Repair and removal of azoxymethane-induced O6-methylguanine in rat colon by O6-methylguanine DNA methyltransferase and apoptosis

Azoxymethane (AOM) is an alkylating agent that generates mutagenic and carcinogenic O⁶-methylguanine (O⁶meG) adducts in DNA. O⁶meG has been detected in human colonic DNA; hence, understanding the innate cellular events occurring in response to the formation of O⁶meG is important in developing preventive strategies for colorectal cancer. We explored the time-course, dose-response, and kinetics of O⁶meG formation and its removal by the DNA repair protein, O⁶-methylguanine DNA methyltransferase (MGMT), and apoptosis. In rats given AOM (10mg/kg), the formation of O⁶meG occurs within 2h of exposure, accompanied by rapid depletion of MGMT activity and followed by the induction of an acute apoptotic response that peaks at 6-8h. MGMT repair and apoptosis are dependent on AOM dose and O⁶meG load. Apoptosis is initiated only when a high O⁶meG load is present and MGMT activity is fully depleted. AOM, 10mg/kg, overwhelms MGMT repair for about 96h and renewed MGMT activity is only observed once O⁶meG is no longer detectable. A threshold for apoptosis is observed at 6h after 6mg/kg AOM, when a high O⁶meG persists and MGMT activity is very low. These data suggest that apoptosis is probably triggered by O⁶meG, but only once the capacity of MGMT to repair O⁶meG is exhausted. In the colonic epithelium, apoptosis may be complementary to MGMT, in terms of minimising potentially mutagenic events and maintaining a healthy genome.