Attenuated Salmonella Engineered with an Apoptosis-inducing Factor (AIF) Eukaryotic Expressing System Enhances Its Anti-tumor Effect in Melanoma in Vitro and in Vivo

VNP20009, an attenuated mutant of Salmonella, is potentially applied for tumor therapy due to its specific accumulation and proliferation in the hypoxic zone of tumor. However, studies have shown that human immunity system and the associated toxicities of attenuated Salmonella evidently alleviated the anti-tumor effect when tumor is reduced. As apoptosis-inducing factor (AIF) can directly induce nuclear apoptosis in the absence of caspases to avoid unwished apoptosis in normal cells, therefore, a eukaryotic expressing VNP20009-AbVec-Igκ-AIF (V-A-AIF) strain was constructed in the present study, and its anti-melanoma effects were evaluated in vitro and in vivo. The results showed that AIF expressed by the V-A-AIF strain significantly enhanced the apoptosis of B16F10 cells in vitro, seen as remarkable decrease of tumor volume, formation of larger necrotic areas, and prolongation of the lifespan in a melanoma-bearing mouse model. Furthermore, we observed that the colonization of the V-A-AIF strain and the massive expression of AIF in tumors significantly promoted apoptosis of tumor cells by upregulating the expression ratio of Bcl-2-associated X protein/B cell lymphoma-2 (Bax/Bcl-2), suppressed the inflammatory response by downregulating toll-like receptor-4/nuclear factor kappa-B (TLR-4/NFκB) signaling pathway, seen as reduction of the expressions of phosphorylated phosphoinositide 3 kinase (PI3K) and protein kinase B (AKT), and decrease of the production of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). Our study demonstrated that the colonization of the V-A-AIF strain in tumor triggers a decent anti-tumor effect in vivo and in vitro, suggesting that the engineered strain may provide a potential reagent for cancer therapy.