Pathogenesis Of Metabolic Bone Diseases Research Articles

Bone health in children undergoing solid organ transplantation.

Pediatric solid organ transplant recipients are a unique and growing patient population who are at risk for metabolic bone disease both before and after transplantation. The odds of sustaining a fracture in adulthood are significantly higher if an individual has sustained at least one childhood fracture, therefore, close monitoring before and after transplant is essential. Emerging data in patients with chronic kidney disease mineral and bone disorder (CKD-MBD) and hepatic osteodystrophy highlights the role of fibroblast growth factor 23 in the pathogenesis of metabolic bone disease in these conditions. While dual X-ray absorptiometry (DXA) is the most widely used imaging modality for assessment of bone mass in children, quantitative computer tomography (QCT) is an emerging modality, especially for patients with glucocorticoid-induced osteoporosis. Solid organ transplantation improves organ function and quality of life; however, bone mineral density can decline following transplantation, particularly during the first three to six months. Immunosuppressive medications, including glucocorticoids, are a major contributing factor. Following transplant, treatment should be tailored to achieve mineral homeostasis, correct nutritional deficiencies, and improve physical conditioning. In summary, early identification and treatment of metabolic bone disease can improve the bone health status of pediatric transplant recipients as they enter adulthood. http://links.lww.com/MOP/A71.

A Quantification Method for Disorganized Bone Components: Application to the Femoral Shaft.

Based on the current paradigm, a healthy bone is one with adequate mass without microarchitectural decay. However, these two features may not be sufficient to ensure that a bone is healthy. In addition, components must be correctly assembled and aligned. This ensures "the right amount of bone, at the right place" and thus, an optimal cohesion or interplay between constituents. Disorganization may be an independent contributor to bone abnormalities including fragility fractures. Indeed, many bone diseases may be characterized by the presence of disorganized bone, including osteogenesis imperfecta, hypophosphatasia, and atypical femur fractures (AFFs). Despite its likely importance, currently, there are no tools to quantify disorganization in vivo. We address this unmet need by describing a novel method for quantifying bone disorganization from X-ray images. Disorganization is quantified as variations in the orientation of bone components in relation to a target reference point. True disorganization created by disarranging (misplacing) pixels within the bone served as "gold standard." To further validate the method in clinical settings, we compared disorganization in three groups of femurs: (i) femurs of women with AFFs (n=9); (ii) fracture-free femurs contralateral to AFFs (n=9); and (iii) fracture-free femurs from controls (n=25). There was excellent agreement between measured disorganization and "gold standard," with R 2 values ranging from 0.84 to 0.99. Precision error ranged from 1.72% to 4.69%. Disorganization produced by abnormalities associated with AFFs was accurately captured. Disorganization level was lowest in fracture-free control femurs, higher in fracture-free contralateral femurs to AFFs, and highest in femurs with AFFs (all p < 0.0001). Quantification of disorganization, a novel biomarker, may provide novel insights into the pathogenesis of metabolic bone diseases beyond that provided by bone mineral density (BMD) or microarchitecture. We provide evidence that measurement of disorganization is likely to help identify patients at risk for fractures, especially in those poorly explained by BMD or microarchitecture such as AFFs. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Biologics Treatment Limits Disease Activity and Bone Metabolism in Patients with Rheumatoid Arthritis

Introduction: Generalized bone loss in rheumatoid arthritis (RA) is multi-factorial, and can be divided into erosions of the joint and osteoporosis. Biomarkers of bone remodeling provide a dynamic view of the remodeling process and they contribute to a better understanding of bone physiology and the pathogenesis of metabolic bone diseases. To examine whether treatment with biologics agents reduces of disease activity and changes the markers of bone metabolism, including osteocalcin (OC), N-terminal propeptide of type I procollagen (PINP), cross-linked Cterminal telopeptide of type I collagen (CTX), receptor activator of the NF-κB ligand (RANKL) and osteoprotegerin (OPG). Materials and methods: 125 patients with active RA, who were treated with DMARDs (44 patients) and biologics agents including 49 patients on Etanercept, 16 patients on Adalibumab/Golibumab/Infliksimab and 16 patients on Rytuksymab/Tocilizumab were included in this study. Clinical and laboratory parameters of disease activity were measured at baseline and after 90 days of treatment and an index of disease activity (DAS-28) calculated. Changes in bone markers levels were measured by ELISA before start the treatment and after. Results: As a result of treatment of the active RA with biologics agents, after 90 days, parameters such as DAS-28, VAS, number of swollen/tender joints, CRP and ERS decreased in the biologics group compared to the DMARDs group. The sRANKL/OPG ratio, and carboxylated (GLA)–OC and CTX levels were higher in RA patients compared to healthy subjects (p<0.001, p=0.02 and p=0.002, respectively). DMARDs therapy decreased the CTX level and sRANKL/OPG ratio in RA patients (p=0.006 and p=0.06, respectively). OPG was increased, whereas the sRANKL/OPG ratio was decreased in RA patients after 90 days biologics treatment. Conclusion: The biologics therapy has protective effect against clinical disease activity as well as joint damage compared to conventional DMARDs in patients with RA.

Effects of receptor activator of NF-κB ligand gene silencing on the human osteoblast-like MG63 cells

The osteoprotegerin (OPG)/receptor activator of nuclear factor-κB ligand (RANKL)/receptor activator of NF-κB (RANK) system plays an important role in the pathogenesis of metabolic bone diseases. This study is aimed to investigate effects and mechanisms of RANKL gene silencing on the function of human osteoblast-like MG63 cells by RNA interference using a lentivirus-based small hairpin RNA (vshRNA) delivery system. After RANKL-specific vshRNAs were designed, constructed and transfected into MG63 cells, changes in the expression levels of RANKL mRNA and protein were determined by Western blot and RT-PCR, respectively; changes in cell activity and cell cycle distribution were examined by thiazolyl blue tetrazolium bromide assay and flow cytometry. The expression levels of RANKL mRNA and protein in MG63 cells were reduced by transferring RANKL-specific vshRNAs. Compared to cells infected with negative control virus, the proliferation of cells infected with the recombinant virus was more likely to be inhibited. Furthermore, the cell cycle of MG63 was altered, with the number of G1 phase cells decreasing significantly (P < 0.05). RANKL-specific vshRNAs can significantly inhibit the expression of the target gene in MG63 cells. RANKL gene silencing can inhibit the proliferation and alter the cell cycle of MG63 cells. Our findings suggest that RANKL might play an important role in the regulation of growth and cell cycle of MG63 cells.

Perspective: Quantifying Osteoblast and Osteocyte Apoptosis: Challenges and Rewards

Since the initial demonstration of the phenomenon in murine and human bone sections approximately 10 yr ago, appreciation of the biologic significance of osteoblast apoptosis has contributed greatly not only to understanding the regulation of osteoblast number during physiologic bone remodeling, but also the pathogenesis of metabolic bone diseases and the pharmacology of some of the drugs used for their treatment. It is now appreciated that all major regulators of bone metabolism including bone morphogenetic proteins (BMPs), Wnts, other growth factors and cytokines, integrins, estrogens, androgens, glucocorticoids, PTH and PTH-related protein (PTHrP), immobilization, and the oxidative stress associated with aging contribute to the regulation of osteoblast and osteocyte life span by modulating apoptosis. Moreover, osteocyte apoptosis has emerged as an important regulator of remodeling on the bone surface and a critical determinant of bone strength, independently of bone mass. The detection of apoptotic osteoblasts in bone sections remains challenging because apoptosis represents only a tiny fraction of the life span of osteoblasts, not unlike a 6-mo-long terminal illness in the life of a 75-yr-old human. Importantly, the phenomenon is 50 times less common in human bone biopsies because human osteoblasts live longer and are fewer in number. Be that as it may, well-controlled assays of apoptosis can yield accurate and reproducible estimates of the prevalence of the event, particularly in rodents where there is an abundance of osteoblasts for inspection. In this perspective, we focus on the biological significance of the phenomenon for understanding basic bone biology and the pathogenesis and treatment of metabolic bone diseases and discuss limitations of existing techniques for quantifying osteoblast apoptosis in human biopsies and their methodologic pitfalls.

An update on biomarkers of bone turnover and their utility in biomedical research and clinical practice

Maintenance of the structural and functional integrity of the skeleton is a critical function of a continuous remodeling driven by highly associated processes of bone resorption and synthetic activities driven by osteoclasts and osteoblasts, respectively. Acceleration of bone turnover, accompanied with a disruption of the coupling between these cellular activities, plays an established role in the pathogenesis of metabolic bone diseases, such as osteoporosis. During the past decades, major efforts have been dedicated to the development and clinical assessment of biochemical markers that can reflect the rate of bone turnover. Numerous studies have provided evidence that serum levels or urinary excretion of these biomarkers correlate with the rate of bone loss and fracture risk, proving them as useful tools for improving identification of high-risk patients. The aim of the present review is to give an update on biomarkers of bone turnover and give an overview of their applications in epidemiological and clinical research. Special attention is given to their utility in clinical trials testing the efficacy of drugs for the treatment of osteoporosis and how they supplement bone mass measurements. Recent evidence suggests that biochemical markers may provide information on bone age that may have indirectly relates to bone quality; the latter is receiving increasing attention. A more targeted use of biomarkers could further optimize identification of high-risk patients, the process of drug discovery, and monitoring of the efficacy of osteoporosis treatment in clinical settings.

Nutritional and metabolic issues in inflammatory bowel disease.

This article describes the clinical papers published in 2002 and early 2003 on nutritional and metabolic derangement in inflammatory bowel disease. Insulin-like growth factor 1 and Insulin-like growth factor binding protein 3 are decreased in inflammatory bowel disease, and disease therapy hardly reverses this situation. There are promising data on recombinant human growth hormone therapy in paediatric inflammatory bowel disease. Several papers have added some fuel to the debate on the prevalence and pathogenesis of metabolic bone disease in inflammatory bowel disease. Articles have been published investigating the role of dietary fat in the therapeutic action of enteral feeding in Crohn's disease. Low-fat diets are particularly useful, and adding medium-chain triglycerides does not impair the effectiveness of these diets. Balanced amounts of saturated, monounsaturated and polyunsaturated fat should probably be used. Relevant contributions on the usefulness of probiotic preparations (VSL#3) in the treatment and prevention of pouchitis have been published. Other papers deal with the effects of medical and surgical therapy on body composition and metabolism in the inflammatory bowel disease, the treatment of oxidative stress of these patients, and the possible role of some vitamin deficiencies on thrombotic risk in the condition. Inflammatory bowel disease therapy hardly reverses growth hormone-insulin-like growth factor 1 disturbances of patients. The role of inflammation and steroid therapy of metabolic bone disease in inflammatory bowel disease is still controversial. Low-fat diets, with added amounts of medium-chain triglycerides, are useful in decreasing gut inflammation in the condition. The search for the optimal dietary fatty acid composition deserves further investigations. The use of probiotics and prebiotics opens new therapeutic perspectives for the disease.

Role of receptor activator of nuclear factor-kappaB ligand and osteoprotegerin in bone cell biology.

Receptor activator of nuclear factor (NF-kappaB) ligand (RANKL), its cellular receptor, receptor activator of NF-kappaB (RANK), and the decoy receptor osteoprotegerin (OPG) constitute a novel cytokine system. RANKL produced by osteoblastic lineage cells and activated T lymphocytes is the essential factor for osteoclast formation, fusion, activation, and survival, thus resulting in bone resorption and bone loss. RANKL activates its specific receptor, RANK located on osteoclasts and dendritic cells, and its signaling cascade involves stimulation of the c-jun, NF-kappaB, and serine/threonine kinase PKB/Akt pathways. The effects of RANKL are counteracted by OPG which acts as a soluble neutralizing receptor. RANKL and OPG are regulated by various hormones (glucocorticoids, vitamin D, estrogen), cytokines (tumor necrosis factor alpha, interleukins 1, 4, 6, 11, and 17), and various mesenchymal transcription factors (such as cbfa-1, peroxisome proliferator-activated receptor gamma, and Indian hedgehog). Transgenic and knock-out mice with excessive or defective production of RANKL, RANK, and OPG display the extremes of skeletal phenotypes, osteoporosis and osteopetrosis. Abnormalities of the RANKL/OPG system have been implicated in the pathogenesis of postmenopausal osteoporosis, rheumatoid arthritis, Paget's disease, periodontal disease, benign and malignant bone tumors, bone metastases, and hypercalcemia of malignancy, while administration of OPG has been demonstrated to prevent or mitigate these disorders in animal models. RANKL and OPG are also important regulators of vascular biology and calcification and of the development of a lactating mammary gland during pregnancy, indicating a crucial role for this system in extraskeletal calcium handling. The discovery and characterization of RANKL, RANK, and OPG and subsequent studies have changed the concepts of bone and calcium metabolism, have led to a detailed understanding of the pathogenesis of metabolic bone diseases, and may form the basis of innovative therapeutic strategies.

Osteoprotegerin and its cognate ligand: a new paradigm of osteoclastogenesis.

Osteoprotegerin (OPG) has recently been identified as a new member of the tumor necrosis factor (TNF)receptor (TNF-R) superfamily (1–3). It is a glycoprotein with seven structural domains, including four TNF-R motifs at the N-terminus (domains 1–4), two death domain homologous regions (domains 5 and 6) which mediate cytotoxicity, and a C-terminus (domain 7) which contains a cysteine residue for homodimer formation (4). In contrast with other known members of the TNF-R superfamily, OPG (or osteoclastogenesis inhibitory factor, OCIF) lacks transmembrane domains, suggesting that it acts as a secreted cytokine receptor (1–4). Enthusiasm for this new cytokine receptor emerged from basic scientists and clinicians alike after it was shown that, by suppressing osteoclastogenesis, OPG is a potent inhibitor of bone resorption (1, 2, 5, 6). Moreover, overexpression of OPG in transgenic mice resulted in osteopetrosis (generalized increase in bone mass) (1) and prevented bone loss following ovariectomy (1, 2). The potent antiresorptive effects of OPG nurtured the hope that deciphering its mode of action would provide detailed insights into osteoclastogenesis, an integral part of the pathogenesis of metabolic bone diseases. However, while the effect of OPG was impressive and comparable with the most potent class of antiresorptive drugs, the bisphosphonates, the cognate ligand for OPG had remained unidentified, rendering OPG another orphan receptor and leaving a major piece of the puzzle of osteoclastogenesis missing. One year later, the cognate ligand for OPG (OPGL) has been identified independently by two groups (7, 8). After screening of OPG-binding cell surface proteins with an OPG immunoprobe, OPGL was expression-cloned and the human OPGL cDNA was isolated from lymph node (7) and stromal cell (8) cDNA libraries. Human OPGL (or osteoclast differentiation factor, ODF) consists of 317 amino acids and belongs to the class of type II transmembrane proteins (7, 8). OPGL exists in a cellbound and a soluble C-terminal form, both of which are bound by OPG (7). It is highly expressed in lymphoid tissues (lymph node, thymus, spleen) and trabecular bone, and to a lesser extent in bone marrow as well as in some bone marrow stromal and osteosarcoma cell lines (7, 8). OPGL mRNA expression in the murine marrow stromal cell line, ST2, is upregulated by vitamin D. Furthermore, vitamin D, interleukin-11, prostaglandin E2 and parathyroid hormone increased OPGL mRNA expression in primary mouse osteoblasts (8). Within fetal bone, OPGL is mainly expressed in the cartilaginous anlagen and in hypertrophic chondrocytes, while in adult bone its expression is most abundant in the growth plate and the metaphysis, all of which are areas of active bone remodeling during growth (7). OPGL binds to the cell surface of osteoclastic lineage cells, and, in the presence of the permissive factor colony-stimuating factor-1 (CSF-1), it induces osteoclastogenesis in vitro in the absence of stromal cells, glucocorticoids and vitamin D, factors previously considered essential for osteoclastogenesis (7, 8). Importantly, OPGL stimulated bone resorption both in vitro (7, 8) and in vivo (7). The latter (after subcutaneous injection of recombinant OPGL into normal mice) was associated with an increased size and nuclearity of osteoclasts, massive bone loss and profound rapid-onset hypercalcemia (7). These studies unequivocally establish OPGL and OPG as a cybernetic couple that regulates bone mass by modulating osteoclastogenesis. OPGL appears to be the endogenous ‘master’ cytokine, which is the conditio sine qua non for normal osteoclast differentiation and activation, whereas OPG is a naturally occurring soluble receptor that counterbalances the effects of OPGL and preserves bone mass (Fig. 1). This concept is further supported by increased osteoclastogenesis and severe osteopenia in OPG knock-out mice (as cited in (7)). Sequence comparison of the OPG ligand (7, 8) has revealed that it is identical with two previously reported novel members of the TNF-R superfamily, TNF-related activation-induced cytokine (TRANCE) (9) and receptor activator of NF-kB ligand (RANKL) (10), which are essential for T-cell and dendritic cell activation. TRANCE was identified as an immediate early gene that is upregulated by T-cell receptor stimulation. Its expression is restricted to lymphoid tissue (thymus, lymph nodes) and T-cells, and it induces activation of the c-jun N-terminal kinase in T-cells (9). RANKL was identified by screening a thymoma cell line which highly expressed RANK, its cognate receptor, and was found to be mainly expressed in lymph node, with lower expression in other tissues such as placenta, bone marrow and thyroid (10). Subsequently, RANKL was European Journal of Endocrinology (1998) 139 152–154 ISSN 0804-4643

Cancer-induced bone diseases.

Science research in the field of bone and calcium, on the one hand, and in cancer, on the other, has developed along separate paths and they have become established as independent disciplines. Table 1 shows a partial list of research objectives in each discipline. In the bone and calcium field, cell differentiation, especially that of osteoclasts, has been intensively studied and intracellular signalling, oncogenes (Src, Myc), cell-cell and cell-matrix interactions and apoptosis are currently topics of major interest. Similar approaches are being pursued in relation to the development of osteoblasts. The cytokines listed in the table are also being examined in an attempt to understand the pathophysiology of various metabolic bone diseases. Hormones too are a major focus of the work in the bone and calcium field in which calcium regulatory hormones such as parathormone (PHT), active vitamin D and calcitonin have been studied in detail. Gonadal hormones, oestrogen and androgen have attracted keen interest in relation to bone development and to the pathogenesis of metabolic bone diseases.

Increased expression of TGF-beta 2 in osteoblasts results in an osteoporosis-like phenotype.

The development of the skeleton requires the coordinated activities of bone-forming osteoblasts and bone-resorbing osteoclasts. The activities of these two cell types are likely to be regulated by TGF-beta, which is abundant in bone matrix. We have used transgenic mice to evaluate the role of TGF-beta 2 in bone development and turnover. Osteoblast-specific overexpression of TGF-beta 2 from the osteocalcin promoter resulted in progressive bone loss associated with increases in osteoblastic matrix deposition and osteoclastic bone resorption. This phenotype closely resembles the bone abnormalities seen in human hyperparathyroidism and osteoporosis. Furthermore, a high level of TGF-beta 2 overexpression resulted in defective bone mineralization and severe hypoplasia of the clavicles, a hallmark of the developmental disease cleidocranial dysplasia. Our results suggest that TGF-beta 2 functions as a local positive regulator of bone remodeling and that alterations in TGF-beta 2 synthesis by bone cells, or in their responsiveness to TGF-beta 2, may contribute to the pathogenesis of metabolic bone disease.

Aluminum in parenteral solutions revisited—again

It has been a dozen years since aluminum was first shown to contaminate parenteral nutrition solutions and to be a contributing factor in the pathogenesis of metabolic bone disease in parenteral nutrition patients as well as in uremic patients. However, there are no regulations in place to effectively reduce aluminum contamination of various parenterally administered nutrients, drugs, and biologic products. The purpose of this review is fourfold: 1) to summarize our knowledge of the adverse effects of aluminum on bone formation and mineralization in parenteral nutrition patients; 2) to discuss the possible role of aluminum in the osteopenic bone disease of preterm infants; 3) to show how lack of regulations covering aluminum content of parenteral solutions can lead to vulnerability of new groups of patients to aluminum toxicity, the example being given here is that of burn patients; and 4) to trace the development of efforts at regulating the aluminum contamination of large- and small-volume parenteral drug products and to point out what still needs to be done in this regard.

Vitamin D Deficiency and Bone Disease in Gastrointestinal Disorders

Advances in knowledge of vitamin D metabolism in the past decade have provided an expanded base for understanding the pathogenesis of metabolic bone disease in digestive disorders and have suggested new approaches to treatment. 1 The purpose of this article is to reemphasize the various digestive diseases that are complicated by bone disease and to discuss the pathogenesis, diagnosis, and therapy of vitamin D deficiency in these conditions. VITAMIN D ABSORPTION AND METABOLISM Vitamin D is derived from dietary sources and is produced endogenously in the skin from the action of ultraviolet light on precursor 7-dehydrocholesterol. Vitamin D is absorbed predominantly in the proximal jejunum. Bile salts are required for absorption of this fat-soluble vitamin, and delivery into the circulation is mainly through intestinal lymphatics. 2,3 The first step in bioactivation occurs in the liver where vitamin D is converted to 25-hydroxyvitamin D (25OHD 3 ) by hepatic 25-hydroxylase. The major