Clinical application of fluorescence endoscopic imaging using hypericin for the diagnosis of human oral cavity lesions.

BACKGROUND: Diagnosis of oral cancer is conventionally carried out using white light endoscopy and histopathology of biopsy samples. However, oral tumours are mostly superficial and the lesion and its margins can be difficult to visualise under white light. We present clinical data on fluorescence diagnostic imaging of oral lesions using hypericin, a plant-based photosensitiser. METHODS: Fluorescence images of lesions and normal tissue were captured using an endoscope after hypericin administration. The images were analysed to extract their colour parameters, which, along with the red-to-blue intensity ratios, were analysed and used to discriminate between tissue types. The results were correlated with those from histopathology. RESULTS: The red-to-blue intensity ratio increased from normal to hyperplastic to cancerous tissue and was a good parameter to discriminate between these tissue types, with sensitivity and specificity levels of 90% and above. CONCLUSION: Our results show that hypericin fluorescence imaging has the potential to be used for the clinical diagnosis of oral cancer. Further study to enhance the clinical potential of this technique includes the development of a real-time image processing and analysis system interfaced to the endoscope to enable same-day cancer diagnosis and demarcation of lesion margins in a clinical setting.

Identification of a thyroid hormone receptor that is pituitary-specific.

Three cellular homologs of the v-erbA oncogene were previously identified in the rat; two of them encode high affinity receptors for the thyroid hormone triiodothyronine (T3). A rat complementary DNA clone encoding a T3 receptor form of the ErbA protein, called r-ErbA beta-2, was isolated. The r-ErbA beta-2 protein differs at its amino terminus from the previously described rat protein encoded by c-erbA beta and referred to as r-ErbA beta-1. Unlike the other members of the c-erbA proto-oncogene family, which have a wide tissue distribution, r-erbA beta-2 appears to be expressed only in the anterior pituitary gland. In addition, thyroid hormone downregulates r-erbA beta-2 messenger RNA but not r-erbA beta-1 messenger RNA in a pituitary tumor-derived cell line. The presence of a pituitary-specific form of the thyroid hormone receptor that may be selectively regulated by thyroid hormone could be important for the differential regulation of gene expression by T3 in the pituitary gland.

Calcitonin messenger RNA encodes multiple polypeptides in a single precursor.

Recombinant DNA techniques were used to analyze the structure of the messenger RNA encoding a precursor of calcitonin, a small calcium-regulating hormone of 32 amino acids. Analyses of the nucleotide sequences of cloned complementary DNA's comprising the entire coding sequence of the messenger RNA revealed that calcitonin is flanked at both its amino and carboxyl termini by peptide extensions linked to the hormone by short sequences of basic amino acids. The location of glycine next to the carboxyl terminal prolinamide of calcitonin is consistent with indications that glycine is required for the enzymatic amidation of proline to the prolinamide. During cellular biosynthesis, calcitonin arises from a large precursor protein by cleavages at both amino and carboxyl terminal residues of the hormone. These findings raise questions concerning the regulation of these cleavages and the potential biological functions of the precursor extensions derived from these cleavages.

Regulation of follistatin messenger ribonucleic acid levels in the rat pituitary.

Follistatin is a glycoprotein, originally isolated from the gonads, that specifically inhibits follicle-stimulating hormone (FSH) biosynthesis and secretion. We have previously detected follistatin mRNA in rat pituitary gonadotropes. To assess the potential physiologic role of follistatin in the rat pituitary, we have investigated the effects of gonadectomy (GDX) and of sex steroid replacement on pituitary follistatin gene expression. Follistatin mRNA levels in individual rat pituitaries were measured by a quantitative reverse transcription-polymerase chain reaction assay. Female and male rats 21 d old underwent surgical GDX and were then killed 21 d after GDX. Follistatin mRNA levels in rat pituitary increased 3.2 +/- 1.5-fold (P < 0.01) in GDX female rats and 8.2 +/- 2.0-fold (P < 0.005) in GDX male rats, compared with intact female and male controls, respectively. Replacement therapy with 17 beta-estradiol-3-benzoate (10 micrograms/100 g body weight) subcutaneously daily for 7 d in GDX female rats resulted in a slight further increase in follistatin mRNA levels compared to GDX females. In contrast, therapy with testosterone propionate (500 micrograms/100 g body weight) subcutaneously daily for 7 d in GDX male rats resulted in a decrease in follistatin mRNA levels, towards but not completely back to baseline levels in intact males. Time-course studies in adult male and female rats showed that the increase in follistatin mRNA levels after GDX is rapid, with significant increases occurring within 24 h after GDX, and parallels or precedes increases in FSH beta mRNA levels and FSH secretion. The regulation of follistatin mRNA levels in the rat pituitary by GDX and by sex steroids suggests that follistatin may be important as an autocrine or paracrine factor in the regulation of FSH.

Differential and tissue-specific regulation of the multiple rat c-erbA messenger RNA species by thyroid hormone.

Thyroid hormone (T3) has been shown to regulate the level of its receptor in a number of tissues and cell lines. Recently, proteins encoded by the protooncogene c-erbA have been identified as T3 receptors. In the rat, four c-erbA gene products have been isolated, three of which, r-erbA alpha-1, r-erbA beta-1, and r-erbA beta-2, encode biologically active T3 receptors; the fourth, r-erbA alpha-2, may play an inhibitory role in T3 action. The present work examines the molecular nature of T3 receptor autoregulation using probes specific for each c-erbA mRNA. Rats were rendered hypothyroid with propylthiouracil and then treated with either saline or T3. Northern blot analyses reveal marked tissue-specific and differential regulation of the multiple c-erbA mRNAs by T3. In the pituitary the levels of r-erbA beta-1 mRNA increase, whereas the levels of the pituitary-specific r-erbA beta-2 mRNA decrease with T3 treatment. In heart, kidney, liver, and brain the levels of r-erbA beta-1 are unaffected by thyroidal status. The levels of both r-erbA alpha mRNAs decrease with T3 treatment in all tissues examined except for the brain, where there is no change. In addition, we find that changes in the mRNAs encoding specific subpopulations of T3 receptors do not always parallel changes in total nuclear T3 binding. Differential regulation of the specific c-erbA mRNA species could have important consequences for T3 action.

Urea signaling in cultured murine inner medullary collecting duct (mIMCD3) cells involves protein kinase C, inositol 1,4,5-trisphosphate (IP3), and a putative receptor tyrosine kinase.

Urea, in concentrations unique to the renal medulla, increases transcription and protein expression of several immediate-early genes (IEGs) including the zinc finger-containing transcription factor, Egr-1. In the present study, the proximal 1.2 kb of the murine Egr-1 5' -flanking sequence conferred urea-responsiveness to a heterologous luciferase reporter gene when transiently transfected into renal medullary mIMCD3 cells,and this effect was comparable with that of the extremely potent immediate-early gene inducer, O-tetradecanoylphorbol 13-acetate (TPA). Urea inducibility of Egr-1 expression was protein kinase C (PKC)-dependent because staurosporine and calphostin C abrogated the urea effect, and down-regulation of PHC through chronic TPa treatment inhibited both urea-inducible Egr-1 protein expression and gene transcription. In addition, hyperosmotic urea increased inositol 1,4,5-trisphosphate (IP3) release from mIMCD3 cells and induced tyrosine phosphorylation of the receptor tyrosine kinase-specific phospholipase C (PLC) isoform, PLC-gamma. Importantly, urea-inducible Egr-1 expression was strongly genistein-sensitive, to a much greater extent than the comparable TPA-inducible Egr-1 expression. These data suggest that urea-inducible Egr-1 expression is a consequence of sequential PLC-gamma activation, IP3 release, and PKC activation. Urea-inducible PLC-gamma activation, in conjunction with the genistein-sensitivity of urea-inducible Egr-1 expression suggest the possibility of a cell surface or cytoplasmic urea-sensing receptor tyrosine kinase.

Regulation of rat luteinizing hormone subunit messenger ribonucleic acids by gonadal steroid hormones.

Little is known about the hormonal regulation of luteinizing hormone (LH) biosynthesis. We have studied the regulation of LH messenger RNA (mRNA) levels by gonadal-steroid hormones in the rat. In one set of experiments, male and female rats were surgically gonadectomized (GDX) and killed 1, 3, 7, 14, 22, and 31 d postoperatively. In another set of experiments, male and female rats were surgically GDX and were injected subcutaneously with testosterone propionate (500 micrograms/100 g body wt per d) or 17 beta-estradiol 3-benzoate (10 micrograms/100 g body wt per d), respectively, beginning 3 wk postoperatively. Levels of serum LH were determined by radioimmunoassay and levels of LH subunit mRNAs in single pituitary glands were determined by blot hybridization analysis using labeled synthetic oligodeoxyribonucleotide probes that correspond to portions of the coding regions of the rat alpha- and LH beta-subunit mRNAs. 4 wk after gonadectomy, serum LH levels rose nine- and 20-fold, while alpha-subunit mRNA levels rose six- and 10-fold, and LH beta-subunit levels rose seven- and 14-fold, compared with controls in males and females, respectively. In gonadal-steroid hormone-treated male and female GDX rats, serum LH levels fell to 8 and 36% of control values, while alpha-subunit mRNA levels declined to 22 and 19%, and LH beta-subunit mRNA levels declined to 6 and 10% of control values, 48 h after injections were initiated, in males and females, respectively. We conclude that gonadal-steroid hormones negatively regulate the levels of both subunit mRNAs in GDX rats in a pattern that parallels the changes in serum LH values. These data suggest that gonadal-steroid hormone regulation of LH biosynthesis occurs, at least in part, at the level of LH subunit mRNAs due to effects at the transcriptional and/or RNA stability levels.

New insights on the mechanism(s) of the dominant negative effect of mutant thyroid hormone receptor in generalized resistance to thyroid hormone.

Generalized resistance to thyroid hormone (GRTH) is a syndrome of hyposensitivity to triiodothyronine (T3) that displays autosomal dominant inheritance. The genetic defect commonly lies in the ligand-binding domain of one of the TR beta alleles. Since there are two major thyroid hormone receptor (TR) isoforms, TR alpha and TR beta, it is not known how the mutant receptor mediates a dominant negative effect. Previously, we showed that T3 caused dissociation of TR homodimers and TR alpha/TR beta dimers from several thyroid hormone response elements (TREs). Hence, we used the electrophoretic mobility shift assay to compare the effect of T3 on the DNA binding of mutant TR beta-1 (Mf-1) from a kindred with GRTH with normal TR beta. Mf-1 bound better as a homodimer than TR beta, but dissociated from DNA only at high T3 concentrations. Both receptors heterodimerized with nuclear auxiliary proteins. They also dimerized with TR alpha and with each other. Surprisingly, T3 disrupted the DNA binding of the Mf-1/TR isoform dimers. Thus, mechanisms for the dominant negative effect by mutant TRs likely involve either increased binding to TREs by mutant homodimers that cannot bind T3 (hence cannot dissociate from DNA) and/or the formation of inactive mutant TR/nuclear protein heterodimers.

Sex steroid hormone regulation of follicle-stimulating hormone subunit messenger ribonucleic acid (mRNA) levels in the rat.

Follicle-stimulating hormone (FSH) beta, luteinizing hormone (LH) beta, and alpha subunit messenger RNA (mRNA) levels were examined in rats after castration and sex-steroid replacement. Subunit mRNAs were determined by blot hybridization using rat FSH beta genomic DNA, and alpha and LH beta complementary DNA (cDNA). Rat FSH beta mRNA is 1.7 kilobase in size. After ovariectomy, female FSH beta mRNA levels increased fourfold, whereas those of LH beta and alpha increased twenty- and eightfold, respectively. With estradiol, all subunits returned toward normal levels. Male LH beta and alpha mRNA levels rose eight- and fourfold, respectively, 40 d postcastration, but FSH beta mRNA levels increased minimally. After 7 d of testosterone propionate, LH beta and alpha mRNAs declined to normal levels, whereas FSH beta mRNA increased slightly. We conclude that in female rats FSH beta is negatively regulated by gonadal steroids, but to a lesser extent than LH beta or alpha mRNAs, and there is a differential regulation of FSH beta mRNA levels in males as compared with females at the time points examined.

Transient elevation of messenger RNA encoding gastrin-releasing peptide, a putative pulmonary growth factor in human fetal lung.

Gastrin-releasing peptide (GRP), the mammalian homologue of the amphibian peptide bombesin, is present in pulmonary neuroendocrine cells and appears to be a growth factor for both normal and neoplastic pulmonary cells. Previously we have reported the cloning of the messenger RNAs (mRNAs) and gene that encode human GRP. We now report that GRP mRNAs are markedly elevated in human fetal lung during the canalicular phase of pulmonary development (from approximately 16 to 30 wk gestation). By RNA blot and in situ hybridization analyses, GRP mRNAs were first detectable in fetal lung at 9-10 wk, plateaued at levels 25-fold higher than in adult lungs from 16 to approximately 30 wk and then declined to near adult levels by 34 wk gestation. By contrast, GRP peptide levels remain elevated until several months after birth. Consistent with this, in situ hybridization and immunohistochemical studies showed that GRP mRNA and peptide consistently colocalized in early gestation lung but that in neonatal lung, many cells that contained GRP peptide no longer contained GRP mRNA. The transient expression of high levels of GRP mRNAs during an approximately 12-wk phase of fetal lung development suggests that the secretion of GRP or its COOH-terminal peptides from pulmonary neuroendocrine cells may play a role in normal lung development.

Direct effects of leptin on brown and white adipose tissue.

Leptin is thought to exert its actions on energy homeostasis through the long form of the leptin receptor (OB-Rb), which is present in the hypothalamus and in certain peripheral organs, including adipose tissue. In this study, we examined whether leptin has direct effects on the function of brown and white adipose tissue (BAT and WAT, respectively) at the metabolic and molecular levels. The chronic peripheral intravenous administration of leptin in vivo for 4 d resulted in a 1.6-fold increase in the in vivo glucose utilization index of BAT, whereas no significant change was found after intracerebroventricular administration compared with pair-fed control rats, compatible with a direct effect of leptin on BAT. The effect of leptin on WAT fat pads from lean Zucker Fa/ fa rats was assessed ex vivo, where a 9- and 16-fold increase in the rate of lipolysis was observed after 2 h of exposure to 0.1 and 10 nM leptin, respectively. In contrast, no increase in lipolysis was observed in the fat pads from obese fa/fa rats, which harbor an inactivating mutation in the OB-Rb. At the level of gene expression, leptin treatment for 24 h increased malic enzyme and lipoprotein lipase RNA 1.8+/-0.17 and 1.9+/-0.14-fold, respectively, while aP2 mRNA levels were unaltered in primary cultures of brown adipocytes from lean Fa/fa rats. Importantly, however, no significant effect of leptin was observed on these genes in brown adipocytes from obese fa/fa animals. The presence of OB-Rb receptors in adipose tissue was substantiated by the detection of its transcripts by RT-PCR, and leptin treatment in vivo and in vitro activated the specific STATs implicated in the signaling pathway of the OB-Rb. Taken together, our data strongly suggest that leptin has direct effects on BAT and WAT, resulting in the activation of the Jak/STAT pathway and the increased expression of certain target genes, which may partially account for the observed increase in glucose utilization and lipolysis in leptin-treated adipose tissue.

Ligand-dependent, Pit-1/growth hormone factor-1 (GHF-1)-independent transcriptional stimulation of rat growth hormone gene expression by thyroid hormone receptors in vitro.

The expression of the rat growth hormone (rGH) gene in the anterior pituitary gland is modulated by Pit-1/GHF-1, a pituitary-specific transcription factor, and by other more widely distributed factors, such as the thyroid hormone receptors (TRs), Sp1, and the glucocorticoid receptor. Thyroid hormone (T3)-mediated transcriptional stimulation of rGH gene expression has been extensively studied in vivo and in vitro including the measurements of (i) rGH mRNA by blot hybridization, (ii) transcriptional rate of rGH gene by nuclear run-on, and (iii) reporter gene expression in which a chimeric plasmid containing 5'-flanking sequences of the rGH gene linked to a reporter gene has been transfected either stably or transiently into pituitary and/or nonpituitary cells. From these studies, it has been suggested that the Pit-1/GHF-1 binding site is necessary for full T3 action. We developed a cell-free in vitro transcription system to examine further the roles of the TRs and Pit-1/GHF-1 in rGH gene activation. Using GH3 nuclear extract as a source of TRs and Pit-1/GHF-1, this in vitro transcription assay showed that T3 stimulation of rGH promoter activity is dependent on the addition of T3 to the GH3 nuclear extract. This transcriptional stimulation was augmented with increasing concentrations of ligand and was T3, but not T4 or reverse T3, specific. T3-mediated stimulation of rGH promoter activity was completely abolished by preincubation of the nuclear extract with rGH-thyroid hormone response element (-200 to -160) but not with Pit-1/GHF-1 (-137 to -65) oligonucleotides. Further, neither deletion of both Pit-1/GHF-1 binding sites nor mutation of the proximal Pit-1/GHF-1 binding site from the rGH promoter abrogated the T3 effect. These results provide evidence that T3-stimulated rGH promoter activity is independent of Pit-1/GHF-1 and raise the possibility that the stimulation of rGH gene expression by T3 might involve direct interaction of TRs with the general transcriptional apparatus.

A novel member of the thyroid/steroid hormone receptor family is encoded by the opposite strand of the rat c-erbA alpha transcriptional unit.

A cDNA encoding a novel member of the thyroid/steroid hormone receptor superfamily, called Rev-ErbA alpha, has been isolated from a rat GH3 cell library. Rev-ErbA alpha is an approximately 56-kilodalton protein most similar in structure to the thyroid hormone receptor (c-erbA) and the retinoic acid receptor, but it does not bind either thyroid hormone or retinoic acid. The mRNA encoding Rev-ErbA alpha is present in many tissues and is particularly abundant in skeletal muscle and brown fat. A genomic DNA fragment containing the entire Rev-ErbA alpha cDNA sequence was isolated and characterized. Remarkably, this DNA fragment also contained a portion of the c-erbA alpha gene. r-erbA alpha-1 and r-erbA alpha-2 are alternative splice products of the c-erbA alpha gene and are members of the receptor superfamily. The genes encoding Rev-ErbA alpha and r-erbA alpha-2 overlap, with their coding strands oriented opposite one another. A 269-base-pair segment of the bidirectionally transcribed region is exonic in both the Rev-ErbA alpha and r-erbA alpha-2 genes, resulting in complementary mRNAs. Thus, through alternative splicing and opposite-strand transcription, a single genomic locus codes for three different members of the thyroid/steroid hormone receptor superfamily. Potential implications of this unusual genomic arrangement are discussed.

Thyroid hormone action and brain development.

Thyroid hormone (TH) plays a crucial role in brain development. Developing rodent cerebellum might be an excellent model for studying the molecular mechanisms of TH action in the brain because perinatal hypothyroidism greatly affects its ontogeny. Although the TH-regulated genes that play crucial roles in cerebellar development have not yet been fully characterized, recent studies have provided novel insights into TH action in brain development.

New advances in understanding the molecular mechanisms of thyroid hormone action.

Thyroid hormone regulation o f gene transcription is a complex process. There are multiple thyroid hormone receptors (TRs) encoded on separate genes that bind to thyroid hormone-response elements (TREs) of target genes containing different orientation and spacing of half-sites. Additionally, there are multiple TR complexes-monomers, homodimers, and heterodimers with other related nuclear proteins-which bind to TREs and may play important roles in gene transcription. Recently, it has been shown that DNA binding of these TR complexes can be differentially regulated by either ligand or TR phosphorylation. Diversity among TR complexes and TREs, as well as mechanisms for regulating TR binding to TREs, may enable sensitive and precise transcriptional control of target genes.

Regulation of two c-erbA messenger ribonucleic acids in rat GH3 cells by thyroid hormone.

There is recent evidence suggesting that c-erbA is the thyroid hormone nuclear receptor, and that there may be multiple c-erbA genes. We investigated the effect of T3 on two c-erbA mRNAs present in GH3 cells. A partial cDNA was isolated from rat GH3 cells which is nearly identical (99.6% nucleotide identity) to rat c-erbA alpha, except for a unique 3'-region corresponding to the carboxyl terminal region of the predicted protein sequence. This cDNA (c-erbA alpha-2), like rat c-erbA alpha, hybridizes to a 2.6 kilobase (kb) mRNA which is distinct from a 6.2 kb species that hybridizes to c-erbA beta. Since nuclear T3-binding is down-regulated by T3, we hypothesized that one or both c-erbA mRNAs might be regulated by T3. GH3 cells were treated with 10 nM T3 for up to 24 h, a manipulation known to decrease nuclear T3 binding by approximately 2-fold in GH cells. Both the 6.2 kb and 2.6 kb mRNA species decreased to nearly 50% of control values at 24 h. These data indicate that these two c-erbA mRNAs are regulated by T3 and suggest that the T3 effect on T3 binding-activity in GH cells may be mediated, in part, by down-regulation of c-erbA mRNA levels.

Differential thyroid hormone-regulated rat thyrotropin beta gene expression detected by blot hybridization.

In the rat, there is a single TSH beta-subunit gene represented by three exons interrupted by two introns. This gene contains two promoters which determines the synthesis of two mRNAs with 5'-untranslated regions that differ by 43 base pairs. This study evaluates the steady state levels of these TSH beta mRNAs in various thyroidal states. Blot hybridization analyses of pituitary mRNA with synthetic probes designed to detect either one or both TSH beta mRNAs were performed. One probe corresponds to 24 bases in the 5'-untranslated region of mRNA1 and a second corresponds to 25 nucleotides in the coding region and detects both mRNA1 and mRNA2. These studies indicate the presence of TSH beta mRNA species of indistinguishable size consistent with the presence of two TSH beta mRNAs that contain slightly different 5'-untranslated regions. Comparison of pituitary RNA obtained from normal and hypothyroid rats reveals that the shorter mRNA (mRNA2) is increased approximately 6- to 8-fold with hypothyroidism while the abundance of the longer mRNA (mRNA1) is relatively unchanged. Treatment of either normal or hypothyroid animals with T3 decreases the abundance of mRNA2 while again mRNA1 is relatively unaffected. Thus, although both mRNAs are detected, only one mRNA is dramatically altered by thyroidal status. Therefore, the single rat TSH beta gene is transcribed into two mRNAs via the use of alternative promoters of which only one is markedly regulated by thyroid hormones.

Reconstitution of ligand-mediated glucocorticoid receptor activity by trans-acting functional domains.

Glucocorticoid receptors (GRs) are ligand-inducible transcription factors that contain several functional domains. We tested whether GR activity can be reconstituted using domains expressed in separate molecules. Hence, we developed a general approach in which proteins can be individually expressed but interact specifically through the leucine zippers of c-Jun and c-Fos fused to each protein. The GR was divided into two different fragments, one encoding the N-terminal trans-activation and DNA-binding domains and conferring constitutive activity to a glucocorticoid-responsive reporter gene, and one containing the C-terminal, ligand-binding domain. Coexpression of the trans-activation-DNA-binding domain and the ligand-binding domain fragments leads to reconstituted ligand-regulated GR activity that is completely dependent on the presence of compatible zippers. These results suggest that, in GRs and perhaps other members of the steroid/thyroid hormone receptor superfamily, ligand-mediated function does not require that these domains be present in cis, but that they can also function in trans. This, together with the absence of interdomain dimerization signals, also suggests that these domains possibly evolved from separate genes.

Thyroid hormones regulate rat thyrotropin beta gene promoter activity expressed in GH3 cells.

Thyroid hormones suppress the synthesis of TSH in part by decreasing the rate of alpha and TSH beta gene transcription. Cis-acting DNA sequences present in the rat TSH beta subunit gene that are induced in transcriptional regulation by thyroid hormone have been identified by deletion-mutation and transient expression studies. Plasmid expression vectors were constructed including 2900, 900, 204, 77, 17 base pairs (bp) of 5'-flanking sequence and exon (5'-untranslated sequence, transcriptional start sites) fused to the coding region of the bacterial chloramphenicol acetyltransferase (CAT) gene. The transfected chimaeric plasmids demonstrated expression (with TSH beta DNA sequences in the 5'- to -3'-but not 3'- to -5'-orientation) in both a clonal pituitary cell line, GH3, and primary pituitary cell cultures, both of which are responsive to thyroid hormones. T3 (10(-11) M to 10(-7) M) treatment of transfected cells produced a dose-dependent decrease in CAT expression with a maximal 70% decrease at 10(-8) M. While a decrease in the basal level of expression was noted with progressive removal of both 5'-flanking and intronic sequences adjacent to exon 1, the fold-decrease in response to T3 was equivalent even in the 57 bp construct. In contrast, T3 had no effect on CAT expression directed by the promoter of the herpes simplex virus thymidine kinase gene. Thus, the rat TSH beta gene 5'-flanking region can direct heterologous gene expression in GH3 cells and contains sequences which have properties of a putative cis-active T3 responsive regulatory element(s).2+he

Binding of thyroid hormone receptors to the rat thyrotropin-beta gene.

Negative regulation of rat TSH beta gene expression by thyroid hormone is mediated largely by decreased transcription of the gene. This is apparently mediated by a cis-acting element which has been localized to a 57-basepair fragment spanning the second transcriptional start site of the rat TSH beta gene. We have investigated whether thyroid hormone receptors bind specifically to DNA sequences in this region of the gene. We compared binding of native T3 receptor to the TSH beta gene sequences and to the rat GH (rGH) gene T3 response element (TRE), and examined the ability of two different forms of in vitro synthesized T3 receptor to bind to the TSH beta gene. The avidin-biotin complex DNA binding assay was used to examine sequence-specific binding of the receptor. [125I]T3-labeled receptor in GH3 cell nuclear extracts bound to a site within the first exon of TSH beta and also to a region immediately upstream of the second transcriptional start site of the gene. In addition, the Hc-erbA beta and r-erbA alpha-1 forms of the T3 receptor each bound to TSH beta and rGH sequences, demonstrating that both alpha- and beta-forms of T3 receptor can bind to TREs exerting either positive or negative transcriptional regulation. Competition experiments showed that both native and in vitro synthesized T3 receptor bound to the first exon of TSH beta with an affinity slightly less than that for the rGH TRE. The two receptor-binding sites of the rTSH beta gene show sequence similarity to adjacent regions of the rGH TRE. These data indicate that negative regulation of rat TSH beta gene transcription may be effected by direct binding of the T3-receptor complex to one or both of the binding sites flanking the second transcriptional start site.