(Taken from the application): Our objective is to understand the contribution of apoptosis in determining bone mass and skeletal structure, in particular if apoptosis of osteoblasts contributes to the decrease in bone mass found with high dose glucocorticoids. We have shown that glucocorticoids induce apoptosis and estrogen prevents glucocorticoid-induced apoptosis in osteoblasts in vivo and in vitro. Glucocorticoids were shown to decrease bcl-2 and up-regulate bax levels creating a dose-dependent decrease in the bcl-2/bax ratio that resulted in cell death. We propose to study the effect of bcl-2 overexpression on, osteoblasts responses to glucocorticoids in vitro by transfection of osteoblasts. The extent of bcl-2 expression, the rate of apoptosis and proliferation, and the expression of bone markers will be assayed. We will then determine if bcl-2 overexpression in vivo will prevent the glucocorticoid-induced decrease in bone mass. A transgenic mouse will be constructed that overexpresses bcl-2 targeted to osteoblasts by the 2.3 kDa promoter region of the Type I collagen gone (Co12.3Bc1-2) and its bone phenotype will be characterized. Then these mice and their wild-type litter mates will be treated with and without glucocorticoids. Static and dynamic histomorphometry will be performed along with assays for apoptosis. Our hypothesis is that bcl-2 overexpression will partially prevent glucocorticoid-induced osteopenia. The specific osteoblast functions that are protected from glucocorticoids action in the Co12.3Bc1-2 mouse, will be studied. Finally, we will determine the time course and extent of apoptosis in bone cells after ovariectomy in wild-type and Co12.3Bc1-2 transgenic mice. Bone mass and apoptosis will be assessed. Since little is known about the role of apoptosis in determining skeletal structure and mass, these studies will test the hypothesis that systemic hormones, such as glucocorticoids, affect bone cell function and ultimately bone mass through the regulation of programmed cell death. We also seek to elucidate the contribution of apoptosis to glucocorticoid-induced osteoporosis. If apoptosis is an important determinant of bone mass, then our long-range goal is to develop novel strategies to prolong osteoblast survival and function during glucocorticoid therapy, and after menopause in patients at risk for osteoporosis.
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