Quercetin Attenuates Atherosclerotic Inflammation by Inhibiting Galectin-3-NLRP3 Signaling Pathway

SCOPE

Atherosclerosis is the underlying pathogenesis of cardiovascular events caused by inflammation, and dietary intervention has been recommended as one fundamental prevention strategy. Herein, the anti-arteriosclerotic properties of quercetin are investigated by modulating galectin-3 (Gal-3)-NLR family, pyrin domain-containing 3 (NLRP3) pathway.

METHODS AND RESULTS

Plaques from ApoE^-/- mice fed by high-fat diet (HFD) with or without quercetin (100 mg (kg·bw)^-1 ) for 16 weeks, and carotid plaques from patients with carotid stenosis are collected for histopathological examinations and molecular mechanism assays. Quercetin significantly alleviates atherosclerotic lesions and reduces lipid retention caused by HFD. Proteomic technology identified Gal--3 increased by HFD but lowered by quercetin. Furthermore, immunofluorescence and immunohistochemistry exhibit higher expressions of Gal-3 and NLRP3 in carotid plaques and plaques from HFD-fed mice, which are concurrently down-regulated by quercetin. Similar to TD139, quercetin dramatically suppresses NLRP3 inflammasome activation in oxidized low-density lipoprotein-laden macrophages, and accordingly alleviates cellular steatosis and IL-1β secretion, which is abolished by recombinant Gal-3. Co-immunoprecipitation shows Gal-3 binding to NLRP3 promotes inflammasome activation.

CONCLUSION

Gal-3 initiates inflammatory lesions by activating NLRP3 inflammasome which functions as a candidate target of quercetin exerting favorable anti-atherogenic effects. The findings highlight a promising strategy for atherosclerosis prevention and treatment by naturally-occurring quercetin.

Flavonoids and cellular stress: a complex interplay affecting human health

Flavonoid consumption has beneficial effects on human health, however, clinical evidence remains often inconclusive due to high interindividual variability. Although this high interindividual variability has been consistently observed in flavonoid research, the potential underlying reasons are still poorly studied. Especially the knowledge on the impact of health status on flavonoid responsiveness is limited and merits more investigation. Here, we aim to highlight the bidirectional interplay between flavonoids and cellular stress. First, the state-of-the-art concerning inflammatory stress and mitochondrial dysfunction is reviewed and a comprehensive overview of recent in vitro studies investigating the impact of flavonoids on cellular stress, induced by tumor necrosis factor α, lipopolysaccharide and mitochondrial stressors, is given. Second, we critically discuss the influence of cellular stress on flavonoid uptake, accumulation, metabolism and cell responses, which has, to our knowledge, never been extensively reviewed before. Next, we advocate the innovative insight that stratification of the general population based on health status can reveal subpopulations that benefit more from flavonoid consumption. Finally, suggestions are given for the development of future cell models that simulate the physiological micro-environment, including interindividual variability, since more mechanistic research is needed to establish scientific-based personalized food recommendations for specific subpopulations.

Intracellular quercetin accumulation and its impact on mitochondrial dysfunction in intestinal Caco-2 cells

PURPOSE

Flavonoid bioavailability and bioactivity is associated with interindividual variability, which is partially due to differences in health status. Previously, it was demonstrated that cellular stress, especially mitochondrial stress, increases intracellular quercetin uptake and this is associated with beneficial health effects. Here, the impact of quercetin on mitochondrial dysfunction, induced by stressors targeting different sites of the electron transport chain, is investigated. The influence of the mitochondrial stress on quercetin uptake and subcellular location is studied and the accumulated quercetin metabolites in intestinal Caco-2 cells and mitochondria are characterized.

PRINCIPAL RESULTS

It was observed that quercetin counteracted (i) the carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP)-induced decrease in maximum oxygen consumption, (ii) the valinomycin-, oligomycin- and FCCP-induced reactive oxygen species production and (iii) the valinomycin-induced disruption of mitochondrial membrane potential. Using confocal microscopy, it was found that upon mitochondrial stress, the intracellular quercetin accumulation increased and was partially located in the mitochondria. Finally, it was demonstrated that quercetin was present as O-methyl, O-methylglucuronide and O-methylsulfate conjugates in the cell lysate and mitochondria-enriched fraction.

MAJOR CONCLUSIONS

This study shows that quercetin can partially restore, especially FCCP-induced, mitochondrial dysfunction and this protective effect was linked with an intracellular quercetin accumulation in the mitochondria of intestinal cells.

In vitro bioavailability and cellular bioactivity studies of flavonoids and flavonoid-rich plant extracts: questions, considerations and future perspectives

In vitro techniques are essential in elucidating biochemical mechanisms and for screening a wide range of possible bioactive candidates. The number of papers published reporting in vitro bioavailability and bioactivity of flavonoids and flavonoid-rich plant extracts is numerous and still increasing. However, even with the present knowledge on the bioavailability and metabolism of flavonoids after oral ingestion, certain inaccuracies still persist in the literature, such as the use of plant extracts to study bioactivity towards vascular cells. There is therefore a need to revisit, even question, these approaches in terms of their biological relevance. In this review, the bioavailability of flavonoid glycosides, the use of cell models for intestinal absorption and the use of flavonoid aglycones and flavonoid-rich plant extracts in in vitro bioactivity studies will be discussed. Here, we focus on the limitations of current in vitro systems and revisit the validity of some in vitro approaches, and not on the detailed mechanism of flavonoid absorption and bioactivity. Based on the results in the review, there is an apparent need for stricter guidelines on publishing data on in vitro data relating to the bioavailability and bioactivity of flavonoids and flavonoid-rich plant extracts.