Hypoxia enhances FGF2- and VEGF-stimulated human placental artery endothelial cell proliferation: roles of MEK1/2/ERK1/2 and PI3K/AKT1 pathways.

Placental development occurs under a low oxygen (2-8% O(2)) environment, which is critical for placental development and angiogenesis. In this study, we examined if hypoxia affected fibroblast growth factor-2 (FGF2)- and vascular endothelial growth factor (VEGF)-stimulated cell proliferation via the mitogen-activated protein kinase kinase 1/2 (MEK1/2)/extracellular signal-regulated kinases 1/2 (ERK1/2) and phosphatidylinositol-3 kinase (PI3K)/v-akt murine thymomaviral oncogene homologue (AKT1) pathways in human placental artery endothelial (HPAE) cells. We observed that under normoxia (approximately 20% O(2)), FGF2 and VEGF dose-dependently stimulated cell proliferation. Hypoxia (3% O(2)) significantly promoted FGF2- and VEGF-stimulated cell proliferation as compared to normoxia. Under both normoxia and hypoxia, FGF2 rapidly induced ERK1/2 and AKT1 phosphorylation, while VEGF-induced ERK1/2, but not AKT1 phosphorylation. However, hypoxia did not significantly alter FGF2- and VEGF-induced ERK1/2 and AKT1 phosphorylation as compared to normoxia. PD98059 (a MEK1/2 inhibitor) at 20microM and LY294002 (a PI3K inhibitor) at 5microM attenuated FGF2- and VEGF-induced phosphorylation of ERK1/2 and AKT1, respectively. PD98059, even at doses that drastically inhibited FGF2-induced ERK1/2 phosphorylation (20microM) and caused cell loss (40microM), did not affect FGF2-stimulated cell proliferation, which was confirmed by U0126 (another potent MEK1/2 inhibitor). PD98059, however, dose-dependently inhibited VEGF-stimulated cell proliferation. Conversely, LY294002 dose-dependently inhibited FGF2-, but not VEGF-stimulated cell proliferation. These data suggest that in the MEK1/2/ERK1/2 and PI3K/AKT1 pathways differentially mediate FGF2- and VEGF-stimulated HPAE cell proliferation. These results also indicate that hypoxia promotes FGF2- and VEGF-stimulated cell proliferation without further activation of the PI3K/AKT1 and MEK1/2/ERK1/2, respectively.