Characterization of insulin-like growth factor binding protein-3 in chronic renal failure serum

IGF-binding protein-3 (IGFBP-3), usually found as glycosylated 41- and 38-kD forms, is the major serum IGFBP during extrauterine life. In normal serum IGFBP-3 binds one IGF peptide and one acid-labile (α) subunit in a high-molecular-weight (MW) complex of 150 kD. By RIA, an excess of IGFBP-3 is present in chronic renal failure (CRF) serum, where it reportedly accumulates at low MW (25-55 kD) rather than as part of the 150-kD complex. To further evaluate IGFBP-3 forms in CRF, sera were obtained from seven healthy adolescents and seven adolescents with CRF. By RIA, IGFBP-3 levels were higher in CRF than normal sera (15.4 ± 2.2 versus 10.1 ± 2.1 µg/mL). High-MW (150-kD) fractions of CRF and normal sera, obtained by neutral size-exclusion chromatography, had equal amounts of IGFBP-3 by RIA. However, a second RIA peak of IGFBP-3, present in low-MW (35-kD) fractions of CRF but not normal sera, could account for the higher IGFBP-3 levels of CRF serum. [^I25I]IGF ligand blots of whole serum and serum fractions, either with or without prior precipitation by IGFBP-3 antiserum, found levels of 41- and 38-kD IGFBP-3 forms to be similar between CRF and normal whole sera and located these forms in the high-MW (150-kD) fractions of CRF and normal sera. [^125IIGF ligand blots also identified excess IGFBP in low-MW CRF serum fractions; cross-linking these IGFBP with [¹²⁵I]IGF-I, followed by precipitation with IGFBP-1, -2, and -3 antibodies, identified high levels of unsaturated IGFBP-1, IGFBP-2, and 19- and 14-kD forms of IGFBP-3 in CRF serum. These studies indicate that 1) normal levels of functional 41- and 38-kD IGFBP-3 forms are present in CRF serum, and these forms can be incorporated into the 150-kD serum complex; and 2) high RIA levels of IGFBP-3 in low-MW fractions of CRF serum are at least in part due to 19- and 14-kD IGFBP-3 forms. These studies suggest that the excess unsaturated IGFBP of CRF serum are small enough to enter interstitial tissue spaces where they may modulate IGF-I-mediated mito-genic, metabolic, and differentiative effects.