BackgroundBreast cancer has recently been linked to cadmium exposure. Although not uniformly supported, it is hypothesized that cadmium acts as a metalloestrogenic carcinogen via the estrogen receptor (ER). Thus, we studied the effects of chronic exposure to cadmium on the normal human breast epithelial cell line MCF-10A, which is ER-negative but can convert to ER-positive during malignant transformation.MethodsCells were continuously exposed to low-level cadmium (2.5 μM) and checked in vitro and by xenograft study for signs of malignant transformation. Transformant cells were molecularly characterized by protein and transcript analysis of key genes in breast cancer.ResultsOver 40 weeks of cadmium exposure, cells showed increasing secretion of matrix metalloproteinase-9, loss of contact inhibition, increased colony formation, and increasing invasion, all typical for cancer cells. Inoculation of cadmium-treated cells into mice produced invasive, metastatic anaplastic carcinoma with myoepithelial components. These cadmium-transformed breast epithelial (CTBE) cells displayed characteristics of basal-like breast carcinoma, including ER-α negativity and HER2 (human epidermal growth factor receptor 2) negativity, reduced expression of BRCA1 (breast cancer susceptibility gene 1), and increased CK5 (cytokeratin 5) and p63 expression. CK5 and p63, both breast stem cell markers, were prominently overexpressed in CTBE cell mounds, indicative of persistent proliferation. CTBE cells showed global DNA hypomethylation and c-myc and k-ras overexpression, typical in aggressive breast cancers. CTBE cell xenograft tumors were also ER-α negative.ConclusionsCadmium malignantly transforms normal human breast epithelial cells—through a mechanism not requiring ER-α—into a basal-like cancer phenotype. Direct cadmium induction of a malignant phenotype in human breast epithelial cells strongly fortifies a potential role in breast cancer.

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). ...