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1 Division of Pediatrics and 2 Department of Molecular Genetics, University of Texas M. D. Anderson Cancer Center, Houston, Texas
Requests for reprints: Eugenie S. Kleinerman, Division of Pediatrics, University of Texas M. D. Anderson Cancer Center, Houston, TX 77030. Phone: 713-792-8110; Fax: 713-794-5042; E-mail: ekleiner{at}mdanderson.org.
| Abstract |
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Key Words: osteosarcoma osterix osteolysis lesion lung metastasis transcription factor
| Introduction |
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| Materials and Methods |
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Cell Transfection. K7M2 cells were transfected with either an osterix expression vector (pTriEx-1.1 hygro-osx) or the control vector pTriEx-1.1 hygro (Novagen, EMD Biosciences, Inc., San Diego, CA) using FuGene 6 (Roche Applied Sciences, Indianapolis, IN) and selected in 100 µg/mL hygromycin (Roche Applied Sciences). The resulting K7M2 clones (K7M2-osx-1 and K7M2-osx-2, respectively) were then maintained in complete medium containing 100 µg/mL hygromycin for 3 months and then screened by Northern blot analysis to assess osterix expression.
Northern Blot Analysis. Total RNA was isolated from cells using a TRIZOL RNA isolation kit (Life Technologies, Inc., Grand Island, NY). Twenty micrograms of total RNA was separated by electrophoresis under denaturing conditions and then transferred to a Hybond N+ membrane. Mouse and human osx cDNAs (6) were labeled with 32P using the Rediprime DNA labeling system (Amersham Biosciences, Piscataway, NJ), respectively. After prehybridization with 10 mL Rapidhyb buffer (Amersham Biosciences) at 65°C for 2 hours, hybridization was done overnight. The blot was subsequently washed in 2% (v/v) SSC, 0.1% (w/v) SDS at room temperature for 20 minutes, and then in 0.5% SSC, 0.1% SDS at 65°C for 20 minutes. Densitometric analysis was done using the Personal Densitometer SI(Molecular Dynamics, Sunnyvale, CA) and adjusted by glyceraldehyde-3-phosphate dehydrogenase internal control.
3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide Assay. Cells (5 x 103) were plated in 96-well culture plates in 0.2 mL culture medium. At 24, 48, and 72 hours after plating, 60 µL 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (2.5 mg/mL, Sigma Chemical Co., St. Louis, MO) was added to each well. Cells were cultured for an additional 2 hours, the supernatant was removed, and 100 µL dimethyl sulfoxide was added to each well. After shaking for 30 seconds, absorbance was measured at 570 nm.
In vivo Analyses. K7M2, K7M2-neo, K7M2-osx-1, and K7M2-osx-2 cells in separate single-cell suspensions (1 x 105) in HBSS (4°C) were injected into the left tibia of the mice. Briefly, mice were anesthetized with Nembutal anesthetic mix, and a 30G1/2 needle was inserted into the proximal end of the left tibia followed by injection of 10 µL cell suspension. Animals were sacrificed 35 days after tumor inoculation. Tumor incidence was determined using flatradiographs taken with a MX-20 Specimen Radiograph System (Faxitron X-ray Co., Wheeling, IL). Tumor size was measured with calipers. The tumor volume was calculated using the following formula: volume = length x width2/2. The grading scheme for quantification of bone lysis has been described previously (8). The lungs were resected and fixed in 10% formalin for 2 days, dissected into five lobes, paraffin embedded, divided into 5 µm sections, and stained with H&E to evaluate metastatic foci.
In vitro Migration Assay. The migration assay was done in 24-well chambers with an 8 µm pore polycarbonate filter (Corning, Inc., Corning,NY). Cells from each cell line were seeded into the top chamber. The bottom chamber was filled with 10% fetal bovine serum containing DMEM or lung extract. The lung extract was prepared from lungs of normal mice homogenized in complete DMEM. The chambers were incubated at 37°C for 4 hours. The cells were then fixed and visualized using a Hema-Diff staining kit (Statlab Medical Products, Inc., Lewisville, TX). The number of cells that migrated to the bottom of the filter were counted (five high-power fields per membrane). Experiments were set up in triplicate, and the average ± SD was determined.
Histologic Assessment of Bone. Bone specimens were fixed in 10% formalin for 24 hours, then decalcified for 14 days using 10% EDTA (pH 7.4). The specimens were then paraffin embedded, divided into 5 µm sections, and stained with H&E to for histologic assessment.
Reverse Transcription-PCR. Total RNA was extracted from different cells. The cDNA was synthesized using a Reverse Transcription System (Promega Corp., Madison, WI). Reverse transcription products were amplified by PCR using specific primers for murine receptor activator of NF-
B ligand (RANKL; sense 5'-GGTCGGGCAATTCTGAATT-3'; antisense 5'-GGGAATTACAAAGTGCACCAG-3'). The initial denaturation was done at 94°C for 5 minutes. Then, the products were subjected to denaturation at 94°C for 1 minute, annealing at 58°C for 1 minute, extension at 72°C for 1 minute for 28 cycles, and a final elongation at 72°C for 10 minutes. The PCR products were subjected to electrophoresis on 1% agarose gel with ethidium bromide and visualized under UV light. The glyceraldehyde-3-phosphate dehydrogenase primers and competimers (Ambion, Austin,TX) were used as the internal controls.
Statistics. After performing a one-way ANOVA to compare the means, comparisons among groups were made using the two-tailed, unpaired Student's t test. A difference was considered significant at P < 0.05.
| Results |
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We next investigated whether osterix expression affected tumor cell growth in vivo. K7M2-osx-1, K7M2-osx-2, K7M2-neo, and K7M2 cells were injected intratibially. After 35 days, the tumor incidence was determined by radiography. The mice injected with K7M2-osx-1 or K7M2-osx-2 cells had a lower incidence of tumor development than the mice injected with the vector control and parental cells (Fig. 2A). The tumors that developed in mice injected with K7M2-osx-1 and K7M2-osx-2 were substantially smaller (Fig. 2B).
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Osterix Expression Suppressed Lung Metastasis In vivo. K7M2 cells were derived from the K7 cell line (10) by repeated cycling of cells from pulmonary metastases into the orthotopic site (7). K7M2 cells are more aggressive locally and have a greater potential to metastasize to lung than K7 cells (11). Because our data showed that the expression of osterix was decreased in K7 and near absent in K7M2 cells (Fig. 1A), a possible link with metastasis as well as tumor formation was hypothesized. We therefore assessed the metastatic potential of the osterix-transfected clones. Mice were sacrificed 35 days after the intratibial injection of tumor cells. The lungs were resected and examined for lung metastasis. Based on the histologic analysis, 60% and 68% of mice injected with K7M2 or K7M2-neo cells, respectively, developed lung metastases (Table 1). By contrast, lung metastases developed in only 30% to 35% of mice inoculated with K7M2-osx-1 or K7M2-osx-2 cells (Table 1). The incidence of metastasis was similar to that in mice injected with the poorly metastatic K7 cells (i.e., 33%). Therefore, transfection of osterix also decreased the metastatic potential of K7M2 cells.
Osterix Suppressed the Migration of K7M2 to Lung Conditioned Medium. The process of metastases involves the migration of cells from the primary tumor to distant organ sites. Because we showed the decreased metastatic potential of the osterix-transfected cells to the lung, we did an in vitro migration assay using lung homogenate as the chemotactic stimulant. The migration of K7M2-osx-1 and K7M2-osx-2 cells to medium containing a lung homogenate was significantly less (18 cells ± 1.5/5 high-power fields and 20 cells ± 0.71/5 high-power fields, respectively) than that of vector control (48 cells ± 3.62/5 high-power fields) and parental K7M2 cells (50 cells ± 5.74/5 high-power fields).
| Discussion |
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In addition to inhibiting tumorigenicity, transfection of osterix into K7M2 cells altered the osteolytic morphology of the tumors. In comparison with K7M2 and K7M2neo-transfected cells, K7M2-osx cells did not result in lytic bone destruction. Osterix expression inhibited the infiltration of the tumor cells into the bone cortex, thus preserving the bone trabecula architecture (Fig. 3B and C).
Osteosarcoma, a primary malignant bone tumor, destroys the cancellous and cortical areas of bone as the tumor grows. This bone destruction is thought to be mediated by osteoclast activity (9). Decreased osterix expression may, therefore, play a role not only in tumor development but also in the tumor phenotype. Decreased osterix expression would result in decreased osteoblast differentiation and increased osteoclast activity leading to lytic destruction as the tumor cells invade the normal bone. Therefore, induction of osteoblastic differentiation by osterix and subsequent inhibition of osteolysis may be one mechanism by which osterix inhibits tumor growth. The mechanism of reduced osteolysis following osx gene transfection is unclear. RANKL has been shown to be critical to osteoclast formation. The expression of RANKL is high in undifferentiated osteoblasts compared with more differentiated osteoblasts. Higher expression of RANKL in osteosarcoma cells may stimulate more osteoclasts in the bone, resulting in increased osteolysis. We were, however, unable to show elevated RANKL expression in K7M2 cells compared with normal mouse osteoblasts. Furthermore, transfection of osterix did not inhibit RANKL expression.
Osteosarcoma is the most common primary bone tumor in both adults and children. Unfortunately, the 60% to 65% disease-free survival rate has remained stagnant for the past 15 to 20 years. The majority of cases of osteosarcoma are "sporadic," with there being no familial history and no consistent genetic alterations. Abnormalities in Rb and p53 have been described in some cases but more often patient tumors have normal Rb and p53 (3). Understanding the genes involved in tumor development, progression, and metastases are crucial to designing better and more specific therapeutic targets. Our data indicate that abnormalities in osterix may contribute to the tumorigenic phenotype of osteosarcoma. Restoration of osterix expression in osteosarcoma cells led to reduced tumor growth, decreased bone destruction, and fewer lung metastasis. Together, these data suggest that osterix might be a therapeutic target for osteosarcoma.
| Acknowledgments |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 6/16/04. Revised 11/23/04. Accepted 12/ 8/04.
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