Tumorigenic conversion of a non-tumorigenic rat urothelial cell line by overexpression of H₂O₂-generating peroxisomal fatty acyl-coa oxidase

Hydrogen peroxide (H₂O₂) and some cytokines that are released during the inflammatory process are important factors for the development of urinary bladder carcinoma and for its growth. Sustained induction of H₂O₂-generating peroxisomal fatty acyl-CoA oxidase (ACOX) in the liver of rats and mice by non-genotoxic peroxisome proliferators leads to the development of liver tumors. To examine the role of ineracellular H₂O₂ generated by ACOX during urinary bladder carcinogenesis, we overexpressed rat ACOX in a non-tumorigenic rat urothelial cell line, MYP3, under the control of the cytomegalovirus promoter. The clones overexpressing rat ACOX, when exposed to a fatty-acid substrate (150 μM linoleic acid), demonstrated strikingly higher levels of intracellular H₂O₂ (P > 0.001) and formed colonies in soft agar in proportion to the duration of exposure to linoleic acid. Furthermore, all the transformants, which were selected at random from soft agar, demonstrated an accelerated growth potential on a plastic surface, as well as tumorigenicity in athymic nude mice. In addition, the growth of these transformants was stimulated by cytokines, interleukin-6 and tumor necrosis factor-cc, better than the growth of ACOX-overexpressing, but non-transformed cells or that of the parental cells. Our results clearly demonstrate that H₂O₂ induced by ACOX acts as a carcinogen on urothelial cells, and that transformed cells have acquired an advantage for growth over nonneoplastic cells because of their selective response to the stimulatory action of several cytokines.