Reversion by hypotonic medium of the shutoff of protein synthesis induced by encephalomyocarditis virus

M A Alonso, L Carrasco
1981 Journal of Virology  
Infection of human HeLa cells by picornaviruses produces a drastic inhibition of host protein synthesis. Treatment of encephalomyocarditis virus-infected HeLa cells with hypotonic medium reversed this inhibition; no viral protein synthesis was detected. The blockade of viral translation by hypotonic conditions was observed for a wide range of multiplicities of infection. However, only with low virus-to-cell ratios did cellular protein synthesis resume. The ratio of cellular to viral mRNA
more » ... o viral mRNA translation was strongly influenced by the concentration of monovalent ions present in the culture medium: a high concentration of NaCl or KCl favored the translation of viral mRNA and strongly inhibited cellular protein synthesis, whereas the opposite was true when NaCl was omitted from the culture medium. Once viral protein synthesis had been blocked by hypotonic medium treatment, it resumed when the infected cells were placed in a normal or hypertonic medium, indicating that the viral components synthesized in the infected cells were not destroyed by this treatment. These observations reinforced the idea that ions play a role in discriminating between viral and cellular mRNA translation in virus-infected animal cells. Infection of animal cells by a cytolytic virus induces a specific inhibition of host macromolecular synthesis. The inhibition of protein synthesis in picornavirus-infected cells, the so-called shutoff phenomenon (1, 7, 28), was first described 20 years ago (8). Since then, studies in a number of laboratories have established that the bulk of viral protein synthesis takes place when cellular mRNA translation is drastically inhibited. Several theories have been proposed to explain the specific regulation of translation observed in animal cells infected with viruses (2, 7). These theories can be divided in two main groups: some suggest the generation of a specific inhibitor of host protein synthesis after viral infection (10, 16), whereas others explain the discrimination between host and viral mRNA translation by changes in the protein-synthesizing machinery of the cell (12, 15, 24, 29, 30) . We advanced the hypothesis that the modification of the cell membrane after viral infection could alter the ionic milieu in the cellular cytoplasm, and, in turn, this change would differentially influence cellular and viral protein syntheses. In support of this model, we know that the in vitro translation of cellular mRNA's is inhibited by monovalent ions at the same time that viral protein synthesis is stimulated (5, 6). A similar discrimination exerted by monovalent ions on translation has also been described in cells infected by viruses (for a review, see reference 7). Moreover, the permeability of virusinfected cells to ions and metabolites changes when the shutoff of host protein synthesis takes place (3, 9, 11, 20) . These changes in membrane permeability probably reflect the interaction of a virion component with the cell membrane and are observed early during viral attachment, as well as late in infection when massive amounts of viral coat protein are synthesized. In any event, it is clear that changes in the intracellular cation concentration of a magnitude sufficient to selectively inhibit host translation by the same mechanism as that which occurs in vitro occur late in the course of infection (9). In this study, we show that the removal of sodium chloride from the culture medium of encephalomyocarditis (EMC) virus-infected cells stopped viral protein synthesis; at the same time, host mRNA translation resumed. MATERIALS AND METHODS CeUs and virus. HeLa ceUs were propagated in culture flasks (Falcon Plastics) containing 6 ml of Eagle medium as modified by Dulbecco (E4D), supplemented with 10% newborn calf serum (GIBCO Laboratories; E4D1o), and incubated at 37°C in a 5% CO2 atmosphere. EMC virus was grown on L-929 cells in a mixture of Eagle medium, phosphate-buffered saline, and E4D (80:15:5), supplemented with 1% newborn calf serum (E4D1). The fraction obtained after the removal of cell 535 on May 9, 2020 by guest
doi:10.1128/jvi.37.2.535-540.1981 fatcat:2aitrrs7inetvo6dpilz3xsufm