THE LOCATION AND THE REGULATION OF THE TYPE I-IODOTHYRONINE 5'-MONODEIODINASE (TYPE I-MD) IN THE RAT THYROID : STUDIES USING A SPECIFIC ANTI-TYPE I-MD ANTIBODY

Abstract Type I-iodothyronine monodeiodinase (type I-MD) is abundant in thyroid tissue and contributes to the generation of T3 secreted by the gland. The availability of a specific antibody against rat type I-MD (type I-MD Ab) allowed us to directly identify this enzyme in rat thyroid glands, and in a differentiated strain of rat thyroid cells maintained in continuous culture (FRTL-5 cells). FRTL-5 cells maintain many differentiated functions of thyroid cells, including the expression of TSH receptor and thyroid peroxidase. Using an immunohistochemical technique on rat thyroid sections, a clear staining for type I-MD was demonstrated in follicular cells. The degree of immunoreactivity was greater in small follicles containing little amounts of colloid compared to large follicles lined by functionally inactive cells. Using immunofluorescence (IFL), a strong staining for type I-MD was observed in FRTL-5 cells grown in medium containing TSH. Both in vivo and in culture the staining for type I-MD was localised in the cytoplasm of thyroid cells, while nuclei were negative. Interestingly, no surface staining was shown when viable FRTL-5 cells were submitted to the same IFL procedure. TSH deprivation for 7 days was followed by the disappearance of type I-MD. Immunoreactivity for type I-MD was recovered by addition of TSH, forskolin or thyroid stimulating antibody (TSAb) to TSH-deprived FRTL-5 cells. The effect of TSH was prevented by cycloheximide. There was no induction of type I-MD when IGF-I was added to FRTL-5 cells. In conclusion, our results indicate that: (i) type I-MD is abundant in intracytoplasmic membranes of rat thyroid cells, both in vivo and in culture, while it is not expressed on the outer side of the plasma membrane; (ii) the synthesis of type I-MD is TSH-dependent through pathways that involve cAMP production, while it is not influenced by IGF-I which activates the tyrosine protein kinase pathway; (iii) TSAb mimics TSH in inducing the expression of type I-MD in FRTL-5 cells, and may thus influence the intrathyroidal generation of T3 in patients with Graves' disease.