Mechanisms underlying arsenic carcinogenesis: hypersensitivity of mice exposed to inorganic arsenic during gestation

Induction of sarcomas by a single subcutaneous injection of 7,12-dimethylbenz[a]anthracene into neonatal male Sprague-Dawley rats: histopathological and immunohistochemical analyses

Animal experiments have shown that carcinogenicity of chemicals is higher in fetal or neonatal periods than adult. We investigated sensitivities to a carcinogen in peri-neonatal rats with a model of sarcomas-induction by a subcutaneous injection of chemo-carcinogen that has rarely done in neonatal rats. Neonatal male SD rats were injected with 7,12-DMBA 10, 100, and 500 microg, which resulted in sarcomas-induction in 0, 62, and 94% of rats. Male SD rats were injected with DMBA 500 microg at 0, 3, 7, 14, and 21 days, which resulted in sarcomas-induction in 94, 70, 64, 50, and 44% of rats. Although the induced sarcomas were occasionally in mixed morphological feature as previous reports for sarcomas of rat, each was immunohistochemically in almost monotonous pattern, and classification was feasible. The incidence of rhabdomyosarcomas was higher in rats neonatally injected with a higher dose of DMBA than a lower dose, and in rats injected at peri-neonatal periods than later periods. In histological observations for the site of injection before overt sarcomas develop, clusters of atypical mesenchymal cells emerged as previous studies, but also those were immunohistochemically differentiated into rhabdomyocytes and other mesenchymal cells. We consider these findings may contribute a little to elucidation of process of sarcomas-induction in rats.

Free radicals and antioxidants in normal physiological functions and human disease

Reactive oxygen species (ROS) and reactive nitrogen species (RNS, e.g. nitric oxide, NO(*)) are well recognised for playing a dual role as both deleterious and beneficial species. ROS and RNS are normally generated by tightly regulated enzymes, such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. Overproduction of ROS (arising either from mitochondrial electron-transport chain or excessive stimulation of NAD(P)H) results in oxidative stress, a deleterious process that can be an important mediator of damage to cell structures, including lipids and membranes, proteins, and DNA. In contrast, beneficial effects of ROS/RNS (e.g. superoxide radical and nitric oxide) occur at low/moderate concentrations and involve physiological roles in cellular responses to noxia, as for example in defence against infectious agents, in the function of a number of cellular signalling pathways, and the induction of a mitogenic response. Ironically, various ROS-mediated actions in fact protect cells against ROS-induced oxidative stress and re-establish or maintain "redox balance" termed also "redox homeostasis". The "two-faced" character of ROS is clearly substantiated. For example, a growing body of evidence shows that ROS within cells act as secondary messengers in intracellular signalling cascades which induce and maintain the oncogenic phenotype of cancer cells, however, ROS can also induce cellular senescence and apoptosis and can therefore function as anti-tumourigenic species. This review will describe the: (i) chemistry and biochemistry of ROS/RNS and sources of free radical generation; (ii) damage to DNA, to proteins, and to lipids by free radicals; (iii) role of antioxidants (e.g. glutathione) in the maintenance of cellular "redox homeostasis"; (iv) overview of ROS-induced signaling pathways; (v) role of ROS in redox regulation of normal physiological functions, as well as (vi) role of ROS in pathophysiological implications of altered redox regulation (human diseases and ageing). Attention is focussed on the ROS/RNS-linked pathogenesis of cancer, cardiovascular disease, atherosclerosis, hypertension, ischemia/reperfusion injury, diabetes mellitus, neurodegenerative diseases (Alzheimer's disease and Parkinson's disease), rheumatoid arthritis, and ageing. Topics of current debate are also reviewed such as the question whether excessive formation of free radicals is a primary cause or a downstream consequence of tissue injury.

Arsenic speciation transported through the placenta from mother mice to their newborn pups

The primary goal of the present study was to confirm the arsenic species that can be transferred from the mother to the bodies of newborn pups through the placenta and the speciated arsenic distribution in the liver and brain of newborn mice after gestational maternal exposure to inorganic arsenic (iAs). Mother mice were exposed to iAsIII and iAsV in drinking water during gestation. The livers and brains of the mother mice and their newborn pups were taken. Contents of iAs, monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), and trimethylarsenic (TMA) compound were detected using the HG-AAS method. Contents of iAs, MMA, and DMA in the liver of mother mice increased with the concentration of arsenite or arsenate in their drinking water. However, only DMA increased with the concentration of arsenate or arsenite in the drinking water in the brain of mother mice. On the other hand, contents of both iAs and DMA in the liver and brain of newborn mice increased with the concentration of arsenate or arsenite administered to their mother orally. Contents of arsenic species in the liver and brain of both mother mice and their newborn pups were significantly lower in the 10 ppm iAsV group than in the 10 ppm iAsIII group. Ratios of iAs or DMA levels between the brain and the liver of newborn mice were larger than 1, whereas those in mother mice were much smaller than 1. iAs taken from drinking water was distributed and metabolized mainly in the liver of mother mice. iAsIII in low levels may be taken up and metabolized easily in the liver compared to iAsV. Both iAs and DMA are transferred from the mother through the placenta and cross the immature blood-brain barrier (BBB) easily. Compared to that in the liver of newborn mice, DMA as an organic metabolite is prevalent in brain, a lipidic organ, if the BBB is not matured enough to prevent it from entering the brain.

Free radicals, metals and antioxidants in oxidative stress-induced cancer

Oxygen-free radicals, more generally known as reactive oxygen species (ROS) along with reactive nitrogen species (RNS) are well recognised for playing a dual role as both deleterious and beneficial species. The "two-faced" character of ROS is substantiated by growing body of evidence that ROS within cells act as secondary messengers in intracellular signalling cascades, which induce and maintain the oncogenic phenotype of cancer cells, however, ROS can also induce cellular senescence and apoptosis and can therefore function as anti-tumourigenic species. The cumulative production of ROS/RNS through either endogenous or exogenous insults is termed oxidative stress and is common for many types of cancer cell that are linked with altered redox regulation of cellular signalling pathways. Oxidative stress induces a cellular redox imbalance which has been found to be present in various cancer cells compared with normal cells; the redox imbalance thus may be related to oncogenic stimulation. DNA mutation is a critical step in carcinogenesis and elevated levels of oxidative DNA lesions (8-OH-G) have been noted in various tumours, strongly implicating such damage in the etiology of cancer. It appears that the DNA damage is predominantly linked with the initiation process. This review examines the evidence for involvement of the oxidative stress in the carcinogenesis process. Attention is focused on structural, chemical and biochemical aspects of free radicals, the endogenous and exogenous sources of their generation, the metal (iron, copper, chromium, cobalt, vanadium, cadmium, arsenic, nickel)-mediated formation of free radicals (e.g. Fenton chemistry), the DNA damage (both mitochondrial and nuclear), the damage to lipids and proteins by free radicals, the phenomenon of oxidative stress, cancer and the redox environment of a cell, the mechanisms of carcinogenesis and the role of signalling cascades by ROS; in particular, ROS activation of AP-1 (activator protein) and NF-kappaB (nuclear factor kappa B) signal transduction pathways, which in turn lead to the transcription of genes involved in cell growth regulatory pathways. The role of enzymatic (superoxide dismutase (Cu, Zn-SOD, Mn-SOD), catalase, glutathione peroxidase) and non-enzymatic antioxidants (Vitamin C, Vitamin E, carotenoids, thiol antioxidants (glutathione, thioredoxin and lipoic acid), flavonoids, selenium and others) in the process of carcinogenesis as well as the antioxidant interactions with various regulatory factors, including Ref-1, NF-kappaB, AP-1 are also reviewed.

Mutually exclusive subsets of BH3-only proteins are activated by the p53 and c-Jun N-terminal kinase/c-Jun signaling pathways during cortical neuron apoptosis induced by arsenite

The c-Jun N-terminal protein kinase (JNK)/c-Jun and p53 pathways form distinct death-signaling modules in neurons that culminate in Bax-dependent apoptosis. To investigate whether this signaling autonomy is due to recruitment of particular BH3-only proteins, we searched for a toxic signal that would activate both pathways in the same set of neurons. We show that arsenite activates both the JNK/c-Jun and p53 pathways in cortical neurons, which together account for >95% of apoptosis, as determined by using the mixed-lineage kinase (JNK/c-Jun) pathway inhibitor CEP11004 and p53-null mice. Despite the coexistence of both pathways in at least 30% of the population, Bim mRNA and protein expression was increased only by the JNK/c-Jun signaling pathway, whereas Noxa and Puma mRNA and Puma protein expression was entirely JNK/c-Jun independent. About 50% of Puma/Noxa expression was p53 dependent, with the remaining signal being independent of both pathways and possibly facilitated by arsenite-induced reduction in P-Akt. However, functionally, Puma was predominant in mediating Bax-dependent apoptosis, as evidenced by the fact that more than 90% of apoptosis was prevented in Puma-null neurons, although Bim was still upregulated, while Bim- and Noxa-null neurons died similarly to wild-type neurons. Thus, the p53 and JNK/c-Jun pathways can activate mutually exclusive subclasses of BH3-only proteins in the same set of neurons. However, other factors besides expression may determine which BH3-only proteins mediate apoptosis.

Interactions of Sodium Selenite, Glutathione, Arsenic Species, and Omega Class Human Glutathione Transferase

Human monomethylarsenate reductase [MMA(V) reductase] and human glutathione S-transferase omega 1-1 (hGSTO1-1) [because MMA(V) reductase and hGSTO1-1 are identical proteins, the authors will utilize the designation "hGSTO1-1"] are identical proteins that catalyze the reduction of arsenate, monomethylarsenate [MMA(V)], and dimethylarsenate [DMA(V)]. Sodium selenite (selenite) inhibited the reduction of each of these substrates by the enzyme in a concentration-dependent manner. The kinetics indicated a noncompetitive inhibition of the MMA(V), DMA(V), or arsenate reducing activity of hGSTO1-1. The inhibition of the MMA(V) reducting activity of hGSTO1-1 by selenite was reversed by 1 mM DL-dithiothreitol (DTT) but not by reduced glutathione (GSH), which is a required substrate for the enzyme. Neither superoxide anion nor hydrogen peroxide was involved in the selenite inhibition of hGSTO1-1. MALDI-TOF and MS/MS analysis demonstrated that five molecules of GSH were bound to one monomer of hGSTO1-1. Four of the five cysteines of the monomer were glutathionylated. Cys-32 in the active center, however, exists mostly in the sulfhydryl form since it was alkylated consistently by iodoacetamide. MALDI-TOF mass spectra analysis of hGSTO1-1 after reaction with GSH and sodium selenite indicated that selenium was integrated into hGSTO1-1 molecules. Three selenium were found to be covalently bonded to the monomer of hGSTO1-1 with three molecules of GSH. It is proposed that the reaction products of the reduction of selenite inhibited the activity of hGSTO1-1 by reacting with disulfides of glutathionylated cysteines to form bis (S-cysteinyl)selenide and S-selanylcysteine and had little or no interaction with the sulfhydryl of Cys-32 in the active site of the enzyme.