Iodide-induced inhibition of adenylate cyclase activity in horse and dog thyroid

Investigation of adrenal and thyroid gland dysfunction in dogs with ultrasonographic diagnosis of gallbladder mucocele formation

Gallbladder mucocele formation is an emerging disease in dogs characterized by increased secretion of condensed granules of gel-forming mucin by the gallbladder epithelium and formation of an abnormally thick mucus that can culminate in obstruction of the bile duct or rupture of the gallbladder. The disease is associated with a high morbidity and mortality and its pathogenesis is unknown. Affected dogs have a significantly increased likelihood of concurrent diagnosis of hyperadrenocorticism, hypothyroidism, and hyperlipidemia. Whether these endocrinopathies represent coincidental primary disease processes that exacerbate gallbladder mucocele formation in predisposed dogs or reflect a concurrent disruption of endocrine and lipid metabolism is unclear. In this study, we investigated a hypothesis that dogs with gallbladder mucocele formation would have a high prevalence of occult and atypical abnormalities in adrenal cortical and thyroid gland function that would suggest the presence of endocrine disruption and provide deeper insight into disease pathogenesis. We performed a case-control study of dogs with and without ultrasonographic diagnosis of gallbladder mucocele formation and profiled adrenal cortical function using a quantitative mass spectrometry-based assay of serum adrenal-origin steroids before and after administration of synthetic cosyntropin. We simultaneously profiled serum thyroid hormone concentrations and evaluated iodine sufficiency by measurement of urine iodine:creatinine ratios (UICR). The studies were complemented by histological examination of archival thyroid tissue and measurements of thyroid gland organic iodine from dogs with gallbladder mucocele formation and control dogs. Dogs with gallbladder mucocele formation demonstrated an exaggerated cortisol response to adrenal stimulation with cosyntropin. A prevalence of 10% of dogs with gallbladder mucocele formation met laboratory-based criteria for suspect or definitive diagnosis of hyperadrenocorticism. A significantly greater number of dogs with gallbladder mucocele formation had basal serum dehydroepiandrosterone (DHEAS) increases compared to control dogs. A high percentage of dogs with gallbladder mucocele formation (26%) met laboratory-based criteria for diagnosis of hypothyroidism, but lacked detection of anti-thyroglobulin antibodies. Dogs with gallbladder mucocele formation had significantly higher UICRs than control dogs. Examination of thyroid tissue from an unrelated group of dogs with gallbladder mucocele formation did not demonstrate histological evidence of thyroiditis or significant differences in content of organic iodine. These findings suggest that dogs with gallbladder mucocele formation have a greater capacity for cortisol synthesis and pinpoint DHEAS elevations as a potential clue to the underlying pathogenesis of the disease. A high prevalence of thyroid dysfunction with absent evidence for autoimmune thyroiditis suggest a disrupted thyroid hormone metabolism in dogs with gallbladder mucocele formation although an influence of non-thyroidal illness cannot be excluded. High UICR in dogs with gallbladder mucocele formation is of undetermined significance, but of interest for further study.

Moderate doses of iodide in vivo inhibit cell proliferation and the expression of thyroperoxidase and Na+/I- symporter mRNAs in dog thyroid

The function and the growth of adult thyroid gland is controlled by the opposite actions of thyrotropin (TSH) and iodide, the main substrate of the gland. Iodide deprivation leads to stimulation of the thyroid, improving the efficiency of iodide transport for hormone biosynthesis. We have investigated cell proliferation and thyroid specific gene expression 24 and 48 h after administering KI to dogs previously treated with goitrogens and perchlorate. In the hypothyroid dogs T3 and T4 serum levels decreased from 53 +/- 4 to < 30 ng/dl and from 1.6 +/- 0.6 to < 1 microg/dl respectively; TSH concentration increased from 0.16 +/- 0.02 to 2.7 +/- 0.4 ng/ml. After a 24 h moderate KI treatment (300 microg KI/dog of +/- 10 kg) serum T3 concentrations rose higher than the initial normal values, while T4 concentrations increased to reach values equivalent to the normal level. The high TSH concentration did not change significantly. The hyperplasia of the chronically stimulated thyroid resulting from goitrogens/NaClO4 treatment was not modified by this short term treatment with KI. In contrast, KI decreased the weight of the total gland and the level of cell proliferation, as determined by the fraction of cells incorporating BrdU. The effect of acute administration of KI on the expression of four major thyroid genes, the TSH receptor (TSHr), thyroglobulin (Tg), thyroperoxidase (TPO), and Na+/I- symporter (NIS) was analyzed by Northern blot. Tg, TPO and NIS mRNA expressions were up-regulated by chronic stimulation. The expression of the mRNAs of TSHr and Tg did not significantly differ between hyperstimulated and KI-treated dogs while TPO and NIS mRNA expression decreased after a 48 h KI treatment. TPO and NIS are therefore the only of these four genes whose expression is acutely modulated by iodide in vivo. Under TSH stimulation low doses of iodide resulted in: (1) decreased cell proliferation, (2) reestablished synthesis and secretion of thyroid hormones, (3) diminished TPO and NIS mRNA expression. Notably low doses of iodide under the same conditions had no effect on Tg and TSHr mRNA expression.

Biosynthesis and metabolism of 2-iodohexadecanal in cultured dog thyroid cells

2-Iodohexadecanal (2-IHDA) is a major thyroid iodolipid. It mimics the main regulatory effects of iodide on thyroid metabolism: inhibition of H2O2 production and of adenylyl cyclase. The biosynthesis of 2-IHDA and its metabolism have been investigated in cultured dog thyroid cells maintained in a differentiated state by forskolin. Incubation of these cells with [9,10-3H]hexadecan-1-ol or [9,10-3H]palmitic acid labeled several phospholipids, but [9, 10-3H]hexadecan-1-ol was selectively incorporated into plasmenylethanolamine. In the presence of an exogenous H2O2 generating system (glucose oxidase), iodide induced the production of [9,10-3H]2-IHDA from [9,10-3H]hexadecan-1-ol-labeled cells but not from [9,10-3H]palmitic acid-labeled cells. 2-IHDA was also generated during the lactoperoxidase-catalyzed iodination of brain and heart plasmalogens, and of ethyl hexadec-1-enyl ether, a synthetic vinyl ether-containing compound. Taken together, these results show that thyroid 2-IHDA is derived from plasmenylethanolamine via an attack of reactive iodine on the vinyl ether group. 2-Iodohexadecan-1-ol (2-IHDO) was also detected in these studies; it was formed later than 2-IHDA, and thyroid cells converted exogenous 2-IHDA into 2-IHDO in a time-dependent way. The ratio of 2-IHDO/2-IHDA increased with H2O2 production and decreased as a function of iodide concentration. An aldehyde-reducing activity was detected in subcellular fractions of the horse thyroid. No formation of 2-iodohexadecanoic acid could be detected. Reduction into the biologically inactive 2-IHDO is thus a major metabolic pathway of 2-IHDA in dog thyrocytes.

Inhibition of human thyroid adenylyl cyclase by 2-iodoaldehydes

2-Iodohexadecanal (IHDA), which can be formed upon addition of iodine to the vinyl ether group of plasmalogens, has been identified as a major thyroid iodolipid (Pereira et al. (1990) J. Biol. Chem. 265, 17018-17025). In this study, we have investigated the possibility that it would be a mediator of the inhibitory effect of iodide on thyroid adenylyl cyclase. In human thyroid membranes, IHDA inhibited the adenylyl cyclase activity stimulated by thyrotropin (TSH), GTP-gamma-S or forskolin (FSK), whereas it did not decrease the specific binding of TSH to its receptors. The inhibitory effect on the cyclase reached a maximum after a 1-h-pre-incubation of the membranes with IHDA at 30 degrees C and was poorly reversible. It was also observed following a 4-h incubation with IHDA at 4 degrees C, a condition in which adenylyl cyclase is protected against heat inactivation. IHDA decreased the Vmax of adenylyl cyclase, but had no effect on the Km for ATPMg2-.IHDA also inhibited the FSK-stimulated adenylyl cyclase activity in liver and kidney cortex membranes, but had no effect on the Mg(2+)-ATPase activity of thyroid membranes. The inhibitory effect of IHDA has also been demonstrated in intact cells. As in membranes, IHDA decreased the rise in cAMP induced by TSH in cultured dog thyroid cells and this inhibition was maintained following pretreatment of the cells with pertussis toxin. In order to evaluate the specificity of the IHDA action, various analogs have been synthesized. This study has permitted the identification of two major structural features required for the inhibition of human thyroid adenylyl cyclase; the terminal aldehyde function and an iodine atom at C2, other halogens being ineffective. In conclusion, we have shown that IHDA exerts a direct inhibitory effect at or near adenylyl cyclase; all the properties of this effect characterized so far are identical to those of the adenylyl cyclase inhibition obtained following the exposure of thyroid tissue to iodide.

Inhibition of H2O2 production by iodoaldehydes in cultured dog thyroid cells

2-Iodohexadecanal (IHDA) has been identified as a major thyroid iodolipid which can be formed upon addition of iodine to the vinyl ether group of plasmalogens (Pereira et al., 1990). In order to test whether IHDA plays a role in the thyroid autoregulation by iodide, we have investigated its effects on the production of H2O2 by cultured dog thyroid cells. IHDA inhibited the formation of H2O2 in dog thyroid cells stimulated by carbamylcholine (CCHOL). In the presence of BSA, which potentiated its action, the effect of IHDA was maximal after 2 h and had an IC50 around 5 microM. The effect of IHDA was not decreased by methimazole, which abolished the inhibition by iodide. IHDA also inhibited the stimulatory effect of bradykinin, but had only a marginal effect on the production of H2O2 induced by ionomycin or phorbol 12-myristate 13-acetate (PMA). The accumulation of inositol phosphates in CCHOL-stimulated thyroid cells was decreased by IHDA. As evaluated by measurements of 51Cr release and [3H]thymidine incorporation into DNA, IHDA had no adverse effect on thyroid cell viability. Several analogs of IHDA, of which the synthesis is described, have been tested for their inhibitory activity. This allowed the identification of two major structural features required for the biological activity: the carbonyl group at C1 and an halogen atom at C2, with iodine conferring a greater activity than bromine, while chlorine and fluorine were inactive. In conclusion, IHDA inhibits the production of H2O2 in CCHOL-stimulated dog thyroid cells by decreasing the phospholipase C cascade activity. This effect involves both the aldehyde function and the iodine atom. These results suggest that IHDA might be the mediator of some of the regulatory actions of iodide on the thyroid gland.

Early adaptation of thyrotropin and thyroglobulin secretion to experimentally decreased iodine supply in man

Five healthy male volunteers (aged 25 to 28 years) were studied both after 4 weeks of treatment with 200 micrograms iodine/d orally (PO) and following experimental iodine depletion by treatment with 3 x 300 mg perchlorate/d PO over a 4-week period, in an attempt to better define the early adaptive responses to an alteration in iodine supply in thyroid function. Intrathyroidal iodine, serum triiodothyronine (T3), free T3 (FT3), thyroxine (T4), free T4 (FT4), reverse T3 (rT3), thyroxine-binding globulin (TBG), thyroglobulin (Tg), and thyrotropin (TSH) levels (10-minute sampling over 24 hours) were measured at the end of iodine administration and at the end of perchlorate treatment. Thyroid volume was determined by sonography, and iodine content was determined by fluorescence scintigraphy. TSH pulses were analyzed by computer-assisted programs. Comparing both experimental situations, perchlorate treatment significantly reduced intrathyroidal iodine concentration (4.0 +/- 1.3 to 3.0 +/- 1.2 nmol/mL, P less than .05), but thyroid volume and total serum T4, T3, FT3, and TBG levels were not altered. Mean 24-hour serum TSH levels (1.8 +/- 0.3 to 1.0 +/- 0.3 mU/L, P less than .001), amount of TSH secreted/pulse (0.5 +/- 0.1 to 0.3 +/- 0.1 mU/L, P less than .001), and FT4 levels (15.7 +/- 1.7 to 14.3 +/- 1.4 pmol/L, P less than .005) were significantly diminished, whereas Tg levels (18.6 +/- 10.0 to 35.1 +/- 14.0 ng/mL, P less than .01) were significantly increased. Thyroid-specific antibodies were normal and were not altered by treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of the cyclic AMP cascade as an oncogenic mechanism: the thyroid example

Three cascades activate thyroid cell proliferation: the EGF-protein tyrosine kinase pathway, the phorbol ester-protein kinase C pathway and the thyrotropin-cyclic AMP pathway. While the first 2 cascades converge early, they remain distinct from the cyclic AMP cascade until very late in G1. The cyclic AMP cascade is characterized by an early and transient expression of c-myc, which may explain why it induces proliferation and differentiation expression. Constitutive activation of this cascade causes growth and hyperfunction, ie, hyperfunctioning adenomas. The various possible defects that could lead to such a constitutive activation are discussed.

Stimulation of generation of inositol phosphates by carbamoylcholine and its inhibition by phorbol esters and iodide in dog thyroid cells

The action of carbamoylcholine (Cchol), NaF and other agonists on the generation of inositol phosphates (IPs) was studied in dog thyroid slices prelabelled with myo-[2-3H]inositol. The stimulation by Cchol (0.1 microM-0.1 mM) of IPs accumulation through activation of a muscarinic receptor [Graff, Mockel, Laurent, Erneux & Dumont (1987) FEBS Lett. 210, 204-210] was pertussis- and cholera-toxin insensitive. Ins(1,4,5)P3, Ins(1,3,4)P3 and InsP4 were generated. NaF (5-20 mM) also increased IPs generation (Graff et al., 1987); this effect was potentiated by AlCl3 (10 microM) and unaffected by pertussis toxin. Although phorbol dibutyrate (5 microM) abolished the cholinergic stimulation of IPs generation (Graff et al., 1987), it did not affect the fluoride-induced response. Cchol and NaF did not require extracellular Ca2+ to exert their effect, and neither KCl-induced membrane depolarization nor ionophore A23187 (10 microM) had any influence on basal IPs levels, or on cholinergic stimulation. However, more stringent Ca2+ depletion with EGTA (0.1 or 1 mM) decreased basal IPs levels as well as the amplitude of the stimulation by Cchol without abolishing it. Dibutyryl cyclic AMP, forskolin, cholera toxin and prostaglandin E1 had no effect on basal IPs levels and did not decrease the response to Cchol. Iodide (4 or 40 microM) also strongly decreased the cholinergic action on IPs, this inhibition being relieved by methimazole (1 mM). Our data suggest that Cchol activates a phospholipase C hydrolysing PtdIns(4,5)P2 in the dog thyroid cell in a cyclic AMP-independent manner. This activation requires no extracellular Ca2+ and depends on a GTP-binding protein insensitive to both cholera toxin and requires no extracellular Ca2+ and depends on a GTP-binding protein insensitive to both cholera toxin and pertussis toxin. The data are consistent with a rapid metabolism of Ins(1,4,5)P3 to Ins(1,3,4)P3 via the Ins(1,4,5)P3 3-kinase pathway, followed by dephosphorylation by a 5-phosphomonoesterase. Indeed, a Ca2+-sensitive InsP3 3-kinase activity was demonstrated in tissue homogenate. Stimulation of protein kinase C and an organified form of iodine inhibit the Cchol-induced IPs generation. The negative feedback of activated protein kinase C could be exerted at the level of the receptor or of the receptor-G-protein interaction.

Inhibition by iodide of iodide binding to proteins: the "Wolff-Chaikoff" effect is caused by inhibition of H2O2 generation

H2O2 generation is limiting the oxidation and binding to proteins of iodide. In dog thyroid slices thyrotropin and carbamylcholine greatly enhance protein iodination and H2O2 generation. The action of thyrotropin is mimicked by dibutyryl cyclic AMP and forskolin which suggests that it is mediated by cyclic AMP. The action of carbamylcholine was mimicked by ionomycin and by phorbol myristate ester. This suggests that the effect of carbamylcholine is mediated by the two intracellular signals generated by the Ca++ phosphatidylinositol cascade: Ca++ and diacylglycerol. The Wolff-Chaikoff effect is the inhibition by iodide of its own organification. In dog thyroid slices, iodide greatly inhibited H2O2 generation stimulated by thyrotropin and by carbamylcholine. Iodide decreased the production of intracellular signals induced by TSH and carbamylcholine but it also inhibited the action of probes of these intracellular signals (dibutyryl cAMP, forskolin, ionomycin, phorbol-myristate ester) on the H2O2 generating system itself. These effects were suppressed by methimazole an inhibitor of iodide oxidation.