Cloning and characterization of a corepressor and potential component of the nuclear hormone receptor repression complex

Nuclear hormone receptors are potent repressors of transcription in the unliganded state. We describe here the cloning of a nuclear receptor corepressor that we call SUN-CoR (Small Unique Nuclear receptor CoRepressor), which shows no homology to previously described nuclear hormone receptor corepressors, N-CoR, or SMRT. SUN-CoR is a highly basic, 16-kDa nuclear protein that is expressed at high levels in adult tissues and is induced during adipocyte and myogenic differentiation. SUN-CoR potentiates transcriptional repression by thyroid hormone receptor and RevErb in vivo, represses transcription when fused to a heterologous DNA binding domain, and interacts with RevErb as well as with thyroid hormone receptor in vitro. SUN-CoR also interacts with N-CoR and SMRT in vitro and with endogenous N-CoR in cells. We conclude that SUN-CoR is a corepressor and may function as an additional component of the complex involved in transcriptional repression by unliganded and orphan nuclear hormone receptors.

Activated lymphocytes increase expression of 5-lipoxygenase and its activating protein in THP-1 cells

The aim of this study was to investigate the regulation of the 5-lipoxygenase pathway of arachidonic acid metabolism by lymphocytes using the monocyte-like cell line, THP-1. When THP-1 cells were incubated over 4-7 days in 10% supernatant from lectin-activated human lymphocytes, their capacity to synthesize 5-lipoxygenase products was significantly increased. In contrast, the supernatant from nonactivated lymphocytes had no effect. The increase in capacity to synthesize 5-lipoxygenase products was mimicked by the addition of either granulocyte macrophage colony-stimulating factor (GM-CSF) or interleukin-3. These increases in synthetic capacity reflected increased enzymatic activity. Increased immunoreactive protein and mRNA for the enzymes 5-lipoxygenase and 5-lipoxygenase-activating protein were also found in cells conditioned with activated lymphocyte supernatants. Furthermore, the increase in mRNA for both enzymes was not blocked by cycloheximide, suggesting that the effect on steady-state mRNA levels does not require the synthesis of new protein. The increase in mRNA could be reproduced by GM-CSF. We conclude that lymphocytes can regulate the expression of 5-lipoxygenase in THP-1 cells over a period of days via the release of soluble factors.

The transcriptional co-activator p/CIP binds CBP and mediates nuclear-receptor function

The functionally conserved proteins CBP and p300 act in conjunction with other factors to activate transcription of DNA. A new factor, p/CIP, has been discovered that is present in the cell as a complex with CBP and is required for transcriptional activity of nuclear receptors and other CBP/p300-dependent transcription factors. The highly related nuclear-receptor co-activator protein NCoA-1 is also specifically required for ligand-dependent activation of genes by nuclear receptors. p/CIP, NCoA-1 and CBP all contain related leucine-rich charged helical interaction motifs that are required for receptor-specific mechanisms of gene activation, and allow the selective inhibition of distinct signal-transduction pathways.

Differential effects of nuclear receptor corepressor (N-CoR) expression levels on retinoic acid receptor-mediated repression support the existence of dynamically regulated corepressor complexes

Thyroid hormone and retinoic acid receptors are members of the nuclear receptor superfamily of ligand-dependent transcription factors that stimulate the transcription of target genes in the presence of activating ligands and repress transcription in their absence. Transcriptional repression by the thyroid hormone and retinoic acid receptors has been proposed to be mediated by the nuclear receptor corepressor, N-CoR, or the related factor, SMRT (silencing mediator of retinoic acid and thyroid hormone receptors). Recent studies have suggested that transcriptional repression by N-CoR involves a corepressor complex that also contains mSin3A/B and the histone deacetylase, RPD3. In this manuscript, we demonstrate that transcriptional repression by the retinoic acid receptor can be either positively or negatively regulated by changes in the levels of N-CoR expression, suggesting a relatively strict stoichiometric relationship between N-CoR and other components of the corepressor complex. Consistent with this interpretation, overexpression of several functionally defined domains of N-CoR also relieve repression by nuclear receptors. N-CoR is distributed throughout the nucleus in a nonuniform pattern, and a subpopulation becomes concentrated into several discrete dot structures when highly expressed. RPD3 is also widely distributed throughout the nucleus in a nonuniform pattern. Simultaneous imaging of RPD3 and N-CoR suggest that a subset of each of these proteins colocalize, consistent with the existence of coactivator complexes containing both proteins. In addition, a substantial fraction of both N-CoR and mSin3 A/B appear to be independently distributed. These observations suggest that interactions between RPD3 and Sin3/N-CoR complexes may be dynamically regulated.

A complex containing N-CoR, mSin3 and histone deacetylase mediates transcriptional repression

Transcriptional repression by nuclear receptors has been correlated to binding of the putative co-repressor, N-CoR. A complex has been identified that contains N-CoR, the Mad presumptive co-repressor mSin3, and the histone deacetylase mRPD3, and which is required for both nuclear receptor- and Mad-dependent repression, but not for repression by transcription factors of the ets-domain family. These data predict that the ligand-induced switch of heterodimeric nuclear receptors from repressor to activator functions involves the exchange of complexes containing histone deacetylases with those that have histone acetylase activity.

Regulation of scavenger receptor expression in smooth muscle cells by protein kinase C: a role for oxidative stress

Phorbol esters increase scavenger-receptor mRNA expression and receptor activity in smooth muscle cells (SMCs). Our present results demonstrate that activation of protein kinase C (PKC) mediates this increase in receptor expression. This conclusion is based on the findings that (1) phorbol esters induced translocation of PKC-alpha from the cytosol to the membrane fraction; (2) PKC inhibitors blocked the effect of phorbol esters on receptor expression; (3) diacylglycerol, a physiological PKC agonist, enhanced scavenger-receptor activity; and (4) in cotransfected human SMCs, constitutively active PKC-alpha stimulated the expression of a reporter gene under control of the scavenger-receptor promoter. Phorbol ester treatment of SMCs increased intracellular reactive oxygen, and the increase in receptor activity was reduced 30% by the antioxidant N-acetyl cysteine (NAC), suggesting a role for reactive oxygen in phorbol ester-mediated receptor regulation. Furthermore, direct treatment of SMCs with reactive oxygen species increased scavenger-receptor activity. In rabbit SMCs, 100 micromol/L H2O2 alone slightly increased scavenger-receptor mRNA and protein expression. In combination, 100 micromol/L H2O2 and 10 micromol/L vanadate, which promotes formation of OH and enhances the inhibition of protein tyrosine phosphatase by H2O2, increased scavenger-receptor mRNA expression 25-fold in rabbit SMCs and 8-fold in human SMCs. NAC reduced the effect of H2O2 and vanadate by 93%. The increase in SMC scavenger-receptor expression occurs at the level of gene transcription. Receptor mRNA half-life was unchanged after treatment with either phorbol esters or reactive oxygen (approximately 14.5 hours), and induction by phorbol esters increased SMC scavenger-receptor mRNA transcription, as determined by nuclear run-on assay. Multiple cytokines and growth factors that contribute to the generation of reactive oxygen species are present in atherosclerotic lesions. These factors may all contribute to the upregulation of SMC scavenger-receptor activity and therefore to the formation of smooth muscle foam cells.

A new approach to determining the rates of recruitment of circulating leukocytes into tissues: application to the measurement of leukocyte recruitment into atherosclerotic lesions

Recruitment of circulating monocytes into the artery wall is an important feature of early atherogenesis. In vitro studies have identified a number of adhesion molecules and chemokines that may control this process but very little work has been done to evaluate their relative importance in vivo, in part because there have been no methods available of sufficient sensitivity and reliability. This paper proposes a new approach in which advantage is taken of naturally occurring or transgenically induced mutations to "mark" donor cells and to follow their fate in recipient animals using highly sensitive PCR methods. The feasibility of the approach is demonstrated by preliminary studies of monocyte recruitment into atherosclerotic lesions. However, the method should in principle be applicable to the study of any of the circulating leukocytes and their rate of entry into any tissue or tissues of interest.

Nuclear receptor coactivators

Retinoic acid, steroid and thyroid hormones regulate complex programs of gene expression by binding to intracellular receptors that are members of the nuclear receptor superfamily of ligand-dependent transcription factors. Recent studies have led to the identification and cloning of genes encoding coactivator molecules that appear to play important roles in mediating ligand-dependent transcription by members of this family. The identification of these coactivator molecules suggests a point of entry into the general transcriptional machinery that is common to several other classes of regulated transcription factors.

Peroxisome proliferator-activated receptors and retinoic acid receptors differentially control the interactions of retinoid X receptor heterodimers with ligands, coactivators, and corepressors

As the obligate member of most nuclear receptor heterodimers, retinoid X receptors (RXRs) can potentially perform two functions: cooperative binding to hormone response elements and coordinate regulation of target genes by RXR ligands. In this paper we describe allosteric interactions between RXR and two heterodimeric partners, retinoic acid receptors (RARs) and peroxisome proliferator-activated receptors (PPARs); RARs and PPARs prevent and permit activation by RXR-specific ligands, respectively. By competing for dimerization with RXR on response elements consisting of direct-repeat half-sites spaced by 1 bp (DR1 elements), the relative abundance of RAR and PPAR determines whether the RXR signaling pathway will be functional. In contrast to RAR, which prevents the binding of RXR ligands and recruits the nuclear receptor corepressor N-CoR, PPAR permits the binding of SRC-1 in response to both RXR and PPAR ligands. Overexpression of SRC-1 markedly potentiates ligand-dependent transcription by PPARgamma, suggesting that SRC-1 serves as a coactivator in vivo. Remarkably, the ability of RAR to both block the binding of ligands to RXR and interact with corepressors requires the CoR box, a structural motif residing in the N-terminal region of the RAR ligand binding domain. Mutations in the CoR box convert RAR from a nonpermissive to a permissive partner of RXR signaling on DR1 elements. We suggest that the differential recruitment of coactivators and corepressors by RAR-RXR and PPAR-RXR heterodimers provides the basis for a transcriptional switch that may be important in controlling complex programs of gene expression, such as adipocyte differentiation.

Nuclear integration of JAK/STAT and Ras/AP-1 signaling by CBP and p300

We report that interferon gamma (IFN-gamma) inhibits transcription of the macrophage scavenger receptor gene by antagonizing the Ras-dependent activities of AP-1 and cooperating ets domain transcription factors, apparently as a result of competition between AP-1/ets factors and activated STAT1 for limiting amounts of CBP and p300. Consistent with this model, STAT1 alpha interacts directly with CBP in cells, and microinjection of anti-CBP and anti-p300 antibodies blocks transcriptional responses to IFN-gamma. Cells lacking STAT1 fail to inhibit AP-1/ets activity, and overexpression of CBP both potentiates IFN-gamma-dependent transcription and relieves AP-1/ets repression. Thus, CBP and p300 integrate both positive and negative effects of IFN-gamma on gene expression by serving as essential coactivators of STAT1 alpha, modulating gene-specific responses to simultaneous activation of two or more signal transduction pathways.

Constitutive retinoid receptors expressed from adenovirus vectors that specifically activate chromosomal target genes required for differentiation of promyelocytic leukemia and teratocarcinoma cells

Sufficient knowledge of transcription factor structure and function has accumulated to allow attempts at the rational design of novel transcription factors for the study of gene regulation and potential application in gene therapy. In the present studies, we have systematically evaluated the function of chimeric retinoid receptors generated by fusion with the transactivation domain of VP16 and expression in adenovirus vectors. By varying the location of fusion of the VP16 transactivation domain with the retinoic acid receptor (RAR) or retinoid X receptor (RXR), marked differences in the specificity of gene activation were obtained. Although several chimeric proteins activated both RAR and RXR target genes, fusion of the NT16 transactivation domain to the N terminus of RAR permitted specific activation of reporter genes containing retinoic acid response elements. In contrast, fusion of the VP16 transactivation domain to the C terminus of RXR permitted specific activation of reporter genes containing RXR response elements. When tested for their ability to activate chromosomal targets, the chimera consisting of VP16 linked to the N terminus of PAR was much more active in promoting the differentiation of HL-60 cells and NTera-2 cells than the chimera consisting of VP16 linked to the C terminus of RXR. These observations support the existence of two distinct retinoid signalling pathways predicted on the basis of biochemical and pharmacologic studies and provide direct evidence that the programs of differentiation elicited by retinoic acid in these cells are mediated by a specific subset of binding sites for RAR-RXR heterodimers. VP16-RAR and VP16-RXR fusion proteins should be of further use in dissecting the relative contributions of RARs and RXRs to specific programs of gene expression. Constitutive retinoid receptors may also be considered for use as novel tumor suppressor genes for genetically based treatment of retinoid-responsive cancers.

A nuclear hormone receptor corepressor mediates transcriptional silencing by receptors with distinct repression domains

Ligand-independent transcriptional repression is an important function of nuclear hormone receptors. An interaction screen with the repression domain of the orphan receptor RevErb identified N-CoR, the corepressor for thyroid hormone receptor (TR) and retinoic acid receptor (RAR). N-CoR is likely to be a bona fide transcriptional corepressor for RevErb because (i) RevErb interacts with endogenous N-CoR, (ii) ectopic N-CoR potentiates RevErb-mediated repression, and (iii) transcriptional repression by RevErb correlates with its ability to bind N-CoR. Remarkably, a region homologous to the CoR box which is necessary for TR and RAR to interact with N-CoR is not required for RevErb. Rather, two short regions of RevErb separated by approximately 200 amino acids are required for interaction with N-CoR. The primary amino acid sequence of the N-terminal region of RevErb essential for N-CoR interaction is not homologous to that of TR or RAR, whereas similarities exist among the C-terminal domains of the receptors. N-CoR contains two adjacent but distinct interaction domains, one of which binds tightly to both RevErb and TR whereas the other binds more weakly and differentially interacts with the nuclear receptors. These results indicate that multiple nuclear receptors, utilizing different primary amino acid sequences, repress transcription by interacting with N-CoR.

Some new twists in the regulation of gene expression by thyroid hormone and retinoic acid receptors

Recent studies indicate that heterodimeric nuclear receptors utilize several novel mechanisms for increasing the complexity of transcriptional responses to hormonal stimuli. By binding as heterodimers, these receptors can potentially respond to more than one activating ligand. Allosteric interactions between the ligand binding domains of RXR and its heterodimeric partners regulate the binding of RXR ligands, resulting in either selective or dual transcriptional responses. Regulation of the relative levels of expression of different heterodimeric partners that permit signaling through RXR is likely further to expand the patterns of transcriptional responses that can occur through a given response element. Heterodimeric nuclear receptors also bind to asymmetric response elements with specific polarities that result from the formation of cooperative interfaces between DNA binding domains. The DNA binding interface serves to determine the response element specificity of different heterodimers based on the spacing between half sites. The specific polarity of DNA binding has also been shown to provide a mechanism for regulating the transcriptional responses of retinoic acid receptors to activating ligands through the differential control of co-repressor interactions. The identification and characterization of co-activator and co-repressor molecules is likely to provide a very interesting next chapter to the mechanisms of steroid hormone action.

A CBP integrator complex mediates transcriptional activation and AP-1 inhibition by nuclear receptors

Nuclear receptors regulate gene expression by direct activation of target genes and inhibition of AP-1. Here we report that, unexpectedly, activation by nuclear receptors requires the actions of CREB-binding protein (CBP) and that inhibition of AP-1 activity is the apparent result of competition for limiting amounts of CBP/p300 in cells. Utilizing distinct domains, CBP directly interacts with the ligand-binding domain of multiple nuclear receptors and with the p160 nuclear receptor coactivators, which upon cloning have proven to be variants of the SRC-1 protein. Because CBP represents a common factor, required in addition to distinct coactivators for function of nuclear receptors, CREB, and AP-1, we suggest that CBP/p300 serves as an integrator of multiple signal transduction pathways within the nucleus.

The humoral hypercalcemia of benignancy. A newly appreciated syndrome

It has become recently appreciated that the hypercalcemia of malignancy is commonly caused by the increased production of parathyroid hormone-related protein (PTHrP) by the cancer. In fact, the demonstration of increased PTHrP production in a patient with hypercalcemia is regarded as pathognomonic of malignancy. The authors describe a patient with a benign ovarian lesion that produced PTHrP and caused hypercalcemia. They identify other reports of hypercalcemia associated with hypercalcemia and benign tumors, and refer to this syndrome as the humoral hypercalcemia of benignancy. Although apparently rare, a benign PTHrP-producing tumor should be considered in the differential diagnosis of hypercalcemia.

Positive regulation of cdc2 gene activity by protein phosphatase type 2A

Several lines of evidence indicate that serine/threonine protein phosphatases may act as negative regulators of cellular growth. For example, treatment of cells with the tumor-promoter okadaic acid, an inhibitor of certain types of these phosphatases, resulted in the increased expression of several proto-oncogenes, indicating a negative role of the respective phosphatases in gene regulation. However, it was puzzling to find that okadaic acid-treated cells, even in the presence of highly expressed proto-oncogenes, did not proliferate, but were arrested at certain points of the cell cycle. To further analyze this discrepancy, we investigated the involvement of protein phosphatases in the control of other cell cycle regulatory genes, such as cdc2 which encodes an essential cell cycle regulatory kinase. We found that cdc2 gene expression was blocked by okadaic acid, but stimulated by protein phosphatase 2A. Protein phosphatase 2A is shown to be a positive regulator of cdc2 gene activity and to be required for cdc2 expression. Thus, our findings identify protein phosphatase 2A as a positive regulator of a major cell cycle regulatory gene and therefore suggest a stimulatory role of this enzyme in this aspect of cellular growth control.

Ligand-independent repression by the thyroid hormone receptor mediated by a nuclear receptor co-repressor

Thyroid-hormone and retinoic-acid receptors exert their regulatory functions by acting as both activators and repressors of gene expression. A nuclear receptor co-repressor (N-CoR) of relative molecular mass 270K has been identified which mediates ligand-independent inhibition of gene transcription by these receptors, suggesting that the molecular mechanisms of repression by thyroid-hormone and retinoic-acid receptors are analogous to the co-repressor-dependent transcriptional inhibitory mechanisms of yeast and Drosophila.

Polarity-specific activities of retinoic acid receptors determined by a co-repressor

Retinoic acid receptors (RARs) and retinoid-X receptors (RXRs) activate or repress transcription by binding as heterodimers to DNA-response elements that generally consist of two direct repeat half-sites of consensus sequence AGGTCA. On response elements consisting of direct repeats spaced by five base pairs (DR + 5 elements), RAR/RXR heterodimers activate transcription in response to RAR-specific ligands, such as all-trans-retinoic acid (RA). In contrast, on elements consisting of direct repeats spaced by one base pair (DR + 1 elements), RAR/RXR heterodimers exhibit little or no response to activating ligands and repress RXR-dependent transcription. Here we show that ligand-dependent transactivation by RAR on DR + 5 elements requires the dissociation of a new nuclear receptor co-repressor, N-CoR, and recruitment of the putative co-activators p140 and p160. Surprisingly, on DR + 1 elements, N-CoR remains associated with RAR/RXR heterodimers even in the presence of RAR ligands, resulting in constitutive repression. These observations indicate that DNA-response elements can allosterically regulate RAR-co-repressor interactions to determine positive or negative regulation of gene expression.

Pit-1 binding to specific DNA sites as a monomer or dimer determines gene-specific use of a tyrosine-dependent synergy domain

Transcriptional activation of the prolactin and growth hormone genes, occurring in a cell-specific fashion, requires short-range synergistic interactions between the pituitary-specific POU domain factor Pit-1 and other transcription factors, particularly nuclear receptors. Unexpectedly, we find that these events involve the gene-specific use of alternative Pit-1 synergy domains. Synergistic activation of the prolactin gene by Pit-1 and the estrogen receptor requires a Pit-1 amino-terminal 25-amino-acid domain that is not required for analogous synergistic activation of the growth hormone promoter. The action of this Pit-1 synergy domain is dependent on the presence of two of three tyrosine residues spaced by 6 amino acids and can be replaced by a comparable tyrosine-dependent trans-activation domain of an unrelated transcription factor (hLEF). The gene-specific utilization of this tyrosine-dependent synergy domain is conferred by specific Pit-1 DNA-binding sites that determine whether Pit-1 binds as a monomer or a dimer. Thus, the critical DNA site in the prolactin enhancer, where this domain is required, binds Pit-1 as a monomer, whereas the Pit-1 sites in the growth hormone gene, which do not utilize this synergy domain, bind Pit-1 as a dimer. The finding that the sequence of specific DNA sites dictates alternative Pit-1 synergy domain utilization based on monomeric or dimeric binding suggests an additional regulatory strategy for differential target gene activation in distinct cell types.

Scavenger receptor A gene regulatory elements target gene expression to macrophages and to foam cells of atherosclerotic lesions

Transcription of the macrophage scavenger receptor A gene is markedly upregulated during monocyte to macrophage differentiation. In these studies, we demonstrate that 291 bp of the proximal scavenger receptor promoter, in concert with a 400-bp upstream enhancer element, is sufficient to direct macrophage-specific expression of a human growth hormone reporter in transgenic mice. These regulatory elements, which contain binding sites for PU.1, AP-1, and cooperating ets-domain transcription factors, are also sufficient to mediate regulation of transgene expression during the in vitro differentiation of bone marrow progenitor cells in response to macrophage colony-stimulating factor. Mutation of the PU.1 binding site within the scavenger receptor promoter severely impairs transgene expression, consistent with a crucial role of PU.1 in regulating the expression of the scavenger receptor gene. The ability of the scavenger receptor promoter and enhancer to target gene expression to macrophages in vivo, including foam cells of atherosclerotic lesions, suggests that these regulatory elements will be of general utility in the study of macrophage differentiation and function by permitting specific modifications of macrophage gene expression.

Identification of a cell-type-specific and E2F-independent mechanism for repression of cdc2 transcription

Human myeloid leukemia cells, such as HL60, U937, and THP1 cells, undergo macrophage differentiation and growth arrest following treatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Surprisingly, we find that growth of a significant percentage of THP1 cells is arrested in the G2 phase of the cell cycle. G2 arrest correlates with cell-specific repression of the gene encoding p34cdc2, a crucial regulator of G2/M progression. Intriguingly, TPA-mediated repression of the cdc2 promoter was independent of the transcription factor E2F, distinguishing this pathway from mechanisms responsible for repression of cdc2 transcription in response to serum starvation. The region of the cdc2 promoter required for repression was located from bp -22 to -2 from the major transcriptional start site. This sequence, which we term the R box, directs the uncoupling of the basal promoter from upstream activators following TPA treatment. Analysis of THP1 nuclear proteins revealed a 55-kDa protein that was induced by TPA and interacted with the cdc2 promoter in an R-box-dependent manner. These observations provide evidence for the existence of cell-type- and promoter-specific pathways for the assembly of stable transcriptional initiation complexes that function to differentially regulate the expression of cell cycle control genes in mammalian cells.

Neutrophils and monocytes express high levels of PU.1 (Spi-1) but not Spi-B

PU.1 (the Spi-1 oncogene) and Spi-B are closely related members of the ets transcription factor family, sharing similar DNA binding specificities mediated by similar DNA binding domains. PU.1 and Spi-B have been previously described as being predominantly expressed coordinately in macrophages and B cells, but their expression in early hematopoietic stages and during the course of myeloid differentiation to monocytes and macrophages or to neutrophils has not been extensively investigated. Here, we report that PU.1 mRNA is upregulated during myeloid differentiation of human purified CD34+ cells and murine multipotential FDCP-mix A4 cells, suggesting that PU.1 is upregulated as an early event during differentiation of multipotential progenitor cells. PU.1 expression is maintained at stable levels during differentiation of myeloid cell lines U937 and HL-60 to monocytic and neutrophilic cells. PU.1 is expressed at highest levels in mature human monocytes and human peripheral blood neutrophils. In contrast to PU.1, significant levels of Spi-B mRNA and protein are found only in some B-cell lines and spleen but are not found in myeloid cell lines, neutrophils, or macrophages. In vitro translated Spi-B protein can bind to PU.1 binding sites in myeloid promoters and transactivate these promoters in nonmyeloid cells. Therefore, although PU.1 and Spi-B may bind to similar DNA control elements and have redundancy of transactivation function in vitro, the lack of significant levels of Spi-B in myeloid cells makes it unlikely that Spi-B plays a significant role in myeloid lineage development and gene expression. In contrast, PU.1 is expressed at high levels not only in monocytes and macrophages but also in neutrophils, indicating that PU.1 can activate gene expression in both major myeloid lineages.