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J. Biol. Chem., Vol. 261, Issue 27, 12555-12561, Sep, 1986
DW Back, SB Wilson, SM Morris Jr and AG Goodridge
Mechanisms involved in stimulation of the synthesis of malic enzyme by
insulin and triiodothyronine and in inhibition of synthesis by glucagon
have been investigated by assessing levels and rates of synthesis of malic
enzyme mRNA in chick embryo hepatocytes in culture. Insulin alone had no
effect on the level of malic enzyme mRNA, whereas triiodothyronine by
itself caused a 7-fold increase. Insulin plus triiodothyronine caused an
11-fold increase. Glucagon caused a 93% decrease in the accumulation of
malic enzyme mRNA caused by insulin plus triiodothyronine. Although the
relative changes in mRNA level are smaller in magnitude, they are
qualitatively similar to the effects of these hormones on synthesis of
malic enzyme, suggesting that control is exerted primarily at
pretranslational steps. After addition of triiodothyronine, malic enzyme
mRNA accumulated with sigmoidal kinetics, approaching a new steady state at
36-48 h after adding hormone. Puromycin, an inhibitor of protein synthesis,
blocked the effect of triiodothyronine if added 30 min prior to the hormone
and inhibited further accumulation of malic enzyme mRNA if added 24 h after
triiodothyronine. However, puromycin had no effect on the level of beta-
tubulin mRNA (t1/2 = 3-5 h), suggesting that the effect of triiodothyronine
on malic enzyme mRNA required synthesis of a peptide. Triiodothyronine
increased transcription of the malic enzyme gene by 2- fold and level of
its mRNA by 11-14-fold, indicating regulation is primarily at a
post-transcriptional step. Glucagon caused malic enzyme mRNA to decay with
a half-life of 1.5 h, whereas alpha-amanitin or actinomycin D, inhibitors
of transcription, caused the mRNA to decay with a half-life of 8-11 h. The
effect of glucagon was entirely post- transcriptional because the hormone
had no effect on transcription. Taken together, these results suggest a
model in which triiodothyronine regulates production of a peptide that
stabilizes malic enzyme transcripts in the cytoplasm and/or nucleus.
Glucagon may inhibit activity of the peptide induced by triiodothyronine.
Hormonal regulation of lipogenic enzymes in chick embryo hepatocytes in culture. Thyroid hormone and glucagon regulate malic enzyme mRNA level at post-transcriptional steps
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