The osteopontin-CD44 survival signal involves activation of the phosphatidylinositol 3-kinase/Akt signaling pathway

We have recently demonstrated that the gene encoding the osteopontin (OPN) protein is activated both by interleukin-3 and granulocyte-macrophage colony-stimulating factor signaling pathways and that, through binding to the cell surface receptor CD44, OPN contributes to the survival activities of interleukin (IL)-3 and GM-CSF (Lin, Y.-H., Huang, C.-J., Chao, J.-R., Chen, S.-T., Lee, S.-F., Yen, J. J.-Y., and Yang-Yen, H.-F. (2000) Mot. Cell. Biol. 20, 2734 -2742). In this report, we demonstrate that the CD44-binding domain of OPN involves a region containing amino acid residues from 121 to 140 and that both threonine and serine at positions 137 and 147, respectively, are essential for the survival stimulatory effect of OPN. Substitution of either residue with alanine results into a dominant negative mutant that overrides the survival effect of IL-3. Upon binding to the CD44 receptor, the wild-type OPN but not the inactive mutant induces activation of phosphatidylinositol 3-kinase and Akt. Last, we demonstrate that two waves of Akt activation are detected in IL-3-treated cells and that the survival promoting effect of OPN is mediated predominantly through the phosphatidylinositol 3-kinase/Akt signaling pathway. Together, our results suggest that a positive autoregulatory loop is involved in the survival pathway of IL-3.