ABSTRACT
Osteopontin (Latin for bone bridging molecule) is a ubiquitous extracellular matrix protein. It is involved in bone remodeling by mediating adhesion or migration of bone cells and by regulating calcification. Osteopontin is also excreted by the kidneys into urine, where its calcium binding capacity may counteract stone formation. Furthermore, the same protein, independently denominated 2ar, is secreted by malignant tumors. There it may mediate migration and homing, and it may confer protection against killing by macrophages through down-regulation of inducible nitric oxide synthetase. Finally, the cloning of genes that are selectively expressed in activated CD4+ T-lymphocytes has identified a cytokine termed Eta-1 (for early T-lymphocyte activation-1) as the predominant transcript, which is also identical to Osteopontin. The cytokine properties expand the physiologic role of the protein beyond the known functions in cell adhesion and spreading, calcium homeostasis, and down-regulation of inducible nitric oxide synthetase to specific regulation of immune responses (Denhardt et al., 1993; Patarca et al., 1993).
