Expression and activities of a recombinant basic fibroblast growth factor-saporin fusion protein

A fusion protein containing the full-length sequences of the mitogen, basic fibroblast growth factor (FGF-2), and the ribosome-inactivating protein, saporin (SAP), has been expressed in E. coli. As expected, it binds with high affinity to heparin-Sepharose like FGF-2 and can displace the binding of radiolabeled FGF-2 to its high affinity receptor. In contrast, the fusion protein only has much lower ribosome-inactivating activity than free saporin, although full ribosome-inactivating protein activity can be generated by proteolytic removal of the FGF-2 moiety. Cytotoxicity experiments with B16-F10 mouse melanoma cells establish that the fusion protein is active as a chemical conjugate against these intact cells. Presumably these cells have the ability to activate the SAP component of the fusion protein through an intra-cellular metabolism of the fusion protein. Because we also show the fusion protein has tumor growth inhibition properties and antimetastatic activity in in vivo models of melanoma, the findings support the hypothesis that FGF-based ligand-mediated cytotoxicity can serve to target cytotoxic agents in vivo.