Chronic
inflammation is a significant risk factor for the development of
urinary bladder cancer. We showed previously that
inflammation induced by killed Escherichia coli strikingly enhanced
N-methyl-N-nitrosourea (MNU)-initiated rat bladder
carcinogenesis. We also demonstrated a marked increase in several
cytokines, including
TNF-alpha, in aspirates from bladders treated with killed E. coli. In the present investigation, we tested the hypothesis that
TNF-alpha released during
inflammation was causally related to the development of
bladder cancer. Using growth in soft
agar and tumorigenicity in athymic nude mice as indices of transformation, we examined the effect of
TNF-alpha on the enhancement of H2O2-initiated transformation of
MYP3 cells;
MYP3 is an anchorage-dependent nontumorigenic rat urothelial cell line. We have already demonstrated that H2O2 is a potent transforming agent which is released during the inflammatory process.
MYP3 cells pretreated with H2O2 were exposed to
TNF-alpha (0 to 100 ng/ml) for 1 week in monolayer culture and were then subjected to growth in soft
agar. A marked increase in the number of colonies was observed in the cells that were first treated with H2O2 and subsequently exposed to
TNF-alpha, as compared with the untreated control (p < 0.001). In addition, treatment with
TNF-alpha alone caused colony formation and was associated with a 6.5- to 8.7-fold increase in intracellular H2O2 (p < 0.001). Addition of an
antioxidant,
alpha-tocopherol, resulted in a significant reduction in the number of colonies induced by
TNF-alpha (p < 0.001). The transformants induced by
TNF-alpha have acquired the potential of anchorage-independent growth and tumorigenicity in athymic nude mice. Our results suggest that
TNF-alpha-induced transformation in urothelial cells is due to induction of H2O2, and that this may be one of the mechanisms involved in the
carcinogenesis in vivo associated with chronic
urinary tract infection.