VEGF-induced retinal angiogenic signaling is critically dependent on Ca²⁺ signaling by Ca²⁺/calmodulin-dependent protein kinase II.

PURPOSE: The authors conducted an in vitro investigation of the role of Ca(2+)-dependent signaling in vascular endothelial growth factor (VEGF)-induced angiogenesis in the retina.

METHODS: Bovine retinal endothelial cells (BRECs) were stimulated with VEGF in the presence or absence of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM; intracellular Ca(2+) chelator), U73122 (phospholipase C (PLC) inhibitor), xestospongin C (Xe-C), and 2-aminoethoxydiphenyl borate (2APB) (inhibitors of inositol-1,4,5 triphosphate (IP(3)) signaling). Intracellular Ca(2+) concentration ([Ca(2+)](i)) was estimated using fura-2 Ca(2+) microfluorometry, Akt phosphorylation quantified by Western blot analysis, and angiogenic responses assessed using cell migration, proliferation, tubulogenesis, and sprout formation assays. The effects of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN93 were also evaluated on VEGF-induced Akt signaling and angiogenic activity.

RESULTS: Stimulation of BRECs with 25 ng/mL VEGF induced a biphasic increase in [Ca(2+)](i), with an initial transient peak followed by a sustained plateau phase. VEGF-induced [Ca(2+)](i) increases were almost completely abolished by pretreating the cells with BAPTA-AM, U73122, Xe-C, or 2APB. These agents also inhibited VEGF-induced phosphorylation of Akt, cell migration, proliferation, tubulogenesis, and sprouting angiogenesis. KN93 was similarly effective at blocking the VEGF-induced activation of Akt and angiogenic responses.

CONCLUSIONS: VEGF increases [Ca(2+)](i) in BRECs through activation of the PLC-IP(3) signal transduction pathway. VEGF-induced phosphorylation of the proangiogenic protein Akt is critically dependent on this increase in [Ca(2+)](i) and the subsequent activation of CaMKII. Pharmacologic inhibition of Ca(2+)-mediated signaling in retinal endothelial cells blocks VEGF-induced angiogenic responses. These results suggest that the PLC/IP(3)/Ca(2+)/CaMKII signaling pathway may be a rational target for the treatment of angiogenesis-related disorders of the eye.