Involvement of independent mechanism upon poly(ADP-ribose) polymerase (PARP) activation in methylmercury cytotoxicity in rat cerebellar granule cell culture

Glutathione depletion induces oxidative injury and apoptosis via TRPM2 channel activation in renal collecting duct cells

Oxidative stress (OS)-induced glutathione (GSH) depletion plays an essential role in several kidney diseases such as chronic kidney disease and nephrotoxicity. The OS-dependent activation of TRPM2 cation channel in several neurons and cells were modulated by the concentration of intracellular GSH. However, the effects of GSH alteration on TRPM2 activation, OS, and apoptosis in the cortical collecting duct (mpkCCD_c14) cells still remain elusive. We investigated the effects of GSH supplementation on OS-induced TRPM2 activation, mitochondrial oxidative stress, and apoptosis in the human embryonic kidney 293 (HEK293) and mpkCCD_c14 cells treated with buthionine-sulfoximine (BSO), a GSH synthase inhibitor. The HEK293 and mpkCCD_c14 cells were divided into five groups as control, GSH (10 mM for 2 h), BSO (0.5 mM for 6 h), BSO + GSH, and BSO + TRPM2 channel blockers. Apoptosis, cell death, mitochondrial OS, caspase -3, caspase -9, cytosolic free Zn²⁺, and Ca²⁺ concentrations were increased in the BSO group of the TRPM2 expressing mpkCCD_c14 cells, although they were diminished by the treatments of GSH, PARP-1 inhibitors (PJ34 and DPQ), and TRPM2 blockers (ACA and 2-APB). The BSO-induced decreases in the levels of cell viability and cytosolic GSH were increased by the treatments of GSH, ACA, and 2-APB. However, the effects of BSO and GSH were not observed in the non-TRPM2 expressing HEK293 cells. Current results show that maintaining GSH homeostasis is not only important for quenching OS in the cortical collecting duct cells but equally critical to modulate TRPM2 activation. Thus, suppressing apoptosis and mitochondrial OS responses elicited by oxidant action of GSH depletion.

Potentiation of Methylmercury-Induced Death in Rat Cerebellar Granular Neurons Occurs by Further Decrease of Total Intracellular GSH with BDNF via TrkB in Vitro

Brain-derived neurotrophic factor (BDNF) is a principal factor for neurogenesis, neurodevelopment and neural survival through a BDNF receptor, tropomyosin-related kinase (Trk) B, while BDNF can also cause a decrease in the intracellular glutathione (GSH) level. We investigated the exacerbation of methylmercury-induced death of rat cerebellar granular neurons (CGNs) by BDNF in vitro. Since methylmercury can decrease intracellular GSH levels, we hypothesized that a further decrease of the intracellular GSH level is involved in the process of the exacerbation of neuronal cell death. In the present study, we established that in CGN culture, a decrease of the intracellular GSH level was further potentiated with BDNF in the process of the methylmercury-induced neuronal death and also in GSH reducer-induced neuronal death. BDNF treatment promoted the decrease in GSH levels induced by methylmercury and also by L-buthionine sulfoximine (BSO) and diethyl maleate (DEM). The promoting effect of BDNF was observed in a TrkB-vector transformant of the rat neuroblastoma B35 cell line but not in the mock-vector transformant. These results indicate that the exacerbating effect of BDNF on methylmercury-induced neuronal death in cultures of CGNs includes a further decrease of intracellular GSH levels, for which TrkB is essential.

Synthesis and biological evaluation of isoindoloisoquinolinone, pyroloisoquinolinone and benzoquinazolinone derivatives as poly(ADP-ribose) polymerase-1 inhibitors

A series of novel fused isoquinolinones with isoindoloisoquinolinone, pyroloisoquinolinone, and benzoquinalizinone skeletons were synthesized from corresponding phenethylimides. The isoquinolinone derivatives were evaluated for their protective effect on chicken erythrocytes subjected to oxidative damage. The effect of isoquinolinone derivatives were analysed by estimation of cell viability, antioxidant enzyme activities, DNA damage (comet assay), PARP-1 inhibition assay and molecular docking of the compounds with PARP-1 active site. The compounds CRR-271, CRR-288 and CRR-224+225 showed significant protective effect at 100 μM concentration. The PARP-1 inhibition assay revealed the IC50 values of CRR-271, CRR-288 and CRR-224+225 as <200 nM, further molecular docking studies shows higher binding energies with PARP-1 active site. Interesting findings in this study suggest that the novel isoquinolinone derivatives inhibit PARP-1 activity and protect cells against oxidative DNA damage, which could be implemented in the treatment of inflammatory diseases.

TrkB is involved in the mechanism by which BDNF accelerates the glutamate-induced death of rat neuroblastoma B35 cells

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) binds to its high-affinity binding receptor, tropomyosin-related kinase (Trk) B, and can induce neuronal differentiation and survival. BDNF also accelerates neuronal cell death in a glutamate-induced model; however, it has been unknown whether the mechanism involves TrkB. In the current study, to determine the role of TrkB in neuronal cell death, we investigated TrkB involvement in BDNF acceleration of glutamate-induced neuronal death.

METHODS

A TrkB-stable transformant of rat neuroblastoma B35 (B35(TrkB)) cells was utilized to investigate whether TrkB is involved in BDNF acceleration of neuronal death. The cell viability of the B35(TrkB) cells was compared to that of mock vector-transgened B35 (B35(mock)) cells after treatment with/without BDNF and glutamate.

RESULTS

In both B35(TrkB) and B35(mock) cells, glutamate treatment decreased the cell viability. BDNF treatment further accelerated the decrease in the viability of B35(TrkB) cells, but not that in the viability of B35(mock) cells. At glutamate concentrations that did not significantly decrease cell viability, BDNF increased the cell viability of B35(TrkB), but not that of B35(mock). A mitogen-activated protein kinase (MAPK) inhibitor, U0126, suppressed BDNF's accelerating effect on cell death. Although B35 parental cells endogenously express other neurotrophin receptors such as TrkA, nerve growth factor β (a ligand of TrkA and p75(NTR)) could not influence the viability of B35(TrkB) or B35(mock) cells.

CONCLUSION

These results indicate that TrkB is an intermediator for the trophic and toxicity-exacerbating effects of BDNF against cell viabilities at non-cytotoxic and cytotoxic glutamate concentrations, respectively.

Short time exposure to hydrogen peroxide induces sustained glutathione export from cultured neurons

Hydrogen peroxide is a normal by-product of cellular metabolism that in higher concentrations can cause oxidative stress. Cultured cerebellar granule neurons efficiently disposed of micromolar concentrations of hydrogen peroxide with half-times in the minute range in a process that predominately involved catalase. Application of up to 100 µM hydrogen peroxide did not affect the cell viability for up to 4h, but caused a time- and concentration-dependent increase in the extracellular glutathione (GSH) content that was accompanied by a matching decrease in the cellular GSH content. Hydrogen peroxide at 100 µM stimulated maximally the GSH export from viable neurons, but did not affect GSH export from cultured astrocytes. The peroxide-induced extracellular GSH accumulation from neurons was lowered by 70% in the presence of MK571, an inhibitor of multidrug resistance protein (Mrp) 1. The extracellular GSH content determined after 4h of incubation was already significantly increased after a 5-min exposure of neurons to hydrogen peroxide and became maximal after 15 min of peroxide application. These data demonstrate that just a short exposure of viable cerebellar granule neurons to micromolar concentrations of hydrogen peroxide stimulates a prolonged Mrp1-mediated export of cellular GSH. This process may compromise the antioxidative potential of neurons and increase their sensitivity toward drugs and toxins.

Comparative toxicogenomic responses of mercuric and methyl-mercury

Background

Mercury is a ubiquitous environmental toxicant that exists in multiple chemical forms. A paucity of information exists regarding the differences or similarities by which different mercurials act at the molecular level.

Results

Transcriptomes of mixed-stage C. elegans following equitoxic sub-, low- and high-toxicity exposures to inorganic mercuric chloride (HgCl₂) and organic methylmercury chloride (MeHgCl) were analyzed. In C. elegans, the mercurials had highly different effects on transcription, with MeHgCl affecting the expression of significantly more genes than HgCl₂. Bioinformatics analysis indicated that inorganic and organic mercurials affected different biological processes. RNAi identified 18 genes that were important in C. elegans response to mercurial exposure, although only two of these genes responded to both mercurials. To determine if the responses observed in C. elegans were evolutionarily conserved, the two mercurials were investigated in human neuroblastoma (SK-N-SH), hepatocellular carcinoma (HepG2) and embryonic kidney (HEK293) cells. The human homologs of the affected C. elegans genes were then used to test the effects on gene expression and cell viability after using siRNA during HgCl₂ and MeHgCl exposure. As was observed with C. elegans, exposure to the HgCl₂ and MeHgCl had different effects on gene expression, and different genes were important in the cellular response to the two mercurials.

Conclusions

These results suggest that, contrary to previous reports, inorganic and organic mercurials have different mechanisms of toxicity. The two mercurials induced disparate effects on gene expression, and different genes were important in protecting the organism from mercurial toxicity.

Biochemical factors modulating cellular neurotoxicity of methylmercury

Methylmercury (MeHg), an environmental toxicant primarily found in fish and seafood, poses a dilemma to both consumers and regulatory authorities, given the nutritional benefits of fish consumption versus the possible adverse neurological damage. Several studies have shown that MeHg toxicity is influenced by a number of biochemical factors, such as glutathione (GSH), fatty acids, vitamins, and essential elements, but the cellular mechanisms underlying these complex interactions have not yet been fully elucidated. The objective of this paper is to outline the cellular response to dietary nutrients, as well as to describe the neurotoxic exposures to MeHg. In order to determine the cellular mechanism(s) of toxicity, the effect of pretreatment with biochemical factors (e.g., N-acetyl cysteine, (NAC); diethyl maleate, (DEM); docosahexaenoic acid, (DHA); selenomethionine, SeM; Trolox) and MeHg treatment on intercellular antioxidant status, MeHg content, and other endpoints was evaluated. This paper emphasizes that the protection against oxidative stress offered by these biochemical factors is among one of the major mechanisms responsible for conferring neuroprotection. It is therefore critical to ascertain the cellular mechanisms associated with various dietary nutrients as well as to determine the potential effects of neurotoxic exposures for accurately assessing the risks and benefits associated with fish consumption.

Vitamin K has the potential to protect neurons from methylmercury-induced cell death in vitro

Vitamin K (VK) has a protective effect on neural cells. Methylmercury is a neurotoxicant that directly induces neuronal death in vivo and in vitro. Therefore, in the present study, we hypothesized that VK inhibits the neurotoxicity of methylmercury. To prove our hypothesis in vitro, we investigated the protective effects of VKs (phylloquinone, vitamin K(1); menaquinone-4, vitamin K(2) ) on methylmercury-induced death in primary cultured neurons from the cerebella of rat pups. As expected, VKs inhibited the death of the primary cultured neurons. It has been reported that the mechanisms underlying methylmercury toxicity involve a decrement of intracellular glutathione (GSH). Actually, treatment with GSH and a GSH inducer, N-acetyl cysteine, inhibited methylmercury-induced neuronal death in the present study. Thus, we investigated whether VKs also have protective effects against GSH-depletion-induced cell death by employing two GSH reducers, L-buthionine sulfoximine (BSO) and diethyl maleate (DEM), in primary cultured neurons and human neuroblastoma IMR-32 cells. Treatment with VKs affected BSO- and DEM-induced cell death in both cultures. On the other hand, the intracellular GSH assay showed that VK(2), menaquinone-4, did not restore the reduced GSH amount induced by methylmercury or BSO treatments. These results indicate that VKs have the potential to protect neurons against the cytotoxicity of methylmercury and agents that deplete GSH, without increasing intracellular GSH levels. The protective effect of VKs may lead to the development of treatments for neural diseases involving GSH depletion.