Glutathione depletion overcomes resistance to arsenic trioxide in arsenic-resistant cell lines

Genetic polymorphisms in MTHFR 677 and 1298, GSTM1 and T1, and metabolism of arsenic

Methylation is the primary route of metabolism of inorganic arsenic in humans, and previous studies showed that interindividual differences in arsenic methylation may have important impacts on susceptibility to arsenic-induced cancer. To date, the factors that regulate arsenic methylation in humans are mostly unknown. Urinary arsenic methylation patterns and genetic polymorphisms in methylenetetrahydrofolate reductase (MTHFR) and glutathione S-transferase (GST) were investigated in 170 subjects from an arsenic-exposed region in Argentina. Previous studies showed that subjects with the TT/AA polymorphisms at MTHFR 677 and 1298 have lower MTHFR activity than others. In this study, it was found that subjects with the TT/AA variant of MTHFR 677/1298 excreted a significantly higher proportion of ingested arsenic as inorganic arsenic and a lower proportion as dimethylarsinic acid. Women with the null genotype of GSTM1 excreted a significantly higher proportion of arsenic as monomethylarsonate than women with the active genotype. No associations were seen between polymorphisms in GSTT1 and arsenic methylation. This is the first study to report (1) associations between MTHFR and arsenic metabolism in humans, and (2) gender differences between genetic polymorphisms and urinary arsenic methylation patterns. Overall, this study provides evidence that MTHFR and GSTM1 are involved in arsenic metabolism in humans, and polymorphisms in the genes that encode these enzymes may play a role in susceptibility to arsenic-induced cancer.

Buthionine sulfoximine enhancement of arsenic trioxide-induced apoptosis in leukemia and lymphoma cells is mediated via activation of c-Jun NH2-terminal kinase and up-regulation of death receptors

The mechanism of apoptosis induced by treatment with As(2)O(3) alone or in combination with buthionine sulfoximine (BSO) was studied in NB4, U937, Namalwa, and Jurkat cells. As(2)O(3) at concentrations <2 micromol/L induced apoptosis in NB4 cells and Namalwa cells but not in U937 and Jurkat cells. As(2)O(3)-induced apoptosis in NB4 cells and Namalwa cells correlated with increase of H(2)O(2) and caspase activation without activation of c-Jun NH(2)-terminal kinase (JNK). BSO (10 micromol/L) depleted the reduced form of intracellular glutathione without inducing apoptosis but synergized with 1 micromol/L As(2)O(3) to induce apoptosis in all four cell lines. This synergy correlated with JNK activation. Treatment with As(2)O(3) plus BSO, but not with As(2)O(3) alone, increased the levels of death receptor (DR) 5 protein and caspase-8 cleavage. The JNK inhibitor SP600125 inhibited the increase in DR5 protein and attenuated apoptosis induced by treatment with As(2)O(3) plus BSO. These observations suggest that a DR-mediated pathway activated by JNK is involved in apoptosis induced by treatment with As(2)O(3) plus BSO.

Efficacy and safety results with the combination therapy of arsenic trioxide, dexamethasone, and ascorbic acid in multiple myeloma patients: a phase 2 trial

BACKGROUND

Single-agent arsenic trioxide has shown promising results in patients with relapsed or refractory multiple myeloma (MM). Because preclinical data suggested greater activity with dexamethasone and ascorbic acid, a phase 2 trial of the combination of arsenic trioxide, dexamethasone, and ascorbic acid in patients with relapsed or refractory MM was conducted.

METHODS

Twenty patients in whom no more than two previous therapies had failed were enrolled. The mean age was 62 yr, and 55% of the patients had refractory disease. The regimen consisted of 14- or 15-wk cycles, with the first cycle considered induction, followed by one or two consolidation cycles with a reduced steroid schedule and then a maintenance cycle in responding patients.

RESULTS

The overall response rate was 30%, with at least stable disease in 80% of patients. Median progression- free survival was 316 d in all patients and 584 d in those with a response. The regimen was well tolerated, with most adverse events being mild or moderate.

CONCLUSIONS

This study showed the clinical efficacy and tolerability of the combination of arsenic trioxide, dexamethasone, and ascorbic acid. Further study is warranted.

Role of the p38 mitogen-activated protein kinase pathway in the generation of arsenic trioxide-dependent cellular responses

Arsenic trioxide (As(2)O(3)) induces differentiation and apoptosis of leukemic cells in vitro and in vivo, but the precise mechanisms that mediate such effects are not known. In the present study, we provide evidence that the kinases MAPK kinase 3 (Mkk3) and Mkk6 are activated during treatment of leukemic cell lines with As(2)O(3) to regulate downstream engagement of the p38 mitogen-activated protein kinase. Using cells with targeted disruption of both the Mkk3 and Mkk6 genes, we show that As(2)O(3)-dependent activation of p38 is defective in the absence of Mkk3 and Mkk6, establishing that these kinases are essential for As(2)O(3)-dependent engagement of the p38 pathway. Pharmacologic inhibition of p38 enhances As(2)O(3)-dependent activation of the c-jun NH(2)-terminal kinase (JNK) and subsequent induction of apoptosis of chronic myelogenous leukemia (CML)- or acute promyelocytic leukemia (APL)-derived cell lines. In addition, in APL blasts, inhibition of p38 enhances myeloid cell differentiation in response to As(2)O(3), as well as suppression of Bcl-2 expression and loss of mitochondrial membrane potential. Similarly, induction of As(2)O(3)-dependent apoptosis is enhanced in mouse embryonic fibroblasts (MEF) with targeted disruption of both the Mkk3 and Mkk6 genes, establishing a key role for this pathway in the regulation of As(2)O(3)-induced apoptosis. In other studies, we show that the small-molecule p38 inhibitors SD-282 and SCIO-469 potentiate As(2)O(3)-mediated suppression of myeloid leukemic progenitor growth from CML patients, indicating a critical regulatory role for p38 in the induction of antileukemic responses. Altogether, our data indicate that the Mkk3/6-p38 signaling cascade is activated in a negative regulatory feedback manner to control induction of As(2)O(3)-mediated antileukemic effects.

Increased cellular glutathione and protection by bone marrow stromal cells account for the resistance of non-acute promylocytic leukemia acute myeloid leukemia cells to arsenic trioxide in vivo

Arsenic trioxide (ATO) is a novel agent for acute promylocytic leukemia (APL). Studies performed in vitro have demonstrated that ATO also induces cell-cycle arrest and apoptosis in multiple cancers, including non-APL acute myeloid leukemia (AML). To explore the potential use of ATO on non-APL AML, we treated the leukemic cells in vivo using a NOD/SCID animal model. Mice harboring HL-60 or NB-4 leukemia or primary AML-M2 cells were treated daily with 5 mug/g ATO intraperitoneally for a maximum of 6 weeks. Although ATO initially appeared to be effective on HL-60 cells, it failed to decrease the leukemic cells in bone marrow (BM) after the extended treatment (52.2 +/- 10.7% vs. 62.2 +/- 2.6% in the controls; P = 0.51); whereas the same treatment to NB-4 leukemic mice significantly decreased the percentage of leukemic cells in BM. ATO also failed to eradicate the primary AML cells in vivo. The reason for the treatment failure was that HL-60 cells quickly developed resistance in vivo. The drug resistance could be partly attributable to the increase of cellular glutathione as a result of compensatory response to ATO treatment because depletion of glutathione with buthionine sulfoximine reversed the drug resistance in vitro. Meanwhile, BM stromal cells also contributed to the drug resistance. Leukemic cells grown on an adherent layer of MS-5 stromal cells in the presence of ATO were more proliferative and less apoptotic and had increased expression cyclin D1, Bcl-xL and Bcl-2 and decreased expression of p21, likely protecting the leukemic cells from ATO cytotoxicity. Therefore, our study suggests that strategies to inhibit the compensatory increase of glutathione and block the interaction between leukemic cells and BM stromal cells should be employed before applying ATO to non-APL hematologic malignancies.

The emerging role of arsenic trioxide as an immunomodulatory agent in the management of multiple myeloma

Multiple myeloma is a clonal disorder of plasma cells which is considered incurable with currently available therapies. Substantial advances have been achieved in the past decade with the identification of cellular mechanisms that confer drug resistance. This has resulted in newer agents such as arsenic trioxide (Trisenoxt), lenalidomide (Revlimid) and bortezomib (Velcade) with promising activity in this disease. In this review article we will outline the history, mechanisms of action, pharmacology, and clinical trials of arsenic trioxide in multiple myeloma.

Flavonoid-induced glutathione depletion: potential implications for cancer treatment

The ability of a number of flavonoids to induce glutathione (GSH) depletion was measured in lung (A549), myeloid (HL-60), and prostate (PC-3) human tumor cells. The hydroxychalcone (2'-HC) and the dihydroxychalcones (2',2-, 2',3-, 2',4-, and 2',5'-DHC) were the most effective in A549 and HL-60 cells, depleting more than 50% of intracellular GSH within 4 h of exposure at 25 microM. In contrast, the flavones chrysin and apigenin were the most effective in PC-3 cells, depleting 50-70% of intracellular GSH within 24 h of exposure at 25 microM. In general, these flavonoids were more effective than three classical substrates of multidrug resistance protein 1 (MK-571, indomethacin, and verapamil). Prototypic flavonoids (2',5'-DHC and chrysin) were subsequently tested for their abilities to potentiate the toxicities of prooxidants (etoposide, rotenone, 2-methoxyestradiol, and curcumin). In A549 cells, 2',5'-DHC potentiated the cytotoxicities of rotenone, 2-methoxyestradiol, and curcumin, but not etoposide. In HL-60 and PC-3 cells, chrysin potentiated the cytotoxicity of curcumin, cytotoxicity that was attenuated by the catalytic antioxidant manganese(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP). Assessments of mitochondrial GSH levels mitochondrial membrane potential and cytochrome c release showed that the potentiation effects induced by 2',5'-DHC and chrysin involve mitochondrial dysfunction.

Role of GSTP1-1 in mediating the effect of As2O3 in the Acute Promyelocytic Leukemia cell line NB4

Arsenic trioxide (As2O3) is a highly effective agent in the treatment of acute promyelocytic leukemia (APL), whereas other hematopoietic tumors are less responsive to this agent and mechanisms underlying As2O3,-resistance are poorly understood. To better understand the complex network of GSH-related pathways in As2O3 sensitivity, we investigated the role of GSH and GSH-relevant enzymes in an APL cell line sensitive to As2O3 (NB4) and in a resistant subclone (AsR). Cell proliferation, viability, and apoptosis were investigated in NB4 cells before and after treatment with 1 muM As2O3 and in AsR cells. In these experimental cell models, GSTP1-1, JNK1 and JNK2 proteins were analyzed by immunoblotting, and a kinase assay for JNK1 was performed. GSH levels as well as the activities of the enzymes glutathione peroxidase, glutathione transferase, gamma-Glutamylcysteynilsinthetase and superoxide dismutase were measured. NB4 cells treated with As2O3 showed a high level of oxidative stress and an increase of GSH levels. GSTP1-1 polymerization and JNK1 activation were detectable after 24 h and were followed by an increase of the apoptotic rate starting at 72 h. Neither GSTP1-1 polymerization nor JNK activation was found in AsR cells that showed a very low apoptotic rate. Our results suggest that APL sensitivity to As2O3 might be, at least in part, mediated by the balance between association and dissociation of JNK from GSTP1-1, depending on the redox status of the cell. Further investigation is warranted to find a way to interfere with this balance, whenever it might represent a mechanism of drug resistance.

Non-irradiation-derived reactive oxygen species (ROS) and cancer: therapeutic implications

Owing to their chemical reactivity, radicals have cytocidal properties. Destruction of cells by irradiation-induced radical formation is one of the most frequent interventions in cancer therapy. An alternative to irradiation-induced radical formation is in principle drug-induced formation of radicals, and the formation of toxic metabolites by enzyme catalysed reactions. Although these developments are currently still in their infancy, they nevertheless deserve consideration. There are now numerous examples known of conventional anti-cancer drugs that may at least in part exert cytotoxicity by induction of radical formation. Some drugs, such as arsenic trioxide and 2-methoxy-estradiol, were shown to induce programmed cell death due to radical formation. Enzyme-catalysed radical formation has the advantage that cytotoxic products are produced continuously over an extended period of time in the vicinity of tumour cells. Up to now the enzymatic formation of toxic metabolites has nearly exclusively been investigated using bovine serum amine oxidase (BSAO), and spermine as substrate. The metabolites of this reaction, hydrogen peroxide and aldehydes are cytotoxic. The combination of BSAO and spermine is not only able to prevent tumour cell growth, but prevents also tumour growth, particularly well if the enzyme has been conjugated with a biocompatible gel. Since the tumour cells release substrates of BSAO, the administration of spermine is not required. Combination with cytotoxic drugs, and elevation of temperature improves the cytocidal effect of spermine metabolites. The fact that multidrug resistant cells are more sensitive to spermine metabolites than their wild type counterparts makes this new approach especially attractive, since the development of multidrug resistance is one of the major problems of conventional cancer therapy.

Role of γ-glutamyl cysteine synthetase (γ-GCS) gene expression as marker of drug sensitivity in acute myeloid leukemias

BACKGROUND

Elevated levels of glutathione (GSH) have been reported to play an important role in mediating chemoresistance in tumor cells. The regulation of gamma-glutamylcysteine synthetase (gamma-GCS) is one of the major determinants of GSH homeostasis. The aim of our study was to investigate gamma-GCS gene expression in patients affected by acute myeloid leukemia (AML).

METHODS

A total of 64 AML samples, including 23 acute promyelocytic leukemia (APL or M3) cases, were included in the study. gamma-GCS mRNA levels were determined by real-time quantitative RT-PCR. All patients were evaluated at diagnosis, whereas post-treatment gamma-GCS mRNA levels were assessed at the end of the consolidation therapy in 16 cases.

RESULTS

Our data showed that variable degrees of gamma-GCS expression were detectable in AML, likely reflecting disease heterogeneity; in particular, APL cases, compared to the other AML subsets, showed both significantly lower basal levels of gamma-GCS mRNA at presentation and significantly increased mRNA levels after treatment.

CONCLUSIONS

Decreased levels of gamma-GCS leading to reduced GSH may at least in part explain the higher sensitivity of APL to chemotherapy.

A phase II trial of arsenic trioxide in patients with metastatic melanoma

BACKGROUND

Arsenic trioxide induces growth inhibition and apoptosis in human melanoma cell lines. Therefore, a Phase II trial was conducted to evaluate the efficacy and toxicity of single-agent arsenic trioxide in patients with Stage IV melanoma.

METHODS

Twenty patients, 10 with metastatic melanoma of cutaneous origin and 10 with metastatic melanoma of choroidal origin, received arsenic trioxide 0.25 mg/kg/day for 5 days, followed by a maintenance dose of 0.35 mg/kg/day twice a week. All patients with melanoma of cutaneous origin and four patients with melanoma of choroidal origin had received prior therapy.

RESULTS

Single-agent arsenic trioxide did not induce clinical response in this patient population. Eight patients (five with melanoma of cutaneous origin, and three with melanoma of choroidal origin) had disease stabilization for at least six weeks. The median overall survival duration for patients with melanoma of cutaneous origin was 7.9 months, and that of patients with melanoma of choroidal origin has not been reached at a median follow-up duration of 11.8 months. Grade 3 toxicity included neutropenia, fatigue, abdominal pain, and arthralgia. Grade 4 toxicity did not occur.

CONCLUSIONS

Single-agent arsenic trioxide was generally well tolerated; however, no tumor regression was observed in this patient population. Future clinical trials should evaluate arsenic trioxide in combination with other anticancer drugs that may improve its clinical activity in melanoma.

Induction of endogenous Nrf2/small maf heterodimers by arsenic-mediated stress in placental choriocarcinoma cells

Exposure to inorganic arsenic has been associated with various forms of cancer, nervous system pathogenesis, and vascular diseases, as well as reproductive and developmental toxicity. Here, the effect of inorganic arsenic on placental JAR choriocarcinoma cells was assessed. The nuclear protein levels of the CNC transcription factor Nrf2 were strongly induced in the presence of arsenic. Dosage response experiments showed that 0.5 microM of arsenic is sufficient to augment Nrf2 levels. The expression of the Nrf2 dimerization partners MafG and MafK appeared not to be modulated by arsenic, whereas MafF protein levels were slightly increased. Arsenic also induced the binding of endogenous Nrf2/small Maf DNA-binding complexes to a stress response element (StRE) recognition site. In addition, arsenic caused oxidative stress in the choriocarcinoma cell model as evidenced by an increase in intracellular H2O2 levels. Expression of the enzyme heme oxygenase-1 (HO-1), a known Nrf2 target gene, was upregulated by exposure of JAR cells to arsenic. These results suggest that Nrf2/small Maf heterodimers may play an important role in the response to arsenic-mediated stress in placental cells.

Antimony trioxide-induced apoptosis is dependent on SEK1/JNK signaling

Very little is known concerning the toxicity of antimony, despite its commercial use as a flame retardant and medical use as a treatment for parasitic infections. Our previous studies show that antimony trioxide (Sb(2)O(3)) induces growth inhibition in patient-derived acute promyelocytic leukemia (APL) cell lines, a disease in which a related metal, arsenic trioxide (As(2)O(3)), is used clinically. However, signaling pathways initiated by Sb(2)O(3) treatment remain undefined. Here, we show that Sb(2)O(3) treatment of APL cells is associated with increased apoptosis as well as differentiation markers. Sb(2)O(3)-induced reactive oxygen species (ROS) correlated with increased apoptosis. In addition, when we decreased the buffering capacity of the cell by depleting glutathione, ROS production and apoptosis was enhanced. Arsenic-resistant APL cells with increased glutathione levels exhibited increased cross-resistance to Sb(2)O(3). Based on studies implicating c-jun kinase (JNK) in the mediation of the response to As(2)O(3), we investigated the role for JNK in Sb(2)O(3)-induced apoptosis. Sb(2)O(3) activates JNK and its downstream target, AP-1. In fibroblasts with a genetic deletion in SEK1, an upstream regulator of JNK, Sb(2)O(3)-induced growth inhibition as well as JNK activation was decreased. These data suggest roles for ROS and the SEK1/JNK pathway in the cytotoxicity associated with Sb(2)O(3) exposure.

NF-kappaB and FLIP in arsenic trioxide (ATO)-induced apoptosis in myelodysplastic syndromes (MDSs)

Tumor necrosis factor (TNF)-alpha, a potent stimulus of nuclear factor-kappaB (NF-kappaB), is up-regulated in myelodysplastic syndrome (MDS). Here, we show that bone marrow mononuclear cells (BMMCs) and purified CD34+ cells from patients with low-grade/early-stage MDS (refractory anemia/refractory anemia with ring sideroblasts [RA/RARS]) have low levels of NF-kappaB activity in nuclear extracts comparable with normal marrow, while patients with RA with excess blasts (RAEB) show significantly increased levels of activity (P = .008). Exogenous TNF-alpha enhanced NF-kappaB nuclear translocation in MDS BMMCs above baseline levels. Treatment with arsenic trioxide (ATO; 2-200 microM) inhibited NF-kappaB activity in normal marrow, primary MDS, and ML1 cells, even in the presence of exogenous TNF-alpha (20 ng/mL), and down-regulated NF-kappaB-dependent antiapoptotic proteins, B-cell leukemia XL (Bcl-XL), Bcl-2, X-linked inhibitor of apoptosis (XIAP), and Fas-associated death domain (FADD)-like interleukin-1beta-converting enzyme (FLICE) inhibitory protein (FLIP), leading to apoptosis. However, overexpression of FLIP resulted in increased NF-kappaB activity and rendered ML1 cells resistant to ATO-induced apoptosis. These data are consistent with the observed up-regulation of FLIP and resistance to apoptosis with advanced MDS, where ATO as a single agent may show only limited efficacy. However, the data also suggest that combinations of ATO with agents that interfere with other pathways, such as FLIP autoamplification via NF-kappaB, may have considerable therapeutic activity.

Elimination of the differential chemoresistance between the murine B-cell lymphoma LY-ar and LY-as cell lines after arsenic (As2O3) exposure via the overexpression of gsto1 (p28)

PURPOSE

Arsenic, in the form of As(2)O(3), has gained therapeutic importance because it has been shown to be very effective clinically in the treatment of acute promyelocytic leukemia (APL). Via numerous pathways arsenic induces cellular alterations such as induction of apoptosis, inhibition of cellular proliferation, stimulation of differentiation, and inhibition of angiogenesis. Responses vary depending on cell type, dose and the form of arsenic. GSTO1, a member of the glutathione S-transferase superfamily omega, has recently been shown to be identical to the rate-limiting enzyme, monomethyl arsenous (MMA(V)) reductase which catalyzes methylarsonate (MMA(V)) to methylarsenous acid (MMA(III)) during arsenic biotransformation. In this study, we investigated whether arsenic trioxide (As(2)O(3)) induces apoptosis in both chemosensitive and chemoresistant cell lines that varied in their expression of p28 (gsto1), the mouse homolog of GSTO1.

METHODS

The cytotoxicity of arsenic in the gsto1- and bcl-2-expressing chemoresistant and radioresistant LY-ar mouse lymphoma cell line, was compared with that of the LY-ar's parental cell line, LY-as. LY-as cells are radiosensitive, apoptotically permissive, and do not express gsto1 or bcl-2. Cell survival, glutathione (GSH) levels, mitochondrial membrane potential, and stress-activated kinase status after arsenic treatment were examined in these cell lines.

RESULTS

As(2)O(3) induced an equivalent dose- and time-dependent increase in apoptosis in these cell lines. Cellular survival, as measured after a 24-h exposure, was also the same in each cell line. Reduced GSH was modulated in a similar time- and dose-dependent manner. Apoptosis was preceded by loss of mitochondrial membrane potential that triggered caspase-mediated pathways associated with apoptosis. With a prolonged exposure of As(2)O(3), both cell lines showed decreased activation of ERK family members, ERK1, ERK2 and ERK5. As(2)O(3) enhanced the death signals in LY-ar cells through a decrease in GSH, loss of mitochondrial membrane potential, and abatement of survival signals. This effect is similar to that seen when LY-ar cells are treated with thiol-depleting agents or by the removal of methionine and cysteine (GSH precursor) from the growth medium. This response is also completely contrary to that seen for radiation, actinomycin D, VP-16 and other agents, where LY-ar cells do not succumb to apoptosis.

CONCLUSIONS

The overexpression of gsto1 in normally chemoresistant and radioresistant LY-ar cells renders them vulnerable to the cytotoxic effects of As(2)O(3), despite the 30-fold overexpression of the survival factor bcl-2. Gsto1 and its human homolog, GSTO1, may serve as a marker for arsenic sensitivity, particularly in cells that are resistant to other chemotherapeutic agents.

Arsenic trioxide for the treatment of myelodysplastic syndromes

The impressive activity of arsenic trioxide in acute promyelocytic leukaemia (APL) has renewed the interest in this old compound. Arsenic trioxide targets the sulfhydryl groups present in many proteins involved in oncogenesis and has a broad spectrum of biological activities. This article will review the mechanisms of action of the drug and their relevance to the treatment of myelodysplastic syndrome (MDS), a disease for which no standard treatment currently exists. The early clinical experience has confirmed the activity of arsenic trioxide in MDS. The preliminary results of ongoing Phase II studies conducted in patients with MDS suggest that arsenic trioxide produces haematological improvement including durable transfusion independence in approximately 30% of patients. The current data are presented and discussed in this review.

Simultaneous and ultrarapid determination of reactive oxygen species and reduced glutathione in apoptotic leukemia cells by microchip electrophoresis

A microchip electrophoresis method coupled with laser-induced fluorescence (LIF) detection was established for simultaneous determination of two kinds of intracellular signaling molecules (reactive oxygen species, ROS, and reduced glutathione, GSH) related to apoptosis and oxidative stress. As the probe dihydrorhodamine-123 (DHR-123) can be converted intracellularly by ROS to the fluorescent rhodamine-123 (Rh-123), and the probe naphthalene-2,3-dicarboxaldehyde (NDA) can react quickly with GSH to produce a fluorescent adduct, rapid determination of Rh-123 and GSH was achieved on a glass microchip within 27 s using a 20 mM borate buffer (pH 9.2). The established method was tested to measure the intracellular ROS and GSH levels in acute promyelocytic leukemia (APL)-derived NB4 cells. An elevation of intracellular ROS and depletion of GSH were observed in apoptotic NB4 cells induced by arsenic trioxide (As(2)O(3)) at low concentration (1-2 microM). Buthionine sulfoximine (BSO), in combination with As(2)O(3) enhanced the decrease of reduced GSH to a great extent. The combined treatment of As(2)O(3) and hydrogen peroxide (H(2)O(2)) led to an inverse relationship between the concentrations of ROS and GSH obtained, showing the proposed method can readily evaluate the generation of ROS, which occurs simultaneously with the consumption of the inherent antioxidant.

Trolox selectively enhances arsenic-mediated oxidative stress and apoptosis in APL and other malignant cell lines

Although arsenic trioxide (As2O3) is an effective therapy in acute promyelocytic leukemia (APL), its use in other malignancies is limited by the toxicity of concentrations required to induce apoptosis in non-APL tumor cells. We looked for agents that would synergize with As2O3 to induce apoptosis in malignant cells, but not in normal cells. We found that trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), a widely known antioxidant, enhances As2O3-mediated apoptosis in APL, myeloma, and breast cancer cells. Treatment with As2O3 and trolox increased intracellular oxidative stress, as evidenced by heme oxygenase-1 (HO-1) protein levels, c-Jun terminal kinase (JNK) activation, and protein and lipid oxidation. The synergistic effects of trolox may be specific to As2O3, as trolox does not add to toxicity induced by other chemotherapeutic drugs. We explored the mechanism of this synergy using electron paramagnetic resonance and observed the formation of trolox radicals when trolox was combined with As2O3, but not with doxorubicin. Importantly, trolox protected nonmalignant cells from As2O3-mediated cytotoxicity. Our data provide the first evidence that trolox may extend the therapeutic spectrum of As2O3. Furthermore, the combination of As2O3 and trolox shows potential specificity for tumor cells, suggesting it may not increase the toxicity associated with As2O3 monotherapy in vivo.

Anthraquinones sensitize tumor cells to arsenic cytotoxicity in vitro and in vivo via reactive oxygen species-mediated dual regulation of apoptosis

Cellular oxidation/reduction state affects the cytotoxicity of a number of chemotherapeutic agents, including arsenic trioxide. Reactive oxygen species (ROS), the major intracellular oxidants, may be a determinant of cellular susceptibility to arsenic. Our previous studies showed that a naphthoquinone and an anthraquinone (emodin) displayed the capability of producing ROS and facilitating arsenic cytotoxicity in both leukemia and solid tumor cell lines. We therefore attempted to test emodin and several other kinds of anthraquinone derivatives on EC/CUHK1, a cell line derived from esophageal carcinoma, and on a nude mouse model, with regard to their effects and mechanisms. Results showed that anthraquinones could produce ROS and sensitize tumor cells to arsenic both in vivo and in vitro. The combination of emodin and arsenic promoted the major apoptotic signaling events, i.e., the collapse of the mitochondrial transmembrane potential, the release of cytochrome c, and the activation of caspases 9 and 3. Meanwhile a combination of emodin and arsenic suppressed the activation of transcription factor NF-kappaB and downregulated the expression of a NF-kappaB-specific antiapoptotic protein, survivin. These two aspects could be antagonized by the antioxidant N-acetyl-L-cysteine. Therefore anthraquinones exert their effects via a ROS-mediated dual regulation, i.e., the enhancement of proapoptosis and the simultaneous inhibition of antiapoptosis. In vivo study showed that emodin made the EC/CUHK1 cell-derived tumors more sensitive to arsenic trioxide with no additional systemic toxicity and side effects. Taken together, these results suggest an innovative and safe chemotherapeutic strategy that uses natural anthraquinone derivatives as ROS generators to increase the susceptibility of tumor cells to cytotoxic therapeutic agents.

ROS stress in cancer cells and therapeutic implications

Reactive oxygen species (ROS) are constantly generated and eliminated in the biological system, and play important roles in a variety of normal biochemical functions and abnormal pathological processes. Growing evidence suggests that cancer cells exhibit increased intrinsic ROS stress, due in part to oncogenic stimulation, increased metabolic activity, and mitochondrial malfunction. Since the mitochondrial respiratory chain (electron transport complexes) is a major source of ROS generation in the cells, the vulnerability of the mitochondrial DNA to ROS-mediated damage appears to be a mechanism to amplify ROS stress in cancer cells. The escalated ROS generation in cancer cells serves as an endogenous source of DNA-damaging agents that promote genetic instability and development of drug resistance. Malfunction of mitochondria also alters cellular apoptotic response to anticancer agents. Despite the negative impacts of increased ROS in cancer cells, it is possible to exploit this biochemical feature and develop novel therapeutic strategies to preferentially kill cancer cells through ROS-mediated mechanisms. This article reviews ROS stress in cancer cells, its underlying mechanisms and relationship with mitochondrial malfunction and alteration in drug sensitivity, and suggests new therapeutic strategies that take advantage of increased ROS in cancer cells to enhance therapeutic activity and selectivity.

The potential of arsenic trioxide in the treatment of malignant disease: past, present, and future

Arsenic trioxide (As2O3) is an effective therapy for acute promyelocytic leukemia (APL), and there has been promising activity noted in other hematologic and solid tumors. The mechanism of action of As2O3 such as differentiation and apoptosis has prompted study into combination therapy. Furthermore, the connection of the sensitivity of diseases such as APL and multiple myeloma to oxidative damage has allowed the investigation of pharmacologic modulation of the cellular redox state for potentiation of As2O3. Continued study of As2O3 as a single-agent and in combination therapy will allow identification of the safest and most effective treatment regimens for malignant disease.

A P53 target gene, PIG11, contributes to chemosensitivity of cells to arsenic trioxide

The tumor suppressor p53 regulates the expression of various genes that promote apoptosis. PIG11 (P53-induced gene 11), also referred to as TP53I11 (tumor protein p53 inducible protein 11), is a direct p53 target gene. Recent data demonstrated that PIG11 was up-regulated markedly in arsenic trioxide induced apoptosis by DDRT-PCR, suggesting a new class of p53 target genes that sensitize cells to the effects of chemotherapeutic agents. In this study, through the construction of a recombinant GFP-PIG11 expression vector and transfection of HEK293 cells with GFP or GFP-PIG11, the role of PIG11 in apoptosis was analyzed. Results demonstrated that the percentage (11.38%) of apoptotic cells with GFP-PIG11 transfection was higher than that (7.28%) of with only GFP transfection (P<0.05). At 24 h after 1 microM of arsenic trioxide treatment, apoptotic cells exhibited a significant increase in the expression of GFP-PIG11 (36.67%+/-2.78), in contrast, 10.50%+/-2.03 only GFP and 5.25%+/-0.96 vehicle control (P<0.01). In addition, we showed that intracellular content of reactive oxygen species (ROS) was 9.66+/-0.52 in GFP-PIG11 transfection, higher than 5.21+/-0.08 in GFP only and 5.99+/-0.45 in vehicle control (P<0.01). The above results suggest that overexpression of PIG11 could induce cell apoptosis in the low levels and enhanced the apoptotic effects of arsenic trioxide. The process could be involved in intracellular generation of ROS.

Topoisomerase I-DNA complexes contribute to arsenic trioxide-induced apoptosis

Topoisomerase I is an essential enzyme that relaxes DNA supercoiling by forming covalent DNA cleavage complexes, which are normally transient. Topoisomerase I-DNA complexes can be trapped by anticancer drugs (camptothecins) as well as by endogenous and exogenous DNA lesions. We show here that arsenic trioxide (a potent inducer of apoptosis that induces the intracellular accumulation of reactive oxygen species and targets mitochondria) induces cellular topoisomerase I cleavage complexes. Bcl-2 overexpression and quenching of reactive oxygen species, which prevent arsenic trioxide-induced apoptosis, also prevent the formation of topoisomerase I-DNA complexes, whereas enhancement of reactive oxygen species accumulation promotes these complexes. The caspase inhibitor, benzyloxycarbonyl-VAD partially prevents arsenic trioxide-induced topoisomerase I-DNA complexes and apoptosis, suggesting that activated caspases further maintain intracellular levels of reactive oxygen species that induce the formation of topoisomerase I-DNA complexes. Down-regulation of topoisomerase I expression decreases arsenic trioxide-induced apoptotic DNA fragmentation. Thus, we propose that arsenic trioxide induces topoisomerase I-DNA complexes that participate in chromatin fragmentation and programmed cell death during apoptosis.

Nitric oxide prodrugs and metallochemotherapeutics: JS-K and CB-3-100 enhance arsenic and cisplatin cytolethality by increasing cellular accumulation

Development of chemotherapeutic resistance is a major cause of pharmacologic failure in cancer treatment. One mechanism of resistance in tumor cells is the overexpression of glutathione S-transferases (GSTs) that serve two distinct roles in the development of drug resistance via the formation of glutathione conjugates with drugs for their cellular efflux, and the inhibition of the mitogen-activated protein kinase pathway. To target GST-based resistance to chemotherapeutics, a series of nitric oxide (NO)-releasing diazeniumdiolates was synthesized and shown to release NO on reaction with GST and/or glutathione. Two diazeniumdiolates, JS-K [O2-(2,4-dinitrophenyl) 1-[(4-ethoxycarbonyl)piperazin-1-yl]diazen-1-ium-1,2-diolate] and CB-3-100 [O2-(2,4-dinitrophenyl) 1-[4-(N,N-diethylcarboxamido)piperazin-1-yl]diazen-1-ium-1,2-diolate], were studied on their ability in reversing arsenic and cisplatin resistance in a rat liver cell line that is tumorigenic and shows acquired tolerance to arsenic and cisplatin, with overexpression of GSTs. The enhanced cytolethality produced by the NO donors was accompanied by increased accumulation of arsenic and platinum within cells and by enhanced activation of mitogen-activated protein kinase members c-jun-NH-kinase and extracellular signal-regulated kinase. Our data indicate that JS-K and CB-3-100 are promising lead compounds for the possible development of a novel class of adjuvant chemotherapeutic agents potentially capable of reversing arsenic and cisplatin resistance in certain tumor cells.

Death by arsenic: implications of PML sumoylation

PML is a multifunctional protein that plays an important role in programmed cell death, albeit by mechanisms that remain unclear. In this issue of Cancer Cell, Hayakawa and Privalsky associate a MAP kinase pathway that mediates As(2)O(3)-induced PML phosphorylation with sumoylation and increased apoptotic activity of PML. Thus, specific MAP kinases may potentiate apoptosis in response to As(2)O(3), a compound that has dramatic activity against acute promyelocytic leukemia (APL) cells. This novel mechanism may have important implications for use of As(2)O(3) as a chemotherapeutic agent, especially in malignancies less sensitive to As(2)O(3) than APL.

JNK activation is a mediator of arsenic trioxide-induced apoptosis in acute promyelocytic leukemia cells

Arsenic trioxide induces c-jun N-terminal kinase (JNK) activation and apoptosis in acute promyelocytic leukemia (APL), where it has major clinical activity, but whether JNK is necessary to induce apoptosis is unknown. To clarify this necessity, we established 2 arsenic trioxide (As(2)O(3))-resistant subclones of the APL cell line, NB4. Both resistant lines showed little activation of JNK1 following treatment with As(2)O(3), even at doses sufficient to elicit robust activation in NB4 cells. One mechanism of resistance in these cells is up-regulated glutathione (GSH) content, and GSH depletion by l-buthionine-[S,R]-sulfoximine (BSO) restores JNK activation and As(2)O(3) sensitivity. This correlation between JNK activation and apoptosis led us to test whether inhibition of JNK would protect cells from As(2)O(3)-induced apoptosis. SEK1(-/-) mouse embryo fibroblasts (MEFs) showed diminished JNK activation following As(2)O(3) treatment and were protected from As(2)O(3)-induced but not doxorubicin-induced apoptosis. Furthermore, treatment of arsenic trioxide-sensitive APL cells with the JNK inhibitor, dicumarol, significantly increased growth and survival in response to As(2)O(3) but did not protect cells from doxorubicin. Together, these data support an essential role for JNK signaling in the induction of growth inhibition and apoptosis by As(2)O(3) and suggest that activating JNK may provide a therapeutic advantage in the treatment of cancers that do not respond to arsenic alone.