Epigallocatechin Gallate Inhibits the HIV Reverse Transcription Step

Background:

Epigallocatechin gallate (EGCG), the most abundant catechin in green tea, has been reported to inhibit HIV-1 replication prior to its integration into host DNA via various proposed mechanisms; however, the specific main target(s) of EGCG remain unclear. In this study, we investigated a number of these proposed detailed mechanism(s) using a cell-based model.

Methods:

Multinuclear activation of galactosidase indicator assays were used for all experiments, including examination of the time of addition and the synergisms with a nucleoside reverse transcriptase inhibitor, 3′-azido-3′-deoxythymidine (AZT).

Results:

The experiments revealed that EGCG suppressed both HIV-1IIIB and HIV-2EHO infection in HeLa-CD4-LTR-β-gal cells, with relatively low 50% effective concentrations of 1.6 and 2.0 μM, respectively. The inhibitory profile of EGCG generated using a time-of-addition assay was identical to that of a non-nucleoside reverse transcriptase inhibitor (NNRTI), MKC-442. Furthermore, synergistic inhibition was observed in EGCG with AZT.

Conclusions:

Based on our findings, EGCG appears to act mainly as an allosteric reverse transcriptase inhibitor with mechanisms different from those of currently approved NNRTIs that directly interact with the NNRTI binding pocket. Thus, EGCG is a good candidate for use as an additional or supportive anti-HIV agent derived from natural plants.

The effects of cladribine and fludarabine on DNA methylation in K562 cells

The effects of the antileukemic adenosine analogues, 2-chloro-2'-deoxyadenosine (cladribine) and 9-beta-D-arabinosyl-2-fluoroadenine (fludarabine), on DNA methylation were studied in a cell line K562. It was previously found that both drugs inactivated SAH hydrolase, an enzyme which participates in the "active methyl" cycle. The study examined the effects of these drugs on three aspects of DNA methylation: (i) activity of endogenous C-5 DNA methyltransferase; (ii) capacity of genomic DNA (gDNA) to accept methyl groups, transferred from S-adenosylmethionine by the bacterial methyltransferase, SssI; (iii) estimation of changes of methylated cytosine levels in gDNA, using methylation-dependent restriction analysis. Cladribine and fludarabine inhibited C-5 DNA methyltransferase, with ED(50) values of 3.5 and 47.0 microM, respectively, after 24hr cell growth in the presence of the drugs. After 48 hr growth of cells with cladribine (0.1 microM) or fludarabine (3 microM), the capacity of DNA to accept methyl groups, in the presence of exogenous bacterial SssI methylase, increased by approximately 1.8 and 1.6 times, respectively, compared to control DNA. Digestion of gDNA with endonucleases HpaII and BssHII followed by SssI DNA methylation, indicated that cladribine (0.1 microM) reduced the level of methylated cytosines in both CpG islands and CCGG sequences, sensitive to HpaII restriction enzyme. Inhibition of DNA methylation by fludarabine was observed mainly in CpG dinucleotide located within sequences sensitive to HpaII. The perturbation of DNA methylation was considered as a complex process. Our findings for cladribine and fludarabine should be regarded as an extra element of their antileukemic efficacy.

Enhancement of anti-herpetic activity of glycyrrhizic acid by physiological proteins

Some enzymes present in biological fluids, such as lysozyme (LYS) and lactoferrin (LAC), are known to possess antibacterial and antiviral activity, against herpesviruses in particular. It will be shown in this paper that their combination with a natural triterpene, namely glycyrrhizic acid (GLA), gives significant results in enhancing the antagonistic activity on HSV1 in in vitro assays. Data elaboration was carried out by calculation of the FIC index (fractional inhibitory concentration) for each combination of the three compounds and by a three-dimensional evaluation of the inhibiting combinatory effects, which indicated the percentage of the synergistic action. A FIC index equal to or below 0.5 demonstrated a significant synergistic effect between two substances. Considering each single compound, the 50% inhibiting doses on viral replication (ID50) were 252+/-53 microg/ml for LAC, 497+/-165 microg/ml for LYS and 740+/-125 microg/ml for GLA. The combination of LAC and GLA showed a clear synergistic effect, with a FIC index of 0.08 and a potentiating activity which, for some doses, was up to 1.5 log10 of difference (from about 5.5x10(6) to 10(5) pfu/ml). The combinations of GLA and LYS, and LYS and LAC showed a less significant synergistic activity. These findings led to the conclusion that some physiological proteins, even at concentrations usually present in some body fluids, may enhance the anti-herpetic activity of a natural compound such as GLA.

Enhancement of antiherpetic activity of cytarabine by hydroxyurea in human embryonic skin-muscle fibroblasts

Pharmacological induction of low deoxyribonucleoside triphosphate (dNTP) levels is a novel approach to combine inhibition of virus replication. In accordance with this concept, the alteration of antiherpes activity of cytarabine (AraC) in combination with hydroxyurea (HU), an inhibitor of ribonucleotide reductase, was studied. The results were confirmed by virus yield assays in human embryonic skin-muscle fibroblasts (HESMF). It was demonstrated that HU significantly enhanced the antiviral activity of AraC against HSV-1 and HSV-2 in HESMF cultures and this inhibition was reversed by an excess of deoxycytidine (dCyt). The observations suggest that the reduction in dCTP level accounts for the potentiating effect of HU on the antiviral activity of AraC.

A new series of cyclic amino acids as inhibitors of S-adenosyl L-methionine synthetase

Optically active 3-amino-3-(tetrahydrofuranyl) carboxylic acid, 3-amino-3-(tetrahydrothienyl) carboxylic acid and their corresponding six membered ring analogues have been synthesised and examined as potential inhibitors of the enzyme S-adenosylmethionine (AdoMet) synthetase. The kinetic behaviour of these compounds was studied using recombinant rat liver AdoMet synthetase (alpha-isoform) fractionated from E. coli transformed with the plasmid pSSRL-T7N. All the compounds tested were competitive inhibitors of the enzyme with respect to L-methionine.

Synergistic effects of triterpenic compounds with prostaglandin A1 on vaccinia virus infected L929 cells

Triterpenic compounds, such as glycyrrhizic acid (GRa) and carbenoxolone (CBX), have a synergistic effect with prostaglandin A1 on the inhibition of vaccinia virus (VV) replication in L929 cells. The fractional inhibitory concentration (FIC) values for GRa and CBX were 0.5 and 0.25, respectively. In the supernatant of triterpene treated cells, increased production of some prostaglandins was shown, whilst cell-associated prostaglandins and prostaglandins of the A series were only slightly influenced by the presence of triterpenes. From these findings there is no evidence that prostaglandin production and metabolism could be involved in the antiviral activity of triterpenes.

Synergistic anti-influenza virus A (H1N1) activities of PM-523 (polyoxometalate) and ribavirin in vitro and in vivo

A Kegin-type polyoxometalate, PM-523, in combination with ribavirin, was tested for its therapeutic effectiveness against influenza virus (FluV) A (H1N1) infection in tissue culture and in mice. PM-523 [(PriNH3)6H [PTi2W10O38(O2)2] x H2O, where Pri is isopropanol] and ribavirin individually inhibited FluV A-induced cytopathic effects in Madin-Darby canine kidney (MDCK) cells at median effective concentrations (EC50s) of 30 and 34 microM, respectively, and at 70% effective concentrations (EC70s) of 48 and 72 microM, respectively. On the other hand, a combination of PM-523 and ribavirin at a ratio of 1:16 exhibited lower EC50s and EC70s than each compound used singly, and combination indices were less than 1. A wide range of combinations of PM-523 and ribavirin at ratios of from 1:128 to 1:1 exhibited additive or synergistic anti-FluV effects in MDCK cells. When these compounds were tested for their anti-FluV A activities in vivo by aerosol exposure of mice which had been infected with a lethal dose of FluV A by an intranasal route, a 1:16 combination of PM-523 and ribavirin was found to have a significantly better therapeutic effect than a single dose of either compound used singly with respect to both the survival rate of the mice and the virus titer in the lungs of the infected mice. PM-523 was effective for the treatment of experimental FluV infection, and in combination with ribavirin, PM-523 exhibited enhanced anti-FluV effects in vitro and in vivo compared with the effect of PM-523 alone.

Effects of novel anti-viral adenosine analogues on the activity of S-adenosylhomocysteine hydrolase from human liver

Some adenosine analogues have been found previously to inhibit S-adenosylhomocysteine (SAH)-hydrolase activity in erythrocyte lysates. In this study, the enzyme was purified 500-fold from human liver and its M(r) found to be 190,000. Its kinetics in the synthase direction were studied, the Km for adenosine being determined as 32 microM. Several purine nucleoside analogues currently used in antitumour and antiviral therapy were tested for their influence on SAH-hydrolase activity. The results confirmed our previous findings for the unpurified human erythrocyte enzyme, and demonstrated that the most potent inhibitors of human liver SAH-hydrolase were neplanocin A, 7-deaza-adenosine (tubercidin), 2'-deoxyadenosine, and 9-beta-D-arabino-furanosyladenine. Analogues showing intermediate inhibition were 2'3'-dideoxyadenosine (2'3'-ddAdo), 5'-deoxy-5'-methyl-thioadenosine, 3'-deoxy-adenosine, 2-chloroadenosine, 1,2,4-triazole-3-carboxamide riboside (ribavirin), delta-adenosylornithine (sinefungin), S-adenosylmethionine, 5-amino-4-imidazole carboxamide riboside (AICAR), and 5'-iodo-5'-deoxyadenosine (5'I,5'-dAdo). Weak or no inhibition was noted with 5'-deoxyadenosine, 5-hydroxyimidazole-4-carboxamideriboside (bredinin), inosine and its deoxy analogues, and acyclovir. Our results show that drugs such as 2'3'-dideoxyadenosine (used in HIV therapy) and ribavirin (an inhibitor of inosinate dehydrogenase), in addition to their other known mechanisms of action, have an inhibitory effect on SAH-hydrolase activity, which may be of significance in their antiviral action.