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Background: Low bone mass is prevalent in ambulatory multiple sclerosis (MS) patients even shortly after clinical onset. The mechanism is not known, but could involve shared etiological risk factors between MS and low bone mass such as hypovitaminosis D operating before disease onset, or increased bone loss after disease onset. The aim of this study was to explore the mechanism of the low bone mass in early-stage MS patients., Methodology/principal Findings: We performed a population-based case-control study comparing bone turnover (cross-linked N-terminal telopeptide of type 1 collagen; NTX, bone alkaline phosphatase; bALP), metabolism (25-hydroxy- and 1, 25-dihydroxyvitamin D, calcium, phosphate, and parathyroid hormone), and relevant lifestyle factors in 99 patients newly diagnosed with clinically isolated syndrome (CIS) or MS, and in 159 age, sex, and ethnicity matched controls. After adjustment for possible confounders, there were no significant differences in NTX (mean 3.3; 95% CI -6.9, 13.5; p = 0.519), bALP (mean 1.6; 95% CI -0.2, 3.5; p = 0.081), or in any of the parameters related to bone metabolism in patients compared to controls. The markers of bone turnover and metabolism were not significantly correlated with bone mass density, or associated with the presence of osteoporosis or osteopenia within or between the patient and control groups. Intake of vitamin D and calcium, reported UV exposure, and physical activity did not differ significantly., Conclusions/significance: Bone turnover and metabolism did not differ significantly in CIS and MS patients with prevalent low bone mass compared to controls. These findings indicate that the bone deficit in patients newly diagnosed with MS and CIS is not caused by recent acceleration of bone loss, and are compatible with shared etiological factors between MS and low bone mass.

Bone remodeling is a tightly regulated process securing repair of microdamage (targeted remodeling) and replacement of old bone with new bone through sequential osteoclastic resorption and osteoblastic bone formation. The rate of remodeling is regulated by a wide variety of calcitropic hormones (PTH, thyroid hormone, sex steroids etc.). In recent years we have come to appreciate that bone remodeling proceeds in a specialized vascular structure,--the Bone Remodeling Compartment (BRC). The outer lining of this compartment is made up of flattened cells, displaying all the characteristics of lining cells in bone including expression of OPG and RANKL. Reduced bone turnover leads to a decrease in the number of BRCs, while increased turnover causes an increase in the number of BRCs. The secretion of regulatory factors inside a confined space separated from the bone marrow would facilitate local regulation of the remodeling process without interference from growth factors secreted by blood cells in the marrow space. The BRC also creates an environment where cells inside the structure are exposed to denuded bone, which may enable direct cellular interactions with integrins and other matrix factors known to regulate osteoclast/osteoblast activity. However, the denuded bone surface inside the BRC also constitutes an ideal environment for the seeding of bone metastases, known to have high affinity for bone matrix. Circulating osteoclast- and osteoblast precursor cells have been demonstrated in peripheral blood. The dominant pathway regulating osteoclast recruitment is the RANKL/OPG system, while many different factors (RUNX, Osterix) are involved in osteoblast differentiation. Both pathways are modulated by calcitropic hormones.

In recent years, we have come to appreciate that the close association between bone and vasculature plays a pivotal role in the regulation of bone remodeling and fracture repair. In 2001, Hauge et al. characterized a specialized vascular structure, the bone remodeling compartment (BRC), and showed that the outer lining of this compartment was made up of flattened cells, displaying all the characteristics of lining cells in bone. A decrease in bone turnover leads to a decrease in surfaces covered with remodeling compartments, whereas increased turnover causes an increase. Immunoreactivity for all major osteotropic growth factors and cytokines including osteoprotegerin (OPG) and RANKL has been shown in the cells lining the BRC, which makes the BRC the structure of choice for coupling between resorption and formation. The secretion of these factors inside a confined space separated from the bone marrow would facilitate local regulation of the remodeling process without interference from growth factors secreted by blood cells in the marrow space. The BRC creates an environment where cells inside the structure are exposed to denuded bone, which may enable direct cellular interactions with integrins and other matrix factors known to regulate osteoclast/osteoblast activity. However, the denuded bone surface inside the BRC also constitutes an ideal environment for the seeding of bone metastases, known to have high affinity for bone matrix. Reduction in BRC space brought about by antiresorptive therapies such as bisphosphonates reduce the number of skeletal events in advanced cancer, whereas an increase in BRC space induced by remodeling activators like PTH may increase the bone metastatic burden. The BRC has only been characterized in detail in trabecular bone; there is, however, evidence that a similar structure may exist in cortical bone, but further characterization is needed.

Unlabelled: Transiliac bone biopsies were obtained from 55 women treated with teriparatide or placebo for 12-24 months. We report direct evidence that modeling bone formation at quiescent surfaces was present only in teriparatide-treated patients and bone formation at remodeling sites was higher with teriparatide than placebo., Introduction: Recombinant teriparatide [human PTH(1-34)], a bone formation agent for the treatment of osteoporosis when given once daily subcutaneously, increases biochemical markers of bone turnover and activation frequency in histomorphometry studies., Materials and Methods: We studied the mechanisms underlying this bone-forming action of teriparatide at the basic multicellular unit by the appearance of cement lines, a method used to directly classify surfaces as modeling or remodeling osteons, and by the immunolocalization of IGF-I and IGF-II. Transiliac bone biopsies were obtained from 55 postmenopausal women treated with teriparatide 20 or 40 microg or placebo for 12-24 months (median, 19.8 months) in the Fracture Prevention Trial., Results: A dose-dependent relationship was observed in modeling and mixed remodeling/modeling trabecular hemiosteons. Trabecular and endosteal hemiosteon mean wall thicknesses were significantly higher in both teriparatide groups than in placebo. There was a dose-dependent relationship in IGF-II immunoreactive staining at all bone envelopes studied. The greater local IGF-II presence after treatment with teriparatide may play a key role in stimulating bone formation., Conclusions: Direct evidence is presented that 12-24 months of teriparatide treatment induced modeling bone formation at quiescent surfaces and resulted in greater bone formation at remodeling sites, relative to placebo.

Unlabelled: An 18-month randomized double-blind study was conducted in postmenopausal women with osteoporosis to compare the effects of once-daily teriparatide 20 microg with alendronate 10 mg on bone histomorphometry. Biopsies were obtained from 42 patients. Indices of bone formation were significantly higher after 6 or 18 months of teriparatide compared with alendronate treatment., Introduction: Alendronate and teriparatide increased BMD, assessed by DXA, by different mechanisms of action, supported by changes in biochemical markers of bone turnover. The purpose of this cross-sectional study was to explore the differential effects of these two osteoporosis treatments at the bone tissue level by examining bone histomorphometric parameters of bone turnover after either 6 or 18 months of treatment., Materials and Methods: Patients were a cohort from a randomized parallel double-blind study conducted to compare the effects of once-daily teriparatide 20 microg and alendronate 10 mg in postmenopausal women with osteoporosis. Transiliac crest bone biopsies were obtained after tetracycline double labeling from 42 patients treated for 6 months (n = 23) or 18 months (n = 14); 5 additional patients were biopsied from contralateral sides at 6 and 18 months. Biopsy specimens adequate for quantitative analysis were analyzed by 2D histomorphometry from 17 patients at 6 months (teriparatide, n = 8; alendronate, n = 9) and 15 patients at 18 months (teriparatide, n = 8; alendronate, n = 7). Data were analyzed by two-sample tests., Results: Histomorphometric indices of bone formation were significantly and markedly greater in the teriparatide group than in the alendronate group at 6 and 18 months, whereas indices of bone resorption were only significantly greater in the teriparatide group than in the alendronate group at 6 months. Bone formation and activation frequency were significantly lower at 18 months compared with 6 months in the teriparatide group, returning to levels comparable with untreated postmenopausal women. In the teriparatide group, the peak in histomorphometric bone formation indices coincided with peak levels for N-terminal propeptide of type I collagen, a biochemical marker of bone formation. The degree of mineralization was lower at 18 months than at 6 months with treatment in both groups but was not different between groups., Conclusions: These results confirm the opposite mechanisms of action of teriparatide and alendronate on bone remodeling and confirm the bone formation effect of teriparatide.

The chronic excessive hypersecretion of parathyroid hormone (PTH) in primary hyperparathyroidism (PHPT) has significant impact on bone remodeling. Bone turnover increases by about 50%, leading to increased resorption at the endosteal envelope, increased cortical porosity, and thinning of cortical bone. In cancellous bone a different response is seen. Despite stimulation of bone resorption and formation at the tissue level, osteoclastic resorption and osteoblastic bone formation at individual bone multicellular units (BMUs) are reduced. This causes reduced erosion depth, reduced bone formation rate, and decreased thickness of bone structural units (BSUs), and bone balance at individual BMUs is preserved and may even improve. Thus, PHPT causes cortical bone loss. At the same time, however, cancellous bone structure remains unchanged or even improved, which may offset cortical bone loss. This probably explains the preservation of bone mass demonstrated in long-term follow-up studies of patients with mild PHPT (S-Ca2+ < 2.80 mM). In severe cases of PHPT (S-Ca2+ > 3.00 mM), the negative effects on cortical bone may override the positive impact on cancellous bone, and lead to bone loss and increased risk of fracture.

Previous studies have demonstrated that the administration of zoledronic acid (ZOL) once yearly for 3 years or once over 3 years, yields similar antifracture efficacy. Bone turnover markers can predict the antifracture efficacy of antiresorptive agents, with procollagen type 1 N-terminal propeptide (P1NP) being the most useful marker. In this retrospective cohort study, we explored the effects of intravenous dosing of ZOL guided by serum (S)-P1NP assessment on bone mineral density (BMD) and fractures. Consenting patients (N  = 202, mean age 68.2 years) with osteoporosis were treated with ZOL for an average of 4.4 (range 2-8) years. S-P1NP and BMD were measured at baseline and every 1-2 years. We assessed the number of subsequent vertebral and nonvertebral fractures in the 2-year time periods. The number of patients assessed was 202, 147, 69, and 29 at years 1-2, 3-4, 5-6, and 7-8, respectively. A new ZOL infusion was given if S-P1NP exhibited values above 35 μg/L. BMD increased by 6.2% (SD 4.0) over the first 2 years and stabilized in years 2-8 (P  <.05). Median S-P1NP exhibited an initial reduction from 58.0 to 31.3 μg/L at year 2 and then increased to 39.0 μg/L at years 7-8. Compared with fractures observed in the last 2 years before baseline, fracture rates exhibited consistent reductions, for vertebral fractures odds ratio (OR) [95% confidence interval] = 0.61 [0.47, 0.80], P  <.001 and for nonvertebral fractures OR = 0.23 [0.18, 0.31], P  <.001. In conclusion, intermittent dosing of intravenous ZOL based on the assessment of S-P1NP with cut-off at 35 μg/L resulted in an initial increase followed by a stable BMD, suppression of S-P1NP, and stable reduction of fractures for 8 years. Only 39% of patients needed more than one infusion. This approach reduces healthcare costs and might also reduce the risk of rare side effects such as osteonecrosis of the jaw and atypical femoral fracture. [ABSTRACT FROM AUTHOR]

Background. Bone turnover markers (BTMs) have emerged as a useful tool for monitoring bone remodeling activity in the skeleton, and their serum levels correlate with bone loss rates in osteoporotic and normal individuals. Whether the same holds for other metabolic bone diseases is still subject to discussion. Methods. We analyzed the relation between levels of BTMs and TSH in 79 females on thyroid hormone substitution therapy for hypothyroidism. Based on the reference range for TSH (0.2–4.0 mU/L) in our lab, we assessed BTMs in five different groups of patients based on the following criteria: (1) hypothyroidism (TSH >4.0); (2) TSH in the high normal range (1.0–4.0); (3) TSH in the low normal range (0.2–1.0); (4) TSH below the normal range (0.01–0.2); (5) TSH undetectable (<0.01). We studied the relationship between TSH and four different bone markers: procollagen type 1 N-terminal propeptide (PINP), C-terminal cross-linking telopeptide of type 1 collagen (CTX), osteocalcin (OC), and bone specific alkaline phosphatase (BSAP). In a subgroup of patients, bone mineral density was assessed by a DXA scan. Results. PINP emerged as the most sensitive and dynamic BTM for assessment of bone turnover in this patient group, achieving significant rho values on nonparametric correlation analysis for both TSH (rho −0.47; p = 0.0001) and FT4 (rho 0.27; p = 0.018). CTX and OC also revealed significant correlations to TSH, albeit with lower rho values (−0.37 and −0.24, respectively). Categorical analysis showed that bone turnover increased significantly, albeit with pronounced interindividual variability for TSH values below the lower limit of normal (0.2 mU/l), with the most severe affected being women exhibiting suppression of TSH. Further analysis of loss rates by DXA in a limited subgroup of patients showed that this was accompanied by accelerated bone loss. Conclusion. PINP is the most sensitive marker of bone turnover in thyroid disorders. TSH values below the lower limit of normal are associated with increased bone turnover and accelerated bone loss, however, with pronounced interindividual variations. Assessment of PINP may be a valuable tool in cases where there is concern about possible adverse effects of thyroid hormone substitution therapy on bone. [ABSTRACT FROM AUTHOR]

Osteoanabolic therapy with parathyroid hormone (PTH(1–84)) or the PTH analogues teriparatide (PTH(1–34), TPTD) and abaloparatide induces a positive remodeling balance and increases modeling and remodeling activity on bone surfaces. As the anabolic action of PTH is primarily remodeling based increased bone turnover maximizes bone accrual. Increased remodeling, however, also increases cortical porosity and reduces mineralization of newly formed bone, which may cause initial reductions in BMD, particularly at sites rich in cortical bone. Increased cortical porosity may also have negative consequences for bone strength. Consequently, an interest developed in concurrent therapies offsetting the potential early negative cortical bone effects developed, and several studies using varying concurrent combinations of TPTD or PTH(1–84) with various antiresorptive (anti-catabolic) agents (estrogen, SERMs, bisphosphonates and denosumab) have been published. This commentary addresses the discrepancy between changes in areal bone mineral density (BMD) and bone turnover markers (BTM) in concurrent therapy studies leading to possible misinterpretations of the results. In studies of concurrent therapies increases in BMD are generally accompanied by decreases in biochemical markers of bone turnover. This includes Procollagen Type I N-Terminal Propetide (PINP), which has emerged as a reliable marker of bone formation during osteoanabolic therapy. We therefore want to submit, that the larger increases in BMD seen initially in patients on concurrent therapy mask the potential for later reduced osteoanabolic action of PTH. This notion is corroborated by: 1) the lesser impairment of bone anabolism seen with milder antiresorptive modalities like hormone replacement therapy (HRT) or Selective Estrogen Receptor Modulators (SERMs); 2) the changes in BMD seen in extension studies where treatment naïve patients previously treated with PTH alone are crossed over to antiresorptive drugs. We therefore advise against a general use of concurrent therapy with PTH and antiresorptive agents, as it entails blunting of osteoanabolic action of PTH in the long run. [ABSTRACT FROM AUTHOR]

The article presents the study that investigates the close association between bone and vasculature which play a vital role in the regulation of bone remodeling and fracture repair. The researchers found a decrease in bone turnover that lead to a decrease in surfaces covered with remodeling compartments. The reduction of bone remodeling compartment space brought about by antiresorptive therapies reduce the number of skeletal events in advanced cancer.