Normal endonuclease activities for damaged DNA during hepatocarcinogenesis

Wavelength dependence of DNA incision by a human ultraviolet endonuclease

The wavelength dependence of an ultraviolet irradiation of the DNA substrate for a human endonuclease was determined. Sites of DNA incision for all UVB and UVC wavelengths examined were at cytosines which were neither cyclobutane pyrimidine dimers nor 6-4'-(pyrimidin-2-one)pyrimidines. The optimal wavelengths for formation of these cytosine photoproducts were between 270 and 295 nm. This human endonuclease therefore has a similar ultraviolet substrate specificity to endonuclease III.

Cytosine photoproduct-DNA glycosylase in Escherichia coli and cultured human cells

Ultraviolet irradiation of DNA produces a variety of pyrimidine base damages. The activities of Escherichia coli endonuclease III and a human lymphoblast endonuclease that incises ultraviolet-irradiated DNA at modified cytosine moieties were compared. Both the bacterial and human enzymes release this cytosine photoproduct as a free base. These glycosylase activities are linear with times of reaction, quantities of enzyme, and irradiation dosages of the substrates. Both enzyme activities are similarly inhibited by the addition of monovalent and divalent cations. Analysis by DNA sequencing identified loci of endonucleolytic incision as cytosines. These are neither cyclobutane pyrimidine dimers, 6-(1,2-dihydro-2-oxo-4-pyrimidinyl)-5-methyl-2,4(1H,3H)-pyrimidinediones, nor apyrimidinic sites. This cytosine photoproduct is separable from unmodified cytosine by high-performance liquid chromatography. This separation should facilitate identification of this modified cytosine and elucidation of its biological significance.

Chromatin solubilization in rapidly growing hepatomas

The presence of substantial amounts of DNA in the cytosol fractions prepared from rapidly growing transplanted rat hepatomas was confirmed. Agarose gel electrophoresis of this DNA from the Morris hepatomas 7288CTC, 7777, and 7800 suggested that it corresponds primarily to nucleosome monomers with smaller amounts of nucleosome dimers and trimers. Despite the evidence for solubilization of chromatin in rapidly growing hepatomas the potential DNA endonuclease activity was less in nuclei from these tumors than in nuclei from more slowly growing hepatomas or host livers. It was suggested that focal necrosis in rapidly growing solid tumors may be associated with the influx of divalent cations and the activation of DNA endonuclease activity.

Potentiation of dimethylnitrosamine genotoxicity in rat hepatocytes isolated following ethanol treatment in vivo

Unscheduled DNA synthesis (UDS), following exposure to dimethylnitrosamine (DMN), was potentiated in cultured hepatocytes isolated following treatment of rats for 14 or 28 days with 20% ethanol/5% sucrose solution. Ethanol treatment was associated with increased UDS, a concomitant increase in hepatic microsomal protein concentration and DMN N-demethylase activity. Increased aniline hydroxylase activity of hepatic microsomes from ethanol-treated rats preceded the measured increase in microsomal protein content or DMN metabolism. The increase in metabolism of DMN in vitro and potentiation of DMN-induced UDS associated with ethanol treatment may contribute to a synergistic effect of ethanol on DMN hepatotoxicity and carcinogenicity. In contrast, ethanol pretreatment did not increase the cytotoxicity of DMN as characterized by enzyme release.

UV-irradiated SV40 minichromosomes as substrates for DNA repair endonucleases

UV-irradiated SV40 minichromosomes have been shown to be a substrate for a purified DNA repair endonuclease. A UV-repair endonuclease activity was also found to be associated with the isolated SV40 minichromosomes themselves. It appeared to have similar properties to the enzymes described from other mammalian sources.

Differential inhibition of rat liver DNA polymerases in vitro by direct-acting carcinogens and the protective effect of a thiol reducing agent

The direct-acting carcinogens acetoxyacetylaminofluorene, methylnitrosourea, and N-methyl-N'-nitro-N-nitrosoguanidine were tested for their ability to inhibit rat liver DNA polymerase-alpha, -beta, and -gamma activity in vitro. DNA polymerase-alpha was the most sensitive, polymerase-beta was the most resistant, and polymerase-gamma exhibited an intermediate response. When the reactions were reassayed in the presence and absence of dithiothreitol, a thiol reducing agent, it was shown that the inhibition by carcinogens was generally reversible with increasing dithiothreitol, except that polymerase-beta recovered only 80-90% of control values. These and binding data suggest that DNA polymerase-beta, the putative repair enzyme, is highly resistant to carcinogen damage. This resistance may contribute to the retention of normal function and fidelity of the repair enzyme during carcinogen exposure in vivo and to a normal cellular repair.

On the substrate specificity of a damage-specific DNA binding protein from human cells

The nature of the lesion recognized by a damage-specific DNA binding protein from human cells was investigated by examining the substrate specificity of this protein. Protein-recognizable damage was introduced into both T7 DNA and Poly d(A-T) at relatively low UV fluences. In addition, the protein demonstrated binding to both nitrous acid- and bisulfite-treated DNA, but not to DNA crosslinked with trioxsalen plus near-UV nor to non-irradiated uracil-containing DNA. These results suggest that this protein could be recognizing minor helix distortions in DNA rather than any one single lesion.

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.