Large hepatitis delta antigen is not a suppressor of hepatitis delta virus RNA synthesis once RNA replication is established.
Moderation of hepatitis delta virus (HDV) replication is a likely prerequisite in the establishment of chronic infections and is thought to be mediated by the intracellular accumulation of large hepatitis delta antigen (L-HDAg). The regulatory role of this protein was suggested from several studies showing that cotransfection of plasmid cDNAs expressing both L-HDAg and HDV RNA results in a potent inhibition of HDV RNA replication. However, since this approach differs significantly from natural HDV infections, where HDV RNA replication is initiated from an RNA template, and L-HDAg appears only late in the replication cycle, it remains unclear whether L-HDAg can modulate HDV RNA replication in the natural HDV replication cycle. In this study, we investigated the effect of L-HDAg, produced as a result of the natural HDV RNA editing event, on HDV RNA replication. The results showed that following cDNA-free HDV RNA transfection, a steady-state level of RNA was established at 3 to 4 days posttransfection. The same level of HDV RNA was reached when a mutant HDV genome unable to make L-HDAg was used, suggesting that L-HDAg did not play a role. The rates of HDV RNA synthesis, as measured by metabolic labeling experiments, were identical at 4 and 8 days posttransfection and in the wild type and the L-HDAg-deficient mutant. We further examined the effect of overexpression of L-HDAg at various stages of the HDV replication cycle, showing that HDV RNA synthesis was resistant to L-HDAg when it was overexpressed 3 days after HDV RNA replication had initiated. Finally, we showed that, contrary to conventional thinking, L-HDAg alone, at a certain molar ratio with HDV RNA, can initiate HDV RNA replication. Thus, L-HDAg does not inherently inhibit HDV RNA synthesis. Taken together, these results indicated that L-HDAg affects neither the rate of HDV RNA synthesis nor the final steady-state level of HDV RNA and that L-HDAg is unlikely to act as an inhibitor of HDV RNA replication in the natural HDV replication cycle.
Macnaughton
Thomas B
TB
Department of Molecular Microbiology and Immunology, University of Southern California Keck School of Medicine, Los Angeles, California 90033-1054, USA.
Lai
Michael M C
MM
D018963:Hepatitis Antigens / Q000502:physiology*; D003698:Hepatitis Delta Virus / Q000235:genetics / Q000502:physiology*; D035923:Hepatitis delta Antigens; D006801:Humans; D012367:RNA, Viral / Q000096:biosynthesis*; D014162:Transfection; D014407:Tumor Cells, Cultured; D014779:Virus Replication
D018963:Hepatitis Antigens; D035923:Hepatitis delta Antigens; D012367:RNA, Viral; C442244:hepatitis delta virus large antigen