Evaluation of the components of insulin-like growth factor (IGF)-IGF binding protein (IGFBP) system in adolescents with type 1 diabetes and persistent microalbuminuria: relationship with increased urinary excretion of IGFBP-3 18 kD N-terminal fragment.

OBJECTIVE: IGFs and their binding proteins (IGFBPs) have an important role in controlling glucose homeostasis and there is evidence to support their involvement in complications related to type I diabetes. The aim of this study was to evaluate the components of the IGF-IGFBP system in adolescents with type 1 diabetes that had developed persistent microalbuminuria (MA). DESIGN AND PATIENTS: A cohort of 49 adolescents with type 1 diabetes were enrolled in the study. Patients were evaluated at baseline and 1 year later (follow-up). Twenty-six patients with persistent urinary albumin excretion (UAE) of more than 20 microg/min/1.73 m2 (21.6-109. 4 microg/min/1.73 m2) in three different nocturnal urinary collections within 6 months were considered to have MA (baseline mean: 41.9 +/- 22.3 microg/min/1.73 m2; follow-up: 55.9 +/- 24.8 microg/min/1.73 m2). Twenty-three patients with UAE of less than 20 microg/min/1.73 m2 were assigned to the group without MA (baseline mean: 8.6 +/- 3.7 microg/min/1.73 m2; follow-up: 11.8 +/- 4.2 microg/min/1.73 m2). Fasting serum levels of IGFBP-1, IGFBP-2, IGFBP-3, IGF-I and free-IGF-I were determined using appropriate immunoenzymatic, radioimmuno- or immunoradiometric assays. Overnight 12-h urinary collections were obtained and assessed for IGFBP-3 levels, determined by immunoradiometric assay. Urinary and circulating immunoreactive IGFBP-3 forms were determined by Western-immunoblotting (WIB) analysis using a specific polyclonal antibody and monoclonal antibodies directed against N-terminal and C-terminal epitopes of IGFBP-3. IGFBP-3 protease activity was determined using protease assay and by analysis of the intact over the fragmented immunoreactive forms of IGFBP-3 determined by WIB analysis.
RESULTS: Patients with MA showed higher levels of urinary IGFBP-3 (649 +/- 440 ng/h/m2) than patients without MA (398 +/- 229 ng/h/m2; P < 0.05). Urinary levels of IGFBP-3 were directly correlated to UAE (P < 0.001). WIB analysis, using monoclonal antibodies directed against characterized N-terminal and C-terminal IGFBP-3 epitopes, determined that the immunoreactive form of IGFBP-3 found in urine from patients with diabetes was an N-terminal 18 kD fragment. Serum IGFBP-3 levels were lower in patients with MA (baseline: 3613 +/- 598 microg/l; one year follow-up: 3347 +/- 624 microg/l) compared with patients without MA (baseline: 4701 +/- 1484 microg/l; follow-up: 4177 +/- 703 microg/l; P < 0.001). In serum from patients with MA, intact IGFBP-3 was decreased, as indicated by WIB analysis. Conversely, IGFBP-3 proteolysis was increased in patients with MA (baseline: 131 +/- 21% of control; follow-up: 130 +/- 23% of control), compared to patients with normal UAE (baseline: 96 +/- 23% of control; follow-up: 96 +/- 14% of control; P < 0.001). Serum IGFBP-3 protease activity was directly correlated to urinary IGFBP-3 levels (P < 0.001). Serum IGFBP-1 levels were increased in patients with MA (baseline: 36 +/- 20 microg/l; follow-up: 36 +/- 17 microg/l) compared with patients without MA (baseline: 17 +/- 11 microg/l; follow-up: 18 +/- microg/l; P < 0.05). Serum IGFBP-2 levels were also persistently increased in patients with MA (baseline: 503 +/- 134 microg/l; follow-up: 484 +/- 166 microg/l) compared with patients without MA (baseline: 375 +/- 83 microg/l; follow-up: 390 +/- 85 microg/l; P < 0.05). On the other hand, free IGF-I levels were decreased in patients with MA (baseline: 2.3 +/- 1. 5 microg/l; follow-up: 2.5 +/- 1. (ABSTRACT TRUNCATED)