Androgens and bone

Discovery and therapeutic promise of selective androgen receptor modulators

Androgens are essential for male development and the maintenance of male secondary characteristics, such as bone mass, muscle mass, body composition, and spermatogenesis. The main disadvantages of steroidal androgens are their undesirable physicochemical and pharmacokinetic properties. The recent discovery of nonsteroidal selective androgen receptor modulators (SARMs) provides a promising alternative for testosterone replacement therapies with advantages including oral bioavailability, flexibility of structural modification, androgen receptor specificity, tissue selectivity, and the lack of steroid-related side effects.

[Physiopathology of osteoporosis]

PURPOSE

The pathophysiology of osteoporosis has seen many recent progress especially with the use of genetically modified animal models.

CURRENT KNOWLEDGE AND KEY POINTS

Among many discoveries, one can notice the crucial role of LRP5, GH, IGF-1 and the sex hormones receptors in the acquisition of the peak bone mass, the control of bone remodeling by the sympathetic nervous system and his implication as a transmitter of mechanical loading in bone. Also, the role of estrogen and androgen receptors as well as the aromatase is specified according to sexes. The role of growth plate's chondrocytes in the installation of the trabecular bone network is better and better demonstrated. The greater periosteal apposition in men, mediated by androgens receptor, seems to explain the greatest radial growth and so the greatest bone resistance to mechanical strains like a lower fracture rate in men compared to women. The bone microarchitecture and quality explain an important part of the mechanical properties of bones and why considering the same bone mass one bone is breaking and another one not.

FUTURE PROSPECTS AND PROJECTS

Many therapeutic applications should finalize the discovery of these new bone cells signalisation pathways.

Effects of combined estrogen/testosterone therapy on bone and body composition in oophorectomized women

OBJECTIVE

To evaluate the effect of adding testosterone undecanoate 40 mg daily to estrogen therapy on bone markers, bone mineral density and body composition in oophorectomized women.

METHODS

Fifty women, 45-60 years old, who had undergone a hysterectomy and bilateral salpingo-oophorectomy for benign disorders, were randomly assigned to oral treatment with testosterone undecanoate 40 mg plus estradiol valerate 2 mg daily or placebo plus estradiol valerate 2 mg daily. Twenty-four weeks later, cross-over was performed to the other treatment regimen. Forty-four women completed the study. Their serum concentrations of insulin-like growth factor (IGF)-I, IGF binding protein (IGFBP)-3, osteocalcin, carboxyterminal telopeptide aminoterminal (ICTP), of type I collagen propeptide of type I procollagen (PICP) and interleukin (IL)-1 receptor antagonist were measured at baseline and after 24 weeks of both treatments, as were also their body mass index (BMI) and blood pressure. Bone mineral density of the total body, spine and hip and total body fat, total lean body mass, trunk fat and trunk lean mass were determined by dual-energy X-ray absorptiometry measurements at baseline and after 24 weeks of both regimens.

RESULTS

During treatment, the addition of testosterone counteracted the decrease in IGF-I and PICP seen with estrogen therapy alone. Osteocalcin and ICTP were significantly reduced to the same extent by both therapies. No change ocurred in the IL-1 receptor antagonist. A significant increase was seen in total lean body mass with the estrogen/testosterone regimen, but the total fat mass, trunk lean or fat mass remained unchanged after 24 weeks of both treatments. No effect was detected on total, hip or spinal bone mineral density after treatment with estrogen alone or estrogen/testosterone. Likewise, BMI and blood pressure were unaffected.

CONCLUSIONS

The addition of testosterone to oral estrogen might have positive effects on bone as suggested by the fact that it counteracted the decline in IGF-I and PICP levels. An anabolic effect on muscle was reflected by an increase in the total lean body mass. No adverse effects were noted on BMI, fat distribution or blood pressure during the 6-month treatment with oral testosterone undecanoate.

Inhibition of the serotonin (5-hydroxytryptamine) transporter reduces bone accrual during growth

Selective serotonin-reuptake inhibitors (SSRIs) antagonize the serotonin (5-hydroxytryptamine) transporter (5-HTT), and are frequently prescribed to children and adolescents to treat depression. However, recent findings of functional serotonergic pathways in bone cells and preliminary clinical evidence demonstrating detrimental effects of SSRIs on bone growth have raised questions regarding the effects of these drugs on the growing skeleton. The current work investigated the impact of 5-HTT inhibition on the skeleton in: 1) mice with a null mutation in the gene encoding for the 5-HTT; and 2) growing mice treated with a SSRI. In both models, 5-HTT inhibition had significant detrimental effects on bone mineral accrual. 5-HTT null mutant mice had a consistent skeletal phenotype of reduced mass, altered architecture, and inferior mechanical properties, whereas bone mineral accrual was impaired in growing mice treated with a SSRI. These phenotypes resulted from a reduction in bone formation without an increase in bone resorption and were not influenced by effects on skeletal mechanosensitivity or serum biochemistries. These findings indicate a role for the 5-HTT in the regulation of bone accrual in the growing skeleton and point to a need for further research into the prescription of SSRIs to children and adolescents.

Lifetime high calcium intake increases osteoporotic fracture risk in old age

Caloric restriction prolongs life span. Calcium restriction may preserve bone health. In osteoporosis, bone mineral density (BMD) has significantly decreased, due to a lack of osteoblast bone formation. Traditional osteoporosis prevention is aimed at maximizing BMD, but the lifetime effects of continuously maintaining a high BMD on eventual bone health in old age, have not been studied. Strikingly, in countries with a high mean BMD, fracture rates in the elderly are significantly higher than in countries with a low mean BMD. Studies show that this is not based on genetic differences. Also, in primary hyperparathyroidism, on the brink of osteoporosis, BMD levels may be significantly higher than normal. Maybe, BMD does not represent long term bone health, but merely momentary bone strength. And maybe, maintaining a high BMD might actually wear out bone health. Since osteoporosis particularly occurs in the elderly, and because in osteoporotic bone less osteoblasts are available, the underlying process may have to do with ageing of osteoblastic cells. In healthy subjects, osteoblastic bone cells respond to the influx of calcium by composing a matrix upon which calcium precipitates. In the process of creating this matrix, 50-70% of the involved osteoblasts die. The greater the influx of calcium, the greater osteoblast activity, and the greater osteoblast apoptosis rate. An increased osteoblast apoptosis rate leads to a decrease in the age-related osteoblast replicative capacity (ARORC). In comparison to healthy bone, in osteoporotic bone the decrease in the replicative capacity of osteoblastic cells is greater. Due to the eventual resulting lack of osteoblast activity, micro-fractures cannot be repaired. Continuously maintaining a high BMD comes with continuously high bone remodeling rates, which regionally exhaust the ARORC, eventually leading to irreparable microfractures. Regarding long time influences on bone health, adequate estrogen levels are known to be protective against osteoporosis. This is generally attributed to its inhibiting influence on osteoclast activity. Instead, its net effects on osteoblast metabolism may be the key to osteoporosis prevention. Adequate estrogen levels inhibit osteoblast activity, calcium apposition and osteoblast apoptosis rate, preserving the ARORC.

CONCLUSION

Regarding osteoporosis prevention, ARORC better than BMD represents bone health. Regarding ARORC, adequate estrogen levels are protective, opposing the similar effects of hyperparathyroidism and a high calcium diet. Tests need to be performed in mice to assess the lifetime effects of a high versus a low calcium diet, on eventual bone fracture toughness.

Additive protective effects of estrogen and androgen treatment on trabecular bone in ovariectomized rats

Both ER and AR activation regulates trabecular bone mass. We show that combined estrogen and androgen treatment results in additive protection of trabecular bone in OVX rats. This may in part be attributable to the effect of AR activation to attenuate the inhibitory effect of ER activation on bone formation.

INTRODUCTION

Sex steroids are important regulators of trabecular bone mass. Both estrogen receptor (ER) and androgen receptor (AR) activation results in increased trabecular bone mass. The aim of this study was to investigate if combined estrogen and androgen treatment might be beneficial in the treatment of trabecular bone loss.

MATERIALS AND METHODS

Twelve-week-old female rats were ovariectomized (OVX) and treated with vehicle (V), 17beta-estradiol (E2; ER activation), dihydrotestosterone (DHT; AR activation), or the combination (E2 + DHT) for 6 weeks. The skeletal phenotype was analyzed by pQCT, microCT, histomorphometry of growth plates, and serum levels of biochemical bone markers.

RESULTS

Both E2 (+121% over V) and DHT (+34%) preserved the trabecular volumetric BMD (tvBMD) in OVX rats. The effect of E2 and DHT on tvBMD was additive, resulting in a 182% increase over V in the rats given E2 + DHT. MicroCT analyses of the trabecular bone microstructure revealed that the effect of E2 and DHT was additive on the number of trabeculae. E2 treatment reduced serum markers of both bone resorption (collagen C-terminal telopeptide) and bone formation (osteocalcin), indicating reduced bone turnover. Addition of DHT to E2 treatment did not modulate the effects of E2 on the marker of bone resorption, whereas it attenuated the inhibitory effect of E2 on the bone formation marker, which might explain the additive protective effect of E2 and DHT on trabecular bone mass. In contrast, DHT partially counteracted the suppressive effect of E2 on longitudinal bone growth and the E2-induced alterations in growth plate morphology.

CONCLUSIONS

These findings show that combined estrogen and androgen treatment results in additive protective effects on trabecular bone in OVX rats. Our data suggest that a combined treatment with selective ER and AR modulators might be beneficial in the treatment of osteoporosis.

Role of the androgen receptor in skeletal homeostasis: the androgen-resistant testicular feminized male mouse model

The role of androgen receptor-mediated androgen action on bone was investigated in testicular feminized male (Tfm) mice. Cortical bone was found to be unresponsive to testosterone (T) in orchidectomized Tfm mice, whereas cortical thickness as well as trabecular BMD and structure were fully maintained by T in the corresponding Tabby control mice. These data show an essential role for androgen receptor-mediated androgen action in periosteal bone formation.

INTRODUCTION

Androgens can affect the male skeleton both directly-through activation of the androgen receptor (AR)-and indirectly-through stimulation of estrogen receptors after aromatization. We assessed the importance of AR-mediated androgen action on bone in a mouse model of androgen resistance.

MATERIALS AND METHODS

Eight-week-old androgen-resistant testicular feminized male (Tfm) and Tabby control mice were orchidectomized (ORX) and treated for 4 weeks with a slow-release testosterone (T) pellet (delivering 167 microg/day) or a placebo pellet. A comprehensive analysis of the skeletal effects of androgen deficiency and replacement was performed using histomorphometry, QCT, and biochemical assessment of bone turnover.

RESULTS

As expected, T increased trabecular BMD, volume, number, and width in ORX Tabby mice. In ORX Tfm mice, however, T had less effect on trabecular BMD and no effect on trabecular bone structure. T action on trabecular bone was associated with opposite changes in bone turnover: trabecular and endocortical bone turnover and serum levels of osteocalcin were all reduced by T in ORX Tabby mice, but not in ORX Tfm mice. T also increased cortical thickness (+16%), area, and density in ORX Tabby mice, but not in Tfm mice, resulting in greater bone strength in the Tabby control strain. The positive effects of T on cortical bone reflected a stimulatory effect on periosteal bone formation (+137%), which was again absent in Tfm mice.

CONCLUSIONS

These data show that, in male mice, AR-mediated T action is essential for periosteal bone formation and contributes to trabecular bone maintenance.

Androgen Replacement Therapy

The major goal of androgen substitution is to replace testosterone at levels as close to physiological levels as is possible. For some androgen-dependent functions testosterone is a pro-hormone, peripherally converted to 5alpha-dihydrotestosterone (DHT) and 17beta-estradiol (E2), of which the levels preferably should be within normal physiological ranges. Furthermore, androgens should have a good safety profile without adverse effects on the prostate, serum lipids, liver or respiratory function, and they must be convenient to use and patient-friendly, with a relative independence from medical services. Natural testosterone is viewed as the best androgen for substitution in hypogonadal men. The reason behind the selection is that testosterone can be converted to DHT and E2, thus developing the full spectrum of testosterone activities in long-term substitution. The mainstays of testosterone substitution are parenteral testosterone esters (testosterone enantate and testosterone cipionate) administered every 2-3 weeks. A major disadvantage is the strongly fluctuating levels of plasma testosterone, which are not in the physiological range at least 50% of the time. Also, the generated plasma E2 is usually supraphysiological. A major improvement is parenteral testosterone undecanoate producing normal plasma levels of testosterone for 12 weeks, with normal plasma levels of DHT and E2 also. Subcutaneous testosterone implants provide the patient, depending on the dose of implants, with normal plasma testosterone for 3-6 months. However, their use is not widespread. Oral testosterone undecanoate dissolved in castor oil bypasses the liver via its lymphatic absorption. At a dosage of 80 mg twice daily, plasma testosterone levels are largely in the normal range, but plasma DHT tends to be elevated. For two decades transdermal testosterone preparations have been available and have an attractive pharmacokinetic profile. Scrotal testosterone patches generate supraphysiological plasma DHT levels, which is not the case with the nonscrotal testosterone patches. Transdermal testosterone gel produces fewer skin irritations than the patches and offers greater flexibility in dosage. Oromucosal testosterone preparations have recently become available. Testosterone replacement is usually of long duration and so patient compliance is of utmost importance. Therefore, the patient must be involved in the selection of type of testosterone preparation. Administration of testosterone to young individuals has almost no adverse effects. With increasing age the risk of adverse effects on the prostate, the cardiovascular system and erythropoiesis increases. Consequently, short-acting testosterone preparations are better suited for aging androgen-deficient men.