Biomedical subjects

V M Nikodem

Publications and source records attributed to V M Nikodem.

At least 37 records · Page 2Linked to original sources

PubMed1994

Recombinant thyroid hormone receptor and retinoid X receptor stimulate ligand-dependent transcription in vitro.

The thyroid hormone and retinoid X receptors form a heterodimer with each other and mediate thyroid hormone (T3)-dependent transcription. Retinoid X receptor, in addition, forms a homodimer and mediates 9-cis-retinoic acid-dependent transcription. Here, recombinant thyroid hormone receptor and recombinant retinoid X receptor beta expressed from baculovirus vectors have been studied for ligand-mediated activation of transcription in vitro. We show that the two recombinant receptors, most likely as a heterodimer, cooperatively enhance transcription in vitro from a template containing functional T3 responsive elements. The enhancement was specific for the T3 responsive element and was greatest when T3 was added to the reaction (approximately 14-fold increase). Albeit to a lesser degree, the two receptors also directed transcription in the absence of T3. Template competition experiments suggest that the two receptors enhance formation of the preinitiation complex and that activation by T3 occurs when the ligand binds the receptor prior to (or during), but not after, the formation of the preinitiation complex. Although 9-cis-retinoic acid had no effect on the T3-dependent transcription, this ligand activated transcription in vitro directed by recombinant retinoic X receptor beta, most likely as a homodimer. This activation was observed when using nuclear extracts from embryonal carcinoma cells as a source of basal transcription factors, but not those from B lymphocytes. These results demonstrate that transcriptional activation mediated by T3 and 9-cis-retinoic acid can be reconstituted in vitro with the respective recombinant receptors.

Animals↗

PubMed1994

Developmental regulation of the thyroid hormone receptor alpha 1 mRNA expression in the rat testis.

The multiplicity of thyroid hormone (TH) effects appears to be mediated by two TH receptors (THRs) encoded by two genes, alpha and beta, and, perhaps, by their various isoforms. The expression of THR beta is correlated with the presence of high affinity binding sites for TH, and all the mutations which cause the syndrome of generalized thyroid hormone resistance occur in THR beta. The function of THR alpha has not been clearly defined as yet. Another enigma in TH action is the effect on the testis. It has been shown that the testis of the adult rat does not respond to TH as measured by an increase in oxygen consumption. Furthermore, it has not been possible to demonstrate the presence of a nuclear high affinity binding site for TH in adult testis. To resolve these problems were measured the levels of THR alpha, its nonhormone binding variant, and THR beta mRNA in the testis at various stages of development. We discovered that the beta-message is absent at all times, whereas the alpha-message is expressed only from fetal through prepubertal stages and is absent in adult testis. THR alpha, but not the beta-mRNA, was detected in immature Sertoli cells in culture, and neither was found in adult Sertoli cell-enriched cultures. Furthermore, THR alpha and its variant mRNA was found, using in situ hybridization, in the seminiferous cords and seminiferous tubules of fetal and prepubertal testis.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Divergent effects of 9-cis-retinoic acid receptor on positive and negative thyroid hormone receptor-dependent gene expression.

The thyroid hormone (TH)-inducible expression of some genes has recently been shown to be enhanced by 9-cis-retinoic acid (9-cis-RA) receptor (RXR). This effect appears to be at least partially elicited by the ability of RXR to heterodimerize with TH receptor (THR) and enhance its binding to the cis-acting thyroid hormone responsive elements (TREs) found within those genes. However, whether RXR beta enhances TH/THR-mediated transactivation of all Tre-containing genes, and if RXR has any effect on TH-dependent negative regulation are not known. In the present study, we show that the TH/THR-inducible expression of the myelin basic protein (MBP) gene is not enhanced by RXR beta, despite high affinity binding of the RXR beta.THR alpha heterodimer to the MBP-TRE. We also demonstrate that RXR beta reverses the TH/THR-dependent down-regulation mediated by the negative TRE found within the promoter of the mouse thyroid stimulating hormone gene (TSH). The ligand for RXR beta (9-cis-RA), either alone or in combination with TH, did not enhance the transcription mediated by either the MBP-TRE, TSH-TRE, or the malic enzyme (ME)-TRE. However, the ME-TRE is known to confer RXR beta-dependent enhancement of TH-induced gene expression. Thus, the capacity of RXR beta to modulate TH-dependent transcriptional regulation depends upon the nature of the TRE.

3T3 Cells↗

PubMed1992

DNA binding and heteromerization of the Drosophila transcription factor chorion factor 1/ultraspiracle.

The Drosophila chorion factor 1/ultraspiracle (CF1/USP) transcription factor, a homologue of the retinoid X receptor, is a developmentally important member of the family of nuclear (steroid) hormone receptors. Using newly developed monoclonal antibodies and a full-length bacterially produced protein, we have studied in detail the in vitro DNA-binding properties of this factor and aspects of its distribution in vivo. During oogenesis, CF1/USP is present both in germline cells and in the somatic follicular epithelium. We have determined the optimal binding site of partially purified bacterially produced CF1/USP by an in vitro selection procedure and also have characterized its binding to the follicular-specific chorion s15 promoter. In vitro this bacterially produced factor is unusual in binding to a single element ("half-site"); simultaneous but noncoordinate binding to a second half-site is possible if these repeated elements are organized in direct orientation and spaced adequately. However, the factor interacts synergistically with several other nuclear hormone receptors: notably, it can form in vitro heteromers with mammalian thyroid and retinoic acid receptors, binding to two half-sites that are organized in either direct or inverted orientation. In vivo the factor most probably functions as a heterodimer, but its partner(s) remains to be determined.

Animals↗

PubMed1992

Characterization of myelin basic protein thyroid hormone response element and its function in the context of native and heterologous promoter.

In this report we have characterized further the myelin basic protein (MBP) gene thyroid hormone response element (TRE) by functional and binding analysis. Mutation and deletion experiments revealed that this TRE, confined to the sequences -184 to -167 of the MBP promoter, is able to function as a classical regulatory element in the context of the native and a heterologous promoter. It is comprised of two regions, containing a motif that is highly conserved among other TREs: AGGACA, arranged as an inverted palindrome. Any mutation within the footprinted region impaired receptor binding and function. Moreover, the deletion of sequences outside of the receptor footprinted region (MBP-TRE-18) resulted in a higher triiodothyronine responsiveness and a concomitant increase in receptor-dependent, hormone-independent repression. Results of transfection assays showed that both receptors alpha and beta elicit indistinguishable triiodothyronine responses when the MBP-TRE functions as a regulator of a heterologous promoter activity. However, a preferential beta receptor transactivation was observed when the MBP-TRE was placed in the context of its native promoter.

3T3 Cells↗

PubMed1992

Heterodimerization of thyroid hormone (TH) receptor with H-2RIIBP (RXR beta) enhances DNA binding and TH-dependent transcriptional activation.

Steroid/TH receptors mediate transcriptional induction of promoters containing hormone response elements (HREs) through an unclear mechanism that involves receptor binding to both hormone and a HRE. Here we demonstrate that both HRE binding and the transcriptional inducing activities of one member of this family, TH receptor, were markedly enhanced by heterodimerization with H-2RIIBP, a non-TH-binding member of the steroid hormone receptor superfamily. H-2RIIBP, the mouse homologue of human retinoic acid-related receptor, was shown to form stable heterodimers with the TH receptor either in solution or when bound to a TH response element. The results presented indicate that it might be necessary for the TH receptor or other members of this superfamily to have specific partners for heterodimer formation to elicit maximal hormone-specific gene regulation from particular HREs.

Animals↗

PubMed1992

Developmental expression of mRNAs from a rat C-erbA genomic locus.

The thyroid hormone receptor alpha gene is alternatively spliced to give the alpha receptor and a variant which does not bind thyroid hormones. An additional alpha-like receptor (Rev-erbA alpha) is transcribed on the opposite strand and overlaps the unique variant sequence. The alpha receptor in rat brain reaches peak levels 5-15 days after birth. The variant remains high from 2 days before birth to old age. The Rev-erbA alpha mRNA increases beginning 5 days after birth, maximizing in adulthood. Thyroid hormone decreases and hypothyroidism increases the level of these 3 messages. We conclude that the levels of these messages are similarly controlled by thyroid hormone but have different ontogenetic controls. No functional results can be seen in vivo from potential anti sense pairing of Rev-erbA alpha with the variant.

Aging↗

PubMed1992

H-2RIIBP (RXR beta) heterodimerization provides a mechanism for combinatorial diversity in the regulation of retinoic acid and thyroid hormone responsive genes.

H-2RIIBP (RXR beta) is a member of the nuclear hormone receptor superfamily that activates transcription of MHC class I genes in response to retinoic acid (RA). Using chemical cross-linking, co-immunoprecipitation, gel mobility shift and streptavidin-biotin DNA precipitation assays, we show that H-2RIIBP formed heterodimers with thyroid hormone (T3) and RA receptors (T3R alpha and RAR alpha). H-2RIIBP heterodimer formation required a conserved sub-domain of its C-terminal region, occurred independently of target DNA and was much more efficient than either T3R alpha/RAR alpha heterodimer or H-2RIIBP homodimer formation. Heterodimers displayed enhanced binding to target DNA elements and contacted DNA in a manner distinct from that of homodimers. A functional role for heterodimers in vivo was demonstrated by synergistic enhancement of MHC class I transcription following co-transfection of H-2RIIBP with T3R alpha or RAR alpha. We provide biochemical evidence that H-2RIIBP formed heterodimers with several naturally occurring nuclear proteins. The results suggest that H-2RIIBP, by virtue of its ability to heterodimerize, enhances combinatorial diversity and versatility in gene regulation mediated by nuclear hormone receptors.

Amino Acid Sequence↗

PubMed1991

Molecular basis of thyroid hormone regulation of myelin basic protein gene expression in rodent brain.

Regulation of myelin basic protein (MBP) gene expression by thyroid hormone has been investigated in rodent brain. Quantitation of the 4 major alternatively spliced transcripts by RNase protection assay showed that the individual mRNAs, corresponding to MBP isoforms 21.5, 18.5, 17, and 14 kDa, were decreased from 2- to 17-fold at all ages studied (4-60 days) in hypothyroid animals when compared to euthyroid, but the timing of onset of expression was not altered. MBP mRNA was also reduced in young adult rats thyroidectomized at the age of 5-6 weeks and was restored to normal by thyroxine administration. Nuclear run-off assays showed that the rate of MBP gene transcription is dependent on thyroid state. Co-transfection of MBP (-256/+1)-chloramphenicol acetyltransferase chimeric gene with a plasmid expressing thyroid hormone receptor alpha, and in the presence of 3,5,3'-triiodothyronine, into NIH3T3 or NG108-15, increased chloramphenicol acetyltransferase expression 4-fold. Using a footprinting technique and Spodoptera frugiperda 9 (Sf9) nuclear extract infected with baculovirus expressing TR alpha, we have identified a single DNA-binding site (-186/-163) for the receptor. A part of this region contains the AGGACA sequence found in thyroid hormone-responsive elements of other 3,5,3'-triiodothyronine-regulated genes. Our finding of a specific hormone-receptor interaction with the MBP promoter region is the first direct demonstration of a thyroid hormone-responsive element in a brain-specific gene.

Animals↗

PubMed1991

Functional characterization and receptor binding studies of the malic enzyme thyroid hormone response element.

We previously showed that the 5'-flanking region of the malic enzyme (ME) gene contains a cis-regulatory element (-281 to -261) that binds thyroid hormone receptors and confers triiodothyronine (T3) inducibility of transcription to the ME promoter (Petty, K.J., Desvergne, B., Mitsuhashi, T., and Nikodem, V. M. (1990) J. Biol. Chem. 265, 7395-7400). In this report, we have used deletion and mutation analyses of the ME thyroid hormone response element (TRE) to evaluate the roles of several subregions of TRE in T3 binding and transactivation. ME TRE was shown to act as an enhancer conferring T3 responsiveness to a heterologous promoter thymidine kinase. Although T3 treatment induced the promoter activity, the absence of hormone resulted in repression as measured by the level of chloramphenicol acetyltransferase expression in the NIH 3T3 transient expression system in the presence of overexpressed receptor. The degree of repression was similar to the degree of T3 induction observed for the same TRE mutants. Mutation and deletion analyses indicated that the functional TRE is comprised of discrete regions that are not contiguous, with a dominant role of a cluster of G residues and an AGGACA sequence. Both functions, induction and repression of transcription, correlated with receptor binding to the ME TRE as determined by competition binding assays using wild type and mutated TRE as competitors.

Animals↗

PubMed1990

Identification of a thyroid hormone response element in the malic enzyme gene.

We have studied the rat malic enzyme gene as a model for thyroid hormone (triiodothyronine (T3)) regulation of transcription. Our previous studies showed that transcription of this gene is controlled by T3 in vivo. In this study, we used COS-7 cells in culture to determine the location of a T3 response element (TRE) within this gene. Cotransfection of the rat T3 alpha-receptor cDNA with a reporter gene linked to 5'-deletion mutants of the malic enzyme gene 5'-flanking region revealed the presence of a TRE between positions -315 and -248. Using T3 receptors synthesized in vitro from their cDNAs, we have identified, through the use of gel shift assays and footprinting, a single DNA-binding site (positions -281 to -261) for the receptor within the rat malic enzyme gene TRE. The site was capable of binding either the rat alpha- or human beta-receptor with similar affinities. Competition binding studies indicated that the apparent affinity of receptor binding to the malic enzyme gene was similar to that of the rat alpha-myosin heavy chain gene TRE (positions -151 to -122) and was significantly greater than that of the rat growth hormone TRE (positions -192 to -163). These results indicate that both the alpha- and beta-forms of the nuclear T3 receptor are capable of binding directly to the malic enzyme gene 5'-flanking region at a site which functions as a hormone-inducible cis-regulatory element. In conjunction with our previous finding that T3 induces or activates essential transcription factors which bind to the promoter, it appears that the regulation of transcription of the malic enzyme gene by thyroid hormone involves at least two regulatory pathways.

Animals↗

PubMed1990

Thyrotropin regulation of malic enzyme in FRTL-5 rat thyroid cells.

TSH-induced increases in malic enzyme mRNA levels in FRTL-5 rat thyroid cells are paralleled by increases in malic enzyme activity and are mimicked by 8-bromo-cAMP. Apparent approximately 4 h after TSH challenge and maximal after 16 h, they decline by 24 h and are at basal levels by 48 h. The increase occurs in the absence of a measurable effect of TSH on DNA synthesis related to cell growth, since [3H] thymidine incorporation into DNA is still at basal levels 24 h after TSH challenge and is maximal only at 48 h. A protein(s) whose formation is inhibited by cycloheximide appears to be critical to the ability of TSH to increase malic enzyme mRNA levels. Thus, cycloheximide given 30 min before TSH prevents the hormone-induced increase in malic enzyme mRNA; also, when given 24 h after TSH, cycloheximide accelerates the loss of the TSH-induced increase in malic enzyme mRNA. In neither case does cycloheximide affect beta-actin mRNA levels. A second factor(s) whose formation is prevented by actinomycin-D appears to be important for the decrease in malic enzyme mRNA levels seen 24 and 48 h after TSH challenge. Thus, in experiments in which it is given 24 h after TSH, actinomycin-D preserves the hormone-induced increase in malic enzyme mRNA levels rather than accelerating the decrease, as does cycloheximide. In the same experiment, beta-actin mRNA levels decrease to less than 10-20% of control values over the same period; this factor also, therefore, appears to exhibit some degree of specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1989

Thyroid hormone-mediated transcriptional activation of the rat liver malic enzyme gene by dehydroepiandrosterone.

Dehydroepiandrosterone (DHEA), a naturally occurring steroid secreted from the adrenal, has been reported to decrease the body weight gain in rodents without suppressing food intake and to stimulate malic enzyme activity in liver (Tepperman, H. M., de la Garza, S. A., and Tepperman, J. (1968) Am. J. Physiol. 214, 1126-1132). Herrin, we demonstrate that DHEA induces hepatic malic enzyme activity by increasing the rate of transcription of the malic enzyme gene. This transcriptional activation of the malic enzyme gene is dose dependent, i.e. the treatment of euthyroid male rats with daily doses of 17.5 and 35 mg of DHEA/100 g of body weight for 7 days elevated the rate of malic enzyme gene transcription in liver above the basal levels 4-5- and 8-9-fold, respectively. The levels of nuclear malic enzyme RNA, cytoplasmic malic enzyme mRNA, and enzyme activity were increased correspondingly. Malic enzyme stimulation by DHEA was liver specific, i.e. malic enzyme activity in brain, heart, kidney, and testis was unchanged. Thyroid hormone is required for the induction of hepatic malic enzyme activity by DHEA since in hypothyroid animals, DHEA was without effect. However, stimulatory effects of thyroid hormone and DHEA on malic enzyme expression are additive in euthyroid rat livers at both levels of gene transcription and enzyme activity.

Animals↗

PubMed1989

Thyroid hormone regulation of transcription factors involved in malic enzyme gene expression.

The mechanisms by which triiodothyronine (T3) regulates gene transcription are inadequately understood. In order to examine the effect of T3 on transcriptional mechanisms, we utilized several techniques to characterize trans-acting factors which interact with cis-regulatory elements of the promoter for the T3-responsive rat malic enzyme gene. Transcription of deletion mutants of the promoter by HeLa extracts revealed the existence of three cis-regulatory elements at -144 to -123, -70 to -50, and -30 to +5 (relative to the cap site at +1). DNase I footprinting disclosed the presence of proteins which bound to each of these regions. Gel mobility shift assays showed that specific binding of liver nuclear proteins to two of these regions (-144/-114 and -76/-47) was diminished in hypothyroid and elevated in hyperthyroid rats. Similar but less marked effects of T3 on the binding of kidney (a T3-responsive tissue) nuclear proteins to the -76/-47 and cap site regions were observed while binding of testis (a T3-nonresponsive tissue) nuclear proteins was unaffected by T3 treatment. Changes in the magnitude and time course of binding of liver proteins to this promoter correlated closely with previously documented T3-induced increases in the hepatic malic enzyme gene transcription rate. These results are consistent with a model in which the regulation of malic enzyme gene transcription by T3 in responsive tissues is mediated in part by hormone-induced increases in the binding of trans-acting factors to cis-regulatory elements.

Animals↗

PubMed1989

Regulation of expression of the alternative mRNAs of the rat alpha-thyroid hormone receptor gene.

The rat alpha-thyroid hormone receptor gene encodes through alternative splicing at least three protein isoforms with different functions, and three mRNA species (2.6, 5.4, 6.8 kilobase (kb) in size) are detected using alpha gene-specific probes (Mitsuhashi, T., Tennyson, G. E., Nikodem, V. M. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 5804-5808). In the present study, the identities of these mRNAs were analyzed by Northern analysis, and it was demonstrated that in rat brain the receptor protein is encoded by the minor 5.4- and 6.8-kb mRNAs and the variant proteins are encoded by the major 2.6-kb mRNA. Relative quantities of these mRNAs were determined by RNase protection assay, and the ratio of the receptor mRNAs to the variant mRNAs was estimated to be 1:6 in adult brain. The ratio between the mRNAs was regulated in both a tissue-specific and developmental stage-specific manner. The receptor mRNA levels were also regulated by the thyroid state of the animal showing an increased level in hypothyroid rat liver while those in brain were not affected. Analysis of the alpha-thyroid hormone receptor gene suggested that the choice between two poly-adenylation sites and subsequent RNA processing appear to generate the 3' heterogeneity of these alternative mRNAs.

Animals↗

PubMed1989

Structural characterization of the rat malic enzyme gene.

We have identified and characterized lambda bacteriophage clones containing genomic DNA encoding rat malic enzyme [(S)-malate:NADP+ oxidoreductase (oxaloacetate-decarboxylating); EC 1.1.1.40]. The malic enzyme gene is unexpectedly large, spanning at least 95 kilobases. It is divided into 14 exons that range in size from 76 to 1513 base pairs. The sizes and boundaries of the exons were determined by Southern blotting and DNA sequencing. The sequences at the 5' and 3' ends of each intron conformed to the consensus sequence for mammalian introns. S1 nuclease and primer-extension assays showed that transcription of the malic enzyme gene initiates at multiple sites, the strongest one at position -31 relative to the ATG. "TATA and CCAAT box" homologies are not present in the proximal promoter region. Analysis of the 3' end of the gene showed that the utilization of alternate polyadenylylation signals in exon 14 results in two mRNAs with 3' untranslated regions of 345 and 1345 nucleotides, respectively.

Amino Acid Sequence↗

PubMed1989

Thyrotropin increases malic enzyme messenger ribonucleic acid levels in rat FRTL-5 thyroid cells.

The addition of TSH to FRTL-5 thyroid cells induces a 7- to 8-fold increase in the steady state level of malic enzyme [L-malate-NADP+ oxidoreductase (decarboxylating); EC 1.1.1.40] mRNA, but does not alter beta-actin mRNA levels. Insulin alone or together with TSH has no effect on malic enzyme mRNA. The effect of TSH is not the result of thyroid hormone formation, since the addition of T3 in the presence or in the absence of TSH and the addition of 5% serum (which includes T3 and T4) have no effect. Forskolin (10(-6) M) reproduces the TSH effect, suggesting that cAMP is involved.

Actins↗

PubMed1989

Coding nucleotide sequence of rat malic enzyme mRNA and tissue specific regulation by thyroid hormone.

A nearly full length ME cDNA has been obtained and sequenced. The identity has been established by comparison of the translated nucleotide sequence with the amino acid sequence of 7 tryptic peptides from purified ME. Northern analysis with this cDNA shows that ME mRNA consists of two different messages of about 27S and 21S. The size difference between two ME mRNAs (approximately equal to 27S and 21S) is attributed to the differences in the 3' noncoding regions. The relative ratios of the two ME mRNAs differ in various tissues examined (liver, heart, kidney, brain, lung, spleen, and testis). Their regulation by T3 is tissue specific with coordinate stimulation of both mRNAs in liver, heart and kidney, suggesting a single promoter for both mRNAs and no stimulation of either in the other tissues. T3 regulates ME mRNA synthesis via a dual-tissue specific mechanism by increasing the rate of transcription in liver and heart and stabilizing nuclear ME RNA sequences only in liver.

Amino Acid Sequence↗