The
insulin-like growth factor (IGF) system has been implicated in the development of experimental
diabetic nephropathy. IGF-binding protein-3 (IGFBP-3) modulates IGF actions, and proteolysis decreases its binding affinity for IGFs. The aim of this study was to explore the possibility that proteolysis of
IGFBP-3 may be altered in
diabetic nephropathy and may therefore modify the intrarenal effects of IGFs.
IGFBP-3 proteolysis in urine from diabetic patients with normo- [
albumin excretion rate (AER), <20 microg/min], micro- (AER, 20-200 microg/min), and macroalbuminuria (AER, >200 microg/min) was studied in 34 patients with
noninsulin-dependent diabetes mellitus (
NIDDM), 14 patients with
insulin-dependent diabetes mellitus, and 9 controls. Urine samples were analyzed by Western
ligand blotting and
IGFBP-3 immunoblotting.
Protease activity was quantitated using [125I]
IGFBP-3 as a substrate. WLB showed three main bands (40-46, 35, and 26 kDa) in control urine and a fainter 18-kDa band. All but the 35-kDa band were immunoreactive with the
IGFBP-3 antiserum. The same pattern of IGFBPs was seen in urine from normoalbuminuric diabetic patients. However, the urine of diabetic patients with micro- 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 than that in normoalbuminuric patients (29 +/- 9%; P = 0.0001). Similar results were observed in patients with
insulin-dependent diabetes mellitus. Proteolytic activity in diabetic urine was due to a
serine protease. In conclusion,
diabetic nephropathy was associated with
IGFBP-3 proteolysis in urine. As similar changes were not observed in patients' sera, this is likely to reflect changes in the kidney or urinary tract, resulting in increased local IGF bioavailability, and therefore may contribute to the structural changes of
diabetic nephropathy.