title: Regulation of start site usage in the leader exons of the rat insulin-like growth factor-I gene by development, fasting, and diabetes.
abstract: Rat insulin-like growth factor-I (IGF-I) mRNAs with different 5'-untranslated region/prepeptide coding sequences result from transcription initiation in one of two leader exons. While not altering the mature IGF-I coding sequence, these different leaders potentially encode two distinct IGF-I prepeptides, one of 48 amino acids (exon 1) and one of 32 amino acids (exon 2). Within exon 1, transcription initiation is dispersed (i.e. occurs over a approximately 350-basepair region), while within exon 2, it is highly localized. A fourth exon 1 start site, residing only approximately 30 basepairs from its 3' end, is suggested on the basis of RNase protection assays; its use would produce an mRNA encoding a third distinct IGF-I leader peptide of 22 amino acids. We have determined that during postnatal development, and as a result of insulinopenic diabetes and fasting, choice of transcription start sites within exon 1 in the liver is coordinately regulated, i.e. use of all start sites increased during development and decreased in the two catabolic states. Transcription initiation at the single major site within exon 2 was also reduced in diabetes and fasting. Insulin replacement therapy and refeeding restored the levels of all transcripts coordinately. During postnatal development, however, transcripts initiating within exon 2 exhibited a different developmental profile than did exon 1 transcripts, increasing especially at the onset of GH-dependent linear growth. In liver, therefore, negative regulation of exon 1 and exon 2 transcription start site usage occurs in catabolic states, while in development, differential regulation of exon 1 and exon 2 transcription start sites occurs.
authors.affiliation: Section on Molecular and Cellular Physiology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: M L
authors.initials: ML
authors.lastname: Adamo
authors.forename: H
authors.initials: H
authors.lastname: Ben-Hur
authors.forename: C T
authors.initials: CT
authors.lastname: Roberts
authors.forename: D
authors.initials: D
authors.lastname: LeRoith
mesh_terms: D000375:Aging; D000818:Animals; D016375:Antisense Elements (Genetics); D001483:Base Sequence; D002872:Chromosome Deletion; D003001:Cloning, Molecular; D003921:Diabetes Mellitus, Experimental / Q000235:genetics*; D005091:Exons; D005215:Fasting / Q000502:physiology*; D005333:Fetus; D005786:Gene Expression Regulation; D015723:Gene Library; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D008099:Liver / Q000254:growth & development / Q000502:physiology* / Q000503:physiopathology; D008297:Male; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides; D021382:Protein Sorting Signals / Q000235:genetics*; D015347:RNA Probes; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000235:genetics / Q000302:isolation & purification; D051381:Rats; D011919:Rats, Inbred Strains; D015183:Restriction Mapping; D014158:Transcription, Genetic
chemical_list: D016375:Antisense Elements (Genetics); D009838:Oligodeoxyribonucleotides; D021382:Protein Sorting Signals; D015347:RNA Probes; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I