Potentiation of methylmercury toxicity by combined metal exposure: In vitro and in vivo models of a restricted metal exposome.

Methylmercury (MeHg) is a prevalent toxic metal that readily modifies protein thiols. Reactive persulfides that play a role in redox homeostasis are able to inactivate this metal through sulfur adduct formation. Although humans are exposed to other metals that could consume reactive persulfides on a daily basis, the health effects of combined exposure to MeHg and other metals remain unexplored. This study aimed to examine potential MeHg toxicity during exposure to MeHg with other metals capable of consuming reactive persulfides. We designed a simple system to assess the risk of combined exposure to metals based on reactivity to reactive persulfides and mercury accumulation. Among the metals examined in a cell-free system, copper, cadmium, nickel, and MeHg consumed Na2S2, used as a model of reactive persulfides, whereas zinc, iron, lithium, strontium, tin, and aluminum did not. In HepG2 cells, binary exposure to MeHg and copper, but not aluminum, increased the consumption of extracellular reactive persulfides. Binary exposure exacerbated MeHg-induced cytotoxicity by promoting the modification of intracellular proteins by MeHg. In a mouse model, binary exposure to MeHg and copper resulted in elevated mercury accumulation in the fetuses and placenta of pregnant mice, as well as the brain and liver of non-pregnant mice. Our study suggests that MeHg sensitivity can be increased by combined exposure with other electrophilic metals. In particular, binary exposure to MeHg and copper during pregnancy exacerbated mercury accumulation in offspring.

Activation of PTP1B/EGFR signaling and cytotoxicity during combined exposure to ambient electrophiles in A431 cells.

Chemical modification of the thiol group on protein tyrosine phosphatase (PTP) 1B triggers an activation of epidermal growth factor receptor (EGFR) signaling that is mimicked by environmental electrophiles through S-modification of PTP1B. While activation of PTP1B/EGFR by a single exposure to an electrophile has been established, the effects of combined exposure to electrophiles are unknown. Here, we examined the activation of EGFR signaling by combined exposure to ambient electrophiles in human epithelial carcinoma A431 cells. Simultaneous exposure to 1,2- and 1,4-naphthoquinone (NQ) augmented the S-modification of endogenous and recombinant human PTP1B (hPTP1B). Combined exposure of hPTP1B or A431 cells to 1,2- and 1,4-NQ escalated the inactivation of PTP compared with individual exposure. Phosphorylation of EGFR and its downstream kinase extracellular signal-regulated kinase (ERK) 1/2 by 1,2-NQ exposure was facilitated by simultaneous exposure to 1,2-NQ with 10 µM 1,4-NQ. An EGFR inhibitor diminished the phosphorylation of ERK1/2, indicating that ERK was phosphorylated following EGFR activation by the NQ cocktail. The combined exposure to NQs also accelerated cell death in A431 cells compared with each NQ alone. While no EGFR/ERK activation was seen following 1,4-benzoquinone (BQ) treatment, exposure to 1,4-NQ in the presence of 1,4-BQ increased 1,4-NQ-mediated activation of EGFR. This suggests that the enhancement of 1,4-NQ-dependent EGFR activation by 1,4-BQ is caused by a different mechanism than 1,2-NQ with 1,4-NQ. These results suggest that combined exposure to ambient electrophiles, even at low concentrations, can induce stronger activation of redox signaling than individual exposure. Our findings indicate that combining different electrophiles may produce unexpected effects.

Adverse effects of methylmercury on gut bacteria and accelerated accumulation of mercury in organs due to disruption of gut microbiota.

Methylmercury (MeHg), an environmental electrophile, binds covalently to the cysteine residues of proteins in organs, altering protein function and causing cytotoxicity. MeHg has also been shown to alter the composition of gut microbes. The gut microbiota is a complex community, the disturbance of which has been linked to the development of certain diseases. However, the relationship between MeHg and gut bacteria remains poorly understood. In this study, we showed that MeHg binds covalently to gut bacterial proteins via cysteine residues. We examined the effects of MeHg on the growth of selected Lactobacillus species, namely, L. reuteri, L. gasseri, L. casei, and L. acidophilus, that are frequently either positively or negatively correlated with human diseases. The results revealed that MeHg inhibits the growth of Lactobacillus to varying degrees depending on the species. Furthermore, the growth of L. reuteri, which was inhibited by MeHg exposure, was restored by Na2S2 treatment. By comparing mice with and without gut microbiota colonization, we found that gut bacteria contribute to the production of reactive sulfur species such as hydrogen sulfide and hydrogen persulfide in the gut. We also discovered that the removal of gut bacteria accelerated accumulation of mercury in the cerebellum, liver, and lungs of mice subsequent to MeHg exposure. These results accordingly indicate that MeHg is captured and inactivated by the hydrogen sulfide and hydrogen persulfide produced by intestinal microbes, thereby providing evidence for the role played by gut microbiota in reducing MeHg toxicity.

Repression of mercury accumulation and adverse effects of methylmercury exposure is mediated by cystathionine γ-lyase to produce reactive sulfur species in mouse brain.

Reactive sulfur species (RSS), such as hydropersulfides and hydropolysulfides with high nucleophilicity, contain mobilized sulfur that readily captures xenobiotic electrophiles, leading to their sulfur adducts. We have previously reported that RSS produced by cystathionine γ-lyase (CSE) captures the electrophilic metal methylmercury (MeHg) to form inert sulfur adducts, which in turn play a critical role in the protection against MeHg-induced motor impairment in mice. However, the mechanism underlying this neuroprotective effect is not fully understood. Here, we addressed this using CSE-knockout mice. The cerebellum of CSE-knockout mice was more susceptible to MeHg than that of wild type mice. Moreover, these CSE-deficient mice exhibited a higher level of mercury accumulation in the brain. However, co-treatment with sodium tetrasulfide, an RSS able to capture MeHg, leading to the formation of its sulfur adducts, blocked the increased accumulation of mercury, motor dysfunction and mortality caused by CSE deficiency. Our findings suggest that capturing MeHg by RSS in association with its sulfur adduct formation is involved in the repression of the brain distribution and deleterious effects of MeHg.

[Evaluation of Educational Effect of Problem-Posing System in Nursing Processing Study].

The nursing processing study is generally difficult, because it is important for nursing college students to understand knowledge and utilize it. We have developed an integrated system to understand, utilize, and share knowledge. We added a problem-posing function to this system, and expected that students would deeply understand the nursing processing study through the new system. This system consisted of four steps: create a problem, create an answer input section, create a hint, and verification. Nursing students created problems related to nursing processing by this system. When we gave a lecture on the nursing processing for second year students of A university, we tried to use the creating problem function of this system. We evaluated the effect by the number of problems and the contents of the created problem, that is, whether the contents consisted of a lecture stage or not. We also evaluated the correlation between those and regular examination and report scores. We derived the following: 1. weak correlation between the number of created problems and report score (r=0.27), 2. significant differences between regular examination and report scores of students who created problems corresponding to the learning stage, and those of students who created problems not corresponding to it (P<0.05). From these results, problem-posing is suggested to be effective to fix and utilize knowledge in the lecture of nursing processing theory.