Role of cytokines in the regulation of bone resorption

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Abstract
The process of bone remodeling involves complex interactions between the osteoclast, the primary bone-resorbing cell, and other cells in its microenvironment. These interactions can regulate bone resorption through two processes: (1) effects on the number of osteoclasts present at a given site and (2) effects on the bone-resorbing capacity of individual osteoclasts. Cells present in the osteoclast microenvironment include marrow stromal cells, osteoblasts, macrophages, T-lymphocytes, and marrow cells. These cells, as well as the osteoclast itself, produce cytokines that can affect osteoclast formation and osteoclast activity.In vitro model systems using rodent organ cultures or long-term marrow culture systems, andin vivo models have demonstrated that cytokines such as interleukin-1, M-CSF, tumor necrosis factor, and interleukin-6 can stimulate the formation and bone-resorbing capacity of osteoclasts. In contrast, cytokines such as interleukin-4, γ-interferon, and transforming factor-β inhibit both osteoclast formation and osteoclast activity. The relative proportions of these cytokines in the marrow microenvironment may play a critical role in regulating osteoclast activity. Knowledge of cytokines that affect osteoclast formation and activity and their capacity to modulate the bone-resorbing process should provide critical insights into normal calcium homeostasis and disorders of bone turnover such as osteoporosis and Paget's disease of bone.