Critical Role of CIB1 in Endothelial Cell Function and In Vivo Ischemia-Induced Angiogenesis.

CIB1, a 22kDa EF-hand containing calcium binding protein, was originally identified in a yeast two-hybrid screen as a binding partner for the cytoplasmic tail of the platelet integrin αIIb. CIB1 also associates with a number of kinases and modulates their activity, suggesting that CIB1 is an important regulatory molecule. Recently, we found that CIB1 is expressed in multiple endothelial cell (EC) types. We therefore tested the role of CIB1 in EC function in vitro, and in angiogenesis both ex vivo and in vivo. To test the role of CIB1 in EC function in vitro, we reduced endogenous CIB1 levels in ECs by RNA interference with an shRNA-delivered by lentivirus. CIB1 depletion significantly decreased EC haptotaxis on fibronectin and EC vascular tube formation on growth factor-reduced Matrigel. Treatment with FGF-2, an angiogenic factor, did not counter the observed inhibition of haptotaxis and tube formation by shRNA against CIB1. However, CIB1 overexpression enhanced FGF-2-induced EC haptotaxis relative to control cells. Similarly, ECs derived from CIB1 null mice exhibited a significant decrease in haptotaxis, tube formation, and proliferation compared to ECs isolated from wild-type littermate controls. In ex vivo aortic ring and tibialis anterior muscle culture assays, CIB1 null cultures supplemented with serum or FGF-2 demonstrated reduced blood vessel sprouting compared to wild-type littermate control cultures. Finally, in vivo assays for hyperoxic retinal angiogenesis and hind-limb induced-ischemia revealed a decrease in post-ischemia retinal neovascularization and Doppler hind-limb blood perfusion recovery, although developmental retinal angiogenesis in CIB1 null mice appeared normal. In conclusion, these findings support a critical role for CIB1 in EC function that appears to be important for ischemia-induced angiogenesis.