Cellular reactions of O6-methylguanine, a product of some alkylating carcinogens

Abnormal hepatic nucleotide pools in sparse fur (spf) mutant mice deficient in ornithine transcarbamylase

Sparse fur hemizygous male mice are over 90% deficient in ornithine transcarbamylase and exhibit increased synthesis of orotic acid. Because our earlier studies have demonstrated that orotic acid is a liver tumor promoter in the rat, it was of interest to determine whether this genetic disorder also increases the risk of tumor promotion. The results revealed that the livers of mutant mice showed a fourfold increase in uridine nucleotides and a 50% decrease in adenosine nucleotides compared to corresponding controls, a pattern of nucleotide pool imbalance similar to that seen in the livers of rats exposed to orotic acid under promoting conditions. Creation of such an imbalance appears to be important for orotic acid to exert its promotional effects. Sparse fur mutant mouse may, therefore, be an ideal animal model to study the tumor-promoting effects of orotate.

Metabolic fate of N1-methyladenine in yeast auxotrophic to adenine

The metabolic fate of N1-methyladenine in yeast with respect to its incorporation into RNA has been studied. Chromatographic analysis of the PCA-soluble and -insoluble fractions of cells grown in the combined presence of adenine and 3H-labeled N1-methyladenine show that (a) N1-methyladenine can enter the cells, (b) however, it is very poorly utilized by the salvage pathway for nucleic acid synthesis and (c) the inhibition occurs probably at the first stage of conversion of the methylated base to the corresponding nucleotide.

Possible depletion of a DNA repair enzyme in human lymphoma cells by subversive repair

Mex+ human lymphoma cell lines contain O6-methylguanine-DNA methyltransferase, a DNA repair enzyme that undergoes suicide inactivation on interaction with its substrate. The cells are therefore competent to remove the alkylation lesion O6-methylguanine from their DNA. However, several repair-deficient lymphoma cell lines (Mex-) are also known. It is shown here that Mex+ cells can be converted temporarily to a Mex- phenotype by growth in nontoxic concentrations of free O6-methylguanine. The depletion of methyltransferase activity is not a result of O6-methylguanine incorporation into DNA and subsequent demethylation by the enzyme. It is proposed that O6-methylguanine is mistakenly incorporated into tRNA molecules by means of a post-transcriptional ribosyl transfer reaction. The demethylation of such bases in tRNA has been demonstrated by using bacterial and human DNA repair enzymes. The existence of such a subversive repair of a methylated base in tRNA raises the possibility of competition between DNA and RNA for cellular DNA repair enzymes. Furthermore, it is proposed that the known aberrant methylation of tRNA in certain transformed cells, together with subversive tRNA repair, could account for the Mex- phenotype.

Biological effects of incorporation of O6-methyldeoxyguanosine into Chinese hamster V79 cells

Analysis of the biological effects of specific DNA alkylations by simple alkylating agents is complicated by the variety of sites involved. It is, therefore, of value to be able to incorporate into cellular DNA nucleosides alkylated in a single position, e.g., O6-methyldeoxyguanosine. Such cellular incorporation is particularly difficult to achieve because this nucleoside is rapidly demethylated by adenosine deaminase. We have attempted to achieve such incorporation into the DNA of V79 cells by using coformycin, an inhibitor of adenosine deaminase, and by forcing the cells to depend on exogenous purines by the use of medium containing aminopterin. The DNA of V79 cells exposed to O6-methyl-[8-3H]deoxyguanosine (2.4 microM, sp. act. 14500 Ci/mole) showed an incorporation level of 4 X 10(-8) nucleotides. When 1000-fold higher concentrations were employed (3-15 mM, sp. act. 1.6 Ci/mole), significant cytotoxicity and inhibition of DNA synthesis was observed. However, because it was not economically feasible to administer high specific activity O6-methyldeoxyguanosine to the cells at these concentrations, we could not determine the amount of labeled nucleoside incorporated into DNA. Examination of the frequency of 6-thioguanine-resistant cells in these treated populations showed no significant increase above the background level. Comparison of the cytotoxic effect of O6-methyldeoxyguanosine with deoxyadenosine showed that the toxicity induced by O6-methyldeoxyguanosine could have resulted from mimicry of deoxyadenosine, rather than by incorporation of the alkylated nucleoside itself.

The incorporation of O6-methyldeoxyguanosine and O4-methyldeoxythymidine monophosphates into DNA by DNA polymerases I and alpha

The modified nucleoside 5'-triphosphates O6-MedGTP ad O4-MedTTP have been synthesised and their acceptability as DNA-precursors investigated using DNA polymerases I and alpha in an in vitro assay. O6-MedGMP is only incorporated into newly-synthesized DNA-like material in the presence of templates containing thymine bases. Similarly O4-MedTMP is only incorporated in the presence of templates containing guanine bases. The results confirm the promutagenic nature and base-pairing properties of O6-MeG ad O4-MeT.

Metabolism of O6-alkyldeoxyguanosines and their effect on removal of O6-methylguanine from rat liver DNA

O6-Methyldeoxyguanosine and O6-ethyldeoxyguanosine are weak inhibitors (of approximately equal potency) of the removal of O6-methylguanine from methylated DNA by a rat liver enzyme in vitro. When administered to rats, O6-ethyldeoxyguanosine retarded the removal from liver DNA of the O6-methylguanine which had been produced by pretreatment with dimethylnitrosamine, but the effect was short lived. O6-Methyldeoxyguanosine was much less effective. When cells in culture were grown in a medium containing radioactive O6-methylguanine or O6-methyldeoxyguanosine there was negligible incorporation of the methylated base into DNA, but substantial conversion to guanine which was incorporated. When these substances were injected into rats after partial hepatectomy, a very small incorporation of O6-methylguanine into DNA apparently occurred. Both O6-ethyldeoxyguanosine and O6-methyldeoxyguanosine were dealkylated by rat liver extracts, but the methylated derivative was metabolized much more rapidly. O6-Methylguanosine and O6-ethylguanosine were also dealkylated by rat liver extracts, but the corresponding bases were not attacked. This reaction was probably carried out by the adenosine deaminase in the extracts because it could be prevented by addition of erythro-9-(2-hydroxy-3-nonyl)adenine, a potent adenosine deaminase inhibitor, and could also be effected by purified calf intestinal adenosine deaminase. The Km for the demethylation of O6-methyldeoxyguanosine by calf intestinal adenosine deaminase was comparable to that for adenosine, whereas the Km for O6-ethyldeoxyguanosine was ten times greater. The V for O6-methyldeoxyguanosine was about 11% that for adenosine, but that for O6-ethyldeoxyguanosine was only 0.3%. The higher Km and the slower V for O6-ethyldeoxyguanosine may contribute to the slower dealkylation of this nucleoside by liver extracts and could account for its greater effect on slowing O6-methylguanine excision from DNA in vivo.

The action of rat cytosol enzymes on some methylated nucleic acid components produced by the carcinogenic N-nitroso compounds

The stabilities of several alkylated nucleic acid components have been examined in the presence of cytosol extracts from a variety of rat tissues. An activity capable of demethylating O6-methyldeoxyguanosine was readily detectable in all tissues examined; arranged in approximate order of decreasing specific activity these are as follows: small intestine, spleen, kidney, lung, liver, skin, heart and brain. The in vitro requirements for the activity derived from liver and the observations that O6-methylguanine and its deoxynucleoside 5'-monophosphate are insensitive to the action of these extracts suggests that this activity may be due to an enzyme which resembles adenosine deaminase. In contrast to the ready degradation of O6-methyldeoxyguanosine the corresponding ethyl derivative was degraded very much more slowly but there was no evidence for other activities against the O4- and O2-methyldeoxythymidines. Similarly, no demethylation of the N-substituted deoxynucleosides, 3-methyldeoxycytidine 3-methldeoxythymidine, 1-methyldeoxyadenosine and 7-methyldeoxyguanosine, was detected.

Theophylline incorporation into the nucleic acids of theophylline-stimulated melanoma cells

Theophyllin, an inhibitor of cAMP-degrading phosphodiesterase, stimulates melanin biosynthesis in cultures of RPMI 3460 hamster melanoma cells. Although theophylline does produce an initial transient elevation of intracellular cAMP levels, long-term treatment with theophylline produces a significant decrease in cAMP content. There is an inhibition of the theophylline stimulation by dibutyryl-cAMP; this is apparently caused by interference of dibutyryl-cAMP with the uptake and incorporation of theophylline, as shown by experiments with 3H-theophylline. An alternative theory is that theophylline, being a methylxanthine compound, is metabolized by the cell and possibly causes melanotic stimulation by becoming incorporated into cellular nucleic acids or by altering the normal nucleic acid metabolism. The following observations are consistent with this theory: (u) 3H-theophylline was incorporated into both trichloroacetic acid (TCA)-soluble and TCA-insoluble cell fractions; most of the insoluble label became soluble after digestion with ribonuclease and deoxyribonuclease. (2) These nuclease digests of the 3H-theophylline-labeled TCA-insoluble cell fractions contained 3H-labeled material that chromatographed differently from normal nucleotides on ion exchange thin layer sheets. (3) The acid-soluble pool of 3H label disappeared rapidly while both the insoluble label and the induction of melanogenesis remained stable for 50 hr after the removal of exogenous 3H-theophylline.

Formation and subsequent removal of O6-methylguanine from deoxyribonucleic acid in rat liver and kidney after small doses of dimethylnitrosamine

1. The amounts of 7-methylguanine and O(6)-methylguanine present in the DNA of liver and kidney of rats 4h and 24h after administration of low doses of dimethylnitrosamine were measured. 2. O(6)-Methylguanine was rapidly removed from liver DNA so that less than 15% of the expected amount (on the basis of 7-methylguanine found) was present within 4h after doses of 0.25mg/kg body wt. or less. Within 24h of administration of dimethylnitrosamine at doses of 1mg/kg or below, more than 85% of the expected amount of O(6)-methylguanine was removed. Removal was most efficient (defined in terms of the percentage of the O(6)-methylguanine formed that was subsequently lost within 24h) after doses of 0.25-0.5mg/kg body wt. At doses greater or less than this the removal was less efficient, even though the absolute amount of O(6)-methylguanine lost during 24h increased with the dose of dimethylnitrosamine over the entire range of doses from 0.001 to 20mg/kg body wt. 3. Alkylation of kidney DNA after intraperitoneal injections of 1-50mug of dimethylnitrosamine/kg body wt. occurred at about one-tenth the extent of alkylation of liver DNA. Removal of O(6)-methylguanine from the DNA also took place in the kidney, but was slower than in the liver. 4. After oral administration of these doses of dimethylnitrosamine, the alkylation of kidney DNA was much less than after intraperitoneal administration and represented only 1-2% of that found in the liver. 5. Alkylation of liver and kidney DNA was readily detectable when measured 24h after the final injection in rats that received daily injections of 1mug of [(3)H]dimethylnitrosamine/kg for 2 or 3 weeks. After 3 weeks, O(6)-methylguanine contents in the liver DNA were about 1% of the 7-methylguanine contents. The amount of 7-methylguanine in the liver DNA was 10 times that in the kidney DNA, but liver O(6)-methylguanine contents were only twice those in the kidney. 6. Extracts able to catalyse the removal of O(6)-methylguanine from alkylated DNA in vitro were isolated from liver and kidney. These extracts did not lead to the loss of 7-methylguanine from DNA. 7. The possible relevance of the formation and removal of O(6)-methylguanine in DNA to the risk of tumour induction by exposure to low concentrations of dimethylnitrosamine is discussed.

[Transplacental induction of neurogenic tumors in rabbits (author's transl)]

The oncogenic effect of ethylnitrosourea (ENU) on rabbits during the organogenesis was studied. ENU, dissolved in sodium phosphate (pH 6.0), was injected in a 50 mg/kg dosis intravenously in pregnant dams between the 8th and 10th day of gestation. From 12 offspring 8 grew up normally and appeared unremarkably until 30--36 months of age when neurologic signs first developed. Six rabbits showed one or more tumors along the peripheral nerves. Two rabbits were bearing brain gliomas in the vicinity of the 3rd ventricle. The tumors of the peripheral nervous system (PNS) were of variable size and soft consistency; they had a cut surface of spongy and even cystic appearance. Histologically, the PNS tumors resemble Schwannomas of the Antoni B type. An ultrastructural study of these neoplasms disclosed cells featuring a basal lamina type of lining characteristics of Schwann cells. These findings demonstrate that the exposure of rabbits to ENU during organogenesis results in the induction of neurogenic tumors which become clinically manifest late in life. It seems, therefore, that the selective oncogenic effect of ENU on the nervous tissues of the exposed animals depends exclusively on the stage of organogenesis at which the drug is brought to bear upon the target organ anlage.