Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.6.1.44 (AGT)
 770 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

O6-Alkylguanine-DNA alkyltransferase (O6-AGT), a constitutively expressed DNA repair protein, removes alkyl groups from the O6-position of guanine in DNA. Tumor cells with high O6-AGT activity are resistant to nitrosoureas and other agents that form toxic O6-alkyl adducts. We evaluated O6-benzylguanine (O6-BG) for its activity to inhibit O6-AGT and potentiate 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in O6-AGT-positive human gastric adenocarcinoma cell line, BGC-823 and its tumor xenograft. The sensitivity of BGC-823 cells to BCNU was increased by pretreatment for 2 hours with 1.5 to 6.0 microg/mL O6-benzylguanine. O6-benzylguanine (0.75-6.0 microg/mL) completely and rapidly suppressed the O6-AGT activity of cells for up to 12 hours. When given i.p. 2 hours before BCNU (25 mg/kg) to animals bearing s.c. tumors, O6-BG (90 mg/kg) produced a growth delay of 38.6 days in human gastric adenocarcinoma xenograft. Furthermore, O6-BG significantly inhibited the O6-AGT activity of tumor tissue and induced evident apoptosis. These results suggest that combination of O6-BG with BCNU may have a significant therapeutic effect in the treatment of mer + tumor.
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PMID:Potentiation of BCNU anticancer activity by O6-benzylguanine: a study in vitro and in vivo. 1090 10

The ability of O(6)-benzylguanine (BG) to inactivate alkyltransferase (AGT) to potentiate the antitumor efficacy of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) is being tested in clinical trials. As of now, there are no examples of acquired resistance to BG+BCNU in the clinical setting. However, we hypothesized that genetically unstable tumors might develop resistance to the combination after repeated drug-exposures to achieve therapeutic efficacy. To evaluate this possibility, we treated three colon cancer cell lines that are either proficient in mismatch repair (MMR) [SW480 (MMR wild type)] or deficient in MMR [HCT116 (hMLH1 mutant) and HCT15 (hMSH6 mutant)] with three cycles of BG+BCNU. After drug-treatments, HCT116 and HCT15 were completely resistant to BG-potentiated cytotoxicity of BCNU. In these two cell lines, the acquired BG resistance resulted from two de novo and different mutations at amino acid 165 in AGT: 165-lysine (K) to glutamic acid (E) (K165E in HCT116), and 165-lysine to asparagine (N) (K165N in HCT15). Both K165-mutated AGTs had markedly decreased enzymatic activity because of unstable AGT protein but were remarkably resistant to BG inactivation. FISH analysis showed that only one copy of MGMT gene exists in HCT116 cells, and the status of promoter methylation of MGMT in HCT15 showed that one allele of the MGMT promoter has an aberrant methylation. Thus, the MGMT gene expressing AGT either from one copy (HCT116) or from unmethylated allele (HCT15) was mutated because of the exposure to BG+BCNU in these two MMR-deficient cell lines. Conversely, MMR-proficient SW480 cells, treated with three cycles of BG+BCNU, maintained wt AGT and the sensitivity to BG-potentiated BCNU-cytotoxicity. To confirm that K165-mutated AGT proteins were responsible for resistance to BG+BCNU, we transfected K165E and K165N MGMT cDNAs into Chinese hampster ovary (CHO) cells. Transfected CHO cells had low AGT activity but increased IC(50) for either BCNU or temozolomide (TMZ), compared with parental CHO cells. BG did not potentiate the cytotoxicity of these two alkylating agents at concentrations up to 200 microM; in contrast, BG, at 25 microM, sensitized CHO-AGT (transfected with wt MGMT cDNA) cells to BCNU or TMZ-cytotoxicity by 3-4 fold. These results suggest that K165 AGT mutants arising in MMR-deficient tumor cells after treatment with chemotherapeutic agents are both resistant to BG-inactivation and are active in the repair of alkylated DNA adducts.
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PMID:Chemotherapy-induced O(6)-benzylguanine-resistant alkyltransferase mutations in mismatch-deficient colon cancer. 1203 16

Administration of 1,3-bis(2-chloroethyl)-N-nitrosourea (BCNU) inhibits hepatic activities of glutathione reductase (GR) comparably in both adult male Fischer-344 (F344) and Sprague-Dawley (SD) rats in vivo. BCNU pretreatment greatly exacerbates the hepatic necrosis caused by diquat in F344 rats, but does not similarly potentiate liver injury in SD rats. The primary purpose of the present studies was to test the hypothesis that BCNU pretreatment would exhibit differences between the two strains in inhibition of GR activities in hepatic subcellular compartments that would correlate with the differing effects on diquat-induced hepatic necrosis. In the present studies, 16 h after administration of 80 mg/kg of BCNU, GR activities in the hepatic homogenates were 20-30% of activities in vehicle-treated controls. Neither the extents of inhibition nor the GR activities in hepatic cytosol, microsomes. mitochondria, or purified nuclei isolated by differential centrifugation were different between SD and F344 rats treated with BCNU. The results indicate that differences between SD and F344 rats in the effects of BCNU on susceptibilities to diquat-induced hepatic necrosis are not readily attributable to compartmentally selective inhibition of GR. In addition, hepatic O6-alkylguanine-DNA alkyltransferase (AGT, MGMT) levels were almost completely depleted in BCNU-treated rats of both strains, thus indicating that MGMT-dependent pathways are unlikely to be critical determinants of the effects of BCNU on this model of acute cell death mediated by reactive oxygen species in vivo.
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PMID:Compartmental inhibition of hepatic glutathione reductase activities by 1,3-bis(2-chloroethyl)-N-nitrosourea (BCNU) in Sprague-Dawley and Fischer-344 rats. 1510 13