The role of thiol oxidation in Cobalt(II)-induced toxicity in hamster lung

The oxidation of glutathione by cobalt/tungsten carbide contributes to hard metal-induced oxidative stress

The occupational exposure to cobalt/tungsten carbide (Co/WC) dusts causes asthma and interstitial fibrosis. The International Agency for Research on Cancer (IARC) recently classified the mixture Co/WC as probably carcinogenic to humans (group 2A). The mechanism of action of Co/WC involves particle driven generation of Reactive Oxygen Species (ROS) with consequent oxidative damage. The present study evaluates the reactivity of Co/WC dust toward glutathione (GSH) and cysteine (Cys). Co/WC oxidized thiols through a mechanism involving the generation of sulphur-centred radicals. The results are consistent with the oxidation taking place at surface active sites, a part of which is accessible only to Cys S-H groups, but not to GSH ones. Such a reaction, with consequent irreversible depletion of antioxidant defenses of cells, will potentiate the oxidative stress caused by particle and cell generated ROS.

Hypoxia post-translationally activates iron-regulatory protein 2

Iron-regulatory proteins 1 and 2 (IRP1 and IRP2) are RNA-binding proteins that post-transcriptionally regulate the expression of mRNAs that code for proteins involved in the maintenance of iron and energy homeostasis. Here we show that hypoxia differentially regulates the RNA binding activities of IRP1 and IRP2 in human 293 and in mouse Hepa-1 cells. In contrast to IRP1, where hypoxic exposure decreases IRP1 RNA binding activity, hypoxia increases IRP2 RNA binding activity. The hypoxic increase in IRP2 RNA binding activity results from increased IRP2 protein levels. Cobalt, which mimics hypoxia by activation of hypoxia-inducible factor 1 (HIF-1), also increases IRP2 protein levels; however, cobalt-induced IRP2 lacks RNA binding activity. Addition of a reductant to cobalt-treated extracts restored IRP2 RNA binding activity. Hypoxic activation of IRP2 is not because of an increase in transcriptional activation by HIF-1, because IRP2 accumulates in Hepa-1 cells lacking a functional HIF-1beta subunit, nor is it because of an increase in IRP2 mRNA stability. Rather, our data indicate that hypoxia increases IRP2 levels by a post-translational mechanism involving protein stability. Differential regulation of IRP1 and IRP2 during hypoxia may regulate specific IRP target mRNAs whose expression is required for hypoxic adaptation. Furthermore, these data imply mechanistic parallels between the hypoxia-induced post-transcriptional regulation of IRP2 and HIF-1alpha.

Human toxicity of cobalt-containing dust and experimental studies on the mechanism of interstitial lung disease (hard metal disease)

In the industry, the potential for exposure to cobalt metal dust is particularly important during the production of cobalt powder and the processing and use of hard metals and other cobalt-containing alloys. The different adverse health effects reported in these workers are reviewed. One of the main target organs is the respiratory tract, and this article concentrates on the lung parenchymal reactions induced by cobalt-containing dust. Clinical and epidemiological data indicate that this manifestation is rarely, if ever, induced by pure cobalt metal dust alone, but requires the concomitant inhalation of other compounds such as tungsten carbide in the hard metal industry (hard metal disease). Experimental studies demonstrate that cobalt metal and metallic carbides interact to produce an elective lung toxicity. Recent work on the mechanism of this interaction, which is based on the production of activated oxygen species, is reviewed. A practical implication in industrial hygiene should be that permissible exposure levels to Co dust might have to be different when exposure is to pure Co particles or an association with carbides.

Cobalt and possible oxidant-mediated toxicity

The occurrence of interstitial lung disease similar to hard metal lung disease in diamond polishers who had been exposed to cobalt (in the absence of tungsten carbide) through the use of polishing disks containing microdiamonds sintered with cobalt, led us to experimentally test the hypothesis that cobalt has pro-oxidant activity in lung tissue. Several experiments were carried out in which we measured indices of oxidant stress, mainly changes in the oxidation state of glutathione and in the activity of the pentose phosphate pathway, upon exposure of hamster pulmonary tissue to CoCl2 in vivo by intratracheal instillation, or in vitro by incubating lung slices. These experiments indicated that cobalt ions are capable of causing thiol oxidation in lung tissue as an early manifestation of oxidant stress, but more studies are needed to establish the relevance of this mechanism in the causation of lung disease in subjects exposed to cobalt-containing dusts.

Potentiation of oxidant-induced toxicity in hamster lung slices by dimethylthiourea

Dimethylthiourea (DMTU) is an effective scavenger of reactive oxygen metabolites. This property has been successfully exploited, experimentally, in the protection of cells and tissues against oxidative damage. In this study, however, we have observed that levels of nonprotein sulfhydryls (NPSH) in hamster lung slices were markedly decreased by incubation with 10 or 40 mM DMTU. These changes were associated with morphological signs of injury, increased levels of oxidised glutathione (GSSG), and an increased activity of the pentose phosphate pathway (PPP), suggesting that the loss of NPSH was due to their oxidation. Incubation with 40 mM, but not 10 mM DMTU, also resulted in a decreased ability to oxidise [6-14C]glucose or to synthesise proteins, suggesting that at the high concentration, DMTU may cause functional impairment of the tissue. Furthermore, the ability of the slices to accumulate putrescine decreased after incubation with the oxidative toxins paraquat (PQ), tert-butyl hydroperoxide (t-BOOH) or hydrogen peroxide (H2O2) and was further decreased by co-incubation with DMTU. Putrescine uptake, a function specific to the alveolar type I and II epithelial cells, was not affected by incubation with DMTU alone. DMTU did not exacerbate the effect of the nonoxidative toxin iodoacetamide (IAA) on putrescine uptake but it did affect markers of general cell damage or dysfunction. We suggest, therefore, that the toxicity of oxidants toward lung tissue is potentiated in alveolar epithelial cells by DMTU.