Chemical carcinogenesis follows a multistep process involving both mutation and increased cell proliferation. Oxidative stress can occur through overproduction of reactive oxygen and nitrogen species through either endogenous or exogenous insults. Important to carcinogenesis, the unregulated or prolonged production of cellular oxidants has been linked to mutation (induced by oxidant-induced DNA damage), as well as modification of gene expression. In particular, signal transduction pathways, including AP-1 and NFkappaB, are known to be activated by reactive oxygen species, and they lead to the transcription of genes involved in cell growth regulatory pathways. This review examines the evidence of cellular oxidants' involvement in the carcinogenesis process, and focuses on the mechanisms for production, cellular damage produced, and the role of signaling cascades by reactive oxygen species.

Carcinogenesis is a multistep process involving mutation and the subsequent selective clonal expansion of the mutated cell. Chemical and physical agents including those that induce reative oxygen species can induce and/or modulate this multistep process. Several modes of action by which carcinogens induce cancer have been identified, including through production of reactive oxygen species (ROS). Oxidative damage to cellular macromolecules can arise through overproduction of ROS and faulty antioxidant and/or DNA repair mechanisms. In addition, ROS can stimulate signal transduction pathways and lead to activation of key transcription factors such as Nrf2 and NF-kB. The resultant altered gene expression patterns evoked by ROS contribute to the carcinogenesis process. Recent evidence demonstrates an association between a number of single nucleotide polymorphisms (SNPs) in oxidative DNA repair genes and antioxidant genes with human cancer susceptibility. These aspects of ROS biology will be discussed in the context of their relationship to carcinogenesis.

Oxidative stress results when the balance between the production of reactive oxygen species (ROS) overrides the antioxidant capability of the target cell; oxidative damage from the interaction of reactive oxygen with critical cellular macromolecules may occur. ROS may interact with and modify cellular protein, lipid, and DNA, which results in altered target cell function. The accumulation of oxidative damage has been implicated in both acute and chronic cell injury including possible participation in the formation of cancer. Acute oxidative injury may produce selective cell death and a compensatory increase in cell proliferation. This stimulus may result in the formation of newly initiated preneoplastic cells and/or enhance the selective clonal expansion of latent initiated preneoplastic cells. Similarly, sublethal acute oxidative injury may produce unrepaired DNA damage and result in the formation of new mutations and, potentially, new initiated cells. In contrast, sustained chronic oxidative injury may lead to a nonlethal modification of normal cellular growth control mechanisms. Cellular oxidative stress can modify intercellular communication, protein kinase activity, membrane structure and function, and gene expression, and result in modulation of cell growth. We examined the role of oxidative stress as a possible mechanism by which nongenotoxic carcinogens may function. In studies with the selective mouse liver carcinogen dieldrin, a species-specific and dose-dependent decrease in liver antioxidant concentrations with a concomitant increase in ROS formation and oxidative damage was seen. This increase in oxidative stress correlated with an increase in hepatocyte DNA synthesis. Antioxidant supplementation prevented the dieldrin-induced cellular changes. Our findings suggest that the effect of nongenotoxic carcinogens (if they function through oxidative mechanisms) may be amplified in rodents but not in primates because of rodents' greater sensitivity to ROS. These results and findings reported by others support a potential role for oxidative-induced injury in the cancer process specifically during the promotion stage. Environ Health Perspect 1 06(Suppl 1):289-295 (1998). http.//ehpnetl.niehs.nih.gov/docs/1998/Suppl-1/ 289-295klaunig/abstract.html

Kupffer cells are resident macrophages of the liver and play an important role in its normal physiology and homeostasis as well as participating in the acute and chronic responses of the liver to toxic compounds. Activation of Kupffer cells directly or indirectly by toxic agents results in the release of an array of inflammatory mediators, growth factors, and reactive oxygen species. This activation appears to modulate acute hepatocyte injury as well as chronic liver responses including hepatic cancer. Understanding the role Kupffer cells play in these diverse responses is key to understanding mechanisms of liver injury. Idiosyncratic drug-induced liver disease results in morbidity and mortality, impacting severely on the development of new pharmacological agents. Modulation of the response of Kupffer cells by drugs has been suggested as a cause for the idiosyncratic response. Similarly, liver damage seen in chronic ethanol consumption appears to be modulated by Kupffer cell activation. More recent evidence has noted a contributory role of Kupffer cell activation in the process of hepatic carcinogenesis. Several nongenotoxic carcinogens, for example, activate Kupffer cells resulting in the release of cytokines and/or reactive oxygen species that induce hepatocyte cell proliferation and may enhance clonal expansion of preneoplastic cells leading to neoplasia. Kupffer cells therefore appear to play a central role in the hepatic response to toxic and carcinogenic agents. Taken together, the data presented in this symposium illustrate to the toxicologist the central role played by Kupffer cells in mediating hepatotoxicity.

Neuroimaging studies have begun to uncover the neural substrates of cancer and treatment-related cognitive dysfunction, but the time course of these changes in the years following chemotherapy is unclear. This study analyzed multimodality 3T MRI scans to examine the structural and functional effects of chemotherapy and post-chemotherapy interval (PCI) in a cohort of breast cancer survivors (BCS; n=24; PCI mean 6, range 3–10 y) relative to age- and education-matched healthy controls (HC; n=23). Assessments included voxel-based morphometry (VBM) for gray matter density (GMD) and fMRI for activation profile during a 3-back working memory task. The relationships between brain regions associated with PCI and neuropsychological performance, self-reported cognition, and oxidative and direct DNA damage as measured in peripheral lymphocytes were assessed in secondary analyses. PCI was positively associated with GMD and activation on fMRI in the right anterior frontal region (Brodmann Areas 9 and 10) independent of participant age. GMD in this region was also positively correlated with global neuropsychological function. Memory dysfunction, cognitive complaints, and oxidative DNA damage were increased in BCS compared to HC. Imaging results indicated lower fMRI activation in several regions in the BCS group. BCS also had lower GMD than HC in several regions, and in these regions GMD was inversely related to oxidative DNA damage and learning and memory neuropsychological domain scores. This is the first study to show structural and functional effects of PCI and to relate oxidative DNA damage to brain alterations in BCS. The relationship between neuroimaging and cognitive function indicates the potential clinical relevance of these findings. The relationship with oxidative DNA damage provides a mechanistic clue warranting further investigation.

Ca2+ accumulates in the nucleus and DNA undergoes enzymatic cleavage into internucleosome-length fragments before acetaminophen and dimethylnitrosamine produce hepatic necrosis in vivo and toxic cell death in vitro. However, Ca(2+)-endonuclease fragmentation of DNA is characteristic of apoptosis, a type of cell death considered biochemically and functionally distinct from toxic cell death. The present studies investigate DNA fragmentation as a critical event in toxic cell death by testing whether the Ca(2+)-calmodulin antagonist chlorpromazine and the Ca2+ channel blocker verapamil prevent acetaminophen-induced hepatic necrosis by inhibiting Ca2+ deregulation and DNA damage. Acetaminophen overdose in mice produced accumulation of Ca2+ in the nucleus (358% of control) and fragmentation of DNA (250% of control) by 6 h, with peak release of ALT occurring at 12-24 h (38,000 U/l). Pretreatment with chlorpromazine prevented increases in nuclear Ca2+ and DNA fragmentation and nearly abolished biochemical evidence of toxic cell death. Verapamil pretreatment also decreased Ca2+ accumulation and DNA damage while attenuating liver injury. The Ca2+ antagonists did not protect against toxic cell death through hypothermia because neither produced the delay in toxicity that is customarily associated with hypothermia. Nor did chlorpromazine or verapamil protect through inhibiting acetaminophen bioactivation. Chlorpromazine failed to diminish glutathione depletion in whole liver and isolated nuclei. Verapamil (250 microM) also failed to alter glutathione depletion in whole liver and had no effect on acetaminophen-glutathione adduct formation by mouse liver microsomes and by cultured mouse hepatocytes. Collectively, these results support the hypothesis that Ca(2+)-induced DNA fragmentation plays a significant role in cell necrosis produced by acetaminophen and may contribute to toxic cell death caused by other alkylating hepatotoxins.

Recent studies have examined and demonstrated the potential cancer chemopreventive activity of freeze-dried berries including strawberries and black raspberries. Although ellagic acid, an abundant component in these berries, has been shown to inhibit carcinogenesis both in vivo and in vitro, several studies have reported that other compounds in the berries may also contribute to the observed inhibitory effect. In the present study, freeze-dried strawberries (Fragara ananassa, FA) or black raspberries (Rubus ursinus, RU) were extracted, partitioned and chromatographed into several fractions (FA-F001, FA-F003, FA-F004, FA-F005, FA-DM, FA-ME from strawberries and RU-F001, RU-F003, RU-F004, RU-F005, RU-DM, RU-ME from black raspberries). These extracts, along with ellagic acid, were analyzed for anti-transformation activity in the Syrian hamster embryo (SHE) cell transformation model. None of the extracts nor ellagic acid by themselves produced an increase in morphological transformation. For assessment of chemopreventive activity, SHE cells were treated with each agent and benzo[a]pyrene (B[a]P) for 7 days. Ellagic acid, FA-ME and RU-ME fractions produced a dose-dependent decrease in transformation compared with B[a]P treatment only, while other fractions failed to induce a significant decrease. Ellagic acid, FA-ME and RU-ME were further examined using a 24 h co-treatment with B[a]P or a 6 day treatment following 24 h with B[a]P. Ellagic acid showed inhibitory ability in both protocols. FA-ME and RU-ME significantly reduced B[a]P-induced transformation only when co-treated with B[a]P for 24 h. These results suggest that a methanol extract from strawberries and black raspberries may display chemopreventive activity. The possible mechanism by which these methanol fractions (FA-ME, RU-ME) inhibited cell transformation appear to involve interference of uptake, activation, detoxification of B[a]P and/or intervention of DNA binding and DNA repair.

SummaryBackground and objectives Obesity precedes and is strongly linked to the development of type 2 diabetic nephropathy in most patients, yet little is known about the effects of weight reduction on this disease. This study aimed to establish proof of concept for the hypothesis that weight reduction ameliorates diabetic nephropathy.Design, setting, participants, & measurements Six obese individuals with advanced diabetic nephropathy (estimated GFR ,40 ml/min per 1.73 m 2 , urine albumin excretion .30 mg/d) currently taking a reninaldosterone axis inhibitor underwent a 12-week very low calorie ketogenic weight reduction diet with encouragement of exercise between March and September 2012. Albuminuria and other parameters of kidney health were the main outcome measures.Results There was a 12% reduction in weight (median 118.5 versus 104.3 kg, P=0.03). The intervention was associated with a 36% reduction in albuminuria that did not reach statistical significance (2124 versus 1366 mg/24 h, P=0.08) and significant reductions in the filtration markers serum creatinine (3.54 versus 3.13 mg/dl, P,0.05) and cystatin C (2.79 versus 2.46 mg/l, P,0.05). Improvements were also noted for the diabetes markers fasting glucose (166 versus 131 mg/dl, P,0.05), fasting insulin (26.9 versus 10.4 mU/ml, P,0.05), and insulin resistance (9.6 versus 4.2, P=0.03). Physical function, general health, and the number of diabetes medications also showed statistically significant signs of improvement.Conclusions After a short-term intensive weight reduction intervention in patients with advanced diabetic nephropathy, improvements were observed in markers of glomerular filtration, diabetes status, and risk factors for kidney disease progression, as well as other general indicators of health and well-being.