Transcriptional regulation of thyrotropin subunit genes by thyrotropin-releasing hormone and dopamine in pituitary cell culture.

Thyrotropin (TSH) is an anterior pituitary glycoprotein hormone which modulates thyroid hormone production by the thyroid gland. TSH and its two subunits, TSH beta and alpha-subunit, are in turn regulated by the thyroid hormones and several stimulatory and inhibitory factors produced by the hypothalamus. Hypothalamic hormones such as thyrotropin-releasing hormone (TRH) and dopamine have profound effects on TSH secretion and may also modulate TSH synthesis. We investigated the ability of these two hormones to directly influence the expression of the TSH subunit genes by measuring subunit mRNA synthesis rates in treated and untreated pituitary cell cultures from hypothyroid rats. In this system, basal rates of mRNA synthesis were similar for the two subunits, with 95 ppm for alpha-subunit and 85 ppm for TSH beta. Treatment with 10(-5) M dopamine, an inhibitor of TSH secretion, decreased both alpha-subunit and TSH beta mRNA synthesis within 15 min, and maximal decreases of approximately 75% for both subunits were obtained after 30 min. The inhibitory effects of dopamine were overcome by co-incubation of the cells with dopamine plus 5 X 10(-4) M dibutyryl cyclic AMP (cAMP), indicating that the actions of dopamine may have been mediated by decreasing intracellular cAMP levels. Treatment with 10(-7) M TRH, a potent stimulator of TSH secretion, increased transcription of both TSH subunit genes 3- to 5-fold. Maximal stimulation occurred after 30 min of treatment; however, 2-fold increases were maintained for up to 4 h and at 48 h. Cytoplasmic levels of TSH beta and alpha-subunit mRNA after 48 h were also depressed by dopamine (50-70%) and increased by TRH (200-280%) treatment. TRH-stimulated TSH beta and alpha-subunit mRNA synthesis was partially inhibited by dopamine. In a 4-h treatment period, TSH beta mRNA synthesis was decreased from 250 to 73% of control and alpha-subunit was decreased from 333 to 182% of control values. Thus, both dopamine and TRH can act to modulate TSH subunit gene transcription, although they differ in the direction of the response and the relative effects on TSH beta and alpha-subunit. The magnitude of the transcriptional response for either TRH (200 to 500%) or dopamine (75%) is similar to the observed changes in cytoplasmic levels of TSH subunit mRNAs and indicates that transcriptional control of the TSH genes may be an important component of hypothalamic regulation of TSH.