ACUTE APOPTOTIC RESPONSE INDUCED BY THE COLON CARCINOGEN AOM IS DEPENDENT ON P53 GENE and NOT THE APC GENE

Background/objective  Apoptosis is disordered in tumourigensis, however, the importance of apoptosis in relation to DNA damage created at the time of initiation by genotoxic carcinogens, i.e. the acute apoptotic response to genotoxic carcinogens (AARGC), has hardly been explored. p53 and APC are tumor suppressor genes known to be altered frequently in colon cancer, however, it remains unclear whether AARGC is dependent on the function of p53 or APC. p53 –/–, p53 ± and APC Min/+ mice provide an excellent model to test the biological significance of AARGC in colon in terms of its ability to delete genetically damaged cells that might progress to cancer. Thus, we have tested the hypothesis that p53 and APC play a critical role in AARGC, by studying AARGC in p53+/– , p53 –/– mice and APC Min/+ 0. Methods  p53 knockout mice were produced by breeding male p53+/– with female C57BL/J mice or interbreeding p53+/– mice. APC Min/+ mice were produced by breeding male APC Min/+ mice with female C57BL/J mice. Mice geno‐typing were confirmed by PCR. At 10–12 weeks age, 44 mice were given a single subcutaneous azoxymethane (AOM 10 mg/kg) injection to induce AARGC, and killed 6 h later (the time of the maximal response). There were eight p53–/– mice, 11 p53+/– mice, nine p53+/+mice, 12 APC Min/+ mice, and six APC+/+ mice. Three p53–/– mice, four p53+/– mice, seven p53+/+ mice, two APC Min/+ , and six APC+/+ mice without AOM injection were used as controls. Apoptosis in colon was measured by classic morphological H & E criteria. Results  In p53+/+ mice, AOM induced a significant increase in apoptosis (4.70 ± 0.35, SEM, apoptotic cells per crypt column) in the distal colon, located almost exclusively in the proliferative compartment. In comparison to the pattern of apoptosis observed in the p53+/+ mice, the apoptotic response of p53–/– mice was almost nonexistent (0.12 ± 0.06) while in p53+/– mice it was significantly suppressed by approximately 50% (2.26 ± 0.28); P  < 0.01. In contrast to the importance of p53 gene on AARGC, absence of the APC gene had no obvious effect on AARGC: APC Min/+ mice (5.07 ± 0.30) and APC+/+ (5.50 ± 0.33); P  > 0.05. Conclusion  p53 function appears to be critically important for carcinogen‐induced apoptosis in colon, while APC homeostasis appears not to be involved in this type of apoptosis. The loss of just one allele of p53, interferes with its function. Further studies are required to determine whether defective AARGC in p53 knockout mice puts them at increased risk of subsequent events in tumorigensis, and whether AARGC can be regulated by known protective agents.