The effects of hormone replacement therapy on cortical bone in postmenopausal women. A histomorphometric study

Altered distributions of bone tissue mineral and collagen properties in women with fragility fractures

Heterogeneity of bone tissue properties is emerging as a potential indicator of altered bone quality in pathologic tissue. The objective of this study was to compare the distributions of tissue properties in women with and without histories of fragility fractures using Fourier transform infrared (FTIR) imaging. We extended a prior study that examined the relationship of the mean FTIR properties to fracture risk by analyzing in detail the widths and the tails of the distributions of FTIR properties in biopsies from fracture and non-fracture cohorts. The mineral and matrix properties of cortical and trabecular iliac crest tissue were compared in biopsies from women with a history of fragility fracture (+Fx; n=21, age: mean 54±SD 15y) and with no history of fragility fracture (-Fx; n=12, age: 57±5y). A subset of the patients included in the -Fx group were taking estrogen-plus-progestin hormone replacement therapy (HRT) (-Fx+HRT n=8, age: 58±5y) and were analyzed separately from patients with no history of HRT (-Fx-HRT n=4, age: 56±7y). When the FTIR parameter mean values were examined by treatment group, the trabecular tissue of -Fx-HRT patients had a lower mineral:matrix ratio (M:M) and collagen maturity (XLR) than that of -Fx+HRT patients (-22% M:M, -18% XLR) and +Fx patients (-17% M:M, -18% XLR). Across multiple FTIR parameters, tissue from the -Fx-HRT group had smaller low-tail (5th percentile) values than that from the -Fx+HRT or +Fx groups. In trabecular collagen maturity and crystallinity (XST), the -Fx-HRT group had smaller low-tail values than those in the -Fx+HRT group (-16% XLR, -5% XST) and the +Fx group (-17% XLR, -7% XST). The relatively low values of trabecular mineral:matrix ratio and collagen maturity and smaller low-tail values of collagen maturity and crystallinity observed in the -Fx-HRT group are characteristic of younger tissue. Taken together, our data suggest that the presence of newly formed tissue that includes small/imperfect crystals and immature crosslinks, as well as moderately mature tissue, is an important characteristic of healthy, fracture-resistant bone. Finally, the larger mean and low-tail values of mineral:matrix ratio and collagen maturity noted in our -Fx+HRT vs. -Fx-HRT biopsies are consistent with greater tissue age and greater BMD arising from decreased osteoclastic resorption in HRT-treated patients.

Estrogen deficiency heterogeneously affects tissue specific stem cells in mice

Postmenopausal disorders are frequently observed in various organs, but their relationship with estrogen deficiency and mechanisms remain unclear. As tissue-specific stem cells have been found to express estrogen receptors, we examined the hypothesis that estrogen deficiency impairs stem cells, which consequently contributes to postmenopausal disorders. Six-week-old C57BL/6 female mice were ovariectomized, following which they received 17β-estradiol replacement or vehicle (control). Sham-operated mice were used as healthy controls. All mice were killed for evaluation 2 months after treatments. Compared with the healthy control, ovariectomy significantly decreased uterine weight, which was partially recovered by 17β-estradiol replacement. Ovariectomy significantly increased the numbers of c-kit-positive hematopoietic stem/progenitor cells in bone marrow, but impaired their capacity to grow mixed cell-type colonies in vitro. Estrogen replacement further increased the numbers of c-kit-positive hematopoietic stem/progenitor cells in bone marrow, without significantly affecting colony growth in vitro. The number of CD105-positive mesenchymal stem cells in bone marrow also significantly decreased after ovariectomy, but completely recovered following estrogen replacement. Otherwise, neither ovariectomy nor estrogen replacement changed the number of Pax7-positive satellite cells, which are a skeletal muscle-type stem cell. Estrogen deficiency heterogeneously affected tissue-specific stem cells, suggesting a likely and direct relationship with postmenopausal disorders.

Ginsenoside Rb1 inhibits osteoclastogenesis by modulating NF-κB and MAPKs pathways

Ginsenosides (GSS), the main active components of ginseng, have been reported possessing anti-osteoporosis activity in ovariectomized rats. However, the active ingredient and the mechanisms underlying the anti-osteoporosis activity of GSS have not been clearly elucidated. In the present study, we determined the effect of ginsenoside Rb1, a major component of ginsenosides, on receptor activator of NF-κB ligand (RANKL)-induced osteoclast formation. Ginsenoside Rb1 inhibited RANKL-induced osteoclast differentiation from Raw264.7 cells without cytotoxicity. Ginsenoside Rb1 also inhibited RANKL-induced TNFα mRNA expression in Raw264.7 cells. Pretreatment with ginsenoside Rb1 significantly inhibited RANKL-induced the gene expression of c-Fos and nuclear factor of activated T-cells c1 (NFATc1), which are two essential and crucial transcription factors for osteoclast formation. Rb1 inhibited RANKL-induced nucleus translocation and activation of NF-κB, the upstream factor of c-Fos and NFATc1. Among the three well known mitogen-activated protein kinases (MAPKs), Rb1 inhibited RANKL-induced JNK and p38 phosphorylation, but not ERK1/2. Taken together, our data suggest that ginsenoside Rb1 is one of the effective components of GSS for the anti-osteoporosis activity and can inhibit osteoclastogenesis by suppressing RANKL-induced activation of both JNK and p38 MAPKs and NF-κB pathways, and consequently down-regulating the gene expression of c-Fos and NFATc1 in osteoclast precursors.

Bone quality and osteoporosis therapy

Although BMD measured by DXA is a useful clinical tool for osteoporosis diagnosis, changes resulting from osteoporosis treatment only partially explain the observed reduction in fractures. Several other bone properties that influence its resistance to fractures and explain this discrepancy have been defined as "bone quality". Bone quality is determined by its structural and material properties and orchestrated by bone turnover, a continuous process of renewal through which old or damaged bone is replaced by a mechanically healthy bone and calcium homeostasis is maintained. Bone structural properties include its geometry (size and shape) and microarchitecture (trabecular architecture and cortical porosity), while bone material properties include its mineral and collagen composition as well as microdamage and its repair. This review aims to update concepts surrounding bone quality and how drugs employed to treat osteoporosis might influence them.

Comparative effects of teriparatide and strontium ranelate in the periosteum of iliac crest biopsies in postmenopausal women with osteoporosis

The periosteum contains osteogenic cells that regulate the outer shape of bone and contribute to determine its cortical thickness, size and position. We assessed the effects of subcutaneous injections of teriparatide (TPTD, 20μg/day) or oral strontium ranelate (SrR, 2g/day) in postmenopausal women with osteoporosis on new bone formation activity at the periosteal and endosteal bone surfaces using dynamic histomorphometric measurements. Evaluable tetracycline-labeled transiliac crest bone biopsies were analyzed from 27 patients in the TPTD group, and 22 in the SrR group after six months of treatment. Measurements were conducted on the thicker and thinner cortices separately, and comparisons between the thicker, thinner and combined cortices were carried out. At the combined periosteal cortex, the mineralization surface as a percent of bone surface (MS/BS%) was greater for TPTD (mean±SE: 8.08±1.22%) than SrR (3.22±1.05%) (p<0.005). The difference in mineral apposition rate (MAR) between TPTD (0.35±0.06μm/day) and SrR (0.14±0.06μm/day) was also significant (p<0.05), while that of bone formation rate per bone surface (BFR/BS) between TPTD (0.014±0.004 mm(3)/mm(2)/year) and SrR (0.004±0.003 mm(3)/mm(2)/year) was not (p=0.057). Statistically significant differences between the two treatments were also observed for MS/BS%, BFR/BS, MAR and the double-labeled perimeter in the periosteum of the thicker, but not thinner, iliac crest cortices. The comparison between the thicker and thinner cortices of both periosteal and endosteal surfaces showed statistically significant differences for MAR and the double-labeled perimeter for TPTD treated women. There were no statistically significant differences in any bone formation dynamic measurements between the two cortices in the SrR group. In conclusion, most of the bone formation and mineralization variables were significantly higher for TPTD- than SrR-treated women at both the periosteal and endosteal combined cortices. The response to TPTD for dynamic bone formation measurements in the periosteal surface was greater for the thicker than thinner cortex, but this difference was not significant in SrR treated patients. This may reflect a greater ability of TPTD to enhance responsiveness of bone to the mechanical loading environment. These effects on bone formation may underlie the improvement in bone quality in patients with osteoporosis treated with TPTD.

Mechanical implications of estrogen supplementation in early postmenopausal women

Whereas the structural implications of drug intervention are well established, there are few data on the possible mechanical consequences of treatment. In this work we examined the changes in elastic and shear moduli (EM and SM) in a region of trabecular bone in the distal radius and distal tibia of early postmenopausal women on the basis of MRI-based micro-finite-element (microFE) analysis. Whole-section axial stiffness (AS) encompassing both trabecular and cortical compartments was evaluated as well. The study was conducted on previously acquired high-resolution images at the two anatomic sites. Images were processed to yield a 3D voxel array of bone-volume fraction (BVF), which was converted to a microFE model of hexahedral elements in which tissue modulus was set proportional to voxel BVF. The study comprised 65 early postmenopausal women (age range 45 to 55 years), of whom 32 had chosen estrogen supplementation (estradiol group); the remainder had not (control group). Subjects had been scanned at baseline and 12 and 24 months thereafter. At the distal tibia, EM and SM were reduced by 2.9% to 5.5% in the control group (p < .05 to <.005), but there was no change in the estradiol subjects. AS decreased 3.9% (4.0%) in controls (p < .005) and increased by 5.8% (6.2%) in estradiol group subjects (p < .05) at 12 (24) months. At the distal radius, EM and SM changes from baseline were not significant, but at both time points AS was increased in estradiol group subjects and decreased in controls (p < .005 to <.05), albeit by a smaller margin than at the tibia. EM and SM were strongly correlated with BV/TV (r(2) = 0.44 to 0.92) as well as with topologic parameters expressing the ratio of plates to rods (r(2) = 0.45 to 0.82), jointly explaining up to 96% of the variation in the mechanical parameters. Finally, baseline AS was strongly correlated between the two anatomic sites (r(2) = 0.58), suggesting that intersubject variations in the bone's mechanical competence follows similar mechanisms. In conclusion, the results demonstrate that micro-MRI-based microFE models are suited for the study of the mechanical implications of antiresorptive treatment. The data further highlight the anabolic effect of short-term estrogen supplementation.

Effect of long-term, high-dose estrogen treatment on prolactin levels: a retrospective analysis

BACKGROUND

Exogenous estrogen administration causes prolactinoma formation in rats and there are anecdotal reports of estrogen treatment and prolactinoma formation in human transsexuals with male-to-female gender reassignment. It remains unclear whether chronic exposure to high-dose estrogen in women is linked with hyperprolactinemia. Aim The aim of this study was to determine whether high-dose, long-term subcutaneous estrogen treatment affected prolactin and gonadotropin levels.

DESIGN

This was a retrospective, observational and descriptive analysis of 101 women treated for up to 15 years following a combined hysterectomy and oophorectomy.

METHOD

A total of 101 women who were receiving estradiol implants 50-100 mg 4-6 monthly, on demand for 5-15 years following abdominal hysterectomy and bilateral salphingo-oophorectomy were studied. The main outcome measures were levels of serum prolactin, follicle stimulating hormone (FSH) and luteinizing hormone (LH). Results The 101 patients received an average cumulative estrogen dose of (mean +/- SEM) 1239 +/- 12 mg per person, 18.96 +/- 1.2 mg/kg body weight, over a period of 5-15 years. The trough estradiol levels after withholding implants for 6 months were high (1047.9 +/- 52 pmol/l). Their average prolactin level of 88.88 +/- 9.2 mU/l was normal and their FSH (1.96 +/- 0.42 IU/l) and LH (2.97 +/- 0.88 IU/l) levels were relatively suppressed.

CONCLUSION

These data are reassuring that, even in women exposed to very high doses of estrogen for a prolonged period of time, hyperprolactinemia is unlikely to occur.

Insulin receptor substrate 2 plays important roles in 17beta-estradiol-induced bone formation

BACKGROUND

Discovering the mechanisms of the estrogen effects on the osteoblasts is very important for the development of new agents which have the clear-cut beneficial effects of estrogen while free of adverse effect.

AIM

The aim of this study was to investigate the differential gene expression of 17beta-estradiol (E2)-treated osteoblast-like cells, and the effect of E2 on the insulin receptor substrate 2 (IRS- 2) expression in human cultured osteoblast-like cells and the osteoblasts of ovariectomized (OVX) rats.

MATERIAL AND METHODS

The differential gene expression of E2-treated osteoblast- like cells was analyzed by cytokine expression array and validated by RT-PCR and Western blot analysis. The protein expression and phosphorylation of one of the differentially expressed gene, IRS-2, treated at different times with E2 were analyzed. The Sprague-Dawley rats were ovariectomized and then treated with E2, the IRS-2 expression was analyzed by immunohistochemistry analysis.

RESULTS

E2 upregulated the mRNA expression of IRS-2, bone morphogenetic protein 9, and connective tissue growth factor expression, down-regulated the mRNA expression of matrix metalloproteinase 15 and some tumor suppressor genes. Peak expression of IRS-2 was observed at 12-24 h of treatment by 10-8M E2. E2 can also increase the phosphorylation of IRS-2. The IRS-2 expression was down-regulated in the osteoblasts and bone marrow cells of the OVX rats, which had lower bone mineral density (BMD) than the normal rats. However, both BMD and IRS-2 expression can be rescued by 10-8M E2 in the OVX rats.

CONCLUSION

IRS-2 in osteoblast is up-regulated by E2 and plays important roles in the estrogen- induced bone formation.

Osteoporosis prevention and therapy: preserving and building strength through bone quality

While bone mineral density measurements play a central role in osteoporosis management, the degree to which increases in this parameter contribute to bone fragility, fracture risk, and the therapeutic efficacy of osteoporosis agents is controversial. Indeed, bone strength is also significantly dependent upon bone turnover and bone quality, including microarchitecture, mineralization, and geometry. Given the critical role of these factors, it is essential to understand how they are affected by therapeutic agents. Even though a number of technological advances, such as microcomputed tomography, magnetic resonance imaging, and computerized analysis of radiographic patterns, help to provide critical information toward a more comprehensive assessment of bone turnover and bone quality, clinical trials addressing these factors are scarce. This article provides a review of studies relating to how osteoporosis therapies impact parameters of bone strength and quality.

Effect of 17beta-oestradiol and ginsenoside on osteoporosis in ovariectomised rats

To study the anti-osteoporosis effects and mechanism of action of oestradiol (E2) and ginsenoside (tR), we measured the bone mineral densities (BMD) of lumbar vertebra and tibia and analysed the tibia histological morphological data, as well observed the activity and the number of osteoblasts and the activity of alkaline phosphatase (ALP) and the concentration of cAMP. Results showed that E2 (400 microg kg- 1 week- 1) and tR (10, 20, 30 mg kg- 1 day- 1) were able to countervail the decreasing in BMDs of lumbar vertebra and tibia induced by OVX in rats (P<0.05); E2 (0.1 micromol l- 1) and ginsenoside Rg1 (1 micromol l- 1 and 10 micromol l- 1) were able to increase the number of osteoblasts, the activity of ALP and the concentration of intercellular cAMP in cultured osteoblast cells. The present findings suggest that E2 and tR have an anti-osteoporosis effect in ovariectomised rats.

Long-term ovariectomy decreases ovine compact bone viscoelasticity

Changes in bone mineral density associated with estrogen depletion in humans do not account for all of the associated change in fracture risk, and it is possible that some of this variation may lie in changes of other aspects of bone quality. The purpose of this study was to investigate changes in viscoelastic behavior of compact bone that may be associated with estrogen depletion. Changes in compact bone viscoelastic properties associated with three years of ovariectomy were investigated with dynamic mechanical analysis (low-amplitude 3-point bending at frequencies of 1-20 Hz) using beams milled from the diaphysis of the ovine radius. The viscoelastic storage modulus was significantly (5.2%) lower at the higher frequencies for the ovariectomized animals. The general anatomic variation in storage modulus, in which cranial sectors had higher values than caudal sectors, did not change with ovariectomy. The loss tangent (tandelta, a measure of damping) was also greatly decreased (up to 83%) at high frequencies in the ovariectomized animals. Anatomic variation in tandelta at low (6-12 Hz) frequencies (cranial and caudal sectors having higher values than lateral or medial sectors) was enhanced with ovariectomy. Changes in viscoelastic properties associated with long-term estrogen depletion could be responsible for a significant reduction in the toughness or strength of a bone without concomitant changes in screening modalities used to evaluate bone quality (e.g., DXA, QCT, QUA).

Influence of estrogen therapy at conventional and high doses on the degree of mineralization of iliac bone tissue: a quantitative microradiographic analysis in postmenopausal women

The beneficial skeletal effects of menopausal estrogen replacement therapy (HRT) are well documented. The role of secondary mineralization of bone as a determinant of bone quality is now well established in postmenopausal women treated with bisphosphonates or SERMs. The aim of present study was to investigate the effect of conventional and high doses of estrogen on the main parameters reflecting the degree of mineralization of bone (DMB). Bone biopsies were obtained from 20 women with osteopenia or osteoporosis before and after 24 months (18 to 38 months) of conventional HRT, and from 19 women who had received high doses of estradiol (implant 100 mg every 3-6 months for 1.5-20 years). DMB parameters (mean DMB, DMB Freq. Max. and Heterogeneity Index of the individual distributions of DMB) were measured using quantitative microradiography in cortical, cancellous, and total bone and expressed as g mineral/cm(3) bone. Values obtained in women before HRT were lower than those reported in pre- and postmenopausal control women. After conventional HRT, there was an increase in mean DMB (total bone) of 4.4 +/- 1.9% (mean +/- SEM) versus pre-treatment values (4.1 +/- 2.1% in cortical bone, 4.5 +/- 2.3% in cancellous bone); these differences did not reach statistical significance (P = 0.055). Results were similar for DMB Freq. Max. but Heterogeneity Index was not significantly changed. After high dose estradiol therapy, mean DMB (total bone) was 6.9 +/- 1.9% higher than in untreated women (8.6 +/- 2.1% in cortical bone, 6.5 +/- 2.1% in cancellous bone); this difference was statistically significant (P </= 0.03). Results were similar for DMB Freq. Max. but once again Heterogeneity Index was not significantly modified. The increases in mean DMB were due to a shift of the curves towards high DMB with a decrease of the low DMB values, as confirmed by the absence of changes in the Heterogeneity Index. Estrogen therapy is associated with an increased degree of mineralization of bone induced by a prolongation of secondary mineralization, similar to that observed with other antiresorptive agents. However, this increase was about two-fold lower than that observed after alendronate therapy (10 mg/day/3 years) in postmenopausal osteoporotic women.

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.