Osteoporosis in experimental postmenopausal polyarthritis: the relative contributions of estrogen deficiency and inflammation

Generalized osteoporosis in postmenopausal rheumatoid arthritis (RA) is caused both by estrogen deficiency and by the inflammatory disease. The relative importance of each of these factors is unknown. The aim of this study was to establish a murine model of osteoporosis in postmenopausal RA, and to evaluate the relative importance and mechanisms of menopause and arthritis-related osteoporosis. To mimic postmenopausal RA, DBA/1 mice were ovariectomized, followed by the induction of type II collagen-induced arthritis. After the mice had been killed, paws were collected for histology, one femur for bone mineral density (BMD) and sera for analyses of markers of bone resorption (RatLaps; type I collagen cross-links, bone formation (osteocalcin) and cartilage destruction (cartilage oligomeric matrix protein), and for the evaluation of antigen-specific and innate immune responsiveness. Ovariectomized mice displayed more severe arthritis than sham-operated controls. At termination of the experiment, arthritic control mice and non-arthritic ovariectomized mice displayed trabecular bone losses of 26% and 22%, respectively. Ovariectomized mice with arthritis had as much as 58% decrease in trabecular BMD. Interestingly, cortical BMD was decreased by arthritis but was not affected by hormonal status. In addition, markers of bone resorption and cartilage destruction were increased in arthritic mice, whereas markers of bone formation were increased in ovariectomized mice. This study demonstrates that the loss of endogenous estrogen and inflammation contribute additively and equally to osteoporosis in experimental postmenopausal polyarthritis. Markers of bone remodeling and bone marrow lymphocyte phenotypes indicate different mechanisms for the development of osteoporosis caused by ovariectomy and arthritis in this model.     

Novel paradigm on the effect of estrogen on bone

According to prevailing unitary model of involutional osteoporosis, female postmenopausal bone loss can be divided into two separate phases: the accelerated, transient phase, which is most distinct over the subsequent decade after the menopause and accounts for 20-30% of the cancellous bone loss and 5-10% of the cortical bone loss (type I osteoporosis), and the following gradual, continuous bone loss (type II osteoporosis). Estrogen deficiency is currently quite unanimously accepted as the primary cause of type I osteoporosis, as well as also a major determinant of type II osteoporosis, and quite plausibly, the quest to uncover the origin of type I (and II) osteoporosis has focused on the estrogen withdrawal-related skeletal changes at and around the menopause. However, given that the cyclical secretion of estrogen begins normally in early adolescence and continues over the entire fertile period (excluding the potential periods of pregnancy) until the eventual cessation of female reproductive capability, one could argue that this menopause-oriented approach is limited in scope. In this review, some classic findings of the pubertal effects of estrogen on female bones are presented, findings that were paramount to Fuller Albright when he first described the disease called postmenopausal osteoporosis in 1940, but studies/findings that have failed to attract the attention they deserve. When these findings are incorporated with the primary function of the axial skeleton and long bones, the locomotion, an alternative, novel explanation for the function of estrogen and accordingly, the origin of the accelerated phase of postmenopausal bone loss, is proposed: estrogen packs mechanically excess bone/mineral into the female skeleton at puberty, a bone stock that later serves as the origin of the type I postmenopausal osteoporosis.     

Effect of salcatonin given intranasally on early postmenopausal bone loss.

OBJECTIVE--To study the effect of salmon calcitonin (salcatonin) given intranasally on calcium and bone metabolism in early postmenopausal women. DESIGN--Double blind, placebo controlled, randomised group comparison. SETTING--Outpatient clinic for research into osteoporosis. SUBJECTS--52 Healthy women who had had a natural menopause two and a half to five years previously. INTERVENTIONS--The 52 women were allocated randomly to two years of treatment with either salcatonin 100IU given intranasally (n = 26) or placebo (n = 26). Both groups received a calcium supplement of 500 mg daily. Seven of the women receiving salcatonin and six of those receiving placebo left the study before its end. MAIN OUTCOME MEASURES--Bone mineral content in the spine, the total skeleton, and the forearms after two years of treatment. RESULTS--Bone mineral content in the spine was significantly higher in the women who had received salcatonin than in those who had received placebo both after one year and after two years of treatment. After one year the difference was 3.8% (95% confidence interval 0.0 to 7.6%) and after two years it was 8.2% (3.8 to 12.6%). In contrast, the bone mineral content in the distal and proximal forearms and in the total skeleton declined similarly in both groups by about 2% each year, and after two years of treatment the differences between the groups were not significant. Biochemical estimates of bone turnover were not affected by salcatonin. CONCLUSION--The results suggest that salcatonin given intranasally in the dose used prevents bone loss in the spine of early post menopausal women but does not affect the peripheral skeleton.     

Accelerated vertebral bone loss in relation to the menopause: a cross-sectional study on lumbar bone density in 286 women of 46 to 55 years of age

Bone mineral density (BMD) of the lumbar spine was measured in 286 women (46–55 years of age) using dual photon absorptiometry. The women were classified in three categories: premenopausal, perimenopausal and postmenopausal. The postmenopausal group was subdivided according to the number of years since the last uterine bleeding. with multiple linear regression analysis of lumbar BMD on age and menopausal status, an acceleration of bone loss was observed during the perimenopausal period and the following first two postmenopausal years. No significant bone loss was detected in relation to age or during the later postmenopausal years. Applying both an additive and a multiplicative model of bone loss, the mean perimenopausal bone loss was 0.061 gramequivalents hydroxyapatite (geqHA)/cm² and 6.4%, respectively. In the first 2 postmenopausal years the mean bone loss was 0.044 geqHA/cm² and 5.1% per year. These results suggest a substantial menopause related acceleration of lumbar bone loss in a relatively short time span with its onset in the perimenopausal period.     

Impaired estrogen sensitivity in bone by inhibiting both estrogen receptor alpha and beta pathways

Although it is well established that estrogen deficiency causes osteoporosis among the postmenopausal women, the involvement of estrogen receptor (ER) in its pathogenesis still remains uncertain. In the present study, we have generated rats harboring a dominant negative ERalpha, which inhibits the actions of not only ERalpha but also recently identified ERbeta. Contrary to our expectation, the bone mineral density (BMD) of the resulting transgenic female rats was maintained at the same level with that of the wild-type littermates when sham-operated. In addition, ovariectomy-induced bone loss was observed almost equally in both groups. Strikingly, however, the BMD of the transgenic female rats, after ovariectomized, remained decreased even if 17beta-estradiol (E(2)) was administrated, whereas, in contrast, the decrease of littermate BMD was completely prevented by E(2). Moreover, bone histomorphometrical analysis of ovariectomized transgenic rats revealed that the higher rates of bone turnover still remained after treatment with E(2). These results demonstrate that the prevention from the ovariectomy-induced bone loss by estrogen is mediated by ER pathways and that the maintenance of BMD before ovariectomy might be compensated by other mechanisms distinct from ERalpha and ERbeta pathways.     

Role of peak bone mass and bone loss in postmenopausal osteoporosis: 12 year study.

OBJECTIVE--To examine the role of peak bone mass and subsequent postmenopausal bone loss in the development of osteoporosis and the reliability of identifying women at risk from one bone mass measurement and one biochemical assessment of the future bone loss. DESIGN--Population based study. SETTING--Outpatient clinic for research into osteoporosis. SUBJECTS--178 healthy early postmenopausal women who had participated in a two year study in 1977. 154 of the women underwent follow up examination in 1989, of whom 33 were excluded because of diseases or taking drugs known to affect calcium metabolism. MAIN OUTCOME MEASURES--Bone mineral content of the forearm and values of biochemical markers of bone turnover. RESULTS--The average reduction in bone mineral content during 1977-89 was 20%, but the fast losers had lost 10.0% more than had the slow loser group (mean loss 26.6% in fast losers and 16.6% in slow losers; p less than 0.001). Prediction of future bone mineral content using baseline bone mineral content and estimated rate of loss gave results almost identical with the actual bone mineral content measured in 1989. Seven women had had a Colles'' fracture and 20 a spinal compression fracture. The group with Colles'' fracture had low baseline bone mineral content (34.7 (95% confidence interval 31.3 to 38.1) units v 39.4 (38.1 to 40.8) units in women with no fracture) whereas the group with spinal fracture had a normal baseline bone mineral content (38.1 (35.0 to 41.1) units) but an increased rate of loss (-2.4 (-3.5 to -1.3)%/year v -1.8 (-2.1 to -1.5)%/year in women with no fracture). CONCLUSIONS--One baseline measurement of bone mass combined with a single estimation of the rate of bone loss can reliably identify the women at menopause who are at highest risk of developing osteoporosis later in life. The rate of loss may have an independent role in likelihood of vertebral fracture.     

Calcitonin, estrogens and the bone

Estrogen deficiency following natural or surgical menopause, is thought to be the main factor leading to postmenopausal bone loss. Furthermore, after estrogen failure a significant reduction of intestinal calcium absorption and a negativization of calcium balance has been observed. The mechanism of estrogen effect on skeletal tissue is not yet fully elucidated. Recently, specific receptors for estrogens in osteoblastic cells have been described; however their low density does not give a full explanation about their functional role. Therefore estrogens act, at least in part, indirectly through calciotropic hormones. In order to further elucidate this issue, we performed some studies in postmenopausal osteoporotic patients and in fertile oophorectomized women. In the first double blind placebo controlled study, after a 1-year estrogen treatment period we observed an increase in bone mineral content in the hormone-treated patients. Furthermore, in all treated patients an improvement of intestinal calcium absorption was detected, while 1,25-dihydroxy-vitamin D serum levels did not show significant changes. To further analyse the relationship between estrogens (E) and calcitonin (CT) in postmenopausal osteoporosis, we performed a double blind placebo controlled study to evaluate the effects of 1-yr estro-progestative treatment on CT secretory reserve, evaluated by calcium infusion test. Blood levels of CT showed a progressive increase during the study period in the hormone-treated group, with a significant increase in the CT response to calcium stimulation test, suggesting a modulation of CT secretion by E. Recently, we performed two studies in fertile oophorectomized women. In the first, we followed longitudinally 24 fertile women for 1 yr. In these patients we measured, before and after oophorectomy, biochemical indexes of bone metabolism and bone mass. During the observation period a significant increase in bone resorption and a significant drop in intestinal calcium absorption was observed. In the second study, performed on 14 women before and 6 months after oophorectomy, a treatment with conjugated estrogens allowed the correction of the primary intestinal defect responsible for the reduced calcium absorption.(ABSTRACT TRUNCATED AT 400 WORDS)     

In vivo anti-osteoporotic activity of isotaxiresinol, a lignan from wood of Taxus yunnanensis

Isotaxiresinol, the main lignan isolated from the water extract of wood of Taxus yunnanensis, was investigated for its effect on bone loss, on serum biochemical markers for bone remodeling and on uterine tissue, using ovariectomized (OVX) rats as the model of postmenopausal osteoporosis. After oral administration of isotaxiresinol (50 and 100mg/kg/d) for 6 weeks, bone mineral content (BMC) and bone mineral density (BMD) in total and cortical bones were increased as compared to those of OVX control rats, and decreases of three bone strength indexes induced by OVX surgery were prevented. Serum biochemical markers for bone remodeling revealed that isotaxiresinol slightly increased bone formation and significantly inhibited bone resorption without side effect on uterine tissue. These results suggest that isotaxiresinol may be useful for treatment of postmenopausal osteoporosis, especially for prevention of bone fracture induced by estrogen deficiency.     

Relationship between serum albumin and bone mineral density in postmenopausal women and in patients with hypoalbuminemia.

Some discrepancies exist about the relationship between serum albumin level and the pathogenesis of osteoporosis; moreover, most of the studies available have especially concerned patients with osteoporosis, often associated with fractures. Our study, therefore, aims to investigate the presence of a relationship between serum albumin level and bone mineral density in a group of healthy women (n=650; mean age 59.0 +/- 7.4 years) who voluntarily underwent screening for osteoporosis only because they were menopausal (11.2 +/- 7.4 years since menopause) and, for comparison, in a group of outpatients (n = 44; mean age 57.6 +/- 7.0 years; 9.1 +/- 6.7 years since menopause) with hypoalbuminemia associated with diseases. The results show a lack of any relationship in healthy women between serum albumin value and bone mineral density; the lack of correlation was also shown when the postmenopausal women were down into normal, osteopenic and osteoporotic (WHO criteria) or in hypo, normal and hyperalbuminemic. The only significant parameters associated with lower bone mineral density, in fact, were age and years since menopause (p<0.0001 and p<0.0001 respectively at lumbar spine and p<0.02 and p<0.001 at femoral neck level). In the group of patients with hypoalbuminemia associated with diseases, on the other hand, a relationship between reduced bone mineral density and hypoalbuminemia was found (p<0.01 and p<0.05 respectively at lumbar spine and femoral neck). In conclusion, in healthy postmenopausal women the serum albumin level does not play a significant role in the pathogenesis of bone density reduction, which is mainly due to the number of years since menopause and advancing age. The hypoalbuminemia may be related to the reduction of bone mass only in the subjects affected by diseases associated with a significant albumin reduction.     

Bone mineral density in Addison's disease: evidence for an effect of adrenal androgens on bone mass.

It is unknown whether replacement doses of cortisone acetate and the absence of the small amounts of androgens secreted by the adrenal cortex may cause osteoporosis. This was studied in 35 patients (12 men and 23 women) suffering from primary adrenocortical failure and taking cortisone acetate 25-37.5 mg and fludrocortisone 50-100 micrograms daily. Bone mineral density was measured by single photon absorptiometry at the midshaft of the radius, representing cortical bone, and at the distal part of the radius, a site with a significant trabecular component. The bone mineral density was normal in premenopausal female patients as well as in male patients, showing that replacement doses of cortisone acetate do not affect bone mass. By contrast, in postmenopausal patients there was a dramatic bone loss in addition to the physiological postmenopausal decrease in bone mass. This loss, combined with the low plasma concentrations of androstenedione, dehydroepiandrosterone, and testosterone (and low concentrations of oestrone of adrenal origin), indicates that adrenal androgens may be essential for the maintenance of bone mass in postmenopausal women with Addison''s disease. In addition, these data indicate that the small amounts of androgens secreted by the adrenal cortex have a role in the maintenance of bone mass in normal postmenopausal women.     

Effects of exercise practice on the maintenance of radius bone mineral density in postmenopausal women

The effect of exercise practice on the radius bone mineral density (BMD) was investigated on 480 women at the age of perimenopause and later. The BMD at the 1/3 distal site of the radius on a non-dominant hand was measured using the Dual Energy X-ray Absorptiometry method. The subjects were divided into group E with regular exercising and group C without regular exercising. The difference of mean radius BMD values was not statistically significant between groups E and C. Each of the groups E and C was further classified into a perimenopause group and groups with stratified postmenopausal years (postmenopausal years less than 5, less than 10 and more than 10). Then, the mean radius BMD value of the perimenopause subgroup was taken as 100% for each of the groups E and C, and the relative radius BMD values of subgroups were compared between groups E and C. The relative BMD values for the subgroup of postmenopausal years more than 10 were higher in E group than those in C group, while no difference was observed in below postmenopausal 10 years between the two groups. To clarify the BMD difference among exercise events, group E was further divided into the following three subgroups: the subgroup practising events which give high-impact to arm (HI group), such as tennis, ping-pong, golf and volleyball, the subgroup practising swimming (SW group), and the subgroup practising events which give low-impact to arm (LI group), such as walking, running, aerobic dance and light gymnastics. No significant differences were observed in BMD values among the above sports event-stratified subgroups when postmenopausal years were less than 10 years, but the relative BMD values for the subgroup of postmenopausal years more than 10 were higher in HI and LI group than those in control group. These results suggest the possibility that exercise practice suppresses a postmenopausal decline in bone density for preventing osteoporosis from developing.     

Estrogen plus estrogen receptor antagonists alter mineral production by osteoblasts in vitro

In early postmenopausal women, estrogen withdrawal is associated with increased bone turnover leading to bone loss and increased risk of fracture. Recent studies have suggested that the remaining bone tissue is significantly stronger, stiffer and has an increased tissue-level mineral content. Such changes may occur to compensate for bone loss or as a direct result of estrogen deficiency. To date many details of the physiology of osteoblastic cells during estrogen deficiency are vague. In this study we test the hypothesis that osteoblastic matrix mineralisation is altered at the onset of estrogen deficiency. In vitro cell culture experiments were carried out up to 28 days to compare the mineral production of MC3T3-E1 osteoblastic cells subject to estrogen deficiency (fulvestrant), enhanced estrogen supplementation (17-β-estradiol) or a combination of both. Mineralisation was detected using von Kossa staining and was quantified with alizarin red absorbance readings. The expression of osteocalcin and osteopontin proteins, markers of osteoblast differentiation and mineralisation, was monitored using immunohistochemistry. Our results demonstrate that estrogen enhancement improves matrix mineralisation by MC3T3 cells in vitro. Furthermore this study found a significant reduction in the level of mineralisation when cells were treated with a combination of estrogen and fulvestrant. In an estrogen deficient environment mineralisation by osteoblastic cells was not altered. These findings suggest that altered tissue mineralisation following estrogen deficiency is not a direct result of estrogen deficiency on osteoblasts. Rather, we propose that altered tissue mineralisation may be a compensatory mechanism by bone to counter bone loss and reduced strength.     

Relation of common allelic variation at vitamin D receptor locus to bone mineral density and postmenopausal bone loss: cross sectional and longitudinal population study.

OBJECTIVE: To determine whether common allelic variation at the vitamin D receptor locus is related to bone mineral density and postmenopausal bone loss. DESIGN: Cross sectional and longitudinal population study. SETTING: Outpatient clinic in research centre. SUBJECTS: 599 healthy women aged 27 to 72 and 125 women with low bone mass aged 55-77 had bone mineral density measured once in the cross sectional study. 136 women aged 45-54 were followed up for 18 years in the longitudinal study. MAIN OUTCOME MEASURES: Bone mineral density measured at the lumbar spine, hip, and forearm and rate of bone loss at different times over 18 years in relation to vitamin D receptor genotype as defined by the endonucleases ApaI, EsmI, and TaqI. RESULTS: Vitamin D receptor genotype was not related to bone mineral density at any site. The maximum difference between homozygotes was 1.3% (P = 0.33, n = 723). Women with low bone mineral density had almost the same genotype frequencies as the women with normal bone mineral densities. Vitamin D receptor genotype was not related to early postmenopausal bone loss from age 51 to 53 (mean (SD) total loss at the lower forearm -3.6% (3.6%)), late postmenopausal bone loss from age 63 to 69 (at the hip-6.2% (8.7%)), or to long term postmenopausal loss from age 51 to 69 (at the lower forearm-24.5% (11.4%)). CONCLUSION: Common allelic variation at the vitamin D receptor locus as defined by the endonucleases ApaI, EsmI, and TaqI is related neither to bone mineral density nor to the rate of bone loss in healthy postmenopausal Danish women.     

The individual age at menopause — An appropriate indicator of postmenopausal body composition?

The interaction between age at menopause and postmenopausal body composition development was tested with in 178 Viennese women aged 47 to 68 years (x=55.4 yr). Postmenopausal body composition was described using dual energy x-ray absorptiometry by absolute fat and lean mass and bone mineral content of the whole body, the arms, legs, the trunk and the head. Upper and lower amount of body fat, the fat percentages of the individual body compartments and the fat distribution index were calculated. Postmenopausal body fat and lean soft tissue mass and postmenopausal bone mineral content were significantly associated with the age at menopause. Women whose menopause occurred late showed the highest amount of body fat (31.2+/−7.7kg) and lean body mass (41.2+/−4.4 kg) postmenopausally, while women with an early menopause exhibited the lowest amount of body fat (27.5+/−8.9kg) and lean body mass (38.4+/−5.4 kg) during the postreproductive phase of life (p<0.05). Women whose menopause occurred later than 51 had a significant higher postmenopausal bone mass (2.26+/−0.9kg versus 2.09+/−0.3 kg; p<0.05). A late menopause was associated with a significantly higher value in fat mass, lean body mass and in bone mineral content. Therefore age at menopause may be assumed as an indicator for body fat and bone mineral content during postmenopause and postmenopausal fat distribution patterns.     

Association of a polymorphism of the CC chemokine receptor-2 gene with bone mineral density

The possible association of the 190G-->A (Val64Ile) polymorphism of the CC chemokine receptor-2 gene (CCR2) with bone mineral density (BMD) was examined in 2215 subjects (1125 men, 1090 women), all of whom were community-dwelling individuals aged 40 to 79 years. Among men aged < 60 years, BMD for the distal radius, lumbar spine, or Ward's triangle was significantly greater in those with the AA genotype than in those with the GG or GA genotypes. For postmenopausal women, BMD for the distal radius or femoral neck was significantly greater in those with the AA genotype than in those with the GG or GA genotypes. In contrast, for men aged > or =60 years and for premenopausal women, BMD was not associated with the CCR2 genotype. These results suggest that CCR2 may be a new candidate for a susceptibility locus for bone mass in middle-aged men and postmenopausal women.     

France Verus Wales

Assessment of roentgenographic measurements of cortical bone of the radius in 196 elderly females, including 63 diabetics, revealed that: (1) in the non-diabetic group there was a significant loss of cortical bone relative to the number of years after the menopause and to body weight; (2) although there was a significant loss of cortical bone relative to years postmenopausal in a group of diabetic patients the cortex in the diabetic group was better preserved than in those non-diabetic controls in whom no vertebral compressions were diagnosed in the roentgenograms; no correlation between bone loss and body weight was found among the diabetics; (3) the thinnest cortical bone and the lowest average body weight was found in the 34 non-diabetics with vertebral compression deformities. It thus appears that involutional osteoporosis will be less prevalent among old women suffering from diabetes mellitus than in comparable non-diabetic subjects, and more prevalent among non-diabetics of low body weight than in old women who are obese or of normal weight.     

Effect of thiazide on rates of bone mineral loss: a longitudinal study.

OBJECTIVE--To determine the effect of thiazide diuretic drugs on rates of bone mineral loss. DESIGN--Longitudinal, observational study with a mean follow up of five years. SETTING--Hawaii Osteoporosis Center, Honolulu. SUBJECTS--1017 Japanese-American men born between 1900 and 1920, of whom 378 were treated for hypertension (study group) and 639 did not have hypertension (control group). INTERVENTION--Thiazide diuretics were taken by 325 men for a mean of 11.9 years; 53 men took antihypertensive drugs other than thiazides. MAIN OUTCOME MEASURE--Rate of bone loss estimated from serial photon absorptiometric scanning at three skeletal sites (calcaneus, distal radius, and proximal radius). RESULTS--Rates of bone loss at all three sites were significantly reduced among thiazide users when compared with controls. The reductions in loss rate ranged from 28.8% (p = 0.02) (distal radius) to 49.2% (p = 0.0005) (calcaneus) relative to the controls. At all three sites the men taking other antihypertensive drugs had faster loss rates (22.6-43.1%) than those of the controls but the difference was significant only for the distal radius. CONCLUSION--Thiazide diuretics slow the rate of bone loss in elderly men.     

Perspectives on using nonhuman primates to understand the etiology and treatment of postmenopausal osteoporosis

The reproductive physiology and skeletal anatomy of nonhuman primates are very similar to those of women and these similarities have prompted studies of the effects of ovariectomy in monkeys on bone metabolism. Following ovariectomy, monkey bone exhibits increases in remodeling activity resulting in bone loss. Since similar bone changes occur after menopause in women, ovariectomized monkeys provide an excellent model of the early skeletal events following menopause and have been employed to study the skeletal actions of drugs designed to treat postmenopausal osteoporosis. This review describes the motivations for examining monkeys, practical aspects of working with monkeys, comparisons of human and monkey bone anatomy, endocrinological aspects of monkey bone metabolism, and the available data obtained in monkeys related to postmenopausal and other forms of osteoporosis.     

Hormone replacement therapy improves distal radius bone structure by endocortical mineral deposition

Hormone replacement therapy (HRT) produces a small increase in bone mineral density (BMD) when measured by dual energy X-ray absorptiometry (DXA). The corresponding decrease in fracture risk is more impressive, implying that other factors that contribute to bone strength are favourably modified by HRT. We investigated, using peripheral quantitated computed tomography (pQCT), the changes produced by HRT in both the distribution of mineral between cortical and trabecular bone and the changes produced by HRT in the apparent structure of trabecular bone, expressed as average hole area and apparent connectivity. Twenty-one postmenopausal women starting HRT and 32 control women were followed for 2 years, with distal radius pQCT measurements every 6 months. HRT prevented the loss of total bone mass seen in controls (p < 0.02). HRT also produced an apparent rapid loss of trabecular bone mass within the first 6 months of the study (p < 0.02), with an associated rapid loss in the apparent connectivity (p = 0.034). Average hole area also increased but not to a statistically significant extent. Exogenous estrogen apparently fills small marrow pores close to the endocortical surface, such that the pQCT-defined boundary between trabecular and cortical bone is shifted in favour of cortical bone. Trabecular bone structure indices are adversely affected, as the central, poorly interconnected trabecular bone with greater than average marrow spaces constitutes a greater fraction of the remaining trabecular bone. This study suggests that the improvements in fracture risk resulting from HRT are explained by a reversal of net endocortical resorption of bone.     

Prolyl Hydroxylase Inhibitors Protect from the Bone Loss in Ovariectomy Rats by Increasing Bone Vascularity

The hypoxia-inducible factor-1α (HIF-1α)/vascular endothelial growth factor (VEGF) pathway is involved in skeletal development, bone repair, and postmenopausal osteoporosis. Inhibitors of prolyl hydroxylases (PHD) enhance vascularity, increase callus formation in a stabilized fracture model, and activate the HIF-1α/VEGF pathway. This study examined the effects of estrogen on the HIF-1α/VEGF pathway in osteoblasts and whether PHD inhibitors can protect from bone loss in postmenopausal osteoporosis. Osteoblasts were treated with estrogen, and expressions of HIF-1α and VEGF were measured at mRNA (qPCR) and protein (Western blot) levels. Further, osteoblasts were treated with inhibitors of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway, and levels of VEGF mRNA and protein expression were detected. In addition, ovariectomized rats were treated with PHD inhibitors, and bone microarchitecture and bone mechanical strength were assessed using micro-CT and biomechanical analyses (lower ultimate stress, modulus, and stiffness). Blood vessel formation was measured with India Ink Perfusion and immunohistochemistry. Estrogen, in a dose- and time-dependent manner, induced VEGF expression at both mRNA and protein levels and enhanced HIF-1α protein stability. Further, the estrogen-induced VEGF expression in osteoblasts involved the PI3K/Akt pathway. PHD inhibitors increased bone mineral density, bone microarchitecture and bone mechanical strength, and promoted blood vessel formation in ovariectomized rats. In conclusion, estrogen and PHD inhibitors activate the HIF-1α/VEGF pathway in osteoblasts. PHD inhibitors can be utilized to protect bone loss in postmenopausal osteoporosis by improving bone vascularity and angiogenesis in bone marrow.