Action of androgens versus estrogens in male skeletal homeostasis

Inhibitory effect of chrysin on estrogen biosynthesis by suppression of enzyme aromatase (CYP19): A systematic review

The cytochrome P450 enzyme functions as the rate-limiting enzyme in changing androgens to estrogens. Inhibition of aromatase is one of the significant objectives of treatment of hormone-dependent diseases such as breast cancer, especially in post-menopausal women. Natural compounds like chrysin, as a flavor that has a high concentration in honey and propolis, are rich sources of them can be useful in inhibiting aromatase for chemoprevention following treatment or in women at risk of acquiring breast cancer. This study intended to summarize the existing evidence on the effect of chrysin on aromatase activity. We systematically searched Science Direct, PubMed and Google Scholar and hand searched the reference lists of identified relevant articles, up to 5 February, 2019. Articles with English abstracts that reported the effect of chrysin on aromatase inhibition and without publication date restriction were investigated. Twenty relevant articles were chosen from a total of 1721 articles. Only one study was performed on humans and two studies were assayed on rats, while other studies were evaluated in vitro. All the studies except one showed that chrysin had the potency of aromatase inhibition; however, only one study performed on endometrial stromal cells showed that chrysin and naringenin did not indicate aromatase inhibitory properties. Various assay methods and experimental conditions were the important aspects leading to different results between the studies. Chrysin has potency in inhibition of the aromatase enzyme and thus can be useful in preventing and treating the hormone-dependent breast cancer and as an adjuvant therapy for estrogen-dependent diseases.

Food restriction causes low bone strength and microarchitectural deterioration in exercised growing male rats

The pathogenesis of bone disorders in young male athletes has not been well understood. We hypothesized that bone fragility is caused by low energy availability, due to insufficient food intake and excessive exercise energy expenditure in young male athletes. To examine this hypothesis, we investigated the influence of food restriction on bone strength and bone morphology in exercised growing male rats, using three-point bending test, dual-energy X-ray absormetry, and micro-computed tomography. Four-week-old male Sprague-Dawley rats were divided randomly into the following groups: the control (Con) group, exercise (Ex) group, food restriction (R) group, and food restriction plus exercise (REx) group after a 1-wk acclimatization period. Thirty-percent food restriction in the R and REx groups was carried out in comparison with that in the Con group. Voluntary running exercise was performed in the Ex and REx groups. The experimental period lasted 13 wk. At the endpoint of this experiment, the bone strength of the femurs and tibial BMD in the REx group were significantly lower than those in the Con group. Moreover, trabecular bone volume and cortical bone volume in the REx group were also significantly lower than those in the Con group. These findings indicate that food restriction causes low bone strength and microarchitectural deterioration in exercised growing male rats.

Eurycoma longifolia: Medicinal Plant in the Prevention and Treatment of Male Osteoporosis due to Androgen Deficiency

Osteoporosis in elderly men is now becoming an alarming health issue due to its relation with a higher mortality rate compared to osteoporosis in women. Androgen deficiency (hypogonadism) is one of the major factors of male osteoporosis and it can be treated with testosterone replacement therapy (TRT). However, one medicinal plant, Eurycoma longifolia Jack (EL), can be used as an alternative treatment to prevent and treat male osteoporosis without causing the side effects associated with TRT. EL exerts proandrogenic effects that enhance testosterone level, as well as stimulate osteoblast proliferation and osteoclast apoptosis. This will maintain bone remodelling activity and reduce bone loss. Phytochemical components of EL may also prevent osteoporosis via its antioxidative property. Hence, EL has the potential as a complementary treatment for male osteoporosis.

Daidzein administration positively affects thyroid C cells and bone structure in orchidectomized middle-aged rats

SUMMARY

Thyroid C cells hormone, calcitonine, inhibits bone resorption. We have demonstrated that daidzein treatment of orchidectomized rats (model for osteoporosis) stimulated C cells and increased trabecular bone mass. These results suggest that, besides direct action, daidzein may also affect bone structure indirectly through enhancement of thyroid C cell activity.

INTRODUCTION

Thyroid C cells produce calcitonin (CT) which acts as an inhibitor of bone resorption. In this study, the influence of daidzein treatment on thyroid C cells, bone structure, and bone function in orchidectomized (Orx) middle-aged rats was investigated.

METHODS

Sixteen-month-old Wistar rats were divided into Orx and sham-operated (SO) groups. Half the Orx rats were given subcutaneous injections of daidzein (30 mg/kg b.w./day) for 3 weeks. CT-immunopositive thyroid C cells were morphometrically analyzed. The metaphyseal region of the proximal tibia was measured histomorphometrically, and cancellous bone area (B.Ar), trabecular thickness (Tb.Th), trabecular number (Tb.N), and trabecular separation (Tb.Sp) were calculated. Serum samples were analyzed for CT and osteocalcin (OC), calcium (Ca) and phosphorus concentrations, and urine samples for Ca levels.

RESULTS

Treatment of Orx animals with daidzein significantly increased volume of C cells compared to the Orx rats. Daidzein also enhanced B.Ar, Tb.Th, and Tb.N and reduced Tb.Sp. The serum OC and urinary Ca concentrations decreased significantly in comparison with the Orx group.

CONCLUSIONS

These findings indicate that daidzein treatment stimulates thyroid C cells, increase trabecular bone mass, and decrease bone turnover in Orx middle-aged rats, which is the model of male osteoporosis.

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.

The effects of long-term raloxifene and estradiol treatments on bone in a patient with congenital aromatase deficiency

INTRODUCTION

In adult aromatase-deficient men, estrogen treatment has always resulted in a rapid skeletal maturation with epiphyseal closure and improved BMD. Raloxifene is a SERM with proven estrogen agonist action on bone that leads to an improvement in BMD and a reduction in bone turnover. The present study reports the effects of raloxifene and transdermal estradiol treatment, respectively, on epiphyseal closure and BMD in an aromatase-deficient man, over a 24-month follow-up, with the aim of obtaining further insight into the role of estrogens in the male skeletal homeostasis.

MATERIALS AND METHODS

A 25-year-old Caucasian man with aromatase deficiency, a bone age of 15.3 years, unfused epiphyses and an impaired BMD was initially administered raloxifene (60 mg/day per os) for 12 months, while transdermal estradiol (25 microg twice weekly) was administered for the subsequent 12 months. During the follow-up, the effects of the two treatments on epiphyseal closure, BMD and bone turnover markers were investigated. An iliac crest bone biopsy was performed only before and after the raloxifene treatment, but it was not repeated after transdermal estradiol treatment.

RESULTS

No changes in bone age were observed after raloxifene therapy, whereas a complete epiphyseal closure was achieved with transdermal estradiol treatment. Compared with baseline values, raloxifene treatment led to improved BMD both at the ultradistal forearm and 33% radius; the transdermal estradiol treatment resulted in a further slight increase in BMD at the 33% radius, but not at the ultradistal forearm. The baseline bone biopsy showed elevated bone remodelling in trabecular bone, while the second biopsy following raloxifene treatment revealed a decrease in remodelling.

DISCUSSION

This study shows that the management of aromatase deficiency in the male cannot consider raloxifene as a first choice treatment, but should be still based on estrogen replacement treatment since in this patient the completion of bone maturation has only been obtained once estradiol substitution was performed. The present case also demonstrates that raloxifene is able to improve BMD in aromatase-deficient men.

Orchiectomy upregulates free soluble RANKL in bone marrow of aged rats

The osteoprotegerin (OPG)/receptor activator of NF-kappaB ligand (RANKL) axis is thought to be involved in the upregulation of bone turnover following sex steroid deficiency. Here, we investigated the effects of orchiectomy (ORX) on bone turnover and free soluble RANKL (sRANKL) in aged rats. Free, bioactive sRANKL is a critical determinant and key mediator for survival and activity of mature osteoclasts. Thirty-three 9-month-old male Fischer-344 rats were either ORX or sham-operated (SHAM). Following in vivo fluorochrome labeling, vehicle (ricinus oil/benzyl benzoate)-treated SHAM and vehicle- or testosterone undecanoate (T, 6 mg/kg s.c. once weekly)-treated ORX rats (n=8-9 each) were killed 2 months after surgery. Vehicle-treated ORX rats showed lower seminal vesicle weight, loss of proximal tibial trabecular bone mineral density, and reduced cortical thickness at the tibial shaft as measured by peripheral quantitative computed tomography relative to SHAM controls. Bone loss in vehicle-treated ORX rats was associated with enhanced bone turnover as evidenced by increases in tibial cancellous bone formation rate, osteoclast numbers, urinary excretion of calcium and deoxypyridinoline, and serum osteocalcin. T treatment of ORX rats restored seminal vesicle weight to SHAM control levels, and completely protected against post-ORX bone loss by suppressing bone turnover. Free sRANKL concentrations in bone marrow supernatants harvested from the proximal femur were about 3-fold higher in vehicle-treated ORX relative to SHAM rats, and returned to SHAM control levels in T-treated ORX rats. mRNA abundance of matrix metalloproteinase-14 (MMP-14) in bone marrow was 4-fold higher in vehicle-treated ORX rats relative to SHAM rats. T treatment of ORX rats suppressed MMP-14 mRNA expression to SHAM control levels. We conclude that orchiectomy increases the concentration of free sRANKL in bone marrow of aged rats. In addition, increased shedding of membrane-bound RANKL by MMP-14 may be a pivotal mechanism resulting in augmented free sRANKL concentrations in the bone marrow environment after androgen withdrawal.

Sex steroids, periodontal health, and tooth loss in older men

Sex steroids have a significant effect on skeletal biology in men, with reduced levels being associated with lower skeletal bone mass and cortical thickness. The purpose of this study was to determine if sex steroids are associated with periodontitis and tooth loss in a cohort of 1210 older dentate men followed for 3 years. Periodontal measures included attachment loss, pocket depth, gingival bleeding, and number of teeth. Baseline serum testosterone and estradiol were measured by radioimmunoassay. Severe periodontitis was common at baseline (38%), and progression occurred in 32% of the cohort. Incident tooth loss occurred in 22% of the cohort. Testosterone, estradiol, and sex hormone binding globulin (SHBG) concentrations were not related to baseline periodontal status or number of teeth. Moreover, there was no relationship between sex steroid levels and periodontitis progression or incident tooth loss. Although periodontitis, progression of periodontitis, and tooth loss are common in older men, they were not associated with sex steroids.

Bone properties in patients with acromegaly: quantitative ultrasound of the heel

Growth hormone (GH) deficiency and acromegaly serve as good models for investigating the effects of GH on bone remodeling. However, the results from various studies are rather conflicting. The aim of our study was to estimate the potential role of gender, disease activity, and duration on both calcaneus quantitative ultrasound (QUS) parameters and bone turnover markers in patients with acromegaly. Thirty-six acromegalic patients (17 men, 19 women) and 3 age- and gender-adjusted controls for every patient were included in the study. The disease was active in 22 patients, and was considered cured in 14 of them. In each subject, QUS of the heel and parameters of bone turnover (bone alkaline phosphatase, beta-crosslaps, and osteocalcin) were measured. The results demonstrated lower QUS values in acromegalic patients compared with the controls. When stratified by gender, the differences in QUS parameters were significant in men, but not in women. Male patients with active disease had significantly lower QUS values than those in remission. Such differences were not observed among women. Multiple regression model indicated strong association between disease activity and the QUS parameters. The group of patients with active disease had a higher level of serum beta-crosslaps, whereas osteocalcin concentration was significantly increased only in male patients with active disease. The results of our study suggest significantly lower QUS values and increased bone turnover in male patients with active acromegaly. The disease activity is the strongest predictor of the QUS parameters in acromegalic patients.

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

The relative contribution of each sex steroid (i.e. estrogen and androgen) on bone in men and the relationships among sex steroids and changes in BMD and bone strength are still unknown. A defective BMD of bone tissue is constantly present in men with aromatase deficiency. This study evaluates the effects of different regimens of treatment with sex steroids over 7.3 years follow-up on BMD in an adult man affected by aromatase deficiency and by a concomitant mild hypogonadism, as previously described. The aim of the study is to provide additional data on the relative roles of androgens and estrogens in male bone metabolism. The effects of testosterone (T) treatment alone and estrogen (tE(2)) treatment alone as well as the effects of the combined treatment with testosterone and estradiol (T plus tE(2)) on areal BMD (aBMD) at dual-energy X-ray absorptiometry (DXA) and the effects of T plus tE(2) on volumetric BMD (vBMD), particular at cortical site, measured by peripheral quantitative computed tomography (pQCT), are investigated. Hormones and markers of bone turnover were monitored during all phases of the study. Treatment with tE(2) normalized serum estradiol, but only the combined treatment with T plus tE(2) normalized both serum estradiol and testosterone. Markers of bone turnover reached a pattern close to normality during T plus tE(2). The aBMD was little modified by T, but increased more during tE(2). T plus tE(2) resulted in a further increase in both aBMD at DXA and vBMD at pQCT. Cortical thickness increased during T plus tE(2) both in radius and tibia. Only the combined treatment led to optimal parameters of aBMD suggesting that testosterone needs estrogens as a permissive factor for a direct androgen anabolic action on bone in men.

The diagnosis and treatment of male osteoporosis: Defining, assessing, and preventing skeletal fragility in men

Male osteoporosis is associated with a significant burden in terms of morbidity, mortality, and economic cost. Despite recent advances in the understanding of the male osteoporotic syndrome, the evaluation and treatment of men suffering from osteoporosis remains a clinical challenge. In men with osteoporosis, it remains particularly critical to exclude underlying pathological causes as these are much more likely to be present than in women. There is increasing evidence that the approaches developed to diagnose and treat the disorder in women may be equally useful in men. The available evidence suggests that the anti-fracture efficacy of treatment with alendronate, risedronate, or teriparatide is similar in both sexes. Additional research is warranted to prospectively address the usefulness of BMD measurements to predict fracture risk, to identify those men who are likely to benefit the most from therapy, and to monitor individual responses to therapy.

Influence of soy aglycon isoflavones on bone-related traits and lens protein characteristics of ovariectomized rats and bioactivity performance of osteoprogenitor cells

Health benefits of soy isoflavones have attracted the concern of the public and the interest of health-care professionals. In this study, two trials were conducted in characterizing bone-related traits and lens proteins as affected by supplementation of soy aglycon isoflavones (SAI). In trial 1, an in vivo study, 20 Sprague-Dawley rats were ovariectomized (OVX) and randomly distributed into OVX and OVX+SAI (135 mg of SAI/kg of feed; 8.33 mg/kg body weight; 2.5 mg/day) groups. Another group containing 10 rats with a sham operation was control (Sham). The experiment period was 3 months, and the rats were subjected to bone-related traits and lens protein characterization. In trial 2, an in vitro study, osteoprogenitor cells (UMR-106) were divided into SAI-supplemented (0.5 mg of SAI/mL of medium) and unsupplemented groups. Results of the in vivo study indicated that daily BW gains in the OVX and OVX+SAI groups were greater than that of the Sham group (p < 0.05). Bone ash and Ca contents of the Sham and OVX+SAI groups were higher than those of the OVX group (p < 0.05), while bone density, strength, and phosphorus contents among groups varied insignificantly (p > 0.05). When the lens proteins were extracted and analyzed with size-exclusion HPLC, the contents of beta- and gamma-crystallins were lowest in the OVX group and the protein solubility decrease could be recovered by dietary SAI supplementation (shown by OVX+SAI group). Based on Raman spectra of the isolated lens proteins, disulfide bonds were observed more in OVX lens than in the Sham and OVX+SAI lens. Results of in vitro study with osteoprogenitor cells revealed that cell viability, alkaline phosphatase activity, osteocalcin, and Ca contents of the SAI-supplemented group were higher than those of the unsupplemented group (p < 0.05). The likely potency to enhance bone and lens health by SAI supplementation is worth pointing out.

Estrogen and androgen play distinct roles in bone turnover in male mice before and after reaching sexual maturity

Aromatase is the sole enzyme which converts androgen into estrogen. We have reported that aromatase-knockout (ArKO) mice showed bone loss by increased bone resorption not only in female but also in male mice, suggesting essential roles of estrogen in bone metabolism in both sexes. However, loss of testicular androgen by orchidectomy (ORX) could induce bone loss in male mice. To clarify the relationship between estrogen and androgen in bone metabolism in male mice, 7-week-old ArKO mice were orchidectomized (ORX) to induce a double deficiency of estrogen and androgen. Bone loss in ORX/ArKO mice was more severe than that in ORX/wild-type and sham/ArKO mice because of advanced bone resorption, indicating that androgen and estrogen individually regulate bone mass by suppressing bone resorption in male mice after reaching sexual maturity. Cortical bone formation was elevated in sham/ArKO mice, but ORX did not influence cortical bone formation in the adult male mice. To examine the influence of androgen deficiency in weaning stage, 3-week-old wild-type mice were orchidectomized. Four weeks after operation, periosteal bone formation in the femur was markedly reduced in ORX mice. Since cortical bone in the same age of ArKO mice was normal, testicular androgen is indispensable for cortical bone formation especially at puberty in male mice. Therefore, estrogen and androgen may play distinct roles in bone turnover of male mice before and after reaching sexual maturity.

Androgen-induced mineralization by MC3T3-E1 osteoblastic cells reveals a critical window of hormone responsiveness

Despite their clinical importance for skeletal growth and homeostasis, the actions of androgens on osteoblastic cells are not well understood. MC3T3-E1 cells, a nontransformed murine preosteoblastic cell line, that traverse the stages of osteoblastic differentiation within 30 days in vitro, were exposed to mibolerone (an androgen receptor (AR) agonist) or 5alpha-dihydroxytestosterone (DHT) from days 3 to 30 post-plating. Cells exposed to this hormonal regimen exhibited a significant increase in mineralization (calcium deposition) compared to vehicle-treated cells. Delaying treatment for 4-11 days (treatment still completed on day 30 post-plating) enhanced mineralization further. Within 2 days post-plating, AR protein increased 7.2-fold in androgen-treated cells and 2.5-fold in vehicle-treated cells. MC3T3-E1 cells transfected with an androgen- and glucocorticoid-responsive reporter construct on day 1 post-plating followed by a 2 day exposure to DHT, mibolerone, or dexamethasone (dex; a glucocorticoid receptor agonist) exhibited reporter gene activation only with dex treatment. In contrast, delaying transfection and treatment for at least 1 day resulted in comparable androgen- and dex-mediated reporter gene transactivation. Therefore, the ability of MC3T3-E1 cells to respond to androgens is dependent on the timing of androgen administration.

Hormones and bone health in postmenopausal women

Although it has been known for some time that estrogen deficiency is a major pathogenetic factor for osteoporosis related fractures among postmenopausal women, the capability of estrogen (with or without a progestin) to prevent fractures has often been questioned. The publication of the data from the two hormone clinical trials of the Women's Health Initiative lays that discussion to rest. In both studies what have been considered a standard dose of conjugated estrogen with or without medroxyprogesterone acetate significantly reduced the risk of all fractures, including clinical vertebral fractures and hip fracture, in a population of postmenopausal women, average age 63 yr, not selected for osteoporosis by BMD. These results are particularly impressive given the difficulty of finding a fracture benefit in lower risk populations with other anti-resorptive agents. Surrogate data on lower doses of hormone therapy suggest a fracture benefit would be seen if studies were to be done. The other outcomes in WHI make it important to define appropriate clinical guidelines for use of hormone therapy for prevention of fractures in postmenopausal women.

Idiopathic or Hypogonadal Osteoporosis in Men

Osteoporosis is being recognized increasingly in men, and represents a substantial public health problem. As the male population ages and lives longer, the incidence of osteoporotic fractures is expected to increase. The current lifetime risk for a fragility fracture is approximately 27% in men aged 50 years or more, and will increase further over the next 20 years. A major problem with osteoporosis in men is that it continues to be unrecognized, and the majority of men with fragility fractures due to osteoporosis are not being treated. A higher level of awareness is required amongst both general practitioners and the general public that osteoporosis is a treatable condition that can affect men. Secondary causes for osteoporosis are more common in men than in women, and require rigorous exclusion and treatment. Undiagnosed clinical hypogonadism is a common cause of osteoporosis in men, and is readily treatable. The cause of primary osteoporosis in men is unknown, but it results in an osteoblast defect. Genetic factors are likely to be important. In some but not all men, relative estrogen deficiency contributes to rapid rates of age-related bone loss and fractures. An adequate calcium intake, regular weight-bearing exercise, and normal vitamin D status are all very important, particularly with increasing age. The role of testosterone in treating eugonadal men with osteoporosis is currently unclear, and larger prospective studies will be required to carefully evaluate the benefits and risks of therapy. First-line treatment of osteoporosis in hypogonadal or eugonadal men is with bisphosphonates. Alendronate increases bone density and reduces vertebral fractures measured using a semiquantitative method in eugonadal or hypogonadal men with osteoporosis. In the near future, it is likely that subcutaneous human parathyroid hormone (1-34) or teriparatide will also be available as an important new anabolic treatment for men with osteoporosis. Teriparatide treatment also increases bone density in men. Selective estrogen receptor modulating drugs require further evaluation in men, but would appear to theoretically benefit men, especially those with low estradiol levels. In the future, selective androgen receptor modulating drugs may be useful in the prevention and treatment of osteoporosis, and in increasing lean body mass in men, without having adverse effects on prostate and breast tissue.

Signaling cross-talk from Gbeta4 subunit to Elk-1 in the rapid action of androgens

Androgens act on transcription via intracellular androgen receptors (ARs), but they also have rapid AR-independent effects. We have identified the multistep processes involved in the rapid actions of androgens in male osteoblasts, which also possess the classical AR. Incubating cells with 5alpha-dihydroxytestosterone (100 pm, DHT) rapidly increased (1 min) the phosphorylation of the transcription factor Elk-1, and this was inhibited by pertussis toxin (PTX). DHT activated ERK1/2, a substrate of Elk-1, within 15 s but had no effect on p38 MAPK or JNK/SAPK. The inhibitors PD98059 (MEK1/2); Gö6976, Gö6983, and chelerythrine (protein kinase C); wortmannin and LY294002 (phosphatidylinositol 3-kinase); PP1 (Src); and PTX all blunted the DHT-stimulated phosphorylation of ERK1/2. DHT increased the phosphorylation of c-Raf-1 within 5 s; this was blocked by conventional protein kinase C and phosphatidylinositol 3-kinase inhibitors. The first activated membrane protein was the PTX-sensitive Gbeta(4) subunit coupled to phospholipase C-beta2, which triggered a rapid (5 s) increase in intracellular calcium and diacylglycerol formation. The androgen antagonist cyproterone acetate did not modify the responses to DHT. Lastly an anti-AR antibody directed against the ligand binding domain recognized a protein at the plasma membrane. The cascade of rapid effects triggered by androgens may involve the classical AR at the plasma membrane or an uncharacterized form of AR that is insensitive to nuclear antagonists.

Colles fracture, spine fracture, and subsequent risk of hip fracture in men and women. A meta-analysis

BACKGROUND

In postmenopausal women, a history of any fracture is an important risk factor for a future hip fracture. Whether similar findings apply to aging men remains to be established. We conducted a systematic review and meta-analysis of the literature to compare men and women with respect to the relative risk of hip fracture after a wrist or spine fracture.

METHODS

Studies published in full from January 1982 through September 2002 in English, French, or German were identified from the PubMed database and from reference lists of retrieved articles. We included cohort studies that reported fractures associated with minimal trauma of the wrist or spine as a risk factor for a subsequent hip fracture among (white) women and men who were fifty years old or older. Data were extracted by two independent reviewers and were checked for accuracy in a second review. Differences in assessments were resolved by consensus of the two reviewers.

RESULTS

Nine cohort studies were included in this meta-analysis: five studies were conducted in the United States and four, in Europe. After homogeneity of association was demonstrated across all studies, a fixed-effects meta-analysis was used to calculate pooled relative risks with 95% confidence intervals. Among postmenopausal women, the relative risks for a future fracture of the hip after a fracture of the wrist or spine were 1.53 (95% confidence interval, 1.34 to 1.74; p < 0.001) and 2.20 (95% confidence interval, 1.92 to 2.51; p < 0.001), respectively. In older men, these relative risks were 3.26 (95% confidence interval, 2.08 to 5.11; p < 0.001) and 3.54 (95% confidence interval, 2.01 to 6.23; p < 0.001), respectively. Fractures of the distal part of the radius increased the relative risk of hip fracture significantly more in men than in women (p = 0.002). The impact of a spine fracture, conversely, did not differ between genders (p = 0.11). Sensitivity analyses with use of random-effects methodology confirmed these findings to be robust.

CONCLUSIONS

This meta-analysis suggests that a previous spine fracture has an equally important impact on the risk of a subsequent hip fracture in both genders. The prospective association between a Colles fracture and a subsequent hip fracture, however, is significantly stronger among men than among postmenopausal women. Men with a Colles fracture are at high risk for a future hip fracture and should be evaluated as candidates for preventive measures.

A osteoporose e os distúrbios endócrinos da tireóide e das gônadas

Apesar da dedicação incessante dos pesquisadores no estudo da osteoporose, muito ainda necessita ser elucidado. A deficiência dos esteróides sexuais, principalmente a de estrógeno, é considerada a principal causa de osteoporose, embora existam inúmeros outros fatores envolvidos. O hipertireoidismo, por exemplo, é considerado um dos fatores de risco para indução ou agravamento da osteoporose e tem despertado o interesse para o estudo dos efeitos de T3 e T4 sobre o metabolismo ósseo. Embora o hipotireoidismo e a afuncionalidade das gônadas seja uma associação freqüente na mulher, a hipofunção da tireóide não é considerada fator de risco para a osteoporose da menopausa. Assim, o estudo da inter-relação entre os distúrbios endócrinos, tão comuns na idade avançada, e a osteoporose é fundamental, pois deste conhecimento poderão advir meios de controle e tratamento adequados, bem como a definição da real natureza do distúrbio ósseo. O objetivo desta revisão é apresentar e discutir alguns aspectos da osteoporose e sua inter-relação com os distúrbios endócrinos da tireóide e das gônadas.

Kinase-mediated regulation of common transcription factors accounts for the bone-protective effects of sex steroids

It has been found that 4-estren-3alpha,17beta-diol, a synthetic ligand for the estrogen receptor (ER) or androgen receptor (AR), which does not affect classical transcription, reverses bone loss in ovariectomized females or orchidectomized males without affecting the uterus or seminal vesicles, demonstrating that the classical genotropic actions of sex steroid receptors are dispensable for their bone-protective effects, but indispensable for their effects on reproductive organs. We have now investigated the mechanism of action of this compound. We report that, identically to 17beta-estradiol or dihydrotestosterone, but differently from raloxifene, estren alters the activity of Elk-1, CCAAT enhancer binding protein-beta (C/EBPbeta), and cyclic adenosine monophosphate-response element binding protein (CREB), or c-Jun/c-Fos by an extranuclear action of the ER or AR, resulting in activation of the Src/Shc/ERK pathway or downregulation of JNK, respectively. All of these effects are non-sex specific, require only the ligand-binding domain of the receptor, and are indispensable for the antiapoptotic action of these ligands on osteoblastic and HeLa cells. Moreover, administration of 17beta-estradiol or 4-estren-3alpha,17beta-diol to ovariectomized mice induces phosphorylation of ERKs, Elk-1, and C/EBPbeta, downregulates c-Jun, and upregulates the expression of egr-1, an ERK/SRE target gene. Kinase-initiated regulation of commonly used transcription factors offers a molecular explanation for the profound skeletal effects of sex steroid receptor ligands, including synthetic ones that are devoid of classical transcriptional activity.

Men, bone and estrogen: unresolved issues

Gonadal function has long been known to be important for skeletal health in men. Prepubertal hypogonadism is clearly associated with impairment in peak bone mass development and adult-onset hypogonadism with accelerated bone loss. Gonadal failure results in deficits in both androgen and estrogen action, but traditionally androgens were assumed to have the most important skeletal effect in men. Recently that model has been reconsidered as a variety of kinds of evidence have appeared to document a critical role for estrogen in bone physiology. As a result of this fresh perspective a host of interesting new dilemmas and hypotheses are being examined, including those related to the mechanisms of sex steroid action in bone, the origins of gender differences in skeletal morphology and physiology, and the role of estrogen in diagnostic and therapeutic strategies in men with metabolic bone disorders.

Sex steroids, ANGELS and osteoporosis

Osteoporosis is characterized by reduced bone density and strength. Bone mass peaks between age 30 and 40 and then declines. This can be accelerated by factors including menopause and insufficient dietary calcium. Hormone replacement therapy (HRT) is currently the standard treatment for osteoporosis. However, growing concern over potential side effects of HRT has driven a search for alternative therapies. A recent report 1 reveals a potential alternative to HRT: a gender-neutral synthetic steroid that increases bone mass and strength without affecting reproductive organs. This compound acts via a novel extranuclear sex steroid receptor signaling mechanism that has important implications for nuclear receptor biology and human health.

Oestrogen receptor specificity in oestradiol-mediated effects on B lymphopoiesis and immunoglobulin production in male mice

Oestrogen treatment down-regulates B lymphopoiesis in the bone marrow of mice. Meanwhile it up-regulates immunoglobulin production. To understand better the oestrogen action on bone marrow male mice lacking oestrogen receptor alpha (ERalpha; ERKO mice), lacking ERbeta (BERKO mice), lacking both receptors (DERKO mice) or wild-type (wt) littermates were castrated and treated for 2.5 weeks with 30 microg/kg 17beta-oestradiol (E2) or vehicle oil as controls. The B lymphopoiesis in the bone marrow was examined by flow cytometry and mature B-cell function was studied using an ELISPOT assay enumerating the B cells in bone marrow and spleen that were actively producing immunoglobulins. In wt mice the frequency of B-lymphopoietic (B220+) cells in the bone marrow decreased from 15% to 5% upon E2 treatment. In ERKO and BERKO mice significant reduction was seen but not of the same magnitude. In DERKO mice no reduction of B lymphopoiesis was seen. In addition, our results show that E2 mediated reduction of different steps in B lymphopoiesis require only ERalpha or both receptors. In wt and BERKO mice E2 treatment resulted in significantly increased levels of B cells actively producing immunoglobulin, while in ERKO and DERKO mice no such change was seen. Similar results were found in both bone marrow and spleen. In conclusion our results clearly show that both ERalpha and ERbeta are required for complete down-regulation of B lymphopoiesis while only ERalpha is needed to up-regulate immunoglobulin production in both bone marrow and spleen.

The structural and hormonal basis of sex differences in peak appendicular bone strength in rats

To identify the structural and hormonal basis for the lower incidence of fractures in males than females, sex differences in femoral mid-shaft geometry and breaking strength were studied in growth hormone (GH)-replete and -deficient male and female rats. Sexual dimorphism appeared during growth. Cortical thickening occurred almost entirely by acquisition of bone on the outer (periosteal) surface in males and mainly on the inner (endocortical) surface in females. By 8 months of age, males had 22% greater bone width and 33% greater breaking strength than females. Gonadectomy (Gx) at 6 weeks reduced sex differences in bone width to 7% and strength to 21% by halving periosteal bone formation in males and doubling it in females. Gx had no net effect on the endocortical surface in males but abolished endocortical bone acquisition in females. GH deficiency halved periosteal bone formation and had no net effect on the endocortical surface in males, but abolished bone acquisition on both surfaces in females, leaving males with 17% greater bone width and 44% greater breaking strength than females. Sex hormone deficiency produces greater bone fragility in males than females by removing a stimulator of periosteal growth in males and removing an inhibitor of periosteal growth in females. GH deficiency produces less bone fragility in males than females because males retain androgen-dependent periosteal bone formation while bone acquisition on both surfaces is abolished in females. Thus, periosteal growth is independently and additively stimulated by androgens and GH in males, inhibited by estrogen, and stimulated by GH in females. The hormonal regulation of bone surfaces establishes the amount and spatial distribution of bone and so the sexual dimorphism in its strength.

Bone loss associated with the use of LHRH agonists in prostate cancer

Hypogonadism is a recognised cause of osteoporosis in men. When patients with advanced prostate cancer are treated with luteinising hormone releasing hormone (LHRH) agonist analogues their circulating testosterone levels decline and these patients may develop fractures.We have undertaken a cross-sectional study on a cohort of patients treated with goserelin (n=41) and compared their bone density and bone turnover with patients with prostate cancer not on goserelin and elderly patients living in the community.There was no difference in bone density between the patients on treatment and those living in the community and there was a similar incidence of osteoporosis (50 and 42%, respectively). The bone marker measurements were higher in the treated patients: urine N-telopeptide (NTX) 80.1 (9) (mean (s.e.)) BCE/mmol, compared to 30.1 (2.9), P<0.001 in elderly patients; and bone alkaline phosphatase 41.9 (6.1) u/l in treated patients and 20.7 (1.5) in untreated prostate cancer patients, P<0.002. Patients on treatment with radionuclide scan evidence of metastases did not have higher bone marker values than those with negative scans.As increased bone turnover and low bone density are associated with enhanced risk of osteoporotic fractures, we suggest that patients on LHRH agonist analogues should receive advice and possibly anti-bone resorptive treatment with bisphosphonates to prevent further bone loss and fractures.Prostate Cancer and Prostatic Diseases (2001) 4, 161-166.

Evidence from the aged orchidectomized male rat model that 17beta-estradiol is a more effective bone-sparing and anabolic agent than 5alpha-dihydrotestosterone

This study was designed to evaluate the impact of estrogen versus androgen action on orchidectomy (ORX)-induced bone loss and associated changes in body composition. During an experimental period of 4 months, aged (12-month-old) ORX rats were treated with 17beta-estradiol (E2; 0.75 microg/day) or different doses of the nonaromatizable androgen 5alpha-dihydrotestosterone (DHT; 45, 75, and 150 microg/day, respectively), via subcutaneous (sc) silastic implants. Low doses of DHT and E2 inhibited the ORX-induced rise of bone turnover markers (serum osteocalcin and urinary deoxypyridinoline [DPD]) to a similar extent. High-dose DHT prevented the ORX-induced decrease of trabecular bone density but had no significant effect on cortical thinning as assessed by peripheral quantitative computed tomography (pQCT). This bone-sparing action of DHT occurred at the expense of hypertrophy of the ventral prostate and seminal vesicles. On the other hand, E2 restored both trabecular bone density and cortical thickness in ORX rats and even prevented age-related bone loss. In contrast to DHT, E2 increased lean body mass and inhibited the ORX-associated increase of fat mass, as measured by DXA. Administration of E2 was associated with increased serum concentrations of insulin-like growth factor (IGF) I and decreased circulating levels of leptin. We conclude that, in the aged ORX rat model, E2 is more effective in preventing ORX-induced bone loss than DHT. Additionally, E2 has anabolic effects on muscle tissue and prevents the ORX-related increase of fat mass. Overall, these data suggest that androgen action on bone and body composition is dependent on stimulation of both androgen receptors (ARs) and estrogen receptors (ERs).

Sex steroid metabolism in the tibial growth plate of the rat

To assess whether growth plate-specific production of sex steroids is possible, we have surveyed the presence of several key-enzymes involved in androgen and estrogen metabolism in the tibial growth plate of female and male rats during development. Using in situ hybridization, mRNAs of aromatase p450, type I and II 17beta-hydroxysteroid dehydrogenase (HSD), steroid sulfatase (STS), and 5alpha-reductase were detected in proliferating and hypertrophic chondrocytes of the growth plate. The former three were strongly up-regulated around sexual maturation (7 wk), whereas the latter two were expressed at a relatively constant level during development. These data were supported by measuring aromatase, type I 17beta-HSD, and STS enzyme activities in chondrocytes collected from tibial growth plates at 1 and 7 wk of age. Of the enzymes studied, there were minor differences between the sexes in aromatase and 5alpha-reductase expression only. In conclusion, our findings clearly indicate the presence of various enzymes involved in sex steroid metabolism in the tibial growth plate, especially in sexually maturing rats, a timepoint at which sex steroids have major effects on longitudinal growth. Our data suggest that intracrinology in the rat growth plate can occur and may be a major source of local sex steroid delivery.

Genistein, a soybean isoflavone, affects bone marrow lymphopoiesis and prevents bone loss in castrated male mice

Soybean isoflavones exhibit selective effects on bone metabolism in postmenopausal women as well as in ovariectomized animals. Recently, the role of estrogen in bone metabolism in men has also received attention, because a man with a mutated estrogen receptor-alpha (ER(alpha)) gene will exhibit osteoporotic phenotypes. To examine the possible role of genistein, a soybean isoflavone, in bone marrow hemopoiesis and bone metabolism in men, male mice were orchidectomized (orx) and treated with genistein (0.4-0.8 mg/day) or 17beta-estradiol (E(2); 0.03 microg/day) subcutaneously for 3 weeks. In orx mice, seminal vesicle weight decreased markedly, and it was not affected by the administration of genistein or E(2). The number of bone marrow cells was markedly increased after orx, and the majority was B-220 weakly positive pre-B cells. Increased B-lymphopoiesis was restored completely by E(2) or genistein administration. In orx mice, bone mineral density of the femur decreased markedly, and this bone loss was prevented to a significant extent by treatment with genistein as well as E(2). Histomorphometry showed that the trabecular bone volume in the femoral distal metaphysis decreased markedly after orx, and genistein and E(2) prevented this bone loss. These results suggest that soybean isoflavones prevent bone loss due to androgen deficiency in males.

Genetics of osteoporosis: role of steroid hormone receptor gene polymorphisms

Osteoporosis is a common skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue with a consequent increase in bone fragility and susceptibility to fracture. In the past years, twin and family study have shown that this disease recognizes a strong genetic component and that genetic factors play an important role in regulating bone mineral density (BMD). While in few isolate conditions osteoporosis can be inherited in a simple Mendelian pattern, due to single gene mutations, in the majority of cases has to be considered a multifactorial polygenic disease in which genetic determinants are modulated by hormonal, environmental and nutritional factors. Given the important role that steroid hormones play in bone cell development and in the maintenance of normal bone architecture, polymorphisms at receptor of the steroid/thyroid hormone receptor superfamily, such as estrogen receptor alpha (ERalpha) and Vitamin D receptor (VDR) have been thoroughly investigated in the last years and appeared to represent important candidate genes. The individual contribution of these genetic polymorphisms to the pathogenesis of osteoporosis remains to be universally confirmed and an important aim in future work will be to define their functional molecular consequences and how these polymorphisms interact with each other and with the environment to cause the osteoporotic phenotype. A further promising application of genetic studies in osteoporosis comes from their pharmacogenomic implications, with the possibility to give a better guidance for therapeutic agents commonly used to treat this invalidating disorder or to identify target molecules for new therapeutic agents.

Clinical review 144: Estrogen and the male skeleton

Because estrogen (E) and T are the major sex steroids in women and men, respectively, the traditional view had been that E primarily regulated bone turnover in women and T played the analogous role in men. The description of ER- deficient and aromatase-deficient males, however, initiated a major shift in our thinking on the relative roles of T and E in regulating the male skeleton, because these individuals all had unfused epiphyses, high bone turnover, and osteopenia. Similar, albeit less striking, findings were noted in mouse models with knock-out of either the ER-alpha or the aromatase genes. Although these human experiments of nature and mouse knock-out models clearly demonstrated an important role for E in the growth and maturation of the male skeleton, they did not define the role of E vs. T in regulating the adult male skeleton. The past several years have witnessed an accumulation of evidence from observational as well as direct interventional studies that now clearly indicates that E plays a major, and likely dominant, role in bone metabolism in men. These data also suggest that a threshold level of bioavailable (or non-SHBG bound) E is needed for skeletal E sufficiency in the male, and that with aging, an increasing percentage of elderly men begin to fall below this level. It is this subset of men who may be at greatest risk for the development of age-related bone loss and osteoporosis. Moreover, these men may also be the ones most likely to respond favorably to treatment with selective E receptor modulators, or perhaps even to T replacement, because the skeletal effects of the latter may be mediated largely via aromatization to E.

Inverse association between bone turnover rate and bone mineral density in community-dwelling men >70 years of age: no major role of sex steroid status

Bone loss is accelerated in elderly men. Little is known about the pathophysiology of senile bone loss or about the role played by relative sex steroid deficiency in the determination of bone turnover in elderly men. In a population-based sample of 283 healthy, ambulatory men, aged 71-86 years, we sought to determine whether lower bone mineral density (BMD; using dual-energy X ray absorptiometry at the hip and the forearm) is associated with higher bone turnover, and we assessed the impact of sex steroid status on bone turnover. Indices of bone formation, serum osteocalcin (s-Oc), and bone-specific alkaline phosphatase (s-bAP) and indices of bone resorption, serum and urinary telopeptide of type I collagen (s-CTx and u-CTx), and urinary free deoxypyridinoline (u-Dpd) were intercorrelated (r = 0.29-0.76, p < 0.001). Bone turnover indices were negatively associated with BMD (r = -0.17 to -0.34, p < 0.01). In univariate analyses, there was a trend toward weak negative associations of bone turnover markers with serum free testosterone (FT), significant only for s-Oc and s-CTx (r = -0.16 and -0.14, p < 0.01), and with serum free estradiol (FE(2)), significant only for u-CTx and s-CTx (r = -0.18 and -0.19; p < 0.01). The lower quartile for FE(2) was associated with higher values of u-CTx (p = 0.003) and s-CTx (p < 0.001). However, in multivariate models, for the individual markers of bone turnover a negative association between estradiol (E(2)) or FE(2) and s-CTx was the only remaining (marginally) significant association (p < 0.05) for the relationship between sex steroids and any of the bone turnover indices assessed. In community-dwelling men age >70 years, bone turnover rate, as determined by biochemical markers, is a significant negative determinant of prevalent BMD. However, the findings do not support the view that relative differences in sex steroid status, as observed among healthy elderly men, have a major impact on bone turnover.

Androgens suppress osteoclast formation induced by RANKL and macrophage-colony stimulating factor

Androgen deficiency in males leads to an increase in osteoclastic bone resorption and a progressive decrease in bone mineral density. In the current studies, we examined the ability of 5 alpha-dihydrotestosterone to suppress osteoclast formation induced by receptor activator of NF-kB ligand (RANKL) and macrophage-colony stimulating factor in vitro. 5 alpha-Dihydrotestosterone suppressed the differentiation of bone marrow monocytes into osteoclasts from both sham-operated and orchidectomized mice. Androgen deficiency also led to an increase in the number of hematopoietic precursors capable of forming osteoclasts and increased the relative responsiveness of these cells to androgens in vitro. Interestingly, E2 was as effective as 5 alpha-dihydrotestosterone in suppressing osteoclast formation in bone marrow monocytes from both sham and orchidectomized mice. As with bone marrow monocytes, 5 alpha-dihydrotestosterone also suppressed RANKL-induced osteoclast formation in the monocyte-macrophagic cell line RAW264.7. In RAW264.7 cells, androgens appear to block RANKL-induced osteoclast formation through selective regulation of c-JUN: Accordingly, 5 alpha-dihydrotestosterone suppressed RANKL-induced c-Jun N-terminal kinase activation and reduced c-Jun expression levels. These effects resulted in a reduction in RANKL-induced activator protein-1 DNA binding activity and a corresponding suppression in activator protein-1-mediated transcriptional activation. These studies indicate that both E and androgens can suppress osteoclast formation via a direct, stromal cell-independent action on osteoclast precursors to block key transcription factors such as c-Jun essential for osteoclast differentiation.

Does body size account for gender differences in femur bone density and geometry?

The extent to which greater bone strength in men is caused by proportionately greater bone mass versus bigger bone size is not clear, primarily because the larger overall body size of men has made direct comparisons of skeletal measures difficult. We examined gender differences in femur neck (FN) areal bone mineral density (BMD) values collected from 5,623 non-Hispanic whites aged 20+ years in the third National Health and Nutrition Examination Survey (NHANES III, 1988-1994) before and after correction for measured height and weight. We supplemented the conventional areal BMD data (Hologic QDR 1000) with measurements of areal BMD and geometric properties (subperiosteal width, section modulus, and cortical thickness) made at narrow "cross-sectional" regions traversing the FN and the proximal shaft using a structural analysis program. Before body size adjustment, men had significantly higher values than women for all variables at the three measurement sites (p < 0.0001). Adjustment for body size reduced the differences between the sexes for all variables but had a greater effect on BMD (1-8% higher in men) than on geometry (5-17% higher in men). When examined by age, the sex discrepancy was significantly greater in the older group for all variables except subperiosteal widths. We conclude that although body size difference may account for most of the areal BMD difference between men and women, male bones are still bigger in ways that suggest greater bone strength. These differences may contribute importantly to lower fracture risk in men.

Lasofoxifene (CP-336,156) protects against the age-related changes in bone mass, bone strength, and total serum cholesterol in intact aged male rats

The purpose of this study was to evaluate if long-term (6 months) treatment with lasofoxifene (LAS), a new selective estrogen receptor modulator (SERM), can protect against age-related changes in bone mass and bone strength in intact aged male rats. Sprague-Dawley male rats at 15 months of age were treated (daily oral gavage) with either vehicle (n = 12) or LAS at 0.01 mg/kg per day (n = 12) or 0.1 mg/kg per day (n = 11) for 6 months. A group of 15 rats was necropsied at 15 months of age and served as basal controls. No significant change was found in body weight between basal and vehicle controls. However, an age-related increase in fat body mass (+42%) and decrease in lean body mass (-8.5%) was observed in controls. Compared with vehicle controls, LAS at both doses significantly decreased body weight and fat body mass but did not affect lean body mass. No significant difference was found in prostate wet weight among all groups. Total serum cholesterol was significantly decreased in all LAS-treated rats compared with both the basal and the vehicle controls. Both doses of LAS treatment completely prevented the age-related increase in serum osteocalcin. Peripheral quantitative computerized tomography (pQCT) analysis at the distal femoral metaphysis indicated that the age-related decrease in total density, trabecular density, and cortical thickness was completely prevented by treatment with LAS at 0.01 mg/kg per day or 0.1 mg/kg per day. Histomorphometric analysis of proximal tibial cancellous bone showed an age-related decrease in trabecular bone volume (TBV; -46%), trabecular number (Tb.N), wall thickness (W.Th), mineral apposition rate, and bone formation rate-tissue area referent. Moreover, an age-related increase in trabecular separation (Tb.Sp) and eroded surface was observed. LAS at 0.01 mg/kg per day or 0.1 mg/kg per day completely prevented these age-related changes in bone mass, bone structure, and bone turnover. Similarly, the age-related decrease in TBV and trabecular thickness (Tb.Th) and the age-related increase in osteoclast number (Oc.N) and osteoclast surface (Oc.S) in the third lumbar vertebral cancellous bone were completely prevented by treatment with LAS at both doses. Further, LAS at both doses completely prevented the age-related decrease in ultimate strength (-47%) and stiffness (-37%) of the fifth lumbar vertebral body. These results show that treatment with LAS for 6 months in male rats completely prevents the age-related decreases in bone mass and bone strength by inhibiting the increased bone resorption and bone turnover associated with aging. Further, LAS reduced total serum cholesterol and did not affect the prostate weight in these rats. Our data support the potential use of a SERM for protecting against the age-related changes in bone and serum cholesterol in elderly men.

Sex- and age-related response to aromatase deficiency in bone

Deficiency of sex steroids causes osteoporosis, but the relationship between estrogen and androgen is not clear because androgen is converted into estrogen by aromatase. In this study, we characterized bone metabolism in the aromatase-deficient (ArKO) mouse. At 9 weeks old, a marked loss of cancellous bone due to increased bone resorption was observed not only in female ArKO mice but also in males. The degree of bone loss in ArKO males was similar to that in females, and treatment with 17beta-estradiol completely restored the bone mass in both sexes. At 32 weeks old, female ArKO mice showed severe loss of cancellous and cortical bone. Male ArKO mice of this age also showed reduced bone mass, but the degree of bone loss in females was more marked than that in males. Here, we report sex- and age-related responses to aromatase deficiency in bone.

Sex steroids and bone

Sex steroids are essential for skeletal development and the maintenance of bone health throughout adult life, and estrogen deficiency at menopause is a major pathogenetic factor in the development of osteoporosis in postmenopausal women. The mechanisms by which the skeletal effects of sex steroids are mediated remain incompletely understood, but in recent years there have been considerable advances in our knowledge of how estrogens and, to a lesser extent androgens, influence bone modeling and remodeling in health and disease. New insights into estrogen receptor structure and function, recent discoveries about the development and activity of osteoclasts, and lessons learned from human and animal genetic mutations have all contributed to increased understanding of the skeletal effects of estrogen, both in males and females. Studies of untreated and treated osteoporosis in postmenopausal women have also contributed to this knowledge and have provided unequivocal evidence for the potential of high-dose estrogen therapy to have anabolic skeletal effects. The development of selective estrogen receptor modulators has provided a new approach to the prevention of osteoporosis and other major diseases of menopause and has implications for the therapeutic use of other steroid hormones, including androgens. Further elucidation of the mechanisms by which sex steroids affect bone thus has the potential to improve the clinical management not only of osteoporosis, both in men and women, but also of a number of other diseases related to sex hormone status.

Skeletal effects of estrogen deficiency as induced by an aromatase inhibitor in an aged male rat model

Aromatization of androgens into estrogens may be important for maintenance of the male skeleton. To address this hypothesis, we evaluated the skeletal effects of selective estrogen deficiency as induced by the aromatase inhibitor vorozole (Vor), with or without 17beta-estradiol (E(2)) administration (1.35 microg/day), in aged (12-month-old) male rats. A baseline group was killed at the start of the experiment (Base). The control group (Control), the group treated with vorozole alone (Vor), the group treated with E(2) alone (E(2)), or the group with a combination of both (Vor + E(2)) were killed 15 weeks later. Vorozole significantly increased serum testosterone (T) and reduced serum E(2) compared with Control. Body weight gain and serum insulin-like growth factor-I (IGF-I) were also lower in Vor, whereas significant weight loss and decrease of serum IGF-I occurred as a result of E(2) administration. Bone formation as assessed by serum osteocalcin was unaffected but osteoid surface in the proximal metaphysis of the tibia was increased in Vor-treated rats. Bone resorption as evaluated by urinary deoxypyridinoline excretion was increased in Vor. Biochemical parameters of bone turnover were reduced significantly in all E(2) treated rats. Premature closure of the growth plates and decreased osteoid and mineralizing surfaces were also observed in E(2) and Vor + E(2). Apparent bone density of lumbar vertebrae and femur, as measured by dual-energy X-ray absorptiometry (DXA), was significantly reduced in Vor. Vorozole decreased femoral bone density mainly in the distal femur (trabecular and cortical region). This decrease of bone density was not present in E(2) and Vor + E(2). Similar findings were observed when bone density was assessed by peripheral quantitative computed tomography (pQCT); that is, trabecular density of the distal femur, the proximal tibia, and the distal lumbar vertebra were all lower in Vor. This decrease in density was not observed in all E(2)-treated animals. In conclusion, administration of the aromatase inhibitor, vorozole, to aged male rats induces net trabecular bone loss in both the appendicular and axial skeleton, despite a concomitant increase in serum testosterone. E(2) administration is able to prevent this trabecular bone loss in vorozole-treated animals.

Androgen deficiency induces high turnover osteopenia in aged male rats: a sequential histomorphometric study

Hypogonadism is considered to be one of the major risk factors for osteoporosis in men. However, the mechanisms of bone loss caused by androgen deficiency are still unclear. In the present study, we sequentially investigated the skeletal and hormonal effects of androgen deficiency in aged orchiectomized (ORX) rats over a time period of 9 months. One hundred seventy 13-month-old male Fischer-344 rats were either ORX or sham-operated (SHAM). Eight rats served as baseline controls. After in vivo fluorochrome labeling, groups of 8-15 SHAM and ORX rats each were killed at 2 weeks and at 1, 2, 3, 4, 6, and 9 months postsurgery. As expected, ORX induced a fall in serum total and free testosterone levels, but also reduced serum estradiol concentrations. Cancellous bone area (BAr) in the proximal tibia but not in the first lumbar vertebral body showed an age-dependent decline in SHAM rats. Relative to SHAM controls, ORX rats had significantly reduced cancellous BAr after 2 weeks post-ORX in the tibia and after 2 months post-ORX in the vertebral body. Thereafter, vertebral and tibial cancellous BAr continued to decline in ORX animals throughout the study. Osteoclast number (NOc), osteoblast surface, bone formation rate (BFR), and activation frequency were increased in ORX animals from 1 month postsurgery until the end of the trial. Moreover, in close temporal association with the histomorphometric findings, serum osteocalcin and urinary excretion of collagen cross-links and calcium were elevated in ORX rats. In a stepwise model of multiple regression analysis using estradiol and free and total testosterone as independent variables, estradiol was the only significant predictor of histomorphometric indices of bone formation and bone resorption in SHAM and ORX rats. These data show that androgen deficiency induces substantial loss of cancellous bone in the axial and appendicular skeleton of aged male rats and that this osteopenia is associated with a sustained increase in bone turnover. Thus, the skeletal effects of androgen withdrawal in aged male rats appear to resemble those induced by estrogen withdrawal in female rats. Furthermore, our study suggests that estradiol may act as a physiological suppressor of bone remodeling in aged male rats.

Osteoporosis and osteoporotic fractures in men: a clinical perspective

The lifetime risk of any fracture of the hip, spine or distal forearm in men aged 50 years has been estimated to be 13%, compared with 40% in women. Although the overall incidence of osteoporosis is less in men than in women, the disease still represents an important public health problem. In particular, hip fractures are associated with substantial mortality and morbidity, even more so than in women. In male patients presenting with osteoporotic fractures, major causes of skeletal fragility, such as hypogonadism, glucocorticoid excess, primary hyperparathyroidism and alcohol abuse, can often be identified. In as many as 50% of osteoporotic men, however, no aetiology can be found: these men suffer from a syndrome commonly referred to as idiopathic osteoporosis, which is presumably related to some type of osteoblast dysfunction. Recent evidence indicates that the loss of skeletal integrity in ageing men may be partially related to endocrine deficiencies, including vitamin D, androgen and/or oestrogen deficiency. While the consequences of vitamin D or oestrogen deficiency in women have been well established, the skeletal impact of these (partial) age-related deficiencies in men remains to be clarified. Osteoporosis in elderly men is a multifactorial disease, as it is in women. The prevention of osteoporosis should therefore focus not only on increasing the bone strength, but also on decreasing the risk of falls. However, the prevention and therapy of osteoporotic disorders in men are virtually unexplored. To date, the use of specific osteoporotic drugs in osteoporotic men is still based on reasonable but untested assumptions.

Estrogen replacement therapy in a man with congenital aromatase deficiency: effects of different doses of transdermal estradiol on bone mineral density and hormonal parameters

The effects of different doses of transdermal estradiol (TE) on bone mineral density (BMD) in a man with aromatase deficiency were evaluated. The study protocol was divided in the following four phases: phase 1, before estradiol treatment; phase 2, 50 microg TE twice weekly for 6 months; phase 3, 25 microg TE twice weekly for 9 months; and phase 4, 12.5 microg TE twice weekly for 9 months. X-rays of hands, legs, and pelvis were performed, and BMD of the lumbar spine, hormonal parameters (LH, FSH, testosterone, and estradiol), and markers of bone turnover were determined during each phase. BMD in phase 1 was 0.933 g/cm2 and increased to 1.051 and 1.173 g/cm2 after 4 and 7 months of TE, respectively. In phase 3, BMD reached the maximum value (1.275 g/cm2). In phase 4, BMD decreased to 1.180 g/cm2 and was 1.029 g/cm2 at the end of the study protocol. A bilateral necrosis of femoral heads was also detected by x-ray films. In phase 1 serum testosterone was in the normal range, whereas serum estradiol was undetectable. During the 24-month period of treatment with TE (phases 2-4), estradiol was directly related to the amount of TE, whereas LH was inversely related to estradiol serum levels. Estradiol and gonadotropins reached optimal values only in phase 3, when FSH also was near normal; serum testosterone concentrations were normal in phases 3 and 4. This study confirms the role of estrogens in achieving and maintaining bone mineral content in the human male, providing further clinical tools useful in the management of bone loss in aromatase deficiency in the male. We suggest that the adequate substitutive dose of TE for maintaining both bone mass and normal estradiol serum levels in adult men with aromatase deficiency may be 25 microg twice weekly (0.47 microg/kg weekly).

Androgens and osteoporosis

Androgen receptors are present in relevant numbers in osteoblasts. Stimulation of androgen receptors in osteoblastic bone marrow stromal cells inhibits the differentiation of osteoclasts in the bone marrow cavity. Androgens not only inhibit osteoclastogenesis but also increase cortical bone formation mainly by stimulating periosteal bone formation. Clinically, androgen action is crucial for the gain of bone mass during puberty and the maintenance of bone mass after puberty. Therefore, androgen replacement is necessary in hypogonadal men. However, the role of androgen replacement in partial androgen deficiency still remains unclear. Thus far, only testosterone has established its role in androgen replacement. However, further clinical and basic research should better define the selective role of androgen versus oestrogen receptor stimulation in male skeletal homeostasis.

Lasofoxifene (CP-336,156), a selective estrogen receptor modulator, prevents bone loss induced by aging and orchidectomy in the adult rat

It has been well documented that selective estrogen receptor modulators (SERMs) can prevent bone loss in ovariectomized rats and postmenopausal women. The purposes of this study were to determine the effects of a potent and orally active SERM, lasofoxifene (CP-336,156), on bone mass, bone strength, total serum cholesterol, prostate weight, and histology in adult male orchidectomized (ORX) rats. Sprague Dawley male rats at 10 months of age were divided into 6 groups, with 10 rats/group. The first group was necropsied on day 0 and served as basal controls. The remaining rats were either sham operated (n = 10) and treated orally with vehicle, or ORX (n = 40) and treated with either vehicle or lasofoxifene at 1, 10, or 100 microg/kg x day for 60 days. Total serum cholesterol, prostate weight and histology, distal femoral bone mineral density (DFBMD) by dual energy x-ray absorptiometry, and static and dynamic bone histomorphometry of the third lumbar vertebral body were determined. Maximal load and stiffness of the fifth lumbar vertebral body were also determined by compression tests. Age-related decreases in DFBMD (-9%) and trabecular bone volume (TBV; -13%) of the third lumbar vertebral body were found in sham-operated rats compared with basal controls. ORX induced significant increases in total serum cholesterol (+31%), eroded surface (+48%), activation frequency of bone turnover (+103%) and significant decreases in prostate weight (-89%), DFBMD (-14%), TBV (-23%), and maximal load (-17%) compared with basal controls. Compared with sham controls, ORX induced significant increases in eroded perimeter and activation frequency. Lasofoxifene decreased body weight in all dose groups compared with both sham and ORX control values. Compared with ORX controls, ORX rats treated with lasofoxifene at 10 or 100 microg/kg x day had significantly lower percent eroded perimeter activation frequency and significantly higher DFBMD, TBV, and maximal load. Further, lasofoxifene at 10 and 100 microg/kg x day significantly decreased total serum cholesterol by 46% and 68% in ORX rats, whereas no effect was found in prostate weight and histology parameters compared with ORX control values. These data showed that lasofoxifene prevented bone loss by inhibiting bone turnover associated with aging and orchidectomy in 10-month-old male rats. Further, lasofoxifene decreased total serum cholesterol and did not affect the prostate in these rats. These results suggest that SERMs such as lasofoxifene may be useful therapeutic agents for preventing bone loss in elderly men with some degree of hypogonadism.

Bone has a sexually dimorphic response to aromatase deficiency

Aromatase synthesizes estrogen from androgen precursors. To better understand the role of estrogen in skeletal metabolism and growth, we have assessed long bone growth and histomorphometry in aromatase-deficient (ArKO) mice. The age range for the animals was 5-7 months. At this age mice have already achieved peak bone density but continue slow bone growth. Femur length, an index of long bone growth, showed decreased growth in ArKO males compared with wild-type (wt) littermates but no significant difference in females. Radiographically, compared with age- and sex- matched littermates both ArKO males and females showed osteopenia in the lumbar spine. Histologically, both ArKO males and females showed an osteoporotic-type picture, characterized by significant decreases in trabecular bone volume and trabecular thickness. However, compared with wt littermates female ArKO animals showed a bone remodeling picture consistent with increased bone turnover, much like early postmenopausal osteoporosis in humans. On the other hand, male ArKO animals showed decreases in both osteoblastic and osteoclastic surfaces compared with wt littermates, similar to age-related osteopenia. These findings suggest that osteoporosis seen in aromatase-deficient mice may arise from different bone remodeling activities between males and females. These results also show that the ArKO model exhibits the expected results of estrogen deficiency and may be a good model for investigating sex-specific responses to estrogen deficiency. Furthermore, they imply that estrogen is important for attaining peak bone mass in male as well as in female mice.