Methylmercury directly modifies the 105th cysteine residue in oncostatin M to promote binding to tumor necrosis factor receptor 3 and inhibit cell growth

We previously found that methylmercury induces expression of oncostatin M (OSM), which is released extracellularly and binds to tumor necrosis factor receptor 3 (TNFR3), possibly enhancing its own toxicity. However, the mechanism by which methylmercury causes OSM to bind to TNFR3 rather than to its known receptors, OSM receptor and LIFR, is unknown. In this study, we aimed to elucidate the effect of methylmercury modification of cysteine residues in OSM on binding to TNFR3. Immunostaining of TNFR3-V5-expressing cells suggested that methylmercury promoted binding of OSM to TNFR3 on the cell membrane. In an in vitro binding assay, OSM directly bound to the extracellular domain of TNFR3, and this binding was promoted by methylmercury. Additionally, the formation of a disulfide bond in the OSM molecule was essential for the binding of both proteins, and LC/MS analysis revealed that methylmercury directly modified the 105th cysteine residue (Cys105) in OSM. Next, mutant OSM, in which Cys105 was replaced by serine or methionine, increased the binding to TNFR3, and a similar effect was observed in immunoprecipitation using cultured cells. Furthermore, cell proliferation was inhibited by treatment with Cys105 mutant OSMs compared with wildtype OSM, and this effect was cancelled by TNFR3 knockdown. In conclusion, we revealed a novel mechanism of methylmercury toxicity, in which methylmercury directly modifies Cys105 in OSM, thereby inhibiting cell proliferation via promoting binding to TNFR3. This indicates a chemical disruption in the interaction between the ligand and the receptor is a part of methylmercury toxicity.

Exploring the biological domain of the neural embryonic stem cell test (ESTn): Morphogenetic regulators, Hox genes and cell types, and their usefulness as biomarkers for embryotoxicity screening

Animal-free assessment of compound-induced developmental neurotoxicity will most likely be based on batteries of multiple in vitro tests. The optimal battery is built by combining tests with complementary biological domains that together ideally cover all relevant toxicity pathways. Thus, biological domain definition, i.e. which biological processes and cell types are represented, is an important assay characteristic for determining the place of assays in testing strategies. The murine neural embryonic stem cell test (ESTn) is employed to predict the developmental neurotoxicity of compounds. The aim of this study was to explore the biological domain of ESTn according to three groups of biomarker genes of early (neuro)development: morphogenetic regulators, Hox genes and cell type markers for the ectodermal and neural lineages. These biomarker groups were selected based on their crucial regulatory role in (neuro)development. Analysis of these genes in a series of previously generated whole transcriptome datasets of ESTn showed that at day 7 in culture cell differentiation resembled hindbrain/branchial/thoracic development between E6.5-E12.5 in vivo, with subsequent development into a mixed cell culture containing different neural subtypes, astrocytes and oligodendrocytes by day 13. In addition, the selected biomarkers showed common and distinct responses to compound exposure. Monitoring the biological domain of ESTn through gene expression patterns of morphogenetic regulators, Hox genes and cell type markers proved instrumental in providing mechanistic understanding of compound effects on neural differentiation in ESTn, and can aid in positioning of the test in a battery of complementary in vitro tests in integrated approaches to testing and assessment.

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.

Knockdown of the gene for homeobox protein HOXB13 reduces toxicity of oxidative-stress inducers in HEK293 cells

HEK293 cells transfected with a double-stranded siRNA to suppress expression of the homeobox gene HOXB13 were highly resistant to oxidative stress-inducing agents, such as hydrogen peroxide, N-ethylmaleimide (NEM), and paraquat. This finding suggests that HOXB13 might enhance cell injury caused by oxidative stress.