The effect of Hyperbaric Oxygen Therapy on osteoclast and osteoblast function
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Bone remodelling, the process by which the skeleton adapts to environmental changes, is dependent on the actions of osteoclasts that resorb bone and osteoblasts which make new bone matrix. Aberrant remodelling underpins bone loss in several debilitating skeletal diseases such as osteoporosis, metastatic breast cancer and multiple myeloma. Changes in remodelling activity can also arise as a consequence of therapeutic intervention for instance intravenous bisphosphonate treatment is associated with osteochemonecrosis of the jaw and localised osteoradionecrosis is a common side effect of radiotherapy. Hyperbaric oxygen is often used as an adjunctive therapy in the treatment of these disorders. HBO involves the administration of 100% oxygen at atmospheric pressures greater than one in sealed chambers. The following studies aimed to evaluate the effect of HBO, hyperoxia, and pressure on RANKL-induced osteoclast differentiation and bone resorption from RAW264.7 and human peripheral blood mononuclear cells (PBMC), and osteoblast differentiation in vitro. The study also aimed to further examine the effect of HBO on ex vivo osteoclast formation from peripheral blood monocytes obtained from patients undergoing HBO. Daily exposure to HBO for ninety minutes significantly suppressed osteoclast differentiation and bone resorption in mouse and human monocytes in normoxic and hypoxic conditions in vitro. The suppressive action of HBO on osteoclast formation was associated with a significant reduction in HIF-1α and RANK mRNA expression and HBO also caused a significant reduction in NFATc1 and DC-STAMP expression. This study has for the first time shown that HBO is able to reduce the ability of precursors to form bone resorbing osteoclast. HBO also suppressed the ability of peripheral blood monocytes to develop into RANKL-induced resorptive osteoclasts. In an ex vivo culture system the suppressive effect of HBO was meditated by an action prior to activation of osteoclast differentiation by RANKL and must therefore be an inhibitory effect on the ability of precursors to differentiate along the osteoclastic lineage. HBO also accelerates the rate of osteoblast differentiation and augments early stages of mineralization and has a more pronounced effect than hyperoxia or pressure alone. HBO enhanced bone nodule formation and ALP activity in human osteoblasts. Furthermore HBO promoted the expression of type I collagen and Runx-2 in both normoxic and hypoxic conditions. HBO had a greater effect on these key markers of osteoblast differentiation than hyperoxia or pressure alone. This study suggests that HBO suppresses osteoclast activity and promotes osteoblastic bone formation, which may at least in part mediate its beneficial effects on necrotic bone. This provides evidence supporting the use of HBO as an adjunctive therapy to prevent osteoclast formation in a range of skeletal disorders associated with low oxygen partial pressure. The study also provides further support for the use of HBO in the treatment of skeletal disorders associated with excessive resorption such as osteomyelitis, and also provides a potential mechanism through which short term HBO may help fracture healing.
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