Assignment of type II iodothyronine deiodinase gene (DIO2) to human chromosome band 14q24.2-->q24.3 by in situ hybridization

Thyroid hormone deiodinases and cancer

Deiodinases constitute a group of thioredoxin fold-containing selenoenzymes that play an important function in thyroid hormone homeostasis and control of thyroid hormone action. There are three known deiodinases: D1 and D2 activate the pro-hormone thyroxine (T4) to T3, the most active form of thyroid hormone, while D3 inactivates thyroid hormone and terminates T3 action. A number of studies indicate that deiodinase expression is altered in several types of cancers, suggesting that (i) they may represent a useful cancer marker and/or (ii) could play a role in modulating cell proliferation - in different settings thyroid hormone modulates cell proliferation. For example, although D2 is minimally expressed in human and rodent skeletal muscle, its expression level in rhabdomyosarcoma (RMS)-13 cells is threefold to fourfold higher. In basal cell carcinoma (BCC) cells, sonic hedgehog (Shh)-induced cell proliferation is accompanied by induction of D3 and inactivation of D2. Interestingly a fivefold reduction in the growth of BCC in nude mice was observed if D3 expression was knocked down. A decrease in D1 activity has been described in renal clear cell carcinoma, primary liver cancer, lung cancer, and some pituitary tumors, while in breast cancer cells and tissue there is an increase in D1 activity. Furthermore D1 mRNA and activity were found to be decreased in papillary thyroid cancer while D1 and D2 activities were significantly higher in follicular thyroid cancer tissue, in follicular adenoma, and in anaplastic thyroid cancer. It is conceivable that understanding how deiodinase dysregulation in tumor cells affect thyroid hormone signaling and possibly interfere with tumor progression could lead to new antineoplastic approaches.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Pretranslational regulation of type 2 deiodinase

Type 2 deiodinase (D2) is a highly potent selenoenzyme that catalyzes the first step of thyroid hormone action and its expression is under close, multilevel control. D2 expression is tissue specific and can be regulated by transcriptional and posttranscriptional mechanisms. There is now compelling evidence that D2 is critically important in triiodothyronine (T3) homeostasis as well as in several biologic processes in which thyroid hormone is involved. A host of factors are known to influence dio2 mRNA including the transcription factors that regulate the dio2 gene in different cell types. In particular, cyclic adenosine monophosphate (cAMP) and two homeodomain- containing proteins are relevant for its defined and highly dynamic expression pattern. Single nucleotide polymorphisms of the dio2 gene and other pre-translational events such as alternative splicing can potentially modulate D2 expression and ultimately T(3) generation. Given the wealth of data accumulated over the past two decades, we have attempted in this review to provide a comprehensive update on the pretranslational regulation of D2.

The DNA sequence and analysis of human chromosome 14

Chromosome 14 is one of five acrocentric chromosomes in the human genome. These chromosomes are characterized by a heterochromatic short arm that contains essentially ribosomal RNA genes, and a euchromatic long arm in which most, if not all, of the protein-coding genes are located. The finished sequence of human chromosome 14 comprises 87,410,661 base pairs, representing 100% of its euchromatic portion, in a single continuous segment covering the entire long arm with no gaps. Two loci of crucial importance for the immune system, as well as more than 60 disease genes, have been localized so far on chromosome 14. We identified 1,050 genes and gene fragments, and 393 pseudogenes. On the basis of comparisons with other vertebrate genomes, we estimate that more than 96% of the chromosome 14 genes have been annotated. From an analysis of the CpG island occurrences, we estimate that 70% of these annotated genes are complete at their 5' end.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

Expression and regulation of type II iodothyronine deiodinase in human thyroid gland

We have studied the expression of type II iodothyronine deiodinase (DII) in human thyroid tumors and cultured human thyroid cells to elucidate the mechanisms involved in the regulation of DII expression in human thyroid gland. Three cases with hyperfunctioning thyroid adenoma, including a case that showed an activating mutation of G(s)alpha with a constitutive activation of cAMP production in cultured cells, and six cases with papillary thyroid carcinoma were analyzed in the present study. Free T(3) was increased, whereas free T(4) was within the normal range in all patients with hyperfunctioning thyroid adenoma. Thyroid tumor tissue and surrounding nontumor tissue were obtained at the time of surgery, and DII expression was compared between tumor tissue and nontumor tissue in each case. Northern analysis demonstrated the presence of DII messenger RNA (mRNA) approximately 7.5 kb in size in all of the tumor and nontumor tissues. DII mRNA and DII activity in hyperfunctioning thyroid adenoma were significantly increased compared with those in nontumor tissue in each case. In contrast, DII mRNA and DII activity in papillary thyroid carcinoma were decreased compared with those in nontumor tissue in each case. DII mRNA and DII activity in cultured human thyroid cells were significantly stimulated by TSH in a dose-dependent manner. The promoter activity of the human DII gene including the complete cAMP response element, transfected to cultured human thyroid cells, was stimulated by (Bu)(2)cAMP. In summary, these results suggest that DII expression in human thyroid gland is regulated at the transcriptional level through the TSH receptor-G(s)alpha-cAMP regulatory cascade, which may be related to the increase in circulating T(3) level in patients with Graves' disease and hyperfunctioning thyroid adenoma.