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Abstract

Estrogen deficiency in rats is responsible for increased osteoclastic resorption and a subsequent rapid bone loss. TGF-β, which is known to have acute effects on bone resorption in several in vitro models, has been shown to be secreted by osteoblastic cells in vitro in response to 17β-estradiol, but little is known about its in vivo effects on bone resorption. We therefore decided to investigate the short-term effect of TGF-β1 on bone resorption in ovariectomized rats. TGF-β1 (0.04–20 ng/injection), or vehicle, was injected daily directly into the bone marrow space, through a thin catheter implanted in the distal end of the right femur, during 4 consecutive days, starting 14 days after the ovariectomy. Bone histomorphometry was performed in the secondary spongiosa of the metaphysis of injected femurs and compared with vehicle-injected femurs of sham ovariectomized rats. Ovariectomy was associated with a marked increase in the resorption surface, a 2-fold increase in the number of osteoclasts, and no change in the number of TRAP-positive marrow cells distant from bone surfaces. Bone resorption was significantly lower in the TGF-β1-injected bones of ovariectomized rats, as compared with vehicle injected bones: the osteoclast surface and the number of osteoclasts were, respectively, 11.0 ± 5.1% versus 20.8 ± 1.3% and 287 ± 41 versus 505 ± 53, in bones injected with 0.2 ng of TGF-β1 as compared with vehicle-injected bones (mean ± SE, p < 0.05). The bone formation rate, assessed by double tetracycline labeling, was increased in ovariectomized rats, but was significantly decreased in TGF-β1-injected bones (2 and 20 ng/injection) of ovariectomized rats. The trabecular bone volume was decreased in ovariectomized animals, but was not modified in TGF-β1-injected bones in this short-term experiment. We conclude that, when injected into bones of ovariectomized rats, TGF-β1 attenuates the stimulation of osteoclastic resorption induced by estrogen deficiency. These data therefore suggest that a local failure of TGF-β secretion could be partly responsible for the stimulation of bone resorption and subsequent bone loss due to the cessation of the ovarian secretion.