A Study of Differential Actions of Thyroid Hormone in Its Target

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Thyroid hormone receptors are known as one of the nuclear receptor family which is important in the cellular proliferation and differentation. We have isolated the NADPH-dependent cytosolic thyroid hormone binding protein, and characterized this protein in previous studies. In the series of our study, we have shown that the protein plays a role not only as a thyroid hormone reservoir but also as a differentiation regulatory factor in the target cells of thyroid hormone.We found that the protein could be induced by the hormones whose receptors are present as c-erb A superfamily, thyroid hormone receptor and vitamin D receptor. The expression induced by these hormones was prominent in the malignant cells rather than in normal cells. In the malignant cells, this protein induced by vitamin D3 actively promoted the differentiation which is related to the function as an inhibitor for intracellular thyroid hormone reservoir.Thus we concluded that the receptor of thyroid hormone plays a multifunctional signal transducer in target cells, at least, in the regulation of differentiation and proliferation.