Physiology of Bone Metabolism in an Aging Population

To better understand the causes of age-related bone loss.

Study 1 – Pathophysiology of osteoporosis in aging men

Osteoporosis in women has recognized for some time now as an enormous public health problem in women. It is now clear that men also develop the disease. We and others have previously implicated that estrogen deficiency plays a significant role in age-related bone loss not only in women, but also in men. It is more difficult to study sex steroid effects on bone in aging men because they only have partial sex steroid deficiency wheras women are almost completely deficient. We have developed a powerful methodology that produces a reversible, short term sex steroid deficiency in elderly men and allows for selective estrogen and/or testosterone replacement.

We are combining this methodology with novel techniques we have recently developed in our laboratories that allow us to assess protein and gene expression of key factors regulating bone resorption by bone marrow cells in vivo. We are studying three important areas where estrogen has been shown to play an important role in regulating bone metabolism in women, but where there is still a lack of data on estrogen ( or testosterone) effects in men. There are: (1) the mechanism(s) of the direct actions of sex steroids on bone resorption; (2) possible extra-skeletal effects of estrogen and/or testosterone on the intestine and the kidney; and (3) the modulation of estrogen and/or testosterone, of the pro-resorptive effects of parathyroid hormone (PTH) on bone. These studies will improve our understanding of sex steroid regulation of the male skeleton and of age-related bone loss in men.

Study 2 – Pathophysiology of osteoporosis in aging women

Postmenopausal women undergo two distinct phases of bone loss – a rapid, transient phase that begins at menopause (due to loss of the direct suppressive effects of estrogen on bone cell function) and a slow, subsequent phase that continues indefinitely (due mainly to loss of estrogen effects on peripheral calcium metabolism leading to secondary hyperparathyroidism (HPT) and , indirectly, to bone loss.

Although much has been learned in recent years about the pathophysiology of bone loss in aging women, the most important remaining research need is to define the mechanisms by which these skeletal effects produce bone loss. Although there have been extensive studies using in vitro systems and experimental animals there are very limited data in women. We will apply novel methodological approaches that will allow us to study of these mechanism directly in humans. We will apply a new method that we have developed to isolate osteoblast and osteoclast precursors from bone marrow, to study their differentiation and function, and to assess their content of mRNA for putative regulatory cytokines and cytokine receptors. These methods utilize 2-color flow cytometry/FACS, intensity of fluorescent probes, and real time RT-PCR. This study will also help explain the molecular mechanisms for the newly approved anabolic regimen of intermittent PTH (1-34) treatment for osteoporosis.

Study 3 – Estrogen receptor coactivators in bone metabolism

This study extends our human clinical studies described above to animal models, with the overall goal of probing deeper into the mechanisms of estrogen action on bone. Recently, a host of coregulators for estrogen receptor (ER) action and of compounds with the ability to activate estrogen signalling in a tissue specific manner have been identified. In this study we will focus on two of the most important coactivators for ER action, steroid receptor coactivator (SRC)-1 and the closely related SRC-2. Following activation of the ER by ligand, these coactivators bind to the ER and are critical in mediating estrogen effects on target tissues. However, little is known about the role of these coactivators in mediating estrogen 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 selected ER modulators, (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 estrogen, particularly in cancellous bone.

We will use the SRC-1 and SRC-2 KO mice to test the hypothesis that: (1) Both SRC-1 and SRC-2 are important in mediating estrogen effects on bone; (2) That either SRC-1 or SRC-2 can, at least partially, compensate for the absence of the other; (3) While estrogen can have at least partial effects on bone in the setting of SRC-1 or SRC-2 deficiency, SERMs will be completely ineffective; and (4) Estrogen action on bone (or at least that component that requires SRC-1) is mediated via effects on cells in both osteoblastic and osteoclastic lineages. These studies will help to provide a comprehensive picture of the role of these important coactivators in estrogen and SERM action on bone, which is also likely to have significant clinical implications.