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Pre-neoplastic lesions in rodent liver often express high levels of MDM2 and lack a p53 response to DNA damage. The question we posed was whether there is a liver-specific regulation of the p53/MDM2 feedback loop and if it can be related to the development of pre-neoplastic lesions, referred to as enzyme altered foci (EAF) in rats. Acute responses of p53 and MDM2 to diethylnitrosamine (DEN) were characterized by employing immunohistochemistry, western blotting, RT-PCR and in situ hybridization. A single dose of DEN induced a centrilobular p53 response that peaked at 24 h. It was associated with transcriptional activation of MDM2 and signs of apoptosis. However, in midzonal hepatocytes, which constitutively expressed high levels of cytoplasmic MDM2, there was a rapid-onset but transient p53 response. It was terminated at 24 h and there were no signs of apoptosis. The rapidly declining p53 levels in midzonal areas was preceded by a transient peak in MDM2 mRNA levels at 6 h. Rats pre-treated with repeated low or high weekly doses of DEN exhibited EAF and these lesions expressed high levels of cytoplasmic MDM2. Using MDM2 as a marker for EAF gave similar results as using glutathione transferase-P (GST-P) as a marker. Furthermore, small EAF, elicited by low doses of DEN, were preferentially localized to midzonal areas. It is concluded that in centrilobular areas DEN-induced alterations in p53/MDM2 levels are compatible with a previously described feedback loop. An attenuated p53 response in midzonal hepatocytes can be related to a high constitutive expression of MDM2 in these cells. The localization of small EAF to midzonal areas, and the fact that EAF cells expressed high levels of MDM2, indicates that MDM2 expression is a factor governing initiation and early development of EAF. The data support the hypothesis that EAF hepatocytes are initiated via epigenetic mechanisms.

Characterizing the role of MDM2 in diethylnitrosamine induced acute liver damage and development of pre-neoplastic lesions