A digitized-fluorescence-imaging study of mitochondrial Ca2+ increase by doxorubicin in cardiac myocytes

Major obstacles to doxorubicin therapy: Cardiotoxicity and drug resistance

Background

Doxorubicin is one of the most commonly prescribed and time-tested anticancer drugs. Although being considered as a first line drug in different types of cancers, the two main obstacles to doxorubicin therapy are drug-induced cardiotoxicity and drug resistance.

Method

The study utilizes systemic reviews on publications of previous studies obtained from scholarly journal databases including PubMed, Medline, Ebsco Host, Google Scholar, and Cochrane. The study utilizes secondary information obtained from health organizations using filters and keywords to sustain information relevancy. The study utilizes information retrieved from studies captured in the peer-reviewed journals on “doxorubicin-induced cardiotoxicity” and “doxorubicin resistance.”

Discussion and results

The exact mechanisms of cardiotoxicity are not known; various hypotheses are studied. Doxorubicin can lead to free radical generation in various ways. The commonly proposed underlying mechanisms promoting doxorubicin resistance are the expression of multidrug resistance proteins as well as other causes.

Conclusion

In this review, we have described the major obstacles to doxorubicin therapy, doxorubicin-induced cardiotoxicity as well as the mechanisms of cancer drug resistance and in following the treatment failures.

Changes in mitochondrial redox state, membrane potential and calcium precede mitochondrial dysfunction in doxorubicin-induced cell death

Mitochondria play central roles in cell life as a source of energy and in cell death by inducing apoptosis. Many important functions of mitochondria change in cancer, and these organelles can be a target of chemotherapy. The widely used anticancer drug doxorubicin (DOX) causes cell death, inhibition of cell cycle/proliferation and mitochondrial impairment. However, the mechanism of such impairment is not completely understood. In our study we used confocal and two-photon fluorescence imaging together with enzymatic and respirometric analysis to study short- and long-term effects of doxorubicin on mitochondria in various human carcinoma cells. We show that short-term (<30 min) effects include i) rapid changes in mitochondrial redox potentials towards a more oxidized state (flavoproteins and NADH), ii) mitochondrial depolarization, iii) elevated matrix calcium levels, and iv) mitochondrial ROS production, demonstrating a complex pattern of mitochondrial alterations. Significant inhibition of mitochondrial endogenous and uncoupled respiration, ATP depletion and changes in the activities of marker enzymes were observed after 48 h of DOX treatment (long-term effects) associated with cell cycle arrest and death.

Dietary ascorbate intake affects steady state tissue concentrations in vitamin C-deficient mice: tissue deficiency after suboptimal intake and superior bioavailability from a food source (kiwifruit)

BACKGROUND

Humans acquire vitamin C (ascorbate) from their diet, and optimal tissue concentrations are required to maintain its enzyme cofactor and antioxidant activities. How dietary intake affects tissue concentrations is difficult to monitor and has generally been based on the measurement of plasma concentrations.

OBJECTIVE

We aimed to determine the effect of various ascorbate intakes on tissue concentrations in the Gulo mouse model of vitamin C deficiency and to compare the effectiveness of delivery when ascorbate was added to the drinking water or obtained through a fruit source (kiwifruit).

DESIGN

Gulo(-/-) mice were fed various amounts of ascorbate for 1 mo, either in their drinking water or as a kiwifruit gel. Tissue vitamin C content was measured and compared with concentrations in wild-type mice.

RESULTS

Ascorbate concentrations in serum, liver, kidney, heart, and white blood cells were extremely labile and were well below concentrations observed in the wild-type mice when serum concentrations were below saturation. All tissues except for brain were rapidly depleted when intake was stopped. Consumption of a preparation of fresh kiwifruit (either green or gold varieties) resulted in up to 5 times more effective delivery to tissues than when ascorbate was administered via the drinking water.

CONCLUSIONS

Subsaturation concentrations of plasma ascorbate resulted in severe deficiency in many tissues, and saturating amounts were required to achieve tissue concentrations similar to those found in wild-type animals. It is possible that the bioavailability of ascorbate is superior from some foods, such as kiwifruit. These results have important implications for human nutrition.

Carvedilol-mediated antioxidant protection against doxorubicin-induced cardiac mitochondrial toxicity

The cardiotoxicity associated with doxorubicin (DOX) therapy limits the total cumulative dose and therapeutic success of active anticancer chemotherapy. Cardiac mitochondria are implicated as primary targets for DOX toxicity, which is believed to be mediated by the generation of highly reactive free radical species of oxygen from complex I of the mitochondrial electron transport chain. The objective of this study was to determine if the protection demonstrated by carvedilol (CV), a beta-adrenergic receptor antagonist with strong antioxidant properties, against DOX-induced mitochondrial-mediated cardiomyopathy [Toxicol. Appl. Pharmacol. 185 (2002) 218] is attributable to its antioxidant properties or its beta-adrenergic receptor antagonism. Our results confirm that DOX induces oxidative stress, mitochondrial dysfunction, and histopathological lesions in the cardiac tissue, all of which are inhibited by carvedilol. In contrast, atenolol (AT), a beta-adrenergic receptor antagonist lacking antioxidant properties, preserved phosphate energy charge but failed to protect against any of the indexes of DOX-induced oxidative mitochondrial toxicity. We therefore conclude that the cardioprotective effects of carvedilol against DOX-induced mitochondrial cardiotoxicity are due to its inherent antioxidant activity and not to its beta-adrenergic receptor antagonism.

Apoptosis and necrosis in health and disease: role of mitochondria

Mitochondria play an important role in both the life and death of cells. Mitochondria are the powerhouse of the cell, providing over 90% of adenosine triphosphate (ATP) consumed by the cell. Mitochondrial energy production, however, is disrupted in various pathological situations leading to cellular Injury. The mechanisms causing the injury are turning out to be more complex than originally expected. For instance, calcium, oxidant chemicals, ischemia/ reperfusion, and a range of other agents promote onset of the mitochondrial permeability transition in mitochondria from liver, heart, and other tissues. Often the consequence of this event is ATP depletion, ion deregulation, mitochondrial and cellular swelling, activation of degradative enzymes, plasma membrane failure, and cell lysis. This is referred to as necrotic cell death. The mitochondrial permeability transition is also involved in apoptotic cell death. In this mode of death, the role of the permeability transition is to release proapoptotic proteins from mitochondria into the cytosol where with the aid of cellular ATP they complete the apoptotic cascade. Therefore, mitochondria contribute to both apoptotic and necrotic death.

Doxorubicin-induced cardiac mitochondrionopathy

Doxorubicin (Adriamycin) is a potent and broad-spectrum antineoplastic agent prescribed for the treatment of a variety of cancers, including both solid tumours and leukaemias. Unfortunately, despite its broad effectiveness, long-term therapy with doxorubicin is associated with a high incidence of a cumulative and irreversible dilated cardiomyopathy. Numerous mechanisms have been proposed to account for this toxicity. Although there is general consensus that doxorubicin undergoes redox cycling to generate free radicals that are responsible for mediating the various cytopathologies associated with drug exposure, the source and subcellular targets continue to be debated. This short review provides a synopsis of the evidence implicating cardiac mitochondria as key intracellular targets, both as sites of generation of highly reactive free radical intermediates as well as targets for the interference with cell calcium regulation and bioenergetic failure that are hallmarks of doxorubicin-induced cardiac failure.

Amlodipine inhibits doxorubicin-induced apoptosis in neonatal rat cardiac myocytes

OBJECTIVES

We examined whether amlodipine, a calcium channel antagonist with potent antioxidant activity, inhibits doxorubicin-induced apoptosis in cultured neonatal rat cardiac myocytes.

BACKGROUND

Recent studies have shown that doxorubicin induces apoptosis as well as necrosis in myocytes through generation of reactive oxygen species.

METHODS

The effects of amlodipine and several other antioxidants on doxorubicin-induced oxidative stress and mitochondria-mediated apoptosis were examined.

RESULTS

Treatment of myocytes with doxorubicin (10(-6) mol/l) for 14 h increased the number of cells with elevated peroxides, as histochemically estimated by 2',7'-dichlorofluorescin (DCF) diacetate, and the percentage of apoptotic myocytes, as estimated by Hoechst 33258 nuclear staining, compared with control myocytes (25.0 +/- 1.6% vs. 5.2 +/- 1.2%). Moreover, doxorubicin-induced myocyte apoptosis was also confirmed by annexin V-fluorescein isothiocyanate binding assay. Doxorubicin induced a reduction in myocyte adenosine 5'-triphosphate content, a loss of mitochondrial membrane potential, cytochrome c release from the mitochondria into the cytosol, and caspase-3 activation to 1.9-fold of control. Amlodipine significantly attenuated increased DCF fluorescence, inhibited the mitochondria-mediated apoptotic responses described earlier, and decreased apoptosis in the doxorubicin-treated myocytes in a dose-dependent fashion. Amlodipine at 10(-6) mol/l significantly decreased apoptosis to 15.4 +/- 0.7%, and this antiapoptotic action was more effective than that seen with other antioxidants, including probucol, ascorbic acid, and alpha-tocopherol. In contrast, the calcium channel antagonist nifedipine (10(-6) mol/l) did not inhibit apoptosis. Catalase, glutathione, and N-acetylcysteine, but not mannitol or superoxide dismutase, significantly decreased DCF fluorescence and attenuated myocyte apoptosis induced by doxorubicin to 18.7 +/- 1.2%, 19.1 +/- 1.7%, and 18.7 +/- 0.6%, respectively.

CONCLUSIONS

Amlodipine significantly inhibits doxorubicin-induced myocyte apoptosis by suppressing the mitochondrial apoptotic pathway. This effect is attributed to the antioxidant properties of amlodipine, affecting mainly hydrogen peroxide.

Differential regulation of phosphatidylserine externalization and DNA fragmentation by caspases in anticancer drug-induced apoptosis of rat mammary adenocarcinoma MTLn3 cells

Caspase activation is a central event in the execution phase of apoptosis and is associated with phosphatidylserine (PS) externalization and DNA fragmentation. We investigated the role of caspase activity in anticancer drug-induced PS externalization and DNA fragmentation in MTLn3 cells. Caspase activation (DEVD-AMC cleavage) occurred in a time- and concentration-dependent manner after exposure to doxorubicin, in association with cleavage of poly(ADP) ribose polymerase and protein kinase C delta, two caspase-3 substrates. Caspase activation was closely followed by oligonucleosomal DNA fragmentation and PS externalization as determined by flow cytometric analysis. Similar observations were made for etoposide and cisplatin. Inhibition of caspases with zVAD-fmk inhibited almost completely doxorubicin-induced DNA fragmentation as well as proteolysis of protein kinase C delta. In contrast, PS externalization induced by doxorubicin was only partly affected by caspase inhibition. Flow cytometric cell sorting demonstrated that DNA fragmentation in the remaining PS positive cells after doxorubicin treatment in the presence of zVAD-fmk was fully blocked. In conclusion, these data indicate that while DNA fragmentation in anticancer drug-induced apoptosis of MTLn3 cells is fully dependent on caspase activity, PS externalization is controlled by both caspase-dependent and caspase-independent pathways.

Contribution of increased mitochondrial free Ca2+ to the mitochondrial permeability transition induced by tert-butylhydroperoxide in rat hepatocytes

Previously, we showed that the oxidant chemical, tert-butylhydroperoxide (t-BuOOH), induces a mitochondrial permeability transition (MPT) in intact hepatocytes, causing lethal cell injury. Here, we investigated the role of mitochondrial free Ca2+ in t-BuOOH cytotoxicity to 1-day-cultured rat hepatocytes using confocal microscopy of autofluorescence and parameter-indicating fluorophores. t-BuOOH (100 micromol/L) caused an early increase of mitochondrial free Ca2+, as assessed by confocal microscopy of Rhod-2 fluorescence. Increased mitochondrial Ca2+ was followed by onset of the MPT, as evidenced by permeation of cytosolic calcein into mitochondria and loss of the mitochondrial membrane potential-indicating dye, tetramethylrhodamine methylester. Preincubation with an intracellular Ca2+ chelator (BAPTA-AM and its derivatives) partially blocked the late phase of mitochondrial NAD(P)H oxidation after t-BuOOH, but failed to prevent the early oxidation of mitochondrial NAD(P)H. Ca2+ chelation also prevented the increase of mitochondrial Ca2+, generation of mitochondrial reactive oxygen species (ROS), onset of the MPT, and subsequent cell death. Confocal images showed that protection occurred when loading of the Ca2+ chelator was predominantly mitochondrial. The antioxidant, desferal, also diminished increased mitochondrial Ca2+ after t-BuOOH and prevented cell death. We conclude that oxidative stress induced by t-BuOOH enhances mitochondrial Ca2+ uptake, leading to increased matrix Ca2+, increased ROS formation, onset of the MPT, and cell death.

Simultaneous measurements of cytosolic and mitochondrial Ca2+ transients in HT29 cells

Loading of HT29 cells with the Ca2+ dye fura-2/AM resulted in an nonhomogeneous intracellular distribution of the dye. Cellular compartments with high fura-2 concentrations were identified by correlation with mitochondrial markers, cellular autofluorescence induced by UV, and dynamic measurement of autofluorescence after inhibition of oxidative phosphorylation. Stimulation with carbachol (10(-4) mol/liter) increased cytosolic, nuclear, and mitochondrial Ca2+ activity ([Ca2+]c, [Ca2+]n, and [Ca2+]m, respectively) measured by UV confocal and conventional imaging. Similar results were obtained with a prototype two-photon microscope (Zeiss, Jena, Germany) allowing for fura-2 excitation. The increase of [Ca2+]m lagged behind that of [Ca2+]c and [Ca2+]n by 10-20 s, and after removing the agonist, [Ca2+]m also decreased with a delay. A strong increase of [Ca2+]m occurred only when a certain threshold of [Ca2+]c (around 1 micromol/liter) was exceeded. In a very similar way, ATP, neurotensin, and thapsigargin increased [Ca2+]c and [Ca2+]m. Carbonyl cyanide p-trifluoromethoxyphenylhyrdrazone reversibly reduced the increase of [Ca2+]m. The source of the mitochondrial Ca2+ increase had intra- and extracellular components, as revealed by experiments in low extracellular Ca2+. We conclude that agonist-induced Ca2+ signals are transduced into mitochondria. 1) Mitochondria could serve as a Ca2+ sink, 2) mitochondria could allow the modulation of [Ca2+]c and [Ca2+]n signals, and 3) [Ca2+]m may serve as a stimulatory metabolic signal when a cell is highly stimulated.

In vivo prevention of adriamycin cardiotoxicity by cyclosporin A or FK506

The use of adriamycin, an antitumour agent, is restricted by its cardiotoxicity. The objective of this study was to investigate the role of mitochondrial Ca2+ in adriamycin-induced cardiotoxicity and the effect of either cyclosporin A (CsA) or tacrolimus (FK506) on that cardiotoxicity. A single dose of adriamycin (10 mg/kg body weight) caused myocardial damage that was manifested by elevation of serum enzymes, glutamate-oxaloacetate transaminase (GOT), glutamate-pyruvate transaminase (GPT), lactate dehydrogenase isoenzyme (LDH-iso) and creatine phosphokinase isoenzyme (CPK2-MB). The permeability of heart inner mitochondrial membrane of adriamycin-treated rats was examined. Tetraphenyl phosphonium ion (TPP+) uptake, estimated with a TPP+-sensitive electrode was used to monitor changes in heart inner mitochondrial membrane potential. Ca2+ efflux was measured spectrophotometrically with the Ca2+ indicator arsenazo III. The ability of heart mitochondria isolated from adriamycin treated rats to retain accumulated Ca2+ or TPP+ was sharply reduced. The increase of diagnostic serum enzymes and isoenzymes and the reduced ability to retain Ca2+ or TPP+ by heart mitochondria were restored to almost the normal levels when (500 microg/kg body weight) of CsA or FK506 were injected with adriamycin. The data suggested that adriamycin cardiotoxicity might be due to the increase of inner membrane permeability in heart mitochondria as a result of increasing the sensitivity of a Ca2+ dependent-pore of the inner mitochondrial membrane to calcium, leading to dissipation of membrane potential and release of pre-accumulated Ca2+. Suitable antagonists of Ca2+-dependent pore formation such as CsA or FK506 may improve heart tolerance to adriamycin.

Cardiotoxicity of doxorubicin: effects of 21-aminosteroids

The purposes of this study were to investigate in vivo the effects of two lazaroids,U-74389G (21-[4-(2,6-di-1-pyrrolidinyl-4-pyrimidinyl)-1-piperazinyl]-pregna-1,4,9 (11)-triene-3,20-dione (2)-2-butenenedionate) and U-83836E (-)-2-[[4-(2,6-di-1-pyrrlidinyl-4-pyrimidinyl)-1-piperazinyl]methy l]-3,4-dihydro-2,5,7,8-tetramethyl-2H-1-benzopyran-6-ol, dihydrochloride against the cardiotoxicity induced by doxorubicin in rat and the mechanisms underlying such a toxicity. Doxorubicin (DXR) administered intraperitoneally (5 mg/kg 4 times per week for 1 week) induced significant decrease of body weight, ECG alterations and 100% mortality. The lazaroids used in this study did not protect from DXR-induced cardiotoxicity. Our results showed that the compound U-74389G delayed, but did not reduce DXR-induced mortality, and did not prevent body weight loss and ECG changes. The compound U-83836E was unable to modify any toxic effects induced by DXR. These data indicate that oxygen free radicals and the subsequent increase in intracellular calcium are only steps of DXR progressive general toxicity that leads to cardiac injury. In conclusion, we propose that the 21-aminosteroids, potent inhibitors of membrane lipid peroxidation, alone are not enough to protect from DXR toxic effects.

Relation between accumulation of phospholipase A2 reaction products and Ca2+ release in isolated liver mitochondria

A Ca(2+)-dependent stimulation of mitochondrial phospholipase A2 is often assumed to play a role in mitochondrial Ca2+ release. We sought to clarify this relation by measuring Ca2+ transport and determining phospholipase A2 reaction products from the same sample of isolated, incubated rat liver mitochondria. When mitochondria had accumulated and spontaneously released again Ca2+, most probably by membrane permeability transition, there was no increase of phospholipase A2 reaction products. However, when the incubation was continued after Ca2+ release, significant increases of the content of lysophosphatidylcholine and unesterified fatty acids could be seen. Quinacrine, an inhibitor of phospholipase A2 activity, prevented Ca2+ release and p-hydroxymercuribenzoic acid, an inhibitor of lysophospholipid reesterification, induced a fast release of Ca2+ from isolated mitochondria. Such effects are usually taken as indirect evidence for a participation of phospholipase A2 in mitochondrial Ca2+ release, but analysis of the mitochondrial lipids revealed that no significant changes of the mass of phospholipase A2 reaction products had occurred. These experiments suggest that the accumulation of phospholipase A2 reaction products in mitochondria is the consequence rather than the cause of the membrane permeability transition. Exogenous phospholipase A2 products, lysophosphatidylcholine and arachidonic acid, induced mitochondrial Ca2+ release after a time lag, which decreased with aging of the mitochondrial preparation. The amount of lysophosphatidylcholine taken up by the mitochondria from the incubation medium during these experiments was measured and compared to the amount of lysophosphatidylcholine produced endogenously by mitochondrial phospholipase A2. From these data it appears likely that the amount of lysophosphatidylcholine generated in the mitochondria after the permeability transition is sufficient to sustain the permeable state. An accumulation of mitochondrially generated phospholipase A2 reaction products after the permeability transition could thus be a decisive factor for the limited reversibility of the membrane permeability transition.

Cocaine-induced mitochondrial dysfunction in primary cultures of rat cardiomyocytes

Morphological alterations of cardiac mitochondria have been observed in rats chronically treated with cocaine. Whether cocaine directly causes heart mitochondrial dysfunction remains unclear. The present study was undertaken to investigate the effect of cocaine on mitochondrial function in cultured myocardial cells. Cells were incubated with cocaine (10(-5)-10(-3) M) for 3-72 h, using either a repeated or single exposure protocol. Cocaine (10(-3) M) produced severe cytotoxicity after repeated exposure (24-72 h), as elevated by leakage of lactate dehydrogenase. Treatment of the cultures with a single exposure protocol (10(-5)-10(-3) M for 24 h or less) produced a very modest cytotoxic effect, as shown by a small increase in LDH leakage. However, cellular ATP levels showed a time-dependent decline in cultures treated with the single exposure protocol. Experiments using a digitized fluorescence imaging system revealed that cocaine (single exposure protocol) caused a dose- and time-dependent decrease in mitochondrial membrane potential, and the decline in membrane potential occurred prior to manifestation of cytotoxicity shown with the repeated exposure protocol. Cytosolic and mitochondrial calcium levels, as determined by fura-2, were not affected during treatment with cocaine. Our results suggest that cocaine may compromise cardiac mitochondrial function and may lead to cardiotoxicity.

Mitochondrial free calcium transients during excitation-contraction coupling in rabbit cardiac myocytes

Mitochondrial free Ca2+ may regulate mitochondrial ATP production during cardiac exercise. Here, using laser scanning confocal microscopy of adult rabbit cardiac myocytes co-loaded with Fluo-3 to measure free Ca2+ and tetramethylrhodamine methylester to identify mitochondria, we measured cytosolic and mitochondrial Ca2+ transients during the contractile cycle. In resting cells, cytosolic and mitochondrial Fluo-3 signals were similar. During electrical pacing, transients of Fluo-3 fluorescence occurred in both the cytosolic and mitochondrial compartments. Both the mitochondrial and the cytosolic transients were potentiated by isoproterenol. These experiments show directly that mitochondrial free Ca2+ rises and falls during excitation-contraction coupling in cardiac myocytes and that changes of mitochondrial Ca2+ are kinetically competent to regulate mitochondrial metabolism on a beat-to-beat basis.

Interactions of intracellular pH and intracellular calcium in primary cultures of rabbit corneal epithelial cells

Homeostasis of intracellular calcium ([Ca++]i) and pH (pHi) is important in the cell's ability to respond to growth factors, to initiate differentiation and proliferation, and to maintain normal metabolic pathways. Because of the importance of these ions to cellular functions, we investigated the effects of changes of [Ca++]i and pHi on each other in primary cultures of rabbit corneal epithelial cells. Digitized fluorescence imaging was used to measure [Ca++]i with fura-2 and pHi with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Resting pHi in these cells was 7.37 +/- 0.05 (n = 20 cells) and resting [Ca++]i was 129 +/- 10 nM (n = 35 cells) using a nominally bicarbonate-free Krebs Ringer HEPES buffer (KRHB), pH 7.4. On exposure to 20 mM NH4Cl, which rapidly alkalinized cells by 0.45 pH units, an increase in [Ca++]i to 215 +/- 14 nM occurred. Pretreatment of the cells with 100 microM verapamil or exposure to 1 mM ethylene bis-(oxyethylenenitrilo)-tetraacetic acid (EGTA) without extracellular calcium before addition of 20 mM NH4Cl did not abolish the calcium increase, suggesting that the source of the calcium transient was from intracellular calcium stores. On removal of NH4Cl or addition of 20 mM sodium lactate, there were minimal changes in calcium even though pHi decreased. Treatment of CE cells with the calcium ionophores, ionomycin and 4-bromo A23187, increased [Ca++]i, but produced a biphasic change in pHi. Initially, there was an acidification of the cytosol, and then an alkalinization of 0.10 to 0.11 pH units above initial values. When [Ca++]i was decreased by treating the cells with 5 mM EGTA and 20 microM ionomycin, pHi decreased by 0.35 +/- 0.02 units. We conclude that an increase in pHi leads to an increase in [Ca++]i in rabbit corneal epithelial cells; however, a decrease in pHi leads to minor changes in [Ca++]i. The ability of CE cells to maintain proper calcium homeostasis when pHi is decreased may represent an adaptive mechanism to maintain physiological calcium levels during periods of acidification, which occur during prolonged eye closure.

A digitized fluorescence imaging study of intracellular Ca2+, pH, and mitochondrial function in primary cultures of rabbit corneal epithelial cells exposed to sodium dodecyl sulfate

Primary cultures of rabbit corneal epithelial cells have been developed as an in vitro system to predict irritancy potential and delayed cytotoxicity of surfactants in our laboratory. The objective of this study was to evaluate the effects of the surfactant sodium dodecyl sulfate (SDS), a common ingredient in consumer products, on intracellular Ca2+, pH, and mitochondrial function in this culture system. Ca2+ and pH were measured in single living corneal epithelial cells by ratio imaging of fura-2 and 2,'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein fluorescence, respectively. Mitochondrial function was examined by probing mitochondrial membrane potential with the fluorescent dye rhodamine 123 and by measuring the ratio of ATP to ADP with an HPLC method. Cell viability was determined by fluorescence imaging of propidium iodide in single cells and LDH leakage assay in populations of cells. SDS (40 micrograms/ml) increased intracellular Ca2+ from 180 +/- 28nM to 453 +/- 86 nM within 2 min, and induced intracellular acidification (pHi dropped 0.3 units in 15 min). Treatment of the cultures with SDS also resulted in dissipation of the mitochondrial membrane potential and decrease of intracellular ATP/ADP. SDS-induced Ca2+ elevation and intracellular acidification preceded the loss of cell viability observed 20 min after exposure. However, SDS-induced cell injury does not appear to be triggered by extracellular Ca(2+)-influx, as absence of extracellular Ca2+ did not attenuate SDS-induced cytotoxicity while it completely blocked ionomycin-induced cytotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Indenoindole depresses lipofuscin formation in cultured neonatal rat myocardial cells

Lipofuscin accumulation in cultured rat myocardial cells is considered an index of intra-lysosomal oxidative reactions and was registered by autofluorescence measurements. Lipofuscinogenesis in secondary lysosomes is thought to be a consequence of Fenton reactions, and is much enhanced by oxidative stress obtained by culturing the cells in an atmosphere containing 40% oxygen. The influence of the synthetic antioxidant indenoindole (DHII), as compared to control cells, was a dose-responsive depression of lipofuscinogenesis to a degree of 19% and 17% with 20 microM DHII and to 25% and 23% with 40 microM DHII after 7 and 14 days in culture, respectively. This demonstrates a significant quenching of oxidative stress and suggests the therapeutic value of DHII and related antioxidants in protecting against oxygen radical-related diseases. It is also suggested that neonatal cardiac myocytes in culture are a suitable model system for the evaluation of oxygen radical-induced myocardial damage.

Mitochondrial Ca2+ overload in primary cultures of rat renal cortical epithelial cells by cytotoxic concentrations of cyclosporine: a digitized fluorescence imaging study

Cyclosporine (CsA) has been reported to disrupt Ca2+ efflux from mitochondria, which suggests that CsA interference with Ca2+ homeostasis may be related to its nephrotoxicity. Therefore, the purpose of this study was (1) to determine intracellular free Ca2+ concentration ([Ca2+]i) and mitochondrial free Ca2+ concentration ([Ca2+]m) after primary cultures of rat renal cortical epithelial cells were exposed to cytotoxic concentrations of CsA; and (2) to explore the role of disruption of intracellular and mitochondrial Ca2+ homeostasis in CsA-induced cytotoxicity. [Ca2+]i in single kidney cells was examined by digitized fluorescence imaging (DFI) of the Ca2+ fluorescent probe, fura-2, and [Ca2+]m in single cells was observed by DFI of fura-2 entrapped in mitochondria after selective permeabilization of plasma membrane and other non-mitochondrial organelles by digitonin. Mitochondrial membrane potential (delta psi) in single kidney cells was examined by rhodamine 123 (Rh-123) with DFI. Intracellular ATP in kidney cells was determined by a HPLC method. CsA resulted in an elevation in [Ca2+]i and [Ca2+]m, dissipation of delta psi and depletion of ATP in a dose- and time-dependent manner. The elevation of [Ca2+]i and [Ca2+]m and depletion of ATP preceded CsA-induced cytotoxicity in kidney cells as measured by lactate dehydrogenase (LDH) leakage. We conclude that CsA-induced alterations in mitochondrial Ca2+ homeostasis and a subsequent loss of energy supply may play a key role in CsA-induced cytotoxicity in primary cultures of rat renal cortical epithelial cells.

Mitochondrial free Ca2+ concentration in living cells

Evidence has accrued during the past two decades that mitochondrial Ca2+ plays an important role in the regulation of numerous cell functions such as energy metabolism. This implies that mitochondrial Ca2+ transport systems might be able to relay the changes of cytosolic Ca2+ concentration ([Ca2+]c) into mitochondrial matrix for regulating biochemical activities. To substantiate this idea, measurements of intramitochondrial free Ca2+ concentration ([Ca2+]m) become essential. In this article, we review the results from recent studies attempting to measure [Ca2+]m in living cells. In addition, the significance of each study is discussed.

Further characterization of the events involved in mitochondrial Ca2+ release and pore formation by prooxidants

Addition of the prooxidant 3,5-dimethyl-N-acetyl-p-benzoquinone imine (3,5(Me)2NAPQI) to Ca(2+)-loaded mitochondria caused a rapid and extensive release of the sequestered Ca2+. Ca2+ release was accompanied by irreversible NAD(P)H oxidation and was followed by the release of adenine and pyridine nucleotides into the extramitochondrial medium; this is evidence of the opening of the pore in the inner mitochondrial membrane. Preincubation of the mitochondria with ADP, cyclosporin A (CSA), m-iodobenzylguanidine (MIBG) or Mg2+ inhibited the prooxidant-induced Ca2+ release and prevented pore-opening. When mitochondria were preincubated with ruthenium red, Ca2+ release was only minimally stimulated by 3,5(Me)2NAPQI. However, increasing the concentration of the prooxidant caused release of an increasing fraction of the sequestered Ca2+. Alternatively, increasing the intramitochondrial Ca2+ load resulted in a lowering of the concentration of 3,5(Me)2NAPQI required for near complete Ca2+ release to occur. In the presence of ruthenium red, 3,5(Me)2NAPQI-induced Ca2+ release was accompanied by irreversible pyridine nucleotide oxidation and followed by the release of nucleotides into the extramitochondrial medium, events which were prevented on preincubation with CSA. Similarly, the addition of CSA, ADP or MIBG during 3,5(Me)2NAPQI-induced Ca2+ release arrested further Ca2+ release. In addition to their inhibitory effect on the 3,5(Me)2NAPQI-induced Ca2+ release, CSA, ADP or MIBG also decreased the rate of the basal, ruthenium red-induced mitochondrial Ca2+ release by 45-70%. It is proposed that the basal, ruthenium red-induced and the prooxidant-induced mitochondrial Ca2+ release occur through a common component that is sensitive to inhibition by CSA, ADP and MIBG and that is involved in mitochondrial pore formation. Furthermore, 3,5(Me)2NAPQI-induced pore opening does not involve Ca(2+)-cycling, but rather involves a site(s) that is (are) synergistically activated by Ca2+ and the prooxidant.

Mitochondrial K+ as modulator of Ca(2+)-dependent cytotoxicity in hepatocytes. Novel application of the K(+)-sensitive dye PBFI (K(+)-binding benzofuran isophthalate) to assess free mitochondrial K+ concentrations

In isolated rat hepatocytes a sustained high increase in intracellular free Ca2+ ([Ca2+]i), induced by extracellular ATP, is associated with mitochondrial dysfunction and cell death. The Ca(2+)-induced effects are Pi-dependent and less severe when the intracellular K+ content is low. In this study, the involvement of mitochondrial K+ processing in Ca(2+)-induced loss of mitochondrial membrane potential (MMP) and viability was investigated. The recently introduced K(+)-sensitive dye PBFI (K(+)-binding benzofuran isophthalate) has been used in combination with video-microscopy to assess intramitochondrial free K+ concentration ([K+]mito) in rat liver mitochondria in situ. After rapid permeabilization of the plasma membrane to remove cytosolic PBFI, the remaining PBFI was localized in mitochondria, and a 'resting' [K+]mito of approx. 15 mM could be measured. Increased [K+]mito levels were measured after induction of a prolonged increase in [Ca2+]i by ATP. Much lower [K+]mito, more comparable with control levels, were observed when intracellular K+ was depleted by omission of extracellular K+. In permeabilized cells the Ca(2+)-induced, Pi-dependent, dissipation of the MMP was markedly delayed in the absence of K+. These observations suggest involvement of [K+]mito as modulating agent in Ca(2+)-induced cytotoxicity in hepatocytes.

Distribution of electrical potential, pH, free Ca2+, and volume inside cultured adult rabbit cardiac myocytes during chemical hypoxia: a multiparameter digitized confocal microscopic study

Exploiting the optical sectioning capabilities of laser scanning confocal microscopy and using parameter-specific fluorescent probes, we determined the distribution of pH, free Ca2+, electrical potential, and volume inside cultured adult rabbit cardiac myocytes during ATP depletion and reductive stress with cyanide and 2-deoxyglucose ("chemical hypoxia"). During normoxic incubations, myocytes exhibited a cytosolic pH of 7.1 and a mitochondrial pH of 8.0 (delta pH = 0.9 units). Sarcolemmal membrane potential (delta psi) was -80 mV, and mitochondrial delta psi was as high as -100 mV, yielding a mitochondrial protonmotive force (delta p) of -155 mV (delta P = delta psi - 60 delta pH). After 30 min of chemical hypoxia, mitochondrial delta pH decreased to 0.5 pH units, but mitochondrial delta psi remained essentially unchanged. By 40 min, delta pH was collapsed, and mitochondrial and cytosolic free Ca2+ began to increase. Mitochondrial and sarcolemmal delta psi remained high. as Ca2+ rose, myocytes shortened, hypercontracted, and blebbed with a 30% decrease of cell volume. After hypercontraction, extensive mitochondrial Ca2+ loading occurred. After another few minutes, mitochondrial depolarized completely and released their load of Ca2+. After many more minutes, the sarcolemmal permeability barrier broke down, and viability was lost. These studies demonstrate a sequence of subcellular ionic and electrical changes that may underlie the progression to irreversible hypoxic injury.

Fluorescence imaging of intracellular Ca2+

The measurement of intracellular Ca2+ concentrations ([Ca2+]i) is of critical importance, because many cellular functions are tightly regulated by [Ca2+]i. The fluorescent indicator, fura-2, has been used frequently to measure [Ca2+]i because of its sensitivity and specificity, and because it can be loaded into living cells with little disruption of function. Most importantly, the peak excitation wavelength of fura-2 changes when it binds Ca2+. As a consequence, measurements of fluorescence at two excitation wavelengths can be used to obtain an estimate of [Ca2+]i that is independent of dye concentration and cell thickness. Fura-2 acetoxymethyl ester (AM) is a lipid-soluble derivative that is often used because of its ability to pass through cell membranes. There are, however, several problems with the use of fura-2 AM such as intracellular compartmentation and incomplete deesterification. The availability of low-light-level cameras and computer hardware for the digitization of fluorescent images has made quantitative fluorescence microscopy possible. This technique has shown a striking spatial heterogeneity of [Ca2+]i in a variety of cell types, and has revealed substantial new information on dynamic intracellular biochemistry and signal transduction. However, the current imaging technology is not fully developed because of dye and instrumentation limitations. Further development of techniques and new probes are required to improve temporal and spatial resolution.

A digitized fluorescence imaging study of intracellular free calcium, mitochondrial integrity and cytotoxicity in rat renal cells exposed to ionomycin, a calcium ionophore

The objective of this study was to explore the role of extracellular Ca2+ and mitochondrial integrity in ionomycin-induced cytotoxicity in primary cultures of rat kidney cortical epithelial cells using digitized fluorescence imaging (DFI), which is a powerful tool for continuously observing the dynamic intracellular biochemistry of single living cells. Using DFI, intracellular free calcium ion concentration ([Ca2+]i), mitochondrial membrane potential and loss of cell viability in individual rat renal cortical epithelial cells were examined temporally by fura-2, rhodamine 123 (Rh-123) and propidium iodide (PI), respectively. Images were taken within 10 min after exposure to 5 and 10 microM ionomycin. These three parameters, [Ca2+]i, mitochondrial membrane potential and cell viability, were also measured in populations of cells by a multiwell fluorescence scanner with fluo-3, Rh-123 and PI, respectively. Cytotoxicity was also assessed by two colorimetric cytotoxicity tests (LDH leakage and mitochondrial MTT reduction). Using DFI, the fluorescence scanner and the colorimetric cytotoxicity tests, we found that exposure of primary cultures of rat kidney cortical epithelial cells to high concentrations of ionomycin (5 and 10 microM) caused a rapid and sustained rise in [Ca2+]i, which preceded dissipation of the mitochondrial membrane potential and loss of cell viability and that chelation of extracellular Ca2+ with EGTA attenuated these responses. We demonstrated the value of using DFI to continuously observe the dynamic intracellular biochemistry of single living cells by establishing a sequence of elevated [Ca2+]i, dissipation of mitochondrial membrane potential and cytotoxicity. We conclude that a combination of the influx of extracellular Ca2+ and loss of mitochondrial integrity may be responsible for the cytotoxicity observed in individual renal cells and populations of renal cells after treatment with ionomycin.