Ameloblastin Regulates Osteogenic Differentiation by Inhibiting Src Kinase via Cross Talk between Integrin β1 and CD63

ABSTRACT Ameloblastin, the most abundant nonamelogenin enamel matrix protein, plays a role in ameloblast differentiation. Here, we found that ameloblastin was expressed in osteosarcoma cells; to explore the potential functions of ameloblastin in osteoblasts, we investigated whether this protein is involved in osteogenic differentiation and bone formation on the premise that CD63, a member of the transmembrane-4 glycoprotein superfamily, interacts with integrins in the presence of ameloblastin. Ameloblastin bound to CD63 and promoted CD63 binding to integrin β1. The interaction between CD63 and integrin β1 induced Src kinase inactivation via the binding of CD63 to Src. The reduction of Src activity and osteogenic differentiation mediated by ameloblastin were abrogated by treatment with anti-CD63 antibody and overexpression of constitutively active Src, respectively. Therefore, our results suggest that ameloblastin is expressed in osteoblasts and functions as a promoting factor for osteogenic differentiation via a novel pathway through the interaction between CD63 and integrin β1.

Ameloblastin (AMBN), also known as sheathlin or amelin, is the most abundant nonamelogenin enamel matrix protein (4,5,14) and a member of the secretory calcium-binding phosphoprotein (SCPP) gene cluster of evolutionarily related molecules that regulate skeletal mineralization (13). Further, AMBN induces cell attachment, proliferation, and differentiation of periodontal ligament cells in vitro (34). In AMBN-null mice, ameloblasts are detached from the matrix, lose cell polarity, and resume proliferation. However, protein expression was not completely inactivated, and truncated RNA missing a portion of exons 5 and 6 is still translated in AMBN knockout (KO) mice (30). Therefore, it is conceivable that exons 5 and 6 of AMBN play a role in ameloblast differentiation (7,30). In this mouse model, structural change was shown in the alveolar bone (30): the alveolar bone exhibited more porosity in truncated-AMBN-expressing mice than in wild-type mice. The changes in alveolar bone in mice lacking exons 5 and 6 of AMBN (AMBN ⌬5-6 ) cannot be directly related to the protein as they could arise from other factors such as changes in occlusal forces in teeth without enamel (30). On the other hand, it has recently been reported that AMBN is expressed in osteoblasts during craniofacial development (25). Although AMBN may play a significant role in not only in tooth development but also bone formation, the role of AMBN in bone formation is still unclear.
AMBN has been shown to interact with CD63 via a yeast two-hybrid assay (29). CD63 is a member of the transmem-brane-4 glycoprotein superfamily, also known as the tetraspanin family (26,32). Most of these proteins are cell surface proteins that are characterized by the presence of four hydrophobic domains and two extracellular domains (26,32). CD63 mediates signal transduction events in the regulation of cell survival, development, activation, growth, and motility (12,16,32). In particular, cell surface CD63 is known to complex with integrins (2) and is involved in the control of integrin outside-in signaling activity (9,12).
Integrins are heterodimeric adhesion receptors for extracellular matrix proteins consisting of ␣and ␤-subunits. Osteoblast differentiation is induced by the aggregation of cell surface integrin ␣2␤1 with matrix type I collagen (COL I) (11,27,31), an event triggered by the stimulation of integrins through the association of their cytoplasmic domains with focal adhesion kinase, Src, and other cytosolic nonreceptor tyrosine kinases. Src is a nonreceptor tyrosine kinase found in a wide variety of tissues (22) and has two important phosphorylation sites in Tyr at 416 and 527 (23). Src phosphorylated at Tyr527 interacts with the Src homology 2 (SH2) domain at the intramolecular level and exhibits a kinase constitutively negative. On the other hand, Tyr416 of Src is present within a kinase domain, and phosphorylation of Tyr416 augments kinase activity. Src-deficient mice have an osteopetrosis phenotype (24), and the reduction of Src activity stimulates osteoblast differentiation and bone formation (17). In this study, to explore further the potential functions of AMBN in osteoblasts, we investigated whether this protein is involved in osteogenic differentiation and bone formation according to the hypothesis that CD63 interacts with integrins in the presence of AMBN.

Reagents and antibodies.
Monensin for inhibiting secretion was obtained from Sigma (St. Louis, MO). A constitutively active mutant Src vector, Src-Y527F, was donated by Addgene, Inc. (Cambridge, MA). For detecting endogenous AMBN by immunohistochemistry, an anti-AMBN polyclonal antibody (W59) was generated in rabbits by immunization with a synthetic peptide (EHETQQYEYS) corresponding to residues 93 to 102 of human AMBN. The human AMBN amino acid sequence (93 to 102) shows homology to mouse and rat AMBN. Commercial antibodies were purchased from the following suppliers: anti-FLAG monoclonal antibody (M2) and anti-␤-actin monoclonal antibody were from Sigma; anti-Src polyclonal antibody and polyclonal antibody specific to phospho-Tyr416 Src were from Cell Signaling Technology (Danvers, MA); anti-integrin ␤1 (M-106) polyclonal antibody and anti-CD63 (MX-49.129.5) monoclonal antibody were from Santa Cruz Biotechnology (Santa Cruz, CA). CD63 blocking antibody (H-193) was purchased from Santa Cruz Biotechnology, and integrin ß1 blocking antibody (MAB2253Z) was purchased from Chemicon (Temecula, CA).
Cell culture of fetal rat calvaria cells. Calvaria cells from 21-day-old Wistar rat fetuses were isolated by sequential collagenase digestion, which resulted in five cell fractions. Briefly, calvariae were dissected free from loosely adherent connective tissues, minced, and sequentially digested in collagenase (type I; Sigma-Aldrich Corp., St. Louis, MO) solution. Cells from the last four of five digestion fractions were separately grown in alpha-minimal essential medium (␣MEM) supplemented with 10% fetal bovine serum (Cansera, Etobicoke, Canada) and antibiotics. After 24 h, cells were trypsinized, pooled, and grown in 24-well plates or 35-mm dishes (0.3 ϫ 10 4 cells/cm 2 ) in the same medium supplemented additionally with 50 g/ml ascorbic acid. All cultures were maintained at 37°C in a humidified atmosphere with 5% CO 2 , and medium was changed every second or third day.
Cell lines and tissue samples. Human osteosarcoma cell lines (NOS-1, SaOS-2, MG63, and HOS) were provided by Cell Bank, RIKEN BioResource Center (Ibaraki, Japan). They were maintained in RPMI 1640 medium or Dulbecco's modified Eagle's medium (Nissui Pharmaceutical Co., Tokyo, Japan) supplemented with 10% heat-inactivated fetal bovine serum (FBS; Invitrogen, Carlsbad, CA) and 100 U/ml penicillin-streptomycin (Invitrogen) under 5% CO 2 in air at 37°C. Tissue samples of osteosarcoma were retrieved from the Surgical Pathology Registry of Hiroshima University Hospital, after approval by the Ethical Committee of Hiroshima University Hospital. Six osteosarcoma tissue samples were used for the immunohistochemical analysis.
Immunohistochemical staining. Immunohistochemical detection of AMBN in the tissue samples was performed on 4.5-m sections mounted on silicon-coated glass slides by using a streptavidin-biotin peroxidase technique as described previously (28). For the immunohistochemical study, the anti-AMBN polyclonal antibody (W59) was used.
Immunofluorescence. NOS-1 and SaOS-2 cells as well as AMBN-overexpressing SaOS-2 cells were seeded on coverslips and grown to approximately 80% confluence. Before cells were fixed, monensin was added to the NOS-1 cells at a final concentration of 5 M for 4 h. The cells were fixed in 4% paraformaldehyde for 10 min at room temperature, rinsed three times with ice-cold phosphatebuffered saline (PBS), and then permeabilized in 0.1% Triton X-100 in PBS for 15 min at room temperature. After three rinses with PBS, the coverslips were incubated with the primary antibodies in PBS. We used anti-AMBN polyclonal antibody (W59) as the primary antibody and anti-rabbit Alexa Fluor 488 (Molecular Probes) as the secondary antibody. DNA was visualized by 4Ј,6-diamidino-2-phenylindole (DAPI) staining. Immunostaining of the cell preparations was recorded by using an epifluorescence Zeiss Axioplan 2 microscope (Zeiss, Inc., Thornwood, NY) attached to a charge-coupled-device (CCD) camera.
Generation of AMBN-overexpressing SaOS-2 cells. Human AMBN cDNA was isolated from the cDNA of NOS-1 cells by reverse transcription-PCR (RT-PCR) using sense and antisense primers. The cDNA was then subcloned by insertion into the EcoRI/BamHI restriction site of pcDNA3.1, and the vector was modified by insertion of a FLAG tag sequence behind the signal peptide sequence using a mutagenesis kit (Stratagene, La Jolla, CA). The AMBN-pcDNA3.1 plasmid or the vector alone was introduced into SaOS-2 cells, and stable clones were obtained by G418 selection (500 g/ml; Invitrogen) in the culture medium. Cell transfection was performed by using FuGENE 6HD (Roche, Basel, Switzerland) according to the manufacturer's instructions. Conditioned medium from control and AMBN-overexpressing cells was collected after 48 h of plating.
Silencing by siRNA. Logarithmically growing NOS-1 cells were seeded at a density of 10 5 cells/dish (6 cm) and transfected with oligonucleotides twice (at 24 and 48 h after replating) by using Oligofectamine (Invitrogen). Forty-eight hours and 96 h after the last transfection, the cells were prepared and analyzed by real-time RT-PCR and mineralization assay, respectively. The small interfering RNA (siRNA) is a 19-bp duplex oligoribonucleotide with a sense strand corresponding to nucleotides 1205 to 1223 of the human AMBN mRNA sequence: 5Ј-UGACCACAUCCGUGGAUUU-3Ј. A scrambled sequence without significant homology to rat, mouse, or human gene sequence was used as a control.
RT-PCR and real-time RT-PCR analysis. Total RNA was isolated from cultures of confluent cells by using an RNeasy Mini Kit (Qiagen, Hilden, Germany) according to the manufacturer's instructions. The preparations were quantified, and their purity was determined by standard spectrophotometric methods. cDNA was synthesized from 1 g of total RNA using a ReverTra Dash kit (Toyobo Biochemicals, Tokyo, Japan). The RT-PCR oligonucleotide primers for human, mouse, or rat AMBN, CD63, runt-related transcription factor 2 (RUNX2), bone morphogenetic protein 2 (BMP2), alkaline phosphatase (ALP), type 1 collagen (COL I), bone sialoprotein (BSP), osteocalcin (OCN), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are listed in Table 1. Aliquots of total cDNA were amplified with 1.25 U of rTaq DNA polymerase (Qiagen) in a PC701 thermal cycler (Astec, Fukuoka, Japan) for 25 to 30 cycles after denaturation for 30 s at 94°C, annealing for 30 s at 60°C, and extension for 1 min at 72°C in the case of all the primers. The amplified products were resolved on 1.5% agarose-TAE (Tris-acetate-EDTA) gels (Nacalai Tesque, Inc., Kyoto, Japan), electrophoresed at 100 mV, and visualized by ethidium-bromide staining. For real-time PCR, aliquots of total cDNA were amplified with Fast SYBR green Master Mix (Applied Biosystems, Foster City, CA). Data acquisition and analysis were performed with a Step One Real-Time PCR System using Step One Software, version 2.1 (Applied Biosystems).
Generation of recombinant human AMBN. The full-length human AMBN cDNA-inserted FLAG tag sequence was subcloned into pEU vector (CellFree Sciences, Yokohama, Japan), which was then transcribed into mRNA. The total mRNA product was translated by using a WEPRO 3240 kit according to the manufacturer's instructions (CellFree Sciences).  Mineralization assay. Mineral nodule formation was detected by the Dahl method for calcium (3). Cells were placed in a 24-well or 6-well plate at a subconfluent density per well and cultured in medium supplemented with 10% FBS, 50 g/ml ascorbic acid, and 5 mM sodium ␤-glycerophosphate at 37°C for a week. The cells were fixed in 70% ethanol for at least 1 h and then stained with alizarin red S (ALZ). After mineral deposits were stained and photographed, the staining was destained with 10% (wt/vol) cetylpyridinium chloride (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan), and the concentrations were determined by the absorbance measured at 562 nm on a multiplate reader.
Western blot and immunoprecipitation analyses. Western blotting was carried out as described previously (28). Thirty micrograms of protein was subjected to 10% polyacrylamide gel electrophoresis, followed by electroblotting onto a ni-

AMBN expression in osteosarcoma samples and cell lines.
To determine the expression of AMBN during the proliferation-differentiation-maturation sequence of osteogenic cells in vitro, rat embryonic calvaria cell populations were obtained and examined for AMBN expression by RT-PCR analysis (Fig.  1A). Under growth conditions, AMBN mRNA was detected in exponentially growing cells, but its expression was clearly downregulated when cells reached confluence (7 day) (Fig.  1A). From early osteogenic differentiation to late maturation stage, the osteoblast markers ALP, a relatively early marker of osteoblast differentiation, and OCN, a later marker of osteoblast differentiation, were upregulated (Fig. 1A). These findings suggest that AMBN may be involved in the early stage of osteogenesis.
Osteosarcoma cell lines are often used for studying the mechanisms that control osteoblast-specific gene expression. To investigate the detailed role of AMBN in osteogenesis, we used osteosarcoma cells. We found that the osteosarcoma cell line NOS-1 expressed AMBN mRNA (Fig. 1B). NOS-1 cells also showed the expression of RUNX2, ALP, COL I, BSP, and OCN mRNAs (Fig. 1B). In NOS-1 cells, the expression of AMBN in the cytoplasm was detected by immunofluorescence analysis in the presence of monensin for inhibiting the secretion (Fig. 1C). We also examined the immunohistochemical expression of AMBN in six paraffin-embedded osteosarcoma samples. We checked the specificity of AMBN antibody in ameloblasts of a 7-day-old mouse. The sequence specificity of AMBN antibody used in this study was conserved between mice and humans. The AMBN antibody specifically recognized AMBN in ameloblasts (Fig. 1D). By using this antibody, we detected the expression of AMBN in two of the samples (cases 1 and 2) (Fig. 1D). Therefore, we demonstrated that the osteosarcoma cells expressed AMBN.
Involvement of AMBN in osteogenic differentiation and mineralization. To explore the involvement of AMBN in osteoblastic activity, we examined osteogenic differentiation and mineralization in NOS-1 cells treated with AMBN siRNA. In the AMBN siRNA-treated cells, AMBN mRNA expression was reduced ( Fig. 2A). Interestingly, remarkable downregulation of ALP, COL I, and BSP mRNA was observed in AMBN-knockdown cells in comparison with the control cells ( Fig. 2A). Moreover, when the control and AMBN-knockdown cells were cultured for 4 days in the presence of osteogenic medium containing ␤-glycerophosphate and ascorbic acid after transfection with AMBN siRNA for 48 h, matrix mineralization was found to be decreased in the AMBN-knockdown cells (Fig. 2B).
To elucidate the detailed role of AMBN in mineralization, we examined ectopic overexpression of AMBN by transfection of the pcDNA3.1-AMBN vector with insertion of FLAG tag sequences behind signal peptide sequences into SaOS-2 cells without AMBN expression (Fig. 3A). We obtained SaOS-2 cells with stable AMBN overexpression (Fig. 3B). In the conditioned medium from AMBN-overexpressing SaOS-2 cells, several bands were detected by Western blotting using anti-FLAG antibody, indicating that AMBN might have been modified after secretion (Fig. 3C). As expected, the upregulation of RUNX2 and BSP mRNA expression (Fig. 3D) and promotion of matrix mineralization were observed in the AMBN-overexpressing SaOS-2 cells (Fig. 3E). We also confirmed that treatment with recombinant human FLAG-AMBN enhanced matrix mineralization in the SaOS-2 cells, as seen in AMBN overexpression ( Fig. 3F and G).
Regulation of Src kinase activity via interaction between integrin and CD63. We next investigated the mechanism of AMBN for promoting osteogenic differentiation and mineralization. On the basis of our hypothesis that CD63 interacts with integrins in the presence of AMBN, we first examined the expression of CD63 and integrin ␤1 in the osteosarcoma cell lines. These cell lines expressed CD63, and SaOS-2 cells in particular strongly expressed CD63 (Fig. 4A). The CD63 protein, being heavily glycosylated, exhibited diffuse distribution on SDS-PAGE gels, as previously reported (12,26). To determine the interaction between CD63 and AMBN, we performed immunoprecipitation using anti-FLAG and anti-CD63 antibodies in FLAG-AMBN-overexpressing SaOS-2 cells. As shown in Fig. 4B, ectopic AMBN bound to CD63 in the ascorbic acid and 5 mM sodium ␤-glycerophosphate. After mineral deposits were stained and photographed, the staining was destained by using 10% (wt/vol) cetylpyridinium chloride, and the concentrations were determined by the absorbance measured at 562 nm on a multiplate reader. The picture shows mineral deposits at 15 days. The absorbance of the control cells at 10 days was defined as 1. The graph shows the relative ALZ levels. (F) Recombinant AMBN promotes mineralization in osteosarcoma cells. AMBN full-length protein including the FLAG tag (rhAMBN) was produced by a wheat germ cell-free system and was examined by Western blotting using anti-FLAG tag antibody. We also used control recombinant protein (rCtrl). (G) SaOS-2 cells cultured for 10 days in the presence of 50 mg/ml ascorbic acid and 5 mM sodium ␤-glycerophosphate. Cells were also cultured on a noncoating dish (control), control recombinant protein-coated dish (rCtrl), and on a dish coated with 10 g/ml recombinant AMBN (rAMBN). After mineral deposits were stained and photographed, the staining was destained by 10% (wt/vol) cetylpyridinium chloride, and the concentrations were determined by absorbance measurement at 562 nm on a multiplate reader. The absorbance of control cells at 10 days is defined as 1. Graph shows relative ALZ levels.
VOL. 31,2011 AMELOBLASTIN REGULATES OSTEOGENIC DIFFERENTIATION 787 ment on COL I-coated dishes. The phosphorylation levels of Src were found to be as low in these AMBN-overexpressing cells as in those under normal conditions (Fig. 4C). We also examined the phosphorylation levels of Src at Tyr416 in AMBN siRNA-transfected NOS-1 cells. AMBN knockdown upregulated the phosphorylation level of Tyr416 of Src (Fig.  4D). To investigate the cross talk among integrin ␤1, CD63, and Src, we performed immunoprecipitation with anti-CD63 antibody in control and AMBN-overexpressing cells after 48 h of attachment on COL I-coated dishes. Only in the AMBNoverexpressing cells did CD63 bind to integrin ␤1 and Src (Fig.  4E). These findings suggested that the interaction between CD63 and integrin ␤1 inhibited Src activity in the presence of AMBN.
To support these established roles of AMBN in regulating osteogenesis via CD63, we examined the inhibition of the osteogenic function of CD63 using CD63 blocking antibody in control and AMBN-overexpressing cells. A previous report showed that AMBN interacts with amino acids 103 to 205 of the human CD63 protein, a region that is hydrophobic and accessible to the external environment, as demonstrated by a yeast two-hybrid assay (35). CD63 blocking antibody used in this study was raised against amino acids 45 to 238 of the human CD63 protein. Indeed, we confirmed that CD63 blocking antibody interrupted the interaction of CD63 with AMBN (data not shown). In Western blot analysis using anti-phospho-Tyr416 Src antibody, reduced phosphorylation levels of Src in AMBN-overexpressing cells were observed without anti-CD63 treatment, but pretreatment with anti-CD63 antibody induced the upregulation of the phosphorylation levels (Fig. 5A). In- terestingly, pretreatment with anti-CD63 antibody blocked the promotion of mineralization in SaOS-2 cells treated with the conditioned medium from AMBN-overexpressing cells (Fig.  5B). To know the involvement of integrin ß1 in the inhibition of Src activity by AMBN overexpression, we treated the integrin ß1 blocking antibody in AMBN-overexpressing cells. Treatment with integrin ß1 blocking antibody upregulated phosphorylation levels of Src at Tyr416 in AMBN-overexpressing cells (Fig. 5C). Interestingly, treatment with integrin ß1 blocking antibody decreased the expression of RUNX2, ALP, COL I, BSP, and OCN mRNAs (Fig. 5D). Moreover, we examined whether Src activation inhibits AMBN-mediated mineralization by transfection of a constitutively active mutant of Src (Y527F) in AMBN-overexpressing cells. Transfection of the constitutively active mutant of Src upregulated the phosphorylation level of Src at Tyr416 (Fig.  6A). Interestingly, matrix mineralization was inhibited by Src activation in the AMBN-overexpressing cells (Fig. 6B). Further, RUNX2, ALP, and BSP were downregulated by Src activation in these cells (Fig. 6C). These findings indicated that the AMBN-CD63 axis inhibited Src activation, which in turn promoted mineralization and osteogenic differentiation.
We examined the effects of AMBN and/or CD63 in a normal mouse osteoblastic cell line, MC3T3-E1. CD63 was expressed in osteosarcoma cell lines but not in the normal mouse osteoblastic cell lines ST2 and MC3T3-E1 (Fig. 7A). We transiently transfected AMBN and/or CD63 into MC3T3-E1 cells (Fig.  7B). Although mineral deposition was not remarkably pro- FIG. 5. AMBN induced osteoblast mineralization via CD63 and Src interaction. (A) After suspension culture with or without CD63 blocking antibody (ab), lysates of control and AMBN-overexpressing cells plated on COL 1-coated dish for 60 min were immunoblotted with anti-phospho-Tyr416. The asterisk indicates IgG of CD63 antibody, and the arrowhead indicates Src phosphorylation at Tyr416 (Src-pY416). ␤-Actin was used as a loading control. (B) SaOS-2 cells were cultured for 10 days after presuspension at 37°C for 2 h in the presence of 50 g/ml ascorbic acid and 5 mM sodium ß-glycerophosphate with or without CD63 blocking antibody and conditioned medium (CM) from control or AMBN-overexpressing cells. ALZ staining was performed to visualize mineral deposition. ALZ was destained by using 10% (wt/vol) cetylpyridinium chloride, and the concentrations were determined by measuring the absorbance at 562 nm. The values are expressed as the means Ϯ standard deviations (n ϭ 3). moted by ectopic AMBN and AMBN/CD63 (data not shown), ectopic expression of AMBN upregulated RUNX2, BSP, and OCN, and ectopic expression of AMBN/CD63 upregulated BSP (Fig. 7C).

DISCUSSION
AMBN is an ameloblast-specific glycoprotein, and AMBN is localized to human chromosome 4q21 within the autosomal dominant amelogenesis imperfecta locus (18), suggesting that this protein is important for enamel formation. AMBN is present in the secretory stage of enamel formation, as demonstrated by immunostaining of ameloblastin (21). It has recently been revealed that AMBN is also expressed in osteoblasts during craniofacial skeletal development and in an osteosarcoma cell line, SaOS-2, cultivated on biosilica matrices (19,25). However, the function of AMBN in osteoblasts is unknown. Here, we found that AMBN promoted mineralization through the inhibition of Src activation in osteosarcoma. This is the first report of the function of AMBN in osteogenesis. We found an osteosarcoma cell line with high expression of AMBN. Interestingly, knockdown of AMBN inhibited the formation of mineralization deposits in this cell line. In fact, AMBN was found to induce osteogenic differentiation and promote mineralization by ectopic AMBN overexpression and recombinant human AMBN treatment. In particular, RUNX2, ALP, and BSP mRNA expression was upregulated by AMBN overexpression and downregulated by AMBN knockdown. RUNX2, ALP, and BSP are osteogenic markers and enhance osteogenic differentiation and matrix mineralization (1, 8).
Therefore, we suggest that increased expression of RUNX2, ALP, and BSP by AMBN overexpression may promote osteogenic differentiation. AMBN, a 65-to 70-kDa protein, is rapidly processed into several lower-molecular-mass proteins ranging from 52 kDa to 13 kDa (21). AMBN is cleaved by MMP-20 (enamelysin), which is an ameloblast-specific gene (10). The cells of bone lineage do not express any traces of MMP-20 (data not shown). Defects of MMP-20 function cause autosomal-recessive amelogenesis imperfecta (20). In this study, we found that secreted AMBN protein was cleaved in the conditioned medium from AMBN-overexpressing osteoblastic cells (Fig. 3C). It is likely that AMBN is cleaved by proteases other than MMP-20 in osteosarcoma cells. Moreover, osteogenic differentiation mediated by AMBN may be promoted by cleaved functional AMBN fragments in osteosarcoma.
Although AMBN has been shown to interact with CD63 (29), the physiological role of this interaction is still unclear. As we found an interaction between AMBN and CD63 in osteosarcoma cells, the function of this interaction in osteogenesis was examined. In osteosarcoma cells with AMBN expression, CD63 blocking antibody suppressed mineralization. Previous studies have shown that cell surface CD63 forms a complex with integrins (2) and that CD63 binds with Src and increases the phosphorylation of Src at Tyr416 (15). In this study, we found that CD63 bound to Src and integrin ␤1 in the presence of AMBN. Interestingly, AMBN stimulation inhibited Src activation, and AMBN-promoted osteogenic differentiation and mineralization were disrupted by constitutively active Src over- expression. Indeed, AMBN knockdown suppressed osteogenic differentiation through Src activation ( Fig. 2A and B and 4D). Moreover, treatment with CD63 blocking antibody or integrin ß1 blocking antibody induced Src activation ( Fig. 5A to D).
These findings suggest that the AMBN-CD63-integrin axis may promote osteogenesis through Src inactivation (Fig. 8). Previous reports have demonstrated that Src-deficient mice develop osteopetrosis by accelerated osteogenic differentiation (17,24)  and that osteoblast differentiation mediated by the reduction of Src kinase activity is regulated by osteocalcin expression via the interaction between RUNX2 and Yes-associated protein (33). Thus, our findings provide evidence for the novel mechanism of AMBN-mediated osteogenic differentiation. Therefore, AMBN can be a therapeutic modality for bone regeneration. Osteosarcoma cells present osteoblastic differentiation and form tumoral bone in a manner similar to that of osteoblasts. In the present study, CD63 was expressed in osteosarcoma cell lines but not in the normal mouse osteoblastic cell lines ST2 and MC3T3-E1 (Fig. 7A). CD63 is reportedly expressed in human mesenchymal stem cells undergoing osteogenic differentiation (6). As neoplastic cells have a potential to undergo divergent differentiation, CD63 expression may be observed in osteosarcoma cells. The effects of AMBN on osteoblasts may be regulated by spatiotemporal interactions between AMBN and CD63 during bone development and the bone-healing process. In fact, ectopic expression of AMBN upregulated RUNX2, BSP, and OCN, and ectopic expression of AMBN/CD63 upregulated BSP in a normal mouse osteoblastic cell line, MC3T3-E1 (Fig. 7C). However, mineral deposition was not remarkably promoted by ectopic AMBN and AMBN/CD63 in MC3T3-E1 cells (data not shown). These findings suggest that additional factors may be required for AMBN-promoted osteogenic differentiation in normal osteoblasts.
In conclusion, our study has revealed an important role of AMBN in osteogenic differentiation via the AMBN-CD63integrin-Src axis. Our findings may help to clarify the pathway involved in osteogenesis.