Several lines of evidence have recently been presented indicating that
proteins previously referred to as Mr 62,000-67,000 transformation-associated secreted
phosphoprotein, 2ar,
osteopontin,
bone sialoprotein I, and 44K Dal bone
phosphoprotein are all very likely encoded by the same gene. We have found that markedly elevated expression of this
protein, which henceforth is referred to as secreted
phosphoprotein I (Sppl) closely correlates with neoplastic transformation of a variety of cell types and that patients with advanced stage metastatic
cancers have elevated levels of Sppl in their blood. Moreover, Sppl expression is induced in mouse epidermal cells in vitro and mouse epidermis in vivo with the
tumor promoter TPA (Craig et al, J. Biol. Chem. 264: 9682-9689, 1989). Sppl amino acid sequence deduced from
cDNA nucleotide sequence (Oldberg et al, Proc. Natl. Acad Sci. U.S.A. 83: 8819-8823, 1986) contains the
GRGDS cell-binding sequence which is known to be important for cell attachment to several adhesive
proteins found in extracellular matrices. Because of the presence of the
GRGDS cell-binding sequence in Sppl, it is probable that abnormally high expression of this soluble
protein by
tumor cells has important consequences for interactions between
tumor cells and the host tissue matrix.