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ABSTRACT
The effects of histone subfractions on rat liver thyroid hormone receptor–DNA interaction were examined using an in-vitro DNA-cellulose binding assay. H1 histones bound to DNA showed reversible and potent inhibition of receptor–DNA binding without affecting receptor–hormone binding. Poly-lysine, bovine serum albumin, ovalbumin and cytochrome c did not alter receptor–DNA binding. H1 histone subfractions (calf thymus lysine-rich histone (CTL)-1, CTL-2 and CTL-3) showed potent inhibition of receptor–DNA binding indistinguishable from each other. The quantity of H1 histone subfractions bound to DNA was the same. Although each subfraction has different functional properties, inhibition of receptor–DNA binding was a common feature of all the H1 histone subfractions, which is important for the non-random distribution of the receptor in chromatin.
Binding of the receptor to core histones was investigated; it was found to bind to core histones more potently than to other proteins (H1 histone, ovalbumin and cytochrome c). Among core histone subfractions, H4 histone bound to the receptor most potently and is the candidate to be one of the acceptor sites of the receptor in chromatin.
Journal of Endocrinology (1989) 121, 337–341
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ABSTRACT
An aqueous two-phase partitioning study of partially purified nuclear thyroid hormone receptor from rat liver was performed. Stability of 3,5,3′-tri-iodo-l-thyronine (T3)–receptor complex and T3-binding activity in the presence of dextran or polyethylene glycol were assessed in order to determine the amount of occupied or unoccupied receptors in each phase. Partition coefficients were calculated as the ratio of receptor concentration in the upper polyethylene glycol-rich phase H2O and that in the lower dextranrich phase H2O. The partition coefficient was a sensitive function of the salt at pH above 6·1 and below 5·1. The salt had no effect on the partition coefficient at pH around 5·6. These results suggest that the isoelectric point of the thyroid hormone receptor is about 5·6, confirming previous determinations using isoelectric focusing. The partition coefficient of the receptor decreased upon T3 binding, regardless of the salt composition. In contrast, the partition coefficient of thyroxine-binding globulin increased upon T3 binding. Free T3 preferentially partitioned into the upper polyethylene glycol-rich phase and gave a partition coefficient higher than 1·0. These results strongly suggest that the decrease in the partition coefficient of the receptor upon hormone binding reflects conformational changes or changes in electrostatic properties of the receptor upon hormone binding. Such an alteration may be involved in biological activation of the receptor upon hormone binding.
J. Endocr. (1988) 119, 431–437
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ABSTRACT
Changes in the amount of cytosolic 3,5,3′-tri-iodo-l-thyronine (T3)-binding protein (CTBP) and its activator during administration of l-thyroxine (T4) to thyroidectomized rats were investigated. Thyroidectomy decreased the amount of CTBP in the kidney, whereas the activator was not significantly modified by thyroidectomy. The activator was increased by administration of T4 to thyroidectomized rats. The amount of CTBP was also increased by administration of T4. The activator increased the maximal binding capacity (MBC) without changes in the affinity constant for T3 binding in CTBP. A T4-induced increase in MBC in cytosol inhibited nuclear T3 binding in vitro by competition of T3 binding between CTBP and the nuclear receptor.
These results suggest that thyroid hormone increases the capacity for cytosolic T3 binding through increasing the amount of CTBP and its activator, and that these increases play a role in regulating the amount of T3 that binds to its nuclear receptor.
Journal of Endocrinology (1989) 123, 99–104
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Abstract
In order to study whether peripheral action of thyroid hormones is altered in insulin deficiency and to elucidate the biological consequences of alteration of the cytosolic 3,5,3′-tri-iodo-l-thyronine (T3) binding protein (CTBP), we measured malic enzyme, T3-responsive nuclear n protein, CTBP and nuclear thyroid hormone receptor in the liver and kidney of streptozotocin (STZ)-induced diabetic rats that were treated with or without insulin and/or a receptor-saturating dose of T3. The following results were obtained. 1. Induction of malic enzyme by T3 was apparently diminished in diabetic rats. However, supplementary injection of insulin enabled previously given T3 to take effect in diabetic rats. 2. T3-responsiveness of other hepatic proteins (n protein and CTBP) was not altered by insulin in diabetic rats. 3. The level of n protein was increased by insulin in diabetic rats in vivo and in perfused rat liver, indicating that the hepatic n protein is a novel insulin-responsive protein. T3 and insulin increased the level of n protein non-synergistically in diabetic rat liver. 4. Hepatic nuclear receptor levels were not altered in diabetic rats. 5. Hepatic CTBP levels were decreased in diabetic rats. This was not due to the toxic effect of STZ. Low CTBP level was only partially increased by insulin after 30 days of diabetic period. Renal CTBP levels were not altered in diabetic rats with or without insulin treatment. These results indicate that reduction of CTBP did not influence the hepatic response to a receptor-saturating dose of T3, although CTBP may regulate the nuclear T3 transport, and that fundamental action of a receptor-saturating dose of T3 was not attenuated in diabetic rat liver.
Journal of Endocrinology (1994) 143, 55–63
