Transcriptional regulation by triiodothyronine requires synergistic action of the thyroid receptor with another trans-acting factor

T3 enhances thyroid cancer cell proliferation through TRβ1/Oct-1-mediated cyclin D1 activation

Several studies have demonstrated that thyroid hormone T3 promotes cancer cell growth, even though the molecular mechanism involved in such processes still needs to be elucidated. In this study we demonstrated that T3 induced proliferation in papillary thyroid carcinoma cell lines concomitantly with an up-regulation of cyclin D1 expression, that is a critical mitogen-regulated cell-cycle control element. Our data revealed that T3 enhanced the recruitment of the TRβ1/Oct-1 complex on Octamer-transcription factor-1 site within cyclin D1 promoter, leading to its transactivation. In addition, silencing of TRβ1 or Oct-1 expression by RNA interference reversed both increased cell proliferation and up-regulation of cyclin D1, underlying the important role of both transcriptional factors in mediating these effects. Finally, T3-induced increase in cell growth was abrogated after knocking down cyclin D1 expression. All these findings highlight a new molecular mechanism by which T3 promotes thyroid cancer cell growth.

Liganded thyroid hormone receptor inhibits phorbol 12-O-tetradecanoate-13-acetate-induced enhancer activity via firefly luciferase cDNA

Thyroid hormone receptor (TR) belongs to the nuclear hormone receptor (NHR) superfamily and regulates the transcription of its target genes in a thyroid hormone (T3)-dependent manner. While the detail of transcriptional activation by T3 (positive regulation) has been clarified, the mechanism of T3-dependent repression (negative regulation) remains to be determined. In addition to naturally occurring negative regulations typically found for the thyrotropin β gene, T3-bound TR (T3/TR) is known to cause artificial negative regulation in reporter assays with cultured cells. For example, T3/TR inhibits the transcriptional activity of the reporter plasmids harboring AP-1 site derived from pUC/pBR322-related plasmid (pUC/AP-1). Artificial negative regulation has also been suggested in the reporter assay with firefly luciferase (FFL) gene. However, identification of the DNA sequence of the FFL gene using deletion analysis was not performed because negative regulation was evaluated by measuring the enzymatic activity of FFL protein. Thus, there remains the possibility that the inhibition by T3 is mediated via a DNA sequence other than FFL cDNA, for instance, pUC/AP-1 site in plasmid backbone. To investigate the function of FFL cDNA as a transcriptional regulatory sequence, we generated pBL-FFL-CAT5 by ligating FFL cDNA in the 5' upstream region to heterologous thymidine kinase promoter in pBL-CAT5, a chloramphenicol acetyl transferase (CAT)-based reporter gene, which lacks pUC/AP-1 site. In kidney-derived CV1 and choriocarcinoma-derived JEG3 cells, pBL-FFL-CAT5, but not pBL-CAT5, was strongly activated by a protein kinase C activator, phorbol 12-O-tetradecanoate-13-acetate (TPA). TPA-induced activity of pBL-FFL-CAT5 was negatively regulated by T3/TR. Mutation of nt. 626/640 in FFL cDNA attenuated the TPA-induced activation and concomitantly abolished the T3-dependent repression. Our data demonstrate that FFL cDNA sequence mediates the TPA-induced transcriptional activity, which is inhibited by T3/TR.

Regulation of fibroblast growth factor receptor-1 (FGFR1) by thyroid hormone: identification of a thyroid hormone response element in the murine Fgfr1 promoter

T(3) is essential for normal skeletal development, acting mainly via the TRalpha1 nuclear receptor. Nevertheless, the mechanisms of T(3) action in bone are poorly defined. Fibroblast growth factor receptor-1 (FGFR1) is also essential for bone formation. Fgfr1 expression and activity are positively regulated by T(3) in osteoblasts, and in mice that harbor a dominant negative PV mutation targeted to TRalpha1 or TRbeta, Fgfr1 expression is sensitive to skeletal thyroid status. To investigate mechanisms underlying T(3) regulation of FGFR1, we obtained primary calvarial osteoblasts from wild-type and TRbeta(PV/PV) littermate mice. T(3) treatment increased Fgfr1 expression 2-fold in wild-type cells, but 8-fold in TRbeta(PV/PV) osteoblasts. The 4-fold increased T(3) sensitivity of TRbeta(PV/PV) osteoblasts was associated with a markedly increased ratio of TRalpha1:TRbeta1 expression that resulted from reduced TRbeta1 expression in TRbeta(PV/PV) osteoblasts compared with wild-type. Bioinformatics and gel shift studies, and mutational analysis, identified a specific TR binding site 279-264 nucleotides upstream of the murine Fgfr1 promoter transcription start site. Transient transfection analysis of a series of Fgfr1 promoter 5'-deletion constructs, of a mutant reporter construct, and a series of heterologous promoter constructs, confirmed that this region of the promoter mediates a TR-dependent transcriptional response to T(3). Thus, in addition to indirect regulation of FGFR1 expression by T(3) reported previously, T(3) also activates the Fgfr1 promoter directly via a thyroid hormone response element located at positions -279/-264.

A thyroid hormone response unit formed between the promoter and first intron of the carnitine palmitoyltransferase-Ialpha gene mediates the liver-specific induction by thyroid hormone

Carnitine palmitoyltransferase-I (CPT-I) catalyzes the rate-controlling step of fatty acid oxidation. CPT-I converts long-chain fatty acyl-CoAs to acylcarnitines for translocation across the mitochondrial membrane. The mRNA levels and enzyme activity of the liver isoform, CPT-Ialpha, are greatly increased in the liver of hyperthyroid animals. Thyroid hormone (T3) stimulates CPT-Ialpha transcription far more robustly in the liver than in non-hepatic tissues. We have shown that the thyroid hormone receptor (TR) binds to a thyroid hormone response element (TRE) located in the CPT-Ialpha promoter. In addition, elements in the first intron participate in the T3 induction of CPT-Ialpha gene expression, but the CPT-Ialpha intron alone cannot confer a T3 response. We found that deletion of sequences in the first intron between +653 and +744 decreased the T3 induction of CPT-Ialpha. Upstream stimulatory factor (USF) and CCAAT enhancer binding proteins (C/EBPs) bind to elements within this region, and these factors are required for the T3 response. The binding of TR and C/EBP to the CPT-Ialpha gene in vivo was shown by the chromatin immunoprecipitation assay. We determined that TR can physically interact with USF-1, USF-2, and C/EBPalpha. Transgenic mice were created that carry CPT-Ialpha-luciferase transgenes with or without the first intron of the CPT-Ialpha gene. In these mouse lines, the first intron is required for T3 induction as well as high levels of hepatic expression. Our data indicate that the T3 stimulates CPT-Ialpha gene expression in the liver through a T3 response unit consisting of the TRE in the promoter and additional factors, C/EBP and USF, bound in the first intron.

Hormonal regulation of multiple promoters of the rat mitochondrial glycerol-3-phosphate dehydrogenase gene: identification of a complex hormone-response element in the ubiquitous promoter B

Rat mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is regulated by multiple promoters in a tissue-specific manner. Here, we demonstrate that thyroid hormone (3,5,3'-tri-iodo-L-thyronine) and steroid hormone but not the peroxisome proliferator clofibrate and retinoic acid stimulate the activation of the ubiquitous promoter B in a receptor-dependent manner, whereas the more tissue-restricted promoters A and C are not inducible by these hormones. Thyroid hormone action is mediated by a direct repeat +4 (DR+4) hormone-response element as identified by deletion and mutation analyses of promoter B in transient transfection analyses. The DR+4 element was able to bind to an in vitro translated thyroid hormone receptor in band-shift and supershift experiments. The hormone-response element comaps with a recognition site for the transcription factor Sp1, suggesting complex regulation of this sequence element. Mutation of this Sp1-recognition site reduces the basal promoter B activity dramatically in HepG2 and HEK293 cells in transient transfection and abolishes the binding of Sp1 in band-shift experiments. As demonstrated by Western-blot experiments, administration of tri-iodothyronine to euthyroid rats increases hepatic mGPDH protein concentrations in vivo. As it has recently been reported that human mGPDH promoter B is not regulated by tri-iodothyronine, this is the first example of a differentially tri-iodothyronine-regulated orthologous gene promoter in man and rat.

Thyroid hormone regulates carnitine palmitoyltransferase Ialpha gene expression through elements in the promoter and first intron

Carnitine palmitoyltransferase I (CPT-I) catalyzes the transfer of long chain fatty acyl groups from CoA to carnitine for translocation across the mitochondrial inner membrane. CPT-Ialpha is a key regulatory enzyme in the oxidation of fatty acids in the liver. CPT-Ialpha is expressed in all tissues except skeletal muscle and adipose tissue, which express CPT-Ibeta. Expression of CPT-Ialpha mRNA and enzyme activity are elevated in the liver in hyperthyroidism, fasting, and diabetes. CPT-Ialpha mRNA abundance is increased 40-fold in the liver of hyperthyroid compared with hypothyroid rats. Here, we examine the mechanisms by which thyroid hormone (T3) stimulates CPT-Ialpha gene expression. Four potential T3 response elements (TRE), which contain direct repeats separated by four nucleotides, are located 3000-4000 base pairs 5' to the start site of transcription in the CPT-Ialpha gene. However, only one of these elements functions as a TRE. This TRE binds the T3 receptor as well as other nuclear proteins. Surprisingly, the first intron of the CPT-Ialpha gene is required for the T3 induction of CPT-Ialpha expression, but this region of the gene does not contain a TRE. In addition, we show that CPT-Ialpha is induced by T3 in cell lines of hepatic origin but not in nonhepatic cell lines.

Expression of the rat liver carnitine palmitoyltransferase I (CPT-Ialpha) gene is regulated by Sp1 and nuclear factor Y: chromosomal localization and promoter characterization

Carnitine palmitoyltransferase (CPT)-I catalyses the transfer of long-chain fatty acids from CoA to carnitine for translocation across the mitochondrial inner membrane. Expression of the 'liver' isoform of the CPT-I gene (CPT-Ialpha) is subject to developmental, hormonal and tissue-specific regulation. To understand the basis for control of CPT-Ialpha gene expression, we have characterized the proximal promoter of the CPT-Ialpha gene. Here, we report the sequence of 6839 base pairs of the promoter and the localization of the rat CPT-Ialpha gene to region q43 on chromosome 1. Our studies show that the first 200 base pairs of the promoter are sufficient to drive transcription of the CPT-Ialpha gene. Within this region are two sites that bind both Sp1 and Sp3 transcription factors. In addition, nuclear factor Y (NF-Y) binds the proximal promoter. Mutation at the Sp1 or NF-Y sites severely decreases transcription from the CPT-Ialpha promoter. Other protein binding sites were identified within the first 200 base pairs of the promoter by DNase I footprinting, and these elements contribute to CPT-Ialpha gene expression. Our studies demonstrate that CPT-Ialpha is a TATA-less gene which utilizes NF-Y and Sp proteins to drive basal expression.

Role of CCAAT enhancer-binding protein beta in the thyroid hormone and cAMP induction of phosphoenolpyruvate carboxykinase gene transcription

Transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK) is stimulated by thyroid hormone (T3) and cAMP. Two DNA elements in the PEPCK promoter are required for T3 responsiveness including a thyroid hormone response element and a binding site called P3(I) for the CCAAT enhancer-binding protein (C/EBP). Both the alpha and beta isoforms of C/EBP are highly expressed in the liver. C/EBPalpha contributes to the liver-specific expression and cAMP responsiveness of the PEPCK gene. In this study, we examined the ability of C/EBPbeta when bound to the P3(I) site to regulate PEPCK gene expression. We report that C/EBPbeta can stimulate basal expression and participate in the induction of PEPCK gene transcription by T3 and cAMP. The cAMP-responsive element-binding protein and AP1 proteins that contribute to the induction by cAMP are not involved in the stimulation by T3. A small region of the transactivation domain of C/EBPbeta is sufficient for the stimulation of basal expression and cAMP responsiveness. Our results suggest that C/EBPalpha and C/EBPbeta are functionally interchangeable when bound to the P3(I) site of the PEPCK promoter.

Cell-specific regulation of transcription of the malic enzyme gene: characterization of cis-acting elements that modulate nuclear T3 receptor activity

Stimulation of malic enzyme transcription by triiodothyronine (T3) is robust (> 60-fold) in chick embryo hepatocytes, weak (5-fold) in chick embryo fibroblasts that stably overexpress the nuclear T3 receptor-alpha, and still weaker (1-fold) in chick embryo fibroblasts which contain nuclear T3 receptor levels that are similar to those of chick embryo hepatocytes. Using DNase I hypersensitivity, functional transfection, and in vitro DNA-binding analyses, four cis-acting elements were identified in the malic enzyme 5'-flanking DNA that conferred differences in nuclear T3 receptor activity between chick embryo hepatocytes and chick embryo fibroblasts. These cell-specific regulatory elements are located at -3895/-3890, -3761/-3744, -3703/-3686, and -3474/-2715 bp and overlap with DNase I hypersensitive sites that are observed in chromatin of chick embryo hepatocytes. Each element enhances T3 responsiveness of the malic enzyme promoter in chick embryo hepatocytes but has no effect on T3 responsiveness in chick embryo fibroblasts. Three of the cell-specific regulatory elements flank a previously identified DNA fragment (-3889 to -3769 bp; Hodnett et al., Arch. Biochem. Biophys. 334, 309-324, 1996) that contains one major and four minor T3 response elements. The cell-specific regulatory element at -3703/-3686 bp binds to the liver-enriched factor, CCAAT/enhancer-binding protein-alpha, whereas cell-specific regulatory elements at -3895/-3890 and -3761/-3744 bp bind proteins of unknown identity. While the cell-specific regulatory element at -3761/-3744 bp contains sequences that resemble binding sites for CCAAT/enhancer-binding protein, activator protein-1, cyclic AMP response element binding protein, and NF-1, none of these proteins appear to bind to this DNA fragment. These data suggest that cell-specific differences in T3 responsiveness of the malic enzyme gene are mediated in large part by nonreceptor proteins that augment the transcriptional activity of the nuclear T3 receptor in hepatocytes.

The thyroid hormone down-regulates the mouse alpha-foetoprotein promoter

The thyroid hormone (T3) was shown to down regulate the level of alpha-foetoprotein (AFP) mRNA in hepatoma cells HepG2. Recombinant plasmids containing segments from the mouse AFP gene promoter were transfected in HepG2 cells and transient expression assays showed that the T3 inhibitory effect depends on the sequence limited by positions -80 and -38, upstream from the TATA box. This sequence is able to confer T3 sensitivity to a heterologous promoter and contains a putative T3-responsive element, as well as likely CEBP- and HNF1-responsive elements. These observations suggest that T3 is a good candidate for hormonal control of the AFP gene expression and especially for the neonatal shut off of the gene.

The CCAAT box binding factor, NF-Y, is required for thyroid hormone regulation of rat liver S14 gene transcription

Triiodothyronine (T3) activates rat liver S14 gene transcription through T3 receptors (TRbeta) binding distal thyroid hormone response elements located between -2.8 and -2.5 kilobase pairs upstream from the transcription start site. Previous studies suggested that proximal promoter elements located between -220 to -80 base pairs upstream from the 5' end of the S14 gene were involved in hormone activation of the S14 gene. This report identifies an inverted CCAAT box (or Y box) at -104ATTGG-100 as a core cis-regulatory element. Gel shift studies using rat liver nuclear proteins show that at least three CCAAT-binding factors interact with this region as follows: NF-Y and c/EBP-related proteins formed major complexes, whereas NF-1/CTF forms a minor complex in gel shift assay. Mutation of the Y box indicated that loss of NF-Y binding, but not c/EBP or NF-1, correlated closely with a decline in basal activity and a loss of T3-mediated transactivation. Substitution of the S14 Y box in reporter genes with elements binding only NF-Y elevated basal activity and T3-mediated transactivation, whereas substitution with elements binding c/EBP-related proteins or SP1 displayed low basal activity and T3-mediated transactivation. These studies indicate that NF-Y and TRbeta functionally interact to confer T3 control to the S14 gene.

Thyroid hormone inhibits the human prolactin gene promoter by interfering with activating protein-1 and estrogen stimulations

Transcription of the human PRL (hPRL) gene in the pituitary is subject to tissue-specific and multihormonal regulation involving two main regulatory regions, a proximal promoter and a distal enhancer. In this report we show that thyroid hormone inhibits the expression of the hPRL gene in rat pituitary cells. Transient expression experiments show that thyroid hormone regulation involves a strong inhibitory element, located in the proximal (-164/-35) promoter, which is modulated by a more distal stimulatory response control region. Gel retardation experiments reveal that the thyroid hormone receptor does not bind to the proximal negative element. We show the existence of an activating protein-1 (AP-1) response element located at positions -61 to -54 of the proximal promoter, conferring AP-1 stimulation to the hPRL promoter. This AP-1 induction is abolished when hormone-bound thyroid hormone receptor is present, indicating that there is an interference between the thyroid hormone receptor and AP-1 regulatory pathways. Furthermore, using the complete hPRL upstream region, we show that estrogen induction is abolished by simultaneous thyroid hormone treatment.

Characterization of the promoter of the rat sarcoplasmic endoplasmic reticulum Ca2+-ATPase 1 gene and analysis of thyroid hormone responsiveness

Relaxation of skeletal muscle requires the re-uptake of Ca2+, which is mediated by the sarcoplasmic reticulum Ca2+-ATPase (SERCA). Thyroid hormone (T3) stimulates the expression of the SERCA1 isoform, which is essential for fast skeletal muscle fiber phenotype. We have cloned and studied the first 962 base pairs of the 5'-flanking region of the rat SERCA1 gene. This sequence was tested for T3-regulated expression in transient transfection experiments using COS7 cells and for binding of thyroid hormone receptor (TR) alpha in mobility shift assays. A construct of the 5'-flanking region and a reporter gene was unresponsive to T3 in the absence of co-transfected thyroid hormone receptor. In the presence of TRalpha, a T3 induction ratio of almost 4.0 was found, and this induction ratio was doubled with co-transfection of an RXR expression plasmid. Analysis of progressive 5'-deletion fragments of the sequence indicated multiple regions involved in T3 responsiveness. Three regions, R1, R2, and R3, were identified that bound TR complexes in mobility shift assays and conferred T3 responsiveness to a heterologous promoter. The most potent of these thyroid hormone response elements, R3, increased the 2-fold background T3 stimulation of the thymidine kinase promoter to nearly 6-fold. Detailed analysis of this element showed that four TR-binding half-sites, comprising two independent thyroid hormone response elements, interact cooperatively to give the maximal T3 response. T3 regulation of SERCA1 expression is mediated by a complex thyroid hormone response element that may serve to provide a greater range of response in interaction with nuclear receptor partners or cell-specific transcription factors.

Retinoic acid induction of human tissue-type plasminogen activator gene expression via a direct repeat element (DR5) located at -7 kilobases

All-trans-retinoic acid (RA) and retinoids induce synthesis of tissue-type plasminogen activator (t-PA) in endothelial and neuroblastoma cells in vitro and in rats in vivo. In HT1080 fibrosarcoma cells, induction of t-PA-related antigen secretion and t-PA mRNA steady state levels by RA were found to depend on de novo protein and mRNA synthesis. Fragments derived from the 5'-flanking region of the t-PA gene (+197 to -9578 base pairs (bp)) were linked to the chloramphenicol acetyltransferase gene. Transfection studies demonstrated that the region spanning bp -7145 to -9578 mediated induction by RA. A functional retinoic acid response element (RARE), consisting of a direct repeat of the GGGTCA motif spaced by 5 nucleotides (t-PA/DR5), was localized at -7.3 kilobases. The t-PA/DR5 element interacted with the heterodimer composed of retinoic acid receptor alpha and retinoid X receptor alpha in vitro, whereas its mutation abolished induction by RA in transient expression. In human EA.hy926 hybrid endothelial and in SK-N-SH neuroblastoma cells, the activity of t-PA/DR5 was found to be independent of the intervening sequence (-632 to -7144 bp) and of its distance from the transcription initiation site. Staurosporine, an inhibitor of protein kinase activity, inhibited induction by RA, suggesting that it required protein phosphorylation.

An N-terminal deletion mutant of estrogen receptor exhibits increased synergism with upstream activators and enhanced binding to the estrogen response element

To study the role of the N-terminal region of the estrogen receptor (ER) in transcription activation and in DNA binding, we constructed a mutant of the Xenopus laevis ER which lacks amino acids 1-159 (XER160/586). In transient transfections, XER160/586 exhibited < 10% of the activity of wild-type XER on a synthetic promoter containing two estrogen response elements (EREs). To examine transcriptional synergism by XER and by XER160/586, we determined the activity of promoters containing EREs and binding sites for either the vitellogenin activator, NF1, or AP1 upstream activator protein. For the three promoters transcription by XER was 2.8-fold greater than expected for additive activities, and transcription by XER160/586 was 6.2-fold greater. These data demonstrate that an upstream activator protein bound near the promoter can partially compensate for the loss of the internal N-terminal (AF1) transactivation domain in XER160/586. Using a promoter interference assay to study the intracellular interaction between ER and the estrogen response element, we found that XER160/586 exhibited a significant increase in affinity for the ERE. Its low basal activity and enhanced affinity for the ERE make XER160/586 an effective dominant negative mutant. When co-expressed with wild-type XER at 1:1 and 5:1 ratios, XER160/586 suppressed the activity of wild-type XER by 57% and > 80%, respectively.

Transcriptional mechanisms in anterior pituitary cell differentiation

Development of the anterior pituitary gland involves the establishment of five distinct cell lineages which are each characterized by the expression of specific trophic hormone genes. Recent studies of the thyrotrope, somatotrope, and lactotrope cell types have investigated the molecular decisions responsible for the commitment and differentiation of these cell types and have characterized the regulatory mechanisms that govern cell-specific expression of individual hormone genes. In particular, elucidation of the molecular basis of heritable dwarf phenotypes lacking particular pituitary cell lineages, such as the Snell, Jackson, and little dwarf mice, and studies of the regulation of trans-acting factors, including Pit-1, involved in pituitary cell restricted gene activation have begun to delineate the pathways responsible for development of this organ.

Physiology of the steroid-thyroid hormone nuclear receptor superfamily

Glucocorticoids, other steroid hormones, thyroid hormones and vitamin-derived hormones (including retinoids) all exert their effects by the regulation of hormone-responsive target genes within the cell nucleus. These hormones bind to a series of specific nuclear receptor proteins that function as hormone-inducible transcription factors. The receptors are structurally homologous, are related to the avian erythroblastosis oncogene v-erbA, and exhibit remarkable evolutionary conservation. Together they form the steroid-thyroid hormone nuclear receptor superfamily. This chapter describes the structure and functions of the various family members and highlights the differences and similarities that occur between individual receptor proteins. Type I receptors, which include glucocorticoid receptor and other steroid receptor proteins, interact as homodimers with target sequences of DNA containing two receptor binding sites arranged as a palindrome. Type II receptors, which include receptors for retinoids, thyroid hormone and vitamin D3, bind as heterodimers (or homodimers) to DNA sequences in which two or more receptor-binding sites are arranged as a direct repeat or as other more complex configurations. The complexity of both receptor-DNA and receptor-receptor interactions predicts the potential for considerable cross-talk between various hormone-activated pathways. Thus, the specificity of hormone action and its regulation is discussed in relation to the structural and functional characteristics of the receptors and their molecular mechanisms of action. Finally, potential sites of regulation of hormone action, from circulating hormone levels in the periphery to their delivery to the cell and final site of action in the nucleus, are highlighted to provide a perspective for the following chapters in this volume and to indicate their clinical significance.

Negative regulation of the glycoprotein hormone alpha gene promoter by thyroid hormone: mutagenesis of a proximal receptor binding site preserves transcriptional repression

Transcription of the glycoprotein hormone alpha gene is repressed by the thyroid hormone receptor (TR) in a hormone dependent manner. Previous studies identified a TR binding site immediately downstream of the TATA box. Site directed mutagenesis and transient gene expression studies were used to evaluate the role of this TR binding site as a negative thyroid response element (nTRE). Mutagenesis of the putative negative thyroid response element (nTRE) site eliminated TR binding but failed to eliminate negative regulation by T3. A mutation which converted the putative nTRE to a higher affinity palindromic element did not enhance repression, but rather eliminated thyroid hormone dependent negative regulation. Proximal alpha promoter sequences between -100 and +44 were replaced with a heterologous thymidine kinase promoter resulting in a construct that was not repressed by T3 treatment. This finding confirmed that repression required proximal alpha promoter sequences and also indicated that repression did not occur by interference with the function of upstream the alpha gene enhancers. These studies indicate that TR binding adjacent to the TATA box is not required for T3 mediated repression of the alpha promoter and suggest that negative regulation may involve protein-protein interactions with promoter-specific transcription factors.