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Skeletal changes accompanying aging are associated with both increased risk of fractures and impaired fracture healing, which, in turn, is due to compromised bone regeneration potential. These changes are associated with increased serum levels of selected proinflammatory cytokines, e.g., tumor necrosis factor alpha (TNF-alpha). We have used a unique model of bone regeneration to demonstrate (1) that aged-related deficits in direct bone formation can be restored to young mice by treatment with TNF blockers and (2) that the cyclin-dependent kinase inhibitor p21 is a candidate for mediation of the osteoinhibitory effects of TNF. It has been hypothesized recently that TNF antagonists may represent novel anabolic agents, and we believe that the data presented here represent a successful test of this hypothesis., (2010 American Society for Bone and Mineral Research)

Unlabelled: DO was used in an aged mouse model to determine if systemically and/or locally administered rhIGF-I improved osteoblastogenesis and new bone formation. Local and systemic rhIGF-I treatment increased new bone formation. However, only systemic delivery produced measurable concentrations of rhIGF-I in the circulation., Introduction: Human and rodent research supports a primary role for IGF-I in bone formation. Significant roles for both endocrine and paracrine/autocrine IGF-I have been suggested for normal osteoblastogenesis and bone formation. We have assessed, using a mouse model of distraction osteogenesis (DO), the impact of continuous administration of recombinant human (rh)IGF-I, delivered either locally to the distraction site or absorbed systemically, on bone formation in an aged mouse model., Materials and Methods: DO was performed in aged mice (18-month-old C57BL/6 male mice), which were distracted at 0.15 mm daily. At the time of osteotomy, miniosmotic pumps were inserted subcutaneously to (1) deliver vehicle or rhIGF-I subcutaneously for systemic delivery or (2) deliver vehicle or rhIGF-I directly to the newly forming bone through infusion tubing routed subcutaneously from the pump to the distraction site. Serum concentrations of mouse IGF-I, human IGF-I, and osteocalcin were determined at the end of the study., Results: New bone formation observed in DO gaps showed a significant increase in new bone formation in rhIGF-I-treated mice, irrespective of delivery route. However, detectable levels of human IGF-I were found only in the serum of animals receiving rhIGF-I systemically. Osteocalcin levels did not differ between controls and rhIGF-I-treated groups., Conclusions: Locally and systemically delivered rhIGF-I both produce significant increases in new bone formed in an aged mouse model in which new bone formation is normally markedly impaired, suggesting that rhIGF-I may improve senile osteoporosis. Because systemic administration of IGF-I can result in untoward side effects, including an increased risk for cancer, the findings that locally delivered IGF-I improves bone regeneration without increasing circulating IGF-I levels suggests that this delivery route may be preferable in an at-risk, aged population.

The effect of age on bone formation in the limb lengthening model of distraction osteogenesis (DO) was investigated in two studies using Sprague-Dawley (SD) rats from two colonies at various ages (CAMM: 9 vs 24 months, Harlan: 4 vs 24 months). External fixators were placed on the right tibiae of 30 male SD rats (20 CAMM, 10 Harlan) and mid-diaphyseal osteotomies were performed. Distraction was performed at 0.2 mm bid for 20 days (CAMM) or 14 days (Harlan). The experimental (DO) and control (contra-lateral) tibiae were removed for high-resolution radiography and decalcified histology. Videomicroscopy was used to quantitate radiodensity, histology (matrix type) and relative areas of cell proliferation, which was identified by proliferating cell nuclear antigen (PCNA) immunochemistry. Both studies demonstrated an age-related decrease in the percent mineralized bone (radiodensity) in the distraction gap (CAMM 9 vs 24 months: 68% vs 51%, P < 0.003; Harlan 4 vs 24 months: 95% vs 36%, P < 0.001) and no significant colony or distraction time-specific difference was seen between the two colonies of 24-month-old rats. Histology was performed on the Harlan rats. The DO gaps in the 24-month-old rats demonstrated less endosteal new bone compared to the 4-month-old rats (P < 0.01), but equivalent periosteal new bone. In 4-month-old rats, PCNA-immunostained cells were organized along the primary matrix front (where the first deposition of osteoid occurs) extending across both periosteal and endosteal surfaces. In 24-month-old rats, PCNA+ cells were organized in zones along the periosteal new bone fronts only and irregularly scattered throughout the endosteal gap within a fibrovascular non-ossifying matrix. These results indicate that 24-month-old rats have a relative deficit in endosteal bone formation which may not be related to cell proliferation but rather to cell organization. This model reflects the clinical situation where radiographic findings in older patients demonstrate significant delays in mineralization during DO. We believe this model of DO in aged rats presents unique in vivo opportunities to test hypotheses concerning (1) the effects of aging on bone repair, (2) the effects of pharmacological agents on bone repair in a geriatric setting, and (3) to study the mechanisms underlying DO.