The flavonoid quercetin induces changes in mitochondrial permeability by inhibiting adenine nucleotide translocase

Exploring the Therapeutic Potential of Flavonoids in the Management of Cancer

Flavonoids are a class of polyphenolic compounds that can be classified into six distinct categories, namely isoflavonoids, flavanones, flavanols, flavonols, flavones, and anthocyanidins. These compounds are naturally occurring and can be found in a diverse range of plant species. Flavonoids, a class of bioactive compounds, are mostly obtained through the consumption of vegetables, fruits and plant-derived beverages such as wine, cocoa-based products and green tea. Flavonoids have been demonstrated to exhibit a diverse range of anticancer properties. These include the modulation of activities of enzymes involved in scavenging reactive oxygen species, involvement in cell cycle arrest, induction of apoptosis and autophagy, as well as suppression of cancer cell proliferation and invasiveness. Flavonoids exhibit a dual role in maintaining reactive oxygen species balance. They function as antioxidants in regular physiological conditions, while also demonstrating significant pro-oxidant properties in cancer cells. This prooxidant activity induces apoptotic pathways and downregulates pro-inflammatory signalling pathways. The paper explores the biochemical characteristics, bioavailability, anticancer efficacy, and modes of action of flavonoids.

Functional activity and morphology of isolated rat cardiac mitochondria under calcium overload. Effect of naringin

The function of mitochondria as a regulator of myocyte calcium homeostasis has been extensively discussed. The aim of the present work was further clarification of the details of modulation of the functional activity of rat cardiac mitochondria by exogenous Ca2+ ions either in the absence or in the presence of the plant flavonoid naringin. Low free Ca2+ concentrations (40-250 nM) effectively inhibited the respiratory activity of heart mitochondria, remaining unaffected the efficacy of oxygen consumption. In the presence of high exogenous Ca2+ ion concentrations (Ca2+ free was 550 µM), we observed a dramatic increase in mitochondrial heterogeneity in size and electron density, which was related to calcium-induced opening of the mitochondrial permeability transition pores (MPTP) and membrane depolarization (Ca2+free ions were from 150 to 750 µM). Naringin partially prevented Ca2+-induced cardiac mitochondrial morphological transformations (200 µM) and dose-dependently inhibited the respiratory activity of mitochondria (10-75 µM) in the absence or in the presence of calcium ions. Our data suggest that naringin (75 µM) promoted membrane potential dissipation, diminishing the potential-dependent accumulation of calcium ions by mitochondria and inhibiting calcium-induced MPTP formation. The modulating effect of the flavonoid on Ca2+-induced mitochondria alterations may be attributed to the weak-acidic nature of the flavonoid and its protonophoric/ionophoric properties. Our results show that the sensitivity of rat heart mitochondria to Ca2+ ions was much lower in the case of MPTP opening and much higher in the case of respiration inhibition as compared to liver mitochondria.

The multifaceted role of quercetin derived from its mitochondrial mechanism

Quercetin is a flavonoid with promising therapeutic applications; nonetheless, the phenotype exerted in some diseases is contradictory. For instance, anticancer properties may be explained by a cytotoxic mechanism, whereas antioxidant-related neuroprotection is a pro-survival process. According to the available literature, quercetin exerts a redox interaction with the electron transport chain (ETC) in the mitochondrion, affecting its membrane potential. It also affects ATP generation by oxidative phosphorylation, where ATP deprivation could partly explain its cytotoxic effect. Moreover, quercetin may support the generation of free radicals through redox reactions, causing a prooxidant effect. The nutrimental stress and prooxidant effect induced by quercetin might promote pro-survival properties such as antioxidant processes. Thus, in this review, we discuss the evidence supporting that quercetin redox interaction with the ETC could explain its beneficial and toxic properties.

Single-cell analysis identifies conserved features of immune dysfunction in simulated microgravity and spaceflight

Microgravity is associated with immunological dysfunction, though the mechanisms are poorly understood. Here, using single-cell analysis of human peripheral blood mononuclear cells (PBMCs) exposed to short term (25 hours) simulated microgravity, we characterize altered genes and pathways at basal and stimulated states with a Toll-like Receptor-7/8 agonist. We validate single-cell analysis by RNA sequencing and super-resolution microscopy, and against data from the Inspiration-4 (I4) mission, JAXA (Cell-Free Epigenome) mission, Twins study, and spleens from mice on the International Space Station. Overall, microgravity alters specific pathways for optimal immunity, including the cytoskeleton, interferon signaling, pyroptosis, temperature-shock, innate inflammation (e.g., Coronavirus pathogenesis pathway and IL-6 signaling), nuclear receptors, and sirtuin signaling. Microgravity directs monocyte inflammatory parameters, and impairs T cell and NK cell functionality. Using machine learning, we identify numerous compounds linking microgravity to immune cell transcription, and demonstrate that the flavonol, quercetin, can reverse most abnormal pathways. These results define immune cell alterations in microgravity, and provide opportunities for countermeasures to maintain normal immunity in space.

Unlocking mitochondrial drug targets: The importance of mitochondrial transport proteins

A disturbed mitochondrial function contributes to the pathology of many common diseases. These organelles are therefore important therapeutic targets. On the contrary, many adverse effects of drugs can be explained by a mitochondrial off-target effect, in particular, due to an interaction with carrier proteins in the inner membrane. Yet this class of transport proteins remains underappreciated and understudied. The aim of this review is to provide a deeper understanding of the role of mitochondrial carriers in health and disease and their significance as drug targets. We present literature-based evidence that mitochondrial carrier proteins are associated with prevalent diseases and emphasize their potential as drug (off-)target sites by summarizing known mitochondrial drug-transporter interactions. Studying these carriers will enhance our knowledge of mitochondrial drug on- and off-targets and provide opportunities to further improve the efficacy and safety of drugs.

Phytochemical Targeting of Mitochondria for Breast Cancer Chemoprevention, Therapy, and Sensitization

Breast cancer is a common and deadly disease that causes tremendous physical, emotional, and financial burden on patients and society. Early-stage breast cancer and less aggressive subtypes have promising prognosis for patients, but in aggressive subtypes, and as cancers progress, treatment options and responses diminish, dramatically decreasing survival. Plants are nutritionally rich and biologically diverse organisms containing thousands of metabolites, some of which have chemopreventive, therapeutic, and sensitizing properties, providing a rich source for drug discovery. In this study we review the current landscape of breast cancer with a central focus on the potential role of phytochemicals for treatment, management, and disease prevention. We discuss the relevance of phytochemical targeting of mitochondria for improved anti-breast cancer efficacy. We highlight current applications of phytochemicals and derivative structures that display anti-cancer properties and modulate cancer mitochondria, while describing future applicability and identifying areas of promise.

Mitochondrial Uncoupling Proteins (UCP1-UCP3) and Adenine Nucleotide Translocase (ANT1) Enhance the Protonophoric Action of 2,4-Dinitrophenol in Mitochondria and Planar Bilayer Membranes

2,4-Dinitrophenol (DNP) is a classic uncoupler of oxidative phosphorylation in mitochondria which is still used in “diet pills”, despite its high toxicity and lack of antidotes. DNP increases the proton current through pure lipid membranes, similar to other chemical uncouplers. However, the molecular mechanism of its action in the mitochondria is far from being understood. The sensitivity of DNP’s uncoupling action in mitochondria to carboxyatractyloside, a specific inhibitor of adenine nucleotide translocase (ANT), suggests the involvement of ANT and probably other mitochondrial proton-transporting proteins in the DNP’s protonophoric activity. To test this hypothesis, we investigated the contribution of recombinant ANT1 and the uncoupling proteins UCP1-UCP3 to DNP-mediated proton leakage using the well-defined model of planar bilayer lipid membranes. All four proteins significantly enhanced the protonophoric effect of DNP. Notably, only long-chain free fatty acids were previously shown to be co-factors of UCPs and ANT1. Using site-directed mutagenesis and molecular dynamics simulations, we showed that arginine 79 of ANT1 is crucial for the DNP-mediated increase of membrane conductance, implying that this amino acid participates in DNP binding to ANT1.

Kaempferol Induces Cell Death and Sensitizes Human Head and Neck Squamous Cell Carcinoma Cell Lines to Cisplatin

Cisplatin is a first-line chemotherapeutic drug commonly used to treat patients with head and neck cancer; nevertheless, cisplatin resistance poses a main challenge for its clinical efficacy. Recent studies have shown that kaempferol, a natural flavonoid found in various plants and foods, has an anticancer effect. The following study evaluated the cytotoxic effects of kaempferol on head and neck tumor cells and their mechanism of action, evaluating the effects on proliferation, the oxygen consumption rate, transmembrane potential, tumor cell migration and induction of apoptosis. Moreover, we determined the effects of a combination of kaempferol and cisplatin on head and neck tumor cells. We found that kaempferol inhibited the oxygen consumption rate and decreased the intracellular ATP content in tumor cells. This novel mechanism may inhibit the migratory capacity and promote antiproliferative effects and apoptosis of tumor cells. Additionally, our in vitro data indicated that kaempferol may sensitize head and neck tumor cells to the effects of cisplatin. These effects provide new evidence for the use of a combination of kaempferol and cisplatin in vivo and their future applications in head and neck cancer therapy.

Characterization of drug-induced human mitochondrial ADP/ATP carrier inhibition

An increasing number of commonly prescribed drugs are known to interfere with mitochondrial function, causing cellular toxicity, but the underlying mechanisms are largely unknown. Although often not considered, mitochondrial transport proteins form a significant class of potential mitochondrial off-targets. So far, most drug interactions have been reported for the mitochondrial ADP/ATP carrier (AAC), which exchanges cytosolic ADP for mitochondrial ATP. Here, we show inhibition of cellular respiratory capacity by only a subset of the 18 published AAC inhibitors, which questions whether all compound do indeed inhibit such a central metabolic process. This could be explained by the lack of a simple, direct model system to evaluate and compare drug-induced AAC inhibition. Methods: For its development, we have expressed and purified human AAC1 (hAAC1) and applied two approaches. In the first, thermostability shift assays were carried out to investigate the binding of these compounds to human AAC1. In the second, the effect of these compounds on transport was assessed in proteoliposomes with reconstituted human AAC1, enabling characterization of their inhibition kinetics. Results: Of the proposed inhibitors, chebulinic acid, CD-437 and suramin are the most potent with IC50-values in the low micromolar range, whereas another six are effective at a concentration of 100 μM. Remarkably, half of all previously published AAC inhibitors do not show significant inhibition in our assays, indicating that they are false positives. Finally, we show that inhibitor strength correlates with a negatively charged surface area of the inhibitor, matching the positively charged surface of the substrate binding site. Conclusion: Consequently, we have provided a straightforward model system to investigate AAC inhibition and have gained new insights into the chemical compound features important for inhibition. Better evaluation methods of drug-induced inhibition of mitochondrial transport proteins will contribute to the development of drugs with an enhanced safety profile.

Development and Application of a Chemical Probe Based on a Neuroprotective Flavonoid Hybrid for Target Identification Using Activity-Based Protein Profiling

Alzheimer's disease (AD) is the most common form of dementia, and up to now, there are no disease-modifying drugs available. Natural product hybrids based on the flavonoid taxifolin and phenolic acids have shown a promising pleiotropic neuroprotective profile in cell culture assays and even disease-modifying effects in vivo. However, the detailed mechanisms of action remain unclear. To elucidate the distinct intracellular targets of 7-O-esters of taxifolin, we present in this work the development and application of a chemical probe, 7-O-cinnamoyltaxifolin-alkyne, for target identification using activity-based protein profiling. 7-O-Cinnamoyltaxifolin-alkyne remained neuroprotective in all cell culture assays. Western blot analysis showed a comparable influence on the same intracellular pathways as that of the lead compound 7-O-cinnamoyltaxifolin, thereby confirming its suitability as a probe for target identification experiments. Affinity pulldown and MS analysis revealed adenine nucleotide translocase 1 (ANT-1) and sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) as intracellular interaction partners of 7-O-cinnamoyltaxifolin-alkyne and thus of 7-O-esters of taxifolin.

Flavonoids modulate liposomal membrane structure, regulate mitochondrial membrane permeability and prevent erythrocyte oxidative damage

In the present work, we investigated the interaction of flavonoids (quercetin, naringenin and catechin) with cellular and artificial membranes. The flavonoids considerably inhibited membrane lipid peroxidation in rat erythrocytes treated with tert-butyl hydroperoxide (700 μM), and the IC₅₀ values for prevention of this process were equal to 9.7 ± 0.8 μM, 8.8 ± 0.7 μM, and 37.8 ± 4.4 μM in the case of quercetin, catechin and naringenin, respectively, and slightly decreased glutathione oxidation. In isolated rat liver mitochondria, quercetin, catechin and naringenin (10-50 μM) dose-dependently increased the sensitivity to Ca²⁺ ions - induced mitochondrial permeability transition. Using the probes TMA-DPH and DPH we showed that quercetin rather than catechin and naringenin strongly decreased the microfluidity of the 1,2-dimyristoyl-sn-glycero-3-phosphocholine liposomal membrane bilayer at different depths. On the contrary, using the probe Laurdan we observed that naringenin transfer the bilayer to a more ordered state, whereas quercetin dose-dependently decreased the order of lipid molecule packing and increased hydration in the region of polar head groups. The incorporation of the flavonoids, quercetin and naringenin and not catechin, into the liposomes induced an increase in the zeta potential of the membrane and enlarged the area of the bilayer as well as lowered the temperature and the enthalpy of the membrane phase transition. The effects of the flavonoids were connected with modification of membrane fluidity, packing, stability, electrokinetic properties, size and permeability, prevention of oxidative stress, which depended on the nature of the flavonoid molecule and the nature of the membrane.

Flavonoids as Anticancer Agents

Flavonoids are polyphenolic compounds subdivided into 6 groups: isoflavonoids, flavanones, flavanols, flavonols, flavones and anthocyanidins found in a variety of plants. Fruits, vegetables, plant-derived beverages such as green tea, wine and cocoa-based products are the main dietary sources of flavonoids. Flavonoids have been shown to possess a wide variety of anticancer effects: they modulate reactive oxygen species (ROS)-scavenging enzyme activities, participate in arresting the cell cycle, induce apoptosis, autophagy, and suppress cancer cell proliferation and invasiveness. Flavonoids have dual action regarding ROS homeostasis—they act as antioxidants under normal conditions and are potent pro-oxidants in cancer cells triggering the apoptotic pathways and downregulating pro-inflammatory signaling pathways. This article reviews the biochemical properties and bioavailability of flavonoids, their anticancer activity and its mechanisms of action.

Nanoencapsulated Quercetin Improves Cardioprotection during Hypoxia-Reoxygenation Injury through Preservation of Mitochondrial Function

The effective delivery of antioxidants to the cells is hindered by their high metabolization rate. In this work, quercetin was encapsulated in poly(lactic-co-glycolic) acid (PLGA) nanoparticles. They were characterized in terms of its physicochemical properties (particle size distribution, ζ-potential, encapsulation efficiency, quercetin release and biological interactions with cardiac cells regarding nanoparticle association, and internalization and protective capability against relevant challenges). A better delivery of quercetin was achieved when encapsulated versus free. When the cells were challenged with antimycin A, it resulted in lower mitochondrial O₂⁻ (4.65- vs. 5.69- fold) and H₂O₂ rate production (1.15- vs. 1.73- fold). Similarly, under hypoxia-reoxygenation injury, a better maintenance of cell viability was found (77 vs. 65%), as well as a reduction of thiol groups (~70 vs. 40%). Therefore, the delivery of encapsulated quercetin resulted in the preservation of mitochondrial function and ATP synthesis due to its improved oxidative stress suppression. The results point to the potential of this strategy for the treatment of oxidative stress-based cardiac diseases.

Quercetin preserves redox status and stimulates mitochondrial function in metabolically-stressed HepG2 cells

Hyperglycemia augments formation of intracellular reactive oxygen species (ROS) with associated mitochondrial damage and increased risk of insulin resistance in type 2 diabetes. We examined whether quercetin could reverse chronic high glucose-induced oxidative stress and mitochondrial dysfunction. Following long-term high glucose treatment, complex I activity was significantly decreased in isolated mitochondria from HepG2 cells. Quercetin dose-dependently recovered complex I activity and lowered cellular ROS generation under both high and normal glucose conditions. Respirometry studies showed that quercetin could counteract the detrimental increase in inner mitochondrial membrane proton leakage resulting from high glucose while it increased oxidative respiration, despite a decrease in electron transfer system (ETS) capacity, and lower non-ETS oxygen consumption. A quercetin-stimulated increase in cellular NAD⁺/NADH was evident within 2 h and a two-fold increase in PGC-1α mRNA within 6 h, in both normal and high glucose conditions. A similar pattern was also found for the mRNA expression of the repulsive guidance molecule b (RGMB) and its long non-coding RNA (lncRNA) RGMB-AS1 with quercetin, indicating a potential change of the glycolytic phenotype and suppression of aberrant cellular growth which is characteristic of the HepG2 cells. Direct effects of quercetin on PGC-1α activity were minimal, as quercetin only weakly enhanced PGC-1α binding to PPARα in vitro at higher concentrations. Our results suggest that quercetin may protect mitochondrial function from high glucose-induced stress by increasing cellular NAD⁺/NADH and activation of PGC-1α-mediated pathways. Lower ROS in combination with improved complex I activity and ETS coupling efficiency under conditions of amplified oxidative stress could reinforce mitochondrial integrity and improve redox status, beneficial in certain metabolic diseases.

On the causes and consequences of the uncoupler-like effects of quercetin and dehydrosilybin in H9c2 cells

Quercetin and dehydrosilybin are polyphenols which are known to behave like uncouplers of respiration in isolated mitochondria. Here we investigated whether the effect is conserved in whole cells. Following short term incubation, neither compound uncouples mitochondrial respiration in whole H9c2 cells below 50μM. However, following hypoxia, or long term incubation, leak (state IV with oligomycin) oxygen consumption is increased by quercetin. Both compounds partially protected complex I respiration, but not complex II in H9c2 cells following hypoxia. In a permeabilised H9c2 cell model, the increase in leak respiration caused by quercetin is lowered by increased [ADP] and is increased by adenine nucleotide transporter inhibitor, atractyloside, but not bongkrekic acid. Both quercetin and dehydrosilybin dissipate mitochondrial membrane potential in whole cells. In the case of quercetin, the effect is potentiated post hypoxia. Genetically encoded Ca++ sensors, targeted to the mitochondria, enabled the use of fluorescence microscopy to show that quercetin decreased mitochondrial [Ca++] while dehydrosilybin did not. Likewise, quercetin decreases accumulation of [Ca++] in mitochondria following hypoxia. Fluorescent probes were used to show that both compounds decrease plasma membrane potential and increase cytosolic [Ca++]. We conclude that the uncoupler-like effects of these polyphenols are attenuated in whole cells compared to isolated mitochondria, but downstream effects are nevertheless apparent. Results suggest that the effect of quercetin observed in whole and permeabilised cells may originate in the mitochondria, while the mechanism of action of cardioprotection by dehydrosilybin may be less dependent on mitochondrial uncoupling than originally thought. Rather, protective effects may originate due to interactions at the plasma membrane.

Selected polyphenols potentiate the apoptotic efficacy of glycolytic inhibitors in human acute myeloid leukemia cell lines. Regulation by protein kinase activities

Background

The glycolysis inhibitor 2-deoxy-d-glucose (2-DG) is a safe, potentially useful anti-tumour drug, but its efficacy is normally low when used alone. Recent studies indicated that 2-DG stimulates the PI3K/Akt and MEK/ERK defensive pathways, which limits the apoptotic efficacy in tumour cell lines. We hypothesized that co-treatment with selected polyphenols could improve 2-DG-provoked apoptosis by preventing defensive kinase activation.

Methods

Cell proliferation was measured by cell counting or the MTT assay. Cell cycle, apoptosis and necrosis were determined by propidium iodide staining and/or annexin V labeling followed by flow cytometry. Mitochondria pore transition and depolarization were determined by calcein-ATM or rhodamine 123 labeling followed flow cytometry. Intracellular reactive oxygen species and GSH were determined by dichlorodihydrofluorescein diacetate or monochlorobimane labeling followed by flow cytometry or fluorimetry. Expression and phosphorylation of protein kinases were analyzed by the Western blot.

Results

(i) 2-DG-provoked apoptosis was greatly potentiated by co-treatment with the sub-lethal concentrations of the flavonoid quercetin in human HL60 acute myeloblastic leukemia cells. Allowing for quantitative differences, apoptosis potentiation was also obtained using NB4 promyelocytic and THP-1 promonocytic cells, using curcumin or genistein instead of quercetin, and using lonidamine instead of 2-DG, but not when 2-DG was substituted by incubation in glucose-free medium. (ii) Quercetin and 2-DG rapidly elicited the opening of mitochondria pore transition, which preceded the trigger of apoptosis. (iii) Treatments did not affect GSH levels, and caused disparate effects on reactive oxygen species generation, which did not match the changes in lethality. (iv) 2-DG and lonidamine stimulated defensive Akt and ERK phosphorylation/activation, while glucose starvation was ineffective. Polyphenols prevented the stimulation of Akt phosphorylation, and in some cases also ERK phosphorylation. In addition, quercetin and 2-DG stimulated GSK-3α,β phosphorylation/inactivation, although with different isoform specificity. The use of pharmacologic inhibitors confirmed the importance of these kinase modifications for apoptosis.

Conclusions

The present in vitro observations suggest that co-treatment with low concentrations of selected polyphenols might represent a manner of improving the poor anti-tumour efficacy of some glycolytic inhibitors, and that apoptosis potentiation may be at least in part explained by the regulation of defensive protein kinase activities.

Electronic supplementary material

The online version of this article (doi:10.1186/s12935-016-0345-y) contains supplementary material, which is available to authorized users.

Coadjuvants in the Diabetic Complications: Nutraceuticals and Drugs with Pleiotropic Effects

Because diabetes mellitus (DM) is a multifactorial metabolic disease, its prevention and treatment has been a constant challenge for basic and clinical investigators focused on translating their discoveries into clinical treatment of this complex disorder. In this review, we highlight recent experimental and clinical evidences of potential coadjuvants in the management of DM, such as polyphenols (quercetin, resveratrol and silymarin), cultured probiotic microorganisms and drugs acting through direct/indirect or pleiotropic effects on glycemic control in DM. Among several options, we highlight new promising therapeutic coadjuvants, including chemical scavengers, the probiotic kefir and the phosphodiesterase 5 inhibitors, which besides the reduction of hyperglycemia and ameliorate insulin resistance, they reduce oxidative stress and improve endothelial dysfunction in the systemic vascular circulation. In the near future, experimental studies are expected to clear the intracellular pathways involving coadjuvants. The design of clinical trials may also contribute to new strategies with coadjuvants against the harmful effects of diabetic complications.

Synergistic neuroprotection by epicatechin and quercetin: Activation of convergent mitochondrial signaling pathways

In view of evidence that increased consumption of epicatechin (E) and quercetin (Q) may reduce the risk of stroke, we have measured the effects of combining E and Q on mitochondrial function and neuronal survival following oxygen-glucose deprivation (OGD). Relative to mouse cortical neuron cultures pretreated (24h) with either E or Q (0.1-10μM), E+Q synergistically attenuated OGD-induced neuronal cell death. E, Q and E+Q (0.3μM) increased spare respiratory capacity but only E+Q (0.3μM) preserved this crucial parameter of neuronal mitochondrial function after OGD. These improvements were accompanied by corresponding increases in cyclic AMP response element binding protein (CREB) phosphorylation and the expression of CREB-target genes that promote neuronal survival (Bcl-2) and mitochondrial biogenesis (PGC-1α). Consistent with these findings, E+Q (0.1 and 1.0μM) elevated mitochondrial gene expression (MT-ND2 and MT-ATP6) to a greater extent than E or Q after OGD. Q (0.3-3.0μM), but not E (3.0μM), elevated cytosolic calcium (Ca(2+)) spikes and the mitochondrial membrane potential. Conversely, E and E+Q (0.1 and 0.3μM), but not Q (0.1 and 0.3μM), activated protein kinase B (Akt). Nitric oxide synthase (NOS) inhibition with L-N(G)-nitroarginine methyl ester (1.0μM) blocked neuroprotection by E (0.3μM) or Q (1.0μM). Oral administration of E+Q (75mg/kg; once daily for 5days) reduced hypoxic-ischemic brain injury. These findings suggest E and Q activate Akt- and Ca(2+)-mediated signaling pathways that converge on NOS and CREB resulting in synergistic improvements in neuronal mitochondrial performance which confer profound protection against ischemic injury.

The protective effects of oral low-dose quercetin on diabetic nephropathy in hypercholesterolemic mice

Aims: Diabetic nephropathy (DN) is one of the most important causes of chronic renal disease, and the incidence of DN is increasing worldwide. Considering our previous report (Gomes et al., 2014) indicating that chronic treatment with oral low-dose quercetin (10 mg/Kg) demonstrated anti-oxidative, anti-apoptotic and renoprotective effects in the C57BL/6J model of DN, we investigated whether this flavonoid could also have beneficial effects in concurrent DN and spontaneous atherosclerosis using the apolipoprotein E-deficient mouse (apoE^−/−).

Methods: Streptozotocin was used to induce diabetes (100 mg/kg/day, 3 days) in male apoE^−/− mice (8 week-old). After 6 weeks, the mice were randomly separated into DQ: diabetic apoE^−/− mice treated with quercetin (10 mg/kg/day, 4 weeks, n = 8), DV: diabetic ApoE^−/− mice treated with vehicle (n = 8) and ND: non-treated non-diabetic mice (n = 8).

Results: Quercetin treatment diminished polyuria (~30%; p < 0.05), glycemia (~25%, p < 0.05), normalized the hypertriglyceridemia. Moreover, this bioflavonoid diminished creatininemia (~30%, p < 0.01) and reduced proteinuria but not to normal levels. We also observed protective effects on the renal structural changes, including normalization of the index of glomerulosclerosis and kidney weight/body weight.

Conclusions: Our data revealed that quercetin treatment significantly reduced DN in hypercholesterolemic mice by inducing biochemical changes (decrease in glucose and triglycerides serum levels) and reduction of glomerulosclerosis. Thus, this study highlights the relevance of quercetin as an alternative therapeutic option for DN, including in diabetes associated with dyslipidemia.

Polyphenols as mitochondria-targeted anticancer drugs

Mitochondria are the respiratory and energetic centers of the cell where multiple intra- and extracellular signal transduction pathways converge leading to dysfunction of those organelles and, consequently, apoptotic or/and necrotic cell death. Mitochondria-targeted anticancer drugs are referred to as mitocans; they have recently been classified by Neuzil et al. (2013) according to their molecular mode of action into: hexokinase inhibitors; mimickers of the Bcl-2 homology-3 (BH3) domains; thiol redox inhibitors; deregulators of voltage-dependent anionic channel (VDAC)/adenine nucleotide translocase (ANT) complex; electron redox chain-targeting agents; lipophilic cations targeting the mitochondrial inner membrane; tricarboxylic acid cycle-targeting agents; and mitochondrial DNA-targeting agents. Polyphenols of plant origin and their synthetic or semisynthetic derivatives exhibit pleiotropic biological activities, including the above-mentioned modes of action characteristic of mitocans. Some of them have already been tested in clinical trials. Gossypol has served as a lead compound for developing more efficient BH3 mimetics such as ABT-737 and its orally available structural analog ABT-263 (Navitoclax). Furthermore, mitochondriotropic derivatives of phenolic compounds such as quercetin and resveratrol have been synthesized and reported to efficiently induce cancer cell death in vitro.

Natural Compounds Modulating Mitochondrial Functions

Mitochondria are organelles responsible for several crucial cell functions, including respiration, oxidative phosphorylation, and regulation of apoptosis; they are also the main intracellular source of reactive oxygen species (ROS). In the last years, a particular interest has been devoted to studying the effects on mitochondria of natural compounds of vegetal origin, quercetin (Qu), resveratrol (RSV), and curcumin (Cur) being the most studied molecules. All these natural compounds modulate mitochondrial functions by inhibiting organelle enzymes or metabolic pathways (such as oxidative phosphorylation), by altering the production of mitochondrial ROS and by modulating the activity of transcription factors which regulate the expression of mitochondrial proteins. While Qu displays both pro- and antioxidant activities, RSV and Cur are strong antioxidant, as they efficiently scavenge mitochondrial ROS and upregulate antioxidant transcriptional programmes in cells. All the three compounds display a proapoptotic activity, mediated by the capability to directly cause the release of cytochrome c from mitochondria or indirectly by upregulating the expression of proapoptotic proteins of Bcl-2 family and downregulating antiapoptotic proteins. Interestingly, these effects are particularly evident on proliferating cancer cells and can have important therapeutic implications.

Renoprotective, anti-oxidative and anti-apoptotic effects of oral low-dose quercetin in the C57BL/6J model of diabetic nephropathy

Background

Diabetic nephropathy (DN) is one of the major causes of end-stage renal disease in diabetic patients. Increasing evidence from studies in the rodents has suggested that this disease is associated with increased oxidative stress due to hyperglycemia. In the present study, we evaluated the renoprotective, anti-oxidative and anti-apoptotic effects of the flavonoid quercetin in C57BL/6J model of DN.

Methods

DN was induced by streptozotocin (STZ, 100 mg/kg/day, for 3 days) in adult C57BL/6J mice. Six weeks later, mice were divided into the following groups: diabetic mice treated with quercetin (DQ, 10 mg/kg/day, 4 weeks), diabetic mice treated with vehicle (DV) or non-treated non-diabetic (ND) mice.

Results

Quercetin treatment caused a reduction in polyuria (~45%) and glycemia (~35%), abolished the hypertriglyceridemia and had significant effects on renal function including, decreased proteinuria and high plasma levels of uric acid, urea and creatinine, which were accompanied by beneficial effects on the structural changes of the kidney including glomerulosclerosis. Flow cytometry showed a decrease in oxidative stress and apoptosis in DN mice.

Conclusion

Taken together, these data show that quercetin effectively attenuated STZ-induced cytotoxicity in renal tissue. This study provides convincing experimental evidence and perspectives on the renoprotective effects of quercetin in diabetic mice and outlines a novel therapeutic strategy for this flavonoid in the treatment of DN.

Polyphenols and mitochondria: an update on their increasingly emerging ROS-scavenging independent actions

Polyphenols, ubiquitously present in fruits and vegetables, have been traditionally viewed as antioxidant molecules. Such contention emerged, mainly from their well established in vitro ability to scavenge free radicals and other reactive oxygen species (ROS). During the last decade, however, increasing evidence has emerged supporting the ability of certain polyphenols to also exert numerous ROS-scavenging independent actions. Although the latter can comprise the whole cell, particular attention has been placed on the ability of polyphenols to act, whether favorably or not, on a myriad of mitochondrial processes. Thus, some particular polyphenols are now recognized as molecules capable of modulating pathways that define mitochondrial biogenesis (i.e., inducing sirtuins), mitochondrial membrane potential (i.e., mitochondrial permeability transition pore opening and uncoupling effects), mitochondrial electron transport chain and ATP synthesis (i.e., modulating complexes I to V activity), intra-mitochondrial oxidative status (i.e., inhibiting/inducing ROS formation/removal enzymes), and ultimately mitochondrially-triggered cell death (i.e., modulating intrinsic-apoptosis). The present review describes recent evidence on the ability of some polyphenols to modulate each of the formerly mentioned pathways, and discusses on how, by acting on such mitochondrial processes, polyphenols may afford protection against those mitochondrial damaging events that appear to be key in the cellular toxicity induced by various xenobiotics as well as that seen during the development of several ROS-related diseases.

Dietary quercetin supplementation in mice increases skeletal muscle PGC1α expression, improves mitochondrial function and attenuates insulin resistance in a time-specific manner

AIMS/HYPOTHESIS

High fat diet (HFD)-induced insulin resistance (IR) is partially characterized by reduced skeletal muscle mitochondrial function and peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC1α) expression. Our previous study showed that a high dose of the bioflavonoid quercetin exacerbated HFD-induced IR; yet, others have demonstrated that quercetin improves insulin sensitivity. The aim of this study was to investigate whether differing doses of quercetin act in a time-dependent manner to attenuate HFD-induced IR in association with improved skeletal muscle mitochondrial function and PGC1α expression.

METHODS

C57BL/6J mice were fed HFD for 3 or 8 wks, with or without a low (50 ug/day; HF+50Q) or high (600 ug/day, HF+600Q) dose of quercetin. Whole body and metabolic phenotypes and insulin sensitivity were assessed. Skeletal muscle metabolomic analysis of acylcarnitines and PGC1α mRNA expression via qRT-PCR were measured.

RESULTS

Quercetin at 50 ug/day for 8 wk attenuated HFD-induced increases in fat mass, body weight and IR and increased PGC1α expression, whereas 600 ug/day of quercetin exacerbated fat mass accumulation without altering body weight, IR or PGC1α. PGC1α expression correlated with acylcarnitine levels similarly in HF and HF+600Q; these correlations were not present in HF+50Q. At both time points, energy expenditure increased in HF+50Q and decreased in HF+600Q, independent of PGC1α and IR.

CONCLUSIONS/INTERPRETATION

Chronic dietary quercetin supplementation at low but not higher dose ameliorates the development of diet-induced IR while increasing PGC1α expression in muscle, suggesting that skeletal muscle may be an important target for the insulin-sensitizing effects of a low dose of quercetin.

The cardioprotectant 3',4'-dihydroxyflavonol inhibits opening of the mitochondrial permeability transition pore after myocardial ischemia and reperfusion in rats

The study aimed to determine the effect of 3',4'-dihydroxyflavonol (DiOHF) on mitochondrial function, in particular opening of the mitochondrial permeability transition pore (mPTP), respiratory function and reactive oxygen species (ROS) production, in isolated cardiac mitochondria after coronary artery occlusion and reperfusion in vivo. Opening of the mPTP, oxygen consumption and ROS production (assessed by measurement of H2O2) was determined in mitochondria isolated from normal hearts or from the ischemic zone of rat hearts subjected to 30min coronary artery occlusion and 15min reperfusion. Treatment of sham rats with DiOHF (10mgkg(-1) iv) significantly increased the concentration of Ca(2+) required to stimulate mPTP opening. This was accompanied by increased state 3 oxygen consumption and decreased H2O2 release. Ischemia and reperfusion (IR) significantly decreased the concentration of Ca(2+) required to stimulate mPTP opening, decreased state 3 oxygen consumption and increased H2O2 release, when pyruvate plus malate was provided as a substrate. Treatment with DiOHF prevented IR-induced changes in mPTP opening, state 3 oxygen consumption and H2O2 release so that there was no difference compared to sham. In isolated cardiac mitochondria from normal rats DiOHF had no effect on mPTP opening or on state 3 respiration but caused a small increase in state 4 respiration and decreased the respiratory control ratio. DiOHF, administered during ischemia just before reperfusion, inhibits mPTP opening and preserves mitochondrial function through a mechanism likely to be independent of its antioxidant activity or any direct effect on the mPTP.

Dehydrosilybin attenuates the production of ROS in rat cardiomyocyte mitochondria with an uncoupler-like mechanism

Reactive oxygen species (ROS) originating from mitochondria are perceived as a factor contributing to cell aging and means have been sought to attenuate ROS formation with the aim of extending the cell lifespan. Silybin and dehydrosilybin, two polyphenolic compounds, display a plethora of biological effects generally ascribed to their known antioxidant capacity. When investigating the cytoprotective effects of these two compounds in the primary cell cultures of neonatal rat cardiomyocytes, we noted the ability of dehydrosilybin to de-energize the cells by monitoring JC-1 fluorescence. Experiments evaluating oxygen consumption and membrane potential revealed that dehydrosilybin uncouples the respiration of isolated rat heart mitochondria albeit with a much lower potency than synthetic uncouplers. Furthermore, dehydrosilybin revealed a very high potency in suppressing ROS formation in isolated rat heart mitochondria with IC(50) = 0.15 μM. It is far more effective than its effect in a purely chemical system generating superoxide or in cells capable of oxidative burst, where the IC(50) for dehydrosilybin exceeds 50 μM. Dehydrosilybin also attenuated ROS formation caused by rotenone in the primary cultures of neonatal rat cardiomyocytes. We infer that the apparent uncoupler-like activity of dehydrosilybin is the basis of its ROS modulation effect in neonatal rat cardiomyocytes and leads us to propose a hypothesis on natural ischemia preconditioning by dietary polyphenols.

Adenine nucleotide translocase family: four isoforms for apoptosis modulation in cancer

Mitochondria have important functions in mammalian cells as the energy powerhouse and integrators of the mitochondrial pathway of apoptosis. The adenine nucleotide translocase (ANT) is a family of proteins involved in cell death pathways that perform distinctly opposite functions to regulate cell fate decisions. On the one hand, ANT catalyzes the adenosine triphosphate export from the mitochondrial matrix to the intermembrane space with the concomitant import of ADP from the intermembrane space to the matrix. On the other hand, during periods of stress, ANT could function as a lethal pore and trigger the process of mitochondrial membrane permeabilization, which leads irreversibly to cell death. In human, ANT is encoded by four homologous genes, whose expression is not only tissue specific, but also varies according to the pathophysiological state of the cell. Recent evidence revealed a differential role of the ANT isoforms in apoptosis and a deregulation of their expression in cancer. In this review, we introduce the current knowledge of ANT in apoptosis and cancer cells and propose a novel classification of ANT isoforms.

Juice of Bryophyllum pinnatum (Lam.) inhibits oxytocin-induced increase of the intracellular calcium concentration in human myometrial cells

The use of preparations from Bryophyllum pinnatum in tocolysis is supported by both clinical (retrospective comparative studies) and experimental (using uterus strips) evidence. We studied here the effect of B. pinnatum juice on the response of cultured human myometrial cells to stimulation by oxytocin, a hormone known to be involved in the control of uterine contractions by increasing the intracellular free calcium concentration ([Ca2+]i). In this work, [Ca2+]i was measured online during stimulation of human myometrial cells (hTERT-C3 and M11) with oxytocin, which had been pre-incubated in the absence or in the presence of B. pinnatum juice. Since no functional voltage-gated Ca2+ channels could be detected in these myometrial cells, the effect of B. pinnatum juice was as well studied in SH-SY5Y neuroblastoma cells, which are known to have such channels and can be depolarised with KCl. B. pinnatum juice prevented the oxytocin-induced increase in [Ca2+]i in hTERT-C3 human myometrial cells in a dose-dependent manner, achieving a ca. 80% inhibition at a 2% concentration. Comparable results were obtained with M11 human primary myometrial cells. In hTERT-C3 cells, prevention of the oxytocin-induced increase in [Ca2+]i was independent of the extracellular Ca2+ concentration and of voltage-dependent Ca2+-channels. B. pinnatum juice delayed, but did not prevent the depolarization-induced increase in [Ca2+]i in SH-SY5Y cells. Taken together, the data suggest a specific and concentration-dependent effect of B. pinnatum juice on the oxytocin signalling pathway, which seems to corroborate its use in tocolysis. Such a specific mechanism would explain the rare and minor side-effects in tocolysis with B. pinnatum as well as its high therapeutic index.