Laurence Huc

Alterations of intracellular pH homeostasis in apoptosis: origins and roles.

AbstractIntracellular 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.

Cell death and diseases related to oxidative stress: 4-hydroxynonenal (HNE) in the balance.

During the last three decades, 4-hydroxy-2-nonenal (HNE), a major α,β-unsaturated aldehyde product of n-6 fatty acid oxidation, has been shown to be involved in a great number of pathologies such as metabolic diseases, neurodegenerative diseases and cancers. These multiple pathologies can be explained by the fact that HNE is a potent modulator of numerous cell processes such as oxidative stress signaling, cell proliferation, transformation or cell death. The main objective of this review is to focus on the different aspects of HNE-induced cell death, with a particular emphasis on apoptosis. HNE is a special apoptotic inducer because of its abilities to form protein adducts and to propagate oxidative stress. It can stimulate intrinsic and extrinsic apoptotic pathways and interact with typical actors such as tumor protein 53, JNK, Fas or mitochondrial regulators. At the same time, due to its oxidant status, it can also induce some cellular defense mechanisms against oxidative stress, thus being involved in its own detoxification. These processes in turn limit the apoptotic potential of HNE. These dualities can imbalance cell fate, either toward cell death or toward survival, depending on the cell type, the metabolic state and the ability to detoxify.

Low concentrations of bisphenol A induce lipid accumulation mediated by the production of reactive oxygen species in the mitochondria of HepG2 cells.

Bisphenol A (BPA) is an endocrine-disrupting chemical that leaches from polycarbonate plastics that consequently leads to low-dose human exposure. In addition to its known xenoendocrine action, BPA exerts a wide variety of metabolic effects, but no data are available on its actions on the functions of liver mitochondrial. To assess these effects, HepG2 cells were exposed to BPA (10(-4)-10(-12)M) and physiological parameters were measured by flow cytometry. We demonstrated a significant mitochondrial dysfunction including ROS production, ΔΨ(M) hyperpolarization, lipid accumulation, lipoperoxidation and the release of pro-inflammatory cytokines. In conclusion, we showed that low concentrations of BPA promote lipid accumulation in hepatic cells triggered by disturbances in mitochondrial function, alterations in lipid metabolism and by inflammation that can therefore contribute to steatosis.

Specific disintegration of complex II succinate:ubiquinone oxidoreductase links pH changes to oxidative stress for apoptosis induction

The formation of reactive oxygen species (ROS) and the change of the intracellular pH (pHi) are common phenomena during apoptosis. How they are interconnected, however, is poorly understood. Here we show that numerous anticancer drugs and cytokines such as Fas ligand and tumour necrosis factor α provoke intracellular acidification and cause the formation of mitochondrial ROS. In parallel, we found that the succinate:ubiquinone oxidoreductase (SQR) activity of the mitochondrial respiratory complex II is specifically impaired without affecting the second enzymatic activity of this complex as a succinate dehydrogenase (SDH). Only in this configuration is complex II an apoptosis mediator and generates superoxides for cell death. This is achieved by the pHi decline that leads to the specific dissociation of the SDHA/SDHB subunits, which encompass the SDH activity, from the membrane-bound components of complex II that are required for the SQR activity.

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

The ubiquitous environmental pollutants polycyclic aromatic hydrocarbons are responsible for important carcinogenic and apoptotic effects, whose mechanisms are still poorly understood, owing to the multiplicity of possible cellular targets. Among these mechanisms, alterations of ionic homeostasis have been suggested. In this work, the effects of benzo(a)pyrene [B(a)P] on pHi were tested in the rat liver F258 epithelial cell line, using the fluoroprobe carboxy‐SNARF‐1. After a 48‐h treatment, B(a)P (50 nM) induced an alkalinization, followed by an acidification after 72 h and the development of apoptosis. Determinations of pHi recovery following an acid load showed an increased acid efflux at 48 h. Cariporide inhibited both the early alkalinization and the increased acid efflux, thus suggesting the involvement of Na+/H+ exchanger 1 (NHE1). Besides, α‐naphtoflavone (α‐NF), an inhibitor of CYP1A1‐mediated B(a)P metabolism, prevented all pHi changes, and NHE1 activation was blocked by the antioxidant thiourea, which inhibited CYP1A1 metabolism‐dependent H2O2 production. Regarding B(a)P‐induced apoptosis, this was prevented by α‐NF and bongkrekic acid, an inhibitor of mitochondria‐dependent apoptosis. Interestingly, apoptosis was significantly reduced by cariporide. Taken together, our results indicate that B(a)P, via H2O2 produced by CYP1A1‐dependent metabolism, induces an early activation of NHE1, resulting in alkalinization; this appears to play a significant role in mitochondria‐dependent B(a)P‐induced apoptosis.

Adverse effects of long-term exposure to bisphenol A during adulthood leading to hyperglycaemia and hypercholesterolemia in mice.

Bisphenol A (BPA) is a suspected endocrine disruptor highly prevalent in our environment since it is used as monomer of polycarbonate plastics and epoxy resins. Recent epidemiological and animal studies have suggested that BPA exposure may influence the development of obesity and related pathologies such as type 2 diabetes, and cardiovascular diseases. However, experimental studies have often focused on short-term exposures. In this study, we investigated the effect of several months of BPA exposure on hepatic and plasma metabolic markers in adult mice. Male CD1 mice were exposed during 8 months to five different BPA doses below or equivalent to the current no observed adverse effect level (NOAEL: 5000 μg/kg/day) through drinking water. Plasma lipid profiles and liver transcriptomic analysis were performed in control and BPA-treated animals. We report a specific impact of BPA exposure on glycaemia, glucose tolerance and cholesterolemia. Consistent with the hypercholesterolemia in BPA-treated animals, RT-qPCR performed on hepatic mRNA from same animals demonstrated an overexpression of key genes involved in cholesterol biosynthesis, namely, Mvd, Lss Hmgcr, and Sqle. BPA also induced the expression of the sterol regulatory element-binding proteins 2, a master regulator of hepatic cholesterol biosynthesis. As shown by the plasma lathosterol to cholesterol ratio, a surrogate marker for cholesterol biosynthesis, whole body cholesterol de novo synthesis was also increased in BPA-exposed animals. These original results are consistent with many epidemiological studies reporting on a link between BPA exposure and the onset of cardiovascular diseases.

4-Hydroxy-2(E)-nonenal metabolism differs in Apc(+/+) cells and in Apc(Min/+) cells: it may explain colon cancer promotion by heme iron.

Animal and epidemiological studies suggest that dietary heme iron would promote colorectal cancer. Oxidative properties of heme could lead to the formation of cytotoxic and genotoxic secondary lipid oxidation products, such as 4-hydroxy-2(E)-nonenal (HNE). This compound is more cytotoxic to mouse wild-type colon cells than to isogenic cells with a mutation on the adenomatous polyposis coli (APC) gene. The latter thus have a selective advantage, possibly leading to cancer promotion. This mutation is an early and frequent event in human colorectal cancer. To explain this difference, the HNE biotransformation capacities of the two cell types have been studied using radiolabeled and stable isotope-labeled HNE. Apc-mutated cells showed better biotransformation capacities than nonmutated cells did. Thiol compound conjugation capacities were higher for mutated cells, with an important advantage for the extracellular conjugation to cysteine. Both cells types were able to reduce HNE to 4-hydroxynonanal, a biotransformation pathway that has not been reported for other intestinal cells. Mutated cells showed higher capacities to oxidize 4-hydroxynonanal into 4-hydroxynonanoic acid. The mRNA expression of different enzymes involved in HNE metabolism such as aldehyde dehydrogenase 1A1, 2 and 3A1, glutathione transferase A4-4, or cystine transporter xCT was upregulated in mutated cells compared with wild-type cells. In conclusion, this study suggests that Apc-mutated cells are more efficient than wild-type cells in metabolizing HNE into thiol conjugates and 4-hydroxynonanoic acid due to the higher expression of key biotransformation enzymes. These differential biotransformation capacities would explain the differences of susceptibility between normal and Apc-mutated cells regarding secondary lipid oxidation products.

Use of reconstituted metabolic networks to assist in metabolomic data visualization and mining.

Metabolomics experiments seldom achieve their aim of comprehensively covering the entire metabolome. However, important information can be gleaned even from sparse datasets, which can be facilitated by placing the results within the context of known metabolic networks. Here we present a method that allows the automatic assignment of identified metabolites to positions within known metabolic networks, and, furthermore, allows automated extraction of sub-networks of biological significance. This latter feature is possible by use of a gap-filling algorithm. The utility of the algorithm in reconstructing and mining of metabolomics data is shown on two independent datasets generated with LC–MS LTQ-Orbitrap mass spectrometry. Biologically relevant metabolic sub-networks were extracted from both datasets. Moreover, a number of metabolites, whose presence eluded automatic selection within mass spectra, could be identified retrospectively by virtue of their inferred presence through gap filling.

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.

Sarcolemmal localisation of Na+/H+ exchange and Na+–HCO3− co‐transport influences the spatial regulation of intracellular pH in rat ventricular myocytes

•  Acid extrusion from ventricular myocytes typically occurs via Na+/H+ exchange (NHE1) and Na+–HCO3− co‐transporters (NBC). This maintains intracellular pH at ∼7.2: The membrane distribution of these transporters is uncertain. •  Immunofluorescence indicates that: NBC isoforms are located in lateral sarcolemma, intercalated discs and transverse tubules, whereas NHE1 is densely expressed at intercalated discs. •  Functional experiments with detubulated myocytes indicate reduced acid extrusion on NBC but no effect on NHE1 activity, confirming exclusion of NHE1 function from transverse tubules. •  Stimulating NHE1 activity induces sub‐sarcolemmal [H+]i depletion (forming local pHi microdomains), particularly at intercalated discs, while stimulating NBC activity induces no pHi microdomains. •  Our results provide the first demonstration that pHi in ventricular myocytes is locally controlled through selective trafficking of membrane ion transporters. NHE1 preferentially controls pHi at intercalated discs, where cell‐to‐cell gap‐junctional channels are located, while NBC influences pHi adjacent to transverse tubules, where key proteins for excitation–contraction coupling are located.