Hypothyroid patients and mice have been shown to have circulating TSH
that is more highly sialylated than their euthyroid counterparts. To l
earn about the underlying cellular mechanisms responsible for this inc
reased sialylation of TSH, we used in situ hybridization to examine th
e beta-galactoside alpha-2,6-sialyltransferase (STase) mRNA content in
thyrotrophs and corticotrophs of euthyroid and hypothyroid mice. Mice
were treated with or without 0.05% propylthiouracil for 1, 2, 3, 4, o
r 6 weeks, then pituitaries were removed, and 5-mu m slices were immun
ocytochemically stained for TSH and ACTH. Adjacent sections were used
for in situ hybridization. A 48-mer deoxynucleotide probe to rat STase
and two control probes were labeled with S-35, and autoradiography wa
s performed. There was an approximately 140% increase in STase mRNA in
hypothyroid thyrotrophs compared to euthyroid thyrotrophs by the firs
t week, with a mean increase of 170% in weeks 1-6, whereas corticotrop
hs exhibited no change in STase mRNA. The increase in hybridization of
the STase probe in hypothyroid thyrotrophs may be due to an increased
transcription of the STase gene, stabilization of the STase mRNA, or
both. Thus, modulation of the STase mRNA levels occurs in thyrotrophs
and represents one important mechanism by which the oligosaccharides o
f TSH are altered under different physiological conditions.