Zinc ejection as a new rationale for the use of cystamine and related disulfide-containing antiviral agents in the treatment of AIDS

Comparison of force fields to study the zinc-finger containing protein NPL4, a target for disulfiram in cancer therapy

Molecular dynamics (MD) simulations are a powerful approach to studying the structure and dynamics of proteins related to health and disease. Advances in the MD field allow modeling proteins with high accuracy. However, modeling metal ions and their interactions with proteins is still challenging. NPL4 is a zinc-binding protein and works as a cofactor for p97 to regulate protein homeostasis. NPL4 is of biomedical importance and has been proposed as the target of disulfiram, a drug recently repurposed for cancer treatment. Experimental studies proposed that the disulfiram metabolites, bis-(diethyldithiocarbamate)‑copper and cupric ions, induce NPL4 misfolding and aggregation. However, the molecular details of their interactions with NPL4 and consequent structural effects are still elusive. Here, biomolecular simulations can help to shed light on the related structural details. To apply MD simulations to NPL4 and its interaction with copper the first important step is identifying a suitable force field to describe the protein in its zinc-bound states. We examined different sets of non-bonded parameters because we want to study the misfolding mechanism and cannot rule out that the zinc may detach from the protein during the process and copper replaces it. We investigated the force-field ability to model the coordination geometry of the metal ions by comparing the results from MD simulations with optimized geometries from quantum mechanics (QM) calculations using model systems of NPL4. Furthermore, we investigated the performance of a force field including bonded parameters to treat copper ions in NPL4 that we obtained based on QM calculations.

Cryo-ET of Env on intact HIV virions reveals structural variation and positioning on the Gag lattice

HIV-1 Env mediates viral entry into host cells and is the sole target for neutralizing antibodies. However, Env structure and organization in its native virion context has eluded detailed characterization. Here, we used cryo-electron tomography to analyze Env in mature and immature HIV-1 particles. Immature particles showed distinct Env positioning relative to the underlying Gag lattice, providing insights into long-standing questions about Env incorporation. A 9.1-Å sub-tomogram-averaged reconstruction of virion-bound Env in conjunction with structural mass spectrometry revealed unexpected features, including a variable central core of the gp41 subunit, heterogeneous glycosylation between protomers, and a flexible stalk that allows Env tilting and variable exposure of neutralizing epitopes. Together, our results provide an integrative understanding of HIV assembly and structural variation in Env antigen presentation.

NMR Studies of Retroviral Genome Packaging

Nearly all retroviruses selectively package two copies of their unspliced RNA genomes from a cellular milieu that contains a substantial excess of non-viral and spliced viral RNAs. Over the past four decades, combinations of genetic experiments, phylogenetic analyses, nucleotide accessibility mapping, in silico RNA structure predictions, and biophysical experiments were employed to understand how retroviral genomes are selected for packaging. Genetic studies provided early clues regarding the protein and RNA elements required for packaging, and nucleotide accessibility mapping experiments provided insights into the secondary structures of functionally important elements in the genome. Three-dimensional structural determinants of packaging were primarily derived by nuclear magnetic resonance (NMR) spectroscopy. A key advantage of NMR, relative to other methods for determining biomolecular structure (such as X-ray crystallography), is that it is well suited for studies of conformationally dynamic and heterogeneous systems—a hallmark of the retrovirus packaging machinery. Here, we review advances in understanding of the structures, dynamics, and interactions of the proteins and RNA elements involved in retroviral genome selection and packaging that are facilitated by NMR.

Inhibition of histone lysine methyltransferases G9a and GLP by ejection of structural Zn(II)

Histone lysine methyltransferases G9a and GLP are validated targets for the development of new epigenetic drugs. Most, if not all, inhibitors of G9a and GLP target the histone substrate binding site or/and the S-adenosylmethionine cosubstrate binding site. Here, we report an alternative approach for inhibiting the methyltransferase activity of G9a and GLP. For proper folding and enzymatic activity, G9a and GLP contain structural zinc fingers, one of them being adjacent to the S-adenosylmethionine binding site. Our work demonstrates that targeting these labile zinc fingers with electrophilic small molecules results in ejection of structural zinc ions, and consequently inhibition of the methyltransferase activity. Very effective Zn(II) ejection and inhibition of G9a and GLP was observed with clinically used ebselen, disulfiram and cisplatin.

NCp7: targeting a multitasking protein for next-generation anti-HIV drug development part 1: covalent inhibitors

The major internal component of the HIV virion core is the nucleocapsid protein 7 (NCp7), a small, highly basic protein that is essential for multiple stages of the viral replicative cycle, and whose structure is preserved in all viral strains, including clinical isolates from therapy-experienced patients. This key protein is recognised as a potential target for an effective next-generation antiretroviral therapy, because it could offer the possibility to develop broad-spectrum agents that are less prone to select for resistant strains. Here, we provide a comprehensive overview of the covalent NCp7 inhibitors that have emerged over the past 25 years of drug discovery campaigns, emphasising, where possible, their structure-activity relationships (SARs) and pharmacophoric features.

2-Methyl-4/5-nitroimidazole derivatives potentiated against sexually transmitted Trichomonas: Design, synthesis, biology and 3D-QSAR study

Trichomoniasis is the most prevalent, non-viral sexually transmitted diseases (STD) caused by amitochondriate protozoan Trichomonas vaginalis. Increased resistance of T. vaginalis to the marketed drug Metronidazole necessitates the development of newer chemical entities. A library of sixty 2-methyl-4/5-nitroimidazole derivatives was synthesized via nucleophilic ring opening reaction of epoxide and the efficacies against drug-susceptible and -resistant Trichomonas vaginalis were evaluated. All the molecules except two were found to be active against both susceptible and resistant strains with MICs ranging 8.55-336.70 μM and 28.80-1445.08 μM, respectively. Most of the compounds were remarkably more effective than the standard Metronidazole. This study analyzes the in vitro and in vivo activities of the new 5-nitroimidazoles, which were found to be safe against human cervical HeLa cells with good selectivity index. The exploration of SAR by the synthesis of four different prototypes and 3D-QSAR study has shown the importance of prototype 1 over other prototypes.

Disulfiram deregulates HIF-α subunits and blunts tumor adaptation to hypoxia in hepatoma cells

AIM

Disulfiram is an aldehyde dehydrogenase inhibitor that was used to treat alcoholism and showed anticancer activity, but its anticancer mechanism remains unclear. The aim of this study was to investigate the effects of disulfiram on the hypoxia-inducible factor (HIF)-driven tumor adaptation to hypoxia in vitro.

METHODS

Hep3B, Huh7 and HepG2 hepatoma cells were incubated under normoxic (20% O2) or hypoxic (1% O2) conditions for 16 h. The expression and activity of HIF-1α and HIF-2α proteins were evaluated using immunoblotting and luciferase reporter assay, respectively. Semi-quantitative RT-PCR was used to analyze HIF-mediated gene expression. Endothelial tubule formation assay was used to evaluate the anti-angiogenic effect.

RESULTS

Hypoxia caused marked expression of HIF-1α and HIF-1α in the 3 hepatoma cell lines, dramatically increased HIF activity and induced the expression of HIF downstream genes (EPO, CA9, VEGF-A and PDK1) in Hep3B cells. HIF-2α expression was positively correlated with the induction of hypoxic genes (CA9, VEGF-A and PDK1). Moreover, hypoxia markedly increased VEGF production and angiogenic potential of Hep3B cells. Disulfiram (0.3 to 2 μmol/L) inhibited hypoxia-induced gene expression and HIF activity in a dose-dependent manner. Disulfiram more effectively suppressed the viability of Hep3B cells under hypoxia, but it did not affect the cell cycle. Overexpression of HIF-2α in Hep3B cells reversed the inhibitory effects of disulfiram on hypoxia-induced gene expression and cell survival under hypoxia.

CONCLUSION

Disulfiram deregulates the HIF-mediated hypoxic signaling pathway in hepatoma cells, which may contribute to its anticancer effect. Thus, disulfiram could be used to treat solid tumors that grow in a HIF-dependent manner.

Structural determinants and mechanism of HIV-1 genome packaging

Like all retroviruses, the human immunodeficiency virus selectively packages two copies of its unspliced RNA genome, both of which are utilized for strand-transfer-mediated recombination during reverse transcription-a process that enables rapid evolution under environmental and chemotherapeutic pressures. The viral RNA appears to be selected for packaging as a dimer, and there is evidence that dimerization and packaging are mechanistically coupled. Both processes are mediated by interactions between the nucleocapsid domains of a small number of assembling viral Gag polyproteins and RNA elements within the 5'-untranslated region of the genome. A number of secondary structures have been predicted for regions of the genome that are responsible for packaging, and high-resolution structures have been determined for a few small RNA fragments and protein-RNA complexes. However, major questions regarding the RNA structures (and potentially the structural changes) that are responsible for dimeric genome selection remain unanswered. Here, we review efforts that have been made to identify the molecular determinants and mechanism of human immunodeficiency virus type 1 genome packaging.

Beyond plasma membrane targeting: role of the MA domain of Gag in retroviral genome encapsidation

The MA (matrix) domain of the retroviral Gag polyprotein plays several critical roles during virus assembly. Although best known for targeting the Gag polyprotein to the inner leaflet of the plasma membrane for virus budding, recent studies have revealed that MA also contributes to selective packaging of the genomic RNA (gRNA) into virions. In this Review, we summarize recent progress in understanding how MA participates in genome incorporation. We compare the mechanisms by which the MA domains of different retroviral Gag proteins influence gRNA packaging, highlighting variations and similarities in how MA directs the subcellular trafficking of Gag, interacts with host factors and binds to nucleic acids. A deeper understanding of how MA participates in these diverse functions at different stages in the virus assembly pathway will require more detailed information about the structure of the MA domain within the full-length Gag polyprotein. In particular, it will be necessary to understand the structural basis of the interaction of MA with gRNA, host transport factors and membrane phospholipids. A better appreciation of the multiple roles MA plays in genome packaging and Gag localization might guide the development of novel antiviral strategies in the future.

Inactivation of hantaviruses by N-ethylmaleimide preserves virion integrity

Thiol groups of cysteine residues are crucial for the infectivity of various enveloped viruses, but their role in the infectivity of viruses of the family Bunyaviridae has thus far not been studied. This report shows that thiol groups are essential to the infectivity of hantaviruses. Alkylation of the thiol functional groups using the membrane-permeable compound N-ethylmaleimide (NEM) and membrane-impermeable compound 5,5′-dithio-bis-(2-nitrobenzoic acid) (DTNB) showed NEM to be a highly effective inactivator of Puumala and Tula hantaviruses. The NEM-inactivated hantavirus maintained the buoyant density of the wild-type virus. Furthermore, the antigenicity of glycoproteins and the cell attachment capacity of virions were retained at NEM concentrations that totally abolished virus infectivity. These results signified preservation of virion integrity following inactivation with NEM, making chemically inactivated virions valuable research antigens. It was demonstrated with biotin-conjugated maleimide, a mechanistic analogue of NEM, that all the structural proteins of hantavirus were sensitive towards thiol alkylation. In contrast to hantaviruses, NEM did not abolish Uukuniemi phlebovirus infectivity to the same extent. This indicates differences in the use of free thiols in virus entry among members of the family Bunyaviridae.

Biophysical characterization of DNA binding from single molecule force measurements

Single molecule force spectroscopy is a powerful method that uses the mechanical properties of DNA to explore DNA interactions. Here we describe how DNA stretching experiments quantitatively characterize the DNA binding of small molecules and proteins. Small molecules exhibit diverse DNA binding modes, including binding into the major and minor grooves and intercalation between base pairs of double-stranded DNA (dsDNA). Histones bind and package dsDNA, while other nuclear proteins such as high mobility group proteins bind to the backbone and bend dsDNA. Single-stranded DNA (ssDNA) binding proteins slide along dsDNA to locate and stabilize ssDNA during replication. Other proteins exhibit binding to both dsDNA and ssDNA. Nucleic acid chaperone proteins can switch rapidly between dsDNA and ssDNA binding modes, while DNA polymerases bind both forms of DNA with high affinity at distinct binding sites at the replication fork. Single molecule force measurements quantitatively characterize these DNA binding mechanisms, elucidating small molecule interactions and protein function.

Oxyhalogen–sulfur chemistry — Kinetics and mechanism of the bromate oxidation of cysteamine

The kinetics and mechanism of the oxidation of the biologically important molecule, cysteamine, by acidic bromate and molecular bromine have been studied. In excess acidic bromate conditions, cysteamine is oxidized to N-brominated derivatives, and in excess cysteamine the oxidation product is taurine according to the following stoichiometry: BrO3–+ H2NCH2CH2SH → H2NCH2CH2SO3H + Br–. There is quantitative formation of taurine before N-bromination commences. Excess aqueous bromine oxidizes cysteamine to give dibromotaurine: 5Br2+ H2NCH2CH2SH + 3H2O → Br2NCH2CH2SO3H + 8Br–+ 8H+, while excess cysteamine conditions gave monobromotaurine. The oxidation of cysteamine by aqueous bromine is effectively diffusion-controlled all the way to the formation of monobromotaurine. Further formation of dibromotaurine is dependent on acid concentrations, with highly acidic conditions inhibiting further reaction towards formation of dibromotaurine. The formation of the N-brominated derivatives of taurine is reversible, with taurine regenerated in the presence of a reducing agent such as iodide. This feature makes it possible for taurine to moderate hypobromous acid toxicity in the physiological environment.Key words: cysteamine, hypobromous acid, toxicities, antioxidant.

Short communication: rapid preparation of preventive and therapeutic whole-killed retroviral vaccines using the microbicide taurine chloramine

A current urgent priority is to develop microbicides and vaccines to combat retroviruses like human immunodeficiency virus (HIV). We show that the cysteine-selective natural compound, taurine chloramine (T-NCl), can be effective in this task. A number of proteins in all retroviruses contain highly conserved cysteine-rich regions that are essential for infection and replication. Our data show that by targeting these essential cysteine residues, T-NCl (2 or 5 mM) acts as a highly effective and safe microbicide that fully blocks the infectivity of high HIV-1 titers (10(6) TCID(50) units/ml) but is not injurious to eukaryotic cells. We also demonstrate that T-NCl can be used to prepare a highly effective whole-killed vaccine against murine AIDS (MAIDS) that shows both preventive and therapeutic efficacy. The vaccine consists of a T-NCl-inactivated retrovirus suspension in host cell lysate. The novelty of our approach lies in the ease and speed of vaccine preparation and its avoidance of harsh inactivation or purification steps that can alter native viral conformation. Our approach is therefore likely to overcome a number of intractable obstacles to the preparation of an effective whole-killed HIV vaccine, such as surviving infective viral particles, rapid viral mutation rates, numerous viral strains, and harsh purification steps. Our approach may also permit the rapid preparation of autologous, or custom-made, vaccines for individual patients.

Disulfiram suppresses invasive ability of osteosarcoma cells via the inhibition of MMP-2 and MMP-9 expression

Cancer cells, characterized by local invasion and distant metastasis, are very much dependent on the extracellular matrix. The expression of matrix metalloproteinases (MMPs) has been implicated in the invasion and metastasis of cancer cells. In this study, we reported the effects of disulfiram, a clinically used anti-alcoholism drug, on tumor invasion suppression, as well as its effects on the activity of MMP-2 and MMP-9 in human osteosarcoma cells (U2OS). Disulfiram has been used for alcohol aversion therapy. However, recent reports have shown that disulfiram may have potential in the treatment of human cancers. Herewith, we showed that the anti-tumor effects of disulfiram, in an invasion assay using U2OS cells and that disulfiram has a type IV collagenase inhibitory activity that inhibits expression of genes and proteins responsible for both cell and non-cell mediated invasion on pathways. In conclusion, disulfiram inhibited expression of MMP-2 and MMP-9 and it regulated the invasion of human osteosarcoma cells. These observations raise the possibility of disulfiram being used clinical for the inhibition of cancer invasion.

ESET/SETDB1 gene expression and histone H3 (K9) trimethylation in Huntington's disease

Chromatin remodeling and transcription regulation are tightly controlled under physiological conditions. It has been suggested that altered chromatin modulation and transcription dysfunction may play a role in the pathogenesis of Huntington's disease (HD). Increased histone methylation, a well established mechanism of gene silencing, results in transcriptional repression. ERG-associated protein with SET domain (ESET), a histone H3 (K9) methyltransferase, mediates histone methylation. We show that ESET expression is markedly increased in HD patients and in transgenic R6/2 HD mice. Similarly, the protein level of trimethylated histone H3 (K9) was also elevated in HD patients and in R6/2 mice. We further demonstrate that both specificity protein 1 (Sp1) and specificity protein 3 (Sp3) act as transcriptional activators of the ESET promoter in neurons and that mithramycin, a clinically approved guanosine-cytosine-rich DNA binding antitumor antibiotic, interferes with the DNA binding of these Sp family transcription factors, suppressing basal ESET promoter activity in a dose dependent manner. The combined pharmacological treatment with mithramycin and cystamine down-regulates ESET gene expression and reduces hypertrimethylation of histone H3 (K9). This polytherapy significantly ameliorated the behavioral and neuropathological phenotype in the R6/2 mice and extended survival over 40%, well beyond any existing reported treatment in HD mice. Our data suggest that modulation of gene silencing mechanisms, through regulation of the ESET gene is important to neuronal survival and, as such, may be a promising treatment in HD patients.

Single DNA molecule stretching measures the activity of chemicals that target the HIV-1 nucleocapsid protein

We develop a biophysical method for investigating chemical compounds that target the nucleic acid chaperone activity of HIV-1 nucleocapsid protein (NCp7). We used an optical tweezers instrument to stretch single lambda-DNA molecules through the helix-coil transition in the presence of NCp7 and various chemical compounds. The change in the helix-coil transition width induced by wild-type NCp7 and its zinc finger variants correlates with in vitro nucleic acid chaperone activity measurements and in vivo assays. The compound-NC interaction measured here reduces NCp7's capability to alter the transition width. Purified compounds from the NCI Diversity set, 119889, 119911, and 119913 reduce the chaperone activity of 5 nM NC in aqueous solution at 10, 25, and 100 nM concentrations respectively. Similarly, gallein reduced the activity of 4 nM NC at 100 nM concentration. Further analysis allows us to dissect the impact of each compound on both sequence-specific and non-sequence-specific DNA binding of NC, two of the main components of NC's nucleic acid chaperone activity. These results suggest that DNA stretching experiments can be used to screen chemical compounds targeting NC proteins and to further explore the mechanisms by which these compounds interact with NC and alter its nucleic acid chaperone activity.

Zinc coupling potentiates anti-HIV-1 activity of baicalin

Baicalin (BA) has been shown with anti-HIV-1 activity. Zinc is a nutrient element. The anti-HIV-1 activity of zinc complex of baicalin (BA-Zn) in vitro was studied and compared with the anti-HIV-1 activities between BA and BA-Zn in the present study. Our results suggested that BA-Zn has lower cytotoxicity and higher anti-HIV-1 activity compared with those of BA in vitro. The CC50s of BA-Zn and BA were 221.52 and 101.73 microM, respectively. The cytotoxicity of BA-Zn was about 1.2-fold lower than that of BA. The BA and BA-Zn inhibited HIV-1 induced syncytium formation, HIV-1 p24 antigen and HIV-1 RT production. The EC50s of BA-Zn on inhibiting HIV-1 induced syncytium formation (29.08 microM) and RT production (31.17 microM) were lower than those of BA (43.27 and 47.34 microM, respectively). BA-Zn was more effective than BA in inhibiting the activities of recombinant RT and HIV-1 entry into host cells. Zinc coupling enhanced the anti-HIV-1 activity of baicalin.

Inhibition of invasion and angiogenesis by zinc-chelating agent disulfiram

Cell invasion and angiogenesis are crucial processes in cancer metastasis that require extracellular matrix (ECM) degradation. Proteolytic degradation of the ECM components is a central event of invasion and angiogenesis processes. During these processes, matrix metalloproteinases (MMPs) seem to be primarily responsible for much of the ECM degradation. Disulfiram is frequently used in the treatment of alcoholism and has been reported to possess antiretroviral activity and can eject intrinsic zinc out of human immunodeficiency virus (HIV) nucleocapsid protein. In this report, we show that disulfiram inhibited invasion and angiogenesis in both tumor and endothelial cells at nontoxic concentrations. The 3H-labeled type IV collagen degradation assay suggested that disulfiram has type IV collagenase inhibitory activity, and this inhibition was responsible for blocking invasion and angiogenesis through cell-mediated and non-cell-mediated pathways. However, the mechanisms underlying cell-mediated signal pathways are not fully characterized. Our data demonstrate that the non-cell-mediated pathway is dominant. Thus, disulfiram could directly interact with MMP-2 and MMP-9 and inhibit their proteolytic activity through a zincchelating mechanism. Addition of zinc could reverse the inhibition of invasiveness and collagenase inhibition through disulfiram treatment. This finding implies that MMP-2 and MMP-9 may be the inhibitory targets for a potential disulfiram treatment. These observations raise the possibility clinical therapeutic applications for disulfiram used as a potential inhibitor of metastatic cell invasion and angiogenesis.

A system of protein target sequences for anti-RNA-viral chemotherapy by a vitamin B6-derived zinc-chelating trioxa-adamantane-triol

The synthesis of the structurally unusual heterotricyclic compound 1-[3-hydroxy-5-(hydroxymethyl)-2-methyl-4-pyridinyl]-2,8,9-trioxaadamantane-3,5,7-triol (trivially named bananin, BN) from pyridoxylidenephloroglucinol and a theoretical prospect on possible biological activities of BN are presented in this report. Pyridoxylidenephloroglucinol is synthesized by Knoevenagel condensation of the vitamin B6 aldehyde pyridoxal with phloroglucinol. Pyridoxylidenephloroglucinol rearranges to light-yellow (4'RS)-1',4'-dihydrobananin by refluxing in 5M hydrochloric acid. Air oxidation subsequently forms BN in the heat which immediately yields orange-yellow (4'RS)-4'-chloro-1',4'-dihydrobananin by 1,4-addition of hydrogen chloride. This intermediate could be isolated but, interestingly, not a BN hydrochloride. Brown BN is finally achieved by base-catalyzed elimination of hydrogen chloride from (4'RS)-4'-chloro-1',4'-dihydrobananin. Regarding possible biological activities, it was demonstrated that BN acts as zinc (Zn2+) chelator. Therefore, a target of interest could be the human immunodeficiency virus type 1 (HIV-1) zinc finger HIV-1 RNA-binding nucleocapsid protein p7 (NCp7). Through suggested zinc ejection from HIV-1 genomic RNA psi-element-binding and HIV-1-RNA-duplex packaging NCp7 by BN, thus rendering NCp7 functionally obsolete, it is deduced that HIV-1 replication and effective infectious virion encapsidation could be inhibited by BN. Furthermore, theoretical and structural considerations propose that BN is converted into bananin 5'-monophosphate (BNP) by the cell type-ubiquitous human enzyme pyridoxal kinase (EC 2.7.1.35). Together with the putative antilentiviral retinoid vitamin A-vitamin B6 conjugate analogue B6RA (Kesel, A. J. Biochem. Biophys. Res. Comm. 2003, 300, 793), BNP is postulated to serve as effector in a system of protein target sequences RX(D/E) of RNA virus components. Human immunodeficiency Retroviridae (HIVs) could possibly be influenced by B6RA and BNP. In addition, candidate targets of B6RA and BNP could be adsorption, transcription and/or viral RNA replication of an interestingly wide RNA virus selection including Picornaviridae (poliovirus, human coxsackievirus, hepatitis A virus), Flaviviridae (yellow fever virus, Dengue virus, West Nile virus, Kunjin virus, St. Louis encephalitis virus, hepatitis C virus), Togaviridae (rubella virus), Coronaviridae (human coronavirus, human SARS-associated coronavirus), Rhabdoviridae (rabies virus), Paramyxoviridae (human parainfluenza virus, measles virus, human respiratory syncytial virus), Filoviridae (Marburg virus, Ebola virus), Bornaviridae (Borna disease virus), Bunyaviridae (Hantaan virus), Arenaviridae (Lassa virus), and Reoviridae (human rotavirus). The postulated scope of 'metabolically trapped' BNP might resemble the antiviral spectrum of the RNA-viral virustatic ribavirin.

Metal and redox modulation of cysteine protein function

In biological systems, the amino acid cysteine combines catalytic activity with an extensive redox chemistry and unique metal binding properties. The interdependency of these three aspects of the thiol group permits the redox regulation of proteins and metal binding, metal control of redox activity, and ligand control of metal-based enzyme catalysis. Cysteine proteins are therefore able to act as "redox switches," to sense concentrations of oxidative stressors and unbound zinc ions in the cytosol, to provide a "storage facility" for excess metal ions, to control the activity of metalloproteins, and to take part in important regulatory and signaling pathways. The diversity of cysteine's multiple roles in vivo is equally as fascinating as it is promising for future biochemical and pharmacological research.

Therapeutic effects of cystamine in a murine model of Huntington's disease

The precise cause of neuronal death in Huntington's disease (HD) is unknown. Proteolytic products of the huntingtin protein can contribute to toxic cellular aggregates that may be formed in part by tissue transglutaminase (Tgase). Tgase activity is increased in HD brain. Treatment in R6/2 transgenic HD mice, using the transglutaminase inhibitor cystamine, significantly extended survival, improved body weight and motor performance, and delayed the neuropathological sequela. Tgase activity and N(Sigma)-(gamma-L-glutamyl)-L-lysine (GGEL) levels were significantly altered in HD mice. Free GGEL, a specific biochemical marker of Tgase activity, was markedly elevated in the neocortex and caudate nucleus in HD patients. Both Tgase and GGEL immunoreactivities colocalized to huntingtin aggregates. Cystamine treatment normalized transglutaminase and GGEL levels in R6/2 mice. These findings are consistent with the hypothesis that transglutaminase activity may play a role in the pathogenesis of HD, and they identify cystamine as a potential therapeutic strategy for treating HD patients.

Reactive sulfur species: an emerging concept in oxidative stress

The ingredients of oxidative stress include a variety of reactive species such as reactive oxygen and reactive nitrogen species (ROS, RNS). While sulfur is usually considered as part of cellular antioxidant systems there is mounting evidence that reactive sulfur species (RSS) with stressor properties similar to the ones found in ROS are formed under conditions of oxidative stress. Thiols as well as disulfides are easily oxidised to sulfur species with sulfur in higher oxidation states. Such agents include thiyl radicals, disulfides, sulfenic acids and disulfide-S-oxides. They rapidly oxidise and subsequently inhibit thiol-proteins and enzymes and can be considered as a separate class of oxidative stressors providing new antioxidant drug targets.

Inhibition of spontaneous metastases formation by amifostine

Amifostine was investigated for its ability to inhibit spontaneous metastases formation using the well-characterized murine sarcoma, Sa-NH. Amifostine was administered intraperitoneally at a dose of 50 mg/kg every other day for 6 days to C3Hf/Kam mice until tumors reached an average size of 8-8.5 mm in diameter. Amifostine was again administered immediately after surgical removal of the tumor-bearing limbs by amputation, and then once more 2 days later. Twenty-one days later, animals were evaluated for the presence of spontaneously developed pulmonary metastases. Nontumor-bearing control animals were sham treated using the same dosing and surgery schedules. Treatment with amifostine appeared to slightly delay tumor growth, that is, 13 vs. 12 days for tumors to reach an average diameter of 8 mm. Amifostine reduced both the incidence of pulmonary metastases formed in experimental animals from 77% to 57% (p < 0.05), and their average number per animal from 12.8 +/- 5.4 (SEM) to 2.9 +/- 1.1 (SEM). The effect of amifostine exposure on serum levels of the angiogenesis inhibitor angiostatin was also determined using Western blot analysis. Consistent with the antimetastatic effect, exposure of animals to 50 mg/kg of amifostine resulted in a 4-fold enhanced serum level of angiostatin above control levels. This phenomenon occurred in tumor-bearing and nontumor-bearing animals. The effects of amifostine on matrix metalloproteinase (MMP) enzymatic activity was also determined using gelatin zymography. Conditioned growth medium collected from Sa-NH cells grown to confluency was exposed to various concentrations of SH, i.e., 2-[(aminopropyl)amino]ethane-thiol (WR-1065), the active thiol form of amifostine, for either 30 min or 18 hr. WR-1065, as a function of increasing dose and time, inhibited the enzymatic activities of MMP-2 and MMP-9. At a concentration and time of exposure likely to be achieved in vivo, that is, 40 microM and 30 min, MMP-2 and MMP-9 activities were reduced to between 30% and 40% of control values. Consistent with these affects, WR-1065 was also found to be effective in inhibiting the ability of Sa-NH cells to migrate through Matrigel membranes. After an 18-hr exposure under in vitro conditions, WR-1065 at concentrations of 4, 40 and 400 microM, and 4 mM, inhibited Sa-NH migration to 11%, 44%, 81% and 97% of control values, respectively. The abilities of amifostine and its active thiol WR-1065 to stimulate angiostatin production in mice, and to inhibit the MMP enzymatic activities and invasion ability of Sa-NH cells under in vitro conditions, are consistent with the observed antimetastatic effects exhibited against Sa-NH tumors growing in vivo.

Experimental determination and calculations of redox potential descriptors of compounds directed against retroviral zinc fingers: Implications for rational drug design

A diverse set of electrophilic compounds that react with cysteine thiolates in retroviral nucleocapsid (NC) proteins and abolish virus infectivity has been identified. Although different in chemical composition, these compounds are all oxidizing agents that lead to the ejection of Zn(II) ions bound to conserved structural motifs (zinc fingers) present in retroviral NC proteins. The reactivity of a congeneric series of aromatic disulfides toward the NC protein of the human immunodeficiency virus type 1 (HIV-1), NCp7, has been characterized by HPLC separation of starting reagents from reaction products. We calculated the absolute redox potentials of these compounds in the gas phase and in aqueous solvent, using a density functional theory method and a continuum solvation model. Pulsed polarography experiments were performed and showed a direct correlation between calculated and experimentally determined redox propensities. A dependence between protein reactivity and redox potential for a specific compound was shown: Reaction with NCp7 did not take place below a threshold value of redox potential. This relationship permits the distinction between active and nonactive compounds targeted against NCp7, and provides a theoretical basis for a scale of reactivity with retroviral zinc fingers. Our results indicate that electrophilic agents with adequate thiophilicity to react with retroviral NC fingers can now be designed using known or calculated electrochemical properties. This may assist in the design of antiretroviral compounds with greater specificity for NC protein. Such electrophilic agents can be used in retrovirus inactivation with the intent of preparing a whole-killed virus vaccine formulation that exhibits unaffected surface antigenic properties.

The secondary amine/nitric oxide complex ion R(2)N[N(O)NO](-) as nucleophile and leaving group in S9N)Ar reactions

Ions of structure R(2)N[N(O)NO](-) and their alkylation products have seen increasing use as nitric oxide (NO)-generating agents for biomedical research applications. Here we show that such diazeniumdiolate anions can readily displace halide from a variety of electrophilic aza- or nitroaromatic substrates to form O(2)-arylated derivatives of structure R(2)N-N(O)=N-OAr. The site of arylation and the cis arrangement of the oxygens were confirmed by X-ray crystallography. Displacement by various nucleophiles showed R(2)N[N(O)NO](-) to be a reasonably good leaving group, with rate constants for displacement by hydroxide, methoxide, and isopropylamine that were between those of chloride and fluoride in the S(N)Ar reactions we surveyed. The Meisenheimer intermediate could be spectrally observed. These O(2)-aryl diazeniumdiolates proved capable of reacting with the nucleophilic sulfur of the HIV-1 p7 nucleocapsid protein's zinc finger assembly to eject the zinc, disrupting a structural motif critical to viral replication and suggesting possible utility in the drug discovery realm.

Amifostine: mechanisms of action underlying cytoprotection and chemoprevention

Amifostine is an important drug in the new field of cytoprotection. It was developed by the Antiradiation Drug Development Program of the US Army Medical Research and Development Command as a radioprotective compound and was the first drug from that Program to be approved for clinical use in the protection of dose limiting normal tissues in patients against the damaging effects of radiation and chemotherapy. Its unique polyamine-like structure and attached sulfhydryl group give it the potential to participate in a range of cellular processes that make it an exciting candidate for use in both cytoprotection and chemoprevention. Amifostine protects against the DNA damaging effects of ionizing radiation and chemotherapy drug associated reactive species. It possesses anti-mutagenic and anti-carcinogenic properties. At the molecular level, it has been demonstrated to affect redox sensitive transcription factors, gene expression, chromatin stability, and enzymatic activity. At the cellular level it has important effects on growth and cell cycle progression. This review focuses on relating its unique chemical design to mechanisms of action that underlie its broad usefulness as both a cytoprotective and chemopreventive agent for use in cancer therapy.

Binding of human immunodeficiency virus type 1 nucleocapsid protein to psi-RNA-SL3

The interaction of the nucleocapsid protein NCp7, from the pNL4-3 isolate of HIV-1, with psi-RNA-SL3, with the sequence 5'-GGACUAGCGGAGGCUAGUCC, was studied using non-denaturing gel electrophoresis. Two kinds of experiments were performed, using buffered solutions of radiolabeled RNA and unlabeled protein. In the 'dilution' experiments, the total RNA concentration, RT, was varied for a series of solutions, but kept equal to the total protein concentration, PT, In the 'titration' experiments, solutions having RT constant but with varying PT were analyzed. The solutions were electrophoresed and the autoradiographic spot intensities, proportional to the amounts of the different species present, were measured. The intensities were fit to a number of equilibrium models, differing in species stoichiometries, by finding the best values of the binding constants. It was shown that NCp7 protein and SL3 RNA combine to form at least two complexes. When PT is below approximately 10 microM, a complex that contains two RNAs and one protein forms. Increasing PT to approximately 100 microM causes the 2:1 complex to oligomerize, forming a species having eight RNAs and four proteins. For the dilution experiments, run at 5 degrees C at an ionic strength of 31 mM, we found K1 for the 2:1 complex is approximately 10(11) M(-2) and K2 for the 8:4 complex is approximately 10(16) M(-3). The titration experiments returned K1 approximately 10(7) M(-2) (poorly determined) and K2 approximately 10(19) M(-3). The analysis was complicated by the loss of RNA at higher protein concentrations, due to formation of an insoluble species containing both RNA and protein, which does not enter the gel. Correcting for this changes the calculated values of equilibrium constants, but not the molecularities determined by our analysis. The observation that a small complex can oligomerize to form a larger species is consistent with the fact that NCp7 organizes and condenses the genome in the virus particle.

Characterization of a novel degradation product of 2,2'-dithiobis[N-isoleucylbenzamide], an inhibitor of HIV nucleocapsid protein zinc fingers

Zinc finger motifs have been found to be important in a variety of protein structures including transcription factors and viral nucleocapsid proteins. Recently, it was demonstrated that various aromatic disulfides effectively remove the metal ion from the zinc finger, resulting in an alteration of tertiary structure in this region of the protein, thereby inhibiting transcription. Among these compounds, 2,2'-dithiobis[N-isoleucylbenzamide] exhibits activity against human immunodeficiency virus (HIV)-type 1 in vitro and has been selected for preclinical development as an anti-HIV agent. Analysis of this agent by reversed-phase high-performance liquid chromatography (HPLC) indicated a significant quantity of two additional compounds. Identifying the parent disulfide was accomplished by scanning eluting peaks with positive ion thermospray ionization (TSP) mass spectrometry (MS). Solution-induced disproportionation of the disulfide into its sulfhydryl monomer was demonstrated by treating the drug with dithiothreitol (DTT) prior to HPLC analysis. TSP-MS analysis of the remaining chromatographic peak suggested a molecular weight of 265, which, with 1H-nuclear magnetic resonance (NMR) data of the isolated material, allowed us to elucidate the chemical structure as N-isoleucyl-benzisothiazolone. Contact with stainless steel, such as that employed in an HPLC system, was found to accelerate degradation of the parent disulfide to the benzisothiazolone.

NMR structure of the HIV-1 nucleocapsid protein bound to stem-loop SL2 of the psi-RNA packaging signal. Implications for genome recognition

The RNA genome of the human immunodeficiency virus type-1 (HIV-1) contains a approximately 120 nucleotide Psi-packaging signal that is recognized by the nucleocapsid (NC) domain of the Gag polyprotein during virus assembly. The Psi-site contains four stem-loops (SL1-SL4) that possess overlapping and possibly redundant functions. The present studies demonstrate that the 19 residue SL2 stem-loop binds NC with affinity (K(d)=110(+/-50) nM) similar to that observed for NC binding to SL3 (K(d)=170(+/-65) nM) and tighter than expected on the basis of earlier work, suggesting that NC-SL2 interactions probably play a direct role in the specific recognition and packaging of the full-length, unspliced genome. The structure of the NC-SL2 complex was determined by heteronuclear NMR methods using (15)N,(13)C-isotopically labeled NC protein and SL2 RNA. The N and C-terminal "zinc knuckles" (Cys-X(2)-Cys-X(4)-His-X(4)-Cys; X=variable amino acid) of HIV-1 NC bind to exposed guanosine bases G9 and G11, respectively, of the G8-G9-U10-G11 tetraloop, and residues Lys3-Lys11 of the N-terminal tail forms a 3(10) helix that packs against the proximal zinc knuckle and interacts with the RNA stem. These structural features are similar to those observed previously in the NMR structure of NC bound to SL3. Other features of the complex are substantially different. In particular, the N-terminal zinc knuckle interacts with an A-U-A base triple platform in the minor groove of the SL2 RNA stem, but binds to the major groove of SL3. In addition, the relative orientations of the N and C-terminal zinc knuckles differ in the NC-SL2 and NC-SL3 complexes, and the side-chain of Phe6 makes minor groove hydrophobic contacts with G11 in the NC-SL2 complex but does not interact with RNA in the NC-SL3 complex. Finally, the N-terminal helix of NC interacts with the phosphodiester backbone of the SL2 RNA stem mainly via electrostatic interactions, but does not bind in the major groove or make specific H-bonding contacts as observed in the NC-SL3 structure. These findings demonstrate that NC binds in an adaptive manner to SL2 and SL3 via different subsets of inter and intra-molecular interactions, and support a genome recognition/packaging mechanism that involves interactions of two or more NC domains of assembling HIV-1 Gag molecules with multiple Psi-site stem-loop packaging elements during the early stages of retrovirus assembly.

Cyclic zinc-dithiocarbamate-S,S'-dioxide blocks CXCR4-mediated HIV-1 infection

To test the anti-human immunodeficiency virus type-1 (HIV-1) activity of 3,6,9,12-tetraazatetradecane-1,14-diylbis(zinc dithiocarbamate)-S,S'-dioxide (cyclic zinc-dithiocarbamate-S, S'-dioxide), MAGI and MAGIC-5 cells were used; the former express CXCR4 and the latter express both CXCR4 and CCR5, which are HIV-1 coreceptors. The compound markedly inhibited HIV-1 X4 (CXCR4-using) viral replication in both MAGI and MAGIC-5 cells. On the other hand, the replication of HIV-1 R5X4 (both CXCR4-and CCR5-using) in MAGI cells but not MAGIC-5 cells was inhibited by the compound. The compound was found to specifically inhibit HIV-1 (X4) envelope-mediated cell-to-cell fusion, binding of anti-CXCR4 monoclonal antibody (12G5) to CXCR4 expressed on the surface of cells, and calcium flux induced by stromal-derived factor-1alpha (SDF-1alpha) bound to CXCR4. The results suggest that the compound inhibited CXCR4-mediated HIV-1 infection by influencing to the HIV-1 coreceptor activity of CXCR4.

Identification of a key target sequence to block human immunodeficiency virus type 1 replication within the gag-pol transframe domain

Although the full sequence of the human immunodeficiency virus type 1 (HIV-1) genome has been known for more than a decade, effective genetic antivirals have yet to be developed. Here we show that, of 22 regions examined, one highly conserved sequence (ACTCTTTGGCAACGA) near the 3' end of the HIV-1 gag-pol transframe region, encoding viral protease residues 4 to 8 and a C-terminal Vpr-binding motif of p6(Gag) protein in two different reading frames, can be successfully targeted by an antisense peptide nucleic acid oligomer named PNA(PR2). A disrupted translation of gag-pol mRNA induced at the PNA(PR2)-annealing site resulted in a decreased synthesis of Pr160(Gag-Pol) polyprotein, hence the viral protease, a predominant expression of Pr55(Gag) devoid of a fully functional p6(Gag) protein, and the excessive intracellular cleavage of Gag precursor proteins, hindering the processes of virion assembly. Treatment with PNA(PR2) abolished virion production by up to 99% in chronically HIV-1-infected H9 cells and in peripheral blood mononuclear cells infected with clinical HIV-1 isolates with the multidrug-resistant phenotype. This particular segment of the gag-pol transframe gene appears to offer a distinctive advantage over other regions in invading viral structural genes and restraining HIV-1 replication in infected cells and may potentially be exploited as a novel antiviral genetic target.

The interaction between HIV-1 Tat and human cyclin T1 requires zinc and a critical cysteine residue that is not conserved in the murine CycT1 protein

HIV-1 Tat activates transcription through binding to human cyclin T1, a regulatory subunit of the TAK/P-TEFb CTD kinase complex. Here we show that the cyclin domain of hCycT1 is necessary and sufficient to interact with Tat and promote cooperative binding to TAR RNA in vitro, as well as mediate Tat transactivation in vivo. A Tat:TAR recognition motif (TRM) was identified at the carboxy-terminal edge of the cyclin domain, and we show that hCycT1 can interact simultaneously with Tat and CDK9 on TAR RNA in vitro. Alanine-scanning mutagenesis of the hCycT1 TRM identified residues that are critical for the interaction with Tat and others that are required specifically for binding of the complex to TAR RNA. Interestingly, we find that the interaction between Tat and hCycT1 requires zinc as well as essential cysteine residues in both proteins. Cloning and characterization of the murine CycT1 protein revealed that it lacks a critical cysteine residue (C261) and forms a weak, zinc-independent complex with HIV-1 Tat that greatly reduces binding to TAR RNA. A point mutation in mCycT1 (Y261C) restores high-affinity, zinc-dependent binding to Tat and TAR in vitro, and rescues Tat transactivation in vivo. Although overexpression of hCycT1 in NIH3T3 cells strongly enhances transcription from an integrated proviral promoter, we find that this fails to overcome all blocks to productive HIV-1 infection in murine cells.

Inactivation of human immunodeficiency virus type 1 infectivity with preservation of conformational and functional integrity of virion surface proteins

Whole inactivated viral particles have been successfully used as vaccines for some viruses, but procedures historically used for inactivation can denature virion proteins. Results have been inconsistent, with enhancement of disease rather than protection seen in some notable instances following vaccination. We used the compound 2,2'-dithiodipyridine (aldrithiol-2; AT-2) to covalently modify the essential zinc fingers in the nucleocapsid (NC) protein of human immunodeficiency virus type 1 (HIV-1) or simian immunodeficiency virus (SIV) virions, thereby inactivating infectivity. The inactivated virus was not detectably infectious in vitro (up to 5 log units of inactivation). However, in contrast to virions inactivated by conventional methods such as heat or formalin treatment, viral and host cell-derived proteins on virion surfaces retained conformational and functional integrity. Thus, immunoprecipitation of AT-2-treated virions was comparable to precipitation of matched untreated virus, even when using antibodies to conformational determinants on gp120. AT-2 inactivated virions bound to CD4(+) target cells and mediated virus-induced, CD4-dependent "fusion from without" comparably to native virions. However, viral entry assays demonstrated that the viral life cycle of AT-2-treated virions was arrested before initiation of reverse transcription. The major histocompatibility complex (MHC) class II molecules on the surface of AT-2-treated virions produced from MHC class II-expressing cells retained the ability to support class II-dependent, superantigen-triggered proliferative responses by resting T lymphocytes. These findings indicate that inactivation via this method results in elimination of infectivity with preservation of conformational and functional integrity of virion surface proteins, including both virally encoded determinants and proteins derived from the host cells in which the virus was produced. Such inactivated virions should provide a promising candidate vaccine antigen and a useful reagent for experimentally probing the postulated involvement of virion surface proteins in indirect mechanisms of HIV-1 pathogenesis.

Structure of the HIV-1 nucleocapsid protein bound to the SL3 psi-RNA recognition element

The three-dimensional structure of the human immunodeficiency virus-type 1 (HIV-1) nucleocapsid protein (NC) bound to the SL3 stem-loop recognition element of the genomic Psi RNA packaging signal has been determined by heteronuclear magnetic resonance spectroscopy. Tight binding (dissociation constant, approximately 100 nM) is mediated by specific interactions between the amino- and carboxyl-terminal CCHC-type zinc knuckles of the NC protein and the G7 and G9 nucleotide bases, respectively, of the G6-G7-A8-G9 RNA tetraloop. A8 packs against the amino-terminal knuckle and forms a hydrogen bond with conserved Arg32, and residues Lys3 to Arg10 of NC form a 310 helix that binds to the major groove of the RNA stem and also packs against the amino-terminal zinc knuckle. The structure provides insights into the mechanism of viral genome recognition, explains extensive amino acid conservation within NC, and serves as a basis for the development of inhibitors designed to interfere with genome encapsidation.