The adult skeleton regenerates by temporary cellular structures that comprise teams of juxtaposed osteoclasts and osteoblasts and replace periodically old bone with new. A considerable body of evidence accumulated during the last decade has shown that the rate of genesis of these two highly specialized cell types, as well as the prevalence of their apoptosis, is essential for the maintenance of bone homeostasis; and that common metabolic bone disorders such as osteoporosis result largely from a derangement in the birth or death of these cells. The purpose of this article is 3-fold: 1) to review the role and the molecular mechanism of action of regulatory molecules, such as cytokines and hormones, in osteoclast and osteoblast birth and apoptosis; 2) to review the evidence for the contribution of changes in bone cell birth or death to the pathogenesis of the most common forms of osteoporosis; and 3) to highlight the implications of bone cell birth and death for a better understanding of the mechanism of action and efficacy of present and future pharmacotherapeutic agents for osteoporosis.

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Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis.

Bone tissue is continuously remodeled through the concerted actions of bone cells, which include bone resorption by osteoclasts and bone formation by osteoblasts, whereas osteocytes act as mechanosensors and orchestrators of the bone remodeling process. This process is under the control of local (e.g., growth factors and cytokines) and systemic (e.g., calcitonin and estrogens) factors that all together contribute for bone homeostasis. An imbalance between bone resorption and formation can result in bone diseases including osteoporosis. Recently, it has been recognized that, during bone remodeling, there are an intricate communication among bone cells. For instance, the coupling from bone resorption to bone formation is achieved by interaction between osteoclasts and osteoblasts. Moreover, osteocytes produce factors that influence osteoblast and osteoclast activities, whereas osteocyte apoptosis is followed by osteoclastic bone resorption. The increasing knowledge about the structure and functions of bone cells contributed to a better understanding of bone biology. It has been suggested that there is a complex communication between bone cells and other organs, indicating the dynamic nature of bone tissue. In this review, we discuss the current data about the structure and functions of bone cells and the factors that influence bone remodeling.

/pdf/biology-of-bone-tissue-structure-function-and-factors-that-4que250efz.pdf

Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells

Growth factors in bone matrix. Isolation of multiple types by affinity chromatography on heparin-Sepharose

BONE differs from reproductive tissues in that many aspects of skeletal growth, turnover, and function occur in the absence of gonadal hormones. Nevertheless, sex steroids have an important impact on bone physiology: they participate in the sexual dimorphism of the skeleton, play a role in maintenance of mineral homeostasis during reproduction, and are essential to maintaining bone balance in adults. Insufficient levels of certain sex steroids predispose the human skeleton to bone loss and to osteoporotic fractures. Estrogen appears to be the most important sex steroid in preventing osteoporosis in women and is the subject of this review. Serious difficulties must be overcome to understand the action of estrogen on mineralized tissues. The skeleton is heterogeneous and contains many cell types. The cells that synthesize (osteoblasts), lie within (osteocytes), and degrade (osteoclasts) bone matrix are present in small numbers, are difficult to isolate as pure populations, and probably do not proliferate.

Skeletal Effects of Estrogen

https://www.cell.com/trends/molecular-medicine/pdf/S1471-4914(04)00313-2.pdf

Osteoclast-derived activity in the coupling of bone formation to resorption

We have developed an in vitro system, using embryonic chicken tibiae grown in a serum-free medium, which exhibits simultaneous bone formation and resorption. Tibiae from 8-day embryos increased in mean (+/- SD) length (4.0 +/- 0.4 to 11.0 +/- 0.3 mm) and dry weight (0.30 +/- 0.04 to 0.84 +/- 0.04 mg) during 12 days in vitro. There was increased incorporation of [3H]proline into hydroxyproline (120 +/- 20 to 340 +/- 20 cpm/mg of bone per 24 hr) as a measure of collagen synthesis, as well as a 62 +/- 5% increase in total calcium and 45Ca taken up as an indication of active mineralization. A physiologic concentration (1 pM) of parathyroid hormone was found to stimulate bone resorption over control levels in this system. Parathyroid hormone stimulated the release of [3H]hydroxyproline from the bone shafts but not from the cartilage ends, indicating the specificity of the response. With 1 pM parathyroid hormone we observed an acute inhibition of bone formation, followed (after 12-16 hr) by a chronic stimulation of bone formation during the 12-day incubation. Both mineral uptake and matrix formation were enhanced at approximately the same rate during the 12-day incubation. The chronic enhancement of formation required parathyroid hormone only for the initial 8-10 hr of incubation. These results could be explained by the production or release of a factor from bone to stimulate formation in response to the acute increase in resorption--a "coupling factor." Indeed, dialyzed culture medium conditioned by actively resorbing bones stimulated bone formation over controls when added to organ cultures at a 1:20 dilution. The factor is larger than 12,000 daltons as determined by dialysis. The factor is specific for the bone shaft and did not affect the cartilage ends.

https://www.pnas.org/content/pnas/78/5/3204.full.pdf

Parathyroid hormone stimulates bone formation and resorption in organ culture: evidence for a coupling mechanism.

Cells were dispersed from rat kidney after enzymatic digestion of the extracellular matrix. When the cells were suspended in a serum-free medium and incubated with 3H-labeled 25-hydroxyvitamin D3 (25-OH-D3) several polar metabolites, including 1,25-(OH)2[3H]D3 and 24,25-(OH)2[3H]D3 were produced. The specific activities of the 25-OH-D3:1- and 24-hydroxylases in isolated rat kidney cells were 10-100 times greater than in avian kidney homogenates. The rates of production of 1,25-(OH)2D3 and 24,25-(OH)2D3 were linear over a wide range in cell densities (0.65-5.0 × 106 cells per ml) and substrate concentrations (3.5-70 nM). The rate of production of 24,25-(OH)2[3H]D3 from 25-OH-[3H]D3 by cells isolated from rats fed control diet was linear with time for up to 30 min, while the synthesis of 1,25-(OH)2[3H]D3 was linear for over 90 min. The specific activity of the 25-OH-D3:1-hydroxylase was increased in kidney cells from vitamin D-deficient rats (11.5 fmol/min per 106 cells) as well as calcium-deficient rats (8.1 fmol/min per 106 cells) when compared to cells from rats fed the control diet (2.0 fmol/min per 106 cells). Also, the specific activity of the 25-OH-D3:24-hydroxylase was reduced in cells from the vitamin D-deficient rats (<0.2 fmol/min per 106 cells) and calcium-deficient rats (5.1 fmol/min per 106 cells) compared to the controls (15.2 fmol/min per 106 cells). On the basis of these results, as well as previous in vivo studies, we conclude that the metabolism of 25-OH-D3 by freshly isolated rat kidney cells reflects the in vivo activities of the renal vitamin D-metabolizing enzymes and may prove useful as an assay.

https://www.pnas.org/content/pnas/77/3/1537.full.pdf

Brian L. Bottemiller

Papers

Parathyroid hormone stimulates bone formation and resorption in organ culture: evidence for a coupling mechanism.

In vitro metabolism of 25-hydroxyvitamin D3 by isolated rat kidney cells.

In Vitro Regulation of 25-Hydroxyvitamin D3 Metabolism by Avian Kidney Cells in Serum-Free Medium