Absence of acute apoptotic response to genotoxic carcinogens in p53‐deficient mice is associated with increased susceptibility to azoxymethane‐induced colon tumours

Abstract

Acute apoptotic response to genotoxic carcinogens (AARGC) might be important for controlling the consequences of mutational load in the colon. It has been shown to occur in parallel with activation of DNA repair mechanisms. Inadequate AARGC might allow development of mutated clones with the potential to progress to cancer. In this study, we tested if p53 levels were important for AARGC in the colon and whether defective AARGC was associated with increased risk for colorectal oncogenesis. Apoptosis was measured in colonic epithelium of mice from each p53 genotype ( p53 -/-, p53 +/-, wild-type) without and 8 hr following a single injection of azoxymethane (AOM). To determine risk for carcinogen-induced colorectal cancer (CRC), groups of mice from each p53 genotype received 3 weekly injections of AOM and colons were examined for tumour 20 weeks later. Rates of spontaneous apoptosis in colon were not affected by p53 level. However, AARGC was absent in p53 -/-mice and reduced by 50% in p53 +/-mice (both p < 0.01) compared to wild-type mice. AOM induced tumours in 30% of wild-type mice (average multiplicity 1.0 tumours/mouse) compared to 72% of p53 +/-mice (2.0 tumours/ mouse, p < 0.01) and 100% of p53 -/-mice (2.8 tumours/mouse, p < 0.01). Without AOM, significantly fewer mice in all groups had tumours. Rates of apoptosis in tumours were independent of p53 status. p53 dysfunction puts intestinal epithelia at increased risk of genotoxin-induced oncogenesis due to impairment of apoptotic response mechanisms. p53 levels do not appear, however, to be important for spontaneous apoptosis in normal epithelium or apoptosis in tumours. Subsequent studies are now warranted to test the converse, namely, that enhanced apoptotic response to carcinogen reduces risk for colorectal oncogenesis. ' 2005 Wiley-Liss, Inc.Key words: apoptosis; carcinogen; intestinal epithelia; colorectal cancer; p53Clinical and animal studies indicate that CRC development is characterized by progressive genomic instability with multiple genetic alterations. p53 is commonly affected as >50% of CRCs show abnormality in both alleles. p53 plays an important role by recognizing DNA damage, triggering repair and apoptosis and inducing cell cycle arrest. It maintains genomic stability and prevents accumulation of multiple genetic changes characteristic of the development of neoplasia. 1 Attenuated apoptosis and accelerated tumour growth correlate with loss of p53, suggesting that loss of p53-mediated apoptosis is the rate-limiting step in tumour progression. 2,3 Whether p53 is important in regulating genetic events early in oncogenesis, such as initiating mutations, is unclear.Genotoxic carcinogens such as AOM are colorectum-selective carcinogens in rodents that alkylate DNA and initiate oncogenesis by forming DNA adducts. 4 Initiating events such as these may be relevant for humans as mutations consistent with alkylating genotoxin-induced damage have been described in human gastrointestinal tissues 5 and specifically in K-ras and p53. [6][7][8] In response to ...

Sulindac corrects defective apoptosis and suppresses azoxymethane‐induced colonic oncogenesis in p53 knockout mice

“…[56][57][58] In contrast to the requirement for p53 to mount a normal acute apoptotic response to DNA damage, we found previously that apoptosis in tumors is not dependent on p53 status. 30 In our present study, sulindac treatment tended to increase apoptosis in tumors, even when one allele or both alleles of p53 were lost, although the effect was not significant. Others have shown that certain NSAID increase levels of apoptosis in tumors.…”

“…36,38,39 Our previous study showed that in the presence of p53 haploinsufficiency, the apoptotic response to carcinogen is defective and is associated with an increased risk of progression to cancer after carcinogen administration. 30 Sulindac reduces this risk as we found that progression to colonic tumour formation was reduced in both wild-type and p53 1/2 mice by 53% and 41% respectively. In effect, p53 1/2 mice were not at increased risk for AOM-induced oncogenesis when given sulindac.…”

“…18 Because p53 dysfunction has been shown to play a role in susceptibility to several types of tumors, including our previous study, 30,43-45 we tested whether sulindac would be protective in this human-relevant genetic context. Our results show that not only does sulindac overcome the colonic apoptotic defect found in p53 haploinsufficiency, it also reverses the enhanced risk of progression to cancer.…”

“…30 Tumors were removed and embedded in paraffin (5 lm) for histopathological analysis. All tumors were examined histologically and evaluated by an independent observer based on the criteria described previously.…”

“…We have found previously that the acute apoptotic response to carcinogen (AARGC) in the mouse colon is p53-dependent with a gene-dosage effect and that decreased AARGC in p53 1/2 and p53 2/2 mice is associated with increased susceptibility to carcinogen-induced colorectal oncogenesis. 30 This model presents the opportunity to determine whether chemopreventive agents can overcome the defects in homeostatic control mechanisms subsequent upon p53 dysfunction, including mechanisms controlling mutational load such as AARGC.We test the ability of sulindac to reverse these p53-dependent defects. We tested whether it could restore the abnormal AARGC seen in p53 1/2 and p53 2/2 mice and determined if this was associated with protection against the consequent enhanced colorectal oncogenesis due to defective p53 function.…”