Introduction
An excellent approach for spatiotemporal targeting of tumors, regardless of their origin or location, is application of genetically modified non-pathogenic anaerobic bacteria (e.g. SalmonellaE. coliClostridium, Bifidobacterium ) which can be preferentially localized and proliferative in tumor site (1). Contrary to the conventional cancer therapies, bacteria can also penetrate easily in tumor site and be engineered in a way to respond in conditions specific to the tumors (2). Despite this, rendering these genera of bacteria in to non-toxic forms for human results in attenuation of their toxicity against cancer cells (3). Therefore, numerous strategies have been applied for further improving their anti-tumoral activities. One such a strategy is to engineer bacteria to secret specific group of cytolytic proteins such as toxins (e.g. Staphylococcus aureus  alpha hemolysin) or enzymes with capability of converting non-toxic prodrugs in to cytotoxic ones (e.g. E. coli  cytosine deaminase, converting 5-Fluorocytosine in to 5-Fluorouracil) (4).
An even more specific cancer targeted therapy could be achieved through the application of specific promoters capable of controlling gene expression in response to specific inducers such as hypoxia. As a common feature of most aggressive tumors, hypoxia is clinically associated with poor clinical outcomes and development of resistance to most chemotherapeutics and radiotherapy (5). Recently, Nasr and Eidgahi have successfully developed an engineered nirB promoter without any responding region for nitrite and nitrate, making it preferable and selective for expression only under hypoxic tumor condition but not in the presence of chemical inducers (6).
Long acting natriuretic peptide (LANP) together with kaliuretic peptide (KP) and vessel dilator (VDL) comprise three of the four cardiac peptides which have shown to suppress the growth of up to 97% of cancer cells in vitro. 80% of human pancreatic adenocarcinomas together with 86% of human small-cell lung carcinomas (SCLC) have shown to be successfully eradicated in athymic mice through the application of these peptides. Furthermore, administration of cardiac peptides significantly reduced the number of human breast adenocarcinoma cells within 24h of application in vitro. Among these peptides, Vessel dilator appears to hold the strongest anticancer property, reducing up to 97% of human prostate cancer cell throughout the first 24 hours (7-9).
In presented study, we developed a novel spatiotemporal cancer targeted therapy applying engineeredE.coli bacteria with capability of expressing cardiac peptides under hypoxic condition of tumor. Creating a hypoxia-inducible system, the cardiac peptide genes were coupled to the synthetic nirB promoter sequence reported by Nasr and Eghbali. The stimulation of synthetic nirB promoter was demonstrated using fluorometry and the expression and function of cardiac peptides were demonstrated by immunoblotting and cell culturing. A murine breast cancer model was used to investigate the potency of hypoxia-inducible cardiac peptide secretion using E. coli in suppressing tumor growth.