Mechanical loading and Δ12prostaglandin J2 induce bone morphogenetic protein-2, peroxisome proliferator-activated receptor γ-1, and bone nodule formation in an osteoblastic cell line

Background and Objective: We have previously reported that mechanical strain applied at a 1 % level to an osteoblastic cell line induces the transcription of prostaglandin D 2 synthase and increases the levels of prostaglandin D 2 and its Δ 12 prostaglandin J 2 metabolite. Mechanical strain also induces the expression of peroxisome proliferator-activated receptor γ-1 and bone nodule formation. We hypothesized that mechanical load induces bone formation via Δ 12 prostaglandin J 2 -dependent synthesis of bone morphogenetic proteins. Our goal was to investigate the molecular events involved in osteogenesis induced by mechanical loading and Δ 12 prostaglandin J 2, namely the induction of bone morphogenetic proteins and peroxisome proliferator-activated receptor γ-1, a nuclear receptor for Δ 12 prostaglandin J 2 . Material and Methods: Osteoblast monolayers were stretched for 1 h with a 1-h resting period and stretched for another hour at 1 Hz with 1% elongation. Cells were collected 0, 1, 6 and 16 h after stretching. Cyclooxygenase inhibitors and A 12 prostaglandin J 2 were added in some experiments. Relative quantitative reverse transcriptase-polymerase chain reaction was used to examine whether the mRNA of bone morphogenetic protein-2, -4, -6, -7 and peroxisome proliferator-activated receptor γ-1 was induced. Immunohistochemistry was used to evaluate bone morphogenetic protein expression in cells. Results: Mechanical strain significantly increased the mRNA expression of bone morphogenetic protein-2, -6, -7 and of peroxisome proliferator-activated receptor γ-1, but not of bone morphogenetic protein-4. In stretched cells, bone morphogenetic protein-2 and peroxisome proliferator-activated receptor γ-1 expression was blocked by cyclooxygenase inhibitors, but restored by exogenous Δ 12 prostaglandin J 2 . Δ 12 Prostaglandin J 2 significantly enhanced bone nodule formation and bone morphogenetic protein-2 expression when added alone to resting osteoblasts. Conclusion: These results suggest that the osteoblastic biomechanical pathways that trigger bone formation involve cyclooxygenase and prostaglandin D 2 synthase activation, induction of Δ 12 prostaglandin J 2 and its nuclear receptor, peroxisome proliferator-activated receptor γ-1, and increased expression of bone morphogenetic protein-2. These data suggest that the Δ 12 prostaglandin J 2 /peroxisome proliferator-activated receptor γ-1/bone morphogenetic protein-2 pathway plays an important role in osteogenesis.