Glial metabolism of quercetin reduces its neurotoxic potential

A Screen of Plant-Based Natural Products Revealed That Quercetin Prevents Pyroglutamylated Amyloid-β (Aβ3(pE)-42) Uptake in Astrocytes As Well As Resulting Astrogliosis and Synaptic Dysfunction

Two connected histopathological hallmarks of Alzheimer's disease (AD) are chronic neuroinflammation and synaptic dysfunction. The accumulation of the most prevalent posttranslationally modified form of Aβ1-42, pyroglutamylated amyloid-β (Aβ3(pE)-42) in astrocytes is directly linked to glial activation and the release of proinflammatory cytokines that in turn contribute to early synaptic dysfunction in AD. At present, the mechanisms of Aβ3(pE)-42 uptake to astrocytes are unknown and pharmacological interventions that interfere with this process are not available. Here we developed a simple screening assay to identify substances from a plant extract library that prevent astroglial Aβ3(pE)-42 uptake. We first show that this approach yields valid and reproducible results. Second, we show endocytosis of Aβ3(pE)-42 oligomers by astrocytes and that quercetin, a plant flavonol, is effective to specifically block astrocytic buildup of oligomeric Aβ3(pE)-42. Importantly, quercetin does not induce a general impairment of endocytosis. However, it efficiently protects against early synaptic dysfunction following exogenous Aβ3(pE)-42 application.

An Update of Kaempferol Protection against Brain Damage Induced by Ischemia-Reperfusion and by 3-Nitropropionic Acid

Kaempferol, a flavonoid present in many food products, has chemical and cellular antioxidant properties that are beneficial for protection against the oxidative stress caused by reactive oxygen and nitrogen species. Kaempferol administration to model experimental animals can provide extensive protection against brain damage of the striatum and proximal cortical areas induced by transient brain cerebral ischemic stroke and by 3-nitropropionic acid. This article is an updated review of the molecular and cellular mechanisms of protection by kaempferol administration against brain damage induced by these insults, integrated with an overview of the contributions of the work performed in our laboratories during the past years. Kaempferol administration at doses that prevent neurological dysfunctions inhibit the critical molecular events that underlie the initial and delayed brain damage induced by ischemic stroke and by 3-nitropropionic acid. It is highlighted that the protection afforded by kaempferol against the initial mitochondrial dysfunction can largely account for its protection against the reported delayed spreading of brain damage, which can develop from many hours to several days. This allows us to conclude that kaempferol administration can be beneficial not only in preventive treatments, but also in post-insult therapeutic treatments.

A critical examination of human data for the biological activity of quercetin and its phase-2 conjugates

This critical review examines evidence for beneficial effects of quercetin phase-2 conjugates from clinical intervention studies, volunteer feeding trials, and in vitro work. Plasma concentrations of quercetin-3-O-glucuronide (Q3G) and 3'-methylquercetin-3-O-glucuronide (3'MQ3G) after supplementation may produce beneficial effects in macrophages and endothelial cells, respectively, especially if endogenous deglucuronidation occurs, and lower blood uric acid concentration via quercetin-3'-O-sulfate (Q3'S). Unsupplemented diets produce much lower concentrations (<50 nmol/l) rarely investigated in vitro. At 10 nmol/l, Q3'S and Q3G stimulate or suppress, respectively, angiogenesis in endothelial cells. Statistically significant effects have been reported at 100 nmol/l in breast cancer cells (Q3G), primary neuron cultures (Q3G), lymphocytes (Q3G and3'MQ3G) and HUVECs (QG/QS mixture), but it is unclear whether these translate to a health benefit in vivo. More sensitive and more precise methods to measure clinically significant endpoints are required before a conclusion can be drawn regarding effects at normal dietary concentrations. Future requirements include better understanding of inter-individual and temporal variation in plasma quercetin phase-2 conjugates, their mechanisms of action including deglucuronidation and desulfation both in vitro and in vivo, tissue accumulation and washout, as well as potential for synergy or antagonism with other quercetin metabolites and metabolites of other dietary phytochemicals.

Neuroprotective Potentials of Flavonoids: Experimental Studies and Mechanisms of Action

Neurological and neurodegenerative diseases, particularly those related to aging, are on the rise, but drug therapies are rarely curative. Functional disorders and the organic degeneration of nervous tissue often have complex causes, in which phenomena of oxidative stress, inflammation and cytotoxicity are intertwined. For these reasons, the search for natural substances that can slow down or counteract these pathologies has increased rapidly over the last two decades. In this paper, studies on the neuroprotective effects of flavonoids (especially the two most widely used, hesperidin and quercetin) on animal models of depression, neurotoxicity, Alzheimer's disease (AD) and Parkinson's disease are reviewed. The literature on these topics amounts to a few hundred publications on in vitro and in vivo models (notably in rodents) and provides us with a very detailed picture of the action mechanisms and targets of these substances. These include the decrease in enzymes that produce reactive oxygen and ferroptosis, the inhibition of mono-amine oxidases, the stimulation of the Nrf2/ARE system, the induction of brain-derived neurotrophic factor production and, in the case of AD, the prevention of amyloid-beta aggregation. The inhibition of neuroinflammatory processes has been documented as a decrease in cytokine formation (mainly TNF-alpha and IL-1beta) by microglia and astrocytes, by modulating a number of regulatory proteins such as Nf-kB and NLRP3/inflammasome. Although clinical trials on humans are still scarce, preclinical studies allow us to consider hesperidin, quercetin, and other flavonoids as very interesting and safe dietary molecules to be further investigated as complementary treatments in order to prevent neurodegenerative diseases or to moderate their deleterious effects.

Application of quercetin in neurological disorders: from nutrition to nanomedicine

Quercetin is a polyphenolic flavonoid, which is frequently found in fruits and vegetables. The antioxidant potential of quercetin has been studied from subcellular compartments, that is, mitochondria to tissue levels in the brain. The neurodegeneration process initiates alongside aging of the neurons. It appears in different parts of the brain as Aβ plaques, neurofibrillary tangles, Lewy bodies, Pick bodies, and others, which leads to Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and other diseases. So far, no specific treatment has been identified for these diseases. Despite common treatments that help to prevent the development of disease, the condition of patients with progressive neurodegenerative diseases usually do not completely improve. Currently, the use of flavonoids, especially quercetin for the treatment of neurodegenerative diseases, has been expanded in animal models. It has also been used to treat animal models of neurodegenerative diseases. In addition, improvements in behavioral levels, as well as in cellular and molecular levels, decreased activity of antioxidant and apoptotic proteins, and increased levels of antiapoptotic proteins have been observed. Low bioavailability of quercetin has also led researchers to construct various quercetin-involved nanoparticles. The treatment of animal models of neurodegeneration using quercetin-involved nanoparticles has shown that improvements are observed in shorter periods and with use of lower concentrations. Indeed, intranasal administration of quercetin-involved nanoparticles, constructing superparamagnetic nanoparticles, and combinational treatment using nanoparticles such as quercetin and other drugs are suggested for future studies.

Conjugation of Oxidized Betanidin and Gomphrenin Pigments from Basella alba L. Fruits with Glutathione

Formation of glutathionic conjugates with quinonoid forms generated through oxidation of betanidin and gomphrenin obtained from fruits of Basella alba L. was studied by high-performance liquid chromatography coupled with triple quadrupole tandem mass spectrometry (HPLC-DAD-ESI-MS/MS) and ion-trap time-of-flight high-resolution mass spectrometry (LCMS-IT-TOF). The conjugates were studied for the aim of trapping the formed quinonoids by glutathione which would indicate a presence of specific quinonoid structures in reaction products of the pigments with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radicals. The structure of betanidin conjugate, which was formed with high efficiency, was established by NMR analysis. In the case of gomphrenin conjugate, its structure was tentatively indicated as analogous to betanidin conjugate by MS ⁿ fragmentation paths. In contrast, no detectable glutathionic conjugate of betanin quinonoid (quinone methide) was present in similar betanin reaction mixtures. As a result of additional experiments performed during oxidation of gomphrenin by ABTS cation radicals in the absence of glutathione, except for decarboxylated and dehydrogenated gomphrenin derivatives, generation of betanidin and its derivatives was observed which indicated that the subsequent dopachromic intermediate rearrangement affected hydrolysis of the glucosidic bond. This is in contrast to betanin which is not deglucosylated in the same conditions during the oxidation. The obtained results shed some light on the oxidation pathways of various glycosylated betacyanins with gomphrenin being presumably the most potent antioxidant ascertained in this group of pigments.

Quercetin and isorhamnetin aglycones are the main metabolites of dietary quercetin in cerebrospinal fluid

SCOPE

Reports on the protective effect of certain foods on brain functions are numerous; however, the permeability of the brain barriers by food components is still hardly recognised. There have been in vitro studies aimed at demonstrating this possibility, but not much is known about this phenomenon in in vivo systems. The objective of the study was to determine the metabolites of dietary quercetin (Q) in urine, blood plasma and cerebrospinal fluid (CSF) after intra-rumen administration of Q rich onion dry skin in an animal model.

METHODS AND RESULTS

Eleven sheep had permanently implanted cannulas in the third ventricle of the brain as the means for CSF collection. The animals were administered Q at the dose of 10 mg/kg bwt. For 12 h the concentration of Q metabolites was measured in urine, blood plasma, and CSF. It was demonstrated that while in blood plasma Q and isorhamnetin mono-glucuronides or mono-sulphates were the main metabolites (80%), in CSF their aglycones were the dominating ones (88%).

CONCLUSION

Q and IR aglycones are the main Q metabolites present in CSF after dietary Q intake. Their passive transport through blood-CSF barrier or a de-conjugating mechanism within that barrier may be involved.

The intracellular metabolism of isoflavones in endothelial cells

Data from epidemiological and human intervention studies have highlighted potential cardiovascular benefits of soy isoflavone-containing foods. In humans, genistein and daidzein are extensively metabolized after absorption into glucuronides and sulfate metabolites. However, limited data exist on isoflavone cellular metabolism, in particular in endothelial cells. We investigated the uptake and cellular metabolism of genistein, daidzein and its major in vivo microbial metabolite, equol, in human endothelial (HUVEC), liver (HepG2) and intestinal epithelial cells (Caco-2 monolayer). Our results indicate that genistein and daidzein are taken up by endothelial cells, and metabolized into methoxy-genistein-glucuronides, methoxy-genistein-sulfates and methoxy-daidzein-glucuronides. In contrast, equol was taken up but not metabolized. In HepG2 and Caco-2 cells, glucuronide and sulfate conjugates of genistein and daidzein and a sulfate conjugate of equol were formed. Our findings suggest that endothelial cell metabolism needs to be taken into account when investigating the cardioprotective mechanisms of action of isoflavones.

Comparison between the effects of quercetin on seizure threshold in acute and chronic seizure models

Flavonoids are important constituents of food and beverages, and several studies have shown that they have neuroactive properties. Many of these compounds are ligands for γ-aminobutyric acid type A receptors in the central nervous system. This study aimed to investigate the anticonvulsant effects of quercetin (3,3',4',5,7-pentahydroxyflavone), which is a flavonoid found in plants, in rats treated with pentylenetetrazole in acute and chronic seizure models. Single intraperitoneal administration of quercetin did not show anticonvulsive effects against acute seizure. Similarly, multiple oral pretreatment with quercetin did not have protective effects against acute seizure. However, multiple intraperitoneal administration of quercetin (25 and 50 mg/kg) significantly increased time to death compared with the control (p < 0.001). However, quercetin pretreatment had no significant effects on the pattern of convulsion development during all periods of kindling. But on the test day, quercetin (100 mg/kg) could significantly increase generalized tonic-clonic seizure onset (GTCS) and decrease GTCS duration compared with the control (p < 0.01, p < 0.05). We conclude that quercetin has a narrow therapeutic dose range for anticonvulsant activities in vivo, and it has different effects on the seizure threshold. The different effects of quercetin on seizure threshold may occur through several mechanisms.

Identification of brain-targeted bioactive dietary quercetin-3-O-glucuronide as a novel intervention for Alzheimer's disease

Epidemiological and preclinical studies indicate that polyphenol intake from moderate consumption of red wines may lower the relative risk for developing Alzheimer's disease (AD) dementia. There is limited information regarding the specific biological activities and cellular and molecular mechanisms by which wine polyphenolic components might modulate AD. We assessed accumulations of polyphenols in the rat brain following oral dosage with a Cabernet Sauvignon red wine and tested brain-targeted polyphenols for potential beneficial AD disease-modifying activities. We identified accumulations of select polyphenolic metabolites in the brain. We demonstrated that, in comparison to vehicle-control treatment, one of the brain-targeted polyphenol metabolites, quercetin-3-O-glucuronide, significantly reduced the generation of β-amyloid (Aβ) peptides by primary neuron cultures generated from the Tg2576 AD mouse model. Another brain-targeted metabolite, malvidin-3-O-glucoside, had no detectable effect on Aβ generation. Moreover, in an in vitro analysis using the photo-induced cross-linking of unmodified proteins (PICUP) technique, we found that quercetin-3-O-glucuronide is also capable of interfering with the initial protein-protein interaction of Aβ(1-40) and Aβ(1-42) that is necessary for the formation of neurotoxic oligomeric Aβ species. Lastly, we found that quercetin-3-O-glucuronide treatment, compared to vehicle-control treatment, significantly improved AD-type deficits in hippocampal formation basal synaptic transmission and long-term potentiation, possibly through mechanisms involving the activation of the c-Jun N-terminal kinases and the mitogen-activated protein kinase signaling pathways. Brain-targeted quercetin-3-O-glucuronide may simultaneously modulate multiple independent AD disease-modifying mechanisms and, as such, may contribute to the benefits of dietary supplementation with red wines as an effective intervention for AD.

Kinetic studies of the effects of Temodal and quercetin on astrocytoma cells

The aim of the present study was to investigate the kinetics of the effects exerted by Temodal and quercetin on the survival of the human astrocytoma MOGGCCM cell line. Our results indicate that quercetin was toxic and induced necrosis, whereas Temodal induced autophagy-mediated cell death most effectively. The amount of cell death directly correlated with drug concentration and length of exposure. During combined administration of both drugs, Temodal attenuated the cytotoxic effects of quercetin. Combinations of both drugs were effective in inducing programmed cell death, but the type of cell death was concentration-dependent. Co-administration of Temodal (100 μM) with a low quercetin concentration (5 μM) resulted in a very significant induction of autophagy; however, after treatment with quercetin at a higher concentration (30 μM), apoptosis became the primary mechanism of cell death. The sequence of drug administration was also important. The highest number of dead cells was observed after simultaneous administration of both drugs or after pre-incubation with Temodal followed by treatment with quercetin. Apoptosis was identified through activation of the mitochondrial pathway including cleavage of caspase-3 and release of cytochrome c. Autophagy was identified through increased levels of LC3II. Our results indicate that Temodal and quercetin are synergistic inducers of programmed cell death, better together than applied separately. This drug combination appears to be a potent and promising therapeutic relevant to the treatment of gliomas.

Platelet-mediated metabolism of the common dietary flavonoid, quercetin

Background

Flavonoid metabolites remain in blood for periods of time potentially long enough to allow interactions with cellular components of this tissue. It is well-established that flavonoids are metabolised within the intestine and liver into methylated, sulphated and glucuronidated counterparts, which inhibit platelet function.

Methodology/Principal Findings

We demonstrate evidence suggesting platelets which contain metabolic enzymes, as an alternative location for flavonoid metabolism. Quercetin and a plasma metabolite of this compound, 4′-O-methyl quercetin (tamarixetin) were shown to gain access to the cytosolic compartment of platelets, using confocal microscopy. High performance liquid chromatography (HPLC) and mass spectrometry (MS) showed that quercetin was transformed into a compound with a mass identical to tamarixetin, suggesting that the flavonoid was methylated by catechol-O-methyl transferase (COMT) within platelets.

Conclusions/Significance

Platelets potentially mediate a third phase of flavonoid metabolism, which may impact on the regulation of the function of these cells by metabolites of these dietary compounds.

The effect of pretreatment or combined treatment of quercetin on menadione toxicity in rat primary mixed glial cells in vitro

Neurons and glia are highly susceptible to reactive oxygen species that play a key role in various neurodegenerative diseases. Menadione, a synthetic derivative of vitamin K, induces reactive oxygen generation. Quercetin one of the most ubiquitous bioflavonoids in food of plant origin, has strong antioxidant activities on different cell types, however recent studies demonstrated that it has also prooxidant and cytotoxic potentials. We examined the action of pre- and co-treatment of quercetin on menadione induced glial toxicity. The primary mixed glial cells obtained from 1 to 3 day old rat brain were pretreated with 10, 25, 100 or 250 muM quercetin for 1 h, washed out and 10, 25, 50, 75 or 100 muM menadione was added for 6 h. The other group of cells was treated with respective doses of quercetin combined simultaneously with the same doses of menadione for 6 h. The cells were washed and incubated for additional 24 h for recovery period and the viability was measured by using MTT assay. Menadione was dose-dependently toxic to glia cells and pretreatment with respective quercetin doses for 1 h could not eliminate this toxicity. Although 10 and 25 muM quercetin combined with 10 and 25 muM menadione could not change, 100 and 250 muM quercetin together with 10 or 25 muM menadione for 6 h increased further the menadione induced toxicity. We conclude that when combined with menadione, quercetin at high doses could be toxic to primary rat glia cells in culture.

The multiple faces of quercetin in neuroprotection

This review discusses the most recent data on the potential of quercetin to confer neuroprotection. Unfortunately, most of the in vitro studies have used quercetin aglycone, which is not detectable in the plasma or in the brain after oral intake. Moreover, quercetin metabolites and glycosides seem to be less neuroprotective and penetrate the BBB less efficiently than aglycone. Surprisingly, quercetin has beneficial effects on various in vivo models of neural disorders, particularly in cerebrovascular insults; contrasting data also do exist. This may be due to an increase of BBB permeability, described in many of these animal models, which would facilitate quercetin brain penetration. Although quercetin causes no significant toxicity in several animal studies, the risk for neurotoxicity is not negligible because of its narrow therapeutic dose-range in vitro. Notably, this risk may be even higher in the case of increased quercetin access to the brain, which may occur pathologically or artificially (e.g., by liposomal preparations). Based on the referred literature, we doubt that quercetin possesses any significant efficacy in neurodegenerative disorders. Instead, therapeutic trials should focus more on the quercetin efficacy in cerebrovascular insults rather than neurodegeneration.

Extra virgin olive oil phenolics: absorption, metabolism, and biological activities in the GI tract

Olive oil, a typical ingredient of the Mediterranean diet, possesses many beneficial health effects. The biological activities ascribed to olive oil consumption are associated in part to its phenolics constituents, and mainly linked to the direct or indirect antioxidant activity of olive oil phenolics and their metabolites, which are exerted more efficiently in the gastrointestinal (GI) tract, where dietary phenolics are more concentrated when compared to other organs. In this regard, we present a brief overview of the metabolism, biological activities, and anticancer properties of olive oil phenolics in the GI tract.