Mitochondrial respiratory chain. For mammals, the respiratory chain consists of four enzyme complexes (complexes I – IV) and two intermediary substrates (coenzyme Q and cytochrome c). The NADH+H + and FADH 2 produced by the intermediate metabolism are oxidized further by the mitochondrial respiratory chain to establish an electrochemical gradient of protons, which is finally used by the F1F0-ATP synthase (complex V) to produce ATP, the only form of energy used by the cell. In this simple representation of the respiratory chain, the supramolecular organization (supercomplexes, dimers) is not shown. 

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... the inner membrane and the ultimate phosphorylation of ADP by the F F -ATP synthase (complex V). In 1963, mtDNA (Figure 1) was characterized by Nass and Schatz and then entirely sequenced by Sanger and co-workers in 1981. By that time, L. Margulis had proposed the endosymbiotic theory of mitochondrial evolution, which states that aerobic bacteria, upon symbiosis, transferred most of their DNA to the nucleus of the protocells and gained the selective advantage of being able to consume oxygen while producing energy in the form of ATP. By examining the polymorphisms in mtDNA, H. Blanc discovered its maternal inheritance; others discovered that it had a mutation rate 10 times higher than that of nuclear DNA. Furthermore, some mitochondrial DNA genes were found to be localized in the chromosomes, but are silent. In 1979, Barell found differences in the genetic code as compared with the universal one. Two years later, the entire sequence of the human mitochondrial DNA was established by F. Sanger and collaborators. The first diseases associated with mtDNA deletions were reported in early 1988; these disorders are associated with several clinical symptoms, such as myopathy, ptosis, external ophthalmoplegia and cardiac block branch (for an extensive review, see (2)). The first point mutation identified was associated with Leber’s disease, which is mainly characterized by visual loss. During the 1990s, the three- dimensional structures of the cytochrome c oxidase (complex IV) and of the ATP synthase were determined by the groups of S. Yoshikawa and J. Walker, respectively. The identification of proapoptotic factors that are released by this organelle indicated that mitochondria play a pivotal role in deciding cell survival or death through intracellular signaling. Apoptosis can be divided in three phases: induction, execution and degradation (Figure 2). In this review, we will not cover the downregulation of mitochondrial apoptotic mechanisms in cancer cells. We will focus more on the particularities of their energy metabolism in relationship with metabolic signaling and tumor biology. In the last decade, much progress has been made in deciphering the link between mitochondrial structure and function with the discovery (consideration) of a networked and dynamic organization of the mitochondrion in living human cells of diverse origin (Figure 3A). Studies examining the mitochondrial interior by electron tomography helped to define the "cristae-junction model" (3), which separates the inner boundary membrane (IBM) closely opposed to the outer membrane from the cristae membrane that projects into the matrix compartment (Figure 3B). These two components may be connected by tubular structures called cristae junctions of relatively uniform size. Recent advances in mitochondrial research revealed that morphological changes of the mitochondria, both overall organization and internal structure, are intimately linked with bioenergetics and apoptosis (for reviews, see (4) and (5), respectively). The additional discovery of pathogenic mutations in genes that are essential for fusion and fission of the mitochondrial network has added complexity to the physiopathological mechanisms of these diseases and the regulation of organellar functions as diverse as energetics, signaling, biosyntheses or apoptosis (6). Prior to discussing the abnormalities of cancer cells energetics it is necessary to give a brief introduction on the metabolic pathways by which glucose is converted in ATP, the only form of energy considered by the cell (Figure 4). Briefly, glucose enters the cells through specific transporters and becomes phosphorylated by hexokinase to G 6 P which is further cleaved in two molecules of glyceraldehyde-3-phosphate and dephosphorylated by the pyruvate kinase in the presence of ADP to yield pyruvate. These reactions produce two molecules of ATP and are collectively known as glycolysis (anaerobic and cytosolic) in contrast with respiration (aerobic and mitochondrial). The end product of glycolysis pyruvate can be transformed in lactate by the lactate dehydrogenase in the presence of an excess of NADH + H + during poor respiration, or it can enter the mitochondria to undergo an oxidative decarboxylation in the presence of coenzymes (NAD + , FAD) which will fuel the respiratory chain to consume oxygen and yield ATP and H 2 O. Inside the mitochondrion, the intermediate metabolism encompasses the Krebs cycle and the beta-oxidation which provide acyl-CoA and acetyl- CoA but also NAD (P)H + H + and FADH 2 . Mitochondria contain other enzymes involved in catabolism (urea cycle) or anabolism with the synthesis of amino acids and uridine. They also play a pivotal role in the phosphorylation of nucleotide diphosphates, notably thymidine. The five complexes of the respiratory chain are embedded within the inner boundary membrane and the cristae membrane (ratio 1 to 6). Only 13 out of the hundred of proteins of the entire OXPHOS system (Figure 5) are encoded by the mtDNA which uses a specific genetic code. The inner membrane is impermeable to protons so that a proton electrochemical gradient can be formed upon the proton extrusion from the matrix to the intermal membrane space by the respiratory chain complexes I, III and IV. According to the chemiosmotic theory of P. Mitchell (7), the resulting electric membrane potential (deltaPsi) and pH difference (deltapH) caused by this process create a strong proton- motive force that is used for the condensation of ADP and Pi by the ATP synthetase to produce ATP. ATP is then exported to the cytoplasm by the adenine nucleotide translocator (ANT) in exchange with cytosolic ADP. Aerobic respiration from glucose produces 38 moles of ATP, which is 19 times more than what glycolysis can produce. However, the rate of ATP production by the OXPHOS system is slower than glycolysis (8). O. Warburg noticed that a significant number of tumors use glycolysis more than respiration to produce the vital ATP, even under normal O 2 pressure. As a result the extracellular medium is acidified. The genetic, biochemical and molecular basis of this phenomenon, which is called the "Warburg effect", still remain puzzling. Its occurrence in all tumors is also challenged by the discovery of cancers that utilize more the mitochondrion to produce ATP. An elevated uptake of glucose (by a factor of 10) is generally observed in human living tumoral tissues by positron- emission tomography (PET) using the isotope analog glucose tracer 2- ( 18 F) fluoro-2-deoxy-D-glucose (FDG). This index was further correlated with increased malignancy and invasiveness (9, 10); for some tumors, a close correlation was observed between the degree of differentiation, growth rate and glucose accumulation (1, 11). Such clinical observations suggest that a large amount of glucose enters tumors, but the contribution of glycolysis and OXPHOS to glucose catabolism cannot be determined with this technique. To do so, bioenergetic studies are required on tissue sample or tumor derived cell lines. In this manner the extent of ATP produced by glycolysis was measured on various cancer cell lines and revealed that Warburg predictions were not systematic. For instance the bioenergetic analysis of two human lung cancer cell lines verified Warburg's theory but also precised that OXPHOS is not inactive per se : it operates at a low-capacity and is repressed by the presence of glucose (Crabtree effect) (12). In striking contrast, an analysis of MCF-7 cells originating from a mammary gland epithelial adenocarcinoma revealed that ATP production is 80 % oxidative in glucose medium (13). Likewise, in hepatoma cells mitochondrial respiration was found to be coupled to ADP phosphorylation and produced 40% of the total cellular ATP in glucose medium (14). More recently, Rodriguez-Enriquez et al. revealed that in HeLa and Hek293 young-spheroids, the contribution of OXPHOS to the total ATP supply was 60% (15). They further studied this feature and evidenced a class of tumor cell lines in which the oxidative metabolism prevails over glycolysis. This situation has been extensively reviewed in (16), and the authors even conclude that "high glycolysis" is not a prerequisite of all cancer cells but could be acquired during the highest proliferative activity and/or in response to stringent micro-environmental conditions, such as intermittent hypoxia and/or glucose limitation. Accordingly, a critical review of numerous studies comparing cancer cells with normal tissues concluded that several tumors derive most of their ATP from mitochondrial oxidative phosphorylation, in striking contrast to Warburg's hypothesis (17). These divergent observations highlight the fundamental question of how variable is the metabolic reprogramming of cancer cells (Figure 6) and what are the relative contributions of oncogenesis, tumor environment, proliferative activity and experimental conditions. The examination of cancer cells metabolic profile should be considered for tumor caracterisation and classification as specific alterations in mitochondrial and glycolytic protein expression were reported in association with changes in cancer cell bioenergetics or apoptosis (18-20). In a large variety of tumors, extensive modifications of the mitochondrion have further been described, including a decrease in organellar biogenesis (11, 21), the alteration of respiratory chain complexes specific activity (22, 23), inhibition of the pyruvate dehydrogenase (PDH) complex (24), truncation of the Krebs cycle with citrate extrusion (25), an increase in the binding of hexokinase II to the mitochondrion (26), variegated changes in organellar shape and size (27) and the accumulation of diverse mutations in mitochondrial DNA (28, 29). More recently, it was proposed that metabolic reprogramming could be used to enhance tumors cells biosynthetic pathway, as required by the deregulated growth and needs. This is ...