Neuroprotective Strategies Against Brain Disorders Cause

National Institute Of Mental Health

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Abstract

A hormone therapy has been prescribed for postmenopausal women to control osteoporosis, hot flashes, moodiness, loss of memory, and other symptoms. Several years ago NIH started the research program entitled Women's Health Initiative evaluating presumed neuroprotective effects of estrogen. Recent reports of clinical trials of hormone replacement therapy concluded that hormone therapy's risks out weighed their benefits. According to these studies, hormone therapy fails to improve cognition function or stop progression of Alzheimer's symptoms in pre-selected 65-year and older postmenopausal women groups. These clinical reports are at odds with most of the recent preclinical studies, which infer that 17beta-estradiol may produce beneficial actions in the brain. These negative findings surprised doctors who have enthusiastically been prescribing hormone therapy for menopausal symptoms, cardiac protection, and cognitive improvement to millions of menopausal women. With these negative findings in mind, NIH halted all trials evaluating the efficacy of hormone therapy except ongoing clinical trials with unopposed estrogen therapy in ovariectomized women whose outcomes are not known. Increased risks of breast cancer, stroke, and blood clots have been previously documented in clinical trials of contraceptive pills containing estrogen and progestin. Despite profound side effects, most young women are willing to take risks over unwanted pregnancies; older women suffering from severe menopausal symptoms may also need to make this kind of difficult decision for their own well being. In addition to applying a drug holiday treatment schedule, lower doses of ethinyl estradiol plus drospirenone (a progesterone analog) for the shortest possible time may be prescribed to alleviate severe menopausal syndromes. Alterative remedies such as isoflavones or phytoestrogens may also be applied. Progesterone suppresses endometriosis or uterine cancer induced by estrogen. However, cumulative findings suggest that most side effects of current hormone therapy may be due to the known biological actions of progesterone. Progestin may increase venous thromboembolism leading to ischemic coronary disease, stroke, and death. A recent clinical study suggests that lower dose 17beta-estradiol does not increase breast cancer risk whereas combination hormone therapy does. Estrogen actions in the brain are mediated by nuclear estrogen receptors ERalpha and ERbeta. Mice with the knockout of the ERbeta gene and thus its receptors, two years or older, develop severe neuronal loss in different brain regions. Additional research and well designed clinical trials may reveal whether unopposed estrogen therapy is either protective or harmful to the aged brain; this information is also needed to guide women so they can make a well-informed personal decision regarding hormone replacement therapy. With this mission in mind, we employed 17beta-estradiol, which is the most potent endogenous ligand of estrogen, to see whether estrogen protects against neurodegeneration in both cell and animal models. Our basic results show that estrogen-mediated neuroprotection is probably via both receptor-independent and -dependent mechanisms. The antioxidative potency of 17beta-estradiol is rather weak. In addition to the induction of the neuronal nitric oxide synthase (NOS1), physiological concentrations of 17beta-estradiol activate nuclear estrogen receptors (ERbeta>ERalpha) and up-regulate cGMP-dependent thioredoxin (Trx) expression. Estrogen-mediated gene induction of Trx may play a pivotal role in the promotion of neuroprotection because Trx is a multifunctional anti-oxidative and anti-apoptotic redox protein. Others have recently reported that 17beta-estradiol can suppress interleukin-induced inflammation and induce Bcl-2, which provides neuroprotection in ischemia/stroke animal models. For managing progressive neurodegeneration such as Alzheimer's dementia, our proposal on the novel estrogen-induced signaling pathway of NOS1-cGMP-PKG in increasing cytoprotective genes may foster research and development of new estrogen ligands devoid of hormonal side effects such as endometriosis. Furthermore, it would be very important to create an international research team to address the critical issue in Women's Health Initiative such as menopausal moodiness and forgetfulness. Our collaborative integrated research project on preconditioning, neuronal adaptation, gene expression, and redox proteomics has made a new discovery supporting the concept that estrogen-induced NOS1 gene expression may result in cyto- and neuro-protection via an additional induction of both antioxidative and anti-apoptotic proteins. Human SH-SY5Y cells contain relatively low levels of Trx; thus, they serve favorably as a model for studying oxidative stress-induced apoptosis. When these neurotrophic cells were subjected to non-lethal 2-hour serum deprivation, their NOS1 and Trx were up-regulated, and the cells became more tolerant of oxidative stress, indicating that nitric oxide (NO) may compensatory protect cells from serum deprivation-induced apoptosis. In the present study, the hormesis mechanism by which NO exerts its preconditioning neuroprotective effects was investigated. Our results revealed that in SH-SY5Y cells, NO inhibits apoptosis through its ability to activate guanylate cyclase, which in turn activates the cGMP-dependent protein kinase (PKG). The activated PKG is required to protect cells from lipid peroxidation and apoptosis, to inhibit caspase-9 and caspase-3 activation, and to elevate the levels of Trx peroxidase-1 and Trx, which subsequently induces the expression of Bcl-2. Consistently, PKG activators promote the elevation of c-Jun, phosphorylated MAPK/ERK1/2, and c-Myc. Elevation of Trx and Trx peroxidase-1 or Mn(II)-superoxide dismutase would reduce hydrogen peroxide and superoxide anion, respectively. Thus, the cytoprotective effect of NO in SH-SY5Y cells appears to proceed via the cGMP/PKG-mediated pathway. This observed hormesis is associated with the elevation of NOS1, NO, and cGMP, and the biosynthesis of the redox protein Trx. Furthermore, exogenously added and endogenously induced Trx increased the synthesis of Bcl-2 and Mn(II)-superoxide dismutase and suppressed oxidative stress-induced hydroxyl radical, lipid peroxidation, and apoptotic cell death. Nitroxyl anions produced by Angeli's salt but not NO are toxic to midbrain substantia nigra dopaminergic neurons both in vitro and in vivo. Our and other studies have recently demonstrated that Trx induction leads to brain tolerance against oxidative stress caused by neurotoxin producing parkinsonism. We are now investigating novel neuroprotective drugs that can induce multiple cyto- and neuro-protective genes and their proteins. In addition to antioxidants and free radical scavengers, the proposed neuroprotection study should also consider hypothermia therapy, anti-inflammatory therapeutics, iron chelators, protease inhibitors, and drugs that up-regulate antioxidative cellular defense systems, neurotrophins, and anti-apoptotic proteins. For achieving neuroprotection, multiple synergistic therapeutics may be more effective than using a single drug treatment with large dosage.