Nanotechnology is a rapidly developing field in the 21(st) century, and the commercial use of nanomaterials for novel applications is increasing exponentially. To date, the scientific basis for the cytotoxicity and genotoxicity of most manufactured nanomaterials are not understood. The mechanisms underlying the toxicity of nanomaterials have recently been studied intensively. An important mechanism of nanotoxicity is the generation of reactive oxygen species (ROS). Overproduction of ROS can induce oxidative stress, resulting in cells failing to maintain normal physiological redox-regulated functions. This in turn leads to DNA damage, unregulated cell signaling, change in cell motility, cytotoxicity, apoptosis, and cancer initiation. There are critical determinants that can affect the generation of ROS. These critical determinants, discussed briefly here, include: size, shape, particle surface, surface positive charges, surface-containing groups, particle dissolution, metal ion release from nanometals and nanometal oxides, UV light activation, aggregation, mode of interaction with cells, inflammation, and pH of the medium.

Pyrrolizidine alkaloid-containing plants are widely distributed in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Because of their abundance and potent toxicities, the mechanisms by which pyrrolizidine alkaloids induce genotoxicities, particularly carcinogenicity, were extensively studied for several decades but not exclusively elucidated until recently. To date, the pyrrolizidine alkaloid-induced genotoxicities were revealed to be elicited by the hepatic metabolism of these naturally occurring toxins. In this review, we present updated information on the metabolism, metabolizing enzymes, and the mechanisms by which pyrrolizidine alkaloids exert genotoxicity and tumorigenicity.

Ginkgo biloba leave extract is among the most widely sold herbal dietary supplements in the United States. Its purported biological effects include: scavenging free radical; lowering oxidative stress; reducing neural damages, reducing platelets aggregation; anti-inflammation; anti-tumor activities; and anti-aging. Clinically, it has been prescribed to treat CNS disorders such as Alzheimer's disease and cognitive deficits. It exerts allergy and changes in bleeding time. While its mutagenicity or carcinogenic activity has not been reported, its components, quercetin, kaempferol and rutin have been shown to be genotoxic. There are no standards or guidelines regulating the constituent components of Ginkgo biloba leave extract nor are exposure limits imposed. Safety evaluation of Ginkgo biloba leave extract is being conducted by the U.S. National Toxicology Program.

Pyrrolizidine alkaloid-containing plants are the most common poisonous plants affecting livestock, wildlife, and humans. The U.S. National Toxicology Program (NTP) classified riddelliine, a tumorigenic pyrrolizidine alkaloid, as "reasonably anticipated to be a human carcinogen" in the NTP 12th Report on Carcinogens in 2011. We previously determined that four DNA adducts were formed in rats dosed with riddelliine. The structures of the four DNA adducts were elucidated as (i) a pair of epimers of 7-hydroxy-9-(deoxyguanosin-N(2)-yl)dehydrosupinidine adducts (termed as DHP-dG-3 and DHP-dG-4) as the predominant adducts; and (ii) a pair of epimers of 7-hydroxy-9-(deoxyadenosin-N(6)-yl)dehydrosupinidine adducts (termed as DHP-dA-3 and DHP-dA-4 adducts). In this study, we selected a nontumorigenic pyrrolizidine alkaloid, platyphylliine, a pyrrolizidine alkaloid N-oxide, riddelliine N-oxide, and nine tumorigenic pyrrolizidine alkaloids (riddelliine, retrorsine, monocrotaline, lycopsamine, retronecine, lasiocarpine, heliotrine, clivorine, and senkirkine) for study in animals. Seven of the nine tumorigenic pyrrolizidine alkaloids, with the exception of lycopsamine and retronecine, are liver carcinogens. At 8-10 weeks of age, female F344 rats were orally gavaged for 3 consecutive days with 4.5 and 24 μmol/kg body weight test article in 0.5 mL of 10% DMSO in water. Twenty-four hours after the last dose, the rats were sacrificed, livers were removed, and liver DNA was isolated for DNA adduct analysis. DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4 adducts were formed in the liver of rats treated with the individual seven hepatocarcinogenic pyrrolizidine alkaloids and riddelliine N-oxide. These DNA adducts were not formed in the liver of rats administered retronecine, the nontumorigenic pyrrolizidine alkaloid, platyphylliine, or vehicle control. These results indicate that this set of DNA adducts, DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4, is a common biological biomarker of pyrrolizidine alkaloid-induced liver tumor formation. To date, this is the first finding that a set of exogenous DNA adducts are commonly formed from a series of tumorigenic xenobiotics.

Pyrrolizidine alkaloid-containing plants are widespread in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Pyrrolizidine alkaloids require metabolism to exert their genotoxicity and tumorigenicity. We have determined that the metabolism of a series of tumorigenic pyrrolizidine alkaloids in vitro or in vivo generates a common set of (+/-)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts that are responsible for tumor induction. The identification and quantitation of the DHP-derived DNA adducts formed in vivo and in vitro were accomplished previously by (32)P-postlabeling/HPLC methodology. In this article, we report the development of a sensitive and specific liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ES-MS/MS) method to detect DHP-derived DNA adducts formed in vitro and in vivo. The method is used to quantify the levels of DHP-2'-deoxyguanosine (dG) and DHP-2'-deoxyadenosine (dA) adducts by multiple reaction monitoring (MRM) analysis in the presence of known quantities of isotopically labeled DHP-dG and DHP-dA internal standards. This HPLC-ES-MS/MS method is accurate and precise. When applied to liver samples from rats treated with the pyrrolizidine alkaloids riddelliine and monocrotaline, the method provided significant new information regarding the mechanism of DNA adduct formation.

Plants that contain pyrrolizidine alkaloids are widely distributed in the world.Although pyrrolizidine alkaloids have been shown to be genotoxic and tumorigenic in experimental animals, the mechanisms of actions have not been fully understood. The results of our recent mechanistic studies suggest that pyrrolizidine alkaloids induce tumors via a genotoxic mechanism mediated by 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adduct formation. This mechanism may be general to most carcinogenic pyrrolizidine alkaloids, including the retronecine-, heliotridine-, and otonecinetype pyrrolizidine alkaloids. It is hypothesized that these DHP-derived DNA adducts are potential biomarkers of pyrrolizidine alkaloid tumorigenicity. The mechanisms that involve the formation of DNA cross-linking and endogenous DNA adducts are also discussed.

Polycyclic aromatic hydrocarbons (PAHs) are a class of mutagenic and tumorigenic environmental contaminants. Although the mechanisms by which PAHs induce cancer in experimental animals have been extensively studied and the metabolic activation pathways have been determined, the environmental fate of PAHs and the phototoxicity exerted by PAHs, as well as their photoreaction products formed in the environment, have received much less attention. In this review, the formation of oxygenated PAHs, PAH quinones, nitro-PAHs, and halogenated PAHs from photoreaction of environmental PAHs are addressed. Upon light irradiation, PAHs and all PAH photoreaction products can absorb light energy to reach photo-excited states, which react with molecular oxygen, medium, and coexisting chemicals to produce reactive oxygen species (ROS) and other reactive intermediates, such as oxygenated PAHs and free radicals. These intermediates, including ROS, induce lipid peroxidation, and DNA damage including DNA strand breakage, oxidation to 8-oxo-2'-deoxyguanosine, and DNA-adducts. Since these toxicological endpoints are associated with age-related diseases, including cancer, environmental PAHs concomitantly exposed to sunlight may potentially promote human skin damage, leading to ageing and skin cancers. Thus, we suggest that (i) in addition to the widely recognized metabolic pathways, more attention must be paid to photoreaction as an important activation pathway for PAHs, (ii) risk assessment of environmental PAHs should take into consideration the complex photochemical reactions leading to mixtures of products that are also phototoxic; and (iii) the study of structure-toxicity relationships should be expanded to cover the complex photoreactions and extrinsic factors that affect phototoxicity endpoints.

Pyrrolizidine alkaloid-containing plants are widespread in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Pyrrolizidine alkaloids are among the first chemical carcinogens identified in plants. Previously, we determined that metabolism of pyrrolizidine alkaloids in vivo and in vitro generated a common set of DNA adducts that are responsible for tumor induction. Using LC-ESI/MS/MS analysis, we previously determined that four DNA adducts (DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4) were formed in rats dosed with riddelliine, a tumorigenic pyrrolizidine alkaloid. Because of the lack of an adequate amount of authentic standards, the structures of DHP-dA-3 and DHP-dA-4 were not elucidated, and the structural assignment for DHP-dG-4 warranted further validation. In this study, we developed an improved synthetic methodology for these DNA adducts, enabling their full structural elucidation by mass spectrometry and NMR spectroscopy. We determined that DHP-dA-3 and DHP-dA-4 are a pair of epimers of 7-hydroxy-9-(deoxyadenosin-N(6)-yl) dehydrosupinidine, while DHP-dG-4 is 7-hydroxy-9-(deoxyguanosin-N(2)-yl)dehydrosupinidine, an epimer of DHP-dG-3. With the structures of these DNA adducts unequivocally elucidated, we conclude that cellular DNA preferentially binds dehydropyrrolizidine alkaloid, for example, dehydroriddelliine, at the C9 position of the necine base, rather than at the C7 position. We also determined that DHP-dA-3 and DHP-dA-4, as well as DHP-dG-3 and DHP-dG-4, are interconvertible. This study represents the first report with detailed structural assignments of the DNA adducts that are responsible for pyrrolizidine alkaloid tumor induction on the molecular level. A mechanism of tumor initiation by pyrrolizidine alkaloids is consequently fully determined.