Hypoxia in combination with FGF-2 induces tube formation by human microvascular endothelial cells in a fibrin matrix: involvement of at least two signal transduction pathways

Hypoxia in combination with a growth factor is a strong inducer of angiogen esis. Among several effects, hypoxia can activate endothelial cells directl y, but the mechanism by which it acts is not fully elucidated. In vitro, hu man microvascular endothelial cells (hMVEC) form capillary-like tubules in fibrin solely after stimulation with a combination of fibroblast growth fac tor (FGF)-2 or vascular endothelial growth factor (VEGF) and the cytokine t umour necrosis factor (TNF)alpha. We show in this paper that in hypoxic con ditions, FGF-2-stimulated hMVEC form tube-like structures in a fibrin matri x in the absence of TNF alpha. Hypoxia/FGF-2-stimulated cells express more urokinase-type plasminogen activator (u-PA) receptor than normoxia/FGF-2-st imulated cells and display a slightly higher turnover of u-PA. This small i ncrease in u-PA activation probably cannot fully explain the hypoxia/FGF-2- induced tube formation. Hypoxia activated at least two signal pathways that may contribute to the enhanced angiogenic response. In hypoxia/FGF-2-stimu lated hMVEC the transcription factor p65 was activated and translocated to the nucleus, whereas in normoxia/FGF-2-stimulated cells p65 remained inacti ve. Furthermore, in hypoxic conditions, the amounts of phosphorylated mitog en-activated protein kinases ERK1/2 were increased compared to normoxic con ditions. We conclude that hypoxia is able to activate different signal path ways in FGF-2-stimulated human endothelial cells, which may be involved in hypoxia-induced angiogenesis.