Mitochondrial membrane cholesterol, the voltage dependent anion channel (VDAC), and the Warburg effect

Improved Highly Mobile Membrane Mimetic Model for Investigating Protein-Cholesterol Interactions

Cholesterol (CHL) plays an integral role in modulating the function and activity of various mammalian membrane proteins. Due to the slow dynamics of lipids, conventional computational studies of protein-CHL interactions rely on either long-time scale atomistic simulations or coarse-grained approximations to sample the process. A highly mobile membrane mimetic (HMMM) has been developed to enhance lipid diffusion and thus used to facilitate the investigation of lipid interactions with peripheral membrane proteins and, with customized in silico solvents to replace phospholipid tails, with integral membrane proteins. Here, we report an updated HMMM model that is able to include CHL, a nonphospholipid component of the membrane, henceforth called HMMM-CHL. To this end, we had to optimize the effect of the customized solvents on CHL behavior in the membrane. Furthermore, the new solvent is compatible with simulations using force-based switching protocols. In the HMMM-CHL, both improved CHL dynamics and accelerated lipid diffusion are integrated. To test the updated model, we have applied it to the characterization of protein-CHL interactions in two membrane protein systems, the human β2-adrenergic receptor (β2AR) and the mitochondrial voltage-dependent anion channel 1 (VDAC-1). Our HMMM-CHL simulations successfully identified CHL binding sites and captured detailed CHL interactions in excellent consistency with experimental data as well as other simulation results, indicating the utility of the improved model in applications where an enhanced sampling of protein-CHL interactions is desired.

Mitochondrial Proteomes in Neural Cells: A Systematic Review

Mitochondria are ancient endosymbiotic double membrane organelles that support a wide range of eukaryotic cell functions through energy, metabolism, and cellular control. There are over 1000 known proteins that either reside within the mitochondria or are transiently associated with it. These mitochondrial proteins represent a functional subcellular protein network (mtProteome) that is encoded by mitochondrial and nuclear genomes and significantly varies between cell types and conditions. In neurons, the high metabolic demand and differential energy requirements at the synapses are met by specific modifications to the mtProteome, resulting in alterations in the expression and functional properties of the proteins involved in energy production and quality control, including fission and fusion. The composition of mtProteomes also impacts the localization of mitochondria in axons and dendrites with a growing number of neurodegenerative diseases associated with changes in mitochondrial proteins. This review summarizes the findings on the composition and properties of mtProteomes important for mitochondrial energy production, calcium and lipid signaling, and quality control in neural cells. We highlight strategies in mass spectrometry (MS) proteomic analysis of mtProteomes from cultured cells and tissue. The research into mtProteome composition and function provides opportunities in biomarker discovery and drug development for the treatment of metabolic and neurodegenerative disease.

E as in Enigma: The Mysterious Role of the Voltage-Dependent Anion Channel Glutamate E73

The voltage-dependent anion channel (VDAC) is the main passageway for ions and metabolites over the outer mitochondrial membrane. It was associated with many physiological processes, including apoptosis and modulation of intracellular Ca²⁺ signaling. The protein is formed by a barrel of 19 beta-sheets with an N-terminal helix lining the inner pore. Despite its large diameter, the channel can change its selectivity for ions and metabolites based on its open state to regulate transport into and out of mitochondria. VDAC was shown to be regulated by a variety of cellular factors and molecular partners including proteins, lipids and ions. Although the physiological importance of many of these modulatory effects are well described, the binding sites for molecular partners are still largely unknown. The highly symmetrical and sleek structure of the channel makes predictions of functional moieties difficult. However, one residue repeatedly sticks out when reviewing VDAC literature. A glutamate at position 73 (E73) located on the outside of the channel facing the hydrophobic membrane environment was repeatedly proposed to be involved in channel regulation on multiple levels. Here, we review the distinct hypothesized roles of E73 and summarize the open questions around this mysterious residue.

Proteomic characterization of a candidate polygenic driver of metabolism in non-small cell lung cancer

Proteome analysis revealed signatures of co-expressed upregulated metabolism proteins highly conserved between primary and non-small cell lung cancer (NSCLC) patient-derived xenograft tumors (Li et al. 2014, Nat. Communications 5:5469). The C10 signature is encoded by seven genes (ADSS, ATP2A2, CTPS1, IMPDH2, PKM2, PTGES3, SGPL1) and DNA alterations in C10-encoding genes are associated with longer survival in a subset of NSCLC. To explore the C10 signature as an oncogenic driver and address potential mechanisms of action, C10 protein expression and protein-protein interactions were determined. In independent NSCLC cohorts, the coordinated expression of C10 proteins was significant and mutations in C10 genes were associated with better outcome. Affinity purification-mass spectrometry and in vivo proximity-based biotin identification defined a C10 interactome involving 667 proteins including candidate drug targets and clusters associated with glycolysis, calcium homeostasis, and nucleotide and sphingolipid metabolism. DNA alterations in genes encoding C10 interactome components were also found to be associated with better survival. These data support the notion that the coordinated upregulation of the C10 signature impinges metabolic processes that collectively function as an oncogenic driver in NSCLC.

Aberrant Cholesterol Metabolism in Ovarian Cancer: Identification of Novel Therapeutic Targets

Cholesterol is an essential substance in mammalian cells, and cholesterol metabolism plays crucial roles in multiple biological functions. Dysregulated cholesterol metabolism is a metabolic hallmark in several cancers, beyond the Warburg effect. Reprogrammed cholesterol metabolism has been reported to enhance tumorigenesis, metastasis and chemoresistance in multiple cancer types, including ovarian cancer. Ovarian cancer is one of the most aggressive malignancies worldwide. Alterations in metabolic pathways are characteristic features of ovarian cancer; however, the specific role of cholesterol metabolism remains to be established. In this report, we provide an overview of the key proteins involved in cholesterol metabolism in ovarian cancer, including the rate-limiting enzymes in cholesterol biosynthesis, and the proteins involved in cholesterol uptake, storage and trafficking. Also, we review the roles of cholesterol and its derivatives in ovarian cancer and the tumor microenvironment, and discuss promising related therapeutic targets for ovarian cancer.

Cell-free electrophysiology of human VDACs incorporated into nanodiscs: An improved method

Voltage-dependent anion-selective channel (VDAC) is one of the main proteins of the outer mitochondrial membrane of all eukaryotes, where it forms aqueous, voltage-sensitive, and ion-selective channels. Its electrophysiological properties have been thoroughly analyzed with the planar lipid bilayer technique. To date, however, available results are based on isolations of VDACs from tissue or from recombinant VDACs produced in bacterial systems. It is well known that the cytosolic overexpression of highly hydrophobic membrane proteins often results in the formation of inclusion bodies containing insoluble aggregates. Purification of properly folded proteins and restoration of their full biological activity requires several procedures that considerably lengthen experimental times. To overcome these restraints, we propose a one-step reaction that combines in vitro cell-free protein expression with nanodisc technology to obtain human VDAC isoforms directly integrated in a native-like lipid bilayer. Reconstitution assays into artificial membranes confirm the reliability of this new methodological approach and provide results comparable to those of VDACs prepared with traditional protein isolation and reconstitution protocols. The use of membrane-mimicking nanodisc systems represents a breakthrough in VDAC electrophysiology and may be adopted to further structural studies.

Warburg's Ghost-Cancer's Self-Sustaining Phenotype: The Aberrant Carbon Flux in Cholesterol-Enriched Tumor Mitochondria via Deregulated Cholesterogenesis

Interpreting connections between the multiple networks of cell metabolism is indispensable for understanding how cells maintain homeostasis or transform into the decontrolled proliferation phenotype of cancer. Situated at a critical metabolic intersection, citrate, derived via glycolysis, serves as either a combustible fuel for aerobic mitochondrial bioenergetics or as a continuously replenished cytosolic carbon source for lipid biosynthesis, an essentially anaerobic process. Therein lies the paradox: under what conditions do cells control the metabolic route by which they process citrate? The Warburg effect exposes essentially the same dilemma—why do cancer cells, despite an abundance of oxygen needed for energy-generating mitochondrial respiration with citrate as fuel, avoid catabolizing mitochondrial citrate and instead rely upon accelerated glycolysis to support their energy requirements? This review details the genesis and consequences of the metabolic paradigm of a “truncated” Krebs/TCA cycle. Abundant data are presented for substrate utilization and membrane cholesterol enrichment in tumors that are consistent with criteria of the Warburg effect. From healthy cellular homeostasis to the uncontrolled proliferation of tumors, metabolic alterations center upon the loss of regulation of the cholesterol biosynthetic pathway. Deregulated tumor cholesterogenesis at the HMGR locus, generating enhanced carbon flux through the cholesterol synthesis pathway, is an absolute prerequisite for DNA synthesis and cell division. Therefore, expedited citrate efflux from cholesterol-enriched tumor mitochondria via the CTP/SLC25A1 citrate transporter is fundamental for sustaining the constant demand for cytosolic citrate that fuels the elevated flow of carbons from acetyl-CoA through the deregulated pathway of cholesterol biosynthesis.

"The Loss of Golden Touch": Mitochondria-Organelle Interactions, Metabolism, and Cancer

Mitochondria represent the energy hub of cells and their function is under the constant influence of their tethering with other subcellular organelles. Mitochondria interact with the endoplasmic reticulum, lysosomes, cytoskeleton, peroxisomes, and nucleus in several ways, ranging from signal transduction, vesicle transport, and membrane contact sites, to regulate energy metabolism, biosynthetic processes, apoptosis, and cell turnover. Tumorigenesis is often associated with mitochondrial dysfunction, which could likely be the result of an altered interaction with different cell organelles or structures. The purpose of the present review is to provide an updated overview of the links between inter-organellar communications and interactions and metabolism in cancer cells, with a focus on mitochondria. The very recent publication of several reviews on these aspects testifies the great interest in the area. Here, we aim at (1) summarizing recent evidence supporting that the metabolic rewiring and adaptation observed in tumors deeply affect organelle dynamics and cellular functions and vice versa; (2) discussing insights on the underlying mechanisms, when available; and (3) critically presenting the gaps in the field that need to be filled, for a comprehensive understanding of tumor cells’ biology. Chemo-resistance and druggable vulnerabilities of cancer cells related to the aspects mentioned above is also outlined.

Expression of voltage-dependent anion channels in endometrial cancer and its potential prognostic significance

The exchange of metabolites between mitochondria and cytosol occurs through pores formed by voltage-dependent anion channel proteins. Voltage-dependent anion channels appear to be master regulators of mitochondrial bioenergetics and the intracellular flow of energy. Deregulation of voltage-dependent anion channels expression is thought to be related to mitochondrial dysfunction in cancer. The aim of this study was to investigate the mRNA and protein expression levels of VDAC1, VDAC2, and VDAC3 in relation to clinicopathological characteristics of endometrial cancer as well as the prognostic significance of voltage-dependent anion channels expression for overall survival. VDAC1 and VDAC3 expressions were significantly higher in cancer compared to normal tissues. Kaplan-Meier analysis indicated that high expression of all VDAC genes or high VDAC2 protein level predicted poor overall survival. Multivariate analysis identified the VDAC1 and VDAC2 mRNA levels as well as VDAC2 protein level as independent prognostic factors. Our results suggest that increased expression of voltage-dependent anion channels correlates with tumor progression and may serve as a potential prognostic biomarker in endometrial cancer.

Intestinal Lipid Metabolism Genes Regulated by miRNAs

MicroRNAs (miRNAs) crucial roles in translation repression and post-transcriptional adjustments contribute to regulate intestinal lipid metabolism. Even though their actions in different metabolic tissues have been elucidated, their intestinal activity is yet unclear. We aimed to investigate intestinal miRNA-regulated lipid metabolism-related genes, by creating an intestinal-specific Dicer1 knockout (Int-Dicer1 KO) mouse model, with a depletion of microRNAs in enterocytes. The levels of 83 cholesterol and lipoprotein metabolism-related genes were assessed in the intestinal mucosa of Int-Dicer1 KO and Wild Type C57BL/6 (WT) littermates mice at baseline and 2 h after an oral lipid challenge. Among the 18 genes selected for further validation, Hmgcs2, Acat1 and Olr1 were found to be strong candidates to be modulated by miRNAs in enterocytes and intestinal organoids. Moreover, we report that intestinal miRNAs contribute to the regulation of intestinal epithelial differentiation. Twenty-nine common miRNAs found in the intestines were analyzed for their potential to target any of the three candidate genes found and validated by miRNA-transfection assays in Caco-2 cells. MiR-31-5p, miR-99b-5p, miR-200a-5p, miR-200b-5p and miR-425-5p are major regulators of these lipid metabolism-related genes. Our data provide new evidence on the potential of intestinal miRNAs as therapeutic targets in lipid metabolism-associated pathologies.

VDAC1 as Pharmacological Target in Cancer and Neurodegeneration: Focus on Its Role in Apoptosis

Cancer and neurodegeneration are different classes of diseases that share the involvement of mitochondria in their pathogenesis. Whereas the high glycolytic rate (the so-called Warburg metabolism) and the suppression of apoptosis are key elements for the establishment and maintenance of cancer cells, mitochondrial dysfunction and increased cell death mark neurodegeneration. As a main actor in the regulation of cell metabolism and apoptosis, VDAC may represent the common point between these two broad families of pathologies. Located in the outer mitochondrial membrane, VDAC forms channels that control the flux of ions and metabolites across the mitochondrion thus mediating the organelle's cross-talk with the rest of the cell. Furthermore, the interaction with both pro-apoptotic and anti-apoptotic factors makes VDAC a gatekeeper for mitochondria-mediated cell death and survival signaling pathways. Unfortunately, the lack of an evident druggability of this protein, since it has no defined binding or active sites, makes the quest for VDAC interacting molecules a difficult tale. Pharmacologically active molecules of different classes have been proposed to hit cancer and neurodegeneration. In this work, we provide an exhaustive and detailed survey of all the molecules, peptides, and microRNAs that exploit VDAC in the treatment of the two examined classes of pathologies. The mechanism of action and the potential or effectiveness of each compound are discussed.

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.

Mitochondria-Associated Membranes As Networking Platforms and Regulators of Cancer Cell Fate

The tight cross talk between two essential organelles of the cell, the endoplasmic reticulum (ER) and mitochondria, is spatially and functionally regulated by specific microdomains known as the mitochondria-associated membranes (MAMs). MAMs are hot spots of Ca2+ transfer between the ER and mitochondria, and emerging data indicate their vital role in the regulation of fundamental physiological processes, chief among them mitochondria bioenergetics, proteostasis, cell death, and autophagy. Moreover, and perhaps not surprisingly, it has become clear that signaling events regulated at the ER-mitochondria intersection regulate key processes in oncogenesis and in the response of cancer cells to therapeutics. ER-mitochondria appositions have been shown to dynamically recruit oncogenes and tumor suppressors, modulating their activity and protein complex formation, adapt the bioenergetic demand of cancer cells and to regulate cell death pathways and redox signaling in cancer cells. In this review, we discuss some emerging players of the ER-mitochondria contact sites in mammalian cells, the key processes they regulate and recent evidence highlighting the role of MAMs in shaping cell-autonomous and non-autonomous signals that regulate cancer growth.

The Voltage-Dependent Anion Channels (VDAC) of Mycobacterium avium phagosome are associated with bacterial survival and lipid export in macrophages

Mycobacterium avium subsp. hominissuis is associated with infection of immunocompromised individuals as well as patients with chronic lung disease. M. avium infects macrophages and actively interfere with the host killing machinery such as apoptosis and autophagy. Bacteria alter the normal endosomal trafficking, prevent the maturation of phagosomes and modify many signaling pathways inside of the macrophage by secreting effector molecules into the cytoplasm. To investigate whether M. avium needs to attach to the internal surface of the vacuole membrane before releasing efferent molecules, vacuole membrane proteins were purified and binding to the surface molecules present in intracellular bacteria was evaluated. The voltage-dependent anion channels (VDAC) were identified as components of M. avium vacuoles in macrophages. M. avium mmpL4 proteins were found to bind to VDAC-1 protein. The inactivation of VDAC-1 function either by pharmacological means or siRNA lead to significant decrease of M. avium survival. Although, we could not establish a role of VDAC channels in the transport of known secreted M. avium proteins, we demonstrated that the porin channels are associated with the export of bacterial cell wall lipids outside of vacuole. Suppression of the host phagosomal transport systems and the pathogen transporter may serve as therapeutic targets for infectious diseases.

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.

VDAC1 functions in Ca2+ homeostasis and cell life and death in health and disease

In the outer mitochondrial membrane (OMM), the voltage-dependent anion channel 1 (VDAC1) serves as a mitochondrial gatekeeper, controlling the metabolic and energy cross-talk between mitochondria and the rest of the cell. VDAC1 also functions in cellular Ca²⁺ homeostasis by transporting Ca²⁺ in and out of mitochondria. VDAC1 has also been recognized as a key protein in mitochondria-mediated apoptosis, contributing to the release of apoptotic proteins located in the inter-membranal space (IMS) and regulating apoptosis via association with pro- and anti-apoptotic members of the Bcl-2 family of proteins and hexokinase. VDAC1 is highly Ca²⁺-permeable, transporting Ca²⁺ to the IMS and thus modulating Ca²⁺ access to Ca²⁺ transporters in the inner mitochondrial membrane. Intra-mitochondrial Ca²⁺ controls energy metabolism via modulating critical enzymes in the tricarboxylic acid cycle and in fatty acid oxidation. Ca²⁺ also determines cell sensitivity to apoptotic stimuli and promotes the release of pro-apoptotic proteins. However, the precise mechanism by which intracellular Ca²⁺ mediates apoptosis is not known. Here, the roles of VDAC1 in mitochondrial Ca²⁺ homeostasis are presented while emphasizing a new proposed mechanism for the mode of action of pro-apoptotic drugs. This view, proposing that Ca²⁺-dependent enhancement of VDAC1 expression levels is a major mechanism by which apoptotic stimuli induce apoptosis, position VDAC1 oligomerization at a molecular focal point in apoptosis regulation. The interactions of VDAC1 with many proteins involved in Ca²⁺ homeostasis or regulated by Ca²⁺, as well as VDAC-mediated control of cell life and death and the association of VDAC with disease, are also presented.

VDAC in cancer

The voltage-dependent anion channel (VDAC) is a pore located at the outer membrane of the mitochondrion. It allows the entry and exit of numerous ions and metabolites between the cytosol and the mitochondrion. Flux through the pore occurs in an active way: first, it depends on the open or closed state and second, on the negative or positive charges of the different ion species passing through the pore. The flux of essential metabolites, such as ATP, determines the functioning of the mitochondria to a noxious stimulus. Moreover, VDAC acts as a platform for many proteins and in so doing supports glycolysis and prevents apoptosis by interacting with hexokinase, or members of the Bcl-2 family, respectively. VDAC is thus involved in the choice the cells make to survive or die, which is particularly relevant to cancer cells. For these reasons, VDAC has become a potential therapeutic target to fight cancer but also other diseases in which mitochondrial metabolism is modified. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Novel Biomarker Proteins in Chronic Lymphocytic Leukemia: Impact on Diagnosis, Prognosis and Treatment

In many cancers, cells undergo re-programming of metabolism, cell survival and anti-apoptotic defense strategies, with the proteins mediating this reprogramming representing potential biomarkers. Here, we searched for novel biomarker proteins in chronic lymphocytic leukemia (CLL) that can impact diagnosis, treatment and prognosis by comparing the protein expression profiles of peripheral blood mononuclear cells from CLL patients and healthy donors using specific antibodies, mass spectrometry and binary logistic regression analyses and other bioinformatics tools. Mass spectrometry (LC-HR-MS/MS) analysis identified 1,360 proteins whose expression levels were modified in CLL-derived lymphocytes. Some of these proteins were previously connected to different cancer types, including CLL, while four other highly expressed proteins were not previously reported to be associated with cancer, and here, for the first time, DDX46 and AK3 are linked to CLL. Down-regulation expression of two of these proteins resulted in cell growth inhibition. High DDX46 expression levels were associated with shorter survival of CLL patients and thus can serve as a prognosis marker. The proteins with modified expression include proteins involved in RNA splicing and translation and particularly mitochondrial proteins involved in apoptosis and metabolism. Thus, we focused on several metabolism- and apoptosis-modulating proteins, particularly on the voltage-dependent anion channel 1 (VDAC1), regulating both metabolism and apoptosis. Expression levels of Bcl-2, VDAC1, MAVS, AIF and SMAC/Diablo were markedly increased in CLL-derived lymphocytes. VDAC1 levels were highly correlated with the amount of CLL-cancerous CD19+/CD5+ cells and with the levels of all other apoptosis-modulating proteins tested. Binary logistic regression analysis demonstrated the ability to predict probability of disease with over 90% accuracy. Finally, based on the changes in the levels of several proteins in CLL patients, as revealed from LC-HR-MS/MS, we could distinguish between patients in a stable disease state and those who would be later transferred to anti-cancer treatments. The over-expressed proteins can thus serve as potential biomarkers for early diagnosis, prognosis, new targets for CLL therapy, and treatment guidance of CLL, forming the basis for personalized therapy.

MicroRNA-7 downregulates the oncogene VDAC1 to influence hepatocellular carcinoma proliferation and metastasis

Recent studies have been shown that voltage-dependent anion channel 1 (VDAC1) plays an important role in carcinogenesis. However, its molecular biological function in hepatocellular carcinoma (HCC) has not been entirely clarified. This study investigated the expression of VDAC1 in HCC and its prognostic value for HCC patients. Furthermore, we also identify the relevant VDAC1 direct target. Western blot, real-time quantitative PCR (qRT-PCR), and immunohistochemical (IHC) staining were performed to detect the expression of VDAC1 in HCC. Furthermore, the relationship between the VDAC1 level and clinicopathological features and prognostic values was explored. The effects of VDAC1 on HCC cell proliferation, migration, and invasion were also investigated in vitro. Predicted target gene of VDAC1 was determined by dual-luciferase reporter assay, qRT-PCR, and Western blot analyses. Our results revealed elevated VDAC1 messenger RNA (mRNA) (P = 0.0020) and protein (P = 0.0035) expression in tumor tissue samples compared with paired adjacent non-tumorous tissue samples. High VDAC1 expression was correlated with distant metastasis (P = 0.025), differentiation (P = 0.002), and advanced tumor stage (P = 0.004) in HCC patients. Kaplan-Meier survival analysis demonstrated that high expression of VDAC1 was significantly correlated with a poor prognosis for HCC patients (P < 0.001). The multivariate analysis revealed that VDAC1 expression was an independent prognostic factor of the overall survival rate of HCC patients. Furthermore, knockdown of VDAC1 inhibits HCC cell proliferation, migration, and invasion in vitro. Moreover, further study revealed that miR-7 was a putative target of VDAC1. Our study suggested that miR-7 suppressed the expression of VDAC1. VDAC1 plays an important role in tumor progression and may be used as a potential role in the prognosis of HCC patients.

The vitamin D receptor inhibits the respiratory chain, contributing to the metabolic switch that is essential for cancer cell proliferation

We recently described the mitochondrial localization and import of the vitamin D receptor (VDR) in actively proliferating HaCaT cells for the first time, but its role in the organelle remains unknown. Many metabolic intermediates that support cell growth are provided by the mitochondria; consequently, the identification of proteins that regulate mitochondrial metabolic pathways is of great interest, and we sought to understand whether VDR may modulate these pathways. We genetically silenced VDR in HaCaT cells and studied the effects on cell growth, mitochondrial metabolism and biosynthetic pathways. VDR knockdown resulted in robust growth inhibition, with accumulation in the G0G1 phase of the cell cycle and decreased accumulation in the M phase. The effects of VDR silencing on proliferation were confirmed in several human cancer cell lines. Decreased VDR expression was consistently observed in two different models of cell differentiation. The impairment of silenced HaCaT cell growth was accompanied by sharp increases in the mitochondrial membrane potential, which sensitized the cells to oxidative stress. We found that transcription of the subunits II and IV of cytochrome c oxidase was significantly increased upon VDR silencing. Accordingly, treatment of HaCaT cells with vitamin D downregulated both subunits, suggesting that VDR may inhibit the respiratory chain and redirect TCA intermediates toward biosynthesis, thus contributing to the metabolic switch that is typical of cancer cells. In order to explore this hypothesis, we examined various acetyl-CoA-dependent biosynthetic pathways, such as the mevalonate pathway (measured as cholesterol biosynthesis and prenylation of small GTPases), and histone acetylation levels; all of these pathways were inhibited by VDR silencing. These data provide evidence of the role of VDR as a gatekeeper of mitochondrial respiratory chain activity and a facilitator of the diversion of acetyl-CoA from the energy-producing TCA cycle toward biosynthetic pathways that are essential for cellular proliferation.

Mitochondrial cholesterol: mechanisms of import and effects on mitochondrial function

Mitochondria require cholesterol for biogenesis and membrane maintenance, and for the synthesis of steroids, oxysterols and hepatic bile acids. Multiple pathways mediate the transport of cholesterol from different subcellular pools to mitochondria. In steroidogenic cells, the steroidogenic acute regulatory protein (StAR) interacts with a mitochondrial protein complex to mediate cholesterol delivery to the inner mitochondrial membrane for conversion to pregnenolone. In non-steroidogenic cells, several members of a protein family defined by the presence of a StAR-related lipid transfer (START) domain play key roles in the delivery of cholesterol to mitochondrial membranes. Subdomains of the endoplasmic reticulum (ER), termed mitochondria-associated ER membranes (MAM), form membrane contact sites with mitochondria and may contribute to the transport of ER cholesterol to mitochondria, either independently or in conjunction with lipid-transfer proteins. Model systems of mitochondria enriched with cholesterol in vitro and mitochondria isolated from cells with (patho)physiological mitochondrial cholesterol accumulation clearly demonstrate that mitochondrial cholesterol levels affect mitochondrial function. Increased mitochondrial cholesterol levels have been observed in several diseases, including cancer, ischemia, steatohepatitis and neurodegenerative diseases, and influence disease pathology. Hence, a deeper understanding of the mechanisms maintaining mitochondrial cholesterol homeostasis may reveal additional targets for therapeutic intervention. Here we give a brief overview of mitochondrial cholesterol import in steroidogenic cells, and then focus on cholesterol trafficking pathways that deliver cholesterol to mitochondrial membranes in non-steroidogenic cells. We also briefly discuss the consequences of increased mitochondrial cholesterol levels on mitochondrial function and their potential role in disease pathology.

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.

Silencing VDAC1 Expression by siRNA Inhibits Cancer Cell Proliferation and Tumor Growth In Vivo

Alterations in cellular metabolism and bioenergetics are vital for cancer cell growth and motility. Here, the role of the mitochondrial protein voltage-dependent anion channel (VDAC1), a master gatekeeper regulating the flux of metabolites and ions between mitochondria and the cytoplasm, in regulating the growth of several cancer cell lines was investigated by silencing VDAC1 expression using small interfering RNA (siRNA). A single siRNA specific to the human VDAC1 sequence at nanomolar concentrations led to some 90% decrease in VDAC1 levels in the lung A549 and H358, prostate PC-3, colon HCT116, glioblastoma U87, liver HepG2, and pancreas Panc-1 cancer cell lines. VDAC1 silencing persisted 144 hours post-transfection and resulted in profound inhibition of cell growth in cancer but not in noncancerous cells, with up to 90% inhibition being observed over 5 days that was prolonged by a second transfection. Cells expressing low VDAC1 levels showed decreased mitochondrial membrane potential and adenoside triphosphate (ATP) levels, suggesting limited metabolite exchange between mitochondria and cytosol. Moreover, cells silenced for VDAC1 expression showed decreased migration, even in the presence of the wound healing accelerator basic fibroblast growth factor (bFGF). VDAC1-siRNA inhibited cancer cell growth in a Matrigel-based assay in host nude mice. Finally, in a xenograft lung cancer mouse model, chemically modified VDAC1-siRNA not only inhibited tumor growth but also resulted in tumor regression. This study thus shows that VDAC1 silencing by means of RNA interference (RNAi) dramatically inhibits cancer cell growth and tumor development by disabling the abnormal metabolic behavior of cancer cells, potentially paving the way for a more effective pipeline of anticancer drugs.

Direct modulation of the outer mitochondrial membrane channel, voltage-dependent anion channel 1 (VDAC1) by cannabidiol: a novel mechanism for cannabinoid-induced cell death

Cannabidiol (CBD) is a non-psychoactive plant cannabinoid that inhibits cell proliferation and induces cell death of cancer cells and activated immune cells. It is not an agonist of the classical CB1/CB2 cannabinoid receptors and the mechanism by which it functions is unknown. Here, we studied the effects of CBD on various mitochondrial functions in BV-2 microglial cells. Our findings indicate that CBD treatment leads to a biphasic increase in intracellular calcium levels and to changes in mitochondrial function and morphology leading to cell death. Density gradient fractionation analysis by mass spectrometry and western blotting showed colocalization of CBD with protein markers of mitochondria. Single-channel recordings of the outer-mitochondrial membrane protein, the voltage-dependent anion channel 1 (VDAC1) functioning in cell energy, metabolic homeostasis and apoptosis revealed that CBD markedly decreases channel conductance. Finally, using microscale thermophoresis, we showed a direct interaction between purified fluorescently labeled VDAC1 and CBD. Thus, VDAC1 seems to serve as a novel mitochondrial target for CBD. The inhibition of VDAC1 by CBD may be responsible for the immunosuppressive and anticancer effects of CBD.

Mitochondrial membrane lipid remodeling in pathophysiology: a new target for diet and therapeutic interventions

Mitochondria are arbiters in the fragile balance between cell life and death. These organelles present an intricate membrane system, with a peculiar lipid composition and displaying transverse as well as lateral asymmetry. Some lipids are synthesized inside mitochondria, while others have to be imported or acquired in the form of precursors. Here, we review different processes, including external interventions (e.g., diet) and a range of biological events (apoptosis, disease and aging), which may result in alterations of mitochondrial membrane lipid content. Cardiolipin, the mitochondria lipid trademark, whose biosynthetic pathway is highly regulated, will deserve special attention in this review. The modulation of mitochondrial membrane lipid composition, especially by diet, as a therapeutic strategy for the treatment of some pathologies will be also addressed.

Overexpression of a key regulator of lipid homeostasis, Scap, promotes respiration in prostate cancer cells

Prostate metabolism is unique, characterised by cholesterol accumulation and reduced respiration. Are these related? We modulated cholesterol levels and despite changes in mitochondrial cholesterol content, we saw no effects on lactate production or respiration. Instead, these features may be related via sterol regulatory element-binding protein 2 (SREBP-2), the master transcriptional regulator of cholesterol synthesis. SREBP-2 diverts acetyl-CoA into cholesterol synthesis and may thus reduce respiration. We examined LNCaP cells overexpressing the SREBP-2 regulator, Scap: although having higher SREBP-2 activity, these cells displayed higher respiration. This striking observation warrants further investigation. Given that SREBP-2 and Scap are regulated by factors driving prostate growth, exploring this observation further could shed light on prostate carcinogenesis.

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.

Ouabain targets the unfolded protein response for selective killing of HepG2 cells during glucose deprivation

Ouabain is a cardiotonic steroid and specific inhibitor of the Na(+)/K(+)-ATPase. The relationship between ouabain treatment and the unfolded protein response (UPR) in cells is not precisely understood. Therefore, we studied the possible effects of ouabain on proliferation, apoptosis, and the UPR. HepG2 cells were cultured overnight and then treated with various concentrations of ouabain (0.75 to 750 nM) in the absence or presence of 10 mM 2-deoxyglucose (2-DG) for 48 hours. We also used real-time polymerase chain reaction to obtain quantitative measurements of expression levels of Grp78, Grp94, CHOP, MTJ-1, HKII, MDR-1, MRP-1, HO-1, and Par-4. Cell number, viability, and proliferation of HepG2 cells were monitored with a real-time cell analyzer system (xCELLigence). We show that ouabain modulates the UPR transcription program and induces cell death in glucose-deprived tumor cells. Ouabain at all concentrations showed no cytotoxicity whereas all concentrations were very effective under 2-DG stress conditions. Our findings show that disruption of the UPR during glucose deprivation could be an attractive approach for selective cancer cell killing and could provide a chemical basis for developing UPR-targeting drugs against solid tumors. Ouabain use as an adjunct to conventional cancer therapy also warrants vigorous investigation.

(+)α-Tocopheryl succinate inhibits the mitochondrial respiratory chain complex I and is as effective as arsenic trioxide or ATRA against acute promyelocytic leukemia in vivo

The vitamin E derivative (+)α-tocopheryl succinate (α-TOS) exerts pro-apoptotic effects in a wide range of tumors and is well tolerated by normal tissues. Previous studies point to a mitochondrial involvement in the action mechanism; however, the early steps have not been fully elucidated. In a model of acute promyelocytic leukemia (APL) derived from hCG-PML-RARα transgenic mice, we demonstrated that α-TOS is as effective as arsenic trioxide or all-trans retinoic acid, the current gold standards of therapy. We also demonstrated that α-TOS induces an early dissipation of the mitochondrial membrane potential in APL cells and studies with isolated mitochondria revealed that this action may result from the inhibition of mitochondrial respiratory chain complex I. Moreover, α-TOS promoted accumulation of reactive oxygen species hours before mitochondrial cytochrome c release and caspases activation. Therefore, an in vivo antileukemic action and a novel mitochondrial target were revealed for α-TOS, as well as mitochondrial respiratory complex I was highlighted as potential target for anticancer therapy.

Systems-scale analysis reveals pathways involved in cellular response to methamphetamine

Background

Methamphetamine (METH), an abused illicit drug, disrupts many cellular processes, including energy metabolism, spermatogenesis, and maintenance of oxidative status. However, many components of the molecular underpinnings of METH toxicity have yet to be established. Network analyses of integrated proteomic, transcriptomic and metabolomic data are particularly well suited for identifying cellular responses to toxins, such as METH, which might otherwise be obscured by the numerous and dynamic changes that are induced.

Methodology/Results

We used network analyses of proteomic and transcriptomic data to evaluate pathways in Drosophila melanogaster that are affected by acute METH toxicity. METH exposure caused changes in the expression of genes involved with energy metabolism, suggesting a Warburg-like effect (aerobic glycolysis), which is normally associated with cancerous cells. Therefore, we tested the hypothesis that carbohydrate metabolism plays an important role in METH toxicity. In agreement with our hypothesis, we observed that increased dietary sugars partially alleviated the toxic effects of METH. Our systems analysis also showed that METH impacted genes and proteins known to be associated with muscular homeostasis/contraction, maintenance of oxidative status, oxidative phosphorylation, spermatogenesis, iron and calcium homeostasis. Our results also provide numerous candidate genes for the METH-induced dysfunction of spermatogenesis, which have not been previously characterized at the molecular level.

Conclusion

Our results support our overall hypothesis that METH causes a toxic syndrome that is characterized by the altered carbohydrate metabolism, dysregulation of calcium and iron homeostasis, increased oxidative stress, and disruption of mitochondrial functions.

The autophagic tumor stroma model of cancer: Role of oxidative stress and ketone production in fueling tumor cell metabolism

A loss of stromal Cav-1 in the tumor fibroblast compartment is associated with early tumor recurrence, lymph-node metastasis, and tamoxifen-resistance, resulting in poor clinical outcome in breast cancer patients. Here, we have used Cav-1 (-/-) null mice as a pre-clinical model for this "lethal tumor micro-environment." Metabolic profiling of Cav-1 (-/-) mammary fat pads revealed the upregulation of numerous metabolites (nearly 100), indicative of a major catabolic phenotype. Our results are consistent with the induction of oxidative stress, mitochondrial dysfunction, and autophagy/mitophagy. The two most prominent metabolites that emerged from this analysis were ADMA (asymmetric dimethyl arginine) and BHB (beta-hydroxybutyrate; a ketone body), which are markers of oxidative stress and mitochondrial dysfunction, respectively. Transcriptional profiling of Cav-1 (-/-) stromal cells and human tumor stroma from breast cancer patients directly supported an association with oxidative stress, mitochondrial dysfunction, and autophagy/mitophagy, as well as ADMA and ketone production. MircoRNA profiling of Cav-1 (-/-) stromal cells revealed the upregulation of two key cancer-related miR's, namely miR-31 and miR-34c. Consistent with our metabolic findings, these miR's are associated with oxidative stress (miR-34c) or activation of the hypoxic response/HIF1a (miR-31), which is sufficient to drive authophagy/mitophagy. Thus, via an unbiased comprehensive analysis of a lethal tumor micro-environment, we have identified a number of candidate biomarkers (ADMA, ketones, and miR-31/34c) that could be used to identify high-risk cancer patients at diagnosis, for treatment stratification and/or for evaluating therapeutic efficacy during anti-cancer therapy. We propose that the levels of these key biomarkers (ADMA, ketones/BHB, miR-31, and miR-34c) could be (1) assayed using serum or plasma from cancer patients, or (2) performed directly on excised tumor tissue. Importantly, induction of oxidative stress and autophagy/mitophagy in the tumor stromal compartment provides a means by which epithelial cancer cells can directly "feed off" of stromal-derived essential nutrients, chemical building blocks (amino acids, nucleotides), and energy-rich metabolites (glutamine, pyruvate, ketones/BHB), driving tumor progression and metastasis. Essentially, aggressive cancer cells are "eating" the cancer-associated fibroblasts via autophagy/mitophagy in the tumor micro-environment. Lastly, we discuss that this "Autophagic Tumor Stroma Model of Cancer Metabolism" provides a viable solution to the "Autophagy Paradox" in cancer etiology and chemo-therapy.

The Akt-SREBP nexus: cell signaling meets lipid metabolism

Phosphatidylinositol 3'-kinase (PI3K) and Akt are signaling kinases involved in cell survival and proliferation. Recent evidence suggests that PI3K/Akt activates the sterol-regulatory element-binding proteins (SREBPs), master transcriptional regulators of lipid metabolism. The precise molecular mechanisms are controversial and differ between SREBP isoforms; proposed mechanisms include increased trafficking and processing of SREBP, reduced degradation, and involvement of the downstream signaling hub, mammalian target of rapamycin complex 1 (mTORC1). In this report, we explore the various mechanistic links between Akt and SREBP. We consider this relationship in diseases where Akt and lipids play crucial roles, including diabetes, viral infections and cancer, suggesting that this Akt-SREBP link provides fresh insights into human health and disease.

Cholesterol as an evolutionary response to living with oxygen

Although often considered in a negative light, cholesterol is an essential molecule with unusually diverse functions. Cholesterol and related sterols (ergosterol in yeast, phytosterols in plants) is considered a hallmark of eukaryotes, and may even have triggered the evolution of multicellular organisms. Synthesis of cholesterol is an extremely oxygen-intensive process and requires sufficient terrestrial oxygen to proceed. In turn, several lines of evidence support the argument that cholesterol evolved at least in part as an adaptation to the hazards of oxygen. This evolutionary perspective usefully informs medical research on cholesterol to address health-related issues, as illustrated by examples drawn from three prominent human diseases: cataracts, heart disease, and cancer.

Examination of the brain mitochondrial lipidome using shotgun lipidomics

Contamination from subcellular organelles and myelin has hindered attempts to characterize the lipidome of brain mitochondria. A high degree of mitochondrial purity is required for accurate measurements of the content and molecular species composition of mitochondrial lipids. We devised a discontinuous Ficoll and sucrose gradient procedure for the isolation and purification of brain mitochondria free from any detectable contamination. Shotgun lipidomics was used to analyze the lipid composition of the brain mitochondria. These procedures can be used to determine whether intrinsic lipid abnormalities underlie mitochondrial dysfunction associated with neurological and neurodegenerative diseases.

Special relationship between sterols and oxygen: were sterols an adaptation to aerobic life?

A fascinating link between sterols and molecular oxygen (O(2)) has been a common thread running through the fundamental work of Konrad Bloch, who elucidated the biosynthetic pathway for cholesterol, to recent work supporting a role of sterols in the sensing of O(2). Synthesis of sterols by eukaryotes is an O(2)-intensive process. In this review, we argue that increased levels of O(2) in the atmosphere not only made the evolution of sterols possible, but that these sterols may in turn have provided the eukaryote with an early defence mechanism against O(2). The idea that nature crafted sterols as a feedback loop to adapt to, or help protect against, the hazards of O(2) is novel and enticing. We marshal several lines of evidence to support this thesis: (1) coincidence of atmospheric O(2) and sterol evolution; (2) sterols regulate O(2) entry into eukaryotic cells and organelles; (3) sterols act as O(2) sensors across eukaryotic life; (4) sterols serve as a primitive cellular defence against O(2) (including reactive oxygen species). Therefore, sterols may have evolved in eukaryotes partially as an adaptive response to the rise of terrestrial O(2), rather than merely as a consequence of it.

A proteomic study reveals unspecific apoptosis induction and reduction of glycolytic enzymes by the phosphorothioate antisense oligonucleotide oblimersen in human melanoma cells

The question of specificity and the elucidation of the exact molecular mechanism of action of post-transcriptional gene silencing agents are two major challenges for their establishment as therapeutics. A proteomic off-target effect study (2-DE with MS) in combination with DIGE comparing the phosphorothioate antisense oligonucleotide oblimersen (Genasense, G3139) to a Bcl-2-targeting siRNA-sequence on human melanoma cells showed that additional off-target effects contribute to the apoptotic effect of oblimersen. When both oligonucleotides were transfected with lipofectamine 2000, only oblimersen increased apoptosis as determined by annexin staining and caspase activity measurement. In contrast to the highly specific siRNA, the expression level of a number of proteins was found to be altered after oblimersen treatment. Several proteins linked to apoptosis and stress response, among those galectin-1, cofilin-1, GRP78, HSP60, nucleophosmin, and peroxiredoxins, were identified and found to be down-regulated after oblimersen treatment. A down-regulation of enolase-1 and three other glycolytic enzymes indicates a reversion of the cancer-related Warburg effect. The observed effects may be caused by a phosphorothioate mediated blockage of the mitochondrial voltage dependent anion channel (VDAC).