Induction of nuclear protein factors specific for hormone-responsive region during activation of thyroglobulin gene by thyrotropin in rat thyroid FRTL-5 cells

In vitro characterization of the thyroidal uptake of O-(2-[18F]fluoroethyl)-l-tyrosine

OBJECTIVES

Positron emission tomography (PET) using O-(2-[(18)F]fluoroethyl)-l-tyrosine (FET) has been successfully employed in the diagnostic workup of brain tumors. Knowledge on the mechanisms of the uptake of radiolabeled amino acids into thyroidal tissues and well-differentiated thyroid carcinomas is limited. We therefore studied several factors potentially governing the uptake of FET in the rat thyroid cell line FRTL-5 in comparison with thyroid tumor cell lines of human origin.

METHODS

FET uptake was determined in thyroid-stimulating hormone (TSH)-stimulated and TSH-deprived FRTL-5 cells, as well as in the cell lines U-138 MG (human glioblastoma), Onco DG-1 (human papillary thyroid carcinoma) and ML-1 (human follicular thyroid carcinoma). The TSH responsiveness of cells was measured by the incubation of TSH-treated and untreated control cells with 2-[(18)F]fluoro-2-deoxyglucose (FDG). All cellular tracer uptake values were related to total protein mass and expressed as percentage per milligram. For countertransport studies, FRTL-5 cells were exposed to 10-300 microM tyrosine methyl ester. TSH-stimulated and TSH-deprived FRTL-5 cells were incubated with 100 kBq/ml FET for 20 min. 2-Aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH), alpha-(methylamino)-isobutyric acid, L-serine and tryptophan were used as competitive inhibitors of FET uptake. All inhibition experiments were repeated with the human thyroid carcinoma cell lines to obtain comparative FET uptake values.

RESULTS

The FET uptake was 155+/-30%/mg in FRTL-5 cells (n=6), 108+/-14%/mg in U-138 MG cells (n=6), 194+/-60%/mg in ML-1 cells (n=9) and 64+/-23%/mg in Onco DG-1 cells (n=6) under identical incubation conditions. Preloading with tyrosine methyl ester increased cellular FET uptake dose dependently in FRTL-5 cells (165+/-25%, n=6). While TSH increased the uptake of FDG in FRTL-5 cells by sixfold, there was no TSH effect on FET accumulation. FET uptake by TSH-treated FRTL-5 cells was sodium independent and significantly inhibited by BCH (91.4+/-3.0%, n=9), tryptophan (94.8+/-1.6%, n=8) and serine (83.2+/-10.8%, n=12). TSH-starved FRTL-5 cells had a sodium-dependent component with a similar inhibition pattern. Onco DG-1 mainly confirmed the inhibition pattern of FET uptake in FRTL-5 cells, reflecting System-L-mediated FET uptake that was blocked by BCH and serine (72-85%, n=9). ML-1 cells revealed a pronounced sodium-dependent FET uptake that was inhibited by tryptophan (70+/-10%, n=9, P<.05) in the presence and in the absence of sodium, suggesting a contribution of alternative amino acid carriers.

CONCLUSION

FET uptake by FRTL-5 cells is not TSH dependent. FET uptake by FRTL-5 cells seems to be mainly mediated by a carrier exhibiting the characteristics of the System L amino acid transporter. FET uptake in thyroid cells and thyroid carcinoma cells was in the same range as that in a glioblastoma cell line. This encourages further research efforts towards the clinical evaluation of FET for the diagnostic workup of well-differentiated thyroid carcinomas.

Feline hyperthyroidism: advances towards novel molecular therapeutics

Feline hyperthyroidism is the most common endocrine disorder of the elderly cat. Traditionally, the disease is treated by surgical thyroidectomy, medical management with antithyroid drugs or radiation therapy using iodine-131. However, none of these treatments is ideal and molecular therapeutics may offer novel methods of treating the disease. This article reviews the background of, and preliminary investigations into, the development of a transcriptionally targeted somatic gene therapy strategy for the treatment of this feline condition.

Characterization of the feline thyroglobulin promoter

The feline thyroglobulin promoter was identified by a combination of standard polymerase chain reaction (PCR) techniques, using primers designed according to regions of homology in published sequences from other species, then adaptor ligated PCR. A 310 bp fragment of the feline thyroglobulin promoter was generated, including 8 nucleotides of adaptor sequence at the 5' end and, based on the putative transcription start site, 36 nucleotides of the thyroglobulin mRNA (untranslated portion). The homology between the feline promoter sequence (from 193 bp upstream to the putative cap site) and canine, bovine and human sequences was 89%, 81% and 78%, respectively. Transient transfection studies, using reporter constructs in which the feline promoter controlled expression of chloramphenicol acetyl transferase, demonstrated promoter activity in thyroid cells, but no activity in non-thyroid cells. The data presented here demonstrate that the feline thyroglobulin promoter may provide a targeting mechanism for somatic gene therapy of feline thyroid disease.

Transcriptional control of the forkhead thyroid transcription factor TTF-2 by thyrotropin, insulin, and insulin-like growth factor I

The hormonal regulation of both thyroglobulin and thyroperoxidase promoter activity in FRTL-5 thyroid cells takes place, at least in part, through a hormone-responsive element to which the thyroid transcription factor TTF-2 binds. The TTF-2 cDNA, encoded by the titf2 locus, has recently been cloned and classified as a member of the forkhead transcription factor family. Here, we demonstrate that TTF-2 mRNA levels become undetectable in FRTL-5 thyroid cells cultured for 4 days in 0.2% serum and in the absent of thyrotropin (TSH) and insulin. Addition of TSH, insulin or insulin-like growth factor I (IGF-I) to the culture medium increases the levels of this transcription factor in a dose- and time- dependent manner and requires ongoing protein synthesis. The TSH effect is greater than that produced by insulin or IGF-I and is similar to the effect produced by the cAMP analog forskolin. The TSH and insulin effects are additive. In all cases, the mRNA levels increase is accompanied by an increase in transcription rate, as demonstrated by run-off assays. These data demonstrate that the TTF-2 mRNA is under tight hormonal control. This is consistent with an important role for TTF-2 as a mediator of the transcriptional activation of thyroid-specific genes (thyroglobulin and thyroperoxidase) by TSH via cAMP and by insulin through the IGF-I receptor.

Characterization of the 5'-flanking region of the gene for the cAMP-inducible protein kinase A subunit, RIIbeta, in Sertoli cells

Activation of cyclic AMP-dependent protein kinases (protein kinase A, PKA) by gonadotropins and cyclic AMP (cAMP) plays an important role in the regulation of testicular functions. A regulatory subunit, RIIbeta, of PKA is transcriptionally induced in rat Sertoli cells in response to treatment with cAMP. The present study addresses regulatory mechanisms leading to increased transcription of the rat RIIbeta gene. We have localized a footprint which overlaps one of the major transcription initiation sites in the basal promoter (-293 to -123). One of the proteins binding this sequence belongs to the NF-1 family of transcription factors. We also observed binding to a basic helix-loop-helix (bHLH) response element. Furthermore, transfection studies of various 5'-deletions of the rat RIIbeta gene in primary cultures of rat Sertoli cells and in peritubular cells revealed the presence of an upstream region (-723 to -395, cAMP-responsive region) inhibiting basal expression from the rat RIIbeta gene only in Sertoli cells. This region was found to enhance cAMP responsiveness in Sertoli cells but not in peritubular cells. Interactions with downstream elements seemed to be important for the function of the cAMP-responsive region. Although some short stretches reveal homology to the cAMP-responsive regions of other slowly cAMP-responding genes, and an AP-1-like element is present, no strong resemblance to any known regulatory element responsive to cAMP is found.

Quantitative analysis of rat thyroglobulin messenger RNA in FRTL-5 cells by competitive polymerase chain reaction with human thyroglobulin messenger RNA

To measure relative expression level of mRNA in a small number of cultured rat thyroid cells (FRTL-5), we developed a system of a quantitative reverse transcription-polymerase chain reaction (RT-PCR) assay. Human thyroglobulin mRNA in total RNA extracted from a human thyroid tissue was used as an internal control. FRTL-5 cells in a 24 well dish were lysed with denaturing solution containing human RNA. Total RNA was extracted followed by reverse transcription and polymerase chain reaction. After digestion with a restriction enzyme, PCR products were separated by electrophoresis and stained with Sybr Green I, then their fluorescence was measured with fluorescent image analyser. Increase of thyroglobulin mRNA in FRTL-5 cells stimulated by thyroid stimulating hormone (TSH) was observed by this technique. Because this method does not require a large number of cells or radioactive isotopes, it is as useful for the analysis of the relative expression level of mRNAs in the cells as the conservative methods such as Northern Blot.

The alpha-subunit mRNAs for Gs and Go2 are differentially regulated by protein kinase A and protein kinase C in rat Sertoli cells

In the present study, we have examined regulatory effects of protein kinase A and protein kinase C activation by 8-CPTcAMP and TPA, respectively, on mRNAs for various G protein alpha-subunits and corresponding immunoreactive proteins in rat Sertoli cells. Gs alpha and Go alpha mRNA levels were transiently increased 1.5-fold and 4-fold, respectively, by 8-CPTcAMP in cultured Sertoli cells. This up-regulation of mRNAs for Gs alpha and Go alpha was also observed when Sertoli cells were incubated in the presence of FSH. When protein synthesis was inhibited by cycloheximide, the cAMP-mediated stimulation of Gs alpha mRNA was abolished, whereas Go alpha mRNA was superinduced to a 50- to 100-fold higher level than basal. Activation of protein kinase C with TPA had a strong, synergistic effect on cAMP-mediated stimulation of Gs alpha mRNA, whereas the cAMP-mediated stimulation of Go alpha mRNA was completely blocked. Surprisingly, changes in mRNA levels were not accompanied by any alterations in the levels of immunoreactive Gs alpha and Go alpha proteins.

The thyrotropin receptor

This chapter has outlined the complex process required for thyroid growth and function. Both events are regulated by TSHR via a multiplicity of signals, with the aid of and requirement for a multiplicity of hormones that regulate the TSHR via receptor cross-talk: insulin, IGF-I, adrenergic receptors, and purinergic receptors. Cross-talk appears to regulate G-protein interactions or activities induced by TSH as well as TSHR gene expression. The TSHR structure and its mechanism of signal transduction is being rapidly unraveled in several laboratories, since the recent cloning of the receptor. In addition, the epitopes for autoantibodies against the receptor that can subvert the normal regulated synthesis and secretion of thyroid hormones, causing hyper- or hypofunction, have been defined. Studies of regulation of the TSHR minimal promotor have uncovered a better understanding of the mechanisms by which TSH regulates both growth and function of the thyroid cell. A key novel component of this phenomenon involves TSH AMP positive and negative regulation of the TSHR. Negative transcriptional regulation is a common feature of MHC class I genes in the thyroid. Subversion of negative regulation or too little negative regulation is suggested to result in autoimmune disease. Methimazole and iodide at autoregulatory levels may be important in reversing this process and returning thyroid function to normal. Their action appears to involve factors that react with the IREs on both the TSHR and the TG promoter. Too much negative regulation, as in the case of ras transformation, results in abnormal growth without function. TTF-1 is implicated as a critical autoregulatory component in both positive and negative regulation of the TSHR and appears to be the link between TSH, the TSHR, TSHR-mediated signals, TG and TPO biosynthesis, and thyroid hormone formation. Differentially regulated expression of the TSHR and TG by cAMP and insulin depend on differences in the specificity of the TTF-1 site, that is, the lack of Pax-8 interactions with the TSHR, and the IRE sites. Single-strand binding proteins will become important in determining how TSHR transcription is controlled mechanistically.

Thyrotropin regulates thyroglobulin mRNA splicing and differential processing

Rat thyroid tissue and cultured rat thyrocyte lines contain two thyroglobulin (Tg) mRNAs: a 9 kb rTg-1 mRNA encoding the 330, kDa Tg monomer and a recently described 0.95 kb rTg-2 mRNA. These transcripts have identical 5' coding sequences (641 nucleotides); however, the 3' end of rTg-2 is comprised of coding and non-coding sequences not present in rTg-1. To determine if a single Tg gene encoded both mRNA species, a genomic clone was isolated which spanned the full-length rTg-2 cDNA sequence. The promoter sequence and restriction map were the same as for the previously characterized rTg-1 gene, indicating that rTg-1 and rTg-2 mRNAs are splicing variants derived from the same Tg gene. The unique 3' end of rTg-2 mRNA comprised a single exon which was intronic with respect to rTg-1 mRNA formation. The level of rTg-2 in cultured rat thyrocytes was more sensitive to thyrotropin (TSH) regulation than was rTg-1. rTg-2 mRNA was rapidly (and reversibly) depleted to nearly undetectable levels after TSH removal, unlike rTg-1. Conversely, TSH rapidly restored control levels of rTg-2 mRNA in such depleted cells. The data thus support a model of TSH-induced splicing and regulation of the two Tg mRNAs in the rat.

Thyroid-specific and hormone-dependent expression of rat thyroglobulin promoter fused with bacterial chloramphenicol acetyltransferase gene in transgenic mice

The minimal promoter of rat thyroglobulin (TG) gene (168 bp) was fused with bacterial chloramphenicol acetyltransferase (CAT) gene, and transgenic mice carrying the TGCAT gene were produced. The minimal promoter is sufficient for thyroid-specific and hormone-dependent expression of TGCAT in transgenic mice. Deletion of a region between -128 and -92 bp (TGII), which is not required for the expression of TGCAT in transient expression assays but whose sequence is most extensively conserved among different species, appears to decrease frequency of the expression of TGCAT in transgenic mice. However, the same deletion apparently has no significant effect on TG promoter activity in stably transformed rat FRTL-5 cells.

The tissue-specific expression of the thyroglobulin gene requires interaction between thyroid-specific and ubiquitous factors

Thyroid-specific expression of the rat thyroglobulin gene is mediated by transcriptional control. Sufficient DNA sequence information to confer thyroid-specific expression to a heterologous gene is contained between positions -168 and +39. DNA-binding studies have demonstrated that this region interacts with two thyroid-specific factors (TTF-1 and TTF-2), and a ubiquitous factor (UFA). Here we have characterized three elements within the promoter, A, K, and C, which are important for promoter activity in thyroid cells. We have shown by mutational analysis that the interaction of TTF-1 with the A and C regions. UFA with the A region, and TTF-2 with the K region are required for full promoter activity. The complex interactions in the A region can be replaced by the substitution of the UFA/TTF-1-binding site with a high-affinity TTF-1 binding site. There is a correlation between the presence of TTF-1 and TTF-2 DNA-binding activities and the expression of thyroglobulin, which implies that the mechanism restricting thyroglobulin expression to thyroid cells is mediated through the control of the expression, or the activity, of TTF-1 and TTF-2.