen-responsive elements (ERE) upstream of the yeast proximal CYC1 or URA3 promoters fused to the lacZ gene of Escherichia coli coding for beta-galactosidase. The induction of beta-galactosidase activity for all reporter genes was strictly dependent on the presence of rtER and estrogens. The activation of transcription mediated by rtER responded in an estradiol-dose-dependent manner as in animal cells. However, compared to hER, the estradiol concentration necessary to achieve maximal activation was 10-fold higher. This is probably a consequence of the lower estradiol-affinity for rtER compared to hER. The levels of induction of the reporter genes containing two or three ERE were strongly enhanced compared to the one ERE construct. This is in agreement with the synergistic effect previously described for multiple ERE. The magnitudes of transcriptional induction mediated by rtER and hER were similar when the reporter gene containing three ERE was used but changed when the one ERE construct was used. In this case transcriptional activation indicated by rtER was 10-20 fold lower. This suggests that rtER requires protein/protein interaction for its stabilization on DNA. Antiestrogens were able to bind rtER and promote gene transcription. However, to produce effects comparable to those obtained with estrogens, much higher concentrations were required. This may imply nonetheless that antihormones were capable of provoking efficient interactions of rtER with the transcriptional machinery.

Estrogen and progesterone receptors in salivary gland adenoid cystic carcinoma

Adenoid cystic carcinomas of salivary glands occur more frequently in women and bear remarkable similarity to adenoid cystic carcinomas of the breast. In addition, breast carcinomas express estrogen and progesterone receptors that impact prognostic significance. This suggests a possible role for sex steroid hormones in the development and progression of salivary gland adenoid cystic carcinoma. On this basis, 12 samples of formalin-fixed, paraffin-embedded salivary gland adenoid cystic carcinomas and 12 samples of normal salivary gland tissue were immunohistochemically evaluated for estrogen and progesterone receptor protein expression. Estrogen receptors were not detected in either group; however, a significantly higher progesterone receptor level was evident in the neoplastic group compared with normal tissue (p < 0.01). These data confirm the presence of progesterone receptors within normal and neoplastic salivary gland tissue. Progesterone receptor expression may be of possible prognostic and therapeutic value in some cases of adenoid cystic carcinoma.

The structure of glucocorticoid receptors

The glucocorticoid receptor of mouse thymic lymphoma cells was investigated. The receptor-hormone complex in cytosolic extracts has a Stokes' radius of 82 A and Mw approximately 330 kDa. In the presence of salt at high concentrations, however, the receptor-complex has a Stokes' radius of 60 A and Mw approximately 120 kDa. This receptor form is able to interact with DNA. Chemical cross-linking was used to stabilize the high molecular weight receptor structure against subunit dissociation and this was found to prevent receptor activation to DNA binding. The affinity labeled receptor was submitted to progressive cross-linking and the intermediate cross-linked forms were analyzed. This led to the conclusion that the high molecular weight receptor structure is a hetero-tetramer consisting of one receptor polypeptide of approximately 100 kDa, two molecules of the 90 kDa heat shock protein hsp90 and an additional protein subunit. The latter was unequivocally identified by immunochemical techniques as the 59 kDa protein p59. The 70 kDa heat shock protein was found not to be a bona fide receptor component but was a contaminant of our immunopurification procedure. Cross-linking studies also showed that the receptor exists in the high molecular weight form in intact cells and in the absence of hormone.

Subunit structure of the glucocorticoid receptor and activation to the DNA-binding state

Glucocorticoid receptors of S49.1 mouse lymphoma cells were analyzed under a variety of conditions. The complexes with an agonist or a steroidal antagonist can be formed in cytosolic extracts, they are of high molecular weight, Mr approximately 330,000 and have a Stokes radius of 82 A. Cross-linking by several agents stabilized this structure against subunit dissociation which produces the activated receptor form of 60 A and DNA-binding ability. Careful analysis of intermediate cross-linked forms lead to the conclusion that the large receptor structure is a hetero-tetramer consisting of one hormone-bearing polypeptide of Mr approximately 94,000, two 90 kDa subunits and a protein component of Mr approximately 50,000. The 90 kDa subunits are the heat shock protein hsp90. The high molecular weight receptor form also exists in intact cells as revealed again by cross-linking. The cytosolic complex with the antagonist can become activated to the DNA-binding form upon warming but simultaneously looses the ligand. Ligand rebinding does not occur subsequent to receptor dissociation. Upon incubation of intact cells at 37 degrees C with agonist or antagonist the respective receptor-ligand complexes are formed. The agonist complex is immediately activated, however, the antagonist complex remains stable in the undissociated state. This explains the biological effect of the antagonist.

Glutathione-S-transferases are major cytosolic thyroid hormone binding proteins

Thyroid hormone binding proteins of rat liver cytosol were characterized. Glutathione-S-transferases were identified among major cytosolic proteins adsorbed by thyroxine affinity matrices. The Ya and Yb subunits of the glutathione-S-transferases were also principal proteins of cytosol covalently labeled with 3,3',5-triiodo-L-thyronine (T3) or 3,3',5,5'-tetraiodo-L-thyronine (T4) by photoaffinity methods. T3 and T4, but not L-thyronine or iodinated tyrosines, were bound with high affinity to purified glutathione-S-transferases and were potent inhibitors of their enzymatic activities. These results suggest that glutathione-S-transferases have the potential to function in the intracellular binding and transport of thyroid hormones. The proteins provide a means for regulating the action and metabolism of thyroid hormones by acting as high capacity binding components.

Thyroid hormone action and the erbA oncogene family

In this review, we discuss the biological action and biochemical function of the v-erbA oncogene product, and the role of c-erbA proto-oncogene products as thyroid hormone receptors, as related to the molecular structure and function of the nuclear hormone receptors at large.

Nuclear acceptor sites for estrogen-receptor complexes in the liver of the turtle, Chrysemys picta. I. Sexual differences, species specificity and hormonal dependency

Hepatic estrogen receptors (ERs) of the female turtle, Chrysemys picta, when complexed with [3H]estradiol ([3H]E2), were shown to bind specifically to liver chromatin isolated from the same species. The binding of the [3H]E2 receptor complex to chromatin requires both the steroid ligand and the receptor protein. Maximal binding occurred within 60-70 min of incubation at 4 degrees C in a Tris buffer containing 0.1 M KCl. The binding of the [3H]E2 receptor complex to intact chromatin was saturable, whereas the binding to turtle or calf thymus DNA remained linear. Scatchard analyses revealed more estrogen receptor binding sites on hepatic chromatin isolated from female turtles than that prepared from the males (binding capacities: female chromatin = 67.9 +/- 6.8 fmol/mg DNA equivalent; male chromatin = 28.5 +/- 2.5 fmol/mg DNA equivalent). Furthermore, the [3H]E2 receptor complex was bound with a higher affinity to female chromatin than to male chromatin (association constants: female chromatin = 11.7 +/- 2.7 X 10(10) M-1; male chromatin = 2.5 +/- 0.7 X 10(10) M-1). In contrast to turtle hepatic [3H]E2 receptors, ERs in rat liver or mouse uterine cytosol exhibited little binding affinity for hepatic chromatin isolated from the turtle. Tissue specificity was demonstrated in the interaction of the [3H]E2 receptor complex and chromatin; high affinity, saturable binding of the [3H]E2 receptor complex was only observed on chromatin isolated from the liver but not on those prepared from the heart, kidney and muscle. A 3- to 4-fold increase in the number of hepatic chromatin [3H]E2 receptor binding sites was observed in 21-day ovariectomized or hypophysectomized female (capacities = 209.3 +/- 6.1 and 270 +/- 10.1 fmol/mg DNA equivalent, respectively). It is postulated that [3H]E2 receptor binding sites on the chromatin of intact females are partially 'masked', and removal of a gonadal and/or pituitary factor(s) unveils additional binding sites on the female chromatin. This paper is first to report the presence of high affinity, species- and tissue-specific acceptor sites on the liver chromatin of a reptilian species. The fact that the levels and properties of these acceptor sites are dependent on the sex and hormonal state of the animal suggests that they may play a role in the regulation of hepatic estrogen responsiveness and vitellogenesis in this species.

Chemical cross-linking of heteromeric glucocorticoid receptors

Glucocorticoid receptors of wild-type and nti ("increased nuclear transfer") mutant S49.1 mouse lymphoma cells exist in extracts under low-salt conditions predominantly as high molecular weight species (Mr greater than or equal to 300,000). These receptor-hormone complexes are unable to bind to DNA. High salt (300 mM KCl) produces dissociated receptors of Mr 116,000 and 60-A Stokes radius (wild type) and Mr 60,000 and 38-A Stokes radius (nti mutant), both of which bind to DNA. We used reaction with bifunctional N-hydroxysuccinimide esters as well as oxidation with Cu2+/o-phenanthroline to stabilize the high molecular weight structures. These cross-linked complexes do not interact with DNA, but reductive cleavage again produces the dissociable receptor forms and restores their ability to bind to DNA. The protein modifying reagents iodoacetamide and diethyl pyrocarbonate also produce stabilized high molecular weight receptor complexes. Cross-linking of the high molecular weight receptor forms can also be achieved in intact cells. Immunochemical techniques were used to prove that the complexes cross-linked either in vivo or in cell extracts do contain the heat shock protein of Mr 90,000 as a common constituent. The data show that the high molecular weight receptor complexes are preexisting in intact cells and that dissociation generates DNA binding ability.

Finger proteins and DNA-specific recognition: distinct patterns of conserved amino acids suggest different evolutionary modes

Finger proteins, the first example of which was Xenopus TFIIIA, share Zn2+ finger-like folded domains capable of binding to nucleic acids. A large number of this type of protein have been characterised from diverse organisms, indicating a wide evolutionary spread of the DNA-binding fingers. At least two classes of finger proteins may be distinguished. Class I proteins contain variable numbers of the tandemly repeating TFIIIA-like finger motif, (Y/F-X-C-X2-4-C-X3-F-X5-L-X2-H-X3-H). Class II finger proteins display a single (C-X2-C-X13-C-X2-C) motif and a facultative second putative finger. The relation between the structure of finger proteins and their recognised DNA sequences is discussed.

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.

Modulation of DNA binding of glucocorticoid receptors

Glucocorticoid receptors of several rodent and human cell lines were subjected to mild proteolysis with several proteases. A hormone binding fragment of Mr approximately 40,000 was generated which had increased affinity for DNA as revealed by DNA-cellulose chromatography. It behaved similar to the truncated nti ('increased nuclear transfer') receptor of mutant mouse lymphoma cells. These data led to the view that wild-type receptors of Mr approximately 94,000 contain in addition to the functional domains for hormone binding and interaction with DNA a third domain ('modulation domain') which is essential for biological activity. Monoclonal antibodies against wild-type receptors were used in DNA binding experiments and increased affinity for DNA was observed. The data suggest that reacting the receptor with antibody leads to functional elimination of the modulation domain as if it were cleaved off by mild proteolysis. Antibody treatment neither caused nor inhibited receptor activation to a DNA binding form.

Potentiation of glucocorticoid-induced lysis in refractory and resistant leukemia cells by inhibitors of ADP-ribosylation

Meta-iodo-benzylguanidine (MIBG; 3 x 10(-5) M), a novel inhibitor of mono(ADP-ribosylation)-and the general ribosylation inhibitor nicotinamide (NA; 5-20 mM) both stimulated the glucocorticoid-mediated lysis of sensitive L1210 leukemia cells and even induced susceptibility in various human and murine lines refractory or resistant to dexamethasone (DEX). Potentiation and induction of DEX-sensitivity by ADP-ribosylation inhibitors was accompanied by an increase in saturable 3H-DEX binding sites and by a 2-3 fold increase in the affinity of intracellular receptors for hormone binding. Moreover, the ribosylation inhibitors converted the glucocorticoid antagonist RU-486 into a potent agonist for cytolysis of L1210 cells. We conclude that the cytolytic action of glucocorticoid hormones in leukemic cells is negatively controlled by (mono)ADP-ribosylation of receptor proteins.