Division of Molecular Genetics, Institute for Enzyme Research, The University of Tokushima, Kuramoto-cho, Tokushima, Japan.
OBJECTIVE: Insulin binds to the alpha-subunit of the insulin receptor (IRalpha) and subsequently exerts its effects in the cells. The soluble ectodomains of several receptors have been found to circulate in the plasma. Therefore, we hypothesized that soluble human insulin receptor (hIR) ectodomain (alpha-subunit and a part of beta-subunit) may exist in the plasma of diabetic patients. RESEARCH DESIGN AND METHODS: We identified soluble hIR ectodomain in human plasma by a two-step purification followed by immunoblotting and gel-filtration chromatography. Furthermore, we established an hIRalpha-specific enzyme-linked immunosorbent assay and measured the plasma IRalpha levels in patients with diabetes. We also investigated this phenomenon in streptozotocin-induced diabetic hIR transgenic mice. RESULTS: Soluble hIRalpha, but not intact hIRbeta or whole hIR, exists in human plasma. The plasma IRalpha levels were significantly higher in type 1 (2.00 +/- 0.60 ng/ml; n = 53) and type 2 (2.26 +/- 0.80; n = 473) diabetic patients than in control subjects (1.59 +/- 0.40 ng/ml; n = 123 (P < 0.001 vs. control). Plasma IRalpha level was positively correlated with blood glucose level, and 10-20% of the insulin in plasma bound to hIRalpha. In the in vivo experiments using diabetic hIR transgenic mice, hyperglycemia was confirmed to increase the plasma hIRalpha level and the half-life estimated to be approximately 6 h. CONCLUSIONS: We propose that the increased soluble IR ectodomain level appears to be a more rapid glycemic marker than A1C or glycoalbumin.