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Expression of Vascular Endothelial Growth Factor and the Effects on Bone Remodeling during Experimental Tooth MovementDepartment of Orthodontics, Hiroshima University Faculty of Dentistry, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8553, Japan; Correspondence: *corresponding author, mkaku{at}hiroshima-u.ac.jp
Vascular endothelial growth factor (VEGF) has an ability to induce functional osteoclasts as well as neovascularization. We recently reported that the number of osteoclasts was enhanced by the injection of recombinant human VEGF (rhVEGF) with the application of mechanical force for experimental tooth movement. In this study, the expression of VEGF was detected in osteoblasts on the tension side of the alveolar bone. Moreover, the rate of tooth movement was significantly increased in the rhVEGF injection groups compared with the controls. These results suggested that VEGF, highly expressed by mechanical stimuli, enhances the number of osteoclasts as a paracrine factor, and that the amount of tooth movement is accelerated by both endogenous VEGF and injected rhVEGF.
Key Words: VEGF osteoclast osteoblast bone remodeling experimental tooth movement
Vascular endothelial growth factor (VEGF) is the most important and essential mediator for angiogenesis (Leung et al., 1989). It was reported that VEGF expression was detected in the vascularization process of developing embryos (Dumont et al., 1995), and the pathophysiologic process of inflammation and wound healing (Dvorak et al., 1995). As the specific receptors for VEGF, two receptor tyrosine kinases, fms-like tyrosine kinase (Flt-1) and fetal liver kinase (Flk-1), have been identified (Shibuya et al., 1990; de Vries et al., 1992; Terman et al., 1992; Millauer et al., 1993). In addition, it has been demonstrated that VEGF exerts various biological functions such as vascular permeability (Senger et al., 1983) and migration of human monocytes (Barleon et al., 1996; Clauss et al., 1996). Orthodontic tooth movement and the related tissue remodeling are achieved by osteoclastic bone resorption and osteoblastic new bone formation. It was also reported that osteoblasts produce VEGF (Harada et al., 1994), and it is well-understood from these findings that experimental tooth movement is a useful in vivo model for investigating the expression and role of VEGF in the periodontal tissues with active remodeling. Meanwhile, in a recent study, recombinant human (rh)VEGF induced many osteoclasts in osteopetrotic (op/op) mice via Flt-1 as well as macrophage-colony-stimulating factor (M-CSF) (Niida et al., 1999). In the following study, the number of osteoclasts induced by local administration of rhM-CSF was substantially greater than that induced by rhVEGF in op/op mice (Kaku et al., 2000). In addition, we have demonstrated that the local administration of rhVEGF significantly enhances the number of osteoclasts induced by experimental tooth movement (Kaku et al., 2001). However, it was still unknown whether rhVEGF injection and mechanical stimuli might increase the amount of tooth movement. Thus, we conducted this study to investigate the expression of VEGF within periodontal tissues during experimental tooth movement using an immunohistochemical approach. Furthermore, we investigated the effect of rhVEGF injection on the rate of tooth movement and the comparison of the numbers of osteoclasts induced by the injection of rhVEGF and rhM-CSF.
Experimental Animals and Immunohistochemical Examination for the Expression of VEGF Thirty-day-old C57BL/6J mice, each weighing about 15 g, were obtained from Jackson Laboratory (Bar Harbor, ME, USA) for use in this experiment. All animals were treated under ethical regulations for animal experiments, defined by the Ethics Committee, Hiroshima University Faculty of Dentistry. An experimental appliance for tooth movement exerts an initial force of 1.0 g applied in the distal direction. The experimental procedures were performed according to methods described in our previous study (Kaku et al., 2001). Experimental tooth movement of the upper incisors was continued for 10 days. The mice were killed under general anesthesia with sodium pentobarbital. They were fixed in 4% paraformaldehyde and rinsed in distilled water. Then, the specimens were decalcified in 14% EDTA (pH 7.4) for 14 days and embedded in paraffin. The premaxillary bones, including the upper incisors, were cut into frontal sections of 4 µm thickness. The expression of VEGF on the tension side was examined on these sections immunohistochemically stained with rabbit anti-mouse VEGF polyclonal antibody (LAB VISION, Fremont, CA, USA), by means of a Vectastain ABC-GO KIT (Vector Laboratories, Burlingame, CA, USA), and counterstained with methyl green. Normal rabbit IgG (Santa Cruz Biotechnology, Santa Cruz, CA, USA) was used as the control for the polyclonal antibody.
Effects of VEGF on the Amount of Experimental Tooth Movement On days 3, 7, 14, and 21 after initiating the experiment, we investigated the changes in the numbers of osteoclasts in the groups (n = 5). The histological sections were stained with tartrate-resistant acid phosphatase (TRAP) and counterstained with hematoxylin. Osteoclasts were identified as TRAP-positive, multi-nucleated cells located on the bone surface. The number of osteoclasts that appeared in the PDL space on the pressure side of the incisors was counted on 5 sections at 35-µm intervals for each specimen.
Comparison of the Number of Osteoclasts Induced by the Injection of rhVEGF and rhM-CSF
Statistical Treatment
Expression of VEGF Fig. 1
Many osteoblasts were detected on the tension side in the PDL space. In the osteoblasts, VEGF was highly expressed, as noted by the intense immunostaining with VEGF antibody (Fig. 1A
Changes in the Numbers of Osteoclasts and the Amount of Tooth Movement
An average of 10 osteoclasts was found on the pressure side of the alveolar bone treated with rhVEGF injection at 3 and 7 days after the beginning of the experiment. On day 14, about 18 osteoclasts were detected. Many resorption lacunae were also detected on the pressure side in the PDL space in rhVEGF-treated animals (Figs. 2A, 3A, 3B
On day 15, the amount of tooth movement was about 0.48 mm in the rhVEGF injection group, greater than 0.37 mm in the controls, but the difference was not statistically significant between the two groups. Subsequently, the rhVEGF treatment group exhibited a substantial increase in tooth movement from 15 to 18 days. On day 21, the amount of tooth movement was about 0.78 mm in the rhVEGF injection group, whereas it was 0.42 mm in the controls. There was a significant difference between the two groups on days 18 and 21 (Fig. 2A
Throughout the experimental period, the width of the midpalatal suture was quite small, and the mean amount of sutural expansion in the rhVEGF injection group and controls was about 0.045 mm (Figs. 2A, 2B
Changes in the Numbers of Osteoclasts Expressed by rhVEGF or rhM-CSF Injection
The numbers of osteoclasts were significantly larger in groups VT and MT than in group T (p < 0.05). The number of osteoclasts in group VT was similar to that in group MT, and the number of osteoclasts was significantly larger in group VMT than in groups T (p < 0.01), VT, and MT (p < 0.05) (Fig. 4B
Furthermore, the numbers of osteoclasts in the experimental tooth movement groups were significantly greater than in the groups without experimental tooth movement (VT vs. V, MT vs. M, p < 0.05). However, no significant differences in the numbers of osteoclasts were found between groups VM and VMT (Fig. 4C
VEGF is one of the crucial regulators of angiogenesis, and its mRNA is expressed in various tissues, such as lung and heart (Berse et al., 1992; Ferrara et al., 1992). Previous studies showed that VEGF mRNA was expressed in osteoblasts in in vitro experiments (Harada et al., 1994), suggesting that osteoblastic activity may be closely related to angiogenesis, which is the main function of VEGF. In this study, many osteoblasts were detected on the tension side in the PDL space 10 days after the beginning of the experiment, and VEGF was highly expressed in these osteoblasts. Thus, it is assumed that VEGF expression in osteoblasts was promoted by mechanical stimuli to the periodontium, suggesting that VEGF may participate in the regulation of bone metabolism and wound healing during orthodontic tooth movement. VEGF has various biological functions, such as vascular permeability and migration of human monocytes (Senger et al., 1983; Barleon et al., 1996; Clauss et al., 1996). A recent study reported that VEGF had an osteoclast differentiation function in op/op mice, and the responses of osteoclast precursor cells to VEGF were directly mediated via Flt-1 (Niida et al., 1999). Very recently, we demonstrated that local administration of rhVEGF enhanced the number of osteoclasts during experimental tooth movement, and the number of nuclei per osteoclast in rhVEGF injection mice was significantly larger than that in control mice (Kaku et al., 2001). In this study, the amount of tooth movement in the rhVEGF injection group was larger than that in controls. Especially, from 15 to 18 days, the rate of tooth movement showed a significant increase. The reason may be because a large number of osteoclasts induced by rhVEGF appeared at 14 days and produced a large amount of bone resorption, leading to a substantial increase in tooth movement from 15 to 18 days after the beginning of the experiments. Meanwhile, the number of osteoclasts in group VT decreased at 21 days. We hypothesized that this was because (1) of the insufficient number of precursor cells of osteoclasts in the PDL space, and (2) apotosis of osteoclasts occurred in the experimental group to maintain homeostasis. Considering that VEGF may play a role in removing necrotic tissue by accelerating the angiogenesis in the compression area, we can conclude that a combination of rhVEGF injection and orthodontic force application is more efficient for tooth movement. The previous study demonstrated that a severe deficiency of osteoclasts in op/op mice can be cured by injections of rhM-CSF (Felix et al., 1990; Kodama et al., 1991a; Wiktor-Jedrzejczak et al., 1991; Sundquist et al., 1995). Direct action of M-CSF on osteoclast lineage cells was exhibited by the expression of the receptor for M-CSF, c-fms, in osteoclasts both in vitro (Kodama et al., 1991b) and in vivo (Hofstetter et al., 1992). From these findings, it was indicated that M-CSF plays an essential role in the differentiation of osteoclasts. In addition, it became clear that M-CSF supports osteoclast differentiation in cooperation with osteoclast differentiation factor (ODF)/osteoprotegerin ligand (OPGL)/TNF-related activation-induced cytokine (TRANCE)/RANKL (Lacey et al., 1998; Yasuda et al., 1998). The present study showed that the number of osteoclasts induced by rhVEGF was almost equivalent to that induced by rhM-CSF, regardless of mechanical force application. However, it was reported that the number of osteoclasts recruited by rhVEGF was less than that by rhM-CSF in op/op mice (Kaku et al., 2000). From these findings, it is suggested that an excess of exogenous rhM-CSF injection is not required for differentiation of osteoclasts in normal mice with sufficent endogenous M-CSF. The number of osteoclasts was significantly greater in group VM than in groups V and M. The number of osteoclasts in group VMT was significantly larger than that in groups VT and MT. These results suggested that VEGF and M-CSF might function independently for the recruitment of osteoclasts. Thus, we expect that simultaneous injection of rhVEGF and rhM-CSF is more efficient for orthodontic tooth movement. The number of osteoclasts in groups VT and MT was significantly larger than that in groups V and M, respectively. However, there was no significant difference in the numbers of osteoclasts between groups VM and VMT. For this reason, we hypothesized that there were insufficient numbers of precursor cells of osteoclasts in the PDL space of group VMT. Several chemical mediators such as prostaglandin and 1,25-(OH)2D3 can accelerate the rate of tooth movement by means of similar mechanisms to increase the number of osteoclasts (Yamasaki et al., 1984; Collins and Sinclair, 1988; Takano-Yamamoto et al., 1992). In comparison with these factors, VEGF is suggested to be more appropriate in terms of wound-healing function through induction of angiogenesis. Therefore, VEGF has possible clinical applications for achieving efficient orthodontic treatment.
This study was funded by a Grant-in-Aid (No. 14771179) from the Ministry of Education, Science, Sports and Culture of Japan. Received for publication February 12, 2002. Revision received October 30, 2002. Accepted for publication November 22, 2002.
Journal of Dental Research, Vol. 82, No. 3,
177-182 (2003) This article has been cited by other articles:
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