Regulation of the Akt/Glycogen synthase kinase-3 axis by insulin-like growth factor-II via activation of the human insulin receptor isoform-A

Insulin‐like growth factor II (IGF‐II) plays a key role in mitogenesis during development and tumorigenesis and is believed to exert its mitogenic functions mainly through the IGF‐I receptor. Recently, we identified the insulin receptor isoform A (IRA) as an additional high affinity receptor for IGF‐II in both fetal and cancer cells. Here we investigated the mitogenic signaling of IGF‐II via the Akt/Glycogen synthase kinase 3 (Gsk3) axis employing R‐IRA cells that are IGF‐I receptor null mouse embryonic fibroblasts expressing the human IRA. IGF‐II induced activation of the proto‐oncogenic serine kinase Akt, reaching maximal at 5–10 min. IGF‐II also caused the rapid and sustained deactivation of glycogen synthase kinase 3‐beta (Gsk3β), reaching maximal at 1–3 min, shortly preceding, therefore, maximal activation of Akt. Under our conditions, IGF‐II and insulin induced 70–80% inhibition of Gsk3βactivity. In these cells IGF‐II also deactivated Gsk3α although less effectively than Gsk3β. In parallel experiments, we found that IGF‐II induced transient activation of extracellular‐signal‐regulated kinases (Erk) reaching maximal at 5–10 min and decreasing thereafter. Time courses and potencies of regulation of both mitogenic pathways (Akt/Gsk3β and Erk) by IGF‐II via IRA were similar to those of insulin. Furthermore, IGF‐II like insulin effectively stimulated cell cycle progression from the G0/G1 to the S and G2/M phases. Interestingly, AP‐1‐mediated gene expression, that was reported to be negatively regulated by Gsk3β was only weakly increased after IGF‐II stimulation. Our present data suggest that the coordinated activation or deactivation of Akt, Gsk3β, and Erk may account for IGF‐II mitogenic effects and support an active role for IRA in IGF‐II action. J. Cell. Biochem. 82: 610–618, 2001. © 2001 Wiley‐Liss, Inc.