Protective activity of aromatic amines and imines against oxidative nerve cell death.

Oxidative stress is a widespread phenomenon in the pathology of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Neuronal cell death due to oxidative stress may causally contribute to the pathogeneses of these diseases. Therefore, neuroprotective antioxidants are considered to be a promising approach to slow down disease progression. We have investigated different aromatic amine and imine compounds for neuroprotective antioxidant functions in cell culture, and found that these compounds possess excellent cytoprotective potential in diverse paradigms of oxidative neuronal cell death, including clonal cell lines, primary cerebellar neurons, and organotypic hippocampal slice cultures. Aromatic amines and imines are effective against oxidative glutamate toxicity, glutathione depletion, and hydrogen peroxide toxicity. Their mode of action as direct antioxidants was experimentally confirmed by electron spin resonance spectroscopy, cell-free brain lipid peroxidation assays, and intracellular peroxide measurements. With half-maximal effective concentrations of 20-75 nM in different neuroprotection experiments, the aromatic imines phenothiazine, phenoxazine, and iminostilbene proved to be about two orders of magnitude more effective than common phenolic antioxidants. This remarkable efficacy could be directly correlated to calculated properties of the compounds by means of a novel, quantitative structure-activity relationship model. We conclude that bridged bisarylimines with a single free NH-bond, such as iminostilbene, are superior neuroprotective antioxidants, and may be promising lead structures for rational drug development.

Cytoprotective antioxidant function of tyrosine and tryptophan residues in transmembrane proteins.

The transmembrane domains of integral membrane proteins show an astounding accumulation of tyrosine and tryptophan residues, especially in the region of the highest lipid density. We found that these residues perform vital antioxidant functions inside lipid bilayers and protect cells from oxidative destruction. First, tyrosine- and tryptophan-containing peptides representing stretches from the transmembrane domains of different integral membrane proteins, including presenilin and the cystic fibrosis transmembrane conductance regulator, prevent oxidative lysis in clonal and primary cells. Second, long-chain acylated tyrosine and tryptophan, but not phenylalanine or short-chain acylated derivatives, are potent inhibitors of lipid peroxidation and oxidative cell death. The antioxidant functions of tyrosine and tryptophan may provide a specific explanation for (a) their unique transmembrane distribution pattern and (b) the high vulnerability of low-protein neuronal membranes to oxidative stress, as seen in neurodegenerative disorders.

The female sex hormone oestrogen as neuroprotectant: activities at various levels.

The female sex hormone oestradiol (oestrogen) is a steroidal compound that binds to specific intracellular receptors which act as transcription factors. Oestrogen displays many of its effects by the classical mode of action through receptor binding, transactivation and binding to consensus oestrogen response elements on DNA. Although the primary role of oestrogen as an ovarian steroid was thought to be the regulation of sex differentiation and maturation, since oestrogen receptors are expressed in a variety of other tissues besides sex organs, oestrogen is believed to exert multiple activities in several target sites throughout the body, including the nervous system. In the brain oestrogens have multiple activities. Potential neuroprotective functions of oestrogens are being intensively studied and it is becoming increasingly clear that oestrogens are (1) neuroprotective hormones acting via oestrogen receptor-dependent pathways at the genomic level and (2) neuroprotective steroidal structures acting independently of the activation of specific oestrogen receptors. One striking activity of the molecule oestradiol is its intrinsic antioxidant activity which makes it a potential chemical shield for neurons. Nerve cells frequently encounter oxidative challenges during the normal physiology, but also under pathophysiological conditions. Oxidative stress has been implicated in a variety of neurodegenerative disorders including amyotrophic lateral sclerosis, Parkinson's disease and Alzheimer's disease. It is important to stress that the antioxidant neuroprotective activity of oestrogens is independent of oestrogen receptor activation, since oestrogen derivatives and aromatic alcohols that do not bind to oestrogen receptors share the same antioxidant neuroprotective activity. Although this effect of oestrogens can clearly be separated from oestrogen receptor binding, oestrogens may interact with intracellular signalling pathways, such as the mitogen activated protein kinase, cyclic AMP pathways, and with the activity of the redox-sensitive transcription factor NF-kappa B.

Dietary Phenols: Antioxidants for the Brain?

Human diet contains numerous phenolic compounds which have been shown to exert protective antioxidant effects in cellular paradigms of oxidative cell death relevant to neurodegenerative disorders. Since reliable in vivo data are scarce, the question whether dietary phenols may act as beneficial neuroprotective agents in the human brain can only be estimated from the chemical composition of the diet with respect to phenolic compounds, their resorption, their metabolic fate, and their ability to cross the blood-brain barrier. We conclude that antioxidant neuroprotection by natural phenolic compounds is highly questionable. Therefore, dietary supplementation with specifically designed phenolic antioxidants has to be in the center of interest. We outline some chemical structural principles of such designer molecules, focusing on a decreased impact on hormone receptors and the absence of prooxidant side-effects.

The antioxidant neuroprotective effects of estrogens and phenolic compounds are independent from their estrogenic properties.

Among the family of steroidal molecules, only estrogens have the capability of preventing neuronal cell death caused by increased oxidative burden. Employing neuronal cell lines, brain membrane, and low density lipoprotein oxidation assays, we show that the antioxidant and neuroprotective effects of estrogens are dependent not on their genomic properties as hormones but rather on their basic chemical properties as hydrophobic phenolic molecules. Concentrations of 17beta-estradiol of 0.1-500 nM, which confer maximum estrogen receptor-dependent gene transcription in vitro as well as maximum estrogen receptor binding, respectively, do not show antioxidant or neuroprotective effects. In contrast, phenolic compounds such as 2,4,6-trimethylphenol, N-acetylserotonin, and 5-hydroxyindole exhibit neuroprotective effects without any estrogenicity. Comparing various natural and synthetic mono- and polyphenolic compounds, no correlation between their antioxidant cytoprotective effect and their estrogenic potency can be seen. These results call into question the idea of a general correlation between the intended pharmacological effects of estrogens and phenolic compounds and their effect on estrogen receptor-dependent pathways. Furthermore, they may open the door toward the rational design of neuroprotective antioxidants with decreased hormonal side effects.

Aromatic alcohols as neuroprotectants.

Free radicals and oxidative stress-induced neuronal cell death have been implicated in various neurological disorders including neurodegenerative diseases. We have recently shown that estrogens can protect neurons against oxidative stress due to their antioxidant potential. Here, we report that also other aromatic alcohols with intact phenolic groups and different phenol derivatives can protect neurons against oxidative cell death as induced by glutamate and hydrogen peroxide. Starting with melatonin, in this study, we degraded the chemical structure step by step and tested various aromatic alcohols as well as phenol derivatives for their potential antioxidant activity. We found that aromatic alcohols with intact phenolic groups protect neurons effectively against oxidative damage and cell death and that this neuroprotective activity is independent of the time the compound is added before the toxin. Methylation of the phenolic hydroxyl group led to a decrease or loss in neuroprotection. Moreover, the tested compounds directly inhibited peroxidation reactions suggesting that the neuroprotection is mediated by antioxidant properties. Our result may have some impact on the design of future drugs for the antioxidant treatment or prevention of oxidative stress-associated pathological conditions.

Neuroprotective potential of aromatic alcohols against oxidative cell death.

Estrogens can protect neurons against oxidative stress-induced cell death due to their antioxidant potential. Here, we report that other aromatic alcohols with intact phenolic groups and different phenol derivatives can also protect neurons against oxidative cell death induced by glutamate and hydrogen peroxide in vitro. This neuroprotective activity was independent of the time the compound was added before the toxin. Methylation of the phenolic hydroxyl group led to a decrease or loss in neuroprotection. Moreover, the tested compounds directly inhibited peroxidation reactions, suggesting that neuroprotection is mediated by their antioxidant properties.