Mammalian NADH:ubiquinone oxidoreductase (Complex I) and nicotinamide nucleotide transhydrogenase (Nnt) together regulate the mitochondrial production of H2O2—Implications for their role in disease, especially cancer

Abstract

Mammalian NADH:ubiquinone oxidoreductase (Complex I) in the mitochondrial inner membrane catalyzes the oxidation of NADH in the matrix. Excess NADH reduces nine of the ten prosthetic groups of the enzyme in bovine-heart submitochondrial particles with a rate of at least 3,300 s −1 . This results in an overall NADH→O 2 rate of ca. 150 s −1 . It has long been known that the bovine enzyme also has a specific reaction site for NADPH. At neutral pH excess NADPH reduces only three to four of the prosthetic groups in Complex I with a rate of 40 s −1 at 22°C. The reducing equivalents remain essentially locked in the enzyme because the overall NADPH→O 2 rate (1.4 s −1 ) is negligible. The physiological significance of the reaction with NADPH is still unclear. A number of recent developments has revived our thinking about this enigma. We hypothesize that Complex I and the Δp-driven nicotinamide nucleotide transhydrogenase (Nnt) co-operate in an energy-dependent attenuation of the hydrogen-peroxide generation by Complex I. This co-operation is thought to be mediated by the NADPH/NADP + ratio in the vicinity of the NADPH site of Complex I. It is proposed that the specific H 2 O 2 production by Complex I, and the attenuation of it, is of importance for apoptosis, autophagy and the survival mechanism of a number of cancers. Verification of this hypothesis may contribute to a better understanding of the regulation of these processes.

The Role of Mitochondrial NADPH-Dependent Isocitrate Dehydrogenase in Cancer Cells

“…RCG utilizes reductive carboxylation of 2OG by the reverse reaction of mitochondrial IDH2 at the expense of NADPH, followed by the reverse aconitase reaction and citrate efflux from the matrix [1–3, 21–23]. NADPH is provided by the malic enzyme converting malate to pyruvate and might also be provided by the mitochondrial transhydrogenase [24]. The OXPHOS independence of this mode means that it may proceed at any level of hypoxia and even at anoxia, thus increasing malignancy [1, 3].…”

“…The consumption of NADPH in the matrix is a consequence of the altered homeostasis of reactive oxygen species (ROS) in cancer cells (see Section 4.2) and is also possible due to NADPH production by the mitochondrial malic enzyme [1, 5] and transhydrogenase [24, 63, 67]. It is not known whether SIRT3-based activation also affects this reverse (NADPH-dependent) IDH2 reaction.…”

“…Isocitrate formed by the reductive carboxylation reaction of IDH2 is processed back to 2OG by IDH3. Although in non-malignant cells Complex I regenerates NADH to NAD + and NADP + could be regenerated to NADPH by, for example, mitochondrial malic enzyme, with increasing malignancy (more dormant state of mitochondria and hence decreasing respiration), the mitochondrial inner membrane H + transhydrogenase [24, 63, 67] may alternatively transfer electrons from NADH and NADP + to NAD + and NADPH at the expense of the proton-motive force [24]. However, it remains to be determined, whether this cycle is possible with d -2-hydroxyglutarate.…”

“…If d -2-hydroxyglutarate was metabolized by IDH3 in the canonical Krebs cycle, then the cycle would be automatically induced by the appearance of d -2-hydroxyglutarate at simultaneously active H + transhydrogenase. Nevertheless OXPHOS cannot be completely dormant, since proton-motive force would be required for this normal “forward” transhydrogenase reaction [24]. …”

Régulation de l’autophagie par la leucine

“…Produits majoritairement au niveau de la chaîne respiratoire mitochondriale, ils permettent l'induction de l'autophagie par la carence nutritionnelle en augmentant l'état d'oxydoréduction d'ATG4B, une protéine impliquée dans la formation des autophagosomes [5]. Lors de la déshydrogénation du glutamate en α-KG, GLUD1 génère un puissant anti-oxydant, le NADPH, qui pourrait être utilisé pour diminuer le taux de ROS via le système glutathion/glutathion réductase et/ou limiter la production de ROS au niveau du complexe I de la chaîne respiratoire mitochondriale (via la Nnt [nicotinamide nucleotide transhydrogenase]) [6]. D'autre part, GLUD1 contrôle l'autophagie en activant MTORC1 par divers mécanismes impliquant l'α-KG (Figure 2).…”