Osteopontin (OPN) is a phosphorylated acidic (Arg-Gly-Asp) RGD-containing
glycoprotein, which exists both as an immobilized extracellular matrix component and as a
soluble molecule. The biological functions of OPN are extensively regulated on the posttranscriptional
and post-translational levels and many of the signaling pathways mediated by
secreted OPN are activated by ligation of the integrin and CD44 families of receptors. Such a
multifaceted glycoprotein, that is expressed by numerous different cells and tissues, is expected
to exert pleiotropic functions. Indeed, OPN is implicated in tumor metastases, tissue
remodeling, inflammation, and cell-mediated immunity. Recently, substantial evidence
suggests that OPN positively regulates angiogenesis. However, the mechanisms that define the
role of this molecule in angiogenesis are incompletely understood. The following review will
discuss the biochemical and biological properties of OPN in the context of its role in the
modulation of angiogenesis.
