2 Review of the literature
2.4 FEMALE SEX STEROID HORMONES
Primary female sex-specific hormones are estrogen and progesterone. They are small, hydrophobic molecules carried through serum globulin in the bloodstream. Sex hormone production is regulated by the hypothalamic pituitary gonadal axis through tightly controlled hormonal and neural signals between the central nervous system, the pituitary and the ovaries respectively. The following hormones are part of the hypothalamic pituitary gonadal axis: 1. Gonadotrophin-releasing hormone; 2. Gonadotrophins i.e. luteinizing
hormone and follicle stimulating hormone (Henry, Norman 2014). Low levels of circulating estrogens and progesterone trigger the release of gonadotrophin releasing hormone from hypothalamus which then stimulates the pituitary to release follicle stimulating hormone and luteinizing hormone, both of which act on the developing follicles to produce estrogens and progesterone (Henry and Norman 2014, Blair et al., 2015b).
2.4.1 Estrogens and progesterone
Estrogen is produced mainly in the reproductive organs (ovaries) during reproductive life and from non-reproductive sites (liver, brain, bone, adipose tissue, muscle, heart) before puberty and after menopause though its level are rather low in comparison with the ovarian estrogens. There are 3 types of estrogen endogenously present in females: estradiol (17β estradiol), estrone, and estriol. Estradiol is the most potent and most prevalent during the reproductive years; estrone is synthesized from adipose tissue mainly after menopause, while estriol is prevalent during pregnancy and is produced by the placenta (Cui et al., 2013).
Sex hormones exert their biological actions through cell and tissue specific receptors. The concept of the estrogen receptor was proposed in the late 1950s (Jensen and Jacobson 1962) and the estrogen receptor α was identified in the 1960s (Toft, Gorski 1966). The gene for human estrogen receptor α was cloned in 1986 (Greene et al., 1986) and that for estrogen receptor β was discovered in the late 1990s. Both receptors are widely distributed in brain and body (Henry and Norman 2014).
Estrogen exerts its specific effects primarily through its nuclear receptors i.e. estrogen receptor α or β (genomic), but also by binding to its membrane bound receptors (non-genomic) (Hewitt et al., 2016, Blair et al., 2015b). The estrogen receptor α is primarily expressed in reproductive organs whereas estrogen receptor β is expressed in a wide variety of tissues. Both estrogen receptors are present in various brain regions including amygdala, cortex, hippocampus, hypothalamus, substantia nigra, stria terminalis, and preoptic area (Cui et al., 2013).
The other important female sex steroid hormone is progesterone, which in reproductive life is produced in the ovaries (corpus luteum), placenta, and adrenal gland (cortex). There are two types of receptors for progesterone, i.e. progesterone receptors A and B; the B-form of the progesterone receptor is more potent than the A-form (Henry and Norman 2014). The activity of the progesterone receptor is dependent on estrogen as well as the properties of the estrogen receptor in target tissues; progesterone receptors also act as a transcription factor similarly as the estrogen receptors. Estrogen receptors are more widely distributed than progesterone receptors, which are limited to uterus, ovary in the periphery, but they are also present in several areas of the brain including pituitary. The presence of progesterone receptors throughout brain means that it should not simply be considered as a reproductive hormone.
2.4.2 Mechanisms of neuroprotective effects of estrogen and progesterone
In vivo animal and human autopsy studies have demonstrated the wide distribution of the estrogen receptor α throughout the hypothalamus, forebrain, and amygdala (Osterlund and
Hurd 2001, Mitra et al., 2003) and the estrogen receptor β in hippocampus and cerebral cortex (Mitterling et al., 2010, Ostlund et al., 2003) emphasizing the role of estrogen in brain functions. A brief summary of some of the neuroprotective mechanisms of estrogen and progesterone is presented in Table 1.
Estrogen can confer neuroprotection through induction and regulation of synaptic activity in hippocampus and the formation of excitatory synapses through N-methyl-D-aspartate receptors (McEwen and Alves 1999, Hao et al., 2003, Jacome et al., 2016) and estrogen receptor mediated gene transcription to regulate hippocampal function (Han et al. 2013).
Estrogen may increase synaptic plasticity whereas progesterone antagonizes this protective effect of estrogen (Baudry et al. 2013) and increases neuronal spine density in prefrontal cortex and hippocampus (Tang-Wai et al. 2004, Shanmugan, Epperson 2014).
Estrogen enhances hippocampal learning through neurogenesis although this may depend upon the type, duration, and time of initiation of HT. Estrogen increases neurite growth and synaptic plasticity among healthy neurons, but not in aged neurons which are less responsive to estrogen. Moreover, a longer duration of naturally occurring estrogen (estradiol) use encompassing the critical time period when neurons are still healthy is another important factor determining the effect of HT on cognition (Duarte-Guterman et al., 2015, Brinton 2008c, Rapp et al., 2003a).
There are several major pathways through which estrogen may exert neuroprotection; up-regulation of cholinergic activity in basal forebrain and hippocampus, especially when neurons are healthy (Gibbs and Aggarwal 1998, Gibbs 2010, Newhouse and Dumas 2015);
increasing the level of neprilysin (an enzyme which degrades amyloid β) (Huang et al., 2004); acting as an antioxidant in brain; and increasing blood flow and glucose transport in brain (Brinton 2008b, Rettberg et al., 2014); decreasing tauopathy (Grimm et al., 2016);
decreasing levels of pro-inflammatory cytokines and also by attenuating the stress induced levels of glucocorticoids (Shivers et al., 2015, Herrera and Mather 2015); it can also activate prefrontal cortex and medial temporal lobe, thus improving cognition (spatial working memory, object recognition, and reference memory) (Rapp et al., 2003a, Markowska and Savonenko 2002).
Estrogen mediated neuroprotection in cognitive tasks involving white matter (such as reasoning, learning, planning) is selective (Pompili et al., 2012) i.e. estrogen related neuroprotection is mediated by an increase in levels of brain derived neurotropic factor and nerve growth factor as well as improving glucose metabolism and cerebral blood flow and diminishing the amounts of free radicals through its anti-oxidant properties (Monk and Brodaty 2000).
Table 1: Summary of mechanisms of neuroprotection exerted by estrogen and progesterone Type of
estrogen
Mechanism of neuroprotection mediation Reference
Estrogen Through increased functional activity of cholinergic neuronal projections to hippocampus and cerebral cortex
(Gibbs and Aggarwal 1998) Estradiol Estrogen increases neuronal plasticity in hippocampus
through enhanced cholinergic activity in basal forebrain
(Gibbs 2010) Estradiol Through estradiol-cholinergic interactions (Newhouse and
Dumas 2015) Estradiol Maintaining and increasing the levels of neprilysin in rat
brain to normal levels after ovariectomy
(Huang et al., 2004) Estrogen Estrogen enhances mitochondrial function (aerobic
glycolysis) in brain
(Brinton 2008a) Estrogen Acts as an antioxidant and regulates glucose transport in
brain and improves cerebral blood flow function (increased ATP production) in cellular models of AD
(Grimm et al., 2016) Estradiol Estrogen has an anti-inflammatory effect (decrease in
tumor necrosis factor and interleukin in female rat brain
(Shivers et al., 2015)
Estradiol Estradiol attenuates glucocorticoid mediated damage to cognition
(Herrera and Mather 2015) Estradiol Estrogen activates multiple areas of brain including
prefrontal cortex and medial temporal lobe in ovarectomized rhesus monkeys
(Rapp et al., 2003a) Estrogen Cyclic provision of estrogen improved working memory
among young but not older rats
(Markowska and Savonenko 2002) Estradiol Estrogen increased spine density in hippocampus of
ovarectomized monkeys only when administered cyclically
(Hao et al., 2003) Estradiol,
progesterone
Estradiol and progesterone exerted neuroprotection by decreasing cholinergic deficits, apoptosis and astrogliosis in hippocampus of ovarectomized rat model of AD
(Hu et al., 2016)
Progesterone can confer neuroprotection via several mechanisms including classic genomic pathways to regulate expression of neurotrophins such as brain derived nerve factor and non-genomic mechanisms by affecting various signaling pathways (Hu et al., 2016), and by acting through its own active metabolites i.e. allopregnanolone (Brinton et al. 2008).
Moreover progesterone decreases neural injury, blood brain barrier leakage, and inflammation in response to ischemia and increases myelination of neurons (de Lignieres 1999).
2.4.3 Sex steroid hormones and aging brain
AD is characterized by an accumulation of amyloid plaques (amyloidopathy) outside the nerve cells and intracellular tau tangles (tauopathy). Figure 3 depicts the pathway of metabolism for amyloid precursor protein. Estrogen exerts neuroprotection by increasing
APP metabolism through the non-amyloidogenic pathway, thus decreasing the Aβ load and it also increases Aβ clearance through up-regulating neprilysin and transthyretin levels (Huang et al., 2004, Barron and Pike 2012, Simpkins et al., 2009).
[Type here]
Figure 3: Pathways of Amyloid precursor protein’s metabolism (Adapted from Barron and Pike. 2012)
Estrogen +
The presence of estrogen receptors in those areas of brain which are affected primarily in AD and dementia led to the hypothesis that loss of neuroprotection at menopause could be a possible mechanism for the higher incidence of AD among women than men (Alzheimer's Association 2016). Aging related high levels of sex hormone-binding globulins and alterations of hypothalamic pituitary gonadal axis result in high peripheral and low brain luteinizing hormone levels which may be linked with the cognitive decline (Blair et al., 2015b, Blair et al., 2015a, Morrison et al., 2006).
The menopause related hormonal decline is often associated with central obesity which promotes chronic inflammation and might account for the high incidence of AD among women (Christensen and Pike 2015, Au et al., 2016). After induced or natural menopause, low insulin sensitivity and low leptin levels in conjunction with impaired lipid and glucose metabolism predispose postmenopausal women to a higher risk of inflammation (Boonyaratanakornkit and Pateetin 2015). Certain estrogen receptor polymorphisms (Cheng et al., 2014), a decrease in expression of estrogen receptors (Bean et al., 2014) and decreased synthesis of estrogen and sex hormone-binding globulin as seen in Down’s syndrome (Chace et al., 2012) have all been reported to be associated with an increased risk of AD.
Moreover, an increased AD risk was observed among women with estrogen receptor β polymorphism (Zhao et al., 2013, Zhao et al., 2011).
2.4.4 Types of commercially available hormone therapy (HT) and their implications There are various types of commercially available estrogens. Conjugated equine estrogen (CEE) is the most commonly used post-menopausal HT in the United States. The other commonly used estrogen preparation is 17β estradiol which is the most potent form of natural estrogen found in premenopausal women. CEE is composed of estrone sulphate and
>10 other compounds (Espeland et al., 2004a). In Finland, women with an intact uterus are administered combination HT which contains estradiol supplemented with norethisterone acetate or levonorgesterel, while women after a hysterectomy use mainly estradiol only (Pentti et al., 2006). Estradiol has a higher binding affinity for both estrogen receptors and
also for membrane receptor mediated actions than that of estrone (Harman et al., 2005, Henderson 2006).
Orally administered CEE undergoes metabolism in liver and yields various estrone to estradiol ratio ranging from 5:1 to 7:1 in contrast to transdermal estradiol which bypasses liver metabolism and yields an estrone to estradiol in ratio of 1:1 which is similar to that observed in the premenopausal period (Hodis et al., 2001, Wharton et al., 2013). Oral CEE increases the secretion of pro-coagulant proteins and C-reactive protein levels from liver, a property not shared with transdermal estradiol i.e. this latter dosage form does not cause the thromboembolic events associated with CEE and which might underlie some of the unwanted effects encountered in clinical trials (Hogervorst and Bandelow 2009, Lakryc et al., 2015). CEE, but not transdermal estradiol, increases the production of sex hormone- binding globulin, thus estradiol results in higher plasma levels of free estradiol. Moreover, estradiol but not estrone has been associated with improved neuronal survival and activation of hippocampus, thus accounting for the differences in the properties of these different HT formulations (McClure et al., 2013).
Naturally occurring progesterone produces an active metabolite called allopregnanolone which has been mainly implicated in progesterone mediated neuroprotection.
Medroxyprogesterone acetate (MPA), a synthetic progesterone mostly used in research settings, differs from natural progesterone in multiple ways: MPA does not undergo first pass metabolism unlike naturally occurring progesterone; MPA inhibits the secretion of brain derived nerve factor which is involved in neuroprotection; and MPA exhibits many non-progestagenic effects such as binding to androgen and glucocorticoid receptors unlike natural progesterone. Moreover, MPA also inhibits the beneficial effects of estradiol in cell cultures and prevents the conversion of natural progesterone to its neuroprotective metabolite allopregnanolone. MPA could not increase levels of antiapoptotic B cell lymphoma 2 (Bcl-2) and also inhibited the estrogen mediated increase in Bcl-2. In contrast, natural progesterone has been reported to exert neuroprotection in cerebral cortical and hippocampal neurons through gene regulation and up-regulation of Bcl-2 (Singh and Su 2013). All these mechanisms indicate that naturally occurring progesterone, but not MPA, is neuroprotective.
The term phytoestrogens refers to plant derived non-steroidal structural analogs of mammalian estrogens, e.g. resveratrol; these provide a potential alternative to the use of HT; it has been claimed that they exert the same effect as HT but without any significant side effects as observed with regular HT use (Zhao et al., 2013, Soni et al., 2014). Resveratrol has been reported to improve mood and cognition in postmenopausal women through its vasodilating effects and increase in cerebral blood flow (Evans et al., 2016).
Oral contraceptives represent another potential marker of lifetime HT use by women during their midlife; which have been associated with the cognition (Warren et al., 2014).
Selective estrogen receptor modulators (SERMs) are commercially available therapeutic agents, which are tissue-selective in their estrogen receptor mediated actions (Frick 2012, Walf et al. 2011). Estrogens and antiestrogens act uniformly as agonists and antagonists respectively in target tissues. In contrast, SERMs possess an unusual pharmacology, such that in some tissues (liver, cardiovascular system, bone) they act as agonists, whereas they function as antagonists in other tissues (brain and breast tissue). The therapeutic potential of SERMs is promising, perhaps they can confer the beneficial effects of estrogen in bone (osteoporosis) and heart, while at the same time preventing the peripheral harmful effects associated with estrogens such as breast and endometrial cancer by acting as estrogen antagonists in these tissues (Riggs and Hartmann 2003, Lewis and Jordan 2005). An association of HT use with a higher risk of cardiovascular disease, thromboembolic events, gall bladder disease, dementia, and breast cancer has been observed in the past (Hulley et
al., 2002, Majoribanks et al., 2012, Boardman et al., 2015); this may be reduced by treatment with SERMs which can mimick estrogen’s agonistic properties in brain while preventing systemic estrogen related harmful effects such as breast and endometrial cancer etc.
Currently available SERMs, such as tamoxifen and raloxifene, both of which are used as chemopreventive agents in estrogen receptor positive breast cancer as well as prophylaxis to prevent fractures among postmenopausal women (Lewis and Jordan 2005). However, both raloxifene and tamoxifene have not shown promising effects on cognition among older women >65 years (Espeland et al. 2010).
2.5 MENOPAUSE
Menopause is a physiological state in a woman’s life characterized by cessation of menstrual cycles and is marked by senescence of ovarian hormones (estrogens and progesterone). The average age at menopause is 51 years and is considered a midlife event due to marked increase in life span compared to previous century (Rocca et al. 2009, Armstrong et al. 2004).
2.5.1 Types and stages of menopause
Menopause occurs either naturally among most females or can be induced at any age after menarche and before the onset of natural menopause. There are several reasons of induced menopause: removal of ovaries, or uterus and ovaries due to benign or malignant conditions; premature ovarian insufficiency; or chemotherapy or radiotherapy in pelvic area (Shuster et al., 2010, Podfigurna-Stopa et al., 2016). Induced menopause is categorized as early menopause or premature menopause if it occurs between 40-45 years of age or <40 years respectively. Natural menopause results in a gradual depletion of sex hormones over a period of certain years whereas induced menopause results in an abrupt cessation of hormone synthesis.
Climacteric symptoms characterize the imminent menopause e.g. vasomotor symptoms caused by disturbances in hypothalamic thermoregulation due to estrogen depletion; sleep disturbances, mood disorders, anxiety, hot flashes, fatigue, and depression (Roberts and Hickey 2016). Currently, HT is the only effective pharmacotherapy available to ease the vasomotor symptoms (Abdi et al., 2016). There are some alternative therapies (herbals, ginseng, acupuncture, yoga etc) to treat vasomotor symptoms, but these are not as effective as HT (Kim et al., 2015). The severity of experiencing menopausal symptoms differs between women, depending upon many sociodemographic, psychological, and lifestyle related factors, such as education, socioeconomic status, occupation, smoking, relationship status, physical activity, history of oophorectomy, stress, and body mass index (Makara-Studzinska et al., 2015).
Menopause has been categorized into following phases based on guidelines devised at a workshop on reproductive aging: perimenopause, menopausal transition, menopause, and post-menopause (Soules et al., 2001, Harlow et al., 2012). Perimenopause is an important stage where a woman’s midlife health status is determining her future health (ESHRE Capri Workshop Group 2011). The treatment of menopausal symptoms requires both a timely evaluation and a diagnosis of stage of menopause along with the exclusion of other differential diagnoses. The National Collaboration Center for Women’s and Children’s Health recommends an individual approach in the management of all stages of menopause.
The woman’s own choice should be considered after informing her about potential harms of HT and any previous and current use of HT should be inquired. HT can be offered as estrogen alone in women without a uterus and combined with progesterone in those with
an intact uterus for short period of time to relieve vasomotor symptoms and also to improve quality of life and the overall health status (Rockville 2015).
2.5.2 Short term and long term implications of menopause
Menopause exposes women to a compromised state of health due to the depletion of previously available sex steroid hormones. Induced menopause, or more specifically premature menopause, increases the risk of heart and neurological diseases (Podfigurna-Stopa et al., 2016). It can be due to elevated sensitivity of the brain to the hormonal loss and the related stress; it can be avoided by provision of HT immediately after induced menopause until the age of natural menopause (Scott et al., 2014). Bilateral oophorectomy was associated with an increased risk of all-cause mortality in the Nurses’ Health Study and similarly decreased endogenous estrogen levels were related to high serum lipid levels and a higher risk of atherosclerosis (Parker et al., 2009). It has been argued that the menopause related decline in the production of sex steroid hormones increases the risk of oxidative stress which may be a factor triggering the higher incidence of dementia among women as they age, and thus it can be attenuated by provision of estrogens (Cervellati and Bergamini 2016).
The presence of estrogen and progesterone receptors in blood vessels and heart forms the foundation for the potential role of these hormones in cardiovascular health through vasodilatation and nitric oxide production, decreasing atherosclerosis, vascular injury and smooth muscle cell growth while promoting endothelial cell growth (Mendelsohn and Karas 1999). The depletion of sex steroid hormones after menopause might account for the higher incidence of heart disease among females (Mendelsohn and Karas 2005). Since the association of vascular and metabolic disorders with AD is plausible, one could argue that they might share a common mechanism (Craft 2009). In the Women’s Health Initiative-Coronary Artery Calcium study, estrogen therapy reduced coronary artery calcification among younger menopausal women (average age 55) but not in older women. This differential effect may be due to the differences occurring in the expression of estrogen receptors and gene expression in calcium homeostasis as the woman ages (Mendelsohn and Karas 2007).
2.6 HORMONE THERAPY AND RISK OF AD, DEMENTIA AND COGNITIVE