Expression of the gene for the beta subunit of mouse thyrotropin results in multiple mRNAs differing in their 5′-untranslated regions

HPT axis‑independent TSHβ splice variant regulates the synthesis of thyroid hormone in mice

Thyroid stimulating hormone (TSH) consists of an α‑subunit and a unique β‑subunit. The first in‑frame TSHβ splice variant produced by the cells of immune system was identified in 2009. The TSHβ splice variant and native TSHβ exhibit different expression profiles, and research has been conducted to elucidate the role of the TSHβ splice variant in different diseases. However, understanding of the fundamental physiological characteristics of the TSHβ splice variant is currently limited. To verify whether the TSHβ splice variant has the potential to induce thyroid follicular cells to synthesize thyroid hormone, in vivo and in vitro stimulation experiments were conducted in the present study. A total of 60 C57BL/6 mice were divided into control‑, 5 and 10 µg TSHβ splice variant‑treated groups at random. Mice were sacrificed at 0.5, 1 and 4 h after intraperitoneal injection, and serum levels of tri‑iodothyronine (T3) and thyroxine (T4) were determined using a radioimmunoassay. Thyroid follicular cells were isolated from the thyroids of mice, and stimulated with 2 µg/ml TSHβ splice variant. Supernatants were collected, and the levels of T3 and T4 were detected. The protein expression levels of the sodium‑iodide symporter, thyroperoxidase and thyroglobulin in thyroid follicular cells were quantified using western blot analysis. To verify whether the TSHβ splice variant expression was regulated by the hypothalamus‑pituitary‑thyroid (HPT) axis, similar to native TSHβ, a total of 60 C57BL/6 mice were equally divided into control, 2 mg/kg T3 intraperitoneal injection and 0.05 mg/kg thyroid‑releasing hormone intraperitoneal injection groups at random. Mice were sacrificed at 1 and 4 h after injection. Alterations in the expression of the TSHβ splice variant in the pituitary, thyroid, peripheral blood leukocytes and spleen tissues were detected using western blot analysis. The present study demonstrated that the TSHβ splice variant is not regulated by the HPT axis and may affect thyroid hormone synthesis. Modifications in the expression of the TSHβ splice variant may occur in a uniquely regulated manner to provide peripheral immunological compartments with a source of activated cells, particularly under immune stress.

Bone marrow cells produce a novel TSHbeta splice variant that is upregulated in the thyroid following systemic virus infection

Although cells of the immune system can produce thyroid-stimulating hormone (TSH), the significance of that remains unclear. Using 5' rapid amplification of cDNA ends (RACE), we show that mouse bone marrow (BM) cells produce a novel in-frame TSHbeta splice variant generated from a portion of intron 4 with all of the coding region of exon 5, but none of exon 4. The TSHbeta splice variant gene was expressed at low levels in the pituitary, but at high levels in the BM and the thyroid, and the protein was secreted from transfected Chinese hamster ovary (CHO) cells. Immunoprecipitation identified an 8 kDa product in lysates of CHO cells transfected with the novel TSHbeta construct, and a 17 kDa product in lysates of CHO cells transfected with the native TSHbeta construct. The splice variant TSHbeta protein elicited a cAMP response from FRTL-5 thyroid follicular cells and a mouse alveolar macrophage (AM) cell line. Expression of the TSHbeta splice variant, but not the native form of TSHbeta, was significantly upregulated in the thyroid during systemic virus infection. These studies characterize the first functional splice variant of TSHbeta, which may contribute to the metabolic regulation during immunological stress, and may offer a new perspective for understanding autoimmune thyroiditis.

Cloning and sequencing of feline thyrotropin (fTSH): heterodimeric and yoked constructs

The genes encoding the mature common glycoprotein alpha (CGA) and hormone-specific beta subunits of feline thyroid stimulating hormone (fTSH) were cloned and sequenced. The feline CGA gene was cloned from RNA extracted from the feline pituitary gland by the reverse transcription polymerase chain reaction (RT-PCR). The gene fragment that encodes mature TSHbeta was cloned from feline genomic DNA after direct polymerase chain reaction (PCR). In both cases, primers were based on the consensus sequences from TSH in other species. The resulting 510 bp PCR product for the CGA-subunit included the full coding sequence for the 96 amino acid mature subunit preceded by a 24 amino acid signal peptide. The 850 bp sequence of fTSHbeta genomic DNA consisted of two coding exons, an intron of 418 bp, and a 60 bp signal sequence. The octapeptide immunoaffinity tag FLAG was added to 3' end of the alpha gene to facilitate detection and purification. Both genes were cloned independently downstream from the EF1alpha promoter of the PEAK transfer vector to facilitate co-expression studies in PEAK cells (modified human embryonic kidney (HEK) cells). A single-chain analogue of fTSH termed yoked fTSH (yfTSH) was developed by fusing the nucleotides encoding the C-terminus of the beta-subunit fused to the N-terminus of the alpha-subunit with DNA encoding the C-terminal peptide (CTP) of human chorionic gonadotropin beta-subunit as a linker peptide. The resulting single-chain analogue encoded from N-terminus to C-terminus: beta-CTP-alpha-FLAG. The resulting DNA sequence was cloned, sequenced, ligated and recloned into expression vector PEAK. This report constitutes the first cloning and sequencing of the genes encoding the subunits of feline thyrotropin.

Characterization of novel anti-mouse thyrotropin monoclonal antibodies

Mouse hybridoma cell lines have been raised to a peptide of the mouse thyrotropin-beta (thyroid stimulating hormone [TSH]beta) subunit of the TSH glycoprotein hormone molecule. Using an enzyme-linked immunosorbent assay (ELISA) capture technique with two anti-TSHbeta monoclonal antibodies (MAbs), these reagents were found to have strong reactivity in a titration-dependent manner against normal mouse serum, and to precipitate under reducing conditions a 13-kDa product, the correct molecular size of the TSHbeta component, from mouse sera. These MAbs had minimal reactivity in ELISA to bovine and rat TSH and slight reactivity to human TSH, demonstrating overall strong species specificity for mouse TSH. Due to the large number of inbred and genetically manipulated mice now available for experimental research, it is anticipated that these reagents will be of considerable value for in-depth studies into TSH-related physiological processes in ways that have not been feasible here-to-fore.

Canine thyrotropin beta-subunit gene: cloning and expression in Escherichia coli, generation of monoclonal antibodies, and transient expression in the Chinese hamster ovary cells

The gene encoding the mature beta subunit of canine thyroid stimulating hormone (cTSH beta) was cloned, sequenced and expressed in Escherichia coli and in Chinese hamster ovary (CHO) cells, and monoclonal antibodies against the recombinant cTSH beta purified from E. coli were generated. The gene fragment that encodes mature TSH beta was cloned from the canine genomic DNA by direct polymerase chain reaction (PCR) using primers that were designed based on the consensus sequences from other species. The resulting 891 basepairs (bp) of genomic DNA consisted of two coding exons of the canine TSH beta gene and an intron of 450 bp. The two exons, which encode the mature cTSH beta subunit, was joined together by an overlap PCR and was expressed in E. coli as 6xHis-tagged protein. The purified recombinant cTSH beta with a molecular weight of about 15 kDa was recognized by the polyclonal antibodies prepared against the native canine TSH in Western blot. Monoclonal antibodies were raised against the purified cTSH beta and subsequently characterized. For transient expression in CHO cells that are permanently transfected with the bovine common alpha gene, a 60-oligonucleotide signal peptide coding sequence was added to the 5' end of the cTSH beta gene before it was cloned into the mammalian expression vector pRSV and used to transfect CHO cells. The medium from these transfected cells, presumably containing the bovine alpha and canine TSH beta in heterodimeric confirmation, exhibited TSH bioactivity as indicated by the stimulation of cAMP production in the cultured FRTL-5 thyrocytes.

cDNA cloning of canine common alpha gene and its co-expression with canine thyrotropin beta gene in baculovirus expression system

The common alpha gene of the canine glycoprotein hormones was cloned, sequenced and co-expressed with the canine thyrotropin beta (TSH beta) gene in the baculovirus expression system, and a bioactive recombinant canine TSH was purified. The canine common alpha gene was cloned from the total RNA extracted from the canine pituitary gland by the reverse transcription polymerase chain reaction (RT-PCR) using primers that were designed based on the consensus sequences from other species. The resulting 476 bp PCR product is consisted of the full coding sequence for the 96 amino acid mature alpha subunit, and a sequence encoding a 24 amino acid signal peptide. Homology analysis with other species revealed that the canine common alpha subunit potentially contains five disulfide bonds and two oligosaccharide chains N-linked to Asn residues located at positions 56 and 82. For expression in the baculovirus expression system, the common alpha gene was cloned downstream of the p10 promoter of the pAcUW51 transfer vector, and the previously cloned canine TSH beta gene was inserted under the polyhedrin promoter of the same vector. The recombinant virus containing both alpha and beta genes was generated and propagated before being used to transfect the Sf9 insect cells for expression. The medium from the Sf9 cultures, presumably containing canine TSH alpha and beta in native heterodimer confirmation, exhibited TSH bioactivity as indicated in the cAMP stimulation assay in FRTL-5 cells. The expressed recombinant protein was purified from the culture medium with an affinity column that was coupled with IgG purified from the polyclonal antibodies generated against the partially purified native canine TSH.

Thyrotropin expression in hypophyseal pars tuberalis-specific cells is 3,5,3'-triiodothyronine, thyrotropin-releasing hormone, and pit-1 independent

The expression of TSH subunit genes (TSH alpha and -beta) in pituitary thyrotropes is primarily regulated via circulating thyroid hormone levels (T3) and the hypothalamic TRH. Hypophyseal pars tuberalis (PT)-specific cells also express both hormonal subunits of TSH, but do not resemble thyrotropes of the pars distalis (PD) with respect to their distinct morphology, secretion, and direct modulation of TSH expression by photoperiodic inputs and melatonin. To investigate whether this distinct regulation of TSH is related to a different molecular structure or different signaling cascades, we analyzed PT-specific TSH and its transcriptional regulation in ovine PT-specific cells. After construction of PT- and PD-specific complementary DNA (cDNA) libraries, the cloning and sequencing of several TSH alpha and -beta subunit clones revealed identical sizes and sequences for the translated and untranslated regions in both hypophyseal compartments. Transcription start site analysis also displayed three identical start sites for the transcription of TSH beta in PT and PD. After cloning of the ovine TRH receptor cDNA and a partial T3 receptor cDNA, in situ hybridization. Northern blot analysis, and PCR experiments showed that TRH and T3 receptors are not expressed in specific cells of the PT. The transcription factor Pit-1 that is involved in TSH expression of thyrotropes could only be detected in the PD. In additional experiments rats were treated with T4 or TRH, and subsequent in situ hybridization studies showed that TSH beta messenger RNA (mRNA) formation was not altered in the PT. In the PD, however, TSH beta mRNA was significantly reduced in the T4-treated group, but was enhanced in the TRH-treated group. We conclude that PT-specific cells of the pituitary are characterized by the transcription of TSH subunits that are identical to TSH expressed in thyrotropes of the PD. The absence of TRH, T3 receptor mRNA, and Pit-1, respectively, as well as the different reactions compared to PD thyrotropes in in vivo experiments lead to the conclusion that the expression of TSH in PT-specific cells of the pituitary is not regulated via the classical thyrotrope receptors and their intracellular pathways, but through a novel, photoperiod-dependent mechanism.

Thyroid hormone regulation of thyrotropin gene expression

Thyroid hormones suppress the synthesis and release of thyrotropin from thyrotropes in the anterior pituitary gland, a feature that is critical in the classic negative-feedback loop of the pituitary-thyroid endocrine axis. The major effect of thyroid hormones in this system is exerted at the transcriptional level. The molecular mechanisms by which there is negative regulation of TSH subunit gene expression by thyroid hormone have been elucidated. The TSH subunit genes have isolated and characterized. Structure-function analyses using fusion genes and DNA transfection approaches have defined the putative negative TREs among the promoters of the rat, mouse, and human alpha and TSH beta genes. These sequences are either largely overlapping direct TRE half-sites, TRE half-sites as direct repeats gapped by two nucleotides, or single TRE half-sites. These arrangements are distinct from those seen in positive TREs. Recent knowledge regarding the molecular mechanisms of thyroid action in general forces consideration of multiple TR isoforms, TR heterodimer partners (TRAPs), and thyroid hormones in the ultimate mechanisms of negative action. Several models have been proposed, but none has yet been proved. In addition, the role of thyroid hormone in the regulation of gene expression at the posttranscriptional level is beginning to be addressed. Future work should continue to illuminate these important facets of gene regulation.

Thyroid hormone regulates expression of the thyrotropin beta-subunit gene from both transcription start sites in the mouse and rat

Thyroid hormones suppress transcription of the gene for the beta-subunit of thyrotropin (TSH beta). Since the TSH beta gene in both the mouse and the rat contains two start sites of transcription in exon 1, we have investigated whether expression of the gene from each start site is differentially regulated by thyroid hormones in each species. RNase protection analysis was used to assay the levels of mRNA specifically transcribed from the upstream (TSS 1) and downstream (TSS 2) transcription start sites in the mouse and rat pituitary. In euthyroid and hypothyroid pituitaries there was an approximately 5-fold and 2-fold greater abundance of mRNA derived from TSS 2 than TSS 1, respectively. Hypothyroidism induced an 18- and a 9-fold increase in TSH beta gene expression from TSS 1 and TSS 2, respectively. Treatment of hypothyroid animals for 1 day with triiodothyronine (T3) reduced expression from both start sites by about 50%; after 4 days of T3 treatment, TSH beta mRNAs derived from both start sites were below detectable levels. These results were confirmed in the rat by primer extension analysis. Expression from TSS 1 in the mouse was also shown to be dependent on thyroid status using the polymerase chain reaction (PCR) technique. In contrast to previous results from primer extension studies, PCR analysis demonstrated that alternative splicing of the TSH beta RNA primary transcript can occur when transcription is initiated at the upstream start site. We conclude that, in both the mouse and the rat pituitary, expression of the TSH beta gene from both transcription start sites is regulated by thyroid hormones.

TSH subunit gene promoters from a murine alpha-subunit producing tumor function normally

The murine thyrotropic MGH101A tumor is characterized by absent thyrotropin (TSH) beta gene expression and altered thyroid hormone (T3) regulation of the alpha-subunit. Comparison of the promoter structures of both alpha and TSH beta subunit genes from MGH101A with the promoter in expressing TtT-97 thyrotropes revealed no detectable differences. Transfection of the TSH beta promoter from MGH101A linked to luciferase showed minimal expression in primary or cloned MGH101A cells, or L-cells. However, a 6- to 10-fold increase in expression was exhibited in transfected thyrotropes. For the alpha gene, promoter activity was highest in thyrotropes and in cloned MGH101A cells, 5-fold lower in MGH101A tumors, and 10-fold lower in L-cells. Both promoters were not substantially affected by T3 treatment in MGH101A cells. In thyrotropes, promoter activity was inhibited 62.5% and 57.7% by 10 nM T3 treatment for the TSH beta and alpha genes, respectively. DNase I protection showed that factors from TtT-97 but not from MGH101A cells interacted with regions in the TSH beta promoter, while nuclear extracts from each tumor demonstrated at least one protein-DNA interaction with the alpha-subunit promoter. These studies suggest that the molecular defects in the MGH101A tumor are related to the absence of trans-acting factors and are not a result of altered primary gene structure.

Isolation and characterization of the gene encoding the beta-subunit of rat follicle-stimulating hormone

Follicle-stimulating hormone (FSH) is a pituitary glycoprotein hormone that is comprised of two dissimilar subunits, alpha and beta, encoded by separate genes. We have isolated the gene encoding the beta-subunit of rat FSH by screening a rat genomic DNA-library by filter hybridization with bovine FSH-beta cDNA. Southern blot analysis of rat genomic DNA suggests that there is a single copy of the FSH-beta gene per haploid genome in the rat. The nucleotide sequence of the rat FSH-beta gene was determined and the amino acid sequence of the subunit was deduced. The gene is composed of three exons and two introns. The predicted amino acid sequence reveals that there is a 20-amino-acid signal peptide followed by a mature protein of 110 amino acid residues. Exons I, II, and III are 36, 187, and 1221 bp, respectively. Intron 1 (640 bp) interrupts the 5'-untranslated (UT) region (61 bp) and intron 2 (approximately 1 kb) interrupts the coding region between amino acid residues +34 and +35. Comparison of the amino acid sequence to those of the human and bovine FSH-beta subunits reveals 80% similarity to both species. The "CAGY" sequence, or the Cys-Ala-Gly-Tyr quartet of amino acids encoded by exon II is present in every other glycoprotein beta-subunit sequenced thus far, is altered in rat FSH-beta, with the Ala residue replaced by Glu. Primer extension analysis demonstrated that there is a single transcriptional start site. The nucleotide sequence of the 5'-flanking region (1 kb) was determined and compared to the nucleotide sequences of corresponding regions in the bovine and human genes. This analysis revealed that there are three regions in the 5'-flanking region of the rat FSH-beta gene that display greater than 80% sequence similarity to regions in the bovine and human genes. The second of these regions also shares similarity to segments within the 5'-flanking regions of the rat alpha- and LH beta-subunit genes. The characterization of the rat FSH-beta gene will enable further study of the regulation of rat FSH using in vitro systems.

Molecular cloning of cDNAs for precursors of porcine pituitary glycoprotein hormone common alpha-subunit and of thyroid stimulating hormone beta-subunit

cDNA clones encoding precursors of glycoprotein hormone common alpha-subunit (pre-alpha) and of thyroid stimulating hormone beta-subunit (pre-TSH beta) were isolated from a porcine anterior pituitary cDNA library using DNA probes, and the nucleotide sequences were determined. The nucleotide sequence of pre-alpha cDNA contained an entire coding region (360 bases) including 5' and 3' untranslated regions. The pre-alpha mRNA was about 900 bases long. The predicted amino acid sequence consisted of a signal peptide of 24 amino acid residues and a mature alpha-subunit protein of 96 residues. Six amino acid residues at the amino terminus of the predicted mature protein had not been found by direct amino acid sequencing of the purified protein. The nucleotide sequence of pre-TSH beta cDNA contained an entire coding region and a 3' untranslated region which has two polyadenylation signals. The length of the pre-TSH beta mRNA was about 500 bases long. The predicted amino acid sequence consisted of a signal peptide of 20 amino acid residues, a mature protein of 112 residues and an additional extension of six amino acid residues at the carboxyl terminus, which had not been found in the amino acid sequence of the purified protein. The coding sequences of the cDNAs showed high homologies with those of other mammalian species (84-93% for pre-alpha and 81-94% for pre-TSH beta). Comprehensive data of our serial molecular cloning for porcine glycoprotein hormones revealed low but significant homologies (34-40%) among three beta-subunits. Upon comparison of frequency of (U)n A sequence in 3' untranslated region, porcine pre-alpha and pre-TSH beta mRNAs were grouped into a moderate class of mRNA stability whereas porcine pre-FSH beta and pre-LH beta were grouped into unstable and stable classes, respectively.

The structure of the human thyrotropin beta-subunit gene

We have determined and characterized the entire structure of the human thyrotropin beta-subunit (TSH beta) gene. It is 4.5 kb in size and separated into three exons. Primer extension analysis showed one transcription start point, and S1 mapping analysis showed two blocks of polyadenylation sites in normal pituitaries. The same transcripts were observed in TSH-producing adenomas. The 5'-flanking region contains two 'TATA' boxes, two 'CAAT' boxes, and two sequences similar to the cyclic AMP-responsive elements.

The human thyrotropin beta-subunit gene differs in 5' structure from murine TSH-beta genes

The gene encoding the beta-subunit of human thyrotropin (hTSH-beta) was isolated, and its nucleotide sequence was determined. The gene is 4.3 kb in length, consists of three exons and two introns, and is present as a single copy as determined by Southern blot analysis of total genomic DNA. The protein coding portion of the gene, which includes exons 2 and 3, was isolated from a human genomic phage library, while exon 1, which encodes only 5' untranslated mRNA sequence, was isolated from a plasmid library of size-selected genomic DNA fragments. Here we describe the isolation of the 5' untranslated exon of the hTSH-beta subunit and 5'-flanking region. The structure of the hTSH-beta gene is very similar to the previously characterized TSH-beta genes from mouse and rat. The genes from all three species have two distinct promoter regions, but while both promoters are utilized by the murine TSH-beta genes, the human TSH-beta gene apparently utilizes only the proximal promoter for transcription initiation. A striking difference in hTSH-beta gene structure compared to the murine genes is that exon 1 of the human gene is 36 nucleotides. An analysis of the mouse, rat, and human exon 1 and 5'-flanking region shows a high percentage of sequence homology, with the exception of a 9-nucleotide insertion 13 bases 3' from the proximal TATA box found in the human gene but not found in the other two species. We propose that this insertion results in the additional length of human exon 1 compared to the mouse and rat genes. By isolating the promoter region of the hTSH-beta gene, we can begin to identify specific sequences involved in the regulation of hTSH gene expression.

Organization and nucleotide sequence of the mouse alpha-subunit gene of the pituitary glycoprotein hormones

We have isolated and determined the nucleotide sequence of the mouse alpha-subunit gene of the pituitary glycoprotein hormones. No detectable rearrangements were observed from the cloned gene when compared with total genomic DNA. The gene is approximately 13.5 kb in size and consists of 4 exons and 3 introns. The 5' untranslated region is encoded by exon 1 and 14 bp of exon 2. A large first intron of about 10 kb interrupts the 5' untranslated region. The coding region is present in exons 2, 3, and 4, while the 3' untranslated region is contained entirely within exon 4. A number of mouse B1 and B2 repeats are present in the 5'-flanking region, the first and third introns, as well as in the 3'-flanking region. In contrast to the mouse beta-subunit gene of thyrotropin, primer extension analysis revealed that the alpha-subunit gene has a single transcriptional start site and that the primary transcript does not exhibit alternative exon splicing. The transcriptional start site is identical in both mouse pituitaries and in TtT97 thyrotropic tumors and is responsive to thyroidal status. The 5'-flanking region contains a sequence that is homologous to a single copy of the duplicated 18-bp cyclic AMP-responsive element of the human alpha-subunit gene. Sequences homologous to putative thyroid-responsive and estrogen responsive elements are present in the 5'-flanking region and may be important in the multihormonal regulation of the alpha-subunit gene in throtrophs and gonadotrophs, respectively.

Transcriptional activation of lck by retrovirus promoter insertion between two lymphoid-specific promoters

p56lck, a member of the src family of cytoplasmic tyrosine protein kinases, is expressed primarily in lymphoid cells. Previous RNase protection data demonstrated the existence of at least two lck mRNAs (type I and type II) with different 5' untranslated regions in most T cells. These have been found here to arise from two separate promoters. S1 nuclease analysis and primer extension were used to locate the site of initiation of type I lck mRNA. The nucleotide sequence of the region upstream of this start site contains no classical promoter motifs. A cDNA clone of type II lck mRNA was isolated. The promoter of this mRNA must be more than 10 kilobases upstream of the type I promoter region. In two murine thymoma cell lines, LSTRA and Thy19, lck is expressed at elevated levels as a result of Moloney murine leukemia virus retrovirus promoter insertion. p56lck is encoded in these cells by a hybrid virus-lck mRNA containing the 5' untranslated region of Moloney virus mRNA. The structures and the sites of integration of the proviruses upstream of lck in these cells were examined by molecular cloning and Southern analysis. A truncated and rearranged provirus, flanked by 554 nucleotides (nt) of duplicated cellular sequences, was found 962 nt upstream of the start site for type I lck mRNA in LSTRA cells. What appears to be a Moloney mink cytopathic focus-forming provirus was found between 584 to 794 nt upstream of the start site for type I lck mRNA in Thy19 cells. Thus in both tumor cell lines, viral DNA is present between the promoters for type I and type II lck mRNAs. Comparison of the sequences of the 5' ends of the lck and c-src genes suggests that divergence of these two genes involved exon shuffling and that a homolog of the neuronal c-src(+) exon is not present in lck.