Persistence of carcinogen-altered cell population in rat urothelium which can be promoted to tumors by chronic inflammatory stimulus.

Chronic inflammation of the urinary tract is a significant risk factor for the development of urinary bladder cancer in man. Previously we have shown that acute and chronic inflammation induced by repeated intravesical instillation of killed Escherichia coli (KEC) strikingly enhanced N-methyl-N-nitrosourea (MNU)-initiated bladder carcinogenesis in our heterotopically transplanted rat urinary bladder model. We conducted the present study to determine whether delayed onset of KEC treatment can still enhance carcinogenesis of the MNU-initiated urothelium and whether continuous KEC treatment is necessary for the development of tumors. After the initiation of carcinogenesis in heterotopically transplanted bladders by the instillation of a single dose (0.25 mg) of MNU, animals were divided into several groups, for which weekly KEC treatment (5 x 10(8) cells suspended in 0.5 ml of phosphate-buffered 2.1% NaCl solution) was begun 1, 5, and 18 weeks later and continued until termination of the experiment at 31 weeks. In addition, animals received 4-week KEC treatment, which was started 1 or 5 weeks after MNU administration. Treatment with KEC alone or MNU alone induced few tumors. Maximal tumor development was demonstrated in the group receiving KEC treatment continuously throughout the experimental period. Delaying the onset of continuous KEC treatment by 4 weeks resulted in a significant decrease in the number of tumors (P = 0.006). However, a substantial number of tumors were induced even when KEC treatment was delayed as many as 18 weeks, as compared to tumor development in the group receiving MNU only (P = 0.007). The tumor volume (size) was not different between continuous and short-term KEC treatment groups. We conclude that (a) although a large number of cells undergo promutagenic DNA damage by a single dose of MNU, the amounts are reduced quickly during the subsequent 4 weeks; but that (b) a substantial number of genetically altered cells remain for a long time and can be promoted to tumors when stimulated by a chronic inflammatory stimulus; and that (c) the duration of KEC treatment determines the number, but not the volume, of tumors.