Alteration of glutathione level in human melanoma cells: effect of N-acetyl-L-cysteine and its analogues

A novel methylated analogue of L-Mimosine exerts its therapeutic potency through ROS production and ceramide-induced apoptosis in malignant melanoma

Melanoma is an aggressive and highly metastatic type of skin cancer where the design of new therapies is of utmost importance for the clinical management of the disease. Thus, we have aimed to investigate the mode of action by which a novel methylated analogue of L-Mimosine (e.g., L-SK-4) exerts its therapeutic potency in an in vitro model of malignant melanoma. Cytotoxicity was assessed by the Alamar Blue assay, oxidative stress by commercially available kits, ROS generation, caspase 3/7 activation and mitochondrial membrane depolarisation by flow cytometry, expression of apoptosis-related proteins by western immunoblotting and profiling of lipid biosynthesis by a metabolomic approach. Overall, higher levels of ROS, sphingolipids and apoptosis were induced by L-SK-4 suggesting that the compound's therapeutic potency is mediated through elevated ROS levels which promote the upregulation of sphingolipid (ceramide) biosynthesis thus leading to the activation of both extrinsic and intrinsic apoptosis, in an experimental model of malignant melanoma.

Glutathione and cysteines suppress cytotoxicity of gas phase of cigarette smoke by direct reacting with unsaturated carbonyl compounds in the gas phase

Unsaturated carbonyl compounds, such as acrolein (ACR) and methyl vinyl ketone (MVK), are environmental pollutants, and are contained in smoke, automobile exhaust, and heated oil. We have previously reported that major cytotoxic factors in the gas phase of cigarette smoke are ACR and MVK. ACR and MVK induce cell damage by reactive oxygen species generation via protein kinase C and NADPH oxidases, and antioxidants, such as glutathione (GSH) and N-acetylcysteine (NAC), can effectively suppress their cytotoxic activities. In this study, we attempted to elucidate the molecular mechanism(s) for suppression of ACR- and MVK-induced cytotoxic activities by these antioxidants. GSH, NAC, L- and D-cysteines completely suppressed cell damage induced by gas phase extract of cigarette smoke. The results of HPLC and mass spectrometry showed that GSH and NAC directly reacted with ACR and MVK. Cysteines and cysteine derivatives suppressed ACR-induced GAPDH carbonylation, a representative protein for carbonylation. The current results suggest that GSH, NAC, and cysteines directly reacted with ACR and MVK, and suppressed these unsaturated carbonyl compounds-induced cell damage by inhibition of protein carbonylation.

N-Acetyl-l-Cysteine Protects Astrocytes against Proteotoxicity without Recourse to Glutathione

N-acetyl-l-cysteine (NAC) exhibits protective properties in brain injury models and has undergone a number of clinical trials. Most studies of NAC have focused on neurons. However, neuroprotection may be complemented by the protection of astrocytes because healthier astrocytes can better support the viability of neurons. Here, we show that NAC can protect astrocytes against protein misfolding stress (proteotoxicity), the hallmark of neurodegenerative disorders. Although NAC is thought to be a glutathione precursor, NAC protected primary astrocytes from the toxicity of the proteasome inhibitor MG132 without eliciting any increase in glutathione. Furthermore, glutathione depletion failed to attenuate the protective effects of NAC. MG132 elicited a robust increase in the folding chaperone heat shock protein 70 (Hsp70), and NAC mitigated this effect. Nevertheless, three independent inhibitors of Hsp70 function ablated the protective effects of NAC, suggesting that NAC may help preserve Hsp70 chaperone activity and improve protein quality control without need for Hsp70 induction. Consistent with this view, NAC abolished an increase in ubiquitinated proteins in MG132-treated astrocytes. However, NAC did not affect the loss of proteasome activity in response to MG132, demonstrating that it boosted protein homeostasis and cell viability without directly interfering with the efficacy of this proteasome inhibitor. The thiol-containing molecules l-cysteine and d-cysteine both mimicked the protective effects of NAC, whereas the thiol-lacking molecule N-acetyl-S-methyl-l-cysteine failed to exert protection or blunt the rise in ubiquitinated proteins. Collectively, these findings suggest that the thiol group in NAC is required for its effects on glial viability and protein quality control.

The toxin biliatresone causes mouse extrahepatic cholangiocyte damage and fibrosis through decreased glutathione and SOX17

Biliary atresia, the most common indication for pediatric liver transplantation, is a fibrotic disease of unknown etiology affecting the extrahepatic bile ducts of newborns. The recently described toxin biliatresone causes lumen obstruction in mouse cholangiocyte spheroids and represents a new model of biliary atresia. The goal of this study was to determine the cellular changes caused by biliatresone in mammalian cells that ultimately lead to biliary atresia and extrahepatic fibrosis. We treated mouse cholangiocytes in three-dimensional (3D) spheroid culture and neonatal extrahepatic duct explants with biliatresone and compounds that regulate glutathione (GSH). We examined the effects of biliatresone on SOX17 levels and determined the effects of Sox17 knockdown on cholangiocytes in 3D culture. We found that biliatresone caused disruption of cholangiocyte apical polarity and loss of monolayer integrity. Spheroids treated with biliatresone had increased permeability as shown by rhodamine efflux within 5 hours compared with untreated spheroids, which retained rhodamine for longer than 12 hours. Neonatal bile duct explants treated with the toxin showed lumen obstruction with increased subepithelial staining for α-smooth muscle actin and collagen, consistent with fibrosis. Biliatresone caused a rapid and transient decrease in GSH, which was both necessary and sufficient to mediate its effects in cholangiocyte spheroid and bile duct explant systems. It also caused a significant decrease in cholangiocyte levels of SOX17, and Sox17 knockdown in cholangiocyte spheroids mimicked the effects of biliatresone.

CONCLUSION

Biliatresone decreases GSH and SOX17 in mouse cholangiocytes. In 3D cell systems, this leads to cholangiocyte monolayer damage and increased permeability; in extrahepatic bile duct explants, it leads to disruption of the extrahepatic biliary tree and subepithelial fibrosis. This mechanism may be important in understanding human biliary atresia. (Hepatology 2016;64:880-893).

Stereoisomers of cysteine and its analogs Potential effects on chemo- and radioprotection strategies

The thiol-containing amino acid, cysteine, and its analogs are useful for a variety of protective applications, including protecting normal tissues against the unwanted side effects of cancer chemotherapeutic agents and radiation treatment. The protection can result from both direct action of the amino acid and/or after its conversion to glutathione (GSH), sulfate, or other sulfur-based protective substances. Unfortunately, high GSH levels have been implicated in the problematic development of tumor cells' resistance to therapy. Due to numerous differences in the metabolic processing of the cysteine stereoisomers, chemo- and radioprotective strategies might be developed using the D-form of the amino acid, which can participate in protection directly, but which cannot be used to support GSH biosynthesis. In this way, protection of normal tissue may be achieved, while the potential development of resistance in tumor cells is minimized. Greatly enhanced therapeutic efficacy of cancer treatment regimens may be the result.

Cytotoxicity of dopamine-derived tetrahydroisoquinolines on melanoma cells

Tetrahydroisoquinolines (TIQs) are endogenous alkaloid compounds detected in urine, central nervous system and some peripheral tissues in Mammalia. No data are at present available on TIQ levels in skin, although in vitro biochemical evidences indicate that they may undergo auto-oxidation with production of reactive oxygen species or may be enzymatically converted into melanin pigments. The effect of two catechol-bearing TIQs, salsolinol (SAL) and tetrahydropapaveroline (THP), on the viability of melanotic or amelanotic melanoma cell lines was investigated. Both SAL and THP were well tolerated up to roughly 30 microM and became overtly toxic at higher concentrations, with SAL being better tolerated than THP. Intracellular activity of some antioxidant enzymes, tyrosinase and alpha-ketoglutarate dehydrogenase was also evaluated to assess the cell response to oxidative and metabolic challenge of TIQs treatment. Catalase and superoxide dismutase pre-treatment only partially prevented TIQs toxicity while a complete protection was obtained with N-acetylcysteine and GSH. TIQs were able to provoke upregulation of the scavenging enzymes catalase and DT-diaphorase and to determine a decrease of the alpha-ketoglutarate dehydrogenase activity. SAL and THP enhanced tyrosinase activity and melanin production, suggesting that they were indeed tyrosinase substrates leading to melanin formation. The results support the evidence that TIQs were toxic toward melanoma cells, leading to their death by necrosis. TIQs toxicity was likely due to increased oxidative stress by generation of reactive oxygen species and oxidative metabolites. Our study represents an intent to furnish an additional contribution for the comprehension of catechol cytotoxicity.

5-(Pentafluorobenzoylamino)fluorescein: A selective substrate for the determination of glutathione concentration and glutathione S-transferase activity

5-(Pentafluorobenzoylamino)fluorescein (PFB-F), a new thiol-reactive molecule was synthesized to improve the detection limits and specificity of the assays for glutathione S-transferase (GST) activity and glutathione (GSH). A rapid assay method to measure GSH concentration or GST activity and the simultaneous analysis of multiple samples is possible because the glutathione adduct, GS-TFB-F, is separated from PFB-F by thin-layer chromatography (TLC) and can be quantitated by a fluorescence scanner. The detection limits for GSH and for GST activity using TLC were found to be as low as 10 pmol/microl and 1 ng/microl using equine liver GST, respectively. Determination of GSH concentration or GST activity in bovine pulmonary artery endothelial (BPAE) cell lysates gave a linear response for samples corresponding to 500-2500 cells. PFB-F could also measure GST activities of GST fusion proteins and prove to be a suitable substrate for determining the activities of human GST isozymes and other sources of mammalian GST. The selectivity of PFB-F with GSH was proven by comparing trace amount of the adducts that formed with cysteine and beta-galactosidase to that formed with GSH. The HPLC profile of a reaction mixture where cell lysate was used in place of purified GST, also shows only two main peaks, corresponding to GS-TFB-F and unreacted PFB-F. The selectivity of PFB-F for GSH was further confirmed by exposing BPAE cells to dl-buthionine-[S,R]-sulfoximine (BSO). Our results of GS-TFB-F determination indicate that 12-, 24-, or 36-h incubations with BSO caused 2-, 6-, or 7.6-fold reductions in GSH levels, respectively.

Alpha-lipoic acid blocks HIV-1 LTR-dependent expression of hygromycin resistance in THP-1 stable transformants

Gene expression of human immunodeficiency virus (HIV) depends on a host cellular transcription factors including nuclear factor-kappaB (NF-kappaB). The involvement of reactive oxygen intermediates (ROI) has been implicated as intracellular messengers in the inducible activation of NF-kappaB. In this study, we compared the efficacy of two antioxidants, alpha-lipoic acid (LA) and N-acetylcysteine (NAC), which are widely recognized NF-kappaB inhibitors. Here, we demonstrate that LA has a more potent activity in inhibiting NF-KappaB-mediated gene expression in THP-1 cells that have been stably transfected with a plasmid bearing a hygromycin B resistance gene under the control of HIV-1 long terminal repeat (LTR) promoter. The spontaneous activation of NF-kappaB in this cell culture system leads to expression of the hygromycin phosphotransferase gene hence rendering the cells resistance to hygromycin B. In this study, the effect of the test compounds against transcriptional activity of HIV-1 LTR was evaluated based on the degree of cellular toxicity due to the inhibitory activity on the expression of hygromycin B resistance gene in the presence of hygromycin B. We also found that 0.2 mM LA could cause 40% reduction in the HIV-1 expression from the TNF-alpha-stimulated OM 10.1, a cell line latently infected with HIV-1. On the other hand, 10 mM NAC was required to elicit the same effect. Furthermore, the initiation of HIV-1 induction by TNF-alpha was completely abolished by 1 mM LA. These findings confirm the involvement of ROI in NF-kappaB-mediated HIV gene expression as well as the efficacy of LA as a therapeutic regimen for HIV infection and acquired immunodeficiency syndrome (AIDS). Moreover, this study validates the applicability of our present assay system which we primarily designed for the screening of candidate drugs against HIV-1 gene expression.

Glutathione plays a key role in the depigmenting and melanocytotoxic action of N-acetyl-4-S-cysteaminylphenol in black and yellow hair follicles

This study examined the effect of glutathione on the in vivo depigmenting potency of N-acetyl-4-S-cysteaminylphenol (N-acetyl-4-S-CAP) in black and yellow mice after multiple intraperitoneal injections on 10 consecutive days. In black mice (C57BL/6J, a/a), N-acetyl-4-S-CAP showed dose-dependent depigmenting potency (0.5, 1.0, and 2.0 mmol/kg), which was in parallel to the tissue eumelanin content (98%, 28%, and 3% of controls, respectively) and to the tissue glutathione content (94%, 85%, and 76%, respectively). In lethal yellow mice (C57BL/6J, Ay/a), only a dose of 2.0 mmol/kg showed the color change of hair to dark, not to white as seen in black mice. This was reflected by the decrease of pheomelanin content (56%) and the increase of eumelanin content (28% of black mice). The simultaneous administration of N-acetyl-cysteine, which up-regulated glutathione content, completely abolished the depigmenting potency of N-acetyl-4-S-CAP, whereas administration of buthionine sulfoximine, which depleted the tissue glutathione content, enhanced the depigmenting potency of N-acetyl-4-S-CAP in black hair. In yellow mice, the darkening of hair follicles by 2.0 mmol/kg of N-acetyl-4-S-CAP was completely abolished by the combined administration of N-acetyl-cysteine, with the resulting hair color the same as in controls, whereas combined administration with buthionine sulfoximine caused some whitening of yellow hair follicles. Our data indicate that the tissue content of glutathione regulates melanocytotoxicity and depigmenting potency of N-acetyl-4-S-CAP and that this alteration of glutathione content may switch the melanogenesis type from pheomelanin to eumelanin.

Radioprotective effect of N-acetylcysteine in vitro using the induction of DNA breaks as end-point

Cultures of Chinese hamster cells were treated with N-acetylcysteine (NAC) and irradiated under hypoxic or aerobic conditions. Protection factors (PFs) were calculated using the yield of DNA breaks as a measure of the radiation response. With NAC up to 50 mmol dm-3, the PF values were smaller for aerobically- than for hypoxically-irradiated cells. In contrast with NAC concentrations > 100 mmol dm-3, the PF values for aerobically-irradiated cells were larger than those for hypoxic cells. This reversal is in agreement with the predictions of a modified competition model, the 'X-model'. The value of X (the ratio of reparable to non-reparable damage) was calculated to be close to 5 for the NAC-treated cells, i.e. about 18% of the radiation damage was not reparable. The increase of the total non-protein bound thiols (NPSH) and GSH content after NAC treatment was measured both in whole cells and isolated nuclei. Measurement of the cellular NPSH content showed a continuous increase with increasing NAC concentrations in the medium, independent of treatment time. In contrast, the amount of intracellular and intranuclear GSH increased with prolonged incubation time with NAC. No quantitative relationship was found between PF values and cellular NPSH or GSH content.