Skeletal effects of long-term estrogen and testosterone replacement treatment in a man with congenital aromatase deficiency: evidences of a priming effect of estrogen for sex steroids action on bone

Bone Mass and Osteoblast Activity Are Sex-Dependent in Mice Lacking the Estrogen Receptor α in Chondrocytes and Osteoblast Progenitor Cells

While estrogen receptor alpha (ERα) is known to be important for bone development and homeostasis, its exact function during osteoblast differentiation remains unclear. Conditional deletion of ERα during specific stages of osteoblast differentiation revealed different bone phenotypes, which were also shown to be sex-dependent. Since hypertrophic chondrocytes can transdifferentiate into osteoblasts and substantially contribute to long-bone development, we aimed to investigate the effects of ERα deletion in both osteoblast and chondrocytes on bone development and structure. Therefore, we generated mice in which the ERα gene was inactivated via a Runx2-driven cyclic recombinase (ERαfl/fl; Runx2Cre). We analyzed the bones of 3-month-old ERαfl/fl; Runx2Cre mice by biomechanical testing, micro-computed tomography, and cellular parameters by histology. Male ERαfl/fl; Runx2Cre mice displayed a significantly increased cortical bone mass and flexural rigidity of the femurs compared to age-matched controls with no active Cre-transgene (ERαfl/fl). By contrast, female ERαfl/fl; Runx2Cre mice exhibited significant trabecular bone loss, whereas in cortical bone periosteal and endosteal diameters were reduced. Our results indicate that the ERα in osteoblast progenitors and hypertrophic chondrocytes differentially contributes to bone mass regulation in male and female mice and improves our understanding of ERα signaling in bone cells in vivo.

Role of sex steroids hormones in the regulation of bone metabolism in men: Evidence from clinical studies

Sex steroids regulate bone metabolism in young men during growth and consolidation. Their deficit during growth compromises longitudinal and radial growth of bones and has a negative impact on body height, bone width, peak areal bone mineral density (aBMD) and bone microarchitecture. In older men, the deficit of sex steroid hormones (mainly 17β-oestradiol) contributes to high bone turnover rate, low aBMD, poor bone microarchitecture, low estimated bone strength, accelerated bone loss and rapid decline of bone microarchitecture. The role of 17β-oestradiol is confirmed by the case of men with congenital oestrogen receptor deficit and with congenital aromatase deficiency. 17β-oestradiol inhibits bone resoption, whereas both hormones regulate bone formation. However, the associations are weak. Prospective data on the utility of blood 17β-oestradiol or testosterone for fracture risk assessment are inconsistent. Men with hypogonadism have decreased aBMD and poor bone microarchitecture. In men with hypogonadism, testosterone replacement therapy increases aBMD and improves bone microarchitecture. In men with prostate cancer, androgen deprivation therapy (gonadoliberin analogues) induces rapid bone loss and severe deterioration of bone microarchitecture.

Critical Role of Estrogens on Bone Homeostasis in Both Male and Female: From Physiology to Medical Implications

Bone is a multi-skilled tissue, protecting major organs, regulating calcium phosphate balance and producing hormones. Its development during childhood determines height and stature as well as resistance against fracture in advanced age. Estrogens are key regulators of bone turnover in both females and males. These hormones play a major role in longitudinal and width growth throughout puberty as well as in the regulation of bone turnover. In women, estrogen deficiency is one of the major causes of postmenopausal osteoporosis. In this review, we will summarize the main clinical and experimental studies reporting the effects of estrogens not only in females but also in males, during different life stages. Effects of estrogens on bone involve either Estrogen Receptor (ER)α or ERβ depending on the type of bone (femur, vertebrae, tibia, mandible), the compartment (trabecular or cortical), cell types involved (osteoclasts, osteoblasts and osteocytes) and sex. Finally, we will discuss new ongoing strategies to increase the benefit/risk ratio of the hormonal treatment of menopause.

Bone geometry and trabecular bone score in transgender people before and after short- and long-term hormonal treatment

BACKGROUND

Gender-affirming hormonal treatment (HT) in adult transgender people influences bone mineral density (BMD). Besides BMD, bone geometry and trabecular bone score are associated with fracture risk. However, it is not known whether bone geometry and TBS changes during HT.

PURPOSE

To investigate the bone geometry and TBS in adult transgender people at different time points, up to 25 years, of HT.

METHODS

A total of 535 trans women and 473 trans men were included, who were divided into three groups at time of their DXA: 20-29 years, 30-39 years, and 40-59 years. Subsequently, each group was divided into different HT durations: baseline, or after 5, 15, or 25 years of HT. Hip structure analysis was performed to measure subperiosteal width, endocortical diameter, average cortical thickness, and section modulus. TBS was calculated based on lumbar spine DXA images.

RESULTS

In trans women in all age groups and in young trans men, no differences were observed in periosteal width, endocortical diameter, average cortical thickness, and section modulus for different durations of HT. In trans men aged 40-59 years, subperiosteal width, endocortical diameter, and section modulus were slightly higher in the groups who were using HT compared to the (peri- or postmenopausal) baseline group. In younger trans women, TBS tended to be higher in those using HT compared to the baseline groups, and in older trans women TBS was higher in those using HT for 25 years versus baseline (+0.04, 95%CI +0.00; +0.08). In younger trans men, TBS tended to be lower in those who used HT compared to the baseline groups, and in older trans men TBS was lower in those using 5 years HT versus baseline (-0.05, 95%CI -0.08; -0.01).

CONCLUSION

No differences in cortical bone geometry parameters were found during different HT-durations. TBS increased in trans women and decreased in trans men, indicating that estrogens have positive effects on TBS. These data may be helpful in determining what sex reference values for calculating T-scores and Z-scores in adult transgender people should be used.

Cross-sex testosterone therapy in ovariectomized mice: addition of low-dose estrogen preserves bone architecture

Cross-sex hormone therapy (XHT) is widely used by transgender people to alter secondary sex characteristics to match their desired gender presentation. Here, we investigate the long-term effects of XHT on bone health using a murine model. Female mice underwent ovariectomy at either 6 or 10 wk and began weekly testosterone or vehicle injections. Dual-energy X-ray absorptiometry (DXA) was performed (20 wk) to measure bone mineral density (BMD), and microcomputed tomography was performed to compare femoral cortical and trabecular bone architecture. The 6-wk testosterone group had comparable BMD with controls by DXA but reduced bone volume fraction, trabecular number, and cortical area fraction and increased trabecular separation by microcomputed tomography. Ten-week ovariectomy/XHT maintained microarchitecture, suggesting that estrogen is critical for bone acquisition during adolescence and that late, but not early, estrogen loss can be sufficiently replaced by testosterone alone. Given these findings, we then compared effects of testosterone with effects of weekly estrogen or combined testosterone/low-dose estrogen treatment after a 6-wk ovariectomy. Estrogen treatment increased spine BMD and microarchitecture, including bone volume fraction, trabecular number, trabecular thickness, and connectivity density, and decreased trabecular separation. Combined testosterone-estrogen therapy caused similar increases in femur and spine BMD and improved architecture (increased bone volume fraction, trabecular number, trabecular thickness, and connectivity density) to estrogen therapy and were superior compared with mice treated with testosterone only. These results demonstrate estradiol is critical for bone acquisition and suggest a new cross-sex hormone therapy adding estrogens to testosterone treatments with potential future clinical implications for treating transgender youth or men with estrogen deficiency.

Sex hormones and quantitative ultrasound parameters at the heel in men and women from the general population

Purpose/introduction

The present study investigates potential associations between liquid chromatography-mass spectrometry (LC-MS) measured sex hormones, dehydroepiandrosterone sulphate, sex hormone-binding globulin (SHBG) and bone ultrasound parameters at the heel in men and women from the general population.

Methods

Data from 502 women and 425 men from the population-based Study of Health in Pomerania (SHIP-TREND) were used. Cross-sectional associations of sex hormones including testosterone (TT), calculated free testosterone (FT), dehydroepiandrosterone sulphate (DHEAS), androstenedione (ASD), estrone (E1) and SHBG with quantitative ultrasound (QUS) parameters at the heel, including broadband ultrasound attenuation (BUA), speed of sound (SOS) and stiffness index (SI) were examined by analysis of variance (ANOVA) and multivariable quantile regression models.

Results

Multivariable regression analysis showed a sex-specific inverse association of DHEAS with SI in men (Beta per SI unit = − 3.08, standard error (SE) = 0.88), but not in women (Beta = − 0.01, SE = 2.09). Furthermore, FT was positively associated with BUA in men (Beta per BUA unit = 29.0, SE = 10.1). None of the other sex hormones (ASD, E1) or SHBG was associated with QUS parameters after multivariable adjustment.

Conclusions

This cross-sectional population-based study revealed independent associations of DHEAS and FT with QUS parameters in men, suggesting a potential influence on male bone metabolism. The predictive role of DHEAS and FT as a marker for osteoporosis in men warrants further investigation in clinical trials and large-scale observational studies.

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Population-based data of healthy men and women from the general population

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Sex hormone panel measured by liquid chromatography-mass spectrometry (LC-MS)

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Associations of dehydroepiandrosterone sulphate and free testosterone with bone ultrasound parameters in men

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Estrone, androstenedione and SHBG were not associated with bone ultrasound parameters in both sexes.

Estrogens in Male Physiology

Estrogens have historically been associated with female reproduction, but work over the last two decades established that estrogens and their main nuclear receptors (ESR1 and ESR2) and G protein-coupled estrogen receptor (GPER) also regulate male reproductive and nonreproductive organs. 17β-Estradiol (E2) is measureable in blood of men and males of other species, but in rete testis fluids, E2 reaches concentrations normally found only in females and in some species nanomolar concentrations of estrone sulfate are found in semen. Aromatase, which converts androgens to estrogens, is expressed in Leydig cells, seminiferous epithelium, and other male organs. Early studies showed E2 binding in numerous male tissues, and ESR1 and ESR2 each show unique distributions and actions in males. Exogenous estrogen treatment produced male reproductive pathologies in laboratory animals and men, especially during development, and studies with transgenic mice with compromised estrogen signaling demonstrated an E2 role in normal male physiology. Efferent ductules and epididymal functions are dependent on estrogen signaling through ESR1, whose loss impaired ion transport and water reabsorption, resulting in abnormal sperm. Loss of ESR1 or aromatase also produces effects on nonreproductive targets such as brain, adipose, skeletal muscle, bone, cardiovascular, and immune tissues. Expression of GPER is extensive in male tracts, suggesting a possible role for E2 signaling through this receptor in male reproduction. Recent evidence also indicates that membrane ESR1 has critical roles in male reproduction. Thus estrogens are important physiological regulators in males, and future studies may reveal additional roles for estrogen signaling in various target tissues.

Estrogens and Androgens in Skeletal Physiology and Pathophysiology

Estrogens and androgens influence the growth and maintenance of the mammalian skeleton and are responsible for its sexual dimorphism. Estrogen deficiency at menopause or loss of both estrogens and androgens in elderly men contribute to the development of osteoporosis, one of the most common and impactful metabolic diseases of old age. In the last 20 years, basic and clinical research advances, genetic insights from humans and rodents, and newer imaging technologies have changed considerably the landscape of our understanding of bone biology as well as the relationship between sex steroids and the physiology and pathophysiology of bone metabolism. Together with the appreciation of the side effects of estrogen-related therapies on breast cancer and cardiovascular diseases, these advances have also drastically altered the treatment of osteoporosis. In this article, we provide a comprehensive review of the molecular and cellular mechanisms of action of estrogens and androgens on bone, their influences on skeletal homeostasis during growth and adulthood, the pathogenetic mechanisms of the adverse effects of their deficiency on the female and male skeleton, as well as the role of natural and synthetic estrogenic or androgenic compounds in the pharmacotherapy of osteoporosis. We highlight latest advances on the crosstalk between hormonal and mechanical signals, the relevance of the antioxidant properties of estrogens and androgens, the difference of their cellular targets in different bone envelopes, the role of estrogen deficiency in male osteoporosis, and the contribution of estrogen or androgen deficiency to the monomorphic effects of aging on skeletal involution.

The association between serum dehydroepiandrosterone Sulphate (DHEA-S) level and bone mineral density in Korean men

CONTEXT

Many lines of evidence indicate that dehydroepiandrosterone (DHEA) plays a distinct role in bone metabolism and that its sulphated form (DHEA-S), which is easily measured in blood, may be a potential biomarker of osteoporosis-related phenotypes. However, most previous epidemiologic studies focused on postmenopausal women and reported conflicting results.

OBJECTIVE

We aimed to investigate the association between the serum DHEA-S level and bone mass in men.

DESIGN AND METHODS

This large cross-sectional study included 1089 healthy Korean men who participated in a routine health screening examination. Bone mineral density (BMD) at the lumbar spine, total femur, femur neck, and trochanter and serum DHEA-S level were obtained in all subjects.

RESULTS

After adjustment for age, body mass index, lifestyle factors and serum levels of calcium, phosphorus, testosterone, 25-OH-vitamin D3 and cortisol, higher serum DHEA-S concentrations were associated with higher BMD values at all skeletal sites. Consistently, compared to the subjects in the highest DHEA-S quartile (Q4), those in the lowest DHEA-S quartile (Q1) showed significantly lower BMD values. Multiple logistic regression analyses revealed that the odds ratios for the risk of lower BMD (T-score <-1) increased in a dose-dependent manner across decreasing DHEA-S quartiles and the odds for the risk of lower BMD were 2·59-fold higher in Q1 than in Q4.

CONCLUSION

These findings support previous evidences that DHEA-S has favourable effects on bone mass in men and suggest that a low serum DHEA-S level may be a potential risk factor for male osteoporosis.

The endocrine role of estrogens on human male skeleton

Before the characterization of human and animal models of estrogen deficiency, estrogen action was confined in the context of the female bone. These interesting models uncovered a wide spectrum of unexpected estrogen actions on bone in males, allowing the formulation of an estrogen-centric theory useful to explain how sex steroids act on bone in men. Most of the principal physiological events that take place in the developing and mature male bone are now considered to be under the control of estrogen. Estrogen determines the acceleration of bone elongation at puberty, epiphyseal closure, harmonic skeletal proportions, the achievement of peak bone mass, and the maintenance of bone mass. Furthermore, it seems to crosstalk with androgen even in the determination of bone size, a more androgen-dependent phenomenon. At puberty, epiphyseal closure and growth arrest occur when a critical number of estrogens is reached. The same mechanism based on a critical threshold of serum estradiol seems to operate in men during adulthood for bone mass maintenance via the modulation of bone formation and resorption in men. This threshold should be better identified in-between the ranges of 15 and 25 pg/mL. Future basic and clinical research will optimize strategies for the management of bone diseases related to estrogen deficiency in men.

Sex steroid actions in male bone

Sex steroids are chief regulators of gender differences in the skeleton, and male gender is one of the strongest protective factors against osteoporotic fractures. This advantage in bone strength relies mainly on greater cortical bone expansion during pubertal peak bone mass acquisition and superior skeletal maintenance during aging. During both these phases, estrogens acting via estrogen receptor-α in osteoblast lineage cells are crucial for male cortical and trabecular bone, as evident from conditional genetic mouse models, epidemiological studies, rare genetic conditions, genome-wide meta-analyses, and recent interventional trials. Genetic mouse models have also demonstrated a direct role for androgens independent of aromatization on trabecular bone via the androgen receptor in osteoblasts and osteocytes, although the target cell for their key effects on periosteal bone formation remains elusive. Low serum estradiol predicts incident fractures, but the highest risk occurs in men with additionally low T and high SHBG. Still, the possible clinical utility of serum sex steroids for fracture prediction is unknown. It is likely that sex steroid actions on male bone metabolism rely also on extraskeletal mechanisms and cross talk with other signaling pathways. We propose that estrogens influence fracture risk in aging men via direct effects on bone, whereas androgens exert an additional antifracture effect mainly via extraskeletal parameters such as muscle mass and propensity to fall. Given the demographic trends of increased longevity and consequent rise of osteoporosis, an increased understanding of how sex steroids influence male bone health remains a high research priority.

Androgens and estrogens in skeletal sexual dimorphism

Bone is an endocrine tissue expressing androgen and estrogen receptors as well as steroid metabolizing enzymes. The bioactivity of circulating sex steroids is modulated by sex hormone-binding globulin and local conversion in bone tissue, for example, from testosterone (T) to estradiol (E2) by aromatase, or to dihydrotestosterone by 5α-reductase enzymes. Our understanding of the structural basis for gender differences in bone strength has advanced considerably over recent years due to increasing use of (high resolution) peripheral computed tomography. These microarchitectural insights form the basis to understand sex steroid influences on male peak bone mass and turnover in cortical vs trabecular bone. Recent studies using Cre/LoxP technology have further refined our mechanistic insights from global knockout mice into the direct contributions of sex steroids and their respective nuclear receptors in osteoblasts, osteoclasts, osteocytes, and other cells to male osteoporosis. At the same time, these studies have reinforced the notion that androgen and estrogen deficiency have both direct and pleiotropic effects via interaction with, for example, insulin-like growth factor 1, inflammation, oxidative stress, central nervous system control of bone metabolism, adaptation to mechanical loading, etc., This review will summarize recent advances on these issues in the field of sex steroid actions in male bone homeostasis.

Bone mineral density in young women with primary ovarian insufficiency: results of a three-year randomized controlled trial of physiological transdermal estradiol and testosterone replacement

CONTEXT

Women with primary ovarian insufficiency have significantly lower serum estradiol and T levels compared with regularly menstruating women. They also have significantly reduced bone mineral density (BMD).

OBJECTIVE

The objective of the study was to evaluate the efficacy of hormone replacement in maintaining BMD in these young women.

DESIGN AND SETTING

This was a randomized, double-blind, single-center, placebo-controlled clinical trial at the National Institutes of Health clinical center (Bethesda, Maryland).

PARTICIPANTS

Young women with primary ovarian insufficiency participated in the study.

INTERVENTIONS

We compared the effect of estradiol and progestin replacement (n = 72) vs estradiol, progestin, and T replacement (n = 73) on BMD. We also compared findings with a contemporaneous control group of normal women (n = 70). All patients received transdermal estradiol (100 μg/d) plus oral medroxyprogesterone acetate 10 mg/d (12 d/mo) for a 3-month run-in period before being randomized in a double-blinded fashion to the addition of transdermal T (150 μg/d) or placebo.

MAIN OUTCOME MEASURE

Change in BMD at the femoral neck was measured by dual-energy x-ray absorptiometry.

RESULTS

At screening, patients had significantly lower femoral neck BMD compared with control women (0.77 vs 0.81 g/cm(2), P = .001) and did not differ in body mass index, age at menarche, or education level. Normal control women lost femoral neck BMD over the study period, whereas patients on estradiol and progestin therapy gained BMD; and at the end of the study period, femoral neck BMD of patients on estradiol and progestin therapy did not differ from that of control women (0.80 g/cm(2) in both groups, P = .9). The addition of T showed no further benefit (percentage change in BMD 3.9 vs 2.4, respectively, P = .9). Nonetheless, using a repeated-measures model, the T group achieved a mean BMD in the femoral neck 0.015 g/cm(2) higher than the placebo group at 3 years (95% confidence interval -0.005 to 0.034, P = .13). Similar findings were observed in the lumbar spine BMD as well.

CONCLUSION

Long-term physiological transdermal estradiol replacement in combination with oral medroxyprogesterone acetate restores mean femoral neck BMD to normal in young women with spontaneous 46,XX primary ovarian insufficiency. However, the addition of physiological transdermal T replacement did not provide additional benefit.

Association between sex steroid levels and bone microarchitecture in men: the STRAMBO study

CONTEXT

Data on the association between bone microarchitecture assessed by high-resolution peripheral quantitative computed tomography (HR-pQCT) and sex steroids in men are scarce.

OBJECTIVE

Our aim was to determine the association between serum sex steroids and bone microarchitecture assessed by HR-pQCT in men.

DESIGN

This is a cross-sectional analysis in the Structure of the Aging Men's Bones cohort.

SETTING

The cohort was recruited from the general population.

PARTICIPANTS

We examined 1169 male volunteers aged 20-87 years. No specific exclusion criteria were used.

INTERVENTIONS

We collected blood samples and performed HR-pQCT at the distal radius and distal tibia.

MAIN OUTCOME MEASURES

We tested the hypothesis that low sex steroid levels are associated with poor bone microarchitecture in men.

RESULTS

Men aged younger than 65 years with bioavailable 17β-estradiol (bio-17β-E2) levels of14.4 pmol/L or less had higher cross-sectional and trabecular areas vs men with bio-17β-E2 greater than 14.4 pmol/L. In men aged 65 years or older, the higher the apparent free T concentration (AFTC), the higher was the distal tibia cortical density (P < .05). Cortical density and thickness as well as total and trabecular density increased with higher bio-17β-E2 levels. Similar results were found after adjustment for limb length and body height. Men with low AFTC and low bio-17β-E2 levels had lower cortical density and thickness at both skeletal sites compared with the reference group. In men with AFTC less than 272 pmol/L, those with low bio-17β-E2 less than 25 pmol/L had lower cortical density and thickness at both skeletal sites vs men having higher bio-17β-E2 levels.

CONCLUSION

In men aged 65 years and older, low bio-17β-E2 levels were associated with poor cortical bone status and, to smaller extent, lower trabecular density.

The role of estrogen and androgen receptors in bone health and disease

Mouse models with cell-specific deletion of the estrogen receptor (ER) α, the androgen receptor (AR) or the receptor activator of nuclear factor κB ligand (RANKL), as well as cascade-selective estrogenic compounds have provided novel insights into the function and signalling of ERα and AR. The studies reveal that the effects of estrogens on trabecular versus cortical bone mass are mediated by direct effects on osteoclasts and osteoblasts, respectively. The protection of cortical bone mass by estrogens is mediated via ERα, using a non-nucleus-initiated mechanism. By contrast, the AR of mature osteoblasts is indispensable for the maintenance of trabecular bone mass in male mammals, but not required for the anabolic effects of androgens on cortical bone. Most unexpectedly, and independently of estrogens, ERα in osteoblast progenitors stimulates Wnt signalling and periosteal bone accrual in response to mechanical strain. RANKL expression in B lymphocytes, but not T lymphocytes, contributes to the loss of trabecular bone caused by estrogen deficiency. In this Review, we summarize this evidence and discuss its implications for understanding the regulation of trabecular and cortical bone mass; the integration of hormonal and mechanical signals; the relative importance of estrogens versus androgens in the male skeleton; and, finally, the pathogenesis and treatment of osteoporosis.

Testicular function and bone metabolism--beyond testosterone

Findings in the past few years have advanced understanding of the crosstalk between testis and bone and could contribute to defining an improved clinical approach to the biochemical diagnosis and therapeutic management of hypogonadism and male osteoporosis. This Review focuses on the Leydig cells of the testis. Other than being responsible for steroidogenesis and production of testosterone, the function of these cells is fundamental to bone health in at least two other ways: Leydig cells produce insulin-like 3 (INSL3), which has a role in osteoblast function, and they contribute to 25-hydroxylation of vitamin D. Impairment of testicular function leads to low levels of testosterone, INSL3 and 25-hydroxyvitamin D and consequently to an increased risk of osteopenia and osteoporosis.

Estrogens in men: clinical implications for sexual function and the treatment of testosterone deficiency

INTRODUCTION

The role of estrogens in male sexual function and the pathogenesis of testosterone deficiency remain controversial and poorly understood.

AIMS

To review the distribution of estrogens in normal and testosterone deficient men, their potential role in sexual function, and the clinical implications of elevated estrogens during testosterone therapy.

METHODS

A comprehensive, broad-based literature review was conducted on the role of estrogens in male sexual function and testosterone deficiency.

RESULTS

Estrogens elicit a variety of physiological responses in men and may contribute to modulation of sexual function. In the absence of testosterone deficiency, elevations in estrogens do not appear to be harmful and estrogens may help maintain some, but not all, sexual function in castrated men. While the therapeutic use of estrogens at pharmacologic doses has been used to suppress serum testosterone, naturally occurring elevations of estrogens do not appear to be a cause of low testosterone. During testosterone replacement, estrogens may rise and occasionally reach elevated levels. There is a lack of evidence that treatment of elevated estrogen levels during testosterone replacement has benefit in terms of male sexuality.

CONCLUSION

Further research on the importance of estrogens in male sexual function is needed. Current evidence does not support a role of naturally occurring estrogen elevations in testosterone deficiency or the treatment of elevated estrogens during testosterone therapy.

Aromatase activity and bone loss in men

Aromatase is a specific component of the cytochrome P450 enzyme system responsible for the transformation of androgen precursors into estrogens. This enzyme is encoded by the CYP19A1 gene located at chromosome 15q21.2, that is, expressed in ovary and testis, but also in many extraglandular sites such as the placenta, brain, adipose tissue, and bone. The activity of aromatase regulates the concentrations of estrogens with endocrine, paracrine, and autocrine effects on target issues including bone. Importantly, extraglandular aromatization of circulating androgen precursors is the major source of estrogen in men. Clinical and experimental evidences clearly indicate that aromatase activity and estrogen production are necessary for longitudinal bone growth, the attainment of peak bone mass, pubertal growth spurt, epiphyseal closure, and normal bone remodeling in young individuals. Moreover, with aging, individual differences in aromatase activity may significantly affect bone loss and fracture risk in men.

Effects of the methanol extract of Basella alba L (Basellaceae) on steroid production in Leydig cells

In this study, Leydig cells were purified from 70 day-old Sprague Dawley male rats and incubated with 10 and 100 μg/mL of methanol extract of Basella alba (MEBa) for 4 hours followed by the evaluation of cell viability, steroid (testosterone and estradiol) production, and the level of aromatase mRNA. Results showed that MEBa did not affect Leydig cell viability. At the concentration of 10 μg/mL, MEBa significantly stimulated testosterone and estradiol production (p < 0.01 and p < 0.03, respectively), and enhanced aromatase mRNA level (p < 0.04). These observations suggest that MEBa directly stimulated testosterone, estradiol and aromatase mRNA levels in isolated Leydig cells.

Aromatase activity and bone loss

Aromatase is a specific component of the cytochrome P450 enzyme system that is responsible for the transformation of C19 androgen precursors into C18 estrogenic compounds. This enzyme is encoded by the CYP19A1 gene located at chromosome 15q21.2, that is expressed in ovary and testis not only but also in many extraglandular sites such as the placenta, brain, adipose tissue, and bone. The regulation of the level and activity of aromatase determines the levels of estrogens that have endocrine, paracrine, and autocrine effects on target issues including bone. Importantly, extraglandular aromatization of circulating androgen precursors is the major source of estrogen not only in men (since only 15% of circulating estradiol is released directly by the testis) but also in women after the menopause. Several lines of clinical and experimental evidence now clearly indicate that aromatase activity and estrogen production are necessary for longitudinal bone growth, attainment of peak bone mass, the pubertal growth spurt, epiphyseal closure, and normal bone remodeling in young individuals. Moreover, with aging, individual differences in aromatase activity and thus in estrogen levels may significantly affect bone loss and fracture risk in both genders.

Methods to quantify sex steroid hormones in bone: applications to the study of androgen ablation and administration

Bone may contain an intraskeletal reservoir of sex steroids that is capable of producing biological effects. The purposes of these experiments were to 1) establish and validate methods to extract and measure intraskeletal sex hormones, 2) compare serum and intraskeletal sex hormone abundance, and 3) determine the impact of testosterone-enanthate administration and orchiectomy on intraskeletal sex hormone concentrations. Tibiae from male F344 rats were crushed, suspended in an aqueous buffer, disrupted mechanically and sonically, extracted with organic solvents, dried, and reconstituted in assay buffer appropriate for measurement of testosterone, dihydrotestosterone, and estradiol by immunoassay. Prior to extraction, bone homogenate was spiked with [³H]testosterone, [³H]dihydrotestosterone, or [³H]estradiol, and >80% of each ³H-labeled sex hormone was recovered. Extracted bone samples were also assayed with and without known amounts of unlabeled sex hormones, and >97% of the expected hormone concentrations were measured. Administration of testosterone-enanthate increased intraskeletal testosterone 11-fold and intraskeletal dihydrotestosterone by 82% without altering intraskeletal estradiol (P < 0.01). Conversely, orchiectomy did not alter intraskeletal testosterone or estradiol but increased intraskeletal dihydrotestosterone by 39% (P < 0.05). In intact rats, intraskeletal testosterone and dihydrotestosterone were directionally higher than in serum, whereas intraskeletal estradiol was directionally lower than serum. Serum androgens were positively correlated with intraskeletal androgens (r = 0.74-0.96, P < 0.001); however, neither serum nor intraskeletal androgens nor serum estradiol were correlated with intraskeletal estradiol. We report the validation of a novel method for measuring intraskeletal sex hormones. Our findings demonstrate that the intraskeletal sex steroid reservoirs are modifiable and only partially influenced by circulating sex hormones.

Sex hormones and bone health in males

Sex steroids play a key role in maintaining skeletal integrity lifelong, through a complex variety of endocrine, but also paracrine and possibly autocrine actions. The current knowledge that androgens may act as pro-hormones for estrogens has seriously challenged many traditional views, so that, at least for their skeletal actions, these can no longer be considered exclusively "male" or "female" hormones.

Regulation of adult bone turnover by sex steroids

Recent reports reveal increasing complexity of mechanisms underlying the bone sparing effects of sex steroids. This review focuses on mechanisms by which sex steroids attenuate endocortical and trabecular adult bone turnover, perhaps their most important property as bone mass regulators. Clearly, estrogen withdrawal increases osteoclast number and bone resorption; however, important open questions are the extent to which osteoblasts and their precursors are involved, and the relative contributions of the RANK/RANKL/OPG system, Fas ligand and Runx2. In addition to reviewing these aspects of estrogen action, we also discuss proskeletal effects of androgens on the adult male skeleton, including aromatization to estrogens and male-specific mechanisms. Detailed understanding of skeletal site- and gender-dependent mechanisms by which sex steroids protect the adult skeleton will provide the foundation for improved risk assessment, prevention and management of osteoporosis.

Rates of bone loss in young adult males

Osteoporosis-related fractures occur more frequently in women compared with men, but mortality is greater in men compared with women. Peak bone mass is a significant predictor of osteoporosis and fracture risk; therefore, it is important to optimize peak bone mass during young adulthood. Several recent longitudinal studies, which are summarized in this article, have investigated bone changes among young men. Cortical bone loss does not appear to be significant until individuals reach their mid-30s and is associated with decreased sex hormone concentrations. Significant trabecular bone loss in young men aged in their 20s has been reported and is associated with reduced lean mass and activity levels, especially among former athletes. Whether changes in activity levels among nonathletes lead to bone loss among young men requires further investigation.

Recent experimental and clinical findings in the skeleton associated with loss of estrogen hormone or estrogen receptor activity

Studies on rodent models and rare human disorders of estrogen production or response have revealed an increased complexity of the actions of estrogen on bone. ERalpha disruption in human males results in delayed epiphyseal maturation, tall stature, trabecular thinning, marked cortical thinning, genu valgum and significantly reduced cortical vBMD, but trabecular number is preserved and there is normal to increased periosteal expansion. Aromatase deficiency results overall in a similar phenotype, although less is known about skeletal architecture. Importantly, estrogen replacement in these individuals, even if provided late in the third decade, may normalize aBMD. Less certain is whether there is complete recovery of normal skeletal architecture and strength. Rodent models, in general, are consistent with the human phenotype but are confounded by inherent differences between mouse and human physiology and issues regarding the completeness of the different knock-out lines. Both human and rodent studies suggest that residual effects of estrogen through ERbeta, truncated ERalpha forms or nonclassical estrogen receptors might account for different phenotypes in the hERKO man, aromatase deficient subjects and rodents. Importantly, androgen, particularly by preserving trabecular number and augmenting both periosteal and epiphyseal growth, also has significant actions on bone.

Sex steroids and the male skeleton: a tale of two hormones

Traditionally, the stronger male skeleton was considered to result from higher androgen levels in men compared to women. However, the regulation of male bone growth by sex steroids appears more complex than originally anticipated. Based on clinical observations and studies in animal models, not only androgens and androgen receptor (AR), but also estrogens and estrogen receptor-alpha (not ERbeta) are required for optimal bone mineral acquisition during male growth. In addition, both sex steroids are involved in the maintenance of male skeletal health. In fact, bone loss and fracture risk have been associated with estrogen exposure in elderly men. Overall, a compelling body of evidence suggests that both androgens and estrogens are crucial for male skeletal growth and maintenance.

Aromatase deficiency in men: a clinical perspective

Human aromatase deficiency is a very rare syndrome characterized by congenital estrogen deprivation that is caused by loss-of-function mutations in CYP19A1, which encodes aromatase. Here, we review the presentation, diagnosis and treatment of aromatase deficiency in men to provide useful advice for clinical management of the condition. At presentation, all men with aromatase deficiency have tall stature, delayed bone maturation, osteopenia or osteoporosis and eunuchoid skeletal proportions. Diagnosis of the condition is supported by the presence of unfused epiphyses and undetectable serum estradiol levels; the condition can be further substantiated by genetic sequencing of CYP19A1. Transdermal estradiol treatment at a daily dose of about 25 microg might be adequate for lifelong replacement therapy. BMD and levels of serum estradiol, luteinizing hormone and testosterone should be monitored carefully and considered powerful biochemical markers of adequate estrogen substitution in clinical practice. Early diagnosis is important to initiate estrogen therapy as soon after puberty as possible to avoid the skeletal complications that are associated with this condition.

Assessment of material, structural, and functional properties of the human skeleton by pQCT systems

Peripheral quantitative computed tomography (pQCT) systems measure bone parameters noninvasively using low radiation doses. This limits image resolution but is practical for the diagnosis and quantitative monitoring of the properties of the peripheral human skeleton. pQCT determines volumetric bone mineral density separately in trabecular and cortical bone. It may combine densitometry determinations with geometric estimates and use strain-stress indexes, and it may be used to analyze muscle variables in some areas, allowing the study of regional fragility. Experimental and clinical ex vivo studies show that pQCT variables correlate with biomechanical predictors of fragility and/or fractures. Since pQCT was approved by the US Food and Drug Administration in 1997, new skeletal regions (human femur and mandible) have been considered in the development of the system. Basically, pQCT explores intraindividual and interindividual variations in greater detail and compares the impact of skeletal diseases, risk factors, and anabolic and catabolic treatments within a given bone cross section.

Increases in bone mineral density in response to oral dehydroepiandrosterone replacement in older adults appear to be mediated by serum estrogens

CONTEXT

The mechanisms by which dehydroepiandrosterone (DHEA) replacement increases bone mineral density (BMD) in older adults are not known.

OBJECTIVE

The aims were to determine the effects of DHEA therapy on changes in sex hormones and IGF-I and their associations with changes in BMD.

DESIGN, SETTING, AND PARTICIPANTS

A randomized, double-blinded, placebo-controlled trial was conducted at an academic research institution. Participants were 58 women and 61 men, aged 60-88 yr, with low serum DHEA sulfate (DHEAS) levels.

INTERVENTION

The intervention was oral DHEA 50 mg/d or placebo for 12 months.

MAIN OUTCOME MEASURES

BMD and serum DHEAS, testosterone, estradiol (E(2)), estrone (E(1)), SHBG, IGF-I, and IGF binding protein 3 were measured before and after intervention. Free testosterone and estrogen (FEI) indices were calculated.

RESULTS

The average changes in hip and spine BMD (DHEA vs. placebo) ranged from 1.1 to 1.6%. Compared with placebo, DHEA replacement increased serum DHEAS, testosterone, free testosterone index, E(1), E(2), FEI, and IGF-I (all P < 0.001) and decreased SHBG (P = 0.02) in women and, in men, increased DHEAS, E(1), FEI (all P < 0.001), and E(2) (P = 0.02) and decreased SHBG (P = 0.037). The changes in total and regional hip BMD were associated with 12-month E(2) (all P <or= 0.001) and FEI (all P <or= 0.013). The effects of DHEA treatment were eliminated by adjustment for 12-month E(2).

CONCLUSIONS

The significant increases in hip BMD in older adults undergoing DHEA replacement were mediated primarily by increases in serum E(2) rather than direct effects of DHEAS.

Supraphysiological testosterone enanthate administration prevents bone loss and augments bone strength in gonadectomized male and female rats

High-dose testosterone enanthate (TE) may prevent hypogonadism-induced osteopenia. For this study, 3-mo-old male and female Fisher SAS rats underwent sham surgery, gonadectomy (GX), or GX plus 28 days TE administration (7.0 mg/wk). GX reduced serum sex hormones (i.e., testosterone, dihydrotestosterone, and estradiol) (P < 0.05) in both sexes and bone concentrations of testosterone (males only), and estradiol (females only). GX also elevated urine deoxypyridinoline/creatinine in both sexes and serum osteocalcin (females only), findings that are consistent with high-turnover osteopenia. GX reduced cancellous bone volume (CBV) and increased osteoid surfaces in tibia of both sexes. GX males also experienced reduced trabecular number and width and increased trabecular separation, whereas GX females experienced increased osteoblast and osteoid surfaces. Bone biomechanical characteristics remained unaffected by GX, except that femoral stiffness was reduced in females. In contrast, TE administration to GX rats elevated serum and bone androgens to supraphysiological concentrations in both sexes but altered neither serum nor bone estradiol in males. Additionally, TE did not prevent GX-induced reductions in serum or bone estradiol in females. TE also reduced markers of high-turnover osteopenia in both sexes. In males, TE prevented GX-induced changes in trabecular number and separation, CBV, and osteoid surfaces while diminishing osteoblast and osteoclast surfaces; however, these changes were not fully prevented in females. In both sexes, TE increased femoral length and femoral maximal strength to above that of Sham and GX animals while preventing the loss of femoral stiffness in females. In conclusion, TE administration appears protective of cancellous bone in male rats and augments cortical bone strength in both sexes.

Sex hormones, their receptors and bone health

Sex steroids regulate skeletal maturation and preservation in both men and women, as already recognized in the 1940s by Albright and Reifenstein. The impact of gonadal insufficiency on skeletal integrity has been widely recognized in adult men and women ever since. In the context of their skeletal actions, androgens and estrogens are no longer considered as just male and female hormones, respectively. Androgens can be converted into estrogens within the gonads and peripheral tissues and both are present in men and women, albeit in different concentrations. In the late 1980s, sex steroid receptors were discovered in bone cells. However, the understanding of sex steroid receptor activation and translation into biological skeletal actions is still incomplete. Due to the complex metabolism, sex steroids may have not only endocrine but also paracrine and/or autocrine actions. Also, circulating sex steroid concentrations do not necessarily reflect their biological activity due to strong binding to sex hormone binding globulin (SHBG). Finally, sex steroid signaling may include genomic and non-genomic effects in bone and non-bone cells. This review will focus on our current understanding of gonadal steroid metabolism, receptor activation, and their most relevant cellular and biological actions on bone.

A novel mutation in the human aromatase gene: insights on the relationship among serum estradiol, longitudinal growth and bone mineral density in an adult man under estrogen replacement treatment

OBJECTIVE

Here we report on a new case of human aromatase deficiency in a man of 26 years of age and present the results of five year follow-up during trandermal estradiol (tE2) substitution, focusing on bone growth and mineralization. The lack of patient's compliance to tE2 treatment, resulting in low but detectable serum estradiol levels, provides helpful information about the physiological estradiol needed in serum to guarantee a complete bone maturation and mineralization.

DESIGN

Clinical case report study.

METHODS

Genetic, biochemical and hormonal evaluations and the study of bone health were performed before and during estrogen treatment.

RESULTS

Eunuchoid body proportions, unfused epiphyses, tall stature, osteopenia, increase fasting insulin, mild astenozoospermia and a history of right cryptorchidism were present. Baseline serum FSH was slightly above the normal range and estradiol was undetectable. Genetic analysis revealed a pattern of compound heterozygosity due to 23 bp deletion in exon IV and a point mutation in the first nucleotide of intron IX of the CYP19A1 gene, respectively. The closure of epiphyseal cartilage, the normalization of bone BMD and bone turnover markers, and the improvement of insulin levels were reached during tE2 only when serum estradiol raised above 73 pmol/L. Sperm parameters and overweight did not improve with substitutive therapy.

CONCLUSIONS

This new case of aromatase deficiency underlines the role of estrogen on skeletal maturation, BMD, metabolic abnormalities and gonadal axis. It provides evidence on the need not only of a continuous estrogen replacement, but also of ensuring adequate estradiol levels in serum in order to ensure a complete bone maturation and mineralization and to prevent the worsening of body skeletal proportions. The comprehension of this physiological aspect has relevant clinical significance especially for the development of new therapeutic strategies useful to treat growth disorders by targeting serum estradiol in men.

Impact on bone of an estrogen receptor-alpha gene loss of function mutation

CONTEXT

The kindred described is the only known instance of a germ line loss of function mutation of estrogen receptor (ER)-alpha.

OBJECTIVE

Our objective was to assess the impact of a loss of function mutation in the ER-alpha gene on histomorphometry, bone volumetric density, bone geometry and skeletal growth, and ER-alpha heterozygosity on spine density and adult height in an extended pedigree.

DESIGN AND PARTICIPANTS

A longitudinal follow-up of the propositus with homozygous loss of function mutation of ER-alpha and single contact evaluation of the kindred were performed.

MAIN OUTCOME MEASURES

Iliac crest bone biopsy and peripheral quantitative computed tomography of propositus with serial measures of areal spine bone mineral density (aBMD) by dual-energy x-ray absorptiometry and bone age were performed. Members of pedigree were evaluated for ER-alpha mutation carrier status and spine aBMD.

RESULTS

Bone biopsy revealed marked osteopenia (cortex: 641 microm), low trabecular volume (10.6%), decreased thickness (76.2 microm), normal trabecular number, and low activation frequency (0.099/yr). Radial periosteal circumference was similar, endosteal circumference larger, and trabecular and cortical volumetric bone mineral density markedly lower (158 and 1092 mg/cm(3), respectively) than controls. Spine aBMD at age 28.5 yr (0.745 g/cm(2)) decreased to 0.684 g/cm(2) (Z score -3.85) in 3.5 yr. Bone age advanced from 15-17.5 yr. Kindred analysis revealed that gene carriers had spine aBMD Z scores less than zero (P = 0.003), but carriers and nonmutant members were similar (-0.84 +/- 0.26 vs. -0.64 +/- 0.16).

CONCLUSION

Homozygous ER-alpha disruption markedly affects bone growth, mineral content, and structure but not periosteal circumference. ER-alpha heterozygosity appears to not impair spine aBMD.

Androgens and bone

PURPOSE OF REVIEW

The present review will focus on the most important recent findings with respect to skeletal androgen action. Many studies have indicated that part of the androgen action may be related to the conversion of androgens into estrogens. Therefore, some of the most recent findings of skeletal estrogen action relevant for male skeletal physiology will also be discussed.

RECENT FINDINGS

Androgens and estrogens stimulate bone formation and inhibit bone resorption. Sex steroids may interact with different receptors, target cells and other bone anabolic pathways. Androgen receptor and estrogen receptor signalling appear to be important for male bone formation during growth. Sex steroid signalling may involve genomic and nongenomic pathways, interaction with mechanical loading, the growth hormone/insulin-like growth factor-I axis and/or other bone anabolic pathways. Estrogen receptor alpha in osteoclasts appears to regulate bone resorption in women but not men, whereas androgen receptor signalling in osteoblasts may only partly regulate bone resorption in males.

SUMMARY

The latest developments indicate that androgens and estrogens are important for male bone metabolism and homeostasis and therefore selective estrogen receptor alpha and androgen receptor signalling remain interesting drug targets for the stimulation of bone formation and male skeletal integrity.