The relationship between DNA strand-scission and DNA synthesis inhibition in HeLa cells treated with neocarzinostatin

DNA double strand breaks but not interstrand crosslinks prevent progress through meiosis in fully grown mouse oocytes

There is some interest in how mammalian oocytes respond to different types of DNA damage because of the increasing expectation of fertility preservation in women undergoing chemotherapy. Double strand breaks (DSBs) induced by ionizing radiation and agents such as neocarzinostatin (NCS), and interstrand crosslinks (ICLs) induced by alkylating agents such as mitomycin C (MMC), are toxic DNA lesions that need to be repaired for cell survival. Here we examined the effects of NCS and MMC treatment on oocytes collected from antral follicles in mice, because potentially such oocytes are readily collected from ovaries and do not need to be in vitro grown to achieve meiotic competency. We found that oocytes were sensitive to NCS, such that this ionizing radiation mimetic blocked meiosis I and caused fragmented DNA. In contrast, MMC had no impact on the completion of either meiosis I or II, even at extremely high doses. However, oocytes treated with MMC did show γ-H2AX foci and following their in vitro maturation and parthenogenetic activation the development of the subsequent embryos was severely compromised. Addition of MMC to 1-cell embryos caused a similarly poor level of development, demonstrating oocytes have eventual sensitivity to this ICL-inducing agent but this does not occur during their meiotic division. In oocytes, the association of Fanconi Anemia protein, FANCD2, with sites of ICL lesions was not apparent until entry into the embryonic cell cycle. In conclusion, meiotic maturation of oocytes is sensitive to DSBs but not ICLs. The ability of oocytes to tolerate severe ICL damage and yet complete meiosis, means that this type of DNA lesion goes unrepaired in oocytes but impacts on subsequent embryo quality.

Single chemical modifications of the C-1027 enediyne core, a radiomimetic antitumor drug, affect both drug potency and the role of ataxia-telangiectasia mutated in cellular responses to DNA double-strand breaks

The radiomimetic enediyne C-1027 induces almost exclusively DNA double-strand breaks (DSB) and is extremely cytotoxic. Unique among radiomimetics, ataxia-telangiectasia mutated (ATM) is dispensable for cellular responses to C-1027-induced DNA damage. This study explores the biological activity of three recently bioengineered C-1027 analogues: 7''-desmethyl-C-1027 (desmethyl), 20'-deschloro-C-1027 (deschloro), and 22'-deshydroxy-C-1027 (deshydroxy). Each compound maintains the characteristic ability of radiomimetics to cleave DNA in cell-free systems, varying in activity from 2-fold (deschloro) to 55-fold (desmethyl) less than C-1027. The induction of cellular DNA breaks based on pulsed field gel electrophoresis, comet analysis, and gammaH2AX activation was in the same rank order as cell-free DNA break induction, although the amount of breaks induced by desmethyl is greatly reduced compared with the other analogues. Despite the disparity in inducing DNA DSBs, all of the analogues produced G2-M cell cycle arrest and activated DNA DSB damage response proteins, such as p53-Ser15 and Chk2-Thr68, at concentrations in concordance with their ability to inhibit cell growth. Interestingly, of the three analogues, only the desmethyl-induced DNA damage response was similar to C-1027, as it did not cause hypersensitive cell growth inhibition in the absence of ATM nor require the kinase to phosphorylate p53 or Chk2. These findings show that simple modifications of the chromophore of C-1027 can result in varied induction of, and cellular response to, DNA DSBs.

Poly(ADP-ribose) polymerase (PARP-1) is not involved in DNA double-strand break recovery

BACKGROUND

The cytotoxicity and the rejoining of DNA double-strand breaks induced by gamma-rays, H2O2 and neocarzinostatin, were investigated in normal and PARP-1 knockout mouse 3T3 fibroblasts to determine the role of poly(ADP-ribose) polymerase (PARP-1) in DNA double-strand break repair.

RESULTS

PARP-1-/- were considerably more sensitive than PARP-1+/+ 3T3s to induced cell kill by gamma-rays and H2O2. However, the two cell lines did not show any significant difference in the susceptibility to neocarzinostatin below 1.5 nM drug. Restoration of PARP-1 expression in PARP-1-/- 3T3s by retroviral transfection of the full PARP-1 cDNA did not induce any change in neocarzinostatin response. Moreover the incidence and the rejoining kinetics of neocarzinostatin-induced DNA double-strand breaks were identical in PARP-1+/+ and PARP-1-/- 3T3s. Poly(ADP-ribose) synthesis following gamma-rays and H2O2 was observed in PARP-1-proficient cells only. In contrast neocarzinostatin, even at supra-lethal concentration, was unable to initiate PARP-1 activation yet it induced H2AX histone phosphorylation in both PARP1+/+ and PARP-1-/- 3T3s as efficiently as gamma-rays and H2O2.

CONCLUSIONS

The results show that PARP-1 is not a major determinant of DNA double-strand break recovery with either strand break rejoining or cell survival as an endpoint. Even though both PARP-1 and ATM activation are major determinants of the cell response to gamma-rays and H2O2, data suggest that PARP-1-dependent poly(ADP-ribose) synthesis and ATM-dependent H2AX phosphorylation, are not inter-related in the repair pathway of neocarzinostatin-induced DNA double-strand breaks.

Sequential 1H NMR assignments and secondary structure of aponeocarzinostatin in solution

Sequential assignments and secondary structural analysis have been accomplished for the 113-residue apoprotein of the antitumor drug neocarzinostatin (NCS) from Streptomyces carzinostaticus. A total of 98% of the main-chain and 77% of the side-chain resonances have been sequence specifically assigned by use of information from coherence transfer experiments and by sequential and interstrand NOEs. Because of the complexity of the NCS spectrum, several sequential assignment strategies were employed to complete the analysis. Apo-NCS consists of three antiparallel beta-sheeted domains by NMR analysis. There is an extensive four-strand antiparallel beta-sheet, and two two-stranded domains. One of the two-strand domains is contiguous, S72-N87, with chain reversal occurring through the region L77-R82. The other two-stranded domain has the section G16-A24 antiparallel with respect to the region S62-R70. This secondary structure is consistent with the crystal structure of holo-NCS at 2.8-A resolution.

Neocarzinostatin and auromomycin preferentially cleave simian virus 40 DNA and chromosomes at a number of discrete locations

Neocarzinostatin and auromomycin were shown to cleave simian virus 40 (SV40) DNA with preference for distinct regions of the viral genome. The positions cut by neocarzinostatin and auromomycin were similar, while micrococcal nuclease cleaved at positions other than those recognized by neocarzinostatin and auromomycin. Breaks were distributed throughout the viral genome and were not associated with any single type of genetic element. The limited number of locations in SV40 DNA that were preferentially cut by neocarzinostatin and auromomycin suggests that drug attack is directed by DNA structures other than the known trinucleotide sequence specificity of the drugs. Neocarzinostatin and auromomycin cut purified, cell-free, nuclear and intracellular chromosomal SV40 DNA at similar regions. The data indicate that there are regions in DNA that are hypersensitive to the drugs; the hypersensitivity may be determined by the microstructure of the DNA. The conformational change associated with the packing of the DNA into nucleosomes did not affect the microstructure of the sensitive region, nor did the shielding effect of nuclear proteins affect the drug's access to it. In addition, intracellular drug metabolism or other cellular factors did not alter the ability of drugs to interact at these sensitive regions.

Hypersensitivity and reduced inhibition of DNA synthesis in ataxia telangiectasia lymphoblasts treated with low levels of neocarzinostatin

The effects of neocarzinostatin (NCS) on lymphoblastoid cell lines (LCLs) established from ataxia telangiectasia (A-T) were determined. A-T lymphoblasts were found to be hypersensitive to low levels of NCS as measured by cell growth and cell survival. On the other hand, A-T lymphoblasts failed to postpone DNA synthesis to the same degree as normal lymphoblasts following treatment with NCS. LCLs established from Nijmegen breakage syndrome (NBS) could be distinguished from ataxia and normal cell lines by their intermediate level of survival following exposure to NCS.

Differential inhibition of DNA and RNA biosynthesis in HeLa S3 cells by tetaine, a dipeptide antibiotic

A dipeptide antibiotic, tetaine, was found to diminish the rate of incorporation of 3H-labelled precursors into nucleic acids of intact and permeabilized HeLa S3 cells with concomitant negligible effect on protein synthesis. Comparison of the inhibitory effects of tetaine indicates that the antibiotic at 0.03-0.1 mM is a selective inhibitor of cellular DNA biosynthesis and, at higher concentration, of DNA and RNA biosynthesis. Tetaine is also an inhibitor of DNA and RNA polymerase reactions in a cell-free system, as determined using partially purified extracts from HeLa S3 cells that served as a source of the enzymes. The pretreatment experiments showed that tetaine inactivated the polymerases without affecting DNA template function. The tetaine effect on biosynthesis of nucleic acids in HeLa S3 cells can be attributed rather to the intact antibiotic than to the product of its enzymatic cleavage, anticapsin.

Poly(deoxyadenylic-deoxythymidylic acid) damage by radiolytically activated neocarzinostatin

The anaerobic reaction of poly(deoxyadenylic-deoxythymidylic acid) with neocarzinostatin activated by the carboxyl radical CO2-, an electron donor generated from gamma-ray radiolysis of nitrous oxide saturated formate buffer, has been characterized. DNA damage includes base release and strand breaks. Few strand breaks are formed prior to alkaline treatment; they bear 3'-phosphoryl termini. In contrast, most (66%) of the base release occurs spontaneously. DNA damage is highly (95%) specific for thymidine sites. Neither DNA-drug covalent adduct nor nucleoside 5'-aldehyde, which are major products in the DNA-nicking reaction initiated by mercaptans and oxygen, is formed in this reaction. Data are presented to show that the CO2(-)-activated neocarzinostatin intermediate is a short-lived free radical able to abstract hydrogen atoms from the C-1' and C-5' positions of deoxyribose. Attack occurs mostly (68%) at the C-1' position, producing a lesion whose properties are consistent with those of (oxidized) apyrimidinic sites.

Gamma-radiolysis study of the reductive activation of neocarzinostatin by the carboxyl radical

The activation of the antitumor protein antibiotic neocarzinostatin (NCS) by the carboxyl radical CO-2, a one-electron donor obtained selectively from gamma-ray irradiation of nitrous oxide-saturated formate buffer, has been investigated in the presence and in the absence of DNA at pH 4.7 and pH 7.0. The reaction of NCS with CO-2 in the absence of DNA is followed by a marked red shift (420----441 nm) and a pronounced increase (X 8.8) of the fluorescence emission corresponding to the naphthalene moiety of the NCS chromophore. The light absorption spectrum shows in parallel a hypochromic change with considerable fine structure throughout the 250-400 nm wavelength range. When DNA is present, the fluorescence intensity at completion of the reaction is slightly reduced (by 5 to 15 per cent) and the maximum emission wavelength shifted to 436-438 nm. However, the bulk rate of reaction is not altered by DNA and is independent of the pH, of the temperature and of the concentration of NCS. The NCS concentration-independence of the reaction rate is consistent with a high intrinsic rate (k greater than 10(8)M-1 . s-1) for the reaction of CO-2 with the NCS chromophore. Complete reduction of the NCS chromophore involves a total of three electron-equivalents. The final product does not react with oxygen, shows no odd electron spin, and is unable to induce DNA strand scission. Its molecular state, however, is fundamentally different when gamma-ray irradiations are performed with DNA. This bears evidence of short-lived one electron or two-electrons reduced intermediates decaying via non-identical routes depending on the presence of the acceptor DNA. Actually, dose-related strand breaks appear in DNA exposed to the action of NCS and CO-2. Some NCS chromophore-DNA covalent adducts are also found. DNA strand breakage by CO-2-activated NCS is correlated with thymine release and is inhibited by a redox-stable intercalating agent. The DNA-nicking process thus bears resemblance to that reported by other authors using mercaptans to initiate reductive activation of the NCS chromophore. However, some spectral differences are observed between the CO-2-reacted and the thiol-treated chromophores. Moreover, thymine release and strand scission in DNA incubated with CO-2 and NCS proceed under anaerobic conditions. It is proposed that the strict oxygen requirement for DNA damage by NCS in the presence of mercaptans is due, at least partly, to competition between oxygen and thiols for reaction with the same primary deoxyribose radical resulting from DNA attack by the reductively activated NCS chromophore.

Mode of inhibition of DNA replication in neocarzinostatin-treated HeLa cells

The effect of antitumor antibiotic neocarzinostatin on DNA replication in HeLa cells was studied by pulse-labeling of DNA with [3H]thymidine and sedimentation analysis of the DNA with alkaline sucrose gradients. The drug, which produced DNA damage, primarily inhibited the replicon initiation in the cells at low doses (less than or equal to 0.1 microgram/ml), and at high doses (greater than or equal to 0.5 microgram/ml) inhibited the DNA chain elongation. An analysis of the number of single-strand breaks of parental DNA, induced by neocarzinostatin, indicated that inhibition of the initiation occurred with introduction of single-strand breaks of less than 1.5 . 10(4)/cell, while inhibition of the elongation occurred with introduction of single-strand breaks of more than 7.5 . 10(4)/cell. Assuming that the relative molecular mass of DNA/HeLa cell was about 10(13) Da, the target size of DNA for inhibition of replicon initiation was calculated to be about 10(9) Da, such being close to an average size of loop DNA in the cell and for inhibition of chain elongation, 1-2 . 10(8) Da which was of the same order of magnitude as the size of replicons. Recovery of inhibited DNA replication by neocarzinostatin occurred during post-incubation of the cells and seemed to correlate with the degree of rejoining of the single-strand breaks of parental DNA. Caffeine and theophylline enhanced the recovery of the inhibited replicon initiation, but did not aid in the repair of the breaks in parental DNA.

Distribution and specificity of mutations induced by neocarzinostatin in the lacI gene of Escherichia coli

Although neocarzinostatin (NCS) attacks DNA almost exclusively at adenine and thymine residues in vitro, exposure of Escherichia coli to this antitumor drug resulted in a high frequency of mutations at guanine:cytosine base pairs in the lacI gene. Thus, NCS-induced base substitution mutations do not appear to result from the major DNA lesions that have been biochemically characterized. The overall distribution of nonsense mutations produced by NCS was distinctly nonrandom, consisting in part of a few "hotspots" and a large number of "coldspots." The existence of these coldspots implies that untargeted mutagenesis does not make a significant contribution to the mutations induced by this SOS-dependent mutagen.

Reduced inhibition of replicon initiation and chain elongation by neocarzinostatin in skin fibroblasts from patients with ataxia telangiectasia

Cells from patients with the genetic disease ataxia telangiectasia are hypersensitive to the DNA-breaking agents X-rays, bleomycin and neocarzinostatin, and show reduced inhibition of DNA synthesis after treatment with these agents, as compared to normal cells. The rate of replicon initiation and chain elongation was measured shortly after brief exposure of two normal and two ataxia telangiectasia fibroblast strains to low doses (0.10-0.30 microgram/ml) of neocarzinostatin, by means of alkaline sucrose gradient analysis. Neocarzinostatin was found to inhibit both initiation and elongation, and both components of DNA synthesis were more resistant to this inhibition in the A-T strains.

On the mechanism of reductive activation in the mode of action of some anticancer drugs

Bioactivation of a number of DNA-specific antitumor drugs depends on oxidoreduction. Bleomycin, neocarzinostatin and anthracycline glycosides are the best known among such drugs in terms of reductive activation processes. Their reduction results in short-lived radical or electrophilic intermediates attacking DNA stereospecifically. The physico-chemical properties of these drugs and the nature of DNA damage are reviewed. Models for DNA-intercalation, electron-donor systems involved in drug metabolisation, and the role of oxygen in radical reactions, are discussed in the light of recent reports.

Degradation of HeLa cell chromatin by neocarzinostatin and its chromophore

Chromatin is the in vivo target site for neocarzinostatin, a DNA strand scission antitumor drug. The effect of neocarzinostatin and its active chromophore component on HeLa cell chromatin is described here. Chromatin consisting of a mixture of mono-, di-, tri- and larger nucleosome fragments is prepared by micrococcal nuclease digestion of HeLa cell nuclei. Drug-induced conversion of chromatin to smaller sized fragments is measured by electrophoresis of the DNA on non-denaturing 4% polyacrylamide gels. Chromatin breakdown measured under these conditions is double-stranded in nature. In the presence of 2 mM dithiothreitol, neocarzinostatin causes degradation of large chromatin fragments and a loss of distinct nucleosome peaks. Detection of chromatin breakdown by neocarzinostatin is dependent upon the concentration of chromatin in the assay. When chromatin is increased from 14 to 70 micrograms/ml, changes in the larger fragments caused by 100 micrograms/ml neocarzinostatin become less obvious are are almost undetectable at 140 micrograms/ml chromatin. No change is observed when chromatin is treated with either neocarzinostatin or its chromophore in the absence of dithiothreitol. For detectable levels of chromatin degradation, 10 micrograms/ml neocarzinostatin is required compared to only 2.5 microgram/ml chromosome (expressed in microgram equivalent neocarzinostatin). Such degradation also occurs more rapidly with chromophore than with neocarzinostatin. Digestion of chromatin with neocarzinostatin continues for at least 30 min at 37 degrees C, while similar degradation caused by chromophore is complete in 1 min. Neocarzinostatin levels which actively degrade isolated chromatin can also effect release of soluble chromatin from intact nuclei. The released chromatin can serve as a substrate for micrococcal nuclease digestion. Such chromatin studies should prove useful in characterizing the mechanism of action of DNA reactive drugs such as neocarzinostatin.

Binding of cesalin, an antitumor protein, to cultured mammalian cells

125-I-labelled cesalin binds to KB cells and plasma membranes in a specific and saturable manner. At 0 degrees C the cesalin specifically bound to cells is completely displaceable by excess unlabelled cesalin, but at 37 degrees C only 50% can be removed after incubation for 2 h. The extent of binding to plasma membranes has the following characteristics: it is increased following treatment of membranes with cholate; treatment with trypsin has no effect on binding and neither is the bound 125-I-labelled cesalin removed following digestion with trypsin; binding is not inhibited by several carbohydrates but is decreased to about one half by concanavalin A. In addition it is found that some degradation of cesalin occurs with KB cells, the specific binding to which is not enhanced by chloroquine, a lysosomotropic agent. No loss of binding in cells is seen after 4 h exposure to cesalin, suggesting no reduction in the receptors by internalization. The data are consistent with a mechanism in which 125I-labelled cesalin is rapidly bound at 37 degrees C to a receptor on cell membranes through which the biological activity is effected. Slowly, some change in the bound cesalin occurs that prevents its complete displacement from the cells.

Effects of inhibitors of DNA synthesis on spontaneous and ultraviolet light-induced sister-chromatid exchanges in Chinese hamster cells

Effects of inhibitors of DNA synthesis on spontaneous and ultraviolet light (UV)-induced sister-chromatid exchanges (SCE) were examined in a Chinese hamster cell line, V79 B-1. The inhibitors used were hydroxyurea (HU), 1-beta-D-arabinofuranosylcytosine (ara-C), aphidicolin (APC), 2',3'-dideoxythymidine triphosphate (ddTTP), neocarzinostatin (NCS), novobiocin (NB) and cycloheximide (CHX). HU, ara-C, and APC increased spontaneous SCE frequency, and had a synergistic effect on UV-induced SCE frequency. DdTTP, NCS and NB failed to show any statistically significant effect on either spontaneous or UV-induced SCE frequencies, though NCS and NB did slightly increase both spontaneous and UV-induced SCE frequencies. On the contrary, CHX decreased spontaneous SCE frequency, and more drastically, also UV-induced SCE frequency. These results are interpreted with respect to the replicating fork of DNA, a structure postulated to be involved in the formation of spontaneous and UV-induced SCE. A new model for SCE formation is proposed.

A model for the activation and inactivation of neocarzinostatin, an antitumor protein

Sulfhydryl compounds specifically activate and inactive neocarzinostatin as measured by the in vitro strand scission of T2 DNA. This effect, and evidence for strained disulfides in the molecule, leads us to propose a model of activation and inactivation. We suggest that neocarzinostatin reacts with sulfhydryl reagents to produce a short-lived active form which may react with DNA or proceed irreversibly via a conformational change to an inactive form (preneocarzinostatin). Three parameters of neocarzinostatin activity (inactivation rate, single strand break plateau position, and initial single strand break rate), have been measured for various sulfhydryl concentrations, and the observed results agree well with values expected from a simplified mathematical treatment of the model, thus supporting the assumptions made.

Contrasts in the actions of protein antibiotics on deoxyribonucleic acid structure and function

The protein antibiotics neocarzinostain (NCS), macromomycin (MCR), and auromomycin (AUR), which is closely related to MCR, have been compared for their in vitro and in vivo actions on deoxyribonucleic acid (DNA). NCS, markedly stimulated by 2-mercaptoethanol, is much more active in inducing strand scissions in superhelical pMB9 and linear duplex lambda DNA than AUR, which is slightly inhibited by 2-mercaptoethanol. Purified MCR, even at very high levels, does not give any significant amount of cutting with either DNA substrate. 2-Propanol stimulates the activity of NCS but inhibits that of AUR. On the other hand, the antioxidant alpha-tocopherol strongly inhibits DNA breakage by both drugs. The intercalating drugs ethidium bromide, daunorubicin, proflavin, and actinomycin D at low concentrations inhibit DNA scission by AUR. The levels of intercalators required to inhibit NCS activity to comparable levels are about 10 times higher than those for AUR. Although MCR has virtually no in vitro DNA cutting activity, it is, like AUR and NCS, cytotoxic, as measured by the inhibition of DNA synthesis and induction of DNA strand breakage in HeLa cells.

Strand scission of superhelical and linear duplex DNAs by the antitumor protein macromomycin. Relationship of in vitro DNA damage to cell growth inhibition

Macromomycin, a protein antitumor drug, was found to cause strand scissions in vitro in superhelical PM2 and SV40 DNA as well as linear duplex lambda DNA. DNA damage appeared to be single rather than double-strand scissions, and there is an indication that DNA breaks occur at some preferential base sites. The DNA breaks were predominantly true single-strand scissions as opposed to alkali-labile bonds. The cutting reaction was inhibited by low temperature (0 degrees C) and reached a maximum at 45 degrees C. The reaction was not affected by 2-mercaptoethanol, although EDTA did cause a slight decrease in the reaction rate. MgCl2 was found to be an effective inhibitor of the strand scission activity of the drug. The rate of DNA cutting was linear over a wide range of DNA substrate levels. There appeared to be a correlation between the drug's ability to damage DNA and to inhibit cell growth in that similar losses of these two activities occurred as the drug was thermally denatured.

An uptake of fluorescein isothiocyanate labeled neocarzinostatin into the cancer and normal cells

An uptake of fluorescein isothiocyanate labeled neocarzinostatin into normal and cancerous epithelial cells from bladder was investigated. Results showed that neocarzinostatin traversed the cell membrane into cytosol and nuclei, and it appeared to have a preferential cytotoxicity for the cancer cell.

DNA damage and repair in relation to cell killing in neocarzinostatin-treated HeLa cells

To elucidate the mechanism of the cell killing activity of neocarzinostatin on mammalian cells, the drug-induced damage of DNA and its repair were examined. Very low doses of neocarzinostatin, at which high survival of cells was observed, clearly produced single-strand breaks of DNA and decomposition of the 'DNA complex', but these damages appeared to be repaired almost completely. At higher doses of neocarzinostatin, single-strand breaks were repaired to a considerable extent while double-strand breaks seemed not to be repaired. The number of non-repairable single-strand breaks was about twice that of double-strand breaks. This implies that single-strand breaks are repaired except for those constituting double-strand breaks. Although at low levels of neocarzinostatin repair of double-strand breaks may occur, the correlation existing between the colony-forming ability of cells treated with neocarzinostatin and non-repairable DNA breakage suggests that production of a small number of critical non-repairable double-strand breaks per cell may be responsible for the cell killing activity of the drug.

Neocarzinostatin induction of DNA repair synthesis in HeLa cells and isolated nuclei

The antitumor antibiotic neocarzinostatin that causes DNA strand breaks in vivo and in vitro is shown to induce DNA repair synthesis in HeLa S3 cells. In the repair assay, the parental DNA was prelabeled with 32P and a density label (bromodeoxyuridine) was introduced into the new synthesized DNA. Quantitation of the repair synthesis as measured by the incorporation of [3H]thymidine into the light parental DNA at varying doses of the drug indicate that there is a significant repair response at low levels of the drug (0.2--0.5 microgram/ml) which cause DNA strand breakage and inhibition of DNA synthesis. In isolated HeLa nuclei neocarzinostatin stimulates the incorporation of dTMP many-fold. This enhancement of dTMP incorporation, which requires the presence of a sulfhydryl agent, is a consequence of the drug-induced DNA strand breakage and is in the parental DNA. These results suggest that an intact cell membrane is not required for DNA strand breakage and its subsequent repair.

Activation and inactivation of neocarzinostatin-induced cleavage of DNA

The possible role of free radicals in the mechanism of neocarzinostatin (NCS) action was studied. While mercaptene markedly stimulate the ability of NCS to degrade DNA, they also rapidly inactivate the antibiotic in a preincubation and at higher concentration inhibit the degradation reaction. The radiation protector S,2-aminoethylisothiuronium bromide-HBr is the most potent compound tested. Scavengers of diffusible OH radicals, O2- or H2O2 do not result in significant inhibition of the oxygen-dependent cleavage of DNA by NCS; in fact, alcohols and other organic solvents stimulate the reaction several-fold. By contrast, the potent peroxyl free radical scavenger, alpha-tocopherol, blocks the reaction 50% at 50 micron.

Single-strand nicking of DNA in vitro by neocarzinostatin and its possible relationship to the mechanism of drug action

Neocarzinostatin, a protein antibiotic with anti-tumor activity was found to place single-strand scissions in DNA in an in vitro reaction. The drug's cutting activity was strongly dependent on the presence of 2-mercaptoethanol or dithiothreitol but some cutting did take place in the absence of reducing agent at very high drug levels and prolonged incubation. The requirement for reducing agents could not be replaced with NAD+, FAD, NADH or H2O2 and the strand-scission reaction was not affected by Mg2+, EDTA or intercalating agents. Similar profiles of heat-inactivation of neocarzinostatin were found whether activity was measured by the scission of DNA strand either in vitro or in HeLa cells treated with the drug. Furthermore, both of these parameters corresponded closely with the ability of the modified drug to inhibit DNA synthesis and growth of HeLa cells. By column isoelectric focusing it was shown that all four activities are associated with the same protein band (pH 3.28). From these data we conclude that the cytotoxic activity of neocarzinostatin and the nicking of DNA strands in vitro appear to reside in the same protein.