A conjugate between human urokinase and saporin, a type-1 ribosome- inactivating protein, is selectively cytotoxic to urokinase receptor- expressing cells

Urokinase-type plasminogen activator (uPA) confers invasive potential to transformed cells. Cancer cells express high numbers of uPA receptors (uPARs), which concentrate uPA activity at the invasive edge of cancer cells and the tumor mass. We synthesized a conjugate between human uPA and saporin (SAP), a ribosome-inactivating protein produced by Saponaria officinalis. Results of cell-killing assays showed that uPA is very effective at targeting saporin specifically to uPAR-expressing cells, whereas cell lines devoid of uPARs were not affected by the conjugate. Receptor-bound uPA is internalized only upon formation of a complex with one of its inhibitors (PAIs). However, our conjugate was highly cytotoxic even when the interaction between uPA and PAIs was prevented. Moreover, the α₂-macroglobulin receptor, which has been reported to mediate the internalization of uPA·PAI complexes, seems not to be involved in cell killing caused by the uPA·SAP conjugate. Thus, uPA·SAP might follow a mechanism of internalization different from that of unconjugated uPA complexed to PAIs, although still uPAR-mediated. Our results also suggest that α₂-macroglobulin and/or its receptor could mediate the internalization and cytotoxicity of unconjugated saporin, as it has been shown for other toxins.