Regulation of thyrotropin biosynthesis. Discordant effect of thyroid hormone on alpha and beta subunit mRNA levels.

J A Gurr,I A Kourides

doi:10.1016/s0021-9258(17)44441-3

Abstract

We have studied the regulation of the biosynthesis of thyrotropin (TSH) and its alpha and beta subunits by thyroid hormone in thyrotropic tumors carried in hypothyroid mice. Treatment with 3,5,3'-triiodo-L-thyronine (T3) (20 micrograms/100 g, body weight) daily for 4 or 10 days reduced serum TSH to 3 and 0.3% of control, respectively. Serum levels of free alpha subunit were reduced to 60 and 11% of control at 4 days and 10 days, respectively, and serum free TSH-beta was undetectable at both time points. There was no significant decrease in tumor TSH content after 4 days of treatment and, after 10 days, TSH content was reduced to 15% of control levels. There was no significant effect of T3 on tumor alpha subunit levels at either 4 or 10 days. In contrast, tumor TSH-beta content was markedly reduced after 4 days and 10 days of T3 treatment, to 29 and 10% of control levels, respectively. Translation of tumor poly(A) mRNA in a rabbit reticulocyte lysate system showed that thyroid hormone decreased translatable TSH-beta mRNA to undetectable levels at both 4 and 10 days, whereas translatable alpha mRNA was reduced strikingly only at 10 days in one of two tumors. RNA blot hybridization with 32P-labeled plasmid probes containing alpha or TSH-beta cDNAs showed that TSH-beta mRNA was reduced to less than 10% of control after both 4 and 10 days of T3 treatment, whereas, again, alpha mRNA was only reduced in one of two tumors at 10 days. Our data thus show that thyroid hormone affects alpha and TSH-beta mRNA and protein levels discordantly and suggest that regulation of TSH biosynthesis may occur predominantly at the level of TSH-beta mRNA.

Highlights

We have studied the regulation of the biosynthesis of thyrotropin (TSH) and its a and j3 subunits by thyroid hormone in thyrotropic tumors carried in hypothyroid mice
There was no significant decrease in tumor TSH content after 4 days of treatment and, after 10 days, TSH content was reduced to 15%of control levels
T o determine themolecular basis for the discordant effect of thyroid hormones on TSH subunit biosynthesis, we have investigated the regulation of a and TSH-PmRNA levels by T3using mouse a and TSH-/3 cDNAs, synthesizedand cloned in this laboratory [15, 16], as specific probes

Summary

Effectof ThyroHidormoonne a and p Thyrotropin mRNAs

Poly(A)mRNA was translated in a cell-free reticulocyte lysate system [19]containing [35S]methionine(1000Ci/mmol, New England. Peptides were separated by electrophoresis on 12-20% polyacrylamide slab gels containing 0.1% sodium dodecyl sulfate [20]. After treatment for fluorography (2l), thedried gels were exposed at -70 "C to Kodak XAR-2 x-ray film. Blot Hybridization of 01 and TSH-8 mRNA-For RNA gel blot hybridization, poly(A) mRNA was denatured with glyoxal [22], fractionated by electrophoresis on 2% agarose gels in 10 mM sodium phosphate, pH 7.0, [22] and transferred tnoitrocellulose, as described by Thomas [23]. Quantitation-The intensity of bandsonautoradiograms was quantitated by scanning ina densitometer with a peak area integrator (Quick Scan Jr., Helena Laboratories)

RESULTS

We have shownthat treatmentof mice bearing thyrotropic

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DISCUSSION