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Pre-synaptic norepinephrine (NE) and dopamine (DA) transporters (NET and DAT) terminate catecholamine synaptic transmission through reuptake of released neurotransmitter. Recent studies reveal that NET and DAT are tightly regulated by receptor and second messenger-linked signaling pathways. Common approaches for studying these transporters involve use of radiolabeled substrates or antagonists, methods possessing limited spatial resolution and that bear limited opportunities for repeated monitoring of living preparations. To circumvent these issues, we have explored two novel assay platforms that permit temporally resolved quantitation of transport activity and transporter protein localization. To monitor the binding and transport function of NET and DAT in real-time, we have investigated the uptake of the fluorescent organic compound 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (ASP+). We have extended our previous single cell level application of this substrate to monitor transport activity via high-throughput assay platforms. Compared to radiotracer uptake methods, acquisition of ASP+ fluorescence is non-isotopic and allows for continuous, repeated transport measurements on both transfected and native preparations. Secondly, we have extended our application of small-molecule-conjugated fluorescent CdSe/ZnS nanocrystals, or quantum dots (Qdots), to utilize antibody and peptide ligands that can identify surface expressed transporters, receptors and other membrane proteins in living cell systems. Unlike typical organic fluorophores, Qdots are highly resistant to bleaching and can be conjugated to multiple ligands. They can also be illuminated by conventional light sources, yet produce narrow, gaussian emission spectra compatible with multiple target visualization (multiplexing). Together, these approaches offer novel opportunities to investigate changes in transporter function and distribution in real-time with superior spatial and temporal resolution.
BACKGROUND - Although multiple pathological processes have been associated with oxidative stress, the causative relation between oxidative stress and arterial lesion progression remains unclear.
METHODS AND RESULTS - To test the effect of creating arterial wall oxidative stress, we compared progression of mouse carotid lesions induced by flow cessation in the wild-type (WT) versus transgenic mice (Tg(p22vsmc)), in which overexpression of p22phox, a critical component of NAD(P)H oxidase was targeted to smooth muscle cell (SMC). Compared with WT mice, arterial lesions grew significantly larger in Tg(p22vsmc) (P<0.001) and demonstrated elevated hydrogen peroxide (H2O2) and vascular endothelial growth factor (VEGF) levels at all time points examined (P<0.001, n=4 animals per time point), probably related to increased expression of hypoxia inducible factor (HIF)-1alpha via SMC oxidative stress in the Tg(p22vsmc) arteries, both basally (203+/-12% versus WT, P<0.001, n=3) and after lesion formation. Interestingly, Tg(p22vsmc) lesions were complicated by extensive neointimal angiogenesis. In vitro experiments confirmed SMCs isolated from Tg(p22vsmc) to be the source for increased H2O2, VEGF, and HIF-1alpha and their capacity to induce angiogenic cord-like structures when cocultured with endothelial cells. The antioxidant ebselen inhibited SMC activities in vitro and intralesion angiogenesis and lesion progression in vivo.
CONCLUSIONS - We have demonstrated a novel pathway by which oxidative stress can trigger in vivo an angiogenic switch associated with experimental plaque progression and angiogenesis. This pathway may be related to human atheroma progression and destabilization through intraplaque hemorrhage.
The combination of synchrotron radiation and a variety of atoms or ions (either covalently attached to the biomolecule prior to crystallization or soaked into crystals) that serve as anomalous scatterers constitutes a powerful tool in the X-ray crystallographer's repertoire of structure determination techniques. Phosphoroselenoates in which one of the nonbridging phosphate oxygens in the backbone is replaced by selenium offer a simplified means for introducing an anomalous scatterer into oligonucleotides by conventional solid-phase synthesis. Unlike other methods that are used to derivatize DNA or RNA by covalent attachment of a heavy atom (i.e., bromine at the C5 position of pyrimidines), tedious synthesis of specialized nucleosides is not required. Introduction of selenium is readily accomplished in solid-phase oligonucleotide synthesis by replacing the standard oxidation agent with a solution of potassium selenocyanide. This results in a diastereomeric mixture of phosphoroselenoates that can be separated by strong anion-exchange HPLC. As a test case, all 10 DNA hexamers of the sequence CGCGCG containing a single phosphoroselenoate linkage (PSe) were prepared. Crystals were grown for a subset of them, and the structure of [d(C(PSe)GCGCG)](2) was determined by the multiwavelength anomalous dispersion technique and refined to 1.1 A resolution.
Selenium was incorporated into an oligodeoxynucleotide in the form of 2'-methylseleno-uridine (U(Se)). The X-ray crystal structure of the duplex left open bracket d(GCGTA)U(Se)d(ACGC) right open bracket (2) was determined by the multiwavelength anomalous dispersion (MAD) technique and refined to a resolution of 1.3 A, demonstrating that selenium can selectively substitute oxygen in DNA and that the resulting compounds are chemically stable. Since derivatization at the 2'-alpha-position with selenium does not affect the preference of the sugar for the C3'-endo conformation, this strategy is suitable for incorporating selenium into RNA. The availability of selenium-containing nucleic acids for crystallographic phasing offers an attractive alternative to the commonly used halogenated pyrimidines.
We have developed a route for the synthesis of 2'-selenium uridine analogues and oligonucleotides containing selenium labels, and have demonstrated for the first time a new strategy to covalently derivatize nucleotides with selenium for phase and structure determination in X-ray crystallography.
Selenium is present in plasma and tissues in specific and non-specific forms. The experiments reported here were carried out to clarify some factors that affect these forms of the element in plasma. A selenium-replete human subject was given 400 microg of selenium daily for 28 days as selenomethionine and, in a separate experiment, as selenate. The selenomethionine raised plasma and albumin selenium concentrations. Selenate did neither. The molar ratio of methionine to selenium in albumin was approximately 8000 under basal and selenate-supplemented conditions but 2800 after selenomethionine supplementation. This demonstrates that selenium from selenomethionine, but not selenium from selenate, can be incorporated into albumin, presumably as selenomethionine in the methionine pool. Selenocysteine incorporation into albumin was studied in rats using (75)Se-selenocysteine. No evidence was obtained for incorporation of (75)Se into albumin after exogenous administration or endogenous synthesis of (75)Se-selenocysteine. Thus, selenocysteine does not appear to be incorporated non-specifically into proteins as is selenomethionine. These findings are in support of selenomethionine being a non-specific form of selenium that is metabolized as a constituent of the methionine pool and is unaffected by specific selenium metabolic processes. No evidence was found for non-specific incorporation of selenium into plasma proteins when it was administered as selenate or as selenocysteine. These forms of the element appear to be metabolized by specific selenium metabolic processes.
Recycling of ascorbic acid from its oxidized forms is required to maintain intracellular stores of the vitamin in most cells. Since the ubiquitous selenoenzyme thioredoxin reductase can recycle dehydroascorbic acid to ascorbate, we investigated the possibility that the enzyme can also reduce the one-electron-oxidized ascorbyl free radical to ascorbate. Purified rat liver thioredoxin reductase catalyzed the disappearance of NADPH in the presence of low micromolar concentrations of the ascorbyl free radical that were generated from ascorbate by ascorbate oxidase, and this effect was markedly stimulated by selenocystine. Dehydroascorbic acid is generated by dismutation of the ascorbyl free radical, and thioredoxin reductase can reduce dehydroascorbic acid to ascorbate. However, control studies showed that the amounts of dehydroascorbic acid generated under the assay conditions used were too low to account for the observed loss of NADPH. Electron paramagnetic resonance spectroscopy directly confirmed that the reductase decreased steady-state ascorbyl free radical concentrations, as expected if thioredoxin reductase reduces the ascorbyl free radical. Dialyzed cytosol from rat liver homogenates also catalyzed NADPH-dependent reduction of the ascorbyl free radical. Specificity for thioredoxin reductase was indicated by loss of activity in dialyzed cytosol prepared from livers of selenium-deficient rats, by inhibition with aurothioglucose at concentrations selective for thioredoxin reductase, and by stimulation with selenocystine. Microsomal fractions prepared from rat liver showed substantial NADH-dependent ascorbyl free radical reduction that was not sensitive to selenium depletion. These results suggest that thioredoxin reductase can function as a cytosolic ascorbyl free radical reductase that may complement cellular ascorbate recycling by membrane-bound NADH-dependent reductases.
The effects of synthetic chemopreventive organoselenium compounds 1,2-, 1,3-, and 1,4-phenylenebis(methylene)selenocyanate (o-, m-, and p-XSC, respectively), benzyl selenocyanate (BSC), and dibenzyl diselenide (DDS) and inorganic sodium selenite on the oxidation of xenobiotics and procarcinogens by human cytochrome P450 (P450 or CYP) enzymes were determined in vitro. Spectral studies showed that BSC and three XSC compounds (but not sodium selenite or DDS) induced type II difference spectrum when added to the suspension of liver microsomes isolated from beta-naphthoflavone-treated rats, with m-XSC being the most potent in inducing spectral interactions with P450 enzymes; m-XSC also produced a type II spectral change with human liver microsomes. o-, m-, and p-XSC inhibited 7-ethoxyresorufin O-deethylation catalyzed by human liver microsomes when added at concentrations below 1 microM levels, but BSC and DDS were less effective. All of these compounds inhibited the oxidation of model substrates for human P450s to varying extents. We studied the effects of these compounds on the activation of procarcinogens by recombinant human CYP1A1, 1A2, and 1B1 enzymes using Salmonella typhimurium NM2009 tester strain for the detection of DNA damage. The three XSCs were found to be very potent inhibitors of metabolic activation of 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole, 2-amino-3,5-dimethylimidazo[4,5-f]quinoline, and 2-aminoanthracene, catalyzed by CYP1A1, 1A2, and 1B1, respectively. The potency of inhibition of m-XSC on CYP1B1-dependent activation of 2-aminoanthracene was compatible to those of alpha-naphthoflavone. These inhibitory actions may, in part, account for the mechanisms responsible for cancer prevention by organoselenium compounds in laboratory animals.
Oxidant substances such as hydrogen peroxide are postulated to cause cardiac dysfunction and injury in a number of pathological conditions. Selenium is an essential nutrient which serves as an oxidant defense through the selenoenzyme glutathione peroxidase. This enzyme metabolizes hydrogen peroxide; its activity in rat heart is reduced to 5% of control by selenium deficiency. Left ventricular function of selenium-deficient and control rat hearts was studied in a Langendorff preparation under isovolumic conditions. A stabilization period of 20 min was followed by a 70 min infusion of hydrogen peroxide at 375 or 1500 nmol/min. When no hydrogen peroxide was infused, perfusion for 90 min had no effect on systolic or diastolic function and no effect of selenium deficiency was detected. Hydrogen peroxide infusion into selenium-deficient hearts at 375 nmol/min led to impaired isovolumic relaxation and a substantial increase in end-diastolic pressure after 45 min which worsened progressively until the experiment was terminated. By contrast no effect was observed on systolic contractile function as assessed by peak pressure or developed pressure. Infusion of this dose of hydrogen peroxide into control hearts had no significant effect on diastolic or systolic function. However, infusion of 1500 nmol hydrogen peroxide/min into control hearts caused diastolic dysfunction after 30 min without affecting systolic function. These results indicate that hydrogen peroxide injury to the perfused rat heart is manifested by diastolic dysfunction before systolic dysfunction occurs. Selenium deficiency lowers the dose of hydrogen peroxide needed to cause diastolic dysfunction. This suggests that the selenoenzyme glutathione peroxidase protects the heart against hydrogen peroxide injury.