Urokinase receptor expression on human microvascular endothelial cells is increased by hypoxia: implications for capillary-like tube formation in a fibrin matrix

Hypoxia stimulates angiogenesis, the formation of new blood vessels. This s tudy evaluates the direct effect of hypoxia (1% oxygen) on the angiogenic r esponse of human microvascular endothelial cells (hMVECs) seeded on top of a 3-dimensional fibrin matrix. hMVECs stimulated with fibroblast growth fac tor-2 (FGF-2) or vascular endothelial growth factor (VEGF) together with tu mor necrosis factor-alpha (TNF-alpha) formed 2- to 3-fold more tubular stru ctures under hypoxic conditions than in normoxic (20% oxygen) conditions. I n both conditions the in-growth of capillary-like tubular structures into f ibrin required cell-bound urokinase-type plasminogen activator (uPA) and pl asmin activities. The hypoxia-induced increase in tube formation was accomp anied by a decrease in uPA accumulation in the conditioned medium. This dec rease in uPA level was completely abolished by uPA receptor-blocking antibo dies. During hypoxic culturing uPA receptor activity and messenger RNA (mRN A) were indeed increased. This increase and, as a consequence, an increase in plasmin formation contribute to the hypoxia-induced stimulation of tube formation. A possible contribution of VEGF-A to the increased formation und er hypoxic conditions is unlikely because there was no increased VEGF-A exp ression detected under hypoxic conditions, and the hypoxia-induced tube for mation by FGF-2 and TNF-alpha was not inhibited by soluble VEGFR-1 (sVEGFR- 1), or by antibodies blocking VEGFR-2, Furthermore, although the alpha(v)-i ntegrin subunit was enhanced by hypoxia, blocking antibodies against alpha( v)beta(3)- and alpha(v)beta(5)-integrins had no effect on hypoxia-induced t ube formation. Hypoxia increases uPA association and the angiogenic respons e of human endothelial cells in a fibrin matrix; the increase in the uPA re ceptor is an important determinant in this process. (Blood, 2000;96:2775-27 83) (C) 2000 by The American Society of Hematology.