Hepatitis C virus co-opts innate immunity component for lipid droplet formation
Authors: Daniel J. Felmlee, Thomas F. Baumert
Journal: Journal of Hepatology (2013)

COMMENTARY ON: Hepatitis C virus infection activates an innate pathway involving IKK-α in lipogenesis and viral assembly. Li Q, Pène V, Krishnamurthy S, Cha H, Liang TJ. Nat Med. 2013 Jun;19(6):722-9. Copyright (2012). Abstract reprinted by permission from Macmillan Publishers Ltd. http://www.ncbi.nlm.nih.gov/pubmed/23708292 Hepatitis C virus (HCV) interacts extensively with host factors to not only establish productive infection but also trigger unique pathological processes. Our recent genome-wide siRNA screen demonstrated that IκB kinase-α (IKK-α) is a crucial host factor for HCV. Here we describe a new nuclear factor κB (NF-κB)-independent and kinase-mediated nuclear function of IKK-α in HCV assembly. HCV, through its 3′ untranslated region, interacts with DEAD box polypeptide 3, X-linked (DDX3X) to activate IKK-α, which translocates to the nucleus and induces a CBP/p300-mediated transcriptional program involving sterol regulatory element-binding proteins (SREBPs). This innate pathway induces lipogenic genes and enhances core-associated lipid droplet formation to facilitate viral assembly. Chemical inhibitors of IKK-α suppress HCV infection and IKK-α-induced lipogenesis, offering a proof-of-concept approach for new HCV therapeutic development. Our results show that HCV uses a novel mechanism to exploit intrinsic innate responses and hijack lipid metabolism, which may contribute to high chronicity rates and the pathological hallmark of steatosis in HCV infection. © European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved One of the pathological hallmarks of hepatitis C virus (HCV) infection includes the accumulation of cytosolic lipid droplets resulting in steatosis. Upon infection, HCV utilizes lipid droplets as platforms for viral assembly [[1]Miyanari Y. Atsuzawa K. Usuda N. Watashi K. Hishiki T. Zayas M. et al.The lipid droplet is an important organelle for hepatitis C virus production.Nat Cell Biol. 2007; 9: 1089-1097Crossref PubMed Scopus (1007) Google Scholar]. Another feature of HCV is its interaction with innate immunity pathways [[2]Yamane D. McGivern D.R. Masaki T. Lemon S.M. Liver injury and disease pathogenesis in chronic hepatitis C.Curr Top Microbiol Immunol. 2013; 369: 263-288Crossref PubMed Scopus (59) Google Scholar]. Viral pathogen-associated molecular patterns (PAMPs) activate inflammatory responses including the induction of interferons. Qisheng Li and colleagues from Jake Liang's laboratory at the Liver Diseases Branch, NIDDK, National Institutes of Health, Bethesda have revealed a new connection between innate response and lipid accumulation, unexpectedly involving a protein in the nuclear factor κB (NF-κB) pathway [[3]Li Q. Pene V. Krishnamurthy S. Cha H. Liang T.J. Hepatitis C virus infection activates an innate pathway involving IKK-alpha in lipogenesis and viral assembly.Nat Med. 2013; 19: 722-729Crossref PubMed Scopus (157) Google Scholar]. Through a previous two-step siRNA screen that distinguished HCV entry and replication from production, the authors identified IκB kinase-α (IKK-α) as being critical in HCV infection [[4]Li Q. Brass A.L. Ng A. Hu Z. Xavier R.J. Liang T.J. et al.A genome-wide genetic screen for host factors required for hepatitis C virus propagation.Proc Natl Acad Sci U S A. 2009; 106: 16410-16415Crossref PubMed Scopus (313) Google Scholar]. Genetic knockdown of IKK-α dramatically inhibited HCV propagation primarily at the level of HCV assembly, while IKK-α overexpression boosted the quantity of HCV produced. Through using multiple strategies, the authors confirmed that IKK-α is specifically critical for HCV assembly. The canonical NF-κB pathway involves retention of NF-κB in the cytosol in complex with inhibitors of NF-κB (IκB) [[5]Perkins N.D. Integrating cell-signalling pathways with NF-kappaB and IKK function.Nat Rev Mol Cell Biol. 2007; 8: 49-62Crossref PubMed Scopus (1998) Google Scholar]. Phosphorylation of IκB by the IκB kinase (IKK) complex target IκB for degradation, releasing NF-κB to translocate to the nucleus and transcriptionally activate target genes (Fig. 1). The canonical pathway is driven by the kinase activity of IKK-β, whereas NEMO has a regulatory function, and IKK-α predominantly functions in an alternate, non-canonical pathway [[5]Perkins N.D. Integrating cell-signalling pathways with NF-kappaB and IKK function.Nat Rev Mol Cell Biol. 2007; 8: 49-62Crossref PubMed Scopus (1998) Google Scholar]. However, recent evidence suggests that IKK-α additionally has NF-κB independent functions, which are an ongoing intense area of research. HCV assembly and production rely on lipid droplets and HCV infection results in an increased quantity of these organelles. Li et al. showed that knockdown of IKK-α reversed this increase in lipid storage, whereas overexpression boosted lipid droplet count over and above HCV infection alone, indicative of IKK-α being a critical mediator of HCV-induced lipid accumulation [[3]Li Q. Pene V. Krishnamurthy S. Cha H. Liang T.J. Hepatitis C virus infection activates an innate pathway involving IKK-alpha in lipogenesis and viral assembly.Nat Med. 2013; 19: 722-729Crossref PubMed Scopus (157) Google Scholar]. Knowing that there are PAMPs present in the 3′ untranslated region (UTR) of the HCV genome, the authors found that this region activates lipid droplet formation through phosphorylation and activation of IKK-α. Surprisingly, this role of IKK-α is not affected when the other elements of the NF-κB pathway are either silenced or activated; IKK-α mediates HCV lipid droplet induction independently of other NF-κB pathway components. To define the connection between the HCV 3′UTR, IKK-α, and lipid droplet induction, the authors looked to a recently identified PAMP recognition receptor (PRR) important in innate responses and HCV infection, DDX3X. Interestingly, the presence of the HCV 3′UTR induced a complex formation including the 3′UTR, DDX3X, and IKK-α. The formation of this complex results in the phosphorylation, nuclear translocation, and activation of IKK-α [[3]Li Q. Pene V. Krishnamurthy S. Cha H. Liang T.J. Hepatitis C virus infection activates an innate pathway involving IKK-alpha in lipogenesis and viral assembly.Nat Med. 2013; 19: 722-729Crossref PubMed Scopus (157) Google Scholar]. The above discoveries already represent a substantial advance in HCV biology, but the remaining piece of the puzzle is how all of this mechanism results in lipid droplet accumulation. The investigators examined IKK-α transcriptional targets by coupling siRNA of IKK-α with microarray and found a dramatic downregulation of sterol response element binding proteins (SREBPs) and their target genes. SREBPs are transcription factors that are master regulators of lipid synthesis [[6]Horton J.D. Goldstein J.L. Brown M.S. SREBPs: activators of the complete program of cholesterol and fatty acid synthesis in the liver.J Clin Invest. 2002; 109: 1125-1131Crossref PubMed Scopus (3963) Google Scholar]. HCV infection increased SREBP mRNA levels, a transcriptional activation that could be reversed by knockdown of IKK-α. The authors then revealed that SREBP-1 transcriptional activation was not only facilitated by IKK-α binding the SREBP-1 promoter, but also through IKK-α activation of another critical transcription factor, CREB binding protein (CBP/p300) [[7]Huang W.C. Ju T.K. Hung M.C. Chen C.C. Phosphorylation of CBP by IKKalpha promotes cell growth by switching the binding preference of CBP from p53 to NF-kappaB.Mol Cell. 2007; 26: 75-87Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar]. This study represents an astounding advance of understanding HCV infection, innate responses, and lipid metabolism. The cellular response of HCV PAMP recognition ultimately results in induction of lipogenesis, in turn stimulating HCV viral production. Co-opting factors of innate immunity for viral propagation is a dramatic adaptation that likely facilitates HCV being so effective in establishing chronic infection. Aside from providing a possible mechanism for the clinical observations of HCV induced steatosis, the study reveals a full and complex pathway, rich in possible therapeutic targets. As an increasing population is taking lipid-modulating drugs including statins, these results may inform unwanted side-effects of these therapies in HCV-infected individuals. Indeed, statins increase SREBP-1 transcription [[8]DeBose-Boyd R.A. Ou J. Goldstein J.L. Brown M.S. Expression of sterol regulatory element-binding protein 1c (SREBP-1c) mRNA in rat hepatoma cells requires endogenous LXR ligands.Proc Natl Acad Sci U S A. 2001; 98: 1477-1482Crossref PubMed Scopus (206) Google Scholar], yet evidence indicates that statins act synergistically with pegylated interferon and ribavirin to inhibit infection [[9]McHutchison J.G. Lawitz E.J. Shiffman M.L. Muir A.J. Galler G.W. McCone J. et al.Peginterferon alfa-2b or alfa-2a with ribavirin for treatment of hepatitis C infection.N Engl J Med. 2009; 361: 580-593Crossref PubMed Scopus (1133) Google Scholar]. The complex interaction of HCV with lipids will continue to be an important consideration as the next generation of direct acting antiviral agents are tested and utilized. Although the screen did not reveal IKK-α as critical for HIV-1, Dengue, West Nile, or influenza infection [[3]Li Q. Pene V. Krishnamurthy S. Cha H. Liang T.J. Hepatitis C virus infection activates an innate pathway involving IKK-alpha in lipogenesis and viral assembly.Nat Med. 2013; 19: 722-729Crossref PubMed Scopus (157) Google Scholar], the role of the DDX3X-IKK-α complex will be assertively investigated for its role in other pathogenic and inflammatory diseases. Interestingly, a single nucleotide polymorphism associated with non-alcoholic fatty liver disease has been located in the gene encoding IKK-α [[10]Romeo S. Kozlitina J. Xing C. Pertsemlidis A. Cox D. Pennacchio L.A. et al.Genetic variation in PNPLA3 confers susceptibility to nonalcoholic fatty liver disease.Nat Genet. 2008; 40: 1461-1465Crossref PubMed Scopus (2441) Google Scholar]. The recent investigation by Li et al. promises to act as a springboard to a new paradigm of liver disease, lipid metabolism, and inflammation. The authors acknowledge support by Inserm, ANRS (13081), University of Strasbourg, the European Union (INTERREG-IV-Rhin Supérieur-FEDER-Hepato-Regio-Net 2009 and 2012, FP7 HEPAMAB GAN 305600), ERC AdG HEPCENT 233130, Direction Générale de l'Offre de Soins, and the Laboratoire d'excellence HEPSYS (ANR-10-LAB-28). D.J.F. acknowledges support from an EASL and an ANRS fellowship. The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.