Effects of U-71,184 and several other CC-1065 analogues on cell survival and cell cycle of Chinese hamster ovary cells

CC-1065 is a very potent antitumor antibiotic which selectively binds in the minor groove of DNA with alkylation at N-3 of adenine. Since therapeutic doses of CC-1065 caused delayed deaths in mice, analogues were synthesized, some of which had significant antitumor activity. The effects of several of these analogues on inhibition of CHO cell survival, cell progression, and their phase-specific toxicity are reported. CC-1065, U-66,664, U-66,819, U-66,694, and U-71,184 all have a left hand segment with an intact cyclopropyl group but have different tail segments. Lethality of these compounds after 2 h drug exposure was in the following order (50% lethal dose in nM in parentheses): CC-1065 (0.06) greater than U-71,184 (1.3) greater than U-66,694 (3.2) greater than U-68,819 (171) greater than U-66,664 (greater than 1200). In general, these compounds did not inhibit progression from G1 to S but slowed progression through S and blocked cells in G2-M. The phase-specific toxicity of U-71,184 and U-66,694 was different from that of CC-1065. CC-1065 was most cytotoxic to cells in M and early G1 and toxicity decreased as cells entered late G1 and S. In contrast, U-66,694 and U-71,184 were most toxic to cells in late G1. The biochemical and cellular effects of U-71,184 were then studied in detail since it was the most active among these analogues. After a 2-h exposure to 3 ng/ml U-71,184, 90% cell kill or growth inhibition was observed whereas 100 ng/ml was needed for similar inhibition of DNA and RNA synthesis. This discrepancy between the doses suggested that inhibition of nucleic acid synthesis may not be causally related to lethality. Further studies showed that when drug was removed after 2 h exposure, DNA synthesis continued to be inhibited whereas RNA and protein synthesis reached levels higher than the control. Therefore, it is likely that at cytotoxic doses the low level of inhibition of DNA synthesis combined with the stimulation of RNA and protein synthesis leads to unbalanced growth and cell death.     

Lethality, DNA alkylation, and cell cycle effects of adozelesin (U-73975) on rodent and human cells.

Adozelesin (U-73975) is an extremely potent cytotoxic agent which causes 90% lethality, after 2 h exposure in vitro, of Chinese hamster ovary and lung (CHO and V79), mouse melanoma (B16), and human ovarian carcinoma (A2780) cells at 0.33, 0.19, 0.2, and 0.025 ng/ml, respectively. Under similar conditions, Adriamycin and cisplatin had 90% lethality values in CHO cells of 150 ng/ml (= 249 nM) and 6800 ng/ml (= 2266 nM), respectively. The relative drug sensitivity of the cell lines (A2780 > V79, B16, CHO) was correlated to the relative amounts of [3H]adozelesin alkylated to DNA. The greater sensitivity of A2780 was due to (a) greater DNA alkylation at different drug doses and (b) greater intrinsic sensitivity of A2780 which resulted in greater cell kill at comparable DNA alkylation. Phase specific toxicity studies show that adozelesin was least lethal to CHO cells in mitosis and very early G1. Lethality increased as cells progressed through G1 and was maximal in late G1 and early S. Mitotic cells had lower drug uptake and correspondingly less drug binding to DNA than G1 or S-phase cells. However, based on the amount of drug alkylated per micrograms of DNA, cells in M, G1, and S were equally sensitive. Therefore, the lower sensitivity of M-phase cells was due to lower drug uptake. Adozelesin had three different effects on progression of CHO, V79, B16, and A2780 through the cell cycle: (a) slowed progression through S which resulted in significantly increasing the percentage of S-phase cells. This effect was transient; (b) cell progression was blocked in G2 for a long time period; (c) the response of the cell lines to the G2 block differed. CHO and V79 cells escaped G2 block by dividing and entered the diploid DNA cycle or did not undergo cytokinesis and became tetraploid. On the contrary, B16 and A2780 cells remained blocked in G2 and did not become tetraploid. Cell progression was inhibited in a similar manner when a synchronized population of M, G1, or S-phase cells were exposed to adozelesin.     

Inhibition of thymidine incorporation into epidermal and dermal DNA of HRS/J mice after a Colcemid pulse.

The introduction of a Colcemid pulse, which is known to inhibit microtubulin assembly and thereby influence cellular events dependent on the assembly, delayed the normal passage of G1 cells to S-phase. Experimental support for this hypothesis was obtained by the pulsing of cultured inbred HRS/J mouse skin for 30 minutes with 20 micrograms Colcemid/ml and by measurement of thymidine (dThd) incorporation for a 4-hour incubation period at different intervals after the pulse. Incorporation of dThd into epidermal and dermal DNA was maximally inhibited 9.5 and 17.5 hours after the Colcemid pulse. Maximal inhibition and minimal inhibition of dThd incorporation were observed 17.5 hours after different concentrations of Colcemid (1--30 micrograms/ml) were pulsed. Maximal inhibition of dThd incorporation occurred after pulsing with 20 micrograms Colcemid/ml. The acceleration of G0 or G1 events to S-phase in proliferative cells caused by mild epidermal stripping was also inhibited after the addition of the Colcemid pulse. The data suggest a time relationship between the Colcemid pulse, the subsequent disruption of microtubulin structure, and the inhibition of dThd incorporation into epidermal and dermal DNA. The intact microtubulin structure is apparently essential for the normal intracellular dependency among the cell membrane, the cytoskeleton structure, and the cell nucleus and occurs when the cells transit from G1 to S-phase.     

Effects of 5-aza-2'-deoxycytidine on survival and cell cycle progression of L1210 leukemia cells

The in-vitro effects of the antileukemic agent 5-aza-2'-deoxycytidine (5-aza-dCyd), on DNA synthesis, growth, cloning in agar, and cell cycle traverse of L1210 leukemia cells were studied. 5-Aza-dCyd at 0.1 microgram/ml for 10 hr (cytotoxic concentration) did not inhibit DNA synthesis but produced a very potent growth inhibition, and changed markedly the DNA flow cytometric histograms. A 5-h continuous exposure to the drug at concentrations ranging from 0.1 to 10 micrograms/ml caused an accumulation of cells in the S portion of the DNA histograms indicating a slowing of the progression of cells in the S phase. A longer exposure time (10 h) at the same concentrations led to a bimodal DNA distribution (peaks at G1 and G2-M) and a depletion of the S phase. When the exposure time to 5-aza-dCyd (0.1 microgram/ml) was extended to 15 and 20 h, there was a decrease in the G2-M peak and an augmentation of the G1 peak. To determine if 5-aza-dCyd produced a block in cell cycle progression, L1210 cells were treated for 10 h with colcemid and 5-aza-dCyd simultaneously for 10 h. Colcemid alone, or colcemid in combination with 5-aza-dCyd produced an accumulation of cells under a single G2-M peak. This indicates that 5-aza-dCyd did not block the progression of L1210 cells through S phase, but only produced a slowing down of this event. These results, indicating that 5-aza-dCyd does not block cell cycle progression and that its cytotoxic action is not self-limiting, are of importance for designing future clinical trials.     

Effects of chartreusin on cell survival and cell cycle progression

Chartreusin was lethal to both L1210 and P388 cells in culture with 90% of the cells being killed after a 24-hr exposure to 1.1 and 2.6 microgram/ml, respectively. The lethality of the drug increased in direct proportion to dose and exposure time. Both L1210 and Chinese hamster ovary cells in S phase were more sensitive to the lethality of the drug than were their corresponding non-S-phase cells. L1210 cells were partially synchronized by exposing an asynchronous culture to [methyl-3H]thymidine (20 Ci/mmol) and Colcemid for 3 hr. Synchronous culture of Chinese hamster ovary cells was established by planting mitotic cells. The progression of cells through the cell cycle was studied with flow microfluorometry both in the presence of the drug and after the drug had been washed off. In the presence of chartreusin the progression of mitotic cells into G1 was not affected. The movement of G1 cells into S was slower, and the movement of G2 cells into mitosis was blocked. When the drug was removed, the G2 to M block persisted for at least 4 hr but the progression of G1 cells to S was no longer inhibited.     

Cell kinetic effects of low doses of the skin carcinogen 7,12-dimethylbenz[a]antracene on hairless mouse epidermis

Groups of hairless (hr/hr) mice were given a single, topicalskin application of either 50, 5 or 0.5 µg 7, 12-dimethylbenz[a]anthracene(DMBA). At different times up to 3 days after treatment epidermalDNA distribution patterns were determined by flow cytometry,and sp. act. of DNA and labeling indices were obtained basedon incorporation of [³H]thymidine Mitotic rates were determinedby the Colcemid method, and the number of basal and suprabasalcells were scored in histological sections. All three dosesof DMBA led to an early depression in the uptake of [³H]thymidine,associated with an accumulation of cells with S phase DNA contentpeaking at 16 h. By combining the methods for studying DNA synthesis,it can be concluded that the alterations observed probably weredue to a slow rate of DNA synthesis in the S phase cells, ratherthan to a block at the entrance of cells into the S phase. Threedays after the application, DMBA still maintained an effecton the cell cycle progression, at least in the S phase. Therewas an obvious dose-response relationship in the inhibitionof epidermal DNA synthesis. Six hours after application of thetwo highest doses of DMBA an early increase in the mitotic ratewas observed. This short-lasting high mitotic rate was followedby a transient, very brief increase in the number of suprabasalcells. Thereafter a decrease in both the mitotic rate and thenumber of suprabasal cells occurred, probably caused by thealterations in DNA synthesis. After the lowest dose there wasno such early increase in the mitotic rate and no initial, short-lastingincrease in the number of suprabasal cells. Hence, this studyshows that decreasing doses of DMBA provoke decreasing degreesof the same type of cell kinetic perturbations in the epidermalcell cycle.     

In vitro biochemical and cytotoxicity studies with 1-beta-D-arabinofuranosylcytosine and 5-azacytidine in combination.

The effect of 1-beta-D-arabinofuranosylcytosine (ara-C) and 5-azacytidine (5-aza-C), alone and in combination, on DNA synthesis and cytotoxicity in hamster fibrosarcoma cells has been studied. After a 2-hr exposure of S-phase cells to ara-C at concentrations of 2 to 200 muM, the cells required about 4 to 6 hr to recover from inhibition of DNA synthesis. When 2 exposures to ara-C were used, maximal cytotoxicity occurred when the 2nd dose of ara-C was administered at the time when the cells recovered from the inhibition of DNA synthesis. When the S-phase cells were exposed to ara-C, the maximal killing effect of 5-aza-C occurred when this agent was administered 6 hr later, at the time when the cells had recovered from the inhibition of DNA synthesis. When S-phase cells were exposed to 5-aza-C, the maximal cell kill produced by ara-C also occurred 5 to 6 hr later. When the S-phase cells were exposed simultaneously to both ara-C and 5-aza-C, significant antagonism with respect to cytotoxicity was observed between these 2 agents. When cells in G1 were exposed to 5-aza-C, the cytotoxicity produced by ara-C on these cells when they entered S phase was additive with respect to the cytotoxicity produced by 5-aza-C exposure alone.     

Detection of S-phase overreplication following chronic hypoxia using a monoclonal anti-BrdUrd

We have examined cytokinetic perturbations induced in Chinese hamster V-79 cells in vitro during and following exposure to chronic hypoxia employing simultaneous flow cytometric measurement of incorporated BrdUrd and DNA content. These data indicate hypoxia inhibited G1 progression into S-phase, but did not significantly delay G2M division and progression into G1. Also, upon reaeration after 20 hr in hypoxia, cells originally in G1 exhibited significant kinetic delay. BrdUrd pulse/chase and pulse/fix data indicated DNA replication was reduced, but not completely inhibited during hypoxia. Also, between 6 and 20 hr of chronic hypoxia and following reaeration, a subset of the original S-phase cells overreplicated their DNA, such that these cells had greater than 2C DNA content. This subpopulation was estimated on the average to comprise approximately 20% of the total population (30% of the treated S-phase subpopulation) by 24 hr following reaeration after 20 hr hypoxia. These results are discussed in light of the similarities between overreplication and gene amplification observed under certain conditions with other agents, which like chronic hypoxia, are used to transiently disrupt DNA synthesis.     

Lethal and cytokinetic effects of anguidine on a human colon cancer cell line

Anguidine is a fungal metabolite with antitumor activity in a murine colon cancer model. Because of disappointing results in clinical trials, we analyzed the lethal and cytokinetic effects of anguidine on cultured human colon cancer cells. The studies revealed a moderate reduction in survival only after prolonged drug exposure. Continuous incubation with anguidine for longer than 48 hr produced a moderate increase in the percentage of S-phase cells and a slight decrease in the proportion of cells in G1/0, by pulse cytophotometry. An immediate reduction in the cumulative labeling index for cells continuously exposed to tritiated thymidine and anguidine and a rapid decrease in the cumulative mitotic index for cells continuously exposed to Colcemid and anguidine indicated a block at the G1 into S and G2 into mitosis transitions. Tumoricidal activity of anguidine in a cultured human colon cancer line is poor and requires prolonged exposure. The kinetic data reflect an almost frozen state of the cell cycle.     

Pol eta is required for DNA replication during nucleotide deprivation by hydroxyurea

Hydroxyurea reduces DNA replication by nucleotide deprivation, whereas UV damage generates DNA photoproducts that directly block replication fork progression. We show that the low fidelity class Y polymerase Pol eta is recruited to proliferating cell nuclear antigen at replication forks both by hydroxyurea and UV light. Under nucleotide deprivation, Pol eta allows cells to accumulate at the G1/S boundary by facilitating slow S-phase progression and promotes apoptosis. Normal cells consequently enter apoptosis at a faster rate than Pol eta-deficient cells. Coincident with hydroxyurea-induced S-phase delay, Pol eta-deficient cells undergo more replication fork breakage and accumulate more foci of the Mre11/Rad50/Nbs1 complex and phosphorylated histone H2AX. We conclude that under conditions of nucleotide deprivation, Pol eta is required for S-phase progression but is proapoptotic. However, as Pol eta is reported to require higher nucleotide concentrations than class B replicative polymerases, its recruitment by hydroxyurea requires it to function under suboptimal conditions. Our results suggest that hydroxyurea-induced apoptosis occurs at the G1/S boundary and that initiation of the S-phase requires greater nucleotide concentrations than does S-phase progression.     

Characterization of B16 melanoma cells resistant to the CC-1065 analogue U-71,184

U-71,184 is a CC-1065 analogue which is highly cytotoxic in vitro and has a broad spectrum of antitumor activity in vivo. Against B16 cells, U-71,184 was 8-fold and 253-fold more potent than Actinomycin D and Adriamycin, respectively. U-71,184 killed 90% of B16 cells at 0.01 ng/ml levels of drug in the medium, which was equivalent to an intracellular concentration of about 8 pg/10(6) cell (= 2 x 10(-8) pmol/cell). A B16 cell line resistant to U-71,184 developed after 3 months of in vitro exposure to gradually increasing concentrations of the drug. The sensitive and resistant cell lines were cloned and a B16/R clone was selected which was 60 to 100 times more resistant to U-71,184 than the cloned sensitive parent (B16/S). Cells grown in the absence of U-71,184 for 2 months retained resistance to the drug. B16/R was slightly cross-resistant only to Adriamycin but not to Actinomycin D, vinblastine, or colchicine. Among alkylating agents, it was slightly cross-resistant to Melphalan but not to 1,3-bis(2-chloroethyl)-1-nitrosourea or cisplatin. B16/R did not overexpress mdr mRNA. Therefore, this cell line does not exhibit the multidrug-resistant phenotype. Most karyotypes of B16/R had a marker chromosome which carried an aberrantly staining region apparently containing repetitive replication of the same segment. Resistance can be partly accounted for by the approximately 10-fold lesser uptake of [3H]-U-71,184 in B16/R, as compared to B16/S. B16/R was cross-resistant in varying degrees to several other CC-1065 analogues. The ratio of the 50% lethal dose of U-71,184 for B16/R, as compared to B16/S, was about 60 (i.e., R/S = 60). In comparison, the following compounds had an R/S ratio of less than 20 (i.e., modest level of cross-resistance to U-71,184): U-68,819, U-73,975, U-75,500, U-75,559, and CC-1065. In contrast, the following compounds had an R/S ratio greater than 20 (i.e., highly cross-resistant to U-71,184): U-71,184 analogues U-71,185, U-73,903, and U-75,012; U-73,975 analogues U-75,613, U-75,032, and U-73,896; and CC-1065 enantiomer U-76,915. We cannot yet explain the difference in the level of cross-resistance between these compounds in vitro. B16/S and B16/R cells were tumorigenic in mice and B16/R was resistant to U-71,184 in vivo. There was no clear indication of cross-resistance of B16/R in vivo to Adriamycin, Actinomycin D, cisplatin, or Melphalan. However, U-73,975, a compound with modest cross-resistance in vitro, was significantly cross-resistant in vivo.     

Effects of tamoxifen on cell cycle progression of synchronous MCF-7 human mammary carcinoma cells

The effects of tamoxifen on cell cycle progression and clonogenic survival have been examined using synchronized cultures of MCF-7 human mammary carcinoma cells. Cell synchrony was induced by mitotic selection. Subsequent cell cycle analyses, using DNA flow cytometry, showed that 85% of synchronized cells had a mean cell cycle time of 21.3 hr with mean phase durations of 9 hr for G0-G1, 9.3 hr for S, and 3 hr for G2 + M. A slowly cycling or noncycling subpopulation comprising 15% of the total population was also observed. Exposure to tamoxifen (5 to 12.5 microM) resulted in a dose-dependent reduction in the number of cells progressing through G0-G1 and entering S phase. Those cells which were not retained in G0-G1, however, appeared to traverse G0-G1 and the remainder of the cell cycle at a rate only slightly less than that of untreated controls. Further experiments demonstrated that the major sensitivity to tamoxifen in terms of both inhibition of cell cycle progression and drug cytotoxicity was restricted to a short interval in the middle of G0-G1. This 2- to 4-hr period of maximum drug sensitivity began approximately 4 hr after mitotic selection, with drug exposures outside this time frame having markedly fewer effects. The significance of these observations in the light of previous studies with asynchronous populations of MCF-7 cells is discussed.     

Flow microfluorimetric analysis of sensitive and resistant leukemia L1210 following 1-beta-D-arabinofuranosylcytosine in vivo.

Two groups of BALB/c X DBA/S F1 mice with sensitive and 1-beta-D-arabinofuranosylcytosine-resistant L1210 ascites, respectively, were used to study the cytokinetic effects of a single dose of 750 mg of 1-beta-D-arabinofuranosylcytosine per kg. Sequential DNA histograms, labeling indices, and mitotic indices were obtained from each of five mice at timed intervals from 0 to 72 hr. Each histogram was obtained by flow microfluorimetry, using the DNA-specific fluorochrome, mithramycin. The histograms were then integrated for cell cycle analysis. Cytokinetic perturbations occurred in both groups, but they were greater in the sensitive population where there was a relative accumulation of cells, mainly in early S phase, at 16 hr. This was followed by a relative depletion of S-phase cells. In the resistant population, there were relative accumulation of cells in early S phase at 8 and 24 hr and in mid-S phase at 32 hr, but there was no subsequent relative depletion of S-phase cells. The labeling index was rapidly reduced in both groups but was recovered in the resistant population within 4 hr. In the sensitive population, there was a transient rise in the labeling index at 8 and 16 hr. Sensitive and resistant populations of L1210 cells were rapidly and reliably distinguished by DNA content cell cycle analysis of a single sample taken between 24 and 72 hr following a large dose of 1-beta-D-arabinofuranosylcytosine. This technique has potential clinical application in the rational design and monitoring of chemotherapy.     

Flow cytometric analysis by bromodeoxyuridine/DNA assay of cell cycle perturbation of methotrexate-treated mouse L1210 leukemia cells

The in vitro effects of methotrexate (MTX) on cell cycle progression and DNA synthesis of L1210 leukemia cells were studied by the bromodeoxyuridine (BrdUrd)/DNA analysis technique. Low dose (10(-8) M) MTX, which slightly inhibits clonal replication of the cells, delays progress across the S phase, and treatment for 24 h results in a slight increase of the S-phase population. Much higher doses (10(-7) M and 10(-6) M) of MTX, which strongly reduce the clonogenicity, prevented the progression of cells at the G1-S boundary and across the S phase, but not in the other phases. The cells arrested at the G1-S boundary were able to incorporate BrdUrd in the medium for 6-12 h after the start of treatment and then lost the ability to incorporate BrdUrd. By determining the colony inhibitory activity of MTX, it could be shown that not only S-phase cells but non-S-phase cells are susceptible to cytotoxicity of MTX. MTX-induced S-phase arrest is closely associated with an alteration in the distribution of BrdUrd-labeled cells, and MTX apparently inhibits BrdUrd incorporation into L1210 cells as the dose and duration of treatment increase. These results suggest that MTX-induced cell cycle perturbation is related to inhibition of DNA synthesis.     

Interaction of CC-1065 and its analogues with mouse DNA and chromatin

CC-1065 is a potent antitumor antibiotic which is cytotoxic to P388 and L1210 leukemia cells in vitro and in vivo. CC-1065 covalently binds to calf thymus DNA preferentially to adenine-thymine regions at N3 of adenine. Here, we compare CC-1065 interaction with P388-derived chromatin, DNA, and histones as measured by electronic absorption and circular dichroism. Two CC-1065 analogues (U-71,184 and its enantiomer, U-71,185) which show different biological activities from CC-1065 were also studied. The shape and temporal behavior of the induced circular dichroism curves generated by CC-1065 or its analogues bound to chromatin were similar to CC-1065 plus DNA. This suggested that CC-1065 and its analogues bind to the minor groove of chromatin DNA in a manner similar to calf thymus DNA. However, the binding of CC-1065 and its analogues to DNA induced a more intense circular dichroism band than binding to chromatin. The order of interaction for both chromatin and DNA was CC-1065 greater than U-71,184 greater than U-71,185. In contrast to the essentially irreversible binding to DNA after 24-h incubation, binding to chromatin was primarily a reversible interaction, the degree of reversibility being U-71,185 greater than U-71,184 = CC-1065. CC-1065 binds weakly and nonspecifically to histones.     

Cyclic AMP, the cytoskeleton and control of cell replication. I. Relationship of cyclic AMP, microtubules and density-dependent growth

CHO cells after being treated by Bt2cAMP (1M), would reverse to the fibroblastoid cell morphologically. The cell body appears spindle, more orientational and polarized. The Bt2cAMP, rendering the cells grow were slowly, did not block them from G1 to S phase. The "contact inhibition" was restored to some degree. The most saturated density of the cells was reduced to 47.4% as compared with the control CHO cells. When CHO cells were treated by Bt2cAMP added with Colcemid (0.05 micrograms/ml), the microtubules (MT) were destroyed. Still, the cells were able to reach the "contact inhibition". The signal of "contact inhibition" may not approach the nucleus via MT.     

Etoposide-induced cell cycle delay and arrest-dependent modulation of DNA topoisomerase II in small-cell lung cancer cells.

As an approach to the rational design of combination chemotherapy involving the anti-cancer DNA topoisomerase II poison etoposide (VP-16), we have studied the dynamic changes occurring in small-cell lung cancer (SCLC) cell populations during protracted VP-16 exposure. Cytometric methods were used to analyse changes in target enzyme availability and cell cycle progression in a SCLC cell line, mutant for the tumour-suppressor gene p53 and defective in the ability to arrest at the G1/S phase boundary. At concentrations up to 0.25 microM VP-16, cells became arrested in G2 by 24 h exposure, whereas at concentrations 0.25-2 microM G2 arrest was preceded by a dose-dependent early S-phase delay, confirmed by bromodeoxyuridine incorporation. Recovery potential was determined by stathmokinetic analysis and was studied further in aphidicolin-synchronised cultures released from G1/S and subsequently exposed to VP-16 in early S-phase. Cells not experiencing a VP-16-induced S-phase delay entered G2 delay dependent upon the continued presence of VP-16. These cells could progress to mitosis during a 6-24 h period after drug removal. Cells experiencing an early S-phase delay remained in long-term G2 arrest with greatly reducing ability to enter mitosis up to 24 h after removal of VP-16. Irreversible G2 arrest was delimited by the induction of significant levels of DNA cleavage or fragmentation, not associated with overt apoptosis, in the majority of cells. Western blotting of whole-cell preparations showed increases in topoisomerase II levels (up to 4-fold) attributable to cell cycle redistribution, while nuclei from cells recovering from S-phase delay showed enhanced immunoreactivity with an anti-topoisomerase II alpha antibody. The results imply that traverse of G1/S and early S-phase in the presence of a specific topoisomerase II poison gives rise to progressive low-level trapping of topoisomerase II alpha, enhanced topoisomerase II alpha availability and the subsequent irreversible arrest in G2 of cells showing limited DNA fragmentation. We suggest that protracted, low-dose chemotherapeutic regimens incorporating VP-16 are preferentially active towards cells attempting G1/S transition and have the potential for increasing the subsequent action of other topoisomerase II-targeted agents through target enzyme modulation. Combination modalities which prevent such dynamic changes occurring would act to reduce the effectiveness of the VP-16 component.     

Cellular mechanisms associated with the lack of chronic thermotolerance expression in HeLa S3 cells.

Chronic thermotolerance is an operational definition for that resistance to cell killing by heat which develops during a protracted exposure at temperatures generally in the range of 41.5-42.5 degrees C which is usually observed as a reduction in the slope of the survival curve. While Chinese hamster ovary (CHO) cells are generally more sensitive to high-temperature heat shock than HeLa cells, studies of cells maintained in suspension culture at 41.5 degrees C demonstrated CHO cells to be more resistant to cell killing at this temperature than HeLa cells, due to the expression of chronic thermotolerance in the hamster cell line and the corresponding lack of chronic thermotolerance expression in the HeLa cell line. Experiments were conducted in the two cell lines while heating under identical conditions, in order to detect any cell line-specific changes in heat-induced perturbation of cell cycle progression and the expression of chronic thermotolerance. Our results showed that CHO cells exhibited a G1 block which lasted throughout the course of the 32-h heating period. HeLa cells, however, failed to accumulate in G1, progressing instead into S phase where spontaneous premature chromosome condensation and nuclear fragmentation were observed. This accumulation of cells with condensed chromatin possessing S-phase DNA content exhibited a linear, one-to-one functional relationship with the fraction of dead cells. Previous studies (M.A. Mackey and W.C. Dewey, Int. J. Hyperthermia, 5:405-415, 1989) demonstrated that synchronized S-phase CHO cells heated at 41.5 degrees C and 42 degrees C were unable to express chronic thermotolerance. Therefore, we hypothesize that progression of cells out of G1 phase into S and G2-M phases leads to lethal processes that prevent the expression of chronic thermotolerance in the HeLa cell line. This hypothesis is strengthened by the observed correlation between the accumulation of "mitotic-like" cells and decreased survival, suggesting that the G1 block observed in CHO cells is causally connected with the expression of chronic thermotolerance.     

Mutagenicity of the antitumor antibiotic CC-1065 and its analogues in mammalian (V79) cells and bacteria

CC-1065 is a very potent antitumor antibiotic which binds in the minor groove of DNA with alkylation at N-3 of adenine. Since CC-1065 caused delayed deaths in mice at therapeutic doses, analogues were prepared whose antitumor and biochemical activities have been reported. In this study, the mutagenicity for V79 cells (6-thioguanine resistance) and Salmonella (histidine auxotrophy or azaguanine resistance) of selected analogues was compared to DNA-binding activity and the structure-activity relationship was determined. CC-1065, U-62,736, U-66,866, U-66,694, U-67,786, and U-68,415 all have an A segment with an intact cyclopropyl group and different B segments. The cyclopropyl group is absent from U-66,226 and U-63,360. Elimination of the cyclopropyl ring diminished the cytotoxic and mutagenic potency of the compounds such that U-63,360 was nearly three orders of magnitude less potent than CC-1065 in V79 cells. For the compounds with an intact cyclopropyl group, the order of cytotoxic and mutagenic potency (molar basis) in V79 cells generally correlated with binding to calf thymus DNA, and increased with the length of the B segment. Thus, the order of cytotoxicity was CC-1065 greater than U-68,415 greater than U-66,694 greater than U-66,866 greater than U-62,736. U-67,786 fell outside this pattern since it was more cytotoxic and mutagenic than U-66,694, although it was of a similar size and had similar DNA-binding activity. These results show that an electrophilic carbon afforded by an intact cyclopropyl group of this type is necessary but not sufficient to account for the high cytotoxic and mutagenic potency of CC-1065 and U-68,415. The size and characteristics of the B segment also affect the potency. At an equitoxic (10 or 50% lethal dose) dose, an inverse relationship exists between cytotoxic and mutagenic potency such that at the 50% lethal dose, the least cytotoxic compound (U-62,736) was more mutagenic than the most cytotoxic compound (CC-1065). We speculate that the more cytotoxic analogues are less mutagenic (at an equitoxic dose) because they may have greater structure-directed binding to less mutable DNA sites in the minor groove.