Stability of nuclear triiodothyronine binding sites

Area of Science:

• Endocrinology
• Molecular Biology
• Cell Biology

Background:

• Rat liver nuclei possess limited, high-affinity binding sites for triiodothyronine (T3).
• Only a fraction of these nuclear T3 binding sites are occupied in vivo.
• Understanding the dynamics and regulation of these binding sites is crucial for thyroid hormone action.

Purpose of the Study:

• To investigate the stability and turnover of triiodothyronine nuclear binding sites in rat liver.
• To determine if unoccupied binding sites are lost during incubation.
• To assess the impact of protein and RNA synthesis inhibition on T3 nuclear binding.

Main Methods:

• Incubation of isolated rat liver nuclei at different temperatures (4°C and 37°C) in the absence of triiodothyronine.
• Measurement of triiodothyronine binding capacity and dissociation rates.
• Administration of cycloheximide and actinomycin D to rats prior to nuclear isolation and in vitro binding assays.

Main Results:

• Preincubation of nuclei without triiodothyronine led to a rapid loss of binding capacity, faster at 37°C than 4°C.
• Once binding equilibrium was achieved, the level of bound triiodothyronine remained stable.
• The rate of triiodothyronine dissociation from nuclei was slower than the rate of binding site loss during preincubation.
• Administration of cycloheximide or actinomycin D decreased the in vitro binding of triiodothyronine to liver nuclei.

Conclusions:

• Unoccupied triiodothyronine nuclear binding sites appear to be unstable and are lost during preincubation.
• The observed loss of binding sites is distinct from the dissociation of already bound hormone.
• Protein and RNA synthesis are involved in maintaining the pool of functional triiodothyronine nuclear binding sites.