Broad-spectrum inhibition of common respiratory RNA viruses by a pyrimidine synthesis inhibitor with involvement of the host antiviral response

Our previous screening of 50 240 structurally diverse compounds led to the identification of 39 influenza A virus infection inhibitors (Kao R.Y., Yang D., Lau L.S., Tsui W.H., Hu L. et al. Nat Biotechnol 2010;28:600-605). Further screening of these compounds against common respiratory viruses led to the discovery of compound FA-613. This inhibitor exhibited low micromolar antiviral activity against various influenza A and B virus strains, including the highly pathogenic influenza A strains H5N1 and H7N9, enterovirus A71, respiratory syncytial virus, human rhinovirus A, SARS- and MERS-coronavirus. No significant cellular toxicity was observed at the effective concentrations. Animal studies showed an improved survival rate in BALB/c mice that received intranasal FA-613 treatments against a lethal dose infection of A/HK/415742Md/2009 (H1N1). Further cell-based assays indicated that FA-613 interfer with the de novo pyrimidine biosynthesis pathway by targeting the dihydroorotate dehydrogenase. Surprisingly, FA-613 lost its antiviral potency in the interferon-deficient Vero cell line, while maintaining its inhibitory activity in an interferon-competent cell line which showed elevated expression of host antiviral genes when infected in the presence of FA-613. Further investigation of the specific connection between pyrimidine synthesis inhibition and the induction of host innate immunity might aid clinical development of this type of drug in antiviral therapies. Therefore, in acute cases of respiratory tract infections, when rapid diagnostics of the causative agent are not readily available, an antiviral drug with properties like FA-613 could prove to be very valuable.

Fluorescence assay of dihydroorotate dehydrogenase that may become a cancer biomarker

We developed an assay method for measuring dihydroorotate dehydrogenase (DHODH) activity in cultured HeLa cells and fibroblasts, and in stage III stomach cancer and adjacent normal tissues from the same patient. The assay comprised enzymatic reaction of DHODH with a large amount of dihydroorotic acid substrate, followed by fluorescence (FL) detection specific for orotic acid using the 4-trifluoromethyl-benzamidoxime fluorogenic reagent. The DHODH activities in the biologically complex samples were readily measured by the assay method. Our data indicate significantly higher DHODH activity in HeLa cells (340 ± 25.9 pmol/10⁵ cells/h) than in normal fibroblasts (54.1 ± 7.40 pmol/10⁵ cells/h), and in malignant tumour tissue (1.10 ± 0.19 nmol/mg total proteins/h) than in adjacent normal tissue (0.24 ± 0.11 nmol/mg total proteins/h). This is the first report that DHODH activity may be a diagnostic biomarker for cancer.

On dihydroorotate dehydrogenases and their inhibitors and uses

Proper nucleosides availability is crucial for the proliferation of living entities (eukaryotic cells, parasites, bacteria, and virus). Accordingly, the uses of inhibitors of the de novo nucleosides biosynthetic pathways have been investigated in the past. In the following we have focused on dihydroorotate dehydrogenase (DHODH), the fourth enzyme in the de novo pyrimidine nucleosides biosynthetic pathway. We first described the different types of enzyme in terms of sequence, structure, and biochemistry, including the reported bioassays. In a second part, the series of inhibitors of this enzyme along with a description of their potential or actual uses were reviewed. These inhibitors are indeed used in medicine to treat autoimmune diseases such as rheumatoid arthritis or multiple sclerosis (leflunomide and teriflunomide) and have been investigated in treatments of cancer, virus, and parasite infections (i.e., malaria) as well as in crop science.

Identification of broad-spectrum antiviral compounds and assessment of the druggability of their target for efficacy against respiratory syncytial virus (RSV)

The search for novel therapeutic interventions for viral disease is a challenging pursuit, hallmarked by the paucity of antiviral agents currently prescribed. Targeting of viral proteins has the inextricable challenge of rise of resistance. Safe and effective vaccines are not possible for many viral pathogens. New approaches are required to address the unmet medical need in this area. We undertook a cell-based high-throughput screen to identify leads for development of drugs to treat respiratory syncytial virus (RSV), a serious pediatric pathogen. We identified compounds that are potent (nanomolar) inhibitors of RSV in vitro in HEp-2 cells and in primary human bronchial epithelial cells and were shown to act postentry. Interestingly, two scaffolds exhibited broad-spectrum activity among multiple RNA viruses. Using the chemical matter as a probe, we identified the targets and identified a common cellular pathway: the de novo pyrimidine biosynthesis pathway. Both targets were validated in vitro and showed no significant cell cytotoxicity except for activity against proliferative B- and T-type lymphoid cells. Corollary to this finding was to understand the consequences of inhibition of the target to the host. An in vivo assessment for antiviral efficacy failed to demonstrate reduced viral load, but revealed microscopic changes and a trend toward reduced pyrimidine pools and findings in histopathology. We present here a discovery program that includes screen, target identification, validation, and druggability that can be broadly applied to identify and interrogate other host factors for antiviral effect starting from chemical matter of unknown target/mechanism of action.

Drosophila melanogaster dihydroorotate dehydrogenase: the N-terminus is important for biological function in vivo but not for catalytic properties in vitro

Dihydroorotate dehydrogenase (DHODH, EC 1.3.99.11), the fourth enzyme of pyrimidine de novo synthesis, is an integral flavoprotein of the inner mitchondrial membrane and is functionally connected to the respiratory chain. Here, experiments have been directed toward determining the roles of the N-terminal sequence motifs both in enzymatic properties of insect DHODH produced in vitro and the in vivo function of the protein. Full-length and three N-terminal truncated derivatives of the Drosophila melanogaster enzyme were expressed in Escherichia coli and purified. For identification on Western blots of recombinant DHODH as well as the native enzyme from flies polyclonal anti-DHODH immunoglobulins were generated and affinity-purified. The enzymatic characteristics of the four versions of DHODH were very similar, indicating that the N-terminus of the enzyme does not influence its catalytic function or its susceptibility to prominent DHODH inhibitors: A77-1726, brequinar, dichloroallyl-lawsone and redoxal. Whereas the efficacy of A77-1726 and dichloroallyl-lawsone were similar with Drosophila and human DHODH, that of brequinar and redoxal differed significantly. The differences in responses of insect DHODH and the enzyme from other species may allow the design of new agents that will selectively control insect growth, due to pyrimidine nucleotide limitation. In vivo expression of the full-length and N-truncated DHODHs from engineered transgenes revealed that the truncated proteins could not support normal de novo pyrimidine biosynthesis during development of the fly (i.e., failure to complement dhod-null mutations), apparently due to instability of the truncated proteins. It is concluded that the proper intracellular localization, directed by the N-terminal targeting and transmembrane motifs, is required for stability and subsequent proper biological function in vivo.

Redoxal as a new lead structure for dihydroorotate dehydrogenase inhibitors: a kinetic study of the inhibition mechanism

Mitochondrial dihydroorotate dehydrogenase (DHOdehase; EC 1.3.99.11) is a target of anti-proliferative, immunosuppressive and anti-parasitic agents. Here, redoxal, (2,2'-[3,3'-dimethoxy[1, 1'-biphenyl]-4,4'-diyl)diimino]bis-benzoic acid, was studied with isolated mitochondria and the purified recombinant human and rat enzyme to find out the mode of kinetic interaction with this target. Its pattern of enzyme inhibition was different from that of cinchoninic, isoxazol and naphthoquinone derivatives and was of a non-competitive type for the human (K(ic)=402 nM; K(iu)=506 nM) and the rat enzyme (K(ic)=116 nM; K(iu)=208 nM). The characteristic species-related inhibition of DHOdehase found with other compounds was less expressed with redoxal. In human and rat mitochondria, redoxal did not inhibit NADH-induced respiration, its effect on succinate-induced respiration was marginal. This was in contrast to the sound effect of atovaquone and dichloroallyl-lawsone, studied here for comparison. In human mitochondria, the IC(50) value for the inhibition of succinate-induced respiration by atovaquone was 6.1 microM and 27.4 microM for the DHO-induced respiration; for dichlorallyl-lawsone, the IC(50) values were 14.1 microM and 0.23 microM.

Expression, purification, and characterization of histidine-tagged rat and human flavoenzyme dihydroorotate dehydrogenase

Mitochondrially bound dihydroorotate-ubiquinone oxidoreductase (dihydroorotate dehydrogenase, EC 1.3.99.11) catalyzes the fourth sequential step in the de novo synthesis of uridine monophosphate. Based on the recent functional expression of the complete rat dihydroorotate dehydrogenase by means of the baculovirus expression vector system in Trichoplusia ni cells, a procedure is described that allows the purification of baculovirus expressed enzyme protein fused to a carboxy-terminal tag of eight histidines. Extracts from mitochondria of Spodoptera frugiperda cells infected with the recombinant virus using Triton X-100 were loaded onto Ni2+-nitrilotriacetic acid agarose and histidine-tagged rat protein was selectively eluted with imidazole-containing buffer. In view of our previously published work, the quality of the electrophoretic homogenous rat enzyme was markedly improved; specific activity was 130-150 micromol dihydroorotate/min per milligram; and the stoichiometry of flavin content was 0.8-1.1 mol/mol protein. Efforts to generate mammalian dihydroorotate dehydrogenases with low production costs from bacteria resulted in successful overexpression of the carboxy-terminal-modified rat and human dihydroorotate dehydrogenase in XL-1 Blue cells. By employing the metal chelate affinity chromatography under native conditions, the histidine-tagged human enzyme was purified with a specific activity of 150 micromol/min/mg and the rat enzyme with 83 micromol/min/mg, respectively, at pH 8.0-8.1 optimum. Kinetic constants of the recombinant histidine-tagged rat enzyme from bacteria (dihydroorotate, Km = 14.6 micromol electron acceptor decylubiquinone, Km = 9.5 micromol) were close to those reported for the enzyme from insect cells, with or without the affinity tag. HPLC analyses identified flavin mononucleotide as cofactor of the rat enzyme; UV-vis and fluorometric analyses verified a flavin/protein ratio of 0.8-1.1 mol/mol. By spectral analyses of the functional flavin with the native human enzyme, the interaction of the pharmacological inhibitors Leflunomide and Brequinar with their target could be clarified as interference with the transfer of electrons from the flavin to the quinone. The combination of the bacterial expression system and metal chelate affinity chomatography offers an improved means to purify large quantities of mammalian membrane-bound dihydroorotate dehydrogenases which, by several criteria, possesses the same functional activities as non-histidine-tagged recombinant enzymes.

Rat dihydroorotate dehydrogenase: isolation of the recombinant enzyme from mitochondria of insect cells

Mammalian dihydroorotate dehydrogenase (EC 1.3.99.11), the fourth enzyme of pyrimidine de novo synthesis is located in the mitochondrial inner membrane with functional connection to the respiratory chain. From the cDNA of rat liver dihydroorotate dehydrogenase cloned in our laboratory the first complete sequence of a mammalian enzyme was deduced. Two hydrophobic stretches centered around residues 20 and 357, respectively, and a short N-terminal mitochondrial targeting sequence of 10 amino acids was proposed. A recombinant baculovirus containing the rat liver cDNA for dihydroorotate dehydrogenase was constructed and used for virus infection and protein expression in Trichoplusia ni cells. The targeting of the recombinant protein to mitochondria of the insect cells was monitored by activity determination of dihydroorotate dehydrogenase in subcellular compartments in comparison to succinate dehydrogenase activity (EC 1.3.5.1), which is a specific marker enzyme of the inner mitochondrial membrane. The results of subcellular distribution were verified by Western blotting with anti-dihydroorotate dehydrogenase immunoglobulins. The activity of the recombinant enzyme in the mitochondria of infected insect cells was found to be about 570-fold above the level of dihydroorotate dehydrogenase in rat liver mitochondria. By cation exchange chromatography of the Triton X-114 solubilisate of mitochondria, dihydroorotate dehydrogenase was purified to give a specific activity of 15 U/mg at pH 8.0. This was a marked progress over the six-step purification procedure of the enzyme from rat liver which resulted in a specific activity of 0.7 U/mg at pH 8.0. The characteristic flavin absorption spectrum obtained with the recombinant enzyme gave strong evidence that the rodent enzyme is a flavoprotein. By enzyme kinetic studies K(m) values for dihydroorotate and ubiquinone were 6.4 and 9.9 microM with the recombinant enzyme, and were 5.0 and 19.7 microM, respectively, with the rat liver enzyme. After expression of only truncated forms of human dihydroorotate dehydrogenase, the present successful generation of the complete rodent enzyme using insect cells and the efficient procedure will promote structure and function studies of the eukaryotic dihydroorotate dehydrogenases in comparison to the microbial enzyme.

Antiviral effects of 6-diazo-5-oxo-L-norleucin on replication of herpes simplex virus type 1

An L-glutamine antagonist, 6-diazo-5-oxo-L-norleucin (L-DON), inhibits replication of vesicular stomatitis virus, poliovirus and paramyxoviruses in cultured cells. We tested the antiviral activity of L-DON against different strains of herpes simplex virus type 1 (HSV-1) in Vero cells. In the presence of a physiological plasma concentration of L-glutamine (0.5mM) L-Don inhibited 50% production of virus plaques at concentrations ranging from 7.9 to 16 microM. At concentrations of 40 microM L-Don inhibited infectious virus yield by 99%. The antiviral activity of L-DON decreased with increasing L-glutamine concentrations. A concentration of 5000 microM of L-Don had no significant effects on the viability of Vero cells. Transmission electron microscopical investigations showed that L-DON prevented mainly envelopment of viral nucleocapsids in the cytoplasm. The immunoprecipitation experiments demonstrated selective inhibition of synthesis of HSV-1 glycoproteins in L-DON treated cells. The results showed that L-DON inhibits HSV-1 replication at a late stage in the virus replication cycle, probably the cytoplasmic maturation of virions and subsequent virion egress from the cells.

Functional expression of a fragment of human dihydroorotate dehydrogenase by means of the baculovirus expression vector system, and kinetic investigation of the purified recombinant enzyme

Human mitochondrial dihydroorotate dehydrogenase (the fourth enzyme of pyrimidine de novo synthesis) has been overproduced by means of a recombinant baculovirus that contained the human cDNA fragment for this protein. After virus infection and protein expression in Trichoplusia ni cells (BTI-Tn-5B1-4), the subcellular distribution of the recombinant dihydroorotate dehydrogenase was determined by two distinct enzyme-activity assays and by Western blot analysis with anti-(dihydroorotate dehydrogenase) Ig. The targeting of the recombinant protein to the mitochondria of the insect cells was verified. The activity of the recombinant enzyme in the mitochondria of infected cells was about 740-fold above the level of dihydroorotate dehydrogenase in human liver mitochondria. In a three-step procedure, dihydroorotate dehydrogenase was purified to a specific activity of greater than 50 U/mg. Size-exclusion chromatography showed a molecular mass of 42 kDa and confirmed the existence of the fully active enzyme as a monomeric species. Fluorimetric cofactor analysis revealed the presence of FMN in recombinant dihydroorotate dehydrogenase. By kinetics analysis, Km values for dihydroorotate and ubiquinone-50 were found to be 4 microM and 9.9 microM, respectively, while Km values for dihydroorotate and decylubiquinone were 9.4 microM and 13.7 microM, respectively. The applied expression system will allow preparation of large quantities of the enzyme for structure and function studies. Purified recombinant human dihytdroorotate dehydrogenase was tested for its sensitivity to a reported inhibitor A77 1726 (2-hydroxyethyliden-cyanoacetic acid 4-trifluoromethyl anilide), which is the active metabolite of the isoxazole derivative leflunomide [5-methyl-N-(4-trifluoromethyl-phenyl)-4-isoxazole carboximide]. An IC50 value of 1 microM was determined for A77 1726. Detailed kinetics experiments revealed uncompetitive inhibition with respect to dihydroorotate (Kiu = 0.94 microM) and non-competitive inhibition with respect to decylubiquinone (Kic = 1.09 microM, Kiu = 1.05 microM). These results suggest that the immunomodulating agent A77 1726 (currently in clinical phase III studies for the treatment of rheumatoid arthritis) is a very good inhibitor of human dihydroorotate dehydrogenase.