Clinical application of metabolic bone markers for fracture - effects of fracture on metabolic bone markers -

The process of fracture healing can be divided into three distinct stages - inflammatory, reparative and remodeling stages. The changes of bone formation and bone resorption in the process of fracture healing are expected to be more dynamic than those changes which occur in the remodeling cycle alone because of aging. Bone formation and bone resorption markers increased 1 or 2 weeks after fracture. Bone resorption markers returned to the baseline level at 24 weeks after fracture, while values of bone formation marker were still higher compared to the baseline level at 24 weeks after fracture. It is suggested that bone metabolism is still activated at 24 weeks after fracture. In the acute phase after fracture, many factors such as bed rest, skin incision, intramedullay reaming during operation, could modify the values of bone resorption and bone formation markers. Therefore, clinical application of metabolic bone markers for fracture might be useful in the remodeling stage after fracture.     

Focal bone involvement in inflammatory arthritis: the role of IL17

Conditions such as rheumatoid arthritis (RA) and spondyloarthritis (SpA, such as psoriatic arthritis, PsA, and ankylosing spondylitis, AS) are characterized by an imbalance between osteoclast (OC) bone resorption and osteoblast (OB) bone formation. The two conditions present substantial differences in bone involvement, which is probably related to the different expression of IL17 and TNFα, two cytokines that strongly promote osteoclastogenesis and focal bone erosions. TNFα is the major inflammatory cytokine in RA. It acts by both triggering OC bone erosion via the RANK–RANKL system, and suppressing OB bone formation through the overexpression of DKK1, a powerful inhibitor of the WNT bone anabolic signaling pathway. Differing from TNFα, IL17 promotes also osteogenesis, particularly at inflamed sites undergoing mechanical stress, such as entheses. Therefore, in RA, where overexpression of TNFα is higher than IL17, OC bone resorption largely prevails upon bone formation. In PsA and AS, the prevailing inflammatory cytokine is IL17, which promotes also osteogenesis. Given the prevalent involvement of entheses poor of OC, excess bone formation may even prevail over excess bone resorption. The results of clinical trials support the different pathophysiology of bone involvement in chronic arthritis. Inflammation control through anti-TNFα agents has not resulted in incomparable effects on radiographic progression and excess bone formation in both AS and PsA. Clinical trials investigating IL17 inhibitors, such as secukinumab, in patients with psoriatic disease are underway. The preliminary results on inflammation and symptoms appear positive, while long-term studies are required to demonstrate an effect on excess bone formation.     

Mechanisms of pathologic new bone formation

Pathologic new bone formation occurs in response to a variety of stimuli. Heterotopic and orthotopic bone formation can interfere with the normal function of the joint and can contribute to disability in inflammatory joint diseases. Syndesmophyte formation and progressive ankylosis are characteristic features of spondyloarthropathies, including psoriatic arthritis and ankylosing spondylitis, and they can be regarded as abnormal bone remodeling. Successful blocking of inflammation in patients with spondyloarthropathy apparently fails to halt progression of ankylosis in cohort studies. This suggests that though they may be linked in some way, bone formation and inflammation are largely independent phenomena. Indeed, new bone formation also occurs in diseases such as osteoarthritis and diffuse idiopathic skeletal hyperostosis. Therefore, therapeutic strategies in spondyloarthropathy ideally should control both inflammation and bone formation.     

Mechanism of arterial calcification with regards to atherosclerotic calcification and medial artery calcification

Vascular calcification is a mechanism of active process, which includes inflammation and metabolism of bone formation. There are two different types of vascular calcification; atherosclerotic calcification and medial artery calcification. As seen in bone metabolism, vascular calcification is reported to be through enchondral ossification through BMP-2-Cbfa1 signal pathway, and through intramembranous ossification through BMP-2-Msx2 signal pathway. In the tissue of vascular calcification, several proteins which are related to bone and bone matrix metabolism have been shown to be present, such as bone morphogenic protein, osteoprotegerin, matrix gla protein, and osteopontin.     

Beta-catenin in the race to fracture repair: in it to Wnt

The Wnt/beta-catenin pathway regulates multiple biological events during embryonic development, including bone formation. Fracture repair recapitulates some of the processes of normal bone development, such as the formation of bone from a cartilaginous template, and many cell-signaling pathways that underlie bone development are activated during the repair process. The Wnt/beta-catenin signaling pathway is activated during fracture repair, and dysregulation of this pathway alters the normal bone-healing response. In early pluripotent mesenchymal stem cells, Wnt/beta-catenin signaling needs to be precisely regulated to facilitate the differentiation of osteoblasts; by contrast, beta-catenin is not needed for chondrocyte differentiation. Once mesenchymal stem cells are committed to the osteoblast lineage, activation of Wnt/beta-catenin signaling enhances bone formation. This activity suggests that the Wnt/beta-catenin pathway is a therapeutic target during bone repair. Indeed, treatments that activate Wnt/beta-catenin signaling, such as lithium, increase bone density and also enhance healing.     

cAMP/PKA pathway activation in human mesenchymal stem cells in vitro results in robust bone formation in vivo

Tissue engineering of large bone defects is approached through implantation of autologous osteogenic cells, generally referred to as multipotent stromal cells or mesenchymal stem cells (MSCs). Animal-derived MSCs successfully bridge large bone defects, but models for ectopic bone formation as well as recent clinical trials demonstrate that bone formation by human MSCs (hMSCs) is inadequate. The expansion phase presents an attractive window to direct hMSCs by pharmacological manipulation, even though no profound effect on bone formation in vivo has been described so far using this approach. We report that activation of protein kinase A elicits an immediate response through induction of genes such as ID2 and FosB, followed by sustained secretion of bone-related cytokines such as BMP-2, IGF-1, and IL-11. As a consequence, PKA activation results in robust in vivo bone formation by hMSCs derived from orthopedic patients.     

A soluble bone morphogenetic protein type IA receptor increases bone mass and bone strength

Diseases such as osteoporosis are associated with reduced bone mass. Therapies to prevent bone loss exist, but there are few that stimulate bone formation and restore bone mass. Bone morphogenetic proteins (BMPs) are members of the TGFβ superfamily, which act as pleiotropic regulators of skeletal organogenesis and bone homeostasis. Ablation of the BMPR1A receptor in osteoblasts increases bone mass, suggesting that inhibition of BMPR1A signaling may have therapeutic benefit. The aim of this study was to determine the skeletal effects of systemic administration of a soluble BMPR1A fusion protein (mBMPR1A-mFc) in vivo. mBMPR1A-mFc was shown to bind BMP2/4 specifically and with high affinity and prevent downstream signaling. mBMPR1A-mFc treatment of immature and mature mice increased bone mineral density, cortical thickness, trabecular bone volume, thickness and number, and decreased trabecular separation. The increase in bone mass was due to an early increase in osteoblast number and bone formation rate, mediated by a suppression of Dickkopf-1 expression. This was followed by a decrease in osteoclast number and eroded surface, which was associated with a decrease in receptor activator of NF-κB ligand (RANKL) production, an increase in osteoprotegerin expression, and a decrease in serum tartrate-resistant acid phosphatase (TRAP5b) concentration. mBMPR1A treatment also increased bone mass and strength in mice with bone loss due to estrogen deficiency. In conclusion, mBMPR1A-mFc stimulates osteoblastic bone formation and decreases bone resorption, which leads to an increase in bone mass, and offers a promising unique alternative for the treatment of bone-related disorders.     

Myeloma bone disease and proteasome inhibition therapies

Bone disease is one of the most debilitating manifestations of multiple myeloma. A complex interdependence exists between myeloma bone disease and tumor growth, creating a vicious circle of extensive bone destruction and myeloma progression. Proteasome inhibitors have recently been shown to promote bone formation in vitro and in vivo. Preclinical studies have demonstrated that proteasome inhibitors, including bortezomib, which is the first-in-class such agent, stimulate osteoblast differentiation while inhibiting osteoclast formation and bone resorption. Clinical studies are confirming these observations. Bortezomib counteracts the abnormal balance of osteoclast regulators (receptor activator of nuclear factor-kappaB ligand and osteoprotegerin), leading to osteoclast inhibition and decreased bone destruction, as measured by a reduction in markers of bone resorption. In addition, bortezomib stimulates osteoblast function, possibly through the reduction of dickkopf-1, leading to increased bone formation, as indicated by the elevation in bone-specific alkaline phosphatase and osteocalcin. The effect of bortezomib on bone disease is thought to be direct and not only a consequence of the agent's antimyeloma properties, making it an attractive agent for further investigation, as it may combine potent antimyeloma activity with beneficial effects on bone. However, the clinical implication of these effects requires prospective studies with specific clinical end points.     

Spontaneous release of interleukin 1 from human blood monocytes reflects bone formation in idiopathic osteoporosis.

Osteoporosis is a state of reduced skeletal mass characterized by various rates of bone remodeling. Multiple locally elaborated factors have been identified that appear to influence the cellular events in bone remodeling. The possible role(s) of these factors in the pathogenesis of osteoporosis is unknown. One such factor, interleukin 1 (IL-1), is of particular interest, as this protein is known to stimulate bone resorption and perhaps formation. Consequently, we have measured the spontaneous secretion of IL-1 activity by cultured peripheral blood monocytes obtained from 22 osteoporotic patients and 14 age-matched control subjects. Monocytes from osteoporotic patients produced more IL-1 than did monocytes from control subjects. When patients were grouped according to monocyte-produced IL-1 activity, dynamic parameters of bone formation, as judged by quantitative histomorphometric analysis of iliac crest bone biopsies and by circulating levels of bone 4-carboxyglutamic acid protein (BGP)--a marker of bone formation--were higher in subjects with elevated IL-1 activity; whereas, indices of bone resorption and static indices of bone formation were similar in subjects with either high or normal IL-1 activity. IL-1 activity released by peripheral blood monocytes appears to reflect bone formation rate in osteoporotic patients and may be of pathogenetic significance in a subset of individuals with osteoporosis.     

Bone remodeling and calcium homeostasis in patients with spinal cord injury: a review

Patients with spinal cord injury exhibit early and acute bone loss with the major functional consequence being a high incidence of pathological fractures. The bone status of these patients is generally investigated by dual-energy x-ray absorptiometry, but this technique does not reveal the pathophysiological mechanism underlying the bone loss. Bone cell activity can be indirectly evaluated by noninvasive techniques, including measurement of specific biochemical markers of bone formation (such as osteocalcin or bone-alkaline phosphatase) and resorption (such as procollagen type I N- or C-terminal propeptide). The bone loss in spinal cord injury is clearly due to an uncoupling of bone remodeling in favor of bone resorption, which starts just after the injury and peaks at about 1 to 4 months. Beyond 6 months, bone resorption activity decreases progressively but remains elevated for many years after injury. Conversely, bone formation is less affected. Antiresorptive treatment induces an early and acute reduction in bone resorption markers. Level of injury and health-related complications do not seem to be implicated in the intensity of bone resorption. During the acute phase, the hypercalcemic status is associated with the suppression of parathyroid hormone and vitamin D metabolites. The high sensitivity of these markers after treatment suggests that they can be used for monitoring treatment efficacy and patient compliance. The concomitant use of bone markers and dual-energy x-ray absorptiometry may improve the physician's ability to detect patients at risk of severe bone loss and subsequent fractures.     

Parathyroid hormone and bone cells

Parathyroid hormone has dual effects on bone cells, stimulating both bone resorption and bone formation. Almost all of the known effects of PTH on bone cells appear to be mediated through the PTH1 receptor (PTHR1). PTHR1 is found on several different cell types, including osteoblasts, osteoclasts, poorly-defined of receptor activator of osteoblastic NF-κB (RANKL) and osteoprotegerin (OPG) and induction of PGE, SDF-1 and perhaps IL-6 that acting with or through RANKL stimulate the differentiation and activity of osteoclasts. The bone formation effect results from PTH activating cells lining the bone surface to lay down new bone matrix increasing the proliferation of osteoblast precursors via release of growth factors FGF-2, TGF-β and IGF-1, increasing the differentiation of sub-populations of osteoblasts, and delaying the apoptosis of mature, matrix-secreting osteoblasts. Osteoclasts have also been implicated in mediating the bone-forming response to PTH through bone resorption-dependent and-independent mechanisms. Signaling of PTH in target cells is primarily through PTHR1 coupling to cAMP and Ca/PKC, but other pathways such as ERK/MAPK, Wnt and mechanical loading pathways may be activated or enhanced secondarily. Desensitization of PTHR1 has been proposed as an explanation for the time- and dose-dependence of PTHs catabolic and anabolic actions, a puzzle that has also been approached using RNA microarrays. PTH can have divergent effects on different bone envelopes i.e. trabecular, endosteal and periosteal, but these are incompletely explained by the current paradigms.     

Clinical significance of close relationship between bone and vasculature

Recently, evidences are accumulating that there is a close relationship between bone and vasculature and both organs are regulated by common factors. Thus, matrix Gla protein (MGP) , which is a non-collagen bone matrix protein was shown to be an important regulator of calcification both in the cartilage and blood vessel. We have recently shown that natriuretic peptides are very potent regulator of endochondral bone formation. Vascular "calcification" resembles the process of bone formation in many aspects. These observations suggest the possibility that osteoporosis and vascular lesions such as atherosclerosis and vascular calcification, are based on common factors, as in the case of multiple risk factor syndrome. Some drugs, currently used for the treatment of osteoporosis, has potential benefits for vascular lesions also.     

Effects of parathyroid hormone on bone formation in a rat model for chronic alcohol abuse

BACKGROUND: Alcoholism is a risk factor for osteoporosis and it is not clear whether the detrimental effects of alcohol on bone are reversible. Parathyroid hormone (PTH) is a potent stimulator of bone matrix synthesis and is being investigated as a therapeutic agent to reverse bone loss. The present investigation was designed to determine the effects of PTH on bone formation in a rat model for chronic alcohol abuse. METHODS AND RESULTS: Alcohol was administered in the diet of female rats (35% caloric intake) for 2 weeks. Human (1-34) PTH (80 microg/kg/day) was administered subcutaneously during the second week of the study. Alcohol resulted in a transient reduction in steady-state mRNA levels for the bone matrix proteins type 1 collagen, osteocalcin, and osteonectin compared with rats that were fed an alcohol-free (control) diet. As expected, alcohol decreased and PTH increased histologic indices of bone formation. Additionally, two-way ANOVA demonstrated that alcohol antagonized PTH-induced bone formation. Despite antagonism, bone formation and mRNA levels for bone matrix proteins in alcohol-fed rats treated with PTH greatly exceeded the values in rats fed the control diet. CONCLUSIONS: The results of this study contribute to a growing body of evidence that alcohol-induced bone loss is primarily due to reduced bone formation. We conclude that alcohol does not prevent the stimulatory effects of PTH on bone formation. This is evidence that the effects of alcohol on the skeleton are reversible. Additionally, the positive effects on bone formation in rats that consumed high concentrations of alcohol suggested that PTH may be useful as an intervention to treat alcohol-induced osteoporosis.     

Nck influences preosteoblastic/osteoblastic migration and bone mass

Migration of the cells in osteoblastic lineage, including preosteoblasts and osteoblasts, has been postulated to influence bone formation. However, the molecular bases that link preosteoblastic/osteoblastic cell migration and bone formation are incompletely understood. Nck (noncatalytic region of tyrosine kinase; collectively referred to Nck1 and Nck2) is a member of the signaling adaptors that regulate cell migration and cytoskeletal structures, but its function in cells in the osteoblastic lineage is not known. Therefore, we examined the role of Nck in migration of these cells. Nck is expressed in preosteoblasts/osteoblasts, and its knockdown suppresses migration as well as cell spreading and attachment to substrates. In contrast, Nck1 overexpression enhances spreading and increases migration and attachment. As for signaling, Nck double knockdown suppresses migration toward IGF1 (insulin-like growth factor 1). In these cells, Nck1 binds to IRS-1 (insulin receptor substrate 1) based on immunoprecipitation experiments using anti-Nck and anti–IRS-1 antibodies. In vivo, Nck knockdown suppresses enlargement of the pellet of DiI-labeled preosteoblasts/osteoblasts placed in the calvarial defects. Genetic experiments indicate that conditional double deletion of both Nck1 and Nck2 specifically in osteoblasts causes osteopenia. In these mice, Nck double deficiency suppresses the levels of bone-formation parameters such as bone formation rate in vivo. Interestingly, bone-resorption parameters are not affected. Finally, Nck deficiency suppresses repair of bone injury after bone marrow ablation. These results reveal that Nck regulates preosteoblastic/osteoblastic migration and bone mass.Bone is a metabolically dynamic tissue, as it is continuously remodeled based on repetition of bone resorption and bone formation (1). Under normal conditions, bone formation and bone resorption are coupled and balanced by the activities of osteoblasts and osteoclasts, respectively (2). Bone remodeling occurs first by osteoclastic bone resorption. Then, preosteoblasts or their precursors migrate into the resorption cavities and attach to the bottom of the cavities, followed by osteoblastic bone formation to start to fill the bone cavity through producing bone matrix (3). Therefore, preosteoblastic migration and attachment during bone remodeling are critical events to maintain bone mass (4–6). Regarding cell migration, most of the important motility and migration in remodeling is undergone by precursors of osteoblasts that are shown to be recruited by TGF-beta1, and these cells differentiate under the control of IGF1 (insulin-like growth factor 1) at the sites of remodeling (7, 8). Migration and attachment of the cells in the lineage of osteoblastic cells are also important in the case of repair after bone injury, as these cells migrate into the bone injury site and start to proliferate and to produce bone. These cellular events are considered to be critical in understanding the pathological state in bone, such as osteoporosis. However, the molecular bases of preosteoblastic/osteoblastic migration with respect to cytoskeletal regulation and its relevance to bone mass determination are still incompletely understood.The key steps in the migration and attachment of the cells include extension of the cell membrane, remodeling of actin cytoskeleton, formation of adhesion complex, and organization of stress fibers. Remodeling of actin cytoskeleton is a process of dynamic assembly and disassembly of filamentous actin. Such reorganization of actin cytoskeleton governs essential aspects of cell motility and attachment that are required for the formation of cellular structures such as lamellipodia, filopodia, stress fibers, and focal adhesions (9, 10). Preosteoblasts and osteoblasts are known to be capable of migrating toward chemo-attractants such as anabolic cytokines (7, 8). However, the key molecules involved in control of cytoskeleton that regulates osteoblastic cell migration and their relevance to bone mass regulation have yet to be elucidated.Nck (noncatalytic region of tyrosine kinase) adaptor proteins are cytosolic effectors that regulate remodeling of the actin cytoskeleton (11, 12). Mammals carry two closely related Nck genes, Nck1 and Nck2 (collectively termed Nck), that contain three N-terminal Src homology 3 (SH3) domains and a single C-terminal SH2 domain. Although actin cytoskeleton plays a critical role in cells and Nck is one of the possible factors affecting polymeric actin dynamics, the function of Nck in osteoblastic cells and in regulation of bone mass is incompletely understood. Therefore, we examined the role of Nck in the migration of bone cells and its relevance to the regulation of bone mass.     

Osteoporosis. Pathogenesis, diagnosis, and treatment in older adults

Osteoporosis is a major cause of disability and excess mortality in older men and women. Hip fracture incidence accelerates approximately 10 years after menopause in women and after age 70 in men. Approximately 1 million Americans suffer fragility fractures each year at a cost of over 14 billion dollars. The disability, mortality, and cost of hip and vertebral fractures are substantial in the rapidly growing, aging population so that prevention of osteoporosis is a major public health concern. BMD is used to make the diagnosis of osteoporosis before incident fracture and predict fracture risk. Recommendations for treatment and prevention of osteoporosis based on BMD score have been published by the World Health Organization and the National Osteoporosis Foundation. In a process that continues throughout life, bone repairs itself by the coupled action of bone resorption followed by bone formation, sometimes referred to as bone turnover. Osteoblasts and osteoclasts are the primary cells involved in bone formation and resorption, respectively. The process of bone turnover is regulated by hormones, such as PIH and local factors such as IL-1 and prostaglandins. Following attainment of peak bone mass at age 25, bone loss begins, accelerates in women at menopause and slows again but continues into advanced years at a rate of 1% to 2% per year, similar to premenopausal bone loss rate. The leading theories of the mechanism of bone loss in older individuals is calcium deficiency leading to secondary hyperparathyroidism and sex hormone deficiency. Risk factors such as age, gender, ethnic background, smoking, exercise, and nutrition, and medical conditions associated with osteoporosis should be evaluated and modified when possible to prevent further bone loss. Osteoporosis treatment and prevention include weight-bearing exercise, calcium and vitamin D supplementation, estrogen replacement, bisphosphonates, selective estrogen receptor antagonists, and calcitonin. Although there is no currently approved treatment for osteoporosis in men, many of the treatments approved for osteoporosis in women hold promise to be beneficial in men.     

The Michael Mason Prize Essay 1997. Nitric oxide and bone: what a gas!

Nitric oxide (NO) is an important signalling molecule in bone which is produced in response to diverse stimuli such as pro-inflammatory cytokines, mechanical strain and sex hormones. Recent work suggests that NO exerts biphasic effects on bone cell activity: high concentrations of NO inhibit bone resorption by inhibiting osteoclast formation and by inhibiting the resorptive function of mature osteoclasts, whereas lower NO concentrations potentiate cytokine- induced bone resorption and may be essential for normal osteoclast function. Similarly, growth and differentiation of osteoblasts are inhibited by high concentrations of NO which may partly be responsible for the inhibitory effects of pro-inflammatory cytokines on bone formation. In contrast, lower amounts of NO produced by constitutive nitric oxide synthase (NOS) enzymes may play a role in regulating normal osteoblast growth and in mediating the effects of oestrogens on bone formation. Evidence of inducible nitric oxide synthase (iNOS) expression has been found in the rheumatoid joint and patients with active rheumatoid arthritis (RA) have raised levels of NO breakdown products in blood and urine. This indicates that NO may be involved in the pathogenesis of bone disease and tissue damage associated with inflammatory conditions such as RA, and raises the possibility that iNOS inhibitors may be of therapeutic value in this situation. The observation that both oestrogen and mechanical strain increase NO production by activating constitutive NOS further suggests that bone loss associated with oestrogen deficiency and immobilization may be related to production of NO and may hence be amenable to treatment with pharmacological NO donors.