Abstract
The herpes simplex virus thymidine kinase/ganciclovir (HSV-sr39tk/GCV) system is a well-established prodrug system used in cancer gene therapy. However, this system is currently not effective enough to eradicate malignant tumors completely. This study aimed to evaluate whether co-expression of interleukin-3 (IL-3) could enhance the anti-tumor activity of HSV-sr39tk/GCV prodrug gene therapy using a murine TRAMP-C1 prostate tumor model. In vitro results demonstrated that HSV-sr39tk-transfected cells exhibited enhanced sensitivity to the GCV prodrug, which was not affected by co-expression of the mIL-3 gene. However, in vivo studies showed that co-expression of the mIL-3 gene significantly increased the HSV-sr39tk/GCV-induced tumor growth delay and even cured the tumor. The TRAMP-C1-specific immune response of spleen lymphocytes from mice bearing HSV-sr39tk- and IL-3-expressing TRAMP-C1 tumors was measured by ELISA. Results showed that IL-3-activated IL-4-dominant lymphocytes became IFN-γ- dominant lymphocytes after combined HSV-sr39tk/GCV therapy. The efficacy of combined therapies on tumor regression was reduced when macrophages populations were depleted by carrageenan or NO production was inhibited by administration of the iNOS inhibitor, L-NAME. These results suggest that utilizing a bicistronic vector to express HSV-sr39tk and the IL-3 gene induced an enhanced macrophage- or NO-dependent anti-tumor effect.
Highlights
Proposed by Moolten in 1986, the activation of a suicide gene encoding an enzyme protein that is nontoxic to genetically modified cells has become an extensively adopted approach for prodrug gene therapies designed to treat cancer [1,2]
This indicates that the sr39tk protein was expressed in both TRAMP-C1/sr39tk and TRAMP-C1/mIL3-sr39tk cells and the response to GCV was not modified by co-expression of the murine IL-3 (mIL-3) gene
We demonstrate that bicistronic expression of IL-3 and HSV-sr39tk/GCV suicide gene system exerts a synergistic effect inhibiting the growth of TRAMP-C1 prostate cancer cells in vivo, but not in vitro
Summary
Proposed by Moolten in 1986, the activation of a suicide gene encoding an enzyme protein that is nontoxic to genetically modified cells has become an extensively adopted approach for prodrug gene therapies designed to treat cancer [1,2]. The bystander effect promotes tumor cell death, inefficient activation of GCV by HSV-tk and prodrug-associated negative side effects limit the clinical efficacy of this system. Dm30-tk and sr39-tk, have been extensively studied in both in vitro and in vivo models These studies have demonstrated that cells transfected with either of the mutant enzymes were more sensitive to the cytotoxic effects of GCV and acyclovir (ACV) when compared to cells transfected with wild-type HSV-tk. In addition to improving GCV activation, combining the HSV-tk/GCV system with other strategies, such as cytokine therapy, has been demonstrated in several tumor systems to be more effective than using a single treatment [10,11,12,13]
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