Androgens and skin
Androgens relevant to the skin
Among the circulating androgens, dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEA-S)
are predominantly produced in the adrenal cortex. Androstenedione is produced approximately
equally by the adrenal cortex and the ovaries, and somewhat less by the testes [4]. These are weak prohormones that act only after conversion to the more potent androgens,
testosterone and 5α-dihydrotestosterone (DHT). Testosterone is mainly secreted by
the testes in males beginning at puberty; in reproductive-age females it arises almost
equally from the ovary and adrenal cortex by a combination of secretion and conversion
of androstenedione in peripheral organs. DHT is mainly synthesized in peripheral organs,
including skin, in both genders.
Testosterone, particularly free testosterone, is the major circulating androgen because
of its concentration and potency [3]. The less potent DHEA-S is the androgen with by far the highest serum concentration
in both sexes and is related to sebum production prepubertally [5] and to cystic acne in adults [6]. Androstenedione and DHEA have also been shown to stimulate sebum secretion in humans
[7]. Male levels of testosterone stimulate full axillary and pubic hair growth, but
in the absence of DHT formation do not suffice to stimulate male beard growth and
scalp hair recession [8].
The androgen receptor and skin
Testosterone and DHT act through a single nuclear receptor, the androgen receptor
(AR), with DHT being the more active ligand [9]. AR is an X-chromosome-encoded, ligand-activated, intracellular transcription factor
that belongs to the steroid/nuclear receptor superfamily [2]
[10]. Like all nuclear receptors, AR is a soluble molecule with a proclivity for employing
transcriptional regulation as a means of promoting its biological effects. In common
with other steroid receptors, AR is compartmentalized in the cytoplasm, where it exists
in polymeric complexes that include the heat shock proteins hsp 90, hsp 70, and hsp
56. Association of androgens with AR results in dissociation of the heat shock proteins.
This in turn exposes a nuclear translocation signal previously buried in the receptor
structure and initiates transport of the ligand-receptor complex to the nucleus. There,
AR occupies androgen response elements in the promoter regions of androgen-regulated
genes to initiate the signaling cascade.
AR is present in epidermal and follicular keratinocytes, sebocytes, sweat gland cells,
dermal papilla cells, dermal fibroblasts, endothelial cells, and genital melanocytes
[11]
[12]. It is stabilized by ligand binding and is up-regulated in genital skin fibroblasts
and sebocytes [13]
[14].
Polymorphisms that confer enhanced receptor activity have been variably associated
with the androgen-dependent skin disorders. Studies of CAG trinucleotide repeats have
shown them to have been inconsistently related to hirsutism [9]
[15]. Failure to account for possible skewed X-inactivation of the affected alleles,
for which there are likewise conflicting data, contributes to some of the disparities.
A recent linkage study has reported that genetic variability in a shorter GGN repeat
of the AR gene, which confers a higher gene effect, has an etiological fraction of
0.46 for early-onset familial male-pattern baldness, confirming an earlier study [16]. Thus, this may prove to be a more clear-cut determinant of sensitivity to androgen
than the CAG repeat.
Androgen metabolism and its transcriptional regulation in the skin
The skin can be regarded as a peripheral organ that locally synthesizes significant
amounts of androgens with intracrine or paracrine actions [3]
[11]. Having recognized the key effects of biologically active androgens on skin, their
local synthesis and degradation have gained special interest. The local level of each
sex steroid depends upon the expression of each of the androgen- and estrogen-synthesizing
enzymes in each cell type, with sebaceous glands and sweat glands being the major
contributors [9]
[17].
The skin, especially sebocytes, is capable of synthesizing cholesterol, which is utilized
in cell membranes, formation of the epidermal barrier and sebum; however, sebocytes
express very little of the key enzyme, cytochrome P450c17, necessary for synthesis
of the androgenic prohormones DHEA and androstenedione [18]. However, DHEA and androstenedione, and possibly DHEA-S, can be converted by sebocytes
and sweat glands, and probably also by dermal papilla cells, into more potent androgens
like testosterone and DHT [2]. Five major enzymes are involved in the activation and deactivation of androgens
in skin [19]. In a first step, steroid sulfatase hydrolyzes DHEA-S to DHEA in skin [20]. The sebaceous gland has been suggested to carry out this reaction since strong
steroid sulfatase immunoreactivity was found in acne skin, primarily associated with
the monocytes infiltrating the lesions [21], but further evidence is required to corroborate this preliminary report. This enzyme
activity has also been detected in the dermal papillae of human terminal hair follicles
[22]. Subsequently, 3β-hydroxysteroid dehydrogenase/Δ5-4-isomerase (3β-HSD) converts DHEA to androstenedione. Two isoforms of this enzyme
have been described. Human skin seems to express exclusively the type 1 isoform. Several
studies led to the conclusion that type 1 3β-HSD is mainly located in the sebaceous
glands [17].
In a further step, androstenedione is activated by conversion to testosterone through
androgenic 17β-hydroxysteroid dehydrogenase (17β-HSD). The cutaneous expression of
17β-HSD was mainly demonstrated in the pilosebaceous unit and epidermal keratinocytes.
To date, twelve isoforms of this enzyme have been identified [19]
[23]. 17β-HSD types 3 and 5 form the active androgen testosterone from androstenedione,
whereas the oxidative reaction of 17β-HSD types 2 and 4 is in the reverse direction,
inactivating the potent sexual steroids. The human sebaceous gland possesses the cellular
machinery needed to transcribe the genes for 17β-HSD types 1-5; among them a strong
17β-HSD2 and 17β-HSD5 expression have been reported [17]
[18]
[24]. The predominance of the strongly pro-oxidative 17β-HSD2 suggests its protective
role against the effects of locally excessive amounts of potent androgens [18]. Greater reductive activity of androgenic17β-HSD types 3 and 5 was noted in sebaceous
glands from the facial than from non-acne prone areas, suggesting an increased net
production of potent androgens in facial areas. In addition, 17β-HSD3 expression was
detected in human sebocytes, but not in keratinocytes, further indicating the importance
of the sebaceous gland in cutaneous androgen activation [17]. However, scrotal skin 17β-HSD3 and 17β-HSD5 expression levels seem to be age-dependent,
in that 17β-HSD3 mRNA is highly expressed before 2 years of age and again during the
teenage years, whereas 17β-HSD5 mRNA is predominantly expressed between the age 2
and 13 years and again after 30 years of age [24]. In hair follicles, 17β-HSD is localized in outer root sheath cells, where it primarily
inactivates androgens: early studies showed androstenedione as the major metabolite
of cultured human hair follicle keratinocytes incubated with radiolabeled testosterone
[19]. Anagen hairs mainly express high levels of type 2 and moderate levels of type 1
17β-HSD [25]. 17β-HSD enzyme activity has also been shown in cultured epidermal keratinocytes
and in the microdissected apocrine sweat glands.
5α-Reductase irreversibly converts testosterone to DHT, the most potent naturally
occurring androgen in tissue [26]. Two isoforms have been described; type 1 dominates in the skin [27]. Expression of this enzyme is predominant in sebaceous and sweat glands, with lesser
activity in epidermal cells and hair follicles [9]. Type 2 activity predominates in beard hair follicles. Finally, two isoenzymes of
3α-hydroxysteroid dehydrogenase (3α-HSD) catabolize active androgens to compounds
that do not bind the androgen receptor (three isozymes of 3α-HSD were initially identified,
but the type 2 isozyme has been found to predominantly form testosterone and has been
renamed 17β-HSD type 5) [17]
[28]. By glucuronidation or sulfation, water soluble steroid metabolites are eliminated
through the kidney. Alternatively, aromatase may convert testosterone and androstenedione
to estrogens in sebaceous glands, outer as well as inner root sheath cells of anagen
terminal hair follicles, and dermal papilla cells [17]
[29].
More recently, transcription factors regulating steroidogenesis in the classical steroidogenic
organs have been demonstrated in the skin. Thus, a review of current literature provides
a hint that SOX-9 may potentiate steroidogenesis by way of steroidogenic factor-1
(SF-1) to activate the steroidogenic acute regulatory protein (StAR), while DAX-1
exhibits an antagonizing function, and WT-1 plays a bimodal tuning role by up-regulating
DAX-1 [30].
SF-1 and DAX-1 are detected in skin and its appendages with a distinctive expression
pattern. Prominent expression of DAX-1 has been confined to the epidermis, sebaceous
glands, sweat glands, and outer root sheath of the hair follicles with weaker expression
in the inner root sheath, matrix cells, and dermal papilla cells. Similarly, SF-1
has been detected in the epidermis, but displayed a scattered nuclear pattern across
all layers [31]
[32]. SOX-9 and WT-1 were also detected in the skin [30]. These data demonstrate that important regulators of steroidogenesis are present
in human skin and its appendages. These transcription factors may have a role in cutaneous
steroidogenesis and thus be involved in hair follicle pathologies associated with
steroids.
Androgens and the sebaceous gland
Sebaceous gland enlargement and production of sebum are dependent upon androgens,
and at puberty, male sebum production modestly increases over that of females [9]. Skin in acne produces higher rates of testosterone and DHT than in healthy individuals.
In addition, isolated elevations of plasma DHT and 3α-androstenediol glucuronide,
postulated by some to be biochemical markers of cutaneous androgen metabolism and
action, have been found in female patients with acne [33], but there is considerable evidence that these results primarily reflect adrenal
steroid metabolites [9]. Androgens stimulate sebocyte proliferation, an effect dependent on the area of
skin from which the sebaceous glands are obtained; facial sebocytes are mostly affected
[34]. In contrast, androgens as single compounds seem to be unable to modify sebocyte
differentiation, which is stimulated by co-incubation with peroxisome proliferator
activated receptor (PPAR) ligands [35]
[36].
Androgens and the hair follicle
Androgens have strong effects on hair growth and appear to act through type 2 5α-reductase
and the AR on dermal papilla cells [9]
[37]. Single nucleotide polymorphisms of the AR have been associated with hirsutism in
women [38] and androgenetic alopecia in men [16]. Dermal papilla cells appear to mediate the growth-stimulating signals of androgens
by releasing growth factors that act in a paracrine fashion on the other cells of
the follicle [39]. Androgens cause enlargement of hair follicles in androgen-dependent areas (beard
in male adolescents, axillary and pubic hair), but in scalp follicles of susceptible
men paradoxically androgens foster miniaturization and shortage of hair in the anagen
stage leading to common baldness [3]. These contradictory effects may be explained by genetically determined differences
in the response of papilla cells to androgens at different body areas [9]. Notably AR mRNA was reported to be expressed in beard and axillary hair dermal
papilla cells for both sexes, but only at a low level in those from occipital scalp
hair [40]. In addition, very high doses of testosterone and DHT (10 μM) were shown to induce
apoptosis of dermal papilla cells in association with activation of the bcl-2 pathway
[41]. The skin of hirsute women forms excessive DHT, but it is unclear whether this is
formed by the hair follicles themselves or by the associated sebaceous gland hyperplasia
[42]. Men with a deficiency of type 2 5α-reductase provided the initial clue that conversion
of testosterone to the more potent DHT by this enzyme enhances androgenic effects
on hair follicles. These individuals produce little or no beard growth and do not
develop androgenetic alopecia [43]. Subsequently the inhibition of type 2 5α-reductase by finasteride has been proven
to slow or even reverse the progression of androgenetic alopecia [44]. Currently, higher levels of StAR and type 1 3β-HSD were detected in the scalp of
men with androgenetic alopecia [45].
Further effects of androgens on human skin
Androgens appear to promote perspiration since males sweat at a greater rate than
females in similar situations [46]. This difference between the sexes arises during puberty. Sweat glands contain over
half of skin 5α-reducatase activity and express the enzymes necessary to form DHT
from DHEA, as well as AR. However, the sweat gland secretion rate is not directly
influenced by androgens: androgen treatment has not stimulated sweat production in
adult women, and anti-androgen application to the skin of males has not decreased
the sweat rate. Therefore, androgens have been postulated to initiate the factors
required for the different sweat secretion rate between the sexes during puberty,
but do not maintain the function of the glands. It is likely that the androgen effect
is exerted on the differentiation of the apoeccrine sweat glands. This type of sweat
glands, a hybrid of this eccrine and apocrine glands, develops during puberty from
eccrine or eccrine-like precursor glands, but the secretory rate is seven-fold higher
in response to similar innervation. Given that these glands comprise up to 45% of
the axillary glands in patients with hyperhidrosis, they probably play a major role
in the pathophysiology of this condition. Hyperhidrosis has been reported to be the
sole skin manifestation of androgen excess. A few studies have indicated that the
apocrine gland of patients with excessive or abnormal odor (osmidrosis), irrespective
of sex, is a typical androgen target organ [47]. Type 1 5α-reductase predominates in the apocrine sweat glands of such patients
[48].
Adult male skin is thicker and drier than female skin. This is in part because androgen
stimulates epidermal hyperplasia and suppresses epidermal barrier function in fetal
and adult human skin [49]. Testosterone replacement treatment has been reported to have a similar effect on
barrier function in castrated adult mice and an adult man with hypopituitarism. Ashcroft
and Mills [50] observed in a hairless mouse model that endogenous testosterone inhibition of cutaneous
wound healing was AR-mediated. However, further work, especially in humans, is required
to better understand the enhanced inflammatory response suggested by these and other
authors [51]
[52].
Androgens, seborrhea and acne
The obligatory role of androgens in the pathophysiology of acne has long been recognized
and corroborated by clinical and experimental observations and therapeutic experience.
Clinical observations supporting the pathogenic role of androgens in acne, include
close association between the normal onset of microcomedonal acne in prepubertal children
with the adrenarcheal rise in circulating DHEA-S levels [53], acne formation in small children with virilizing tumors or congenital adrenal hyperplasia
(CAH) [54], hyperandrogenism identified in women with sudden exacerbation of acne, persistent
acne beyond 30 years of age and therapy-refractory acne, absence or rarity of acne
in men with androgen insensitivity syndrome or early castration before puberty [55], induction of acne by systemic or topical administration of androgens or anabolic
steroids [56], and positive associations between serum androgen levels and acne lesion counts
in men and women [57]. In vitro studies using sebaceous gland organ culture, rat prepucial glands, and primary culture
or immortalized human sebocytes have all demonstrated the expression of the enzymes
necessary for the synthesis and metabolism of androgens [17]
[27]
[58]
[59]. However, the in vitro effect of supplemented androgens on the cell division and lipogenesis varies, depending
partly on the culture conditions [34]
[35]
[58]
[60]
[61]. Hormonal treatment of acne in female patients using various methods of suppressing
androgen secretion or action suppresses sebum production by 12.5-65% and is beneficial
as monotherapy for female patients with mild to moderate acne [62]
[63].
In addition to stimulation of sebum production, indirect evidence also suggests the
importance of androgens on comedogenesis and inflammation: higher activity of type
1 5α-reductase in the follicular infrainfundibulum, indicating increased capacity
for producing androgens [64] and significant association between inflammatory lesions in adult women with acne
and serum androgens [9]
[65]. On the other hand, in vitro findings indicated that stress factors, such as the corticotrophin- releasing hormone,
increases 3β-HSD mRNA levels, implying that stress and inflammation may also augment
androgenesis in sebocytes [66].
Functional studies are needed to prove the significance of the expression of the steroidogenic
enzymes identified and localized in human skin, especially in sebaceous glands. Moreover,
quantitative differences of the enzyme activities between normal healthy and acne-prone
skin should be precisely determined before the design of potential drugs and the advancement
of their clinical use.
Expression of steroidogenic enzymes in acne patients
The distribution and strong activities of various hydroxysteroid dehydrogenases in
human sebaceous glands in acne-prone as compared to non-acne skin have long been observed
[67]. While there was no difference in the rates of enzymatic hydrolysis of steroid sulfatase
between the freshly obtained epidermal tissue of acne-prone and normal skin [20], a novel non-estrogenic inhibitor of steroid sulfatase, 6-[2-(adamantylidene)hydroxybenzoxazole]-O-sulfamate, was shown to effectively block the enzyme in the skin leading to a reduction
of sebum secretion in animal studies [68]. However, although an exclusive predominance of type 1 5α-reductase has been demonstrated
in sebaceous glands, with higher enzyme activity in facial skin than in nonacne-prone
skin [27]
[69], there seems to be no relationship between the activity of 5α-reductase or 17β-HSD
in sebaceous glands and the presence or absence of acne in both sexes [70]. Moreover, in a 3-month, multicenter, randomized, placebo-controlled clinical trial,
the use of a potent selective inhibitor of type 1 5α-reductase alone or in combination
with systemic minocycline was not associated with clinical improvement of acne [71]. On the other hand, enhanced expression of type 2 5α-reductase was revealed in the
hair follicle but not in the sebaceous glands in inflammatory acne lesions [72]. Greater activity of type 2 17β-HSD, the isozyme working in the opposite oxidizing
direction to inactivate androgens [73], was found in sebaceous glands from nonacne-prone areas as compared to sebaceous
glands obtained from facial skin [18].
Androgen excess
Cutaneous manifestations - seborrhea, acne, hirsutism, male-pattern alopecia, i.e.
, SAHA syndrome when full-blown [74] - are prominent symptoms of peripheral androgen excess. Androgen excess occurs by
increased circulating androgens (hyperandrogenemia) or by increased intracellular
levels of androgens in the skin (hyperandrogenism). Hyperandrogenemia arises from
ovarian or adrenal dysfunction or tumors, disturbed peripheral metabolism of androgen
precursors, or exogenous androgenic medications [3]. Polycystic ovary syndrome, a polymorphic disorder, accounts for over 90 percent
of cases. This hyperandrogenic disorder is often associated with insulin resistance
and acanthosis nigricans in obese women, a combination termed HAIR-AN syndrome [75]. Other important causes, in order of decreasing frequency, are CAH, hyperprolactinemia,
Cushing's syndrome, gonadal or adrenal neoplasms, disorders of sexual differentiation,
and corpus luteum dysfunctions of pregnancy. Exogenous (iatrogenic) causes of androgen
excess include testosterone, anabolic steroids, the androgenic progestin danazol,
and valproic acid [3].
In CAH, defects of enzymes involved in adrenal cortisol synthesis result in inefficient
cortisol biosynthesis [76]. This is compensated by an increased pituitary secretion of adrenocorticotropic
hormone. Thus, normal cortisol blood levels may be achieved, but at the cost of an
excess production of adrenal androgens, which is responsible for clinical signs. The
enzyme most often defective in adrenal hyperplasia (over 95%) is 21-hydroxylase. In
a few instances, other intermediary enzymes are responsible (11β-hydroxylase, 3β-HSD
or P450 oxidoreductase) [76]
[77]. 21-Hydroxylase deficiency is an autosomal recessive disorder. Various mutations
affecting both alleles of the gene lead to variable degrees of impairment of 21-hydroxylase
activity [76]. Severe defects produce classic forms, which manifest themselves in infancy (virilization,
genital ambiguity in girls, and, if mineralocorticosteroids are also deficient, salt
wasting) or in childhood (pseudoprecocious puberty). Mild defects, one of the most
common genetic disorders in man, cause nonclassic, late onset presentations, in which
less severe signs occur later in childhood or during or after puberty including premature
pubarche, acne, hirsutism, or irregular menses; this form may even remain asymptomatic
(cryptic form).
There may be a tendency to underestimate the role of androgen excess in men with acne.
Indeed, a few published studies indicate the relevance of this phenomenon. Men with
persistent acne had significantly higher serum levels of androgens than age-matched
controls [78], and excess androgens of adrenal origin were frequently detected in men with severe
(cystic) acne [6].
Hypogonadism
Skin signs of hypogonadism are thin, weak hair with a reduced number of terminal hair
follicles on the face and the axilla and a feminine pattern of pubic hair, as well
as lack of seborrhoea, acne and androgenetic alopecia, along with penis and testicular
atrophy [79].
Treatment of androgen excess-associated disorders
The major thrust of drug design for treatment of androgen-associated disorders, so
far, has been directed against several levels of androgen function and metabolism
[19]. However, only partial effectiveness has been achieved either by androgen depletion,
inhibition of androgen metabolism or blockade of the AR. In addition, major adverse
events can occur, since effectiveness is only associated with systemic application
of such compounds.
Acne, hirsutism, and androgenetic alopecia of female pattern as manifestations of
systemic or local androgen excess are best treated by eliminating the cause (e.g.,
tumors, drugs) or by interfering with androgen secretion or action. Oral contraceptives
are used in women with polycystic ovary syndrome or idiopathic hirsutism [41]; both estrogens and progestins contribute to the androgen-suppressive effect [62]
[80]. Anti-androgens such as high-dose cyproterone acetate or spironolactone exhibit
the strongest anti-androgenic activity among progestins [41]; they must be used with contraception because anti-androgens interfere with the
differentiation of the genitalia of the fetal male. The combination of drospirenone
with a low dose estrogen seems to be as effective as the combination low-dose cyproterone
acetate and estrogen [80]
[81]. In male acne patients with CAH, low-dose glucocorticoids (e.g., methylprednisolone
4 mg every other day at bedtime) have been used to suppress ACTH-mediated adrenal
androgen production [82]. These hormonal treatments are best combined with other anti-acne regimens for a
quicker relief [83]. Finasteride is the first selective androgen- metabolizing enzyme inhibitor introduced,
targeting androgenetic alopecia in men [19].
Estrogens and skin
Estrogens and human skin
A number of studies have shown that estrogens have many important beneficial and protective
roles in skin physiology [84]
[85]. Estrogens can delay or prevent skin aging manifestations by reducing epidermal
thinning and maintaining skin thickness and hydration [86]
[87]
[88]. In postmenopausal women skin collagen content decreases at the rate of 2% per year
[89], while estradiol treatment can significantly increase hydroxyproline content [90]. Skin elasticity also correlates negatively with years since menopause, while hormone
replacement therapy increases elasticity by 5% over a year [91]. Estrogens also accelerate cutaneous wound healing [92], and many women notice an improvement in inflammatory skin disorders, such as psoriasis
during pregnancy [93]
[94]
[95]. Epidemiological studies indicate that the mortality rates from non-melanoma skin
cancer [96] and melanoma [97] are significantly lower in women.
Binding studies on whole human skin homogenates have demonstrated the existence of
estrogen-binding sites, although receptor levels vary with body site, with higher
numbers in facial skin compared to thigh or breast [98]
[99]. Recent studies have begun to localize the molecular and cellular basis of these
findings.
The estrogen receptors
Two distinct intracellular estrogen receptors (ER), ERα and ERβ, have been identified
that belong to the superfamily of nuclear hormone receptors [100]. ERα and ERβ share approximately 60% homology in the ligand binding domain, but
bind estradiol with a similar affinity [101]. More recently it has been demonstrated that estrogens can also act independently
of their classical genomic pathway [102]. Rapid cellular responses to estrogens, much faster than that can be attributed
to genomic signaling, provide evidence that cell membrane estrogen receptors exist.
These receptors can activate signaling cascades via conventional second messengers,
including adenylate cyclase, cAMP, phospholipase C, protein kinase C, mitogen-activated
protein kinase and ligand or voltage-gated ion channels [103]
[104]
[105]
[106]
[107]
[108]. In some cells ERβ counteracts ERα, in some cases acting as an ERα heterodimer to
inhibit the transactivating function of ERα, and in other cases acting as a homodimer
to regulate specific genes, many of which are anti-proliferative [109].
Estrogens and epidermal keratinocytes
Recently, it has been demonstrated by immunohistochemistry that ERβ is the predominant
estrogen receptor in adult human scalp skin, where it is strongly expressed in the
stratum basale and stratum spinosum of the epidermis [110]
[111]
[112]. A recent study using semi-quantitative RT-PCR has confirmed ERβ mRNA expression
in the skin of the midgestational human fetus [113].
Further work has demonstrated that human epidermal keratinocytes express only ERβ
mRNA and protein by RT-PCR [114] and western blotting [115]. In contrast, an investigation of the high affinity estrogen-binding sites in human
neonatal foreskin epidermal keratinocytes showed expression of both ERα and ERβ by
immunocytochemistry and northern blotting [116]. The difference between these studies of ER expression may be due to culture conditions:
since keratinocytes expressing both receptors were cultured in media containing phenolred,
which has estrogenic activity [117], and estradiol can up-regulate ERα expression in cultured keratinocytes [116], skin ERα may be up-regulated by estrogenic compounds. Recent studies have also
demonstrated that estradiol conjugated with bovine serum albumin can stimulate both
epidermal keratinocyte proliferation and DNA synthesis [118], and estradiol increases phosphorylation levels of ERK1 and ERK2 kinases within
15 minutes in such cells [116]; both are indications that estradiol can activate non-genomic signaling pathways
in the epidermis.
Estrogens and dermal fibroblasts
Primary cultures of human dermal fibroblasts from female skin have been shown to express
both mRNA and protein for ERα and ERβ [119]. Although they co-express both receptors, immunocytochemistry showed some variation
in their expression. ERβ was predominately nuclear, while ERα was expressed in both
the cytoplasm and the nucleus. Furthermore, mRNA levels for ERβ were higher than levels
of ERα. The same group also demonstrated that estradiol up-regulates ERβ expression
in dermal fibroblasts cultured from postmenopausal women [120].
Estrogens and the pilosebaceous unit
The hair follicle
Estrogens appear to stimulate hair growth in man [9]. They are thought to prolong the anagen phase of scalp hair growth by increasing
cell proliferation rates and postponing their transition to the telogen phase. In
an organ culture system, estradiol stimulates hair shaft elongation in fronto-temporal
male hair follicles [121]. Estrogens in low dosage modestly stimulate pubic and axillary hair growth of hypogonadal
girls, independently of changes in androgen levels. It is possible that this effect
of estrogens on hair growth is mediated in part by induction of androgen receptors,
as is the case in brain, or by increase in insulin-like growth factor-I levels. In
late pregnancy, when estrogen levels are high, a high proportion of scalp hair follicles
remain in anagen. Postpartum, a large number of hair follicles simultaneously advance
into telogen phase, causing loss of a large number of hairs. This postpartum telogen
effluvium has been postulated to be caused by the rapid decrease of estrogens at the
time of delivery causing a large number of hair follicles to simultaneously advance
into telogen. On the other hand, estrogens significantly inhibit hair growth in a
number of other mammalian species [84]
[85]. The estradiol-induced delay in the transition from the telogen to anagen phase
in the mouse hair growth cycle appears to be mediated by the ERα of dermal papilla
cells [122]. ER-null and aromatase-null mice are virilized and so have not proven to be a useful
model for understanding the role of estrogens in hair and skin. In situ, immunohistochemical studies have shown that ERβ is strongly expressed in human scalp
anagen hair follicles in contrast to ERα [29]
[112]. While there are no reports of ER expression in human telogen follicles, in the
murine hair cycle ERα expression is maximal in the telogen follicle [123]. Human hair follicles in culture express ERβ, although the distribution pattern
by immunohistochemistry appears to be gender-specific [124]: ERβ immunoreactivity predominated in the female follicular dermal papilla, which
appears to determine the type of hair produced [125], and although basal levels were much lower in male follicles, they could be up-regulated
by estradiol. Dermal papilla cells cultured from female follicles expressed mRNA for
both ERα and ERβ. Immunocytochemistry has demonstrated co-expression of ERα and ERβ
although there was some variation; ERβ was predominately nuclear, while ERα was expressed
in the cytoplasm and the nucleus [29]. This is similar to the observation of ER expression in dermal fibroblasts [119]; however, in contrast to mRNA levels in dermal fibroblasts [120], in cultured follicular dermal papilla cells mRNA levels for ERα were approximately
2-fold higher than ERβ [29]. In addition, the expression of ERα mRNA in cultured follicular dermal papilla cells
was down-regulated by dexamethasone, while ERβ expression remained unaffected [29].
The sebaceous gland
Estrogens suppress sebaceous gland size and function, both indirectly and directly,
by pituitary-gonadal suppression of androgen production [9]. The estrogen effect is clear at the 35 μg dose of ethynylestradiol in oral contraceptive
pills. The dose of estrogen required to suppress sebum production appears to be greater
than the dose required to suppress ovulation [32]. In vitro studies with the addition of estradiol failed to show a regulatory effect on cell proliferation [35]
[58], while the inhibition of lipogenesis varied in different animal studies [35]
[125]
[126]. When administered systemically, estrogens produce a reduction in size and secretion
of sebaceous glands in both human sexes [127].
Immunostaining of human skin has shown that ERα and ERβ are co-localized in different
sebocytes [110]
[111]
[112], while basal sebocytes only display ERα [128]; the significance of this finding remains to be determined. The human epidermis,
unlike most steroidogenic organs abundant in ERα, mainly expresses ERβ.
The apocrine gland
Similar to the sebaceous gland, the apocrine gland develops from the hair follicle
and remains attached to it, increasing in size and activity with sexual maturity.
A recent study has confirmed by both immunohistochemistry and RT-PCR that ERβ is expressed
in the human axillary apocrine gland [129]. Although the apocrine secretory epithelium exhibited strong nuclear and cytoplasmic
staining for ERβ, there was no expression of ERα, as also confirmed by RT-PCR [129].
The eccrine glands
Although the role of estrogens in the eccrine glands is unclear, recent immunohistochemical
studies by two separate laboratories have demonstrated the presence of ERβ, but not
ERα in human eccrine glands [110]
[112].
Estrogens and aromatase
Aromatase, the product of the CYP19 gene, catalyzes three consecutive hydroxylation reactions converting C19 androgens
to C18 estrogens [130]. Aromatase is also present in various extra-gonadal tissues and its expression is
regulated in part by means of tissue-specific promoters through the alternative splicing
mechanism on multiple exon 1 variants[131]. At least six variants of exon 1 have been described; exons 1a, 1b, 1c, 1d, 1e and
1f that are specific for expression in the placenta, skin fibroblasts/fetal liver/adipose
tissue/vascular tissue, ovary, ovary/prostate, placenta, and fetal brain, respectively
[132]
[133].
By immunohistochemical examination, aromatase was found in the outer root sheath of
anagen, terminal hair follicles and in sebaceous glands, but rarely in telogen hair
follicles [134]. The higher expression of aromatase in the scalp hair follicles of women than men,
particularly on the occiput, has suggested that local estrogen formation from testosterone
may play a role in protecting them from alopecia [134]. Aromatase activity has been reported in the pilosebaceous unit [135], keratinocytes cultured in serum-free medium [136], fibroblasts from both genital and non-genital skin [137] and fibroblasts from adipose tissue [138]. However, in a recent study by Chen et al. (unpublished data), the mRNA expression
of aromatase was under the detection limit in the total scalp extract from either
bald or occipital area of men with androgenetic alopecia.
Estrogens and acne
Although the efficacy of ethynylestradiol-containing oral contraceptives has been
confirmed and approved in acne treatment, very little is known about the role of estrogens
in pathogenesis of acne formation. The quantitative difference of ERα and ERβ between
normal and acne skin remains to be determined, although significant differences in
the number of estrogen receptors between normal and acne-bearing skin was found to
exist in both sexes [127].
Progestins and skin
Human skin and progestins
All progestins have the unique effect of increasing body core temperature. Natural
progesterone has no known influence on human skin other than exerting this effect
at normal luteal phase levels. This progestin action results from raising the thermoregulatory
set-point at which sweating occurs [150]; evidence for a direct effect on cutaneous vasomotor tone is inconclusive [151]. However, synthetic progestins have varying degrees of clinically significant androgenic,
anti-androgenic, anti-mineralocorticoid and glucocorticoid side-effects ([Table 1]). Nevertheless, androgenic progestins such as norethindrone and levonorgestrel have
been important in cutaneous medicine in combination with estrogen in the treatment
of hirsutism and acne. Consequently, pharmaceutical development efforts have centered
on developing new generations of synthetic progestins such as drospirenone with selectively
improved anti-androgenic and anti-mineralocorticoid profiles.
Table 1 Endocrine profiles of commonly used progestins. Potency estimates are semiquantitative
composites of clinical and animal studies
<TD VALIGN="TOP">
Type of progestin
</TD><TD VALIGN="TOP">
Anti-androgenic activity
</TD><TD VALIGN="TOP">
Androgenic activity
</TD><TD VALIGN="TOP">
Anti-mineralocorticoid activity
</TD><TD VALIGN="TOP">
Glucocorticoid activity
</TD>
<TD VALIGN="TOP">
Progesterone-derived
</TD><TD VALIGN="TOP">
</TD><TD VALIGN="TOP">
</TD><TD VALIGN="TOP">
</TD><TD VALIGN="TOP">
</TD>
<TD VALIGN="TOP">
Chlormadinone acetate
</TD><TD VALIGN="TOP">
±
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
±
</TD><TD VALIGN="TOP">
+
</TD>
<TD VALIGN="TOP">
Cyproterone acetate
</TD><TD VALIGN="TOP">
++
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
+
</TD>
<TD VALIGN="TOP">
Drospirenone
</TD><TD VALIGN="TOP">
+
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
+
</TD><TD VALIGN="TOP">
0
</TD>
<TD VALIGN="TOP">
Medroxyprogesterone acetate
</TD><TD VALIGN="TOP">
±
</TD><TD VALIGN="TOP">
±
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
+
</TD>
<TD VALIGN="TOP">
Progesterone
</TD><TD VALIGN="TOP">
±
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
+
</TD><TD VALIGN="TOP">
±
</TD>
<TD VALIGN="TOP">
Nortestosterone-derived
</TD><TD VALIGN="TOP">
</TD><TD VALIGN="TOP">
</TD><TD VALIGN="TOP">
</TD><TD VALIGN="TOP">
</TD>
<TD VALIGN="TOP">
Desogestrel
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
±
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
0
</TD>
<TD VALIGN="TOP">
Levonorgestrel
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
++
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
0
</TD>
<TD VALIGN="TOP">
Norethindrone
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
+
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
0
</TD>
<TD VALIGN="TOP">
Norgestimate
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
±
</TD><TD VALIGN="TOP">
0
</TD><TD VALIGN="TOP">
0
</TD>
<TD VALIGN="TOP">
++ Indicates the most potent side-effect of its class, + indicates a small but clinically
significant effect of doses used therapeutically, and±indicates an equivocal effect
</TD>
Mode of action and effects of progestins
All progestins have corticoid, anti-corticoid, androgenic, and anti-androgenic effects.
Discovery of these properties dates to discovery of the anti-mineralocorticoid effect
of progesterone itself [152], which led to the development of the progesterone analogue spironolactone as an
anti-mineralocorticoid [153] with little progestational activity [154]. Subsequently, the progesterone analogue cyproterone acetate was found to be a potent
anti-androgen [80]
[82]
[155]. The anti-androgenicity of spironolactone was later recognized [156] and served as the basis for the recent development of the unique progestin drospirenone
[80]
[81]. Meanwhile, the search for orally active progestins led to the discovery of the
progestational effects of the 19-nortestosterone analogues that are the gestogenic
components of most oral contraceptives [157]. They are structurally androgenic and generate estrogenic or anti-estrogenic metabolites
to varying degrees.
The molecular basis of the relatively promiscuous pattern of signaling by progestins
is due to high homology among the DNA binding domains (75%) and modest homology among
the ligand binding domains (≥50%) of progestin, mineralocorticoid, glucocorticoid
and androgen receptors [158]
[159]. A new weakly gestogenic 19-nortestosterone derivative, tibolone, which has estrogenic
effects on climacteric symptoms and bone and significant androgenic effects, is now
marketed in Europe [160]
[161]. A new generation of “pure” progestins based on 19-norprogesterone is under development.
The clinically important endocrine properties of commonly used gestogens are summarized
in[Table 1] [80]
[81]
[152]
[153]
[154]
[156]
[162]
[163]
[164]
[165]
[166].
The effects of progestins are not necessarily direct or genomic. Combination with
estrogen in oral contraceptives enhances gonadotropin suppression of ovarian androgen
production. Progesterone itself also complements estrogen effects on fluid retention
by lowering the osmotic threshold for vasopressin release [150]. Unique structural features of individual agents are responsible for other actions,
such as inhibition of 5α-reductase and inactivation of cytochrome P-450 by spironolactone
[151]
[167] or 3β-HSD inhibition by cyproterone acetate [17]
[80]
[155]. Medroxyprogesterone acetate, but not progesterone itself, inhibits estrogen-induced
vasodilation, which involves both genomic and non-genomic activation of nitric oxide
synthase, while norethisterone has a non-genomic vasodilator effect [165]
[168]
[169].
Although 3β-HSD, the enzyme capable of converting pregnelolone to progesterone, has
been characterized and localized in human skin, the production of progesterone has
not yet been directly demonstrated in sebaceous glands [170]. Progesterone receptors are detected in nuclei of human sebaceous gland cells, and
the effect of progesterone on sebum production in animal models varies in different
studies, depending on the species and sex of the animals [127]
[171]
[172]
[173]
[174].
Progestins and skin disorders
The major use of progestins in skin disorders is in the treatment of hirsutism and
acne vulgaris, where they are prescribed as components of estrogen-progestin combination
pills and as anti-androgens [175]. Estrogen-progestin treatment can reduce the need for shaving by half and arrest
progression of hirsutism of various etiologies, but does not necessarily reverse it.
In contrast, they are effective in reducing acne. It is unclear whether those combinations
with non-androgenic progestins are more effective in reducing lesions. However, they
are to be favored because they do not inhibit the beneficial effect of estrogen on
the serum lipid profile; those with drospirenone have the additional advantage of
counteracting estrogen-induced sodium retention [63]
[176]
[177]. Cyproterone acetate and spironolactone are similarly effective as anti-androgens
in reducing hirsutism, although there is wide variability in individual responses.
Whether progestins play a direct role in hair cycling is unknown. Although telogen
effluvium has been attributed to high estrogen or prolactin levels, the settings in
which these hormones are incriminated (pregnancy and oral contraceptive use) [178]
[179] are related to high progestin states as well.
Progestins are effective in reducing post-menopausal hot flashes [161]. This is likely to mainly result from action on the central nervous system [180].
