The FASEB Journal, Vol 11, 1169-1176, Copyright © 1997 by The Federation of American Societies for Experimental Biology

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The amino-terminal fragment of human urokinase directs a recombinant chimeric toxin to target cells: internalization is toxin mediated

MS Fabbrini, D Carpani, I Bello-Rivero and MR Soria
Dibit-Department of Biological and Technological Research, San Raffaele Scientific Institute, Milano, Italy. fabbris@dibit.hsr.it

In contrast to two-chain urokinase (uPA), a chemical conjugate between uPA and native saporin (a cytotoxic plant seed ribosome-inactivating protein) did not require plasminogen activator inhibitors to be internalized. To dissect this pathway, we constructed a chimera consisting of the amino-terminal fragment (ATF) of human urokinase fused to a saporin isoform (SAP-3). The chimeric ATF-SAP toxin was expressed in Escherichia coli, purified, and characterized for its ribosome-inactivating activity. Besides being a potent inhibitor of protein synthesis in cell-free assays, ATF-SAP was specifically cytotoxic toward cells expressing human uPAR. Competition experiments indicated that both the human uPAR and the LDL-related receptor protein are involved in mediating the cell killing ability of ATF-SAP. We conclude that neither plasminogen activator inhibitors nor the catalytic moiety of urokinase are necessary to initiate these internalization pathways. Thus, saporin may play a role similar to plasminogen activator inhibitors in its ability to trigger internalization of uPAR-bound ligands through endocytic receptors.

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