[CANCER RESEARCH 46, 5511-5517, November 1986]
`Differences
`in DNA Damage Produced by Incorporation of 5-Aza-2'-deoxycytidine
`
`or 5,6-Dihydro-5-azacytidine
`
`into DNA of Mammalian Cells
`
`Joseph M. Covey,1 Maurizio D'Incaici,2 Eugene J. Tilchen, Daniel S. Zaharko, and Kurt W. Kohn
`
`Laboratory of Molecular Pharmacology [J. M. C., M. D'I, E. J. T., K. W. K.], and Developmental Therapeutics Program, Division of Cancer Treatment [J. M. C.,
`M. D'I., E. J. T., D. S. Z., K. W. K.], National Cancer Institute, NIH, Bethesda, Maryland 20892
`
`ABSTRACT
`(aza-dCyd) and 5,6-dihydro-5-
`The effects of 5-aza-2'-deoxycytidine
`azacytidine (H2-aza-Cyd) on the integrity of DNA from several mam
`malian cell
`lines were compared using the alkaline elution technique.
`While both compounds have been shown to inhibit DNA methylation, a
`direct comparison of their effects on DNA structure has not previously
`been reported. Exposure of I 121(1cells to ll,-:i/a-( yd (1-100 fig/ml)
`and simultaneous labeling with [I4C|thymidine for 24 h resulted in the
`production of single-strand breaks
`in DNA, which were significantly
`repaired when cells were incubated in drug-free medium for an additional
`24 h. This differed from our previous findings for aza-dCyd, confirmed
`here in parallel experiments, which showed that this compound produces
`alkali-labile lesions that persist for 48 h. The DNA effects of both drugs
`were significantly reduced when cells were prelabeled with [l4C]thymidine,
`indicating that production of DNA lesions requires incorporation of the
`anomalous base. Studies utilizing pulse-labeled DNA indicated that aza-
`dCyd has little effect on the rate of DNA elongation, whereas H2-aza-
`Cyd produced a complete inhibition for at least 6 h after drug removal.
`The contrasting pattern of DNA damage induced by these compounds in
`LI 210 was also observed in two human lymphoblastoid cell lines, one of
`which was derived from a patient with xeroderma pigmentosum. We had
`previously concluded that alkali-labile sites in DNA from aza-dCyd-
`treated cells probably arise due to the chemical instability of aza-dCyd.
`In contrast,
`incorporated H2-aza-Cyd is chemically stable. The single-
`strand breaks produced in H2-aza-Cyd treated cells were not of the alkali-
`labile type, and may represent an accumulation of DNA replication
`fragments and/or intermediates
`in an excision repair process. Thus,
`the
`DNA lesions produced by the two drugs have markedly different char
`acteristics, and H2-aza-Cyd should not be considered to be merely a stable
`pharmacological congener of aza-dCyd.
`
`by cytidine kinase from
`(15). This compound is phosphorylated
`L1210 and HeLa cells, although less efficiently than is azacy
`tidine (16, 17), yet is incorporated into LI210 nuclear RNA at
`levels equal
`to or exceeding those of azacytidine (18). Studies
`demonstrating
`effects of H2-aza-dCyd on gene expression sug
`gest that
`it is also incorporated
`into DNA and inhibits DNA
`methylation ( 19), although it may be less active than aza-dCyd.
`While recent
`reports have demonstrated
`a correlation
`be
`tween inhibition of DNA methylation and aza-dCyd cytotox-
`icity (20), it is not certain that
`this is the unique mechanism of
`action for these compounds. We have shown that aza-dCyd
`treatment
`of LI210
`leukemia cells /// vitro results
`in alkali-
`labile sites in DNA as demonstrated
`by the alkaline elution
`technique
`(21). These DNA lesions increase during 24-48 h
`following drug removal and are then slowly removed over the
`next 48 h. It was proposed that
`this DNA damage results from
`the chemical
`instability of aza-dCyd incorporated
`into DNA.
`In this paper we compare
`and contrast
`the DNA lesions
`produced by aza-dCyd and the stable analogue H2-aza-Cyd in
`LI210 leukemia and in two human lymphoblastoid cell lines as
`measured by alkaline elution. We report here differences
`in
`both the nature of the lesions induced by these two drugs and
`in their persistence in DNA. It appears
`therefore that H2-aza-
`Cyd should not be considered a pharmacological
`analogue of
`aza-dCyd.
`
`MATERIALS AND METHODS
`
`INTRODUCTION
`aza-dCyd3 ( 1) has been shown to have cytotoxic and antineo-
`plastic activities
`in a number of experimental
`tumor
`systems
`(2-5) and has undergone clinical
`trial (6). aza-dCyd is anabol-
`ized to the nucleoside triphosphate
`(7) and is incorporated into
`DNA (8-10) but not into RNA (9). The presence of azacytosine
`moieties in DNA (even a small percentage substitution of DNA
`cytosines)
`results in inactivation of DNA-cytosine-methyltrans-
`ferase, an enzyme which catalyzes
`the transfer of a methyl
`group to the 5-position
`of cytosines
`in DNA (10-12). The
`hypomethylation
`of DNA produced by aza-dCyd and related
`compounds
`have been shown to result
`in activation of latent
`cellular genes (13).
`Since azacytidine and aza-dCyd spontaneously decompose in
`aqueous solution (14) leading to difficulties in pharmaceutical
`formulation,
`the stable analogue H2-aza-dCyd was synthesized
`
`accepted 8/6/86.
`revised 4/24/86;
`Received 6/3/85;
`The costs of publication of this article were defrayed in part by the payment
`of page charges. This article must
`therefore be hereby marked advertisement
`in
`accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
`1To whom requests for reprints should be addressed, at Laboratory of Molec
`ular Pharmacology, Developmental Therapeutics Program, National Cancer In
`stitute, NIH, Building 37, Room 5A19, Bethesda, MD 20892.
`2On leave from Istituto di Ricerche Farmacologiche
`"Mario Negri," Milan
`Italy.
`3The abbreviations used are: aza-dCyd, 5-aza-2'-deoxycytidine; H2-aza-Cyd,
`5,6,-dihydro-5-azacytidine;
`ara-C,
`l-/3-D-arabinofuranosylcytosine;
`XP, xero
`derma pigmentosum.
`
`Cell Culture. Mouse 11210 leukemia cells were grown at 37*C in
`suspension culture in RPMI 1630 (HEM Research, Inc.) supplemented
`with 20% heat-inactivated horse serum (GIBCO, Grand Island, NY),
`0.84 HIML-glutamine, penicillin (82 units/ml),
`streptomycin (82 ng/
`ml) (all from the Media Unit, NIH), and SO/<\i mercaptoethanol
`(Sigma
`Chemical Co., St. Louis, MO). Stock cultures were maintained
`in
`exponential growth at a density between 0.5 x 10s and 1 x 10' cells/
`ml.
`Two human lymphoblastoid cell lines were also used in these studies
`(Institute for Medical Research, Camden, NJ). One line (GM 3714)
`was derived from a clinically normal
`female, while the other
`(GM
`2345A) was obtained from an 8-year-old female with xeroderma pig
`mentosum (complementation group A). Both lines were maintained in
`vitro at 37°Cin RPMI 1630 medium (GIBCO) supplemented with 15%
`heat-inactivated fetal calf serum.
`Labeling. In most experiments cells were labeled for 24 h concurrent
`with drug treatment using [2-14C]thymidine (specific activity, 52-58
`mCi/mmol; New England Nuclear) at a concentration of 0.02 uC\/m\.
`For some experiments, LI210 cells were labeled for 24 h ([2-uC]-
`thymidine, 0.02 ¿iCi/ml),washed 2 times with fresh medium, and then
`treated with H2-aza-Cyd or aza-dCyd for 24 h.
`Drug Treatment. aza-dCyd was provided by Dr. D. DeVos, Phar-
`machemie B. V. (Haarlem, Holland), and by the Drug Synthesis and
`Chemistry Branch, National Cancer
`Institute (Bethesda, MD), which
`also provided H2-aza-Cyd. aza-dCyd is unstable in aqueous solution
`and decomposes in a temperature-
`and pH-dependent manner
`(14) to
`form several noncytotoxic products (4). In RPMI 1630 medium at 37"C
`aza-dCyd decomposed in a first-order
`fashion with a fi/2of 17.5 h. H2-
`aza-Cyd has been shown to be stable for weeks in aqueous solution at
`room temperature (15). For in vitro experiments, both compounds were
`CELGENE 2074
`5511
`APOTEX v. CELGENE
`IPR2023-00512
`
`

`

`DNA DAMAGE PRODUCED BY Aza-dCyd AND H2-aza-Cyd
`
`pH 12.1
`
`1.0
`0.9
`0.8
`0.7
`0.6
`
`0.5
`
`0.4
`
`0.3
`
`0.2
`
`0.1
`0'.9
`
`OO t
`
`oc
`I
`
`0.8
`9
`=* 0.7
`H,
`O 0.6
`
`5
`
`0.5
`
`0.4
`
`0.3
`
`0.2
`
`pH 12.1
`24 Ht posi drug incubation
`
`pH 12.6
`24 Hi posi drug ncubation
`
`\ W•i
`
`300 Radi
`
`0.1
`O
`
`5.0
`
`10
`
`5.0
`
`10
`
`15
`
`O
`15
`TIME OF ELUTION (Hr)
`Fig. 1. Alkaline elution profiles of [14C]DNA from L1210 cells treated for 24
`h with 11;-a/;i i 'yd at the indicated concentrations
`(/tg/ml). Elutions were carried
`out at pH 12.1 (I and Q or pH 12.6 (B and /') either
`immediately after drug
`washout (A and ÃŒI)or following 24 h postincubation in drug-free medium (C and
`D). Labeling with ["CJthymidine
`(0.02 /iCi/ml) and treatment with H2-aza-Cyd
`were simultaneous. Points, mean of two to three values; bars, range;
`, elution
`of [14C]DNA from untreated L1210 after exposure to 300 rads in the cold.
`
`saline (0.15 M NaCl-10.6 min
`dissolved in sterile phosphate-buffered
`KH2PO4-5.6 HIMNa2HPO4) and then immediately added to cell sus
`pensions
`(~5.0 x IO5 cells/ml)
`to give a final drug concentration
`between 0.1 and 100 Me/ml. After drug treatment,
`the cells were washed
`twice by centrifugation and resuspended in fresh medium. Cell density
`was determined using a Coulter Counter
`(Model ZBI, Coulter Elec
`tronics, Hialeah, FL), and aliquots of cells were removed for alkaline
`elution analysis. In some experiments,
`remaining cells were diluted as
`necessary to maintain cell density below S x 10s cells/ml and incubated
`in drug-free medium for an additional 24 h. At the end of this period,
`appropriate aliquots were again removed for alkaline elution.
`Alkaline Elution. The method of alkaline elution has been reviewed
`in detail (22). In brief, approximately
`IO1'cells were resuspended in
`cold phosphate-buffered saline and layered on polycarbonate filters, 0.8
`ftm pore size, 25 mm diameter
`(Nuclepore Corp., Pleasanton, CA).
`Cells were then lysed with a solution containing 2% sodium dodecyl
`sulfate-0.02 M disodium EDTA-0.1 M glycine, pH 10.0 (lysis solution),
`which was allowed to flow through the filter by gravity. After connecting
`the outlet of the filter holders to the pumping system, 2 ml of proteinase
`K, 0.5 mg/ml (EM Labs, Darmstadt, West Germany), dissolved in the
`lysis solution, were added to a reservoir over the polycarbonate
`filters
`and pumped for approximately
`l h at 2 ml/h. DNA was eluted from
`the filters by pumping 0.02 M EDTA solution adjusted to pH 12.1 or
`12.6 with tetrapropylammonium
`hydroxide (RSA Corp., Elmsford,
`NY) containing 0.1% sodium dodecyl sulfate through the filters at
`approximately 2 ml/h. Fractions were collected at 3-h intervals with
`fractions and filters processed as previously described (22). In most
`experiments, L1210 cells labeled overnight with [mer/ry/-3H]thymidine
`(specific activity, 20 Ci/mmole; 0.1 ¿iCi/mlcells; New England Nuclear)
`and irradiated with 300 nuls of y-radiation were added to suspensions
`of l4C-labeled experimental cells to serve as internal elution standards.
`Elongation Experiments. Exponentially growing L1210 cells (5-6 x
`IO5cells/ml) were pulse labeled for 15 min with [iwe/A}'/-3H]thymidine
`(1.0 /iCi/ml cells), washed twice by centrifugation,
`and resuspended in
`warm RPMI 1630 medium. aza-dCyd, H2-aza-Cyd, or ara-C was added
`and the cells were incubated at 37°Cfor 1 h. Cells were then washed,
`resuspended, and maintained at 37°C.Aliquots were removed at various
`times and analyzed by alkaline elution as described above, except
`that
`[l4C]thymidine (0.02 ¿iCi/ml)-IabeIedcells were used as internal stand
`ard.
`Flow Cytometry. LI210 cells were treated with aza-dCyd or H2-aza-
`Cyd as described above. The analysis of DNA by flow cytometry was
`performed at the end of 24 h treatment and after 24 h postincubation
`in drug-free medium. Before analysis, cells were washed and suspended
`in Hanks' solution. Cells were stained by adding 3 ml propidium iodide
`(Calbiochem-Behring, San Diego, CA) (propidium iodide, 50 Mg/ml in
`0.1% sodium citrate plus 30 n\ Nonidet P-40) to 1(10/iI cell suspension
`and were stored at 4"C for 30 min before DNA analysis. Absence of
`aggregates
`and the suitability of the preparation were checked by
`fluorescence microscopy before the samples were run. DNA analysis
`was performed using a 30L cytofluorograph (Ortho Instruments, West-
`wood, MA). The fluorescence pulses were detected in a spectral range
`between 580 and 780 nm (to exclude the overlapping region of excita
`tion and emission spectra of unbound propidium iodide) and then
`integrated. Each cytofluorometric
`assay was performed with 5 x IO4
`cells. The percentage of cells in each cycle phase was calculated by the
`method of Maisch et al. (23).
`
`RESULTS
`
`strand breaks must occur in regions of DNA which were syn
`thesized while drug was available for incorporation. Production
`of single-strand breaks was concentration
`dependent up to 100
`tig/ml. Treatment with H2-aza-Cyd concentrations
`above this
`level caused increasing
`cell
`lysis during drug washout
`and
`resulted in elution curves with markedly reduced retention in
`the first fraction (data not shown). Incubation of cells in drug-
`free medium for 24 h resulted in some repair of the single-
`strand breaks, as evidenced by the reduced elution rates shown
`in Fig. 1, C and D. Repair was incomplete, with damage
`produced by the higher H2-aza-Cyd concentrations
`diminishing
`to near the level initially produced by 5 tig/ml. Single-strand
`breaks produced by the latter drug concentration were only
`slightly reduced after 24 h drug-free incubation. This pattern
`of repair
`is in contrast
`to the results obtained with aza-dCyd
`treated cells (Fig. 2); 24 h postdrug incubation resulted in either
`little change or an increased elution rate, with no significant
`repair at any drug concentration
`over this time period.
`In addition,
`the elution profiles demonstrated
`a fundamental
`difference in the nature of DNA damage produced by these two
`agents. The marked increase in elution rate with increasing pH
`following aza-dCyd treatment,
`coupled with the characteristic
`convex shape of the elution curves (21, 22), demonstrates
`the
`formation
`of alkali-labile
`sites in the DNA.
`In contrast,
`the
`elution curves obtained after H2-aza-Cyd treatment were essen
`tially linear and showed only a minimal pH dependency.
`The effects of aza-dCyd and H2-aza-Cyd on L1210 DNA
`
`elution profiles (at pH 12.1 and 12.6) of DNA
`Representative
`from LI210 cells treated for 24 h with H2-aza-Cyd (5-100 fig/
`ml) are shown in Fig. 1. The data in Fig. 1, A and B were
`obtained at the end of drug treatment, whereas those in Fig. 1,
`C and n were obtained following an additional 24 h incubation
`of the cells in drug-free medium. The increased elution rate of
`DNA from H2-aza-Cyd-treated
`cells is indicative of single-
`strand breaks in the DNA. Since cells were exposed to [14C]
`thymidine and H2-aza-Cyd simultaneously,
`the observed single-
`5512
`
`

`

`DNA DAMAGE PRODUCED BY Aza-dCyd AND H2-aza-Cyd
`
`0.9
`0.8
`0.7
`0.6
`
`0.5
`
`0.4
`
`0.3
`
`Å’ 0.2
`
`pH 12.1
`
`\ 300 Rad»
`
`pH 126
`
`0
`
`1.0
`
`10
`H a/a C („gml)
`
`100
`
`24+24 Hi, pH 12 6
`
`24+24 Hr, pH 12.1
`24 Hi. Pre-tabeted,
`pH 12.6
`24 Hi. Pre-latated.
`pH 12.1
`
`24+24 Hr, pH 12.6
`
`0.1
`
`0.9
`0.8
`0.7
`0.6
`
`0.5
`
`0.4
`
`0.3
`
`0.2
`
`24 Hi, pH 12.6
`
`/I
`
`/
`
`/
`
`/
`
`)<
`J
`
`'
`
`24+24 Hi, pH
`
`12.1
`
`24 Hi, pH 12.1
`
`24 Hi, Pre-labeted.
`pH 12.1
`
`pH 12.1
`24 HI pott-drug
`
`pH 12.6
`24 Ht pDrKkug
`
`0
`
`0.1
`
`1.0
`Aza-dC (fig/ml)
`
`10
`
`0.1
`
`5.0
`
`10
`
`0
`15
`5.0
`TIME
`OF ELUTION
`IHrl
`Fig. 2. Alkaline elution pronies of [UC]DNA from L1210 cells treated for 24
`h with aza-dCyd at the indicated concentrations
`(¿ig/ml).Elutions were carried
`out at pH 12.1 (I and C) and pH 12.6 (B and /') either immediately after drug
`washout
`(I and B) or following 24 h postincubation in drug-free medium (C and
`/»).Labeling with 0.02 ,,< i ml [>4C]thymidine and treatment with aza-dCyd were
`simultaneous. Points, mean of three values; bars, range;
`, elution of ['HJDNA
`from untreated LI210 after irradiation with 300 rads in the cold; 0.1 .if 'i |'l(|
`thymidine for 24 h was used to label these cells; points, mean of 25 values; bars,
`SE.
`
`10
`
`15
`
`Fig. 3. Effect of elution pH and treatment protocol on DNA elution kinetics
`from LI210 cells treated with Hj-aza-Cyd (//j-aza-C)
`(A) or aza-dCyd (Aza-dC)
`(B). —Log(retention)
`is proportional
`to DNA strand length, where (retention)
`represents
`the fraction of [MC]DNA retained on the filter at
`the time when
`internal
`standard (|'Il|l)NA)
`retention is equal
`to 0.2. The duration of drug
`treatment and simultaneous labeling with ["CJthymidine was 24 h and the elutions
`were carried out at pH 12.1 (O, •)or pH 12.6 (D. •)either
`immediately after
`drug washout (•,•)or after 24 h postincubation in drug-free medium (O, O). C,
`C,
`-log
`retention
`of [MC]DNA from cells that were prelabeled with [I4C]-
`thymidine,
`then exposed to drug for 24 h and eluted immediately. Points, mean
`of two to six values; bars, range, except in A (•,•)for which SE is shown.
`
`instability of the incorporated molecule
`quence of the chemical
`(aza-dCyd) or from an accumulation
`of strand scission inter
`mediates in an excision repair process (H2-aza-Cyd; see below).
`in Figs. 1 and 2 as well as data from additional
`depicted
`It might also be proposed that
`incorporation
`of anomalous
`experiments
`are summarized
`in Fig. 3, which shows the rela
`bases in the growing DNA strand produces an inhibition of
`tionship between drug concentration
`and the DNA elution rate
`further elongation (chain termination)
`and the resultant accu
`(plotted
`as the negative
`logarithm of the fraction of DNA
`mulation of small size DNA fragments. The latter mechanism
`retained
`on the filter at a fixed point of internal
`standard
`has been proposed to explain the DNA damage observed in
`elution). Both drugs caused a dose-dependent
`increase in DNA
`cells treated with ara-C (24).
`elution rate following a 24-h exposure with simultaneous
`label
`To investigate
`the effects of aza-dCyd and H2-aza-Cyd on
`ing. aza-dCyd-induced
`lesions demonstrated
`a large pH-de-
`DNA elongation, pulse-labeled cells were treated with drug for
`pendent
`increase (Fig. 3Ä),whereas there was much less pH
`1 h. After drug washout, DNA size was followed using alkaline
`dependence
`in the case of H2-aza-Cyd-treated
`cells (Fig. 3A).
`elution. The results of two separate representative
`experiments
`Repair of H2-aza-Cyd-induced
`lesions was evident, while 24 h
`are shown in Fig. 4. At the end of pulse labeling (r ,). control
`of postincubation
`following aza-dCyd treatment
`resulted in a
`DNA was small but increased towards normal
`length over 5-6
`clear increase in DNA damage for concentrations
`above 0.1 ¿¿g/
`h of incubation. Treatment
`of cells with aza-dCyd (1-25 fig/
`ml. We previously reported (21) that aza-dCyd lesions persist
`ml) resulted in only slight slowing of elongation during and
`for at least 48 h after drug removal with no evidence of repair.
`immediately following drug exposure; by 4 h, treated and con
`In contrast,
`the present data indicate that many of the H2-aza-
`trol DNA were nearly equivalent
`in size. In contrast, ara-C, a
`Cyd lesions are cleared from the DNA within 24 h, probably
`compound with known chain-terminating
`activity (25), pro
`by a repair process. When cells were labeled with [l4C]thymidine
`duced marked inhibition of elongation. Somewhat surprisingly,
`before aza-dCyd or H2-aza-Cyd exposure, only a small
`increase
`H2-aza-Cyd blocked further chain elongation completely and
`in DNA elution rate was produced by either compound (Fig.
`irreversibly for at
`least 6 h after drug removal. These data
`suggest
`that whereas
`aza-dCyd produces
`little or no chain
`3). This suggests
`that significant DNA damage is limited to
`termination, H2-aza-Cyd may induce failure of DNA elongation
`DNA strand segments synthesized in the presence of drug.
`The observed reductions
`in DNA strand length could result
`and/or
`ligation.
`from strand cleavage at sites of drug incorporation
`as a conse-
`The effects of aza-dCyd and H2-aza-Cyd on LI 210 cell cycle
`5513
`
`

`

`DNA DAMAGE PRODUCED BY Aza-dCyd AND H2-aza-Cyd
`
`late S phase and G2 + M was increased compared to control,
`whereas the percentage in G, was reduced. After an additional
`24 h incubation in drug-free medium,
`there appears
`to have
`been some renewed cell cycle progression at 0.1 jug/ml but not
`at
`the higher drug concentrations.
`In contrast, H2-aza-Cyd
`treatment produced no cell cycle effects during 24 h of exposure.
`After a 24-h drug-free period, control and H2-aza-Cyd treated
`cells showed an increase in (.;,. probably because a plateau
`density was achieved. Again, a tendency for 100 fig/ml and
`higher
`levels of H2-aza-Cyd to cause cell fragmentation was
`observed, yielding a nonevaluable DNA histogram.
`and repair
`In an attempt
`to further
`elucidate
`the nature
`mechanisms of H2-aza-Cyd- and aza-dCyd-induced DNA dam
`age, studies were done using two human lymphoblastoid
`cell
`lines. One (GM2345A) was derived from a patient with XP,
`while the other (GM3714) was from a clinically normal subject.
`It was anticipated
`that comparison
`of the DNA damage pro
`duced in these two cell
`lines would indicate
`the degree of
`dependency of H2-aza-Cyd- and aza-dCyd-associated DNA re
`pair on the excision repair mechanism absent
`in XP cells
`(complimentation
`group A). Fig. 5 presents elution profiles for
`DNA from the normal
`(GM3714)
`cell
`line. Fig. 5, A and C
`show the elution of DNA from H2-aza-Cyd-treated
`cells after
`24 h exposure
`(.-I) and after an additional
`24 h of drug-free
`incubation (("). Fig. 5, B and D present analogous data for aza-
`dCyd-treated
`cells. The characteristics
`of DNA damage pro-
`
`1.0
`0.9
`0.8
`0.7
`0.6
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`0.5
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`0.4
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`0.3
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`0.2
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`0.1
`
`I.Or.
`0.9 P
`0.8
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`0.2
`
`AM OC
`24 Hr poet-dug
`
`f
`
`0
`0.4
`0.8
`1.2
`
`1.6
`
`2.0
`2.4
`
`0
`0.4
`0.8
`1.2
`1.6
`
`2.0
`2.4
`
`-101
`
`23456
`
`HOURS AFTER END OF DRUG TREATMENT
`
`Fig. 4. Effect of cytidine analogues on DNA elongation. LI210 cells were
`pulse labeled for 15 min, washed twice, and then incubated for l h with either no
`drug (•)or A: aza-dCyd, 1 Mg/ml (•);ara-C, 0.5 /ig/ml fT); H2-aza-Cyd, 50 »ig/
`ml (A); B: aza-dCyd, 10 »<g/ml(O); aza-dCyd, 25 i/g/ml (A); ara-C, 12 »ig/ml(V).
`Cells were then washed and incubated at 37'C. Aliquots were removed at
`the
`indicated times and analyzed by alkaline elution. —Log(retention)
`is proportional
`to DNA strand length, where (retention)
`is the fraction of ['I I|l >NA remaining
`on the fÃ-lterwhen internal
`standard retention ((UC]DNA)
`is equal
`to 0.2. The
`ordinale is such that average single-strand length increases with height. Points,
`mean of two determinations.
`
`Table 1 Cell cycle distribution of LI210 cells treated with aza-dCyd
`
`inConcentration
`
`% of cells
`
`S
`phase12
`
`S
`phase +
`G2-M30
`phase1316
`
`45
`44
`43
`
`18
`
`33
`43
`42
`
`29
`31
`24Early
`
`10
`6
`12Mid-S
`
`1922Late
`
`00
`
`.1
`1.0
`10.0G,45
`
`64
`
`38
`34
`32
`
`0.1
`1.0
`10.0
`
`16
`9
`10
`
`13
`14
`16
`
`(Mg/ml)After
`24 h treatment
`
`After an additional 24
`h postdrug inculi.!
`
`Table 2 Cell cycle distribution of LI 210 treated with H2-aza-Cyd
`
`inAfter
`
`24 h treatmentConcentrationGig/ml)0
`1050
`
`100G,45
`
`% of cells
`
`S
`phase1211
`
`S
`phase +
`G2-M30
`phase1315
`
`45
`51
`39Early
`
`64
`
`13
`25Mid-S
`
`1115Late
`
`29
`24
`22
`
`18
`
`After an additional
`24 h postdrug incu
`bation
`
`' NE, not évaluable.
`
`0.1
`
`10
`
`10
`
`15
`
`23
`12
`NE
`
`99
`
`NE
`
`89
`
`NE
`
`60
`70
`NE°
`
`10
`50
`100
`
`5.0
`0
`15
`TIME OF ELUTION (Hr)
`Fig. 5. Alkaline elution profiles of [UC]DNA from human lymphoblasts (GM
`3714) treated with H2-aza-Cyd (H2-aza-Q (A and Q or aza-dCyd (Aza-dQ (B
`and /') at the indicated concentrations Gig/ml). Elutions were carried out at pH
`12.6 either immediately after drug washout
`(I and B) or following 24 h postin
`progression were examined using flow cytometry. These results
`cubation in drug-free medium (C and D). Labeling and drug treatment were
`are tabulated in Tables 1 (aza-dCyd) and 2 (H2-aza-Cyd). Fol
`simultaneous. Points, mean of two values; bars, range;
`, elution of [WC]DNA
`lowing 24 h of aza-dCyd exposure,
`the percentage of cells in
`from untreated 11210 after exposure to 300 rads in the cold.
`5514
`
`

`

`DNA DAMAGE PRODUCED BY Aza-dCyd AND H2-aza-Cyd
`
`in each cell line. While these results do not exclude the possi
`bility that
`the H2-aza-Cyd-induced
`lesions were removed by
`excision repair,
`it appears
`that
`the specific mechanism that
`is
`deficient
`in XP cells plays no role in the observed repair of aza-
`dCyd- and H2-aza-Cyd-induced DNA damage.
`
`DISCUSSION
`
`duced in this cell line was similar to that seen with LI210 cells
`(Figs. 1-3). H2-aza-Cyd produced linear elution curves, whereas
`aza-dCyd curves were convex, again suggesting that
`the latter
`drug produces alkali-labile sites. It is noteworthy that H2-aza-
`Cyd treatment
`resulted in a reduced DNA retention in the first
`fraction (Fig. 5/4). This pattern is generally indicative of non
`specific cell death and lysis, suggesting that
`these human cell
`lines are more sensitive than L1210 cells to the postulated lytic
`effect of H2-aza-Cyd.
`aza-dCyd-induced DNA
`After 24 h of drug-free incubation,
`lesions showed evidence of repair only at the lowest dose tested
`(0.1 jig/ml).
`In contrast, H2-aza-Cyd-induced
`lesions were re
`paired even at drug doses up to 100 /¿g/ml.These results are
`summarized in Fig. 6A, which again relates drug concentration
`to a measure of DNA retention. The overall degree of DNA
`damage produced by H2-aza-Cyd and aza-dCyd in GM 3714
`was less than that produced in LI210 with identical concentra
`tions and exposure periods. The present experiments are unable
`to distinguish whether
`this difference results from more exten
`sive incorporation
`of the drugs
`into LI210 DNA (resulting
`from greater anabolic activity and/or cell cycle effects), differ
`ences in the kinetics of DNA repair
`in the two cell
`lines, or
`both.
`performed in the XP
`Experiments were also simultaneously
`cell line (GM2345A). Elution curves for both H2-aza-Cyd- and
`aza-dCyd-treated
`cells were essentially identical
`to those ob
`tained with the normal cell line. The results are summarized in
`Fig. 6/;. The quantitative
`similarity of DNA damage produced
`in the two cell lines is evident (Fig. 6, A versus B). DNA strand
`breaks produced by both drugs were repaired to a similar extent
`
`Human Lymphoblastoid
`Cell Line GM3714
`
`B
`
`Human Lymphoblastoid (XPI
`Celi Une GM2345A
`
`Aza-dC. 24 Hr
`Aza-dC. 24+24 HI
`H,-az»-C.
`
`24 Hf
`
`the DNA alterations
`In this report we have investigated
`produced by incorporation
`of the cytidine analogues aza-dCyd
`and H2-aza-Cyd into DNA of LI 210 murine leukemia and two
`human lymphoblastoid
`cell
`lines. When cells are exposed to
`[l4C]thymidine
`and H2-aza-Cyd simultaneously
`for 24 h, a
`significant frequency of DNA single-strand breaks was observed
`in the labeled DNA by means of the alkaline elution technique.
`The lack of dependence of the frequency of these lesions on the
`pH of the eluting solution (Fig. 3/1) suggests that
`they are true
`single-strand breaks and not a consequence of alkali-labile sites
`in the DNA.
`In contrast,
`the effect of aza-dCyd in simultane
`ously ['"CJthymidine-labeled
`cells was to produce an alkaline
`elution profile characteristic
`of alkali-labile
`lesions: a marked
`pH dependence of the elution rate and elution curves which
`bend downward (22). The markedly reduced amount of DNA
`damage seen in prelabeled cells suggests that
`lesions are pro
`duced only in DNA which was synthesized in the presence of
`aza-dCyd or H2-aza-Cyd and which has therefore incorporated
`the abnormal bases. Moreover,
`this observation eliminates
`the
`possibility that a significant
`fraction of the DNA damage seen
`in simultaneously
`labeled and treated cells resulted from non
`specific toxicity and cell lysis, which would be expected to also
`damage preexisting DNA.
`Glazer and Knode (9), using alkaline agarose gel electropho-
`resis, demonstrated
`that aza-dCyd exposure resulted in forma
`tion of low molecular weight
`fragments of newly synthesized
`DNA, whereas previously synthesized DNA remained intact.
`They postulated that aza-dCyd inhibited DNA elongation. Our
`studies suggest
`that aza-dCyd has little effect on DNA elonga
`tion (Fig. 4) and that
`the observed damage results
`from the
`presence of alkali-labile sites in the DNA which are converted
`to single-strand breaks under the conditions of elution or elec-
`•trophoresis.These effects of aza-dCyd are in contrast
`to those
`demonstrated
`for ara-C. ara-C has been shown to possess a
`partial, concentration-dependent
`DNA chain-terminating
`activ
`ity (25) and to produce alkaline elution profiles
`showing no
`evidence of alkali-labile sites (24). This is consistent with our
`demonstration
`that ara-C markedly slows chain elongation but
`that
`the block is incomplete
`and gradual growth to full size
`DNA can occur. When these observations
`are considered to
`gether with the finding that ara-C potently inhibits overall DNA
`synthesis
`(25), whereas aza-dCyd does not (2), it can be con
`cluded that
`these two compounds
`have contrasting
`cellular
`effects and produce DNA damage through different mecha
`nisms.
`Following a 1-h treatment of L1210 with H2-aza-Cyd (50 ng/
`ml), further DNA elongation was completely inhibited for at
`least 6 h (Fig. 4). This is somewhat
`surprising in light of the
`near normal
`elongation which occurred during the 1 h of
`exposure (f0in Fig. 4A). Moreover,
`it was demonstrated in Figs.
`1 and 3 that rapidly eluting DNA present after 24 h of simul
`taneous H2-aza-Cyd and label will increase in size if cells are
`incubated for an additional 24 h in the absence of drug. These
`observations are consistent with a model
`in which H2-aza-Cyd
`incorporation
`does not produce
`chain termination within a
`single replicón but prevents subsequent
`ligation of replicons to
`5515
`
`to -Log (retention)] of [MC]DNA from
`Fig. 6. Strand length [proportional
`normal human lymphoblasts
`(A) or xeroderma pigmentosum lymphoblasts (B) as
`a function of Hj-aza-Cyd (H2-aza-C) (•,O) or aza-dCyd (Aza-dC) (•.D) concen
`tration.
`(Retention),
`fraction of DNA retained on the filter after 15 h of elution
`(fifth fraction) considering the retention after collection of the first fraction to be
`1.0. Duration of drug treatment and simultaneous
`labeling with ["Cjthymidine
`was 24 h, with elutions carried out at pH 12.6 immediately after drug washout
`(•,•)or following 24 h postincubation
`in drug-free medium (O, D). Points,
`the
`mean of two values; bars, range.
`
`

`

`DNA DAMAGE PRODUCED BY Aza-dCyd AND H2-aza-Cyd
`
`analysis conditions
`
`in cell culture methods and/or
`differences
`were involved.
`of aza-dCyd and H2-aza-
`When the range of concentrations
`Cyd used for alkaline elution and flow cytometry studies are
`compared relative to effects on LI210 cell viability, a contrast
`between the two compounds
`is again observed. As measured by
`soft-agar colony forming ability of treated cells, 24 h exposure
`to aza-dCyd yields a 1-Iog cell kill at 0.1 tig/ml and a maximum
`of 3 logs at 1-100 ¿ig/ml(4). H2-aza-Cyd exposure at concen
`trations
`less than 5 /¿g/mlproduces
`little or no cytotoxicity,
`while 100 Mg/ml yields only a 1- to 1.5-log cell kill (27). Thus
`H2-aza-Cyd appears both less potent and less efficacious than
`aza-dCyd in vitro, a pattern which is repeated in the results of
`therapy studies with LI210 bearing mice in vivo (5, 27). While
`these findings are consistent with the lower potency of H2-aza-
`Cyd for producing DNA lesions and with the more extensive
`reversal of H2-aza-Cyd-induced
`damage,
`the exact relationship
`between the cytotoxicity
`of these compounds
`and the DNA
`damage produced remains uncertain.
`It has been reported that
`aza-dCyd cytotoxicity is also correlated with the inhibition of
`DNA methylation (20). Moreover, Jones and Taylor
`(28) have
`shown H2-aza-Cyd to be less active than aza-dCyd in inhibiting
`methylation. Since this process and the production
`of DNA
`lesions appear
`to require anabolism to the nucleotide triphos-
`phate and incorporation of the drugs into DNA (10), differences
`in cytotoxicity
`between H2-aza-Cyd and aza-dCyd may ulti
`mately be related to the extent of their metabolic activation.
`Nevertheless,
`the contrasting nature of DNA damage produced
`by these two compounds
`suggests that the biochemical basis for
`their cytotoxic activity may be quite different.
`
`ACKNOWLEDGMENTS
`The authors wish to thank Susan Hurst-Calderone for technical
`assistance, YvesPommier and Donna Kerrigan for valuablediscussions,
`and Madie Tyler for typing the manuscript.
`
`If the drug-free incubation time is
`form large DNA strands.
`sufficiently long,
`it appears
`that
`this block can be partially
`overcome.
`H2-aza-Cyd and aza-dCyd are related compounds differing in
`the sugar moiety, and in that
`the azapyrimidine
`ring of H2-aza-
`Cyd is 5,6-saturated. This latter
`feature confers stability to the
`H2-aza-Cyd molecule: aza-dCyd undergoes
`spontaneous
`ring
`opening in aqueous
`solution (fi/2 = 17 h at 37°C,pH 7.4)
`ultimately
`forming
`l-0-D-2'-deoxyribofuranosyl-3-guanylurea
`(4, 14), whereas H2-aza-Cyd demonstrates
`no decomposition
`under
`similar conditions
`(15). We have previously proposed
`(21) that
`the formation of DNA alkali-labile sites in aza-dCyd-
`treated cells is related to the incorporation
`of the chemically
`unstable azacytosine base into DNA. It is uncertain if aza-dCyd
`incorporated
`into DNA undergoes
`spontaneous
`ring