Effects of 2-mercaptoethanol and buthionine sulfoximine on cystine metabolism by and proliferation of mitogen-stimulated human and mouse lymphocytes

A Mimic of the Tumor Microenvironment: A Simple Method for Generating Enriched Cell Populations and Investigating Intercellular Communication

Understanding the early heterotypic interactions between cancer cells and the surrounding non-cancerous stroma is important in elucidating the events leading to stromal activation and establishment of the tumor microenvironment (TME). Several in vitro and in vivo models of the TME have been developed; however, in general these models do not readily permit isolation of individual cell populations, under non-perturbing conditions, for further study. To circumvent this difficulty, we have employed an in vitro TME model using a cell growth substrate consisting of a permeable microporous membrane insert that permits simple generation of highly enriched cell populations grown intimately, yet separately, on either side of the insert's membrane for extended co-culture times. Through use of this model, we are capable of generating greatly enriched cancer-associated fibroblast (CAF) populations from normal diploid human fibroblasts following co-culture (120 hr) with highly metastatic human breast carcinoma cells, without the use of fluorescent tagging and/or cell sorting. Additionally, by modulating the pore-size of the insert, we can control for the mode of intercellular communication (e.g., gap-junction communication, secreted factors) between the two heterotypic cell populations, which permits investigation of the mechanisms underlying the development of the TME, including the role of gap-junction permeability. This model serves as a valuable tool in enhancing our understanding of the initial events leading to cancer-stroma initiation, the early evolution of the TME, and the modulating effect of the stroma on the responses of cancer cells to therapeutic agents.

Human CD4+ T cells require exogenous cystine for glutathione and DNA synthesis

Adaptive immune responses require activation and expansion of antigen-specific T cells. Whereas early T cell activation is independent of exogenous cystine (Cys2), T cell proliferation is dependent of Cys2. However, the exact roles of Cys2 in T cell proliferation still need to be determined. The aim of this study was to elucidate why activated human T cells require exogenous Cys2 in order to proliferate. We activated purified naïve human CD4+ T cells and found that glutathione (GSH) levels and DNA synthesis were dependent on Cys2 and increased in parallel with increasing concentrations of Cys2. Vice-versa, the GSH synthesis inhibitor L-buthionine-sulfoximine (BSO) and inhibition of Cys2 uptake with glutamate inhibited GSH and DNA synthesis in parallel. We further found that thioredoxin (Trx) can partly substitute for GSH during DNA synthesis. Finally, we show that GSH or Trx is required for the activity of ribonucleotide reductase (RNR), the enzyme responsible for generation of the deoxyribonucleotide DNA building blocks. In conclusion, we show that activated human T cells require exogenous Cys2 to proliferate and that this is partly explained by the fact that Cys2 is required for production of GSH, which in turn is required for optimal RNR-mediated deoxyribonucleotide synthesis and DNA replication.

Does metabolic reprogramming underpin age-associated changes in T cell phenotype and function?

T cells are required for an effective adaptive immune response. The principal function of T cells is to promote efficient removal of foreign material by identifying and mounting a specific response to nonself. A decline in T cell function in aging is thought to contribute to reduced response to infection and vaccination and an increase in autoimmunity. This may in part be due to the age-related decrease in naïve CD4(+) T cells and increase in antigen-experienced CD4(+) T cells, loss of redox homeostasis, and impaired metabolic switching. Switching between subsets is triggered by the integration of extracellular signals sensed through surface receptors and the activation of discrete intracellular metabolic pathways. This article explores how metabolic programming and loss of redox homeostasis during aging may contribute to age-associated changes in T cell phenotype and function.

Review: 2-mercaptoethanol alteration of in vitro immune functions of species other than murine

Descriptions that organosulfurs could alter biologically relevant cellular functions began some 40 years ago when cell mediated and humoral murine in vitro immune responses were reported to be dramatically enhanced by any of four xenobiotic, sulfhydryl compounds—2-mercaptoethanol (2-ME), dithiothreitol, glutathione, and l-cysteine; the most effective of the four was 2-ME. These findings triggered a plethora of reports defining 2-ME benefits for a multitude of immunological processes, primarily with murine models. This led to investigations on 2-ME alterations of (a) immune functions in other species, (b) activities of other cell-types, and (c) in situ diseases. In addition, the early findings may have been instrumental in the identification of the previously undefined anticarcinogenic chemicals in specific foods as organosulfurs. Outside the plant organosulfurs, there are no comprehensive reviews of these areas to help define mechanisms by which organosulfurs function as well as identify potential alternative uses. Therefore, the present review will focus on 2-ME alterations of in vitro immune functions in species other than murine; namely, fish, amphibian, reptile, avian, whales, dolphins, rat, hamster, rabbit, guinea pig, feline, canine, porcine, ovine, bovine, and human. Processes, some unique to a given species, were in general, enhanced and in some cases dependent upon the presence of 2-ME. The largest benefits occurred in media that were serum free, followed by those in autologous serum and then fetal bovine serum supplemented medium. Concentrations of 2-ME were generally in the low μM range, with exceptions of those for salamander (20 mM), turtles (70 mM) and dolphins (7 mM). The few studies designed to assess mechanisms found that changes induced by 2-ME were generally accompanied by alterations of reduced/oxidized glutathione cellular concentrations. The major benefit for most studies, however, was to increase the sensitivity of the culture environment, which permitted a specific process to be more easily dissected.

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2-ME altered in vitro immune functions of species other than murine.

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Benefits were found for species from fish to humans.

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Enhancement occurred in serum-free and in autologous or fetal bovine serum.

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Generally, optimal concentrations of 2-ME were in the low uM range.

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Concentration exceptions were salamander (20 mM), turtles (70 mM), and dolphins (7 mM).

Healthy ageing and depletion of intracellular glutathione influences T cell membrane thioredoxin-1 levels and cytokine secretion

BACKGROUND

During ageing an altered redox balance has been observed in both intracellular and extracellular compartments, primarily due to glutathione depletion and metabolic stress. Maintaining redox homeostasis is important for controlling proliferation and apoptosis in response to specific stimuli for a variety of cells. For T cells, the ability to generate specific response to antigen is dependent on the oxidation state of cell surface and cytoplasmic protein-thiols. Intracellular thiols are maintained in their reduced state by a network of redox regulating peptides, proteins and enzymes such as glutathione, thioredoxins and thioredoxin reductase. Here we have investigated whether any relationship exists between age and secreted or cell surface thioredoxin-1, intracellular glutathione concentration and T cell surface thioredoxin 1 (Trx-1) and how this is related to interleukin (IL)-2 production.

RESULTS

Healthy older adults have reduced lymphocyte surface expression and lower circulating plasma Trx-1 concentrations. Using buthionine sulfoximine to deplete intracellular glutathione in Jurkat T cells we show that cell surface Trx-1 is lowered, secretion of Trx-1 is decreased and the response to the lectin phytohaemagglutinin measured as IL-2 production is also affected. These effects are recapitulated by another glutathione depleting agent, diethylmaleate.

CONCLUSION

Together these data suggest that a relationship exists between the intracellular redox compartment and Trx-1 proteins. Loss of lymphocyte surface Trx-1 may be a useful biomarker of healthy ageing.

Obesity, longevity, quality of life: alteration by dietary 2-mercaptoethanol

Previous investigations demonstrated that optimization of murine immunological reactivity in tissue culture required a sulfhydryl compound; the most effective being 2-mercaptoethanol (2-Me). Since these reports, 2-Me was found beneficial for both growth/function of other cell-types in vitro, including those of other species, and when fed orally, it impeded and/or reversed some in situ physiological changes associated with aging. More recently, thiol-containing compounds possessing oxidation-reduction potentials weaker than 2-Me were found to impart beneficial effects for many other, including human, diseases. Based on these effects, the research herein addressed the question: What consequences might dietary 2-Me impart on health and disease of mice other than those associated with aging? The main parameters monitored over the lifetime of individual animals exposed to dietary 10⁻³ M 2-Me in their drinking water were: quality of life (obesity and development of recumbent, emaciated and/or cachectic health); longevity; and appearance of tumors. Instead of anticipated toxic attributes, the following unique benefits were found; mean survival of a moderately-lived strain (A/J) was increased 40.8%, high-fat-diet obesity was curtailed in C57BL/10 mice, and a goal of aging intervention protocols, namely preventing loss of quality of life during aging (recumbent, emaciated and/or cachectic) was achieved. Various mechanisms are discussed as they pertain to these findings.

Adaptive responses to low-dose/low-dose-rate gamma rays in normal human fibroblasts: the role of growth architecture and oxidative metabolism

To investigate low-dose/low-dose-rate effects of low-linear energy transfer (LET) ionizing radiation, we used gamma-irradiated cells adapted to grow in a three-dimensional architecture that mimics cell growth in vivo. We determined the cellular, molecular and biochemical changes in these cells. Quiescent normal human fibroblasts were irradiated with single acute or chronic doses (1-10 cGy) of (137)Cs gamma rays. Whereas exposure to an acute dose of 10 cGy increased micronucleus formation, protraction of the dose over 48 h reduced micronucleus frequency to a level similar to or lower than what occurs spontaneously. The protracted treatment also up-regulated the cellular content of the antioxidant glutathione. These changes correlated with modulation of phospho-TP53 (serine 15), a stress marker that was regulated by doses as low as 1 cGy. The DNA damage that occurred after exposure to an acute dose of 10 cGy was protected against in two ways: (1) up-regulation of cellular antioxidant enzyme activity by ectopic overexpression of MnSOD, catalase or glutathione peroxidase, and (2) inhibition of superoxide anion generation by flavin-containing oxidases. These results support a significant role for oxidative metabolism in mediating low-dose radiation effects and demonstrate that cell culture in three dimensions is ideal to investigate radiation-induced adaptive responses. Expression of connexin 43, a constitutive protein of gap junctions, and the G(1) checkpoint were more sensitive to regulation by gamma rays in cells maintained in a three-dimensional than in a two-dimensional configuration.

Exogenous cysteine and cystine promote cell proliferation in CaCo-2 cells

Previous studies have shown that intracellular glutathione, a ubiquitous intracellular thiol, is related to cell proliferation and that cysteine or its disulphide form, cystine, also induces cell proliferation. Cysteine is a thiol containing amino acid and a rate-limiting precursor of glutathione. Therefore, it is still unresolved as to whether the proliferative effect of cysteine or cystine is entirely mediated by a change in the intracellular glutathione status. The objective of this study was to delineate the relationship among cysteine/cystine (thereafter referred to as cyst(e)ine), intracellular glutathione and cell proliferation in the human colon cancer CaCo-2 cell line. CaCo-2 cells were cultured in cyst(e)ine-free Dulbecco's Modified Eagle Medium without serum, and treated with 200 microm cysteine and/or 200-400 microm cystine for 24 h. In the presence of DL-buthionine-[S, R]-sulfoximine (BSO), a glutathione synthesis inhibitor, exogenously administered cyst(e)ine did not change the intracellular glutathione content, but increased the intracellular cysteine as well as cystine level. Addition of exogenous cyst(e)ine following 5 mm BSO treatment significantly increased cell proliferation as measured by 3H-thymidine incorporation and protein content. Cell cycle analyses revealed that cyst(e)ine promoted cell progression from the G1 phase to the S phase. Correspondingly, cyst(e)ine treatment induced expression of cyclin D1 and phosphorylation of retinoblastoma protein (Rb). In conclusion, these data indicate that both cysteine and cystine have proliferative effects in CaCo-2 cells independent of an increase in intracellular glutathione. Induction of cyclin D1, phosphorylation of Rb, and subsequent facilitation of G1-to-S phase transition were involved in the proliferative effect of exogenous cyst(e)ine.

In vitro proliferative response of human peripheral blood mononuclear cells to concanavalin A

The proliferation response of human peripheral blood mononuclear cells (PBMC) to concanavalin A (con A) was tested in a medium with or without addition of fetal calf serum (FCS) or a serum substitute. The time profile of the proliferative response of PBMC to con A was different in cells cultured in a medium supplemented with or without FCS. This different pattern occurred for the con A-induced DNA-synthesis as well as for the RNA- or protein-synthesis. The peak proliferation rate and the stimulation index of proliferation of human PBMC to con A was also higher when the cells were cultured in a medium with 10% FCS compared to cells cultured in a FCS-free medium. Substitution of the fetal calf serum by a serum substitute even induced a profound inhibition of the de novo synthesis of DNA in human PBMC. The results indicate that a lymphocyte proliferation response to con A can still be obtained in a culture medium where no FCS was added, although a weaker stimulation occurred in comparison to a culture medium with FCS. However, addition of a serum substitute caused a marked inhibition of the lymphocyte proliferation rate.

Immunomodulatory and protective effects of N-acetylcysteine in mitogen-activated murine splenocytes in vitro

N-Acetylcysteine (NAC) is a pro-glutathione drug used to treat chronic lung disorders and because of its anti-AIDS virus activity in vitro, has been proposed for AIDS therapy. The effect of NAC on mitogen-activated-lymphocyte blastogenesis in C57B1/6 mouse splenocytes and ability of NAC to protect lymphocytes against mitogen-induced cytotoxicity was examined in vitro. NAC increased splenocyte proliferation in the presence of optimal and suboptimal concentrations of concanavalin A (Con A) and lipopolysaccharide (LPS). Stimulatory and costimulatory effects of NAC on mitogen-induced responses were also evident. The dose-response relationship describing the effects of NAC on lymphocyte proliferation with Con A-induced responses were enhanced in a dose-dependent manner, whereas the corresponding LPS-induced responses increased to a maximum level followed by decline in responses at higher concentrations of NAC. When splenocytes were incubated with inhibitory supraoptimal concentrations of Con A (10 microg/ml) or LPS (150 microg/ml), NAC partially enhanced the Con A-induced response but completely prevented the inhibitory effect of supraoptimal concentrations of LPS on splenocyte blastogenesis. Optimal and supraoptimal concentrations of Con A caused activation-induced cell death in the splenocytes whereas comparable concentrations of LPS did not produce a similar effect. Splenocyte cell death produced by the optimal mitogenic concentrations of Con A was completely blocked by the addition of NAC to cultures. Immunomodulation and protective effects of NAC were observed in mitogen-activated lymphocytes in vitro.

Reducing environment protects sinusoidal lymphocytes isolated from normal human liver from apoptosis

BACKGROUND/AIMS

We previously reported that the populations of lymphocytes and the expression of activated antigens in human sinusoidal mononuclear cells were different from those in peripheral blood mononuclear cells. Attempts to culture these cells for further study failed because they died rapidly under standard culture conditions in vitro after isolation from the liver. In this study, we evaluated the characteristics of cell death and the effects of various culture conditions on the viability of these cells.

METHODS

Sinusoidal mononuclear cells were isolated from University of Wisconsin solution that had been perfused through the portal veins of normal healthy human livers harvested for transplantation into living related recipients.

RESULTS

70% of sinusoidal mononuclear cells cultured in vitro were nonviable within 48 h after isolation, while only 10% of peripheral blood mononuclear cells died under the same conditions. Sinusoidal mononuclear cells showed DNA ladder formation of DNA on electrophoresis and characteristic morphological pattern on electron microscopic examination that suggested they had died in an apoptotic manner. The addition of human liver extracts or 2-mercaptoethanol and reduced glutathione to the cultures rescued the sinusoidal mononuclear cells from apoptosis. Furthermore, diamide, a sulfhydryl group specific oxidant, negated the effect of the liver extract.

CONCLUSION

In comparison with peripheral blood mononuclear cells, human sinusoidal mononuclear cells were more subject to death by apoptosis ex vivo, which was reversed by exogenous agents producing reducing conditions. These results suggested that hepatic sinusoidal mononuclear cells might express a different sensitivity to redox environment than peripheral blood mononuclear cells.

Differential in vitro effects of physiological and atmospheric oxygen tension on normal human peripheral blood mononuclear cell proliferation, cytokine and immunoglobulin production

Since the early work of Mischell & Dutton (Science 153, 1004-1008, 1966), it has been recognized that certain lymphocyte cultures are exquisitely sensitive to the harsh effects of atmospheric oxygen tension. The influence of oxygen partial pressure (pO2) on normal human peripheral blood mononuclear cell (PBMC) phenotype, proliferative ability, cytokine, immunoglobulin production, and redox status was examined by culturing PBMC under ambient oxygen (high pO2) or a more physiological pO2 (5% O2; low pO2). Low pO2 conditions promoted a significant increase in overall viable PBMC number and enhanced Concanavalin A (Con A)- or pokeweed mitogen (PWM)-stimulated PBMC proliferation by approximately 30% and 50%, respectively. No differential pO2 effects were apparent on phytohemagglutinin (PHA)- or staphylococcal enterotoxin B (SEB)-induced proliferation. Both resting and Con A-stimulated lymphocytes incubated for 24 h under high pO2 had a greater baseline carboxy-2',7'-dichlorofluorescin (C-DCF) fluorescence, and were less able to quench the effect of H2O2 treatment compared to lymphocytes cultured under low pO2 conditions. Supernatant gamma-IFN, IL-2, and IL-4 concentrations were elevated 50-65% when PBMC were stimulated with Con A for 24 h under low pO2; however, lipopolysaccharide (LPS)-stimulated IL-1 beta production was reduced by over 75%. PWM-stimulated IgM production by PBMC was significantly reduced in day 7 cultures incubated under low pO2, whereas IgG and IgA production remained relatively unaltered. Immunophenotyping analyses did not reveal any significant alterations in cell subset or marker distribution at the time points examined; however, an interesting trend of increased CD69 expression was observed for Con A-stimulated PBMC incubated under low pO2. These results demonstrate that O2 is a critical parameter for the in vitro culture of lymphocytes, and suggests that varying pO2 may differentially alter PBMC functionality.

N-acetylcysteine-promoted survival of PC12 cells is glutathione-independent but transcription-dependent

Our prior work established that comparable concentrations of N-acetylcysteine (NAC) both block the proliferation of PC12 cells and prevent death of trophic factor-deprived sympathetic neurons and PC12 cells. The present work addresses several aspects of the mechanisms of these actions. NAC increases intracellular levels of glutathione (GSH) by approximately 10-fold in PC12 cells. However, blockade of this increase by treatment with buthionine sulfoximine did not affect either promotion of survival or inhibition of DNA synthesis. Thus, these actions of NAC are independent of its effects on intracellular GSH. NAC's actions in our system do not appear to be dependent on its anti-oxidant/radical scavenger properties, but may be due to its activity as a reductant. Consistent with this, several other reducing agents, the most effective of which was 2,3-dimercaptopropanol, mimicked NAC in blocking DNA synthesis and suppressing death of PC12 cells and sympathetic neurons. Finally, we observed that in striking contrast to nerve growth factor and a number of other trophic agents, the survival-promoting effects of NAC on PC12 cells are blocked by actinomycin D. This suggests that NAC may act by inducing specific gene expression.

N-ras mRNA expression is unaffected in glutathione-depleted cells of hematopoietic origin

Glutathione (GSH) depletion in mitogen-stimulated T lymphocytes has been shown to markedly inhibit their proliferative response. This block in proliferation is associated with a significant reduction in total RNA and DNA synthesis; however, the specific mechanism involved in this inhibition of proliferation is unknown. Miller et al. have reported that lowering intracellular GSH levels by greater than 30%, in murine and human tumor cell lines of non-hematopoietic origin, leads to down-regulation of HA-, Ki- and N-ras oncogene expression [Miller. A.C., Gafner, J., Clark, E.P. and Samid, D. (1993) Mol. Cell Biol., 13, 4416-4422]. The reduction in ras transcript levels correlated with the extent of GSH depletion and was independent of the specific mode of oncogene activation. Since the activity of p21(ras) is thought to be involved in pathways of T cell activation, we set out to determine whether down-regulation of ras expression in T cells could be the mechanism by which T cell proliferation was inhibited in GSH-depleted T lymphocytes. Despite reducing the GSH level of concanavalin A-activated human peripheral blood mononuclear cells by 66%, no effect on ras mRNA expression was observed. Similarly, no reduction of ras transcript levels were detected in a human T cell line (Jurkat) or in a human monocytic cell line (THP-1) depleted of glutathione. Our results demonstrate that the mechanism by which GSH depletion inhibits T cell proliferation does not appear to involve a decrease in ras mRNA expression. In addition, our results suggest that differences in the regulation of ras mRNA expression may exist between lymphoid/monocytic cells of non-hematopoietic origin.

Nucleotide changes in oxidatively stressed lymphocytes

Similar to HIV-1-induced suppression of thymus-derived lymphocytes (T cells), oxidatively stressed T cells show inhibited DNA synthesis and proliferation. The influence of oxidative stress on nucleotide pools was explored using 3H-uridine addition to OKT3-stimulated peripheral blood lymphocytes. The cells were preincubated and stimulated in the presence of 1 mM buthionine sulfoximine to inhibit GSH synthesis. This treatment gave rise to a significant reduction in dUDP and TTP biosynthesis following 18-32 hours stimulation, indicating possible impairment of ribonucleotide reductase activity.

Phospholipid metabolism of lymphocytes with inhibited glutathione synthesis using L-buthionine-S,R-sulfoximine

The potential role of phospholipid metabolism in restricting lymphocyte proliferation under conditions of oxidative stress was investigated using [1-14C]-arachidonic acid (14C-AA) and 32P-orthophosphoric acid. Human peripheral blood lymphocytes (PBL) and PBL depleted of glutathione with L-buthionine-S,R-sulfoximine (BSO-PBL) were compared. The relative uniformity of glutathione depletion in the PBL population was assessed by flow cytometry. BSO-PBL were 40 to 90% depleted of glutathione 1 to 3 days after activation, respectively, and the BSO-PBL had unimpaired early activation events based on 32P-phosphatidylinositol levels. However, unlike stimulated PBL, which showed a progressive decrease in radioactivity incorporated into phosphatidylcholine and a corresponding increase into phosphatidylethanolamine, no significant differences occurred with BSO-PBL. Prelabeled BSO-PBL showed considerably more 14C radioactivity in the supernatant following 72-120 h stimulation with anti-CD3 than control PBL, which was mostly in the form of unmetabolized 14C-AA. Higher levels of leukotriene B4, 5-hydroxyeicosatetraenoate and 12-hydroxy-5,8,10-heptadecatrienoate also were observed with L-buthionine-S,R-sulfoximine treatment, which could explain the impaired proliferation obtained with a depletion of cellular glutathione. Both lysophosphatidylcholine and liberated free 14C-AA increased with L-buthionine-S,R-sulfoximine treatment following 72 h stimulation, suggesting functional impairment in the reacylating enzymes. The increased release of 14C-AA by BSO-PBL also may contribute to the imparied proliferation that occurs with loss of glutathione.