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

The secreted phosphoprotein osteopontin (OPN) is strongly associated with t he process of neoplastic transformation, based both on its pattern of expre ssion in vivo and in vitro and on functional analyses. We have used 3T3 cel ls derived from wildtype and OPN-deficient mice and transformed by transfec tion with oncogenic ras to assess the role of OPN in transformation in vitr o and in tumorigenesis in vivo. There was no effect of an absence of OPN on the ability or the cells to undergo immortalization or to form morphologic ally transformed foci following ras transfection. Wildtype and OPN-deficien t cell lines were established from such foci, and lines with similar ras mR NA levels selected for further analysis. Ras-transformed cell lines from bo th wildtype and OPN-deficient mice could form colonies in soft agar indicat ing 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 t o wildtype cell lines. Tumorigenesis in syngeneic and nude mice was assesse d 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 transformat ion by ras requires OPN expression, and implicate increased OPN expression as an important effector of the transforming activity of the ras oncogene. (C) 2000 Cancer Research Campaign.