Chemically-induced apoptosis: p21 and p53 as determinants of enterotoxin activity

The relationship between toxin-induced apoptosis and longer-term (>72 h) in testinal toxicity was investigated in vivo using p53 wild type ((+/+)) and 'knockout' ((-/-)) mice. The enterotoxic antimetabolite 5-fluorouracil (5-F U) induced acute p53-dependent apoptosis in the crypts of both small intest ine and midcolon. Although the amount of apoptosis was the same order of ma gnitude at its peak (24 h) at both 40 and 400 mg/kg 5-FU, only 400 mg/kg 5- FU brought about changes in the integrity of the gut after 96 h. These were characterised by the loss of epithelial cells from crypts and villi. Only after 400 mg/kg 5-FU were mitotic index and DNA synthesis significantly sup pressed in both small intestinal and midcolonic crypts. This correlated wit h a prolonged, p53-dependent expression of p21(waf-1/cip1). In p53 null ((- /-)) mice significant reductions in 5-FU-induced apoptosis and relief from the inhibition of cell cycle progression permitted retention of crypt integ rity after 5-FU. Thus, quantitative measures of acute apoptosis in vivo did not accurately predict subsequent pathological changes in the gut. Rather, p53-dependent inhibition of cell cycle progression together with cell loss by apoptosis caused a loss of crypt integrity. Importantly, the tissue tox icity of 5-fluorouracil was genetically determined at a locus (p53) separat e from that directly associated with toxin action. The selectivity of toxin action is therefore also determined by events 'downstream' of those associ ated with the direct mode of action of the toxin. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.