αB-crystallin regulation of angiogenesis by modulation of VEGF

alpha B-crystallin is a chaperone belonging to the small heat shock protein family. Herein we show attenuation of intraocular angiogenesis in alpha B-crystallin knockout (alpha B-crystallin(-/-)) mice in 2 models of intraocular disease: oxygen-induced retinopathy and laser-induced choroidal neovascularization. Vascular endothelial growth factor A (VEGF-A) mRNA and hypoxia inducible factor-1 alpha protein expression were induced during retinal angiogenesis, but VEGF-A protein expression remained low in alpha B-crystallin(-/-) retina versus wild-type mice, whereas VEGF-R2 expression was not affected. Both alpha B-crystallin and its phosphorylated serine59 formwere expressed, and immunoprecipitation revealed alpha B-crystallin binding to VEGF-A but not transforming growth factor-beta in cultured retinal pigment epithelial (RPE) cells. alpha B-crystallin and VEGF-A are colocalized in the endoplasmic reticulum in RPE cells under chemical hypoxia. alpha B-crystallin(-/-) RPE showed low VEGF-A secretion under serum-starved conditions compared with wild-type cells. VEGF-A is polyubiquitinated in control and alpha B-crystallin siRNA treated RPE; however, mono-tetra ubiquitinated VEGF-A increases with alpha B-crystallin knockdown. Endothelial cell apoptosis in newly formed vessels was greater in alpha B-crystallin(-/-) than wild-type mice. Proteasomal inhibition in alpha B-crystallin(-/-) mice partially restores VEGF-A secretion and angiogenic phenotype in choroidal neovascularization. Our studies indicate an important role for alpha B-crystallin as a chaperone for VEGF-A in angiogenesis and its potential as a therapeutic target. (Blood. 2010; 115(16): 3398-3406)