Identification of a 57-Kilodalton Selenoprotein in Human Thyrocytes as Thioredoxin Reductase and Evidence That Its Expression Is Regulated through the Calcium-Phosphoinositol Signaling Pathway

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Abstract
Human thyrocytes incubated with the phorbol ester, phorbol 12- myristate 13-acetate (PMA; 1025-1028 mol/L) and the calcium iono- phore A23187 (1025-1028 mol/L) showed a marked increase in the expression of a 57-kDa selenoprotein identified as thioredoxin reduc- tase (TR). After the addition of A23187 with PMA, a significant in- duction in TR expression was observed after 6 h, with maximal in- duction occurring by 24 h. The addition of 8-bromo-cAMP (1024 mol/L) or TSH (10 U/L) alone had no effect on TR expression, nor did these agents influence the induction of TR brought about by the addition of A23187 and PMA. These data show that the calcium-phosphoinositol second messenger cascade that controls hydrogen peroxide genera- tion in the human thyrocyte is also an important stimulator of TR expression. The role of TR in the thyrocyte is unclear, but the sel- enoenzyme has a high capacity to detoxify compounds, such as hy- drogen peroxide and lipid hydroperoxides, that are produced in high concentration during thyroid hormone synthesis. (J Clin Endocrinol Metab 83: 2052-2058, 1998)

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