Truncated midkine induces transformation of cultured cells and short latency of tumorigenesis in nude mice

To test whether truncated midkine (tMK), an alternative spliced form of exon 3, induces the transformation of mammalian cells, tMK cDNA was transfected into SW-13 cells. Although, the growth of MK transfectant (SW-13/MK) cells was close to those of the parent SW-13 and vector transfectant (SW-13/empty) cells, the growth of tMK transfectant (SW-13/tMK) cells was significantly promoted compared with that of the above three cells. Both SW-13/tMK and SW-13/MK formed colonies in 0.35% soft agar, indicating that tMK and MK induce mammalian cell transformation. SW-13/tMK frequently formed solid tumor after its subcutaneous injection into nude mice. Additionally, in SW-13/tMK and SW-13/MK-injected mice, advanced visible tumors were detected compared with that in the case of SW-13/empty-injected mice as control. These findings indicate that tMK induces mammalian cell transformation and promotes tumor establishment in vivo.