Saporin is a type I
ribosome-inactivating protein that is often appended with a cell-binding domain to specifically target and kill
cancer cells.
Urokinase plasminogen activator (uPA)-
saporin, for example, is an anticancer toxin that consists of a chemical conjugate between the human uPA and native
saporin. Both
saporin and uPA-
saporin enter the target cell by endocytosis and must then escape the endomembrane system to reach the cytosolic ribosomes. The latter process may represent a rate-limiting step for intoxication and would therefore directly affect toxin potency. In the present study, we document two treatments (
shock with
dimethylsulfoxide and lipopolyamine coadministration) that generate substantial cellular sensitization to
saporin/uPA-
saporin. With the use of lysosome-endosome X (LEX)1 and LEX2 mutant cell lines, an endosomal trafficking step preceding cargo delivery to the late endosomes was identified as a major site for the
dimethylsulfoxide-facilitated entry of
saporin into the cytosol.
Dimethylsulfoxide and lipopolyamines are known to disrupt the integrity of endosome membranes, so these
reagents could facilitate the rapid movement of toxin from permeabilized endosomes to the cytosol. However, the same pattern of toxin sensitization was not observed for
dimethylsulfoxide- or lipopolyamine-treated cells exposed to
diphtheria toxin,
ricin, or the catalytic A chain of
ricin. The sensitization effects were thus specific for
saporin, suggesting a novel mechanism of
saporin translocation by endosome disruption. Lipopolyamines have been developed as in vivo gene therapy vectors; thus, lipopolyamine coadministration with uPA-
saporin or other
saporin conjugates could represent a new approach for anticancer toxin treatments.