Flavivirus Infection Activates the XBP1 Pathway of the Unfolded Protein Response To Cope with Endoplasmic Reticulum Stress

Abstract

The unfolded protein response (UPR) is a coordinated change in gene expression triggered by perturbations in functions of the endoplasmic reticulum (ER). XBP1, a key transcription factor of the UPR, is activated by an IRE1-mediated splicing event, which results in a frameshift and encodes a protein with transcriptional activity. Here, we report that XBP1 was activated during flaviviral infection, as evidenced by XBP1 mRNA splicing and protein expression, as well as induction of the downstream genes ERdj4, EDEM1, and p58(IPK) in Japanese encephalitis virus (JEV)-and dengue virus serotype 2 (DEN-2)-infected cells. Reporter systems based on IRE1-mediated XBP1 splicing were established, and several flaviviral proteins associated with the ER, including glycoproteins and small hydrophobic membrane-anchored proteins, were found to trigger the splicing event. Notably, nonstructural protein NS2B-3 of DEN-2, but not of JEV, was a potent inducer of XBP1 splicing through an unclear mechanism(s). Reduction of XBP1 by a small interfering RNA had no effect on cells' susceptibility to the two viruses but exacerbated the flavivirus-induced cytopathic effects. Overall, flaviviruses trigger the XBP1 signaling pathway and take advantage of this cellular response to alleviate virus-induced cytotoxicity.

Dengue Virus Modulates the Unfolded Protein Response in a Time-dependent Manner

“…Additionally, we find that mid-course during DENV infection (24 -36 h), the IRE1-XBP1 pathway is activated, as demonstrated by the presence of the sXbp1 mRNA and nuclear localization of XBP1, in agreement with previous reports for Japanese encephalitis virus and DENV (17). Activation of the IRE1-XBP1 pathway occurs during the phase of the viral life cycle in which protein synthesis and viral particle formation are taking place, causing both ER stress and phospholipid depletion.…”

“…Other reports using the pestivirus bovine viral diarrhea virus have shown that induction of PERK and subsequent eIF2␣ phosphorylation induces the apoptotic pathway (16). Infection with Japanese encephalitis virus and DENV-2 was shown to induce the IRE-XBP1 pathway and a subset of XBP1-activated genes involved in the ER-associated degradation pathway, protecting cells from virus-induced cytopathic effects (17). In contrast, another group suggested that DENV induction of IRE1-XBP1 and ATF6 pathways contributed to dengue pathogenesis (18), whereas WNV induction of the UPR was shown to lead to CHOP-mediated apoptosis (19).…”

Role of the Endoplasmic Reticulum-associated Degradation (ERAD) Pathway in Degradation of Hepatitis C Virus Envelope Proteins and Production of Virus Particles

“…Maintenance of moderately low levels of HCV in the infected liver may contribute to the persistence of HCV infection, often associated with a lengthy asymptomatic phase that can last for decades. A range of viruses, including flaviviruses such as JEV, dengue virus, and West Nile virus, have been reported to induce XBP1 mRNA splicing triggered by ER stress (2,3,24). However, we demonstrate here that, in contrast to HCV, the envelope protein of JEV, which causes acute encephalitis, is not recognized by EDEMs, and the ERAD pathway does not control JEV production.…”

“…Quality control of proteins, such as the elimination of misfolded proteins, is largely connected with the endoplasmic reticulum (ER), 2 which is an organelle responsible for the folding and distribution of secretory proteins to their sites of action. This pathway is termed ER-associated degradation (ERAD) and is triggered by ER stress.…”

Morphological Studies of Hepatic Tissue of Mice Reinfected With Dengue Virus Serotypes 1 or 2

“…It also contains enzymes involved in drug and steroid metabolism. Several endogenous imbalances in cells, such as massive protein production, loss of calcium homeostasis, inhibition of N-linked glycosylation, and accumulation of mutant protein, often contribute to its malfunction (Yu et al 2006). One of the major morphological changes in Flavivirus-infected cells is the proliferation and hypertrophy of endoplasmic reticulum membranes, where virus particles accumulate inside (Barth 2000, Burke & Monath 2001, Hase et al 1992.…”