Cultured thyroid cell adenosine 3',5'-cyclic monophosphate response to thyrotropin: loss and restoration of sensitivity to iodide inhibition

Apoptosis of NOD.H2 h4 thyrocytes by low concentrations of iodide is associated with impaired control of oxidative stress

BACKGROUND

Enhanced iodide intake in NOD.H2(h4) mice accelerates the incidence and severity of spontaneous autoimmune thyroiditis (SAT) via an unknown mechanism. A plausible hypothesis is that iodide-induced apoptosis of thyrocytes can create imbalances in antigenic load and/or disruption of immunoregulatory mechanisms that facilitate activation of autoreactive T cells in cervical lymph nodes draining the thyroid.

METHODS

We examined whether NOD.H2(h4) thyrocytes, exposed to low NaI concentrations in vitro, are more susceptible to apoptosis compared to thyrocytes from CBA/J mice, which are resistant to iodide-accelerated SAT (ISAT). We also looked, at the transcriptional level, for differential activation of genes involved in apoptosis or oxidative stress pathways that may account for potential differences in iodide-mediated apoptosis between NOD.H2(h4) and CBA/J thyrocytes.

RESULTS

We report that NOD.H2(h4) thyrocytes, cultured for 24 h at very low (4-8 μM) concentrations of NaI, exhibit high levels (40-55%) of apoptosis, as assessed microscopically following staining with fluorescent caspase inhibitors. Similar treatment of thyrocytes from CBA/J mice, which are resistant to ISAT, yielded significantly lower (10-20%) apoptotic rates. Expression analysis by real-time polymerase chain reaction using arrays of apoptosis- and oxidative stress-related genes showed that NaI intake upregulates the expression of 22 genes involved in ROS metabolism and/or antioxidant function in CBA/J thyrocytes, whereas only two of these genes were upregulated in NOD.H2(h4) thyrocytes. Among the set of overexpressed genes were those encoding thyroid peroxidase (Tpo; 5.77-fold), glutathione peroxidases (Gpx2, Gpx4, Gpx7; 2.03-3.14-fold), peroxiredoxins (Prdx1, Prdx2, Prdx5; 2.27-2.97-fold), superoxide dismutase 1 (Sod1; 3.57-fold), thioredoxin 1 (Txn1; 2.13-fold), and the uncoupling proteins 2 and 3 (Ucp2, Ucp3; 2.01-2.15-fold).

CONCLUSIONS

The results demonstrate that an impaired control of oxidative stress mechanisms is associated with the observed high susceptibility of NOD.H2(h4) thyrocytes to NaI-mediated apoptosis, and suggest a contributing factor for the development of ISAT in this strain.

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.

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

The characteristics of the iodide-induced inhibition of cyclic AMP accumulation in dog thyroid slices have been previously described [Van Sande, J., Cochaux, P. and Dumont, J. E. (1985) Mol. Cell. Endocrinol. 40, 181-192]. In the present study we investigated the characteristics of the iodide-induced inhibition of adenylate cyclase activity in dog and horse thyroid. The inhibition of cyclic AMP accumulation by iodide in stimulated horse thyroid slices was similar to that observed in dog thyroid slices. The inhibition was observed in slices stimulated by thyroid-stimulating hormone, cholera toxin and forskolin. Increasing the concentration of the stimulators did not overcome the iodide-induced inhibition. Adenylate cyclase activity, assayed in crude homogenates or in plasma-membrane-containing particulates (100,000 x g pellets), was lower in homogenates or in particulates prepared from iodide-treated slices than from control slices. This inhibition was observed on the cyclase activity stimulated by forskolin, fluoride or guanosine 5'-[beta, gamma-imino]triphosphate, but also on the basal activity. It was relieved when the homogenate was prepared from slices incubated with iodide and methimazole. Similar results were obtained with dog thyroid. The inhibition persisted when the particulate fraction was washed three times during 1 h at 100,000 x g, in the presence of bovine serum albumin or increasing concentration of KCl. It was similar whatever the duration of the cyclase assay, in a large range of protein concentration. These results indicate that a stable modification of adenylate cyclase activity, closely related to the plasma membrane, was induced when slices were incubated with iodide. Iodide inhibition did not modify the affinity of adenylate cyclase for its substrate (MgATP), but induced a decrease of the maximal velocity of the enzyme. The percentage inhibition was slightly decreased when Mg2+ concentration increased, and markedly decreased when Mn2+ concentration increased. A detectable adenylate cyclase activity was demonstrated when intact slices were incubated in the presence of [alpha-32P]ATP, probably because of the presence of broken cells produced during the slicing. Iodide had no direct effect on this cyclase system, which confirms that iodide needs the integrity of the cell to induce the inhibition and suggests that the inhibition is not transmitted between cells.

Desensitization of the thyroid cyclic AMP response to thyroid stimulating immunoglobulin: comparison with TSH

Studies were conducted to examine the characteristics of thyroid cell cAMP stimulation by thyroid stimulating immunoglobulins (TSI) and to compare the cAMP response to TSI and TSH in desensitized human thyroid cells. In terms of cAMP production, preexposure (eight hours) of the cells to TSI induced a desensitization very similar to TSH-induced desensitization: both TSH- and TSI-desensitized cells showed a normal response to cholera toxin and forskolin stimulation; TSH and TSI desensitization was interchangeable in that desensitization by either stimulator affected the action of the other; the time of recovery from either TSH and TSH desensitization was identical; the cycloheximide (10(-4) mol/L) prevented both TSI- and TSH-induced desensitization; preexposure of the cells to iodine, which affects mainly the adenylate cyclase catalytic unit, or to epinephrine, which activate the inhibitory regulatory protein Ni by the alpha 2-adrenergic stimulation, induced a similar inhibition of the subsequent stimulation by both TSH or TSI. The remarkable similarities between TSH and TSI in stimulating and desensitizing thyroid cells strongly support the concept that TSI activates thyroid adenylate cyclase by interacting with the TSH receptor and not through an allosteric mechanism.

Further characterization of the iodide inhibitory effect on the cyclic AMP system in dog thyroid slices

Iodide inhibits cyclic AMP accumulation in the thyroid by a process which is prevented by inhibition of iodide uptake and of thyroid peroxidase. By a similar process, it also exerts other independent effects such as the enhancement of iodinated protein release. Iodide inhibited the stimulation of adenylate cyclase by prostaglandin E1, cholera toxin and forskolin. The action of iodide was not relieved by phosphodiesterase inhibitors and was not additive with the effect of norepinephrine or adenosine. Iodide did not decrease the cellular level of ATP. The data are compatible with an inhibition of adenylate cyclase beyond the level of the receptor, presumably at the level of the catalytic unit or its interaction with the positive transducing unit NS. The effect of iodide required TSH for its expression but not for its installation. It was decreased under all conditions in which iodide organification was decreased: decreased iodide or increased methimazole concentration, absence of calcium in the medium, etc. However, the relation between iodide binding to proteins and effect was not linear. The effect was not relieved by washing in the absence of iodide and in the presence of perchlorate, but it was partly reversible in the presence of methimazole propylthiouracyl or thiourea. It was not relieved by cooling to 20 degrees C and cytochalasin b, which block stimulated thyroglobulin hydrolysis and iodothyronine release, nor by actinomycin D, cycloheximide, puromycin, mepacrine or indomethacin. The data suggest that iodide binds to a saturable cell component by a reaction which is reversible only in the presence of thiol-containing drugs.

Functional properties of porcine-thyroid follicles in suspension

Porcine thyroid follicle cells were isolated (about 10(7) cells per gram of tissue) and cultured in small aggregates in agarose-coated culture dishes. The aggregates became arranged into follicle-like structures capable of iodide uptake and organification. In the presence of TSH (0.2 mU/ml), the aggregation of follicles was enhanced, and iodide uptake as well as TSH-stimulated organification of iodide was increased compared with that in the control. In culture, the active iodide metabolism was gradually lost over a 7-day period. This was not due to a disappearance of the TSH-adenylate cyclase system, since cAMP production was retained and stimulated by TSH (half-maximal effect at about 1 mU/ml). Acutely TSH stimulated iodide efflux and iodide organification (half-maximal effect at about 20 microU/ml). The stimulatory effect on organification was transient: within an hour further organification proceeded as in the absence of hormone. The effects on efflux and organification were already maximal at low TSH concentrations, whereas cAMP production was stimulated with up to 50-fold higher TSH levels, i.e. the findings were typical of spare receptors. In the continued presence of epidermal growth factor, a potent mitogen for thyroid cells, the follicles increased in size and contained one single large lumen. Their capability to take up and organify iodide was reduced.

Feeding supplemental iodine to mink: reproductive and histopathologic effects

Adult mink were fed various concentrations of supplemental iodine, ranging from 10 to 320 ppm, for 1 or 7 mo before breeding. Long-term, low-level (10-20 ppm) iodine supplementation was beneficial for both reproduction and lactation. Supplemental iodine in excess of 80 ppm, however, resulted in a reduction in the number of females that whelped, a decrease in litter size, and an increase in kit mortality. Thyroid glands of kits whelped and nursed by dams fed more than 20 ppm supplemental iodine, both short-term and long-term, showed hypertrophy marked by follicular cell hyperplasia and a decreased amount of colloid. Similar histopathologic lesions were observed in the thyroids of adults that received 80 ppm or more supplemental iodine; also observed were numerous lesions in the gallbladder.

Interaction between iodothyronines and thyrotropin receptor in human cultured thyroid cells

In order to verify the existence of a "short-loop" negative feedback between iodothyronines and adenylate cyclase system of human thyroid, we have studied the effect of preincubation with iodothyronines, iodotyrosines, iodothyronine analogues and iodide on TSH-induced cAMP cellular accumulation in normal human thyroid cells in primary culture. Iodide did not produce an inhibitory effect on TSH-dependent adenylate cyclase system both in normal human thyroid plasma membranes and cultured cells. Iodothyronines at a 30-40 microM concentration did not inhibit the TSH-dependent adenylate cyclase activity of human thyroid plasma membranes; however at a 1 microM concentration they were able to inhibit the TSH-dependent cAMP accumulation by cultured cells. Preincubation with iodotyrosines and iodothyronine analogues failed to inhibit the TSH-responsive cAMP accumulation in human thyroid cultured cells.

Biochemistry of thyroid regulation under normal and abnormal conditions

Perhaps in an oversimplified view, abnormal thyroid growth can be classified into two main categories: a) those cases due to excess of thyroid stimulators extrinsic to the gland; b) situations in which an intrinsic alteration in the gland occurs: Extrinsic (excess thyroid stimulation) Iodide deficiency with elevated TSH Goitrogens Graves' immunoglobulins Thyroid stimulating factors produced by tumors Dishormonogenesis with hypothyroidism Intrinsic (normal TSH) Increased sensitivity to TSH (iodine depletion) Altered autoregulation (?) Abnormal TSH receptor Other biochemical abnormalities From the studies performed in animals it can be concluded that since goiter appears before a detectable increase in serum TSH occurs, an intrinsic alteration in the thyroid gland would be responsible for the onset of growth. Under these conditions TSH would play a permissive role in promoting and maintaining the gland enlargement. In some aspects this situation is similar to that of certain endemic goiter areas. It may be postulated that under a mild iodine deficiency a decrease in thyroidal iodine concentration occurs (and/or in certain iodocompounds), thus rendering the gland more sensitive to the stimulatory action of TSH, and leading to the appearance of goiter. If this mechanism is able to maintain an euthyroid status no further alterations will occur. In more severely iodine deficient areas, or when additional factors such as dietary goitrogens are present, hypothyroidism develops and TSH is clearly elevated. A similar localized mechanism can be postulated for the development of nodular goiter. It is more difficult to explain the pathogenesis of goiter and tumors in nonendemic areas, since the biochemical findings so far reported are not conclusive. It seems likely that an alteration of the TSH receptor is a common factor to many tumors in man and animals. However, some contradictory results would preclude us from making a general statement. The wide variety of biochemical alterations reported would perhaps indicate, that there is not a single cause for the rise of abnormal thyroid growth and that different factors may play a role in the regulation of growth under such circumstances. It is to be hoped that future studies will provide a better comprehension of this problem.

Discontinuity of thyroid gland response to hormonal stimulation: effect of TSH and cAMP on iodide organification

The action of TSH on the process of iodide organification was studied in rat thyroid glands under different experimental in vitro incubation conditions. The effects on glands of both short and prolonged exposure to TSH were evaluated using two different procedures: continuous and pulse labelling of thyroids with radioactive iodide. It was demonstrated that during prolonged contact with thyroid cells, TSH stimulates iodide organification periodically. This periodic effect is cyclic and is composed of a stimulatory and an inhibitory phase. Each cycle lasts 30-45 min, and several cycles follow one another in a regular manner. Furthermore, it has been shown that the periodic effect of THS is due to an intrinsic property of the thyroid cell to respond in a cyclic manner to hormonal stimulation. TSH stimulated the accumulation of organic iodide only when introduced at a precise phase of the cycle. The same type of discontinuous thyroid cell response was obtained when TSH was replaced by its intracellular mediator, cAMP. This indicates that the initiation of the cyclic response to hormonal stimulation is localized at the steps after that of cAMP formation. It seems, therefore, that this cyclic response of thyroid glands is not related to the recently observed phenomenon of desensitization. This phenomenon, characterized by the development of resistance in many target organs and cells toward their respective hormonal stimulators, is due to modifications in steps preceding those of cAMP formation. The discontinuity of thyroid gland response to both TSH and cAMP described in this work seems to be a new phenomenon, whose physiological significance and possible molecular mechanisms are discussed.