Identification of a new class of steroid hormone receptors

Identification of an epitope shared by the DNA-binding domain of glucocorticoid receptor and the B chain of insulin

A monoclonal antibody (8G11-C6) generated by an auto-anti-idiotypic route and directed to a site near the ligand-binding site of the glucocorticoid receptor also binds to native insulin and the B chain of insulin but not to the A chain of insulin. The glucocorticoid receptor and the B chain of insulin, therefore, share a cross-reacting epitope. Examination of the primary sequences of the two proteins revealed a limited number of regions of identity or close homology. Several peptides representative of those regions were synthesized. A heptapeptide sequence of the B chain of insulin with homology to a sequence in the first "zinc finger" of the DNA-binding domain of the glucocorticoid receptor was identified as the cross-reactive epitope. This heptapeptide sequence is restricted to and highly conserved among insulins of various species. Homologous sequences are found in the DNA-binding domains of most steroid receptors and related DNA-binding proteins. Consistent with this is the finding that 8G11-C6 inhibits the binding of glucocorticoid receptor to DNA-cellulose.

A novel member of the thyroid/steroid hormone receptor family is encoded by the opposite strand of the rat c-erbA alpha transcriptional unit

A cDNA encoding a novel member of the thyroid/steroid hormone receptor superfamily, called Rev-ErbA alpha, has been isolated from a rat GH3 cell library. Rev-ErbA alpha is an approximately 56-kilodalton protein most similar in structure to the thyroid hormone receptor (c-erbA) and the retinoic acid receptor, but it does not bind either thyroid hormone or retinoic acid. The mRNA encoding Rev-ErbA alpha is present in many tissues and is particularly abundant in skeletal muscle and brown fat. A genomic DNA fragment containing the entire Rev-ErbA alpha cDNA sequence was isolated and characterized. Remarkably, this DNA fragment also contained a portion of the c-erbA alpha gene. r-erbA alpha-1 and r-erbA alpha-2 are alternative splice products of the c-erbA alpha gene and are members of the receptor superfamily. The genes encoding Rev-ErbA alpha and r-erbA alpha-2 overlap, with their coding strands oriented opposite one another. A 269-base-pair segment of the bidirectionally transcribed region is exonic in both the Rev-ErbA alpha and r-erbA alpha-2 genes, resulting in complementary mRNAs. Thus, through alternative splicing and opposite-strand transcription, a single genomic locus codes for three different members of the thyroid/steroid hormone receptor superfamily. Potential implications of this unusual genomic arrangement are discussed.

The GLI-Kruppel family of human genes

Previous characterization of GLI, a gene found to be amplified and expressed in a subset of human brain tumors, revealed the presence of five tandem zinc fingers related to those of Krüppel (Kr), a Drosophila segmentation gene of the gap class. We have used the GLI cDNA as a molecular probe to isolate related sequences from the human genome. Partial characterization of six related loci, including sequence determination, expression studies, and chromosome localization, revealed that each locus could encode a separate finger protein. The predicted proteins all had similar H-C links, i.e., a conserved stretch of 9 amino acids connecting the C-terminal histidine of one finger to the N-terminal cysteine of the next. On the basis of amino acid sequence and intron-exon organization, the genes could be placed into one of two subgroups: the GLI subgroup (with the consensus finger amino acid sequence [Y/F]XCX3GCX3[F/Y]X5LX2HX3-4H[T/S]GEKP) or the Kr subgroup (with the consensus finger amino acid sequence [Y/F]XCX2CX3FX5LX2HXRXHTGEKP). Unlike GLI or Kr, most of the newly isolated genes were expressed in many adult tissues. The predicted proteins probably control the expression of other genes and, by analogy with Kr and GLI, may be important in human development, tissue-specific differentiation, or neoplasia.

The structure and function of steroid receptor proteins

This review has highlighted several topics in the study of steroid hormone action. The unanswered questions regarding the mechanism of ligand-controlled LRF activity, the extent of evolutionary conservation and specificity of DNA binding, and the validity of various models of transcriptional regulation mediated through gene networks point to the future direction of research in this field. Steroid hormones are used extensively in clinical treatments, especially glucocorticoids. Our laboratory is attempting to determine which gene networks are responsible for some of these clinical phenotypes. Figure 5 points out that the study of glucocorticoid action holds a unique position because it spans both the basic sciences and the field of applied molecular biology. Now that we have a fundamental knowledge of the necessary elements required for steroid-dependent regulation of gene expression, we can better investigate the clinical responses to steroid therapy (which include devastating side effects) by isolating and characterizing the important target gene(s). In this author's opinion, future directions in the study of steroid responsiveness will have to include a systematic approach toward deciphering a variety of these LRF-regulated gene networks in experimentally feasible systems. Hopefully, work in this area may be revealing and perhaps beneficial to ongoing clinical studies. In addition, the study of mechanisms of transcriptional induction and repression, using the model system of LRFs, could be applicable to many gene regulatory systems which are controlled by such processes as development and differentiation.

Isolation of human cDNA clones of myb-related genes, A-myb and B-myb

cDNA clones of the myb-related genes A-myb and B-myb were obtained by screening human cDNA libraries. The predicted open reading frame of B-myb could encode a protein of 700 amino acid residues. Although the C-terminal end has not been cloned yet, an almost entire coding region of A-myb, which is 745 amino acid long, was determined. The A-myb and B-myb proteins are highly homologous with the myb protein in three regions. Domain I, which is 161 amino acid long, is well conserved in the myb gene family. The homology between human-myb and A-myb in domain I is 90% at the amino acid level. Domain II, which is about 85 amino acid long, is less well conserved. Although it is a short stretch, domain III is found in the C-terminal region. The mRNAs of A-myb and B-myb were 5.0 and 2.6 kb, respectively. The mRNA expression pattern of the myb gene family in various tumors is presented.

Physicochemical characterization of the nuclear form of Ah receptor from mouse hepatoma cells exposed in culture to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Molecular properties of nuclear aromatic hydrocarbon (Ah) receptor from Hepa-1c1c9 (Hepa-1) cells were assessed by velocity sedimentation on sucrose gradients and by gel permeation chromatography on Sephacryl S-300. Nuclear Ah receptor was obtained by exposing intact cells to [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for 1 h at 37 degrees C in culture followed by extraction of receptor from nuclei with buffers containing 0.5 M KCl. The nuclear Ah receptor was compared to the cytosolic Ah receptor from the same cells. Under conditions of low ionic strength, the Ah receptor from Hepa-1 cytosol sedimented as a single 9.4 +/- 0.63 S binding peak that had a Stokes radius of 7.1 +/- 0.12 nm and an apparent relative molecular mass of 271,000 +/- 16,000. After prolonged (24 h) exposure to high ionic strength (0.5 M KCl), cytosol labeled with [3H]TCDD exhibited two specific binding peaks. The large form of cytosolic Ah receptor seen under high ionic strength conditions sedimented at 9.4 +/- 0.46 S, had a Stokes radius of 6.9 +/- 0.19 nm, and an apparent Mr 267,000 +/- 15,000. The smaller ligand-binding subunit generated by exposing cytosol to 0.5 M KCl sedimented at 4.9 +/- 0.62 S, had a Stokes radius of 5.0 +/- 0.14 nm, and an apparent Mr 104,000 +/- 12,000. Nuclear Ah receptor, analyzed under high ionic strength conditions, sedimented at 6.2 +/- 0.20 S, had a Stokes radius of 6.8 +/- 0.19 nm, and an apparent Mr 176,000 +/- 7000. Nuclear Ah receptor from rat H4IIE hepatoma cells was analyzed and found to have physicochemical characteristics identical to those of nuclear Ah receptor from the mouse Hepa-1 cells. The molecular mass of Hepa-1 nuclear Ah receptor was found to be statistically different from both the Mr approximately 267,000 cytosolic Ah receptor and the Mr approximately 104,000 subunit which were present in cytosol under high ionic strength conditions. Hepa-1 nuclear Ah receptor could not be converted to a smaller ligand-binding subunit by treatment with alkaline phosphatase, ribonuclease, or sulfhydryl-modifying reagents or prolonged exposure to 1.0 M KCl. Cytosolic Ah receptor from Hepa-1 cells was "transformed" by heating at 25 degrees C in vitro into a form with high affinity for DNA-cellulose. The transformed cytosolic Ah receptor, when analyzed under conditions of high ionic strength, sedimented at approximately 6 S, had a Stokes radius of approximately 6.7 nm, and an apparent Mr approximately 167,000.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient administration of estradiol-17 beta establishes an autoregulatory loop permanently inducing estrogen receptor mRNA

A single transient dose of estradiol-17 beta is sufficient to elicit the permanent induction of hepatic estrogen receptor mRNA, which is induced 18-fold (from 0.13 to 2.4 molecules per cell) and then remains fully induced for at least 125 days. In primary liver cultures, extremely low concentrations of estradiol-17 beta, which are below the Kd of the Xenopus laevis estrogen receptor, maintain persistent induction of estrogen receptor mRNA but not of estrogen-inducible vitellogenin mRNA. These data and the ability of the antiestrogen, hydroxytamoxifen, to reverse persistent induction of estrogen receptor mRNA, support a model in which transient doses of estradiol-17 beta induce the estrogen receptor and thereby establish an autoregulatory loop. The low levels of estradiol-17 beta normally circulating in male X. laevis and the elevated level of receptor provide sufficient hormone-receptor complex to permanently maintain the induced level of expression of the estrogen receptor gene. The permanent induction of the estrogen receptor may be the regulatory switch that results in the persistent expression of a recently identified class of proteins that exhibit long-term responses to estrogen.

Identification of a new member of the steroid receptor super-family by cloning and sequence analysis

We have isolated a human testis complementary DNA clone on the basis of homology to the DNA binding domain of steroid receptors. Expression of this complementary DNA, which we call TR2, produces a 52 kd DNA binding protein that does not bind significantly to any known steroids. Northern blot analysis has shown that TR2 mRNA is about 2.5 kilobases (kb) and is relatively abundant in androgen-sensitive organs, such as ventral prostate and seminal vesicle. Dot blot hybridization indicates that TR2 mRNA levels increased after castration of rats, and this increase is reversed by 5 alpha-dihydrotestosterone injection. This evidence suggests that TR2 mRNA levels are negatively controlled by androgen in the rat ventral prostate.

The GLI-Kruppel family of human genes

Previous characterization of GLI, a gene found to be amplified and expressed in a subset of human brain tumors, revealed the presence of five tandem zinc fingers related to those of Krüppel (Kr), a Drosophila segmentation gene of the gap class. We have used the GLI cDNA as a molecular probe to isolate related sequences from the human genome. Partial characterization of six related loci, including sequence determination, expression studies, and chromosome localization, revealed that each locus could encode a separate finger protein. The predicted proteins all had similar H-C links, i.e., a conserved stretch of 9 amino acids connecting the C-terminal histidine of one finger to the N-terminal cysteine of the next. On the basis of amino acid sequence and intron-exon organization, the genes could be placed into one of two subgroups: the GLI subgroup (with the consensus finger amino acid sequence [Y/F]XCX3GCX3[F/Y]X5LX2HX3-4H[T/S]GEKP) or the Kr subgroup (with the consensus finger amino acid sequence [Y/F]XCX2CX3FX5LX2HXRXHTGEKP). Unlike GLI or Kr, most of the newly isolated genes were expressed in many adult tissues. The predicted proteins probably control the expression of other genes and, by analogy with Kr and GLI, may be important in human development, tissue-specific differentiation, or neoplasia.

Organisation of the entire rabbit progesterone receptor mRNA and of the promoter and 5' flanking region of the gene

cDNA clones corresponding to the 3' and 5' non coding regions of the rabbit progesterone receptor (rPR) mRNA and genomic clones corresponding to the promoter and 5' flanking region of this gene were isolated and sequenced up to nucleotide -2761. The 3' non coding region is very long (3058-3553 nucleotides) and contains three different polyadenylation sites. Primer extension experiments and S1 mapping showed the existence of 2 transcription initiation sites 699 and 712 bp upstream from the initiator ATG. The promoter region contains two modified TATA boxes: TAGAAA at -17 and TAGA at -37bp. A CAACT sequence is present at position -100 and one consensus binding site for the transcription factor Sp1 is found at position -51. A 317 bp sequence was observed (positions -2590 to -2273) which belongs to the C family of the short interspersed repeats of the rabbit. Sequences resembling the consensus for estrogen and progesterone responsive elements are observed at several locations in the 5' flanking region. The progesterone receptor is present in tissue extracts mainly as a mixture of two molecular species (110 and 79 kDa) whose origin remains currently debated. By Northern blot analysis we have shown, using rabbit and human mRNAs, that these receptor species are not derived from separate mRNAs. Transcription-translation experiments also showed that, at least in vitro, they are not derived by use of different translation initiation sites on the same messenger RNA.

The steroid and thyroid hormone receptor superfamily

Analyses of steroid receptors are important for understanding molecular details of transcriptional control, as well as providing insight as to how an individual transacting factor contributes to cell identity and function. These studies have led to the identification of a superfamily of regulatory proteins that include receptors for thyroid hormone and the vertebrate morphogen retinoic acid. Although animals employ complex and often distinct ways to control their physiology and development, the discovery of receptor-related molecules in a wide range of species suggests that mechanisms underlying morphogenesis and homeostasis may be more ubiquitous than previously expected.

Cloning of human androgen receptor complementary DNA and localization to the X chromosome

The androgen receptor (AR) mediates the actions of male sex steroids. Human AR genomic DNA was cloned from a flow-sorted human X chromosome library by using a consensus nucleotide sequence from the DNA-binding domain of the family of nuclear receptors. The AR gene was localized on the human X chromosome between the centromere and q13. Cloned complementary DNA, selected with an AR-specific oligonucleotide probe, was expressed in monkey kidney (COS) cells and yielded a high-affinity androgen-binding protein with steroid-binding specificity corresponding to that of native AR. A predominant messenger RNA species of 9.6 kilobases was identified in human, rat, and mouse tissues known to contain AR and was undetectable in tissues lacking AR androgen-binding activity, including kidney and liver from androgen-insensitive mice. The deduced amino acid sequence of AR within the DNA-binding domain has highest sequence identity with the progesterone receptor.