Arsenical-induced Reactive Oxygen Species Lead to Altered Cellular Signaling and Phenotypic Alterations in Human Bladder Cells

Abstract

Arsenical-induced carcinogenesis in human bladder has been established through epidemiological evidence, but unfortunately, no mode of action had been determined for this phenomenon. UROtsa cells, a normal, immortalized cell culture model of human urothelium does not form tumors when injected into immuno-compromised mice nor does it have anchorage-independent growth. UROtsa cells were shown to be malignantly transformed following low-level exposure to both arsenite [As(III)] and its more toxic metabolite, monomethylarsonous acid [MMA(III)] providing additional models for studying arsenical-induced carcinogenesis of the bladder. These transformed cell lines allow researchers the ability to investigate the process of urothelial tumorigenesis at multiple time points of arsenical exposure. In the studies discussed here in, environmentally relevant levels of As(III) and MMA(III) were chosen. UROtsa cells were exposed to As(III) and MMA(III) both acutely and chronically to begin investigations into signaling pathway alterations that can lead to carcinogenesis in the human bladder upon exposure to arsenicals. In acute studies, it was shown that As(III) and MMA(III) generate oxidative stress response in UROtsa at low, environmentally relevant levels. The ROS generated by MMA(III) led to an increased 8-oxo-dG formation after 30 min, supporting the importance of MMA(III) in damage caused in the bladder by arsenicals. Because ROS has been linked to MAPK signaling, it was shown that 50 nM MMA(III) and 1 µM As(III) induce MAPK signaling following acute exposures and this increase is dependent on the production of ROS.Next, it was necessary to begin to look at changes that occur during transformation of UROtsa with MMA(III). Chronic exposure to 50 nM MMA(III) constitutively increases the amounts of EGFR, activated Ras, and COX-2 protein in MSC cells. Chronic upregulation of COX-2 in MSC52 cells is due to increased levels of ROS. Phenotypic changes seen in MSC52 cells (hyperproliferation and anchorage independent growth) are dependent on the secondary generation of excess ROS in MSC52 cells. These data clearly present evidence supporting a role for ROS in both acute and chronic toxicities associated with low-level arsenical exposure, and gives evidence that ROS are important in cellular transformation following MMA(III) exposure.