Hepatocyte Nuclear Factor 4α Contributes to Thyroid Hormone Homeostasis by Cooperatively Regulating the Type 1 Iodothyronine Deiodinase Gene with GATA4 and Krüppel-Like Transcription Factor 9

Abstract

Type 1 iodothyronine deiodinase (Dio1), a selenoenzyme catalyzing the bioactivation of thyroid hormone, is highly expressed in the liver. Dio1 mRNA and enzyme activity levels are markedly reduced in the livers of hepatocyte nuclear factor 4␣ (HNF4␣)-null mice, thus accounting for its liver-specific expression. Consistent with this deficiency, serum T 4 and rT 3 concentrations are elevated in these mice compared with those in HNF4␣-floxed control littermates; however, serum T 3 levels are unchanged. Promoter analysis of the mouse Dio1 gene demonstrated that HNF4␣ plays a key role in the transactivation of the mouse Dio1 gene. Deletion and substitution mutation analyses demonstrated that a proximal HNF4␣ site (direct repeat 1 [TGGACAAA GGTGC]; HNF4␣-RE) is crucial for transactivation of the mouse Dio1 gene by HNF4␣. Mouse Dio1 is also stimulated by thyroid hormone signaling, but a direct role for thyroid hormone receptor action has not been reported. We also showed that thyroid hormone-inducible Krüppel-like factor 9 (KLF9) stimulates the mouse Dio1 promoter very efficiently through two CACCC sequences that are located on either side of HNF4␣-RE. Furthermore, KLF9 functions together with HNF4␣ and GATA4 to synergistically activate the mouse Dio1 promoter, suggesting that Dio1 is regulated by thyroid hormone in the mouse through an indirect mechanism requiring prior KLF9 induction. In addition, we showed that physical interactions between the C-terminal zinc finger domain (Cf) of GATA4 and activation function 2 of HNF4␣ and between the basic domain adjacent to Cf of GATA4 and a C-terminal domain of KLF9 are both required for this synergistic response. Taken together, these results suggest that HNF4␣ regulates thyroid hormone homeostasis through transcriptional regulation of the mouse Dio1 gene with GATA4 and KLF9.Thyroid hormone plays important roles in growth, development, differentiation, and the basal metabolic rate in vertebrates. Synthesis of thyroid hormone occurs exclusively in the thyroid gland, whose predominant secretory product is the prohormone thyroxin (T 4 ) and which produces only a small amount of the biologically active hormone 3,5,3Ј-triiodothyronine (T 3 ) (29). The majority of plasma T 3 derives from extrathyroid tissues via outer-ring deiodination of T 4 (9). This activation is catalyzed by two different deiodinases, type 1 iodothyronine deiodinase (Dio1) and type 2 iodothyronine deiodinase (Dio2). Dio1 is one of a family of selenoenzymes extensively expressed in the liver, kidney, thyroid, and pituitary in mammals (5, 6). Unlike Dio2, Dio1 can catalyze both activation of T 4 by outer-ring deiodination (5ЈD) to generate T 3 and inactivation of T 4 by inner-ring deiodination to produce 3,3Ј,5Ј-triiodothyronine (rT 3 ) (5D) (9). The expression and activity of Dio1 are modulated by a variety of hormonal, nutritional, and developmental factors, the most potent being thyroid hormone. Thyroid hormone-induced Dio1 expression contributes to the T 3 excess commonly found in hyperthyroidism. Propylthiour...

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