title: Intermittent PTH stimulates periosteal bone formation by actions on post-mitotic preosteoblasts.
abstract: Intermittent administration of parathyroid hormone (PTH) stimulates bone formation on the surface of cancellous and periosteal bone by increasing the number of osteoblasts. Previous studies of ours in mice demonstrated that intermittent PTH increases cancellous osteoblast number at least in part by attenuating osteoblast apoptosis, but the mechanism responsible for the anabolic effect of the hormone on periosteal bone is unknown. We report that daily injections of 100 ng/g of PTH(1-34) to 4-6 month old mice increased the number of osteoblasts on the periosteum of lumbar vertebrae by 2-3 fold as early as after 2 days. However, the prevalence of apoptotic periosteal osteoblasts was only 0.2% in vehicle treated animals, which is approximately 20-fold lower than is the case for cancellous osteoblasts. Moreover, PTH did not have a discernable effect on periosteal osteoblast apoptosis. Administration of BrdU for 4 days failed to label periosteal osteoblasts under either basal conditions or following administration of PTH. Cancellous osteoblasts, on the other hand, were labeled under basal conditions, but PTH did not increase the percentage of BrdU-positive cells. Thus, intermittent PTH does not increase cancellous or periosteal osteoblast number by stimulating the proliferation of osteoblast progenitors. Consistent with high turnover of cancellous osteoblasts as compared to that of periosteal osteoblasts, ganciclovir-induced ablation of replicating osteoblast progenitors in mice expressing thymidine kinase under the control of the 3.6 kb rat Col1A1 promoter resulted in disappearance of osteoblasts from cancellous bone over a 7-14 day period, whereas periosteal osteoblasts were unaffected. However, 14 days of pre-treatment with ganciclovir prevented PTH anabolism on periosteal bone. We conclude that in cancellous bone, attenuation of osteoblast apoptosis by PTH increases osteoblast number because their rate of apoptosis is high, making this effect of the hormone profound. However, in periosteal bone where the rate of osteoblast apoptosis is low, PTH must exert pro-differentiating and/or pro-survival effects on post-mitotic pre-osteoblasts. Targeting the latter cells is an effective mechanism for increasing osteoblast number in periosteal bone where the production of osteoblasts from replicating progenitors is slow.
authors.affiliation: Division of Endocrinology and Metabolism, Center for Osteoporosis and Metabolic Bone Diseases, Central Arkansas Veterans Healthcare System, Slot 587, University of Arkansas for Medical Sciences, 4301 W. Markham, Little Rock, AR, USA. rljilka@uams.edu
authors.forename: Robert L
authors.initials: RL
authors.lastname: Jilka
authors.forename: Charles A
authors.initials: CA
authors.lastname: O'Brien
authors.forename: A Afshan
authors.initials: AA
authors.lastname: Ali
authors.forename: Paula K
authors.initials: PK
authors.lastname: Roberson
authors.forename: Robert S
authors.initials: RS
authors.lastname: Weinstein
authors.forename: Stavros C
authors.initials: SC
authors.lastname: Manolagas
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D002452:Cell Count; D024042:Collagen Type I / Q000378:metabolism; D004261:DNA Replication / Q000187:drug effects; D005260:Female; D015774:Ganciclovir / Q000494:pharmacology; D008297:Male; D051379:Mice; D008938:Mitosis / Q000187:drug effects*; D008954:Models, Biological; D010006:Osteoblasts / Q000166:cytology* / Q000187:drug effects*; D010012:Osteogenesis / Q000187:drug effects*; D010281:Parathyroid Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D010521:Periosteum / Q000166:cytology* / Q000187:drug effects*; D013234:Stem Cells / Q000166:cytology / Q000187:drug effects; D013937:Thymidine Kinase / Q000378:metabolism; D013997:Time Factors
chemical_list: D024042:Collagen Type I; D010281:Parathyroid Hormone; D013937:Thymidine Kinase; D015774:Ganciclovir