Antitumor immunotoxin expression is enhanced by Escherichia coli csrB-promoter activity

In bacterial oncotherapy, tumor-targeting bacteria deliver cytotoxins that induce cancer-cell apoptosis, requiring exogenous cues to induce such cytotoxins. We mined the part of the Escherichia coli genome regulating immunotoxin (anticancer protein) to maximize tumor-specific activity. E. coli was introduced into a mouse tumor model, and RNA-seq analysis was performed. csrB, encoding small regulatory RNA, was highly upregulated in tumors. Genes controlled by csrB participate in acetate metabolism, enriched in the tumor microenvironment. qPCR of in vitro bacterial culture revealed that csrB expression depended on acetate levels. The csrB-promoter regulated acetate-controlled expression of β-galactosidase. For E. coli-mediated oncotherapy, we therefore selected the csrB promoter to regulate a recombinant form of immunotoxin, psp-TGFα-PE38, comprising TGFα, Pseudomonas exotoxin A, with a secretion tag (psp). Under csrB-promoter control, TP was notably expressed when acetate was present. Tumor-cell viability was dramatically reduced following treatment with the TP-containing bacterial-culture supernatant. TP was continuously present in the tumors of CT26-tumor-bearing mice administered TP-expressing E. coli. When exogenous stimuli were absent and TP was expressed by E. coli, tumor growth was substantially retarded, and the survival period increased. Tumor-colonizing bacteria thus offer promise in sensing tumor conditions and altering antitumor protein expression, potentially improving outcomes.