Deficiency of antibody response to hypervariable region of hepatitis C virus in patients with chronic hepatitis C

Interaction of heterogeneous nuclear ribonucleoprotein A1 with cytochrome P450 2A6 mRNA: implications for post-transcriptional regulation of the CYP2A6 gene

The human xenobiotic-metabolizing enzyme cytochrome P450, CYP2A6, catalyzes the bioactivation of a number of carcinogens and drugs and is overexpressed in cases of liver diseases, such as cirrhosis, viral hepatitis, and parasitic infestation, and in certain tumor cells. This suggests that CYP2A6 may be a major liver catalyst in pathological conditions. In the present study, we have addressed molecular mechanisms underlying the regulation of the CYP2A6 gene. We present evidence of several proteins present in human hepatocytes that interact specifically with the 3'-untranslated region (UTR) of CYP2A6 mRNA. Biochemical and immunological evidence show that the RNA-protein complex of highest intensity contains the heterogeneous nuclear ribonucleoprotein (hnRNP) A1 or a closely related protein. Mapping of the hnRNP A1 binding site within CYP2A6 3'-UTR reveals that the smallest portion of RNA supporting significant binding consists of 111 central nucleotides of the 3'-UTR. Our studies also indicate that hnRNPA1 from HepG2 cancer cells exhibits modified binding characteristics to the CYP2A6 3'-UTR compared with primary hepatocytes. We found that the level of CYP2A6 mRNA remains high in conditions of impaired transcription in primary human hepatocytes, showing that CYP2A6 expression can be affected post-transcriptionally in conditions of cellular stress. Our results indicate that the post-transcriptional regulation involves interaction of the hnRNP A1 protein with CYP2A6 mRNA. The present data suggest that hnRNPA1 is a critical regulator of expression of the human CYP2A6 gene and support the notion that this P450 isoform may be of particular significance in stressed human liver cells.

Endoplasmic reticulum stress due to altered cellular redox status positively regulates murine hepatic CYP2A5 expression

Murine hepatic cytochrome P450 2A5 (CYP2A5) is uniquely induced by a variety of agents that cause liver injury and inflammation, conditions that are typically associated with downregulation of P450s. We hypothesized that induction of CYP2A5 occurs in response to hepatocellular damage resulting in endoplasmic reticulum (ER) stress. Treatment of mice in vivo and mouse hepatocytes in primary culture with the CYP2A5 inducer pyrazole resulted in overexpression of the ER stress biomarker glucose-regulated protein (GRP) 78. Treatment of primary hepatocytes with ER stress activators thapsigargin, tunicamycin, and trans-4,5-dihydroxy-1,2-dithiane (DTT(ox)) and the calcium ionophore A23187 (calcimycin) resulted in elevated GRP78 mRNA levels; however, only the reducing agent DTT(ox) induced levels of CYP2A5 mRNA, protein, and coumarin 7-hydroxylase activity. To test the hypothesis that CYP2A5 induction is due to liver injury resulting from altered cellular redox status, we demonstrated that CYP2A5 induction, elevated serum alanine aminotransferase, and oxidative protein damage occur concurrently in pyrazole-treated mice. Pyrazole also induced the expression of cytosolic alpha and mu class glutathione S-transferase expression both in vivo and in primary mouse hepatocytes. Moreover, treatment of hepatocytes with the redox cycling quinone menadione resulted in overexpression of CYP2A5 and GSTM1 mRNA. Finally, pretreatment of hepatocytes with the antioxidants N-acetylcysteine and vitamin E attenuated pyrazole-mediated increases in CYP2A5 mRNA levels. These findings clearly indicate that induction of mouse hepatic CYP2A5 during liver injury occurs via a novel mechanism involving ER stress due to altered cellular redox status.

Sequence evolution and cross-reactive antibody responses to hypervariable region 1 in acute hepatitis C virus infection

Hepatitis C virus (HCV) infection may result in acute resolving or chronic infection. Patients that clear the infection have a more vigorous cellular immune response and an early humoral response to the hypervariable region 1 (HVR1) of the E2 envelope protein. To analyse further the properties of the early anti-HVR1 response, cross-reactivity of anti-HVR1 responses was assessed in five patients with acute HCV infection, who were infected by the same virus strain during a nosocomial outbreak. The sequence evolution of HVR1 was examined in sequential serum samples up to 37 months post infection. Peptides were synthesised corresponding to the obtained HVR1 sequences and unrelated HVR1 sequences, and antibody reactivity to the peptides in sequential sera was investigated by ELISA. The results suggest an association between specific gaps in humoral immunity and the HVR1 sequence evolution during early infection. Possible interpretations of this phenomenon include immune escape mechanisms or suppression of specific anti-HVR1 antibodies.

Hepatitis B infection and aflatoxin biomarker levels in Gambian children

OBJECTIVES

To examine the relationship between hepatitis B virus (HBV) infection and biomarkers of aflatoxin exposure in West African children.

METHODS

Sera from 444 children aged 3-4 years who were selected to be representative of their communities were analysed for aflatoxin-albumin (AF-alb) adducts and markers of hepatitis B infection.

RESULTS

There was large interindividual variation in adduct levels (range: 2.2 to 459 pg AF-lysine eq./mg albumin). Adduct level was strongly correlated with season, with an approximately twofold higher mean level in the dry season than the wet. Geometric mean adduct levels in uninfected children, chronic carriers and acutely infected children were 31.6 (n = 404), 44.9 (n = 34) and 96.9 (n = 6) pg/mg, respectively. The relationship of AF-alb level to ethnicity, month of sampling and HBV status was examined in a multiple regression model. Month of obtaining the blood sample (P = 0.0001) and HBV status (P = 0.0023) each made a highly significant contribution to the model; the high AF-alb levels were particularly associated with acute infection. Elevated serum transaminase levels were significantly (P < 0.002) associated with HBV status, with acutely infected children having the highest levels. Ethnicity was not significantly associated with AF-alb adduct levels in the model (P = 0.09).

CONCLUSIONS

HBV infection and month of sampling both significantly influence AF-alb adduct levels. The effect of seasonality on adducts was also observed in a previous study of 347 Gambian adults, although there was no correlation between adduct level and HBV status in that population. This difference between children and adults may reflect a more severe effect of HBV infection, particularly acute infection, in childhood on hepatic AF metabolism.

Modulation of murine phenobarbital-inducible CYP2A5, CYP2B10 and CYP1A enzymes by inhibitors of protein kinases and phosphatases

Phenobarbital causes a multitude of effects in hepatocytes, including increased cell proliferation, inhibition of apoptosis and upregulation of xenobiotic and endobiotic metabolizing enzymes. In this study, the involvement of several protein kinase and phosphatase pathways on constitutive and phenobarbital-induced activities of CYP2A5, CYP2B10 and CYP1A1/2 in primary mouse hepatocytes was determined using well-defined chemical modulators of intracellular protein phosphorylation and desphosphorylation events. A 48-h treatment of the hepatocytes with 2-aminopurine, a nonspecific serine/threonine kinase inhibitor, elicited dose-dependent increases in both basal and phenobarbital-induced CYP2A5 catalytic activity (assayed as coumarin 7-hydroxylation), the maximal induction being 60-fold greater than the control value upon cotreatment with 1.5 mM phenobarbital and 10 mM 2-aminopurine. In contrast, phenobarbital induction of CYP2B10 (pentoxyresorufin O-deethylase) and CYP1A1/2 (ethoxyresorufin O-deethylase) activities were blocked by 2-aminopurine. Increases in CYP2A5 activity were also observed after exposure of the hepatocytes to other protein kinase inhibitors affecting the cell cycle, i.e. roscovitine, K-252a and rapamycin. Inhibitors of protein kinases A and C, as well as tyrosine kinases, did not appreciably affect CYP2A5 activity levels. The serine/threonine phosphatase inhibitors tautomycin, calyculin A and okadaic acid all reduced both basal and phenobarbital-induced CYP2A5, CYP2B10 and CYP1A1/2 activities. These results further strengthen the concept that hepatic CYP2A5 is regulated in a unique way compared with CYP2B10 and CYP1A.

Evolution of the hypervariable region of hepatitis C virus

Despite being the most intensively studied part of the hepatitis C virus genome, our understanding of the function of the hypervariable region (HVR) at the NH2-terminus of the E2 protein remains very limited. During chronic infection the HVR usually changes over time, with changes usually mirrored by those of HVR-specific antibodies. Evidence is presented suggesting that the HVR is subject to constraint in its length, amino acid composition, and in the amino acid replacements that are tolerated at different positions. These constraints, and the pattern of HVR variation during chronic infection, suggest that it has an important role in virus infection. A causal relationship between HVR variation and persistence of HCV remains to be demonstrated.

Altered expression of hepatic carcinogen metabolizing enzymes with liver injury in HBV transgenic mouse lineages expressing various amounts of hepatitis B surface antigen

AIMS/BACKGROUND

The objective of this work was to evaluate the possible modulation of carcinogen metabolizing enzymes in relation to chronic infection by hepatitis B virus (HBV). This was to test whether enzyme level is altered in association with HBV gene expression per se or only when that expression was associated with an induction of liver injury.

METHODS

For this purpose, we studied four different HBV transgenic mouse lineages (23.3, 45.2, 50.4 and 107.5) that express the transgene encoding for the large envelope protein (HBsAg) at different levels. These lineages exhibit an associated liver injury which progresses with age and is positively correlated with the degree of accumulation of HBsAg in the hepatocytes. The modulation of levels of cytochrome P450 (1a, 2a-5, 2b, 2c, 3A4 and 2E1) and glutathione S-transferases (GST alpha and pi) involved in carcinogen metabolism was examined by immunohistochemistry in these lineages.

RESULTS

While we observed an increase in staining intensity of P450s 1-a and 2a-5 in lineages expressing cytopathic amounts of HBsAg (lineages 50.4 and 45.2), we only observed minor changes or no changes at all for the other lineages (23.3 and 107.5). Staining with antibodies to cytosolic pi class GST demonstrated an increase in older mice, although no major alterations were observed for GST alpha.

CONCLUSIONS

These results suggest that liver cell injury induced by accumulation of HBV antigens can result in the induction of some carcinogen metabolizing enzymes and this may be one mechanism of chemical-viral interaction in hepatocarcinogenesis.

Viral persistence, antibody to E1 and E2, and hypervariable region 1 sequence stability in hepatitis C virus-inoculated chimpanzees

The relationship of viral persistence, the immune response to hepatitis C virus (HCV) envelope proteins, and envelope sequence variability was examined in chimpanzees. Antibody reactivity to the HCV envelope proteins E1 or E2 was detected by enzyme-linked immunosorbent assay (ELISA) in more than 90% of a human serum panel. Although the ELISAs appeared to be sensitive indicators of HCV infection in human serum panels, the results of a cross-sectional study revealed that a low percentage of HCV-inoculated chimpanzees had detectable antibody to E1 (22%) and E2 (15%). Viral clearance, which was recognized in 28 (61%) of the chimpanzees, was not associated with an antibody response to E1 or E2. On the contrary, antibody to E2 was observed only in viremic chimpanzees. A longitudinal study of animals that cleared the viral infection or became chronically infected confirmed the low level of antibody to E1, E2, and the HVR-1. In 10 chronically infected animals, the sequence variation in the E2 hypervariable region (HVR-1) was minimal and did not coincide with antibody to E2 or to the HVR-1. In addition, low nucleotide and amino acid sequence variation was observed in the E1 and E2 regions from two chronically infected chimpanzees. These results suggest that mechanisms in addition to the emergence of HVR-1 antibody escape variants are involved in maintaining viral persistence. The significance of antibodies to E1 and E2 in the chimpanzee animal model is discussed.

Long-term evolution of the hypervariable region of hepatitis C virus in a common-source-infected cohort

The long-term evolution of the hepatitis C virus hypervariable region (HVR) and flanking regions of the E1 and E2 envelope proteins have been studied in a cohort of women infected from a common source of anti-D immunoglobulin. Whereas virus sequences in the infectious source were relatively homogeneous, distinct HVR variants were observed in each anti-D recipient, indicating that this region can evolve in multiple directions from the same point. Where HVR variants with dissimilar sequences were present in a single individual, the frequency of synonymous substitution in the flanking regions suggested that the lineages diverged more than a decade previously. Even where a single major HVR variant was present in an infected individual, this lineage was usually several years old. Multiple lineages can therefore coexist during long periods of chronic infection without replacement. The characteristics of amino acid substitution in the HVR were not consistent with the random accumulation of mutations and imply that amino acid replacement in the HVR was strongly constrained. Another variable region of E2 centered on codon 60 shows similar constraints, while HVR2 was relatively unconstrained. Several of these features are difficult to explain if a neutralizing immune response against the HVR is the only selective force operating on E2. The impact of PCR artifacts such as nucleotide misincorporation and the shuffling of dissimilar templates is discussed.

Broadly reactive antibodies to hypervariable region 1 in hepatitis C virus-infected patient sera: relation to viral loads and response to interferon

To clarify the nature of serum anti-hypervariable region 1 (HVR1) antibodies in patients infected with hepatitis C virus (HCV), we assessed the reactivity of 21 patients' sera with 42 HVR1 proteins by Western blot. HVR1 was expressed as fusion proteins with glutathione S-transferase (GST). The patients' sera reacted with variable percentages of the HVR1 proteins, and always reacted with HVR1 proteins of the different genotype. In the genotype-1b-infected patients, the percentage of genotype-1b HVR1 proteins reactive with serum correlated significantly with viral loads; the sera reactive with the higher percentages of HVR1 proteins contained the larger viral loads. In addition, it was significantly lower in the responders of interferon (IFN) therapy than in nonresponders. The competition assays indicated that multiple fractions of anti-HVR1 antibodies with different specificity in a serum reacted with different HVR1 proteins, and that, additionally, a single fraction of antibodies often reacted with more than one HVR1 protein through a similar amino acid sequence. In conclusion, serum anti-HVR1 antibodies were broadly reactive by the mechanism of both the cross-reactivity of a single fraction of anti-HVR1 antibodies with more than one HVR1 protein and the presence of multiple fractions of anti-HVR1 antibodies with different specificity in a serum. In genotype-1b-infected patients, the broad reactivity of serum anti-HVR1 antibodies correlated with viral loads and response to IFN. Further studies are necessary to elucidate the correlation among the broad reactivity of sera with multiple HVR1 proteins and clinical features of chronic hepatitis C patients.

Hepatocellular carcinoma: from gene to public health

Liver diseases associated with chronic hepatitis B virus (HBV) infection, including hepatocellular carcinoma, account for more than 1 million deaths annually worldwide. In addition to HBV infection, other risk factors are involved in the etiology of hepatocellular carcinoma and, among these, dietary exposure to the carcinogenic aflatoxins is of particular importance in certain regions of southeast Asia and sub-Saharan Africa. The relative contributions of these two risk factors and the mechanism of the interaction between them in the pathogenesis of hepatocellular carcinoma are still poorly understood. The recently developed individual biochemical and molecular markers of aflatoxin exposure, i.e., aflatoxin-albumin adducts in blood and a specific GC to TA transversion mutation in codon 249 of the p53 gene (249ser p53 mutation) in hepatocellular carcinomas, permit a better quantitative estimation of aflatoxin exposure in different populations of the world. A comprehensive summary of the data from our laboratory and the literature, based on a large number (>1000) of individual cases of hepatocellular carcinoma, is presented here and shows the following: 1) A high level and high prevalence of exposure to aflatoxins occur in West Africa, Mozambique, and some regions of China; 2) a high prevalence of the 249ser p53 mutation is detected in these countries; and 3) hepatocellular carcinomas from countries with low or no exposure to aflatoxins show a very low prevalence of the 249ser p53 mutation and distinctly different p53 mutation spectra, probably indicating different etiologies. Experimental and epidemiologic studies demonstrate an interaction between HBV infection and aflatoxins in hepatocarcinogenesis. The relevance of the biochemical/molecular markers of aflatoxin exposure, HBV vaccination, and the reduction of aflatoxin exposure, in addition to the interaction between HBV infection and other risk factors in liver carcinogenesis, are discussed with regard to the implementation of measures for primary prevention.