Original article Insulin-like growth factor binding protein-3 protease activity in the urine of children with chronic renal failure

The insulin-like growth factors, IGF-1 and IGF- II, are polypeptides that potentiate cellular growth. In addi- tion to binding to specific cell surface receptors, the IGFs bind with high affinity to a family of proteins, the insulin- like growth factor binding proteins (IGFBPs). Serum and urine IGFBP patterns are altered in individuals with chronic renal failure (CRF). We recently reported that the urinary IGFBP pattern of CRF patients is unique for increased insulin-like growth factor binding protein-1 (U-IGFBP-1) levels. In this study, we used western ligand blotting (WLB), western immunoblotting (WIB), and radioimmunoassay (RIA) to further evaluate serum and urine IGFBP profiles of children with CRF (n = 14). Five patients with CRF displayed decreased serum IGFBP-3 profiles by WLB. Serum IGFBP-3 WIB profiles were re- markable for 30- and 20-kDa fragments of IGFBP-3 not seen in control serum. Serum IGFBP-3 levels, as deter- mined by RIA, were slightly elevated. Serum levels of IGFBP-2 also were increased, although not at a level reach- ing statistical significance. WLB of CRF urine revealed a large increase in U-IGFBP-1 and a complete absence of urinary IGFBP-3. Recent studies of serum from pregnant women and seminal plasma have demonstrated a similar absence of intact IGFBP-3, due to the presence of a specif- ic IGFBP-3 protease. To evaluate whether an IGFBP-3 protease accounts for the absence of intact U-IGFBP-3 in children with CRF, urine and serum samples from in- dividuals with CRF and controls were tested. An IGFBP-3 protease assay using concentrated urine revealed the pres- ence of two distinct proteases found only in the CRF urines. Serum IGFBP-3 protease activity was no greater in patients with CRF than in controls. We conclude that the decrease in intact urinary IGFBP-3 is due to proteolysis by an IGFBP-3 protease.