Triiodothyronine binding to isolated liver cell nuclei

The mechanisms by which thyroid hormones interact with liver nuclei remain incompletely understood. Prior research has shown that thyroid hormones influence gene expression through nuclear binding. However, the specific conditions under which triiodothyronine (T3) binds to liver nuclei are not fully characterized. This uncertainty drove the need for a more detailed investigation into the binding behavior of T3 in isolated nuclei. Existing studies have explored hormone-receptor interactions in other tissues, but liver-specific nuclear binding remains understudied. The role of nuclear integrity in T3 accumulation is not well defined. Environmental factors such as temperature and pH may affect binding dynamics. The influence of various ions and reagents on nuclear function is also unclear. This gap motivated the current study to examine T3 binding under controlled in vitro conditions.

This study aimed to investigate the binding of triiodothyronine to isolated nuclei from rat liver. The researchers sought to determine the conditions under which T3 accumulates in these nuclei. They wanted to assess the specificity of T3 binding and the factors influencing it. Understanding whether cytosolic proteins are required for binding was a key goal. The study also aimed to identify the nature of the binding complex formed. Researchers were interested in the effects of various reagents on T3 accumulation. They wanted to compare the binding affinities of different thyroid hormone analogues. The ultimate goal was to clarify the mechanisms of T3 interaction with liver nuclei.

The researchers isolated nuclei from euthyroid rat liver using sucrose sedimentation. They used a 2.3M sucrose solution with or without a Triton wash. Incubation conditions included 0.32M sucrose, 1mM MgCl2, and 0.02M Tris-Cl buffer at pH 7.4 or 7.85. Specific T3 binding was measured at concentrations from 10 to 1,000 pM. They tested the effects of calcium, EDTA, and phosphate ions on nuclear integrity and T3 accumulation. The influence of dithiothreitol (DTT) and anion exchange resin was also analyzed. Researchers used RNAse, DNAse, and pronase to assess the role of nucleic acids. They extracted the NTBP-T3 complex using 0.4M KCl and measured binding affinity and capacity.

T3 binding to isolated nuclei was observed at concentrations from 10 to 1,000 pM. Specific binding sites were saturated by excess unlabeled T3 at 0.15 muM. Binding was maximal after 203 hours at 20°C and proportional to the amount of nuclei. Calcium ions enhanced nuclear integrity but reduced T3 accumulation. EDTA and phosphate ions damaged nuclei but increased T3 accumulation. Binding was not affected by energy-dependent reactions or RNA synthesis. DTT significantly increased T3 binding under certain conditions. Pronase obliterated binding, but RNAse and DNAse had no effect. The NTBP-T3 complex formed in vitro resembled that formed in vivo. T3 in the complex resisted accumulation by anion exchange resin at 0-2°C but bound after 20 minutes at 37°C. The apparent Ka was 0.2 times 10^-10 M^-1 at pH 7.85 with 5 mM DTT. The binding capacity was 508 pg T3/g wet tissue or 53 times 10^-15 moles T3/100 mug DNA.

The study suggests that T3 binding to liver nuclei is influenced by several factors. Specific binding sites are saturated by excess unlabeled T3, indicating high affinity. Calcium ions support nuclear integrity but limit T3 accumulation. EDTA and phosphate ions damage nuclei but enhance T3 binding. DTT increases binding under certain conditions. The NTBP-T3 complex formed in vitro mirrors in vivo conditions. Pronase disrupts binding, but nucleases do not. The binding capacity measured may not reflect total capacity. It may instead represent the amount of T3 dissociated during incubation. The data suggest that binding is not mediated by cytosolic proteins. The study highlights the role of environmental factors in T3-nucleus interactions. The results may inform future research on thyroid hormone action in liver cells.