High-resolution identification of human adiponectin oligomers and regulation by pioglitazone in type 2 diabetic patients

Adiponectin is an adipokine with insulin-sensitizing, anti-inflammatory, and cardiac protective actions. It homo-oligomerizes into trimers, hexamers, and higher molecular weight (HMW) species, which are not fully characterized. We describe high-resolution separation of adiponectin oligomers under native conditions in polyacrylamide gel coupled with methods for producing standards to provide facile and accurate identification of the oligomers. Using these procedures, adiponectin trimers in human and rodent plasma were found to migrate as two distinct populations. Distributions of these two populations are linearly proportional in plasma from type 2 diabetic patients before (R2 = 0.903, P < 0.001) and after (R2 = 0.960, P < 0.0001) 12 weeks of treatment with pioglitazone as well as from control subjects (R2 = 0.891, P < 0.0001). In addition, HMW adiponectin could be separated into three distinct oligomers: nonamer (9mer), dodecamer (12mer), and the previously characterized octadecamer (18mer). Plasma concentrations of all oligomers increased on pioglitazone treatment, with the largest fold increase being observed in 9mers and 12mers compared with baseline. Increasing concentrations of adiponectin during oligomerization in vitro led to a disproportionate increase in 18mers. The difference between in vivo and in vitro observations suggests that higher total adiponectin protein concentration contributes to pioglitazone's ability to enhance HMW adiponectin levels, but additional factors likely affect oligomer assembly or turnover independently.