Many viruses are known to trigger endoplasmic reticulum (ER) stress, whereas the infected cells may develop the defensive unfolded protein response (UPR), leading, depending on conditions, to either cell survival or programmed cell death. One of the UPR branches is mediated by up-regulation of Xbp1 transcription factor caused by unconventional cytoplasmic splicing of its mRNA. Phosphorylated Ire1 normally mediates the splicing of the Xbp1 mRNA leading to production of its shorter spliced form. We studied the role of different representatives of Picornaviridae family in modulating unfolded protein response.
We show upregulation of phosphorylated Ire1 protein level in HeLa cells on the early stages in poliovirus infection and its consequent downregulation in the course of viral reproductive cycle. Even though Ire1 phosphorylation did occur on early stages in poliovirus infection, spliced products of Xbp1 mRNA were not detected throughout the viral reproduction cycle. Moreover we showed that IRE1 mediated Xbp1 mRNA splicing was repressed in infected cells. The addition of chemical inducers of ER stress (dithiothreitol, tunicamycin or thapsighargin) to infected cells led to a markedly lower accumulation of spliced Xbp1 mRNA as compared to the level of this mRNA in inducer treated mock-infected cells. The inhibition of Ire1-mediated Xbp1 mRNA splicing during poliovirus infection was time-dependent and observed in different cell lines. Mengovirus infection also causes upregulation of phosphorylated Ire1 in HeLa cells. In contrast to the wild-type mengovirus, its mutant deficient in activity of the leader protein (L, a “security” protein) neither affected the content of IRE1 in infected cells nor suppressed the inducer-elicited activation of the IRE1/Xbp1 pathway, suggesting that it is L that plays a key role in this suppression.
Thus, our results demonstrate the ability of picornaviruses to suppress still another defensive activity of the host cell and add one more item to the growing list of counter-defensive functions of picornaviral security proteins.