Thrombin-cleaved COOH-terminal osteopontin peptide binds with cyclophilin C to CD147 in murine breast cancer cells

Osteopontin is a glycoprotein that has been linked to metastatic function in breast, lung, and prostate cancers. However, the mechanism by which osteopontin acts to induce metastatic properties is largely unknown. One intriguing feature of osteopontin is the presence of a conserved thrombin cleavage site that is COOH-terminal from a well-characterized RGD domain. Although the COOH-terminal fragment may bind to cell surface CD44 receptors, little is known about the COOH-terminal osteopontin fragment. In the current study, we use the murine mammary epithelial tumor cell lines 4T1 and 4T07; these cells are thioguanine-resistant sublines derived from the parental population of 410.4 cells from Balb/cfC3H mice. Using flow cytometry and Forster resonance energy transfer, we show that the COOH-terminal fragment of osteopontin binds with another marker of metastatic function (cyclophilin C or rotamase) to the CD147 cell surface glycoprotein (also known as extracellular matrix metalloproteinase inducer), to activate Akt1/2 and matrix metalloproteinase-2. In in vitro assays, thrombin cleavage of osteopontin to generate short COOH-terminal osteopontin in the presence of cyclophilin C increases migration and invasion of both 4T07 and 4T1 cells. This interaction between osteopontin peptide and cyclophilin C has not been previously described but assigns a heretofore unknown function for the thrombin-cleaved osteopontin COOH-terminal fragment.