Unscheduled DNA synthesis was observed in primary rat liver cell cultures treated with members of five different classes of chemical procarcinogens requiring enzymatic activation as well as with a direct-acting carcinogen. In total, ten carcinogens and one related analog not commonly accepted as carcinogenic were active, while one weak carcinogen and four noncarcinogens were inactive. The production of unscheduled DNA synthesis by this spectrum of chemical carcinogens indicates that these cultures have substantially retained the metabolic capability of liver for activating diverse procarcinogens. Thus, such cultures may be useful for detecting the ability of chemicals to interact with DNA and, thereby, assigning them priority for consideration as potential cancer-causing agents.

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Detection of chemical carcinogens by unscheduled DNA synthesis in rat liver primary cell cultures.

The nervous system specificity of the carcinogenic effect of N-ethyl-N-nitrosourea in rats at the perinatal age is of particular interest because the formation of the ultimate reactant, an electrophilic ethyl cation, occurs nonenzymatically and, hence, is not tissue specific. Indeed, similar initial degrees of DNA ethylation were found in the DNA of target (brain) and nontarget tissue (liver), in terms of the molar fractions of O6-ethylguanine, N7-ethylguanine, and N3-ethyladenine 1 hr after a pulse of [1-(14)C]ethylnitrosourea. However, over a 240-hr period of observation, the elimination rate from DNA of O6-ethylguanine (a modified base likely to cause anomalous base pairing during DNA replication) was strikingly slower in brain (half-life, about 220 hr) as compared to liver (about 30 hr), and also much slower than the elimination rates from brain DNA of N7-ethylguanine (about 90 hr) and N3-ethyladenine (about 16 hr). The data suggest that the rate of elimination from DNA of O6-ethylguanine may be an important factor (with the requirement for DNA replication of the target cell) in neoplastic transformation by ethylnitrosourea.

Persistence of O6-ethylguanine in rat-brain DNA: correlation with nervous system-specific carcinogenesis by ethylnitrosourea.

Cis platinum (II) diammino-dichloride, cis Pt(II)(NH3)2Cl2 (A), is very effective against sarcoma 180 and leukaemia L 1210 in mice1,2 as well as Dunning ascites leukaemia and Walker 256 carcinosarcoma tumours in rats3. This and related compounds also reversibly inhibit cell division and induce cell elongation in E. coli B4,5. At a concentration below 5 µ, cis Pt(II)(NH3)2Cl2 selectively inhibits DNA synthesis in human amnion AV3 cells6, and there is a correlation between the relative effectiveness of a series of compounds and their capacity to inhibit DNA synthesis6. Support for the suggestion that in this cell line the effective platinum compounds acted by binding directly to DNA was obtained by Howie and Gale7. They found that DNA synthesis was selectively and persistently inhibited in Ehrlich ascites tumour cells removed from rats up to 4 days after treatment with a single injection of 10 mg/kg of A.

Cross-linking of complementary strands of DNA in mammalian cells by antitumour platinum compounds.

Publisher Summary This chapter discusses different repair processes for photochemical damage in mammalian cells. Eukaryotic chromosomes respond to DNA and protein damage in different ways. Damage to DNA alone has minimal effect on overall chromosomal structure that is maintained by protein. Damage to protein results in disruption of the chromosome and breaks in DNA. Repair of damage to each chromosomal component is different. Protein damage does not initially involve loss of genetic information, so repair can be affected through normal protein synthesis pathways that are active at all stages of the cell cycle. Repair of DNA involves novel pathways, such as photo reactivation and excision repair that are distinct from normal DNA synthesis and insensitive to many inhibitors that block semi-conservative DNA replication. During normal replication, there is an additional recovery system, S-phase recovery, which enables DNA to be replicated on damaged templates.

Repair Processes for Photochemical Damage in Mammalian Cells

The ethylation of nucleic acids by the nervous system-specific pulse-carcinogen N-ethyl-N-nitrosourea (ENU) was investigated in different tissues of the fetal, 10-day-old, and adult BD IX rat, and compared with data on the ethylation of rat-liver nucleic acids by the hepatocarcinogen diethylnitrosamine-1-14C. One hour after a pulse of ENU-1-14C, the molar fractions of N7-ethylguanine (N7-EG), O6-ethylguanine (O6-EG), N3-ethyladenine (N3-EA), and N7-ethyladenine (N7-EA) were similar in the DNA of “high-risk” (fetal or 10-day-old brain) and “low-risk” tissues (e.g. liver), and independent of the fraction of cells engaged in DNA replication at the time of the ENU pulse. The average relative yields of these ethylated bases after ethylation of DNA by ENU-1-14C in vivo were 54% (N7-EG), 28% (O6-EG), 17% (N3-EA) and ≦ 2% (N7-EA). Similar values were obtained after incubation of rat liver DNA with ENU-1-14C in vitro. The initial degree of base ethylation in the DNA of different tissues does not provide an explanation for the nervous system-specificity of the carcinogenic effect of ENU. It was, therefore, of interest to find a significant difference between brain and other tissues with regard to the elimination rate from DNA of O6-EG. While the respective elimination rates of N7-EG and N 3-EA were similar, O6-EG was removed from brain DNA much more slowly (t
1/2∼229 h) than from liver DNA (t1/2∼36 h) or from the DNA of other pooled tissues (t1/2∼54 h). The capacity of the target cell to eliminate O6-EG from its DNA, together with the frequency of DNA replication may be an important factor in determining the probability of neoplastic transformation.

Molecular and cellular mechanisms associated with pulse-carcinogenesis in the rat nervous system by ethylnitrosourea: ethylation of nucleic acids and elimination rates of ethylated bases from the DNA of different tissues

Quantitative aspects of the repair of alkylated DNA in cultured mammalian cells: II. Non-semiconservative DNA synthesis (‘repair synthesis’) in HeLa and Chinese hamster cells following treatment with alkylating agents

B A Smith

Papers

Quantitative aspects of the repair of alkylated DNA in cultured mammalian cells: II. Non-semiconservative DNA synthesis (‘repair synthesis’) in HeLa and Chinese hamster cells following treatment with alkylating agents