Proteolysis of insulin-like growth factor-binding protein-3 is increased in urine from patients with diabetic nephropathy

The insulin-like growth factor (IGF) system has been implicated in the deve lopment of experimental diabetic nephropathy. IGF-binding protein-3 (IGFBP- 3) modulates IGF actions, and proteolysis decreases its binding affinity fo r IGFs. The aim of this study was to explore the possibility that proteolys is of IGFBP-3 may be altered in diabetic nephropathy and may therefore modi fy the intrarenal effects of IGFs. IGFBP-3 proteolysis in urine from diabet ic patients with normo- [albumin excretion rate (AER), <20 mu g/min], micro - (AER, 20-200 mu g/min), and macroalbuminuria (AER, >200 mu g/min) was stu died in 34 patients with noninsulin-dependent diabetes mellitus (NIDDM), 14 patients with insulin-dependent diabetes mellitus, and 9 controls. Urine s amples were analyzed by Western ligand blotting and IGFBP-3 immunoblotting. Protease activity was quantitated using [I-125]IGFBP-3 as a substrate. WLB showed three main bands (40-46, 35, and 26 kDa) in control urine and a fai nter 18-kDa band. All but the 35-kDa band were immunoreactive with the IGFB P-3 antiserum. The same pattern of IGFBPs was seen in urine from normoalbum inuric diabetic patients. However, the urine of diabetic patients with micr o- and macroalbuminuria contained little or no intact 40- to 46-kDa IGFBP-3 . In patients with noninsulin-dependent diabetes mellitus, urinary IGFBP-3 protease activity in micro- (n = 13) and macroalbuminuric patients (n = 12; mean +/- SD[SCAP], 75 +/- 25% and 84 +/- 24%) was significantly higher tha n that in normoalbuminuric patients (29 +/- 9%; P = 0.0001). Similar result s were observed in patients with insulin-dependent diabetes mellitus. Prote olytic activity in diabetic urine was due to a serine protease. In conclusi on, diabetic nephropathy was associated with IGFBP-3 proteolysis in urine. As similar changes were not observed in patients' sera, this is likely to r eflect changes in the kidney or urinary tract, resulting in increased local IGF bioavailability, and therefore may contribute to the structural change s of diabetic nephropathy.