The Mechanism of Arsenic-Induced Cell Transformation

Arsenic is a naturally existing element which has been recognized as a potent carcinogen. Humans are exposed to it via both environmental contamination and occupational exposure. Epidemiological studies have indicated that people exposed to high levels of arsenic are prone to develop skin, bladder, liver and lung cancers. Although the etiology of arsenic tumorigenesis remains to be elucidated, oxidative stress induced by accumulation of excessive reactive oxygen species (ROS) has been known to be involved in mediating arsenic deleterious effects. On the other hand, autophagy, first described a half century ago, has recently emerged as an important cellular process involved in various cellular functions. It is an important cellular self-defense mechanism and functions in clearing intracellular microbes, misfolded proteins and damaged organelles. The impairment of autophagy leads to accumulation of dysfunctional organelles, a factor well known to induce genotoxic stress. Arsenic-induced cell transformation, a process which converts normal cells to a cancer-like state of uncontrolled division, is a critical step in arsenic tumorigenesis. However, it remains unclear in terms of the role of ROS, autophagy and their interaction in the transformation. Hypothesis: Our preliminary work has shown that chronic low dose arsenic exposure alters intracellular ROS levels and autophagy activity, and induces human lung epithelial cells’ transformation. We hypothesize that the genomic damage caused by increased ROS levels and inadequate autophagy protection is responsible for arsenic-induced cell transformation. Specific Aims: (1) To determine the interaction between ROS and autophagy upon arsenic exposure in BEAS-2B cells; (2) To determine the mechanism underlying arsenic-induced alteration of autophagy activity and whether autophagy inhibits arsenic-induced DNA damage and chromosomal aberration; and (3) To investigate whether autophagy inhibits arsenic-induced cell transformation by performing soft agar assay in vitro and tumorigenecity assay in vivo. Study Design: we will first determine the interaction between ROS and autophagy after arsenic exposure. Then, we will determine whether there is a protective effect of autophagy on arsenic-induced DNA damage and chromosomal aberration. To further address the issue whether autophagy protects cells from arsenic-induced oxidative stress and transformation, we will investigate whether down-regulation of autophagy enhances arsenic-induced transformation by performing soft agar assay in vitro and tumorigenecity assay in vivo. Cancer relevance: The proposed study will determine the mechanism underlying arsenic-induced cell transformation, a process which converts normal cells into a cancer-like state of uncontrolled division. The cell transformation is a critical step in arsenic-induced cancers and has significant scientific as well as clinical importance. It is also an innovative study since it is the first investigation of the role of autophagy (a cellular protective mechanism) and its interaction with oxidative stress (caused by excessive reactive oxygen species which induce cellular damages) in arsenic-induced cell transformation. Data collected from this study will provide valuable information for our understanding the mechanism underlying arsenic-induced cancers.