Potent inhibition of specific diadenosine polyphosphate hydrolases by suramin

Development of Nucleoside Diphosphate-Bearing Fragile Histidine Triad-Imaging Fluorescence Probes with Well-Tuned Hydrophobicity for Intracellular Delivery

Fluorescence-guided cancer surgery can dramatically improve recurrence rates and postoperative quality of life of patients by accurately distinguishing the boundary between normal and cancer tissues during surgery, thereby minimizing excision of normal tissue. One promising target in early stage cancer is fragile histidine triad (FHIT), a cancer suppressor protein with dinucleoside triphosphate hydrolase activity. In this study, we have developed fluorescence probes containing a nucleoside diphosphate moiety, which dramatically improves the reactivity and specificity for FHIT, and a moderately lipophilic ester moiety to increase the membrane permeability. The ester moiety is cleaved by ubiquitous intracellular esterases, and then, FHIT in the cells specifically cleaves nucleoside monophosphate. The remaining phosphate moiety is rapidly cleaved by ubiquitous intracellular phosphatases to release the fluorescent dye. We confirmed that this probe can detect FHIT activity in living cells. A comprehensive evaluation of the effects of various ester moieties revealed that probes with CLogP = 5-7 showed good membrane permeability and were good substrates of the target enzyme; these findings may be helpful in the rational design of other multiple phosphate-containing probes targeting intracellular enzymes.

Synthesis of Fluorescent Probes Targeting Tumor-Suppressor Protein FHIT and Identification of Apoptosis-Inducing FHIT Inhibitors

For the early diagnosis of cancer, leading to a better chance of full recovery, marker genes whose expression is already altered in precancerous lesions are desirable, and the tumor-suppressor gene FHIT is one candidate. The gene product, FHIT protein, has a unique dinucleoside triphosphate hydrolase (AP₃Aase) activity, and in this study, we designed and synthesized a series of FHIT fluorescent probes utilizing this activity. We optimized the probe structure for high and specific reactivity with FHIT and applied the optimized probe in a screening assay for FHIT inhibitors. Screening of a compound library with this assay identified several hits. Structural development of a hit compound afforded potent FHIT inhibitors. These inhibitors induce apoptosis in FHIT-expressing cancers via caspase activation. Our results support the idea that FHIT binders, no matter whether inhibitors or agonists of AP₃Aase activity, might be promising anticancer agents.

Tanshinones induce tumor cell apoptosis via directly targeting FHIT

The liposoluble tanshinones are bioactive components in Salvia miltiorrhiza and are widely investigated as anti-cancer agents, while the molecular mechanism is to be clarified. In the present study, we identified that the human fragile histidine triad (FHIT) protein is a direct binding protein of sodium tanshinone IIA sulfonate (STS), a water-soluble derivative of Tanshinone IIA (TSA), with a Kd value of 268.4 ± 42.59 nM. We also found that STS inhibited the diadenosine triphosphate (Ap3A) hydrolase activity of FHIT through competing for the substrate-binding site with an IC₅₀ value of 2.2 ± 0.05 µM. Notably, near 100 times lower binding affinities were determined between STS and other HIT proteins, including GALT, DCPS, and phosphodiesterase ENPP1, while no direct binding was detected with HINT1. Moreover, TSA, Tanshinone I (TanI), and Cryptotanshinone (CST) exhibited similar inhibitory activity as STS. Finally, we demonstrated that depletion of FHIT significantly blocked TSA's pro-apoptotic function in colorectal cancer HCT116 cells. Taken together, our study sheds new light on the molecular basis of the anti-cancer effects of the tanshinone compounds.

Small-Molecule Inhibitors of the Tumor Suppressor Fhit

The tumor suppressor Fhit and its substrate diadenosine triphosphate (Ap₃ A) are important factors in cancer development and progression. Fhit has Ap₃ A hydrolase activity and cleaves Ap₃ A into adenosine monophosphate (AMP) and adenosine diphosphate (ADP); this is believed to terminate Fhit-mediated signaling. How the catalytic activity of Fhit is regulated and how the Fhit⋅Ap₃ A complex might exert its growth-suppressive function remain to be discovered. Small-molecule inhibitors of the enzymatic activity of Fhit would provide valuable tools for the elucidation of its tumor-suppressive functions. Here we describe the development of a high-throughput screen for the identification of such small-molecule inhibitors of Fhit. Two clusters of inhibitors that decreased the activity of Fhit by at least 90 % were identified. Several derivatives were synthesized and exhibited in vitro IC₅₀ values in the nanomolar range.

Structural and functional attributes of malaria parasite diadenosine tetraphosphate hydrolase

Malaria symptoms are driven by periodic multiplication cycles of Plasmodium parasites in human red blood corpuscles (RBCs). Malaria infection still accounts for ~600,000 annual deaths, and hence discovery of both new drug targets and drugs remains vital. In the present study, we have investigated the malaria parasite enzyme diadenosine tetraphosphate (Ap4A) hydrolase that regulates levels of signalling molecules like Ap4A by hydrolyzing them to ATP and AMP. We have tracked the spatial distribution of parasitic Ap4A hydrolase in infected RBCs, and reveal its unusual localization on the infected RBC membrane in subpopulation of infected cells. Interestingly, enzyme activity assays reveal an interaction between Ap4A hydrolase and the parasite growth inhibitor suramin. We also present a high resolution crystal structure of Ap4A hydrolase in apo- and sulphate- bound state, where the sulphate resides in the enzyme active site by mimicking the phosphate of substrates like Ap4A. The unexpected infected erythrocyte localization of the parasitic Ap4A hydrolase hints at a possible role of this enzyme in purinerigic signaling. In addition, atomic structure of Ap4A hydrolase provides insights for selective drug targeting.

Direct Monitoring of Nucleotide Turnover in Human Cell Extracts and Cells by Fluorogenic ATP Analogs

Nucleotides containing adenosine play pivotal roles in every living cell. Adenosine triphosphate (ATP), for example, is the universal energy currency, and ATP-consuming processes also contribute to posttranslational protein modifications. Nevertheless, detecting the turnover of adenosine nucleotides in the complex setting of a cell remains challenging. Here, we demonstrate the use of fluorogenic analogs of ATP and adenosine tetraphosphate to study nucleotide hydrolysis in lysates of human cell lines and in intact human cells. We found that the adenosine triphosphate analog is completely stable in lysates of human cell lines, whereas the adenosine tetraphosphate analog is rapidly turned over. The observed activity in human cell lysates can be assigned to a single enzyme, namely, the human diadenosine tetraphosphate hydrolase NudT2. Since NudT2 has been shown to be a prognostic factor for breast cancer, the adenosine tetraphosphate analog might contribute to a better understanding of its involvement in cancerogenesis and allow the straightforward screening for inhibitors. Studying hydrolysis of the analogs in intact cells, we found that electroporation is a suitable method to deliver nucleotide analogs into the cytoplasm and show that high FRET efficiencies can be detected directly after internalization. Time-dependent experiments reveal that adenosine triphosphate and tetraphosphate analogs are both processed in the cellular environment. This study demonstrates that these nucleotide analogs indeed bear the potential to be powerful tools for the exploration of nucleotide turnover in the context of whole cells.

Discovery of inhibitors of lupin diadenosine 5',5'''-P(1),P(4)-tetraphosphate hydrolase by virtual screening

Novel inhibitors of lupin diadenosine 5',5'''-P(1),P(4)-tetraphosphate (Ap(4)A) hydrolase have been identified by in silico screening of a large virtual chemical library. Compounds were ranked on the basis of a consensus from six scoring functions. From the top 100 ranked compounds six were selected and initially screened for inhibitory activity using a single concentration isothermal titration calorimetry assay. Two of these compounds that showed excellent solubility properties were further analyzed, but only one [NSC51531; 2-((8-hydroxy-4-(4-methyl-2-sulfoanilino)-9,10-dioxo-9,10-dihydro-1-anthracenyl)amino)-5-methylbenzenesulfonic acid] exhibited competitive inhibition with a K(i) of 1 microM. A structural analogue of this compound also exhibited competitive inhibition with a comparable K(i) of 2.9 microM. (1)H, (15)N NMR spectroscopy was used to map the binding site of NSC51531 on lupin Ap(4)A hydrolase and demonstrated that the compound bound specifically in the substrate-binding site, consistent with the competitive inhibition results. Binding of NSC51531 to the human form of Ap(4)A hydrolase is nonspecific, suggesting that this compound may represent a useful lead in the design of specific inhibitors of the plant-like form of Ap(4)A hydrolases.

Biochemical and immunochemical characterisation of human diadenosine triphosphatase provides evidence for its identification with the tumour suppressor Fhit protein

We describe here the purification and characterisation of the human enzyme diadenosine triphosphatase isolated from human platelets and leukocytes, offering biochemical and immunochemical evidence to identify this enzyme with the novel tumour suppressor Fhit protein, a homodimer composed of approximately 17 kDa monomers. It catalyses the Mg(2+)-dependent hydrolysis of diadenosine triphosphate, Ap(3)A, to AMP+ADP. The fluorogenic substrate di-ethenoadenosine triphosphate, epsilon-(Ap(3)A), and Fhit antibodies were used for enzymatic and immunochemical characterisations, respectively. Human Ap(3)Aase presents a native molecular mass of approximately 32 kDa and no significant differences were found in K(m) values (2 microM), activating effects by Mg(2+), Ca(2+), and Mn(2+), optimum pH (7.0-7.2) or inhibition by Zn(2+) and diethyl pyrocarbonate between the human enzyme and the recombinant Fhit protein. Suramin is a very potent competitive inhibitor of both human Ap(3)Aase and Fhit protein with K(i) values in the range 20-30 nM. Both human and rat Ap(3)Aase activity co-purifies with Fhit immunoreactivity under gel filtration, ion-exchange and affinity chromatography. Homogeneous human Ap(3)Aase preparations analysed by SDS-PAGE and Western blot analysis with Fhit antibodies elicit immunochemical responses corresponding to a approximately 17 kDa polypeptide, indicating a dimeric structure for the enzyme Ap(3)Aase. The strong inhibition of Fhit enzyme by the drug suramin, supports the need to investigate the therapeutic potential of Fhit-Ap(3)Aase mediated by its interaction with suramin or related drugs.

Specific and nonspecific enzymes involved in the catabolism of mononucleoside and dinucleoside polyphosphates

This review concerns enzymes that can degrade nucleoside 5'-tetra- and pentaphosphates (p(4)N and p(5)N) and those that can degrade various dinucleoside polyphosphates (Np(3-6)N'). Most of these enzymes are hydrolases, and they occur in all types of organisms. Certain fungi and protozoa also possess specific Np(n)N' phosphorylases. Specific p(4)N hydrolases have been demonstrated in mammals and in plants. In yeast, p(4)N and p(5)N are hydrolyzed by exopolyphosphatases. Among other hydrolases that can degrade these minor mononucleotides are phosphatases, apyrase, and (asymmetrical) Np(4)N' hydrolase, as well as the nonspecific adenylate deaminase. Np(n)N's are good substrates for Type I phosphodiesterases and nucleotide pyrophosphatases, and diadenosine polyphosphates are easily deaminated to diinosine polyphosphates by nonspecific adenylate deaminases. Specific Np(3)N' hydrolases occur in both prokaryotes and eukaryotes. Interestingly, the human fragile histidine triad (Fhit) tumor suppressor protein appears to be a typical Np(3)N' hydrolase. Among the specific Np(4)N' hydrolases are asymmetrically cleaving ones, which are typical of higher eukaryotes, and symmetrically cleaving enzymes found in Physarum polycephalum and in many bacteria. An enzyme that hydrolyzes both diadenosine tetraphosphate and diadenosine triphosphate has been found in the fission yeast Schizosaccharomyces pombe. Its amino acid sequence is similar to that of the human Fhit/Np(3)N' hydrolase. Very recently, a typical (asymmetrical) Np(4)N' hydrolase has been demonstrated for the first time in a bacterium-the pathogenic Bartonella bacilliformis. Another novelty is the discovery of diadenosine 5', 5"'-P(1),P 6-hexaphosphate hydrolases in budding and fission yeasts and in mammalian cells. These enzymes and the (asymmetrical) Np(4)N' hydrolases have the amino acid motif typical of the MutT (or Nudix hydrolase) family. In contrast, the Schizosaccharomyces pombe Ap(4)A/Ap(3)A hydrolase, the human Fhit protein, and the yeast Np(n)N' phosphorylases belong to a superfamily GAFH, which includes the histidine triad proteins.

Separation of diadenosine polyphosphates by capillary electrophoresis

The influence of buffer composition and pH on the electrophoretic behavior of diadenosine polyphosphates with a phosphate chain ranging from two to five phosphate groups has been examined. The electrophoretic mobility in carbonate buffer increases according to the number of phosphates, whereas in borate buffer the mobility changes in an irregular way as a function of pH. This finding can be rationalized by a well-known interaction of borate with ribose rings, which modifies the charge and the hydrodynamic radius of each diadenosine polyphosphate in a different way. Our study shows that the best separation of diadenosine polyphosphates can be achieved at the highest pH values of the range examined both in borate and carbonate buffers.

Fluorimetric detection of enzymatic activity associated with the human tumor suppressor Fhit protein

The human tumor suppressor Fhit protein exhibits diadenosine triphosphatase activity, hydrolyzing Ap(3)A to AMP and ADP. We report that Fhit protein efficiently cleaves the fluorogenic Ap(3)A analog diethenoadenosine triphosphate giving support to establish a simple fluorimetric assay for quantification of Fhit enzyme. Fluorimetric assays were initially tested to demonstrate that diethyl pyrocarbonate and suramin inhibit Fhit enzyme.

The polysulfonated compound suramin blocks adsorption and lateral difusion of herpes simplex virus type-1 in vero cells

Several polysulfonate compounds have been shown to have the potential to inhibit the replication of herpesviruses by blocking binding and penetration of the host cell. We analyzed the actions of the polysulfonate compound suramin on the replication of herpes simplex virus type 1 (HSV-1) and compared them with the actions of heparin. We used the expression of a reporter gene (beta-galactosidase) recombined into the latency-associated transcript region of the 17syn+ strain of HSV-1 to quickly evaluate productive cycle activity and have shown that it can be directly correlated with virus replication under the conditions used. We find that suramin, like heparin, blocks the binding of HSV-1 to the cell membrane. Also, suramin efficiently blocks the cell-to-cell spread of the virus; this effect has not been previously reported. Our control experiments demonstrate that heparin also has some effect on intercellular spread of HSV-1 but to a significantly lesser degree than does suramin. We suggest that suramin and related polysulfonate compounds have potential for developing of antiherpes treatments.

The gene, ialA, associated with the invasion of human erythrocytes by Bartonella bacilliformis, designates a nudix hydrolase active on dinucleoside 5'-polyphosphates

ialA, one of two genes associated with the invasion of human red blood cells by Bartonella bacilliformis, the causative agent of several diseases, has been cloned and expressed in Escherichia coli. The protein, IalA, contains an amino acid array characteristic of a family of enzymes, the Nudix hydrolases, active on a variety of nucleoside diphosphate derivatives. IalA has been purified, identified, and characterized as an enzyme catalyzing the hydrolysis of members of a class of signaling nucleotides, the dinucleoside polyphosphates, with its highest activity on adenosine 5'-tetraphospho-5'-adenosine (Ap4A), but also hydrolyzing Ap5A, Ap6A, Gp4G, and Gp5G. In each case, a pyrophosphate linkage is cleaved yielding a nucleoside triphosphate and the remaining nucleotide moiety.