Identification of Na+/H+exchange as a new target for toxic polycyclic aromatic hydrocarbons in liver cells

Lung cancer associated with combustion particles and fine particulate matter (PM2.5) - The roles of polycyclic aromatic hydrocarbons (PAHs) and the aryl hydrocarbon receptor (AhR)

Air pollution is the leading cause of lung cancer after tobacco smoking, contributing to 20% of all lung cancer deaths. Increased risk associated with living near trafficked roads, occupational exposure to diesel exhaust, indoor coal combustion and cigarette smoking, suggest that combustion components in ambient fine particulate matter (PM2.5), such as polycyclic aromatic hydrocarbons (PAHs), may be central drivers of lung cancer. Activation of the aryl hydrocarbon receptor (AhR) induces expression of xenobiotic-metabolizing enzymes (XMEs) and increase PAH metabolism, formation of reactive metabolites, oxidative stress, DNA damage and mutagenesis. Lung cancer tissues from smokers and workers exposed to high combustion PM levels contain mutagenic signatures derived from PAHs. However, recent findings suggest that ambient air PM2.5 exposure primarily induces lung cancer development through tumor promotion of cells harboring naturally acquired oncogenic mutations, thus lacking typical PAH-induced mutations. On this background, we discuss the role of AhR and PAHs in lung cancer development caused by air pollution focusing on the tumor promoting properties including metabolism, immune system, cell proliferation and survival, tumor microenvironment, cell-to-cell communication, tumor growth and metastasis. We suggest that the dichotomy in lung cancer patterns observed between smoking and outdoor air PM2.5 represent the two ends of a dose-response continuum of combustion PM exposure, where tumor promotion in the peripheral lung appears to be the driving factor at the relatively low-dose exposures from ambient air PM2.5, whereas genotoxicity in the central airways becomes increasingly more important at the higher combustion PM levels encountered through smoking and occupational exposure.

Extracellular vesicles released by polycyclic aromatic hydrocarbons-treated hepatocytes trigger oxidative stress in recipient hepatocytes by delivering iron

A growing body of evidences indicate the major role of extracellular vesicles (EVs) as players of cell communication in the pathogenesis of liver diseases. EVs are membrane-enclosed vesicles released by cells into the extracellular environment. Oxidative stress is also a key component of liver disease pathogenesis, but no role for hepatocyte-derived EVs has yet been described in the development of this process. Recently, some polycyclic aromatic hydrocarbons (PAHs), widespread environmental contaminants, were demonstrated to induce EV release from hepatocytes. They are also well-known to trigger oxidative stress leading to cell death. Therefore, the aim of this work was to investigate the involvement of EVs derived from PAHs-treated hepatocytes (PAH-EVs) in possible oxidative damages of healthy recipient hepatocytes, using both WIF-B9 and primary rat hepatocytes. We first showed that the release of EVs from PAHs -treated hepatocytes depended on oxidative stress. PAH-EVs were enriched in proteins related to oxidative stress such as NADPH oxidase and ferritin. They were also demonstrated to contain more iron. PAH-EVs could then induce oxidative stress in recipient hepatocytes, thereby leading to apoptosis. Mitochondria and lysosomes of recipient hepatocytes exhibited significant structural alterations. All those damages were dependent on internalization of EVs that reached lysosomes with their cargoes. Lysosomes thus appeared as critical organelles for EVs to induce apoptosis. In addition, pro-oxidant components of PAH-EVs, e.g. NADPH oxidase and iron, were revealed to be necessary for this cell death.

Pathophysiology of hepatic Na+/H+ exchange (Review)

Na+/H+ exchangers (NHEs) are a family of membrane proteins that contribute to exchanging one intracellular proton for one extracellular sodium. The family of NHEs consists of nine known members, NHE1-9. Each isoform represents a different gene product that has unique tissue expression, membrane localization, physiological effects, pathological regulation and sensitivity to drug inhibitors. NHE1 was the first to be discovered and is often referred to as the 'housekeeping' isoform of the NHE family. NHEs are not only involved in a variety of physiological processes, including the control of transepithelial Na+ absorption, intracellular pH, cell volume, cell proliferation, migration and apoptosis, but also modulate complex pathological events. Currently, the vast majority of review articles have focused on the role of members of the NHE family in inflammatory bowel disease, intestinal infectious diarrhea and digestive system tumorigenesis, but only a few reviews have discussed the role of NHEs in liver disease. Therefore, the present review described the basic biology of NHEs and highlighted their physiological and pathological effects in the liver.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Polycyclic aromatic hydrocarbons can trigger hepatocyte release of extracellular vesicles by various mechanisms of action depending on their affinity for the aryl hydrocarbon receptor

Extracellular vesicles (EVs) are membrane enclosed nanostructures released by cells into the extracellular environment. As major actors of physiological intercellular communication, they have been shown to be pathogenic mediators of several liver diseases. EVs also appear to be potential actors of drug-induced liver injury, but nothing is known concerning environmental pollutants. We aimed to study the impact of polycyclic aromatic hydrocarbons (PAHs), major contaminants, on hepatocyte-derived EV production, with a special focus on hepatocyte death. Three PAHs were selected, based on their presence in food and their affinity for the aryl hydrocarbon receptor (AhR): benzo(a)pyrene (BP), dibenzo(a,h)anthracene (DBA), and pyrene (PYR). Treatment of primary rat and WIF-B9 hepatocytes by all three PAHs increased the release of EVs, mainly comprised of exosomes, in parallel with modifying exosome protein marker expression and inducing apoptosis. Moreover, PAH treatment of rodents for three months also led to increased EV levels in plasma. The EV release involved CYP metabolism and the activation of the transcription factor, the AhR, for BP and DBA and another transcription factor, the constitutive androstane receptor (CAR), for PYR. Furthermore, all PAHs increased cholesterol levels in EVs but only BP and DBA were able to reduce the cholesterol content of total cell membranes. All cholesterol changes very likely participated in the increase in EV release and cell death. Finally, we studied changes in cell membrane fluidity caused by BP and DBA due to cholesterol depletion. Our data showed increased cell membrane fluidity, which contributed to hepatocyte EV release and cell death.

Disturbances in H+ dynamics during environmental carcinogenesis

Despite the improvement of diagnostic methods and anticancer therapeutics, the human population is still facing an increasing incidence of several types of cancers. According to the World Health Organization, this growing trend would be partly linked to our environment, with around 20% of cancers stemming from exposure to environmental contaminants, notably chemicals like polycyclic aromatic hydrocarbons (PAHs). PAHs are widespread pollutants in our environment resulting from incomplete combustion or pyrolysis of organic material, and thus produced by both natural and anthropic sources; notably benzo[a]pyrene (B[a]P), i.e. the prototypical molecule of this family, that can be detected in cigarette smoke, diesel exhaust particles, occupational-related fumes, and grilled food. This molecule is a well-recognized carcinogen belonging to group 1 carcinogens. Indeed, it can target the different steps of the carcinogenic process and all cancer hallmarks. Interestingly, H⁺ dynamics have been described as key parameters for the occurrence of several, if not all, of these hallmarks. However, information regarding the role of such parameters during environmental carcinogenesis is still very scarce. The present review will thus mainly give an overview of the impact of B[a]P on H⁺ dynamics in liver cells, and will show how such alterations might impact different aspects related to the finely-tuned balance between cell death and survival processes, thereby likely favoring environmental carcinogenesis. In total, the main objective of this review is to encourage further research in this poorly explored field of environmental molecular toxicology.

Role for the ATPase inhibitory factor 1 in the environmental carcinogen-induced Warburg phenotype

Most tumors undergo metabolic reprogramming towards glycolysis, the so-called Warburg effect, to support growth and survival. Overexpression of IF1, the physiological inhibitor of the F0F1ATPase, has been related to this phenomenon and appears to be a relevant marker in cancer. Environmental contributions to cancer development are now widely accepted but little is known about the underlying intracellular mechanisms. Among the environmental pollutants humans are commonly exposed to, benzo[a]pyrene (B[a]P), the prototype molecule of polycyclic aromatic hydrocarbons (PAHs), is a well-known human carcinogen. Besides apoptotic signals, B[a]P can also induce survival signals in liver cells, both likely involved in cancer promotion. Our previous works showed that B[a]P elicited a Warburg-like effect, thus favoring cell survival. The present study aimed at further elucidating the molecular mechanisms involved in the B[a]P-induced metabolic reprogramming, by testing the possible involvement of IF1. We presently demonstrate, both in vitro and in vivo, that PAHs, especially B[a]P, strongly increase IF1 expression. Such an increase, which might rely on β2-adrenergic receptor activation, notably participates to the B[a]P-induced glycolytic shift and cell survival in liver cells. By identifying IF1 as a target of PAHs, this study provides new insights about how environmental factors may contribute to related carcinogenesis.

Environmental carcinogenesis and pH homeostasis: Not only a matter of dysregulated metabolism

According to the World Health Organization, around 20% of all cancers would be due to environmental factors. Among these factors, several chemicals are indeed well recognized carcinogens. The widespread contaminant benzo[a]pyrene (B[a]P), an often used model carcinogen of the polycyclic aromatic hydrocarbons' family, has been suggested to target most, if not all, cancer hallmarks described by Hanahan and Weinberg. It is classified as a group I carcinogen by the International Agency for Research on Cancer; however, the precise intracellular mechanisms underlying its carcinogenic properties remain yet to be thoroughly defined. Recently, the pH homeostasis, a well known regulator of carcinogenic processes, was suggested to be a key actor in both cell death and Warburg-like metabolic reprogramming induced upon B[a]P exposure. The present review will highlight those data with the aim of favoring research on the role of H⁺ dynamics in environmental carcinogenesis.

The environmental carcinogen benzo[a]pyrene induces a Warburg-like metabolic reprogramming dependent on NHE1 and associated with cell survival

Cancer cells display alterations in many cellular processes. One core hallmark of cancer is the Warburg effect which is a glycolytic reprogramming that allows cells to survive and proliferate. Although the contributions of environmental contaminants to cancer development are widely accepted, the underlying mechanisms have to be clarified. Benzo[a]pyrene (B[a]P), the prototype of polycyclic aromatic hydrocarbons, exhibits genotoxic and carcinogenic effects, and it is a human carcinogen according to the International Agency for Research on Cancer. In addition to triggering apoptotic signals, B[a]P may induce survival signals, both of which are likely to be involved in cancer promotion. We previously suggested that B[a]P-induced mitochondrial dysfunctions, especially membrane hyperpolarization, might trigger cell survival signaling in rat hepatic epithelial F258 cells. Here, we further characterized these dysfunctions by focusing on energy metabolism. We found that B[a]P promoted a metabolic reprogramming. Cell respiration decreased and lactate production increased. These changes were associated with alterations in the tricarboxylic acid cycle which likely involve a dysfunction of the mitochondrial complex II. The glycolytic shift relied on activation of the Na⁺/H⁺ exchanger 1 (NHE1) and appeared to be a key feature in B[a]P-induced cell survival related to changes in cell phenotype (epithelial-to-mesenchymal transition and cell migration).

Sulfur dioxide and benzo(a)pyrene trigger apoptotic and anti-apoptotic signals at different post-exposure times in mouse liver

There is considerable concern that exposure to PAHs in combination with other air pollutants may lead to cancer or apoptosis in different cells. This study investigated the interaction effects between SO2 and BaP in mouse liver after long-term exposure. Mice were exposed to BaP for 5days or SO2 inhalation for 4weeks alone or together. The mitochondrial membrane potential (MMP) was assessed using the lipophilic cationic probe JC-1. The mRNA and protein level of several mitochondrial respiratory complex subunits and apoptosis-related genes were analyzed by real-time RT-PCR and/or western blot, respectively. We observed the pathology change of the mouse liver after 4-week treatments. It was revealed that MMP was reduced after co-exposure of SO2 and BaP after a 4-week treatment (1day post-exposure, p.e. 1d), with the suppression of the mRNA expression of complexes IV and V subunits, CO1, CO4, and ATP6. Co-exposure of SO2 and BaP appeared to be able to cause apoptotic signals, as judged by the suppression of bcl-2 and the bcl-2/bax ratio and the elevation of bax, caspase 3 activation, p53 accumulation and phosphorylation 1d post-exposure to SO2 and BaP, while the anti-apoptotic signal was detected by the elevation of bcl-2 and the bcl-2/bax ratio as well as the suppression of bax and p53 expression after a 13-week post-exposure (p.e. 13w) of SO2 and BaP. These results indicate that co-exposure to SO2 and BaP appears to lead to apoptotic as well as anti-apoptotic signals at different post-exposure times.

Protective action of n-3 fatty acids on benzo[a]pyrene-induced apoptosis through the plasma membrane remodeling-dependent NHE1 pathway

Plasma membrane is an early target of polycyclic aromatic hydrocarbons (PAH). We previously showed that the PAH prototype, benzo[a]pyrene (B[a]P), triggers apoptosis via DNA damage-induced p53 activation (genotoxic pathway) and via remodeling of the membrane cholesterol-rich microdomains called lipid rafts, leading to changes in pH homeostasis (non-genotoxic pathway). As omega-3 (n-3) fatty acids can affect membrane composition and function or hamper in vivo PAH genotoxicity, we hypothesized that addition of physiologically relevant levels of polyunsaturated n-3 fatty acids (PUFAs) might interfere with B[a]P-induced toxicity. The effects of two major PUFAs, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), were tested on B[a]P cytotoxicity in the liver epithelial cell line F258. Both PUFAs reduced B[a]P-induced apoptosis. Surprisingly, pre-treatment with DHA increased the formation of reactive B[a]P metabolites, resulting in higher levels of B[a]P-DNA adducts. EPA had no apparent effect on B[a]P metabolism or related DNA damage. EPA and DHA prevented B[a]P-induced apoptotic alkalinization by affecting Na(+)/H(+) exchanger 1 activity. Thus, the inhibitory effects of omega-3 fatty acids on B[a]P-induced apoptosis involve a non-genotoxic pathway associated with plasma membrane remodeling. Our results suggest that dietary omega-3 fatty acids may have marked effects on the biological consequences of PAH exposure.

Control of cellular Bcl-xL levels by deamidation-regulated degradation

Deamidation of two asparagines activates a conditional PEST sequence to target Bcl-xL for degradation.

The cellular concentration of Bcl-x_L is among the most important determinants of treatment response and overall prognosis in a broad range of tumors as well as an important determinant of the cellular response to several forms of tissue injury. We and others have previously shown that human Bcl-x_L undergoes deamidation at two asparaginyl residues and that DNA-damaging antineoplastic agents as well as other stimuli can increase the rate of deamidation. Deamidation results in the replacement of asparginyl residues with aspartyl or isoaspartyl residues. Thus deamidation, like phosphorylation, introduces a negative charge into proteins. Here we show that the level of human Bcl-x_L is constantly modulated by deamidation because deamidation, like phosphorylation in other proteins, activates a conditional PEST sequence to target Bcl-x_L for degradation. Additionally, we show that degradation of deamidated Bcl-x_L is mediated at least in part by calpain. Notably, we present sequence and biochemical data that suggest that deamidation has been conserved from the simplest extant metazoans through the human form of Bcl-x_L, underscoring its importance in Bcl-x_L regulation. Our findings strongly suggest that deamidation-regulated Bcl-x_L degradation is an important component of the cellular rheostat that determines susceptibility to DNA-damaging agents and other death stimuli.

Cellular levels of the pro-survival protein Bcl-x_L are an important determinant of cellular susceptibility to many death stimuli, including most cancer therapies. We previously showed that human Bcl-x_L undergoes deamidation – the conversion of two neutral asparaginyl side-chains into negatively charged aspartyl side-chains – a process that occurs spontaneously but is accelerated by the treatment of tumor cells with DNA-damaging agents. Here, we show that deamidation activates a hitherto undetected signal sequence within Bcl-x_L that targets it for degradation by a pathway involving the proteolytic enzyme calpain. This increased degradation of Bcl-x_L, and the consequent enhanced cellular susceptibility to programmed cell death, may contribute to the ability of DNA-damaging agents to kill tumors. We also demonstrate that deamidation of Bcl-x_L has likely been conserved from the simplest metazoans to humans, underscoring the importance of deamidation in the regulation of Bcl-x_L.

Exposure to polycyclic aromatic hydrocarbons: bulky DNA adducts and cellular responses

Environmental and dietary carcinogens such as polycyclic aromatic hydrocarbons (PAHs) have been intensively studied for decades. Although the genotoxicity of these compounds is well characterized (i.e., formation of bulky PAH-DNA adducts), molecular details on the DNA damage response triggered by PAHs in cells and tissues remain to be clarified. The conversion of hazardous PAHs into carcinogenic intermediates depends on enzyme-catalyzed biotransformation. Certain cytochrome P450-dependent monooxygenases (CYPs) play a pivotal role in PAH metabolism. In particular, CYP1A1 and 1B1 catalyze oxidation of PAHs toward primary epoxide species that can further be converted into multiple follow-up products, both nonenzymatically and enzymatically. Distinct functions between these major CYP enzymes have only been appreciated since transgenic animal models had been derived. Electrophilic PAH metabolites are capable of forming stable DNA adducts or to promote depurination at damaged nucleotide sites. During the following DNA replication cycle, bulky PAH-DNA adducts may be converted into mutations, thereby affecting hot spot sites in regulatory important genes such as Ras, p53, and others. Depending on the degree of DNA distortion and cell cycle progression, PAH-DNA adducts trigger nucleotide excision repair (NER) and various DNA damage responses that might include TP53-dependent apoptosis in certain cell types. In fact, cellular responses to bulky PAH-DNA damage are complex because distinct signaling branches such as ATM/ATR, NER, TP53, but also MAP kinases, interact and cooperate to determine the overall outcome to cellular injuries initiated by PAH-DNA adducts. Further, PAHs and other xenobiotics can also confer DNA damage via an alternative route of metabolic activation, which leads to the generation of PAH semiquinone radicals and reactive oxygen species (ROS). One-electron oxidations mediated by peroxidases or other enzymes can result in PAH radical cations that mainly form unstable DNA adducts subjected to depurination. In addition, generation of ROS can also trigger multiple cellular signaling pathways not directly related to mutagenic or cytotoxic effects, including those mediated by NFκB, SAPK/JNK, and p38. In recent years, it became clear that PAHs may also be involved in inflammatory diseases, autoimmune disorders, or atherosclerosis. Further research is under way to better characterize the significance of such newly recognized systemic effects of PAHs and to reconsider risk assessment for human health.

Myristic acid increases dihydroceramide Δ4-desaturase 1 (DES1) activity in cultured rat hepatocytes

Dihydroceramide Δ4-desaturase 1 (DES1) catalyzes the last step of the de novo ceramide biosynthesis, which consists of the introduction of a trans Δ4-double bond in the carbon chain of the dihydroceramide. It was previously observed that myristic acid binds DES1 through N-myristoylation. This N-terminal modification significantly increased the activity of the recombinant DES1 in COS-7 cells and targeted part of the enzyme initially present in the endoplasmic reticulum to the mitochondrial outer membrane, leading to an increase in ceramide levels. Since these results were obtained in a recombinant COS-7 cell model with high expression of rat DES1, the purpose of the present study was to investigate if the native DES1 enzyme was really upregulated by its N-myristoylation in cultured rat hepatocytes. We first showed that DES1 was the main dihydroceramide desaturase isoform expressed in rat hepatocytes. In this model, the wild-type myristoylable recombinant form of rat DES1 was found in both the endoplasmic reticulum and the mitochondria whereas the mutated non-myristoylable recombinant form (N-terminal glycine replaced by an alanine) was almost exclusively localized in the endoplasmic reticulum, which evidenced the importance of the myristoylation. Then, we showed that compared to other fatty acids, myristic acid was the only one to increase native DES1 activity, in both total cell lysates and mitochondrial fractions. The myristic acid-associated increase in DES1 activity was not linked to elevated mRNA or protein expression but more likely to its N-terminal myristoylation. Finally, the myristic acid-associated increase in DES1 activity slightly enhanced the number of apoptotic cells.

Identification of the couple GSK3α/c-Myc as a new regulator of hexokinase II in benzo[a]pyrene-induced apoptosis

The early apoptotic events induced by environmental pollutants with carcinogenic properties are poorly understood. Here, we focus on the early cytotoxic effects of benzo[a]pyrene (B[a]P). In F258 rat hepatic epithelial cells, B[a]P induces intrinsic apoptosis via a mitochondrial dysfunction characterized by the release of hexokinase II (HKII) from the mitochondria. Cancer cells often have an anomalous cell energy metabolism; since HKII dysfunction regulates B[a]P-induced apoptosis in F258 cells, but may also alter cell energy metabolism, HKII release from the mitochondria may represent an important B[a]P-related carcinogenic issue. Thus in the present study, we aimed at deciphering the mechanisms underlying HKII dysfunction upon B[a]P exposure. We show that while glycogen synthase kinase 3 beta (GSK3β) regulated the expression of HKII at the transcriptional level, glycogen synthase kinase 3 alpha (GSK3α) was involved in B[a]P-induced apoptosis via a decrease in c-Myc expression. The reduced level of c-Myc caused the relocation of HKII from the mitochondria to the cytosol, thereby being involved in the formation of reactive oxygen species and apoptosis. In conclusion, we show that the couple GSK3α/c-Myc plays a key role in B[a]P-induced early apoptotic cell signaling via HKII dysfunction.

DNA damage and DNA damage response in human bronchial epithelial BEAS-2B cells following exposure to 2-nitrobenzanthrone and 3-nitrobenzanthrone: role in apoptosis

Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) are mutagenic and carcinogenic environmental pollutants found in diesel exhaust and on urban air pollution particles. In the present study, human bronchial epithelial BEAS-2B cells were exposed to 2-nitrobenzanthrone (2-NBA) and 3-nitrobenzanthrone (3-NBA). DNA damage responses were compared to those observed after exposure to 1-nitropyrene (1-NP) and benzo[a]pyrene (B[a]P). Examination by microscopy revealed that 3-NBA was the most potent toxic compound while weaker responses were observed with 1-NP and B[a]P. Most interestingly, 2-NBA did not induce cell death or any other stress-related responses. 3-NBA induced a typical apoptotic cell death judged by nuclear condensation and little plasma membrane damage as well as cleavage of caspase 3 and poly-(ADP-ribose) polymerase (PARP). Exposure to 3-NBA resulted in an accumulation of cells in S-phase, and further analysis by Western blotting, immunocytochemistry and flow cytometry revealed that 3-NBA induced a DNA damage response characterized by phosphorylation of ATM (ataxia-telangiectasia mutated), checkpoint kinase (Chk) 2/Chk1, H2AX and p53. The p53 inhibitor pifithrin-α inhibited 3-NBA-induced apoptosis while small effects were seen using pifithrin-μ, suggesting that 3-NBA-induced cell death is a result of transcriptional activation of p53. In conclusion, 3-NBA is a potent inducer of apoptosis, which seemed to be triggered by the DNA damage response. Furthermore, a change of the nitro-group to the second position (i.e. 2-NBA) dramatically changed the cellular reactivity of the compound.

Ionic dependence of sulphur mustard cytotoxicity

The effect of ionic environment on sulphur mustard (bis 2-chloroethyl sulphide; HD) toxicity was examined in CHO-K1 cells. Cultures were treated with HD in different ionic environments at constant osmolar conditions (320 mOsM, pH 7.4). The cultures were refed with fresh culture medium 1h after HD exposure, and viability was assessed. Little toxicity was apparent when HD exposures were carried out in ion-free sucrose buffer compared to LC(50) values of approximately 100-150 microM when the cultures were treated with HD in culture medium. Addition of NaCl to the buffer increased HD toxicity in a salt concentration-dependent manner to values similar to those obtained in culture medium. HD toxicity was dependent on both cationic and anionic species with anionic environment playing a much larger role in determining toxicity. Substitution of NaI for NaCl in the treatment buffers increased HD toxicity by over 1000%. The activity of the sodium hydrogen exchanger (NHE) in recovering from cytosolic acidification in salt-free and in different chloride salts did not correlate with the HD-induced toxicity in these buffers. However, the inhibition by HD of intracellular pH regulation correlated with its toxicity in NaCl, NaI and sucrose buffers. Analytical chemical studies and the toxicity of the iodine mustard derivative ruled out the role of chemical reactions yielding differentially toxic species as being responsible for the differences in HD toxicity observed. This work demonstrates that the early events that HD sets into motion to cause toxicity are dependent on ionic environment, possibly due to intracellular pH deregulation.

Membrane remodeling, an early event in benzo[a]pyrene-induced apoptosis

Benzo[alpha]pyrene (B[alpha]P) often serves as a model for mutagenic and carcinogenic polycyclic aromatic hydrocarbons (PAHs). Our previous work suggested a role of membrane fluidity in B[alpha]P-induced apoptotic process. In this study, we report that B[alpha]P modifies the composition of cholesterol-rich microdomains (lipid rafts) in rat liver F258 epithelial cells. The cellular distribution of the ganglioside-GM1 was markedly changed following B[alpha]P exposure. B[alpha]P also modified fatty acid composition and decreased the cholesterol content of cholesterol-rich microdomains. B[alpha]P-induced depletion of cholesterol in lipid rafts was linked to a reduced expression of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase). Aryl hydrocarbon receptor (AhR) and B[alpha]P-related H(2)O(2) formation were involved in the reduced expression of HMG-CoA reductase and in the remodeling of membrane microdomains. The B[alpha]P-induced membrane remodeling resulted in an intracellular alkalinization observed during the early phase of apoptosis. In conclusion, B[alpha]P altered the composition of plasma membrane microstructures through AhR and H(2)O(2) dependent-regulation of lipid biosynthesis. In F258 cells, the B[alpha]P-induced membrane remodeling was identified as an early apoptotic event leading to an intracellular alkalinization.

N-Myristoylation targets dihydroceramide Delta4-desaturase 1 to mitochondria: partial involvement in the apoptotic effect of myristic acid

This study was designed to analyze the effect of myristic acid on ceramide synthesis and its related lipoapoptosis pathway. It was previously observed that myristic acid binds dihydroceramide Delta4-desaturase 1 (DES1) through N-myristoylation and activates this enzyme involved in the final de novo ceramide biosynthesis step. In the present study, we show first by immunofluorescence microscopy and subcellular fractionation that DES1 myristoylation targets part of the recombinant protein to the mitochondria in COS-7 cells. In addition, native dihydroceramide Delta4-desaturase activity was found in both the endoplasmic reticulum and mitochondria in rat hepatocytes. Dihydroceramide conversion to ceramide was increased in COS-7 cells expressing DES1 and incubated with myristic acid. The expression of the wild-type myristoylable DES1-Gly alone, but not the expression of the unmyristoylable mutant DES1-Ala, induced apoptosis of COS-7 cells. Finally, myristic acid alone also increased the production of cellular ceramide and had an apoptotic effect. This effect was potentiated on caspase activity when the myristoylable form of DES1 was expressed. Therefore, these results suggest that the myristoylation of DES1 can target the enzyme to the mitochondria leading to an increase in ceramide levels which in turn contributes to partially explain the apoptosis effect of myristic acid in COS-7 cells.

Environmental toxicity, oxidative stress and apoptosis: ménage à trois

Apoptosis is an evolutionary conserved homeostatic process involved in distinct physiological processes including organ and tissue morphogenesis, development and senescence. Its deregulation is also known to participate in the etiology of several human diseases including cancer, neurodegenerative and autoimmune disorders. Environmental stressors (cytotoxic agents, pollutants or toxicants) are well known to induce apoptotic cell death and to contribute to a variety of pathological conditions. Oxidative stress seems to be the central element in the regulation of the apoptotic pathways triggered by environmental stressors. In this work, we review the established mechanisms by which oxidative stress and environmental stressors regulate the apoptotic machinery with the aim to underscore the relevance of apoptosis as a component in environmental toxicity and human disease progression.

Apoptosis-induced alkalinization by the Na+/H+ exchanger isoform 1 is mediated through phosphorylation of amino acids Ser726 and Ser729

Apoptosis is a complex process essential for normal tissue development and cellular homeostasis. While biochemical events that occur late in the apoptotic process are better characterized, early physiological changes that initiate the progression of cell death remain poorly understood. Previously, we observed that lymphocytes, undergoing apoptosis in response to growth factor withdrawal, experienced a rapid and transient rise in cytosolic pH. We found that the protein responsible was the pH-regulating, plasma membrane protein Na(+)/H(+) exchanger isoform 1 (NHE1), and that its activity was impeded by inhibition of the stress-activated kinase, p38 MAP kinase. In the current study, we examined how NHE1 is activated during apoptosis. We identified the phosphorylation sites on NHE1 that regulate its alkalinizing activity in response to a cell death stimulus. Performing targeted mutagenesis, we observed that substitution of Ser726 and Ser729 for alanines produced a mutant form of NHE1 that did not alkalinize in response to an apoptotic stimulus, and expression of which protected cells from serum withdrawal- induced death. In contrast, substitution of Ser726 and Ser729 for glutamic acids raised the basal pH and induced susceptibility to death. Analysis of serine phosphorylation showed that phosphorylation of NHE1 during apoptosis decreased upon mutation of Ser726 and Ser729. Our findings thus confirm a necessary function for NHE1 during apoptosis and reveal the critical regulatory sites that when phosphorylated mediate the alkalinizing activity of NHE1 in the early stages of a cell death response.

Regulation of Na+/H+ exchanger 1 allosteric balance by its localization in cholesterol- and caveolin-rich membrane microdomains

The Na+/H+ exchanger 1, which plays an essential role in intracellular pH regulation in most tissues, is also known to be a key actor in both proliferative and apoptotic processes. Its activation by H+ is best described by the Monod-Wyman-Changeux model: the dimeric NHE-1 oscillates between a low and a high affinity conformation, the balance between the two forms being defined by the allosteric constant L(0). In this study, influence of cholesterol- and caveolin-rich microdomains on NHE-1 activity was examined by using cholesterol depleting agents, including methyl-beta-cyclodextrin (MBCD). These agents activated NHE-1 by modulating its L(0) parameter, which was reverted by cholesterol repletion. This activation was associated with NHE-1 relocation outside microdomains, and was distinct from NHE-1 mitogenic and hormonal stimulation; indeed MBCD and serum treatments were additive, and serum alone did not change NHE-1 localization. Besides, MBCD activated a serum-insensitive, constitutively active mutated NHE-1 ((625)KDKEEEIRK(635) into KNKQQQIRK). Finally, the membrane-dependent NHE-1 regulation occurred independently of Mitogen Activated Protein Kinases, especially Extracellular Regulated Kinase activation, although this kinase was activated by MBCD. In conclusion, localization of NHE-1 in membrane cholesterol- and caveolin-rich microdomains constitutes a novel physiological negative regulator of NHE-1 activity.

Regulation of cell survival by Na+/H+ exchanger-1

Na(+)/H(+) exchanger-1 (NHE1) is a ubiquitous plasma membrane Na(+)/H(+) exchanger typically associated with maintenance of intracellular volume and pH. In addition to the NHE1 role in electroneutral Na(+)/H(+) transport, in renal tubular epithelial cells in vitro the polybasic, juxtamembrane NHE1 cytosolic tail domain acts as a scaffold, by binding with ezrin/radixin/moesin (ERM) proteins and phosphatidylinositol 4,5-bisphosphate, which initiates formation of a signaling complex that culminates in Akt activation and opposition to initial apoptotic stress. With robust apoptotic stimuli renal tubular epithelial cell NHE1 is a caspase substrate, and proteolytic cleavage may permit progression to apoptotic cell death. In vivo, genetic or pharmacological NHE1 loss of function causes renal tubule epithelial cell apoptosis and renal dysfunction following streptozotocin-induced diabetes, ureteral obstruction, and adriamycin-induced podocyte toxicity. Taken together, substantial in vivo and in vitro data demonstrate that NHE1 regulates tubular epithelial cell survival. In contrast to connotations of NHE1 as an unimportant "housekeeping" protein, this review highlights that NHE1 activity is critical for countering tubular atrophy and chronic renal disease progression.

Cisplatin-induced apoptosis involves membrane fluidification via inhibition of NHE1 in human colon cancer cells

We have previously shown that cisplatin triggers an early acid sphingomyelinase (aSMase)-dependent ceramide generation concomitantly with an increase in membrane fluidity and induces apoptosis in HT29 cells. The present study further explores the role and origin of membrane fluidification in cisplatin-induced apoptosis. The rapid increase in membrane fluidity following cisplatin treatment was inhibited by membrane-stabilizing agents such as cholesterol or monosialoganglioside-1. In HT29 cells, these compounds prevented the early aggregation of Fas death receptor and of membrane lipid rafts on cell surface and significantly inhibited cisplatin-induced apoptosis without altering drug intracellular uptake or cisplatin DNA adducts formation. Early after cisplatin treatment, Na+/H+ membrane exchanger-1 (NHE1) was inhibited leading to intracellular acidification, aSMase was activated, and ceramide was detected at the cell membrane. Treatment of HT29 cells with Staphylococcus aureus sphingomyelinase increased membrane fluidity. Moreover, pretreatment with cariporide, a specific inhibitor of NHE1, inhibited cisplatin-induced intracellular acidification, aSMase activation, ceramide membrane generation, membrane fluidification, and apoptosis. Finally, NHE1-expressing PS120 cells were more sensitive to cisplatin than NHE1-deficient PS120 cells. Altogether, these findings suggest that the apoptotic pathway triggered by cisplatin involves a very early NHE1-dependent intracellular acidification leading to aSMase activation and increase in membrane fluidity. These events are independent of cisplatin-induced DNA adducts formation. The membrane exchanger NHE1 may be another potential target of cisplatin, increasing cell sensitivity to this compound.

pH-dependent toxicity of sulphur mustard in vitro

The dependence of sulphur mustard (HD) toxicity on intracellular (pH(i)) and extracellular pH was examined in CHO-K1 cells. HD produced an immediate and significant concentration-dependent decline in cytosolic pH, and also inhibited the mechanisms responsible for restoring pH(i) to physiological values. The concentration-response of HD-induced cytosolic acidification, closely paralleled the acidification of the extracellular buffer through HD hydrolysis. A viability study was carried out in order to assess the importance of HD-induced cytosolic acidification. Cultures were exposed to HD for 1 h in media that were adjusted through a pH range (pH 5.0-10), and the 24 h LC(50) values were assessed using the viability indicator dye alamarBlue. The toxicity of HD was found to be dependent on extracellular pH, with a greater than eight-fold increase in LD(50) obtained in cultures treated with HD at pH 9.5, compared to those treated at pH 5.0. Assays of apoptotic cell death, including morphology, soluble DNA, caspase-3 activity and TUNEL also showed that as pH was increased, much greater HD concentrations were required to cause cell death. The modest decline in HD half-life measured in buffers of increasing pH, did not account for the protective effects of basic pH. The early event(s) that HD initiates to eventually culminate in cell death are not known. However, based on the data obtained in this study, we propose that HD causes an extracellular acidification through chemical hydrolysis and that this, in both a concentration and temporally related fashion, results in cytosolic acidification. Furthermore, HD also acts to poison the antiporter systems responsible for maintaining physiological pH(i), so that the cells are unable to recover from this insult. It is this irreversible decline in pH(i) that initiates the cascade of events that results in HD-induced cell death.

c-Jun NH2-terminal kinase-related Na+/H+ exchanger isoform 1 activation controls hexokinase II expression in benzo(a)pyrene-induced apoptosis

Regulation of the balance between survival, proliferation, and apoptosis on carcinogenic polycyclic aromatic hydrocarbon (PAH) exposure is still poorly understood and more particularly the role of physiologic variables, including intracellular pH (pH(i)). Although the involvement of the ubiquitous pH(i) regulator Na(+)/H(+) exchanger isoform 1 (NHE1) in tumorigenesis is well documented, less is known about its role and regulation during apoptosis. Our previous works have shown the primordial role of NHE1 in carcinogenic PAH-induced apoptosis. This alkalinizing transporter was activated by an early CYP1-dependent H(2)O(2) production, subsequently promoting mitochondrial dysfunction leading to apoptosis. The aim of this study was to further elucidate how NHE1 was activated by benzo(a)pyrene (BaP) and what the downstream events were in the context of apoptosis. Our results indicate that the mitogen-activated protein kinase kinase 4/c-Jun NH(2)-terminal kinase (MKK4/JNK) pathway was a link between BaP-induced H(2)O(2) production and NHE1 activation. This activation, in combination with BaP-induced phosphorylated p53, promoted mitochondrial superoxide anion production, supporting the existence of a common target for NHE1 and p53. Furthermore, we showed that the mitochondrial expression of glycolytic enzyme hexokinase II (HKII) was decreased following a combined action of NHE1 and p53 pathways, thereby enhancing the BaP-induced apoptosis. Taken together, our findings suggest that, on BaP exposure, MKK4/JNK targets NHE1 with consequences on HKII protein, which might thus be a key protein during carcinogenic PAH apoptosis.

Different mechanisms involved in apoptosis following exposure to benzo[a]pyrene in F258 and Hepa1c1c7 cells

The present study compares and elucidates possible mechanisms why B[a]P induces different cell signals and triggers apparently different apoptotic pathways in two rather similar cell lines (hepatic epithelial cells of rodents). The rate and maximal capacity of metabolic activation, as measured by the formation of B[a]P-tetrols and B[a]P-DNA adducts, was much higher in mouse hepatoma Hepa1c1c7 cells than in rat liver epithelial F258 cells due to a higher induced level of cyp1a1. B[a]P increased intracellular pH in both cell lines, but this change modulated the apoptotic process only in F258 cells. In Hepa1c1c7 cells reactive oxygen species (ROS) production appeared to be a consequence of toxicity, unlike F258 cells in which it was an initial event. The increased mitochondrial membrane potential found in F258 cells was not observed in Hepa1c1c7 cells. Surprisingly, F258 cells cultured at low cell density were somewhat more sensitive to low (50nM) B[a]P concentrations than Hepa1c1c7 cells. This could be explained partly by metabolic differences at low B[a]P concentrations. In contrast to the Hepa1c1c7 model, no activation of cell survival signals including p-Akt, p-ERK1/2 and no clear inactivation of pro-apoptotic Bad was observed in the F258 model following exposure to B[a]P. Another important difference between the two cell lines was related to the role of Bax and cytochrome c. In Hepa1c1c7 cells, B[a]P exposure resulted in a "classical" translocation of Bax to the mitochondria and release of cytochrome c, whereas in F258 cells no intracellular translocation of these two proteins was seen. These results suggest that the rate of metabolism of B[a]P and type of reactive metabolites formed influence the resulting balance of pro-apoptotic and anti-apoptotic cell signaling, and hence the mechanisms involved in cell death and the chances of more permanent genetic damage.

Membrane Fluidity Changes Are Associated with Benzo[a]Pyrene-Induced Apoptosis in F258 Cells: Protection by Exogenous Cholesterol

Polycyclic aromatic hydrocarbons (PAHs) such as benzo[a]yrene (B[a]P) constitute a widely distributed class of environmental pollutants, responsible for highly toxic effects. Elucidating the intracellular mechanisms of this cytotoxicity thus remains a major challenge. Besides the activation of the p53 apoptotic pathway, we have previously found in F258 hepatic cells that the B[a]P (50 nM)-induced apoptosis was also dependent upon the transmembrane transporter NHE1, whose activation might result from membrane alterations in our model. We here demonstrate that: (1) B[a]P induces a membrane fluidization surprisingly linked to NHE1 activation; (2) membrane stabilization by exogenous cholesterol protects cells from B[a]P-induced apoptosis, via an effect on late acidification and iron uptake.

Protective effect of monosialoganglioside GM1 against chemically induced apoptosis through targeting of mitochondrial function and iron transport

Exogenous treatment with monosialoganglioside GM1 has been described to afford protection against different apoptotic insults. However, the underlying mechanisms remain to be determined. In this study, we focused on the effect of GM1 on the apoptotic cascade induced by benzo[a]pyrene (B[a]P) in rat hepatic F258 epithelial cells. We first demonstrated that a co-treatment with GM1 (80 microM) reduced B[a]P (50 nM)-induced apoptosis as evidenced by a decrease of both cell population exhibiting nuclear fragmentation and caspase 3 cleavage and activity. We next showed that the p53 phosphorylation and nuclear translocation as well as the intracellular alkalinization related to Na+/H+ exchanger 1 (NHE1) activation, two early events of the apoptosis induced by B[a]P, were not inhibited by GM1. In contrast, the late mitochondria-dependent acidification elicited by B[a]P was inhibited by GM1 co-treatment, and an inhibition of the oxidative stress was also observed. Because GM1 has been shown to reduce the low-molecular weight iron content related to ethanol-induced oxidative stress, we finally investigated the involvement of iron under our conditions. Using the two iron chelators deferiprone and desferrioxamine, we clearly showed that iron played an important role in B[a]P-induced apoptosis in F258 cells, and that B[a]P-treatment resulted in a significant GM1-sensitive increase in (55)Fe uptake. In conclusion, our results indicate that exogenous GM1 partly prevents B[a]P-induced apoptosis by interfering with mitochondria-related intracellular acidification and iron transport.

Multiple apoptotic pathways induced by p53-dependent acidification in benzo[a]pyrene-exposed hepatic F258 cells

Polycyclic aromatic hydrocarbons (PAH), such as benzo[a]pyrene (B[a]P), are ubiquitous genotoxic environmental pollutants. Their DNA-damaging effects lead to apoptosis induction, through similar pathways to those identified after exposure to other DNA-damaging stimuli with activation of p53-related genes and the involvement of the intrinsic apoptotic pathway. However, at a low concentration of B[a]P (50 nM), our previous results pointed to the involvement of intracellular pH (pHi) variations during B[a]P-induced apoptosis in a rat liver epithelial cell line (F258). In the present work, we identified the mitochondrial F0F1-ATPase activity reversal as possibly responsible for pHi decrease. This acidification not only promoted executive caspase activation, but also activated leucocyte elastase inhibitor/leucocyte-derived DNase II (LEI/L-DNase II) pathway. p53 appeared to regulate mitochondria homeostasis, by initiating F0F1-ATPase reversal and endonuclease G (Endo G) release. In conclusion, a low dose of B[a]P induced apoptosis by recruiting a large panel of executioners apparently depending on p53 phosphorylation and, for some of them, on acidification.

Arsenic Trioxide Induces Apoptosis of Human Monocytes during Macrophagic Differentiation through Nuclear Factor-κB-Related Survival Pathway Down-Regulation

Arsenic trioxide (As(2)O(3)) is known to be toxic toward leukemia cells. In this study, we determined its effects on survival of human monocytic cells during macrophagic differentiation, an important biological process involved in the immune response. As(2)O(3) used at clinically relevant pharmacological concentrations induced marked apoptosis of human blood monocytes during differentiation with either granulocyte-macrophage colony-stimulating factor or macrophage colony-stimulating factor. Apoptosis of monocytes was associated with increased caspase activities and decreased DNA binding of p65 nuclear factor-kappaB (NF-kappaB); like As(2)O(3), the selective NF-kappaB inhibitor (E)-3-[(4-methylphenyl)-sulfonyl]-2-propenenitrile (Bay 11-7082) strongly reduced survival of differentiating monocytes. The role of NF-kappaB in arsenic toxicity was also studied in promonocytic U937 cells during phorbol 12-myristate 13-acetate-induced macrophagic differentiation. In these cells, As(2)O(3) first reduced DNA binding of p65 NF-kappaB and subsequently induced apoptosis. In addition, overexpression of the p65 NF-kappaB subunit, following stable infection with a p65 retroviral expressing vector, increased survival of As(2)O(3)-treated U937 cells. As(2)O(3) specifically decreased protein levels of X-linked inhibitor of apoptosis protein and FLICE-inhibitory protein, two NF-kappaB-regulated genes in both U937 cells and blood monocytes during their differentiations. Finally, As(2)O(3) was found to inhibit macrophagic differentiation of monocytic cells when used at cytotoxic concentrations; however, overexpression of the p65 NF-kappaB subunit in U937 cells reduced its effects toward differentiation. In contrast to monocytes, well differentiated macrophages were resistant to low concentrations of As(2)O(3). Altogether, our study demonstrates that clinically relevant concentrations of As(2)O(3) induced marked apoptosis of monocytic cells during in vitro macrophagic differentiation likely through inhibition of NF-kappaB-related survival pathways.

Role of cell signaling in B[a]P-induced apoptosis: characterization of unspecific effects of cell signaling inhibitors and apoptotic effects of B[a]P metabolites

Here we show that several cell signaling inhibitors have effect on cyp1a1 expression and the metabolism of benzo[a]pyrene (B[a]P) in Hepa1c1c7 cells. The CYP1A1 inhibitor alpha-naphthoflavone (alpha-NF), the p53 inhibitor pifithrin-alpha (PFT-alpha), the ERK inhibitors PD98059 and U0126, and the p38 MAPK inhibitors SB202190 and PD169316 induced the expression and level of cyp1a1 protein. On the other hand, during the first h the inhibitors appeared to reduce the metabolism of B[a]P as measured by the generation of tetrols and by covalent binding of B[a]P to macromolecules. In contrast, the phosphatidylinositol-3 (PI-3) kinase inhibitor wortmannin, had neither an effect on the cyp1a1 expression nor the B[a]P-metabolism. In order to avoid these unspecific effects, we characterized the mechanisms involved in the apoptotic effects of B[a]P-metabolites. B[a]P and the B[a]P-metabolites B[a]P-7,8-DHD and BPDE-I induced apoptosis, whereas B[a]P-4,5-DHD had no effect. B[a]P, B[a]P-7,8-DHD and BPDE-I induced an accumulation and phosphorylation of p53, while the Bcl-2 proteins Bcl-xl, Bad and Bid were down-regulated. Interestingly, the levels of anti-apoptotic phospho-Bad were up-regulated in response to B[a]P as well as to B[a]P-7,8-DHD and BPDE-I. Both p38 MAPK and JNK were activated, but the p38 MAPK inhibitors were not able to inhibit BPDE-I-induced apoptosis. PFT-alpha reduced the BPDE-I-induced apoptosis, while both the PI-3 kinase inhibitor and the ERK inhibitors increased the apoptosis in combination with BPDE-I. BPDE-I also triggered apoptosis in primary cultures of rat lung cells. In conclusion, often used cell signaling inhibitors both enhanced the expression and the level of cyp1a1 and more directly acted as inhibitors of cyp1a1 metabolism of B[a]P. However, studies with the B[a]P-metabolite BPDE-I supported the previous suggestion that p53 has a role in the pro-apoptotic signaling pathway induced by B[a]P. Furthermore, these studies also show that the reactive metabolites of B[a]P induce the anti-apoptotic signals, Akt and ERK. Neither the induction nor the activity of p38 MAPK and JNK seems to be of major importance for the B[a]P-induced apoptosis.

Leber Hereditary Optic Neuropathy in 2 of 4 Siblings with 11778 mtDNA Mutation: Clinical Variability or Effect of Toxic Environmental Exposure?

Although mitochondrial (mt) DNA mutation at nucleotide position 11778 accounts for most cases of Leber's hereditary optic neuropathy (LHON), the phenotypic expression may vary greatly even in different members of the same family. The possible influence of exogenous toxicity on phenotypic expression is still debated in LHON. Here we describe 4 siblings carrying the 11778 mtDNA mutation with a different phenotype. The index case developed an atypical optic neuropathy at the age of 60 years after a long history of occupational exposure to polycyclic aromatic hydrocarbons (PAHs). This report underlines a number of unanswered questions about phenotypic variability of LHON including the possible influence of PAH toxicity.

Alterations of intracellular pH homeostasis in apoptosis: origins and roles

Intracellular pH (pHi) has an important role in the maintenance of normal cell function, and hence this parameter has to be tightly controlled within a narrow range, largely through the activity of transporters located at the plasma membrane. These transporters can be modulated by endogenous or exogenous molecules as well as, in some pathological situations, leading to pHi changes that have been implicated in both cell proliferation and cell death. Whereas intracellular alkalinization seems to be a common feature of proliferative processes, the precise role of pHi in apoptosis is still unclear. The present review gathers the most recent advances along with previous data on both the origin and the role of pHi alterations in apoptosis and highlights the major concerns that merit further research in the future. Special attention is given to the possible role played by pHi-regulating transporters.