Identification of a Truncated Dual Oxidase 2 (DUOX2) Messenger Ribonucleic Acid (mRNA) in Two Rat Thyroid Cell Lines. Insulin and Forskolin Regulation of DUOX2 mRNA Levels in FRTL-5 Cells and Porcine Thyrocytes

The Duox2 flavoprotein is strongly expressed in the thyroid gland, where it plays a critical role in the synthesis of thyroid hormones likely by providing thyroperoxidase with H(2)O(2). A truncated DUOX2 mRNA was isolated from the rat thyroid cell line FRTL-5. The cDNA sequence predicted an open reading frame of 1458 bp, encoding a polypeptide of 486 amino acids corresponding to the carboxyl fragment of the Duox2 flavoprotein. The truncated form of DUOX2 mRNA, expressed in another rat thyroid cell line, the PC Cl3 cell line, was absent from Fischer rat thyroid glands. Although it was expressed in both cell lines to a greater extent than normal mRNA, it failed to support protein synthesis in an in vitro translation system. Insulin increased the levels of both normal and truncated DUOX2 mRNA in FRTL-5 cells grown in TSH-free medium containing a low concentration of serum. The stimulating effect of insulin on DUOX2 mRNA expression was reproduced in pig thyroid follicles in primary culture. The presence of insulin in the culture medium converted forskolin from a stimulator to an inhibitor in FRTL-5 cells maintained in low serum conditions, but not in porcine thyrocytes in primary culture.