In many medical fields, ultrasound (US) has proven to be an effective diagnostic or therapeutic tool [1
]. With respect to the therapeutic applications, low-intensity pulsed US (LIPUS) has been used extensively as an adjuvant to fracture healing [3
]. It has been well-established that fracture healing involves four events: inflammation, soft callus formation, hard callus formation and bone remodeling. Pre-clinical work has shown LIPUS to be effective in bone fracture models of small [7
] and large animals [19
]. Duatre [7
] previously demonstrated that LIPUS accelerated bone fracture union in a rabbit femoral fracture model. Azuma and coworkers [9
] also clearly demonstrated that LIPUS acted effectively at each phase of the healing process in a rat femoral fracture model. The results of a study using a sheep osteotomy model similarly demonstrated that LIPUS can significantly accelerate the fracture-healing process, increase the cortical bone mineral density and improve the lateral bending strength of the healing fracture [19
]. These therapeutic effects of LIPUS have since been confirmed in randomized clinical trials [22
The use of in vitro
cell culture systems has been of central importance for research at the cellular and molecular levels. A number of investigations have indicated that LIPUS is effective for the treatment of different cell types that play important roles in the processes of fracture healing. Most in vitro
studies have examined the US-associated bone formative response by use of osteoblastic cells from mammals [24
]. We recently demonstrated that the osteoblast activity in goldfish scales responds sensitively to LIPUS and may be important in promoting bone formation [31
]. LIPUS has been proven effective in bone marrow stromal cells, which are thought to be one of the cell types involved in fracture healing [32
]. In addition, LIPUS is reported to stimulate the growth and synthesis of matrix proteins in chondrocytes [34
]. LIPUS was demonstrated to exert its effects through integrin receptors; that is, LIPUS promotes cell proliferation via the activation of integrin receptors in human skin fibroblasts [36
]; and LIPUS activates α5
integrin and promotes cell differentiation in mouse mandibular osteoblasts [30
]. Moreover, LIPUS induces RANKL and chemokines via the angiotensin II type 1 receptor in MC3T3-E1 cells [25
]. LIPUS has been shown to stimulate cell proliferation, proteoglycan synthesis and expression of growth factor-related genes in human nucleus pulposus HNPSV-1 cells [37
]. LIPUS is reported to regulate the proliferation and differentiation of osteoblasts through osteocytes [38
]. As described above, the effects of LIPUS are evident; however, the detailed mechanisms by which LIPUS promotes bone fracture healing at the cellular or molecular level are largely unclear.
With the help of novel transcript profiling technology, a view of the genome-wide expression profiles can be assayed simultaneously, allowing scanning differential expression of a large number of genes. This technology has been used to analyze the expression of genes in response to LIPUS in human osteoblastic osteosarcoma MG-63 [28
] and SaOS-2 cells [29
]. While it is important to identify individual genes that are differentially expressed, there is also a need to move beyond this level of analysis. Recently, we have used pathway analysis technologies to map gene expression data into relevant gene networks on the basis of their functional annotation and known molecular interactions [39
]. Using these technologies, unique gene networks that are associated with cellular development and cell death were identified in human lymphoma U937 cells treated with LIPUS [39
In the present study, we investigated the changes in gene expression in MC3T3-E1 preosteoblast cells treated with LIPUS by using a GeneChip® microarray analysis system in order to better understand the molecular mechanisms underlying cellular responses to this stress.
Within the fields of orthopedics, LIPUS has been used to promote bone fracture healing [3
]. Although numerous studies have been conducted on this issue [7
], the molecular mechanisms by which LIPUS affects bone cells have not been well understood. The use of in vitro
cell culture systems has been of central importance for research at the cellular and molecular levels. The present study employed high-density oligonucleotide microarray analysis combined with computational gene expression analysis tools to demonstrate, for the first time, that many genes were responsive to LIPUS in MC3T3-E1 preosteoblast cells.
A number of previous reports [24
] clearly indicated that LIPUS had positive effects on differentiation, gene expression, mineralization and proliferation under in vitro
cell culture conditions, although the degree of these effects differed among studies. It is assumed that differences in the experimental conditions, such as the intensity of LIPUS, exposure period, cell type and culture conditions, contributed to discrepancies in the biological effects of LIPUS reported. Unsworth et al.
] demonstrated that the ALP activity was significantly increased at days 6 to 10, but not at days 2 and 4 in MC3T3-E1 cells treated with once daily LIPUS (30 mW/cm2
for 20 min). Using rat osteoblastic ROS 17/2.8 cells, Takayama et al.
] found that ALP activity was increased at day 7 of culture after transient LIPUS (30 mW/cm2
for 20 min) stimulation without affecting the cell proliferation. A single LIPUS (30 mW/cm2
for 20 min) treatment induces RANKL and chemokines via the angiotensin II type 1 receptor, while this stimulation does not affect the expressions of ALP and Bglap in MC3T3-E1 cells [25
]. In mouse bone-marrow-derived stromal ST2 cells, once LIPUS (30 mW/cm2
for 20 min) exposure induced significant increases in the expressions of Bglap
, as well as FBJ osteosarcoma oncogene (Fos
), an immediate-early gene [32
]. In this study, although neither cell growth nor ALP activity was influenced by a single exposure of MC3T3-E1 cells to LIPUS (30 mW/cm2
for 20 min), significant elevation of the mRNA level of Bglap
was observed in the LIPUS-treated cells.
In the present study, many of the 38 and 37 genes that were upregulated or downregulated by 1.5-fold or more, respectively, were identified in the cells after LIPUS treatment. Seventeen genes were validated by real-time quantitative PCR assay, which was consistent with the microarray data. Of these 75 genes, Mmp13
are indicated to be related to LIPUS [27
]. The expression level of Mmp13
was reported to be affected by LIPUS; LIPUS (30 mW/cm2
for 20 min per day) significantly increased the expression of Mmp13
mRNA in MC3T3-E1 cell culture at day 10 [27
] and in nucleus pulposus HNPSV-1 cell culture at day 3 [37
], whereas LIPUS (7.5, 30 and 120 mW/cm2
) significantly and dose-dependently inhibited Mmp13
expression in isolated rat chondrocytes [42
]. Recent findings demonstrated no statistically significant difference between the control and LIPUS (30 mW/cm2
)-treated groups in the expression of Dmp1
on a human tooth slice organ culture [43
]. To the best of our knowledge, 73 genes out of 75 genes were not previously reported as related to the LIPUS response.
In addition, to study the molecular functions and gene networks, the microarray data were analyzed using Ingenuity®
Pathway Analysis tools. Of particular interest in this study was the identification of gene network U, which contained many upregulated genes that were principally associated with bone morphology. Six of the genes in this network, Dmp1
] and Thbs1
], have previously been shown to be involved in bone morphology. In addition, a number of genes in the gene network U, including Cd200
, have been shown to be expressed in osteoblasts [50
] and to participate in the development and/or differentiation of osteoblasts. For example, the Cd200-Cd200 receptor axis was shown to be a possible regulator of bone mass, via the formation of osteoclasts in this study using Cd200-knockout mice [51
]. A significant increase in the expression of Dmp1
, an intermediate and late marker of bone cell differentiation, was detected in fully differentiated osteoblasts [50
], and the addition of Dmp1 protein to osteoblast cells markedly induced cell differentiation [52
]. We also identified gene network D, which included many downregulated genes. Interestingly, this network contained the Id genes (other designation: inhibitor of differentiation), Id1
, all of which belong to the helix-loop-helix (HLH) transcription factors and can form heterodimers among the basic HLH transcription factors [53
]. All Id family members are reported to decrease the differentiation of bone cells, including osteoblasts [53
]. The genes that were differentially expressed and/or belonged to gene networks are likely to be involved in the acceleration of fracture healing by LIPUS. However, further experiments will be needed to confirm this.
There are two research papers investigating the effects of LIPUS on gene expression in osteoblastic cells using global-scale microarray analysis. Leskinen et al.
] indicated that 377 genes were regulated two-fold by a single LIPUS (1.035 MHz; mean acoustic peak pressure, 128–510 kPa; for 30 min) in human osteoblastic osteosarcoma MG-63 cells. They also suggested that LIPUS affects the genes involved with the cellular membrane and regulation of transcription, but not with osteoblast differentiation [28
]. Lu et al.
] identified 165 genes whose expressions were changed more than two-fold in human osteoblastic osteosarcoma SaOS-2 cells treated with a single LIPUS (30 mW/cm2
for 20 min). These genes belonged to more than ten protein families, including integrins and cytoskeleton genes, the transforming growth factor-beta family and the insulin-like growth factor family [29
]. Moreover, Kobayashi et al.
] previously showed using cDNA microarrays that LIPUS (30 mW/cm2
for 20 min) once daily for three days significantly induced 114 genes that were differentially expressed, and these genes included growth factor-related proteins, proteoglycans, collagens and matrix metallopeptidases. We identified 193 genes that were downregulated more than 1.5-fold and 201 genes that were upregulated more than 1.5-fold using a GeneChip microarray system in human lymphoma U937 cells treated with a single LIPUS (300 mW/cm2
for 1 min) [39
]. Here, a total of 75 genes were differentially expressed in preosteoblast MC3T3-E1 cells at 24 h after LIPUS (30 mW/cm2
for 20 min) exposure. However, almost all of the genes that were differentially expressed and identified here were not reported in these four microarray studies [28
]. As in the case of the biological effects of LIPUS, this discrepancy may have been due to the different experimental conditions, particularly with respect to the cell origin. Elucidation of the detailed mechanisms underlying the changes in gene expression responsive to LIPUS remains for further investigations.
4. Experimental Section
4.1. Cell Culture
The mouse preosteoblast cell line, MC3T3-E1, was provided by the: RIKEN BioResource Center (RIKEN BRC) through the National Bio-Resource Project of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT; Tsukuba, Japan). The cells were routinely cultured in minimum essential medium with alpha modification (MEMα; Wako Pure Chemical Industries, Ltd., Osaka, Japan) supplemented with 10% fetal bovine serum (FBS; Equitech-Bio, Inc., Kerrville, TX, USA) at 37 °C in humidified air with 5% CO2.
4.2. LIPUS Treatment
LIPUS treatment was applied by a sonic accelerated fracture-healing system (SAFHS) apparatus (SAFHS 4000J; Teijin Pharma, Ltd., Tokyo, Japan). This apparatus is the same as Exogen’s SAFHS apparatus (Exogen Inc., Piscataway, NJ, USA). The signal had a spatial average-temporal average (SATA) intensity of 30 mW/cm2, with a frequency of 1.5 MHz in a pulsed-wave mode (0.2-s burst sine waves repeated at 1.0 kHz).
The cells were cultivated in MEMα supplemented with 10% FBS, 0.3 mM L-ascorbic acid (Wako Pure Chemical Industries, Ltd., Osaka, Japan) and 10 mM β-glycerophosphate (Calbiochem, La Jolla, CA, USA) at 37 °C for 14 days. After trypsinization with 0.25% trypsin-0.91 mM EDTA solution, the cells (5 × 105
cells) were seeded on a 35-mm plastic culture dish (ASAHI GLASS Co., Ltd., Tokyo, Japan) with 2 mL of culture medium and cultured at 37 °C for 24 h. It has been demonstrated that LIPUS is applied to cells from the upper [26
] or lower side [25
] of the culture dish. As demonstrated in Figure 8
, LIPUS was transmitted through the bottom of the culture dish with standard US coupling gel (Teijin Pharma, Ltd., Tokyo, Japan). Attached cells in the dish were sonicated for 20 min at 37 °C in a CO2
incubator. After sonication, the cells were incubated at 37 °C in a CO2
incubator for the indicated period before the experiments.
4.3. Measurement of Cell Number
The cells were exposed to LIPUS followed by 0, 6, 24 and 48 h culture at 37 °C, were washed once with Ca2+/Mg2+-free phosphate buffered saline (PBS) and, then, treated with 0.25% trypsin-0.91 mM EDTA solution. The number of detached cells was counted using a Burker Turk hemocytometer.
4.4. Measurement of ALP Activity
The cells were exposed to LIPUS followed by 0, 6, 24 and 48 h culture at 37 °C, were washed once with Ca2+/Mg2+-free PBS and scraped using a plastic policeman. Cellular material was placed into 50 mM Tris-HCl buffer (pH 7.2) containing 0.01% Triton X-100 and homogenated by an ultrasonic disruptor (UD-200, TOMY SEIKO Co., Tokyo, Japan). The protein concentration was measured using a BCA Protein Assay Kit (Pierce; Rockford, IL, USA). ALP activity of the cell homogenate was measured by using a protocol supplied by Sigma (Sigma-Aldrich, Co., St. Louis, MO, USA).
4.5. RNA Isolation
The cells were exposed to LIPUS followed by 6 and 24 h culture at 37 °C. Total RNA was extracted from cells using an RNeasy Total RNA Extraction Kit (Qiagen K.K., Tokyo, Japan) along with on-column DNase I treatment (RNase-free DNase kit, Qiagen K.K., Tokyo, Japan). RNA quality was analyzed using a Bioanalyzer 2100 (Agilent Technologies, Inc., Santa Clara, CA, USA). RNA samples that had RNA integrity number (RIN) values above 9.5 were considered acceptable.
4.6. High-Density Oligonucleotide Microarray and Computational Gene Expression Analyses
Gene expression was analyzed using a GeneChip®
system with a Mouse Genome 430 2.0 array (Affymetrix, Inc., Santa Clara, CA, USA) spotted with 45,101 probe sets. Samples for array hybridization were prepared as described in the Affymetrix GeneChip®
Expression Technical Manual. In short, 500 ng of total RNA were used to synthesize cRNA with a GeneChip®
3′ IVT Express Kit (Affymetrix, Inc., Santa Clara, CA, USA). After fragmentation, biotin-labeled cRNA was hybridized to the array at 45 °C for 16 h. The arrays were washed, stained with streptavidin-phycoerythrin and scanned using a probe array scanner. The obtained hybridization intensity data were further analyzed using GeneSpring®
GX (Agilent Technologies, Inc., Santa Clara, CA, USA) to extract the significant genes. To examine gene ontology, including biological processes, cellular components, molecular functions and gene networks, the obtained data were analyzed using Ingenuity®
Pathway Analysis tools (Ingenuity Systems, Inc., Mountain View, CA, USA), a web-delivered application that enables the identification, visualization and exploration of molecular interaction networks in gene expression data [39
4.7. Real-Time Quantitative PCR Assay
Complementary DNA was produced from total RNA using an ExScript RT Reagent Kit (Takara Bio Inc., Shiga, Japan) with a random 6 mers and an oligo dT primer. Real-time quantitative PCR was performed on an Mx3005P real-time PCR system (Agilent Technologies, Inc., Santa Clara, CA, USA) using SYBR®
PreMix ExTaq (Takara Bio Inc., Shiga, Japan). The specific primer and probe sequences are listed in Table S3
. The temperature cycling conditions for each primer consisted of 10 s at 95 °C followed by 40 cycles of 15 s at 95 °C and 40 s at 60 °C. The dissociation analysis was carried out over a range from 55 °C to 95 °C by monitoring SYBR green fluorescence, and PCR-specific products were determined as a single peak in the melting curves at more than 80 °C. In addition, the specificity of primers was confirmed as a single band with the correctly amplified fragment size through an agarose gel electrophoresis of the PCR products. Glyceraldehyde-3-phosphate dehydrogenase (Gapdh
) was used as an internal control.
4.8. Statistical Analysis
Results were expressed as the means ± standard deviation (SD). Differences in the means were analyzed using the Student’s t-test, and p values < 0.05 were regarded as statistically significant.