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Biomedical subjects

M L Privalsky

Publications and source records attributed to M L Privalsky.

At least 37 records · Page 2Linked to original sources
PubMed1997

SMRT corepressor interacts with PLZF and with the PML-retinoic acid receptor alpha (RARalpha) and PLZF-RARalpha oncoproteins associated with acute promyelocytic leukemia.

Retinoic acid receptors (RARs) are hormone-regulated transcription factors that control key aspects of normal differentiation. Aberrant RAR activity may be a causal factor in neoplasia. Human acute promyelocytic leukemia, for example, is tightly linked to chromosomal translocations that fuse novel amino acid sequences (denoted PML, PLZF, and NPM) to the DNA-binding and hormone-binding domains of RARalpha. The resulting chimeric receptors have unique transcriptional properties that may contribute to leukemogenesis. Normal RARs repress gene transcription by associating with ancillary factors denoted corepressors (also referred to as SMRT, N-CoR, TRAC, or RIP13). We report here that the PML-RARalpha and PLZF-RARalpha oncoproteins retain the ability of RARalpha to associate with corepressors, and that this corepressor association correlates with certain aspects of the leukemic phenotype. Unexpectedly, the PLZF moiety itself can interact with SMRT corepressor. This interaction with corepressor is mediated, in part, by a POZ motif within PLZF. Given the presence of POZ motifs in a number of known transcriptional repressors, similar interactions with SMRT may play a role in transcriptional silencing by a variety of both receptor and nonreceptor transcription factors.

DNA-Binding Proteins↗
PubMed1997

Nuclear receptor coactivator ACTR is a novel histone acetyltransferase and forms a multimeric activation complex with P/CAF and CBP/p300.

We report here the identification of a novel cofactor, ACTR, that directly binds nuclear receptors and stimulates their transcriptional activities in a hormone-dependent fashion. ACTR also recruits two other nuclear factors, CBP and P/CAF, and thus plays a central role in creating a multisubunit coactivator complex. In addition, and unexpectedly, we show that purified ACTR is a potent histone acetyltransferase and appears to define a distinct evolutionary branch to this recently described family. Thus, hormonal activation by nuclear receptors involves the mutual recruitment of at least three classes of histone acetyltransferases that may act cooperatively as an enzymatic unit to reverse the effects of histone deacetylase shown to be part of the nuclear receptor corepressor complex.

Acetyltransferases↗
PubMed1997

Retinoid X and retinoic acid receptors interact with transcription factor II-B by distinct mechanisms.

Nuclear hormone receptors are believed to modulate target gene expression by interacting with the general transcriptional machinery of the cell. We demonstrate here that two otherwise closely related members of the nuclear hormone receptor family, retinoid acid receptors (RARs) and retinoid X receptors (RXRs), exhibit significant differences in their interactions with the transcriptional machinery. RARs display a strong constitutive interaction with transcription factor II-B (TFIIB) that requires the TFIIB C-terminus, whereas RXR exhibits a weaker, hormone-stimulated interaction with the TFIIB that maps outside of the TFIIB C-terminus. Use of a dominant-negative mutant of TFIIB suggests that the TFIIB interaction is essential for full transcriptional activation by RXR.

Animals↗
PubMed1997

Plasticity of tetramer formation by retinoid X receptors. An alternative paradigm for DNA recognition.

Retinoid X receptors (RXRs) are transcription factors that traditionally have been thought to bind DNA as protein dimers. Recently, however, it has been recognized that RXRs can also bind to DNA as protein tetramers. Receptor tetramers form cooperatively on response elements containing suitably reiterated half-sites, and play an important role in determining the specificity of DNA recognition by different nuclear receptors. We report here that RXR tetramers exhibit significant functional plasticity, and form on response elements possessing diverse half-site orientations and spacings. This ability of RXRs to form tetramers and related oligomers appears to contribute to the synergistic transcriptional activation observed when multiple, spatially separated response elements are introduced into a single promoter. Oligomerization may therefore be a common paradigm for DNA recognition and combinatorial regulation by several different classes of transcription factors.

Animals↗
PubMed1997

A conformational switch in nuclear hormone receptors is involved in coupling hormone binding to corepressor release.

Nuclear hormone receptors are ligand-regulated transcription factors that modulate gene expression in response to small, hydrophobic hormones, such as retinoic acid and thyroid hormone. The thyroid hormone and retinoic acid receptors typically repress transcription in the absence of hormone and activate it in the presence of hormone. Transcriptional repression is mediated, in part, through the ability of these receptors to physically associate with ancillary polypeptides called corepressors. We wished to understand the mechanism by which corepressors are recruited to unliganded nuclear hormone receptors and are released on the binding of hormone. We report here that an alpha-helical domain located at the thyroid hormone receptor C terminus appears to undergo a hormone-induced conformational change required for release of corepressor and that amino acid substitutions that abrogate this conformational change can impair or prevent corepressor release. In contrast, retinoid X receptors appear neither to undergo an equivalent conformational alteration in their C termini nor to release corepressor in response to cognate hormone, consistent with the distinct transcriptional regulatory properties displayed by this class of receptors.

Amino Acid Sequence↗
PubMed1997

Thyroid hormone resistance syndrome manifests as an aberrant interaction between mutant T3 receptors and transcriptional corepressors.

Nuclear hormone receptors are hormone-regulated transcription factors that play critical roles in chordate development and homeostasis. Aberrant nuclear hormone receptors have been implicated as causal agents in a number of endocrine and neoplastic diseases. The syndrome of Resistance to Thyroid Hormone (RTH) is a human genetic disease characterized by an impaired physiological response to thyroid hormone. RTH is associated with diverse mutations in the thyroid hormone receptor beta-gene. The resulting mutant receptors function as dominant negatives, interfering with the actions of normal thyroid hormone receptors coexpressed in the same cells. We report here that RTH receptors interact aberrantly with a newly recognized family of transcriptional corepressors variously denoted as nuclear receptor corepressor (N-CoR), retinoid X receptor interacting protein-13 (RIP-13), silencing mediator for retinoid and thyroid hormone receptors (SMRT), and thyroid hormone receptor-associating cofactor (TRAC). All RTH receptors tested exhibit an impaired ability to dissociate from corepressors in the presence of thyroid hormone. Two of the RTH mutations uncouple corepressor dissociation from hormone binding; two additional RTH mutants exhibit an unusually strong interaction with corepressor under all hormone conditions tested. Finally, artificial mutants that abolish corepressor binding abrogate the dominant negative activity of RTH mutants. We suggest that an altered corepressor interaction is likely to play a critical role in the dominant negative potency of RTH mutants and may contribute to the variable phenotype in this disorder.

Cell Line↗
PubMed1996

DNA recognition by normal and oncogenic thyroid hormone receptors. Unexpected diversity in half-site specificity controlled by non-zinc-finger determinants.

The nuclear hormone receptors regulate target gene expression in response to hormones of extracellular origin. The DNA binding specificity of these receptors therefore plays the critical role of defining the precise repertoire of target genes that respond to a given hormone. We report here an analysis of the DNA binding specificity of the thyroid hormone receptor (c-ErbA protein) and that of an oncogenic derivative, the v-ErbA protein. These otherwise closely similar proteins exhibit quite divergent DNA sequence specificities at multiple positions within the DNA binding site. The thyroid hormone receptor (c-ErbA protein exhibits a particularly broad DNA specificity, whereas the v-ErbA protein is comparatively quite specific. Intriguingly, these differences in DNA recognition largely map to an N-terminal receptor domain not traditionally implicated in DNA binding, and are further influenced by heterodimer formation with retinoid X receptors. We propose that the N terminus of nuclear hormone receptors plays an critical role in DNA recognition by altering the conformation of the receptor domains that make the actual base-specific contacts.

Amino Acid Sequence↗
PubMed1996

Identification of TRACs (T3 receptor-associating cofactors), a family of cofactors that associate with, and modulate the activity of, nuclear hormone receptors.

Nuclear hormone receptors are the largest known family of eukaryotic transcription factors and serve as critical effectors of vertebrate homeostasis, growth, and differentiation. The precise transcriptional response mediated by a given nuclear hormone receptor is dictated by hormone, promoter, and cellular context, and many nuclear hormone receptors function bimodally as both transcriptional activators and repressors. We report here the identification of a family of proteins, denoted TRACs (T3 receptor-associating cofactors), which physically interact with nuclear hormone receptors and can modulate the transcriptional properties of these receptors. TRACs associate with retinoic acid, retinoid X, and thyroid hormone receptors, as well as the PML-RAR alpha and v-Erb A oncoproteins. Certain TRAC forms attenuate target gene expression and may serve as corepressors, whereas other TRAC family members appear to counteract these effects. We suggest that TRACs and related cofactors may participate in dictating the pleiotropic transcriptional capacities of the nuclear hormone receptors.

Alternative Splicing↗
PubMed1995

Cooperative formation of high-order oligomers by retinoid X receptors: an unexpected mode of DNA recognition.

Nuclear hormone receptors are ligand-regulated transcription factors that function in metazoan homeostasis and differentiation. We report here that DNA recognition by at least one class of nuclear receptors, the retinoid X receptors (RXRs), can occur through an unanticipated mechanism involving the cooperative formation of protein tetramers and other high-order oligomers. Formation of these oligomeric complexes enables RXRs alpha and gamma, but not beta, to efficiently regulate transcription through response elements poorly recognized by RXR dimers. Thus, oligomer formation plays an important role in determining the specificity of DNA recognition by nuclear receptors and contributes to isoform-dependent differences in gene regulation.

Animals↗
PubMed1995

Role of the N terminus in DNA recognition by the v-erb A protein, an oncogenic derivative of a thyroid hormone receptor.

The v-erb A oncogene is a mutated derivative of a normal cellular locus (c-erb A alpha) encoding a thyroid hormone receptor. Although both the v-erb A protein and thyroid hormone receptor bind to DNA, the DNA sequence specificity of the viral oncoprotein is altered from that of the normal cellular receptor. Intriguingly, amino acid differences in both the zinc-finger domain and in a less-characterized N-terminal region of the v- and c-erb A polypeptides are jointly responsible for these differences in DNA specificity. We demonstrate here that this newly recognized N-terminal determinant of DNA specificity appears to function by restricting the DNA sequence repertoire manifested by the zinc-finger domain itself. The unique presence of a tyrosine in the N terminus of the thyroid hormone receptor acts to abrogate this restriction, thereby permitting this receptor to utilize response elements containing nonconsensus half-sites. The ability of the N terminus to modulate the DNA recognition properties of nuclear hormone receptors extends to the retinoic acid receptor and contributes to the distinct DNA specificities displayed by the retinoic acid and thyroid hormone receptors.

Amino Acid Sequence↗
PubMed1994

Reconstitution of thyroid hormone receptor and retinoic acid receptor function in the fission yeast Schizosaccharomyces pombe.

We report here a characterization of the thyroid hormone receptors (T3Rs), retinoic acid receptors (RARs), and retinoid X receptors (RXRs) by reconstituting their actions in the fission yeast Schizosaccharomyces pombe. S. pombe provide a well defined and readily manipulated genetic background devoid of known endogenous nuclear hormone receptors. All the receptors tested, when introduced exogenously into S. pombe, induced high levels of reporter gene activation in response to physiological concentrations of hormone ligand. In these properties, the S. pombe system exhibits significant advantages over the previously employed Saccharomyces cerevisiae system. Use of the S. pombe system permitted the elucidation of previously undescribed differences in the DNA sequence recognition properties of different isoforms of the RXR and RARs, and the identification of apparently novel forms of response element for RXRs and RARs. Intriguingly, the v-erb A allele of T3R, a transcriptional repressor in vertebrate cells, acts as a transcriptional activator both in S. cerevisiae and in the evolutionarily highly divergent S. pombe, underscoring the importance of cellular factors in the regulation of receptor transcriptional activity.

Base Sequence↗
PubMed1994

DNA sequence specificity of the v-erb A oncoprotein/thyroid hormone receptor: role of the P-box and its interaction with more N-terminal determinants of DNA recognition.

The viral erb A oncogene is a mutated allele of a normal cell gene for a thyroid hormone receptor. The DNA recognition properties of the v-erb A protein are altered from those of the thyroid hormone receptor, due in part to a point mutation in the P-box of the zinc-finger domain of the viral allele. We report here the effects of systematically varying this P-box codon; our results suggest that this P-box amino acid contributes to DNA specificity not by promoting recognition of the appropriate response elements, but rather by excluding binding of the erb A protein to inappropriate half-sites. In this manner, DNA recognition by the v-erb A protein appears to differ from that by the glucocorticoid receptor. A variety of P-box amino acids were compatible with recognition of the prototypic AGGTCA half-site; intriguingly, several of these mutant erb A proteins could also recognize a variety of alternative half-site sequences. Recognition of these alternative half-sites required a compatible amino acid sequence in the N terminus of the erb A protein. Our results begin to define a code by which the identity of the amino acids in the zinc-finger and N-terminal domains is reflected in the DNA recognition properties of the receptor.

Amino Acid Sequence↗
PubMed1993

Reconstitution of retinoid X receptor function and combinatorial regulation of other nuclear hormone receptors in the yeast Saccharomyces cerevisiae.

The nuclear hormone receptor family of transcription factors regulates gene expression via a complex combinatorial network of interactions. Of particular interest is the ability of retinoid X receptors (RXRs) to form heterodimers with retinoic acid receptors (RARs) and thyroid hormone receptors (TRs), thereby modifying their activities. We report here that RXR, RAR, and TR function can be reconstituted in the yeast Saccharomyces cerevisiae and demonstrate that the combinatorial regulation seen in vertebrate cells can be reproduced in the yeast background. Using this system, we have shown that RARs respond to a wide variety of retinoid ligands but that RXRs are specific for the 9-cis isomer of retinoic acid. RXR enhanced the activity of RARs and TRs on a variety of hormone response elements without demonstrably altering their DNA specificity. Interestingly, the ability of RXR to potentiate gene activation by RARs and by TRs varied for different receptor isoforms.

Base Sequence↗
PubMed1993

The erbA oncogene represses the actions of both retinoid X and retinoid A receptors but does so by distinct mechanisms.

Genetic lesions that function as dominant negative mutations in microbial systems have long been recognized. It is only relatively recently, however, that similar dominant negative mutations have been implicated as a basis for genetic and neoplastic disorders in vertebrates. We describe here a dissection of the actions of the erbA oncogene protein, an aberrant form of thyroid hormone receptor that acts as a dominant negative inhibitor of other nuclear hormone receptors. We demonstrate that the ErbA oncoprotein interferes with thyroid hormone and trans-retinoic acid receptors by competing for binding to the corresponding response elements. Heterodimerization of the ErbA oncoprotein with these receptors does not play an observable role in repression. In contrast, however, the ErbA oncoprotein does efficiently form a heterodimer with the retinoid X receptor (RXR) class of nuclear hormone receptors; complex formation enhances the DNA-binding properties of the ErbA protein but dramatically interferes with the ability of the RXR component to activate gene expression. Our results indicate that the erbA oncogene may play a previously unanticipated role in neoplasia by interfering with RXR function.

Animals↗
PubMed1993

Nuclear hormone receptors involved in neoplasia: erb A exhibits a novel DNA sequence specificity determined by amino acids outside of the zinc-finger domain.

The erb A oncogene is a dominant negative allele of a thyroid hormone receptor gene and acts in the cancer cell by encoding a transcriptional repressor. We demonstrate here that the DNA sequence recognition properties of the oncogenic form of the erb A protein are significantly altered from those of the normal thyroid hormone receptors and more closely resemble those of the retinoic acid receptors; this alteration appears to play an important role in defining the targets of erb A action in neoplasia. Unexpectedly, the novel DNA recognition properties of erb A are encoded by an N-terminal region not previously implicated as playing this function in current models of receptor-DNA interaction. Two N-terminal erb A amino acids in particular, histidine 12 and cysteine 32, contribute to this phenomenon, acting in conjunction with amino acids in the zinc finger domain. The effects of the N-terminal domain can be observed at the level of both DNA binding and transcriptional modulation. Our results indicate that unanticipated determinants within the nuclear hormone receptors participate in DNA sequence recognition and may contribute to the differential target gene specificity displayed by different receptor forms.

Amino Acid Sequence↗
PubMed1993

v-erbA and citral reduce the teratogenic effects of all-trans retinoic acid and retinol, respectively, in Xenopus embryogenesis.

Treatment of late blastula/early gastrula stage Xenopus embryos with all-trans retinoic acid results in disruption of the primary body axis through effects on both mesoderm and neuroectoderm. This effect of retinoic acid, coupled with the known presence of retinoic acid in Xenopus embryos has led to the proposal that retinoic acid may be an endogenous morphogen providing positional information in early development. To further elucidate the role of retinoic acid in early Xenopus development, we have attempted to interfere with the retinoic acid signalling pathway both at the level of retinoic acid formation, by treatment with citral (3,7-dimethy-2,6-octadienal), and at the level of nuclear retinoic acid receptor function, by microinjection of v-erbA mRNA. The feasibility of this approach was demonstrated by the ability of citral treatment and v-erbA mRNA injection to reduce the teratogenic effects of exogenous retinol and retinoic acid, respectively, in early Xenopus development. Interestingly, v-erbA mRNA injection and citral treatment of gastrula stage embryos resulted in tadpoles with a similar set of developmental defects. The defects were chiefly found in tissues that received a contribution of cells from the neural crest, suggesting that at least a subset of neural crest cells may be sensitive to the endogenous level of retinoic acid. In accord with this proposal, it was found that the expression patterns of two early markers of cranial neural crest cells, Xtwi and XAP-2, were altered in embryos injected with v-erbA mRNA. These results indicate that structures in addition to the primary axis are regulated by retinoic acid signalling during early Xenopus development.

Abnormalities, Drug-Induced↗
PubMed1993

Functional domains of the v-erbA protein necessary for oncogenesis are required for transcriptional activation in Saccharomyces cerevisiae.

The v-erbA oncogene is a retrovirus-transduced and altered copy of a cellular gene (c-erbA-alpha) for a thyroid hormone receptor. In this paper we show that the v-erbA domains required for transcriptional activation in yeast and for oncogenic function in animal cells are closely congruent. We conclude that the behavior of the v-erbA protein as a transcriptional activator in yeast appears to closely reflect the same biochemical requirements that are necessary for v-erbA function in the neoplastic vertebrate cell. Intriguingly, parallel analyses of c-erbA-alpha and -beta demonstrated unexpected differences in the activities of the two thyroid hormone receptor isoforms in the yeast, perhaps reflecting different functions of these genes in vertebrates. Furthermore, results obtained by analysis of chimeric v-/c-erbA genes suggest that the basal and the hormone-induced transcriptional activity of the nuclear hormone receptors can be modulated independently by distinct structural features within the protein molecule.

Cell Transformation, Neoplastic↗