Steroid Receptors and the Mechanism of the Specificity of Glucocorticoid Responsiveness of Somatic Cell Hybrids between Hepatoma Tissue Culture Cells and Mouse Fibroblasts

Regulated expression of human atrial natriuretic polypeptide gene in mouse L cells

To investigate whether the human atrial natriuretic polypeptide (hANP) gene is responsive to glucocorticoid, we co-introduced the hANP gene (with SV40 enhancer) with HSV-tk gene into mouse tk- L cells. The transformants with hANP gene with SV40 enhancer expressed hANP specific RNAs. The administration of 1 microM dexamethasone reduced the expressed hANP specific RNAs, especially those that had a physiological initiation site. These results suggest that the hANP gene is really a glucocorticoid responsive gene and may be negatively regulated by glucocorticoid.

Analysis of androgen-sensitivity in rat prostate X mouse kidney cell hybrids

Variant androgen-sensitive cell lines were produced by fusing freshly isolated epithelial cells from the rat ventral prostate with a line of murine renal tumor (RAG) cells. The properties of the cloned lines of the prostate X RAG hybrids can be summarized as follows: (1) the modal chromosome number of the hybrid cell lines ranged from 68 to 176; (2) the cells had doubling times of 7.6-49.5 h; and (3) epitheloid, ameboid and intermediate morphologies were observed among the various lines. The proliferative response of various hybrid lines to treatment with 10 nM 5 alpha-dihydrotestosterone was used to classify the hybrids as either very sensitive (greater than 40% reduction in cell doubling time), sensitive (greater than 10% reduction in doubling time) to androgens, or insensitive (less than 10% reduction in doubling time) to androgens. There was no direct relationship between the androgen-sensitivity of the cells and their androgen receptor content, suggesting that these variant cell lines may be useful for the study of the genetic factors involved in cellular responses to androgens.

Cystic fibrosis and normal fibroblasts have identical glucocorticoid receptor profiles and induced protein responses

The possibility of differences in glucocorticoid sensitivity between normal and cystic fibrosis fibroblasts was studied in cultured cells derived from forearm skin biopsies. Both normal and cystic fibrosis cells bound tritiated dexamethasone with high affinity (Kd 37 degrees C approximately equal to 10 nmol/1) to a limited class of sites (concentration approximately equal to 50,000/cell); no difference between cystic fibrosis and normal cells was seen in the nuclear transfer of receptor-bound tritiated dexamethasone. After 4 h exposure to dexamethasone and analysis of 35S-methionine labelled proteins by 2-dimensional gel electrophoresis, both normal and cystic fibrosis cells showed increased abundance of protein kappa (molecular weight approximately equal to 41,000; pKi approximately equal to 6.5) over non-steroid exposed gels. After 16 h exposure, in both cystic fibrosis and normal cells exposed to dexamethasone a decrease in protein omega (mol. wt approximately equal to 42,000; pKi approximately equal to 5.4) was seen compared with control. We interpret these data as evidence against a difference in glucocorticoid receptor of effector mechanisms between cystic fibrosis and normal fibroblasts.

How do steroid hormones function to induce the transcription of specific genes?

On the whole, the question as to how steroid hormones act to induce the transcription of specified genes is not much more understood today than it was 15 years ago, but considerable progress has been made during this time to bring us a step closer to unravelling the mysteries surrounding the mechanism of hormonal gene activation. Hopefully, further studies in the three fields described (i.e. DNA/receptor binding experiments, gene-transfer experiments, and in vitro transcriptional studies) will bring in enough information for a complete story to be told one day. As to the questions asked in the introduction of this review, there appear to be sequences around inducible genes that render them sensitive to hormone administration. These sequences could be arranged in appropriate chromosomal structures flanking the gene region particularly at the 5′ end. As to whether these sequences bind the hormone/receptor complex, the experiments with the MMTV proviral DNA (17, 21–23) and the ovalbumin gene (27) appear to provide an affirmative answer. But the answer to whether this binding is responsible for the gene activation must await future experimentation. In vitro transcriptional studies specially designed with the hormone/receptor/DNA complex may be useful in answering this question.

Regulated expression of human growth hormone genes in mouse cells

We have asked whether there are sequences around the human growth hormone gene that render this gene responsive to induction by glucocorticoid hormones. Recombinant clones encoding human growth hormone were introduced into the chromosome of murine fibroblasts by cotransformation. Exposure of cotransformants to glucocorticoids results in a three to five fold induction of human growth hormone mRNA and a similar induction in secreted human growth hormone protein. The DNA sequences required for induction reside within 500 nucleotides of 5'-flanking DNA. Fusion of this segment of 5'-flanking DNA to the structural gene sequences of a hormone-insensitive gene, such as thymidine kinase, now renders this gene responsive to glucocorticoid induction.

Glucocorticoids and melanoma: receptor properties of dexamethasone sensitive and resistant tumors

We have grown solid tumors using dexamethasone-sensitive (clone 6) and -resistant (clone 5) cells cloned from RPMI 3460 Syrian hamster melanoma. Clone 6 but not clone 5 tumor growth was retarded by dexamethasone, indicating that these tumors retain the growth sensitivity to the hormone characteristic of the cells from which they are derived. Both tumor types contain cytosolic glucocorticoid receptors (significantly higher levels in clone 5 tumors) with similar affinity and steroid specificity characteristics and these can exist as stable, activated (nuclear binding) complexes. Despite these similarities the receptor in the two tumor types differ by some physiochemical criteria. By sucrose gradient analysis, cytosols from both tumors contain 7S receptor complexes but clone 6 contains an additional 13S form. Activated receptors isolated by DEAE-cellulose chromatography from both clone 5 and 6 tumor cytosols sediment as a single peak at 4-5S. However, the DEAE-cellulose profiles indicate that clone 6 but not clone 5 activated complexes (Peak I) appear heterogeneous with respect to charge. Interestingly, DNA-cellulose chromatography indicates that activated receptors from clone 5 tumor cytosols may bind more tightly to DNA than those from clone 6. We are investigating these receptor differences in more detail to determine more precisely the role and pathways of action of glucocorticoid hormones in melanoma.

The mouse metallothionein-I gene is transcriptionally regulated by cadmium following transfection into human or mouse cells

Recombinant vectors containing the mouse metallothionein-I gene (MT-I) and the Escherichia coli xanthine-guanine phosphoribosyltransferase gene (gpt) were used to transfect human hgprt- HeLa cells. Transfected MT-I genes are transcriptionally regulated by cadmium but not by glucocorticoids. S1 mapping indicates that the transcripts from transfected MT-I genes begin at the correct transcription initiation site. We also transfected mouse tk- L cells with a vector containing the mouse MT- I gene and the herpes simplex virus-I thymidine kinase gene. MT-I gene transcription is regulated by cadmium but not by glucocorticoids in this homologous system as well. Finally, we fused the MT-I gene promoter/regulatory region to the thymidine kinase structural gene. Thymidine kinase activity is regulated by cadmium when this fusion gene is transfected into mouse tk- L cells. Deletion mapping experiments indicate that the DNA sequences necessary for regulation of the MT-I gene by cadmium lie within 148 bp of its transcription start site.

Hormonal inducibility of rat alpha 2u globulin genes in transfected mouse cells

Two different genes coding for the hormonally regulated rat liver protein alpha 2u globulin were introduced into mouse Ltk- cells through co-transfection with the HSV-1 thymidine kinase gene. Three to ten copies of the alpha 2u globulin genes were detected several tk+ clones, over 50% of which could be induced with dexamethasone, the produce alha 2u globulin mRNA and protein. This suggests that the information necessary for hormonal response is contained in the DNA fragment used for transfer.

Comparative evaluation of skin atrophy in man induced by topical corticoids

In this in-patient study, mild skin atrophy was observed clinically in 6 out of a total of 22 patients after fluocinolone acetonide and in one patient after flumethasone pivalate ointment applications (P less than 0,05). Investigations were discontinued prematurely on days 14 and 17 in 2 patients because they developed early atrophy due to fluocinolone acetonide applications. Histological findings indicating either moderate or marked skin atrophy were evident in 15 patients after flucinolone acetonide as against one patient after flumethasone pivalate ointment applications (P less than 0.001). In comparison with the controls the mean decrease in epidermal thickness was more marked after fluocinolone acetonide applications, namely 30.5% vs 21.3% after flumethasone pivalate applications. The ultrastructural tissue changes were less marked at the sites to which flumethasone pivalate was applied. In comparison with controls, the percentages of mean decrease in diameters of collagen fibrils measured in six volunteer patients ranged from 5.1% to 27.6% after fluocinolone acetonide and from 0% to 12.3% following flumethasone pivalate ointment applications, This difference was statistically significant (range P less than 0,01 to less than 0.001). This experimental study has demonstrated that flumethasone pivalate displays only a mild atrophogenic effect and it is clinically and histologically significantly less atrophogenic than fluocinolone acetonide. Flumethasone pivalate can therefore be reckoned as a suitable topical corticosteroid especially for the long-term treatment of corticoid-responsive dermatoses.

Somatic cell fusion in the study of glucocorticoid action

The basic phenomena of cell fusion and hybrid cell formation are briefly described and the potential of somatic cell hybridization in studies on the expression of differentiated cellular functions is discussed. The technique of cell hybridization has been applied to two types of cellular responses to glucocorticoids. The induction of specific proteins has been investigated in hybrids of inducible cells with uninducible cells. Most studies dealt with the liver-specific enzyme tyrosine aminotransferase, whose inducibility was extinguished in the majority of the hybrids between hepatoma and nonliver cells. However, upon chromosome segregation, inducibility reappeared in some of these hybrid cells. The current ideas about cellular control of inducibility are discussed. The other major glucocorticoid-responsive system investigated in cell hybridization studies consists of lymphoid cells which are killed when exposed to the steroid. Such sensitive cells were hybridized with several types of glucocorticoid-resistant lymphoid lines, and sensitivity was found to be dominant over resistence. Hybrids between sensitive and resistant lymphoid cells, however, showed an increase in the frequency at which resistance occurred as compared to the rate observed with the wild-type parental cells. No complementation to steroid sensitivity was found in hybrids between different types of resistant cells with defects in the glucocorticoid-specific receptor system.

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

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

Isolation and characterization of L-cell variants with altered sensitivity to glucocorticoids

Stable glucocorticoid-resistant receptor-containing (R+) variants of L cells have been obtained and characterized. Five clones of such cells were obtained after chemical mutagenesis of the A9HT cell line. These clones and an additional, spontaneously resistant subclone of the LB82 cell line all contain glucocorticoid receptors which appear normal with regard to quantity, affinity for steroids, and in vivo localization in subcellular compartments. Somatic cell hybrids were formed between two of these R+-resistant clones, a previously isolated R-resistant clone, and three different sensitive L cell parents. In no case was steroid resistance dominant. Thus, these resistant L cells appear to be candidates for complementation analysis. The R+-resistant clones may be hindered in as yet undefined steps of glucocorticoid action.

Binding of the partially purified glucocorticoid receptor of rat liver to chromatin and DNA

The binding of the glucocorticoid receptor of rat liver to chromatin and DNA has been studied with crude and partially purified preparations of cytosol receptor labelled with [3H]-triamcinolone acetonide in vitro. The use of crude preparations of receptor and increasing protein concentrations leads to an apparent saturation of chromatin and DNA, suggesting a limited number of high affinity nuclear acceptor sites for the receptor. Appropriate controls indicate that the observed saturability of chromatin acceptor sites is due to the presence in crude receptor preparations of heat-stable protein factors which interfere with the binding of the receptor to the genome; whereas the apparent saturation of DNA is due to contamination with deoxyribonucleases. If the activated complex of receptor and triamcinolone acetonide (R-TA) is partially purified to a step where it is free from nucleases and inhibitors, its binding to both chromatin and DNA is linearly dependent on the concentration of free (R-TA) in the incubation medium. There is no absolute specificity with respect to the source of DNA or chromatin, although liver chromatin has considerably higher receptor binding capacity than chromatin from avian erythrocytes. The rate kinetics of association and dissociation for the binding of (R-TA) to DNA and chromatin are very similar, but DNA exhibits a 10-fold higher receptor binding capacity than chromatin. These data, in conjunction with the effect of poly-(D)-lysine and and NaCl on the binding of (R-TA) to chromatin and DNA, suggest that most of the receptor molecules bound to chromatin in vitro interact with the "accessible" DNA stretches. Although a small population of receptor molecules may bind specifically to target tissue genome, the detection of these specific sites against the background of unspecific binding is not possible with unfractionated chromatin or DNA preparations.