Vascular actions of estrogen and Alzheimer's disease

Estrogen Signaling in Alzheimer's Disease: Molecular Insights and Therapeutic Targets for Alzheimer's Dementia

Estrogens play a crucial physiological function in the brain; however, debates exist concerning the role of estrogens in Alzheimer's disease (AD). Women during pre-, peri-, or menopause periods are more susceptible for developing AD, suggesting the connection of sex factors and a decreased estrogen signaling in AD pathogenesis. Yet, the underlying mechanism of estrogen-mediated neuroprotection is unclarified and is complicated by the existence of estrogen-related factors. Consequently, a deeper analysis of estrogen receptor (ER) expression and estrogen-metabolizing enzymes could interpret the importance of estrogen in age-linked cognitive alterations. Previous studies propose that hormone replacement therapy may attenuate AD onset in postmenopausal women, demonstrating that estrogen signaling is important for the development and progression of AD. For example, ERα exerts neuroprotection against AD by maintaining intracellular signaling cascades and study reported reduced expression of ERα in hippocampal neurons of AD patients. Similarly, reduced expression of ERβ in female AD patients has been associated with abnormal function in mitochondria and improved markers of oxidative stress. In this review, we discuss the critical interaction between estrogen signaling and AD. Moreover, we highlight the potential of targeting estrogen-related signaling for therapeutic intervention in AD.

Androgen receptor gene and sex-specific Alzheimer's disease

Women are at a 2-fold risk of developing late-onset Alzheimer's disease (AD) (onset at 65 years of age or older) compared with men. During perimenopausal years, women undergo hormonal changes that are accompanied by metabolic, cardiovascular, and inflammatory changes. These all together have been suggested as risk factors for late-onset AD. However, not all perimenopausal women develop AD; we hypothesize that certain genetic factors might underlie the increased susceptibility for developing AD in postmenopausal women. We investigated the Androgen Receptor gene (AR) in a clinical cohort of male and female AD patients and normal control subjects by sequencing all coding exons and evaluating the length and distribution of the CAG repeat in exon 1. We could not establish a correlation between the repeat length, sex, and the disease status, nor did we identify possible pathogenic variants. AR is located on the X chromosome; to assess its role in AD, X-inactivation patterns will need to be studied to directly correlate the actual expressed repeat length to a possible sex-specific phenotypic effect.

Neuronal death in Alzheimer's disease and therapeutic opportunities

Alzheimer’s disease (AD) is an age-related neurodegenerative disease that affects approximately 24 million people worldwide. A number of different risk factors have been implicated in AD; however, neuritic (amyloid) plaques are considered as one of the defining risk factors and pathological hallmarks of the disease. In the past decade, enormous efforts have been devoted to understand the genetics and molecular pathogenesis leading to neuronal death in AD, which has been transferred into extensive experimental approaches aimed at reversing disease progression. Modern medicine is facing an increasing number of treatments available for vascular and neurodegenerative brain diseases, but no causal or neuroprotective treatment has yet been established. Almost all neurological conditions are characterized by progressive neuronal dysfunction, which, regardless of the pathogenetic mechanism, finally leads to neuronal death. The particular emphasis of this review is on risk factors and mechanisms resulting in neuronal loss in AD and current and prospective opportunities for therapeutic interventions. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of AD.

The benefits of oestrogen following menopause: why hormone replacement therapy should be offered to postmenopausal women

Recently, two major epidemiological studies found that hormone replacement therapy (HRT) in postmenopausal women increased the risk of breast cancer. One of the studies also found that HRT increased the risk of cardiovascular disease and thrombosis. As a consequence, women were advised to cease this therapy. However, detailed analysis of these studies suggests that the conclusions may be erroneous. Other studies suggest that the timing of initiation of HRT for healthy women is critical to achieving a beneficial outcome. When begun within 5 years of menopause in healthy women, oestrogen-based HRT results in far greater benefits than adverse outcomes. There is substantial objective evidence that the benefits of HRT include: Reduced distressing symptoms of menopause. Reduced risk of osteoporotic fractures, dementia and colorectal cancer. Improved wellbeing, quality of life; improved vaginal epithelium, sexual enjoyment and bladder capacity. Improved cardiovascular system, with reduced myocardial ischaemia and cardiovascular-related death. Increased longevity. The adverse effects of HRT include: Oral HRT doubles the risk of thromboembolism. HRT promotes growth of pre-existing breast cancer.

Menopausal transition: A possible risk factor for brain pathologic events

BACKGROUND AND OBJECTIVE

Incidence and prevalence of Alzheimer's disease (AD) are higher in postmenopausal women than in age-matched men. Since at menopause the endocrine system and other biological paradigms undergo substantial changes, we thought to be of interest studying whether (and how) the balance between some biological parameters allegedly neuroprotective (e.g. related to estrogen, dehydroepiandrosterone and CD36 functions) and others considered pro-neurotoxic (e.g. related to glucocorticoid and interleukin-6 activities) vary during lifespan in either sex in either normalcy or neurodegenerative disorders.

SUBJECTS AND METHODS

Along with this aim, we evaluated the gene expression levels of estrogen receptors (ERs), glucocorticoid receptors (HGRs), interleukin-6 (IL-6) and CD36, a scavenger receptor of class B allegedly playing a key role in the proinflammatory events associated with AD, in a population of 209 healthy subjects (73M, 106F, 20-91-year old) and 85 AD patients (36M, 49F, 65-89-year old). Results obtained were related to plasma titers of estrogens, cortisol and dehydroepiandrosterone sulfate (DHEAS). Studies were performed in peripheral leukocytes, since these cells (1) are easily obtainable by a simple blood sampling, (2) express many molecules and multiple receptors which are under the same regulatory mechanisms as those operative in the brain and (3) some of them, e.g. monocytes, share many functions with microglial cells.

RESULTS

In healthy men all the study parameters were quite stable during lifespan. In women, instead, at menopausal transition, some changes that may predispose to neurodegeneration occurred. In particular, there was (1) an up-regulation of ERs, and a concomitant increase of IL-6 gene expression, events likely due to the loss of the inhibitory control exerted by estradiol (E(2)); (2) an increase of HGR alpha:HGR beta ratio, indicative of an augmented cortisol activity on HGR alpha not sufficiently counteracted by the inhibitory HGR beta function; (3) a reduced CD36 expression, directly related to the increased cortisol activity; and (4) an augmented plasma cortisol:DHEAS ratio, widely recognized as an unfavorable prognostic index for the risk of neurodegeneration. In AD patients of both sexes, the expression of the study parameters was similar to that found in sex- and age-matched healthy subjects, thus indicating their unrelatedness to the disease, and rather a better correlation with biological events.

CONCLUSIONS

Menopausal transition is a critical phase of women's life where the occurrence of an unfavorable biological milieu would predispose to an increased risk of neurodegeneration. Collectively, the higher prevalence of AD in the female population would depend, at least in part, on the presence of favoring biological risk factors, whose contribution to the development of the disease occurs only in the presence of possible age-dependent triggers, such as beta-amyloid deposition.

Appropriate use of hormones should alleviate concerns of cardiovascular and breast cancer risk

Since the publication of several recent randomized trials in the United States, prescriptions for hormonal therapy have dropped precipitously. This has been due, in large part, to the concerns about the increased risk of cardiovascular (CV) disease and breast cancer among the hormone users. This review takes the perspective that the appropriate use of hormones largely alleviates these concerns. The appropriate use of hormones pertains to treating younger, healthy women who have menopausal symptoms as well as using low-doses of hormones. In the randomized trials, suggesting an increased CV risk, the older women were largely asymptomatic and had other CV risk factors. Data are presented to suggest that there is no increased CV risk with hormonal therapy in younger, healthy women within 5 years of menopause. Moreover, a model is presented to attempt to explain the potential of preventing CV disease when estrogen is begun early, and the relative hazard associated with later use. The risk of breast cancer with hormonal therapy is put into perspective with the realization that this risk is related to hormonal dose and duration of use, and that the absolute risk remains small. Use of progestogens, in particular, appears to enhance this risk. The appropriate use of hormones also pertains to using lower-doses. Here data are presented showing efficacy with lower-doses and improved safety. With the use of lower-doses of estrogens, the progestogen dose, as required in women with a uterus, can be minimized.

Estrogen receptors and metabolic activity in the human tuberomamillary nucleus: changes in relation to sex, aging and Alzheimer’s disease

The human tuberomamillary nucleus (TMN), that is the sole source of histamine in the brain, is involved in arousal, learning and memory and is impaired in Alzheimer's disease (AD) as shown by the presence of cytoskeletal alterations, a reduction in the number of large neurons, a diminished neuronal metabolic activity and decreased histamine levels in the hypothalamus and cortex. Experimental data and the presence of sex hormone receptors suggest an important role of sex steroids in the regulation of the function of TMN neurons. Therefore, we investigated sex-, age- and Alzheimer-related changes in estrogen receptor alpha and beta (ERalpha and ERbeta) in the TMN. In addition, metabolic activity changes of TMN neurons were determined by measuring Golgi apparatus (GA) and cell size. In the present study, ERalpha immunocytochemical expression in AD patients did not differ from that in elderly controls. However, a larger amount of cytoplasmic ERbeta was found in the TMN cells of AD patients. Earlier studies, using the GA size as a parameter, have shown a clearly decreased metabolic activity in the TMN neurons in AD. In the present study, the size of the GA did not change during aging, indicating the absence of strong metabolic changes. Cell size of the TMN neurons appeared to increase during normal aging in men but not in women. Concluding, the enhanced cytoplasmic expression of ERbeta in the TMN may be involved in the diminished neuronal metabolism of these neurons in AD patients.

Estrogen and Alzheimer's disease: the story so far

The ovarian hormone estrogen has long been used to treat the physical symptoms of menopause and to aid in the prevention of osteoporosis in postmenopausal women. Cumulative evidence from basic science and clinical research suggests that estrogen also plays a significant neuromodulatory and neuroprotective role. The numerous estrogenic effects in the brain include the modulation of synaptogenesis, increased cerebral blood flow, mediation of important neurotransmitters and hormones, protection against apoptosis, anti-inflammatory actions, and antioxidant properties. These multiple actions in the central nervous system support estrogen as a potential treatment for the cognitive decline associated with Alzheimer's disease (AD), the most common form of dementia. Evidence from epidemiological studies supports enhanced cognitive function in women with AD taking estrogen replacement therapy (ERT) as well as a reduced risk for developing AD in healthy women receiving ERT. Additional clinical evidence suggests that estrogen may modulate specific cognitive functions such as working memory and verbal learning and memory. However, results from more recent controlled trials have not consistently shown a beneficial effect of estrogen on the cognitive function of women with AD. Future research should focus on examining the influence of multiple potential mediators of ERT including the route of estrogen administration, form of estrogen (conjugated estrogens vs estradiol), duration of treatment, opposed versus unopposed estrogen and the use of estrogen analogues. Further, sensitive neuropsychological measures may provide more detailed information concerning the specific effects of estrogen on cognitive function. These important issues must be addressed in order to establish the role of estrogen for the prevention and treatment of AD in women.

Alzheimer's disease: insights from epidemiology

While a complete understanding of the pathogenesis of Alzheimer's disease (AD) remains elusive, many conclusions can be drawn from the numerous epidemiological studies undertaken to date. Prevalence and incidence estimates show consistency, following a roughly exponential pattern with a doubling of both parameters roughly every five years after age 65. Roughly 7% of the population aged 65 and over has AD. The clinical course of the disease is reasonably well established and mortality rates rise with increasing levels of cognitive deficit. Four risk factors for AD are firmly established: increasing age, the presence of the apolipoproteinE-epsilon4 allele, familial aggregation of cases, and Down's syndrome. Numerous other associations have been shown in some studies, but not in others. For example, women generally appear at higher risk than men, as do people with lower levels of education; depression is probably prodromal; head injury is an established risk factor, and may interact with the apoE gene; several occupational exposures appear hazardous, and exposure to aluminum in the water supply confers excess risk. Hypertension and other vascular symptoms appear to predispose to AD, which is now seen as nosologically closer to vascular dementia than was previously believed. Several apparently protective factors have been identified, although preventive trials based on these have so far shown minimal effectiveness. The use of non-steroidal anti-inflammatory drugs to treat arthritis is associated with a reduced risk of AD, as is estrogen use by post-menopausal women. Physical activity appears beneficial, as does a diet with high levels of vitamins B6, B12 and folate. while red wine in moderate quantities appears protective. This review concludes with a discussion of the strengths and limitations of current epidemiological methods for studying Alzheimer's disease.