Androgens play a vital role in maintaining women�s sexual health and overall well-being as basic science research and clinical trials have begun to delineate the role of androgens in women throughout the reproductive cycle. The pathophysiologic mechanism of androgen receptor expression has been documented in numerous tissue sites including the central nervous system, vagina, pelvic floor, lower urinary and reproductive tracts; as well as in breast tissue, bone, muscle and the cardiovascular system. The impact of advancing age and estrogens on androgen bioavailability, as well as the psychophysiologic role of androgen receptor expression on sexual function demands further research to better define treatment paradigms for improving the quality of life for women across the reproductive life cycle. A. Introduction
In recent years, increased attention to women�s sexual health has propelled basic science research and clinical trials exploring paradigms for improving quality of life amidst a growing population of menopausal women. As the prevalence of female sexual dysfunction has become manifest, the knowledge of the intricate pathophysiologic role of androgens has fostered a clearer understanding of the biosynthesis pathways and mechanism of action of the androgen receptor and its impact on numerous tissue and cellular sites throughout the body. Understanding androgen physiology and the pathways by which advancing age and medical conditions can alter androgen production will aid in the comprehension of the impact of androgens on female sexual health.

B. Androgen biosynthesis pathways in women
Androgens are important for the development of reproductive function and hormonal homeostasis, and are the precursors for the biosynthesis of estrogens. The physiological role of androgens in women is confounded by the fact that synthesis and metabolism of androgens take place in three compartments: the ovary, adrenal and peripheral tissues which suggest a complex regulation of androgen synthesis by various organs, tissues and enzymes involved in biotransformation. The major androgens in women, listed in descending order of serum concentration, include dehydroepiandrosterone sulphate (DHEAS), dehydroepiandrosterone (DHEA), androstenedione (A), testosterone (T), and 5 α-dihydrotestosterone (DHT). Androgen secretion is regulated by stimulation from adrenocorticotropic hormone (ACTH) to the adrenal glands, and by luteinizing hormone (LH) to the ovary, along with other intraglandular autocrine and paracrine mechanisms.1 T and DHT have the most potent biological activity of the androgenic steroids. In women, approximately 25% of androgen biosynthesis occurs in the ovaries, 25% is produced by the adrenal gland, and the remainder is produced in the peripheral tissue. Circulating T functions as a prohormone, converting to DHT or estradiol in target tissues. Furthermore, T can be synthesized in target tissue on demand; consequently resulting in levels of plasma T which may not provide the critical information on the availability of other metabolites. DHEAS, DHEA, and A are the major sources of peripheral androgen production in women.

Androgens have been shown to regulate the development, growth, and maintenance of secondary sex characteristics as well as modulate the physiological function of multiple receptors and tissue sites including the central nervous system, bone, breast, pilosebaceous unit, skeletal muscle, adipose and genital organs and tissues. Androgens not only have direct effects on target sites, but also their effects on these tissues may be mediated by its conversion to estrogens. Hence an imbalance in androgen biosynthesis or metabolism may have ill-effects on female general health, well-being and sexual function.

C. Biochemical mechanism of androgen action
Androgens enter target and nontarget cells by passive diffusion, and once inside the cell, the biologically active androgens (T, DHT) bind to a specific, soluble intracellular receptor protein molecule localized in the cytoplasm or in the nucleus.2-5 The binding energy of the hormone to its receptor results in physiochemical changes in the receptor complex, converting the receptor from a biologically inactive form to a biologically active state.2-5 These reactions initiated by hormone binding lead to interaction of the hormone receptor complex with unique and specific DNA enhancer elements referred to as androgen response elements (AREs).3, 6 The interaction of the activated androgen receptor complex with the AREs results in recruitment and binding of transcriptional factors, coactivators, or corepressors into the transcriptional complex and regulation of androgen-dependent gene transcription.3, 7

D. Structural and functional domains of the androgen receptor
Androgens exert their effects by first binding to androgen receptors (AR). The AR gene is located on the X chromosome with no corresponding allele on the Y, so it functions solely as a single-copy gene, as shown by the complete loss of androgen effect in XY individuals with an inactivating mutation of the AR.8 The AR, comprised of 918 amino acids, with an estimated molecular weight of 110 kDa 3, is a ligand-dependent nuclear transactivating factor and a member of the steroid-thyroid hormone-retinoic acid nuclear receptor superfamily.2, 4-7 The members of this superfamily are characterized by distinct functional domains comprised within a unique protein structure which includes: the hormone-binding domain, DNA-binding domain, and amino terminal domain encompassing a transactivation function, the nuclear localization domain, and the dimerization domain.3

The hormone-binding domain is located near the carboxyl terminal region and is comprised of a hydrophobic region, which forms the hormone-binding site.3 The two predominant and naturally occurring androgens that bind to the AR are T and DHT. In vivo and in vitro studies have demonstrated that DHT binds with greater affinity to the AR than T and is more potent in inducing biological responses.9, 10 The higher potency of DHT is attributed to DHT binding to the AR with greater affinity and thereby dissociating more slowly, and the AR-DHT complex being more stable.11

The AR gene shares significant homology with both the estrogen receptor and progesterone receptor genes. Two isoforms of the AR have been identified in a variety of human tissues and are similar in structure to the isoforms of the progesterone receptor. Despite the differences in structure and abundance the two AR isoforms do not appear to differ in their regulation or in their ability to bind with ligands and activate target genes.12

Recent studies have suggested that different genes may be activated by different ligands and that this may be reconciled by the presence of cell-specific and limiting transcription factors and coactivators. In addition, the local concentration of the androgen hormone depends on the expression and activity of steroid-converting enzymes such as dehydrogenases and reductases; the latter of which play important roles in the peripheral conversion of androgens to active or inactive metabolites. Furthermore, the tissue- and cell-specific expression of different coactivators or corepressors in different target cells plays an important role in specific gene expression by androgens in various target tissues. The conformational changes in the hormone receptor complex are induced by ligand binding of T versus DHT. The energy of binding and the conformational changes in the protein determine which transcriptional factors, coactivators, and corepressors are to be assembled in the transcription complex and determine which genes are regulated by androgens and therefore act as a discriminatory mechanism of gene activation.3

The DNA-binding domain, a region highly conserved among all members of this superfamily, is comprised of 68 amino acids. This region folds into a tertiary structure resulting in the formation of two distinct zinc fingers that bind to the DNA in the major groove. The first zinc finger confers specificity, while the second zinc finger contributes to an increased binding affinity for DNA.2-6 The AR binds to the palindromic ARE in a dimer form and in a cooperative manner, which is suited to interaction with the ARE half-sites of the palindromic response elements.3

Several other regions of the AR protein contribute to stabilization of the dimer molecule, among which include the ligand-binding domain and the loop of the second zinc finger.2-6 Considerable homology exists between the DNA-binding domain of AR and that of progesterone, glucocorticoid, and mineralocorticoid receptors. Hence, regulation of gene expression by these proteins involves a complex mechanism that requires interaction of the transactivation domains with other accessory factors such as transcriptional factors, coactivators and corepressors.2-7

The transactivational functional domain (TAF1) of the AR is localized in the amino terminal region of which unique sequences within this region interact with transcription factors and other accessory coactivators and corepressors3 A second transactivation functional domain (TAF2) is localized to the hormone-binding region. Consequently, regulation of gene expression by the AR and the physiological response to androgens which is observed in various target tissues may be modulated by one or all of the following factors: binding of a specific ligand (T versus DHT) to the AR, tissue-specific expression of the AR, differential binding of AR to AREs, or tissue- and cell-specific expression of accessory transcriptional factors (coactivators and corepressors) necessary for interactions with the transactivation domain of the AR.3

E. Effect of age on androgen status
Advancing age has a much larger impact on androgen status than menopause. Several studies have examined androgen changes across the menopausal transition. Total testosterone does not change appreciably until women are much older (71-95 years of age), while androstenedione levels decrease much earlier.13 Mean 24-hour levels of testosterone decrease in women from age 20 to age 50, however this decline reflects aging, in that the ratio of DHEAS/T is constant over this time span.14 DHEA and DHEAS concentrations begin to decline in the second decade of life, and by the age of 80, serum levels are approximately 20% to 30% of peak levels.15 Data from a large longitudinal study demonstrate a 13% decline in mean DHEAS and a 46% decline in mean T levels between ages 42 and 50.16 These data confirm that although aging affects levels of testosterone, these levels are not much different before and after the menopause transition, and the small reduction in ovarian production is thought to result from declines in androstenedione.13

F. Androgen function in the postmenopausal ovary
Androgens are produced by ovarian theca lutein cells, are present in ovarian follicular fluid, and are the principal sex steroid of growing follicles. AR are found in the normal surface epithelium of the ovaries, suggesting that androgens are active in the organ. It is currently believed that after menopause, the ovaries are a major site of androgen production.17-22 In the postmenopausal ovary, the loss of ovarian follicles and granulosa cells eliminates its estrogen-producing ability.23 However, secondary interstitial cells and hilar cells may persist in the postmenopausal ovary;24 which for many years were thought to remain continuously activated by the high levels of circulating LH, and thus remained steroidogenically active.19 Furthermore, one of the most convincing data which suggested an important contribution of the postmenopausal ovary to steroidogenesis came from the analysis of ovarian and peripheral vein hormone levels.25 However, postmenopausal ovaries are atrophic with limited blood flow. Hence ovarian vein sampling may be difficult and cross-contamination of adrenal venous blood can occur at the sampling site. Thus, herein lies the difficulty in assessing true hormonal activity in the postmenopausal population.26
The reduction in postmenopausal ovarian androgen production is not precipitous. Instead, ovarian testosterone production decreases slowly over the 5 to 10 years following the last menstrual period, whereas ovarian androstenedione production decreases substantially more at the time of menopause than does testosterone production.27 Couzinet, et al 23 has recently concluded that the commonly held belief of a consistent and significant androgenic capability of the postmenopausal ovary is false. He demonstrated that in the absence of adrenal function, postmenopausal women averaging 12 years after menopause had no detectable circulating androgens and that their postmenopausal ovaries were devoid of gonadotropin receptors and steroidogenic enzymes.23 These observations suggest that the postmenopausal ovary as early as 5 years after menopause is not a source of androgens. Instead, postmenopausal androgens are derived primarily from an adrenal rather than ovarian source.16

Furthermore, it is important to also consider that most of the androgens in women, particularly after menopause are synthesized in peripheral tissues from DHEAS and DHEA.28 In this fashion, DHEA and DHEAS are converted into more potent androgens or estrogens in peripheral target tissues, and they exert their action in the same cells in which their synthesis took place without significant diffusion into the circulation; a process defined as intracrine production.26 Consequently, this process may limit the interpretation of serum levels of active sex steroids as the sex steroids made in peripheral tissues may never enter the circulation, but are instead inactivated locally into more water-soluble compounds which then diffuse into the general circulation where they can be measured.26

On the other hand, realization of the precursor role of circulating androgenic steroids leads to the prediction that lower than average levels of these steroids could lead to inadequate synthesis of estradiol in peripheral tissues such as bone and brain. Changes that have been traditionally considered ensuing sequelae of estrogen insufficiency, such as loss of bone mineralization and possibly changes in brain-derived functions, may paradoxically turn out to be a consequence of insufficiency of circulating androgenic steroids.29

In addition, genetic and ethnic variation has been demonstrated in postmenopausal ovarian androgenic activity as recent studies have shown that DHEAS levels are related to ovarian function in older women which varies with ethnicity.16, 30 In defining the relationship of adrenal steroid production during declining ovarian function, Lasley, et al demonstrated that log circulating DHEAS concentrations were highest among Chinese and Japanese women, and lowest among African-American and Hispanic women in a prospective cohort of 3,029 women between the ages of 42 and 54 across five ethnic groups;16 and this pattern persisted after adjustment for age, smoking, and log body mass index (BMI).

F. The impact of estrogen on androgen bioavailability
Estrogens (E) play an important role in maintaining genital sensation, blood flow and function; as vaginal wall thickness, rugae and lubrication have been shown to be estrogen dependent.3, 31-34 Low E levels are associated with sexual complaints during menopause, particularly vaginal dryness and dyspareunia,35 due to thinning of the vaginal walls, diminished vaginal acidity with resultant vulnerability to infection, trauma and decreased ability to heal.36 E may affect smooth muscle cell growth in the vagina and the clitoris, regulate connective tissue metabolism and nitric oxide synthesis, and may be important in maintaining the functional integrity of vaginal and clitoral smooth muscle function.3
In premenopausal women, the ovaries are the principal source of E, which functions as a circulating hormone to act on distal target tissues.29 However in postmenopausal women, the primary source of E comes from the aromatization of DHEA, A and T to estrone and estradiol in the peripheral tissues which include: adipose tissues, osteoblasts and chondrocytes of bone, the vascular epithelium and aortic smooth muscle cells, and numerous sites in the brain. This circulating E originates in extragonadal sites where it acts locally, and enters the circulation if it escapes local metabolism;29 consequently reflecting, instead of directing, E action in postmenopausal women. Consequently, circulating levels of T, A, DHEA and DHEAS become extremely important in terms of providing adequate substrate for estrogen biosynthesis in these sites. The increased aromatase activity following menopause results in the peripheral tissues taking on a greater role in the production of estrogen compared with this process in younger women.27 This increased aromatase activity is due to the progressive increase in body fat with aging, and an increase in aromatase activity per unit of fat with decreased endogenous E.37 Increased total body fat has an inverse effect on SHBG, in that the greater the BMI, the lower the SHBG concentration,38 which has significant implications for the bioavailability of androgens.27

Estrogen therapy has been shown to provide significant relief of menopausal somatic symptoms, such as hot flashes, night sweats and vaginal dryness.27 However, it often does not provide adequate restoration of sexual desire, potentially because of its effect on SHBG and androgens. Estrogen replacement therapy, particularly at higher doses, and when administered orally (as oral contraceptives or hormone replacement therapy), increases SHBG thereby increasing the binding of T; and decreases the endogenous production, metabolism, and bioavailability, of both ovarian and adrenal androgens.27, 39-41

G. The effect of androgens on the central nervous system, mood and psychosexual function
Androgens appear to play a key role in the psychophysiology of women before and after menopause. The effects of androgens on the brain are mediated through androgen receptors as well as by the aromatization of testosterone to estrogen. The cortical and pituitary actions of androgens are mediated through the androgen receptor. Androgen receptors have been identified in the cortex, pituitary, hypothalamus, preoptic region, thalamus, amygdala and brain stem. Hypothalamic and limbic system aromatization leads to estrogen receptor-mediated actions. Androgen effects in the brain influence sexual behavior, libido, temperature control, sleep control, assertiveness, cognitive function, and learning capacities, including visual-spatial skills and language fluency.42

The relationship been mood and symptoms of menopause including depression, mood swings, irritability, lethargy, difficulty concentration, insomnia, anxiety and loss of sexual function has been studied extensively. The biologic factors influencing mood disorders and menopause are based on the premise that alterations in reproductive hormone activity cause changes in mood and behavior as a result of their impact on central neurotransmitter release.43 In addition, psychosocial factors also have an impact on mood in postmenopausal women as there may be variation in sensitivity to sudden (oophorectomy) versus gradual (natural menopause) decline in ovarian function as symptoms of depression may significantly increase after surgical menopause.44 Furthermore, women who have undergone surgical menopause have been shown to demonstrate lower levels of androgens than age-matched, naturally menopausal women.45
Androgens play an important role in women�s sexual functioning, particularly sexual desire, as the psychological significance of loss of sexual function can have profound impact on a woman�s psychosexual health. Many studies have sought to delineate the role of androgens in maintaining sexual function. Increasing evidence suggests that women with androgen insufficiency experience alleviation of their psychologic symptoms as well as note improvement in concentration, energy, fatigue, libido, sexual response and well-being with androgen replacement therapy.46
H. Androgen receptor expression in vaginal tissue
Although androgens influence clitoral, labial and vaginal physiology, its role in female sexual function is controversial and poorly understood. There is limited biochemical and physiological data on the role of androgens in regulating female genital tissue structure and in modulating female genital sexual response. However, improved libido and orgasmic response as well as increased sexual satisfaction have been reported in women undergoing androgen therapy to alleviate menopausal symptoms.

Hemodynamic events during genital sexual arousal are regulated by estrogens and enhanced by androgen supplementation.33, 47 Furthermore, genital sexual arousal in women which is characterized by an increase in genital blood flow, leads to vasocongestion of the vagina, vulva and clitoris, and increased genital sensation, vaginal lengthening and lubrication;3 while changes in the hormonal milieu can alter these physiological processes. Vaginal lubrication is a combination of basal mucin production and vaginal vascular transudate, which constitutes the major estrogen-dependent lubrication component during genital arousal;3 while mucin production and proliferation of vaginal epithelial cells are regulated by androgens.48, 49

The immunohistochemical detection of androgen and estrogen receptors in vaginal tissues has been reported.50 In animal models, the labia majora, labia minora, and vagina stain positive for the androgen receptor and vaginal epithelium responds to T replacement in a similar manner to estrogen replacement, even in the absence of estrogen.51 Although ARs are present in the human vagina, it is unclear whether T acts directly on the receptor or by conversion to DHT or aromatization to estrogen. The enzymes necessary for metabolism of T, aromatase and 5-α reductase, have been found to be expressed in the human vagina, which suggests that they play a role in the conversion of T to DHT and estrogen in the vagina. The presence of aromatase mRNA indicates that some of the effects of T in the vagina are also mediated through conversion to E. In E-depleted women, this residual source of E could be beneficial. Varying levels of aromatase in the vagina may help to explain why postmenopausal women receiving hormone therapy present with different degrees of vaginal maturation and atrophy, sometimes requiring the addition of topical E therapy.51

Berman et al51 confirmed the presence of ARs, mediating both T and DHT actions, in the human vagina, with their density affected by age, menopausal status, and E replacement. Postmenopausal women receiving oral or transdermal E replacement had lower vaginal androgen receptor densities than those who were not. This suggests that E replacement may down-regulate vaginal ARs. Fewer ARs in vaginal subepithelium of women on hormone therapy may results from estrogenic stimulation of sex hormone-binding globulin (SHBG), leading to less free T and therefore, less production of ARs.51 Furthermore, the current study revealed that androgen density is lower in the mucosa of postmenopausal women, irrespective of type or route of hormone therapy. Hence a reduction of androgen receptors in postmenopausal women combined with a gradual age-related decline in serum androgen levels in women may further decrease the androgen responsiveness of vaginal tissue.

Nitric Oxide (NO), vasoactive intestinal peptide (VIP), and serotonin are among several biochemical factors implicated in the signaling pathway of genital smooth muscle relaxation. Nitric oxide, which is a product of the conversion of arginine by nitric oxide synthase, has been recognized as an important molecule with a broad range of functions in the lower urinary tract and vagina. Estrogens may affect smooth muscle cell growth in the vagina and the clitoris as well as regulate connective tissue metabolism and nitric oxide synthesis, which may be important in maintaining the functional integrity of vaginal and clitoral smooth muscle function.3 Thickness, rugae of the vaginal wall, and vaginal lubrication have been shown to be estrogen dependent as estrogen deficiency results in thin vaginal walls which are more susceptible to trauma, impaired healing and a less acidic environment predisposed to infection. Vaginal epithelium of ovariectomized mice treated with T or aromatase inhibitors demonstrate an increased number of layers, thickness, and mitotic rates compared to controls.51 In addition, estrogen replacement therapy has been shown to increase pelvic blood flow in menopausal women and in women with surgical or medical oophorectomy.36

The physiologic response in vaginal tissues to androgens is also mediated by specific androgen receptors. These effects may be related to maintenance of nonvascular smooth muscle function in the vagina and of vascular smooth muscle in the clitoris. Traish et al3 characterized androgen expression in proximal and distal vaginal tissues from control and ovariectomized animals treated with or without estrogen and/or androgen replacement therapy. It was demonstrated that androgens enhance nitric oxide synthase expression and activity and down-regulated arginase (a substrate for nitric oxide synthase) activity in the proximal vagina, which may be manifested in facilitation of vaginal smooth muscle relaxation to electric field stimulation and VIP in androgen-treated animals. Estrogens, on the other hand, down-regulate nitric oxide synthase activity and increase arginase activity which may result in attenuation of vaginal tissue relaxation to electric field stimulation and to VIP.3 These observations suggest that androgens play an important role in modulating the physiology of vaginal tissue and may contribute to modulation of the genital sexual response in women.

I. Androgen receptor expression in the female pelvic floor and lower urinary tract
Disorders of the pelvic floor, including urinary and fecal incontinence as well as pelvic organ prolapse are major health problems for women, as the incidence of these disorders increase with age, particularly in postmenopausal women. Muscles of the pelvic floor and lower urinary tract are involved in the support of the pelvic organs and micturition, and damage to these muscles or lack of hormonal stimulation may cause pelvic organ prolapse and/or urinary incontinence in women.

The presence of androgen receptors in levator ani muscle in animal rat models is well documented and have been used widely as a bioassay of androgenic activity.52-54 In fact, higher levels of androgen receptor expression have been found in the levator ani muscle than in other skeletal muscles of the rat. Furthermore, androgen responsiveness depends not only on the androgen receptor expression within the particular muscle, but also on the particular cell type within that muscle55 Androgen receptors have also been found in the urethral and trigonal epithelium, detrusor muscle and smooth muscle of the urethra in rabbits.56 In addition, studies on male rats have demonstrated that androgen receptors and β-estrogen receptors were coexpressed in the urothelium, neurons, bladder smooth muscle cells, and proximal urethra striated muscle cells, suggesting that androgens may play an important role in the regulation of voiding function either by direct effects and/or indirect effects through interaction with estrogen in the lower urinary tract.52, 57

Nnodim demonstrated the marked changes in muscle mass of the levator ani that occurs after castration and T supplementation in rats.53, 54 In the group of denervated levator ani muscle but gonad-intact rats, myofiber cross-sectional area was markedly diminished and satellite cell nuclei increased significantly. In the group of castrated rats, pronounced myofiber atrophy was observed but satellite cells were not affected. The combination of castration and denervation of levator ani produced the same degree of myofiber atrophy as denervation alone but had no impact on the satellite cells. These results demonstrated that removal of androgen source caused an atrophic effect on levator ani in a similar fashion as denervation as well as diminished the muscle�s ability to recruit and proliferate satellite cells. The administration of exogenous T to the castrated adult rats restored the myofiber cross-sectional areas and increased satellite cell proliferation. Hence the anabolic effects of androgen play an important role in the levator ani of rats.
In human studies, the expression of AR in levator ani muscle and its fascia has been shown. Copas et al demonstrated the expression of androgen and progesterone receptors in the striated muscle fibers, stromal cells and fascia of levator ani. Estrogen receptors were also present in the stromal cells and fascia of the levator ani, but not in the muscle fibers.58 Ewies et al compared the expression of androgen receptors in human cardinal ligaments of prolapsed uteri with non-prolapsed controls59 and demonstrated that the cardinal ligaments of women with uterine prolapse expressed three to four times more androgen receptors than women without prolapse, implying that androgens may play a role in pelvic organ prolapse.

Androgens may also play a role in stress urinary incontinence as urinary levels of androgens have been demonstrated to be significantly higher in postmenopausal patients with stress urinary incontinence compared to postmenopausal women without incontinence. Concentrations of androgen metabolites in the urine of patients with incontinence were positively related to bladder neck descencus as measured by perineal ultrasound.60

J. Androgen receptor expression in endometrial cancer
Androgens are aromatized to estrogens in several tissues, including the endometrium. During menopause, extraglandular production of estrogens may play a role in the development of certain forms of endometrial carcinoma that depend on estrogenic stimulation for their growth. Therefore, local intra-tumor aromatization of androgens may play a critical role in stimulating the growth of estrogen-dependent endometrial carcinomas.

Maia et al demonstrated immunohistochemical presence of AR in the stroma of non-malignant endometrium which is in accordance with previous studies indicating that the levels of mRNA transcripts for AR are much higher in the endometrial stromal cells than in the glandular epithelium.61 However in endometrioid endometrial adenocarcinoma, there is a shift of AR towards the malignant epithelium with little staining of the intervening stroma, which may suggest a role for the AR in the mechanism of cellular growth in estrogen-dependent endometrial carcinomas. Previous studies have shown that the stimulatory effects of testosterone on the growth of certain cell lines of endometrial carcinoma in vitro were only observed in tumors that were capable of responding to estrogens.62 This stimulatory effect was dependent on the presence of an aromatase enzymatic system capable of converting androgens into estrogens. Thus intra-tumor aromatization of androgen precursors may therefore generate a hyper-estrogenic milieu in the carcinoma, stimulating tumor growth during menopause despite the presence of low plasma levels of estrogens. Consequently during menopause, as most estrogens originate from the extraglandular conversion of androgen precursors, patients with estrogen-dependent endometrial carcinoma will demonstrate strong immunohistochemical staining for testosterone receptors in the glandular epithelium.61

K. Androgen receptor expression in ovarian cancer
Epidemiological evidence supports the possibility of an androgen-ovarian cancer link as most ovarian cancers express AR, and antiandrogens inhibit ovarian cancer growth.12 Oral contraceptives, the most effective chemoprotective agent against ovarian cancer, suppress ovarian testosterone production by 35-70%.12 In contrast, there is evidence that the AR gene may have an ovarian tumor suppressor function. Androgen receptor mRNA and protein are down-regulated in ovarian cancer. Furthermore, loss of heterozygosity in the region containing the gene has been reported in approximately 40% of ovarian cancers. In addition, nonrandom X-inactivation has been reported in invasive ovarian cancer, with expression potentially favoring the allele producing the less active receptor protein.12 While androgens, acting through AR have been implicated in the disease, progestins, acting through progesterone receptors, may provide protection against the disease. Based on mounting evidence in support of the role of androgens and progestins in ovarian cancer, polymorphisms in the androgen and progesterone receptor genes may act as risk factors for ovarian cancer and/or as modifiers of risk associated with exposure to hormonal factors including oral contraceptive use, parity, and BRCA 1/2 mutation status.12 Future studies across large, diverse populations are necessary to identify more precisely the contribution of genetic factors and/or environmental exposures to the etiology of ovarian cancer.
L. Androgen receptor expression in breast cancer
Numerous studies indicate that 70-80% of primary breast tumors express AR, as well as 75% of breast cancer metastases, and it is the sole sex steroid receptor expressed in approximately 25% of metastatic disease.63 However it is unknown at this time whether there is any relationship between exogenous androgen therapy and the incidence of breast cancer, as epidemiological studies have shown both positive and negative associations between endogenous androgen levels and breast cancer risk.8, 64 For instance, experimental data suggest that conventional estrogen treatment regimens, as oral contraceptives or hormone replacement therapy, may upset the normal estrogen-androgen balance and promote unopposed estrogenic stimulation of mammary epithelial proliferation which may potentiate breast cancer risk.8 In addition, endogenous androgen activity may be suppressed as oral estrogen therapy reduces free androgens by stimulating hepatic production of SHBG and suppressing LH, thereby inhibiting ovarian androgen production.8 A recent study found that a low-dose oral contraceptive induced robust mammary epithelial proliferation in rats but addition of methyltestosterone to the therapy significantly suppressed the proliferation.8 In addition, testosterone added to estrogen therapy significantly inhibits estrogen-induced mammary epithelial proliferation in ovariectomized rhesus monkeys.8 There is also the theoretical risk of aromatization of androgens into estrogens in target tissue which may have potential deleterious impact on women with a history of breast cancer or any estrogen-dependent neoplasia.

Androgens have been shown in vitro to have either inhibitory or stimulatory effects on the growth of human breast cancer cells, and androgens either up- or downregulate AR mRNA expression in breast cancer cell lines.65 AR have also been associated with longer survival in women with operable breast cancer and a favorable response to hormone treatment in advanced disease.66, 67 Although androgens have not been used as a primary hormonal treatment for breast cancer since the 1960s due to their masculinizing side-effects (i.e., hirsutism, acne), androgens such as fluoxymesterone have a therapeutic efficacy comparable to current hormonal therapies such as tamoxifen.68 In fact, clinical observations and experimental data indicate that androgens inhibit mammary growth and have been used with success similar to that of tamoxifen to treat breast cancer. 8

In vivo studies using the hormone-dependent DMBA rat mammary tumor model have shown that treatment with testosterone results in tumor regression and a concomitant reduction in estrogen receptor (ER) levels.65 One possible mechanism for this decrease in ER levels in the tumors is that androgen directly regulates ER expression. An alternative explanation is that pharmacological doses of testosterone may be aromatized to estrogen resulting in autologous down-regulation of ER. Thirdly, androgens bind with a low affinity to ER, which may result in an apparent reduction in ER levels following treatment with high doses of testosterone propionate due to interference with ligand binding in biochemical assays for ER.65

Further studies are needed to evaluate the efficacy of supplementing hormone therapy with androgens and ensuing breast cancer risk. Furthermore, as current forms of estrogen in oral contraceptives and oral estrogen replacement therapy suppress endogenous androgen activity, there is a need for future studies on the efficacy of supplementing both oral contraceptive and estrogen replacement therapy with physiological replacement androgen, in a nonaromatizable form, to maintain the natural estrogen-androgen ratios typical of normal women.

M. Androgen receptor expression in bone
Sex steroids are directly involved in bone remodeling. Androgens, acting either directly or via aromatization to estrogen, have profound impact on the physiology and preservation of bone mineral density in women. Androgens also increase muscle mass and strength and induce mechanical factors that alter the balance between bone resorption and formation in favor of formation, with the net result an increase in bone mass and strength.69

ARs are found in all three bone cells: osteoblasts, osteoclasts, and osteocytes. However ARs are predominantly expressed in active osteoblasts and to a greater degree in cortical rather than cancellous bone at the site of bone formation. ARs are also found in bone marrow cells that regulate osteoclastogenesis. Osteoclast function is regulated by estrogen primarily, although indirect aromatization of T to estrogen can also occur. ARs can also be found embedded in osteocytes within the bone matrix.69
Androgens have an important function in regulating bone matrix production and organization. ARs are up-regulated by androgens in bone and also by exposure to glucocorticoids, estrogen and vitamin D. Androgen exposure enhances osteoblast differentiation and the synthesis of extracellular matrix proteins. Androgens also stimulate bone mineralization and influence bone cells� function through the effect on local and systemic factors that control the bone cells� microenvironment. The synthesis of transforming growth factor (TGF) β, a potent osteoblast mitogen, as well as insulin-like growth factor (IGF) II and fibroblast growth factor are all influenced by DHT and T. Androgens also decrease osteoclast genesis by inhibiting the production of interleukin-6 (IL-6) in the stromal cells of the bone marrow, resulting in diminished maturation and development of osteoclast precursors and osteoclasts.69 The reduction of IL-6 has been demonstrated for T, DHT, A and DHEA. Furthermore, T and DHT also regulate osteoclast activity, thereby reducing bone turnover by inhibiting both parathyroid hormone and IL-1-stimulated prostaglandin E2 production.69

N. Anabolic effects of androgen
There are clear associations among muscle mass, muscle strength and bone density in that muscle exerts a greater load on bone than does weight-dependent gravity. Mechanical loading when combined with estrogen, results in a greater osteogenic response than either separately. This is probably the result of estrogen�s antiresorptive effect and of the stimulation of bone formation with exercise.69 Approximately 4% of muscle mass is lost during the first 3 years after menopause, which is associated with a significant decline in muscle strength.70 In men, muscle strength is preserved until age 60 years and reaches levels found in menopausal women at about 75 years of age, which may explain the greater tendency for falls in older women.71

Androgens have direct anabolic effects on skeletal muscle as T increases lean body mass and decreases fat mass in a dose- and concentration-dependent fashion. In addition, the administration of androgens is associated with the up-regulation of androgen receptors and resultant increased responsiveness of skeletal muscle.72 The action of T on muscle involves multiple mechanisms including its effects on inducing protein synthesis, recruiting satellite cells, and modulating the commitment of pluripotent mesenchymal cells to myogenic lineage.52 T supplementation has been shown to increase muscle mass in older men and HIV-infected men with low testosterone, chronic debilitative illnesses and healthy, but hypogonadal men, with the anabolic effects on muscle mass dependent on dose and plasma concentration.52, 73 The T-induced muscle fiber hypertrophy was also associated with a dose-dependent increase in myonuclear number inside the muscle fiber.72 In addition to the stimulation of muscle protein synthesis, T also affected satellite cells, which are quiescent precursors of skeletal muscle. In response to T, satellite cells proliferated and then fused subsequently with the muscle fibers resulting in an increase in myonuclear number and muscle fiber hypertrophy. The observed increase in the number of satellite cells were seen in men who were treated with supraphysiological doses of T.52, 72
Administration of T and DHT have been shown to be associated with increasing muscle mass and decreasing fat mass. Bhasin et al74 demonstrated that testosterone supplementation in older men, young hypogonadal men, and middle-aged men with visceral obesity resulted in a decrease in fat mass which was proportional to the administered testosterone doses. Hence supraphysiologic doses of T may produce a strong anabolic effect and T may also influence additional steps in the myogenic and adipogenic pathways, muscle-protein synthesis, and satellite cell replication.52

O. Androgen effects on the cardiovascular system
In recent years, there has been a dramatic increase in research into androgen effects on the cardiovascular system. Whereas previously androgens were considered harmful (and estrogens protective) for the cardiovascular system, current evidence suggests that androgens have beneficial or neutral cardiovascular effects and that they exert different effects at early (plaque formation) and late (rupture, thrombosis, vasospasm) stages of atherosclerosis.75 An increasing number of studies demonstrate protective effects of androgens on cardiovascular function. However these findings derive almost entirely from male patients, and hold undetermined relevance for women�s cardiovascular health. Nonetheless, limited human data does suggest that testosterone exposure does not shorten life span in either gender, and oral estrogen treatment increases the risk of cardiovascular death in men as it does in women. Patterns of age-specific cardiovascular death rates provide little support for the gender gap being due to estrogen protection. Rather, androgen exposure in early life (perinatal androgen imprinting) may predispose males to earlier onset of atherosclerosis, but the subsequent tempo of atherosclerotic progression is similar in men and women.75 Future research on women�s cardiovascular health will promote a better understanding of AR coregulators, nongenomic androgen effects, tissue-specific metabolic activation of androgens, and androgen sensitivity.

P. Conclusion
The importance of androgens in women is now being recognized as an essential component in maintaining sexual health and overall well-being. Extensive research has documented the physiologic role of androgen receptors throughout the body as well as its involvement in cancers of the reproductive tract. However additional basic science and clinical trials are needed to further assess the role of androgens in premenopausal women as well as during the natural decline that occurs with aging and menopause, and the abrupt losses that occur with surgical menopause. In addition to quality of life and sexual outcomes, the impact of androgens on the pelvic floor and genital tract, muscle, bone, cognitive and cardiovascular function require improved characterization with future studies.

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