Structure of the chicken insulin-like growth factor I gene reveals conserved promoter elements

Insulin-like growth factor I plays a major role in cellular growth by mediating many of the actions of growth hormone. In mammals, IGF-I gene expression is complicated, as the single-copy gene is transcribed and processed into multiple mRNAs that encode at least two peptide precursors. Recent cDNA cloning studies have suggested that in chickens the organization of the gene might be simpler, and that comparative mapping might define essential features of IGF-I that are common to vertebrates. To achieve this goal, we have cloned and characterized the chicken IGF-I gene. The chicken gene is more compact than its mammalian homologues. It is composed of 4 exons which are distributed over 50 kilobases (kb) of chromosomal DNA and are transcribed and processed into mRNAs of 1.9 and 2.6 kb. As in the rat and human genes, the mature 70-amino acid IGF-I molecule is encoded within exons 2 and 3, while exons 1 and 4 contain NH2- and COOH-terminal extension peptides, respectively, and untranslated sequences. Both ends of the chicken gene are structurally heterogeneous. IGF-I mRNAs were found to have two regions of polyadenylation which mapped to exon 4 and were separated by approximately 0.7 kb. At the 5' end, several transcription initiation sites were identified within a 74-nucleotide portion of exon 1 that was found to be highly conserved between chicken and mammalian IGF-I genes. To test promotor function, chimeric genes were constructed linking fragments of chicken IGF-I 5'-flanking DNA to a promoterless reporter plasmid. Hybrid genes containing at least 0.6 kb of IGF-I enhanced expression of luciferase after transfection into SK-N-MC cells, a human line that synthesizes IGF-I mRNA. These observations represent the first thorough characterization of an IGF-I gene and provide an impetus for discerning the mechanisms by which IGF-I expression is regulated during growth, development, and differentiation.