Physiological bone remodeling is a highly coordinated process responsible for bone resorption and formation and is necessary to repair damaged bone and to maintain mineral homeostasis. In addition to the traditional bone cells (osteoclasts, osteoblasts, and osteocytes) that are necessary for bone remodeling, several immune cells have also been implicated in bone disease. This minireview discusses physiological bone remodeling, outlining the traditional bone biology dogma in light of emerging osteoimmunology data. Specifically discussed in detail are the cellular and molecular mechanisms of bone remodeling, including events that orchestrate the five sequential phases of bone remodeling: activation, resorption, reversal, formation, and termination.Bone is a dynamic tissue that undergoes continual adaption during vertebrate life to attain and preserve skeletal size, shape, and structural integrity and to regulate mineral homeostasis. Two processes, remodeling and modeling, underpin development and maintenance of the skeletal system. Bone modeling is responsible for growth and mechanically induced adaption of bone and requires that the processes of bone formation and bone removal (resorption), although globally coordinated, occur independently at distinct anatomical locations. Bone remodeling is responsible for removal and repair of damaged bone to maintain integrity of the adult skeleton and mineral homeostasis. This tightly coordinated event requires the synchronized activities of multiple cellular participants to ensure that bone resorption and formation occur sequentially at the same anatomical location to preserve bone mass. This work will review the cellular participants and molecular mechanisms that coordinate the five distinct phases of bone remodeling and includes an appraisal of immune cells and their role in regulating normal bone physiology.

Bone remodeling is essential for adult bone homeostasis. It comprises two phases: bone formation and resorption. The balance between the two phases is crucial for sustaining bone mass and systemic mineral homeostasis. This review highlights recent work on physiological bone remodeling and discusses our knowledge of how systemic and growth factors regulate this process.

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The clinical findings that alendronate blunted the anabolic effect of human parathyroid hormone (PTH) on bone formation suggest that active resorption is involved and enhances the anabolic effect. PTH signals via its receptor on the osteoblast membrane, and osteoclasts are impacted indirectly via the products of osteoblasts. Microarray with RNA from rats injected with human PTH or vehicle showed a strong association between the stimulation of monocyte chemoattractant protein-1 (MCP-1) and the anabolic effects of PTH. PTH rapidly and dramatically stimulated MCP-1 mRNA in the femora of rats receiving daily injections of PTH or in primary osteoblastic and UMR 106-01 cells. The stimulation of MCP-1 mRNA was dose-dependent and a primary response to PTH signaling via the cAMP-dependent protein kinase pathway in vitro. Studies with the mouse monocyte cell line RAW 264.7 and mouse bone marrow proved that osteoblastic MCP-1 can potently recruit osteoclast monocyte precursors and facilitate receptor activator of NF-B ligand-induced osteoclastogenesis and, in particular, enhanced fusion. Our model suggests that PTH-induced osteoblastic expression of MCP-1 is involved in recruitment and differentiation at the stage of multinucleation of osteoclast precursors. This information provides a rationale for increased osteoclast activity in the anabolic effects of PTH in addition to receptor activator of NF-B ligand stimulation to initiate greater bone remodeling. Parathyroid hormone (PTH)2 is a principal hormone regulating bone remodeling via its actions on both bone formation and bone resorption. The finding that the anabolic effect of human PTH-(1-34) on bone formation was blunted when bone resorption was inhibited indicated that active resorption is involved and enhances the in vivo anabolic effect of PTH (1-3). However, the molecular basis of the osteoclast activation in this process has not been fully studied. In order to identify the key mediators for the anabolic effects of PTH, we performed microarray using RNA from the femora of 3-month-old Sprague-Dawley female rats injected daily with hPTH-(1-34) to increase bone formation or vehicle. This was compared with rats receiving hPTH-(1-34) in a catabolic protocol by continuous infusion. PTH-(1-34) regulated numerous genes (ϳ1,000), but differentially, in both regimes (see accompanying article, Ref. 4). Notably, in the anabolic protocol, a number of cytokines and chemokines, RANKL, interleukin-6, CXCL1, and CCL2 (MCP-1), were highly induced. In contrast, none of these were significantly increased, by microarray, in the catabolic protocol. The chemokine, monocyte chemoattractant protein-1 (MCP-1, CCL2), was found to be the most highly stimulated gene from 14-day intermittent hPTH-(1-34) or hPTH-(1-31) administration (both cause over 100-fold changes). With continuous infusion of PTH, the MCP-1 mRNA was elevated less than 2-fold, and this difference indicates a potentially important role for MCP-1 in the anabolic effect of PTH.Chemokines are bioactive peptides that regulate leuk...

To understand the mechanisms by which glucocorticoids promote differentiation of fetal rat calvaria derived osteoblasts to produce bone-like mineralized nodules in vitro, a panel of osteoblast growth and differentiation related genes that characterize development of the osteoblast phenotype has been quantitated in glucocorticoid-treated cultures. We compared the mRNA levels of osteoblast expressed genes in control cultures of subcultivated cells where nodule formation is diminished, to cells continuously (35 days) exposed to 10(-7) M dexamethasone, a synthetic glucocorticoid, which promotes nodule formation to levels usually the extent observed in primary cultures. Tritiated thymidine labelling revealed a selective inhibition of internodule cell proliferation and promotion of proliferation and differentiation of cells forming bone nodules. Fibronectin, osteopontin, and c-fos expression were increased in the nodule forming period. Alkaline phosphatase and type I collagen expression were initially inhibited in proliferating cells, then increased after nodule formation to support further growth and mineralization of the nodule. Expression of osteocalcin was 1,000-fold elevated in glucocorticoid-differentiated cultures in relation to nodule formation. Collagenase gene expression was also greater than controls (fivefold) with the highest levels observed in mature cultures (day 35). At this time, a rise in collagen and TGF beta was also observed suggesting turnover of the matrix. Short term (48 h) effects of glucocorticoid on histone H4 (reflecting cell proliferation), alkaline phosphatase, osteopontin, and osteocalcin mRNA levels reveal both up or down regulation as a function of the developmental stage of the osteoblast phenotype. A comparison of transcriptional levels of these genes by nuclear run-on assays to mRNA levels indicates that glucocorticoids exert both transcriptional and post-transcriptional effects. Further, the presence of glucocorticoids enhances the vitamin D3 effect on gene expression. Those genes which are upregulated by 1,25(OH)2D3 are transcribed at an increased rate by dexamethasone, while those genes which are inhibited by vitamin D3 remain inhibited in the presence of dexamethasone and D3. We propose that the glucocorticoids promote changes in gene expression involved in cell-cell and cell-extracellular matrix signaling mechanisms that support the growth and differentiation of cells capable of osteoblast phenotype development and bone tissue-like organization, while inhibiting the growth of cells that cannot progress to the mature osteoblast phenotype in fetal rat calvarial cultures.