Inhibition of O(6)-methylguanine-DNA methyltransferase increases azoxymethane-induced colonic tumors in rats.
Azoxymethane (AOM) causes O(6)-methylguanine adduct formation which leads to G-->A transitions. Their repair is carried out by O(6)-methylguanine-DNA methyltransferase (MGMT). To evaluate the importance of this repair event in AOM-induced carcinogenesis, we examined the effect of O(6)-benzylguanine (BG), a potent inhibitor of MGMT, on colonic tumor development. Rats were treated weekly for 2 weeks at 0 and 24 h with BG (60 mg/kg body wt i.p.) or vehicle (40% polyethylene glycol, PEG-400), followed 2 h after the first dose of BG with AOM (15 mg/kg body wt) or vehicle (saline) i.p. Rats were killed 35 weeks later and tumors harvested and DNA extracted. In the AOM-treated groups, BG caused a significant increase in tumor incidence with tumors in 65.9%, versus 30.8% in the AOM/PEG-treated group (P < 0.05). In the BG/AOM group there was also a significant increase in tumor multiplicity, with 2.3 tumors/tumor-bearing rat, versus 1.6 tumors/tumor- bearing rat in the AOM/PEG group (P < 0.05). Since O(6)-methylguanine adducts can cause activating mutations in the K-ras and beta-catenin genes, we examined the effects of BG on these mutations. In the BG group there were seven mutations in codon 12 or 13 of exon 1 of the K-ras gene in 51 tumors examined, compared with no K-ras mutations in 17 tumors analyzed in the AOM/PEG group (P = 0.12). In the BG/AOM group there were 10 mutations in exon 3 of the beta-catenin gene among 48 tumors evaluated, compared with six mutations in 16 tumors analyzed in the PEG/AOM group (P = 0.16). In summary, MGMT inhibition increases AOM-induced colonic tumor incidence and multiplicity in rats.
Wali
R K
RK
Department of Medicine, University of Chicago, Chicago, IL 60637, USA.
Skarosi
S
S
Hart
J
J
Zhang
Y
Y
Dolan
M E
ME
Moschel
R C
RC
Nguyen
L
L
Mustafi
R
R
Brasitus
T A
TA
Bissonnette
M
M
D000818:Animals; D001397:Azoxymethane / Q000633:toxicity*; D002273:Carcinogens / Q000633:toxicity*; D003110:Colonic Neoplasms / Q000139:chemically induced* / Q000201:enzymology* / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D011905:Genes, ras; D006147:Guanine / Q000031:analogs & derivatives / Q000494:pharmacology; D007700:Kinetics; D009154:Mutation; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000037:antagonists & inhibitors*; D051381:Rats
D002273:Carcinogens; D004791:Enzyme Inhibitors; C064976:O(6)-benzylguanine; D006147:Guanine; D019853:O(6)-Methylguanine-DNA Methyltransferase; D001397:Azoxymethane