Thyroid Hormone Control of Thyrotropin Gene Expression in Rat Anterior Pituitary Cells*

Abstract

The relationship between thyroid hormone and TSH subunit mRNA levels and gene transcription in rat pituitaries was investigated. Synthesis of alpha-subunit and TSH-beta mRNAs were measured by incorporation of [32P]UTP into isolated nuclei, followed by hybridization of labeled transcripts to specific cDNAs. Steady state levels of mRNA were measured by Northern blot analysis of mRNA followed by hybridization with labeled TSH beta and alpha-subunit cDNAs. Hybridizing bands were quantitated by densitometry. TSH subunit mRNA synthesis was suppressed by T3 treatment in a manner directly related to T3 receptor occupancy; this effect occurred both in vivo and in cell culture. In the euthyroid rat, alpha-subunit mRNA had a higher level of synthesis [126 parts/million (ppm)] than did TSH beta mRNA (24 ppm). In the hypothyroid animal this situation was reversed, and the synthesis of both alpha-subunit and TSH beta mRNAs was increased to 230 and 331 ppm, respectively. T3 administration to hypothyroid animals suppressed mRNA synthesis maximally to 69 ppm for alpha-subunit and 10 ppm for TSH beta. The alpha-subunit to TSH beta mRNA synthesis ratio can thus vary from 0.7 in the hypothyroid animal to 6.9 in the hyperthyroid animal. The transcription rate for both genes decreased in linear fashion with increased T3 nuclear receptor occupancy. A 50% decrease in transcription occurred at a T3 dose (10(-9) M) similar to the Kd of the thyroid receptor. This agrees with our previous findings in thyrotropic tumors indicating a time- and dose-dependent suppression of the TSH subunit genes with increasing levels of nuclear receptor-bound T3. In primary rat pituitary cultures, T3 directly regulated steady state levels and synthesis of TSH subunit mRNA. With euthyroid pituitary cultures, alpha-subunit mRNA synthesis (50 ppm) was greater than that of TSH beta (12 ppm), but with hypothyroid pituitary cultures both mRNAs were synthesized at nearly equal rates (alpha-subunit, 120 ppm; TSH beta, 105 ppm). T3 (10(-8) M) treatment of hypothyroid cultures for 3 days decreased alpha-subunit (60% of control) and TSH beta (35% of control) mRNAs. Thus, mRNA synthesis is changed coordinately for both TSH subunits, but TSH beta is altered to a greater extent by thyroid status.