Yutong Wang

Date of Graduation

2001

Document Type

Thesis

Abstract

Triiodothyronine (T3) stimulates a 70-fold or greater increase in transcription of the malic enzyme gene in chick embryo hepatocytes. Previous work has shown that the effects of T3 on malic enzyme transcription are mediated in large part by sequence elements located within a liver-specific and T3-inducible DNase I hypersensitive region in the 5′-flanking DNA of the malic enzyme gene. Within this DNase I hypersensitive region is a cluster of nuclear T3 receptor (TR) binding sites that together constitute a T3 response unit (TAU). Flanking the T3RU are accessory elements that enhance T3 regulation conferred by the T3RU in chick embryo hepatocytes. In this dissertation, I have characterized the proteins that bind TR accessory elements in the malic enzyme gene. I demonstrate that the homeodomain proteins, Pbx and Meis1, interact with a strongly active TR accessory element designated as region E. In addition, using a dominant-negative approach, I have developed functional evidence demonstrating that the liver-enriched transcription factor, CCAAT/Enhancer-binding protein (C/EBP), interacts with another TR accessory element referred to as region F. Results from transfection and protein-protein interaction assays suggest that the Pbx-Meis1 bound to region E and C/EBP bound to region F stimulate T3 responsiveness of the malic enzyme gene by directly interacting with TR bound to the T3RU. In further work, I have identified two new TR accessory elements that enhance T3 regulation of malic enzyme transcription in chick embryo hepatocytes. One element (designated as region G) binds chicken ovalbumin upstream promoter-transcription factor and the other element (designated as region H) binds an unidentified E-box binding protein. In summary, a wide variety of transcription factors mediate the potent effects of T3 on malic enzyme transcription in hepatocytes.

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