In Vitro Infection with Hepatitis B Virus Using Differentiated Human Serum Culture of Huh7.5-NTCP Cells without Requiring Dimethyl Sulfoxide.

An estimated two billion people worldwide have been infected with hepatitis B virus (HBV). Despite the high infectivity of HBV in vivo, a lack of easily infectable in vitro culture systems hinders studies of HBV. Overexpression of the sodium taurocholate co-transporting polypeptide (NTCP) bile acid transporter in hepatoma cells improved infection efficiency. We report here a hepatoma cell culture system that does not require dimethyl sulfoxide (DMSO) for HBV infection. We overexpressed NTCP in Huh7.5 cells and allowed these cells to differentiate in a medium supplemented with human serum (HS) instead of fetal bovine serum (FBS). We show that human serum culture enhanced HBV infection in Huh7.5-NTCP cells, e.g., in HS cultures, HBV pgRNA levels were increased by as much as 200-fold in comparison with FBS cultures and 19-fold in comparison with FBS+DMSO cultures. Human serum culture increased levels of hepatocyte differentiation markers, such as albumin secretion, in Huh7.5-NTCP cells to similar levels found in primary human hepatocytes. N-glycosylation of NTCP induced by culture in human serum may contribute to viral entry. Our study demonstrates an in vitro HBV infection of Huh7.5-NTCP cells without the use of potentially toxic DMSO.

In Vitro Hepatitis C Virus Infection and Hepatic Choline Metabolism.

Choline is an essential nutrient required for normal neuronal and muscular development, as well as homeostatic regulation of hepatic metabolism. In the liver, choline is incorporated into the main eukaryotic phospholipid, phosphatidylcholine (PC), and can enter one-carbon metabolism via mitochondrial oxidation. Hepatitis C virus (HCV) is a hepatotropic positive-strand RNA virus that similar to other positive-strand RNA viruses and can impact phospholipid metabolism. In the current study we sought to interrogate if HCV modulates markers of choline metabolism following in vitro infection, while subsequently assessing if the inhibition of choline uptake and metabolism upon concurrent HCV infection alters viral replication and infectivity. Additionally, we assessed whether these parameters were consistent between cells cultured in fetal bovine serum (FBS) or human serum (HS), conditions known to differentially affect in vitro HCV infection. We observed that choline transport in FBS- and HS-cultured Huh7.5 cells is facilitated by the intermediate affinity transporter, choline transporter-like family (CTL). HCV infection in FBS, but not HS-cultured cells diminished CTL1 transcript and protein expression at 24 h post-infection, which was associated with lower choline uptake and lower incorporation of choline into PC. No changes in other transporters were observed and at 96 h post-infection, all differences were normalized. Reciprocally, limiting the availability of choline for PC synthesis by use of a choline uptake inhibitor resulted in increased HCV replication at this early stage (24 h post-infection) in both FBS- and HS-cultured cells. Finally, in chronic infection (96 h post-infection), inhibiting choline uptake and metabolism significantly impaired the production of infectious virions. These results suggest that in addition to a known role of choline kinase, the transport of choline, potentially via CTL1, might also represent an important and regulated process during HCV infection.

Establishing normal metabolism and differentiation in hepatocellular carcinoma cells by culturing in adult human serum.

Tissue culture medium routinely contains fetal bovine serum (FBS). Here we show that culturing human hepatoma cells in their native, adult serum (human serum, HS) results in the restoration of key morphological and metabolic features of normal liver cells. When moved to HS, these cells show differential transcription of 22-32% of the genes, stop proliferating, and assume a hepatocyte-like morphology. Metabolic analysis shows that the Warburg-like metabolic profile, typical for FBS-cultured cells, is replaced by a diverse metabolic profile consistent with in vivo hepatocytes, including the formation of large lipid and glycogen stores, increased glycogenesis, increased beta-oxidation and ketogenesis, and decreased glycolysis. Finally, organ-specific functions are restored, including xenobiotics degradation and secretion of bile, VLDL and albumin. Thus, organ-specific functions are not necessarily lost in cell cultures, but might be merely suppressed in FBS. The effect of serum is often overseen in cell culture and we provide a detailed study in the changes that occur and provide insight in some of the serum components that may play a role in the establishment of the differentiated phenotype.

The use of human umbilical cord blood serum is beneficial for the continuous production of hepatitis C virus.

In this study we investigated if human umbilical cord blood serum (CBS) is a suitable replacement for foetal bovine serum (FBS) in cultures of human hepatoma cell line Huh7.5, particularly regarding its capacity to maintain high growth rates, differentiation status and its ability to support robust hepatitis C virus (HCV) infection. Generally, CBS-cultured Huh7.5 cells remained comparable to FBS-cultured cells, and proliferated equally well. Albumin secretion, a hepatocyte differentiation marker, had increased 8x in CBS; however, most other hepatocyte markers we tested had not changed. Surprisingly, CBS-cultured cells were able to sustain very high levels of HCV production, and HCV infection in CBS-cultured cells did not induce cell lysis, which is typically seen in HCV-infected cells cultured in FBS. We discuss some of the differences between CBS, adult human serum and FBS that may explain the differences observed.

MicroRNAs regulate the immunometabolic response to viral infection in the liver.

Immune regulation of cellular metabolism can be responsible for successful responses to invading pathogens. Viruses alter their hosts' cellular metabolism to facilitate infection. Conversely, the innate antiviral responses of mammalian cells target these metabolic pathways to restrict viral propagation. We identified miR-130b and miR-185 as hepatic microRNAs (miRNAs) whose expression is stimulated by 25-hydroxycholesterol (25-HC), an antiviral oxysterol secreted by interferon-stimulated macrophages and dendritic cells, during hepatitis C virus (HCV) infection. However, 25-HC only directly stimulated miR-185 expression, whereas HCV regulated miR-130b expression. Independently, miR-130b and miR-185 inhibited HCV infection. In particular, miR-185 significantly restricted host metabolic pathways crucial to the HCV life cycle. Interestingly, HCV infection decreased miR-185 and miR-130b levels to promote lipid accumulation and counteract 25-HC's antiviral effect. Furthermore, miR-185 can inhibit other viruses through the regulation of immunometabolic pathways. These data establish these microRNAs as a key link between innate defenses and metabolism in the liver.

Investigating the antiviral role of cell death-inducing DFF45-like effector B in HCV replication.

Cell-death-inducing DFF45-like effector B (CIDEB) is an apoptotic host factor, which was recently found to also regulate hepatic lipid homeostasis. Herein we delineate the relevance of these dual roles of CIDEB in apoptosis and lipid metabolism in the context of hepatitis C virus (HCV) replication. We demonstrate that HCV upregulates CIDEB expression in human serum differentiated hepatoma cells. CIDEB overexpression inhibits HCV replication in HCV replicon expressing Huh7.5 cells, while small interfering RNA knockdown of CIDEB expression in human serum differentiated hepatoma cells promotes HCV replication and secretion of viral proteins. Furthermore, we characterize a CIDEB mutant, KRRA, which is deficient in lipid droplet clustering and fusion and demonstrate that CIDEB-mediated inhibition of HCV is independent of the protein's lipid droplet fusogenic role. Our results suggest that higher levels of CIDEB expression, which favour an apoptotic role for the host factor, inhibit HCV. Collectively, our data demonstrate that CIDEB can act as an anti-HCV host factor and contribute to altered triglyceride homeostasis.

Hepatitis C virus-induced cytoplasmic organelles use the nuclear transport machinery to establish an environment conducive to virus replication.

Hepatitis C virus (HCV) infection induces formation of a membranous web structure in the host cell cytoplasm where the viral genome replicates and virions assemble. The membranous web is thought to concentrate viral components and hide viral RNA from pattern recognition receptors. We have uncovered a role for nuclear pore complex proteins (Nups) and nuclear transport factors (NTFs) in the membranous web. We show that HCV infection leads to increased levels of cytoplasmic Nups that accumulate at sites enriched for HCV proteins. Moreover, we detected interactions between specific HCV proteins and both Nups and NTFs. We hypothesize that cytoplasmically positioned Nups facilitate formation of the membranous web and contribute to the compartmentalization of viral replication. Accordingly, we show that transport cargo proteins normally targeted to the nucleus are capable of entering regions of the membranous web, and that depletion of specific Nups or Kaps inhibits HCV replication and assembly.

Human serum activates CIDEB-mediated lipid droplet enlargement in hepatoma cells.

Human hepatocytes constitutively express the lipid droplet (LD) associated protein cell death-inducing DFFA-like effector B (CIDEB). CIDEB mediates LD fusion, as well as very-low-density lipoprotein (VLDL) maturation. However, there are limited cell culture models readily available to study CIDEB's role in these biological processes, as hepatoma cell lines express negligible levels of CIDEB. Recent work has highlighted the ability of human serum to differentiate hepatoma cells. Herein, we demonstrate that culturing Huh7.5 cells in media supplemented with human serum activates CIDEB expression. This activation occurs through the induced expression of PGC-1α, a positive transcriptional regulator of CIDEB. Coherent anti-Stokes Raman scattering (CARS) microscopy revealed a correlation between CIDEB levels and LD size in human serum treated Huh7.5 cells. Human serum treatment also resulted in a rapid decrease in the levels of adipose differentiation-related protein (ADRP). Furthermore, individual overexpression of CIDEB was sufficient to down-regulate ADRP protein levels. siRNA knockdown of CIDEB revealed that the human serum mediated increase in LD size was CIDEB-dependent. Overall, our work highlights CIDEB's role in LD fusion, and presents a new model system to study the PGC-1α/CIDEB pathway's role in LD dynamics and the VLDL pathway.

Human serum leads to differentiation of human hepatoma cells, restoration of very-low-density lipoprotein secretion, and a 1000-fold increase in HCV Japanese fulminant hepatitis type 1 titers.

UNLABELLED: In this study, we differentiated the human hepatoma cell line Huh7.5 by supplementing tissue culture media with human serum (HS) and examined the production of hepatitis C virus (HCV) by these cells. We compared the standard tissue culture protocol, using media supplemented with 10% fetal bovine serum (FBS), to media supplemented with 2% HS. Cells cultured in HS undergo rapid growth arrest, have a hepatocyte-like morphology, and increase the expression of hepatocyte differentiation markers. In addition, expression of cell adhesion proteins claudin-1, occludin, and e-cadherin are also increased. The lipid droplet content of these cells is highly increased, as are key lipid metabolism regulators liver X receptor alpha, peroxisome proliferator-activated receptor (PPAR)-α, and PPAR-γ. Very-low-density lipoprotein secretion, which is absent in FBS-grown cells, is restored in Huh7.5 cells that are cultured in HS. All these factors have been implicated in the life cycle of HCV. We show that viral production of Japanese fulminant hepatitis type 1 increases 1,000-fold when cells are grown in HS, compared to standard FBS culture conditions. The virus produced under these conditions is associated with apolipoprotein B, has a lower density, higher specific infectivity, and has a longer half-life than virus produced in media supplemented with FBS. CONCLUSION: We describe a convenient, cost-effective method to produce hepatocyte-like cells, which produce large amounts of virus that more closely resemble HCV present in serum of infected patients.

Lipoprotein profiles in SCID/uPA mice transplanted with human hepatocytes become human-like and correlate with HCV infection success.

Although multiple determinants for hepatitis C virus (HCV) infection are known, it remains partly unclear what determines the human specificity of HCV infection. Presumably, the presence of appropriate entry receptors is essential, and this may explain why HCV is unable to infect nonhuman hepatocytes. However, using mice with chimeric human livers, we show in this study that the presence of human hepatocytes, and therefore human entry receptors, is not sufficient for HCV infection. In successfully transplanted SCID/Alb-uPA mice, infection with HCV is reliable only when ∼70-80% of the liver consists of human hepatocytes. We show that chimeric mice, which are hard to infect with HCV, have significant groups of human hepatocytes that are readily infected with hepatitis B virus. Thus it is unlikely that the lack of infection with HCV can simply be attributed to low hepatocyte numbers. We investigated whether the humanization of lipoprotein profiles is positively associated with infection success. We show that the lipoprotein profiles of chimeric mice become more human-like at high levels of engraftment of human hepatocytes. This and expression of markers of human lipoprotein biosynthesis, human apolipoprotein B (ApoB) and cholesterol ester transfer protein (CETP), show a strong positive correlation with successful infection. Association of HCV in the blood of chimeric mice to ApoB-containing lipoproteins is comparable to association of HCV in patient serum and provides further support for a critical role for ApoB-containing lipoproteins in the infectious cycle of HCV. Our data suggest that the weakest link in the HCV infection chain does not appear to be the presence of human hepatocytes per se. We believe that HCV infection also depends on the presence of sufficient levels of human lipoproteins.

Activity-based protein profiling identifies a host enzyme, carboxylesterase 1, which is differentially active during hepatitis C virus replication.

Hepatitis C virus (HCV) relies on many interactions with host cell proteins for propagation. Successful HCV infection also requires enzymatic activity of host cell enzymes for key post-translational modifications. To identify such enzymes, we have applied activity-based protein profiling to examine the activity of serine hydrolases during HCV replication. Profiling of hydrolases in Huh7 cells replicating HCV identified CES1 (carboxylesterase 1) as a differentially active enzyme. CES1 is an endogenous liver protein involved in processing of triglycerides and cholesterol. We observe that CES1 expression and activity were altered in the presence of HCV. The knockdown of CES1 with siRNA resulted in lower levels of HCV replication, and up-regulation of CES1 was observed to favor HCV propagation, implying an important role for this host cell protein. Experiments in HCV JFH1-infected cells suggest that CES1 facilitates HCV release because less intracellular HCV core protein was observed, whereas HCV titers remained high. CES1 activity was observed to increase the size and density of lipid droplets, which are necessary for the maturation of very low density lipoproteins, one of the likely vehicles for HCV release. In transgenic mice containing human-mouse chimeric livers, HCV infection also correlates with higher levels of endogenous CES1, providing further evidence that CES1 has an important role in HCV propagation.

Phospholipid homeostasis in phosphatidylserine synthase-2-deficient mice.

Phosphatidylserine (PS) is synthesized in mammalian cells by two distinct serine-exchange enzymes, phosphatidylserine synthase-1 and -2. We recently demonstrated that mice lacking PS synthase-2 develop normally and exhibit no overt abnormalities [Bergo et al., (2002) J. Biol. Chem. 277:47701-47708]. We now show that PS synthase-2 mRNA levels are up to 80-fold higher in livers of embryos than in adults. Despite reduced serine-exchange activity in several tissues of PS synthase-2 deficient mice, the phospholipid composition of mitochondria and microsomes from these tissues is normal. Although PS synthase-2 is highly expressed in neurons, axon extension of cultured sympathetic neurons is not impaired by PS synthase-2 deficiency. We hypothesized that mice compensate for PS synthase-2 deficiency by modifying their phospholipid metabolism. Our data show that the rate of PS synthesis in hepatocytes is not reduced by PS synthase-2 deficiency but PS synthase-1 activity is increased. Moreover, PS degradation is decreased by PS synthase-2 deficiency, probably as a result of decreased PS degradation via phospholipases rather than decreased PS decarboxylation. These experiments underscore the idea that cellular phospholipid composition is tightly controlled and show that PS synthase-2-deficient hepatocytes modify phospholipid metabolism by several compensatory mechanisms to maintain phospholipid homeostasis.

Disruption of the phosphatidylserine decarboxylase gene in mice causes embryonic lethality and mitochondrial defects.

Most of the phosphatidylethanolamine (PE) in mammalian cells is synthesized by two pathways, the CDP-ethanolamine pathway and the phosphatidylserine (PS) decarboxylation pathway, the final steps of which operate at spatially distinct sites, the endoplasmic reticulum and mitochondria, respectively. We investigated the importance of the mitochondrial pathway for PE synthesis in mice by generating mice lacking PS decarboxylase activity. Disruption of Pisd in mice resulted in lethality between days 8 and 10 of embryonic development. Electron microscopy of Pisd-/- embryos revealed large numbers of aberrantly shaped mitochondria. In addition, fluorescence confocal microscopy of Pisd-/- embryonic fibroblasts showed fragmented mitochondria. PS decarboxylase activity and mRNA levels in Pisd+/- tissues were approximately one-half of those in wild-type mice. However, heterozygous mice appeared normal, exhibited normal vitality, and the phospholipid composition of livers, testes, brains, and of mitochondria isolated from livers, was the same as in wild-type littermates. The amount and activity of a key enzyme of the CDP-ethanolamine pathway for PE synthesis, CTP:phosphoethanolamine cytidylyltransferase, were increased by 35-40 and 100%, respectively, in tissues of Pisd+/- mice, as judged by immunoblotting; PE synthesis from [3H]ethanolamine was correspondingly increased in hepatocytes. We conclude that the CDP-ethanolamine pathway in mice cannot substitute for a lack of PS decarboxylase during development. Moreover, elimination of PE production in mitochondria causes fragmented, misshapen mitochondria, an abnormality that likely contributes to the embryonic lethality.

Metabolism and functions of phosphatidylserine.

Phosphatidylserine (PS) is a quantitatively minor membrane phospholipid that is synthesized by prokaryotic and eukaryotic cells. In this review we focus on genes and enzymes that are involved in PS biosynthesis in bacteria, yeast, plants and mammalian cells and discuss the available information on the regulation of PS biosynthesis in these organisms. The enzymes that synthesize PS are restricted to endoplasmic reticulum membranes in yeast and mammalian cells, yet PS is widely distributed throughout other organelle membranes. Thus, mechanisms of inter-organelle movement of PS, particularly the transport of PS from its site of synthesis to the site of PS decarboxylation in mitochondria, are considered. PS is normally asymmetrically distributed across the membrane bilayer, thus the mechanisms of transbilayer translocation of PS, particularly across the plasma membrane, are also discussed. The exposure of PS on the outside surface of cells is widely believed to play a key role in the removal of apoptotic cells and in initiation of the blood clotting cascade. PS is also the precursor of phosphatidylethanolamine that is made by PS decarboxylase in bacteria, yeast and mammalian cells. Furthermore, PS is required as a cofactor for several important enzymes, such as protein kinase C and Raf-1 kinase, that are involved in signaling pathways.

Defining the importance of phosphatidylserine synthase 2 in mice.

Phosphatidylserine synthase 1 (Pss1) and phosphatidylserine synthase 2 (Pss2) produce phosphatidylserine by exchanging serine for the head groups of other phospholipids. Pss1 and Pss2 are structurally similar (approximately 32% amino acid identity) but differ in their substrate specificities, with Pss1 using phosphatidylcholine for the serine exchange reaction and Pss2 using phosphatidylethanolamine. Whether Pss1 and Pss2 are both required for mammalian growth and development is not known, and no data exist on the relative contributions of the two enzymes to serine exchange activities in different tissues. To address those issues and also to define the cell type-specific expression of Pss2, we generated Pss2-deficient mice in which a beta-galactosidase marker is expressed from Pss2 regulatory sequences. Histologic studies of Pss2-deficient mice revealed very high levels of beta-galactosidase expression in Sertoli cells of the testis and high levels of expression in brown fat, neurons, and myometrium. The ability of testis extracts from Pss2-deficient mice to catalyze serine exchange was reduced by more than 95%; reductions of approximately 90% were noted in the brain and liver. However, we found no perturbations in the phospholipid content of any of these tissues. As judged by Northern blots, the expression of Pss1 was not up-regulated in Pss2-deficient cells and tissues. Testis weight was reduced in Pss2-deficient mice, and some of the male mice were infertile. We conclude that Pss2 is responsible for the majority of serine exchange activity in in vitro assays, but a deficiency in this enzyme does not cause perturbations in phospholipid content or severe developmental abnormalities.