Chapter 5 THROMBIN (105-126 ok

Thrombin-related peptide 508 (TP508) accelerates bone regeneration during distraction osteogenesis. Here we examined the effect of TP508 on bone regeneration during distraction osteogenesis by immunolocalization of RUNX2 protein, a marker of osteoblast differentiation, osteopontin (OPN) and bone sialoprotein (BSP), two late markers of the osteoblast lineage. Distraction was performed in tibiae of rabbits over a period of 6 days. TP508 (30 or 300 μg) or vehicle was injected into the distraction gap at the beginning and the end of the distraction period. Two weeks after active distraction, tissue samples were harvested and processed for immunohistochemical analysis. We also tested the in vitro effect of TP508 on Runx2 mRNA expression in osteoblast-like (MC3T3-E1) cells by PCR analysis. RUNX2 and OPN protein were observed in preosteoblasts, osteoblasts, osteocytes of newly formed bone, in blood vessel cells and many fibroblast-like cells of the soft connective tissue. Immunostaining for BSP was more restricted to osteoblasts and osteocytes. Significantly more RUNX2-and OPN-expressing cells were seen in the group treated with 300 μg TP508 than in the control group injected with saline or with 30 μg TP508. However, TP508 failed to increase Runx2 mRNA levels in MC3T3-E1 cells after 2-3 days of exposure. Our data suggest that TP508 enhances bone regeneration during distraction osteogenesis by increasing the proportion of cells of the osteoblastic lineage. TP508 may stimulate bone regeneration indirectly, by enhancing blood vessel growth. Clinically, TP508 may shorten the healing time during distraction osteogenesis, which may be a benefit when bone regeneration is slow.