FN and Coll I integrin receptor expression in c-kit⁺ cells and reversion of EPC differentiation by integrin antagonists in culture. (A-B) FACS analysis and quantification of α₄, α₅, and α₂ integrin expression in c-kit⁺ cells (n = 3). (C) Plating c-kit⁺ cells onto different FN subfragments (40-120 kDa) revealed a different contribution of α₄ and α₅ integrin receptors to EPC differentiation. Onto both fragments, SDF-1 enhanced EPC differentiation compared with controls, although to a different extent. In fact, the amount of differentiated EPCs was significantly higher onto 40-kDa FN fragment, containing the CS-1 epitope for α₄ binding (filled bars), compared with the 120-kDa FN fragment, containing the RGD motif, specific for α₅ binding (open bars). *P < .05; n = 4. C indicates control-untreated cells. (D-F) Reversion of EPC differentiation using FN or Coll I adhesion antagonists. CS-1 (D), RGD (E), and DGEA (F) peptides antagonizing binding of EPCs to FN via α₄ and α₅ integrins, or Coll I via α₂β₁, were added in culture. Cell counts showed that these antagonists reverted SDF-1–enhanced differentiation revealing a cell adhesion–dependent mechanism. *P < .05; n = 4. Error bars indicate standard errors.