Characterization of urokinase plasminogen activator-activated recombinant anthrax toxin for targeted cancer therapy

Abstract

Anthrax lethal toxin (LeTx), secreted by Bacillus anthracis, consists of protective antigen (PrAg) and lethal factor (LF). Following binding to its ubiquitously expressed cell surface receptors tumor endothelium marker 8 and capillary morphogenesis gene-2, PrAg is cleaved by cell-surface furin and furin like proteases, leading to the release of a 20 kDa fragment. The resulting C-terminal 63-kDa fragment (PrAg63) then heptamerizes, binds up to three molecules of LF, and induces receptor-mediated endocytosis, followed by the release of LF into the cytosol. The urokinase plasminogen activator (uPA) and its receptor (uPAR) are overexpressed in most tumors while being absent from most normal cells under normal physiological conditions. In order to exploit these characteristics, we developed a urokinase-activated recombinant anthrax lethal toxin by replacing the furin cleavage site of PrAg with a uPA cleavage site (PrAgU2), and by fusing LF amino acids 1-254 to the ADP-ribosylation domain of Pseudomonas exotoxin A (FP59). The resulting molecule binds to all cells but should only be activated on tumor cells overexpressing uPA/uPAR. We tested the in vitro efficacy of PrAgU2/FP59 on a panel of human tumor cell lines from different tumor types using a 3H-thymidine incorporation inhibition assay. PrAgU2/FP59 was potent against a majority of non-small cell lung cancer and pancreatic cancer cell lines (80% and 75% of sensitive cell lines respectively). On the other hand, most colon, prostate, thyroid and breast cancer cell lines were not sensitive to PrAgU2/FP59. Furthermore, none of a panel of 7 normal human cells was sensitive to PrAgU2/FP59, indicating high in vitro tumor specificity for this toxin. To understand the molecular mechanisms underlying PrAgU2/FP59 toxicity, we determined total uPA levels in our cell line panels using ELISA kits. Total uPA levels correlated with cell line sensitivity to PrAgU2/FP59 in each tumor type tested. Furthermore, blocking the uPA/uPAR protease system using a monoclonal antibody directed against the active site of uPA blocked the potency of PrAgU2/FP59 by 200-fold, demonstrating dependence on high uPA/uPAR activity on cell surface for PrAgU2/FP59 potency. The above results indicate that PrAgU2/FP59 is a new agent for the potential targeted therapy of several types of cancer.