Quercetin: A Flavonoid with Diverse Chemo Preventive Properties against Cancer

Abstract

Quercetin, an exceptional and extraordinary flavonoid possessing bioactive properties, presents a plethora of benefits for the promotion of good health. The anti-tumor characteristics of quercetin have been well-documented in various in vitro and in vivo investigations, encompassing a wide range of cell lines and animal models. Quercetin, through the activation of caspase-3 and inhibition of the phosphorylation of Akt, mTOR, and ERK, as well as the reduction of β-catenin and stabilization of HIF-1α, augments apoptosis and autophagy in cancer. Additionally, quercetin curbs cancer cell metastasis by decreasing MMP and VEGF secretion. Significantly, the potent cytotoxicity of quercetin against cancer cells is accompanied by minimal or no adverse effects or harm to healthy cells.

1. Introduction

As the second greatest cause of death worldwide, cancer is a complicated illness [1]. In actuality, the survival rate for cancer patients is still quite low despite advancements in diagnostic and treatment protocols, which include surgery, radiation, chemotherapy, and target and gene therapy [2]. The recurrence of cancer after months or years due to metastases—the spread of cancer cells from a primary lesion to distant sites—is most likely the true cause of the high cancer death rate [3]. The cancer and metastatic treatments that are now on the market usually cause substantial levels of toxicity and are linked to a variety of side effects that are frequently unanticipated and unexplained [4]. Therefore, it is highly recommended to develop new and more effective cancer treatment and metastasis prevention measures [5]. Phytochemicals are among the current class of therapeutic agents for cancer because of their low toxicity and capacity to target numerous cell signaling such as VEGF [6]. Quercetin, derived from quercetum (oak woodland) and called after Quercus, has been used since 1857. It is abundant in plants such as apples, berries, brassica vegetables, capers, grapes, onions, spring onions, tea, and tomatoes, as well as many seeds, nuts, flowers, bark, and leaves. However, quercetin is found in medicinal plants such as Ginkgo biloba, Hypericum perforatum, and elderberry and is mostly obtained from onions, apples, and tea. The molecule of quercetin has a ketocarbonyl group, and the oxygen atom on the first carbon is basic and can form salts with strong acids. It has four active groups in its molecular structure: a dihydroxy group between the A ring, o-dihydroxy group B, C ring C2, C3 double bond, and 4-carbonyl. The presence of a phenolic hydroxyl group and double bonds confers significant antioxidant action on quercetin. Its antioxidant and anti-inflammatory qualities have been linked to the prevention and treatment of cardiovascular disease and cancer. Furthermore, in vivo and in vitro investigations have revealed that quercetin has antibacterial activity and efficiently inhibits biofilm formation by blocking the expression of associated genes, antitumor activity, antiangiogenic activity, and so on. Furthermore, quercetin has a role [7]. Quercetin has numerous anti-inflammatory, antioxidant, and anticancer properties. Quercetin has been shown in both in vivo and in vitro studies to have anti-tumor actions by modifying cell cycle progression, reducing cell proliferation, inducing apoptosis, limiting angiogenesis and metastatic progression, and influencing autophagy. This study outlines the data regarding quercetin pharmacological potential and inhibition of malignancies, arguing that it should be investigated as a therapeutic agent against a variety of cancers [8]. The metabolite of quercetin, known as quercetin-3-glucuronide (Q3G), is one of the flavonoid classes present in fruits, vegetables, and medicinal plants. Due to its quick conjugation with glucuronic acid upon absorption from the small intestine, Q3G is also the main quercetin conjugate in human plasma, where quercetin is either missing or undetectable. Even though research has shown quercetin to have anti-viral, anti-microbial, anti-thrombotic, anti-carcinogenic, and neuroprotective properties, due to its short biological half-life, low water solubility, and chemical instability, it has poor bioavailability, which limits its usefulness in the food and pharmaceutical industries. Better than the previously listed, most quercetin in food and its metabolites are linked to a sugar molecule; this can boost bioavailability since sugar molecules are more soluble in water [9].

2. Therapeutic activity of quercetin in cancer

Numerous disorders have demonstrated the substantial positive effects of quercetin. More and more scientists are focusing on quercetin’s potential as a tumor treatment because, at appropriate dosages, it does not appear to have any harmful side effects on normal cells. Quercetin significantly prevents the cell cycle, promotes apoptosis, and inhibits angiogenesis and metastasis in vitro, as demonstrated by some studies that have confirmed its ability to exert anti-tumor functions in a variety of mechanisms. These results have been confirmed in both in vitro and in vivo models of various tumors, with encouraging outcomes. According to the findings of in vivo research, quercetin at the chosen dosage effectively inhibits the growth of xenograft tumor models [8]. Moreover, quercetin has undergone clinical trials; these have revealed no toxicity or adverse effects on the general public [10]. Quercetin’s capacity to reduce inflammation and lower the incidence of prostate cancer has been shown in clinical research [11, 12]. Phase I clinical trials demonstrated quercetin’s anticancer efficacy and verified the drug’s safety when administered intravenously. Quercetin-based clinical trials have demonstrated its ability to reduce blood pressure and improve anemia [13].

3. In vitro studies

3.1 Effect of quercetin on the cell cycle of cancer cells

The four main phases of the cell cycle are G1 (pre-DNA synthesis), S (DNA synthesis phase), G2 (late DNA synthesis phase), and M (mitotic phase). Cyclin, cyclin-dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CKI) are primarily responsible for controlling the cell cycle. The regulatory pathway of the pRb network primarily controls the cell cycle (Figure 1) [15].

Figure 1.

How quercetin affects the cell cycle process by modulating several signaling molecules (^√downregulation and ˣupregulation by quercetin) [14]. Copyright Elsevier (2023).

Cyclin binds to its corresponding CDK to create a complex that phosphorylates Rb and releases cAb1 and E2F. E2F then reaches the nucleus to encourage the transition from the G1 phase to the S phase for cell-autonomous division. INK4 (which includes p15, p16, p18, and p19) is one type of CKI that competes with cyclin D1 for binding to CDK4/CDK6, which blocks the development of the cell cycle and phosphorylating Rb [16]. Consuming quercetin may be important for controlling the cell cycle, promoting apoptosis, and preventing metastasis. Moreover, quercetin inhibits the growth of colon cancer cells by modifying the expression of the anti-aging genes SIRT-6 and Klotho. Numerous molecular targets that may actively regulate cell proliferation, apoptosis, and cellular senescence were identified by the miRNA expression profile and functional enrichment analysis. Apart from the control of aging-associated miRNA and anti-aging protein activities like SIRT-6 and Klotho, telomerase activity suppression resulting in telomere length reduction established an intriguing path for the development of future colon cancer treatments. Therefore, a combination therapy combining quercetin and epigenetic modifiers in addition to currently available chemotherapeutic medications may assist in controlling the course of colon cancer and minimize its side effects [17]. To control cell cycle division, p53 primarily regulates another significant route that stimulates the expression of p21, GADD45, and Bax [18]. On the other hand, aberrant cyclin expression has been linked to aberrant cell cycle activity and unchecked tumor cell multiplication in research on tumor development. Quercetin has been demonstrated to induce cell cycle arrest at G2/M in human leukemia U937 cells. Cyclin D, E, and E2F levels decreased in tandem with the buildup of G2/M, while cyclin B levels significantly increased [19, 20, 21, 22]. Comparable outcomes were also observed in ovarian cancer (SKOV3), where the cell was halted in the S and G2/M phases, and there was a decrease in cyclin D1 [23]. Furthermore, quercetin can alter the G0/G1 phase of HOS osteosarcoma cells and 232B4 chronic lymphocytic leukemia cells [24, 25]. Numerous research investigations have also examined the impact of quercetin on p53-related pathways and the advancement of malignant cell cycles. According to the study, quercetin can raise ER pressure, which in turn encourages p53’s release. This inhibition of p53, CDK2, cyclin A, and cyclin B activity leads to the arrest of MCF-7 breast cancer cells at the S phase [26, 27, 28, 29]. Quercetin caused MDA-MB-453 cells in a different study team’s investigation of the breast cancer cell line to enter the G2/M phase arrest. Additionally, they discovered a noteworthy rise in p53 expression in their investigation [30, 31]. Quercetin can also impact the advancement of the cell cycle in other ways. A specific dose of quercetin-6-C-beta-d-glucopyranoside administration causes cell cycle arrest at the G0/G1 phase in the prostate cancer cell lines PC-3 and DU-145. Down-regulation of cyclin E and D, increased expression of p21 and p27, and protein expression of PNCA and CDK2 may be linked to this occurrence [32, 33]. Additionally, researchers discovered that 7-O-Geranylquercetin, a novel O-alkylated derivative of quercetin, could stop the G2/M cell cycle of MGC-803 and SGC-7901 gastric cancer cell lines [34, 35]. In YD10B and YD38 OSCC cells, quercetin reduced cell viability and caused G1 cell cycle arrest. Furthermore, quercetin significantly upregulated the expression of a CDK inhibitor while downregulating the expression of proteins that promote cell cycle progression. In both kinds of OSCC cells, quercetin also markedly and dose-dependently boosted the number of annexin-V-positive cells. Because of quercetin apoptotic potential, PARP was cleaved, which activated the p38 MAPK-signaling pathway [21].

3.2 Effect of quercetin on apoptosis of cancer cells

The mechanism by which living organisms eliminate diseased or unnecessary cells from their bodies is widely recognized as apoptosis, or the process of cellular demise. It primarily encompasses two distinct apoptotic pathways, namely the extrinsic (also referred to as death receptor) and intrinsic (also known as mitochondrial) pathways (Figure 2) [36].

Figure 2.

Quercetin impact on the different apoptotic pathways (quercetin ^√downregulation and ˣoverexpression, respectively) [14]. Copyright Elsevier (2023).

The extrinsic pathway is initiated by TRAILR and FAS death receptors, which collectively activate a cascade of proteases, including caspase-3, -6, -7, -8, -9, and -10, ultimately leading to cell death. On the other hand, the intrinsic pathway is triggered by the liberation of cytochrome c (cyt c) from the mitochondria. It is widely acknowledged that tumor cells possess the ability to evade death and proceed with tumor formation. Hence, the induction of apoptosis in tumor cells has become a significant breakthrough in the realm of anticancer therapy [37]. Quercetin has been shown to influence the apoptotic pathway and cause tumor cells to die. It has been demonstrated that a suitable dosage of quercetin increases the expression of pro-apoptotic proteins and lowers the expression of anti-apoptotic proteins. A375SM melanoma cells [38], A2780S ovarian cancer cells, and HL-60 acute myeloid leukemia cells can all undergo apoptosis when treated with quercetin [39]. Cancer stem cells are essential to the disease’s development. It has recently been established that quercetin can cause gastric cancer stem cells to undergo apoptosis [35, 40, 41]. Quercetin promotes the production of Bax and Bad during the intrinsic apoptotic pathway and downregulates the anti-apoptotic proteins Bcl-XL, Bcl-2, and Mcl-1. It also activates caspase-3, -8, and -9. Quercetin concurrently promoted cytochrome c release. Quercetin downregulated Akt, PLK-1, cyclin-B1, cyclin-A, CDC-2, CDK-2, and Bcl-2 while upregulating Bax, caspase-3, and p21. Furthermore, it has been observed to decrease STAT3 activation and promote STAT3 protein degradation in liver cancer cells [42]. Quercetin glycosides also have pro-apoptotic effects. Quercetin glycosides inhibited the expression of proteins linked to apoptosis and produced ROS and cyt c more quickly, all of which eventually caused cancer cells to undergo apoptosis [34]. In HepG2 cells, quercetin glycosides can also trigger caspase-3-induced apoptosis [43].

3.3 Effect of quercetin on angiogenesis and metastasis of cancer cells

The process of forming new capillaries is known as angiogenesis, and it is aided by endostatin, adhesion molecules, growth factors, and other substances. Physiological angiogenesis is linked to the development of the reproductive system and the healing of injuries. Tumor growth and metastasis are impacted by dysregulated angiogenesis, which is intimately linked to neoplastic disorders [44]. The intricate process of tumor angiogenesis involves the interaction of tumor cells with endothelial cells. VEGF, or vascular endothelial growth factor, is crucial for priming. A crucial molecule for endothelial cell growth and survival, VEGF can enhance vascular permeability, induce a signaling pathway that is dependent on the VEGF receptor 2 (VEGFR2), induce extravasation of plasma fibrin, and cause tumor angiogenesis. By preventing the formation of new blood vessels, quercetin can also have anti-tumor effects. Quercetin inhibits the expression of the downstream regulatory factor AKT, targets the VEGFR-2-mediated angiogenesis pathway in prostate and breast malignancies, and limits tumor growth (Figure 3) [45].

Figure 3.

Quercetin regulates the pathways involved in angiogenesis, autophagy, and metastasis (show quercetin respective effects on √downregulation and ˣ overexpression) [14]. Copyright Elsevier (2023).

Additionally, quercetin can boost the effectiveness of anticancer medications and reduce the angiogenesis of drug-resistant cells [46]. Angiogenesis is the first step in the spread of most malignant cancers. The greater metastatic potential and lower survival rate of malignant tumors are correlated with the number of tumor micro-vessels. Neovascularization gives tumor cells food and a place to hide from the body. A sequence of circulating pathways is followed by tumor cells to infiltrate and establish secondary malignancies [47]. The process of epithelial-to-mesenchymal transition (EMT) is crucial for the spread of cancer. E-cadherin, MUC1, and other epithelial-type proteins are downregulated throughout the EMT process, while mesenchymal markers including N-cadherin, Vimentin, Snail, and others are acquired [48]. In several malignancies, quercetin can prevent EMT by upregulating the expression of E-cadherin and downregulating the N-cadherin, Vimentin, and Snail protein family. Furthermore, zinc-dependent extracellular matrix (ECM) remodeling de-endopeptidase family members known as matrix metalloproteinases (MMPs) are involved in various stages of cancer. Research has demonstrated that MMPs play a major role in the invasion and metastasis of cancer [49]. Quercetin can prevent tumor cell invasion and migration in both head and neck squamous cell carcinoma and colorectal cancer [50]. Quercetin blocks the Snail-dependent AKT activation pathway, which prevents lung cancer cells from invasively growing and metastasizing. N-cadherin, vimentin, ADAM9, and MMP-related protein expression were all markedly downregulated, whereas E-cadherin expression was markedly upregulated following quercetin administration [51, 52, 53]. Researchers employed TGF-β1 to generate EMT in cancer cells in colorectal cancer cell lines. To prevent EMT, quercetin administration can suppress Twist1 and control the expression of E-cadherin [54]. In PANC-1 and PATU-8988 pancreatic cancer cells, quercetin can reduce the expression of MMP2 and MMP7, which are intimately linked to the EMT process. It was also noted by researchers that quercetin can reverse the malignant progression brought on by IL-6 and prevent pancreatic cancer cells from invasively spreading by blocking the STAT3 signaling pathway [55]. By preventing c-Met’s activation and the subsequent activation of Gabl, FAK, and PAK, quercetin can prevent HGF-induced melanoma cell migration [56]. Moreover, quercetin glycosides can prevent cancer from spreading. It can block the ERK1/2 and FAK pathways as well as reduce the migratory viability of pancreatic cancer cells [57].

3.4 Effect of quercetin on autophagy of cancer cells

Under conditions of starvation and energy deficit, autophagy is the process by which cells generate new macromolecules and ATP through a sequence of processes, thereby maintaining the cells’ regular metabolism and ability to survive. One important stage of autophagy is the production of autophagosomes, which is dependent on the ATG1/ULK complex’s positive regulator, which is made up of ATG1, ATG13, and ATG17. Subsequently, the class II PI3K complex is triggered, and the elongation of autophagic membranes is promoted by the ATG5-12 conjugate with 16. This leads to the formation of the autophagosome marker LC3II [58, 59]. Research has shown that autophagy plays a key role in the development of some tumor disorders and that many tumor cells have altered autophagic activity. Autophagy was once believed to be an inhibitor of tumor formation in the field of oncology; however, additional research indicates that autophagy may facilitate the evolution of tumors [60]. Autophagic vacuoles and acidic vesicular organelles (AVOs) were generated upon treating gastric cancer cell lines with quercetin. LC3I was transformed to LC3II, which was then attracted to autophagosomes to activate autophagy genes and start the protective autophagy progression in gastric cancer cells. Quercetin has a protective autophagous impact on gastric cancer cells by blocking AKT-mTOR signaling and increasing the expression of HIF-α [61]. Quercetin-treated glioblastoma and colon cancer cell lines also exhibited the accumulation of LC3II and the creation of AVOs, confirming quercetin stimulation of tumor cell protective autophagy [62, 63]. Nonetheless, quercetin suppression of proliferation and encouragement of apoptosis can both be enhanced by pretreatment with the autophagy inhibitor chloroquine [61].

3.5 Effects of quercetin on the transcription factors

Transcriptional factors are known to have key roles in tumor initiation and development, and their significance in connecting chronic inflammation to various cancers is well-established, even when it comes to distinct molecular targets [64]. Transcription is a highly regulated process in normal cells, and it is essential for normal cellular processes like differentiation and proliferation [65]. Conversely, signaling proteins that regulate distinct transcription factors are often dysfunctional in malignant cells and are thought to be the primary source of some oncogenic conversions in the function of these proteins, such as aberrant cell division and proliferation, anti-apoptosis, invasion, angiogenesis, metastasis, and resistance to chemotherapy and radiation [66]. Quercetin, like most phytochemicals, has been shown in numerous studies to target various cancer-associated molecules and pathways, including EGFR (epithelial growth factor receptor), ERK (extracellular signal-regulated kinase), MAPK (mitogen-activated protein kinase), NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), STAT (signal transducer and activator of transcription), TGF-β (transforming growth factor-beta 1), TNF (tumor necrosis factor), Wnt/β-catenin (wingless-type MMTV integration site family), and so on. Quercetin can be considered a promising anticancer agent [67]. Numerous studies have shown that dysregulation of various oncogenic transcription factors, including NF-κB, Wnt, Notch, MAPK, PI3K, and TGF, occurs in various cancer types, resulting in the induction, proliferation, and development of tumor cells. Consequently, these transcription molecules may be used to target malignancy.

3.5.1 Effects of quercetin on NF-κB

A transcription factor called NF-κB regulates the expression of several proteins involved in the control of important physiological processes, including angiogenesis (VEGF), apoptosis (Bcl-xL, cIAP1/2, FLICE, survivin, and XIAP), cell invasion (MMP-2, MMP-9, intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion protein-1 (ICAM-1), inflammation (interleukin-6 (IL-6), IL-1β, cyclooxygenase-2 (COX-2) and tumor necrosis factor (TNF)), metastasis (C-X-C chemokine receptor type-4 (CXCR)-4, TWIST), and proliferation (cyclin D1 and MYC) [68]. RelA (p65), RelB, c-Rel, NF-κB1/p105, and NF-κB2/p100 are the five primary transcription factors that make up NF-κB. These transcription factors are primarily inactive in the cytoplasm of most cells, but they can be triggered by a variety of substances, including growth factors, ROS, cytokines (IL-1β), and oncogenic stress [69, 70]. The stimulant parameters stated above activate the IKK complex’s inhibitors, which are made up of three initial parts: IKK1/IKKα, IKK2/IKKβ, and NEMO (NF-κB essential modifier)/IKKγ. NF-κB activity is then regulated as a result [71]. When the IKK complex is activated, the IκB inhibitor proteins are phosphorylated, which leads to their destruction. Additionally, NF-κB signaling is activated to target gene expression [72]. The major signaling route that modulates NF-κB activation is PI3K/AKT. Quercetin (40 μM) treatment of PC-3 cell lines lacking p53 was shown to dramatically activate the PI3K/AKT pathway, leading to the phosphorylation of several molecules, including AKT (p-Thr308 and Ser473) and NF-κB-p105/p50 (p-Ser893) [22, 73, 74, 75, 76].

3.5.2 Effects of quercetin on tyrosine kinase signaling

Two signaling pathways that contribute to the development of cancer include growth factor signaling and receptor tyrosine kinase (RTK) signaling. The human genome contains around 100 growth factors, of which approximately 90 genes encode proteins with tyrosine kinase activity. Furthermore, 58 of the 90 genes that make up the RTK are categorized into 20 distinct subfamilies. RTKs are cell surface receptors that control cell homeostasis; any malfunction can lead to a variety of clinical diseases. Five of the twenty groups listed— EGFR/ErbB, insulin receptors, PDGFR, FGFR, VEGFR, and CCK—are implicated in the initiation and spread of cancer [77]. RTKs are divided into three separate structural sections: the cytoplasmic, transmembrane, and extracellular regions. The extracellular portion of every cancer-related RTK is glycosylated, and the N-terminal region has many disulfide bonds. RTKs’ extracellular portion possesses the ability to bind to ligands, perhaps causing TRK dimerization. RTKs are typically auto phosphorylated by trans-phosphorylating their kinase portion, which activates the cytoplasmic kinase portion of the protein and stabilizes by attaching to a dimeric ligand. The cytoplasmic portion consists of the juxtamembrane and C-terminal domains. The second one has tyrosine kinase, which catalyzes the phosphorylation of the receptor [78]. By reducing the expression of ErbB receptors, quercetin inhibited the growth and survival of prostate cancer cells that were androgen-dependent and -independent. In a 2005 study on colon cancer cell lines, it was shown that quercetin (0–100 μM) reduced the expression of ErbB2, ErbB3, and AKT signaling cascades in HT-29 cells, which in turn increased apoptosis [79, 80].

3.5.3 Effects of quercetin on Wnt signaling pathway

Nineteen genes in the human genome encode Wnt proteins, which control some human developmental events, such as cell division, death, migration, polarity, and destiny [14, 81]. The frizzled (FZD) receptors on Wnts—secreted glycoproteins rich in cysteines—identify them. LRP5 (LDL receptor-related proteins) or LRP6 are examples of phosphorylatable co-receptors that are necessary for the activation of FZDs, a class of seven transmembrane component surface cell receptors. These processes result in the complexation of FZDs with the cytoplasmic phosphor-protein disheveled (DVL). There are three known DVLs in humans: β-catenin, DVL1, and DVL2 [82]. A kind of basic Wnt, the Wnt/β-catenin pathway regulates several processes, including morphogenesis, embryogenesis, and the proliferation or differentiation of stem cells. Adenomatous polyposis coli (APC), a cytoplasmic multi-protein complex, destabilizes β-catenin to induce it to dissociate in the proteasomal processes in the absence of any stimuli. The Wnt pathway was inhibited by low intracellular concentrations of β-catenin through the transcription factor family TCF/LEF (T cell factor/lymphoid enhancer factor). β-catenin interaction with Wnt receptors is followed by suppressed subsequences of phosphorylation and degradation. Under these circumstances, the cytoplasmic presence of free β-catenin and its translocation into the nucleus cause the transcription of TCF/LEF-related genes, either stimulating or inhibiting apoptosis [83]. Treatment of prostate cancer (PC-3) cell lines with quercetin (0–60 μM) has been reported to upregulate E-cadherin and decrease TGF-β-triggered production of N-cadherin, Slug, Snail, Twist, and vimentin. Furthermore, quercetin decreased the development of prostate cancer by downregulating the two major Wnt signaling components, β-catenin and cyclin D1. This suggests that quercetin controlled the Wnt signaling axis and TGF-β-triggered EMT. Using A375 melanoma cells as a model, additionally, quercetin stimulated apoptosis in A375 cell lines by declining the regulation of Bcl-2 and activating caspase 3/7 via PARP cleavage [84].

3.5.4 Effects of quercetin on Notch signaling

The notch pathway, like the Wnt-β catenin cascade, controls multiple developmental pathways and defines the kind of stem cell differentiation. DLS (delta-serrate-Lag2) recognizes and activates Notch receptors as part of the notch pathway. After interacting with DLL (delta-like ligands) and JAG (jagged) DLS ligands, notch receptors become active [85]. Quercetin-ionizing radiation (IR) treatment of colon cancer stem cells (CSCs) had more anticancer effects than either IR or QU treatment alone. In xenograft mice-produced human colon cancer, quercetin-IR co-treatment (20 μM and 5 Gy, respectively) dramatically reduced CSC indicators and downregulated the expression of Notch-1 signaling molecules. Furthermore, in both DLD-1 and HT-29 cell lines, this co-treatment reduced the regulation of all five γ-secretase complex protein moieties (APH1, nicastrin, PEN2, presenilin-1, and presenilin-2). According to a different study, exposing breast cancer cell lines, such as MDA-MB-231, MCF-7, and T47D, to Quercetin-3-methyl ether (0–20 μM) decreased Notch-1 expression, which in turn prevented the EZH2 (enhancer of zeste homolog 2) pathway and PI3K and AKT phosphorylation, which in turn caused apoptosis [86].

3.5.5 Effects of quercetin on MAPK signaling

Differentiation, growth, migration, proliferation, apoptosis, and other cellular functions are all regulated by the mitogen-activated protein kinase (MAPK) pathway. Humans have been found to have six different types of MAPKs: ERK1/2, ERK3/4, ERK5, ERK7/8, JNK 1/2/3, and isoforms of p38 a/b/g (ERK6)/d [87]. MAPKs are thought to operate downstream of many membrane receptors, including the family of EGF receptors. Different MAPKs that primarily operate as transcription factors of genes are activated, and this regulates DNA replication, cell cycle progression, and division in addition to inflammatory and stress responses [88]. Quercetin (20 μM) treatment of MDA-MB-231 cells increased Foxo3a expression through pJNK/JNK upregulation, which in turn triggered cell cycle arrest and apoptosis [89].

3.5.6 Effects of quercetin on PI3K

The serine/threonine kinase group includes phosphatidylinositol 3-kinases (PI3Ks), which are triggered when growth factors interact with TRK. RTKs are phosphorylated at the tyrosine residue upon ligand binding. Through an adaptor protein, the phosphotyrosine can interact with PI3K directly or indirectly. Like MAPKs, PI3K activates a number of downstream kinases that significantly impact cell survival after activation [90]. Quercetin (0–200 μM) was found to promote apoptosis in leukemia cells by decreasing AKT phosphorylation and reducing the expression of PI3K and Bcl-2 proteins in an HL-60 cell research. Consequently, it reduced the viability and multiplication of leukemia cells. In an additional investigation, administering quercetin (0–200 μM) to DBTRG-05 and U-251 glioma cells resulted in a decrease in AKT protein levels. Additionally, quercetin Qu increased the effectiveness of temozolomide compared to conventional chemotherapy methods by blocking the PI3K/AKT signaling axis [91, 92].

3.5.7 Effects of quercetin on transforming growth factor-beta

The TGF-β family consists of around 40 members, which include Nodal, Activin, TGF-β, and bone morphogenetic proteins (BMGPs). TGF-β molecules are membrane proteins that carry out a variety of signaling functions from the cell membrane to the nucleus. This route encourages cell cycle arrest in the early stages of carcinogenesis, but as tumors grow, it also causes angiogenesis, invasion, and metastatic processes. Apart from SMAD-4, which plays a crucial role in modifying TGF-β’s action, other signaling cascades that affect this one include MAPK, PI3K/AKT, and Wnt/β-catenin [93, 94]. The study conducted on colon cancer (SW-480 cells) showed that Qu (0–100 μM) inhibited TGF-β1-promoted EMT by upregulating E-cadherin through the downregulation of Twist1. Increased SHH expression in pancreatic cancer cells triggered the TGF-β1/Smad2/3 signaling pathway, which in turn promoted Snail and Zeb2 levels and EMT [94, 95].

3.6 Effects of quercetin on non-coding RNAs

3.6.1 Effects of quercetin on micro-RNAs (miRNAs)

Numerous studies have shown that one of the primary causes of breast cancer incidence is miRNA malfunction [96, 97]. Through the overexpression of p53, quercetin increased the expression of miR-34a, which in turn inhibited the function of SIRT1 (silent information regulator 1). As a result, increased p53 acetylation encouraged p53 stability and p53-associated apoptosis. Class III nuclear acetylase SIRT1 regulates multiple genomic processes through transcription factors and histone modification. It has been discovered that SIRT1 deacetylates a critical lysine residue, which dysregulates p53. Furthermore, miR-34a can inhibit SIRT1’s mRNA by attaching to its 3′ UTR region [98, 99]. Quercetin may successfully stop the growth and invasion of lung cancer by targeting miRNAs. The expression of miRNAs in lung cancer cases was influenced by a quercetin-rich diet; the most notable miRNAs were oncogene miRNAs, such as the miR-17 and miR-146 families, and tumor suppressor miRNAs, such as the let-7 and miR-26 families. Integral membrane proteins of the claudin family regulate the functioning of tight junctions (TJs), and several cancer types have been linked to TJ malfunction [100]. A study conducted on lung adenocarcinoma A549 cell lines showed that quercetin (2.5–100 μM) promoted the expression of miR-16 in a time- and dose-dependent manner. This, in turn, reduced the stability of claudin-2 mRNA and slowed the growth of lung adenocarcinoma [101].

3.6.2 Effects of quercetin on long non-coding RNAs (lnc-RNAs) and circular RNAs (circ-RNAs)

It has been revealed that in NSCLC cells, miR-34a-5p downregulates lncRNA-SNHG7 (small nuclear RNA host gene 7), while lncRNA-SNHG7 regulation increased. Quercetin (0–100 μM) downregulated SNHG7, which led to the upregulation of miR-34a-5p. Therefore, quercetin inhibited the proliferation, viability, invasion, and migration of NSCLC cells by targeting quercetin/SNHG7/miR-34a-5p. One of the characteristics of prostate cancer is elevated expression of the lncRNA MALAT1. Research demonstrated that by modifying the regulation of EMT, PI3K/AKT, and apoptosis-associated molecules, quercetin (0–50 μM) reduced the expression of MALAT1, suppressing proliferation, growth, migration, and invasion in PC-3 cells [102]. Another family of single-strand, non-coding RNAs, known as circular RNAs (circRNAs), are produced from back-spliced exons of mRNAs and antisense RNAs. Because of their covalently closed structures, circRNAs are immune to RNAse R’s breakdown. Applying Qu (0–200 μM) to HCT-116 colon cancer cells inhibited cell division by encouraging apoptosis. Thus, compared to untreated cells, quercetin-treated HCT-116 cells had differently regulated 240 lncRNA, 131 circRNA, 83 miRNA, and 1415 mRNA, according to the whole transcription sequencing technique. Furthermore, it was discovered that Quercetin inhibited leucine α-2-glycoprotein-1 (LRG1) regulation in HCT-116 cells to achieve its anticancer effects. Subsequent research revealed that Qu reduced the expression of LRG1 by downregulating the lncRNAs in a competitive manner. A different study using HeLa cancer cells showed that quercetin (0–50 μM) changed the regulation of approximately 10 miRNAs, 1 lncRNA (MALAT1), and 71 circRNAs [103].

3.7 In vivo studies

It has been demonstrated that quercetin inhibits the growth of several malignancies in different xenograft models. After receiving quercetin treatment, the tumor volume was drastically reduced, and the survival rate of animal models bearing tumors was greatly elevated. The encouragement of apoptosis and suppression of proliferation, angiogenesis, and metastasis are examples of how quercetin inhibits these processes in xenograft animal models [8]. In the models of breast cancer and leukemia cell xenograft, the inhibition of the AKT/mTOR pathway by varying doses of quercetin can induce apoptosis and impact the cell cycle [104]. Moreover, the effects of quercetin on tumor growth can be observed in vivo through the promotion of graft angiogenesis and metastasis. One of the earlier discovered anti-angiogenic factors, Thrombospondin-1 (TSP-1), has been found to inhibit tumor growth. A recent investigation has revealed that quercetin can upregulate the expression of TSP-1, leading to the inhibition of tumor growth in a prostate cancer model using mice [105]. Additionally, in a BALB/c mice model of breast cancer, Zhao et al. discovered that the administration of 34 mg/kg of quercetin can inhibit angiogenesis by targeting the calcineurin/NFAT pathway [106]. In vivo experiments have further substantiated the inhibitory impact of quercetin on the metastasis of tumors. Following the administration of 50 mg/kg quercetin, the incidence of colorectal lung metastasis was notably diminished [68]. The administration of quercetin has also been observed to deter epithelial-mesenchymal transition (EMT) by influencing the EGFR signaling pathway and curtailing the expression of VEGF [107]. Moreover, the suppressive effect of quercetin on the growth of allogeneic tumors has been identified in tumor cell models, including lung cancer and pancreatic cancer. Furthermore, quercetin exhibits the ability to synergize with other compounds in imparting anti-tumor activity [108]. For instance, the co-encapsulation of vincristine and quercetin within liposomal agents enhances the therapeutic efficacy of breast cancer treatment [109]. It was shown that the injection of MCF-7 cells into female BALB/c nude mice resulted in a decrease in the expression of VEGF, VEGFR, and NFATc3 in the tumor tissue under Qu (34 mg/kg/day) treatment. These findings revealed that quercetin targeted calcineurin to decrease angiogenesis in the MCF-7 cells of xenograft mice [106]. Additionally, it has been observed that quercetin (0–100 μM) decreased VEGFR expression in retinoblastoma cells (Y79) in a dose-dependent manner, indicating its anti-angiogenic effect [110].

4. Conclusions and perspectives

Quercetin, a kind of polyphenolic flavonoid, has anticancer properties. It has been demonstrated that quercetin exerts its anticancer effects by modulation of many dysregulated signaling pathways, including apoptosis and autophagy. Quercetin can stop the metastatic cascade at its start. It prevents extracellular matrix breakdown, tumor-associated angiogenesis, and Epithelial-mesenchymal transition. Except for a very small number of human clinical investigations that are not related to cancer, most quercetin-recognized properties have only been studied in vitro or in animal models. Furthermore, quercetin has not yet been the subject of any clinical trials as a cancer treatment. Quercetin displays anticancer actions via regulating multiple signaling pathways such as PI3K/AKT, NF-B, P53, Wnt/−catenin, MAPK, JAK/STAT, and Hedgehog. Quercetin inhibits many intracellular signaling molecules, including TNF-, Bax, Bcl-2, caspases, and VEGF. Quercetin has been researched for its anticancer properties in a variety of cancers, including breast cancer, prostate cancer, ovarian cancer, lung cancer, colon cancer, hepatocellular carcinoma, lymphoma, and pancreatic cancer. However, most new quercetin anticancer research is focused on cancer in humans. Quercetin has negligible adverse effects and negligible toxicity on normal cells while inducing death in malignant cells via both the intrinsic pathway and receptor-mediated extrinsic route by targeting various cellular signals. Furthermore, quercetin is an effective adjuvant to flavonoids, such as quercetin related-glycosylated derivatives, in addition to being able to overcome drug resistance.

Abbreviations

ABC	ATP-binding cassette
AIF	apoptosis-inducing factor-1
AMPK	adenosine monophosphate-activated protein kinase
BAK	BCL-2homologous antagonist killer
BAX	BCL-2 associated X protein
BCL-2	B-cell lymphoma-2
BIK	BCL-2 interacting killer
CCND	cyclin D
COX-2	cyclooxygenase-2
DNA	deoxyribonucleic acid
EGFR	epithelial growth factor receptor
EMT	endothelial to mesenchymal transition
ErbB	ErbB receptor tyrosine kinase 4
ERK	extracellular signal-regulated kinase
EZH2	enhancer of zeste homolog 2
H2AX	H2A histone family member X
HCC	hepatocellular carcinoma
HIF-1α	hypoxia-inducible factor-1α
hnRNP	heterogeneous nuclear ribonucleoprotein
IGF-R1	insulin-like growth factor type 1 receptor
IL	interleukin
JAK	Janus kinase
JNK	C-Jun N-terminal kinas
MALAT1	metastasis-associated lung adenocarcinoma transcript 1
MAPK	mitogen-activated protein kinase
MMP	matrix metalloproteinase
mTOR	mammalian target of rapamycin
ncRNA	non-coding RNA
NF-κB	nuclear factor kappa-light-chain-enhancer of activated B cells
PCNA	proliferating cell nuclear antigen
PI3K	phosphoinositide 3-kinases
Akt	protein kinase B
MUC4	Mucin 4
PKB	protein kinase B
PTEN	phosphatase and tensin homolog
PUMA	P53 upregulated modulator of apoptosis
ROS	reactive oxygen species
SIRT1	silent information regulator1
Sox2	sex-determining region Y-box2
c-Src	proto-oncogene tyrosine-protein kinase Src
STAT	signal transducer and activator of transcription
TGF-β	transforming growth factor beta 1
TNF	tumor necrosis factor
TWIST1	twist family BHLH transcription factor 1
VEGF	vascular endothelial growth factor
Wnt/β-catenin	wingless-type MMTV integration site family
XIAP	X-linked inhibitor of apoptosis