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First Consensus Meeting on Menopause in the East Asian Region

Hormone replacement therapy and risk of Alzheimer’s disease

Ge Qin-sheng and Tian Qin-jie
Division of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynaecology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences, Beijing, China

Introduction

Alzheimer’s disease is one of the most common and severe progressive neurodegenerative diseases of ageing, accounting for a significant number of cases of senile dementia. It is characterized clinically by progressive loss of memory and other cognitive functions, resulting in severe dementia. Alzheimer’s disease is responsible for two-thirds of all cases of senile dementia, while vascular dementia accounts for only about one-quarter in America, Europe and, according to recent reports, China [1]. This recent figure differs from past reports of dementia in China [2] and in Japan [3] in which the main type was vascular dementia (Alzheimer’s disease/vascular dementia, China 1:4, Japan 1:2). In addition to being a major cause of mortality, it is expensive to treat and uniquely distressing for both patients and their families. Alzheimer’s disease is more common and serious than was appreciated only two decades ago.

Dementia is defined as a progressive loss of memory and cognitive function over an extended period of time, i.e. months and years. Although senile dementia has been the term applied to progressive cognitive decline with onset after 65 years of age, there appear to be no reliable distinctions between senile and presenile forms of the disease as initially described by Alzheimer in 1907. The diagnosis of Alzheimer’s disease is one of exclusion as there is no non-invasive diagnostic test. Hence, diagnosis of Alzheimer’s disease during life is based on a careful neurological examination, neuropsychological testing, blood studies, and radiological evaluation to exclude chronic infection, tumour, vascular disease, vitamin or other nutritional deficiencies, or alternations in intracranial pressure such as hydrocephalus. Diagnostic criteria consisting of: (1) proof of dementia, (2) meeting established clinical criteria, and (3) staging of severity, have been proposed and validated by clinicopathological correlation. Some neuroimaging techniques and laboratory investigations are promising diagnostic tests.

The aetiology of Alzheimer’s disease is poorly understood. Evidence accumulated during recent years, however, suggests a polyaetiological nature. It is likely that both genetic and environmental factors are at the root of the disorder. In a number of previously reported risk factors for Alzheimer’s disease, it has been demonstrated to be associated with: (1) age, (2) gender (female), (3) family history, (4) low education and socioeconomic status, and (5) head injury. Putative risk factors include occupational exposure to electromagnetic fields, toxic effects of trace metals or organic solvents, physical and social inactivity, immunologic disorders, infectious diseases (e.g. chronic virus infection), and thyroid disease. There are some possible factors including higher education, smoking, exogenous oestrogen and use of anti-inflammatory drugs, wich seem to be inversely associated with the development of Alzheimer’s disease.

Pathophysiology of Alzheimer’s disease

Our understanding of the pathophysiology of Alzheimer’s disease has been significantly increased in recent years. Genetic mutations associated with familial cases of the disease have been identified and the involved genes localized on chromosomes 1, 14 and 21. The apolipoprotein E (apo E) genotype has been found to affect the risk of developing the disease, i.e. those homozygous for the apo E4 allele are much more prone to develop Alzheimer’s disease. The definitive diagnosis of the disease still depends on the demonstration of characteristic neuropathological lesions, i.e. neurofibrillary tangles and senile plaques, the number of which correlates with the severity of the dementia. Other lesions include neuronal and synaptic loss, amyloid angiopathy and a severe decrease in the level of cortical acetylcholine. Neurofibrillary tangles have been found to be composed of the microtubule-associated protein tau in a highly phosphorylated state. Accumulation of these phosphorylated tau proteins is thought to be associated with disturbances of intracellular transport of molecules and organelles in affected neurons, leading to cell dysfunction and death. An imbalance in the activities of selected protein kinases and phosphatases is also thought to generate these highly phosphorylated tau species. The major component of senile plaques is the A4/beta-amyloid peptide, generated by proteolysis of the amyloid peptide precursor (APP), a transmembrane protein. When aggregated into amyloid fibrils, the A4/beta-amyloid peptide is thought to be neurotoxic. An abnormal metabolism of the APP is often considered as a central physiopathological mechanism of the disease [4].

There is a quantitative loss of cortical neurons in specific brain regions of Alzheimer’s disease sufferers [5, 6]. Schechter et al. [5] reported a 25% loss of small neurons in the frontal cortex and a 45% decline of larger neurons in both frontal and superior temporal cortices. Degeneration of cholinergic neurons and concomitant impairment of cortical and hippocampal neurotransmission lead to cognitive and memory deficits.

Disturbance of the cholinergic neurotransmitter system is the most significant change in all neurochemical pathologies of Alzheimer’s disease [7–9]. Acetylcholine is presently considered the most important neurotransmitter for memory, learning and cognitive functions. Many investigators have reported a marked decrease in the activity of choline acetyltransferase (ChAT), an enzyme involved in the synthesis of acetylcholine in the cerebral cortex, striatum and hippocampus of the brains of patients with Alzheimer’s disease [8]. A decrease in nicotinic acetylcholine receptors in the cerebral cortex of Alzheimer’s disease brains has also been reported [10, 11].

Amyloid (primarily beta-amyloid) formation in the brain is diagnostic of Alzheimer’s disease. Several lines of evidence suggest that neurotoxic A4/beta-amyloid deposits are of primary importance in the pathogenesis of Alzheimer’s disease. The ability to form stable cross-beta fibrils is an intrinsic physicochemical characteristic of the human beta-amyloid peptide (A-beta), which forms the brain amyloid of Alzheimer’s disease. Apo E accelerates precipitation of beta-amyloid. Human apo E, long known for its prominent role in cholesterol transport and plasma lipoprotein metabolism, has recently emerged as a major genetic risk factor for Alzheimer’s disease. In a variety of populations worldwide, one of the three common alleles of apo E, apo E4, is overrepresented in Alzheimer’s subjects compared with age- and sex-matched controls. The genetic and epidemiological evidence suggests that apo E is a major susceptibility gene for Alzheimer’s disease; it likely accounts for a major portion of the genetic heterogeneity of the disease. Based on an understanding of the structure and function of apo E in lipid transport and cellular metabolism, it is suggested that apo E is involved in a final common pathway of neuronal repair and remodelling: apo E3 (the most common allele) supporting effective repair and remodelling after neuronal injury by noxious agents, and apo E4 being less effective in these processes [12].

Mutations in the APP gene can cause early-onset autosomal dominant Alzheimer’s disease. In vitro studies indicate that cells expressing mutant APPs overproduce pathogenic forms of the A-beta peptide, the major component of Alzheimer’s disease amyloid. However, mutations in the APP gene are responsible for 5% or less of all cases of early-onset familial Alzheimer’s disease. A locus on chromosome 14 is responsible for Alzheimer’s disease in other families with early-onset Alzheimer’s disease and represents the most severe form of the disease in terms of age of onset and rate of decline of cognitive function. In late-onset Alzheimer’s disease, the apo E gene allele epsilon 4 is a risk factor for Alzheimer’s disease. This allele appears to act as a dose-dependent age-of-onset modifier. The epsilon 2 allele of this gene may be protective.

The microtubule-associated protein tau is abnormally hyperphosphorylated in the brain of patients with Alzheimer’s disease, and the abnormal tau is the major protein subunit of paired helical filaments (PHF). The abnormal phosphorylation of tau probably precedes its polymerization into PHF. The abnormal tau does not bind to tubulin, but competes with tubulin in binding to normal tau and thereby inhibits the assembly of microtubules in the affected neurons [13].

Epidemiology

Alzheimer’s disease has become a major public health problem. Because of the current increase in the geriatric population throughout the world, its prevalence is doubling approximately every 5.1 years. The Chinese population presently numbers more than 1.2 billion, 10% of whom are elderly, comprising approximately 100 million people. The incidence of Alzheimer’s disease is high, slightly below cardiovascular and cerebral vascular diseases and cancer. The incidence of dementia in China is 2.57% among those over 55 years, 3.46% among those over 60 years and 4.61% among those over 65 years, of which Alzheimer’s disease made up 1.50%, 2.05% and 2.90%, respectively, in Shanghai during 1987–1988 [1]. The age-matched prevalence of female Alzheimer’s disease is about twice that of male Alzheimer’s disease in the age range 55–85 years.

The finding of an increased prevalence in elderly women suggests that oestrogen deficiency may play a role in the development of Alzheimer’s disease. Firstly, prevalence of Alzheimer‘s disease is greater in women than in men of a comparable age, women aged 50–64 having a 1.7-times higher incidence of Alzheimer’s disease compared with men of the same age [14]. Secondly, oestrogen replacement can reduce the incidence of coronary heart disease in postmenopausal women, and women who have had a heart attack are more likely to develop dementia than women with a negative history. Thirdly, lower levels of circulating oestrogen are likely to be associated with lower body weight in postmenopausal women, and Alzheimer’s disease tends to occur in thinner women. In China, postmenopausal women have a prevalence rate of Alzheimer’s disease that is twice that of men of the same age (i.e. 2% and over) [1]. In 1994, 14.1% of Chinese women were aged 55 or over, and this proportion is estimated to grow with increasing life expectancy in the future. It was estimated that Alzheimer’s disease affected approximately 4,300,000 Chinese women aged 55 and over in 1994.

With Alzheimer’s disease emerging as a major public health concern, identification of factors that may prevent it is crucial. Oestrogen could be one such candidate. There are contentions that postmenopausal oestrogen replacement therapy (ERT) may be associated with a decreased risk of Alzheimer’s disease and that oestrogen replacement may improve the cognitive performance of women with this illness.

The earliest study of hormone replacement therapy (HRT) and Alzheimer’s disease was published in 1984 by Heyman et al. [15]. Sherwin demonstrated that scores on several cognitive measures in surgically postmenopausal women changed concurrently with changes in serum concentration of oestradiol [16]. Honjo et al. [17] found that serum levels of oestrogen in women with Alzheimer’s disease were lower than those in senile women. The favourable effects of ERT on cognitive function and prevention of senile dementia in old age now represent a revitalized area of clinical research. Studies in women who have undergone surgical menopause have demonstrated that the menopause is associated with subclinical cognitive and affective dysfunction, which is improved by ERT. Oestrogen loss results in cognitive disorders, including menopausal cognitive dysfunction and senile dementia, in later life [18].

During a case-control study of ERT, Henderson et al. [19] found that when cognitive performances were compared between oestrogen users and non-users with Alzheimer’s disease, those with Alzheimer’s disease were significantly less likely to be taking oestrogen replacement than control subjects (7 vs 18%); the mean performance on a cognitive screening instrument was significantly better (Mini Mental State Examination [MMSE] scores of 14.9 vs 6.5) in HRT users.

However, in a population-based case-control study in Seattle, Brenner et al. [20] compared the exposure to ERT of 107 female Alzheimer’s disease patients with 120 age- and sex-matched controls by using computerized pharmacy data. When logistic regression was applied, ever-use of oestrogen was not associated with Alzheimer’s disease (adjusted OR 1.1, 95% CI 0.6–1.8). This study suggests that ERT has no impact on the risk of Alzheimer’s disease in women. The study suffers, however, from the bias that compliance was not directly tested, and this makes the conclusions questionable at least.

Paganini-Hill and Henderson [21] carried out a case-control study nested within a prospective cohort study comprising 8877 women and found that the risk of Alzheimer’s disease and related dementia was significantly reduced in oestrogen users compared with non-users (OR 0.65, 95% CI 0.49–0.88). The risk was reduced for both oral and non-oral (i.e. injections and/or creams) routes of administration. The risk decreased significantly with both increasing dosages (p = 0.01) and increasing duration (p = 0.01) of oral therapy with conjugated equine oestrogen. Within each dose category, the risk decreased with increasing duration of therapy, with the lowest observed risk in long-term users who received high doses (OR 0.48, 95% CI 0.19–1.17). Risk was also associated with variables related to endogenous oestrogen levels, increasing with increasing age at menarche and (although not statistically significant) decreasing with increasing weight. These results suggested that ERT may be useful for preventing or delaying the onset of Alzheimer’s disease in postmenopausal women.

To investigate whether the use of oestrogen during the postmenopausal period affects the risk of Alzheimer’s disease, Tang et al. [22] studied 1124 elderly women who were initially free of Alzheimer’s disease and who were followed up for 1–5 years using standard annual clinical assessments and criterion-based diagnoses. The age at onset of Alzheimer’s disease was significantly later in those who had taken oestrogen than in those who had not and the relative risk of the disease was significantly reduced (9/156 [5.8%] oestrogen users vs 158/968 [16.3%] non-users; OR 0.40, 95% Cl 0.22–0.85, p < 0.01), even after adjustment for differences in education, ethnic origin and apo E genotype. Women who had used oestrogen for longer than 1 year had a greater reduction in risk; none of 23 women who were taking oestrogen at study enrolment developed Alzheimer’s disease. This demonstrates that oestrogen use in postmenopausal women may delay the onset and decrease the risk of Alzheimer’s disease.

Efficacy of hormone replacement therapy in Alzheimer’s disease

In order to investigate the therapeutic efficacy of oestrogen in female Alzheimer’s patients, Ohkura et al. [23] treated 15 Alzheimer’s patients, with a mean age of 72 years, with 0.625 mg conjugated equine oestrogen orally, twice a day, for 6 weeks. The effects of oestrogen on Alzheimer’s disease patients were evaluated by psychometric assessments, behaviour rating scales, regional cerebral blood flow (rCBF) measurement and quantitative electroencephalogram (EEG) analysis. During ERT, the mean MMSE score increased significantly from 11.6 ± 1.9 to 13.2 ± 2.0 at 3 weeks (p < 0.01) and 13.8 ± 2.0 at 6 weeks (p < 0.001). The mean Hasegawa Dementia Scale (HDS) score increased significantly from 8.6 ± 2.1 to 11.5 ± 2.3 at 3 weeks (p < 0.001) and 11.6 ± 2.6 at 6 weeks (p < 0.01). The improvements were observed in questions related to orientation in time and space, recent and remote events, calculation, and repeating digits in reverse order. Significant improvements in the mean scores of the Gottfries-Brane-Steen geriatric rating scale and Hamilton Depression Rating Scale were also observed in the oestrogen-treated group, but not in the untreated control group. ERT increased the mean rCBF significantly in the lower frontal region (p < 0.01) and primary motor area (p < 0.02) of the right hemisphere. The mean absolute power delta band values in both left and right frontal EEG (Fp1 and Fp2) (p < 0.01) and theta band values in Fp2 (p < 0.05) decreased significantly during ERT. Three weeks after the termination of HRT, the mean MMSE and HDS scores returned almost to pretreatment levels. Mild-to-moderate Alzheimer’s disease patients responded better to HRT than did severe cases. It is inferred that ERT significantly improves cognitive function, dementia symptoms, rCBF and EEG activity in female patients with Alzheimer’s disease.

The results among oestrogen users in the cohort study of Paganini-Hill and Henderson [21] showed that the relative risk decreased with increasing doses: 0.59 for £0.625 mg and 0.46 for ³1.25 mg (p = 0.02). They also found a reduced relative risk with increasing years of use: 0.74 for <7 years and 0.49 for ³7 years (p = 0.01).

Honjo et al. [24] administered oestrogen (1.25 mg/day) for 7 weeks to 13 women with Alzheimer’s disease; 2.5 mg medroxyprogesterone acetate (MPA) was added during weeks 4–7. The HDS scores were improved in the third week, but were slightly decreased in the sixth week compared with the third week. Ohkura et al. [25] treated seven women with mild-to-moderate Alzheimer’s disease with oestrogen for 5 and 45 months, giving four patients 5 mg/day MPA during the last 10–12 days of each 28-day cycle of conjugated equine oestrogen (0.625 mg/day). HRT was found to be very effective in four cases, moderately effective in two cases and ineffective in one case. The physical condition of four patients worsened during the time they were receiving MPA.

Mechanism of hormone replacement therapy in Alzheimer’s disease

The mechanism of oestrogen on Alzheimer’s disease remains to be clarified. It may act via several mechanisms. Honjo et al. [24] suggested that: (1) an antidepressive effect, (2) an improvement in rCBF, (3) direct stimulation of the neurons, (4) development of gliacyte, and (5) suppression of apo E may be the reasons for its effects. Some mechanisms may be combined, contributing to the beneficial effects on clinical symptoms. New evidence shows that oestrogen therapy may also have substantial neurochemical effects, direct effects on the vasculature, and effects on the generation of free radicals, which may be neurotoxic.

The present collective evidence [26] confirms that: (1) oestrogen helps maintain cholinergic function in the brain in which the loss of nerve cells that use the neurotransmitter acetylcholine is central to the striking memory impairment in Alzheimer’s disease; (2) oestrogen may promote the breakdown of the APP to fragments less likely to aggregate as beta-amyloid; (3) oestrogen has antioxidant properties that may act to diminish the cytotoxic effects of beta-amyloid; (4) other important oestrogen actions include reducing the acute-phase inflammatory response, increasing rCBF, augmenting cerebral glucose utilization, and blunting the deleterious effects of stress on the brain.

Studies in experimental animal models provide a convincing rationale for a role for ERT in the treatment and prevention of dementia. These studies have established the role of oestrogen in the regeneration and preservation of neuronal elements within the central nervous system that are analogous to those regions of the brain most sensitive to the neurodegenerative changes associated with Alzheimer’s disease. Furthermore, behavioural studies in these animals have established a correlation between the hormone-dependent changes in the neuronal architecture and learning and memory. However, extrapolation of these studies to postmenopausal women must be done with caution. Surgical and natural loss of ovarian function does not result in a clinically relevant decline in cognitive function over the short term (one to two decades), or ever, in some women. The modest changes that are observed may relate to hormonal effects on neurotransmitter levels or their receptors. Although Singh and Muldoon [27] noted changes in neurotransmitter concentrations 5 weeks after ovariectomy, changes in cognitive performance in their rat model did not become significant until 28 weeks after ovariectomy — the equivalent of approximately 20 years of human life. Except for the familial forms of the disease, Alzheimer’s disease is rarely seen in the first two decades after the menopause. However, by the third decade after the menopause, 50% of women can be expected to manifest the histopathological changes of Alzheimer’s disease. Approximately half of these women are without clinical evidence of disease. Thus, the neurodegenerative process of Alzheimer’s disease probably precedes by many years its age of onset. We can only speculate that oestrogen may play some role in modifying this process. Data from experimental animal models suggest that oestrogen deficiency would selectively increase the vulnerability of oestrogen-responsive neural elements, such as the cholinergic neurons of the basal forebrain and hippocampus, mediated perhaps by the reduced expression of neurotrophic factors, decreased clearance of the amyloid protein, and/or reduced rCBF that are associated with oestrogen deficiency. The brain’s ability to adapt to the neuronal loss by stimulating axonal and synaptic regeneration would also be impaired by oestrogen deficiency as suggested by oestrogen’s ability to restore the synaptic density of lesioned brains of ovariectomized animals. Thus, oestrogen deficiency, like the apo E4 allele, can be considered not a cause of Alzheimer’s disease but one of several factors that may modify the neuronal injury and loss leading to Alzheimer’s disease [28]. The limited epidemiological data and intervention trials currently available are consistent with this interpretation.

Hagino [29] assumed the presence of specific neuronal elements in the region between the optic chiasma and the paraventricular nuclei which are sensitive to oestrogen and may be inhibited (or excited) by the oestrogen level of the blood and in turn regulate (increase or decrease) the secretion of gonadotropin. Oestrogen promotes the growth and survival of cholinergic neurons and could decrease cerebral amyloid deposition, both of which may delay the onset of, or prevent, Alzheimer’s disease.

There is a marked decrease in the activity of ChAT in Alzheimer’s disease. Gibbs [30] postulated that oestrogen replacement can significantly affect the expression of ChAT and nerve growth factor (NGF) receptors in specific basal forebrain cholinergic neurons. NGF is a polypeptide that is necessary for the survival and growth of developing sympathetic and sensory neurons as well as basal forebrain cholinergic neurons in the brain. The effects of NGF are mediated by the binding of the factor to its specific receptor present on the surface of NGF-responsive cells [31]. The time-course of the effects is consistent with a direct upregulation of ChAT followed by either a direct or indirect downregulation of p75NGFR and trkA NGF receptors, possibly due to increased cholinergic activity in the hippocampal formation and cortex and a decrease in hippocampal levels of NGF. Current evidence suggests that ChAT, p75NGFR, trkA and NGF all play a role in regulating cholinergic function in the hippocampal formation and cortex. In addition, all have been implicated in the maintenance of normal learning and memory processes as well as in changes in cognitive function associated with ageing and with neurodegenerative disease. It is possible that oestrogen may affect cognitive function via its effects on NGF-related systems and basal forebrain cholinergic neurons. Indirect evidence suggests that oestrogen interacts with NGF-related systems and that changes in circulating levels of oestrogen can contribute to age-related changes in hippocampal levels of NGF. These findings have important implications for consideration of ERT in pre- and postmenopausal women.

The senile plaques in Alzheimer’s disease consist of amyloid, primarily beta-amyloid. Apo E accelerates precipitation of beta-amyloid. Apo E has been found to be higher in women with serum oestrogen <20 mg/ml than in women with higher oestrogen levels, and is suppressed with HRT [24, 32].

Jaffe et al. [33], using a cell line that contains high levels of oestrogen receptors, tried to investigate the possible effect of oestrogen on the metabolism of the Alzheimer APP and found that treatment with physiological concentrations of 17b-oestradiol is associated with accumulation in the conditioned medium of an amino-terminal cleavage product of APP (soluble APP or protease nexin-2), indicative of non-amyloidogenic processing. There were no obvious changes in the levels of intracellular immature or mature APP holoproteins, suggesting that oestrogen may increase the secretory metabolism of APP.

The potential antioxidant activity of 17b-oestradiol and other steroid hormones in neuronal cells was investigated in 1995 by studying oxidative stress-induced cell death caused by the neurotoxins amyloid beta protein, hydrogen peroxide and glutamate in the clonal mouse hippocampal cell line HT22 [34]. Preincubation of the cells with 10-5 M 17b-oestradiol prior to addition of the neurotoxins prevented oxidative stress-induced cell damage and ultimately cell death, as detected with cell viability and cell lysis assays. At the DNA level, 17b-oestradiol blocked the DNA degradation caused by glutamate. Other steroid hormones, such as progesterone, aldosterone, corticosterone and the steroid precursor cholesterol, did not protect the cells. The neuronal protection afforded by 17b-oestradiol was oestrogen-receptor-independent. These data demonstrate a potent neuroprotective activity of the antioxidant 17b-oestradiol, which may have implications for the prevention and treatment of Alzheimer’s disease.

Schneider et al. [35] reported that there was greater improvement in women completing a trial of ERT and tacrine (a proven drug for controlling and treating Alzheimer’s disease) who had been taking ERT beforehand than in those who had not, demonstrating that continuing ERT may enhance the response to tacrine in women with Alzheimer’s disease.

The study of Ohkura et al. [23] showed that ERT improved not only rCBF, but also EEG activity in female patients with Alzheimer’s disease. Oestrogen also acts as a neurotrophic factor which stimulates neurite growth and synapse formation [36]. In addition, oestrogen may play a role in the reparative neuronal response to injury [37].

Conclusion

Oestrogen is useful in the prevention and treatment of Alzheimer’s disease through a number of mechanisms. Because of the urgency and enormity of the problem of dementia in our ageing populations, it is essential to allocate the resources needed to conduct the appropriate clinical trials to determine oestrogen’s efficacy in both the treatment and prevention of this devastating condition. Trials are needed so that women and their physicians can adequately weigh the risks and benefits of hormone replacement for the treatment and, more importantly, the prevention of dementia.

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