In vitro activity of the transcription activation functions of the progesterone receptor. Evidence for intermediary factors

Isoform-specific degradation of PR-B by E6-AP is critical for normal mammary gland development

E6-associated protein (E6-AP), which was originally identified as an ubiquitin-protein ligase, also functions as a coactivator of estrogen (ER-α) and progesterone (PR) receptors. To investigate the in vivo role of E6-AP in mammary gland development, we generated transgenic mouse lines that either overexpress wild-type (WT) human E6-AP (E6-AP(WT)) or ubiquitin-protein ligase-defective E6-AP (E6-AP(C833S)) in the mammary gland. Here we show that overexpression of E6-AP(WT) results in impaired mammary gland development. In contrast, overexpression of E6-AP(C833S) or loss of E6-AP (E6-AP(KO)) increases lateral branching and alveolus-like protuberances in the mammary gland. We also show that the mammary phenotypes observed in the E6-AP transgenic and knockout mice are due, in large part, to the alteration of PR-B protein levels. We also observed alteration in ER-α protein level, which might contribute to the observed mammary phenotype by regulating PR expression. Furthermore, E6-AP regulates PR-B protein levels via the ubiquitin-proteasome pathway. Additionally, we also show that E6-AP impairs progesterone-induced Wnt-4 expression by decreasing the steady state level of PR-B in both mice and in human breast cancer cells. In conclusion, we present the novel observation that E6-AP controls mammary gland development by regulating PR-B protein turnover via the ubiquitin proteasome pathway. For the first time, we show that the E3-ligase activity rather than the coactivation function of E6-AP plays an important role in the mammary gland development, and the ubiquitin-dependent PR-B degradation is not required for its transactivation functions. This mechanism appears to regulate normal mammogenesis, and dysregulation of this process may be an important contributor to mammary cancer development and progression.

Biochemical analyses of nuclear receptor-dependent transcription with chromatin templates

Chromatin, the physiological template for transcription, plays important roles in gene regulation by nuclear receptors (NRs). It can (1) restrict the binding of NRs or the transcriptional machinery to their genomic targets, (2) serve as a target of regulatory posttranslational modifications by NR coregulator proteins with histone-directed enzymatic activities, and (3) function as a binding scaffold for a variety of transcription-related proteins. The advent of in vitro or "cell-free" systems that accurately recapitulate ligand-dependent transcription by NRs with chromatin templates has allowed detailed analyses of these processes. Biochemical studies have advanced our understanding of the mechanisms of gene regulation, including the role of ligands, coregulators, and nucleosome remodeling. In addition, they have provided new insights about the dynamics of NR-mediated transcription. This chapter reviews the current methodologies for assembling, transcribing, and analyzing chromatin in vitro, as well as the new information that has been gained from these studies.

Nuclear hormone receptor degradation and gene transcription: An update

The ubiquitin-proteasome pathway (UPP) is known to degrade short-lived and misfolded proteins. Its role in cell cycle regulation and signal transduction is well established. However, the importance of the UPP in nuclear hormone receptor-regulated gene transcription is relatively new. Nuclear hormone receptors (NHRs) are degraded by the UPP both in the presence or absence of their cognate ligands. In recent years, it has become evident that NHR degradation and NHR-dependent transcription are interdependent processes. The link between these two processes has become stronger with the discovery of a number of ubiquitin-pathway enzymes and components of the proteasome acting as modulators of NHR function. Also, UPP enzymes and components of the proteasome are recruited to the promoters of NHR-responsive genes. Interestingly both coactivators and corepressors (coregulators) of NHRs are also targeted to the UPP for degradation. Furthermore, additional evidence also indicates that the UPP may be involved in the turnover of transcription complexes, thereby facilitating proper gene transcription. In this review we discuss and provide an update on the role of UPP in NHR-dependent gene regulation.

The ubiquitin-conjugating enzyme UBCH7 acts as a coactivator for steroid hormone receptors

We investigated the role of the ubiquitin-conjugating enzyme UBCH7 in nuclear receptor transactivation. Using transient transfection assays, we demonstrated that UBCH7 modulates the transcriptional activity of progesterone receptor (PR) and glucocorticoid, androgen, and retinoic acid receptors in a hormone-dependent manner and that the ubiquitin conjugation activity of UBCH7 is required for its ability to potentiate transactivation by steroid hormone receptors (SHR). However, UBCH7 showed no significant effect on the transactivation functions of p53 and VP-16 activation domain. Depletion of endogenous UBCH7 protein by small interfering RNAs suggests that UBCH7 is required for the proper function of SHR. Furthermore, a chromatin immunoprecipitation assay demonstrated the hormone-dependent recruitment of UBCH7 onto estrogen receptor- and PR-responsive promoters. Additionally, we show that UBCH7 and E6-associated protein (E6-AP) synergistically enhance PR transactivation. We also demonstrate that UBCH7 interacts with steroid receptor coactivator 1 (SRC-1) and that UBCH7 coactivation function is dependent on SRC-1. Taken together, our results reveal the possible role of UBCH7 in steroid receptor transactivation and provide insights into the mechanism of action of UBCH7 in receptor function.

The MN1 oncoprotein synergizes with coactivators RAC3 and p300 in RAR-RXR-mediated transcription

The t(12;22) creates an MN1-TEL fusion gene leading to acute myeloid leukemia. The fusion partner TEL (ETV6) is a member of the ETS family of transcription factors. The nature of the other fusion partner, MN1, has not been investigated in detail until now. We recently described that MN1 activates the transcription activity of the moloney sarcoma virus long terminal repeat, indicating that this protein itself may act as a transcription factor. We show here that MN1 comprises multiple transcription activating domains. A search for a bound DNA sequence revealed that MN1 has affinity for retinoic acid responsive elements. A DR5 retinoic acid responsive element was observed in the LTR. The combination of MN1 and ligand-activated retinoic acid receptor leads to a synergistic induction of expression directed by the LTR. Cotransfection of MN1 with RAC3 or p300, known coactivators of retinoic acid receptors, leads to a further synergistic induction of transcription. In addition, the effect of MN1 can be inhibited by the wild-type adenovirus ElA protein that inhibits p300 function, but not by an E1A mutant lacking the p300-binding site. GAL4-MN1-mediated transcription can be enhanced directly by RAC3 and p300. Taken together, our results indicate that MN1 is a transcription coactivator rather than a sequence-specific transcription factor, and that it may stimulate RAR/RXR-mediated transcription through interaction with p160 and p300.

The inhibitory function in human progesterone receptor N termini binds SUMO-1 protein to regulate autoinhibition and transrepression

Although most studies of progesterone receptors (PR) and their two isoforms, PR-A and PR-B, focus on transcriptional stimulation, the receptors exhibit important inhibitory properties. Autoinhibition refers to an inhibitory function located in the PR N terminus, whose deletion increases transcriptional activity at least 6-10-fold. Transrepression refers to the ability of PR-A to suppress the transcriptional activity of PR-B and other nuclear receptors, including estrogen receptors. Self-squelching refers to the observation in transient transfection assays that increasing receptor concentrations paradoxically decrease transcriptional activity. Using a series of N-terminal deletion mutants constructed in both PR isoforms, we have mapped their autoinhibitory and transrepressor activities to a small ubiquitin-like modifier (SUMO-1) protein consensus-binding motif, (387)IKEE, located in the N terminus upstream of AF1. Self-squelching does not involve this site. SUMO-1 binds PR covalently at (387)IKEE, but only if the C-terminal, liganded, hormone-binding domain is also present. A single point K388R mutation within the (387)IKEE motif in either PR-A or PR-B leads to a loss of autoinhibitory and transrepressor functions of the liganded, full-length receptors. We conclude that autoinhibition and transrepression involve N-terminal sumoylation combined with intramolecular N/C-terminal communication.

Comparative properties of two peptide-antibody interactions as deduced from epitope delineation

The linear epitope recognized by three closely related antibodies specific for the E6 oncoprotein of papillomavirus type 16 was delineated by phage display, spot peptide synthesis on cellulose membranes, and kinetic measurements with antigenic variants using a BIACORE. The same approaches, recently applied to an antibody specific for tobacco mosaic virus protein, led to the clear-cut delineation of a functional epitope comprising four key positions with well defined physico-chemical properties. In contrast, the E6 system is characterized by a non-essential contribution to binding of various factors, so that combinations of alternative properties are compatible with measurable binding activity.

The C-terminal half of Hsp90 is responsible for its cytoplasmic localization

With some exceptions, research so far has shown heat shock protein (Hsp) 90 to be a cytoplasmic protein. Here, we studied the sequence determinants which dictate the subcellular localization of Hsp90. By constructing hybrid molecules between a nuclear protein, progesterone receptor (PR), and parts of Hsp90, we demonstrated that the C-terminal but not the N-terminal half of Hsp90 can prevent nuclear translocation of the PR. Studies with an antibody raised against a region which contains the major nuclear localization signal (NLS) of the PR suggest that the inhibition of nuclear localization is not due to steric hindrance of the NLS of the PR by Hsp90 sequences in hybrid molecules. In order to characterize further the cytoplasmic anchoring of Hsp90 we constructed four chimeric molecules between the C-terminal half of Hsp90 and estrogen receptor (ER) with different numbers of nuclear localization protosignals (proto-NLS). When the C-terminal half of Hsp90 was fused with ER containing no or one proto-NLS, the hybrid molecule was located exclusively in the cytoplasm. When the nuclear translocation signal was strengthened by adding two or three protosignals, the hybrid molecule was exclusively nuclear. These results suggest that the C-terminal half of Hsp90 contains a sequence which is responsible for the cytoplasmic localization of the protein. Further deletions of the molecule suggested that the cytoplasmic anchoring signal is located between amino acids 333 and 664.

Delineation of a linear epitope by multiple peptide synthesis and phage display

Two different approaches, the phage display technique and the Spot peptide synthesis on cellulose membranes, were used to identify sequences recognized by Fab 57P, specific for tobacco mosaic virus protein (TMVP), and define the preferred chemical composition of a functional epitope. Kinetic measurements of the interaction between peptide variants and the antibody fragment were used to further refine the molecular basis of binding activity. Our results show that the functional epitope of Fab 57P requires precise physico-chemical properties at a limited number of positions, and that residues flanking these key residues can influence binding affinity. The phage display and Spot synthesis methods allowed the straightforward localization of the binding region and the identification of residues that are essential for recognition. However, these methods yielded slightly different views of accessory factors that are able to influence antibody binding. The influence on binding activity of these factors can only be assessed through quantitative affinity measurements.

Ligand-dependent activation of transcription in vitro by retinoic acid receptor alpha/retinoid X receptor alpha heterodimers that mimics transactivation by retinoids in vivo

All-trans and 9-cis retinoic acids (RA) signals are transduced by retinoic acid receptor/retinoid X receptor (RAR/RXR) heterodimers that act as functional units controlling the transcription of RA-responsive genes. With the aim of elucidating the underlying molecular mechanisms, we have developed an in vitro transcription system using a chromatin template made up of a minimal promoter and a direct repeat with 5-spacing-based RA response element. RARalpha and RXRalpha were expressed in and purified from baculovirus-infected Sf9 cells, and transcription was carried out by using naked DNA or chromatin templates. Transcription from naked templates was not affected by the presence of RA and/or RAR/RXR heterodimers. In contrast, very little transcription occurred from chromatin templates in the absence of RA or RAR/RXR heterodimers whereas their addition resulted in a dosage-dependent stimulation of transcription that never exceeded that occurring on naked DNA templates. Most importantly, the addition of synthetic agonistic or antagonistic retinoids to the chromatin transcription system mimicked their stimulatory or inhibitory action in vivo, and activation by a RXR-specific retinoid was subordinated to the binding of an agonist ligand to the RAR partner. Moreover, the addition of the p300 coactivator generated a synergistic enhancement of transcription. Thus, the dissection of this transcription system ultimately should lead to the elucidation of the molecular mechanisms by which RAR/RXR heterodimers control transcription in a ligand-dependent manner.

The Angelman syndrome-associated protein, E6-AP, is a coactivator for the nuclear hormone receptor superfamily

In this study, we found that the E6-associated protein (E6-AP/UBE3A) directly interacts with and coactivates the transcriptional activity of the human progesterone receptor (PR) in a hormone-dependent manner. E6-AP also coactivates the hormone-dependent transcriptional activities of the other members of the nuclear hormone receptor superfamily. Previously, it was shown that E6-AP serves the role of a ubiquitin-protein ligase (E3) in the presence of the E6 protein from human papillomavirus types 16 and 18. Our data show that the ubiquitin-protein ligase function of E6-AP is dispensable for its ability to coactivate nuclear hormone receptors, showing that E6-AP possesses two separable independent functions, as both a coactivator and a ubiquitin-protein ligase. Disruption of the maternal copy of E6-AP is correlated with Angelman syndrome (AS), a genetic neurological disorder characterized by severe mental retardation, seizures, speech impairment, and other symptoms. However, the exact mechanism by which the defective E6-AP gene causes AS remains unknown. To correlate the E6-AP coactivator function and ubiquitin-protein ligase functions with the AS phenotype, we expressed mutant forms of E6-AP isolated from AS patients and assessed the ability of each of these mutant proteins to coactivate PR or provide ubiquitin-protein ligase activity. This analysis revealed that in the majority of the AS patients examined, the ubiquitin-protein ligase function of E6-AP was defective whereas the coactivator function was intact. This finding suggests that the AS phenotype results from a defect in the ubiquitin-proteosome protein degradation pathway.

Reconstitution of the N-terminal transcription activation function of human mineralocorticoid receptor in a defective human glucocorticoid receptor

N-terminal sequences involved in transcription activation by the human mineralocorticoid receptor (hMR) have yet to be defined. We have addressed this issue and generated overlapping internal deletion mutants hMRDelta59-162, hMRDelta59-247, hMRDelta59-328, hMRDelta162-247, hMRDelta247-328, hMRDelta247-382, and hMRDelta328-382 with intact DNA-binding and hormone-binding domains. A second set of mutant receptors with unique BglII sites was generated to facilitate the isolations of fragments. Immunodetection with anti-hMR peptide antibodies and hormone-binding assays showed that the mutations did not affect the expression of the receptors or ability to bind aldosterone. Distribution of aldosterone binding activity of wild type and deletion mutants expressed in HeLa cells was predominantly nuclear. Furthermore, deletion of sequences between 59 and 390 did not affect DNA binding activity. Transfection studies with HeLa cells revealed a region around residue 247 that was crucial for normal receptor function. Deletion of amino acids 59-162 did not affect the transcriptional activity of the hMR. However, deletion of sequences 247-382 and 328-382 markedly decreased the transcription activation function. The induction of the reporter gene by the chimera hGRDelta71-262/hMR328-382 was 2-fold higher than with the wild type hGR, but 200-fold when compared with hGRDelta71-262, indicating that the AF-1 domain is located between positions 328 and 382 in the hMR.

The steroid receptor coactivator-1 contains multiple receptor interacting and activation domains that cooperatively enhance the activation function 1 (AF1) and AF2 domains of steroid receptors

Steroid receptors are ligand-inducible transcription factors, and their association with steroid receptor coactivators (SRCs) upon binding to DNA is necessary for them to achieve full transcriptional potential. To understand the mechanism of SRC-1 action, its ability to interact and enhance the transcriptional activity of steroid receptors was analyzed. First, we show that SRC-1 is a modular coactivator that possesses intrinsic transcriptional activity when tethered to DNA and that it harbors two distinct activation domains, AD1 and AD2, needed for the maximum coactivation function of steroid receptors. We also demonstrate that SRC-1 interacts with both the amino-terminal A/B or AF1-containing domain and the carboxyl-terminal D/E or AF2-containing domain of the steroid receptors. These interactions are carried out by multiple regions of SRC-1, and they are relevant for transactivation. In addition to the inherent histone acetyltransferase activity of SRC-1, the presence of multiple receptor-coactivator interaction sites in SRC-1 and its ability to interact with components of the basic transcriptional machinery appears to be, at least in part, the mechanism by which the individual activation functions of the steroid receptors act cooperatively to achieve full transcriptional activity.

Induction of cell-free, in vitro transcription by recombinant androgen receptor peptides

An in vitro, cell-free transcription system, based on prostate-derived transcriptional machinery and very powerful androgen response elements (AREs), has been developed. Multiple (p(ARR3)LovTATA) AREs from the androgen-regulated probasin gene were linked to G-free cassettes and used in nuclear extracts prepared from prostate carcinoma cell lines (PC3 and LNCaP cells) to test specific induction of transcription by full-length AR and by glutathione-S-transferase (GST)-fusion peptides in which the androgen receptor (AR) DNA-binding domain alone (AR524-649), or together with the ligand-binding domain (AR524-902), or a portion of the NH2-terminal domain (AR232-649) were incorporated. In the presence of AR, nuclear extracts from PC3 cells had greater activity in supporting transcription than those from LNCaP cells; and lower background activity than those from HeLa cells. All of the AR forms correctly initiated in vitro transcription of ARE-templates in an androgen-independent manner. The amount of specific, inducible transcript was dependent on the concentration of AR peptide present. AR524-902 was the most potent transactivator tested, with the maximal level of specific transcript over 900-fold higher than the minimal level. At all concentrations this peptide was three to four times more active than either AR524-649 or AR232-649. In conclusion, we have developed a very specific and sensitive cell-free transcription system for delineating trans-activational regions of the AR.

Distinct effects of ATP on transcription complex formation and initiation in a yeast in vitro transcription system

The stages and kinetics of transcription complex formation in a Saccharomyces cerevisiae in vitro transcription system were analysed using the anionic detergent sarkosyl. In contrast to findings from other systems, we were not able to distinguish between a fully formed pre-initiation complex and a 'rapid start' complex to which nucleotides were added. A further increase in resistance of transcription to sarkosyl was only observed 12 min after transcription initiation, by which time elongation was underway. Low concentrations of ATP, dATP or, surprisingly, the non-hydrolysable analogue ATPgammaS selectively stimulated transcription when present during assembly of transcription complexes, although the level of stimulation dropped when ATP was added progressively later. The effect of ATP did not correlate with the kinetics of template commitment, signifying that it functioned at a later stage than this, but prior to the full assembly of sarkosyl-resistant pre-initiation complexes. ATP also altered the sarkosyl resistance of initiating transcription complexes possibly by affecting a rate-limiting step leading to earlier appearance of elongated transcripts. This effect was antagonised by ATPgammaS, thus providing evidence that the stimulatory effect of ATP on pre-initiation complex formation and its effect on the lag between initiation and elongation phases are distinct.

Interaction of steroid hormone receptors with transcription factors involves chromatin remodelling

The mechanism by which steroid hormones modulate promoter utilization is not clear. Evidence from transfection studies and cell-free assays points to an interaction of the hormone receptors with general transcription factors, as well as with sequence-specific transcription factors. Moreover co-activators or transcription intermediary factors, have been identified which could mediate some of the transcriptional effects of the hormone-receptor complex. However, in addition to this interaction of receptors with proteins directly involved in transcription, a participation of chromatin structure in gene regulation by steroid hormones is becoming increasingly evident. In the case of the MMTV promoter, the nucleosomal organization seems to be responsible for transcriptional repression prior to hormonal stimulation. This effect is due to occlusion by a nucleosome positioned on the MMTV promoter sequences in such a way that essential transcription factors cannot access their recognition sites. Following hormone induction, a remodelling of the nucleosome structure takes place which enables a whole complement of sequence specific transcription factors to assemble on the promoter. Since a complete occupancy of binding sites does not take place when the promoter is present as naked DNA, the nucleosomal organization appears to be required for the proper synergism between transcription factors following hormonal induction. According to this model, the positioning of a nucleosome sets the stage for constitutive repression and hormone induction of the MMTV promoter.

Retinoid-dependent in vitro transcription mediated by the RXR/RAR heterodimer

The effects of retinoids on gene regulation are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Here, we provide the first biochemical evidence that, in vitro, ligand governs the transcriptional activity of RXR alpha/RAR alpha by inducing conformational changes in the ligand-binding domains. Using limited proteolytic digestion we show that binding of the cognate ligand causes a conformational change in the carboxy-terminal part of the receptor. We also show that recombinant RXR alpha/RAR alpha is partially active in the absence of exogenously added ligand. Trans-activation depends critically on the ligand-dependent transcriptional activation function AF-2 of RAR alpha. Full activation by recombinant RXR alpha/RAR alpha, however, requires the addition of either all-trans RA, 9-cis RA, or other RAR-specific agonists, whereas an RAR alpha-specific antagonist abolishes trans-activation. Intriguingly, the ligand-dependent AF-2 of RXR does not contribute to the level of transcription from the RAR beta 2 promoter in vitro even when the cognate ligand (9-cis RA) is bound. Thus, the major role of RXR in trans-activation of the RAR beta 2 promoter is to serve as an auxiliary factor required for the binding of RAR which, in turn, is directly responsible for transcriptional activity.

Bacterially expressed Fabs of monoclonal antibodies neutralizing tumour necrosis factor alpha in vitro retain full binding and biological activity

Antibody fragments specific for the human tumour necrosis factor alpha (TNF alpha) have been cloned from lambda combinatorial expression libraries using total RNA obtained from three different hybridoma cell lines of therapeutic interest. The previously described bacteriophage lambda vectors, lambda HC2 and lambda LC1, were modified to create unique antibody cloning sites in the combinatorial construct and a novel tag peptide was inserted at the C-terminal end of the expressed Fd chain. Sequence analysis of the cloned Fabs indicated that two of them were derived from a single B cell. Expression in E. coli showed that the amount of recovered Fab in the bacterial culture medium was related to the sequences of the variable coding regions. Hybrid Fabs created by chain exchange of similar antibodies were as active as the originally paired Fabs in binding assays. The relative affinities and the capacities of the bacterially expressed Fabs to neutralize TNF alpha cytotoxicity in vitro were identical to those of the parental antibodies. The results demonstrate that, using an in vitro approach, it is possible to generate from existing hybridoma cell lines high affinity Fabs which retain antigen specificity. The cloning sites incorporated into the C-terminal parts of these Fabs will now permit their further modification to include additional functional characteristics not possible with the original hybridoma antibodies.

Transcription activation by nuclear receptors

Nuclear receptors constitute a superfamily of ligand-inducible transcription factors which respond to endocrine, paracrine and, possibly, autocrine signals. Multiple regulatory mechanisms assure that signal transduction results in an accurate regulation of the respective gene networks. Apart from selective expression of the cognate receptor and its binding to specific hormone response elements of target genes, additional mechanisms are responsible for the cell- and promoter-specific transcription activation. They are based on the "interpretation" of the signal by the multiple functional modules of a given receptor and involve a specific interplay with various factors binding to complex target gene promoters and cell-specific intermediary transcription factors that mediate the activity of the two receptor transcription activation functions, as well as homo- and heterodimerization, and interference with other signalling pathways. Moreover, a single ligand may initiate different gene programs due to the differential target gene specificities of nuclear receptor isoforms. Thus, signal transduction by nuclear receptors involves a multitude of interactive elements, as could have been expected from the central role of these signals in homeostasis, embryonic development and differentiation. Two distinct mechanisms are involved in anti-hormone action. Type I anti-hormones impair the activity of the transcription activation function, while type II antagonists impair DNA binding. Experiments aimed at an understanding of these mechanisms are discussed.

Functional cross-talk between receptors for peptide and steroid hormones

Communication between a cell surface peptide hormone receptor and an intracellular steroid hormone receptor can take various routes, as dictated by the physiology of a particular cell type. There is increasing evidence for a novel route which requires that a peptide hormone receptor pathway converge on a steroid hormone receptor, leading to its activation. One consequence of such a process can be signal amplification for the peptide hormone receptor agonist. This is exemplified by the self-potentiating action of GnRH, which is a critical component in events leading to a surge in LH secretion and ovulation. One signaling pathway stimulated by the GnRH receptor may entail a phosphorylation cascade resulting in progesterone-independent modulation of progesterone receptor activity.