Osteopontin is required for full expression of the transformed phenotype by the ras oncogene

The secreted phosphoprotein osteopontin (OPN) is strongly associated with the process of neoplastic transformation, based both on its pattern of expression in vivo and in vitro and on functional analyses. We have used 3T3 cells derived from wildtype and OPN-deficient mice and transformed by transfection with oncogenic ras to assess the role of OPN in transformation in vitro and in tumorigenesis in vivo. There was no effect of an absence of OPN on the ability of the cells to undergo immortalization or to form morphologically transformed foci following ras transfection. Wildtype and OPN-deficient cell lines were established from such foci, and lines with similar ras mRNA levels selected for further analysis. Ras-transformed cell lines from both wildtype and OPN-deficient mice could form colonies in soft agar indicating that this process can occur in the absence of OPN. However, the ability of the OPN-deficient cell lines to form colonies was reduced as compared to wildtype cell lines. Tumorigenesis in syngeneic and nude mice was assessed for a subset of cell lines that formed colonies efficiently in soft agar. Cell lines unable to make OPN formed tumors in these mice much more slowly than wildtype cells, despite similar growth of the cells on plastic and in soft agar. Taken together, these results indicate that maximal transformation by ras requires OPN expression, and implicate increased OPN expression as an important effector of the transforming activity of the ras oncogene.