Expression of alpha- and beta-subunits and activity of Na(+)K(+)ATPase in pig thyroid cells in primary culture: modulation by thyrotropin and thyroidhormones

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 e xpression of the alpha- and beta-subunits and activity of Na(+)K(+)ATPase w ere subjected to variations in response, (a) to TSH, that controls the expr ession of differentiation in thyroid cells and (b) to thyroid hormones as p otential autocrine factors. Studies were carried out on pig thyroid cells c ultured (a) without TSH to obtain thyroid cell monolayers (TCM) in basal st ale or (b) with TSH in the form of cell monolayers (TCM-T) or as reconstitu ted thyroid follicles (RTF). Iodide uptake activity, thyroperoxidase protei n and thyroglobulin mRNA taken as parameters of thyroid cell differentiatio n were 6 to 25-fold higher in RTF and TCM-T than in TCM. Western blot analy ses of Na+K+ ATPase subunits revealed that the alpha-subunit (105 kDa) cont ent of TCM-T and RTF was similar but 8-fold higher than that of TCM. In con trast, the beta-subunit (50 kDa) content of TCM-T and RTF was only about tw ice that of TCM. Similar relative variations were observed at the mRNA leve l for both alpha- and beta-subunits. Na+K+ ATPase activity was only 40% hig her in RTF and TCM-T than in TCM, A 48 h treatment of RTF by either T4 or T 3 (1-100nM) induced a 3-fold increase of the alpha-subunit but did neither alter the beta-subunit nor the Na(+)K(+)ATPase activity. In conclusion, NaK+ ATPase activity and the level of expression of its beta-subunit, known t o control the assembly and targetting of alpha-beta oligomers and thus the amount of functional sodium pump at the plasma membrane, are only moderatel y altered when thyroid cells undergo major changes in their differentiation status. Our data show that the expression of the alpha-subunit of Na+ K+ A TPase by thyroid cells is up-regulated by TSH and thyroid hormones. (C)1998 Elsevier Science Ireland Ltd. All rights reserved.