Mitochondrial electron-transport-chain inhibitors of complexes I and II induce autophagic cell death mediated by reactive oxygen species

XAS and XFM studies of selenium and copper speciation and distribution in the kidneys of selenite-supplemented rats

Se and Cu were colocalised in the kidneys of selenite-fed rats, but there was no evidence of Se–Cu bonding.

Condurango glycoside-rich components stimulate DNA damage-induced cell cycle arrest and ROS-mediated caspase-3 dependent apoptosis through inhibition of cell-proliferation in lung cancer, in vitro and in vivo

Chemotherapeutic potential of Condurango glycoside-rich components (CGS) was evaluated in NSCLC, in vitro and in BaP-intoxicated rats, in vivo. NSCLC cells were treated with different concentrations of CGS to test their effect on cell viability. Cellular morphology, DNA-damage, AnnexinV-FITC/PI, cell cycle regulation, ROS-accumulation, MMP, and expressions of related signalling genes were critically analysed. 0.22 μg/μl CGS (IC₅₀ dose at 24 h) was selected for the study. CGS-induced apoptosis via DNA damage was evidenced by DNA-ladder formation, increase of AnnexinV-positive cells, cell cycle arrest at subG0/G1 and differential expressions of apoptotic genes. ROS-elevation and MMP-depolarization with significant caspase-3 activation might lead to apoptotic cell death. Anti-proliferative activity was confirmed by EGFR-expression modulation. ROS accumulation and DNA-nick formation with tissue damage-repair activity after post-cancerous CGS treatment, in vivo, supported the in vitro findings. Overall results advocate considerable apoptosis-inducing potential of CGS against NSCLC, validating its use against lung cancer by CAM practitioners.

Understanding the non-canonical pathways involved in p53-mediated tumor suppression

In the last three decades since the discovery of p53, it has become increasingly apparent that p53 plays a very important role in tumor suppression. Previously, it was thought that the tumor suppressive functions lied solely in the canonical p53-mediated apoptosis, cell cycle arrest and senescence. However, more recent research has shown that anti-oncogenic activity of p53 can still occur in the absence of these downstream functions. These results suggest that more non-canonical roles of p53 may have a much larger impact on other p53-regulated programs then initially anticipated. Recently, the non-canonical activities of p53 such as cell metabolism, autophagy and necrosis have been the subject of intense study. p53 affects many aspects of cellular metabolism including catabolism, anabolism and reactive oxygen species levels. p53 has a dual role in autophagy regulation. Initiation of autophagy occurs through direct transcription of pro-autophagy genes and inhibition transpires through a transcription-independent mechanism. The role of p53 in these cellular processes is quite complex and evidence suggests that p53 can play both a pro- and anti-oncogenic role in these non-conical pathways. Despite of more than 60,000 publications on p53 in the literature, the mechanisms for p53-mediated tumor suppression apparently needs to be further elucidated.

PPARγ activation rescues mitochondrial function from inhibition of complex I and loss of PINK1

Parkinson's disease has long been associated with impaired mitochondrial complex I activity, while several gene defects associated with familial Parkinson's involve defects in mitochondrial function or 'quality control' pathways, causing an imbalance between mitochondrial biogenesis and removal of dysfunctional mitochondria by autophagy. Amongst these are mutations of the gene for PTEN-induced kinase 1 (PINK1) in which mitochondrial function is abnormal. Peroxisome proliferator-activated receptor gamma (PPARγ), a nuclear receptor and ligand-dependent transcription factor, regulates pathways of inflammation, lipid and carbohydrate metabolism, antioxidant defences and mitochondrial biogenesis. We have found that inhibition of complex I in human differentiated SHSY-5Y cells by the complex I inhibitor rotenone irreversibly decrease mitochondrial mass, membrane potential and oxygen consumption, while increasing free radical generation and autophagy. Similar changes are seen in PINK1 knockdown cells, in which potential, oxygen consumption and mitochondrial mass are all decreased. In both models, all these changes were reversed by pre-treatment of the cells with the PPARγ agonist, rosiglitazone, which increased mitochondrial biogenesis, increased oxygen consumption and suppressed free radical generation and autophagy. Thus, rosiglitazone is neuroprotective in two different models of mitochondrial dysfunction associated with Parkinson's disease through a direct impact on mitochondrial function.

How Parkinsonian toxins dysregulate the autophagy machinery

Since their discovery, Parkinsonian toxins (6-hydroxydopamine, MPP+, paraquat, and rotenone) have been widely employed as in vivo and in vitro chemical models of Parkinson's disease (PD). Alterations in mitochondrial homeostasis, protein quality control pathways, and more recently, autophagy/mitophagy have been implicated in neurotoxin models of PD. Here, we highlight the molecular mechanisms by which different PD toxins dysregulate autophagy/mitophagy and how alterations of these pathways play beneficial or detrimental roles in dopamine neurons. The convergent and divergent effects of PD toxins on mitochondrial function and autophagy/mitophagy are also discussed in this review. Furthermore, we propose new diagnostic tools and discuss how pharmacological modulators of autophagy/mitophagy can be developed as disease-modifying treatments for PD. Finally, we discuss the critical need to identify endogenous and synthetic forms of PD toxins and develop efficient health preventive programs to mitigate the risk of developing PD.

Dichloroacetate induces protective autophagy in LoVo cells: involvement of cathepsin D/thioredoxin-like protein 1 and Akt-mTOR-mediated signaling

Dichloroacetate (DCA) is an inhibitor of pyruvate dehydrogenase kinase (PDK), and recently it has been shown as a promising nontoxic antineoplastic agent. In this study, we demonstrated that DCA could induce autophagy in LoVo cells, which were confirmed by the formation of autophagosomes, appearance of punctate patterns of LC3 immunoreactivity and activation of autophagy associated proteins. Moreover, autophagy inhibition by 3-methyladenine (3-MA) or Atg7 siRNA treatment can significantly enhance DCA-induced apoptosis. To determine the underlying mechanism of DCA-induced autophagy, target identification using drug affinity responsive target stability (DARTS) coupled with ESI-Q-TOF MS/MS analysis were utilized to profile differentially expressed proteins between control and DCA-treated LoVo cells. As a result, Cathepsin D (CTSD) and thioredoxin-like protein 1 (TXNL1) were identified with significant alterations compared with control. Further study indicated that DCA treatment significantly promoted abnormal reactive oxygen species (ROS) production. On the other hand, DCA-triggered autophagy could be attenuated by N-acetyl cysteine (NAC), a ROS inhibitor. Finally, we demonstrated that the Akt-mTOR signaling pathway, a major negative regulator of autophagy, was suppressed by DCA treatment. To our knowledge, it was the first study to show that DCA induced protective autophagy in LoVo cells, and the potential mechanisms were involved in ROS imbalance and Akt-mTOR signaling pathway suppression.

Autophagic vacuolation induced by excess ROS generation in HABP1/p32/gC1qR overexpressing fibroblasts and its reversal by polymeric hyaluronan

The ubiquitous hyaladherin, hyaluronan-binding protein 1 (HABP1/p32/gC1qR) upon stable overexpression in normal fibroblasts (F-HABP07) has been reported to induce mitochondrial dysfunction, growth retardation and apoptosis after 72 h of growth. HABP1 has been observed to accumulate in the mitochondria resulting in generation of excess Reactive Oxygen Species (ROS), mitochondrial Ca⁺⁺ efflux and drop in mitochondrial membrane potential. In the present study, autophagic vacuolation was detected with monodansylcadaverin (MDC) staining from 36 h to 60 h of culture period along with elevated level of ROS in F-HABP07 cells. Increased expression of autophagic markers like MAP-LC3-II, Beclin 1 and autophagic modulator, DRAM confirmed the occurrence of the phenomenon. Reduced vacuole formation was observed upon treatment with 3-MA, a known PI3 kinase inhibitor, only at 32 h and was ineffective if treated later, as high ROS level was already attained. Treatment of F111 and F-HABP07 cells with bafilomycin A1 further indicated an increase in autophagosome formation along with autophagic degradation in HABP1 overexpressed fibroblasts. Comparison between normal fibroblast (F111) and F-HABP07 cells indicate reduced level of polymeric HA, its depolymerization and perturbed HA-HABP1 interaction in F-HABP07. Interestingly, supplementation of polymeric HA, an endogenous ROS scavenger, in the culture medium prompted reduction in number of vacuoles in F-HABP07 along with drop in ROS level, implying that excess ROS generation triggers initiation of autophagic vacuole formation prior to apoptosis due to overexpression of HABP1. Thus, the phenomenon of autophagy takes place prior to apoptosis induction in the HABP1 overexpressing cell line, F-HABP07.

Autophagy prevents irradiation injury and maintains stemness through decreasing ROS generation in mesenchymal stem cells

Stem cells were characterized by their stemness: self-renewal and pluripotency. Mesenchymal stem cells (MSCs) are a unique type of adult stem cells that have been proven to be involved in tissue repair, immunoloregulation and tumorigenesis. Irradiation is a well-known factor that leads to functional obstacle in stem cells. However, the mechanism of stemness maintenance in human MSCs exposed to irradiation remains unknown. We demonstrated that irradiation could induce reactive oxygen species (ROS) accumulation that resulted in DNA damage and stemness injury in MSCs. Autophagy induced by starvation or rapamycin can reduce ROS accumulation-associated DNA damage and maintain stemness in MSCs. Further, inhibition of autophagy leads to augment of ROS accumulation and DNA damage, which results in the loss of stemness in MSCs. Our results indicate that autophagy may have an important role in protecting stemness of MSCs from irradiation injury.

Piperlongumine induces autophagy by targeting p38 signaling

Piperlongumine (PL), a natural product isolated from the plant species Piper longum L., can selectively induce apoptotic cell death in cancer cells by targeting the stress response to reactive oxygen species (ROS). Here we show that PL induces cell death in the presence of benzyloxycarbonylvalyl-alanyl-aspartic acid (O-methyl)-fluoro-methylketone (zVAD-fmk), a pan-apoptotic inhibitor, and in the presence of necrostatin-1, a necrotic inhibitor. Instead PL-induced cell death can be suppressed by 3-methyladenine, an autophagy inhibitor, and substantially attenuated in cells lacking the autophagy-related 5 (Atg5) gene. We further show that PL enhances autophagy activity without blocking autophagy flux. Application of N-acetyl-cysteine, an antioxidant, markedly reduces PL-induced autophagy and cell death, suggesting an essential role for intracellular ROS in PL-induced autophagy. Furthermore, PL stimulates the activation of p38 protein kinase through ROS-induced stress response and p38 signaling is necessary for the action of PL as SB203580, a p38 inhibitor, or dominant-negative p38 can effectively reduce PL-mediated autophagy. Thus, we have characterized a new mechanism for PL-induced cell death through the ROS-p38 pathway. Our findings support the therapeutic potential of PL by triggering autophagic cell death.

Cellular metabolic and autophagic pathways: traffic control by redox signaling

It has been established that the key metabolic pathways of glycolysis and oxidative phosphorylation are intimately related to redox biology through control of cell signaling. Under physiological conditions glucose metabolism is linked to control of the NADH/NAD redox couple, as well as providing the major reductant, NADPH, for thiol-dependent antioxidant defenses. Retrograde signaling from the mitochondrion to the nucleus or cytosol controls cell growth and differentiation. Under pathological conditions mitochondria are targets for reactive oxygen and nitrogen species and are critical in controlling apoptotic cell death. At the interface of these metabolic pathways, the autophagy-lysosomal pathway functions to maintain mitochondrial quality and generally serves an important cytoprotective function. In this review we will discuss the autophagic response to reactive oxygen and nitrogen species that are generated from perturbations of cellular glucose metabolism and bioenergetic function.

Association of Egr-1 and autophagy-related gene polymorphism in men with chronic obstructive pulmonary disease

BACKGROUND/PURPOSE

Autophagy is important in cellular homeostasis and control of inflammatory immune response. Increased autophagy has recently been associated with increased cell death and chronic obstructive pulmonary disease (COPD) pathogenesis. Two autophagy regulator genes have been identified: Egr-1 (early growth response), associated with different phenotype expressions in asthma; and, Atg16L1 (autophagy related 16-like 1), a candidate gene responsible for susceptibility to chronic inflammatory diseases. We will explore the role of the Egr-1 and Atg16L1 gene polymorphisms in COPD.

METHODS

The genotypes of 151 male smoking patients with COPD and 100 male smoking controls were evaluated by polymerase chain reaction followed by restriction fragment length polymorphism analysis of the Egr-1 (-4071 A → G) rs7729723 and Atg16L-1 (T300A) rs2241880 variants.

RESULTS

The G allele of the Egr-1 gene polymorphism was associated with an increased risk of developing COPD [odds ratio (OR), 2.05; 95% confidence interval (CI), 1.15-3.72], and participants with the G allele polymorphism (GG and GA genotypes) had a 2.56-fold higher risk (OR, 2.56; 95% CI, 1.31-5.16) of having COPD than those homozygous for the A allele [35.8% (54/151) vs. 24.0% (24/100); p = 0.007]. Participants with the A allele of the Atg16L1 gene polymorphism (AA and AG genotypes) had a 3.34-fold higher risk (OR, 3.34; 95% CI, 1.32-8.97) of having COPD than those homozygous for the G allele [93.4% (141/151) vs. 81.0% (81/100); p = 0.013].

CONCLUSION

The Egr-1 and Atg16L1 genes' polymorphisms were significant risk factors for susceptibility to COPD. These results demonstrate that autophagy regulator genetic mutations are associated with COPD in male smokers.

Cadmium and cellular signaling cascades: interactions between cell death and survival pathways

Cellular stress elicited by the toxic metal Cd(2+) does not coerce the cell into committing to die from the onset. Rather, detoxification and adaptive processes are triggered concurrently, allowing survival until normal function is restored. With high Cd(2+), death pathways predominate. However, if sublethal stress levels affect cells for prolonged periods, as in chronic low Cd(2+) exposure, adaptive and survival mechanisms may deregulate, such that tumorigenesis ensues. Hence, death and malignancy are the two ends of a continuum of cellular responses to Cd(2+), determined by magnitude and duration of Cd(2+) stress. Signaling cascades are the key factors affecting cellular reactions to Cd(2+). This review critically surveys recent literature to outline major features of death and survival signaling pathways as well as their activation, interactions and cross talk in cells exposed to Cd(2+). Under physiological conditions, receptor activation generates 2nd messengers, which are short-lived and act specifically on effectors through their spatial and temporal dynamics to transiently alter effector activity. Cd(2+) recruits physiological 2nd messenger systems, in particular Ca(2+) and reactive oxygen species (ROS), which control key Ca(2+)- and redox-sensitive molecular switches dictating cell function and fate. Severe ROS/Ca(2+) signals activate cell death effectors (ceramides, ASK1-JNK/p38, calpains, caspases) and/or cause irreversible damage to vital organelles, such as mitochondria and endoplasmic reticulum (ER), whereas low localized ROS/Ca(2+) levels act as 2nd messengers promoting cellular adaptation and survival through signal transduction (ERK1/2, PI3K/Akt-PKB) and transcriptional regulators (Ref1-Nrf2, NF-κB, Wnt, AP-1, bestrophin-3). Other cellular proteins and processes targeted by ROS/Ca(2+) (metallothioneins, Bcl-2 proteins, ubiquitin-proteasome system, ER stress-associated unfolded protein response, autophagy, cell cycle) can evoke death or survival. Hence, temporary or permanent disruptions of ROS/Ca(2+) induced by Cd(2+) play a crucial role in eliciting, modulating and linking downstream cell death and adaptive and survival signaling cascades.

Chelerythrine induces reactive oxygen species-dependent mitochondrial apoptotic pathway in a murine T cell lymphoma

Chelerythrine is a well-known protein kinase C inhibitor and potential antiproliferative and antitumor pharmacological agent. Chelerythrine inhibits/suppresses the HSF1 phosphorylation by inhibiting PKC and blocks the nuclear migration and subsequent synthesis of hsp70 leading to reduced cell viability and activation of apoptotic machinery. Chelerythrine is also known to enhance the production of reactive oxygen intermediate that is strong activator of apoptosis in high concentration. Therefore, the present study intended to investigate the role of chelerythrine-induced reactive oxygen intermediate on the viability and apoptosis of Dalton's lymphoma cells. Enhanced production of reactive oxygen species in Dalton's lymphoma (DL) cells was observed upon treatment of chelerythrine only which was seen completely abolished on treatment of mitochondrial complex inhibitors rotenone and malonate, and anti-oxidant, N-acetyl-L-cysteine. Increased number of DL cells undergoing apoptosis, as observed by fluorescent microscopy and flow cytometry analysis, in chelerythrine only-treated group was seen that was significantly inhibited on treatment of mitochondrial complex inhibitors and anti-oxidants. Staurosporine, on the other hand, does not lead to enhanced production of reactive oxygen intermediate in DL cells.

Rotenone upregulates alpha-synuclein and myocyte enhancer factor 2D independently from lysosomal degradation inhibition

Dysfunctions of chaperone-mediated autophagy (CMA), the main catabolic pathway for alpha-synuclein, have been linked to the pathogenesis of Parkinson's disease (PD). Since till now there is limited information on how PD-related toxins may affect CMA, in this study we explored the effect of mitochondrial complex I inhibitor rotenone on CMA substrates, alpha-synuclein and MEF2D, and effectors, lamp2A and hsc70, in a human dopaminergic neuroblastoma SH-SY5Y cell line. Rotenone induced an upregulation of alpha-synuclein and MEF2D protein levels through the stimulation of their de novo synthesis rather than through a reduction of their CMA-mediated degradation. Moreover, increased MEF2D transcription resulted in higher nuclear protein levels that exert a protective role against mitochondrial dysfunction and oxidative stress. These results were compared with those obtained after lysosome inhibition with ammonium chloride. As expected, this toxin induced the cytosolic accumulation of both alpha-synuclein and MEF2D proteins, as the result of the inhibition of their lysosome-mediated degradation, while, differently from rotenone, ammonium chloride decreased MEF2D nuclear levels through the downregulation of its transcription, thus reducing its protective function. These results highlight that rotenone affects alpha-synuclein and MEF2D protein levels through a mechanism independent from lysosomal degradation inhibition.

p38(MAPK)/p53-Mediated Bax induction contributes to neurons degeneration in rotenone-induced cellular and rat models of Parkinson's disease

Rotenone is an environmental neurotoxin that induces degeneration of dopaminergic (DA) neurons in substantia nigra pars compacta (SNpc), which ultimately results in parkinsonism, but the molecular mechanisms of selective degeneration of nigral DA neurons are not fully understood. In the present study, we investigated the induction of p38(MAPK)/p53 and Bax in SNpc of Lewis rats after chronic treatment with rotenone and the contribution of Bax to rotenone-induced apoptotic commitment of differentiated PC12 cells. Lewis rats were subcutaneously treated with rotenone (1.5mg/kg) twice a day for 50days and the loss of tyrosine hydroxylase (THase), motor function impairment, and expression of p38(MAPK), P-p38(MAPK), p53, and Bax were assessed. After differentiated PC cells were treated with rotenone (500nM) for 6-36h, protein levels of p38(MAPK) and P-p38(MAPK), p53 nuclear translocation, Bax induction and cell death were measured. The results showed that rotenone administration significantly reduced motor activity and caused a loss of THase immunoreactivity in SNpc of Lewis rats. The degeneration of nigral DA neurons was accompanied by the increases in p38(MAPK), P-p38(MAPK), p53, and Bax protein levels. In cultured PC12 cells, rotenone also induced an upregulation of p38(MAPK), P-p38(MAPK), p53 and Bax. Pharmacological inhibition of p38(MAPK) with SB203580 (25μM) blunted rotenone-induced cell apoptosis. Treatment with SB203580 prevented the p53 nuclear translocation and upregulation of Bax. Inhibition of p53 with pifthrin-alpha or Bax with siRNAs significantly reduced rotenone-induced Bax induction and apoptotic cell death. These results suggest that the p38(MAPK)/p53-dependent induction of Bax contributes to rotenone's neurotoxicity in PD models.

Autophagy contributes to ING4-induced glioma cell death

Previous studies suggest that ING4, a novel member of ING (inhibitor of growth) family, can inhibit brain tumor growth. However, whether autophagy is involved in ING4-induced cell death still remains unknown. In this study, we found that in addition to apoptosis, autophagy also contributed to cell death induced by ING4. Autophagy levels were elevated following the exposure to Ad-ING4, including enhanced fluorescence intensity of monodansylcadervarine (MDC), a specific in vivo marker for autophagic vacuoles, and increased expression levels of the LC3-II and Beclin-1, wheras the autophagic levels were attenuated following the pretreatment of 3-MA, the inhibitor of autophagy, which significantly decreased the Ad-ING4-induced cell death compared with caspase inhibitor zVAD. Furthermore, ING4 also induced mitochondrial dysfunction, such as mitophagy, collapse of mitochondrial membrane potential and the intracellular ROS, which indicated that mitochondria might be associated with the process of autophagic cell death of glioma cells. Finally, the relationship among Bax, Bcl-2, Beclin-1 and caspase family proteins levels were analyzed in glioma cells U251MG and LN229 infected with Ad-ING4 or Ad-lacZ. It is suggested that both autophagy and apoptosis could contribute to ING4-induced glioma cell death, and mitochondria might play an important role in this process. Our findings reveal novel aspects of the autophagy in glioma cells that underlie the cytotoxic action of ING4, possibly providing new insights in the development of combinatorial therapies for gliomas.

Rheb regulates mitophagy induced by mitochondrial energetic status

Mitophagy has been recently described as a mechanism of elimination of damaged organelles. Although the regulation of the amount of mitochondria is a core issue concerning cellular energy homeostasis, the relationship between mitochondrial degradation and energetic activity has not yet been considered. Here, we report that the stimulation of mitochondrial oxidative phosphorylation enhances mitochondrial renewal by increasing its degradation rate. Upon high oxidative phosphorylation activity, we found that the small GTPase Rheb is recruited to the mitochondrial outer membrane. This mitochondrial localization of Rheb promotes mitophagy through a physical interaction with the mitochondrial autophagic receptor Nix and the autophagosomal protein LC3-II. Thus, Rheb-dependent mitophagy contributes to the maintenance of optimal mitochondrial energy production. Our data suggest that mitochondrial degradation contributes to a bulk renewal of the organelle in order to prevent mitochondrial aging and to maintain the efficiency of oxidative phosphorylation.

The protective roles of autophagy in ischemic preconditioning

Autophagy, a process for the degradation of protein aggregates and dysfunctional organelles, is required for cellular homeostasis and cell survival in response to stress and is implicated in endogenous protection. Ischemic preconditioning is a brief and nonlethal episode of ischemia, confers protection against subsequent ischemia-reperfusion through the up-regulation of endogenous protective mechanisms. Emerging evidence shows that autophagy is associated with the protective effect of ischemic preconditioning. This review summarizes recent progress in research on the functions and regulations of the autophagy pathway in preconditioning-induced protection and cellular survival.

Mechanistic toxicity study of perfluorooctanoic acid in zebrafish suggests mitochondrial dysfunction to play a key role in PFOA toxicity

The aquatic environment is an important site for perfluorooctanoic acid (PFOA) deposit. Nevertheless, the exact mode of action and its resulting toxicological effects in aquatic organisms remain largely unknown. To gain a better understanding of the mode of action of teleost PFOA toxicity, transcriptomics, proteomics, biochemical parameters and reproduction were integrated in this study. Male and female zebrafish were exposed to nominal concentrations of 0.1, 0.5 and 1 mg L(-1) PFOA for 4 and 28 d resulting in PFOA accumulation which was higher in males than in females. These gender-related differences were likely caused by different elimination rates due to distinct hormone levels and differences in transport activity by solute carriers. The general mode of action of PFOA was described as an increase of the mitochondrial membrane permeability followed by an impairment of aerobic ATP production. Depletion of liver glycogen stores together with altered expression levels of transcripts involved in carbohydrate metabolism, with emphasis on anaerobic metabolism, was probably a means of compensating for this decreased aerobic efficiency. The mitochondrial dysfunction further resulted in effects on oxidative stress and apoptosis at the gene transcript and protein level. As a consequence, evidence for the replacement of the affected cells and organelles to sustain tissue homeostasis was found at the transcript level, resulting in an even greater glycogen depletion. Despite this increase in metabolic expenditure, no effects on reproduction were found indicating that the fish seemed to cope with exposure to the tested concentrations of PFOA during the exposure period of 1 month.

Physiological roles of mitochondrial reactive oxygen species

Historically, mitochondrial reactive oxygen species (mROS) were thought to exclusively cause cellular damage and lack a physiological function. Accumulation of ROS and oxidative damage have been linked to multiple pathologies, including neurodegenerative diseases, diabetes, cancer, and premature aging. Thus, mROS were originally envisioned as a necessary evil of oxidative metabolism, a product of an imperfect system. Yet few biological systems possess such flagrant imperfections, thanks to the persistent optimization of evolution, and it appears that oxidative metabolism is no different. More and more evidence suggests that mROS are critical for healthy cell function. In this Review, we discuss this evidence following some background on the generation and regulation of mROS.

Mitochondria and reactive oxygen species: physiology and pathophysiology

The air that we breathe contains nearly 21% oxygen, most of which is utilized by mitochondria during respiration. While we cannot live without it, it was perceived as a bane to aerobic organisms due to the generation of reactive oxygen and nitrogen metabolites by mitochondria and other cellular compartments. However, this dogma was challenged when these species were demonstrated to modulate cellular responses through altering signaling pathways. In fact, since this discovery of a dichotomous role of reactive species in immune function and signal transduction, research in this field grew at an exponential pace and the pursuit for mechanisms involved began. Due to a significant number of review articles present on the reactive species mediated cell death, we have focused on emerging novel pathways such as autophagy, signaling and maintenance of the mitochondrial network. Despite its role in several processes, increased reactive species generation has been associated with the origin and pathogenesis of a plethora of diseases. While it is tempting to speculate that anti-oxidant therapy would protect against these disorders, growing evidence suggests that this may not be true. This further supports our belief that these reactive species play a fundamental role in maintenance of cellular and tissue homeostasis.

Cell death, dysglycemia and myocardial infarction

Dysglycemia (hyper- and hypoglycemia) has been associated with higher mortality among patients suffering from myocardial infarction (MI). Moreover, dysglycemia may induce cell death. Cell death (necrosis, apoptosis and autophagy) is a ubiquitous process that characterizes the course of several diseases, including MI, and occurs in diverse forms varying in mechanism, pattern and consequence. Therefore, cell death is a potential pathway through which dysglycemia affects the outcome of MI and it is essential to regulate myocardial cell death in the treatment of patients with MI caused by dysglycemia. In this review, we summarized the mechanisms of MI at the cellular level and the regulatory effects of dysglycemia on myocardial cell death. The ability to modulate myocardial cell death may be a promising target of new treatments aimed at limiting MI caused by dysglycemia. However, further research is required to elucidate the mechanisms underlying cell death regulation in MI caused by dysglycemia.

Cerebrospinal fluid biomarkers in Parkinson disease

Clinical diagnosis of Parkinson disease (PD) is difficult in early stages of disease, with high risk of misdiagnosis. The long preclinical phase of PD provides the possibility for early therapeutic intervention once disease-modifying therapies have been developed, but lack of biomarkers for early diagnosis and monitoring of disease progression represents a major obstacle to achievement of this goal. Accordingly, research efforts aimed at identification of novel biomarkers have been increasing in the past 5 years. Cerebrospinal fluid (CSF) is an accessible source of brain-derived proteins, which mirror molecular changes that take place in the CNS. In this Review, we discuss evidence from numerous studies that have focused on identification of candidate CSF biomarkers for PD. Notably, molecular pathways related to α-synuclein, tau and β-amyloid peptides have received considerable attention. CSF levels of the protein DJ-1 are also of interest, although further investigation of this candidate marker is required. These studies support the usefulness of a combination of various CSF biomarkers of PD to increase diagnostic accuracy during early phases of the disease, and to differentiate PD from other neurodegenerative disorders.

Excessive autophagy contributes to neuron death in cerebral ischemia

AIMS

To determine the extent to which autophagy contributes to neuronal death in cerebral hypoxia and ischemia.

METHODS

We performed immunocytochemistry, western blot, cell viability assay, and electron microscopy to analyze autophagy activities in vitro and in vivo.

RESULTS

In both primary cortical neurons and SH-SY5Y cells exposed to oxygen and glucose deprivation (OGD)for 6 h and reperfusion (RP) for 24, 48, and 72 h, respectively, an increase of autophagy was observed as determined by the increased ratio of LC3-II to LC3-I and Beclin-1 (BECN1) expression. Using Fluoro-Jade C and monodansylcadaverine double-staining, and electron microscopy we found the increment in autophagy after OGD/RP was accompanied by increased autophagic cell death, and this increased cell death was inhibited by the specific autophagy inhibitor, 3-methyladenine. The presence of large autolysosomes and numerous autophagosomes in cortical neurons were confirmed by electron microscopy. Autophagy activities were increased dramatically in the ischemic brains 3-7 days postinjury from a rat model of neonatal cerebral hypoxia/ischemia as shown by increased punctate LC3 staining and BECN1 expression.

CONCLUSION

Excessive activation of autophagy contributes to neuronal death in cerebral ischemia.

Regulation of autophagy by metabolic and stress signaling pathways in the heart

Autophagy is an essential process for the maintenance of cellular homeostasis in the heart under both normal and stress conditions. Autophagy is a key degradation pathway and acts as a quality control sensor. It protects myocytes from cytotoxic protein aggregates and dysfunctional organelles by quickly clearing them from the cell. It also responds to changes in energy demand and mechanical stressors to maintain contractile function. The autophagic-lysosomal pathway responds to serum starvation to ensure that the cell maintains its metabolism and energy levels when nutrients run low. In contrast, excessive activation of autophagy is detrimental to cells and contributes to the development of pathological conditions. A number of signaling pathways and proteins regulate autophagy. These include the 5'-AMP-activated protein kinase/mammalian target of rapamycin pathway, FoxO transcription factors, Sirtuin 1, oxidative stress, Bcl-2 family proteins, and the E3 ubiquitin ligase Parkin. In this review, we will discuss how this diverse cast of characters regulates the important autophagic process in the myocardium.

Rotenone inhibits autophagic flux prior to inducing cell death

Rotenone, which selectively inhibits mitochondrial complex I, induces oxidative stress, α-synuclein accumulation, and dopaminergic neuron death, principal pathological features of Parkinson's disease. The autophagy-lysosome pathway degrades damaged proteins and organelles for the intracellular maintenance of nutrient and energy balance. While it is known that rotenone causes autophagic vacuole accumulation, the mechanism by which this effect occurs has not been thoroughly investigated. Treatment of differentiated SH-SY5Y cells with rotenone (10 μM) induced the accumulation of autophagic vacuoles at 6 h and 24 h as indicated by Western blot analysis for microtubule associated protein-light chain 3-II (MAP-LC3-II). Assessment of autophagic flux at these time points indicated that autophagic vacuole accumulation resulted from a decrease in their effective lysosomal degradation, which was substantiated by increased levels of autophagy substrates p62 and α-synuclein. Inhibition of lysosomal degradation may be explained by the observed decrease in cellular ATP levels, which in turn may have caused the observed concomitant increase in acidic vesicle pH. The early (6 h) effects of rotenone on cellular energetics and autophagy-lysosome pathway function preceded the induction of cell death and apoptosis. These findings indicate that the classical mitochondrial toxin rotenone has a pronounced effect on macroautophagy completion that may contribute to its neurotoxic potential.

Chemotherapeutic induction of mitochondrial oxidative stress activates GSK-3α/β and Bax, leading to permeability transition pore opening and tumor cell death

Survival of tumor cells is favored by mitochondrial changes that make death induction more difficult in a variety of stress conditions, such as exposure to chemotherapeutics. These changes are not fully characterized in tumor mitochondria, and include unbalance of the redox equilibrium, inhibition of permeability transition pore (PTP) opening through kinase signaling pathways and modulation of members of the Bcl-2 protein family. Here we show that a novel chemotherapeutic, the Gold(III)-dithiocarbamato complex AUL12, induces oxidative stress and tumor cell death both favoring PTP opening and activating the pro-apoptotic protein Bax of the Bcl-2 family. AUL12 inhibits the respiratory complex I and causes a rapid burst of mitochondrial superoxide levels, leading to activation of the mitochondrial fraction of GSK-3α/β and to the ensuing phosphorylation of the mitochondrial chaperone cyclophilin D, which in turn facilitates PTP opening. In addition, following AUL12 treatment, Bax interacts with active GSK-3α/β and translocates onto mitochondria, where it contributes to PTP induction and tumor cell death. These findings provide evidence that targeting the redox equilibrium maintained by mitochondria in tumor cells allows to hit crucial mechanisms that shield neoplasms from the toxicity of many anti-tumor strategies, and identify AUL12 as a promising chemotherapeutic compound.

Increased levels of reduced cytochrome b and mitophagy components are required to trigger nonspecific autophagy following induced mitochondrial dysfunction

Mitochondria are essential organelles producing most of the energy required for the cell. A selective autophagic process called mitophagy removes damaged mitochondria, which is critical for proper cellular homeostasis; dysfunctional mitochondria can generate excess reactive oxygen species that can further damage the organelle as well as other cellular components. Although proper cell physiology requires the maintenance of a healthy pool of mitochondria, little is known about the mechanism underlying the recognition and selection of damaged organelles. In this study, we investigated the cellular fate of mitochondria damaged by the action of respiratory inhibitors (antimycin A, myxothiazol, KCN) that act on mitochondrial respiratory complexes III and IV, but have different effects with regard to the production of reactive oxygen species and increased levels of reduced cytochromes. Antimycin A and potassium cyanide effectively induced nonspecific autophagy, but not mitophagy, in a wild-type strain of Saccharomyces cerevisiae; however, low or no autophagic activity was measured in strains deficient for genes that encode proteins involved in mitophagy, including ATG32, ATG11 and BCK1. These results provide evidence for a major role of specific mitophagy factors in the control of a general autophagic cellular response induced by mitochondrial alteration. Moreover, increased levels of reduced cytochrome b, one of the components of the respiratory chain, could be the first signal of this induction pathway.

Role of reduced manganese superoxide dismutase in ischemia-reperfusion injury: a possible trigger for autophagy and mitochondrial biogenesis?

Excessive generation of superoxide and mitochondrial dysfunction has been described as being important events during ischemia-reperfusion (I/R) injury. Our laboratory has demonstrated that manganese superoxide dismutase (MnSOD), a major mitochondrial antioxidant that eliminates superoxide, is inactivated during renal transplantation and renal I/R and precedes development of renal failure. We hypothesized that MnSOD knockdown in the kidney augments renal damage during renal I/R. Using newly characterized kidney-specific MnSOD knockout (KO) mice the extent of renal damage and oxidant production after I/R was evaluated. These KO mice (without I/R) exhibited low expression and activity of MnSOD in the distal nephrons, had altered renal morphology, increased oxidant production, but surprisingly showed no alteration in renal function. After I/R the MnSOD KO mice showed similar levels of injury to the distal nephrons when compared with wild-type mice. Moreover, renal function, MnSOD activity, and tubular cell death were not significantly altered between the two genotypes after I/R. Interestingly, MnSOD KO alone increased autophagosome formation, mitochondrial biogenesis, and DNA replication/repair within the distal nephrons. These findings suggest that the chronic oxidative stress as a result of MnSOD knockdown induced multiple coordinated cell survival signals including autophagy and mitochondrial biogenesis, which protected the kidney against the acute oxidative stress following I/R.

Manganese superoxide dismutase interacts with a large scale of cellular and mitochondrial proteins in low-dose radiation-induced adaptive radioprotection

The cellular adaptive response to certain low-level genotoxic stresses, including exposure to low-dose ionizing radiation (LDIR), shows promise as a tool to enhance radioprotection in normal cells but not in tumor cells. Manganese superoxide dismutase (MnSOD), a fundamental mitochondrial antioxidant in mammalian cells, plays a key role in the LDIR-induced adaptive response. In this study, we aimed to elucidate the signaling network associated with MnSOD-induced radiation protection. A MnSOD-interacting protein profile was established in LDIR-treated human skin cells. Human skin keratinocytes (HK18) were irradiated with a single dose of LDIR (10 cGy X-ray) and the cell lysates were immunoprecipitated using α-MnSOD and applied to two different gel-based proteomic experiments followed by mass spectrometry for protein identification. Analysis of the profiles of MnSOD-interacting partners before and after LDIR detected various patterns of MnSOD protein-protein interactions in response to LDIR. Interestingly, many of the MnSOD-interacting proteins are known to have functions related to mitochondrial regulation of cell metabolism, apoptosis, and DNA repair. These results provide evidence indicating that in addition to the enzymatic action of detoxifying superoxide, the antioxidant MnSOD may function as a signaling regulator in stress-induced adaptive protection through cell survival pathways.

A critical role for Mnt in Myc-driven T-cell proliferation and oncogenesis

Mnt (Max's next tango) is a Max-interacting transcriptional repressor that can antagonize both the proproliferative and proapoptotic functions of Myc in vitro. To ascertain the physiologically relevant functions of Mnt and to help define the relationship between Mnt and Myc in vivo, we generated a series of mouse strains in which Mnt was deleted in T cells in the absence of endogenous c-Myc or in the presence of ectopic c-Myc. We found that apoptosis caused by loss of Mnt did not require Myc but that ectopic Myc expression dramatically decreased the survival of both Mnt-deficient T cells in vivo and Mnt-deficient MEFs in vitro. Consequently, Myc-driven proliferative expansion of T cells in vitro and thymoma formation in vivo were prevented by the absence of Mnt. Consistent with T-cell models, mouse embryo fibroblasts (MEFs) lacking Mnt were refractory to oncogenic transformation by Myc. Tumor suppression caused by loss of Mnt was linked to increased apoptosis mediated by reactive oxygen species (ROS). Thus, although theoretically and experimentally a Myc antagonist, the dominant physiological role of Mnt appears to be suppression of apoptosis. Our results redefine the physiological relationship between Mnt and Myc and requirements for Myc-driven oncogenesis.

Reduction of STAT3 expression induces mitochondrial dysfunction and autophagy in cardiac HL-1 cells

Signal transducer and activator of transcription 3 (STAT3) is an important mediator of cardiac survival pathways. Reduced levels of STAT3 in patients with end-stage heart failure suggest a clinical relevance of STAT3 deficiency for cardiac disease. The recent identification of STAT3 as a mitochondrial protein which is important for full activity of mitochondrial complex I has opened a new field for the investigation of how STAT3 functions in cardioprotection. The goal of this study was to establish a cell culture model with a reduced STAT3 expression, and to use this model for the investigation of mitochondrial and mitochondrial-associated functions under STAT3 deficiency. In the murine cardiomyogenic cell line HL-1, the expression of STAT3 was silenced by lentiviral transduction with anti-STAT3 shRNA (STAT3 KD cells). STAT3 mRNA and protein levels were significantly reduced in HL-1 STAT3 KD cells compared to HL-1 cells transduced with a control shRNA. Spectrophotometric and polarographic assays with mitochondrial enriched fractions and intact cells showed reduced activities of respiratory chain complexes I, II, III and IV in HL-1 STAT3 KD cells. At ultrastructural level, a severe damage of mitochondrial integrity was observed, combined with a significant increase in autophagolysosomes in STAT3-deficient HL-1 cells. Our results demonstrate that the HL-1 STAT3 KD cell line is a good model to study cellular consequences of STAT3 deficiency. Moreover, this is the first study to show that STAT3 deficiency leads to a disruption of mitochondrial ultrastructure and increased autophagy.

Autophagy in toxicology: cause or consequence?

Research on autophagy and its effects on cell metabolism and physiology has increased dramatically during recent years. Multiple forms of autophagy have been characterized, and many of the genes involved in the regulation of this process have been identified. The importance of autophagy for embryonic development and maintenance of tissue homeostasis in the adult organism has been demonstrated convincingly, and several human diseases have been linked to deficiencies in autophagy. Most often, autophagy serves as a protective mechanism, but persistent activation of autophagy can result in cell death. This is true for many toxic agents. In fact, there are ample examples of cross talk between autophagy and other modes of cell death after exposure to toxicants. However, the relative contribution of autophagy to the overall toxicity of these compounds is not always clear, and further research is needed to clarify the toxicological significance of this process.

Mitochondrial c-Src regulates cell survival through phosphorylation of respiratory chain components

Mitochondrial protein tyrosine phosphorylation is an important mechanism for the modulation of mitochondrial functions. In the present study, we have identified novel substrates of c-Src in mitochondria and investigated their function in the regulation of oxidative phosphorylation. The Src family kinase inhibitor PP2 {amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo [3,4d] pyrimidine} exhibits significant reduction of respiration. Similar results were obtained from cells expressing kinase-dead c-Src, which harbours a mitochondrial-targeting sequence. Phosphorylation-site analysis selects c-Src targets, including NDUFV2 (NADH dehydrogenase [ubiquinone] flavoprotein 2) at Tyr(193) of respiratory complex I and SDHA (succinate dehydrogenase A) at Tyr(215) of complex II. The phosphorylation of these sites by c-Src is supported by an in vivo assay using cells expressing their phosphorylation-defective mutants. Comparison of cells expressing wild-type proteins and their mutants reveals that NDUFV2 phosphorylation is required for NADH dehydrogenase activity, affecting respiration activity and cellular ATP content. SDHA phosphorylation shows no effect on enzyme activity, but perturbed electron transfer, which induces reactive oxygen species. Loss of viability is observed in T98G cells and the primary neurons expressing these mutants. These results suggest that mitochondrial c-Src regulates the oxidative phosphorylation system by phosphorylating respiratory components and that c-Src activity is essential for cell viability.

Reactive oxygen species regulation of autophagy in cancer: implications for cancer treatment

Reactive oxygen species (ROS) are important in regulating normal cellular processes, but deregulated ROS contribute to the development of various human diseases including cancers. Autophagy is one of the first lines of defense against oxidative stress damage. The autophagy pathway can be induced and upregulated in response to intracellular ROS or extracellular oxidative stress. This leads to selective lysosomal self-digestion of intracellular components to maintain cellular homeostasis. Hence, autophagy is the survival pathway, conferring stress adaptation and promoting viability under oxidative stress. However, increasing evidence has demonstrated that autophagy can also lead to cell death under oxidative stress conditions. In addition, altered autophagic signaling pathways that lead to decreased autophagy are frequently found in many human cancers. This review discusses the advances in understanding of the mechanisms of ROS-induced autophagy and how this process relates to tumorigenesis and cancer therapy.

Salinomycin induces autophagy in colon and breast cancer cells with concomitant generation of reactive oxygen species

BACKGROUND

Salinomycin is a polyether ionophore antibiotic that has recently been shown to induce cell death in human cancer cells displaying multiple mechanisms of drug resistance. The underlying mechanisms leading to cell death after salinomycin treatment have not been well characterized. We therefore investigated the role of salinomycin in caspase dependent and independent cell death in colon cancer (SW480, SW620, RKO) and breast cancer cell lines (MCF-7, T47D, MDA-MB-453).

METHODOLOGY/PRINCIPAL FINDINGS

We detected features of apoptosis in all cell lines tested, but the executor caspases 3 and 7 were only strongly activated in RKO and MDA-MB-453 cells. MCF-7 and SW620 cells instead presented features of autophagy such as cytoplasmic vacuolization and LC3 processing. Caspase proficient cell lines activated autophagy at lower salinomycin concentrations and before the onset of caspase activation. Salinomycin also led to the formation of reactive oxygen species (ROS) eliciting JNK activation and induction of the transcription factor JUN. Salinomycin mediated cell death could be partially inhibited by the free radical scavenger N-acetyl-cysteine, implicating ROS formation in the mechanism of salinomycin toxicity.

CONCLUSIONS

Our data indicate that, in addition to its previously reported induction of caspase dependent apoptosis, the initiation of autophagy is an important and early effect of salinomycin in tumor cells.

Spatiotemporal autophagic degradation of oxidatively damaged organelles after photodynamic stress is amplified by mitochondrial reactive oxygen species

Although reactive oxygen species (ROS) have been reported to evoke different autophagic pathways, how ROS or their secondary products modulate the selective clearance of oxidatively damaged organelles is less explored. To investigate the signaling role of ROS and the impact of their compartmentalization in autophagy pathways, we used murine fibrosarcoma L929 cells overexpressing different antioxidant enzymes targeted to the cytosol or mitochondria and subjected them to photodynamic (PD) stress with the endoplasmic reticulum (ER)-associated photosensitizer hypericin. We show that following apical ROS-mediated damage to the ER, predominantly cells overexpressing mitochondria-associated glutathione peroxidase 4 (GPX4) and manganese superoxide dismutase (SOD2) displayed attenuated kinetics of autophagosome formation and overall cell death, as detected by computerized time-lapse microscopy. Consistent with a primary ER photodamage, kinetics and colocalization studies revealed that photogenerated ROS induced an initial reticulophagy, followed by morphological changes in the mitochondrial network that preceded clearance of mitochondria by mitophagy. Overexpression of cytosolic and mitochondria-associated GPX4 retained the tubular mitochondrial network in response to PD stress and concomitantly blocked the progression toward mitophagy. Preventing the formation of phospholipid hydroperoxides and H(2)O(2) in the cytosol as well as in the mitochondria significantly reduced cardiolipin peroxidation and apoptosis. All together, these results show that in response to apical ER photodamage ROS propagate to mitochondria, which in turn amplify ROS production, thereby contributing to two antagonizing processes, mitophagy and apoptosis.

Effect of resveratrol on oxygen consumption by Philasterides dicentrarchi, a scuticociliate parasite of turbot

The phytoalexin resveratrol (RESV) displays antiparasitic activity against Philasterides dicentrarchi, a scuticociliate pathogen of turbot, and causes oxidative stress, inhibition of antioxidant enzyme activity and morphological alterations in the parasite mitochondria. In this study, we analysed the mitochondrial biology of P. dicentrarchi and assessed the effect of RESV on mitochondrial metabolism. We found that RESV caused dose-dependent inhibition of mitochondrial electron transport and O₂ consumption in ciliates permeabilized with digitonin. Although the RESV molecule has a high capacity for antiradical and antioxidant activity, it induced a high level of pro-oxidant activity against the ciliate, thus causing a significant increase in intracellular ROS production. The increased ROS production was accompanied by mitochondrial collapse and dysfunction of mitochondrial membrane potential (ΔΨm) and by a significant increase in intracellular Ca⁺² levels. RESV inhibited parasite growth in a similar way to antimycin A, an inhibitor of mitochondrial electron transport and ROS generator. The findings confirm the mitochondria as a target in the potential development of effective antiparasitic treatments.

Defective autophagy in Parkinson's disease: role of oxidative stress

Parkinson's disease (PD) is a paradigmatic example of neurodegenerative disorder with a critical role of oxidative stress in its etiopathogenesis. Genetic susceptibility factors of PD, such as mutations in Parkin, PTEN-induced kinase 1, and DJ-1 as well as the exposure to pesticides and heavy metals, both contribute to altered redox balance and degeneration of dopaminergic neurons in the substantia nigra. Dysregulation of autophagy, a lysosomal-driven process of self degradation of cellular organelles and protein aggregates, is also implicated in PD and PD-related mutations, and environmental toxins deregulate autophagy. However, experimental evidence suggests a complex and ambiguous role of autophagy in PD since either impaired or abnormally upregulated autophagic flux has been shown to cause neuronal loss. Finally, it is generally believed that oxidative stress is a strong proautophagic stimulus. However, some evidence coming from neurobiology as well as from other fields indicate an inhibitory role of reactive oxygen species and reactive nitrogen species on the autophagic machinery. This review examines the scientific evidence supporting different concepts on how autophagy is dysregulated in PD and attempts to reconcile apparently contradictory views on the role of oxidative stress in autophagy regulation. The complex relationship between autophagy and oxidative stress is also considered in the context of the ongoing search for a novel PD therapy.

Immunohistochemical detection of cytoplasmic LC3 puncta in human cancer specimens

Autophagy is an evolutionarily conserved catabolic process that involves the entrapment of cytoplasmic components within characteristic vesicles for their delivery to and degradation within lysosomes. Alterations in autophagic signaling are found in several human diseases including cancer. Here, we describe a validated immunohistochemical protocol for the detection of LC3 puncta in human formalin-fixed, paraffin-embedded cancer specimens that can also be applied to mouse tissues. In response to systemic chemotherapy, autophagy-competent mouse tumors exhibited LC3 puncta, which did not appear in mouse cancers that had been rendered autophagy-deficient by the knockdown of Atg5 or Atg7. As compared with normal tissues, LC3 staining was moderately to highly elevated in the large majority of human cancers studied, albeit tumors of the same histological type tended to be highly heterogeneous in the number and intensity of LC3 puncta per cell. Moreover, tumor-infiltrating immune cells often were highly positive for LC3. Altogether, this protocol for LC3 staining appears suitable for the specific detection of LC3 puncta in human specimens, including tissue microarrays. We surmise that this technique can be employed for retrospective or prospective studies involving large series of human tumor samples.

Mitochondrial metabolism in Parkinson's disease impairs quality control autophagy by hampering microtubule-dependent traffic

Abnormal presence of autophagic vacuoles is evident in brains of patients with Parkinson's disease (PD), in contrast to the rare detection of autophagosomes in a normal brain. However, the actual cause and pathological significance of these observations remain unknown. Here, we demonstrate a role for mitochondrial metabolism in the regulation of the autophagy-lysosomal pathway in ex vivo and in vitro models of PD. We show that transferring mitochondria from PD patients into cells previously depleted of mitochondrial DNA is sufficient to reproduce the alterations in the autophagic system observed in PD patient brains. Although the initial steps of this pathway are not compromised, there is an increased accumulation of autophagosomes associated with a defective autophagic activity. We prove that this functional decline was originated from a deficient mobilization of autophagosomes from their site of formation toward lysosomes due to disruption in microtubule-dependent trafficking. This contributed directly to a decreased proteolytic flux of α-synuclein and other autophagic substrates. Our results lend strong support for a direct impact of mitochondria in autophagy as defective autophagic clearance ability secondary to impaired microtubule trafficking is driven by dysfunctional mitochondria. We uncover mitochondria and mitochondria-dependent intracellular traffic as main players in the regulation of autophagy in PD.

The neurogenic basic helix-loop-helix transcription factor NeuroD6 enhances mitochondrial biogenesis and bioenergetics to confer tolerance of neuronal PC12-NeuroD6 cells to the mitochondrial stressor rotenone

The fundamental question of how and which neuronal specific transcription factors tailor mitochondrial biogenesis and bioenergetics to the need of developing neuronal cells has remained largely unexplored. In this study, we report that the neurogenic basic helix-loop-helix transcription factor NeuroD6 possesses mitochondrial biogenic properties by amplifying the mitochondrial DNA content and TFAM expression levels, a key regulator for mitochondrial biogenesis. NeuroD6-mediated increase in mitochondrial biogenesis in the neuronal progenitor-like PC12-NEUROD6 cells is concomitant with enhanced mitochondrial bioenergetic functions, including increased expression levels of specific subunits of respiratory complexes of the electron transport chain, elevated mitochondrial membrane potential and ATP levels produced by oxidative phosphorylation. Thus, NeuroD6 augments the bioenergetic capacity of PC12-NEUROD6 cells to generate an energetic reserve, which confers tolerance to the mitochondrial stressor, rotenone. We found that NeuroD6 induces an adaptive bioenergetic response throughout rotenone treatment involving maintenance of the mitochondrial membrane potential and ATP levels in conjunction with preservation of the actin network. In conclusion, our results support the concept that NeuroD6 plays an integrative role in regulating and coordinating the onset of neuronal differentiation with acquisition of adequate mitochondrial mass and energetic capacity to ensure energy demanding events, such as cytoskeletal remodeling, plasmalemmal expansion, and growth cone formation.

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.

Regression of pathological cardiac hypertrophy: signaling pathways and therapeutic targets

Pathological cardiac hypertrophy is a key risk factor for heart failure. It is associated with increased interstitial fibrosis, cell death and cardiac dysfunction. The progression of pathological cardiac hypertrophy has long been considered as irreversible. However, recent clinical observations and experimental studies have produced evidence showing the reversal of pathological cardiac hypertrophy. Left ventricle assist devices used in heart failure patients for bridging to transplantation not only improve peripheral circulation but also often cause reverse remodeling of the geometry and recovery of the function of the heart. Dietary supplementation with physiologically relevant levels of copper can reverse pathological cardiac hypertrophy in mice. Angiogenesis is essential and vascular endothelial growth factor (VEGF) is a constitutive factor for the regression. The action of VEGF is mediated by VEGF receptor-1, whose activation is linked to cyclic GMP-dependent protein kinase-1 (PKG-1) signaling pathways, and inhibition of cyclic GMP degradation leads to regression of pathological cardiac hypertrophy. Most of these pathways are regulated by hypoxia-inducible factor. Potential therapeutic targets for promoting the regression include: promotion of angiogenesis, selective enhancement of VEGF receptor-1 signaling pathways, stimulation of PKG-1 pathways, and sustention of hypoxia-inducible factor transcriptional activity. More exciting insights into the regression of pathological cardiac hypertrophy are emerging. The time of translating the concept of regression of pathological cardiac hypertrophy to clinical practice is coming.

Macroautophagy and cell responses related to mitochondrial dysfunction, lipid metabolism and unconventional secretion of proteins

Macroautophagy has important physiological roles and its cytoprotective or detrimental function is compromised in various diseases such as many cancers and metabolic diseases. However, the importance of autophagy for cell responses has also been demonstrated in many other physiological and pathological situations. In this review, we discuss some of the recently discovered mechanisms involved in specific and unspecific autophagy related to mitochondrial dysfunction and organelle degradation, lipid metabolism and lipophagy as well as recent findings and evidence that link autophagy to unconventional protein secretion.

Arctigenin preferentially induces tumor cell death under glucose deprivation by inhibiting cellular energy metabolism

Selectively eradicating cancer cells with minimum adverse effects on normal cells is a major challenge in the development of anticancer therapy. We hypothesize that nutrient-limiting conditions frequently encountered by cancer cells in poorly vascularized solid tumors might provide an opportunity for developing selective therapy. In this study, we investigated the function and molecular mechanisms of a natural compound, arctigenin, in regulating tumor cell growth. We demonstrated that arctigenin selectively promoted glucose-starved A549 tumor cells to undergo necrosis by inhibiting mitochondrial respiration. In doing so, arctigenin elevated cellular level of reactive oxygen species (ROS) and blocked cellular energy metabolism in the glucose-starved tumor cells. We also demonstrated that cellular ROS generation was caused by intracellular ATP depletion and played an essential role in the arctigenin-induced tumor cell death under the glucose-limiting condition. Furthermore, we combined arctigenin with the glucose analogue 2-deoxyglucose (2DG) and examined their effects on tumor cell growth. Interestingly, this combination displayed preferential cell-death inducing activity against tumor cells compared to normal cells. Hence, we propose that the combination of arctigenin and 2DG may represent a promising new cancer therapy with minimal normal tissue toxicity.