Thyroid hormone receptors

Abstract

The thyroid hormones exert their actions mainly through two isoforms of receptors which are nuclear receptors. Nongenomic actions are not described in this review although there is growing evidence of their existence and importance. The thyroid hormone receptors regulate the expression of target genes, by binding to thyroid hormone response elements (specific DNA sequences) together with cofactors: corepressors in the absence of the hormone and coactivators when ligand-bound. The thyroid hormone receptors bind either as monomers, but mainly as homodimers and even more frequently as heterodimers with the RXR retinoid receptor. The different active isoforms alpha-1, beta-1 and beta-2 are produced from two genes as well as other isoforms through alternate splicing. The alpha-2 isoform is unable to bind the thyroid hormone due to a different ligand binding domain and may have a dominant negative effect on the active isoforms, by engaging in inactive dimers with them. The role of this dominant negative effect is unknown. In addition, some non DNA-binding isoforms are also produced with no definitive role known either/without their definitive role being known either? Intensive drug design research has been realized to selectively target the beta receptors in order to improve metabolic parameters and, at the same time, avoid deleterious cardiac and bone effects. Inactivating mutations of beta receptor, and more recently of the alpha receptor, have proven to be the cause of thyroid hormone resistance syndromes, very rare conditions with different phenotypes depending on the mutated receptor.