Osteopontin/OPN: Proteins and Enzymes

Osteopontin (OPN), also known as bone sialoprotein (BSP), is a secreted SIBLING family protein that can be variably modified by O- and N-glycosylation, sulfation, phosphorylation, and transglutamination. OPN is widely expressed and is prominent in mineralized tissues. It inhibits bone mineralization and kidney stone formation and promotes inflammation, cell ad­hesion, and migration. Its expression is upregulat­ed during inflammation, obesity, atherosclerosis, cancer, and tissue damage and contributes to the pathophysiology of these conditions. The central region of OPN contains RGD and non-RGD binding sites for multiple integrins. Adjacent to the RGD motif is the sequence SLAYGLR (SVVYGLR in human) which serves as a cryptic binding site for additional integrins: it is masked in full length OPN but is exposed following OPN cleavage by multiple proteases in tumors and sites of tissue injury.