Changes in osteoprotegerin/RANKL system, bone mineral density, and bone biochemicals markers in patients with recent spinal cord injury

A cut-off value of plasma osteoprotegerin level may predict the presence of coronary artery calcifications in chronic kidney disease patients

BACKGROUND

Expression of bone proteins resulting from transdifferentiation of vascular smooth muscle cells into osteoblasts suggests that vascular calcifications are a bioactive process. Osteoprotegerin (OPG) could play a key role in bone-vascular calcification imbalance and could be a marker of vascular calcification extent and progression. The purpose of this study was to evaluate relationships between vascular risk biomarkers (including classic risk factors and OPG) and coronary artery calcification (CAC) extent in chronic kidney disease (CKD) patients and to establish within the markers the appropriate cut-off value to predict CAC.

METHODS

A total of 133 non-dialyzed CKD patients at various stages of kidney disease [75 males/58 females, median age: 69.9 (27.4-94.6)] were enrolled, excluding extrarenal replacement therapy patients. All underwent chest multidetector computed tomography for CAC scoring. Blood samples were collected for measurement of vascular risk markers (kidney disease, inflammation, nutrition, calcium phosphate and OPG). A potential relationship between CAC and these biological markers was investigated, and a receiver-operating characteristic (ROC) curve was designed thereafter to identify a cut-off value of involved markers that best predicted the presence of CAC.

RESULTS

After adjustment for age, diabetes, smoking and gender, among biological markers, only low-estimated glomerular filtration rate using Modification of Diet in Renal Disease [OR = 3.63 (1.10-12.02)], high FEPO(4) [OR = 3.99 (1.17-13.6)] and high OPG levels [OR = 8.54 (2.14-34.11)] were associated with the presence of CAC. A protective effect of 1.25(OH)(2) vitamin D [OR = 0.20 (0.05-0.79)] and LDL cholesterol [OR = 0.27 (0.08-0.94)] on CAC was also observed. ROC curve analysis showed that the OPG best cut-off value predicting CAC was 757.7 pg/mL.

CONCLUSION

These results suggest that a CAC increase is strongly associated with a plasma OPG increase in CKD patients. The values of OPG >757.7 pg/mL allow us to predict the presence of CAC in these patients.

The response of bone to mechanical loading and disuse: fundamental principles and influences on osteoblast/osteocyte homeostasis

Bone's response to increased or reduced loading/disuse is a feature of many clinical circumstances, and our daily life, as habitual activities change. However, there are several misconceptions regarding what constitutes loading or disuse and why the skeleton gains or loses bone. The main purpose of this article is to discuss the fundamentals of the need for bone to experience the effects of loading and disuse, why bone loss due to disuse occurs, and how it is the target of skeletal physiology which drives pathological bone loss in conditions that may not be seen as being primarily due to disuse. Fundamentally, if we accept that hypertrophy of bone in response to increased loading is a desirable occurrence, then disuse is not a pathological process, but simply the corollary of adaptation to increased loads. If adaptive processes occur to increase bone mass in response to increased load, then the loss of bone in disuse is the only way that adaptation can fully tune the skeleton to prevailing functional demands when loading is reduced. The mechanisms by which loading and disuse cause bone formation or resorption are the same, although the direction of any changes is different. The osteocyte and osteoblast are the key cells involved in sensing and communicating the need for changes in mass or architecture as a result of changes in experienced loading. However, as those cells are affected by numerous other influences, the responses of bone to loading or disuse are not simple, and alter under different circumstances. Understanding the principles of disuse and loading and the mechanisms underlying them therefore represents an important feature of bone physiology and the search for targets for anabolic therapies for skeletal pathology.

The role of the receptor activator of nuclear factor-kappaB ligand/osteoprotegerin cytokine system in primary hyperparathyroidism

CONTEXT

The mechanisms of action of PTH on bone in vivo remain incompletely understood. The objective of this investigation was to examine changes in serum levels of receptor activator of nuclear factor-kappaB ligand and osteoprotegerin (OPG) in primary hyperparathyroidism and their relationship to bone loss.

PATIENTS AND METHODS

Twenty-nine patients with primary hyperparathyroidism had baseline circulating soluble receptor activator of nuclear factor-kappaB ligand (sRANKL) and OPG measured. The relationship to biochemical markers of bone turnover and changes in bone mineral density over 2 yr was examined.

RESULTS

Baseline sRANKL levels were elevated (1.7+/-0.1 pmol/liter), whereas OPG remained in the normal range (5.6+/-0.4 pmol/liter). Circulating sRANKL did not correlate with PTH but did correlate with markers of bone resorption (urine deoxypyridinoline cross-links: r=0.51, P<0.01; serum N-telopeptide of type I collagen: r=0.37, P<0.05). Furthermore, sRANKL correlated with both IL-6 and IL-6 soluble receptor (IL-6sR) (r=0.47, P<0.05 and r=0.55, P<0.005, respectively). Serum sRANKL levels also correlated with bone loss at the total femur (r=-0.53, P<0.01). Lastly, a high value of sRANKL in combination with values of IL-6 and IL-6sR in the upper quartile (sRANKL>or=1.81 pg/ml, IL -6>or=11.8 pg/ml, and IL-6sR>or=45.6 ng/ml) defined a group of four women with significantly greater rates of bone loss at the total femur than the remaining patients (-2.7+/-1.7% vs. +0.5+/-0.3%; n=4 vs. n=19, P<0.05).

CONCLUSION

Determination of circulating levels of sRANKL may be useful in identifying patients with mild primary hyperparathyroidism at greater risk for bone loss. The fact that circulating sRANKL did not correlate with PTH but did correlate with markers of bone resorption suggests that skeletal responsiveness to PTH may differ in this disease.

Effects of spinal cord injury on osteoblastogenesis, osteoclastogenesis and gene expression profiling in osteoblasts in young rats

INTRODUCTION

Spinal cord injury (SCI) causes a significant amount of bone loss in the sublesional area in animals and humans, and this type of bone loss is different from other forms of osteoporosis such as disuse osteoporosis and postmenopausal osteoporosis. However, no data is available on the cellular and molecular changes of osteoblastogenesis and osteoclastogenesis during SCI-induced bone loss.

METHODS

SCI and SHAM rats were used in this study to investigate osteoblastogenesis and osteoclastogenesis in bone-marrow culture. We also measured bone mass and bone histomorphometry, as well as the expression of alkaline phosphatase (ALP), core binding factor alpha1 (Cbfa-1), osterix, receptor activator of NF-kappaB ligand (RANKL) and osteoprotegerin (OPG) in osteoblast-like cells in bone-marrow culture obtained from SCI and SHAM rats.

RESULTS

Bone mineral density (BMD) measurement showed serious bone loss in the tibial ephiphyses and metaphyses of SCI rats compared with SHAM rats. In addition, bone histomorphometry analysis of the tibial metaphyses of SCI rats demonstrated that bone microarchitecture in SCI rats deteriorated further than in SHAM rats, and increased eroded surfaces and bone formation rates were observed in SCI rats. The number of osteoclasts that developed from bone marrow of SCI rats at equal density was significantly increased compared with SHAM rats, and the area of the resorption pits formed in the bone marrow culture from SCI rats was significantly greater than SHAM rats, whereas the number of CFU-F and CFU-OB was similar in both groups. RANKL mRNA and protein levels in osteoblast-like cells in culture obtained from SCI rats were significantly higher than those from the SHAM rats, whereas OPG levels decreased slightly. The ratios of RANKL to OPG expression in SCI rats were significantly higher than those in SHAM rats. However, osteogenic gene profiling of Cbfa-1, ALP and osterix in SCI rats remained similar with SHAM rats.

CONCLUSION

These changes favor increased osteoclast activity over osteoblast activity, and may explain, in part, the imbalance in bone formation and resorption following SCI.

Mechanisms of osteoporosis in spinal cord injury

Osteoporosis is a known complication of spinal cord injury (SCI), but its mechanism remains unknown. The pathogenesis of osteoporosis after SCI is generally considered disuse. However, although unloading is an important factor in the pathogenesis of osteoporosis after SCI, neural lesion and hormonal changes also seem to be involved in this process. Innervation and neuropeptides play an important role in normal bone remodelling. SCI results in denervation of the sublesional bones and the neural lesion itself may play a pivotal role in the development of osteoporosis after SCI. Although upper limbs are normally loaded and innervated, bone loss also occurs in the upper extremities in patients with paraplegia, indicating that hormonal changes may be associated with osteoporosis after SCI. SCI-mediated hormonal changes may contribute to osteoporosis after SCI by different mechanisms: (1) increased renal elimination and reduced intestinal absorption of calcium leading to a negative calcium balance; (2) vitamin D deficiency plays a role in the pathogenesis of SCI-induced osteoporosis; (3) SCI antagonizes gonadal function and inhibits the osteoanabolic action of sex steroids; (4) hyperleptinaemia after SCI may contribute to the development of osteoporosis; (5) pituitary suppression of TSH may be another contributory factor to bone loss after SCI; and (6) bone loss after SCI may be caused directly, at least in part, by insulin resistance and IGFs. Thus, oversupply of osteoclasts relative to the requirement for bone resorption and/or undersupply of osteoblasts relative to the requirement for cavity repair results in bone loss after SCI. Mechanisms for the osteoporosis following SCI include a range of systems, and osteoporosis after SCI should not be simply considered as disuse osteoporosis. Unloading, neural lesion and hormonal changes after SCI result in severe bone loss. The aim of this review is to improve understanding with regard to the mechanisms of osteoporosis after SCI. The understanding of the pathogenesis of osteoporosis after SCI can help in the consideration of new treatment strategies. Because bone resorption after SCI is very high, intravenous bisphosphonates and denosumab should be considered for the treatment of osteoporosis after SCI.

Bone loss in spinal cord-injured patients: from physiopathology to therapy

STUDY DESIGN

Review article on bone metabolism and therapeutic approach on bone loss in patients with spinal cord injury (SCI).

OBJECTIVE

The first part aims to describe the process of bone demineralization and its effects on bone mass in patients with SCI. The second part describes and discusses the therapeutic approaches to limiting the alteration in bone metabolism related to neurological lesions.

SETTING

Propara Rehabilitation Center, Montpellier, France.

RESULTS

During the first 24 months postinjury, demineralization occurs exclusively in the sublesional areas and predominantly in weight-bearing skeletal sites such as the distal femur and proximal tibia, both of which are trabecular-rich sites. Reduced bone mass, in association with a modified bone matrix property and composition, is very likely at the origin of pathological fractures after minor trauma to which these patients are frequently exposed. Since these fractures may be asymptomatic yet may lead to complications, preventing and managing 'neurological osteoporosis' remains a considerable challenge. Two main approaches are considered: the first consists in applying a mechanical stimulus to the bone tissue by standing, orthotically aided walking or functional electrical stimulation (FES). The second uses medications, particularly antiresorptive drugs such as calcitonin or diphosphonates.

CONCLUSION

To develop well-adapted treatments, a more precise understanding of bone loss etiology is needed. The current rehabilitation programs are based on the idea that the bone physiological changes observed in patients with SCI are due to immobility, but results indicate that alterations inherent to neurological damage may play an even greater role in inducing osteoporosis.

Parathyroid hormone may maintain bone formation in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis

Mechanical unloading of bone causes an imbalance in bone formation and resorption leading to bone loss and increased fracture risk. Black bears(Ursus americanus) are inactive for up to six months during hibernation, yet bone mineral content and strength do not decrease with disuse or aging. To test whether hibernating bears have biological mechanisms to prevent disuse osteoporosis, we measured the serum concentrations of hormones and growth factors involved in bone metabolism and correlated them with the serum concentration of a bone formation marker (osteocalcin). Serum was obtained from black bears over a 7-month duration that included periods of activity and inactivity. Both resorption and formation markers increased during hibernation, suggesting high bone turnover occurred during inactivity. However, bone formation appeared to be balanced with bone resorption. The serum concentration of parathyroid hormone (PTH) was higher in the hibernation(P=0.35) and post-hibernation (P=0.006) seasons relative to pre-hibernation levels. Serum leptin was lower (P&lt;0.004)post-hibernation relative to pre-hibernation and hibernation periods. Insulin-like growth factor I (IGF-I) decreased (P&lt;0.0001) during hibernation relative to pre-hibernation and reached its highest value during remobilization. There was no difference (P=0.64) in 25-OH vitamin D between the three seasons. Serum osteocalcin (bone formation marker) was significantly correlated with PTH, but not with leptin, IGF-I or 25-OH vitamin D. Osteocalcin and PTH were positively correlated when samples from all seasons were pooled and when only hibernation samples were considered, raising the possibility that the anabolic actions of PTH help maintain bone formation to prevent disuse osteoporosis. Prostaglandin E2 (PGE2)release from MC3T3 osteoblastic cells was significantly affected by treatment with bear serum from different seasons (i.e. hibernation versus active periods). The seasonal changes in PGE2 release showed trends similar to the seasonal changes in serum IGF-I. Since both PGE2 and IGF-I are associated with collagenous bone formation, it is possible that seasonal changes in a circulating factor influence IGF-I levels in vivo in bears and PGE2 release in osteoblastic cells in vitro. The significant decrease in serum leptin following arousal from hibernation may promote bone formation during remobilization, assuming there is a similar decrease in intracerebroventricular leptin. These findings support the idea that seasonal changes in the concentration of circulating molecules help regulate bone formation activity and may be important for preventing disuse osteoporosis in bears.

Physiological effects of lower extremity functional electrical stimulation in early spinal cord injury: lack of efficacy to prevent bone loss

STUDY DESIGN

Controlled, repeat-measures study.

OBJECTIVES

To determine if functional electrical stimulation (FES) can affect bone atrophy in early spinal cord injury (SCI), and the safety, tolerance and feasibility of this modality in bone loss remediation.

SETTING

Spinal Injuries Units, Royal Adelaide Hospital and Hampstead Rehabilitation Centre, South Australia.

METHODS

Patients with acute SCI (ASIA A-D) were allocated to FES (n=23, 28+/-9 years, C4-T10, 13 Tetra) and control groups (CON, n=10, 31+/-11 years, C5-T12, four Tetra). The intervention group received discontinuous FES to lower limb muscles (15 min sessions to each leg twice daily, over a 5-day week, for 5 months). Dual energy X-ray absorptiometry (DEXA) measured total body bone mineral density (tbBMD), hip, spine BMD and fat mass (FM) within 3 weeks, and 3 and 6 months postinjury.

RESULTS

FES and CON groups' tbBMD differed significantly at 3 months postinjury (P<0.01), but not thereafter. Other DEXA measures (hip, spine BMD, FM) did not differ between groups at any time. No adverse events were identified.

CONCLUSION

Electrically stimulated muscle activation was elicited, and tetanic effects were reproducible; however, there were no convincing trends to suggest that FES can play a clinically relevant role in osteoporosis prevention (or subsequent fracture risk) in the recently injured patient. The lack of an osteogenic response in paralysed extremities to electrically evoked exercise during subacute and rehabilitation/recovery phases cannot be fully explained, and may warrant further evaluation.

Plasma Osteoprotegerin Is Associated with Mortality in Hemodialysis Patients

Expression of bone proteins resulting from transdifferentiation of vascular smooth muscle cells into osteoblasts suggests that vascular calcifications are a bioactive process. Regulating molecules such as osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL) could play a key role in bone-vascular calcification imbalance. This study investigated the contribution of these proteins as well as mineral metabolism disorders in hemodialysis (HD) patient outcome. A total of 185 HD patients were followed up prospectively for 2 yr. In addition to clinical characteristics, mineral metabolism markers as well as OPG and soluble RANKL (sRANKL) were measured at baseline. After 2 yr, survival rates were described with Kaplan-Meier and compared with Cox regression analyses; 50 patients died (27 from cardiovascular diseases). Calcium, phosphate, and calcium x phosphate product were not associated with mortality. Both hyperparathyroidism (parathyroid hormone > or =300 pg/ml) and hypoparathyroidism (parathyroid hormone <150 pg/ml) were poorly associated with all-cause and cardiovascular mortality. By contrast, elevated OPG levels predicted all-cause (relative risk [RR] 2.67; 95% confidence interval [CI] 1.32 to 5.41; P = 0.006) and cardiovascular mortality (RR 3.15; 95% CI 1.14 to 8.69; P = 0.03). Low levels of sRANKL were associated with a protective effect for all-cause mortality (RR 0.45; 95% CI 0.21 to 0.94; P = 0.03). The association of OPG with all-cause mortality was stronger in patients with C-reactive protein > or =12.52 mg/L. In this condition, both highest (RR 5.68; 95% CI 1.48 to 22.73; P = 0.01) and lowest tertiles (RR 5.37; 95% CI 147 to 1968; P = 0.01) significantly predicted poor outcome. These results show that regulating-bone molecules, especially OPG, are strong predictors of mortality in HD patients, suggesting that OPG is a vascular risk factor, in particular in patients who have high C-reactive protein levels. OPG determination therefore should be added to the biologic follow-up of these patients.