Arsenic Exposure Transforms Human Epithelial Stem/Progenitor Cells into a Cancer Stem-like Phenotype

BACKGROUND: Inorganic arsenic is a ubiquitous environmental carcinogen affecting millions of people worldwide. Evolving theory predicts that normal stem cells (NSCs) are transformed into cancer stem cells (CSCs) that then drive oncogenesis. In humans, arsenic is carcinogenic in the urogenital system (UGS), including the bladder and potentially the prostate, whereas in mice arsenic induces multiorgan UGS cancers, indicating that UGS NSCs may represent targets for carcinogenic initiation. However, proof of emergence of CSCs induced by arsenic in a stem cell population is not available. METHODS: We continuously exposed the human prostate epithelial stem/progenitor cell line WPE-stem to an environmentally relevant level of arsenic (5 mu M) in vitro and determined the acquired cancer phenotype. RESULTS: WPE-stem cells rapidly acquired a malignant CSC-like phenotype by 18 weeks of exposure, becoming highly invasive, losing contact inhibition, and hypersecreting matrix metalloproteinase-9. When hetero-transplanted, these cells (designated As-CSC) formed highly pleomorphic, aggressive tumors with immature epithelial- and mesenchymal-like cells, suggesting a highly pluripotent cell of origin. Consistent with tumor-derived CSCs, As-CSCs formed abundant free-floating spheres enriched in CSC-like cells, as confirmed by molecular analysis and the fact that only these floating cells formed xenograft tumors. An early loss of NSC self-renewal gene expression (p63, ABCG2, BMI-1, SHH, OCT-4, NOTCH-1) during arsenite exposure was subsequently reversed as the tumor suppressor gene PTEN was progressively suppressed and the CSC-like phenotype acquired. CONCLUSIONS: Arsenite transforms prostate epithelial stem/progenitor cells into CSC-like cells, indicating that it can produce CSCs from a model NSC population.