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Intermittent Force Induces High RANKL Expression in Human Periodontal Ligament Cells
K. Nakao1,
T. Goto2,*,
K.K. Gunjigake1,
T. Konoo1,
S. Kobayashi2 and
K. Yamaguchi1
1 Division of Orofacial Functions and Orthodontics, and
2 Division of Anatomy, Kyushu Dental College, 2-6-1 Manazuru, Kokurakita-ku, Kitakyushu, 803-8580, Japan
Correspondence: * corresponding author, tgoto{at}kyu-dent.ac.jp
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ABSTRACT
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Intermittent compressive force stimulates bone resorption in orthodontic treatment. This study examined the molecular mechanism in human periodontal ligament (PDL) cells stimulated by an intermittent force. PDL cells were subjected to compressive force (2.0 or 5.0 g/cm2) for 2–4 days. Continuous or intermittent force was applied all day or for 8 hrs per day, respectively. At days 3 and 4, cell damage was less with intermittent force than with continuous force. At day 4, RANKL and IL-1β expressions were greater with intermittent force than with continuous force. An IL-1 receptor antagonist inhibited the compressive force-induced RANKL expression. These findings indicate that IL-1β is an autocrine factor regulating compressive force-induced RANKL expression in PDL cells, and that intermittent force can effectively induce RANKL in PDL cells with less cell damage.
Key Words: periodontal ligament cell • intermittent force • RANKL • IL-1β
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INTRODUCTION
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Periodontal ligament (PDL) cells initiate bone remodeling in response to the load applied during orthodontic tooth movement (Middleton et al., 1996). On the compression side, osteoclasts resorb bone. Osteoclast formation and differentiation are regulated by a balance between the receptor activator of nuclear factor kappa B ligand (RANKL) and osteoprotegerin (OPG) (Suda et al., 1999; Hofbauer et al., 2000). PDL cells express both RANKL and OPG (Kanzaki et al., 2001; Hasegawa et al., 2002; Zhang et al., 2004; Yamaguchi et al., 2006).
PDL cells produce interleukin-1β (IL-1β) under mechanical stress (Shimizu et al., 1994). IL-1β-treated PDL cells show increased bone-resorbing activity compared with unstimulated PDL cells. IL-1β mediates osteoclastogenesis by enhancing stromal cell expression of RANKL and directly stimulating the differentiation of osteoclast precursors (Wei et al., 2005). Therefore, IL-1β probably modulates the bone remodeling caused by orthodontic compression forces.
Intermittent force was found to be suitable for tooth movement, because it provides a rest period that allows for reconstruction of the periodontal tissue (Oppenheim, 1944). A recent study showed that mechanical loading protocols are more osteogenic when the load cycles are divided into several hours, than when the cycles are applied in a single bout (Robling et al., 2002). The amount of tooth movement achieved by intermittent force exceeded the expected value predicted by the duration of the force application (Konoo et al., 2001; Hayashi et al., 2004). Although intermittent force is effective for orthodontic tooth movement, the mechanism of osteoclastogenesis during intermittent force is still unclear.
Various methods have been developed for the application of compression forces in vitro, including weighted glass cylinders (Kanzaki et al., 2002), hydrostatic pressure (Angele et al., 2003), and the release of a pre-stretched membrane (He et al., 2004). Since compression force induces apoptosis (Goga et al., 2006), cell damage must also be considered during force application.
In this study, to clarify the mechanism of intermittent force in PDL cells, we investigated the expression of RANKL, OPG, and IL-1β mRNA in PDL cells stimulated by intermittent force or continuous force. Furthermore, we examined the relationship between compressive force and RANKL expression via the IL-1β signaling pathway in PDL cells under compressive force.
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MATERIALS & METHODS
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Cell Culture
Human PDL cells were isolated from extracted premolars obtained for orthodontic reasons at Kyushu Dental College Hospital. All of the experiments followed the guidelines of the Research Ethic Committee of Kyushu Dental College, and informed consent was obtained from all participants (three males and five females).
We isolated the cells by scraping the ligament tissue, mincing the tissue with a scalpel, and digesting it with 2 mg/mL collagenase type IV (Sigma, St. Louis, MO, USA) in phosphate-buffered saline (PBS) for 30 min at 37°C. The cells were collected and suspended in a-minimal essential medium ( MEM; Gibco-BRL, Grand Island, NY, USA) containing 10% fetal bovine serum (FBS; Gibco-BRL) with antibodies (167 U/mL penicillin G, 50 mg/mL gentamicin, and 0.25 mg/mL fungizone).
Compressive Force Application
PDL cells were seeded at 5.0 x 104 cells per round coverglass ( 22 mm). The coverglasses were placed in culture dishes, and 1.5 mL of -MEM containing 10% FBS and antibiotics were added. The cells were pre-cultured for 3 days. To apply a compressive force, we placed the coverglasses on an appliance that we had designed. We applied compressive force continuously or intermittently by changing the amount of medium (Fig. 1A ); force was applied all day, and intermittent force was applied for 8 hrs per day, according to procedures previously described (Hayashi et al., 2004). PDL cells were subjected to 0.17 (control), 2.0, or 5.0 g/cm2 compressive force for 2, 3, or 4 days. When the medium of intermittent-force groups was changed, the medium of continuous-force groups was also changed.
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Figure 1. Compressive force and cell damage. (A) Isolated cells were seeded in culture dishes and incubated at 37°C in an atmosphere of 5% CO2. We applied compression by changing the amount of medium. The gravity of medium was 1.01 g/mL and the cross-section area of the cylindrical tube was 5.7 cm2. The forces produced by the medium were 0.17 g/cm2 (0.96 mL; control), 2.0 g/m2 (11.3 mL), and 5.0 g/cm2 (28.3 mL). Control force was the same as the force by 1.5 mL medium in a 35-mm dish. (B) Phase-contrast images of PDL cells under compressive force for 4 days. The cells were cultured (a) without additional medium (control) or with (b) 2.0 g/cm2 intermittent force (2.0 g i-force), (c) 2.0 g/cm2 continuous force (2.0 g c-force), (d) 5.0 g/cm2 i-force, or (e) 5.0 g/cm2 c-force. Atrophic cells appear as bright cells (arrows). Bars = 20 µm. (C) Cell damage assessed based on the activity of LDH released from the cells. N = 15 samples each. Data are presented as means ± SD. *Significant difference (p < 0.01).
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Cell Morphology
PDL cells that had been subjected to compressive force were examined under a phase-contrast microscope (ULWCD 0.30, Olympus Optical, Tokyo, Japan), equipped with an Olympus SC35 type 12 camera (Olympus).
Cell Damage
We assessed cell damage by measuring the activity of lactic dehydrogenase (LDH) released from the cells, using an LDH assay kit (Sigma). Medium without cells was used as a control for LDH activity. A low control value was determined for cells and medium in the absence of compressive force. A high control value was determined for cells dissolved with detergent (1% TritonX-100; Sigma), so that the maximum quantity of LDH could be determined. After the application of compressive force, the cells were collected, and a supernatant sample (100 µL) from each group was transferred and mixed with 100 µL of LDH assay mixture. The optical density (OD) was determined at a wavelength of 490 nm, with the use of a micro-plate reader (Model 550, Bio-Rad, Herts, UK).
After subtracting the background control OD from the OD for each well, we calculated cell damage as a percentage according to the following formula:
Semi-quantitative RT-PCR
Total RNA was purified by means of a Total RNA Extraction Miniprep system (Viogene, Sunnyvale, CA, USA) according to the manufacturer’s instructions. cDNA was synthesized from 2.0 µg of total RNA in 30 µL of reaction buffer composed of 500 µM dNTPs, 20 U ribonuclease inhibitor (Promega, Madison, WI, USA), and 200 U Superscript II reverse transcriptase (Invitrogen Life Technology, Carlsbad, CA, USA).
Oligonucleotide primer sequences were designed for the reverse-transcriptase/polymerase chain-reaction (RT-PCR). The following primers were used for amplification: for RANKL, 5' TCA GAA GAT GGC ACT CAC TG 3' and 5' AAC ATC TCC CAC TGG CTG TA 3'; for OPG, 5' AGG CCC TTC AAG GTGTCT TGG TC 3' and 5' GTG GTG CAA GCT GGA ACC CCA G 3'; for GAPDH, 5' TGG TAT CGT GGA AGG ACT CAT G 3' and 5' TCT CTT CCT CTT GAG CTC TTG C 3'; for IL-1β, 5' GAT SAA GCC CAC TCT ACA GCT 3' and 5' ATT GGC CCT GAA AGG AGA GA 3'; and for IL-1 receptor I, 5' GGA CTC CAG GAT TCA TCA GC 3' and 5' AGG ACA TAC GGC ATA TAT AG 3'. Each cycle consisted of a denaturation step at 94.0°C for 60 sec, an annealing step (RANKL, 54°C for 60 sec; OPG, 64.0°C for 40 sec; GAPDH, 60.0°C for 60 sec; IL-1β, 60.0°C for 60 sec; IL-1 receptor I, 54.0°C for 60 sec), and an extension step at 72.0°C for 60 sec. The PCR products underwent electrophoresis in 2% agarose gels and were visualized with ethidium bromide.
Western Blotting
Cell lysate from PDL cells was boiled and separated by 7.5% SDS-PAGE. After electrophoresis, proteins were transferred to polyvinylidene difluoride membranes (Millipore, Billerica, MA, USA). Blots were probed with rabbit polyclonal antibody against IL-1 receptor I (IL-1RI) (1:200; Santa Cruz Biotechnology, Santa Cruz, CA, USA), and incubated with horseradish peroxidase-tagged secondary antibody (1:5000; GE Healthcare UK Ltd, Buckinghamshire, UK).
Statistical Analysis
We used one-way ANOVA to analyze the cell damage, followed by individual post hoc comparisons (Scheffé).
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RESULTS
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The in vitro application of a four-day compressive force altered the state of PDL cells (Fig. 1B ). With the application of greater compressive forces, more atrophic cells were observed. There were fewer atrophic cells in the intermittent-force groups than in the continuous-force groups.
We assessed cell damage by measuring the activity of LDH released from the cells. The damage to PDL cells under a compressive force of 2.0 g/cm2 was less than that under a 5.0 g/cm2 at each day (Fig. 1C). At 3 and 4 days, the damage to PDL cells due to an intermittent force of 2.0 g/cm2 was significantly less than that due to a continuous force of 2.0 g/cm2.
To evaluate differences in the expression of bone-related genes, we measured the expression of OPG, RANKL, and IL-1β mRNA by RT-PCR. An optimal number of PCR cycles was determined by the cycle-dependency of PCR products (Fig. 2A). The highest OPG mRNA expression level was detected in unstimulated PDL cells, and expression was maintained at a similar level from day 2 to day 4 (Figs. 2B, 2C). PDL cells under compression expressed less OPG mRNA, in a force- and time-dependent manner. However, the obvious difference in the OPG mRNA level between cells subjected to intermittent force and those subjected to continuous force was not marked. Only after 3 days of compression was the OPG mRNA level lower under 2.0 g/cm2 intermittent force than under 2.0 g/cm2 continuous force.
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Figure 2. PCR analysis of OPG, RANKL, and IL-1β mRNA expression with compressive forces. Specificities of each primer were confirmed by a BLAST-assisted Internet search. (A) PCR products of OPG mRNA in unstimulated PDL cells by amplification for 20, 25, 30, 35, or 40 cycles. Since the amount of OPG mRNA expression was dependent on the cycle number between 25 and 35, 30-cycle amplification was used at an optimal cycle for semi-quantitative analysis of OPG mRNA expression. (B) OPG, RANKL, and IL-1β mRNA expressions in PDL cells under intermittent force (i-force) or continuous force (c-force). Force was applied at 2.0 or 5.0 g/cm2 for 2 to 4 days. Total RNA was analyzed by RT-PCR. MW indicates molecular-weight markers. (C) OPG, RANKL, and IL-1β mRNA expression relative to GAPDH expression was analyzed with the use of NIH Image software. The band intensities of OPG expression in the two-day control and RANKL or IL-1β expression at 2 days with 2.0 g/cm2 i-force were assigned values of 1.
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RANKL mRNA expression was not detected in unstimulated human PDL cells. Compression induced RANKL mRNA expression. The intermittent force induced greater RANKL mRNA expression than did the continuous force at both 2.0 and 5.0 g/cm2. At 2.0 g/cm2 intermittent force, the RANKL mRNA level increased in a time-dependent manner, reaching a maximum at 4 days in this experiment. An intermittent force of 5.0 g/cm2 induced a high expression level of RANKL mRNA, whereas a continuous force of 5.0 g/cm2 induced little RANKL mRNA expression.
Similarly, there was no IL-1β mRNA expression without force loading, and compression induced IL-1β mRNA expression. The intermittent force induced more IL-1β mRNA than did the continuous force at both 2.0 and 5.0 g/cm2. At 3 and 4 days, IL-1β mRNA expression was greater with 2.0 g/cm2 intermittent force than with 5.0 g/cm2 compression. IL-1β mRNA expression was highest at 4 days, with an intermittent compression of 2.0 g/cm2.
To clarify the relationship between IL-1β and RANKL mRNA expression with compression, we examined the expression of the IL-1 receptor and the effects of an IL-1 receptor antagonist. PDL cells constitutively expressed IL-1 receptor I mRNA (Fig. 3A) and IL-1 receptor I protein at 80 kDa, as has been reported (Tseng et al., 2006) (Fig. 3B). The treatment with the IL-1 receptor antagonist (100 ng/mL) inhibited RANKL mRNA up-regulation at 4 days with 2.0 g/cm2 intermittent force (Fig. 3C).
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Figure 3. RANKL expressions through IL-1 receptor 1. (A) IL-1 receptor I mRNA expression was confirmed by RT-PCR with primers specific for IL-1 receptor I. (B) Exhibition of IL-1 receptor I protein (80 kDa) in PDL cells was confirmed by Western blot analysis. Bands were detected with the use of the chemiluminescent horseradish peroxidase substrate. (C) To examine the involvement of IL-1β and the IL-1β receptor in the expression of RANKL in cells under compression force, we added 100 ng/mL IL-1 receptor antagonist (IL-1Ra) (ProSpec-Tany, TechnoGene, Rehovot, Israel) to the culture medium when the compressive force was applied. The effects of the IL-1 receptor antagonist on RANKL expression in cells subjected to an intermittent force (i-force) (8 hrs per day for 4 days) are shown. Neither control cells nor cells subjected to an i-force in the presence of IL-1 receptor antagonist expressed RANKL. MW indicates molecular-weight markers.
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DISCUSSION
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In this study, we demonstrated that 2.0 g/cm2 intermittent force induced greater RANKL and IL-1β mRNA expression and less cell damage than did continuous force in PDL cells, and that RANKL expression in PDL cells with a compression force is modulated by autocrine IL-1β.
We used a primary cell culture system to analyze the signal pathway from mechanical stress to the expression of RANKL and OPG. Generally, PDL cells are a heterogenous population, consisting mainly of fibroblasts (Lekic and McCulloch, 1996). We designed a new method for applying compression by increasing the amount of medium, to create hydraulic pressure. This method applied a uniform compressive force in vitro, while providing sufficient nutrition for the cells. Using our method, we could easily change the magnitude and timing of the force to be applied equally to all cells. In preliminary experiments, we varied the amount of medium from 0.17 to 7.0 g/cm2, and found that 2.0 and 5.0 g/cm2 were suitable as low and high pressures, respectively. There were fewer atrophic cells and less damage to PDL cells in the intermittent-force groups than in the continuous-force groups. With excessive compressive force, the cells became aligned irregularly and cell viability decreased, indicating that the compressive force caused cell death. Robling et al (2001) reported that mechanosensitivity began to decline soon after the loading was initiated, and that cells needed a couple of hours to restore mechanosensitivity. Though there exists the difference between cell damage and mechanosensitivity, we believe that the recovery from cell damage after loading might result in less cell damage. In this study, we used only 8 hrs per day force-application for intermittent force. This duration is based on the experiment by Hayashi et al.(2004), and we believe that 8 hrs of force-application is suitable for the simulation of clinical conditions.
We performed semi-quantitative RT-PCR to analyze the mRNA expression levels for target genes in PDL cells subjected to continuous and intermittent forces. PDL cells subjected to compression expressed less OPG mRNA, in a force- and time-dependent manner. At 4 days, the OPG mRNA level was lower in cells subjected to 2.0 g/cm2 intermittent force than in those subjected to 2.0 g/cm2 continuous force. The inactivation of OPG may play a key role in osteoclast differentiation (Hasegawa et al., 2002). Previous studies have indicated that mechanical stress (tensile force) up-regulated OPG mRNA in a force-dependent manner (Tsuji et al., 2004), whereas OPG expression remained constant under compressive force (Kanzaki et al., 2002). Although compressive force reduced OPG expression in the present study, its effect was less on the OPG mRNA level than on the mRNA level of RANKL and IL-1β.
Compression induced RANKL mRNA in PDL cells, with intermittent force having a greater effect than continuous force. Our results are consistent with those from a previous report of the time-dependent up-regulation of RANKL expression by compressive force in PDL cells (Kanzaki et al., 2002). Compared with OPG expression, RANKL expression in PDL cells is likely to be affected to a greater degree by compression.
IL-1 is one of the factors that stimulate bone resorption (Gowen et al., 1983; Heath et al., 1985; Shimizu et al., 1992). Similar to RANKL expression, IL-1β mRNA expression was induced by compression, with intermittent force inducing greater IL-1β mRNA expression than continuous force. Although a previous study demonstrated that cell elongation induced a significant increase in the production of IL-1β (Shimizu et al., 1994), we observed greater IL-1β mRNA expression under 2.0 g/cm2 than under 5.0 g/cm2 compressive force. Therefore, optimal compressive force with less cell damage is needed for IL-1β expression in PDL cells.
The similarity between the effects of compression on RANKL and IL-1β expression in PDL cells suggests a relationship between RANKL and IL-1β expression. Although PGE2, IL-6, and IL-8 have been shown to stimulate RANKL expression under compressive force (Kanzaki et al., 2002; Yamamoto et al., 2006), another previous study has indicated that IL-1 mediates RANKL expression (Wei et al., 2005). In our study, PDL cells expressed IL-1β receptor, and treatment with an IL-1 receptor antagonist inhibited RANKL mRNA up-regulation by intermittent force. Based on our findings, we propose the following as a possible mechanism by which a compressive force could induce osteoclastogenesis in PDL cells (Fig. 4): An intermittent force promotes IL-1β expression, the released IL-1β binds to the IL-1β receptor, and this signal enhances RANKL expression, inducing osteoclastogenesis. Thus, a weak intermittent force could effectively induce the expression of RANKL via IL-1β expression in human PDL cells under optimal compression forces.
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Figure 4. Schematic of a proposed mechanism by which compressive force induces osteoclastogenesis in PDL cells. First, compressive force stimulates IL-1β expression. Second, IL-1β is released and binds to the IL-1β receptor on the cells. Third, IL-1β acts through its receptor to signal the induction of RANKL expression. Up-regulated RANKL expression and down-regulated OPG expression then induce osteoclastogenesis in osteoclast progenitors.
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ACKNOWLEDGMENTS
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This work was supported by the Ministry of Education, Science, Sports and Culture of Japan (Grant 15591941 to Dr. T. Goto).
Received for publication September 1, 2006.
Revision received February 13, 2007.
Accepted for publication March 4, 2007.
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REFERENCES
|
|---|
- Angele P, Yoo JU, Smith C, Mansour J, Jepsen KJ, Nerlich M, et al. (2003). Cyclic hydrostatic pressure enhances the chondrogenic phenotype of human mesenchymal progenitor cells differentiated in vitro. J Orthop Res 21:451–457.[CrossRef][Medline]
[Order article via Infotrieve]
- Goga Y, Chiba M, Shimizu Y, Mitani H (2006). Compressive force induces osteoblast apoptosis via caspase-8. J Dent Res 85:240–244.
- Gowen M, Wood DD, Ihrie EJ, McGuire MK, Russell RG (1983). An interleukin 1 like factor stimulates bone resorption in vitro. Nature 306:378–380.[CrossRef][Medline]
[Order article via Infotrieve]
- Hasegawa T, Yoshimura Y, Kikuiri T, Yawaka Y, Takeyama S, Matsumoto A, et al. (2002). Expression of receptor activator of NF-kappa B ligand and osteoprotegerin in culture of human periodontal ligament cells. J Periodontal Res 37:405–411.[CrossRef][Medline]
[Order article via Infotrieve]
- Hayashi H, Konoo T, Yamaguchi K (2004). Intermittent 8-hour activation in orthodontic molar movement. Am J Orthod Dentofacial Orthop 125:302–309.[Medline]
[Order article via Infotrieve]
- He Y, Macarak EJ, Korostoff JM, Howard PS (2004). Compression and tension: differential effects on matrix accumulation by periodontal ligament fibroblasts in vitro. Connect Tissue Res 45:28–39.[CrossRef][Medline]
[Order article via Infotrieve]
- Heath JK, Saklatvala J, Meikle MC, Atkinson SJ, Reynolds JJ (1985). Pig interleukin 1 (catabolin) is a potent stimulator of bone resorption in vitro. Calcif Tissue Int 37:95–97.[Medline]
[Order article via Infotrieve]
- Hofbauer LC, Khosla S, Dunstan CR, Lacey DL, Boyle WJ, Riggs BL (2000). The roles of osteoprotegerin and osteoprotegerin ligand in the paracrine regulation of bone resorption. J Bone Miner Res 15:2–12.[CrossRef][Medline]
[Order article via Infotrieve]
- Kanzaki H, Chiba M, Shimizu Y, Mitani H (2001). Dual regulation of osteoclast differentiation by periodontal ligament cells through RANKL stimulation and OPG inhibition. J Dent Res 80:887–891.
- Kanzaki H, Chiba M, Shimizu Y, Mitani H (2002). Periodontal ligament cells under mechanical stress induce osteoclastogenesis by receptor activator of nuclear factor kappaB ligand up-regulation via prostaglandin E2 synthesis. J Bone Miner Res 17:210–220.[CrossRef][Medline]
[Order article via Infotrieve]
- Konoo T, Kim YJ, Gu GM, King GJ (2001). Intermittent force in orthodontic tooth movement. J Dent Res 80:457–460.
- Lekic P, McCulloch CA (1996). Periodontal ligament cell population: the central role of fibroblasts in creating a unique tissue. Anat Rec 245:327–341.[CrossRef][Medline]
[Order article via Infotrieve]
- Middleton J, Jones M, Wilson A (1996). The role of the periodontal ligament in bone modeling: the initial development of a time-dependent finite element model. Am J Orthod Dentofacial Orthop 109:155–162.[CrossRef][Medline]
[Order article via Infotrieve]
- Oppenheim A (1944). A possibility for physiologic orthodontic movement. Am J Orthod 30:277–328.
- Robling AG, Burr DB, Turner CH (2001). Recovery periods restore mechanosensitivity to dynamically loaded bone. J Exp Biol 204:3389–3399.[Abstract/Free Full Text]
- Robling AG, Hinant FM, Burr DB, Turner CH (2002). Improved bone structure and strength after long-term mechanical loading is greatest if loading is separated into short bouts. J Bone Miner Res 17:1545–1554.[CrossRef][Medline]
[Order article via Infotrieve]
- Shimizu N, Ogura N, Yamaguchi M, Goseki T, Shibata Y, Abiko Y, et al. (1992). Stimulation by interleukin-1 of interleukin-6 production by human periodontal ligament cells. Arch Oral Biol 37:743–748.[CrossRef][Medline]
[Order article via Infotrieve]
- Shimizu N, Yamaguchi M, Goseki T, Ozawa Y, Saito K, Takiguchi H, et al. (1994). Cyclic-tension force stimulates interleukin-1 beta production by human periodontal ligament cells. J Periodontal Res 29:328–333.[CrossRef][Medline]
[Order article via Infotrieve]
- Suda T, Takahashi N, Udagawa N, Jimi E, Gillespie MT, Martin TJ (1999). Modulation of osteoclast differentiation and function by the new members of the tumor necrosis factor receptor and ligand families. Endocr Rev 20:345–357.[Abstract/Free Full Text]
- Tseng J, Do J, Widdicombe JH, Machen TE (2006). Innate immune responses of human tracheal epithelium to Pseudomonas aeruginosa flagellin, TNF-alpha, and IL-1beta. Am J Physiol Cell Physiol 290:C678–C690.[Abstract/Free Full Text]
- Tsuji K, Uno K, Zhang GX, Tamura M (2004). Periodontal ligament cells under intermittent tensile stress regulate mRNA expression of osteoprotegerin and tissue inhibitor of matrix metalloprotease-1 and -2. J Bone Miner Metab 22:94–103.[CrossRef][Medline]
[Order article via Infotrieve]
- Wei S, Kitaura H, Zhou P, Ross FP, Teitelbaum SL (2005). IL-1 mediates TNF-induced osteoclastogenesis. J Clin Invest 115:282–290.[CrossRef][Medline]
[Order article via Infotrieve]
- Yamaguchi M, Aihara N, Kojima T, Kasai K (2006). RANKL increase in compressed periodontal ligament cells from root resorption. J Dent Res 85:751–756.
- Yamamoto T, Kita M, Kimura I, Oseko F, Terauchi R, Takahashi K, et al. (2006). Mechanical stress induces expression of cytokines in human periodontal ligament cells. Oral Dis 12:171–175.[Medline]
[Order article via Infotrieve]
- Zhang D, Yang YQ, Li XT, Fu MK (2004). The expression of osteoprotegerin and the receptor activator of nuclear factor kappa B ligand in human periodontal ligament cells cultured with and without 1alpha, 25-dihydroxyvitamin D3. Arch Oral Biol 49:71–76.[Medline]
[Order article via Infotrieve]
Journal of Dental Research, Vol. 86, No. 7,
623-628 (2007)
DOI: 10.1177/154405910708600708
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ABSTRACT
INTRODUCTION