Abstract LB-367: Bioluminescent transposon screen identifies cancer cell specific promoters in Salmonella typhimurium

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL The ability of Salmonella Typhimurium to specifically localize to malignant tumors in vivo has been well documented and offers a powerful avenue for targeted anti-cancer therapies. S. Typhimurium, a common gastrointestinal pathogen, utilizes a complex array of type III secretion protein complexes, effector molecules and transcription factors during its normal pathophysiology. The expression of these genes is highly regulated in a spatiotemporal manner in response to the host environment. Akin to other host environments, such as the gastrointestinal tract and phagosomes, tumors have specific extracellular properties such as low pH and hypoxia. Identifying S. Typhimurium genes and their promoters that respond specifically to these tumor environmental properties has served as the basis for several lines of research with the hope of utilizing S. Typhimurium for anti-cancer based therapies. To identify such genes that are regulated in a tumor specific manner, we engineered a bioluminescent transposon reporter-trap to screen a S. Typhimurium library for genes specifically regulated by co-culture with malignant cells in vitro. Five previously undiscovered genes were identified (adiY, yohJ, STM1787, STM1791, and STM1793) and their promoter sequences were found to be specifically activated by the acidic microenvironment associated with cancer cells in vitro and tumors in vivo. Three of the genes (STM1787, STM1791, and STM1793) are controlled by a single promoter, which contains a recently discovered conserved tusp motif. We have uncovered that transcriptional regulation of this motif is more complex as it is sensitive to low pH in addition to hypoxia as was originally ascribed. The genes identified herein are highly expressed in an acidic pH tumor environment, but are not required for bacterial tumor targeting. Therefore, the promoters regulating these genes may be ideal candidates for utilization in therapeutic gene, pro-drug or toxin delivery studies. To address this inquiry, we utilized the cancer cell-activated STM1787 promoter sequence to regulate the expression of a toxic tumor transgene in a wild type strain of S. Typhimurium. Shiga-like toxin 2 (Stx2) is a holoenzyme produced by some strains of E.coli. It has been extensively studied for the potent translational inhibition characteristics of the A subunit in addition to the high tumor cell binding-specificity of the B subunit. Conditionally regulating the tumor-specific expression of Stx2 by STM1787's promoter resulted in striking cancer cell death in vitro, and shows promise in mouse xenografts in vivo. In conclusion, we have identified novel tumor-specific genes expressed by S. Typhimurium, identified the acidic pH-responsiveness of the tusp motif, and demonstrated that these promoters can be successfully engineered for targeted anti-cancer therapy using live S. Typhimurium. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-367. doi:1538-7445.AM2012-LB-367