[APPL1-mediated effects of leptin on activity of GSK-3β in C2C12 cell]

To observe the effects of leptin on activity of GSK-3β and explore its mechanism.

C2C12 myoblasts differentiated for 3 days into myotubes in differentiation medium. Myotubes were stimulated by leptin (100 nmol/L) for 0, 5, 15 or 30 min respectively. Western blot was used to detect the expression levels of GSK-3β and phospho-GSK-3β (ser-9). Co-immunoprecipitation (CO-IP) was performed to determine the relationship among APPL1, leptin receptor and GSK-3β in the presence or absence of leptin. The expression level of GSK-3β at phospho-GSK-3β (ser-9) was detected in APPL1-suppressed C2C12 myotube while that of APPL1 at phospho-APPL1 (ser-401) determined in GSK-3β overexpressed/inhibited C2C12 cell.

Leptin time-dependently increased the phosphorylation level of GSK-3β at ser-9 in C2C12 cell, and the pGSK-3β level in cells incubated by leptin for 30 min was as 4.08 times as which in control cells (P < 0.01). The triple complex of APPL1, leptin receptor and GSK-3β, in the presence of leptin, the binding capacity between APPL1 and GSK-3β was stronger. The level of phospho-GSK-3β was significantly lower in APPL1-suppressed C2C12 cell compared with that in control cells. And the phosphorylation of APPL1 at ser-401 could be induced by GSK-3β.

Leptin promotes muscle glycogen synthesis by inducing phosphorylation of GSK-3β in C2C12 cell. Such a function may be mediated by the triple complex of APPL1, leptin receptor and GSK-3β. Meanwhile, GSK-3β can also increase the phosphorylation of APPL1 at ser-401.