The influence of collagen gels on the orientation of the polarity of epithelial thyroid cells in culture was studied under four different conditions . (a) Isolated cells cultured on the surface of a collagen gel formed a monolayer. The apical pole was in contact with the culture medium and the basal membrane was attached to the substratum .(b) Isolated cells embedded inside the gel organized within 8 d into follicles . The basal pole was in contact with collagen and the apical pole was oriented towards the interior of the follicular lumen .(c) Cells were first organized into floating vesicles, structures in which the apical surface is in contact with the culture medium, and the vesicles were embedded inside the collagen gel . After 3 d, cell polarity was inverted, the apical pole being oriented towards the cavity encompassed by cells . Vesicles had been transformed into follicles.(d) Monolayers formed on collagen gels as in a were overlaid with a second layer of collagen, which was polymerized in contact with the apical cell surface. A disorganization of the continuous pavement occurred within 24 h; cells attached to the upper layer of collagen and reorganized into follicles in the collagen sandwich within 4-8 d .A similar process occurred when the monolayer was grown on plastic and overlaid with collagen, or grown on collagen and covered with small pieces of glass cover slips. No reorganization was observed between two glass surfaces .In conclusion, first, a basal pole was always formed in the area of contact between the cell membrane and an adhesive surface and, second, the interaction of a preformed apical pole with an adhesive surface was not compatible with the stability of this domain of the plasma membrane . The interaction of the cell membrane with extracellular components having adhesive properties appears to be a determinant factor in the orientation and stabilization of epithelial cell polarity .The formation of a polarized epithelial cell monolayer involves two types of events : (a) the formation of intercellular junctions, the most typical for epithelia being the tight junction (14, 36) ; and (b) cell polarization by an asymmetrical distribution of membrane components between the apical and basolateral domains (8,21) and by a polar distribution of intracellular

In the thyroid glands, thyroglobulin (Tg) is specifically synthesized by follicular cells and then secreted into the apical lumen where it is concentrated and used as a substrate for thyroid hormone synthesis. The presence of Tg in the circulation has been reported in normal and pathological situations. To determine the domains of the plasma membrane, apical and/or basolateral, involved in Tg secretion, porcine thyroid epithelial cells were cultured as a monolayer on the porous bottom of a culture chamber in which both apical and basal media are independently accessible. Control experiments using labeled Tg ascertained the tightness of the monolayer and showed that within 48 h only 0.2-0.5% of the Tg introduced in the apical medium was transferred through the cell layer into the basal compartment. For kinetic studies of Tg synthesis and secretion, monolayers were cultured for up to 72 h in the presence of 35S-methionine and with or without 100 microU/ml thyrotropin (TSH) in the basal medium. Labeled Tg was measured by double immunoprecipitation and by fluorography of polyacrylamide gel electrophoresis. We showed that 80-95% of total secreted Tg was recovered in the apical medium. The remainder was secreted through the basolateral membranes in the basal medium. The amount of tg secreted into the apical compartment was stimulated two- to threefold by TSH whereas no TSH effect was observed on secretion in the basal compartment. Moreover, measuring apical and basal volumes, we observed a net water flow from the apical to the basal side. It was stimulated threefold by TSH, increasing the Tg concentration in the apical compartment of the stimulated cell layer. During the culture time, the amount of Tg synthesized and secreted was increased by TSH, as was the Tg mRNA content, as determined by the dot-blot hybridization method.

When cultured in polystyrene dishes subjected to previous treatment and supplied with a serum-containing medium, hog thyroid cells form monolayers displaying dome-like arrangements after three to four days. Cells involved in formation of "domes" are morphologically polarized; the apical microvilli of these cells point toward the culture medium. When the tissue is cultured in untreated polystyrene dishes, thyroid cells remain in suspension; their aggregates swell progressively and form hollow spheres encompassed by a single layer of cells. The polarity of the cells forming such spheres is inverse in comparison to the condition characteristic of the intact thyroid gland. When culture medium is supplemented with TSH, PGE1, PGE2 or dBC, structures resembling true follicles are formed in both types of cultures. Gelatin, added to suspension cultures, is also capable of promoting follicle formation. Cultured thyroid cells regularly form an epithelial layer as a result of the interaction of cellular processes. However, the polarization of this layer depends on culture conditions. Thus, structures with either a normal follicle-like polarization of their cells or showing an inverted type of polarization can be obtained.

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 receptoradenylate cyclase complex are present only on the basolateral domain of thyroid cell plasma membranes.In vivo, thyroid epithelial cells are organized into follicles. They concentrate and organify circulating iodide and respond within minutes to acute thyrotropin stimulation by increased cAMP synthesis, iodide effiux, and formation of apical pseudopods (10). As both iodide and thyroid-stimulating hormone (TSH) present in the blood have access to the basal surface of follicular cells, it is likely that the iodide concentration mechanism and the TSH receptor-adenyl cyclase complex are present on the basolateral domain of the plasma membrane of the thyroid epithelial cell. No direct evidence of an asymmetrical distribution of these components between the apical and basolateral domain of the plasma membrane has yet been reported.In vitro, when cultured at high cell density on glass or polystyrene substrates, isolated porcine thyroid cells form polarized monolayers 04,20,26). As in other epithelial systems, the basal surface of the cell layer is in contact with the substrate, 1172 whereas the apical pole of the cells is oriented towards the culture medium. Under these culture conditions, ceils are unable to concentrate iodide and the intracellular cAMP level is not changed upon addition of TSH to the culture medium. This loss of iodide concentration activity and of responsiveness to acute thyrotropin stimulation might be due, at least partly, to the inaccessibility of the basolateral domain of the plasma membrane to molecules present in the culture medium.We used two different experimental procedures to overcome the inaccessibility of the basal surface of the cell layer. (a) Thyroid cells were cultured in petri dishes coated with a th...

A small conductance chloride channe1 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 chioride over gluconate and iodide, and is impermeable to Nat, K+ and ~traethylammo~~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 S-nitro-2-(3-phenylpropyiamino)~benzoate (NPPB) did not affect the activity of the thyroid Cl-channels. Treatment of thyroid cells with 8-(4chlorophenylthio)adenosine-3',5'-cyclic monophosphate (I-chloro-CAMP) (0.5 mM) prior to giga-seal formation increased Cl-channel activity in the apical membrane of thyroid cells.

Isolated porcine thyroid cells cultured in suspension in Eagle Minimum Essential Medium supplemented with calf serum (5-20%) reorganize to form vesicles, i.e. closed structures in which all cells have an inverted polarity as compared to that found in follicles: the apical membranes are bathed by the culture medium. Under these conditions, cells neither concentrate iodide nor respond to acute thyrotropin (TSH) stimulation. When embedded in collagen gel, these vesicles undergo polarity reversal to form follicles. We describe here the change in the orientation of cell polarity and the subsequent reappearance of specific thyroid functions. Six hr after embedding, membrane areas in contact with collagen fibers show basal characteristics. At this time, cells begin to concentrate iodide and to respond to acute TSH stimulation (iodide efflux and increased cAMP levels). Most cells form follicles 24 hr after embedding, but 48 hr are required for the transformation of all vesicles into follicles. This occurs without opening of the tight junctions. Iodide organification is detected 24 hr after embedding, when periodic acid-Schiff positive material, identified as thyroglobulin by immunofluorescence, accumulates in the lumen. Iodide concentration and organification, as well as response to TSH stimulation reach maximal levels after 3 days in the collagen matrix. After a 5-day culture in the collagen matrix in the absence of TSH, cell activity can be stimulated by chronic treatment with low hormone concentrations (10-100 microU/ml). As shown with thyroid cells grown in monolayer on permeable substrates (Chambard M., et al., 1983, J. Cell Biol. 96, 1172-1177), iodide uptake and cAMP-mediated TSH responses are expressed when the halogen and the hormone have direct access to the basal membrane. Organification, on the contrary, requires a closed apical compartment.

Isolated porcine thyroid cells, cultured in the presence of thyrotropin ( 2 0.25 mU/ml) or prostaglandin EZ ( 2 0.1 pM), showed decreased adenosine 3': 5'-monophosphate (cyclic AMP) response to further thyrotropin or prostaglandin EZ stimulation, respectively.Kinetics of the refractory process to thyrotropin and prostaglandin EZ are different : ( a) maximal refractoriness to prostaglandin EZ was attained after 2 -6 h exposure to prostaglandin EZ while refractoriness to thyrotropin was maximal only after 12-24 h ; (b) the degree of refractoriness to prostaglandin Ez was much greater than that to thyrotropin.Refractoriness to thyrotropin or prostaglandin EZ is characterized : by specificity for each thyroid stimulator; by dependence upon the dose of thyrotropin or prostaglandin E2 in culture, e.g. induction of high degree of refractoriness with 0.5 mU/ml thyrotropin (or 1 pM prostaglandin Ez), which elicits only a small cyclic AMP increase; by time requirement for induction; by partial effect; by changes of maximum activation of cyclic AMP response; by reversibility.This refractoriness of the cyclic AMP response was not induced by dibutyryl adenosine 3': 5'-monophosphate. It was not attributed to increased cyclic AMP-phosphodiesterase activity, but to alterations in the receptor-adenylate cyclase system. Prevention of refractoriness to thyrotropin or prostaglandin EZ by incubation of cells in the presence of actinomycin D, puromycin and cycloheximide suggests that new RNA and protein syntheses are required for the development of the refractory state.

Two different independent processes are operating in cultured thyroid cells to regulate adenylate cyclase/cyclic AMP responsiveness to thyroid stimulators (thyrotropin and prostaglandin Ez) :firstly, refractoriness or negative regulation [preceding paper], which is specific for each thyroid stimulator, is not mediated by cyclic AMP and is not accompanied by alteration of adenylate cyclase activity; secondly, positive regulation which is characterized by an augmentation of the cyclic AMP response stimulated by thyrotropin and prostaglandin E2. This process is not specific for each thyroid stimulator and is a state of increased susceptibility of cyclic AMP synthesis to stimulation, accompanied by increased activity of the catalytic subunit of adenylate cyclase. Positive regulation is apparently mediated by increased intracellular cyclic AMP levels. It is a time-dependent and dose-dependent process. Very low concentrations ( 5 -50 pU/ml) of thyrotropin augmented cyclic AMP synthesis stimulated by thyrotropin and prostaglandin E2 whereas higher concentrations (above 0.1 mU/ml) augmented prostaglandin El stimulation but induced refractoriness to thyrotropin. Prostaglandin E2 (0.1 to 10 pM) augmented thyrotropin stimulation and dibutyryl adenosine 3' : 5'-monophosphate (0.3 to 2 mM) augmented thyrotropin and prostaglandin Ez stimulation. Positive regulation is a slow process which develops within days and increases up to day 5 in culture. Experiments using inhibitors suggested that protein synthesis is required for the full expression of the increase in adenylate cyclase activity induced by the studied thyroid stimulators.We have recently shown that the cyclic AMP response to thyrotropin of thyroid cells in culture was under a dual regulation by thyrotropin : chronic exposure of the cells to physiological concentrations of the hormone augmented the subsequent cyclic AMP response to thyrotropin (positive regulation), whereas moderate to high concentrations of thyrotropin induced refractoriness to further thyrotropin stimulation [l].The conditions for the development of refractoriness to thyrotropin and prostaglandin Ez have been described in a preceding paper [2]. In this report, we analyze the mechanism of the positive regulation of the acute cyclic AMP response to thyrotropin and prostaglandin E2 as influenced by chronic treatment of cultured thyroid cells with these stimulators. MATERIALS AND METHODSThe methods and the materials used have been described in the preceding paper [ 2 ] . Cells cultured in the presence of thyrotropin (10 mU/ml), prostaglandin EZ (1 pM) or dibutyryl cyclic AMP (0.4 mM) will be referred to further as thyrotropin-treated, prostaglandin E2-treated or BtzcAMP-treated cells, respectively. Cells cultured in the absence of these compounds will be referred to as control cells.Cyclic AMP synthesis using prelabeling technique with 14C-labeled adenine [3] was measured as reported previously [4] with minor modifications. Duration of preincubation with [14C]adenine was 40 rnin. To preserve cell viab...