| Background: Colorectal carcinoma is one of the most common cancer in American and Western Europe and the incidence is high in recent years, as well as in China, the advanced colorectal cancer is common and many patients died of recurrent or nonresectabale tumors. Suicide genes have played an important role in gene therapy against tumor. Many reports have demonstrated that after transfering suicide gene (HSV-tk, CD, dCK) into carcinoma cells, obvious carcinoma killing effects though prodrug transforming has been observed. However, the key to gene therapy of suicide gene for carcinoma is targeting expression in tumor cells. If suicide genes expressed in normal cells, it might create side effects through pro-drug transformation just as chemotherapy did. It was suggested that transcriptional regulatory sequences of house-keeping gene such as CEA gene could make foreign gene express constantly at a high level in some special tissues which CEA producing and such expression would produce an obvious target-killing effect on the tumor which CEA producing. But what about this target killing effect on the tumor tissue which CEA producing low compare to the tumor tissue which CEA producing high? Aims: To investigate the target killing effect of CEA (carcinoembryonic antigen) tissue-specific CD ( cytosine deaminase ) / 5-FC( 5-fluorocytosine ) system on human colorectal cancinoma cells in which CEA registered a different expression. Methods: After amplification, the Recombinant retro viral vector GlCEACDNa, in which the CD gene was controlled under the CEA promoter, was digested by Sal I and retroviral vector pCD2 was digested by BamH I and EcoR I respectively. The recombinant retroviral vectors were packaged in PAS 17 cells with liposome technique and then filters which diameter were 0.45 um were used to harvest the viral supernatant. Hella cell line was transfected to determine the CFU (colony forming unit ) of the viral supernatant. Then the recombinant retroviral vector GlCEACDNa and retroviral vector pCD2 were introduced through liposome technique respectively to the human colorectal carcinoma cell lines LoVo and SW480 ( in which CEA was expressed highly and low respectively), and then the cells were selectively cultured in G418. The proliferated colonies were treated with 5-FC. Then 5 106 parental cells and the cells transfected with pCD2 retroviral vectors and GlCEACDNa retroviral vectors were injected into flanking Balb/c nude mice respectively. 500 mg / kg 5-FC were given ip daily for 21ds when the rumors were palpable. All the mice were sacrificed at the end of the treatment. Then RT-PCR was performed to detect the expression of targeted gene in carcinoma tissue and pathological analysis was made. The human colorectal carcinoma cell lines LoVo and SW480 were injected into flanking Balb/c nude mice respectively. 0.2 mL viral supernatant were injected into tumors daily for 3 d and 500 mg / kg 5-FC were given ip daily ( 21 d ) when the tumors were palpable. RT-PCR was performed to detect the expression of targeted gene in carcinoma tissue and pathological analysis was made too. Results: After transfection, LoVo-CEACD cells and LoVo-CD cells were more sensitive to 5-FC than their parental cells (P < 0.01, t =5.688, n =9; P<0.01, t =3.136, n =9) , and SW480-CEACD cells and SW480-CD cells were more sensitive too than their parental cells (P<0.01, t =3.437, n =9; P<0.01, t =3.516, n =9) . Furthermore, the LoVo-CEACD cells were more sensitive to 5-FC than the LoVo-CD cells (P<0.05, t=2.183, n =9) while the SW480-CEACD cells were less sensitive than SW480-CD cells. The CEA tissue-specific CD/5-FC system displayed a higher anti-tumor effect on LoVo-CEACD cells than that on SW480-CEACD cells in vitro. Bystander effect in all cells transfected with CD gene were conveyed in our study. The targeted genes were detected in tumor tissues. An obvious antitumor effect was observed in nude mice bearing tumors transfected with ceacd or cd genes than that in the mice bearing tumors not transfected with targeted genes i...
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