Serum levels of insulin-like growth factor system components and relationship to bone metabolism in Type 1 and Type 2 diabetes mellitus patients

Osteopenia has been ascribed to diabetics without residual insulin secretion and high insulin requirement. However, it is not known if this is partially due to disturbances in the IGF system, which is a key regulator of bone cell function. To address this question, we performed a cross-sectional study measuring serum levels of IGF-I, IGF-binding protein-1 (IGFBP-1), IGFBP-3, IGFBP-4 and IGFBP-5 by specific immunoassays in 52 adults with Type I (n=27) and Type 2 (n=25) diabetes mellitus and 100 age- and sex-matched healthy blood donors. In the diabetic patients, we further determined serum levels of proinsulin, intact parathyroid hormone (PTH), 25-hydroxyvitamin D-3, 1,25-dihydroxyvitamin D-3 and several biochemical bone markers, including osteocalcin (OSC), bone alkaline phosphatase CB-ALP), carboxy-terminal propeptide of type I procollagen (PICP), and type I collagen cross-linked carboxy-terminal telopeptide (ICTP). Urinary albumin excretion was ascertained as a marker of diabetic nephropathy. Bone mineral density (BMD) of hip and lumbar spine was determined by dual-energy X-ray absorptiometry. Data are presented as means +/- S.E.M. Differences between the experimental groups were determined by performing a one-way analysis of variance (ANOVA), followed by Newman-Keuls test. Correlations between variables were assessed using univariate linear regression analysis and partial correlation analysis. Type 1 diabetics showed significantly lower IGF-I (119 +/- 8 ng/ml) and IGFBP-3 (2590 +/- 104 ng/ml) but higher IGFBP-1 levels (38 +/- 10 ng/ml) compared with Type 2 patients (170 +/- 13, 2910 +/- 118, 11 +/- 3 respectively; P<0.05) or healthy controls (169 +/- 5, 4620 +/- 192, 3.5 +/- 0.4 respectively; P<0.01). IGFBP-5 levels were markedly lower in both diabetic groups (Type 1, 228 +/- 9; Type 2, 242 +/- 11 ng/ml) than in controls (460 +/- 7 ng/ml, P<0.01), whereas IGFBP-4 levels were similar in diabetics and controls. IGF-I correlated positively with IGFBP-3 and IGFBP-5 and negatively with IGFBP-1 and IGFBP-4 in all subjects. Type 1 patients showed a lower BMD of hip (83 +/- 2%, Z-score) and lumbar spine (93 +/- 2%) than Type 2 diabetics (93 +/- 5%, 101 +/- 5% respectively), reaching significance in the female subgroups (P<0.05). In Type 1 patients, BMD of hip correlated negatively with IGFBP-1 (r = - 0.34, P<0.05) and IGFBP-4 (r= - 0.3, P<0.05) but positively with IGFBP-5 (r=0.37, P<0.05), which was independent of age, diabetes duration, height, weight and body mass index, as assessed by partial correlation analysis. Furthermore, biochemical markers indicating bone loss (ICTP) and increased bone turnover (PTH, OSC) correlated positively with IGFBP-1 and IGFBP-4 but negatively with IGF-I, IGFBP-3 and IGFBP-5, while the opposite was observed with bone formation markers (PICP, B-ALP) and vitamin D-3 metabolites. In 20 Type 2 patients in whom immunoreactive proinsulin could be detected, significant positive correlations were found between proinsulin and BMD of hip (r=0.63, P<0.005), IGF-I (r=0.59, P<0.01) as well as IGFBP-3 (r=0.49, P<0.05). Type 1 and Type 2 patients with macroalbuminuria showed a lower BMD of hip, lower IGFBP-5 but higher IGFBP-4 levels, suggesting that diabetic nephropathy may contribute to bone loss by a disturbed IGF system. In conclusion, the findings of this study support the hypothesis that the imbalance between individual IGF system components and the lack of endogenous proinsulin may contribute to the lower BMD in Type 1 diabetics.