Methylmercury induces the expression of chemokine CCL4 via SRF activation in C17.2 mouse neural stem cells

Diepoxybutane induces the p53-dependent transactivation of the CCL4 gene that mediates apoptosis in exposed human lymphoblasts

Diepoxybutane (DEB) is the most toxic metabolite of the environmental chemical 1,3-butadiene. We previously demonstrated the occurrence of DEB-induced p53-mediated apoptosis in human lymphoblasts. The p53 protein functions as a master transcriptional regulator in orchestrating the genomic response to a variety of stress signals. Transcriptomic analysis indicated that C-C chemokine ligand 4 (CCL4) gene expression was elevated in a p53-dependent manner in DEB-exposed p53-proficient TK6 cells, but not in DEB-exposed p53-deficient NH32 cells. Thus, the objective of this study was to determine whether the CCL4 gene is a transcriptional target of p53 and deduce its role in DEB-induced apoptosis in human lymphoblasts. Endogenous and exogenous wild-type p53 transactivated the activity of the CCL4 promoter in DEB-exposed lymphoblasts, but mutant p53 activity on this promoter was reduced by ∼80% under the same experimental conditions. Knockdown of the upregulated CCL4 mRNA levels in p53-proficient TK6 cells inhibited DEB-induced apoptosis by ∼45%-50%. Collectively, these observations demonstrate for the first time that the CCL4 gene is upregulated by wild-type p53 at the transcriptional level, and this upregulation mediates apoptosis in DEB-exposed human lymphoblasts.

Potential Association between Methylmercury Neurotoxicity and Inflammation

Methylmercury (MeHg) is the causal substrate of Minamata disease and a major environmental toxicant. MeHg is widely distributed, mainly in the ocean, meaning its bioaccumulation in seafood is a considerable problem for human health. MeHg has been intensively investigated and is known to induce inflammatory responses and neurodegeneration. However, the relationship between MeHg-induced inflammatory responses and neurodegeneration is not understood. In the present review, we first describe recent findings showing an association between inflammatory responses and certain MeHg-unrelated neurological diseases caused by neurodegeneration. In addition, cell-specific MeHg-induced inflammatory responses are summarized for the central nervous system including those of microglia, astrocytes, and neurons. We also describe MeHg-induced inflammatory responses in peripheral cells and tissue, such as macrophages and blood. These findings provide a concept of the relationship between MeHg-induced inflammatory responses and neurodegeneration, as well as direction for future research of MeHg-induced neurotoxicity.

Methylmercury toxic mechanism related to protein degradation and chemokine transcription

Methylmercury is an environmental pollutant that causes neurotoxicity. Recent studies have reported that the ubiquitin-proteasome system is involved in defense against methylmercury toxicity through the degradation of proteins synthesizing the pyruvate. Mitochondrial accumulation of pyruvate can enhance methylmercury toxicity. In addition, methylmercury exposure induces several immune-related chemokines, specifically in the brain, and may cause neurotoxicity. This summary highlights several molecular mechanisms of methylmercury-induced neurotoxicity.

Increased putrescine levels due to ODC1 overexpression prevents mitochondrial dysfunction-related apoptosis induced by methylmercury

AIMS

We had previously reported that addition of putrescine to the culture medium was reported to reduce methylmercury toxicity in C17.2 neural stem cells. Here, we have examined the inhibition of methylmercury-induced cytotoxicity by putrescine using ODC1-overexpressing C17.2 cells.

MATERIALS AND METHODS

We established stable ODC1-overexpressing C17.2 cells and evaluated methylmercury-induced apoptosis by examining the TUNEL assay and cleaved caspase-3 levels. Mitochondria-mediated apoptosis was also evaluated by reduction of mitochondrial membrane potential and recruitment of Bax and Bak to the mitochondria.

KEY FINDINGS

ODC is encoded by ODC1 gene, and putrescine levels in ODC1-overexpressing cells were significantly higher than in control cells. Overexpression of ODC1 and addition of putrescine to the culture medium suppressed DNA fragmentation and caspase-3 activation, which are observed when apoptosis is induced by methylmercury. Moreover, mitochondrial dysfunction and reactive oxygen species (ROS) generation, caused by methylmercury, were also inhibited by the overexpression of ODC1 and putrescine; pretreatment with ODC inhibitor, however, promoted both ROS generation and apoptosis by methylmercury. Finally, we found that Bax and Bak, the apoptosis-promoting factors, to be increased in mitochondria, following methylmercury treatment, and the same was inhibited by overexpression of ODC1. These results suggest that overexpression of ODC1 may prevent mitochondria-mediated apoptosis by methylmercury via increase of putrescine levels.

SIGNIFICANCE

Our findings provide important clues to clarify mechanisms involved in the defense against methylmercury toxicity and suggest novel biological functions of putrescine.

The Nuclear Protein HOXB13 Enhances Methylmercury Toxicity by Inducing Oncostatin M and Promoting Its Binding to TNFR3 in Cultured Cells

Homeobox protein B13 (HOXB13), a transcription factor, is related to methylmercury toxicity; however, the downstream factors involved in enhancing methylmercury toxicity remain unknown. We performed microarray analysis to search for downstream factors whose expression is induced by methylmercury via HOXB13 in human embryonic kidney cells (HEK293), which are useful model cells for analyzing molecular mechanisms. Methylmercury induced the expression of oncostatin M (OSM), a cytokine of the interleukin-6 family, and this was markedly suppressed by HOXB13 knockdown. OSM knockdown also conferred resistance to methylmercury in HEK293 cells, and no added methylmercury resistance was observed when both HOXB13 and OSM were knocked down. Binding of HOXB13 to the OSM gene promoter was increased by methylmercury, indicating the involvement of HOXB13 in the enhancement of its toxicity. Because addition of recombinant OSM to the medium enhanced methylmercury toxicity in OSM-knockdown cells, extracellularly released OSM was believed to enhance methylmercury toxicity via membrane receptors. We discovered tumor necrosis factor receptor (TNF) receptor 3 (TNFR3) to be a potential candidate involved in the enhancement of methylmercury toxicity by OSM. This toxicity mechanism was also confirmed in mouse neuronal stem cells. We report, for the first time, that HOXB13 is involved in enhancement of methylmercury toxicity via OSM-expression induction and that the synthesized OSM causes cell death by binding to TNFR3 extracellularly.