Gene regulation by steroid hormones.

GENE regulation by steroid hormones is mediated by binding of the hormone ligand to the corresponding receptor that triggers a complex set of interactions of the hormone receptors with each other, with DNA in chromatin, and with a variety of other proteins. In this review we attempt to summarize what is known about these interactions using as the main example the regulation of mouse mammary tumor virus transcription by glucocorticoids and progestins. We describe in some detail the interaction of monomers and homodimers of the steroid receptors with their recognition sequences, and the molecular mechanism used to discriminate between the responsive elements for glucocorticoids/progestins and estrogens. We then review the interactions between homologous and heterologous hormone receptors on complex hormone regulatory regions, before devoting some attention to the synergistic and inhibitory interactions of hormone receptors with other transcription factors. Finally we briefly summarize some of the possible m...

Steroid hormone receptors: interaction with deoxyribonucleic acid and transcription factors

I. Introduction REGULATION of gene expression by steroid hormones is a traditional field of molecular endocrinology. The hormones act by binding to intracellular receptors, which themselves orchestrate the transcriptional response. After the cloning of the hormone receptors and the identification of their target sequences on DNA at the end of the 1980s, the question of how steroid hormones modulate the activity of various genes in target cells seemed to be solved. It was clear that steroid hormone receptors are ligand-activated transcriptional modulators, which in most cases act through binding to specific sequences on DNA called hormone-responsive elements (HREs) (1). Careful mutational analysis identified a modular structure of the receptors composed of a DNA-binding domain, nuclear localization signals, a ligand-binding domain, and several transactivation domains (2). Interactions of the receptors with components of the basal transcriptional machinery and with sequence-specific transcription factors we...

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Interaction of Steroid Hormone Receptors with the Transcription Initiation Complex

We reported previously that the lymphocyte-derived octamer transcription factor 2A (Oct-2A or OTF-2A) activated both natural immunoglobulin promoters and synthetic promoters which contain the 'octamer' site, but was unable by itself to stimulate transcription from a remote enhancer position. Here we examine a larger set of transcription factors with respect to their proximal versus remote activation. Since a transcription factor may contain more than one activation domain, we have chosen to study the potential of individual activation domains in the context of fusion proteins that contain the DNA binding domain of GALA. We have identified at least two distinct functional classes of transcriptional activation domains. 'Proximal' activation domains, exemplified by glutamine-rich domains of Oct-1, Oct-2A and Sp1, stimulate transcription only from a position close to the TATA box, usually in response to a remote enhancer. 'General' activation domains, derived from VP16, GAL4, p65 (NF-chi B), TFE3, ITF-1 and ITF-2, can activate transcription from remote as well as proximal positions. These domains contain many acidic amino acids and/or other features such as clusters of serine and threonine. The proline-rich activation domains of AP-2 and CTF/NF1 may represent a third class with considerable promoter activity and low but significant enhancer activity. Furthermore, activation domains of both the acidic and glutamine-rich types seem to have a modular structure, since duplicated subdomains can substitute for the entire domain.

Different activation domains stimulate transcription from remote ('enhancer') and proximal ('promoter') positions.

Stimulation of T lymphocytes through their antigen receptor leads to the appearance of several transcription factors, including NF-AT and NF-kappa B, which are involved in regulating genes required for immunologic activation. To investigate the activity of a single transcription factor in individual viable cells, we have applied an assay that uses the fluorescence-activated cell sorter to quantitate beta-galactosidase (beta-gal). We have analyzed the distribution of NF-AT transcriptional activity among T cells undergoing activation by using a construct in which three tandem copies of the NF-AT-binding site directs transcription of the lacZ gene. Unexpectedly, stimulation of cloned stably transfected Jurkat T cells leads to a bimodal pattern of beta-gal expression in which some cells express no beta-gal and others express high levels. This expression pattern cannot be accounted for by cell-cycle position or heritable variation. Further results, in which beta-gal activity is correlated with NF-AT-binding activity, indicate that the concentration of NF-AT must exceed a critical threshold before transcription initiates. This threshold likely reflects the NF-AT concentration-dependent assembly of transcription complexes at the promoter. Similar constructs controlled by NF-kappa B or the entire interleukin-2 enhancer show bimodal expression patterns during induction, suggesting that thresholds set by the concentration of transcription factors may be a common property of inducible genes.

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Single cell assay of a transcription factor reveals a threshold in transcription activated by signals emanating from the T-cell antigen receptor.

Typical cell type-specific or inducible mammalian genes are under the control of one or more remote enhancers which transmit their effect to the promoter region located at the initiation site of transcription. Both enhancers and promoters are composed of multiple binding sites for transcription factors. To study the requirements for promoter and enhancer function, we have used a reporter gene that is completely dependent on a single DNA-binding transcription factor in vivo. This factor is a truncated, hormone-independent form of the glucocorticoid receptor which interacts strongly with a palindromic binding site. After transfection into HeLa cells, transcription of a reporter gene with one, two or four copies of the binding site upstream of the TATA box is enhanced less than 10, at least 100 and greater than 1000-fold respectively, in the presence of the receptor. Even when the TATA box is deleted, the four upstream binding sites confer receptor-dependent transcription, though from scattered initiation sites. When four copies of the palindromic binding site are placed downstream of the transcription unit, they form a very strong receptor-dependent enhancer. This enhancer can activate comparably well promoters containing binding sites for either glucocorticoid receptor, Sp1 factor, or octamer factor. Our data show that a single defined DNA-binding factor can mediate both promoter and enhancer activity, and that it can co-operate functionally both with itself and with seemingly unrelated transcription factors.

https://www.embopress.org/doi/pdf/10.1002/j.1460-2075.1990.tb08134.x

M D Schatt

Papers

A single DNA-binding transcription factor is sufficient for activation from a distant enhancer and/or from a promoter position.