Dissociation between insulin-mediated signaling pathways and biological effects in placental cells: Role of protein kinase B and MAPK phosphorylation

Beyond the presence of insulin receptors, little is known of the mechanisms underlying the biological effects of insulin in the placenta. We show that phosphorylation of MAPK and protein kinase B were enhanced 286 +/- 23% and 393 +/- 17% upon insulin stimulation of JAr placental cells. MAPK activati on was prevented by pretreatment with PD98059 but was unaffected by wortman nin. Insulin stimulation of protein kinase B phosphorylation was abolished by pretreatment with wortmannin, suggesting that it is dependent on phospha tidylinositol 3-kinase activation. Despite protein kinase B phosphorylation , GLUT4 translocation, glucose uptake, and glycogen synthesis were not stim ulated by insulin. By contrast, glycogen synthesis was stimulated 20-fold i n cells incubated with 11 mm glucose. Mitogenesis assessed by incorporation of [H-3]thymidine into DNA was enhanced 1.9-fold in response to insulin. S timulation of DNA synthesis was inhibited by pretreatment with PD98059 but was insensitive to wortmannin. These results indicate that stimulation of m itogenesis is one major biological effect of insulin in placenta cells that implicates the MAPK signaling pathway. Phosphatidylinositol 3-kinase-depen dent protein kinase B activation is not sufficient to stimulate glucose tra nsport and glycogen synthesis, highlighting the placenta as a nonclassic ta rget of insulin for the regulation of glucose metabolism.