Severe osteoporosis affects over 10 million Americans. Over 300,000 hip fractures are caused by osteoporosis annually. Recombinant parathyroid hormone (PTH) 1-34 (teriparatide) is currently the only FDA-approved anabolic agent for treatment of osteoporosis in the United States. Intermittent administration of PTH increases bone formation, whereas continuous infusion of PTH (1-34) results in bone resorption. However, the roles of distinct signaling pathways activated by PTH receptor (PTH1R) that generates this skeletal response remain incompletely understood. The ubiquitin-proteasome pathway plays an important role in regulating and controlling bone metabolism. PTH1R ubiquitination is a reversible process. Deubiquitination of modified PTH1R is mediated by ubiquitin specific protease 2 (USP2). The expression of USP2 is rapidly increased by acute exposure to PTH in osteoblasts and returns to control levels with prolonged treatment. Other proteins, such as, regulator of G protein signaling 2 (RGS2), runt-related transcription factor 2 (Runx2), and b-catenin, the activities of which can be induced by PTH and involved in osteogenesis, are also down regulated by a ubiquitin-sensitive pathway. Osteoclasts do not express PTH1R, and the response to PTH is primarily mediated by receptor activator of NFkB ligand (RANKL). The engagement of RANKL binding to RANK leads to induction of gene expression involved in osteoclastogenesis. Proteasomal inhibitor enhances osteoblastogenesis by increasing Runx2 activity and stabilization of b-catenin protein, and it inhibits osteoclastogenesis by decreasing RANKL-induced signaling cascades. These findings suggest that there is a central hypothesis that PTH exerts bimodal effects on bone metabolism mediated by the ubiquitin-proteasome pathway, and proteasomal inhibitor enhances the anabolic effects of PTH on bone.
Two specific aims are developed to test this hypothesis.
Aim 1 will characterize how bimodal effects of PTH on osteoblastogenesis are mediated by ubiquitin-proteasome pathway in vitro.
In Aim 2, we will define the in vivo role of intermittent and continuous PTH administration in combination with proteasomal inhibitor on bone mass. The information gained through these studies will not only contribute to understanding the mechanisms of PTH treatment of osteoporosis, but also provide important insight into designing pharmacological agents for treatment of metabolic bone diseases such as osteoporosis.
The goal of this research plan is to characterize how ubiquitin-proteasome pathway controls parathyroid hormone effect on bone metabolism. The information gained through these studies will contribute to understanding the mechanisms of PTH treatment of osteoporosis. The long-term objective is to identify new compounds that can be used to treat osteoporosis.