BRF-1 is Critical for Bone Remodeling

Objectives: Osteoporosis is a major cause of tooth loss in elderly populations. Physical activity is shown to prevent bone loss by upregulating bone strengthening; however, its mechanisms remain unclear. Here, we identified a systemic molecule, Bone Regenerative Factor-1 (BRF-1; gene name FSTL3, NM_005860), that may drive mechanical load-dependent bone remodeling.

Methods: Sprague Dawley rat femurs (females, 10-12 weeks old) were loaded by treadmill walking (12m/min/day). mRNA and protein expression in trabecular bone (TB) and bone marrow (BM) were determined by qrt-PCR, Western blots and immunofluorescence. BRF-1 knockouts and wild type mice were used to understand the role of BRF-1. Fluorohistochemistry determined bone mineral apposition rate (MAR) and three-point bending tested mechanical strength. One-way ANOVA and post hoc Tukey tests were used for statistical analysis.

Results: BRF-1 mRNA (5.8±1.3-fold) and protein (8.3±2.0-fold) expression were significantly upregulated in TB and BM following exercise. BRF-1 was required for load-dependent bone formation. BRF-1-/- mice exhibited weaker (ultimate strength of FSTL3+/+ femurs 3.2±0.4% vs. FSTL3-/- 2.4±0.3%, n =7, p=0.003) and more brittle bones (fracture strain of FSTL3+/+ femurs 7.7±3.4% vs. FSTL3-/- 5.0±1.3%, n=7, p=0.037). BRF-1-/- mice failed to synthesize bone in response to exercise versus BRF-1+/+ mice (MAR in BRF+/+ TB 21.8±0.9 vs. BRF-1-/- TB 7.1±0.4). Strikingly, circulating levels of BRF-1 increased in mice, humans and rats following 2 days (5.2±1.3-fold, rat, n=6) and 5 days (6.4±1.1-fold, rat, n=6) exercise, suggesting systemic control of bone formation. Additionally, mice afflicted with glucocorticoid-induced osteoporosis exhibited a 28-32% reduction in circulating BRF-1 in comparison to healthy mice.

Conclusions: BRF-1 is the first molecule identified that regulates bone remodeling in response to exercise. This provides a new paradigm for understanding the regulation of load-dependent bone formation in osteopenia-associated dental problems and other diseases.

Support: This research was funded by The Ohio State University College of Dentistry Student Research Program.