This Project extends the clinical investigative studies to animal models, with the overall goal of probing deeper into the mechanisms of estrogen (E) action on bone. Recently, a host of coregulators for E receptor (ER) action and of compounds with the ability to activate E signaling in a tissue specific manner (selective ER modulators, SERMs) have been identified. In the proposed studies, we will focus on two of the most important coactivators for ER action, steroid receptor coactivator (SRC)-I and the closely related SRC-2. Following activation of the ER by ligand, these coactivators bind to the ER and are critical in mediating E effects on target tissues. However, little is known about the role of these coactivators in mediating E action in bone, whether either can, at least partially, compensate for loss of the other, or how these coactivators may modulate the tissue specific actions of SERMs such as tamoxifen or raloxifene. In preliminary studies, we have shown that SRC-1 knock out (KO) mice have an impaired skeletal response to E, particularly in cancellous bone. We will use these and the SRC-2 KO mice to test the hypotheses that: (1) Both SRC-1 and -2 are important in mediating E effects on bone; (2) In the setting of loss of SRC-1 or -2, each coactivator can, at least partially, compensate for the absence of the other; (3) While E can have at least partial effects on bone in the setting of SRC-1 or -2 deficiency, SERMs will be completely ineffective; and (4) E action on bone (or least that component that requires SRC-1) is mediated via effects on cells in both the osteoblastic and osteoclastic lineages. Our Specific Aims, therefore, are: (1) To better define the role of SRC-1 and -2, and their interactions, in mediating E action on the female mouse skeleton in vivo; (2) Using the SRC-1 and -2 KO mice, characterizing the role of these coactivators in SERM action in bone; and (3) Defining the extent to which selective replacement of SRC-1 in cells of either the osteoblastic or the osteoclastic lineages can correct the deficit in the skeletal effects of E that are present in the SRC-1 KO mice. When combined with the molecular studies on these same coactivators in Project 5, the proposed in vivo studies will help to provide a comprehensive picture of the role of these important coactivators in E and SERM action on bone, which is also likely to have significant clinical implications.