Effect of polyamine depletion on caspase activation: a study with spermine synthase-deficient cells

Activation of the caspase proteases represents a central point in apoptosis. The requirement for spermine for the processes leading to caspase activation has been studied in transformed embryonic fibroblasts obtained from gyro (Gy) mutant male mice. These cells lack spermine synthase activity and thus provide a valuable model to study the role of spermine in cell processes. Gy fibroblasts do not contain spermine and have a higher spermidine content. However, when compared with fibroblasts obtained from normal male littermates (N cells), Gy fibroblasts were observed to grow normally. The lack of spermine did not affect the expression of Bcl-2, and caspases 3 and 9 were activated by etoposide in both N and Gy cells, indicating that spermine is dispensable for caspase activation. Spermine deficiency did not significantly influence caspase activity in cells treated with etoposide, cycloheximide or staurosporine, but sensitized the cells to UV irradiation, which triggered significantly higher caspase activity in Gy cells compared with N cells. alpha-Difluoromethylornithine (DFMO), an inhibitor of polyamine synthesis that is able to deplete cells of putrescine and spermidine, but usually does not influence spermine content, was able to produce a more complete polyamine depletion in Gy cells. This depletion, which included spermine deficiency, dramatically increased caspase activation and cell death in Gy fibroblasts exposed to UV irradiation. On the other hand, in either N or Gy cells, DFMO treatment did not influence caspase activity triggered by staurosporine, but inhibited it when the inducers were cycloheximide or etoposide. In Gy cells depleted of polyamines by DFMO, polyamine replenishment with either spermidine or spermine was sufficient to restore caspase activity induced by etoposide, indicating that, in this model, polyamines have an interchangeable role in supporting caspase activation. Therefore, spermine is not required for such activation, and the effect and specificity of polyamine depletion on caspase activity may be very different, depending on the role of polyamines in the specific death pathways engaged by different stimuli. Some inducers of apoptosis, for example etoposide, absolutely require polyamines for caspase activation, yet the lack of polyamines, particularly spermine, strongly increases caspase activation when induced by UV irradiation.

Polyamines, NO and cGMP mediate stimulation of DNA synthesis by tumor necrosis factor and lipopolysaccharide in chick embryo cardiomyocytes

OBJECTIVE

We have recently shown that tumor necrosis factor-alpha (TNFalpha) and lipopolysaccharide (LPS) stimulate DNA synthesis in chick embryo cardiomyocytes (CMs). The aim of the present research was to investigate the pathways involved in this mitogenic response.

METHODS

CMs were isolated from 10-day-old chick embryos and grown to confluence. After 20 h of serum starvation the cells were treated with TNFalpha and LPS, and/or specific agonists and antagonists to manipulate the levels of polyamines, NO, cGMP and their biosynthetic enzymes ornithine decarboxylase (ODC), nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC). ODC, NOS, sGC activities and cGMP contents were determined by radiochemical procedures. DNA synthesis was determined by incorporation of [3H]-thymidine.

RESULTS

Treatment of CMs with TNFalpha and LPS increased cell number and [3H]-thymidine incorporation. Addition of TNFalpha and LPS provoked an induction of ODC, with consequent polyamine accumulation, and a more delayed enhancement of NOS activity, which appeared to be independent of the activation of the ODC-polyamine system. TNFalpha and LPS treatment also enhanced cGMP level in CMs and both polyamine and NO biosyntheses appeared to be required. Experiments with specific inhibitors of ODC and NOS, as well as with inhibitors of sGC and cGMP-dependent protein kinase (PKG), showed that polyamine-, NO- and cGMP-dependent pathways are required for the mitogenic action of TNFalpha and LPS. Moreover, addition of exogenous polyamines to untreated cells raised the cGMP level in a NO-dependent fashion, and enhanced [3H]-thymidine incorporation. The latter effect was inhibited by sGC or PKG inhibitors. Treatment of quiescent cells with NO donors, 8-bromo-cGMP or YC-1, an sGC activator, also promoted DNA synthesis. Furthermore, putrescine and NO donor can additively activate sGC in cell-free extracts.

CONCLUSION

TNFalpha and LPS stimulate DNA synthesis in chick embryo CMs and this effect is mediated by polyamines, NO and intracellular cGMP.

Polyamines directly induce release of cytochrome c from heart mitochondria

Cytochrome c release from mitochondria to the cytosol represents a critical step in apoptosis, correlated to the activation of the caspase cascade. In this report, we show that addition of micromolar concentrations of polyamines to isolated rat heart mitochondria induces the release of cytochrome c. Spermine, which is effective at concentrations of 10-100 microM, is more potent than spermidine, whereas putrescine has no effect up to 1 mM. The release of cytochrome c caused by spermine is a rapid, saturable and selective process that is independent of mitochondria damage. Spermine, unlike polylysine, is able to release a discrete amount of cytochrome c from intact, functional mitochondria. The cytochrome c-releasing power of spermine is not affected by cyclosporin A, differently from the effect of permeability transition inducers. In a cardiac cell-free model of apoptosis, the latent caspase activity of cytosolic extracts from cardiomyocytes could be activated by cytochrome c released from spermine-treated heart mitochondria. These data indicate a novel mechanism of cytochrome c release from the mitochondrion, and suggest that prolonged and sustained elevation of polyamines, characteristic of some pathologies such as heart hypertrophy, could be involved in the development of apoptosis.

Molecular mechanisms of response to low oxygen tension in the vascular wall

A number of human diseases are linked to local reduced oxygen availability. Hypoxemia, the condition in which oxygen partial pressure in blood falls below 40 mmHg, generates a distress which leads the cells in the vascular wall to activate a genetic program inducing a homeostatic response. The effectiveness of this response is conditioned by the degree and duration of the hypoxic stress and depends on the equilibrium among several factors which are worked out mainly in the vascular endothelial cell layer. Among them are vasoconstrictors such as angiotensin II, endothelins, prostaglandins and thromboxans, and vasodilators such as nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor. A present challenge of the research is understanding the physiological and pathophysiological relevance of the growing body of data collected, disclosing the potential therapeutical application of the basic knowledge in this field.

p44/42 mitogen-activated protein kinase is involved in the expression of ornithine decarboxylase in leukaemia L1210 cells

The involvement of p44/42 mitogen-activated protein kinase (MAPK) in the induction of ornithine decarboxylase (ODC) was investigated by using PD98059, a specific MAPK-kinase (MEK1/2) inhibitor, and other signal-transduction inhibitors. In d,l-alpha-difluoromethylornithine (DFMO)-resistant L1210 cells stimulated to grow from quiescence, treatment with PD98059 inhibited p44/42 MAPK phosphorylation and the induction of ODC activity and protein. A marked reduction of the accumulation of mature ODC mRNA and its intron-containing precursor was observed, whereas ODC turnover was hardly affected. PD98059 also reduced the content of antizyme, but not that of antizyme mRNA. U0126, a novel and more potent inhibitor of MEK1/2, provoked a dose-dependent inhibition of ODC induction at lower concentrations with respect to PD98059. Other effective inhibitors of ODC induction proved to be genistein, manumycin A, herbimycin A, LY294002, wortmannin and KT5823, suggesting the involvement of other key proteins of signal-transduction pathways, i.e. Ras, Src, phosphatidylinositol 3-kinase and cGMP-dependent protein kinase, which may have a positive impact on MAPK. Cells kept in a DFMO-free medium, and thus containing high levels of putrescine and spermidine, showed enhanced MAPK phosphorylation and lower sensitivity to PD98059, compared with cells maintained in the presence of DFMO. In conclusion, these results indicate that the activation of p44/42 MAPK may favour the expression of ODC, and that polyamines, in turn, may affect the phosphorylation state of MAPK.

Nitric oxide can function as either a killer molecule or an antiapoptotic effector in cardiomyocytes

Caspase enzymes are a family of cysteine proteases that play a central role in apoptosis. Recently, it has been demonstrated that caspases can be S-nitrosylated and inhibited by nitric oxide (NO). The present report shows that in chick embryo heart cells (CEHC), NO donor molecules such as S-nitroso-N-acetylpenicillamine (SNAP), S-nitrosoglutathione, spermine-NO or sodium nitroprusside inhibit caspase activity in both basal and staurosporine-treated cells. However, the inhibitory effect of NO donors on caspase activity is accompanied by a parallel cytotoxic effect, that precludes NO to exert its antiapoptotic capability. N-Acetylcysteine (NAC) at a concentration of 10 mM blocks depletion of cellular glutathione and cell death in SNAP-treated CEHC, but it poorly affects the ability of SNAP to inhibit caspase activity. Consequently, in the presence of NAC, SNAP attenuates not only caspase activity but also cell death of staurosporine-treated CEHC. These data show that changes in the redox environment may inhibit NO-mediated toxicity, without affecting the antiapoptotic capability of NO, mediated by inhibition of caspase enzymes. NO may thus be transformed from a killer molecule into an antiapoptotic agent.

Spermine triggers the activation of caspase-3 in a cell-free model of apoptosis

Polyamines are ubiquitous organic cations required for cell proliferation. However, some evidence suggested that their excessive accumulation can induce apoptosis. We show here that, in a post-nuclear extract from U937 cells, the addition of spermine triggers the death program, represented by cytochrome c exit from mitochondria, the dATP-dependent processing of pro-caspase-3 and the onset of caspase activity. Spermine is more effective than spermidine, whereas putrescine has no effect. Polyamine acetylation abolishes their pro-apoptotic power. These data demonstrate a direct mechanism responsible for polyamine toxicity and also suggest that an excessive elevation of free polyamines could be involved in the transduction of a death signal.

Nitric oxide mediates either proliferation or cell death in cardiomyocytes. Involvement of polyamines

Nitric oxide (NO) is a molecule involved in several signal transduction pathways leading either to proliferation or to cell death. Induction of ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis, represents an early event preceding DNA synthesis. In some cell types increased ODC activity seems to be involved in cytotoxic response. We investigated the role of NO and ODC induction on the events linked to cell proliferation or to cell death in cultured chick embryo cardiomyocytes. Exposure of cardiomyocytes to tumor necrosis factor (TNF) and lipopolysaccharide (LPS) caused NO synthase (NOS) and ODC induction as well as increased incorporation of [3H]-thymidine. This last effect was blocked by a NOS inhibitor and was strongly reduced by difluoromethylornithine (DFMO), an irreversible inhibitor of ODC. Sodium nitroprusside (SNP), an exogenous NO donor, inhibited the increases of NOS and ODC activities and abolished the mitogenic effect of TNF and LPS. Moreover, SNP alone caused cell death in a dose dependent manner. The cytotoxicity of SNP was not affected by DFMO while it was prevented by antioxidants. The results suggest that different pathways would mediate the response of cardiomyocytes to NO: they can lead either to ODC induction and DNA synthesis when NO is formed through NOS induction or to growth inhibition and cell death, when NO is supplied as NO donor. Increased polyamine biosynthesis would mediate the proliferative response of NO, while the cytotoxicity of exogenous NO seems to involve some oxidative reactions and to depend on the balance between NO availability and cellular redox mechanisms.

Spermine causes caspase activation in leukaemia cells

Exposure of several leukaemia cell types to the polyamine spermine triggered caspase activation. In HL60 cells, the onset of caspase activity correlated with the accumulation of spermine, and was accompanied by the processing of the caspase-3 precursor and the digestion of the substrate proteins PARP and gelsolin. Spermine also induced the accumulation of cytochrome c in the cytosol. Caspase activation triggered by spermine was not blocked by antioxidants or inhibition of polyamine oxidase. The deregulation of polyamine uptake strongly sensitised the cells to spermine-induced caspase activation. These data show that an excessive intracellular level of spermine triggers caspase activation that is not mediated by oxidative mechanisms, and suggest a model where elevated free cytosolic polyamines may act as transducers of a death message.

Inhibition of etoposide-induced apoptosis with peptide aldehyde inhibitors of proteasome

Recent investigations have indicated the involvement of proteasome in programmed cell death. The present studies show that although peptide aldehyde inhibitors of proteasome are by themselves weak inducers of apoptosis, they inhibit the apoptotic effect of the anticancer drug etoposide in rat thymocytes. Acetyl-Leu-Leu-norvalinal (LLnV-al) and other related peptide aldehydes inhibited the increase in caspase activity and DNA fragmentation that followed treatment with etoposide and their effect was related to their potency as proteasome inhibitors. To inhibit etoposide-induced apoptosis, LLnV-al must be present within 3 h of treatment with etoposide, in the same way as the inhibitor of protein synthesis cycloheximide must be. Etoposide caused a rapid accumulation of p53 protein that was not inhibited by LLnV-al, which was also a strong inducer of p53. Peptide aldehydes were also weak activators of caspase activity, suggesting that the same mechanism, i.e. the blocking of proteasome function, both triggers apoptosis and inhibits the effect of etoposide. These results are consistent with a model in which proteasome is selectively involved in the pathway used by etoposide to induce cell suicide.

Simultaneous detection of reduced and oxidized glutathione in tissues and mitochondria by capillary electrophoresis

We have developed a rapid and precise method for glutathione quantitation by capillary electrophoresis, that allows a low amount of both redox forms to be measured. Small fragments of rat heart or liver tissues (20 mg wet weight) and the corresponding mitochondria (1 mg protein) were homogenized in 1% perchloric acid and the acid-soluble phase ultrafiltered by centrifugation with a microconcentrator (Mr cut-off 3000 Da). The analysis was performed at a constant temperature (28 degrees C) using a Beckman P/ACE System 2100, equipped with a UV absorbance detector set to 200 nm. The limit of quantitation in heart tissue was 1.8 microM for GSH and 1.2 microM for GSSG. Myocardial concentrations of GSH and GSSG were 8.1 +/- 2.6 and 0.45 +/- 0.15 (nmol/mg protein +/- S.D.), respectively. The ratio of GSH to GSSG was 17.8 +/- 1.3 for heart tissue, whereas it was much higher (>100) in the mitochondria. An oxidative stress decreased the myocardial tissue GSH/GSSG ratio, indicating that the CE analysis of both glutathione forms is also a useful method to study biological redox modification.

Increase of neuronal nitric oxide synthase in rat skeletal muscle during ageing

Nitric oxide synthases (NOS) are different widely expressed enzymes which produce the molecular messenger nitric oxide. The neuronal isoform of NOS (nNOS) is involved in several processes of the cell metabolism, most of which are, at present, not fully understood (neurotransmission, smooth muscle motility, myoblast and myocyte biology and others). In skeletal muscle nNOS is present mainly at the plasmalemma, where it is attached to the dystrophin-related proteins; in fact, in pathologies involving dystrophin, nNOS is altered as well. We report that in aged rats the nNOS amount in skeletal muscle increases both in the soluble and microsomal fractions and that an additional intracytoplasmic localisation appears.

Inhibition of glucocorticoid-induced apoptosis with 5-aminoimidazole-4-carboxamide ribonucleoside, a cell-permeable activator of AMP-activated protein kinase

The AMP-activated protein kinase (AMPK) is related to a growing family of protein kinases that are believed to protect cells against environmental and nutritional stress. In the present study the hypothesis of a protective role for AMPK against thymocyte apoptosis has been tested. It is shown that AMPK is expressed in rat thymocytes that contain the transcript for the a1 isoform of the AMPK catalytic subunit and can be activated by treatment with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a well-established activator of AMPK. AICAR is not toxic and prevents glucocorticoid-induced apoptosis in the same concentration range used to activate AMPK. At concentrations higher than 1 mM, AICAR fully restores cell viability and inhibits DNA laddering in dexamethasone-treated thymocytes. Furthermore, AICAR blocks the dexamethasone-induced activation of caspase 3-like enzymes, which are believed to play a pivotal role in apoptotic cell death. Activation of AMPK by oligomycin, which depletes thymocytes of ATP, is also correlated to inhibition of caspase 3-like activity in dexamethasone-treated cells. However, AICAR and oligomycin do not exert any protective action when apoptosis is induced by staurosporine. These results indicate that AICAR is a powerful inhibitor of glucocorticoid-induced apoptosis and suggest that AMPK activation may interfere with a step in the apoptotic cascade triggered by dexamethasone.

Phosphatidylinositol 3-kinase is required for the induction of ornithine decarboxylase in leukemia cells stimulated to growth

The involvement of phosphatidylinositol 3-kinase (PI3K) in the induction of ornithine decarboxylase (ODC) was investigated by using specific PI3K inhibitors. In difluoromethylornithine-resistant L1210 cells stimulated to growth from quiescence, treatment with LY294002 inhibited cell growth and provoked a complete block of the induction of ODC activity (IC50 approximately 2 microM) and ODC protein. Some reduction in the accumulation of ODC mRNA was also observed, whereas ODC turnover was not affected significantly. Wortmannin, another specific inhibitor of PI3K, structurally unrelated to LY294002, also inhibited ODC induction with an IC50 of about 10 nM. These results indicate that PI3K activity is required for the induction of ODC, possibly affecting both ODC mRNA level and translation. Since p70 S6 kinase (p70S6K) is considered an important mediator of PI3K action in several experimental systems, the effect of rapamycin, which can lead to selective inhibition of p70S6K, was also investigated. Rapamycin inhibited p70S6K activity and produced ODC inhibiting effects similar to those elicited by LY294002. However, LY294002 and wortmannin at concentrations which inhibited almost completely PI3K activity did not decrease p70S6K activity, suggesting that p70S6K does not mediate the PI3K effects on ODC, but may lie on a separate pathway in this experimental model.

ATP depletion inhibits glucocorticoid-induced thymocyte apoptosis

In quiescent thymocytes, mitochondrial de-energization was not correlated to apoptotic death. In fact, thymocytes treated with oligomycin, a highly specific inhibitor of ATP synthase, alone or with atractyloside to block ATP translocation from the cytoplasm, were alive, even if their mitochondria were depolarized, as revealed by flow cytometry after Rhodamine 123 staining. Furthermore, oligomycin was a powerful inhibitor of apoptosis induced in rat thymocytes by dexamethasone and, to a lesser extent, by the calcium ionophore A23187 and etoposide, but was without effect when apoptosis was induced by staurosporine, and increased cell death in mitogen-treated thymocytes. The inhibition of apoptosis was confirmed by morphological criteria, inhibition of inter-nucleosomal DNA fragmentation and inhibition of the loss of membrane integrity. The anti-apoptotic effect of oligomycin in cells treated with A23187 or etoposide was correlated to the inhibition of protein synthesis, while inhibition of apoptosis induced by dexamethasone, already evident at an oligomycin concentration of 10 ng/ml, was instead strictly correlated to the effect exerted on the cellular ATP level. Thymocyte apoptosis triggered by dexamethasone was blocked or delayed by inhibitors of respiratory-chain uncouplers, inhibitors of ATP synthase and antioxidants: a lasting protection from dexamethasone-induced apoptosis was always correlated to a drastic and rapid reduction in ATP level (31-35% of control), while a delay in the death process was characterized by a moderate decrease in ATP (73-82% of control). Oligomycin inhibited the specific binding of radioactive corticosteroid to thymocyte nuclei, confirming the inhibitory effect of ATP depletion on glucocorticoid binding and suggesting that ATP depletion is a common mediator of the anti-apoptotic action of different effectors in glucocorticoid-induced apoptosis. In conclusion, the reported data indicate that ATP may act as a cellular modulator of some forms of apoptosis, depending on the death trigger, and that in quiescent cells the de-energization of mitochondria is not necessarily linked to apoptosis.

Presence of a DNA-4236 bp deletion and 8-hydroxy-deoxyguanosine in mouse cardiac mitochondrial DNA during aging

A particular mitochondrial DNA (mtDNA) deletion, the so-called "common deletion", accumulates progressively with age in human post-mitotic cells. We investigated the presence of age-related mtDNA deletions in mouse heart and, according to the free-radical theory of aging, the potential involvement of reactive oxygen species (ROS). Hearts from young, adult and old male Balb/c mice were homogenized and centrifuged in order to discard nuclear DNA. The supernatant was then utilized to prepare mtDNA by SDS-proteinase K digestion. The presence of a mtDNA4236 deletion was estimated by PCR analysis, by separating the amplificated segment on agarose gel. The incidence of the mtDNA4236 deletion was 16%, 28% and 78% in young, adult and old mice, respectively. 8-hydroxy-2'-deoxyguanosine (8-OH-dG), a marker of DNA oxidation, was also determined by HPLC-electrochemical analysis. 8-OH-dG was not detectable in young mice, while its concentrations (moles 8-OH-dG/10(6) moles dG; mean +/- SD) were 59.0 +/- 1.41 and 31.0 +/- 4.24 (p < 0.02) in adult and old mice, respectively. These data indicate that a mtDNA4236 deletion is progressively associated with aging in mouse hearts, and that oxidative damage to mtDNA is greater in middle-aged than senescent animals.

Role of reactive oxygen species in cardiovascular aging

Biochemical and structural changes occurring in the myocardium with aging are mainly resulting from the association of a general tissue atrophy with the hypertrophy of the remaining myocytes. Whilst hypertrophy seems to be a compensatory process to the loss of cardiomyocytes and to a mild systolic hypertensive condition that accompanies elderly people, atrophy should be the modification more closely related to aging 'per se.' In support to the free radical theory of aging, several signs of oxidative damage have been shown in the aged heart, such as lipofuscin accumulation, decreased phospholipid unsaturation index, greater formation of both hydrogen peroxide and 8-hydroxy-2'deoxyguanosine. As a compensatory reaction, the activities of the main oxygen-radical scavenger enzymes are stimulated in the mitochondria of aged rat heart. Endothelium-mediated vasoregulation is more susceptible to oxidative stress in aged with respect to young rats, suggesting that also the vasculature can be negatively influenced by the oxygen free radicals generated during aging. The possible primary role of oxygen free radicals in the development of myocardial atrophy is also discussed.

Alpha-tocopherol pretreatment improves endothelium-dependent vasodilation in aortic strips of young and aging rats exposed to oxidative stress

Acetylcholine-induced, endothelium-dependent relaxation of norepinephrine-precontracted aortic strips, was severely impaired after exposure to a hypoxanthine/xanthine oxidase reaction generating oxygen radicals. This effect was more evident in aortic strips of aging rats (24 months old) in comparison to young rats (3 months old). The addition of authentic .NO (1 microM) completely relaxed aortic strips exposed to oxidative stress both in young and aging rats. In vitro EPR measurements showed that the .NO signal was reduced by enzymatic O2.- generating reaction. The activity of a partial purified preparation of constitutive NO synthase from rat cerebellum was significantly decreased after exposure to exogenous oxygen radicals. Pretreatment of aortic strips with 100 microM alpha-tocopherol-phosphate, produced a significant improvement of acetylcholine-dependent relaxation in the aortic strips exposed to oxidative stress, particularly in the aged vessel. The content of malondialdehyde in aortic tissue did not change after oxidative stress or alpha-tocopherol pretreatment. Alpha-tocopherol was unable to recover the NO synthase activity depressed in vitro by hypoxanthine/xanthine oxidase reaction. This study confirms that an oxidative stress impairs the endothelium-mediated vasodilation. Alpha-tocopherol pretreatment protects the vessel against this damage. The mechanism of action of alpha-tocopherol is unknown, but seems unrelated to an antioxidant activity.

Induction of nitric oxide synthase mRNA expression. Suppression by exogenous nitric oxide

The reactive nitrogen species, nitric oxide (NO), plays an important role in the pathogenesis of neurodegenerative diseases. The suppression of NO production may be fundamental for survival of neurons. Here, we report that pretreatment of human ramified microglial cells with nearly physiological levels of exogenous NO prevents lipopolysaccharide (LPS)/tumor necrosis factor alpha (TNF alpha)-inducible NO synthesis, because by affecting NF-kappa B activation it inhibits inducible Ca(2+)-independent NO synthase isoform (iNOS) mRNA expression. Using reverse transcriptase polymerase chain reaction, we have found that both NO donor sodium nitroprusside (SNP) and authentic NO solution are able to inhibit LPS/TNF alpha-inducible iNOS gene expression; this effect was reversed by reduced hemoglobin, a trapping agent for NO. The early presence of SNP during LPS/TNF alpha induction is essential for inhibition of iNOS mRNA expression. Furthermore, SNP is capable of inhibiting LPS/TNF alpha-inducible nitrite release, as determined by Griess reaction. Finally, using electrophoretic mobility shift assay, we have shown that SNP inhibits LPS/TNF alpha-elicited NF-kappa B activation. This suggests that inhibition of iNOS gene expression by exogenous NO may be ascribed to a decreased NF-kappa B availability.

Oxygen tension influences DNA fragmentation and cell death in glucocorticoid-treated thymocytes

Internucleosomal DNA fragmentation and cell death induced by dexamethasone in rat thymocytes were inhibited when cells were cultured in 95% N2/5% CO2 atmosphere, in which oxygen was rapidly reduced to under 0.5%. DNA fragmentation was delayed by a less severe hypoxia in 5% oxygen whilst in cell cultured in high oxygen atmosphere (95% O2) cell death was increased. On the other hand, prolonged oxygen deprivation caused an increase of spontaneous apoptotic cell death. Hypoxia also inhibited DNA fragmentation induced by calcium ionophore A23187, but not by topoisomerase inhibitor camptothecin. These data support the hypothesis of the involvement of oxygen reactive species in calcium-mediated apoptosis and suggest a complex role of oxygen in the modulation of programmed cell death.

Protective effect of spermine on DNA exposed to oxidative stress

Pathological conditions that cause oxidative stress can affect DNA integrity. The aim of this research was to study the protective effect of spermine against DNA damage induced by an oxygen-radical generating system. Deoxyguanosine and DNA were separately dissolved in phosphate buffer and incubated for 1 h at 40 degrees C in the presence of 50 mM H2O2/10 mM ascorbic acid. Single nucleosides and their products of oxidation were then obtained by enzymatic digestion of DNA. The compounds were separated by micellar electrokinetic capillary chromatography (MECC) with SDS-modified mobile phase and detected at 254 nm. Two major products of DNA oxidation have been identified as derivatives of deoxyguanosine with electrophoretic properties different from 8-hydroxy-2'-deoxyguanosine. When the oxidation of DNA was carried out in the presence of 0.1 mM spermine, the formation of the two by-products of deoxyguanosine was markedly reduced. On the contrary, spermine did not prevent the oxidation of deoxyguanosine alone, suggesting that the polyamine should be bound to the DNA strands to exert its antioxidative effect.