The role(s) of Src kinase and Cbl proteins in the regulation of osteoclast differentiation and function

FGFR2-Cbl interaction in lipid rafts triggers attenuation of PI3K/Akt signaling and osteoblast survival

Fibroblast growth factor receptor (FGFR) signaling plays an important role in skeletogenesis. The molecular mechanisms triggered by activated FGFR in bone forming cells are however not fully understood. In this study, we identify a role for phosphatidylinositol 3-kinase (PI3K) signaling in cell apoptosis induced by FGFR2 activation in osteoblasts. We show that FGFR2 activation leads to decrease PI3K protein levels, resulting in attenuation of PI3K signaling in human osteoblasts. Biochemical and molecular analyses revealed that the attenuated PI3K signaling induced by FGFR2 activation is due to increased Cbl-PI3K molecular interaction mediated by the Cbl Y731 residue, which results in increased PI3K ubiquitination and proteasome degradation. Biochemical and immunocytochemical analyses showed that FGFR2 and Cbl interact in raft micro-domains at the plasma membrane. FGFR2 activation increases FGFR2 and Cbl recruitment in micro-domains, resulting in increased molecular interactions. Consistently, functional analyses showed that the attenuation of PI3K/Akt signaling triggered by FGFR2 activation results in increased osteoblast apoptosis. These results identify a functional molecular mechanism by which activated FGFR2 recruits Cbl in raft micro-domains to trigger PI3K ubiquitination and proteasome degradation, and reveal a novel role for PI3K/Akt attenuation in the control of osteoblast survival by FGFR2 signaling.

Podosome-type adhesions and focal adhesions, so alike yet so different

Cell-matrix adhesions are essential for cell migration, tissue organization and differentiation, therefore playing central roles in embryonic development, remodeling and homeostasis of tissues and organs. Matrix adhesion-dependent signals cooperate with other pathways to regulate biological functions such as cell survival, cell proliferation, wound healing, and tumorigenesis. Cell migration and invasion are integrated processes requiring the continuous, coordinated assembly and disassembly of integrin-mediated adhesions. An understanding of how integrins regulate cell migration and invasiveness through the dynamic regulation of adhesions is fundamental to both physiological and pathological situations. A variety of cell-matrix adhesions has been identified, namely, focal complexes, focal adhesions, fibrillar adhesions, podosomes, and invadopodia (podosome-type adhesions). These adhesion sites contain integrin clusters able to develop specialized structures, which are different in their architecture and dynamics although they share almost the same proteins. Here we compare recent advances and developments in the elucidation of the organization and dynamics of focal adhesions and podosome-type adhesions, in order to understand how such subcellular sites - though closely related in their composition - can be structurally and functionally different. The underlying question is how their respective physiological or pathological roles are related to their distinct organization.

A quantitative study of the recruitment potential of all intracellular tyrosine residues on EGFR, FGFR1 and IGF1R

Receptor tyrosine kinases transmit and process extracellular cues by recruiting intracellular signaling proteins to sites of tyrosine phosphorylation. Using protein microarrays comprising virtually every human SH2 and PTB domain, we generated quantitative protein interaction maps for three well-studied receptors--EGFR, FGFR1 and IGF1R--using phosphopeptides derived from every intracellular tyrosine residue on each receptor, regardless of whether or not they are phosphorylated in vivo. We found that, in general, peptides derived from physiological sites of tyrosine phosphorylation bind to substantially more SH2 or PTB domains than do peptides derived from nonphysiological sites, supporting the idea that kinases and interaction domains co-evolve and suggesting that new sites arise predominantly through selection favoring advantageous interactions, rather than through selection disfavoring unwanted interactions. We also found substantial qualitative overlap in the recruitment profiles of these three receptors, suggesting that their different biological effects arise, at least in part, from quantitative differences in their affinities for the proteins they recruit.

Functional grouping of osteoclast genes revealed through microarray analysis

We describe for the first time functional clusters of genes that are modulated during the differentiation of osteoclasts. Pathway analysis was applied to gene array data generated from affymetrix chips hybridized to RNA isolated from RAW264.7 cells exposed to RANK-ligand (RANK-L) for 5 days. This analysis revealed major functional gene clusters that were either up- or down-regulated during osteoclastogenesis. Some of the genes within the clusters have known functions, while others do not. We discuss herein the relevance of these functional gene clusters and their modulation to biological processes underlying the formation, function, and fate of osteoclasts.

Calcitonin induces podosome disassembly and detachment of osteoclasts by modulating Pyk2 and Src activities

Osteoclasts (OCs) attach to the extracellular matrix via specialized attachment structures called podosomes, which form a prominent F-actin-rich ring that is thought to correspond to the sealing zone of resorbing OCs. Calcitonin (CT), a 32-amino acid polypeptide, inhibits bone resorption by decreasing motility, inducing retraction, disassembling podosome, and disrupting the actin-ring structure of OCs. However, the detailed mechanisms of how CT induces the disassembly of podosome and disruption of the adhesive structures in OCs are not well characterized. Pyk2 localizes in the sealing zone of OCs. It is activated by ligation of integrins, and then activates Src, an important signaling molecule for bone resorption. Thus, the Pyk2/Src complex in podosome could be a potential target for the CT-induced signaling. Using interference reflection, phase contrast, and confocal microscopy, CT effects on the dynamic changes of peripheral adhesive structure in living OCs were examined. CT induced dephosphorylation at Tyr(402) of Pyk2 and decreased its labeling at peripheral adhesion region, which would prevent formation of the Pyk2/Src complex in this region. CT induced increase of intracellular phosphorylation of Tyr(402) Pyk2 and increase of dephosphorylation at Tyr(527) of Src and Pyk2/Src colocalization in the central region of OCs. This evidence suggested that Src might function as an adaptor protein that competes for Pyk2 and relocates it from peripheral adhesive zone to the central region of OCs. In conclusion, CT may induce podosome reassembly and peripheral adhesive zone detachment by modulating Pyk2 and Src phosphorylation state and their intracellular distribution in OCs.

Identification of candidate molecular markers predicting sensitivity in solid tumors to dasatinib: rationale for patient selection

Dasatinib is a multitargeted kinase inhibitor that was recently approved for the treatment of chronic myelogenous leukemia and Philadelphia chromosome-positive acute lymphoblastic leukemia with resistance or intolerance to prior therapy. It is also in clinical trials for treating patients with solid tumors. The identification of molecular markers predictive of response to dasatinib could assist in clinical development by selecting patients most likely to derive clinical benefit. Using baseline gene expression profiling of a panel of 23 breast cancer cell lines, we identified genomic signatures highly correlated with in vitro sensitivity to dasatinib. The ability of these signatures to predict dasatinib sensitivity was further confirmed and validated in independent test cell lines. A six-gene model was used to correctly predict dasatinib sensitivity in 11 out of 12 (92%) additional breast and 19 out of 23 (83%) lung cancer cell lines. Quantitative real-time PCR and immunohistochemical assays further confirmed the differential expression pattern of selected markers. Finally, these gene signatures were observed in a subset of primary breast, lung, and ovarian tumors suggesting potential utility in patient selection. The subset of breast cancer patients expressing the dasatinib-sensitive signature includes a distinct clinical and molecular subgroup: the so-called "triple negative" (i.e., estrogen receptor-negative, progesterone receptor-negative, and HER2-negative) or "basal" breast cancer subtype. This patient population has a poor prognosis and currently has few effective treatment options. Our results implicate that dasatinib may represent a valuable treatment option in this difficult-to-treat population. To test this hypothesis, clinical studies are now under way to determine the activity of dasatinib in these patients.

The molecular understanding of osteoclast differentiation

Osteoclasts are multinucleated cells of monocyte/macrophage origin that degrade bone matrix. The differentiation of osteoclasts is dependent on a tumor necrosis factor (TNF) family cytokine, receptor activator of nuclear factor (NF)-kappaB ligand (RANKL), as well as macrophage colony-stimulating factor (M-CSF). Congenital lack of osteoclasts causes osteopetrosis, investigation of which has provided insights into the essential molecules for osteoclastogenesis, including TNF receptor-associated factor (TRAF) 6, NF-kappaB and c-Fos. In addition, genome-wide screening techniques have shed light on an additional set of gene products such as nuclear factor of activated T cells (NFAT) c1. Here we summarize the efforts to understand the sequential molecular events induced by RANKL during osteoclast differentiation. RANKL binds to its receptor RANK, which recruits adaptor molecules such as TRAF6. TRAF6 activates NF-kappaB, which is important for the initial induction of NFATc1. NFATc1 is activated by calcium signaling and binds to its own promoter, thus switching on an autoregulatory loop. An activator protein (AP)-1 complex containing c-Fos is required for the autoamplification of NFATc1, enabling the robust induction of NFATc1. Finally, NFATc1 cooperates with other transcriptional partners to activate osteoclast-specific genes. NFATc1 autoregulation is controlled by an epigenetic mechanism, which has profound implications for an understanding of the general mechanism of irreversible cell fate determination. From the clinical point of view, RANKL signaling pathway has promise as a strategy for suppressing the excessive osteoclast formation characteristic of a variety of bone diseases.

Activating and inhibitory functions of DAP12

When associated with different receptors, the signalling adaptor DAP12 has been shown to both potentiate and attenuate the activation of leukocytes. But how can a protein with a single signalling motif elicit qualitatively different cellular responses? We describe a model of DAP12 function, whereby the quality of the cellular response (activation or inhibition) is modulated by the avidity of the interaction between the DAP12-associated receptor and its ligand. This model extends from previous studies of inhibitory signalling mediated by other adaptors, such as the Fc-receptor gamma-chain and CD3zeta, and provides a potential mechanism for the conflicting phenotypes observed in studies of DAP12-deficient mice.

Intraarticular interferon-beta gene therapy ameliorates adjuvant arthritis in rats

Interferon (IFN)-beta has significant immunomodulatory properties and has received much interest as a potentially therapeutic agent for rheumatoid arthritis (RA). Systemic IFN-beta treatment of patients with RA was not effective, probably because of pharmacokinetic issues. Therefore, we studied the effect of local IFN-beta production by adenovirus-mediated gene transfer to the ankle joints of arthritic rats. Adjuvant arthritis (AA) in rats was used as a model to study intraarticular gene therapy with an adenoviral vector encoding the rat IFN-beta gene (Ad.IFN-beta). The effect on paw swelling was measured by water displacement plethysmometry. Synovial tissue of the hind paws was examined by immunohistochemistry. Bone destruction was analyzed on the basis of radiographs. In addition, quantitative real-time polymerase chain reaction was used to assess IFN-beta expression. Levels of IFN-beta mRNA and protein peaked 2 days after intraarticular injection and declined thereafter. Local delivery of Ad.IFN-beta after the onset of disease reduced paw swelling significantly. This was accompanied by a reduction in synovial inflammation. The clinical effects in rat AA lasted up to 9 days. Strikingly, Ad.IFN-beta treatment protected bone from erosion, reduced levels of c-Cbl and Cbl-b (both signaling molecules essential for osteoclast activity), and reduced the matrix metalloproteinase-3:tissue inhibitor of metalloproteinase-1 ratio in the joint. Immunohistochemical analysis of the synovial tissue revealed a clear shift toward a more antiinflammatory cytokine profile. Local overexpression of IFN-beta inhibits arthritis progression and protects against bone destruction in rat AA. These findings validate IFN-beta as a therapeutic molecule for intraarticular gene therapy of arthritis.

Down-regulation of ubiquitin ligase Cbl induced by twist haploinsufficiency in Saethre-Chotzen syndrome results in increased PI3K/Akt signaling and osteoblast proliferation

Genetic mutations of Twist, a basic helix-loop-helix transcription factor, induce premature fusion of cranial sutures in Saethre-Chotzen syndrome (SCS). We report here a previously undescribed mechanism involved in the altered osteoblastogenesis in SCS. Cranial osteoblasts from an SCS patient with a Twist mutation causing basic helix-loop-helix deletion exhibited decreased expression of E3 ubiquitin ligase Cbl compared with wild-type osteoblasts. This was associated with decreased ubiquitin-mediated degradation of phosphatidyl inositol 3 kinase (PI3K) and increased PI3K expression and PI3K/Akt signaling. Increased PI3K immunoreactivity was also found in osteoblasts in histological sections of affected cranial sutures from SCS patients. Transfection with Twist or Cbl abolished the increased PI3K/Akt signaling in Twist mutant osteoblasts. Forced overexpression of Cbl did not correct the altered expression of osteoblast differentiation markers in Twist mutant cells. In contrast, pharmacological inhibition of PI3K/Akt, but not ERK signaling, corrected the increased cell growth in Twist mutant osteoblasts. The results show that Twist haploinsufficiency results in decreased Cbl-mediated PI3K degradation in osteoblasts, causing PI3K accumulation and activation of PI3K/Akt-dependent osteoblast growth. This provides genetic and biochemical evidence for a role for Cbl-mediated PI3K signaling in the altered osteoblast phenotype induced by Twist haploinsufficiency in SCS.

Identification and functional characterization of an Src homology domain 3 domain-binding site on Cbl

Cbl is an adaptor protein and ubiquitin ligase that binds and is phosphorylated by the nonreceptor tyrosine kinase Src. We previously showed that the primary interaction between Src and Cbl is mediated by the Src homology domain 3 (SH3) of Src binding to proline-rich sequences of Cbl. The peptide Cbl RDLPPPPPPDRP(540-551), which corresponds to residues 540-551 of Cbl, inhibited the binding of a GST-Src SH3 fusion protein to Cbl, whereas RDLAPPAPPPDR(540-551) did not, suggesting that Src binds to this site on Cbl in a class I orientation. Mutating prolines 543-548 reduced Src binding to the Cbl 479-636 fragment significantly more than mutating the prolines in the PPVPPR(494-499) motif, which was previously reported to bind Src SH3. Mutating Cbl prolines 543-548 to alanines substantially reduced Src binding to Cbl, Src-induced phosphorylation of Cbl, and the inhibition of Src kinase activity by Cbl. Expressing the mutated Cbl in osteoclasts induced a moderate reduction in bone-resorbing activity and increased amounts of Src protein. In contrast, disabling the tyrosine kinase-binding domain of full-length Cbl by mutating glycine 306 to glutamic acid, and thereby preventing the previously described binding of the tyrosine kinase-binding domain to the Src phosphotyrosine 416, had no effect on Cbl phosphorylation, the inhibition of Src activity by full-length Cbl, or bone resorption. These data indicate that the Cbl RDLPPPP(540-546) sequence is a functionally important binding site for Src.

Association of leupaxin with Src in osteoclasts

Leupaxin (LPXN), which belongs to the paxillin extended family of adaptor proteins, was previously identified as a component of the sealing zone in osteoclasts. LPXN was found to associate with several podosomal proteins, such as the protein tyrosine kinase Pyk2, the protein-tyrosine phosphatase-PEST (PTP-PEST), actin-binding proteins, and regulators of actin cytoskeletal reorganization. It was previously demonstrated that inhibition of LPXN expression resulted in reduced osteoclast-mediated resorption. In the current study, overexpression of LPXN in murine osteoclasts resulted in both enhanced resorptive activity and cell adhesion, as assessed by in vitro resorption assays. The overexpression of LPXN resulted in an increased association of Pyk2 with LPXN. In an attempt to determine an additional biochemical basis for the observed phenomenon in increased osteoclast activity, a coimmunoprecipitation screen for additional binding partners revealed that Src, a protein tyrosine kinase that is critical to both podosome formation and osteoclast function, was also associated with LPXN. After exposure to the pro-inflammatory and osteoclastogenic cytokine TNF-alpha, there was an increase in the level of Src that coimmunoprecipitated with LPXN. Our data indicate that association of the scaffold protein LPXN with Src adds further complexity to the organization of the podosomal signaling complex in osteoclasts.

Molecular regulation of osteoclast activity

Osteoclasts are multinucleated cells derived from hematopoietic precursors that are primarily responsible for the degradation of mineralized bone during bone development, homeostasis and repair. In various skeletal disorders such as osteoporosis, hypercalcemia of malignancy, tumor metastases and Paget's disease, bone resorption by osteoclasts exceeds bone formation by osteoblasts leading to decreased bone mass, skeletal fragility and bone fracture. The overall rate of osteoclastic bone resorption is regulated either at the level of differentiation of osteoclasts from their monocytic/macrophage precursor pool or through the regulation of key functional proteins whose specific activities in the mature osteoclast control its attachment, migration and resorption. Thus, reducing osteoclast numbers and/or decreasing the bone resorbing activity of osteoclasts are two common therapeutic approaches for the treatment of hyper-resorptive skeletal diseases. In this review, several of the key functional players involved in the regulation of osteoclast activity will be discussed.

Osteoclastic differentiation and function regulated by old and new pathways

The osteoclast is a specialized multinucleated variant of the macrophage family. It degrades mineralized tissue, and is required for modeling and remodeling of bone. The osteoclast has long been known to require vitamin D for its differentiation and to be regulated by parathyroid hormone via circulating Ca(2+) levels. Two local signals important in osteoclast survival and differentiation, CSF-1 and RANKL, were characterized by the mid-1990 s. A basic framework of specialized cell attachment and resorption molecules was also clear by that time, including the alpha(v)beta(3) integrin, the key adhesion molecule of the mature osteoclast, the highly expressed vacuolar-type H(+)-ATPase that drives acid secretion to dissolve mineral, and cathepsin K, the predominant acid proteinase for collagenolysis. Recently, additional detail has been added to this framework, showing that the osteoclast has more complex regulation than was previously believed. These include the findings that one component of the V-H(+)-ATPase is unique to the osteoclast, that chloride transport and probably Cl(-)/H(+) exchange are also required for mineral degradation, and that additional receptors besides RANK and Fms regulate osteoclast formation and survival. Additional receptors include estrogen receptor-alpha, TNF-family receptors other than RANK, and, at least in some cases, glycoprotein hormone receptors including the TSH-R and the FSH-R. Challenges in understanding osteoclast biology include how the signalling mechanisms function cooperatively. Recent findings suggest that there is a network of cytoplasmic adapters, including Gab-2 and BCAR1, which are modified by multiple signalling mechanisms and which serve to integrate the signalling pathways.

Critical role of cortactin in actin ring formation and osteoclastic bone resorption

Tyrosine kinase c-Src plays an essential role in ruffled border formation and bone resorption in osteoclasts; however, it is unclear how c-Src controls ruffled border formation during bone resorption. To address this question, we investigated the role of cortactin, a c-Src substrate, in osteoclasts. We found that cortactin showed colocalization with c-Src and actin rings in osteoclasts. Overexpression of cortactin stimulated actin ring formation in RAW 264.7 cells. In contrast, overexpression of Csk inhibited tyrosine phosphorylation of cortactin and binding of cortactin to c-Src. More importantly, overexertion of a mutant cortactin strongly suppressed actin ring formation and bone resorbing activity in osteoclasts. Collectively, our data indicate that cortactin controls osteoclastic bone resorption by regulating actin organization.