Isolation of a laccase with HIV-1 reverse transcriptase inhibitory activity from fresh fruiting bodies of the Lentinus edodes (Shiitake mushroom)

A laccase with a molecular mass of 67 kDa and inhibitory activity toward HIV-1 reverse transcriptase (IC50 = 7.5 microM) was isolated from fresh fruiting bodies of the Lentinus edodes (Shiitake mushroom). Its characteristics were compared with those of laccases from cultured mushroom mycelia reported earlier. The laccase was unadsorbed on DEAE-cellulose, Affi-gel blue gel and CM-cellulose, but was adsorbed on Con A-Sepharose. About 50-fold purification was achieved with a 19.2% yield of the enzyme. The activity of the enzyme increased steadily from 20 degrees C to 70 degreesC. The activity disappeared after exposure to the boiling temperature for 10 min. Its optimal pH was 4 and very little enzyme activity remained at and above pH 10. The laccase inhibited HIV-1 reverse transcriptase with an IC50 of 7.5 microM, but did not demonstrate any antifungal or anti-proliferative activity.

Raltegravir is a substrate for SLC22A6: a putative mechanism for the interaction between raltegravir and tenofovir

The identification of transporters of the HIV integrase inhibitor raltegravir could be a factor in an understanding of the pharmacokinetic-pharmacodynamic relationship and reported drug interactions of raltegravir. Here we determined whether raltegravir was a substrate for ABCB1 or the influx transporters SLCO1A2, SLCO1B1, SLCO1B3, SLC22A1, SLC22A6, SLC10A1, SLC15A1, and SLC15A2. Raltegravir transport by ABCB1 was studied with CEM, CEM(VBL100), and Caco-2 cells. Transport by uptake transporters was assessed by using a Xenopus laevis oocyte expression system, peripheral blood mononuclear cells, and primary renal cells. The kinetics of raltegravir transport and competition between raltegravir and tenofovir were also investigated using SLC22A6-expressing oocytes. Raltegravir was confirmed to be an ABCB1 substrate in CEM, CEM(VBL100), and Caco-2 cells. Raltegravir was also transported by SLC22A6 and SLC15A1 in oocyte expression systems but not by other transporters studied. The K(m) and V(max) for SLC22A6 transport were 150 μM and 36 pmol/oocyte/h, respectively. Tenofovir and raltegravir competed for SLC22A6 transport in a concentration-dependent manner. Raltegravir inhibited 1 μM tenofovir with a 50% inhibitory concentration (IC(50)) of 14.0 μM, and tenofovir inhibited 1 μM raltegravir with an IC(50) of 27.3 μM. Raltegravir concentrations were not altered by transporter inhibitors in peripheral blood mononuclear cells or primary renal cells. Raltegravir is a substrate for SLC22A6 and SLC15A1 in the oocyte expression system. However, transport was limited compared to endogenous controls, and these transporters are unlikely to have a great impact on raltegravir pharmacokinetics.

Novel fold and carbohydrate specificity of the potent anti-HIV cyanobacterial lectin from Oscillatoria agardhii

Oscillatoria agardhii agglutinin (OAA) is a recently discovered cyanobacterial lectin that exhibits potent anti-HIV activity. Up to now, only its primary structure and carbohydrate binding data have been available. To elucidate the structural basis for the antiviral mechanism of OAA, we determined the structure of this lectin by x-ray crystallography at 1.2 Å resolution and mapped the specific carbohydrate recognition sites of OAA by NMR spectroscopy. The overall architecture of OAA comprises 10 β-strands that fold into a single, compact, β-barrel-like domain, creating a unique topology compared with all known protein structures in the Protein Data Bank. OAA sugar binding was tested against Man-9 and various disaccharide components of Man-9. Two symmetric carbohydrate-binding sites were located on the protein, and a preference for Manα(1-6)Man-linked sugars was found. Altogether, our structural results explain the antiviral activity OAA and add to the growing body of knowledge about antiviral lectins.

Development of modified nucleosides that have supremely high anti-HIV activity and low toxicity and prevent the emergence of resistant HIV mutants

An idea to use 4'-C-substituted-2'-deoxynucleoside derivatives was proposed based on a working hypothesis to solve the problems of existing acquired immune deficiency syndrome chemotherapy (highly active antiretroviral therapy). Subsequent studies have successfully proved the validity of the idea and resulted in the development of 2'-deoxy-4'-C-ethynyl-2-fluoroadenosine and 2'-deoxy-4'-C-ethynyl-2-chloroadenosine, nucleoside reverse transcriptase inhibitors, which have supremely high activity against all human immunodeficiency viruses including multidrug-resistant HIV and low toxicity.

Fc-glycosylation influences Fcγ receptor binding and cell-mediated anti-HIV activity of monoclonal antibody 2G12

Interactions between the Fc segment of IgG and FcγRs on a variety of cells are likely to play an important role in the anti-HIV activity of Abs. Because the nature of the glycan structure on the Fc domain is a critical determinant of Fc-FcγR binding, proper Fc glycosylation may contribute to Ab-mediated protection. We have generated five different glycoforms of the broadly HIV-1-neutralizing mAb 2G12 in wild-type and glycoengineered plants and Chinese hamster ovary cells. Plant-derived 2G12 exhibited highly homogeneous glycosylation profiles with a single dominant N-glycan species. Using flow cytometry with FcγR-expressing cell lines, all 2G12 glycoforms demonstrated similar binding to FcγRI, FcγRIIa, and FcγRIIb. In contrast, two glycoforms derived from glycoengineered plants that lack plant-specific xylose and core α1,3-fucose, and instead carry human-like glycosylation with great uniformity, showed significantly enhanced binding to FcγRIIIa compared with Chinese hamster ovary or wild-type plant-derived 2G12. Using surface plasmon resonance, we show that binding of 2G12 to FcγRIIIa is markedly affected by core fucose, irrespective of its plant-specific α1,3 or mammalian-type α1,6 linkage. Consistent with this finding, 2G12 glycoforms lacking core fucose (and xylose) mediated higher antiviral activity against HIV-1 or simian immunodeficiency virus as measured by Ab-dependent cell-mediated virus inhibition. This is, to our knowledge, the first demonstration that specific alterations of Fc glycosylation can improve antiviral activity. Such alterations may result in better immunotherapeutic reagents. Moreover, biasing vaccine-induced immune responses toward optimal Fc glycosylation patterns could result in improved vaccine efficacy.

Protein adducts as prospective biomarkers of nevirapine toxicity

Nevirapine (NVP) is a non-nucleoside reverse transcriptase inhibitor used against human immunodeficiency virus type-1 (HIV-1), mostly to prevent mother-to-child HIV-1 transmission in developing countries. Despite its clinical efficacy, NVP administration is associated with a variety of toxic responses that include hepatotoxicity and skin rash. Although the reasons for the adverse effects of NVP administration are still unclear, increasing evidence supports the involvement of metabolic activation to reactive electrophiles. In particular, Phase II activation of the NVP metabolite 12-hydroxy-NVP is thought to mediate NVP binding to bionucleophiles, which may be at the onset of toxicity. In the present study, we investigated the nature and specific locations of the covalent adducts produced in human serum albumin and human hemoglobin by reaction in vitro with the synthetic model electrophile 12-mesyloxy-NVP, used as a surrogate for the Phase II metabolite 12-sulfoxy-NVP. Multiple sites of modification were identified by two different mass spectrometry-based methodologies, liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) and matrix-assisted laser desorption ionization tandem mass spectrometry (MALDI-TOF-TOF-MS). These two distinct methodologies, which in some instances afforded complementary information, allowed the identification of multiple adducts involving cysteine, lysine, tryptophan, histidine, serine, and the N-terminal valine of hemoglobin. Tryptophan, which is not a common site of covalent protein modification, was the NVP-modified amino acid residue detected in the two proteins and consistently identified by both LC-ESI-MS/MS and MALDI-TOF-TOF-MS. The propensity of tryptophan to react with the NVP-derived electrophile is further emphasized by the fact that human serum albumin possesses a single tryptophan residue, which suggests a remarkable selectivity that may be useful for biomonitoring purposes. Likewise, the NVP adduct with the terminal valine of hemoglobin, detected by LC-ESI-MS/MS after N-alkyl Edman degradation, appears as an easily assessed marker of NVP binding to proteins. Our results demonstrate the merits and complementarity of the two MS-based methodologies for the characterization of protein binding by NVP and suggest a series of plausible biomarkers of NVP toxicity that should be useful in the monitoring of toxicity effects in patients administered NVP.

Efflux transporters- and cytochrome P-450-mediated interactions between drugs of abuse and antiretrovirals

Multidrug regimens and corresponding drug interactions cause many adverse reactions and treatment failures. Drug efflux transporters: P-gp, MRP, BCRP in conjunction with metabolizing enzymes (CYPs) are major factors in such interactions. Most effective combination antiretrovirals (ARV) therapy includes a PI or a NNRTI or two NRTI. Coadministration of such ARV may induce efflux transporters and/or CYP3A4 resulting in sub-therapeutic blood levels and therapeutic failure due to reduced absorption and/or increased metabolism. A similar prognosis is true for ARV-compounds and drugs of abuse combinations. Morphine and nicotine enhance CYP3A4 and MDR1 expression in vitro. A 2.5 fold rise of cortisol metabolite was evident in smokers relative to nonsmokers. Altered functions of efflux transporters and CYPs in response to ARV and drugs of abuse may result in altered drug absorption and metabolism. Appropriate in vitro models can be employed to predict such interactions. Influence of genetic polymorphism, SNP and inter-individual variation in drug response has been discussed. Complexity underlying the relationship between efflux transporters and CYP makes it difficult to predict the outcome of HAART as such, particularly when HIV patients taking drugs of abuse do not adhere to HAART regimens. HIV(+) pregnant women on HAART medications, indulging in drugs of abuse, may develop higher viral load due to such interactions and lead to increase in mother to child transmission of HIV. A multidisciplinary approach with clear understanding of mechanism of interactions may allow proper selection of regimens so that desired therapeutic outcome of HAART can be reached without any side effects.

Deriving four functional anti-HIV siRNAs from a single Pol III-generated transcript comprising two adjacent long hairpin RNA precursors

Several different approaches exist to generate expressed RNA interference (RNAi) precursors for multiple target inhibition, a strategy referred to as combinatorial (co)RNAi. One such approach makes use of RNA Pol III-expressed long hairpin RNAs (lhRNAs), which are processed by Dicer to generate multiple unique short interfering siRNA effectors. However, because of inefficient intracellular Dicer processing, lhRNA duplexes have been limited to generating two independent effective siRNA species. In this study, we describe a novel strategy whereby four separate anti-HIV siRNAs were generated from a single RNA Pol III-expressed transcript. Two optimized lhRNAs, each comprising two active anti-HIV siRNAs, were placed in tandem to form a double long hairpin (dlhRNA) expression cassette, which encodes four unique and effective siRNA sequences. Processing of the 3' position lhRNA was more variable but effective multiple processing was possible by manipulating the order of the siRNA-encoding sequences. Importantly, unlike shRNAs, Pol III-expressed dlhRNAs did not compete with endogenous and exogenous microRNAs to disrupt the RNAi pathway. The versatility of expressed lhRNAs is greatly expanded and we provide a mechanism for generating transcripts with modular lhRNAs motifs that contribute to improved coRNAi.

Recent advances on the use of the CXCR4 antagonist plerixafor (AMD3100, Mozobil™) and potential of other CXCR4 antagonists as stem cell mobilizers

AMD3100 was originally discovered as an anti-HIV agent effective in inhibiting the replication of HIV in vitro at nanomolar concentrations. We found it to be a potent and selective antagonist of CXCR4, the receptor for the chemokine SDF-1 (now called CXCL12). AMD3100 was then developed, and marketed, as a stem cell mobilizer, and renamed plerixafor (Mozobil™). The path to the discovery of Mozobil™ as a stem cell mobilizer was described in Biochem. Pharmacol. 77: 1655-1664 (2009). Here I review the recent advances that have consolidated the role of plerixafor in mobilizing hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) from the bone marrow into the blood circulation. Plerixafor acts synergistically with granulocyte colony-stimulating factor (G-CSF), and its usefulness has been proven particularly for the mobilization of HSCs and HPCs for autologous stem cell transplantation in patients with non-Hodgkin's lymphoma (NHL) or multiple myeloma (MM). Plerixafor also has great potential for the treatment of hematological malignancies other than NHL and MM, and non-hematological malignancies, and, eventually, several other diseases depending on the CXCL12-CXCR4 interaction. Various AMD3100 analogs have been described (i.e. AMD11070, AMD3465, KRH-3955, T-140, and 4F-benzoyl-TN14003), primarily as potential anti-HIV agents. They are all strong CXCR4 antagonists. Their role in stem cell mobilization remains to be assessed.

[Metabolism of 3-cyanomethyl-4-methyl-DCK, a new anti-HIV candidate, in human intestinal microsomes]

The biotransformation, CYP reaction phenotyping, the impact of CYP inhibitors and enzyme kinetics of 3-cyanomethyl-4-methyl-DCK (CMDCK), a new anti-HIV preclinical candidate belonging to DCK analogs, were investigated in human intestinal microsomes and recombinant cytochrome P450 (CYP) enzymes. CMDCK (4 micromol L(-1)) was incubated with a panel of rCYP enzymes (CYP1A2, 2C9, 2C19, 2D6 and 3A4) in vitro. The remaining parent drug in incubates was quantitatively analyzed by a LC-MS method. CYP3A4 was identified as the principal CYP isoenzyme responsible for its metabolism in intestinal microsomes. The major metabolic pathway of CMDCK was oxidation and a number of oxidative metabolites were screened with LC-MS. The Km, Vmax, CLint and T1/2 of CMDCK obtained from human intestinal microsome were 45.6 micromol L(-1), 0.33 micromol L(-1) min(-1), 12.1 mL min(-1) kg(-1) and 25.7 min, respectively. Intestinal clearance of CMDCK was estimated from in vitro data to be 3.3 mL min(-1) kg(-1), and was almost equal to the intestinal blood flow rate (4.6 mL min(-1) kg(-1)). The selective CYP3A4 inhibitors, ketoconazole, troleandomycin and ritonavir demonstrated significant inhibitory effects on CMDCK intestinal metabolism, which suggested that co-administration of CMDCK with potent CYP3A inhibitors, such as ritonavir, might decrease its intestinal metabolic clearance and subsequently improve its bioavailability in body.

Acaconin, a chitinase-like antifungal protein with cytotoxic and anti-HIV-1 reverse transcriptase activities from Acacia confusa seeds

From the seeds of Acacia confusa, a chitinase-like antifungal protein designated as acaconin that demonstrated antifungal activity toward Rhizoctonia solani with an IC₅₀ of 30±4 µM was isolated. Acaconin demonstrated an N-terminal sequence with pronounced similarity to chitinases and a molecular mass of 32 kDa. It was isolated by chromatography on Q-Sepharose, SP-Sepharose and Superdex 75 and was not bound by either ion exchanger. Acaconin was devoid of chitinase activity. The antifungal activity against Rhizoctonia solani was completely preserved from pH 4 to 10 and from 0°C to 70°C. Congo Red staining at the tips of R. solani hyphae indicated inhibition of fungal growth. However, there was no antifungal activity toward Mycosphaerella arachidicola, Fusarium oxysporum, Helminthosporium maydis, and Valsa mali. Acaconin inhibited proliferation of breast cancer MCF-7 cells with an IC₅₀ of 128±9 µM but did not affect hepatoma HepG2 cells. Its IC₅₀ value toward HIV-1 reverse transcriptase was 10±2.3 µM. The unique features of acaconin include relatively high stability when exposed to changes in ambient pH and temperature, specific antifungal and antitumor actions, potent HIV-reverse transcriptase inhibitory activity, and lack of binding by strongly cationic and anionic exchangers.

A dynamic model of HIV integrase inhibition and drug resistance

Human immunodeficiency virus type 1 (HIV-1) integrase is one of three virally encoded enzymes essential for replication and, therefore, a rational choice as a drug target for the treatment of HIV-1-infected individuals. In 2007, raltegravir became the first integrase inhibitor approved for use in the treatment of HIV-infected patients, more than a decade since the approval of the first protease inhibitor (saquinavir, Hoffman La-Roche, 1995) and two decades since the approval of the first reverse transcriptase inhibitor (retrovir, GlaxoSmithKline, 1987). The slow progress toward a clinically effective HIV-1 integrase inhibitor can at least in part be attributed to a poor structural understanding of this key viral protein. Here we describe the development of a restrained molecular dynamics protocol that produces a more accurate model of the active site of this drug target. This model provides an advance on previously described models as it ensures that the catalytic DDE motif makes correct, monodentate interactions with the two active-site magnesium ions. Dynamic restraints applied to this coordination state create models with the correct solvation sphere for the metal ion complex and highlight the coordination sites available for metal-binding ligands. Application of appropriate dynamic flexibility to the core domain allowed the inclusion of multiple conformational states in subsequent docking studies. These models have allowed us to (1) explore the effects of key drug resistance mutations on the dynamic flexibility and conformational preferences of HIV integrase and to (2) study raltegravir binding in the context of these dynamic models of both wild type and the G140S/Q148H drug-resistant enzyme.

Nevirapine hypersensitivity

Treatment of HIV-1 infections with nevirapine is associated with skin and liver toxicity. These two organ toxicities range from mild to severe, in rare cases resulting in life-threatening liver failure or toxic epidermal necrolysis. The study of the mechanistic steps leading to nevirapine-induced skin rash has been facilitated by the discovery of an animal model in which nevirapine causes a skin rash in rats that closely mimics the rash reported in patients. The similarity in characteristics of the rash between humans and rats strongly suggests that the basic mechanism is the same in both. The rash is clearly immune-mediated in rats, and partial depletion of CD4(+) T cells, but not CD8(+) T cells, is protective. We have demonstrated that the rash is related to the 12-hydroxylation of nevirapine rather than to the parent drug. This is presumably because the 12-hydroxy metabolite can be converted to a reactive quinone methide in skin, but that remains to be demonstrated. Although the rash is clearly related to the 12-hydroxy metabolite rather than the parent drug, cells from rechallenged animals respond ex vivo to the parent drug by producing cytokines such as interferon-gamma with little response to the 12-hydroxy metabolite, even when the rash was induced by treatment with the metabolite rather than the parent drug. This indicates that the response of T cells in vitro cannot be used to determine what caused an immune response. We are now studying the detailed steps by which the 12-hydroxy metabolite induces an immune response and skin rash. This animal model provides a unique tool to study the mechanistic details of an idiosyncratic drug reaction; however, it is likely that there are significant differences in the mechanisms of different idiosyncratic drug reactions, and therefore the results of these studies cannot safely be generalized to all idiosyncratic drug reactions.

Ribosomal frameshifting: an emerging drug target for HIV

Combating HIV currently remains one of the key public health challenges. While immense effort has been invested in the development of HIV-targeted therapeutic agents, interfering with protein synthesis is a relatively unexplored area. HIV has an absolute requirement for a -1 ribosomal frameshift event during translation that constitutes a potentially attractive target for interfering with the viral life cycle. Research suggests that while a considerable amount of investigation is still required, targeting frameshifting in HIV with small molecules, peptides and oligonucleotides is feasible.

Maraviroc: pharmacokinetics and drug interactions

Maraviroc is a potent selective CCR5 antagonist and is the first of this new class of oral agents to be approved for the treatment of CCR5-tropic HIV type-1. Maraviroc is extensively metabolized by CYP3A4, with renal clearance accounting for approximately 23% of total clearance. The half-life of maraviroc is approximately 16 h. Maraviroc does not inhibit any of the major CYP450 enzymes at clinically relevant doses and it has not shown any clinically relevant effects on plasma concentrations of other agents; hence, no dose adjustments of coadministered agents are required. Maraviroc exposure is altered by agents that modulate the activity of CYP3A4 and, in some circumstances, maraviroc dose adjustment is necessary. This article aims to review all pharmacokinetic and drug interaction data available for maraviroc, and to provide a comprehensive summary of the dose adjustment recommendations for maraviroc when coadministered with agents from all classes of antiretroviral therapy as well as other commonly coadministered agents.

Fractional mass filtering as a means to assess circulating metabolites in early human clinical studies

Recent changes in the regulatory environment have led to a need for new methods to assess circulating human drug metabolites in early clinical studies with respect to their potential toxicological impact. The specific goals of such studies are to determine if the metabolites present in human plasma following administration of a drug candidate also are observed in plasma from the animal studies employed for preclinical toxicological evaluation, and to estimate corresponding exposure margins (animal:human) for the major metabolites. Until recently, the accepted best practice for the characterization of circulating drug metabolites utilized liquid chromatography/tandem mass spectrometry (LC/MS/MS)-based methodologies, in conjunction with authentic chemical standards, for the detection and quantitative analyses of metabolites predicted from both animal studies and experiments with human liver preparations in vitro. While this approach is satisfactory for anticipated biotransformation products, metabolites that were not expected to circulate in human plasma frequently escape detection. Current accurate mass instruments enable the use of the technique of fractional mass filtering to detect both expected and unexpected metabolites in a rapid, less resource-intensive and more robust manner. Application of this technology to several clinical development programs at Merck Research Laboratories has demonstrated the value of fractional mass filtering in the assessment of circulating drug metabolites in early clinical trials.

Acyclovir is activated into a HIV-1 reverse transcriptase inhibitor in herpesvirus-infected human tissues

For most viruses, there is a need for antimicrobials that target unique viral molecular properties. Acyclovir (ACV) is one such drug. It is activated into a human herpesvirus (HHV) DNA polymerase inhibitor exclusively by HHV kinases and, thus, does not suppress other viruses. Here, we show that ACV suppresses HIV-1 in HHV-coinfected human tissues, but not in HHV-free tissue or cell cultures. However, addition of HHV-6-infected cells renders these cultures sensitive to anti-HIV ACV activity. We hypothesized that such HIV suppression requires ACV phosphorylation by HHV kinases. Indeed, an ACV monophosphorylated prodrug bypasses the HHV requirement for HIV suppression. Furthermore, phosphorylated ACV directly inhibits HIV-1 reverse transcriptase (RT), terminating DNA chain elongation, and can trap RT at the termination site. These data suggest that ACV anti-HIV-1 activity may contribute to the response of HIV/HHV-coinfected patients to ACV treatment and could guide strategies for the development of new HIV-1 RT inhibitors.

[Pharmacogenetic testing: soon before every prescription?]

Genetic polymorphisms have currently been described in more than 200 systems affecting pharmacological responses (cytochromes P450, conjugation enzymes, transporters, receptors, effectors of response, protection mechanisms, determinants of immunity). Pharmacogenetic testing, i.e. the profiling of individual patients for such variations, is about to become largely available. Recent progress in the pharmacogenetics of tamoxifen, oral anticoagulants and anti-HIV agents is reviewed to discuss critically their potential impact on prescription and contribution/limits for improving rational and safe use of pharmaceuticals. Prospective controlled trials are required to evaluate large-scale pharmacogenetic testing in therapeutics. Ethical, social and psychological issues deserve particular attention.

Characterization of a peptide domain within the GB virus C NS5A phosphoprotein that inhibits HIV replication

Background

GBV-C infection is associated with prolonged survival in HIV-infected people and GBV-C inhibits HIV replication in co-infection models. Expression of the GBV-C nonstructural phosphoprotein 5A (NS5A) decreases surface levels of the HIV co-receptor CXCR4, induces the release of SDF-1 and inhibits HIV replication in Jurkat CD4+ T cell lines.

Methodology/Principal Findings

Jurkat cell lines stably expressing NS5A protein and peptides were generated and HIV replication in these cell lines assessed. HIV replication was significantly inhibited in all cell lines expressing NS5A amino acids 152–165. Substitution of an either alanine or glycine for the serine at position 158 (S158A or S158G) resulted in a significant decrease in the HIV inhibitory effect. In contrast, substituting a phosphomimetic amino acid (glutamic acid; S158E) inhibited HIV as well as the parent peptide. HIV inhibition was associated with lower levels of surface expression of the HIV co-receptor CXCR4 and increased release of the CXCR4 ligand, SDF-1 compared to control cells. Incubation of CD4+ T cell lines with synthetic peptides containing amino acids 152–167 or the S158E mutant peptide prior to HIV infection resulted in HIV replication inhibition compared to control peptides.

Conclusions/Significance

Expression of GBV-C NS5A amino acids 152–165 are sufficient to inhibit HIV replication in vitro, and the serine at position 158 appears important for this effect through either phosphorylation or structural changes in this peptide. The addition of synthetic peptides containing 152–167 or the S158E substitution to Jurkat cells resulted in HIV replication inhibition in vitro. These data suggest that GBV-C peptides or a peptide mimetic may offer a novel, cellular-based approach to antiretroviral therapy.

The efficacy of generating three independent anti-HIV-1 siRNAs from a single U6 RNA Pol III-expressed long hairpin RNA

RNA Interference (RNAi) effectors have been used to inhibit rogue RNAs in mammalian cells. However, rapidly evolving sequences such as the human immunodeficiency virus type 1 (HIV-1) require multiple targeting approaches to prevent the emergence of escape variants. Expressed long hairpin RNAs (lhRNAs) have recently been used as a strategy to produce multiple short interfering RNAs (siRNAs) targeted to highly variant sequences. We aimed to characterize the ability of expressed lhRNAs to generate independent siRNAs that silence three non-contiguous HIV-1 sites by designing lhRNAs comprising different combinations of siRNA-encoding sequences. All lhRNAs were capable of silencing individual target sequences. However, silencing efficiency together with concentrations of individual lhRNA-derived siRNAs diminished from the stem base (first position) towards the loop side of the hairpin. Silencing efficacy against HIV-1 was primarily mediated by siRNA sequences located at the base of the stem. Improvements could be made to first and second position siRNAs by adjusting spacing arrangements at their junction, but silencing of third position siRNAs remained largely ineffective. Although lhRNAs offer advantages for combinatorial RNAi, we show that good silencing efficacy across the span of the lhRNA duplex is difficult to achieve with sequences that encode more than two adjacent independent siRNAs.

[Advances in the study of molecular mechanism of APOBEC3G anti-HIV-1]

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide 3 protein G (APOBEC3G) is part of the innate immune system of host cells and has cytidine deaminase activity. It specifically incorporates into the virion during HIV-1 replication. The incorporation of APOBEC3G needs its interaction with HIV-1 Gag. In the HIV-1 reverse transcription process, APOBEC3G deaminates dC to dU in the first minus strand cDNA, and then induces extensive hypermutation in the viral genome. Besides deamination, APOBEC3G also inhibits HIV-1 by some kinds of non-deamination mechanisms which need to be further elucidated. HIV-1 Vif counteracts the activity of APOBEC3G by an ubiquitin-proteasome-mediated degradation of APOBEC3G. As a broad spectrum inhibitor of viruses, APOBEC3G also inhibits various retroviruses, retrotransposons and other viruses like HBV. Upregulating the expression of APOBEC3G or blocking the Vif-mediated degradation of APOBEC3G might be novel strategies to treat HIV-1 infection in the future.

HIV integrase: a target for drug discovery

Current antiviral strategies against HIV rely on structure-function analysis of HIV reverse transcriptase (RT) and protease (PR). The third viral pol gene product, HIV integrase (IN), is also a good target for drug discovery, since IN is essential for retroviral replication and, moreover, it has no obvious functional analogue in the host. IN forms a ternary complex with metal ions and DNA and has a mechanism of catalysis common with other polynucleotidyl transferases. Although there is no structural information for full-length IN available, structures of all three functional IN domains have been determined by X-ray crystallography and NMR spectroscopy. The N-terminal domain has a novel zinc-binding fold, the catalytic domain shares a common structural motif with other polynucleotidyl transferases, and the C-terminal DNA-binding domain has a Src-homology-3-like fold. This structural information provides the basis for drug development. In turn, increasing numbers of IN inhibitors identified so far may serve structure-function analysis of IN. The final goal is the development of new classes of anti-HIV drugs, which can be added to the repertoire of anti-RT and anti-PR drugs.