Ras Signalling Pathway: A Gateway for HSV-1 Infection

The key role of Ras in diverse cellular functions qualifies it as a central divergence point for many signal transduction pathways, the most extensively studied of which include the ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase), and p38-kinase pathways[1,2]. Stimulation of these pathways results in the activation of different nuclear transcription factors (including SRF, ELK-1, JUN, NFκB, and ATF-2), which in turn leads to a variety of biological functions regulating cell growth, differentiation, apoptosis, migration, and survival[3,4]. In this study, a new role for Ras signaling pathway as key determinant of host cell permissiveness to viral infection was investigated. Herpes simplex virus-1 (HSV-1) was chosen because of its importance as a human pathogen and its potential use in cancer therapy[5,6,7,8,9]. 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 (e.g., v-erbB, sos, and ras). Only transformed cells demonstrate significant morphological changes as a result of infection (such as cell rounding and clumping), increased viral protein synthesis, and a high titer of viral progeny. Interestingly, this was specific to the cells with an overactivated 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 protein synthesis. The demonstration that inhibitors of Ras (farnesyl transferase inhibitors or FTIs)[10] and a blocker of MEK1/2 (a downstream molecule in ERK pathway) named PD98059[11], prevented this increase in HSV-1 infection further proved that an activated Ras pathway is important for infection. As Ras is a key regulator of several signalling pathways, mutant versions of Ras that preferentially activate only one of several known pathways downstream of Ras (RAF/ERK, PI3K, or RAL/GDS) were used to find out which pathway was involved in permissiveness to infection. The authors showed that the RAF/ERK pathway was the only one whose activation significantly increased susceptibility to HSV-1. To further elucidate the role(s) of the Ras pathway in HSV-1 infection, it was important to identify the step at which HSV-1 infection is blocked in the nonpermissive NIH-3T3 cells. Since virus binding and internalization are comparable between permissive and nonpermissive cells, transcription of viral genes were considered to be a logical target for investigation. Herpes genes are transcribed in a sequential manner with α genes being the first to be expressed and required for the synthesis of subsequent polypeptide groups β and γ [5]. Accordingly, the relative amounts of HSV-1 transcripts (including the α gene α27, β genes UL29 and UL30, γ1 gene γ134.5, and γ2 gene UL44) generated in NIH-3T3 cells and their Ras-transformed version (named H-ras cells) were compared after amplification of these transcripts by quantitative RT-PCR. The results show