Hepatitis C virus and its complex interplay with hepatic glucose and lipid metabolism
Authors: Birke Bartosch
Journal: Journal of Hepatology (2009)

HCV replication suppresses cellular glucose uptake through down-regulation of cell surface expression of glucose transportersJournal of HepatologyVol. 50Issue 5PreviewPersistent infection with hepatitis C virus (HCV) causes extrahepatic diseases, including diabetes. We investigated the possible effect(s) of HCV replication on cellular glucose uptake and expression of the facilitative glucose transporter (GLUT) 2 and 1. Full-Text PDF Chronic hepatitis C, which affects over 3% of the world's population, is characterized by inflammatory liver lesions, steatosis and progressive fibrosis of variable intensity. Each year, 4–5% of chronic hepatitis C patients develop cirrhosis and hepatocellular carcinoma (HCC). Hepatitis C has become the main indication for liver transplantation in industrialised countries and will soon be the leading cause of HCC in Western Europe [[1]Alter M.J. Epidemiology of hepatitis C virus infection.World J Gastroenterol. 2007; 13: 2436-2441Crossref PubMed Scopus (1008) Google Scholar]. Treatment options, which are based on interferon treatment in combination with the nucleoside analogue ribavirin, remain limited and treatment success varies depending on the viral genotype. Insulin resistance and type 2 diabetes are common complications of all liver diseases, especially at the advanced stage. Chronic hepatitis C virus (HCV) infection is associated with an increased risk of developing glucose intolerance and diabetes and both clinical and experimental observations suggest that HCV may directly interfere with glucose homeostasis already at early stages of infection [2Fartoux L. Poujol-Robert A. Guechot J. Wendum D. Poupon R. Serfaty L. et al.Insulin resistance is a cause of steatosis and fibrosis progression in chronic hepatitis C.Gut. 2005; 54: 1003-1008Crossref PubMed Scopus (338) Google Scholar, 3Hui J.M. Sud A. Farrell G.C. Bandara P. Byth K. Kench J.G. et al.Insulin resistance is associated with chronic hepatitis C virus infection and fibrosis progression [corrected].Gastroenterology. 2003; 125: 1695-1704Abstract Full Text Full Text PDF PubMed Scopus (668) Google Scholar, 4Moucari R. Asselah T. Cazals-Hatem D. Voitot H. Boyer N. Ripault M.P. et al.Insulin resistance in chronic hepatitis C: association with genotypes 1 and 4, serum HCV RNA level, and liver fibrosis.Gastroenterology. 2008; 134: 416-423Abstract Full Text Full Text PDF PubMed Scopus (464) Google Scholar]. HCV has been shown to directly interfere with the insulin signaling pathway by different molecular mechanisms in a genotype dependent manner, and all HCV genotypes have been reported to induce insulin resistance. Furthermore, some studies have reported positive correlations between the level of insulin resistance and HCV replication levels [5Shintani Y. Fujie H. Miyoshi H. Tsutsumi T. Tsukamoto K. Kimura S. et al.Hepatitis C virus infection and diabetes: direct involvement of the virus in the development of insulin resistance.Gastroenterology. 2004; 126: 840-848Abstract Full Text Full Text PDF PubMed Scopus (671) Google Scholar, 6Bernsmeier C. Duong F.H. Christen V. Pugnale P. Negro F. Terracciano L. et al.Virus-induced over-expression of protein phosphatase 2A inhibits insulin signalling in chronic hepatitis C.J Hepatol. 2008; 49: 429-440Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar, 7Duong F.H. Christen V. Berke J.M. Penna S.H. Moradpour D. Heim M.H. Upregulation of protein phosphatase 2Ac by hepatitis C virus modulates NS3 helicase activity through inhibition of protein arginine methyltransferase 1.J Virol. 2005; 79: 15342-15350Crossref PubMed Scopus (72) Google Scholar, 4Moucari R. Asselah T. Cazals-Hatem D. Voitot H. Boyer N. Ripault M.P. et al.Insulin resistance in chronic hepatitis C: association with genotypes 1 and 4, serum HCV RNA level, and liver fibrosis.Gastroenterology. 2008; 134: 416-423Abstract Full Text Full Text PDF PubMed Scopus (464) Google Scholar]. For these reasons, insulin resistance is considered to be an important determinant for the establishment and/or maintenance of chronic HCV infection and its roles in HCV replication and associated disease are therefore important areas of research. The paper by Kasai et al. [[8]Kasai D. Adachi T. Deng L. Nagano-Fujii M. Sada K. Ikeda M. et al.HCV replication suppresses cellular glucose uptake through down-regulation of cell surface expression of glucose transporters.J Hepatol. 2009; 50: 883-894Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar] in this issue investigates the effects HCV infection on hepatic glucose signaling, which is closely linked to insulin signaling cascades; the authors report that HCV suppresses liver glucose uptake and correlate this finding with downregulation or modulation of the cell surface expression of the hepatic glucose transporters GLUT1 and GLUT2. Glucose is cleared from the bloodstream by a family of facilitative transporters (GLUTs), which catalyze the transport of glucose down its concentration gradient and into cells of target tissues. Currently, several established functional facilitative glucose transporter isoforms have been identified [[9]Zhao F.Q. Keating A.F. Functional properties and genomics of glucose transporters.Curr Genomics. 2007; 8: 113-128Crossref PubMed Scopus (417) Google Scholar], but only two with expression and activity in the liver. GLUT1 is ubiquitously expressed, but present at particularly high levels in human erythrocytes and in the endothelial cells of barrier tissues such as the blood–brain barrier [[10]Mueckler M. Hresko R.C. Sato M. Structure, function and biosynthesis of GLUT1.Biochem Soc Trans. 1997; 25: 951-954Crossref PubMed Scopus (33) Google Scholar]. It is responsible for the low-level of basal glucose uptake required to sustain respiration in all cells. GLUT2 is a low affinity transporter for glucose. Its expression occurs mainly in the liver, pancreas, kidney and in intestinal absorptive epithelial cells where it is located in the basolateral membrane. In the liver its expression levels have been shown to directly correlate with blood glucose levels. Thus, in hepatocytes, GLUT2 mediates influx of glucose from the blood into the liver, as well as efflux of glucose that has been synthesized by gluconeogenesis in the liver into the blood [11Wright E.M. Turk E. The sodium/glucose cotransport family SLC5.Pflugers Arch. 2004; 447: 510-518Crossref PubMed Scopus (28) Google Scholar, 12Kellett G.L. Brot-Laroche E. Apical GLUT2: a major pathway of intestinal sugar absorption.Diabetes. 2005; 54: 3056-3062Crossref PubMed Scopus (286) Google Scholar, 13Guillam M.T. Hummler E. Schaerer E. Yeh J.I. Birnbaum M.J. Beermann F. et al.Early diabetes and abnormal postnatal pancreatic islet development in mice lacking Glut-2.Nat Genet. 1997; 17: 327-330Crossref PubMed Scopus (344) Google Scholar, 14Bady I. Marty N. Dallaporta M. Emery M. Gyger J. Tarussio D. et al.Evidence from glut2-null mice that glucose is a critical physiological regulator of feeding.Diabetes. 2006; 55: 988-995Crossref PubMed Scopus (108) Google Scholar, 15Uldry M. Ibberson M. Hosokawa M. Thorens B. GLUT2 is a high affinity glucosamine transporter.FEBS Lett. 2002; 524: 199-203Abstract Full Text Full Text PDF PubMed Scopus (235) Google Scholar]. Besides liver GLUT2, additional pathways for uptake and efflux of newly synthesized glucose into the blood have been put into evidence, but remain to be identified [14Bady I. Marty N. Dallaporta M. Emery M. Gyger J. Tarussio D. et al.Evidence from glut2-null mice that glucose is a critical physiological regulator of feeding.Diabetes. 2006; 55: 988-995Crossref PubMed Scopus (108) Google Scholar, 16Guillam M.T. Burcelin R. Thorens B. Normal hepatic glucose production in the absence of GLUT2 reveals an alternative pathway for glucose release from hepatocytes.Proc Natl Acad Sci USA. 1998; 95: 12317-12321Crossref PubMed Scopus (100) Google Scholar, 17Hosokawa M. Thorens B. Glucose release from GLUT2-null hepatocytes: characterization of a major and a minor pathway.Am J Physiol Endocrinol Metab. 2002; 282: E794-E801Crossref PubMed Scopus (38) Google Scholar]. Kasai et al. [[8]Kasai D. Adachi T. Deng L. Nagano-Fujii M. Sada K. Ikeda M. et al.HCV replication suppresses cellular glucose uptake through down-regulation of cell surface expression of glucose transporters.J Hepatol. 2009; 50: 883-894Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar] investigated the effect of HCV infection on cellular glucose transport and report that in the hepatoma cell line Huh7.5, glucose uptake is reduced by 50 to 60% either upon infection with HCV of genotype 2a or upon introduction of genomic or subgenomic HCV replicons of genotype 1b. The reduction in glucose uptake was restored to wild-type levels upon elimination of HCV virus or replicons from the cells via interferon treatment. Thus reduction in glucose uptake seemed not only to be specifically induced by HCV, but was observed to similar extents in the context of genotype 2a as well as 1b, suggesting that the modulation of glucose transport by HCV may be conserved across the different HCV genotypes. Kasai et al. [[8]Kasai D. Adachi T. Deng L. Nagano-Fujii M. Sada K. Ikeda M. et al.HCV replication suppresses cellular glucose uptake through down-regulation of cell surface expression of glucose transporters.J Hepatol. 2009; 50: 883-894Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar] furthermore reported a reduced surface staining of the glucose transporters GLUT1 and 2 in HCV replicon bearing cells. Upon infection with HCV virus, GLUT2 surface expression levels also significantly decreased, while GLUT1 expression was barely affected, suggesting that in the context of an infection, the inhibitory effects of HCV on glucose uptake are mainly mediated by changes in GLUT2 surface expression. The authors demonstrate furthermore, that reduced cell surface expression of GLUT2 and GLUT1 was not due to proteasomal degradation, but in the case of GLUT2 correlated well with a ca 2-fold reduction of its mRNA levels. Transcription of the GLUT2 gene returned to normal upon interferon treatment, and the use of a reporter construct containing the GLUT2 promoter further confirmed a suppressive effect of HCV on GLUT2 transcription. GLUT1 transcription was not modified by presence of HCV replicons or virus, suggesting that the downregulation of GLUT1 from the cell surface by HCV replicons occurs by modification of its intracellular localization. The authors were furthermore able to confirm HCV-induced GLUT2 downregulation in vivo. In liver biopsies from HCV infected individuals GLUT2 specific immunostaining was weaker than in livers from healthy patients. GLUT2 downregulation in vivo was also observed in liver biopsies from chronic hepatitis B patients. In chronic hepatitis C, one important aspect of insulin resistance is thought to be due to its regulatory effect on genes and pathways involved in lipogenesis [18Yang W. Hood B.L. Chadwick S.L. Liu S. Watkins S.C. Luo G. et al.Fatty acid synthase is up-regulated during hepatitis C virus infection and regulates hepatitis C virus entry and production.Hepatology. 2008; 48: 1396-1403Crossref PubMed Scopus (156) Google Scholar, 19Jackel-Cram C. Babiuk L.A. Liu Q. Up-regulation of fatty acid synthase promoter by hepatitis C virus core protein: genotype-3a core has a stronger effect than genotype-1b core.J Hepatol. 2007; 46: 999-1008Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar, 20Oem J.K. Jackel-Cram C. Li Y.P. Zhou Y. Zhong J. Shimano H. et al.Activation of sterol regulatory element-binding protein 1c and fatty acid synthase transcription by hepatitis C virus non-structural protein 2.J Gen Virol. 2008; 89: 1225-1230Crossref PubMed Scopus (93) Google Scholar]. Lipogenesis and concomitant stimulation of triglyceride synthesis and storage in lipid droplets as well as increased vLDL assembly and secretion are thought to stimulate or even be required for the HCV assembly process [21Miyanari 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 (987) Google Scholar, 22Moradpour D. Penin F. Rice C.M. Replication of hepatitis C virus.Nat Rev Microbiol. 2007; 5: 453-463Crossref PubMed Scopus (1113) Google Scholar]. While SREBP-1c has emerged as a major mediator of insulin action on cholesterol and lipogenic gene regulation and is frequently found upregulated in HCV infection, it is also required for transcriptional activation of enzymes that convert glucose into glucose 6-phosphate (G6P). G6P is the precursor for glycolysis, a process that provides carbon for subsequent de novo lipid synthesis, but is also the form in which glucose exerts its transcriptional activity. Indeed transcription of glycolytic as well as lipogenic genes such as acetyl CoA carboxylase (ACC) and fatty acid synthase (FAS) has been shown to be regulated by insulin signaling pathways/ SREBP-1c but also by glucose [[23]Dentin R. Girard J. Postic C. Carbohydrate responsive element binding protein (ChREBP) and sterol regulatory element binding protein-1c (SREBP-1c): two key regulators of glucose metabolism and lipid synthesis in liver.Biochimie. 2005; 87: 81-86Crossref PubMed Scopus (278) Google Scholar]. Glucose stimulates lipogenesis by activating the glucose-responsive basic/helix-loop-helix/leucine zipper transcription factor carbohydrate responsive element binding protein (ChREBP). In particular, glucose regulates the entry of ChREBP from the cytosol into the nucleus in order to allow its interaction with carbohydrate responsive elements (ChoRE) present in the promoter regions of glycolytic and lipogenic genes [[24]Denechaud P.D. Dentin R. Girard J. Postic C. Role of ChREBP in hepatic steatosis and insulin resistance.FEBS Lett. 2008; 582: 68-73Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar]. Indeed, ChREBP silencing in hepatocytes and mice has been shown to cause a significant reduction in lipid synthesis [[25]Dentin R. Pegorier J.P. Benhamed F. Foufelle F. Ferre P. Fauveau V. et al.Hepatic glucokinase is required for the synergistic action of ChREBP and SREBP-1c on glycolytic and lipogenic gene expression.J Biol Chem. 2004; 279: 20314-20326Crossref PubMed Scopus (349) Google Scholar]. Thus, ChREBP acts in synergy with SREBP-1c in response to glucose and insulin, respectively, to induce lipogenic genes such as FAS and ACC. The downregulation of GLUT1 and GLUT2 by HCV observed by Kasai et al. [[8]Kasai D. Adachi T. Deng L. Nagano-Fujii M. Sada K. Ikeda M. et al.HCV replication suppresses cellular glucose uptake through down-regulation of cell surface expression of glucose transporters.J Hepatol. 2009; 50: 883-894Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar] raises the interesting scenario, where HCV may modulate activity of lipogenic enzymes not only by dys-regulating insulin signaling, such as by modifying SREBP-1c induced transcription, but also by dys-regulating glucose sensitive transcription factors like ChREBP. The exact regulation of ChREBP activation is not yet elucidated and it is thought that besides glucose, additional factors stimulate its nuclear translocation [[26]Davies M.N. O'Callaghan B.L. Towle H.C. Glucose activates ChREBP by increasing its rate of nuclear entry and relieving repression of its transcriptional activity.J Biol Chem. 2008; 283: 24029-24038Crossref PubMed Scopus (84) Google Scholar]. Indeed, the lipogenic activity of ChREBP is also under control of protein kinase A, AMP-activated protein kinase as well as protein phosphatase 2A [[27]Iizuka K. Horikawa Y. ChREBP: a glucose-activated transcription factor involved in the development of metabolic syndrome.Endocr J. 2008; 55: 617-624Crossref PubMed Scopus (168) Google Scholar], enzymes that in turn have been shown to be modulated by HCV in a fashion that favors lipogenesis [6Bernsmeier C. Duong F.H. Christen V. Pugnale P. Negro F. Terracciano L. et al.Virus-induced over-expression of protein phosphatase 2A inhibits insulin signalling in chronic hepatitis C.J Hepatol. 2008; 49: 429-440Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar, 28Borowski P. Heiland M. Oehlmann K. Becker B. Kornetzky L. Feucht H. et al.Non-structural protein 3 of hepatitis C virus inhibits phosphorylation mediated by cAMP-dependent protein kinase.Eur J Biochem. 1996; 237: 611-618Crossref PubMed Scopus (45) Google Scholar, 29Aoubala M. Holt J. Clegg R.A. Rowlands D.J. Harris M. The inhibition of cAMP-dependent protein kinase by full-length hepatitis C virus NS3/4A complex is due to ATP hydrolysis.J Gen Virol. 2001; 82: 1637-1646Crossref PubMed Scopus (22) Google Scholar]. But how a cellular microenvironment that favors lipogenesis, is compatible with an HCV-induced decrease of intracellular glucose, raises the important question where the carbon required for lipogenesis would originate from. While it remains to be seen whether HCV-induced suppression of glucose uptake in Huh7.5 cells holds true under more physiologically relevant conditions, such as in primary human hepatocytes, overall not much is known regarding the efficacy of hepatic glucose uptake in insulin resistant states. The study by Kasai et al. [[8]Kasai D. Adachi T. Deng L. Nagano-Fujii M. Sada K. Ikeda M. et al.HCV replication suppresses cellular glucose uptake through down-regulation of cell surface expression of glucose transporters.J Hepatol. 2009; 50: 883-894Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar] in this issue certainly adds a new level of complexity to the interactions of HCV with the hepatic glucose and lipid metabolism, and underlines their importance in the viral life cycle and its potential pathogenic sequelae. The burning questions that remain are, whether GLUT2 downregulation is a requirement for HCV replication and if so, what are the consequences of GLUT2 downregulation on lipogenesis. It will furthermore be interesting to find out whether the previously reported association of GLUT2 with the insulin receptor [[30]Eisenberg M.L. Maker A.V. Slezak L.A. Nathan J.D. Sritharan K.C. Jena B.P. et al.Insulin receptor (IR) and glucose transporter 2 (GLUT2) proteins form a complex on the rat hepatocyte membrane.Cell Physiol Biochem. 2005; 15: 51-58Crossref PubMed Scopus (42) Google Scholar] is of any importance in these processes. Answers to these questions may help us to delineate not only the molecular mechanisms underlying HCV replication but will also help to elucidate the roles of GLUTs in glucose and insulin signaling. The author thanks Dr. Thierry Buronfosse for critical reading of the paper.