Potential mechanisms of quercetin in cancer prevention: focus on cellular and molecular targets

Various physiological processes such as cell proliferation, differentiation, stemness, migration, and apoptosis are controlled by the Wnt signalling system, also known as APC/β-catenin/Tcf pathway [33].

There are two different Wnt/β-catenin signalling pathways: canonical and non-canonical. Canonical pathway employ β-catenin regulating gene expression, in contrast, non-canonical is a β-catenin-independent pathway [34]. To be more specific, in the canonical pathway, Wnt ligand (Wnt1 and Wnt3a) binds to Frizzled and Lrp5/6 receptor complexes which lead to stabilization and transcriptional activation of cytoplasmic β-catenin while non-canonical Wnt signalling pathway is represented as a response to Wnt ligands which is independent of β-catenin stabilization [35]. The loss of Wnt signalling regulation is frequently correlated with tumor progression and metastasis [36]. Various studies have demonstrated that the anti-tumor effect of quercetin on the Wnt/β-catenin pathway has a multifunctional impact. Cells from the NT2/D1 line were studied by Mojsin et al. (human in vitro model of teratocarcinoma). In NT2/D1 cells, they discovered that quercetin inhibits SOX2, Nanog, and Oct4 expression, as well as β-catenin nuclear translocation, resulting in a decrease in β-catenin-dependent transcriptional activity [37]. In research by Haesung et al., quercetin caused apoptosis in 4T1 cells and showed dose-dependent anticancer efficacy (murine mammary cancer cell line). After treatment with 20 µM quercetin, Dickkopf-related protein 1 (DKK1), 2 and 3, which are regulators of Wnt signalling, are increased and cell viability is reduced, according to the researchers [38]. The Wnt signalling pathway was studied in human colon cancer cells (SW480) by Shan et al., who found that quercetin inhibits the production of cyclin D1 and survivin, which are involved in the cell cycle regulation and death, in the cells [39]. Quercetin's influence on GSK3 has been examined in a recent study on HT29 colon cancer cells (key elements of the Wnt pathway). Quercetin at doses of 10 to 75 µM did not significantly inhibit GSK3 and GSK3, and as a result, the total β-catenin level in HT29 cells was mostly unaffected. Various experimental setups and biological settings can produce different kinds of reactions [40].

For AKT to translocation to the plasma membrane, phosphatidylinositol-3-kinase (PI3K) has to be present in the cell. Biochemical processes such as cell cycle progression, differentiation, cell survival, and cell proliferation are all influenced by the PI3K/AKT pathway [41]. The Bcl-2 protein family and Bax are controlled by the PI3K/AKT pathway, which has anti-apoptotic properties (a pro-apoptotic gene) [42]. Deregulation of this pathway could be a key event in cancer pathogenesis [41]. Several studies explored the quercetin effect on PI3K/AKT pathway. Using the HCC1937 PTEN-null cancer cell line, Gulati et al. observed that 25 µM quercetin may decrease active AKT/PKB phosphorylation and dramatically limit cell proliferation of PTEN-null cancer cells, which is consistent with previous studies [43].

In a study on HL-60 leukaemia cell lines, Yuan et al. investigated the quercetin mechanism and discovered that quercetin decreased p-AKT and Bcl-2 levels, as well as triggered apoptosis, in a manner that was associated with the inhibitory actions of PI3K/AKT. It was revealed that at 150 µM quercetin, the inhibitory effects were the most pronounced [42, 44, 45].

In interconnected networks in a cell, the JAK/STAT (Janus kinase/signal transducers and activators of transcription) signalling pathway detects stimulus signals from outside the cell and transmits its message to the cell nucleus and activates several transcription factors [46‐48]. The JAK/STAT pathway is involved in regulating processes such as immune maintenance, cell division and growth, cell death, and tumor formation. JAK/STAT pathway components are regulated by other paths such as ERK MAPK and PI3K [49‐51]. Skin, immune system, and cancer disorders are caused by JAK/STAT pathway disruptions [52].

In line with this, Qin et al. examined the effect of quercetin on leptin and its receptor in MGC-803 cells via the JAK/STAT signalling pathway. They found that quercetin caused the arrest of cells in the G2/M stage of the cell cycle through the p-STAT3 pathway and celled to apoptosis and necrosis. On the other hand, the flavonoid compound reduces the expression of leptin and its receptor [53].

It has been shown that quercetin can inhibit IL-6-induced glioblastoma cell growth and migration by regulating the STAT3 signalling pathway, which affects the expression of this protein in glioblastoma cells. In T98G and U-87 cell lines, quercetin blocked the IL-6-induced STAT3 pathway, reducing the expression of GP130, JAK1, and STAT3. As a result, there is a reduction in the proliferation and migration of cancer cells [54].

The effects of quercetin and epigalactochin-3-gallate (EGCG) on cholangiocarcinoma cells were investigated by Senggunprai et al. IL-6 and IFN-gamma were found to regulate JAK/STAT (STAT1/3 phosphorylation) pathways in cholangiocarcinoma cells, and the results suggested that these two chemicals can be employed as chemopreventive agents against these cells. Lastly, these compounds were able to suppress KKU100 cancer cell proliferation and migration [55, 56].

Quercetin suppresses clonogenic survival in BT-474 cells, triggers apoptosis via caspase 3, 8, and PARP cleavage, and causes cell cycle arrest in the sub-G0/G1 phase, according to a study by Seo et al., which also found that quercetin reduces p-JAK1 and p-STAT3 expression and inhibits MMP-9 secretion. When it comes to HER-2-expressing breast cancer, this flavonoid can both prevent and treat the disease [57]. Luo et al. examined the processes of apoptosis, autophagy, and quercetin proliferation in cervical cancer cells. Using quercetin-conjugated gold nanoparticles, they found that the complex shows a similar role in suppressing JAK2, a protein expressed in cervical cancer cells, which suppresses proliferation, invasion, and migration processes. They also found that apoptosis and autophagy processes occurred through caspase-3 in cancer cells inhibited by JAK2, and that cyclin D1 and mTOR were suppressed by the STAT3/5 and PI3K/AKT signalling pathways [58, 59].

Mitogen-Activated Protein Kinase (MAPK) fall into three main categories: ERKs (stimulated by mitogens and growth factors), JNK/SAPK, and p38s (stimulated by cellular stress and inflammatory cytokines) [60‐62]. Cell proliferation, differentiation, gene expression, cell survival, mitosis, and apoptosis are all affected by MAPK, which has been found in MAPK14, MAPK 7, and MAPK 12 [63]. Due to the association between the MAPK pathway and many types of cancer, some bioactive natural compounds, such as quercetin, have been found to prevent cancer by altering the MAPK signalling pathway. Chen et al. observed that stimulating the p38 MAPK signalling pathway boosted the expression of ABCG2, MDR1, and pHSP27 genes in multidrug-resistant spheres of oral cancer cells. Through the suppression of pHSP27 expression and the progressive modification of EMT, they discovered that quercetin might promote apoptosis in cancer spheres. Quercetin and cisplatin, when combined, can inhibit the development of oral cancers and diminish the resistance of the malignancies to treatment [64].

The isocourestine chemical isolated from Bidens bipinnata L. was examined by Huang et al. and was proven to inhibit liver tumour growth and progression. Activation of caspases 3, 8, and 9 leads to an increase in apoptosis, which in turn triggers JNK phosphorylation, suppresses the production of ERK and p38 MAPK and lowers the level of PKC, all of which lead to the cell cycle arrest at G1. Isoquercetin, on the other hand, inhibits the development of transplanted tumours in naked mice when tested in vivo [65‐67]. Zhu et al., studied the molecular effect of a quercetin derivative called 7-O-granuestine quercetin (GQ) on gastric cancer cells (SGC-7901 and MGC-803). They found that the compound induced cancer cell apoptosis and cell arrest in the G2/M phase. Its molecular mechanism is that GQ increases ROS production, activates the p38 and JNK pathways, disrupts the regulation of Bcl-2, Bcl-xl, and Bax proteins, releases cytochrome c, and ultimately induces apoptosis [68, 69]. ERK and AKT were shown to be connected with the advancement of esophageal squamous cell carcinoma in a study conducted by Zhao et al. The phosphorylated form of these proteins was found to be associated with the progression of the disease. Quercetin-3-methyl ether was employed to limit the kinase activity of these proteins, which in turn inhibited the proliferation and advancement of esophageal cancer cells, according to the researchers [70].

Another study on prostate cancer (PC3 and LNCaP cell line) by Erdogan et al., showed that quercetin inhibited the proliferation of PC3 and CD44⁺ / CD133⁺ cells. On the other hand, co-administration of siRNA and quercetin induces apoptosis and induces cell entry into G1 and cell arrest, and finally inhibits the phosphorylation of AKT, PI3K, and ERK1/2 proteins and the expression of p38, ABCG2 proteins, and NF-κB was reduced [71‐73].

The apoptotic effects of quercetin on canine osteosarcoma cells (D17 and DSN) were examined by Ryu et al., Apoptosis, cell cycle, ROS content, mitochondrial membrane depolarization, intracellular calcium concentrations, and quercetin's antiproliferative effect were all shown to be affected by quercetin. S6, AKT, and p70S6K proteins can be inhibited by quercetin whereas ERK1/2, p38, and c-JNK proteins can be elevated. Osteosarcoma cells die as a result of the combination of these events [74].

In human CRC cell lines obtained from patients with microsatellite instability (MSI), quercetin stimulated 5-fluorouracil-induced apoptosis in a p53-dependent way [75]. Bcl-2 protein expression was reduced in CO115 cells when quercetin and 5-FU were combined. This suggests that quercetin and 5-FU synergism is dependent on the apoptotic mitochondrial pathway [75]. HCT15 (p53 mutant) cells, on the other hand, lacked this synergy, indicating that quercetin's effects are p53-dependent [75]. Quercetin also accelerated the apoptosis generated by doxorubicin (DOX) in hepatoma cells in a p53-dependent manner by downregulating Bcl-xl expression [76]. In line with this result, the quercetin impact on DOX-mediated apoptosis was reduced upon the applying pifithrin-a (p53 inhibitor), Z-VAD fmk (caspase inhibitor), or Bcl-xl expression vector [76]. Moreover, when Bcl-xl expression is inhibited, quercetin promoted p53 protein expression, Bax translocation, transcriptional activity and DOX-induced PUMA (p53 upregulated modulator of apoptosis) expression [76]. Quercetin promoted the expressions of cell death-related genes including Bax, p53, caspase-3 and cytochrome-c in cervical cancer cells, while significantly down-regulated AKT and Bcl-2 expression [77]. Quercetin promoted mitochondrial cytochrome-c release and the accumulation of ROS, resulting in apoptosis and cell cycle arrest at G2/M phases [77]. It has been shown that quercetin strongly promoted p53, Sestin 2, and activated AMPK protein expressions while suppressing mTOR in a dose-dependent manner [78]. Quercetin generated intracellular ROS, leading to the induction of apoptosis and the regulation of the Sestrin 2/AMPK/mTOR pathway in a p53- independent manner [78]. It has been reported that quercetin significantly enhanced Trichostatin A (TSA)-induced apoptosis and growth arrest in A549 cells; the transfection of p53 siRNA reduced its enhancing impacts [79]. However, since p53 silencing does not entirely limit quercetin's effect on TSA-induced apoptosis in A549 cells, it has been postulated that the p53-independent route may potentially contribute to quercetin's boosting effect [79]. Quercetin promoted the antitumor effect of TSA in a xenograft mouse model of lung cancer [79]. The combination of TSA and quercetin treatment of tumors in mice increased p53 and apoptosis levels [79]. It has been reported that quercetin, regardless of p53 status, markedly suppresses cell proliferation, and promotes sub-G1 and apoptotic cell populations in lung cancer cells [80]. Besides, upon treatment of H460 cells with quercetin, genes related to death pathways such as the c-Jun Nterminal kinase (JNK) pathway (JNK, MEKK1, MKK4), death receptors (TNFR1, TRAILR, FAS), the interleukin-1 receptor pathway (IRAK, IL1, IL1R), the NF-κB pathway (IκBα) and the caspase cascade (dFF45, caspase-10) were upregulated, while genes associated with cell survival (NF-κB, AKT, IKK) and genes involved in cell proliferation (SCF, CdKs, SKP2, cyclins) were downregulated [80, 81]. Moreover, quercetin promoted the expression of GAdd45, involved in cell cycle arrest [80].

Quercetin generates intracellular ROS production, decreases mitochondrial membrane potential, and promotes sestrin 2 expression via the AMPK/p38 pathway, thus leading to apoptosis in HT-29 colon cancer cells in a p53-independent pathway [82]. Nano-quercetin (NQ) has been shown to activate p53-ROS crosstalk and cause apoptosis in HepG2 cells; in addition, it triggers epigenetic modifications that lead to cell cycle arrest in the sub-G phase, decreased cell populations in the mitotic and synthetic phases, and suppression of proliferation [83]. In addition, MET containing a variety of phenolics such as quercetin, homoorientin, naringin, and isorhamnetin has been found to induce apoptosis in HCCSCs without relying on p53 by increasing the Bax/Bcl-2 ratio; cleaved PARP protein expression was used to indicate apoptosis [84, 85].

Quercetin caused a marked cell cycle arrest in HT-29 cells in the S-phase [86]. A reduction in phosphorylated-AKT and CSN6 protein expression, as well as a decrease in Bcl-2 and Myc protein expression, was seen after quercetin administration [86]. Moreover, CSN6 overexpression omitted the impacts of quercetin treatment on HT-29 cells, indicating the involvement of the AKT-CSN6-Myc signalling axis in quercetin-induced apoptosis in the cells [86]. The synergistic effect of rutin (a glycoside with 5-FU( in the induction of apoptosis in PC3 cells line has been reported [87]. The combination of rutin/5-FU promoted p53 gene expression and decreased Bcl-2 protein expression [87]. A quercetin-supplemented diet promoted the antitumor effect of TSA in nude mice which bears lung cancer in a dose-dependent manner via the upregulation of p53 [88].

Cell cycle arrest and apoptosis have been seen in MDA-MB-231 breast cancer cells after treatment with quercetin, which was decreased by knocking down the protein Foxo3a [89]. Furthermore, quercetin-induced Foxo3a activity was eliminated by using a JNK inhibitor [89]. The activities of p21, p53 and GADD45 signalling pathways, activated by quercetin, were decreased as a result of Foxo3a knockdown and the suppression of JNK activity [89]. Quercetin exhibited a time- and dose-dependent impact on cell survival in prostate cancer cells [90]. Disturbing the ROS homeostasis and affecting mitochondrial integrity, quercetin caused apoptotic and necrotic cell death in the cells [90]. DU-145 prostate cancer cells with mutated p53 and increased ROS levels displayed a marked decrease in pro-survival AKT pathway activation even though Raf/MEK were stimulated, upon quercetin treatment [90]. However, in the PC-3 prostate cancer cell line, which lacks p53 and PTEN, and bears low ROS levels, vivid activation of AKT and NF-κB pathways were found [90]. Through controlling ROS, AKT, and NF-κB pathways, quercetin employs its anti-cancer impact [90].

Pretreatment with quercetin could make ovarian cancer cells susceptible to radiation-induced cell death in p53 dependent manner [91]; the combinational treatment of irradiation and quercetin could exacerbate DNA damages, cause apoptotic cell death, promote Bax and p21 expressions and reduce Bcl-2 expression in the cells [91]. However, all of these events could be reversed upon knocking down of p53 [91]. The combinational treatment of irradiation and quercetin in the human ovarian cancer xenograft model could restrict tumor growth while activating p53, γ-H2AX and CCAAT/enhancer-binding protein homologous protein [91].

As Caspase-3 is activated in the cancer cells, quercetin ruthenium complex dramatically up-regulates the p53 and Bax, and Caspase-3 expression downregulates the Bcl-2 proteins in HT-29 cell lines [92]. The results of in vivo study showed that the complex treatment could significantly decrease the Bcl-2 expression in colon cancerous tissues while significantly increasing p53 and Bax expression [92]. The results also revealed that the complex caused apoptosis in colon cancer cells via p53-mediated activation of Bax, caspase-3, and Bcl-2 downregulation as well as the suppression of the mTOR/AKT pathway [92]. The vanadium–quercetin combination increased the expression of p53, caspase-9, caspase-3, and Bax and decreased the expression of Bcl-2, mTOR, VEGF, and AKT, resulting in cell cycle arrest and death in rat mammary carcinogenesis and the MCF-7 cell line [93].

When AGS human gastric cancer cells were treated with quercetin, anti-apoptotic Bcl-2, Mcl-1, and Bcl-x protein expression were decreased, and pro-apoptotic proteins such as Bad, Bid, and Bax protein expression was increased [94]. In addition, CDK10 (cyclin-dependent kinase 10), VEGFB (vascular endothelial growth factor B), and KDELC2 (KDEL [Lys-Asp-Glu-Leu] containing 2) gene expressions, associated with apoptosis pathways, were decreased after quercetin treatment [94]. However, quercetin promoted TP53INP1 (tumor protein p53 inducible nuclear protein 1), TNFRSF10D (Tumor necrosis factor receptor superfamily, member 10d, decoy with truncated death domain), and JUNB (jun B proto-oncogene) gene expressions [94, 95].

Quercetin has been shown to promote trichostatin A (TSA) induced apoptosis in H1299 cells, and p53 null cancer cells, in p53-independent manners, which may be less effective than the p53-dependent pathway [79, 96]. The protein expression of p53 significantly was increased in cervical cancer cells including in HeLa, HFF, and SiHa cells after quercetin treatment. Besides, quercetin has also been shown to increase the p53 nuclear signal in SiHa and HeLa cells [97]. Quercetin also increased the transcript level of p21 and the level of Bax protein in HeLa and SiHa cells, famous target genes of p53, suggesting the induction of the transcriptional activity of p53 by quercetin [97]. While increasing p53 and its nuclear signal, quercetin caused apoptosis and cell cycle arrest in the G2 phase in HeLa and SiHa cells [97].

Tamoxifen and quercetin have been reported to control several apoptosis-related gene expressions including p53, Bax, p21, and Bcl-2 in breast cancer cells, leading to cell apoptosis regulation [98]. The results revealed that the impact of quercetin on tamoxifen-induced cell apoptosis is via the p53 signalling pathways [98]. Through suppression of NF-κB and phosphorylation of AKT, the combination of quercetin and curcumin synergistically reduced cell growth and proliferation in K562 chronic myeloid leukemia cells [99]. Additionally, the combination via stimulation of the p53 signalling pathway and FasL triggered apoptotic cell death [99]. Upon exposure to the increasing concentrations of quercetin, p53 expression was increased while cyclin D1 expression was decreased in HepG2 and PANC-1 cells [100]. Furthermore, Quercetin triggered cell cycle arrest in the S phase in GEM-resistant cell lines [100].

Quercetin has been shown to enhance the efficacy of anti-cancer medications such as gemcitabine and doxorubicin, as measured by an increase in the proportion of dead cells [101]. In addition, the results revealed that the combinational treatment of quercetin with anti-cancer drugs decreased the expression of HIF-1α and promoted the expression of cleaved caspase 3 and p53, the regulator of apoptosis [101].

Table 1 presents numerous case studies demonstrating the impact of quercetin on various malignancies via signalling pathways.

Autophagy is the process by which cells, under conditions of starvation and lack of energy, synthesize new macromolecules and ATP through a series of reactions, and maintain normal metabolism and cell survival, respectively [102‐104]. Autophagosome formation is an essential step in autophagy and is based on the positive regulator of the ATG1/ULK complex consisting of ATG1, ATG13, and ATG17. The class III PI3K complex is then activated, ATG5-12 conjugated with 16 promoting autophagy membrane elongation, followed by autophagosome LC3II marker formation [105]. Dual membrane autophagosome formation, characterized by PI3 kinase cascade type III-Atg6/Beclin 1, is a major feature of autophagy [106]. Autophagous expansion is performed by two conjugate systems such as ubiquitin: the Atg12-Atg5 conjugate system and the Atg8/LC3-phosphatidyl ethanolamine conjugate system [107]. Signalling autophagy can be combined with multiple signalling pathways in response to various types of cell stress including hypoxia, radiation, starvation, or active chemical insults [108]. From them, the AKT-mTOR classic signalling path is considered as a normal negative regulator to start the formation of two-sided membranes [109], while according to signalling positively, the accumulation of HIF-1α typically activates the progression of autophagy by suppressing the mTOR1 complex or induction of BNIP3/ BNIP3L that can disrupt the interaction of Beclin 1 with Bcl-2/Bcl-xL [110, 111]. The molecular interference between the autophagic signalling pathway and apoptotic is complex and both routes are similar or attached genes that are very important for their respective performance [112]. It has been reported that Atg5, which is essential in the fusion system such as autophagic can be broken in response to death stimuli and then change autophagy to apoptosis [113]. In addition, Bcl-2 and Bcl-xL, two negative apoptosis regulators can connect to the BH3 domain of Beclin 1 and ensure the progress of autophagy [114].

Treatment of stomach cancer cells with quercetin, autophagic vacuoles and acids vaginal organs (AVOs) was developed, LC3I, and A focus on the autophagosome-recruiting LC3II led to the induction of protective autophagy in stomach cancer cells. By decreasing AKT-mTOR signalling and increasing HIF-expression, quercetin prevents the growth of gastric cancer cells [115]. LC3II accumulation and AVOs formation were also found in quercetin-treated glioblastoma cells and colorectal cancer, all of which induced quercetin-protective tumor cell autophagy [116‐118]. However, before treatment with chloroquine, an autophagy inhibitor, it can increase apoptosis and inhibit quercetin proliferation [115, 116].

Based on studies, the development of some tumor diseases is strongly related to autophagy, and there are changes in autophagy in a large number of tumor cells. In the oncology field, autophagy initially appears to be a tumor development inhibitor while further research shows that autophagy can upgrade the progression of the tumor [119, 120].

The Discovery of PTCH1 gene mutation in people with Gorlin syndrome and sporadic forms of carcinoma basal cell carcinoma (CBC) has led to the importance of the hedgehog (HH) signalling pathway in human carcinogenesis [121]. Mutations in the PTCH1 gene or one of the components in the hedgehog (HH) signalling pathway is involved in the etiology and the development of some forms of cancers such as—medulloblastoma, breast cancer, colon cancer, pancreatic cancer, esophageal adenocarcinomas and in small cell lung cancer [122]. The hedgehog (HH) signalling pathway is one of the regulatory pathways found in humans preserved since Drosophila Melanogaster. Three counterparts of Drosophila hedgehog (HH) have been identified: Sonic Hedgehog (SHH), Desert Hedgehog (DHH) and Indian Hedgehog (IHH) [123]. Sonic Hedgehog Protein (SHH) is the most important in the development of basal cell carcinoma [124]. PTCH1 is the receptor for all forms of hedgehog (HH) in humans and is part of a cell surface receptor complex consisting of two transmembrane proteins: PTCH1 and SMO (smoothened) [123]. The mechanism by which activation of the hedgehog (HH) pathway leads to carcinogenesis is unknown. Later, hedgehog (HH) is a secretory protein that binds to PTCH1 to activate the signalling pathway. When the hedgehog (HH) binds to the PTCH1 receptor complex, SMO inhibition occurs and the signal is transduced. This is happening through the interaction of a series of proteins, including SUFU (suppressor of fused) which leads to the activation of gli transcription (glioma-associated oncogene) [125]. Gli1 always leads to transcriptional activation, while Gli2 and Gli3 may cause activation or suppression. The regulators of the cell cycle are the target genes WNT, TGF-b, PTCH1 and Gli1. Hedgehog interacting protein (HIP) binds to the hedgehog (HH) and acts as a negative regulator in the signalling pathway [125].

Only a few pharmacological studies are proving the anticancer mechanisms of Quercetin by acting on the hedgehog signalling pathway. In a recent study by Mousavi et al., quercetin nanoparticles were tested in vitro at concentrations of 10, 20, 40, 80, and 100 μM) on LNCaP prostate cancer cells. The results showed that activation of the hedgehog signalling pathway at 40 mM concentrations is the mechanism underlying the anticancer and antiproliferative action [126]. In another study by Slusarz et al., the anticancer effect of Quercetin on PC3 human prostate cancer lines and TRAMP-C2 mouse lines at IC₅₀ 1–25 mol/L was demonstrated by inhibiting Gli1 mRNA and hedgehog signalling, respectively [127].

The most representative signalling pathways affected by quercetin during cancer prevention are illustrated in Fig. 1.

Part of the quercetin anticancer effect is exerted by regulating the expression of miRNAs (Additional file 1) and the interaction between quercetin and miRs is an important subject that has been investigated in many studies (Fig. 2).

Quercetin is a flavonoid and chemically it is a phenyl-substituted chromone composed of a basic skeleton of fifteen carbon atoms, composed of a chromium nucleus formed by the benzo ring A and the heterocyclic ring C, and in the aromatic ring B has a phenyl substitution [147]. Data have shown that the different substituents in rings A and B are responsible for the pharmacological activities of quercetin [148].

Although many researchers have attempted to highlight and understand the possible structure-anticancer activity correlation of quercetin, there is a scarcity of data to explain the relationship between chemical structure-cytotoxic and antitumor effect [149]. As a potential correlation, the hydroxylation pattern of the B ring in the chemical structure of quercetin would be responsible for its antiproliferative effects and inhibition of protein kinase B (AKT) [148, 150] (Fig. 3).

Many studies show that quercetin in combination with anticancer drugs and other anti-cancer bioactive compounds can have a greater effect [151].

The use of quercetin in combination with temozolomide, a standard care chemotherapy treatment for brain tumors, significantly improved the inhibitory effect of temozolomide on human glioblastoma/brain cancer cells and suppressed the survival of glioblastoma cells [152].

The benefits of quercetin in a study by Singhal et al. [153], showed its effects on the management of recurrent cases of breast cancer that is not surgical. In breast cancer cells, quercetin with doxorubicin was administered, and it became clear that the anti-tumor effects of synthetic drugs strengthen. In addition, quercetin combines the two substances to reduce the side effects of synthetic drugs on non-tumor cells. Therefore, it is considered a promising factor in the development of chemotherapy compounds in the treatment of breast cancer [154].

Another study showed that the use of quercetin may increase the anticancer effects of doxorubicin chemotherapy on liver cancer cells while protecting normal liver cells [76]. Li at el. evaluated the impact of the use of quercetin in combination with cisplatin chemotherapy in human oral squamous cell carcinoma (OSCC) cell lines, as well as in oral cancer-induced mice. The study found that the combination of quercetin and cisplatin increased apoptosis in human oral cancer cells, as well as inhibited the growth of cancer in mice, suggesting the therapeutic potential of the combination of quercetin and cisplatin in oral cancer [155].

A combination of hyperoside and quercetin (QH; 1:1) is shown to have synergic anticancer effects. QH treatment in PC3 prostate cancer cell line can downregulate miR-21 expression and therefore, can reduce the inhibitory effect of this miR on its targets PDCD4 and MARCKS, which ultimately leads to decreased invasion and increased apoptosis [156‐158]. In addition, QH treatment in renal cancer cells can inhibit the expression of specificity protein (Sp) transcription factors and survive in expression which is controlled by SPs. Further studies show that this inhibition is exerted via down-regulating the oncomiR-27 and up-regulating the zinc finger protein ZBTB10 [157].

Singh et al. demonstrated in vivo in mice with transgenic prostate adenocarcinoma TRAMP that the combination of supplements of quercetin and resveratrol, two antioxidants found abundantly in grapes, had anti-cancer benefits in this mouse model of prostate cancer [159].

The combination of sulforaphane or quercetin with green tea catechins (GTCs) prevents the progression of PDA by upregulating miR-let-7a and downregulating K-ras. The combinations have much more anti-cancer benefits than any of these components alone [160]. Combination of resveratrol and quercetin (RQ) has a similar effect on colon cancer [161].

The synergistic combination of arctigenin and quercetin has been studied in two prostate cancer cell lines, LAPC-4 and LNCaP. The results indicate that this combination can inhibit the expression of oncogenic miRs miR21, miR-19b and miR-148a in LAPC-4 and miR21 and miR-148a in LNCaP cell line way more effectively than quercetin alone [162‐164].

Multidrug resistance (MDR) is the most important cause of cancer treatment failure. Deciphering the mechanisms of multidrug resistance could contribute to the efficiency of therapeutic strategies for the treatment of neoplastic diseases, and also mitigate the side effects of drugs. [8, 165]. The reason for this is the increased activity of ATP-binding cassette family transporters (ABC) [166].

Quercetin can reverse the resistance mechanism of anticancer drugs through the inhibition of the group P function and ABCB1 gene expression in many cell lines [167, 168]. However, many studies have focused on the anti-cancer properties of this bioactive compound. Several pathways have been identified which can affect quercetin in various cancers [169, 170].