Persistent bone-sparing effect of interleukin-1 receptor antagonist: A hypothesis on the role of IL-1 in ovariectomy-induced bone loss

The recent finding that treatment with the interleukin-1 (IL-1) inhibitor, interleukin-1 receptor antagonist (IL-1ra) decreases bone loss and bone resorption in ovariectomized rats, strongly suggested that IL-1 mediates, at least in part, the effects of estrogen deficiency on bone resorption. Although in vitro studies have shown that IL-1 activates mature osteoclasts and stimulates osteoclastogenesis, the two main mechanisms by which estrogen deficiency stimulates bone resorption, it is still unclear whether IL-1 mediates both effects of estrogen deficiency in vivo. To investigate this matter, we have examined the changes in bone mineral density (BMD) which occur in ovariectomized rats after completion of 1 month of estrogen or IL-1ra treatment begun at the time of ovariectomy. Ovariectomy caused a marked decreased in BMD which was blocked by 17 estradiol and decreased by IL-1ra. Cessation of estrogen therapy was followed by a rapid induction of bone loss, indicating that estrogen blocks the activation and utilization of mature osteoclasts without depleting the bone microenvironment of osteoclast precursors and mature, inactive osteoclasts. In contrast, ovariectomized rats treated with IL-1ra maintained a stable bone density for the first 4 weeks after completion of the treatment. In these rats, bone loss resumed not earlier than 6 weeks after discontinuation of the IL-1ra treatment. Estrogen deficiency was necessary to unveil the bone-sparing effect of IL-1ra because in a control experiment in which rats were treated with IL-1ra for the 4 weeks before ovariectomy, BMD began to decrease immediately after ovariectomy. Based on these results we propose the hypothesis that in conditions of estrogen deficiency, the main effect of IL-1ra is to block the proliferation and differentiation of osteoclast precursors, an event that results in the depletion of mature, rapidly responsive osteoclasts. We also suggest that estrogen may have important direct effects on the regulation of osteoclast activity.