Nuclear receptors for thyroid hormone: evidence for nonrandom distribution within chromatin.

Abstract

Chromatin receptor proteins appear to mediate some actions of thyroid hormone. In this study, sheared mammalian chromatin containing [125Itriiodo-thyronine (T3) bound by these receptors was separated using sucrose gradient velocity sedimentation. T3-receptor complexes were distributed throughout the chromatin fractions, but were enriched in the slowly sedimenting fractions. The latter contain most of the template capacity for RNA synthesis and most of the endogenous RNA polymerase activity but a minor portion of the total DNA. Formaldehyde treatment of chromatin containing receptor-bound [1261IT3 resulted in fixation of radioactivity, as evidenced by its migration with chromatin after equilibrium density gradient sedimentation in both cesium chloride and Conray. This fixation implies that the T3 receptor protein is closely associated with chromatin. These results suggest that proteins involved in the regulation of gene function may be nonrandomly distributed within chromatin subfractions, and are consistent with a direct role for thyroid hormone in regulating genetic expression.Thyroid hormones play a key role in vertebrate development and maintenance (1-3). Recently, high-affinity, limitedcapacity binding sites for triiodothyronine (T3), and thyroxine (T4), which appear to be receptors for these hormones, have been found in target cell nuclei (4-15). The concentrations of T3 or T4 required for binding to these receptors and for bioreactivity are similar (6), and there is a good correlation between the ability of thyroid hormone analogs to inhibit T3-receptor binding and to promote the biological effect (15). Studies of thyroid hormone analogs also suggest that T3 and T4 function through physical interactions, such as with receptors, rather than through chemical reactions involving iodine or quinones (16).The thyroid hormone receptors appear to be salt-extractable, acidic chromatin proteins (5,8,10,11). The possibility that thyroid hormones act at the chromatin level is also consistent with findings that these hormones influence RNA synthesis (3,17,18). Thus, the receptors may represent an identifiable acidic chromatin protein which possibly regulates the expression of specific genes. In the present studies, the T3 receptor association with chromatin and distribution within sheared chromatin subfractions (19,20)

Regulation of growth hormone messenger RNA by thyroid and glucocorticoid hormones.

“…Because thyroid hormone receptors and glucocorticoidreceptor complexes are found in the chromatin (3,35,36), it is possible that the changes in mRNA levels are due to hormonal regulation of transcription, as appears to be the case with sex steroids (37,38). However, it is not known, particularly for thyroid hormones, whether control of mRNA is generally responsible for other actions of the hormones.…”

Thyroid hormone receptors bind to defined regions of the growth hormone and placental lactogen genes.

“…The presence of thyroid hormone was required for binding of its receptor to the growth hormone gene promoter, suggesting that thyroid hormone renders the receptor capable of recognizing specific gene regions. L-3,5,3'-Triiodothyronine (T3), the active form of thyroid hormone, modulates the expression of a number of genes (1,2), and this is ascribed to the association of T3 with receptors localized in chromatin (3). In rat pituitary tumor cells the concentration of growth hormone (GH) mRNA increases in response to physiological concentrations ofT3 (refs.…”

Intranuclear Incorporation and Binding to Chromosomal DNA of Epinephrine

“…H ) It was reported that specific receptor of thyroid hormones was also present nonhistone protein in chromatin. 15 ) These hormones were assumed to function by interaction with non-histone protein which regulated the gene expression.…”