Hydrogen peroxide is essential for estrogen-deficiency bone loss and osteoclast formation.

We recently found that estrogen deficiency leads to a lowering of thiol antioxidant defenses in rodent bone. Moreover, administration of agents that increase the concentration in bone of glutathione, the main intracellular antioxidant, prevented estrogen-deficiency bone loss, whereas depletion of glutathione by buthionine sulfoximine administration provoked substantial bone loss. To analyze further the mechanism by which antioxidant defenses modulate bone loss, we have now compared expression of the known antioxidant enzymes in osteoclasts. We found that glutathione peroxidase 1 (Gpx), the enzyme primarily responsible for the intracellular degradation of hydrogen peroxide, is overwhelmingly the predominant antioxidant enzyme expressed by osteoclasts and that its expression was increased in bone marrow macrophages by receptor activator of nuclear factor-kappaB ligand (RANKL) and in osteoclasts by 17beta-estradiol. We therefore tested the effect of overexpression of Gpx in osteoclasts by stable transfection of RAW 264.7 (RAW) cells, which are capable of osteoclastic differentiation in response to RANKL, with a Gpx-expression construct. Osteoclast formation was abolished. The Gpx expression construct also suppressed RANKL-induced nuclear factor-kappaB activation and increased resistance to oxidation of dihydrodichlorofluorescein by exogenous hydrogen peroxide. We therefore tested the role of hydrogen peroxide in the loss of bone caused by estrogen deficiency by administering pegylated catalase to mice. We found that catalase prevented ovariectomy-induced bone loss. These results suggest that hydrogen peroxide is the reactive oxygen species responsible for signaling the bone loss of estrogen deficiency.

A crucial role for thiol antioxidants in estrogen-deficiency bone loss.

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.

CCL9/MIP-1gamma and its receptor CCR1 are the major chemokine ligand/receptor species expressed by osteoclasts.

Although much has been learned recently of the mechanisms by which the differentiation of osteoclasts is induced, less is known of the factors that regulate their migration and localization, and their interactions with other bone cells. In related cell types, chemokines play a major role in these processes. We therefore systematically tested the expression of RNA for chemokines and their receptors by osteoclasts. Because bone is the natural substrate for osteoclasts and may influence osteoclast behavior, we also tested expression on bone slices. Quantitative RT-PCR using real-time analysis with SYBR Green was therefore performed on RNA isolated from bone marrow cells after incubation with macrophage-colony stimulating factor (M-CSF) with/without receptor-activator of NFkappaB ligand (RANKL), on plastic or bone. We found that RANKL induced expression of CCL9/MIP-1gamma to levels comparable to that of tartrate-resistant acid phosphatase (TRAP), a major specialized product of osteoclasts. CCL22/MDC, CXCL13/BLC/BCA-1, and CCL25/TECK were also induced. The dominant chemokine receptor expressed by osteoclasts was CCR1, followed by CCR3 and CX3CR1. Several receptors expressed on macrophages and associated with inflammatory responses, including CCR2 and CCR5, were down-regulated by RANKL. CCL9, which acts through CCR1, stimulated cytoplasmic motility and polarization in osteoclasts, identical to that previously observed in response to CCL3/MIP-1alpha, which also acts through CCR1 and is chemotactic for osteoclasts. These results identify CCL9 and its receptor CCR1 as the major chemokine and receptor species expressed by osteoclasts, and suggest a crucial role for CCL9 in the regulation of bone resorption.