Apoptosis in Murine Norovirus-Infected RAW264.7 Cells Is Associated with Downregulation of Survivin

Noroviruses (NVs) are recognized as a major cause of nonbacterial gastroenteritis in humans. Studies of the human NVs continue to be hampered by the inability to propagate them in any cell culture system. Until recently, most data concerning NV replication were derived from studies of feline calicivirus and rabbit hemorrhagic disease virus, which are cultivable members of the family Caliciviridae. From such studies, it was proposed that caliciviruses induce apoptosis to facilitate the dissemination of viral progeny in the host. The discovery that MNV type 1 (MNV-1) grows in RAW264.7 cells provided the first cell culture system for use in studying the role of apoptosis in NV infection. We first showed that MNV-1 replication triggered apoptosis in infected RAW264.7 cells and then demonstrated that cell death was associated with activation of caspase-9 and caspase-3 through the mitochondrial pathway. This process was dependent on virus replication, since inactivated virus failed to induce signs of apoptosis. In order to better understand the apoptotic process induced by MNV-1 infection of RAW264.7 cells, we investigated the expression profiles of MNV-1-infected versus mock-infected cells. Survivin, a member of the inhibitor of apoptosis protein family, was found to be significantly downregulated in an inverse relationship with the virus genome replication. This study showed that, unlike other viruses that upregulate survivin, MNV-1 is the first virus found to downregulate the levels of survivin. We observed that MNV-1 replication in RAW264.7 cells activated caspases, resulting in apoptosis through the mitochondrial pathway, possibly as a result of downregulation of survivin.