Evaluation of cross-sectional and longitudinal changes in volumetric bone mineral density in postmenopausal women using single- versus dual-energy quantitative computed tomography

Central quantitative computed tomography (QCT) is increasingly used in clinical trials and practice to assess bone mass or strength and to evaluate longitudinal changes in response to drug treatment. Current studies utilize single-energy (SE) QCT scans, which may be confounded both by the amount of bone marrow fat at baseline and changes in marrow fat over time. However, the extent to which marrow fat changes either underestimate volumetric BMD (vBMD) measurements at baseline or under-/overestimate longitudinal changes in vivo in humans remains unclear. To address this issue, 197 early postmenopausal women [median age (IQR) 56.7 (54.4-58.7) years] underwent spine and hip QCT scans at baseline and 3 years using a 128-slice dual-source dual-energy (DE) scanner. The scans were analyzed as either SE scans (100 kVp) or DE scans (100 kVp and 140 kVp), with the latter accounting for bone marrow fat. At baseline, vertebral trabecular vBMD was (median) 17.6% lower (P < 0.001) while femur neck (FN) cortical vBMD was only 3.2% lower (P < 0.001) when assessed by SE vs DE scanning. SE scanning overestimated the 3 year rate of bone loss for trabecular bone at the spine by 24.2% (P < 0.001 vs DE rates of loss) but only by 8.8% for changes in FN cortical vBMD (P < 0.001 vs DE rates of loss). The deviation between SE and DE rates of bone loss in trabecular vBMD became progressively greater as the rate of bone loss increased. These findings demonstrate that SE QCT scans underestimate trabecular vBMD and substantially overestimate rates of age-related bone loss due to ongoing conversion of red to yellow marrow. Further, the greater the rate of bone loss, the greater the overestimation of bone loss by SE scans. Although our findings are based on normal aging, recent evidence from animal studies demonstrates that the skeletal anabolic drugs teriparatide and romosozumab may markedly reduce marrow fat, perhaps accounting for the disproportionate increases in trabecular vBMD by SE QCT as compared to dual-energy X-ray absorptiometry with these agents. As such, future studies using recently available DE scanning technology that has satisfactory precision and radiation exposure are needed to evaluate changes in trabecular vBMD independent of changes in marrow fat with aging and drugs that may alter marrow fat composition.

Effects of Age and Estrogen on Skeletal Gene Expression in Humans as Assessed by RNA Sequencing

Precise delineation of the specific genes and pathways altered with aging and estrogen (E) therapy may lead to new skeletal biomarkers and the development of novel bone therapeutics. Previous human bone studies, however, have been limited by only examining pre-specified genes and pathways. High-throughput RNA sequencing (RNAseq), on the other hand, offers an unbiased approach to examine the entire transcriptome. Here we present an RNAseq analysis of human bone samples, obtained from iliac crest needle biopsies, to yield the first in vivo interrogation of all genes and pathways that may be altered in bone with aging and E therapy in humans. 58 healthy women were studied, including 19 young women (mean age ± SD, 30.3 ± 5.4 years), 19 old women (73.1 ± 6.6 years), and 20 old women treated with 3 weeks of E therapy (70.5 ± 5.2 years). Using generally accepted criteria (false discovery rate [q] < 0.10), aging altered a total of 678 genes and 12 pathways, including a subset known to regulate bone metabolism (e.g., Notch). Interestingly, the LEF1 transcription factor, which is a classical downstream target of the Wnt/β-catenin signaling pathway, was significantly downregulated in the bones from the old versus young women; consistent with this, LEF1 binding sites were significantly enriched in the promoter regions of the differentially expressed genes in the old versus young women, suggesting that aging was associated with alterations in Wnt signaling in bone. Further, of the 21 unique genes altered in bone by E therapy, the expression of INHBB (encoding for the inhibin, beta B polypeptide), which decreased with aging (by 0.6-fold), was restored to young adult levels in response to E therapy. In conclusion, our data demonstrate that aging alters a substantial portion of the skeletal transcriptome, whereas E therapy appears to have significant, albeit less wide-ranging effects. These data provide a valuable resource for the potential identification of novel biomarkers associated with age-related bone loss and also highlight potential pathways that could be targeted to treat osteoporosis.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02349113.

Global transcriptional profiling using RNA sequencing and DNA methylation patterns in highly enriched mesenchymal cells from young versus elderly women

Age-related bone loss in humans is associated with a decrease in bone formation relative to bone resorption, although the mechanisms for this impairment in bone formation with aging are not well understood. It is known that the precursors for the bone-forming osteoblasts reside in the mesenchymal cell population in bone marrow. Thus, in an effort to identify relevant genetic pathways that are altered with aging, we examined the gene expression and DNA methylation patterns from a highly enriched bone marrow mesenchymal cell population from young (mean age, 28.7 years) versus old (mean age, 73.3 years) women. Bone marrow mononuclear cells from these women were depleted of hematopoietic lineage (lin) and endothelial cells using a combination of magnetic- and fluorescence-activated cell sorting, yielding a previously characterized mesenchymal cell population (lin-/CD34-/CD31- cells) that is capable of osteoblast differentiation. Whole transcriptome RNA sequencing (RNAseq) of freshly isolated cells (without in vitro culture) identified 279 differentially expressed genes (p < 0.05, false discovery rate [q]< 0.10) between the young and old subjects. Pathway analysis revealed statistically significant (all p < 0.05) alterations in protein synthesis and degradation pathways, as well as mTOR, gap junction, calcium, melatonin and NFAT signaling pathways. Further, Reduced Representational Bisulphite sequencing (RRBS DNA methylation sequencing) revealed significant differences in methylation between the young and old subjects surrounding the promoters of 1528 target genes that also exhibited significant differences in gene expression by RNAseq. In summary, these studies provide novel insights into potential pathways affected by aging in a highly enriched human mesenchymal cell population analyzed without the confounding effects of in vitro culture. Specifically, our finding of alterations in several genes and pathways leading to impaired protein synthesis and turnover with aging in bone marrow mesenchymal cells points to the need for further studies examining how these changes, as well as the other alterations with aging that we identified, may contribute to the age-related impairment in osteoblast formation and/or function.

Body composition during childhood and adolescence: relations to bone strength and microstructure

CONTEXT

Numerous studies have examined the association of body composition with bone development in children and adolescents, but none have used micro-finite element (μFE) analysis of high-resolution peripheral quantitative computed tomography images to assess bone strength.

OBJECTIVE

This study sought to examine the relations of appendicular lean mass (ALM) and total body fat mass (TBFM) to bone strength (failure load) at the distal radius and tibia.

DESIGN, PARTICIPANTS, AND SETTING

This was a cross-sectional study of 198 healthy 8- to <15-year-old boys (n = 109) and girls (n = 89) performed in a Clinical Research Unit.

RESULTS

After adjusting for bone age, height, fracture history, ALM, and TBFM, multiple linear regression analyses in boys and girls, separately, showed robust positive associations between ALM and failure loads at both the distal radius (boys: β = 0.92, P < .001; girls: β = 0.66, P = .001) and tibia (boys: β = 0.96, P < .001; girls: β = 0.66, P < .001). By contrast, in both boys and girls the relationship between TBFM and failure load at the distal radius was virtually nonexistent (boys: β = -0.07; P = .284; girls: β = -0.03; P = .729). At the distal tibia, positive, albeit weak, associations were observed between TBFM and failure load in both boys (β = 0.09, P = .075) and girls (β = 0.17, P = .033).

CONCLUSIONS

Our data highlight the importance of lean mass for optimizing bone strength during growth, and suggest that fat mass may have differential relations to bone strength at weight-bearing vs non-weight-bearing sites in children and adolescents. These observations suggest that the strength of the distal radius does not commensurately increase with excess gains in adiposity during growth, which may result in a mismatch between bone strength and the load experienced by the distal forearm during a fall. These findings may explain, in part, why obese children are over-represented among distal forearm fracture cases.

Diminished bone strength is observed in adult women and men who sustained a mild trauma distal forearm fracture during childhood

Children and adolescents who sustain a distal forearm fracture (DFF) owing to mild, but not moderate, trauma have reduced bone strength and cortical thinning at the distal radius and tibia. Whether these skeletal deficits track into adulthood is unknown. Therefore, we studied 75 women and 75 men (age range, 20 to 40 years) with a childhood (age < 18 years) DFF and 150 sex-matched controls with no history of fracture using high-resolution peripheral quantitative computed tomography (HRpQCT) to examine bone strength (ie, failure load) by micro-finite element (µFE) analysis, as well as cortical and trabecular bone parameters at the distal radius and tibia. Level of trauma (mild versus moderate) was assigned using a validated classification scheme, blind to imaging results. When compared to sex-matched, nonfracture controls, women and men with a mild trauma childhood DFF (eg, fall from standing height) had significant reductions in failure load (p < 0.05) of the distal radius, whereas women and men with a moderate trauma childhood DFF (eg, fall while riding a bicycle) had values similar to controls. Consistent findings were observed at the distal tibia. Furthermore, women and men with a mild trauma childhood DFF had significant deficits in distal radius cortical area (p < 0.05), and significantly lower dual-energy X-ray absorptiometry (DXA)-derived bone density at the radius, hip, and total body regions compared to controls (all p < 0.05). By contrast, women and men with a moderate trauma childhood DFF had bone density, structure, and strength that did not differ significantly from controls. These findings in young adults are consistent with our observations in children/adolescents with DFF, and they suggest that a mild trauma childhood DFF may presage suboptimal peak bone density, structure, and strength in young adulthood. Children and adolescents who suffer mild trauma DFFs may need to be targeted for lifestyle interventions to help achieve improved skeletal health.

In vivo assessment of bone quality in postmenopausal women with type 2 diabetes

Although patients with type 2 diabetes (T2D) are at significant risk for well-recognized diabetic complications, including macrovascular disease, retinopathy, nephropathy, and neuropathy, it is also clear that T2D patients are at increased risk for fragility fractures. Furthermore, fragility fractures in patients with T2D occur at higher bone mineral density (BMD) values compared to nondiabetic controls, suggesting abnormalities in bone material strength (BMS) and/or bone microarchitecture (bone "quality"). Thus, we performed in vivo microindentation testing of the tibia to directly measure BMS in 60 postmenopausal women (age range, 50-80 years) including 30 patients diagnosed with T2D for >10 years and 30 age-matched, nondiabetic controls. Regional BMD was measured by dual-energy X-ray absorptiometry (DXA); cortical and trabecular bone microarchitecture was assessed from high-resolution peripheral quantitative computed tomography (HRpQCT) images of the distal radius and tibia. Compared to controls, T2D patients had significantly lower BMS: unadjusted (-11.7%; p<0.001); following adjustment for body mass index (BMI) (-10.5%; p<0.001); and following additional adjustment for age, hypertension, nephropathy, neuropathy, retinopathy, and vascular disease (-9.2%; p=0.022). By contrast, after adjustment for confounding by BMI, T2D patients had bone microarchitecture and BMD that were not significantly different than controls; however, radial cortical porosity tended to be higher in the T2D patients. In addition, patients with T2D had significantly reduced serum markers of bone turnover (all p<0.001) compared to controls. Of note, in patients with T2D, the average glycated hemoglobin level over the previous 10 years was negatively correlated with BMS (r=-0.41; p=0.026). In conclusion, these findings represent the first demonstration of compromised BMS in patients with T2D. Furthermore, our results confirm previous studies demonstrating low bone turnover in patients with T2D and highlight the potential detrimental effects of prolonged hyperglycemia on bone quality. Thus, the skeleton needs to be recognized as another important target tissue subject to diabetic complications. © 2014 American Society for Bone and Mineral Research.

Bone strength and structural deficits in children and adolescents with a distal forearm fracture resulting from mild trauma

Although distal forearm fractures (DFFs) are common during childhood and adolescence, it is unclear whether they reflect underlying skeletal deficits or are simply a consequence of the usual physical activities, and associated trauma, during growth. Therefore, we examined whether a recent DFF, resulting from mild or moderate trauma, is related to deficits in bone strength and cortical and trabecular bone macro- and microstructure compared with nonfracture controls. High-resolution peripheral quantitative computed tomography was used to assess micro-finite element-derived bone strength (ie, failure load) and to measure cortical and trabecular bone parameters at the distal radius and tibia in 115 boys and girls with a recent (<1 year) DFF and 108 nonfracture controls aged 8 to 15 years. Trauma levels (mild versus moderate) were assigned based on a validated classification scheme. Compared with sex-matched controls, boys and girls with a mild-trauma DFF (eg, fall from standing height) showed significant deficits at the distal radius in failure load (-13% and -11%, respectively; p < 0.05) and had higher ("worse") fall load-to-strength ratios (both +10%; p < 0.05 for boys and p = 0.06 for girls). In addition, boys and girls with a mild-trauma DFF had significant reductions in cortical area (-26% and -23%, respectively; p < 0.01) and cortical thickness (-14% and -13%, respectively; p < 0.01) compared with controls. The skeletal deficits in the mild-trauma DFF patients were generalized, as similar changes were present at the distal tibia. By contrast, both boys and girls with a moderate-trauma DFF (eg, fall from a bicycle) had virtually identical values for all of the measured bone parameters compared with controls. In conclusion, DFFs during growth have two distinct etiologies: those owing to underlying skeletal deficits leading to fractures with mild trauma versus those owing to more significant trauma in the setting of normal bone strength.

Effects of age on bone mRNA levels of sclerostin and other genes relevant to bone metabolism in humans

Although aging is associated with a decline in bone formation in humans, the molecular pathways contributing to this decline remain unclear. Several previous clinical studies have shown that circulating sclerostin levels increase with age, raising the possibility that increased production of sclerostin by osteocytes leads to the age-related impairment in bone formation. Thus, in the present study, we examined circulating sclerostin levels as well as bone mRNA levels of sclerostin using quantitative polymerase chain reaction (QPCR) analyses in needle bone biopsies from young (mean age, 30.0years) versus old (mean age, 72.9years) women. In addition, we analyzed the expression of genes in a number of pathways known to be altered with skeletal aging, based largely on studies in mice. While serum sclerostin levels were 46% higher (p<0.01) in the old as compared to the young women, bone sclerostin mRNA levels were no different between the two groups (p=0.845). However, genes related to notch signaling were significantly upregulated (p=0.003 when analyzed as a group) in the biopsies from the old women. In an additional analysis of 118 genes including those from genome-wide association studies related to bone density and/or fracture, BMP/TGFβ family genes, selected growth factors and nuclear receptors, and Wnt/Wnt-related genes, we found that mRNA levels of the Wnt inhibitor, SFRP1, were significantly increased (by 1.6-fold, p=0.0004, false discovery rate [q]=0.04) in the biopsies from the old as compared to the young women. Our findings thus indicate that despite increases in circulating sclerostin levels, bone sclerostin mRNA levels do not increase in elderly women. However, aging is associated with alterations in several key pathways and genes in humans that may contribute to the observed impairment in bone formation. These include notch signaling, which represents a potential therapeutic target for increasing bone formation in humans. Our studies further identified mRNA levels of SFRP1 as being increased in aging bone in humans, suggesting that this may also represent a viable target for the development of anabolic therapies for age-related bone loss and osteoporosis.

Effects of estrogen on bone mRNA levels of sclerostin and other genes relevant to bone metabolism in postmenopausal women

CONTEXT

Studies in postmenopausal women have shown that estrogen reduces circulating sclerostin levels, but effects of estrogen on skeletal sclerostin mRNA levels are unknown.

OBJECTIVE

The objective of the study was to evaluate the effects of short-term estrogen treatment on bone mRNA levels of sclerostin and other genes relevant to bone metabolism.

DESIGN, SETTING, AND PATIENTS

Needle bone biopsies were obtained from 20 postmenopausal women treated with transdermal estrogen for 3 weeks and 20 untreated controls. Quantitative PCR analyses were used to examine the expression of sclerostin and other genes related to bone metabolism, including 71 additional genes linked to bone density/fracture from genome-wide association studies.

RESULTS

Estrogen treatment was associated with lower bone sclerostin mRNA levels (by 48%, P<.05) and with lower (by 54%, P<.01) mRNA levels of the sclerostin-related protein, sclerostin domain-containing protein 1 (SOSTDC1), which is also a Wnt/bone morphogenetic protein inhibitor. Consistent with studies in mice showing that ovariectomy increased nuclear factor-κB (NF-κB) activation, we found that estrogen treatment was associated with a significant reduction in inflammatory genes as a group (P=.028), with bone mRNA levels of NFKB2 and RELB (both encoding proteins in the NF-κB transcription factor complex) being significantly reduced individual genes. Eight of the 71 genome-wide association study-related genes examined were modulated by estrogen (P<.05, false discovery rate<0.10).

CONCLUSION

In humans, estrogen-induced decreases in two key inhibitors of Wnt/bone morphogenetic protein signaling, sclerostin and SOSTDC1, along with reductions in NF-κB signaling, may be responsible for at least part of the protective effects of estrogen on bone.

Three-dimensional structural analysis of the proximal femur in an age-stratified sample of women

INTRODUCTION

Aging is associated with worsening bone structure and increasing risk of hip fracture. However, the commonly used clinical tool, dual-energy x-ray absorptiometry, does not provide information on changes with age or disease separately in trabecular versus cortical bone or in bone geometry. Here we used 3D quantitative computed tomography (QCT) to analyze age-related changes in femoral volumetric bone mineral density (vBMD) and structure in a well characterized, population-based cohort of Rochester, Minnesota women.

METHODS

MIAF-Femur (MIAF: medical image analysis framework) was used for the analysis of CT datasets from 358 women age 20 to 97 years. Integral, "apparent" cortical (rather than true cortical vBMD, due to volume averaging effects) and trabecular vBMD, volume, and bone mineral content (BMC) as well as cortical thickness of the femur head, neck, trochanter, inter-trochanteric, and proximal shaft volumes of interest (VOIs) were measured. In addition, changes in vBMD in the superior, inferior, posterior and anterior quadrants of the femur neck were assessed.

RESULTS

Cross-sectional percent decreases in vBMD across life were 2- to 5-fold higher in trabecular versus cortical bone at all sites in the femur, although absolute changes in the trabecular and cortical bone were fairly similar. In addition, the slopes of the relationships of trabecular vBMD with age were generally similar in pre- and postmenopausal women, whereas apparent cortical vBMD in the femur neck, trochanter, inter-trochanteric region, and proximal shaft remained relatively stable in premenopausal women but decreased significantly with age following the menopause. Bone volume increased at all sites, more so in pre- compared to postmenopausal women. Age-related BMC changes were not significant in premenopausal women, but BMC losses were highly significant in postmenopausal women. Detailed analyses of femur neck cortical bone showed that percent apparent vBMD decreases in the superior quadrants were 2- to 3-fold greater than in the inferior quadrants; changes in absolute values were most different (~2-fold) between the superior-posterior and inferior-posterior quadrants.

CONCLUSIONS

These data demonstrate that patterns of changes with age within the femur differ in the trabecular versus cortical bone. In the cortical compartment which, due to limitations in spatial resolution, contains some subcortical bone and should be regarded as an "apparent" cortical VOI, the superior quadrants in the femur neck undergo the greatest decreases. These findings may have important implications for understanding the structural basis for increased hip fracture risk with aging.

Effects of bisphosphonate treatment on circulating osteogenic endothelial progenitor cells in postmenopausal women

OBJECTIVE

To evaluate whether bisphosphonates modulate vascular calcification by a modification in endothelial progenitor cells (EPCs) coexpressing osteoblastic surface markers and genes.

PATIENTS AND METHODS

We performed a double-blind, randomized study of 20 healthy, early postmenopausal women (from February 1, 2008, through July 31, 2008) treated with placebo or risedronate sodium (35 mg/wk) for 4 months. Peripheral blood was collected at baseline and 4 months to determine serum inflammatory markers, osteoprotegerin, and receptor activator of nuclear factor-κB ligand levels and bone turnover markers. Peripheral blood mononuclear cells were stained for EPC surface markers (CD34, CD133, and vascular endothelial growth factor receptor/kinase insert domain receptor) and osteoblast markers (osteocalcin, alkaline phosphatase, and Stro-1).

RESULTS

Risedronate treatment resulted in a significant down-regulation of gene sets for osteoblast differentiation and proliferation in EPCs with a trend of decreasing EPCs coexpressing osteocalcin.

CONCLUSION

Our findings indicate that bisphosphonate treatment down-regulates the expression of osteogenic genes in EPCs and suggest a possible mechanism by which bisphosphonates may inhibit vascular calcification.

Relationship of sympathetic activity to bone microstructure, turnover, and plasma osteopontin levels in women

CONTEXT

Studies in rodents have demonstrated that sympathetic activity reduces bone formation and bone mass; these effects are mediated by the noncollagenous matrix protein, osteopontin.

OBJECTIVE

The objective of the study was to relate sympathetic activity (measured using microneurography at the peroneal nerve) to bone microstructure (assessed by high resolution peripheral quantitative computed tomography), bone turnover, and plasma osteopontin levels.

DESIGN, SETTING, AND PATIENTS

Twenty-three women aged 20-72 yr (10 premenopausal and 13 postmenopausal) were studied in the Clinical Research Unit.

RESULTS

Sympathetic activity (bursts per 100 heart beats) was 2.4-fold higher in postmenopausal as compared with premenopausal women (P < 0.001). In the two groups combined and after age adjustment, sympathetic activity was inversely correlated with trabecular bone volume fraction (r = -0.55, P < 0.01) and thickness (r = -0.59, P < 0.01) and positively correlated with trabecular separation (r = 0.45, P < 0.05). Sympathetic activity was negatively correlated with serum amino-terminal propeptide of type I collagen in postmenopausal women (r = -0.65, P = 0.015), with a similar trend in premenopausal women (r = -0.58, P = 0.082). Sympathetic activity was also negatively correlated with plasma osteopontin levels (r = -0.43, P = 0.045), driven mainly by the correlation in postmenopausal women (r = -0.76, P = 0.002).

CONCLUSION

These findings represent the first demonstration in humans of a relationship between sympathetic activity and bone microstructure and circulating levels of amino-terminal propeptide of type I collagen and osteopontin. Given the critical role of osteopontin in mediating the effects of β-adrenergic signaling on bone, the inverse association between sympathetic activity and plasma osteopontin levels may reflect a negative feedback loop to limit the deleterious effects of sympathetic activity on bone metabolism. Based on the higher sympathetic activity observed in postmenopausal women, additional human studies are needed to define the role of increased sympathetic activity in mediating postmenopausal bone loss.

Sclerostin levels during growth in children

Serum sclerostin levels are associated with cortical porosity, suggesting that changes in sclerostin production during growth may play a role in defining cortical structure.

INTRODUCTION

Sclerostin, produced by osteocytes, is a potent inhibitor of Wnt signaling and bone formation. While sclerostin levels increase with age in adults and are higher in men compared to women, there is currently no information on changes in circulating sclerostin levels during growth in humans.

METHODS

We measured serum sclerostin levels in 6- to 21-year-old girls (n = 62) and boys (n = 56) and related these to trabecular and cortical bone microarchitectural parameters using high-resolution peripheral quantitative computed tomography and to markers of bone turnover.

RESULTS

Serum sclerostin levels were higher in boys as compared to girls and declined in both sexes following the onset of puberty. There was no consistent relationship between sclerostin levels and trabecular bone parameters in either sex. However, serum sclerostin levels were inversely associated with cortical volumetric bone mineral density and cortical thickness in girls and positively associated with the cortical porosity index in both girls and boys. Bone turnover markers were positively correlated with serum sclerostin levels in both sexes.

CONCLUSION

The gender difference in serum sclerostin levels appears to be established during puberty, and sclerostin levels tend to decline in late puberty in both girls and boys. Serum sclerostin levels are associated with cortical porosity, suggesting that changes in sclerostin production during growth may play a role in defining cortical structure.

Characterization of mesenchymal progenitor cells isolated from human bone marrow by negative selection

Studies on the pathogenesis of osteoporosis and other metabolic bone diseases would be greatly facilitated by the development of approaches to assess changes in gene expression in osteoblast/osteoprogenitor populations in vivo without the potentially confounding effects of in vitro culture and expansion of the cells. While positive selection to identify a progenitor population in human marrow can be used to select for cells capable of osteoblast differentiation, each of the markers that have been used to identify marrow mesenchymal populations (alkaline phosphatase [AP], Stro-1, CD29, CD49a, CD73, CD90, CD105, CD166, CD44, CD146 and CD271) may be expressed on distinct subsets of marrow mesenchymal cells. Thus, positive selection with one or more of these markers could exclude a possibly relevant cell population that may undergo important changes in various clinical conditions. In the present report, we describe the isolation and characterization of human osteoprogenitor cells obtained by depletion of bone marrow cells of all hematopoietic lineage/hematopoietic stem cells and endothelial/endothelial precursor cells (lin-/CD34/CD31-). The yield of lin-/CD34/CD31- cells from ~10 mL of bone marrow (~80 million mononuclear cells) was ~80,000 cells (0.1% of mononuclear cells). While not selected on the basis of expression for the mesenchymal marker, Stro-1, 68% of these cells were Stro-1+. Using linear whole transcriptome amplification followed by quantitative polymerase chain reaction (QPCR) analysis, we also demonstrated that, compared to lin- cells (which are already depleted of hematopoietic cells), lin-/CD34/31- cells expressed markedly lower mRNA levels for the endothelial/hematopoietic markers, CD34, CD31, CD45, and CD133. Lin-/CD34/31- cells were also enriched for the expression of mesenchymal/osteoblastic markers, with a further increase in runx2, osterix, and AP mRNA expression following in vitro culture under osteogenic conditions. Importantly, lin-/CD34/31- cells contained virtually all of the mineralizing cells in human marrow: while these cells displayed robust calcium deposition in vitro, lin-/CD34/31+ cells demonstrated little or no mineralization when cultured under identical osteogenic conditions. Lin-/CD34/31- cells thus represent a human bone marrow population highly enriched for mesenchymal/osteoblast progenitor cells that can be analyzed without in vitro culture in various metabolic bone disorders, including osteoporosis and aging.

Effects of intermittent parathyroid hormone treatment on osteoprogenitor cells in postmenopausal women

Intermittent parathyroid hormone (PTH) 1-34 treatment stimulates bone formation, but the molecular mechanisms mediating this effect have not been previously studied in humans. Thus, we used magnetic activated cell sorting to isolate hematopoietic lineage negative (lin-)/alkaline phosphatase positive (AP+) osteoprogenitor cells from bone marrow of 20 postmenopausal women treated with PTH (1-34) for 14 days and 19 control subjects. Serum PINP and CTX increased in PTH-treated subjects (by 97% and 30%, respectively, P<0.001). Bone marrow lin-/AP+ cells from PTH-treated subjects showed an increase in the RANKL/OPG mRNA ratio (by 7.5-fold, P=0.011) and in the mRNAs for c-fos (a known PTH-responsive gene, by 42%, P=0.035) and VEGF-C (by 57%, P=0.046). Gene Set Enrichment Analysis (GSEA, testing for changes in pre-specified pathways) demonstrated that PTH had no effect on osteoblast proliferation, apoptosis, or differentiation markers. However, PTH treatment resulted in a significant decrease (GSEA P-value, 0.005) in a panel of BMP target genes in the lin-/AP+ cells. Our findings thus identify several future directions for studying mechanisms of PTH action in humans. First, given the increasing evidence that PTH induces angiogenesis, the role of increased VEGF-C production by bone marrow osteoprogenitor cells in mediating this effect and the anabolic response to PTH warrants further study. Second, while the observed inhibition of BMP target gene expression by PTH is not consistent with the anabolic effects of PTH on bone and requires further validation, these data do generate the hypothesis that an inhibition of BMP signaling by PTH may, over time, limit the availability of mature osteoblasts on bone surfaces and thereby contribute to the observed waning of the anabolic response to PTH.

Effects of estrogen on osteoprogenitor cells and cytokines/bone-regulatory factors in postmenopausal women

Decreases in estrogen levels contribute not only to early postmenopausal bone loss but also to bone loss with aging. While estrogen is critical for the maintenance of bone formation, the mechanism(s) of this effect remain unclear. Thus, we assessed the effects of 4months of transdermal estradiol treatment (0.05mg/day) of postmenopausal women as compared to no treatment (n=16 per group) on the expression of genes in pre-specified pathways in freshly isolated bone marrow osteoprogenitor cells (hematopoietic lineage [lin]-/Stro1+). We also evaluated whether estrogen treatment modulated peripheral blood or bone marrow plasma levels of the Wnt antagonists, sclerostin and DKK1, as well as serotonin, OPG, RANKL, adiponectin, oxytocin, and inflammatory cytokines (TNFα, IL-1β, and IL-6), as each of these molecules have recently been shown to play an important role in regulating osteoblast function and/or being responsive to estrogen. We observed a significant decrease in the expression of several proliferation markers (cyclin B1, cyclin E1, E2F1) and increase in adhesion molecules (N-cadherin) in bone marrow lin-/Stro1+ cells from estrogen-treated compared to control women. None of the peripheral blood or bone marrow plasma marker levels differed between the two groups, with the exception of sclerostin levels, which were significantly lower in the estrogen-treated as compared to the control women in peripheral serum (by 32%, P=0.009) and in bone marrow plasma (by 34%, P=0.017). There were significant differences in bone marrow versus peripheral plasma levels of several factors: sclerostin and OPG levels were higher in bone marrow as compared to peripheral plasma, whereas serotonin and adiponectin levels were higher in peripheral as compared to bone marrow plasma. In summary, our data directly assessing possible regulation by estrogen of osteoprogenitor cells in humans indicate that, consistent with previous studies in mice, estrogen suppresses the proliferation of human bone marrow lin-/Stro1+ cells, which likely represent early osteoprogenitor cells. Further animal and human studies are needed to define the role of the changes we observed in mRNAs for adhesion molecules in these cells and in local sclerostin production in bone in mediating the effects of estrogen on bone metabolism in humans.

Trabecular bone deficits among Vietnamese immigrants

SUMMARY

Compared to white women, lower areal bone mineral density (aBMD) in middle-aged Vietnamese immigrants is due to reduced trabecular volumetric bone mineral density (vBMD), which in turn is associated with greater trabecular separation along with lower estrogen levels.

INTRODUCTION

The epidemiology of osteoporosis in Asian populations is still poorly known, but we previously found a deficit in lumbar spine aBMD among postmenopausal Southeast Asian women, compared to white women, that persisted after correction for bone size. This issue was revisited using more sophisticated imaging techniques.

METHODS

Twenty Vietnamese immigrants (age, 44-79 years) were compared to 162 same-aged white women with respect to aBMD at the hip, spine and wrist, vBMD at the hip and spine by quantitative computed tomography and vBMD and bone microstructure at the ultradistal radius by high-resolution pQCT. Bone turnover and sex steroid levels were assessed in a subset (20 Vietnamese and 40 white women).

RESULTS

The aBMD was lower at all sites among the Vietnamese women, but femoral neck vBMD did not differ from middle-aged white women. Significant differences in lumbar spine and ultradistal radius vBMD in the Vietnamese immigrants were due to lower trabecular vBMD, which was associated with increased trabecular separation. Bone resorption was elevated and bone formation depressed among the Vietnamese immigrants, although trends were not statistically significant. Serum estradiol was positively associated with trabecular vBMD in the Vietnamese women, but their estrogen levels were dramatically lower compared to white women.

CONCLUSIONS

Although reported discrepancies in aBMD among Asian women are mainly an artifact of smaller bone size, we identified a specific deficit in the trabecular bone among a sample of Vietnamese immigrants that may be related to low estrogen levels and which needs further study.

Relation of age, gender, and bone mass to circulating sclerostin levels in women and men

Sclerostin is a potent inhibitor of Wnt signaling and bone formation. However, there is currently no information on the relation of circulating sclerostin levels to age, gender, or bone mass in humans. Thus we measured serum sclerostin levels in a population-based sample of 362 women [123 premenopausal, 152 postmenopausal not on estrogen treatment (ET), and 87 postmenopausal on ET] and 318 men, aged 21 to 97 years. Sclerostin levels (mean ± SEM) were significantly higher in men than women (33.3 ± 1.0 pmol/L versus 23.7 ± 0.6 pmol/L, p < .001). In pre- and postmenopausal women not on ET combined (n = 275) as well as in men, sclerostin levels were positively associated with age (r = 0.52 and r = 0.64, respectively, p < .001 for both). Over life, serum sclerostin levels increased by 2.4- and 4.6-fold in the women and men, respectively. Moreover, for a given total-body bone mineral content, elderly subjects (age ≥ 60 years) had higher serum sclerostin levels than younger subjects (ages 20 to 39 years). Our data thus demonstrate that (1) men have higher serum sclerostin levels than women, (2) serum sclerostin levels increase markedly with age, and (3) compared with younger subjects, elderly individuals have higher serum sclerostin levels for a given amount of bone mass. Further studies are needed to define the cause of the age-related increase in serum sclerostin levels in humans as well as the potential role of this increase in mediating the known age-related impairment in bone formation.

Effects of estrogen and testosterone on resting energy expenditure in older men

The mechanisms by which sex hormones cause changes in body composition are unclear. Sex steroid deficiency might directly reduce energy expenditure/fat oxidation and thereby predispose to increased body fat. Alternatively, sex steroid deficiency could result in lean tissue loss and thus reduced energy expenditure. Our objective was to examine the independent and combined effects of acute testosterone and estrogen withdrawal on respiratory exchange ratio (RER) and resting energy expenditure (REE) in men. The objective of the study was to examine the independent and combined effects of acute estrogen and testosterone withdrawal on RER and REE in men. A total of 54 men aged 50-80 years, BMI range of 17-35 kg/m(2) underwent a 3-week eugonadal run-in hormone-treatment period involving suppression of endogenous sex steroids using letrozole and leuprolide acetate (Lupron) while sex steroid concentrations were maintained with transdermal testosterone (T) and estradiol (E). A second Lupron injection was then given and participants were randomized to one of the following four 3-week treatment groups: group A (-T, -E), group B (-T, +E), group C (+T, -E), and group D (+T, +E). REE and RER were measured via indirect calorimetry before and after the 3-week treatment period. Three-week suppression and/or repletion of estrogen or testosterone did not produce changes in RER or REE within or between groups. We conclude that abrupt changes in sex steroids does not change resting substrate oxidation, indicating that changes that can be observed after more prolonged periods of deficiency are most likely due to direct effects of sex steroids on body composition.

Effects of parathyroid hormone treatment on circulating sclerostin levels in postmenopausal women

CONTEXT

Intermittent PTH treatment stimulates bone formation, but the mechanism(s) of this effect remain unclear. Sclerostin is an inhibitor of Wnt signaling, and animal studies have demonstrated that PTH suppresses sclerostin production.

OBJECTIVE

The objective of the study was to test whether intermittent PTH treatment of postmenopausal women alters circulating sclerostin levels.

DESIGN

Prospective study.

SETTING

The study was conducted at a clinical research unit.

PARTICIPANTS AND INTERVENTIONS

Participants included 27 postmenopausal women treated with PTH (1-34) for 14 d and 28 control women.

MAIN OUTCOME MEASURES

Serum sclerostin levels were measured.

RESULTS

Circulating sclerostin levels decreased significantly in the PTH-treated subjects, from (mean ± SEM) 551 ± 32 to 482 ± 31 pg/ml (-12.7%, P < 0.0001) but did not change in the control women (baseline, 559 ± 34 pg/ml; end point, 537 ± 40 pg/ml, P = 0.207; P = 0.017 for difference in changes between groups). Bone marrow plasma was obtained in a subset of the control and PTH-treated subjects (n = 19 each) at the end of the treatment period, and marrow plasma and peripheral serum sclerostin levels were significantly correlated (R = 0.64, P < 0.0001). Marrow plasma sclerostin levels were 24% lower in PTH-treated compared with control women, but perhaps due to the smaller sample size, this difference was not statistically significant (P = 0.173).

CONCLUSIONS

Circulating sclerostin levels correlate with bone marrow plasma levels and are reduced by intermittent PTH therapy in postmenopausal women. Further studies are needed to assess the extent to which decreases in sclerostin production contribute to the anabolic skeletal response to PTH.

Effects of suppression of follicle-stimulating hormone secretion on bone resorption markers in postmenopausal women

CONTEXT

It has recently been proposed that the increase in bone resorption after the menopause may not be due principally to estrogen deficiency but rather to the concomitant increase in circulating FSH levels.

OBJECTIVE

The objective of the study was to test whether suppression of FSH secretion in postmenopausal women reduces levels of bone resorption markers.

DESIGN

This was a prospective study.

SETTING

The study was conducted at a clinical research unit.

PARTICIPANTS AND INTERVENTIONS

Postmenopausal women were treated with a GnRH agonist (leuprolide acetate, 7.5 mg im every 28 d; n = 21) or placebo injections (control; n = 20). Both groups received the aromatase inhibitor, letrozole, 2.5 mg/d, to eliminate variations in endogenous estrogen levels as a confounder.

MAIN OUTCOME MEASURES

Serum FSH and bone resorption markers [serum C-terminal telopeptide of type I collagen (CTX) and tartrate-resistant acid phosphatase 5b (TRAP5b)] at d 105 (3.5 months) of treatment as compared with baseline.

RESULTS

Compared with baseline, serum FSH levels did not change significantly in controls (+6%) but were reduced (-86%, into the premenopausal range) in the GnRH group. Due to the aromatase inhibitor-induced reduction in estrogen production, serum CTX and TRAP5b levels increased significantly in controls (+20 and +10%, respectively). In the GnRH group, suppression of FSH secretion did not reduce serum CTX or TRAP5b levels; rather, both markers also increased in these women (+34 and +15%, respectively; P = 0.161 and 0.266 for comparison of percent changes between groups).

CONCLUSIONS

This direct interventional study demonstrates that FSH does not regulate bone resorption in postmenopausal women.

Effect of blockade of TNF-alpha and interleukin-1 action on bone resorption in early postmenopausal women

After acute estrogen withdrawal in postmenopausal women, administration of anakinra or etanercept, specific blockers of IL-1 and TNF-alpha, respectively, reduced the rise in bone resorption markers to about one half of that in controls. This is consistent with an important role for these immune cytokines in mediating the effect of estrogen deficiency on bone.

INTRODUCTION

Studies in rodents have implicated increased production of interleukin (IL)-1 beta and TNF-alpha as mediators of bone loss after ovariectomy, but their roles are unclear in humans whose immune system differs markedly from that of rodents.

MATERIALS AND METHODS

We administered transdermal estradiol, 0.1 mg/d, for 60 days to 42 early postmenopausal women. Estrogen treatment was discontinued, and subjects were randomly assigned to intervention groups receiving 3 wk of injections with 0.9% saline, anakinra 100 mg/d, or etanercept 25 mg/twice weekly. Bone turnover was assessed by measuring serum carboxyl-terminal telopeptide of type 1 collagen (CTX) and amino-terminal telopeptide of type 1 collagen (NTX), markers for bone resorption, and serum amino-terminal propeptide of type 1 collagen (P1NP), a marker for bone formation. Results were expressed as percent change in markers from baseline (last 2 days of estrogen treatment and days 20 and 21 of intervention).

RESULTS

The percent changes from baseline during intervention for serum CTX, urine NTX, and serum PINP, respectively, were 43.3 +/- 8.0%, 12.0 +/- 7.1%, and -41.0 +/- 2.5% for the control group; 25.9 +/- 6.3%, 9.5 +/- 4.0%, and -37.8 +/- 3.0% for the anakinra group; and 21.7 +/- 5.0%, 0.32 +/- 3.82%, and -34.5 +/- 3.9% for the etanercept group. Compared with the control group, the blunting of the increase in serum CTX fell just below the level of significance (p=0.10) after anakinra treatment, whereas the blunting of the increase in serum CTX (p=0.034) and in urine NTX (p=0.048) were significant after etanercept treatment. Other changes were not significant.

CONCLUSIONS

The data are consistent with a role for TNF-alpha, and possibly for IL-1 beta, in mediating increased bone resorption during estrogen deficiency in women. Although either cytokine blocker reduced serum CTX by about one half, the effect of combined blockade could not be tested because of concerns about toxicity. The data do not exclude direct or indirect contributory roles for RANKL or for other cytokines.