Dithiolethiones D3T and ACDT Protect Against Iron Overload-Induced Cytotoxicity and Serve as Ferroptosis Inhibitors in U-87 MG Cells.

Iron overload-induced oxidative stress is implicated in various neurodegenerative disorders. Given the numerous adverse effects associated with current iron chelators, natural antioxidants are being explored as alternative therapeutic options. Dithiolethiones found in cruciferous vegetables have emerged as promising candidates against a wide range of toxicants owing to their lipophilic and cytoprotective properties. Here, we test the dithiolethiones 3H-1,2-dithiole-3-thione (D3T) and 5-amino-3-thioxo-3H-(1,2) dithiole-4-carboxylic acid ethyl ester (ACDT) against ferric ammonium citrate (FAC)-induced toxicity in U-87 MG astrocytoma cells. Exposure to 15 mM FAC for 24 h resulted in 54% cell death. A 24-h pretreatment with 50 µM D3T and ACDT prevented this cytotoxicity. Both dithiolethiones exhibited antioxidant effects by activating the nuclear factor erythroid 2-related factor-2 (Nrf2) transcription factor and upregulating levels of intracellular glutathione (GSH). This resulted in the successful inhibition of FAC-induced reactive oxygen species, lipid peroxidation, and cell death. Additionally, D3T and ACDT upregulated expression of the Nrf2-mediated iron storage protein ferritin which consequently reduced the total labile iron pool. A 24-h pretreatment with D3T and ACDT also prevented cell death induced by the ferroptosis inducer erastin by upregulating the transmembrane cystine/glutamate antiporter (xCT) expression. The resulting increase in intracellular GSH and alleviation of lipid peroxidation was comparable to that caused by ferrostatin-1, a specific ferroptosis inhibitor. Collectively, our findings demonstrate that dithiolethiones may show promise as potential therapeutic options for the treatment of iron overload disorders.

Reversal of genetic brain iron accumulation by N,N'-bis(2-mercaptoethyl)isophthalamide, a lipophilic metal chelator, in mice.

N,N'-bis(2-mercaptoethyl)isophthalamide (NBMI) is a novel lipophilic metal chelator and antioxidant used in mercury poisoning. Recent studies have suggested that NBMI may also bind to other metals such as lead and iron. Since NBMI can enter the brain, we evaluated if NBMI removes excess iron from the iron-loaded brain and ameliorates iron-induced oxidative stress. First, NBMI exhibited preferential binding to ferrous (Fe2+) iron with a negligible binding affinity to ferric (Fe3+) iron, indicating a selective chelation of labile iron. Second, NBMI protected SH-SY5Y human neuroblastoma cells from the cytotoxic effects of high iron. NBMI also decreased cellular labile iron and lessened the production of iron-induced reactive oxygen species in these cells. Deferiprone (DFP), a commonly used oral iron chelator, failed to prevent iron-induced cytotoxicity or labile iron accumulation. Next, we validated the efficacy of NBMI in Hfe H67D mutant mice, a mouse model of brain iron accumulation (BIA). Oral gavage of NBMI for 6 weeks decreased iron accumulation in the brain as well as liver, whereas DFP showed iron chelation only in the liver, but not in the brain. Notably, depletion of brain copper and anemia were observed in BIA mice treated with DFP, but not with NBMI, suggesting a superior safety profile of NBMI over DFP for long-term use. Collectively, our study demonstrates that NBMI provides a neuroprotective effect against BIA and has therapeutic potential for neurodegenerative diseases associated with BIA.

N,N'bis-(2-mercaptoethyl) isophthalamide (NBMI) exerts neuroprotection against lead-induced toxicity in U-87 MG cells.

N,N'-bis(2-mercaptoethyl)isophthalamide (NBMI) is a novel lipophilic heavy metal chelator and thiol redox antioxidant. This study was designed to investigate the neuroprotective activity of NBMI in U-87 MG cells exposed to lead acetate (PbAc). Cells were pretreated with NBMI for 24 h prior to a 48 h exposure to PbAc. Cell death (55%, p < 0.0001) and reduction of intracellular GSH levels (0.70-fold, p < 0.005) induced by 250 µM Pb were successfully attenuated by NBMI pretreatment at concentrations as low as 10 µM. A similar pretreatment with the FDA-approved Pb chelator dimercaptosuccinic acid (DMSA) proved ineffective, indicating a superior PKPD profile for NBMI. Pretreatment with NBMI successfully counteracted Pb-induced neuroinflammation by reducing IL-1ß (0.59-fold, p < 0.05) and GFAP expression levels. NBMI alone was also found to significantly increase ferroportin expression (1.97-fold, p < 0.05) thereby enhancing cellular ability to efflux heavy metals. While no response was observed on the apoptotic pathway, this study demonstrated for the first time that necrotic cell death induced by Pb in U-87 MG cells is successfully attenuated by NBMI. Collectively these data demonstrate NBMI to be a promising neuroprotective compound in the realm of Pb poisoning.

Disubstituted Dithiolethione ACDT Exerts Neuroprotective Effects Against 6-Hydroxydopamine-Induced Oxidative Stress in SH-SY5Y Cells.

Parkinson's disease (PD) is a prevalent, progressive, neurodegenerative disorder with no known cure. Oxidative stress has been found to play a significant role in its etiology, and the search for novel neuroprotective compounds that actively prevent disease progression is currently ongoing. Dithiolethiones are a group of sulfur-containing heterocyclic compounds found in cruciferous vegetables. Using the 6-hydroxydopamine (6-OHDA) model of PD, we tested a previously identified disubstituted dithiolethione 5-amino-3-thioxo-3H-(1,2) dithiole-4-carboxylic acid ethyl ester (ACDT) for its neuroprotective potential. Pretreatment of SH-SY5Y cells with ACDT led to a time- and concentration-dependent induction of the antioxidant glutathione (GSH). ACDT also diminished 6-OHDA-induced cell death, lactate dehydrogenase release, elevation of caspase 3/7 activity, and increase in levels of reactive oxygen species. Inhibition of the GSH-synthesizing enzyme glutamate-cysteine ligase catalytic subunit (GCLC) led a corresponding dissipation of ACDT's neuroprotective effects, hence underlining the importance of GSH in ACDT's neuroprotective response. ACDT caused the stabilization and nuclear translocation of nuclear factor erythroid-2 related factor (Nrf2), resulting in increased protein expression of the phase II enzyme NADPH:quinone oxidoreductase 1 (NQO1), and the excitatory amino acid cysteine membrane transporter (EAAT3). Interestingly, no changes in the levels of other Nrf2-dependent molecules including GCLC were observed, indicating the possible involvement of additional alternate mechanisms behind ACDT's GSH-inducing property. Collectively, the data demonstrated ACDT to be a promising new dithiolethione for the treatment of PD, with two modifiable functional groups offering additional avenues for enhanced pharmacological application.

Synthesis and structure-activity relationships study of dithiolethiones as inducers of glutathione in the SH-SY5Y neuroblastoma cell line.

Parkinson's disease is a neurodegenerative disorder that involves the degeneration of nigrostriatal dopaminergic neurons. Elevated levels of reactive oxygen species have been shown to deplete cellular levels of the ubiquitous antioxidant glutathione, leading to oxidative stress and eventual neuronal cell death. Dithiolethiones, a class of sulfur-containing heterocyclic molecules, have been shown to induce cellular production of glutathione in a variety of tissues, but have not been extensively evaluated in neurons. Herein, we report the synthesis and preliminary structure-activity relationships study of several substituted dithiolethiones. Three molecules were identified (D3T, CPDT, and 2d) that potently induced cellular glutathione in the SH-SY5Y neuroblastoma cell line. Furthermore, these compounds were found to provide neuroprotection in the 6-hydroxydopamine model of neurotoxicity. This study suggests that dithiolethione-mediated neuroprotection may have potential as a disease-modifying antiparkinsonian therapy.

Neuroprotective effects of disubstituted dithiolethione ACDT against manganese-induced toxicity in SH-SY5Y cells.

Dithiolethiones are lipophilic, organosulfur compounds that activate the Nrf2 transcription factor causing an upregulation of various phase II antioxidant enzymes. A disubstituted dithiolethione 5-amino-3-thioxo-3H-(1,2) dithiole-4-carboxylic acid ethyl ester (ACDT) retains the functional pharmacophore while also containing modifiable functional groups. Neuroprotection against autoimmune encephalomyelitis in vivo and 6-hydroxy dopamine (a model for Parkinson's disease) in vitro have been previously reported with ACDT. Manganese (Mn) is a metal essential for metabolic processes at low concentrations. Overexposure and accumulation of Mn leads to a neurological condition called manganism which shares pathophysiological sequelae with parkinsonism. Here we hypothesized ACDT to be protective against manganese-induced cytotoxicity. SH-SY5Y human neuroblastoma cells exposed to 300 µM MnCl2 displayed approximately 50% cell death, and a 24-h pretreatment with 75 µM ACDT significantly reversed this cytotoxicity. ACDT pretreatment was also found to increase total GSH levels (2.18-fold) and the protein levels of NADPH:quinone oxidoreductase-1 (NQO1) enzyme (6.33-fold), indicating an overall increase in the cells' antioxidant defense stores. A corresponding 2.32-fold reduction in the level of Mn-induced reactive oxygen species was also observed in cells pretreated with ACDT. While no changes were observed in the protein levels of apoptotic markers Bax and Bcl-2, pretreatment with 75 µM ACDT led to a 2.09-fold downregulation of ZIP14 import transporter, indicating a potential reduction in the cellular uptake of Mn as an additional neuroprotective mechanism. These effects did not extend to other transporters like the divalent metal transporter 1 (DMT1) or ferroportin. Collectively, ACDT showed substantial neuroprotection against Mn-induced cytotoxicity, opening a path for dithiolethiones as a potential novel therapeutic option against heavy metal neurotoxicity.

Influence of iron metabolism on manganese transport and toxicity.

Although manganese (Mn) is critical for the proper functioning of various metabolic enzymes and cofactors, excess Mn in the brain causes neurotoxicity. While the exact transport mechanism of Mn has not been fully understood, several importers and exporters for Mn have been identified over the past decade. In addition to Mn-specific transporters, it has been demonstrated that iron transporters can mediate Mn transport in the brain and peripheral tissues. However, while the expression of iron transporters is regulated by body iron stores, whether or not disorders of iron metabolism modify Mn homeostasis has not been systematically discussed. The present review will provide an update on the role of altered iron status in the transport and toxicity of Mn.

Further structure-activity relationships study of substituted dithiolethiones as glutathione-inducing neuroprotective agents.

BACKGROUND: Parkinson's disease is a neurodegenerative disorder associated with oxidative stress and glutathione depletion. The induction of cellular glutathione levels by exogenous molecules is a promising neuroprotective approach to limit the oxidative damage that characterizes Parkinson's disease pathophysiology. Dithiolethiones, a class of sulfur-containing heterocyclic molecules, are known to increase cellular levels of glutathione; however, limited information is available regarding the influence of dithiolethione structure on activity. Herein, we report the design, synthesis, and pharmacological evaluation of a further series of dithiolethiones in the SH-SY5Y neuroblastoma cell line. RESULTS: Our structure-activity relationships data show that dithiolethione electronic properties, given as Hammett σp constants, influence glutathione induction activity and compound toxicity. The most active glutathione inducer identified, 6a, dose-dependently protected cells from 6-hydroxydopamine toxicity. Furthermore, the protective effects of 6a were abrogated by the inhibitor of glutathione synthesis, buthionine sulfoximine, confirming the importance of glutathione in the protective activities of 6a. CONCLUSIONS: The results of this study further delineate the relationship between dithiolethione chemical structure and glutathione induction. The neuroprotective properties of analog 6a suggest a role for dithiolethiones as potential antiparkinsonian agents.

Neurobehavioral effects of lead and manganese individually and in combination in developmentally exposed rats.

The behavioral effects of exposure to environmentally relevant low levels of the known neurotoxicants lead (Pb) and manganese (Mn) are of current interest, especially when exposure occurs in the period of early development. The incidence of real life neurodevelopmental toxicity is dependent on co-exposure to multiple neurotoxicants present in the environment, and it is therefore important to study these in combination. Pregnant rats were given ad libitum access to drinking water solutions containing Pb (10 µg/mL), Mn (2mg/mL) or a mixture, from beginning of gestation, through lactation and until weaning. Metal-exposed pups were born with significantly higher body weights compared to controls. While female pups exposed to metal mixtures displayed the highest body weights through two months of age, differences were not significant among male pups after the first week. Mixture-exposed pups exhibited the fastest righting times when tested between postnatal days (PNDs) 1 and 10. Learning and memory were assessed in five-day Morris water maze (MWM) tests, ending in probe trials on PNDs 25 and 60. On PND 24, mixture-exposed males performed significantly better than individual metal-exposed males, but this effect was no longer evident between PNDs 56 and 60. Females exposed to Pb or Mn displayed deficits at both behavioral time points. In the open-field test, Mn-exposed males were hypoactive and displayed greater anxiety than controls on PND 24, and showed similar trends when the test was repeated on PND 59. No such differences were observed in female pups or in pups exposed to metal mixture. Metal levels analyzed in blood, brain and milk samples suggested a pharmacokinetic interaction between Pb and Mn. Therefore, developmental exposures to low levels of Pb and Mn can produce gender-specific neurobehavioral deficits, and this study underlines the importance of studying toxicants in combination.