Growth-associated changes in glutathione content correlate with liver metastatic activity of B16 melanoma cells

Involvement of the modulation of cancer cell redox status in the anti-tumoral effect of phenolic compounds

The association between the anti-tumoral properties of phenolics, the generation of ROS in culture medium and modulation of redox homeostasis was analyzed. In AGS cells, the anti-proliferative effect of quercetin was not reverted by catalase or SOD.

Glucocorticoid receptor knockdown decreases the antioxidant protection of B16 melanoma cells: an endocrine system-related mechanism that compromises metastatic cell resistance to vascular endothelium-induced tumor cytotoxicity

We previously reported an interorgan system in which stress-related hormones (corticosterone and noradrenaline), interleukin-6, and glutathione (GSH) coordinately regulate metastatic growth of highly aggressive B16-F10 melanoma cells. Corticosterone, at levels measured in tumor-bearing mice, also induces apoptotic cell death in metastatic cells with low GSH content. In the present study we explored the potential role of glucocorticoids in the regulation of metastatic cell death/survival during the early stages of organ invasion. Glucocorticoid receptor (GCR) knockdown decreased the expression and activity of γ-glutamylcysteine synthetase (γ-GCS), the rate-limiting step in GSH synthesis, in metastatic cells in vivo independent of the tumor location (liver, lung, or subcutaneous). The decrease in γ-GCS activity was associated with lower intracellular GSH levels. Nrf2- and p53-dependent down-regulation of γ-GCS was associated with a decrease in the activities of superoxide dismutase 1 and 2, catalase, glutathione peroxidase, and glutathione reductase, but not of the O₂⁻-generating NADPH oxidase. The GCR knockdown-induced decrease in antioxidant protection caused a drastic decrease in the survival of metastatic cells during their interaction with endothelial cells, both in vitro and in vivo; only 10% of cancer cells attached to the endothelium survived compared to 90% survival observed in the controls. This very low rate of metastatic cell survival was partially increased (up to 52%) in vivo by inoculating B16-F10 cells preloaded with GSH ester, which enters the cell and delivers free GSH. Taken together, our results indicate that glucocorticoid signaling influences the survival of metastatic cells during their interaction with the vascular endothelium.

RedOx status, proteasome and APEH: insights into anticancer mechanisms of t10,c12-conjugated linoleic acid isomer on A375 melanoma cells

This study describes the investigation of the efficiency of conjugated linoleic acid (CLA) isomers in reducing cancer cells viability exploring the role of the oxidative stress and acylpeptide hydrolase (APEH)/proteasome mediated pathways on pro-apoptotic activity of the isomer trans10,cis12 (t10,c12)-CLA. The basal activity/expression levels of APEH and proteasome (β-5 subunit) were preliminarily measured in eight cancer cell lines and the functional relationship between these enzymes was clearly demonstrated through their strong positive correlation. t10,c12-CLA efficiently inhibited the activity of APEH and proteasome isoforms in cell-free assays and the negative correlation between cell viability and caspase 3 activity confirmed the pro-apoptotic role of this isomer. Finally, modulatory effects of t10,c12-CLA on cellular redox status (intracellular glutathione, mRNA levels of antioxidant/detoxifying enzymes activated through NF-E2-related factor 2, Nrf2, pathway) and on APEH/β-5 activity/expression levels, were investigated in A375 melanoma cells. Dose- and time-dependent variations of the considered parameters were established and the resulting pro-apoptotic effects were shown to be associated with an alteration of the redox status and a down-regulation of APEH/proteasome pathway. Therefore, our results support the idea that these events are involved in ROS-dependent apoptosis of t10,c12-CLA-treated A375 cells. The combined inhibition, triggered by t10,c12-CLA, via the modulation of APEH/proteasome and Nrf2 pathway for treating melanoma, is suggested as a subject for further in vivo studies.

Role of glutathione in cancer progression and chemoresistance

Glutathione (GSH) plays an important role in a multitude of cellular processes, including cell differentiation, proliferation, and apoptosis, and disturbances in GSH homeostasis are involved in the etiology and progression of many human diseases including cancer. While GSH deficiency, or a decrease in the GSH/glutathione disulphide (GSSG) ratio, leads to an increased susceptibility to oxidative stress implicated in the progression of cancer, elevated GSH levels increase the antioxidant capacity and the resistance to oxidative stress as observed in many cancer cells. The present review highlights the role of GSH and related cytoprotective effects in the susceptibility to carcinogenesis and in the sensitivity of tumors to the cytotoxic effects of anticancer agents.

Stress hormones promote growth of B16-F10 melanoma metastases: an interleukin 6- and glutathione-dependent mechanism

BACKGROUND

Interleukin (IL)-6 (mainly of tumor origin) activates glutathione (GSH) release from hepatocytes and its interorgan transport to B16-F10 melanoma metastatic foci. We studied if this capacity to overproduce IL-6 is regulated by cancer cell-independent mechanisms.

METHODS

Murine B16-F10 melanoma cells were cultured, transfected with red fluorescent protein, injected i.v. into syngenic C57BL/6J mice to generate lung and liver metastases, and isolated from metastatic foci using high-performance cell sorting. Stress hormones and IL-6 levels were measured by ELISA, and CRH expression in the brain by in situ hybridization. DNA binding activity of NF-κB, CREB, AP-1, and NF-IL-6 was measured using specific transcription factor assay kits. IL-6 expression was measured by RT-PCR, and silencing was achieved by transfection of anti-IL-6 small interfering RNA. GSH was determined by HPLC. Cell death analysis was distinguished using fluorescence microscopy, TUNEL labeling, and flow cytometry techniques. Statistical analyses were performed using Student's t test.

RESULTS

Plasma levels of stress-related hormones (adrenocorticotropin hormone, corticosterone, and noradrenaline) increased, following a circadian pattern and as compared to non-tumor controls, in mice bearing B16-F10 lung or liver metastases. Corticosterone and noradrenaline, at pathophysiological levels, increased expression and secretion of IL-6 in B16-F10 cells in vitro. Corticosterone- and noradrenaline-induced transcriptional up-regulation of IL-6 gene involves changes in the DNA binding activity of nuclear factor-κB, cAMP response element-binding protein, activator protein-1, and nuclear factor for IL-6. In vivo inoculation of B16-F10 cells transfected with anti-IL-6-siRNA, treatment with a glucocorticoid receptor blocker (RU-486) or with a β-adrenoceptor blocker (propranolol), increased hepatic GSH whereas decreased plasma IL-6 levels and metastatic growth. Corticosterone, but not NORA, also induced apoptotic cell death in metastatic cells with low GSH content.

CONCLUSIONS

Our results describe an interorgan system where stress-related hormones, IL-6, and GSH coordinately regulate metastases growth.

Glutathione synthesis

BACKGROUND

Glutathione (GSH) is present in all mammalian tissues as the most abundant non-protein thiol that defends against oxidative stress. GSH is also a key determinant of redox signaling, vital in detoxification of xenobiotics, and regulates cell proliferation, apoptosis, immune function, and fibrogenesis. Biosynthesis of GSH occurs in the cytosol in a tightly regulated manner. Key determinants of GSH synthesis are the availability of the sulfur amino acid precursor, cysteine, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL), which is composed of a catalytic (GCLC) and a modifier (GCLM) subunit. The second enzyme of GSH synthesis is GSH synthetase (GS).

SCOPE OF REVIEW

This review summarizes key functions of GSH and focuses on factors that regulate the biosynthesis of GSH, including pathological conditions where GSH synthesis is dysregulated.

MAJOR CONCLUSIONS

GCL subunits and GS are regulated at multiple levels and often in a coordinated manner. Key transcription factors that regulate the expression of these genes include NF-E2 related factor 2 (Nrf2) via the antioxidant response element (ARE), AP-1, and nuclear factor kappa B (NFκB). There is increasing evidence that dysregulation of GSH synthesis contributes to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary and liver fibrosis, alcoholic liver disease, cholestatic liver injury, endotoxemia and drug-resistant tumor cells.

GENERAL SIGNIFICANCE

GSH is a key antioxidant that also modulates diverse cellular processes. A better understanding of how its synthesis is regulated and dysregulated in disease states may lead to improvement in the treatment of these disorders. This article is part of a Special Issue entitled Cellular functions of glutathione.

Disturbances in the glutathione/ophthalmate redox buffer system in the woodchuck model of hepatitis virus-induced hepatocellular carcinoma

Purpose. The incidence of liver tumors is rising in USA. The purpose of this study was to evaluate liver oxido-reductive status in the presence of chronic liver disease and hepatocellular carcinoma (HCC). Methods. Glutathione species and ophthalmate (OA) concentrations were measured by LC-MS in processed plasma and red blood cells (RBC) from infected Woodchuck with hepatitis virus (WHV). Blood samples were obtained from: (i) infected animals with tumors (WHV+/HCC+), (ii) infected animals without tumors (WHV+/HCC−) and (iii) healthy animals (WHC−/HCC−). Results. The concentration of reduced glutathione (GSH) and the ratio GSH/GSG were lower in plasma from WHV+/HCC+ animals when compared to WHV+/HCC− and WHV−/HCC− (P < 0.01). In contrast, the concentration of oxidized glutathione (GSSG) was found to be higher in plasma from WHV+/HCC+ animals when compared to WHV+/HCC− and WHV−/HCC− (P < 0.01). The Glutathione species and its ratio from the RBC compartment were similar among all groups. OA concentration in both plasma and RBC was significantly higher from WHV+/HCC+ when compared to WHV+/HCC− and WHV−/HCC− (P < 0.01). Conclusions. Disturbances of the glutathione redox buffer system and higher concentrations of OA were found in the WCV+/HCC+ animal model. The role of these compounds as biomarkers of early tumor development in patients with end stage liver disease remains to be determined.

Electrophoretic fingerprint metallothionein analysis as a potential prostate cancer biomarker

Prostate-specific antigen (PSA) is a routinely used marker of prostate cancer; however, the cut-off values for unambiguous positive/negative prostate cancer diagnoses are not defined. Therefore, despite the best effort, certain percentage of misdiagnosed cases is being recorded every year. For this reason, search for more specific diagnostic markers is of great interest. In this study, systematic comparison of PSA and metallothionein (MT) levels in blood serum of 46 prostate cancer-diagnosed patients is presented. It is clearly demonstrated that PSA levels vary significantly and despite normal total PSA values in the range of 0 - 4 ng/mL were obtained in over 36.9% of cases, positive prostate cancer was diagnosed by biopsy. In contrary, MT levels were considerably elevated in all tested samples and no significant variations were observed. These results are indicating the potential of MT as an additional prostate cancer marker reducing, in combination with PSA, the probability of false positive/negative diagnosis. To increase the throughput of the screening, chip-based capillary electrophoresis was suggested as a rapid and effective method for the fingerprinting analysis of prostate cancer from diseased blood sera.

Glutathione in cancer cell death

Glutathione (L-γ-glutamyl-L-cysteinyl-glycine; GSH) in cancer cells is particularly relevant in the regulation of carcinogenic mechanisms; sensitivity against cytotoxic drugs, ionizing radiations, and some cytokines; DNA synthesis; and cell proliferation and death. The intracellular thiol redox state (controlled by GSH) is one of the endogenous effectors involved in regulating the mitochondrial permeability transition pore complex and, in consequence, thiol oxidation can be a causal factor in the mitochondrion-based mechanism that leads to cell death. Nevertheless GSH depletion is a common feature not only of apoptosis but also of other types of cell death. Indeed rates of GSH synthesis and fluxes regulate its levels in cellular compartments, and potentially influence switches among different mechanisms of death. How changes in gene expression, post-translational modifications of proteins, and signaling cascades are implicated will be discussed. Furthermore, this review will finally analyze whether GSH depletion may facilitate cancer cell death under in vivo conditions, and how this can be applied to cancer therapy.

Intertissue flow of glutathione (GSH) as a tumor growth-promoting mechanism: interleukin 6 induces GSH release from hepatocytes in metastatic B16 melanoma-bearing mice

B16 melanoma F10 (B16-F10) cells with high glutathione (GSH) content show high metastatic activity in vivo. An intertissue flow of GSH, where the liver is the main reservoir, can increase GSH content in metastatic cells and promote their growth. We have studied here possible tumor-derived molecular signals that could activate GSH release from hepatocytes. GSH efflux increases in hepatocytes isolated from mice bearing liver or lung metastases, thus suggesting a systemic mechanism. Fractionation of serum-free conditioned medium from cultured B16-F10 cells and monoclonal antibody-induced neutralization techniques facilitated identification of interleukin (IL)-6 as a tumor-derived molecule promoting GSH efflux in hepatocytes. IL-6 activates GSH release through a methionine-sensitive/organic anion transporter polypeptide 1- and multidrug resistance protein 1-independent channel located on the sinusoidal site of hepatocytes. Specific siRNAs were used to knock down key factors in the main signaling pathways activated by IL-6, which revealed a STAT3-dependent mechanism. Our results show that IL-6 (mainly of tumor origin in B16-F10-bearing mice) may facilitate GSH release from hepatocytes and its interorgan transport to metastatic growing foci.

The expression and activity of cystathionine-γ-lyase and 3-mercaptopyruvate sulfurtransferase in human neoplastic cell lines

The expression and activity of cystathionine γ-lyase (CST) and 3-mercaptopyruvate sulfurtransferase (MPST) were investigated in the human neoplastic cells lines: astrocytoma U373, neuroblastoma SH-SY5Y, melanoma A375, and melanoma WM35. Gene expression analysis demonstrated that the investigated neoplastic cells showed the expression of MPST and what is particularly interesting, the expression of CST. The presence of CST in these cells was confirmed using RT-PCR and western blot analysis. However, in U373 cells, a very low activity of CST was detected. In all the investigated cell lines, the activity of MPST was higher than that of CST, which suggests that in these cells, the main pathway of sulfane sulfur formation is the MPST-catalyzed reaction. RP-HPLC analysis showed a large disparity between the level of cystathionine and GSH in the investigated neoplastic cells. In SH-SY5Y cells, the low level of GSH and low GSH/GSSG ratio corresponded with the highest CST activity. Further investigations could aim at verifying whether the stimulation of CST, at the level of protein or gene expression, could change the proliferation of neoplastic cells.

Oxidative and nitrosative stress in the metastatic microenvironment

Metastases that are resistant to conventional therapies are the main cause of most cancer-related deaths in humans. Tumor cell heterogeneity, which associates with genomic and phenotypic instability, represents a major problem for cancer therapy. Additional factors, such as the attack of immune cells or organ-specific microenvironments, also influence metastatic cell behavior and the response to therapy. Interaction of cancer and endothelial cells in capillary beds, involving mechanical contact and transient adhesion, is a critical step in the initiation of metastasis. This interaction initiates a cascade of activation pathways that involves cytokines, growth factors, bioactive lipids and reactive oxygen and nitrogen species (ROS and RNS) produced by either the cancer cell or the endothelium. Vascular endothelium-derived NO and H₂O₂ are cytotoxic for the cancer cells, but also help to identify some critical molecular targets that appear essential for survival of invasive metastatic cell subsets. Surviving cancer cells that extravasate and start colonization of an organ or tissue can still be attacked by macrophages and be influenced by specific intraorgan microenvironment conditions. At all steps; from the primary tumor until colonization of a distant organ; metastatic cells undergo a dynamic process of constant adaptations that may lead to the survival of highly resistant malignant cell subsets. In this sequence of molecular events both ROS and RNS play key roles.

Reactive oxygen species and melanoma: an explanation for gender differences in survival?

Epidemiological research consistently shows a female advantage in melanoma survival. So far, no definite candidate for the explanation of this phenomenon has emerged. We propose that gender differences in oxidative stress caused by radical oxygen species (ROS) underlie these survival differences. It is known that males express lower amounts of anti-oxidant enzymes, resulting in more oxidative stress than females. The primary melanoma environment is characterized by high ROS levels, from exogenous sources as well as ROS production within melanoma cells themselves. ROS are known to be able to promote metastasis through a wide variety of mechanisms. We hypothesize that the higher levels of ROS in men enhance selection of ROS-resistance in melanoma cells. Subsequently, ROS can stimulate the metastatic potential of melanoma cells. In addition, due to the lower anti-oxidant defenses in men, ROS produced by melanoma cells cause more damage to healthy tissues surrounding the tumor, further stimulating metastasis. Therefore, ROS may explain the observed differences between males and females in melanoma survival.

Glutathione in Cancer Biology and Therapy

The glutathione (GSH) content of cancer cells is particularly relevant in regulating mutagenic mechanisms, DNA synthesis, growth, and multidrug and radiation resistance. In malignant tumors, as compared with normal tissues, that resistance associates in most cases with higher GSH levels within these cancer cells. Thus, approaches to cancer treatment based on modulation of GSH should control possible growth-associated changes in GSH content and synthesis in these cells. Despite the potential benefits for cancer therapy of a selective GSH-depleting strategy, such a methodology has remained elusive up to now. Metastatic spread, not primary tumor burden, is the leading cause of cancer death. For patient prognosis to improve, new systemic therapies capable of effectively inhibiting the outgrowth of seeded tumor cells are needed. Interaction of metastatic cells with the vascular endothelium activates local release of proinflammatory cytokines, which act as signals promoting cancer cell adhesion, extravasation, and proliferation. Recent work shows that a high percentage of metastatic cells with high GSH levels survive the combined nitrosative and oxidative stresses elicited by the vascular endothelium and possibly by macrophages and granulocytes. ?-Glutamyl transpeptidase overexpression and an inter-organ flow of GSH (where the liver plays a central role), by increasing cysteine availability for tumor GSH synthesis, function in combination as a metastatic-growth promoting mechanism. The present review focuses on an analysis of links among GSH, adaptive responses to stress, molecular mechanisms of invasive cancer cell survival and death, and sensitization of metastatic cells to therapy. Experimental evidence shows that acceleration of GSH efflux facilitates selective GSH depletion in metastatic cells.

Regulation of glutathione synthesis

Glutathione (GSH) is a ubiquitous intracellular peptide with diverse functions that include detoxification, antioxidant defense, maintenance of thiol status, and modulation of cell proliferation. GSH is synthesized in the cytosol of all mammalian cells in a tightly regulated manner. The major determinants of GSH synthesis are the availability of cysteine, the sulfur amino acid precursor, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL). GCL is composed for a catalytic (GCLC) and modifier (GCLM) subunit and they are regulated at multiple levels and at times differentially. The second enzyme of GSH synthesis, GSH synthase (GS) is also regulated in a coordinated manner as GCL subunits and its up-regulation can further enhance the capacity of the cell to synthesize GSH. Oxidative stress is well known to induce the expression of GSH synthetic enzymes. Key transcription factors identified thus far include Nrf2/Nrf1 via the antioxidant response element (ARE), activator protein-1 (AP-1) and nuclear factor kappa B (NFkappaB). Dysregulation of GSH synthesis is increasingly being recognized as contributing to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary fibrosis, cholestatic liver injury, endotoxemia and drug-resistant tumor cells. Manipulation of the GSH synthetic capacity is an important target in the treatment of many of these disorders.

The prometastatic microenvironment of the liver

The liver is a major metastasis-susceptible site and majority of patients with hepatic metastasis die from the disease in the absence of efficient treatments. The intrahepatic circulation and microvascular arrest of cancer cells trigger a local inflammatory reaction leading to cancer cell apoptosis and cytotoxicity via oxidative stress mediators (mainly nitric oxide and hydrogen peroxide) and hepatic natural killer cells. However, certain cancer cells that resist or even deactivate these anti-tumoral defense mechanisms still can adhere to endothelial cells of the hepatic microvasculature through proinflammatory cytokine-mediated mechanisms. During their temporary residence, some of these cancer cells ignore growth-inhibitory factors while respond to proliferation-stimulating factors released from tumor-activated hepatocytes and sinusoidal cells. This leads to avascular micrometastasis generation in periportal areas of hepatic lobules. Hepatocytes and myofibroblasts derived from portal tracts and activated hepatic stellate cells are next recruited into some of these avascular micrometastases. These create a private microenvironment that supports their development through the specific release of both proangiogenic factors and cancer cell invasion- and proliferation-stimulating factors. Moreover, both soluble factors from tumor-activated hepatocytes and myofibroblasts also contribute to the regulation of metastatic cancer cell genes. Therefore, the liver offers a prometastatic microenvironment to circulating cancer cells that supports metastasis development. The ability to resist anti-tumor hepatic defense and to take advantage of hepatic cell-derived factors are key phenotypic properties of liver-metastasizing cancer cells. Knowledge on hepatic metastasis regulation by microenvironment opens multiple opportunities for metastasis inhibition at both subclinical and advanced stages. In addition, together with metastasis-related gene profiles revealing the existence of liver metastasis potential in primary tumors, new biomarkers on the prometastatic microenvironment of the liver may be helpful for the individual assessment of hepatic metastasis risk in cancer patients.

Effects of hepatocyte growth factor on glutathione synthesis, growth, and apoptosis is cell density-dependent

Hepatocyte growth factor (HGF) is a potent hepatocyte mitogen that exerts opposing effects depending on cell density. Glutathione (GSH) is the main non-protein thiol in mammalian cells that modulates growth and apoptosis. We previously showed that GSH level is inversely related to cell density of hepatocytes and is positively related to growth. Our current work examined whether HGF can modulate GSH synthesis in a cell density-dependent manner and how GSH in turn influence HGF's effects. We found HGF treatment of H4IIE cells increased cell GSH levels only under subconfluent density. The increase in cell GSH under low density was due to increased transcription of GSH synthetic enzymes. This correlated with increased protein levels and nuclear binding activities of c-Jun, c-Fos, p65, p50, Nrf1 and Nrf2 to the promoter region of these genes. HGF acts as a mitogen in H4IIE cells under low cell density and protects against tumor necrosis factor alpha (TNFalpha)-induced apoptosis by limiting JNK activation. However, HGF is pro-apoptotic under high cell density and exacerbates TNFalpha-induced apoptosis by potentiating JNK activation. The increase in cell GSH under low cell density allows HGF to exert its full mitogenic effect but is not necessary for its anti-apoptotic effect.

Bcl-2 and glutathione depletion sensitizes B16 melanoma to combination therapy and eliminates metastatic disease

PURPOSE

Advanced melanoma resists all current therapies, and metastases in the liver are particularly problematic. Prevalent resistance factors include elevated glutathione (GSH) and increased expression of bcl-2 in melanoma cells. GSH has pleiotropic effects promoting cell growth and broad resistance to therapy, whereas Bcl-2 inhibits the activation of apoptosis and contributes to elevation of GSH. This study determined the in vivo efficacy of combination therapies administered while GSH and Bcl-2 were individually and simultaneously decreased in metastatic melanoma lesions.

EXPERIMENTAL DESIGN

Highly metastatic murine B16 melanoma (B16M-F10) cells have elevated levels of both GSH and Bcl-2. B16M-F10 cells were injected i.v. to establish metastatic lesions in vivo. GSH was decreased using an L-glutamine--enriched diet and administration of verapamil and acivicin, whereas Bcl-2 was reduced using oligodeoxynucleotide G3139. Paclitaxel, X-rays, tumor necrosis factor-alpha, and IFN-gamma were administered as a combination therapy.

RESULTS

Metastatic cells were isolated from liver to confirm the depletion of GSH and Bcl-2 in vivo. Reduction of Bcl-2 and GSH, combined with partial therapies, decreased the number and volume of invasive B16M-F10 foci in liver by up to 99% (P<0.01). The full combination of paclitaxel, X-rays, and cytokines eliminated B16M-F10 cells from liver and all other systemic disease, leading to long-term survival (>120 days) without recurrence in 90% of mice receiving the full therapy. Toxicity was manageable; the mice recovered quickly, and hematology and clinical chemistry data were representative of accepted clinical toxicities.

CONCLUSIONS

Our results suggest a new strategy to induce regression of late-stage metastatic melanoma.

Ruthenium complexes can target determinants of tumour malignancy

Metastases are more decisive for tumour prognosis than primary lesions, because of their multiple locations, low accessibility to surgery and/or radiotherapy, and generally poor responsiveness to chemotherapy. The metastasis should therefore be the primary target for drug therapy. Among ruthenium complexes, NAMI-A is a leading compound that shows selective effects for solid tumour metastases related to a mechanism of action involving the inhibition of the processes of tumour invasiveness. NAMI-A opens an avenue to new perspectives in cancer chemotherapy. This includes novel compounds directed at targets selectively expressed by tumour metastases, thus reducing the typical side effects of the current metal-based drugs that are active via their unselective DNA interaction.

Importance of the trans-sulfuration pathway in cancer prevention and promotion

The trans-sulfuration pathway is a biochemical mechanism that links methionine metabolism to the biosynthesis of cellular redox-controlling molecules, like cysteine, glutathione, and taurine. While there is some knowledge about the metabolic intermediates and enzymes that participate in trans-sulfuration, little is known about the physiological importance of this mechanism. Deficiencies within the trans-sulfuration pathway induces (i) the generation of reactive species of oxygen (ROS) and halogens (RHS), (ii) homocyst(e)ine accumulation, and (iii) the synthesis of proinflammatory molecules by macrophages, and contribute to humans pathologies like atherosclerosis and tumor development. In this review we outline the role of this biochemical pathway in tumor development and analyze current findings on the role of trans-sulfuration in mammalian physiology. The potential relationship between chronic inflammation, and tumor and atherosclerotic development are discussed.

Bcl-2 and Mn-SOD antisense oligodeoxynucleotides and a glutamine-enriched diet facilitate elimination of highly resistant B16 melanoma cells by tumor necrosis factor-alpha and chemotherapy

Mitochondrial glutathione (mtGSH) depletion increases sensitivity of Bcl-2-overexpressing B16 melanoma (B16M)-F10 cells (high metastatic potential) to tumor necrosis factor-alpha (TNF-alpha)-induced oxidative stress and death in vitro. In vivo, mtGSH depletion in B16M-F10 cells was achieved by feeding mice (where the B16M-F10 grew as a solid tumor in the footpad) with an L-glutamine (L-Gln)-enriched diet, which promoted in the tumor cells an increase in glutaminase activity, accumulation of cytosolic L-glutamate, and competitive inhibition of GSH transport into mitochondria. L-Gln-adapted B16M-F10 cells, isolated using anti-Met-72 monoclonal antibodies and flow cytometry-coupled cell sorting, were injected into the portal vein to produce hepatic metastases. In l-Gln-adapted invasive (iB16M-Gln+) cells, isolated from the liver by the same methodology and treated with TNF-alpha and an antisense Bcl-2 oligodeoxynucleotide, viability decreased to approximately 12%. iB16M-Gln+ cell death associated with increased generation of O2*- and H2O2, opening of the mitochondrial permeability transition pore complex, and release of proapoptotic molecular signals. Activation of cell death mechanisms was prevented by GSH ester-induced mtGSH replenishment. The oxidative stress-resistant survivors showed an adaptive response that includes overexpression of manganese-containing superoxide dismutase (Mn-SOD) and catalase activities. By treating iB16M-Gln+ cells with a double anti- antisense therapy (Bcl-2 and SOD2 antisense oligodeoxynucleotides) and TNF-alpha, metastatic cell survival decreased to approximately 1%. Chemotherapy (taxol plus daunorubicin) easily removed this minimum percentage of survivors. This contribution identifies critical molecules that can be sequentially targeted to facilitate elimination of highly resistant metastatic cells.

Association between pterostilbene and quercetin inhibits metastatic activity of B16 melanoma

Inhibition of cancer growth by resveratrol (trans-3,5,4'-trihydroxystilbene; RESV), a phytoalexin present in many plant species, is limited by its low bioavailability. Pterostilbene (3,5-dimethoxy-4'-hydroxystilbene; PTER) and quercetin (3,3',4',5,6-pentahydroxyflavone; QUER), two structurally related and naturally occurring small polyphenols, show longer half-life in vivo. In vitro growth of highly malignant B16 melanoma F10 cells (B16M-F10) is inhibited (56%) by short-time exposure (60 min/day) to PTER (40 microm) and QUER (20 microm) (approximate mean values of plasma concentrations measured within the first hour after intravenous administration of 20 mg/kg each polyphenol). Intravenous administration of PTER and QUER (20 mg/kg per day) to mice inhibits (73%) metastatic growth of B16M-F10 cell in the liver, a common site for metastasis development. The anti-metastatic mechanism involves: 1) a PTER-induced inhibition of vascular adhesion molecule 1 expression in the hepatic sinusoidal endothelium, which consequently decreases B16M-F10 cell adhesion to the endothelium through very late activation antigen 4; and 2) a QUER- and PTER-induced inhibition of Bcl-2 expression in metastatic cells, which sensitizes them to vascular endothelium-induced cytotoxicity. Our findings demonstrate that the association of PTER and QUER inhibits metastatic melanoma growth and extends host survival.

Antioxidants and cancer therapy: furthering the debate

The consideration of whether to use antioxidants concomitantly with chemotherapy and radiation therapy has evolved into a heated debate. This special theme issue brings together several contributors to this debate, whose perspectives enlarge our views of the questions at hand, pointing out several very relevant ideas. First, the early hypotheses of the role of antioxidants in carcinogenesis gave a simplified and often inaccurate picture of the physiological effects of specific antioxidants. Antioxidants can have protective effects that have nothing to do with oxidation; on the other hand, they can under some circumstances develop prooxidant properties and promote carcinogenesis. During treatment, however, their role is far from clear and may be either quite positive or potentially negative. A number of clinical studies have already demonstrated beneficial effects of antioxidants in ameliorating side effects of chemotherapy. More theoretical work on the chemistry of antioxidants and chemotherapy drugs suggests that antioxidants might improve therapeutic efficacy of antineoplastics by counteracting aldehydes that impede the passage of cells through the cell cycle. However, detailed clinical study also makes it clear that we are only at the very beginning of understanding the dynamics of antioxidants and oxidant damage in the body during conventional treatment. Nevertheless, research is under way on radioprotective and chemoprotective substances, some of them rooted in traditional medicine and others in our understanding of dietary antioxidants, that may eventually lead to antioxidant-based supplements that support tolerability and efficacy of treatment, without protecting tumors through interference from antineoplastic treatment.

Glutamine regulates amino acid transport and glutathione levels in a human neuroblastoma cell line

Both amino acid transport and glutathione play a key role in regulating cancer cell growth. Glutamine can serve as an important ATP source for cancer cells, and it can supply glutamate, a precursor for the synthesis of glutathione, by the hydrolysis of glutamine. We examined the effects of glutamine concentrations [2 mM (control), 400 microM, 200 microM, and 0 microM] on cell growth, amino acid transport, and glutathione levels in a human neuroblastoma cell line, SK-N-SH, by using cell culture technique. Cell growth rates were dependent on glutamine concentrations in culture media. Glutamate transport significantly increased in glutamine-deprived groups, and this increase was remarkable in lower glutamine groups (200 microM and 0 microM glutamine). Glutamine deprivation resulted in a significant decrease in glutathione levels by 20% compared with control, but glutathione in 0 microM glutamine was maintained with the same levels found in 400 microM and 200 microM glutamine. DNA and protein synthesis correlated directly with glutamine concentrations in culture media. Our results suggest that glutamine mediates neuroblastoma cell proliferation by regulating amino acid transport and glutathione synthesis, both when sufficient nutrients are present and when key nutrients such as glutamine are in limited supply.

Butyrate reduces liver metastasis of rat colon carcinoma cells in vivo and resistance to oxidative stress in vitro

Injection of the rat colon carcinoma cell line CC531 into spleen of syngeneic rats results in considerable amounts of liver metastases within 14 days. We investigated whether preincubation of the cells with butyrate reduced their metastatic ability in vivo and whether this was accompanied by reduction in related properties such as secretion of metalloproteinases and their ability to withstand oxidative stress. Butyrate incubation reduced cell growth rate and initiated apoptosis in a dose- and time-related manner, but proliferation was retrieved when cultivation was continued in medium without butyrate. Splenic injection of butyrate treated, proliferating cells resulted in significantly reduced amounts of tumor mass compared to untreated cells. The butyrate treated cells were more susceptible to oxidative stress than control cells, as demonstrated by increased number of apoptotic cells and reduced cell growth after exposure to menadione. A reduction in cellular glutathione was found after prolonged incubation with butyrate. Butyrate appeared not to alter the secretion of active metalloproteinases from the cells although an apparent increase in proforms was demonstrated. Neither did butyrate alter the synthesis of metalloproteinase inhibitors. Lastly, a reduced adhesion of the tumor cells to collagen coated matrix was found after butyrate treatment. Thus, the inhibitory effects of butyrate on tumor malignancy are caused by a diversity of mechanisms.

Polyamine depletion switches the form of 2-deoxy-D-ribose-induced cell death from apoptosis to necrosis in HL-60 cells

Our previous studies demonstrated that intracellular polyamine depletion blocked HL-60 cell apoptosis triggered by exposure to 2-deoxy-d-ribose (dRib). Here, we have characterized the intracellular events underlying the apoptotic effects of dRib and the involvement of polyamines in these effects. Treatment of HL-60 cells with dRib induces loss of mitochondrial transmembrane potential, radical oxygen species production, intracellular glutathione depletion and translocation of Bax from cytosol to membranes. These effects are followed by cell death. However, the mode of cell death caused by dRib depends on intracellular levels of polyamines. d-Rib-treated cells with normal polyamine levels, progressing through the G(1) into the S and G(2)/M phases, undergo apoptosis, while in polyamine-depleted cells, being blocked at the G(1) phase, cell death mechanisms are switched to necrosis. The present study points to a relationship between the cell cycle distribution and the mode of cell death, and suggests that the level of intracellular spermidine, essential to cell cycle progression, may determine whether a cell dies by apoptosis or necrosis in response to a death stimulus.

Acceleration of glutathione efflux and inhibition of gamma-glutamyltranspeptidase sensitize metastatic B16 melanoma cells to endothelium-induced cytotoxicity

Highly metastatic B16 melanoma (B16M)-F10 cells, as compared with the low metastatic B16M-F1 line, have higher GSH content and preferentially overexpress BCL-2. In addition to its anti-apoptotic properties, BCL-2 inhibits efflux of GSH from B16M-F10 cells and thereby may facilitate metastatic cell resistance against endothelium-induced oxidative/nitrosative stress. Thus, we investigated in B16M-F10 cells which molecular mechanisms channel GSH release and whether their modulation may influence metastatic activity. GSH efflux was abolished in multidrug resistance protein 1 knock-out (MRP-/-1) B16M-F10 transfected with the Bcl-2 gene or in MRP-/-1 B16M-F10 cells incubated with l-methionine, which indicates that GSH release from B16M-F10 cells is channeled through MRP1 and a BCL-2-dependent system (likely related to an l-methionine-sensitive GSH carrier previously detected in hepatocytes). The BCL-2-dependent system was identified as the cystic fibrosis transmembrane conductance regulator, since monoclonal antibodies against this ion channel or H-89 (a protein kinase A-selective inhibitor)-induced inhibition of cystic fibrosis transmembrane conductance regulator gene expression completely blocked the BCL-2-sensitive GSH release. By using a perifusion system that mimics in vivo conditions, we found that GSH depletion in metastatic cells can be achieved by using Bcl-2 antisense oligodeoxynucleotide- and verapamil (an MRP1 activator)-induced acceleration of GSH efflux, in combination with acivicin-induced inhibition of gamma-glutamyltranspeptidase (which limits GSH synthesis by preventing cysteine generation from extracellular GSH). When applied under in vivo conditions, this strategy increased tumor cytotoxicity (up to approximately 90%) during B16M-F10 cell adhesion to the hepatic sinusoidal endothelium.

Potentiation of antimetastatic activity of pentoxifylline in B16F10 and B16F1 melanoma cells through inhibition of glutathione content

PURPOSE

The aim of this work is to evaluate the antioxidative activity of antimetastatic agent pentoxifylline (PTX) in B16F10 and B16F1 melanoma cells.

MATERIALS AND METHODS

B16F10 and B16F1 cell lines were treated with PTX with different doses at different time intervals. Reduced glutathione, glutathione-S-transferase, and lipid peroxides were estimated to evaluate the action of PTX.

RESULTS

The activity of pentoxifylline treatment showed glutathione depletion, increased glutathione-S-transferase activity with upregulation of lipid peroxidation in B16F10 and B16F1 melanoma cells. However, the effect on GSH, GST, and LPOs was dependent on dose concentrations and time intervals.

CONCLUSIONS

Pentoxifylline, an effective membrane modifier, showed significant depletion in glutathione level in conjunction with increased lipid peroxidation. The results suggested an antimetastatic property by PTX at a nontoxic dose in B16F10 and B16F1 melanoma cells.

Down-regulation of glutathione and Bcl-2 synthesis in mouse B16 melanoma cells avoids their survival during interaction with the vascular endothelium

B16 melanoma (B16M) cells with high GSH content show high metastatic activity. However, the molecular mechanisms linking GSH to metastatic cell survival are unclear. The possible relationship between GSH and the ability of Bcl-2 to prevent cell death was studied in B16M cells with high (F10) and low (F1) metastatic potential. Analysis of a Bcl-2 family of genes revealed that B16M-F10 cells, as compared with B16M-F1 cells, overexpressed preferentially Bcl-2 (approximately 5.7-fold). Hepatic sinusoidal endothelium-induced B16M-F10 cytotoxicity in vitro increased from approximately 19% (controls) to approximately 97% in GSH-depleted B16M-F10 cells treated with an antisense Bcl-2 oligodeoxynucleotide (Bcl-2-AS). l-Buthionine (S,R)-sulfoximine-induced GSH depletion or Bcl-2-AS decreased the metastatic growth of B16M-F10 cells in the liver. However, the combination of l-buthionine (S,R)-sulfoximine and Bcl-2-AS abolished metastatic invasion. Bcl-2-overexpressing B16M-F1/Tet-Bcl-2 and B16M-F10/Tet-Bcl-2 cells, as compared with controls, showed an increase in GSH content, no change in the rate of GSH synthesis, and a decrease in GSH efflux. Thus, Bcl-2 overexpression may increase metastatic cell resistance against oxidative/nitrosative stress by inhibiting release of GSH. In addition, Bcl-2 availability regulates the mitochondrial GSH (mtGSH)-dependent opening of the permeability transition pore complex. Death in B16M-F10 cells was sharply activated at mtGSH levels below 30% of controls values. However, this critical threshold increased to approximately 60% of control values in Bcl-2-AS-treated B16M-F10 cells. GSH ester-induced replenishment of mtGSH levels (even under conditions of cytosolic GSH depletion) prevented cell death. Our results indicate that survival of B16M cells with high metastatic potential can be challenged by inhibiting their GSH and Bcl-2 synthesis.

Tumor cytotoxicity by endothelial cells. Impairment of the mitochondrial system for glutathione uptake in mouse B16 melanoma cells that survive after in vitro interaction with the hepatic sinusoidal endothelium

High GSH content associates with high metastatic activity in B16-F10 melanoma cells cultured to low density (LD B16M). GSH homeostasis was investigated in LD B16M cells that survive after adhesion to the hepatic sinusoidal endothelium (HSE). Invasive B16M (iB16M) cells were isolated using anti-Met-72 monoclonal antibodies and flow cytometry-coupled cell sorting. HSE-derived NO and H(2)O(2) caused GSH depletion and a decrease in gamma-glutamylcysteine synthetase activity in iB16M cells. Overexpression of gamma-glutamylcysteine synthetase heavy and light subunits led to a rapid recovery of cytosolic GSH, whereas mitochondrial GSH (mtGSH) further decreased during the first 18 h of culture. NO and H(2)O(2) damaged the mitochondrial system for GSH uptake (rates in iB16M were approximately 75% lower than in LD B16M cells). iB16M cells also showed a decreased activity of mitochondrial complexes II, III, and IV, less O(2) consumption, lower ATP levels, higher O(2) and H(2)O(2) production, and lower mitochondrial membrane potential. In vitro growing iB16M cells maintained high viability (>98%) and repaired HSE-induced mitochondrial damages within 48 h. However, iB16M cells with low mtGSH levels were highly susceptible to TNF-alpha-induced oxidative stress and death. Therefore depletion of mtGSH levels may represent a critical target to challenge survival of invasive cancer cells.

Increased resistance towards oxidative stress accompanies enhancement of metastatic potential obtained by repeated in vivo passage of colon carcinoma cells in syngeneic rats

The colon carcinoma cell line CC531 is metastatic to liver after splenic injection in syngeneic rats. After repeated in vivo passages, a subline was selected that produced liver metastases at a considerably higher rate than the original cell line. These cells were characterized by increased intracellular glutathione, proliferation and ability to restore glutathione after exposure to oxidative stress, thus indicating an elevated resistance to oxidative stress. Furthermore, the increased metastatic ability was also accompanied by increased proliferation rate, adhesion to extracellular matrix proteins and endothelial cells, and secretion of a 60 kD matrix metalloproteinase. When cultured in vitro for a prolonged time (more than 30 trypsinizations), the cells showed a reduced in vivo metastatic ability, reduced secretion of three metalloproteinases including the 60 kD proteinase, and reduced intracellular glutathione. These results indicate that metastatic ability can be influenced through several adaptive mechanisms, and that the cell's ability to resist oxidative stress and maintain intracellular glutathione are of central importance.

Inhibition of cancer growth by resveratrol is related to its low bioavailability

The relationship between resveratrol (RES) bioavalability and its effect on tumor growth was investigated. Tissue levels of RES were studied after i.v. and oral administration of trans-resveratrol (t-RES) to rabbits, rats, and mice. Half-life of RES in plasma, after i.v. administration of 20 mg t-RES/kg b.wt., was very short (e.g., 14.4 min in rabbits). The highest concentration of RES in plasma, either after i.v. or oral administration (e.g., 2.6 +/- 1.0 microM in mice 2.5 min after receiving 20 mg t-RES/kg orally), was reached within the first 5 min in all animals studied. Extravascular levels (brain, lung, liver, and kidney) of RES, which paralleled those in plasma, were always < 1 nmol/g fresh tissue. RES measured in plasma or tissues was in the trans form (at least 99%). Hepatocytes metabolized t-RES in a dose-dependent fashion (e.g., 43 nmol of t-RES/g x min in the presence of 20 microM tRES), which means that the liver can remove circulating RES very rapidly. In vitro B16 melanoma (B16M) cell proliferation and generation of reactive oxygen species (ROS) was inhibited by t-RES in a concentration-dependent fashion (100% inhibition of tumor growth was found in the presence of 5 microM t-RES). Addition of 10 microM H(2)O(2) to B16M cells, cultured in the presence of 5 microM t-RES, reactivated cell growth. Oral administration of t-RES (20 mg/kg twice per day; or included in the drinking water at 23 mg/l) did not inhibit growth of B16M inoculated into the footpad of mice (solid growth). However, oral administration of t-RES (as above) decreased hepatic metastatic invasion of B16M cells inoculated intrasplenically. The antimetastatic mechanism involves a t-RES (1 microM)-induced inhibition of vascular adhesion molecule 1 (VCAM-1) expression in the hepatic sinusoidal endothelium (HSE), which consequently decreased in vitro B16M cell adhesion to the endothelium via very late activation antigen 4 (VLA-4).

Glutamine and its relationship with intracellular redox status, oxidative stress and cell proliferation/death

Glutamine is a multifaceted amino acid used for hepatic urea synthesis, renal ammoniagenesis, gluconeogenesis in both liver and kidney, and as a major respiratory fuel for many cells. Decreased glutamine concentrations are found during catabolic stress and are related to susceptibility to infections. Besides, glutamine is not only an important energy source in mitochondria, but is also a precursor of the brain neurotransmitter glutamate, which is likewise used for biosynthesis of the cellular antioxidant glutathione. Reactive oxygen species, such as superoxide anions and hydrogen peroxide, function as intracellular second messengers activating, among others, apoptosis, whereas glutamine is an apoptosis suppressor. In fact, it could contribute to block apoptosis induced by exogenous agents or by intracellular stimuli. In conclusion, this article shows evidences for the important role of glutamine in the regulation of the cellular redox balance, including brain oxidative metabolism, apoptosis and tumour cell proliferation.

Modeling of the anticancer action for radical derivatives of nitroazoles: quantitative structure-activity relationship (QSAR) study

A QSAR analysis of the anti-tumor, anti-metastasis and anti-colony formation (for metastatic colonies) activities of eighteen nitroazoles (including metronidazole and hypoxic radiosensitizers RP-170, KU-2285 and sanazole (drug AK-2123)) and their nitro and nitroso anion radical derivatives against melanoma B16 in mice has been performed. The QSAR models were built with the use of the frontal polygon method. This approach has features of different 3D QSAR methodologies and belongs to the group of "indirect" methods. The procedure allows to build robust models with high predictive ability even in series of diverse and conformationally flexible compounds. Key atomic characteristics accompany the geometrical requirements in the analysis of local 3D molecular similarity. By variation of weight coefficients for hydrophobicity, refraction increments, and partial charge it is possible to achieve better quality of QSAR and evaluate the importance of each characteristic for biological activity under consideration. It was found that hydrophobicity, molar refraction and charge characteristics of nitro anion radical derivatives are more significant for interaction with molecular targets than those of the parent compounds and of the nitroso anion radical derivatives. Size and hydrophobic properties of substituents in nitro anion radicals play significant role for ligand-target interaction in the processes of inhibition of metastatic spreading and growth of metastatic colonies by nitroazoles. A scheme of competitive interaction of parent nitroazoles and their anion radicals with a target in organism is suggested. It can be concluded that the step of one-electron reduction of nitroazoles can be important for anticancer activity of these drugs.

Extra-cellular thiol metabolism in clones of human metastatic melanoma with different gamma-glutamyl transpeptidase expression: implications for cell response to platinum-based drugs

Thiol redox status can affect important functions both intracellularly and extracellularly. The plasma membrane enzyme gamma-glutamyl transpeptidase (GGT), which plays a crucial role in cellular handling of thiols, is often expressed in malignant tumors, including melanoma, although its expression levels may vary widely among different tumors or cells of the same tumor. In an attempt to better understand the functional significance of GGT overexpression, we have examined the relationships between intra- and extra-cellular thiol metabolism and GGT expression. Intra- and extra-cellular distribution of glutathione and other low mol. wt. thiols and disulfides was investigated in two different Me665/2 human melanoma clones that originated from the same metastasis, but exhibiting high (2/60 clone) and low (2/21 clone) GGT activity. Intracellular content of glutathione was lower in GGT-rich 2/60 cells, in spite of high GGT expression. A lower utilization of extracellular cystine was also observed in these cells. In both clones, a direct secretion of cysteine in the extracellular medium was detected, which was independent of GGT-mediated catabolism of extracellular glutathione. Substantial amounts of glutathione, GSSG and glutathione-cysteine disulfide were accumulated extracellularly only in the case of GGT-poor 2/21 cells, while the same event was apparent in 2/60 cells only after the following inhibition of GGT activity. When exposed to the trinuclear platinum compound BBR 3464 or hydrogen peroxide, which are very reactive for sulfur-containing nucleophiles, the 2/60 clone showed higher sensitivity than the 2/21 clone to both agents. These results suggest that the clone-specific balance between transport of sulfur aminoacids and GGT activity results in profound differences in the capability of each clone to modify the thiol redox status of the extracellular milieu. The finding may have important implications in tumor cell behavior with particular reference to chemosensitivity, since thiols are recognized factors in modulation of cell sensitivity to platinum-based anticancer drugs.

gamma-Glutamyl transpeptidase overexpression increases metastatic growth of B16 melanoma cells in the mouse liver

B16 melanoma (B16M) cells with high glutathione (GSH) content show rapid proliferation in vitro and high metastatic activity in the liver in vivo. gamma-Glutamyl transpeptidase (GGT)-mediated extracellular GSH cleavage and intracellular GSH synthesis were studied in vitro in B16M cells with high (F10) and low (F1) metastatic potential. GGT activity was modified by transfection with the human GGT gene (B16MF1/Tet-GGT cells) or by acivicin-induced inhibition. B16MF1/Tet-GGT and B16MF10 cells exhibited higher GSH content (35 +/- 6 and 40 +/- 5 nmol/10(6) cells, respectively) and GGT activity (89 +/- 9 and 37 +/- 7 mU/10(6) cells, respectively) as compared (P <.05) with B16MF1 cells (10 +/- 3 nmol GSH and 4 mU GGT/10(6) cells). Metastasis (number of foci/100 mm(3) of liver) increased in B16MF1 cells pretreated with GSH ester ( approximately 3-fold, P <.01), and decreased in B16MF1/Tet-GGT and B16MF10 cells pretreated with the GSH synthesis inhibitor L-buthionine (S,R)-sulphoximine ( approximately 5-fold and 2-fold, respectively, P <.01). Liver, kidney, brain, lung, and erythrocyte GSH content in B16MF1/Tet-GGT- or B16MF10-bearing mice decreased as compared with B16MF1- and non-tumor-bearing mice. Organic anion transporting polypeptide 1-independent sinusoidal GSH efflux from hepatocytes increased in B16MF1/Tet-GGT- or B16MF10-bearing mice ( approximately 2-fold, P <.01) as compared with non-tumor-bearing mice. Our results indicate that tumor GGT activity and an intertissue flow of GSH can regulate GSH content of melanoma cells and their metastatic growth in the liver.

Tumoricidal activity of endothelial cells. Inhibition of endothelial nitric oxide production abrogates tumor cytotoxicity induced by hepatic sinusoidal endothelium in response to B16 melanoma adhesion in vitro

The mechanism of NO- and H(2)O(2)-induced tumor cytotoxicity was examined during B16 melanoma (B16M) adhesion to the hepatic sinusoidal endothelium (HSE) in vitro. We used endothelial nitric-oxide synthetase gene disruption and N(G)-nitro-l-arginine methyl ester-induced inhibition of nitric-oxide synthetase activity to study the effect of HSE-derived NO on B16M cell viability. Extracellular H(2)O(2) was removed by exogenous catalase. H(2)O(2) was not cytotoxic in the absence of NO. However, NO-induced tumor cytotoxicity was increased by H(2)O(2) due to the formation of potent oxidants, likely ( small middle dot)OH and (-)OONO radicals, via a trace metal-dependent process. B16M cells cultured to low density (LD cells), with high GSH content, were more resistant to NO and H(2)O(2) than B16M cells cultured to high density (HD cells; with approximately 25% of the GSH content found in LD cells). Resistance of LD cells decreased using buthionine sulfoximine, a specific GSH synthesis inhibitor, whereas resistance increased in HD cells using GSH ester, which delivers free intracellular GSH. Because NO and H(2)O(2) were particularly cytotoxic in HD cells, we investigated the enzyme activities that degrade H(2)O(2). NO and H(2)O(2) caused an approximately 75% (LD cells) and a 60% (HD cells) decrease in catalase activity without affecting the GSH peroxidase/GSH reductase system. Therefore, B16M resistance to the HSE-induced cytotoxicity appears highly dependent on GSH and GSH peroxidase, which are both required to eliminate H(2)O(2). In agreement with this fact, ebselen, a GSH peroxidase mimic, abrogated the increase in NO toxicity induced by H(2)O(2).