Structure and function of a human insulin-like growth factor-I gene promoter

We have identified and characterized a promoter regulatory region for the human insulin-like growth factor-I (IGF-I) gene. The 5′-ends of IGF-I mRNAs were first mapped to a series of sites within a 158-nucleotide portion of exon 1 that was found to be structurally similar to the recently delineated transcription start region of the chicken IGF-I gene. In both species multiple initiation sites are probably a reflection of the absence of typical transcriptional regulatory signals, such as a TATA or CCAAT box, in the proximal promoter, although neither gene sequence resembles a GC-rich housekeeping promoter, which also controls a dispersed initiation region. To test promoter function, hybrid genes were constructed linking human IGF-I DNA to a promoterless reporter plasmid. Fusion genes containing from 385-4300 nucleotides of the IGF-I 5′- flanking region enhanced luciferase activity after transfection into the IGF-I-producing SK-N-MC cell line. A plasmid with 1630 nucleotides of 5′-DNA gave maximal activity and directed accurate initiation of the hybrid gene, while the same promoter fragment inserted in the reverse orientation was inert. Although cognate recognition sequences were identified for several transcription factors in the 1630 nucleotides 5′ to the transcription start region, no sites were found that resembled the putative GH response element recently mapped to the proximal promoter of the rat Spi2.1 gene. This study underscores the diversity of mechanisms contributing to IGF-I gene expression by demonstrating that heterogeneous transcription initiation accounts for IGF-I mRNAs with different 5′-ends and provides a starting point for elucidating the ways in which GH and other trophic agents induce IGF-I gene transcription.