Insulin-like growth factor I action on rat anterior pituitary cells: effects of intracellular messengers on growth hormone secretion and messenger ribonucleic acid levels.
We have previously shown that insulin-like growth factor (IGF-I) suppresses basal and GHRH-induced GH gene transcription. cAMP is a putative intracellular mediator of GHRH action. We, therefore, studied the mechanism of IGF-I action on the somatotroph with or without cAMP activators. Primary rat pituitary cells growing in serum-free medium were treated with IGF-I. GH secretion was measured by RIA, and mRNA levels were measured by hybridization to [32P]GH cDNA. 8-Bromo-cAMP (8-Br-cAMP; 0.625 mM) stimulated GH mRNA levels after 72 h by 238%. IGF-I (6.5 nM) caused a 64% inhibition of 8-Br-cAMP-stimulated GH mRNA levels and a similar inhibition of GH secretion. This inhibition was time and dose dependent, with maximal (71%) suppression of cAMP-induced GH achieved with 13 nM IGF-I after 72 h. Forskolin (1 microM), a stimulator of adenylate cyclase, stimulated GH secretion (198%) which was inhibited by IGF-I by 42%. 12-O-Tetradecanoylphorbol 13-acetate, (a phorbol ester; 50 nM), a potent activator of protein kinase C, strongly stimulated GH secretion (347%), which was similarly suppressed by IGF-I by 51%. The suppressive action of IGF-I on somatotroph gene expression is unimpaired by direct activation of both cAMP and protein kinase C, suggesting that IGF-I acts upon the GH gene by a mechanism that is not altered by these second messengers. The negative feedback inhibition of physiological concentrations of IGF-I on GH, therefore, appears to override the potent stimulation of GH by these intracellular messengers.
Morita
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Department of Medicine, Cedars-Sinai Medical Center-University of California School of Medicine, Los Angeles 90048.
Yamashita
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Melmed
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D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D002478:Cells, Cultured; D013006:Growth Hormone / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D007700:Kinetics; D009693:Nucleic Acid Hybridization; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D012333:RNA, Messenger / Q000187:drug effects / Q000235:genetics*; D051381:Rats; D011919:Rats, Inbred Strains; D011994:Recombinant Proteins / Q000494:pharmacology*; D013002:Somatomedins / Q000494:pharmacology*
D012333:RNA, Messenger; D011994:Recombinant Proteins; D013002:Somatomedins; D015124:8-Bromo Cyclic Adenosine Monophosphate; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone