Removal of cell-bound lipoproteins: a crucial step for the efficient infection of liver cells with hepatitis C virus in vitro

Generalized substitution of isoencoding codons shortens the duration of papillomavirus L1 protein expression in transiently gene-transfected keratinocytes due to cell differentiation

Recently we reported that gene codon composition determines differentiation-dependent expression of the PV L1 genes in mouse primary keratinocytes (KCs) in vitro and in vivo (Zhao et al. 2005, Mol. Cell Biol. 25:8643–8655). Here, we investigated whether generalized substitution of isoencoding codons affects the duration of expression of PV L1 genes in mouse and human KCs in day 1 culture transiently transfected with native (Nat) and codon modified (Mod) L1 genes. Following transient transfection, KC continuously transcribed both Nat and Mod PV L1 genes for at least 12 days, with the levels of L1 mRNAs from the Mod L1 genes significantly higher than those from the Nat L1 genes. However, continuous L1 protein expression at day 9 post-transfection was observed for both mouse and human KCs transfected with the Nat L1 genes only. Further, aa-tRNAs prepared from D8 KC cultures enhanced translation of two PV Nat L1 DNAs in RRL lysate and PV Nat L1 mRNAs in D0 cell-free lysate, whereas aa-tRNAs from D0 KCs enhanced translation of PV Mod L1 mRNAs in D8 cell-free lysate. It appears that aa-tRNAs in less-differentiated and differentiated KCs differentially match the PV Nat and Mod L1 mRNAs to regulate their translations in vitro.

How does hepatitis C virus enter cells?

Hepatitis C virus (HCV) exists in different forms in the circulation of infected people: lipoprotein bound and lipoprotein free, enveloped and nonenveloped. Viral particles with the highest infectivity are associated with lipoproteins, whereas lipoprotein-free virions are poorly infectious. The detection of HCV's envelope proteins E1 and E2 in lipoprotein-associated virions has been challenging. Because lipoproteins are readily endocytosed, some forms of HCV might utilize their association with lipoproteins rather than E1 and E2 for cell attachment and internalization. However, vaccination of chimpanzees with recombinant envelope proteins protected the animals from hepatitis C infection, suggesting an important role for E1 and E2 in cell entry. It seems possible that different forms of HCV use different receptors to attach to and enter cells. The putative receptors and the assays used for their validation are discussed in this review.

Potential cellular receptors involved in hepatitis C virus entry into cells

Hepatitis C virus (HCV) infects hepatocytes and leads to permanent, severe liver damage. Since the genomic sequence of HCV was determined, progress has been made towards understanding the functions of the HCV-encoded proteins and identifying the cellular receptor(s) responsible for adsorption and penetration of the virus particle into the target cells. Several cellular receptors for HCV have been proposed, all of which are associated with lipid and lipoprotein metabolism. This article reviews the cellular receptors for HCV and suggests a general model for HCV entry into cells, in which lipoproteins play a crucial role.

Stimulation of Fas agonistic antibody-mediated apoptosis by heparin-like agents suppresses Hsp27 but not Bcl-2 protective activity

We report that in Jurkat T cells or freshly isolated T lymphocytes, physiological concentrations of high-molecular weight sulfated polysaccharides such as heparin, heparan sulfate, and dextran sulfate significantly increased the percentage of cell death induced by Fas IgM agonistic antibody. The phenomenon was caspase dependent and P53 independent and correlated with an increased accessibility of cell surface Fas receptors. We also observed that the Fas IgM agonistic antibody-dependent formation of sodium dodecyl sulfate (SDS)-resistant large structures containing Fas receptor was decreased in the presence of heparin-like agents. In contrast, the different agents had no effect when cell death was triggered by FasL, the natural ligand of Fas that does not generate SDS-resistant forms of Fas. Interestingly, the synergistic effect of heparin-like agents toward Fas IgM agonistic antibody-mediated cell death abolished Hsp27 antiapoptotic activity but did not alter much the protection generated by Bcl-2 expression.

Serum anti-cholesterol antibodies in chronic hepatitis-C patients during IFN-alpha-2b treatment

Previously we detected more than 3 times higher anti-cholesterol antibody (ACHA) levels in HIV positive patients compared to healthy individuals, however, this level significantly decreased during highly active anti-retroviral therapy (HAART). In our present study we examined whether these findings could also be detected in patients with chronic hepatitis C (CHC). We calculated the correlation between the ACHA levels and the C5b-9 complement activation product. 39 patients with CHC were treated with IFN-alpha-2b (Schering-Plough) 5 MU daily for 6 weeks, followed by 5 MU TIW. Serum levels of ACHA and complement activation products were measured with ELISA. Serum HCV RNA was measured by a highly sensitive branched DNA technique before and 3, 6 and 12 months after the beginning of IFN-alpha-2b therapy. 52 healthy persons served as controls. At the onset of treatment ACHA level was significantly (p = 0.0062) higher in patients (40 (24-69) AU/ml) (median (interquartile range)) than in control sera (26 (20-35) AU/ml). In the 26 responder patients ACHA levels decreased to the normal level during the therapy, but no change was observed in the 13 non-responders. In patients with a sustained response ACHA levels remained low till the end of the 12 months IFN treatment. ACHA levels were significantly (p = 0.0422) higher in the patients with low (< 4.0 mmol/l) than in those with normal (> or = 4.0 mmol/l) cholesterol concentrations. The ACHA level before the therapy strongly correlated (r = 0.5499, p = 0.0014) with C5b-9 serum levels. ACHA levels are elevated in CHC, but this elevation is not as high as in HIV. Decrease of viral load by IFN-alpha-2b treatment in the responders results in normalization of ACHA concentration. High ACHA levels in patients with low serum cholesterol concentration suggest that high ACHA levels may contribute to the decrease in cholesterol levels. The correlation between the ACHA and C5b-9 levels indicate, that the ACHA may play a role in the complement activation in CHC.

Bovine viral diarrhea virus as a surrogate model of hepatitis C virus for the evaluation of antiviral agents

The identification and development of new antiviral agents that can be used to combat hepatitis C virus (HCV) infection has been complicated by both technical and logistic issues. There are few, if any, robust methods by which HCV virions can be grown in vitro. The development of HCV RNA replicons has been a great breakthrough that has allowed for the undertaking of significant screening efforts to identify inhibitors of HCV intracellular replication. However, since replicons do not undergo a complete replication cycle, drug screening programs and mechanism of action studies based solely on these assays will not identify compounds targeting either early (virion attachment, entry, uncoating) or late (virion assembly, egress) stages of the viral replication cycle. Drugs that negatively affect the infectivity of new virions will also not be identified using HCV RNA replicons. Bovine viral diarrhea virus (BVDV) shares a similar structural organization with HCV, and both viruses generally cause chronic long-term infections in their respective hosts. The BVDV surrogate model is attractive, since it is a virus-based system. It is easy to culture the virus in vitro, molecular clones are available for genetic studies, and the virus undergoes a complete replication cycle. Like HCV, BVDV utilizes the LDL receptor to enter cells, uses a functionally similar internal ribosome entry site (IRES) for translation, uses an NS4A cofactor with its homologous NS3 protease, has a similar NS3 helicase/NTPase, a mechanistically similar NS5B RNA-dependent RNA polymerase, and a seemingly equivalent mechanism of virion maturation, assembly and egress. While the concordance between drugs active in either BVDV or HCV is largely unknown at this time, BVDV remains a popular model system with which drugs can be evaluated for potential antiviral activity against HCV and in studies of drug mechanism of action.

CD81 engineered with endocytotic signals mediates HCV cell entry: implications for receptor usage by HCV in vivo

Although CD81 has been shown to bind HCV E2 protein, its role as a receptor for HCV remains controversial. In this study, we constructed two CD81 chimeras by linking the cytoplasmic domains of recycling surface receptors, low-density lipoprotein receptor (LDLR), and transferrin receptor (TfR), respectively, to CD81 and compared their internalization properties to wild-type CD81. Binding experiments with anti-hCD81 antibody showed that cell-surface CD81 chimeric receptors were internalized much more efficiently than wild-type CD81. In addition, CD81 chimeras, but not wild-type CD81, could internalize recombinant E2 protein and E2-enveloped viral particles from the serum of HCV-infected patients into Huh7 liver cells. The latter resulted in persistent positive-strand viral RNA and accumulation of replication intermediates, negative-strand viral RNA, in the infected cells, suggesting that the internalized viruses have undergone replication. Therefore, it appeared that CD81, possibly in association with a liver-specific endocytotic protein(s), represents one of the pathways by which HCV can infect hepatocytes.