We measured the rate of generation of reactive oxygen species (ROS) [hydroxyl radicals (OH) and hydrogen peroxide (HO)] catalyzed by ambient particulate matter (PM) in the dithiothreitol (DTT) assay. To understand the mechanism of ROS generation, we tested several redox-active substances, such as 9,10-phenanthrenequinone (PQ), 5-hydroxy-1,4-naphthoquinone (5H-1,4NQ), 1,2-naphthoquinone (1,2-NQ), 1,4-naphthoquinone (1,4-NQ), copper(II), manganese(II), and iron (II and III). Both pure compounds and their mixtures show different patterns in DTT oxidation versus ROS generation. The quinones, known to oxidize DTT in the efficiency order of PQ > 5H-1,4NQ > 1,2-NQ > 1,4-NQ, show a different efficiency order (5H-1,4NQ > 1,2-NQ ≈ PQ > 1,4-NQ) in the ROS generation. Cu(II), a dominant metal in DTT oxidation, contributes almost negligibly to the ROS generation. Fe is mostly inactive in DTT oxidation, but shows synergistic effect in OH formation in the presence of other quinones (mixture/sum> 1.5). Ten ambient PM samples collected from an urban site were analyzed, and although DTT oxidation was significantly correlated with HO generation (Pearson's r = 0.91), no correlation was observed between DTT oxidation and OH formation. Our results show that measuring both DTT consumption and ROS generation in the DTT assay is important to incorporate the synergistic contribution from different aerosol components and to provide a more inclusive picture of the ROS activity of ambient PM.

We assessed the interactions among the particulate matter (PM) components in generating the reactive oxygen species (ROS) based on a dithiothreitol (DTT) assay. We started with the standard solutions of known redox-active substances, i.e., quinones (9,10-phenanthraquinone, 1,2-naphthoquinone, 1,4-naphthoquinone, and 5-hydroxy-1,4-naphthoquinone) and metals [Fe (II), Mn (II), and Cu (II)]. Both DTT consumption and hydroxyl radical (·OH) generation were measured in the DTT assay. The interactions of Fe were additive with quinones in DTT consumption but strongly synergistic in ·OH generation. Cu showed antagonistic interactions with quinones in both DTT consumption and ·OH generation. Mn interacted synergistically with quinones in DTT oxidation but antagonistically in ·OH generation. The nature of the interactions of these metals (Fe, Mn, and Cu) with ambient humic-like substances (HULIS) resembled that with quinones, although the intensity of interactions were weaker in DTT consumption than ·OH generation. Finally, we demonstrated that the DTT consumption capability of ambient PM can be well explained by HULIS, three transition metals (Fe, Mn, and Cu), and their interactions, but ·OH generation involves a contribution (∼50%) from additional compounds (aliphatic species or metals other than Fe, Mn, and Cu) present in the hydrophilic PM fraction. The study highlights the need to account for the interactions between organic compounds and metals, while apportioning the relative contributions of chemical components in the PM oxidative potential.

Reactive oxygen species (ROS) play a central role in aqueous-phase processing and health effects of atmospheric aerosols. Although hydroxyl radical (•OH) and hydrogen peroxide (H2O2) are regarded as major oxidants associated with secondary organic aerosols (SOA), the kinetics and reaction mechanisms of superoxide (O2 •–) formation are rarely quantified and poorly understood. Here, we demonstrate a dominant formation of O2 •– with molar yields of 0.01–0.03% from aqueous reactions of biogenic SOA generated by •OH photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of •OH and O2 •– formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, •OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflect a relative abundance of organic hydroperoxides and alcohols contained in SOA. These findings and mechanistic understanding have important implications on the atmospheric fate of SOA and particle-phase reactions of highly oxygenated organic molecules as well as oxidative stress upon respiratory deposition.

Carcinoma cells hijack the epithelial-mesenchymal transition (EMT) for tumor dissemination. Paired-related homeobox 1 (PRRX1) has been identified as a new EMT inducer. However, the function of PRRX1 in gastric cancer has not been elucidated. In this study, we observed that PRRX1 expression levels were upregulated and positively correlated with metastasis and EMT markers in human gastric cancer specimens. PRRX1 overexpression had distinct effects on the cell morphology, proliferation, migration and invasion of BGC823 and SGC7901 gastric cancer cells both in vitro and in xenografts. PRRX1 overexpression resulted in the regulation of the EMT molecular markers N-cadherin, E-cadherin and vimentin as well as the levels of intranuclear β-catenin and the Wnt/β-catenin target c-Myc. Furthermore, the inhibition of the Wnt/β-catenin pathway by XAV939 offset the effects of PRRX1 overexpression. These findings demonstrate that PRRX1 promotes EMT in gastric cancer cells through the activation of Wnt/β-catenin signaling and that PRRX1 upregulation is closely correlated with gastric cancer metastasis.

Peroxiredoxin 2 (Prdx2) is a member of the peroxiredoxin family, which is responsible for neutralizing reactive oxygen species. Prdx2 has been found to be elevated in several human cancer cells and tissues, including colorectal cancer (CRC), and it influences diverse cellular processes involving cells' survival, proliferation, and apoptosis, which suggests a possible role for Prdx2 in the maintenance of cancer cell. However, the mechanism by which Prdx2 modulates CRC cells' survival is unknown. The current study aimed to determine the effect of elevated Prdx2 on CRC cells and to further understand the underlying mechanisms. The results of this study showed that Prdx2 was upregulated in CRC tissues compared with the matched noncancer colorectal mucosa tissues and that Prdx2 expression was positively associated with tumor metastasis and the TNM stage. In the LoVo CRC cell line, Prdx2 was upregulated at both the RNA and protein levels compared with the normal FHC colorectal mucosa cell line. In addition, the LoVo CRC cell line was significantly more resistant to hydrogen peroxide (H₂O₂)-induced apoptosis because of a failure to activate pro-apoptotic pathways in contrast to Prdx2 knockdown cells. Suppression of Prdx2 using a lentiviral vector-mediated Prdx2-specific shRNA in the LoVo cell line restored H₂O₂ sensitivity. Our results suggested that Prdx2 has an essential role in regulating oxidation-induced apoptosis in CRC cells. Prdx2 may have potential as a therapeutic target in CRC.

We investigated the complexation state of atmospheric iron and copper and its impact on the oxidative potential (OP) of ambient PM2.5 (PM, particulate matter). A novel fractionation scheme was developed to segregate three different fractions of Fe and Cu present in ambient PM2.5: (i) complexed with hydrophobic organic compounds, (ii) complexed with hydrophilic organic compounds, and (iii) free or inorganic metal fraction. A solid phase extraction (C-18) column was used to separate these fractions. The fractionation scheme applied to the ambient PM2.5 samples collected from an urban site showed that up to 70–90% of water-soluble Fe and Cu were complexed with the organic compounds. The capability of the complexes of Fe­(II) and Cu­(II) with Suwanee river fulvic acid (SRFA), a proxy for the atmospheric organic compounds, to generate reactive oxygen species (ROS) (·OH and H2O2) in a surrogate lung fluid (SLF) was measured. The complex of Fe–SRFA showed a strong synergistic effect in the generation of ·OH in SLF, while that of Cu–SRFA showed an additive effect. Overall, our results indicate that organic complexation of the metals in ambient PM could significantly alter their OP and needs to be considered while assessing their health impacts.

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Respiratory deposition of secondary organic aerosols (SOA) and iron may lead to the generation of reactive oxygen species and free radicals in lung fluid to cause oxidative stress, but their underlying mechanism and formation kinetics are not well understood. Here we demonstrate substantial formation of organic radicals in surrogate lung fluid (SLF) by mixtures of Fe2+ and SOA generated from photooxidation of isoprene, α-terpineol, and toluene. The molar yields of organic radicals by SOA are measured to be 0.03–0.5% in SLF, which are 5–10 times higher than in water. We observe that Fe2+ enhances organic radical yields dramatically by a factor of 20–80, which can be attributed to Fe2+-facilitated decomposition of organic peroxides, in consistency with a positive correlation between peroxide contents and organic radical yields. Ascorbate mediates redox cycling of iron ions to sustain organic peroxide decomposition, as supported by kinetic modeling reproducing time- and concentration-dependence of organic radical formation as well as additional experiments observing the formation of Fe2+ and ascorbate radicals in mixtures of ascorbate and Fe3+. •OH and superoxide are found to be scavenged by antioxidants efficiently. These findings have implications on the role of organic radicals in oxidative damage and lipid peroxidation.