Triiodothyronine Rapidly Decreases Transcription of the Thyrotropin Subunit Genes in Thyrotropic Tumor Explants*

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Abstract
We have investigated the direct effect of the thyroid hormone T3 on the TSH subunit genes in tissue explants. Minces of TtT 97 thyrotropic tumor were treated with 5 nM T3 for varying periods of time. Nuclei were then isolated from the tumor cells and allowed to continue RNA synthesis in the presence of [.alpha.-32P]UTP. Newly synthesized RNA sequences were quantified by hybridization to immobilized cloned cDNAs containing sequences specific for either TSH.beta. or .alpha.-subunit mRNA. Basal TSH.beta. and .alpha.-subunit mRNA synthesis rates were both approximately 300-400 parts/million, the same as in vivo values. After 15 min of T3 treatment, TSH.beta. mRNA synthesis was significantly decreased by 42% and was maximally decreased by 95% after 1 or more hours of T3. Synthesis of .alpha.-subunit mRNA was decreased by 38% after 30 min of T3 treatment and by 78% after 1 h or more of T3. The suppressive effects of T3 on transcription correlated with the time course of T3 binding to its nuclear receptors. These changes are quantitatively similar to those observed after in vivo T3 treatment. Decreases in mRNA synthesis preceded significant decreases in tissue steady state mRNA levels or subunit protein levels. The presence of the protein synthesis inhibitor cycloheximide (25 .mu.g/ml) during a 4-h incubation with T3 did not change the T3-mediated decreases in TSH.beta. or .alpha.-subunit mRNA synthesis or the decreases in cellular mRNA levels. Therefore T3 can act directly on the thyrotope to suppress TSH.beta. and .alpha.-subunit mRNA synthesis and protein synthesis is not necessary for the T3-mediated decreases in gene transcription. The data suggest that T3 may act directly at the level of the TSH subunit genes to modulate their expression.