Small molecular compounds inhibit HIV-1 replication through specifically stabilizing APOBEC3G

Discovery of L15 as a novel Vif PROTAC degrader with antiviral activity against HIV-1

Viral infectivity factor (Vif) has been recognized as a new therapeutic target for human immunodeficiency virus-1 (HIV-1) infected patients. In our previous work, we have synthesized a novel class of Vif inhibitors with 2-amino-N-(5-hydroxy-2-methoxyphenyl)-6-((4-nitrophenyl)thio)benzamide scaffold, which show obvious activity in HIV-1 infected cells and are also effective against drug-resistant strains. Proteolytic targeting chimera (PROTAC) utilizes the ubiquitin-proteasome system to degrade target proteins, which is well established in the field of cancer, but the antiviral PROTAC molecules are rarely reported. In order to explore the effectiveness of PROTAC in the antiviral area, we designed and synthesized a series of degrader of HIV-1 Vif based on 2-amino-N-(5-hydroxy-2-methoxyphenyl)-6-((4-nitrophenyl)thio)benzamide scaffold. Among them, L15 can degrade Vif protein obviously in a dose-dependent manner and shows certain antivirus activity. Meanwhile, molecular dynamics simulation indicated that the ternary complex formed by L15, Vif, and E3 ligase adopted a reasonable binding mode and maintained a stable interaction. This provided a molecular basis and prerequisite for the selective degradation of the Vif protein by L15. This study reports the HIV-1 Vif PROTAC for the first time and represents the proof-of-concept of PROTACs-based antiviral drug discovery in the field of HIV/ acquired immune deficiency syndrome (AIDS).

Interactions between HIV proteins and host restriction factors: implications for potential therapeutic intervention in HIV infection

Different host proteins target different HIV proteins and antagonize their functions, depending on the stage of the HIV life cycle and the stage of infection. Concurrently, HIV proteins also target and antagonize various different host proteins to facilitate HIV replication within host cells. The preceding quite specific area of knowledge in HIV pathogenesis, however, remains insufficiently understood. We therefore propose, in this review article, to examine and discuss the HIV proteins that counteract those host restriction proteins which results directly in increased infectivity of HIV. We elaborate on HIV proteins that antagonize host cellular proteins to promote HIV replication, and thus HIV infection. We examine the functions and mechanisms via which Nef, Vif, Vpu, Env, Vpr, and Vpx counteract host proteins such as Ser5, PSGL-1, IFITMS, A3G, tetherin, GBP5, SAMHD1, STING, HUSH, REAF, and TET2 to increase HIV infectivity. Nef antagonizes three host proteins, viz., Ser5, PSGL1, and IFITIMs, while Vpx also antagonizes three host restriction factors, viz., SAMHD1, STING, and HUSH complex; therefore, these proteins may be potential candidates for therapeutic intervention in HIV infection. Tetherin is targeted by Vpu and Env, PSGL1 is targeted by Nef and Vpu, while Ser5 is targeted by Nef and Env proteins. Finally, conclusive remarks and future perspectives are also presented.

Design of Vif-Derived Peptide Inhibitors with Anti-HIV-1 Activity by Interrupting Vif-CBFβ Interaction

One of the major functions of the accessory protein Vif of human immunodeficiency virus type 1 (HIV-1) is to induce the degradation of APOBEC3 (A3) family proteins by recruiting a Cullin5-ElonginB/C-CBFβ E3 ubiquitin ligase complex to facilitate viral replication. Therefore, the interactions between Vif and the E3 complex proteins are promising targets for the development of novel anti-HIV-1 drugs. Here, peptides are designed for the Vif-CBFβ interaction based on the sequences of Vif mutants with higher affinity for CBFβ screened by a yeast surface display platform. We identified two peptides, VMP-63 and VMP-108, that could reduce the infectivity of HIV-1 produced from A3G-positive cells with IC50 values of 49.4 μM and 55.1 μM, respectively. They protected intracellular A3G from Vif-mediated degradation in HEK293T cells, consequently increasing A3G encapsulation into the progeny virions. The peptides could rapidly enter cells after addition to HEK293T cells and competitively inhibit the binding of Vif to CBFβ. Homology modeling analysis demonstrated the binding advantages of VMP-63 and VMP-108 with CBFβ over their corresponding wild-type peptides. However, only VMP-108 effectively restricted long-term HIV-1 replication and protected A3 functions in non-permissive T lymphocytes. Our findings suggest that competitive Vif-derived peptides targeting the Vif-CBFβ interaction are promising for the development of novel therapeutic strategies for acquired immune deficiency syndrome.

Broad-spectrum antiviral strategy: Host-targeting antivirals against emerging and re-emerging viruses

Viral infections are amongst the most prevalent diseases that pose a significant threat to human health. Targeting viral proteins or host factors represents two primary strategies for the development of antiviral drugs. In contrast to virus-targeting antivirals (VTAs), host-targeting antivirals (HTAs) offer advantages in terms of overcoming drug resistance and effectively combating a wide range of viruses, including newly emerging ones. Therefore, targeting host factors emerges as an extremely promising strategy with the potential to address critical challenges faced by VTAs. In recent years, extensive research has been conducted on the discovery and development of HTAs, leading to the approval of maraviroc, a chemokine receptor type 5 (CCR5) antagonist used for the treatment of HIV-1 infected individuals, with several other potential treatments in various stages of development for different viral infections. This review systematically summarizes advancements made in medicinal chemistry regarding various host targets and classifies them into four distinct catagories based on their involvement in the viral life cycle: virus attachment and entry, biosynthesis, nuclear import and export, and viral release.

Design, synthesis and biological evaluation of biaryl amide derivatives against SARS-CoV-2 with dual-target mechanism

The COVID-19 pandemic highlights the urgent need to develop effective small-molecule antivirals. Thirty-three novel biaryl amide derivatives were synthesized and evaluated for anti-coronaviral activity. Some significant SARs were uncovered and the intensive structure modifications led to the most active compounds 8b and 8h. The broad-spectrum anti-coronaviral effects of 8h were validated at RNA and protein levels. 8h inhibits coronavirus replication at multiple stages, from virus entry to virus dsRNA synthesis. The mechanism of action showed that 8h may simultaneously act on 3CLpro and TMPRSS2 to display anti-coronaviral effects. 8h combined with RdRp inhibitor showed synergistic inhibitory activity against coronavirus. This study confirmed that biaryl amide derivatives may be a new class of potential therapeutic agents against coronavirus with multiple target effect, worthy of further investigation.

Mammalian Genomic Manipulation with Orthogonal Bxb1 DNA Recombinase Sites for the Functional Characterization of Protein Variants

The Bxb1 bacteriophage serine DNA recombinase is an efficient tool for engineering recombinant DNA into the genomes of cultured cells. Generally, a single engineered "landing pad" site is introduced into the cell genome, permitting the integration of transgenic circuits or libraries of transgene variants. While sufficient for many studies, the extent of genetic manipulation possible with a single recombinase site is limiting and insufficient for more complex cell-based assays. Here, we harnessed two orthogonal Bxb1 recombinase sites to enable alternative avenues for using mammalian synthetic biology to characterize transgenic protein variants. By designing plasmids flanked by a second pair of auxiliary recombination sites, we demonstrate that we can avoid the genomic integration of undesirable bacterial DNA elements using the same starting cells engineered for whole-plasmid integration. We also created "double landing pad" cells simultaneously harboring two orthogonal Bxb1 recombinase sites at separate genomic loci, allowing complex cell-based genetic assays. Integration of a genetically encoded calcium indicator allowed for the real-time monitoring of intracellular calcium signaling dynamics, including kinetic perturbations that occur upon overexpression of the wild-type or variant version of the calcium signaling relay protein STIM1. A panel of missense mutants of the HIV-1 accessory protein Vif was paired with various paralogs within the human Apobec3 innate immune protein family to identify combinations capable or incapable of interacting within cells. These cells allow transgenic protein variant libraries to be readily paired with assay-specific protein partners or biosensors, enabling new functional readouts for large-scale genetic assays for protein function.

Various strategies for developing APOBEC3G protectors to circumvent human immunodeficiency virus type 1

Host restriction factor APOBEC3G (A3G) efficiently restricts Vif-deficient HIV-1 by being packaged with progeny virions and causing the G to A mutation during HIV-1 viral DNA synthesis as the progeny virus infects new cells. HIV-1 expresses Vif protein to resist the activity of A3G by mediating A3G degradation. This process requires the self-association of Vif in concert with A3G proteins, protein chaperones, and factors of the ubiquitination machinery, which are potential targets to discover novel anti-HIV drugs. This review will describe compounds that have been reported so far to inhibit viral replication of HIV-1 by protecting A3G from Vif-mediated degradation.

Compound IMB-Z inhibits hepatitis B virus replication through increasing APOBEC3G expression and incorporation into viral nucleocapsids

OBJECTIVES

As a host restriction factor, apolipoprotein B messenger RNA-editing enzyme catalytic polypeptide-like 3G (APOBEC3G or A3G) has been shown to suppress the replication of several viruses including hepatitis B virus (HBV). Recently, we reported that IMB-Z, a N-phenylbenzamide derivative, could inhibit Enterovirus 71 replication, and A3G mediated its antiviral activity. Whether IMB-Z exhibits an inhibitory effect on HBV replication has not been investigated.

MATERIAL AND METHODS

HBV DNA, pregenomic RNA (pgRNA), core protein, and capsid levels were determined by a qPCR assay or Southern blot, Northern blot, Western blot, and particle gel assay, respectively. Mutation analysis of HBV DNAs was conducted by a differential DNA denaturation PCR assay. A3G encapsidation into HBV nucleocapsids was examined by Western blot analysis after ultracentrifugation and a co-immunoprecipitation (IP) assay between HBV core and A3G proteins.

RESULTS

In the present study, we found that IMB-Z could considerably inhibit HBV replication in HepAD38 cells. Interestingly, IMB-Z did not alter the HBV pgRNA production but could reduce the level of core protein, viral nucleocapsids, and core-associated DNA, as well as cccDNA intracellular amplification. Similar to the action of IMB-Z's inhibition of Enterovirus 71 replication, we found that IMB-Z's inhibition of HBV replication was associated with increased level of A3G. Mechanistically, we demonstrated that the inhibitory effect of IMB-Z is independent of the cytidine deaminase activity of A3G and is exerted by increasing its incorporation into viral nucleocapsids.

CONCLUSIONS

Our results indicate that IMB-Z inhibits HBV through pharmacological induction A3G expression and incorporation into HBV nucleocapsids.

Immune-Related Gene Profile in HIV-Infected Patients with Discordant Immune Response

Background

Background: In spite of many reports on persistent low CD4 T cell counts and change in immune-related gene expression level in patients with HIV infection, there is still uncertainty about significant association between gene expression level and HIV infection in patients with and without discordant immune response (DIR). The aim of this study was to compare the expression level of CD4, CCL5, IFN-γ, STAT1, APOBEC3G, CD45, and ICAM-1 genes in HIV-1-positive patients with and without DIR.

Methods

Methods: In this study, 30 HIV-1-positive patients (15 patients with and 15 patients without DIR [control group]) were included. PBMCs of the patients were collected through density radient centrifugation with Ficoll-Hypaque. RNeasy Plus Mini kit was used to extract RNA. Relative expression levels of CD4, CCL5, IFN-γ, STAT1, APOBEC3G, CD45, and ICAM-1 genes were evaluated by real-time PCR. The data were analyzed using one-way ANOVA.

Results

Results: CD4 T cell counts were significantly lower in DIR patients than the control group (p < 0.01). While there was no significant difference in the relative expression levels of CD4, CCL5, IFN-γ, STAT1, CD45, and ICAM-1 between patients with DIR and control group, APOBEC3G expression level was significantly higher in the patients with DIR as compare to the control group (p < 0.01).

Conclusion

Conclusion: Our findings suggest a significantly higher APOBEC3G expression level in patients with DIR, suggesting the potential role of APOBEC3G in patients with immunological discordance besides its suppressing role in HIV-1 infection. Confirmation of this hypothesis requires further research.

Aromatic disulfides as potential inhibitors against interaction between deaminase APOBEC3G and HIV infectivity factor

APOBEC3G (A3G) is a member of cytosine deaminase family with a variety of innate immune functions. It displays activities against retrovirus and retrotransposon by inhibition of virus infectivity factor (Vif)-deficient HIV-1 replication. The interaction between A3G N-terminal domain and Vif directs the cellular Cullin 5 E3-ubiquitin ligase complex to ubiquitinate A3G, and leads to A3G proteasomal degradation, which is a potential target for anti-HIV drug. Currently, there are very few reports about stable small molecules targeting the interaction between A3G and Vif. In this study, we screened two series of small molecules containing carbamyl sulfamide bond or disulfide bond as bridges of two different aromatic rings. Five asymmetrical disulfides were successfully identified against interaction between A3G and Vif with the IC ₅₀ values close to or smaller than 1 μM, especially, not through covalently binding with A3G or Vif. They restore the A3G expression in the presence of Vif by inhibiting Vif-induced A3G ubiquitination and degradation. This study opens a way to the discovery of new anti-HIV drugs.

The current toolbox for APOBEC drug discovery

Mutational processes driving genome evolution and heterogeneity contribute to immune evasion and therapy resistance in viral infections and cancer. APOBEC3 (A3) enzymes promote such mutations by catalyzing the deamination of cytosines to uracils in single-stranded DNA. Chemical inhibition of A3 enzymes may yield an antimutation therapeutic strategy to improve the durability of current drug therapies that are prone to resistance mutations. A3 small-molecule drug discovery efforts to date have been restricted to a single high-throughput biochemical activity assay; however, the arsenal of discovery assays has significantly expanded in recent years. The assays used to study A3 enzymes are reviewed here with an eye towards their potential for small-molecule discovery efforts.

Synthesis and structure-activity relationship study of new biaryl amide derivatives and their inhibitory effects against hepatitis C virus

A series of novel biaryl amide derivatives were synthesized and evaluated for anti-HCV virus activity. Some significant SARs were uncovered. The intensive structural modifications led to fifteen novel compounds with more potent inhibitory activity compared to the hit compounds IMB 26 and IMB1f. Among them, compound 80 was the most active, with EC₅₀ values almost equivalent to the clinical drug telaprevir (EC₅₀ = 15 nM). Furthermore, it also had a good safety and in vitro and oral pharmacokinetic (oral bioavailability in rats: 34%) profile, suggesting a highly drug-like nature. Compound 80represents a more promising scaffold for anti-HCV virus activity for further study.

Host restriction factor A3G inhibits the replication of Enterovirus D68 through competitively binding 5' UTR with PCBP1

The host restriction factor APOBEC3G (A3G) presents extensively inhibition on a variety of viruses, including retroviruses, DNA and RNA viruses. Our recent study showed that A3G inhibits enterovirus 71 (EV71) and coxsackievirus A16 (CA16) via competitively binding 5'UTR with the host protein poly(C)-binding protein 1 (PCBP1) that is required for multiple EVs replication. However, in addition to EV71 and CA16, whether A3G inhibits other EVs has not been investigated. Here, we demonstrate that A3G could inhibit EVD68 replication, which needs PCBP1 for its replication, but not CA6 that PCBP1 is dispensable for CA6 replication. Further investigation revealed that nucleic acid binding activity of A3G is required for EVD68 restriction, which is similar to the mechanism presented in EV71 restriction. Mechanistically, A3G competitively binds to the cloverleaf (1-123) and the stem-loop IV (234-446) domains of EVD68 5'UTR with PCBP1, thereby inhibiting the 5'UTR activity of EVD68, whereas A3G doesn't interact with CA6 5'UTR results in no effect on CA6 replication. Moreover, non-structural protein 2C encoded by EVD68 overcomes A3G suppression through inducing A3G degradation via the autophagy-lysosome pathway. Our finding revealed that A3G might have broad spectrum antiviral activity against multiple EVs through the general mechanism, which might provide important information for the development of anti-EVs strategy. Importance As the two major pathogens causing hand, food, and mouth disease (HFMD), EV71 and CA16 attract more attention for the discovery of pathogenesis, the involvement of cellular proteins and so on. However, other EVs such as CA6 or EVD68 constantly occurred sporadic or might spread widely in recent years worldwide. Therefore, more information related to these EVs needs to be further investigated so as to develop broad-spectrum anti-EVs inhibitor. In this study, we first reveal that PCBP1 involved in PV and EV71 virus replication, also is required for the replication of EVD68 but not CA6. Then we found that the host restriction factor A3G specifically inhibits the replication of EVD68 but not CA6 via competitively binding to the 5'UTR of EVD68 with PCBP1. Our findings broaden the knowledge related to EVs replication and the interplay between EVs and host factors.

Lead optimization to improve the antiviral potency of 2-aminobenzamide derivatives targeting HIV-1 Vif-A3G axis

The viral infectivity factor (Vif)-apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G (APOBEC3G) axis has been recognized as a valid target for developing novel small-molecule therapies for acquired immune deficiency syndrome (AIDS) or for enhancing innate immunity against viruses. Our previous work reported the novel Vif antagonist 2-amino-N-(2-methoxyphenyl)-6-((4-nitrophenyl)sulfonyl)benzamide (2) with strong antiviral activity. In this work, through optimizations of ring C of 2, we discovered the more potent compound 6m with an EC₅₀ of 0.07 μM in non-permissive H9 cells, reflecting an approximately 5-fold enhancement of antiviral activity compared to that of 2. Western blotting indicated that 6m more strongly suppressed the defensive protein Vif than 2 at the same concentration. Furthermore, 6m suppressed the replication of various clinical drug-resistant HIV strains (FI, NRTI, NNRTI, IN and PI) with relatively high efficacy. These results suggested that compound 6m is a more potent candidate for treating AIDS.

The Role of APOBECs in Viral Replication

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC) proteins are a diverse and evolutionarily conserved family of cytidine deaminases that provide a variety of functions from tissue-specific gene expression and immunoglobulin diversity to control of viruses and retrotransposons. APOBEC family expansion has been documented among mammalian species, suggesting a powerful selection for their activity. Enzymes with a duplicated zinc-binding domain often have catalytically active and inactive domains, yet both have antiviral function. Although APOBEC antiviral function was discovered through hypermutation of HIV-1 genomes lacking an active Vif protein, much evidence indicates that APOBECs also inhibit virus replication through mechanisms other than mutagenesis. Multiple steps of the viral replication cycle may be affected, although nucleic acid replication is a primary target. Packaging of APOBECs into virions was first noted with HIV-1, yet is not a prerequisite for viral inhibition. APOBEC antagonism may occur in viral producer and recipient cells. Signatures of APOBEC activity include G-to-A and C-to-T mutations in a particular sequence context. The importance of APOBEC activity for viral inhibition is reflected in the identification of numerous viral factors, including HIV-1 Vif, which are dedicated to antagonism of these deaminases. Such viral antagonists often are only partially successful, leading to APOBEC selection for viral variants that enhance replication or avoid immune elimination.

A novel HIV-1 inhibitor that blocks viral replication and rescues APOBEC3s by interrupting vif/CBFβ interaction

HIV remains a health challenge worldwide, partly because of the continued development of resistance to drugs. Therefore, it is urgent to find new HIV inhibitors and targets. Apolipoprotein B mRNA-editing catalytic polypeptide-like 3 family members (APOBEC3) are important host restriction factors that inhibit HIV-1 replication by their cytidine deaminase activity. HIV-1 viral infectivity factor (Vif) promotes proteasomal degradation of APOBEC3 proteins by recruiting the E3 ubiquitin ligase complex, in which core-binding factor β (CBFβ) is a necessary molecular chaperone. Interrupting the interaction between Vif and CBFβ can release APOBEC3 proteins to inhibit HIV-1 replication and may be useful for developing new drug targets for HIV-1. In this study, we identified a potent small molecule inhibitor CBFβ/Vif-3 (CV-3) of HIV-1 replication by employing structure-based virtual screening using the crystal structure of Vif and CBFβ (PDB: 4N9F) and validated CV-3's antiviral activity. We found that CV-3 specifically inhibited HIV-1 replication (IC50 = 8.16 µm; 50% cytotoxic concentration >100 µm) in nonpermissive lymphocytes. Furthermore, CV-3 treatment rescued APOBEC3 family members (human APOBEC3G (hA3G), hA3C, and hA3F) in the presence of Vif and enabled hA3G packaging into HIV-1 virions, which resulted in Gly-to-Ala hypermutations in viral genomes. Finally, we used FRET to demonstrate that CV-3 inhibited the interaction between Vif and CBFβ by simultaneously forming hydrogen bonds with residues Gln-67, Ile-102, and Arg-131 of CBFβ. These findings demonstrate that CV-3 can effectively inhibit HIV-1 by blocking the interaction between Vif and CBFβ and that this interaction can serve as a new target for developing HIV-1 inhibitors.

Synthesis and Bioactivity of N-(4-Chlorophenyl)-4-Methoxy-3-(Methylamino) Benzamide as a Potential Anti-HBV Agent

Introduction

Hepatitis B virus (HBV) is a global health concern that can cause acute and chronic liver diseases. Thus, there is an urgent need to research novel anti-HBV agents. Our previous reports show that N-phenylbenzamide derivatives exert broad-spectrum antiviral effects against HIV-1, HCV, and EV71 by increasing intracellular levels of APOBEC3G (A3G). As A3G is capable of inhibiting the replication of HBV, we screened the N-phenylbenzamide derivatives against HBV.

Methods

In this study, a new derivative, N-(4-chlorophenyl)-4-methoxy-3-(methylamino) benzamide (IMB-0523), was synthesized and its anti-HBV activity was evaluated in vitro and in vivo. The acute toxicity and pharmacokinetic profiles of IMB-0523 were also investigated.

Results

Our results show that IMB-0523 has higher anti-HBV activity in both wild-type HBV (IC₅₀: 1.99 µM) and drug-resistant HBV (IC₅₀: 3.30 µM) than lamivudine (3TC, IC₅₀: 7.37 µM in wild-type HBV, IC₅₀: >440 µM in drug-resistant HBV). The antiviral effect of IMB-0523 against HBV may be due to an increased level of intracellular A3G. IMB-0523 also showed low acute toxicity (LD₅₀: 448 mg/kg) in mice and promising PK properties (AUC_0-t: 7535.10±2226.73 µg·h/L) in rats. Further, IMB-0523 showed potent anti-HBV activity in DHBV-infected ducks.

Conclusion

Thus, IMB-0523 may be a potential anti-HBV agent with different mechanisms than current anti-HBV treatment options.

Mechanistic Insight into Antiretroviral Potency of 2'-Deoxy-2'-β-fluoro-4'-azidocytidine (FNC) with a Long-Lasting Effect on HIV-1 Prevention

In preclinical and phase I and II clinical studies, 2'-deoxy-2'-β-fluoro-4'-azidocytidine (FNC) displays a potent and long-lasting inhibition of HIV-1 infection. To investigate its mechanism of action, we compared it with the well-documented lamivudine (3TC). Pharmacokinetic studies revealed that the intracellular retention of FNC triphosphate in peripheral blood mononuclear cells was markedly longer than that of the 3TC triphosphate. FNC selectively enters and is retained in HIV target cells, where it exerts long-lasting prevention of HIV-1 infection. In addition to inhibition of HIV-1 reverse transcription, FNC also restores A3G expression in CD4⁺ T cells in FNC-treated HIV-1 patients. FNC binds to the Vif-E3 ubiquitin ligase complex, enabling A3G to avoid Vif-induced ubiquitination and degradation. These data reveal the mechanisms underlying the superior anti-HIV potency and long-lasting action of FNC. Our results also suggest a potential clinical application of FNC as a long-lasting pre-exposure prophylactic agent capable of preventing HIV infection.

Human cytomegalovirus protein pUL36: A dual cell death pathway inhibitor

Human cytomegalovirus (HCMV) is an important human pathogen and a paradigm of intrinsic, innate, and adaptive viral immune evasion. Here, we employed multiplexed tandem mass tag-based proteomics to characterize host proteins targeted for degradation late during HCMV infection. This approach revealed that mixed lineage kinase domain-like protein (MLKL), a key terminal mediator of cellular necroptosis, was rapidly and persistently degraded by the minimally passaged HCMV strain Merlin but not the extensively passaged strain AD169. The strain Merlin viral inhibitor of apoptosis pUL36 was necessary and sufficient both to degrade MLKL and to inhibit necroptosis. Furthermore, mutation of pUL36 Cys¹³¹ abrogated MLKL degradation and restored necroptosis. As the same residue is also required for pUL36-mediated inhibition of apoptosis by preventing proteolytic activation of procaspase-8, we define pUL36 as a multifunctional inhibitor of both apoptotic and necroptotic cell death.

Medicinal chemistry strategies toward host targeting antiviral agents

Direct‐acting antiviral agents (DAAs) represent a class of drugs targeting viral proteins and have been demonstrated to be very successful in combating viral infections in clinic. However, DAAs suffer from several inherent limitations, including narrow‐spectrum antiviral profiles and liability to drug resistance, and hence there are still unmet needs in the treatment of viral infections. In comparison, host targeting antivirals (HTAs) target host factors for antiviral treatment. Since host proteins are probably broadly required for various viral infections, HTAs are not only perceived, but also demonstrated to exhibit broad‐spectrum antiviral activities. In addition, host proteins are not under the genetic control of viral genome, and hence HTAs possess much higher genetic barrier to drug resistance as compared with DAAs. In recent years, much progress has been made to the development of HTAs with the approval of chemokine receptor type 5 antagonist maraviroc for human immunodeficiency virus treatment and more in the pipeline for other viral infections. In this review, we summarize various host proteins as antiviral targets from a medicinal chemistry prospective. Challenges and issues associated with HTAs are also discussed.

Viral Manipulations of the Cullin-RING Ubiquitin Ligases

Cullin-RING ubiquitin ligases (CRLs) are efficient and diverse toolsets of the cells to regulate almost every biological process. However, these characteristics have also been usurped by many viruses to optimize for their replication. CRLs are often at the forefront of the arms races in the coevolution of viruses and hosts. Here we review the modes of actions and functional consequences of viral manipulations of host cell CRLs. We also discuss the therapeutic applications to target these viral manipulations for treating viral infections.

Human cytomegalovirus interactome analysis identifies degradation hubs, domain associations and viral protein functions

Human cytomegalovirus (HCMV) extensively modulates host cells, downregulating >900 human proteins during viral replication and degrading ≥133 proteins shortly after infection. The mechanism of degradation of most host proteins remains unresolved, and the functions of many viral proteins are incompletely characterised. We performed a mass spectrometry-based interactome analysis of 169 tagged, stably-expressed canonical strain Merlin HCMV proteins, and two non-canonical HCMV proteins, in infected cells. This identified a network of >3400 virus-host and >150 virus-virus protein interactions, providing insights into functions for multiple viral genes. Domain analysis predicted binding of the viral UL25 protein to SH3 domains of NCK Adaptor Protein-1. Viral interacting proteins were identified for 31/133 degraded host targets. Finally, the uncharacterised, non-canonical ORFL147C protein was found to interact with elements of the mRNA splicing machinery, and a mutational study suggested its importance in viral replication. The interactome data will be important for future studies of herpesvirus infection.

Ubiquitination and SUMOylation in HIV Infection: Friends and Foes

As intracellular parasites, viruses hijack the cellular machinery to facilitate their replication and spread. This includes favouring the expression of their viral genes over host genes, appropriation of cellular molecules, and manipulation of signalling pathways, including the post-translational machinery. HIV, the causative agent of AIDS, is notorious for using post-translational modifications to generate infectious particles. Here, we discuss the mechanisms by which HIV usurps the ubiquitin and SUMO pathways to modify both viral and host factors to achieve a productive infection, and also how the host innate sensing system uses these post-translational modifications to hinder HIV replication.

APOBEC3G is a restriction factor of EV71 and mediator of IMB-Z antiviral activity

Enterovirus 71 (EV71), a single-stranded positive-sense RNA virus, is the causative agent of hand, foot, and mouth disease (HFMD), for which no effective antiviral therapy is currently available. Apolipoprotein B messenger RNA-editing enzyme catalytic polypeptide-like 3G (APOBEC3G or A3G) is a cytidine deaminase that inhibits the replication of several viruses, such as human immunodeficiency virus-1, hepatitis B virus and hepatitis C virus. In our efforts toward understanding the antiviral spectrum and mechanism of A3G, we found that ectopic expression of A3G inhibited EV71 replication, whereas knockdown of endogenous A3G expression promoted EV71 replication. Moreover, inhibition of EV71 replication by IMB-Z, a N-phenylbenzamide derivative, is associated with increased levels of intracellular A3G, but reducing the level of A3G by RNA interference diminished the antiviral activity of IMB-Z. Mechanistically, we obtained evidence suggesting that the cytidine deaminase activity is not required for A3G inhibition of EV71 replication. Instead, we demonstrated that A3G can interact with viral 3D RNA-dependent RNA polymerase (RdRp) and viral RNA and be packaged into progeny virions to reduce its infectivity. Taken together, our results indicate that A3G is a cellular restriction factor of EV71 and mediator of the antiviral activity of IMB-Z. Pharmacological induction and/or stabilization of A3G is a potential therapeutic approach to treat diseases caused by EV71 infection, such as HFMD.

Restriction Factors: From Intrinsic Viral Restriction to Shaping Cellular Immunity Against HIV-1

Antiviral restriction factors are host cellular proteins that constitute a first line of defense blocking viral replication and propagation. In addition to interfering at critical steps of the viral replication cycle, some restriction factors also act as innate sensors triggering innate responses against infections. Accumulating evidence suggests an additional role for restriction factors in promoting antiviral cellular immunity to combat viruses. Here, we review the recent progress in our understanding on how restriction factors, particularly APOBEC3G, SAMHD1, Tetherin, and TRIM5α have the cell-autonomous potential to induce cellular resistance against HIV-1 while promoting antiviral innate and adaptive immune responses. Also, we provide an overview of how these restriction factors may connect with protein degradation pathways to modulate anti-HIV-1 cellular immune responses, and we summarize the potential of restriction factors-based therapeutics. This review brings a global perspective on the influence of restrictions factors in intrinsic, innate, and also adaptive antiviral immunity opening up novel research avenues for therapeutic strategies in the fields of drug discovery, gene therapy, and vaccines to control viral infections.

Identification of small molecule compounds targeting the interaction of HIV-1 Vif and human APOBEC3G by virtual screening and biological evaluation

Human APOBEC3G (hA3G) is a restriction factor that inhibits human immunodeficiency 1 virus (HIV-1) replication. The virally encoded protein Vif binds to hA3G and induces its degradation, thereby counteracting the antiviral activity of hA3G. Vif-mediated hA3G degradation clearly represents a potential target for anti-HIV drug development. Herein, we have performed virtual screening to discover small molecule inhibitors that target the binding interface of the Vif/hA3G complex. Subsequent biochemical studies have led to the identification of a small molecule inhibitor, IMB-301 that binds to hA3G, interrupts the hA3G-Vif interaction and inhibits Vif-mediated degradation of hA3G. As a result, IMB-301 strongly inhibits HIV-1 replication in a hA3G-dependent manner. Our study further demonstrates the feasibility of inhibiting HIV replication by abrogating the Vif-hA3G interaction with small molecules.

A New Class of Antiretroviral Enabling Innate Immunity by Protecting APOBEC3 from HIV Vif-Dependent Degradation

The infectivity of HIV depends on overcoming APOBEC3 (A3) innate immunity, predominantly through the expression of the viral protein Vif, which induces A3 degradation in the proteasome. Disruption of the functional interactions of Vif enables A3 mutagenesis of the HIV genome during viral replication, which can result in a broadly neutralizing antiviral effect. Vif function requires self-association along with interactions with A3 proteins, protein chaperones, and factors of the ubiquitination machinery and these are described here as a potential platform for novel antiviral drug discovery. This Review will examine the current state of development of Vif inhibitors that we believe to have therapeutic and functional cure potential.

Recent updates for designing CCR5 antagonists as anti-retroviral agents

The healthcare system faces various challenges in human immunodeficiency virus (HIV) therapy due to resistance to Anti-Retroviral Therapy (ART) as a consequence of the evolutionary process. Despite the success of antiretroviral drugs like Zidovudine, Zalcitabine, Raltegravir WHO ranks HIV as one of the deadliest diseases with a mortality of one million lives in 2016. Thus, there emerges an urgency of developing a novel anti-retroviral agent that combat resistant HIV strains. The clinical development of ART from a single drug regimen to current triple drug combination is very slow. The progression in the structural biology of the viral envelope prompted the discovery of novel targets, which can be demonstrated a proficient approach for drug design of anti-retroviral agents. The current review enlightens the recent updates in the structural biology of the viral envelope and focuses on CCR5 as a validated target as well as ways to overcome CCR5 resistance. The article also throws light on the SAR studies and most prevalent mutations in the receptor for designing CCR5 antagonists that can combat HIV-1 infection. To conclude, the paper lists diversified scaffolds that are in pipeline by various pharmaceutical companies that could provide an aid for developing novel CCR5 antagonists.

Retroviral restriction: nature's own solution

PURPOSE OF REVIEW

The present review will discuss recent advances in the development of anti-HIV therapies inspired by studies of the mechanisms of host restriction factor-mediated resistance to HIV infection.

RECENT FINDINGS

Manipulating the interplay between host cell restriction factors and viral accessory factors that overcome them can potentially be therapeutically useful. Preliminarily successful therapies - some of which are entering clinical trials - either inhibit the ability of virus to evade restriction factor-mediated immunity, or promote intracellular levels of restriction factors. These aims are achieved by multiple means, which are discussed.

SUMMARY

Many restriction factors appear to provide potentially useful targets for anti-HIV therapies, so time and interest should be invested in investigating ways to successfully therapeutically manipulate restriction factor-mediated immunity.

APOBEC Enzymes as Targets for Virus and Cancer Therapy

Human DNA cytosine-to-uracil deaminases catalyze mutations in both pathogen and cellular genomes. APOBEC3D, APOBEC3F, APOBEC3G, and APOBEC3H restrict human immunodeficiency virus 1 (HIV-1) infection in cells deficient in the viral infectivity factor (Vif), and have the potential to catalyze sublethal levels of mutation in viral genomes in Vif-proficient cells. At least two APOBEC3 enzymes, and in particular APOBEC3B, are sources of somatic mutagenesis in cancer cells that drive tumor evolution and may manifest clinically as recurrence, metastasis, and/or therapy resistance. Consequently, APOBEC3 enzymes are tantalizing targets for developing chemical probes and therapeutic molecules to harness mutational processes in human disease. This review highlights recent efforts to chemically manipulate APOBEC3 activities.

Anti-HIV Activities and Mechanism of 12-O-Tricosanoylphorbol-20-acetate, a Novel Phorbol Ester from Ostodes katharinae

APOBEC3G is a member of the human cytidine deaminase family that restricts Vif-deficient viruses by being packaged with progeny virions and inducing the G to A mutation during the synthesis of HIV-1 viral DNA when the progeny virus infects new cells. HIV-1 Vif protein resists the activity of A3G by mediating A3G degradation. Phorbol esters are plant-derived organic compounds belonging to the tigliane family of diterpenes and could activate the PKC pathway. In this study, we identified an inhibitor 12-O-tricosanoylphorbol-20-acetate (hop-8), a novel ester of phorbol which was isolated from Ostodes katharinae of the family Euphorbiaceae, that inhibited the replication of wild-type HIV-1 and HIV-2 strains and drug-resistant strains broadly both in C8166 cells and PBMCs with low cytotoxicity and the EC₅₀ values ranged from 0.106 μM to 7.987 μM. One of the main mechanisms of hop-8 is to stimulate A3G expressing in HIV-1 producing cells and upregulate the A3G level in progeny virions, which results in reducing the infectivity of the progeny virus. This novel mechanism of hop-8 inhibition of HIV replication might represents a promising approach for developing new therapeutics for HIV infection.

Identification and characterization of loop7 motif and its role in regulating biological function of human APOBEC3G through molecular modeling and biological assay

Human APOBEC3G (hA3G) is a cytidine deaminase which inhibits HIV-1 replication. The HIV-1 accessory protein viral infectivity factor (Vif) counteracts with hA3G by targeting it for proteasomal degradation. In this work, we constructed and optimized molecular models of the hA3G dimer and the hA3G–Vif complex. The molecular modeling study revealed that the loop7 motif of hA3G appears on the interfaces of both the hA3G–Vif complex and the hA3G dimer. Biochemical analysis provided evidence suggesting that binding of Vif to hA3G results in steric blocking of hA3G dimerization, implying that monomeric hA3G serves as a substrate for Vif-mediated degradation. Furthermore, we presented evidence for the important roles of the loop7 motif, especially the central residues within the region, in hA3G dimerization, hA3G--Vif interaction, Vif-mediated hA3G degradation as well as subcellular localization of hA3G. This work highlights a multiple-task interface formed by loop7 motif, which regulates biological function of hA3G, thus providing the feasibility of the strategy of blocking Vif-mediated A3G degradation by targeting the putative site around loop7.

Conserved Interaction of Lentiviral Vif Molecules with HIV-1 Gag and Differential Effects of Species-Specific Vif on Virus Production

The virion infectivity factor (Vif) open reading frame is conserved among most lentiviruses. Vif molecules contribute to viral replication by inactivating host antiviral factors, the APOBEC3 cytidine deaminases. However, various species of lentiviral Vif proteins have evolved different strategies for overcoming host APOBEC3. Whether different species of lentiviral Vif proteins still preserve certain common features has not been reported. Here, we show for the first time that diverse lentiviral Vif molecules maintain the ability to interact with the human immunodeficiency virus type 1 (HIV-1) Gag precursor (Pr55^Gag) polyprotein. Surprisingly, bovine immunodeficiency virus (BIV) Vif, but not HIV-1 Vif, interfered with HIV-1 production and viral infectivity even in the absence of APOBEC3. Further analysis revealed that BIV Vif demonstrated an enhanced interaction with Pr55^Gag compared to that of HIV-1 Vif, and BIV Vif defective for the Pr55^Gag interaction lost its ability to inhibit HIV-1. The C-terminal region of capsid (CA) and the p2 region of Pr55^Gag, which are important for virus assembly and maturation, were involved in the interaction. Transduction of CD4⁺ T cells with BIV Vif blocked HIV-1 replication. Thus, the conserved Vif-Pr55^Gag interaction provides a potential target for the future development of antiviral strategies.IMPORTANCE The conserved Vif accessory proteins of primate lentiviruses HIV-1, simian immunodeficiency virus (SIV), and BIV all form ubiquitin ligase complexes to target host antiviral APOBEC3 proteins for degradation, with different cellular requirements and using different molecular mechanisms. Here, we demonstrate that BIV Vif can interfere with HIV-1 Gag maturation and suppress HIV-1 replication through interaction with the precursor of the Gag (Pr55^Gag) of HIV-1 in virus-producing cells. Moreover, the HIV-1 and SIV Vif proteins are conserved in terms of their interactions with HIV-1 Pr55^Gag although HIV-1 Vif proteins bind Pr55^Gag less efficiently than those of BIV Vif. Our research not only sheds new light on this feature of these conserved lentiviral Vif proteins but also provides a formerly unrecognized target for the development of antiviral strategies. Since increasing the Vif-Pr55^Gag interaction could potentially suppress virus proliferation, this approach could offer a new strategy for the development of HIV inhibitors.

An analog of camptothecin inactive against Topoisomerase I is broadly neutralizing of HIV-1 through inhibition of Vif-dependent APOBEC3G degradation

Camptothecin (CPT) is a natural product discovered to be active against various cancers through its ability to inhibit Topoisomerase I (TOP1). CPT analogs also have anti-HIV-1 (HIV) activity that was previously shown to be independent of TOP1 inhibition. We show that a cancer inactive CPT analog (O2-16) inhibits HIV infection by disrupting multimerization of the HIV protein Vif. Antiviral activity depended on the expression of the cellular viral restriction factor APOBEC3G (A3G) that, in the absence of functional Vif, has the ability to hypermutate HIV proviral DNA during reverse transcription. Our studies demonstrate that O2-16 has low cytotoxicity and inhibits Vif-dependent A3G degradation, enabling A3G packaging into HIV viral particles that results in A3G signature hypermutations in viral genomes. This antiviral activity was A3G-dependent and broadly neutralizing against sixteen HIV clinical isolates from groups M (subtypes A-G), N, and O as well as seven single and multi-drug resistant strains of HIV. Molecular modeling predicted binding near the PPLP motif crucial for Vif multimerization and activity. O2-16 also was active in blocking Vif degradation of APOBEC3F (A3F). We propose that CPT analogs not active against TOP1 have novel therapeutic potential as Vif antagonists that enable A3G-dependent hypermutation of HIV.

Models of Viral Population Dynamics

Models of viral population dynamics have contributed enormously to our understanding of the pathogenesis and transmission of several infectious diseases, the coevolutionary dynamics of viruses and their hosts, the mechanisms of action of drugs, and the effectiveness of interventions. In this chapter, we review major advances in the modeling of the population dynamics of the human immunodeficiency virus (HIV) and briefly discuss adaptations to other viruses.

SupT1 Cell Infusion as a Possible Cell-Based Therapy for HIV: Results from a Pilot Study in Hu-PBMC BRGS Mice

In a previous in vitro study, the SupT1 cell line was explored as a decoy target for HIV-1, proposing SupT1 cell infusion as a possible cell-based therapy for HIV. In the present work, the previous in vitro model was translated into an in vivo setting. Specifically, Hu-PBMC BRGS mice were infected with a high input of HIV-1 LAI (100,000 TCID₅₀), and 40 million 30 Gy-irradiated SupT1 cells were infused weekly for 4 weeks as a therapy. Blood samples were taken to monitor CD4+ T cell count and viral load, and mice were monitored daily for signs of illness. At the earliest time point analyzed (Week 1), there was a significantly lower plasma viral load (~10-fold) in all animals treated with SupT1 cell infusion, associated with a higher CD4+ T cell count. At later time points, infection proceeded with robust viral replication and evident CD4+ T cell depletion, except in one mouse that showed complete suppression of viral replication and preservation of CD4+ T cell count. No morbidity or mortality was associated with SupT1 cell infusion. The interesting tendencies observed in the generated data suggest that this approach should be further investigated as a possible cell-based HIV therapy.

A small molecule compound IMB-LA inhibits HIV-1 infection by preventing viral Vpu from antagonizing the host restriction factor BST-2

Human BST-2 inhibits HIV-1 replication by tethering nascent virions to the cell surface. HIV-1 codes Vpu that counteracts BST-2 by down-regulating this restriction factor from the cell surface. This important function makes Vpu a potential therapeutic target. Yet, no agents have been reported to block Vpu from antagonizing BST-2. In this study, we report a small molecule compound IMB-LA that abrogates the function of Vpu and thereby strongly suppresses HIV-1 replication by sensitizing the virus to BST-2 restriction. Further studies revealed that IMB-LA specifically inhibits Vpu-mediated degradation of BST-2 and restores the expression of BST-2 at the cell surface. Although IMB-LA does not prevent Vpu from interacting with BST-2 or β-TrCP2-containing ubiquitin E3 ligase, sorting of BST-2 into lysosomes in Vpu-expressing cells is blocked by IMB-LA. Most importantly, HIV-1 release and infection is inhibited by IMB-LA only in BST-2-expressing cells. In summary, results herein demonstrated that IMB-LA could specifically inhibit the degradation of BST-2 induced by Vpu, and impair HIV-1 replication in a BST-2 dependent manner, suggesting the feasibility of utilizing small molecule compounds to disable the antagonist function of Vpu and thereby expose HIV-1 to the restriction by BST-2.

RNA binding to APOBEC3G induces the disassembly of functional deaminase complexes by displacing single-stranded DNA substrates

APOBEC3G (A3G) DNA deaminase activity requires a holoenzyme complex whose assembly on nascent viral reverse transcripts initiates with A3G dimers binding to ssDNA followed by formation of higher-order A3G homo oligomers. Catalytic activity is inhibited when A3G binds to RNA. Our prior studies suggested that RNA inhibited A3G binding to ssDNA. In this report, near equilibrium binding and gel shift analyses showed that A3G assembly and disassembly on ssDNA was an ordered process involving A3G dimers and multimers thereof. Although, fluorescence anisotropy showed that A3G had similar nanomolar affinity for RNA and ssDNA, RNA stochastically dissociated A3G dimers and higher-order oligomers from ssDNA, suggesting a different modality for RNA binding. Mass spectrometry mapping of A3G peptides cross-linked to nucleic acid suggested ssDNA only bound to three peptides, amino acids (aa) 181-194 in the N-terminus and aa 314-320 and 345-374 in the C-terminus that were part of a continuous exposed surface. RNA bound to these peptides and uniquely associated with three additional peptides in the N- terminus, aa 15-29, 41-52 and 83-99, that formed a continuous surface area adjacent to the ssDNA binding surface. The data predict a mechanistic model of RNA inhibition of ssDNA binding to A3G in which competitive and allosteric interactions determine RNA-bound versus ssDNA-bound conformational states.

Identification of an HIV-1 replication inhibitor which rescues host restriction factor APOBEC3G in Vif-APOBEC3G complex

HIV-1 Vif protein is one of the most crucial accessory proteins for viral replication. It efficiently counteracts the important host restriction factor APOBEC3G (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G, A3G) which is lethal to HIV-1 by causing G to A mutation of viral genome. Vif protein mediates degradation of APOBEC3G via the complicated protein-protein interactions of Vif, APOBEC3G, Elongin C/B and Cullin 5. The importance of Vif-APOBEC3G complex makes it a good potential target to develop new therapeutics of HIV-1. We identified a potent HIV-1 replication inhibitor (ZBMA-1, IC50 = 1.01 μM) that efficiently protected APOBEC3G protein by targeting Vif-APOBEC3G complex. The co-immunoprecipitation and docking studies indicated that compound ZBMA-1 affected the binding of Elongin C with Vif protein.

Redoxal, an inhibitor of de novo pyrimidine biosynthesis, augments APOBEC3G antiviral activity against human immunodeficiency virus type 1

APOBEC3G (A3G) is a cytidine deaminase that restricts HIV-1 replication by inducing G-to-A hypermutation in viral DNA; deamination-independent mechanisms are also implicated. HIV-1 Vif protein counteracts A3G by inducing its proteasomal degradation. Thus, the Vif-A3G axis is a potential therapeutic target. To identify compounds that inhibit Vif:A3G interaction, a 307,520 compound library was tested in a TR-FRET screen. Two identified compounds, redoxal and lomofungin, inhibited HIV-1 replication in peripheral blood mononuclear cells. Lomofungin activity was linked to A3G, but not pursued further due to cytotoxicity. Redoxal displayed A3G-dependent restriction, inhibiting viral replication by stabilizing A3G protein levels and increasing A3G in virions. A3G-independent activity was also detected. Treatment with uridine or orotate, intermediates of pyrimidine synthesis, diminished redoxal-induced stabilization of A3G and antiviral activity. These results identify redoxal as an inhibitor of HIV-1 replication and suggest its ability to inhibit pyrimidine biosynthesis suppresses viral replication by augmenting A3G antiviral activity.

ASK1 restores the antiviral activity of APOBEC3G by disrupting HIV-1 Vif-mediated counteraction

APOBEC3G (A3G) is an innate antiviral restriction factor that strongly inhibits the replication of human immunodeficiency virus type 1 (HIV-1). An HIV-1 accessory protein, Vif, hijacks the host ubiquitin–proteasome system to execute A3G degradation. Identification of the host pathways that obstruct the action of Vif could provide a new strategy for blocking viral replication. We demonstrate here that the host protein ASK1 (apoptosis signal-regulating kinase 1) interferes with the counteraction by Vif and revitalizes A3G-mediated viral restriction. ASK1 binds the BC-box of Vif, thereby disrupting the assembly of the Vif–ubiquitin ligase complex. Consequently, ASK1 stabilizes A3G and promotes its incorporation into viral particles, ultimately reducing viral infectivity. Furthermore, treatment with the antiretroviral drug AZT (zidovudine) induces ASK1 expression and restores the antiviral activity of A3G in HIV-1-infected cells. This study thus demonstrates a distinct function of ASK1 in restoring the host antiviral system that can be enhanced by AZT treatment.

The human protein APOBEC3G (A3G) inhibits HIV-1 replication, but the viral protein Vif counteracts by inducing A3G degradation. Here Miyakawa et al. show that the antiretroviral drug AZT restores A3G function in vitro by stimulating expression of a host protein, ASK1, which interferes with the action of Vif.

Host APOBEC3G protein inhibits HCV replication through direct binding at NS3

Human APOBEC3G (hA3G) is a cytidine deaminase that restricts replication of certain viruses. We have previously reported that hA3G was a host restriction factor against hepatitis C virus (HCV) replication, and hA3G stabilizers showed a significant inhibitory activity against HCV. However, the molecular mechanism of hA3G against HCV remains unknown. We show in this study that hA3G’s C-terminal directly binds HCV non-structural protein NS3 at its C-terminus, which is responsible for NS3’s helicase and NTPase activity. Binding of hA3G to the C-terminus of NS3 reduced helicase activity, and therefore inhibited HCV replication. The anti-HCV mechanism of hA3G appeared to be independent of its deamination activity. Although early stage HCV infection resulted in an increase in host hA3G as an intracellular response against HCV replication, hA3G was gradually diminished after a long-term incubation, suggesting an unknown mechanism(s) that protects HCV NS3 from inactivation by hA3G. The process represents, at least partially, a cellular defensive mechanism against HCV and the action is mediated through a direct interaction between host hA3G and HCV NS3. We believe that understanding of the antiviral mechanism of hA3G against HCV might open an interesting avenue to explore hA3G stabilizers as a new class of anti-HCV agents.

Identification of a novel HIV-1 inhibitor targeting Vif-dependent degradation of human APOBEC3G protein

APOBEC3G (A3G) is a cellular cytidine deaminase that restricts HIV-1 replication by inducing G-to-A hypermutation in viral DNA and by deamination-independent mechanisms. HIV-1 Vif binds to A3G, resulting in its degradation via the 26 S proteasome. Therefore, this interaction represents a potential therapeutic target. To identify compounds that inhibit interaction between A3G and HIV-1 Vif in a high throughput format, we developed a homogeneous time-resolved fluorescence resonance energy transfer assay. A 307,520 compound library from the NIH Molecular Libraries Small Molecule Repository was screened. Secondary screens to evaluate dose-response performance and off-target effects, cell-based assays to identify compounds that attenuate Vif-dependent degradation of A3G, and assays testing antiviral activity in peripheral blood mononuclear cells and T cells were employed. One compound, N.41, showed potent antiviral activity in A3G(+) but not in A3G(-) T cells and had an IC50 as low as 8.4 μM and a TC50 of >100 μM when tested against HIV-1Ba-L replication in peripheral blood mononuclear cells. N.41 inhibited the Vif-A3G interaction and increased cellular A3G levels and incorporation of A3G into virions, thereby attenuating virus infectivity in a Vif-dependent manner. N.41 activity was also species- and Vif-dependent. Preliminary structure-activity relationship studies suggest that a hydroxyl moiety located at a phenylamino group is critical for N.41 anti-HIV activity and identified N.41 analogs with better potency (IC50 as low as 4.2 μM). These findings identify a new lead compound that attenuates HIV replication by liberating A3G from Vif regulation and increasing its innate antiviral activity.

Factores solubles con actividad antiviral: en búsqueda de nuevos blancos terapéuticos para la infección por el VIH-1

Los mecanismos innatos antivirales han resultado de gran interés debido a su uso potencial para la prevención y tratamiento de la infección por el VIH. En particular, los factores solubles antivirales han sido objeto de múltiples investigaciones por su capacidad de inhibir diferentes pasos del ciclo replicativo viral y de potenciar la respuesta inmune del hospedero. Entre estos factores solubles se destacan TRIM-5α, APOBEC3G, SAMHD1, ELAFIN, SERPINA1 y SLPI, que actúan directamente sobre la partícula viral o la célula, o promueven la producción de moléculas involucradas en la respuesta inmune contra el virus. Algunos de ellos se han correlacionado con un bajo riesgo de adquirir la infección por el VIH o con una lenta progresión a sida. La exploración de los mecanismos antivirales de estas proteínas es requisito para el desarrollo de nuevas alternativas terapéuticas.

Cullin E3 ligases and their rewiring by viral factors

The ability of viruses to subvert host pathways is central in disease pathogenesis. Over the past decade, a critical role for the Ubiquitin Proteasome System (UPS) in counteracting host immune factors during viral infection has emerged. This counteraction is commonly achieved by the expression of viral proteins capable of sequestering host ubiquitin E3 ligases and their regulators. In particular, many viruses hijack members of the Cullin-RING E3 Ligase (CRL) family. Viruses interact in many ways with CRLs in order to impact their ligase activity; one key recurring interaction involves re-directing CRL complexes to degrade host targets that are otherwise not degraded within host cells. Removal of host immune factors by this mechanism creates a more amenable cellular environment for viral propagation. To date, a small number of target host factors have been identified, many of which are degraded via a CRL-proteasome pathway. Substantial effort within the field is ongoing to uncover the identities of further host proteins targeted in this fashion and the underlying mechanisms driving their turnover by the UPS. Elucidation of these targets and mechanisms will provide appealing anti-viral therapeutic opportunities. This review is focused on the many methods used by viruses to perturb host CRLs, focusing on substrate sequestration and viral regulation of E3 activity.

Suppression of APOBEC3-mediated restriction of HIV-1 by Vif

The APOBEC3 restriction factors are a family of deoxycytidine deaminases that are able to suppress replication of viruses with a single-stranded DNA intermediate by inducing mutagenesis and functional inactivation of the virus. Of the seven human APOBEC3 enzymes, only APOBEC3-D, -F, -G, and -H appear relevant to restriction of HIV-1 in CD4+ T cells and will be the focus of this review. The restriction of HIV-1 occurs most potently in the absence of HIV-1 Vif that induces polyubiquitination and degradation of APOBEC3 enzymes through the proteasome pathway. To restrict HIV-1, APOBEC3 enzymes must be encapsidated into budding virions. Upon infection of the target cell during reverse transcription of the HIV-1 RNA into (-)DNA, APOBEC3 enzymes deaminate cytosines to form uracils in single-stranded (-)DNA regions. Upon replication of the (-)DNA to (+)DNA, the HIV-1 reverse transcriptase incorporates adenines opposite to the uracils thereby inducing C/G to T/A mutations that can functionally inactivate HIV-1. APOBEC3G is the most studied APOBEC3 enzyme and it is known that Vif attempts to thwart APOBEC3 function not only by inducing its proteasomal degradation but also by several degradation-independent mechanisms, such as inhibiting APOBEC3G virion encapsidation, mRNA translation, and for those APOBEC3G molecules that still become virion encapsidated, Vif can inhibit APOBEC3G mutagenic activity. Although most Vif variants can induce efficient degradation of APOBEC3-D, -F, and -G, there appears to be differential sensitivity to Vif-mediated degradation for APOBEC3H. This review examines APOBEC3-mediated HIV restriction mechanisms, how Vif acts as a substrate receptor for a Cullin5 ubiquitin ligase complex to induce degradation of APOBEC3s, and the determinants and functional consequences of the APOBEC3 and Vif interaction from a biological and biochemical perspective.

Structure-guided analysis of the human APOBEC3-HIV restrictome

Human APOBEC3 (A3) proteins are host-encoded intrinsic restriction factors that inhibit the replication of many retroviral pathogens. Restriction is believed to occur as a result of the DNA cytosine deaminase activity of the A3 proteins; this activity converts cytosines into uracils in single-stranded DNA retroviral replication intermediates. A3 proteins are also equipped with deamination-independent means to restrict retroviruses that work cooperatively with deamination-dependent restriction pathways. A3 proteins substantially bolster the intrinsic immune system by providing a powerful block to the transmission of retroviral pathogens; however, most retroviruses are able to subvert this replicative restriction in their natural host. HIV-1, for instance, evades A3 proteins through the activity of its accessory protein Vif. Here, we summarize data from recent A3 structural and functional studies to provide perspectives into the interactions between cellular A3 proteins and HIV-1 macromolecules throughout the viral replication cycle.