Osteoporosis is a disease that is characterized by low bone mass that results in insidious deterioration of the skeleton leaving it fragile and vulnerable to fracture. Osteoporosis is due to multiple genetic, hormonal, nutritional, and physical factors that act alone or together to diminish skeletal integrity. Estrogen replacement blunts the loss of bone by blocking osteoclast-mediated bone demineralization. In this setting, parathyroid hormone (PTH) is able to mediate bone growth. We recently made a series of observations that suggest another potentially important mechanism by which estrogen deficiency may contribute to osteoporosis. Specifically, we found that amino-terminally truncated PTH fragments that are thought to be inactive promoted rapid internalization of the PTH receptor (PTH1R) in a cell-specific manner in bone. These otherwise biologically inactive PTH fragments reduced the abundance of PTH1R on the cell surface without first activating the receptor. This uncoupling of PTH1R activation and inactivation is due to the presence of a cytoplasmic adaptor protein, termed EBP50 (ezrin-binding protein, 50 kDa). EBP50 contains an estrogen response element. Experimental animals lacking EBP50 have osteopenia and develop osteoporosis. We hypothesize that estrogens increase EBP50 expression, thereby protecting the osteoblast PTH1R against the inactivating effects of PTH fragments.
The Specific Aims of this project are: (1) To examine gonadal hormone regulation of EBP50, and (2) to determine if estrogen induction of EBP50 expression contributes to restoring the anabolic action of PTH on bone. The effects of gonadal hormones on EBP50 expression will be determined in osteoblastic human and rat cell lines. The second group of experiments consists of two parts. In the first, ectopic ossicles prepared from bone marrow of EBP50-null mice will be used as a model system to analyze the effects of PTH and estrogens on bone, independent of systemic effects or the donor animal. The second part will examine the effects of estradiol administration alone or in combination with PTH on restoring bone in ovariectomized EBP50-null mice. The proposed studies will lead to a better understanding of the hormonal events that impact bone health and importantly influence the response to bone loading. Novel predictive measures of fragility and potential avenues for therapeutic intervention may be developed. The goal of these new studies is to characterize how the effect of female hormones on bone may be regulated by a certain adapter protein. The long-term objective is to identify novel compounds that can be used to treat post-menopausal osteoporosis. ? ? ?
