Molecular cloning, expression and purification of truncated midkine and its growth stimulatory activity on Wilms’ tumor (G401) cells

Midkine (MK) is a heparin binding growth factor identified as a product of a retinoic acid-responsive gene; it is frequently expressed at high levels in many human carcinomas. Although the expression of the mRNA encoding truncated MK (tMK) in unique human cancer cells has been reported, the tMK polypeptide itself has not yet been identified. In order to clarify the biological role of tMK, recombinant tMK was expressed in Escherichia coli and purified. Recombinant tMK was purified as a single band in SDS-PAGE under reducing conditions showing an apparent molecular mass of 10 kDa. Purified recombinant tMK showed the same extent of proliferative activity towards Wilms’ tumor (G401) cells as full length human MK. These results suggest that the structure of this recombinant tMK is same as the native polypeptide.