Overview

Novel Receptor-Based Countermeasures to Microgravity-Induced Bone Loss

Principal Investigator:
Bert W. O'Malley, M.D.

Organization:
Baylor College of Medicine

Technical Summary

The biological actions mediated by the estrogen receptor (ER), vitamin D receptor (VDR) and extracellular Ca²⁺_o-sensing receptor (CaR) play key roles in the normal control of bone growth and skeletal turnover that is necessary for skeletal health. These receptors act by controlling the differentiation and/or function of osteoblasts and osteoclasts, and other cell types within the bone and bone marrow microenvironment. The appropriate use of selective ER modulators (SERMs) which target bone, vitamin D analogs that favor bone formation over resorption, and CaR agonists that may both stimulate osteoblastogenesis and inhibit osteoclastogenesis as well as the function of mature osteoclasts, should make it possible to prevent the reduction in bone formation and increase in bone resorption that normally contribute to the bone loss induced by weightlessness. Indeed, there may be synergistic interactions among these receptors that enhance the actions of any one used alone. Therefore, we proposed to: 1) assess the in vitro ability of novel ER, VDR and CaR agonists, alone or in combination, to modulate osteoblastogenesis and mature osteoblast function under conditions of 1g and simulated microgravity; 2) assess the in vitro ability of novel ER, VDR and CaR agonists, alone or in combination, to modulate osteoclastogenesis and bone resorption under conditions of 1g and simulated microgravity; and 3) carry out baseline studies on the skeletal localization of the CaR in normal rat bone as well as the in vivo actions of our novel ER- and VDR-based therapeutics in the rat in preparation for their use, alone or in combination, in well-established ground-based models of microgravity and eventually in space flight.

We have found that the CaR is expressed in osteoblastic cells as well as in bovine, murine and rat bone and that activation of this receptor in osteoblastic cells leads to activation of an outward K⁺ channel and chemotaxis of calvarial osteoblasts in response to elevated Ca²⁺_o. The CaR is also present in osteoclast precursors and in osteoclast-like cells formed in vitro and plays roles in regulating osteoclastogenesis and osteoclast chemotaxis. In our VDR studies, we have examined the ability of the VDR agonist, EB1089, which is less calcemic than calcitriol, to regulate osteoblastic gene expression and find that it is more potent than calcitriol. In addition, gene expression in the MG-63 osteoblastic cell line was characterized in the Slow Turning Lateral Vessel (STLV) culture system, which approximates many aspects of microgravity. Many genes were down-regulated in comparison to monolayer cultures grown at unit gravity, and responses to VDR agonists were less robust. In ongoing hindlimb suspension studies in male rats, EB1089 was able to prevent unloading-induced bone loss measured at the proximal tibia, while calcitriol was able to increase bone mineral density. However, increases in serum calcium in calcitriol-treated animals, not observed in EB1089-treated rats, indicate that the latter is a superior countermeasure. EB1089 is also a less potent stimulator of osteoclast formation in comparison to calcitriol. Finally, our ER studies have revealed that osteoblastic gene expression patterns induced by estradiol and the SERMs idoxifene and raloxifene, are distinct even though all agents are capable of inhibiting bone loss due to sex steroid depletion. Raloxifene does not reduce bone mineral density in normal female rats and has only modest effects on biochemical markers of bone turnover suggesting its use in gonad-intact populations should not increase the risk of bone loss via inhibiting endogenous estrogens. In ongoing hindlimb suspension studies in ovariectomized female rats, raloxifene and estradiol, individually, appear able to prevent loss of bone mineral density. Since raloxifene does not exert undesirable, estrogen-like effects in reproductive tissues, it has the potential to be an acceptable countermeasure to disuse-induced bone loss. Ongoing studies will continue to examine the use of EB1089 and raloxifene, alone and in combination, to prevent bone loss in male and female rats induced by hindlimb suspension.

Collectively these studies suggest that manipulation of VDR and ER activity has the potential to reduce the risk of bone loss resulting from the microgravity environment encountered during Space travel. Importantly, our data also suggest that novel ligands for these two receptors that significantly attenuate the negative side effects of the natural ligands can be effectively employed to reduce unloading-induced bone loss, and the ensuing risks of bone fracture.

This project's funding ended in 2000