Molecular Mechanisms of Reovirus Oncolysis
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Mammalian reovirus is a naturally benign virus that preferentially replicates in cancer cells (reovirus oncolysis) and has been tested as a potential cancer therapy in vitro, in vivo and in clinical trials for treating cancers from a wide range of origins. Reovirus-induced apoptosis has been shown to be important for reovirus oncolysis. The tumor suppressor protein p53 plays vital roles in mediating host apoptosis but its effect on reovirus oncolysis has not been fully understood and hence investigated. Data here show that p53 does not affect reovirus replication or reovirus-induced apoptosis in human cancer cells. However, significant enhancement of the reovirus-mediated apoptosis is induced by addition of p53 accumulators/activators such as a MDM2 antagonist Nutlin-3a or sub-lethal concentrations of chemotherapy drugs. This enhanced cell death is p53-dependent, requires NF-kappaB activation and p53 target genes p21 and bax. Furthermore, a combination of reovirus and p53 accumulators/activators directly results in significantly higher level of reovirus dissemination and spread. One of the hurdles for current chemotherapy in patients is the side effects caused by high concentrations of cytotoxic drugs. Hence, a therapeutic regime using the combination of reovirus and sub-lethal (lower concentrations of) chemotherapeutics that induce p53 activation/accumulation potentially can both enhance tumor regression and reduce the side effects in patients. Ras mutation, one of the most prevalent mutations in human cancer, has been implicated to determine the susceptibility of cancer cells to reovirus infection. However, the underlying mechanism of reovirus preferential replication needs to be further delineated. Quantitative analysis was used to compare individual steps of reovirus replication between non-transformed and Ras-transformed NIH 3T3 cells. Contrary to previous reports, reoviral protein synthesis is shown to be comparable between non-Ras and Ras-transformed cells. Meanwhile, although reovirus binding and internalization is not affected by Ras-transformation, reovirus uncoating is enhanced in Ras-transformed cells. Ras-transformation also enables reovirus to better spread to neighboring cells through apoptosis. Furthermore, reovirus infection of Ras effector mutant cells that activate specific Ras effector pathways indicate that sub-Ras pathways play different roles in enhancing reovirus preferential replication in Ras-transformed cells and therefore provide additional targets for cancer therapy.