Thyroid hormone regulation of thyrotropin alpha- and beta-subunit gene transcription

We studied the effect of thyroid hormone on the transcription of the genes for the alpha- and beta-subunits of thyrotropin (TSH) in thyrotropic tumors (IAK 109D and 109F) carried in hypothyroid mice. Gene transcription was measured in isolated nuclei by allowing completion of RNA chains initiated in vivo in the presence of [alpha-32P]UTP and by hybridization of labeled RNA transcripts to filter-bound plasmids containing alpha or TSH-beta cDNA sequences. Treatment of animals carrying tumor IAK 109D with 3,5,3'-triiodo-L-thyronine (T3) (5 micrograms/100 g body weight) for 2 hr reduced TSH-beta gene transcription to less than 10% of control levels, whereas alpha RNA synthesis was reduced to 59% of control. The inhibition of TSH-beta gene activity was maintained after 6 hr of T3 treatment, whereas alpha gene transcription rose slightly to 77% of control. The tumor content of alpha and TSH-beta mRNA, determined by dot blot hybridization with 32P-labeled plasmid probes containing alpha or TSH-beta cDNAs, was unchanged after 2 hr of T3 treatment, and each was reduced by approximately 25% at 6 hr. These untreated tumors contained approximately equal amounts of alpha and TSH-beta mRNA. However, the basal rate of TSH-beta gene transcription was threefold greater than that of alpha gene transcription. Treatment of animals bearing tumor IAK 109F with the same dose of T3 for 30 min did not significantly affect alpha or TSH-beta gene transcription, but at 2 hr alpha and TSH-beta RNA synthesis had decreased to 50% and 10% of control values, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)