Cathepsin K, osteoclastic resorption, and osteoporosis therapy

Cathepsin K inhibitor–polymer conjugates: potential drugs for the treatment of osteoporosis and rheumatoid arthritis

The role of the newly discovered cysteine protease, cathepsin K, in osteoporosis and rheumatoid arthritis is reviewed. The current development of cathepsin K inhibitors and their targeted delivery using synthetic polymer carriers are discussed. Future challenges and possible strategies to improve these delivery systems are addressed.

Hormonal Therapies and Osteoporosis

Osteoporosis, now defined as a disease characterized by low bone mass and a microarchitectural deterioration of bone tissue leading to enhanced bone fragility and fracture risk, is a major public health problem. Classic hormonal therapies to prevent and treat osteoporosis associated with menopause have recently been questioned due to the risk/benefit ratio of prolonged treatment. There is a critical need for safe and effective alternative therapeutics for this disease. Nonhuman primates have been used as models to assess bone changes associated with estrogen deficiency because their trabecular and cortical bone remodeling processes, monthly menstrual cycles, and reproductive-hormone patterns are similar to those of humans. The ovariectomized nonhuman primate has become the preferred model in which to study effects on bone remodeling, particularly with regard to bone mass, architecture, and strength, in fulfillment of studies required by international guidelines for the development of antiosteoporotic drugs. The nonhuman primate is amenable to several methodologies that assess bone quantity and quality, including dual energy x-ray absorptiometry (DXA), quantitative computed tomography (QCT), histology, static and dynamic histomorphometry, and biomechanical testing, as well as assays developed for clinical use, which serve as biomarkers of bone metabolic processes. The use of the nonhuman primate model in the assessment of osteoporosis therapeutics, both hormonal (sex steroids and their analogues, parathyroid hormone) and nonhormonal (bisphosphonates), has provided valuable information on the safety and efficacy as well as the mechanisms of bone loss associated with estrogen deficiency that is directly applicable to the human situation.

Genetic regulation of osteoclast development and function

Osteoclasts are the principal, if not exclusive, bone-resorbing cells, and their activity has a profound impact on skeletal health. So, disorders of skeletal insufficiency, such as osteoporosis, typically represent enhanced osteoclastic bone resorption relative to bone formation. Prevention of pathological bone loss therefore depends on an appreciation of the mechanisms by which osteoclasts differentiate from their precursors and degrade the skeleton. The past five years have witnessed important insights into osteoclast formation and function. Many of these discoveries have been made through genetic experiments that involved the rare hereditary disorder osteopetrosis.

Multinucleated giant cells in various forms of giant cell containing lesions of the jaws express features of osteoclasts

BACKGROUND

The nature and the mechanism involved in the formation of the multinucleated giant cells (MGCs) in various giant cell-containing lesions of the jaws are not fully understood. The aim of this study is to clarify the osteoclastic features of the MGCs in central giant cell granuloma (CGCG), peripheral giant cell granuloma (PGCG), cherubism, and aneurysmal bone cyst (ABC), and the mechanism underlying the interrelations between cellular components in the formation of the MGCs.

METHODS

Immunohistochemical study with a panel of antibodies including vacuolar H+-ATPase (V-ATPase), carbonic anhydrase II (CA II), Cathepsin K, matrix metalloproteinases-9 (MMP-9), CD68, and proliferating cell nuclear antigen (PCNA), and enzyme histochemical staining for tartarate-resistant acid phosphatase (TRAP) were applied on a total number of 53 cases of giant cell-containing lesions including CGCG (n = 34), PGCG (n = 6), cherubism (n = 7), and ABC (n = 6). In situ hybridization was also carried out to detect the mRNA expression of the receptor activator of NF-kappaB ligand (RANKL), a newly identified cytokine that is shown to be essential in the osteoclastogenesis, its receptor RANK (receptor activator of NF-kappaB ligand), and its decoy receptor OPG (osteoprotegerin) in these four types of lesions.

RESULTS

Immunohistochemical and enzyme histochemical studies showed that both the MGCs and a fraction of mononuclear cells in these lesions were strongly positive for TRAP, V-ATPase, CA II, Cathepsin K, MMP-9, and CD68, while the spindle-shaped mononuclear cells were positive for PCNA. The results with in situ hybridization indicated that RANKL mRNA was mainly expressed in the spindle mononuclear cells while OPG was extensively distributed in both the MGCs and the mononuclear cells. RANK mRNA was expressed in the MGCs and some round mononuclear cells.

CONCLUSIONS

These results suggest that MGCs in the four types of giant cell-containing lesions of the jaws show characteristics of the osteoclast phenotype. The mononuclear stromal cells, which show TRAP positively, may be the precursors of the MGCs. RANKL, OPG, and RANK expressed in these lesions may play important roles in the formation of the MGCs. The similar characteristics and mechanisms in the differentiation of MGCs in these lesions also suggest that there might be a similar kind of pathogenesis involved in the formation of the MGCs in these lesions

Molecular mechanisms underlying osteoclast formation and activation

Osteoporosis is one of the leading causes of morbidity in the elderly and is characterized by a progressive loss of total bone mass and bone density. Bone loss in osteoporosis is due to the persistent excess of osteoclastic bone resorption over osteoblastic bone formation. Receptor activator of NFkappaB ligand (RANKL) critically regulates both osteoclast differentiation and activation. TRAFs appear to be central coupling molecules in the signal transduction pathways that regulate osteoclastogenesis, cathepsin K is the major mediator of osteoclastic bone resorption, and sex steroids and aging also affect osteoclastogenesis and osteoclast activity. However, bone homeostasis depends upon the intimate coupling of bone formation and bone resorption, wherein both osteoclasts and osteoblasts exert vital stimulatory and inhibitory effects upon each other via molecules such as RANKL, TGFbeta, PDGF, BMP2, and Mim-1. This review will highlight some of the major features of the complex circuit of cytokines, growth factors, and hormones that underlies the formation and function of osteoclasts and the dynamic equilibrium that marks the interaction between osteoclasts and osteoblasts.

Osteoclastogenesis, bone resorption, and osteoclast-based therapeutics

Over the past decade, advances in molecular tools, stem cell differentiation, osteoclast and osteoblast signaling mechanisms, and genetically manipulated mice models have resulted in major breakthroughs in understanding osteoclast biology. This review focuses on key advances in our understanding of molecular mechanisms underlying the formation, function, and survival of osteoclasts. These include key signals mediating osteoclast differentiation, including PU.1, RANK, CSF-1/c-fms, and src, and key specializations of the osteoclast including HCl secretion driven by H+-ATPase and the secretion of collagenolytic enzymes including cathepsin K and matrix metalloproteinases (MMPs). These pathways and highly expressed proteins provide targets for specific therapies to modify bone degradation. The main outstanding issues, basic and translational, will be considered in relation to the osteoclast as a target for antiresorptive therapies.