Pituitary cells respond to thyroid hormone by discrete, gene-specific pathways

Area of Science:

• Biochemistry
• Molecular Biology
• Endocrinology

Background:

• Type 1 iodothyronine deiodinase (D1) is crucial for converting thyroxine (T4) to the active thyroid hormone, triiodothyronine (T3).
• Previous research indicated T3 regulates Type 1 deiodinase (dio1) gene expression independently of new protein synthesis in liver and thyroid cells.
• Certain T3-regulated genes, like rat growth hormone (GH), show T3-induced transcription that is inhibited by protein synthesis inhibitors.

Purpose of the Study:

• To investigate whether the cycloheximide blockade of T3 effects on gene expression is cell-type or gene-specific.
• To compare the regulation of two positively T3-regulated genes, dio1 and GH, within the same cell type (somatotrope tumor cell lines).

Main Methods:

• Utilized somatotrope tumor cell lines expressing both dio1 and GH.
• Compared the effects of T3, cycloheximide, retinoic acid, and dexamethasone on dio1 and GH mRNA synthesis.

Main Results:

• T3 stimulation of dio1 mRNA synthesis was not blocked by cycloheximide.
• T3's effect on GH mRNA synthesis was eliminated by cycloheximide.
• Retinoic acid modulated GH expression and synergized with T3, but did not affect dio1.
• Dexamethasone enhanced T3's effect on both dio1 and GH mRNA levels.

Conclusions:

• The T3 induction of mRNA synthesis differs between genes within the same cell.
• Distinct molecular pathways mediate the regulation of dio1 and GH gene expression by T3.