IGF-I exerts a wide spectrum of endocrine, paracrine or autocrine actions on multiple cells and tissues. In rat and other mammalian species, a complex family of IGF-I mRNAs derived from a single gene. Our central hypothesis is that the heterogeneity of IGF-I mRNAs and encoded precursors is integral to tbe ability of IGF-I to exert its diversity of actions. Different IGF-I precursors encoded by different IGF-I mRNAs could determine targeting of IGF-I to intracellular or extracellular sites of action or could influence IGF-I interactions with receptors or binding proteins. Proposed studies will test these possibilities by (i) defining the structures of the IGF-I that result from translation of different mRNAs (ii) analysing intracellular localiation, processing and secretion of IGF-I precursors in cultured cell systems (iii) using model cell systems and transgenic mice to assess the biological actions of IGF-I precursors. Transgenic mice represent unique models to study the developmental and long term consequences of perturbations in the synthesis of specific IGF-I precursors.
We aim to target growth hormone (GH) independent expression of specific IGF-I pecursors to liver, the major source of circulating IGF-I and analyse whether such targeting can correct growth retardation in GH and IGF-I deficient mice. This will address two central questions in IGF-I physiology: the role of IGF-I in stimulating growth independent of GH and the contribution of circulating versus locally synthesized IGF-I in stimulating somatic and tissue growth during development. Transcriptional and post-transcriptional mechanisms that regulate the expression of different IGF-I mRNAs could determine the sites, levels and time course of IGF-I expression and thereby the locus and time course of IGF-I action. We will analysc these mechanisms by (i) characterizing the GH dependent promotor (s) that regulate IGF-I synthesis at the level of transcription (ii) assessing post-transcriptional control of IGF-I synthesis at the level of mRNA stability. We have demonstrated that large molecular forms of IGF-I mRNA with long 3'untranslated regions (3'UTs) are less stable than smaller forms. We will therefore define the sequences that mediate instability of these mRNAs, establish whether glucocorticoids destabilize these mRNAs and whether this results in reduced IGF-I synthesis.
