1 INSERM U 606, Lariboisière Hospital, 2 rue Ambroise Paré, 75010 Paris, Université Paris 7, Paris, France
2 LEDAC, UMR CNRS/UJF 5538, Institut Albert Bonniot, Faculté de Médecine, 38706 La Tronche CEDEX, France
* Author for correspondence (e-mail: pierre.marie{at}larib.inserm.fr)
Accepted 8 December 2004
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Summary |
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Key words: Osteoblast, 5 integrin, FGFR2, Cbl, Apoptosis, Ubiquitination
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Introduction |
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Integrin receptors are coupled to growth factor receptors and thereby regulate multiple biological functions (Miranti and Brugge, 2002). Convergence of integrin and growth factor signaling pathways have been described in focal adhesion complexes (Plopper et al., 1995
). Integrins were shown to collaborate with growth factors in triggering tyrosine phosphorylation of growth factor receptors such as epidermal growth factor, platelet derived growth factor and fibroblast growth factor (FGF) receptors (FGFRs) (Miyamoto et al., 1996
). FGFs bind to and activate FGFRs, which are members of a family of tyrosine kinase receptors (Jaye et al., 1992
; Schlessinger, 2000
). FGFR activation leads to intracellular signaling and activation of genes involved in cell proliferation, migration, differentiation and survival. In bone, FGF/FGFR signaling plays an important role in regulating osteoblasts and osteogenesis (Hurley et al., 2002
; Marie, 2003
). This is exemplified by the observation in humans that constitutive activation of FGFR2 by genetic mutation activates several signaling pathways that result in increased osteoblast differentiation (Lomri et al., 1998
; Lemonnier et al., 2001a
; Ornitz and Marie, 2002
) and apoptosis (Mansukhani et al., 2000
; Lemonnier et al., 2001b
). The increased osteoblast gene expression induced by FGFR2 involves increased expression of the adhesion molecule N-cadherin, revealing a link between FGFR2 signaling, cell-cell adhesion and osteoblast gene expression (Lemonnier et al., 2001a
). The role of FGFR signaling in integrin expression and cell attachment in osteoblasts has however not been characterized.
The ubiquitin ligase Cbl acts as an adaptor protein that is phosphorylated and recruited to activated receptor tyrosine kinases (reviewed by Sanjay et al., 2001). Cbl downregulates receptor tyrosine kinases by mediating ubiquitination resulting in proteasome-mediated degradation of these molecules after ligand binding. The mechanism by which Cbl negatively regulates these molecules involves Cbl recruitment, which allows polyubiquitination of activated receptors such as EGF, PDGF and FGFR (Joazeiro et al., 1999
; Levkowitz et al., 1999
; Yokouchi et al., 1999
). This involves the phosphotyrosine-binding (PTB) domain of Cbl, which binds to the activated receptor, and the RING domain that is required for ubiquitination (Yokouchi et al., 1999
; Thien and Langdon, 2001
). Cbl is also known to interact with receptor tyrosine kinase associated proteins such as Src proteins, and to regulate Src activity (Andoniou et al., 2000
; Sanjay et al., 2001
). We recently showed that constitutive FGFR2 activation in human osteoblasts results in Cbl-mediated Src protein ubiquitination by the proteasome, which contributes to the increased osteoblast differentiation induced by FGFR2 activation (Kaabeche et al., 2004
). The role of Cbl in integrin regulation in osteoblasts is at present unknown.
In the present study, we show that FGFR2 associates with Cbl and the 5 integrin subunit to promote Cbl-mediated ubiquitination of
5, resulting in reduced cell attachment and osteoblast apoptosis. This establishes for the first time a functional link between FGFR2 signaling and Cbl-mediated integrin expression resulting in altered cell attachment and apoptosis in osteoblasts.
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Materials and Methods |
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Cell adhesion
For the dose-dependent attachment assay on fibronectin, a titration curve of human fibronectin was used at concentrations ranging from 0 to 0.15 µg/ml coated on 96-well plates. The non-adhesive surface was blocked using 3% BSA in PBS for 1 hour at room temperature. FGFR2 mutant and control cells (50,000 cells) were allowed to adhere for 1 hour at 37°C in DMEM serum-free medium. Wells were extensively washed in PBS and stained with Crystal Violet. Dye was homogenized using acetic acid (0.1%) and OD at 570 nm was measured using a plate reader. For the time-course attachment assay, FGFR2 mutant and control cells were plated at 100,000 cells/well in six-well plates coated with fibronectin or type I collagen (BioCat Cellware, Beckton Dickinson, Le Pont de Claix, France) and cultured for 30, 60, 120 or 240 minutes in DMEM with 10% FCS. Adherent cells were collected using trypsin/EDTA (0.1%) and counted using a Coulter counter ZM (Beckman-Coulter).
Plasmids and transfection
FGFR2 mutant and control cells plated at 2500 cells/cm2 the day before transfection were co-transfected with the plasmid (2.5 µg/cm2 dish) and pSV-ß-galactosidase (50 ng ß-gal) control vector (Promega) in DMEM with 1% FCS. Cells were incubated with empty vector (pcDNA3) or 5 integrin, 70Z-Cbl or Cbl-G306E vectors and Exgen 500 (Euromedex) according to the manufacturer's directions. Efficiency of transfection was controlled by determination of ß-gal activity (ß-gal reporter gene assay, Roche). The number of ß-gal+ cells was counted 24 hours post-transfection. For the cell adhesion assay after
5 integrin transfection, FGFR2 mutant cells plated on fibronectin-coated plates were transfected with the
5 plasmid or the empty vector and cultured for 48 hours before counting as described above.
Inhibition of proteasome
To determine whether 5 integrin subunit is degraded by the proteasome, FGFR2 mutant cells were treated with 10 µM lactacystin (Calbiochem), a specific proteasome inhibitor that binds covalently to the active-site N-terminal threonine residue in proteasome ß-subunits (Fenteany and Schreiber, 1998
). The cells were treated for 24 hours with lactacystine or solvent and
5 protein levels were determined by immunoprecipitation and western blot analysis.
RT-PCR analysis
The expression of integrin transcripts was examined in FGFR2 mutant and control cells by reverse-transcription polymerase chain reaction (RT-PCR) analysis. Confluent cells were washed with PBS, and total cellular RNA was extracted using the Extract-All reagent (Eurobio, France) according to the manufacturer's protocol. 3 µg total RNA were reverse transcribed at 37°C for 1 hour. cDNA samples were amplified (30 cycles for integrins, 23 cycles for GAPDH) using specific primers (Table 1). Southern blots were performed by running aliquots of amplified cDNAs on 1% agarose gels followed by transfer onto nylon membranes (Appligene-Oncor) according to the manufacturer's protocol. Hybridization of blots was carried out overnight at 50°C with [-32P]ATP-labelled internal antisense primers. Membranes were then washed and exposed to Kodak X-ray films at 80°C with intensifying screens and the signal for each gene was related to that of GAPDH.
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Immunohistochemistry, immunoprecipitation and western blot analysis
For immunohistochemistry, coronal sutures obtained from a normal fetus were fixed in formaldehyde, dehydrated in ethanol and embedded in paraffin as described previously (Lemonnier et al., 2000) and used for immunohistochemical detection of the
5 integrin subunit. Non specific sites were saturated using serum (10% FCS) for 24 hours at 4°C, then the sections were incubated with anti-
5 antibody or non-specific goat serum (1:1000), and the signal was amplified and revealed using the golden bead amplification system described previously (Lemonnier et al., 2000
).
For immunocytochemistry, FGFR2 mutant and control cells were plated on uncoated glass coverslips overnight in DMEM with 10% FCS and immunostaining was performed essentially as described previously (Fournier et al., 2002) using antibodies against Cbl, FGFR2 and
5 integrin (Santa Cruz Biotechnology, Santa Cruz, CA) diluted 1:1000.
Western blot analysis of integrins in confluent FGFR2 mutant and control cells cultured on fibronectin-coated wells was performed. Cell extracts were scraped in ice-cold lysis buffer containing protease inhibitors (Boehringer, Mannheim, Germany). Lysates were clarified by centrifugation at 12,000 g for 10 minutes at 4°C. The protein content of the resulting supernatants was determined by the DC Protein assay (BioRad Laboratories, CA). Proteins were then resolved by 6% SDS-PAGE and transferred onto PVDF Hybond-P membranes (Amersham). The membranes were incubated overnight with 10% blocking solution (Roche) and then with goat anti-5 or anti-
v, mouse anti-ß1 antibodies (Santa Cruz), or ß-actin (Sigma) diluted 1:500. Membranes were washed and incubated with specific peroxidase-coupled appropriate secondary antibodies and the signal was visualized using a chemiluminescence detection system (Perbio-Science, Erembodegem, Belgium).
For immunoprecipitation analysis, equal aliquots (300 µg) of protein lysates were immunoprecipitated using 2.5 µg specific antibodies (anti-5, anti-Cbl or anti-ubiquitin; Santa Cruz) and incubated overnight at 4°C in a rotating device. After 24 hours, 20 µl protein A/G agarose (Santa Cruz) were added and incubated for 1 hour at 4°C. Immunoprecipitates were then collected by centrifugation at 1200 g for 3 minutes, the pellets were washed four times with lysis buffer, and resuspended in 50 µl running buffer. Aliquots were then subjected to electrophoresis as described above, and membranes were reacted with
5, Cbl, ubiquitin, Bax or Bcl-2 antibodies (Santa Cruz). Immunoblots were probed with peroxidase-coupled specific secondary antibodies as indicated, and visualized by enhanced chemiluminescence.
Caspase activity
Caspase-3, -6, -7 and -9 activities were determined essentially as described with minor modifications (Lemonnier et al., 2001b).
Data analysis
The results presented are representative of three to four different experiments. Differences between the mean±s.e.m. were analysed using the statistical package super-ANOVA (Macintosh, Abacus concepts, Berkeley, CA) with a minimal significance of P<0.05.
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Results |
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As cell-matrix interactions are mediated by integrins, we investigated whether the reduced cell attachment induced by FGFR2 activation in osteoblasts was associated with alteration of specific integrins. We restricted analysis to the most important integrins (5,
V, ß1) expressed in vivo in human osteoblasts and that mediate cell adhesion on fibronectin or type I collagen. Analysis at the translational level by western blotting showed that
5 protein levels were lower in FGFR2 mutants compared to control cells (Fig. 2A). In contrast,
v and ß1 protein levels were higher in mutant cells compared to control cells cultured in basal culture conditions, indicating that FGFR2 activation induces a specific decrease in
5 integrin subunit. Analyses carried out at the transcriptional (reverse transcription-polymerase chain reaction) level showed that mRNA levels for
5,
v and ß1 integrin subunits did not differ markedly in FGFR2 mutant compared to control cells (Fig. 2B), suggesting that a post-transcriptional mechanism is responsible for the reduced
5 protein levels induced by FGFR2 activation. To validate the in vivo relevance of these in vitro data, we examined expression of the
5 integrin subunit by immunohistochemical analysis of the cranial suture. Immunoreactivity of
5 integrin was detected in osteoblasts along the bone matrix in the cranial suture, illustrating the in vivo expression of this integrin subunit in normal human cranial bone suture (Fig. 2C). Control sections showed no specific staining. These results show that the
5 integrin subunit is expressed by osteoblasts in vitro and in vivo in human calvarial osteoblasts and that FGFR2 activation reduces
5 expression at the protein, but not the RNA level in these cells.
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As human calvarial osteoblasts express FGFR2 in vitro and in vivo (Lemonnier et al., 2000; Kaabeche et al., 2004
) and FGFR2 activation reduces
5 integrin subunit expression (Fig. 2A), we investigated whether FGFR2 might interact with
5 integrin in mutant osteoblasts. Immunoprecipitation analysis showed that
5 integrin co-immunoprecipitated with FGFR2 in control and FGFR2 mutant osteoblasts cultured on fibronectin (Fig. 3A). FGFR2 coimmunoprecipitated with
5 integrin in control and FGFR2 mutant osteoblasts, confirming the interaction between the two proteins (Fig. 3B). Both FGFR2 levels and
5 integrin levels were reduced in mutant cells, confirming the reduced
5 integrin expression (Fig. 2A) and the reduced FGFR2 expression in mutant osteoblasts (Kaabeche et al., 2004
). To confirm that FGFR2 interacts with
5 integrin in osteoblasts at the cellular level, we performed an immunocytochemical analysis and found that FGFR2 mutant osteoblasts displayed lower adhesion and reduced cell spreading compared to control cells (Fig. 3C). Consistent with the biochemical results, the immunocytochemical analysis showed decreased
5 integrin levels in mutant cells. Moreover, we found that
5 integrin colocalized with FGFR2 at the leading edge of the cell in membrane ruffle regions (Fig. 3C), confirming the immunoprecipitation analysis. Overall, these data strongly indicate that
5 integrin subunit interacts with FGFR2 in human calvarial osteoblasts.
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As 5 integrin mRNA levels are unchanged in FGFR2 mutant osteoblasts (Fig. 2B), we postulated that the reduced expression of the integrin might result from protein downregulation induced by FGFR2 activation. Ubiquitin-dependent proteasome degradation is an important process involved in protein downregulation. We therefore investigated whether the decreased
5 integrin levels in FGFR2 mutant cells may result from ubiquitin-mediated proteasome degradation. Treatment with the specific proteasome inhibitor lactacystin (10 µM) increased
5 integrin to normal levels in mutant osteoblasts and actually restored normal
5 integrin levels (Fig. 4A). This strongly suggests that the decreased expression of
5 integrin induced by FGFR2 activation results from increased proteasome-mediated degradation. The E3 ubiquitin ligase Cbl is known to interact with several molecules such as Src and tyrosine kinase receptors, resulting in their ubiquitination and proteasome degradation (Sanjay et al., 2001
). Our recent data showed that FGFR2 activation induces Cbl recruitment, which contributes to Fyn, Lyn and FGFR2 degradation in osteoblasts (Kaabeche et al., 2004
). We therefore hypothesized that Cbl recruitment induced by FGFR2 activation might also mediate
5 integrin proteasome degradation in mutant osteoblasts. As shown in Fig. 4B, immunoprecipitation analysis revealed that the ubiquitin ligase Cbl immunoprecipitated with
5 integrin in mutant cells, indicating an interaction between the two proteins. Activation of Cbl results in recruitment of ubiquitin, resulting in proteasome degradation. We therefore investigated whether
5 integrin can interact with ubiquitin in mutant osteoblasts and found that
5 integrin immunoprecipitates with ubiquitin, indicating that ubiquitin is associated with
5 integrin in this context (Fig. 4C). Moreover, the level of
5 integrin associated with ubiquitin was higher in FGFR2 mutant cells compared to control cells, suggesting increased ubiquitin-
5 integrin recruitment in mutant osteoblasts. To confirm the interaction between
5 integrin and Cbl, we performed an immunocytochemical analysis. We found that Cbl colocalized with
5 integrin at the leading edge of the cells, further indicating interactions between the two proteins (Fig. 5). Overall, these results show that
5 integrin subunit interacts with both FGFR2 and the ubiquitin ligase Cbl and that the
5 integrin subunit is targeted to proteasome degradation once associated with Cbl in mutant osteoblasts.
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The RING finger of Cbl directs recruitment of the ubiquitin system and is important for protein ubiquitination induced by tyrosine kinase receptors (Waterman et al., 1999; Yokouchi et al., 1999
). To determine whether Cbl-mediated
5 integrin ubiquitination in FGFR2 mutant osteoblasts is dependent on the RING domain of Cbl, FGFR2 mutant cells were transfected with 70Z-Cbl in which the RING finger is disrupted (Yokouchi et al., 2001
) and changes in
5 integrin protein levels were determined. Transfection of FGFR2 mutant osteoblasts with 70Z-Cbl resulted in increased
5 integrin protein levels in mutant cells, which were restored to control levels, as revealed by immunoprecipitation analysis (Fig. 6). This reflects the inhibitory effect of 70Z-Cbl on ubiquitin recruitment and proteasome degradation of the
5 integrin. Moreover, this shows that the increased Cbl-dependent
5 integrin degradation induced by FGFR2 activation requires the presence of the RING finger domain of Cbl. The PTB domain in the N-terminal half of Cbl is another domain of Cbl that binds to activated tyrosine kinase receptors. We investigated the role of the Cbl PTB domain in Cbl-mediated
5 integrin degradation induced by FGFR2 activation in osteoblasts. Transfection of FGFR2 mutant cells with the Cbl G306E mutant in which a point mutation abolishes the binding ability of the Cbl PTB domain to the activated receptor increased
5 integrin protein levels in FGFR2 mutant cells (Fig. 6). This indicates that Cbl-mediated ubiquitination of
5 integrin is dependent on interaction with the PTB domain of Cbl which interacts with the activated FGFR2.
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The above results indicate that FGFR2 activation reduces 5 integrin levels and osteoblast attachment as a consequence of Cbl-dependent degradation by the proteasome. We therefore assessed the functional implication of the observed
5 integrin downregulation induced by FGFR2 activation in osteoblasts. To this aim, FGFR2 mutant cells were transfected with an
5 integrin plasmid in order to rescue
5 integrin expression, and changes in
5 integrin levels and cell attachment on fibronectin were determined. Transfection of FGFR2 mutant osteoblasts with the
5 plasmid resulted in increased
5 integrin levels upon immunoprecipitation analysis (Fig. 7A). Interestingly, forced expression of
5 integrin in FGFR2 mutant cells fully rescued the defective cell attachment on fibronectin (Fig. 7B). This indicates that the reduced osteoblast attachment induced by FGFR2 activation is functionally related to
5 integrin downregulation.
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Alteration of integrin-mediated cell attachment is associated with apoptosis in anchorage-dependent cells (Frisch and Screaton, 2001). Our previous data indicate that FGFR2 activation results in caspase-dependent apoptosis in human calvarial osteoblasts (Lemonnier et al., 2001b
). We therefore investigated the implication of
5 integrin in caspase-dependent apoptosis induced by FGFR2 activation. To this goal, we examined the changes in Bax, a pro-apoptotic protein that is known to be involved in caspase-dependent apoptosis (Reed et al., 1996
). Western blot analysis after immunoprecipitation showed a marked increase in Bax levels in FGFR2 mutant cells compared to control cells (Fig. 8A). Transfection of FGFR2 mutant osteoblasts with
5 integrin plasmid abolished the increased Bax levels in mutant cells, indicating that restoration of
5 integrin levels corrected the abnormal Bax levels in FGFR2 mutant osteoblasts. We also looked at Bcl-2 levels that are known to be involved in the protection against apoptosis mediated by
5ß1 integrin in other cells (Zhang et al., 1995
). Bcl-2 levels were slightly decreased in FGFR2 mutant osteoblasts compared to control cells, and forced expression of
5 integrin rescued Bcl-2 protein levels in mutant cells (Fig. 8B). These results indicate that the decreased
5 integrin expression can largely account for the alteration in Bax/Bcl-2 levels induced by FGFR2 activation in osteoblasts.
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To further investigate the downstream mechanisms by which decreased 5 integrin expression may mediate apoptosis in mutant osteoblasts, we performed biochemical analyses to determine caspase-9 activity that is altered by changes in Bax/Bcl-2 ratio during apoptosis (Reed et al., 1996
). Caspase-9 activity was increased in FGFR2 mutant osteoblasts compared to control cells, and transfection with the
5 integrin plasmid decreased caspase-9 activity which was restored to normal levels (Fig. 8C). To confirm the implication of
5 integrin in osteoblast apoptosis induced by FGFR2 activation, we investigated the changes in caspase-3, a key caspase involved in DNA fragmentation that is activated by caspase-9 (Reed et al., 1996
). Caspase-3 activity was increased in FGFR2 mutant cells and transfection with the
5 integrin plasmid abolished the increased caspase-3 activity induced by FGFR2 activation (Fig. 8D). We conclude that rescue of
5 integrin expression in FGFR2 mutant osteoblasts restores cell attachment, corrects Bax/Bcl-2 to normal levels and abolishes the increased caspase-9 and caspase-3 activity induced by FGFR2 activation. This supports a functional role for
5 integrin downregulation in cell detachment and apoptosis induced by FGFR2 activation in osteoblasts.
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Discussion |
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In other cell types, 5ß1 expression is regulated by several molecular mechanisms such as transcriptional activation (Delcommenne and Streuli, 1995
), mRNA stability (Xu and Clark, 1996
) and translational control (Harwood et al., 1999
). In the present study, we show that
5 integrin mRNA levels were unchanged whereas
5 protein levels were decreased in FGFR2 mutant osteoblasts, suggesting that FGFR2 activation results in
5 integrin degradation. Ubiquitin-mediated proteasome degradation is an important mechanism controlling the degradation of many proteins (Hochstrasser, 1995
). Our finding that specific inhibition of proteasome activity by lactacystin rescued
5 protein levels strongly indicates that
5 integrin downregulation induced by FGFR2 activation occurs through proteasome degradation. Ubiquitin-dependent degradation of proteins involves the ubiquitination of the target protein followed by its degradation by the proteasome (Ciechanover, 1998
). The ubiquitin ligase Cbl plays a major role in protein degradation through the proteasome pathway (Sanjay et al., 2001
). Here we show that the
5 integrin subunit interacts with both the ubiquitin ligase Cbl and ubiquitin in FGFR2 mutant osteoblasts, which indicates that FGFR2 activation induces Cbl-mediated
5 integrin recruitment, ubiquitination and subsequent degradation via the proteasome. Our finding that transfection with 70Z-Cbl, which lacks the RING domain required for Cbl interaction with ubiquitin restored
5 integrin protein levels, further indicates that Cbl-mediated ubiquitination plays an essential role in proteasomal degradation of the
5 integrin subunit. Because the G306E Cbl mutant, that inactivates the PTB domain of Cbl, rescued
5 protein expression in FGFR2 mutant cells, it appears that the PTB domain of Cbl is involved in the Cbl-mediated downregulation of the
5 subunit in response to ligand-independent, constitutive activation of FGFR2. These data indicate that constitutive FGFR2 activation in osteoblasts activates the ubiquitin ligase activity of Cbl, resulting in ubiquitination and proteasome degradation of the
5 subunit integrin. This provides a Cbl-dependent mechanism by which the
5 integrin protein is downregulated in response to activation of FGFR2 in osteoblasts.
Loss of cell attachment to the extracellular matrix triggers apoptosis through several signaling mechanisms in anchorage-dependent cells (Frisch and Screaton, 2001). It was therefore of interest to determine if the Cbl-dependent downregulation of
5 integrin and subsequent reduction in cell attachment may contribute functionally to osteoblast apoptosis induced by FGFR2 activation. One mechanism of apoptosis involves alteration of Bax/Bcl-2, which triggers cytochrome c release from the mitochondria and activates caspase-9 (Reed et al., 1996
). We found that the decreased expression of
5 integrin subunit in FGFR2 mutant osteoblasts was associated with an increased Bax/Bcl-2 ratio and increased caspase-9 and caspase-3 activities, indicating that the alteration of
5 integrin-mediated cell attachment triggers caspase-dependent apoptosis in osteoblasts. This is consistent with the finding in other cells that the
5ß1 integrin supports cell survival on fibronectin by increasing Bcl-2 protein transcription (Zhang et al., 1995
; Matter and Ruoslahti, 2001
). Our finding that forced expression of
5 integrin restored the Bax/Bcl-2 ratio and corrected caspase-9 and caspase-3 activities in FGFR2 mutant osteoblasts strongly indicates that the selective alteration of
5 integrin induced by FGFR2 activation governs apoptotic signals in osteoblasts through Bax/Bcl-2 and activation of the caspase-9-caspase-3 cascade. Thus, Cbl-mediated ubiquitination of the
5 integrin subunit appears to play a major role in the induction of apoptosis induced by FGFR2 activation in osteoblasts. Phosphatidylinositol 3-kinase (PI3K) was shown to be involved in cell survival mediated by
5ß1 expression in epithelial cells (Lee and Juliano, 2000
). It is interesting to note that activated Cbl forms complexes with PI3K following integrin-mediated cell adhesion (Ojaniemi et al., 1997
; Zell et al., 1998
) and is involved in PI3K-dependent cell signaling (Meng and Lowell, 1998
; Anzai et al., 1999
; Finkelstein and Shimizu, 2000
). In bone, we recently showed that FGF2 induces cell survival through PI3K signaling in human calvarial osteoblasts (Debiais et al., 2004
). The elucidation of the role of this and other signaling molecules acting downstream of FGFR2-Cbl-
5 integrin to trigger osteoblast apoptosis may have important implications with regard to the control of osteogenesis by FGFR signaling.
In summary, our data indicate that Cbl recruitment induced by FGFR2 activation triggers 5 integrin proteasome degradation, which results in reduced osteoblast attachment on fibronectin- and caspase-dependent apoptosis. This supports a functional role of the
5 integrin subunit in the induction of apoptosis triggered by FGFR2 activation. Furthermore, the data point to a novel Cbl-dependent mechanism involved in the coordinate regulation of cell apoptosis induced by
5 integrin degradation in response to FGFR2 signaling in osteoblasts.
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Acknowledgments |
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