Osteopontin.

Although osteopontin (OP) has been shown to play a role in bone mineralization and to mediate bone cell adhesion, its function in other tissues is not yet known. The sequences of OP from seven species have been reported; some of the sequences that are conserved in all seven species and their functions are mentioned. The biochemical structure of OP and the functional properties of its motifs make OP a strong candidate for regulating mineralization and/or mediating local cell dynamics. In addition to its role in mineralization, OP has also been shown to promote migration of smooth muscle cells and macrophages. OP expression is high in many tumors, and it correlates with the metastatic potential in some instances. Abundant OP has also been found in human tissue specimens from patients with clinical tuberculosis and in other granulomatous diseases. Experimental approaches in the authors' laboratory have focused on the role of OP as an autocrine motility factor in osteoclasts and human melanoma cell lines; their results suggest that posttranslational modification (phosphorylation) of OP is important in its biological functions.