Short-term effects of high dose estrogen on tibiae of growing male rats

Insulin-like growth factors: actions on the skeleton

The discovery of the growth hormone (GH)-mediated somatic factors (somatomedins), insulin-like growth factor (IGF)-I and -II, has elicited an enormous interest primarily among endocrinologists who study growth and metabolism. The advancement of molecular endocrinology over the past four decades enables investigators to re-examine and refine the established somatomedin hypothesis. Specifically, gene deletions, transgene overexpression or more recently, cell-specific gene-ablations, have enabled investigators to study the effects of the Igf1 and Igf2 genes in temporal and spatial manners. The GH/IGF axis, acting in an endocrine and autocrine/paracrine fashion, is the major axis controlling skeletal growth. Studies in rodents have clearly shown that IGFs regulate bone length of the appendicular skeleton evidenced by changes in chondrocytes of the proliferative and hypertrophic zones of the growth plate. IGFs affect radial bone growth and regulate cortical and trabecular bone properties via their effects on osteoblast, osteocyte and osteoclast function. Interactions of the IGFs with sex steroid hormones and the parathyroid hormone demonstrate the significance and complexity of the IGF axis in the skeleton. Finally, IGFs have been implicated in skeletal aging. Decreases in serum IGFs during aging have been correlated with reductions in bone mineral density and increased fracture risk. This review highlights many of the most relevant studies in the IGF research landscape, focusing in particular on IGFs effects on the skeleton.

Effect of GH/IGF-1 on Bone Metabolism and Osteoporsosis

Background. Growth hormone (GH) and insulin-like growth factor (IGF-1) are fundamental in skeletal growth during puberty and bone health throughout life. GH increases tissue formation by acting directly and indirectly on target cells; IGF-1 is a critical mediator of bone growth. Clinical studies reporting the use of GH and IGF-1 in osteoporosis and fracture healing are outlined. Methods. A Pubmed search revealed 39 clinical studies reporting the effects of GH and IGF-1 administration on bone metabolism in osteopenic and osteoporotic human subjects and on bone healing in operated patients with normal GH secretion. Eighteen clinical studies considered the effect with GH treatment, fourteen studies reported the clinical effects with IGF-1 administration, and seven related to the GH/IGF-1 effect on bone healing. Results. Both GH and IGF-1 administration significantly increased bone resorption and bone formation in the most studies. GH/IGF-1 administration in patients with hip or tibial fractures resulted in increased bone healing, rapid clinical improvements. Some conflicting results were evidenced. Conclusions. GH and IGF-1 therapy has a significant anabolic effect. GH administration for the treatment of osteoporosis and bone fractures may greatly improve clinical outcome. GH interacts with sex steroids in the anabolic process. GH resistance process is considered.

Short-term and long-term orthopaedic issues in patients with fragility fractures

BACKGROUND

Patients with impaired bone quality who suffer a fragility fracture face substantial challenges in both their short- and long-term care. In addition to poor bone quality, many of these patients have multiple medical comorbidities that alter their surgical risk and affect their ultimate functional recovery. Some medical issues can contribute to the altered bone quality and must be addressed to prevent future fractures.

QUESTIONS/PURPOSES

This review summarizes the modifications in perioperative management and fracture fixation in patients with common fragility fractures who have impaired bone quality. It also summarizes the postoperative diagnosis and treatment of secondary causes of impaired bone quality in these patients.

METHODS

We performed a PubMed search, and literature published after 2000 was prioritized, with the exception of benchmark clinical trial studies published before 2000.

RESULTS

Patients with altered bone quality require rapid perioperative management of multiple medical comorbidities. Implant selection in patients with poor quality bone should permit early weightbearing, and constructs should maximize surface area contact with the remaining bone. Long-term diagnosis and treatment of other disease states contributing to poor bone quality (vitamin D deficiency/insufficiency, hypothyroidism, hyperthyroidism, hyperparathyroidism, Cushing's disease, and hypogonadism) must occur to minimize the chances of future fractures.

CONCLUSIONS

Recognition of patients with impaired bone quality and proper treatment of their special needs in both the short and long term are essential for their best opportunity for maximal functional recovery and prevention of future fractures.

Structural adaptation and intracortical bone turnover in an ovine model of osteoporosis

Compact bone makes up approximately 80% of the human skeletal mass. This study examines the effect of estrogen deficiency on compact bone turnover and associated geometrical structural adaptation over a 31-month period in a large animal model. Twenty-seven skeletally mature sheep were divided into control (n = 16) and ovariectomy group (OVX, n = 11). Animals were administered five different fluorochrome dyes to label intracortical bone turnover, and sacrificed at 31 months. Compact bone samples were analyzed for cortical geometry, intracortical turnover at five time points, resorption cavities, porosity, and compressive strength. Intracortical bone turnover was significantly increased in OVX, which demonstrated seasonal variation. Cross-sectional area in OVX was significantly greater than control and was associated with an increased section modulus. Intracortical porosity was significantly increased in OVX, however, there was no significant difference in ultimate compressive strength between the groups. Our results demonstrate increased intracortical bone turnover, resportion spaces, and porosity in OVX, without adversely affecting compressive strength. Our results also support the hypothesis of geometrical adaptation of compact bone in response to estrogen deficiency. These results suggest an early structural compensatory response in compact bone, despite increased intracortical turnover.

Bisphosphonates do not inhibit periosteal bone formation in estrogen deficient animals and allow enhanced bone modeling in response to mechanical loading

The suppressive effects of bisphosphonates (BPs) on bone remodeling are clear yet there is conflicting data concerning the effects of BPs on modeling (specifically formation modeling on the periosteal surface). The normal periosteal expansion that occurs during aging has significant benefits to maintaining/improving the bones' mechanical properties and thus it is important to understand whether BPs affect this bone surface. Therefore, the purpose of this study was to determine the effects of BPs on periosteal bone formation modeling induced by ovariectomy (OVX) and mechanical loading. Six-month-old Sprague-Dawley OVX rats (n=60; 12/group) were administered vehicle, risedronate, alendronate, or zoledronate at doses used clinically for treatment of post-menopausal osteoporosis. Three weeks after initiating BP treatment, all animals underwent in vivo ulnar loading of the right limb every other day for 1 week (3 total sessions). Periosteal surface mineral apposition rate, mineralizing surface, and bone formation rate were determined at the mid-diaphysis of both loaded (right) and non-loaded (left) ulnae. There was no significant effect of any of the BPs on periosteal bone formation parameters compared to VEH-treated animals in the non-loaded limb, suggesting that BP treatment does not compromise the normal periosteal expansion associated with estrogen loss. Mechanical loading significantly increased BFR in the loaded limb compared to the non-loaded limb in all BP-treated groups, with no difference in the magnitude of this effect among the various BPs. Collectively, these data show that BP treatment, at doses comparable to those used for treatment of post-menopausal osteoporosis, (1) does not alter the periosteal formation activity that occurs in the absence of estrogen and (2) allows normal stimulation of periosteal bone formation in response to the anabolic stimulation of mechanical loading.

Morphological alterations in the growth plate cartilage of ovariectomized mice

The effects of ovariectomy on growth and estrogen receptor (ER) expression level in the epiphyseal growth plate in mice have been estimated by histomorphometry and immunohistochemistry. Twelve female ddY mice, 8-9 weeks of age, were subjected to bilateral ovariectomy and 12 others were sham operated. They were then killed 8 weeks later. Ovariectomy significantly increased the total thickness of the distal femoral and proximal tibial growth plate cartilage. Ovariectomy caused a 1.4-fold increase in the thickness of the proliferative layer in the distal growth plate of the femur and a 1.3-fold increase in the thickness of the proliferative layer in the proximal growth plate of the tibia. ERalpha and ERbeta immunoreactivity was detected in chondrocytes of the growth plate and the expression level of ERs in epiphyseal plates was increased in ovariectomized mice compared with controls. These data suggest that ERalpha and ERbeta are coexpressed in the growth plates of the mice and that the cartilage growth and the level of expression of ERs in these tissues are hormonally regulated.

Low-dose estrogen treatment suppresses periosteal bone formation in response to mechanical loading

Estrogen and exercise influence cortical bone formation. Both affect bone during growth, but with complex interactions. We hypothesized that estrogen reduces the osteogenic response caused by exercise at the periosteal surface of bone, while it enhances bone formation on the endocortical surface. To test our hypothesis, 16 young (8 weeks old) male Sprague-Dawley rats were randomized into two groups: (1) low-dose 17-alpha ethynylestradiol treatment+bone loading (EE2) or (2) vehicle-treated+bone loading (vehicle). We applied controlled loading to the right ulna at a peak force of 17 N, 2 min/day, 3 days/week for 5 weeks to simulate exercise. The left nonloaded ulna served as an internal control for loading. Mechanical loading increased cortical area (7.7%) and bone mineral content (8%) in the vehicle-treated group (P < 0.05) but only slightly increased cortical area in the EE2 group (P = 0.08). Histomorphometry showed 1 week of mechanical loading increased periosteal bone formation rate by 29% in the vehicle group and this response was reduced (P < 0.05) to only 15% in the EE2 group. At the endocortical surface, there were no differences in the loading response between the vehicle and EE2-treated groups. We conclude low-dose EE2 suppresses the mechanical loading response on the periosteal surface of long bones, but had no effect on the loading response at the endocortical bone surface in growing male rats.

Knee ligament mechanical properties are not influenced by estrogen or its receptors

Women are at greater risk of tearing their knee anterior cruciate ligament (ACL) than men participating in similar athletic activities. There is currently no conclusive explanation for this disparity; however, as ACL injuries in women have been linked with estrogen fluctuations during the menstrual cycle, one hypothesis is that estrogen has a direct detrimental effect on knee ligament mechanical properties. This study investigated the influence of estrogen and its receptors (ER alpha and ER beta) on knee ligament mechanical properties. This was achieved by testing the viscoelastic and tensile mechanical properties of knee medial collateral ligaments (MCL) and ACLs from: 1) male Sprague-Dawley rats treated with either estrogen (17alpha-ethynylestradiol; 0.03 mg/kg) or an ER alpha-specific agonist (propyl pyrazole triol; 2 mg/kg), and 2) female mice with a null mutation of the gene encoding for ER beta. Estrogen treatment had no significant effects on the viscoelastic or tensile mechanical properties of the rat MCL or ACL. Similarly, pharmacological stimulation of ER alpha using a selective agonist in rats and genetic modulation of ER beta by null mutation of its gene in mice did not influence MCL or ACL properties. These data indicate that estrogen does not have a major direct effect on ligament mechanical properties. Energies for the prevention of the disproportionately high rate of knee ligament injuries in women may be better spent focusing on more established and modifiable risk factors, such as abnormalities in neuromuscular control about the knee.

Genomic and non-genomic actions of sex steroids in the growth plate

Sex steroids, and particularly estrogens, are important regulators of bone growth and bone mass accrual. For a long time, it was thought that these effects were mainly caused by their modulatory effects on the somatotrophic axis. Data gathered in the past years have challenged this view and it is now widely accepted that many of the effects of sex steroids on growth and bone mass accrual are caused by direct effects on target cells in the growth plate and bone. This review summarizes and discusses some of our latest findings on the expression of sex steroid receptors in the growth plate, the source of the ligands activating these receptors, and their putatitive mechanism of action predominantly focusing on observations in the rat.

Periosteum: biology, regulation, and response to osteoporosis therapies

Periosteum contains osteogenic cells that regulate the outer shape of bone and work in coordination with inner cortical endosteum to regulate cortical thickness and the size and position of a bone in space. Induction of periosteal expansion, especially at sites such as the lumbar spine and femoral neck, reduces fracture risk by modifying bone dimensions to increase bone strength. The cell and molecular mechanisms that selectively and specifically activate periosteal expansion, as well as the mechanisms by which osteoporosis drugs regulate periosteum, remain poorly understood. We speculate that an alternate strategy to protect human bones from fracture may be through targeting of the periosteum, either using current or novel agents. In this review, we highlight current concepts of periosteal cell biology, including their apparent differences from endosteal osteogenic cells, discuss the limited data regarding how the periosteal surface is regulated by currently approved osteoporosis drugs, and suggest one potential means through which targeting periosteum may be achieved. Improving our understanding of mechanisms controlling periosteal expansion will likely provide insights necessary to enhance current and develop novel interventions to further reduce the risk of osteoporotic fractures.

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.

Detecting and tracking local changes in the tibiae of individual rats: a novel method to analyse longitudinal in vivo micro-CT data

In this study we present the analysis of in vivo micro-CT scans using a new method based on image registration that accurately evaluates longitudinal micro-CT studies. We tested if detailed changes in the bone architecture could be detected and tracked in individual animals. A prototype in vivo micro-CT scanner (Skyscan 1076) was developed in which tibiae of rats that are lying on a bed under gas anaesthesia were scanned. For this study, three female Wistar rats were used: a sham-operated rat, an ovariectomised (OVX) rat and one rat that served as a reproducibility control. The reproducibility control rat was scanned twice in 1 day. The other animals were scanned at week 0, just before surgery, at week 4 and at week 14 after surgery. Architectural changes over time were detected by overlaying two data sets made at different time points using an algorithm that uses mutual information for optimal registration. The scans were segmented into binary data sets using a local thresholding algorithm. The reproducibility test showed small errors of less than 3% in bone volume measurements and errors less than 0.5% in measurements of trabecular thickness. The sham-operated rat showed no changes in total bone volume, though thinning and eventual loss of some small trabeculae could be detected, which could be related to the age of the animal. The OVX rat lost much trabecular bone volume, especially in the metaphysis (60% at week 4, 75% at week 14). The remaining trabeculae slowly increased in thickness. Following the different scans in time showed the forming of new trabecular structures. Additionally, small longitudinal growth at the growth plate could be detected after the first 4 weeks. Further, the OVX rat showed extensive modelling at the proximal endosteal lateral cortex. We have shown a new method that can detect and track changes in the local bone architecture and individual trabeculae in time, in an individual living animal. This method enables longitudinal in vivo micro-CT studies and has the potential to greatly contribute to experimental rat or mouse studies on pharmacological intervention and transgenic models.

Men, bone and estrogen: unresolved issues

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

Effects of locally-delivered human macrophage products and estrogen on murine inflammatory bone resorption

The objective of this study was to use an in vivo model of periodontitis (mouse calvaria) to quantify the effects of local release of secreted human macrophage products, 17beta-estradiol (E2), and proinflammatory lipopolysaccharide (LPS) on histologic bone resorption. Human THP-1 monocytes (106) were converted to macrophage phenotype by 500 ng/ml phorbol 12-myristate- 13-acetate (PMA) and treated as follows: no stimulation or Escherichia coli LPS (10 microg/ml) alone or in combination with a physiologic dose of E2 (100 pg/ml) for 24 h in RPMI/10% FBS, washed extensively, then incubated for 24 h in serum-free media. Supernatant products were concentrated and incorporated into a 4% (w/v) methylcellulose gel. Separate gels were incorporated with the following: LPS (500 microg/animal) alone, high dose of E2 (10 ng/animal) alone, a combination of LPS + E2, or gel only (controls). Loaded or control gels were placed into a polylactic acid occlusive dome, inserted subcutaneously over the calvaria of mature ovariectomized ICR Swiss mice (8 mice x 7 groups x 2 times [5/14 days] = 112 animals), then calvaria were evaluated histologically. Macrophage stimulation with LPS alone, but not LPS in combination with E2, produced supernatants which upregulated osteoclast numbers in the suture area compared to gel controls at 5 days (p = 0.009). The addition of LPS directly to the local delivery gels significantly upregulated osteoclasts in endosteal surfaces compared to gel controls at 5 days (p = 0.024) and at 14 days (p = 0.025). The addition of E2 to LPS down-regulated resorption to a level not different from gel controls at 14 days. This in vivo model appears effective in studying inflammatory bone resorption, which may be inhibited by E2 directly or through its influence on secreted macrophage products.

The contribution of reduced peak accrual of bone and age-related bone loss to osteoporosis at the spine and hip: insights from the daughters of women with vertebral or hip fractures

The genetic hypothesis states that a daughter will resemble her mother by about 50% in a given trait because she shares, on average, half her genes. We used this trait resemblance in mothers and daughters to determine whether abnormalities in volumetric bone mineral density (vBMD) or bone size in women with fractures originate in growth or aging. vBMD and volume of the third lumbar vertebra and femoral neck were estimated using posteroanterior (PA) scanning by dual-energy X-ray absorptiometry (DXA). Vertebral volume was estimated as (scan area)(3/2) and femoral neck volume was pi* (width/2)(2)* height. vBMD was bone mineral content (BMC)/volume. The data were expressed as age-specific SD or Z scores (mean +/- SEM). Vertebral vBMD was reduced by -0.98 +/- 0.14 SD (p < 0.001) in 34 women with vertebral fractures, and by -0.36 +/- 0.13 SD (p < 0.05) in their 44 premenopausal daughters. The vBMD deficit in the daughters (relative to age-matched controls) was no different from one-half their mothers' deficit (relative to their age-matched controls). Vertebral volume was reduced in the women with vertebral fractures relative to age-matched controls (-0.77 +/- 0.15 SD; p < 0.001), but not in their daughters (-0.17 +/- 0.13 SD, NS). The 31 women with hip fractures and their 41 premenopausal daughters had no deficits in vertebral volume or vBMD. Femoral neck vBMD was reduced in the women with hip fractures (-1.24 +/- 0.12 SD; p < 0.001) but not in their daughters (-0.17 +/- 0.13 SD, NS). Femoral neck volume was increased by 0.98 +/- 0.30 SD (p < 0.05) in women with hip fractures (relative to age-matched controls) and by 0.54 +/- 0.14 SD (p < 0.001) in their daughters (relative to age-matched controls); that is, about one-half that of their mothers. We propose that women with vertebral fractures have reduced vertebral vBMD because of, in large part, reduced accrual of bone during growth (because the deficit in their daughters was almost one-half their mothers' deficit); reduced vertebral volume in women with vertebral fractures is caused by reduced periosteal apposition during aging (because their daughters have no deficit in vertebral volume). Women with hip fractures have reduced vBMD because of age-related bone loss (because their daughters have no deficit in vBMD) but the increased femoral neck volume is growth related (because their daughters' femoral neck size is increased by one-half as much). The pathogenesis of bone fragility at the axial and appendicular skeleton is heterogeneous and has its origins in growth and aging.

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.