The Human Type 2 Iodothyronine Deiodinase Is a Selenoprotein Highly Expressed in a Mesothelioma Cell Line

Types 1 and 3 iodothyronine deiodinases are known to be selenocysteine-containing enzymes. Although a putative human type 2 iodothyronine deiodinase (D2) gene (hDio2) encoding a similar selenoprotein has been identified, basal D2 activity is not selenium (Se)-dependent nor has D2 been labeled with75Se. A human mesothelioma cell line (MSTO-211H) has recently been shown to have ∼40-fold higher levels ofhDio2 mRNA than mesothelial cells. Mesothelioma cell lysates activate thyroxine (T4) to 3,5,3′-triiodothyronine with typical characteristics of D2 such as low K m (T4), 1.3 nm, resistance to propylthiouracil, and a short half-life (∼30 min). D2 activity is ∼30-fold higher in Se-supplemented than in Se-depleted medium. An antiserum prepared against a peptide deduced from theDio2 mRNA sequence precipitates a 75Se protein of the predicted 31-kDa size from 75Se-labeled mesothelioma cells. Bromoadenosine 3′5′ cyclic monophosphate increases D2 activity and 75Se-p31 ∼2.5-fold whereas substrate (T4) reduces both D2 activity and 75Se-p31 ∼2–3-fold. MG132 or lactacystin (10 µm), inhibitors of the proteasome pathway by which D2 is degraded, increase both D2 activity and 75Se-p31 3–4-fold and prevent the loss of D2 activity during cycloheximide or substrate (T4) exposure. Immunocytochemical studies with affinity-purified anti-hD2 antibody show a Se-dependent increase in immunofluorescence. Thus, human D2 is encoded by hDio2 and is a member of the selenodeiodinase family accounting for its highly catalytic efficiency in T4 activation. Types 1 and 3 iodothyronine deiodinases are known to be selenocysteine-containing enzymes. Although a putative human type 2 iodothyronine deiodinase (D2) gene (hDio2) encoding a similar selenoprotein has been identified, basal D2 activity is not selenium (Se)-dependent nor has D2 been labeled with75Se. A human mesothelioma cell line (MSTO-211H) has recently been shown to have ∼40-fold higher levels ofhDio2 mRNA than mesothelial cells. Mesothelioma cell lysates activate thyroxine (T4) to 3,5,3′-triiodothyronine with typical characteristics of D2 such as low K m (T4), 1.3 nm, resistance to propylthiouracil, and a short half-life (∼30 min). D2 activity is ∼30-fold higher in Se-supplemented than in Se-depleted medium. An antiserum prepared against a peptide deduced from theDio2 mRNA sequence precipitates a 75Se protein of the predicted 31-kDa size from 75Se-labeled mesothelioma cells. Bromoadenosine 3′5′ cyclic monophosphate increases D2 activity and 75Se-p31 ∼2.5-fold whereas substrate (T4) reduces both D2 activity and 75Se-p31 ∼2–3-fold. MG132 or lactacystin (10 µm), inhibitors of the proteasome pathway by which D2 is degraded, increase both D2 activity and 75Se-p31 3–4-fold and prevent the loss of D2 activity during cycloheximide or substrate (T4) exposure. Immunocytochemical studies with affinity-purified anti-hD2 antibody show a Se-dependent increase in immunofluorescence. Thus, human D2 is encoded by hDio2 and is a member of the selenodeiodinase family accounting for its highly catalytic efficiency in T4 activation. thyroxine 3,5,3′-triiodothyronine type 1 iodothyronine deiodinase selenocysteine type 2 iodothyronine deiodinase propylthiouracil human type 3 iodothyronine deiodinase immunoprecipitation cycloheximide 8-bromoadenosine 3′5′ cyclic monophosphate fetal bovine serum human embryonic kidney polyacrylamide gel electrophoresis phosphate-buffered saline Sec insertion sequence A group of three specific deiodinases monodeiodinate thyroxine (T4)1 to 3,5,3′-triiodothyronine (T3), the active thyroid hormone, or 3,3′,5′-triiodothyronine, an inactive metabolite. The first of these enzymes to be cloned was the type 1 iodothyronine deiodinase (D1), which revealed a rare structural characteristic,i.e. the presence of the selenocysteine (Sec) codon, UGA, in the active center of the enzyme conferring an ∼200-fold higher catalytic efficiency than sulfur in the deiodination reaction (1Berry M.J. Banu L. Larsen P.R. Nature. 1991; 349: 438-440Crossref PubMed Scopus (744) Google Scholar). It also revealed a requirement for a stem loop structure in the 3′-untranslated region, the Sec insertion sequence (SECIS) element (2Berry M.J. Banu L. Chen Y.Y. Mandel S.J. Kieffer J.D. Harney J.W. Larsen P.R. Nature. 1991; 353: 273-276Crossref PubMed Scopus (515) Google Scholar), which is found in all eukaryotic selenoprotein mRNAs described to date (3Low S.C. Berry M.J. Trends Biochem. Sci. 1996; 21: 203-208Abstract Full Text PDF PubMed Scopus (389) Google Scholar). The most recently cloned deiodinase is the type 2 (D2), a propylthiouracil (PTU)-resistant, low K m(T4) obligate outer ring deiodinase that catalyzes T4 to T3 conversion (4Croteau W. Davey J.C. Galton V.A. St Germain D.L. J. Clin. Invest. 1996; 98: 405-417Crossref PubMed Scopus (331) Google Scholar, 5Salvatore D. Bartha T. Harney J.W. Larsen P.R. Endocrinology. 1996; 137: 3308-3315Crossref PubMed Scopus (227) Google Scholar). The major open reading frame of the hD2 cDNA encodes a putative ∼31-kDa protein with high similarity of the Sec-containing active center of the putative D2 enzyme with those of D1 and the other member of this group, type 3 iodothyronine deiodinase (D3) (6St. Germain D.L. Schwartzman R.A. Croteau W. Kanamori A. Wang Z. Brown D.D. Galton V.A. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 7767-7771Crossref PubMed Scopus (119) Google Scholar). Furthermore, a SECIS element has been identified in the extreme 3′-untranslated region of both the human and highly homologous chicken Dio2 genes (7Buettner C. Harney J.W. Larsen P.R. J. Biol. Chem. 1998; 273: 33374-33378Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar,8Gereben B. Bartha T. Tu H.M. Harney J.W. Rudas P. Larsen P.R. J. Biol. Chem. 1999; 274: 13768-13776Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). Human, mouse, rat, chicken, and frog D2 all contain a putative in-frame UGA codon in the deduced amino acid sequence of the active center (position 133 in hD2) and therefore all are considered to be selenoenzymes (8Gereben B. Bartha T. Tu H.M. Harney J.W. Rudas P. Larsen P.R. J. Biol. Chem. 1999; 274: 13768-13776Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). Despite this evidence, there is vigorous disagreement that D2 is a selenoprotein, with some investigators claiming that Dio2encodes a “virtual” or artificial selenoprotein that is not expressed in humans or rats (9Safran M. Farwell A.P. Leonard J.L. J. Biol. Chem. 1991; 266: 13477-13480Abstract Full Text PDF PubMed Google Scholar, 10Safran M. Farwell A.P. Rokos H. Leonard J.L. J. Biol. Chem. 1993; 268: 14224-14229Abstract Full Text PDF PubMed Google Scholar, 11Farwell A.P. Safran M. Dubord S. Leonard J.L. J. Biol. Chem. 1996; 271: 16369-16374Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar, 12Safran M. Farwell A.P. Leonard J.L. J. Biol. Chem. 1996; 271: 16363-16368Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar, 13Leonard J.L. Leonard D.M. Safran M. Wu R. Zapp M.L. Farwell A.P. Endocrinology. 1999; 140: 2206-2215Crossref PubMed Scopus (27) Google Scholar, 14Stachelek S.J. Kowalik T.F. Farwell A.P. Leonard J.L. J. Biol. Chem. 2000; 275: 31701-31707Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar). Three major arguments support this position, namely 1) the failure of Se deficiency to reduce D2 catalytic activity either in vivo or in vitro (9Safran M. Farwell A.P. Leonard J.L. J. Biol. Chem. 1991; 266: 13477-13480Abstract Full Text PDF PubMed Google Scholar, 15Chanoine J.P. Safran M. Farwell A.P. Tranter P. Ekenbarger D.M. Dubord S. Alex S. Arthur J.R. Beckett G.J. Braverman L.E. Leonard J.L. Endocrinology. 1992; 130: 479-484Crossref Scopus (63) Google Scholar, 16Chanoine J.P. Safran M. Farwell A.P. Dubord S. Alex S. Stone S. Arthur J.R. Braverman L.E. Leonard J.L. Endocrinology. 1992; 131: 1787-1792Crossref PubMed Scopus (49) Google Scholar), 2) the inability to identify a 75Se-labeled protein of the expected size in cells expressing D2 (13Leonard J.L. Leonard D.M. Safran M. Wu R. Zapp M.L. Farwell A.P. Endocrinology. 1999; 140: 2206-2215Crossref PubMed Scopus (27) Google Scholar), and 3) the inability to identify immunoreactive protein by Western analysis, immunoprecipitation (IP), or immunocytochemistry using antibodies prepared against peptides deduced from the sequence of the putative D2 mRNA (4Croteau W. Davey J.C. Galton V.A. St Germain D.L. J. Clin. Invest. 1996; 98: 405-417Crossref PubMed Scopus (331) Google Scholar). An alternative scenario put forward is that D2 is a large protein complex (200 kDa) containing one or more 29-kDa substrate-binding subunits (p29), an ∼60-kDa cAMP-induced activation protein, and one or more catalytic subunits (12Safran M. Farwell A.P. Leonard J.L. J. Biol. Chem. 1996; 271: 16363-16368Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar). Rat p29 was identified in astrocytes byN-bromoacetyl-T4 labeling (17Farwell A.P. Leonard J.L. J. Biol. Chem. 1989; 264: 20561-20567Abstract Full Text PDF PubMed Google Scholar, 18Safran M. Leonard J.L. J. Biol. Chem. 1991; 266: 3233-3238Abstract Full Text PDF PubMed Google Scholar, 19Leonard D.M. Stachelek S.J. Safran M. Farwell A.P. Kowalik T.F. Leonard J.L. J. Biol. Chem. 2000; 275: 25194-25201Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar), has no enzymatic activity, and is highly similar to Dickkopf-3 (20Glinka A. Wu W. Delius H. Monaghan A.P. Blumenstock C. Niehrs C. Nature. 1998; 391: 357-362Crossref PubMed Scopus (1331) Google Scholar). Although many of these apparent discrepancies could be explained by low expression of a highly efficient enzyme, reservations as to the identity of D2 are expressed even in recent reviews of the subject (21Kohrle J. Cell. Mol. Life Sci. 2000; 57: 1853-1863Crossref PubMed Scopus (169) Google Scholar,22Kohrle J. Acta Med. Austriaca. 2000; 27: 1-7Crossref PubMed Scopus (35) Google Scholar). A mesothelioma cell line, MSTO-211H, was recently shown to express large amounts of Dio2 mRNA by microarray analysis (accession number U53506), offering a potential system to resolve the issue of D2 identity (23Rihn B.H. Mohr S. McDowell S.A. Binet S. Loubinoux J. Galateau F. Keith G. Leikauf G.D. FEBS Lett. 2000; 480: 95-100Crossref PubMed Scopus (70) Google Scholar). MG132 and lactacystin were obtained from Calbiochem (La Jolla, CA) and dissolved in Me2SO. MG132 is a reversible proteasome inhibitor. Lactacystin blocks proteasome activity by targeting the catalytic β-subunit and covalently inhibiting the chymotrypsin- and trypsin-like activities. T4, cycloheximide (CX), and 8-bromoadenosine 3′5′ cyclic monophosphate (8-Br-cAMP) were from Sigma and were dissolved in 40 mm NaOH (T4) or Me2SO. Protein G plus/protein A-agarose solution was obtained from Oncogene Research Products (Boston, MA). Outer ring-labeled [125I]T4 (specific activity, 4400 Ci/mmol) was from PerkinElmer Life Sciences. Na2[75Se]O3 was kindly provided by the University of Missouri Research Reactor, courtesy of Drs. Marla Berry and Dolph L. Hatfield. All other reagents were of analytical grade. Mesothelioma (MSTO-211H) and mesothelial (MeT-5A) cell lines were obtained from American Type Culture Collection (ATCC; Manassas, VA) and made available through Dr. Rihn. Cells were plated in 60-mm dishes and grown until confluence in RPMI or M-199, respectively, and supplemented with 0–10% fetal bovine serum (FBS). Human embryonic kidney epithelial cells (HEK-293) cells were incubated in Dulbecco's modified Eagle's medium with 0 or 10% fetal bovine serum. Each experiment was performed with duplicate dishes for each condition, and control plates contained the respective vehicles, 0.2% Me2SO and/or 0.6 mm NaOH. At the appropriate times, cells were harvested and processed for D2 activity in the presence of 1 mm PTU as described previously (24Steinsapir J. Harney J. Larsen P.R. J. Clin. Invest. 1998; 102: 1895-1899Crossref PubMed Scopus (86) Google Scholar). The results are reported as fmol T4deiodinated/mg protein·min ± S.D. In some experiments D2 was transiently expressed in HEK-293. Cells were transfected by the CaPO4 method as described previously (25Steinsapir J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar) with plasmids containing wild type D2 (KD2-SelP), a D2 mutant in which the Sec-133 (but not Sec-266) was replaced by Cys (CysD2), or a CysD2 in which the SECIS element was deleted (CysD2ΔXba). This was performed as described previously (25Steinsapir J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar) after cells were labeled with 4–6 µCi of Na2[75Se]O3/dish. The lysis buffer contained 25 mm Tris-HCl (pH 7.4), 300 mm NaCl, 1 mm CaCl2, 0.5% Triton X-100, 10 µg/ml leupeptin, 0.2 units/ml aprotinin, 1 mmphenylmethylsulfonyl fluoride. The 2,000-rpm supernatant of each cell lysate was incubated for 16 h at 4 °C with D2 rabbit antisera 85254 at final dilution of 1:100 (25Steinsapir J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar). 30 µl of protein G plus/protein A-agarose solution were added per tube and incubated under slow agitation for 2 h at 4 °C. The IP pellets were washed once with lysis buffer and once with 25 mm Tris-HCl (pH 7.4), 140 mm NaCl, 1 mm CaCl2. Pellets were re-suspended in sample loading buffer and analyzed in 12% SDS-PAGE. In some experiments IP was processed in unlabeled cells to allow measurement of D2 activity. For the affinity purification of the antiserum 45618 (25Steinsapir J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar) the keyhole limpet hemocyanin conjugate of the hD2 peptide (EVRSWLEKNFSKR; residues 253 to 265) used to immunize the rabbits was coupled to agarose resin using Amino-Link agarose gel (Pierce) following the manufacture's instructions. The column was regenerated by washing with 3 m sodium acetate (pH 4.5) and equilibrated with phosphate-buffered saline (PBS) prior to each cycle. The D2 antiserum (45618) was passed through the column five times and then washed 10 times with PBS. antibodies were with 3 acetate (pH and were against at 4 °C for 2 This affinity-purified antiserum was used in the and Western analysis of or cells as described B. C. Harney J.W. Larsen P.R. Bianco A.C. Mol. 2000; PubMed Scopus Google Scholar). This was performed as described previously B. Harney J.W. Larsen P.R. Bianco A.C. Endocrinology. 2000; 141: PubMed Scopus Google Scholar). and cells were grown in for with in (pH 7.4), with 0.5% Triton in for 10 and then for 30 with bovine serum in PBS. rabbit D2 antibody was used at by with µg/ml In the the affinity-purified antibody was used with This was by with and with Mesothelioma (MSTO-211H) cell T3 with typical of namely a K m for T4 of and to by 1 mm PTU as shown in the D1 nor were nor the mesothelial cells (MeT-5A) express D2 activity D2 activity in cells was highly activity was by the of containing 1 these are by Se deficiency and not by other in is from the that Se D2 activity at each of from at 10% to ∼30-fold no was 1 The from cells incubated in the of with or for h an increase in no in the K m 1 A of D2 activity that the is obtained at A that an of Se is after 2 the h of Se to increase D2 activity were in cells transiently expressing D2 to for h D2 activity by this is 1 that the cells express a selenoprotein containing a deduced D2 cells were labeled with Na2[75Se]O3 for h and processed for D2 IP using an anti-hD2 peptide antiserum (25Steinsapir J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar). (MeT-5A) cells were used as of and (but not kDa) are in the of cell lysates of or cells 1 are by with in cells are in 1 analysis of the IP of the cell lysate revealed an ∼31-kDa 75Se-labeled of the predicted size of the of which is in the presence of and A is hD2 cDNA is transiently expressed of the presence of a UGA at the of the open reading frame that be either as a Sec or as a codon D. Harney J.W. Larsen P.R. 1999; Scopus Google Scholar). The of all other 75Se-labeled is in the IP pellets 1 IP of cell lysates D2 activity and this activity be from the washed IP of the antiserum to the cell lysate not D2 activity nor to serum or protein Despite these IP were to D2 by Western analysis of cell lysates using affinity-purified antibody The hD2 gene a to the most T. D. B. Tu H.M. Harney J.W. Rudas P. Larsen P.R. Endocrinology. 2000; 141: PubMed Scopus (89) Google Scholar). D2 activity increases in cells for h with in a 2 there is a D2 activity and cells with 1 mm were labeled with 75Se and processed for D2 The of ∼2.5-fold in cells 2 of or D2 activity were not and transiently expressed D2 have a that is by to such as or T4 (24Steinsapir J. Harney J. Larsen P.R. J. Clin. Invest. 1998; 102: 1895-1899Crossref PubMed Scopus (86) Google Scholar, J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar, J.L. Larsen P.R. PubMed Scopus Google Scholar, Germain D.L. Endocrinology. PubMed Scopus Google Scholar, J.L. Biochem. PubMed Scopus Google Scholar). This is explained by and of the D2 (25Steinsapir J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar, B. C. Harney J.W. Larsen P.R. Bianco A.C. Mol. 2000; PubMed Scopus Google Scholar). of cells with for 1 h in a loss of D2 activity, with a D2 half-life of The in D2 activity was by the proteasome MG132 (10 2 Furthermore, to MG132 D2 activity to with similar results obtained with 10 lactacystin 2 not other labeled was also by with MG132 2 to substrate T4 in 10% for 1 h a in D2 activity of to 2 which was by MG132 2 This was by a in the 75Se-p31 and cells were also analyzed by immunocytochemistry using an affinity-purified antibody In cells incubated in medium for the for D2 with for h the of the the 3 was in cells incubated with antibody 3 or in cells incubated with affinity-purified antibody the of expressed cells were with D2 antibody and with antibody against the protein 3 shown previously to with transiently expressed D2 in and cells B. Harney J.W. Larsen P.R. Bianco A.C. Endocrinology. 2000; 141: PubMed Scopus Google Scholar). was by of 3 The Se-supplemented cells have the D2 activity reported to date in a human higher than in thyroid D. Tu H. Harney J.W. Larsen P.R. J. Clin. Invest. 1996; 98: PubMed Scopus (169) Google Scholar). This high D2 expression the that hD2 is a selenoprotein encoded by the Dio2 gene D. Bartha T. Harney J.W. Larsen P.R. Endocrinology. 1996; 137: 3308-3315Crossref PubMed Scopus (227) Google Scholar). The results a number of the identity of hD2 as the of the Dio2 Furthermore, allow that the human enzyme as predicted from a number of studies using transiently expressed The first to D2 as a selenoprotein is to its Se studies that the D2 in of rats is not by Se deficiency J.P. Safran M. Farwell A.P. Tranter P. Ekenbarger D.M. Dubord S. Alex S. Arthur J.R. Beckett G.J. Braverman L.E. Leonard J.L. Endocrinology. 1992; 130: 479-484Crossref Scopus (63) Google Scholar) and that cells of Se for not have basal D2 activity (9Safran M. Farwell A.P. Leonard J.L. J. Biol. Chem. 1991; 266: 13477-13480Abstract Full Text PDF PubMed Google Scholar). Although there was of the D2 activity in after of Se basal D2 activity was not S. M. Croteau W. M. F. Germain D.L. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). the system is known to Se with high which could D2 is not by Se deficiency D. H. S. H. W. PubMed Scopus Google Scholar). In the other Se for h basal D2 activity by This in D2 activity was by Se In of Se D2 activity by ∼30-fold in Se-depleted in a and a that also in cells transiently expressing D2 A of cells also reduces the levels of selenoprotein, Biochem. J. 271: PubMed Scopus Google Scholar). A by the studies is the 75Se labeling and of A recent could not identify the protein by IP using antibodies that were against the of D2 or against the catalytic of the enzyme (13Leonard J.L. Leonard D.M. Safran M. Wu R. Zapp M.L. Farwell A.P. Endocrinology. 1999; 140: 2206-2215Crossref PubMed Scopus (27) Google Scholar). This is not the in the studies 1 and 2 from potential in the antisera per which be an the D2 activity in or are of that in cells. of the higher D2 IP of D2 activity, as as of a 75Se-labeled protein of the predicted size kDa) from cells 1 This is the first of 31-kDa protein is by in D2 activity A in D2 activity and the of 75Se-p31 is also during with proteasome inhibitors or D2 substrate identity were obtained by studies using the affinity-purified anti-hD2 was in cells and was by Se Furthermore, D2 with the as found previously in and cells transiently expressing hD2 B. Harney J.W. Larsen P.R. Bianco A.C. Endocrinology. 2000; 141: PubMed Scopus Google Scholar). Despite IP of the D2 protein and activity, were to D2 by Western analysis of low antibody affinity for protein, low D2 or A short half-life and the of D2 by of cells to in cells and in cells transiently expressing D2 are in these A short half-life is of D2 in all cells A.P. Safran M. Dubord S. Leonard J.L. J. Biol. Chem. 1996; 271: 16369-16374Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar, J. Harney J. Larsen P.R. J. Clin. Invest. 1998; 102: 1895-1899Crossref PubMed Scopus (86) Google Scholar, Germain D.L. Endocrinology. PubMed Scopus Google Scholar) and is explained by the that D2 is the of and by the system (24Steinsapir J. Harney J. Larsen P.R. J. Clin. Invest. 1998; 102: 1895-1899Crossref PubMed Scopus (86) Google Scholar, J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar, B. C. Harney J.W. Larsen P.R. Bianco A.C. Mol. 2000; PubMed Scopus Google Scholar). This is also the in in which D2 activity and protein levels were to inhibitors of the pathway It is that in and transfected with inhibitors for 1 h increases D2 activity by (24Steinsapir J. Harney J. Larsen P.R. J. Clin. Invest. 1998; 102: 1895-1899Crossref PubMed Scopus (86) Google Scholar, J. Bianco A.C. Buettner C. Harney J. Larsen P.R. Endocrinology. 2000; 141: 1127-1135Crossref PubMed Scopus (89) Google Scholar) whereas in the a similar in a 3–4-fold increase in activity 2 This be explained by a more of the enzyme in mesothelioma cells. Furthermore, as in cells and transfected to T4, the D2 the and a that be by with inhibitors results that the targeting of D2 to the system and the of its are of this protein, of the cell type in which is or is transiently expressed or In the in the that hD2 is a selenoprotein, the of the human Dio2 as as the homologous chicken, mouse, and are also with reported recently that of the Dio2 gene D2 activity in and M. S. Germain D.L. Galton V.A. and Scholar). a is to that this is the of high levels of a selenodeiodinase in human cells from a that not express the other the high levels of in S.A. Tu H.M. Harney J.W. M. S.J. Larsen P.R. J. Med. 2000; PubMed Scopus Google Scholar). It be of to also express high levels of D2 and of of T4 to T3 conversion have in these cells. Dr. for cell lysates for activity.