Effects of Thyroid Hormones and Thyrotropin-Releasing Hormone on Thyrotropin Biosynthesis by Mouse Pituitary Tumor Cells in Vitro*

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Abstract
Mouse thyrotropic tumor cells grown in primary culture were shown to synthesize TSH and proteins, as determined by the incorporation of radioactive proline into immunoprecipitable TSH and trichloroacetic acid-precipitable proteins. The net TSH content of the cells and medium determined by RIA is also increased during 24 h of incubation, and newly formed hormone is detected in the medium within 1 h after the addition of proline tracer. To study the effect of T4 and T3 on TSH synthesis, cultures were pulse-labeled with [3H]proline after they had been exposed to either T3 or T4. After 48 but not 24 h, exposure to either T3 or T4 was followed by inhibition. When studied after 48 h of incubation, T4, (10-13 M) or T3 (10-11 M) at the lowest concentration tested, was inhibitory to TSH synthesis. At concentrations of T4 and T3 greater than 10-9 M, the inhibitory effects on TSH synthesis were partially reversed, suggesting a biphasic response. Incubation in TRH (10-7 M) for 24 h led to a significant increase in TSH synthesis, total protein, acid-precipitable protein, and total DNA. The effect of TRH on TSH biosynthesis was a function of the logarithm of its concentration over the range of 10-11-10-7 M. The inhibitory action of 10-6 M T3 on TSH synthesis was reversed by exposure to 10-l0 or 10-7 M TRH.