Regulation of thyrotropin receptor gene expression in rat FRTL-5 thyroid cells.
TSH receptor mRNA levels in FRTL-5 thyroid cells are autoregulated at a transcriptional level by the same hormones required for the growth and function of the cells: TSH, insulin, and insulin-like growth factor-I (IGF-I). Thus, the ability of TSH, via its cAMP signal, to down-regulate steady state receptor mRNA levels is preceded by the action of TSH to decrease pre-mRNA levels in nuclear run-on assays to the same quantitative level as evident in Northern analyses. In contrast, the receptor mRNA half-life is shown not to change when down-regulation is reversed by withdrawing TSH in the presence or absence of actinomycin-D. Evidence is additionally provided that TSH receptor mRNA levels are increased by insulin, IGF-I, or calf serum in both Northern and run-on assays. This action cannot be duplicated by hydrocortisone and is evident at more than 20-fold lower concentrations of IGF-I than insulin. Moreover, insulin, IGF-I, and/or calf serum are required for the autoregulatory negative transcriptional regulation of the TSH receptor by TSH/cAMP, as is the case for thyroglobulin. This occurs despite the opposite actions of TSH/cAMP on the two genes, positive in the case of thyroglobulin and negative with TSH receptor. The positive and negative regulatory actions, respectively, of insulin/IGF-I and TSH on receptor gene expression are associated with coincident increases or decreases in cell surface receptors measured by [125I]TSH binding. The autoregulation additionally involves the interplay of a second cAMP-modulated regulatory factor, one which up-regulates TSH receptor mRNA levels rather than causing down-regulation. Thus, cycloheximide inhibits the transcriptional action of both TSH/cAMP and insulin/IGF-I/serum within 4 h, i.e. a rapidly synthesized protein is an intermediate in both cases. The presence of cycloheximide for as little as 1 h, however, uncovers the ability of TSH/cAMP to increase TSH receptor mRNA levels. This activity is the result of the action of a stable cAMP-induced activator which can be detected physiologically, i.e. in the absence of cycloheximide. For example, low levels of a cAMP analog (0.2 mM), as opposed to high levels (greater than 1 mM), can increase TSH receptor RNA levels. Low levels also accelerate the insulin/IGF-I-dependent return of receptor mRNA to normal levels after TSH withdrawal.(ABSTRACT TRUNCATED AT 400 WORDS)
Saji
M
M
Section on Cell Regulation, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Akamizu
T
T
Sanchez
M
M
Obici
S
S
Avvedimento
E
E
Gottesman
M E
ME
Kohn
L D
LD
D000818:Animals; D001790:Blood Physiological Phenomena; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000494:pharmacology; D003513:Cycloheximide / Q000494:pharmacology; D015536:Down-Regulation; D005786:Gene Expression Regulation; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D011989:Receptors, Thyrotropin / Q000235:genetics*; D013961:Thyroid Gland / Q000378:metabolism*; D013972:Thyrotropin / Q000378:metabolism / Q000494:pharmacology; D014158:Transcription, Genetic; D015854:Up-Regulation
D007328:Insulin; D012333:RNA, Messenger; D011989:Receptors, Thyrotropin; D007334:Insulin-Like Growth Factor I; D013972:Thyrotropin; D003513:Cycloheximide; D000242:Cyclic AMP