Estrogen induces tissue specific changes in the chromatin conformation of the vitellogenin genes in Xenopus

Environmental signaling: what embryos and evolution teach us about endocrine disrupting chemicals

The term "endocrine disrupting chemicals" is commonly used to describe environmental agents that alter the endocrine system. Laboratories working in this emerging field-environmental endocrine research-have looked at chemicals that mimic or block endogenous vertebrate steroid hormones by interacting with the hormone's receptor. Environmental chemicals known to do this do so most often with receptors derived from the steroid/thyroid/retinoid gene family. They include ubiquitous and persistent organochlorines, as well as plasticizers, pharmaceuticals, and natural hormones. These chemicals function as estrogens, antiestrogens, and antiandrogens but have few, if any, structural similarities. Therefore, receptor-based or functional assays have the best chance of detecting putative biological activity of environmental chemicals. Three nuclear estrogen receptor forms-alpha, beta, and gamma-as well as multiple membrane forms and a possible mitochondrial form have been reported, suggesting a previously unknown diversity of signaling pathways available to estrogenic chemicals. Examples of environmental or ambient estrogenization occur in laboratory experiments, zoo animals, domestic animals, wildlife, and humans. Environmentally estrogenized phenotypes may differ depending upon the time of exposure-i.e., whether the exposure occurred at a developmental (organizational and irreversible) or postdevelopmental (activational and reversible) stage. The term "estrogen" must be defined in each case, since steroidal estrogens differ among themselves and from synthetic or plant-derived chemicals. An "estrogen-like function" seems to be an evolutionarily ancient signal that has been retained in a number of chemicals, some of which are vertebrate hormones. Signaling, required for symbiosis between plants and bacteria, may be viewed, therefore, as an early example of hormone cross-talk. Developmental feminization at the structural or functional level is an emerging theme in species exposed, during embryonic or fetal life, to estrogenic compounds. Human experience as well as studies in experimental animals with the potent estrogen diethylstilbestrol provide informative models. Advances in the molecular genetics of sex differentiation in vertebrates facilitate mechanistic understanding. Experiments addressing the concept of gene imprinting or induction of epigenetic memory by estrogen or other hormones suggest a link to persistent, heritable phenotypic changes seen after developmental estrogenization, independent of mutagenesis. Environmental endocrine science provides a new context in which to examine the informational content of ecosystem-wide communication networks. As common features come to light, this research may allow us to predict environmentally induced alterations in internal signaling systems of vertebrates and some invertebrates and eventually to explicate environmental contributions to human reproductive and developmental health.

Determinants of vitellogenin B1 promoter architecture. HNF3 and estrogen responsive transcription within chromatin

The liver-specific vitellogenin B1 promoter is efficiently activated by estrogen within a nucleosomal environment after microinjection into Xenopus laevis oocytes, consistent with the hypothesis that significant nucleosome remodeling over this promoter is not a prerequisite for the activation by the estrogen receptor (ERalpha). This observation lead us to investigate determinants other than ERalpha of chromatin structure and transcriptional activation of the vitellogenin B1 promoter in this system and in vitro. We find that the liver-enriched transcription factor HNF3 has an important organizational role for chromatin structure as demonstrated by DNase I-hypersensitive site mapping. Both HNF3 and the estrogen receptor activate transcription synergistically and are able to interact with chromatin reconstituted in vitro with three positioned nucleosomes. We propose that HNF3 is the cellular determinant which establishes a promoter environment favorable to a rapid transcriptional activation by the estrogen receptor.

Androgen responsiveness of the murine beta-glucuronidase gene is associated with nuclease hypersensitivity, protein binding, and haplotype-specific sequence diversity within intron 9

The tissue specificity and genetic variability of the murine beta-glucuronidase (GUS) response to androgen provide useful markers for identifying elements which underlie this responsiveness. While GUS is expressed constitutively in all examined cell types, kidney epithelial cells uniquely exhibit a manyfold yet slow rise in GUS mRNA and enzyme levels when stimulated by androgens. Three major phenotypes of this androgen response have been described among inbred strains of mice: (i) a strong response in strains of the Gusa haplotype, (ii) a reduced response in strains of the Gusb and Gush haplotypes, and (iii) no response, as observed in Gusor mice. These response variants define a cis-active element(s) which is tightly linked to the GUS structural gene. Nuclease hypersensitivity scans of kidney chromatin within and surrounding the structural gene revealed an androgen-inducible hypersensitive site in intron 9 of the gene in Gusa but not in Gusor mice. When a radiolabeled fragment of Gusa DNA containing this hypersensitive site was incubated with kidney nuclear extracts and then subjected to gel electrophoresis, two shifted bands were observed whose levels were dramatically higher in extracts of androgen-treated than in those of untreated Gusa mice. The shifted bands reflect binding of a kidney-specific factor(s) to a 57-bp region of complex dyad symmetry in Gusa and Gusor mice which is partially deleted in Gusb and Gush mice. This binding site is located approximately 130 bp downstream of a glucocorticoid response element sequence motif which is totally deleted in [Gus]or mice. Taken together, our results suggest that the androgen responsiveness of GUS in murine kidney epithelial cells is controlled by elements within the proximal end of intron 9 of the GUS structural gene.

Androgen responsiveness of the murine beta-glucuronidase gene is associated with nuclease hypersensitivity, protein binding, and haplotype-specific sequence diversity within intron 9

The tissue specificity and genetic variability of the murine beta-glucuronidase (GUS) response to androgen provide useful markers for identifying elements which underlie this responsiveness. While GUS is expressed constitutively in all examined cell types, kidney epithelial cells uniquely exhibit a manyfold yet slow rise in GUS mRNA and enzyme levels when stimulated by androgens. Three major phenotypes of this androgen response have been described among inbred strains of mice: (i) a strong response in strains of the Gusa haplotype, (ii) a reduced response in strains of the Gusb and Gush haplotypes, and (iii) no response, as observed in Gusor mice. These response variants define a cis-active element(s) which is tightly linked to the GUS structural gene. Nuclease hypersensitivity scans of kidney chromatin within and surrounding the structural gene revealed an androgen-inducible hypersensitive site in intron 9 of the gene in Gusa but not in Gusor mice. When a radiolabeled fragment of Gusa DNA containing this hypersensitive site was incubated with kidney nuclear extracts and then subjected to gel electrophoresis, two shifted bands were observed whose levels were dramatically higher in extracts of androgen-treated than in those of untreated Gusa mice. The shifted bands reflect binding of a kidney-specific factor(s) to a 57-bp region of complex dyad symmetry in Gusa and Gusor mice which is partially deleted in Gusb and Gush mice. This binding site is located approximately 130 bp downstream of a glucocorticoid response element sequence motif which is totally deleted in [Gus]or mice. Taken together, our results suggest that the androgen responsiveness of GUS in murine kidney epithelial cells is controlled by elements within the proximal end of intron 9 of the GUS structural gene.

DNAase I sensitivities in chromatin of the Xenopus laevis somatic and oocyte 5 S DNAs

The DNAase I sensitivities of the somatic-type 5 S DNA and oocyte-type 5 S DNA have been compared in nuclei from adult somatic tissues of Xenopus laevis. Neither of these Type III genes is expressed in mature erythrocytes and only somatic-type 5 S DNA is expressed in liver. We find that somatic-type 5 S DNA is DNAase-I-sensitive in liver nuclei and less sensitive in erythrocyte nuclei, while oocyte-type 5 S DNA is insensitive in both tissues. The DNAase I sensitivity appears to be uniform across each active somatic-type 5 S DNA repeat. Two regions slightly hypersensitive to DNAase I are found only in liver somatic-type 5 S DNA. One of these regions is within the gene, overlapping with the binding site of the transcription factor (TF III A) required for 5 S RNA synthesis. Thus, the correlation between DNAase I sensitivity and gene activity previously seen for protein-coding genes also holds for these Type III genes. Our data lead us to suggest that the fully DNAase-I-sensitive chromatin conformation on 5 S DNA requires the presence both of transcription factors and RNA polymerase.

Chromatin structural transitions and the phenomenon of vitellogenin gene memory in chickens

We have previously shown that the steroid hormone-mediated transcriptional activation of the chicken vitellogenin II gene (VTGII) in the liver is accompanied by a series of chromatin structural changes, including the formation of two sets of 5'-proximal nuclease-hypersensitive sites and the demethylation of a single 5'-flanking MspI site which lies within a region of DNA that recently has been shown by Jost and co-workers to specifically bind the estrogen receptor complex in vitro. To assay the stability and possible functional significance of these induced structural changes, we transiently activated the VTGII gene during embryonic development and then allowed the chickens to hatch and grow for various periods of time before analyzing their livers. By 7 weeks posthatching all of the induced 5'-flanking hypersensitive sites had decayed. Moreover, the loss of these sites occurred without consequence to the "memory effect," that is, these structural features did not need to be present in hormone withdrawn birds to allow this gene to be activated more rapidly in response to a secondary presentation of estradiol. Although the demethylation was more stable, it also appeared not to be the basis of the memory phenomenon. The birds that still exhibited memory after 25 weeks of hormone withdrawal were not more extensively demethylated within the receptor-binding site than were the birds which failed to show memory at this age. A similar uncoupling of these two parameters was also observed when embryos were first injected with submaximal doses of estradiol and then assayed 1 week after hatching; the chickens which acquired memory were not demethylated to any greater extent than those which did not acquire memory. Other parameters that may be relevant to memory are discussed.

Chromatin structural transitions and the phenomenon of vitellogenin gene memory in chickens

We have previously shown that the steroid hormone-mediated transcriptional activation of the chicken vitellogenin II gene (VTGII) in the liver is accompanied by a series of chromatin structural changes, including the formation of two sets of 5'-proximal nuclease-hypersensitive sites and the demethylation of a single 5'-flanking MspI site which lies within a region of DNA that recently has been shown by Jost and co-workers to specifically bind the estrogen receptor complex in vitro. To assay the stability and possible functional significance of these induced structural changes, we transiently activated the VTGII gene during embryonic development and then allowed the chickens to hatch and grow for various periods of time before analyzing their livers. By 7 weeks posthatching all of the induced 5'-flanking hypersensitive sites had decayed. Moreover, the loss of these sites occurred without consequence to the "memory effect," that is, these structural features did not need to be present in hormone withdrawn birds to allow this gene to be activated more rapidly in response to a secondary presentation of estradiol. Although the demethylation was more stable, it also appeared not to be the basis of the memory phenomenon. The birds that still exhibited memory after 25 weeks of hormone withdrawal were not more extensively demethylated within the receptor-binding site than were the birds which failed to show memory at this age. A similar uncoupling of these two parameters was also observed when embryos were first injected with submaximal doses of estradiol and then assayed 1 week after hatching; the chickens which acquired memory were not demethylated to any greater extent than those which did not acquire memory. Other parameters that may be relevant to memory are discussed.

DNAaseI-hypersensitive minichromosomes of SV40 possess an elastic torsional strain in DNA

Previously, we have shown that DNA in a small fraction (2-5%) of SV40 minichromosomes was torsionally strained and could be relaxed by treating minichromosomes with topoisomerase I. This fraction was enriched with endogeneous RNA polymerase II (Luchnik et al., 1982, EMBO J., 1, 1353). Here we show that one and the same fraction of SV40 minichromosomes is hypersensitive to DNAase I and is relaxable by topoisomerase I. Moreover, this fraction completely loses its hypersensitivity to DNAase I upon relaxation. The possibility that this fraction of minichromosomes can be represented by naked DNA is ruled out by the results of studying the kinetics of minichromosome digestion by DNAase I in comparison to digestion of pure SV40 DNA and by measuring the buoyant density of SV40 chromatin in equilibrium CsCl gradient. Our data obtained with SV40 minichromosomes may be relevant to the mechanism responsible for DNAase I hypersensitivity in the loops or domains of cellular chromatin.

Transcriptionally active chromatin

Eukaryotic chromatin has a dynamic, complex hierarchical structure. Active gene transcription takes place on only a small proportion of it at a time. While many workers have tried to characterize active chromatin, we are still far from understanding all the biochemical, morphological and compositional features that distinguish it from inactive nuclear material. Active genes are apparently packaged in an altered nucleosome structure and are associated with domains of chromatin that are less condensed or more open than inactive domains. Active genes are more sensitive to nuclease digestions and probably contain specific nonhistone proteins which may establish and/or maintain the active state. Variant or modified histones as well as altered configurations or modifications of the DNA itself may likewise be involved. Practically nothing is known about the mechanisms that control these nuclear characteristics. However, controlled accessibility to regions of chromatin and specific sequences of DNA may be one of the primary regulatory mechanisms by which higher cells establish potentially active chromatin domains. Another control mechanism may be compartmentalization of active chromatin to certain regions within the nucleus, perhaps to the nuclear matrix. Topological constraints and DNA supercoiling may influence the active regions of chromatin and be involved in eukaryotic genomic functions. Further, the chromatin structure of various DNA regulatory sequences, such as promoters, terminators and enhancers, appears to partially regulate transcriptional activity.

Persistence, methylation and expression of vitellogenin gene derivatives after injection into fertilized eggs of Xenopus laevis

We report the fate of different derivatives of the vitellogenin genes after injection into fertilized eggs of Xenopus. We injected a constructed minigene as well as a 5' fragment of the A2 vitellogenin gene. The minigene survives in embryogenesis much better than the 5' A2 fragment and is retained more frequently and at a higher level in frog tissues. The mosaic distribution of the foreign DNA in different frog tissues indicates that no integration occurred before the first cleavage stage. The persisting DNA may be partially integrated but is mostly found in an episome-like form. This unintegrated form is not supercoiled and is rearranged. Methylation of the Hpa II sites prior to injection has no influence on the survival of the injected sequences and the Hpa II sites of the surviving DNA are unmethylated irrespective whether the injected DNA was methylated or not. Whereas the derivatives are transcribed in embryos, they cannot be activated by estrogen in the liver of young frogs.

Sequence-specific binding of glucocorticoid receptor to MTV DNA at sites within and upstream of the transcribed region

Glucocorticoid receptor protein stimulates transcription initiation within murine mammary tumor virus (MTV) DNA sequences in vivo, and interacts selectively with MTV DNA in vitro. We mapped and compared five regions of MTV DNA that are bound specifically by purified receptor; one resides upstream of the transcription start site, and the others are distributed within transcribed sequences between 4 and 8 kb from the initiation site. Each region contains at least two strong binding sites for receptor, which itself appears to be a tetramer of 94,000 dalton hormone-binding subunits. Three of the five binding regions contain nine nuclease footprints that lack extensive homology, although a family of related octanucleotides can be discerned. Receptor interacts with the different regions with similar efficiencies, suggesting that receptor affinity for upstream and internal regions may differ by less than one order of magnitude. Moreover, each region appears to be bound independent of the others. A restriction fragment containing four footprint sequences from one of the regions has previously been shown to act in vivo as a receptor-dependent transcriptional enhancer element, implying that the binding sites detected in vitro may be biologically functional.

DNA sequences bound specifically by glucocorticoid receptor in vitro render a heterologous promoter hormone responsive in vivo

Glucocorticoids stimulate transcriptional initiation within integrated mammary tumor virus (MTV) DNA sequences in infected cells. We report here that production of herpes simplex virus thymidine kinase (tk) RNA is stimulated as much as 50-fold by the glucocorticoid, dexamethasone, when sequences from a particular region of MTV DNA are fused upstream of the normally constitutive tk promoter region. Three cloned fragments of MTV DNA were tested, each itself lacking the sequences required for transcription initiation. We monitored the effects of dexamethasone on the efficiency with which these recombinants transfect a tk- rat cell line to a tk+ phenotype, and measured tk enzymatic activity, the size and abundance of tk mRNA, and the 5' termini of tk transcripts in the transfectants. We conclude that the MTV promoter region contains a "glucocorticoid response element" that can be separated from a second element essential for MTV transcription initiation. The hormone response element maps within a 340-base pair MTV DNA fragment that contains specific binding sites for purified glucocorticoid receptor protein in vitro, implying that receptor binding at these sites in vivo may mediate hormone responsiveness. Comparison of several different constructions indicates that the location and orientation of the glucocorticoid response element relative to the transcription start site is not rigidly constrained.

In contrast to other Xenopus genes the estrogen-inducible vitellogenin genes are expressed when totally methylated

The methylation-sensitive restriction enzymes Hha I and Hpa II were used to analyze the methylation pattern of four Xenopus laevis genes in DNA of embryos, of erythrocytes, and of untreated and estrogen-treated hepatocytes. Within these four genes all sites tested are fully modified in embryonic DNA. However, the adult beta 1-globin gene is unmethylated in DNA of erythrocytes, where it is expressed, and the 68 kd albumin gene, active only in hepatocytes, is specifically hypomethylated in hepatic DNA. The vitellogenin genes A1 and A2, in hepatocytes simultaneously expressed upon estrogen treatment, are heavily methylated in all adult tissues, irrespective of expression. Our results reveal that specific genes can be actively transcribed even when they are fully methylated and that changes in the methylation pattern are not a general prerequisite for gene activation.

Differential sensitization to deoxyribonuclease I of Xenopus vitellogenin and albumin genes during primary and secondary induction of vitellogenesis by oestradiol

The sensitivity to DNAase (deoxyribonuclease) I (which preferentially digests transcribed sequences) of vitellogenin and albumin genes in liver and erythrocytes of male Xenopus after primary and secondary induction of vitellogenesis by oestrogen was measured by hybridization to cDNA (complementary DNA) of the residual DNA after enzymic digestion of isolated nuclei. Vitellogenin sequences were rendered selectively more sensitive to limited DNAase-I digestion (15-20% of DNA rendered acid-soluble) during primary hormonal activation (5 days) of vitellogenin genes in liver, but not erythrocyte, nuclei. Hormone withdrawal (25 days after first injection) did not result in reversion to a pre-activation gene configuration, nor did secondary hormonal stimulation (5 days after second and 25 days after first injection) augment the sensitivity of the genes to digestion by the nuclease. Similar hormone treatment did not affect the sensitivity of the constitutively expressed albumin genes in liver nuclei, nor their insensitivity in erythrocyte nuclei. Under the same conditions, globin genes remained indigestible in liver nuclei. It is concluded that primary induction of vitellogenesis in male Xenopus liver is accompanied by relatively long-lasting (3-4 weeks) change in the configuration of vitellogenin genes in hepatic nuclei which is not reversed or further modified during short-term oestrogen withdrawal or upon secondary stimulation.

Responses to androgens of rat ventral prostate nuclear androgen-binding sites sensitive and resistant to micrococcal nuclease

Rat ventral prostate nuclei were separated into three major fractions by mild digestion with micrococcal nuclease and two fractions by extensive digestion. All fractions contained androgen-binding sites. Almost 50% of nuclear binding sites were resistant to enzymic digestion when only 5-15% of total DNA was resistant. Under milder digestion conditions, 21% of nuclear binding sites were associated with an intermediate fraction, representing 16% of total nuclear DNA, which was enriched in specific androgen-regulated gene sequences. This fraction was rapidly degraded by more extensive digestion. The nuclease sensitivity of these particular genes was markedly influenced by castration and the administration of dihydrotestosterone to castrated animals. The nuclear content of both nuclease-resistant and -sensitive androgen-binding sites was decreased by castration. Whereas the administration of androgen to animals castrated 1 day previously preferentially replenished nuclease-resistant sites, nuclease-sensitive sites, including those associated with transcriptionally active regions, had apparent priority when androgen was supplied to animals castrated 7 days previously. The significance of these observations to the regulation of nuclear processes and the possible interrelationships of nuclease-sensitive and -insensitive sites are discussed.