Cells that are infected with a virus recognize the invader's presence by their innate immunity machinery and switch on a variety of defensive mechanisms. The infecting virus, on the other hand, may possess tools capable of interfering with host antiviral responses. The outcome of the infection, both in terms of the efficiency of virus growth and the extent of host pathology, depends on the trade-off between these defensive and counterdefensive measures.
Cellular innate immunity involves multiple pathways, and one powerful defense is apoptosis, or the programmed self-sacrifice of the infected cell, potentially limiting viral reproduction and spread (10
). However, many viruses are able to suppress this defensive mechanism (14
). Remarkably, virus-elicited pathology may be specific for a given type of cells and a given virus. Unraveling the interplay between pathways leading to the death or survival of the infected cells is an important task that may provide clues to understanding viral pathogenesis and, possibly, may indicate new directions for searching for antiviral drugs.
Picornaviruses are a family of small nonenveloped animal viruses that includes important human and animal pathogens such as polioviruses, rhinoviruses, hepatitis A virus, foot-and-mouth disease viruses, and many others (89
). Their genome is represented by a single-stranded 7.2- to 8-kb RNA molecule of positive polarity encoding about a dozen mature proteins (generated by the limited proteolysis of a single polyprotein precursor), nearly all of which are directly involved in the replication of the viral RNA and formation of virions (1
The first picornavirus demonstrated to interact with the host cell apoptotic machinery by both triggering and suppressing the apoptotic response was poliovirus (95
). Since then, a wealth of data has been accumulated that shows that the activation of apoptotic pathways is a widespread, though not universal, response to picornavirus infection. Thus, apoptosis-inducing capacity was reported for coxsackieviruses B3, B4, and B5 (22
), enteroviruses 70 and 71 (25
), human rhinoviruses 1B, 9, 14, and 16 (32
), foot-and-mouth disease virus (53
), avian encephalomyelitis virus (62
), and hepatitis A virus (16
) and was the subject of several recent reviews (15
). The antiapoptotic activity of picornaviruses was studied predominantly by using poliovirus (3
) and coxsackievirus B3 (21
The present study is focused on the interaction of cardioviruses, which are representatives of a genus in the picornavirus family, with the apoptotic machinery of infected cells. Our interest in this topic stemmed from the fact that these viruses, e.g., encephalomyocarditis virus (EMCV) and its strain mengovirus (MV), as well as the less-related Theiler's murine encephalomyelitis virus (TMEV), while sharing major features of genome organization and reproductive strategy with other family members, encode a unique protein that is not found in other picornaviruses. Indeed, the leader (L) protein, a derivative of the N-terminal portion of the viral polyprotein (55
), appears to be a major player in controlling the virus-host interaction. On the one hand, it is devoid of any known enzymatic activity, and L-lacking mutants are viable, at least in certain cultured cells (19
). On the other hand, the L protein appears to inhibit host translation (35
), suppresses interferon production (46
), and impairs nucleocytoplasmic traffic (11
). It has been hypothesized that cardiovirus L protein also is involved in the interaction with defensive apoptotic machinery.
Previous studies have demonstrated that TMEV infection may induce apoptosis, especially in partially restrictive cells (50
). EMCV also exerted a similar effect in certain cell lines (87
). The reason(s) underlying variability in the apoptosis-inducing effects of cardioviruses remains unexplained. Here, we demonstrate that the productive cardiovirus infection of susceptible HeLa cells resulted in their cytopathic death, which was not accompanied by clear signs of apoptosis. On the contrary, the infected cells acquired an antiapoptotic state, as evidenced by their failure to develop an apoptotic response to viral and nonviral apoptosis inducers. However, the antiapoptotic state failed to develop in cells infected with a mutant virus with inactivated L, and this mutant instead elicited caspase-dependent apoptosis preceded by cytochrome c
efflux. These data suggest that the wild-type (wt) L protein is involved, directly or otherwise, in the control of viral antiapoptotic function(s).