Osteopontin (OPN) is an inducible cell attachment protein which binds alphavbeta3-integrin and CD44 receptors to promote tumor metastasis. We hypothesized that OPN alters expression of its CD44 receptor to promote neoplastic cell migration.RAW264.7 cells were stimulated with OPN (0-10 nM) for 0-12 hours to determine the time- and concentration-dependence of CD44 protein and mRNA expression. In selected instances, a competitive ligand for the alphavbeta3-integrin, GRGDSP (50 nM), or an inhibitor of protein synthesis, anisomycin (10 microg/ml), was added. Cell adhesion to hyaluronan was assayed with the crystal violet assay.OPN upregulates plasma membrane total CD44 protein in a concentration-(ANOVA P = 0.001) and time-dependent (ANOVA P = 0.001) fashion. CD44v6 is not altered. Cell adhesion to hyaluronate increases in parallel with CD44 expression. Steady state mRNA levels for CD44 are not altered by OPN. 5 nM OPN increases CD44 protein half-life from 105 +/- 11 minutes to 278 +/- 15 minutes. (P < 0.03) Blockade of either alphavbeta3-integrin ablates the OPN-dependent increase in CD44.These data indicate that OPN increases plasma membrane CD44 expression and cell adhesion by binding to its alphavbeta3-integrin receptor. We conclude that OPN may promote tumor metastatic behavior by CD44 expression.