Inhibition of O6-alkylguanine-DNA alkyltransferase and DNase I activities in vitro by some alkylating substances and antineoplastic agents

O6-alkylguanine-DNA alkyltransferase: low pKa and high reactivity of cysteine 145

The active site cysteine of human O(6)-alkylguanine-DNA alkyltransferase (hAGT), Cys145, was shown to be highly reactive with model electrophiles unrelated to substrates, including 1-chloro-2,4-dinitrobenzene. The high reactivity suggested that the Cys145 thiolate anion might be stable at neutral pH. The pK(a) was estimated from plots of UV spectra (A(239)) and reactivity toward 4,4'-dithiopyridine vs pH. The estimated pK(a) for hAGT was 4-5, depending upon the method used, and near that of the extensively characterized papain Cys25. Rates of reaction with 4,4'-dithiopyridine were similar for the thiolate forms of hAGT, papain, glutathione, and the bacterial hAGT homologue Ogt (the pK(a) of the latter was 5.4). Bound Zn(2+) has previously been shown to be required for the catalytic activity of hAGT (Rasimas, J. J. et al. (2003) Biochemistry 42, 980-990). Zn(2+) was shown to be required for the low pK(a) of hAGT. The high reactivity of hAGT Cys145 is postulated to be important in normal catalytic function, in cross-linking reactions involving bis-electrophiles, and in inhibition of the DNA repair function of hAGT by electrophiles.

Expression of yeast apurinic/apyrimidinic endonuclease (APN1) protects lung epithelial cells from bleomycin toxicity

Bleomycin is a well-established anti-tumor drug. Its major untoward effect, pulmonary toxicity, has limited its usage. In this study, we used a DNA repair protein, yeast apurinic/apyrimidinic endonuclease (APN1) to reduce the toxicity of bleomycin on lung cells. A549 cells, an alveolar epithelial cell line, were transduced by MIEG3 retroviral vector encoding both enhanced green fluorescent protein (EGFP) and APN1. Transduced cells were sorted by fluorescent-activated cell sorter (FACS) analysis and were cloned. The APN1 expression of transduced A549 cell population and four selected clones expressing different levels of EGFP was confirmed by Northern, Western, and apurinic/apyrimidinic (AP) endonuclease activity analyses. The expression of APN1 was positively correlated with the expression of EGFP. The protective effect of APN1 against bleomycin was determined by single cell gel electrophoresis/Comet assay and by clonogenic survival assay following bleomycin treatment. The A549 population expressing APN1 showed a significant reduction of DNA damage in the presence of 20, 50, and 100 microg/ml bleomycin; similarly, the APN1-expressing A549 population also demonstrated increased survival in the presence of bleomycin compared with the vector-transduced A549 population. In selected clones, three of four APN1-expressing clones resulted in significantly improved cell survival. The current study suggests that the yeast DNA repair protein, APN1, can reduce bleomycin toxicity to target lung cells.

Formation and persistence of N7- and O6-methyl-guanine in DNA of chick embryo brain cells in ovo following administration of N-nitroso-N-methylurea

Previously, O6-methyl-guanine-DNA alkyltransferase (AT) of the chicken embryo has been investigated in vitro. In the present studies, the effect of in vivo (in ovo) treatment with methylnitrosourea (MNU) was examined at a developmental stage of 15 days and doses of 1.25-20 mg/egg, yielding about 1-16 mmol MNU/kg embryo weight. At intervals of 1-24 h, DNA of the brain was prepared. N7-methylguanine and O6-methylguanine were quantified by combining a rapid method of DNA isolation, high-pressure-liquid-chromatography (HPLC) and electrochemical detection of the guanine-alkyl adducts. In parallel, the AT activity of brain homogenates was determined. Within the range of the detection limits (N7-methylguanine: 16 nM, O6-methylguanine: 2.5 nM), no repair of the guanine adducts, being about 500 nmol O6-methyl- and 1800 nmol N7-methyl-adducts per mmol guanine 1 h following administration of 10 mg MNU/egg, was evident. The rather low acute toxicity of MNU in the chicken embryo at the 15th day of development DL50/24 h being > 4 mg MNU/embryo, argues, therefore, for an additional repair mechanism, e.g. cell replacement repair.

The in vitro influence of sulfated bis-lactobionic acid amides on O6-alkylguanine-DNA alkyltransferase, DNase I, nucleic acid synthesis and chromatin structure

The influence of four sulfated bis-lactobionic acid amides (BLAA) of molecular weights between 2388 and 2514 on O6-alkylguanine-DNA alkyltransferase (AT), DNase I and nucleic acid synthesis as well as on nucleoid sedimentation and the viscosity of alkaline lysates of chicken embryo cells was studied in vitro. The activities of AT and DNase I were inhibited by BLAA in a dose-dependent manner. Depending on the polyanion used, concentrations depleting AT activity by 50% ranged between 3.5 and 7.0 microM, whereas BLAA concentrations of almost 250-320 microM were needed to halve DNase I activity. At concentrations above 8 microM, BLAA decreased scheduled DNA synthesis in a dose-dependent fashion whereas RNA synthesis remained unchanged even at the highest BLAA concentrations used (2 mM). In chicken embryo brain cells BLAA exerted a biphasic effect on the nucleoid sedimentation and the viscosity of alkaline cell lysates reflecting a decrease in chromatin compactness at lower BLAA concentrations (10-100 microM) and an increase in chromatin compactness at higher polyanion concentrations (> or = 200 microM). The remarkably high sensitivity of the nuclear enzyme AT deserves further investigation in regard to the fate of the polyanions within cells and tissues.

Toxicological studies with primary cultures of chick embryo cells: DNA fragmentation under the influence of DNase I-inhibitors

Chicken embryo brain and liver cells in vitro exhibited spontaneous DNA fragmentation as determined by viscometry of alkaline cell lysates. Ca2+ and Mg2+ enhanced, while Zn2+, the Ca2+ chelator ethylenglycol-bis(beta-aminoethyl-ether)-N,N,N'-tetraacetic acid (EGTA), spermine and--to a lesser extent--spermidine and Hoechst 33,258 inhibited spontaneous DNA fragmentation. Under the same conditions chromatin condensation, as assessed by nucleoid sedimentation, increased. Exposure of chicken embryo cells to various genotoxic agents, i.e. doxorubicin, bleomycin, methyl methanesulfonate, thiyl radicals, H2O2, UV light, and X-rays, increased DNA fragmentation in a dose dependent manner. Zn2+ or EGTA diminished DNA fragmentation in cells exposed to bleomycin, thiyl radicals, H2O2 and UV light. An apparent sensitisation to X-irradiation has been observed in Zn2+ or EGTA-pretreated cells. It is suggested by the present investigations that, with agent specific peculiarities, apoptotic phenomena are implicated when nucleotoxicity is assessed in chicken embryo cells by physico-chemical short-term tests in vitro.