1,721,024 research outputs found
Brain adenosine receptors as targets for therapeutic intervention in neurodegenerative diseases
No full textAdenosine acts as a neurotransmitter in the brain through the activation of four specific G-protein-coupled receptors (the A1, A2A, A2B, and A3 receptors). The A1 receptor has long been known to mediate neuroprotection, mostly by blockade of Ca2+ influx, which results in inhibition of glutamate release and reduction of its excitatory effects at a postsynaptic level. However, the development of selective A1 receptor agonists as antiischemic agents has been hampered by their major cardiovascular side effects. More recently, apparently deleterious effects have been reported following the activation of other adenosine receptor subtypes, namely, the A2A and the A3 receptors. In particular, selective A2A receptor antagonists have been demonstrated to markedly reduce cell death associated with brain ischemia in the rat, suggesting that the cerebral A2A receptor may indeed contribute to the development of ischemic damage. The beneficial effects evoked by A2A antagonists may be due to blockade of presynaptic A2A receptors (which are stimulatory on glutamate release) and/or to inhibition of A2A receptor-mediated activation of microglial cells. Even more puzzling data have been reported for the A3 receptor subtype, which can indeed mediate both cell protection and cell death, simply depending upon the degree of receptor activation and/or specific pathophysiological conditions. In particular, a mild subthreshold activation of this receptor has been associated with a reinforcement of the cytoskeleton and reduction of spontaneous apoptosis, which may play a role in "ischemic preconditioning" of the brain, according to which a short ischemic period may protect the brain from a subsequent, sustained ischemic insult that would be lethal. In contrast, a robust and prolonged activation of the A3 receptor has been shown to trigger cell death by either necrosis or apoptosis. Such apparently opposing actions may be reconciled by hypothesizing that adenosine-mediated cell killing during ischemia may be aimed at isolating the most damaged areas to favor those parts of the brain that still retain a chance for functional recovery. In fact, both A3 receptor-mediated cell death and A2A receptor-mediated actions may be viewed as an attempt to selectively kill irreversibly damaged cells in the "core" ischemic area, in order to save space and energy for the surrounding live cells in the "pneumbra" area. Hence, the pharmacological modulation of the A2A and A3 receptors via selective ligands may represent a novel strategy in the therapeutic approach to pathologies characterized by acute or chronic neurodegenerative events
Cellular effects of ornithine decarboxylase induction in cells maintained with a salts/glucose medium
No full textCultures preincubated in a growth restricted salts/glucose medium in the presence and absence of ornithine decarboxylase (ODC) activating factors were then incubated under ideal growth conditions to study the influence of these factors on cell growth. Incubation of confluent cultures in a salts/glucose medium alone did not induce ODC or change the other biochemical parameters investigated. However, if cultures were incubated in the salts/glucose medium supplemented with asparagine (ASN) and agents that increase cellular cAMP levels then ODC was induced after 6-8 h. This primary induction in the salts/glucose medium resulted in altered and delayed ODC induction during growth stimulation and also caused a delay in (3H) thymidine incorporation without affecting (3H) uridine and (3H) leucine incorporation. These results demonstrate that incubation of cultures in a salts/glucose media with ASN and dibutyryl cAMP (dBcAMP) causes refractory ODC induction and altered (3H) thymidine incorporation upon growth challenge with complete medium. These effects were not observed when cells were preincubated in a salts/glucose medium alone
Factors influencing the phagocytosis, neoplastic transformation, and cytotoxicity of particulate nickel compounds in tissue culture systems
No full textSeven particulate nickel compounds were studied for their cell transformation activity using cultured Syrian hamster embryo cells and for their phagocytotic activity in cultured Chinese hamster ovary cells. The crystalline nickel compounds (αNi3S2, αNiS, and Ni3Se2) had significantly more cell transforming activity and were more actively phagocytized than the other nickel compounds examined (amorphous NiS, metallic Ni, Ni3O2, and NiO). Therefore, the crystalline structure of nickel compounds is one factor influencing their toxic activity upon biological systems. A second influencing factor was the particle size of the water-insoluble nickel compounds. Particles of crystalline αNiS ranging from 2 to 4 μm were phagocytized six times more than αNiS particles having mean diameters of 5-6 μm. Differences in amorphous NiS particle size had little effect on its already low susceptibility to be phagocytized by cells and ability to cause a reduction of cell plating efficiency. The presence of Mn dust inhibited the neoplastic transformation of crystalline nickel sulfide and also reduced the phagocytosis of crystalline αNiS and αNi3S2 particles by cultured cells. The phagocytosis of crystalline NiS particles was inhibited by the presence of amorphous NiS, Mn or MnCl2. Therefore, the presence of noncarcinogenic metals which are not themselves actively phagocytized diminishes the transforming effects of crystalline metal compounds probably by reducing their internalization. Various metabolic inhibitors such as dansylcadaverine, cycloheximide, and actinomycin D reduced the phagocytosis of crystalline αNiS
The phagocytosis and transforming activity of crystalline metal sulfide particles are related to their negative surface charge
No full textCrystalline nickel sulfide (αNiS) and cobalt sulfide (CoS2) particles can cause greater cell transformation and cellular toxicity than the respective amorphous metal sulfide particles. Cultured mammalian cells phagocytose the crystalline metal sulfide particles more readily than the amorphous ones. In the case of the nickel sulfides, the crystalline metal sulfide particles had negatively charged surfaces (Zeta potential: -27.012 mV) in contrast to the amorphous particles, which were positively charged (Zeta potential: +9.174 mV). X-ray photoelectron spectroscopy analysis of amorphous and crystalline NiS particles revealed that the outermost surface (1-4 nm) of the two particles had striking differences in Ni/S ratios and in their sulfur oxidation states. Rendering particles' surfaces more negative by reduction with lithium aluminum hydride enhanced their phagocytosis, and in the case of amorphous NiS chemical reduction resulted in an incidence of morphological transformation of Syrian hamster embryo cells comparable to that observed with untreated crystalline αNiS
Cytoplasmic dissolution of phagocytized crystalline nickel sulfide particles: a prerequisite for nuclear uptake of nickel
No full textThe intracellular fate of particulate crystalline αNiS, an inducer of neoplastic transformation which is readily phagocytized by cultured cells, was compared with that of particulate amorphous NiS, which does not have these properties. Amorphous and crystalline NiS both dissolve slowly in complete medium; phagocytized αNiS particles remain in the cytoplasm, where they dissolve more rapidly than extracellular particles. Thus the selective phagocytosis of αNiS accounts for both high intracellular particle accumulation and high levels of soluble Ni relative to the surrounding medium. Since phagocytized αNiS particles do not enter the nucleus, dissolution in the cytoplasm may represent an activation step in carcinogenesis, forming soluble Ni which diffuses into the nucleus. Dissolution products from phagocytized αNiS were detected in subcellular fractions isolated from treated cells; the highest levels were found in the nuclei, mitochondria, and lysosomes. That the Ni in the subcellular fractions was dissolved is suggested by the fact that dissolution products from phagocytized αNiS were detected in nuclei after centrifugation on sucrose pads, which substantially reduced contamination from cytoplasmic αNiS particles. Cytoplasmic dissolution of αNiS was enhanced by prior exposure of cells to the same compound. Loss of visible particles from cells was compared with loss of total Ni by use of α63NiS particles; the particles disappeared from almost half the cells during the first 2 d of treatment, while the total radioactivity associated with the cells and the total number of cells in the monolayer remained the same. The accelerated dissolution of αNiS after exposure to the same particles may be due to enhancement of lysosomal enzyme activity by particle phagocytosis. A 20-30% increase in intracellular acid phosphatase activity was observed after treatment with crystalline, but not amorphous, NiS, suggesting enhanced lysosomal activity
Modulation of cyclooxygenase-2 and brain reactive astrogliosis by purinergic P2 receptors
No full textAstroglial cells respond to trauma and ischemia with reactive gliosis, a reaction characterized by increased astrocytic proliferation and hypertrophy. Although beneficial to a certain extent, excessive gliosis may be detrimental, contributing to neuronal death in neurodegenerative diseases. We have tested the hypothesis that ATP may act as a trigger of reactive gliosis in an in vitro model (rat brain primary astrocytes) where reactive astrogliosis can be quantified as elongation of astrocytic processes. Challenge of cells with the ATP analog alpha,beta methyleneATP (alpha,beta meATP) resulted in concentration dependent elongation of astrocytic processes, an effect that was fully counteracted by the non-selective ATP/P2 receptor antagonists suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). Signalling studies revealed that alpha,beta meATP-induced gliosis is mediated by a novel G-protein-coupled receptor (a P2Y receptor) coupled to an early release of arachidonic acid. Challenge of cells with alpha,beta meATP also resulted in an increase of inducible cyclooxygenase-2 (COX-2), the activity of which has been reported to be pathologically increased in a variety of neurodegenerative diseases characterized by inflammation and astrocytic activation. Induction of COX-2 by alpha,beta meATP was causally related to reactive astrogliosis, since the selective COX-2 inhibitor NS-398 prevented both the purine-induced elongation of astrocytic processes and the associated COX-2 increase. Preliminary data on the putative receptor-to-nucleus pathways responsible for purine-induced gliosis suggest that induction of the COX-2 gene may occur through the protein kinase C/mitogen activated protein kinase system, and may involve the formation of activated AP-1 transcription complexes. We speculate that antagonists selective at this novel P2Y receptor subtype may represent a novel class of neuroprotective agents able to slow down neurodegeneration by counteracting the inflammatory events contributing to neuronal cell death
A key role for caspase-2 and caspase-3 in the apoptosis induced by 2-chloro-2'-deoxy-adenosine (cladribine) and 2-chloro-adenosine in human astrocytoma cells
No full textBoth the anticancer agent 2-chloro-2'-deoxy-adenosine (Cladribine) and its derivative 2-chloro-adenosine induce apoptosis of human astrocytoma cells (J Neurosci Res 60:388-400, 2000). In this study, we have analyzed the involvement of caspases in these effects. Both compounds produced a gradual and time-dependent activation of "effector" caspase-3, which preceded the appearance of the nuclear signs of apoptosis, suggesting a temporal correlation between these two events. Moreover, the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-dl-Asp-fluoromethylketone (fmk) suppressed both caspase-3 activation and apoptosis induction. "Initiator" caspase-9 and caspase-8 were only marginally activated at later times in the apoptotic process. Accordingly, at concentrations that selectively inhibit these caspases, neither N-benzyloxycarbonyl-Leu-Glu-His-Asp-fmk nor N-benzyloxycarbonyl-Ile-Glu-Thr-Asp-fmk could prevent adenosine analog-induced cell death. To definitively rule out a role for the caspase-9/cytochrome c-dependent mitochondrial pathway of cell death, neither adenosine analog had any effect on mitochondrial membrane potential, which was instead markedly reduced by other apoptotic stimuli (e.g., deoxyribose, NaCN, and betulinic acid). Consistently, although the latter triggered translocation of mitochondrial cytochrome c to the cytoplasm, no cytosolic accumulation of cytochrome c was detected with adenosine analogs. Conversely, 1 to 7 h after addition of either adenosine analog (i.e., before the appearance of caspase-3 activation), caspase-2 activity was surprisingly and markedly increased. The selective caspase-2 inhibitor N-benzyloxy carbonyl-Val-Asp-Val-Ala-Asp-fmk significantly reduced both adenosine analogs-induced caspase-2 activation and the associated cell death. We conclude that adenosine analogs induce the apoptosis of human astrocytoma cells by activating an atypical apoptotic cascade involving caspase-2 as an initiator caspase, and effector caspase-3. Therefore, these compounds could be effectively used in the pharmacological manipulation of tumors characterized by resistance to cell death via either the mitochondrial or caspase-8/death receptor pathways
Methylazoxymethanol microencephaly in rats: neurochemical characterization and behavioral studies with the nootropic oxiracetam
No full textThe administration of methylazoxymethanol (MAM) to pregnant rats induced a marked reduction in the weight of the offspring's brain. This reduction was due to aplasia of the cortex and hippocampus, whose thicknesses were 50% of those of control animals. A significant reduction was also observed in the striatum. This aplasia could be ascribed to the antimitotic effect of MAM, which, when given at gestational day 15, prevented the development of neurons in the three brain areas mentioned. Indeed, we infer here that the total number of GABA-receptor complexes, as measured by [3H]muscimol and [3H]flunitrazepam binding, was reduced to the same degree as was the weight of the cortex. Similarly, total [3H]haloperidol binding sites were reduced in the striatum. From the behavioral point of view, offspring of MAM-treated rats (MAM rats) showed impaired acquisition in the water-maze and pole-climbing tests, indicating that this brain aplasia had disrupted cognitive processes. In contrast, these animals showed normal growth, and grossly their behavior appeared normal. Oxiracetam, a new compound that belongs to the recently described class of nootropic drugs, was able to restore acquisition processes in MAM rats. We propose therefore that MAM rats might become an interesting and quite simple animal model for evaluation of new acquisition-enhancing drugs. Moreover, this model could also be useful to study the neurochemical correlates of cognitive processes
- …
