Binding of adiponectin and C1q in human serum, and clinical significance of the measurement of C1q–adiponectin / total adiponectin ratio

ObjectiveAdiponectin and C1q have similar sequences, exist abundantly in blood, and are produced by adipose tissues. The aim of this study was to examine whether adiponectin and C1q form protein-complex in blood and to know the clinical significance of the C1q–adiponectin (C1q–APN) complex in serum.MethodsThe direct interaction between adiponectin and C1q was investigated by far western blotting and co-immunoprecipitation. The relationship between serum C1q–APN and various clinical features was analyzed in 329 Japanese men who underwent health check-up, including measurements of visceral (VFA) and subcutaneous fat area (SFA) by computed tomography (Victor-J study).ResultsAdiponectin bound to C1q in vitro and C1q–APN complex existed in human blood. C1q–APN complexes were identified in high- and middle-molecular weight forms of adiponectin in human serum by gel-filtration chromatography. Stepwise multiple regression analysis identified body mass index, VFA and SFA as significant determinants of serum C1q–APN level. Serum C1q–APN/Total-APN ratio correlated positively with cardiovascular risk factor accumulation in subjects with VFA ≥ 100 cm2.ConclusionsThese results indicate that high- and middle-molecular forms of adiponectin partly consist of adiponectin-complex with other proteins including C1q and that the blood C1q–APN/Total-APN ratio may serve as a biomarker of the metabolic syndrome in general male subjects.