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Laboratory for Cancer Medicine and University Department of Medicine, Royal Perth Hospital, Perth, Western Australia 6001, Australia
Address all correspondence and requests for reprints to: Peter J. Leedman, M.D., Ph.D., University Department of Medicine, Royal Perth Hospital, Box X2213 GPO, Perth, Western Australia 6001, Australia. E-mail: peterl{at}cyllene.uwa.edu.au
Thyroid hormone (T3) negatively regulates TSH ß-subunit
(TSHß) messenger RNA (mRNA) gene expression in whole rat pituitary,
in part at the level of mRNA stability. However, the regulation of
TSHß mRNA turnover by T3 in pure populations of
thyrotropes and in other species is unknown. To further investigate
this, we used murine thyrotropic TtT97 tumor cells. Using primary
cultures of TtT97 cells, T3 down-regulated TSHß mRNA to
35% of the control level by 8 h. Actinomycin D chase revealed
that T3 destabilized TSHß mRNA, reducing the half-life
from 24 to 7 h, and was accompanied by a decrease in TSHß
mRNA size. Ribonuclease H analysis revealed that this
T3-induced decrease in size was due to a shortening of
poly(A) tail from 160 to 30 nucleotides and was specific for
TSHß mRNA. Cycloheximide mimicked the poly(A) tail effect observed
with T3. In the absence of T3, actinomycin D
deadenylated TSHß mRNA without inducing rapid decay. We conclude that
T3 reduces the steady state half-life of TSHß mRNA in
murine TtT97 thyrotropic tumor cells accompanied by a reduction in
poly(A) tail length. However, in the absence of T3,
deadenylation alone is not sufficient to induce TSHß mRNA decay.
Together with the high degree of sequence conservation in the
3'-untranslated region of murine and rat TSHß mRNA sequences and the
similarities of the T3 effect, these data provide the first
evidence for a highly conserved posttranscriptional mechanism operative
across species. We propose a model in which T3 coordinately
regulates shortening of the poly(A) tail and the activity of a
trans-acting RNA-binding protein and/or an exonuclease
to accelerate TSHß mRNA turnover.
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