■ PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR δ: THE COLON TUMOR CONNECTION

Numerous correlations have been found between chronic inflammation, such as that encountered in cases of inflammatory boweldisease, and malignancies, including colorectal cancer (CRC). Factors implicated in both of these pathologies include innateimmune system responses as well as involvement of the peroxisome proliferator-activated receptor δ (PPARδ), a transcriptionfactor very highly expressed in the gastrointestinal tract. A recent study by Raymond N. DuBois and co-workers has helped toclarify the nebulous role of PPARδ in immune cell recruitment and CRC progression ((2014) PNAS, 111, 7084−7089).The research team demonstrated in two different mouse models that PPARδ deletion reduced colonic chronic inflammation

and tumor growth. Relative to wild-type mice, PPARδ deletion mice also showed lower levels of pro-inflammatory chemokinesand cytokines as well as lower expression of the pro-inflammatory enzyme cyclooxygenase 2 (COX-2) during chronicinflammation. Their results, along with those of previous studies, allowed the authors to propose that PPARδ activation inducesan upregulation of COX-2, which enables the production of prostaglandin E2 (PGE2) in colonic epithelial cells. In turn, PGE2stimulates macrophages to produce proinflammatory chemokines and cytokines that cause chronic inflammation and elevate therisk of CRC development.Heidi Dahlmann

■ CELLULAR PERSULFIDES AND POLYSULFIDES:ABUNDANT AND REACTIVE

Adapted from Ida et al. (2014) PNAS, 111, 7606−7611.Copyright 2014 Ida et al.

Current common knowledge dictates that the enzymes cystathioneβ-synthase (CSB) and cystathionine γ-lyase (CSE) producehydrogen sulfide (H2S) during the metabolism of sulfide-containing amino acids and that H2S acts as a major cellularsignaling molecule in mammals. However, new resultspresented by Jon M. Fukuto, Takaaki Akaike, and co-workersprovide a strong challenge to these assumptions ((2014) PNAS,111, 7606−7611). The research team developed a method forlabeling and detecting persulfides and polysulfides by HPLC-MSand then explored the enzymatic production, cellular abundance,and reactivity of these classes of molecules.CSE and CSB were each able to metabolize cysteine (CysSSCys)

to an unstable Cys hydropersulfide (CysSSH), as determined instudies with isolated enzymes and in cells. CysSSH in the cellspresumably reacted with both oxidized and reduced glutathione(GSSG and GSH, respectively) to produce GSSH as well as otherglutathione-derived per- and polysulfides such as GSSSG. Thesepersulfide species were quantified in mouse tissues at 50−100 μMconcentrations and were detected in human tissue and plasmasamples; protein-bound polythiolated species generated in cells wereidentified as well. The reactivity of persulfides such as GSSH toward8-nitro-cGMP, an endogenously produced electrophilic signalingmolecule, was much higher than that of H2S or GSH. Thus, theauthors suggest that H2S, which can be released through persulfidedegradation, may simply be a biomarker of persulfide formation andthat the persulfides themselves may have a primary role in cellularsignaling and regulatory pathways.Heidi Dahlmann

■ ETOPOSIDE METABOLITE COVALENTLY BINDSHUMAN TOPOISOMERASE IIβ

Adapted from Smith et al. (2014) Biochemistry, 53, 3229−3236.Copyright 2014 American Chemical Society.

A common pharmaceutical approach to fighting cancer is totreat patients with drugs that interfere with enzymes that pro-cess DNA, such as topoisomerase II. This enzyme generatestransient double-strand breaks in order to relax supercoiledDNA and remove knots and tangles from the genome.Etoposide is used to treat a broad spectrum of cancers. Thedrug intercalates into cleaved DNA and blocks topoisomeraseII-mediated ligation, which causes toxic accumulation ofdouble-stranded breaks in the genome. Unfortunately, etopo-side treatment also comes with a 2−3% chance of developingtherapy-related acute myeloid leukemia (t-AML), an eventlinked to the ability of cells to oxidize the phenolic moiety ofthe drug to form etoposide quinone.Topoisomerase IIβ is the human isoform implicated in

t-AML development. To elucidate whether etoposide quinoneinterferes with this isoform, Neil Osheroff and co-workers re-cently characterized the impact of the metabolite on topo-isomerase IIβ-mediated DNA cleavage and ligation ((2014)Biochemistry, 53, 3229−3236). They found that DNA treatedwith topoisomerase IIβ and etoposide quinone sustained 4-foldhigher levels of cleavage and a greater proportion of double-to single-stranded breaks than did DNA incubated with

Published: July 21, 2014

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topoisomerase IIβ and etoposide. These findings suggest thatthe parent drug and metabolite act by different mechanisms.The research team also discovered that preincubating etoposidequinone with topoisomerase IIβ inhibited enzymatic activityand that incubating the quinone with reducing agents greatlyimpaired drug activity. On the basis of these results, the authorspropose that etoposide quinone alters the activity of topo-isomerase IIβ by covalently binding to the enzyme.Heidi Dahlmann

■ NICOTINAMIDE NUCLEOTIDETRANSHYDROGENASE REQUIRED FOR BRAINMITOCHONDRIAL-MEDIATED H2O2 REMOVAL

This figure is adapted from that originally published in J. Biol.Chem. Lopert, P. and Patel, M. Nicotinamide NucleotideTranshydrogenase (Nnt) Links the Substrate Requirement inBrain Mitochondria for Hydrogen Peroxide Removal to theThioredoxin/Peroxiredoxin (Trx/Prx) System. J. Biol. Chem.2014; 289, 15611−15620. Copyright The American Society forBiochemistry and Molecular Biology.

Mitochondrial reactive oxygen species (ROS) are important cellsignaling molecules, but when their levels get too high, varioustypes of cellular damage can result. In fact, many neurologicaldisorders, such as Parkinson’s disease, are linked to dysregula-tion of ROS in the brain. Normally, there are several enzymaticpathways by which ROS are detoxified; for example, H2O2 can beconverted to H2O by the thioredoxin/peroxiredoxin (Trx/Prx)pathway or by the glutathione (GSH) peroxidase pathway. It wasknown that H2O2 metabolism by brain mitochondria appeared tobe highly dependent on respiration, a metabolic process thatgenerates the enzymatic cofactor NADH. Furthermore, in brainmitochondria, roughly 80% of H2O2 catabolism is mediated by theTrx/Prx pathway, which is dependent on the cofactor NADPH.Recently, Pamela Lopert and Manisha Patel have elucidated themolecular link between respiration and Trx/Prx-mediated H2O2detoxification ((2014) J. Biol. Chem., 289, 15611−15620).Aware that the enzyme nicotinamide nucleotide trans-

hydrogenase (Nnt) utilizes the proton gradient generated byrespiration to covert NADH to NADPH, the research pairdemonstrated that pharmacologically inhibiting or knocking downNnt expression significantly decreased H2O2 metabolism inisolated brain mitochondria and brain neuronal cells. Cellsdeficient in Nnt also had decreased NADPH levels and anincreased ratio of oxidized Prx3 relative to Nnt-proficient cells.The authors note that while Nnt mutation or deficiency has beenassociated with diseases such as diabetes, obesity, andglucocortocoid defiency, it has never previously been linked toneurodegeneration.Heidi Dahlmann

■ PUMPING UP PEROXIREDOXIN VIAPOST-TRANSLATIONAL MODIFICATION

Peroxiredoxin (Prx) enzymes, which function as homodimers,each containing two closely spaced cysteine residues, mediate

redox signaling in cells by converting hydrogen peroxide toH2O. The catalytic cycle begins with the H2O2-mediated oxida-tion of a cysteine residue on one Prx subunit to form a sulfenicacid derivative. This moiety can then react with a resolvingcysteine on the other Prx subunit to generate a disulfide bondbetween the two subunits that is reduced back by thethioredoxin/thioredoxin reductase system at the expense ofNADPH. The sulfenic acid derivative can also be overoxidizedby another molecule of H2O2 to form a sulfinic acid moiety thatinactivates the enzyme. The inactivation of Prx by over-oxidation can be slowly reversed by sulfiredoxin in an ATP-dependent mechanism. The collective activity of peroxidasescontrols levels of H2O2 in cells. However, post-translationalmodifications such as tyrosine nitration can throw off thebalance of redox homeostasis.A group led by Ana Denicola has recently explored the

impact of nitration on the activity of human Prx2, the mostabundant peroxidase in erythrocytes ((2014) J. Biol. Chem.,289, 15536−15543). The research team discovered that ex-posure of Prx2 to an excess of peroxynitrite mainly inducedmononitration or dinitration of the enzyme, particularly at Tyr-193, which is located in a loop of Prx that controls the con-formation of the homodimer and is known to play a role in thesusceptibility of Prx to overoxidation. The nitrated Pxr2 was notonly faster at reducing H2O2 than was nontreated Pxr2, but itwas also less susceptible to overoxidation. The authorsreasoned that tyrosine nitration stabilizes the homodimer in aconformation that favors rapid disulfide bond formation,precluding overoxidation.Heidi Dahlmann

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