The deiodination of thyroid hormones in extrathyroidal tissues plays an important role in modulating thyroid hormone action. The type II deiodinase (DII) converts thyroxine to the active hormone 3,5,3 Ј -triiodothyronine, and in the rat is expressed in the brain, pituitary gland, and brown adipose tissue (BAT). Complementary DNAs (cDNAs) for the types I and III deiodinases (DI and DIII, respectively) have been isolated and shown to code for selenoproteins. However, information concerning the structure of the mammalian DII remains limited, and the pattern of its expression in human tissues is undefined. We report herein the identification and characterization of rat and human DII cDNAs. Both code for selenoproteins and exhibit limited regions of homology with the DI and DIII. In the rat pituitary and BAT, DII mRNA levels are altered more than 10-fold by changes in the thyroid hormone status of the animal. Northern analysis of RNA derived from human tissues reveals expression of DII transcripts in heart, skeletal muscle, placenta, fetal brain, and several regions of the adult brain. These studies demonstrate that: ( a ) the rat and human DII are selenoproteins, ( b ) DII expression in the rat is regulated, at least in part, at the pretranslational level in some tissues, and ( c ) DII is likely to be of considerable physiologic importance in thyroid hormone economy in the human fetus and adult. ( J. Clin. Invest. 1996. 98:405-417.)

Type 3 iodothyronine deiodinase (D3) catalyzes the conversion of T4 and T3 to inactive metabolites. It is highly expressed in placenta and thus can regulate circulating fetal thyroid hormone concentrations throughout gestation. We have cloned and expressed a 2.1-kb human placental D3 cDNA which encodes a 32-kD protein with a Km of 1.2 nM for 5 deiodination of T3 and 340 nM for 5' deiodination of reverse T3. The reaction requires DTT and is not inhibited by 6n-propylthiouracil. We quantitated transiently expressed D3 by specifically labeling the protein with bromoacetyl [12"'I T3. The KatIK. ratio for 5 deiodination of T3 was over 1,000-fold that for 5' deiodination of reverse T3. Human D3 is a selenoenzyme as evidenced by (a) the presence of an in frame UGA codon at position 144, (b) the synthesis of a 32-kD 75Se-labeled protein in D3 cDNA transfected cells, and (c) the presence of a selenocysteine insertion sequence element in the 3' untranslated region of the mRNA which is required for its expression. The D3 selenocysteine insertion sequence element is more potent than that in the type 1 deiodinase or glutathione peroxidase gene, suggesting a high priority for selenocysteine incorporation into this enzyme. The conservation of this enzyme from Xenopus laevis tadpoles to humans implies an essential role for regulation of thyroid hormone inactivation during embryological development. (J. Clin. Invest. 1995. 96:2421-2430

V-domain Ig suppressor of T cell activation (VISTA) is a novel checkpoint regulator with limited homology to other B7 family members. The constitutive expression of VISTA on both the myeloid and T lymphocyte lineages coupled to its important role in regulating innate and adaptive immune responses, qualifies VISTA to be a promising target for immunotherapeutic intervention. Studies have shown differential impact of agonistic and antagonistic targeting of VISTA, providing a unique landscape for influencing the outcome of cancer and inflammatory diseases.

Thyroid hormone (TH) is critical for cardiac development and heart function. In heart disease, TH metabolism is abnormal, and many biochemical and functional alterations mirror hypothyroidism. Although TH therapy has been advocated for treating heart disease, a clear benefit of TH has yet to be established, possibly because of peripheral actions of TH. To assess the potential efficacy of TH in treating heart disease, type 2 deiodinase (D2), which converts the prohormone thyroxine to active triiodothyronine (T3), was expressed transiently in mouse hearts by using the tetracycline transactivator system. Increased cardiac D2 activity led to elevated cardiac T3 levels and to enhanced myocardial contractility, accompanied by increased Ca 2؉ transients and sarcoplasmic reticulum (SR) Ca 2؉ uptake. These phenotypic changes were associated with up-regulation of sarco(endo)plasmic reticulum calcium ATPase (SERCA) 2a expression as well as decreased Na ؉ ͞Ca 2؉ exchanger, ␤-myosin heavy chain, and sarcolipin (SLN) expression. In pressure overload, targeted increases in D2 activity could not block hypertrophy but could completely prevent impaired contractility and SR Ca 2؉ cycling as well as altered expression patterns of SERCA2a, SLN, and other markers of pathological hypertrophy. Our results establish that elevated D2 activity in the heart increases T3 levels and enhances cardiac contractile function while preventing deterioration of cardiac function and altered gene expression after pressure overload.calcium ͉ cardiac hypertrophy ͉ sarcoplasmic reticulum ͉ deiodinase ͉ transgenic mice T hyroid hormone (TH) is essential for normal development in vertebrates (1), with TH levels rising postnatally and peaking in the third week of life (2). This surge is critical for fetal-to-adult switch in the cardiac gene program and is responsible for changes in Ca 2ϩ homeostasis, myosin isozyme content [␣-myosin heavy chain (MHC)-to-␤-MHC switch], and action potential profile (3, 4). Intriguingly, the genetic and functional changes associated with heart failure, such as reduced Ca 2ϩ transients and ␣-MHC-to-␤-MHC shifts, which recapitulate the fetal gene program, are also observed in hypothyroidism (5, 6). In addition, TH metabolism and signaling are abnormal in heart failure (5, 7, 8). For example, circulating and cardiac triiodothyronine (T3) levels (i.e., active TH) are reduced in advanced heart disease, after acute myocardial infarction, and in patients with cardiopulmonary bypass. These T3 changes occur in association with decreased peripheral conversion of thyroxine (T4) into T3 (5, 8) and elevated cardiac deiodinase type 3 (D3) activity (9). TH receptor expression is also altered in pathological hypertrophy (10), and myocardial contractility is impaired in mice lacking TH receptors (11). Not surprisingly, TH and its analogue 3,5-diiodothyropropionic acid (DITPA) have been advocated for treating heart failure (12, 13) and for reversing cardiac dysfunction in patients with hypothyroidism (5), although TH use has been limited by car...

The type III iodothyronine deiodinase metabolizes the active thyroid hormones thyroxine and 3,5,3'-triiodothyronine to inactive compounds. Recently, we have characterized a Xenopus laevis cDNA (XL-15) that encodes a selenoprotein with type III deiodinase activity (St. Germain, D.L., Schwartzman, R., Croteau, W., Kanamori, A., Wang, Z., Brown, D.D., and Galton, V.A. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 7767-7771). Using the XL-15 as a probe, we screened a rat neonatal skin cDNA library. Among the clones isolated was one (rNS43-1) which contained a 2.1-kilobase pair cDNA insert that manifested significant homology to both the XL-15 and the G21 rat type I deiodinase cDNAs, including the presence of an in-frame TGA codon. Expression studies demonstrated that the rNS43-1 cDNA encodes a protein with 5-, but not 5'-, deiodinase activity that is resistant to inhibition by propylthiouracil and aurothioglucose. Northern analysis demonstrated a pattern of tissue expression in the rat consistent with that of the type III deiodinase and site directed mutagenesis confirmed that the TGA triplet codes for selenocysteine. We conclude that the rNS43-1 cDNA encodes the rat type III deiodinase and that the types I and III deiodinases present in amphibians and mammals constitute a family of conserved selenoproteins important in the metabolism of thyroid hormones.