Credit: Image courtesy of AP

Viruses are cunning — they use altered aspects of transformed cells to facilitate cell infection. This is now backfiring, however, as researchers are exploiting this property to develop anti-cancer therapeutics. To further this research, it is important to know which features of transformed cells aid permissiveness to infection. To this end, Lee and co-workers, in the August issue of Nature Cell Biology, showed that an activated Ras pathway (found in many cancer cells) switches off the host-cell anti-viral mechanism, thus increasing infection by herpes simplex virus 1 (HSV-1).

The authors initially compared the level of viral infection in untransformed NIH-3T3 cells with that in cells transformed with oncogenes that activate the Ras pathway — v-erbB, sos and ras. Only transformed cells show significant morphological changes related to infection, such as cell rounding and clumping, and high levels of viral protein synthesis and viral progeny. Interestingly, this was specific to the Ras pathway and was not a general feature of the transformed phenotype, as cells transformed with c-myc did not have an increase in viral output. The demonstration that inhibitors of Ras and Mek1/2 — a downstream effector in the Erk pathway — prevented this increase in infection by HSV-1 further proved that an activated Ras pathway is important for infection.

As Ras is a key regulator of several signalling pathways, Ras effector mutants that activate just one pathway — Raf/Erk, phosphatidylinositol 3-kinase and RALGDS — were used to see which pathway was involved in permissiveness to infection. The authors showed that activation of the Raf/Erk pathway was the only one that markedly increased susceptibility to HSV-1.

To identify where Ras acts in the infection cycle, the authors used quantitative polymerase chain reaction to investigate which of the genes were transcribed. The early α-gene transcripts were present at equal levels in both ras-transformed and untransformed cells, but the later β- and γ-gene transcripts were present at higher levels in the ras-transformed cells. As the α-proteins are needed for transcription of the β- and γ-proteins, it is possible that the α-proteins are not translated in the untransformed cells; this was confirmed by Western blots.

A well-known anti-viral strategy of cells is the phosphorylation of the double-stranded RNA-activated protein kinase (PKR), which leads to inhibition of the translation initiation factor eIF2α, and subsequently translation. In vitro kinase assays were used to show that PKR phosphorylation is more pronounced in untransformed cells as compared with ras-transformed cells. The Ras pathway can therefore inactivate the PKR mechanism of the cell to facilitate HSV-1 infection.

HSV-1 mutants, such as G207, are now being tested in clinical trials for anti-cancer activity. R3616, which is similar to G207, is an HSV-1 mutant that is deleted for the viral anti-PKR gene, γ134.5. The R3616 mutant infects ras-transformed — but not untransformed — cells, which suggests that the Ras-signalling pathway compensates for the loss of the virus's own anti-PKR mechanism. Confirming the role of PKR in this pathway, mouse embryonic fibroblasts (MEFs) that were deleted for PKR could be infected by the mutant virus whereas parental MEFs remained nonpermissive.

HSV-1, therefore, predominantly uses the host anti-PKR mechanism to infect transformed cells, and its selectiveness for transformed cells makes it a good candidate for an anti-cancer therapeutic. This study not only provides knowledge of the mechanism behind host-cell permissiveness, but also indicates which patients with cancer might benefit from treatment with oncolytic viruses.