Transcription of eukaryotic protein-encoding genes by RNA polymerase II is modulated by two distinct classes of transcription factors. The first class comprises general transcription factors which are necessary for accurate initiation of transcription. These factors include TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIG/J, and TFIIH (11, 82). TFIID is a multiprotein complex consisting of TATA-binding protein (TBP) complexed with a number of TBP-associated factors (73). The binding of TFIID is thought to be the first step in transcriptional initiation (11, 82). The subsequent binding of TFIIB is thought to be involved in bringing RNA polymerase II to the complex through the association of TFIIF (34, 37). This leads to the recruitment of TFIIE and TFIIH and possibly other factors, which ultimately results in the initiation of transcription (11,75,82).The magnitude of transcriptional activity is greatly affected by the second class of transcription factors that generally bind to distal control DNA elements (52). These sequence-specific factors act to either promote or inhibit the formation of an active transcriptional initiation complex. Recent in vitro transcription studies suggest that the entry of TFIIB may be rate limiting for transcriptional initiation and that several transcriptional activators act to recruit or stabilize the interaction of TFIIB with the initiation complex (14, 62). TFIIB contains a potential N-terminal zinc finger structure (amino acids 14 to 36) that may be important for interaction with RNA polymerase II and TFIIF (34), a more C-terminal amphipathic ␣ helix (amino acids 184 to 201), and two imperfect repeats (amino acids 124 to 201 and 218 to 294) (34,37,62) (Fig. 1A). The amphipathic ␣ helix appears to be important for interaction with TBP (34, 37) and several transcriptional activators such as the herpes simplex virus VP16 protein (62).Members of the steroid/thyroid hormone receptor gene family are sequence-specific DNA-binding proteins that play important roles in gene regulation. The steroid hormone receptor subfamily includes the receptors which mediate the effects of glucocorticoids, progestins, mineralocorticoids, androgens, and estrogens (12, 16). The thyroid/retinoid receptor subfamily (16, 21) includes receptors that mediate the effects of thyroid hormone (3,5,3Ј-triiodo-L-thyronine [T3]), all-trans retinoic acid, 9-cis retinoic acid, and 1,25-dihydroxyvitamin D 3 as well as several orphan receptors (e.g., COUP-TF and c-ErbA␣2) whose ligands, if any, are unknown (26,27,44). Steroid/thyroid receptors bind to specific DNA sequences known as hormone response elements (HREs) and mediate ligand-dependent ac-

Many nuclear receptors are capable of recognizing similar DNA elements. The molecular event(s) underlying the functional specificities of these receptors (in regulating the expression of their native target genes) is a very important issue that remains poorly understood. Here we report the cloning and analysis of a novel nuclear receptor coactivator (designated NRIF3) that exhibits a distinct receptor specificity. Fluorescence microscopy shows that NRIF3 localizes to the cell nucleus. The yeast two-hybrid and/or in vitro binding assays indicated that NRIF3 specifically interacts with the thyroid hormone receptor (TR) and retinoid X receptor (RXR) in a ligand-dependent fashion but does not bind to the retinoic acid receptor, vitamin D receptor, progesterone receptor, glucocorticoid receptor, or estrogen receptor. Functional experiments showed that NRIF3 significantly potentiates TR- and RXR-mediated transactivation in vivo but has little effect on other examined nuclear receptors. Domain and mutagenesis analyses indicated that a novel C-terminal domain in NRIF3 plays an essential role in its specific interaction with liganded TR and RXR while the N-terminal LXXLL motif plays a minor role in allowing optimum interaction. Computer modeling and subsequent experimental analysis suggested that the C-terminal domain of NRIF3 directly mediates interaction with liganded receptors through an LXXIL (a variant of the canonical LXXLL) module while the other part of the NRIF3 protein may still play a role in conferring its receptor specificity. Identification of a coactivator with such a unique receptor specificity may provide new insight into the molecular mechanism(s) of receptor-mediated transcriptional activation as well as the functional specificities of nuclear receptors.

The ligand-binding domains (LBDs) of the thyroid/retinoid receptor gene subfamily contain a series of heptad motifs important for dimeric interactions. This subfamily includes thyroid hormone receptors (T3Rs), all-trans retinoic acid (RA) receptors (RARs), 9-cis RA receptors (RARs and retinoid X receptors [RXRs]), the 1,25-dihydroxyvitamin D 3 receptor (VDR), and the receptors that modulate the peroxisomal ␤-oxidation pathway (PPARs). These receptors bind to their DNA response elements in vitro as heterodimers with the RXRs. Unliganded receptors in vivo, in particular the T3Rs, can mediate gene silencing and ligand converts these receptors into a transcriptionally active form. The in vivo interactions of these receptors with RXR were studied by using a GAL4-RXR chimera containing the yeast GAL4 DNA-binding domain and the LBD of RXR␤. GAL4-RXR activates transcription from GAL4 response elements in the presence of 9-cis RA. Unliganded T3R, which does not bind or activate GAL4 elements, represses the activation of GAL4-RXR by 9-cis RA in HeLa cells. However, addition of T3 alone leads to transcriptional activation. These findings suggest that T3R can repress or activate transcription while tethered to the LBD of GAL4-RXR and that heterodimerization can occur in vivo without stabilization by hormone response elements. Similar ligand-dependent activation was observed in HeLa cells expressing RAR, VDR, or PPAR and in GH4C1 cells from endogenous receptors. Replacement of the last 17 amino acids of the LBD of RXR␤ with the 90-amino-acid transactivating domain of the herpes simplex virus VP16 protein leads to a GAL4 constitutive activator that is repressed by wild-type T3R but not by a ninth heptad mutant that does not form heterodimers. This finding suggests that the ninth heptad of T3R is important for gene silencing and that the LBD of RXR does not exhibit silencing activity. This conclusion was verified with GAL4-LBD chimeras and with wild-type receptors in assays using appropriate response elements. These studies indicate that the LBD has diverse functional roles in gene regulation.

We report that thyroid hormone (T3) receptor (T3R) can activate the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR). Purified chick T3R-al (cT3R-al) binds as monomers and homodimers to a region in the LTR (nucleotides -104 to -75 [-104/-75]) which contains two tandem NF-cB binding sites and to a region (-80/-45) which contains three Spl binding sites. In contrast, human retinoic acid receptor a (RAR-a) and mouse retinoid X receptor 1B (RXR-,B) do not bind to these elements. However, RXR-1 binds to these elements as heterodimers with cT3R-cl and to a lesser extent with RAR-. Gel mobility shift assays also revealed that purified NF-cB p50/65 or p50/50 can bind to one but not both NF-KB sites simultaneously. Although the binding sites for p50/65, p50/50, and T3R, or Spl and T3R, overlap, their binding is mutually exclusive, and with the inclusion of RXR-3, the major complex is the RXR-P-cT3R-a1 heterodimer. The NF-KB region of the LTR and the NF-KB elements from the c light chain enhancer both function as T3 response elements (TREs) when linked to a heterologous promoter. The TREs in the HIV-1 NF-KB sites appear to be organized as a direct repeat with an 8-or 10-bp gap between the half-sites. Mutations within the NF-KB motifs which eliminate binding of cT3R-al also abolish stimulation by T3, indicating that cT3R-al binding to the Spl region does not independently mediate activation by T3. The Spl region, however, is converted to a functionally strong TRE by the viral tat factor. These studies indicate that the HIV-1 LTR contains both tat-dependent and tat-independent TREs and reveal the potential for T3R to modulate other genes containing NF-cB-and Spi-like elements. Furthermore, they indicate the importance of other transcription factors in determining whether certain T3R DNA binding sequences can function as an active TRE.Receptors which mediate the action of thyroid hormone (T3R), all trans-retinoic acid receptors (RAR), and 1,25-dihydroxy-vitamin D3 receptor (VDR) are closely related members of the steroid receptor superfamily (13,17,20 (42,54) and Spl (-77/-46) (39). Stimulation of HIV-1 genes by the viral protein tat transactivator requires the TAR element (+19/+44) (37,63), and the effects of tat are enhanced by the Spl element and to a lesser degree by the NF-KB element (5,6,71). Although the mechanism of action of tat has not been fully resolved, two models have been proposed to explain the mechanism of stimulation of HIV-1 transcripts by tat: a feedback model in which tat increases the rate of transcription initiation (5, 71) and an RNA processivity model in which tat relieves premature termination at the TAR site (41). In the present investigation, we analyzed the mechanism of regulation of the HIV-1 LTR by T3R. Our results (i) indicate that T3R stimulates the HIV-1 LTR by binding to and activating T3 response elements (TREs) contained within the viral NF-KB and Spl DNA motifs, indicating that T3R has the potential to regulate a variety of genes containing NF-KB and Spl DNA el...

The mdm2 gene is positively regulated by p53 through a p53-responsive DNA element in the first intron of the mdm2 gene. mdm2 binds p53, thereby abrogating the ability of p53 to activate the mdm2 gene, and thus forming an autoregulatory loop of mdm2 gene regulation. Although the mdm2 gene is thought to act as an oncogene by blocking the activity of p53, recent studies indicate that mdm2 can act independently of p53 and block the G 1 cell cycle arrest mediated by members of the retinoblastoma gene family and can activate E2F1/DP1 and the cyclin A gene promoter. In addition, factors other than p53 have recently been shown to regulate the mdm2 gene. In this article, we report that thyroid hormone (T3) receptors (T3Rs), but not the closely related members of the nuclear thyroid hormone/retinoid receptor gene family (retinoic acid receptor, vitamin D receptor, peroxisome proliferation activation receptor, or retinoid X receptor), regulate mdm2 through the same intron sequences that are modulated by p53. Chicken ovalbumin upstream promoter transcription factor I, an orphan nuclear receptor which normally acts as a transcriptional repressor, also activates mdm2 through the same intron region of the mdm2 gene. Two T3R-responsive DNA elements were identified and further mapped to sequences within each of the p53 binding sites of the mdm2 intron. A 10-amino-acid sequence in the N-terminal region of T3R␣ that is important for transactivation and interaction with TFIIB was also found to be important for activation of the mdm2 gene response element. T3 was found to stimulate the endogenous mdm2 gene in GH4C1 cells. These cells are known to express T3Rs, and T3 is known to stimulate replication of these cells via an effect in the G 1 phase of the cell cycle. Our findings, which indicate that T3Rs can regulate the mdm2 gene independently of p53, provide an explanation for certain known effects of T3 and T3Rs on cell proliferation. In addition, these findings provide further evidence for p53-independent regulation of mdm2 which could lead to the development of tumors from cells that express low levels of p53 or that express p53 mutants defective in binding to and activating the mdm2 gene.

Thyroid hormone receptor (T3R) is a member of the steroid hormone receptor gene family of nuclear hormone receptors. In most cells T3R activates gene expression only in the presence of its ligand, L-triiodothyronine (T3). However, in certain cell types (e.g., GH4C1 cells) expression of T3R leads to hormone-independent constitutive activation. This activation by unliganded T3R occurs with a variety of gene promoters and appears to be independent of the binding of T3R to specific thyroid hormone response elements (TREs). Previous studies indicate that this constitutive activation results from the titration of an inhibitor of transcription. Since the tumor suppresser p53 is capable of repressing a wide variety of gene promoters, we considered the possibility that the inhibitor is p53. Evidence to support this comes from studies indicating that expression of p53 blocks T3R-mediated constitutive activation in GH4C1 cells. In contrast with hormoneindependent activation by T3R, p53 had little or no effect on T3-dependent stimulation which requires TREs. In addition, p53 mutants which oligomerize with wild-type p53 and interfere with its function also increase promoter activity. This enhancement is of similar magnitude to but is not additive with the stimulation mediated by unliganded T3R, suggesting that they target the same factor. Since p53 mutants are known to target wild-type p53 in the cell, this suggests that T3R also interacts with p53 in vivo and that endogenous levels of p53 act to suppress promoter activity. Evidence supporting both functional and physical interactions of T3R and p53 in the cell is presented. The DNA binding domain (DBD) of T3R is important in mediating constitutive activation, and the receptor DBD appears to functionally interact with the N terminus of p53 in the cell. In vitro binding studies indicate that the T3R DBD is important for interaction of T3R with p53 and that this interaction is reduced by T3. These findings are consistent with the in vivo studies indicating that p53 blocks constitutive activation but not ligand-dependent stimulation. These studies provide insight into mechanisms by which unliganded nuclear hormone receptors can modulate gene expression and may provide an explanation for the mechanism of action of the v-erbA oncoprotein, a retroviral homolog of chicken T3R␣.Thyroid hormone (L-triiodothyronine [T3]) receptors (T3Rs) are members of a subfamily of the nuclear receptor gene family which includes the retinoic acid receptors, the retinoid X receptors, and the vitamin D receptor (20,25,48,57). Two T3R genes (T3R␣ and T3R␤ genes) have been identified in mammals (74, 95). The T3R␣ gene in humans and rats expresses one functional receptor (T3R␣1), while the T3R␤ gene expresses two receptors (T3R␤1 and T3R␤2) which differ in their most-N-terminal regions as a result of alternative splicing (38,48). The chicken T3R␣ (cT3R␣) is a 408-amino-acid protein which is also the cellular homolog of the v-erbA oncoprotein of avian erythroblastosis virus (74). Figure 1A illustrates the...

The effects of thyroid hormone (T3) treatment on liver Na,K-adenosine triphosphatase (Na,K-ATPase) at the levels of subunit messenger RNA (mRNA), enzymatic activity, and enzyme content were studied in euthyroid rats injected for 5 consecutive days with T3. Northern and slot blot analyses of polyadenylated mRNA revealed that T3 treatment coordinately increases the level of mRNA encoding the alpha 1- and beta 1-subunits, approximately 4- and 3-fold, respectively, above basal levels. To determine whether this increase in the subunit mRNA consequently results in an increase in the synthesis of the enzyme, a modified liver cell fractionation procedure was developed, and the subcellular fractions from control and T3-treated livers were examined biochemically. Western blot analysis and Na,K-ATPase assay demonstrated that T3 treatment resulted in a 2-fold increase in both the amount and activity of the enzyme. Furthermore, the Western blot analysis of endoglycosidase-H-treated membrane fractions revealed an increase in the amount of the precursor beta-subunit in the T3-treated liver rough microsomal fraction, suggesting that an increase in subunit synthesis contributes at least partially to the increase in the rat liver Na,K-ATPase by T3 treatment.

Thyroid hormone (T3) receptor (T3R) regulates the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) by binding to and activating thyroid hormone response elements (TREs) embedded within the viral NF-B and Sp1 motifs. The TREs within the NF-B sites are necessary for activation by T3 in the absence of Tat, while those in the Sp1 motifs function as TREs only when Tat is expressed, suggesting that Tat and T3R interact in the cell. Transactivation of the HIV-1 LTR by T3R␣ and several receptor mutants revealed that the 50-amino-acid N-terminal A/B region of T3R␣, known to interact with the basal transcription factor TFIIB, is critical for activation of both Tat-dependent and Tat-independent responsive sequences of the LTR. A single amino acid change in the highly conserved i region in the ligand-binding domain of T3R␣ eliminates Tat-independent but not Tat-dependent activation of the HIV-1 LTR by T3. Ro 5-3335 [7-chloro-5-(2-pyrryl)-3H-1,4-benzodiazepin-2(H)-one], which inhibits Tat-mediated transactivation of HIV-1, also inhibits the functional interaction between Tat and T3R␣. Binding studies with glutathione-S-transferase fusion proteins and Western (immunoblot) analysis indicate that T3R␣ interacts withTat through amino acids within the DNA-binding domain of T3R␣. Mutational analysis revealed that amino acid residues in the basic and C-terminal regions of Tat are required for the binding of Tat to T3R␣, while the N terminus of Tat is not required. These studies provide functional and physical evidence that stimulation of the HIV-1 LTR by T3 involves an interaction between T3R␣ and Tat. Our results also suggest a model in which multiple domains of T3R␣ interact with Tat and other factors to form transcriptionally important complexes.