Impaired glycemic control is often linked with obesity or type 2 diabetes. The action of many anti-hyperglycemic agents involves the use of insulin signaling pathway. However, the proximal insulin signaling is often impaired in these conditions. Hence, a therapeutic agent that improves hyperglycemia independent of proximal insulin signaling would be highly desirable.The E4orf1 peptide (E4) derived from human adenovirus 36 (Ad36) promotes cellular glucose uptake in vitro and in vivo, independent of insulin. In contrast to insulin-induced signaling, E4 bypasses proximal insulin signaling to upregulate the phosphorylation of AKT and AS160, and the membrane translocation of Glut4, thereby increasing cellular glucose uptake. It is unknown if E4 activity also affects other targets of AKT that insulin influences, such asphosphorylation of GSK3α/β at serine 9/21 , p70S6Kinase at serine 371, and FOXO1 at serine 256. Among other functions,GSK3α/β is involved in insulin resistance, and insulin inactivates it by phosphorylation. In response to insulin, p70S6Kinase phosphorylation promotes cell growth and proliferation, and FOXO1 phosphorylation upregulates the process of adipocyte differentiation.
3T3-L1 preadipocytes inducibly expressing E4 or a null vector (NV)were exposed to doxycycline for 24 h and serum starved for 2 h. E4 and NV cells was treated with 0 nM or 100nM insulin for 30 min. Protein extracted from cellswereanalyzed by western blotting.
Similar to insulin treated cells, E4 cells without insulin exposure also significantly increased phosphorylation of GSK3α/β, p70S6Kinase, and FOXO1 compared to NV without insulin exposure.Insulin treatment of E4 cells did not have additive effect in phosphorylation, which suggests that E4 alone can influence other targets of AKT activation similar to those induced by insulin.
Therefore, E4 offers a promising template to improve glycemic control, particularly when the proximal insulin signaling is impaired.