Carbohydrate reactivation of thyroxine 5'-deiodinase (type II) in cultured mouse neuroblastoma cells is dependent upon new protein synthesis

Folate deficiency may increase the risk for elevated TSH in patients with type 2 diabetes mellitus

BACKGROUND

Type 2 diabetes mellitus (T2DM) and thyroid dysfunction (TD) are two common chronic endocrine disorders that often coexist. Folate deficiency has been reported to be related with the onset and development of T2DM. However, the relationship between folate deficiency and TD remains unclear. This study aims to investigate the association of serum folate with TD in patients with T2DM.

METHODS

The study used data on 268 inpatients with T2DM in the Beijing Chao-yang Hospital, Capital Medical University from October 2020 to February 2021. Thyroid stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4), and serum folate were measured with chemiluminescence immunoassay (CLIA), and folate deficiency was defined as a serum folate concentration < 4.4 ng/mL. Ordinary least squares regression models were used to assess the association of serum folate with TSH concentration. Multivariable logistic regression models were performed to explore the correlation of folate deficiency and the risk for elevated TSH.

RESULTS

15.3% of T2DM patients had TD. Among those patients with TD, 80.5% had elevated TSH. Compared with the normal-TSH and low-TSH groups, the prevalence of folate deficiency was significantly higher in the elevated-TSH group (P < 0.001). Serum folate level was negatively associated with TSH (β=-0.062, 95%CI: -0.112, -0.012). Folate deficiency was associated with the higher risk for elevated TSH in patients with T2DM (OR = 8.562, 95%CI: 3.108, 23.588).

CONCLUSIONS

A low serum folate concentration was significantly associated with a higher risk for elevated TSH among T2DM patients.

Association between Lower Normal Free Thyroxine Concentrations and Obesity Phenotype in Healthy Euthyroid Subjects

We investigated whether thyroid function could identify obesity phenotype in euthyroid subjects. A cross-sectional analysis was performed among nondiabetic, euthyroid subjects. We stratified subjects into four groups by BMI and insulin resistance (IR). Of 6241 subjects, 33.8% were overweight or obese (OW/OB) and 66.2% were normal weight (NW). Free thyroxine (FT4) levels were negatively associated with body mass index, waist circumference, triglyceride, c-reactive protein, and HOMA-IR and positively with high-density lipoprotein cholesterol in both genders. In multivariate regression analysis, FT4 level, a continuous measurement, was negatively correlated with HOMA-IR (β = -0.155, P < 0.001 in men; β = -0.175, P < 0.001 in women). After adjustment for age, sex, metabolic, and life style factors, subjects in the lowest FT4 quartile had an odds ratio (OR) for IR of 1.99 (95% confidence interval 1.61-2.46), as compared to those in the highest quartile. The association between low FT4 and IR remained significant in both NW and OW/OB subgroups. In conclusion, low normal FT4 levels were independently related to IR in NW and OW/OB euthyroid subjects. Further studies are needed to investigate the mechanisms by which low FT4 levels are linked to high IR in euthyroid ranges.

High-normal TSH values in obesity: is it insulin resistance or adipose tissue's guilt?

OBJECTIVE

Clinical evidences reported subclinical alterations of thyroid function in obesity, although the relationship between thyroid status and obesity remains unclear. We cross-sectionally investigated the influence of metabolic features on hypothalamic-pituitary-thyroid axis in obesity.

DESIGN AND METHODS

We enrolled 60 euthyroid subjects with no history of type 2 diabetes mellitus and assessed the relationship of thyroid function with insulin resistance, measured using euglycemic clamp, and abdominal fat volume, quantified by computed tomography scan (CT scan). Thyroid stimulating hormone (TSH) correlated with BMI (r = 0.46; P = 0.02), both visceral (r = 0.58; P = 0.02) and subcutaneous adipose tissue volumes (r = 0.43; P = 0.03) and insulin resistance (inverse relationship with insulin sensitivity-glucose uptake: r = -0.40; P = 0.04).

RESULTS

After performing multivariate regression, visceral adipose tissue volume was found to be the most powerful predictor of TSH (β = 3.05; P = 0.01), whereas glucose uptake, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, subcutaneous adipose tissue volume, and triglycerides were not. To further confirm the hypothesis that high-normal TSH values could be dependent on adipose tissue, and not on insulin resistance, we restricted our analyses to moderately obese subjects' BMI ranging 30-35 kg/m(2). This subgroup was then divided as insulin resistant and insulin sensitive according to the glucose uptake (≤ or >5 mg · kg(-1) · min(-1), respectively). We did not find any statistical difference in TSH (insulin resistant: 1.62 ± 0.65 µU/ml vs. insulin sensitive: 1.46 ± 0.48; P = not significant) and BMI (insulin resistant: 32.2 ± 1.6 kg/m(2) vs. insulin sensitive: 32.4 ± 1.4; P = not significant), thus confirming absence of correlation between thyroid function and insulin sensitivity per se.

CONCLUSION

Our study suggests that the increase in visceral adipose tissue is the best predictor of TSH concentration in obesity, independently from the eventual concurrent presence of insulin resistance.

Relationship of thyroid function with body mass index and insulin-resistance in euthyroid obese subjects

BACKGROUND AND AIMS

It is recognized that overt thyroid dysfunction is associated with weight changes, but the influence of a minor alteration of thyroid function remains unclear. This study aimed to further investigate the relationship between obesity and thyroid function and to examine the possible role of insulin resistance on the hypothalamic-pituitary- thyroid axis.

METHODS AND RESULTS

Serum TSH and free T4 (FT4) levels, anthropometric and metabolic parameters were evaluated in 581 obese patients. In all patients TSH values progressively increased according to the severity of obesity and were positively correlated with body mass index (p=0.001, r=0.13) and waist circumference (p=0.02, r=0.11). Patients with insulin resistance showed higher TSH (1.8±1.0 vs 1.6±0.9 μUI/l; p=0.03) and lower FT4 levels (13.8±2.3 vs 15.0±2.2 pmol/l; p<0.001), as compared with patients with normal insulin sensitivity. Moreover, TSH was positively correlated with fasting insulin (p<0.001, r=0.152) and homeostasis model assessment of insulin resistance (HOMA-IR; p<0.001, r=0.148), and negatively correlated with Quantitative Insulin Sensitivity Check Index (QUICKI; p<0.001, r=-0.148); FT4 was negatively associated with fasting insulin (p<0.001, r=-0.287) and HOMA-IR (p<0.001, r=-0.295), and positively associated with QUICKI (p<0.001, r=0.295).

CONCLUSIONS

A relationship between thyroid function and overweight/ obesity condition seems to exist, mainly influenced by insulin resistance. Whether variations in TSH and/or thyroid hormones, within a normal range, can influence body weight or whether obesity per se can alter thyroid function cannot be stated so far. Further studies are needed to assess the link between thyroid function and body weight, by considering not only changes in thyroid hormones, but also body fat distribution, obesity duration and low-grade inflammation.

Plasma concentrations of free triiodothyronine predict weight change in euthyroid persons

BACKGROUND

Factors that influence energy metabolism and substrate oxidation, such as thyroid hormones (THs), may be important regulators of body weight.

OBJECTIVE

We investigated associations of THs cross-sectionally with obesity, energy expenditure, and substrate oxidation and prospectively with weight change.

DESIGN

Euthyroid, nondiabetic, healthy, adult Pima Indians (n = 89; 47 M, 42 F) were studied. Percentage body fat (%BF) was measured by using dual-energy X-ray absorptiometry; sleeping metabolic rate (SMR), respiratory quotient, and substrate oxidation rates were measured in a respiratory chamber. Thyroid-stimulating hormone (TSH), free thyroxine (T(4)), free triiodothyronine (T(3)), and leptin concentrations were measured in fasting plasma samples.

RESULTS

TSH, but neither free T(3) nor free T(4), was associated with %BF and leptin concentrations (r = 0.27 and 0.29, respectively; both: P <or= 0.01). In multiple regression analyses adjusted for age, sex, fat mass, and fat-free mass, free T(3) was a positive predictor of SMR (P = 0.02). After adjustment for age, sex, %BF, and energy balance, free T(3) was a negative predictor of 24-h respiratory quotient (P < 0.05) and a positive predictor of 24-h lipid oxidation rate (P = 0.006). Prospectively, after an average follow-up of 4 +/- 2 y, the mean increase in weight was 3 +/- 9 kg. Baseline T(3) concentrations were associated with absolute and annual percentage of changes in weight (r = -0.27, P = 0.02, and r = -0.28, P = 0.009, for the age- and sex-adjusted associations, respectively).

CONCLUSIONS

In euthyroid Pima Indians, lower free T(3) but not free T(4) concentrations were an independent predictor of SMR and lipid oxidation and a predictor of weight gain. This finding indicates that control of T(4)-to-T(3) conversion may play a role in body weight regulation.

Thyroid function in humans with morbid obesity

Morbidly obese subjects may present with abnormal thyroid function tests but the reported data are scarce. Therefore, we studied the thyroid parameters in 144 morbidly obese patients, 110 females and 34 males, to assess the prevalence of hypothyroidism. Eleven percent (11.8%) carried the diagnosis of hypothyroidism and were undergoing levothyroxine (LT4) replacement therapy, 7.7% had newly diagnosed subclinical hypothyroidism, 0.7% had subclinical hyperthyroidism and 7.7% were euthyroid with positive antibodies (anti-thyroid peroxidase antibodies [TPOAb]). From the 144 subjects, we selected a cohort of 78 euthyroid subjects with negative TPOAb, who did not receive LT4 replacement or suppression therapy (the experimental group) and compared them to 77 normal-weight euthyroid subjects, TPOA-negative, matched for age and gender who served as controls. The experimental group had higher serum levels of triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), and thyrotropin (TSH) compared to the control group. Serum TSH concentration was associated with fasting serum insulin levels and insulin resistance but not with serum leptin levels, body mass index (BMI), fat mass, and lean body mass. In conclusion, in morbidly obese individuals, the prevalence of overt and subclinical hypothyroidism was high (19.5%). The morbidly obese subjects have higher levels of T3, FT3, T4, and TSH, probably the result of the reset of their central thyrostat at higher level.

Expression of type 2 iodothyronine deiodinase in corticotropin-secreting mouse pituitary tumor cells is stimulated by glucocorticoid and corticotropin-releasing hormone

We identified the presence of iodothyronine deiodinase in AtT-20 mouse pituitary tumor cells that secrete corticotropin. Iodothyronine deiodinating activity in AtT-20 cells fulfills all the characteristics of type 2 iodothyronine deiodinase (D2), including the inhibition by thyroid hormones, the insensitivity to inhibition by 6-propyl-2-thiouracil, and the low Michaelis-Menten constant value for T4. Northern analysis using mouse D2 cRNA probe demonstrated the hybridization signal of approximately 7.0 kb in size in AtT-20 cells. D2 activity and D2 mRNA were stimulated by glucocorticoid in a dose-dependent manner but were not stimulated by testosterone or beta-estradiol. D2 expression was stimulated by (Bu)2cAMP, and CRH in a dose-dependent manner in the presence of dexamethasone. These results suggest the previously unrecognized role of local thyroid hormone activation by D2 in the regulation of pituitary corticotrophs.

Cyclic adenosine 3',5'-monophosphate and glucose stimulate thyroxine 5'-deiodinase type II in cultured mouse neuroblastoma cells

Nutrient modulation increases mouse neuroblastoma (NB) T4-5'-deiodinase II (T4-5'-D II) activity. Carbohydrates are more potent than either amino acids or glycerol as nutrient sources. Glucose rapidly (2 to 4 hours) enhances NB enzyme activity and the response is dependent on new protein synthesis. The present study was performed to further characterize this glucose effect and explore its relationship to the cyclic adenosine monophosphate (cAMP) system in these cells. NB T4-5'-D II activity reached a maximum level (sixfold) in response to glucose (10 mmol/L) at 16 hours and thereafter remained constant up to 22 hours before reverting back to basal level between 24 and 30 hours. This pattern of response allowed the performance of detailed studies on maximum glucose activated NB T4-5'-D II under transient equilibrium conditions during the 16- to 22-hour period. Addition of dibutyryl cAMP (dbcAMP) (1 mmol/L) at this stage significantly increased enzyme activity (twofold at 2 hours and fourfold at 4 and 6 hours) compared with glucose alone. There was an additive response to dbcAMP under these maximum glucose-activated conditions. Nonactivated NB T4-5'-D II showed a twofold response to dbcAMP (1 mmol/L) at 4 hours in a glucose-free medium. Under these conditions, glucose (10 mmol/L) also increased enzyme activity twofold. Combined studies with dbcAMP and glucose increased enzyme activity fourfold at 4 hours. Subsequent studies were performed with forskolin (10 mumol/L) and cholera toxin (1 nmol/L), modulators of endogenous cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of 5'-deiodinase activity in rat astroglial cells by acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) induced a large increase in the type II 5'-deiodinase (5'D) activity in astroglial cells. This required a time lag of about 4 h. Half-maximal stimulation was obtained with about 7 ng/ml aFGF. This factor at 20 ng/ml induced several times more 5'D activity than did 20 ng/ml basic fibroblast growth factor (bFGF) after 8 h incubation. aFGF (20 ng/ml) produced a 10-50-fold increase in 5'D activity after 24 h, whereas the effect of 20 ng/ml bFGF had disappeared after 24 h. Heparin (17 micrograms/ml) potentiated the 5'D response to natural and recombinant aFGF. Glucocorticoids amplified the aFGF-induction of 5'D activity. This is the first demonstration in astroglial cells that a growth factor can regulate the 5'D activity.

Glucose-evoked recovery of hepatic thyroxine 5'-deiodinase independent of de novo protein synthesis in fasted rat

The glucose-evoked recovery of Type I thyroxine 5'-deiodinase activity in the hepatic microsomes of fasted rat was not inhibited by either cycloheximide, puromycin or actinomycin D during 3 h after glucose feeding; however, [3H]-leucine uptake by the liver or the hepatic microsomal fraction was significantly inhibited by cycloheximide and puromycin but not by actinomycin D. These results indicate that the glucose-evoked recovery of deiodinase activity may be independent of de novo protein synthesis.

Molecular Cloning and Characterization of c-erb-A mRNA Species in Mouse Neuroblastoma Cells

Abstract The mouse neuroblastoma cell line is an excellent model in which to study thyroid hormone action and metabolism, particularly in neural tissue. We therefore undertook the molecular cloning and characterization in these cells of putative thyroid hormone receptors related to c-erb-A. Since rat brain tissue contains multiple cell types, and because of possible subtle differences between species (mouse and rat), we therefore screened a new cDNA library constructed from mouse neuroblastoma cell mRNA with synthetic oligonucleotide probes based on the published nucleotide sequence of the c-erb-A gene in whole rat brain. Despite the fact that this rat brain cDNA sequence is now recognized to represent a c-erb-A alpha 1 form, the cDNA clones that we isolated were all members of the newly-recognized c-erb-A alpha 2 form. This identification was made on the basis of nucleotide sequence divergence downstream of the nucleotide corresponding to amino-acid residue 370 in the predicted coding region. The two longest mouse neuroblastoma cDNA clones, clone 29 (1796 bp) and clone 32 (1410 bp), were 93% to 94% homologous with the c-erb-A alpha 2 and c-erb-A alpha 1 forms in their DNA binding and thyroid hormone binding domains (up to amino-acid residue 370 in the latter). Both clones 29 and 32 were incomplete in that they terminated at their 3'ends at an internal Eco R1 site. Fortunately this 120 bp (40 derived amino-acid) truncation was downstream of the reported thyroid hormone binding domain. The 5'untranslated end of clone 29 (446 bp) was of interest because a region of its nucleotide sequence (279 bp) revealed a high degree of homology (87%) with rat brain c-erb-A alpha 1. This highly conserved region in clone 29 appeared to be important in the regulation of translation because only clone 32, in which this region was truncated, efficiently translated protein in a cell-free system. The protein product of clone 32 did not bind thyroid hormone. Northern blot analysis of mouse neuroblastoma mRNA with site-specific synthetic oligonucleotides revealed that the c-erb-A alpha 2 species was dominant (major band of 2.4 kb), with a lesser amount of the c-erb-A alpha1 species (major band of 1.8 kb).

IN CONCLUSION

1) We report the molecular cloning of c-erb-A variants in a specific neural tissue cell type, namely mouse neuroblastoma cells; 2) The cells contained predominantly the c-erb-A alpha 2 subtype; 3) This c-erb-A alpha 2 form was unique up to bp 167 at its 5'untranslated end, and was then followed, up to the ATG initiation codon, by a highly conserved region common to all c-erb-A a species; 4) The 5'untranslated region appeared to play a role in the translational efficiency of this mRNA; 5) The protein product of the c-erb-A alpha 2 mRNA in these cells did not appear to bind thyroid hormone. In view of this finding, the physiological role of the c-erb-A alpha 2 protein remained speculative and may involve a represser function at the thyroid hormone responsive element.