Inhibition of liver metastasis of colon cancer in mice by oral DNA vaccine pcDNA3.1(+)/flk-1_((n1-7))

Background and purpose:Vascular endothelial growth factor (VEGF)and vascular endothelial growth factor receptor-2(VEGFR-2, mouse flk-1)play an important role in tumor-associated angiogenesis in both tumor growth and metastasis, This study was done to test the impacts of oral DNA vaccine pcDNA3.1(+)/flk-1_ (n1-7) on tumor metastasis by inhibiting angiogenesis and investigate its mechanism in vivo. Methods:① Extracellular domain of flk-1 was amplified by RT-PCR, and then the flk-1_ (n1-7) cDNA was inserted into eucaryotic expression vector pcDNA3.1(+) so that the recombinant plasmids pcDNA3.1(+)/flk-1_ (n1-7) was constructed.② The recombinant plasmid pcDNA3.1(+)/flk-1_ (n1-7) was transformed into attenuated Salmonella typhimurium SL3261 in order to develop an oral DNA vaccine against the extracellular domain of flk-1.③ Mice were divided into 3 groups randomly(vaccine group, empty-vector group and saline group)and were respectively orally immunized with either SL3261 transformed with the recombinant plasmid, pcDNA3.1+ or saline for 3 times with 2-wk intervals apart from each other Mice were inoculated with colonic adenocarcinoma cells (CT-26) by intrasplenic injection 2 weeks after the last immunization. Mice were sacrificed and liver metastasis were analyzed fifteen days after tumor cell inoculation.Results:① We successfully constructed a recombinant plasmid DNA encoding extracellular domain of flk-1. ② We developed an oral DNA vaccine against the extracellular domain of flk-1 carried by attenuated Salmonella typhimurium SL3261.③ There were much more liver metastasises in the vector group and saline group than in the vaccine group (P0.05).Conclusions:We successfully constructed the recombinant plasmid pcDNA3.1(+)/flk-1_ (n1-7) and developed an oral DNA vaccine against the extracellular domain of flk-1 carried by attenuated Salmonella typhimurium SL3261. The oral DNA vaccine can reduce the possibility of liver metastasises of colon cancer.