Osteopontin OPN] has originally been identified from various sources, but
initially research focused on its role in mineralization and bone homeostasis and
remodelling. This work has established a critical role for OPN in regulating the
mineralization of calcium phosphate, the inorganic component of most mineralized
tissues such as bones and teeth, but also in bone resorption and in pathological
calcifications such as atherosclerosis or urinary and kidney stone. The role of posttranslational
modifications and proteolytic processing has been studied in some detail,
and OPN has been recognized as an integrin-binding protein that is involved in multiple
biological processes such as arterial calcification, cardiovascular disease, inflammation
and cancer. An intracellular form of OPN, which may mediate distinct biological
functions of the molecule, has been discovered. OPN was found to be directly involved
in regulating inflammatory and immune responses in a vast number of pathologies, as
well as the progression and metastasis of many cancers. Spurred by discoveries in the
last decade, therapeutic, diagnostic and prognostic approaches have been developed,
and OPN is arguably the SIBLING that receives the most attention from many areas in
biomedical research outside the mineralized tissues. In this chapter we attempt to
provide a general overview of the many new roles of this non-collagenous protein.
Keywords: Osteopontin, mineralization, pathological calcification, integrinbinding
protein, inflammation, cancer.
