Contribution of gene-specific lesions, DNA-replication-associated damage, and subsequent transcriptional inhibition in topoisomerase inhibitor-mediated apoptosis in lymphoma cells

Liposomal delivery of MicroRNA-7-expressing plasmid overcomes epidermal growth factor receptor tyrosine kinase inhibitor-resistance in lung cancer cells

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI) have been strikingly effective in lung cancers harboring activating EGFR mutations. Unfortunately, the cancer cells eventually acquire resistance to EGFR-TKI. Approximately 50% of the acquired resistance involves a secondary T790M mutation. To overcome the resistance, we focused on EGFR suppression using microRNA-7 (miR-7), targeting multiple sites in the 3'-untranslated region of EGFR mRNA. Two EGFR-TKI-sensitive cell lines (PC-9 and H3255) and two EGFR-TKI-resistant cell lines harboring T790M (RPC-9 and H1975) were used. We constructed miR-7-2 containing miR-7-expressing plasmid. After transfection of the miR-7-expressing plasmid, using cationic liposomes, a quantitative PCR and dual luciferase assay were conducted to examine the efficacy. The antiproliferative effect was evaluated using a cell count assay and xenograft model. Protein expression was examined by Western blotting. The miR-7 expression level of the transfectants was approximately 30-fold higher, and the luciferase activity was ablated by 92%. miR-7 significantly inhibited cell growth not only in PC-9 and H3255 but also in RPC-9 and H1975. Expression of insulin receptor substrate-1 (IRS-1), RAF-1, and EGFR was suppressed in the four cell lines. Injection of the miR-7-expressing plasmid revealed marked tumor regression in a mouse xenograft model using RPC-9 and H1975. EGFR, RAF-1, and IRS-1 were suppressed in the residual tumors. These findings indicate promising therapeutic applications of miR-7-expressing plasmids against EGFR oncogene-addicted lung cancers including T790M resistance by liposomal delivery.

Arsenic trioxide cooperates with all trans retinoic acid to enhance mitogen-activated protein kinase activation and differentiation in PML-RARalpha negative human myeloblastic leukemia cells

Arsenic trioxide (ATO) synergistically promotes all trans retinoic acid (ATRA)-induced differentiation of PML-RARalpha negative HL-60 myeloblastic leukemia cells. In PML-RARalpha positive myeloid leukemia cells, ATO is known to cause degradation of PML-RARalpha with subsequent induced myeloid differentiation. We found that ATO by itself does not cause differentiation of the PML-RARalpha negative HL-60 cells, but enhances ATRA's capability to cause differentiation. ATO augmented ATRA-induced RAF/MEK/ERK axis signaling, expression of CD11b and p47(PHOX), and inducible oxidative metabolism. ATO enhanced ATRA-induced population growth retardation without evidence of apoptosis or enhanced G1/G0 growth arrest. Compared to ATRA-treated cells, the ATRA plus ATO-treated cells progressed more slowly through the cell cycle as detected by a slower rate of accumulation in G2/M following nocodazole treatment. Hoechst/PI staining showed that low-dose ATO did not induce apoptosis. In summary, our results indicate that ATO in conjunction with ATRA is of potential chemotherapeutic use in PML-RARalpha negative leukemias.

Targeting RNA polymerase I with an oral small molecule CX-5461 inhibits ribosomal RNA synthesis and solid tumor growth

Deregulated ribosomal RNA synthesis is associated with uncontrolled cancer cell proliferation. RNA polymerase (Pol) I, the multiprotein complex that synthesizes rRNA, is activated widely in cancer. Thus, selective inhibitors of Pol I may offer a general therapeutic strategy to block cancer cell proliferation. Coupling medicinal chemistry efforts to tandem cell- and molecular-based screening led to the design of CX-5461, a potent small-molecule inhibitor of rRNA synthesis in cancer cells. CX-5461 selectively inhibits Pol I-driven transcription relative to Pol II-driven transcription, DNA replication, and protein translation. Molecular studies demonstrate that CX-5461 inhibits the initiation stage of rRNA synthesis and induces both senescence and autophagy, but not apoptosis, through a p53-independent process in solid tumor cell lines. CX-5461 is orally bioavailable and demonstrates in vivo antitumor activity against human solid tumors in murine xenograft models. Our findings position CX-5461 for investigational clinical trials as a potent, selective, and orally administered agent for cancer treatment.

Evaluation for safety of antioxidant chemopreventive agents

Antioxidants are considered as the most promising chemopreventive agents against various human cancers. However, some antioxidants play paradoxical roles, acting as "double-edged sword." A primary property of effective and acceptable chemopreventive agents should be freedom from toxic effects in healthy population. Miscarriage of the intervention by beta-carotene made us realize the necessity for evaluation of safety before recommending use of antioxidant supplements for chemoprevention. We have evaluated the safety of antioxidants on the basis of reactivity with DNA. Our results revealed that phytic acid, luteolin, and retinoic acid did not cause DNA damage under the experimental condition. Furthermore, phytic acid inhibited the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine, an indicator of oxidative DNA damage, in cultured cells treated with a H(2)O(2)-generating system. Thus, it is expected that these chemopreventive agents can safely protect humans against cancer. On the other hand, some chemopreventive agents with prooxidant properties (alpha-tocopherol, quercetin, catechins, isothiocyanates, N-acetylcysteine) caused DNA damage via generation of reactive oxygen species in the presence of metal ions and endogenous reductants under some circumstances. Furthermore, other chemopreventive agents (beta-carotene, genistein, daidzein, propyl gallate, curcumin) exerted prooxidant properties after metabolic activation. Therefore, further studies on safety should be required when antioxidants are used for cancer prevention.

Inhibition of ischemia/reperfusion-induced damage by dexamethasone in isolated working rat hearts: the role of cytochrome c release

We investigated the contribution of dexamethasone treatment on the recovery of postischemic cardiac function and the development of reperfusion-induced arrhythmias in ischemic/reperfused isolated rat hearts. Rats were treated with 2 mg/kg of intraperitoneal injection of dexamethasone, and 24 hours later, hearts were isolated according to the 'working' mode, perfused, and subjected to 30 min global ischemia followed by 120 min reperfusion. Cardiac function including heart rate, coronary flow, aortic flow, and left ventricular developed pressure were recorded. After 60 min and 120 min reperfusion, 2 mg/kg of dexamethasone significantly improved the postischemic recovery of aortic flow and left ventricular developed pressure from their control values of 10.7 +/- 0.3 ml/min and 10.5 +/- 0.3 kPa to 22.2 +/- 0.3 ml/min (p < 0.05) and 14.3 +/- 0.5 kPa (p < 0.05), 19.3 +/- 0.3 ml/min (p < 0.05) and 12.3 +/- 0.5 kPa (p < 0.05), respectively. Heart rate and coronary flow did not show a significant change in postischemic recovery after 60 or 120 min reperfusion. In rats treated with 0.5 mg/kg of actinomycin D injected i.v., one hour before the dexamethasone injection, suppressed the dexamethasone-induced cardiac protection. Electrocardiograms were monitored to determine the incidence of reperfusion-induced ventricular fibrillation. Dexamethasone pretreatment significantly reduces the occurrence of ventricular fibrillation. Cytochrome c release was also observed in the cytoplasm. The results suggest that the inhibition of cytochrome c release is involved in the dexamethasone-induced cardiac protection.

Apoptotic pathways of epothilone BMS 310705

OBJECTIVE

BMS 310705 is a novel water-soluble analog of epothilone B currently in phase I clinical evaluation in the treatment of malignancies such as ovarian, renal, bladder, and lung carcinoma. Using an early passage cell culture model derived from the ascites of a patient clinically refractory to platinum/paclitaxel therapy, we evaluated the pathway of caspase-mediated apoptosis.

METHODS

Cells were treated for 1 h and subsequently evaluated for apoptosis, survival, and caspase activity. Apoptosis was determined by fluorescent microscopy. Caspase-3, -8, and -9 activities were determined by fluorometry using target tetrapeptide substrates. Mitochondrial release of cytochrome c was determined by immunoblot analysis.

RESULTS

After treatment with BMS 310705, apoptosis was confirmed in >25% of cells at 24 h. Survival was significantly lower (P < 0.02) in cells treated with 0.05 micro M BMS 310705 vs paclitaxel. Analysis revealed an increase of caspase-9 and -3 activity; no caspase -8 activity was observed. Release of cytochrome c was detected at 12 h following treatment. SN-38 and topotecan failed to induce apoptosis.

CONCLUSIONS

BMS 310705 induces significant apoptosis, decreases survival, and utilizes the mitochondrial-mediated pathway for apoptosis in this model.

Cross-linking of surface IgM in the Burkitt's lymphoma cell line ST486 provides protection against arsenite- and stress-induced apoptosis that is mediated by ERK and phosphoinositide 3-kinase signaling pathways

The ST486 cell line, derived from a human Burkitt's lymphoma, is a model for antigen-induced clonal deletion in germinal center B-lymphocytes, with apoptosis induced upon cross-linking of surface IgM. Moreover, this cell line is highly sensitive to the induction of apoptosis by many chemicals, including sodium arsenite, a significant environmental contaminant with immunotoxic activity. In contrast to arsenite and other chemicals, surface IgM cross-linking induces apoptosis in ST486 cells with delayed kinetics. Moreover, the initial signaling events following IgM stimulation are associated with cell survival and proliferation and include activation of the extracellular-signal regulated kinase (ERK) and the phosphoinositide 3-kinase (PI3K) pathways. We examined the question of whether IgM-mediated activation of the ERK and PI3K pathways can influence the apoptotic response of ST486 cells following exposure to arsenite and selected drugs with different molecular targets, including cycloheximide, etoposide, and camptothecin, and a physical stress, hyperthermia. Our findings show that IgM-stimulated cells are significantly protected against arsenite and drug-induced apoptosis during a window of several hours after surface IgM cross-linking, as evidenced by an inhibition of cleavage of poly(ADP-ribose) polymerase and lack of morphological changes indicative of apoptosis. Significantly, surface IgM cross-linking also protects against arsenite-induced mitochondrial depolarization as well as caspase-9 cleavage. Furthermore, we demonstrate that this IgM-mediated protection requires the activation of the ERK and PI3K pathways, because inhibition of either pathway blocks the ability of antigen receptor activation to protect against apoptosis. Our study also provides evidence for p90(S6) ribosomal kinase as a point of convergence between the two signaling pathways resulting in the phosphorylation of the pro-apoptotic Bcl-2 family member Bad at serine 112. This investigation demonstrates, for the first time, that specific signals transduced by activation of the B-cell receptor protect cells at a common point of regulation in the apoptotic pathways for diverse stresses.

Using CTLL-2 and CTLL-2 bcl2 cells to avoid interference by apoptosis in the in vitro micronucleus test

In vitro assays for chromosome aberrations (i.e., in vitro micronucleus and in vitro metaphase analysis tests) frequently produce false-positive or exaggerated-positive results. Our previous work suggested that apoptosis interferes with these tests, producing misleading results. These previous studies were conducted by performing the in vitro micronucleus test in CTLL-2 cells and a CTLL-2 cell derivative stably transfected with the apoptosis inhibitor gene bcl2. In the present study, these previous observations were extended by examining micronucleus induction with a larger number of compounds in both CTLL-2 and CTLL-2 bcl2 cells and measuring apoptosis with annexin V-FITC. Both cell lines were treated with different classes of compounds that were anticipated to be exclusively apoptosis inducers, or compounds known to be clastogens or aneugens, some of which were anticipated to be both genotoxic and apoptotic. We were able to confirm that compounds that are only apoptogenic induced micronuclei in CTLL-2 but not CTLL-2 bcl2 cells, indicating that the positive responses are due to apoptosis in CTLL-2 cells. Some genotoxins (clastogens and aneugens) did not produce apoptosis by the annexin V assay and gave similar responses in CTLL-2 and CTLL-2 bcl2 cells. Finally, higher responses were induced in CTLL-2 cells compared to CTLL-2 bcl2 cells that were treated with aneugens or clastogens that were also apoptosis inducers, suggesting that the greater response in CTLL-2 cells is a consequence of both genotoxicity and apoptosis. Finally, it was demonstrated that just eliminating CTLL-2 cells having three or more micronuclei from scoring was not adequate for correctly evaluating agents that only produce apoptosis. The results indicate that coupling the in vitro micronucleus test in both CTLL-2 and CTLL-2 bcl2 cells with the measurement of apoptosis is able to distinguish the genotoxic effects of a test compound from its apoptotic potential and is able to avoid interference from apoptosis in the in vitro micronucleus test. These observations may provide the basis for a useful genotoxicity assay.

In vitro cytotoxicity assay to screen compounds for apoptosis-inducing potential on lymphocytes and neutrophils

In vitro cytotoxicity assay to screen compounds for apoptosis-inducing potential on lymphocytes and neutrophils was investigated. Mouse, rat, dog, and human whole blood were incubated for 4 and 6 hr with actinomycin D, camptothecin, cortisone acetate, cycloheximide, doxorubicin, etoposide, 5-FU, mitomycin C and puromycin. Apoptotic lymphocytes and neutrophils were counted. All test compounds induced in vitro apoptosis of lymphocytes and/or neutrophils, but there were different potencies among the test compounds and there were also species differences in susceptibility. To investigate the in vivo effects of etoposide and cycloheximide which induced apoptosis of rat lymphocytes and that of rat lymphocytes and neutrophils, respectively, in in vitro assay, rats were intravenously administered either etoposide at 12.5, 25 or 50 mg/kg or cycloheximide at 1.25, 2.5 or 5 mg/kg. Etoposide caused decreases of circulating lymphocytes at 3 hr after administration in a dose-dependent manner, -16, -25 and -51%. Although cycloheximide caused neither decreased lymphocyte nor neutrophil counts, apoptosis in 30% of neutrophils was observed in rats receiving 5 mg/kg at 3 hr after administration. Etoposide at 50 mg/kg and cycloheximide at 5 mg/kg caused lymphocyte apoptosis in the spleen, thymus, mesenteric lymph nodes, bone marrow, and Peyer's patch from 1 to 6 hr after administration, with the maximum changes at 3 hr. In addition to apoptosis of these organs, cycloheximide at 5 mg/kg caused apoptosis of polymorphonuclear cells in the lamina propria of the small intestine. Therefore, it was found that the changes seen in the in vivo experiments considerably reflected the changes seen in the in vitro experiments. From these results, apoptosis is probably one of the major mechanisms for leukocyte toxicity induced by cytotoxic compounds, and the in vitro assay to screen compounds for acute apoptosis-inducing potential on lymphocytes and neutrophils would be useful as a primary screening method for animal toxicity studies. It may also be useful for risk assessments in advance of clinical trials.

Differential induction of apoptosis and MAP kinase signaling by mitochondrial toxicants in drug-sensitive compared to drug-resistant B-lineage lymphoid cell lines

A panel of human B-lineage lymphoma cell lines differing in cancer drug-resistance status and Bcl-2/Bax expression were used to study the contribution of mitochondrial-based perturbations and regulation in differential induction of apoptosis. Mitochondrial dysfunction was induced in cells by the uncoupler carbonyl cyanide m-chlorophenylhydrazone (mClCCP) and the respiratory chain inhibitor antimycin A. Cells were then assayed for early changes in MAP kinase signaling and subsequent induction of apoptosis. The cancer drug-resistant cell lines EW36 and CA46, overexpressing Bcl-2 and deficient in Bax, respectively, were both resistant to mitochondrial toxicant-induced cleavage of poly(ADP-ribose) polymerase (PARP) and morphologically detectable apoptotic cell death. In contrast, cancer drug-sensitive ST486 cell line, with low Bcl-2 expression, was sensitive to PARP cleavage and apoptosis engagement. Interestingly, mClCCP induced twofold more apoptosis than antimycin A in the ST486 cells. Exposure to the mitochondrial toxicants resulted in the early and preferential activation of the ERK and p38 MAP kinase pathways in only the drug-sensitive ST486 cell line, with mClCCP more potent than antimycin A. Specific inhibition of the p38 pathway augmented baseline and mClCCP-induced apoptosis. These results show that multi-drug-resistant and -sensitive B-lineage cells are also resistant and sensitive to compounds inducing mitochondrial dysfunction. The differential sensitivity to mitochondrial toxicant effects involved regulation by MAP kinases, since ERK and p38 were found to be preferentially activated only in the drug-sensitive B-lineage cells. Modulation of the p38 signaling pathway altered the sensitivity of cells to mitochondrial stress and may play a more general role in regulating the sensitivity of B-lineage cells to drugs and environmental toxicants.

Polyomavirus small t antigen prevents retinoic acid-induced retinoblastoma protein hypophosphorylation and redirects retinoic acid-induced G0 arrest and differentiation to apoptosis

Polyomavirus small t antigen (ST) impedes late features of retinoic acid (RA)-induced HL-60 myeloid differentiation as well as growth arrest, causing apoptosis instead. HL-60 cells were stably transfected with ST. ST slowed the cell cycle, retarding G2/M in particular. Treated with RA, the ST transfectants continued to proliferate and underwent apoptosis. ST also impeded the normally RA-induced hypophosphorylation of the retinoblastoma tumor suppressor protein consistent with failure of the cells to arrest growth. The RA-treated transfectants expressed CD11b, an early cell surface differentiation marker, but inducible oxidative metabolism, a later and more mature functional differentiation marker, was largely inhibited. Instead, the cells underwent apoptosis. ST affected significant known components of RA signaling that result in G0 growth arrest and differentiation in wild-type HL-60. ST increased the basal amount of activated ERK2, which normally increases when wild-type cells are treated with RA. ST caused increased RARalpha expression, which is normally down regulated in RA-treated wild-type cells. The effects of ST on RA-induced myeloid differentiation did not extend to monocytic differentiation and G0 arrest induced by 1,25-dihydroxy vitamin D3, whose receptor is also a member of the steroid-thyroid hormone superfamily. In this case, ST abolished the usually induced G0 arrest and retarded, but did not block, differentiation without inducing apoptosis, thus uncoupling growth arrest and differentiation. In sum, the data show that ST disrupted the normal RA-induced program of G0 arrest and differentiation, causing the cells to abort differentiation and undergo apoptosis.

Cdc42 is a substrate for caspases and influences Fas-induced apoptosis

Fas-mediated apoptosis results in the activation of caspases, which subsequently cleave cellular substrates that are essential for normal cell viability. In the present study, we show that the Ras-related GTP-binding protein Cdc42 is susceptible to caspase-catalyzed proteolysis in a number of cell lines, including NIH3T3 fibroblasts, human breast cancer cells (e.g. T47D), and COS-7 cells. Both caspase-3 and caspase-7 were able to catalyze the cleavage of Cdc42, whereas caspase-6 and caspase-8 were without effect. The susceptibility to the caspase-stimulated degradation is specific; although Rac can also serve as a caspase substrate, neither Rho nor Ras is degraded. Caspase sensitivity is conferred by a consensus sequence (DXXD) that lies immediately upstream of the Rho insert regions (residues 122-134) of Cdc42 and Rac. The removal of a stretch of residues (120) that includes the insert region or site-directed mutagenesis of either aspartic acid 118 or 121 within a constitutively active background (i.e. Cdc42(F28L)) as well as a wild-type Cdc42 background yields Cdc42 molecules that provide a marked protection against Fas ligand-induced apoptosis. Overall, these results are consistent with a model in which Cdc42 acts downstream of Fas, perhaps to influence the rate of apoptosis, with the ultimate caspase-mediated degradation of Cdc42 then allowing for a maximal apoptotic response.

Roles of NF-kappaB and 26 S proteasome in apoptotic cell death induced by topoisomerase I and II poisons in human nonsmall cell lung carcinoma

Activation of signaling pathways after DNA damage induced by topoisomerase (topo) poisons can lead to cell death by apoptosis. Treatment of human nonsmall cell lung carcinoma (NSCLC-3 or NSCLC-5) cells with the topo I poison SN-38 or the topo II poison etoposide (VP-16) leads to activation of NF-kappaB before induction of apoptosis. Inhibiting the degradation of IkappaBalpha by pretreatment with the proteasome inhibitor MG-132 significantly inhibited NF-kappaB activation and apoptosis but not DNA damage induced by SN-38 or VP-16. Transfection of NSCLC-3 or NSCLC-5 cells with dominant negative mutant IkappaBalpha (mIkappaBalpha) inhibited SN-38 or VP-16 induced transcription and DNA binding activity of NF-kappaB without altering drug-induced apoptosis. Regulation of apoptosis by mitochondrial release of cytochrome c and activation of pro-caspase 9 followed by cleavage of poly(ADP-ribose) polymerase by effector caspases 3 and 7 was similar in neo and mIkappaBalpha cells treated with SN-38 or VP-16. In contrast to pretreatment with MG-132, exposure to MG-132 after SN-38 or VP-16 treatment of neo or mIkappaBalpha cells decreased cell cycle arrest in the S/G2 + M fraction and enhanced apoptosis compared with drug alone. In summary, apoptosis induced by topoisomerase poisons in NSCLC cells is not mediated by NF-kappaB but can be manipulated by proteasome inhibitors.

Simultaneous measurement of the frequencies of intrachromosomal recombination and chromosome gain using the yeast DEL assay

The yeast DEL assay measures the frequency of intrachromosomal recombination between two partially-deleted his3 alleles on chromosome XV. The his3Delta alleles share approximately 400bp of overlapping homology, and are separated by an intervening LEU2 sequence. Homologous recombination between the his3Delta alleles results in deletion of the intervening LEU2 sequence (DEL), and reversion to histidine prototrophy. In this study we have attempted to further extend the use of the yeast DEL assay to measure the frequency of chromosome XV gain events. Reversion to His(+)Leu(+) in the haploid yeast DEL tester strain RSY6 occurs upon non-disjunction of chromosome XV sister chromatids, coupled with a subsequent DEL event. Here we have tested the ability of the yeast DEL assay to accurately predict the aneugenic potential of the diversely-acting, known or suspected aneugens actinomycin D, benomyl, chloral hydrate, ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS), and methotrexate. Actinomycin D and benomyl strongly induced aneuploidy. EMS and methotrexate modestly induced aneuploidy, while chloral hydrate and MMS failed to illicit any significant induction. In addition, by FACS-analysis of DNA content it was shown that the majority of both spontaneous- and chemically-induced His(+)Leu(+) revertants were heterodiploid. Thus, our results indicate endoreduplication of almost entire chromosome sets as a major mechanism of aneuploidy induction in haploid Saccharomyces cerevisiae.

Distinct mechanisms of DNA damage in apoptosis induced by quercetin and luteolin

Quercetin has been reported to have carcinogenic effects. However, both quercetin and luteolin have anti-cancer activity. To clarify the mechanism underlying the carcinogenic effects of quercetin, we compared DNA damage occurring during apoptosis induced by quercetin with that occuring during apoptosis induced by luteolin. Both quercetin and luteolin similarly induced DNA cleavage with subsequent DNA ladder formation, characteristics of apoptosis, in HL-60 cells. In HP 100 cells, an H2O2-resistant clone of HL-60 cells, the extent of DNA cleavage and DNA ladder formation induced by quercetin was less than that in HL-60 cells, whereas differences between the two cell types were minimal after treatment with luteolin. In addition, quercetin increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), an indicator of oxidative DNA damage, in HL-60 cells but not in HP 100 cells. Luteolin did not increase 8-oxodG formation, but inhibited topoisomerase II (topo II) activity of nuclear extract more strongly than quercetin and cleaved DNA by forming a luteolin-topo II-DNA ternary complex. These results suggest that quercetin induces H2O2-mediated DNA damage, resulting in apoptosis or mutations, whereas luteolin induces apoptosis via topo II-mediated DNA cleavage. The H2O2-mediated DNA damage may be related to the carcinogenic effects of quercetin.

Topoisomerase II cleavable complex formation within DNA loop domains

The distribution of VM-26 (Teniposide)-stabilized cleavable complexes within DNA loops bound to the nuclear matrix was determined to provide further insights into the mode of DNA synthesis inhibition by VM-26. Covalent binding of [(3)H]VM-26 was 9-fold greater per milligram of nuclear matrix protein compared with high salt-soluble nonmatrix protein of CEM cells. The ratio declined from 9-fold in CEM cells to 4-fold in drug-resistant VM-1/C2 cells, which have decreased nuclear matrix DNA topoisomerase IIalpha. VM-26 induced a concentration-dependent increase in the frequency of cleavable complex formation with actively replicating matrix DNA. At 25 microM VM-26, the frequency was 32 +/- 2 (SEM) complexes per 10(6) bp of replicating matrix DNA compared with 13 +/- 2 (SEM) complexes per 10(6) bp of nonreplicating DNA in the matrix fraction. VM-26 at concentrations as high as 25 microM stabilized less than 3 complexes per 10(6) bp in the various nonmatrix DNA domains, since the nonmatrix DNA comprises the DNA loop domains that are distal to the matrix-bound replication sites. A negligible frequency of cleavable complex formation was detected in both the matrix and nonmatrix DNA domains of drug-resistant VM-1/C2 cells. Compared with untreated control cells, VM-26 induced an accumulation of nascent DNA in the nuclear matrix fraction of CEM cells but decreased the amount of nascent DNA in the nonmatrix fraction. The extensive cleavable complex formation on matrix replicating DNA stalled most of the replication forks within 1 kb of the replication sites on the nuclear matrix. The results provide evidence that nascent DNA bound to the nuclear matrix is an important site of VM-26 cleavable complex formation, and that these complexes inhibit DNA synthesis by blocking the movement of nascent DNA away from replication sites on the nuclear matrix.

Telomerase activity is not related to apoptosis in leukemic cell lines

Any deregulation of apoptosis or an escape from cellular senescence will drive the cells to neoplasia. It remains unclear whether there is a direct linkage between apoptosis and telomerase activity particularly in transformed cell lines. In the present study, we investigated the telomerase activities in three leukemic cell lines (HL-60, U937 and K562) after treating these cells with various doses of antitumor drugs, puromycin or actinomycin D (ActD). Our results showed that HL-60 cells underwent apoptosis rapidly when treated with either 20 microg/ml of puromycin or 5 microg/ml of Act D with more than 60% of the cells becoming apoptotic at 6 hrs and almost 100% at 12 hrs. But telomerase activity analyzed by TRAP assay in these apoptotic cells remained unchanged as compared with the untreated control cells suggesting that whether the cells were apoptotic or not, it had no effect on telomerase activity. However, if lower dosages of the drugs were used, that is, 0.5-1.5 microg/ml of puromycin or 0.01-0.5 microg/ml of Act D, a decrease in telomerase activity was observed at 24-48 hrs, and was completely undetectable at 72 hrs. This decrease in telomerase activity was dose- and time-dependent. The suppression of telomerase activity by low doses of these two drugs is probably due to the inhibitory effect of the drugs on protein translation or RNA transcription rather than direct inhibition of the telomerase activity. Flow cytometry analysis of the cell cycle of the drug-treated cells showed that these drugs unselectively induced apoptosis at all phases of the cell cycle and was unrelated to the changes in telomerase activity. Similar results were observed in U937 and K562 cells except that K562 cells underwent apoptosis more slowly than the former two cell lines.

Expression of cell death regulatory genes and limited apoptosis induction in avian blastodermal cells

Apoptosis is a well-established cellular mechanism for selective cell deletion during development. However, little is known about the expression of an apoptotic pathway and its role in determining the relative sensitivity of the early, pre-gastrula, avian embryo to stress-induced cell death. We examined the sensitivity of avian blastodermal cells to engage in apoptosis upon exposure to etoposide, a topoisomerase II-inhibitor that rapidly and efficiently induces apoptosis in many cell types. We found that while the blastodermal cells are capable of engaging in apoptosis, they are highly resistant to such induction with respect to both concentration of drug required and length of exposure, even when compared to a tumor cell line with a known multi-drug resistant phenotype. Additionally, we assessed the expression of several candidate regulatory genes in blastodiscs from infertile eggs (i.e., maternal RNA transcripts), blastodermal cells immediately following oviposition, and various stages of early development up to gastrulation. This analysis revealed that several genes whose products have anti-apoptotic activity, including bcl-2, bcl-xL, hsp70, grp78 and the glutathione S-transferases, are expressed as early as the stage 1 embryo in the newly oviposited egg. These transcripts are also found in the infertile blastodisc, suggesting a role for maternally derived transcripts in the protection of the oocyte and zygote. Significantly, constitutive levels of hsp70 mRNA exceeded those of the other anti-apoptotic genes in the blastodermal cells. These results contribute to an emerging picture of stress resistance at the earliest stages of avian embryo development which involves multiple anti-apoptotic genes that act at different regulatory points in the apoptotic cascade.