Electrophysiological correlates of fluid transport in cultured porcine thyroid cells

Local thyroid renin-angiotensin system in experimental breast cancer

An association between breast cancer and thyroid dysfunction exists although the underlying mechanisms remain to be elucidated. Numerous studies have characterized the role of thyroid hormones in controlling the synthesis and secretion of renin-angiotensin system (RAS) components, but little information is available on the putative role of the local RAS on thyroid function.

AIMS

Here we analyze several soluble and membrane-bound RAS-regulating aminopeptidase activities in thyroid gland from rats with mammary tumors and the relationship with the circulating levels of thyroid stimulating hormone (TSH) and free thyroxin (fT4).

MAIN METHODS

We analyze soluble and membrane-bound RAS-regulating aminopeptidase activities fluorometrically using their corresponding aminoacyl-β-naphthylamide as the substrate.

KEY FINDINGS

We have found in rats with mammary tumors a concomitant change of thyroid RAS-regulating enzymes and thyroid hormone production.

SIGNIFICANCE

We suggest that existence of alterations in the regulatory mechanisms mediated by the angiotensins of the local tissue RAS as a consequence of the carcinogenic process which could act alone or in combination with alterations at a higher level of regulation such as the hypothalamus-pituitary axis.

Altered ion transport by thyroid epithelia from CFTR(-/-) pigs suggests mechanisms for hypothyroidism in cystic fibrosis

Subclinical hypothyroidism has been linked to cystic fibrosis, and the cystic fibrosis transmembrane conductance regulator (CFTR) shown to be expressed in the thyroid. The thyroid epithelium secretes Cl⁻ and absorbs Na(+) in response to cAMP. Chloride secretion may provide a counter-ion for the SLC26A4 (pendrin)-mediated I⁻ secretion which is required for the first step of thyroid hormonogenesis, thyroglobulin iodination. In contrast, few models exist to explain a role for Na(+) absorption. Whether CFTR mediates the secretory Cl⁻ current in thyroid epithelium has not been directly addressed. We used thyroids from a novel pig CFTR(-/-) model, generated primary pig thyroid epithelial cell cultures (pThECs), analysed these cultures for preservation of thyroid-specific transcripts and proteins, and monitored the following parameters: (1) the Cl⁻ secretory response to the cAMP agonist, isoprenaline; and (2) the amiloride-sensitive Na(+) current. Baseline short-circuit current (I(sc)) did not differ between CFTR(+/+) and CFTR(-/-) cultures. Serosal isoprenaline increased I(sc) in CFTR(+/+), but not CFTR(-/-), monolayers. Compared with CFTR(+/+) thyroid cultures, amiloride-sensitive Na(+) absorption measured in CFTR(-/-) pThECs represented a greater fraction of the resting I(sc). However, levels of transcripts encoding epithelial sodium channel (ENaC) subunits did not differ between CFTR(+/+) and CFTR(-/-) pThECs. Immunoblot analysis verified ENaC subunit protein expression, but quantification indicated no difference in expression levels. Our studies definitively demonstrate that CFTR mediates cAMP-stimulated Cl⁻ secretion in a well-differentiated thyroid culture model and that knockout of CFTR promotes increased Na(+) absorption by a mechanism other than increased ENaC expression. These findings suggest several models for the mechanism of cystic fibrosis-associated hypothyroidism.

Developmental delays consistent with cochlear hypothyroidism contribute to failure to develop hearing in mice lacking Slc26a4/pendrin expression

Mutations of SLC26A4 cause an enlarged vestibular aqueduct, nonsyndromic deafness, and deafness as part of Pendred syndrome. SLC26A4 encodes pendrin, an anion exchanger located in the cochlea, thyroid, and kidney. The goal of the present study was to determine whether developmental delays, possibly mediated by systemic or local hypothyroidism, contribute to the failure to develop hearing in mice lacking Slc26a4 (Slc26a4(-/-)). We evaluated thyroid function by voltage and pH measurements, by array-assisted gene expression analysis, and by determination of plasma thyroxine levels. Cochlear development was evaluated for signs of hypothyroidism by microscopy, in situ hybridization, and quantitative RT-PCR. No differences in plasma thyroxine levels were found in Slc26a4(-/-) and sex-matched Slc26a4(+/-) littermates between postnatal day 5 (P5) and P90. In adult Slc26a4(-/-) mice, the transepithelial potential and the pH of thyroid follicles were reduced. No differences in the expression of genes that participate in thyroid hormone synthesis or ion transport were observed at P15, when plasma thyroxine levels peaked. Scala media of the cochlea was 10-fold enlarged, bulging into and thereby displacing fibrocytes, which express Dio2 to generate a cochlear thyroid hormone peak at P7. Cochlear development, including tunnel opening, arrival of efferent innervation at outer hair cells, endochondral and intramembraneous ossification, and developmental changes in the expression of Dio2, Dio3, and Tectb were delayed by 1-4 days. These data suggest that pendrin functions as a HCO3- transporter in the thyroid, that Slc26a4(-/-) mice are systemically euthyroid, and that delays in cochlear development, possibly due to local hypothyroidism, lead to the failure to develop hearing.

Mechanism of iodide/chloride exchange by pendrin

We performed an electrophysiological study to investigate ion transport of pendrin and thereby understand the pathogenesis of Pendred syndrome. Using pendrin-transfected COS-7 cells, we could show that pendrin transports both iodide and chloride measured as voltage-dependent inward and outward membrane currents. Chloride in the culture medium, [Cl-]o, was efficiently exchanged with cytoplasmic iodide, [I-]i, under physiological concentrations, indicating that pendrin is important for chloride uptake and iodide efflux. Although exchange of iodide in the medium, [I-]o, with cytoplasmic chloride, [Cl-]i, was observed, a significantly high concentration of iodide (10 mm) was required. In addition, either iodide or chloride was required on both sides of the cell membrane for the anion exchange activity of pendrin, indicating that iodide and chloride activate the exchange activity of pendrin while they are transported. The present study further supports that pendrin is responsible for the iodide efflux in thyroid cells where intracellular iodide concentration is high and that the general function of pendrin in other tissues is to transport chloride through exchange with other anions.

Angiotensin II AT1 receptor stimulates Na+ -K+ATPase activity through a pathway involving PKC-zeta in rat thyroid cells

Angiotensin II (Ang II) receptor subtype 1, AT1, is expressed by the rat thyroid. A relationship between thyroid function and several components of the renin-angiotensin system has also been established, but the Ang II cellular effects in thyrocytes and its transduction signalling remain undefined. The aim of the present paper was to investigate the modulation of the activity of the Na(+)-K(+)ATPase by Ang II and its intracellular transduction pathway in PC-Cl3 cells, an established epithelial cell line derived from rat thyroid. Here we have demonstrated, by RT-PCR analysis, the expression of mRNA for the Ang II AT1 receptor in PC-Cl3 cells; mRNA for the Ang II AT2 receptor was not detected. Ang II was not able to affect the intracellular Ca(2+) concentration in fura-2-loaded cells, but it stimulated the translocation from the cytosol to the plasma membrane of atypical protein kinase C-zeta (PKC-zeta) and -iota (PKC-) isoforms with subsequent phosphorylation of the extracellular signal-regulated kinases 1 and 2 (ERK1 and 2). Translocated atypical PKCs displayed temporally different activations, the activation of PKC-zeta being the fastest. PC-Cl3 cells stimulated with increasing Ang II concentrations showed dose- and time-dependent activation of the Na(+)-K(+)ATPase activity, which paralleled the PKC-zeta translocation time course. Na(+)-K(+)ATPase activity modulation was dependent on PKC activation since the PKC antagonist staurosporine abolished the stimulatory effect of Ang II. The inhibition of the ERK kinases 1 and 2 (MEK1 and 2) by PD098059 (2'-amino-3'-methoxyflavone) failed to block the effect of Ang II on the Na(+)-K(+)ATPase activity. In conclusion, our results suggest that Ang II modulates Na(+)-K(+)ATPase activity in PC-Cl3 cells through the AT1 receptor via activation of atypical PKC-zeta while the Ang II-activated PKC- appears to have other as yet unknown functions.

The role of proteases in fibronectin matrix remodeling in thyroid epithelial cell monolayer cultures

Fischer rat thyroid (FRT) cells organize a matrix of extracellular fibronectin (FN) fibrils, which undergoes extensive remodeling according to cell culture confluence. In non-confluent cells FN forms a fibrillar array associated with the ventral cell surface. However, basal FN is progressively removed in confluent cultures and substituted by non-fibrillar FN deposits at lateral cell domains in regions of cell-cell contacts. FRT cells secrete and expose on the plasma membrane the tissue-type plasminogen activator and, in serum-free cultures, plasminogen induces a rapid loss of FN fibrils. Incubation with plasmin inhibitors greatly reduces this effect. FRT cells also express annexin II, a plasminogen receptor, suggesting that plasmin activity is associated with the pericellular enviroment. This is in agreement with the observation that a great reduction in FN degradation is observed if the cells are pre-incubated with carboxypeptidase B, which prevents plasminogen binding to the cells. A gelatinolytic activity with a molecular weigth equivalent to MMP-2 has been demonstrated by zymography of culture media, and the presence of MMP-2 and MT1-MMP on the cell plasma membrane has been detected by immunofluorescence. These results indicate that in the FN remodeling process, occurring during FRT epithelium maturation, both plasmin-dependent (tPA activated) and plasmin-independent proteolytic activities are involved.

Thyrotropin regulation of basolateral Cl- and I- effluxes in thyroid follicles in culture

This report describes chloride and iodide effluxes across the basolateral membrane of porcine thyroid follicles reconstituted in culture. Basolateral chloride efflux is activated by thyrotropin (TSH). TSH (10 mU/ml) induces a twofold increase in the initial rate of chloride efflux. Forskolin (FSK, 5 microM) which increases intracellular cAMP also stimulates the initial rate of chloride efflux 3.5-fold, whereas an increase in the free cytosolic Ca2+ with the ionophore A23187 or thapsigargin, fails to mimic the TSH effect. The chloride channel blocker 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB) dose dependently inhibits chloride efflux rates with the maximal and half maximal effects observed for 100 microM and 30 microM, respectively. Basolateral chloride efflux rates are also inhibited in the presence of the organic anion transporter blocker probenecid (5 mM) or the Cl-/HCO3- exchanger blocker 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS, 250 microM), respectively, by 60% and 40%, whereas it is not affected by ClO4 (100 microM). The initial rate of iodide efflux is weakly activated (1.4-fold) by TSH (10 mU/ml). TSH effect could be reproduced by agents known to activate Ca(2+)-dependent processes as A23187, ionomycin (1 microM), phorbol 12-myristate 13-acetate (TPA, 0.1 microM) and epidermal growth factor (EGF, 0.1 microM) which increase the initial rate of iodide efflux from 1.2- to 1.8-fold, whereas FSK is without effect. The chloride channel blocker NPPB (500 microM) is required to significantly inhibit the initial rate of iodide efflux by 30%. The initial rate of iodide efflux is also reduced by 30% in the presence of SITS (250 microM) or probenecid (5 mM) whereas it is activated by ClO4 (100 microM). We conclude that basolateral chloride and iodide effluxes are both regulated by TSH, using two different transduction pathways. Chloride efflux regulation may involve a cAMP transduction signal, whereas the regulation of iodide efflux may involve a Ca2+ signal. Furthermore, as the sensitivities of chloride and iodide effluxes for the anion transporter blockers (especially NPPB) are different, it seems likely that chloride and iodide use two different transport pathways.

Activation of sodium transport mediates regulation of thyroid follicle volume in response to hypotonic media

The thyroid epithelium possesses a bidirectional fluid transport system capable of absorbing Na+ and secreting Cl-. In the present studies we have examined its possible role in the regulation of thyroid follicular size. When exposed to hypotonic media (200 mosM) cultured porcine thyroid follicles first swelled and then displayed a regulatory volume decrease (RVD) over 60 min. This was associated with a transient depolarization of the transepithelial potential difference (TEP) and subsequent hyperpolarization with a time course similar to RVD. Phenamil (1 microM), an antagonist of epithelial Na+ channels, did not affect initial swelling but prevented the subsequent follicular RVD. Phenamil abolished hyperpolarization of TEP, but the loop diuretic bumetanide, which inhibits Cl- secretion in thyroid cells, did not prevent it. Exposure to hypotonic medium produced a slow hyperpolarization of the intracellular potential (basolateral membrane potential) consistent with an increase in basolateral membrane K+ conductance. Ba2+ and quinidine, which are known to inhibit K+ channels in epithelia, prevented RVD. Addition of the K+ ionophore valinomycin (1 microM) caused follicle shrinkage that was prevented by phenamil (1 microM). We conclude that cultured follicles respond to hypotonically induced stretch by activating outwardly directed Na+ transport through a mechanism which involves change in the basolateral K+ conductance. This response would be characteristic of a system that controlled follicle volume. However, it is not clear from these studies whether the cells responded primarily to the increase in follicle volume or to the change in cell volume that is expected to accompany hypotonic challenge.

Chloride conductance of apical membrane in cultured porcine thyroid cells activated by cyclic AMP

The thyroid epithelium transports fluid bidirectionally using active transport of Na+ ions from apical to basal poles and active transport of Cl- in the reverse direction. In these studies we sought evidence for cyclic AMP activated Cl- channels on the apical membranes of thyroid cells in monolayer culture. A Cl(-)-dependent basal-positive short-circuit current (ISC) was demonstrated in bicameral chambers after blocking Na+ transport with phenamil, and responded to prostaglandin (PG) E2 with a spike of 5-10 min duration followed by a plateau. The onset of the spike coincided with an increase in the conductance of the epithelium. Application of an external Cl- concentration gradient, by replacing the medium in the apical compartment with Cl(-)-free medium, resulted in an increase in ISC after, but not before, addition of PGE2. Forskolin and thyroid-stimulating hormone (TSH), but not A23187, also stimulated Cl- transport. In conjunction with previous observations that Cl- transport was mediated by a bumetanide-sensitive NaKCl2 symporter on the basal membrane, these observations indicated the presence of a cyclic AMP activated Cl- conductance in the apical membrane of thyroid cells.

Bidirectional ion transport in thyroid: secretion of anions by monolayer cultures that absorb sodium

Cultured porcine thyroid cell monolayers transport Na+ in an apical-to-basal direction, resulting in the development of a basal-positive transepithelial potential difference (TEP) and the formation of domes (fluid-filled elevations of the cell layer above the culture dish substrate). Stimulation by prostaglandin E2 (PGE2) increases the magnitude of the TEP, the short-circuit current (Isc) measured in Transwell Ussing chambers, and the height of domes in cultures grown on impermeable substrates. A phenamil-resistant, PGE2-stimulated component of the Isc in Transwells and of the TEP in monolayers in conventional culture dishes was inhibitable by bumetanide, a diuretic drug that blocks NaKCl2 symporters, mediating active transport of Cl-. The rate of decrease in height of domes in cultures after addition of phenamil, presumably indicative of transport of fluid in a basal-to-apical direction, was also reduced by bumetanide. Studies with Transwells in Cl(-)-free, HCO(3-)-free or Cl(-)- and HCO(3-)-free media indicated that thyroid cells transported HCO3- as well as Cl- in a basal-to-apical direction. It was concluded that the thyroid epithelium is both sodium absorbing and anion secreting.

Regulation of thyroid follicular volume by bidirectional transepithelial ion transport

Previous studies have shown that thyroid cells in monolayer culture exhibit bidirectional ion transport comprising apical-to-basal amiloride-sensitive Na+ transport and oppositely directed bumetanide-sensitive Cl- transport. We have now investigated the role of ion transport in the regulation of thyroid follicular size using follicular primary porcine thyroid cell cultures. Bumetanide (10 microM) added at the beginning of culture inhibited the formation of follicular lumina and caused a fall in follicle height when added to 3-day-old cultures. In contrast, phenamil (1 microM; an amiloride analog) increased follicle size both in freshly isolated and 3-day-old cultures. The effect of bumetanide was prevented by the prior addition of phenamil. Micropuncture studies showed that follicles had a lumen-negative, basal-positive transepithelial potential difference which was progressively reduced in magnitude by the serial addition of bumetanide (10 microM) and phenamil (1 microM). We conclude that thyroid follicles possess a bidirectional ion transport system which transports Na+ in an apical-to-basal direction and Cl- in the opposite direction. The balance between these two processes determines net solute flux and hence follicular size. A physiological role of ion transport in the thyroid may be to regulate follicular volume suggesting that abnormalities of ion transport may be responsible for disorders of follicular size.

Small conductance chloride channels in the apical membrane of thyroid cells

A small conductance chloride channel has been identified on the apical membrane of porcine thyroid cells using the patch-clamp technique. In cell attached membrane patches with NaCl in the pipette, the single channel conductance is 5.5 pS. The channel is highly selective for chloride over gluconate and iodide, and is impermeable to Na+, K+ and tetraethylammonium ions. The open state probability of the channel is not affected by voltage. The channel activity disappears after excision of the patch. The Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) did not affect the activity of the thyroid Cl- channels. Treatment of thyroid cells with 8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphate (8-chloro-cAMP) (0.5 mM) prior to giga-seal formation increased Cl- channel activity in the apical membrane of thyroid cells.

The thyroid cell monolayer in culture. A tight sodium absorbing epithelium

When cultured on collagen coated nitrocellulose filters, thyroid epithelial cells form morphologically and functionally polarized monolayers. The bioelectric parameters of these monolayers were measured after mounting in Ussing chambers; transepithelial potential (Vab), short circuit current (Isc) and transepithelial resistance were respectively 12 +/- 1 mV (apical side negative), 3.8 +/- 0.2 microA cm-2 and 3250 +/- 214 omega cm2 (mean +/- SEM, n = 75). Eighty two percent of the short circuit current was related to sodium absorption as shown by inhibition by apical amiloride (Km = 0.2 microM) and by basal ouabain (K1/2 = 0.3 microM). Amphotericin B (5-25 micrograms/ml) added to the apical bath increased Isc suggesting an apical rate-limiting step. Step by step replacement of choline by Na+ in a Na+-free medium resulted in a progressive increase in Vab and Isc with half maximal effect at 20 +/- 1 mM Na+. Thyrotropin (TSH) increased Isc and Vab in a biphasic way with a transient maximum after 5 min and a plateau after 20 min (about four times the basal level at 100 microU/ml TSH). This increase in sodium transport was also inhibited by apical amiloride. Thus, in culture, the thyroid cell monolayer behaves as a tight sodium absorbing epithelium controlled by TSH, with a rate limiting apical sodium channel as the entry mechanism and a basolateral Na+, K+-ATPase as the electromotive force.