Children with chronic renal failure (CRF) grow poorly. Linear growth occurs in bone growth plates and reflects a balance between actions of growth simulators such as growth hormone (GH) and insulin-like growth factors (IGFs), and growth inhibitors. In normal sera, IGF binds IGF binding protein (IGFBP)-3 and acid-labile subunit (ALS) in a large complex; proteolysis of this IGFBP-3 releases IGFs to act on targets such as growth plates. In CRF sera this process is normal, but excess small IGFBPs may block IGF action by binding released IGF. IGFBP-1 and -2 block IGF action in cell culture; levels of each are high in CRF sera and correlate with growth failure. In CRF children, GH increases growth, the amount of IGFBP-3 in the large complex and the amount of IGF released by the complex; this increased release of IGFs appears to overcome the high level of inhibitory IGFBPs. In this proposal, the first goal is to identify IGFBPs that are growth inhibitors, potentially in CRF. In part one of this goal (Specific Aim #1), newly assayable, intact IGFBP-4 will be measured in CRF sera from children to see if levels correlate with growth failure. Part two of this goal (Specific Aim #2) investigates how excess IGFBPs affect growth. It includes experiments in which chick cartilage will be exposed to IGFBP-2 and IGFBP-3 (fragments), in vitro, and in vivo studies in which rats will be exposed to excess IGFBP-2 by injection, or mice to excess IGFBP-2 or -3 by overexpression of the human transgenes. The second goal (Specific Aim #3) is to see how IGFBP-3 releases IGFs to tissue; purified IGFBP-3 and ALS will be used to identify proteases such as plasminogen that bind to and digest IGFBP-3, and to determine how they do so. The third goal (Specific Aim #4) is to see if high cytokine concentrations lead to high levels of inhibitory IGFBP-1 and -2, perhaps revealing a mechanism for CRF-induced excesses in IGFBPs. The fourth goal (Specific Aim #5) is to grow larger IGFBP-1:IGF-I crystals which should result in an understanding of how IGFBPs bind IGFs. Identifying inhibitory IGFBPs, how they bind IGFs, and how their production is regulated may suggest clinical ways to improve IGF action by either lowering IGFBP production or by blocking IGFBP binding to IGFs. Understanding how IGFBP-3 and proteases interact to release IGFs may suggest new ways to liberate IGFs from IGFBP-3 in order to promote anabolic effects in a clinical setting.
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