ToxSci Advance Access published online on November 8, 2006
Toxicological Sciences, doi:10.1093/toxsci/kfl160
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1 Department of Carcinogenesis, University of TX, M.D. Anderson, TX, Smithville, TX USA
* To whom correspondence should be addressed. Previous studies have shown that human bladder cells (UROtsa), a target of arsenic-induced cancer, can biotransform arsenite to monomethylarsonous acid [MMA(III)], which is more cytotoxic and capable of transforming the UROtsa cells following long-term, low-level exposure. Cyclooxygenase-2 (COX-2) causes hyperplasia in bladder cells and is considered a key biomarker in bladder cancer. To investigate the role of mitogenic pathway stimulation in MMA(III)-induced transformation, UROtsa cells were treated with 50 nM MMA(III) for 12, 24, or 52 weeks and analyzed by western blot for COX-2 expression. Elevations in COX-2 expression were noted following chronic MMA(III) exposure and this induction increased with duration of exposure, suggesting that COX-2 or the signal transduction pathways responsible for COX-2 protein expression may play a role in MMA(III)-induced transformation. Acute exposure studies found MMA(III) treatment (10, 50, and 100 nM, 4 h) induced COX-2 in UROtsa cells with the lowest doses (10 and 50 nM) causing the strongest induction. Using pharmacological inhibitors of various pathways, it was shown that epidermal growth factor receptor (EGFR), extracellular signal-regulated kinase (ERK 1/2), phosphoinositide 3-kinase (PI3K), and src were important in the induction of COX-2 by MMA(III). ERK 2 phosphorylation was verified by western blot analysis with a peak at 15 min and c-jun was translocated to the nucleus following 50 nM MMA(III) treatment. To determine MMA(III) targets, receptors of the ErbB family were further investigated. Chronic MMA(III) exposure led to upregulation of the epidermal growth factor receptor (EGFR), or ErbB1. Short term MMA(III) treatment caused the phosphorylation of ErbB2 in its autophosphorylation site. To verify the importance of these signaling pathways to the growth of the MMA(III) transformed UROtsa cells in soft agar, various inhibitors were used to block pathways and monitor cells growth. Pathways of importance in anchorage independent growth of UROtsa cells chronically exposed to MMA(III) are src, PI3K, and COX 1 and 2. As COX-2 is an important mediator that contributes to carcinogenesis via promotion of cell proliferation, inhibition of cell death, induction of angiogenesis, and facilitation of invasion, and it is highly upregulated both acutely and chronically in the MMA(III)-transformed cells, it is likely that activation of the MAPK pathway and increased COX-2 expression is a plausible mechanism for MMA(III) bladder carcinogenesis. Authors Bredfeldt and Eblin are first authors.
Received August 21, 2006
Accepted October 30, 2006
Environmental Toxicology
Mitogenic Signal Transduction Caused by Monomethylarsonous Acid in Human Bladder Cells: Role in Arsenic-Induced Carcinogenesis
T. G. Bredfeldt 1 *, K. E. Eblin 2 * *, S. Buffington 2, and A. J. Gandolfi 2
2 Department of Pharmacology and Toxicology, University of AZ, Tucson, AZ USA
K. E. Eblin, E-mail: eblin{at}pharmacy.arizona.edu
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