Biomedical subjects

Donald L St Germain

Publications and source records attributed to Donald L St Germain.

9 recordsLinked to original sources

PubMed2005

Effects of rexinoids on thyrotrope function and the hypothalamic-pituitary-thyroid axis.

Retinoid X receptor (RXR)-selective retinoids (rexinoids) can cause central hypothyroidism in humans, and this effect has been confirmed in rodent models. In this report, we characterized the effect of rexinoids on the hypothalamic-pituitary-thyroid axis in mice and TSH regulation in a thyrotrope-derived cell line. The synthetic rexinoid (LG 268) suppressed TSH and T4 levels in mice. Hypothalamic TRH mRNA was unaffected, but steady-state pituitary TSHbeta mRNA levels were significantly lowered, suggesting a direct effect of rexinoids on thyrotropes. LG 268 suppressed TSH protein secretion and TSHbeta mRNA in TalphaT1 thyrotropes as early as 8 h after treatment, whereas the retinoic acid receptor-selective retinoid (TTNPB) had no effect. Type 2 iodothyronine deiodinase (D2) mRNA and activity were suppressed by LG 268 in TalphaT1 cells, whereas only D2 mRNA was suppressed in mouse pituitaries. LG 268 suppressed TSHbeta promoter activity by 42% and the -200 to -149 region accounted for a majority of the LG 268-mediated suppression of promoter activity. The RXRgamma isotype is expressed in thyrotropes. In vitro transfection and in vivo transgenic studies indicate that any RXR isotype can mediate TSH suppression by rexinoids, but the RXRgamma isotype is most efficient at mediating this response. RXRgamma-deficient mice lacked pituitary D2 mRNA suppression by LG 268, but D2 activity remained intact. In summary, RXR-selective retinoids (rexinoids) have multiple effects on the hypothalamic-pituitary-thyroid axis. Rexinoids directly suppress TSH secretion, TSHbeta mRNA levels and promoter activity, and D2 mRNA levels but have no direct effect on hypothalamic TRH levels. Rexinoids also stimulate type 1 iodothyronine deiodinase activity in the liver and pituitary.

Animals↗

PubMed2005

Targeted disruption of the type 1 selenodeiodinase gene (Dio1) results in marked changes in thyroid hormone economy in mice.

The type 1 deiodinase (D1) is thought to be an important source of T3 in the euthyroid state. To explore the role of the D1 in thyroid hormone economy, a D1-deficient mouse (D1KO) was made by targeted disruption of the Dio1 gene. The general health and reproductive capacity of the D1KO mouse were seemingly unimpaired. In serum, levels of T4 and rT3 were elevated, whereas those of TSH and T3 were unchanged, as were several indices of peripheral thyroid status. It thus appears that the D1 is not essential for the maintenance of a normal serum T3 level in euthyroid mice. However, D1 deficiency resulted in marked changes in the metabolism and excretion of iodothyronines. Fecal excretion of endogenous iodothyronines was greatly increased. Furthermore, when compared with both wild-type and D2-deficient mice, fecal excretion of [125I]iodothyronines was greatly increased in D1KO mice during the 48 h after injection of [125I]T4 or [125I]T3, whereas urinary excretion of [125I]iodide was markedly diminished. From these data it was estimated that a majority of the iodide generated by the D1 was derived from substrates other than T4. Treatment with T3 resulted in a significantly higher serum T3 level and a greater degree of hyperthyroidism in D1KO mice than in wild-type mice. We conclude that, although the D1 is of questionable importance to the wellbeing of the euthyroid mouse, it may play a major role in limiting the detrimental effects of conditions that alter normal thyroid function, including hyperthyroidism and iodine deficiency.

Animals↗

PubMed2005

Insights into the role of deiodinases from studies of genetically modified animals.

The deiodinases function at a pre-receptor level in tissues to modulate the concentrations, and thus the actions, of thyroid hormones. Although much has been learned in the last two decades about the biochemical properties and expression patterns of these enzymes, a complete understanding of their physiologic roles requires study of their actions in the intact animal. To date only a limited number of naturally occurring human or animal models exhibiting excessive or deficient deiodinase activity have been defined. In particular, no human genetic models of deiodinase deficiency have been identified. However, several transgenic animal models involving either loss-of function or gain-of-function of deiodinase activity have been devised and are currently being characterized. This review focuses on the progress being made in using these animal models to define the physiologic functions and significance of this important class of enzymes.

Alleles↗

PubMed2004

Hearing loss and retarded cochlear development in mice lacking type 2 iodothyronine deiodinase.

The later stages of cochlear differentiation and the developmental onset of hearing require thyroid hormone. Although thyroid hormone receptors (TRs) are a prerequisite for this process, it is likely that other factors modify TR activity during cochlear development. The mouse cochlea expresses type 2 deiodinase (D2), an enzyme that converts thyroxine, the main form of thyroid hormone in the circulation, into 3,5,3'-triiodothyronine (T3) the major ligand for TRs. Here, we show that D2-deficient mice have circulating thyroid hormone levels that would normally be adequate to allow hearing to develop but they exhibit an auditory phenotype similar to that caused by systemic hypothyroidism or TR deletions. D2-deficient mice have defective auditory function, retarded differentiation of the cochlear inner sulcus and sensory epithelium, and deformity of the tectorial membrane. The similarity of this phenotype to that caused by TR deletions suggests that D2 controls the T3 signal that activates TRs in the cochlea. Thus, D2 is essential for hearing, and the results suggest that this hormone-activating enzyme confers on the cochlea the ability to stimulate its own T3 response at a critical developmental period.

Animals↗

PubMed2004

Complex organization and structure of sense and antisense transcripts expressed from the DIO3 gene imprinted locus.

The human DIO3 gene and its mouse homolog, Dio3, map to chromosomes 14q32 and 12F1, respectively, and code for the type 3 deiodinase, an enzyme that inactivates thyroid hormones and is highly expressed during pregnancy and development. Mouse Dio3 is imprinted and preferentially expressed from the paternal allele in the fetus. We analyzed the human DIO3 genomic region and identified a gene (DIO3OS) that is transcribed in the antisense orientation. Multiple DIO3OS transcripts are expressed in most tissues. The structure of several DIO3OS cDNAs obtained by RT-PCR-based techniques reveals the occurrence of numerous splice variants. The exon-intron structures of DIO3OS are similar in mouse and human, but the homology of the exonic sequence is very low, except for the first exon, and no conserved open reading frame is present. We also detected DIO3 transcripts containing additional 5' untranslated sequence and a potential alternative upstream promoter for mouse Dio3. Exonic sequence of a Dio3os cDNA overlaps with the Dio3 promoter and strong promoter activity in the antisense orientation is detected in a genomic fragment located 3' of mouse and human DIO3 but not in the DIO3 promoter region. These results suggest that the DIO3 gene may lie within the structure of the antisense gene, a complex arrangement often observed in imprinted loci.

Alternative Splicing↗

PubMed2003

Activity and response to serum of the mammalian thyroid hormone deiodinase 3 gene promoter: identification of a conserved enhancer.

Dio3 is an imprinted gene that codes for the type 3 deiodinase (D3), a conserved selenocysteine-containing enzyme that degrades thyroid hormones (TH) into inactive metabolites. D3 is partially responsible for the very low TH levels that are found in utero as it is highly expressed in mammalian uterus, placenta, and fetal tissues. For this reason, it is presumed that D3 protects the developing fetus from high, adult-like levels of TH. To further understand the regulation of D3 expression, we analyzed the promoter activity of 6 kb of 5'-flanking regions of the human and mouse Dio3, by transient transfection studies. Both mouse and human Dio3 promoters are markedly responsive to serum and, to a lesser extent, to phorbol esters and fibroblast growth factor, but only in a cell line in which the endogenous Dio3 mRNA is also responsive to those factors. In addition, we identified a conserved enhancer located 3' of the gene containing putative AP-1 and serum response elements, and that further increases the serum responsiveness of the Dio3 promoter in a cell-specific manner. The cell-specific serum response of the Dio3 promoter and the identified enhancer may play critical roles in the regulation of gene expression at this imprinted locus.

5' Flanking Region↗

PubMed2003

Determinants of iodothyronine deiodinase activities in rodent uterus.

The deiodinase types 2 and 3 (D2, D3), which convert T4 to active and inactive metabolites, respectively, are expressed in the rodent uterus and highly induced during pregnancy. To examine the factors regulating the expression of these enzymes in this tissue, we studied D2 and D3 activity in pregnant rats, in pseudopregnant rats before and after the induction of artificial decidualization, and in ovariectomized rats treated with 17beta-estradiol (E2) and/or progesterone (P). Our results demonstrate that induction of D3 activity begins immediately after implantation and increases markedly over the next 72 h. A similar time course and magnitude of D3 induction is noted in the artificially decidualized uterus in pseudopregnant rats, whereas only minimal increases in activity are observed in the nondecidualized control uterine horns in the same animal. In contrast, D2 activity is not induced by a decidualization stimulus. In spontaneously cycling female rats, both D2 and D3 were observed to be 3- to 8-fold higher in proestrus, compared with diestrus. Furthermore, levels of D2 and D3 activity were greatly increased in ovariectomized rats given E2 and P in various combinations. D2 activity was stimulated primarily by E2, whereas E2 and P acted synergistically to increase D3 activity. These results demonstrate that E2 and P regulate thyroid hormone metabolism in the uterus, and that the implantation process is a potent stimulus for the induction of D3 activity in this organ. Such precise and profound changes in deiodinase expression are likely to play important physiological roles in fetal development and may influence uterine function.

Animals↗

PubMed2003

Thyroid hormone deiodinases: physiology and clinical disorders.

Thyroid hormone action is achieved through the binding of 3,5,3'-triiodothyronine to its nuclear receptor, which results in alterations in gene expression. An impairment in thyroid hormone action during vertebrate development results in severe, irreversible abnormalities in tissue growth, maturation, and function. The deiodinases are a family of selenoproteins expressed in a number of fetal and adult tissues that catalyze the activation and inactivation of thyroid hormones. Their unique biochemical characteristics and tissue and developmental expression patterns suggest that deiodinases may control the concentration of active thyroid hormone available to specific tissues or cell types at certain stages of development. The deiodinases thus appear to play an important role in regulating thyroid hormone action at a prereceptor level. Current research focusing on a better understanding of the biochemistry, regulation, and physiologic role of these enzymes is the focus of this review.

Animals↗

PubMed2002

Dexamethasone inhibits growth factor-induced type 3 deiodinase activity and mRNA expression in a cultured cell line derived from rat neonatal brown fat vascular-stromal cells.

Studies examining the regulation of the type 3 deiodinase (D3) have been hampered by the lack of cell lines that constitutively express this enzyme. To address this issue, a new cell line, designated brown fat vascular-stromal (BVS-1), was generated by continuous subculturing of precursor cells derived from the vascular-stromal fraction of rat neonatal brown fat. BVS-1 cells did not differentiate into adipocytes when cultured for 5 d in DMEM supplemented with 2% newborn calf serum, 4 nM insulin, 2 nM T(3), and 10 nM dexamethasone (DEX). However, when cultured in regular medium, the cells expressed high levels of D3 activity (1-5 pmol/h per milligram protein) and mRNA. D3 mRNA was markedly induced by treatment for 6 h with epidermal growth factor, acid or basic fibroblast growth factors (10 ng/ml), or a 3-h treatment with a phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA), 1 microM] or 10% fetal bovine serum. However, preincubation of cells overnight with 50 nM DEX completely blocked the D3-inducing effects of basic fibroblast growth factor. The DEX effect was partially blocked when a glucocorticoid receptor antagonist was present. Overnight DEX treatment (50 nM) also decreased basal D3 activity by 80%. In summary, we have established BVS-1 cells as a continuous cell line useful for studying the regulation of D3 expression. Furthermore, we have shown that DEX inhibits growth factor-induced D3 expression in these cells.

Adipose Tissue, Brown↗