Low-Protein Diet Regulates a Proximal Nephron Insulin-like Growth Factor Binding Protein

Previous studies have demonstrated that the glomerulus and proximal tubule basolateral membrane possess both insulin-like growth factor (IGF) receptors and IGF-binding proteins (IGFBPs). The purpose of this study is to examine the control of these proteins in defined nephron segments during dietary protein restriction. Animals were fed isocaloric 6% (low-protein [LP]) or 40% (high-protein) protein diets for 7 to 10 days. 125I[IGF] affinity labeling of membranes prepared from isolated glomeruli or proximal tubule basolateral membranes demonstrated two proteins on autoradiograms of 6% polyacrylamide gels with molecular weights of 140,000 d and more than 200,000 d that were blocked by 300 nmol/L unlabeled IGF-I. The lower molecular weight species has been identified previously as the alpha subunit of the IGF-I receptor and was upregulated by a 6% LP diet. The upregulation of the IGF-I receptor was evident on 12% polyacrylamide gels of both glomeruli and basolateral membranes. Another protein with a molecular weight of approximately 38,000 d also was upregulated by LP diet. The protein was evident on 125I-IGF-I ligand blots and was immunostained with IGFBP-5 antibodies. Cytosol prepared from cortical tissue also demonstrated a 31,000-d protein that was immunostained with IGFBP-5 antibodies in animals fed a LP diet, but not in animals fed a high-protein or normal diet. RNA prepared from cortical tissue and hybridized with a IGFBP-5 cDNA probe revealed a 6.0-Kb transcript that was increase by LP diet. These data indicate that the enhanced IGF-I binding observed in proximal nephron structures following a LP diet is a result of increased expression of the IGF-I receptor as well as IGFBP-5. The increase in membrane-associated and soluble binding protein when IGF-I circulating levels are diminished suggests that the protein might function as an extravascular reservoir, limiting degradation and facilitating growth factor interaction with the receptor.