Thyroid hormone influences beta myosin heavy chain (beta MHC) expression

Area of Science:

• Molecular Biology
• Cardiovascular Physiology
• Endocrinology

Background:

• Thyroid hormone plays a crucial role in cardiac development and function.
• Beta-myosin heavy chain (beta-MHC) is a key contractile protein in the heart, and its expression is regulated by thyroid hormone.

Purpose of the Study:

• To investigate the mechanism by which 3,5,3'-Triiodo-L-thyronine (T3) regulates beta-MHC gene expression in rat fetal heart cells.
• To identify the specific DNA sequences within the beta-MHC promoter responsible for T3-mediated regulation.

Main Methods:

• Utilized deletion mutants of rat and human beta-MHC promoter regions fused to a CAT expression vector.
• Assayed chloramphenicol acetyltransferase (CAT) activity to quantify gene expression.
• Performed deletion analysis of promoter constructs to map T3 response elements (TREs).

Main Results:

• T3 significantly inhibited the expression of both rat and human beta-MHC constructs.
• The inhibitory effect of T3 was observed with an IC50 of approximately 1 nM, consistent with EC50 values for beta-MHC mRNA in cardiomyocytes.
• Deletion analysis indicated the presence of a T3 response element (TRE) located near the transcription start site of the beta-MHC gene.

Conclusions:

• Thyroid hormone (T3) negatively regulates beta-MHC gene expression in fetal cardiomyocytes.
• A TRE located proximally to the transcription start site is involved in T3-mediated repression.
• T3-receptor binding to this TRE may impede the formation of the transcriptional initiation complex, thereby inhibiting beta-MHC expression.