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Background: Breast cancer is a very common cancer with significant premature mortality in women. In this study, we show that HKDC1 expression in breast cancer cells is increased significantly. We aim to investigate the detailed mechanism for the regulation of HKDC1 expression and its potential contribution to tumorigenesis. Methods: Gene expression was evaluated by real time PCR, western blotting, and immunohistochemistry. The mechanism for PGC1β/SREBP1-mediated HKDC1 expression was investigated using luciferase reporter assay, chromatin immunoprecipitation, and siRNA techniques. In addition, HKDC1 was overexpressed or knocked down by lentivirus to evaluate the potential effect on in vitro cell proliferation, glucose uptake, mitochondrial function, apoptosis, and reactive oxygen species (ROS) formation. Furthermore, an in vivo xenograft tumor development study was employed to investigate the effect of HKDC1 on tumor growth and mouse survival. Results: HKDC1 is highly expressed in both breast cancer cells and clinical tumor tissues. HKDC1 expression is upregulated and co-activated by PGC1β through SREBP1 binding motif on the HKDC1 promoter. HKDC1 is located on the mitochondrial membrane and regulates the permeability transition pore opening by binding with VDAC1, subsequently modulating glucose uptake and cell proliferation. Overexpression of HKDC1 increases while knockdown of HKDC1 decreases in vitro breast cancer cell proliferation and in vivo tumor growth, metastasis, and mouse survival. Conclusions: PGC1β regulates breast cancer tumor growth and metastasis by SREBP1-mediated HKDC1 expression. This provides a novel therapeutic strategy through targeting the PGC1β/HKDC1 signaling pathway for breast cancer treatment.

The voltage-dependent anion channel (VDAC) is well known as the pathway for passive diffusion of anionic hydrophilic mitochondrial metabolites across the outer membrane, but a more complex functionality of the three isoforms of VDAC has emerged, as addressed in the Frontiers in Oncology Research Topic on “Uncovering the Function of the Mitochondrial Protein VDAC in Health and Disease: from Structure-Function to Novel Therapeutic Strategies.” VDAC as the single most abundant protein in mitochondrial outer membranes is typically involved in isoform-specific interactions of the mitochondrion with its surroundings as, for example, during mitochondria-dependent pathways of cell death. VDAC closure can also act as an adjustable limiter (governator) of global mitochondrial metabolism, as during hepatic ethanol metabolism to promote selective oxidation of membrane-permeant acetaldehyde. In cancer cells, high free tubulin inhibits VDAC1 and VDAC2, contributing to suppression of mitochondrial function in the Warburg phenomenon. Erastin, the canonical inducer of ferroptosis, opens VDAC in the presence of tubulin and hyperpolarizes mitochondria, leading to mitochondrial production of reactive oxygen species, mitochondrial dysfunction, and cell death. Our understanding of VDAC function continues to evolve.

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.

In the outer mitochondrial membrane (OMM), the voltage-dependent anion channel 1 (VDAC1) functions in cellular Ca2+ homeostasis by mediating the transport of Ca2+ in and out of mitochondria. VDAC1 is highly Ca2+-permeable and modulates Ca2+ access to the mitochondrial inter-membrane space. Intra-mitochondrial Ca2+ controls energy metabolism by enhancing the rate of NADH production via modulating critical enzymes in the tricarboxylic acid (TCA) cycle and fatty acid oxidation. Mitochondrial [Ca2‏+] is regarded as an important determinant of cell sensitivity to apoptotic stimuli and was proposed to act as a ‘priming signal’, sensitizing the organelle and promoting the release of pro-apoptotic proteins. However, the precise mechanism by which intracellular Ca2+ ([Ca2+]i) mediates apoptosis is not known. Here, we review the roles of VDAC1 in mitochondrial Ca2+ homeostasis and in apoptosis. Accumulated evidence shows that apoptosis-inducing agents act by increasing [Ca2+]i and that this, in turn, augments VDAC1 expression levels. Thus, a new concept of how increased [Ca2+]i activates apoptosis is postulated. Specifically, increased [Ca2+]i enhances VDAC1 expression levels, followed by VDAC1 oligomerization, cytochrome c release and subsequently, apoptosis. Evidence supporting this new model suggesting that up-regulation of VDAC1 expression constitutes a major mechanism by which apoptotic stimuli induce apoptosis, with VDAC1 oligomerization being a molecular focal point in apoptosis regulation is presented. A new proposed mechanism of pro-apoptotic drug action, namely Ca2+-dependent enhancement of VDAC1 expression, provides a platform for developing a new class of anti-cancer drugs modulating VDAC1 levels via the promoter, and for overcoming the resistance of cancer cells to chemotherapy.

It is becoming increasingly apparent that mitochondria dysfunction plays an important role in the pathogenesis of Huntington’s disease (HD), but the underlying mechanism is still elusive. Thus, there is a still need for further studies concerning the upstream events in the mitochondria dysfunction that could contribute to cell death observed in HD. Taking into account the fundamental role of the voltage-dependent anion-selective channel (VDAC) in mitochondria functioning, it is reasonable to consider the channel as a crucial element in HD etiology. Therefore, we applied inducible PC12 cell model of HD to determine the relationship between the effect of expression of wild type and mutant huntingtin (Htt and mHtt, respectively) on cell survival and mitochondria functioning in intact cells under conditions of undergoing cell divisions. Because after 48 h of Htt and mHtt expression differences in mitochondria functioning co-occurred with differences in the cell viability, we decided to estimate the effect of Htt and mHtt expression lasted for 48 h on VDAC functioning. Therefore, we isolated VDAC from the cells and tested the preparations by black lipid membrane system. We observed that the expression of mHtt, but not Htt, resulted in changes of the open state conductance and voltage-dependence when compared to control cells cultured in the absence of the expression. Importantly, for all the VDAC preparations, we observed a dominant quantitative content of VDAC1, and the quantitative relationships between VDAC isoforms were not changed by Htt and mHtt expression. Thus, Htt and mHtt-mediated functional changes of VDAC, being predominantly VDAC1, which occur shortly after these protein appearances in cells, may result in differences concerning mitochondria functioning and viability of cells expressing Htt and mHtt. The assumption is important for better understanding of cytotoxicity as well as cytoprotection mechanisms of potential clinical application.

Mitochondria are fascinating organelles, at least in part, because two membranes with very different permeability properties and functions surround them. These membranes contribute to the intracellular compartmentation between mitochondria and the cytosol in eukaryotic cells. Thus, lipids (e.g., phospholipids, cholesterol, cardiolipin), membrane proteins [e.g., voltage-dependent anion channel (VDAC), ion exchangers, mitochondrial carriers such as adenine nucleotide translocase (ANT)], as well as soluble intermembrane space proteins cooperate to allow the channeling of metabolites (e.g., ATP, ADP), ions (e.g., calcium, potassium, sodium) and water across mitochondrial membranes. An increasing body of literature suggests that mitochondrial proteins behave also as control point in oncology (1, 2). For example, they can be involved (i) in the metabolic shift of cancer cells from oxidative phosphorylation to glycolysis via its binding to cytosolic proteins such as hexokinase II (i.e., Warburg effect), (ii) in the control of calcium fluxes between the endoplasmic reticulum (ER) and the mitochondrion via interaction with the inositol-3-phosphate receptor (IP3R), (iii) in the control of inner mitochondrial membrane potential (ΔΨm) via interaction with tubulin and finally, (iv) in the regulation of mitochondrial membrane permeability via interaction with Bax/Bcl-2 family members and/or the mitochondrial permeability transition pore complex (PTPC). Moreover, some proteins, via a shift in their expression, appear to be critical for the cancer cell response to chemotherapy (e.g., cisplatin, oxaliplatin, carboplatin). However, how these diverse functions are modulated and coordinated in cancer cells to control the balance between life and death is still largely unknown and requires extensive review and discussion to broaden our view of the role of mitochondrial proteins in oncology, the role of upstream signaling pathways controlling mitochondrial function and how these proteins and/or signaling pathways might be selective target for anti-cancer therapy. Thus, this research topic presents seven manuscripts, which reviewed the current knowledge of mitochondrial proteins and polyprotein complexes as druggable target for anti-cancer therapy and also explored new putative targets. Thus, Suh et al. reviewed the literature on the anti-cancer potential of the modulation of the permeability transition pore, a megachannel in the inner membrane (3). Sutendra and Michelakis proposed to reverse mitochondrial suppression, which is an hallmark of many cancer cells, with metabolic-modulating drugs, like pyruvate dehydrogenase kinase (PDK) inhibitors or M2 isoenzyme of pyruvate kinase (PKM2) activators as a novel anti-cancer strategy (4). In a similar perspective, Rasola and Chiara discussed the mechanistic interactions among oncogenic kinase pathways, glycogen synthase kinase (GSK-3) activity and subsequent modulation of mitochondrial functions that shape the pro-survival phenotype of cancer cells, such as control of redox homeostasis and inhibition of the mitochondrial permeability transition pore (5). In addition, Shoshan-Barmatz et al. review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to cancer (6). Alternatively, Fulda proposed to shift the balance of mitochondrial apoptosis to by pass evasion of apoptosis and treatment resistance in human cancers (7). Ishii et al. reviewed the accumulating knowledge on pyrvinium pamoate, an FDA-approved anthelmintic, as an anti-cancer drug targeting mitochondrial respiration (8). Interestingly, this drug acts synergistically with dexamethasone to block the proliferation if human myeloma cells and might be used in combination. In a more exploratory approach, Kang and Pervaiz explored the crosstalk between Bcl-2 family and Ras family small GTPases (9). They analyzed their work and the recent literature to show how this molecular crosstalk can regulate the cell fate. Previously, they showed a converging role of Rac1 and Bcl-2 to promote a pro-oxidant state, frequently observed in cancer cells. Despite the complexity of GTPases family, they argued that the modulation of the physical interactions between guanine exchange factors (GEF) and GTPases could open new avenues for future redox-based therapeutic designs. We hope that this research topic provides new insights into the role of mitochondrial proteins as major control points in oncology and help the reader to elaborate new strategies for 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.