Flavonoids as Natural Anti-Inflammatory Agents Targeting Nuclear Factor-Kappa B (NFκB) Signaling in Cardiovascular Diseases: A Mini Review

1Cardiovascular diseases (CVDs) represent the major burden of mortality and morbidity in the developed countries. The most common pathogeneses of CVDs are inflammatory processes. Various transcription factors are related to inflammatory responses in CVDs such as T-bet, signal transducer and activator of transcription 3 (STAT3), interferon regulatory factors (IRFs), activator protein 1 (AP-1), and transcription factor Bcl11b. However, the key player in the regulation of inflammation is the transcription factor nuclear factor kappa B (NFκB). The inhibition of NFκB pathway has been demonstrated to show beneficial effect in various CVDs including hypertension, myocardial infraction, and arteriosclerosis. These findings support that targeted inhibition of NFκB appears to be a promising strategy in reducing cardiovascular complications.

2Flavonoids are plant polyphenolic compound derivatives from natural origin found in fruits, grains, vegetables, roots, bark, flowers, stems, tea, and wine. Non-plant natural products such as mushrooms and honey, plant extracts, plant juices, plant powders, and essential oils have shown to possess anti-inflammatory activities and many of these plant natural products have polyphenols as their major compound. However, the protective effects of flavonoid in CVDs via inhibition of NFκB are yet to be reviewed. Therefore, in this mini-review, we focused on the anti-inflammatory actions of flavonoids via inhibition of NFκB mechanism in CVDs.

3Flavonoids are categorized into six subclasses depending on its chemical structures: flavones, flavonols, flavanones, isoflavones, flavan-3-ols, and anthocyanidins.

4Flavones are found abundant in flowers, fruits, and leaves such as red peppers, celery, parsley, chamomile, mint, and ginkgo biloba. The most studied flavones are luteolin, apigenin, and tangeritin.

5Flavonols such as kaempferol, myricetin, quercetin, rutin, fisetin, silymarin, and isorhamnetin are ubiquitous in foods such as saffron, onions, kale, lettuce, tomatoes, apples, grapes, berries, red wine, and tea.

6Flavanones widely present in all citrus fruits, which gives the bitter taste of the juice and its peel. Oranges, lemons, and grapes are rich sources of flavanones and major compounds are hesperitin, naringenin, and eriodictyol.

7Isoflavones are unique in that they resemble estrogen in structure and, therefore, are classified as phytoestrogens. There are found abundantly in soy products such as tofu, roasted soy nuts, and miso.

8Flavan-3-ols, also called as dihydroflavonols, include catechin, epicatechin, gallocatechin, epigallocatechin, epicatechingallate, epigallocatechingallate, and procyanidin. The most commonly associated food with the flavan-3-ol compounds is black and green tea and fruits such as bananas, apples, blueberries, peaches, and pears.

9Anthocyanins are rich in outer cell layers of fruits such as merlot grapes, raspberries, cranberries, red grapes, strawberries, blueberries, bilberries, and blackberries. The most commonly studied anthocyanins are cyanidin, delphinidin, malvidin, pelargonidin, and peonidin.

10There are a few cellular redox pathways involved in the development of the chronic inflammatory CVD, which includes NFκB. NFκB is a transcription factor that activates inhibitor of kappa B (IκB) kinase in the cytosol upon being stimulated by inflammatory stimuli. Subsequent signaling pathways via canonical or non-canonical lead to migration of NFκB toward the nucleus and hence initiates the targeting gene such as pro-inflammatory cells, monocytes, macrophages, and T and B cells ( Figure 1 ).

11The canonical NFκB pathway responds rapidly to stimuli and activates NFκB, which increases pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumor necrosis factor alpha (TNF-α), which results in cell apoptosis. TNF-α receptor signaling plays an important role in the canonical pathway of NFκB in cell death via Jun N-terminal kinases (JNK), p38, and caspase 8 cascades. Furthermore, NFκB also activates angiotensin II, endothelin-1, and phenylephrine as hypertrophic agonist via IκB degradation and p65 nuclear translocation.

12A central signaling component of the non-canonical NFκB pathway is NFκB-inducing kinase, which induces p100 phosphorylation through kinase IKKα in a slow manner. Ligands of a subset of tumor necrosis factor receptor (TNFR) superfamily members are typical inducers of the non-canonical NFκB pathway.

13Quercetin or 3, 3′, 4′, 5, 7-pentahydroxyflvanone that falls into the category of flavonol is widely found in plants such as Ginkgo biloba, Hypericum perforatum, and Sambucus canadensis as well as vegetables such as apples, berries, grapes, onions, shallots, and tomatoes.

14In a clinical study involving patients with chronic systemic inflammation (CSI) in stable coronary artery disease (CAD), quercetin showed anti-inflammatory effects with reduction in indicators of CSI. Quercetin decreased IL-1β and TNF-α levels in blood serum, in addition to decreasing the transcriptional activity of NFкB in blood mononuclear cells. In leptin-induced inflammation model using human umbilical vein endothelial cells (HUVECs), quercetin significantly suppressed the upregulation of Ob-Ra (leptin receptor) expression, ERK1/2 phosphorylation, NFкB, and TNF-α. Furthermore, in a neonatal rat cardiac fibroblast, quercetin inhibited TNF-α, IL-1β, and IL-6 secretion by inhibiting the activation of NFкB and Akt induced by lipopolysaccharide (LPS).

15Luteolin or 3’, 4’, 5, 7-tetrahydroxyflavone is one of the most prevalent flavones widely found in fruits and vegetables such as carrots, cabbages, parsley, broccoli, celery, and apple skins.

16In an in-vivo sodium fluoride-induced hypertensive model, administration of luteolin increased nitric oxide (NO) bioavailability, reversed prolongation of QT and QTc intervals, and reduced the expressions of kidney injury marker 1 (Kim-1), NFκB, and cardiac troponin I (CTnI), which eventually normalized the blood pressure. Previous study in neonatal rat cardiac myocytes exposed to LPS showed luteolin reduced the TNF-α levels in the medium, downregulated the TNF-α mRNA in myocytes, inhibited degradation of IκB-β and nuclear translocation of NFκB, as well as reduced NFκB DNA binding, proposing the therapeutic potential of luteolin the management of inflammation-related myocardial diseases.

17Fisetin or 3, 3c,4c,7-tetrahydroxyflavone is a bioactive molecule found in fruits such as strawberry, apple, persimmon, and grape and vegetables such as onion and cucumber.

18reported the protective effect of fisetin against isoproterol-induced myocardial injury by suppressing myocardial injury markers, creatine kinase-muscle/brain (CK-MB), lactate dehydrogenase (LDH), and inflammatory markers (TNF-α and IL-6) in the blood serum as well as normalization of histological and ultrastructure of the heart. In addition, fisetin regulated the balance between pro- or anti-oxidants and pro- or anti-apoptotics proteins in the myocardial tissue. These protective effects of fisetin are attributed to the downregulation of receptor for advanced glycation end products (RAGE) and NFκB.

19Fisetin attenuated the development of diabetic cardiomyopathy by attenuating the expression of myocardial NFκB and the pro-inflammatory cytokines IL-1β, IL-6, and TNF-α in the heart of diabetic rats. These result in reduction of cardiac function markers such as CK-MB, LDH, and cTn as well as normalization heart morphology.

20Apigenin, a flavone, is found widely available in fruits and vegetables, such as grapefruits, oranges, celeries, and onions.

21In LPS-treated macrophages, apigenin has been shown to reduce toll-like receptor 4 (TLR-4), MyD88, and p-IκB-α expression levels via nuclear NFκB p65 signaling pathway. Similarly, in LPS-challenged apoE-/- mice, treatment with apigenin increased expression of ATP binding cassette A1 (ABCA1), which alleviated extra lipid accumulation, reduced miR-33, TLR-4, and NFκB p65 levels, lessened the macrophages and smooth muscle cell contents in the atherosclerotic region, and improved plasma lipid profile. These results suggested that apigenin attenuates atherogenesis by inhibition of nuclear NFκB p65 that up-regulates ABCA1-mediated cholesterol efflux.

22Apigenin was also shown to improve cardiac dysfunction and fibrosis in diabetic cardiomyopathy. Apigenin blunted the activity of NFκB and downregulated the activity of caspase3 accompanying with decreasing oxidative stress marker, glutathione peroxidase (GSH-Px), malondialdehyde (MAD), and superoxide dismutase (SOD).

23Isoliquiritigenin is extracted from root of licorice and has been used traditionally for the treatment of inflammatory or pulmonary diseases. In HUVECS exposed to TNF-α, isoliquiritigenin blocked the involvement of NFкB at the transcriptional levels, and thus attenuated the downstream expression of VCAM-1, E-selectin, THP-1 monocyte adhesion, IкB-α, and PECAM-1, suggesting the protective effects of isoliquiritigenin through NFкB-dependent mechanisms. In angiotensin II induced hypertension model, isoliquiritigenin attenuated inflammation cytokines including IL-1β and TNF-α, excessive deposition of extracellular matrix, and oxidative stress-induced apoptosis via nuclear factor E2-related factor 2 (Nrf2) and NFκB pathways.

24Rutin is a flavonol that presents in buckwheat and citrus fruits. In a sodium fluoride-induced hypertensive rats, administration of rutin reduced blood pressure elevation by enhancing NO bioavailability via down-regulation of NFκB expression and up-regulation of Nrf2.

25In carfilzomib-induced cardiotoxicity in rat, rutin protected against myocardial hypertrophy by upregulating IκB-α and downregulating NFκB expression, resulting in attenuation of β-myosin heavy chain, reduction in B-type natriuretic peptide mRNA expressions, and normalization of cardiac muscle fiber morphology.

26In addition, rutin increased activities of Nrf, decreased activation of NFκB in human embryonic kidney reporter cell line, and preserved relaxation of fetal placental arteries derived from human chorionic plate.

27In high mobility group box 1 (HMGBI)-induced inflammatory response in HUVECs, rutin attenuated NFκB and ERK1/2, which, in turn, reduced IL-6 and TNF-α levels. Up-regulation of VCAM-1, intercellular adhesion molecule-1 (ICAM-1), and E-selectin induced by HMGB1 were similarly inhibited by rutin, suggesting that the protective effect of rutin on vascular inflammation is by inhibiting the HMGB1 and NFκB pathways.

28In LPS-induced inflammation in HUVECs, rutin reversed barrier disruption, expression of cell adhesion molecules, and adhesion and migration of monocytes in endothelial cells. The barrier protective effects of rutin were linked to a down-regulation of TNF-α, deactivation of NFκB, and reduced phosphorylation of IκB-α.

29Chrysin (5,7-dihydroxyflavone) is a flavone, which is found in the blue passion flower, honey, and propolis. Chrysin prevented doxorubicin (DOX)-induced cardiomyopathy including disturbance of cardiac conduction, increased serum cardiac markers and histopathological alteration in heart of rat via downregulation of NFκB, mitogen-activated protein kinase (MAPK), suppression of AKT pathway and its upstream activator, vascular endothelial growth factor (VEGF).

30In a rat model of monocrotaline-induced pulmonary arterial hypertension (PAH), chrysin reduced right ventricular systolic pressure and mean pulmonary artery pressure. In addition to suppression of right ventricular remodeling, chrysin abolished increased expression of collagen I, collagen III, and NFκB.

31In isoprenaline-induced myocardial injury in rats, chrysin relieved hemodynamic and ventricular dysfunction as well as reduced ultrastructural myocardial damage via inhibition of NFκB, IκKβ expression, and TNF-α level as well as increased peroxisome proliferator-activated receptor-gamma (PPAR-γ) expression.

32In a rat model of myocardial infarction, fibrosis in the interstitial and perivascular regions and expression of collagen was reduced following chrysin treatment. This effect is associated with increased PPAR-γ expression and decreased NFκB expression via inhibition of IκKβ phosphorylation, leading to reduction of matrix metalloproteinase-2 (MMP-2), MMP-9 levels, and suppression of activator protein 1 (AP-1) level.

33Genistein under the subgroup of an isoflavone [4′,5,7-trihydroxyisoflavone,5,7-dihydroxy-3-(4-hydroxyphenyl)-4-H-1-benzopyran-4-one] is primarily found in soy-based foods, legumes, and red clover. In homocysteine-induced endothelial cell inflammatory injury, genistein prevented endothelial damage via blockade of activation of NFκB, expression of inflammatory cytokine and adhesion molecule, IL-6, and ICAM-1.

34explored the effect of genistein on angiotensin II-induced vascular smooth muscle cell inflammation. Angiotensin II-induced expression of NFκB, C-reactive protein (CRP), MMP-9, phosphorylation of ERK1/2 and p-38, which lead to atherosclerotic inflammatory damage, were reversed following genistein treatment. Furthermore, genistein enhanced expression of PPAR-γ, suggesting cardiovascular protective effect by the isoflavone is through regulation of p38/ERK1/2-PPARγ-NFκB signaling pathway.

35Silymarin is a flavonolignan extracted from the milk thistle. Silymarin augmented relaxation of pulmonary arteries isolated from a lung ischemia-reperfusion (I/R) injury model. Vascular protective effect of silymarin is due to inhibition of NFκB, thus suppressing the serum concentration of inflammatory cytokines and reducing protein expression of hypoxia inducible factor-1α (HIF-1α) and iNOS.

36Silibinin, a major active constituent of silymarin, was able to reduce the abnormal size of cardiac myocytes and prevent hypertrophy by alleviating the production of epidermal growth factor receptor (EGFR). Silibilin exerted its anti-inflammatory effect by suppressing the activation of NFκB stimulated by angiotensin II in cardiac myocytes or in the aortic banding male mice. Furthermore, silibilin interfered with the phosphorylation and degradation of IκB-α and activation of IκKβ in vivo.

37Kaempferol (3,4′,5,7-tetrahydroxyflavone) is a flavonol that is present widely in fruits, vegetables, and herbs, including grapes, tomatoes, and tea. In cardiac fibroblasts stimulated with LPS, kaempferol decreased release of pro-inflammatory cytokines by inhibiting AKT phosphorylation and NFκB activation. In isoprenaline-induced cardiac damage, kaempferol improved the hemodynamic and left ventricular functions in male rats, which abated the increased serum concentration of CK-MB and LDH, preserved the morphology of myocardium, and reduced the levels of pro-inflammatory cytokines. Similarly, kaempferol prevented cardiac damage by inhibiting the protein expression of NFκB, p38, and JNK suggesting that cardioprotective and anti-inflammatory action of kaempferol was associated with NFκB signaling pathway.

38Table 1 summarizes the effects and mechanisms of action of flavonoids in CVD.

39The actions of flavonoids in mitigating inflammation by modulation of NFкB offer potential agents for the treatment of CVDs. However, several of these actions reported in vitro may yet to be fully recognized due to their low bioavailabilities following oral administration. Flavonoids have shown promising results in reducing atherosclerosis in several animal experimental models; however, conflicting results were reported in human clinical trials. The low bioavailability and clinical efficacy of flavonoids are attributed to their poor absorption, metabolism by the metabolizing enzymes in the intestine and liver, and structural modifications by the colonic bacteria remain as the major problems. Continuous investigation is required to enhance the bioavailability and efficacy of the flavonoids to tap the full potential of these natural agents.

40All authors contributed to the writing. KC, DM, and MM conceived, designed, and revised the manuscript.

41This study was funded by Government Agency grant GA001-2017, MAHSA University and USIM research project code: PPPI/FPG/0118/051000/15618 Universiti Kebangsaan Malaysia and Universiti Putra Malaysia. The funding agencies played no role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript, which are fully the responsibilities of the authors.

42The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

43The reviewer ZJ declared a shared affiliation, with no collaboration, with one of the authors, XFL, to the handling editor at time of review