Date Published: March 28, 2013
Publisher: Public Library of Science
Author(s): Seong-Jun Kim, Gulam H. Syed, Aleem Siddiqui, Jing-hsiung James Ou.
Hepatitis C Virus (HCV) induces intracellular events that trigger mitochondrial dysfunction and promote host metabolic alterations. Here, we investigated selective autophagic degradation of mitochondria (mitophagy) in HCV-infected cells. HCV infection stimulated Parkin and PINK1 gene expression, induced perinuclear clustering of mitochondria, and promoted mitochondrial translocation of Parkin, an initial event in mitophagy. Liver tissues from chronic HCV patients also exhibited notable levels of Parkin induction. Using multiple strategies involving confocal and electron microscopy, we demonstrated that HCV-infected cells display greater number of mitophagosomes and mitophagolysosomes compared to uninfected cells. HCV-induced mitophagy was evidenced by the colocalization of LC3 puncta with Parkin-associated mitochondria and lysosomes. Ultrastructural analysis by electron microscopy and immunoelectron microscopy also displayed engulfment of damaged mitochondria in double membrane vesicles in HCV-infected cells. The HCV-induced mitophagy occurred irrespective of genotypic differences. Silencing Parkin and PINK1 hindered HCV replication suggesting the functional relevance of mitophagy in HCV propagation. HCV-mediated decline of mitochondrial complex I enzyme activity was rescued by chemical inhibition of mitophagy or by Parkin silencing. Overall our results suggest that HCV induces Parkin-dependent mitophagy, which may have significant contribution in mitochondrial liver injury associated with chronic hepatitis C.
Hepatitis C virus (HCV) infection most often leads to chronic hepatitis, which can progress to steatosis, fibrosis, cirrhosis, and hepatocellular carcinoma . HCV RNA genome encodes a polyprotein, which includes; core, E1, E2, p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B . Viral RNA replicates in the endoplasmic reticulum (ER)-derived modified membranous structures and is subsequently assembled on lipid droplets , . Most of the viral proteins are either associated with or tethered to the ER . These associations and relevant activities overburden the ER and induce an ER stress response exhibited by the unfolded protein response (UPR) .
Autophagy involves clearance of protein aggregates, damaged mitochondria, peroxisosmes and bacteria and viruses . A growing body of literature on autophagy implicates its role in cellular homeostasis, innate immunity, defense mechanisms against lethal entities and in maintenance of persistent viral infections . Pathogens have developed strategies to usurp autophagic pathways and hijack the machinery to favor viral proliferation and persistent infection . Several reports described the HCV-induced events of bulk autophagy and have linked this pathway to aiding viral proliferation , , , , , . The requirement of autophagy was emphatically shown by silencing key components of autophagy (Atg5, Atg7, Beclin-1, Atg8) respectively, and observing a decline in viral translation and replication process , , , , , . Here, for the first time, we demonstrate that HCV induces organelle selective autophagic degradation of mitochondria (mitophagy). This was shown by marked translocation of Parkin to mitochondria (Figures 1, S3, and S11) in HCV-infected cells. Parkin translocation to mitochondria is considered a hallmark of mitophagy . We also observed a significant stimulation of Parkin expression in both HCV-infected Huh7 cells and liver tissues samples obtained from chronic HCV patients (Figure 3). Similarly, HCV also stimulated the expression of PINK1, another key component of mitophagy involved in recruitment of Parkin to damaged mitochondria and its subsequent activation.