Macrostemonoside A promotes visfatin expression in 3T3-L1 cells

The newly identified fat cell-secreted factor, visfatin, is insulin-mimetic and play a positive role in attenuating insulin resistance and diabetes. Natural steroidal saponins including tigogenin saponins have been reported to provide anti-diabetic activity and modulate glucose metabolism, but the mechanism remains unknown. In this study, we examined the effect of macrostemonoside A, a tigogenin steroidal saponin isolated from the bulbs of Allium macrostemon Bung on the expression of visfatin in differentiated 3T3-L1 adipocytes. It was found that macrostemonoside A markedly enhanced the synthesis and secretion of visfatin protein in 3T3-L1 adipocytes, and increased visfatin mRNA in a dose and time dependent manner as well. Moreover, visfatin promoter-driven luciferase expression in cells was elevated by macrostemonoside A, which was blocked by SB-203580, a specific inhibitor of p38 MAPK pathway. Lastly, we found that macrostemonoside A did not affect the expression of PPAR gamma and its DNA-binding ability to visfatin promoter. These results indicate that macrostemonoside A could potently stimulate visfatin expression in 3T3-L1 adipocytes, which occurs at the transcriptional level and is mediated at least partially via p38 MAPK signaling pathway. Its regulation on visfatin in adipocytes may constitute an important element in its improvement of insulin resistance and diabetes.