Mannose-Binding Quinone Glycoside, MBQ: Potential Utility and Action Mechanism

A Pradimicin-Based Staining Dye for Glycoprotein Detection

Pradimicin A (PRM-A) and related compounds constitute an exceptional family of natural pigments that show Ca²⁺-dependent recognition of d-mannose (Man). Although these compounds hold great promise as research tools in glycobiology, their practical application has been severely limited by their inherent tendency to form water-insoluble aggregates. Here, we demonstrate that the 2-hydroxyethylamide derivative (PRM-EA) of PRM-A shows little aggregation in neutral aqueous media and retains binding specificity for Man. We also show that PRM-EA stains glycoproteins in dot blot assays, whereas PRM-A fails to do so, owing to severe aggregation. Significantly, PRM-EA is sensitive to glycoproteins carrying high mannose-type and hybrid-type N-linked glycans, but not to those carrying complex-type N-linked glycans. Such staining selectivity has never been observed in conventional dyes, suggesting that PRM-EA could serve as a unique staining agent for the selective detection of glycoproteins with terminal Man residues.

Paving the Way for Practical Use of Sugar-Binding Natural Products as Lectin Mimics in Glycobiological Research

Pradimicins (PRMs) constitute an exceptional class of natural products that show Ca²⁺ -dependent recognition of d-mannose (Man). In addition to therapeutic uses as antifungal drugs, the application of PRMs as lectin mimics for glycobiological research has been attracting considerable interest, since the emerging biological roles of Man-containing glycans have been highlighted. However, only a few attempts have been made to use PRMs for glycobiological purposes. The limited use of PRMs is primarily due to the early assumption that the readily modifiable carboxyl group of PRMs is involved in Ca²⁺ binding, and thus, not available to prepare research tools. Recently, this assumption has been disproved by structural elucidation of the Ca²⁺ complex of PRMs, which paves the way for designing carboxyl group modified derivatives of PRMs for research use. This article outlines studies related to Ca²⁺ -mediated Man binding of PRMs and discusses their application for glycobiology.

Molecular Basis of Mannose Recognition by Pradimicins and their Application to Microbial Cell Surface Imaging

Naturally occurring pradimicins (PRMs) show specific recognition of d-mannose (d-Man) in aqueous media, which has never been achieved by artificial small molecules. Although the Ca²⁺-mediated dimerization of PRMs is essential for their d-Man binding, the dimeric structure has yet to be elucidated, leaving the question open as to how PRMs recognize d-Man. Thus, we herein report the structural elucidation of the dimer by a combination of X-ray crystallography and solid-state NMR spectroscopy. Coupled with our previous knowledge regarding the d-Man binding geometry of PRMs, elucidation of the dimer allowed reliable estimation of the mode of d-Man binding. Based on the binding model, we further developed an azide-functionalized PRM derivative (PRM-Azide) with d-Man binding specificity. Notably, PRM-Azide stained Candida rugosa cells having mannans on their cell surface through conjugation with the tetramethylrhodamine fluorophore. The present study provides the practical demonstration that PRMs can serve as lectin mimics for use in glycobiological studies.

Pradimicin A, a D-mannose-binding antibiotic, binds pyranosides of L-fucose and L-galactose in a calcium-sensitive manner

Pradimicin A (PRM-A) is a unique antibiotic with a lectin-like ability to bind D-mannose (D-Man) in the presence of Ca(2+) ion. Although accumulated evidences suggest that PRM-A recognizes the 2-, 3-, and 4-hydroxyl groups of D-Man, BMY-28864, an artificial PRM-A derivative, was shown not to bind L-fucose (L-Fuc) and L-galactose (lLGal), both of which share the characteristic array of the three hydroxyl groups with D-Man. To obtain a plausible explanation for this inconsistency, we performed co-precipitation experiments of PRM-A with L-Fuc, L-Gal, and their methyl pyranosides (L-Fuc-OMe, L-Gal-OMe) by taking advantage of aggregate-forming propensity of the binary [PRM-A/Ca(2+)] complex. While L-Fuc and L-Gal were hardly incorporated into the aggregate, L-Fuc-OMe and L-Gal-OMe were found to exhibit significant binding to PRM-A. However, increased Ca(2+) concentration abolished this binding, raising the possibility that poor binding of L-Fuc and L-Gal to PRM-A is attributed to their chelation with Ca(2+) ion. This possibility was partly supported by (1)H NMR analysis that detected interaction of L-Fuc and L-Gal with Ca(2+) ion in aqueous solution. These results collectively indicate that PRM-A binds pyranosides of L-Fuc and L-Gal when Ca(2+) concentration is not excessive to trap these sugars by chelation but sufficient to form the [PRM-A/Ca(2+)] complex.

Design of anticancer prodrugs for reductive activation

Anticancer prodrugs designed to target specifically tumor cells should increase therapeutic effectiveness and decrease systemic side effects in the treatment of cancer. Over the last 20 years, significant advances have been made in the development of anticancer prodrugs through the incorporation of triggers for reductive activation. Reductively activated prodrugs have been designed to target hypoxic tumor tissues, which are known to overexpress several endogenous reductive enzymes. In addition, exogenous reductive enzymes can be delivered to tumor cells through fusion with tumor-specific antibodies or overexpressed in tumor cells through gene delivery approaches. Many anticancer prodrugs have been designed to use both the endogenous and exogenous reductive enzymes for target-specific activation and these prodrugs often contain functional groups such as quinones, nitroaromatics, N-oxides, and metal complexes. Although no new agents have been approved for clinical use, several reductively activated prodrugs are in various stages of clinical trial. This review mainly focuses on the medicinal chemistry aspects of various classes of reductively activated prodrugs including design principles, structure-activity relationships, and mechanisms of activation and release of active drug molecules.

The carbohydrate-binding plant lectins and the non-peptidic antibiotic pradimicin A target the glycans of the coronavirus envelope glycoproteins

Objectives

Many enveloped viruses carry carbohydrate-containing proteins on their surface. These glycoproteins are key to the infection process as they are mediators of the receptor binding and membrane fusion of the virion with the host cell. Therefore, they are attractive therapeutic targets for the development of novel antiviral therapies. Recently, carbohydrate-binding agents (CBA) were shown to possess antiviral activity towards coronaviruses. The current study further elucidates the inhibitory mode of action of CBA.

Methods

Different strains of two coronaviruses, mouse hepatitis virus and feline infectious peritonitis virus, were exposed to CBA: the plant lectins Galanthus nivalis agglutinin, Hippeastrum hybrid agglutinin and Urtica dioica agglutinin (UDA) and the non-peptidic mannose-binding antibiotic pradimicin A.

Results and conclusions

Our results indicate that CBA target the two glycosylated envelope glycoproteins, the spike (S) and membrane (M) protein, of mouse hepatitis virus and feline infectious peritonitis virus. Furthermore, CBA did not inhibit virus–cell attachment, but rather affected virus entry at a post-binding stage. The sensitivity of coronaviruses towards CBA was shown to be dependent on the processing of the N-linked carbohydrates. Inhibition of mannosidases in host cells rendered the progeny viruses more sensitive to the mannose-binding agents and even to the N-acetylglucosamine-binding UDA. In addition, inhibition of coronaviruses was shown to be dependent on the cell-type used to grow the virus stocks. All together, these results show that CBA exhibit promising capabilities to inhibit coronavirus infections.

Antiviral activity of carbohydrate-binding agents against Nidovirales in cell culture

Coronaviruses are important human and animal pathogens, the relevance of which increased due to the emergence of new human coronaviruses like SARS-CoV, HKU1 and NL63. Together with toroviruses, arteriviruses, and roniviruses the coronaviruses belong to the order Nidovirales. So far antivirals are hardly available to combat infections with viruses of this order. Therefore, various antiviral strategies to counter nidoviral infections are under evaluation. Lectins, which bind to N-linked oligosaccharide elements of enveloped viruses, can be considered as a conceptionally new class of virus inhibitors. These agents were recently evaluated for their antiviral activity towards a variety of enveloped viruses and were shown in most cases to inhibit virus infection at low concentrations. However, limited knowledge is available for their efficacy towards nidoviruses. In this article the application of the plant lectins Hippeastrum hybrid agglutinin (HHA), Galanthus nivalis agglutinin (GNA), Cymbidium sp. agglutinin (CA) and Urtica dioica agglutinin (UDA) as well as non-plant derived pradimicin-A (PRM-A) and cyanovirin-N (CV-N) as potential antiviral agents was evaluated. Three antiviral tests were compared based on different evaluation principles: cell viability (MTT-based colorimetric assay), number of infected cells (immunoperoxidase assay) and amount of viral protein expression (luciferase-based assay). The presence of carbohydrate-binding agents strongly inhibited coronaviruses (transmissible gastroenteritis virus, infectious bronchitis virus, feline coronaviruses serotypes I and II, mouse hepatitis virus), arteriviruses (equine arteritis virus and porcine respiratory and reproductive syndrome virus) and torovirus (equine Berne virus). Remarkably, serotype II feline coronaviruses and arteriviruses were not inhibited by PRM-A, in contrast to the other viruses tested.

The alpha(1,2)-mannosidase I inhibitor 1-deoxymannojirimycin potentiates the antiviral activity of carbohydrate-binding agents against wild-type and mutant HIV-1 strains containing glycan deletions in gp120

Exposure of carbohydrate‐binding agents (CBAs) (i.e. the mannose‐specific plant lectins Hippeastrum hybrid agglutinin and Galanthus nivalis agglutinin) to HIV‐1 progressively select for mutant HIV‐1 strains that contain N‐glycan deletions in their envelope gp120. This results in resistance of the mutant virus strains to the CBAs. Exposure of such mutant virus strains to the α(1,2)‐mannosidase I inhibitor 1‐deoxymannojirimycin (DMJ) results in an enhanced suppression of mutant virus‐induced cytopathicity in CEM cell cultures. Moreover, when combined with CBAs at concentrations that showed poor if any suppression of mutant virus replication as single drugs, a synergistic antiviral activity of DMJ was observed. These observations argue for a combined exposure of CBAs and glycosylation inhibitors such as DMJ to HIV to afford a more pronounced suppression of virus replication, prior to, or during, CBA resistance development.

Carbohydrate-binding agents: a potential future cornerstone for the chemotherapy of enveloped viruses?

Carbohydrate-binding agents (CBAs) inhibit HIV-1 and it is proposed that therapy with such agents may have important implications for the future of anti-HIV therapy. Examples of CBAs include the procaryotic cyanovirin-N (CV-N), plant lectins such as HHA, GNA, NPA, CA and UDA, the monoclonal antibody 2G12 directed against a glycan-containing epitope on HIV envelope gp120, and the mannose-specific non-peptidic antibiotic Pradimicin A, which inhibits the entry of HIV-1 into its target cells. CBAs prevent not only virus infection of susceptible cells, but also inhibit syncytia formation between persistently HIV-infected cells and uninfected lymphocytes. In addition, CBAs may also prevent DC-SIGN-mediated transmission of HIV to T-lymphocytes. Therefore, CBAs qualify as potential microbicide drugs. Long-term exposure of HIV to CBAs in cell culture results in the progressive deletion of N-glycans of HIV gpl20 in an attempt of the virus to escape drug pressure. In this respect, the CBAs are endowed with a high genetic barrier. Multiple mutations at N-glycosylation sites are required before pronounced phenotypic drug resistance development becomes evident. CBA treatment of HIV may consist of a novel chemotherapeutic concept with a dual mechanism of antiviral action: a direct antiviral activity by preventing HIV entry and transmission to its target cells, and an indirect antiviral activity by forcing HIV to delete glycans in its gpl20 envelope. The latter phenomenon will result in creating 'holes' in the protective glycan shield of the HIV envelope, whereby the immune system may become triggered to produce neutralizing antibodies against previously hidden immunogenic epitopes of gp120. If this concept can be proven in in vivo, low-molecular-weight non-peptidic CBAs such as Pradimycin A may become the cornerstone for the efficient treatment of infections of those viruses that require a glycosylated envelope (that is, HIV, but also hepatitis C virus) for entry into its target cells. In addition, influenza virus and coronavirus infections may also qualify to be treated by CBAs.

Pradimicin A, a carbohydrate-binding nonpeptidic lead compound for treatment of infections with viruses with highly glycosylated envelopes, such as human immunodeficiency virus

Pradimicin A (PRM-A), an antifungal nonpeptidic benzonaphtacenequinone antibiotic, is a low-molecular-weight (molecular weight, 838) carbohydrate binding agent (CBA) endowed with a selective inhibitory activity against human immunodeficiency virus (HIV). It invariably inhibits representative virus strains of a variety of HIV-1 clades with X4 and R5 tropisms at nontoxic concentrations. Time-of-addition studies revealed that PRM-A acts as a true virus entry inhibitor. PRM-A specifically interacts with HIV-1 gp120 and efficiently prevents virus transmission in cocultures of HUT-78/HIV-1 and Sup T1 cells. Upon prolonged exposure of HIV-1-infected CEM cell cultures, PRM-A drug pressure selects for mutant HIV-1 strains containing N-glycosylation site deletions in gp120 but not gp41. A relatively long exposure time to PRM-A is required before drug-resistant virus strains emerge. PRM-A has a high genetic barrier, since more than five N-glycosylation site deletions in gp120 are required to afford moderate drug resistance. Such mutated virus strains keep full sensitivity to the other known clinically used anti-HIV drugs. PRM-A represents the first prototype compound of a nonpeptidic CBA lead and, together with peptide-based lectins, belongs to a conceptually novel type of potential therapeutics for which drug pressure results in the selection of glycan deletions in the HIV gp120 envelope.