Measurement of Na+-K+ pump in muscle

Glycolytic enzyme upregulation and numbness of mitochondrial activity characterize the early phase of apoptosis in cerebellar granule cells

Alzheimer's disease (AD) and cancer proceed via one or more common molecular mechanisms: a metabolic shift from oxidative phosphorylation to glycolysis-corresponding to the activation of the Warburg effect-occurs in both diseases. The findings reported in this paper demonstrate that, in the early phase of apoptosis, glucose metabolism is enhanced, i.e. key proteins which internalize and metabolize glucose-glucose transporter, hexokinase and phosphofructokinase-are up-regulated, in concomitance with a parallel decrease in oxygen consumption by mitochondria and increase of L-lactate accumulation. Reversal of the glycolytic phenotype occurs in the presence of dichloroacetate, inhibitor of the pyruvate dehydrogenase kinase enzyme, which speeds up apoptosis of cerebellar granule cells, reawakening mitochondria and then modulating glycolytic enzymes. Loss of the adaptive advantage afforded by aerobic glycolysis, which occurs in the late phase of apoptosis, exacerbates the pathological processes underlying neurodegeneration, leading inevitably the cell to death. In conclusion, the data propose that both aerobic, i.e. Warburg effect, essentially due to the protective numbness of mitochondria, and anaerobic glycolysis, rather due to the mitochondrial impairment, characterize the entire time frame of apoptosis, from the early to the late phase, which mimics the development of AD.

Glucose-6-phosphate tips the balance in modulating apoptosis in cerebellar granule cells

A metabolic shift from oxidative phosphorylation to glycolysis (i.e. the Warburg effect) occurs in Alzheimer's disease accompanied by an increase of both activity and level of HK-I. The findings reported here demonstrate that in the early phase of apoptosis VDAC1 activity, but not its protein level, progressively decreases, in concomitance with the physical interaction of HK-I with VDAC1. In the late phase of apoptosis, glucose-6-phosphate accumulation in the cell causes the dissociation of the two proteins, the re-opening of the channel and the recovery of VDAC1 function, resulting in a reawakening of the mitochondrial function, thus inevitably leading to cell death.

Extracellular ADP prevents neuronal apoptosis via activation of cell antioxidant enzymes and protection of mitochondrial ANT-1

Apoptosis in neuronal tissue is an efficient mechanism which contributes to both normal cell development and pathological cell death. The present study explores the effects of extracellular ADP on low [K(+)]-induced apoptosis in rat cerebellar granule cells. ADP, released into the extracellular space in brain by multiple mechanisms, can interact with its receptor or be converted, through the actions of ectoenzymes, to adenosine. The findings reported in this paper demonstrate that ADP inhibits the proapoptotic stimulus supposedly via: i) inhibition of ROS production during early stages of apoptosis, an effect mediated by its interaction with cell receptor/s. This conclusion is validated by the increase in SOD and catalase activities as well as by the GSSG/GSH ratio value decrease, in conjunction with the drop of ROS level and the prevention of the ADP protective effect by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a novel functionally selective antagonist of purine receptor; ii) safeguard of the functionality of the mitochondrial adenine nucleotide-1 translocator (ANT-1), which is early impaired during apoptosis. This effect is mediated by its plausible internalization into cell occurring as such or after its hydrolysis, by means of plasma membrane nucleotide metabolizing enzymes, and resynthesis into the cell. Moreover, the findings that ADP also protects ANT-1 from the toxic action of the two Alzheimer's disease peptides, i.e. Aβ1-42 and NH2htau, which are known to be produced in apoptotic cerebellar neurons, further corroborate the molecular mechanism of neuroprotection by ADP, herein proposed.

l-Lactate metabolism in HEP G2 cell mitochondria due to the l-lactate dehydrogenase determines the occurrence of the lactate/pyruvate shuttle and the appearance of oxaloacetate, malate and citrate outside mitochondria

As part of an ongoing study of l-lactate metabolism both in normal and in cancer cells, we investigated whether and how l-lactate metabolism occurs in mitochondria of human hepatocellular carcinoma (Hep G2) cells. We found that Hep G2 cell mitochondria (Hep G2-M) possess an l-lactate dehydrogenase (ml-LDH) restricted to the inner mitochondrial compartments as shown by immunological analysis, confocal microscopy and by assaying ml-LDH activity in solubilized mitochondria. Cytosolic and mitochondrial l-LDHs were found to differ from one another in their saturation kinetics. Having shown that l-lactate itself can enter Hep G2 cells, we found that Hep G2-M swell in ammonium l-lactate, but not in ammonium pyruvate solutions, in a manner inhibited by mersalyl, this showing the occurrence of a carrier-mediated l-lactate transport in these mitochondria. Occurrence of the l-lactate/pyruvate shuttle and the appearance outside mitochondria of oxaloacetate, malate and citrate arising from l-lactate uptake and metabolism together with the low oxygen consumption and membrane potential generation are in favor of an anaplerotic role for l-LAC in Hep G2-M.

Genistein and daidzein prevent low potassium-dependent apoptosis of cerebellar granule cells

We have investigated the ability of certain dietary flavonoids, known to exert beneficial effects on the central nervous system, to affect neuronal apoptosis. We used cerebellar granule cells undergoing apoptosis due to potassium deprivation in a serum-free medium in either the absence or presence of the flavonoids genistein and daidzein, which are present in soy, and of catechin and epicatechin, which are present in cocoa. These compounds were used in a blood dietary concentration range. We found that genistein and daidzein, but not catechin and epicatechin, prevented apoptosis, with cell survival measured 24h after the induction of apoptosis being higher than that of the same cells incubated in flavonoid free medium (80% and 40%, respectively); there was no effect in control cells. A detailed investigation of the effect of these compounds on certain mitochondrial events that occur in cells en route to apoptosis showed that genistein and daidzein prevented the impairment of glucose oxidation and mitochondrial coupling, reduced cytochrome c release, and prevented both impairment of the adenine nucleotide translocator and opening of the mitochondrial permeability transition pore. Interestingly, genistein and daidzein were found to reduce the levels of reactive oxygen species, which are elevated in cerebellar granule cell apoptosis. These findings strongly suggest that the prevention of apoptosis depends mainly on the antioxidant properties of genistein and daidzein. This could lead to the development of a flavonoid-based therapy in neuropathies.

Transport and metabolism of L-lactate occur in mitochondria from cerebellar granule cells and are modified in cells undergoing low potassium dependent apoptosis

Having confirmed that externally added L-lactate can enter cerebellar granule cells, we investigated whether and how L-lactate is metabolized by mitochondria from these cells under normal or apoptotic conditions. (1) L-lactate enters mitochondria, perhaps via an L-lactate/H+ symporter, and is oxidized in a manner stimulated by ADP. The existence of an L-lactate dehydrogenase, located in the inner mitochondrial compartment, was shown by immunological analysis. Neither the protein level nor the Km and Vmax values changed en route to apoptosis. (2) In both normal and apoptotic cell homogenates, externally added L-lactate caused reduction of the intramitochondrial pyridine cofactors, inhibited by phenylsuccinate. This process mirrored L-lactate uptake by mitochondria and occurred with a hyperbolic dependence on L-lactate concentrations. Pyruvate appeared outside mitochondria as a result of external addition of L-lactate. The rate of the process depended on L-lactate concentration and showed saturation characteristics. This shows the occurrence of an intracellular L-lactate/pyruvate shuttle, whose activity was limited by the putative L-lactate/pyruvate antiporter. Both the carriers were different from the monocarboxylate carrier. (3) L-lactate transport changed en route to apoptosis. Uptake increased in the early phase of apoptosis, but decreased in the late phase with characteristics of a non-competitive like inhibition. In contrast, the putative L-lactate/pyruvate antiport decreased en route to apoptosis with characteristics of a competitive like inhibition in early apoptosis, and a mixed non-competitive like inhibition in late apoptosis.

Caspase-dependent alteration of the ADP/ATP translocator triggers the mitochondrial permeability transition which is not required for the low-potassium-dependent apoptosis of cerebellar granule cells

We investigated ADP/ATP exchange mediated by the adenine nucleotide translocator and opening of the mitochondrial permeability transition pore in homogenates from cerebellar granule cells en route to apoptosis induced by low potassium. We showed that, in the first 3 h of apoptosis, when maximum cytochrome c release had already occurred, adenine nucleotide translocator function was impaired owing to the action of reactive oxygen species, but no permeability transition pore opening occurred. Over 3-8 h of apoptosis, the permeability transition pore progressively opened, owing to caspase action, and further ADP/ATP translocator impairment occurred. The kinetics of transport and permeability transition pore opening were inversely correlated, both in the absence and presence of inhibitors of antioxidant and proteolytic systems. We conclude that, en route to apoptosis, alteration of the adenine nucleotide translocator occurs, resulting in permeability transition pore opening. This process depends on the action of caspase on pore component(s) other than the ADP/ATP translocator, because no change in either amount or molecular weight of the latter protein was noted during apoptosis, as measured by western blotting. Cell death occurs via apoptosis in the presence of cyclosporin A, the permeability transition pore inhibitor, thus showing that permeability transition pore opening, not needed for cytochrome c release, is also unnecessary for apoptosis to occur.

An increase in the ATP levels occurs in cerebellar granule cells en route to apoptosis in which ATP derives from both oxidative phosphorylation and anaerobic glycolysis

Although it is recognized that ATP plays a part in apoptosis, whether and how its level changes en route to apoptosis as well as how ATP is synthesized has not been fully investigated. We have addressed these questions using cultured cerebellar granule cells. In particular, we measured the content of ATP, ADP, AMP, IMP, inosine, adenosine and L-lactate in cells undergoing apoptosis during the commitment phase (0-8 h) in the absence or presence of oligomycin or/and of citrate, which can inhibit totally the mitochondrial oxidative phosphorylation and largely the substrate-level phosphorylation in glycolysis, respectively. In the absence of inhibitors, apoptosis was accompanied by an increase in ATP and a decrease in ADP with 1:1 stoichiometry, with maximum ATP level found at 3 h apoptosis, but with no change in levels of AMP and its breakdown products and with a relatively low level of L-lactate production. Consistently, there was an increase in the cell energy charge and in the ratio ([ATP][AMP])/[ADP](2). When the oxidative phosphorylation was completely blocked by oligomycin, a decrease of the ATP content was found both in control cells and in cells undergoing apoptosis, but nonetheless cells still died by apoptosis, as shown by checking DNA laddering and by death prevention due to actinomycin D. In this case, ATP was provided by anaerobic glycolysis, as suggested by the large increase of L-lactate production. On the other hand, citrate itself caused a small decrease in ATP level together with a huge decrease in L-lactate production, but it had no effect on cell survival. When ATP level was further decreased due to the presence of both oligomycin and citrate, death occurred via necrosis at 8 h, as shown by the lack of DNA laddering and by death prevention found due to the NMDA receptor antagonist MK801. However, at a longer time, when ATP level was further decreased, cells died neither via apoptosis nor via glutamate-dependent necrosis, in a manner similar to something like to energy catastrophe. Our results shows that cellular ATP content increases in cerebellar granule cell apoptosis, that the role of oxidative phosphorylation is facultative, i.e. ATP can also derive from anaerobic glycolysis, and that the type of cell death depends on the ATP availability.

Cytochrome c, released from cerebellar granule cells undergoing apoptosis or excytotoxic death, can generate protonmotive force and drive ATP synthesis in isolated mitochondria

In rat cerebellar granule cells, cytochrome c release takes place during glutamate toxicity and apoptosis due to deprivation of depolarising levels of potassium. We show that, as in necrosis, the released cytochrome c present in the cytosolic fraction obtained from cerebellar granule cells undergoing apoptosis can operate as a reactive oxygen species (ROS) scavenger and as a respiratory substrate. The capability of the cytosolic fraction containing cytochrome c, obtained from cerebellar granule cells undergoing either necrosis or apoptosis, to energise coupled mitochondria isolated by the same cells is also investigated. We show that, in both cases, the cytosolic fraction containing cytochrome c, added to mitochondria, can cause proton ejection, and membrane potential generation and can drive ATP synthesis and export in the extramitochondrial phase, as photometrically measured via the ATP detecting system. Cytochrome c, separated immunologically from the cytosolic fraction of apoptotic cells when added to mitochondria, is found to cause proton ejection to generate membrane potential and to drive ATP synthesis and export in a manner not sensitive to the further addition of the cytosolic fraction depleted of cytochrome c, which failed to do this. In the light of these findings we propose that in apoptosis the released cytochrome c can contribute to provide ATP required for the cell programmed death to occur.