Species Differences in Cardiac Thyroid Hormone Receptor Isoforms Protein Abundance

Little is known about the cardiac expression of different thyroid hormone receptor (TR) isoforms. The aim of the study was to investigate such patterns of TR expression at the protein level in different species and in some human tissues. Western blot analysis with specific polyclonal rabbit antibodies to each TR isoform was performed with samples from myocardium of the left ventricle from man, dog, guinea pig, rat and mouse, as well as with samples from several human tissues such as heart, skeletal muscle, brain, liver and thyroid. The TRα1 isoform was present in all of the species examined. The TRα2 was recognized in human, dog and guinea pig heart, while no such band was recognized in rat and mouse hearts. TRβ1 was not detected in the human heart but in the other species. Simularly to TRα1, TRβ2 was detected in all of the species examined. In the human tissues studied, TRα1 was detected in heart and skeletal muscle, whereas TRα2 was found only in the heart. TRβ1 was not detected in any of the examined human tissues, while TRβ2 was found in all of them. These results revealed unique distributions of TR variants and they demonstrate common epitopes in TR in the different species. For the first time, the presence of a TRβ2 isoform has been shown in human tissues. TR isoforms may have a tissue and species specific role in the regulation of gene expression and may in part explain variable tissue effects of thyroid hormones.