Cadmium generates reactive oxygen- and carbon-centered radical species in rats: insights from in vivo spin-trapping studies

Metallothionein gene expression in liver of rats exposed to cadmium and supplemented with zinc and selenium

Cadmium (Cd), one of the most widely distributed heavy metals, is highly toxic to humans and animals. It is well known that zinc (Zn) and selenium (Se) administration reduce the Cd-induced toxicity and that metallothioneins can have a protective effect to mitigate Cd toxicity in biological systems. In this study we report the expression analysis of the two metallothioneines gene classes MT-1 and MT-2 as well as the total metalloprotein content in the liver of rats exposed to Cd (200 ppm), Cd + Zn (200 ppm + 500 ppm), Cd + Se (200 ppm + 0.1 ppm) or Cd + Zn + Se (200 ppm + 500 ppm + 0.1 ppm) in their drinking water for 35 days. Metals accumulation was quantified in rat liver. Cd decreased significantly the hepatic concentrations of Se and increased those of Zn. The treatment of Cd-exposed rats with Se alone or combined with Zn reversed the Cd-induced depletion of Se concentrations in the liver. However, Zn or Zn + Se administration significantly increased the liver Cd uptake and had no effect on the Cd-induced increase in hepatic concentrations of Zn. The molecular assay showed a decreasing trend of MT-1 relative gene expression levels in animals supplemented with Zn (6.87-fold), Se (3.58-fold), and their combination (1.69-fold) when compared to Cd-treated animals (16.22-fold). Upregulation of the MT-2 expression were recorded in all conditions, although fold induction levels were less pronounced than MT-1 expressions. Our data suggest that the well-established protective effect of Zn and Se against Cd-induced toxicity passes through non-MT gene expression mechanisms, being more dependent on the oxidative stress status of the cell.

The oxidative stress: endoplasmic reticulum stress axis in cadmium toxicity

Cadmium preferentially accumulates in the kidney, the major target for cadmium-related toxicity. Several underlying mechanisms are postulated, and reactive oxygen species (ROS) have been considered as crucial mediators for tissue injuries. In addition to oxidative stress, we recently disclosed that endoplasmic reticulum (ER) stress also plays a critical role. Cadmium causes ER stress in vitro and in vivo and mediates induction of apoptosis in target tissues. In this article, we describe a role for ER stress and involvement of particular branches of the unfolded protein response (UPR) in cadmium-triggered tissue injury, especially nephrotoxicity. We also discuss relationship between oxidative stress and ER stress, and involvement of selective ROS in the induction of pro-apoptotic branches of the UPR.

Evidence for the Existence in Arabidopsis thaliana of the Proteasome Proteolytic Pathway: ACTIVATION IN RESPONSE TO CADMIUM

Heavy metals are known to generate reactive oxygen species that lead to the oxidation and fragmentation of proteins, which become toxic when accumulated in the cell. In this study, we investigated the role of the proteasome during cadmium stress in the leaves of Arabidopsis thaliana plants. Using biochemical and proteomics approaches, we present the first evidence of an active proteasome pathway in plants. We identified and characterized the peptidases acting sequentially downstream from the proteasome in animal cells as follows: tripeptidyl-peptidase II, thimet oligopeptidase, and leucine aminopeptidase. We investigated the proteasome proteolytic pathway response in the leaves of 6-week-old A. thaliana plants grown hydroponically for 24, 48, and 144 h in the presence or absence of 50 mum cadmium. The gene expression and proteolytic activity of the proteasome and the different proteases of the pathway were found to be up-regulated in response to cadmium. In an in vitro assay, oxidized bovine serum albumin and lysozyme were more readily degraded in the presence of 20 S proteasome and tripeptidyl-peptidase II than their nonoxidized form, suggesting that oxidized proteins are preferentially degraded by the Arabidopsis 20 S proteasome pathway. These results show that, in response to cadmium, the 20 S proteasome proteolytic pathway is up-regulated at both RNA and activity levels in Arabidopsis leaves and may play a role in degrading oxidized proteins generated by the stress.

Hydrogen sulfide is a reversible inhibitor of the NADH oxidase activity of synaptic plasma membranes

Hydrogen sulfide is now accepted as a neuromodulator, which can be involved in neuronal defence against oxidative stress insults in the brain. In this work we show that concentrations of H(2)S within the physiological range reported in the brain produce a reversible inhibition of the NADH oxidase activity and coupled superoxide anion production by synaptic plasma membranes from rat brain. At physiological pH 7 the concentration of H(2)S needed for 50% inhibition of the NADH oxidase activity is 5+/-1 microM, which is within the low range of the reported physiological H(2)S concentrations. Thus, the NADH oxidase activity of the neuronal plasma membrane can act as a sensor of local H(2)S depletion in neurones. H(2)S inhibition of the NADH oxidase activity of the neuronal plasma membrane can be accounted for direct reduction by H(2)S of cytochrome b(5). However, H(2)S fails to afford a significant protection against the inhibition of this activity by peroxynitrite. In conclusion, our results point out that H(2)S is more potent as inhibitor of reactive oxygen species formation than as a sacrificial antioxidant.

Protective effects of selenium (Se) and zinc (Zn) on cadmium (Cd) toxicity in the liver of the rat: effects on the oxidative stress

Cadmium (Cd) is a very harmful environmental pollutant that transfers between various levels of the food chain. To study the protective effect of Se and Zn on Cd-induced oxidative stress in livers, male rats received either, tap water, Cd, Cd+Zn, Cd+Se or Cd+Zn+Se in their drinking water, for 35 days. The activities of total superoxide dismutase (SOD), copper, zinc-superoxide dismutase (CuZn SOD), glutathione peroxidase (GPx) and catalase (CAT), malondialdehyde (MDA) level and the ratio of CuZn SOD to GPx activity, were determined in the liver. Exposure to Cd lowered total SOD, CuZn SOD, GPx and CAT activities, while it increased MDA level and the ratio of CuZn SOD to GPx activity, in the organ studied. With Se or Zn administration during exposure to Cd, only partial corrective effects on Cd-induced oxidative stress in the liver have been observed, while Se and Zn together assured a more efficient protection of the organ against the observed oxidative stress.

Role of oxidative stress in cadmium toxicity and carcinogenesis

Cadmium (Cd) is a toxic metal, targeting the lung, liver, kidney, and testes following acute intoxication, and causing nephrotoxicity, immunotoxicity, osteotoxicity and tumors after prolonged exposures. Reactive oxygen species (ROS) are often implicated in Cd toxicology. This minireview focused on direct evidence for the generation of free radicals in intact animals following acute Cd overload and discussed the association of ROS in chronic Cd toxicity and carcinogenesis. Cd-generated superoxide anion, hydrogen peroxide, and hydroxyl radicals in vivo have been detected by the electron spin resonance spectra, which are often accompanied by activation of redox sensitive transcription factors (e.g., NF-kappaB, AP-1 and Nrf2) and alteration of ROS-related gene expression. It is generally agreed upon that oxidative stress plays important roles in acute Cd poisoning. However, following long-term Cd exposure at environmentally-relevant low levels, direct evidence for oxidative stress is often obscure. Alterations in ROS-related gene expression during chronic exposures are also less significant compared to acute Cd poisoning. This is probably due to induced adaptation mechanisms (e.g., metallothionein and glutathione) following chronic Cd exposures, which in turn diminish Cd-induced oxidative stress. In chronic Cd-transformed cells, less ROS signals are detected with fluorescence probes. Acquired apoptotic tolerance renders damaged cells to proliferate with inherent oxidative DNA lesions, potentially leading to tumorigenesis. Thus, ROS are generated following acute Cd overload and play important roles in tissue damage. Adaptation to chronic Cd exposure reduces ROS production, but acquired Cd tolerance with aberrant gene expression plays important roles in chronic Cd toxicity and carcinogenesis.