Dihydromyricetin is a variant of the flavonoid Myricetin. A flavonoid is a naturally occurring compound, found in many fruit and vegetables, known for its anti-oxidant and anti-inflammatory properties. From its Chinese medicinal origin, Dihydromyricetin is also used for its anti-hangover properties.
The history of myricetin (1) extends back to more than a hundred years. It was first isolated in the late eighteenth century from the bark of Myrica nagi Thunb. (Myricaceae), harvested in India, as light yellow-coloured crystals . Isolation was primarily sparked by interest in the dyeing property of the compound. It was well characterised in a further study of Perkin , who established the melting point as 357 C and prepared various bromo, methyl, ethyl and potassium analogues. This report also described myricitrin (2), a myricetin glycoside (myricetin-3-O-rhamnoside), for the first time. In a subsequent study, Perkin  found that myricetin yields a phloroglucinol and gallic acid upon hydrolysis, which served to confirm its chemical structure.
Scavenging activity of myricetin towards various radicals and ions. DPPH, 2,2-diphenyl-1-picrylhydrazyl; TEAC, Tetraethylammonium Chloride; ORAC, Oxygen Radical Absorbance Capacity; FRAP, Ferric Reducing Antioxidant Power; ROS, Reactive Oxygen Species; NO, Nitric Oxide.
The inhibition of ABTS+ and DPPH radicals by myricetin was found to be polyphenol oxidase-dependent . However, Rusak and coworkers  reported that although the compound exerts a strong scavenging activity against DPPH radicals, it does not have activity against ROS in menadione-stressed HL-60 cells. The thiyl radical was reportedly inhibited by as much as 81.5% at a myricetin concentration of 500 µM (160 µg/mL). This radical serves as a catalyst for the cis-trans isomerization of fatty acids. It is generated from thiols and is induced by trans-arachidonic acid (TAA) formation during UV irradiation.
Duthie and coworkers  reported that myricetin, at a concentration of 100 μM, restricts H2O2-induced DNA strand breakage in human lymphocytes, in the absence of genotoxicity. The same research group  proposed that the compound, at an effective concentration of 1 mM, protects against DNA strand breakage in human colonocyte Caco-2 cells resulting from oxidative attack caused by H2O2. Miyajima and coworkers  reported that it has an inhibitory effect on the peroxidation of liposomes. It induces the degradation of nuclear DNA that is concurrent with lipid peroxidation and is enhanced by Fe(III) or Cu(II) . Myricetin-induced lipid peroxidation was inhibited by SOD in the presence of Cu(II), but was increased by CAT and sodium azide in the presence of Fe(III). The compound displayed cytoprotective effects against Fe(III)-induced genotoxicity via stimulation of DNA repair processes. Myricetin at 25, 50 and 100 μM, in the presence of Fe(III), prevented lipid peroxidation and stimulated the release of DNA oxidation bases into culture media . A study by Morel and coworkers  revealed that myricetin at 300 μM is able to inhibit lipid peroxidation in Fe-treated rat hepatocyte cultures. At this concentration, phenoxyl radical intermediates are formed that possibly contribute to the mode of action.
A molecular mechanism-based study by Qin and coworkers  suggested that Nrf2-mediated anti-oxidant response element activation is involved in myricetin-induced expression profiling in hepatic HepG2 cells. They found that of a total of 44,000 gene probes in HepG2 cells, myricetin is able to upregulate the signals of 143 and downregulate 476 of them, twofold or more. At concentrations of 20, 40 and 60 μM, myricetin displayed better in vitro cytoprotective effects against H2O2 or CCl4-induced oxidative injury in human hepatocyte (HL-7702) cells than α-tocopherol (positive control). It also improved cell viability, increased reduced glutathione content in cells, reduced lactate dehydrogenase leakage into culture medium and decreased the formation of malondialdehyde in hepatocyte cells .
Myricetin proved to inhibit the tert-butylhydroperoxide (t-BOOH)-initiated chemiluminescence of mouse liver homogenates as reflected by the obtained IC50 value of 15 mM . These results suggest that the compound may have potential to protect against lipid peroxidation and other free radical-mediated cell injuries. The compound also mitigated t-BOOH-induced increases in the levels of oxidative stress parameters including malondialdehyde and the protein carbonyl group of erythrocytes from Type-2 diabetic patients in vitro . These findings suggest that supplementation of the diet with myricetin or myricetin-rich food may be beneficial to all pathological conditions where the anti-oxidant system of the body is overwhelmed.
Although myricetin displayed moderate cytotoxicity towards human laryngeal carcinoma HEp2 cells, its activity against their drug-resistant CK2 subline was relatively poor . The compound increased the expression of cytochrome CYP1A1 in both cell lines. A mechanism-based study by Xu and coworkers  revealed that myricetin exerts strong inhibitory activity against human prostate cancer PC-3 cells. The effect was found to increase with increasing concentrations of the compound, up to 300 μM. Moreover, combination with myricitrin produced a strong synergistic effect, resulting in a decrease in cell proliferation. Similarly, myricetin alone, or in combination with myricetrin, also induced PC-3 cell apoptosis, which was further enhanced with increasing concentration.
The viability and proliferation of bladder cancer T24 cells was decreased following exposure to myricetin, while the migration of T24 cells was decreased through a reduction in in vitro MMP-9 expression . Moreover, the compound induced apoptosis and promoted cell cycle arrest at G2/M by downregulating cyclin B1 and cyclin-dependent kinase cdc2. The mode of action suggests that myricetin inhibits the phosphorylation of Akt, while increasing the phosphorylation of p38 MAPK. Proliferation of human hepatoma cancer HepG2 cells was decreased and G2/M phase arrest was induced by the compound. It increased protein levels in the p53/p21 cascade, while decreasing Cdc2 and cyclin B1 protein levels in HepG2 cells . Moreover, the upregulation of Thr14/Tyr15 phosphorylated Cdc2 and p27, and the downregulation of CDK7 kinase protein and CDK7-mediated Thr161 phosphorylated Cdc2 were recorded after treatment with myricetin.
The compound was found to exert moderate cytotoxicity, which was mediated by G2/M cell cycle arrest and apoptosis, towards human oesophageal adenocarcinoma OE33 cells . Investigation of the mechanism revealed that G2/M cell cycle arrest by myricetin occurs via up-regulation of GADD45β and 14-3-3σ and down-regulation of cyclin B1 at the mRNA and protein levels. Myricetin was reported to stimulate the expression of PIG3 mRNA and the protein levels in human oesophageal cancer KYSE-510 and OE33 cells [99,100]. Induction of PIG3 caused apoptosis in cancer cells through the mitochondrial pathway in a p53-independent manner . It provided significant anti-proliferative effects against melanocyte B16F10, SK-MEL-1 and Melan-A cells. The C2-C3 double bond and hydroxy substituents in myricetin were found to be responsible for the activity. The inhibition of activity of phosphatidylinositol 3-kinase (PI3K), an enzyme that plays an important role in signal transduction and cell transformation, and the reduction of PKC and tyrosine kinase activity of EGF-R were also attributed to the hydroxy moieties in ring-B and the C2-C3 double bond [89,90].
The compound can be regarded as a potent chemoprotective agent against prostate cancer . It was found to induce cytotoxicity and DNA condensation in human colon cancer HCT-15 cells, and increased the BCL2-associated X protein/B-cell lymphoma 2 ratio in cancer cells . A significant surge in the release of apoptosis-inducing factor from mitochondria was recorded in the presence of myricetin. The compound stimulated the basolateral uptake of the pro-carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), by partially inhibiting the MRP2-mediated excretion of PhIP from intestinal cells back to the lumen .
Moreover, the compound was found to be active against thrombin and neutrophil elastase with IC50 values of 28 and 7 µM, respectively . Its ability to inhibit rabbit platelet aggregation and PAF-induced 5-HT release is reflected by the respective IC50 values of 17.5 and 64.1 μM obtained. However, the compound at a concentration of 7.9 μM had no effect against 5-HT release from platelets . A docking experiment indicated that myricetin has the potential to inhibit thrombin and that the compound could therefore be helpful in the treatment of thrombotic disease .
Myricetin displayed anti-inflammatory activity by inhibiting the production of LPS-induced prostaglandins . The structure-activity relationship suggested that the double-bond at C2-C3 and keto group at C-4 are the most likely factors responsible for the strong inhibitory effect towards COX-2 expression. At a concentration of 10 μM, myricetin inhibited NO production in endotoxin-stimulated RAW264.7 murine macrophages, without cytotoxicity being evident . The compound was also found to inhibit the production of LPS-stimulated NO, pro-inflammatory cytokines, PGE2 production and protein levels of iNOS and COX-2 in RAW 264.7 macrophages . A study by Lee and coworkers  using JB6 P+ mouse epidermal cells revealed that myricetin inhibits phorbol ester-induced COX-2 expression by suppressing activation of NF-κB at concentrations of 10 and 20 μM. It also attenuated the phorbol ester-induced production of PGE2 and blocked the phorbol ester-stimulated DNA binding activity of NF-κB.
Myricetin was found to be active against periodontitis, an infectious inflammatory disease caused by microbes of dental bacterial plaque that affect the connective tissue and supporting bone surrounding the teeth. Activation of ERK-1/2, AKT and p38, and lipoteichoic acid-induced COX-2 expression in human gingival fibroblasts was inhibited by the compound. It also blocked IκB degradation and PGE2 synthesis and expression [127,128]. Myricetin did not have any effect on cell viability, but decreased the mRNA expression and enzyme activity of MMP-1, -2 and -8 in human growth factor (HGF). This compound also inhibited the RANKL-stimulated activation of p-38, ERK and cSrc signalling, and the RANKL-stimulated degradation of IkB in RAW264.7 cells. Moreover, the secretion of LPS-induced TNF-α and IL-1β in RAW264.7 cells was significantly inhibited by myricetin . 041b061a72