Tumorigenic Poxviruses Up-Regulate Intracellular Superoxide To Inhibit Apoptosis and Promote Cell Proliferation

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Vaccinia virus (VACV) produces large plaques consisting of a rapidly expanding ring of infected cells surrounding a lytic core, whereas myxoma virus (MYXV) produces small plaques that resemble a focus of transformed cells. This is odd, because bioinformatics suggests that MYXV carries homologs of nearly all of the genes regulating Orthopoxvirus attachment, entry, and exit. So why does MYXV produce foci? One notable difference is that MYXV-infected cells produce few of the actin microfilaments that promote VACV exit and spread. This suggested that although MYXV carries homologs of the required genes (A33R, A34R, A36R, and B5R), they are dysfunctional. To test this, we produced MYXV recombinants expressing these genes, but we could not enhance actin projectile formation even in cells expressing all four VACV proteins. Another notable difference between these viruses is that MYXV lacks a homolog of the F11L gene. F11 inhibits the RhoA-mDia signaling that maintains the integrity of the cortical actin layer. We constructed an MYXV strain encoding F11L and observed that, unlike wild-type MYXV, the recombinant virus disrupted actin stress fibers and produced plaques up to 4-fold larger than those of controls, and these plaques expanded ϳ6-fold faster. These viruses also grew to higher titers in multistep growth conditions, produced higher levels of actin projectiles, and promoted infected cell movement, although neither process was to the extent of that observed in VACV-infected cells. Thus, one reason for why MYXV produces small plaques is that it cannot spread via actin filaments, although the reason for this deficiency remains obscure. A second reason is that leporipoxviruses lack vaccinia's capacity to disrupt cortical actin. P oxviruses produce two kinds of plaques in culture. The first kind is formed rapidly by viruses like vaccinia virus (VACV), and typically they comprise a ring of infected cells surrounding a large central clearing or lytic zone. The second type of poxvirus plaque is smaller, grows slower, and consists of a clump of virusinfected cells. These plaques look more like a cluster of transformed cells and are sometimes called foci. Such plaques are produced by tumorigenic poxviruses like the leporipoxviruses myxoma virus (MYXV) and Shope fibroma virus. Although it is well established that the appearance of poxvirus plaques is determined by the genetics of both the host and the virus, why MYXV plaques look so different from VACV plaques, even when plated on the same cell type, is not well understood.The process of plaque formation depends (in part) upon how well viruses can engage the host machinery to spread efficiently from cell to cell, processes that are best understood for VACV (reviewed in references 44, 51, and 58). VACV characteristically produces several different forms of infectious virus. Mature viruses (MV) are bounded by just a single lipid bilayer. MV comprise the most abundant infectious form and are probably released by cell lysis. However, some MV migrate away from viral factories, w...

Section: Discussionmentioning

confidence: 99%

Modulation of the Myxoma Virus Plaque Phenotype by Vaccinia Virus Protein F11

Irwin

Evans

2012

Self Cite

“…M131R encodes an inactive homologue of mammalian Zn/Cu superoxide dismutase (SOD) and is predicted to interfere with the activity of cellular SOD (29)(30)(31). Deletion of the M131R gene had no effect on the virulence of MYXV in European rabbits.…”

Section: Resultsmentioning

confidence: 99%

Comparative Analysis of the Complete Genome Sequence of the California MSW Strain of Myxoma Virus Reveals Potential Host Adaptations

Kerr

Rogers

Fitch

et al. 2013

“…Recombinant leporipoxvirus SODs form a stable heterodimer with a cellular copper chaperone protein associated with the host Cu-Zn SODs, leading to the hypothesis that this interaction reduces host Cu-Zn SOD activity by sequestering Cu, thus keeping Cu from the host enzyme (16). Gene knockout experiments indicated that the viral Cu-Zn SOD homolog was involved in upregulating the intracellular ROS level, which inhibited the programmed cell death response; this upregulation was proposed to benefit virus replication (17).…”

mentioning

confidence: 99%

Chlorovirus PBCV-1 Encodes an Active Copper-Zinc Superoxide Dismutase

Kang

Duncan

Kuszynski

et al. 2014

Superoxide dismutases (SODs) are metalloproteins that protect organisms from toxic reactive oxygen species by catalyzing the conversion of superoxide anion to hydrogen peroxide and molecular oxygen. Chlorovirus PBCV-1 encodes a 187-amino-acid protein that resembles a Cu-Zn SOD with all of the conserved amino acid residues for binding copper and zinc (named cvSOD). cvSOD has an internal Met that results in a 165-amino-acid protein (named tcvSOD). Both cvSOD and tcvSOD recombinant proteins inhibited nitroblue tetrazolium reduction of superoxide anion generated in a xanthine-xanthine oxidase system in solution. tcvSOD was chosen for further characterization because it was easier to produce. Recombinant tcvSOD also inhibited a riboflavin photochemical reduction system in a polyacrylamide gel assay, which was blocked by the Cu-Zn SOD inhibitor cyanide but not by azide, which inhibits Fe and Mn SODs. A k cat /K m value for cvSOD was determined by stop-flow spectrophotometry as 1.28 ؋ 10 8 M ؊1 s ؊1 , suggesting that cvSOD-catalyzed O 2 ؊ dismutation was not a diffusion controlled encounter. The cvsod gene was expressed as a late gene, and cvSOD activity was detected in purified virions. Superoxide accumulated rapidly during virus infection, and circumstantial evidence indicates that cvSOD aids its decomposition to benefit virus replication. Cu-Zn SOD homologs have been described to occur in 3 other families of large DNA viruses, poxviruses, baculoviruses, and mimiviruses, which group as a clade. Interestingly, cvSOD does not group in the same clade as the other virus SODs but instead groups in an expanded clade that includes Cu-Zn SODs from many cellular organisms. IMPORTANCEVirus infection often leads to an increase in toxic reactive oxygen species in the host, which can be detrimental to virus replication. Viruses have developed various ways to overcome this barrier. As reported in this article, the chloroviruses often encode and package a functional Cu-Zn superoxide dismutase in the virion that presumably lowers the concentration of reactive oxygen induced early during virus infection.