Kinetics and mechanism of the 1-beta-D-arabinofuranosylcytosine-induced potentiation of cis-diamminedichloroplatinum(II) cytotoxicity.

Certain aspects of the potentiation induced by 1-beta-D-arabinofuranosylcytosine (ara-C) on cis-diamminedichloroplatinum(II) (cis-DDP) cytotoxicity were investigated. The time dependency of additions of ara-C and cis-DDP was established by allowing cells to grow for various intervals in fresh medium following the removal of one agent before adding the second one. ara-C had no potentiating effect on cis-DDP toxicity when given to the cells before the addition of cis-DDP. When the experiment was reversed so that cis-DDP was added first and ara-C second, a slight potentiating effect was observed even if the drugs were added 4 h apart. The optimal toxic effect was obtained when ara-C and cis-DDP were added together. Continuous exposure of cells to concentrations of ara-C and cis-DDP 10 times lower than those used in pulse treatment experiments resulted in an additive rather than a synergistic effect. ara-C, unable to kill cells in pulse treatment, killed 96% of the cells after 24 h of continuous incubation. Thiourea was able to prevent the cytotoxic effect of cis-DDP in a concentration-dependent manner when given to the cells immediately following their treatment with cis-DDP; at 0.1 M thiourea, the cytotoxic effect of cis-DDP was almost completely prevented. Similar results were obtained when the cells were exposed to a combination of cis-DDP and ara-C. In this case, 0.1 M thiourea resulted in over 80% survival of cells treated with the drug combination. Thiourea had to be added to the cells either together with cis-DDP or immediately following removal of the drug in order to completely prevent the cytotoxic effect. A similar time factor was involved when cells were treated with a combination of cis-DDP and ara-C before their exposure to thiourea, but in this case thiourea was only able to prevent completely the cytotoxic effect when added simultaneously with the drug combination. In other experiments, the effect of thiourea on cis-DDP-induced DNA cross-linking was measured by the alkaline elution technique. Thiourea was capable of preventing DNA cross-link formation both in cells treated with cis-DDP alone, and in cells exposed to the combination of cis-DDP and ara-C These observations further support the contention that ara-C potentiates cis-DDP cytotoxicity by increasing the ability of the platinum compound to form earlier, more stable DNA cross-links regardless of whether it is present in free or monoadducted form.(ABSTRACT TRUNCATED AT 400 WORDS)

Mixed function oxidase activities of established human colon carcinoma cell lines in the activation of cyclophosphamide.

Three established human colon carcinoma cell lines (LoVo, SW620, and SW403) with different degrees of phenotype differentiation were investigated for their sensitivity to the cytotoxic effects of cyclophosphamide (CP) and to its active metabolite, 4-hydroxycyclophosphamide (4-OH-CP), and for their mixed function oxidase (MFO) activities. None of the cell lines showed sensitivity to CP as determined by the inhibition of colony formation assay, even after continuous drug treatment at high concentrations (200 microgram/ml) for up to 72 h. CP also had no effect on the cellular doubling time or on the incorporation of [3H]-thymidine. Pretreatment with phenobarbital (PB) plus hydrocortisone (HC) was unable to induce CP cytotoxicity. In contrast, 4-OH-CP, the major metabolite formed from CP by MFO, was highly toxic to the cells. About 90% cell kill was obtained at drug concentrations of 17.5 microgram/ml (LoVo), 15 microgram/ml (SW620), and 55 microgram/ml (SW403) after 1-h incubation at 37 degrees. MFO activities were determined by measuring p-nitroanisole demethylase (PNAD) and arylhydrocarbon hydroxylase (AHH) in microsomes prepared from noninduced cells or from cells treated with benzanthracene or PB plus HC. Intrinsic AHH activities were below the level of detection for all cell lines [less than 1 pmol of 3-hydroxybenzo(a)pyrene (3-OH-BP) formed per min per mg of protein]. Treatment with benzanthracene resulted in AHH activities of 12 to 15 pmol of 3-OH-BP per min per mg of protein, but treatment with PB plus HC failed to induce significant AHH activities. PNAD activities in noninduced cells as well as in cells treated with benzanthracene were 0.05 to 0.08 nmol of p-nitrophenol formed per min per mg of protein; treatment with PB plus HC increased PNAD activities by only 1.5-fold. Thus, in contrast to reports for rat colon and for a single human colon cancer cell line, CP is inactive when applied directly to several other human colon carcinoma cell lines. Because these cells have minimally detectable intrinsic and induced MFO activities, we conclude that CP cannot be successfully metabolized into 4-OH-CP to induce a significant degree of cell kill.

Interactions of S-(2-haloethyl)-mercapturic acid analogs with plasmid DNA.

A series of related S-(2-haloethyl)-L-cysteine analogs were synthesized and their interaction with DNA was studied with plasmid pBR322. Both S-(2-chloroethyl)-L-cysteine (CEC) and S-(2-bromoethyl)-L-cysteine (BrEC) rapidly induced relaxation of the supercoiled plasmid as determined by agarose gel electrophoresis and electron microscopy, whereas S-(2-fluoroethyl)-L-cysteine did not interact with DNA. The relaxation was most probably due to strand scission at alkylated labile sites in the DNA. When 35S-labeled CEC or BrEC was used as the substrate, covalent binding of 35S to DNA was obtained; CEF displayed a somewhat higher binding than BrEC. No binding of 35S was obtained with (2-hydroxyethyl)-L-[35S]cysteine, [35S]cysteine, or [35S]cystine, substrates which did not induce relaxation of the DNA. Esterification of the carboxyl group resulted in a somewhat lower rate of DNA strand scission, whereas N-acetylation prevented the cysteine analogs from inducing DNA strand breaks. S-(2-Chloroethyl)-glutathione (GSH) did not interact with DNA as determined by lack of effect on the superhelicity of DNA, a finding which is in agreement with the hypothesis that the primary amine groups of CEC or BrEC may participate in the formation of reactive intermediates which can interact with DNA. S-(2-Hydroxyethyl)-GSH and S-(2-hydroxyethyl)-L-cysteine were unable to induce DNA strand breaks. Neutral denaturation of supercoiled pBR322 treated with the analogs revealed that compounds which were able to induce DNA strand breaks also interfered with denaturation of double-stranded circular DNA. No such interference was observed when double-stranded linear DNA (obtained by BamH1 restriction digestion) was treated with the analogs prior to denaturation. These data indicate that a marked difference exists between S-(2-chloroethyl)-L-cysteine and S-(2-chloroethyl)-glutathione in their reaction with supercoiled plasmid DNA. Either a major difference exists in the reactivity of the corresponding episulfonium ions of these conjugates or a separate mechanism of alkylation based on a free alpha-amino of the cysteine conjugate is participating in DNA strand breakage and possible crosslinking. In vivo toxic effects of these S-(2-chloroethyl) conjugates are predicted to be distinctly different.

Differences in transformation of repair-deficient mutants of E. coli with BPDE- or chlorozotocin-modified plasmid DNA.

Plasmid pBR322 was alkylated with either chlorozotocin or with r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo-[a]pyrene (BPDE) before it was transformed into various strains of Escherichia coli. Plasmid survival was determined as ability to convert the bacteria to tetracycline and ampicillin resistance. Increased levels of alkylation caused a decrease in transforming activity in all strains studied. This decrease did not seem to be a result of alkylation induced strand scission, but rather some other biochemical or conformational change induced by the alkylating event. In E. coli AB1157 transformation was decreased by 50% with 6 alkylations/plasmid molecule for BPDE and 8-9 alkylations for chlorozotocin. At these levels of alkylation the loss in supercoiled DNA due to strand scission was less than 5%. Alkylated pBR322 was also transformed into repair-deficient strains of E. coli. In strain JC2924 (recA6) the survival of both BPDE- and chlorozotocin-modified DNA was similar to survival in the repair proficient strain AB1157, which would indicate that postreplicational repair of BPDE- or chlorozotocin-modified plasmid DNA was not significant under these conditions. Chlorozotocin-modified pBR322 did not seem to be repaired by the bacterial uvr-endonucleases as determined by plasmid survival in strains AB1884 (uvrC34), AB1885 (uvrB5) and AB1886 (uvrA6). With BPDE-alkylated plasmid DNA the results were strikingly different. Strains AB1884 and AB1886 were more sensitive to BPDE modified DNA than the wild type strain AB1157. Strain AB1885 was similar to AB1157 in sensitivity to BPDE-alkylated plasmid. These findings suggest that bacterial uvr-endonucleases may be able to recognize and repair BPDE-alkylated pBR322. The role of the uvrB protein in repair of alkylated DNA needs to be further investigated.

Effect of 2-chloroethylnitrosoureas on plasmid DNA including formation of strand breaks and interstrand cross-links.

Plasmid [3H]pBR 322 was incubated with various alkylating agents including chlorozotocin, N,N'-bis(2-chloroethyl)-N'-nitrosourea (BCNU), N-ethyl-N-nitrosourea (Enu) and dimethylsulfate (DMS). Formation of DNA strand breaks was followed by separation of the various forms of DNA on agarose gels and liquid scintillation counting of the bands. All alkylating agents examined were capable of rapidly producing strand breaks in time and concentration dependent fashion. Bands migrating as relaxed circular and supercoiled forms of the plasmid disappeared, and extensive alkylation resulted in formation of a band that migrated faster than the linear form of DNA. Electron microscopy of this band showed that it consisted of relaxed circles. Prolonged storage of alkylated plasmid resulted in fragmentation of the DNA, possibly due to strand scission at apurinic sites. A new neutral denaturation technique was developed, which allowed for the detection of DNA interstrand cross-links with minimal effects on other potentially labile sites of the alkylated DNA. The level of alkylation was quantitated by incubating [3H]pBR 322 with [2-chloroethyl-U-14C]chlorozotocin and was shown to be independent of DNA concentration but have a linear relationship with drug concentration. Linear and relaxed circular forms of the plasmid were alkylated to a somewhat higher extent than supercoiled DNA. Alkylation of pBR 322 with defined superhelical densities showed no preferential loss in DNA with a specific superhelical density, indicating that alkylation-induced unwinding is independent of superhelicity under the experimental conditions used.

Microsomal activation of thioacetamide-S-oxide to a metabolite(s) that covalently binds to calf thymus DNA and other polynucleotides.

In the presence of NADPH liver microsomes isolated from phenobarbital-pretreated rats catalyze the conversion of [3H]thioacetamide-S-oxide to a reactive intermediate(s) which covalently binds to calf thymus DNA, calf liver RNA, polyguanylic acid (poly(G)) and polyadenylic acid (poly(A)). The highest level of binding of radioactivity was obtained with poly(G), followed by poly(A), RNA and DNA. The incorporation of radioactivity into DNA was linear for 30 min and there was a requirement for NADPH for time-dependent covalent binding to occur. Performing the microsomal incubations in an atmosphere of 80% CO/20% O2 or adding partially purified anti cytochrome P-450 immune serum to the microsomal incubations inhibited the total metabolism of thioacetamide-S-oxide and had a small, but insignificant, inhibitory effect on binding of radioactivity to calf thymus DNA. Using a reconstituted monooxygenase system containing cytochrome P-450 purified from phenobarbital-treated rats we were unable to detect any metabolism of thioacetamide-S-oxide. Only background levels of radio-activity were incorporated into calf thymus DNA when microsomes isolated from phenobarbital-treated rats were incubated with [3H]thioacetamide in the presence of NADPH. These results suggest that thioacetamide-S-oxide is an obligatory intermediate in the metabolic activation of thioacetamide to a reactive metabolite(s) which binds to calf thumus DNA.

Formation of DNA-binding products from isolated benzo[a]pyrene metabolites in rat liver nuclei.

Liver nuclei from 3-methylcholanthrene-treated rats in the presence of NADPH metabolized 3- and 9-hydroxybenzo[a]pyrene and 7,8-dihydro-7,8-dihydroxybenzo[a]pyrene to products that bound to DNA. Maximal binding was obtained with the dihydrodiol which was approximately 3-fold that with 9-hydroxybenzo[a]pyrene, and 60-fold that with 3-hydroxybenzo[a]pyrene, as substrates. Both 4,5-dihydro-4,5-dihydroxybenzo[a]pyrene and 9,10-dihydro-9,10-dihydroxybenzo[a]pyrene were also extensively metabolized by the nuclear fraction but did not give rise to DNA-binding products. The available evidence suggests that the DNA binding species derived from 9-hydroxy-benzo[a]pyrene is 9-hydroxy-benzo[a]pyrene-4,5-oxide and from 7,8-dihydro-7,8-dihydroxybenzo[a]pyrene, as previously observed in different systems, 7,8-dihydro-7,8-dihydroxy-benzo[a]pyrene-9,10-oxide.

Formation in isolated rat liver microsomes and nuclei of benzo(a)pyrene metabolites that bind to DNA.

The hepatic nuclear fraction isolated from 3-methylcholanthrene (MC)-treated rats contained enhanced levels of cytochrome P-450 and aryl hydrocarbon hydroxylase [benzo(a)pyrene (BP) monooxygenase], whereas the activities of epoxide hydrase and reduced nicotinamide adenine dinucleotide phosphate-cytochrome c reductase and the concentration of cytochrome b5 were not altered. The metabolite pattern of BP was investigated by using high-pressure liquid chromatography and was found to be similar in nuclei and microsomes from MC-treated rats. After incubation of the nuclear fraction with [3H]BP and reduced nicotinamide adenine dinculeotide phosphate, radioactivity was found to be associated with nuclear DNA and the extent of binding was markedly enhanced by pretreatment of the animals with MC. Binding was strongly inhibited by a-napthoflavone but was not influenced by 1,1,1-trichloropropene-2,3-oxide, an inhibitor of epoxide hydrase. In the presence of microsomes from MC-treated rats, increased binding of BP to DNA was observed in nuclei from both control and MC-treated rats; moreover, when the nuclear DNA was replaced by a corresponding amount of calf thymus DNA, the extent of binding was severalfold enhanced. In contrast to nuclei from control rats, the nuclear fraction from MC-treated rats showed an increase in bound radioactivity when incubated with a microsome-free supernatant, obtained by incubating microsomes from MC-treated rats with [3H]BP. The increase in extent of binding was eliminated in the presence of menadione or alpha-naphthoflavone. It is suggested that under the conditions used here the following different processes may have contributed to the total incorporation of BP products into nuclear DNA: (a) formation of DNA-binding products derived from BP by nuclear aryl hydrocarbon hydroxylase; (b) formation of DNA-binding products from microsomal BP metabolites by nuclear aryl hydrocarbon hydroxylase; and (c) direct transfer of reactive microsomal metabolites to nuclear DNA.

The metabolism of benzo(alpha)pyrene in isolated rat liver cells.

Isolated rat liver cells catalyze the metabolism of benzo(alpha)pyrene (BP) with the resulting formation of phenols, dihydrodiols, and conjugates. The rate of the primary oxidative step in the process was similar to that catalyzed by isolated rat liver microsomes in the presence of a reduced nicotinamide adenine dinucleotide phosphate-generating system and responded similarly to various inhibitors, including 2-diethylaminoethyl-2,2-diphenylvalerate, metyrapone, alpha-naphthoflavone, and hexobarbital. The level of cytoplasmic, reduced nicotinamide adenine dinucleotide phosphate was not rate limiting in liver cells isolated from either fed or fasted animals. The conjugates and dihydrodiols formed were readily excreted, whereas low concentrations of phenols accumulated intracellularly. The pattern of metabolites of BP was the same in isolated rat liver cells and in the isolated perfused rat liver. 3-Methylcholanthrene treatment of the rats caused a marked increase in cellular BP metabolism as well as in cytochrome P-450 concentration. The induced hemoprotein revealed characteristics similar to those previously established with isolated liver microsomes, i.e., increase in high-spin form, enhanced affinity for BP as revealed by a lower Michaelis constant, and sensitivity to the inhibitory action of alpha-naphthoflavone. After 3-methylcholanthrene treatment, phenols and dehydrodiols constituted a larger percentage of the total metabolites, indicating a more pronounced stimulation of the oxidative than of the conjugative step of BP metabolism by induction, and the dihydrodiols now tended to accumulate intracellularly.

Recent studies on cytochrome P-450-linked functions in isolated rat liver cells.

In rat liver cells isolated by perfusion in the perfusion in the presence of collagenase, the major portion of cytochrome P-450 is present in the oxidized, nonsubstrate-bound, low spin state. Drug addition to a suspension of liver cells results in the rapid formation of the cytochrome P-450 (Fe3+)-substrate complex which in turn is followed by the appearance of other species with different spectral characteristics before steady state drug monooxygenation is achieved. Cytochrome P-450-linked metabolism of various tested drugs and carcinogenic polycyclic hydrocarbons by isolated rat liver cells is as fast, or faster, as with rat liver microsomes supplemented with a NADPH generating system. Both experimental models respond similarily to phenobarbital or 3-methylcholanthrene pretreatment of the animals and to various of the wellknown inhibitors of drug metabolism. Except with liver cells isolated from fasted, phenobarbital-treated rats, generation of cytosolic NADPH seems sufficient to support optimal drug metabolism even in the absence of added substrates of intermediary metabolism. In isolated liver cells oxidized drug metabolites undergo subsequent metabolic conversion, most often to form the corresponding glucuronides and sulphates. These are readily excreted, whereas non-conjugated products, e.g. free phenols, tend to accumulate intracellularly. Cellular glucuronide formation is strongly inhibited by ethanol-presumably due to an unfavorable effect of the increased NADH/NAD+ ratio on the synthesis of uridine-5'-diphosphoglucuronic acid (UDPGA). In contrast, low concentrations of ethanol have no, or only a slight stimulatory effect on the cytochrome P-450-linked step of drug metabolism and there are indications that the oxidation of low concentrations of ethanol is in fact stimulated by a facilitated reoxidation of cytosolic NADH occuring during drug monooxygenation.