Distinct alterations in chromatin organization of the two IGF-I promoters precede growth hormone-induced activation of IGF-I gene transcription.
Many of the physiological actions of GH are mediated by IGF-I, a secreted 70-residue peptide whose gene expression is induced by GH in the liver and other tissues via mechanisms that remain incompletely characterized but depend on the transcription factor Stat5b. Here we investigate the chromatin landscape of the IGF-I gene in the liver of pituitary-deficient young adult male rats and assess the impact of a single systemic GH injection. Despite minimal ongoing transcription in the absence of GH, both IGF-I promoters appear to reside in open chromatin environments, at least as inferred from relatively high levels of acetylation of core histones H3 and H4 when compared with adjacent intergenic DNA and from enhanced trimethylation of histone H3 at lysine 4. This landscape of open chromatin may reflect maturation of the liver. Surprisingly, in the absence of hormone, IGF-I promoter 1 appears poised to be activated, as evidenced by the presence of the transcriptional coactivator p300 and recruitment of RNA polymerase (Pol) II into a preinitiation complex. By contrast, chromatin surrounding IGF-I promoter 2 is devoid of both p300 and RNA Pol II. Systemic GH treatment causes an approximately 15-fold increase in transcription from each IGF-I promoter within 60 min of hormone administration, leading to a sustained accumulation of IGF-I mRNA. The coordinated induction of both IGF-I promoters by GH is accompanied by hyperacetylation of histones H3 and H4 in promoter-associated chromatin, a decline in monomethylation at lysine 4 of histone H3, and recruitment of RNA Pol II to IGF-I promoter 2. We conclude that GH actions induce rapid and dramatic changes in hepatic chromatin at the IGF-I locus and activate IGF-I gene transcription in the liver by distinct promoter-specific mechanisms: at promoter 1, GH causes RNA Pol II to be released from a previously recruited paused preinitiation complex, whereas at promoter 2, hormone treatment facilitates recruitment and then activation of RNA Pol II to initiate transcription.
Chia
Dennis J
DJ
Department of Pediatrics, Oregon Health and Science University, Portland, Oregon 97239, USA.
Young
Jennifer J
JJ
Mertens
April R
AR
Rotwein
Peter
P
D000107:Acetylation / Q000187:drug effects; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D002843:Chromatin / Q000187:drug effects* / Q000378:metabolism*; D047369:Chromatin Immunoprecipitation; D013006:Growth Hormone / Q000494:pharmacology*; D006657:Histones / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D008297:Male; D008745:Methylation / Q000187:drug effects; D011401:Promoter Regions, Genetic / Q000235:genetics*; D011485:Protein Binding; D012319:RNA Polymerase II / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D020133:Reverse Transcriptase Polymerase Chain Reaction; D050799:STAT5 Transcription Factor / Q000378:metabolism; D015533:Transcriptional Activation / Q000187:drug effects* / Q000235:genetics
D002843:Chromatin; D006657:Histones; D050799:STAT5 Transcription Factor; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D012319:RNA Polymerase II