A muscle secreting protein attractin regulates expression of UCP-2 in adipocytes.

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Prolonged spaceflight and bed-resting cause remark appetite loss. A lot of investigators suggest that this appetite loss is probably due to psychological stress. In the present study, we reasoned that skeletal muscle atrophied by unloading regulated energy consumption via a novel muscle-secreting enzyme, attractin. Our DNA microarray analysis revealed that skeletal muscle atrophied by unloading remarkably expressed the secreted-type attractin. In addition, starvation stimulated expression of attractin in skeletal muscle, indicating that energy intake may regulate attractin expression in the muscle. Our co-culturing system with 3T3-L1 adipocytes and attractin-overexpressing Cos7 cells showed that the secreted attractin from Cos7 cells stimulated expression of uncoupled protein-2 (UCP-2) in the adipocytes. This finding suggests that attractin may stimulate heat conversion in adipocytes and prevent the obesity. Up to now, there is no report about the factors regulating attractin expression. We found that only insulin decreased expression of attractin in skeletal muscle, indicating that blood glucose levels may be involved in attractin expression. Based on these findings, we propose that skeletal muscle may secrete several cytokines (so called myokines), and these myokines may regulate the growth and function of adipose tissues.