Cap-dependent and hepatitis C virus IRES-mediated translation is modulated by phosphorylation of eIF2α under oxidative stress.

Chronic hepatitis C is often associated with oxidative stress. Hepatitis C virus (HCV) utilizes an internal ribosome entry site (IRES) element for translation, in contrast to cap-dependent translation of the majority of cellular proteins. To understand how virus translation is modulated under oxidative stress, HCV IRES-mediated translation was compared with cap-dependent translation using a bicistronic reporter construct and hydrogen peroxide (H2O2) as a stress inducer. In H2O2-sensitive HeLa cells, H2O2 repressed translation in a time- and dose-dependent manner, concomitant with the kinetics of eIF2 alpha phosphorylation. A phosphornimetic of eIF2 alpha, which mimics the structure of the phosphorylated eIF2 alpha, was sufficient to repress translation in the absence of H2O2. In H2O2-resistant HepG2 cells, H2O2 activated both HCV IRES-mediated and cap-dependent translation, associated with an increased level of phospho-eIF2 alpha. It was postulated that H2O2 might stimulate translation in HepG2 cells via an eIF2 alpha-independent mechanism, whereas the simultaneous phosphorylation of eIF2 alpha repressed part of the translational activities. Indeed, the translational repression was released in the presence of a non-phosphorylatable mutant, eIF2 alpha-SA, resulting in further enhancement of both translational activities after exposure to H2O2- In HuH7 cells, which exhibited an intermediate level of sensitivity towards H2O2, both HCV IRES-mediated and cap-dependent translational activities were upregulated after treatment with various doses of H2O2, but the highest level of induction was achieved with a low level of H2O2, which may represent the physiological level of H2O2. At this level, the HCV IRES-mediated translation was preferentially upregulated compared with cap-dependent translation.