The effects of androgen deficiency on murine bone remodeling and bone mineral density are mediated via cells of the osteoblastic lineage

Mitofusin 2 plays a critical role in maintaining the functional integrity of the neuromuscular-skeletal axis

PURPOSE

Mitofusin 2 (Mfn2) is one of two mitofusins involved in regulating mitochondrial size, shape and function, including mitophagy, an important cellular mechanism to limit oxidative stress. Reduced expression of Mfn2 has been associated with impaired osteoblast differentiation and function and a reduction in the number of viable osteocytes in bone. We hypothesized that the genetic absence of Mfn2 in these cells would increase their susceptibility to aging-associated metabolic stress, leading to a progressive impairment in skeletal homeostasis over time.

METHODS

Mfn2 was selectively deleted in vivo at three different stages of osteoblast lineage commitment by crossing mice in which the Mfn2 gene was floxed with transgenic mice expressing Cre under the control of the promoter for Osterix (OSX), collagen1a1, or DMP1 (Dentin Matrix Acidic Phosphoprotein 1).

RESULTS

Mice in which Mfn2 was deleted using DMP1-cre demonstrated a progressive and dramatic decline in bone mineral density (BMD) beginning at 10 weeks of age (n = 5 for each sex and each genotype from age 10 to 20 weeks). By 15 weeks, there was evidence for a functional decline in muscle performance as assessed using a rotarod apparatus (n = 3; 2 males/ 1 female for each genotype), accompanied by a decline in lean body mass. A marked reduction in trabecular bone mass was evident on bone histomorphometry, and biomechanical testing at 25 weeks (k/o: 2 male/1 female, control 2 male/2 female) revealed severely impaired femur strength. Extensive regional myofiber atrophy and degeneration was observed on skeletal muscle histology. Electron microscopy showed progressive disruption of cellular architecture, with disorganized sarcomeres and a bloated mitochondrial reticulum. There was also evidence of neurodegeneration within the ventral horn and roots of the lumbar spinal cord, which was accompanied by myelin loss and myofiber atrophy. Deletion of Mfn2 using OSX-cre or Col1a1-cre did not result in a musculoskeletal phenotype. Where possible, male and female animals were analyzed separately, but small numbers of animals in each group limited statistical power. For other outcomes, where sex was not considered, small sample sizes might still limit the strength of the observation.

CONCLUSION

Despite known functional overlap of Mfn1 and Mfn2 in some tissues, and their co-expression in bone, muscle and spinal cord, deletion of Mfn2 using the 8 kB DMP1 promoter uncovered an important non-redundant role for Mfn2 in maintaining the neuromuscular/bone axis.

Causality between Sex Hormones and Bone Mineral Density in Childhood: Age- and Tanner-Stage-Matched Sex Hormone Level May Be an Early Indicator of Pediatric Bone Fragility

This study aimed to investigate the impact of hypogonadism on bone mineral density (BMD) in children and adolescents with chronic diseases to determine the relationship between sex hormones and BMD. This retrospective study included 672 children and adolescents with chronic diseases such as hemato-oncologic, rheumatoid, gastrointestinal, and endocrinologic diseases. The relationship between the sex- and Tanner-stage-matched Z-scores for sex hormones and the sex- and age-matched lumbar spine BMD (LSBMD) Z-scores was evaluated. Adjustments were made for confounders such as underlying diseases, age at diagnosis, and age- and sex-matched body mass index Z-scores. Patients had a mean LSBMD Z-score of -0.55 ± 1.31. In the multivariate regression analysis, male testosterone showed a positive association with the LSBMD Z-score (p < 0.001), whereas female estradiol, luteinizing hormone, and follicular-stimulating hormone showed no significant association with the LSBMD Z-scores. In the male group, the testosterone level was associated with LSBMD Z-scores > -1.0 (p < 0.001), > -2.0 (p < 0.001), and > -3.0 (p = 0.002), while the estradiol level was associated with LSBMD Z-scores > -2.0 (p = 0.001) and > -3.0 (p = 0.002) in the female group. In conclusion, sex hormones are associated with BMD in children and adolescents with chronic diseases. Therefore, various measures may be necessary to predict future skeletal problems and improve bone health in these patients.

Contrasting bone profiles in PCOS are related to BMI: a systematic review and meta-analysis

CONTEXT

Controversial results have emerged regarding whether PCOS is protective or increases the risk of bone frailty.

OBJECTIVE

This study investigated whether the PCOS condition affects bone parameters of premenopausal women. This is an update for a previous meta-analysis published in 2019.

DATA SOURCES

We searched MEDLINE and Embase.

STUDY SELECTION

Studies were considered eligible for the update if published in English between the 1st of October 2018 and the 31st of December 2023. The diagnosis of PCOS should be based on NIH criteria, the Rotterdam Consensus, AE-PCOS society criteria, or ICD codes in women over 18 years old. Only records with the Newcastle-Ottawa Scale > 6 were selected for data extraction.

DATA EXTRACTION

Data were extracted by two independent reviewers.

DATA SYNTHESIS

We identified 31 studies that met the inclusion criteria for qualitative analysis from 3322 studies in the whole period (1990-2023). Overall, cross-sectional studies included 1822 individuals with PCOS and 1374 controls, while cohort studies incorporated 30305 women with PCOS and 101907 controls. Contrasting profiles emerged after stratification using a BMI cutoff of 27 kg/m2. Individuals with PCOS and a BMI <27 kg/m2 exhibited lower vertebral and non-vertebral bone density, reduced bone turnover marker (osteocalcin), and increased bone resorption marker (CTX) levels. Conversely, individuals with PCOS and a BMI >27 kg/m2 exhibited increased vertebral and non-vertebral BMD, with no significant changes in bone formation and resorption markers (except osteocalcin).

CONCLUSIONS

The findings of this study alert for a low bone mass, low bone formation, and increased bone resorption PCOS with a BMI <27 kg/m2.

Effects of distinct Polycystic Ovary Syndrome phenotypes on bone health

Polycystic Ovary Syndrome (PCOS) is a highly prevalent and heterogenous endocrinopathy affecting 5-18% of women. Although its cardinal features include androgen excess, ovulatory dysfunction, and/or polycystic ovarian morphology, women often display related metabolic manifestations, including hyperinsulinaemia, insulin resistance, and obesity. Emerging data reveal that the hormonal alterations associated with PCOS also impact bone metabolism. However, inconsistent evidence exists as to whether PCOS is a bone-protective or bone-hindering disorder with an accumulating body of clinical data indicating that hyperandrogenism, hyperinsulinaemia, insulin resistance, and obesity may have a relative protective influence on bone, whereas chronic low-grade inflammation and vitamin D deficiency may adversely affect bone health. Herein, we provide a comprehensive assessment of the endocrine and metabolic manifestations associated with PCOS and their relative effects on bone metabolism. We focus principally on clinical studies in women investigating their contribution to the alterations in bone turnover markers, bone mineral density, and ultimately fracture risk in PCOS. A thorough understanding in this regard will indicate whether women with PCOS require enhanced surveillance of bone health in routine clinical practice.

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.

Androgen plays an important role in regulating the synthesis of pheromone in the scent gland of muskrat

The scent (musk) gland is an organ unique to muskrats and other scent-secreting animals, and the pheromones (musk) synthesized and secreted by the scent gland play a role in chemical communication among scent-secreting animals. The musk gland is synchronized with testicular developmental changes; however, little is known regarding androgen secretion from the testis and how this regulates pheromone synthesis and the secretion of scent. To investigate the effect of androgens on the synthesis of pheromones in the musk gland, we established a muskrat castration model by surgical removal of the testis, and analyzed the histomorphology, hormone concentration, gene expression, and changes in the chemical composition of the musk gland in castration and control groups by histomorphological analysis, Enzyme-Linked ImmunoSorbent Assay (ELISA), RNA sequencing (RNA-seq), and gas chromatography-mass spectrometry (GCMS). Histomorphological analysis results showed that after castration, muskrat gland cells underwent significant atrophy (P < 0.05). Hormone measurement results showed that there was a significant decrease in serum testosterone and muskrat musk testosterone (P < 0.05) after muskrat castration. Transcriptome sequencing results showed that 510 differentially expressed transcripts (DETs) were mainly enriched in fatty acid metabolism, terpenoid backbone biosynthesis, fatty acid degradation, PPAR signaling pathway, and fatty acid biosynthesis. GCMS results showed that macrocyclic ketones, steroids, fatty acids, alcohols, and esters in musk were significantly changed (P < 0.05). In conclusion, androgens were found to play an important function in the chemical communication exchange between muskrats through regulating pheromone synthesis in musk cells. This study provides a basis for understanding the mechanism of animal communication influenced by androgens.

Testosterone therapy in children and adolescents: to whom, how, when?

Male production of testosterone is crucial for the development of a wide range of functions. External and internal genitalia formation, secondary sexual characteristics, spermatogenesis, growth velocity, bone mass density, psychosocial maturation, and metabolic and cardiovascular profiles are closely dependent on testosterone exposure. Disorders in androgen production can present during all life-stages, including childhood and adolescence, and testosterone therapy (TT) is in many cases the only treatment that can correct the underlying deficit. TT is controversial in the pediatric population as hypoandrogenism is difficult to classify and diagnose in these age groups, and standardized protocols of treatment and monitorization are still lacking. In pediatric patients, hypogonadism can be central, primary, or a combination of both. Testosterone preparations are typically designed for adults' TT, and providers need to be aware of the advantages and disadvantages of these formulations, especially cognizant of supratherapeutic dosing. Monitoring of testosterone levels in boys on TT should be tailored to the individual patient and based on the anticipated duration of therapy. Although clinical consensus is lacking, an approximation of the current challenges and common practices in pediatric hypoandrogenism could help elucidate the broad spectrum of pathologies that lie behind this single hormone deficiency with wide-ranging implications.

Methylsulfonylmethane Increases the Alveolar Bone Density of Mandibles in Aging Female Mice

Methylsulfonylmethane (MSM) is a naturally occurring anti-inflammatory compound that effectively treats multiple degenerative diseases such as osteoarthritis and acute pancreatitis. Our previous studies have demonstrated the ability of MSM to differentiate stem cells from human exfoliated deciduous (SHED) teeth into osteoblast-like cells. This study examined the systemic effect of MSM in 36-week-old aging C57BL/6 female mice in vivo by injecting MSM for 13 weeks. Serum analyses showed an increase in expression levels of bone formation markers [osteocalcin (OCN) and procollagen type 1 intact N-terminal propeptide (P1NP)] and a reduction in bone resorption markers [tartrate-resistant acid phosphatase (TRAP) and C-terminal telopeptide of type I collag (CTX-I)] in MSM-injected animals. Micro-computed tomographic images demonstrated an increase in trabecular bone density in mandibles. The trabecular bone density tended to be higher in the femur, although the increase was not significantly different between the MSM- and phosphate-buffered saline (PBS)-injected mice. In mandibles, an increase in bone density with a corresponding decrease in the marrow cavity was observed in the MSM-injected mice. Furthermore, immunohistochemical analyses of the mandibles for the osteoblast-specific marker - OCN, and the mesenchymal stem cell-specific marker - CD105 showed a significant increase and decrease in OCN and CD105 positive cells, respectively. Areas of bone loss were observed in the inter-radicular region of mandibles in control mice. However, this loss was considerably decreased due to stimulation of bone formation in response to MSM injection. In conclusion, our study has demonstrated the ability of MSM to induce osteoblast formation and function in vivo, resulting in increased bone formation in the mandible. Hence, the application of MSM and stem cells of interest may be the right combination in alveolar bone regeneration under periodontal or other related diseases that demonstrate bone loss.

Androstenedione (a Natural Steroid and a Drug Supplement): A Comprehensive Review of Its Consumption, Metabolism, Health Effects, and Toxicity with Sex Differences

Androstenedione is a steroidal hormone produced in male and female gonads, as well as in the adrenal glands, and it is known for its key role in the production of estrogen and testosterone. Androstenedione is also sold as an oral supplement, that is being utilized to increase testosterone levels. Simply known as "andro" by athletes, it is commonly touted as a natural alternative to anabolic steroids. By boosting testosterone levels, it is thought to be an enhancer for athletic performance, build body muscles, reduce fats, increase energy, maintain healthy RBCs, and increase sexual performance. Nevertheless, several of these effects are not yet scientifically proven. Though commonly used as a supplement for body building, it is listed among performance-enhancing drugs (PEDs) which is banned by the World Anti-Doping Agency, as well as the International Olympic Committee. This review focuses on the action mechanism behind androstenedione's health effects, and further side effects including clinical features, populations at risk, pharmacokinetics, metabolism, and toxicokinetics. A review of androstenedione regulation in drug doping is also presented.

Tetracycline, an Appropriate Reagent for Measuring Bone-Formation Activity in the Murine Model of the Streptococcus mutans-Induced Bone Loss

Tetracycline is used as a fluorescent reagent to measure bone formation activity in bone histomorphometric analyses. However, there is a possibility to lead a different conclusion when it is used in a bacteria-infected murine model since the tetracycline is considered to work as an antibiotic reagent. There are non-antibiotic fluorescent reagents such as alizarin and calcein for measuring bone formation activity. The purpose of this study was to clarify whether tetracycline could be an appropriate reagent to measure bone formation activity in a murine bacterial model in the same way as a non-antibiotic fluorescent reagent. We used Streptococcus mutans (S. mutans), a normal inhabitant in the oral cavity and tetracycline-sensitive bacteria, for inducing the bacterial model. The murine bacterial model was generated by intravenously inoculating S. mutans to the tail vein, followed immediately by the injection of the first fluorescent reagent, and the second one was injected 2 days prior to euthanization. After one day of inoculation with S. mutans, the subcutaneously injected alizarin had a similar colony count derived from the liver and the bone marrow tissue compared to the phosphate buffered saline (PBS)-injected control group. On the other hand, subcutaneous injection of tetracycline led to a significantly lower colony count from the liver compared to alizarin- or calcein-injected group. However, on day seven, after S. mutans intravenous injections, bone mineral density of distal femurs was significantly reduced by the bacteria inoculation regardless of which fluorescent reagents were injected subcutaneously. Finally, S. mutans inoculation reduced bone-formation-activity indices in both the tetracycline-alizarin double-injected mice and the calcein-alizarin double-injected mice. These results suggested that a one-time injection of tetracycline did not affect bone formation indices in the S. mutans-induced bone loss model. Tetracycline could be used for measuring bone formation activity in the same way as non-antibiotic fluorescent reagent such as calcein and alizarin, even in a tetracycline-sensitive bacterium-infected model.

Peptidomimetic inhibitor of L-plastin reduces osteoclastic bone resorption in aging female mice

L-plastin (LPL) was identified as a potential regulator of the actin-bundling process involved in forming nascent sealing zones (NSZs), which are precursor zones for mature sealing zones. TAT-fused cell-penetrating small molecular weight LPL peptide (TAT- MARGSVSDEE, denoted as an inhibitory LPL peptide) attenuated the formation of NSZs and impaired bone resorption in vitro in osteoclasts. Also, the genetic deletion of LPL in mice demonstrated decreased eroded perimeters and increased trabecular bone density. In the present study, we hypothesized that targeting LPL with the inhibitory LPL peptide in vivo could reduce osteoclast function and increase bone density in a mice model of low bone mass. We injected aging C57BL/6 female mice (36 weeks old) subcutaneously with the inhibitory and scrambled peptides of LPL for 14 weeks. Micro-CT and histomorphometry analyses demonstrated an increase in trabecular bone density of femoral and tibial bones with no change in cortical thickness in mice injected with the inhibitory LPL peptide. A reduction in the serum levels of CTX-1 peptide suggests that the increase in bone density is associated with a decrease in osteoclast function. No changes in bone formation rate and mineral apposition rate, and the serum levels of P1NP indicate that the inhibitory LPL peptide does not affect osteoblast function. Our study shows that the inhibitory LPL peptide can block osteoclast function without impairing the function of osteoblasts. LPL peptide could be developed as a prospective therapeutic agent to treat osteoporosis.

Effect of Steroidal Hormone Pregnenolone on Proliferation and Differentiation of MC3T3-E1 Osteoblast like Cells

Background:

Bone remodeling is a complex process that includes continuous resorption by osteoclast cells and bone formation by osteoblast cells. Bone fragility is a common health issue of the elderly population, particularly in postmenopausal women. It has been established that steroidal hormones have an important role in bone homeostasis. Therefore hormone replacement therapy could have beneficial effects on bone health as compared to other treatments.

Objectives:

An imbalance between the rate of bone formation and bone resorption leads to the fragility of bones. During the current study, we aimed to explore the ability of pregnenolone (1) (PRE), on proliferation and differentiation of MC3T3-E1 cells. We further aimed to investigate the underlying mechanism of action for the anabolic effect of PRE (1).

Methods:

The effects of pregnenolone (1) on proliferation, differentiation, and mineralization of MC3T3 osteoblast-like cells were determined. Cell viability was analyzed using MTT assay and flow cytometry. ALP activity and alizarin staining were employed to evaluate the effect of pregnenolone on osteoblast differentiation. Moreover, western blot for analysis of certain important proteins, crucial for the regulation of bone homeostasis, such as BMP2 and RANKL, was also performed.

Results:

Our results showed that pregnenolone (1) at a concentration of 5 μM caused a significant (p< 0.05) rise in the growth of MC3T3-E1 cells, whereas a comparable effect was observed in osteoblast differentiating assays. A significant decrease in RANKL expression was observed at (0.04 – 1 .M). Our results, therefore, indicated the possible role of pregnenolone (1) in positive regulation of bone homeostasis by suppressing RANKL expression.

Conclusion:

Taken together, our results indicate that pregnenolone (1) has the potential to enhance osteoblast proliferation, as inferred from the increased number of cells. These results demonstrated that pregnenolone (1) could be a potential anabolic agent for the treatment of fragility related disorders.

L-Plastin deficiency produces increased trabecular bone due to attenuation of sealing ring formation and osteoclast dysfunction

Bone resorption requires the formation of complex, actin-rich cytoskeletal structures. During the early phase of sealing ring formation by osteoclasts, L-plastin regulates actin-bundling to form the nascent sealing zones (NSZ). Here, we show that L-plastin knockout mice produce osteoclasts that are deficient in the formation of NSZs, are hyporesorptive, and make superficial resorption pits in vitro. Transduction of TAT-fused full-length L-plastin peptide into osteoclasts from L-plastin knockout mice rescued the formation of nascent sealing zones and sealing rings in a time-dependent manner. This response was not observed with mutated full-length L-plastin (Ser-5 and -7 to Ala-5 and -7) peptide. In contrast to the observed defect in the NSZ, L-plastin deficiency did not affect podosome formation or adhesion of osteoclasts in vitro or in vivo. Histomorphometry analyses in 8- and 12-week-old female L-plastin knockout mice demonstrated a decrease in eroded perimeters and an increase in trabecular bone density, without a change in bone formation by osteoblasts. This decrease in eroded perimeters supports that osteoclast function is attenuated in L-plastin knockouts. Micro-CT analyses confirmed a marked increase in trabecular bone mass. In conclusion, female L-plastin knockout mice had increased trabecular bone density due to impaired bone resorption by osteoclasts. L-plastin could be a potential target for therapeutic interventions to treat trabecular bone loss.

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.

Effect and mechanism of psoralidin on promoting osteogenesis and inhibiting adipogenesis

BACKGROUND

Psoralidin (PL), a prenylated coumestrol, is isolated from Psoralea corylifolia L. (Fabaceae), which is frequently used for treatment of osteoporosis.

PURPOSE

This study was designed to investigate the dual effects and potential mechanism of PL on promoting osteogenesis and inhibiting adipogenesis.

METHODS

Bone marrow mesenchymal stem cells (BMSCs) were used to investigate the effect of PL on stimulating osteogenesis and inhibiting adipogenesis, while preosteoblast MC3T3-E1 cells and preadipocyte 3T3-L1 cells were employed to explore the potential mechanisms. Estradiol (E₂) and ICI 182,780 (ICI) were used as the specific agonist and antagonist of classical estrogen receptors (ER), respectively, to interfere with classical ER signaling. Meanwhile, G-1 and G-15 were introduced as the selective agonist and antagonist of G protein coupled receptor 30 (GRP30, a membrane ER) to further clarify if membrane ER involved in PL mediating osteogenesis and adipogenesis RESULTS: PL not only promoted mineralization, but also inhibited adipocytes formation of BMSCs. In terms of osteogenesis, PL enhanced calcium nodule formation, alkaline phosphatase activity and osteocalcin levels in MC3T3-E1 cells. As for adipogenesis, PL decreased adipocyte formation in 3T3-L1 cells through down-regulating several mRNA expressions and protein synthesis of adipogenesis related factors. ICI completely blocked the effect of PL in promoting osteogenesis, but only partially suppressed its effect in inhibition of adipogenesis, while G-15 partially suppressed the effect of PL on promoting mineralization and inhibiting oil drop formation. Furthermore, during suppression of adipocyte differentiation, PL regulated protein kinase B / glycogen synthase kinase 3β / β-catenin signaling pathway.

CONCLUSION

PL promoted osteogenesis via mediating classical ER pathway, and inhibited adipocytes formation by regulating combined classical and membrane ER pathways. PL might be a potential candidate for the treatment of postmenopausal osteoporosis by modulating the competitive relationship between osteogenesis and adipogenesis of bone marrow mesenchymal stem cells.

Effects of androgen and progestin on the proliferation and differentiation of osteoblasts

Osteoporosis is liable to affect patients with gonadal hormone deficiency, and a supplement of androgens may be used to increase bone density of patients with osteoporosis. Since the androgens currently used may cause severe side effects, it is useful to investigate the effect of other androgens and progestin on bone improvement. The aim of the current study was to investigate the effects of pregnenolone (Preg), androstenedione (AD), etiocholanolone (Etio), androsterone (An), nandrolone (NA) and testosterone (T) on the proliferation and differentiation of osteoblasts for potential clinical applications. Human osteoblasts were cultured and treated with androgens and progestin, including Preg, AD, Etio, An, NA, and T, at concentrations of 0, 10^-10, 10^-8, 10^-6 and 10^-5 mol/l. The levels of cell proliferation, alkaline phosphatase (ALP) activity and osteocalcin content were measured and assessed. Preg, AD, Etio, An, and T at concentrations of 10^-10 and/or 10^-8 mol/l significantly improved osteoblast proliferation. NA at concentrations of 10^-10, 10^-8, 10^-6 and 10^-5 mol/l also significantly improved osteoblast proliferation. Preg, AD, Etio, An, NA, and T significantly increased ALP activity and osteocalcin content. The present study demonstrated, for the first time, that Preg, AD, Etio, An, and NA could improve the proliferation and differentiation of osteoblasts in vitro.

Oxidation-specific epitopes restrain bone formation

Atherosclerosis and osteoporosis are epidemiologically linked and oxidation specific epitopes (OSEs), such as phosphocholine (PC) of oxidized phospholipids (PC-OxPL) and malondialdehyde (MDA), are pathogenic in both. The proatherogenic effects of OSEs are opposed by innate immune antibodies. Here we show that high-fat diet (HFD)-induced bone loss is attenuated in mice expressing a single chain variable region fragment of the IgM E06 (E06-scFv) that neutralizes PC-OxPL, by increasing osteoblast number and stimulating bone formation. Similarly, HFD-induced bone loss is attenuated in mice expressing IK17-scFv, which neutralizes MDA. Notably, E06-scFv also increases bone mass in mice fed a normal diet. Moreover, the levels of anti-PC IgM decrease in aged mice. We conclude that OSEs, whether produced chronically or increased by HFD, restrain bone formation, and that diminished defense against OSEs may contribute to age-related bone loss. Anti-OSEs, therefore, may represent a novel therapeutic approach against osteoporosis and atherosclerosis simultaneously.

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.

Concise Review: Musculoskeletal Stem Cells to Treat Age-Related Osteoporosis

Age‐related (type‐II) osteoporosis is a common and debilitating condition driven in part by the loss of bone marrow (BM) mesenchymal stromal cells (MSC) and their osteoblast progeny, leading to reduced bone formation. Current pharmacological regiments targeting age‐related osteoporosis do not directly treat the disease by increasing bone formation, but instead use bisphosphonates to reduce bone resorption—a treatment designed for postmenopausal (type‐I) osteoporosis. Recently, the bone regenerative capacity of MSCs has been found within a very rare population of skeletal stem cells (SSCs) residing within the larger heterogeneous BM‐MSC pool. The osteoregenerative potential of SSCs would be an ideal candidate for cell‐based therapies to treat degenerative bone diseases such as osteoporosis. However, to date, clinical and translational studies attempting to improve bone formation through cell transplantation have used the larger, nonspecific, MSC pool. In this review, we will outline the physiological basis of age‐related osteoporosis, as well as discuss relevant preclinical studies that use exogenous MSC transplantation with the aim of treating osteoporosis in murine models. We will also discuss results from specific clinical trials aimed at treating other systemic bone diseases, and how the discovery of SSC could help realize the full regenerative potential of MSC therapy to increase bone formation. Finally, we will outline how ancillary clinical trials could be initiated to assess MSC/SSC‐mediated bone formation gains in existing and potentially unrelated clinical trials, setting the stage for a dedicated clinical investigation to treat age‐related osteoporosis. Stem Cells Translational Medicine2017;6:1930–1939

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.

Disruption of c-Kit Signaling in Kit(W-sh/W-sh) Growing Mice Increases Bone Turnover

c-Kit tyrosine kinase receptor has been identified as a regulator of bone homeostasis. The c-Kit loss-of-function mutations in WBB6F1/J-Kit^W/W-v mice result in low bone mass. However, these mice are sterile and it is unclear whether the observed skeletal phenotype is secondary to a sex hormone deficiency. In contrast, C57BL/6J-Kit^W-sh/^W-sh (W^sh/W^sh) mice, which carry an inversion mutation affecting the transcriptional regulatory elements of the c-Kit gene, are fertile. Here, we showed that W^sh/W^sh mice exhibited osteopenia with elevated bone resorption and bone formation at 6- and 9-week-old. The c-Kit W^sh mutation increased osteoclast differentiation, the number of committed osteoprogenitors, alkaline phosphatase activity and mineralization. c-Kit was expressed in both osteoclasts and osteoblasts, and c-Kit expression was decreased in W^sh/W^shosteoclasts, but not osteoblasts, suggesting an indirect effect of c-Kit on bone formation. Furthermore, the osteoclast-derived coupling factor Wnt10b mRNA was increased in W^sh/W^sh osteoclasts. Conditioned medium from W^sh/W^sh osteoclasts had elevated Wnt10b protein levels and induced increased alkaline phosphatase activity and mineralization in osteoblast cultures. Antagonizing Wnt10b signaling with DKK1 or Wnt10b antibody inhibited these effects. Our data suggest that c-Kit negatively regulates bone turnover, and disrupted c-Kit signaling couples increased bone resorption with bone formation through osteoclast-derived Wnt 10 b.

The androgen receptor has no direct antiresorptive actions in mouse osteoclasts

Androgen deficiency or androgen receptor knockout (ARKO) causes high-turnover osteopenia, but the target cells for this effect remain unclear. To examine whether AR in osteoclasts directly suppresses bone resorption, we crossed AR-floxed with cathepsin K-Cre mice. Osteoclast-specific ARKO (ocl-ARKO) mice showed no changes neither in osteoclast surface nor in bone microarchitecture nor in the response to orchidectomy and androgen replacement, indicating that the AR in osteoclasts is not critical for bone maintenance. In line with the lack of a bone phenotype, the levels of AR were very low in osteoclast-enriched cultures derived from bone marrow (BM) and undetectable in osteoclasts generated from spleen precursors. Since tibiae of ubiquitous ARKO mice displayed increased osteoclast counts, the role of AR was further explored using cell cultures from these animals. Osteoclast generation and activity in vitro were similar between ARKO and wildtype control (WT) mice. In co-culture experiments, BM stromal cells (BMSCs) were essential for the suppressive action of AR on osteoclastogenesis and osteoclast activity. Stimulation with 1,25(OH)2 vitamin D3 increased Rankl and decreased Tnfsf11 (osteoprotegerin, Opg) gene expression in BMSCs more than in osteoblasts. This increase in the Rankl/Opg ratio following 1,25(OH)2D3 stimulation was lower, not higher, in ARKO mice. Runx2 expression in BMSCs was however higher in ARKO vs. WT, suggesting that ARKO mice may more readily commit osteoprogenitor cells to osteoblastogenesis. In conclusion, the AR does not seem to suppress bone resorption through direct actions in osteoclasts. BMSCs may however represent an alternative AR target in the BM milieu.

The host microenvironment influences prostate cancer invasion, systemic spread, bone colonization, and osteoblastic metastasis

Prostate cancer (PCa) is the second leading cause of cancer death in men worldwide. Most PCa deaths are due to osteoblastic bone metastases. What triggers PCa metastasis to the bone and what causes osteoblastic lesions remain unanswered. A major contributor to PCa metastasis is the host microenvironment. Here, we address how the primary tumor microenvironment influences PCa metastasis via integrins, extracellular proteases, and transient epithelia-mesenchymal transition (EMT) to promote PCa progression, invasion, and metastasis. We discuss how the bone-microenvironment influences metastasis; where chemotactic cytokines favor bone homing, adhesion molecules promote colonization, and bone-derived signals induce osteoblastic lesions. Animal models that fully recapitulate human PCa progression from primary tumor to bone metastasis are needed to understand the PCa pathophysiology that leads to bone metastasis. Better delineation of the specific processes involved in PCa bone metastasize is needed to prevent or treat metastatic PCa. Therapeutic regimens that focus on the tumor microenvironment could add to the PCa pharmacopeia.

Treatment with resveratrol attenuates sublesional bone loss in spinal cord-injured rats

BACKGROUND AND PURPOSE

Sublesional osteoporosis predisposes individuals with spinal cord injury (SCI) to an increased risk of low-trauma fracture. The aim of the present work was to investigate the effect of treatment with resveratrol (RES) on sublesional bone loss in spinal cord-injured rats.

EXPERIMENTAL APPROACH

Complete SCI was generated by surgical transaction of the cord at the T10-12 level. Treatment with RES (400 mg·kg(-1) body mass per day(-1) , intragastrically) was initiated 12 h after the surgery for 10 days. Then, blood was collected and femurs and tibiae were removed for evaluation of the effects of RES on bone tissue after SCI.

KEY RESULTS

Treatment of SCI rats with RES prevented the reduction of bone mass including bone mineral content and bone mineral density in tibiae, preserved bone structure including trabecular bone volume fraction, trabecular number, and trabecular thickness in tibiae, and preserved mechanical strength including ultimate load, stiffness, and energy in femurs. Treatment of SCI rats with RES enhanced femoral total sulfhydryl content, reduced femoral malondialdehyde and IL-6 mRNA levels. Treatment of SCI rats with RES suppressed the up-regulation of mRNA levels of PPARγ, adipose-specific fatty-acid-binding protein and lipoprotein lipase, and restored mRNA levels of Wnt1, low-density lipoprotein-related protein 5, Axin2, ctnnb1, insulin-like growth factor 1 (IGF-1) and receptor for IGF-1 in femurs and tibiae.

CONCLUSIONS AND IMPLICATIONS

Treatment with RES attenuated sublesional bone loss in spinal-cord-injured rats, associated with abating oxidative stress, attenuating inflammation, depressing PPARγ signalling, and restoring Wnt/β-catenin and IGF-1 signalling.

The relevance of mouse models for investigating age-related bone loss in humans

Mice are increasingly used for investigation of the pathophysiology of osteoporosis because their genome is easily manipulated, and their skeleton is similar to that of humans. Unlike the human skeleton, however, the murine skeleton continues to grow slowly after puberty and lacks osteonal remodeling of cortical bone. Yet, like humans, mice exhibit loss of cancellous bone, thinning of cortical bone, and increased cortical porosity with advancing age. Histologic evidence in mice and humans alike indicates that inadequate osteoblast-mediated refilling of resorption cavities created during bone remodeling is responsible. Mouse models of progeria also show bone loss and skeletal defects associated with senescence of early osteoblast progenitors. Additionally, mouse models of atherosclerosis, which often occurs in osteoporotic participants, also suffer bone loss, suggesting that common diseases of aging share pathophysiological pathways. Knowledge of the causes of skeletal fragility in mice should therefore be applicable to humans if inherent limitations are recognized.

Quantitative trait locus on chromosome X affects bone loss after maturation in mice

Genetic programming is known to affect the peak bone mass and bone loss after maturation. However, little is known about how polymorphic genes on chromosome X (Chr X) modulate bone loss after maturation. We previously reported a quantitative trait locus (QTL) on Chr X, designated Pbd3, which had a suggestive linkage to bone mass, in male SAMP2 and SAMP6 mice. In this study, we aimed to clarify the effects of Pbd3 on the skeletal phenotype. We generated a congenic strain, P2.P6-X, carrying a 45.6-cM SAMP6-derived Chr X interval on a SAMP2 genetic background. The effects of Pbd3 on the bone phenotype were determined by microcomputed tomography (microCT), whole-body dual-energy X-ray absorptiometry (DXA), serum bone turnover markers, and histomorphometric parameters. Both the bone area fraction (BA/TA) on microCT and whole-body DXA revealed reduced bone loss in P2.P6-X compared with that in SAMP2. The serum concentrations of bone turnover markers at 4 months of age were significantly lower in P2.P6-X than in SAMP2, but did not differ at 8 months of age. These results were observed in female mice, but not in male mice. In conclusion, a QTL within a segregated 45.6-cM interval on Chr X is sex-specifically related to the rate of bone loss after maturation.

Sex steroids during bone growth: a comparative study between mouse models for hypogonadal and senile osteoporosis

SUMMARY

In this study, the role of disturbed bone mineral acquisition during puberty in the pathogenesis of osteoporosis was studied. To this end, a mouse model for senile and hypogonadal osteoporosis was used. Longitudinal follow-up showed that bone fragility in both models results from deficient bone build-up during early puberty.

INTRODUCTION

Male osteoporosis may result from impaired bone growth. This study characterizes the mechanisms of deficient peak bone mass acquisition in models for senile (SAMP6) and hypogonadal (orchidectomized SAMR1) osteoporosis.

METHODS

Bone mineral acquisition was investigated longitudinally in SAMP6 and orchidectomized SAMR1 mice (eight to ten animals per group) using peripheral quantitative computed tomography and histomorphometry. Additionally, the effects of long-term 5alpha-dihydrotestosterone (DHT) and 17beta-estradiol (E2) replacement were studied. Statistical analysis was performed using ANOVA and Student's t test.

RESULTS

SAMP6 mice showed an early (4 weeks) medullary expansion of the cortex due to impaired endocortical bone formation (-43%). Despite compensatory periosteal bone formation (+47%), cortical thickness was severely reduced in 20-week-old SAMP6 versus SAMR1. Orchidectomy reduced periosteal apposition between 4 and 8 weeks of age and resulted in high bone turnover and less trabecular bone gain in SAMP6 and SAMR1. DHT and E2 stimulated periosteal expansion and trabecular bone in orchidectomized SAMP6 and SAMR1. E2 stimulated endocortical apposition in SAMP6. Moreover, sex steroid action occurred between 4 and 8 weeks of age.

CONCLUSION

Bone fragility in both models resulted from deficient bone build-up during early puberty. DHT and E2 improved bone mass acquisition in orchidectomized animals, suggesting a role for AR and ER in male skeletal development.

Bone mass is preserved and cancellous architecture altered due to cyclic loading of the mouse tibia after orchidectomy

INTRODUCTION

The study of adaptation to mechanical loading under osteopenic conditions is relevant to the development of osteoporotic fracture prevention strategies. We previously showed that loading increased cancellous bone volume fraction and trabecular thickness in normal male mice. In this study, we tested the hypothesis that cyclic mechanical loading of the mouse tibia inhibits orchidectomy (ORX)-associated cancellous bone loss.

MATERIALS AND METHODS

Ten-week-old male C57BL/6 mice had in vivo cyclic axial compressive loads applied to one tibia every day, 5 d/wk, for 6 wk after ORX or sham operation. Adaptation of proximal cancellous and diaphyseal cortical bone was characterized by muCT and dynamic histomorphometry. Comparisons were made between loaded and nonloaded contralateral limbs and between the limbs of ORX (n = 10), sham (n = 11), and basal (n = 12) groups and tested by two-factor ANOVA with interaction.

RESULTS

Cyclic loading inhibited bone loss after ORX, maintaining absolute bone mass at age-matched sham levels. Relative to sham, ORX resulted in significant loss of cancellous bone volume fraction (-78%) and trabecular number (-35%), increased trabecular separation (67%), no change in trabecular thickness, and smaller loss of diaphyseal cortical properties, consistent with other studies. Proximal cancellous bone volume fraction was greater with loading (ORX: 290%, sham: 68%) than in contralateral nonloaded tibias. Furthermore, trabeculae thickened with loading (ORX: 108%, sham: 48%). Dynamic cancellous bone histomorphometry indicated that loading was associated with greater mineral apposition rates (ORX: 32%, sham: 12%) and smaller percent mineralizing surfaces (ORX: -47%, sham: -39%) in the final week. Loading resulted in greater BMC (ORX: 21%, sham: 15%) and maximum moment of inertia (ORX: 39%, sham: 24%) at the cortical midshaft.

CONCLUSIONS

This study shows that cancellous bone mass loss can be prevented by mechanical loading after hormonal compromise and supports further exploration of nonpharmacologic measures to prevent rapid-onset osteopenia and associated fractures.

Cortical bone loss in androgen-deficient aged male rats is mainly caused by increased endocortical bone remodeling

INTRODUCTION

Hypogonadism is considered to be one of the major risk factors for osteoporosis in men. Here, we sequentially studied the effects of androgen deficiency on cortical bone in aged orchiectomy (ORX) rats.

MATERIALS AND METHODS

One hundred seventy 13-mo-old male Fischer-344 rats were either ORX or sham-operated. After in vivo fluorochrome labeling, groups of 8-15 SHAM and ORX rats each were killed at 2 wk and 1, 2, 3, 4, 6, and 9 mo after surgery. To examine the effects of testosterone replacement therapy, 9-mo-old ORX rats were supplemented with testosterone undecanoate at a weekly dose of 6 mg/kg for 4 mo. Cortical bone changes in the tibial shaft were monitored by pQCT analysis and by bone histomorphometry.

RESULTS

SHAM rats did not show age-related bone loss at the tibial diaphysis. pQCT analysis and bone histomorphometry showed cortical bone osteopenia in ORX rats, beginning from 2 mo after surgery until the end of the study. Androgen deficiency induced a sustained decrease in periosteal bone formation during the first 4 mo after ORX. However, although periosteal expansion of the tibial shaft tended to be slower in ORX rats compared with SHAM controls, the reduction in total cross-sectional area in ORX animals reached statistical significance only at 4 mo after surgery. The major mechanism for cortical bone loss in aged ORX rats was a progressive expansion of the marrow cavity, which was associated with an initial increase in endocortical eroded perimeter at 1 and 2 mo after surgery, followed by a sustained increase in endocortical bone formation until the end of the study. All these changes were prevented in aged ORX rats receiving testosterone supplementation in an insulin-like growth factor system-independent fashion.

CONCLUSIONS

We conclude that androgen deficiency-induced cortical bone loss in aged, nongrowing rats is mainly caused by augmented endocortical bone remodeling.

Receptor activator of nuclear factor kappaB ligand and osteoprotegerin regulation of bone remodeling in health and disease

Osteoclasts and osteoblasts dictate skeletal mass, structure, and strength via their respective roles in resorbing and forming bone. Bone remodeling is a spatially coordinated lifelong process whereby old bone is removed by osteoclasts and replaced by bone-forming osteoblasts. The refilling of resorption cavities is incomplete in many pathological states, which leads to a net loss of bone mass with each remodeling cycle. Postmenopausal osteoporosis and other conditions are associated with an increased rate of bone remodeling, which leads to accelerated bone loss and increased risk of fracture. Bone resorption is dependent on a cytokine known as RANKL (receptor activator of nuclear factor kappaB ligand), a TNF family member that is essential for osteoclast formation, activity, and survival in normal and pathological states of bone remodeling. The catabolic effects of RANKL are prevented by osteoprotegerin (OPG), a TNF receptor family member that binds RANKL and thereby prevents activation of its single cognate receptor called RANK. Osteoclast activity is likely to depend, at least in part, on the relative balance of RANKL and OPG. Studies in numerous animal models of bone disease show that RANKL inhibition leads to marked suppression of bone resorption and increases in cortical and cancellous bone volume, density, and strength. RANKL inhibitors also prevent focal bone loss that occurs in animal models of rheumatoid arthritis and bone metastasis. Clinical trials are exploring the effects of denosumab, a fully human anti-RANKL antibody, on bone loss in patients with osteoporosis, bone metastasis, myeloma, and rheumatoid arthritis.

Biochemical markers in preclinical models of osteoporosis

Although several treatments for osteoporosis exist, further understanding of the mode of action of current treatments, as well as development of novel treatments, are of interest. Thus, preclinical models of osteoporosis are very useful, as they provide the possibility for gaining knowledge about the cellular mechanisms underlying the disease and for studying pharmaceutical prevention or intervention of the disease in simple and strictly controlled systems. In this review, we present a comprehensive collection of studies using biochemical markers of bone turnover for investigation of preclinical models of osteoporosis. These range from pure and simple in vitro systems, such as osteoclast cultures, to ex vivo models, such as cultures of embryonic murine tibiae and, finally, to in vivo models, such as ovariectomy and orchidectomy of rats. We discuss the relevance of the markers in the individual models, and compare their responses to those observed using 'golden standard' methods.

Height and bone mineral density in androgen insensitivity syndrome with mutations in the androgen receptor gene

INTRODUCTION AND HYPOTHESIS

Androgen insensitivity syndrome (AIS) constitutes a natural model to study effects of androgens and estrogens on growth and bone density. We evaluated height and bone density in patients with AIS with mutations in the androgen receptor (AR) gene.

METHODS

A retrospective analysis was conducted of eight subjects with complete AIS (CAIS) and four with partial AIS (PAIS) submitted to gonadectomy followed by estrogen replacement, and three with PAIS who did not undergo gonadectomy. Standing height and bone mineral apparent density (BMAD) by DXA were measured and compared with male (z (m)) and female (z (f)) reference populations. The z-scores were compared with a value of zero using the one-sample t-test.

RESULTS

Final heights of patients with CAIS and PAIS were intermediate between those predicted for females and males. BMAD of the lumbar spine in CAIS and PAIS after gonadectomy and estrogen replacement (z (f) = - 1.56 +/- 1.04, P = 0.006, and z (m) = - 0.75 +/- 0.89, P = 0.04) indicated vertebral bone deficit, whereas BMAD at the femoral neck was normal. No patient reported fractures.

CONCLUSION

Subjects with AIS had mean final height intermediate between mean normal male and female, and decreased bone mineral density in the lumbar spine. These data suggest an important role for androgens in normal male growth and bone density not replaced by estrogens.

Glucocorticoids act directly on osteoclasts to increase their life span and reduce bone density

Glucocorticoid administration to mice results in a rapid loss of bone mineral density due to an imbalance in osteoblast and osteoclast numbers. Whereas excess glucocorticoids reduce both osteoblast and osteoclast precursors, cancellous osteoclast number surprisingly does not decrease as does osteoblast number, presumably due to the ability of glucocorticoids to promote osteoclast life span. Whether glucocorticoids act directly on osteoclasts in vivo to promote their life span and whether this contributes to the rapid loss of bone with glucocorticoid excess remains unknown. To determine the direct effects of glucocorticoids on osteoclasts in vivo, we expressed 11beta-hydroxysteroid dehydrogenase type 2, an enzyme that inactivates glucocorticoids, specifically in the osteoclasts of transgenic mice using the tartrate-resistant acid phosphatase promoter. Bone mass, geometry, and histomorphometry were similar in untreated wild-type and transgenic animals. Glucocorticoid administration for 7 d caused equivalent increases in cancellous osteoblast apoptosis, and equivalent decreases in osteoblasts, osteoid, and bone formation, in wild-type and transgenic mice. In contrast, glucocorticoids stimulated expression of the mRNA for calcitonin receptor, an osteoclast product, in wild-type but not transgenic mice. Consistent with the previous finding that glucocorticoids decrease osteoclast precursors and prolong osteoclast life span, glucocorticoids decreased cancellous osteoclast number in the transgenic mice but not wild-type mice. In accord with this decrease in osteoclast number, the loss of bone density observed in wild-type mice was strikingly prevented in transgenic mice. These results demonstrate for the first time that the early, rapid loss of bone caused by glucocorticoid excess results from direct actions on osteoclasts.

Osteocyte apoptosis is induced by weightlessness in mice and precedes osteoclast recruitment and bone loss

Mechanical stimulation of cultured osteocytic cells attenuates their apoptosis. We report here that, conversely, reduced mechanical forces in the murine model of unloading by tail suspension increases the prevalence of osteocyte apoptosis, followed by bone resorption and loss of mineral and strength.

INTRODUCTION

Mechanical loading is critical for the maintenance of bone mass; weightlessness, as with reduced physical activity in old age, bed rest, or space flight, invariably leads to bone loss. However, the cellular and molecular mechanisms responsible for these phenomena are poorly understood. Based on our earlier findings that physiologic levels of mechanical strain prevent apoptosis of osteocytic cells in vitro, we examined here whether, conversely, reduced mechanical forces increase the prevalence of osteocyte apoptosis in vivo and whether this event is linked to bone loss.

MATERIALS AND METHODS

Swiss Webster mice or OG2-11beta-hydroxysteroid dehydrogenase type 2 (OG2-11beta-HSD2) transgenic mice and wildtype littermates were tail-suspended or kept under ambulatory conditions. Static and dynamic histomorphometry and osteocyte and osteoblast apoptosis by in situ end-labeling (ISEL) were assessed in lumbar vertebra; spinal BMD was measured by DXA; and bone strength was measured by vertebral compression.

RESULTS

We show that within 3 days of tail suspension, mice exhibited an increased incidence of osteocyte apoptosis in both trabecular and cortical bone. This change was followed 2 weeks later by increased osteoclast number and cortical porosity, reduced trabecular and cortical width, and decreased spinal BMD and vertebral strength. Importantly, whereas in ambulatory animals, apoptotic osteocytes were randomly distributed, in unloaded mice, apoptotic osteocytes were preferentially sequestered in endosteal cortical bone--the site that was subsequently resorbed. The effect of unloading on osteocyte apoptosis and bone resorption was reproduced in transgenic mice in which osteocytes are refractory to glucocorticoid action, indicating that stress-induced hypercortisolemia cannot account for these effects.

CONCLUSIONS

We conclude that diminished mechanical forces eliminate signals that maintain osteocyte viability, thereby leading to apoptosis. Dying osteocytes in turn become the beacons for osteoclast recruitment to the vicinity and the resulting increase in bone resorption and bone loss.

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.

Bone morphogenetic protein type IA receptor signaling regulates postnatal osteoblast function and bone remodeling

Bone morphogenetic proteins (BMPs) function during various aspects of embryonic development including skeletogenesis. However, their biological functions after birth are less understood. To investigate the role of BMPs during bone remodeling, we generated a postnatal osteoblast-specific disruption of Bmpr1a that encodes the type IA receptor for BMPs in mice. Mutant mice were smaller than controls up to 6 months after birth. Irregular calcification and low bone mass were observed, but there were normal numbers of osteoblasts. The ability of the mutant osteoblasts to form mineralized nodules in culture was severely reduced. Interestingly, bone mass was increased in aged mutant mice due to reduced bone resorption evidenced by reduced bone turnover. The mutant mice lost more bone after ovariectomy likely resulting from decreased osteoblast function which could not overcome ovariectomy-induced bone resorption. In organ culture of bones from aged mice, ablation of the Bmpr1a gene by adenoviral Cre recombinase abolished the stimulatory effects of BMP4 on the expression of lysosomal enzymes essential for osteoclastic bone resorption. These results demonstrate essential and age-dependent roles for BMP signaling mediated by BMPRIA (a type IA receptor for BMP) in osteoblasts for bone remodeling.

The skeletal effects of glucocorticoid excess override those of orchidectomy in mice

Hypogonadism has been implicated as a contributing factor in glucocorticoid-induced osteoporosis, but evidence for this is limited. Hypogonadism and glucocorticoid excess both cause bone loss, but the cellular mechanisms responsible are distinct. Loss of gonadal steroids causes an increase in bone remodeling by up-regulating osteoblastogenesis and osteoclastogenesis. Glucocorticoid excess, conversely, suppresses remodeling by down-regulating osteoblastogenesis and osteoclastogenesis. Nonetheless, both conditions increase osteoblast apoptosis and decrease osteoclast apoptosis, and both cause bone loss due to an undersupply of osteoblasts relative to the need for cavity repair. To investigate their interactions, we compared the effects of orchidectomy, glucocorticoid excess, or both combined in mice. After 28 d, serum unbound testosterone concentration and seminal vesicle weight were not diminished when prednisolone was administered alone. Vertebral bone mineral density and compression strength decreased to the same extent in animals receiving prednisolone or after orchidectomy, but the changes were not additive. Orchidectomy induced the expected up-regulation of osteoblast and osteoclast progenitors, but these changes were prevented in orchidectomized mice simultaneously receiving glucocorticoids. Likewise, the increase in cancellous osteoid, osteoblasts, osteoclasts, bone formation, and activation frequency caused by orchidectomy were prevented by prednisolone. The prevalence of osteoblast apoptosis increased in the mice receiving prednisolone or after orchidectomy, but the increases were not additive. These data demonstrate that hypogonadism does not occur in or contribute to glucocorticoid-induced osteoporosis and that the adverse skeletal effects of glucocorticoid excess override those of orchidectomy.

Osteopontin deficiency produces osteoclast dysfunction due to reduced CD44 surface expression

Osteopontin (OPN) was expressed in murine wild-type osteoclasts, localized to the basolateral, clear zone, and ruffled border membranes, and deposited in the resorption pits during bone resorption. The lack of OPN secretion into the resorption bay of avian osteoclasts may be a component of their functional resorption deficiency in vitro. Osteoclasts deficient in OPN were hypomotile and exhibited decreased capacity for bone resorption in vitro. OPN stimulated CD44 expression on the osteoclast surface, and CD44 was shown to be required for osteoclast motility and bone resorption. Exogenous addition of OPN to OPN-/- osteoclasts increased the surface expression of CD44, and it rescued osteoclast motility due to activation of the alpha(v)beta(3) integrin. Exogenous OPN only partially restored bone resorption because addition of OPN failed to produce OPN secretion into resorption bays as seen in wild-type osteoclasts. As expected with these in vitro findings of osteoclast dysfunction, a bone phenotype, heretofore unappreciated, was characterized in OPN-deficient mice. Delayed bone resorption in metaphyseal trabeculae and diminished eroded perimeters despite an increase in osteoclast number were observed in histomorphometric measurements of tibiae isolated from OPN-deficient mice. The histomorphometric findings correlated with an increase in bone rigidity and moment of inertia revealed by load-to-failure testing of femurs. These findings demonstrate the role of OPN in osteoclast function and the requirement for OPN as an osteoclast autocrine factor during bone remodeling.

Interleukin-11 as a stimulatory factor for bone formation prevents bone loss with advancing age in mice

Cytokines in interleukin (IL)-11 subfamily participate in the regulation of bone cell proliferation and differentiation. We report here positive effects of IL-11 on osteoblasts and bone formation. Overexpression of human IL-11 gene in transgenic mice resulted in the stimulation of bone formation to increase cortical thickness and strength of long bones, and in the prevention of cortical bone loss with advancing age. Bone resorption and osteoclastogenesis were not affected in IL-11 transgenic mice. In experiments in vitro, IL-11 stimulated transcription of the target gene for bone morphogenetic protein (BMP) via STAT3, leading to osteoblastic differentiation in the presence of BMP-2, but inhibited adipogenesis in bone marrow stromal cells. These results indicate that IL-11 is a stimulatory factor for osteoblastogenesis and bone formation to conserve cortical bone, possibly by enhancing BMP actions in bone. IL-11 may be a new therapeutic target for senile osteoporosis.

The phospholipids sphingosine-1-phosphate and lysophosphatidic acid prevent apoptosis in osteoblastic cells via a signaling pathway involving G(i) proteins and phosphatidylinositol-3 kinase

The naturally occurring phospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have recently emerged as bioactive compounds that exert mitogenic effects in many cell types, including osteoblasts. In the current study, we examined the ability of each of these compounds to influence osteoblast survival. Using terminal deoxynucleotidyl transferase-mediated deoxyuridine 5'-triphosphate nick-end labeling and DNA fragmentation assays, we found that both LPA and S1P dose-dependently inhibited (by at least 50% and 40%, respectively) the apoptosis induced by serum withdrawal in cultures of primary calvarial rat osteoblasts and SaOS-2 cells. The antiapoptotic effects were inhibited by pertussis toxin, wortmannin, and LY294002, implicating G(i) proteins and phosphatidylinositol-3 kinase (PI-3 kinase) in the signaling pathway that mediates phospholipid-induced osteoblast survival. Specific inhibitors of p42/44 MAPK signaling did not block LPA- or S1P-induced osteoblast survival. LPA and S1P induced PI-3 kinase-dependent activation of p70 S6 kinase, but rapamycin, a specific inhibitor of p70 S6 kinase activation, did not prevent phospholipid-induced osteoblast survival. LPA and S1P also inhibited apoptosis in Swiss 3T3 fibroblastic cells in a G(i) protein-dependent fashion. In fibroblastic cells, however, the antiapoptotic effects of S1P were sensitive to inhibition of both PI-3 kinase and p42/44 MAPK signaling, whereas those of LPA were partially abrogated by inhibitors of p42/44 MAPK signaling but not by PI-3 kinase inhibitors. These data demonstrate that LPA and S1P potently promote osteoblast survival in vitro, and that cell-type specificity exists in the antiapoptotic signaling pathways activated by phospholipids.

Promotion of osteoclast survival and antagonism of bisphosphonate-induced osteoclast apoptosis by glucocorticoids

Glucocorticoids depress bone formation by inhibiting osteoblastogenesis and increasing osteoblast apoptosis. However, the role of bone resorption in the initial rapid phase of bone loss characteristic of glucocorticoid-induced osteoporosis is unexplained, and the reason for the efficacy of bisphosphonates in this condition remains unknown. We report that in murine osteoclast cultures, glucocorticoids prolonged the baseline survival of osteoclasts and antagonized bisphosphonate-induced caspase activation and apoptosis by a glucocorticoid receptor-mediated action. Consistent with the in vitro evidence, in a murine model of glucocorticoid-induced osteoporosis, the number of cancellous osteoclasts increased, even though osteoclast progenitor number was reduced. Moreover, in mice receiving both glucocorticoids and bisphosphonates, the expected proapoptotic effect of bisphosphonates on osteoclasts was abrogated, as evidenced by maintenance of osteoclast numbers and, additionally, loss of bone density. In contrast, bisphosphonate administration prevented glucocorticoid-induced osteoblast apoptosis. These results indicate that the early loss of bone with glucocorticoid excess is caused by extension of the life span of pre-existing osteoclasts, an effect not preventable by bisphosphonates. Therefore, the early beneficial effects of these agents must be due, in part, to prolonging the life span of osteoblasts.

Up-regulation of TNF-producing T cells in the bone marrow: a key mechanism by which estrogen deficiency induces bone loss in vivo

In vivo studies have shown T cells to be central to the mechanism by which estrogen deficiency induces bone loss, but the mechanism involved remains, in part, undefined. In vitro, T cells from ovariectomized mice produce increased amounts of tumor necrosis factor (TNF), which augments receptor activator of NF-kappa B ligand (RANKL)-induced osteoclastogenesis. However, both the mechanism and the relevance of this phenomenon in vivo remain to be established. In this study, we found that ovariectomy increased the number of bone marrow T cell-producing TNF without altering production of TNF per T cell. Attesting to the essential contribution of TNF, ovariectomy induced rapid bone loss in wild type (wt) mice but failed to do so in TNF-deficient (TNF(-/-)) mice. Furthermore, ovariectomy induced bone loss, which was absent in T cell-deficient nude mice, was restored by adoptive transfer of wt T cells, but not by reconstitution with T cells from TNF(-/-) mice. These findings demonstrate the key causal role of T cell-produced TNF in the bone loss after estrogen withdrawal. Finally, ovariectomy caused bone loss in wt mice and in mice lacking p75 TNF receptor but failed to do so in mice lacking the p55 TNF receptor. These findings demonstrate that enhanced T cell production of TNF resulting from increased bone marrow T cell number is a key mechanism by which estrogen deficiency induces bone loss in vivo. The data also demonstrate that the bone-wasting effect of TNF in vivo is mediated by the p55 TNF receptor.

Chromosome 13 locus, Pbd2, regulates bone density in mice

Bone density is inherited as a complex polygenic trait. Previously, we identified two quantitative trait loci (QTLs) specifying the peak relative bone mass (bone mass corrected by bone size) on chromosomes (Chrs) 11 and 13 by interval mapping in two mouse strains: SAMP2 and SAMP6. The latter strain is an established murine model of senile osteoporosis and exhibits a significantly lower peak relative bone mass than SAMP2 mice. In this study, we report the effects of the Chr 13 QTL on peak bone density (Pbd2). First, we constructed a congenic strain P6.P2-Pbd2b, which carried a single genomic interval from the Chr 13 of SAMP2 on an SAMP6-derived osteoporotic background, to dissect this polygenic trait into single gene factors. This congenic strain had a higher bone density than the background strain using three measurement methods with different principles for bone density. Next, we measured the peak relative bone mass of the AKR/J strain and the 13 senescence-accelerated mouse (SAM) strains, which are considered to be a series of recombinant-like inbred (RI) strains derived from the AKR/J strain and other unspecified strains. We then determined the microsatellite marker haplotypes of these strains around the Pbd2 locus, in which three strains with a high relative bone mass shared the same haplotype over the 26-centimorgan (cM) region. In the Pbd2 locus, a high relative bone mass was associated with alleles of the unknown strain, whereas a low relative bone mass was associated with the alleles from the AKR/J strain. These results confirmed the existence of a Pbd2 locus regulating bone density in the SAM strains.

The loss of Smad3 results in a lower rate of bone formation and osteopenia through dysregulation of osteoblast differentiation and apoptosis

Smad3 is a well-characterized intracellular effector of the transforming growth factor beta (TGF-beta) signaling pathway and was implicated recently in the potentiation of vitamin D receptor (VDR)-mediated signaling. Given that both TGF-beta and vitamin D are important regulators of bone remodeling, it is expected that Smad3 plays an integral role in normal maintenance of bone. However, the exact mechanisms by which Smad3 functions in bone remodeling are unknown. Here, we show that mice with targeted deletion of Smad3 are osteopenic with less cortical and cancellous bone compared with wild-type littermates. Decreases in bone mineral density (BMD) in Smad3 null mice reflect the inability of osteoblasts to balance osteoclast activity, although osteoclast numbers are normal and vitamin D mediated serum calcium homeostasis is maintained. The osteopenia of Smad3 null mice is attributed to a decreased rate of bone formation associated with increased osteocyte number and apoptosis. These findings are supported by studies with isolated primary osteoblasts that show TGF-beta can no longer inhibit the differentiation of osteoblasts in the absence of Smad3; yet, TGF-beta-stimulated proliferation remains intact. Together these data support a model that a loss of Smad3 increases the osteocyte fate of the osteoblast and decreases the duration of osteoblast function by shortening lifespan, ultimately resulting in osteopenia.

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.

Effects of transdermal testosterone gel on bone turnover markers and bone mineral density in hypogonadal men

OBJECTIVE

Androgen replacement has been reported to increase bone mineral density (BMD) in hypogonadal men. We studied the effects of 6 months of treatment with a new transdermal testosterone (T) gel preparation on bone turnover markers and BMD.

DESIGN

This was a prospective, randomized, multicentre, parallel clinical trial where 227 hypogonadal men, mean age 51 years (range: 19-68 years) were studied in 16 academic and research institutions in the USA. Subjects were randomized to apply 1% T gel containing 50 or 100 mg T (delivering approximately 5-10 mg T/day) or two T patches (delivering 5 mg T/day) transdermally for 90 days. At day 91, depending on the serum T concentration, the T gel dose was adjusted upward or downward to 75 mg T/day until day 180. No dose adjustment occurred in the T patch group.

MEASUREMENTS

Serum T, free T and oestradiol, bone turnover markers and BMD were measured on days 0, 30, 90 and 180 before and after treatment.

RESULTS

Application of T gel 100 mg/day resulted in serum T concentrations 1.4 and 1.9-fold higher than in the T gel 50 mg/day and the T patch groups, respectively. Proportional increases occurred in serum oestradiol. Urine N-telopeptide/creatinine ratio, a marker for bone resorption, decreased significantly (P = 0.0019) only in the T gel 100 mg/day group. Serum bone osteoblastic activity markers (osteocalcin, procollagen and skeletal alkaline phosphatase) increased significantly during the first 90 days of treatment without intergroup differences but declined to baseline thereafter. BMD increased significantly both in the hip (+1.1 +/- 0.3%) and spine (+2.2 +/- 0.5%) only in the T gel 100 mg/day group (P = 0.0001).

CONCLUSIONS

Transdermal testosterone gel application for 6 months decreased bone resorption markers and increased osteoblastic activity markers for a short period, which resulted in a small but significant increase in BMD. Ongoing long-term studies should answer whether the observed increases in BMD are sustained or continue to be dependent on the dose of testosterone administered.

Attenuation of the self-renewal of transit-amplifying osteoblast progenitors in the murine bone marrow by 17 beta-estradiol

In agreement with evidence that estrogens slow the rate of bone remodeling by suppressing the production of both osteoclasts and osteoblasts, loss of estrogens leads to an increase in the number of osteoclast as well as early osteoblast progenitors (CFU-osteoblasts; CFU-OBs) in the murine bone marrow. Here we show that CFU-OBs are early transit-amplifying progenitors, i.e., dividing cells capable of limited self-renewal, and that 17 beta-estradiol acts in vivo and in vitro to attenuate their self-renewal by approximately 50%. Consistent with a direct receptor-mediated action of estrogens on early mesenchymal cell progenitors, anti-estrogen receptor-alpha (anti-ER alpha) Ab's stain a small number of marrow cells that exhibit characteristics of primitive undifferentiated cells, including a high nucleus/cytoplasm ratio and lack of lineage-specific biochemical markers; the effect of 17 beta-estradiol on CFU-OB self-renewal is absent in mice lacking ER alpha. Because both osteoblasts and the stromal/osteoblastic cells that are required for osteoclast development are derived from CFU-OBs, suppression of the self-renewal of this common progenitor may represent a key mechanism of the anti-remodeling effects of estrogens.

Unresolved issues in osteoporosis in men

Fragility fractures in men are a public health problem. The increasing longevity in men is likely to increase the public health burden of fractures in men. This problem remains unrecognized by doctors, the public and governments. About one third of all hip fractures occur in men but the incidence and gender ratio varies from country to country for reasons that are not understood. The prevalence of spine fractures is about half that of women in most studies, but similar to that of women in several other studies. The incidence of spine fractures is uncertain but is likely to be about half that of women except in 80+ year olds, when it appears to be similar. The causes of the higher mortality in men than in women following hip or spine fracture are not well defined. Areal bone mineral density (aBMD) predicts fracture risk in men; the relative risk for spine and hip fracture conferred by a 1 SD lower aBMD, or by a prevalent fracture, is similar in men and women. The age-specific absolute risk (number of cases per 1,000 per year) conferred by a given hip aBMD is similar in men and women. The age-specific absolute risk conferred by aBMD at the calcaneus or radius for spine fracture is similar for men and women. If the absolute and relative risks are similar then the lower incidence of fractures in men than women may reflect the lower proportion of the male population distribution below a given structural determinant of bone fragility. That is, at any age, there may be fewer men than women with smaller bones, lower volumetric bone mineral density (vBMD), thinner trabeculae or cortices, architectural disruption, or higher remodeling rates. Higher mortality and fewer falls may also contribute to the lower incidence of fractures in men. This tail end of the male population distribution (for traits like bone size, vBMD, architecture, and remodeling rates) is the likely source of fracture cases in males. Hypogonadism is a risk factor for osteoporosis. However, the definition, prevalence, causes and structural consequence of hypogonadism are inadequately defined. At what level of testosterone is bone balance negative? What structural determinants of axial and appendicular strength are regulated by testosterone, estrogen, growth hormone (GH), insulin like growth factor 1 (IGF-1) (or their interactions)? Is reduced bone size in men with spine or hip fractures due to failed growth-related or age-related periosteal expansion? If reduced vBMD is due to reduced accrual, is this due to reduced cortical thickness? What factors regulate and coregulate the periosteal and endocortical modeling and remodeling? Are reduced trabecular numbers due to failed formation at the growth plate, excess resorption of primary trabeculae or reduced formation of secondary trabeculae? Is reduced trabecular thickness due to failed prepubertal or pubertal bone formation? Is reduced cortical and trabecular thickness during aging due to excessive endosteal resorption or reduced bone formation? If the former, is this due to increased remodeling sites or increased resorption depth? Most evidence favors reduced bone formation as the cause of bone loss with trabecular bone loss occurring by reduced formation and thinning more than by increased resorption and loss of connectivity. Cortical bone loss is less than in women because endocortical resorption is less and periosteal apposition is greater. If the reduced bone formation is most important, is this due to reduced osteoprogenitors, reduced osteoblast matrix synthesis or early osteoblast apoptosis? Anti-spine-fracture efficacy has been demonstrated in only one randomized heated with alendronate drug in men. The gaps in our knowledge remain large.