Regulation by thyroxine of the mRNA encoding the alpha subunit of mouse thyrotropin

Thyrotropin (TSH), a glycoprotein hormone of the pituitary consisting of two subunits (alpha and beta), regulates thyroxine (T4) production by the thyroid gland. T4, in turn, regulates TSH biosynthesis and release. We have studied the regulation of the messenger RNA encoding the alpha subunit of TSH by T4 in pituitaries and in a transplantable thyrotropic tumor in mice. Hypothyroid male LAF1 mice bearing the TtT 97 thyrotropic tumor were injected daily with T4 for either 0, 1, 5, 12, or 33 days. Levels of TSH and its unassociated alpha (free alpha) and TSH-beta subunits in the plasma of these animals fell to less than 5% of control values after 33 days. Concentrations of TSH and TSH-beta in both tumor and pituitary also fell to low levels (less than 2% of control), while intracellular concentrations of free alpha subunit remained unchanged. Cellular levels of the mRNA encoding the precursor of the alpha subunit or pre-alpha (alpha mRNA) were measured by cell-free translation followed by electrophoretic analysis of immunoprecipitates of pre-alpha subunit and by nucleic acid hybridization to a radiolabeled cDNA probe specific for the alpha mRNA. In the pituitary, translatable and hybridizable alpha mRNA was decreased slightly after 1 day of T4 and decreased 40-50% after 5 and 12 days. In thyrotropic tumors, both translatable and total alpha mRNA showed a 60% decrease by 1 day and a maximum 85% decrease after 5, 12, and 33 days of T4. Therefore, T4 acts rapidly in vivo to decrease steady state alpha mRNA levels in the thyrotrope, and this decrease is maintained for the duration of treatment with thyroid hormone. This regulatory process is reflected in the sharp decreases in levels of TSH and free alpha subunit in plasma and in lower concentrations of the intact TSH in tissue. In contrast, the maintenance of high tissue concentrations of free alpha subunit after T4 treatment may be a reflection of alterations in a post-translational process specific for the free alpha subunit, as opposed to that of the intact TSH.