The p85alpha subunit of class IA phosphatidylinositol 3-kinase regulates the expression of multiple genes involved in osteoclast maturation and migration

Functional Role of Phospholipase D in Bone Metabolism

Phospholipase D (PLD) proteins are major enzymes that regulate various cellular functions, such as cell growth, cell migration, membrane trafficking, and cytoskeletal dynamics. As they are responsible for such important biological functions, PLD proteins have been considered promising therapeutic targets for various diseases, including cancer and vascular and neurological diseases. Intriguingly, emerging evidence indicates that PLD1 and PLD2, 2 major mammalian PLD isoenzymes, are the key regulators of bone remodeling; this suggests that these isozymes could be used as potential therapeutic targets for bone diseases, such as osteoporosis and rheumatoid arthritis. PLD1 or PLD2 deficiency in mice can lead to decreased bone mass and dysregulated bone homeostasis. Although both mutant mice exhibit similar skeletal phenotypes, PLD1 and PLD2 play distinct and nonredundant roles in bone cell function. This review summarizes the physiological roles of PLD1 and PLD2 in bone metabolism, focusing on recent findings of the biological functions and action mechanisms of PLD1 and PLD2 in bone cells.

Phospholipase D2 controls bone homeostasis by modulating M-CSF-dependent osteoclastic cell migration and microtubule stability

Phospholipase D2 (PLD2), a signaling protein, plays a central role in cellular communication and various biological processes. Here, we show that PLD2 contributes to bone homeostasis by regulating bone resorption through osteoclastic cell migration and microtubule-dependent cytoskeletal organization. Pld2-deficient mice exhibited a low bone mass attributed to increased osteoclast function without altered osteoblast activity. While Pld2 deficiency did not affect osteoclast differentiation, its absence promoted the migration of osteoclast lineage cells through a mechanism involving M-CSF-induced activation of the PI3K–Akt–GSK3β signaling pathway. The absence of Pld2 also boosted osteoclast spreading and actin ring formation, resulting in elevated bone resorption. Furthermore, Pld2 deletion increased microtubule acetylation and stability, which were later restored by treatment with a specific inhibitor of Akt, an essential molecule for microtubule stabilization and osteoclast bone resorption activity. Interestingly, PLD2 interacted with the M-CSF receptor (c-Fms) and PI3K, and the association between PLD2 and c-Fms was reduced in response to M-CSF. Altogether, our findings indicate that PLD2 regulates bone homeostasis by modulating osteoclastic cell migration and microtubule stability via the M-CSF-dependent PI3K–Akt–GSK3β axis.

A signaling protein that regulates bone resorption may prove a useful target in treating skeletal conditions such as osteoporosis and rheumatoid arthritis. Bone is synthesized by cells called osteoblasts, while osteoclasts trigger bone resorption, keeping the skeleton healthy. Imbalances in this recycling process are common in bone disorders. Je-Young Choi and Hyun-Ju Kim at Kyungpook National University in Daegu, South Korea, and co-workers demonstrated that phospholipase D2 (PLD2), a membrane protein, directly regulates bone resorption in mice. Mice without the Pld2 gene had increased osteoclast activity, resulting in low bone mass. The absence of PLD2 promotes the migration of osteoclasts via a particular signaling pathway. This increased the organization of microtubules, polymers that help form the cytoskeleton. The results suggest that regulating PLD2 activity could form the basis of a future treatment method.

Osteoclast Signal Transduction During Bone Metastasis Formation

Osteoclasts are myeloid lineage-derived bone-resorbing cells of hematopoietic origin. They differentiate from myeloid precursors through a complex regulation process where the differentiation of preosteoclasts is followed by intercellular fusion to generate large multinucleated cells. Under physiological conditions, osteoclastogenesis is primarily directed by interactions between CSF-1R and macrophage colony-stimulating factor (M-CSF, CSF-1), receptor activator of nuclear factor NF-κB (RANK) and RANK ligand (RANKL), as well as adhesion receptors (e.g., integrins) and their ligands. Osteoclasts play a central role in physiological and pathological bone resorption and are also required for excessive bone loss during osteoporosis, inflammatory bone and joint diseases (such as rheumatoid arthritis) and cancer cell-induced osteolysis. Due to the major role of osteoclasts in these diseases the better understanding of their intracellular signaling pathways can lead to the identification of potential novel therapeutic targets. Non-receptor tyrosine kinases and lipid kinases play major roles in osteoclasts and small-molecule kinase inhibitors are emerging new therapeutics in diseases with pathological bone loss. During the last few years, we and others have shown that certain lipid (such as phosphoinositide 3-kinases PI3Kβ and PI3Kδ) and tyrosine (Src−family and Syk) kinases play a critical role in osteoclast differentiation and function in humans and mice. Some of these signaling pathways shows similarity to immunoreceptor-like receptor signaling and involves important other enzymes (e.g., PLCγ2) and adapter proteins (such as the ITAM−bearing adapters DAP12 and the Fc-receptor γ-chain). Here, we review recently identified osteoclast signaling pathways and their role in osteoclast differentiation and function as well as pathological bone loss associated with osteolytic tumors of the bone. A better understanding of osteoclast signaling may facilitate the design of novel and more efficient therapies for pathological bone resorption and osteolytic skeletal metastasis formation.

Idelalisib inhibits osteoclast differentiation and pre-osteoclast migration by blocking the PI3Kδ-Akt-c-Fos/NFATc1 signaling cascade

Since increased number of osteoclasts could lead to impaired bone structure and low bone mass, which are common characteristics of bone disorders including osteoporosis, the pharmacological inhibition of osteoclast differentiation is one of therapeutic strategies for preventing and/or treating bone disorders and related facture. However, little data are available regarding the functional relevance of phosphoinositide 3-kinase (PI3K) isoforms in the osteoclast differentiation process. To elucidate the functional involvement of PI3Kδ in osteoclastogenesis, here we investigated how osteoclast differentiation was influenced by idelalisib (also called CAL-101), which is p110δ-selective inhibitor approved for the treatment of specific human B cell malignancies. Here, we found that receptor activator of nuclear factor kappa B ligand (RANKL) induced PI3Kδ protein expression, and idelalisib inhibited RANKL-induced osteoclast differentiation. Next, the inhibitory effect of idelalisib on RANKL-induced activation of the Akt-c-Fos/NFATc1 signaling cascade was confirmed by western blot analysis and real-time PCR. Finally, idelalisib inhibited pre-osteoclast migration in the last stage of osteoclast differentiation through down-regulation of the Akt-c-Fos/NFATc1 signaling cascade. It may be possible to expand the clinical use of idelalisib for controlling osteoclast differentiation. Together, the present results contribute to our understanding of the clinical value of PI3Kδ as a druggable target and the efficacy of related therapeutics including osteoclastogenesis.

An abnormal bone marrow microenvironment contributes to hematopoietic dysfunction in Fanconi anemia

Fanconi anemia is a complex heterogeneous genetic disorder with a high incidence of bone marrow failure, clonal evolution to acute myeloid leukemia and mesenchymal-derived congenital anomalies. Increasing evidence in Fanconi anemia and other genetic disorders points towards an interdependence of skeletal and hematopoietic development, yet the impact of the marrow microenvironment in the pathogenesis of the bone marrow failure in Fanconi anemia remains unclear. Here we demonstrated that mice with double knockout of both Fancc and Fancg genes had decreased bone formation at least partially due to impaired osteoblast differentiation from mesenchymal stem/progenitor cells. Mesenchymal stem/progenitor cells from the double knockout mice showed impaired hematopoietic supportive activity. Mesenchymal stem/progenitor cells of patients with Fanconi anemia exhibited similar cellular deficits, including increased senescence, reduced proliferation, impaired osteoblast differentiation and defective hematopoietic stem/progenitor cell supportive activity. Collectively, these studies provide unique insights into the physiological significance of mesenchymal stem/progenitor cells in supporting the marrow microenvironment, which is potentially of broad relevance in hematopoietic stem cell transplantation.

Deletion of FGFR3 in Osteoclast Lineage Cells Results in Increased Bone Mass in Mice by Inhibiting Osteoclastic Bone Resorption

Fibroblast growth factor receptor 3 (FGFR3) participates in bone remodeling. Both Fgfr3 global knockout and activated mice showed decreased bone mass with increased osteoclast formation or bone resorption activity. To clarify the direct effect of FGFR3 on osteoclasts, we specifically deleted Fgfr3 in osteoclast lineage cells. Adult mice with Fgfr3 deficiency in osteoclast lineage cells (mutant [MUT]) showed increased bone mass. In a drilled-hole defect model, the bone remodeling of the holed area in cortical bone was also impaired with delayed resorption of residual woven bone in MUT mice. In vitro assay demonstrated that there was no significant difference between the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts derived from wild-type and Fgfr3-deficient bone marrow monocytes, suggesting that FGFR3 had no remarkable effect on osteoclast formation. The bone resorption activity of Fgfr3-deficient osteoclasts was markedly decreased accompanying with downregulated expressions of Trap, Ctsk, and Mmp 9. The upregulated activity of osteoclastic bone resorption by FGF2 in vitro was also impaired in Fgfr3-deficient osteoclasts, indicating that FGFR3 may participate in the regulation of bone resorption activity of osteoclasts by FGF2. Reduced adhesion but not migration in osteoclasts with Fgfr3 deficiency may be responsible for the impaired bone resorption activity. Our study for the first time genetically shows the direct positive regulation of FGFR3 on osteoclastic bone resorption. © 2016 American Society for Bone and Mineral Research.

Adult-Onset Deletion of β-Catenin in (10kb)Dmp1-Expressing Cells Prevents Intermittent PTH-Induced Bone Gain

β-Catenin (βcat) is a major downstream signaling node in canonical Wingless-related integration site (Wnt) signaling pathway, and its activity is crucial for canonical Wnt signal transduction. Wnt signaling has recently been implicated in the osteo-anabolic response to PTH, a potent calcium-regulating factor. We investigated whether βcat is essential for the anabolic action of intermittent PTH by generating male mice with adult-onset deletion of βcat in a subpopulation of bone cells (osteocytes and late-stage osteoblasts), treating them with an anabolic regimen of PTH, and measuring the skeletal responses. Male (10kb)Dmp1-CreERt2 transgenic mice that also harbored floxed loss-of-function βcat alleles (βcat(f/f)) were induced for Cre activity using tamoxifen, then injected daily with human PTH 1-34 (30 μg/kg) or vehicle for 5 weeks. Mice in which βcat was deleted showed either total lack of bone mineral density (BMD) gain, or BMD loss, and did not respond to PTH treatment. However, bone mass measurements in the trabecular compartment of the femur and spine revealed PTH-induced bone gain whether βcat was deleted or not. PTH-stimulated increases in periosteal and cancellous bone formation rates were not impaired by βcat deletion, but resorption markers and cortical porosity were significantly increased in induced mice, particularly induced mice treated with PTH. These results suggest that βcat is required for net-positive BMD effects of PTH therapy but that the anabolic effects per se of PTH treatment might not require osteocytic/osteoblastic βcat.

Loss of Cbl-PI3K interaction enhances osteoclast survival due to p21-Ras mediated PI3K activation independent of Cbl-b

Cbl family proteins, Cbl and Cbl-b, are E3 ubiquitin ligases and adaptor proteins, which play important roles in bone-resorbing osteoclasts. Loss of Cbl in mice decreases osteoclast migration, resulting in delayed bone development where as absence of Cbl-b decreases bone volume due to hyper-resorptive osteoclasts. A major structural difference between Cbl and Cbl-b is tyrosine 737 (in YEAM motif) only on Cbl, which upon phosphorylation interacts with the p85 subunit of phosphatidylinositol-3 Kinase (PI3K). In contrast to Cbl(-/-) and Cbl-b(-/-) , mice lacking Cbl-PI3K interaction due to a Y737F (tyrosine to phenylalanine, YF) mutation showed enhanced osteoclast survival, but defective bone resorption. To investigate whether Cbl-PI3K interaction contributes to distinct roles of Cbl and Cbl-b in osteoclasts, mice bearing CblY737F mutation in the Cbl-b(-/-) background (YF/YF;Cbl-b(-/-) ) were generated. The differentiation and survival were augmented similarly in YF/YF and YF/YF;Cbl-b(-/-) osteoclasts, associated with enhanced PI3K signaling suggesting an exclusive role of Cbl-PI3K interaction, independent of Cbl-b. In addition to PI3K, the small GTPase Ras also regulates osteoclast survival. In the absence of Cbl-PI3K interaction, increased Ras GTPase activity and Ras-PI3K binding were observed and inhibition of Ras activation attenuated PI3K mediated osteoclast survival. In contrast to differentiation and survival, increased osteoclast activity observed in Cbl-b(-/-) mice persisted even after introduction of the resorption-defective YF mutation in YF/YF;Cbl-b(-/-) mice. Hence, Cbl and Cbl-b play mutually exclusive roles in osteoclasts. Whereas Cbl-PI3K interaction regulates differentiation and survival, bone resorption is predominantly regulated by Cbl-b in osteoclasts.

Natural polyamines inhibit the migration of preosteoclasts by attenuating Ca2+-PYK2-Src-NFATc1 signaling pathways

Natural polyamines have numerous biological activities. Several studies have reported their beneficial role in bone metabolism, but their mode of action is not fully understood. Bone diseases such as osteoporosis, which is characterized by impaired bone structure and low bone mass, are caused by an increased number of osteoclasts and/or overactivation of osteoclastogenesis. Osteoclast differentiation is a multi-complex procedure involving the following sequential steps: differentiation-migration-fusion-resorption. In this study, we found that putrescine, spermidine or spermine inhibited the RANKL-mediated migration of preosteoclasts. Furthermore, the RANKL-mediated activation of the Src-PYK2 signaling axis and of transcription factors such as NF-κB and NFATc1 was prevented by each polyamine. Anti-osteoclastogenic and anti-migration activities of polyamines were confirmed by evaluating their potential to downregulate the mRNA expression levels of osteoclastogenesis-related genes such as OSCAR, TRAP, cathepsin K and c-Src, and genes related to fusion and/or migration of preosteoclasts. Moreover, ATP-mediated elevation of cytosolic free Ca(2+) concentration ([Ca(2+)]i) was strongly inhibited by each polyamine, indicating the involvement of [Ca(2+)]i in the anti-fusion activities of polyamines. In conclusion, polyamines could exhibit anti-osteoclastogenic activity by inhibiting the migration of preosteoclasts via the Ca(2+)-PYK2-Src-NFATc1 signaling axis.

Siglec-15 regulates osteoclast differentiation by modulating RANKL-induced phosphatidylinositol 3-kinase/Akt and Erk pathways in association with signaling Adaptor DAP12

Siglecs are a family of sialic acid-binding immunoglobulin-like lectins that regulate the functions of cells in the innate and adaptive immune systems through glycan recognition. Here we show that Siglec-15 regulates osteoclast development and bone resorption by modulating receptor activator of nuclear factor κB ligand (RANKL) signaling in association with DNAX-activating protein 12 kDa (DAP12), an adaptor protein bearing an immunoreceptor tyrosine-based activation motif (ITAM). Among the known Siglecs expressed in myeloid lineage cells, only Siglec-15 was upregulated by RANKL in mouse primary bone marrow macrophages. Siglec-15-deficient mice exhibit mild osteopetrosis resulting from impaired osteoclast development. Consistently, cells lacking Siglec-15 exhibit defective osteoclast development and resorptive activity in vitro. RANKL-induced activation of phosphatidylinositol 3-kinase (PI3K)/Akt and Erk pathways were impaired in Siglec-15-deficient cells. Retroviral transduction of Siglec-15-null osteoclast precursors with wild-type Siglec-15 or mutant Siglec-15 revealed that the association of Siglec-15 with DAP12 is involved in the downstream signal transduction of RANK. Furthermore, we found that the ability of osteoclast formation is preserved in the region adjacent to the growth plate in Siglec-15-deficient mice, indicating that there is a compensatory mechanism for Siglec-15-mediated osteoclastogenesis in the primary spongiosa. To clarify the mechanism of this compensation, we examined whether osteoclast-associated receptor (OSCAR)/Fc receptor common γ (FcRγ) signaling, an alternative ITAM-mediated signaling pathway to DAP12, rescues impaired osteoclastogenesis in Siglec-15-deficient cells. The ligands in type II collagen activate OSCAR and rescue impaired osteoclastogenesis in Siglec-15-deficient cells when cultured on bone slices, indicating that Siglec-15-mediated signaling can be compensated for by signaling activated by type II collagen and other bone matrix components in the primary spongiosa. Our findings indicate that Siglec-15 plays an important role in physiologic bone remodeling by modulating RANKL signaling, especially in the secondary spongiosa.

The PI3K inhibitor GDC-0941 combines with existing clinical regimens for superior activity in multiple myeloma

The phosphatidylinositol 3'-kinase (PI3K) pathway is dysregulated in multiple myeloma (MM); we therefore tested a highly selective class I PI3K inhibitor, GDC-0941, for anti-myeloma activity. Functional and mechanistic studies were first performed in MM cell lines, then extended to primary MM patient samples cultured in vitro. GDC-0941 was then assessed as a single agent and in various combinations in myeloma tumor xenograft models. We show p110 α and β are the predominant PI3K catalytic subunits in MM and that a highly selective class I PI3K inhibitor, GDC-0941, has robust activity as a single agent to induce cell cycle arrest and apoptosis of both MM cell lines and patient myeloma cells. Mechanistic studies revealed an induction of cell cycle arrest at G0/G1, with decreased phospho-FoxO1/3a levels, decreased cyclin D1 and c-myc expression, and an increase in the cell cycle inhibitor, p27kip. Induction of apoptosis correlated with increased expression of the pro-apoptotic BH3-only protein BIM, cleaved caspase 3 and cleaved poly (ADP-ribose) polymerase (PARP). In vitro, GDC-0941 synergized with dexamethasone (Dex) and lenalidomide (combination index values of 0.3-0.4 and 0.4-0.8, respectively); in vivo GDC-0941 has anti-myeloma activity and significantly increases the activity of the standard of care agents in several murine xenograft tumor models (additional tumor growth inhibition of 37-53% (Dex) and 22-72% (lenalidomide)). These data provide a clear therapeutic hypothesis for the inhibition of PI3K and provide a rationale for clinical development of GDC-0941 in myeloma.

Regulation of osteoclasts by membrane-derived lipid mediators

Osteoclasts are bone-resorbing cells of monocytic origin. An imbalance between bone formation and resorption can lead to osteoporosis or osteopetrosis. Osteoclastogenesis is triggered by RANKL- and IP3-induced Ca²⁺ influx followed by activation of NFATc1, a master transcription factor for osteoclastogenic gene regulation. During differentiation, osteoclasts undergo cytoskeletal remodeling to migrate and attach to the bone surface. Simultaneously, they fuse with each other to form multinucleated cells. These processes require PI3-kinase-dependent cytoskeletal protein activation to initiate cytoskeletal remodeling, resulting in the formation of circumferential podosomes and fusion-competent protrusions. In multinucleated osteoclasts, circumferential podosomes mature into stabilized actin rings, which enables the formation of a ruffled border where intensive membrane trafficking is executed. Membrane lipids, especially phosphoinositides, are key signaling molecules that regulate osteoclast morphology and act as second messengers and docking sites for multiple important effectors. We examine the critical roles of phosphoinositides in the signaling cascades that regulate osteoclast functions.

Class IA phosphatidylinositol 3-kinase regulates osteoclastic bone resorption through protein kinase B-mediated vesicle transport

Class IA phosphatidylinositol 3-kinases (PI3Ks) are activated by growth factor receptors and regulate a wide range of cellular processes. In osteoclasts, they are activated downstream of α(v) β(3) integrin and colony-stimulating factor-1 receptor (c-Fms), which are involved in the regulation of bone-resorbing activity. The physiological relevance of the in vitro studies using PI3K inhibitors has been of limited value, because they inhibit all classes of PI3K. Here, we show that the osteoclast-specific deletion of the p85 genes encoding the regulatory subunit of the class IA PI3K results in an osteopetrotic phenotype caused by a defect in the bone-resorbing activity of osteoclasts. Class IA PI3K is required for the ruffled border formation and vesicular transport, but not for the formation of the sealing zone. p85α/β doubly deficient osteoclasts had a defect in macrophage colony-stimulating factor (M-CSF)-induced protein kinase B (Akt) activation and the introduction of constitutively active Akt recovered the bone-resorbing activity. Thus, the class IA PI3K-Akt pathway regulates the cellular machinery crucial for osteoclastic bone resorption, and may provide a molecular basis for therapeutic strategies against bone diseases.

Caffeic acid 3,4-dihydroxy-phenethyl ester suppresses receptor activator of NF-κB ligand–induced osteoclastogenesis and prevents ovariectomy-induced bone loss through inhibition of mitogen-activated protein kinase/activator protein 1 and Ca2+–nuclear factor of activated T-cells cytoplasmic 1 signaling pathways

Receptor activator of NF-κB ligand (RANKL) stimulation leads to the activation of mitogen-activated protein kinase (MAPK)/AP-1 and Ca2+–nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) signaling pathways in osteoclastogenesis. Targeting these pathways has been an encouraging strategy for bone-related diseases, such as postmenopausal osteoporosis. In this study, we examined the effects of caffeic acid 3,4-dihydroxy-phenethyl ester (CADPE) on osteoclastogenesis. In mouse bone marrow monocytes (BMMs) and RAW264.7 cells, CADPE suppressed RANKL-induced osteoclast differentiation and actin-ring formation in a dose-dependent manner within non–growth inhibitory concentrations at the early stage, while CADPE had no effect on macrophage colony-stimulating factor (M-CSF)-induced proliferation and differentiation. At the molecular level, CADPE inhibited RANKL-induced phosphorylation of MAPKs, including extracellular signal-regulated kinases 1/2 (ERK1/2), p38, and c-Jun N-terminal kinase (JNK), without significantly affecting the NF-κB signaling pathway. CADPE abrogated RANKL-induced activator protein 1 (AP-1)/FBJ murine osteosarcoma viral oncogene homolog (c-Fos) nuclear translocation and activation. Overexpression of c-Fos prevented the inhibition by CADPE of osteoclast differentiation. Furthermore, CADPE suppressed RANKL-induced the tumor necrosis factor receptor associated factor 6 (TRAF6) interaction with c-src tyrosine kinase (c-Src), blocked RANKL-induced the phosphorylation of protein kinase B (AKT), and inhibited RANKL-induced Ca2+ oscillation. As a result, CADPE decreased osteoclastogenesis-related marker gene expression, including NFATc1, TRAP, cathepsin K, and c-Src. To test the effects of CADPE on osteoclast activity in vivo, we showed that CADPE prevented ovariectomy-induced bone loss by inhibiting osteoclast activity. Together, our data demonstrate that CADPE suppresses osteoclastogenesis and bone loss through inhibiting RANKL-induced MAPKs and Ca2+-NFATc1 signaling pathways. CADPE is a novel agent in the treatment of osteoclast-related diseases, such as osteoporosis.

Role of intercellular adhesion molecule-2 in osteoclastogenesis

Osteoclasts, multinucleated bone-resorbing cells, are specialized cells derived from the monocyte/macrophage lineage. Therefore, it is essential for mononuclear precursors to find a fusion partner during its differentiation. Our previous study showed an important role of cell communication via Mac-1 (CD11b/CD18) during osteoclastogenesis. However, the counter receptor of Mac-1 was still unknown. Flow cytometric analysis showed that bone marrow-derived mononuclear cells, used as osteoclast precursors, expressed intercellular adhesion molecule-1 and -2. Quantitative RT-PCR analysis revealed that expression level of ICAM-2 was higher than that of ICAM-1 in bone marrow cells. The osteoclastogenesis induced by receptor activator of NF-kappaB ligand (RANKL) was inhibited by anti-ICAM-2 neutralizing antibody but not by anti-ICAM-1 neutralizing antibody. The inhibitory effect of anti-ICAM-2 antibody on osteoclastogenesis was enhanced by simultaneous treatment of anti-CD11b neutralizing antibody. Furthermore, osteoclastogenesis induced by tumor necrosis factor α (TNFα) was also inhibited by anti-ICAM-2 neutralizing antibody. The involvement of lymphocytes in osteoclastogenesis was excluded, because anti-ICAM-2 antibody inhibited osteoclastogenesis using bone marrow-derived cells from immunodeficiency mice. Immunocytochemical staining demonstrated colocalization of ICAM-2 and Mac-1 during osteoclastogenesis; however, Mac-1 immunoreactivity was lost in differentiated multinucleated osteoclast. These results suggest the important role of ICAM-2/Mac-1 binding in osteoclastogenesis induced by either RANKL or TNFα.

Insulin-like growth factor-binding protein-2 is required for osteoclast differentiation

Global deletion of the Igfbp2 gene results in the suppression of bone turnover. To investigate the role of insulin-like growth factor-binding protein-2 (IGFBP-2) in regulating osteoclast differentiation, we cultured Igfbp2(-/-) bone marrow cells and found a reduction in the number of osteoclasts and impaired resorption. Addition of full-length IGFBP-2 restored osteoclast differentiation, fusion, and resorption. To determine the molecular domains of IGFBP-2 that were required for this effect to be manifest, Igfbp2(-/-) bone marrow cells were transfected with constructs in which the heparin-binding (HBD) or the IGF-binding domains of IGFBP-2 were mutated. We found that both domains were necessary for osteoclastogenesis because expression of the mutated forms of either domain failed to support the formation of functionally mature osteoclasts. To discern the mechanism by which IGFBP-2 regulates osteoclast formation, PTEN abundance and phosphorylation status as well as AKT responsiveness to IGF-I were analyzed. Igfbp2(-/-) cells had elevated levels of PTEN and phospho-PTEN compared with controls. Expression of wild-type IGFBP-2 reduced the level of PTEN to that of wild-type cells. Cells expressing the IGF-binding mutant showed suppression of PTEN and phospho-PTEN equivalent to the wild-type protein, whereas those expressing the IGFBP-2 HBD mutant showed no PTEN suppression. When the ability of IGF-I to stimulate AKT activation, measured by Thr³⁰⁸ and Ser⁴⁷³ phosphorylation, was analyzed, stimulation of Ser⁴⁷³ in response to IGF-I in preosteoclasts required the presence of intact IGFBP-2. This effect was duplicated by the addition of a CK2 inhibitor that prevents the phosphorylation of PTEN. In contrast, in fully differentiated osteoclasts, stimulation of Thr³⁰⁸ phosphorylation required the presence of intact IGFBP-2. We conclude that IGFBP-2 is an important regulator of osteoclastogenesis and that both the heparin- and the IGF-binding domains of IGFBP-2 are essential for the formation of fully differentiated and functional osteoclasts.

Distinguishing the proapoptotic and antiresorptive functions of risedronate in murine osteoclasts: role of the Akt pathway and the ERK/Bim axis

OBJECTIVE

Nitrogen-containing bisphosphonates are one of the most successful therapeutics for osteoporosis. The aim of this study was to elucidate the functional mechanism of one of the typical nitrogen-containing bisphosphonates, risedronate.

METHODS

Osteoclasts generated from murine bone marrow macrophages were treated with risedronate in vitro, and its effects on apoptosis and bone-resorbing activity were examined. The mechanism of action of risedronate was examined by gene induction of constitutively active Akt-1 and constitutively active MEK-1, and by gene deletion of Bim. Bim(-/-) mice, in which osteoclasts were resistant to apoptosis, were treated with risedronate and analyzed radiographically, biochemically, and histologically.

RESULTS

Risedronate induced osteoclast apoptosis through the mitochondria-dependent pathway with an increased expression of Bim, and the proapoptotic effect of risedronate was suppressed by Bim deletion and constitutively active MEK-1 introduction. In contrast, the risedronate-induced suppression of bone resorption was completely reversed by inducing constitutively active Akt-1, but not by Bim deletion or constitutively active MEK-1 introduction. These results suggested that apoptosis and bone-resorbing activity of osteoclasts were regulated through the ERK/Bim axis and the Akt pathway, respectively, both of which were suppressed by risedronate. Although osteoclast apoptosis in response to risedronate administration was suppressed in the Bim(-/-) mice, risedronate treatment increased bone mineral density in Bim(-/-) mice at a level equivalent to that in wild-type mice.

CONCLUSION

Our findings indicate that the antiresorptive effect of risedronate in vivo is mainly mediated by the suppression of the bone-resorbing activity of osteoclasts and not by the induction of osteoclast apoptosis.

Class I(A) PI3Kinase regulatory subunit, p85α, mediates mast cell development through regulation of growth and survival related genes

Stem cell factor (SCF) mediated KIT receptor activation plays a pivotal role in mast cell growth, maturation and survival. However, the signaling events downstream from KIT are poorly understood. Mast cells express multiple regulatory subunits of class 1_A PI3Kinase (PI3K) including p85α, p85β, p50α, and p55α. While it is known that PI3K plays an essential role in mast cells; the precise mechanism by which these regulatory subunits impact specific mast cell functions including growth, survival and cycling are not known. We show that loss of p85α impairs the growth, survival and cycling of mast cell progenitors (MCp). To delineate the molecular mechanism (s) by which p85α regulates mast cell growth, survival and cycling, we performed microarray analyses to compare the gene expression profile of MCps derived from WT and p85α-deficient mice in response to SCF stimulation. We identified 151 unique genes exhibiting altered expression in p85α-deficient cells in response to SCF stimulation compared to WT cells. Functional categorization based on DAVID bioinformatics tool and Ingenuity Pathway Analysis (IPA) software relates the altered genes due to lack of p85α to transcription, cell cycle, cell survival, cell adhesion, cell differentiation, and signal transduction. Our results suggest that p85α is involved in mast cell development through regulation of expression of growth, survival and cell cycle related genes.

Balanced interactions between Lyn, the p85alpha regulatory subunit of class I(A) phosphatidylinositol-3-kinase, and SHIP are essential for mast cell growth and maturation

The growth and maturation of bone marrow-derived mast cells (BMMCs) from precursors are regulated by coordinated signals from multiple cytokine receptors, including KIT. While studies conducted using mutant forms of these receptors lacking the binding sites for Src family kinases (SFKs) and phosphatidylinositol-3-kinase (PI3K) suggest a role for these signaling molecules in regulating growth and survival, how complete loss of these molecules in early BMMC progenitors (MCps) impacts maturation and growth during all phases of mast cell development is not fully understood. We show that the Lyn SFK and the p85α subunit of class I(A) PI3K play opposing roles in regulating the growth and maturation of BMMCs in part by regulating the level of PI3K. Loss of Lyn in BMMCs results in elevated PI3K activity and hyperactivation of AKT, which accelerates the rate of BMMC maturation due in part to impaired binding and phosphorylation of SHIP via Lyn's unique domain. In the absence of Lyn's unique domain, BMMCs behave in a manner similar to that of Lyn- or SHIP-deficient BMMCs. Importantly, loss of p85α in Lyn-deficient BMMCs not only represses the hyperproliferation associated with the loss of Lyn but also represses their accelerated maturation. The accelerated maturation of BMMCs due to loss of Lyn is associated with increased expression of microphthalmia-associated transcription factor (Mitf), which is repressed in MCps deficient in the expression of both Lyn and p85α relative to controls. Our results demonstrate a crucial interplay of Lyn, SHIP, and p85α in regulating the normal growth and maturation of BMMCs, in part by regulating the activation of AKT and the expression of Mitf.

The haploinsufficient hematopoietic microenvironment is critical to the pathological fracture repair in murine models of neurofibromatosis type 1

Germline mutations in the NF1 tumor suppressor gene cause neurofibromatosis type 1 (NF1), a complex genetic disorder with a high predisposition of numerous skeletal dysplasias including short stature, osteoporosis, kyphoscoliosis, and fracture non-union (pseudoarthrosis). We have developed murine models that phenocopy many of the skeletal dysplasias observed in NF1 patients, including reduced bone mass and fracture non-union. We also show that the development of these skeletal manifestations requires an Nf1 haploinsufficient background in addition to nullizygous loss of Nf1 in mesenchymal stem/progenitor cells (MSCs) and/or their progenies. This is replicated in two animal models of NF1, PeriCre⁺;Nf1^flox/− and Col2.3Cre⁺;Nf1^flox/−mice. Adoptive transfer experiments demonstrate a critical role of the Nf1+/− marrow microenvironment in the impaired fracture healing in both models and adoptive transfer of WT bone marrow cells improves fracture healing in these mice. To our knowledge, this is the first demonstration of a non-cell autonomous mechanism in non-malignant NF1 manifestations. Collectively, these data provide evidence of a combinatory effect between nullizygous loss of Nf1 in osteoblast progenitors and haploinsufficiency in hematopoietic cells in the development of non-malignant NF1 manifestations.

Erk1 positively regulates osteoclast differentiation and bone resorptive activity

The extracellular signal-regulated kinases (ERK1 and 2) are widely-expressed and they modulate proliferation, survival, differentiation, and protein synthesis in multiple cell lineages. Altered ERK1/2 signaling is found in several genetic diseases with skeletal phenotypes, including Noonan syndrome, Neurofibromatosis type 1, and Cardio-facio-cutaneous syndrome, suggesting that MEK-ERK signals regulate human skeletal development. Here, we examine the consequence of Erk1 and Erk2 disruption in multiple functions of osteoclasts, specialized macrophage/monocyte lineage-derived cells that resorb bone. We demonstrate that Erk1 positively regulates osteoclast development and bone resorptive activity, as genetic disruption of Erk1 reduced osteoclast progenitor cell numbers, compromised pit formation, and diminished M-CSF-mediated adhesion and migration. Moreover, WT mice reconstituted long-term with Erk1^−/− bone marrow mononuclear cells (BMMNCs) demonstrated increased bone mineral density as compared to recipients transplanted with WT and Erk2^−/− BMMNCs, implicating marrow autonomous, Erk1-dependent osteoclast function. These data demonstrate Erk1 plays an important role in osteoclast functions while providing rationale for the development of Erk1-specific inhibitors for experimental investigation and/or therapeutic modulation of aberrant osteoclast function.

The PI3K pathway drives the maturation of mast cells via microphthalmia transcription factor

Mast cell maturation is poorly understood. We show that enhanced PI3K activation results in accelerated maturation of mast cells by inducing the expression of microphthalmia transcription factor (Mitf). Conversely, loss of PI3K activation reduces the maturation of mast cells by inhibiting the activation of AKT, leading to reduced Mitf but enhanced Gata-2 expression and accumulation of Gr1(+)Mac1(+) myeloid cells as opposed to mast cells. Consistently, overexpression of Mitf accelerates the maturation of mast cells, whereas Gata-2 overexpression mimics the loss of the PI3K phenotype. Expressing the full-length or the src homology 3- or BCR homology domain-deleted or shorter splice variant of the p85α regulatory subunit of PI3K or activated AKT or Mitf in p85α-deficient cells restores the maturation but not growth. Although deficiency of both SHIP and p85α rescues the maturation of SHIP(-/-) and p85α(-/-) mast cells and expression of Mitf; in vivo, mast cells are rescued in some, but not all tissues, due in part to defective KIT signaling, which is dependent on an intact src homology 3 and BCR homology domain of p85α. Thus, p85α-induced maturation, and growth and survival signals, in mast cells can be uncoupled.

p85alpha regulates osteoblast differentiation by cross-talking with the MAPK pathway

Class IA phosphoinositide 3-kinase (PI3K) is involved in regulating many cellular functions including cell growth, proliferation, cell survival, and differentiation. The p85 regulatory subunit is a critical component of the PI3K signaling pathway. Mesenchymal stem cells (MSC) are multipotent cells that can be differentiated into osteoblasts (OBs), adipocytes, and chondrocytes under defined culture conditions. To determine whether p85α subunit of PI3K affects biological functions of MSCs, bone marrow-derived wild type (WT) and p85α-deficient (p85α(-/-)) cells were employed in this study. Increased cell growth, higher proliferation rate and reduced number of senescent cells were observed in MSCs lacking p85α compare with WT MSCs as evaluated by CFU-F assay, thymidine incorporation assay, and β-galactosidase staining, respectively. These functional changes are associated with the increased cell cycle, increased expression of cyclin D, cyclin E, and reduced expression of p16 and p19 in p85α(-/-) MSCs. In addition, a time-dependent reduction in alkaline phosphatase (ALP) activity and osteocalcin mRNA expression was observed in p85α(-/-) MSCs compared with WT MSCs, suggesting impaired osteoblast differentiation due to p85α deficiency in MSCs. The impaired p85α(-/-) osteoblast differentiation was associated with increased activation of Akt and MAPK. Importantly, bone morphogenic protein 2 (BMP2) was able to intensify the differentiation of osteoblasts derived from WT MSCs, whereas this process was significantly impaired as a result of p85α deficiency. Addition of LY294002, a PI3K inhibitor, did not alter the differentiation of osteoblasts in either genotype. However, application of PD98059, a Mek/MAPK inhibitor, significantly enhanced osteoblast differentiation in WT and p85α(-/-) MSCs. These results suggest that p85α plays an essential role in osteoblast differentiation from MSCs by repressing the activation of MAPK pathway.

The loss of Cbl-phosphatidylinositol 3-kinase interaction perturbs RANKL-mediated signaling, inhibiting bone resorption and promoting osteoclast survival

Cbl is an adaptor protein and an E3 ligase that plays both positive and negative roles in several signaling pathways that affect various cellular functions. Tyrosine 737 is unique to Cbl and is phosphorylated by Syk and Src family kinases. Phosphorylated Cbl Tyr(737) creates a binding site for the p85 regulatory subunit of PI3K, which also plays an important role in the regulation of bone resorption by osteoclasts. To investigate the role of Cbl-PI3K interaction in bone homeostasis, we examined the knock-in mice (Cbl(YF/YF)) in which the PI3K binding site in Cbl is ablated due to the mutation in the regulatory tyrosine. We report that in Cbl(YF/YF) mice, despite increased numbers of osteoclasts, bone volume is increased due to defective osteoclast function. Additionally, in ex vivo cultures, mature Cbl(YF/YF) osteoclasts showed an increased ability to survive in the presence of RANKL due to delayed onset of apoptosis. RANKL-mediated signaling is perturbed in Cbl(YF/YF) osteoclasts, and most interestingly, AKT phosphorylation is up-regulated, suggesting that the lack of PI3K sequestration by Cbl results in increased survival and decreased bone resorption. Cumulatively, these in vivo and in vitro results show that, on one hand, binding of Cbl to PI3K negatively regulates osteoclast differentiation, survival, and signaling events (e.g. AKT phosphorylation), whereas on the other hand it positively influences osteoclast function.

Molecular genetic studies of gene identification for osteoporosis: the 2009 update

Osteoporosis is a complex human disease that results in increased susceptibility to fragility fractures. It can be phenotypically characterized using several traits, including bone mineral density, bone size, bone strength, and bone turnover markers. The identification of gene variants that contribute to osteoporosis phenotypes, or responses to therapy, can eventually help individualize the prognosis, treatment, and prevention of fractures and their adverse outcomes. Our previously published reviews have comprehensively summarized the progress of molecular genetic studies of gene identification for osteoporosis and have covered the data available to the end of September 2007. This review represents our continuing efforts to summarize the important and representative findings published between October 2007 and November 2009. The topics covered include genetic association and linkage studies in humans, transgenic and knockout mouse models, as well as gene-expression microarray and proteomics studies. Major results are tabulated for comparison and ease of reference. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis.

Important roles of PI3Kgamma in osteoclastogenesis and bone homeostasis

G protein-coupled receptor-regulated PI3Kgamma is abundantly expressed in myeloid cells and has been implicated as a promising drug target to treat various inflammatory diseases. However, its role in bone homeostasis has not been investigated, despite the fact that osteoclasts are derived from myeloid lineage. We therefore carried out thorough bone phenotypic characterization of a PI3Kgamma-deficient mouse line and found that PI3Kgamma-deficient mice had high bone mass. Our analyses further revealed that PI3Kgamma deficiency did not affect bone formation because no significant changes in osteoblast number and bone formation rate were observed. Instead, the lack of PI3Kgamma was associated with decreased bone resorption, as evidenced by decreased osteoclast number in vivo and impaired osteoclast formation in vitro. The decreased osteoclast formation was accompanied by down-regulated expression of osteoclastogenic genes, compromised chemokine receptor signaling, and an increase in apoptosis during osteoclast differentiation. Together, these data suggest that PI3Kgamma regulates bone homeostasis by modulating osteoclastogenesis. Our study also suggests that inhibition of PI3Kgamma, which is being considered as a potential therapeutic strategy for treating chronic inflammatory disorders, may result in an increase in bone mass.

Inhibitory effects of ZSTK474, a novel phosphoinositide 3-kinase inhibitor, on osteoclasts and collagen-induced arthritis in mice

Introduction

Targeting joint destruction induced by osteoclasts (OCs) is critical for management of patients with rheumatoid arthritis (RA). Since phosphoinositide 3-kinase (PI3-K) plays a critical role in osteoclastogenesis and bone resorption, we examined the effects of ZSTK474, a novel phosphoinositide 3-kinase (PI3-K)-specific inhibitor, on murine OCs in vitro and in vivo.

Methods

The inhibitory effect of ZSTK474 on OC formation was determined and compared with other PI3-K inhibitors by counting tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells after culturing murine bone marrow monocytic OC precursors, and RAW264.7 cells. Activation of Akt and expression of nuclear factor of activated T cells (NFAT) c1 in cultured RAW264.7 cells were examined. The suppressing effect of ZSTK474 on bone resorption was assessed by the pit formation assay. The in vivo effects of ZSTK474 were studied in collagen-induced arthritis (CIA) in the mouse. Oral daily administration of ZSTK474 was started either when more than half or when all mice developed arthritis. Effects of ZSTK474 were evaluated using the arthritis score and histological score of the hind paws.

Results

ZSTK474 inhibited the differentiation of bone marrow OC precursors and RAW264.7 cells in a dose-dependent manner. The inhibitory effect of ZSTK474 was much stronger than that of LY294002, the most commonly used PI3-K inhibitor. In addition, ZSTK474 suppressed the bone resorbing activity of mature OCs. Moreover, oral daily administration of ZSTK474, even when begun after the development of arthritis, ameliorated CIA in mice without apparent toxicity. Histological examination of the hind paw demonstrated noticeable reduction of inflammation and of cartilage destruction in ZSTK474-treated mice. ZSTK474 also significantly decreased OC formation adjacent to the tarsal bone of the hind paw.

Conclusions

These findings suggest that inhibition of PI3-K with ZSTK474 may potentially suppress synovial inflammation and bone destruction in patients with RA.

3BP2 adapter protein is required for receptor activator of NFκB ligand (RANKL)-induced osteoclast differentiation of RAW264.7 cells

The adapter protein 3BP2 (also known as SH3BP2 and Abl SH3-binding protein 2) has been involved in leukocyte signaling and activation downstream immunoreceptors. Genetic studies have further associated 3BP2 mutations to the human disease cherubism and to inflammation and bone dysfunction in mouse. However, how wild type 3BP2 functions in macrophage differentiation remains poorly understood. In this study, using small interfering RNA-mediated silencing of 3BP2 in the RAW264.7 monocytic cell line, we show that 3BP2 was required for receptor activator of NFkappaB ligand (RANKL)-induced differentiation of RAW264.7 cells into multinucleated mature osteoclasts but not for granulocyte macrophage-colony stimulating factor/interleukin-4-induced differentiation into dendritic cells. 3BP2 silencing was associated with impaired activation of multiple signaling events downstream of RANK, including actin reorganization; Src, ERK, and JNK phosphorylation; and up-regulation of osteoclastogenic factors. In addition, 3BP2 knockdown cells induced to osteoclast by RANKL displayed a reduced increase of Src and nuclear factor of activated T cells (NFATc1) mRNA and protein expression. Importantly, 3BP2 interacted with Src, Syk, Vav, and Cbl in monocytic cells, and the introduction of constitutively active mutants of Src and NFATc1 in 3BP2-deficient cells restored osteoclast differentiation. Finally, the expression of a 3BP2 cherubism mutant was found to promote increased Src activity and NFAT-dependent osteoclast formation. Together, this study demonstrates that wild type 3BP2 is a key regulator of RANK-mediated macrophage differentiation into osteoclast through Src and NFATc1 activation.

Implication of the calcium sensing receptor and the Phosphoinositide 3-kinase/Akt pathway in the extracellular calcium-mediated migration of RAW 264.7 osteoclast precursor cells

While the processes involved in the formation, maturation and apoptosis of osteoclasts have been investigated extensively in previous studies, little is known about the mechanisms responsible for the localization and homing of osteoclast precursor cells to the bone environment in order to initiate the bone remodeling process. Recent studies have suggested that the extracellular Ca(2+) (Ca(2+)(o)) concentration gradient present near the bone environment may be one of the participating factors, producing a chemoattractant effect on osteoclast precursors. Using the murine osteoclast precursor cells of the monocyte-macrophage lineage, the RAW 264.7 cell line, we have shown that Ca(2+)(o) increases the migration of these cells in a directional manner. The participation of the calcium sensing receptor (CaR) in this effect was tested by knocking down its expression through RNA interference, which resulted in an abolition of the migratory response. By the use of specific pathway inhibitors and western blot analysis, the phosphoinositide 3-kinase (PI3K)/Akt and phospholipase Cbeta pathways were shown to be implicated in the migratory effect. The implication of the Akt pathway in the Ca(2+)(o)-induced chemoattraction of RAW 264.7 cells was also confirmed by transducing the cells with the fusion protein TAT-dominant negative-Akt, which decreased the migratory effect. In contrast, the MAPK pathways (ERK1/2, p38 and JNK) were not involved in the production of the migratory effect. We conclude that through the activation of the CaR and subsequent signaling via the PI3K/Akt pathway, Ca(2+)(o) produces a chemoattractant effect on the osteoclast precursor RAW 264.7 cells. These results suggest that the Ca(2+)(o) gradient present near the bone may be one of the initiating factors for the homing of osteoclast precursors to bone, thus possibly playing a role in the initiation of bone remodeling.