Generally, the widely accepted theory is that genetic factors, intestinal dysbiosis, environmental factors, and aberrant inflammatory responses are the main inducible factors in the initiation and/or progression of inflammation-related intestinal diseases
[44]. In consideration that environmental factors interact with the intestinal microbiota whose dysbiosis results in the over-activation of inflammatory responses where almost genetic factors are involved
[58][59][60], thus dysfunction of intestinal mucosal immunity represented by uncontrolled inflammatory responses contributes mostly to the onset and development of inflammation-related intestinal diseases. Actually, aberrant innate and adaptive inflammatory responses are common consequences in intestinal diseases, mostly represented by the activation of NF-κB and up-regulated levels of pro-inflammatory cytokines, including tumor necrosis factor α (TNF-α), interferon γ (IFN-γ), and interleukins
[61].
The conclusive signature of biological functions of polyphenols is anti-oxidative stress, achieved through neutralizing free radicals by donating an electron or hydrogen atom from hydroxyl groups
[62], reducing highly reactive hydroxyl radicals-induced oxidation by chelating with Fe
2+ [63], regeneration of essential vitamins
[64], and activation of nuclear factor erythroid 2-related factor 2 (Nrf2) related antioxidant system
[65]. In addition, numerous sources also reported the anti-inflammatory roles of polyphenols. Mechanically, these polyphenols regulate intestinal inflammatory responses by inactivating NF-κB, modulating mitogen-activated protein Kinase (MAPK), and phosphatidylinositide3-kinases/protein kinase B (PI3K/AkT) signaling cascades, thus reducing synthesis and release of pro-inflammatory cytokines
[66].
4.2. Interventional Options of Polyphenols in Inflammation-Related Intestinal Diseases
Functional foods and their extracts rich in natural polyphenols, such as fruits, coffee, vegetables, and whole grains, have been wildly applied in clinical trials. Anthocyanidins are a group of flavonoids that exist in berries. Anthocyanin-rich bilberry extract was demonstrated to ameliorate disease activity in UC patients
[67]. Further study showed anthocyanin-rich bilberry extracts reduced TNF-α and IFN-γ, as well as phosphorylated NF-κB levels, while enhanced levels of IL-22 and IL-10 in colonic biopsies of UC patients
[68]. Another clinical study showed that supplementation with anthocyanin-rich purple corn could improve infliximab-mediated disease remission in IBD
[69]. One study confirmed that green tea extract enriched in EGCG was an effective supplement for the chemoprevention of relapse of metachronous colorectal adenomas
[70]. Resveratrol is a natural polyphone found in grapes, red wine, and berries. A randomized, double-blind, and placebo-controlled pilot study has confirmed that resveratrol capsules treatment increased anti-oxidative capacity, decreased serum malondialdehyde (MDA) level and disease activity, and increased quality of life in patients with UC
[71]. Resveratrol was also reported to potentially improve the therapeutic outcomes in patients suffering from CRC when used either alone or as a combination therapy
[72].
More recently, robust experimental studies have been performed to investigate the protective or preventive effects of polyphenols in inflammation-related intestinal diseases based on the persuasive regulatory roles in oxidative stress, inflammation, and dysbiosis.
Cyanidin-3-Glucoside (C3G) is one of the anthocyanins which can be hydrolyzed into cyanidin (Cy), both of which were reported to improve clinical symptoms and reverse the colonic histological changes in TNBS-challenged mice
[73]. In addition, C3G improved DSS-induced body weight loss, colon length shortening, and morphology of colonic mucosa
[74]. However, intraperitoneal injection with C3G showed no effects against DSS-induced symptoms except for decreases in pro-inflammatory cytokines and an increase in the regulatory T cell (Treg) population in the colon
[75]. In vitro analysis revealed that C3G significantly decreased
TNF-α and
IL-6 mRNA levels by inactivation of NF-κB in THP-1
[76]. Except for C3G, pelargonidin 3-Glucoside (P3G) also showed beneficial roles in inflammatory intestinal diseases. Oral therapy with P3G reversed DSS-induced diarrhea, bloody stools, erosion of mucosal epithelium, crypt atrophy, loss of villi and goblet cells, as well as inflammatory cell infiltration in the colon of rats
[77].
Procyanidin, which is formed from catechin and epicatechin, belongs to proanthocyanin. Several studies have shown the protective effects of procyanidins against DSS-induced murine colitis, which were associated with increased goblet cells, enhanced claudin 1, anti-oxidative enzymes, and short chain fatty acid (SCFA) levels, as well as decreased mRNA levels of pro-inflammatory cytokines
[78][79]. Meanwhile, procyanidin treatment activated the AMPK/mTOR/p70S6K signal pathway, thus alleviating DSS-induced colitis by promoting cell proliferation
[80]. EGCG is a major bioactive polyphenol in green tea. Several studies revealed the critical roles of EGCG in alleviating DSS-induced clinical manifestations, including intestinal permeability, histopathological changes, and inflammatory cells infiltration in the colon
[81], decreasing pro-inflammatory cytokine levels, maintaining Th1/Th2 balance, and inactivating TLR4-NF-κB signaling pathway
[82]. Another study indicated that increased abundance of SCFAs-producing microbiota, such as
Akkermansia, may also be responsible for the beneficial roles
[83]. In addition, two studies revealed the therapeutic effects of EGCG in TNBS-induced murine colitis via inhibiting the activation of NF-κB, mast cells and macrophage activation
[84][85]. Dietary supplementation with EGCG improved acetic acid-induced colitis, as indicated by colon mucosal damage index and histological scores, and decreased levels of NO, MDA, TNF-α, IFN-γ, p65, as well as increased superoxide dismutase (SOD) activity
[86]. EGCG treatment significantly decreased the mean number of aberrant crypt foci and tumor load, as well as increased the abundance of
Bifidobacterium and
Lactobacillus in AOM/DSS-induced CRC
[87].
Oral administration with apigenin alleviated colon length shortening, decreased levels of colonic myeloperoxidase (MPO), alkaline phosphatase (AKP), TNF-α, IL-6, and restored intestinal microbiome in TNBS and DSS colitis models
[88][89].
Hesperidin and naringin are natural flavonoid compounds that occur in citrus fruits. Several studies suggested that both hesperidin and naringin could alleviate DSS-induced colitis in mice by improving the integrity of the colon, decreasing the expression of pro-inflammatory cytokines, and elevating the expression of colonic tight junction (TJ) proteins
[90][91]. Meanwhile, oral administration of hesperidin and naringin reversed the DSS-disturbed microbial community in the colon and increased the ratio of Firmicutes/Bacteroides
[92][93][94][95].
Kaempferol was widely used in DSS-induced colitis due to its anti-inflammatory property. Treatment with kaempferol alleviated DSS-induced body weight loss, bloody stool, shortened colon, colonic morphological damage, and up-regulated pro-inflammatory cytokines. Mechanically, kaempferol decreased serum LPS concentration, inactivated the downstream TLR4-NF-κB signal pathway, and restored microbial community
[96]. The same positive outcomes were also observed in the fecal microbiota from kaempferol-treated mice
[96]. Compared to the LPS group, kaempferol dramatically restored transepithelial resistance (TEER), evaluated the expression of TJ proteins, and inactivated of NF-κB signal pathway in Caco2 cells
[97].
Quercetin is another member of flavanol that exists in vegetables and fruits. Supplementation with dihydroquercetin significantly reversed DSS-induced colitis in mice via down-regulating levels of
IL-1β,
IL-6,
TNF-α, and up-regulating serum IL-10, colonic ZO-1, occludin, and
Lactobacillus levels
[98]. The literature also reported that quercetin alleviated DSS-induced murine colitis by increasing the expression of the glutamate-cysteine ligase catalytic subunit (GCLC) and serum glutathione level
[99]. Supplementation with quercetin attenuated LPS-induced intestinal injury and decreased pro-inflammatory cytokines and oxidative stress indices. Further analysis revealed that quercetin evaluated the expression of intestinal TJ proteins, inhibited apoptosis of intestinal epithelial cells, and increased the abundance of SCFAs-producing bacteria
[100][101]. Quercetin dramatically decreased the number and size of colon tumors in AOM/DSS-induced murine CRC
[102]. The AOM rat model revealed that quercetin inactivated the PI3K-Akt signal pathway to reduce proliferation, increase cell apoptosis, and suppress the formation of early preneoplastic lesions in colon carcinogenesis
[103].
Supplementation with genistein alleviated body weight loss, shortened colon, and inflammation, which skewed M1 macrophages towards M2, and decreased the mRNA levels of pro-inflammatory cytokines, thus attenuating DSS-induced colitis in mice
[104]. Another study reported the beneficial effects against DSS-induced colitis were achieved via ubiquitination of NLRP3 inflammasome
[105].
Oral administration of resveratrol significantly alleviated DSS-induced laboratory symptoms in mice and reduced mRNA levels of pro-inflammatory cytokines, as well as diminished p38 MAPK activation
[106]. Another study showed that resveratrol alleviated DSS-induced colitis by down-regulating protein abundance involved in autophagy and up-regulating levels of phosphorylated mTOR and SIRT1
[107]. Intraperitoneal administration of resveratrol to rats also significantly improved TNBS-induced colon injury, decreased MDA level, and increased glutathione peroxidase (GSH-Px) and catalase (CAT) activity
[108]. Resveratrol was also reported to alleviate LPS-induced enteritis in broilers and ducks via regulation of Nrf2 and NF-κB signaling pathways
[109][110]. An AOM/DSS-induced CRC model indicated the positive outcomes, which were associated with modulating the balance of the colonic microbial community, inhibiting histone deacetylases, and decreasing the populations of Th1 and Th17 cells
[111].