Previous studies suggest that adipocyte-derived exosomes play a role in cell-to-cell communication during the development of metabolic diseases. However, the characteristics and function of exosomes released from adipocytes in vivo remain to be elucidated. Clearly, exosomes released from adipocytes could exist in the circulation. In addition, because the composition of exosomes is heterogenic, depending on the cellular origin of the exosome, adipocyte-derived exosomes could be accompanied by molecules produced specifically in adipocytes. In this context, this study postulated that such molecules associated with exosomes in the serum could be markers for adipocyte-derived exosomes in vivo. This study particularly focused on secretory proteins produced specifically in adipocytes, namely adipocytokines including adiponectin, leptin, and resistin.
Serum adiponectin is partially associated with exosomesBased on western blotting, CD63, a well-known protein marker of exosomes, was concentrated in the pellet of mouse serum after ultracentrifugation, suggesting successful isolation of exosomes. Western blotting detected adiponectin but no leptin and only trace amounts of resistin in the exosome fraction. After ultracentrifugation on a discontinuous gradient, both adiponectin and CD63 were detected in a fraction at a density of 1.17 g/mL, consistent with the density of exosomes. The adiponectin signal in the exosome fraction was decreased by proteinase K treatment and completely quenched by a combination of proteinase K and Triton X-100. These results suggest that a portion of adiponectin exists as a transmembrane protein in the exosomes in mouse serum. showed that the concentration of adiponectin in the serum and the ratio of adiponectin to total protein in the exosome fraction were lower in obese mice than in lean mice.In conclusion, this study showed that serum adiponectin is partially associated with exosomes in mice. Considering that adiponectin is produced exclusively by adipocytes, adiponectin-associated exosomes in serum could be derived from adipocytes. This study proposes that adiponectin could be a marker for exosomes released from adipocytes in vivo.iii