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Triiodothyronine regulation of multiple rat hepatic genes: requirement for ongoing protein synthesis
P S Hamblin1, A Santos, N C Wong
1Department of Medicine, University of Minnesota, Minneapolis 55455.
Abstract:
We have examined the role of rapidly turning over proteins in the T3 regulation of multiple rat hepatic genes. T3 induction of the rapidly responsive mRNA-S14 was markedly inhibited by cycloheximide (1 mg/100 g BW) or emetine (3 mg/100 g) injected ip 30 min before T3 (mRNA-S14 concentration was only 35% of that in T3-treated controls 8.5 h after administration of either protein synthesis inhibitor, P less than 0.01). Cycloheximide exhibited a similar effect on each of five other more slowly responsive T3 regulated genes. When cycloheximide was given 10 h after T3, the expected T3-induced rise of mRNA-S7 activity was completely prevented, and for mRNA-S4 activity the anticipated rise was blunted to 40% of T3-treated control (P less than 0.05). Cycloheximide caused sharp declines in the activity of two other mRNAs, S6 and S8, which because of shorter lag times of response to T3, had already risen when the drug was given. Values for both these mRNAs returned to the baseline hypothyroid level within 6 h of injection of the drug and remained low for a further 8 h (P less than 0.05). The expected deinduction of mRNA-S10 by T3 was also markedly modified. T3 lowered this mRNA to 11% of the hypothyroid control after 8 h, whereas cycloheximide given 30 min before the hormone blunted this fall to only 72% of control (P less than 0.01). Thus there appeared to be a 70% reduction in the rate of T3 induced fall of mRNA-S10. We did not find that cycloheximide caused a generalized decrease in poly (A)+ RNA mass.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Rapidly turning over proteins are crucial for thyroid hormone (T3) regulation of rat liver gene expression. Inhibiting protein synthesis with cycloheximide or emetine significantly alters T3
Area of Science:
- Biochemistry
- Molecular Biology
- Endocrinology
Background:
- Thyroid hormone (T3) regulates gene expression in the liver.
- The role of rapidly turning over proteins in T3-mediated gene regulation is not fully understood.
Purpose of the Study:
- To investigate the involvement of rapidly turning over proteins in T3 regulation of hepatic gene expression.
- To determine the impact of protein synthesis inhibition on T3-induced mRNA changes.
Main Methods:
- Administration of cycloheximide or emetine (protein synthesis inhibitors) before or after T3 treatment in rats.
- Quantification of specific mRNA levels (mRNA-S14, S7, S4, S6, S8, S10) using techniques not specified in the abstract.
- Assessment of poly(A)+ RNA mass.
Main Results:
- Inhibition of protein synthesis before T3 markedly reduced T3 induction of mRNA-S14.
- Cycloheximide similarly affected five other T3-regulated genes.
- Delayed cycloheximide administration prevented or blunted T3-induced increases in mRNA-S7 and S4.
- Cycloheximide caused rapid declines in mRNA-S6 and S8, returning them to baseline.
- The expected T3-induced fall in mRNA-S10 was significantly blunted by cycloheximide.
- No generalized decrease in poly(A)+ RNA mass was observed.
Conclusions:
- Rapidly turning over proteins play a critical role in both the induction and repression of T3-regulated hepatic genes.
- Protein synthesis is essential for the proper temporal and quantitative response of these genes to thyroid hormone.