Different pathways of apoptosis revealed by 2-chloro-adenosine and deoxy-D-ribose in mammalian astroglial cells

Both the adenosine analogue 2-chloro-adenosine (2-CA) and the reducing sugar deoxy-D-ribose (dRib) induce apoptosis of astroglial cells in rat brain primary cultures (Abbracchio et al.: Biochem Biophys Res Commun 213:908-915, 1995). The present study was undertaken to elucidate by both morphological and cytofluorimetric analyses the intracellular mechanism(s) involved in induction of apoptosis by these two agents. The poly(ADP-ribose)polymerase (PARP) inhibitor 3-aminobenzamide did not prevent either 2-CA- or dRib-induced cell death, suggesting that activation of PARP is not critically important for induction of apoptosis in astrocytes. The radical scavenger N-acetyl-cysteine (NAC) strongly inhibited dRib- but not 2-CA-induced cell death, suggesting a differential role for radical formation in apoptosis by these two agents. A time-dependent increase of cells with depolarized mitochondria was observed in dRib-, and to a lesser extent, in 2-CA-treated cultures. NAC also prevented dRib- but not 2-CA-induced mitochondrial changes. We conclude that, in mammalian astrocytes, apoptosis can proceed through diverse and multiple pathways, depending upon the apoptotic stimulus. For dRib, apoptosis likely proceeds through generation of radicals and mitochondrial involvement. An adenosine extracellular receptor linked to an as yet unidentified signaling pathway may instead mediate 2-CA-induced cell death, which may have intriguing implications for both nervous system development and brain response to trauma and ischemia.

Developmental models of brain dysfunctions induced by targeted cellular ablations with methylazoxymethanol

Abnormal brain development represents one of the major causes of neurological disorders in humans, and determining the factors responsible for generating specific brain malformations represents a formidable task for developmental neurobiology. The knowledge of the precise neurogenetic time table and the use of toxins, like methylazoxymethanol, able to interfere with neuroepithelial cells entering their last mitotic cycle, have allowed for targeted neuronal ablations in specific brain areas of the central nervous system (CNS) when administered at different gestational or postnatal days in various animal species. Of particular relevance are the studies in which ablations of neuronal populations of cortex, hippocampus, and cerebellum have been made. The results obtained show that these early ablations induce a number of neuroanatomic, neurochemical, and electrophysiological changes that give us the possibility to unravel the biochemical strategies utilized by surviving neurons to adapt to the perturbated environment. Most striking are the findings that target deprivation does not affect the survival of afferent neurons in the CNS (except for neurons of the lateral geniculate nucleus), in sharp contrast to the notion of target dependence for peripheral nervous system neurons. Animals showing selective ablations in the Ammon's horn of the hippocampus allow us to understand the complex biochemical pathways leading to changes in activity-dependent synaptic plasticity, and the data underscore the fundamental role of diverse Ca(2+)-dependent protein kinases, and their substrates, in modulating pre- and postsynaptic events during induction and maintenance of long-term potentiation (LTP). Because LTP represents a useful model to study molecular substrates of learning and memory, this animal model might be of relevance in understanding cognitive brain dysfunctions.

Low levels of hydrogen peroxide and L-histidine induce DNA double-strand breakage and apoptosis

The results presented in this study demonstrate that L-histidine triggers a lethal response in U937 cells exposed to nontoxic, albeit growth-inhibitory, levels of H2O2. Treatment for 1 h with the cocktail H2O2/L-histidine promotes the formation of a low level of DNA double-strand breaks that are rapidly rejoined, and this process is followed by secondary DNA fragmentation at about 7 h of post-treatment incubation, at which time cells are still viable. The appearance of oligonucleosomal DNA fragments associated with the detection of morphological changes typical of apoptosis strongly suggests that a portion of the cells was undergoing an apoptotic process. The relative level of cells with fragmented chromatin never exceeded 15-20% throughout the 20 h post-treatment incubation. Treatment with high concentrations of H2O2 in the presence of L-histidine was found to trigger necrotic cell death. The results presented in this paper provide further experimental evidence in support of the notion that DNA double-strand breaks mediate the lethal effects of the cocktail H2O2/L-histidine and suggest that this type of DNA lesion can promote both apoptotic and necrotic cell death, depending on the concentration of the oxidant.

Interaction of thiocolchicoside with [3H]strychnine binding sites in rat spinal cord and brainstem

Radioreceptor binding assays and receptor autoradiography were used to investigate the activity of thiocolchicoside on strychnine-sensitive binding sites in rat brain and spinal cord using [3H]strychnine as a ligand. Thiocolchicoside displaced the binding of [3H]strychnine with an affinity similar to that of unlabeled glycine, and showed a Hill coefficient and proportionality parameter (P) less than unity. The activity of thiocolchicoside toward [3H]strychnine binding sites was confirmed in autoradiographic studies. The results suggest that thiocolchicoside behaves as an allosteric compound acting on the strychnine-sensitive glycine receptor in rat brainstem and spinal cord, and that this may provide a possible mechanism for the myorelaxant activity of this colchicoside derivative, the first clinically useful drug acting on this receptor.

Direct excision of 50 kb pair DNA fragments from megabase-sized fragments produced during apoptotic cleavage of genomic DNA

The DNA of U937 cells exposed to two different apoptotic stimuli, namely the cocktail H2O2/3-aminobenzamide (3AB) and etoposide, was analyzed using field inversion gel electrophoresis (FIGE) as well as programmable, autonomously controlled electrode electrophoresis (PACE). The results obtained indicate that FIGE is not appropriate for sizing apoptotic DNA fragments. PACE appears to be more accurate and reliable and the results obtained with this technique strongly suggest that the 50 kb DNA fragments are directly excised from Mb-sized DNA fragments without the intermediate cleavage of 200-300 kb products.

Abnormal pattern of platelet APP isoforms in Alzheimer disease and Down syndrome

OBJECTIVE

To determine if changes in levels of amyloid precursor protein (APP) isoforms in periphery are associated with Alzheimer disease and Down syndrome.

DESIGN

After subjects were grouped according to diagnosis, APP isoform levels in platelets were compared.

SETTING

University medical center.

SUBJECTS

Ten patients who fulfilled diagnostic criteria for probable Alzheimer disease, 22 healthy volunteers, and 7 elderly (mean age, 42.7 years) and 7 young (mean age, 19.0 years) patients with Down syndrome.

MAIN OUTCOME MEASURES

The levels of APP isoforms were evaluated by means of Western blot analysis and immunostaining of whole platelets.

RESULTS

The ratio between the 130- and the 106- to 110-kd APP isoforms was markedly lower in patients with Alzheimer disease and in elderly patients with Down syndrome than in control subjects. In young patients with Down syndrome, the ratio did not significantly differ from that in control subjects.

CONCLUSIONS

A consistent alteration in platelet APP isoforms has been found in Alzheimer disease and Down syndrome. Further studies will determine whether this alteration could provide a peripheral biochemical marker of the disorder and whether it could intervene in the pathogenesis of Alzheimer disease.

Purinoceptors in the Central Nervous System

New exciting developments on the occurrence and functional role of purinoceptors in mammalian brain were presented at the session "Purinoceptors in the central nervous system" chaired by Flaminio Cattabeni and Tom Dunwiddie at the Purines '96 international conference. The focus of the session were topics of recent interest, including the sources and mechanisms involved in ATP and adenosine release during physiological neurotransmission in hippocampus, the brain expression of the recently cloned P2 receptors, and the role of the various adenosine receptor subtypes in brain protection from neurodegeneration associated with trauma-, ischemia-and excessive excitatory amino acid neurotransmission. New important insights into the mechanisms responsible for the formation and release of adenosine into the extracellular space were provided by data obtained by Dunwiddie and coworkers in hippocampal pyramidal neurons. These data may have functional implications for the role of purines in modulation of synaptic plasticity and long-term potentiation in this brain area, and hence in cognitive functions. Buell provided an updated overview on the cloning, molecular characteristics and brain expression of various ligand-gated P2X purinoceptors; although the functional role of these receptors in mammalian brain still awaits elucidation, their widespread distribution in the nervous system strongly suggests that ATP-mediated events are more prevalent and important in brain than expected. Pedata presented data on the functional interrelationships between adenosine and glutamate in the brain, and also provided evidence for alterations of the reciprocal regulation between these two systems in aged brain, which may have important implications for both ischemia-and trauma-associated neurodegenerative events and senescence-associated cognitive impairment. Finally, von Lubitz provided novel data on the molecular mechanisms likely to be at the basis of the brain protective effects associated with the chronic stimulation of the adenosine A3 receptor, further confirming that this receptor represents a crucial target for the development of new antiischemic and antineurodegenerative therapeutic agents.

Modulation of muscarinic receptor-stimulated phosphoinositide breakdown by sulfhydryl group modification is a general response in different rat brain regions and depends on the stage of brain development

In a previous study we reported that muscarinic receptor-stimulated phosphoinositide breakdown in rat cortical slices is significantly reduced by modification of critical thiols located at the post-receptor level. We have now extended this observation by investigating whether this effect is a general response in different brain areas and is differentially regulated, within the cortex, in the neonatal and adult brain. Experimental results have demonstrated that indeed the effect of sulfhydryl group modification on muscarinic receptor-stimulated phosphoinositide breakdown appears to be mediated by a general mechanism operative in different brain regions (cortex, hippocampus and striatum). In addition, this response is more pronounced in the early stages of brain development, when muscarinic receptor agonists lead to the highest inositol phosphate accumulation.

2-(Substituted)amino-2,8-diazaspiro[4,5]decan-1,3-diones as potential muscarinic agonists: synthesis, modeling and binding studies

A series of 2-(acyl)amino-8-substituted-2,8-diazaspiro[4,5]decan-1,3-diones (5a-j), structurally related to the muscarinic agonist RS-86, was synthesized and compounds tested for their affinity towards muscarinic receptors. Though all compounds proved to be less active than the model in binding studies, only three derivatives (5a, b, c) being able to significantly displace 3H-QNB at mM concentration, their behaviour could be interpreted in terms of theoretical molecular descriptors computed on the basis of the suggestions coming from Molecular Dynamics simulations of ligand-receptor complexes.

Pyruvate enhances DNA single-strand break formation while abolishing cytotoxicity in U937 cells exposed to tert-butylhydroperoxide

tert-Butylhydroperoxide (tB-OOH) induces killing and DNA single strand breaks (SSBs) in cultured U937 cells. Pyruvate while increasing the rate of oxygen consumption also increased the magnitude of the DNA scission produced by tB-OOH. Rotenone, an inhibitor of complex I, abolished both effects but did not, however, affect the DNA SSB-frequency observed after treatment with tB-OOH alone. These results collectively suggest that pyruvate potentiates the DNA-damaging activity of tB-OOH via stimulation of oxygen consumption. Importantly, under the same experimental conditions, pyruvate was found to abolish both the decline in nonprotein sulfhydryls (NPSH) and the cytotoxicity induced by tB-OOH. Thus, cells with energized mitochondria are more sensitive to the DNA-damaging effects of tB-OOH and display resistance against its cytotoxic effects. As a consequence, DNA SSBs promoted by tB-OOH do not appear to be toxic for the cell.

The respiratory-chain poison antimycin A promotes the formation of DNA single-strand breaks and reduces toxicity in U937 cells exposed to t-butylhydroperoxide

Antimycin A at levels that abolish oxygen consumption had a slight, although statistically significant, inhibitory effect on the toxicity elicited by t-butylhydroperoxide in U937 cells. The protective effect was observed after 6 h of post-treatment incubation, but was no longer apparent after 24 h. Unexpectedly, these events were associated with a marked accumulation of DNA single-strand breaks produced by low levels of t-butylhydroperoxide. Both an oxygen- and a carbon-centred radical were found to arise during treatment with t-butylhydroperoxide, and their formation was significantly lowered by antimycin A. Thus inhibition of electron transport at the level of complex III appears (a) to decrease the formation of toxic species which mediate, at least partially, the lethal effects elicited by t-butylhydroperoxide, and (b) to enhance the formation of DNA-damaging species generated at low concentrations of t-butylhydroperoxide. Rotenone and cyanide, which respectively inhibit complexes I and IV, did not affect DNA damage elicited by t-butylhydroperoxide. These results suggest that DNA single-strand breaks do not mediate the toxicity of t-butylhydroperoxide, and that specific mitochondrial functions might modulate the formation of the toxic and of DNA-damaging species generated by organic hydroperoxides.

Prevention of necrosis and activation of apoptosis in oxidatively injured human myeloid leukemia U937 cells

A 3 h exposure to 1 mM H2O2 followed by 6 h post-challenge growth in peroxide-free medium induces necrosis in U937 cells. Addition of the poly(ADP-ribose)polymerase inhibitor 3-aminobenzamide during recovery prevents necrosis and triggers apoptosis, as shown by the appearance of apoptotic bodies, extensive blebbing and formation of multimeric DNA fragments as well as 50 kb double stranded DNA fragments. Thus, the same initial damage can be a triggering event for both apoptotic and necrotic cell death. Furthermore, necrosis does not appear to be a passive response to overwhelming damage.

Determination of the endogenous phosphorylation state of B-50/GAP-43 and neurogranin in different brain regions by electrospray mass spectrometry

Electrospray mass spectrometry coupled to liquid chromatography was utilized to measure two PKC neuronal substrates, B-50/GAP-43 and neurogranin, in single rat brain areas. Aliquots of perchloric acid extracts were directly injected and mass spectra recorded. At elution times of 14.2 and 27.0 min two molecular species of MW 7450 and 23 602 Da were observed. These values are in excellent agreement for the expected MW for rat neurogranin and B-50/GAP-43. The presence of molecular species shifted by 80 mass units in both cases indicates that these proteins are present in phosphorylated forms in cortical and hippocampal extracts.

Hydrogen peroxide mediates the killing of U937 tumor cells elicited by pharmacologically attainable concentrations of ascorbic acid: cell death prevention by extracellular catalase or catalase from cocultured erythrocytes or fibroblasts

Pharmacologically attainable concentrations of ascorbic acid are highly toxic for U937 cells (a human promyelocytic cell line), and this response appears to be mediated by H2O2. This inference finds experimental support in the following observations: 1) toxic levels of H2O2 are readily generated upon dissolution of survival-range concentrations of ascorbic acid in the culture medium; 2) the lethal effects elicited by ascorbic acid or reagent H2O2 are prevented by the addition of either catalase or the intracellular iron chelator o-phenanthroline and are characterized by similar temporal dependence; 3) U937 cells resistant to hydrogen peroxide are cross-resistant to ascorbic acid; 4) under the conditions utilized in this study, H2O2 and ascorbate promote similar modes of cell death (i.e., necrosis); and 5) cell killing provoked by H2O2 or ascorbate is an inverse function of cell density and is suppressed by coculturing U937 target cells with human erythrocytes (at a density far below that present in the blood) and human fibroblasts. Cytoprotection was not observed using catalase-depleted erythrocytes. Taken together, these results strongly suggest that H2O2 is entirely responsible for the ascorbate-induced U937 cell killing. We therefore propose that it is unlikely that the vitamin damages or kills tumor cells of normal tissues in vivo via the H2O2 based mechanism, because the oxidant would be removed promptly by the neighboring cells.

Isolation and preliminary characterization of a Chinese hamster ovary cell line with high-degree resistance to hydrogen peroxide

We have isolated and conducted preliminary characterization of a cell line derived from the Chinese hamster ovary cell line AA8, which we have designated AG8 and which is highly resistant to the cytotoxic effects of H2O2 (approximately 17-fold when the H2O2 treatment was at 37 degrees; approximately 11-fold when the H2O2 treatment was at 4 degrees). AG8 cells were moderately (but significantly; P < 0.05) cross-resistant to CdCl2 (approximately 4-fold), NaAsO2 (approximately 2.3-fold), t-butyl hydroperoxide (approximately 2.9-fold), cumene hydroperoxide (approximately 3-fold), menadione (approximately 1.7-fold) and HgCl2 (approximately 1.5-fold), but were not significantly cross-resistant to hyperthermia (43 degrees), 254 nm UV light, 137Cs gamma-rays, and 42-MeV (p-->Be+) fast neutrons. As regards their biochemical status, AG8 and AA8 cells contain similar non-protein sulfhydryl levels per milligram of protein. Catalase activity (assessed by both spectrophotometry and polarography) was significantly higher in AG8 than in AA8 cells irrespective of whether enzyme activity was expressed per 10(6) cells (approximately 3.6-fold increase) or per milligram of protein (approximately 1.6-fold increase). AG8 cells also exhibited significantly greater glutathione reductase activity than wild-type cells when the data were expressed per 10(6) cells (approximately 2.9-fold) or per milligram of protein (approximately 1.3-fold). Glutathione peroxidase activity was immeasurably low in both cell lines. The susceptibility of the two cell lines to H2O2-mediated generation of DNA single-strand breaks (as measured by alkaline elution) indicated a slightly (approximately 1.5-fold) decreased yield in the resistant AG8 cell line. The two cell lines repaired these breaks with similar kinetics. In contrast, no measurable induction of DNA double-strand breaks (as measured by pulsed-field gel electrophoresis) was apparent in either cell line after survival-curve range concentrations of H2O2. On the basis of these data, it appears that the AG8 phenotype involves two previously identified resistance mechanisms, namely an adaptive component that may or may not involve increased antioxidant capacity, and a second component that does involve increased antioxidant (primarily catalase) capacity.

AG8 cells, which are highly resistant to hydrogen peroxide, display collateral sensitivity to the combination of hydrogen peroxide and L-histidine

The results obtained in the present study indicate that AG8 cells, which are highly resistant to H2O2, are not cross-resistant to the combination of H2O2/L-histidine. In fact, once the influence of elevated catalase on the AG8 pheno-type has been circumvented (by treatment of AG8 cells with aminotriazole), AG8 cells display essentially no cross-resistance to the H2O2/L-histidine cocktail while retaining considerable resistance to H2O2 alone (when compared to wild-type AA8 cells). Although H2O2 alone dose not produce DNA double strand breaks (DSBs), this type of lesion was readily detected upon exposure of sensitive or resistant cells to the oxidant in the presence of the amino acid. Interestingly, similar levels of DNA DSBs were detected in AA8 and catalase-depleted AG8 cells. An excellent correlation was found when the cytotoxicity and the level of DNA DSBs obtained in sensitive and resistant cells (with normal or reduced catalase levels) challenged with the cocktail H2O2/L-histidine were compared. This would suggest that DSBs produced on a per cell basis always result in an equal level of toxicity, regardless of the cell type (resistant versus sensitive cell line), the lethality of each of these cell lines being dependent on the number of induced DSBs. In conclusion, the results presented here provide further evidence in support of the hypothesis that cell killing elicited by the combination of H2O2/L-histidine involves a mechanism distinct from that following treatment with H2O2 alone. The fact that H2O2-resistant AG8 cells, which are not cross-resistant to agents promoting cell death via DNA DSB-induction, display collateral sensitivity to the cocktail H2O2/L-histidine, strongly suggests that cell killing triggered by this treatment is mediated by DNA double strand breakage.

Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells

In addition to their well-established roles as neurotransmitters and neuromodulators, growing evidence suggests that nucleotides and nucleosides might also act as trophic factors in both the central and peripheral nervous systems. Specific extracellular receptor subtypes for these compounds are expressed on neurons, glial and endothelial cells, where they mediate strikingly different effects. These range from induction of cell differentiation and apoptosis, mitogenesis and morphogenetic changes, to stimulation of synthesis or release, or both, of cytokines and neurotrophic factors, both under physiological and pathological conditions. Nucleotides and nucleosides might be involved in the regulation of development and plasticity of the nervous system, and in the pathophysiology of neurodegenerative disorders. Receptors for nucleotides and nucleosides could represent a novel target for the development of therapeutic strategies to treat incurable diseases of the nervous system, including trauma- and ischemia-associated neurodegeneration, demyelinating and aging-associated cognitive disorders.

Levels of NGF, p75NGFR and ChAT immunoreactivity in brain of adult and aged microencephalic rats

Methylazoxymethanol (MAM)-induced microencephalic aged animals with reduced cortical mass and unmodified basal nucleus were used to study the relationship between cells that produce and cells that utilize NGF. Total cortical ChAT activity of MAM 2, 19 and 27 month old animals was reduced compared to their age-matched controls. To verify whether the reduction of enzyme activity can be ascribed to changes in or ablation of projecting neurons, we carried out immunohistochemical analysis of ChAT and low affinity NGF receptor (p75NGFR) in the basal nucleus of control and MAM-treated animals. ChAT and p75NGFR immunostaining of basal forebrain cholinergic neurons showed morphological changes in MAM animals, as revealed by cellular atrophy, reduced dendritic arborization and decreased staining intensity. In the cerebral cortex of microencephalic animals, reduced levels of NGF compared to controls were observed at all examined ages. These results suggest that MAM treatment induces long-lasting ablation of cortical NGF-synthesizing cells leading to reduced trophic support to basal forebrain cholinergic neurons, which might be responsible for the cellular atrophy observed in the basal nucleus.

Cytotoxic impact of DNA single vs double strand breaks in oxidatively injured cells

Hydrogen peroxide is a potent inducer of DNA single strand breaks (SSBs) in cultured mammalian cells. These lesions, however, are efficiently repaired and do not appear to mediate the cytotoxic response. This inference is based on the observations that a) inhibiting the rate of SSB-removal does not result in an increased cytotoxicity; b) using different experimental conditions it is possible to dissociate the formation of DNA SSBs from the cytotoxic response; c) the induction/loss of the oxidant-resistant phenotype in cell variants characterized by different levels of resistance to the lethal effect of the oxidant does not correlate with resistance to DNA SSB-induction; d) a much larger accumulation of DNA SSBs can be observed following treatment with H2O2 at 4 degrees C, as compared to 37 degrees C, although the opposite is true in terms of cytotoxicity. In the presence of micromolar levels of L-Histidine, H2O2 also induces DNA double strand breaks (DSBs), a type of lesion which we suggest may mediate the lethal event. This conclusion finds experimental support in the following observations: a) DNA DSBs are generated at survival-range concentrations, and a linear correlation exists between the level of this lesion and cytotoxicity; b) this correlation curve overlaps with the curves generated under similar experimental conditions using different cell lines with different sensitivity to the oxidant alone, or different clones derived from the same cell line, some of which showed a high degree of resistance to H2O2. Finally, the formation of DNA DSBs appears to enhance both apoptotic and necrotic cell death.

Evidence for dissimilar mechanisms of enhancement of inorganic and organic hydroperoxide cytotoxicity by L-histidine

L-Histidine markedly increases inorganic and organic hydroperoxide-induced cytotoxicity and DNA single-strand breaks (SSBs) in Chinese hamster ovary cells. These effects were prevented by the iron chelator o-phenanthroline and were insensitive to the antioxidant N,N'-diphenyl-1,4-phenylenediamine. An excess of L-glutamine, a competitive inhibitor of L-histidine uptake, prevented the L-histidine-mediated enhancement of cytotoxicity induced by both inorganic and organic peroxides. L-Glutamine did not affect the level of DNA SSBs produced by H2O2/L-histidine, although it abolished the enhancement of SSB formation triggered by L-histidine in cells exposed to the organic peroxides. DNA SSBs generated by the organic hydroperoxides either alone or associated with L-histidine were removed with superimposable kinetics, whereas those produced by H2O2 in the presence of the amino acid were repaired more slowly than SSBs produced by the oxidant alone. DNA double-strand breaks, which are considered to be highly cytotoxic, were detected only in cells treated with H2O2 and L-histidine. Finally, L-histidine was shown to markedly increase the extent of mitochondrial damage produced by organic but not by inorganic hydroperoxides.

G protein-dependent activation of phospholipase C by adenosine A3 receptors in rat brain

The recently cloned G protein-coupled adenosine A3 receptor has been proposed to play a role in the pathophysiology of cerebral ischemia. Because phospholipase C activation occurs as a very early response to brain ischemia, we evaluated the ability of A3- selective and nonselective adenosine analogues to elicit phosphoinositide hydrolysis. In myo-[3H]inositol-labeled rat striatal and hippocampal slices, A3 agonists stimulated formation of [3H]inositol phosphates in a concentration-dependent manner. In striatum, the potency order was 2-chloro-N6-(3-iodobenzyl)- adenosine-5'-N-methyluronamide > or = N6-(3-iodobenzyl)- adenosine-5'-N-methyluronamide >> N-methyl-1,3-di-n-butylxanthine-7-beta-D-ribofuronamide > or = 5'-N-ethylcarboxamidoadenosine > or = N6-2-(4-aminophenyl)-ethyladenosine > N6-(p-sulfophenyl)-adenosine = 1,3-dibutylxanthine-7- riboside, which is identical to the potency order in binding studies at cloned rat A3 receptors. Stimulation of phospholipase C activity was abolished by guanosine-5'-O-(2-thiodiphosphate), confirming the involvement of a G protein-coupled receptor. Activation of phospholipase C was higher in the striatum than in the hippocampus, consistent with A3 receptor densities. Stimulation of phospholipase C activity by adenosine analogues was only modestly antagonized by xanthine derivatives and at much higher concentrations than needed for blocking adenosine A1, A2A, and A2b receptors. In the presence of an A1/A2 antagonist, a selective A3 in rat striation. Thus, stimulation of phospholipase C activity agonist only weakly inhibited forskolin-stimulated adenylyl cyclase activity represents a principal transduction mechanism for A3 receptors in mammalian brain, and perhaps A3 receptor-mediated increases of inositol phosphates in the ischemic brain contribute to neurodegeneration by raising intracellular calcium levels.

The L-histidine-mediated enhancement of hydrogen peroxide-induced DNA double strand breakage and cytotoxicity does not involve metabolic processes

The cytotoxic response of Chinese hamster ovary (CHO) cells to challenge with hydrogen peroxide was highly dependent upon the temperature of exposure, being markedly higher at 37 degrees than at 4 degrees C. Increasing intracellular levels of L-histidine prior to challenge with hydrogen peroxide increased the toxicity elicited by the oxidant at both physiologic and ice-bath temperatures. The effect of the amino acid, however, was more pronounced under conditions at 4 degrees C, as compared to 37 degrees C. Indeed, at 4 degrees C the oxidant was nontoxic at submillimolar levels and pre-exposure to L-histidine restored cytotoxicity to levels slightly higher than those observed after treatment at 37 degrees C (in the micromolar range). Pre-exposure to the amino acid increased the production of DNA double-strand breaks (DSBs) elicited by treatment with the oxidant both at 37 degrees and 4 degrees C. A remarkable correlation was found when the level of this lesion was plotted against the cytotoxic response observed using different concentrations of L-histidine or hydrogen peroxide, or treating the cells with the oxidant either at 37 degrees or 4 degrees C, thus suggesting the existence of a cause-effect relationship. The overlapping correlation curves obtained with cells challenged with the oxidant at 4 degrees or 37 degrees C also suggest that similar molecular mechanisms mediate the formation of DNA DSBs under both experimental conditions. Two lines of evidence provide experimental support for this inference: (1) the kinetics of repair of DNA DSBs generated at 37 degrees or 4 degrees C were virtually superimposable; this would suggest that the same repair pathway(s) is/are responsible for the removal of DNA DSBs generated at the two temperatures; and (2) the size distribution of double-stranded DNA fragments produced under the two treatment conditions, resulting in a similar cytotoxic response, was basically identical. This is indicative of remarkable similarities in the topology of chromosomal domains where DSBs are generated. Overall, the results presented in this paper provide further experimental evidence supporting the notion that DNA DSBs are responsible for the L-histidine-mediated enhancement of hydrogen peroxide-induced cytotoxicity, and demonstrate that the mechanism whereby the amino acid enhances the ability of hydrogen peroxide to produce DNA double strand breakage and cell killing does not depend on cellular metabolism and/or energy-dependent reactions.

Effects of ATP analogues and basic fibroblast growth factor on astroglial cell differentiation in primary cultures of rat striatum

We have used primary cultures of rat striatum to study the effects of ATP analogues on the elongation of astrocytic processes, a parameter of astroglial cell differentiation. Parallel studies were performed with basic fibroblast growth factor, a known regulator of astroglial cell function. After three days in culture, both the growth factor and alpha beta-methylene-ATP induced dramatic increases in the mean length of astrocytic processes/cell. For both agents, effects were dose-dependent. The effect of alpha beta-methylene-ATP was antagonized by the trypanoside suramin and mimicked by 2-methyl-thio-ATP, suggesting the involvement of a suramin-sensitive P2-purinoceptor. Neither an additive nor a synergistic effect between alpha beta-methylene-ATP and basic fibroblast growth factor on the elongation of processes was detected in cultures exposed to both agents. Indeed, an inhibition with respect to the effects induced by either agent alone was recorded, suggesting that the growth factor and the purine analogue can modulate astrocytic differentiation by activation of common intracellular pathways. It is concluded that, like basic fibroblast growth factor, ATP can promote the maturation of astrocytes towards a more differentiated phenotype characterized by the presence of longer astrocytic processes. These findings might have interesting implications for astroglial cell differentiation during brain development and for ischemia- and trauma-associated hypergliosis.

The effect of hydrogen peroxide/L-histidine-induced DNA single- vs. double-strand breaks on poly(ADP-ribose)polymerase

L-Histidine markedly increases the ability of hydrogen peroxide to induce DNA cleavage and this effect is associated with a 3-aminobenzamide-inhibitable decline in NAD+ levels, an event which very likely reflects an enhanced stimulation of the enzyme poly(ADP-ribose)polymerase. 3-Aminobenzamide slowed down the removal of alkaline elution-detected strand breaks induced by either H2O2 alone (producing only DNA single-strand breaks) or associated with L-histidine (resulting in the formation of both single-strand breaks and DNA double-strand breaks), and the extent of inhibition was similar under the two experimental conditions. 3-Aminobenzamide did not affect the rate of rejoining of DNA double-strand breaks generated by the cocktail H2O2/L-histidine. The above results suggest that these double-strand breaks have hardly any effect on the induction of poly(ADP-ribose)polymerase activity, a conclusion that is consistent with the observation that the activity of this enzyme appears to be basically identical under conditions that abolish the formation of DNA double-strand breaks, in the absence of measurable variations in the level of induction of DNA single-strand breaks (e.g. in the presence of an excess of L-glutamine, a competitive inhibitor of L-histidine uptake). Finally, 3-aminobenzamide did not affect the toxicity of the oxidant, both in the absence and presence of L-histidine.