CD44 Variants but not CD44s Cooperate with b1-containing Integrins to Permit Cells to Bind to Osteopontin Independently of Arginine-glycine-aspartic Acid, thereby Stimulating Cell Motility and Chemotaxis

The expression of osteopontin (OPN), CD44 variants, and integrins has been correlated with tumorigenesis and metastasis. Here we show that these proteins cooperate to enhance cell motility. First, we demonstrate that several different CD44 variants bind to OPN in an arginine-glycineaspartic acid-independent manner, but that the standard form of CD44 does not. These CD44 variants bind to both the aminoand COOHterminal portions of OPN independently of the arginine-glycine-aspartic acid sequence, suggesting that multiple domains on OPN can be bound by the CD44 variants. Antibodies directed against the integrin b1 subunit are able to inhibit this binding. The binding of CD44 variants to OPN is significantly augmented by both anti-CD44s and anti-CD44v antibodies. This augmentation by anti-CD44 antibodies is OPN specific and, again, can be blocked by anti-b1 antibodies. Finally, we show that OPN binding by CD44 variants/b1-containing integrins promotes cell spreading, motility, and chemotactic behavior.