Hepatitis C virus NS3/4A protease cleaves SPG20, a key regulator of lipid droplet turnover, to promote lipid droplet formation
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Hepatitis C virus (HCV) assembles in close proximity to lipid droplets (LDs), which play important roles in HCV RNA replication. HCV infection often causes the accumulation of large LDs in hepatocytes. However, the molecular mechanism underlying HCV-induced large LD formation is poorly understood. It has been reported that the SPG20/Spartin protein associates with the LD surface and plays a crucial role in LD turnover by recruiting the ubiquitin ligase Itch to promote the ubiquitin-dependent degradation of adipophilin (ADRP), which protects LDs from lipase-mediated degradation. To elucidate the mechanism underlying HCV-induced large LD formation, we investigated the SPG20 protein’s role in LD formation in HCV J6/JFH1-infected Huh-7.5 cells. Immunoblot analysis revealed that HCV infection promoted SPG20 protein cleavage. Transfection of increasing amounts of NS3/4A, but not the inactive NS3/4A mutant, resulted in SPG20 cleavage, implicating the NS3/4A protease in this cleavage. Site-directed mutagenesis suggested that the NS3/4A protease cleaves SPG20 at Cys 504 and Cys 562 . The SPG20 protein was co-immunoprecipitated with the LD-attached protein TIP47. Increasing amounts of NS3/4A protease, but not inactive NS3/4A, decreased the co-precipitation of SPG20 with TIP47. The siRNA-mediated knockdown of Itch in Huh-7.5 cells restored ADRP levels, suggesting that Itch mediates ubiquitylation-dependent ADRP degradation. Immunofluorescence staining of HCV-infected cells revealed that ADRP was localized mainly around LDs in HCV-infected cells, whereas cytosolic ADRP was decreased. We propose that the HCV NS3/4A protease specifically cleaves SPG20 and inhibits Itch-mediated ubiquitin-dependent degradation of LD-associated ADRP, thereby promoting the formation of large LDs.
IMPORTANCE
HCV infection often promotes the formation of large LDs in HCV-infected cells. However, the molecular mechanism underlying large LD formation is poorly understood. LD turnover is regulated by SPG20, Itch E3 ligase, and ADRP. To elucidate the mechanism underlying the formation of large LDs induced by HCV infection, we investigated the roles of SPG20, Itch, and ADRP in large LD formation. The HCV NS3/4A protease specifically cleaves SPG20 and disrupts Itch recruitment to LD-associated ADRP. Therefore, LD-associated ADRP can escape and protects LDs from lipase-mediated degradation, thereby promoting LD growth. We propose that HCV NS3/4A protease-mediated cleavage of SPG20 contributes to a previously uncharacterized mechanism underlying the formation of large LDs in HCV-infected cells. These findings may lead to a better understanding of how the virus forms large LDs in infected cells.
