Differential repair of O⁴-alkylthymidine following exposure to methylating and ethylating hepatocarcinogens

Abstract

Recent experiments have demonstrated that O6-alkylguanine is rapidly removed from hepatocyte DNA following continuous exposure to 1,2-dimethylhydrazine or diethylnitrosamine. In contrast, O4-ethyldeoxythymidine accumulates to concentrations more than 50 times greater than O6-ethyldexyguanosine. Studies on the formation and persistence of O4-methyldeoxythymidine in vivo have not been reported. This study reports the development of sensitive radioimmune assays to O4-methyldeoxythymidine and O4-ethyldeoxythymidine. Utilizing this method, the accumulation and removal of O4-methyldeoxythymidine and O4-ethyldeoxythymidine in liver DNA from rats exposed to 1,2-dimethylhydrazine or diethylnitrosamine were measured. The results demonstrated that O4-methyldeoxythymidine was formed at an O6-methylguanine/O4-methyldeoxythymidine ratio of approximately 100/1 and was repaired with a half-time of approximately 20 h. In contrast, O4-ethyldeoxythymidine removal was 13 times slower with a t 1/2 of approximately 11 days after both pulse dose and cessation of continuous DEN administration. Combined with previously reported data, results presented here suggest that (i) despite a lower rate of formation, O4-methyldeoxythymidine becomes nearly equal in importance to O6-methylguanine as a promutagenic adduct in hepatocytes from continuously exposed rats and (ii) differential repair of O4-alkylthymidine adducts provides a mechanism that may explain in part the superior ability of ethylating versus methylating agents to induce hepatocellular carcinomas in the rat.

Hepatitis B Virus X Protein Acts as a Tumor Promoter in Development of Diethylnitrosamine-Induced Preneoplastic Lesions

“…To examine this possibility, mutant phage derived from DEN-treated ATX and wild-type mice were picked at random, and the cII genes were sequenced (n ϭ 63) to establish the mutation spectrum (Table 2). Compared to the mutation spectrum reported for untreated mice (49), DEN treatment resulted in a relatively large increase in transition and transversion events at A and T base pairs, a finding consistent with the long half-life and mutagenicity of O 4 -ethyldeoxythymine (O 4 -EtT) moieties formed by DEN (t 1/2 ϭ 11 days) (48). In addition, there was an approximately twofold increase in the incidence of transitions from G or C to A or T base pairs in ATX relative to wild-type animals.…”

Relationships between DNA adduct formation and carcinogenesis.

“…Thus, this model for initiation underestimated the latter, suggesting that DNA adducts other than 06-MG might be involved. As will be discussed in greater detail in subsequent portions of this review, it is likely that 04-methyldeoxythymidine also contributes to the initiation of hepatocytes (18).…”

“…into nonparenchymal cells (NPC), consisting of endothelial and Kupffer cells, and hepatocytes by either centrifugal elutriation (13,16) or low-speed centrifugation (14,17,18).…”

“…Polyclonal antibodies to 04-methylthymidine (04-MedThd) were also developed, permitting the first measurements of this minor alkylation product in animals exposed to methylating agents (18). Following a single 20 mg/kg exposure to SDMH, 04-MedThd was present in rat liver at 1/100 the amount of 06-MG. 04-MedThd was removed from rat liver relatively rapidly, having a half-time of -20 hr (Fig.…”

“…Thus, differences in repair rates of 04-MedThd and 04-EtdThd provides a mechanism to explain the greater ability of ethylating versus methylating agents to induce hepatocellular carcinoma in the rat. This is shown in Table 1, where alkylation data from hepatocyte DNA of rats exposed to equimolar doses of SDMH and DEN for up to 28 days were compared (18) …”