Studies on the anti-metastatic effects of genetically engineered mesenchymal stem cells and their working mechanism
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The metastasis of cancer is a leading cause of death, but conventional cancer treatments do not efficiently eliminate established metastatic tumors. Therefore, alternative therapeutic options to eliminate these relapsed and metastatic tumors are required. Recently, mesenchymal stem cells (MSCs) have been evaluated for the targeted delivery of anti-cancer agents, based on their ability to migrate to tumor. MSCs are hypo-immunogenic and exhibited prolonged transgene expression due to long life-span in vivo. These recent findings suggested that MSCs could be utilized as a cellular vehicle for cancer gene therapy and vaccine. Here are two different novel strategies for curing metastatic tumors. First, I engineered MSCs to express dodecameric TNF-related apoptosis-inducing ligand (dTRAIL) and/or herpes simplex virus thymidine kinase (HSV-TK) for the treatment of pulmonary metastatic renal cell carcinoma. MSCs expressing dodecameric TRAIL (MSC/dTRAIL) exerted cytotoxic activity towards two different tumor cell lines, but not towards MSCs themselves, indicating tumor cell specificity. Furthermore, MSCs co-expressing dTRAIL and HSV-TK (MSC/dTRAIL-TK) significantly enhanced the apoptosis of tumor cells after ganciclovir (GCV) treatment, compared to MSC/TK or MSC/dTRAIL. MSCs appeared to migrate more selectively into the tumor nodules within the lung of metastatic tumor-bearing mice, compared to that of control mice, regardless of genetic engineering. In addition, MSC/dTRAIL-TK treatment significantly decreased the number of tumor nodules, to a greater degree than MSC/dTRAIL or MSC/TK, and led to a prolonged survival. More importantly, the anti-metastatic effect of MSC/dTRAIL-TK was markedly enhanced by repeated injection, but not by increased dose, and resulted in 100% survival of tumor-bearing mice after three injections. I added more supporting evidences for the mechanisms involved in the achievement of long-term remission by MSC/dTRAIL-TK. The second approach showed MSCs transduced with interleukin-12M (MSCs/IL-12M) combined with irradiation. One of the alternative strategies is immunotherapy using immune system or their components to attack cancer cells. In cancer immunotherapy, cytokines have been widely tried for its ability to regulate the cross talk between innate and adaptive immunity, leading to induction of antitumor immunity. This combination approach was resulted in anti-metastatic effects with inhibition of tumor growths effectively and significantly enhanced survival rates. I examined a potential strategy to treat the murine malignant hepatoma by combination treatment of IL-12M expressing mesenchymal stem cells (MSC/IL-12M) with irradiation. Irradiation increases the expression of chemokines that could enhance the recruitment of MSC into the tumor microenvironment. IL-12 expression was increased in tumor favorably, and it induced the proliferation of cluster of differentiation (CD) 8+ T-lymphocytes. Increasing apoptotic activity indicated the antitumor effect to suggest a cytotoxicity of immune cells. That is, antitumor activity is efficiently enhanced by combination treatment of MSC/IL-12M with irradiation. Therefore, I suggest that a combination treatment of MSC/IL-12M with irradiation could be the optimal strategy to treat malignant hepatoma through the synergy of both treatments. And also, clinically relevant fractionated irradiation increased the tropism for and localization of MSC in the tumor microenvironment. In conclusion, MSC-mediated sequential combination gene therapy achieved long-term remission of metastatic renal cell carcinoma (RCC) and Hepatocellular carcinoma (HCC) without noticeable toxicity. All of my findings provide innovative therapeutic options by using engineered MSCs alone to completely eradicate metastatic tumors by simple and repeated administrations in chemo- and/or radiation-resistant cancer patients.