Distribution of type V secretory phospholipase A2 expression in human hepatocytes damaged by liver disease

BACKGROUND AND AIM

Type V secretory phospholipase A2 (sPLA2-V) is a key enzyme in the arachidonate cascade. However, the distribution of sPLA2-V in human liver has not yet been investigated. In this study, the significance of sPLA2-V expression in human hepatocytes damaged by liver disease was investigated.

METHODS

Samples of liver tissue from patients with chronic hepatitis B and C, hepatitis virus-related liver cirrhosis, and congestive hepatocyte injury were immunostained with antibodies against sPLA2-V, cyclooxygenase (COX)-2, hepatitis viral antigens, transforming growth factor (TGF)-beta1, interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha.

RESULTS

In chronic hepatitis patients, sPLA2-V-positive hepatocytes were scattered in the liver lobules, while cyclooxygenase-2, IL-1beta, and TNF-alpha were diffusely expressed. Hepatocytes around necroinflammatory lesions were strongly positive for sPLA2-V. Some sPLA2-V-positive hepatocytes were also positive for viral antigens. TGF-beta1 was expressed only in fibrotic lesions. The pattern of distribution of these proteins in liver cirrhosis patients was similar to that in chronic hepatitis patients, but sPLA2-V expression tended to be more intense than in chronic hepatitis. In the congestive liver, sPLA2-V, COX-2, and the two cytokines were diffusely expressed in surviving hepatocytes.

CONCLUSIONS

sPLA2-V expression in hepatocytes is induced by viral infection, fibrosis, and circulatory disturbance. Immunostaining using sPLA2-V antibody is useful for the detection of injured hepatocytes in patients with liver diseases.

Production in Pichia pastoris and characterization of genetic engineered chimeric HBV/HEV virus-like particles

OBJECTIVE

To investigate the presentation of a neutralization epitope-containing peptide antigen of hepatitis E virus (HEV) on chimeric virus-like particles (VLPs) of hepatitis B surface antigen (HBsAg).

METHODS

The gene fragment corresponding to amino acids (aa) 551-607 (HEnAg) of HEV capsid protein, which contains the only neutralization epitope identified to date, was fused via a synthetic glycine linker in frame with the gene of HBsAg. The resulted fusion gene was then integrated through transformation into the genome of Pichia pastoris under the control of a methanol-induced alcohol oxidase 1 (AOX1) promoter and expressed intracellularly. The expression products in the soluble cell extracts were characterized by Western blot, ELISA, CsCl density gradient analysis, and electron microscopic visualization.

RESULTS

The novel fusion protein incorporating HBsAg and the neutralization epitope-containing HEnAg was expressed successfully in Pichia pastoris with an expected molecular weight of approximately 32 kD. It was found to possess the ability to assemble into chimeric HBV/HEV VLPs with immunological physical and morphological characteristics akin to HBsAg particles. Not only did the chimeric VLPs show high activity levels in a HBsAg particle-specific ELISA but they were also strongly immunoreactive with hepatitis E (HE) positive human serum in a HEV specific ELISA, indicating that HEnAg peptide fragments were exposed on VLP surfaces and would be expected to be readily accessible by cells and molecules of the immune system. Similarity between chimeric VLPs to highly immunogenic HBsAg particles may confer good immunogenicity on surface-displayed HEnAg.

CONCLUSION

The chimeric HBV/HEV VLPs produced in this study may have potential to be a recombinant HBV/HEV bivalent vaccine candidate.

Conformational antigenic determinants generated by interactions between a bacterially expressed recombinant peptide of the hepatitis E virus structural protein

A 23 kDa peptide locating to amino acid residues 394 to 604 of the major Hepatitis E Virus (HEV) structural protein was expressed in E. coli. This peptide was found to interact naturally with one another to form homodimers and it was recognized strongly and commonly in its dimeric form by HEV reactive human sera. The antigenic activity associated with the dimeric form was abrogated when the dimer was dissociated into monomer and the activity was reconstituted after the monomer was re-associated into dimer again. The dimeric form of the peptide elicited a vigorous antibody response in experimental animals and the resulting antisera were found to cross-react against HEV, effecting an efficient immune capture of the virus. These results attributed the antigenic activity associated with the dimeric form of the peptide to conformational antigenic determinants generated as a result of interaction between the peptide molecules. It is suggested that some of these antigenic determinants may be expressed by the HEV capsid and raised the possibility of this bacterially expressed peptide as an HEV vaccine candidate.

Baculovirus expression of chimeric hepatitis B virus core particles with hepatitis E virus epitopes and their use in a hepatitis E immunoassay

Two hepatitis B core proteins bearing the immunodominant region of the hepatitis E virus (HEV) capsid protein, one at the C terminus of hepatitis B virus core antigen (HBcAg) and the other within the HBcAg immunodominant loop, were constructed. Both chimeric proteins exhibited HEV reactivity, but only the first construct retained HBcAg reactivity. The second construct was used to develop an anti-HEV test which is equivalent to a commercial test for the detection of anti-HEV immunoglobulin G (IgG) but is more sensitive for the detection of anti-HEV IgM.

Interindividual variability in P450-dependent generation of neoantigens in halothane hepatitis

Halothane hepatitis occurs because susceptible patients mount immune responses to trifluoroacetylated protein antigens, formed following cytochrome P450-mediated bioactivation of halothane to trifluoroacetyl chloride. In the present study, an in vitro approach has been used to investigate the cytochrome P450 isozyme(s) which catalyze neoantigen formation and to explore the protective role of non-protein thiols (cysteine and reduced glutathione). Significant levels of trifluoroacetyl protein antigens were generated when human liver microsomes, and also microsomes from livers of rats pre-treated with isoniazid, phenobarbital or beta-naphtoflavone, were incubated with halothane plus a nicotinamide adenine dinucleotidephosphate (NADPH) generating system. Immunoblotting studies revealed that the major trifluoroacetyl antigens expressed in vitro exhibited molecular masses of 50-55 kDa and included 60 and 80 kDa neoantigens recognized by antibodies from patients with halothane hepatitis. Much lower concentrations of halothane were required to produce maximal antigen generation in isoniazid-induced rat microsomes, as compared with phenobarbital or isosafrole-induced microsomes (0.5 vs 12.5 microl/ml). In isoniazid-induced microsomes, antigen generation was inhibited > 90% by the nucleophiles cysteine and glutathione and by the CYP2E1-selective inhibitors diallylsulfide and p-nitrophenol, but was unaffected by inhibitors of other P450 isozymes (furafylline, sulfaphenazole or triacetyloleandomycin). Neoantigen formation in six human liver microsomal preparations was inhibited in the presence of diallylsulfide, but not by furafylline, sulfaphenazole or triacetyloleandomycin, and exhibited marked variability which correlated with CYP2E1 levels. These results suggest that the balance between metabolic bioactivation by CYP2E1 and detoxication of reactive metabolites by cellular nucleophiles could be an important metabolic risk factor in halothane hepatitis.

Characterization of mRNA for hepatitis delta antigen: exclusion of the full-length antigenomic RNA as an mRNA

Hepatitis delta virus (HDV) encodes a single protein, the hepatitis delta antigen (HDAg), which is thought to be translated from a 0. 8-kb RNA of antigenomic sense. This subgenomic RNA species is present in very small amounts in HDV-infected liver tissues and in cultured cells infected or transfected with HDV, and in some cases it cannot be detected at all. In contrast, HDAg protein is present in large amounts in all natural and experimental models of HDV infection. This study addresses whether other HDV RNA species, such as the antigenomic-sense, genome-size HDV RNA can also serve as the mRNA for HDAg synthesis. Taking advantage of the ability of herpes simplex virus (HSV) to degrade only polyadenylated mRNAs, we examined the effect of HSV coinfection on HDAg synthesis. It was shown that HSV infection did degrade the subgenomic 0.8-kb HDV mRNA but not HDV genome-length RNA. Under such conditions, HDAg synthesis was completely inhibited. Furthermore, the genome-length HDV RNA was found not to be associated with polysomes. Finally, in vitro translation studies demonstrated that HDAg could not be translated directly from the genome-length antigenomic-sense HDV RNA. These results suggest that only the subgenomic RNA species of HDV possesses properties characteristic of the mRNA for HDAg and that the genome-length RNA cannot be used for translating HDAg. In addition, we found that HDV RNA replication did not depend on de novo HDAg synthesis.

Genotype-specific complementation of hepatitis delta virus RNA replication by hepatitis delta antigen

Characterizations of genetic variations among hepatitis delta virus (HDV) isolates have focused principally on phylogenetic analysis of sequences, which vary by 30 to 40% among three genotypes and about 10 to 15% among isolates of the same genotype. The significance of the sequence differences has been unclear but could be responsible for pathogenic variations associated with the different genotypes. Studies of the mechanisms of HDV replication have been limited to cDNA clones from HDV genotype I, which is the most common. To perform a comparative analysis of HDV RNA replication in genotypes I and III, we have obtained a full-length cDNA clone from an HDV genotype III isolate. In transfected Huh-7 cells, the functional roles of the two forms of the viral protein, hepatitis delta antigen (HDAg), in HDV RNA replication are similar for both genotypes I and III; the short form is required for RNA replication, while the long form inhibits replication. For both genotypes, HDAg was able to support replication of RNAs of the same genotype that were mutated so as to be defective for HDAg production. Surprisingly, however, neither genotype I nor genotype III HDAg was able to support replication of such mutated RNAs of the other genotype. The inability of genotype III HDAg to support replication of genotype I RNA could have been due to a weak interaction between the RNA and HDAg. The clear genotype-specific activity of HDAg in supporting HDV RNA replication confirms the original categorization of HDV sequences in three genotypes and further suggests that these should be referred to as types (i.e., HDV-I and HDV-III) rather than genotypes.

Overexpression of hepatitis delta antigen protects insect cells from baculovirus-induced cytolysis

Hepatitis delta virus (HDV) is a human pathogen causing fulminant hepatitis and liver cirrhosis. HDV has a circular single-stranded RNA genome, which encodes only a single protein, hepatitis delta antigen (HDAg), from the antigenomic strand. Although the functional roles of HDAg have been extensively studied, the molecular mechanism of persistent infection and pathogenesis of HDV are not yet understood. Here we report that overexpressed HDAg protects cells from death in baculovirus-infected insect cells. Using both wild-type and recombinant baculovirus-infected insect cells, we have demonstrated that HDAg inhibited wild-type baculovirus-induced cytolysis and thus extended the survival of virus-infected insect cells. By deletion analysis, we show that N-terminal 25 amino acids are essential for this function. From these data, we suggest that HDAg may play a major role in persistent infection of HDV.

Casein kinase II and protein kinase C modulate hepatitis delta virus RNA replication but not empty viral particle assembly

Hepatitis delta virus (HDV) contains two virus-specific delta antigens (HDAgs), large and small forms, which are identical in sequence except that the large one contains 19 extra amino acids at the C terminus. HDAgs are nuclear phosphoproteins with distinct biological functions; the small form activates HDV RNA replication, whereas the large form suppresses this process but is required for viral particle assembly. In this study, we have characterized the phosphorylative property of HDAg in a human hepatoma cell line (HuH-7) and examined the role of phosphorylation in HDAg function. As demonstrated by in vivo labeling and kinase inhibitor experiments, the phosphorylation levels of both HDAgs were diminished by the inhibitor of casein kinase II (CKII). Nevertheless, phosphorylation of only the small form could be markedly reduced by the protein kinase C (PKC) inhibitor, suggesting different phosphorylation properties between the two HDAgs. When these two kinase inhibitors were added separately to the transient-expression system, HDV RNA replication was profoundly suppressed. In contrast, the inhibitors did not affect the assembly of empty HDAg particle from HDAgs and hepatitis B virus surface antigen. To further examine the role of phosphorylation in HDAg function, two conservative CKII recognition sites at Ser-2 and Ser-123 of both HDAgs and one potential PKC recognition site at Ser-210 of the large HDAg were altered to alanine by site-directed mutagenesis. Transfection experiments indicated that mutation at Ser-2, but not Ser-123, significantly impaired the activity of the small HDAg in assisting HDV RNA replication. This property is in accordance with our observation that Ser-2, not Ser-123, was the predominant CKII phosphorylation site in the small HDAg. Our studies also excluded the possibility that the phosphorylation of Ser-2, Ser-123, or Ser-210, had roles in the trans-suppression activity of the large HDAg, in the assembly of empty virus-like HDAg particle, and in the nuclear transport of HDAgs. In conclusion, our results indicate that both CKII and PKC positively modulate HDV RNA replication but not the assembly of empty HDAg particle. The role of CKII in HDV replication may at least in part be accounted for by the phosphorylation of Ser-2 in the small HDAg. The effect of PKC on HDV RNA replication is, however, not to mediate the phosphorylation of the conservative Ser-210 in the large HDAg but rather to act on as-yet-unidentified Ser or Thr residues in the small HDAg or cellular factors. These findings provide the first insight into the roles of phosphorylation of the two HDAgs in the HDV replication cycle.

A novel form of hepatitis delta antigen

Hepatitis delta virus (HDV) is known to express a protein termed the small delta antigen, a structural protein which is also essential for genome replication. During replication, posttranscriptional RNA editing specifically modifies some of the HDV RNA, leading to the production of an elongated form of the delta antigen, the large form, which is essential for virus assembly. The present study showed that yet another form of HDV protein is expressed during genome replication. This novel form is not produced in all infected cells, but it arises during replication in transfected cells and in infected woodchucks, and as was previously reported, patients infected with HDV do make antibodies directed against it. These findings are an indicator of the complexity of gene expression during HDV infection and replication.