Growth hormone rapidly activates insulin-like growth factor I gene transcription in vivo.
Many of the growth-promoting properties of GH are mediated by insulin-like growth factor I (IGF-I), a highly conserved circulating 70-amino acid peptide. Recent studies have shown that multiple mechanisms influence IGF-I gene expression, including transcription from two promoters, alternative RNA splicing, and variable polyadenylation. In order to determine how GH regulates IGF-I gene expression we have analyzed the response of hypophysectomized rats to a single ip injection of recombinant GH. A rise in hepatic IGF-I mRNA was detected within 2 h of GH treatment, with peak values of more than 15-fold above untreated animals by 4 h, and a decline by 16 h. A coordinate increase was seen in all IGF-I mRNA splicing and polyadenylation variants, indicating that neither alternative RNA processing nor differential poly A addition were altered by GH. Transcription run-on experiments using isolated hepatic nuclei and direct analysis of nuclear RNA demonstrated a rise in nascent IGF-I mRNA within 30 min of GH treatment, with peak levels reaching more than 10-fold above background by 2 h and declining by 6 h. As determined by RNase protection assays, transcripts directed by each promoter were coordinately and equivalently activated after GH. A single GH-responsive DNase I hypersensitive site was mapped in chromatin to the second IGF-I intron. This site exhibited rapid kinetics of induction which mirrored the pattern of transcriptional stimulation after GH treatment. These experiments show that GH enhances IGF-I expression in vivo by predominantly transcriptional mechanisms. The rapid kinetics of IGF-I gene activation and the temporally associated chromatin changes demonstrate a direct link between a GH-dependent signal transduction pathway and nuclear events.
Bichell
D P
DP
Department of Medicine, Washington University School of Medicine, Saint Louis, Missouri 63110.
Kikuchi
K
K
Rotwein
P
P
D000818:Animals; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects; D013006:Growth Hormone / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis / Q000235:genetics*; D008099:Liver / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D013268:Stimulation, Chemical; D014158:Transcription, Genetic / Q000187:drug effects*; D015533:Transcriptional Activation
D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone