Expression of Functional Chicken Oviduct Progesterone Receptors in Yeast (Saccharomyces Cerevisiae)

The role of epithelial progesterone receptor isoforms in embryo implantation

The loss of uterine epithelial progesterone receptor (PGR) is crucial for successful embryo implantation in both humans and mice. The two major isoforms PGRA and PGRB have divergent functions under both physiological and pathological conditions. The present study compares phenotypes and gene signatures of PGRA and PGRB in uterine epithelium using uterine epithelial-specific constitutively expressed PGRA or PGRB mouse models. The cistrome and transcriptome analysis reveals substantial overlap between epithelial PGRA and PGRB, and both disrupt embryo implantation through FOXO1 pathways. Constitutive epithelial PGRA and PGRB expression impairs ESR1 occupancy at the promoter of Lif leading to reduced Lif transcription and further exaggerates SGK1 expression leading to enhanced PI3K-SGK1 activities, and both contribute to the decline of nuclear FOXO1 expression. Our study demonstrates that PGRA and PGRB in the uterine epithelium act on a similar set of target genes and commonly regulate the LIF-SGK1-FOXO1 signaling pathway for embryo implantation.

Bioactivity-based screening of antibiotics and hormones

Bioactivity-based screening methods are relatively cheap, quick and easy to use tools. Especially with respect to antimicrobial residues and compounds with hormonal activity, they form a very cost-effective alternative to physical chemical methods in large-scale surveillance and monitoring programs, where their main purpose is to identify samples that require additional chemical confirmation. A major advantage is their intrinsic capability to detect unknown compounds and new hazards. This review shows an overview of the available methods and their potential and limitations for regulatory control.

Detecting ligands and dissecting nuclear receptor-signaling pathways using recombinant strains of the yeast Saccharomyces cerevisiae

This is a general protocol for the identification of natural and xenobiotic ligands of metazoan nuclear receptors (NRs) expressed in yeast. Yeast engineered to express an NR and a response element-driven reporter gene provide a system to detect and quantify ligand-dependent transcriptional activity. Such assays allow researchers to measure different types of ligands and determine dose-dependent activation of NRs. This methodology can also be used to examine the components of signal transduction pathways when conducted with mutant or engineered yeast strains expressing additional proteins or having alternate DNA response elements. This assay typically takes 2-3 d to complete, but most of this time entails cell growth rather than 'hands on' time.

Ubiquitin fusion technique and related methods

The ubiquitin fusion technique, developed in 1986, is still the method of choice for producing a desired N-terminal residue in a protein of interest in vivo. This technique is also used as a tool for protein expression. Over the past two decades, several otherwise unrelated methods were invented that have in common the use of ubiquitin fusions as a component of design. I describe the original ubiquitin fusion technique, its current applications, and other methods that use the properties of ubiquitin fusions.

Ligand-induced heterodimerization between the ligand binding domains of the Drosophila ecdysteroid receptor and ultraspiracle

The insect ecdysteroid receptor consists of a heterodimer between EcR and the RXR-orthologue, USP. We addressed the question of whether this heterodimer, like all other RXR heterodimers, may be formed in the absence of ligand and whether ligand promotes dimerization. We found that C-terminal protein fragments that comprised the ligand binding, but not the DNA binding domain of EcR and USP and which were equipped with the activation or DNA binding region of GAL4, respectively, exhibit a weak ability to interact spontaneously with each other. Moreover, the heterodimer formation is greatly enhanced upon administration of active ecdysteroids in a dose-dependent manner. This was shown in vivo by a yeast two-hybrid system and in vitro by a modified electromobility shift assay. Furthermore, the EcR fragment expressed in yeast was functional and bound radioactively labelled ecdysteroid specifically. Ligand binding was greatly enhanced by the presence of a USP ligand binding domain. Therefore, ecdysteroids are capable of inducing heterodimer formation between EcR and USP, even when the binding of these receptor proteins to cognate DNA response elements does not occur. This capability may be a regulated aspect of ecdysteroid action during insect development.

Expression and purification of homogenous proteins in Saccharomyces cerevisiae based on ubiquitin-FLAG fusion

The construction of an expression vector for increased expression of cytoplasmic proteins in Saccharomyces cerevisiae is described. To enhance the yield of expressed proteins, fusion of ubiquitin to an octapeptide (a FLAG tag) upstream of the respective model genes was applied. During protein maturation ubiquitin is efficiently removed by yeast autologous hydrolases, generating the FLAG octapeptide at the N-terminus. Fusion proteins were recognized by the specific monoclonal antibody M1 directed against the FLAG tag. The FLAG-tagged proteins were purified to homogeneity by immunoaffinity chromatography using an anti-FLAG M1 agarose. Different model proteins, green fluorescent protein, green fluorescent protein-human lysozyme, green fluorescent protein elongation-initiahon factor 5a, green fluorescent protein-rapamycin-selective 25-kDa immunophilin, and green fluorescent protein-heat shock protein 90 beta have been selected to demonstrate the efficiency of the new vector construct.

Reconstitution of hepatitis C virus protease activities in yeast

The hepatitis C virus (HCV) protease genes (NS2/3 and NS3) were expressed in yeast with their natural substrates fused to a ligand-dependent transcriptional activator, the retinoic acid receptor (RARbeta). RARbeta can activate transcription in yeast cells in response to retinoic acids. We hypothesized that cis-cleavage at the NS2-3 or NS3-4A junctions by the appropriate HCV proteases would release RARbeta, thereby activating transcription of a reporter gene. Our results from Western blot analyses and reporter gene activation indicate that the wild-type NS2/3 and NS3 enzymes are catalytically active in yeast cells, whereas mutations in the catalytic domain of NS2(C993V) and NS3(S1165A) lead to inactive enzymes. We conclude that HCV NS2/3 and NS3 protease activities can be reconstituted in yeast.

Characterization of transiently and constitutively expressed progesterone receptors: evidence for two functional states

Activated steroid receptors induce chromatin remodeling events in the promoters of some target genes. We previously reported that transiently expressed progesterone receptor (PR) cannot activate mouse mammary tumor virus (MMTV) promoter when it adopts the form of ordered chromatin. However, when expressed continuously, the PR acquires this ability. In this study we explored whether this gain of function occurs through alterations in nucleoprotein structure at the MMTV promoter or through changes in receptor status. We observed no major structural differences at the MMTV promoter in the presence of constitutively expressed PR and found its mechanism of activation to be very similar to that of the glucocorticoid receptor (GR). However, a systematic comparison of the functional behavior of the transiently and constitutively expressed PR elucidated significant differences. The transiently expressed PR is activated in the absence of ligand by cAMP and by components in FBS and has significantly increased sensitivity to progestins. In contrast, the constitutively expressed PR is refractory to activation by cAMP and serum and has normal sensitivity to its ligand. In addition, while the PR is localized to the nucleus in both cases, a significant fraction of the transiently expressed PR is tightly bound to the nucleus even in the absence of ligand, while the majority of constitutively expressed PR is not. These results strongly suggest that the PR undergoes processing in the cell subsequent to its initial expression and that this processing is important for various aspects of its function, including its ability to productively interact with target genes that require chromatin remodeling for activation.

Two estrogen receptor (ER) isoforms with different estrogen dependencies are generated from the trout ER gene

A characteristic of all estrogen receptors (ER) cloned from fish to date is the lack of the first 37-42 N-terminal amino acids specific to the A domain. Here we report the isolation and characterization from trout ovary of a full-length complementary DNA (cDNA) clone encoding an N-terminal variant form of the rainbow trout ER (rtER). Sequence analysis of open reading frame of this cDNA predicts a 622-amino acid protein. The C-terminal region of this protein, from amino acid position 45 to the end, was very similar to the previously reported rtER (referred to as the short form, or rtER(S)). In contrast, this novel rtER cDNA (referred to as the long form, or rtER(L)) contains an additional in-frame ATG initiator codon that adds 45 residues to the N-terminal region of the protein. This new N-terminal region may represent the A domain of ER found in tetrapod species. The first 227 bp of this new cDNA were similar to the 3'-end intronic sequence of the rtER gene intron 1. These data together with S1 nuclease, primer extension, and RT-PCR experiments demonstrate that the rtER(L) represents a second isoform of rtER that arises from an alternative promoter within the first intron of the gene. Transcripts encoding both rtER forms were expressed in the liver. In vitro translation of the rtER(L) cDNA produced 2 proteins with molecular masses of 71 and 65 kDa, whereas rtER(S) cDNA produced 1 65-kDa protein. Interestingly, Western blot analysis with a specific antibody against the C-terminal region of rtER revealed 2 receptor forms of 65 and 71 kDa in trout liver nuclear extracts, in agreement with the presence of the 2 distinct classes of rtER messenger RNA in this tissue. Functional analysis of both rtER isoforms revealed that although rtER(S) consistently exhibited a basal (estrogen-independent) trans-activation activity that could be further increased in the presence of estrogens, the novel isoform rtER(L) is characterized by a strict estrogen-dependent transcriptional activity. These data suggest that the additional 45 residues at the N-terminal region of rtER(L) clearly modify the hormone-independent trans-activation function of the receptor.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

Coregulatory proteins in steroid hormone receptor action: the role of chromatin high mobility group proteins HMG-1 and -2

To directly activate specific gene expression, the progesterone receptor must bind to specific hormone response elements in target promoters. We have previously reported that progesterone receptor requires a nuclear factor, high mobility group 1 or 2 (HMG-1/-2) for high-affinity interaction with DNA in vitro and for full transcriptional activity in vivo. We have also observed that HMG-1/-2 selectively influences the activity of the steroid hormone class of nuclear receptors but does not affect other classes of nuclear receptors. This report is a summary of our published and unpublished studies to determine the effects of HMG-1/-2 on a broad range of nuclear receptor supergene family members and to define the mechanism for the specific effect of HMG-1/-2 on the steroid class of receptors. Our studies and available structural data suggest a model where the DNA binding domains of nonsteroid nuclear receptors contain a minor groove DNA interface, termed the C-terminal extension, that contributes to high-affinity DNA binding. Steroid receptors lack such a minor groove interface and therefore require an additional protein, HMG-1/-2, that functionally substitutes for the C-terminal extension to facilitate high-affinity interactions with DNA.

Investigation of steroid receptor function in the budding yeast Saccharomyces cerevisiae

Steroid hormones are small lipophilic molecules that control a wide range of responses in both the developing and adult organism. The actions of these molecules are mediated by soluble receptor proteins that function as hormone-activated transcription factors. The first steroid receptors were expressed in the yeast Saccharomyces cerevisiae over 10 years ago, and to date virtually all the classical steroid receptors, together with a number of non-steroid members of the nuclear receptor superfamily, have been expressed in yeast. The ability to reconstitute steroid receptor signalling in yeast cells by co-expression of the receptor protein and a reporter gene driven by the appropriate hormone response element has presented researchers with a powerful model system for investigating receptor action. In this review, the use of yeast-based steroid receptor transactivation assays to investigate the roles of molecular chaperones, the mechanisms of DNA binding and gene activation, and the functional properties of hormone mimics will be discussed.

Bioactive agents from natural sources: trends in discovery and application

About 30% of the worldwide sales of drugs are based on natural products. Though recombinant proteins and peptides account for increasing sales rates, the superiority of low-molecular mass compounds in human diseases therapy remains undisputed mainly due to more favorable compliance and bioavailability properties. In the past, new therapeutic approaches often derived from natural products. Numerous examples from medicine impressively demonstrate the innovative potential of natural compounds and their impact on progress in drug discovery and development. However, natural products are currently undergoing a phase of reduced attention in drug discovery because of the enormous effort which is necessary to isolate the active principles and to elucidate their structures. To meet the demand of several hundred thousands of test samples that have to be submitted to high-throughput screening (HTS) new strategies in natural product chemistry are necessary in order to compete successfully with combinatorial chemistry. Today, pharmaceutical companies have to spend approximately US $350 million to develop a new drug. Currently, approaches to improve and accelerate the joint drug discovery and development process are expected to arise mainly from innovation in drug target elucidation and lead finding. Breakthroughs in molecular biology, cell biology, and genetic engineering in the 1980 s gave access to understanding diseases on the molecular or on the gene level. Subsequently, constructing novel target directed screening assay systems of promising therapeutic significance, automation, and miniaturization resulted in HTS approaches changing the industrial drug discovery process drastically. Furthermore, elucidation of the human genome will provide access to a dramatically increased number of new potential drug targets that have to be evaluated for drug discovery. HTS enables the testing of an increasing number of samples. Therefore, new concepts to generate large compound collections with improved structural diversity are desirable.

CNS1 encodes an essential p60/Sti1 homolog in Saccharomyces cerevisiae that suppresses cyclophilin 40 mutations and interacts with Hsp90

Cyclophilins are cis-trans-peptidyl-prolyl isomerases that bind to and are inhibited by the immunosuppressant cyclosporin A (CsA). The toxic effects of CsA are mediated by the 18-kDa cyclophilin A protein. A larger cyclophilin of 40 kDa, cyclophilin 40, is a component of Hsp90-steroid receptor complexes and contains two domains, an amino-terminal prolyl isomerase domain and a carboxy-terminal tetratricopeptide repeat (TPR) domain. There are two cyclophilin 40 homologs in the yeast Saccharomyces cerevisiae, encoded by the CPR6 and CPR7 genes. Yeast strains lacking the Cpr7 enzyme are viable but exhibit a slow-growth phenotype. In addition, we show here that cpr7 mutant strains are hypersensitive to the Hsp90 inhibitor geldanamycin. When overexpressed, the TPR domain of Cpr7 alone complements both cpr7 mutant phenotypes, while overexpression of the cyclophilin domain of Cpr7, full-length Cpr6, or human cyclophilin 40 does not. The open reading frame YBR155w, which has moderate identity to the yeast p60 homolog STI1, was isolated as a high-copy-number suppressor of the cpr7 slow-growth phenotype. We show that this Sti1 homolog Cns1 (cyclophilin seven suppressor) is constitutively expressed, essential, and found in protein complexes with both yeast Hsp90 and Cpr7 but not with Cpr6. Cyclosporin A inhibited Cpr7 interactions with Cns1 but not with Hsp90. In summary, our findings identify a novel component of the Hsp90 chaperone complex that shares function with cyclophilin 40 and provide evidence that there are functional differences between two conserved sets of Hsp90 binding proteins in yeast.

Bioactive steroids as contaminants of the common carbon source galactose

Most inducible expression vectors for the budding yeast Saccharomyces cerevisiae are based on galactose-inducible promoters. Yeast has been increasingly used to study vertebrate steroid receptors because of its powerful genetics. In principle, both regulatory systems are compatible and can be combined in the same strain. However, we found that commercial galactose can be contaminated by bioactive estrogen and progesterone at concentrations that are sufficient to fully activate their cognate receptors. Since steroids can elicit biological responses in pathogenic fungi and possibly other microorganisms, such contaminants in a commonly used fermentable carbon source may need to be screened for.

Development of a Tier I screening battery for detecting endocrine-active compounds (EACs)

One of the components of our research program is development of a mode-of-action screening battery to detect several different types of endocrine-active compounds (EACs). Our working hypothesis is that a comprehensive short-term in vivo/in vitro battery can be developed to identify endocrine toxicants using a collection of endpoints. The goals of this battery are that it be quick, cost effective, and predictive. The purpose of this battery is to identify potential EACs and to assess their potency in order to prioritize compounds for further study. Two in vivo screens (intact male and ovariectomized female rats) are being evaluated for their ability to detect several different types of endocrine activity. To validate this screen, 15 compounds with known endocrine activities are being used to evaluate a collection of different endpoints for their variability, stability over time, predictiveness, and dose dependency. These positive controls were chosen because they can modulate development, reproduction, or cancer. The advantage of an in vivo screen is that it utilizes a metabolically and physiologically intact system. The male in vivo battery will be used to assess several different types of endocrine activity, primarily by using a comprehensive hormonal battery. The female in vivo battery will be used to identify compounds which are either estrogenic/antiestrogenic or can alter the prolactin pathway. The in vitro portion of the screening battery consists of a yeast transactivation system (YTS). The YTS is being evaluated for its ability to identify compounds which are agonists or antagonists to the estrogen, androgen, or progesterone receptors. The expression of mammalian receptors in yeast allows for assessment of steroid-dependent transcriptional activators. The value of this system is that it can be used as a routine screen for compounds that interact with steroid receptors. Alterations in ligand binding to these receptors can be correlated with alterations in development via masculinization of females and/or feminization of males, decreases in reproductive success, or modulation of cancer incidence from in vivo tests. The in vivo and in vitro screens are designed to be run in parallel with built-in redundancy in order to reduce the probability of false-negative/ positive responses.

Steroid receptor induction of gene transcription: a two-step model

Coactivators, such as steroid receptor coactivator 1 (SRC-1A) and CREB (cAMP response element binding protein)-binding protein (CBP), are required for efficient steroid receptor transactivation. Using an in vitro transcription assay, we found that progesterone receptor (PR)-driven transcription is inhibited by a dominant negative PR ligand-binding domain-interacting region of SRC-1A, indicating that SRC-1A is required for actual transcriptional processes. In addition, these coactivators also possess intrinsic histone acetyltransferase (HAT) activity and bind to each other and another HAT, p300/CBP-associated factor. Here we show that the human PR also interacts with p300/CBP-associated factor in vitro. Recruitment of multiple HATs to target promoters suggests an important role for chromatin remodeling in transcriptional activation of genes by steroid receptors. In transient transfection assays, we found that addition of a histone deacetylase inhibitor, trichostatin A, strongly potentiated PR-driven transcription. In contrast, directing histone deacetylase-1 (HD1) to a promoter using the GAL4 DNA binding domain inhibited transcription. Furthermore, PR transactivation was repressed by recruiting HD1 into the PR-DNA complex by fusing HD1 to a PR ligand-binding domain-interacting portion of SRC-1. Collectively, these results suggest that targeted histone acetylation by recruited HAT cofactors and histone deacetylation are important factors affecting PR transactivation. Recruitment of coactivators and HATs by the liganded PR in vivo may result in (i) remodeling of transcriptionally repressed chromatin to facilitate assembly and (ii) enhanced stabilization of the preinitiation complex by the activation functions of coactivators and the liganded PR itself.

Drosophila ecdysone receptor functions as a constitutive activator in yeast

Transcriptional activation of the Drosophila ecdysone receptor (EcR) was studied in yeast cells, which carry a reporter plasmid containing the ecdysone response element in the absence or presence of its heterodimeric partners, ultraspiracle protein (USP) or human retinoid X receptor (RXRalpha). High constitutive transcriptional activation was detected in the yeast strain expressing EcR, but not USP or RXRalpha in the absence of ponasterone or muristerone A. Incubation of these ligands with yeast cells coexpressing EcR and USP or RXRalpha did not enhance the constitutive transcriptional activity. However, specific ligand binding using [3H]ponasterone A as a radioactive ligand was detected only in yeast extracts prepared from the yeast strain coexpressing EcR and USP, but not from yeast strains expressing only EcR or USP. The ligand binding characteristics of the EcR/USP complexes were similar to those reported in an insect cell line with a Kd value of 1.8 nM for [3H]ponasterone A. These data are in contrast to mammalian cell transfection studies, and indicate that the EcR is the only member of the nuclear receptor superfamily of ligand-activated transcription factors which functions as a constitutive transcriptional activator in yeast, although the EcR/USP complexes exhibit normal ligand binding properties.

Evaluation of chemicals with endocrine modulating activity in a yeast-based steroid hormone receptor gene transcription assay

There is a concern that chemicals in our environment are affecting human health by disrupting normal endocrine function. Much of the concern has focused on chemicals that can interact directly with steroid hormone receptors. We have used a yeast-based assay to assess chemical interactions with the estrogen, androgen, and progesterone receptors. The yeast transformants used in this study contained the human estrogen, androgen, or progesterone receptor along with the appropriate steroid responsive elements upstream of the beta-galactosidase reporter gene. Chemicals were added to yeast cultures in doses ranging from 10(-12) to 10(-4) M and following incubation, the yeasts were then lysed and assayed for beta-galactosidase activity. Diethylstilbesterol and 17-beta estradiol were most active in the estrogen receptor assay, followed by the phytoestrogen, coumestrol. p-Nonylphenol and bisphenol A were approximately 5000- and 15,000-fold less active, respectively, than estradiol. Methoxychlor, DDT and its metabolites, o,p'-DDD, and o,p'-DDE ranged in potency from 5 to 24 X 10(6) less potent than estradiol. Testosterone and dihydrotestosterone were most potent in the androgen receptor assay, followed by estradiol and progesterone. p,p'-DDE was approximately 10(6)-fold less potent than testosterone. None of the industrial chemicals tested interacted with the progesterone receptor. These data demonstrate the utility of using yeast-based receptor assays for detecting chemical interaction with steroid receptors and these assays should serve as a useful component of an in vitro-in vivo strategy to assess the effects of chemicals on endocrine function.

Protein expression using ubiquitin fusion and cleavage

Expressing proteins and polypeptides as fusions to ubiquitin offers the advantage of an often dramatic increase in yield, and the ability to produce any desired amino-terminal residue upon ubiquitin cleavage. The recent availability of cloned ubiquitin-cleaving enzymes has enhanced this technique for both bacterial and eukaryotic host systems.

Differential transcriptional regulation of the apoAI gene by retinoic acid receptor homo- and heterodimers in yeast

Several members of the nuclear receptor superfamily including RXR (retinoid X receptor) bind to a specific retinoic acid response element (site A) of the apoAI promoter. However, transcriptional activation of the apoAI gene by different homo- and heterodimeric forms of RXR or RAR (retinoic acid receptors) cannot be evaluated in mammalian cells, which contain endogenous RXR or RAR. In order to circumvent this limitation, we assessed the DNA-binding activities and transcriptional activation of different homo- and heterodimers of these receptors in yeast. Electrophoretic mobility shift assays (EMSA) demonstrated that yeast expressed RARalpha does not bind to site A of the apoAl promoter, whereas binding of RARbeta to site A is ligand-dependent. Both RARalpha and RARbeta form heterodimers with RXRalpha and bind to site A with high affinity. These DNA-binding studies correlate with the transcriptional data, which indicated that RARbeta but not RARalpha activates transcription from site A in response equally well to 9-cis and all-trans retinoic acids. 9-cis RA is a more potent ligand than all-trans RA to activate transcription via RXR/RAR heterodimers. We conclude that this yeast expression system is a useful tool to elucidate the 'transactivation code' for apoAl site A via specific combinations of different homo and heterodimeric versions of RXR and RAR.

Differential functional activities of rainbow trout and human estrogen receptors expressed in the yeast Saccharomyces cerevisiae

The cDNA of rainbow trout estrogen receptor (rtER), highly and stably expressed in yeast, Saccharomyces cerevisiae, was used to analyse the biological activity of the receptor. The rtER mRNA encoded a 65-kDa protein which was immunorevealed by a specific antibody and migrated with the authentic rtER major protein form detected in trout liver. Yeast rtER bound estradiol with high affinity and the dissociation constant (Kd = 1.35 nM) was very similar to the value measured from trout liver extracts but 3-5-fold higher than the Kd found for human estrogen receptor (hER). This indicates therefore that the rtER has a lower estradiol affinity compared to the human receptor. While the hER Kd remained unchanged at both 4 degrees C or 22 degrees C, it was slightly modified at 30 degrees C. The Kd measured for rtER at 22 degrees C and 30 degrees C were about 2-fold, and 12-fold higher, respectively, than the Kd obtained at 4 degrees C suggesting an alteration of the rtER affinity for its ligand at elevated temperature. To examine the estrogen-receptor-mediated activation of transcription in yeast, reporter plasmids integrated or not in the yeast genome were used. The reporter genes consist of one, two, or three copies of estrogen-responsive elements (ERE) upstream of the yeast proximal CYC1 or URA3 promoters fused to the lacZ gene of Escherichia coli coding for beta-galactosidase. The induction of beta-galactosidase activity for all reporter genes was strictly dependent on the presence of rtER and estrogens. The activation of transcription mediated by rtER responded in an estradiol-dose-dependent manner as in animal cells. However, compared to hER, the estradiol concentration necessary to achieve maximal activation was 10-fold higher. This is probably a consequence of the lower estradiol-affinity for rtER compared to hER. The levels of induction of the reporter genes containing two or three ERE were strongly enhanced compared to the one ERE construct. This is in agreement with the synergistic effect previously described for multiple ERE. The magnitudes of transcriptional induction mediated by rtER and hER were similar when the reporter gene containing three ERE was used but changed when the one ERE construct was used. In this case transcriptional activation indicated by rtER was 10-20 fold lower. This suggests that rtER requires protein/protein interaction for its stabilization on DNA. Antiestrogens were able to bind rtER and promote gene transcription. However, to produce effects comparable to those obtained with estrogens, much higher concentrations were required. This may imply nonetheless that antihormones were capable of provoking efficient interactions of rtER with the transcriptional machinery.

Functional analysis of aryl hydrocarbon receptor nuclear translocator interactions with aryl hydrocarbon receptor in the yeast two-hybrid system

The aryl hydrocarbon receptor (AHR) mediates dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin)-induced transcriptional activation of a battery of genes by interaction with a cofactor, called aryl hydrocarbon receptor nuclear translocator (ARNT) protein. Both AHR and ARNT belong to a family of proteins that includes the Drosophila circadian-rhythm protein and "single-minded" protein. These proteins share a domain called the PAS domain. In addition to the PAS domain, both AHR and ARNT contain basic helix-loop-helix (bHLH) and glutamine (Q)-rich domains. The roles of these domains in the receptor-mediated transcriptional activation are not understood completely. By using the yeast two-hybrid system with the N-terminal half of AHR as a probe, which contains the bHLH and PAS regions, to screen cDNA libraries prepared from human lymphocytes and C57BL mouse liver for clones encoding proteins capable of binding to these regions, we isolated a partial ARNT cDNA clone. These results demonstrated that the N-terminal half of AHR is capable of interacting with ARNT in yeast (probably through the bHLH motif). A fusion protein containing the GAL4 DNA binding domain (DB) linked to the full-length AHR was not capable of activating expression of a reporter gene containing the GAL4 DNA binding site, suggesting that ligand-free AHR alone has no transactivating properties in yeast. However, the C-terminal portion (amino acid residues 580-797) of the AHR, including the Q-rich domain, could confer transactivation of the reporter gene expression in the same system, suggesting that the N-terminal portion of the AHR contains transcription repression properties. In contrast, GAL4(DB)-ARNT fusion protein was able to activate expression of the same reporter gene. Deletion analysis of ARNT revealed that the C-terminal 75 amino acids, including the Q-rich domain, exhibited full transactivation function in yeast and mammalian cells. These results revealed different structural organizations for the transactivation properties between AHR and ARNT, although both contained transactivation domains at the C-termini.

Yeast two-hybrid system demonstrates that estrogen receptor dimerization is ligand-dependent in vivo

Previous studies using in vitro procedures have not clearly established whether the estrogen receptor (ER) acts as a monomer or dimer in the cell. We have used the yeast two-hybrid system as an in vivo approach to investigate the dimerization of the estrogen receptor in the absence and presence of estrogen and anti-estrogens. This system is independent of ER binding to the estrogen response element. Two vectors, expressing GAL4 DNA binding domain-human ER and GAL4 transactivation domain-human ER, were constructed. Control experiments showed that each fusion protein had a high affinity binding site for estradiol-17 beta and could transactivate an ERE-LacZ reporter gene in yeast similar to the wild type ER. The two fusion proteins, GAL4 DB-hER and GAL 4 TA-hER, were expressed in the yeast strain, PCY2, which carries a GAL1 promoter-lacZ reporter. ER dimerization was measured via reconstitution of GAL4 through interaction of the fusion proteins, which transactivates LacZ through the GAL1 promoter. When both ER fusion proteins were expressed, beta-galactosidase activity was estradiol-17 beta-inducible. Furthermore, we showed that both tamoxifen and ICI 182,780 also induced beta-galactosidase activity, albeit lower than that induced by estradiol-17 beta. These results strongly argue that ER dimerization is ligand-dependent and the dimer can be induced by estradiol-17 beta, tamoxifen, or ICI 182,780. We also treated the yeast containing the two fusion proteins with estradiol-17 beta and tamoxifen or ICI 182,780 simultaneously to determine the effects on ER dimerization. beta-Galactosidase activity was lower when the yeast was treated with a higher ratio of tamoxifen or ICI 182,780 to estrogen than estradiol-17 beta alone. Taken together, we conclude that ER dimerization is ligand (estradiol-17 beta, tamoxifen, or ICI 182, 780)-dependent, and we suggest that estradiol-17 beta-induced dimers are destabilized when estradiol-17 beta is used with tamoxifen or ICI 182,780 simultaneously.

Phosphorylation and progesterone receptor function

Four phosphorylation sites have been identified in the chicken progesterone receptor. Two of these sites exhibit basal phosphorylation which is enhanced upon treatment with hormone and two of the sites are phosphorylated in response to hormone. Mutation of one of these hormone dependent sites, Ser530 to Ala530, causes a decrease in transcriptional activation at low concentrations of hormone, but the activity is unaffected at high concentrations. However, the hormone binding of the mutant is unaffected suggesting that phosphorylation of Ser530 plays a role in facilitating the response of the receptor to low concentrations of hormone. The chicken progesterone receptor can be activated by modulators of kinases in the absence of hormone. The finding that signals initiated by tyrosine phosphorylation (through treatment with EGF) or through the dopamine receptor suggests that there are multiple means of activating chicken progesterone receptor. In contrast, the human progesterone receptor does not exhibit ligand independent activation; however, its activity in the presence of the agonist R5020 is enhanced by treatment with 8-Br-cAMP, an activator of protein kinase A, and treatment with 8-Br-cAMP causes the antagonist, RU486, to act as an agonist.

Human dioxin receptor chimera transactivation in a yeast model system and studies on receptor agonists and antagonists

A yeast dioxin receptor chimera model has been developed to study ligand binding and transactivation properties of the human dioxin receptor. Using this new yeast model, the human dioxin receptor chimera was found to possess a constitutive transactivity on a LacZ reporter gene, however, the transactivation by the chimera was enhanced by the addition of several polycyclic aromatic hydrocarbons to the culture medium. The order of best polycyclic aromatic hydrocarbon inducer to worst correlated well with the known in vitro dioxin receptor binding affinities for these polycyclic aromatic hydrocarbons. 7,8-Benzoflavone, a weak dioxin receptor agonist and strong antagonist of the mammalian dioxin receptor also behaved as a weak agonist and strong antagonist of the human dioxin receptor chimera expressed in yeast. The implications for these findings as well as the utility of this new yeast human dioxin receptor chimera model are discussed.

The protooncogene c-jun contains an unusual estrogen-inducible enhancer within the coding sequence

Estrogens have previously been shown to induce c-jun mRNA levels in target cells during hormone induced proliferation, and this appears to be a primary hormonal response involving transcriptional activation. In this report we have now identified an estrogen dependent enhancer within the coding sequence of c-jun. This element has the sequence GCAGAnnnTGACC which is identical to the consensus estrogen response element GGTCAnnnTGACC in the second half site, but varies considerably in the first half site. Synthetic oligodeoxynucleotides containing this jun sequence bind the estrogen receptor in cell-free studies using a competitive band shift assay with the consensus element. The jun element also confers hormone inducibility to reporter plasmids in yeast and mammalian based transcriptional systems. Structure-function studies illustrate that the TGACC half-site and its immediate flanking dinucleotides, but not the GCAGA half-site, are required for estrogen receptor binding. In contrast, both the GCAGA and TGACC half-sites are obligatory for hormone-inducible transcriptional activation. These results suggest a model in which the estrogen receptor functions as a heterodimer to regulate transcription of the c-jun protooncogene. Coupled with reports of estrogen response elements in c-fos and estrogenic induction of c-fos and c-jun in vivo, these findings also support a role for AP-1 components as early response genes in estrogen-induced proliferation.

The yeast SIN3 gene product negatively regulates the activity of the human progesterone receptor and positively regulates the activities of GAL4 and the HAP1 activator

The activation of gene transcription in eukaryotic organisms is regulated by sequence-specific DNA-binding proteins as well as by non-DNA-binding proteins. In this report we describe the modulatory functions of a non-DNA-binding protein, SIN3 (also known as SDI1, UME4, RPD1, and GAM2) on the transactivation properties of the human progesterone receptor (hPR), GAL4, and the HAP1 activator in yeast. Our data suggest that SIN3 is a dual function protein. It negatively regulates the transcriptional activities of hPR-A and hPR-B by affecting the N-terminal activation domain (AF1). SIN3 positively regulates the transcriptional activities of GAL4 and the HAP1 activator. However, it has no effect on the transcriptional activities of the human glucocorticoid receptor (hGR) and GCN4. The SIN3 protein contains four copies of a paired amphipathic helix (PAH) motif. Deletion analysis of the SIN3 PAH motifs shows that the PAH3 motif is essential for SIN3-mediated regulation of hPR, GAL4, and the HAP1 activator. In contrast, the PAH1, PAH2, and PAH4 motifs are not required for SIN3-mediated regulation of these activators. Additionally, we examined the mechanism(s) by which the SIN3 protein modulate the activities of various activators. We are unable to demonstrate the direct interaction of SIN3 protein with these activators using the yeast two-hybrid system or co-immunoprecipitation. These data suggest that SIN3 regulates the transactivation functions of hPR, GAL4, and the HAP1 activator by an indirect mechanism.

Influence of a steroid receptor DNA-binding domain on transcriptional regulatory functions

We have isolated two independent mutations in the DNA-binding domain of the rat glucocorticoid receptor, P493R and S459A, that implicate DNA binding in the control of attached transcriptional activation domains, either that of the receptor itself or of VP16. The mutants are capable of activating transcription normally, but unlike wild-type receptors, they interfere with particular transcriptional activators in yeast and mammalian cells, and inhibit growth when overexpressed in yeast. The mutant residues reside at positions within the three-dimensional structure of the receptor that could, in principle, transduce structural changes from the DNA-binding surface of the receptor to other functional domains. These findings, together with the salt dependence of specific and nonspecific DNA binding by these receptors, suggest that specific DNA acts as an allosteric effector that directs the functional interaction of the receptor with targets of transcriptional activation and that the P493R and S459A mutants mimic the allosteric effect of specific DNA, allowing the receptor to interact with regulatory targets even in the absence of specific DNA binding.

Retinoid X receptor homodimers function as transcriptional activators in yeast

The possibility that different retinoids activate transcription from a specific retinoic acid (RA)-responsive element known as site A via different homo and heterodimeric versions of RA receptors cannot be evaluated in mammalian cells because they contain endogenous RA receptors (RAR). However, this limitation can be overcome by using yeast cells, which do not contain endogenous RAR, to study retinoid signaling pathways. Here, we describe heterologous expression of the human retinoid X receptor (RXR alpha) in yeast and hormone-dependent activation of a reporter construct containing site A upstream from a yeast promoter fused to the lacZ gene of Escherichia coli. Western blot analysis of yeast extracts containing RXR alpha revealed a distinct immunoreactive polypeptide co-migrating with the mammalian-produced RXR alpha. Electrophoretic mobility shift assays demonstrated that RXR alpha produced in yeast binds efficiently to site A in the absence of 9-cis-RA. However, transcription activation experiments showed that RXR alpha transactivates a yeast basal promoter linked to site A only in the presence of 9-cis-RA. We conclude that RXR alpha homodimers bind to site A in the absence of 9-cis-RA, but function as ligand-dependent transactivators in yeast cells. This retinoid-responsive transcription unit created in yeast cells provides a powerful genetic tool for the systemic unraveling of the synergistic interactions between RXR alpha and its heterodimeric partners.

A highly conserved region in the hormone-binding domain of the human estrogen receptor functions as an efficient transactivation domain in yeast

Human estrogen receptor (hER) mutants which activate transcription in the absence of hormone were isolated by random mutagenesis and genetic selection in the yeast Saccharomyces cerevisiae. Twenty constitutive hER mutants defining ten different alleles were selected. All sequence changes resulted in truncations of the receptor within a 123-amino-acid (aa) segment (aa 270 to 393) spanning the D region and the N-terminal part of region E which contains the hormone-binding domain (HBD). Transactivation assays using both the constitutive hER mutants and a series of deleted receptor derivatives generated in vitro revealed that the N-terminal part of region E, between aa 302 and 339, contains an efficient transcriptional activation function which is constitutively active in yeast. The location of this transactivation function in hER is similar to that of the tau 2 activation function of the glucocorticoid receptor and corresponds to a sequence which is highly conserved among the steroid hormone receptors. Thus, a conserved region exists in the HBD of the hER which can function as an autonomous transactivation domain.

Development of improved cell-based assays and screens in Saccharomyces through the combination of molecular and classical genetics

Traditionally, the discovery of pharmaceutical and agrochemical products has largely depended on mass screening. Over the years, screen design and screening programs have evolved in terms of the sensitivity with which active material can be identified, the number of samples that can be tested, and the types of molecular targets and cellular functions that can be conveniently assayed. More recently, screens with desirable properties have been developed for a great variety of molecular targets through the exploitation of Saccharomyces molecular biology and genetics. Recent advances have enabled researchers to develop yeast-based screens for agents acting on a number of new therapeutic targets: G-protein linked receptors, cytoplasmic receptors, ion (potassium) channels, novel fungal cell wall enzymes, fungal sterol biosynthesis enzymes, antiviral targets, immunosuppressive targets, cyclic nucleotide phosphodiesterase, oncogenes and the multiple drug resistance (MDR) protein.

Chicken progesterone receptor expressed in Saccharomyces cerevisiae is correctly phosphorylated at all four Ser-Pro phosphorylation sites

This study describes the phosphorylation of chicken progesterone receptor (cPR) produced in yeast, Saccharomyces cerevisiae, and examines the dependence of specific phosphorylations on hormone and DNA binding. The chicken progesterone receptor is expressed in vivo as two forms, cPRB and a smaller form, cPRA. Characterization of the phosphorylation sites in the cPRB form expressed in yeast shows that progesterone receptor is phosphorylated on the three serines (Ser211, Ser260, and Ser530) reported previously in chicken oviduct. An additional site which was phosphorylated in response to hormone was also detected and was subsequently identified as Ser367. Although cPRB and cPRA are phosphorylated identically in chicken oviduct, cPRA expressed in yeast is phosphorylated on Ser211, Ser260, and Ser367, but phosphorylation of Ser530 is almost undetectable. In contrast, cPRB expressed in yeast is phosphorylated on all four sites. No phosphorylations were found in or near the region required for hormone binding, indicating that phosphorylation is not required for hormone binding. In order to determine whether any of the phosphorylations were DNA-dependent, phosphorylation was also studied using cPRA containing a partial deletion of the DNA binding domain. Two of the sites, Ser211 and Ser367, showed reduced phosphorylation in this mutant, suggesting a possible requirement for DNA binding activity for the phosphorylation of these sites. To our knowledge, this is one of the first demonstrations that a eucaryotic protein expressed in yeast is correctly phosphorylated.

Fusion of GAL4-VP16 to a steroid-binding domain provides a tool for gratuitous induction of galactose-responsive genes in yeast

We demonstrate that the hormone-binding domain (HBD) of the human estrogen receptor (ER) can function as an autonomous regulatory domain in the budding yeast, Saccharomyces cerevisiae. As in mammalian cells, the HBD can subject the activity of a heterologous protein, which is fused to it, to hormonal control. Thus, a chimeric transcriptional activator consisting of (i) the DNA-binding domain of GAL4, (ii) the ER HBD, and (iii) the activation domain of viral protein 16 (VP16) stimulates both episomal and integrated reporter genes exclusively in the presence of steroid hormone. Steroids being gratuitous signals for yeast, this fusion protein is a convenient tool for highly regulated production of proteins of interest. Notably, it can be exploited to activate the commonly used galactose-inducible expression vectors without switching the carbon source.

Creation of an active estrogen-responsive element by a single base change in the flanking sequence of a cellular oncogene: a possible mechanism for hormonal carcinogenesis?

Estrogens are considered to act as promoters in a multistep process of hormonal carcinogenesis, although the molecular mechanisms by which these hormones act in tumorigenesis are unclear at present. Estradiol is known to induce expression of certain proto-oncogenes, and this led us to examine potential regulatory regions of the cellular c-fos oncogene. The 5'-flanking region of the murine c-fos contains a 13-bp palindromic sequence (GGTCTnnnAGACC) with striking homology to the consensus estrogen-responsive element (ERE) GGTCAnnnTGACC. However, the c-fos sequence did not bind the human estrogen receptor or confer hormonal responsiveness in a yeast-based transcriptional test system. Importantly, a single base change in the fifth position of the c-fos sequence (GGTCTnnnAGACC to GGTCA/GnnnAGACC) produced an element that bound the estrogen receptor and conferred estrogen-dependent transcriptional activation of a reporter gene. This suggests a specific hypothesis by which estrogens could act as tumor promoters. In this paradigm, the regulatory region of the cellular oncogenes, tumor suppressor genes, and growth-factor genes contain inactive sequences with close homologies to hormone-responsive elements. Initiation occurs when some agent (e.g., a chemical carcinogen) causes a mutation in such a sequence to create a functional hormone-responsive element. Estrogens, acting through their receptors and the mutated element, can then activate the target gene to stimulate cell proliferation and increase the population of initiated cells.

Progesterone receptor phosphorylation complexities in defining a functional role

All steroid receptors are phosphoproteins and several, including progesterone receptors (PRs), become hyperphosphorylated upon binding of ligand. PR phosphorylation is complex, occurring in different cellular compartments and perhaps requiring multiple serine kinases. A model that is emerging proposes that PR phosphorylation is progressive, occurring in at least a three-stage cascade. However, the functional significance of this phosphorylation cascade remains unclear.

Functional analysis of the human estrogen receptor using a phenotypic transactivation assay in yeast

We have constructed yeast strains in which the expression of the Saccharomyces cerevisiae URA3 gene is induced by the human estrogen receptor (hER). Promoter sequences required for both basal and activated transcription of URA3 were replaced with one or three estrogen-response elements (EREs) positioned upstream of the native TATA box. These constructs were each integrated at the TRP1 locus of a yeast strain in which the natural URA3 gene had been deleted, and the integrants were transformed with low- or high-copy-number shuttle plasmids expressing wild-type or truncated derivatives of hER. Transformants were assayed for growth on uracil-deficient medium plus or minus estradiol (E2), for resistance to 5-fluoroorotic acid (5-FOA) and for activity of OMPdecase (orotidine-5'-monophosphate decarboxylase), the product of the URA3 gene. We show that the growth and 5-FOA-resistance (5-FOAR) phenotypes of these strains are strictly dependent upon the function of the receptor derivatives. Induction of URA3, measured by OMPdecase activity, was observed over a 20- to 2500-fold range depending on the receptor derivative, its expression level and the number of EREs in the responsive promoter. Both one- and three-ERE reporter strains expressing the full-length receptor are completely E2-dependent for growth, and display a 5-FOAR phenotype in the absence of the hormone. We demonstrate that the individual hER transactivation functions, TAF1 and TAF2, are both functional in yeast, and that the hormone-dependent TAF2 is the more potent activator on our reporters. We show that hER displays strong homosynergism in yeast, and discuss the contributions of the two TAFs in hER synergism.(ABSTRACT TRUNCATED AT 250 WORDS)