Decreased cellular uptake and metabolism in Allan-Herndon-Dudley syndrome (AHDS) due to a novel mutation in the MCT8 thyroid hormone transporter.

We report a novel 1 bp deletion (c.1834delC) in the MCT8 gene in a large Brazilian family with Allan-Herndon-Dudley syndrome (AHDS), an X linked condition characterised by severe mental retardation and neurological dysfunction. The c.1834delC segregates with the disease in this family and it was not present in 100 control chromosomes, further confirming its pathogenicity. This mutation causes a frameshift and the inclusion of 64 additional amino acids in the C-terminal region of the protein. Pathogenic mutations in the MCT8 gene, which encodes a thyroid hormone transporter, results in elevated serum triiodothyronine (T3) levels, which were confirmed in four affected males of this family, while normal levels were found among obligate carriers. Through in vitro functional assays, we showed that this mutation decreases cellular T3 uptake and intracellular T3 metabolism. Therefore, the severe neurological defects present in the patients are due not only to deficiency of intracellular T3, but also to altered metabolism of T3 in central neurones. In addition, the severe muscle hypoplasia observed in most AHDS patients may be a consequence of high serum T3 levels.

Potent inhibition of estrogen sulfotransferase by hydroxylated PCB metabolites: a novel pathway explaining the estrogenic activity of PCBs.

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants which exert a variety of toxic effects in animals, including disturbances of sexual development and reproductive function. The estrogenic effects of PCBs may be mediated in part by hydroxylated PCB metabolites (PCB-OHs), but the mechanisms by which they are brought about are not understood. PCBs as well as PCB-Hs show low affinities for both alpha and beta estrogen receptor isoforms. In the present study we demonstrate that various environmentally relevant PCB-OHs are extremely potent inhibitors of human estrogen sulfotransferase, strongly suggesting that they indirectly induce estrogenic activity by increasing estradiol bioavailability in target tissues.

Sulfation of thyroid hormone by estrogen sulfotransferase.

Sulfation is one of the pathways by which thyroid hormone is inactivated. Iodothyronine sulfate concentrations are very high in human fetal blood and amniotic fluid, suggesting important production of these conjugates in utero. Human estrogen sulfotransferase (SULT1E1) is expressed among other tissues in the uterus. Here we demonstrate for the first time that SULT1E1 catalyzes the facile sulfation of the prohormone T4, the active hormone T3 and the metabolites rT3 and 3,3'-diiodothyronine (3,3'-T2) with preference for rT3 approximately 3,3'-T2 > T3 approximately T4. Thus, a single enzyme is capable of sulfating two such different hormones as the female sex hormone and thyroid hormone. The potential role of SULT1E1 in fetal thyroid hormone metabolism needs to be considered.

Characterization of human iodothyronine sulfotransferases.

Sulfation is an important pathway of thyroid hormone metabolism that facilitates the degradation of the hormone by the type I iodothyronine deiodinase, but little is known about which human sulfotransferase isoenzymes are involved. We have investigated the sulfation of the prohormone T4, the active hormone T3, and the metabolites rT3 and 3,3'-diiodothyronine (3,3'-T2) by human liver and kidney cytosol as well as by recombinant human SULT1A1 and SULT1A3, previously known as phenol-preferring and monoamine-preferring phenol sulfotransferase, respectively. In all cases, the substrate preference was 3,3'-T2 >> rT3 > T3 > T4. The apparent Km values of 3,3'-T2 and T3 [at 50 micromol/L 3'-phosphoadenosine-5'-phosphosulfate (PAPS)] were 1.02 and 54.9 micromol/L for liver cytosol, 0.64 and 27.8 micromol/L for kidney cytosol, 0.14 and 29.1 micromol/L for SULT1A1, and 33 and 112 micromol/L for SULT1A3, respectively. The apparent Km of PAPS (at 0.1 micromol/L 3,3'-T2) was 6.0 micromol/L for liver cytosol, 9.0 micromol/L for kidney cytosol, 0.65 micromol/L for SULT1A1, and 2.7 micromol/L for SULT1A3. The sulfation of 3,3'-T2 was inhibited by the other iodothyronines in a concentration-dependent manner. The inhibition profiles of the 3,3'-T2 sulfotransferase activities of liver and kidney cytosol obtained by addition of 10 micromol/L of the various analogs were better correlated with the inhibition profile of SULT1A1 than with that of SULT1A3. These results indicate similar substrate specificities for iodothyronine sulfation by native human liver and kidney sulfotransferases and recombinant SULT1A1 and SULT1A3. Of the latter, SULT1A1 clearly shows the highest affinity for both iodothyronines and PAPS, but it remains to be established whether it is the prominent isoenzyme for sulfation of thyroid hormone in human liver and kidney.

Increased cAMP levels in stimulated neutrophils inhibit their adhesion to human bronchial epithelial cells.

Bronchial epithelial cells express the intercellular adhesion molecule-1 that mediates binding of activated neutrophils via interaction with Mac-1 and/or leukocyte function-associated antigen-1. In this study, we examined whether increased intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) affected neutrophil adhesion to the human bronchial epithelial cells. It was found that the N-formylmethionyl-leucyl-phenylalanine (fMLP)-stimulated neutrophil adhesion was concentration dependently inhibited when the cAMP analogs dibutyryl adenosine 3',5'-cyclic monophosphate or 8-bromoadenosine 3',5'-cyclic monophosphate were present. The beta-adrenergic receptor agonists isoprenaline and salmeterol, in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), were also able to inhibit the fMLP-stimulated adhesion of neutrophils to bronchial epithelial cells. These agonists in combination with IBMX significantly increased the intracellular cAMP level in both neutrophils and epithelial cells. Preincubation of neutrophils with the long-acting beta2-adrenergic receptor agonist salmeterol (in the presence of IBMX) inhibited their fMLP-stimulated adhesion to epithelial cells, whereas pretreatment of epithelial cells did not influence the adhesion process. When ethanol-fixed epithelium was used, salmeterol pretreatment also diminished the adhesion of stimulated neutrophils. Moreover, combinations of salmeterol or isoprenaline with IBMX inhibited fMLP-upregulated Mac-1 expression. Therefore, we conclude from these data that elevation of intracellular cAMP in the neutrophil inhibits stimulated neutrophil adhesion to bronchial epithelial cells via Mac-1.