Original Full Length ArticleLoss of the PGE2 receptor EP1 enhances bone acquisition, which protects against age and ovariectomy-induced impairments in bone strength☆
Introduction
Prostaglandin E2 (PGE2) is produced by arachidonic acid metabolism and is synthesized via the cyclooxygenase (COX) and prostaglandin synthase pathways. COX-2/PGE2 signaling mediates both physiological and pathological effects in a vast array of tissue types, including bone [1], [2], [3], [4]. The broad effects of PGE2 are attributed to the four prostanoid receptors which bind PGE2: EP1, EP2, EP3 and EP4. These four receptors differ in tissue distribution, ligand binding affinity and activation of downstream signaling pathways [1], [3], [5]. EP2, EP3 and EP4 modulate cAMP levels [3], [5]. EP2 and EP4 activation stimulates the production of cAMP through Gαs. In contrast, EP3 activation results in decreased cAMP levels through Gαi, Gαq, or Gαs, depending on the EP3 isoform. Less is known about the EP1 receptor. While the EP1 receptor is involved in regulating intracellular calcium levels, the G protein to which it couples remains to be identified.
In bone, PGE2 exerts both anabolic and catabolic effects [6], [7], [8], [9]. Administration of PGE2 to mice lacking each of the four prostanoid receptors identified EP4 as the primary mediator of PGE2-induced bone formation [10]. While the role of EP1 in osteoblastic differentiation and bone metabolism is not as well defined, selective EP1 agonists have been shown to stimulate the proliferation of osteoblast progenitors, but impair osteoblastic differentiation [11]. Consistent with these findings, we previously demonstrated that loss of EP1 accelerates osteoblastic differentiation and fracture repair [12]. Although there is evidence supporting a role for PGE2/EP1 signaling in osteoblastic differentiation and bone regeneration, the effects of EP1 receptor signaling on bone homeostasis during two critical phases, growth and aging, are poorly understood. EP1 has been implicated in other aging-related pathologies including neurodegenerative disease [13] and hemin-mediated neurotoxicity [14]. In the present study we examined the hypothesis that EP1 acts as a negative regulator of bone formation and skeletal growth, while loss of EP1 promotes maintenance of bone during aging. We report that EP1−/− mice maintain increased bone mineral density and stronger cortical and trabecular bone biomechanical properties with aging. The trabecular bone of the EP1−/− mice is also resistant to and protective against aging-induced ovariectomy-induced bone loss. The altered bone properties of the EP1−/− mice result mainly from an increased bone formation rate. In vitro studies confirmed that the EP1 receptor acts to inhibit bone marrow osteoprogenitor cell differentiation and mineralization.
Section snippets
Methods
Experimental Animals: All animal studies were conducted with the approval of the University Committee on Animal Resources at the University of Rochester. Wild type C57BL/6J (WT) mice were purchased from Jackson Laboratories (Bar Harbor, ME) at 4-weeks of age. EP1−/− (KO) mice [15] are on a C57Bl/6J genetic background and were generously provided Dr. Matthew Breyer (Vanderbilt University). Female mice were used for ovariectomy experiments, while male mice were used for aging experiments.
Loss of EP1 increases bone volume fraction and bone strength
Based on the accelerated fracture healing phenotype of EP1−/− mice and the enhanced osteoblastic differentiation observed in EP1−/− mesenchymal progenitors, relative to WT [17], we sought to determine the effects of EP1 deletion on bone homeostasis. We examined the baseline bone phenotype in WT and EP1−/− mice between 2 and 12 months of age by microCT. Reconstruction of microCT data collected from the distal metaphyseal region of the femur showed progressive loss of trabecular bone as a function
Discussion
In the present study we identified PGE2/EP1 signaling as a negative regulator of osteoblast differentiation and bone formation, and demonstrate for the first time that the EP1 receptor plays a negative role in regulation of postnatal bone homeostasis. We have previously demonstrated that EP1−/− mice exhibit accelerated fracture healing and that EP1−/− primary bone marrow progenitor cells differentiate into osteoblasts at an increased rate relative to WT cells, while osteoclastogenesis is not
Acknowledgments
We would like to thank the Histology, Biochemistry and Molecular Imaging (HBMI) core for technical assistance with histology, and the Biomechanics and Multimodal Tissue Imaging (BMTI) Core for technical assistance with the biomechanical testing.
Supported by grants: This work was supported by NIH/NIAMS R01AR048681-06A1 (to RJO) and NSFC 81200255/H2501 (to MZ). The HBMI and BMTI cores are supported by NIH/NIAMS P30 AR061307 (to EMS).
Authors' roles: Study design: MZ, MF, JHJ, AEL, and RJO; Data
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All authors state that they have no conflicts of interest.