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Introduction
Osteoclasts are large multinuclear cells, formed by fusion of macrophages derived from hematopoietic stem cells (1). Osteoclasts are involved in bone remodeling under normal conditions; osteoblasts build the bone matrix, while osteoclasts absorb the matrix. Dysfunction of osteoblasts or over-activity of osteoclasts leads to a breakdown in the balance of bone remodeling. In inflammation (2), osteoclasts are overactive and upregulated, which leads to an imbalance between bone formation and resorption, causing bone loss (3). Diseases associated with osteoclasts frequently arise in the fields of orthopedics (4), stomatology (5), otorhinolaryngology (5) and rheumatology (6).
In order to investigate diseases associated with osteoclasts, it is necessary to culture osteoclasts in vitro. Osteoclasts are terminally-differentiated cells which lack the ability to proliferate; in addition, it is difficult to separate osteoclasts from the surface of cortical bone in vitro. Therefore, it is not possible to separate osteoclasts from cortical bone to culture for further experiments. At present, there are three kinds of cells commonly used for osteoclast modeling: Murine bone marrow cells (BMCs) (7,8); murine RAW 264.7 cells (9); and, human peripheral blood mononuclear cells (PBMCs) (10). Various stimulators have been used, including receptor activator of nuclear factor κ-B ligand (RANKL), macrophage colony-stimulating factor (MCSF), tumor necrosis factor-α, 1α, 25-dihydroxyvitamin D3, lipopolysaccharide and mercaptopurine (11); only RANKL and MCSF have been identified to be effective.
RAW 264.7 murine monocyte-macrophage cells differentiate into osteoclasts following stimulation with RANKL and MCSF (9); this model is the most commonly used to date.
A combination of tartrate-resistant acid phosphatase (TRAP) staining and a bone resorption test are required to identify osteoclasts, as other osteoclast-like cells may be TRAP-positive (11).
There are few studies using a functional osteoclast model induced from a human cell line (7–10). THP-1 cells are a monocyte cell line derived from a patient with acute leukemia. It remains unknown whether THP-1 cells are able to differentiate into osteoclasts following stimulation with RANKL and MCSF. Suspended THP-1 monocytes will differentiate into adherent macrophages with the stimulation of phorbol-12 myristate-13 acetate (PMA), an activator of the protein kinase C (PKC) signaling pathway (12); macrophages are the intermediate stage between monocytes and osteoclasts.
In order to investigate human osteoclasts, a human osteoclast model is required. The present study aimed to...