Reversal of the antiviral activity of ribavirin against Sindbis virus in Ae. albopictus mosquito cells

mRNA Capping by Venezuelan Equine Encephalitis Virus nsP1: Functional Characterization and Implications for Antiviral Research

Alphaviruses are known to possess a unique viral mRNA capping mechanism involving the viral nonstructural protein nsP1. This enzyme harbors methyltransferase (MTase) and nsP1 guanylylation (GT) activities catalyzing the transfer of the methyl group from S-adenosylmethionine (AdoMet) to the N7 position of a GTP molecule followed by the formation of an m(7)GMP-nsP1 adduct. Subsequent transfer of m(7)GMP onto the 5' end of the viral mRNA has not been demonstrated in vitro yet. Here we report the biochemical characterization of Venezuelan equine encephalitis virus (VEEV) nsP1. We have developed enzymatic assays uncoupling the different reactions steps catalyzed by nsP1. The MTase and GT reaction activities were followed using a nonhydrolyzable GTP (GIDP) substrate and an original Western blot assay using anti-m3G/m(7)G-cap monoclonal antibody, respectively. The GT reaction is stimulated by S-adenosyl-l-homocysteine (Ado-Hcy), the product of the preceding MTase reaction, and metallic ions. The covalent linking between nsP1 and m(7)GMP involves a phosphamide bond between the nucleotide and a histidine residue. Final guanylyltransfer onto RNA was observed for the first time with an alphavirus nsP1 using a 5'-diphosphate RNA oligonucleotide whose sequence corresponds to the 5' end of the viral genome. Alanine scanning mutagenesis of residues H37, H45, D63, E118, Y285, D354, R365, N369, and N375 revealed their respective roles in MT and GT reactions. Finally, the inhibitory effects of sinefungin, aurintricarboxylic acid (ATA), and ribavirin triphosphate on MTase and capping reactions were investigated, providing possible avenues for antiviral research.

IMPORTANCE

Emergence or reemergence of alphaviruses represents a serious health concern, and the elucidation of their replication mechanisms is a prerequisite for the development of specific inhibitors targeting viral enzymes. In particular, alphaviruses are able, through an original reaction sequence, to add to their mRNA a cap required for their protection against cellular nucleases and initiation of viral proteins translation. In this study, the capping of a 5' diphosphate synthetic RNA mimicking the 5' end of an alphavirus mRNA was observed in vitro for the first time. The different steps for this capping are performed by the nonstructural protein 1 (nsP1). Reference compounds known to target the viral capping inhibited nsP1 enzymatic functions, highlighting the value of this enzyme in antiviral development.

A prospective randomized trial of mycophenolate mofetil in liver transplant recipients with hepatitis C

Hepatitis C is the most common indication for liver transplantation (LT) in the United States. Recurrence of hepatitis C virus (HCV) infection post-LT remains a problem for which there is no completely satisfactory treatment. The aim of the present study is to evaluate mycophenolate mofetil (MMF), which has both immunosuppressive and antiviral properties, to determine whether it is associated with a difference in the rate of HCV recurrence and also examine its impact on patient and graft survival. Between August 1995 and May 1998, a total of 106 patients who were HCV positive before LT were randomized to tacrolimus (TAC) and prednisone versus TAC, prednisone, and MMF therapy. The rate of recurrence of HCV, patient and graft survival, incidences of rejection, and histological findings were examined. Fifty six patients were randomized to TAC and steroid therapy (double [D] drug; group D), and 50 patients were randomized to TAC, steroid, and MMF therapy (triple [T] drug; group T). Liver biopsies were performed when liver function was abnormal; protocol liver biopsies were not performed. Mean follow-up was 4.3 +/- 0.8 years. Actuarial patient survivals at 4 years were 72.6% in group D and 73.8% in group T (P = not significant). Actuarial graft survivals at 4 years were 65.6% in group D and 65.4% in group T. One patient in group D and 2 patients in group T underwent a second LT for recurrent HCV. One patient in each group died of recurrent HCV without re-LT. Twenty-six patients in group D (46.4%) and 23 patients in group T (46.0%) showed signs of recurrent HCV. Mean hepatitis activity index (HAI) scores were 7.4 +/- 2.7 in group D and 7.0 +/- 3.4 in group T, and mean fibrosis scores were 2.9 +/- 1.7 in group D and 2.6 +/- 1.1 in group T. The rate of rejection was 0.57/patient in each group for the entire follow-up period. None of these values reached statistical significance. Rates of HCV recurrence, graft loss or death from recurrent HCV, and 4-year actuarial patient and graft survival were not different between the groups. In liver transplant recipients with HCV, MMF has no impact on patient survival, graft survival, rejection, or rate of HCV recurrence based on biochemical changes and histological findings. In addition, there was no difference in HAI or fibrosis score between the two groups. Either MMF has no anti-HCV effect or its immunosuppressive properties overwhelm its antiviral effect in the clinical setting.

Novel nicotinamide adenine dinucleotide analogues as potential anticancer agents: quest for specific inhibition of inosine monophosphate dehydrogenase

Synthetic nicotinamide adenine dinucleotide (NAD) analogues containing 5-beta-D-ribofuranosylnicotinamide (C-NAD), 6-beta-D-ribofuranosylpicolinamide (C-PAD), 3-beta-D-ribofuranosylbenzamide (BAD), and 2-beta-D-ribofuranosylthiazole-4-carboxamide (TAD) in place of the nicotinamide riboside moiety are described and evaluated as potential inhibitors of inosine monophosphate dehydrogenase (IMPDH). TAD and BAD showed potent inhibitory activity against the enzyme in the form of pyrophosphates, as well as metabolically stable methylene- and difluoromethylenebis(phosphonate)s. Fluorination at the C2' (ribo and arabino configuration) and C3' (ribo) of the adenosine moiety of TAD afforded analogues highly potent against IMPDH, but weakly active against alcohol dehydrogenase. With the exception of the methylenebis(phosphonate) analogue of TAD compounds containing a methylene bridge were poor inhibitors of growth of K562 cells. On the other hand, NAD analogues containing difluoromethylene linkage were highly effective in inhibition of K562 cell growth, as well as potent inducers of K562 cell differentiation. Such compounds, therefore, may be of potential therapeutic interest.