Hepatitis C virus envelope proteins regulate CHOP via induction of the unfolded protein response

Abstract

Unfolded protein response (UPR) is a cellular adaptive response that functions to reduce stress caused by malfolded proteins in the endoplasmic reticulum (ER). UPR can be induced under physiological or pathological conditions and is responsible for the pathogenesis of many human diseases. Hepatitis C virus (HCV) is a single-stranded, positive-sense RNA virus causing chronic diseases. Its genome encodes two envelope proteins E1 and E2, which mature in the ER to form a noncovalently bound, native complex and disulfide aggregates and have previously been shown to induce expression of the molecular chaperone immunoglobulin heavy chain binding protein. In this study, we show that HCV envelope protein expression regulates another stress indicator CCAAT/enhancer-binding protein-homologous protein (CHOP). The ER-stress element and the activating transcription factor 4 element in the CHOP promoter were activated to a similar extent by HCV envelope protein expression. Using mouse embryonic fibroblasts deficient in the ER stress kinase RNA-activated protein kinase-like ER-resident kinase (PERK), we showed that PERK was necessary and sufficient for activating the CHOP promoter. Expression of HCV E1 and/or E2 also induced splicing of X-box binding protein 1 and transactivation of the unfolded protein response element, leading to the speculation that HCV E1 and E2 not only regulate the UPR but also ER-associated degradation.

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

“…However, the involvement of ERAD in the life cycle of viruses and infectious diseases remains poorly understood. Until recently, an experimental HCV cell culture infection system has been lacking such that studies evaluating the effect of HCV infection on the ERAD pathway were performed by either using HCV subgenomic replicons which lack structural proteins or by ectopic expression of one or multiple structural proteins (21,22). However, this problem was solved by identifica- tion of an HCV clone, JFH-1, capable of replicating and assembling infectious virus particles in cultured hepatocytes (15).…”

In vivo hepatic endoplasmic reticulum stress in patients with chronic hepatitis C

“…HepG2 cells expressing the HCV core develop ER calcium depletion [20], a mechanism of ER stress [2]. There is indirect evidence of engagement of the UPR (ie ER stress) in cells expressing HCV subgenomic replicons [18,19], structural proteins (ie core [20], E2 alone [21], E1 alone or combined with E2 [22]), or the non-structural protein NS4B [23].…”

“…We then asked whether the expression of genes known to be involved in diffuse processes such as hepatocyte proliferation, liver inflammation, and apoptosis [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] was increased in diseased livers. Thus, we measured the expression mRNA levels of MKi67 (proliferation), JUN (a result of JNK activation involved in proliferation and inflammation), CREB3L3 mRNA (also known as CREBH, encoding CREB-H [31], a hepatocyte-specific bZIP transcription factor of the ATF subfamily which serves as a link between inflammation and the acute-phase response), NF-kappaB-induced anti-apoptotic mRNAs (cFLAR, GADD45B, BCL2A1, and IRE3 ), the short variant of IER3 called IER3S, and p53-inducible pro-apoptotic mRNAs (BBC3, PMAIP1, BAX, and FAS ).…”

“…Envelope proteins E1 and E2 reside in the ER lumen and the viral replicase is assumed to localize on ER-derived membranes [15][16][17], leading to the possibility of inducing the ER-resident stress sensors. In vitro studies have shown that ER stress may be triggered by HCV subgenomic replicons [18,19], structural or non-structural proteins [20][21][22][23]. In vitro studies also suggest that HCV may interfere with the UPR to inhibit it [24].…”

Activation of endoplasmic reticulum stress response by hepatitis viruses up-regulates protein phosphatase 2A

“…For HCV it has been published that the expression of envelope E1 and E2 proteins, 23,24 the expression of HCV core proteins 25 or the transfection of subgenomic replicons 26 ) induce an ER stress response. But there are also publications reporting that HCV disrupts (part of) the ER stress response.…”