K. Fuller

TRANCE (tumor necrosis factor-related activation-induced cytokine) is a recently described member of the tumor necrosis factor superfamily that stimulates dendritic cell survival and has also been found to induce osteoclastic differentiation from hemopoietic precursors. However, its effects on mature osteoclasts have not been defined. It has long been recognized that stimulation of osteoclasts by agents such as parathyroid hormone (PTH) occurs through a hormonal interaction with osteoblastic cells, which are thereby induced to activate osteoclasts. To determine whether TRANCE accounts for this activity, we tested its effects on mature osteoclasts. TRANCE rapidly induced a dramatic change in osteoclast motility and spreading and inhibited apoptosis. In populations of osteoclasts that were unresponsive to PTH, TRANCE caused activation of bone resorption equivalent to that induced by PTH in the presence of osteoblastic cells. Moreover, osteoblast-mediated stimulation of bone resorption was abrogated by soluble TRANCE receptor and by the soluble decoy receptor osteoprotegerin (OPG), and stimulation of isolated osteoclasts by TRANCE was neutralized by OPG. Thus, TRANCE expression by osteoblasts appears to be both necessary and sufficient for hormone-mediated activation of mature osteoclasts, and TRANCE-R is likely to be a receptor for signal transduction for activation of the osteoclast and its survival.

The mechanisms through which estrogen prevents bone loss are uncertain. Elsewhere, estrogen exerts beneficial actions by suppression of reactive oxygen species (ROS). ROS stimulate osteoclasts, the cells that resorb bone. Thus, estrogen might prevent bone loss by enhancing oxidant defenses in bone. We found that glutathione and thioredoxin, the major thiol antioxidants, and glutathione and thioredoxin reductases, the enzymes responsible for maintaining them in a reduced state, fell substantially in rodent bone marrow after ovariectomy and were rapidly normalized by exogenous 17-beta estradiol. Moreover, administration of N-acetyl cysteine (NAC) or ascorbate, antioxidants that increase tissue glutathione levels, abolished ovariectomy-induced bone loss, while l-buthionine-(S,R)-sulphoximine (BSO), a specific inhibitor of glutathione synthesis, caused substantial bone loss. The 17-beta estradiol increased glutathione and glutathione and thioredoxin reductases in osteoclast-like cells in vitro. Furthermore, in vitro NAC prevented osteoclast formation and NF-kappaB activation. BSO and hydrogen peroxide did the opposite. Expression of TNF-alpha, a target for NF-kappaB and a cytokine strongly implicated in estrogen-deficiency bone loss, was suppressed in osteoclasts by 17-beta estradiol and NAC. These observations strongly suggest that estrogen deficiency causes bone loss by lowering thiol antioxidants in osteoclasts. This directly sensitizes osteoclasts to osteoclastogenic signals and entrains ROS-enhanced expression of cytokines that promote osteoclastic bone resorption.

The mechanisms through which estrogen prevents bone loss are uncertain. Elsewhere, estrogen exerts beneficial actions by suppression of reactive oxygen species (ROS). ROS stimulate osteoclasts, the cells that resorb bone. Thus, estrogen might prevent bone loss by enhancing oxidant defenses in bone. We found that glutathione and thioredoxin, the major thiol antioxidants, and glutathione and thioredoxin reductases, the enzymes responsible for maintaining them in a reduced state, fell substantially in rodent bone marrow after ovariectomy and were rapidly normalized by exogenous 17- estradiol. Moreover, administration of N-acetyl cysteine (NAC) or ascorbate, antioxidants that increase tissue glutathione levels, abolished ovariectomy-induced bone loss, while L-buthionine-(S,R)-sulphoximine (BSO), a specific inhibitor of glutathione synthesis, caused substantial bone loss. The 17- estradiol increased glutathione and glutathione and thioredoxin reductases in osteoclast-like cells in vitro. Furthermore, in vitro NAC prevented osteoclast formation and NF-B activation. BSO and hydrogen peroxide did the opposite. Expression of TNF-, a target for NF-B and a cytokine strongly implicated in estrogen-deficiency bone loss, was suppressed in osteoclasts by 17- estradiol and NAC. These observations strongly suggest that estrogen deficiency causes bone loss by lowering thiol antioxidants in osteoclasts. This directly sensitizes osteoclasts to osteoclastogenic signals and entrains ROS-enhanced expression of cytokines that promote osteoclastic bone resorption.

It has previously only been possible to assess osteoclastic bone resorption in intact bone, where other cell types may modify or mediate osteoclastic responses to environmental agents. We have recently developed techniques which enable us to measure bone resorption by osteoclasts extracted from bone and have used these techniques to assess the effects of prostaglandins (PGs) and calcium-regulating hormones on bone resorption by these cells. Osteoclasts were mechanically disaggregated from neonatal rabbit long bones and cultured on slices of devitalized cortical bone for 8 or 24 h. After this time, osteoclasts were associated with the appearance in the scanning electron microscope of characteristic resorption pits, the volume of which was calculated by computer-assisted morphometric and stereophotogrammetric techniques after removal of cells. Salmon calcitonin inhibited osteoclastic bone resorption at concentrations of 1 pg/ml and above, while PTH and 1,25-dihydroxyvitamin D3 were without significant effect. This suggests that the latter hormones do not increase bone resorption in intact bone through a direct effect on osteoclasts. PGI2, PGE1, and PGE2, all of which are known to stimulate resorption when added to intact bone, paradoxically reduced resorption in our cultures. It appears likely that PGs act as direct inhibitors of osteoclastic bone resorption but have an additional effect on other cells in bone, which are induced by PGs to cause osteoclastic stimulation.

A carborundum wheel was used to prepare slices of cortical bone that demonstrate a predictable surface appearance in the scanning electron microscope. Osteoclasts were mechanically disaggregated from neonatal rabbit long bones and settled onto these slices. After 24h in culture osteoclasts were associated with areas of excavation in the bone surface. These excavated areas typically showed a well-defined outline and a distinctive fibrillar base, which resembled the pattern of collagen fibrils in bone. The majority of such concavities were of approximately circular outline and of smaller diameter than the associated osteoclast, but other excavations were elongated or of complex morphology, and may have been produced by osteoclasts that were resorbing bone while they migrated. Irregular concavities tended to be more shallow but to occupy a greater area of the bone surface than circular concavities. Roughening of the bone surface without detectable excavation was also seen adjacent to osteoclasts. Calcitonin and cytochalasin B, which inhibit osteoclastic motility, also inhibited bone resorptive activity by these cells. The techniques described in this paper represent a model system with which to assess the direct and indirect effects of hormones, cells and substrate composition on the induction, stimulation and inhibition of osteoclastic bone resorption and to investigate the mechanisms by which cells degrade extracellular matrices.