Novel Molecular and Cellular Mechanisms for Osteoanabolic Action of Parathyroid Hormone

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Project Summary/Abstract Osteoporosis afflicts the elderly population and results from a disruption of the balance between osteoblastic bone formation and osteoclastic bone resorption. Intermittent parathyroid hormone (PTH) by daily injection increases bone formation, whereas continuous PTH causes bone resorption. However, the mechanisms by which intermittent administration of PTH increases bone mass remain poorly understood; delineation of these mechanisms could enable a refinement of PTH-based osteoporosis treatment. The anabolic PTH effects on bone are mediated mostly by the Gs/cAMP signaling cascades and the canonical beta-catenin pathway that is independent of Wnt, whereas Gq/PLC activation may antagonize these osteoanabolic actions. In addition, other signaling pathways including PTH-stimulated PLD activity, ERK1/2 activation and PI3K/Akt signaling also affect the anabolic PTH actions in bone. We have obtained considerable data suggesting the interaction of PTHR with beta-catenin significantly constrains the anabolic effect of PTHR signaling, and disruption of this interaction with a cell-permeable peptide constitutes a viable therapeutic approach for improving the efficacy of PTH in vivo. The major goal of this proposal is to delineate the multiple elements of PTHR signaling that promote/restrain the anabolic effects of PTH on bone. We will test the hypothesis that PTHR dissociates from beta-catenin and subsequent trafficking of each induces distinct signaling pathways that mediate the anabolic action of PTH in bone, and that a disruptor peptide that promotes PTHR-beta-catenin dissociation has pro-anabolic therapeutic utility in vivo. Three aims are proposed to test this hypothesis. In aim 1, we will elucidate mechanisms whereby beta-catenin coupling to PTHR elicits initial biased signaling of the PTHR, and the role of PTHR trafficking and beta-catenin translocation in sustaining anabolic PTH signaling. Aim 2 will define the role of the disruptor peptide in regulating multiple PTHR signaling pathways. In aim 3, we will directly assess whether disruption of beta- catenin with PTHR increases osteoblast bone formation and reduces osteoclast bone resorption in vitro and in vivo. Successful completion of the proposed research will contribute to a better understanding of the molecular and cellular mechanisms mediating the actions of PTH on bone and advance a novel therapeutic modality to improve PTH-based treatment for osteoporosis.