REVIEwS Estrogen and androgen signaling in the pathogenesis of BPH Clement K. M. Ho and Fouad K. Habib Abstract | Estrogens and androgens have both been implicated as causes of benign prostatic hyperplasia (BPH). Although epidemiological data on an association between serum androgen concentrations and BPH are inconsistent, it is generally accepted that androgens play a permissive role in BPH pathogenesis. In clinical practice, inhibitors of 5α-reductase (which converts testosterone to the more potent androgen dihydrotestosterone) have proven effective in the management of BPH, confirming an essential role for androgens in BPH pathophysiology. To date, multiple lines of evidence support a role for estrogens in BPH pathogenesis. Studies of the two estrogen receptor (ER) subtypes have shed light on their differential functions in the human prostate; ERα and ERβ have proliferative and antiproliferative effects on prostate cells, respectively. Effects of estrogens on the prostate are associated with multiple mechanisms including apoptosis, aromatase expression and paracrine regulation via prostaglandin E2. Selective estrogen receptor modulators or other agents that can influence intraprostatic estrogen levels might conceivably be potential therapeutic targets for the treatment of BPH. Ho, C. K. M. & Habib, F. K. Nat. Rev. Urol. 8, 29–41 (2011); doi:10.1038/nrurol.2010.207
Introduction
the human prostate begins its morphogenesis at about 10–12 weeks of gestation and prostatic growth continues while fetal plasma androgen levels remain high.1 the fetal testis secretes testosterone into the circulation at sufficient levels to stimulate the differentiation and growth of the urogenital sinus tissues, leading to formation of the pros tate gland.2 after birth, plasma testosterone decreases to a low baseline level3 and the prostate does not resume growth until puberty, when large amounts of androgens secreted from the testes stimulate prostatic cells to undergo morphofunctional maturation, giving rise to the various histological zones and functional tubuloalveolar glands. the human prostate reaches its full size of approximately 20 g and mature morphology at 18–20 years of age.4 Benign prostatic hyperplasia (BPH) is a pathological condition characterized by nonmalignant enlargement of the prostate gland, common in elderly men. Patients with BPH typically present with lower urinary tract symptoms (luts), which can be classified as either urinary tract obstructive symptoms such as hesitancy, intermittent stream and straining, or urinary bladder irritation symptoms such as frequency, urgency and urge incontinence. urinary retention, whether acute or chronic, is also common. medical management of BPH and luts typically involves the reduction of prostate size by hormonal treatment, adrenergic blockade of urinary bladder tone or both. Histologically, BPH is characterized by hyperplasia of both epithelial and stromal tissues within the prostate Competing interests The authors declare no competing interests.
gland.5 in a review of studies including more than a thou sand human prostate specimens, 50% of men between 51 and 60 years of age showed pathological features of BPH.6 the percentage of men with histologically identifi able BPH at autopsy increases every year between 41 and 90 years of age.4 autopsy studies performed in differ ent parts of the world demonstrate similar agespecific prevalence of BPH in many countries in europe, asia and the us.7,8 Histological hyperplasia of the prostate is commonly observed in older men, so much so that some urologists and researchers consider it a natural occurrence during the process of prostate development and aging. only two factors are generally considered essential for the development of BPH, namely the presence of a testis, or more precisely androgens, and aging. to date, the exact etiology of BPH remains unresolved despite research data from a plethora of studies. the three main hypotheses—embryonic reawakening, stem cell theory, and various hormonal theories— are briefly summarized below.7 During embryonic development, the prostate develops from the urogenital sinus under the influence of andro gens secreted from the fetal testis, and prostate epithelial cells are directed towards functional differentiation by stromal signaling.9 mcneal10 proposed the embryonic reawakening hypothesis of BPH in 1978, which states that stromaderived factors associated with aging might induce glandular budding and branching, giving rise to new alveoli and eventually hyperplastic nodules.10 epithelium–stroma interactions might also be medi ated by abnormal levels of growth factors (including
nature reviews | urology
Department of Biochemistry, Raigmore Hospital, Old Perth Road, Inverness IV2 3UJ, UK (C. K. M. Ho). Prostate Research Group, Room FU501, Chancellor’s Building, 49 Little France Crescent, University of Edinburgh, Edinburgh EH16 4SB, UK (F. K. Habib). Correspondence to: F. K. Habib
[email protected]
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rEviEwS Key points ■ Androgens play a permissive role in the pathogenesis of benign prostatic hyperplasia (BPH) ■ Inhibition of 5α-reductase activity is currently the mainstay of hormonal treatment of BPH ■ Increasing evidence from epidemiological, animal and in vitro studies supports a role for estrogens in the pathogenesis of BPH ■ Estrogen receptors ERα and ERβ mediate proliferative and antiproliferative effects of estrogens on prostate cells, respectively ■ Some androgens are weak ligands for ERs but might have potent agonistic effects on prostate cells because of high tissue concentrations ■ The prevalence of ERα and ERβ in hyperplastic prostate raises the potential of selective estrogen receptor modulators as potential therapeutic agents for BPH
epidermal growth factor, basic fibroblast growth factor and transforming growth factor β [tGFβ]) from either the epithelial or stromal compartment, resulting in the reawakening of embryonic cellular growth potential, and leading to hyperplasia.11 the latter notion is supported by a transgenic mouse model, in which overexpression of FGF3 resulted in epithelial hyperplasia of the murine prostate gland.12 Furthermore, under hypoxic in vitro conditions, human prostate stromal cells in culture secrete increased levels of growth factors (such as FGF7 and tGFβ) than normoxic cells, suggesting local hypoxia might be one of the triggers of embryonic reawakening of the prostatic stroma.13 in 1989, isaacs and Coffey 7 proposed the stem cell theory of BPH pathogenesis, suggesting that an increase in the number of prostatic stem cells or an increase in the clonal expansion of existing stem cells into transit amplify ing cells might be responsible. Precise distribution of stem cells in the human prostate is currently unclear, although both epithelial and stromal stem cell units are believed to be present in the adult prostate.14 Biomarkers character istic of stem cells have been identified in cultured stromal cells derived from human BPH tissues.15 stem cells in the adult prostate might expand both the stromal and epi thelial compartments in response to as yet unidentified stimuli, resulting in hyperplasia of the prostate. multiple hormonal theories of BPH etiology have been proposed and they are mainly focused on the actions of androgens, estrogens or both. the essential role of androgens in the development of normal prostate is widely accepted. a central role for androgens in the development of BPH was concluded from observations that castration significantly improved the symptoms of BPH. in addition, 5αdihydrotestosterone (DHt) concentrations are significantly higher in hyperplastic than normal prostate tissues, although no difference in androstenedione or testosterone concentrations between the two types of tissues have been detected.16 recently, the abnormal prostate development observed in patients deficient in 5αreductase enzyme has highlighted the importance of 5αreduced androgens, such as DHt, in the normal growth and development of this gland.17 Despite a widely accepted role for androgens in prostate development and growth, it is not entirely clear why BPH develops at a stage in life when plasma levels of androgens
are gradually decreasing. in addition, multiple lines of new evidence from epidemiological, animal and in vitro studies support a role for estrogens in the pathogenesis of BPH. increasing estrogen:androgen ratios (in plasma and possibly also in prostate tissues) with advancing age has also been proposed as a factor for BPH. in this review, we will briefly summarize the bio chemistry of androgens and estrogens in relation to the normal and hyperplastic human prostate. we will then discuss the existing epidemiological data on the associa tion between increased levels of these sex steroids and BPH. the distribution of androgen receptor (ar), estro gen receptor (er), 5αreductase and aromatase in both normal and hyperplastic prostate tissues will be exam ined, and recent data on the mechanisms by which estro gens exert their actions in the prostate will be discussed. Finally, therapeutic inhibition of 5αreductase and aro matase—enzymes directly involved in the metabolism and biosynthesis of androgens and estrogens in the prostate —for the management of BPH will be discussed.
Androgen and estrogen signaling Sex steroids in men sex steroid hormones, including androgens and estro gens, are synthesized from a common sterol precursor, cholesterol, by the concerted actions of multiple enzymes (Figure 1). in the normal male, testes are the major source of circulating androgens and the principal tes ticular androgen is testosterone. Besides testosterone, the testes also secrete small amounts of other sex steroids including androstenedione, DHt, estradiol and estrone. testicular function is regulated by two pituitary gonado tropins—folliclestimulating hormone and luteinizing hormone (lH). upon stimulation by lH, leydig cells in the testis produce testosterone, which is secreted into the bloodstream. Plasma levels of the two gonadotropins are regulated by gonadotropinreleasing hormone from the hypothalamus. the testis exerts an influence on the hypo thalamic–pituitary axis by negative feedback mechanisms involving androgens, estrogens and inhibin B.18 Besides the testes, another important source of cir culating androgens in man is the adrenal gland. major adrenal androgens include dehydroepiandrosterone (DHea), DHeasulfate and androstenedione. although these adrenal androgens are weak androgens compared with testosterone, they can be converted to more active steroid metabolites and thus indirectly alter prostate growth and function. the most potent androgen in men, DHt, is believed to be largely converted from testo sterone in peripheral tissues such as prostate and skin by the action of 5αreductase isozymes (Figure 1). of the circulating estrogens in men, 75–90% are believed to derive from peripheral conversion of testo sterone and androstenedione to estradiol and estrone, respectively (Figure 1), in adipose, brain, bone and other tissues. Conversion in the testes is responsible for 10–25% of circulating estrogens.19 the two natural estrogens in man, estradiol and estrone, can be interconverted by enzymes belonging to the family of 17βhydroxysteroid dehydrogenases.
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rEviEwS
Cholesterol
HO DHEA
Androgens
O
Estrogens
H 3βHSD 5α-Androstanedione
Androstenedione O
O 5αR
O
Estrone O Aromatase
O 17βHSD
17βHSD DHT
17βHSD
Testosterone OH
OH
Estradiol OH Aromatase
5αR
O
HO
O
HO
Figure 1 | Biosynthesis and metabolism of the major androgens and estrogens in human prostate. Cholesterol is the precursor of all steroid hormones. All enzymes required for the conversion of DHEA, which is the most abundant steroid in the bloodstream (mainly secreted by the adrenal gland), to active steroid hormones including estradiol, testosterone and DHT, are expressed in the human prostate. Abbreviations: 5αR, 5α-reductase; DHEA, dehydroepiandrosterone; DHT, dihydrotestosterone; HSD, hydroxysteroid dehydrogenase.
in plasma, most of the circulating steroid hormones are bound to carrier proteins. the major binding pro teins are albumin, sexhormonebinding globulin (sHBG) and cortisolbinding globulin. For example, approximately 2% of circulating testosterone is free or unbound in men, whereas the rest is either tightly bound to sHBG or loosely bound to albumin. Free testosterone is thought to constitute the active hormone and provides a better measure of testosterone status in the body than total testosterone level. measurement of free testosterone by the gold standard method of equilibrium dialysis is laborious and impractical for routine laboratory prac tice. to this end, many equations have been proposed to estimate plasma free testosterone levels in men using the measured concentrations of total testosterone, albumin and sHBG.20,21 steroid hormones, including androgens and estrogens, exert their effects on target gene expression by binding to their cognate intracellular receptors, which function as hormoneinducible transcription factors.
Androgen and estrogen receptors steroid receptors, including the ar and er, share a similar design of structural and functional domains (Figure 2). the nterminal activation function 1 (aF1)
domain of ar is constitutively active in truncated receptor fragments containing no ligandbinding domain (lBD). By contrast, the Cterminal activation function 2 (aF2) domain operates in a liganddependent manner and is more conserved in sequence than aF1 among steroid hormone receptors. 22 length of the ar polypeptide varies because of two trinucleotide repeat polymorphic stretches in exon 1 of the AR gene (Figure 2); a poly glutamine stretch and a polyglycine stretch are encoded by polymorphic CaG and GGC trinucleotide repeats, respectively. the number of CaG repeats, and thus length of glutamine repeats in the ar polypeptide, is inversely related to both basal and ligandinduced transactivation activity of ar.23 ar is expressed in many adult and fetal tissues, with the highest concentrations in reproductive tissues such as testis, prostate and seminal vesicle.24,25 two isoforms of ar protein have been reported; ara and arB are encoded by a single gene. the shorter, nterminally trun cated ara isoform is produced by translation initia tion at an alternate site—the first internal methionine residue (met188) (Figure 2).25 in a study examining ar expression in adult and fetal reproductive tissues includ ing the prostate, fulllength arB was the predominant or only detectable isoform, accounting for 80% or more
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rEviEwS A/B
C
AF-1 1
D
DBD Hinge
Gln(n)
Gly(n)
E/F LBD/AF-2
559
624 676
919
559
624 676
919
180
263 302
AR-B Gly(n)
188
AR-A 1
595
ERα (18%) 1
(97%) (30%) 144
227 255
(59%) 530
ERβ
Figure 2 | Schematic representation of human ARs and ERs. General organization of domains A to F is shared by all steroid receptors including ARs and ERs. The N-terminal A/B domain is involved in transactivation, whereas the C-domain recognizes and binds to DNA sequences of target genes. The C-terminus (E/F domains) mediates multiple functions including ligand binding, dimerization and transactivation. AR-A and AR-B are encoded by a single AR gene with two different translational start sites separated by 187 amino acids. Polyglutamine (Gln) and polyglycine (Gly) tracts of variable lengths at the N-terminus of AR result in variable total lengths of AR-A and AR-B (approximately 732 and 919 amino acids, respectively). ERα and ERβ are products of two different genes. Percentages in parentheses indicate amino acid homology between structural domains of the two ER subtypes. Their nearly identical DBDs indicate that the two ER subtypes can bind to similar target genes, whereas low amino acid homologies in other domains suggest dissimilar ligand preference and transactivation functions between ERα and ERβ. Abbreviations: AF-1/-2, activation function 1/2; AR, androgen receptor; DBD, DNA-binding domain; ER, estrogen receptor; LBD, ligand-binding domain. Compiled using data from studies by williams and Franklin,129 Katzenellenbogen et al.151 and Gregory et al.152
of the total ar immunoreactivity.25 Currently, it is not known whether the two isoforms perform different physiological functions. Both testosterone and DHt can readily bind to ar, while androstenedione and DHea have much lower affinities for ar. the relative binding affinities of the common circulating androgens are shown in table 1. Despite their ability to bind to the same receptor, DHt and testosterone are believed to have distinct roles in normal development. During embryogenesis, testo sterone is essential for the development of wolffian duct structures such as the epididymis and vas deferens, whereas DHt is required for normal prostate forma tion and masculinization of male external genitalia. mechanisms by which the two androgens exert differ ent physiological functions via the same ar are not fully understood. multiple mechanisms have been proposed, including differential ar binding affinity and dissocia tion,26–28 recruitment of different steroid receptor co regulators29,30 and differential response owing to diverse androgen response element sequences.31 erα and erβ are encoded by two distinct genes located on different chromosomes. the two er subtypes have the same arrangement of functional domains with the most conserved region being the Dnabinding domain (97% identity), consistent with the receptors binding to similar Dna response elements.32 However, aF1 domains share
18% homology only, suggesting that they might trans criptionally activate distinct estrogenresponsive genes.33 the lBDs are relatively well conserved (59% identity), which explains their similar affinities for various estro gens and estrogenic compounds. the er subtypes have dissimilar relative binding affinities for a number of naturally occurring estrogenic compounds (table 1). some steroids traditionally considered weak andro gens, such as 5androstenediol and 3βandrostanediol have moderate affinities for both er subtypes (table 1). an important metabolite of DHea in the prostate, 7αhydroxydehydroepiandrosterone (7αoHDHea), has also been shown to bind and activate erβ in vitro; erβ and the enzyme CYP7B, which converts DHea to 7αoHDHea, are both expressed in the prostate epi thelium, suggesting a role for CYP7B in regulating the estrogenic environment in prostate.34
Androgen signaling in BPH The role of androgens androgens are widely accepted to be essential for normal prostate development and constitute an indispensible component of the pathophysiology of BPH. this notion is supported by clinical observations in patients with BPH treated by androgen withdrawal; prostate glands of patients with BPH markedly decreased in size upon either orchiectomy 35 or administration of the anti androgen flutamide.36 Given the androgen dependence of BPH, it is not clear why this condition is common at a stage of life when plasma androgen levels are declining with advancing age. multiple studies have examined pos sible relationships between the risk of BPH and circulat ing levels of androgens including testosterone and DHt but no consistent relationship has been established.37–41 For example, in the third national Health & nutrition examination survey (nHanes iii) plasma total testo sterone and calculated free testosterone concentrations were not different between control subjects and men with luts. However, those patients with luts were reported to have higher plasma levels of androstanediol glucuronide (a metabolite of DHt) than controls.39 By contrast, higher total and bioavailable testosterone levels were associated with reduced BPH risk in the Prostate Cancer Prevention trial.40 Hypogonadism is a relatively common condition in the aging man and provides a useful model for the study of androgen deficiency and prostate physiology. men with untreated androgen deficiency were reported to have reduced total and central prostate volumes mea sured by transrectal ultrasonography compared with agematched controls. Decreased total and central pros tate volumes were also observed in men with treated androgen deficiency despite prolonged restoration of physiological testosterone concentrations.42 in compari son, healthy young men who were longterm anabolic steroid abusers were shown to have approximately 20% higher central prostate volume but similar total prostate volume compared with agematched controls. the ratio of central to peripheral prostate volume was increased by 77% in anabolic steroid abusers, suggesting that the
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rEviEwS central prostate is particularly sensitive to stimulation by supraphysiological concentrations of androgens.43 not all studies to date support the hypothesis that plasma androgen levels are directly proportional to prostate volume or severity of luts. nevertheless, chronic androgen deficiency owing to hypogonadism can be associated with reduced prostate size, whereas supraphysiological androgen levels can lead to enlarge ment of the prostate. although a role for androgens in causing BPH is debatable, it is generally accepted that they play at least a permissive role. also, plasma levels of androgens do not necessarily reflect their concentrations within the prostate gland.44 DHt concentrations in BPH tissues are generally higher than testosterone concentra tions, whereas the reverse is true in plasma.45,46 in fact, the prostate not only responds to androgens derived from plasma but is also an organ rich in androgen metabolizing enzymes including 5αreductase, which converts testosterone to the more potent DHt. an early report of higher intraprostatic DHt concentrations in BPH tissues than in normal tissues led to the hypothesis that local accumulation of DHt within the prostate can cause BPH.16 However, walsh et al.47 demonstrated that DHt levels were not higher in BPH than normal prostate tissues and argued that the previously reported differ ences in prostatic DHt levels were artifacts caused by differences in sample collection methods.47
Androgen metabolism in the prostate a myriad of androgenmetabolizing enzymes have been detected in the human prostate, including 5αreductase, 17βhydroxysteroid dehydrogenase, 3αhydroxysteroid dehydrogenase, 3βhydroxysteroid dehydrogenase and cytochrome P450 19a1 (CYP19a1; commonly referred to as aromatase) (Figure 1); among these, 5αreductase is probably the enzyme most relevant to normal prostate development and the treatment of BPH.48 within the prostate, androgenmetabolizing enzymes expressed in different cell types might function in concert with one another to produce a particular androgen metabolite; this is, to a certain extent, analogous to the zonal arrangement of steroidogenic enzymes in the adrenal gland and ovary. labrie et al.49 proposed that DHea is converted to testo sterone in basal cells of the prostatic epithelium, whereas testosterone is converted to DHt by 5αreductase within basal cells or luminal epithelial cells.49 Both testosterone and DHt can bind to ar, which is predominantly expressed in the luminal epithelium (Figure 3). whether this type of local steroid metabolism can act as a crosstalk signaling mechanism between various compartments of the prostate and thus contribute to the development of hyperplasia awaits further investigation. Ar involvement in BPH many studies have demonstrated ar immunoreactivity in most nuclei of the luminal epithelium in normal and hyperplastic human prostate tissues, compared to only a small number or none of the basal epithelial cells showing immunostaining. some stromal cell nuclei were also immunoreactive (table 2).50–54 to date, no consistent
Table 1 | Relative binding affinities of ligands for AR and ERs29,153–155 Compound
relative binding affinity Ar (rat)
Erα (human) Human
rat
Estradiol
1.9–3.9
100*
100*
100*
Estrone
