Functional characterization and receptor binding studies of the malic enzyme thyroid hormone response element

Area of Science:

• Molecular Biology
• Genetics
• Endocrinology

Background:

• The 5'-flanking region of the malic enzyme (ME) gene contains a cis-regulatory element that binds thyroid hormone receptors.
• This element confers triiodothyronine (T3)-inducibility of transcription to the ME promoter.

Purpose of the Study:

• To evaluate the roles of subregions within the ME thyroid hormone response element (TRE) in T3 binding and transactivation.
• To elucidate the functional significance of discrete regions within the TRE.

Main Methods:

• Deletion and mutation analyses of the ME TRE.
• Transient expression assays using NIH 3T3 cells and chloramphenicol acetyltransferase reporter gene.
• Competition binding assays to assess receptor binding to wild-type and mutated TRE.

Main Results:

• The ME TRE functions as an enhancer, conferring T3 responsiveness to a heterologous promoter.
• T3 treatment induced promoter activity, while hormone absence resulted in repression.
• Functional TRE regions are discrete and non-contiguous, with a cluster of G residues and an AGGACA sequence playing dominant roles.
• Both transcriptional induction and repression correlated with receptor binding.

Conclusions:

• The ME TRE contains distinct functional subregions critical for thyroid hormone receptor interaction and transcriptional regulation.
• The identified functional regions, including a G-rich cluster and AGGACA sequence, are essential for mediating both T3-induced activation and hormone-dependent repression.
• Receptor binding affinity to the TRE directly correlates with its ability to modulate gene transcription in response to thyroid hormone.