Inhibition of both etoposide-induced DNA fragmentation and activation of poly(ADP-ribose) synthesis by zinc ion

Fatal hyperammonemia and carbamoyl phosphate synthetase 1 (CPS1) deficiency following high-dose chemotherapy and autologous hematopoietic stem cell transplantation

Fatal hyperammonemia secondary to chemotherapy for hematological malignancies or following bone marrow transplantation has been described in few patients so far. In these, the pathogenesis of hyperammonemia remained unclear and was suggested to be multifactorial. We observed severe hyperammonemia (maximum 475 μmol/L) in a 2-year-old male patient, who underwent high-dose chemotherapy with carboplatin, etoposide and melphalan, and autologous hematopoietic stem cell transplantation for a neuroblastoma stage IV. Despite intensive care treatment, hyperammonemia persisted and the patient died due to cerebral edema. The biochemical profile with elevations of ammonia and glutamine (maximum 1757 μmol/L) suggested urea cycle dysfunction. In liver homogenates, enzymatic activity and protein expression of the urea cycle enzyme carbamoyl phosphate synthetase 1 (CPS1) were virtually absent. However, no mutation was found in CPS1 cDNA from liver and CPS1 mRNA expression was only slightly decreased. We therefore hypothesized that the acute onset of hyperammonemia was due to an acquired, chemotherapy-induced (posttranscriptional) CPS1 deficiency. This was further supported by in vitro experiments in HepG2 cells treated with carboplatin and etoposide showing a dose-dependent decrease in CPS1 protein expression. Due to severe hyperlactatemia, we analysed oxidative phosphorylation complexes in liver tissue and found reduced activities of complexes I and V, which suggested a more general mitochondrial dysfunction. This study adds to the understanding of chemotherapy-induced hyperammonemia as drug-induced CPS1 deficiency is suggested. Moreover, we highlight the need for urgent diagnostic and therapeutic strategies addressing a possible secondary urea cycle failure in future patients with hyperammonemia during chemotherapy and stem cell transplantation.

Unstabilized DNA breaks in HTLV-1 Tax expressing cells correlate with functional targeting of Ku80, not PKcs, XRCC4, or H2AX

BACKGROUND

Expression of the human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein rapidily induces a significant increase of micronuclei (MN) and unstabilized DNA breaks in cells. Unstabilized DNA breaks can have free 3'-OH ends accessible to in situ addition of digoxygenin (DIG)-labeled dUTP using terminal deoxynucleotidyl transferase. In the present work, we used a GFP-Tax (green fluorescent protein) plasmid, which produces a functionally active GFP-tagged Tax protein, to detect the cellular target(s) for Tax which might mechanistically explain the clastogenic phenomenon. We examined the induction of MN and unstabilized DNA breaks in wild type cells and cells individually knocked out for Ku80, PKcs, XRCC4, and H2AX proteins. We also assessed in the same cells, the signal strengths produced by DIG-dUTP incorporation at the unstable DNA breaks in the presence and absence of Tax.

RESULTS

Cells mutated for PKcs, XRCC4 and H2AX showed increased frequency of MN and unstabilized DNA breaks in response to the expression of Tax, while cells genetically mutated for Ku80 were refractory to Tax's induction of these cytogenetic effects. Moreover, by measuring the size of DIG-dUTP incorporation signal, which indicates the extent of unstable DNA ends, we found that Tax induces larger signals than those in control cells. However, in xrs-6 cells deficient for Ku80, this Tax effect was not seen.

CONCLUSIONS

The data here demonstrate that clastogenic DNA damage in Tax expressing cells is explained by Tax targeting of Ku80, but not PKcs, XRCC4 or H2AX, which are all proteins directly or indirectly related to the non-homologous end-joining (NHEJ) repair system. Of note, the Ku80 protein plays an important role at the initial stage of the NHEJ repair system, protecting and stabilizing DNA-breaks. Accordingly, HTLV-1 Tax is shown to interfere with a normal cellular protective mechanism for stabilizing DNA breaks. These DNA breaks, unprotected by Ku80, are unstable and are subject to erosion or end-to-end fusion, ultimately leading to additional chromosomal aberrations.

Induction of apoptosis by lupeol and mango extract in mouse prostate and LNCaP cells

Prostate cancer (PCA) is one of the most invasive malignancy and second leading cause of cancer related deaths in United States and some other countries. Long latency period makes PCA an ideal disease for pharmacologic or nutritional chemoprevention. Lupeol, a triterpene present in mango and other fruits, has shown to possess anticancer properties in in vivo and in vitro assays. Here, we recorded the apoptogenic activity in mouse prostate by lupeol and mango pulp extract (MPE). Testosterone was injected subcutaneously (5 mg/kg body weight) for 14 consecutive days to male Swiss albino mice. Lupeol/MPE supplementation resulted in arrest of prostate enlargement in testosterone-treated animals. In mouse prostate tissue, lupeol and MPE supplementation resulted in a significantly high percentage of apoptotic cells in the hypodiploid region. The induction of apoptosis in mouse prostate cells was preceded by the loss of mitochondrial transmembrane potential and DNA laddering. In testosterone-induced mouse prostate, upregulation of antiapoptotic B-cell non-Hodgkin lymphoma-2 and downregulation of proapoptotic Bcl-2-associated X protein and caspase-3 were also recorded. We further observed apoptogenic activities of lupeol in an in vitro model using human prostate cancer cells [lymph node carcinoma of the prostate (LNCaP)]. The apoptogenic response of lupeol-induced changes in LNCaP cells can be summarized as early increase of reactive oxygen species followed by induction of mitochondrial pathway leading to cell death. Thus, the results of this study demonstrate that lupeol/MPE is effective in combating testosterone-induced changes in mouse prostate as well as causing apoptosis by modulating cell-growth regulators.

Hepatoprotective effects of lupeol and mango pulp extract of carcinogen induced alteration in Swiss albino mice

Lupeol, a triterpene present in mango and other fruits, is known to exhibit a number of pharmacological properties including antioxidant, antilithiatic, and antidiabetic effects. In the present study, chemopreventive properties of lupeol and mango pulp extract (MPE) were evaluated against 7,12-dimethylbenz(a)anthracene (DMBA) induced alteration in liver of Swiss albino mice. Lupeol (25 mg/kg body weight, bw) or 1 mL of 20% w/v aqueous MPE/mouse were daily given once for 1 wk after a single dose of DMBA (50 mg/kg bw). Lupeol/MPE supplementation effectively influenced the DMBA induced oxidative stress, characterized by restored antioxidant enzyme activities and decrease in lipid peroxidation. A reduction of apoptotic cell population in the hypodiploid region was observed in lupeol and MPE supplemented animals. The inhibition of apoptosis was preceded by decrease in reactive oxygen species (ROS) level and restoration of mitochondrial transmembrane potential followed by decreased DNA fragmentation. In DMBA treated animals, downregulation of antiapoptotic Bcl-2 and upregulation of proapoptotic Bax and Caspase 3 in mouse liver was observed. These alterations were restored by lupeol/MPE, indicating inhibition of apoptosis. Thus, lupeol/MPE was found to be effective in combating oxidative stress induced cellular injury of mouse liver by modulating cell-growth regulators.

Grateloupia longifolia polysaccharide inhibits angiogenesis by downregulating tissue factor expression in HMEC-1 endothelial cells

1. The antiangiogenic and antitumor properties of Grateloupia longifolia polysaccharide (GLP), a new type of polysaccharide isolated from the marine alga, were investigated with several in vitro and in vivo models. Possible mechanisms underlying its antiangiogenic activity were also assessed. 2. GLP dose-dependently inhibited proliferation of human microvascular endothelial cells (HMEC-1) and human umbilical vein endothelial cells (HUVEC), with IC50 values of 0.86 and 0.64 mg ml(-1), respectively. In tube formation and cell migration assays using HMEC-1 cells, noncytotoxic doses of GLP significantly inhibited formation of intact tube networks and reduced the number of migratory cells. Inhibition by GLP was VEGF-independent. 3. In the chick chorioallantoic membrane (CAM) assay, GLP (2.5 microg egg(-1)) reduced new vessel formation compared with the vehicle control. GLP (0.1 mg plug(-1)) also reduced the vessel density in Matrigel plugs implanted in mice. 4. The levels of pan and phosphorylated receptors for VEGF, VEGFR-1 (flt-1) and VEGFR-2 (KDR) were not significantly altered by 5 mg ml(-1) GLP treatment of HMEC-1, although tissue factor (TF) showed significant decreases at both mRNA and protein levels following GLP treatment. 5. In mice bearing sarcoma-180 cells, intravenous administration of GLP (200 mg kg(-1)) decreased tumor weight by 52% without obvious toxicity. Vascular density in sections of the tumor was reduced by 64% after GLP treatment. 6. Collectively, these results indicate that GLP has antitumor properties, associated at least, in part, with the antiangiogenesis induced by downregulation of TF.

Human cytomegalovirus (HCMV) IE1 plays role in resistance to apoptosis with etoposide in cancer cell line by Cdk2 accumulation

Human cytomegalovirus (HCMV) has many strategies to survive the attack of the host. HCMV infection of host cells induces cellular activation and disturbance of the cell cycle. It is possible that HCMV modulates the behavior of certain cancer cells that are susceptible to HCMV infection. This study was performed to identify the possible mechanism of resistance to apoptotic stimuli in some cancer cell lines by HCMV infection. HCMV-infected cancer cells showed resistance to apoptosis induced by the topoisomerase II inhibitor etoposide. UMG1-2, which constitutively expresses HCMV immediate-early protein-1 (IE1), had resistance to apoptosis induced by etoposide as compared with the parental cell line U373MG. Measurement of caspases activity with fluorogenic substrates in etoposide-treated U373MG and UMG1-2 cells and the direct activation of caspase-3 with peptides containing arginine-glycine-aspartate in U373MG and UMG1-2 cells revealed that the inhibition level of apoptosis by HCMV IE1 would be upstream of caspase-3 in the caspase cascade pathway. Cellular expression of Cdk2 was increased in UMG1- 2 after etoposide treatment while the expression of E2F-1 in UMG1-2 was decreased as compared with that in U373MG. The Cdk2 inhibitor, roscovitine, decreased the resistance to apoptosis on etoposide-treated UMG1-2. These results suggest that aberrant HCMV infection confers resistance to anticancer drugs on some cancer cells and protects cells from apoptosis, possibly due to the deregulation of cyclin-dependent kinase by HCMV immediate-early protein.

Zinc inhibits Bax and Bak activation and cytochrome c release induced by chemical inducers of apoptosis but not by death-receptor-initiated pathways

Zinc has been known for many years to inhibit apoptosis but the mechanism remains unclear. Originally thought to inhibit an apoptotic endonuclease, zinc has subsequently been shown to inhibit steps earlier in the pathway. Since many additional steps in apoptosis have now been defined, we have re-evaluated the steps inhibited by zinc. In response to activation of the chemical-mediated death pathway by anisomycin, 0.3 mM zinc inhibited Bax and Bak activation, cytochrome c release, and all of the subsequent steps in apoptosis. In the receptor-mediated death pathway initiated by Fas or tumor necrosis factor, 3 mM zinc was required to inhibit apoptosis as judged by inhibition of caspase 3 activity and DNA digestion, but it failed to inhibit cytochrome c release, activation of Bax and Bak, or upstream signaling events in this pathway. These results are consistent with zinc selectively inhibiting activation of BH3-only proteins required in the chemical pathway but inhibiting downstream caspase activation in the death-receptor pathway.

A macrocyclic zinc(II) fluorophore as a detector of apoptosis

Our originally designed dansylamidoethylcyclen 4 as a biomimetic Zn(2+)-selective fluorophore has been demonstrated to be a good detector of the apoptosis (induced by an anticancer agent, etoposide, and H(2)O(2)) in cancer cells such as HeLa and HL60 cells. The macrocyclic Zn(2+) ligand 4 (mostly as a deprotonated form) is cell-permeable to show weak fluorescence (emission at 550 nm), which forms a strong fluorescent 1:1 Zn(2+) complex 5 (emission at 530 nm) when Zn(2+) is incorporated into the cells by a zinc(II) ionophore pyrithione. Thus formed, Zn(2+) complex 5 is cell-impermeable and remains intact over a few hours. When apoptosis in HeLa or HL60 cells is artificially induced, 4 selectively and strongly stains apoptotic cells only at early stages, which was verified by using the conventional apoptosis detection probe annexin V-Cy3. Detection of the apoptotic cells by 4 was perhaps due to significantly increased free Zn(2+) flux at early stages of apoptosis. Apoptotic detection by 4 has been compared with a presently available Zn(2+) fluorophore, Zinquin 1. We present that 4 has advantages in detection of apoptosis over annexin V-Cy3 and Zinquin 1.

Role of Ca2+ in the intracellular signaling pathway of anti-IgM-induced apoptosis in the human B-cell line, MBC-1, established from Burkitt lymphoma

The role of Ca2+ in the intracellular signal transduction process that causes antibody-induced apoptotic cell death in B-cells is not completely understood. We previously established a B-cell line (MBC-1) from a patient with Burkitt lymphoma at the leukemic stage that demonstrated the typical morphology and internucleosomal DNA fragmentation of apoptosis when treated with anti-immunoglobulin (Ig)M antibody. This antibody-induced cell death was partially inhibited by pretreatment with ethyleneglycol-bis-tetraacetic acid (EGTA) and actinomycin-D. FK506, an immunosupressive agent and calcineurin inhibitor, also partially rescued the anti-IgM antibody-induced death of MBC-1 cells. These results show that the calcium signaling pathway, which leads to a change in gene expression, plays an important role in anti-IgM-induced apoptosis in MBC-1 cells. Flow cytometric measurement of the cytosolic free Ca2+ concentration ([Ca2+]i) showed that nontoxic concentrations of 4-bromo-calcium ionophore A23187 (Ca2+ IP) increased [Ca2+]i more than did the anti-IgM antibody. A brief Ca2+ spike was observed on anti-IgM antibody treatment, but a gradual increase and decrease were observed when the cells were treated with Ca2+ IP at a nontoxic concentration of 1 microg/mL. These findings suggest that interpretations differ for the 2 patterns of calcium signaling and that the brief spiked elevation of Ca2+ produces distinct biological and cellular responses compared to the gradual increase and decrease of [Ca2+]i. Our results support the hypothesis that Ca2+ plays a significant role as a multifunctional second messenger providing specific information to the nucleus in anti-IgM antibody-induced apoptosis in MBC-1 cells.

Reactive oxygen species modulate Zn(2+)-induced apoptosis in cancer cells

Some recent evidence has suggested a protective role of zinc against cancer. The mechanism by which zinc exerts this action has not been defined and, in particular, it has not been clarified whether zinc may directly act on cancer cells and the molecular mechanisms involved in this effect. In this study, we examined the in vitro effect of zinc on the apoptosis of mouse TS/A mammary adenocarcinoma cells, studying the zinc-dependent modulation of the intracellular levels of reactive oxygen species (ROS) and of p53 and Fas/Fas ligand pathways. We showed that zinc concentrations ranging from 33.7 to 75 muM Zn(2+) induced apoptosis in mammary cancer cells. The apoptosis was associated with an increased production of intracellular ROS, and of p53 and Fas/Fas ligand mRNA and protein. Zn(2+) induced a faint metallothionein response in TS/A cells in comparison with mouse lymphocytes. The treatment of tumor cells with the antioxidant N-acetylcysteine was able to prevent Zn(2+)-induced apoptosis, as well as the increase of p53 and Fas ligand protein induced by zinc. The data demonstrate that zinc exerts a direct action on mammary cancer cells inducing ROS-mediated apoptosis and that the effect may be mediated by the ROS-dependent induction of p53 and Fas/Fas ligand.

p53 and redox state in etoposide-induced acute myeloblastic leukemia cell death

We investigated whether p53, being a redox-sensitive protein, has a role in the responsiveness of AML cells to etoposide. Two subclones of the OCI/AML-2 cell line, the etoposide-sensitive (ES) and the etoposide-resistant (ER), were used as models. Sensitivity to etoposide was measured by trypan blue and annexin V assays. Etoposide-induced peroxide formation was associated with the induction of cell death. Evident expression of mutated p53 was observed in both subclones in basal growth conditions as analysed by Western blotting and flow cytometry. After etoposide exposure for up to 24 hours, some nuclear accumulation of p53 was observed in the ER subclone, as analysed by Western blotting. The conformation of p53, however, was not changed from mutated toward wild-type during exposure in either of the subclones as analysed by flow cytometry. In conclusion, etoposide-induced change in cellular redox state was associated with apoptosis, but was not a sufficient stimulus for p53 to make its conformation active. Thus, mutated p53 seems to have no role in etoposide-induced apoptosis.

Induction of mitochondrial manganese superoxide dismutase confers resistance to apoptosis in acute myeloblastic leukaemia cells exposed to etoposide

We investigated the possible roles of mitochondrial manganese superoxide dismutase (MnSOD) and bcl-2 in etoposide-induced cell death in acute myeloblastic leukaemia (AML) using two subclones of the OCI/AML-2 cell line, the etoposide-sensitive (ES) and the etoposide-resistant (ER), as models. Cell death after 24 h exposure to 10 micromol/l etoposide was about 60% and 70% in the ES subclone and about 20% and 25% in the ER subclone, when analysed by trypan blue and annexin V respectively. Cytochrome c efflux from mitochondria to cytosol was observed after 4 h of exposure in both subclones, whereas the activation of caspase-3 was not detectable until after 12 h of exposure in the ES subclone and 24 h of exposure in the ER subclone, using Western blotting. The decrease in mitochondrial membrane potential, when analysed by the JC-1 probe fluorocytometrically, also appeared to take place later in the ER than in the ES subclone. Both subclones showed evident basal expression of MnSOD and bcl-2 by Western blotting. Etoposide caused a potent induction of MnSOD, more than 400% at 12 h, in the ER but not in the ES subclone. No significant change in bcl-2 expression could be observed in either of the subclones during exposure to etoposide when analysed by Western blotting or flow cytometry. In conclusion, we suggest that MnSOD might have a special role in the protection of AML cells against etoposide-induced cell death. Although unable to influence the cytochrome c efflux to cytosol, MnSOD might prevent the disruption of mitochondrial membrane potential, which evidently leads to cell death by releasing various activators of apoptosis.

Inhibitory effect of zinc on human prostatic carcinoma cell growth

BACKGROUND

Normal human prostate accumulates the highest levels of zinc of any soft tissue in the body. In contrast, the zinc level in prostate cancer is markedly decreased from the level detected in nonprostate tissues. Despite these relationships, the possible role of zinc in the growth of normal and malignant prostate has not been determined.

METHODS

Growth inhibition and various regulatory responses were investigated in two human prostate carcinoma cell lines (LNCaP and PC-3), treated with or without zinc.

RESULTS

Incubation of the prostate carcinoma cell lines with physiological levels of zinc resulted in the marked inhibition of cell growth. A lower 50% inhibition of cell growth (IC50) value for zinc (about 100 ng/ml) was detected in LNCaP cells, which are androgen-responsive, whereas androgen-independent PC-3 cells exhibited a higher IC50 for zinc (about 700 ng/ml). Incubation with 1 microg/ml zinc resulted in maximum inhibition of growth in both cell lines. These inhibitory effects of zinc correlated well with the accumulation of zinc in the cells. Simultaneously, cell flow cytometric analyses revealed a dramatic increase of the cell population in G2/M phase, in both LNCaP (2.3-fold vs. control) and PC-3 (1.9-fold vs. control), and a decreased proportion of cells in S phase (LNCaP, -51.4%; PC-3, -23%), indicating a G2/M phase arrest. The cell growth inhibition and G2/M arrest in these cells were accompanied by an increase in apoptosis, as demonstrated by the characteristic cell morphology and further confirmed by cellular DNA fragmentation. The specificity of zinc-induced apoptosis was identified by ethylenediamine-tetraacetic acid (EDTA)-chelation, which abolished the zinc effect on cellular DNA fragmentation. The zinc-induced G2/M phase arrest and apoptosis were accompanied by increased mRNA levels of p21(Waf1/Cip1/Sdi1) in both LNCaP (p53+/+) and PC-3 (p53-/-) cells.

CONCLUSIONS

These results suggest that zinc inhibits human prostatic carcinoma cell growth, possibly due to induction of cell cycle arrest and apoptosis. There now exists strong evidence that the loss of a unique capability to retain high levels of zinc is an important factor in the development and progression of malignant prostate cells.

Inhibition of hepatic cell nuclear DNA fragmentation by zinc in carbon tetrachloride-treated rats

BACKGROUND/AIMS

The aims of this study were to ascertain: 1) whether hepatic cell DNA fragmentation is increased in rats with early stages of liver disease induced by carbon tetrachloride; 2) whether the inhibition of DNA cleavage is involved in the hepatoprotective effects of zinc; and 3) if relationships exist between DNA fragmentation and the onset of fibrosis in this experimental model.

METHODS

Twenty-one treated rats and 23 controls were divided into two groups to receive either a standard diet or one supplemented with zinc. All the animals were sacrificed 1 week later for histological and biochemical assessments, which included a DNA fragmentation index, hepatic zinc and metallothionein concentrations, fibrosis measured by hepatic hydroxyproline concentration and plasma alanine aminotransferase activity.

RESULTS

Hepatic cell DNA fragmentation was increased in rats with early hepatic fibrosis and the increase was independent of hepatocytolysis, as measured by alanine aminotransferase activity. Oral zinc administration inhibited hepatic cell DNA fragmentation in the treated rats and was proportional to the hepatic concentration of the metal. The mechanism of the zinc-mediated decrease in DNA cleavage was related to an increase in the hepatic metallothionein concentration. Hepatic cell DNA fragmentation was related to hydroxyproline concentration.

CONCLUSIONS

Our results suggest that apoptosis may be involved in the early transformations occurring in the liver and which can lead to the initiation of cirrhosis. As such, the potential therapeutic use of zinc supplementation would warrant further investigation.

Terminal differentiation of normal human oral keratinocytes is associated with enhanced cellular TGF-beta and phospholipase C-gamma 1 levels and apoptotic cell death

Subculture of primary normal human oral keratinocytes (NHOK) results in terminal differentiation, leading to cell death. To investigate whether the subculture-induced death of NHOK is due to apoptosis, we studied transferase-mediated dUTP nick end labeling (TUNEL)-positive cells, DNA fragmentation, and expression of several apoptosis-associated genes from NHOK with different passage numbers. We also determined the effect of transforming growth factor beta1 (TGF-beta1) on the induction of apoptosis in NHOK. We were able to subculture primary NHOK up to the fifth passage, at which point cells showed morphological features of differentiation. Appearance of DNA fragmentation concurrently occurred with an increase in the number of TUNEL-positive cells with higher passage numbers. The level of cellular p53 proteins was gradually decreased by the continued passage of cells, whereas the levels of intracellular and secreted TGF-beta and phospholipase C-gamma1 (PLC-gamma1) were significantly elevated by serial subculture. Exogenous TGF-beta1 also induced differentiation and apoptosis of proliferating NHOK. These data indicate that terminal differentiation of NHOK is associated with apoptosis, which is, in part, linked to elevated cellular levels of TGF-beta and PLC-gamma1.

Application of fluorescence microscopy to measure apoptosis in Jurkat T cells after treatment with a new investigational anticancer agent (N.C.1213)

1. Apoptosis as the mechanism of cell death induced by a new cytotoxic and anticancer agent (N.C.1213) was investigated by morphological and biochemical criteria in human Jurkat T leukemia cells. 2. The effect of N.C.1213 on the survival of Jurkat T, LV-50, H-9, and Molt-3 cells was measured. Jurkat T cells exhibited the highest response, with less than 10% of the cells remaining viable after exposure to 10 microM N.C.1213 for a 24 hr period. All other cell cultures were also affected but to a lesser extent. 3. With the use of a fluorescence microscope, several morphological features characteristic of apoptosis such as condensed chromatin and apoptotic bodies were identified in Jurkat T cells after exposure to N.C.1213 and melphalan. The results indicated that melphalan was more cytotoxic than N.C.1213 as shown by the dye exclusion test. However, N.C.1213 showed a greater apoptotic index than melphalan. The IC50 of N.C.1213 in Jurkat T cells was determined to be 3.5 microM. 4. A DNA ladder (fragmentation of DNA into multimers of approximately 200 base pairs), which is one characteristic feature of apoptosis, was not detected when Jurkat T cells were exposed to N.C.1213. Hence it is probable that the key morphological events in apoptosis observed in the present experimental conditions precede the internucleosomal cleavage of DNA.

Topoisomerase II and the response to antileukemic therapy

A number of recent studies have investigated the expression of topoisomerase II in clinical leukemia specimens. Here we outline the rationale for these studies, identify potential pitfalls, summarize recent results, and discuss unanswered questions in this area.

Micromolar zinc affects endonucleolytic activity in hydrogen peroxide-mediated apoptosis

When damaged by hydrogen peroxide, peripheral blood lymphocytes undergo cell death by apoptosis in the absence of internucleosomal DNA cleavage, while, in the same cells, other apoptosis-inducing treatments bring DNA cleavage to completion. However, the formation of internucleosomal DNA fragments is readily obtained if cells are pretreated with a divalent metal chelator, TPEN, at micromolar concentrations. Since the coadministration of equimolar zinc concentrations abrogates the formation of the ladder, a zinc-inhibitable endonucleolytic activity is accounted for the effect. Most notably, subtraction of zinc ions does not increase the percentage of cells undergoing apoptosis, but rather results in a rescue from death.

Zinc metabolism in the brain: relevance to human neurodegenerative disorders

Zinc is an important trace element in biology. An important pool of zinc in the brain is the one present in synaptic vesicles in a subgroup of glutamatergic neurons. In this form it can be released by electrical stimulation and may serve to modulate responses at receptors for a number of different neurotransmitters. These include both excitatory and inhibitory receptors, particularly the NMDA and GABA(A) receptors. This pool of zinc is the only form of zinc readily stained histochemically (the chelatable zinc pool), but constitutes only about 8% of the total zinc content in the brain. The remainder of the zinc is more or less tightly bound to proteins where it acts either as a component of the catalytic site of enzymes or in a structural capacity. The metabolism of zinc in the brain is regulated by a number of transport proteins, some of which have been recently characterized by gene cloning techniques. The intracellular concentration may be mediated both by efflux from the cell by the zinc transporter ZrT1 and by complexing with apothionein to form metallothlonein. Metallothionein may serve as the source of zinc for incorporation into proteins, including a number of DNA transcription factors. However, zinc is readily released from metallothionein by disulfides, increasing concentrations of which are formed under oxidative stress. Metallothionein is a very good scavenger of free radicals, and zinc itself can also reduce oxidative stress by binding to thiol groups, decreasing their oxidation. Zinc is also a very potent inhibitor of nitric oxide synthase. Increased levels of chelatable zinc have been shown to be present in cell cultures of immune cells undergoing apoptosis. This is very reminiscent of the zinc staining of neuronal perikarya dying after an episode of ischemia or seizure activity. Thus a possible role of zinc in causing neuronal death in the brain needs to be fully investigated. intraventricular injections of calcium EDTA have already been shown to reduce neuronal death after a period of ischemia. Pharmacological doses of zinc cause neuronal death, and some estimates indicate that extracellular concentrations of zinc could reach neurotoxic levels under pathological conditions. Zinc is released in high concentrations from the hippocampus during seizures. Unfortunately, there are contrasting observations as to whether this zinc serves to potentiate or decrease seizure activity. Zinc may have an additional role in causing death in at least some neurons damaged by seizure activity and be involved in the sprouting phenomenon which may give rise to recurrent seizure propagation in the hippocampus. In Alzheimer's disease, zinc has been shown to aggregate beta-amyloid, a form which is potentially neurotoxic. The zinc-dependent transcription factors NF-kappa B and Sp1 bind to the promoter region of the amyloid precursor protein (APP) gene. Zinc also inhibits enzymes which degrade APP to nonamyloidogenic peptides and which degrade the soluble form of beta-amyloid. The changes in zinc metabolism which occur during oxidative stress may be important in neurological diseases where oxidative stress is implicated, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Zinc is a structural component of superoxide dismutase 1, mutations in which give rise to one form of familiar ALS. After HIV infection, zinc deficiency is found which may be secondary to immune-induced cytokine synthesis. Zinc is involved in the replication of the HIV virus at a number of sites. These observations should stimulate further research into the role of zinc in neuropathology.

Apoptosis-resistant phenotype selected by alternating exposure to camptothecin and etoposide

We selected an apoptosis-resistant subline (VC-33) in a human promyelocytic leukemia cell line, HL-60, by alternating exposure to camptothecin (CPT) and etoposide (VP-16). When wild-type (WT) and VC-33 cells were incubated with various concentrations of either CPT or VP-16 for 4 h, VC-33 showed several-fold resistance to apoptosis induced by these agents in comparison with WT cells. VC-33 cells also exhibited cross-resistance to apoptosis induced by 1-beta-d-arabinofuranosylcytosine, hydroxyurea, a calcium ionophore (A23187), cycloheximide, or UV irradiation. The levels of protein-DNA cross-linking induced by CPT or VP-16, and the amounts of ara-CTP generation, tended to be smaller in VC-33 cells, but the difference was not sufficient to explain the difference in the sensitivity to apoptosis. The initial rise of intracellular calcium ions with A23187 and the expression of P-glycoprotein, Bcl-2, and Bcl-Xl were comparable between WT and VC-33 cells. This mutant may represent a new phenotype of resistance to apoptosis induced by a variety of agents, and may thus be useful in the study of the mechanisms of apoptosis.

Cell density-dependent apoptosis in HL-60 cells, which is mediated by an unknown soluble factor, is inhibited by transforming growth factor beta1 and overexpression of Bcl-2

We report a novel mode of apoptosis induction observed in human leukemic HL-60 cells. These cells spontaneously underwent apoptosis in the course of proliferation when the cell density became higher than 1 x 10(6)/ml. This occurred under ordinary in vitro culture conditions, with or without fetal calf serum. Even the low density cells were committed to undergo apoptosis if they were cultured under artificially concentrated conditions. Replacement of the culture supernatant of the low density cells by that of the high density ones resulted in apoptosis induction in the former cells. This apoptosis-inducing activity of the high density cell culture supernatant was completely eliminated by the action of trypsin but was fully restored following ultrafiltration by 3-kDa pore-sized membrane. A strong apoptosis-inducing activity was recovered from the culture supernatant of the high density HL-60 cells at a specific fraction in reverse-phase column chromatography. Neither an interleukin-beta converting enzyme inhibitor nor CPP-32 inhibitor blocked the induction of cell density-dependent apoptosis in HL-60 cells, although overexpression of Bcl-2 protein markedly attenuated the induction of this mode. Surprisingly, transforming growth factor-beta1 and activin A did not induce but, rather, inhibited the induction of cell density-dependent apoptosis. These data suggest that HL-60 cells release an unknown low molecular weight peptide-containing factor in response to an increase in cell density to induce apoptosis in an autocrine manner and that the interleukin-beta converting enzyme-independent intracellular machinery for this mode of apoptosis is strongly affected by signaling events through the transforming growth factor-beta1 receptor and by the action of Bcl-2 oncoprotein.

Constitutive endonuclease to induce high molecular weight or internucleosomal DNA fragmentation in freshly isolated leukemia cells

Using an autodigestion method, we investigated endogenous endonuclease(s) in leukemia cells freshly obtained from pediatric patients with various types of leukemia. Endonucleolytic activity was found to cause both high molecular weight and internucleosomal DNA fragmentation at a neutral pH in whole cell lysates of all common acute lymphoblastic leukemia (cALL) blasts, which was Mg2+-dependent and Ca2+-independent. Whole lysates from most acute myeloblastic leukemia (AML) cells possessed similar endonuclease activity, but both Mg2+ and Ca2+ were required for the activity. Our results suggest that leukemia cells of different lineages have distinct constitutive endonucleases, which may play a role in the occurrence of apoptosis in these cells.

On the use of Zn2+ to discriminate endonucleases activated during apoptosis

One approach to discriminate among specific DNases in apoptosis is to use inhibitors specific for each endonuclease. Zn2+ is known to inhibit Ca(2+)- and Mg(2+)-dependent endonuclease enzymatic activities during apoptosis. Acidic DNases were thought to be insensitive to Zn2+. In this paper, we analyse the effects of Zn2+ on activity of DNase II, either purified or in nuclei from lens fiber cells. These cells follow a physiological nuclear degeneration with DNase II accumulation in their nuclei. We show that Zn2+ is able to inhibit also this acidic endonuclease at a concentration of 1-6 mM. At a higher concentration of Zn2+, DNA is extensively degraded during the assay, masking the inhibition of the enzyme. This DNA degradation in the presence of Zn2+ has led to an overestimation of the activity of DNase II in studies of apoptosis. Hence, Zn2+ cannot be used to specifically identify one endonuclease among the different DNases involved in nuclear degradation during programmed cell death.

Temporal phases in apoptosis defined by the actions of Src homology 2 domains, ceramide, Bcl-2, interleukin-1beta converting enzyme family proteases, and a dense membrane fraction

We have begun to explore the mechanisms of apoptosis using a cell-free system based on extracts from Xenopus eggs. Nuclei assembled or placed in these extracts undergo the morphological changes typical of apoptosis and eventually disintegrate. We used this system to investigate the potential involvement in apoptosis of proteins containing Src homology 2 (SH2) domains, which are known to interact with specific tyrosine-phosphorylated ligands. SH2 domains from a number of signaling proteins, including Lck, Src, and Abl, inhibited apoptosis when present at concentrations of 10-100 nM. The inhibition was dependent on specific interaction with endogenous tyrosine-phosphorylated ligands. A synthetic peptide ligand for Src family SH2 domains also inhibited apoptosis in a phosphotyrosine-dependent manner. Kinetic analysis defined three phases in the apoptotic process occurring in this cell-free system. SH2 domains and ceramide act throughout the first 60-90 min of the process (the "initiation" phase). Next, Bcl-2, interleukin-1beta converting enzyme family(CPP32-like) proteases, and the heavy membrane fraction act in a period occurring approximately 90-120 min after the start of incubation (the "sentencing" phase). In the final phase ("execution"), the process of active nuclear destruction ensues.

Zinc and Alzheimer's disease: is there a direct link?

Zinc is an essential trace element in human biology, but is neurotoxic at high concentrations. Several studies show that zinc promotes aggregations of beta-amyloid protein, the main component of the senile plaques typically found in Alzheimer's disease brains. In other neurological disorders where neurons appear to be dying by apoptosis (gene-directed cell death), chelatable zinc accumulates in the perikarya of neurons before, or during degeneration. As there is evidence for apoptotic death of neurons in Alzheimer's disease, an involvement of zinc in this process needs to be investigated. Zinc interacts with enzymes and proteins, including transcription factors, which are critical for cell survival and could be linked to apoptotic processes. While controversial, some studies indicate that total tissue zinc is markedly reduced in several brain regions of Alzheimer's patients. At face value, it seems that a paradox exists between reports of a decrease in zinc in the Alzheimer's brain and the putative link to aberrant high zinc levels promoting plaque formation. An hypothesis to explain this inconsistency is presented. Neuropathological changes mediated by endogenous or exogenous stressors may be relevant factors affecting abnormal zinc metabolism. This paper reviews current investigations that suggest a role of zinc in the etiology of Alzheimer's disease.

Preferential induction of apoptosis in mouse CD4+CD8+ alpha beta TCRloCD3 epsilon lo thymocytes by zinc

High concentrations of zinc salts (500 microM and greater) are known to inhibit apoptosis in a variety of systems. However, closer examination of dose effects revealed that lower concentrations of zinc (80-200 microM) could induce apoptosis in approximately 30-40% of mouse thymocytes following 8 h incubation. The ability of zinc to cause thymocyte apoptosis was detected flow-cytometrically by reduction in propidium iodide DNA fluorescence and forward scatter, both quantitative indicators of apoptotic death. Zinc induced both internucleosomal DNA fragmentation and morphological changes characteristic of apoptosis as determined by gel electrophoresis and electron microscopy, respectively. In addition, transcriptional and translational inhibitors prevented zinc-induced apoptosis, indicating a requirement for de novo mRNA and protein synthesis, another characteristic of apoptotic death. Fluorescent immunophenotype-specific apoptotic analysis indicated that zinc-induced apoptosis occurred primarily in the less mature CD4+CD8+ alpha beta TCRloCD3 epsilon lo thymocyte subset, with lower amounts of death occurring in the other subsets. This lineage specificity was shared with glucocorticoid-induced apoptosis. Taken together, these results indicate that zinc induces true apopotitic death in mouse thymocytes and suggests a role for zinc in the regulation of apoptosis.

Association of high molecular weight DNA fragmentation with apoptotic or non-apoptotic cell death induced by calcium ionophore

Calcium ionophore (A23187)-induced high molecular weight (HMW) and internucleosomal DNA fragmentation were investigated in human leukemia cell lines. An apoptosis-sensitive cell line, HL-60, showed HMW, internucleosomal DNA fragmentation and morphological changes of apoptosis by A23187. MOLT-4, which is resistant to apoptosis, exhibited only HMW DNA fragmentation and died of necrosis under the same conditions. Autodigestion experiments suggested the endonucleolytic activity to cause HMW fragmentation in the cytoplasm of both cell lines. The activity was more dependent on Mg2+ than Ca2+ in HL-60, whereas it was Ca(2+)-dependent in MOLT-4. These results suggest that HMW DNA fragmentation is not specific to apoptosis.

Pyrrolidine dithiocarbamate, a potent inhibitor of nuclear factor kappa B (NF-kappa B) activation, prevents apoptosis in human promyelocytic leukemia HL-60 cells and thymocytes

We examined the effect of pyrrolidine dithiocarbamate (PDTC), which potently blocks the activation of nuclear factor kappa B (NF-kappa B), on the induction of apoptosis by a variety of agents. Treatment of a human promyelocytic leukemia cell line, HL-60, with 10 micrograms/mL etoposide or 2 microM 1-beta-D-arabinofuranosylcytosine induced NF-kappa B activation within 1 hr and subsequently caused apoptosis within 3-4 hr. The simultaneous addition of 50-500 microM PDTC with these agents blocked NF-kappa B activation and completely abrogated both morphologically apoptotic changes and internucleosomal DNA fragmentation for up to 6 hr. However, PDTC failed to inhibit the endonuclease activity contained in the whole cell lysates. The inhibitory effect of PDTC was also observed in etoposide- and dexamethasone-induced apoptosis in human thymocytes at a concentration of 1-10 microM. Since PDTC has both antioxidant and metal-ion chelating activities, we tested the effects of N-acetyl-L-cysteine (NAC) (antioxidant) or o-phenanthroline (OP) (metal-ion chelator) on the induction of apoptosis. Pretreatment of HL-60 cells or thymocytes with 100-500 microM OP for 2 hr, but not 10-60 mM NAC, suppressed subsequent occurrence of apoptosis induced by etoposide. These results suggest that the activation of NF-kappa B plays an important role in the apoptotic process of human hematopoietic cells.

Apoptosis induced by DNA topoisomerase I and II inhibitors in human leukemic HL-60 cells

The induction of apoptosis following topoisomerase inhibitors proceeds in at least three distinct steps: (1) induction of cleavable complexes (potentially lethal damage), (2) topoisomerase-induced DNA damage, and (3) a presently unknown sequence of events that must either lead to cell cycle arrest (G2-block, differentiation) or apoptosis. DNA degradation provides a convenient way to quantify apoptosis in HL-60 cells. Extensive apoptosis can be induced rapidly in undifferentiated HL-60 cells without prevention by cycloheximide or actinomycin D. Therefore, HL-60 cells appear to express constitutively the apoptotic machinery that may be kept under control of a yet unknown repressor. The absence of the tumor suppressor p53 and the presence of bcl-2 are in contrast with the sensitivity of these cells to apoptosis. Agents that modify chromatin structure (zinc, poly[ADPribose] inhibitors, spermine) can block DNA fragmentation without affecting cell survival. By contrast macrophage-like differentiation by phorbol esters suppresses apoptosis without affecting topoisomerase-induced DNA damage. Better understanding of the apoptotic regulation in the widely used and characterized HL-60 cell line should allow the identification of new mechanisms and parameters of cellular sensitivity and resistance to the cytotoxic activity of anticancer agents.

Programmed cell death of the normal human neutrophil: an in vitro model of senescence

The present study provides experimental data which indicate that the neutrophil is ideal for studying programmed cell death or apoptosis in vitro. Neutrophils can be obtained from human peripheral blood in large numbers with minimal experimental manipulation and are easily separated from other leukocytes, providing nearly pure cell suspensions. The neutrophil life span in vitro is sufficiently short to allow observations to be made within eight hours after experimental manipulation. Neutrophils can also be easily maintained in serum-free, chemically defined media which can be systematically altered, thereby defining specific variables that influence the apoptotic process. Since the neutrophils do not need an exogenous trigger to undergo programmed cell death, it is also an excellent model to study senescence. It was determined from this study that neutrophils undergo apoptosis most efficiently at 37 degrees C, a temperature requirement for physiologic cell death. Neutrophils undergo apoptosis at a slightly faster rate and maintain membrane integrity better when incubated in a tissue culture medium (e.g., RPMI 1640) compared with a balanced salt solution (e.g., HBBB). Cycloheximide, an inhibitor of protein synthesis, was shown to accelerate apoptosis in a dose-dependent manner. The presence of Zn++ significantly decreased the rate of apoptosis, whereas the presence of Ca++ and Mg++ had no apparent effect. These studies indicate that the process of senescence, culminating in cell death, is subject to modulation by a variety of agents and experimental conditions. In addition, the ultrastructural features of neutrophils undergoing programmed cell death in vitro were compared in detail to those occurring in vivo and were found to be comparable.

Interleukin-3 and Bcl-2 cooperatively inhibit etoposide-induced apoptosis in a murine pre-B cell line

Murine bone marrow-derived hemopoietic cells, dependent on interleukin (IL)-3 for their growth in culture, undergo programmed cell death, or apoptosis, upon cytokine withdrawal. The topoisomerase II inhibitor etoposide causes a more rapid onset of apoptosis in the IL-3-dependent cell line BAF3, deprived of IL-3. This acceleration of apoptosis by etoposide is prevented by inhibitors of RNA and protein synthesis and by the nucleases inhibitor aurintricarboxylic acid. The presence of IL-3 or overexpression of the oncogene bcl-2 caused a marked delay in the induction of apoptosis by etoposide, acting in a cooperative manner. The time at which the apoptotic program is irreversible is close to the induction of endonuclease activity as indicated by the effect of the delayed addition of either IL-3 or aurintricarboxylic acid on the onset of apoptosis, suggesting the importance of endonuclease activation in the development of apoptosis in hemopoietic cells.

Changes in nuclear chromatin precede internucleosomal DNA cleavage in the induction of apoptosis by etoposide

Etoposide, a DNA topoisomerase II inhibitor, caused a concentration-dependent induction of apoptosis in immature thymocytes. Using a flow cytometric method to separate and quantify normal and apoptotic cells, etoposide-induced apoptosis was inhibited by cycloheximide and actinomycin D but not by zinc. Etoposide induced a marked cleavage of DNA into nucleosomal length fragments or multiples thereof, which was completely inhibited if the thymocytes were also incubated in the presence of zinc. Etoposide, alone, induced the classical ultrastructural features of apoptosis, but in the presence of zinc, the morphological pattern was markedly different and dominated by discrete clumps of condensed chromatin abutting the nuclear membrane. These latter changes resemble those described as the earliest changes in apoptosis. These results support the hypothesis that, in the induction of apoptosis, critical alterations in nuclear chromatin occur prior to endonuclease cleavage of DNA into nucleosomal fragments.

Combined oral administration of etoposide and arabinofuranosylcytosine-5'-stearylphosphate enhances the antitumor effect against P388 ascites tumors

We investigated the antitumor effect of oral administration of etoposide and arabinofuranosylcytosine-5'-stearylphosphate (C18PCA) against P388 ascites tumors in B6D2F1 mice. Etoposide (25 mg/kg) and C18PCA (5 mg/kg) were given orally on days 1-5 after tumor inoculation. The median life span of the mice treated with etoposide or C18PCA alone was 19.5 and 18 days, respectively. The combination of both drugs significantly extended the median life span to 33 days. To clarify this enhancement of the increase in median life span, we examined intracellular deoxyribonucleoside triphosphate (dNTP) pools, cell-cycle distribution, DNA fragmentation, and the time course of the plasma drug concentration. Etoposide had no effect on intracellular dNTP pools in this experimental system, whereas treatment of cells with C18PCA or with the combination of both drugs resulted in a significant increase in dTTP pools to values ranging from 1.8- to 2.0-fold higher than the control levels. There was a significant increase in cells in the S + G2/M phase when cells had been treated with both etoposide and C18PCA. Agarose-gel electrophoresis of the extracted DNA revealed that C18PCA enhanced the fragmentation of DNA, with a length of about 180 bp being induced by etoposide. The plasma peak levels of etoposide (1000 nM) and ara-C (50 nM) were observed at 20 and 30 min after the simultaneous administration of both drugs, respectively. The plasma etoposide level gradually decreased to 10% of the peak level at 240 min after administration. On the other hand, the plasma concentration of ara-C was maintained at above 20 nM at 240 min. These observations suggest that C18PCA and etoposide act on P388 murine leukemic cells by accumulating cells in the S + G2/M phase. Even if the plasma concentration of ara-C is low, the repair of DNA damage by etoposide may be hindered in the presence of ara-C following an increase in DNA fragmentation.

Induction of apoptosis in childhood acute leukemia by chemotherapeutic agents: failure to detect evidence of apoptosis in vivo

This study is designed to investigate whether apoptosis occurs in vivo in pediatric patients with acute leukemia during induction therapy. When patients with common acute lymphoblastic leukemia (cALL) and acute myeloblastic leukemia (AML) were treated with prednisolone (60 mg/m2/day, p.o. or i.v.) and etoposide (150 mg/m2/day, i.v.), respectively, the blast cell counts fell to below 30% and 5%, respectively, in 1 week. However, during this cytoreduction phase, neither morphologically apoptotic cells nor fragmentation of DNA derived from peripheral blast cells were detected at any preparations. On the other hand, cALL but not AML cells spontaneously undergo apoptosis following their culture in vitro. The addition of autologous serum instead of fetal calf serum substantially prevented apoptosis from occurring spontaneously in cALL cells. When cALL and AML cells freshly obtained from patients before therapy were treated in vitro with 10 mumol/l prednisolone and 20 micrograms/ml etoposide, respectively, these cells underwent apoptosis within 6 hours, as determined by a morphological and DNA fragmentation assay. These in vivo and in vitro findings suggest that, although anticancer drugs may induce apoptosis in vivo, these apoptotic cells cannot be detected due to their rapid removal from the circulation.

Apoptosis induced by etoposide in small-cell lung cancer cell lines

The DNA fragmentation, a parameter of apoptosis, in non-small (NSCLC) and small (SCLC) cell lung cancer cell lines (N231 and PC-9) was evaluated. The DNA fragmentation in SCLC lines, but not in NSCLC lines, was observed in overgrown cells without exposure to anticancer drugs. In etoposide (VP-16)-treated N231 but not PC-9 cells, DNA fragmentation continued to increase up to 42 h, and the increase was dependent on the concentration of VP-16. The endonuclease activity of VP-16-treated N231, but not PC-9, cells required both Ca2+ and Mg2+ for full activity. It was elevated in a time- and concentration-dependent manner. As this activity was not affected by addition of cycloheximide, the activation of the endonuclease activity without protein synthesis may be involved in VP-16-induced cytotoxicity in N231.

Aphidicolin potentiates apoptosis induced by arabinosyl nucleosides in human myeloid leukemia cell lines

We investigated the effect of aphidicolin, an inhibitor of DNA polymerase alpha and delta, on the induction of apoptosis by arabinosyl nucleosides in a human promyelocytic leukemia cell line, HL-60. Pretreatment of HL-60 cells with aphidicolin (2 microM) significantly increased the number of morphologically apoptotic cells induced by 1-beta-D arabinofuranosylcytosine (ara-C) during 4 hr of incubation. This is consistent with the appearance of DNA fragmentation as determined quantitatively by diphenylamine or by agarose gel electrophoresis. The inhibition of cell growth on day 3 after drug exposure was correlated with the degree of apoptosis: Such synergistic interaction between aphidicolin and ara-C has also been observed in other human myeloid leukemia cell lines, U937 and KG-1. In addition, the induction of apoptosis by 9-beta-D arabinofuranosyladenine or 9-beta-D arabinofuranosylguanine is augmented by aphidicolin.

Metallothionein and anticancer agents: the role of metallothionein in cancer chemotherapy

Metallothioneins (MTs) are intracellular proteins containing the highest amount of thiol groups within the cytoplasm. These thiol groups are able to bind several cytotoxic agents, such as platinum compounds and alkylating agents. Increased levels of MT are one mechanism of resistance to these anticancer drugs, as intracytoplasmic binding of MT prevents the active molecules from reaching their target, the intranuclear DNA of tumor cells. MT synthesis can easily be induced by physiologic heavy metals such as zinc and copper. Pharmacological modulation of MT levels has been used to increase the MT pool in normal tissues and decrease their susceptibility to the toxicity of anticancer drugs. In the case of tumors arising in the brain, where the inducibility of MT synthesis is low, this approach would allow protection of normal tissues without decreasing the antitumor activity of the cytotoxic agents. The interaction of MT with cytotoxic agents is not limited to covalent binding. A correlation between MT synthesis and amplification of oncogenes such as ras has been reported. Furthermore, the cytotoxic drugs are bound by MT after competition with zinc and copper; these metals are cofactors of numerous metalloenzymes, some of which are involved in the metabolism of nucleic acids. Competitive displacement of these metals might modify nucleic acid metabolism and influence cellular proliferation. On the other hand, increased MT levels could provide a zinc cofactor reserve that increases the cell's reparative potential when faced by DNA damage by cytotoxic agents.(ABSTRACT TRUNCATED AT 250 WORDS)

DNA damage and cell killing by camptothecin and its derivative in human leukemia HL-60 cells

Camptothecin (CPT) has been recognized as a topoisomerase I (Topo I) inhibitor. However, the mechanism of cytotoxicity of this agent remains unknown. In the present study, we analyzed the kinetics of Topo I-mediated DNA single-strand breaks and internucleosomal DNA cleavage produced by CPT and its derivative, 7-ethyl-10-hydroxycamptothecin (SN-38), in HL-60 cells. DNA single-strand breaks were detected using alkaline sucrose gradient centrifugation when HL-60 cells were incubated with 10 microM CPT or 10 microM SN-38 for 30 min. These DNA single-strand breaks were rapidly repaired after drug removal, while the cytotoxic action of these drugs was sustained. Treatment of HL-60 cells with CPT or SN-38 for 3 h produced extensive degradation of DNA. Agarose gel electrophoresis showed a ladder of DNA fragments consisted of multimers of approximately 200 base pairs, characteristic of apoptosis. Interestingly, this type of DNA fragmentation was also induced within 4 h after repair of DNA single-strand breaks, and subsequently loss of cell viability was observed. When zinc ion, a potent inhibitor of endonuclease, was added to drug-free medium after treatment with CPT or SN-38, internucleosomal DNA cleavage was abolished. Furthermore, addition of zinc ion reduced the loss of cell viability. These data suggest that Topo I-mediated DNA single-strand breaks may be necessary but are not sufficient for cell death, and the endonuclease involved in induction of internucleosomal DNA cleavage may play an important role in HL-60 cell death induced by Topo I inhibitor.

Interleukin 3 protects murine bone marrow cells from apoptosis induced by DNA damaging agents

Murine bone marrow-derived cells, dependent on interleukin 3 (IL-3) for their growth in culture, undergo programmed cell, or apoptosis, upon cytokine withdrawal. Here it is reported that a variety of DNA damaging agents cause a more rapid onset of apoptosis in a factor-dependent cell line, BAF3, deprived of IL-3. In contrast, when cultured in the presence of IL-3, or other growth promoting factors, BAF3 cells are highly resistant to X-irradiation and the cytotoxic drugs etoposide and cisplatin. Overexpression of the bcl2 gene product also protects BAF3 cells from DNA damage. The presence of IL-3 is not required during the initial events of DNA damage or its repair. In the absence of IL-3, cells still complete the repair of DNA breaks within 15 min, and continue to cycle for 5 h. At this time, IL-3 is necessary to prevent the accelerated onset of DNA cleavage from a G2 arrest point.

Inhibition of apoptosis by zinc: a reappraisal

Apoptosis--or programmed cell death--is an active type of cell death, occurring in several pathophysiological conditions. One of the most important characteristics of apoptosis is that cell death is preceded by DNA fragmentation, consequent to the activation of nuclear calcium- and magnesium-dependent endonuclease(s). DNA fragmentation can be inhibited by zinc ions. By using several techniques, such as DNA agarose gel electrophoresis, cytofluorimetric analysis of DNA content and of cell cycle, 3H-thymidine incorporation and trypan blue dye exclusion test, we show that zinc, despite completely inhibiting DNA fragmentation and the consequent loss of nuclear DNA content, does not protect rat thymocytes from spontaneous or dexamethasone-induced death. Our data also suggest that DNA fragmentation, although characteristic, is not a critical event for thymocyte death of apoptotic type.

A simple fluorescence method for surface antigen phenotyping of lymphocytes undergoing DNA fragmentation

Apoptosis, a metabolically active process of programmed cell death characterized by DNA fragmentation, is believed to play an important role in development of lymphocyte repertoires and in embryogenesis. Studies of this phenomenon would be greatly facilitated by the development of a simple assay capable of identifying and isolating intact apoptotic cells. A rapid fluorescence assay which identifies relatively small, intact cells containing fragmented DNA is described in this report. Thymocytes in which DNA fragmentation is induced by culture with or without dexamethasone are readily identified by their bright blue fluorescence after a 15 min treatment with Hoechst 33342, a DNA binding fluorochrome which diffuses through cell membranes. Since Hoechst 33342 staining does not require destruction of the cell membrane, it is possible to directly phenotype cell surface antigen expression on Hoechst 33342bright lymphocytes by conventional immunofluorescence techniques and to evaluate membrane integrity of Hoechst 33342bright cells by dye exclusion criteria. The advantages of this system are that it: (1) is rapid and simple, (2) quantitates the percentage of cells fragmenting their DNA and presumably undergoing apoptosis, (3) permits standard immunofluorescence staining of cell surface markers to identify even minor cell subsets of presumably apoptotic cells within heterogeneous populations, (4) provides the tools (fluorescence activated cell sorting) for purifying intact cells containing fragmented DNA for further biochemical studies, and (5) provides a means for identifying cells which exclude vital dyes and in which DNA fragmentation will eventually result in cell death.

Apoptosis induced by anticancer drugs

Most of the cytotoxic anticancer drugs in current use have been shown to induce apoptosis in susceptible cells. The fact that disparate agents, which interact with different targets, induce cell death with some common features (endonucleolytic cleavage of DNA, changes in chromatin condensation) suggests that cytotoxicity is determined by the ability of the cell to engage this so-called 'programmed' cell death. The mechanism of the coupling of a stimulus (drug-target interaction) to a response (cell death) is not known, but modulation of this coupling may affect the outcome of drug treatment. This review surveys the recent evidence which supports the idea that the drug-target interaction per se is not the sole determinant of cellular sensitivity of cytotoxic drugs. Studies of the signals which might engage apoptosis, the genes which modulate it and the biochemical process of drug-induced apoptosis itself are described, where possible, for glucocorticoids, topoisomerase inhibitors, alkylating agents, antimetabolites and antihormones. It is suggested that identification of the gene products which couple the stimulus to the response, and so determine intrinsic cellular sensitivity (and resistance), will be important targets for new types of drugs. These might then allow responses to occur in the major cancers of man, which are chemoresistant.

The role of the nucleus and other compartments in toxic cell death produced by alkylating hepatotoxicants

Hepatocellular necrosis occurs under a wide range of pathological conditions. In most cases, toxic cell death takes place over a finite span of time, delayed from the point of initial injury and accompanied by homeostatic counterresponses that are varied and complex. The present strategies for discovering critical steps in cell death recognize that (1) different toxins produce similar morphologic changes that precede killing in widely varied cell types, and that (2) lethal events are likely to involve one or more compartmentalized functions that are common to most cells. Investigations of the plasma membrane, endoplasmic reticulum, cytoplasm, mitochondrion, and nucleus have greatly advanced our understanding of acute hepatocellular necrosis. This report examines each compartment but emphasizes molecular changes in the nucleus which may explain cell death caused by alkylating hepatotoxicants. Accumulating knowledge about two distinct modes of cell death, necrosis and apoptosis, indicates that loss of Ca2+ regulation and subsequent damage to DNA may be critical steps in lethal damage to liver cells by toxic chemicals.