The functions of crucial cysteine residues in the arsenite methylation catalyzed by recombinant human arsenic (III) methyltransferase

Origins, fate, and actions of methylated trivalent metabolites of inorganic arsenic: progress and prospects

The toxic metalloid inorganic arsenic (iAs) is widely distributed in the environment. Chronic exposure to iAs from environmental sources has been linked to a variety of human diseases. Methylation of iAs is the primary pathway for metabolism of iAs. In humans, methylation of iAs is catalyzed by arsenic (+ 3 oxidation state) methyltransferase (AS3MT). Conversion of iAs to mono- and di-methylated species (MAs and DMAs) detoxifies iAs by increasing the rate of whole body clearance of arsenic. Interindividual differences in iAs metabolism play key roles in pathogenesis of and susceptibility to a range of disease outcomes associated with iAs exposure. These adverse health effects are in part associated with the production of methylated trivalent arsenic species, methylarsonous acid (MAs^III) and dimethylarsinous acid (DMAs^III), during AS3MT-catalyzed methylation of iAs. The formation of these metabolites activates iAs to unique forms that cause disease initiation and progression. Taken together, the current evidence suggests that methylation of iAs is a pathway for detoxification and for activation of the metalloid. Beyond this general understanding of the consequences of iAs methylation, many questions remain unanswered. Our knowledge of metabolic targets for MAs^III and DMAs^III in human cells and mechanisms for interactions between these arsenicals and targets is incomplete. Development of novel analytical methods for quantitation of MAs^III and DMAs^III in biological samples promises to address some of these gaps. Here, we summarize current knowledge of the enzymatic basis of MAs^III and DMAs^III formation, the toxic actions of these metabolites, and methods available for their detection and quantification in biomatrices. Major knowledge gaps and future research directions are also discussed.

Tetraarsenictetrasulfide and Arsenic Trioxide Exert Synergistic Effects on Induction of Apoptosis and Differentiation in Acute Promyelocytic Leukemia Cells

Since arsenic trioxide (As³⁺) has been successfully used in the treatment of acute promyelocytic leukemia (APL), its adverse effects on patients have been problematic and required a solution. Considering the good therapeutic potency and low toxicity of tetraarsenictetrasulfide (As₄S₄) in the treatment of APL, we investigated the effects of combining As₄S₄ and As³⁺ on the apoptosis and differentiation of NB4 and primary APL cells. As₄S₄, acting similarly to As³⁺, arrested the G₁/S transition, induced the accumulation of cellular reactive oxygen species, and promoted apoptosis. Additionally, low concentrations of As₄S₄ (0.1–0.4 μM) induced differentiation of NB4 and primary APL cells. Compared with the As₄S₄- or As³⁺-treated groups, the combination of As₄S₄ and As³⁺ obviously promoted apoptosis and differentiation of NB4 and primary APL cells. Mechanistic studies suggested that As₄S₄ acted synergistically with As³⁺ to down-regulate Bcl-2 and nuclear factor-κB expression, up-regulate Bax and p53 expression, and induce activation of caspase-12 and caspase-3. Moreover, the combination of low concentrations of As₄S₄ and As³⁺ enhanced degradation of the promyelocytic leukemia-retinoic acid receptor α oncoprotein. In summary, As₄S₄ and As³⁺ synergistically induce the apoptosis and differentiation of NB4 and primary APL cells.

Dose-dependent effects of selenite (Se(4+)) on arsenite (As(3+))-induced apoptosis and differentiation in acute promyelocytic leukemia cells

To enhance the therapeutic effects and decrease the adverse effects of arsenic on the treatment of acute promyelocytic leukemia, we investigated the co-effects of selenite (Se⁴⁺) and arsenite (As³⁺) on the apoptosis and differentiation of NB4 cells and primary APL cells. A 1.0-μM concentration of Se⁴⁺ prevented the cells from undergoing As³⁺-induced apoptosis by inhibiting As³⁺ uptake, eliminating As³⁺-generated reactive oxygen species, and repressing the mitochondria-mediated intrinsic apoptosis pathway. However, 4.0 μM Se⁴⁺ exerted synergistic effects with As³⁺ on cell apoptosis by promoting As³⁺ uptake, downregulating nuclear factor-кB, and activating caspase-3. In addition to apoptosis, 1.0 and 3.2 μM Se⁴⁺ showed contrasting effects on As³⁺-induced differentiation in NB4 cells and primary APL cells. The 3.2 μM Se⁴⁺ enhanced As³⁺-induced differentiation by promoting the degradation of promyelocytic leukemia protein–retinoic acid receptor-α (PML–RARα) oncoprotein, but 1.0 μM Se⁴⁺ did not have this effect. Based on mechanistic studies, Se⁴⁺, which is similar to As³⁺, might bind directly to Zn²⁺-binding sites of the PML RING domain, thus controlling the fate of PML–RARα oncoprotein.