VDAC Regulation: A Mitochondrial Target to Stop Cell Proliferation

Abstract

Cancer metabolism is emerging as a chemotherapeutic target. Enhanced glycolysis and suppression of mitochondrial metabolism characterize the Warburg phenotype in cancer cells. The flux of respiratory substrates, ADP, and Pi into mitochondria and the release of mitochondrial ATP to the cytosol occur through voltage-dependent anion channels (VDACs) located in the mitochondrial outer membrane. Catabolism of respiratory substrates in the Krebs cycle generates NADH and FADH₂ that enter the electron transport chain (ETC) to generate a proton motive force that maintains mitochondrial membrane potential (ΔΨ) and is utilized to generate ATP. The ETC is also the major cellular source of mitochondrial reactive oxygen species (ROS). αβ-Tubulin heterodimers decrease VDAC conductance in lipid bilayers. High constitutive levels of cytosolic free tubulin in intact cancer cells close VDAC decreasing mitochondrial ΔΨ and mitochondrial metabolism. The VDAC-tubulin interaction regulates VDAC opening and globally controls mitochondrial metabolism, ROS formation, and the intracellular flow of energy. Erastin, a VDAC-binding molecule lethal to some cancer cell types, and erastin-like compounds identified in a high-throughput screening antagonize the inhibitory effect of tubulin on VDAC. Reversal of tubulin inhibition of VDAC increases VDAC conductance and the flux of metabolites into and out of mitochondria. VDAC opening promotes a higher mitochondrial ΔΨ and a global increase in mitochondrial metabolism leading to high cytosolic ATP/ADP ratios that inhibit glycolysis. VDAC opening also increases ROS production causing oxidative stress that, in turn, leads to mitochondrial dysfunction, bioenergetic failure, and cell death. In summary, antagonism of the VDAC-tubulin interaction promotes cell death by a "double-hit model" characterized by reversion of the proproliferative Warburg phenotype (anti-Warburg) and promotion of oxidative stress.

Keywords: Cancer metabolism; Erastin; Glycolysis; Mitochondria; Oxidative stress; Tubulin; Voltage-dependent anion channel; Warburg effect.

Fig. 1

Mitochondrial metabolism. Flux of metabolites including fatty acids, certain amino acids, pyruvate, ADP, and Pi across the mitochondrial outer membrane occurs through VDAC. Catabolism of respiratory substrates in the Krebs cycle generates NADH and FADH₂, which fuel the electron transport chain (Complexes I–IV) and supports oxidative phosphorylation. The Krebs cycle also produces metabolic intermediaries released to the cytosol for the biosynthesis of proteins and lipids. Proton pumping by the respiratory chain across mitochondrial inner membrane (MIM) generates mitochondrial ΔΨ. Protons moving back across MIM into the matrix drive ATP synthesis from ADP and Pi by the F₁-F₀-ATP synthase (Complex V). Mitochondrial ATP is exported from the matrix by the adenine nucleotide transporter (ANT) and released to the cytosol through VDAC.

Fig. 2

Metabolic flexibility of tumors and VDAC opening. Cancer cells switch between oxidative and glycolytic bioenergetic profiles depending on nutrient availability, tissue oxygenation, intratumor localization, and pharmacological treatments to inhibit glycolysis or to promote mitochondrial metabolism. In cancer cells, constitutive high free tubulin blocks VDAC conductance, suppresses mitochondrial metabolism, and decreases cytosolic ATP/ADP to favor glycolysis. Reversal of the inhibitory effect of tubulin by VDAC–tubulin antagonists leads to VDAC opening and reversal of the Warburg phenotype.

Fig. 3

Mechanisms to promote cell death after VDAC opening. VDAC–tubulin antagonists open VDAC increasing the flux of metabolites into and out of mitochondria leading to increased mitochondrial ΔΨ, increased mitochondrial metabolism, high cytosolic ATP/ADP ratio, and decreased glycolysis (Hit 1: anti-Warburg effect). VDAC opening also increases ROS formation concurrent with the decrease in glycolysis. Increased ROS damage mitochondrial DNA, cardiolipin, and mitochondrial proteins and activate JNK (Hit 2: oxidative stress). Activated JNK translocates to mitochondria causing mitochondrial dys-function, bioenergetic failure, and cell death.

Fig. 4

Erastin-like compound X1 causes cell death in situ and in vivo. Lead compound X1 caused cell death in Huh7 hepatocarcinoma cells in culture and slowed tumor growth in a xenograft model of Huh7 cells in nude mice.