Deletion of the thyroid hormone beta 1 receptor increases basal and triiodothyronine-induced growth hormone messenger ribonucleic acid in GH(3) cells

The conserved diversity, restricted distribution, and differential regulation of the thyroid hormone receptor (TR) isoforms raise the possibility of isoform-specific functions. We have addressed the roles of individual TRs in GH gene expression in GH3 cells by using an isoform-specific antisense RNA to delete TR beta 1. An antisense RNA vector, directed against the isoform-specific coding sequence of the parent TR beta 1 complementary DNA, was constructed. Stable transfected GH(3)-derived cell lines expressing this construct were established. Appropriate control cell lines were established in parallel. Depletion of TR beta 1 in cells expressing the antisense construct was confirmed at both the messenger RNA and protein levels. Total TR expression was maintained in these cells by a reciprocal increase in TR beta 2 levels. This perturbation of the TR population was associated with a 10.5-fold increase in basal and a 5.0-fold increase in T-3-stimulated GH gene expression, but no increase in total T-3 binding of nuclear extracts. In transient cotransfection experiments, there were no differences between control cells and those expressing the antisense construct in either basal or T-3-stimulated expression of reporters containing a Variety of thyroid hormone response elements. Depletion of TR beta 1 in GH(3) cells results in a reciprocal increase in TR beta 2. These changes are associated with increased basal and T-3-stimulated GH gene expression, which are not due to a nonspecific enhancement of basal or hormone-stimulated transcription. We demonstrate that TR beta 1 is not required for T-3 induction of the GH gene in GH(3) cells and that TR beta 1 and TR beta 2 are not equivalent in their effects on basal repression of the GH promoter. The data illustrate the potential for isoform- and promoter-specific dissociation of the repression and activation properties of the TRs.