Stable isotope labeling by essential nutrients in cell culture (SILEC) for accurate measurement of nicotinamide adenine dinucleotide metabolism

Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are conserved metabolic cofactors that mediate reduction-oxidation (redox) reactions throughout all domains of life. The diversity of synthetic routes and cellular processes involving the transfer of reducing equivalents to and from these cofactors makes the accurate quantitation and metabolic tracing of NAD(H) and NADP(H) of broad interest. However, current analytical techniques typically rely on standard curves that do not incorporate confounding effects of the sample matrix. We utilized the essential requirement of niacin and tryptophan for NAD synthesis in mammalian cells to devise a stable isotope labeling by essential nutrients in cell culture (SILEC) method for efficient labeling of intracellular NAD(H) and NADP(H) pools. Coupling this approach with detection by liquid chromatography-high resolution mass spectrometry (LC-HRMS), we demonstrate the utility of incorporating a [¹³C₃¹⁵N₁]-nicotinamide moiety into a library of NAD-derived metabolites for use as internal standards in matrixed samples. Using a two-label system incorporating [¹³C₃¹⁵N₁]-nicotinamide and [¹³C₁₁]-tryptophan, we quantify the relative contribution of salvage and de novo NAD synthesis, respectively, in S. cerevisiae and HepG2 human hepatocellular carcinoma cells under basal conditions. As a further proof-of-principle, we demonstrate an improvement in the linear range for quantification of NAD and apply this method to analysis of NAD(H) in mouse liver. This method demonstrates the generalizability of SILEC, and provides a simple method for generating a library of stable isotope labeled internal standards for quantifying and tracing the metabolism of cellular and tissue NAD(H) and NADP(H).

Abstract 1660: Impact of antioxidants on myeloperoxidase (MPO)-dependent DNA damage and genotoxicity induced by etoposide (VP-16):implications for therapy-induced second malignancies

Previous work (Mol. Pharm. 79: 479-87, 2011) demonstrated that MPO, found in myeloid progenitor cells, oxidized the anticancer agent VP-16 to its phenoxyl radical (VP-O•) and led to enhanced topoisomerase II-mediated strand cleavage through redox cycling resulting in MLL translocations, implicating MPO in VP-16 leukemogenesis. Utilizing shRNA MPO in myeloid leukemia HL60 cells, MPO dependency for VP-16 activity was further established (PAACR 55: 826, 2014). In the present study, we examined the effects of dehydroascorbate (DHA) and trolox on MPO-dependent effects of VP-16, VP-16 catechol (VP-OH) and VP-16 ortho-quinone (VP-oQ, fully oxidized VP-16). In addition, the role of GSH as a mediator of the pro- or anti-oxidant effects of VP-16, its metabolites and several other phenolic agents was evaluated. Using Comet assays, VP-16-induced DNA strand breaks in MPO+ HL60 cells were significantly reduced when cells were pre-incubated with 1 mM DHA; known to act as a reductant of MPO generated VP-O•. In MPO knockdowns, VP-16-induced DNA damage was diminished compared to MPO+ cells. In these MPO knockdowns, DHA did not perturb VP-16-induced DNA damage. Similar results were demonstrated with VP-OH, consistent with MPO-mediated generation of VP-O• and DHA reduction/protection against DNA damage. For VP-oQ, 1,4-benzoquinone, and camptothecin, DNA damage was similar in MPO+ and knockdown cells with no attenuation in the presence of DHA. VP-16 and VP-OH also induced oxidative DNA damage in MPO+ cells which was reduced by DHA. This DNA damage was attenuated in MPO knockdowns with no further effect by DHA. The phenolic vitamin E analog trolox, a known MPO substrate, also reduced oxidative DNA damage induced by VP-16 and VP-OH but not by VP-oQ. Using 3′-(p-hydroxyphenyl) fluorescein (HPF), pro-oxidant activity of VP-16 was demonstrated in MPO+ cells which converted to anti-oxidant effects in MPO knockdowns. Both DHA and trolox protected cells against VP-16-induced pro-oxidant effects in MPO+ cells. When GSH levels were reduced by incubation with buthionine sulfoximine, the pro-oxidant effects of VP-16 in MPO+ cells were eliminated. N-acetyl cysteine (NAC) restored VP-16-induced pro-oxidant activity. In contrast, the pro-oxidant effects of the phenolic agents quercetin and EGCG were found to be independent of MPO and were diminished by addition of NAC. Together, results strongly suggest that MPO-catalyzed oxidation of VP-16 to redox active species leads to enhanced genotoxic events linked to the known leukemogenic action of this anticancer agent. These MPO dependent effects are also dependent on GSH likely through thiyl radical formation and cycling. The protective effects of DHA and trolox further suggest that reduction of MPO-catalyzed VP-16 free radicals may be an effective strategy to prevent drug-induced second malignancies. Support: NIH R01 CA090787.

Citation Format: Jason Goodspeed, Soumendra Karmahapatra, Ragu Kanagasabai, Alex Klausing, Jack C. Yalowich. Impact of antioxidants on myeloperoxidase (MPO)-dependent DNA damage and genotoxicity induced by etoposide (VP-16):implications for therapy-induced second malignancies. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1660. doi:10.1158/1538-7445.AM2015-1660

Abstract 826: Myeloperoxidase (MPO) dependency for DNA damage, genotoxicity and cytotoxicity induced by etoposide (VP-16): Implications for therapy-induced second malignancies

Use of the phenolic anticancer agent etoposide (VP-16), which targets DNA topoisomerase II (topo II), can lead to an increased incidence of acute myeloid leukemia linked to rearrangements in the MLL gene. Previous work (Mol. Pharm. 79: 479-87, 2011) demonstrated that MPO, found in myeloid progenitor cells, oxidized VP-16 to its phenoxyl radical form and led to enhanced DNA topo II-mediated strand cleavage through redox cycling resulting in MLL translocations. In the present study, we utilized MPO shRNA in myeloid leukemia HL-60 cells to further examine MPO dependency for VP-16-, VP-16 catechol (VP-OH)-, and VP-16 ortho-quinone (VP-oQ)-induced: 1) DNA strand breaks; 2) oxidative DNA damage; 3) caspase 3 activation; 4) cytotoxicity. Using Comet assays, VP-16 induced a concentration-dependent increase in DNA strand breaks which was attenuated in MPO knockdown cells, as well as in cells incubated with succinylacetone (SA), a heme synthesis inhibitor which depletes cells of active MPO. Similar MPO dependency was demonstrated using VP-OH, and parthenolide (PTL), a sesquiterpene lactone, whose activity is known to be dependent on MPO. In contrast, DNA damage induced by VP-oQ, a fully oxidized VP-16 metabolite, and by the topo I inhibitor camptothecin (CPT), was independent of MPO. VP-16 and VP-OH also induced DNA abasic sites in MPO-replete HL-60 cells. In MPO depleted cells, VP-16 diminished oxidative DNA damage to levels below those seen in controls. Paradoxically, caspase 3 activation induced by VP-16 and VP-OH was decreased in MPO-replete compared to MPO-depleted cells. By comparison, PTL-induced caspase 3 activation was MPO dependent and CPT-induced caspase 3 activation was MPO independent. Cytotoxicity (trypan blue exclusion) induced by VP-16 and VP-OH was inversely related to, while that of PTL corresponded to, MPO levels and activity. VP-oQ, CPT, and podophyllotoxin (PDT) were equally cytotoxic regardless of MPO levels or activity. Clonogenic assays revealed similar paradoxes where VP-16 cytotoxicity was diminished in MPO-replete compared to MPO knockdown cells. In contrast, the effects of VP-oQ, CPT, and PDT on colony formation were not influenced by expression of MPO. Mass spectrometric analysis demonstrated MPO-dependent production of VP-oQ GSH adducts. The production of these inactive adducts were reduced in MPO depleted cells accounting, in part, for the unexpected increase in VP-16 cytotoxicity in MPO knockdown cells. Together, results strongly suggest that MPO-catalyzed oxidation of VP-16 to redox active species leads to enhanced genotoxic events linked to the known leukemogenic action of this anticancer agent. Future studies will be directed toward use of nutritional antioxidants to diminish formation of genotoxic radical species of VP-16 in MPO-containing myeloid precursors as a strategy to prevent drug-induced second malignancies.

Citation Format: Ragu Kanagasabai, Jason Goodspeed, Soumendra Krishna Karmahapatra, Alex Klausing, Anna Skwarska, Michael Darby, Yuan Zhao, Jiang Wang, Mitchell A. Phelps, Jack C. Yalowich. Myeloperoxidase (MPO) dependency for DNA damage, genotoxicity and cytotoxicity induced by etoposide (VP-16): Implications for therapy-induced second malignancies. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 826. doi:10.1158/1538-7445.AM2014-826

Analyzing habituation responses to novelty in zebrafish (Danio rerio)

Analysis of habituation is widely used to characterize animal cognitive phenotypes and their modulation. Although zebrafish (Danio rerio) are increasingly utilized in neurobehavioral research, their habituation responses have not been extensively investigated. Utilizing the novel tank test, we examine intra- and inter-session habituation and demonstrate robust habituation responses in adult zebrafish. Analyzing the intra-session habituation to novelty further, we also show that selected anxiogenic drugs (caffeine, pentylenetetrazole), as well as stress-inducing alarm pheromone, attenuated zebrafish habituation. Some acute anxiolytic agents, such as morphine and ethanol, while predictably reducing zebrafish anxiety, had no effects on habituation. Chronic ethanol and fluoxetine treatments improved intra-session habituation in zebrafish. In general, our study parallels literature on rodent habituation responses to novelty, and reconfirms zebrafish as a promising model for cognitive neurobehavioral research.