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Abstract
Osteopontin is a secreted glycoprotein with a multidomain structure and functions
characteristic of a matricellular protein. Osteopontin interacts with cell surface
receptors via arginine-glycine-aspartate (RGD)- and non-RGD containing adhesive domains,
in addition to binding to components of the structural extracellular matrix. While
normally expressed in bone and kidney, osteopontin levels are elevated during wound
healing and inflammation in most tissues studied to date. Since 1986, over one thousand
studies have been published on osteopontin, including recent experiments in osteopontin-deficient
mice. These studies reveal osteopontin as a cell adhesive, signaling, migratory, and
survival stimulus for various mesenchymal, epithelial, and inflammatory cells, in
addition to being a potent regulator of osseous and ectopic calcification. Based on
these reports, a general picture of osteopontin as an important regulator of inflammation
and biomineralization is emerging. A common denominator in osteopontin function in
these situations is its ability to regulate the function of macrophage and macrophage-derived
cells (i.e. osteoclasts). While we have learned much about osteopontin and the processes
it appears to regulate over the past decade, many questions regarding this important
multifunctional protein remain unanswered and provide important directions for future
studies.