Abstract
Changes in intracellular calcium [Ca(2+)](i) levels control critical cytosolic and nuclear events that are involved in the initiation and progression of tumor angiogenesis in endothelial cells (ECs). Therefore, the mechanism(s) involved in agonist-induced Ca(2+)(i) signaling is a potentially important molecular target for controlling angiogenesis and tumor growth. Several studies have shown that blood vessels in tumors differ from normal vessels in their morphology, blood flow and permeability. We had previously reported a key role for arachidonic acid (AA)-mediated Ca(2+) entry in the initial stages of tumor angiogenesis in vitro. In this study we assessed the mechanism involved in AA-induced EC migration. We report that TRPV4, an AA-activated channel, is differentially expressed in EC derived from human breast carcinomas (BTEC) as compared with 'normal' EC (HMVEC). BTEC display a significant increase in TRPV4 expression, which was correlated with greater Ca(2+) entry, induced by AA or 4αPDD (a selective TRPV4 agonist) in the tumor-derived ECs. Wound-healing assays revealed a key role of TRPV4 in regulating cell migration of BTEC but not HMVEC. Knockdown of TRPV4 expression completely abolished AA-induced BTEC migration, suggesting that TRPV4 mediates the pro-angiogenic effects promoted by AA. Furthermore, pre-incubation of BTEC with AA induced actin remodeling and a subsequent increase in the surface expression of TRPV4. This was consistent with the increased plasma membrane localization of TRPV4 and higher AA-stimulated Ca(2+) entry in the migrating cells. Together, the data presented herein demonstrate that: (1) TRPV4 is differentially expressed in tumor-derived versus 'normal' EC; (2) TRPV4 has a critical role in the migration of tumor-derived but not 'normal' EC migration; and (3) AA induces actin remodeling in BTEC, resulting in a corresponding increase of TRPV4 expression in the plasma membrane. We suggest that the latter is critical for migration of EC and thus in promoting angiogenesis and tumor growth.