Anticancer drugs targeting the mitochondrial electron transport chain

Is reliance on mitochondrial respiration a "chink in the armor" of therapy-resistant cancer?

A series of recent reports has suggested PGC1α-driven upregulation of mitochondrial oxidative phosphorylation as a selective vulnerability of drug-resistant cancers. Accordingly, chemical inhibitors of respiration led to selective eradication of such cancer cells due to their preferential sensitivity to mitochondrial production of reactive oxygen species. These insights create a timely opportunity for a biomarker guided application of already existing and newly emerging mitochondrial inhibitors in recurrent drug-resistant cancer, including lymphomas, melanomas, and other malignant diseases marked by increased mitochondrial respiration.

Structural re-arrangement and peroxidase activation of cytochrome c by anionic analogues of vitamin E, tocopherol succinate and tocopherol phosphate

Cytochrome c is a multifunctional hemoprotein in the mitochondrial intermembrane space whereby its participation in electron shuttling between respiratory complexes III and IV is alternative to its role in apoptosis as a peroxidase activated by interaction with cardiolipin (CL), and resulting in selective CL peroxidation. The switch from electron transfer to peroxidase function requires partial unfolding of the protein upon binding of CL, whose specific features combine negative charges of the two phosphate groups with four hydrophobic fatty acid residues. Assuming that other endogenous small molecule ligands with a hydrophobic chain and a negatively charged functionality may activate cytochrome c into a peroxidase, we investigated two hydrophobic anionic analogues of vitamin E, α-tocopherol succinate (α-TOS) and α-tocopherol phosphate (α-TOP), as potential inducers of peroxidase activity of cytochrome c. NMR studies and computational modeling indicate that they interact with cytochrome c at similar sites previously proposed for CL. Absorption spectroscopy showed that both analogues effectively disrupt the Fe-S(Met(80)) bond associated with unfolding of cytochrome c. We found that α-TOS and α-TOP stimulate peroxidase activity of cytochrome c. Enhanced peroxidase activity was also observed in isolated rat liver mitochondria incubated with α-TOS and tBOOH. A mitochondria-targeted derivative of TOS, triphenylphosphonium-TOS (mito-VES), was more efficient in inducing H2O2-dependent apoptosis in mouse embryonic cytochrome c(+/+) cells than in cytochrome c(-/-) cells. Essential for execution of the apoptotic program peroxidase activation of cytochrome c by α-TOS may contribute to its known anti-cancer pharmacological activity.

Powerhouse down: Complex II dissociation in the respiratory chain

Complex II of the respiratory chain (RC) recently emerged as a prominent regulator of cell death. In both cancer cells as well as neurodegenerative diseases, mutations in subunits have been found along with other genetic alterations indirectly affecting this complex. Anticancer compounds were developed that target complex II and cause cell death in a tumor-specific way. Our mechanistic understanding of how complex II is activated for cell death induction has recently been made clearer in recent studies, the results of which are covered in this review. This protein assembly is specifically activated for cell death via the dissociation of its SDHA and SDHB subunits from the membrane-anchoring proteins through pH change or mitochondrial Ca(2+) influx. The SDH activity contained in the SDHA/SDHB subcomplex remains intact and then generates, in an uncontrolled fashion, excessive amounts of reactive oxygen species (ROS) for cell death. Future studies on this mitochondrial complex will further elucidate it as a target for cancer treatments and reveal its role as a nexus for many diverse stimuli in cell death signaling.

Bornyl caffeate induces apoptosis in human breast cancer MCF-7 cells via the ROS- and JNK-mediated pathways

AIM

The purpose of the present study was to investigate the anticancer activity of bornyl caffeate in the human breast cancer cell line MCF-7.

METHODS

The cell viability was determined using the MTT assay, and apoptosis was initially defined by monitoring the morphology of the cell nuclei and staining an early apoptotic biomarker with Annexin V-FITC. The mitochondrial membrane potential was visualized by JC-1 under fluorescence microscopy, whereas intracellular reactive oxygen species (ROS) were assessed by flow cytometry. The expression of apoptosis-associated proteins was determined by Western blotting analysis.

RESULTS

Bornyl caffeate induced apoptosis in MCF-7 cells in a dose- and time-dependent manner. Consistently, bornyl caffeate increased Bax and decreased Bcl-xl, resulting in the disruption of MMP and subsequent activation of caspase-3. Moreover, bornyl caffeate triggered the formation of ROS and the activation of the mitogen-activated protein (MAP) kinases p38 and c-Jun N-terminal kinase (JNK). Antioxidants attenuated the activation of MAP kinase p38 but barely affected the activation of JNK. Importantly, the cytotoxicity of bornyl caffeate was partially attenuated by scavenging ROS and inhibited by MAP kinases and caspases.

CONCLUSION

The present study demonstrated that bornyl caffeate induced apoptosis in the cancer cell line MCF-7 via activating the ROS- and JNK-mediated pathways. Thus, bornyl caffeate may be a potential anticancer lead compound.

Discovery of potent broad spectrum antivirals derived from marine actinobacteria

Natural products provide a vast array of chemical structures to explore in the discovery of new medicines. Although secondary metabolites produced by microbes have been developed to treat a variety of diseases, including bacterial and fungal infections, to date there has been limited investigation of natural products with antiviral activity. In this report, we used a phenotypic cell-based replicon assay coupled with an iterative biochemical fractionation process to identify, purify, and characterize antiviral compounds produced by marine microbes. We isolated a compound from Streptomyces kaviengensis, a novel actinomycetes isolated from marine sediments obtained off the coast of New Ireland, Papua New Guinea, which we identified as antimycin A1a. This compound displays potent activity against western equine encephalitis virus in cultured cells with half-maximal inhibitory concentrations of less than 4 nM and a selectivity index of greater than 550. Our efforts also revealed that several antimycin A analogues display antiviral activity, and mechanism of action studies confirmed that these Streptomyces-derived secondary metabolites function by inhibiting the cellular mitochondrial electron transport chain, thereby suppressing de novo pyrimidine synthesis. Furthermore, we found that antimycin A functions as a broad spectrum agent with activity against a wide range of RNA viruses in cultured cells, including members of the Togaviridae, Flaviviridae, Bunyaviridae, Picornaviridae, and Paramyxoviridae families. Finally, we demonstrate that antimycin A reduces central nervous system viral titers, improves clinical disease severity, and enhances survival in mice given a lethal challenge with western equine encephalitis virus. Our results provide conclusive validation for using natural product resources derived from marine microbes as source material for antiviral drug discovery, and they indicate that host mitochondrial electron transport is a viable target for the continued development of broadly active antiviral compounds.

Synthetic phosphoethanolamine has in vitro and in vivo anti-leukemia effects

Background:

We recently showed that synthetic phosphoethanolamine reduces tumour growth and inhibits lung metastasis in vivo. Here, we investigated its anti-leukaemia effects using acute promyelocytic leukaemia (APL) as a model.

Methods:

Cytotoxic effects of Pho-s on leukaemia cells were evaluated by MTT assay. Leukaemic cells obtained from hCG-PML-RARa transgenic mice were transplanted to NOD/SCID mice. After the animals were diagnosed as leukaemic, treatment started with Pho-s using all-trans retinoid acid or daunorubicin as positive control or and saline control. Cell morphology and immunophenotyping were used to detect the undifferentiated blast cells in the spleen, liver and bone marrow. The induction of apoptosis in vitro and in malignant leukaemic clones was evaluated.

Results:

Synthetic phosphoethanolamine is cytotoxic and induces apoptosis through the mitochondrial pathway in vitro to leukaemia cell lines. In vivo Pho-s exhibits anti-proliferative effects in APL model reducing the number of CD117⁺ and Gr-1⁺ immature myeloid cells in the BM, spleen and liver. Synthetic phosphoethanolamine impairs the expansion of malignant clones CD34⁺/CD117⁺, CD34⁺ and Gr-1⁺ in the BM. In addition, Pho-s induces apoptosis of immature cells in the spleen and liver, a notable effect.

Conclusion:

Synthetic phosphoethanolamine has anti-leukaemic effects in an APL model by inhibiting malignant clone expansion, suggesting that it is an interesting compound for leukaemia treatment.

Mitochondrial targeting overcomes ABCA1-dependent resistance of lung carcinoma to α-tocopheryl succinate

α-Tocopheryl succinate (α-TOS) is a promising anti-cancer agent due to its selectivity for cancer cells. It is important to understand whether long-term exposure of tumour cells to the agent will render them resistant to the treatment. Exposure of the non-small cell lung carcinoma H1299 cells to escalating doses of α-TOS made them resistant to the agent due to the upregulation of the ABCA1 protein, which caused its efflux. Full susceptibility of the cells to α-TOS was restored by knocking down the ABCA1 protein. Similar resistance including ABCA1 gene upregulation was observed in the A549 lung cancer cells exposed to α-TOS. The resistance of the cells to α-TOS was overcome by its mitochondrially targeted analogue, MitoVES, that is taken up on the basis of the membrane potential, bypassing the enhanced expression of the ABCA1 protein. The in vitro results were replicated in mouse models of tumours derived from parental and resistant H1299 cells. We conclude that long-term exposure of cancer cells to α-TOS causes their resistance to the drug, which can be overcome by its mitochondrially targeted counterpart. This finding should be taken into consideration when planning clinical trials with vitamin E analogues.

Silencing of doublecortin-like (DCL) results in decreased mitochondrial activity and delayed neuroblastoma tumor growth

Doublecortin-like (DCL) is a microtubule-binding protein crucial for neuroblastoma (NB) cell proliferation. We have investigated whether the anti-proliferative effect of DCL knockdown is linked to reduced mitochondrial activity. We found a delay in tumor development after DCL knockdown in vivo in doxycycline-inducible NB tumor xenografts. To understand the mechanisms underlying this tumor growth retardation we performed a series of in vitro experiments in NB cell lines. DCL colocalizes with mitochondria, interacts with the mitochondrial outer membrane protein OMP25/ SYNJ2BP and DCL knockdown results in decreased expression of genes involved in oxidative phosphorylation. Moreover, DCL knockdown decreases cytochrome c oxidase activity and ATP synthesis. We identified the C-terminal Serine/Proline-rich domain and the second microtubule-binding area as crucial DCL domains for the regulation of cytochrome c oxidase activity and ATP synthesis. Furthermore, DCL knockdown causes a significant reduction in the proliferation rate of NB cells under an energetic challenge induced by low glucose availability. Together with our previous studies, our results corroborate DCL as a key player in NB tumor growth in which DCL controls not only mitotic spindle formation and the stabilization of the microtubule cytoskeleton, but also regulates mitochondrial activity and energy availability, which makes DCL a promising molecular target for NB therapy.

Alpha-tocopheryl succinate inhibits autophagic survival of prostate cancer cells induced by vitamin K3 and ascorbate to trigger cell death

Background

The redox-silent vitamin E analog α-tocopheryl succinate (α-TOS) was found to synergistically cooperate with vitamin K3 (VK3) plus ascorbic acid (AA) in the induction of cancer cell-selective apoptosis via a caspase-independent pathway. Here we investigated the molecular mechanism(s) underlying cell death induced in prostate cancer cells by α-TOS, VK3 and AA, and the potential use of targeted drug combination in the treatment of prostate cancer.

Methodology/Principal Findings

The generation of ROS, cellular response to oxidative stress, and autophagy were investigated in PC3 prostate cancer cells by using drugs at sub-toxic doses. We evaluated whether PARP1-mediated apoptosis-inducing factor (AIF) release plays a role in apoptosis induced by the combination of the agents. Next, the effect of the combination of α-TOS, VK3 and AA on tumor growth was examined in nude mice. VK3 plus AA induced early ROS formation associated with induction of autophagy in response to oxidative stress, which was reduced by α-TOS, preventing the formation of autophagosomes. α-TOS induced mitochondrial destabilization leading to the release of AIF. Translocation of AIF from mitochondria to the nucleus, a result of the combinatorial treatment, was mediated by PARP1 activation. The inhibition of AIF as well as of PARP1 efficiently attenuated apoptosis triggered by the drug combination. Using a mouse model of prostate cancer, the combination of α-TOS, VK3 and AA was more efficient in tumor suppression than when the drugs were given separately, without deleterious side effects.

Conclusions/Significance

α-TOS, a mitochondria-targeting apoptotic agent, switches at sub-apoptotic doses from autophagy-dependent survival of cancer cells to their demise by promoting the induction of apoptosis. Given the grim prognosis for cancer patients, this finding is of potential clinical relevance.

Mitochondrial complex II, a novel target for anti-cancer agents

With the arrival of the third millennium, in spite of unprecedented progress in molecular medicine, cancer remains as untamed as ever. The complexity of tumours, dictating the potential response of cancer cells to anti-cancer agents, has been recently highlighted in a landmark paper by Weinberg and Hanahan on hallmarks of cancer [1]. Together with the recently published papers on the complexity of tumours in patients and even within the same tumour (see below), the cure for this pathology seems to be an elusive goal. Indisputably, the strategy ought to be changed, searching for targets that are generally invariant across the landscape of neoplastic diseases. One such target appears to be the mitochondrial complex II (CII) of the electron transfer chain, a recent focus of research. We document and highlight this particularly intriguing target in this review paper and give examples of drugs that use CII as their molecular target. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Glutamine deprivation enhances antitumor activity of 3-bromopyruvate through the stabilization of monocarboxylate transporter-1

Anticancer drug efficacy might be leveraged by strategies to target certain biochemical adaptations of tumors. Here we show how depriving cancer cells of glutamine can enhance the anticancer properties of 3-bromopyruvate, a halogenated analog of pyruvic acid. Glutamine deprival potentiated 3-bromopyruvate chemotherapy by increasing the stability of the monocarboxylate transporter-1, an effect that sensitized cells to metabolic oxidative stress and autophagic cell death. We further elucidated mechanisms through which resistance to chemopotentiation by glutamine deprival could be circumvented. Overall, our findings offer a preclinical proof-of-concept for how to employ 3-bromopyruvate or other monocarboxylic-based drugs to sensitize tumors to chemotherapy.

Molecular mechanism for the selective impairment of cancer mitochondrial function by a mitochondrially targeted vitamin E analogue

The effects of α-tocopheryl succinate (α-TOS), α-tocopheryl acetyl ether (α-TEA) and triphenylphosphonium-tagged vitamin E succinate (mitochondrially targeted vitamin E succinate; MitoVES) on energy-related mitochondrial functions were determined in mitochondria isolated from AS-30D hepatoma and rat liver, bovine heart sub-mitochondrial particles (SMPs), and in rodent and human carcinoma cell lines and rat hepatocytes. In isolated mitochondria, MitoVES stimulated basal respiration and ATP hydrolysis, but inhibited net state 3 (ADP-stimulated) respiration and Ca(2+) uptake, by collapsing the membrane potential at low doses (1-10μM). Uncoupled mitochondrial respiration and basal respiration of SMPs were inhibited by the three drugs at concentrations at least one order of magnitude higher and with different efficacy: MitoVES>α-TEA>α-TOS. At high doses (>10μM), the respiratory complex II (CII) was the most sensitive MitoVES target. Acting as an uncoupler at low doses, this agent stimulated total O(2) uptake, collapsed ∆ψ(m), inhibited oxidative phosphorylation and induced ATP depletion in rodent and human cancer cells more potently than in normal rat hepatocytes. These findings revealed that in situ tumor mitochondria are preferred targets of the drug, indicating its clinical relevance.

Implications of altered glutathione metabolism in aspirin-induced oxidative stress and mitochondrial dysfunction in HepG2 cells

We have previously reported that acetylsalicylic acid (aspirin, ASA) induces cell cycle arrest, oxidative stress and mitochondrial dysfunction in HepG2 cells. In the present study, we have further elucidated that altered glutathione (GSH)-redox metabolism in HepG2 cells play a critical role in ASA-induced cytotoxicity. Using selected doses and time point for ASA toxicity, we have demonstrated that when GSH synthesis is inhibited in HepG2 cells by buthionine sulfoximine (BSO), prior to ASA treatment, cytotoxicity of the drug is augmented. On the other hand, when GSH-depleted cells were treated with N-acetyl cysteine (NAC), cytotoxicity/apoptosis caused by ASA was attenuated with a significant recovery in oxidative stress, GSH homeostasis, DNA fragmentation and some of the mitochondrial functions. NAC treatment, however, had no significant effects on the drug-induced inhibition of mitochondrial aconitase activity and ATP synthesis in GSH-depleted cells. Our results have confirmed that aspirin increases apoptosis by increased reactive oxygen species production, loss of mitochondrial membrane potential and inhibition of mitochondrial respiratory functions. These effects were further amplified when GSH-depleted cells were treated with ASA. We have also shown that some of the effects of aspirin might be associated with reduced GSH homeostasis, as treatment of cells with NAC attenuated the effects of BSO and aspirin. Our results strongly suggest that GSH dependent redox homeostasis in HepG2 cells is critical in preserving mitochondrial functions and preventing oxidative stress associated complications caused by aspirin treatment.

Hyperactivation of oxidative mitochondrial metabolism in epithelial cancer cells in situ: visualizing the therapeutic effects of metformin in tumor tissue

We have recently proposed a new mechanism for explaining energy transfer in cancer metabolism. In this scenario, cancer cells behave as metabolic parasites, by extracting nutrients from normal host cells, such as fibroblasts, via the secretion of hydrogen peroxide as the initial trigger. Oxidative stress in the tumor microenvironment then leads to autophagy-driven catabolism, mitochondrial dys-function, and aerobic glycolysis. This, in turn, produces high-energy nutrients (such as L-lactate, ketones, and glutamine) that drive the anabolic growth of tumor cells, via oxidative mitochondrial metabolism. A logical prediction of this new "parasitic" cancer model is that tumor-associated fibroblasts should show evidence of mitochondrial dys-function (mitophagy and aerobic glycolysis). In contrast, epithelial cancer cells should increase their oxidative mitochondrial capacity. To further test this hypothesis, here we subjected frozen sections from human breast tumors to a staining procedure that only detects functional mitochondria. This method detects the in situ enzymatic activity of cytochrome C oxidase (COX), also known as Complex IV. Remarkably, cancer cells show an over-abundance of COX activity, while adjacent stromal cells remain essentially negative. Adjacent normal ductal epithelial cells also show little or no COX activity, relative to epithelial cancer cells. Thus, oxidative mitochondrial activity is selectively amplified in cancer cells. Although COX activity staining has never been applied to cancer tissues, it could now be used routinely to distinguish cancer cells from normal cells, and to establish negative margins during cancer surgery. Similar results were obtained with NADH activity staining, which measures Complex I activity, and succinate dehydrogenase (SDH) activity staining, which measures Complex II activity. COX and NADH activities were blocked by electron transport inhibitors, such as Metformin. This has mechanistic and clinical implications for using Metformin as an anti-cancer drug, both for cancer therapy and chemo-prevention. We also immuno-stained human breast cancers for a series of well-established protein biomarkers of metabolism. More specifically, we now show that cancer-associated fibroblasts over-express markers of autophagy (cathepsin B), mitophagy (BNIP3L), and aerobic glycolysis (MCT4). Conversely, epithelial cancer cells show the over-expression of a mitochondrial membrane marker (TOMM20), as well as key components of Complex IV (MT-CO1) and Complex II (SDH-B). We also validated our observations using a bioinformatics approach with data from > 2,000 breast cancer patients, which showed the transcriptional upregulation of mitochondrial oxidative phosphorylation (OXPHOS) in human breast tumors (p < 10(-20)), and a specific association with metastasis. Therefore, upregulation of OXPHOS in epithelial tumor cells is a common feature of human breast cancers. In summary, our data provide the first functional in vivo evidence that epithelial cancer cells perform enhanced mitochondrial oxidative phosphorylation, allowing them to produce high amounts of ATP. Thus, we believe that mitochondria are both the "powerhouse" and "Achilles' heel" of cancer cells.