Increased expression of macrophage migration inhibitory factor during fracture healing in rats

We previously reported that macrophage migration inhibitory factor (MIF) is expressed in osteoblasts in murine calvarial bone, and that MIF upregulates the expression of matrix metalloproteinase (MMP)-13 mRNA in osteoblasts and chondrocytes; however, its pathophysiological functions in bone have not been well understood. In this study, we used a rat femoral fracture model to examine the expression of MIF during the fracture healing process. Semiquantitative reverse transcription-polymerase chain reaction revealed that MIF mRNA was increased throughout the healing process. The level of MIF mRNA reached a maximum at day 4 postfracture, while MMP-13 mRNA became maximal at day 14 postfracture. Immunohistochemical analysis showed that MIF protein was present in the granulation tissues formed at the fracture site on day 4. On days 7 and 10, MIF was detected in the thickened periosteum, in osteoblastic cells that were present within the intramembranously formed bone under the thickened periosteum, and in chondrocytes within the cartilaginous callus. From day 14 to day 28, MIF was present in chondrocytes within the callus, although the level of MIF declined gradually over this period. On days 7 to 14, MMP-13 was also detected in osteoblastic cells within the intramembranously formed bone and in chondrocytes within the cartilaginous callus. The immunoreactivity for MMP-13 within chondrocytes decreased on days 21 and 28. These results suggest the possibility that MIF plays an important role in fracture healing in association with proliferation of stromal cells, and the induction of MMP-13 in osteoblasts or chondrocytes.