Damage and repair of DNA in cultured mammalian cells with N-diazoacetylglycine amide

N-Diazoacetylglycine amide, a diazochetoalkane, has been studied in vitro for DNA damage and repair in cells of a cloned subline from a BALB/c mouse. To our present knowledge, none of these compounds have been investigated for such activities. At nontoxic levels, a prolonged dose-dependent unscheduled DNA synthesis was observed by autoradiography. DNA damage was studied by sedimentation through alkaline sucrose gradients after the cells were lysed on the gradients. Treatment of the cells for 1 hr with nontoxic doses of N-diazoacetylglycine amide resulted in slower sedimentation of DNA. The number of single-strand breaks appeared rather linearly dose dependent for a large range of concentrations. Breaks were at their maximum after 1 hr of treatment, and no further increase in the number of breaks was seen. Some repair of the breaks probably occurs, but repair was sluggish even 68 hr after treatment. A significant part of the breaks was observed after incubation at 4 degrees in an ethylenediaminetetraacetate hypotonic solution. This seems to indicate that the compound does not require metabolic activation. Nontoxic doses of N-diazoacetylglycine amide and other similar derivatives exert mutagenic and carcinogenic activities. The presence of DNA damage and the difficulty in its repair at such doses could be related to both of these biological properties.

A procedure for the assay of DNA damage in mammalian cells by alkaline elution and microfluorometric DNA determination

The microfluorimetric assay of Kissane and Robins (J. Biol. Chem. 233, 184-188, 1958) has been modified to monitor DNA content in alkaline elution fractions. The elution profiles of DNA from treated and control cultured cells demonstrated a good correlation whether DNA in the elution fractions was determined by the fluorometric method or by measuring its radioactivity. The DNA recovery was of about 80% and the measured material was DNase digestible.

Relationships between mutation and transformation frequencies in mammalian cells treated "in vitro" with chemical carcinogens

An analytical quantitative comparison of data from the literature about frequencies of mutations and transformations induced by mutagenic-carcinogenic compounds in mammalian cells was carried out without any selection of unfitting data. The analysis was performed for equitoxic doses and background level. Data on transformation frequency came from 105 experiments performed with 34 carcinogenic compounds: those on mutation frequency came from 66 experiments performed with 26 mutagenic compounds; 7 compounds were assayed for both these activities. The difference in frequency between structural mutations and transformations was about 10(2)-10(3) and it appears statistically extremely significant. These results seem to indicate an absolute difference between structural mutations and transformations. In the framework of other observations it is suggested that structural alterations in a single gene are perhaps only a component of the steps present in the oncogenetic process. We may regard as "epigenetic" type phenomena the other steps involved in this process.

Physicochemical alterations in the conformation of rat liver chromatin induced by carcinogens in vivo

Administration of methylating carcinogens such as methyl methanesulfonate (120 mg/kg), dimethylnitrosamine (5 mg/kg), or methylnitrosourea (80 mg/kg) to rats resulted in an increased ellipticity in circular dichroism spectra and in an enhanced ability to bind ethidium bromide in the liver chromatin. Although shearing of the chromatin preparations increased both the ellipticity and number of binding sites for ethidium bromide, the carcinogen-induced effects were noticeable whether or not chromatin was sheared. Although the doses of the 3 carcinogens used in these studies are equivalent in their ability to induce strand breaks in liver DNA at 4 hr, their effects on the induction of conformational changes in liver chromatin are different. For example, methyl methanesulfonate induced the minimum conformational changes in liver chromatin at 4 hr, whereas methylnitrosourea induced the maximum changes at 4 hr. Methyl methanesulfonate and dimethylnitrosamine, on the other hand, induced maximum changes at 3 days. The conformational changes induced by methyl methanesulfonate and methylnitrosourea, and not by dimethylnitrosamine, tend to be repaired by 14 days.

Alkaline lysis of mammalian cells for sedimentation analysis of nuclear DNA. Conformation of released DNA as monitored by physical, electron microscopic and enzymological techniques

The degree of single strandedness of the DNA released from rat liver nuclei by various alkaline lysing solutions (including some with sodium dodecyl sulfate) was determined both before and after sedimentation in alkaline sucrose gradients employing electron microscopy, melting profiles, circular dichroism measurements, and digestibility by S1 nuclease. Regardless of the technique employed, the results obtained following alkaline sucrose gradient centrifugation of the DNA are consistent. The DNA was completely single stranded as judged by electron microscopy, circular dichroism spectra, and digestibility by S1 nuclease, an enzyme that specifically hydrolyzes single-stranded DNA. This was not true if the DNA was analyzed following alkaline lysis of the nuclei but before centrifugation. Under conditions which gave a complete transition to the single-stranded state, as judged by melting profiles and circular dichroism spectra, only 10-15% of the DNA was hydrolyzed by S1 nuclease. An increase in the susceptibility of the released DNA to S1 nuclease was observed with increases in the pH of the lysing solution. In order to release DNA which was single stranded as judged by both physical and enzymological techniques, the rat liver nuclei were lysed for 30 min with a 0.3 M NaOH lysing solution containing 0.5% dodecyl sulfate, 0.3 M NaCl and 0.03 M EDTA.

An Effective Immunodepressive Treatment for Demonstrating in Allogeneic Mice the Tumorigenic Properties of Cells Grown in Vitro

Short intensive treatment with N-diazoacetylglycine amide (DGA) before the inoculum, followed by prolonged daily administration of L-aspàraginase (Asnase), was tested for its ability to elicit tumorigenic properties of fibroblast-like cells cultured in vitro. With this treatment progressive tumor growth was obtained in allogeneic mice injected with cells of a transformed subline. Results show that combined use of DGA and Asnase affords a higher probability of proving in vivo the tumorigenic properties of injected cells than in newborn or X-irradiated recipients. Experimental data indicate that L-asparagine depletion does not inhibit the in vitro growth of fibroblast-like cells.

A clarification of the complex spectrum observed with the ultraviolet circular dichroism of ethidium bromide bound to DNA

Ethidium bromide intercalation strongly effects the circular dichroism spectrum of DNA in the region of 230-300 mu, in a complex manner. In this report we present a study that quantitizes the relationships of the circular dichroism spectrum in the region of 230-300 mu and the ethidium bromide induced optical activity centered around 308 mu. We present evidence of two hidden cooperative bands that are probably the negative counterparts of the 308 mu band and 330 mu shoulder positive cooperative bands. The hidden band is quantitatively characterized. We confirm that the direct effect of ethidium bromide on the DNA spectrum is simply linearly proportional to the amount of intercalated dye. We also observe that the ethidium bromide enters freely when there is a molecule intercalated for every 3 sites, but that the intercalation is more difficult when the molecule intercalates at every second site.

In vivo repair of rat liver DNA damaged by 3-hydroxyxanthine

Using alkaline and neutral sucrose gradients, it was observed that administration of the hepatocarcinogen, 3-hydroxyxanthine, to rats, resulted in the slower sedimentation of liver DNA in alkaline and neutral sucrose gradients. Slower sedimentation of liver DNA in alkaline sucrose gradients was apparent within 4 h following the administration of the carcinogen at a dose of 50 mug/g body weight. The fragmentation of liver DNA was progressive with an increase in the dose. Such fragmentation caused by a dose of 100 mug/g was largely repaired by 24 h following the administration of the carcinogen. Fragmentation and repair of liver DNA with a dose of 100 mug or more/g body weight was also seen using neutral sucrose gradients. In neutral sucrose gradients significant repair of the fragmented DNA was seen by 24 h after carcinogen. Liver DNA from rats given 3-hydroxyxanthine (100 mug/g for 4 h) or calf thymus DNA incubated in vitro with this carcinogen did not show any change in their melting profile.