Osteoporosis, loss of skeletal mass with aging, is a major health problem in the United States. This process is accelerated in women after the menopause due to the osteoclast stimulating effects of estrogen deficiency. The precise mechanism by which estrogen deficiency increases bone resorption is poorly understood. Although there is convincing evidence to suggest that estrogen withdrawal increases osteoclast formation, the possibility that osteoclast life span is also increased has not been investigated. The increased osteoclast activity seen after the menopause appears to be mediated by cytokines, such as interleukin 6 (IL- 6), interleukin 1 (IL-1), and tumor necrosis factor (TNF), although whether the effects of these cytokines are mediated only on osteoclast generation or also on osteoclast loss (for example, by apoptosis) is unknown. We have found that each of these factors can increase osteoclast generation in vitro and in vivo,, and we have indirect evidence to suggest that IL-1 promotes osteoclast survival. It is generally accepted that osteoclasts are relatively short-lived cells that disappear from the bone surface during the reversal phase of bone remodeling when resorption ceases and formation commences. The fate of osteoclasts at reversal sites is unknown, but clearly acceleration or delay of their disappearance is a potential control mechanism for bone resorption, and could be involved in the pathogenesis of osteoporosis. We have hypothesized that osteoclasts undergo programmed cell death (apoptosis) in bone remodelling units. We have examined sections of calvarial bone from normal mice in which bone resorption was initiated by IL-1 and have observed apoptotic osteoclasts in bone remodelling units, predominantly at reversal sites. We have also reported that osteoclasts in transgenic mice expressing SV40 T antigen driven by the tartrate-resistant acid phosphatase (TRAP) promoter undergo apoptosis. We have since re-examined sections of calvarial bone from normal mice in which bone resorption was initiated by IL-1 and have observed apoptotic osteoclasts in bone remodelling units, predominantly at reversal sites. More recently, we have modified the in vitro mouse bone marrow-derived osteoclast culture system and have detected apoptotic osteoclasts in cytospin preparations of the supernatants after a further 24 or 48 hours of culture. In preliminary experiments with this model, we have found that estradiol and transforming growth factor-beta (TGFbeta) appear to promote osteoclast apoptosis, while 1,25 (OH)2 vitamin D3 and IL-1 prevent it. We now plan 1) to develop and fully characterize an in vitro model to examine the regulation and molecular mechanisms involved in osteoclast apoptosis, 2) to establish whether apoptosis is a determinant of osteoclast function in vitro and in vivo, 3) to determine the effects of estradiol and antibodies to cytokines, such as IL-6, on osteoclast apoptosis and survival in vivo in ovariectomized mice and 4) to determine whether loss of function of cell survival genes, such as Bcl-2 protein, inhibits bone resorption. If apoptosis is an important determinant of osteoclast activity and of bone remodelling, then these studies may provide new molecular targets for regulating rates of bone remodelling with pharmacologic agents.
Showing the most recent 10 out of 80 publications
