Active transport of iodide in isolated porcine thyroid cells. Application to an in vitro bioassay of thyrotropin

Resveratrol increases iodide trapping in the rat thyroid cell line FRTL-5

BACKGROUND

Resveratrol, a polyphenol found in grapes, exhibits several beneficial health effects by its antioxidant, antiinflammatory, and chemopreventive properties. The aim of the present study was to determine the effect of resveratrol on iodide trapping and efflux as well as its mode of action using FRTL-5 cells, having in mind the pivotal role of the natrium iodide symporter (NIS) in the treatment of differentiated thyroid cancers.

METHODS

Cells were treated with resveratrol for various times and doses, in the presence or absence of thyrotropin (TSH). Iodide trapping, iodide efflux, rat NIS (rNIS) protein expression, and cyclic AMP (cAMP) production were evaluated.

RESULTS

Resveratrol increased iodide trapping in a time-dependent (optimal 6 hours) and dose-dependent (100 microM) way in the presence of TSH. It showed an additive effect when concomitantly added with an optimal dose of TSH. Resveratrol (50 microM) increased (threefold) rNIS protein expression. In TSH-deprived cells, resveratrol also provoked an increase in rNIS protein (>3-fold in 6 hours) with an optimum at 40 microM. Resveratrol did not inhibit iodide efflux from FRTL-5 cells. It neither increased intracellular cAMP nor induced the arborization of living cells, two TSH-induced effects. A non-cAMP mode of action is highly suspected.

CONCLUSIONS

Resveratrol increases iodide trapping in FRTL-5 cells, increasing iodide influx and rNIS protein level even in the absence of TSH. It has an additive effect with TSH. Consequently, resveratrol could be a promising molecule for radioiodide therapy in follicular and papillary differentiated thyroid carcinoma in association with recombinant human TSH.

Hexamethylenebisacetamide modulation of thyroglobulin and protein levels in thyroid cells is not mediated by phosphatidylinositol-3-kinase: a study with wortmannin

Hexamethylenebisacetamide (HMBA) induces in murine erythroleukemia cells (MELC) the commitment to terminal differentiation leading to globin gene expression. In the thyroid, HMBA acts as a growth factor and also as a differentiating agent. In the present paper, we studied the effect of HMBA on the very specific thyroid marker thyroglobulin (Tg) in two different thyroid cell systems, i.e., porcine cells in primary culture and ovine cells in long term culture. Using wortmannin, a specific inhibitor of phosphatidylinositol-3-kinase, we investigated whether this enzyme is involved in HMBA mode of action. We found that HMBA is a positive modulator of Tg production in porcine cells, but a negative effector in the OVNIS cell line. As all HMBA effects studied in the present paper, i.e., Tg production and total protein levels, are not inhibited by wortmannin, we suggest the non-involvement of phosphatidylinositol-3-kinase in HMBA mode of action.

DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) increases iodide trapping, inhibits thyroperoxidase and antagonizes the TSH-induced apical iodide efflux in porcine thyroid cells

4,4'-Di-isothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of several anionic channels and transporters including the band 3 protein of the red blood cell membrane was tested on iodide metabolism in cultured porcine thyroid cells. We used three experimental cell culture models: (i) forskolin-stimulated correctly inside-in polarized follicle-associated thyroid cells cultured onto plastic support (ii) suspensions of isolated cells derived from such cultures (iii) polarized monolayers in bicameral chambers. DIDS was observed to increase free-iodide trapping in all conditions. Organification of iodide by follicle-associated cell cultures incubated for 6 h decreased as a function of DIDS concentration with an IC50 of 5 x 10(-5) M. This block in organification is accounted for a block in thyroperoxidase activity as in vitro both purified lactoperoxidase and purified porcine thyroperoxidase were inhibited by DIDS with a similar dose-dependency the IC50 being also of 5 x 10(-5) M. Both control and DIDS-treated cells in suspension, actively trapped iodide and reached a steady concentration in about 50 min; however the plateau was 4.4-fold higher in (10(-3) M) DIDS-treated cells. Acute TSH-stimulation at this plateau of 125I-preloaded cells in suspension in the presence of 2 mM methimazole (MMI) induced a fast release of iodide from these cells as expected (first step of the TSH-biphasic effect). This TSH-induced iodide efflux was however completely inhibited by DIDS (10(-3) M). Furthermore, addition of DIDS to the apical compartment of TSH-prestimulated cell monolayers in bicameral chambers resulted in an increase in intracellular-iodide concentration and in an inhibition of iodide efflux into the apical medium. Taken together, the present results demonstrate that DIDS mainly interacts with two main components of the thyroid apical cell membrane: thyroperoxidase and a cAMP-sensitive iodide channel.

Inhibition of the Na+/I- symporter by harmaline and 3-amino-1-methyl-5H-pyrido(4,3-b)indole acetate in thyroid cells and membrane vesicles

Novel inhibitors of the Na+/I- symporter were identified using rat-thyroid-derived FRTL-5 cells and sealed vesicles from calf thyroid as model systems. Na(+)-dependent 125I- uptake was inhibited by the hallucinogenic drug harmaline and by a chemically related convulsive agent, 3-amino-1-methyl- 5H-pyrido(4,3-b)indole acetate (TRP-P-2). TRP-P-2 (Ki = 0.25 mM) was tenfold more effective as an inhibitor than harmaline (Ki = 4.0 mM). Inhibition by TRP-P-2 was competitive with respect to Na+ and was fully reversible. Although TRP-P-2 is a relatively low-affinity inhibitor, its affinity for the Na+ site of the Na+/I- symporter is over 100 times higher than that of Na+ (Km = 50 mM). 45Ca(2+)-efflux rates in calf thyroid membrane vesicles were not affected by TRP-P-2, indicating that membrane integrity is not disrupted by the drug. These findings show that TRP-P-2 may be a potentially useful tool for the identification and characterization of the Na+/I- symporter.

Preferential formation of triiodothyronine residues in newly synthesized [14C]tyrosine-labeled thyroglobulin molecules in follicles reconstructed in a suspension culture of hog thyroid cells

Early processes of thyroid hormone (T4 and T3) synthesis in thyroglobulin molecules were studied using follicles reconstructed in a primary culture of hog thyroid cells under the influence of TSH. When the reconstructed follicles were incubated with 14C-tyrosine, thyroglobulin containing the labeled tyrosine was newly synthesized and in the presence of iodide, some of the labeled tyrosine residues were iodinated and coupled to produce labeled iodothyronines, T4 and T3. Coupling efficiency, especially the efficiency of T3 production, was much higher than that obtained from the average iodoamino acid composition of mature thyroglobulin from the gland, indicating a preferential iodination of hormonogenic tyrosines and synthesis of T3. The total production of T3 was higher than T4 under the present conditions. However, free labeled T4 released into the medium was more than T3 after 16 h incubation of the labeled follicles with non-labeled tyrosine, suggesting the preferential liberation of T4 from the labeled peptide and/or release from the cells.

The electrogenic, Na+-dependent I- transport system in plasma membrane vesicles from thyroid glands

Using vesicles from the plasma membrane of hog thyroid, we have characterized its Na+-dependent I- transport system. We have found it to be totally Na+ dependent; K+ cannot substitute and Li+ can partially substitute for Na+; the Na+:I- flux ratio is larger than one; the system is electrogenic, being stimulated by a delta psi negative inside the vesicles. A number of large, lipophilic anions are fully-competitive inhibitors of Na+-dependent I- uptake; the closer their atomic radii are to that of iodine, the smaller their Ki values.

Comparative evaluation of cyclic AMP and iodide accumulation responses to thyroid-stimulating immunoglobulins in cultured FRTL-5 cells

The rat thyroid cell strain FRTL-5 was used to investigate the relationship between cyclic AMP and iodide accumulation responses to thyroid-stimulating immunoglobulins (TSIg). Immunoglobulin G-enriched precipitates of sera from 19 of 21 (90%) newly-diagnosed Graves' disease patients gave significant (P less than 0.01) accumulation of iodide (125I), and 16 of these also stimulated intracellular cyclic AMP. Correlation was poor however, with certain TSIg preparations giving widely divergent responses. After initiation of antithyroid treatment, 40% of sera investigated contained TSIg detectable in both bioassay systems, and all but one of the remainder were stimulatory in one of the two bioassays. All patients in remission were devoid of detectable TSIg as determined by iodide accumulation, although a single preparation stimulated cyclic AMP accumulation. LATS-B, a lyophilized reference serum preparation containing high TSIg activity, enhanced iodide accumulation, which showed evidence of correlation with intracellular cyclic AMP at doses above 0.5 mU/ml. At lower doses, iodide accumulation was observed in the absence of detectable cyclic AMP accumulation. TSH and LATS-B-induced iodide accumulation were enhanced, and iodide efflux reduced, by the anion channel blocker 4-4' diisothiocyanate stilbene 2,2' disulphonic acid (DIDS). In contrast, Ig-enriched fractions of normal sera decreased both basal and stimulated iodide accumulation, but were without effect on efflux. TSIg from untreated Graves' sera gave widely-differing iodide accumulation responses which showed poor correlation with both intracellular cyclic AMP and cyclic-AMP-independent iodide efflux. This clear dissociation of responses to serum Ig preparations suggests that iodide uptake in FRTL-5 cells, which do not organify iodide, may be subject to variable effects of non-TSIg components of Graves' sera, on both iodide uptake itself, and as inhibitors of TSIg-induced accumulation of intracellular cyclic AMP.

Polarization of thyroid cells in culture: evidence for the basolateral localization of the iodide "pump" and of the thyroid-stimulating hormone receptor-adenyl cyclase complex

When cultured in collagen gel-coated dishes, thyroid cells organized into polarized monolayers. The basal poles of the cells were in contact with the collagen gel, whereas the apical surfaces were facing the culture medium. Under these culture conditions, thyroid cells do not concentrate iodide nor respond to acute stimulation by thyroid-stimulating hormone (TSH). To allow the free access of medium components to the basal poles, the gel was detached from the plastic dish and allowed to float in the culture medium. After release of the gel, the iodide concentration and acute response to TSH stimulation were restored. Increased cAMP levels, iodide efflux, and formation of apical pseudopods were observed. When the thyroid cells are cultured on collagen-coated Millipore filters glued to glass rings, the cell layer separates the medium in contact with the apical domain of the plasma membrane (inside the ring) from that bathing the basolateral domain (outside the ring). Iodide present in the basal medium was concentrated in the cells, whereas no transport was observed when iodide was added to the luminal side. Similarly, an acute effect of TSH was observed only when the hormone was added to the basal medium. These results show that the iodide concentration mechanism and the TSH receptor-adenylate cyclase complex are present only on the basolateral domain of thyroid cell plasma membranes.

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.

Reorganization of porcine thyroid cells into functional follicles in a chemically defined, serum- and thyrotropin-free medium

In the serum-free, chemically defined medium NCTC 109, freshly isolated porcine thyroid cells aggregate and form functional follicles in culture even in the absence of thyrotropin. The follicular pattern observed under light and electron microscopy express the main morphological characteristics of in vivo thyroid cells. Follicles are large, replete with dense colloid, and the apical pole of cells is characterized by well-developed microvilli and the presence of aminopeptidase N. The index of iodide transport activity (125I-C/M ratio) decreases vs. days of culture to a resting value of about 1 or 2 at day 2. Addition of thyrotropin (200 microU/ml final concentration) at day 4 is followed by a 10-fold increase in iodide transport activity within 24 h and a 40-fold increase 4 d later. Incorporation and organification of iodide are dose dependent between 0 and 250 microU/ml thyrotropin; highest concentrations (4,000--16,000 muU/ml) are significantly inhibitory. In the absence of thyrotropin each cell synthesizes 8.2 pg thyroglobulin/d. Acute stimulation by thyrotropin at day 4 resulted in a slight decrease in the quantity of thyroglobulin present in the cell layer but in an increase in the total amount of thyroglobulin recovered in both cells and medium, reaching 34.3 pg/cell/d. The protein exported into the medium is thyroglobulin, as shown by SDS PAGE and immunological properties. Here we demonstrate that porcine thyroid cells can be maintained in culture as resting, highly differentiated, follicular-associated cells, sensitive to acute stimulation by thyrotropin.

Stimulation by thyrotropin, cholera toxin and dibutyryl cyclic AMP of the multiplication of differentiated thyroid cells in vitro

Primary cultures of dog thyroid cells have been established. The cells originated from follicles and displayed differentiation characteristics of such cells: iodide trapping and organification, responsiveness of iodide organification and cyclic AMP accumulation to thyrotropin (TSH), induction of a two-dimensional follicular structure by TSH. TSH also stimulated the multiplication of these cells. The effect of TSH was detected with concentrations as low as 100 muU/ml and was reproduced with purified TSH. It was reproduced by cholera toxin (10 ng/ml) and dibutyryl cyclic AMP (10(-5) M). The data show that TSH, which stimulates the function of thyroid tissue, in vivo and in vitro, activates the multiplication of differentiated dog-thyroid follicular cells in primary culture, which suggests that this trophic effect is, partly at least, mediated by cyclic AMP.

In vitro conversion of porcine thyroid cells growing in monolayer into functional follicular cells

Porcine thyroid cells in primary cultures form either monolayers or, when they are stimulated by thyrotropin (TSH), follicles. This system, monolayer-follicle associated cell culture, determines two morphofunctional states of the thyroid cell in vitro. These cells divide, when grown in monolayer. In this article we describe the precise conditions which allow the conversion of monolayer cells into follicles. The rate of cell growth was lowered using a serum-free medium. Cells were concentrated, stimulated by TSH and cultured on poly-L-lysine pretreated plastic flasks. Light and electron microscope studies show that cells reorganize into follicles containing thyroglobulin. Active iodide transport by the cells, as well as detection of thyroid hormones in the cell culture media, demonstrate that these follicles are functional. Formation of monolayers from follicles is in vitro a spontaneous phenomenon. It is linked to the loss of cell polarity, iodide transport, synthesis of hormones and to the decrease of the number of TSH receptor-sites. These main characteristics of differentiation may be regained in vitro after conversion of monolayer thyroid cells into active follicles up to at least generation five.

The radiobiological response of the thyroid. Part I: Sheep thyroid cells in culture as a radiobiological model

A system for culturing sheep thyroid cells in vitro is described. The properties of the cultures may be manipulated so that they preserve in vitro the follicular architecture typical of the thyroid, or so that they undertake cellular proliferation. The cultured cells also undertake iodide metabolism. These growth and differentiation characteristics can be preserved for at least three weeks, which is sufficient time for assay of many radiobiological end-points.

Demonstration of growth in porcine thyroid cell culture

Eagle's minimum essential medium supplemented with 20 per cent newborn calf serum (N.C.S.) allows porcine thyroid cell survival but not cell growth in vitro. In NCTC 109 medium supplemented with 20 per cent N.C.S. these cells actively grow and may be serially propagated. Cell population doubling time expressed as DNA doubling value is 3.5 days at 37 degrees C in 95 per cent air-5 per cent CO2. Thyrotropin does not affect porcine thyroid cell multiplication in vitro but stimulates the plating efficiency in primary cultures to about 130 per cent of controls. Cell selection was obtained by replacing media with Earle's balanced salt solution. This operation provoked death of nearly all cells by day 18 but subsequent addition of growth medium resulted in proliferation of epithelial cell clones. From generation 2 to generation 8, cells produce thyroglobulin but they do not actively trap iodide nor form follicles when thyrotropin is added to the media. Cell selection, demonstration of growth, as well as freeze-storage techniques described in this paper permit selection and storage of porcine thyroid cells and the potential constitution of cell collections.

Noninactivation of thyrotropin by cultured porcine thyroid cells

Freshly isolated porcine thyroid cells were cultured in the presence of highly purified porcine thyrotropin. Cells associate into follicles between the second and tenth day of culture and later form a monolayer. The biological and immunological activity of thyrotropin was measured daily in the media. Thyrotropin concentration and biological activity remained unchanged from the onset of the culture up to day 14. Limiting factors influencing thyroglobulin biosynthesis do not appear before day 13. The loss of follicular organization at day 10 cannot be explained by thyrotropin degradation in the medium. Considering the number of receptors per cell and the half life of the thyrotropin . receptor complex in the two dissociation compartments previously demonstrated, it appears in terms of both biological activity and affinity for the receptors that the thyrotropin molecules released from the first compartment do not differ from native molecules. It can be calculated that at least 31% of the molecules released from the second compartment are not inactivated. Thus, it is probable that the catabolism of thyrotropin on the receptor, or near the receptor site, does not play an important role in the regulation of thyroid cell function in vitro.

Thyrotropin-induced plasma membrane protein modifications in porcine thyroid cells

Highly purified plasma membranes were obtained from isolated porcine thyroid cells maintained in conditions of culture in the presence of thyrotropin (stimulated cells) or in their absence (non-stimulated cells). Analyses of both types of membranes by high-resolution sodium dodecylsulfate-polyacrylamide slab gel electrophoresis showed reproducible quantitative differences in protein bands of apparent molecular weight 38,000, 36,000 and inconstantly 96,000. Phosphorylation of membranes by [gamma-32P]ATP was 2-3 times higher in membranes from thyrotropin-stimulated than in membranes from non-stimulated cells. About 20 32P-labeled bands were detected by slab gel electrophoresis in denaturing conditions, among which the catalytic subunit of Na+, K+ ATPase was characterized. In addition, plasma membranes from thyrotropin-stimulated cells contained a firmly bound [14C]glucosamine-containing glycoprotein probably related to an aggregation-promoting factor. 125I-labeled thyroglobulin and components of unknown nature were associated with plasma membranes from thyrotropin-stimulated cells. Whether they participate in the structure and function(s) of the plasma membrane or represent contaminants of the preparation is not clear at the present time.