Melatonin antagonizes the intrinsic pathway of apoptosis via mitochondrial targeting of Bcl-2

We have recently shown that melatonin antagonizes damage-induced apoptosis by interaction with the MT-1/MT-2 plasma membrane receptors. Here, we show that melatonin interferes with the intrinsic pathway of apoptosis at the mitochondrial level. In response to an apoptogenic stimulus, melatonin allows mitochondrial translocation of the pro-apoptotic protein Bax, but it impairs its activation/dimerization The downstream apoptotic events, i.e. cytochrome c release, caspase 9 and 3 activation and nuclear vesiculation are equally impaired, indicating that melatonin interferes with Bax activation within mitochondria. Interestingly, we found that melatonin induces a strong re-localization of Bcl-2, the main Bax antagonist to mitochondria, suggesting that Bax activation may in fact be antagonized by Bcl-2 at the mitochondrial level. Indeed, we inhibit the melatonin anti-apoptotic effect (i) by silencing Bcl-2 with small interfering RNAs, or with small-molecular inhibitors targeted at the BH3 binding pocket in Bcl-2 (i.e. the one interacting with Bax); and (ii) by inhibiting melatonin-induced Bcl-2 mitochondrial re-localization with the MT1/MT2 receptor antagonist luzindole. This evidence provides a mechanism that may explain how melatonin through interaction with the MT1/MT2 receptors, elicits a pathway that interferes with the Bcl-2 family, thus modulating the cell life/death balance.

Sequential phases of Ca2+ alterations in pre-apoptotic cells

The very early events of the intrinsic, damage-induced apoptotic pathway, i.e., upstream to Bax activation, probably consist of physico-chemical alterations (i.e., redox, pH or Ca2+ changes) rather then subtle molecular interactions, and in spite of many studies they remain unclear. One problem is that cells undergo apoptosis in an asynchronous way, leading to heterogeneity in the cell population that impairs the results of bulk analyses. In this study, we present a flow cytometric approach for studying Ca2+ alteration in apoptosis at the single cell level. By means of a multiparametric analysis, we could discriminate different sub-populations, i.e., viable and apoptotic cells and cells in secondary necrosis, and separately analyse static as well as dynamic Ca2+ parameters in each sub-population. With this approach, we have identified a set of sequential Ca2+ changes; two very early ones occur prior to any other apoptotic alterations, whereas a later change coincides with the appearance of apoptosis. Interestingly, the two pre-apoptotic changes occur simultaneously in all treated cells, i.e., at fixed times post-treatment, whereas the later one occurs at varying times, i.e., within a wide time range, concomitantly with the other apoptotic events.

Melatonin antagonizes apoptosis via receptor interaction in U937 monocytic cells

Among the non-neurological functions of melatonin, much attention is being directed to the ability of melatonin to modulate the immune system, whose cells possess melatonin-specific receptors and biosynthetic enzymes. Melatonin controls cell behaviour by eliciting specific signal transduction actions after its interaction with plasma membrane receptors (MT(1), MT(2)); additionally, melatonin potently neutralizes free radicals. Melatonin regulates immune cell loss by antagonizing apoptosis. A major unsolved question is whether this is due to receptor involvement, or to radical scavenging considering that apoptosis is often dependent on oxidative alterations. Here, we provide evidence that on U937 monocytic cells, apoptosis is antagonized by melatonin by receptor interaction rather than by radical scavenging. First, melatonin and a set of synthetic analogues prevented apoptosis in a manner that is proportional to their affinity for plasma membrane receptors but not to their antioxidant ability. Secondly, melatonin's antiapoptotic effect required key signal transduction events including G protein, phospholipase C and Ca(2+) influx and, more important, it is sensitive to the specific melatonin receptor antagonist luzindole.

Non-apoptogenic Ca2+-related extrusion of mitochondria in anoxia/reoxygenation stress

Tumor cells often develop molecular strategies for survival to anoxia/reoxygenation stress as part of tumor progression. Here we describe that the B lymphoma Epstein-Barr-positive cells E2r survive reoxygenation in spite of a very high and long-lasting increase in cytosolic Ca2+ and the loss of about half of their mitochondria due to specific extrusion of the organelles from the cells. The extrusion typically occurs 3 days after reoxygenation, and a regular mitochondrial asset is regained after further 24 h.

The cleavage mode of apoptotic nuclear vesiculation is related to plasma membrane blebbing and depends on actin reorganization

In U937 monocytic cells induced to apoptosis, plasma membrane blebbing of different intensities appears, before the development of nuclear alterations; this latter phenomenon can occur through two major pathways, namely the cleavage and the budding mode (Dini et al., 1996). Strongly blebbing cells develop deep nuclear constrictions leading to nuclear fragmentation according to the cleavage mode, while cells with milder forms of blebbing, or no blebbing at all, undergo nuclear fragmentation along the budding mode. Compounds interfering with different cytoskeletal components affect blebbing, which is completely inhibited by the actin polymerization inhibitors, cytochalasins, while disturbance of tubulin network with taxol limits blebbing to milder forms. At the same time, the cytoskeletal poisons affect the type of nuclear fragmentation, abolishing the cleavage mode, shifting all events into the budding pathway. Adherent cells, which possess a more structured cytoskeleton, do not develop strong blebs and undergo nuclear fragmentation via budding. These observations suggest that the deep cytoskeletal movements that cause the strongest forms of plasma membrane blebbing strangle the nucleus, leading to the constrictions that later evolve into nuclear fragmentation by cleavage. The trigger for the cytoskeletal movements, known to be redox-sensitive, is probably the apoptotic GSH extrusion.

Molecular determinants involved in the increase of damage-induced apoptosis and delay of secondary necrosis due to inhibition of mono(ADP-ribosyl)ation

ADP-ribosylations are reversible posttranslational modifications that regulate the activity of target proteins, catalyzed by two different classes of enzymes, namely poly(ADP-ribosyl)polymerases (PARPs) and mono(ADP-ribosyl)transferases (ADPRTs). It is now emerging that ADP-ribosylation reactions control signal transduction pathways, mostly as a response to cell damage, aimed at both cell repair and apoptosis. Inhibition of ADPRTs, but not PARPs, increases the extent of apoptosis induced by cytocidal treatments, at the same time delaying secondary necrosis, the process leading to plasma membrane collapse in apoptotic cells, and responsible for apoptosis-related inflammation in vivo. Thus, ADPRT inhibitors may be ideal as adjuvants to cytocidal therapies; to this purpose, we investigated the molecular determinant(s) for such effects by probing a set of molecules with similar structures. We found that the apoptosis-modulating effects were mimicked by those compounds possessing an amidic group in the same position as two of the most popular ADPRT inhibitors, namely, 3-aminobenzamide and nicotinamide. This study may provide useful suggestions in designing molecules with therapeutic potential to be used as adjuvant in cytocidal therapies.

Hyperpolarization of plasma membrane of tumor cells sensitive to antiapoptotic effects of magnetic fields

Chemical/physical agents able to prevent apoptosis are receiving much attention for their potential health hazard as tumor promoters. Magnetic fields (MFs), which have been shown to increase the occurrence of some tumors, reduce damage-induced apoptosis by a mechanism involving Ca2+ entry into cells. In order to discover the mechanism of such effect of MFs, we investigated the interference of MFs on cell metabolism and analyzed cell parameters that are involved in apoptotic signaling and regulation of Ca2+ fluxes. Here we show that different types (static and extremely low-frequency, ELF pulsating) of MFs of different intensities alter plasma membrane potential. Interestingly, MFs induce plasma membrane hyperpolarization in cells sensitive to the antiapoptotic effect of MFs, whereas cells that are insensitive showed a plasma membrane depolarization. These opposite effects suggest that protection against apoptosis and membrane potential modulation are correlated, plasma membrane hyperpolarization possibly being part of the signal transduction chain determining MFs' antiapoptotic effect.

Intracellular pro-oxidant activity of melatonin deprives U937 cells of reduced glutathione without affecting glutathione peroxidase activity

It was long believed that melatonin might counteract intracellular oxidative stress because it was shown to potentiate antioxidant endogenous defences, and to increase the activity of many antioxidant enzymes. However, it is now becoming evident that when radicals are measured within cells, melatonin increases, rather than decreasing, radical production. Herein we demonstrate a pro-oxidant effect of melatonin in U937 cells by showing an increase of intracellular oxidative species and a depletion of glutathione (GSH). The activity of glutathione peroxidase is not modified by melatonin treatment as it does occur in other experimental models.

Analysis of Calcium Changes in Endoplasmic Reticulum during Apoptosis by the Fluorescent Indicator Chlortetracycline

Many studies suggest that endoplasmic reticulum (ER) Ca2+ pool rather than cytosolic Ca2+ may play a crucial role in triggering apoptosis. In this study, we performed an image analysis of cells loaded with the fluorescent dye chlortetracycline (CTC) to in situ analyze Ca2+ changes within the ER in apoptosing promonocytic U937 cells. The results, validated through the use of thapsigargin (THG) as ER Ca2+ depletor, confirm the findings that apoptotic cells have a Ca2+-depleted ER, in contrast with treated but still viable cells.

Redox Modulation of the Apoptogenic Activity of Thapsigargin

Thapsigargin (THG), a selective inhibitor of endoplasmic reticulum (ER) Ca2+-ATPases, causes the rapid emptying of ER Ca2+; in some cell types, this is accompanied by apoptosis, whereas other cells maintain viability. In order to understand the molecular determinants of such a different behavior, we explored the role of oxygen versus nitrogen radicals, by analyzing the apoptogenic ability of THG in the presence of inhibitors of glutathione or nitric oxide (NO) synthesis, respectively. We observed that oxygen radicals play a sensitizing role whereas nitrogen radicals prevent THG-dependent apoptosis, showing that the apoptogenic effect of THG is redox sensitive.

NMR exposure sensitizes tumor cells to apoptosis

NMR technology has dramatically contributed to the revolution of image diagnostic. NMR apparatuses use combinations of microwaves over a homogeneous strong (1 Tesla) static magnetic field. We had previously shown that low intensity (0.3-66 mT) static magnetic fields deeply affect apoptosis in a Ca2+ dependent fashion (Fanelli et al., 1999 FASEBJ., 13;95-102). The rationale of the present study is to examine whether exposure to the static magnetic fields of NMR can affect apoptosis induced on reporter tumor cells of haematopoietic origin. The impressive result was the strong increase (1.8-2.5 fold) of damage-induced apoptosis by NMR. This potentiation is due to cytosolic Ca2+ overload consequent to NMR-promoted Ca2+ influx, since it is prevented by intracellular (BAPTA-AM) and extracellular (EGTA) Ca2+ chelation or by inhibition of plasma membrane L-type Ca2+ channels. Three-days follow up of treated cultures shows that NMR decrease long term cell survival, thus increasing the efficiency of cytocidal treatments. Importantly, mononuclear white blood cells are not sensitised to apoptosis by NMR, showing that NMR may increase the differential cytotoxicity of antitumor drugs on tumor vs normal cells. This strong, differential potentiating effect of NMR on tumor cell apoptosis may have important implications, being in fact a possible adjuvant for antitumor therapies.

Oxidative Upregulation of Bcl-2 in Healthy Lymphocytes

In many cell systems, pharmacological glutathione (GSH) depletion with the GSH neosynthesis inhibitor buthionine sulfoximine (BSO) leads to cell death and highly sensitizes tumor cells to apoptosis induced by standard chemotherapeutic agents. However, some tumor cells upregulate Bcl-2 in response to BSO, thus surviving the treatment and failing to be chemosensitized. Cell lines of monocytic and lymphocytic origins respond to BSO treatment in an opposite way, lymphocytes being chemosensitized and unable to transactivate Bcl-2. In this article we investigate the response to BSO of lymphocytes freshly isolated from peripheral blood of healthy donors. After ensuring that standard separation procedures do not alter per se lymphocytes redox equilibrium nor Bcl-2 levels in the first 24 h of culture, we show that BSO treatment promotes the upregulation of Bcl-2, with a mechanism involving the increased radical production consequent to GSH depletion. Thus, BSO treatment may increase the differential cytocidal effect of cytotoxic drugs in tumor versus normal lymphocytes.

Melatonin as an Apoptosis Antagonist

The pineal hormone melatonin (Mel), in addition to having a well-established role as a regulator of circadian rhythms, modulates nonneural compartments by acting on specific plasma membrane receptors (MT1/MT2) present in many different cell types. Mel plays immunomodulatory roles and is an oncostatic and antiproliferative agent; this led to the widespread belief that Mel may induce or potentiate apoptosis on tumor cells, even though no clear indications have been presented so far. Here we report that Mel is not apoptogenic on U937 human monocytic cells, which are known to possess MT1 receptors at the times (up to 48 h) and doses (up to 1 mM) tested. Mel does not even potentiate apoptosis, but instead, significantly reduces apoptosis induced by both cell-damaging agents (intrinsic pathway) and physiological means (extrinsic pathway). The doses required for the antiapoptotic effect (>or=100 microM) are apparently not compatible with receptor stimulation (receptor affinity<1 nM). However, receptor involvement cannot be ruled out, because we discovered that the actual Mel concentration active on cells was lower than the nominal one because of sequestration by fetal calf serum (FCS). Accordingly, in FCS-free conditions, Mel doses required for a significant antiapoptotic effect are much lower.

Magnetic Fields Protect from Apoptosis via Redox Alteration

Magnetic fields (MFs) are receiving much attention in basic research due to their emerging ability to alter intracellular signaling. We show here that static MFs with intensity of 6 mT significantly alter the intracellular redox balance of U937 cells. A strong increase of reactive oxygen species (ROS) and a decrease of glutathione (GSH) intracellular levels were found after 2 h of MF exposure and maintained thereafter. We found that also other types of MFs, such as extremely-low-frequency (ELF) MFs affect intracellular GSH starting from a threshold at 0.09 mT. We previously reported that static MFs in the intensity range of 0.3-60 mT reduce apoptosis induced by damaging agents (Fanelli et al., 1998). Here, we show that ELF-MFs are also able to protect U937 from apoptosis. Interestingly, this ability is limited to the ELF intensities able to alter redox equilibrium, indicating a link between MF's antiapoptotic effect and the MF alteration of intracellular redox balance. This suggests that MF-produced redox alterations may be part of the signaling pathway leading to apoptosis antagonism. Thus, we tested whether MFs may still exert an antiapoptotic action in cells where the redox state was artificially altered in both directions, that is, by creating an oxidative (via GSH depletion with BSO) or a reducing (with DTT) cellular environment. In both instances, MFs fail to affect apoptosis. Thus, a correct intracellular redox state is required in order for MFs to exert their antiapoptotic effect.

Different fates of intracellular glutathione determine different modalities of apoptotic nuclear vesiculation

U937 monocytic cells show two main apoptotic nuclear morphologies, budding and cleavage, that are the result of two independent morphological routes, since they never interconvert one into the other, and are differently modulated by stressing or physiological apoptogenic agents [Exp Cell Res 1996; 223:340-347]. With the aim of understanding which biochemical alterations are at the basis of these alternative apoptotic morphologies, we performed an in situ analysis that showed that in U937 cells intracellular glutathione (GSH) is lost in cells undergoing apoptosis by cleavage, whereas it is maintained in apoptotic budding cells. Lymphoma cells BL41 lose GSH in apoptosis, and show the cleavage nuclear morphology; the same cells latently infected with Epstein Barr Virus (E2r line) undergo apoptosis without GSH depletion and show the budding nuclear morphology. GSH depletion is not only concomitant to, but is the determinant of the cleavage route, since the inhibition of apoptotic GSH efflux with cystathionine or methionine shifts the apoptotic morphology from cleavage to budding. Accordingly, cystathionine or methionine antagonizes apoptosis in the all-cleavage BL41, without affecting the all-budding E2r.

Apoptotic GSH Extrusion Is Associated with Free Radical Generation

Reactive oxygen species (ROS) are involved in many forms of apoptosis and mediate apoptosis in a number of cell types. In this paper, we use a variant of U937 monocytic cells (U937 HX) that show different biochemical features with respect to standard U937. Apoptotic standard U937 extrude reduced glutathione (GSH) and generate free radicals concomitantly with loss of mitochondria transmembrane potential (mt Deltapsi). These events are correlated with the extrusion of intracellular GSH. Conversely, apoptotic U937 HX cells retain GSH, and the loss of mt Deltapsi is not accompanied by generation of free radicals. The perfect inverse correlation between (a) ROS generation and (b) the presence of intracellular GSH during apoptosis suggests novel mechanisms to finely tune ROS generation in apoptosis.

Cytosolic and Endoplasmic Reticulum Ca2+Concentrations Determine the Extent and the Morphological Type of Apoptosis, Respectively

During apoptosis, an increase in cytosolic Ca(2+) concentration ([Ca(2+)](c)) accompanies the depletion of endoplasmic reticulum (ER). The actual roles of each of the two events in apoptosis are difficult to understand. In this work, we have modulated the basal [Ca(2+)](c) and the thapsigargin (THG)-dependent reticular flux (i.e., by chelating extracellular Ca(2+) or by modulating intracellular Ca(2+) by 3-aminobenzamide [3-ABA]). We have found that these treatments alter these Ca(2+) parameters in a differential way and, accordingly, affect apoptosis differentially. We have found that the increase in [Ca(2+)](c) is related to the extent of apoptosis, whereas the ER depletion affects the apoptotic nuclear morphology by shifting it towards the cleavage mode.

[Lack of direct cytotoxic effect of intracellular nanotubes]

Nanotubes have a great therapeutic potential due to their astounding physico-chemical features, the possibility to be funtionalised for ad hoc uses, and the specific interaction of nanotubes as such with life molecules (DNA and proteins). These features recommend a thorough toxicological study before widespread pharmaceutic use. We provide evidence that culture cells with phagocytic potential internalise multi wall nanotubes (10-50 nm average size). This is not accompanied by cytotoxicity in terms of induction of &apoptosis or necrosis at the doses used (up to 125 microg/mI).

Oxidative Bax dimerization promotes its translocation to mitochondria independently of apoptosis

Bax is a cytosolic protein, which in response to stressing apoptotic stimuli, is activated and translocates to mitochondria, thus initiating the intrinsic apoptotic pathway. In spite of many studies and the importance of the issue, the molecular mechanisms that trigger Bax translocation are still obscure. We show by computer simulation that the two cysteine residues of Bax may form disulfide bridges, producing conformational changes that favor Bax translocation. Oxidative, nonapoptogenic treatments produce an up-shift of Bax migration compatible with homodimerization, which is reverted by reducing agents; this is accompanied by translocation to mitochondria. Dimers also appear in pure cytosolic fractions of cell lysates treated with H2O2, showing that Bax dimerization may take place in the cytosol. Bax dimer-enriched lysates support Bax translocation to isolated mitochondria much more efficiently than untreated lysates, indicating that dimerization may promote Bax translocation. The absence of apoptosis in our system allows the demonstration that Bax moves because of oxidations, even in the absence of apoptosis. This provides the first evidence that Bax dimerization and translocation respond to oxidative stimuli, suggesting a novel role for Bax as a sensor of redox imbalance.

Proapoptotic activity of new glutathione S-transferase inhibitors

Selected 7-nitro-2,1,3-benzoxadiazole derivatives have been recently found very efficient inhibitors of glutathione S-transferase (GST) P1-1, an enzyme which displays antiapoptotic activity and is also involved in the cellular resistance to anticancer drugs. These new inhibitors are not tripeptide glutathione-peptidomimetic molecules and display lipophylic properties suitable for crossing the plasma membrane. In the present work, we show the strong cytotoxic activity of these compounds in the following four different cell lines: K562 (human myeloid leukemia), HepG2 (human hepatic carcinoma), CCRF-CEM (human T-lymphoblastic leukemia), and GLC-4 (human small cell lung carcinoma). The LC50 values are in the micromolar/submicromolar range and are close to the IC50 values obtained with GSTP1-1, suggesting that the target of these molecules inside the cell is indeed this enzyme. The cytotoxic mechanism of 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol, the most effective GSTP1-1 inhibitor, has been carefully investigated in leukemic CCRF-CEM and K562 cell lines. Western blot and immunoprecipitation analyzes have shown that 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio)hexanol promotes in both cell lines the dissociation of the GSTP1-1 in a complex with c-jun NH2-terminal kinase (JNK). This process triggers a reactive oxygen species (ROS)-independent activation of the JNK-mediated pathway that results in a typical process of apoptosis. Besides this main pathway, in K562 cells, a ROS-mediated apoptosis partially occurs (about 30%) which involves the p38MAPK signal transduction pathway. The low concentration of this new compound needed to trigger cytotoxic effects on tumor cells and the low toxicity on mice indicate that the new 7-nitro-2,1,3-benzoxadiazole derivatives are promising anticancer agents.

Glutathione depletion up-regulates Bcl-2 in BSO-resistant cells

Glutathione depletion by inhibition of its synthesis with buthionine sulfoximine (BSO) is a focus of the current research in antitumor therapy, BSO being used as chemosensitizer. We had previously shown that two human tumor cell lines (U937 and HepG2) survive to treatment with BSO: BSO can elicit an apoptotic response, but the apoptotic process is aborted after cytochrome c release and before caspase activation, suggesting the development of an adaptive response (FASEB J., 1999, 13, 2031-2036). Here, we investigate the mechanisms of such an adaptation. We found that following BSO, U937 up-regulate Bcl-2 mRNA and protein levels, by a mechanism possibly involving NF-kappaB transcription factor; the increase in protein level is limited by a rapid decay of Bcl-2 in BSO-treated cells, suggesting that redox imbalance speeds up Bcl-2 turnover. BSO-dependent Bcl-2 up-regulation is associated with the ability to survive to BSO. Indeed, 1) its abrogation by CAPE or protein synthesis inhibition sensitizes U937 to BSO; 2) in a panel of four tumor lines, BSO-resistant (U937, HepG2, and HGB1) but not BSO-sensitive (BL41) cells can up-regulate Bcl-2 following GSH depletion; remarkably, only the latter are chemosensitized by BSO.

Rescue of cells from apoptosis by antioxidants occurs downstream from GSH extrusion

Antioxidants-that is, scavengers of free radicals and anaerobic conditions (5%CO(2), 95% N(2))-protect monocytic U937 cells from damage-induced apoptosis. Antioxidants rescue the cells acting on the apoptotic pathway at a step downstream from gluthatione extrusion. Reducing agents, such as DTT, also reduce stress-induced apoptosis. Thus, apoptotic GSH extrusion triggers the downstream events of apoptosis by leaving cells unprotected against thiol\s oxidation and radical production.

Hypoxic stress stably alters apoptotic parameters on U937 cells

Tumor promonocytic U937 cells cultured under a low O(2)/high CO(2) atmosphere display altered characteristics after restoration of normal atmosphere: increased resistance to apoptosis induced by different treatments; apoptotic morphology; lack of glutathione (GSH) extrusion in apoptosis; lack of protection by antioxidants; and lack of Ca(2+) mobilization with thapsigargin. These alterations were stably maintained for many months of culture in normal conditions, originating the stable U937-HX variant. Since the hypoxic treatment did not produce a great selective pressure, the alterations are conceivably the result of stable adaptative response.

Static magnetic fields affect calcium fluxes and inhibit stress-induced apoptosis in human glioblastoma cells

BACKGROUND

Epidemiologic data revealed increased brain tumor incidence in workers exposed to magnetic fields (MFs), raising concerns about the possible link between MF exposure and cancer. However, MFs seem to be neither mutagenic nor tumorigenic. The mechanism of their tumorigenic effect has not been elucidated.

METHODS

To evaluate the interference of MFs with physical (heat shock, HS) and chemical (etoposide, VP16) induced apoptoses, respectively, we exposed a human glioblastoma primary culture to 6 mT static MF. We investigated cytosolic Ca(2+) ([Ca(2+)](i)) fluxes and extent of apoptosis as key endpoints. The effect of MFs on HS- and VP16-induced apoptoses in primary glioblastoma cultures from four patients was also tested.

RESULTS

Static MFs increased the [Ca(2+)](i) from a basal value of 124 +/- 4 nM to 233 +/- 43 nM (P < 0.05). MF exposure dramatically reduced the extent of HS- and VP16-induced apoptoses in all four glioblastoma primary cultures analyzed by 56% (range, 28-87%) and 44% (range, 38-48%), respectively. However, MF alone did not exert any apoptogenic activity. Differences were observed across the four cultures with regard to apoptotic induction by HS and VP16 and to MF apoptotic reduction, with an individual variability with regard to apoptotic sensitivity.

CONCLUSION

The ability of static MFs to reduce the extent of damage-induced apoptosis in glioblastoma cells might allow the survival of damaged and possibly mutated cells.