Specific disintegration of complex II succinate:ubiquinone oxidoreductase links pH changes to oxidative stress for apoptosis induction

Abstract

The formation of reactive oxygen species (ROS) and the change of the intracellular pH (pH i ) are common phenomena during apoptosis. How they are interconnected, however, is poorly understood. Here we show that numerous anticancer drugs and cytokines such as Fas ligand and tumour necrosis factor a provoke intracellular acidification and cause the formation of mitochondrial ROS. In parallel, we found that the succinate:ubiquinone oxidoreductase (SQR) activity of the mitochondrial respiratory complex II is specifically impaired without affecting the second enzymatic activity of this complex as a succinate dehydrogenase (SDH). Only in this configuration is complex II an apoptosis mediator and generates superoxides for cell death. This is achieved by the pH i decline that leads to the specific dissociation of the SDHA/SDHB subunits, which encompass the SDH activity, from the membrane-bound components of complex II that are required for the SQR activity.

Mitochondrial respiratory chain complexes: apoptosis sensors mutated in cancer?

“…This observation was corroborated by various studies showing the proapoptotic effects of specific complex II inhibitors, notably 3-nitropropionate (irreversible inhibitor) and methylmalonate (competitive inhibitor) in neuronal cells (Pang and Geddes, 1997;McLaughlin et al, 1998). Both reagents target the SDHA succinate-binding site and equally inhibit the SDH and SQR activities (Brusque et al, 2002;Lemarie et al, 2011). When the tumour-suppressor gene function of complex II subunits was discovered for SDHD, SDHC and SDHB (Baysal et al, 2000;Niemann and Muller, 2000;Astuti et al, 2001), two mechanisms were evoked to explain the oncogenesis process associated with complex II deficiency: (1) the establishment of a pseudo-hypoxic state leading to HIF1a stabilization in tumours through succinate inhibition of the HIF1a prolyl hydroxylase (PHD), which favours the glycolytic pathway and promotes tumour formation (GimenezRoqueplo et al, 2001;Pollard et al, 2005Pollard et al, , 2006Selak et al, 2005;Lehtonen et al, 2007;Cervera et al, 2008) and (2) the long-term generation of sublethal superoxides at the complex II level contributing to either genomic instability and subsequent tumoral development or HIF1a stabilization (Senoo-Matsuda et al, 2001;Ishii et al, 2005;Slane et al, 2006;Guzy et al, 2008).…”

“…They form the catalytic core that is able to oxidize succinate (which binds to SDHA) and carries electrons from this TCA cycle substrate to the FAD cofactor in SDHA and finally to the 3 [Fe-S] clusters in SDHB Rutter et al, 2010). At this stage, electrons can be captured in vitro by artificial electron acceptors and the corresponding enzymatic activity is called SDH activity (Lemarie et al, 2011). This catalytic core is associated with a hydrophobic tail formed by two transmembrane proteins, SDHC and SDHD, both of which also project a short segment of amino acids into the intermembrane space (Yankovskaya et al, 2003;Sun et al, 2005).…”

“…SDHA to CoQ at the Q P site constitutes the SQR activity of RCC II, which can, just as the SDH activity, be measured in vitro with the appropriate enzymatic assay (Lemarie et al, 2011). The precise molecular mechanisms of ROS production at the complex II level have rarely been explored.…”

“…Indeed, the ability of complex II to generate ROS was mostly neglected in the 1990s as the common dogma considered RCCs I and III to be the principal sources of mitochondrial ROS (McLennan and Degli Esposti, 2000; Ricci et al, 2003b), based on studies of chemical inhibition by rotenone and antimycin A, respectively (Lenaz and Genova, 2010). Nevertheless, different sites for superoxide production have been evoked, for example, at the SDHB/C/D CoQ-binding site (Szeto et al, 2007) possibly through the stabilization of the semiquinone radical (Tran et al, 2006), at the FAD level in SDHA (Yankovskaya et al, 2003) or between the SDHA-embedded FAD group and a downstream blockade (Guzy et al, 2008;Lemarie et al, 2011). These mechanisms appear to depend on the respective targets of the chemical inhibitors used or on the specific sequence of the mutations analysed.…”

Identification of NCF2/p67phox as a novel p53 target gene

“…[21][22][23][24][25] These data suggest that the modulation of survival signaling by ROS is also critical for some types of cancer development, although the genotoxic effect has been mainly emphasized for the role of ROS in tumor formations. [26][27][28][29][30] In pancreatic cancer, instead, Nox4-generated ROS have a protective function against apoptosis through the inhibition of AKT-ASK1 phosphorylation signaling. 22,23,28 ROS are even generated downstream of p53 and p53 family members, p63 and p73, most likely by the transcriptional modulation of genes that regulate the cellular redox state and that directly contribute to p53/p63/p73-mediated cell death.…”