Expression of VEGF gene isoforms in a rat segmental bone defect model treated with EPCs

Angiogenesis and osteogenesis are essential for bone growth, fracture repair, and bone remodeling. Vascular endothelial growth factor (VEGF) has an important role in bone repair by promoting angiogenesis and osteogenesis. In our previous study, endothelial progenitor cells (EPCs) promoted bone healing in a rat segmental bone defect as confirmed by radiological, histological, biomechanical, and micro-CT evaluations. Although EPCs have demonstrated effectiveness in animal models of fracture healing, the mechanism by which EPCs enhance fracture healing remains unclear. We hypothesized a possible paracrine mechanism of action, where the secretion of growth factors critical to the processes of fracture healing (such as VEGF), is responsible for the positive effects of EPC therapy. The purpose of this study was to evaluate VEGF gene expression after local EPC therapy for a rat segmental bone defect.

Rat bone marrow-derived EPCs were isolated by the Ficoll-paque gradient centrifuge technique. The EPCs were cultured for 7-10 days in endothelial cell growth medium with supplements and collected for treatment of the rat segmental bone defect. EPCs were identified by immunocytochemistry staining with primary antibodies for CD34, CD133, fetal liver kinase-1, and Von Willebrand factor. A total of 56 rats were studied. A 5-mL segmental bone defect was created in the middle one-third of each femur followed by miniplate fixation. The treatment group received 1 × 10 EPCs locally at the bone defect on a gelfoam scaffold and control animals received the gelfoam scaffold only. Seven control and 7 EPC-treated rats were included in each group at 1, 2, 3, and 10 weeks. The animals were sacrificed at the end of the treatment period, and specimens from the fracture gap area were collected and immediately frozen. Rat VEGF mRNA was measured by reverse-transcriptase-polymerase chain reaction and quantified by VisionWorksLS. All measurements were performed in triplicate.

Cultured EPCs at 1 week showed positive staining for CD34, CD133, fetal liver kinase-1, and Von Willebrand factor markers. The EPC group had a greater VEGF expression than the control group at weeks 1, 2, and 3, but not at week 10. Three VEGF isoforms were detected in this rat model: VEGF120, VEGF164, and VEGF188. VEGF120 and VEGF164 levels peaked at 2 weeks, whereas VEGF188 levels peaked at 3 weeks. All 3 VEGF isoform levels were low at 10 weeks.

EPC-based therapy for a segmental bone defect results in increased VEGF expression during the early period of fracture repair. In addition, the specific VEGF isoform may be a key regulator of the bone healing process. These findings demonstrate that EPCs may promote fracture healing by increasing VEGF levels and thus stimulating angiogenesis, a process that is essential for early callus formation and bone regeneration.