Hepatocyte nuclear factor 4α contributes to thyroid hormone homeostasis by cooperatively regulating the type 1 iodothyronine deiodinase gene with GATA4 and Kruppel-like transcription factor 9

Type 1 iodothyronine deiodinase (Dio1), a selenoenzyme catalyzing the bioactivation of thyroid hormone, is highly expressed in the liver. Diol mRNA and enzyme activity levels are markedly reduced in the livers of hepatocyte nuclear factor 4 alpha (HNF4 alpha)-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 alpha-floxed control littermates; however, serum T(3) levels are unchanged. Promoter analysis of the mouse Diol gene demonstrated that HNF4 alpha plays a key role in the transactivation of the mouse Dio1 gene. Deletion and substitution mutation analyses demonstrated that a proximal HNF4 alpha site (direct repeat 1 [TGGACAAA GGTGC]; HNF4 alpha-RE) is crucial for transactivation of the mouse Dio1 gene by HNF4 alpha. 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 Kruppel-like factor 9 (KLF9) stimulates the mouse Dio1 promoter very efficiently through two CACCC sequences that are located on either side of HNF4 alpha-RE. Furthermore, KLF9 functions together with HNF4 alpha 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 alpha 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 alpha regulates thyroid hormone homeostasis through transcriptional regulation of the mouse Dio1 gene with GATA4 and KLF9.