Bax increases the pore size of rat brain mitochondrial voltage-dependent anion channel in the presence of tBid

Isogarcinol inhibits nasopharyngeal carcinoma growth through mitochondria-mediated autophagic cell death

BACKGROUND AND AIMS

Isogarcinol, a natural compound extracted from the fruits of Garcinia oblongifolia, has potential chemopreventive activity. This study aimed to elucidate the anti-tumor effects and mechanism of action of isogarcinol on nasopharyngeal carcinoma (NPC).

METHODS

Isogarcinol was isolated from Garcinia oblongifolia by using chromatographic separation. The anti-tumor effects of isogarcinol in NPC cells were tested by MTT assay, flow cytometry, wound healing assay, western blotting, transwell assay, colony formation assay, immunofluorescence, and transmission electron microscopy (TEM). The anti-tumor efficacy in vivo was evaluated in NPC cells xenograft models.

RESULTS

Functional studies revealed that isogarcinol inhibited the proliferation, colony formation, migration and invasion abilities of NPC cells in vitro. Isogarcinol caused mitochondrial damage to overproduce reactive oxygen species through reducing the mitochondrial membrane potential and ΔΨm. Isogarcinol also substantially inhibited NPC cells growth in a xenograft tumor model without any obvious toxicity when compared with paclitaxel (PTX). Mechanistic studies have illustrated that isogarcinol increased the Bax/Bcl-2 ratio, cleaved caspase-3, and cytoplasmic cytochrome C levels to induce mitochondrial apoptosis. The ROS overproduction by isogarcinol could suppress EMT pathway via decreasing the levels of p-Akt and Snail. Furthermore, isogarcinol promoted the conversion of LC3-Ⅰ to LC3-Ⅱ, but increased p62 level to block autophagic flux, resulting in the accumulation of damaged mitochondria to promote autophagic cell death in NPC cells.

CONCLUSION

This study provides a new theoretical foundation for the anti-tumor application of Garcinia oblongifolia and confirms that isogarcinol could be developed as a candidate drug for NPC treatment with low toxicity.

Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

Post-translational modifications and protein quality control of mitochondrial channels and transporters

Mitochondria play a critical role in energy metabolism and signal transduction, which is tightly regulated by proteins, metabolites, and ion fluxes. Metabolites and ion homeostasis are mainly mediated by channels and transporters present on mitochondrial membranes. Mitochondria comprise two distinct compartments, the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM), which have differing permeabilities to ions and metabolites. The OMM is semipermeable due to the presence of non-selective molecular pores, while the IMM is highly selective and impermeable due to the presence of specialized channels and transporters which regulate ion and metabolite fluxes. These channels and transporters are modulated by various post-translational modifications (PTMs), including phosphorylation, oxidative modifications, ions, and metabolites binding, glycosylation, acetylation, and others. Additionally, the mitochondrial protein quality control (MPQC) system plays a crucial role in ensuring efficient molecular flux through the mitochondrial membranes by selectively removing mistargeted or defective proteins. Inefficient functioning of the transporters and channels in mitochondria can disrupt cellular homeostasis, leading to the onset of various pathological conditions. In this review, we provide a comprehensive overview of the current understanding of mitochondrial channels and transporters in terms of their functions, PTMs, and quality control mechanisms.

Pro-Apoptotic and Anti-Cancer Activity of the Vernonanthura Nudiflora Hydroethanolic Extract

The mitochondrial voltage-dependent anion channel 1 (VDAC1) protein is involved in several essential cancer hallmarks, including energy and metabolism reprogramming and apoptotic cell death evasion. In this study, we demonstrated the ability of hydroethanolic extracts from three different plants, Vernonanthura nudiflora (Vern), Baccharis trimera (Bac), and Plantago major (Pla), to induce cell death. We focused on the most active Vern extract. We demonstrated that it activates multiple pathways that lead to impaired cell energy and metabolism homeostasis, elevated ROS production, increased intracellular Ca²⁺, and mitochondria-mediated apoptosis. The massive cell death generated by this plant extract's active compounds involves the induction of VDAC1 overexpression and oligomerization and, thereby, apoptosis. Gas chromatography of the hydroethanolic plant extract identified dozens of compounds, including phytol and ethyl linoleate, with the former producing similar effects as the Vern hydroethanolic extract but at 10-fold higher concentrations than those found in the extract. In a xenograft glioblastoma mouse model, both the Vern extract and phytol strongly inhibited tumor growth and cell proliferation and induced massive tumor cell death, including of cancer stem cells, inhibiting angiogenesis and modulating the tumor microenvironment. Taken together, the multiple effects of Vern extract make it a promising potential cancer therapeutic.

Appraisal of the Possible Role of PPARγ Upregulation by CLA of Probiotic Pediococcus pentosaceus GS4 in Colon Cancer Mitigation

The prevalence of colon cancer (CC) is increasing at the endemic scale, which is accompanied by subsequent morbidity and mortality. Although there have been noteworthy achievements in the therapeutic strategies in recent years, the treatment of patients with CC remains a formidable task. The current study focused on to study role of biohydrogenation-derived conjugated linoleic acid (CLA) of probiotic Pediococcus pentosaceus GS4 (CLA_GS4) against CC, which induced peroxisome proliferator-activated receptor gamma (PPARγ) expression in human CC HCT-116 cells. Pre-treatment with PPARγ antagonist bisphenol A diglycidyl ether has significantly reduced the inhibitory efficacy of enhanced cell viability of HCT-116 cells, suggesting the PPARγ-dependent cell death. The cancer cells treated with CLA/CLA_GS4 demonstrated the reduced level of Prostaglandin E2 PGE₂ in association with reduced COX-2 and 5-LOX expressions. Moreover, these consequences were found to be associated with PPARγ-dependent. Furthermore, delineation of mitochondrial dependent apoptosis with the help of molecular docking LigPlot analysis showed that CLA can bind with hexokinase-II (hHK-II) (highly expressed in cancer cells) and that this association underlies voltage dependent anionic channel to open, thereby causing mitochondrial membrane depolarization, a condition that initiates intrinsic apoptotic events. Apoptosis was further confirmed by annexin V staining and elevation of caspase 1p10 expression. Taken all together, it is deduced that, mechanistically, the upregulation of PPARγ by CLA_GS4 of P. pentosaceus GS4 can alter cancer cell metabolism in association with triggering apoptosis in CC.

Mitochondrial VDAC1: A Potential Therapeutic Target of Inflammation-Related Diseases and Clinical Opportunities

The multifunctional protein, voltage-dependent anion channel 1 (VDAC1), is located on the mitochondrial outer membrane. It is a pivotal protein that maintains mitochondrial function to power cellular bioactivities via energy generation. VDAC1 is involved in regulating energy production, mitochondrial oxidase stress, Ca²⁺ transportation, substance metabolism, apoptosis, mitochondrial autophagy (mitophagy), and many other functions. VDAC1 malfunction is associated with mitochondrial disorders that affect inflammatory responses, resulting in an up-regulation of the body’s defensive response to stress stimulation. Overresponses to inflammation may cause chronic diseases. Mitochondrial DNA (mtDNA) acts as a danger signal that can further trigger native immune system activities after its secretion. VDAC1 mediates the release of mtDNA into the cytoplasm to enhance cytokine levels by activating immune responses. VDAC1 regulates mitochondrial Ca²⁺ transportation, lipid metabolism and mitophagy, which are involved in inflammation-related disease pathogenesis. Many scientists have suggested approaches to deal with inflammation overresponse issues via specific targeting therapies. Due to the broad functionality of VDAC1, it may become a useful target for therapy in inflammation-related diseases. The mechanisms of VDAC1 and its role in inflammation require further exploration. We comprehensively and systematically summarized the role of VDAC1 in the inflammatory response, and hope that our research will lead to novel therapeutic strategies that target VDAC1 in order to treat inflammation-related disorders.

Insight into the interplay between mitochondria-regulated cell death and energetic metabolism in osteosarcoma

Osteosarcoma (OS) is a pediatric malignant bone tumor that predominantly affects adolescent and young adults. It has high risk for relapse and over the last four decades no improvement of prognosis was achieved. It is therefore crucial to identify new drug candidates for OS treatment to combat drug resistance, limit relapse, and stop metastatic spread. Two acquired hallmarks of cancer cells, mitochondria-related regulated cell death (RCD) and metabolism are intimately connected. Both have been shown to be dysregulated in OS, making them attractive targets for novel treatment. Promising OS treatment strategies focus on promoting RCD by targeting key molecular actors in metabolic reprogramming. The exact interplay in OS, however, has not been systematically analyzed. We therefore review these aspects by synthesizing current knowledge in apoptosis, ferroptosis, necroptosis, pyroptosis, and autophagy in OS. Additionally, we outline an overview of mitochondrial function and metabolic profiles in different preclinical OS models. Finally, we discuss the mechanism of action of two novel molecule combinations currently investigated in active clinical trials: metformin and the combination of ADI-PEG20, Docetaxel and Gemcitabine.

Extracellular Signal-Regulated Kinase1 (ERK1)-Mediated Phosphorylation of Voltage-Dependent Anion Channel (VDAC) Suppresses its Conductance

ERK1 is one of the members of the mitogen-activated protein kinases that regulate important cellular functions. VDAC is located at the outer membrane of mitochondria. Here, an interaction between VDAC and ERK1 has been studied on an artificial planar lipid bilayer using in vitro electrophysiology experiments. We report that VDAC is phosphorylated by ERK1 in the presence of Mg²⁺-ATP and its single-channel currents are inhibited on the artificial bilayer membrane. Treatment of Alkaline phosphatase on ERK1 phosphorylated VDAC leads to partial recovery of the single-channel VDAC currents. Later, phosphorylation of VDAC was demonstrated by Pro-Q diamond dye. Mass Spectrometric studies indicate phosphorylation of VDAC at Threonine 33, Threonine 55, and Serine 35. In a nutshell, phosphorylation of VDAC leads to the closure of the channel.

Structural basis of complex formation between mitochondrial anion channel VDAC1 and Hexokinase-II

Complex formation between hexokinase-II (HKII) and the mitochondrial VDAC1 is crucial to cell growth and survival. We hypothesize that HKII first inserts into the outer membrane of mitochondria (OMM) and then interacts with VDAC1 on the cytosolic leaflet of OMM to form a binary complex. To systematically investigate this process, we devised a hybrid approach. First, we describe membrane binding of HKII with molecular dynamics (MD) simulations employing a membrane mimetic model with enhanced lipid diffusion capturing membrane insertion of its H-anchor. The insertion depth of the H-anchor was then used to derive positional restraints in subsequent millisecond-scale Brownian dynamics (BD) simulations to preserve the membrane-bound pose of HKII during the formation of the HKII/VDAC1 binary complex. Multiple BD-derived structural models for the complex were further refined and their structural stability probed with additional MD simulations, resulting in one stable complex. A major feature in the complex is the partial (not complete) blockade of VDAC1's permeation pathway, a result supported by our comparative electrophysiological measurements of the channel in the presence and absence of HKII. We also show how VDAC1 phosphorylation disrupts HKII binding, a feature that is verified by our electrophysiology recordings and has implications in mitochondria-mediated cell death.

VDAC1 at the Intersection of Cell Metabolism, Apoptosis, and Diseases

The voltage-dependent anion channel 1 (VDAC1) protein, is an important regulator of mitochondrial function, and serves as a mitochondrial gatekeeper, with responsibility for cellular fate. In addition to control over energy sources and metabolism, the protein also regulates epigenomic elements and apoptosis via mediating the release of apoptotic proteins from the mitochondria. Apoptotic and pathological conditions, as well as certain viruses, induce cell death by inducing VDAC1 overexpression leading to oligomerization, and the formation of a large channel within the VDAC1 homo-oligomer. This then permits the release of pro-apoptotic proteins from the mitochondria and subsequent apoptosis. Mitochondrial DNA can also be released through this channel, which triggers type-Ι interferon responses. VDAC1 also participates in endoplasmic reticulum (ER)-mitochondria cross-talk, and in the regulation of autophagy, and inflammation. Its location in the outer mitochondrial membrane, makes VDAC1 ideally placed to interact with over 100 proteins, and to orchestrate the interaction of mitochondrial and cellular activities through a number of signaling pathways. Here, we provide insights into the multiple functions of VDAC1 and describe its involvement in several diseases, which demonstrate the potential of this protein as a druggable target in a wide variety of pathologies, including cancer.

Insights Into the Role of Mitochondrial Ion Channels in Inflammatory Response

Mitochondria are the source of many pro-inflammatory signals that cause the activation of the immune system and generate inflammatory responses. They are also potential targets of pro-inflammatory mediators, thus triggering a severe inflammatory response cycle. As mitochondria are a central hub for immune system activation, their dysfunction leads to many inflammatory disorders. Thus, strategies aiming at regulating mitochondrial dysfunction can be utilized as a therapeutic tool to cure inflammatory disorders. Two key factors that determine the structural and functional integrity of mitochondria are mitochondrial ion channels and transporters. They are not only important for maintaining the ionic homeostasis of the cell, but also play a role in regulating reactive oxygen species generation, ATP production, calcium homeostasis and apoptosis, which are common pro-inflammatory signals. The significance of the mitochondrial ion channels in inflammatory response is still not clearly understood and will need further investigation. In this article, we review the different mechanisms by which mitochondria can generate the inflammatory response as well as highlight how mitochondrial ion channels modulate these mechanisms and impact the inflammatory processes.

The TLK1/Nek1 axis contributes to mitochondrial integrity and apoptosis prevention via phosphorylation of VDAC1

The TLK1/Nek1 axis contributes to cell cycle arrest and implementation of the DDR to mediate survival upon DNA damage. However, when the damage is too severe, the cells typically are forced into apoptosis, and the contribution of TLKs in this process has not been investigated. In contrast, it is known that Nek1 may play a role by phosphorylating VDAC1 maintaining proper opening and closure of the channel and thus mitochondrial integrity. We now show that the activating phosphorylation of Nek1-T141 by TLK1 contributes to the phosphorylation and stability of VDAC1 and thereby to mitochondrial permeability and integrity. Treatment of three different cell lines model that overexpress Nek1-T141A mutant with doxorubicin showed exquisite sensitivity to the drug, with implementation of rapid accumulation of cells with subG1 DNA content (apoptotic) and other alterations in the cell cycle. In addition, these cells displayed reduced oxygen consumption under normal conditions and less reliance on mitochondria and more dependence on glycolysis for energy production. Consistent with greater apoptosis, upon treatment with low doses of doxorubicin, cells overexpressing Nek1-T141A displayed leakage of Cyt-C into the cytoplasmic fraction. This suggests that inhibiting the TLK1/Nek1/VDAC1 nexus could sensitize cancer cells to apoptotic killing in combination with an appropriate DNA damaging agent. We in fact have previously reported that Nek1 expression is elevated in advanced Prostate Cancer (PCa) and we now report that VDAC1 expression is elevated and correlated with disease stage, thereby making the TLK1/Nek1/VDAC1 nexus a very attractive target for PCa.

The roles of mitochondria-associated membranes in mitochondrial quality control under endoplasmic reticulum stress

The endoplasmic reticulum (ER) and mitochondria are two important organelles in cells. Mitochondria-associated membranes (MAMs) are lipid raft-like domains formed in the ER membranes that are in close apposition to mitochondria. They play an important role in signal transmission between these two essential organelles. When cells are exposed to internal or external stressful stimuli, the ER will activate an adaptive response called the ER stress response, which has a significant effect on mitochondrial function. Mitochondrial quality control is an important mechanism to ensure the functional integrity of mitochondria and the effect of ER stress on mitochondrial quality control through MAMs is of great significance. Therefore, in this review, we introduce ER stress and mitochondrial quality control, and discuss how ER stress signals are transmitted to mitochondria through MAMs. We then review the important roles of MAMs in mitochondrial quality control under ER stress.

Regulation of VDAC1 contributes to the cardioprotective effects of penehyclidine hydrochloride during myocardial ischemia/reperfusion

Penehyclidine hydrochloride (PHC) preconditioning can alleviate myocardial ischemia/reperfusion (I/R) injury and inhibits the upregulation of voltage-dependent anion channel 1 (VDAC1) during I/R. To validate that VDAC1 is a bona fide target of PHC for the protection against myocardial I/R injury, VDAC1 expression construct was delivered by lentiviruses into rat left ventricular myocardium before PHC preconditioning and myocardial I/R. Overexpression of VDAC1 exacerbated cardiac dysfunction and myocardial injury following I/R, and abolished the cardioprotective effect of PHC during I/R injury. Moreover, VDAC1 overexpression with myocardial I/R further increased cytochrome c release from mitochondria to cytoplasm, elevated the levels of cleaved caspase-3 and Bax, and decreased the level of Bcl-2 as compared with I/R alone, and PHC-mediated inhibition of mitochondria-dependent apoptosis during myocardial I/R was abolished by VDAC1 overexpression. In addition, VDAC1 was overexpressed in H9c2 cardiomyocytes undergoing anoxia/reoxygenation (A/R) with or without PHC pretreatment. The in vitro results showed that overexpression of VDAC1 further reduced mitochondrial membrane potential, increased mitochondrial membrane permeability and enhanced mitochondria-dependent apoptosis in H9c2 cells after A/R, and VDAC1 overexpression abrogated the protective effect of PHC on the mitochondrial function and integrity during A/R. In conclusion, exogenous overexpression of VDAC1 during myocardial I/R inhibits the cardioprotective effects of PHC. These effects may be associated with the suppression of VDAC1 expression.

VDAC1 at the crossroads of cell metabolism, apoptosis and cell stress

This review presents current knowledge related to VDAC1 as a multi-functional mitochondrial protein acting on both sides of the coin, regulating cell life and death, and highlighting these functions in relation to disease. It is now recognized that VDAC1 plays a crucial role in regulating the metabolic and energetic functions of mitochondria. The location of VDAC1 at the outer mitochondrial membrane (OMM) allows the control of metabolic cross-talk between mitochondria and the rest of the cell and also enables interaction of VDAC1 with proteins involved in metabolic and survival pathways. Along with regulating cellular energy production and metabolism, VDAC1 is also involved in the process of mitochondria-mediated apoptosis by mediating the release of apoptotic proteins and interacting with anti-apoptotic proteins. VDAC1 functions in the release of apoptotic proteins located in the mitochondrial intermembrane space via oligomerization to form a large channel that allows passage of cytochrome c and AIF and their release to the cytosol, subsequently resulting in apoptotic cell death. VDAC1 also regulates apoptosis via interactions with apoptosis regulatory proteins, such as hexokinase, Bcl2 and Bcl-xL, some of which are also highly expressed in many cancers. This review also provides insight into VDAC1 function in Ca²⁺ homeostasis, oxidative stress, and presents VDAC1 as a hub protein interacting with over 100 proteins. Such interactions enable VDAC1 to mediate and regulate the integration of mitochondrial functions with cellular activities. VDAC1 can thus be considered as standing at the crossroads between mitochondrial metabolite transport and apoptosis and hence represents an emerging cancer drug target.

VDAC1 deacetylation is involved in the protective effects of resveratrol against mitochondria-mediated apoptosis in cardiomyocytes subjected to anoxia/reoxygenation injury

We have recently demonstrated that Voltage-dependent anion channel 1 (VDAC1), a protein located in the mitochondrial outer membrane, is involved in the effects of resveratrol on the mitochondrial permeability transition pore (mPTP). However, the underlying mechanism of action remains to be elucidated. In the present study, we demonstrated that resveratrol promoted VDAC1 deacetylation in cardiomyocytes in response to anoxia/reoxygenation (A/R) injury. Moreover, silent information regulator of transcription 1 (SIRT1), a NAD⁺-dependent class III histone deacetylase, was up-regulated after pretreatment with resveratrol. Cells that were treated with Ex527, a specific inhibitor of SIRT1, showed a reduction in both SIRT1 expression and VDAC1 deacetylation, indicating that the deacetylation effect of resveratrol on VDAC1 is mediated by SIRT1. Furthermore, the ability deacetylated VDAC1 to bind to Bax was decreased after pretreatment with resveratrol, whereas Bcl-2 expression changed in the opposite direction. As a result, opening of the mPTP was restrained, the mitochondrial membrane potential was reserved, and cytochrome c release was inhibited, which subsequently decreased cardiomyocyte apoptosis. However, the cardioprotective effects observed after treatment of resveratrol could be abrogated by Ex527. In conclusion, resveratrol induces deacetylation of VDAC1 by SIRT1, thereby preventing mitochondria-mediated apoptosis in cardiomyocytes upon A/R injury.

Voltage-Dependent Anion Channel 1 As an Emerging Drug Target for Novel Anti-Cancer Therapeutics

Cancer cells share several properties, high proliferation potential, reprogramed metabolism, and resistance to apoptotic cues. Acquiring these hallmarks involves changes in key oncogenes and non-oncogenes essential for cancer cell survival and prosperity, and is accompanied by the increased energy requirements of proliferating cells. Mitochondria occupy a central position in cell life and death with mitochondrial bioenergetics, biosynthesis, and signaling are critical for tumorigenesis. Voltage-dependent anion channel 1 (VDAC1) is situated in the outer mitochondrial membrane (OMM) and serving as a mitochondrial gatekeeper. VDAC1 allowing the transfer of metabolites, fatty acid ions, Ca²⁺, reactive oxygen species, and cholesterol across the OMM and is a key player in mitochondrial-mediate apoptosis. Moreover, VDAC1 serves as a hub protein, interacting with diverse sets of proteins from the cytosol, endoplasmic reticulum, and mitochondria that together regulate metabolic and signaling pathways. The observation that VDAC1 is over-expressed in many cancers suggests that the protein may play a pivotal role in cancer cell survival. However, VDAC1 is also important in mitochondria-mediated apoptosis, mediating release of apoptotic proteins and interacting with anti-apoptotic proteins, such as B-cell lymphoma 2 (Bcl-2), Bcl-xL, and hexokinase (HK), which are also highly expressed in many cancers. Strategically located in a “bottleneck” position, controlling metabolic homeostasis and apoptosis, VDAC1 thus represents an emerging target for anti-cancer drugs. This review presents an overview on the multi-functional mitochondrial protein VDAC1 performing several functions and interacting with distinct sets of partners to regulate both cell life and death, and highlights the importance of the protein for cancer cell survival. We address recent results related to the mechanisms of VDAC1-mediated apoptosis and the potential of associated proteins to modulate of VDAC1 activity, with the aim of developing VDAC1-based approaches. The first strategy involves modification of cell metabolism using VDAC1-specific small interfering RNA leading to inhibition of cancer cell and tumor growth and reversed oncogenic properties. The second strategy involves activation of cancer cell death using VDAC1-based peptides that prevent cell death induction by anti-apoptotic proteins. Finally, we discuss the potential therapeutic benefits of treatments and drugs leading to enhanced VDAC1 expression or targeting VDAC1 to induce apoptosis.

Putative roles of mitochondrial Voltage-Dependent Anion Channel, Bcl-2 family proteins and c-Jun N-terminal Kinases in ischemic stroke associated apoptosis

There is a constant need for better stroke treatments. Neurons at the periphery of an ischemic stroke affected brain tissue remains metabolically active for several hours or days after stroke onset. They later undergo mitochondrion-mediated apoptosis. It has been found that inhibiting apoptosis in the peripheral ischemic neurons could be very effective in the prevention of stroke progression. During stroke associated apoptosis, cytosolic c-Jun N-terminal Kinases (JNKs) and Bcl-2 family proteins translocate towards mitochondria and promote cytochrome c release by interacting with the outer mitochondrion membrane associated proteins. This review provides an overview of the plausible interactions of the outer mitochondrial membrane Voltage Dependent Anion Channel, Bcl-2 family proteins and JNKs in cytochrome c release in the peripheral ischemic stroke associated apoptotic neurons. The review ends with a note on designing new anti-stroke treatments.

Chrysin inhibited tumor glycolysis and induced apoptosis in hepatocellular carcinoma by targeting hexokinase-2

Background

Hexokinase-2(HK-2) plays dual roles in glucose metabolism and mediation of cell apoptosis, making it an attractive target for cancer therapy. Chrysin is a natural flavone found in plant extracts which are widely used as herb medicine in China. In the present study, we investigated the antitumor activity of chrysin against hepatocellular carcinoma (HCC) and the role of HK-2 played for chrysin to exert its function.

Methods

The expression of HK-2 in HCC cell line and tumor tissue was examined by western blotting and immunohistochemistry staining. The activities of chrysin against HCC cell proliferation and tumor glycolysis were investigated. Chrysin-induced apoptosis was analyzed by flow cytometry. The effect of chrysin on HK-2 expression and the underlying mechanisms by which induced HCC cell apoptosis were studied. In HK-2 exogenous overexpression cell, the changes of chrysin-induced cell apoptosis and glycolysis suppression were investigated. HCC cell xenograft model was used to confirm the antitumor activity of chrysin in vivo and the effect on HK-2 was tested in chrysin-treated tumor tissue.

Results

In contrast with normal cell lines and tissue, HK-2 expression was substantially elevated in the majority of tested HCC cell lines and tumor tissue. Owing to the decrease of HK-2 expression, glucose uptake and lactate production in HCC cells were substantially inhibited after exposure to chrysin. After chrysin treatment, HK-2 which combined with VDAC-1 on mitochondria was significantly declined, resulting in the transfer of Bax from cytoplasm to mitochondria and induction of cell apoptosis. Chrysin-mediated cell apoptosis and glycolysis suppression were dramatically impaired in HK-2 exogenous overexpression cells. Tumor growth in HCC xenograft models was significantly restrained after chrysin treatment and significant decrease of HK-2 expression was observed in chrysin-treated tumor tissue.

Conclusion

Through suppressing glycolysis and inducing apoptosis in HCC, chrysin, or its derivative has a promising potential to be a novel therapeutic for HCC management, especially for those patients with high HK-2 expression.

JNK3 phosphorylates Bax protein and induces ability to form pore on bilayer lipid membrane

Bax is a pro-apoptotic cytosolic protein. In this work native (unphosphorylated) and JNK3 phosphorylated Bax proteins are studied on artificial bilayer membranes for pore formation. Phosphorylated Bax formed pore on the bilayer lipid membrane whereas native one does not. In cells undergoing apoptosis the pore formed by the phosphorylated Bax could be important in cytochrome c release from the mitochondrial intermembrane space to the cytosol. The low conductance (1.5 nS) of the open state of the phosphorylated Bax pore corresponds to pore diameter of 0.9 nm which is small to release cytochrome c (∼3.4 nm). We hypothesized that JNK3 phosphorylated Bax protein can form bigger pores after forming complexes with other mitochondrial proteins like VDAC, t-Bid etc. to release cytochrome c.

Mitochondrial VDAC, the Na+/Ca2+ Exchanger, and the Ca2+ Uniporter in Ca2+ Dynamics and Signaling

Mitochondrial Ca²⁺ uptake and release play pivotal roles in cellular physiology by regulating intracellular Ca²⁺ signaling, energy metabolism, and cell death. Ca²⁺ transport across the inner and outer mitochondrial membranes (IMM, OMM, respectively), is mediated by several proteins, including the voltage-dependent anion channel 1 (VDAC1) in the OMM, and the mitochondrial Ca²⁺ uniporter (MCU) and Na⁺-dependent mitochondrial Ca²⁺ efflux transporter, (the NCLX), both in the IMM. By transporting Ca²⁺ across the OMM to the mitochondrial inner-membrane space (IMS), VDAC1 allows Ca²⁺ access to the MCU, facilitating transport of Ca²⁺ to the matrix, and also from the IMS to the cytosol. Intra-mitochondrial Ca²⁺ controls energy production and metabolism by modulating critical enzymes in the tricarboxylic acid (TCA) cycle and fatty acid oxidation. Thus, by transporting Ca²⁺, VDAC1 plays a fundamental role in regulating mitochondrial Ca²⁺ homeostasis, oxidative phosphorylation, and Ca²⁺ crosstalk among mitochondria, cytoplasm, and the endoplasmic reticulum (ER). VDAC1 has also been recognized as a key protein in mitochondria-mediated apoptosis, and apoptosis stimuli induce overexpression of the protein in a Ca²⁺-dependent manner. The overexpressed VDAC1 undergoes oligomerization leading to the formation of a channel, through which apoptogenic agents can be released. Here, we review the roles of VDAC1 in mitochondrial Ca²⁺ homeostasis, in apoptosis, and in diseases associated with mitochondria dysfunction.

Anion Channels of Mitochondria

Mitochondria are the "power house" of a cell continuously generating ATP to ensure its proper functioning. The constant production of ATP via oxidative phosphorylation demands a large electrochemical force that drives protons across the highly selective and low-permeable mitochondrial inner membrane. Besides the conventional role of generating ATP, mitochondria also play an active role in calcium signaling, generation of reactive oxygen species (ROS), stress responses, and regulation of cell-death pathways. Deficiencies in these functions result in several pathological disorders like aging, cancer, diabetes, neurodegenerative and cardiovascular diseases. A plethora of ion channels and transporters are present in the mitochondrial inner and outer membranes which work in concert to preserve the ionic equilibrium of a cell for the maintenance of cell integrity, in physiological as well as pathophysiological conditions. For, e.g., mitochondrial cation channels KATP and BKCa play a significant role in cardioprotection from ischemia-reperfusion injury. In addition to the cation channels, mitochondrial anion channels are equally essential, as they aid in maintaining electro-neutrality by regulating the cell volume and pH. This chapter focusses on the information on molecular identity, structure, function, and physiological relevance of mitochondrial chloride channels such as voltage dependent anion channels (VDACs), uncharacterized mitochondrial inner membrane anion channels (IMACs), chloride intracellular channels (CLIC) and the aspects of forthcoming chloride channels.

Voltage-Dependent Anion Channel 1(VDAC1) Participates the Apoptosis of the Mitochondrial Dysfunction in Desminopathy

Desminopathies caused by the mutation in the gene coding for desmin are genetically protein aggregation myopathies. Mitochondrial dysfunction is one of pathological changes in the desminopathies at the earliest stage. The molecular mechanisms of mitochondria dysfunction in desminopathies remain exclusive. VDAC1 regulates mitochondrial uptake across the outer membrane and mitochondrial outer membrane permeabilization (MOMP). Relationships between desminopathies and Voltage-dependent anion channel 1 (VDAC1) remain unclear. Here we successfully constructed the desminopathy rat model, evaluated with conventional stains, containing hematoxylin and eosin (HE), Gomori Trichrome (MGT), (PAS), red oil (ORO), NADH-TR, SDH staining and immunohistochemistry. Immunofluorescence results showed that VDAC1 was accumulated in the desmin highly stained area of muscle fibers of desminopathy patients or desminopathy rat model compared to the normal ones. Meanwhile apoptosis related proteins bax and ATF2 were involved in desminopathy patients and desminopathy rat model, but not bcl-2, bcl-xl or HK2.VDAC1 and desmin are closely relevant in the tissue splices of deminopathies patients and rats with desminopathy at protein lever. Moreover, apoptotic proteins are also involved in the desminopathies, like bax, ATF2, but not bcl-2, bcl-xl or HK2. This pathological analysis presents the correlation between VDAC1 and desmin, and apoptosis related proteins are correlated in the desminopathy. Furthermore, we provide a rat model of desminopathy for the investigation of desmin related myopathy.

Clinical implication of voltage-dependent anion channel 1 in uterine cervical cancer and its action on cervical cancer cells

Two-dimensional gel electrophoresis and liquid chromatography-tandem mass spectrometry were performed to investigate the influence of human nonmetastatic clone 23 type 1 (nm23-H1), a metastasis-associated gene on proteomic alterations in cancer cells of the uterine cervix. It was validated by RT-PCR and Western blot analysis. The expression of voltage-dependent anion channel 1 (VDAC1) was increased in nm23-H1 gene silenced SiHa or CaSki cervical cancer cells. The clinical implication was shown that cervical cancer tissues with positive VDAC1 immunoreactivity exhibited deep stromal invasion (>10 mm in depth) and large tumor size (> 4 cm in diameter). Cervical cancer patients with positive VDAC1 immunoreactivity displayed higher recurrence and poorer overall survival than those with negative VDAC1. Silencing of VDAC1 reduced cell proliferation and migratory ability. Mitochondrial membrane potential was decreased and reactive oxygen species generation was increased in the VDAC1 gene-silenced cervical cancer cells. Cell cycle progression and autophagy were not changed in VDAC1 silencing cells. The cytotoxicity of cisplatin was significantly enhanced by knockdown of cellular VDAC1 and the compounds that interfere with hexokinase binding to VDAC. Therapeutic strategies may be offered using VDAC1 as a target to reduce cell growth and migration, enhance the synergistic therapeutic efficacy of cisplatin and reduce cisplatin dose-limiting toxicity.

Growth inhibitory and apoptosis-inducing effects of allergen-free Rhus verniciflua Stokes extract on A549 human lung cancer cells

Evidence suggests that Rhus verniciflua Stokes (RVS) or its extract has the potential to be used for the treatment of inflammatory and neoplastic diseases. However, direct use of RVS or its extract as a herbal medicine has been limited due to the presence of urushiol, an allergenic toxin. In the present study, we prepared an extract of the allergen‑removed RVS (aRVS) based on a traditional method and investigated its inhibitory effect on the growth of various types of human cancer cells, including lung (A549), breast (MCF-7) and prostate (DU-145) cancer cell lines. Notably, among the cell lines tested, treatment with the aRVS extract strongly inhibited proliferation of the A549 cells at a 0.5 mg/ml concentration for 24 h that was not cytotoxic to normal human dermal fibroblasts. Furthermore, aRVS extract treatment largely reduced the survival and induced apoptosis of the A549 cells. At the mechanistic levels, treatment with the aRVS extract led to the downregulation of Bcl-2 and Mcl-1 proteins, the activation of caspase-9/-3 proteins, an increase in cytosolic cytochrome c levels, the upregulation of Bax protein, an increase in phosphorylated p53 protein but a decrease in phosphorylated S6 protein in the A549 cells. Importantly, treatment with z-VAD‑fmk, a pan-caspase inhibitor attenuated aRVS extract-induced apoptosis in the A549 cells. These results demonstrate firstly that aRVS extract has growth inhibitory and apoptosis-inducing effects on A549 human lung cancer cells through modulation of the expression levels and/or activities of caspases, Bcl-2, Mcl-1, Bax, p53 and S6.

Phosphorylation of voltage-dependent anion channel by c-Jun N-terminal Kinase-3 leads to closure of the channel

Stress activated c-Jun N-terminal Kinase-3 (JNK3) has been reported to act on mitochondrion to promote neuronal cell death. Phosphorylation of mitochondrial Voltage-Dependent Anion Channel (VDAC) plays an important role in mitochondria-mediated cell death. Keeping these in view phosphorylation of rat brain VDAC by JNK3 has been studied in vitro. Pro Q Diamond phospho-protein staining experiment demonstrates VDAC is phosphorylated by JNK3. Bilayer electrophysiological experiments show that single-channel conductance of VDAC phosphorylated by JNK3 is significantly lower than that of the native VDAC at a membrane potential. The opening probability of VDAC undergoes massive reduction due to phosphorylation by JNK3. These indicate closure of VDAC due to phosphorylation by JNK3. Treatment of phosphorylated VDAC with alkaline phosphatase reversed the VDAC functional activity as shown by single-channel current and opening probability. The physiological consequence of closure of VDAC as a result of phosphorylation has been attributed to JNK3 dependent mitochondria-mediated apoptosis.

The mitochondrial voltage-dependent anion channel 1 in tumor cells

VDAC1 is found at the crossroads of metabolic and survival pathways. VDAC1 controls metabolic cross-talk between mitochondria and the rest of the cell by allowing the influx and efflux of metabolites, ions, nucleotides, Ca2+ and more. The location of VDAC1 at the outer mitochondrial membrane also enables its interaction with proteins that mediate and regulate the integration of mitochondrial functions with cellular activities. As a transporter of metabolites, VDAC1 contributes to the metabolic phenotype of cancer cells. Indeed, this protein is over-expressed in many cancer types, and silencing of VDAC1 expression induces an inhibition of tumor development. At the same time, along with regulating cellular energy production and metabolism, VDAC1 is involved in the process of mitochondria-mediated apoptosis by mediating the release of apoptotic proteins and interacting with anti-apoptotic proteins. The engagement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space involves VDAC1 oligomerization that mediates the release of cytochrome c and AIF to the cytosol, subsequently leading to apoptotic cell death. Apoptosis can also be regulated by VDAC1, serving as an anchor point for mitochondria-interacting proteins, such as hexokinase (HK), Bcl2 and Bcl-xL, some of which are also highly expressed in many cancers. By binding to VDAC1, HK provides both a metabolic benefit and apoptosis-suppressive capacity that offer the cell a proliferative advantage and increase its resistance to chemotherapy. Thus, these and other functions point to VDAC1 as an excellent target for impairing the re-programed metabolism of cancer cells and their ability to evade apoptosis. Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to both cancer development and therapy. In addressing the recently solved 3D structures of VDAC1, this review will point to structure-function relationships of VDAC as critical for deciphering how this channel can perform such a variety of roles, all of which are important for cell life and death. Finally, this review will also provide insight into VDAC function in Ca2+ homeostasis, protection against oxidative stress, regulation of apoptosis and involvement in several diseases, as well as its role in the action of different drugs. We will discuss the use of VDAC1-based strategies to attack the altered metabolism and apoptosis of cancer cells. These strategies include specific siRNA able to impair energy and metabolic homeostasis, leading to arrested cancer cell growth and tumor development, as well VDAC1-based peptides that interact with anti-apoptotic proteins to induce apoptosis, thereby overcoming the resistance of cancer cell to chemotherapy. Finally, small molecules targeting VDAC1 can induce apoptosis. VDAC1 can thus be considered as standing at the crossroads between mitochondrial metabolite transport and apoptosis and hence represents an emerging cancer drug target. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Lytic and non-lytic permeabilization of cardiolipin-containing lipid bilayers induced by cytochrome C

The release of cytochrome c (cyt c) from mitochondria is an important early step during cellular apoptosis, however the precise mechanism by which the outer mitochondrial membrane becomes permeable to these proteins is as yet unclear. Inspired by our previous observation of cyt c crossing the membrane barrier of giant unilamellar vesicle model systems, we investigate the interaction of cyt c with cardiolipin (CL)-containing membranes using the innovative droplet bilayer system that permits electrochemical measurements with simultaneous microscopy observation. We find that cyt c can permeabilize CL-containing membranes by induction of lipid pores in a dose-dependent manner, with membrane lysis eventually observed at relatively high (µM) cyt c concentrations due to widespread pore formation in the membrane destabilizing its bilayer structure. Surprisingly, as cyt c concentration is further increased, we find a regime with exceptionally high permeability where a stable membrane barrier is still maintained between droplet compartments. This unusual non-lytic state has a long lifetime (>20 h) and can be reversibly formed by mechanically separating the droplets before reforming the contact area between them. The transitions between behavioural regimes are electrostatically driven, demonstrated by their suppression with increasing ionic concentrations and their dependence on CL composition. While membrane permeability could also be induced by cationic PAMAM dendrimers, the non-lytic, highly permeable membrane state could not be reproduced using these synthetic polymers, indicating that details in the structure of cyt c beyond simply possessing a cationic net charge are important for the emergence of this unconventional membrane state. These unexpected findings may hold significance for the mechanism by which cyt c escapes into the cytosol of cells during apoptosis.

Bax induces cytochrome c release by multiple mechanisms in mitochondria from MCF7 cells

Bax, a pro-apoptotic member of the Bcl-2 family of proteins has the ability to form transmembrane pores large enough to allow cytochrome c (Cyt c) release, as well as to activate the mitochondrial permeability transition pore (mPTP); however, no differential study has been conducted to clarify which one of these mechanisms predominates over the other in the same system. In the present study, we treated isolated mitochondria from MCF7 cells with recombinant protein Bax and tested the efficacy of the mPTP inhibitor cyclosporin A (CsA) and of the Bax channel blocker (Bcb) to inhibit cytochrome c release. We also, induced apoptosis in MCF7 cell cultures with TNF-α plus cycloheximide to determine the effect of such compounds in apoptosis induction via mPTP or Bax oligomerization. Cytochrome c release was totally prevented by CsA and partially by Bcb when apoptosis was induced with recombinant Bax in isolated mitochondria from MCF7 cells. CsA increased the number of living cells in cell culture, as compared with the effect of Bax channel blocker. These results indicate that mPTP activation is the predominant pathway for Bax-induced cytochrome c release from MCF7 mitochondria and for apoptosis induction in the whole cell.

IP3, a small molecule with a powerful message

Research conducted over the past two decades has provided convincing evidence that cell death, and more specifically apoptosis, can exceed single cell boundaries and can be strongly influenced by intercellular communication networks. We recently reported that gap junctions (i.e. channels directly connecting the cytoplasm of neighboring cells) composed of connexin43 or connexin26 provide a direct pathway to promote and expand cell death, and that inositol 1,4,5-trisphosphate (IP3) diffusion via these channels is crucial to provoke apoptosis in adjacent healthy cells. However, IP3 itself is not sufficient to induce cell death and additional factors appear to be necessary to create conditions in which IP3 will exert proapoptotic effects. Although IP3-evoked Ca(2+) signaling is known to be required for normal cell survival, it is also actively involved in apoptosis induction and progression. As such, it is evident that an accurate fine-tuning of this signaling mechanism is crucial for normal cell physiology, while a malfunction can lead to cell death. Here, we review the role of IP3 as an intracellular and intercellular cell death messenger, focusing on the endoplasmic reticulum-mitochondrial synapse, followed by a discussion of plausible elements that can convert IP3 from a physiological molecule to a killer substance. Finally, we highlight several pathological conditions in which anomalous intercellular IP3/Ca(2+) signaling might play a role. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

VDAC1: from structure to cancer therapy

Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to cancer. Found at the outer mitochondrial membrane, VDAC1 assumes a crucial position in the cell, controlling the metabolic cross-talk between mitochondria and the rest of the cell. Moreover, its location at the boundary between the mitochondria and the cytosol enables VDAC1 to interact with proteins that mediate and regulate the integration of mitochondrial functions with other cellular activities. As a metabolite transporter, VDAC1 contributes to the metabolic phenotype of cancer cells. This is reflected by VDAC1 over-expression in many cancer types, and by inhibition of tumor development upon silencing VDAC1 expression. Along with regulating cellular energy production and metabolism, VDAC1 is also a key protein in mitochondria-mediated apoptosis, participating in the release of apoptotic proteins and interacting with anti-apoptotic proteins. The involvement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space is discussed, as is VDAC1 oligomerization as an important step in apoptosis induction. VDAC also serves as an anchor point for mitochondria-interacting proteins, some of which are also highly expressed in many cancers, such as hexokinase (HK), Bcl2, and Bcl-xL. By binding to VDAC, HK provides both metabolic benefit and apoptosis-suppressive capacity that offers the cell a proliferative advantage and increases its resistance to chemotherapy. VDAC1-based peptides that bind specifically to HK, Bcl2, or Bcl-xL abolished the cell’s abilities to bypass the apoptotic pathway. Moreover, these peptides promote cell death in a panel of genetically characterized cell lines derived from different human cancers. These and other functions point to VDAC1 as a rational target for the development of a new generation of therapeutics.

Human Bop is a novel BH3-only member of the Bcl-2 protein family

One group of Bcl-2 protein family, which shares only the BH3 domain (BH3-only), is critically involved in the regulation of programmed cell death. Herein we demonstrated a novel human BH3-only protein (designated as Bop) which could induce apoptosis in a BH3 domain-dependent manner. Further analysis indicated that Bop mainly localized to mitochondria and used its BH3 domain to contact the loop regions of voltage dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane. In addition, purified Bop protein induced the loss of mitochondrial transmembrane potential (Δψm) and the release of cytochrome c. Furthermore, Bop used its BH3 domain to contact pro-survival Bcl-2 family members (Bcl-2, Bcl-X(L), Mcl-1, A1 and Bcl-w), which could inhibit Bop-induced apoptosis. Bop would be constrained by pro-survival Bcl-2 proteins in resting cells, because Bop became released from phosphorylated Bcl-2 induced by microtubule-interfering agent like vincristine (VCR). Indeed, knockdown experiments indicated that Bop was partially required for VCR induced cell death. Finally, Bop might need to function through Bak and Bax, likely by releasing Bak from Bcl-X(L) sequestration. In conclusion, Bop may be a novel BH3-only factor that can engage with the regulatory network of Bcl-2 family members to process intrinsic apoptotic signaling.

Targeting malignant mitochondria with therapeutic peptides

The current status of peptides that target the mitochondria in the context of cancer is the focus of this review. Chemotherapy and radiotherapy used to kill tumor cells are principally mediated by the process of apoptosis that is governed by the mitochondria. The failure of anticancer therapy often resides at the level of the mitochondria. Therefore, the mitochondrion is a key pharmacological target in cancer due to many of the differences that arise between malignant and healthy cells at the level of this ubiquitous organelle. Additionally, targeting the characteristics of malignant mitochondira often rely on disruption of protein--protein interactions that are not generally amenable to small molecules. We discuss anticancer peptides that intersect with pathological changes in the mitochondrion.

BID is a critical factor controlling cell viability regulated by IFN-α

Clinical applications of human interferon (IFN)-α have met with varying degrees of success. Nevertheless, key molecules in cell viability regulated by IFN-α have not been clearly identified. Our previous study indicated that IFN (α, β, and ω) receptor (IFNAR) 1/2- and IFN regulatory factor 9-RNA interference (RNAi) completely restored cell viability after IFN-α treatment in human ovarian adenocarcinoma OVCAR3 cells sensitive to IFN-α. In this study, IFNAR1/2- and IFN regulatory factor 9-RNAi inhibited the gene expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), but not of Fas ligand, after IFN-α treatment. In fact, TRAIL but not Fas ligand inhibited the viability of OVCAR3 cells. IFN-α notably upregulated the levels of TRAIL protein in the supernatant and on the membrane of OVCAR3 cells. After TRAIL signaling, caspase 8 inhibitor and BH3 interacting domain death agonist (BID)-RNAi significantly restored cell viability in response to IFN-α and TRAIL in OVCAR3 cells. Furthermore, BID-RNAi prevented both IFN-α and TRAIL from collapsing the mitochondrial membrane potential (ΔΨm). Finally, we provided important evidence that BID overexpression led to significant inhibition of cell viability after IFN-α or TRAIL treatments in human lung carcinoma A549 cells resistant to IFN-α. Thus, this study suggests that BID is crucial for cell viability regulated by IFN-α which can induce mitochondria-mediated apoptosis, indicating a notable potential to be a targeted therapy for IFN-α resistant tumors.

Regulation of mitochondrial function by voltage dependent anion channels in ethanol metabolism and the Warburg effect

Voltage dependent anion channels (VDAC) are highly conserved proteins that are responsible for permeability of the mitochondrial outer membrane to hydrophilic metabolites like ATP, ADP and respiratory substrates. Although previously assumed to remain open, VDAC closure is emerging as an important mechanism for regulation of global mitochondrial metabolism in apoptotic cells and also in cells that are not dying. During hepatic ethanol oxidation to acetaldehyde, VDAC closure suppresses exchange of mitochondrial metabolites, resulting in inhibition of ureagenesis. In vivo, VDAC closure after ethanol occurs coordinately with mitochondrial uncoupling. Since acetaldehyde passes through membranes independently of channels and transporters, VDAC closure and uncoupling together foster selective and more rapid oxidative metabolism of toxic acetaldehyde to nontoxic acetate by mitochondrial aldehyde dehydrogenase. In single reconstituted VDAC, tubulin decreases VDAC conductance, and in HepG2 hepatoma cells, free tubulin negatively modulates mitochondrial membrane potential, an effect enhanced by protein kinase A. Tubulin-dependent closure of VDAC in cancer cells contributes to suppression of mitochondrial metabolism and may underlie the Warburg phenomenon of aerobic glycolysis. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Multistep and multitask Bax activation

Bax is a pro-apoptotic protein allowing apoptosis to occur through the intrinsic, damage-induced pathway, and amplifying that one occurring via the extrinsic, receptor mediated pathway. Bax is present in viable cells and activated by pro-apoptotic stimuli. Activation implies structural changes, consisting of exposure of the N terminus and hydrophobic domains; changes in localization, consisting in migration from cytosol to mitochondria and endoplasmic reticulum membranes; changes in the aggregation status, from monomer to dimer and multimer. Bax has multiple critical domains, namely the N terminus exposed after activation; two hydrophobic stretches exposed for membrane anchorage; two reactive cysteines allowing multimerization; the BH3 domain for interactions with the Bcl-2 family members; alpha helix 1 for t-Bid interaction. Bax has also multiple functions: it releases different mitochondrial factors such as cytochrome c, SMAC/diablo; it regulates mitochondrial fission, the mitochondrial permeability transition pore; it promotes Ca(2+) leakage through ER membrane. Altogether, Bax activation is a complex multi-step phenomenon. Here, we analyze these events as logically separable or alternative steps, attempting to assess their role, timing and reciprocal relation.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

Facilitation of mitochondrial outer and inner membrane permeabilization and cell death in oxidative stress by a novel Bcl-2 homology 3 domain protein

We identified a sequence homologous to the Bcl-2 homology 3 (BH3) domain of Bcl-2 proteins in SOUL. Tissues expressed the protein to different extents. It was predominantly located in the cytoplasm, although a fraction of SOUL was associated with the mitochondria that increased upon oxidative stress. Recombinant SOUL protein facilitated mitochondrial permeability transition and collapse of mitochondrial membrane potential (MMP) and facilitated the release of proapoptotic mitochondrial intermembrane proteins (PMIP) at low calcium and phosphate concentrations in a cyclosporine A-dependent manner in vitro in isolated mitochondria. Suppression of endogenous SOUL by diced small interfering RNA in HeLa cells increased their viability in oxidative stress. Overexpression of SOUL in NIH3T3 cells promoted hydrogen peroxide-induced cell death and stimulated the release of PMIP but did not enhance caspase-3 activation. Despite the release of PMIP, SOUL facilitated predominantly necrotic cell death, as revealed by annexin V and propidium iodide staining. This necrotic death could be the result of SOUL-facilitated collapse of MMP demonstrated by JC-1 fluorescence. Deletion of the putative BH3 domain sequence prevented all of these effects of SOUL. Suppression of cyclophilin D prevented these effects too, indicating that SOUL facilitated mitochondrial permeability transition in vivo. Overexpression of Bcl-2 and Bcl-x(L), which can counteract the mitochondria-permeabilizing effect of BH3 domain proteins, also prevented SOUL-facilitated collapse of MMP and cell death. These data indicate that SOUL can be a novel member of the BH3 domain-only proteins that cannot induce cell death alone but can facilitate both outer and inner mitochondrial membrane permeabilization and predominantly necrotic cell death in oxidative stress.

Hexokinase II detachment from the mitochondria potentiates cisplatin induced cytotoxicity through a caspase-2 dependent mechanism

Cancer cells are frequently glycolytic and overexpress hexokinase II (HXK II). In cancer cells, the majority of hexokinase II is localized to the mitochondria through interaction with the voltage dependent anion channel (VDAC). Disruption in the binding of hexokinase II to the mitochondria has been shown to promote mitochondrial injury provoked by pro-apoptotic proteins. The present study demonstrates that cisplatin induces the PIDD (p53 induced protein with a death domain) dependent activation of caspase-2. In turn, caspase-2 cleaves and activates Bid, resulting in the oligomerization of Bak and the release of cytochrome c. Notably, the detachment of hexokinase II from the mitochondria markedly potentiates the onset of caspase-2 induced mitochondrial damage, thus resulting in a synergistic induction of cisplatin induced cytotoxicity.

Uncovering the role of VDAC in the regulation of cell life and death

Proper cell activity requires an efficient exchange of molecules between mitochondria and cytoplasm. Lying in the outer mitochondrial membrane, VDAC assumes a crucial position in the cell, forming the main interface between the mitochondrial and the cellular metabolisms. As such, it has been recognized that VDAC plays a crucial role in regulating the metabolic and energetic functions of mitochondria. Indeed, down-regulation of VDAC1 expression by shRNA leads to a decrease in energy production and cell growth. VDAC has also been recognized as a key protein in mitochondria-mediated apoptosis through its involvement in the release of apoptotic proteins located in the inter-membranal space and as the proposed target of pro- and anti-apoptotic members of the Bcl2-family and of hexokinase. Questions, however, remain as to if and how VDAC mediates the transfer of apoptotic proteins from the inter-membranal space to the cytosol. The diameter of the VDAC pore is only about 2.5-3 nm, insufficient for the passage of a folded protein like cytochrome c. New work, however, suggests that pore formation involves the assembly of homo-oligomers of VDAC or hetero-oligomers composed of VDAC and pro-apoptotic proteins, such as Bax. Thus, VDAC appears to represent a convergence point for a variety of cell survival and cell death signals. This review provides insight into the central role of VDAC in mammalian cell life and death, emphasizing VDAC function in the regulation of mitochondria-mediated apoptosis and, as such, its potential as a rational target for new therapeutics.