Apoptosis induced by hydrogen peroxide is mediated by decreased superoxide anion concentration and reduction of intracellular milieu

Intracellular acidification triggered by mitochondrial-derived hydrogen peroxide is an effector mechanism for drug-induced apoptosis in tumor cells

We recently showed that two photoproducts of merocyanine 540, C2 and C5, triggered cytochrome C release; however, C5 was inefficient in inducing caspase activity and apoptosis in leukemia cells, unlike C2. Here we show that HL60 cells acidified upon exposure to C2 but not C5. The intracellular drop in pH and caspase activation were dependent upon hydrogen peroxide production, and were inhibited by scavengers of hydrogen peroxide. On the contrary, caspase inhibitors did not block hydrogen peroxide production. In turn, increased intracellular hydrogen peroxide concentration was downstream of superoxide anion produced within 2 h of exposure to C2. Inhibitor of NADPH oxidase diphenyleneiodonium neither inhibited superoxide production nor caspase activation triggered by C2. However, exposure of purified mitochondria to C2 resulted in significantly increased superoxide production. Furthermore, cytochrome C release from isolated mitochondria induced by C2 was completely inhibited in the presence of scavengers of hydrogen peroxide. Contrarily, scavenging hydrogen peroxide had no effect on the cyclosporin A-sensitive mitochondrial permeability transition induced by C5. Our data suggest a scenario where drug-induced hydrogen peroxide production induces intracellular acidification and release of cytochrome C, independent of the inner membrane pore, thereby creating an intracellular environment permissive for caspase activation.

Hydrogen peroxide in the human body

Hydrogen peroxide (H(2)O(2)) is widely regarded as a cytotoxic agent whose levels must be minimized by the action of antioxidant defence enzymes. In fact, H(2)O(2) is poorly reactive in the absence of transition metal ions. Exposure of certain human tissues to H(2)O(2) may be greater than is commonly supposed: substantial amounts of H(2)O(2) can be present in beverages commonly drunk (especially instant coffee), in freshly voided human urine, and in exhaled air. Levels of H(2)O(2) in the human body may be controlled not only by catabolism but also by excretion, and H(2)O(2) could play a role in the regulation of renal function and as an antibacterial agent in the urine. Urinary H(2)O(2) levels are influenced by diet, but under certain conditions might be a valuable biomarker of 'oxidative stress'.

Artifacts in cell culture: rapid generation of hydrogen peroxide on addition of (-)-epigallocatechin, (-)-epigallocatechin gallate, (+)-catechin, and quercetin to commonly used cell culture media

There is considerable current interest in the possible beneficial health effects of quercetin, catechins, epigallocatechins, epigallocatechin gallates, and related phenolic compounds found in teas, wines, and other plant products. As a result, many laboratories are studying the effects of these compounds on cells in culture. The present paper shows that addition of these compounds to commonly used cell culture media leads to generation of substantial amounts of hydrogen peroxide (H(2)O(2)). Dulbecco's modified Eagle medium gives the highest H(2)O(2) level for all the compounds tested, with levels reaching >400 microM within 2 h for addition of 1 mM concentrations of gallic acid, epigallocatechin gallate, and epigallocatechin. Catechin and quercetin produced lower, but still significant, levels of H(2)O(2). McCoy's 5A and RPMI 1640 media also promoted H(2)O(2) production from the above phenolic compounds. This rapid generation of H(2)O(2) could account for some or all of the reported effects of phenolic compounds on cells in culture.

Acute peroxide treatment of rat hippocampal slices induces adenosine-mediated inhibition of excitatory transmission in area CA1

Brief exposure to conditions that generate free radicals inhibits synaptic transmission in hippocampal slices, most likely via a presynaptic mechanism. Because other physiologically stressful conditions that generate free radicals, such as hypoxia or ischemia, stimulate the release of adenosine from brain slices, we determined whether increases in extracellular adenosine mediate the presynaptic inhibition of excitatory transmission induced by peroxide treatment. Simultaneous addition of hydrogen peroxide (0.01%) and ferrous sulfate (100 microM) resulted in a >80% decrease in synaptic potentials recorded in the CA1 region of hippocampal slices of adult male rats. Treatment with theophylline (200 microM), a non-selective adenosine receptor antagonist, or 8-cyclopentyl-1,3-dipropylxanthine (100 nM), a selective adenosine A1 receptor antagonist, prior to and during hydrogen peroxide superfusion prevented the inhibition. These results demonstrate that acute exposure to hydrogen peroxide induces an adenosine-mediated decrease in synaptic transmission in hippocampal slices.

Inhibitors of poly (ADP-ribose) synthetase protect rat proximal tubular cells against oxidant stress

BACKGROUND

The generation of reactive oxygen species (ROS) has been implicated in the pathogenesis of renal ischemia-reperfusion injury. ROS produce DNA strand breaks that lead to the activation of the DNA-repair enzyme poly (ADP-ribose) synthetase (PARS). Excessive PARS activation results in the depletion of its substrate, nicotinamide adenine dinucleotide (NAD) and subsequently of adenosine 5'-triphosphate (ATP), leading to cellular dysfunction and eventual cell death. The aim of this study was to investigate the effect of various PARS inhibitors on the cellular injury and death of rat renal proximal tubular (PT) cells exposed to hydrogen peroxide (H2O2).

METHODS

Rat PT cell cultures were incubated with H2O2 (1 mM) either in the presence or absence of the PARS inhibitors 3-aminobenzamide (3-AB, 3 mM), 1,5-dihydroxyisoquinoline (0.3 mM) or nicotinamide (Nic, 3 mM), or increasing concentrations of desferrioxamine (0.03 to 3 mM) or catalase (0.03 to 3 U/ml). Cellular injury and death were determined using the MTT and lactate dehydrogenase (LDH) assays, respectively. H2O2-mediated PARS activation in rat PT cells and the effects of PARS inhibitors on PARS activity were determined by measurement of the incorporation of [3H]NAD into nuclear proteins.

RESULTS

Incubation of rat PT cells with H2O2 significantly inhibited mitochondrial respiration and increased LDH release, respectively. Both desferrioxamine and catalase reduced H2O2-mediated cellular injury and death. All three PARS inhibitors significantly attenuated the H2O2-mediated decrease in mitochondrial respiration and the increase in LDH release. Incubation with H2O2 produced a significant increase in PARS activity that was significantly reduced by all PARS inhibitors. 3-Aminobenzoic acid (3 mM) and nicotinic acid (3 mM), structural analogs of 3-AB and Nic, respectively, which did not inhibit PARS activity, did not reduce the H2O2-mediated injury and necrosis in cultures of rat PT cells.

CONCLUSION

We propose that PARS activation contributes to ROS-mediated injury of rat PT cells and, therefore, to the cellular injury and cell death associated with conditions of oxidant stress in the kidney.

Reactive oxygen intermediates regulate cellular response to apoptotic stimuli: an hypothesis

Production of reactive oxygen intermediates (ROI) has been thought for a long time to adversely affect the physiology and survival of a cell. There is now a growing body of evidence to suggest that ROI such as superoxide anion (O2*-) and hydrogen peroxide (H2O2) can influence the growth, as well as death, of animal cells in vitro. The observation that cells release O2*- or its dismutation product H2O2, either constitutively in the case of tumor cells or following cytokine stimulation, has led to the speculation that they might possibly serve as intercellular messengers to stimulate proliferation via mechanisms common to natural growth factors. However, as the balance between cell populations in an organism is tightly controlled by the rate of proliferation and death of constituent cells, an increase in cell numbers could reciprocally be viewed as deregulation of cell death. Hence, it is equally important to decipher how ROI influence the response of cells to signals that activate cell death pathway(s). We propose that ROI not only regulate proliferation but also affect cell sensitivity to triggers which activate the cellular suicide program (apoptosis) versus those that cause accidental (necrotic) cell death.