The androgen receptor has no direct antiresorptive actions in mouse osteoclasts

Reproducible preclinical models of androgen receptor driven human prostate cancer bone metastasis

BACKGROUND

Preclinical models recapitulating the metastatic phenotypes are essential for developing the next-generation therapies for metastatic prostate cancer (mPC). We aimed to establish a cohort of clinically relevant mPC models, particularly androgen receptor positive (AR+) bone metastasis models, from LuCaP patient-derived xenografts (PDX) that reflect the heterogeneity and complexity of mPC.

METHODS

PDX tumors were dissociated into single cells, modified to express luciferase, and were inoculated into NSG mice via intracardiac injection. The progression of metastases was monitored by bioluminescent imaging. Histological phenotypes of metastases were characterized by immunohistochemistry and immunofluorescence staining. Castration responses were further investigated in two AR-positive models.

RESULTS

Our PDX-derived metastasis (PDM) model collection comprises three AR+ adenocarcinomas (ARPC) and one AR- neuroendocrine carcinoma (NEPC). All ARPC models developed bone metastases with either an osteoblastic, osteolytic, or mixed phenotype, while the NEPC model mainly developed brain metastasis. Different mechanisms of castration resistance were observed in two AR+ PDM models with distinct genotypes, such as combined loss of TP53 and RB1 in one model and expression of AR splice variant 7 (AR-V7) expression in another model. Intriguingly, the castration-resistant tumors displayed inter- and intra-tumor as well as organ-specific heterogeneity in lineage specification.

CONCLUSION

Genetically diverse PDM models provide a clinically relevant system for biomarker identification and personalized medicine in metastatic castration-resistant prostate cancer.

Hormonal Control of Bone Architecture Throughout the Lifespan: Implications for Fracture Prediction and Prevention

BACKGROUND

Skeletal modeling in childhood and adolescence and continuous remodeling throughout the lifespan are designed to adapt to a changing environment and resist external forces and fractures. The flux of sex steroids in men and women, beginning from fetal development and evolving through infancy, childhood, puberty, young adulthood, peri/menopause transition, and postmenopause, is critical for bone size, peak bone mass, and fracture resistance.

OBJECTIVE

This review will highlight how changes in sex steroids throughout the lifespan affect bone cells and the consequence of these changes on bone architecture and strength.

METHODS

Literature review and discussion.

RESULTS

The contributions of estrogen and testosterone on skeletal development have been difficult to study due to the reciprocal and intertwining contributions of one on the other. Although orchiectomy in men renders circulating testosterone absent, circulating estrogen also declines due to testosterone being the substrate for estradiol. The discovery of men with absent estradiol or resistance to estrogen and the study of mouse models led to the understanding that estrogen has a larger direct role in skeletal development and maintenance in men and women. The mechanistic reason for larger bone size in men is incompletely understood but related to indirect effects of testosterone on the skeleton, such as higher muscle mass leading to larger mechanical loading. Declines in sex steroids during menopause in women and androgen deprivation therapies in men have profound and negative effects on the skeleton. Therapies to prevent such bone loss are available, but how such therapies can be tailored based on bone size and architecture remains an area of investigation.

CONCLUSION

In this review, the elegant interplay and contribution of sex steroids on bone architecture in men and women throughout the lifespan is described.

Attenuation of Bone Mineral Density Decline During Anemia Treatment With Methenolone Acetate in Myelodysplastic Syndrome

In an aging society, addressing the risks and management of osteoporotic fractures is critical to reduce mortality. Similarly, the morbidity of chronic kidney disease and myelodysplastic syndrome increases with aging. The association between chronic kidney disease and fractures is well understood; however, recent reports have indicated an increased risk of incident osteoporosis in patients with prevalent myelodysplastic syndrome. In this case report, we present an older man with stage 4 chronic kidney disease complicated by myelodysplastic syndrome and progressive decline in bone mineral density. He was treated with methenolone acetate and darbepoetin for anemia caused by myelodysplastic syndrome. During anemia treatment, the decline in bone mineral density was attenuated overtime. The case findings suggest the potential association between the use of methenolone acetate as a synthetic anabolic steroid and attenuated decline in bone mineral density.

The Utility of Preclinical Models in Understanding the Bone Health of Transgender Individuals Undergoing Gender-Affirming Hormone Therapy

PURPOSE OF REVIEW

To summarise the evidence regarding the effects of gender-affirming hormone therapy (GAHT) on bone health in transgender people, to identify key knowledge gaps and how these gaps can be addressed using preclinical rodent models.

RECENT FINDINGS

Sex hormones play a critical role in bone physiology, yet there is a paucity of research regarding the effects of GAHT on bone microstructure and fracture risk in transgender individuals. The controlled clinical studies required to yield fracture data are unethical to conduct making clinically translatable preclinical research of the utmost importance. Novel genetic and surgical preclinical models have yielded significant mechanistic insight into the roles of sex steroids on skeletal integrity. Preclinical models of GAHT have the potential inform clinical approaches to preserve skeletal integrity and prevent fractures in transgender people undergoing GAHT. This review highlights the key considerations required to ensure the information gained from preclinical models of GAHT are informative.

Sarcopenia, osteoporosis and frailty

Muscles and bones are intricately connected tissues displaying marked co-variation during development, growth, aging, and in many diseases. While the diagnosis and treatment of osteoporosis are well established in clinical practice, sarcopenia has only been classified internationally as a disease in 2016. Both conditions are associated with an increased risk of adverse health outcomes such as fractures, dysmobility and mortality. Rather than focusing on one dimension of bone or muscle mass or weakness, the concept of musculoskeletal frailty captures the overall loss of physiological reserves in the locomotor system with age. The term osteosarcopenia in particular refers to the double jeopardy of osteoporosis and sarcopenia. Muscle-bone interactions at the biomechanical, cellular, paracrine, endocrine, neuronal or nutritional level may contribute to the pathophysiology of osteosarcopenia. The paradigm wherein muscle force controls bone strength is increasingly facing competition from a model centering on the exchange of myokines, osteokines and adipokines. The most promising results have been obtained in preclinical models where common drug targets have been identified to treat these conditions simultaneously. In this narrative review, we critically summarize the current understanding of the definitions, epidemiology, pathophysiology, and treatment of osteosarcopenia as part of an integrative approach to musculoskeletal frailty.

Cancer Cell-Extrinsic Roles for the Androgen Receptor in Prostate Cancer

Given the central role of the androgen receptor (AR) in prostate cancer cell biology, AR-targeted therapies have been the backbone of prostate cancer treatment for over 50 years. New data indicate that AR is expressed in additional cell types within the tumor microenvironment. Moreover, targeting AR for the treatment of prostate cancer has established side effects such as bone complications and an increased risk of developing cardiometabolic disease, indicating broader roles for AR. With the advent of novel technologies, such as single-cell approaches and advances in preclinical modeling, AR has been identified to have clinically significant functions in other cell types. In this mini-review, we describe new cancer cell-extrinsic roles for AR within the tumor microenvironment as well as systemic effects that collectively impact prostate cancer progression and patient outcomes.

Protein kinase D3 conditional knockout impairs osteoclast formation and increases trabecular bone volume in male mice

Studies using kinase inhibitors have shown that the protein kinase D (PRKD) family of serine/threonine kinases are required for formation and function of osteoclasts in culture. However, the involvement of individual protein kinase D genes and their in vivo significance to skeletal dynamics remains unclear. In the current study we present data indicating that protein kinase D3 is the primary form of PRKD expressed in osteoclasts. We hypothesized that loss of PRKD3 would impair osteoclast formation, thereby decreasing bone resorption and increasing bone mass. Conditional knockout (cKO) of Prkd3 using a murine Cre/Lox system driven by cFms-Cre revealed that its loss in osteoclast-lineage cells reduced osteoclast differentiation and resorptive function in culture. Examination of the Prkd3 cKO mice showed that bone parameters were unaffected in the femur at 4 weeks of age, but consistent with our hypothesis, Prkd3 conditional knockout resulted in 18 % increased trabecular bone mass in male mice at 12 weeks and a similar increase at 6 months. These effects were not observed in female mice. As a further test of our hypothesis, we asked if Prkd3 cKO could protect against bone loss in a ligature-induced periodontal disease model but did not see any reduction in bone destruction in this system. Together, our data indicate that PRKD3 promotes osteoclastogenesis both in vitro and in vivo.

The effects of seeding density and osteoclastic supplement concentration on osteoclastic differentiation and resorption

The bone resorbing osteoclasts are a complex type of cell essential for in vivo bone remodeling. There is no consensus on medium composition and seeding density for in vitro osteoclastogenesis, despite the importance thereof on osteoclastic differentiation and activity. The aim of this study was to investigate the relative effect of monocyte or peripheral blood mononuclear cell (PBMC) seeding density, osteoclastic supplement concentration and priming on the in vitro generation of functional osteoclasts, and to explore and evaluate the usefulness of commonly used markers for osteoclast cultures. Morphology and osteoclast formation were analyzed with fluorescence imaging for tartrate resistant acid phosphatase (TRAP) and integrin β3 (Iβ3). TRAP release was analyzed from supernatant samples, and resorption was analyzed from culture on Corning® Osteo Assay plates. In this study, we have shown that common non-standardized culturing conditions of monocyte or PBMCs had a significant effect on the in vitro generation of functional osteoclasts. We showed how increased osteoclastic supplement concentrations supported osteoclastic differentiation and resorption but not TRAP release, while priming resulted in increased TRAP release as well. Increased monocyte seeding densities resulted in more and large TRAP positive bi-nuclear cells, but not directly in more multinucleated osteoclasts, resorption or TRAP release. Increasing PBMC seeding densities resulted in more and larger osteoclasts and more resorption, although resorption was disproportionally low compared to the monocyte seeding density experiment. Exploration of commonly used markers for osteoclast cultures demonstrated that Iβ3 staining was an excellent and specific osteoclast marker in addition to TRAP staining, while supernatant TRAP measurements could not accurately predict osteoclastic resorptive activity. With improved understanding of the effect of seeding density and osteoclastic supplement concentration on osteoclasts, experiments yielding higher numbers of functional osteoclasts can ultimately improve our knowledge of osteoclasts, osteoclastogenesis, bone remodeling and bone diseases.

Skeletal and gene-regulatory functions of nuclear sex steroid hormone receptors

The wide variety of sex hormone actions underlie bone growth and health, and their actions mediate gene regulation by the cognate nuclear receptors. Nuclear androgen and estrogen receptors (AR, and ERα/ERβ) are hormone-dependent and DNA binding- transcription regulatory factors, and gene regulation by sex hormones often accompany with chromatin remodeling under aid of a number of co-regulators. As sex hormone biosynthesis is under highly regulated systemic and local regulations, the skeletal actions of sex hormones could be inferred from only the phenotypic abnormalities in skeleton in mouse genetic models deficient of nuclear receptors selectively in specific types of bone cells as well as at specific cell differentiation stages. Anabolic androgen actions and anti-bone resorptive estrogen actions are discussed here from the phenotypic abnormalities in such model mice. Though rapid gene regulation by sex hormones may not require chromatin reorganization, dynamic chromatin reconfiguration looks to facilitate profound and long-term hormonal actions. In this review, we focus the recent findings in gene regulation at a chromatin level, particularly of the function of enhancer RNAs transcribed from strong enhancers, and in the role of liquid-liquid phase separation state in transcription initiation through chromatin reconfiguration.

Recent Advances in Osteoclast Biological Behavior

With the progress of the aging population, bone-related diseases such as osteoporosis and osteoarthritis have become urgent problems. Recent studies have demonstrated the importance of osteoclasts in bone homeostasis, implying these will be an important mediator in the treatment of bone-related diseases. Up to now, several reviews have been performed on part of osteoclast biological behaviors such as differentiation, function, or apoptosis. However, few reviews have shown the complete osteoclast biology and research advances in recent years. Therefore, in this review, we focus on the origin, differentiation, apoptosis, behavior changes and coupling signals with osteoblasts, providing a simple but comprehensive overview of osteoclasts for subsequent studies.

Sex Steroid Regulation of Oxidative Stress in Bone Cells: An In Vitro Study

Environmental stimuli, including sex hormones and oxidative stress (OS), affect bone balance, modifying the epigenetic profiles of key osteogenic genes. Nonetheless, the interplay between sex steroids, epigenome and OS has yet be fully elucidated. This paper aims to study in vitro the role of sex steroids in OS-induced alteration in bone cells’ homeostasis, and to assess the possible contribution of epigenetic modifications. Toward this purpose, osteoblast (MC3T3-E1) and osteocyte (MLOY-4) cell lines were exposed to two different sources of free oxygen radicals, i.e., tert-butyl hydroperoxide and dexamethasone, and the protective effect of pre-treatment with androgens and estrogens was evaluated. In particular, we analyzed parameters that reflect bone cell homeostasis such as cell viability, cell migration, transcriptomic profile, transcriptional activity, and epigenetic signature. Our findings indicate that estrogens and androgens counteract OS effects. Using partially overlapping strategies, they reduce OS outcomes regarding cell viability, cell migration, the transcriptomic profile of gene families involved in bone remodeling, and epigenetic profile, i.e., H3K4me3 level. Additionally, we demonstrated that the protective effect of steroids against OS on bone homeostasis is partially mediated by the Akt pathway. Overall, these results suggest that the hormonal milieu may influence the mechanisms of age-related bone disease.

The Development of Molecular Biology of Osteoporosis

Osteoporosis is one of the major bone disorders that affects both women and men, and causes bone deterioration and bone strength. Bone remodeling maintains bone mass and mineral homeostasis through the balanced action of osteoblasts and osteoclasts, which are responsible for bone formation and bone resorption, respectively. The imbalance in bone remodeling is known to be the main cause of osteoporosis. The imbalance can be the result of the action of various molecules produced by one bone cell that acts on other bone cells and influence cell activity. The understanding of the effect of these molecules on bone can help identify new targets and therapeutics to prevent and treat bone disorders. In this article, we have focused on molecules that are produced by osteoblasts, osteocytes, and osteoclasts and their mechanism of action on these cells. We have also summarized the different pharmacological osteoporosis treatments that target different molecular aspects of these bone cells to minimize osteoporosis.

Sexual Dimorphism in Osteoclasts

Osteoclasts are the principal mediators of bone resorption. They form through the fusion of mononuclear precursor cells under the principal influence of the cytokines macrophage colony stimulating factor (M-CSF, aka CSF-1) and receptor activator of NF-κB ligand (RANKL, aka TNFSF11). Sexual dimorphism in the development of the skeleton and in the incidence of skeletal diseases is well described. In general, females, at any given age, have a lower bone mass than males. The reasons for the differences in the bone mass of the skeleton between women and men at various ages, and the incidence of certain metabolic bone diseases, are multitude, and include the actions of sex steroids, genetics, age, environment and behavior. All of these influence the rate that osteoclasts form, resorb and die, and frequently produce different effects in females and males. Hence, a variety of factors are responsible for the sexual dimorphism of the skeleton and the activity of osteoclasts in bone. This review will provide an overview of what is currently known about these factors and their effects on osteoclasts.

Androgen action on renal calcium and phosphate handling: Effects of bisphosphonate treatment and low calcium diet

Renal calcium and phosphate handling is an important contributor to mineral homeostasis and bone health and the androgen receptor (AR) is highly expressed in the kidney. We investigated the short term effects of androgen deprivation on renal calcium and phosphate reabsorption, independent of their effects on bone. Two weeks following orchidectomy (ORX) of adult mice, bone loss occurred along with hypercalciuria, which was similarly prevented by testosterone and dihydrotestosterone supplementation. Treatment with bisphosphonates prior to ORX also inhibited hypercalciuria, indicating that the calcium flux originated from the bone. Renal calcium and phosphate transporter expression was increased post-ORX, independent of bisphosphonates. Furthermore, androgen deprivation appeared to stimulate local synthesis of 1,25(OH)₂D₃. When bisphosphonate-treated mice were fed a low calcium diet, bone resorption was no longer blocked and secondary hyperparathyroidism developed, which was more pronounced in ORX mice than sham-operated mice. In conclusion, this study shows that androgen deprivation increased renal calcium and phosphate transporter expression, independent of bone, and underlines the importance of adequate intestinal calcium supply in circumstances of androgen deprivation and bisphosphonate treatment.

The Selective Androgen Receptor Modulator Ostarine Improves Bone Healing in Ovariectomized Rats

Non-steroidal selective androgen receptor modulators, including ostarine, have been developed as an alternative to steroidal hormones. Ostarine has shown a beneficial effect on bone in experimental studies, but no data regarding the effect of ostarine on bone healing have yet been reported. We investigated effects of ostarine on bone healing in ovariectomized rats. Sprague-Dawley rats (3 months old) were ovariectomized (Ovx, n = 46) or left intact (Non-Ovx, n = 10). After 8 weeks, an osteotomy of the tibia metaphysis was created in all rats, and the Ovx rats were divided into four groups: untreated Ovx (n = 10) and three Ovx groups (each of 12 rats) treated with ostarine at doses of 0.04, 0.4, or 4 mg/kg BW (OS-0.04, OS-0.4, and OS-4 groups). Five weeks later, bone healing was analyzed. The OS-4 dose enhanced callus formation, increased callus density, accelerated bridging time of the osteotomy, and elevated alkaline phosphatase gene expression in callus and its protein expression in serum. In the Ovx group, most of the callus parameters were diminished. All OS treatments increased the weight of the gastrocnemius muscle, but only partly enhanced uterus weight in OS-0.4 and OS-4. Serum cholesterol level was reduced, and serum phosphorus was elevated in OS-0.04 and OS-4. Ostarine appeared to have a positive effect on early bone healing in ovariectomized rats. Considering its favorable effect on non-osteotomized bone and muscle, this treatment could be further explored as a therapy for osteoporosis. However, possible metabolic side effects should first be evaluated.

IGFBP7 acts as a negative regulator of RANKL-induced osteoclastogenesis and oestrogen deficiency-induced bone loss

OBJECTIVES

Insulin-like growth factor-binding protein 7 (IGFBP7) is a low-affinity insulin growth factor (IGF) binder that may play an important role in bone metabolism. We previously reported that IGFBP7 enhanced osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) via the Wnt/β-catenin signalling pathway. In this study, we tried to reveal its function in osteoclast differentiation and osteoporosis.

METHODS

We used both in vitro and in vivo studies to investigate the effects of IGFBP7 on RANKL-induced osteoclastogenesis and osteoporosis, together with the underlying molecular mechanisms of these processes.

RESULTS

We show that IGFBP7 inhibited receptor activation of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclastogenesis, F-actin ring formation and bone resorption, which was confirmed by using recombinant IGFBP7 protein, lentivirus and siRNA. The NF-κB signalling pathway was inhibited during this process. Moreover, in a mouse ovariectomy-induced osteoporosis model, IGFBP7 treatment attenuated osteoporotic bone loss by inhibiting osteoclast activity.

CONCLUSIONS

Taken together, these findings show that IGFBP7 suppressed osteoclastogenesis in vitro and in vivo and suggest that IGFBP7 is a negative regulator of osteoclastogenesis and plays a protective role in osteoporosis. These novel insights into IGFBP7 may facilitate the development of potential treatment strategies for oestrogen deficiency-induced osteoporosis and other osteoclast-related disorders.

Androgens and Androgen Receptor Actions on Bone Health and Disease: From Androgen Deficiency to Androgen Therapy

Androgens are not only essential for bone development but for the maintenance of bone mass. Therefore, conditions with androgen deficiency, such as male hypogonadism, androgen-insensitive syndromes, and prostate cancer with androgen deprivation therapy are strongly associated with bone loss and increased fracture risk. Here we summarize the skeletal effects of androgens—androgen receptors (AR) actions based on in vitro and in vivo studies from animals and humans, and discuss bone loss due to androgens/AR deficiency to clarify the molecular basis for the anabolic action of androgens and AR in bone homeostasis and unravel the functions of androgen/AR signaling in healthy and disease states. Moreover, we provide evidence for the skeletal benefits of androgen therapy and elucidate why androgens are more beneficial than male sexual hormones, highlighting their therapeutic potential as osteoanabolic steroids in improving bone fracture repair. Finally, the application of selective androgen receptor modulators may provide new approaches for the treatment of osteoporosis and fractures as well as building stronger bones in diseases dependent on androgens/AR status.

Androgen Receptor in Neurons Slows Age-Related Cortical Thinning in Male Mice

Androgens via the androgen receptor (AR) are required for optimal male bone health. The target cell(s) for the effects of androgens on cortical bone remain(s) incompletely understood. In females, estrogen receptor alpha in neurons is a negative regulator of cortical and trabecular bone. Whether neuronal AR regulates bone mass in males remains unexplored. Here, we inactivated AR in neurons using a tamoxifen-inducible CreERT2 under the control of the neuronal promoter Thy1. Tamoxifen induced a 70% to 80% reduction of AR mRNA levels in Thy1-CreERT2-positive brain regions cerebral cortex and brainstem as well as in the peripheral nervous tissue of male neuronal AR knockout (N-ARKO) mice. Hypothalamic AR mRNA levels were only marginally reduced and the hypothalamic-pituitary-gonadal axis remained unaffected, as determined by normal levels of serum testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). In contrast to orchidectomy, deletion of neuronal AR did not alter body weight, body composition, hindlimb muscle mass, grip strength, or wheel running. MicroCT analysis of the femur revealed no changes in bone accrual during growth in N-ARKO mice. However, 36- and 46-week-old N-ARKO mice displayed an accelerated age-related cortical involution, namely a more pronounced loss of cortical thickness and strength, which occurred in the setting of androgen sufficiency. Neuronal AR inactivation decreased the cancellous bone volume fraction in L₅ vertebra but not in the appendicular skeleton of aging mice. MicroCT findings were corroborated in the tibia and after normalization of hormonal levels. Serum markers of bone turnover and histomorphometry parameters were comparable between genotypes, except for a 30% increase in osteoclast surface in the trabecular compartment of 36-week-old N-ARKO mice. Cortical bone loss in N-ARKO mice was associated with an upregulation of Ucp1 expression in brown adipose tissue, a widely used readout for sympathetic tone. We conclude that androgens preserve cortical integrity in aging male mice via AR in neurons. © 2018 American Society for Bone and Mineral Research.

Age-related bone loss and sarcopenia in men

Bone and muscle are required for mobility but they also have endocrine and metabolic functions. In ageing as well as in many chronic diseases, bone loss and muscle atrophy occur simultaneously, leading to concomitant osteoporosis and sarcopenia. This occurs in both genders but compared with postmenopausal women, men appear to be better protected against age-related bone and muscle decay. Sex steroids (both androgens like testosterone and oestrogens like estradiol) are mainly responsible for musculoskeletal sexual dimorphism. They stimulate peak bone and muscle mass accretion during puberty and midlife, and prevent subsequent loss in ageing men but not post-menopausal women. Still, recent studies have highlighted the importance of intrinsic ageing mechanisms such as cellular senescence and oxidative stress in both genders. Sarcopenia may predispose to dysmobility, frailty, falls and fractures, but whether so-called osteosarcopenia qualifies as a distinct entity remains debated. Although randomized clinical trials in male osteoporosis are smaller and therefore underpowered for some outcomes like hip fractures, the available evidence suggests that the clinical diagnostic and therapeutic approach to male osteoporosis is largely similar to that in postmenopausal women. There is a clear unmet medical need for effective and safe anabolic drugs to rebuild the ageing skeleton, muscle, and preferably both tissues simultaneously. The Wnt/sclerostin and myostatin/activin receptor signalling pathways appear particularly promising in this regard. In this narrative review, we aim to provide an overview of our current understanding of the pathophysiology and treatment of male osteoporosis and sarcopenia, and interactions between these two diseases.

Gonadal Hormones and Bone

In both sexes, estrogen is one of the most essential hormones for maintaining bone integrity. Also, especially in men, androgen has beneficial effects on bone independent of estrogen. However, estrogen replacement therapy for postmenopausal women increases the risk of developing breast cancer and endometrial cancer, and androgen replacement therapy for partial androgen deficiency of the aging male increases the risk of developing prostate cancer. Various mechanisms have been proposed on the effects of gonadal hormones on bone, such as effects through cytokines including IL-6 and effects on the OPG/RANKL ratio. In addition, large amounts of new information deriving from high-throughput gene expression analysis raise the possibility of multiple other effects on bone cells. Both estrogen and androgen exert their effects via the estrogen receptor (ER) or the androgen receptor (AR), which belongs to the nuclear receptor superfamily. Compounds such as selective estrogen receptor modulators (SERMs) and selective androgen receptor modulators (SARMs) also bind ER and AR, respectively. However, SERMs and SARMs alter the ER or AR structure differently from estrogen or androgen, resulting in other downstream gene responses. As a result they can exert favorable effects on bone while suppressing the undesirable actions of estrogen and androgen. Elucidation of ER and AR ligand-specific and tissue-specific gene regulation mechanisms will also provide information on the signal transduction mechanisms of other nuclear receptors and will be valuable for the development of new therapeutic agents.

Commensal Microbiota Enhance Both Osteoclast and Osteoblast Activities

Recent studies suggest that the commensal microbiota affects not only host energy metabolism and development of immunity but also bone remodeling by positive regulation of osteoclast activity. However, the mechanism of regulation of bone cells by the commensal microbiota has not been elucidated. In this study, 8-week-old specific pathogen-free (SPF) and germ-free (GF) mice were compared in terms of alveolar bones and primary osteoblasts isolated from calvarias. Micro-CT analysis showed that SPF mice had larger body size associated with lower bone mineral density and bone volume fraction in alveolar bones compared with GF mice. Greater numbers of osteoclasts in alveolar bone and higher serum levels of tartrate-resistant acid phosphatase 5b were observed in SPF mice. Tissue extracts from SPF alveolar bone showed higher levels of cathepsin K, indicating higher osteoclast activity. SPF alveolar extracts also showed elevated levels of γ-carboxylated glutamic acid⁻osteocalcin as a marker of mature osteoblasts compared with GF mice. Polymerase chain reaction (PCR) array analysis of RNA directly isolated from alveolar bone showed that in SPF mice, expression of mRNA of osteocalcin, which also acts as an inhibitor of bone mineralization, was strongly enhanced compared with GF mice. Cultured calvarial osteoblasts from SPF mice showed reduced mineralization but significantly enhanced expression of mRNAs of osteocalcin, alkaline phosphatase, insulin-like growth factor-I/II, and decreased ratio of osteoprotegerin/receptor activator of nuclear factor-kappa B ligand compared with GF mice. Furthermore, PCR array analyses of transcription factors in cultured calvarial osteoblasts showed strongly upregulated expression of Forkhead box g1. In contrast, Gata-binding protein 3 was strongly downregulated in SPF osteoblasts. These results suggest that the commensal microbiota prevents excessive mineralization possibly by stimulating osteocalcin expression in osteoblasts, and enhances both osteoblast and osteoclast activity by regulating specific transcription factors.

Sex steroids and the kidney: role in renal calcium and phosphate handling

Calcium and phosphate are vital for the organism and constitute essential components of the skeleton. Serum levels are tightly hormonally regulated and maintained by exchange with three major sources: the intestines, the kidney and the bone. The effects of sex steroids on the bone have been extensively studied and it is well known that sex steroid deficiency induces bone loss, indirectly influencing renal calcium and phosphate homeostasis. However, it is unknown whether sex steroids also directly regulate renal calcium and phosphate handling, hereby potentially indirectly impacting on bone. The presence of androgen receptors (AR) and estrogen receptors (ER) in both human and rodent kidney, although their exact localization within the kidney remains debated, supports direct effects. Estrogens stimulate renal calcium reabsorption as well as phosphate excretion, while the effects of androgens are less clear. Many of the studies performed with regard to renal calcium and/or phosphate homeostasis do not correct for the calcium and phosphate fluxes from the bone and intestines, which complicates the differentiation between the direct effects of sex steroids on renal calcium and phosphate handling and the indirect effects via the bone and intestines. The objective of this study is to review the literature and current insight of the role of sex steroids in calcium and phosphate handling in the kidney.

The biological and clinical advances of androgen receptor function in age-related diseases and cancer [Review]

Hormonal alterations with aging contribute to the pathogenesis of several diseases. Androgens mediate their effects predominantly through binding to the androgen receptor (AR), a member of the ligand-dependent nuclear receptor superfamily. By androgen treatment, AR is recruited to specific genomic loci dependent on tissue specific pioneer factors to regulate target gene expression. Recent studies have revealed the epigenetic modulation by AR-associated histone modifiers and the roles of non-coding RNAs in AR signaling. Androgens are male sex hormone to induce differentiation of the male reproductive system required for the establishment of adult sexual function. As shown by several reports using AR knockout mouse models, androgens also have anabolic functions in several tissues such as bone, muscle and central nervous systems. Notably, AR has a central role in prostate cancer progression. Prostate cancer is the most frequently diagnosed cancer in men. Androgen-deprivation therapy for cancer patients and decline of serum androgen with aging promote several diseases associated with aging and quality of life of older men such as osteoporosis, sarcopenia and dementia. Thus, androgen replacement therapy for treating late onset hypogonadism (LOH) or new epigenetic regulators have the potential to overcome the symptoms caused by the low androgen, although adverse effects for cardiovascular diseases have been reported. Given the increasing longevity and consequent rise of age-related diseases and prostate cancer patients, a more understanding of the AR actions in male health remains a high research priority.

A Case of Male Osteoporosis: A 37-Year-Old Man with Multiple Vertebral Compression Fractures

While the contributing role of testosterone to bone health is rather modest compared to other factors such as estradiol levels, male hypogonadism is associated with low bone mass and fragility fractures. Along with stimulating physical puberty by achieving virilization and a normal muscle mass and improving psychosocial wellbeing, the goals of testosterone replacement therapy in male hypogonadism also include attainment of age-specific bone mineral density. We report on a 37-year-old man who presented with multiple vertebral compression fractures several years following termination of testosterone replacement therapy for presumed constitutional delay in growth and puberty. Here, we discuss the management of congenital hypogonadotropic hypogonadism with hyposmia (Kallmann syndrome), with which the patient was ultimately diagnosed, the role of androgens in the acquisition of bone mass during puberty and its maintenance thereafter, and outline specific management strategies for patients with hypogonadism and high risk for fragility fractures.

Nuclear Receptors in Skeletal Homeostasis

Nuclear receptors are a family of transcription factors that can be activated by lipophilic ligands. They are fundamental regulators of development, reproduction, and energy metabolism. In bone, nuclear receptors enable bone cells, including osteoblasts, osteoclasts, and osteocytes, to sense their dynamic microenvironment and maintain normal bone development and remodeling. Our views of the molecular mechanisms in this process have advanced greatly in the past decade. Drugs targeting nuclear receptors are widely used in the clinic for treating patients with bone disorders such as osteoporosis by modulating bone formation and resorption rates. Deficiency in the natural ligands of certain nuclear receptors can cause bone loss; for example, estrogen loss in postmenopausal women leads to osteoporosis and increases bone fracture risk. In contrast, excessive ligands of other nuclear receptors, such as glucocorticoids, can also be detrimental to bone health. Nonetheless, the ligand-induced osteoprotective effects of many other nuclear receptors, e.g., vitamin D receptor, are still in debate and require further characterizations. This review summarizes previous studies on the roles of nuclear receptors in bone homeostasis and incorporates the most recent findings. The advancement of our understanding in this field will help researchers improve the applications of agonists, antagonists, and selective modulators of nuclear receptors for therapeutic purposes; in particular, determining optimal pharmacological drug doses, preventing side effects, and designing new drugs that are more potent and specific.

Estrogens and Androgens in Skeletal Physiology and Pathophysiology

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

Androgens have antiresorptive effects on trabecular disuse osteopenia independent from muscle atrophy

Aging hypogonadal men are at increased risk of osteoporosis and sarcopenia. Testosterone is a potentially appealing strategy to prevent simultaneous bone and muscle loss. The androgen receptor (AR) mediates antiresorptive effects on trabecular bone via osteoblast-lineage cells, as well as muscle-anabolic actions. Sex steroids also modify the skeletal response to mechanical loading. However, it is unclear whether the effects of androgens on bone remain effective independent of mechanical stimulation or rather require indirect androgen effects via muscle. This study aims to characterize the effects and underlying mechanisms of androgens on disuse osteosarcopenia. Adult male mice received a unilateral botulinum toxin (BTx) injection, and underwent sham surgery or orchidectomy (ORX) without or with testosterone (ORX+T) or dihydrotestosterone (ORX+DHT) replacement. Compared to the contralateral internal control hindlimb, acute trabecular number and bone volume loss was increased by ORX and partially prevented DHT. T was more efficient and increased BV/TV in both hindlimbs over sham values, although it did not reduce the detrimental effect of BTx. Both androgens and BTx regulated trabecular osteoclast surface as well as tartrate-resistant acid phosphatase expression. Androgens also prevented BTx-induced body weight loss but did not significantly influence paralysis or muscle atrophy. BTx and ORX both reduced cortical thickness via endosteal expansion, which was prevented by T but not DHT. In long-term follow-up, the residual trabecular bone volume deficit in sham-BTx hindlimbs was prevented by DHT but T restored it more efficiently to pre-treatment levels. Conditional AR deletion in late osteoblasts and osteocytes or in the satellite cell lineage increased age-related trabecular bone loss in both hindlimbs without influencing the effect of BTx on trabecular osteopenia. We conclude that androgens have antiresorptive effects on trabecular disuse osteopenia which do not require AR actions on bone via muscle or via osteocytes.

Androgen receptors and experimental bone loss - an in vivo and in vitro study

Testosterone is a sex hormone that exhibits many functions beyond reproduction; one such function is the regulation of bone metabolism. The role played by androgen receptors during testosterone-mediated biological processes associated with bone metabolism is largely unknown. This study aims to use a periodontal disease model in vivo in order to assess the involvement of androgen receptors on microbial-induced inflammation and alveolar bone resorption in experimental bone loss. The impact of hormone deprivation was tested through both orchiectomy and chemical blockage of androgen receptor using flutamide (FLU). Additionally, the direct effect of exogenous testosterone, and the role of the androgen receptor, on osteoclastogenesis were investigated. Thirty male adult rats (n=10/group) were subjected to: 1-orchiectomy (OCX); 2-OCX sham surgery; or 3-OCX sham surgery plus FLU, four weeks before the induction of experimental bone loss. Ten OCX sham-operated rats were not subjected to experimental bone loss and served as healthy controls. The rats were euthanized two weeks later, so as to assess bone resorption and the production of inflammatory cytokines in the gingival tissue and serum. In order to study the in vitro impact of testosterone, osteoclasts were differentiated from RAW264.7 cells and testosterone was added at increasing concentrations. Both OCX and FLU increased bone resorption, but OCX alone was observed to increase osteoclast count. IL-1β production was increased only in the gingival tissue of OCX animals, whereas FLU-treated animals presented a decreased expression of IL-6. Testosterone reduced the osteoclast formation in a dose-dependent manner, and significantly impacted the production of TNF-α; FLU partially reversed these actions. When taken together, our results indicate that testosterone modulates experimental bone loss, and that this action is mediated, at least in part, via the androgen receptor.