Interplay between Cellular Methyl Metabolism and Adaptive Efflux during Oncogenic Transformation from Chronic Arsenic Exposure in Human Cells

Abstract

After protracted low level arsenic exposure, the normal human prostate epithelial cell line RWPE-1 acquires a malignant phenotype with DNA hypomethylation, indicative of disrupted methyl metabolism, and shows arsenic adaptation involving glutathione overproduction and enhanced arsenic efflux. Thus, the interplay between methyl and glutathione metabolism during this progressive arsenic adaptation was studied. Arsenic-treated cells showed a time-dependent increase in LC 50 and a marked increase in homocysteine (Hcy) levels. A marked suppression of S-adenosylmethionine (SAM) levels occurred with decreased methionine adenosyltransferase 2A (converts methionine to SAM) expression and increased negative regulator methionine adenosyltransferase B, suggesting reduced conversion of Hcy to SAM. Consistent with Hcy overproduction, activity and expression of S-adenosylhomocysteine hydrolase (converts S-adenosylhomocysteine to Hcy) were both increased. Expression of cystathionine ␤-synthase, a key gene in the transsulfuration pathway, and various glutathione production genes were increased, resulting in a 5-fold increase in glutathione. Arsenic efflux increased along with expression of ATP-binding cassette protein C1, which effluxes arsenic as a glutathione conjugate. Evidence of genomic DNA hypomethylation was observed during early arsenic exposure, indicating that the disruption in methyl metabolism had a potential impact related to oncogenesis. Thus, cellular arsenic adaptation is a dynamic, progressive process that involves decreased SAM recycling and concurrent accumulation of Hcy, which is channeled via transsulfuration to increase glutathione and enhance arsenic efflux but may also impact the carcinogenic process.Arsenic is a metalloid widely distributed in the environment mainly found in inorganic forms. Inorganic arsenic is a human carcinogen inducing cancers of the skin, lung, and urinary bladder and possibly liver, prostate, and kidney (1-4). Indeed, several studies indicate a link between inorganic arsenic exposure and human prostate cancer (1-4). Our laboratory has developed an in vitro prostate cancer model in which the normal human prostate epithelial cell line RWPE-1 is malignantly transformed after chronic, low level arsenite exposure (1, 5). The transformed cells, called CAsE-PE, form malignant carcinoma after inoculation into nude mice and overexpress prostate-specific antigen (1, 5), similar to human prostate carcinoma. Chronic, low level arsenite exposure also induces adaptation to arsenic, in which cells become resistant to acute arsenic toxicity (6 -8). This arsenic adaptation occurs through various metabolic changes, including overexpression of efflux transport proteins and increased glutathione production (6 -8).Arsenic can be enzymatically modified to form mono-and dimethylarsenic compounds via arsenic methyltransferases using SAM 2 as the methyl donating cofactor (9). Because this methylation may generate toxic intermediates, it is no longer thought to be detoxicating of inorganic arsenic (9 -11). ...

Arsenic exposure causes epigenetic dysregulation of IL-8 expression leading to proneoplastic changes in kidney cells

“…Arsenic could affect DNA methylation due to the competitive action of DNA methyltransferases (DNMTs) and arsenite methyltransferases (AS3MT) for methyl group carried by S-adenosyl methionine. AS3MT utilizes SAM to methylate arsenite thus leading to inadequate supply of SAM to DNMTs and causing genome hypomethylation (Caudill et al, 2001;Coppin et al, 2008;Reichard et al, 2007). Some reports also point to the effect of arsenic exposure on histone modification such as H3 histone tail acetylation (Cronican et al, 2013).…”

Protective effects of quercetin against arsenic-induced testicular damage in rats

“…In addition, an increasing number of experimental studies have shown that arsenic causes endocrine disruptions, change in cell cycle kinetics, epigenetic effects and change in transcription (NRC, 2001;Kitchin & Wallace, 2005;Davey et al, 2007;Reichard et al, 2007;Coppin et al, 2008).…”

Toxicogenomic Studies in Human Populations

“…While As-induced global DNA hypomethylation has been observed in mouse and rat liver (Okoji et al, 2002;Chen et al, 2004;Uthus and Davis, 2005) and in mammalian cells in vitro (Sciandrello et al, 2004;BenbrahimTallaa et al, 2005;Reichard et al, 2007;Coppin et al, 2008), results from the only available human population study are contradictory, showing that arsenic exposure through drinking water contamination induced a global DNA hypermethylation in a dose-dependent manner in peripheral blood leukocytes (Pilsner et al, 2007).…”