MOLECULAR MECHANISMS OF ESTROGEN-REGULATED BONE RESORPTION

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The primary objective of this project is to enhance our understanding of the mechanisms of increased bone resorption, which accompanies estrogen deficiency. Bone resorption parameters have been compared between control females six months of age, ovariectomized females (OVX), females receiving hormone replacement therapy (HRT), and females receiving a calcium channel antagonists (Diltiazem, Nifedipine, Verapamil), or females receiving a combination of the two agents. As expected OVX rats displayed an increased rate of bone resorption from both skeletal compartments and HRT reduced that effect. The actions of the various antagonists were antagonistic to that of estrogen on the amorphous compartment and synergistic on the calcified compartment indicating differing mechanisms of action. The effects of the calcium channel antagonists on mean tail arterial pressure have proceeded as hypothesized demonstrating that all three classes of calcium channel antagonists significantly lowered blood pressure as compared to controls even when given in combination with HRT. ELISA assays designed to detect various bone degradation makers (deoxypyridinoline (DPD), pyridinoline (PYD), and helical peptide) in urine have provided an alternate avenue by which to monitor skeletal metabolism. OVX females display a significant increase in the excretion of DPD, PYD, and helical peptide, indicating an increase in bone turnover, while those receiving HRT were not significantly different from controls. The utilization of calcium antagonists decreases the excretion of DPD, PYD, and helical peptide, indicating a decrease in bone turnover and suggesting a similar mode of action as estrogen. In order to determine the mechanism by which estrogen elicits this action in osteoblast-like cells (7F2 & UMR-106), experiments were carried out in an enriched media (OPTI-MEM) in the absence of estrogenic compounds (FBS & phenol red). This protocol did not alter the viability or morphology of the cells as compared to controls grown with FBS. Experimental results indicate that estrogen over a range of concentrations (0.1nM to 1.9mM) stimulates cell proliferation following 24 hr incubation, demonstrating estrogen's osteoblastic activity. Our initial genomic studies suggest that estrogen up regulates the transcription of Transforming Growth Factor Beta Inducible Early Growth Response (TIEG) and osteoprotegerin (OPG). These experimental findings support the hypothesis of an antiresorptive role for estrogen in skeletal turnover.