Mitochondrial Complex II Prevents Hypoxic but Not Calcium- and Proapoptotic Bcl-2 Protein-induced Mitochondrial Membrane Potential Loss

Abstract

Mitochondrial membrane potential loss has severe bioenergetic consequences and contributes to many human diseases including myocardial infarction, stroke, cancer, and neurodegeneration. However, despite its prominence and importance in cellular energy production, the basic mechanism whereby the mitochondrial membrane potential is established remains unclear. Our studies elucidate that complex II-driven electron flow is the primary means by which the mitochondrial membrane is polarized under hypoxic conditions and that lack of the complex II substrate succinate resulted in reversible membrane potential loss that could be restored rapidly by succinate supplementation. Inhibition of mitochondrial complex I and F 0 F 1 -ATP synthase induced mitochondrial depolarization that was independent of the mitochondrial permeability transition pore, Bcl-2 (B-cell lymphoma 2) family proteins, or high amplitude swelling and could not be reversed by succinate. Importantly, succinate metabolism under hypoxic conditions restores membrane potential and ATP levels. Furthermore, a reliance on complex II-mediated electron flow allows cells from mitochondrial disease patients devoid of a functional complex I to maintain a mitochondrial membrane potential that conveys both a mitochondrial structure and the ability to sequester agonist-induced calcium similar to that of normal cells. This finding is important as it sets the stage for complex II functional preservation as an attractive therapy to maintain mitochondrial function during hypoxia.Mitochondria are multifunctional organelles involved in calcium buffering (1-3), apoptosis (4 -9), necrosis (10, 11), reactive oxygen species production (12-15) and nuclear stress signaling (16 -18). However, despite their importance in a number of cellular processes, the primary function of mitochondria remains that of cellular energy production. Although limited energy can be derived from cytosolic enzyme systems, the vast majority of cellular ATP generation is generated by the mitochondrial electrochemical gradient and ATP synthase complex. Structurally, the inner mitochondrial membrane (IMM) 3 and the outer mitochondrial membrane divide the organelle into two well defined compartments, the matrix and the intermembrane space, respectively. Protein complexes within the highly selective IMM facilitate energetically favorable electron transfer from metabolic substrates to the terminal acceptor oxygen. These protein complexes utilize the energy derived from this electron transfer to pump protons across the IMM into the intermembrane space, effectively establishing a proton motive electrochemical gradient, known as the mitochondrial membrane potential (⌬⌿ m ). Controlled proton flux across the IMM through the F 0 F 1 -ATP synthase molecular motor then drives the generation of ATP, a process described as the chemiosmotic theory (19).Although the chemiosmotic theory of mitochondrial energy production is widely accepted, the basic mechanism in which the mitochondria establish the ⌬⌿ m remains poorly under...

Hypoxia and aerobic metabolism adaptations of human endothelial cells

“…In mitochondria from hypoxic endothelial cells, the considerably elevated protein expression and activity of complex II contribute to an increase in mΔΨ and succinate-sustained mROS formation mainly through increased reverse electron transport. It has been previously reported that in endothelial cells, complex II-driven electron flow is the primary means by which the mitochondrial membrane is polarized under hypoxic conditions [10]. Another interesting finding from this study is that, in mitochondria from hypoxic endothelial cells, a reduction of expression and maximal malate-sustained activity of complex I was accompanied by an increase in oxidation of other weaker than malate NAD-linked substrates α-ketoglutarate or isocitrate.…”

Mitochondrial Targeting of Vitamin E Succinate Enhances Its Pro-apoptotic and Anti-cancer Activity via Mitochondrial Complex II

“…This, in turn, stabilizes the ODD domain of the HIF1␣, which in the ODD-GFP construct regulates the stability of GFP. Because we did not observe GFP fluorescence in HCT116 ODD-GFP cells exposed to MitoVE 11 S in less than 6 h (data not shown), whereas significant ROS accumulation occurred within relatively short periods, the pseudohypoxic state is probably secondary to the importance of ROS in promoting the onset of apoptosis in cancer cells exposed to MitoVE 11 S. To corroborate this premise, using live confocal microscopy and cells transfected with the pHyPer-dMito plasmid coding for the redox sensor OxyR (65), we observed generation of ROS in cancer cells as early as in 5 min following the addition of MitoVE 11 S. 3 This further supports the primary role of the CII UbQ-binding site in the molecular action of the agent and is consistent with the notion that inhibition of CII can result in HIF1␣ stabilization (31,32,66,67).…”

Intracoronary Cytoprotective Gene Therapy

“…To determine cytoprotective effects of VEGF-B 167 , neonatal rat cardiomyocytes were isolated and cultured with reactive oxygen specie (ROS) production measured as previously described (19,21,22). They were exposed to VEGF-B 167 , VEGF-A, VEGF-E, and placental growth factor (PlGF) in the absence or in the presence of angiotensin II or norepinephrine (50 M −6 ) (Online Appendix).…”