Quercetin is a member of the flavonoid family with a flavone nucleus, composed of a heterocyclic pyrone ring that links the two benzene rings to form the central nucleus (Russo et al., 2014). The main form of quercetin found in herbs is composed of hydrophilic glycosides (sugar conjugates), which are not easily and directly absorbed by enterocytes. The daily dosage of quercetin for human usage is 10–100 mg, which could be obtained from 500–1,000 mg of purified extract (Brito et al., 2015). Within enterocytes, quercetin undergoes a variety of enzymatic reactions, including methylation, hydrolysis, sulfonylation, and glucuronidation, by specific transferase enzymes (Brito et al., 2015). Following transportation into the intestinal lumen and then the liver, quercetin metabolites (the major quercetin-derived circulating compounds in plasma, quercetin-3-glucuronide and quercetin-3ʹ-sulfate) are formed via other conjugation reactions (Alrawaiq and Abdullah 2014). There are two forms of quercetin: conjugated and non-conjugated; the plasma concentration of each is 3.5–5.0 μmol/L and <0.33 μmol/L, respectively. The form of quercetin which is absorbed by enterocytes is the conjugated form (Alrawaiq and Abdullah 2014). Recently, a new study revealed that the microbes of the gut play an important role in quercetin absorption because they have enzymes which render the quercetin molecules smaller and more absorbable (Xu et al., 2017; Santangelo et al., 2019). Quercetin can be metabolized to glucuronidated, methylated, or sulfated derivatives (D'Andrea 2015). As previously stated, quercetin has several anticancer effects that have been demonstrated in numerous in vitro and in vivo studies. Its antitumor effects include its antioxidant activity, inhibition of angiogenesis, inhibition of the cell cycle and proliferation, and prevention of tumor metastasis. In addition, quercetin has many other beneficial properties that make it an effective supplement in the daily diet, including anti-inflammatory effects, antihypertensive effects, antithrombotic effects, anti-atherosclerosis properties, and anti-arrhythmia activity. Therefore quercetin has attracted attention from researchers as an adjuvant agent to take advantage of its antitumor, antioxidant, cytoprotective, and anti-proliferative properties (Lugli et al., 2009). Many studies have been conducted to evaluate the beneficial antitumor effects of quercetin against the kidney, breast, prostate, lung, ovarian, colorectal, pancreatic, and nasopharyngeal cancer (Lee et al., 2015a; Liu et al., 2017a; Baby et al., 2018; Huang et al., 2018; Li et al., 2018; Polukonova et al., 2018; Yang et al., 2019a). One of the most important features of quercetin is its pro-apoptotic effect, which is caused by increasing pro-apoptotic molecules such as P53, BAX, caspase-3, and caspase-9 or stimulating the mitochondrial apoptosis pathway or decreasing antiapoptotic proteins (Roos and Kaina, 2006; Zhang et al., 2008a; Zhang et al., 2009a; Tan et al., 2009). Quercetin could regulate and inhibit the cell cycle by activating p21 and decreasing D1/Cd4 and E/Cdk2 ratios. Quercetin can arrest the cell cycle at the G1 phase as well as inhibit microtubule polymerization which also affects the cell cycle (Gupta and Panda, 2002; Moon et al., 2003). There are various mechanisms and pathways by which quercetin can regulate the cell cycle, some of which can increase the cell cycle, such as the PI3K/AKT/PKB pathway. Moreover, quercetin can inhibit carcinogenesis and metastasis along with apoptosis induction (JÁF, 2004; Gulati et al., 2006). p53 is a key molecule in the regulation of cell death and cell survival pathways and plays a role in cancer therapy. p53 can act as an antioxidant by protecting the cellular DNA from oxidative damage and by regulating genes for endogenous antioxidants, such as catalase, Gpx1, microsomal GSH homologous transferase PIG12, Mn-SOD2, and aldehyde dehydrogenase ALDH4A1. Many research works have confirmed that quercetin can stabilize p53 levels and increase its phosphorylation (Polyak et al., 1997; Hussain et al., 2004; Yoon et al., 2004; Tanigawa et al., 2008). In malignant cells, p53 genes can be blocked or mutated, causing loss of functions (Gibellini et al., 2011). Another antioxidant function of quercetin is to quench reactive oxygen species (ROS), thereby preventing ROS-mediated DNA damage (Metodiewa et al., 1999; Awad et al., 2000). Therefore, quercetin has the potential to be used in cancer treatment because of its ability to regulate the cell cycle, antioxidant effects, p53 stabilization, and apoptosis induction. The potent anticancer activity and structure of quercetin are illustrated in Figure 1.

Yangyang Guo and others evaluated the effects of quercetin on pancreatic ductal adenocarcinoma (PDA) using different techniques, including a real-time cell analysis, migration, proliferation, and invasion, and used a nude mouse tumor formation assay. They evaluated tumor growth and metastasis as well as colony formation, cell migration, and flow cytometry analysis of SHH signaling in pancreatic cancer cells (PCCs), in vivo and in vitro (Guo et al., 2021a). Quercetin showed an antitumor activity by downregulating c-Myc, leading to inhibition of PCC proliferation. Quercetin reduced the level of TGF-β1 and suppressed the epithelial-mesenchymal transition, thereby blocking migration and invasion. Quercetin induced apoptosis via death receptor pathways and the mitochondrial pathway. In an animal study in nude mice, treatment with quercetin reduced metastasis. The therapeutic effects of quercetin on PDA involve the regulation of SHH activity and are related to Gli2 but not Gli1. The quercetin-mediated suppression of PCC proliferation, migration, and invasion was abrogated when the SHH activity was increased using recombinant Shh protein. Moreover, by promoting the expression of Zeb2 and Snail1, Shh could activate TGF-β1/Smad2/3 signaling and promote EMT, resulting in a partial reversal of the quercetin-mediated suppression of PCC migration and invasion. These findings suggest that quercetin could be used to treat PDA by inhibiting PCC migration, metastasis, and invasion and by inducing apoptosis via antagonizing the TGF-β1/Smad and SHH signaling pathways (55).

Sarah Hassan and her colleagues evaluated the efficacy of a combination of quercetin with the known drugs gemcitabine (GEM) and doxorubicin (DOX) against human pancreatic cancer cells and hepatic cancer cells, respectively. They also evaluated the effects of quercetin on the activity of drug efflux pumps as well as the effects related to hypoxia. Moreover, they investigated the role of multidrug transporters (including MDR) and HIF-1α (Hassan et al., 2020). In their research, they combined quercetin with other anticancer drugs and showed that combination therapy (anticancer drug plus quercetin) had better results than single therapy. In 2D and 3D cultures, the percentage of dead cells increased to 60%. Deeper evaluation revealed that quercetin induced p53-mediated apoptosis via increased levels of the p53 regulator and also downregulated HIF-1α. Moreover quercetin could inhibit the MDR-1 efflux activity. In conclusion, they found quercetin could be administered in combination with GEM or DOX in multidrug resistant pancreatic and hepatic cancers, respectively (Hassan et al., 2020).

Liu et al. (2020) evaluated the effects of quercetin on GEM-resistant PC cells and its mechanism. They evaluated two cell lines from each of pancreatic and hepatocellular carcinoma. The PCC lines were BxPC-3 and PANC-1, and the HCC cell lines were HepG2 and Huh-7. Using a proliferation assay, it was found that quercetin had a cytotoxic effect on HepG2 and PANC-1 (GEM-resistant), and a flow cytometry analysis revealed a pro-apoptotic effect on HepG2 and PANC-1. Quercetin could induce apoptosis via upregulation of antitumor protein p53 and also cyclin-D1 downregulation, as seen by Western blotting. Moreover, it caused cell cycle arrest in the S phase. In conclusion, their data revealed that quercetin could be used in combination with known anticancer drugs against GEM-resistant hepatic and pancreatic cancers (Liu et al., 2020).

Hoca et al. (2020a) evaluated the effect of quercetin and resveratrol on the epithelial-mesenchymal transition (EMT) in pancreatic cancer cells (CD133-negative and CD133-positive). The CD133+ cancer stem cells were purified from PANC-1 cells using the MiniMACS system, and then the three cell variants (CD133+, CD133-, and PANC-1) received different doses of quercetin and resveratrol. Immunocytochemistry with antibodies against vimentin, TNF-α, ACTA-2, N-cadherin, IL-1β, and the MTT assay were employed. In the CD133 + cells that were treated with quercetin, the intensity of N-cadherin, ACTA-2, and IL-1β staining was reduced, compared to CD133 + cells treated with resveratrol. In conclusion, quercetin could reduce N-cadherin expression and prevent EMT and metastasis more than resveratrol in PCC stem cells (Hoca et al., 2020a). Table 1 lists some studies on the therapeutic effects of quercetin in pancreatic cancer.

Cing-Syuan Lei et al. tested whether a combination of quercetin and irinotecan might be effective for decreasing the metastasis of gastric cancer (GC) by measuring gene and protein expression (Lei et al., 2018). Their study compared the effect of low-dose SN-38 (irinotecan metabolite) in combination with quercetin with a high dose of SN-38 alone on β-catenin expression, cell viability, and apoptosis. In vivo xenograft animal models and in vitro studies looked at the effects of quercetin and low-dose irinotecan on GC metastasis. The β-catenin protein levels were lower in AGS cells treated with quercetin and a low dose of SN-38 than the single therapy using quercetin. ITG-β6 and Twist-1 gene expression (two EMT markers) as well as cyclooxygenase-2 gene expression were higher in high-dose irinotecan-treated cells than the combination therapy. In the AGS mouse model, VEGF-A (vascular endothelial growth factor), VEGF-receptor 2, and the percentage of Tie2-expressing monocytes were significantly lower after combined therapy. The data suggested that the treatment of GC with irinotecan could be improved by combining it with quercetin (Lei et al., 2018).

Hemati et al. evaluated the effect of si-RNA targeted against CDC20 (cell division cycle protein 20 homolog) and antiproliferative drugs (quercetin and DOX) against GC. They investigated niosome-encapsulated delivery vehicles for si-RNA and drugs. They found that si-RNA delivery in combination with anticancer drugs led to downregulation of CDC20, and therefore improved GC treatment (Hemati et al., 2019). To optimize the si-RNA loading capacity and physicochemical properties, they varied the cationic lipid content of cationic PEGylated niosomes. Quercetin and DOX as well as anti-CDC20 si-RNA were loaded into the co-delivery system, and physicochemical properties, controlled release, thermosensitivity, rates of apoptosis, and gene silencing efficacy were measured. Intriguingly, the data revealed that the co-delivery system, which was designed for loading si-RNA, had an appropriate high positive charge for drug delivery. They also showed a thermosensitive drug release behavior that successfully silenced CDC20 expression when compared with the single delivery of either si-RNA or the drug. Furthermore, their system effectively inhibited GC cell growth. Their data suggested that PEGylated niosomes co-loaded with CDC20 si-RNA plus anticancer drugs might be used as a novel system for GC treatment (Hemati et al., 2019).

Hai Li et al. evaluated the mechanism and effects of quercetin on metastasis of GC and also searched whether urokinase plasminogen activator and urokinase plasminogen receptor (uPA/uPAR) were involved in the mechanism or not. The uPA/uPAR system plays a key role in GC metastasis, so they planned the study to test whether quercetin could affect this system. In their study, they measured the amount of uPA and uPAR activity in precancerous tissues and compared them with different GC cell lines in terms of migration and invasion (Li and Chen 2018a). The data revealed that in precancerous tissues, uPA and uPAR activities were lower than that in GC cells, and the migration and invasion of GC cell lines were correlated with uPAR expression. uPA and uPAR protein expression levels were reduced along with migration and invasion after GC BGC823 and AGS cells were treated with quercetin (10 µM for 72 h). Quercetin combined with uPAR knockdown decreased matrix metalloproteinase-9 and metalloproteinase-2 activities, thereby inhibiting Pak1-Limk1-cofilin signaling. Quercetin treatment inhibited AMPKα activator, NF-κB, ERK1/2, and PKC-δ, which caused downregulation of uPA and uPAR expression. In conclusion, quercetin could be a novel component in GC therapy for reducing metastasis and invasion (Li and Chen 2018a).

Zeng et al. (2018) evaluated the effects of quercetin on GC. They treated human GC cells (NCI-N87) with 15 μM quercetin for 48 h along with dimethyl sulfoxide as a control. The HiSeq 2500 DNA sequence data were used to compare differentially expressed genes (DEGs) between groups. An advanced analysis was used to assess the protein–protein interaction (PPI) network. The regulatory network of transcription factors (TFs-DEGs) was elucidated using Cytoscape. The DEGS found were Fos proto-oncogene (FOS, degree = 12), aryl hydrocarbon receptor (AHR, degree = 12), Jun proto-oncogene (JUN, degree = 11), and cytochrome P450 family 1 subfamily A member 1 (CYP1A1, degree = 11), which were significantly associated with other proteins in the PPI network with higher degrees. Early growth response 1 (EGR1), FOS like 1 (FOSL1), FOS, and JUN were higher among the five TF-DEGs, whereas AHR was downregulated. The Wnt signaling pathway was also enriched for FOSL1, JUN, and Wnt family member 7B (WNT7B). In the PPI network, CYP1A1 was closely linked to AHR. Therefore, quercetin may have targeted FOS, AHR, JUN, CYP1A1, EGR1, FOSL1, and WNT7B in GC (Zeng et al., 2018).

Shang et al. (2018a) and his colleagues carried out in vitro studies on the potential of quercetin to induce human GC cell death, apoptosis, and alter gene expression. Their data revealed that quercetin could induce GC cell apoptosis and also change gene expression. Flow cytometry revealed that quercetin increased the level of reactive oxygen species (ROS) and led to the destruction of the mitochondrial membrane by reducing certain protein levels, and finally it caused apoptosis in AGS cells. The Western blotting showed that quercetin decreased the level of antiapoptotic proteins, including Bcl-x, Bcl-2, and Mcl-1, while increasing pro-apoptotic proteins, including Bid, Bax, and Bad. Quercetin induced various effects on gene expression. For instance, quercetin decreased the expression of KDELC2F (KDEL [Lys-Asp-Glu-Leu] containing 2), VEGF-B (vascular endothelial growth factor B), and CDK10 (cyclin-dependent kinase 10) but increased the expression of TP53INP1 (tumor protein p53 inducible nuclear protein 1), TNFRSF10D (tumor necrosis factor receptor superfamily 10D, decoy with truncated death domain), JUN-B (jun B proto-oncogene), and TP53INP1. In conclusion, their data revealed the molecular mechanism, gene expression, and signaling pathway involved in quercetin’s ability to inducing apoptosis in human GC cells (Shang et al., 2018a). Table 2 lists some studies on the therapeutic effects of quercetin in GC.

Mostafa et al. carried out a study to design and synthesize multi-target anticancer drugs for CRC treatment. Their agents targeted some essential enzymes for CRC survival and proliferation, including PIM-1, 5-LOX, and COX-2 (117). Compounds 5a-d and 5g inhibited the COX-2 enzyme similar to celecoxib with a high selectivity index. Compound 5b-e inhibited the 5-LOX enzyme to the same extent as quercetin, while 5g, 5f, and 5a showed slightly lower inhibitory activity than quercetin. They concluded that 5e-g and 5a-c were better than Celecoxib, while 5g, 5f, and 5a were better than diclofenac sodium, in an in vivo model. Compounds 5e-g and 5a-c showed a better safety profile than celecoxib in fasted rats. Compounds 5g, 5f, and 5d had the highest potency against CRC cell lines (HCT-116 and Caco-2) at a much lower dosage than against normal human cells. Moreover, compounds 5g and 5e induced apoptosis in CRC cell lines and induced caspase activation. Compounds 5d, 5g, and 5e inhibited PIM1 and PIM2 kinase, which was comparable to the reference staurosporine (El-Miligy et al., 2021).

Ghamedi et al. evaluated the mechanism and effects of a 200 mg dose of quercetin and a 150 mg dose of EGCG (epigallocatechin gallate) at varying proportions on the induction of apoptosis and inhibition of proliferation of human CRC cell line HCT-116 (Al-Ghamdi et al., 2021). The phytomolecules inhibited cell growth, arrested cell cycle, annexin V, and reduced clonogenicity. Colony formation was inhibited by the lowest dose of the tested drugs. Furthermore, a significant increase in annexin V was found at 150 mg dose of quercetin and 100 mg dose of EGCG. The combination therapy induced cell cycle arrest at the G1 phase. In conclusion, the combination of EGCG plus quercetin could be used as effective combination chemotherapy in the future, but more studies are required to establish a suitable dose and any side effects (Al-Ghamdi et al., 2021).

Erdoğan et al. (2021) evaluated the antiangiogenic effects of quercetin and luteolin on colon cancer cells (HT29) and also their anticancer effects compared to the traditional chemotherapy drug 5FU, as well as a combination of 5FU with luteolin or quercetin. They used Western blotting, qRT-PCR, human VEGF ELISA, fluorescence microscopy, and an MTT assay. They used Western blotting to assess the effects on the genes p53, Bax, Bcl-2, P38, MAPK, PTEN, Akt protein, and mTOR. ELISA was used to determine the effects of treatment on angiogenesis and the MTT assay to measure cell viability. Fluorescence microscopy was used to detect apoptosis in HT29 cells. The induction of apoptosis in cells treated with quercetin and 5FU was eight times higher than the control, and it was 10 times higher for luteolin and 5FU. The VEGF level was significantly lower in cells treated with a combination of quercetin or luteolin plus 5FU. They found that quercetin and luteolin could regulate apoptosis in HT-29 cells and also that combination therapy reduced antiapoptotic factors, including Bcl-2, mTOR, and Akt gene expression compared to the control group. P53, P38, MPK, and PTEN gene expression increased faster in groups treated with 5FU and quercetin than in cells treated with 5FU and luteolin. In conclusion, luteolin and quercetin could synergistically increase the anticancer effect of 5FU and could also reduce the toxic effects of 5FU in colorectal cancer (Erdoğan et al., 2021).

In a study by Wenhui Liu et al., they induced constipation (a known colorectal risk factor) by administering loperamid in a rat model, and then evaluated the effects of quercetin on loperamid-induced constipation (Liu and Zhi 2021). The data revealed that in rats that were treated with 25 mg/kg and 50 mg/kg of quercetin, the intestinal transit rate was increased, as well as the concentration of short-chain fatty acids and levels of gastrin, motilin, and substance P. Moreover, quercetin improved intestinal peristalsis movement and reduced somatostatin levels. The expression of aquaporin 3, transient receptor potential vanilloid 1, glial cell line-derived neurotrophic factor, enteric nerve-related factors, nitric oxide synthase, stem cell factor, and c-Kit was evaluated by Western blotting and RT-qPCR. They found that quercetin treatment reduced loperamid-induced constipation by increasing the markers of interstitial cells of Cajal, including stem cell factor, its receptor c-Kit, and AQP3. In conclusion, their data showed that quercetin had protective effects against loperamide-induced constipation, so it could be used to reduce the risk of colorectal cancer (Liu and Zhi 2021).

Iván Benito et al. evaluated the effects of daily supplementation with microencapsulated probiotics alone or in combination with microencapsulated quercetin, for the prevention of colorectal cancer. They used ApcMin/+ mice which spontaneously develop intestinal adenomas and carcinoma. They assessed histological alterations, intestinal bleeding, fat depots, respiratory quotient, body weight, and energy expenditure. Furthermore, they evaluated gene expression involved in the Wnt signaling pathway (Benito et al., 2021). ApcMin/+ mice were administered with Bf and Lg probiotic strains, 10 (7) CFU/100 g of food, or both probiotic strains plus microencapsulated quercetin (15 mg/100 g of food) for 73 days. Then they evaluated the energy metabolism, changes in organ and body weight, colon tissue histology, intestinal microbiota, and Wnt signaling pathway gene expression. The data revealed that the microencapsulated supplement (probiotics plus quercetin) could prevent colorectal cancer progression in ApcMin/+ mice (Benito et al., 2021).

Shree et al. (2021) evaluated the role of quercetin in apoptosis, hyperproliferation, and inflammation and also its mechanism in 1,2-dimethylhydrazine (DMH)-induced carcinogenicity and tumor multiplicity. They administered the rats with quercetin at 25 or 50 mg/kg body weight orally and 20 mg/kg bodyweight of DMH subcutaneously for 15 weeks and then sacrificed them. The DMH produces reactive oxygen species (superoxide) by membrane lipid peroxidation and causes an imbalance in redox homeostasis. DMH also decreases tissue antioxidant levels. Proliferative and inflammatory factors were increased in DMH-induced intestinal carcinogenicity as a result of low Bax/Bcl-2 ratio and apoptosis dysregulation. Quercetin pretreatment reduced the harmful effects of DMH, including preserving the detoxifying enzyme activity and reducing proliferation and early markers (mucin depletion and goblet cell disintegration) in colonic tissue. Quercetin regulated the expression of β-catenin and APC and lowered the incidence and multiplicity of tumors. The histological results further confirmed the beneficial role of quercetin in reducing DMH-induced pathological alterations (Shree et al., 2021). Table 3 lists some studies on the therapeutic effect of quercetin in colorectal cancer.

In a study by Antara Banerjee et al., the effects of a combination of Lycopodium clavatum and quercetin on CRC cells (Colo-320) were evaluated by measuring expression of extracellular matrix proteins, cytotoxicity assay, morphological alterations, and expression of apoptotic genes. Furthermore, the anti-inflammatory, apoptotic, and proliferative responses of cancer cells treated with Lycopodium and quercetin alone or in combination, were evaluated to assess potential synergistic effects and expression of tumor suppressor genes (Banerjee et al., 2020). Their study was mainly conducted to identify whether L. clavatum extract or quercetin or a combination could produce significant anti-inflammatory effects in Colo-320 cells. Gelatin zymmography, toxicity biomarkers, and apoptotic gene expression were measured. The combination of quercetin (50 μm) and L. clavatum extract (10 μL) significantly attenuated cell growth and decreased the proliferation potential and colony formation compared to their separate administration. Antimicrobial assays showed that Lycopodium exerted antimicrobial activity against Pseudomonas aeruginosa and Escherichia coli. The quercetin and L. clavatum extracts contain flavonoids and alkaloids. Gelatin zymography showed a prominent decrease in MMP9 and MMP2 activity following administration of the quercetin and Lycopodium combination. This regimen altered the expression of catalase, cyclin D1, Bcl2, caspase-3, Bax, and Wnt1 genes in colon cancer cells. Quercetin and Lycopodium showed synergistic effects which may effectively attenuate progression of CRC with few adverse effects and no drug resistance (Banerjee et al., 2020).

Yana Li et al. evaluated whether quercetin could sensitize colon cancer cells to radiotherapy, considering the fact that quercetin can inhibit the Notch-1 signaling pathway and thus reduce proliferation of colon cancer cells and CSCs (colon cancer stem cells) (Li et al., 2020). Their study showed that a combination of quercetin with ionizing radiation (IR) had more significant antitumor effects than either IR or quercetin alone by suppressing the Notch-1 signaling pathway in CSCs. These findings were further validated using an in vivo xenograft of human colon cancer in nude mice, which showed a significant reduction in CSC markers and Notch-1 signaling proteins. Combined treatment with low-dose IR and quercetin remarkably decreased the expression of all gamma-secretase complex proteins in DLD-1 and HT-29 cells. Moreover, the combination therapy was partly abrogated by altering the expression of NICD (Notch intracellular domain). Finally, their study suggested that a combination of IR (5 Gy) and quercetin (20 mM) could be a novel therapeutic approach in colon cancer by suppressing the Notch-1 signaling pathway in CSCs. Further studies are required to confirm the therapeutic potential of combined quercetin–radiotherapy in clinical trials (Li et al., 2020).

In a study by Xin et al., it was shown that NF-κB plays a major role in resistance to chemotherapy in esophageal cancer cells treated with 5-FU. Their study mainly focused on the potential benefits of quercetin on the chemosensitivity of human esophageal cancer and identifying the underlying antitumor mechanism (Chuang-Xin et al., 2012a). Their study evaluated the effect of combination of quercetin and traditional chemotherapeutic drugs on esophageal carcinoma. The MMT assay was employed to measure the effects of quercetin on Eca109 and EC9706 cell proliferation. Western blotting was employed to evaluate protein levels, while apoptosis was investigated using FACS (Annexin V-FITC/propidium iodide). Their study showed that the combination of 5-FU and quercetin considerably reduced cell growth and provoked apoptosis in esophageal cancer cell lines (Eca109 and EC9706) in comparison with either 5-FU or quercetin alone. These effects were correlated with the downregulation of phosphorylated inhibitor of NF-κB (pIκBα) in response to treatment with 5-FU alone. In conclusion, the addition of quercetin to the conventional chemotherapeutic drug 5-FU may improve treatment of esophageal cancer (Chuang-Xin et al., 2012a).

Zhao et al. investigated whether quercetin-3-methyl ether (Q3ME), which is a flavonoid frequently found in tea, vegetables, fruits, wine, and tea, could inhibit the formation and progression of preneoplastic esophageal lesions produced by N-nitrosomethylbenzylamine. They studied inflammation and proliferation of esophageal cells in vivo. Q3ME suppressed esophageal cancer cell proliferation as well as the malignant transformation of healthy esophageal cells by inhibiting the MAPK and AKT/mTOR/p70S6K signaling pathways. Q3ME inhibited esophageal tumorigenesis by targeting ERKs and AKT (Zhao et al., 2018a). They found that phosphorylated ERKs (p-ERKs) and p-AKT were overexpressed in esophageal cancer cell lines and in tissue samples from patients with esophageal cancer. Using a human pull-down assay and a phosphokinase array, they showed that Q3ME could interact with ERKs and AKT and inhibit their kinase activity. Mechanistically, Q3ME reduced proliferation of esophageal cancer cells and anchorage-independent growth. According to Western blotting, Q3ME could inhibit the activity of ERK and AKT downstream signaling pathways and subsequently suppress AP-1 (activating protein-1) transcription factor. Interestingly, Q3ME also suppressed the development of preneoplastic lesions caused by NMBA. This inhibitory effect was correlated with attenuated proliferation of esophageal cancer cells and lower inflammation in vivo (Zhao et al., 2018a).

Yue Liu et al. evaluated the effects of quercetin on the angiogenesis and migration of esophageal cancer cells, in addition to the underlying mechanism (Liu et al., 2021). In their study, human esophageal cancer cells (Eca109) received 5 or 10 μg/ml of quercetin. A scratch wound healing assay evaluated cell migration, invasion was examined using a transwell assay, and a colony formation assay was conducted. Human umbilical vein vascular endothelial cells (CLR-1730) were inoculated in Eca109 conditioned medium, and the effects of quercetin were measured by tube formation and wound healing assays. Western blotting measured the levels of MMP2, MMP9, and VEGF-A protein expression. Quercetin (10 μg/ml) decreased colony formation in Eca109 cells; however, no difference was observed between the control group and the 5 μg/ml quercetin group. The group treated with 10 μg/ml quercetin showed reduced cell migration and invasion, while 5 μg/ml only suppressed invasion. Tube formation ability and migration of endothelial cells was inhibited in cells incubated in Eca109 conditioned medium. The group treated with 10 μg/ml quercetin showed reduced levels of MMP2, MMP9, and VEGF-A (Liu et al., 2021). Table 4 lists some studies on the therapeutic effects of quercetin in esophageal cancer.

Yamada et al., reported that myricetin and quercetin could suppress the AKT signaling axis, which may subsequently inhibit TGF-α- and HGF-mediated HuH7 cell migration (Yamada et al., 2020). In their study, the effects of quercetin on the migration of HuH7 cancer cells induced by TGF-α or HGF were investigated. Quercetin prominently inhibited the migration of HuH7 cells mediated by both TGF-α and HGF in a dose-dependent manner. Furthermore, myricetin (another flavonol compound) also prominently suppressed the migration of cancer cells. Neither myricetin nor quercetin affected the autophosphorylation of receptors mediated by TGF-α and HGF. Moreover, quercetin did not affect the phosphorylation of p38 MAPK induced by TGF-α or HGF. On the other hand, both myricetin and quercetin suppressed AKT phosphorylation mediated by the growth factors. Their study showed that quercetin could inhibit HCC cell migration mediated by growth factors by blocking the AKT signaling pathway but not p38 MAPK (Yamada et al., 2020).

Zhao et al., conducted a study to evaluate whether a combination of quercetin plus cisplatin could produce synergistic effects in HCC cells (Zhao et al., 2014). The hepatocellular carcinoma cell line HepG2 was treated with either cisplatin (10 μM) or quercetin (50 μM) or their combination, and apoptosis and cell proliferation were measured. The cisplatin and quercetin combination triggered apoptosis and inhibited proliferation in HepG2 cells compared with either agent alone. The combination of cisplatin and quercetin affected the expression of genes involved in the cell cycle and apoptosis. Administration of quercetin alone led to a significant increase in p16 expression in HepG2 cells. G1 phase cell cycle arrest induced by quercetin and apoptosis in HepG2 cells were partly abrogated by p16 targeted knockdown using RNA interference technology. Therefore, the cell cycle arrest and apoptosis caused by quercetin plus cisplatin likely involved p16 and provided a more prominent antiproliferative and apoptotic effect (Zhao et al., 2014).

Ding and colleagues showed that quercetin could inhibit the proteasome activity and decrease the phosphorylation of ERK1/2. The increased activity of ERK1/2 led to an increase in the proteasome chymotrypsin-like activity, whereas the increased activity of MEK1 resulted in a lower proteasome chymotrypsin-like activity. Administration of quercetin mitigated the expression of proteasome β subunits. Their study showed that the MEK1/ERK1/2 signaling pathway could modulate the expression of proteasome β subunits, leading to a lower proteasome chymotrypsin-like activity (Ding et al., 2018). Caspase and trypsin-like protease activities remained constant in HepG2 cells along with an increased activity of JNK and p38 MAPK and attenuated phosphorylation of ERK1/2. The decreased proteasome activity induced by quercetin may not be reversed following suppression of the JNK and p38 MAPK signaling pathways. Upregulation or downregulation of MEK1 enhanced or attenuated proteasome chymotrypsin-like activity, respectively. The expression of β subunits of proteasome was reduced by both MEK1/ERK1/2 inhibition and by quercetin administration (Ding et al., 2018).

Srisa-Nga et al. designed a study to prepare quercetin magnetic nanocarriers using polymeric micelles to allow monitoring and treatment of hepatocellular carcinoma. These polymeric micelles were characterized based on their morphology, size, and magnetic properties. They evaluated the cellular uptake, cytotoxicity, cell cycle analysis, and potential magnetic targeting in HCC cells transfected with hepatitis B virus (HepG2.2.15) (Srisa-Nga et al., 2019). They co-encapsulated superparamagnetic iron oxide nanoparticles (SPIONs) and quercetin (QCT) into mPEG750-b-OCL-Bz micelles (QCT-SPION micelles) and added methoxy-poly (ethylene glycol)-b-oligo-3-caprolactone in order to inhibit the growth of HepG2.2.15 cells. The QCT-SPION micelles were prepared using the film hydration technique. They had a spherical morphology with an average size of 22–55 nm. The optimized micelles showed a quercetin loading capacity and entrapment efficiency of 3.5 and 70%, respectively. The SPIONs alone had a T2 relaxivity of 137 mM⁻¹ s⁻¹, while the cluster of SPIONs inside the SPION-QCT micelles had a T2 relaxivity value of about 246 mM⁻¹ s⁻¹, implying a higher sensitivity for magnetic resonance imaging. The SPION-QCT micelles were taken up by HepG2.2.15 cells and showed more cytotoxicity than quercetin, along with cell cycle arrest at the G0/G1 phase. The SPION-QCT micelles accumulated in the vicinity of neodymium–iron–boron (NdFeB) magnetic discs, and the application of a strong magnetic field further inhibited tumor cell growth. These mPEG750-b-OCL-Bz micelles acted as a novel multifunctional co-delivery vehicle for SPIONs and quercetin for monitoring and treatment of HCC (Srisa-Nga et al., 2019).

Pasachan et al. designed a study to evaluate the antidiabetic effects of TTE (Tiliacora triandra Colebr) on the gluconeogenesis of liver cells and HCC cells (HepG2). The principal constituents of TTE include quercetin and epicatechin. They investigated the underlying mechanism and the in vivo effects in a type 2 diabetes mellitus (T2DM) rat model. They asked whether TTE could act as a nutritional agent in the management of T2DM and prevent complications such as non-alcoholic steatohepatitis, non-alcoholic fatty liver disease, and eventual cirrhosis (Pasachan et al., 2021a). They used a Diels aqueous extract of TTE to affect glucose synthesis in cells with T2DM and HepG2 cells. The HepG2 cells received TTE and purified quercetin and epicatechin. Hepatic gluconeogenesis was evaluated in rats with T2DM, which received either daily TTE (1,000 mg/kg body weight), metformin (30 mg/kg), or the combination of metformin plus TTE over a period of 12 weeks. Similar to quercetin and epicatechin, TTE stimulated gene expression of catalase, glutathione peroxidase, and copper–zinc superoxide dismutase. TTE lowered the synthesis of new glucose by suppressing phosphoenolpyruvate carboxykinase and glucose-6-phosphatase and stimulating AMP-induced protein kinase phosphorylation in HepG2 cells. Similar to metformin, TTE showed antidiabetic, anti-triglyceridemic, anti-hyperglycemic, and antioxidant effects in rats with T2DM. TTE may be used as a nutraceutical agent in individuals with insulin resistance, obesity, or those receiving antidiabetic medication (Pasachan et al., 2021a). Table 5 lists some studies on the therapeutic effects of quercetin in HCC.

Singh et al. (2020) investigated the effects of a combination of quercetin and resveratrol (at minimum non-toxic concentrations) on oral cancer. In their study, the combination caused cell death in Cal-33 oral cancer cells but not in HEK-293 normal cells. They found that stimulation of cyclin E (a cell cycle regulatory protein) and reduced expression of cyclin A led to cell cycle arrest at the S phase in Cal-33 cells. The comet assay in association with gamma-H2AX foci showed DNA injury, and the overexpression of Bax and PARP1 cleavage showed cellular apoptosis after the combination treatment. Resveratrol treatment, both alone and in combination, could downregulate the expression of histone deacetylase 1 (HDAC), HDAC8, and HDAC3. Collectively, the combined quercetin and resveratrol regimen suppressed cell growth and caused DNA damage and cell cycle arrest in oral cancer cells but not in non-cancer cells (Singh et al., 2020).

Yuan and others designed a study to identify the effects of quercetin on proliferation, migration, apoptosis, and invasion in the vincristine (VCR)-resistant P glycoprotein (P-gp) overexpressing oral cancer cell line, KB/VCR. The VCR sensitivity of KB/VCR cells was increased by inhibiting P-gp expression with quercetin. They found that the proliferation, invasion, and migration of KB/VCR cells were suppressed by quercetin, in addition to enhanced VCR sensitivity and apoptosis (Yuan et al., 2015). The anticancer activity of these agents was assessed in KB/VCR oral cancer cells in vitro. According to their study, 25–100 µM concentrations of quercetin could efficiently block proliferation, invasion, and migration of KB/VCR cells, while at concentration above 50 µM, it resulted in the G1 phase cell cycle arrest. According to apoptosis analysis, 50 or 100 µM of quercetin stimulated apoptosis in KB/VCR cells by inhibiting Bax expression and upregulating Bcl-2 and caspase-3. Dose ranges of 25–100 µM quercetin reduced drug efflux mediated by decreasing the expression of P-gp in KB/VCR cells. A combination of quercetin (50 µM) and vincristin (0.375 µM) was even more effective in inducing apoptosis. Consequently, quercetin may reduce drug resistance as a promising approach for treatment of oral cancer and other tumor types (Yuan et al., 2015).

In another study by Ma et al., the effects of quercetin on the human oral cancer SAS cell line were evaluated. They found that quercetin stimulated apoptosis by mitochondrial and endoplasmic reticulum stress signaling pathways in SAS cells (Ma et al., 2018). The effect of quercetin on cell death was investigated using flow cytometry, Western blotting, confocal laser microscopy, and Annexin V/propidium iodide (PI) double staining, at 6–48 h following quercetin treatment. They found that quercetin reduced cell viability and increased production of Ca2+ and reactive oxygen species. Annexin V/PI staining showed increased apoptosis, with a reduction in mitochondrial membrane potential (ΔΨm), in addition to modified expression of apoptosis-related proteins. Western blotting showed that quercetin increased Fas, Fas ligand, Fas-related protein with death domain, and caspase-8, which were correlated with death receptors on the surface of the cells. Quercetin upregulated ATF-6α (activating transcription factor), gastrin-releasing peptide-78, and ATF-6β, showing endoplasmic reticulum stress. The levels of pro-apoptotic protein BH3-interacting death-domain antagonist in conjunction with reduced expression of antiapoptotic proteins Bcl-2 and Bcl-extra-large protein, all of which reduced mitochondrial membrane potential. In addition, confocal microscopy showed that quercetin upregulated the expression of apoptosis-inducing factor, cytochrome c, and endonuclease G, all of which can stimulate apoptosis. Therefore, quercetin may be a promising anticancer drug in the treatment of oral cancer (Ma et al., 2018).

Huang et al. (2013) explored the antitumor effects of quercetin in human oral cancer cell lines with EGFR upregulation and assessed the role of FOXO1 transcription factor in growth inhibition induced by quercetin. They showed that FOXO1 played a critical role in suppressing cellular growth mediated by quercetin in oral cancer cells with high EGFR expression. Quercetin results caused cell cycle arrest at the G2 phase and stimulated apoptosis in TW206 and HSC-3 oral tumor cells with high EGFR expression. Quercetin activated FOXO1 and blocked EGFR/Akt activation. Silencing of FOXO1 decreased the expression of FasL and p21 mediated by quercetin and further increased apoptosis and G2 cell cycle arrest. Similarly, quercetin inhibited cancer growth in the mouse HSC-3 xenograft model (Huang et al., 2013).

Cheng et al. (2012) focused on the effects of quercetin on overcoming drug resistance and the underlying mechanisms in oral squamous cell carcinoma. They used a non-adhesive cell culture system, called the DRSP (drug-resistant sphere) model, in order to generate drug-resistant cells from oral tumor cell line SCC25. A comparative analysis was conducted between DSRPs and the parental control cells based on the measurement of EMT (epithelial-mesenchymal transition)-related markers, CSC features, and genes related to drug resistance in vitro. They also looked at carcinogenesis and tumor treatment in vivo. Their study on DRSPs showed overexpression of genes involved in drug resistance, including CSC-related markers and ABCG2, such that DRSPs had higher cisplatin (Cis) resistance and more CSC features than those in the control group. Furthermore, upregulation of p-Hsp27 (phosphorylated heat-shock protein 27) due to activation of the p38 MAPK signaling pathway was observed in DRSPs. Hsp27 silencing reduced the resistance to cisplatin and triggered apoptosis in DRSPs. Moreover, quercetin reduced the expression of p-Hsp27 and acted as an Hsp27 inhibitor to reduce the EMT status, resulting in the activation of apoptosis in DRSPs. Enhanced tumorigenesis was verified in DRSPs using a nude mouse xenograft model. Combined treatment by cisplatin and quercetin could reduce tumor growth and decrease drug resistance in OSCC. The p38 MAPK–Hsp27 signaling axis plays a major function in drug resistance induced by CSCs in oral cancer, and this could be inhibited by quercetin and cisplatin treatment (Cheng et al., 2012). Table 6 lists some studies on the therapeutic effects of quercetin in oral cancer.