Metabolic Regulation of IGF-I Gene Expression

Abstract

Insulin-like growth factor-I (IGF-I) is structurally similar to proinsulin and has wide-ranging actions in the promotion of cellular growth, proliferation, and differentiation. Circulating IGF-I is produced mainly in the liver and acts in an endocrine manner at many sites throughout the body, but IGF-I is also produced in a wide variety of organs and tissues, where it has local paracrine and autocrine effects. Consistent with its role as an important regulator of cell growth, IGF-I is tightly modulated by hormonal and nutritional status. In intact animals, reduction in levels of growth hormone or insulin, reduced food intake, and reduction in dietary protein all lead to a fall in circulating IGF-I levels and a parallel fall in hepatic IGF-I expression. Much of this regulation has been confirmed in cultured cell models showing responses to growth hormone, insulin, and availability of amino acids. Regulation takes place at the level of gene transcription since growth hormone, insulin, and dietary protein all increase both hepatic IGF-I expression and rates of transcript elongation in nuclear runon assays. Although the IGF-I gene is complex, extending over 80 kb and including six exons, most transcripts are initiated at several sites in exon 1. We have used the model of streptozotocin-induced diabetes in rats to study mechanisms of metabolic regulation. Diabetic rats exhibit dose-dependent decreases in serum IGF-I levels and hepatic IGF-I expression, preceded by a reduction in IGF-I gene transcription. A similar impact of diabetes is seen with in vitro transcription systems driven by hepatic nuclear extracts from normal and diabetic rats. In such systems, the study of deletion mutants reveals that a region downstream from the major initiation sites in exon 1 is necessary for the diabetes-associated fall in IGF-I gene transcription. We have localized a metabolism-responsive element to a footprinted region ∼ 140 bp downstream and found that mutation of this element blunts the decrease in IGF-I transcription that occurs with diabetic nuclear extracts. Gel mobility shift analysis with oligonucleotides containing this element reveals reduced DNA-protein interactions with diabetic nuclear extracts. Fasting or protein restriction also lead to diminished hepatic IGF-I expression and a fall in IGF-I gene transcription. Although the downstream region is also involved in nutrition-regulated transcription, the sequence elements that confer this control have not yet been identified. Work now in progress is directed toward identification of the transcription factors that mediate IGF-I regulation by alterations in insulin levels and nutritional status. Such understanding should provide further insight into the role and regulation of IGF-I.