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.
Department of Molecular Microbiology and Immunology, University of Southern California Keck School of Medicine, Los Angeles, California 90033-1054, USA.
Thomas B
TB
Macnaughton
Michael M C
MM
Lai
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