Oncolytic Vesicular Stomatitis Virus Induces Apoptosis in U87 Glioblastoma Cells by a Type II Death Receptor Mechanism and Induces Cell Death and Tumor Clearance In Vivo

Vesicular stomatitis virus (VSV) is a potential oncolytic virus for treating glioblastoma multiforme (GBM), an aggressive brain tumor. Matrix (M) protein mutants of VSV have shown greater selectivity for killing GBM cells versus normal brain cells than VSV with wild-type M protein. The goal of this research was to determine the contribution of death receptor and mitochondrial pathways to apoptosis induced by an M protein mutant (M51R) VSV in U87 human GBM tumor cells. Compared to controls, U87 cells expressing a dominant negative form of Fas (dnFas) or overexpressing Bcl-X(L) had reduced caspase-3 activation following infection with M51R VSV, indicating that both the death receptor pathway and mitochondrial pathways are important for M51R VSV-induced apoptosis. Death receptor signaling has been classified as type I or type II, depending on whether signaling is independent (type I) or dependent on the mitochondrial pathway (type II). Bcl-X(L) overexpression inhibited caspase activation in response to a Fas-inducing antibody, similar to the inhibition in response to M51R VSV infection, indicating that U87 cells behave as type II cells. Inhibition of apoptosis in vitro delayed, but did not prevent, virus-induced cell death. Murine xenografts of U87 cells that overexpress Bcl-X(L) regressed with a time course similar to that of control cells following treatment with M51R VSV, and tumors were not detectable at 21 days postinoculation. Immunohistochemical analysis demonstrated similar levels of viral antigen expression but reduced activation of caspase-3 following virus treatment of Bcl-X(L)-overexpressing tumors compared to controls. Further, the pathological changes in tumors following treatment with virus were quite different in the presence versus the absence of Bcl-X(L) overexpression. These results demonstrate that M51R VSV efficiently induces oncolysis in GBM tumor cells despite deregulation of apoptotic pathways, underscoring its potential use as a treatment for GBM.