Potent thyrotropic activity of human chorionic gonadotropin variants in terms of 125I incorporation and de novo synthesized thyroid hormone release in human thyroid follicles

Thyrotropin Causes Dose-dependent Biphasic Regulation of cAMP Production Mediated by Gs and Gi/o Proteins

The thyrotropin (TSH) receptor (TSHR) signals via G proteins of all four classes and β-arrestin 1. Stimulation of TSHR leads to increasing cAMP production that has been reported as a monotonic dose-response curve that plateaus at high TSH doses. In HEK 293 cells overexpressing TSHRs (HEK-TSHR cells), we found that TSHR activation exhibits an "inverted U-shaped dose-response curve" with increasing cAMP production at low doses of TSH and decreased cAMP production at high doses (>1 mU/ml). Since protein kinase A inhibition by H-89 and knockdown of β-arrestin 1 or β-arrestin 2 did not affect the decreased cAMP production at high TSH doses, we studied the roles of TSHR downregulation and of Gi/Go proteins. A high TSH dose (100 mU/ml) caused a 33% decrease in cell-surface TSHR. However, because inhibiting TSHR downregulation with combined expression of a dominant negative dynamin 1 and β-arrestin 2 knockdown had no effect, we concluded that downregulation is not involved in the biphasic cAMP response. Pertussis toxin, which inhibits activation of Gi/Go, abolished the biphasic response with no statistically significant difference in cAMP levels at 1 and 100 mU/ml TSH. Concordantly, co-knockdown of Gi/Go proteins increased cAMP levels stimulated by 100 mU/ml TSH from 55% to 73% of the peak level. These data show that biphasic regulation of cAMP production is mediated by Gs and Gi/Go at low and high TSH doses, respectively, which may represent a mechanism to prevent overstimulation in TSHR-expressing cells. SIGNIFICANCE STATEMENT: We demonstrate biphasic regulation of TSH-mediated cAMP production involving coupling of the TSH receptor (TSHR) to Gs at low TSH doses and to Gi/o at high TSH doses. We suggest that this biphasic cAMP response allows the TSHR to mediate responses at lower levels of TSH and that decreased cAMP production at high doses may represent a mechanism to prevent overstimulation of TSHR-expressing cells. This mechanism could prevent chronic stimulation of thyroid gland function.

The Pathogenic TSH β-subunit Variant C105Vfs114X Causes a Modified Signaling Profile at TSHR

1) Background: Central congenital hypothyroidism (CCH) is a rare endocrine disorder that can be caused by mutations in the β-subunit of thyrotropin (TSHB). The TSHB mutation C105Vfs114X leads to isolated thyroid-stimulating-hormone-(TSH)-deficiency and results in a severe phenotype. The aim of this study was to gain more insight into the underlying molecular mechanism and the functional effects of this mutation based on two assumptions: a) the three-dimensional (3D) structure of TSH should be modified with the C105V substitution, and/or b) whether the C-terminal modifications lead to signaling differences. 2) Methods: wild-type (WT) and different mutants of hTSH were generated in human embryonic kidney 293 cells (HEK293 cells) and TSH preparations were used to stimulate thyrotropin receptor (TSHR) stably transfected into follicular thyroid cancer cells (FTC133-TSHR cells) and transiently transfected into HEK293 cells. Functional characterization was performed by determination of Gs, mitogen activated protein kinase (MAPK) and Gq/11 activation. 3) Results: The patient mutation C105Vfs114X and further designed TSH mutants diminished cyclic adenosine monophosphate (cAMP) signaling activity. Surprisingly, MAPK signaling for all mutants was comparable to WT, while none of the mutants induced PLC activation. 4) Conclusion: We characterized the patient mutation C105Vfs114X concerning different signaling pathways. We identified a strong decrease of cAMP signaling induction and speculate that this could, in combination with diverse signaling regarding the other pathways, accounting for the patient's severe phenotype.

Thyrotoxicosis of Pregnancy

Thyrotoxicosis presenting during pregnancy is a common clinical problem and can be challenging to differentiate between physiologic patterns of thyroid dysfunction during gestation and intrinsic hyperthyroidism. This review provides a summary of the differential diagnosis, clinical presentation, diagnostic options, potential adverse effects of maternal thyrotoxicosis to the fetus, and treatment recommendations for thyrotoxicosis arising in pregnancy.

Ovarian and adrenal androgens and their link to high human chorionic gonadotropin levels: a prospective controlled study

Background. Although the association between human chorionic gonadotropin (hCG) and hyperandrogenism was identified more than 40 years ago, relevant questions remain unanswered. Design and Methods. We conducted a prospective, longitudinal, and controlled study in 23 women with a diagnosis of a complete hydatidiform mole (HM). Results. All participants completed the study. Before HM evacuation mean hCG was markedly higher in the cases than in the control group (P ≤ 0.001). Free testosterone (T) and dehydroepiandrosterone sulfate (DHEA-S) were found to be higher in the cases (2.78 ± 1.24 pg/mL and 231.50 ± 127.20 μ/dL) when compared to the control group (1.50 ± 0.75 pg/mL and 133.59 ± 60.69 μ/dL) (P = 0.0001 and 0.001), respectively. There was a strong correlation between hCG and free T/total T/DHEA-S concentrations (r = 0.78; P ≤ 0.001, r = 0.74;  P ≤ 0.001, and r = 0.71;  P ≤ 0.001), respectively. In the cases group 48 hours after HM evacuation, hCG levels were found to be significantly lower when compared to initial levels (P = 0.001) and free T and DHEA-S declined significantly (P = 0.0002 and 0.009). Conclusion. Before uterus evacuation, hCG, free T, and DHEA-S levels were significantly higher when compared with controls finding a strong correlation between hCG and free T/DHEA-S levels. Forty-eight hours after HM treatment hCG levels declined and the difference was lost. A novel finding of our study is that in cases, besides free T, DHEA-S was also found to be significantly higher and both the ovaries and adrenal glands appear to be the sites of this androgen overproduction.

Persistent perioperative tachycardia and hypertension diagnosed as thyroid storm induced by a hydatidiform mole: a case report

Thyroid storm is a critical complication of molar pregnancy. However, early diagnosis of it is difficult because it is a rare complication and usually presents nonspecific findings. In this case report, we present a woman with molar pregnancy who had persistent tachycardia and hypertension. She was diagnosed initially with preeclampsia and sepsis as complications of molar pregnancy. During dilation and curettage under general anesthesia with sevoflurane and remifentanil, tachycardia and hypertension remained even with continuous infusion of labetalol. The patient was subsequently diagnosed with thyroid storm associated with molar pregnancy. She was restored to a clinically euthyroid state 1 day after the operation, and her thyroid function test and β-hCG values were normal 3 months later. The anesthesiologists should bear in mind the possibility of thyroid storm in patients with molar pregnancies who show persistent tachycardia and hypertension.

Thyroid dysfunction in hyperemesis gravidarum: a study in Turkish pregnant women

OBJECTIVE

In this study we investigate the possible relation of thyroid dysfunction and thyroid antibodies to hyperemesis gravidarum.

MATERIAL AND METHODS

Thirty-seven patients with hyperemesis gravidarum and 33 healthy controls have been included in this study.

RESULTS

Thyroid dysfunction was significantly more common than in controls (38% vs 6%, p=0.002). Thyroglobulin antibodies were also significantly more common in patients with hyperemesis gravidarum than controls (54 IU/mL vs. 14 IU/mL, p=0.03).

CONCLUSION

Hyperemesis gravidarum can be a risk factor for postpartum thyroid dysfunction. Thyroid antibodies must be checked in the hyperemesis population in an endemic goitre region and/or iodine replacement regions.

The superagonistic activity of bovine thyroid-stimulating hormone (TSH) and the human TR1401 TSH analog is determined by specific amino acids in the hinge region of the human TSH receptor

Bovine TSH (bTSH) has a higher affinity to the human TSHR (hTSHR) and a higher signaling activity than human TSH (hTSH). The molecular reasons for these phenomena are unknown. Distinct negatively charged residues (Glu297, Glu303, and Asp382) in the hinge region of the hTSHR are known to be important for bTSH binding and signaling. To investigate the potential relevance of these positions for differences between bTSH and hTSH in the interaction to the hTSHR, we determined bTSH- and hTSH-mediated cAMP production of several substitutions at these three hinge residues. To examine specific variations of hTSH, we also investigated the superagonistic hTSH analog TR1401 (TR1401), whose sequence differs from hTSH by four additional positively charged amino acids that are also present in bTSH. To characterize possible interactions between the acidic hTSHR positions Glu297, Glu303, or Asp382 and the additional basic residues of TR1401, we investigated TR1401 binding and signaling properties. Our data reveal increased cAMP signaling of the hTSHR using TR1401 and bTSH compared with hTSH. Whereas Asp382 seems to be important for bTSH- and TR1401-mediated but not for hTSH-mediated signaling, the substitution E297K exhibits a decreased signaling for all three TSH variants. Interestingly, bTSH and TR1401 showed only a slightly different binding pattern. These observations imply that specific residues of the hinge region are mediators of the superagonistic activity of bTSH and TR1401 in contrast to hTSH. Moreover, the simultaneous localization of binding components in the glycoprotein hormone molecule and the receptor hinge region permits important reevaluation of interacting hormone receptor domains.

Hypothyroidism associated with anti-human chorionic gonadotropin antibodies secondarily produced by gonadotropin therapy in a case of idiopathic hypothalamic hypogonadism

We report a 22-yr-old male patient with idiopathic hypothalamic hypogonadism who showed secondary resistance to gonadotropin (Gn) therapy over 3 yr after successful treatment with hCG combined with human menopausal Gn. The patient simultaneously developed subclinical hypothyroidism. Endocrine examination revealed low levels of testosterone (0.3 ng/ml), free T4 (0.91 ng/dl), and increased levels of TSH (31.1 microU/ml) in the serum. Serum autoantibodies to thyroid gland were all negative. Interestingly, thyroid function was improved after discontinuation of Gn therapy. In vitro assays by immunoprecipitation using 125I-hCG or 125I-TSH elucidated the presence of anti-hCG antibody in the serum 13 months after commencement of Gn therapy but anti-TSH antibody was not detected in the serum. Furthermore, the anti-hCG antibody specifically bound to hCG but not to other glycoproteins including TSH and FSH based on a competitive displacement assay. Bioassays using porcine thyroid cells revealed that the serum gamma-globulin fraction enables the suppression of cyclic AMP (cAMP) synthesis stimulated by TSH. Our findings suggest that anti-hCG and/or anti-idiotypic hCG antibodies induced by hCG therapy impaired TSH-dependent cAMP production through interfering with binding of TSH to its receptor, and this resulted in subclinical hypothyroidism in this patient.

Stimulation by thyroid-stimulating hormone and Grave's immunoglobulin G of vascular endothelial growth factor mRNA expression in human thyroid follicles in vitro and flt mRNA expression in the rat thyroid in vivo

To elucidate the pathogenesis of thyroid gland hypervascularity in patients with Graves' disease, we studied the expression of mRNAs for vascular endothelial growth factor (VEGF) and its receptor, Flt family, using human thyroid follicles in vitro and thiouracil-fed rats in vivo. Human thyroid follicles, cultured in the absence of endothelial cells, secreted de novo-synthesized thyroid hormone in response to thyroid-stimulating hormone (TSH) and Graves' IgG. The thyroid follicles produced VEGF mRNA but not flt-1 mRNA. The expression of VEGF mRNA was enhanced by insulin, tumor-promoting phorbol ester, calcium ionophore, dibutyryl cAMP, TSH, and Graves' IgG. When rats were fed thiouracil for 4 wk, their serum levels of TSH were increased at day 3. VEGF mRNA was also increased on day 3, accompanied by an increase in flt family (flt-1 and KDR/ flk-1) mRNA expression. These in vitro and in vivo findings suggest that VEGF is produced by thyroid follicles in response to stimulators of TSH receptors, via the protein kinase A and C pathways. VEGF, a secretable angiogenesis factor, subsequently stimulates Flt receptors on endothelial cells in a paracrine manner, leading to their proliferation and producing hypervascularity of the thyroid gland, as seen in patients with Graves' disease.