Abstract
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.
Circulating sclerostin levels correlate with bone marrow plasma levels and are reduced by intermittent parathyroid hormone therapy in postmenopausal women.
After decades of investigation, intermittent PTH (1-34) treatment has emerged as the first truly anabolic therapy for osteoporosis (1,2). However, whereas PTH 1-34 stimulates bone formation, the cellular and molecular mechanism(s) for this effect, particularly in humans, remain unclear (2,3). Concurrent with the development of PTH 1-34 as an anabolic agent for bone has been the identification of the Wnt/β-catenin signaling pathway as a major regulator of bone mass (4,5). Activation of this pathway results in an expansion of osteoprogenitor cells as well as reduced apoptosis of mature osteoblasts, leading to increased bone formation and potentially marked increases in bone mass (4,5). The effects of Wnt ligands on the canonical signaling pathway involving β-catenin signaling are mediated by binding of these ligands to a seven-transmembrane domain-spanning frizzled receptor and either of two coreceptors, low-density lipoprotein receptor-related proteins 5 or 6 (4,5).
Given that both PTH and Wnt signaling can increase bone formation, there has been considerable interest in recent years in delineating possible cross talk between these pathways. Much of this interest has focused on regulation by PTH of the potent inhibitor of Wnt signaling, sclerostin (6). Sclerostin is a secreted Wnt antagonist produced by osteocytes that regulates bone mass by binding to low-density lipoprotein receptor-related protein-5 and -6 to inhibit the canonical Wnt/β-catenin signaling pathway. The biological importance of sclerostin in regulating bone mass in humans is highlighted by two genetic disorders associated with markedly increased bone mass, sclerosteosis and van Buchem’s disease (7,8,9,10). Sclerosteosis is caused by a mutation in the gene encoding sclerostin, SOST, which results in an improperly spliced SOST mRNA (7,8), whereas van Buchem’s disease is caused by a deletion of an enhancer element downstream of the SOST gene (9,10). These findings, combined with the demonstration that sclerostin-deficient mice have increased bone mass (11), have led to the development of antisclerostin-neutralizing antibodies as a novel anabolic treatment for osteoporosis (12).
Given the critical role of sclerostin in regulating Wnt signaling and bone mass, Bellido et al. (13) and Keller and Kneissel (14) demonstrated several years ago that PTH treatment of mice led to reduced SOST mRNA and sclerostin protein in bone, specifically in osteocytes. These findings were subsequently confirmed in rats (15). Moreover, transgenic mice expressing a constitutively active PTH receptor only in osteocytes had increased bone mass as well as reduced expression of sclerostin in osteocytes, increased Wnt signaling, increased osteoblast numbers, and decreased osteoblast apoptosis (16), and PTH-induced bone gain was recently shown to be blunted in mice whose sclerostin secretion cannot be modulated because the SOST gene is missing or is overexpressed (17). PTH has also been shown to reduce SOST expression by osteoblastic cells in vitro (13,14), likely through direct transcriptional regulation of the SOST bone enhancer using myocyte enhancer factors 2 transcription factors (18).
Despite these in vitro and rodent studies, there are no data at present regarding possible effects of intermittent PTH 1-34 treatment on sclerostin production in humans. The recent development of serum assays for sclerostin led to the observation by Mirza et al. (19) that serum PTH levels were inversely associated with sclerostin levels in postmenopausal women. However, because correlation does not prove causality, we tested in a prospective study of postmenopausal women treated with PTH 1-34 whether PTH treatment regulated circulating sclerostin levels in humans. In addition, using serum assays, we assessed changes in other possible mediators of the effects of PTH on bone, including another Wnt antagonist, Dickkopf (DKK)-1 (20), as well as receptor activator of nuclear factor-κB ligand (RANKL) (21,22,23,24,25) and osteoprotegerin (OPG) (21,22,23,24,25,26). Finally, in a subset of patients, we obtained bone marrow aspirates and evaluated the possible relationship of these same proteins in bone marrow plasma to levels present in peripheral serum.
Subjects and Methods
Study subjects
Fifty-five untreated postmenopausal women between the ages of 55 and 85 yr were randomized into an open-label, controlled study to receive once-daily PTH 1-34 treatment or no treatment for 14 d. Menopause was defined as the absence of menses for at least 10 yr and serum FSH 30 IU/liter or greater. All of the study subjects were in good health, with no clinically significant abnormalities in any of the screening laboratory serum studies (calcium, phosphorus, alkaline phosphatase, creatinine, TSH, and hemoglobin), and were asked to discontinue all dietary supplements for a period of 10 wk before study enrollment. Subjects were not included if they had been maintained on corticosteroids, anticonvulsant therapy, pharmacological thyroid hormone therapy, bisphosphonates, calcitonin, or estrogen therapy within the past 12 months. Subjects with a clinical history of an osteoporotic fracture (vertebral, hip, or distal forearm) were also excluded. All subjects provided written informed consent, and the study was approved by the Mayo Clinic Institutional Review Board.
Experimental protocol
To ensure vitamin D sufficiency, all subjects were treated with vitamin D3, 50,000 IU twice weekly for 5 wk. After an additional 4 wk (1 wk before initiation of the study), serum 25-hydoxyvitamin vitamin D levels were determined. After the 10-wk run-in period, subjects were randomly assigned to receive either once-daily PTH 1-34 at a dose of 40 μg or no treatment. The PTH 1-34 used was as sold by Eli Lilly (Indianapolis, IN) under the brand name Forteo. The 40-μg/d dose chosen was as used in the original clinical trials and was without any significant adverse effects (1).
All subjects were maintained on a diet containing 600 mg/d calcium during the study and had fasting morning (0800 h) serum samples obtained for measurement of serum sclerostin, bone turnover markers, DKK1, RANKL, and OPG levels on the day before beginning treatment and again on the 14th day of treatment. A subset of these women (n = 19 from the control group and n = 20 from the PTH group) had bone marrow aspirates performed on the final study day for further studies on gene expression by bone marrow hematopoeitic lineage-negative/alkaline phosphatase-positive cells; these data are being reported separately (Drake M. T., B. Srinivasan, U. I. Modder, A. C. Ng, A. H. Undale, M. M. Roforth, J. M. Peterson, L. K. McCready, B. L. Riggs, and S. Khosla, manuscript in preparation). After local anesthesia, bone marrow aspirates were collected in an anticoagulant (EDTA) from the iliac crest using a standard procedure. We were able to isolate sufficient bone marrow plasma from 19 subjects in each group for comparison of matched bone marrow plasma and serum measurements. For the blood and bone marrow plasma samples obtained at the end of the study, PTH was administered 4 h before sampling (i.e. at 0400 h) in all subjects (whether or not a marrow plasma sample was obtained) to maximize our ability to detect changes in gene and protein expression after PTH treatment because previous studies in rodents (17,27) found that early time points after PTH treatment may show more dramatic changes than 24 h after PTH dosing. All peripheral blood serum and bone marrow plasma samples were stored at −70 C in the laboratory until analysis.
Peripheral serum and bone marrow plasma assays
RANKL, OPG, osteocalcin (OCN), DKK1, and sclerostin concentrations were assessed in peripheral serum and bone marrow plasma using single-plex or multiplex Luminex kits (Millipore, Billerica, MA). DKK1 and sclerostin concentrations were assessed with custom-made single-plex Luminex kits (Millipore/Linco, St. Charles MO). In validation studies, the Luminex sclerostin assay used in this study correlated well (r = 0.89, P < 0.0001) with the previously published ELISA developed at Amgen (19), with a range of absolute values that was similar, but the Luminex assay had a greater sensitivity than the ELISA (60 pg/ml for the ELISA vs. 10 pg/ml for the Luminex assay). The following biochemical markers of bone formation and resorption were measured in peripheral serum and bone marrow plasma with immunoassay kits: tartrate-resistant acid phosphatase (TRACP 5B; BoneTRAP kit; Immunodiagnostic Systems, Fountain Hills, AZ); C-terminal telopeptide of type I collagen (CTXI; Serum CrossLaps kit; Immunodiagnostic Systems); bone-specific alkaline phosphatase (BAP; MicroVue BAP kit; Quidel, San Diego, CA); and C-terminal propeptide of type I collagen (CICP; Metra CICP kit; Quidel). The interassay coefficient of variation precision for each assay was as follows: RANKL less than 6%, OPG less than 5%, OCN less than 7%, DKK1 less than 11%, sclerostin less than 12%, TRACP 5B less than 9%, CTXI less than 11%, BAP less than 8%, and CICP less than 7%. Serum amino-terminal propeptide of type I procollagen (PINP) was measured in the peripheral serum samples only (because we did not have sufficient bone marrow plasma remaining for this assay) by RIA (interassay coefficient of variation <10%; Immunodiagnostic Systems). All assays were performed according to manufacturer instructions.
Statistical analyses
All data are reported as the mean ± sem. Within-group pairwise comparisons between the baseline and end point values were done using paired t tests, and the percent change in each variable was compared between the control and PTH groups using nonpaired t tests. Pearson’s correlations were used to test for associations between variables. P < 0.05 was considered significant.
Results
Table 1 shows the baseline anthropometric and biochemical variables in the study subjects. As is evident, the subjects in the two groups were well matched for all of the variables. Due to the vitamin D replacement protocol, all subjects had 25-hydroxyvitamin D levels above 30 ng/ml.
Table 1.
Baseline anthropometric and biochemical variables in the study subjects
| Control | PTH | P value | |
|---|---|---|---|
| n | 28 | 27 | |
| Age (yr) | 64.9 ± 1.3 | 65.0 ± 1.2 | 0.935 |
| Height (cm) | 164.0 ± 1.2 | 163.0 ± 1.1 | 0.521 |
| Weight (kg) | 73.5 ± 1.6 | 72.7 ± 2.1 | 0.783 |
| Body mass index (kg/m2) | 27.6 ± 0.7 | 27.5 ± 0.6 | 0.904 |
| Serum parameters | |||
| Calcium (mg/dl) | 9.8 ± 0.1 | 9.7 ± 0.6 | 0.124 |
| Phosphorus (mg/dl) | 3.8 ± 0.1 | 3.7 ± 0.1 | 0.772 |
| Creatinine (mg/dl) | 0.81 ± 0.03 | 0.81 ± 0.03 | 0.936 |
| 25-Hydroxyvitamin D (ng/ml) | 55.8 ± 1.9 | 60.5 ± 2.4 | 0.129 |
As shown in Fig. 1A, serum sclerostin levels did not change in the control women but decreased significantly in the PTH-treated women. Moreover, the percent change in serum sclerostin levels in the PTH-treated women was significantly different from the percent change in sclerostin levels in the control women (Fig. 1B). Table 2 shows changes in the bone turnover markers and other variables in the two groups. Bone formation markers (CICP, PINP, OCN, BAP) decreased slightly in the control women, but all four markers increased significantly in the PTH-treated women, with highly significant differences in the percent changes between groups. Bone resorption markers (CTX and TRAP 5B) also increased significantly in the PTH-treated women, with significant differences between groups for percent changes in these variables. Changes in serum DKK1 or OPG levels were not different between groups. Serum RANKL levels were not detectable in the majority of the samples and are not shown. We also tested for possible associations between changes in serum sclerostin levels and changes in each of the variables in Table 2 (separately and combined in the two treatment groups). Although none of these achieved statistical significance (data not shown), there was a weak inverse association (R = −0.20, P = 0.146) between the percent change in sclerostin and percent change in CICP in the combined groups; a similar inverse association (R = −0.20, P = 0.139) was noted for percent change in sclerostin and percent change in PINP.
Figure 1.
A, Baseline (BSL) and end point (EP) serum sclerostin levels in the control and PTH-treated women. ***, P < 0.0001 compared with baseline. B, Percent change from baseline in circulating serum sclerostin levels in the control (−4.7 ± 2.6%) and PTH-treated (−12.7 ± 1.9%) women. *, P = 0.017 for comparison with the change in control women.
Table 2.
Baseline, end point, and percent change in serum bone turnover markers and other variables in the two study groups
| Control
|
PTH
|
||||||
|---|---|---|---|---|---|---|---|
| Baseline | End point | Change (%) | Baseline | End point | Change (%) | P value | |
| CICP (ng/ml) | 112 ± 8 | 102 ± 7a | −8 ± 3 | 111 ± 4 | 191 ± 14b | 71 ± 10 | <0.001 |
| PINP (ng/ml) | 53.0 ± 4.4 | 50.4 ± 6.1 | −2.7 ± 3.6 | 59.0 ± 3.8 | 108.3 ± 6.2b | 88 ± 11 | <0.001 |
| OCN (ng/ml) | 12.8 ± 1.8 | 11.3 ± 1.5a | −10 ± 2 | 9.2 ± 1.3 | 11.0 ± 1.3b | 27 ± 4 | <0.001 |
| BAP (U/liter) | 23.3 ± 1.1 | 21.9 ± 1.0b | −6 ± 1 | 24.9 ± 1.5 | 28.2 ± 1.5a | 15 ± 5 | <0.001 |
| CTX (ng/ml) | 0.53 ± 0.04 | 0.48 ± 0.03 | 0.4 ± 9 | 0.67 ± 0.05 | 1.04 ± 0.08b | 59 ± 8 | <0.001 |
| TRAP 5B (U/liter) | 3.6 ± 0.2 | 3.7 ± 0.2c | 5 ± 2 | 4.0 ± 0.2 | 4.5 ± 0.2b | 12 ± 2 | 0.040 |
| DKK1 (ng/ml) | 1.2 ± 0.1 | 1.3 ± 0.1 | 8 ± 7 | 1.2 ± 0.1 | 1.3 ± 0.1 | 11 ± 7 | 0.737 |
| OPG (pg/ml) | 507 ± 31 | 474 ± 26c | −5 ± 2 | 501 ± 22 | 472 ± 23c | −6 ± 2 | 0.861 |
The P value in the last column is for comparison of percent change in the two groups.
P < 0.01 for comparison with the baseline value.
P < 0.001 for comparison with the baseline value.
P < 0.05 for comparison with the baseline value.
To assess the possible relationship between circulating levels of sclerostin and the other markers and their levels in the bone microenvironment, we also compared the correlation between the serum and bone marrow plasma levels of these variables in the subset of subjects in whom we had matched values at the end of the study (Table 3). Serum and bone marrow plasma levels of sclerostin were strongly correlated, and the slope of the regression line was close to 1.0 (0.99) (Fig. 2). Similarly, serum and bone marrow plasma CICP levels were highly correlated. By contrast, the association between serum and bone marrow plasma levels of BAP was considerably weaker and not present for OCN or DKK1. Serum and bone marrow plasma levels of CTX were better correlated than those for TRAP 5B, and serum and bone marrow plasma levels of OPG were modestly correlated.
Table 3.
Pearson correlation coefficients for the relationship between peripheral serum and bone marrow plasma levels of the variables measured
| R value | P value | |
|---|---|---|
| Sclerostin | 0.64 | <0.001 |
| CICP | 0.87 | <0.001 |
| OCN | 0.17 | 0.314 |
| BAP | 0.33 | 0.044 |
| CTX | 0.69 | <0.001 |
| TRAP 5B | 0.39 | 0.017 |
| DKK1 | 0.14 | 0.410 |
| OPG | 0.36 | 0.026 |
As noted in Subjects and Methods, bone marrow plasma was available in 19 subjects from each of the two study groups, and the matched peripheral serum values were used for this analysis.
Figure 2.
Plot showing the correlation between bone marrow plasma and peripheral serum sclerostin levels in the control and PTH subjects combined.
Finally, we compared bone marrow plasma levels of the various biomarkers in the subset of patients from whom we had bone marrow samples (Table 4). The statistical power of these comparisons was limited by the cross-sectional nature of marrow collections and the smaller sample size relative to peripheral blood analyses. Whereas bone marrow plasma sclerostin levels were lower in the PTH-treated compared with the control [by 24%, a larger decrease than that noted in the serum in the PTH treated women in the longitudinal analysis (Fig. 1B)], this difference was not statistically significant. Although we did isolate bone marrow hematopoietic lineage-negative/alkaline phosphatase-positive cells from this subset of patients, these cells, which represent relatively early osteoprogenitor cells (28), did not express the SOST mRNA when analyzed using quantitative RT-PCR (data not shown). Bone marrow plasma CICP, CTX, and TRAP 5B, but not OCN or BAP, levels were significantly higher in the PTH-treated compared with the control women. Interestingly, whereas we did not find significant differences between groups in changes in serum OPG levels (Table 2), bone marrow plasma OPG levels were significantly lower (by 33%) in the PTH-treated compared with the control women.
Table 4.
Bone marrow plasma levels of the various markers in the control and PTH-treated women assessed at the 14-d time point (n = 19 per group)
| Control | PTH | P value | |
|---|---|---|---|
| Sclerostin (pg/ml) | 571 ± 87 | 436 ± 43 | 0.173 |
| CICP (ng/ml) | 209 ± 16 | 420 ± 46 | <0.001 |
| OCN (ng/ml) | 19.1 ± 3.5 | 13.6 ± 1.4 | 0.152 |
| BAP (U/liter) | 37.3 ± 11.3 | 37.7 ± 8.7 | 0.979 |
| CTX (ng/ml) | 0.56 ± 0.05 | 0.82 ± 0.1 | 0.033 |
| TRAP 5B (U/liter) | 2.1 ± 0.2 | 3.6 ± 0.2 | <0.001 |
| DKK1 (ng/ml) | 0.32 ± 0.02 | 0.28 ± 0.02 | 0.172 |
| OPG (pg/ml) | 480 ± 51 | 322 ± 32 | 0.013 |
We also compared the ratio of the mean values of the various markers in the PTH-treated with the control subjects in the subjects in whom we had both the bone marrow plasma and peripheral serum values (n = 19 per group, Supplemental Table 1, published on The Endocrine Society’s Journals Online web site at http://jcem.endojournals.org). Although there were some differences in these ratios between peripheral serum and bone marrow plasma noted for the various markers, the most notable difference was, as expected, for OPG, which had a ratio of 1.01 in the peripheral serum vs. 0.67 in the bone marrow plasma.
Discussion
We demonstrate in the present study that intermittent PTH 1-34 treatment of postmenopausal women is associated with a reduction in circulating sclerostin levels. These findings establish that, similar to rodent and in vitro studies (13,14,15,16,17), PTH regulates sclerostin levels in humans. Even the short course of PTH treatment (14 d) in the present study was associated with marked increases in markers of bone formation. Thus, given the known inhibitory effects of sclerostin on bone formation (6,8,10,12,17), these findings provide support for the hypothesis that in humans, as in rodents, at least part of the anabolic effect of PTH on bone may be mediated by an inhibition of sclerostin production. Additional support for this hypothesis would have come from the demonstration of a significant association between changes in sclerostin levels after PTH treatment and changes in bone formation markers. Our failure to find a clear association between changes in sclerostin and changes in the bone formation markers could be due to the variability of the markers or due to the possibility that changes in sclerostin account for only part of the anabolic effects of PTH on bone, given the known multiple pathways regulated by PTH that impact bone formation (2,3). In addition, it is possible that changes in sclerostin production occur well before changes in bone formation markers, and even sampling sclerostin levels 14 d after PTH treatment may have missed more robust decreases in sclerostin levels early after initiating PTH treatment, which might also have shown stronger associations with the bone formation markers.
We also demonstrate that bone marrow plasma and peripheral blood serum sclerostin concentrations are strongly correlated, consistent with the observation that osteocytes in bone are likely the major source of sclerostin production (13,16) and therefore of the sclerostin present in peripheral blood. Moreover, the magnitude of the difference in bone marrow plasma sclerostin levels in control vs. PTH-treated women was in fact greater in bone marrow plasma than in peripheral blood; however, this difference in marrow plasma sclerostin levels did not reach statistical significance, likely due to the smaller number of subjects in whom we had bone marrow plasma as well as the cross-sectional nature of this analysis, as opposed to the greater power associated with assessing pre- and posttreatment values in peripheral serum. Thus, we would have required double the number of bone marrow plasma samples (n = 38 per group, rather than the 19 per group we had) for the cross-sectional bone marrow plasma data to achieve statistical significance.
In contrast to the relatively high correlation between bone marrow plasma and peripheral serum sclerostin levels, the correlation between marrow plasma and peripheral serum levels of OPG was much weaker. Similar to our findings, previous studies in normal rats found that OPG concentrations within bone correlated poorly with circulating levels (29), likely reflecting the fact that multiple tissues besides bone [e.g. the vasculature (30)] contribute to OPG present in the circulation. Of interest, whereas PTH treatment was not associated with any differences in changes in circulating OPG levels between the control and PTH-treated women, bone marrow plasma OPG levels were reduced (by 33%) in the PTH-treated compared with the control women. These findings are consistent with considerable in vitro data demonstrating suppression of OPG mRNA and protein production by PTH in bone cells (21,22,23,24,25). Furthermore, rodent studies have suggested that the effects of intermittent PTH treatment on suppressing OPG mRNA levels in bone are transient (26), and the fact that we obtained bone marrow plasma aspirates 4 h after the last dose of PTH may have enhanced our ability to observe a suppression of OPG levels by PTH. Concurrent with the decrease in marrow plasma OPG levels, bone resorption markers did increase in the PTH-treated women, suggesting a potential role for suppression of OPG in mediating the increase in bone resorption after PTH treatment. Of note, inhibition of sclerostin by PTH is likely not involved in the observed increase in bone resorption because sclerostin antibody therapy has been found to decrease indices of bone resorption (12).
We recognize that bone marrow plasma samples inevitably are contaminated by peripheral blood, and there is no rigorous way to correct for such contamination. However, had there been substantial contamination of the marrow samples by peripheral blood, we would have seen similar correlations between the marrow plasma and peripheral blood levels of virtually all the markers measured (i.e. the two compartments would have become more or less identical). The fact that some markers (e.g. CICP, sclerostin) showed relatively strong correlations between the marrow plasma and peripheral blood whereas others (e.g. OCN, DKK1) showed no associations indicates that the marrow plasma and peripheral blood samples did in fact represent different compartments. We should also note the caveat that for the markers assessed, the fact that we analyzed plasma in the bone marrow and serum in the peripheral blood could also have had an impact on the specific correlations between bone marrow plasma and peripheral blood that we observed.
Due to our desire to examine possible mechanisms for the effects of intermittent PTH therapy on bone formation, we chose the 40 μg/d (as opposed to the clinically approved 20 μg/d) dose of PTH 1-34 for this study because Neer et al. (1) found somewhat greater effects of the higher PTH 1-34 dose on bone mineral density accrual. Moreover, one of the other objectives of this study was to examine possible effects of PTH treatment on gene expression in hematopoietic lineage-negative/alkaline phosphatase-positive cells (Drake, M. T., B. Srinivasan, U. I. Modder, A. C. Ng, A. H. Undale, M. M. Roforth, J. M. Peterson, L. K. McCready, B. L. Riggs, and S. Khosla, manuscript in preparation). Thus, we wanted to maximize our chances of identifying possible gene expression and protein changes over the relatively short 14-d duration of the study. This higher dose may explain the fact that we observed significant increases in bone resorption markers over the relatively short 14-d treatment period; by contrast, changes in bone resorption markers may take longer to occur at the lower dose (31). Nonetheless, we would stress that the 40-μg/d dose is biologically relevant as an anabolic dose, based on the previously noted effects of this dose on bone mineral density (1).
In summary, we demonstrate that intermittent PTH 1-34 treatment of postmenopausal women results in a decrease in circulating serum sclerostin levels. Our study also provides the first comparison in humans between bone marrow plasma and peripheral serum levels of sclerostin, OPG, DKK1, and bone turnover markers. We demonstrate that bone marrow plasma and peripheral serum sclerostin levels are strongly correlated (with the slope of this relationship close to 1.0), whereas marrow plasma and peripheral serum OPG levels are much less correlated. This is consistent with bone being the major source of sclerostin production, whereas multiple sources (bone, vasculature, other sources) likely contribute to circulating OPG levels. These findings are also important for future studies assessing peripheral serum sclerostin levels in that our data indicate that changes in these levels reflect (on average) changes of similar magnitude in bone marrow plasma sclerostin levels. Overall, our data extend previous in vitro (13,14) and animal studies (13,14,15,17) on regulation of sclerostin production by PTH. Moreover, based on the known effects of sclerostin inhibition on bone formation (6,8,10,11,17), our findings point to the need for further studies testing the hypothesis that in humans, as in rodents, at least part of the anabolic effect of intermittent PTH therapy on bone is mediated by suppression of sclerostin production and subsequent activation of Wnt signaling.
Acknowledgments
We thank Kelley Hoey for processing of the samples.
Footnotes
This work was supported by National Institutes of Health Grants AG004875 and UL1-RR24150 (Center for Translational Science Activities), U.S. Public Health Service.
Disclosure Summary: D.D., M.S., and P.K. are employees of Amgen, Inc. None of the other authors have a conflict to disclose.
First Published Online July 14, 2010
Abbreviations: BAP, Bone-specific alkaline phosphatase; CICP, C-terminal propeptide of type I collagen; CTXI, C-terminal telopeptide of type I collagen; DKK, Dickkopf; OCN, osteocalcin; OPG, osteoprotegerin; PINP, propeptide of type I procollagen; RANKL, receptor activator of nuclear factor-κB ligand; TRACP, tartrate-resistant acid phosphatase.
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