Exogenous H₂O₂ activates endothelial cell–associated MMP-2 and MMP-9. (A) MMP-9 activity. (B) MMP-2 activity. mHEVc cells were incubated with 0, 0.3, 1, 3, and 50 μM H₂O₂ for 10 minutes. The cells were washed, then equal cell numbers were lysed, and MMP activity was examined by zymography. H₂O₂ had no effect on cell viability (data not shown). Data presented are the mean ± SD from 5 experiments. *P < .05 compared with nonstimulated mHEVc cells.