Adenovirus late protein synthesis is resistant to the inhibition of translation induced by poliovirus

Translation of Sindbis virus 26S mRNA does not require intact eukariotic initiation factor 4G

The infection of baby hamster kidney (BHK) cells by Sindbis virus gives rise to a drastic inhibition of cellular translation, while under these conditions the synthesis of viral structural proteins directed by the subgenomic 26S mRNA takes place efficiently. Here, the requirement for intact initiation factor eIF4G for the translation of this subgenomic mRNA has been examined. To this end, SV replicons that contain the protease of human immunodeficiency virus type 1 (HIV-1) or the poliovirus 2A(pro) replacing the sequences of SV glycoproteins have been constructed. BHK cells electroporated with the different RNAs synthesize protein C and the corresponding protease at late times. Notably, the proteolysis of eIF4G by both proteases has little effect on the translation of the 26S mRNA. In addition, recombinant viable SVs were engineered that encode HIV-1 PR or poliovirus 2A protease under the control of a duplicated late promoter. Viral protein synthesis at late times of infection by the recombinant viruses is slightly affected in BHK cells that contain proteolysed eIF4G. The translatability of SV genomic 49S mRNA was assayed in BHK cells infected with a recombinant virus that synthesizes luciferase and transfected with a replicon that expresses poliovirus 2Apro. Under conditions where eIF4G has been hydrolysed significantly the translation of genomic SV RNA was deeply inhibited. These findings indicate a different requirement for intact eIF4G in the translation of genomic and subgenomic SV mRNAs. Finally, the translation of the reporter gene that encodes green fluorescent protein, placed under the control of a second duplicate late promoter, is also resistant to the cleavage of eIF4G. In conclusion, despite the presence of a cap structure in the 5' end of the subgenomic SV mRNA, intact eIF4G is not necessary for its translation.

Heat shock phenocopies E1B-55K late functions and selectively sensitizes refractory tumor cells to ONYX-015 oncolytic viral therapy

ONYX-015 is an E1B-55K-deleted adenovirus that has promising clinical activity as a cancer therapy. However, many tumor cells fail to support ONYX-015 oncolytic replication. E1B-55K functions include p53 degradation, RNA export, and host protein shutoff. Here, we show that resistant tumor cell lines fail to provide the RNA export functions of E1B-55K necessary for ONYX-015 replication; viral 100K mRNA export is necessary for host protein shutoff. However, heat shock rescues late viral RNA export and renders refractory tumor cells permissive to ONYX-015. These data indicate that heat shock and late adenoviral RNAs may converge upon a common mechanism for their export. Moreover, these data suggest that the concomitant induction of a heat shock response could significantly improve ONYX-015 cancer therapy.

Translation of eukaryotic translation initiation factor 4GI (eIF4GI) proceeds from multiple mRNAs containing a novel cap-dependent internal ribosome entry site (IRES) that is active during poliovirus infection

Eukaryotic translation initiation factor 4GI (eIF4GI) is an essential scaffolding protein required to recruit the 43 S complex to the 5'-end of mRNA during translation initiation. We have previously demonstrated that eIF4GI protein expression is translationally regulated. This regulation is mediated by cis-acting RNA elements, including an upstream open reading frame and an IRES that directs synthesis of five eIF4GI protein isoforms via alternative AUG initiation codon selection. Here, we further characterize eIF4GI IRES function and show that eIF4GI is expressed from several distinct mRNAs that vary via alternate promoter use and alternate splicing. Several mRNA variants contain the IRES element. We found that IRES activity mapped to multiple regions within the eIF4GI RNA sequence, but not within the 5'-UTR per se. However, the 5'-UTR enhanced IRES activity in vivo and played a role in initiation codon selection. The eIF4GI IRES was active when transfected into cells in an RNA form, and thus, does not require nuclear processing events for its function. However, IRES activity was found to be dependent upon the presence, in cis, of a 5' m7guanosine-cap. Despite this requirement, the eIF4GI IRES was activated by 2A protease cleavage of eIF4GI, in vitro, and retained the ability to promote translation during poliovirus-mediated inhibition of cap-dependent translation. These data indicate that intact eIF4GI protein is not required for the de novo synthesis of eIF4GI, suggesting its expression can continue under stress or infection conditions where eIF4GI is cleaved.

p53-Independent and -dependent requirements for E1B-55K in adenovirus type 5 replication

The adenovirus type 5 mutant dl1520 was engineered previously to be completely defective for E1B-55K functions. Recently, this mutant (also known as ONYX-015) has been suggested to replicate preferentially in p53(-) and some p53(+) tumor cell lines but to be attenuated in primary cultured cells (C. Heise, A. Sampson-Johannes, A. Williams, F. McCormick, D. D. F. Hoff, and D. H. Kirn, Nat. Med. 3:639-645, 1997). It has been suggested that dl1520 might be used as a "magic bullet" that could selectively lyse tumor cells without harm to normal tissues. However, we report here that dl1520 replication is independent of p53 genotype and occurs efficiently in some primary cultured human cells, indicating that the mutant virus does not possess a tumor selectivity. Although it was not the sole host range determinant, p53 function did reduce dl1520 replication when analyzed in a cell line expressing temperature-sensitive p53 (H1299-tsp53) (K. L. Fries, W. E. Miller, and N. Raab-Traub, J. Virol. 70:8653-8659, 1996). As found earlier for other E1B-55K mutants in HeLa cells (Y. Ho, R. Galos, and J. Williams, Virology 122:109-124, 1982), dl1520 replication was temperature dependent in H1299 cells. When p53 function was restored at low temperature in H1299-tsp53 cells, it imposed a modest defect in viral DNA replication and accumulation of late viral cytoplasmic mRNA. However, in both H1299 and H1299-tsp53 cells, the defect in late viral protein synthesis appeared to be much greater than could be accounted for by the modest defects in late viral mRNA levels. We therefore propose that in addition to countering p53 function and modulating viral and cellular mRNA nuclear transport, E1B-55K also stimulates late viral mRNA translation.

A late adenovirus factor induces eIF-4E dephosphorylation and inhibition of cell protein synthesis

Adenovirus prevents host cell protein synthesis during its late phase of replication in large part by causing the underphosphorylation of translation initiation factor eIF-4E, a component of initiation factor eIF-4F (cap-binding protein complex). Late adenovirus mRNAs are preferentially translated because they possess a reduced requirement for eIF-4F. This study continues the characterization of the mechanism by which adenovirus inhibits cellular protein synthesis. First it is shown that adenovirus blocks the addition of phosphate to eIF-4E rather than enhancing its removal, establishing that the virus impairs a signalling pathway or protein kinase activity involved in eIF-4E phosphorylation. It is then shown that shutoff of cell protein synthesis and translation of late viral mRNAs are uncoupled, in that shutoff actually occurs a short time (1 to 3 h) after late adenovirus mRNAs are already undergoing translation. Finally, by using a variety of genetic mutants stalled at different stages in the viral life cycle, it was found that dephosphorylation of eIF-4E and inhibition of cell translation are not caused by early adenovirus gene products acting at late times or by events related to viral DNA replication. Instead, it is shown that inhibition of eIF-4E phosphorylation and cell translation are mediated upon activation of the viral major late transcription unit. These and other results presented indicate that the adenovirus signal which induces eIF-4E dephosphorylation and shutoff of cell protein synthesis is linked either to an activity of one or more late viral polypeptides, to double-stranded RNA produced by opposition of the early and late viral transcription units, or to both.

Picornavirus inhibitors

Picornaviruses are among the best understood animal viruses in molecular terms. A number of important human and animal pathogens are members of the Picornaviridae family. The genome organization, the different steps of picornavirus growth and numerous compounds that have been reported as inhibitors of picornavirus functions are reviewed. The picornavirus particles and several agents that interact with them have been solved at atomic resolution, leading to computer-assisted drug design. Picornavirus inhibitors are useful in aiding a better understanding of picornavirus biology. In addition, some of them are promising therapeutic agents. Clinical efficacy of agents that bind to picornavirus particles has already been demonstrated.

Dependence of the adenovirus tripartite leader on the p220 subunit of eukaryotic initiation factor 4F during in vitro translation. Effect of p220 cleavage by foot-and-mouth-disease-virus L-protease on in vitro translation

The adenovirus tripartite leader (TPT) 5' untranslated region (5'UTR) allows translation in poliovirus-infected cells, in which the p220 subunit of eukaryotic initiation factor 4F is degraded. This p220-independent translation was investigated by measuring in vitro translation in a reticulocyte lysate of a reporter gene, chloramphenicol acetyltransferase, coupled to the TPT 5'UTR. The p220 subunit was degraded by translation of a foot-and-mouth-disease L-protease construct. Surprisingly, the TPT 5'UTR was dependent on intact p220, as are other naturally capped mRNA species. Translation of encephalomyocarditis virus RNA was p220 independent, as expected from its ability to support internal, cap-independent initiation. In vitro protein-synthesis experiments with purified initiation factors confirmed the dependence of TPT mRNA translation on eukaryotic initiation factor 4F. The relationship between adenovirus TPT-5'UTR-directed translation and poliovirus-induced host cell shut-off is discussed.

Lack of direct correlation between p220 cleavage and the shut-off of host translation after poliovirus infection

Poliovirus induces a drastic inhibition of host protein synthesis soon after infection of susceptible cells. The correlation between this inhibition and the cleavage of p220, a polypeptide that forms part of protein synthesis initiation factor elF-4F, has been examined in detail. Measurements of protein synthesis at half-hourly intervals after infection with poliovirus show the lack of direct correlation between p220 cleavage and the blockade of cellular translation. Moreover, the use of inhibitors of poliovirus RNA synthesis helped to dissociate those two events more clearly. Thus, in the presence of guanidine or Ro 09-0179 when little shut-off was induced by poliovirus extensive proteolytic degradation of p220 took place. When HeLa cells infected with poliovirus are placed at 28 degrees the inhibition of host protein synthesis is prevented and cellular translation continues for at least 8 hr, albeit at a reduced level compared to cells incubated at 37 degrees. At 28 degrees, cleavage of p220 is observed and about 80% of p220 is degraded after 6 hr of incubation at that temperature. Strikingly, when cells in which more than 50% of p220 is cleaved are shifted to 37 degrees, cellular translation recuperates to 100%, in spite of the fact that no detectable p220 is present. Furthermore, if poliovirus-infected cells are incubated for 2 hr at 37 degrees to permit the cleavage of p220 and then are shifted to 28 degrees in the presence of guanidine, cellular proteins are synthesized at the same level as uninfected HeLa cells incubated at 28 degrees. These results show that translation of cellular mRNAs takes place in cells containing a cleaved p220 and indicate that this cleavage is not directly responsible for the shut-off of host translation induced by poliovirus.

The 5'-untranslated region of picornaviral genomes

Picornaviruses are small naked icosahedral viruses with a single-stranded RNA genome of positive polarity. According to current taxonomy, the family includes four genera: Enterouirus (polioviruses, coxsackieviruses, echoviruses, and other enteroviruses), Rhinovirus, Curdiouirus [encephalomyocarditis virus (EMCV), mengovirus, Theiler's murine encephalomyelitis virus (TMEV)], and Aphthouirus [foot-and-mouth disease viruses (FMDV)]. There are also some, as yet, unclassified picornaviruses [e.g., hepatitis A virus (HAW] that should certainly be assessed as a separate genus. Studies on the molecular biology of picornaviruses might be divided into two periods: those before and after the first sequencing of the poliovirus genome. The 5'-untranslated region (5-UTR) of the viral genome was one of the unexpected problems. This segment proved to be immensely long: about 750 nucleotides or ∼10% of the genome length. There were also other unusual features (e.g., multiple AUG triplets preceding the single open reading frame (ORF) that encodes the viral polyprotein). This chapter shows that the picornaviral 5-UTRs are not only involved in such essential events as the synthesis of viral proteins and RNAs that could be expected to some extent, although some of the underlying mechanisms appeared to be quite a surprise, but also may determine diverse biological phenotypes from the plaque size or thermosensitivity of reproduction to attenuation of neurovirulence. Furthermore, a close inspection of the 5-UTR structure unravels certain hidden facets of the evolution of the picornaviral genome. Finally, the conclusions drawn from the experiments with the picornaviral5-UTRs provide important clues for understanding the functional capabilities of the eukaryotic ribosomes.

Degradation of cellular proteins during poliovirus infection: studies by two-dimensional gel electrophoresis

Picornaviruses encode for their own proteinases, which are responsible for the proteolytic processing of the polyprotein encoded in the viral genome to produce the mature viral polypeptides. The two poliovirus proteinases, known as proteins 2A and 3C, use the poliovirus-encoded polyprotein as a substrate. The possibility that these poliovirus proteinases also degrade cellular proteins remains largely unexplored. High-resolution two-dimensional gel electrophoresis indicates that a few cellular proteins disappear after poliovirus infection. Thus, at least nine acidic and five basic cellular proteins, ranging in Mr from 120 to 30 kilodaltons, are clearly degraded during poliovirus infection of HeLa cells. The degradation of these cellular polypeptides is very specific because it does not occur upon infection of HeLa cells with encephalomyocarditis virus or Semliki Forest virus. Moreover, inhibitors of poliovirus replication, such as cycloheximide or 3-methylquercetin, block the disappearance of these polypeptides. These results suggest that the input virions are not responsible for this degradation and that active poliovirus replication is required for the proteolysis to occur. Analysis of the time course of the disappearance of these polypeptides indicates that it does not occur during the first 2 h of infection, clearly suggesting that this phenomenon is not linked to the poliovirus-induced shutoff of host protein synthesis. This conclusion is strengthened by the finding that 3-methylquercetin blocks proteolysis without preventing shutoff of host translation.

Alternatives for the initiation of translation

New evidence of exceptions to the scanning mechanism for the initiation of translation has been recently obtained. These data suggest that ribosomes can bind and initiate internally on certain mRNAs without having to scan from the 5' end.

Initiation of protein synthesis by internal entry of ribosomes into the 5' nontranslated region of encephalomyocarditis virus RNA in vivo

Expression vectors that yield mono-, di-, and tricistronic mRNAs upon transfection of COS-1 cells were used to assess the influence of the 5' nontranslated regions (5'NTRs) on translation of reporter genes. A segment of the 5'NTR of encephalomyocarditis virus (EMCV) allowed translation of an adjacent downstream reporter gene (CAT) regardless of its position in the mRNAs. A deletion in the EMCV 5'NTR abolishes this effect. Poliovirus infection completely inhibits translation of the first cistron of a dicistronic mRNA that is preceded by the capped globin 5'NTR, whereas the second cistron preceded by the EMCV 5'NTR is still translated. We conclude that the EMCV 5'NTR contains an internal ribosomal entry site that allows cap-independent initiation of translation. mRNA containing the adenovirus tripartite leader is also resistant to inhibition of translation by poliovirus.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.