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In tumour cells, elevated levels of mitochondria-bound isoforms of hexokinase (HK-I and HK-II) result in the evasion of apoptosis, thereby allowing the cells to continue proliferating. The molecular mechanisms by which bound HK promotes cell survival are not yet fully understood. Our studies relying on the purified mitochondrial outer membrane protein VDAC (voltage-dependent anion channel), isolated mitochondria or cells in culture suggested that the anti-apoptotic activity of HK-I occurs via modulation of the mitochondrial phase of apoptosis. In the present paper, a direct interaction of HK-I with bilayer-reconstituted purified VDAC, inducing channel closure, is demonstrated for the first time. Moreover, HK-I prevented the Ca(2+)-dependent opening of the mitochondrial PTP (permeability transition pore) and release of the pro-apoptotic protein cytochrome c. The effects of HK-I on VDAC activity and PTP opening were prevented by the HK reaction product glucose 6-phosphate, a metabolic intermediate in most biosynthetic pathways. Furthermore, glucose 6-phosphate re-opened both the VDAC and the PTP closed by HK-I. The HK-I-mediated effects on VDAC and PTP were not observed using either yeast HK or HK-I lacking the N-terminal hydrophobic peptide responsible for binding to mitochondria, or in the presence of an antibody specific for the N-terminus of HK-I. Finally, HK-I overexpression in leukaemia-derived U-937 or vascular smooth muscle cells protected against staurosporine-induced apoptosis, with a decrease of up to 70% in cell death. These results offer insight into the mechanisms by which bound HK promotes tumour cell survival, and suggests that its overexpression not only ensures supplies of energy and phosphometabolites, but also reflects an anti-apoptotic defence mechanism.

Mitochondrial stress releases mitochondrial DNA (mtDNA) into the cytosol, thereby triggering the type Ι interferon (IFN) response. Mitochondrial outer membrane permeabilization, which is required for mtDNA release, has been extensively studied in apoptotic cells, but little is known about its role in live cells. We found that oxidatively stressed mitochondria release short mtDNA fragments via pores formed by the voltage-dependent anion channel (VDAC) oligomers in the mitochondrial outer membrane. Furthermore, the positively charged residues in the N-terminal domain of VDAC1 interact with mtDNA, promoting VDAC1 oligomerization. The VDAC oligomerization inhibitor VBIT-4 decreases mtDNA release, IFN signaling, neutrophil extracellular traps, and disease severity in a mouse model of systemic lupus erythematosus. Thus, inhibiting VDAC oligomerization is a potential therapeutic approach for diseases associated with mtDNA release.

The role of the voltage-dependent anion channel (VDAC) in cell death was investigated using the expression of native and mutated murine VDAC1 in U-937 cells and VDAC inhibitors. Glutamate 72 in VDAC1, shown previously to bind dicyclohexylcarbodiimide (DCCD), which inhibits hexokinase isoform I (HK-I) binding to mitochondria, was mutated to glutamine. Binding of HK-I to mitochondria expressing E72Q-mVDAC1, as compared to native VDAC1, was decreased by B70% and rendered insensitive to DCCD. HK-I and ruthenium red (RuR) reduced the VDAC1 conductance but not that of E72Q-mVDAC1. Overexpression of native or E72Q-mVDAC1 in U-937 cells induced apoptotic cell death (80%). RuR or overexpression of HK-I prevented this apoptosis in cells expressing native but not E72Q-mVDAC1. Thus, a single amino-acid mutation in VDAC prevented HK-I-or RuR-mediated protection against apoptosis, suggesting the direct VDAC regulation of the mitochondria-mediated apoptotic pathway and that the protective effects of RuR and HK-I rely on their binding to VDAC.