Eukaryotic mRNA translation regulation is most often achieved by interference with initiation events (10). Unimpeded initiation occurs upon assembly of eukaryotic initiation factor 4F at the cap, recruitment of the 43S preinitiation complex, scanning, formation of the 48S initiation complex at the initiation codon, and 60S ribosomal subunit joining. Translation initiation repressors binding to 5Ј and 3Ј untranslated regions (UTR) have been reported to interfere with distinct steps of the initiation cascade. These include obstructed eukaryotic initiation factor 4F cap assembly (26), inhibited stable association of the 40S ribosomal subunit (9, 12), and blocked 60S ribosomal subunit joining (31).Some mRNAs, however, can initiate translation by noncanonical mechanisms. Picornaviruses feature uncapped (29) plus-strand RNA genomes whose translation depends on internal ribosome entry sites (IRES) within their highly structured 5Ј UTR for initiation in a 5Ј-end, cap-independent manner (21, 34). Poliovirus (PV), a member of the family Picornaviridae, is responsible for paralytic poliomyelitis, resulting from specific targeting of spinal cord motor neurons in the human central nervous system. Neurological disease is the result of complex interactions determined by the host, e.g., aggravating susceptibility factors (44), viral receptors in the central nervous system (17), and the innate immune response (19), and the virus itself, e.g., capsid structure (43) and the viral RNA-dependent RNA polymerase (33; reviewed in reference 16).An important viral determinant of neuropathogenesis is the IRES. Single point mutations in IRES stem loop domain V of the PV (Sabin) oral vaccine strains have been implicated in neurovirulence attenuation (8,22,37), and deliberate manipulation of the PV IRES reduces its neurovirulent potential (1). Replacement with a heterologous IRES element of human rhinovirus type 2 (HRV2) eliminates PV propagation in cells of neuronal derivation (5, 13, 14) and abolishes neuropathogenicity in mice transgenic for the human PV receptor (13) and in nonhuman primates (14). Despite severely repressed viral propagation in neurons, the chimeric PV-RIPO replicates with wild-type kinetics in malignant glioma (15) and breast cancer (30) cells. These observations suggest that the conditions for HRV2 IRES-mediated translation in neurons differ from those in glioma and other cancer cells. HRV2 IRES-driven translation could require enabling factors in glioma cells that are absent in neuronal cells, or, alternatively, negative regulators in neurons could block PV-RIPO propagation.Neuroblast:glioma heterokaryon analyses revealed that neuron-specific repression is trans dominant over robust PV-RIPO propagation in glioma cells (27). We identified association of double-stranded RNA binding protein 76 (DRBP76), nuclear factor of activated T cells, 45 kDa (NF45), and RNA helicase A (RHA) with the HRV2 IRES by affinity chromatography with lysates from neuron-derived cells (27). These proteins have been reported to associate with e...

Cell adhesion molecules of the immunoglobulin superfamily are aberrantly expressed in malignant glioma. Amongst these, the human poliovirus receptor CD155 provides a molecular target for therapeutic intervention with oncolytic poliovirus recombinants. Poliovirus has been genetically modified through insertion of regulatory sequences derived from human rhinovirus type 2 to selectively replicate within and destroy cancerous cells. Efficacious oncolysis mediated by poliovirus derivatives depends on the presence of CD155 in targeted tumors. To prepare oncolytic polioviruses for clinical application, we have developed a series of assays in high-grade malignant glioma (HGL) to characterize CD155 expression levels and susceptibility to oncolytic poliovirus recombinants. Analysis of 6 HGL cases indicates that CD155 is expressed in these tumors and in primary cell lines derived from these tumors. Upregulation of the molecular target CD155 rendered explant cultures of all studied tumors highly susceptible to a prototype oncolytic poliovirus recombinant. Our observations support the clinical application of such agents against HGL.

Translation of picornavirus plus-strand RNA genomes occurs via internal ribosomal entry at highly structured 5 untranslated regions. In addition to canonical translation factors, translation rate is likely influenced by supplementary host and viral trans-acting factors. We previously reported that insertion of a heterologous human rhinovirus type 2 internal ribosomal entry site (IRES) into the poliovirus (PV) genome, generating the chimeric virus PV-RIPO, selectively abrogates viral translation and propagation in neurons, which eliminate poliovirus's signature neuropathogenicity. While severely deficient in cells of neuronal lineage, the rhinovirus IRES promotes efficient propagation of PV-RIPO in cancer cells. Tumor-specific IRES function can be therapeutically exploited to direct viral cytotoxicity to cancer cells. Neuron-glioma heterokaryon analysis implicates neuronal trans-dominant inhibition in this effect, suggesting that host trans-acting factors repress IRES function in a cell-type-specific manner. We identified a set of proteins from neuronal cells with affinity for the rhinovirus IRES, including double-stranded RNA-binding protein 76 (DRBP76). DRBP76 associates with the IRES in neuronal but not in malignant glioma cells. Moreover, DRBP76 depletion in neuronal cells enhances rhinovirus IRES-driven translation and virus propagation. Our observations suggest that cell-typespecific association of DRBP76 with the rhinovirus IRES represses PV-RIPO translation and propagation in neuronal cells. Posttranscriptional regulation constitutes an important level of control of gene expression. The global rate of protein synthesis is influenced by modification of translation initiation factors. In addition, the translation of many mRNAs is independently regulated by processes targeting their untranslated regions (UTRs). In many instances, specific UTRs bind ribonucleoprotein complexes that alter conditions for translation of individual messages.The plus-strand RNA genomes of Picornaviridae feature complex UTRs involved in the regulation of viral translation and genome replication. Translation of viral RNA occurs in competition with cellular mRNA in infected host cells (3). Accordingly, picornavirus genomes have adopted unconventional features enabling efficient viral translation while limiting host cell protein synthesis. In contrast to cellular mRNAs, picornaviral genomic RNAs are uncapped (34), and their uncommonly large and highly structured 5ЈUTRs contain internal ribosomal entry sites (IRESs) that mediate translation initiation in a 5Ј-end, cap-independent manner (26, 39). IRESmediated translation is unimpeded by virus-induced cleavage of canonical initiation factors eukaryotic initiation factor 4G and poly(A)-binding protein (13,27), indicating that their involvement in initiation at the IRES deviates from capped mRNAs. Moreover, divergent means of translation initiation imply the involvement of noncanonical translation factors or IRES trans-acting factors (ITAFs) (4). Such factors, by virtue of their cell and ...

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