Angiostatin Induces Intracellular Acidosis and Anoikis in Endothelial Cells at a Tumor-Like Low pH

Angiostatin inhibits angiogenesis by binding to endothelial cells (ECs) lining the vasculature of growing tumors. These cells are in a dynamic state during angiogenesis and are thus not firmly attached to the extracellular matrix. This makes them more vulnerable to anoikis, a process resulting in cell death initiated by or promoted by loss of attachment. Another potential source of EC vulnerability during tumor angiogenesis is that tumor extracellular pH is typically lower than in normal tissues. This presents an additional challenge to ECs in terms of maintaining ionic homeostasis. We report here that the lethality of angiostatin is significantly enhanced both by reduced matrix attachment during exposure and lowered extracellular pH (pH e ). Another effect of angiostatin at reduced pH e is a decreased intracellular pH (pH i ). These effects were observed in three model systems: aortic ring sprouts, ECs during tube formation, and ECs in a scratch/migration assay. In these three dynamic assays, angiostatin...