9 research outputs found
The importance of thyroid hormone sulfation during fetal development
Normal fetal development requires the presence of thyroid hormone. Disruption of any of the
processes regulating the bioavailability of thyroid hormone may contribute to congenital
anomalies. This thesis is focussed a) on the importance of thyroid hormone sulfation during fetal
development, and b) on the potential sulfation-disrupting effects of environmental chemicals such
as hydroxylated polychlorinated biphenyls (PCBs), because of potential pathogenetic
consequences of disturbed thyroid hormone sulfation for the development of organs, such as
lungs and brain.
In this general introduction, first some information is given on the development of fetal thyroid
status, and the importance of thyroid hormone for the development of organs such as the brain is
discussed. Secondly, Ihyroid hormone synthesis, transport and metabolism, which are all
processes regulating thyroid hormone bioavailability, are reviewed. Additionally, the role of
sulfation in thyroid hormone metabOlism, especially during fetal development, is addressed.
Furthermore, some general information on PCBs and other polyhalogenated aromatic
hydrocarbons is given, and their potential estrogen and thyroid hormone-disrupting effects are
discussed. Finally, the outline of this thesis is presented
Characterization of iodothyronine sulfatase activities in human and rat liver and placenta
In conditions associated with high serum iodothyronine sulfate
concentrations, e.g. during fetal development, desulfation of these
conjugates may be important in the regulation of thyroid hormone
homeostasis. However, little is known about which sulfatases are involved
in this process. Therefore, we investigated the hydrolysis of
iodothyronine sulfates by homogenates of V79 cells expressing the human
arylsulfatases A (ARSA), B (ARSB), or C (ARSC; steroid sulfatase), as well
as tissue fractions of human and rat liver and placenta. We found that
only the microsomal fraction from liver and placenta hydrolyzed
iodothyronine sulfates. Among the recombinant enzymes only the endoplasmic
reticulum-associated ARSC showed activity toward iodothyronine sulfates;
the soluble lysosomal ARSA and ARSB were inactive. Recombinant ARSC as
well as human placenta microsomes hydrolyzed iodothyronine sulfates with a
substrate preference for 3,3'-diiodothyronine sulfate (3,3'-T(2)S)
approximately T(3) sulfate (T(3)S) >> rT(3)S approximately T(4)S, whereas
human and rat liver microsomes showed a preference for 3,3'-T(2)S > T(3)S
>> rT(3)S approximately T(4)S. ARSC and the tissue microsomal sulfatases
were all characterized by high apparent K(m) values (>50 microM) for
3,3'-T(2)S and T(3)S. Iodothyronine sulfatase activity determined using
3,3'-T(2)S as a substrate was much higher in human liver microsomes than
in human placenta microsomes, although ARSC is expressed at higher levels
in human placenta than in human liver. The ratio of estrone sulfate to
T(2)S hydrolysis in human liver microsomes (0.2) differed largely from
that in ARSC homogenate (80) and human placenta microsomes (150). These
results suggest that ARSC accounts for the relatively low iodothyronine
sulfatase activity of human placenta, and that additional arylsulfatase(s)
contributes to the high iodothyronine sulfatase activity in human liver.
Further research is needed to identify these iodothyronine sulfatases, and
to study the physiological importance of the reversible sulfation of
iodothyronines in thyroid hormone metabolism
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
In vitro profiling of the endocrine-disrupting potency of brominated flame retardants
Over the last years, increasing evidence has become available that some brominated flame retardants (BFRs) may have endocrine disrupting (ED) potencies. The goal of the current study was to perform a systematic in vitro screening of the ED potencies of BFRs (1) to elucidate possible modes of action of BFRs in man and wildlife, and (2) to classify BFRs with similar profiles of ED potencies. A test set of twenty-seven individual BFRs was selected, consisting of nineteen polybrominated diphenylethers (PBDE) congeners, tetrabromobisphenol-A (TBBPA), hexabromocyclododecane (HBCD), 2,4,6-tribromophenol (246-TBP), ortho-hydroxylated BDE-47 (6OH-BDE-47), and TBBPA-bis(2,3)dibromopropylether (TBBPA-DBPE). All BFRs were tested for their potency to interact with the arylhydrocarbon receptor (AhR), androgen receptor (AR), progesterone receptor (PR), and estrogen receptor (ER). In addition, all BFRs were tested for their potency to inhibit estradiol (E2) sulfation by E2-sulfotransferase (E2SULT), to interfere with thyroid hormone 3,3`,5-triiodothyronine (T3) mediated cell proliferation, and to compete with T3-precursor thyroxine (T4) for binding to the plasma transport protein transthyretin (TTR). The results of the in vitro screening indicated that BFRs have ED potencies, some of which had not or only marginally been described before (AR-antagonism, PR-antagonism, E2SULT inhibition, and potentiation of T3-mediated effects). For some BFRs, the potency to induce AR-antagonism, E2SULT inhibition and TTR competition was higher than for natural ligands or clinical drugs used as positive controls. Based on their similarity in ED profiles, BFRs were classified into five different clusters. These findings support further investigation of the potential endocrine disrupting effects of these environmentally relevant BFRs in man and wildlife
Genotype-phenotype relationship in patients with mutations in thyroid hormone transporter MCT8
Loss-of-function mutations in thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to severe X-linked psychomotor retardation and elevated serum T3levels. Most patients, for example those with mutations V235M, S448X, insI189, or delF230, cannot stand, walk, or speak. Patients with mutations L434W, L568P, and S194F, however, walk independently and/or develop some dysarthric speech. To study the relationship between mutation and phenotype, we transfected JEG3 and COS1 cells with wild-type or mutant MCT8. Expression and function of the transporter were studied by analyzing T3and T4uptake, T3metabolism (by cotransfected type 3 deiodinase), Western blotting, affinity labeling with N-bromoacetyl-T3, immunocytochemistry, and quantitative RT-PCR. Wild-type MCT8 increased T3uptake and metabolism about 5-fold compared with empty vector controls. Mutants V235M, S448X, insI189, and delF230 did not significantly increase transport. However, S194F, L568P, and L434W showed about 20, 23, and 37% of wild-type activity.RT-PCR did not show significant differences in mRNA expression between wild-type and mutant MCT8. Immunocytochemistry detected the nonfunctional mutants V235M, insI189, and delF230 mostly in the cytoplasm, whereas mutants with residual function were expressed at the plasma membrane. Mutants S194F and L434W showed high protein expression but low affinity for N-bromoacetyl-T3; L568P was detected in low amounts but showed relatively high affinity. Mutations in MCT8 cause loss of function through reduced protein expression, impaired trafficking to the plasma membrane, or reduced substrate affinity. Mutants L434W, L568P, and S194F showed significant residual transport capacity, which may underlie the more advanced psychomotor development observed in patients with these mutations. Copyrigh
Increased thyroxine sulfate levels in critically ill patients as a result of a decreased hepatic type I deiodinase activity
Introduction: Marked changes in peripheral thyroid hormone metabolism occur in critical illness, resulting in low serum T3 and high rT 3 levels. In this study, we investigated whether T4S levels are increased in patients who died after intensive care and whether T4S levels are correlated with liver type I deiodinase (D1) or sulfotransferase (SULT) activity. Methods: A total of 64 blood samples and 65 liver biopsies were obtained within minutes after death from 79 intensive care patients, randomized for intensive or conventional insulin treatment. Serum T4S and the activities of hepatic D1 and 3,3′-diiodothyronine (T2)-SULT and estrogen-SULT were determined. Results: No differences in T4S or hepatic SULT activities were found between patients treated with intensive or with conventional insulin therapy. T4S levels were significantly elevated compared with healthy references. Furthermore, hepatic D1, but not SULT activity, showed a strong correlation with serum T4S (R = -0.53; P < 0.001) and T4S/T4 ratio (R = -0.62; P < 0.001). Cause of death was significantly correlated with hepatic T 2- and estrogen-SULT activities (P < 0.01), with SULT activities being highest in the patients who died of severe brain damage and lowest in the patients who died of a cardiovascular collapse. A longer period of intensive care was associated with higher levels of T4S (P = 0.005), and high levels of bilirubin were associated with low T2-SULT (P = 0.04) activities and high levels of T4S (P < 0.001). Conclusion: Serum T4S level
Bioactivation of dibrominated biphenyls by cytochrome P450 activity to metabolites with estrogenic activity and estrogen sulfotransferase inhibition capacity
Exposure of humans and wildlife to xenobiotics, such as halogenated biphenyls, that interfere with the endogenous estrogen balance may lead to endocrine disruption. Such compounds may either mimic or block estradiol's action by agonistic or antagonistic action, respectively. They may also affect endogenous estradiol concentrations by induction or inhibition of enzymes that metabolize estradiol. In the present study, we demonstrate that estrogenic metabolites of two brominated biphenyls, 2,2′-dibromobiphenyl (2,2′-DBB) and 4,4′-dibromobiphenyl (4,4′-DBB), are formed by rat liver microsomal cytochrome P450 (CYP) activity. Bioactivation of 2,2′-DBB and 4,4′-DBB yielded various mono- and dihydroxylated bromobiphenyl metabolites, which were collected by preparative HPLC and analyzed by LC/MS. Several of the metabolites bound to the estrogen receptor (ER) activated the ER and inhibited human estrogen sulfotransferase (hEST). Seven monohydroxylated metabolites were positively identified using synthetic monohydroxylated reference compounds. These synthetic monohydroxylated bromobiphenyls also bound to and activated the ER and inhibited hEST. The highest ER affinity was observed for 4-OH-2,2′-DBB, with an E
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