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

Abstract

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.

Keywords: apoptosis; cancer; metabolism; mitochondria; voltage-dependent anion channel 1.

Figure 1

Schematic representation of VDAC1 as a hub protein and its associated proteins. VDAC1 interacting proteins (see Modulation of VDAC1-Mediated Apoptosis and Metabolism via Interacting Proteins), sub-grouped into those associated with metabolism, energy, apoptosis, anti-oxidant, cell receptors, or signal transduction, or localized to mitochondria, endoplasmic reticulum (ER), nucleus, and cell membrane.

Figure 2

Schematic representation of VDAC1 as a multi-functional channel involved in cell survival and cell death. The various functions of VDAC1 include control of the metabolic cross-talk between the mitochondria and the rest of the cell, cellular energy production [by transporting ATP/ADP and reduced nicotinamide adenine dinucleotide (NADH) between the inter-membrane space and the cytosol and by binding hexokinase (HK)], and Ca²⁺ signaling (by transporting Ca²⁺, acyl-CoAs, and cholesterol); the Ca²⁺ influx and efflux transport systems in the OMMs and IMMs and Ca²⁺-mediated regulation of the tricarboxylic acid (TCA) cycle; activation of pyruvate dehydrogenase (PDH), isocitrate dehydrogenase (ICDH), and α-ketoglutarate dehydrogenase (α KGDH) by intra-mitochondrial Ca²⁺, leading to enhanced activity of the TCA cycle; and control of the electron transport chain and the ATP synthase (F_oF₁). VDAC1 in the OMM functions as a Ca²⁺ channel. In the IMM, the uptake of Ca²⁺ into the matrix is mediated by a Ca²⁺-selective transporter, the mitochondrial Ca²⁺ uniporter (MCU), regulated by a calcium-sensing accessory subunit (MICU1). Ca²⁺ efflux is mediated by NCLX, a Na⁺/Ca²⁺ exchanger. The accumulation of high levels of matrix Ca²⁺ triggers the opening of the permeability transition pore (PTP), a fast Ca²⁺ release channel. The role of VDAC1 in the transport of H₂O₂ is also presented. In addition, the figure shows the process of transfer of acyl-CoAs across the OMM by VDAC1 to the intermembrane space, where they are converted by carnitine palmitoyltransferase 1a (CPT1a) into acylcarnitine for further process during β-oxidation and cholesterol transport by a multi-protein complex, the transduceosome, containing steroidogenic acute regulatory protein (StAR)/translocator protein (TSPO)/VDAC1. Molecular fluxes are indicated by arrows.

Figure 3

VDAC1 function in cell death. Different models proposed for the release of apoptogenic proteins, such as Cyto c (gold balls) and apoptosis-inducing factor (AIF), from the mitochondrial inter-membrane space to the cytosol, leading to apoptosis. These models include (a) VDAC1 closure and outer mitochondrial membrane (OMM) rupture serving as the Cyto c release pathway—prolonged VDAC1 closure leads to mitochondrial matrix swelling and OMM rupture, resulting in the appearance of a non-specific release pathway for apoptogenic proteins; (b) a permeability transition pore (PTP) provides the apoptogenic protein release pathway—a large conductance pore-forming complex, the PTP, composed of VDAC1 in the OMM, adenine nucleotide translocase in the inner mitochondrial membrane (IMM), and cyclophilin D (CyD) in the matrix, allows apoptogenic protein release; (c) Bax activation, followed by its oligomerization, results in OMM permeabilization—upon apoptosis induction, Bax becomes associated with mitochondria as a large oligomer/complex forming a Cyto c-conducting channel in the OMM; (d) a pore is formed by oligomerized forms of Bax and Bak—Bax/Bak oligomerization, supposedly activated by BH3-only proteins (e.g., Bid), results in OMM permeabilization and Cyto c release; (e) a Bax- and VDAC1-based hetro-oligomer mediates Cyto c release—the interaction of pro-apoptotic proteins (Bax/Bak) with VDAC1 forms a cytochrome c (Cyto c) release pathway; (f) mitochondrial apoptosis-inducing channel (MAC) as the release pathway—MAC offers a high-conductance channel and a putative Cyto c release channel; (g) oligomeric VDAC1 as a channel for the release of apoptotic proteins—a protein-conducting channel is formed within a VDAC1 homo-oligomer, allowing Cyto c release and apoptotic cell death; (h) mitochondrial Ca²⁺ overload induces apoptosis—following Ca²⁺ overload in the matrix, Ca²⁺ transport mediated by VDAC1 across the OMM and by the mitochondrial Ca²⁺ uniporter (MCU) in the IMM leads to dissipation of the membrane potential, mitochondria swelling, PTP opening, Cyto c release, and apoptotic cell death. PTP opening is also accompanied by an efflux of the accumulated Ca²⁺ into the cytosol.

Figure 4

Apoptosis stimuli induce VDAC1 oligomerization: a proposed Cyto c release channel A. Before apoptosis induction, VDAC1 is in the monomeric state, with the amphipathic α-helix N-terminal region cytoplasmically exposed (169) or positioned within the pore (–49). (B) Upon apoptotic signaling, VDAC1 oligomerizes to form a multimer and the amphipathic α-helix N-terminal region of each VDAC1 molecule flips into the hydrophobic pore formed by the β-barrels, forming a hydrophilic pore capable of conducting Cyto c.

Figure 5

Structure of VDAC1-based peptides. Schematic illustration of D-N-terminus-Antp (D-N-Ter-Antp) (A) and D-Antp-LP4 and Tf-D-LP4 (B) peptides. The VDAC1-derived sequences N-terminus and LP4 are in blue and orange, respectively. The cell-penetrating peptide Antp in green and Tf is in light blue. The loop shape of LP4 stabilized by a tryptophan zipper (Trp) is in purple. The N-terminus of LP4 and Antp is composed of d-amino acids. (C) The sequences of D-LP4 and retro-D-LP4.

Figure 6

Proposed mode of action of VDAC1-based peptides leading to mitochondria-mediated cell death. VDAC1 is overexpressed in the mitochondria of cancer cells and associated with hexokinase (HK) and Bcl-2, Δψ is maintained, the cell remains in homeostasis with respect to energy production and is protected from apoptosis. VDAC1-based peptides interact with anti-apoptotic proteins HK and Bcl-2 causing the proteins to disassociate from VDAC1, and leading to Δψ dissipation, decreased ATP production, mitochondrial dysfunction, VDAC1 oligomerization, and Cyto c release. These events ultimately lead to cell death.