Three distinct neuroprotective functions of myricetin against glutamate-induced neuronal cell death: involvement of direct inhibition of caspase-3

Some natural flavonoids are competitive inhibitors of Caspase-1, -3 and -7 despite their cellular toxicity

A common feature of both apoptosis and inflammation is the activation of caspases. Caspases are aspartate-directed cysteine proteases that have numerous cellular targets. It has been discovered that several flavonoids are inhibitors of caspases. Flavonoids are members of a family of polyphenolic compounds from plants that have many biological properties, one of which is the ability to induce cell death. Some flavonoids are selective inhibitors of particular caspases. Since some of the inhibitory flavonoids are nevertheless cytotoxic, these results suggest that flavonoid-induced cell death may be occurring through a non-classical apoptosis pathway that is not dependent on caspase activity.

Mechanisms underlying the protective effects of myricetin and quercetin following oxygen-glucose deprivation-induced cell swelling and the reduction in glutamate uptake in glial cells

The protective effects of the flavonoid polyphenols, myricetin and quercetin, were investigated on key features of ischemic injury in cultures including cell swelling and the reduction in glutamate uptake. C6 glial cells were exposed to oxygen-glucose deprivation (OGD) for 5 h and cell swelling was determined 90 min after the end of OGD. OGD-induced swelling was significantly blocked by both quercetin and myricetin although higher concentrations were required for quercetin. OGD-induced free radical production, a contributing factor in cell swelling, was significantly reduced by both myricetin and quercetin. However, depolarization of the inner mitochondrial membrane potential (ΔΨ(m)), the blockade of which generally reduces swelling, was significantly diminished by myricetin, but not quercetin. This indicated that quercetin could reduce swelling despite its inability to prevent depolarization of ΔΨ(m) possibly through other signaling pathways. Increased intracellular calcium ([Ca²+](i)) is an important characteristic of ischemic injury and is implicated in swelling. Both myricetin and quercetin attenuated the increase in [Ca²+](i). Further, a reduction in [Ca²+](i), through the use of nifedipine, nimodipine, verapamil, dantrolene, or BAPTA-AM, significantly reduced OGD-induced cell swelling indicating that one possible mechanism by which such flavonoids attenuate cell swelling may be through regulating [Ca²+](i). OGD-induced decrease in glutamate uptake was attenuated by myricetin, but not quercetin. Cyclosporin A, a blocker of the mitochondrial permeability transition (mPT) pore, but not FK506 (that does not block the mPT), attenuated the decline in glutamate uptake after OGD, indicating the involvement of the mPT in glutamate uptake. Our results indicated that while blockade of ΔΨ(m) may be sufficient to reduce swelling, it may not be a necessary factor, and that flavonoids reduce cell swelling by regulating [Ca²+](i). The differential effects of myricetin and quercetin on OGD-induced reduction on glutamate uptake may be due to their differential effects on mitochondria.

Octyl Gallate Inhibits ATP-induced Intracellular Calcium Increase in PC12 Cells by Inhibiting Multiple Pathways

Phenolic compounds affect intracellular free Ca(2+) concentration ([Ca(2+)](i)) signaling. The study examined whether the simple phenolic compound octyl gallate affects ATP-induced Ca(2+) signaling in PC12 cells using fura-2-based digital Ca(2+) imaging and whole-cell patch clamping. Treatment with ATP (100 microM) for 90 s induced increases in [Ca(2+)](i) in PC12 cells. Pretreatment with octyl gallate (100 nM to 20 microM) for 10 min inhibited the ATP-induced [Ca(2+)](i) response in a concentration-dependent manner (IC(50)=2.84 microM). Treatment with octyl gallate (3 microM) for 10 min significantly inhibited the ATP-induced response following the removal of extracellular Ca(2+) with nominally Ca(2+)-free HEPES HBSS or depletion of intracellular Ca(2+) stores with thapsigargin (1 microM). Treatment for 10 min with the L-type Ca(2+) channel antagonist nimodipine (1 microM) significantly inhibited the ATP-induced [Ca(2+)](i) increase, and treatment with octyl gallate further inhibited the ATP-induced response. Treatment with octyl gallate significantly inhibited the [Ca(2+)](i) increase induced by 50 mM KCl. Pretreatment with protein kinase C inhibitors staurosporin (100 nM) and GF109203X (300 nM), or the tyrosine kinase inhibitor genistein (50 microM) did not significantly affect the inhibitory effects of octyl gallate on the ATP-induced response. Treatment with octyl gallate markedly inhibited the ATP-induced currents. Therefore, we conclude that octyl gallate inhibits ATP-induced [Ca(2+)](i) increase in PC12 cells by inhibiting both non-selective P2X receptor-mediated influx of Ca(2+) from extracellular space and P2Y receptor-induced release of Ca(2+) from intracellular stores in protein kinase-independent manner. In addition, octyl gallate inhibits the ATP-induced Ca(2+) responses by inhibiting the secondary activation of voltage-gated Ca(2+) channels.

Myricetin suppresses oxidative stress-induced cell damage via both direct and indirect antioxidant action

We evaluated the cytoprotective effect of myricetin on oxidative stress damaged cells by assessment of the scavenging effect of reactive oxygen species (ROS) and the activities of antioxidant enzymes. Myricetin showed the scavenging effect of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals on intracellular ROS. In addition, myricetin restored the activity and protein expression of cellular antioxidant defense enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) reduced by hydrogen peroxide (H(2)O(2)) treatment. H(2)O(2)-induced cellular DNA and lipid damages, and myricetin was found to prevent the DNA damage shown by inhibition of DNA tail and it decreased nuclear phospho-histone H2A.X expression, which are both markers for DNA strand breakage. Membrane lipid peroxidation was also attenuated as shown by inhibition of TBARS formation and of fluorescence intensity of diphenyl-1-pyrenylphosphine (DPPP). These results suggest that myricetin protects cells against H(2)O(2)-induced cell damage via inhibition of ROS generation and activation of antioxidant enzymes.

Cholinesterase inhibitory effects of the extracts and compounds of Maclura pomifera (Rafin.) Schneider

We have investigated anticholinesterase potential of the methanol extracts from the leaf, wood, flower, twig, and stem bark of the female and male individuals and rhizodermis and fruit from the female tree of Maclura pomifera (Rafin.) Schneider (Moraceae) along with its major isoflavonoids; osajin and pomiferin as well as their semi-synthetic derivatives; iso-osajin and iso-pomiferin. Anticholinesterase activity was determined by Ellman method using ELISA microplate reader. Osajin and pomiferin had a noticeable inhibition of AChE with IC(50) values of 2.239 and 0.096 mM, respectively, while their iso-derivatives were found to display less inhibition towards AChE. The extracts and compounds did not inhibit BChE. The extracts were analyzed for osajin and pomiferin contents by LC-DAD-MS and only the fruits and female flowers contained osajin (fruit: 8.87%, female flowers: 0.19%, w/w) and pomiferin (fruit: 13.6%, female flowers: 0.36%, w/w).