Beta-1,4-galactosyltransferase and alpha-mannosidase-II messenger ribonucleic acid levels increase with different kinetics in thyrotrophs of hypothyroid mice

Abstract

Hypothyroid animals secrete not only more TSH, but also qualitatively different TSH, with altered oligosaccharides. To explore the cellular mechanism of oligosaccharide modulation by thyroid status, 40 mice were treated without or with propylthiouracil (PTU), and pituitaries were removed after 1, 2, 3, 4, and 6 weeks. Serum T4 levels confirmed that mice receiving PTU were hypothyroid even after only 1 week. Thyrotrophs and corticotrophs were identified in 5-microns thick pituitary slices using immunocytochemistry; in situ hybridization was performed using 35S-labeled 48-mer DNA probes to beta-1,4-galactosyltransferase and alpha-mannosidase-II messenger RNAs (mRNAs). A control probe also was used. Autoradiography was performed for 4 weeks. The cells and silver grains were scored, and the differences reported below were significant by analysis of variance. Compared to euthyroid thyrotrophs, the beta-1,4-galactosyltransferase mRNA level in thyrotrophs increased 440% after mice received PTU for 1 week; the mean increase within thyrotrophs over the 6-week period was 173%, whereas there was little change in corticotrophs. Compared to euthyroid thyrotrophs, the alpha-mannosidase-II mRNA level in thyrotrophs remained unchanged while mice received PTU for 4 weeks, but then increased 150% after week 6 of PTU treatment. Thus, thyroid status modulates mRNA levels of two glycosyltransferases in thyrotrophs, perhaps by affecting gene transcription or mRNA stability. Moreover, the kinetics of the modulation of the two glycosyltransferase mRNAs differ. This is the first report of the modulation of these glycosyltransferase mRNA levels in thyrotrophs and may explain in part the mechanism by which different isoforms of TSH are secreted in different physiological states.