TNF‐α is known to inhibit osteoblast differentiation in vitro and yet it is essential for bone fracture repair. Roles of TNF‐α in the bony repair of injured growth plate were examined in young rats treated with a TNF‐α antagonist. The results show that TNF‐α mediates p38 activation, which influences the recruitment, proliferation, and osteoblast differentiation of mesenchymal cells and negatively regulates bone formation at the injured growth plate.

Introduction: TNF‐α inhibits expression of osteoblast differentiation factor cbfa1 and osteoblast differentiation in vitro and yet TNF‐α signaling is essential for bone fracture healing. Roles of TNF‐α in the bony repair of injured growth plate cartilage are unknown.

Materials and Methods: Roles of TNF‐α in the activation of p38 mitogen activated protein (MAP) kinase and the subsequent bony repair of the injured growth plate were examined in young rats receiving the TNF‐α inhibitor ENBREL or saline control. Activation of p38 was determined by Western blot analysis and immunohistochemistry. Inflammatory cell counts on day 1, measurements of repair tissue proportions, and counting of proliferative mesenchymal cells on day 8 at growth plate injury site were carried out (n = 6). Expression of inflammatory cytokines TNF‐α and IL‐1β, fibrogenic growth factor (FGF)‐2, cbfa1, and bone protein osteocalcin at the injured growth plate was assessed by quantitative RT‐PCR. Effects of TNF‐α signaling on proliferation, migration, and apoptosis of rat bone marrow mesenchymal cells (rBMMCs) and the regulatory roles of p38 in these processes were examined using recombinant rat TNF‐α, ENBREL, and the p38 inhibitor SB239063 in cultured primary rBMMCs.

Results: p38 activation was induced in the injured growth plate during the initial inflammatory response, and activated p38 was immunolocalized in inflammatory cells at the injury site and in the adjacent growth plate. In addition, activation of p38 was blocked in rats treated with TNF‐α antagonist, suggesting a role of TNF‐α in p38 activation. Whereas TNF‐α inhibition did not alter inflammatory infiltrate and expression of TNF‐α and IL‐1β at the injured growth plate on day 1, it reduced mesenchymal infiltrate and cell proliferation and FGF‐2 expression on day 8. Consistently, TNF‐α increased proliferation and migration of rBMMCs in vitro, whereas p38 inhibition reduced rBMMC proliferation and migration. At the injured growth plate on day 8, TNF‐α inhibition increased expression of cbfa1 and osteocalcin and increased trabecular bone formation at the injury site. There was a significant inverse correlation between TNF‐α and cbfa1 expression levels, suggesting a negative relationship between TNF‐α and cbfa1 in this in vivo model.

Conclusions: These observations suggest that TNF‐α activates p38 MAP kinase during the inflammatory response at the injured growth plate, and TNF‐α‐p38 signaling seems to be required for marrow mesenchymal cell proliferation and migration at the growth plate injury site and in cell culture. Furthermore, TNF signaling has an inhibitory effect on bone formation at the injured growth plate by suppressing bone cell differentiation and bone matrix synthesis at the injury site.