Nox4-dependent ROS modulation by amino endoperoxides to induce apoptosis in cancer cells

Redox signaling-mediated tumor extracellular matrix remodeling: pleiotropic regulatory mechanisms

BACKGROUND

The extracellular matrix (ECM), a fundamental constituent of all tissues and organs, is crucial for shaping the tumor microenvironment. Dysregulation of ECM remodeling has been closely linked to tumor initiation and progression, where specific signaling pathways, including redox signaling, play essential roles. Reactive oxygen species (ROS) are risk factors for carcinogenesis whose excess can facilitate the oxidative damage of biomacromolecules, such as DNA and proteins. Emerging evidence suggests that redox effects can aid the modification, stimulation, and degradation of ECM, thus affecting ECM remodeling. These alterations in both the density and components of the ECM subsequently act as critical drivers for tumorigenesis. In this review, we provide an overview of the functions and primary traits of the ECM, and it delves into our current understanding of how redox reactions participate in ECM remodeling during cancer progression. We also discuss the opportunities and challenges presented by clinical strategies targeting redox-controlled ECM remodeling to overcome cancer.

CONCLUSIONS

The redox-mediated ECM remodeling contributes importantly to tumor survival, progression, metastasis, and poor prognosis. A comprehensive investigation of the concrete mechanism of redox-mediated tumor ECM remodeling and the combination usage of redox-targeted drugs with existing treatment means may reveal new therapeutic strategy for future antitumor therapies.

Ginkgo biloba extract protects human neuroblastoma SH-SY5Y cells against oxidative glutamate toxicity by activating redoxosome-p66Shc

Ginkgo biloba extract (GBE), a traditional Chinese herbal medicine component, is widely used to alleviate symptoms of neurodegenerative diseases. It has been confirmed that GBE exerts its pharmacological effect mainly due to its antioxidant activity; however, the molecular mechanism responsible for this effect remains unclear. The aim of the present study was to investigate the detailed mechanism of GBE, the main component of Gingko biloba dropping medicine, against oxidative glutamate toxicity in human neuroblastoma SH-SY5Y cells. The SH-SY5Y cells were untreated or pretreated with GBE followed by glutamate stimulation. Cell viability was assessed using an MTT assay. In addition, oxidative stress indexes, including intracellular ROS generation and NADPH oxidase and caspase activity, were also measured. The protein expression of key signaling factors involved in the redoxosome-p66Shc pathway was evaluated to elucidate the neuroprotective effect of GBE. The results showed that GBE treatment significantly attenuated the glutamate-induced cytotoxicity in SH-SY5Y cells by suppressing oxidative stress. A mechanical study revealed that redoxosome-p66Shc activation was associated with glutamate-induced cytotoxicity, which caused mitochondrial dysfunction and cell death. Interestingly, GBE treatment attenuated the activation of redoxosome-p66Shc in a dose-dependent manner, which suggested that the protective effect of GBE on SH-SY5Y cells against oxidative glutamate toxicity may be mediated by the modulation of redoxosome-p66Shc signaling. The current findings contribute to a better understanding of the therapeutic effect of GBE and indicate that redoxosome-p66Shc signaling might be a novel therapeutic target in the prevention and/or treatment of neurodegenerative diseases.

ROS-Mediated Apoptosis and Autophagy in Ovarian Cancer Cells Treated with Peanut-Shaped Gold Nanoparticles

Background

Even with considerable improvement in treatment of epithelial ovarian cancer achieved in recent years, an increasing chemotherapy resistance and disease 5-year relapse is recorded for a majority part of patients that encourages the search for better therapeutic options. Gold nanoparticles (Au NPs) due to plethora of unique physiochemical features are thoroughly tested as drug delivery, radiosensitizers, as well as photothermal and photodynamic therapy agents. Importantly, due to highly controlled synthesis, it is possible to obtain nanomaterials with directed size and shape.

Methods

In this work, we developed novel elongated-type gold nanoparticles in the shape of nanopeanuts (AuP NPs) and investigated their cytotoxic potential against ovarian cancer cells SKOV-3 using colorimetric and fluorimetric methods, Western blot, flow cytometry, and fluorescence microscopy.

Results

Peanut-shaped gold nanoparticles showed high anti-cancer activity in vitro against SKOV-3 cells at doses of 1–5 ng/mL upon 72 hours treatment. We demonstrate that AuP NPs decrease the viability and proliferation capability of ovarian cancer cells by triggering cell apoptosis and autophagy, as evidenced by flow cytometry and Western blot analyses. The overproduction of reactive oxygen species (ROS) was noted to be a critical mediator of AuP NPs-mediated cell death.

Conclusion

These data indicate that gold nanopeanuts might be developed as nanotherapeutics against ovarian cancer.

Nox4 regulates InsP3 receptor-dependent Ca2+ release into mitochondria to promote cell survival

Cells subjected to environmental stresses undergo regulated cell death (RCD) when homeostatic programs fail to maintain viability. A major mechanism of RCD is the excessive calcium loading of mitochondria and consequent triggering of the mitochondrial permeability transition (mPT), which is especially important in post-mitotic cells such as cardiomyocytes and neurons. Here, we show that stress-induced upregulation of the ROS-generating protein Nox4 at the ER-mitochondria contact sites (MAMs) is a pro-survival mechanism that inhibits calcium transfer through InsP3 receptors (InsP3 R). Nox4 mediates redox signaling at the MAM of stressed cells to augment Akt-dependent phosphorylation of InsP3 R, thereby inhibiting calcium flux and mPT-dependent necrosis. In hearts subjected to ischemia-reperfusion, Nox4 limits infarct size through this mechanism. These results uncover a hitherto unrecognized stress pathway, whereby a ROS-generating protein mediates pro-survival effects through spatially confined signaling at the MAM to regulate ER to mitochondria calcium flux and triggering of the mPT.

Pathophysiologic Mechanisms of Concussion, Development of Chronic Traumatic Encephalopathy, and Emerging Diagnostics: A Narrative Review

Pathophysiological mechanisms and cascades take place after a mild traumatic brain injury (mTBI) that can cause long-term sequelae, including chronic traumatic encephalopathy in patients with multiple concurrent TBIs. As diagnostic imaging has become more advanced, microanatomical changes present after mTBI may now be more readily visible. In this narrative review, the authors discuss emerging diagnostics and findings in mTBI through advanced imaging, electroencephalograms, neurophysiologic processes, Q2 biochemical markers, and clinical tissue tests in an effort to help osteopathic physicians to understand, diagnose, and manage the pathophysiology behind mTBI, which is increasingly prevalent in the United States.

NOX4 activation is involved in ROS-dependent Jurkat T-cell death induced by Entamoeba histolytica

AIMS

Entamoeba histolytica can induce host cell death through induction of various intracellular signalling pathways. The responses triggered by E. histolytica are closely associated with tissue pathogenesis and immune evasion. Although E. histolytica can induce reactive oxygen species (ROS) in host cells, which NADPH oxidase (NOX) isoform contributes to amoeba-triggered Jurkat T-cell death is unclear. In this study, we investigated the signalling role of NOX4-derived ROS in E. histolytica-induced Jurkat T-cell death process.

METHODS AND RESULTS

In resting-state Jurkat T cells, NOX4 is strongly expressed. When Jurkat T cells were incubated with live E. histolytica trophozoites, intracellular ROS was significantly increased compared to cells incubated with medium alone. E. histolytica-induced ROS production was inhibited by pretreating Jurkat T cells with a NOX inhibitor. In addition, pretreating Jurkat T cells with a NOX inhibitor (Diphenyleneiodonium chloride) effectively blocked E. histolytica-induced phosphatidylserine (PS) exposure and DNA fragmentation of host cells. Moreover, siRNA-mediated knockdown of NOX4 protein expression in Jurkat T cells prevented E. histolytica-induced ROS generation and DNA fragmentation.

CONCLUSION

These results suggest that NOX4 has a critical role in ROS-dependent cell death process in Jurkat T cells induced by E. histolytica.

NOX4 inhibition protects enteric glial cells against Clostridium difficile toxin B toxicity via attenuating oxidative and Endoplasmic reticulum stresses

Enteric glial cells (EGCs), one main cell population of the enteric nervous system (ENS), play a major role in regulating intestinal barrier function. Clostridium difficile toxin B (TcdB) is the major virulence factor produced by C. difficile and estimated to be toxic to EGCs by inducing cell death, cell cycle arrest, and inflammatory cytokine production; however, the detailed mechanism for such effect is still unclear. In this study, we further evaluated the toxic effect of TcdB on EGCs and the involvement of NADPH oxidases in such process using the rat-transformed EGCs (CRL-2690). The results showed that NOX4 was activated by TcdB in EGCs and functioned as the major factor causing cytotoxicity and cell apoptosis. Mechanically, NOX4-generated H₂O₂ was the inducer of oxidative stress, Ca²⁺ homeostasis disorder, and ER stress in EGCs upon TcdB treatment, and NOX4 inhibition protected EGCs against TcdB toxicity via attenuating these dysfunctions. These findings contribute to our understanding of the mechanism by which TcdB affects EGCs and suggest the potential value of NOX4 inhibition for treatment against C. difficile infection.

Assessment of a bifendate derivative bearing a 6,7-dihydro-dibenzo[c,e]azepine scaffold as a potential anti-metastatic agent

Multidrug resistance (MDR) and metastasis are major causes of mortality in patients with cancer. We recently reported a bifendate derivative bearing a dibenzo[c,e]azepine scaffold (4i) as a P-gp and BCRP-medicated MDR reversal agent. As a continuation of the previous research, its ability to inhibit cancer metastasis was investigated in MDA-MB-231 cells in the present work. Wound-healing and chamber migration assays showed that 4i could significantly attenuate the migration of MDA-MB-231 cells. Additionally, 4i obviously suppressed the invasive activity of MDA-MB-231 cells, thus displaying potential anti-metastasis activity. Preliminary mechanism studies indicated that the anti-metastasis activity of 4i was associated with the inhibitory effect on the activity and expression of MMP-2 and MMP-9. These results, together with the previous findings, suggest that compound 4i could be a promising lead for the development of novel anti-cancer agents with anti-MDR and metastatic activities.

Renoprotective effect of topiroxostat via antioxidant activity in puromycin aminonucleoside nephrosis rats

Topiroxostat is a novel inhibitor of xanthine oxidase, and is postulated to exert a renoprotective effect. Puromycin aminonucleoside nephrosis (PAN) is a rat model of minimal change nephrotic syndrome. In this study, we examined whether topiroxostat ameliorates the kidney injury in PAN rats that was induced by a single intraperitoneal injection of PA (100 mg/kg body weight). Rats were divided into four groups: control rats, PAN rats, control rats treated with topiroxostat (1.0 mg/kg/day), and PAN rats treated with topiroxostat. Topiroxostat significantly reduced the amount of uric acid in the kidney cortex, while serum UA concentration remained unaffected by this treatment. Urinary protein excretion decreased significantly on day 10 in PAN rats upon topiroxostat treatment. Podocyte injury in PAN rats, as indicated by the reduction in WT‐1‐positive cell numbers and podocin immunoreactivity and foot process effacement, was partially yet significantly alleviated with topiroxostat treatment. In the kidney cortex, the increase in oxidative stress markers such as nitrotyrosine and 8‐hydroxy‐2‐deoxyguanosine (8‐OHdG) and the enhanced expressions of xanthine oxidase and NADPH oxidase 4 (NOX4) in PAN rats were significantly ameliorated by topiroxostat. Using cultured podocytes NOX4 expression was upregulated by adding 12 mg/dL UA into the culture medium. These results suggest that topiroxostat ameliorates proteinuria and kidney injury in PAN rats by lowering oxidative stress and tissue UA concentration. The renoprotective effects of topiroxostat could be attributed to its potential to inhibit xanthine oxidase and NOX4 in concert with suppression of intracellular UA production.

Microenvironment-Cell Nucleus Relationship in the Context of Oxidative Stress

The microenvironment is a source of reactive oxygen species (ROS) that influence cell phenotype and tissue homeostasis. The impact of ROS on redox pathways as well as directly on epigenetic mechanisms and the DNA illustrate communication with the cell nucleus. Changes in gene transcription related to redox conditions also influence the content and structure of the extracellular matrix. However, the importance of microenvironmental ROS for normal progression through life and disease development still needs to be thoroughly understood. We illustrate how different ROS concentration levels trigger various intracellular pathways linked to nuclear functions and determine processes necessary for the differentiation of stem cells. The abnormal predominance of ROS that leads to oxidative stress is emphasized in light of its impact on aging and diseases related to aging. These phenomena are discussed in the context of the possible contribution of extracellular ROS via direct diffusion into cells responsible for organ function, but also via an impact on stromal cells that triggers extracellular modifications and influences mechanotransduction. Finally, we argue that organs-on-a-chip with controlled microenvironmental conditions can help thoroughly grasp whether ROS production is readily a cause or a consequence of certain disorders, and better understand the concentration levels of extracellular ROS that are necessary to induce a switch in phenotype.

Overexpression of NOX4 predicts poor prognosis and promotes tumor progression in human colorectal cancer

NADPH oxidase 4 (NOX4), a major source of reactive oxygen species (ROS) production, has been increasingly reported to be involved in tumorigenesis and/or tumor progression, but limited data are available regarding the role of NOX4 in colorectal carcinoma (CRC). We retrieved six independent investigations from Oncomine database and found that NOX4 is highly expressed in CRC tissues compared with corresponding normal controls. Similar results were also found in clinical specimens at both mRNA and protein levels. Immunohistochemical analysis indicated that NOX4 overexpression was highly correlated with T classification, N classification, distant metastasis, and poor prognosis of CRC patients, which was also confirmed by GSE14333 and GSE17536 datasets from the Gene Expression Omnibus. Furthermore, we demonstrated that when NOX4 expression was knocked down by siRNAs, cell proliferation, cell-cycle and apoptosis, migration and invasion were significantly altered in CRC cell lines HCT116 and LOVO. Meanwhile, NOX4 promoted cancer cell proliferation and apoptosis, migration and invasion by regulating the expression of relevant genes. By these approaches we aim to elucidate NOX4 may be a reliable prognostic factor or therapeutic target in CRC.

NOX4-driven ROS formation regulates proliferation and apoptosis of gastric cancer cells through the GLI1 pathway

NADPH Oxidase 4 (NOX4), a member of the NOX family, has emerged as a significant source of reactive oxygen species, playing an important role in tumor cell proliferation, apoptosis, and other physiological processes. However, the potential function of NOX4 in gastric cancer (GC) cell proliferation is yet unknown. The aim of this study was to illustrate whether NOX4 plays a role in regulating gastric cancer cell growth. First, the clinical information from 90 patients was utilized to explore the clinical value of NOX4 as a predictive tool for tumor size and prognosis. Results showed that NOX4 expression was correlated with tumor size and prognosis. In vitro assays confirmed that knockdown of NOX4 expression blocked cell proliferation and the expression of Cyclin D1, BAX, and so on. Interestingly, NOX4 promoted cell proliferation via activation of the GLI1 pathway. GLI1, a well-known transcription factor in the Hedgehog signaling pathway, was overexpressed to test whether NOX4 activates downstream signaling via GLI1. Overexpression of GLI1 reversed the inhibition of proliferation induced by NOX4 knockdown. In addition, overexpression of NOX4 increased GLI1 expression, and depletion of GLI1 expression decreased the effects induced by NOX4 overexpression. Further, ROS generated by NOX4 was required for GLI1 expression, as shown by use of the ROS inhibitor, diphenylene iodonium (DPI). In summary, the findings indicate that NOX4 plays an important role in gastric cancer cell growth and apoptosis through the generation of ROS and subsequent activation of GLI1 signaling. Hence, the targeting of NOX4 may be an attractive therapeutic strategy for blocking gastric cancer cell proliferation.

Redox regulation in tumor cell epithelial-mesenchymal transition: molecular basis and therapeutic strategy

Epithelial–mesenchymal transition (EMT) is recognized as a driving force of cancer cell metastasis and drug resistance, two leading causes of cancer recurrence and cancer-related death. It is, therefore, logical in cancer therapy to target the EMT switch to prevent such cancer metastasis and recurrence. Previous reports have indicated that growth factors (such as epidermal growth factor and fibroblast growth factor) and cytokines (such as the transforming growth factor beta (TGF-β) family) are major stimulators of EMT. However, the mechanisms underlying EMT initiation and progression remain unclear. Recently, emerging evidence has suggested that reactive oxygen species (ROS), important cellular secondary messengers involved in diverse biological events in cancer cells, play essential roles in the EMT process in cancer cells by regulating extracellular matrix (ECM) remodeling, cytoskeleton remodeling, cell–cell junctions, and cell mobility. Thus, targeting EMT by manipulating the intracellular redox status may hold promise for cancer therapy. Herein, we will address recent advances in redox biology involved in the EMT process in cancer cells, which will contribute to the development of novel therapeutic strategies by targeting redox-regulated EMT for cancer treatment.

ANGPTL4 T266M variant is associated with reduced cancer invasiveness

Angiopoietin-like 4 (ANGPTL4) is a secretory protein that can be cleaved to form an N-terminal and a C-terminal protein. Studies performed thus far have linked ANGPTL4 to several cancer-related and metabolic processes. Notably, several point mutations in the C-terminal ANGPTL4 (cANGPTL4) have been reported, although no studies have been performed that ascribed these mutations to cancer-related and metabolic processes. In this study, we compared the characteristics of tumors with and without wild-type (wt) cANGPTL4 and tumors with cANGPTL4 bearing the T266M mutation (T266M cANGPTL4). We found that T266M cANGPTL4 bound to integrin α5β1 with a reduced affinity compared to wt, leading to weaker activation of downstream signaling molecules. The mutant tumors exhibited impaired proliferation, anoikis resistance, and migratory capability and had reduced adenylate energy charge. Further investigations also revealed that cANGPTL4 regulated the expression of Glut2. These findings may explain the differences in the tumor characteristics and energy metabolism observed with the cANGPTL4 T266M mutation compared to tumors without the mutation.

Cancer Progression Prediction Using Gene Interaction Regularized Elastic Net

Different types of genomic aberration may simultaneously contribute to tumorigenesis. To obtain a more accurate prognostic assessment to guide therapeutic regimen choice for cancer patients, the heterogeneous multi-omics data should be integrated harmoniously, which can often be difficult. For this purpose, we propose a Gene Interaction Regularized Elastic Net (GIREN) model that predicts clinical outcome by integrating multiple data types. GIREN conveniently embraces both gene measurements and gene-gene interaction information under an elastic net formulation, enforcing structure sparsity, and the "grouping effect" in solution to select the discriminate features with prognostic value. An iterative gradient descent algorithm is also developed to solve the model with regularized optimization. GIREN was applied to human ovarian cancer and breast cancer datasets obtained from The Cancer Genome Atlas, respectively. Result shows that, the proposed GIREN algorithm obtained more accurate and robust performance over competing algorithms (LASSO, Elastic Net, and Semi-supervised PCA, with or without average pathway expression features) in predicting cancer progression on both two datasets in terms of median area under curve (AUC) and interquartile range (IQR), suggesting a promising direction for more effective integration of gene measurement and gene interaction information.

Syntheses of 7-dehydrocholesterol peroxides and their improved anticancer activity and selectivity over ergosterol peroxide

Three 7-dehydrocholesterol peroxides were photochemically prepared and their anticancer activity was studied.

MicroRNAs associated with the efficacy of photodynamic therapy in biliary tract cancer cell lines

Photodynamic therapy (PDT) is a palliative treatment option for unresectable hilar biliary tract cancer (BTC) showing a considerable benefit for survival and quality of life with few side effects. Currently, factors determining the cellular response of BTC cells towards PDT are unknown. Due to their multifaceted nature, microRNAs (miRs) are a promising analyte to investigate the cellular mechanisms following PDT. For two photosensitizers, Photofrin® and Foscan®, the phototoxicity was investigated in eight BTC cell lines. Each cell line (untreated) was profiled for expression of n=754 miRs using TaqMan® Array Human MicroRNA Cards. Statistical analysis and bioinformatic tools were used to identify miRs associated with PDT efficiency and their putative targets, respectively. Twenty miRs correlated significantly with either high or low PDT efficiency. PDT was particularly effective in cells with high levels of clustered miRs 25-93*-106b and (in case of miR-106b) a phenotype characterized by high expression of the mesenchymal marker vimentin and high proliferation (cyclinD1 and Ki67 expression). Insensitivity towards PDT was associated with high miR-200 family expression and (for miR-cluster 200a/b-429) expression of differentiation markers Ck19 and Ck8/18. Predicted and validated downstream targets indicate plausible involvement of miRs 20a*, 25, 93*, 130a, 141, 200a, 200c and 203 in response mechanisms to PDT, suggesting that targeting these miRs could improve susceptibility to PDT in insensitive cell lines. Taken together, the miRNome pattern may provide a novel tool for predicting the efficiency of PDT and-following appropriate functional verification-may subsequently allow for optimization of the PDT protocol.

Smad3/Nox4-mediated mitochondrial dysfunction plays a crucial role in puromycin aminonucleoside-induced podocyte damage

Podocyte depletion due to apoptosis is the key hallmark of proteinuric kidney disease progression. Recently, several studies reported that mitochondrial (mt) dysfunction is involved in podocyte injury, while the underlying molecular mechanisms remain elusive. This study investigated the potential proximal signaling related to in vitro and in vivo mitochondrial dysfunction in a puromycin aminonucleoside (PA)-induced podocyte injury model. PA time- and dose-dependently resulted in cultured mouse podocyte damage, presenting with an increase of apoptotic cells and induction of activated caspase3/9. PA also caused mitochondrial damage and dysfunction based on the downregulation of the mtDNA level, decrease of transcriptional factors mtTfa and Nrf-1, decrease of CoxI, II and IV, and reduction of the oxygen consumption level and mitochondrial membrane potential level as well as excessive production of cellular ROS. Additionally, antioxidant MnSOD and catalase levels were decreased in mitochondrial fractions, and reduction of complex I and IV activity was also observed in PA-stimulated podocytes. Furthermore, an obvious translocation of p-Smad3 from the cytosol to nuclei and induction of mitochondrial Nox4 were detected following PA application. The PA-induced shift of cytochrome c was observed from mitochondria to the cytoplasm. Induction of Nox4 by PA administration was significantly repressed by Smad3-shRNA, while Nox4-shRNA showed no effect on PA-induced p-Smad3 activation. Notably, both Smad3 and Nox4 silencing significantly prevented the reduction of the mtDNA level, restored mitochondrial function, and decreased cellular apoptosis in PA-stimulated podocytes. A similar mitochondrial dysfunction was obtained in a PA-injected nephropathy rat, which was effectively inhibited by treatment with the antiproteinuric drug prednisone. In addition, Dab2 knockdown decreased albumin uptake and influx whereas it showed no effect on cellular apoptosis in PA-stimulated podocytes. In conclusion, our findings demonstrated that Smad3-Nox4 axis-mediated mitochondrial dysfunction is involved in PA-induced podocyte damage likely via increasing ROS generation and activating the cytochrome c-caspase9-caspase3 apoptotic signaling pathway. Dab2 may be required for the increased permeability of podocytes following injury.

Differential effect of artemisinin against cancer cell lines

The present study aims at defining the differential cytotoxicity effect of artemisinin toward P815 (murin mastocytoma) and BSR (kidney adenocarcinoma of hamster) cell lines. Cytotoxicity was measured by the growth inhibition using MTT assay. These in vitro cytotoxicity studies were complemented by the determination of apoptotic DNA fragmentation and Annexin V- streptavidin-FITC assay. Furthermore, we examined the in vitro synergism between artemisinin and the chemotherapeutic drug, vincristin. The in vivo study was investigated using the DBA2/P815 (H2d) mouse model. While artemisinin acted on both tumor cell lines, P815 was much more sensitive to this drug than BSR cells, as revealed by the respective IC50 values (12 µM for P815 and 52 µM for BSR cells). On another hand, and interestingly, apoptosis was induced in P815 but not induced in BSR. These data, reveal an interesting differential cytotoxic effect, suggesting the existence of different molecular interactions between artemisinin and the studied cell lines. In vivo, our results clearly showed that the oral administration of artemisinin inhibited solid tumor development. Our study demonstrates that artemisinin caused differential cytotoxic effects depending not only on the concentration and time of exposure but also on the target cells.

sp3 C–H oxidation by remote H-radical shift with oxygen- and nitrogen-radicals: a recent update

This review updates on recent advances in aliphatic sp³ C–H bond oxidation by remote H-radical abstraction with oxygen- and nitrogen-radicals classifying by the type of the radical precursors.

Copper-catalyzed aerobic aliphatic C-H oxygenation with hydroperoxides

We report herein Cu-catalyzed aerobic oxygenation of aliphatic C–H bonds with hydroperoxides, which proceeds by 1,5-H radical shift of putative oxygen-centered radicals (O-radicals) derived from hydroperoxides followed by trapping of the resulting carbon-centered radicals with molecular oxygen.