Prevention of cytotoxicity of nickel by quercetin: the role of reactive oxygen species and histone acetylation

Cholinesterase Inhibitory Potential of Quercetin towards Alzheimer's Disease - A Promising Natural Molecule or Fashion of the Day? - A Narrowed Review

Natural substances are known to have strong protective effects against neurodegenerative diseases. Among them, phenolic compounds, especially flavonoids, come to the fore with their neuroprotective effects. Since quercetin, which is found in many medicinal plants and foods, is also taken through diet, its physiological effects on humans are imperative. Many studies have been published up to date on the neuroprotective properties of quercetin, a flavanol derivative. However, there is no review published so far summarizing the effect of quercetin on the cholinesterase (ChE) enzymes related to the cholinergic hypothesis, which is one of the pathological mechanisms of Alzheimer's Disease (AD). However, ChE inhibitors, regardless of natural or synthetic, play a vital role in the treatment of AD. Although the number of studies on the ChE inhibitory effect of quercetin is limited, it deserves to be discussed in a review article. With this sensitivity, the neuroprotective effect of quercetin against AD through ChE inhibition was scrutinized in the current review study. In addition, studies on the bioavailability of quercetin and its capacity to cross the blood-brain barrier and how this capacity and bioavailability can be increased were given. Generally, studies containing data published in recent years were obtained from search engines such as PubMed, Scopus, and Medline and included herein. Consequently, quercetin should not be considered as a fashionable natural compound and should be identified as a promising compound, especially with increased bioavailability, for the treatment of AD.

Quercetin enhances the antitumor activity of trichostatin A through up-regulation of p300 protein expression in p53 null cancer cells

In the present study, we investigated the p53-independent mechanism by which quercetin (Q) increased apoptosis in human lung cancer H1299 cells exposed to trichostatin A (TSA), a histone deacetylase inhibitor. We also investigated the role of Q in increasing the acetylation of histones H3 and H4 and the possible mechanism. Q at 5 μM significantly increased apoptosis by 88% in H1299 cells induced by TSA at 72 h. Q also significantly increased TSA-induced death receptor 5 (DR5) mRNA and protein expression as well as caspase-10/3 activities in H1299 cells. Transfection of DR5 siRNA into H1299 cells significantly diminished the enhancing effects of Q on TSA-induced apoptosis. Furthermore, TSA in combination with Q rather than TSA alone significantly increased p300 expression. Transfection of p300 siRNA in H1299 cells significantly diminished the increase of histone H3/H4 acetylation, DR5 protein expression, caspase-10/3 activity and apoptosis induced by Q. In addition, similar effects of Q were observed when Q was combined with vorinostat, another FDA-approved histone deacetylase inhibitor. These data suggest that the up-regulation of p300 expression, which in turn increases histone acetylation and DR5 expression, plays an important role in the enhancing effect of Q on TSA/vorinostat- induced apoptosis in H1299 cells.

Mechanisms of superoxide signaling in epigenetic processes: relation to aging and cancer

Superoxide is a precursor of many free radicals and reactive oxygen species (ROS) in biological systems. It has been shown that superoxide regulates major epigenetic processes of DNA methylation, histone methylation, and histone acetylation. We suggested that superoxide, being a radical anion and a strong nucleophile, could participate in DNA methylation and histone methylation and acetylation through mechanism of nucleophilic substitution and free radical abstraction. In nucleophilic reactions superoxide is able to neutralize positive charges of methyl donors S-adenosyl-L-methionine (SAM) and acetyl-coenzyme A (AcCoA) enhancing their nucleophilic capacity or to deprotonate cytosine. In the reversed free radical reactions of demethylation and deacetylation superoxide is formed catalytically by the (Tet) family of dioxygenates and converted into the iron form of hydroxyl radical with subsequent oxidation and final eradication of methyl substituents. Double role of superoxide in these epigenetic processes might be of importance for understanding of ROS effects under physiological and pathological conditions including cancer and aging.

NiCl2-down-regulated antioxidant enzyme mRNA expression causes oxidative damage in the broiler(')s kidney

The kidney serves as a major organ of nickel (Ni) excretion and is a target organ for acute Ni toxicity due to Ni accumulation. There are no studies on the Ni or Ni compound-regulated antioxidant enzyme mRNA expression in animals and human beings at present. This study was conducted to investigate the pathway of nickel chloride (NiCl2)-caused renal oxidative damage by the methods of biochemistry, quantitative real-time polymerase chain reaction, and enzyme-linked immunosorbent assay. Two hundred and eighty one-day-old broilers were randomly divided into four groups and fed on a control diet and three experimental diets supplemented with 300, 600, and 900 mg/kg of NiCl2 for 42 days. Dietary NiCl2 elevated the malondialdehyde (MDA), nitric oxide (NO), 8-hydroxy-2'-deoxyguanosine (8-OHdG) contents, and reduced the ability to inhibit hydroxy radical in the NiCl2-treated groups. Also, the renal inducible nitric oxide synthase (iNOS) activity and mRNA expression levels were increased. The total antioxidant (T-AOC) and activities of antioxidant enzymes including copper zinc superoxide dismutase (CuZn-SOD), manganese superoxide dismutase (Mn-SOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione reductase (GR), and glutathione-s-transferase (GST) were decreased, and the glutathione (GSH) contents as well were decreased in the kidney. Concurrently, the renal CuZn-SOD, Mn-SOD, CAT, GSH-Px, GST, and GR mRNA expression levels were decreased. The above-mentioned results showed that dietary NiCl2 in excess of 300 mg/kg caused renal oxidative damage by reducing mRNA expression levels and activities of antioxidant enzymes, and then enhancing free radicals generation, lipid peroxidation, and DNA oxidation.