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P R Larsen

Publications and source records attributed to P R Larsen.

At least 37 records · Page 2Linked to original sources
PubMed1999

Mutation of the Secys residue 266 in human type 2 selenodeiodinase alters 75Se incorporation without affecting its biochemical properties.

Type 2 deiodinase (D2) is a low Km iodothyronine deiodinase that metabolizes thyroxine (T4) to the active metabolite T3. We have recently shown that the cDNA for the human D2 coding region contains two in-frame selenocysteine (TGA) codons. The 3' TGA is seven codons 5' to a universal stop codon, TAA. The human D2 enzyme, transiently expressed in HEK-293 cells, can be in vivo labeled with 75Se as a doublet of approximately 31 kDa. This doublet is consistent with the possibility that the carboxy-terminal TGA codon can either encode selenocysteine or function as a stop codon. To test this hypothesis we mutagenized the second selenocysteine codon to a cysteine (TGC) or to an unambiguous stop codon (TAA). While the selenium incorporation pattern is different between the wild-type and mutant proteins, the deiodination properties of the enzyme are not affected by mutating the 3'TGA codon. Thus, we conclude that neither this residue nor the remaining seven carboxy-terminal amino acids are critical for the deiodination process.

Animals↗
PubMed1999

Regional expression of the type 3 iodothyronine deiodinase messenger ribonucleic acid in the rat central nervous system and its regulation by thyroid hormone.

Type 3 iodothyronine deiodinase (D3) is a selenoenzyme that inactivates thyroid hormone. It is necessary for T3 homeostasis in the central nervous system. D3 activity has been identified in many regions of the brain and parallels thyroid status, but the level at which it is regulated and its specific cellular locations are not known. We evaluated the effect of thyroid status on the expression of the D3 gene within the central nervous system using in situ hybridization histochemistry. D3 messenger RNA (mRNA) was identified throughout, but with high focal expression in the hippocampal pyramidal neurons, granule cells of the dentate nucleus, and layers II-VI of the cerebral cortex. In every region, D3 mRNA abundance was correlated with thyroid status. Four different D3 transcripts were identified by Northern analyses, with evidence for region-specific processing, and D3 mRNA increased 4- to 50-fold from the euthyroid to the hyperthyroid state. D3 mRNA was not detectable in hypothyroid brain. In the central nervous system, the D3 gene is highly T3 responsive, and its focal localization within the hippocampus and cerebral cortex suggests an important role for T3 homeostasis in memory and cognitive functions.

Animals↗
PubMed1999

Polarized targeting of epithelial cell proteins in thyrocytes and MDCK cells.

Polarized trafficking signals may be interpreted differently in different cell types. In this study, we have compared the polarized trafficking of different proteins expressed endogenously in primary porcine thyroid epithelial cells to similar proteins expressed in MDCK cells. As in MDCK cells, NH4Cl treatment of filter-grown thyrocytes caused mis-sorted soluble proteins to exhibit enhanced secretion to the apical medium. In independent studies, thrombospondin 1 (a thyroid basolaterally secreted protein) was secreted basolaterally from MDCK cells. Likewise, the 5'-deiodinase (a thyroid basolateral membrane protein) encoded by the DIO1 gene was also distributed basolaterally in transfected MDCK cells. Consistent with previous reports, when the secretion of human growth hormone (an unglycosylated regulated secretory protein) was examined from transfected MDCK cells, the release was nonpolarized. However, transfected thyrocytes secreted growth hormone apically in a manner dependent upon zinc addition. Moreover, two additional regulated secretory proteins expressed in thyrocytes, thyroglobulin (the major endogenous glycoprotein) and parathyroid hormone (an unglycosylated protein expressed transiently), were secreted apically even in the absence of zinc. We hypothesize that while cellular mechanisms for interpreting polarity signals are generally similar between thyrocytes and MDCK cells, thyrocytes allow for specialized packaging of regulated secretory proteins for apical delivery, which does not require glycosylation but may involve availability of certain ions as well as appropriate intracellular compartmentation.

Animals↗
PubMed1998

The 3'-untranslated region of human type 2 iodothyronine deiodinase mRNA contains a functional selenocysteine insertion sequence element.

Type 2 deiodinase (D2) catalyzes the 5'-deiodination of thyroxine to form 3,5,3'-triiodothyronine. Two mammalian D2 cDNAs have been identified containing 2 kilobases (kb) of the 7-kb mRNA including the complete coding sequence. Both contain in-frame TGA codons, which should serve as selenocysteine codons. However, the selenocysteine insertion sequence (SECIS) elements required for the decoding of UGA as a selenocysteine in the 3'-untranslated region (UTR) of the mRNA are not present. We have identified two overlapping expressed sequence tag clones, which contain the missing 4.4-kb 3'-UTR of the human D2 (hD2) cDNA. Computer analysis predicts a stem loop structure 280 base pairs 5' to the polyadenylation site, which has potent SECIS activity. A fragment containing these sequences hybridizes to D2 mRNA in human thyroid. A G to A mutation in the essential AUGA motif of this element abolished its function. Transfection of the hD2 coding region plus the 3'-UTR results in the expression of D2, and its in vitro transcribed mRNA programs D2 activity in Xenopus oocytes. This is the first identification of a SECIS element in a mammalian D2 cDNA and establishes that hD2 is a bona fide selenoprotein.

3' Untranslated Regions↗
PubMed1998

Type 2 iodothyronine deiodinase in rat pituitary tumor cells is inactivated in proteasomes.

The goal of these studies was to define the rate-limiting steps in the inactivation of type 2 iodothyronine deiodinase (D2). We examined the effects of ATP depletion, a lysosomal protease inhibitor, and an inhibitor of actin polymerization on D2 activity in the presence or absence of cycloheximide or 3,3', 5'-triiodothyronine (reverse T3, rT3) in rat pituitary tumor cells (GH4C1). We also analyzed the effects of the proteasomal proteolysis inhibitor carbobenzoxy- L-leucyl-L-leucyl-L-leucinal (MG132). The half-life of D2 activity in hypothyroid cells was 47 min after cycloheximide and 60 min with rT3 (3 nM). rT3 and cycloheximide were additive, reducing D2 half-life to 20 min. D2 degradation was partially inhibited by ATP depletion, but not by cytochalasin B or chloroquine. Incubation with MG132 alone increased D2 activity by 30-40% for several hours, and completely blocked the cycloheximide- or rT3-induced decrease in D2 activity. These results suggest that D2 is inactivated by proteasomal uptake and that substrate reduces D2 activity by accelerating degradation through this pathway. This is the first demonstration of a critical role for proteasomes in the post-translational regulation of D2 activity.

Adenosine Triphosphate↗
PubMed1998

The guanosine monophosphate reductase gene is conserved in rats and its expression increases rapidly in brown adipose tissue during cold exposure.

Non-shivering thermogenesis is required for survival of rodents during cold stress. Uncoupling protein-1 acts in brown adipose tissue (BAT) to transport protons, thus dissipating the proton gradient across the inner mitochondrial membrane. This permits respiration uncoupled from ATP synthesis. UCP-1 function is inhibited by the binding of purine nucleotides, with GTP/GDP being more potent than ATP/ADP. We used a cDNA subtraction analysis to identify cDNAs rapidly induced by cold exposure. One of these encodes rat guanosine monophosphate reductase (GMP-r). This was surprising in that previous data had suggested that this enzyme was absent in rodents. Rat GMP-r is 96% identical to human GMP-r, and its mRNA is increased 30-fold in BAT within 6 h of cold exposure. The gene is also expressed (but not cold-responsive) in muscle and kidney, but not in white fat. We speculate that the physiological function of the marked increase in BAT GMP-r during cold stress may be to deplete the brown adipocyte of guanine nucleotides, converting them to IMP, thus permitting enhanced UCP-1 function. This is a previously unrecognized regulatory aspect of thermogenesis, an essential physiological response of rodents to cold.

Acclimatization↗
PubMed1998

Studies of the hormonal regulation of type 2 5'-iodothyronine deiodinase messenger ribonucleic acid in pituitary tumor cells using semiquantitative reverse transcription-polymerase chain reaction.

We developed a sensitive competitive RT-PCR technique for quantitating the ratio of D2 to cyclophilin messenger RNA (mRNA) and used this to study type 2 deiodinase (D2) mRNA regulation. Hyperthyroidism in rats causes a 2- to 3-fold reduction in anterior pituitary and medial basal hypothalamus (MBH). Thyroid hormone (T3) withdrawal increased the D2/cyclophilin ratio 2- to 3-fold over 48 h in both GC and GH4C1 cells. T3 additional reduced D2 gene transcription by 50% over 2 h and about 30% over the next 2 h. D2 mRNA half-life is 2 h and is not affected by T3, indicating that its effect is due to suppression of D2 gene transcription. The T3 effect did not require new protein synthesis. Longer treatment with T3 led to a maximum decrease of 70% in D2 mRNA, indicating that there is also a T3-independent transcriptional component of the D2 gene. 3,3',5'-Triiodothyronine (reverse T3) caused a slight increase D2 mRNA over 24 h but an 80-90% decrease in D2 activity, indicating that it acts posttranscriptionally. Dexamethasone, 8 Br-cAMP, and TRH also caused modest increases in D2 mRNA in pituitary tumor cells. We conclude that D2 gene transcription has both T3-dependent and T3-independent components. Thus, posttranscriptional effects of D2 substrates such as T4 will be required for complete feedback inhibition of D2 activity. The short half-life of D2 mRNA and D2 protein explains the rapid response of D2 activity to thyroid hormone administration.

1-Methyl-3-isobutylxanthine↗
PubMed1998

Further characterization of thyroid hormone response elements in the human type 1 iodothyronine deiodinase gene.

The increased type 1 iodothyronine deiodinase expression in hyperthyroid patients increases the fraction of plasma T3 generated from T4 by the propylthiouracil-sensitive pathway. In this study, we extend our analysis of the thyroid hormone response elements (TREs) in the 5' flanking region of the human dio1 gene. The 5' TRE (TRE2), a direct repeat separated by 4 bp (DR+4) at -660 bp, arises from an A to G substitution in an Alu sequence, the first example of this phenomenon. An SP1 binding site immediately 5' to TRE2 increases basal expression of a 430-bp dio1 promoter-chloramphenicol acetyltransferase construct in the presence of unliganded thyroid hormone receptor, thus decreasing T3 responsiveness, but does not do so when this complex is placed in its more 5' wild-type location. The two octameric binding sites of TRE1, a retinoid X-receptor independent DR+10 structure at -90, can be exchanged or inverted without loss of T3 response potency, despite significant changes in thyroid hormone receptor binding, as assessed by gel shift assays. However, the retinoic acid response of the 716-bp dio1 5' flanking region is unaffected by elimination of TRE2 but is lost with mutations in TRE1. These findings indicate the importance of functional analyses of potential ligand-responsive transcription factors, as well as the influence of position, on TRE function and interaction with basal transcription factors. The unusual features of these TREs emphasize the need to consider alternatives to canonical half-site arrangements of receptor binding sites and contexts in the evaluation of T3- and retinoic acid-responsive genes.

Base Sequence↗
PubMed1997

Structure-activity relationships for thyroid hormone deiodination by mammalian type I iodothyronine deiodinases.

The bioactivity of thyroid hormone is determined to a large extent by the monodeiodination of the prohormone T4 by the hepatic selenoenzyme type I iodothyronine deiodinase (IDI), i.e. by outer ring deiodination (ORD) to the active hormone T3' or by inner ring deiodination (IRD) to the inactive metabolite rT3. IDI also catalyzes the IRD of T3 and the ORD of rT3' both to T2, as well as the deiodination of different iodothyronine sulfates, e.g. IRD of T3S and ORD of T2S. Previous studies have indicated important differences in catalytic specificity between dog IDI (dID1) and human ID1 (hID1), in particular with respect to the ORD of rT3. This study was done to investigate the relationship between structure and catalytic function of this enzyme by comparing the deiodination of T4, T3, rT3, T3S, and T2S by native dID1 and hID1 in liver microsomes as well as by recombinant wild-type, chimeric and mutated d/hID1 enzymes expressed in HEK293 cells. With both native and recombinant wild-type enzymes, the substrate specificity was T3S > T2S approximately rT3 approximately T4 > T3 for dID1, and rT3 > > T2S approximately T3S > T4 approximately T3 for hID1. Whereas ORD of T4 and of T4, T3, and T3S showed relatively little variation between the different d/hID1 constructs, large differences were found for the ORD of rT3 and T2S. Both reactions were favored by the presence of the amino acids G, E and, in particular, F, present in hID1 at positions 45, 46, and 65, instead of the dID1 residues N, G, and L, respectively. However, although ORD of rT3 was not affected by the presence (hID1) or absence (dID1) of the TGMTR(48-52) sequence, the ORD of T2S was markedly inhibited by the presence of this sequence. Therefore, we have identified structural elements in ID1 that have substrate-specific impacts on deiodination. Our results suggest the specific interaction of the mono-substituted inner ring of the substrates rT3 and T2S but not the disubstituted inner ring of T3, T3S, or T4, with the aromatic ring of F65 in Id1, perhaps by pi-pi interactions.

Amino Acid Sequence↗
PubMed1997

The role of the active site cysteine in catalysis by type 1 iodothyronine deiodinase.

Type 1 iodothyronine deiodinase (deiodinase 1) is a selenoenzyme that converts the prohormone T4 to the active thyroid hormone T3 by outer ring deiodination or to the inactive metabolite rT3 by inner ring deiodination. Although selenocysteine has been demonstrated to be essential for the biochemical profile of deiodinase 1, the role of a highly conserved, active site cysteine (C124 in rat deiodinase 1) has not been defined. The present studies examined the effects of a Cys124Ala mutation on rat deiodinase 1 enzymatic function and substrate affinity. At a constant 10-mM concentration of dithiothreitol (DTT), the C124A mutant demonstrated a 2-fold lower apparent maximal velocity (Vmax) and Km for rT3 (KmrT3) than the wild type for outer ring deiodination, whereas the Vmax/Km ratio was unchanged. Similarly, the apparent Vmax and KmT3 sulfate for inner ring deiodination were 2-fold lower in the C124A mutant relative to those in the wild type, with no change in the Vmax/Km ratio. The C124A mutant exhibited ping-pong kinetics in the presence of DTT, and substitution of the active site cysteine increased the KmDTT by 14-fold relative to that of the wild-type enzyme, with no significant effects on KmrT3 or Vmax. The C124A mutant was inhibited by propylthiouracil in an uncompetitive fashion and exhibited a 2-fold increase in K(i)propylthiouracil compared with that of the wild type. KmrT3 was also reduced for the C124A mutant when 5 mM reduced glutathione, a potential physiological monothiol cosubstrate, was used in outer ring deiodination assays. These results demonstrate that thiol cosubstrate interactions with C124 in type 1 deiodinase play an important role in enhancing catalytic efficiency for both outer and inner ring deiodination.

Amino Acid Sequence↗
PubMed1997

Regional distribution of type 2 thyroxine deiodinase messenger ribonucleic acid in rat hypothalamus and pituitary and its regulation by thyroid hormone.

To identify the specific locations of type 2 deiodinase (D2) messenger RNA (mRNA) in the hypothalamus and pituitary gland and determine its regulation by thyroid hormone, we performed in situ hybridization histochemistry, Northern analysis, and quantitative RT-PCR in euthyroid, hypothyroid, and hyperthyroid rats. By in situ hybridization histochemistry, silver grains were concentrated over ependymal cells lining the floor and infralateral walls of the third ventricle extending from the rostral tip of the median eminence (ME) to the infundibular recess, surrounding blood vessels in the arcuate nucleus (ARC), and in the ME adjacent to the portal vessels and overlying the tuberoinfundibular sulci. Silver grains also accumulated over distinct cells in the midportion of the anterior pituitary. In hypothyroid animals, an increase in signal intensity was observed in the caudal hypothalamus, and a marked increase in the number of positive cells occurred in the anterior pituitary. Microdissection of the hypothalamus for Northern and PCR analysis established the authenticity of D2 mRNA in the caudal hypothalamus, and confirmed that the majority of D2 mRNA is concentrated in this region. The distribution of D2 mRNA suggests its expression in specialized ependymal cells, termed tanycytes, originating from the third ventricle. Thus, the tanycyte is the source of the high D2 activity previously found in the ARC-ME region of the hypothalamus. The results indicate that tanycytes may have a previously unrecognized integral role in feedback regulation of TSH secretion by T4.

Animals↗
PubMed1996

Characterization of the promoter of the rat sarcoplasmic endoplasmic reticulum Ca2+-ATPase 1 gene and analysis of thyroid hormone responsiveness.

Relaxation of skeletal muscle requires the re-uptake of Ca2+, which is mediated by the sarcoplasmic reticulum Ca2+-ATPase (SERCA). Thyroid hormone (T3) stimulates the expression of the SERCA1 isoform, which is essential for fast skeletal muscle fiber phenotype. We have cloned and studied the first 962 base pairs of the 5'-flanking region of the rat SERCA1 gene. This sequence was tested for T3-regulated expression in transient transfection experiments using COS7 cells and for binding of thyroid hormone receptor (TR) alpha in mobility shift assays. A construct of the 5'-flanking region and a reporter gene was unresponsive to T3 in the absence of co-transfected thyroid hormone receptor. In the presence of TRalpha, a T3 induction ratio of almost 4.0 was found, and this induction ratio was doubled with co-transfection of an RXR expression plasmid. Analysis of progressive 5'-deletion fragments of the sequence indicated multiple regions involved in T3 responsiveness. Three regions, R1, R2, and R3, were identified that bound TR complexes in mobility shift assays and conferred T3 responsiveness to a heterologous promoter. The most potent of these thyroid hormone response elements, R3, increased the 2-fold background T3 stimulation of the thymidine kinase promoter to nearly 6-fold. Detailed analysis of this element showed that four TR-binding half-sites, comprising two independent thyroid hormone response elements, interact cooperatively to give the maximal T3 response. T3 regulation of SERCA1 expression is mediated by a complex thyroid hormone response element that may serve to provide a greater range of response in interaction with nuclear receptor partners or cell-specific transcription factors.

Animals↗
PubMed1996

Type 2 iodothyronine deiodinase is highly expressed in human thyroid.

Type 2 iodothyronine deiodinase (D2) is a recently cloned selenodeiodinase thought to provide intracellular 3,5,3' triiodothyronine (T3) to a restricted group of tissues. We report here the presence of D2 mRNA in human thyroid at levels 50-150-fold higher than in placenta. Surprisingly, while type 1 deiodinase (D1) is known to be present in human thyroid, D2 has not been evaluated previously. D2 mRNA was especially high in thyroids from Graves' patients and in follicular adenomas. Stimulated thyroids had higher D2 to D1 mRNA ratios than normal or multinodular glands suggesting differential regulation of D1 and D2 expression. Microsomes from normal, Graves', and TSH-stimulated thyroids contained low Km D2 activity resistant to propylthiouracil (1 mM) or to inactivation by N-bromoacetyl T3, agents which block or inactivate D1. At 2 nM thyroxine (T4), 100 times the physiological-free T4 levels, 60-80% of T4 to T3 conversion in stimulated, but only 27% of that in normal thyroids, is catalyzed by D2. We conclude that intrathyroidal T4 to T3 conversion by D2 may contribute significantly to the relative increase in thyroidal T3 production in patients with Graves' disease, toxic adenomas, and, perhaps, iodine deficiency.

Enzyme Inhibitors↗
PubMed1996

Is there a negative TRE in the luciferase reporter cDNA?

Transient expression assays using a luciferase (LUC) reporter gene are often used in studies of positive and negative thyroid hormone response elements (TREs) or of wild-type and mutated thyroid hormone receptors (TRs). However, unliganded TR (the beta isoform > than the alpha) increases LUC expression from 2 different TK-LUC vectors in several cell types, especially JEG-3 cells, and 3,5,3'-triiodothyronine (T3) reduces that expression by as much as 80% in a TR-dependent manner. The TR effects require an intact TR DNA-binding domain and the results suggest that there may be a negative TRE in the LUC cDNA. We conclude that great care must be used in the interpretation of studies of thyroid hormone action using LUC expression plasmids especially when these are performed in JEG-3 cells.

Animals↗
PubMed1996

In vivo genomic footprinting of thyroid hormone-responsive genes in pituitary tumor cell lines.

We studied the effects of thyroid hormone (T3) on nuclear protein-DNA interactions by using dimethyl sulfate (DMS) and DNase I ligation-mediated PCR footprinting. We examined an endogenous gene the growth hormone (GH) gene, and a stably transfected plasmid containing the chicken lysozyme silencer (F2) T3 response element (TRE) gene, F2-TRE-TK-CAT, both in pituitary tumor (GC) cells. The 235-1 cell line, which expresses prolactin (PRL) and Pit-1, but not the T3 receptor (TR) or GH, was used as a control. DMS and DNase I footprinting identified protected G residues in the Pit-1, Sp1, and Zn-15 binding sites of the GH gene in GC, but not in 235-1, cells. There was no specific protection of the tripartite GH TRE at -180 bp against either DMS or DNase I in the absence or presence of T3 in either cell line. However, T3 increased protection of the Pit-1 and Sp1 binding sites against DMS in GC cells. In GC cells stably transfected with a plasmid containing F2-TRE-TK-CAT or TRalpha, chloramphenicol acetyltransferase expression was T3 inducible and DMS footprinting revealed both F2 TRE TR-binding half sites in a pattern suggesting the binding of TR homodimers before and during T3 exposure. We conclude that the GH gene is accessible to specific nuclear proteins in GC, but not in 235-1, cells and that T3 enhances this interaction, although there is no evidence of TR binding to the low-affinity rat GH TRE. The presence of TR binding to the high-affinity F2 TRE before and during T3 exposure suggests that reversible interaction of T3 with DNA-bound TRs, rather than transient T3-TR contact with TREs, determines the level of T3-stimulated transcriptional activation.

Animals↗