Hepatitis C Virus Infection Induces Hepatic Expression of NF-κB-Inducing Kinase and Lipogenesis by Downregulating miR-122.

Hepatitis C virus (HCV) harnesses host dependencies to infect human hepatocytes. We previously identified a pivotal role of I kappa B kinase alpha (IKK-alpha) in regulating cellular lipogenesis and HCV assembly. In this study, we defined and characterized NF-kappa B-inducing kinase (NIK) as an IKK-alpha upstream serine/threonine kinase in IKK-alpha-mediated proviral effects and the mechanism whereby HCV exploits this innate pathway to its advantage. We manipulated NIK expression in Huh7.5.1 cells through loss- and gain-of-function approaches and examined the effects on IKK-alpha activation, cellular lipid metabolism, and viral assembly. We demonstrated that NIK interacts with IKK-alpha to form a kinase complex in association with the stress granules, in which IKK-alpha is phosphorylated upon HCV infection. Depletion of NIK significantly diminished cytosolic lipid droplet content and impaired HCV particle production. NIK overexpression enhanced HCV assembly, and this process was abrogated in cells deprived of IKK-alpha, suggesting that NIK acts upstream of IKK-alpha. NIK abundance was increased in HCV-infected hepatocytes, liver tissues from Alb-uPA/Scid mice engrafted with human hepatocytes, and chronic hepatitis C patients. NIK mRNA contains an miR-122 seed sequence binding site in the 3' untranslated region (UTR). miR-122 mimic and hairpin inhibitor directly affected NIK levels. In our hepatic models, miR-122 levels were significantly reduced by HCV infection. We demonstrated that HNF4A, a known transcriptional regulator of pri-miR-122, was downregulated by HCV infection. NIK represents a bona fide target of miR-122 whose transcription is downregulated by HCV through reduced HNF4A expression. This effect, together with the sequestering of miR-122 by HCV replication, results in "derepression" of NIK expression to deregulate lipid metabolism. IMPORTANCE Chronic hepatitis C virus (HCV) infection is a major global public health problem. Infection often leads to severe liver injury that may progress to cirrhosis, hepatocellular carcinoma, and death. HCV coopts cellular machineries for propagation and triggers pathological processes in the liver. We previously identified a pivotal role of IKK-alpha in regulating cellular lipid metabolism and HCV assembly. In this study, we characterized NIK as acting upstream of IKK-alpha and characterized how HCV exploits this innate pathway to its advantage. Through extensive mechanistic studies, we demonstrated that NIK is a direct target of miR-122, which is regulated at the transcription level by HNF4A, a hepatocyte-specific transcription factor. We show in HCV infection that NIK expression is increased while both HNF4A and miR-122 levels are decreased. NIK represents an important host dependency that links HCV assembly, hepatic lipogenesis, and miRNA biology.