Hepatitis C Virus Upregulates Beclin1 for Induction of Autophagy and Activates mTOR Signaling

Abstract

Hepatitis C virus (HCV) induces autophagosome formation in infected human hepatocytes. We have previously reported that HCV exploits autophagic machinery in favor of virus growth and survival in host cells (S. Shrivastava et al., Hepatology 53:406 -414, 2011); however, the mechanisms for autophagy induction is poorly understood. In the present study, we observed that HCV infection transcriptionally upregulates Beclin1, which forms complex with Vps34, the class III phosphatidylinositol 3-kinase, as a first step for autophagy initiation. Although Bcl-2 has an anti-autophagy effect by its association with Beclin1 in nutrient-deprived cells, our studies revealed that HCV-mediated autophagy occurs independent of Beclin1-Bcl-2 dissociation. Mammalian target of rapamycin (mTOR) is a positive regulator of cell growth and is recognized as an inhibitor of autophagy induction. Our results demonstrated that HCV infection enhances phospho-mTOR expression and its downstream target 4EBP1 activation, suggesting that mTOR is not a negative regulator of HCV-induced autophagy. On the other hand, HCV infection in autophagyimpaired cells reduced phospho-mTOR, mTOR, and phospho-4EBP1 expression. Together, these results suggested that HCV induces autophagy by upregulating Beclin1 and activates mTOR signaling pathway, which in turn may promote hepatocyte growth.

Hepatitis C Virus Induces Interleukin-1β (IL-1β)/IL-18 in Circulatory and Resident Liver Macrophages

“…Reactive oxygen species (ROS) production has been implicated in the induction of IL-1␤ (27). Impairment of autophagy in HCV-infected hepatocytes enhances ROS production (28). However, we did not observe an induction of inflammasome mediators in autophagy knockdown HCV-infected hepatocytes, suggesting that ROS may not be an effector mechanism for inflammasome induction.…”

Transcriptional Suppression of miR-181c by Hepatitis C Virus Enhances Homeobox A1 Expression

“…Huh7.5 cells were cotransfected with a luciferase reporter plasmid containing the miR-181c promoter (125 ng) and equal amounts of plasmid DNA containing HCV core, E1E2p7, nonstructural proteins (NS2, NS3/4A, NS4B, NS5A, or NS5B), or the full-length genome (FL-HCV) by using Lipofectamine 2000 (Invitrogen). Luciferase activity was determined as previously described (14).…”

Target of Rapamycin (TOR) and Autophagy in Chronic Hepatitis C Virus Infection: Relation to Disease Activity

“…This unexpected finding indicates that autophagy was induced despite mTOR activation. Previous studies found that autophagy was upregulated in HCV-infected hepatocytes with increased expression of mTOR and vice versa [30,95]. This observation is explained by finding that autophagy induction may be mediated by the class III PI3K, Vps34, which is also known to be involved in activation of mTOR [96].…”

“…This suggests that mTOR was activated in chronic HCV infection. In previous studies, expression of total and phosphorylated-mTOR and its downstream substrates was increased in HCV-infected Huh 7.5 cells [30][31][32]. A recent study has also shown that HCV NS5A activated mTOR and its substrates under serum-starved conditions in Huh7 and HCV subgenomic replicon cells [33].…”

“…Thus, mTOR signaling and autophagy may be inversely inter-related or may act concurrently in HCV infection. A potential explanation of this paradox is likely that HCV infection induces autophagy for establishment of infection, while activates mTOR signaling for hepatocyte growth [30].…”