Ulrike I. Mödder

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 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.

Regulation of circulating sclerostin levels by sex steroids in women and in men

Sex steroids are important regulators of bone turnover, but the mechanisms of their effects on bone remain unclear. Sclerostin is an inhibitor of Wnt signaling, and circulating estrogen (E) levels are inversely associated with sclerostin levels in postmenopausal women. To directly test for sex steroid regulation of sclerostin levels, we examined effects of E treatment of postmenopausal women or selective withdrawal of E versus testosterone (T) in elderly men on circulating sclerostin levels. E treatment of postmenopausal women (n = 17) for 4 weeks led to a 27% decrease in serum sclerostin levels [versus +1% in controls (n = 18), p < .001]. Similarly, in 59 elderly men, we eliminated endogenous E and T production and studied them under conditions of physiologic T and E replacement, and then following withdrawal of T or E, we found that E, but not T, prevented increases in sclerostin levels following induction of sex steroid deficiency. In both sexes, changes in sclerostin levels correlated with changes in bone‐resorption, but not bone‐formation, markers (r = 0.62, p < .001, and r = 0.33, p = .009, for correlations with changes in serum C‐terminal telopeptide of type 1 collagen in the women and men, respectively). Our studies thus establish that in humans, circulating sclerostin levels are reduced by E but not by T. Moreover, consistent with recent data indicating important effects of Wnts on osteoclastic cells, our findings suggest that in humans, changes in sclerostin production may contribute to effects of E on bone resorption.

Osteocalcin Expression by Circulating Endothelial Progenitor Cells in Patients With Coronary Atherosclerosis

OBJECTIVES This study was designed to test whether patients with coronary atherosclerosis have increases in circulating endothelial progenitor cells (EPCs) expressing an osteogenic phenotype. BACKGROUND Increasing evidence indicates a link between bone and the vasculature, and bone marrow and circulating osteogenic cells have been identified by staining for the osteoblastic marker, osteocalcin (OCN). Endothelial progenitor cells contribute to vascular repair, but repair of vascular injury may result in calcification. Using cell surface markers (CD34, CD133, kinase insert domain receptor [KDR]) to identify EPCs, we examined whether patients with coronary atherosclerosis had increases in the percentage of EPCs expressing OCN. METHODS We studied 72 patients undergoing invasive coronary assessment: control patients (normal coronary arteries and no endothelial dysfunction, n = 21) versus 2 groups with coronary atherosclerosis-early coronary atherosclerosis (normal coronary arteries but with endothelial dysfunction, n = 22) and late coronary atherosclerosis (severe, multivessel coronary artery disease, n = 29). Peripheral blood mononuclear cells were analyzed using flow cytometry. RESULTS Compared with control patients, patients with early or late coronary atherosclerosis had significant increases (approximately 2-fold) in the percentage of CD34+/KDR+ and CD34+/CD133+/KDR+ cells costaining for OCN. Even larger increases were noted in the early and late coronary atherosclerosis patients in the percentage of CD34+/CD133-/KDR+ cells costaining for OCN (5- and 2-fold, p < 0.001 and 0.05, respectively). CONCLUSIONS A higher percentage of EPCs express OCN in patients with coronary atherosclerosis compared with subjects with normal endothelial function and no structural coronary artery disease. These findings have potential implications for the mechanisms of vascular calcification and for the development of novel markers for coronary atherosclerosis.

Emerging therapeutic opportunities for skeletal restoration.

Osteoporosis, a syndrome characterized by thin bones and fractures, has become more prevalent in both women and men. Established therapies for treating this disorder consist primarily of drugs that prevent bone loss, such as the bisphosphonates and selective oestrogen receptor modulators. Although these drugs have been shown to reduce fractures in randomized trials, there is an urgent need for treatments that could lower fracture risk further without additional adverse effects. The introduction of parathyroid hormone (teriparatide), which significantly increases bone mineral density, albeit for a relatively short duration, raised expectations that drugs that stimulate bone formation might cure osteoporosis. After outlining current approaches for treating osteoporosis, this Review focuses on emerging therapeutic opportunities for osteoporosis that are based on recent insights into skeletal physiology. Such novel strategies offer promise not only for reducing age-related bone loss and the associated risk of fractures but also for restoring bone mineral density to healthy levels.

Skeletal Effects of Estrogen Are Mediated by Opposing Actions of Classical and Nonclassical Estrogen Receptor Pathways

ERα acts either through classical (ERE‐mediated) or nonclassical (non‐ERE) pathways. The generation of mice carrying a mutation that eliminates classical ERα signaling presents a unique opportunity to study the relative roles of these pathways in bone. This study defines the skeletal phenotype and responses to ovariectomy and estrogen replacement in these mice.

Relation of serum serotonin levels to bone density and structural parameters in women

Recent studies have demonstrated an important role for circulating serotonin in regulating bone mass in rodents. In addition, patients treated with selective serotonin reuptake inhibitors (SSRIs) have reduced areal bone mineral density (aBMD). However, the potential physiologic role of serotonin in regulating bone mass in humans remains unclear. Thus we measured serum serotonin levels in a population‐based sample of 275 women and related these to total‐body and spine aBMD assessed by dual‐energy X‐ray absorptiometry, femur neck total and trabecular volumetric BMD (vBMD) and vertebral trabecular vBMD assessed by quantitative computed tomography (QCT), and bone microstructural parameters at the distal radius assessed by high‐resolution peripheral QCT (HRpQCT). Serotonin levels were inversely associated with body and spine aBMD (age‐adjusted R = −0.17 and −0.16, P < .01, respectively) and with femur neck total and trabecular vBMD (age‐adjusted R = −0.17 and −0.25, P < .01 and < .001, respectively) but not lumbar spine vBMD. Bone volume/tissue volume, trabecular number, and trabecular thickness at the radius were inversely associated with serotonin levels (age‐adjusted R = −0.16, −0.16, and −0.14, P < .05, respectively). Serotonin levels also were inversely associated with body mass index (BMI; age‐adjusted R = −0.23, P < .001). Multivariable models showed that serotonin levels remained significant negative predictors of femur neck total and trabecular vBMD, as well as trabecular thickness at the radius, after adjusting for age and BMI. Collectively, our data provide support for a physiologic role for circulating serotonin in regulating bone mass in humans.

Concise Review: Insights from Normal Bone Remodeling and Stem Cell-Based Therapies for Bone Repair

There is growing interest in the use of mesenchymal stem cells for bone repair. As a major reason for normal bone remodeling is the removal of fatigue microcracks, advances in our understanding of this process may inform approaches to enhance fracture healing. Increasing evidence now indicates that physiological bone remodeling occurs in close proximity to blood vessels and that these vessels carry perivascular stem cells that differentiate into osteoblasts. Similarly, fracture healing is critically dependent on the ingrowth of blood vessels not only for a nutrient supply but also for the influx of osteoblasts. A number of animal and human studies have now shown the potential benefit of bone marrow‐derived mesenchymal stem cells in enhancing bone repair. However, as in other tissues, the question of whether these cells improve fracture healing directly by differentiating into osteoblasts or indirectly by secreting paracrine factors that recruit blood vessels and the accompanying perivascular stem cells remains a major unresolved issue. Moreover, CD34+ cells, which are enriched for endothelial/hematopoietic cells, have also shown efficacy in various bone repair models, at least in part due to the induction of angiogenesis and recruitment of host progenitor cells. Thus, mesenchymal and nonmesenchymal stem/progenitor cells are attractive options for bone repair. It is possible that they contribute directly to bone repair, but it is also likely that they express paracrine factors in the appropriate amounts and combinations that promote and sustain the healing process. STEM CELLS 2010;28:2124–2128

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.

The Endogenous Selective Estrogen Receptor Modulator 27-Hydroxycholesterol Is a Negative Regulator of Bone Homeostasis

Osteoporosis is an important clinical problem, affecting more than 50% of people over age 50 yr. Estrogen signaling is critical for maintaining proper bone density, and the identification of an endogenous selective estrogen receptor (ER) modulator, 27-hydroxycholesterol (27HC), suggests a mechanism by which nutritional/metabolic status can influence bone biology. With its levels directly correlated with cholesterol, a new possibility emerges wherein 27HC links estrogen and cholesterol signaling to bone homeostasis. In these studies, we found that increasing concentrations of 27HC, both by genetic and pharmacological means, led to decreased bone mineral density that was associated with decreased bone formation and increased bone resorption. Upon manipulation of endogenous estrogen levels, many of the responses to elevated 27HC were altered in such a way as to implicate ER as a likely mediator. In a model of postmenopausal bone loss, some pathologies associated with elevated 27HC were exacerbated by the absence of endogenous estrogens, suggesting that 27HC may act both in concert with and independently from classic ER signaling. These data provide evidence for interactions between estrogen signaling, cholesterol and metabolic disease, and osteoporosis. Patients with high cholesterol likely also have higher than average 27HC, perhaps putting them at a higher risk for bone loss and fracture. More studies are warranted to fully elucidate the mechanism of action of 27HC in bone and to identify ways to modulate this pathway therapeutically.