Structure, function and nutritional potential of milk osteopontin

Osteopontin (OPN) is a multifunctional protein present in most tissues and body fluids, with the highest concentrations found in milk. Processes for isolation of OPN from bovine milk for use in infant formula have been developed and studies have investigated the effects of oral administration of milk OPN. At the same time, plasma OPN levels have been shown to be elevated in some types of cancer, and OPN has been suggested as a potential diagnostic marker for cancer. OPN exists in several different isoforms in vivo, of which presumably only a minority is directly or indirectly implicated in cancer related events. In this article, we review the differences between milk-derived OPN and OPN derived from transformed cells and compare the structure of OPN from human and bovine milk. Furthermore, current knowledge about the function of OPN in milk and recent findings about the effect of orally presented OPN is discussed.