The accumulation and toxicity of methylated arsenicals in endothelial cells: important roles of thiol compounds

Excess intake of arsenic is known to cause vascular diseases as well as skin lesions and cancer in humans. Recent reports suggest that trivalent methylated arsenicals, which are intermediate metabolites in the methylation process of inorganic arsenic, are responsible for the toxicity and carcinogenicity of environmental arsenic. We investigated acute toxicity and accumulation of monomethylarsonic acid (MMAV), dimethylarsinic acid (DMAV), trimethylarsine oxide (TMAO), and monomethylarsonous acid diglutathione (MMAIII (GS)2) in rat heart microvessel endothelial (RHMVE) cells. MMAV (LC50 = 36.6 mM) and DMAV (LC50 = 2.54 mM) were less toxic than inorganic arsenicals (cf. LC50 values for inorganic arsenite (iAsIII), and inorganic arsenate (iAsV) was reported to be 36 and 220 μM, respectively, in RHMVE cells. TMAO was essentially not toxic. However, MMAIII (GS)2 was highly toxic (LC50 = 4.1 μM). The order of cellular arsenic accumulation of those four organic arsenic compounds was MMAIII (GS)2 >> MMAV > DMAV > TMAO. MMAIII (GS)2 was efficiently taken up by the cells and cellular arsenic content increased with the concentration of MMAIII (GS)2 in culture medium. N-acetyl-L-cysteine (NAC) reduced cellular arsenic content in DMAV-exposed cells and also decreased the cytotoxicity of DMAV, whereas it changed neither cellular arsenic content nor the viability in MMAV-exposed cells. mRNA levels of heme oxygenase-1 (HO-1) were decreased by NAC in DMAV-exposed, but MMAV-exposed cells. Buthionine sulfoximine (BSO), a cellular glutathione (GSH) depleting agent, enhanced the cytotoxicity of MMAV. However, BSO reduced, rather than enhanced, the cytotoxicity of DMAV. These results suggest that intracellular GSH modulated the toxic effects of arsenic in opposite ways for MMAV and DMAV. Even though intracellular GSH decreased the cytotoxicity of MMAV, extracellularly added GSH enhanced the cytotoxicity of MMAV. The use of high-performance liquid chromatography (HPLC)-inductively coupled plasma mass spectrometric analyses suggested that a small amount of MMAV was converted to MMAIII (GS)2 in the presence of GSH. These results suggest that MMAIII (GS)2 is highly toxic compared to other arsenic compounds because of faster accumulation of this species by cells, in addition to having the toxic nature of methylated trivalent organic arsenics