Ulrich Loos

The promoter of the human sodium/iodide symporter responds to certain phthalate plasticisers

The diesters of benzene-1,2-dicarboxylic (phthalic) acid, the phthalates, are used to make plastics flexible and can comprise 40% of the weight of plastic. Human exposure to phthalates can occur via ingestion, inhalation and dermal routes, as well as through parenteral exposure from medical devices containing phthalates. Since earlier morphological studies showed that some phthalates induced thyroid hyperactivity, we thought it important to investigate possible effects of six major phthalates on the transcriptional activity of sodium/iodide symporter (NIS). Di-isodecyl phthalate (DIDP), benzyl butyl phthalate (BBP) and di-octyl phthalate (DOP) increased the activity of the human NIS promoter construct 2.5-, 2.6- and 2.4-fold, respectively. Likewise, these phthalates also enhanced the rat NIS endogenous mRNA expression ca. 2-fold. No effect was observed for bis-(2-ethylhexyl) phthalate (DEHP) and di-isononyl phthalate (DINP), whereas dibutyl phthalate (DBP) appeared to down-regulate hNIS promoter. Although the demonstrated stimulation of NIS gene transcription by DIDP, BBP and DOP is not very strong, this finding is of great importance as humans are routinely exposed for long periods to phthalate plasticisers, the accumulation of which may contribute to thyroid hyperfunction.

Modulation of iodide uptake by dialkyl phthalate plasticisers in FRTL-5 rat thyroid follicular cells

Plasticisers imparting flexibility to plastics are man-made chemicals abundantly present in the environment. Effects of six different dialkyl phthalates were studied in vitro in the rat thyroid cell line FRTL-5 on their ability to modulate basal iodide uptake mediated by the sodium/iodide symporter (NIS). The present study shows that diisodecyl phthalate (DIDP), dioctyl phthalate (DOP), diisononyl phthalate (DINP) and bis (2-ethylhexyl) phthalate (DEHP) significantly enhance iodide uptake when concentrations in the magnitude between 10(-4) M and 10(-3) M were applied. In this range, these phthalates do not assess toxicity on the cells. Specific inhibiton of NIS demonstrated that enhancement of iodide uptake is due to NIS. In contrast, benzyl butyl phthalate (BBP) also augments iodide uptake at 1mM but this concentration has just exceeded the toxicity threshold and dibutyl phthalate (DBP), the most toxic compound did not modulate iodide uptake at any concentration applied. As we can deduce from our results, plasticisers are capable of significantly modulating NIS mediated iodide uptake activity.

Characterization of a thyroid-specific and cyclic adenosine monophosphate-responsive enhancer far upstream from the human sodium iodide symporter gene.

We describe the cloning and characterization of a human sodium iodide (NIS) upstream enhancer (NUE). This putative enhancer was cloned based on its sequence homology (69% identity) to the rat NUE. A 296 base pair (bp) genomic DNA fragment, which is located 9000 bp upstream from the human hNIS gene, was amplified by polymerase chain reaction (PCR) and inserted into a luciferase reporter gene in front of both the homologous NIS promoter and the heterologous SV40 promoter. No enhancer activity could be found after transfection into HeLa cells, but in FRTL-5 cells representing the thyroid model, a threefold stimulation of the NIS promoter was found. This enhancer activity was present in both directions and was stimulated threefold by thyrotropin (TSH) and 14-fold by the cyclic adenosine monophosphate (cAMP) agonist forskolin. A small element (TGACGCA) in this enhancer was found to be of central importance, because its site-directed mutagenesis abolished the enhancer activity. This element bound specifically to proteins in nuclear extracts from FRTL-5 cells and to a lesser extent also from HeLa cells. In summary, we describe a thyroid-specific and cAMP-responsive enhancer far upstream from the human NIS gene, which is located in the intronic region of another gene coding for a ribosomal protein.

Inhibition of TSH-stimulated radioiodine turnover and release of T4 and T3 in vivo by somatostatin☆

Abstract With respect to the hypothalamic-pituitary-thyroid axis, it has been demonstrated that somatostatin blocks the pituitary thyrotropin (TSH) release stimulated by thyrotropin-releasing hormone (TRH).1,2 Recently, somatostatin has been shown to be present in the thyroid gland.3,4 To elucidate the role of somatostatin in the regulation of thyroid function, the effect of somatostatin on TSH-stimulated thyroidal radioiodine turnover and thyroid hormone release in vivo were studied in euthyroid volunteers.

Iodination of proteins in TPO transfected thyroid cancer cells is independent of NIS.

This study shows that organification of radioiodide into proteins of thyroid cancer cells exogenously co-expressing the thyroid peroxidase (TPO) and the sodium/iodide symporter (NIS) is independent of NIS function. When administering (125) I to cells constitutively expressing either NIS, or TPO or NIS/TPO, next to iodide accumulation due to NIS activity, organification was exclusively observed in TPO expressing/co-expressing cells. The use of specific inhibitors for TPO and NIS showed that organification is strictly dependent of TPO and not of NIS. An identical pattern of iodoproteins migrating between approximately 75 and 200 kDa in all cell lines tested was observed. Among the five major iodoproteins, two polypeptides appear to be related and three are most probably unrelated, according to their peptide pattern. Our results significantly indicate that co-expression of TPO in NIS transfected cells mediates iodination on the one hand but on the other hand does not contribute to augmentation of a putative NIS-based radioiodide concentrator gene therapy.

Thyroid function in term newborn infants with congenital goiter

Eighty-four term newborn infants without goiter and 45 newborn infants with congenital goiter were studied with regard to thyroid function. The radiologic development of the femoral and tibial epiphyses was evaluated in those with goiter. Fifty-eight percent of the patients had retarded bone age, markedly elevated TSH levels, elevated TBI, decreased total T4I, and decreased PBI values. Forty-two percent of newborn infants with congenital goiter had a normal bone age, normal values for TSH, PBI, and total T4I, and elevated values for TBI. It is concluded that the 58% of the newborn infants with congenital goiter had subtle hypothyroidism. They require substitution therapy with thyroid hormones in order to avoid possible retardation of normal brain development. Patients with congenital goiter who have no biochemical evidence of hypothyroidism should also be treated with thyroid hormones to achieve rapid regression of goiter.

Evidence for Peripheral Autoregulation of Thyroxine Conversion

The reciprocal changes of triiodothyronine (T3) serum concentrations and (SC) and thyroxine (T4) SC observed in patients with varying degrees of primary hypothyroidism are well known. T3 SC are often still within normal limits, when T4 SC have already reached low-normal or subnormal levels. The patients do not have clinical evidence of hypothyroidism at that time (1). Considering the other extreme of the spectrum, most athyroid patients have a negative TRH test only if T4 SC are raised to levels beyond the normal range by large doses of T4. This is associated with T3 SC in the upper normal range. These patients do not appear to be clinically hyperthyroid (2). These findings suggest that a non-thyroidal mechanism accounts for the reciprocal changes in T4 SC and T3 SC. The purpose of this clinical study was to clarify the role of these non-thyroidal mechanisms in regulating T3.

[Acute factitious hyperthyroidism--moderate clinical symptoms in 3 cases under beta-blocker treatment].

The clinical and laboratory findings are described in three patients who ingested large amounts of L-thyroxine (two cases) and L-thyroxine together with L-triiodothyronine and who were treated with propranolol. Serum concentrations of thyroxine (maximum values 75 micrograms/dl, 64 micrograms/dl, and 20 micrograms/dl, respectively; normal range 4-12 micrograms/dl), triiodothyronine (maximum values 837 ng/dl, 453 ng/dl, and 566 ng/dl, resp.; normal range 80-180 ng/dl), reverse triiodothyronine (maximum values 235 ng/dl, 190 ng/dl, and 65 ng/dl, resp.; normal range 10-40 ng/dl) as well as free thyroxine equivalent and free triiodothyronine equivalent were monitored daily until they reached the normal range. Statistical analysis of the kinetics of these parameters indicated that the extreme thyroxine conversion was directed toward reverse triiodothyronine, partly due to the treatment with the beta-adrenergic blocker propranolol. The striking discrepancy between the high concentrations of the active hormones and the moderate clinical symptoms was most likely caused by peripheral effects of propranolol.SummaryThe clinical and laboratory findings are described in three patients who ingested large amounts ofl-thyroxine (two cases) andl-thyroxine together withl-triiodothyronine and who were treated with propranolol. Serum concentrations of thyroxine (maximum values 75 µg/dl, 64 µg/dl, and 20 µg/dl, respectively; normal range 4–12 µg/dl), triiodothyronine (maximum values 837 ng/dl, 453 ng/dl, and 566 ng/dl, resp.; normal range 80–180 ng/dl), reverse triiodothyronine (maximum values 235 ng/dl, 190 ng/dl, and 65 ng/dl, resp.; normal range 10–40 ng/dl) as well as free thyroxine equivalent and free triiodothyronine equivalent were monitored daily until they reached the normal range. Statistical analysis of the kinetics of these parameters indicated that the extreme thyroxine conversion was directed toward reverse triiodothyronine, partly due to the treatment with the β-adrenergic blocker propranolol. The striking discrepancy between the high concentrations of the active hormones and the moderate clinical symptoms was most likely caused by peripheral effects of propranolol.

EXPRESSION OF S14-MRNA AND ITS TRANSLATIONAL PRODUCT IN DIFFERENTIATING 3T3-L1 CELLS

During adipogenic conversion of 3T3-L1 cells S14-mRNA increased from undetectable levels in preadipocytes to high levels in differentiated adipocytes (adipocyte-like cells). In vitro translation of hybrid-selected S14-mRNA revealed an identical protein in 3T3-L1 adipocytes and in rat liver with regard to migration properties in two-dimensional gel electrophoresis. No translation product was found in 3T3-L1 preadipocytes. In conclusion, protein-S14 is very similar or even identical in rat lipogenic tissues and in 3T3-L1 adipocytes. Therefore, this cell line can be employed to elucidate further physiological aspects of protein-S14.

Acute hypothyroidism has no effect on pulmonary vascular resistance

SummaryThe effect of acute hypothyroidism on the pulmonary circulation was studied in 9 nonobese athyreotic patients by right heart catheterization at rest and during exercise. The patients were studied while they were hypothyroid 2 weeks after ceasing triiodothyronine treatment and while they were euthyroid on replacement therapy. At rest, pulmonary blood flow [4.0±0.6 l/min vs 5.8±1.0 l/min,p<0.01] and systolic pulmonary artery pressure [18±3 mmHg vs 23±2 mmHg,p<0.01] were lower when the patients were hypothyroid than when they were euthyroid. The mean and diastolic pressures in the pulmonary artery and the pulmonary capillary pressures were not different among the groups. Likewise, thyroid hormone levels had no significant effect on pulmonary vascular resistance [100±25 dyn-s-cm−5 vs 90±23 dyn-s-cm−5]. With supine exercise, pulmonary blood flow [10.1±1.6 l/min vs. 13.2±2.0 l/min,p<0.01], mean pulmonary artery pressure [25±6 mmHg vs 30±6 mmHg,p<0.02], and systolic pulmonary artery pressure [36±6 mmHg vs 44±8 mmHg,p<0.01] were lower when the patients were hypothyroid. The diastolic pulmonary artery pressure and the pulmonary capillary pressure were similar in both thyroid states. Again, thyroid deficiency had no effect on pulmonary vascular resistance [81±23 dyn-s-cm−5 vs 76±24 dyn-s-cm−5]. The lower systolic pressures in the pulmonary artery seen in hypothyroidism are probably due to the decreased systolic volume load of the pulmonary circulation. The data do not suggest that thyroid hormones play a role in the regulation of pulmonary vascular resistance.