Role of the N- and C-terminal Fragments of Parathyroid-Hormone-Related Protein as Putative Therapies to Improve Bone Regeneration Under High Glucocorticoid Treatment Luı ´s Ferna ´ ndes de Castro, B.Sc., 1 Daniel Lozano, B.Sc., 1 Sonia Dapı ´a, Ph.D., 2 Sergio Portal-Nu ´n ˜ ez, Ph.D., 1 Jose ´ R. Caeiro, M.D., 2 Enrique Go ´ mez-Barrena, M.D., Ph.D., 3 and Pedro Esbrit, Ph.D. 1 The parathyroid-hormone-related protein (PTHrP) is an important modulator of bone formation and bone remodeling. High and=or prolonged glucocorticoid (GC) treatments inhibit PTHrP expression in osteoblastic cells and bone formation and repair. We assessed the ability of the N- and C-terminal PTHrP fragments to restore the GC-altered bone regeneration after bone marrow ablation in mice. Animals were administered 3-methylprednisolone or vehicle and PTHrP (1–36) or PTHrP (107–139) every other day, beginning 4 days before marrow ablation in the tibia, and euthanized 12 days later. GC-treated mice showed in the ablated tibia a decrease in bone formation and in osteoblast and sclerostin-positive osteocyte numbers, reduced expression of osteoblastic factors, decreased osteogenesis of bone-marrow-derived cells, an increase in the numbers of mul- tinucleated osteoclasts and adipocytes, and decreased cortical vascularization, as well as altered bone structure (measured by microcomputerized tomography) in the intact femur. These effects were reversed at least in part by either PTHrP peptide. The present novel findings support the use of both PTHrP peptides tested as putative bone regenerative therapies in GC-related bone diseases. Introduction T he parathyroid-hormone (PTH)-related protein (PTHrP) is abundant in bone, where it acts as an im- portant regulator of bone formation. 1 Mice with osteoblast- specific deletion of the PTHrP gene have postnatal osteopenia associated with low bone formation, 2 and a decrease in PTHrP production by osteoblasts occurs in age- and diabetes-related osteopenia. 3,4 Daily administration of the N-terminal fragment of PTHrP, interacting with the PTH receptor 1 (PTH1R) in osteoblasts, increased bone formation in both ovariectomized rats and postmenopausal women. 5–7 In the latter, this anabolic effect was associated with increased plasma osteocalcin (OC) levels with no changes in bone resorption markers. 7 In addi- tion, PTHrP (107–111) (called osteostatin), a highly conserved epitope in the C-terminal PTHrP sequence, was found to re- store the decreased bone mineral density in the femur of ovariectomized rats. 5 Both in vitro and in vivo studies indicate that osteostatin is a potent inhibitor of osteoclastic bone re- sorption. 8–10 Also of note, transient exposure to the putative PTHrP (107–139) fragment induces growth and differentiation in osteoblastic cells in vitro. 11–13 At least part of these effects appear to occur through the vascular endothelial growth factor (VEGF) system, 12,13 which is of interest in view of the key role of angiogenesis in modulating bone formation. 14 Thus, increasing evidence suggests that the bone anabolic action of PTHrP is complex and might not only be ascribed to its N-terminal PTH-like domain. Chronic glucocorticoid (GC) exposure is a major cause of bone mass loss through mechanisms incompletely under- stood. High GC dosing induces an early increase in bone resorption followed by a decrease in bone formation and bone remodeling. 15,16 GCs can act on osteoclasts, both indi- rectly via increasing receptor activator of NF-kB ligand (RANKL) and decreasing osteoprotegerin (OPG) in osteo- blasts, and directly by diminishing their bone-degrading activity. 17–19 However, the bone sparing action of GCs seems to be mainly accounted for by a deficit in the number of 1 Bone and Mineral Metabolism Laboratory, Jime ´nez Dı ´az Foundation (Capio Group), Madrid, Spain. 2 Trabeculae Ò , San Cibrao das Vin ˜ as, Ourense, Spain. 3 Orthopedic Department, Jime ´nez Dı ´az Foundation and Autonomous University of Madrid, Madrid, Spain. Portions of this study were presented at the 35th European Symposia on Calcified Tissues, May 24–28, 2008 (this short oral paper was awarded with an European Calcified Tissue Society Travel Award), and the 30th Annual Meeting of the American Society for Bone and Mineral Research, September 12–16, 2008, in Barcelona (Spain) and Montre ´al (Que ´bec, Canada), respectively. TISSUE ENGINEERING: Part A Volume 16, Number 4, 2010 ª Mary Ann Liebert, Inc. DOI: 10.1089=ten.tea.2009.0355 1157