Specific binding of estrogen receptor to sites upstream and within the transcribed region of the chicken ovalbumin gene

Small-molecule hormones: molecular mechanisms of action

Small-molecule hormones play crucial roles in the development and in the maintenance of an adult mammalian organism. On the molecular level, they regulate a plethora of biological pathways. Part of their actions depends on their transcription-regulating properties, exerted by highly specific nuclear receptors which are hormone-dependent transcription factors. Nuclear hormone receptors interact with coactivators, corepressors, basal transcription factors, and other transcription factors in order to modulate the activity of target genes in a manner that is dependent on tissue, age and developmental and pathophysiological states. The biological effect of this mechanism becomes apparent not earlier than 30-60 minutes after hormonal stimulus. In addition, small-molecule hormones modify the function of the cell by a number of nongenomic mechanisms, involving interaction with proteins localized in the plasma membrane, in the cytoplasm, as well as with proteins localized in other cellular membranes and in nonnuclear cellular compartments. The identity of such proteins is still under investigation; however, it seems that extranuclear fractions of nuclear hormone receptors commonly serve this function. A direct interaction of small-molecule hormones with membrane phospholipids and with mRNA is also postulated. In these mechanisms, the reaction to hormonal stimulus appears within seconds or minutes.

Structural basis for nuclear hormone receptor DNA binding

The gene family of nuclear receptors is characterized by the presence of a typical, well conserved DNA-binding domain. In general, two zinc coordinating modules are folded such that an α-helix is inserted in the major groove of the DNA-helix displaying a sequence similar to one of two hexameric consensus motifs. Both zinc molecules coordinate four cysteines. Although the DNA-binding domains as well as the hormone response elements are very similar, each nuclear receptor will affect transcription of a specific set of target genes. This is in part due to some important receptor-specific variations on the general theme of DNA interaction. For most nuclear receptors, the DNA-binding domain dimerizes on DNA, which explains why most hormone response elements consist of a repeat of two hexamers. The hexamer dimers can be organized either as direct, inverted or everted repeats with spacers of varying lengths. The DNA can be bound by homodimers, heterodimers and for some orphan receptors, as monomer. Another key element for DNA binding by nuclear receptors is the carboxy-terminal extension of the DNA-binding domain extending into the hinge region. This part not only co-determines sequence specificity, but also affects other functions of the receptors like nuclear translocation, intranuclear mobility and transactivation potential. Moreover, allosteric signals passing through towards other receptor domains, explain why to some extent, the DNA elements can also be considered as controlling ligands.

Expression of early genes in estrogen induced phenotypic conversion of neuroblastoma cells

Estrogens are known to modulate the growth rate and differentiation state of a number of cells. In uterine, as well as in mammary tumor cells, estrogen-dependent proliferation and differentiation are correlated to a series of biochemical responses, including increased expression of proto-oncogenes such as: c-fos, c-jun and c-myc. Since estrogens were shown to regulate the proliferation and the differentiation state of cells of nervous origin, the aim of the present study was to investigate whether these effects were associated to changes in the expression of early genes. In the model system utilized, the human cell line SK-ER3, an increase in c-fos mRNA and Fos protein without change of c-jun and related genes mRNA concentration was observed after short term treatment with 17 beta-estradiol (E2). A significant decrease of c-fos, c-jun and jun-D proto-oncogene mRNA levels were found after prolonged hormonal treatment. The exposure to the hormone did not determine any change in N-myc expression. Since the three protooncogene mRNAs are rapidly induced following estrogen treatment in other cell systems and target tissues, it is concluded that the estrogen-induced differentiation of neuroblastoma cells is correlated to a pattern of expression of early genes that might be peculiar for the activity of this hormone in neural cells.

Factor-assisted DNA binding as a possible general mechanism for steroid receptors. Functional heterogeneity among activated receptor-steroid complexes

We previously reported that activated glucocorticoid receptor-steroid complexes from rat HTC cell cytosol exist as at least two sub-populations, one of which requires a low molecular weight (700-3000 Da) factor(s) for binding to DNA. This factor is removed by Sephadex G-50 chromatography and is found predominantly in extracts of crude HTC cell nuclei. We have now determined that factor is not limited to HTC cells since an apparently identical factor(s) was found in nuclear extracts of rat kidney and liver as well as human HeLa and MCF-7 cells. Furthermore, the DNA binding of a sub-population of human glucocorticoid receptors depends on factor. While these results were obtained with agonist (dexamethasone) bound receptors, a sub-population of HTC cell receptors covalently labeled by the antiglucocorticoid dexamethasone 21-mesylate also displayed factor-dependent DNA binding. This receptor heterogeneity was not an artifact of cell-free activation since the cell-free nuclear binding of dexamethasone mesylate labeled complexes was, as in intact cells, less than that for dexamethasone bound complexes. Earlier results suggested that the increased DNA binding with factor involved a direct interaction of receptor with factor(s). We now find that the factor-induced DNA binding is retained by amino terminal truncated (42 kDa) glucocorticoid receptors from HTC cells. Thus the ability of receptor to interact with factor(s) is encoded by the DNA and/or steroid binding domains. Two dimensional gel electrophoresis analysis of dexamethasone-mesylate labeled 98 kDa receptors revealed multiple charged isoforms for both sub-populations but no differences in the amount of the various isoforms in each sub-population. Finally, activated progesterone and estrogen receptor complexes were also found to be heterogeneous, with a similar, if not identical, small molecular weight factor(s) being required for the DNA binding of one sub-population. The observations that functional heterogeneity of receptors is not unique to glucocorticoid receptors, whether bound by an agonist or antagonist, and that the factor(s) is neither species nor tissue specific suggests that factor-assisted DNA binding may be a general mechanism for all steroid receptors.

Identification of an estrogen response element upstream of the human c-fos gene that binds the estrogen receptor and the AP-1 transcription factor

Transcription of the proto-oncogene c-fos is stimulated by 17 beta-estradiol in estrogen responsive human and rat cells. To understand the molecular mechanisms of estrogen regulation of c-fos gene transcription, the human c-fos gene promoter, with 2.25 Kb of 5'-flanking DNA, was cloned upstream of the bacterial CAT gene and tested for estrogen regulation by transient transfection in HeLa cells. When an expression vector coding for the human estrogen receptor was co-transfected with the fos -CAT reporter, the promoter was found to respond to 17 beta-estradiol. An element responsible for estrogen induction was mapped in a 240 bp region localized 1060 to 1300 bases upstream of the startsite of transcription of the gene. Sequence analysis revealed, clustered in a 19 bp sub-region, a sequence corresponding to an imperfectly palindromic ERE: CGGCAGCGTGACC and two sequences: CTGAG and GTGAC, homologous to the core sequence of AP-1 transcription factor binding sites. A synthetic oligonucleotide reproducing this sub-region binds 'in vitro' both the estrogen receptor and AP-1 factor(s) and confers estrogen-responsivity to the HSV-tk gene promoter. Transcriptional activation by the estrogen receptor is prevented by mutations in the fos ERE that hamper binding of the receptor in vitro. Activation of the c-fos gene promoter in HeLa cells requires the DNA binding domain of the estrogen receptor, and can be achieved independently by the TAF-1 and the TAF-2 transcriptional activation functions of this molecule. A receptor mutant lacking the hormone binding domain can activate the c-fos gene promoter in the absence of estrogen.

Localization of an estrogen receptor binding site near the promoter of the uteroglobin gene

By means of a DNA-cellulose competitive binding assay, we have studied the interaction of the estrogen receptor with genomic fragments of the estrogen responsive rabbit uteroglobin gene. The fragments spanned from 3255 bp upstream to 1754 bp downstream of the initiation site. Only a fragment (-396/+8) showed strong affinity for the receptor. Within this fragment a unique palindromic sequence (GGTCAccaTGCCC) was found which is very similar to the canonical consensus sequence for the estrogen receptor. A synthetic oligonucleotide of that structure specifically competed for the binding of the receptor to DNA-cellulose.

Molecular mechanism of steroid hormone action during aging. A review

The application of immunochemistry coupled with genetic engineering techniques has helped greatly in the understanding of the molecular mechanism of steroid hormone action. Particularly, the recent observations on nuclear localization of steroid receptor proteins and the interaction of the steroid-receptor complex with the genome have provided much insight into the whole pathway of steroid hormone action. Despite the large amount of data accumulated over the years on the mechanism of steroid action in general, relatively little is known about the changes occurring in the action of steroid hormones during aging. However, there is some evidence of a decreased responsiveness of target tissues to steroid hormones during senescence. In the light of recent progress in steroid research, an attempt has been made in this article to discuss the alterations that occur at different steps of steroid action as a function of age.

Androgen receptor-binding regions of an androgen-responsive gene

The interaction of 5 alpha-dihydrotestosterone-receptor complexes with purified DNA fragments representing upstream, coding and intervening sequences of the prostate binding protein C3(1) gene was investigated using a DNA-cellulose competition binding assay. The partially purified androgen-receptor complexes which were used in the assay had proven DNA-binding capabilities. Two fragments were identified with relatively high affinity for androgen-receptor complexes. A 300 bp fragment extending from -220 to +80 and a 500 bp fragment derived entirely from the first intron consistently competed most effectively in the system. The presence of a high affinity site or sites in or near the promoter region of the gene is consistent with current models of transcriptional activation of hormone-responsive genes by steroid receptors. High affinity sites for steroid receptors within introns may indicate a role for receptors in regulation of transcription at other stages, or in post-transcriptional modification.