madman
Super Moderator
Revisiting the physiological role of androgens in women (2022)
Elena Rosato, Francesca Sciarra, Eleni Anastasiadou, Andrea Lenzi & Mary Anna Venneri
ABSTRACT
Introduction: Extensive research underlines the critical functions of androgens in females. Nevertheless, the precise mechanisms of their action are poorly understood. Here, we review the existing literature regarding the physiological role of androgens in women throughout life.
Areas covered: Several studies show that androgen receptors (ARs) are broadly expressed in numerous female tissues. They are essential for many physiological processes, including reproductive, sexual, cardiovascular, bone, muscle, and brain health. They are also involved in adipose tissue and liver function. Androgen levels change with the menstrual cycle and decrease in the first decades of life, independently of menopause.
Expert opinion: To date, studies are limited by including small numbers of women, the difficulty of dosing androgens, and their cyclical variations. In particular, whether androgens play any significant role in regulating the establishment of pregnancy is poorly understood. The neural functions of ARs have also been investigated less thoroughly, although it is expressed at high levels in brain structures. Moreover, the mechanism underlying the decline of dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) with age is unclear. Other factors, including estrogen’s effect on adrenal androgen production, reciprocal regulation of ARs, and non-classical effects of androgens, remain to be determined.
1. Introduction
Androgens (from the Greek ‘andro,” meaning male or man) are traditionally considered male sex steroid hormones responsible for the maintenance of male characteristics, while knowledge of the roles of androgens in women is limited. However, it is now established that they play essential roles in women’s physiology and are the most abundant sex hormones in females. Considering pg/ml as a unit of measurement, in women of reproductive age, while estrogens vary from 50 to 300, androgens and their precursors circulate in considerably greater levels: testosterone around 400, androstenedione around 2000, DHEA around 5000 [1].
Furthermore, several studies show that androgen receptors (ARs) are broadly expressed in several female tissues (urogenital tissue, mammary gland, nervous systems, bone, cardiovascular system, adipose tissue, and liver), where they mediate crucial functions and until recently were thought to be only targets of estrogen.
The limiting factors contributing to this ‘knowledge gap’ are the lack of more accurate laboratory methods for dosing androgens and the uncertainty of age and gender-specific reference range.
*This review aims to highlight the most recent findings on the role of androgens in female physiology, which is to have a firm basis for investigating clinically relevant situations of androgen insufficiency or excess throughout a woman’s life (childhood, reproductive age, and menopause).
Hyperandrogenism is a heterogeneous group of disorders characterized by elevated androgen levels that exhibit a typical phenotype. Signs and symptoms of excessive androgen secretion in women include hirsutism, acne, seborrhea, androgenic alopecia, and menstrual disorders [2].
The most common hyperandrogenic disorder in women of reproductive age is polycystic ovary syndrome (PCOS). The latter is a condition caused by an imbalance of sex hormones that can lead to menstrual cycle changes, ovarian cysts, difficulty conceiving, and other health changes, with approximately an 80-85% prevalence among women with androgen excess [3]. Other less frequent conditions include idiopathic hirsutism, androgenic drug intake, Cushing’s syndrome, nonclassical congenital adrenal hyperplasia (NCAH), acanthosis nigricans (HAIRAN), ovarian or adrenal androgen-secreting neoplasms (ASN), and hyperprolactinemia [4,5].
Studies on women suffering from gender dysphoria (female to male trans-sexual subjects) demonstrate the relevance of androgen effects in women: the long-term administration of androgens is associated with cardiovascular risk, malignancies, and mental health problems such as depression and anxiety. However, future studies should aim to explore the long-term outcome of androgens treatment in transgender women [6].
Today we also know that not only an excess of androgens but also a reduction in androgen levels can cause various pathological conditions: a hypoandrogenic state is detrimental to cardiovascular, mental, and sexual health. In this regard, the Endocrine Society recommends using testosterone in women only in the case of HSDD (hypoactive sexual desire disorder) [7]. However, the long-term safety of treatments with testosterone remains to be evaluated.
Androgen insufficiency, affecting not only postmenopausal women but also young women (for example when using contraceptives), is an increasingly debated topic in recent years, especially for the absence of specific cutoffs for female hypoandrogenism.
2. Synthesis and androgen-producing organs in women
2.1. Synthesis of androgens
Androgens, like all steroid hormones, originate from cholesterol, and they are also called ‘C19 steroids’ as they have 19 carbon atoms. Androgens can be divided, based on biosynthetic pathways, into two groups:
- C19-A5-steroids derived from A5 pregnans. The progenitor of C19-A5-steroids is dehydroepiandrosterone (DHEA) and its sulfate, dehydroepiandrosterone sulfate (DHEAS), which is interconverted by the steroid sulfatase (sulfotransferase).
- C19-A4-steroids derived from A4 pregnans: androstenedione (A4) and testosterone (T).
The biosynthesis of androgens requires different enzymatic steps in succession [8,9], which we have summarized in Figure 1.
Based on their metabolic activity, there are three groups of endogenous androgens: precursors androgens (DHEA, DHEAS, and androstenedione), testosterone and its metabolites (such as dihydrotestosterone-DHT and androstenediol).
Testosterone and androstenedione are ‘aromatizable androgens.’ They are metabolized by aromatase to estrogens (estradiol-E2 and estrone-E1, respectively), which then interact with estrogen receptors (ER).
The enzymatic activity of aromatase and the conversion of circulating androgens into estrogens is clinically significant in women and men with aromatase deficiencies, and its regulation in tissues needs to be elucidated [10,11].
2.2. Sources of circulating androgens in women: ‘endocrinology’ and ‘intracrinology.”
In women, the synthesis of androgens takes place in many tissues such as adrenal glands (reticular zone) and ovaries (theca cells of the ovarian follicle) which are considered classical steroidogenic organs. However, many peripheral tissues, including adipose tissue, skin, vagina, endometrial tissue, mammary gland, and nervous systems, are capable of synthesizing androgens [12-16].
While the ovary and adrenal glands contribute less to the amount of circulating active androgen, they produce significant levels of the inactive androgen precursors: DHEA, its sulfate ester DHEAS (only adrenal glands), and androstenedione, which are the most abundant released into the circulation [17].
This androgen production occurs under the pituitary stimulus of luteinizing hormone (LH) and adrenocorticotropic hormone (ACTH), respectively.
On the other hand, most of the active androgens come from peripheral metabolism/conversion that produces 50% of the circulating testosterone, and this is a critical step for women.
This local control of androgen action by metabolic activation of precursors and subsequent inactivation has been termed ‘intracrinology’ and was first conceptualized by Labrie [12,18,19].
The concept of intracrinology also underlines that a substantial amount of testosterone is metabolized from the inactive precursor DHEAS intracellularly in the target tissue, and it is not present in the peripheral blood. Similarly, circulating concentrations of DHT are low, but this is largely due to local metabolism within target tissues at the site of action.
Different tissues can participate in the modulation of the circulating levels of androgens in relation to the quantity and activity of the enzymes available in the tissue, specifically to the ratio between 5a-reductase (which transforms testosterone into DHT and androstenediol) and aromatase (which transforms A4 androgens into estrogens).
Peripheral tissues are able to metabolize and transform androgens into more or less active or completely inactive metabolites. Moreover, unlike the steroidogenic organs, peripheral tissues are not under the control of LH and ACTH, which are able to modulate enzymatic activity and secretion of a specific steroid in circulation.
The importance of androgen distribution and production in women is schematized in Figure 2, according to Simon JA et al. [20].
2.3. Peripheral steroidogenic tissue in women
The synthesis of androgens in women can take place in several peripheral tissues defined as follows:
* The adipose tissue is rich in aromatase which transforms A4 androgens into estrogens, and it has all the necessary enzymes for the activation of androgenic precursors and their subsequent inactivation for excretion, thereby regulating the local androgenic milieu. In female adipose tissue, aldo-ketoreductase 1C3 (AKR1C3, also named 17B-HSD type 5, HSD17B5) converts androstenedione to testosterone and interconverts estrogens and androgens [12].
*Skin has the capability to produce androgens both de novo from cutaneous cholesterol or using ovarian or adrenal circulating precursors, such as DHEA, through specific enzymatic activities. The pilosebaceous unit expresses key enzymes involved in sex hormone syntheses, such as CYP11A1, CYP17A1, 3BHSD, and CYP19A1 [13]. However, the main precursor used to produce steroids is adrenal DHEAS. DHEAS is hydrolyzed to DHEA by the sulfatase located in sebaceous glands and dermal papilla cells in terminal hair follicles, whereas the enzymatic activity of 3B-HSD1 converts DHEA into androstenedione, and 17B-HSD3 converts androstenedione in testosterone [13].
*Vulvovaginal tissue is an androgen-target organ with the ability to synthesize androgens. It should be remembered that DHEA is transformed into both androgens and estrogens in the vagina [21,22]. A recent study has shown the ability of human vagina smooth muscle cells to synthesize androgens from the upstream precursor, DHEA. Increased expression of pro-androgenic steroidogenic enzymes (HSD3B1/B2, HSD17B3/B5), 5a-reductase isoforms, and sulfotransferase mRNAs were detected in the vaginal tissue compared to the ovarian one. In addition, enzymes involved in androgen inactivation were less abundant in the vagina than in the ovaries [14].
*Endometrial tissue has a role in the local synthesis of androgens and signaling via intracrine mechanisms within the endometrium [23]. The expression and activity of androgen-metabolizing enzymes within the endometrium may represent a key mechanism for controlling steroid bioavailability within the tissue and regulation of endometrial functions. These enzymes include CYP11A1 and CYP17A1 [24], 3BHSD, which were detected in glandular epithelial cells in proliferative and secretory phase endometrial samples [25], 17 BHSD and importantly aldo-keto reductase family 1 member C3 (AKR1C3; also known as 17BHSD5) that is the most efficient enzyme for the conversion of androstenedione to testosterone in the endometrium and it is localized into glandular and luminal epithelial cells throughout the menstrual cycle [26,27], CYP19A1, that is induced upon decidualization in endometrial stromal cells (ESCs). Following the induction of aromatase expression in decidualized ESCs, local estrogens regulate immune-mediated vascular remodeling by altering the function of uterine Natural killer (uNK) cells in early pregnancy [28].
Many functional processes in the endometrium, including cell proliferation, apoptosis, resistance to oxidative stress, and cell motility, are under the control of endocrine and intracrine androgens. Furthermore, processes such as decidualization are influenced by the biosynthetic enzymes of androgens. Thus, androgens can be considered key players during the onset of pregnancy, and androgen-metabolizing enzymes might become therapeutic targets for the treatment of infertility associated with endometrial dysfunction [15].
*Mammary gland expresses enzymes involved in androgenic synthesis [8]. The epithelium lining the acini and ducts of the mammary gland is composed of two layers, an inner epithelial layer and an outer discontinuous layer of myoepithelial cells. By immunocytochemistry, 3BHSD is detected in the epithelial cells of acini and ducts as well as in stromal fibroblasts. Also, immunostaining for type 5 17BHSD gives similar results as those seen for 3BHSD, observed in epithelial, stromal cells, and in blood vessel walls [29].
*Nervous system Neurosteroids, which include DHEAS, testosterone, and their metabolites, are synthesized in the central and peripheral nervous systems (CNS-PNS). Neurosteroids interact with non-sex hormone receptors and are able to influence excitability and neuronal function [16], and their synthesis plays a protective and restorative role in survival and neurodegeneration [30].
2.4. Androgens biosynthesis in human placenta
Since the discovery of the fetoplacental unit [31], many previously unknown roles of the placenta have been investigated. The placenta is the major site of steroid hormone synthesis during pregnancy and is considered a ‘classical steroidogenic organ,” along with the adrenal glands and ovaries. The primary role of the placenta is to synthesize and produce estrogen and progesterone, both necessary for the maintenance of pregnancy. It is known that androgens are the substrates for the synthesis of estrogen, thus having a fundamental role in placenta functions (Figure 3).
3. Body circulation: androgen binding proteins
4. Mechanism of action of androgens: the androgen receptor (AR), the genomic and non-genomic mechanism of action
5. Target organs for androgens in women
Androgen receptors mediate crucial functions in several female tissues, such as the female genital system (ovary, endometrial tissue, vulvovaginal tissue), breast, skin, bone, muscle, adipose tissue, liver, platelets, blood vessels, immune system, kidney, and nervous system (Table 1)
5.1. Female genital system
5.1.1. -Ovary
In addition to being one of the principal organs of steroidogenesis, the ovaries are also the target organ of androgens. Androgens have a crucial role in regulating ovarian function and fertility and in all phases of follicular development and ovulation.
5.1.2. -Endometrial tissue
Androgens can control the physiological and pathophysiological conditions of the endometrium through AR-mediated pathways and indirectly as precursors of local estrogen synthesis [71].
5.1.3. -Vulvovaginal tissue
The vagina is both a synthesis and an androgen-target organ. Androgens play an essential role in the differentiation of the vagina and in maintaining trophic and functional actions in postnatal life.
5.2. Breast
Androgens play a crucial role in breast cell proliferation, directly and indirectly, through their aromatization into estrogen [86].
5.3. Skin
The skin constitutes a critical peripheral steroidogenic tissue and an androgen-target organ.
5.4. Nervous system
The nervous system is a place of steroidogenesis and a target of androgenic action. Androgens can increase neuronal cells’ survival, stimulation differentiation, and plasticity [92-94].
ARs are abundantly expressed both in the CNS and in the PNS.
a) Central nervous system (CNS)
b) Peripheral nervous systems (PNS)
5.5. Bone tissue
Current evidence suggests that circulating androgens and estrogens protect bone tissue and structure.
5.6. Muscle
Androgens increase muscle size and strength [108]. This anabolic effect is not only explained by the activation of the myocyte AR, but is also the combined result of many genomic and non-genomic actions [109]. Women with PCOS and higher androgen levels have greater muscle mass and better competitive performance [110,111].
5.7. Adipose tissue
Adipose tissue is both an important peripheral steroidogenic tissue and an androgen-target organ. Androgens are key modulators of body fat distribution in both men and women. Importantly, androgens are responsible for accumulating adipose tissue, especially at the visceral and abdominal levels. It seems that they are also accountable for the increase in the volume of adipocytes [115,116]. This distribution of body fat results from the local transformation of steroids by enzymes specifically expressed in adipose tissue, underlining the fundamental role of intracrinology.
5.8. Liver
Androgens act on the liver in different ways like:
-stimulating the synthesis of lipoproteins (LDL cholesterol) triglycerides [122] and increasing insulin resistance or reducing insulin sensitivity. The mechanisms of how androgens induce hyperinsulinemia and insulin resistance are not clear.
-stimulating the synthesis of coagulation factors, such as fibrinogen and ATIII [125].
5.9. Platelet and vessels
Several studies indicate that androgens can increase Thromboxane A2 (TXA2) receptors to activate platelets, one of the central mechanisms responsible for hemostasis and atherosclerotic processes [126].
However, more recent studies have shown that testosterone can inhibit platelet aggregation with a mechanism dependent on nitric oxide (NO) synthesis [127,128]. Furthermore, testosterone plays a role in blood vessels, acting as a potent vasodilator, both in an endothelium-dependent and an endothelium-independent way. The former mechanism assumes that testosterone leads to the increased synthesis of NO [128,129]. The second endothelium-independent relaxation mechanism has been demonstrated in isolated rabbit coronary arteries and aorta [130].
5.10. Immune system
Sex hormones can regulate several processes in the immune system, and ARs are detected in many different hematopoietic cells. It would appear that androgens play an immunosuppressive role, and some studies show that androgen deprivation therapy can induce expansion and increase the T-cell response [133].
5.11. Androgens and female sexual function
For women, the correlation between circulating androgens and sexual desire is still a matter of debate today. In fact, it is well-known that women's sexuality is influenced by various biological, psychological, and social factors. Moreover, apart from the complex multidimensional nature of sexual desire across the reproductive lifespan, the correlation between measurements of testosterone and specific signs and symptoms has been difficult because most available assays are unreliable [135]. However, despite this, it is believed that androgens play an independent role in women’s arousability and pleasure as well as intensity and ease of orgasm [136). Moreover, the androgen milieu and sexual desire in women seem to be tightly linked because they both decline with age.
As we have seen, there are multiple ways androgens target the brain regions (hypothalamic, limbic, and cortical) involved in sexual function and behavior. Testosterone appears to play a key role in the motivational components of women’s sexuality.
6. Androgens and female pubertal development
During intrauterine life, the fetal adrenal cortex produces DHEA and DHEAS, a substrate for the placental production of estrogen and androgen. However, after birth, the adrenal cortex of the fetus undergoes involution, and the concentrations of DHEA and DHEAS decrease.
During childhood, around 8-9 years, the reticular area of the adrenal cortex produces increased amounts of androgenic precursors, which leads to numerous physiognomic changes. This phase is a fundamental stage of sexual maturation and takes the name of ‘adrenarche’ [142]. Adrenarche is probably a progressive maturation process from early childhood [143].
7. How androgen levels vary in the menstrual cycle
Circulating concentrations of androgens undergo variations throughout the menstrual cycle, exceeding those of estrogens [150,151]. Testosterone is at its lowest concentrations in the early follicular phase of the cycle, rises to a mid-cycle peak, and the luteal phase concentrations are higher than those in the early follicular phase [152]. In the menstrual period, plasma concentrations of testosterone are relatively high, whereas estrogen and progesterone levels progressively decline [153].
Total and free testosterone levels peak at midcycle, coincident with E2, E1 and SHBG levels and follicular development, whereas DHT did not change. These data support the hypothesis that the changes in testosterone across the cycle may reflect changes in SHBG and estrogen [151].
8. How androgen levels vary according to women's age
The serum androgen levels decline steeply in the early reproductive years, with flattening out during middle age (around 40 years old) and a slight tendency to increase in the following years [158] (Figure 4).
9. Conclusion
DHEAS, DHEA, androstenedione, testosterone, and DHT in women are present with decreasing concentration levels. Specifically, DHEAS, DHEA, and androstenedione can be transformed intracellularly into small amounts of active androgens. On the other hand, the same precursors, when inactive, can maintain low contractions within the tissues. This local hormonal control, activation, and inactivation mechanism has been termed ‘intracrinology.’
The central message of this review is that androgens play essential roles in women’s physiology. They act in different target organs, directly, by binding to the ARs, or indirectly, after their aromatization into estrogens.
Androgens in women are essential for reproductive competency, sexual function, cardiovascular health, appropriate bone remodeling, muscle tone, and mass and brain function. In particular, androgens seem to play a beneficial role in follicular development and regulating pregnancy establishment and maintenance. Physiologically, the androgen levels in women change with the menstrual cycle and, unlike estrogens, are reduced already in the first decades of life, largely independent of menopause. This decrease with aging is due to reduced production rather than altered metabolism.
Most importantly, after menopause, DHEA becomes the exclusive source of both estrogens and androgens.
Overall, the picture emerging from our investigations is that physiological levels of androgens are essential for women’s health throughout life. Changes in the availability of circulating androgens may impact the regulation of numerous physiological processes.
In this context, diagnosis of androgen deficiency or excess in women is of clinical relevance because restoring physiological levels of androgens is essential in the prevention and treatment of many diseases.
10. Expert opinion
From a clinical perspective, this review provides physiological data on androgen action in women that are an essential basis for further investigation into female androgen insufficiency or excess.
To unravel the distinctive pathological effects of androgens in women, it is necessary to deeply understand their physiological actions in the various tissues under normal conditions. As we have seen, androgens are known to regulate many processes in female physiology, but the potential role and precise mechanism of action in some of them are poorly understood. The significant factors that contribute to this ‘knowledge gap’ include;
1. The difficulty of dosing androgens with standard laboratory methods,
2. The difficulty of taking into account the diurnal and cyclical variations in androgen levels for blood sampling, and
3. The uncertainty of what is considered normal in serum androgen levels in women of different ages and, in particular, the absence of specific cutoffs for female hypoandrogenism.
Since the bias mentioned above influences the studies on this topic, it would be necessary to perform prospective, longitudinal studies on larger populations [158].
Amongst the mechanisms that need further investigation, is the potential role of androgens in regulating pregnancy. From this perspective, some therapies, such as selective AR modulators, could be used to improve reproductive outcomes. Another topic to investigate is the function of AR in several structures of the CNS involved in cognitive processes. Furthermore, the molecular mechanisms behind decreasing DHEA and DHEAS levels with advancing age are also unclear. Therefore, more studies are needed to elucidate the selective reduction of the adrenal reticular zone, responsible for the production and secretion of androgens, with increasing age.
Finally, existing data do not permit an evaluation of the reciprocal regulation between androgens and estrogens, mainly their receptors and transcription factors. The remedy to this unsatisfactory state of affairs will develop more effective therapeutic protocols for managing androgen insufficiency or excess.
Elena Rosato, Francesca Sciarra, Eleni Anastasiadou, Andrea Lenzi & Mary Anna Venneri
ABSTRACT
Introduction: Extensive research underlines the critical functions of androgens in females. Nevertheless, the precise mechanisms of their action are poorly understood. Here, we review the existing literature regarding the physiological role of androgens in women throughout life.
Areas covered: Several studies show that androgen receptors (ARs) are broadly expressed in numerous female tissues. They are essential for many physiological processes, including reproductive, sexual, cardiovascular, bone, muscle, and brain health. They are also involved in adipose tissue and liver function. Androgen levels change with the menstrual cycle and decrease in the first decades of life, independently of menopause.
Expert opinion: To date, studies are limited by including small numbers of women, the difficulty of dosing androgens, and their cyclical variations. In particular, whether androgens play any significant role in regulating the establishment of pregnancy is poorly understood. The neural functions of ARs have also been investigated less thoroughly, although it is expressed at high levels in brain structures. Moreover, the mechanism underlying the decline of dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) with age is unclear. Other factors, including estrogen’s effect on adrenal androgen production, reciprocal regulation of ARs, and non-classical effects of androgens, remain to be determined.
1. Introduction
Androgens (from the Greek ‘andro,” meaning male or man) are traditionally considered male sex steroid hormones responsible for the maintenance of male characteristics, while knowledge of the roles of androgens in women is limited. However, it is now established that they play essential roles in women’s physiology and are the most abundant sex hormones in females. Considering pg/ml as a unit of measurement, in women of reproductive age, while estrogens vary from 50 to 300, androgens and their precursors circulate in considerably greater levels: testosterone around 400, androstenedione around 2000, DHEA around 5000 [1].
Furthermore, several studies show that androgen receptors (ARs) are broadly expressed in several female tissues (urogenital tissue, mammary gland, nervous systems, bone, cardiovascular system, adipose tissue, and liver), where they mediate crucial functions and until recently were thought to be only targets of estrogen.
The limiting factors contributing to this ‘knowledge gap’ are the lack of more accurate laboratory methods for dosing androgens and the uncertainty of age and gender-specific reference range.
*This review aims to highlight the most recent findings on the role of androgens in female physiology, which is to have a firm basis for investigating clinically relevant situations of androgen insufficiency or excess throughout a woman’s life (childhood, reproductive age, and menopause).
Hyperandrogenism is a heterogeneous group of disorders characterized by elevated androgen levels that exhibit a typical phenotype. Signs and symptoms of excessive androgen secretion in women include hirsutism, acne, seborrhea, androgenic alopecia, and menstrual disorders [2].
The most common hyperandrogenic disorder in women of reproductive age is polycystic ovary syndrome (PCOS). The latter is a condition caused by an imbalance of sex hormones that can lead to menstrual cycle changes, ovarian cysts, difficulty conceiving, and other health changes, with approximately an 80-85% prevalence among women with androgen excess [3]. Other less frequent conditions include idiopathic hirsutism, androgenic drug intake, Cushing’s syndrome, nonclassical congenital adrenal hyperplasia (NCAH), acanthosis nigricans (HAIRAN), ovarian or adrenal androgen-secreting neoplasms (ASN), and hyperprolactinemia [4,5].
Studies on women suffering from gender dysphoria (female to male trans-sexual subjects) demonstrate the relevance of androgen effects in women: the long-term administration of androgens is associated with cardiovascular risk, malignancies, and mental health problems such as depression and anxiety. However, future studies should aim to explore the long-term outcome of androgens treatment in transgender women [6].
Today we also know that not only an excess of androgens but also a reduction in androgen levels can cause various pathological conditions: a hypoandrogenic state is detrimental to cardiovascular, mental, and sexual health. In this regard, the Endocrine Society recommends using testosterone in women only in the case of HSDD (hypoactive sexual desire disorder) [7]. However, the long-term safety of treatments with testosterone remains to be evaluated.
Androgen insufficiency, affecting not only postmenopausal women but also young women (for example when using contraceptives), is an increasingly debated topic in recent years, especially for the absence of specific cutoffs for female hypoandrogenism.
2. Synthesis and androgen-producing organs in women
2.1. Synthesis of androgens
Androgens, like all steroid hormones, originate from cholesterol, and they are also called ‘C19 steroids’ as they have 19 carbon atoms. Androgens can be divided, based on biosynthetic pathways, into two groups:
- C19-A5-steroids derived from A5 pregnans. The progenitor of C19-A5-steroids is dehydroepiandrosterone (DHEA) and its sulfate, dehydroepiandrosterone sulfate (DHEAS), which is interconverted by the steroid sulfatase (sulfotransferase).
- C19-A4-steroids derived from A4 pregnans: androstenedione (A4) and testosterone (T).
The biosynthesis of androgens requires different enzymatic steps in succession [8,9], which we have summarized in Figure 1.
Based on their metabolic activity, there are three groups of endogenous androgens: precursors androgens (DHEA, DHEAS, and androstenedione), testosterone and its metabolites (such as dihydrotestosterone-DHT and androstenediol).
Testosterone and androstenedione are ‘aromatizable androgens.’ They are metabolized by aromatase to estrogens (estradiol-E2 and estrone-E1, respectively), which then interact with estrogen receptors (ER).
The enzymatic activity of aromatase and the conversion of circulating androgens into estrogens is clinically significant in women and men with aromatase deficiencies, and its regulation in tissues needs to be elucidated [10,11].
2.2. Sources of circulating androgens in women: ‘endocrinology’ and ‘intracrinology.”
In women, the synthesis of androgens takes place in many tissues such as adrenal glands (reticular zone) and ovaries (theca cells of the ovarian follicle) which are considered classical steroidogenic organs. However, many peripheral tissues, including adipose tissue, skin, vagina, endometrial tissue, mammary gland, and nervous systems, are capable of synthesizing androgens [12-16].
While the ovary and adrenal glands contribute less to the amount of circulating active androgen, they produce significant levels of the inactive androgen precursors: DHEA, its sulfate ester DHEAS (only adrenal glands), and androstenedione, which are the most abundant released into the circulation [17].
This androgen production occurs under the pituitary stimulus of luteinizing hormone (LH) and adrenocorticotropic hormone (ACTH), respectively.
On the other hand, most of the active androgens come from peripheral metabolism/conversion that produces 50% of the circulating testosterone, and this is a critical step for women.
This local control of androgen action by metabolic activation of precursors and subsequent inactivation has been termed ‘intracrinology’ and was first conceptualized by Labrie [12,18,19].
The concept of intracrinology also underlines that a substantial amount of testosterone is metabolized from the inactive precursor DHEAS intracellularly in the target tissue, and it is not present in the peripheral blood. Similarly, circulating concentrations of DHT are low, but this is largely due to local metabolism within target tissues at the site of action.
Different tissues can participate in the modulation of the circulating levels of androgens in relation to the quantity and activity of the enzymes available in the tissue, specifically to the ratio between 5a-reductase (which transforms testosterone into DHT and androstenediol) and aromatase (which transforms A4 androgens into estrogens).
Peripheral tissues are able to metabolize and transform androgens into more or less active or completely inactive metabolites. Moreover, unlike the steroidogenic organs, peripheral tissues are not under the control of LH and ACTH, which are able to modulate enzymatic activity and secretion of a specific steroid in circulation.
The importance of androgen distribution and production in women is schematized in Figure 2, according to Simon JA et al. [20].
2.3. Peripheral steroidogenic tissue in women
The synthesis of androgens in women can take place in several peripheral tissues defined as follows:
* The adipose tissue is rich in aromatase which transforms A4 androgens into estrogens, and it has all the necessary enzymes for the activation of androgenic precursors and their subsequent inactivation for excretion, thereby regulating the local androgenic milieu. In female adipose tissue, aldo-ketoreductase 1C3 (AKR1C3, also named 17B-HSD type 5, HSD17B5) converts androstenedione to testosterone and interconverts estrogens and androgens [12].
*Skin has the capability to produce androgens both de novo from cutaneous cholesterol or using ovarian or adrenal circulating precursors, such as DHEA, through specific enzymatic activities. The pilosebaceous unit expresses key enzymes involved in sex hormone syntheses, such as CYP11A1, CYP17A1, 3BHSD, and CYP19A1 [13]. However, the main precursor used to produce steroids is adrenal DHEAS. DHEAS is hydrolyzed to DHEA by the sulfatase located in sebaceous glands and dermal papilla cells in terminal hair follicles, whereas the enzymatic activity of 3B-HSD1 converts DHEA into androstenedione, and 17B-HSD3 converts androstenedione in testosterone [13].
*Vulvovaginal tissue is an androgen-target organ with the ability to synthesize androgens. It should be remembered that DHEA is transformed into both androgens and estrogens in the vagina [21,22]. A recent study has shown the ability of human vagina smooth muscle cells to synthesize androgens from the upstream precursor, DHEA. Increased expression of pro-androgenic steroidogenic enzymes (HSD3B1/B2, HSD17B3/B5), 5a-reductase isoforms, and sulfotransferase mRNAs were detected in the vaginal tissue compared to the ovarian one. In addition, enzymes involved in androgen inactivation were less abundant in the vagina than in the ovaries [14].
*Endometrial tissue has a role in the local synthesis of androgens and signaling via intracrine mechanisms within the endometrium [23]. The expression and activity of androgen-metabolizing enzymes within the endometrium may represent a key mechanism for controlling steroid bioavailability within the tissue and regulation of endometrial functions. These enzymes include CYP11A1 and CYP17A1 [24], 3BHSD, which were detected in glandular epithelial cells in proliferative and secretory phase endometrial samples [25], 17 BHSD and importantly aldo-keto reductase family 1 member C3 (AKR1C3; also known as 17BHSD5) that is the most efficient enzyme for the conversion of androstenedione to testosterone in the endometrium and it is localized into glandular and luminal epithelial cells throughout the menstrual cycle [26,27], CYP19A1, that is induced upon decidualization in endometrial stromal cells (ESCs). Following the induction of aromatase expression in decidualized ESCs, local estrogens regulate immune-mediated vascular remodeling by altering the function of uterine Natural killer (uNK) cells in early pregnancy [28].
Many functional processes in the endometrium, including cell proliferation, apoptosis, resistance to oxidative stress, and cell motility, are under the control of endocrine and intracrine androgens. Furthermore, processes such as decidualization are influenced by the biosynthetic enzymes of androgens. Thus, androgens can be considered key players during the onset of pregnancy, and androgen-metabolizing enzymes might become therapeutic targets for the treatment of infertility associated with endometrial dysfunction [15].
*Mammary gland expresses enzymes involved in androgenic synthesis [8]. The epithelium lining the acini and ducts of the mammary gland is composed of two layers, an inner epithelial layer and an outer discontinuous layer of myoepithelial cells. By immunocytochemistry, 3BHSD is detected in the epithelial cells of acini and ducts as well as in stromal fibroblasts. Also, immunostaining for type 5 17BHSD gives similar results as those seen for 3BHSD, observed in epithelial, stromal cells, and in blood vessel walls [29].
*Nervous system Neurosteroids, which include DHEAS, testosterone, and their metabolites, are synthesized in the central and peripheral nervous systems (CNS-PNS). Neurosteroids interact with non-sex hormone receptors and are able to influence excitability and neuronal function [16], and their synthesis plays a protective and restorative role in survival and neurodegeneration [30].
2.4. Androgens biosynthesis in human placenta
Since the discovery of the fetoplacental unit [31], many previously unknown roles of the placenta have been investigated. The placenta is the major site of steroid hormone synthesis during pregnancy and is considered a ‘classical steroidogenic organ,” along with the adrenal glands and ovaries. The primary role of the placenta is to synthesize and produce estrogen and progesterone, both necessary for the maintenance of pregnancy. It is known that androgens are the substrates for the synthesis of estrogen, thus having a fundamental role in placenta functions (Figure 3).
3. Body circulation: androgen binding proteins
4. Mechanism of action of androgens: the androgen receptor (AR), the genomic and non-genomic mechanism of action
5. Target organs for androgens in women
Androgen receptors mediate crucial functions in several female tissues, such as the female genital system (ovary, endometrial tissue, vulvovaginal tissue), breast, skin, bone, muscle, adipose tissue, liver, platelets, blood vessels, immune system, kidney, and nervous system (Table 1)
5.1. Female genital system
5.1.1. -Ovary
In addition to being one of the principal organs of steroidogenesis, the ovaries are also the target organ of androgens. Androgens have a crucial role in regulating ovarian function and fertility and in all phases of follicular development and ovulation.
5.1.2. -Endometrial tissue
Androgens can control the physiological and pathophysiological conditions of the endometrium through AR-mediated pathways and indirectly as precursors of local estrogen synthesis [71].
5.1.3. -Vulvovaginal tissue
The vagina is both a synthesis and an androgen-target organ. Androgens play an essential role in the differentiation of the vagina and in maintaining trophic and functional actions in postnatal life.
5.2. Breast
Androgens play a crucial role in breast cell proliferation, directly and indirectly, through their aromatization into estrogen [86].
5.3. Skin
The skin constitutes a critical peripheral steroidogenic tissue and an androgen-target organ.
5.4. Nervous system
The nervous system is a place of steroidogenesis and a target of androgenic action. Androgens can increase neuronal cells’ survival, stimulation differentiation, and plasticity [92-94].
ARs are abundantly expressed both in the CNS and in the PNS.
a) Central nervous system (CNS)
b) Peripheral nervous systems (PNS)
5.5. Bone tissue
Current evidence suggests that circulating androgens and estrogens protect bone tissue and structure.
5.6. Muscle
Androgens increase muscle size and strength [108]. This anabolic effect is not only explained by the activation of the myocyte AR, but is also the combined result of many genomic and non-genomic actions [109]. Women with PCOS and higher androgen levels have greater muscle mass and better competitive performance [110,111].
5.7. Adipose tissue
Adipose tissue is both an important peripheral steroidogenic tissue and an androgen-target organ. Androgens are key modulators of body fat distribution in both men and women. Importantly, androgens are responsible for accumulating adipose tissue, especially at the visceral and abdominal levels. It seems that they are also accountable for the increase in the volume of adipocytes [115,116]. This distribution of body fat results from the local transformation of steroids by enzymes specifically expressed in adipose tissue, underlining the fundamental role of intracrinology.
5.8. Liver
Androgens act on the liver in different ways like:
-stimulating the synthesis of lipoproteins (LDL cholesterol) triglycerides [122] and increasing insulin resistance or reducing insulin sensitivity. The mechanisms of how androgens induce hyperinsulinemia and insulin resistance are not clear.
-stimulating the synthesis of coagulation factors, such as fibrinogen and ATIII [125].
5.9. Platelet and vessels
Several studies indicate that androgens can increase Thromboxane A2 (TXA2) receptors to activate platelets, one of the central mechanisms responsible for hemostasis and atherosclerotic processes [126].
However, more recent studies have shown that testosterone can inhibit platelet aggregation with a mechanism dependent on nitric oxide (NO) synthesis [127,128]. Furthermore, testosterone plays a role in blood vessels, acting as a potent vasodilator, both in an endothelium-dependent and an endothelium-independent way. The former mechanism assumes that testosterone leads to the increased synthesis of NO [128,129]. The second endothelium-independent relaxation mechanism has been demonstrated in isolated rabbit coronary arteries and aorta [130].
5.10. Immune system
Sex hormones can regulate several processes in the immune system, and ARs are detected in many different hematopoietic cells. It would appear that androgens play an immunosuppressive role, and some studies show that androgen deprivation therapy can induce expansion and increase the T-cell response [133].
5.11. Androgens and female sexual function
For women, the correlation between circulating androgens and sexual desire is still a matter of debate today. In fact, it is well-known that women's sexuality is influenced by various biological, psychological, and social factors. Moreover, apart from the complex multidimensional nature of sexual desire across the reproductive lifespan, the correlation between measurements of testosterone and specific signs and symptoms has been difficult because most available assays are unreliable [135]. However, despite this, it is believed that androgens play an independent role in women’s arousability and pleasure as well as intensity and ease of orgasm [136). Moreover, the androgen milieu and sexual desire in women seem to be tightly linked because they both decline with age.
As we have seen, there are multiple ways androgens target the brain regions (hypothalamic, limbic, and cortical) involved in sexual function and behavior. Testosterone appears to play a key role in the motivational components of women’s sexuality.
6. Androgens and female pubertal development
During intrauterine life, the fetal adrenal cortex produces DHEA and DHEAS, a substrate for the placental production of estrogen and androgen. However, after birth, the adrenal cortex of the fetus undergoes involution, and the concentrations of DHEA and DHEAS decrease.
During childhood, around 8-9 years, the reticular area of the adrenal cortex produces increased amounts of androgenic precursors, which leads to numerous physiognomic changes. This phase is a fundamental stage of sexual maturation and takes the name of ‘adrenarche’ [142]. Adrenarche is probably a progressive maturation process from early childhood [143].
7. How androgen levels vary in the menstrual cycle
Circulating concentrations of androgens undergo variations throughout the menstrual cycle, exceeding those of estrogens [150,151]. Testosterone is at its lowest concentrations in the early follicular phase of the cycle, rises to a mid-cycle peak, and the luteal phase concentrations are higher than those in the early follicular phase [152]. In the menstrual period, plasma concentrations of testosterone are relatively high, whereas estrogen and progesterone levels progressively decline [153].
Total and free testosterone levels peak at midcycle, coincident with E2, E1 and SHBG levels and follicular development, whereas DHT did not change. These data support the hypothesis that the changes in testosterone across the cycle may reflect changes in SHBG and estrogen [151].
8. How androgen levels vary according to women's age
The serum androgen levels decline steeply in the early reproductive years, with flattening out during middle age (around 40 years old) and a slight tendency to increase in the following years [158] (Figure 4).
9. Conclusion
DHEAS, DHEA, androstenedione, testosterone, and DHT in women are present with decreasing concentration levels. Specifically, DHEAS, DHEA, and androstenedione can be transformed intracellularly into small amounts of active androgens. On the other hand, the same precursors, when inactive, can maintain low contractions within the tissues. This local hormonal control, activation, and inactivation mechanism has been termed ‘intracrinology.’
The central message of this review is that androgens play essential roles in women’s physiology. They act in different target organs, directly, by binding to the ARs, or indirectly, after their aromatization into estrogens.
Androgens in women are essential for reproductive competency, sexual function, cardiovascular health, appropriate bone remodeling, muscle tone, and mass and brain function. In particular, androgens seem to play a beneficial role in follicular development and regulating pregnancy establishment and maintenance. Physiologically, the androgen levels in women change with the menstrual cycle and, unlike estrogens, are reduced already in the first decades of life, largely independent of menopause. This decrease with aging is due to reduced production rather than altered metabolism.
Most importantly, after menopause, DHEA becomes the exclusive source of both estrogens and androgens.
Overall, the picture emerging from our investigations is that physiological levels of androgens are essential for women’s health throughout life. Changes in the availability of circulating androgens may impact the regulation of numerous physiological processes.
In this context, diagnosis of androgen deficiency or excess in women is of clinical relevance because restoring physiological levels of androgens is essential in the prevention and treatment of many diseases.
10. Expert opinion
From a clinical perspective, this review provides physiological data on androgen action in women that are an essential basis for further investigation into female androgen insufficiency or excess.
To unravel the distinctive pathological effects of androgens in women, it is necessary to deeply understand their physiological actions in the various tissues under normal conditions. As we have seen, androgens are known to regulate many processes in female physiology, but the potential role and precise mechanism of action in some of them are poorly understood. The significant factors that contribute to this ‘knowledge gap’ include;
1. The difficulty of dosing androgens with standard laboratory methods,
2. The difficulty of taking into account the diurnal and cyclical variations in androgen levels for blood sampling, and
3. The uncertainty of what is considered normal in serum androgen levels in women of different ages and, in particular, the absence of specific cutoffs for female hypoandrogenism.
Since the bias mentioned above influences the studies on this topic, it would be necessary to perform prospective, longitudinal studies on larger populations [158].
Amongst the mechanisms that need further investigation, is the potential role of androgens in regulating pregnancy. From this perspective, some therapies, such as selective AR modulators, could be used to improve reproductive outcomes. Another topic to investigate is the function of AR in several structures of the CNS involved in cognitive processes. Furthermore, the molecular mechanisms behind decreasing DHEA and DHEAS levels with advancing age are also unclear. Therefore, more studies are needed to elucidate the selective reduction of the adrenal reticular zone, responsible for the production and secretion of androgens, with increasing age.
Finally, existing data do not permit an evaluation of the reciprocal regulation between androgens and estrogens, mainly their receptors and transcription factors. The remedy to this unsatisfactory state of affairs will develop more effective therapeutic protocols for managing androgen insufficiency or excess.
