EMPs affect the angiogenic properties of endothelial progenitors in vitro: effects on tube formation in matrigel. EPDC in RPMI/10% FCS and varying amounts of EMPs were plated on matrigel, and tube formation was evaluated after 24 hours of incubation. (A) Biphasic effect of EMPs on tube formation: at low EMP concentrations (≤ 2.10³/well), polygon number and tube formation increased by 20% (P = .012, n = 12) over control (S: EMP last washing supernatant). However, higher amounts of EMPs (≥ 2.10⁵ EMP/well) had the reverse effect and progressively decreased the number of tubes to 50% at 8.10⁵ EMP/well. Stars indicate significant changes (*P < .05, **P < .01, ***P < .001). Results are the mean (± SD) of 12 independent experiments. (B) Inverted light microscope representative images (400 × magnification) showing EPDC tube formation in matrigel in the absence and presence of 2.10³ or 2.10⁵ EMPs per well. Asterisks indicate retracted cell clusters. Slides were viewed with an inverted Nikon light microscope (Nikon, Melville, NY) using an Olympus lens (40×/0.60; Olympus, Tokyo, Japan). Images were acquired using a Nikon model DXM 1200F camera and were processed with Lucia G software (both from Nikon) and Adobe Photoshop version 7.0 software (Adobe Systems, San Jose, CA). (C) Experiments were performed in the presence of inhibitors of either plasmin formation (anti-uPA antibody) or activity (α₂-antiplasmin, α₂-AP). The biphasic effect of EMPs on tube formation (left bar panel) was abolished by the anti-uPA antibody (middle) and by α₂-AP (right). Asterisks indicate significant changes (P < .05). Results are the mean (± SD) of 3 independent experiments.