Expression of α-and β-subunits and activity of Na +K +ATPase in pig thyroid cells in primary culture: modulation by thyrotropin and thyroid hormones

Abstract

Na +K +ATPase located at the basolateral pole of thyroid epithelial cells, contributes to thyroid hormone synthesis by generating the driving force for the uptake of the substrate, iodide. We have investigated whether the expression of the α- and β-subunits and activity of Na +K +ATPase were subjected to variations in response, (a) to TSH, that controls the expression of differentiation in thyroid cells and (b) to thyroid hormones as potential autocrine factors. Studies were carried out on pig thyroid cells cultured (a) without TSH to obtain thyroid cell monolayers (TCM) in basal state or (b) with TSH in the form of cell monolayers (TCM-T) or as reconstituted thyroid follicles (RTF). Iodide uptake activity, thyroperoxidase protein and thyroglobulin mRNA taken as parameters of thyroid cell differentiation were 6 to 25-fold higher in RTF and TCM-T than in TCM. Western blot analyses of Na +K +ATPase subunits revealed that the α-subunit (105 kDa) content of TCM-T and RTF was similar but 8-fold higher than that of TCM. In contrast, the β-subunit (50 kDa) content of TCM-T and RTF was only about twice that of TCM. Similar relative variations were observed at the mRNA level for both α- and β-subunits. Na +K +ATPase activity was only 40% higher in RTF and TCM-T than in TCM. A 48 h treatment of RTF by either T4 or T3 (1–100nM) induced a 3-fold increase of the α-subunit but did neither alter the β-subunit nor the Na +K +ATPase activity. In conclusion, Na +K +ATPase activity and the level of expression of its β-subunit, known to control the assembly and targetting of α– β oligomers and thus the amount of functional sodium pump at the plasma membrane, are only moderately altered when thyroid cells undergo major changes in their differentiation status. Our data show that the expression of the α-subunit of Na +K +ATPase by thyroid cells is up-regulated by TSH and thyroid hormones.