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Cancer cells survive and adapt to many types of stress including hypoxia, nutrient deprivation, metabolic, and oxidative stress. These stresses are sensed by diverse cellular signaling processes, leading to either degradation of mitochondria or alleviation of mitochondrial stress. This review discusses signaling during sensing and mitigation of stress involving mitochondrial communication with the endoplasmic reticulum, and how retrograde signaling upregulates the mitochondrial stress response to maintain mitochondrial integrity. The importance of the mitochondrial unfolded protein response, an emerging pathway that alleviates cellular stress, will be elaborated with respect to cancer. Detailed understanding of cellular pathways will establish mitochondrial stress response as a key mechanism for cancer cell survival leading to cancer progression and resistance, and provide a potential therapeutic target in cancer., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Mitochondria are therapeutic targets in many diseases including cancer, metabolic disorders, and neurodegenerative diseases. Therefore, strategies to deliver therapeutics of interest to mitochondria are important for therapeutic development. As delocalized lipophilic cations (DLCs) preferentially accumulate in mitochondria, DLC-conjugation has been utilized to facilitate therapeutic delivery systems with mitochondrial targeting capability. Here we report that upon DLC-conjugation, anionic polymers exhibit significantly improved mitochondrial targeting when compared to cationic polymers and charge-neutral polymers. Considering that the cell membrane generally bears a net negative charge, the observed phenomenon is unexpected. Notably, the DLC-conjugated anionic polymers circumvent endosomal entrapment. The rapid mitochondrial accumulation of DLC-conjugated anionic polymers is likely a membrane-potential-driven process, along with the involvement of the mitochondrial pyruvate carrier. Moreover, the structural variations on the side chain of DLC-conjugated anionic polymers do not compromise the overall mitochondrial targeting capability, widely extending the applicability of anionic macromolecules in therapeutic delivery systems.

Mitochondrial fission mediated by the GTPase dynamin-related protein 1 (Drp1) is an attractive drug target in numerous maladies that range from heart disease to neurodegenerative disorders. The compound mdivi-1 is widely reported to inhibit Drp1-dependent fission, elongate mitochondria, and mitigate brain injury. Here, we show that mdivi-1 reversibly inhibits mitochondrial complex I-dependent O 2 consumption and reverse electron transfer-mediated reactive oxygen species (ROS) production at concentrations (e.g., 50 μM) used to target mitochondrial fission. Respiratory inhibition is rescued by bypassing complex I using yeast NADH dehydrogenase Ndi1. Unexpectedly, respiratory impairment by mdivi-1 occurs without mitochondrial elongation, is not mimicked by Drp1 deletion, and is observed in Drp1-deficient fibroblasts. In addition, mdivi-1 poorly inhibits recombinant Drp1 GTPase activity (K i > 1.2 mM). Overall, these results suggest that mdivi-1 is not a specific Drp1 inhibitor. The ability of mdivi-1 to reversibly inhibit complex I and modify mitochondrial ROS production may contribute to effects observed in disease models., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Mitochondrial oxidative metabolism plays a key role in meeting energetic demands of cells by oxidative phosphorylation (OxPhos). Here, we have briefly discussed (a) the dynamic relationship that exists among glycolysis, the tricarboxylic acid (TCA) cycle, and OxPhos; (b) the evidence of impaired OxPhos (i.e. mitochondrial dysfunction) in breast cancer; (c) the mechanisms by which mitochondrial dysfunction can predispose to cancer; and (d) the effects of host and environmental factors that can negatively affect mitochondrial function. We propose that impaired OxPhos could increase susceptibility to breast cancer via suppression of the p53 pathway, which plays a critical role in preventing tumorigenesis. OxPhos is sensitive to a large number of factors intrinsic to the host (e.g. inflammation) as well as environmental exposures (e.g. pesticides, herbicides and other compounds). Polymorphisms in over 143 genes can also influence the OxPhos system. Therefore, declining mitochondrial oxidative metabolism with age due to host and environmental exposures could be a common mechanism predisposing to cancer.

Recently, mitochondria have been suggested to act in tumor suppression. However, the underlying mechanisms by which mitochondria suppress tumorigenesis are far from being clear. In this study, we have investigated the link between mitochondrial dysfunction and the tumor suppressor protein p53 using a set of respiration-deficient (Res(-)) mammalian cell mutants with impaired assembly of the oxidative phosphorylation machinery. Our data suggest that normal mitochondrial function is required for γ-irradiation (γIR)-induced cell death, which is mainly a p53-dependent process. The Res(-) cells are protected against γIR-induced cell death due to impaired p53 expression/function. We find that the loss of complex I biogenesis in the absence of the MWFE subunit reduces the steady-state level of the p53 protein, although there is no effect on the p53 protein level in the absence of the ESSS subunit that is also essential for complex I assembly. The p53 protein level was also reduced to undetectable levels in Res(-) cells with severely impaired mitochondrial protein synthesis. This suggests that p53 protein expression is differentially regulated depending upon the type of electron transport chain/respiratory chain deficiency. Moreover, irrespective of the differences in the p53 protein expression profile, γIR-induced p53 activity is compromised in all Res(-) cells. Using two different conditional systems for complex I assembly, we also show that the effect of mitochondrial dysfunction on p53 expression/function is a reversible phenomenon. We believe that these findings will have major implications in the understanding of cancer development and therapy.

Apoptosis-inducing factor (AIF)-deficient harlequin (Hq) mice undergo neurodegeneration associated with a 40-50% reduction in complex I level and activity. We tested the hypothesis that AIF and complex I regulate reactive oxygen species (ROS) production by brain mitochondria. Isolated Hq brain mitochondria oxidizing complex I substrates displayed no difference compared to wild type (WT) in basal ROS production, H2O2 removal, or ROS production stimulated by complex I inhibitors rotenone or 1-methyl-4-phenylpyridinium. In contrast, ROS production caused by reverse electron transfer to complex I was attenuated by approximately 50% in Hq mitochondria oxidizing the complex II substrate succinate. Basal and rotenone-stimulated rates of H2O2 release from in situ mitochondria did not differ between Hq and WT synaptosomes metabolizing glucose, nor did the level of in vivo oxidative protein carbonyl modifications detected in synaptosomes, brain mitochondria, or homogenates. Our results suggest that AIF does not directly modulate ROS release from brain mitochondria. In addition, they demonstrate that in contrast to ROS produced by mitochondria oxidizing succinate, ROS release from in situ synaptosomal mitochondria or from isolated brain mitochondria oxidizing complex I substrates is not proportional to the amount of complex I. These findings raise the important possibility that complex I contributes less to physiological ROS production by brain mitochondria than previously suggested.

There have been several reports on the phosphorylation of various subunits of NADH-ubiquinone oxidoreductase (complex I) in mammalian mitochondria. The effects of phosphorylation on assembly or activity of these subunits have not been investigated directly. The cAMP-dependent phosphorylation of the MWFE and ESSS subunits in isolated bovine heart mitochondria has been recently reported. We have investigated the significance of potential phosphorylation of these two subunits in complex I assembly and function by mutational analysis of the phosphorylation sites. Chinese hamster mutant cell lines missing either the MWFE or the ESSS subunits were transfected and complemented with the corresponding wild type and mutant cDNAs made by site-directed mutagenesis. In MWFE the serine 55 was substituted by alanine, glutamate, glutamine, and aspartate (S55A, S55E, S55Q, and S55D, respectively). The glutamate substitutions might be expected to mimic the phosphorylated state of the protein. With the exception of the MWFE(S55A) mutant protein the assembly of complex I was completely blocked, and no activity could be detected. Various substitutions in the ESSS protein (S2A, S2E, S8A, S8E, T21A, T21E, S30A, S30E) appeared to cause lower levels of mature protein and a significantly reduced complex I activity measured polarographically. The ESSS (S2/8A) double mutant protein caused a complete failure to assemble. These mutational analyses suggest that if phosphorylation occurs in vivo, the effects on complex I activity are significant.

The pathologic activation of NMDA receptors by glutamate is a major contributor to neuronal cell death after stroke. Receptor activation causes a massive influx of calcium into the neuron that is accumulated by the mitochondria. The favored hypothesis is that the calcium loaded mitochondria generate reactive oxygen species that damage and ultimately killed the neuron. In this review this hypothesis is critically re-examined with an emphasis on the role played by deficits in ATP generation. Novel techniques are developed to monitor the bioenergetic status of in situ mitochondria in cultured neurons. Applying these techniques to a model of glutamate excitotoxicity suggests that enhanced reactive oxygen species are a consequence rather than a cause of failed cytoplasmic calcium homeostasis (delayed calcium deregulation, [DCD]), but that prior oxidative damage facilitates DCD by damaging mitochondrial ATP generation. This impacts on current hypotheses relating to the neuroprotective effects of mild mitochondrial uncoupling.

Partial inhibition of mitochondrial respiratory complex I by rotenone reproduces aspects of Parkinson's disease in rodents. The hypothesis that rotenone enhancement of neuronal cell death is attributable to oxidative stress was tested in an acute glutamate excitotoxicity model using primary cultures of rat cerebellar granule neurons. As little as 5 nM rotenone increased mitochondrial superoxide (O2*-) levels and potentiated glutamate-induced cytoplasmic Ca2+ deregulation, the first irreversible stage of necrotic cell death. However, the potent cell-permeant O2*- trap manganese tetrakis (N-ethylpyridinium-2yl) porphyrin failed to prevent the effects of the inhibitor. The bioenergetic consequences of rotenone addition were quantified by monitoring cell respiration. Glutamate activation of NMDA receptors used the full respiratory capacity of the in situ mitochondria, and >80% of the glutamate-stimulated respiration was attributable to increased cellular ATP demand. Rotenone at 20 nM inhibited basal and carbonyl cyanide p-trifluoromethoxyphenylhydrazone-stimulated cell respiration and caused respiratory failure in the presence of glutamate. ATP synthase inhibition by oligomycin was also toxic in the presence of glutamate. We conclude that the cell vulnerability in the rotenone model of partial complex I deficiency under these specific conditions is primarily determined by spare respiratory capacity rather than oxidative stress.

Mitochondrial outer membrane permeabilization and cytochrome c release promote caspase activation and execution of apoptosis through cleavage of specific caspase substrates in the cell. Among the first targets of activated caspases are the permeabilized mitochondria themselves, leading to disruption of electron transport, loss of mitochondrial transmembrane potential (DeltaPsim), decline in ATP levels, production of reactive oxygen species (ROS), and loss of mitochondrial structural integrity. Here, we identify NDUFS1, the 75 kDa subunit of respiratory complex I, as a critical caspase substrate in the mitochondria. Cells expressing a noncleavable mutant of p75 sustain DeltaPsim and ATP levels during apoptosis, and ROS production in response to apoptotic stimuli is dampened. While cytochrome c release and DNA fragmentation are unaffected by the noncleavable p75 mutant, mitochondrial morphology of dying cells is maintained, and loss of plasma membrane integrity is delayed. Therefore, caspase cleavage of NDUFS1 is required for several mitochondrial changes associated with apoptosis.