The function of retinoid X receptors on negative thyroid hormone response elements

Area of Science:

• Molecular Endocrinology
• Nuclear Receptor Signaling
• Genetics and Disease

Background:

• Retinoid X receptors (RXRs) heterodimerize with thyroid hormone receptors (TRs), enhancing DNA binding and T3-mediated transactivation on positive TREs.
• The roles of RXRs on negative TREs and their influence on the dominant negative effects of mutant TRs remain undefined.

Purpose of the Study:

• To elucidate the function of RXRs on negative TREs and their interaction with dominant negative TR mutants.
• To investigate the mechanism by which RXRs affect TR binding and function on specific TREs.

Main Methods:

• Transient cotransfection assays using luciferase reporter genes (alphaLuc and TRH-Luc) with various TR and RXR constructs.
• Electrophoretic mobility shift assays (EMSA) to assess DNA binding affinity of TR-RXR heterodimers.

Main Results:

• RXRgamma enhanced the dominant negative effect of TRbeta1 mutant R316H on the alphaLuc reporter, a negative TRE.
• EMSA revealed that RXRalpha augmented DNA binding of both wild-type and R316H TRs as heterodimers on negative TREs.
• The dominant negative effect of R316H was more pronounced on negative TREs than positive TREs, correlating with PRTH clinical features.

Conclusions:

• RXR-TR heterodimers are involved in basal transactivation and T3 suppression of negatively regulated genes.
• RXRs amplify the dominant negative effects of specific mutant TRs on negative TREs by augmenting DNA binding, not by TR displacement.
• The differential effects of mutant R316H on negative versus positive TREs contribute to the PRTH phenotype.