Myricetin: targeting signaling networks in cancer and its implication in chemotherapy

The tumor micro-environment is diverse and involves complex molecular interactions ranging from cell-to-cell communication to metastasis and invasion [8, 35]. In cancer metastasis, cellular machinery is continuously under stress as a huge number of interfaces are taking place on a wide molecular landscape [65, 66]. A comprehensive understanding of the chain of molecular events taking place simultaneously in tumor cells helps in developing an efficacious drug and delivery system [94, 98]. The current therapeutic regimen for cancer has several drawbacks that make them aggressively toxic for both tumors as well as healthy cells [10].

Studies have found that naturally occurring phytochemicals have the therapeutic potential to treat human chronic diseases [9, 84, 100]. These phytochemicals target human long non-coding RNAs (lncRNAs) via RNAi technology. ROR, NEAT1, HI9, and PVTI are the commonest lncRNAs that are modulated by several phytochemicals including curcumin, sulforaphane, Epigallocatechin gallate (EGCG) and resveratrol. To enhance the efficacy of the treatment, conventional chemotherapeutic agents along with phytochemicals as a combination therapy can also be administered to patients [64, 69].

Myricetin, an isoflavonoid, is found in a wide variety of natural sources and is reported to contribute to alleviating numerous physiological anomalies such as cancers, cardiac diseases, inflammatory disorders, and neurological diseases. Several studies have highlighted the antioxidant and free radical scavenging properties of myricetin [6, 120]. Owing to these properties myricetin has been noted as a potential modulator of immune reactions and hypertension. Additionally, it has also been found to be an effective analgesic, anti-allergic and anti-inflammatory agent [2]. However, to perform the task, it makes molecular interaction with a wide range of proteins involved in cellular pathways related to cell survival, growth, differentiation, motility, homeostasis, and apoptosis. It is reported to alter the cell-to-cell molecular cascade in different human disorders [76, 86, 102, 116]. In healthy tissues, myricetin promotes signaling through the Akt pathway to induce cytoprotection, but in cancers, it suppresses this signaling cascade to induce apoptosis [40, 48]. In addition, it is also reported to promote TGFβ signaling in UV-exposed epidermal cells but TGFβ expression is down-regulated after myricetin treatment in the parasite-infected liver of mice [31, 71]. Thus, the current review aims to shed light on the interplay between myricetin and cellular pathways, its role in the prevention of cancer, current up-to-date knowledge regarding the nano-formulations of myricetin and selected clinical trials.

It has been reported that myricetin is involved in the inhibition of cancer growth via regulating the expression of key enzymes involved in cancer progression and proliferation [60] (Table 1). A study conducted by Kim et al. reported that myricetin induces apoptosis in the HCT-15 cell lines of human colon cancer, in a dose-dependent manner. Myricetin promoted cytotoxicity in HCT-15 cells via activating the apoptotic genes such as the Bcl-2 associated X Protein. Dose-dependent (5-100 µM) presence of myricetin increased the Bax/Bcl-2 ratio ultimately leading to the recruitment of apoptotic machinery and consequently leading to cell death in HCT-15 cells [45]. Myricetin holds tremendous potential to be used as a potential therapeutic solution for colon cancer. Within molecular docking, cancer cell-based assays, and inhibitory mechanism studies it was found that myricetin prevents activation of human flap endonuclease 1 (hFEN1). The hFEN-1 is an endonuclease responsible for DNA replication and repair. The study showed that myricetin at a specific concentration (IC 50, 690 mM) prevents the attachment of the hFEN1 to Arg100 and Lys93 and consequently inhibits the proliferation of colon cancer [61].

Myricetin has also been reported to suppress angiogenesis in ovarian cancer cell lines. A study using in vitro and in vivo approaches showed that myricetin inhibited angiogenesis in a dose-dependent manner in OVCAR-3 cells [29]. The underlying mechanism involved in the suppression of angiogenesis is still unknown. Future investigations unveiling VEGF and other angiogenesis factors modulation through myricetin will enhance understanding of myricetin therapeutic potential.

In another ovarian cancer cell line SKOV-3, myricetin directly influenced cell viability in a dose-dependent manner. At a dose of 40 µg/ml, myricetin triggered apoptosis in SKOV-3 cells via activation of endoplasmic reticulum stress and DNA double-strand breaks [110]. Similar results were found in the study conducted by Zheng et al. in A2780 and OVCAR-3 cell lines of ovarian cancer [125]. From these findings, it can be justified that flavonoid myricetin has significant potential to be used as a potent inhibitor of ovarian cancer.

A recent study reported that myricetin can inhibit the proliferation and invasion of malignant cells in skin cancer. As potential anticancer mechanism, myricetin inhibited the activation of mitogen-activated protein kinase-1 (MEK1), which in turn prevented the activation of the downstream signal of the ERK/RSK AP-1 axis, thereby exerting inhibition of neoplastic transformation of skin cells. [41, 53]. Moreover, it has come to light less lately that myricetin can bind to central kinases such as the PI3K/Akt/JAK1, RAF1, MEK1, MKK4, and FYN. These kinases are involved in the regulation of a plethora of cellular processes and myricetin can directly inhibit such processes. It has also been investigated that myricetin inhibited the expression of tPA and EGF and explicitly inhibited growth in skin cancer cells in a dose-dependent manner [106]. Myricetin inhibits the growth of human lung adenocarcinoma cell line A549 cells in vitro. Myricetin directly targeted ERK signaling pathway and prevented the invasion and migration of A459 cells in a time-dependent manner [101]. It has also been demonstrated that a combination of myricetin and radiotherapy enhanced the sensitivity of tumor cells (A549 and H1299) toward radiation [121]. Another study has shown that a combination of myricetin and chemotherapy (5-fluorouracil) is efficient to prevent growth, invasion, and metastasis in the esophageal cancer cell EC9706 [113]. In addition to this, findings have also demonstrated the role of myricetin in the modulation and inhibition of bladder cancer. In human T24 bladder cancer cells, myricetin inhibited tumor growth and viability in a dose-dependent and time-dependent manner [105].

For centuries, plant extracts are used to treat almost all kinds of incurred illnesses, however, anciently, the source plants were solely chosen from past experiences or on a trial basis [89, 96, 99]. The plant extracts contained flavonoids and had been proved useful in treating various anomalies including cancers [72, 87]. In more recent times, the plant extracts are refined and detailed studies are carried out to further unearth the efficacy of these flavonoids and their specified use in treatment strategies[83, 85]. One such flavonoid, myricetin (3,5,7,3’,4’,5’-hexahydroxyflavone) has been found effective against several diseases including cancers. It is readily found in numerous edible parts of plants, including herbs and teas as well as fruits such as oranges, grapes, and berries. Myricetin is a lipophilic and weak acidic compound that works best at pH 2.0 and with low aqueous solubility (16.60 g/mL), making it insoluble in the gastrointestinal tract and thus limiting its efficacy via oral absorption [102, 117].

Despite clinical advancements in human health and cancer treatment, optimal gut absorption and solubility are the two stumbling blocks that have significantly hampered the drug efficacy in treating cancer. The drug concentration and mode of delivery greatly affect its efficacy, thus various strategies have been proposed [81, 95].

Recent developments have attempted to enhance myricetin bioavailability by designing a drug delivery system based on nanotechnology [115]. Such efforts have been made previously on several natural compounds whose therapeutic efficacy was hampered by poor aqueous solubility [33, 34].

To overcome the low bioavailability and increase oral delivery of myricetin, alternative approaches such as nanoformulation of myricetin are done to improve solubility, drug delivery, and efficacy. In this section, we will discuss different strategies for nanoformulation of myricetin. Many studies have achieved significant improvements to curb various cancers by using myricetin nanoformulations.

Nanoformulation is carried out in numerous ways owing to a wide range of shapes and forms of nanoscale particles [19, 63]. Nanoformulations including nanocrystals, nanoemulsions, polymeric nanoparticles, dendrimers, carbon nanotubes, polymeric micelles, and lipid nanocarriers, are exploited for their ability to circumvent the poor oral bioavailability of insoluble drugs [74]. So various studies have exploited certain advantages of each type and have made a wide range of myricetin-nanoformulations available for use in cancer and other treatments (Fig. 2). The nanoformulation could also provide an added advantage of targeted delivery of drugs. It is achieved by furnishing formulations to target a specific molecule, thus reducing the risk of wastage of dose at one side and protecting other sites from the toxicity and side effects of the drug. Moreover, high efficacy is achieved with low dosage [82]. The synthesis of metal nanoparticles encapsulating drugs including flavonoids is well established and tested practice.

In a recent study, the development of nano-emulsifying drug delivery systems (SNEDDS) of myricetin was achieved, which was formulated in various phases of excipients, constructing pseudo-ternary phase diagrams, and optimizing based on droplet size and emulsification efficacy after drug loading [77].

Another strategy is the microemulsion formulation to improve the issues of drugs. A microemulsion formulation was successfully developed by Guo et al. resulted in efficiently delivering and increased antiproliferative activity by myricetin in human cancer HepG2 cells without damaging normal cells of surroundings. This formulation consisted of Cremophor RH40 (12%), Tween 80 (6%), Transcutol HP (9%), WL 1349 (18%) and distilled water (55%). This microemulsion significantly increased the solubility of myricetin 1225 times more than myricetin alone [24].

Myricetin micelles (Myr-MCs) have also been formulated using a range of micelles compounds for their utilization in the treatment of various anomalies. One such nanocarrier was HS15-Myr micelle based on polyoxyl 15 hydroxystearate micelles encapsulating myricetin. The resulting ultra-small-sized micelles (12.17 ± 0.73 nm) demonstrated higher aqueous stability, storage temperature range, and membrane permeation than the free myricetin solution among the accepted pH ranges for eyedrops. These were also seen to permeate into the cornea of a mouse without any damage and showed anti-inflammatory properties [27].

A recent study has synthesized myricetin-mediated silver nanoparticles (Myr-AgNPs) using a green approach to aim for improved therapeutic efficacy of myricetin. They found that the resulted NPs were spherical and showed promising antibacterial effects [57]. In another example, spherical myricetin-gold nanoparticles (Myr-AuNPs) with sizes less than 50 nm were synthesized by adopting a simple and stable ultrasound-assisted method. Interestingly, the graph-theoretical network analysis revealed mTOR as an effective target for Myr-AuNPs in breast cancer cells which was further confirmed by in silico molecular docking. Further studies to assess the anti-cancerous effect of Myr-AuNPs revealed that it could decrease cell viability by acting as a pro-apoptotic agent and could also depolarize mitochondrial membrane potential and elevate reactive oxygen species [67].

Gaber et al. encapsulated myricetin into Gelucire-based solid lipid nanoparticles (SLNs). They proposed that fat-soluble antioxidant in SLN helps the drug survive at a higher temperature. Moreover, cell media is usually not suitable for the survival of the flavonoid, so physiological buffers, as well as simulated fluids, should be supplemented with stabilizers additives. With all their recommendations, they found 300-fold lesser drug degradation rates and 4500-fold increased half-life of myricetin [21], thus clearly indicating the importance of nanoformulation of myricetin and other flavonoids.

The development of inhalable myricetin solid lipid nanoparticles (SLNs) for lung cancer therapy has produced encouraging results. The formulation could be done in two steps, producing complexation of myricetin with phospholipid Lipoid-S100 and nanoencapsulation in Gelucire-based, surfactant-free SLNs, which could result in 75.98 nm diameter nanoparticles with a zeta-potential of -22.5 mV, and encapsulation efficiency of 84.5%. Subsequent experiments with these inhalable myricetin solid lipid nanoparticles showed enhanced cellular uptake and increased efficacy of myricetin with a threefold reduction in IC₅₀ values [68].

Myricetin-loaded NLCs (nanostructured lipid carriers) have also been utilized in combination with another drug. For example, myricetin-loaded NLCs in combination with docetaxel (DXT) in MDA-MBA231 breast cancer cells increased the percentage of apoptosis. The expression of anti-apoptotic genes, Cyclin B1 and Mcl1 was reduced, while the expression of pro-apoptotic factors such as Bax and Bid was significantly increased. [62].

To increase the low solubility of myricetin, Hong et al. synthesized nanosuspensions with myricetin, the particle size ranging from 300 to 500 nm. The results of the study showed that myricetin nanosuspensions had a higher solubility in vitro and efficacy compared to simple myricetin powder [25].

Folic acid (FA)-conjugated bovine serum albumin (BSA) nanoparticles (NPs) encapsulating myricetin could be targeted to bind to folate receptor (FR) positive breast cancer cells. The Myr-loaded BSA NPs were further assembled by a modified desolvation cross-linking technique. An FA-conjugated carrier, N-hydroxysuccinimide (NHS)-FA ester, was successfully synthesized by Kunjiappan et al. This resulted in NPS non-covalently bound to folate receptors and demonstrated fast release of myricetin in targeted breast cancer cells where they appeared to cause a significant decrease in cell viability of targeted cells [50]. In another study, myricetin-loaded mesoporous silica nanoparticles (MSN) combined with multidrug resistance protein (MRP-1) siRNA showed excellent target accuracy in non-small cell lung cancer (NSCLC) cells. Again, FA-conjugation directed the nanoformulations to target lung cancer cells exclusively and the release of the drug into the cells resulted in a decrease in cell viability in myr-nanoformulations treatment combined with MRP 1 than myricetin alone in A549 and NCI-H1299 cell lines [103].

The bioavailability of myricetin can also be increased by the formulation of nanophytosomes. A group of researchers developed nanophytosomes with myricetin using phosphatidylcholine. Myricetin nanophytosomes were formulated using a thin-layer hydration-sonication method with a varied ratio of myricetin, phosphatidylcholine, and cholesterol, respectively. The results of the study showed that the formulation myricetin: phosphatidylcholine: cholesterol in the ratio 1: 1: 0.4 has the highest bioavailability. [17].

Formulation of polymeric carrier based on chitosan-functionalized Pluronic P123/F68 micelles encapsulating myricetin has also shown promising results against glioblastoma cancer. These myricetin micelles (Myr-MCs) exhibited improved cellular uptake and antitumor activity compared to free myricetin in vitro and in vivo. These micelles could also affect apoptotic proteins such as Bcl-2, Bad and Bax in mice [111].

Cancer is a multifactorial disease and several cellular pathways responsible for cell growth, survival and proliferation are modulated simultaneously in disease development and progression. Effective drugs for cancer must have the potential to target different molecular players belonging to different cell signaling cascades. Myricetin is an isoflavonoid that is usually present as glycoside in different fruits, vegetables, nuts, berries and herbs. It has also been found as an important constituent of wine, tea, and some medicinal plants. Several cellular pathways modulated by myricetin in humans are discussed in detail in this paper. Myricetin has been demonstrated to modulate cell pathways essential for supporting tumor cell survival such as PI3K/Akt pathway, nrf signaling, canonical and non-canonical wnt pathway, mTOR pathway, Ras/Raf pathway and JAK/STAT pathway. The interplay between myricetin and signaling pathways sheds light on its importance as a therapeutic solution for cancer. However, further exploration is still required to sketch out the most probable targeting of the cellular network. The major stumbling block to using myricetin at the clinical level is its poor water solubility, pH and low bioavailability. Plenty of research is in the phase of pre-clinical trials to establish nano-formulations of myricetin that can enhance its bioavailability and absorption but still requires herculean efforts to bring it to clinical trials and human use. Considering the importance of myricetin as an important modulator in-depth analysis of its interaction with non-coding RNAs will bring up a new avenue in the smart targeting of cancer cells. Considering such benefits putting myricetin to use as a therapeutic agent has been challenging because of its limited bioavailability and solubility. Nano formulation at in vitro and in vivo levels has proven its significance in enhancing the bioavailability and treatment efficacy of myricetin. The incorporation of myricetin into nanoformulations can be a promising therapeutic option. Further research delineating the dosage safety of myricetin nanoformulation will be a step forward in its clinical application.