PML/RARalpha fusion protein mediates the unique sensitivity to arsenic cytotoxicity in acute promyelocytic leukemia cells: Mechanisms involve the impairment of cAMP signaling and the aberrant regulation of NADPH oxidase

Transient and permanent changes in DNA methylation patterns in inorganic arsenic-mediated epithelial-to-mesenchymal transition

Chronic low dose inorganic arsenic exposure causes cells to take on an epithelial-to-mesenchymal phenotype, which is a crucial process in carcinogenesis. Inorganic arsenic is not a mutagen and thus epigenetic alterations have been implicated in this process. Indeed, during the epithelial-to-mesenchymal transition, morphologic changes to cells correlate with changes in chromatin structure and gene expression, ultimately driving this process. However, studies on the effects of inorganic arsenic exposure/withdrawal on the epithelial-to-mesenchymal transition and the impact of epigenetic alterations in this process are limited. In this study we used high-resolution microarray analysis to measure the changes in DNA methylation in cells undergoing inorganic arsenic-induced epithelial-to-mesenchymal transition, and on the reversal of this process, after removal of the inorganic arsenic exposure. We found that cells exposed to chronic, low-dose inorganic arsenic exposure showed 30,530 sites were differentially methylated, and with inorganic arsenic withdrawal several differential methylated sites were reversed, albeit not completely. Furthermore, these changes in DNA methylation mainly correlated with changes in gene expression at most sites tested but not at all. This study suggests that DNA methylation changes on gene expression are not clear-cut and provide a platform to begin to uncover the relationship between DNA methylation and gene expression, specifically within the context of inorganic arsenic treatment.

New Strategies to Direct Therapeutic Targeting of PML to Treat Cancers

The tumor suppressor function of the promyelocytic leukemia (PML) protein was first identified as a result of its dysregulation in acute promyelocytic leukemia, however, its importance is now emerging far beyond hematological neoplasms, to an extensive range of malignancies, including solid tumors. In response to stress signals, PML coordinates the regulation of numerous proteins, which activate fundamental cellular processes that suppress tumorigenesis. Importantly, PML itself is the subject of specific post-translational modifications, including ubiquitination, phosphorylation, acetylation, and SUMOylation, which in turn control PML activity and stability and ultimately dictate cellular fate. Improved understanding of the regulation of this key tumor suppressor is uncovering potential opportunities for therapeutic intervention. Targeting the key negative regulators of PML in cancer cells such as casein kinase 2, big MAP kinase 1, and E6-associated protein, with specific inhibitors that are becoming available, provides unique and exciting avenues for restoring tumor suppression through the induction of apoptosis and senescence. These approaches could be combined with DNA damaging drugs and cytokines that are known to activate PML. Depending on the cellular context, reactivation or enhancement of tumor suppressive PML functions, or targeted elimination of aberrantly functioning PML, may provide clinical benefit.

Arsenic speciation in saliva of acute promyelocytic leukemia patients undergoing arsenic trioxide treatment

Arsenic trioxide has been successfully used as a therapeutic in the treatment of acute promyelocytic leukemia (APL). Detailed monitoring of the therapeutic arsenic and its metabolites in various accessible specimens of APL patients can contribute to improving treatment efficacy and minimizing arsenic-induced side effects. This article focuses on the determination of arsenic species in saliva samples from APL patients undergoing arsenic treatment. Saliva samples were collected from nine APL patients over three consecutive days. The patients received 10 mg arsenic trioxide each day via intravenous infusion. The saliva samples were analyzed using high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry. Monomethylarsonous acid and monomethylmonothioarsonic acid were identified along with arsenite, dimethylarsinic acid, monomethylarsonic acid, and arsenate. Arsenite was the predominant arsenic species, accounting for 71.8 % of total arsenic in the saliva. Following the arsenic infusion each day, the percentage of methylated arsenicals significantly decreased, possibly suggesting that the arsenic methylation process was saturated by the high doses immediately after the arsenic infusion. The temporal profiles of arsenic species in saliva following each arsenic infusion over 3 days have provided information on arsenic exposure, metabolism, and excretion. These results suggest that saliva can be used as an appropriate clinical biomarker for monitoring arsenic species in APL patients.

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Synergism between arsenite and proteasome inhibitor MG132 over cell death in myeloid leukaemic cells U937 and the induction of low levels of intracellular superoxide anion

Increased oxygen species production has often been cited as a mechanism determining synergism on cell death and growth inhibition effects of arsenic-combined drugs. However the net effect of drug combination may not be easily anticipated solely from available knowledge of drug-induced death mechanisms. We evaluated the combined effect of sodium arsenite with the proteasome inhibitor MG132, and the anti-leukaemic agent CAPE, on growth-inhibition and cell death effect in acute myeloid leukaemic cells U937 and Burkitt's lymphoma-derived Raji cells, by the Chou-Talalay method. In addition we explored the association of cytotoxic effect of drugs with changes in intracellular superoxide anion (O₂⁻) levels. Our results showed that combined arsenite+MG132 produced low levels of O₂⁻ at 6h and 24h after exposure and were synergic on cell death induction in U937 cells over the whole dose range, although the combination was antagonistic on growth inhibition effect. Exposure to a constant non-cytotoxic dose of 80μM hydrogen peroxide together with arsenite+MG132 changed synergism on cell death to antagonism at all effect levels while increasing O₂⁻ levels. Arsenite+hydrogen peroxide also resulted in antagonism with increased O₂⁻ levels in U937 cells. In Raji cells, arsenite+MG132 also produced low levels of O₂⁻ at 6h and 24h but resulted in antagonism on cell death and growth inhibition. By contrast, the combination arsenite+CAPE showed high levels of O₂⁻ production at 6h and 24 h post exposure but resulted in antagonism over cell death and growth inhibition effects in U937 and Raji cells. We conclude that synergism between arsenite and MG132 in U937 cells is negatively associated to O₂⁻ levels at early time points after exposure.

Treatment of acute promyelocytic leukemia with single-agent arsenic trioxide

It is well recognized that arsenic trioxide (ATO) is an efficacious agent for the treatment of acute promyelocytic leukemia (APL). Use of single agent ATO in the treatment of APL leads to remissions which are durable in the majority. ATO is probably the most effective single agent in the treatment of APL and there have been very few reports of primary resistance. It has been used both as a single agent and in combination with other conventional drugs to treat APL. Use of ATO is the accepted standard of care in the management of relapsed APL, where it is often used effectively as a bridge to a stem cell transplant. However, its role in newly diagnosed APL remains controversial. ATO probably has multiple mechanisms of action. Better understanding of its mechanisms of action/s is likely to lead to more rationale use of this agent or its derivatives either alone or in combination with other drugs. There is limited data on the kinetics of leukemia clearance and normal haematopoietic recovery after the administration of single agent ATO for the treatment of APL, preliminary data suggests that it is likely to be different from conventional therapy. There have been a number of concerns of the potential short and long term toxicity of this agent. Most such concerns arise from the toxicity profile noted in people exposed to long term arsenic exposure in the environment. With the therapeutic doses and schedules of administration of ATO in the treatment of malignancies the overall toxicity profile has been favorable. In a resource constrained environments the use of a single agent ATO based regimen is a realistic and acceptable option to treat almost all patients. In the developed world it has the potential in combination with other agents to improve the clinical outcome with reduction of dose intensity of chemotherapy and remains an option for patients who would not tolerate conventional therapy. In this review we focus on the use of single agent ATO for the treatment of APL and summarize our experience and review the literature.

Oxidative stress, inflammation, and cancer: how are they linked?

Extensive research during the past 2 decades has revealed the mechanism by which continued oxidative stress can lead to chronic inflammation, which in turn could mediate most chronic diseases including cancer, diabetes, and cardiovascular, neurological, and pulmonary diseases. Oxidative stress can activate a variety of transcription factors including NF-κB, AP-1, p53, HIF-1α, PPAR-γ, β-catenin/Wnt, and Nrf2. Activation of these transcription factors can lead to the expression of over 500 different genes, including those for growth factors, inflammatory cytokines, chemokines, cell cycle regulatory molecules, and anti-inflammatory molecules. How oxidative stress activates inflammatory pathways leading to transformation of a normal cell to tumor cell, tumor cell survival, proliferation, chemoresistance, radioresistance, invasion, angiogenesis, and stem cell survival is the focus of this review. Overall, observations to date suggest that oxidative stress, chronic inflammation, and cancer are closely linked.

Targeting mitochondria for cancer therapy

Several recent insights into the roles of mitochondria in cancer have renewed efforts to develop nongenotoxic therapies targeting mitochondrial proteins and functions. Mitochondria are central hubs for intrinsic apoptotic pathways that are activated by cellular stress and injury, and as a consequence, cancers often have defects in these pathways. Bcl-2, the first identified regulator of apoptotic cell deaths, was discovered as an oncogene in human cancers. BCL-2 inhibits mitochondrial pathways of apoptosis through local effects at mitochondrial and endoplasmic reticulum membranes. Increased expression of BCL-2 and the related antiapoptotic proteins BCL-X(L), MCL-1, and BCL-W occurs in significant subsets of common cancer types (Table I) and is generally correlated with poor response. Although incomplete, the emerging understanding of BCL-2 functions through structural, biochemical, and organelle physiology studies has provided paths for targeting BCL-2 with small molecules. Cancer cells also exhibit metabolic differences with their normal cell counterparts, including aerobic glycolysis, known as the Warburg effect, mitochondrial membrane hyperpolarization, and unusual dependence on nutrient substrates such as glucose and glutamine. This knowledge has prompted reexamination of the potential cancer selectivity of previously identified mitochondriotoxic compounds, including approved drugs for other indications, and screening programs to identify new compounds with mitochondrial activities.

Signal transduction pathways and transcription factors triggered by arsenic trioxide in leukemia cells

Arsenic trioxide (As(2)O(3)) is widely used to treat acute promyelocytic leukemia (APL). Several lines of evidence have indicated that As(2)O(3) affects signal transduction and transactivation of transcription factors, resulting in the stimulation of apoptosis in leukemia cells, because some transcription factors are reported to associate with the redox condition of the cells, and arsenicals cause oxidative stress. Thus, the disturbance and activation of the cellular signaling pathway and transcription factors due to reactive oxygen species (ROS) generation during arsenic exposure may explain the ability of As(2)O(3) to induce a complete remission in relapsed APL patients. In this report, we review recent findings on ROS generation and alterations in signal transduction and in transactivation of transcription factors during As(2)O(3) exposure in leukemia cells.

Biological responses to arsenic compounds

Arsenic is a metalloid that generates various biological effects on cells and tissues. Depending on the specific tissue exposed and the time and degree of exposure, diverse responses can be observed. In humans, prolonged and/or high dose exposure to arsenic can have a variety of outcomes, including the development of malignancies, severe gastrointestinal toxicities, diabetes, cardiac arrhythmias, and death. On the other hand, one arsenic derivative, arsenic trioxide (As(2)O(3)), has important antitumor properties. This agent is a potent inducer of antileukemic responses, and it is now approved by the Food and Drug Administration for the treatment of acute promyelocytic leukemia in humans. The promise and therapeutic potential of arsenic and its various derivatives have been exploited for hundreds of years. Remarkably, research focused on the potential use of arsenic compounds in the treatment of human diseases remains highly promising, and it is an area of active investigation. An emerging approach of interest and therapeutic potential involves efforts to target and block cellular pathways activated in a negative feedback manner during treatment of cells with As(2)O(3). Such an approach may ultimately provide the means to selectively enhance the suppressive effects of this agent on malignant cells and render normally resistant tumors sensitive to its antineoplastic properties.

Enhancement of arsenic trioxide cytotoxicity by dietary isothiocyanates in human leukemic cells via a reactive oxygen species-dependent mechanism

Although clearly effective in acute promyelocytic leukemia (APL), arsenic trioxide (ATO) demonstrates little clinical benefit as a single agent in the treatment of non-APL hematological malignancies. We screened a library of 2000 marketed drugs and naturally occurring compounds to identify agents that potentiate the cytotoxic effects of ATO in leukemic cells. Here, we report the identification of three isothiocyanates (sulforaphane, erysolin and erucin) found in cruciferous vegetables as enhancers of ATO cytotoxicity. Both erysolin and sulforaphane significantly enhanced ATO-mediated cytotoxicity and apoptosis in a panel of leukemic cell lines; erucin activity was variable among cell types. Cellular exposure to sulforaphane in combination with ATO resulted in a dramatic increase in levels of reactive oxygen species (ROS) compared to treatment with either agent alone. Sulforaphane, alone or with ATO, decreased intracellular glutathione (GSH) content. Furthermore, addition of the free radical scavenger N-acetyl-l-cysteine (NAC) rescued cells from ATO/isothiocyanate-mediated cytotoxicity. Our data suggest that isothiocyanates enhance the cytotoxic effects of ATO through a ROS-dependent mechanism. Combinatorial treatment with isothiocyanates and ATO might provide a promising therapeutic approach for a variety of myeloid malignancies.