Ovarian androgens protect against bone loss in rats made oestrogen deficient by treatment with ICI 182,780

Mechanical loading-related bone gain is enhanced by tamoxifen but unaffected by fulvestrant in female mice

Accumulating evidence indicates that estrogen receptors (ERs) are involved in the mechano-adaptive mechanisms by which loading influences the mass and architecture of bones to establish and maintain their structural load-bearing competence. In the present study, we assessed the effects of the ER modulators tamoxifen and fulvestrant (ICI 182,780) on loading-related changes in the volume and structure of trabecular and cortical bone in the tibiae of female mice. Ten days after actual or sham ovariectomy, 17-wk-old female C57BL/6 mice were treated with vehicle (peanut oil), tamoxifen (0.02, 0.2, or 2 mg/kg · d), fulvestrant (4 mg/kg · d), or their combination and the right tibiae subjected to a short period of noninvasive axial loading (40 cycles/d) on 5 d during the subsequent 2 wk. In the left control tibiae, ovariectomy, tamoxifen, or fulvestrant did not have any significant effect on cortical bone volume, whereas trabecular bone volume was decreased by ovariectomy, increased by tamoxifen, and unaffected by fulvestrant. In the right tibiae, loading was associated with increases in both trabecular and cortical bone volume. Notably, the medium dose of tamoxifen synergistically enhanced loading-related gain in trabecular bone volume through an increase in trabecular thickness. Fulvestrant had no influence on the effects of loading but abrogated the enhancement of loading-related bone gain by tamoxifen. These data demonstrate that, at least in female mice, the adaptive response to mechanical loading of trabecular bone can be enhanced by ER modulators, in this case by tamoxifen.

Emerging drugs for the management of cancer treatment induced bone loss

AREAS COVERED IN THIS REVIEW

We focus our attention on data on the efficacy of currently available and emerging drugs for the management of cancer treatment induced bone loss (CTIBL) found in a PubMed research from 1997 till today.

IMPORTANCE OF THE FIELD

One of the most common and severe safety issues of the antihormonal therapy in both sexes is the CTIBL and the related fragility fractures. In postmenopausal women with estrogenic receptor positive breast cancer, the third-generation aromatase inhibitors (AIs) are the standard therapy. Observational retrospective studies have found that AIs treated patients had a high rate of bone loss and fracture risk (RR 1.3). Also in men with prostate cancer receiving androgen deprivation therapy, the increase in bone turnover and the consequent bone loss are very rapid and sustained significantly increasing the fracture risk.

WHAT THE READER WILL GAIN

The aim of our review is to provide the current evidences for the management of bone loss and fracture risk in this subpopulation.

TAKE HOME MESSAGE

The very high rate of bone loss and the high incidence of fractures indicate that cancer patients at risk of CTIBL need to be carefully monitored and stratified for fracture risk. Although there is a strong evidence of efficacy in prevention of bone loss and reduction of fracture risk for many drugs approved for postmenopausal osteoporosis (PMO) and male osteoporosis, for CTIBL there are actually no drugs approved for this indication.

Estrogen/estrogen receptor alpha signaling in mouse posterofrontal cranial suture fusion

Background

While premature suture fusion, or craniosynostosis, is a relatively common condition, the cause is often unknown. Estrogens are associated with growth plate fusion of endochondral bones. In the following study, we explore the previously unknown significance of estrogen/estrogen receptor signaling in cranial suture biology.

Methodology/Principal Findings

Firstly, estrogen receptor (ER) expression was examined in physiologically fusing (posterofrontal) and patent (sagittal) mouse cranial sutures by quantitative RT-PCR. Next, the cranial suture phenotype of ER alpha and ER beta knockout (αERKO, βERKO) mice was studied. Subsequently, mouse suture-derived mesenchymal cells (SMCs) were isolated; the effects of 17-β estradiol or the estrogen antagonist Fulvestrant on gene expression, osteogenic and chondrogenic differentiation were examined in vitro. Finally, in vivo experiments were performed in which Fulvestrant was administered subcutaneously to the mouse calvaria. Results showed that increased ERα but not ERβ transcript abundance temporally coincided with posterofrontal suture fusion. The αERKO but not βERKO mouse exhibited delayed posterofrontal suture fusion. In vitro, addition of 17-β estradiol enhanced both osteogenic and chondrogenic differentiation in suture-derived mesenchymal cells, effects reversible by Fulvestrant. Finally, in vivo application of Fulvestrant significantly diminished calvarial osteogenesis, inhibiting suture fusion.

Conclusions/Significance

Estrogen signaling through ERα but not ERβ is associated with and necessary for normal mouse posterofrontal suture fusion. In vitro studies suggest that estrogens may play a role in osteoblast and/or chondrocyte differentiation within the cranial suture complex.

Hormone therapy for breast cancer, with an emphasis on the pure antiestrogen fulvestrant: mode of action, antitumor efficacy and effects on bone health

Breast cancer is a major health problem in women of developed Western countries. Whereas estrogen receptor (ER) may be involved in many cases in breast carcinogenesis, its expression in breast tumors may predict a favorable response to hormone therapy. In this review, we report the role played by ER in breast cancer and compare the effects and mechanisms of action of partial (tamoxifen) and pure (fulvestrant) antiestrogens, as well as of aromatase inhibitors. Moreover, as ER also has a critical role in bone metabolism, we review the beneficial and adverse effects of breast cancer hormone therapy on bone health, with a particular emphasis on fulvestrant, the only pure antiestrogen recently approved by the FDA for Phase III clinical trials. We conclude that, because of its therapeutic efficacy and its seemingly minimal effect on bone integrity, fulvestrant represents a new option for the hormonal treatment of breast cancer that deserves further clinical evaluation.

Bone loss in patients with breast cancer receiving aromatase inhibitors and associated treatment strategies

Hormone-receptor-positive breast cancer in postmenopausal women is treated increasingly with aromatase inhibitors because of increased efficacy and reduced incidence of endometrial cancer compared with tamoxifen. However, aromatase inhibitor therapy increases bone turnover as a result of nearly complete oestrogen depletion, leading to increases in bone loss and fragility fractures that erode patients' functional independence and quality of life. Management of patients with aromatase inhibitor-associated bone loss (AIBL) is currently evolving and intervention strategies are under investigation. Although no treatments are specifically approved for AIBL, bisphosphonates are currently the intervention of choice for patients with low bone mineral density or evidence of rapid bone turnover, along with adequate calcium and vitamin D supplementation and a healthy lifestyle. In this setting, the majority of information available regarding bisphosphonate efficacy is from studies of intravenous zoledronic acid (4 mg) every 6 months. Data from four large international studies (three of identical design in postmenopausal women and one in premenopausal women) indicate that zoledronic acid is effective in the management of AIBL. Treatment algorithms based on risk factors and bone mineral density are under development, and the results of ongoing studies should help define optimal bone health management for patients undergoing aromatase inhibitor treatment for early breast cancer.

Selective Androgen Receptor Modulator (SARM) treatment prevents bone loss and reduces body fat in ovariectomized rats

PURPOSE

This study was conducted to examine the bone and body composition effects of S-4, an aryl-propionamide derived Selective Androgen Receptor Modulator (SARM) in an ovariectomy induced model of accelerated bone loss.

METHODS

One hundred twenty female Sprague-Dawley rats aged to twenty-three weeks were randomly assigned to twelve treatment groups. Drug treatment was initiated immediately following ovariectomy and continued for one hundred twenty days. Whole body bone mineral density (BMD), body composition, and lumbar vertebrae BMD were measured by dual energy x-ray absorptiometry. More stringent regional pQCT and biomechanical strength testing was performed on excised femurs.

RESULTS

We found that S-4 treatment maintained whole body and trabecular BMD, cortical content, and increased bone strength while decreasing body fat in these animals.

CONCLUSIONS

The data presented herein show the protective skeletal effects of S-4. Our previous reports have shown the tissue selectivity and muscle anabolic activity of S-4. Together these data suggest that S-4 could reduce the incidence of fracture via two different mechanisms (i.e., via direct effects in bone and reducing the incidence of falls through increased muscle strength). This approach to fracture reduction would be advantageous over current therapies in these patients which are primarily antiresorptive in nature.

Effects of the steroidal aromatase inhibitor exemestane and the nonsteroidal aromatase inhibitor letrozole on bone and lipid metabolism in ovariectomized rats

PURPOSE

Exemestane (EXE) and letrozole (LET) are third-generation aromatase inhibitors currently prescribed for postmenopausal hormone-dependent breast cancer. The impact on end organs of estrogen depletion in menopausal women is of significant clinical importance. We studied the effects of EXE, its principal metabolite, 17-hydroexemestane (17-H-EXE), and LET on bone and lipid metabolism in ovariectomized (OVX) rats.

EXPERIMENTAL DESIGN

OVX rats were treated by weekly intramuscular injection for 16 weeks with 20, 50, and 100 mg/kg EXE, 20 mg/kg 17-H-EXE, and daily oral gavage of 1 mg/kg LET. At the end of the treatment period, bone mineral density (BMD), the bone resorption marker serum pyridinoline, the bone formation marker serum osteocalcin, bone mechanical properties, histomorphometry, and serum lipid concentrations were determined.

RESULTS

Lumbar vertebral and femoral BMD, bending strength of the femur, compressive strength of the fifth lumbar vertebra, and trabecular bone volume were significantly higher in OVX animals given EXE and 17-H-EXE than in OVX controls. EXE and 17-H-EXE significantly reduced an ovariectomy-induced increase in serum pyridinoline and serum osteocalcin. EXE and 17-H-EXE given to OVX rats caused significant reductions of serum cholesterol and low-density lipoprotein cholesterol. In contrast, OVX rats treated with LET had BMD, bone biomarkers, mechanical failure properties, and lipid levels similar to those of OVX controls.

CONCLUSIONS

EXE and 17-H-EXE significantly prevent bone loss, enhance bone mechanical strength, and lower serum cholesterol and low-density lipoprotein levels in OVX rats. These protective effects on end-organ function are not seen with the nonsteroidal inhibitor LET.

Androgens and bone

Loss of estrogens or androgens increases the rate of bone remodeling by removing restraining effects on osteoblastogenesis and osteoclastogenesis, and also causes a focal imbalance between resorption and formation by prolonging the lifespan of osteoclasts and shortening the lifespan of osteoblasts. Conversely, androgens, as well as estrogens, maintain cancellous bone mass and integrity, regardless of age or sex. Although androgens, via the androgen receptor (AR), and estrogens, via the estrogen receptors (ERs), can exert these effects, their relative contribution remains uncertain. Recent studies suggest that androgen action on cancellous bone depends on (local) aromatization of androgens into estrogens. However, at least in rodents, androgen action on cancellous bone can be directly mediated via AR activation, even in the absence of ERs. Androgens also increase cortical bone size via stimulation of both longitudinal and radial growth. First, androgens, like estrogens, have a biphasic effect on endochondral bone formation: at the start of puberty, sex steroids stimulate endochondral bone formation, whereas they induce epiphyseal closure at the end of puberty. Androgen action on the growth plate is, however, clearly mediated via aromatization in estrogens and interaction with ERalpha. Androgens increase radial growth, whereas estrogens decrease periosteal bone formation. This effect of androgens may be important because bone strength in males seems to be determined by relatively higher periosteal bone formation and, therefore, greater bone dimensions, relative to muscle mass at older age. Experiments in mice again suggest that both the AR and ERalpha pathways are involved in androgen action on radial bone growth. ERbeta may mediate growth-limiting effects of estrogens in the female but does not seem to be involved in the regulation of bone size in males. In conclusion, androgens may protect men against osteoporosis via maintenance of cancellous bone mass and expansion of cortical bone. Such androgen action on bone is mediated by the AR and ERalpha.

Aromatase in bone: roles of Vitamin D3 and androgens

We have mainly focused on the regulatory mechanism of cytochrome P450 aromatize in bone cells. Our previous study demonstrated a strong positive correlation of serum dehydroepiandrosterone sulfate (DHEA-S) and estrone (E1) with BMD in postmenopausal women but no correlation between serum estradiol (E2) and BMD in the same group. In addition, administration of DHEA to ovariectomized rat significantly increased BMD. These in vivo findings strongly suggested that circulating adrenal androgen may be converted to estrogen in osteoblast and may contribute to BMD maintenance. Actually, in cultured human osteoblast cells, DHEA was found to convert to androstenedione by 3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity and then androstenedione to estrone through the apparent aromatase activity. The aromatase activity in cultured human osteoblast cells was significantly increased by dexamethasone (DEX). Interestingly, DEX and 1alpha,25-dihydroxyvitamin D3 (VD3) synergistically enhanced aromatase activity as well as P450arom mRNA expression. A little stronger induction of aromatase activity by DEX and VD3 was observed in cultured human fibroblasts. The increase of the aromatase activity by DEX and VD3 was accompanied with the increase of luciferase activity of fibroblast cells transfected with Exon 1b-promoter-luciferase construct, but not of osteoblasts transfected with the same construct, suggesting a different regulatory mechanism of aromatase by DEX and 1alpha,25-dihydroxyvitamin D3 (VD3) between these two cells despite the same promotor usage. In human bone cells, intracrine mechanism through aromatase activity, together with a positive regulation of aromatase activity by glucocorticoid and VD3, may contribute to the local production of estrogens, thus leading to protective effect against osteoporosis especially after menopause. The effect of sex steroids (estrogen versus testosterone) in bone remodeling was also briefly reviewed based on several recent findings in this field.

The role of estrogens in men and androgens in women

The past years several have witnessed a significant transformation in our understanding of sex steroid action in the male and female skeleton. Data from animal and human studies indicate that sex steroids have important skeletal effects in both genders. It seems from the in vivo human data that estrogen is likely more potent than testosterone in inhibiting bone resorption. Estrogen and testosterone appear to be important for maintaining bone formation. In addition, androgens clearly enhance bone size, likely through effects on periosteal bone formation. How much of this gender cross-talk at the physiological level is caused by "promiscuous" actions of sex steroids at the molecular level, with estrogen acting by way of the androgen receptor (and androgens via the estrogen receptor) is an interesting and important question, the answer to which may well provide additional surprises.

Generation and characterization of androgen receptor knockout (ARKO) mice: an in vivo model for the study of androgen functions in selective tissues

By using a cre-lox conditional knockout strategy, we report here the generation of androgen receptor knockout (ARKO) mice. Phenotype analysis shows that ARKO male mice have a female-like appearance and body weight. Their testes are 80% smaller and serum testosterone concentrations are lower than in wild-type (wt) mice. Spermatogenesis is arrested at pachytene spermatocytes. The number and size of adipocytes are also different between the wt and ARKO mice. Cancellous bone volumes of ARKO male mice are reduced compared with wt littermates. In addition, we found the average number of pups per litter in homologous and heterozygous ARKO female mice is lower than in wt female mice, suggesting potential defects in female fertility and/or ovulation. The cre-lox ARKO mouse provides a much-needed in vivo animal model to study androgen functions in the selective androgen target tissues in female or male mice.

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.

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.

Relative contributions of testosterone and estrogen in regulating bone resorption and formation in normal elderly men

Young adult males who cannot produce or respond to estrogen (E) are osteopenic, suggesting that E may regulate bone turnover in men, as well as in women. Both bioavailable E and testosterone (T) decrease substantially in aging men, but it is unclear which deficiency is the more important factor contributing to the increased bone resorption and impaired bone formation that leads to their bone loss. Thus, we addressed this issue directly by eliminating endogenous T and E production in 59 elderly men (mean age 68 years), studying them first under conditions of physiologic T and E replacement and then assessing the impact on bone turnover of withdrawing both T and E, withdrawing only T, or only E, or continuing both. Bone resorption markers increased significantly in the absence of both hormones and were unchanged in men receiving both hormones. By two-factor ANOVA, E played the major role in preventing the increase in the bone resorption markers, whereas T had no significant effect. By contrast, serum osteocalcin, a bone formation marker, decreased in the absence of both hormones, and both E and T maintained osteocalcin levels. We conclude that in aging men, E is the dominant sex steroid regulating bone resorption, whereas both E and T are important in maintaining bone formation.