Cancer cell expression of autotaxin controls bone metastasis formation in mouse through lysophosphatidic acid-dependent activation of osteoclasts

New insights into the autotaxin/LPA axis in cancer development and metastasis

Lysophosphatidic acid (LPA) is a simple lipid with a single fatty acyl chain linked to a glycerophosphate backbone. Despite the simplicity of its structure but owing to its interactions with a series of at least six G protein-coupled receptors (LPA1-6), LPA exerts pleiotropic bioactivities including stimulation of proliferation, migration and survival of many cell types. Autotaxin (ATX) is a unique enzyme with a lysophospholipase D (lysoPLD) activity that is responsible for the levels of LPA in the blood circulation. Both LPA receptor family members and ATX/LysoPLD are aberrantly expressed in many human cancers. This review will present the more striking as well as novel experimental evidences using cell lines, cancer mouse models and transgenic animals identifying the roles for ATX and LPA receptors in cancer progression, tumor cell invasion and metastasis.

The role and therapeutic potential of the autotaxin-lysophosphatidate signalling axis in breast cancer

ATX (autotaxin) is a secreted lysophospholipase capable of catalysing the formation of the bioactive lipid mediator LPA (lysophosphatidate) from LPC (lysophosphatidylcholine). The ATX-LPA signalling axis plays an important role in both normal physiology and disease pathogenesis, including cancer. In a number of different human cancers, expression of ATX and the G-protein-coupled LPARs (lysophosphatidic acid receptors) have been shown to be elevated and their activation regulates many processes central to tumorigenesis, including proliferation, invasion, migration and angiogenesis. The present review provides an overview of the ATX-LPA signalling axis and collates current knowledge regarding its specific role in breast cancer. The potential manipulation of this pathway to facilitate diagnosis and treatment is also discussed.

Interaction of platelet-derived autotaxin with tumor integrin αVβ3 controls metastasis of breast cancer cells to bone

Autotaxin (ATX), through its lysophospholipase D activity controls physiological levels of lysophosphatidic acid (LPA) in blood. ATX is overexpressed in multiple types of cancers, and together with LPA generated during platelet activation promotes skeletal metastasis of breast cancer. However, the pathophysiological sequelae of regulated interactions between circulating LPA, ATX, and platelets remain undefined in cancer. In this study, we show that ATX is stored in α-granules of resting human platelets and released upon tumor cell-induced platelet aggregation, leading to the production of LPA. Our in vitro and in vivo experiments using human breast cancer cells that do not express ATX (MDA-MB-231 and MDA-B02) demonstrate that nontumoral ATX controls the early stage of bone colonization by tumor cells. Moreover, expression of a dominant negative integrin αvβ3-Δ744 or treatment with the anti-human αvβ3 monoclonal antibody LM609, completely abolished binding of ATX to tumor cells, demonstrating the requirement of a fully active integrin αvβ3 in this process. The present results establish a new mechanism for platelet contribution to LPA-dependent metastasis of breast cancer cells, and demonstrate the therapeutic potential of disrupting the binding of nontumor-derived ATX with the tumor cells for the prevention of metastasis.

LPA receptor signaling: pharmacology, physiology, and pathophysiology

Lysophosphatidic acid (LPA) is a small ubiquitous lipid found in vertebrate and nonvertebrate organisms that mediates diverse biological actions and demonstrates medicinal relevance. LPA's functional roles are driven by extracellular signaling through at least six 7-transmembrane G protein-coupled receptors. These receptors are named LPA1-6 and signal through numerous effector pathways activated by heterotrimeric G proteins, including Gi/o, G12/13, Gq, and Gs LPA receptor-mediated effects have been described in numerous cell types and model systems, both in vitro and in vivo, through gain- and loss-of-function studies. These studies have revealed physiological and pathophysiological influences on virtually every organ system and developmental stage of an organism. These include the nervous, cardiovascular, reproductive, and pulmonary systems. Disturbances in normal LPA signaling may contribute to a range of diseases, including neurodevelopmental and neuropsychiatric disorders, pain, cardiovascular disease, bone disorders, fibrosis, cancer, infertility, and obesity. These studies underscore the potential of LPA receptor subtypes and related signaling mechanisms to provide novel therapeutic targets.

Lysophosphatidic acid stimulates osteoclast fusion through OC-STAMP and P2X7 receptor signaling

Bone is continuously remodeled by bone formation and resorption, and cooperative bone metabolism is precisely regulated to maintain homeostasis. Osteoclasts, which are responsible for bone resorption, are differentiated through multiple steps that include cell fusion at the last step of differentiation, yielding multinuclear cells. However, the factors involved in and the precise mechanism of cell fusion are still unknown. To determine the molecules involved in osteoclast fusion, we examined the effect of lysophosphatidic acid (LPA), which has been reported to participate in the progression of cancer bone metastasis. LPA had no effect on osteoclast formation and bone resorption under receptor activator of nuclear factor kappa B ligand (RANKL) conditions, whereas LPA stimulated osteoclast fusion, thereby causing increased osteoclast diameter and bone resorptive capacity under a RANKL-limited condition. This result encouraged us to assess what molecules are needed for LPA-stimulated osteoclast fusion. Interestingly, LPA stimulated osteoclast stimulatory transmembrane protein (OC-STAMP) and P2X7 receptor mRNA expression during osteoclast fusion under a RANKL limiting condition. siRNA-induced OC-STAMP or P2X7 receptor knockdown significantly suppressed the LPA-stimulated increase in osteoclast diameter and bone resorptive capacity in differentiating cultures. Using cyclosporin A as an inhibitor, we revealed that NF-ATc1 directly regulates OC-STAMP and P2X7 receptor expression during LPA-stimulated osteoclast fusion. These results suggest that LPA is a critical regulator of osteoclast fusion by inducing the OC-STAMP and P2X7 receptor. Therefore, LPA signaling might be useful to help understand their effects on osteoclast formation and as a therapeutic target for patients with pathologically increased osteoclast formation.

Autotaxin in the crosshairs: taking aim at cancer and other inflammatory conditions

Autotaxin is a secreted enzyme that produces most of the extracellular lysophosphatidate from lysophosphatidylcholine, the most abundant phospholipid in blood plasma. Lysophosphatidate mediates many physiological and pathological processes by signaling through at least six G-protein coupled receptors to promote cell survival, proliferation and migration. The autotaxin/lysophosphatidate signaling axis is involved in wound healing and tissue remodeling, and it drives many chronic inflammatory conditions from fibrosis to colitis, asthma and cancer. In cancer, lysophosphatidate signaling promotes resistance to chemotherapy and radiotherapy, and increases both angiogenesis and metastasis. Research into autotaxin inhibitors is accelerating, both as primary and adjuvant therapy. Historically, autotaxin inhibitors had poor bioavailability profiles and thus had limited efficacy in vivo. This situation is now changing, especially since the recent crystal structure of autotaxin is now enabling rational inhibitor design. In this review, we will summarize current knowledge on autotaxin-mediated disease processes including cancer, and discuss recent advancements in the development of autotaxin-targeting strategies. We will also provide new insights into autotaxin as an inflammatory mediator in the tumor microenvironment that promotes cancer progression and therapy resistance.

Lysophosphatidic acid receptor type 1 (LPA1) plays a functional role in osteoclast differentiation and bone resorption activity

Lysophosphatidic acid (LPA) is a natural bioactive lipid that acts through six different G protein-coupled receptors (LPA1-6) with pleiotropic activities on multiple cell types. We have previously demonstrated that LPA is necessary for successful in vitro osteoclastogenesis of bone marrow cells. Bone cells controlling bone remodeling (i.e. osteoblasts, osteoclasts, and osteocytes) express LPA1, but delineating the role of this receptor in bone remodeling is still pending. Despite Lpar1(-/-) mice displaying a low bone mass phenotype, we demonstrated that bone marrow cell-induced osteoclastogenesis was reduced in Lpar1(-/-) mice but not in Lpar2(-/-) and Lpar3(-/-) animals. Expression of LPA1 was up-regulated during osteoclastogenesis, and LPA1 antagonists (Ki16425, Debio0719, and VPC12249) inhibited osteoclast differentiation. Blocking LPA1 activity with Ki16425 inhibited expression of nuclear factor of activated T-cell cytoplasmic 1 (NFATc1) and dendritic cell-specific transmembrane protein and interfered with the fusion but not the proliferation of osteoclast precursors. Similar to wild type osteoclasts treated with Ki16425, mature Lpar1(-/-) osteoclasts had reduced podosome belt and sealing zone resulting in reduced mineralized matrix resorption. Additionally, LPA1 expression markedly increased in the bone of ovariectomized mice, which was blocked by bisphosphonate treatment. Conversely, systemic treatment with Debio0719 prevented ovariectomy-induced cancellous bone loss. Moreover, intravital multiphoton microscopy revealed that Debio0719 reduced the retention of CX3CR1-EGFP(+) osteoclast precursors in bone by increasing their mobility in the bone marrow cavity. Overall, our results demonstrate that LPA1 is essential for in vitro and in vivo osteoclast activities. Therefore, LPA1 emerges as a new target for the treatment of diseases associated with excess bone loss.

Lysophosphatidic acid receptor inhibition as a new multipronged treatment for rheumatoid arthritis

OBJECTIVE

To investigate the effect of lysophosphatidic acid (LPA) receptor inhibition in a mouse model of autoantibody-mediated arthritis.

METHODS

Arthritis was induced in C57BL/6 mice by K/BxN serum transfer. Arthritic mice were treated with the LPA receptor antagonist, Ki16425 and arthritis severity was assessed clinically and histologically. Expression of inflammatory mediators in joints was identified by a mouse cytokine array and validated by western blot and real-time PCR assays. Effects of treatment with LPA receptor antagonist or with small interfering RNA on bone metabolism were assessed by in vitro assays of osteoclastogenesis, bone resorption, osteoblasts differentiation and bone mineralisation.

RESULTS

Mice treated with the LPA receptor antagonist Ki16425 showed attenuated arthritis characterised by reduction of synovial inflammation, cartilage damage and, more markedly, bone erosion. We detected increased apoptosis, reduction of inflammatory mediators and of bone remodelling proteins in arthritic joints from mice treated with Ki16425. In addition, we demonstrated that inhibition or suppression of LPA1 receptor reduces osteoclast differentiation and bone resorption and, on the contrary, it promotes differentiation of osteoblasts and bone mineralisation.

CONCLUSIONS

Pharmacological inhibition of LPA1 receptor in the K/BxN serum-transfer arthritis model led to reduction of severity of arthritis involving multiple mechanisms, increased apoptosis, reduced inflammatory mediators and proteins involved in bone remodelling, that show LPA1 as a very promising target in rheumatoid arthritis treatment.

Integrin-mediated cell surface recruitment of autotaxin promotes persistent directional cell migration

Autotaxin (ATX) is a secreted lysophospholipase D (lysoPLD) that binds to integrin adhesion receptors. We dissected the roles of integrin binding and lysoPLD activity in stimulation of human breast cancer and mouse aortic vascular smooth muscle cell migration by ATX. We compared effects of wild-type human ATX, catalytically inactive ATX, an integrin binding-defective ATX variant with wild-type lysoPLD activity, the isolated ATX integrin binding N-terminal domain, and a potent ATX selective lysoPLD inhibitor on cell migration using transwell and single-cell tracking assays. Stimulation of transwell migration was reduced (18 or 27% of control, respectively) but not ablated by inactivation of integrin binding or inhibition of lysoPLD activity. The N-terminal domain increased transwell migration (30% of control). ATX lysoPLD activity and integrin binding were necessary for a 3.8-fold increase in the fraction of migrating breast cancer cell step velocities >0.7 μm/min. ATX increased the persistent directionality of single-cell migration 2-fold. This effect was lysoPLD activity independent and recapitulated by the integrin binding N-terminal domain. Integrin binding enables uptake and intracellular sequestration of ATX, which redistributes to the front of migrating cells. ATX binding to integrins and lysoPLD activity therefore cooperate to promote rapid persistent directional cell migration.

Necessity of lysophosphatidic acid receptor 1 for development of arthritis

OBJECTIVE

Lysophosphatidic acid (LPA) is a bioactive lipid that binds to a group of cell surface G protein-coupled receptors (LPA receptors 1-6 [LPA1-6 ]) and has been implicated as an important mediator of angiogenesis, inflammation, and cancer growth. This study was undertaken to analyze the effects of LPA1 on the development of arthritis.

METHODS

Expression of LPA receptors on synovial tissue was analyzed by immunohistochemistry and quantitative reverse transcription-polymerase chain reaction. The effects of abrogation of LPA1 on collagen-induced arthritis (CIA) were evaluated using LPA1 -deficient mice or LPA1 antagonist. Migrating fluorescence-labeled CD11b+ splenocytes, which were transferred into the synovium of mice with CIA, were counted. CD4+ naive T cells were incubated under Th1-, Th2-, or Th17-polarizing conditions, and T helper cell differentiation was assessed. Osteoclast formation from bone marrow cells was examined.

RESULTS

LPA1 was highly expressed in the synovium of patients with rheumatoid arthritis (RA) compared with that of patients with osteoarthritis. LPA1 -deficient mice did not develop arthritis following immunization with type II collagen (CII). LPA1 antagonist also ameliorated murine CIA. Abrogation of LPA1 was associated with reductions in cell infiltration, bone destruction in the joints, and interleukin-17 production from CII-stimulated splenocytes. Infiltration of transferred CD11b+ macrophages from LPA1 -deficient mice into the synovium was suppressed compared with infiltration of macrophages from wild-type mice. LPA1 antagonist inhibited the infiltration of macrophages from wild-type mice. Differentiation into Th17, but not Th1 or Th2, and osteoclast formation were also suppressed under conditions of LPA1 deficiency or LPA1 inhibition in vitro.

CONCLUSION

Collectively, these results indicate that LPA/LPA1 signaling contributes to the development of arthritis via cellular infiltration, Th17 differentiation, and osteoclastogenesis. Thus, LPA1 may be a promising target molecule for RA therapy.

Therapeutic potentials of ecto-nucleoside triphosphate diphosphohydrolase, ecto-nucleotide pyrophosphatase/phosphodiesterase, ecto-5'-nucleotidase, and alkaline phosphatase inhibitors

The modulatory role of extracellular nucleotides and adenosine in relevance to purinergic cell signaling mechanisms has long been known and is an object of much research worldwide. These extracellular nucleotides are released by a variety of cell types either innately or as a response to patho-physiological stress or injury. A variety of surface-located ecto-nucleotidases (of four major types; nucleoside triphosphate diphosphohydrolases or NTPDases, nucleotide pyrophosphatase/phosphodiesterases or NPPs, alkaline phosphatases APs or ALPs, and ecto-5'-nucleotidase or e5NT) are responsible for meticulously controlling the availability of these important signaling molecules (at their respective receptors) in extracellular environment and are therefore crucial for maintaining the integrity of normal cell functioning. Overexpression of many of these ubiquitous ecto-enzymes has been implicated in a variety of disorders including cell adhesion, activation, proliferation, apoptosis, and degenerative neurological and immunological responses. Selective inhibition of these ecto-enzymes is an area that is currently being explored with great interest and hopes remain high that development of selective ecto-nucleotidase inhibitors will prove to have many beneficial therapeutic implications. The aim of this review is to emphasize and focus on recent developments made in the field of inhibitors of ecto-nucleotidases and to highlight their structure activity relationships wherever possible. Most recent and significant advances in field of NTPDase, NPP, AP, and e5NT inhibitors is being discussed in detail in anticipation of providing prolific leads and relevant background for research groups interested in synthesis of selective ecto-nucleotidase inhibitors.

Hits of a high-throughput screen identify the hydrophobic pocket of autotaxin/lysophospholipase D as an inhibitory surface

Autotaxin (ATX), a lysophospholipase D, plays an important role in cancer invasion, metastasis, tumor progression, tumorigenesis, neuropathic pain, fibrotic diseases, cholestatic pruritus, lymphocyte homing, and thrombotic diseases by producing the lipid mediator lysophosphatidic acid (LPA). A high-throughput screen of ATX inhibition using the lysophosphatidylcholine-like substrate fluorogenic substrate 3 (FS-3) and ∼10,000 compounds from the University of Cincinnati Drug Discovery Center identified several small-molecule inhibitors with IC₅₀ vales ranging from nanomolar to low micromolar. The pharmacology of the three most potent compounds: 918013 (1; 2,4-dichloro-N-(3-fluorophenyl)-5-(4-morpholinylsulfonyl) benzamide), 931126 (2; 4-oxo-4-{2-[(5-phenoxy-1H-indol-2-yl)carbonyl]hydrazino}-N-(4-phenylbutan-2-yl)butanamide), and 966791 (3; N-(2,6-dimethylphenyl)-2-[N-(2-furylmethyl)(4-(1,2,3,4-tetraazolyl)phenyl)carbonylamino]-2-(4-hydroxy-3-methoxyphenyl) acetamide), were further characterized in enzyme, cellular, and whole animal models. Compounds 1 and 2 were competitive inhibitors of ATX-mediated hydrolysis of the lysophospholipase substrate FS-3. In contrast, compound 3 was a competitive inhibitor of both FS-3 and the phosphodiesterase substrate p-nitrophenyl thymidine 5'-monophosphate. Computational docking and mutagenesis suggested that compounds 1 and 2 target the hydrophobic pocket, thereby blocking access to the active site of ATX. The potencies of compounds 1-3 were comparable to each other in each of the assays. All of these compounds significantly reduced invasion of A2058 human melanoma cells in vitro and the colonization of lung metastases by B16-F10 murine melanoma cells in C57BL/6 mice. The compounds had no agonist or antagonist effects on select LPA or sphingosine 1-phosphate receptors, nor did they inhibit nucleotide pyrophosphatase/phosphodiesterase (NPP) enzymes NPP6 and NPP7. These results identify the molecular surface of the hydrophobic pocket of ATX as a target-binding site for inhibitors of enzymatic activity.

Autotaxin inhibitors: a patent review

INTRODUCTION

Autotaxin (ATX) is a lysophospholipase D enzyme that hydrolyzes lysophosphatidylcholine to lysophosphatidic acid (LPA) and choline. LPA is a bioactive lipid mediator that activates several transduction pathways, and is involved in migration, proliferation and survival of various cells. Thus, ATX is an attractive medicinal target.

AREAS COVERED

The aim of this review is to summarize ATX inhibitors, reported in patents from 2006 up to now, describing their discovery and biological evaluation.

EXPERT OPINION

ATX has been implicated in various pathological conditions, such as cancer, chronic inflammation, neuropathic pain, fibrotic diseases, etc. Although there is an intensive effort on the discovery of potent and selective ATX inhibitors in order to identify novel medicinal agents, up to now, no ATX inhibitor has reached clinical trials. However, the use of ATX inhibitors seems an attractive strategy for the development of novel medicinal agents, for example anticancer therapeutics.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Role of the autotaxin-lysophosphatidate axis in cancer resistance to chemotherapy and radiotherapy

High expression of autotaxin in cancers is often associated with increased tumor progression, angiogenesis and metastasis. This is explained mainly since autotaxin produces the lipid growth factor, lysophosphatidate (LPA), which stimulates cell division, survival and migration. It has recently become evident that these signaling effects of LPA also produce resistance to chemotherapy and radiation-induced cell death. This results especially from the stimulation of LPA(2) receptors, which depletes the cell of Siva-1, a pro-apoptotic signaling protein and stimulates prosurvival kinase pathways through a mechanism mediated via TRIP-6. LPA signaling also increases the formation of sphingosine 1-phosphate, a pro-survival lipid. At the same time, LPA decreases the accumulation of ceramides, which are used in radiation therapy and by many chemotherapeutic agents to stimulate apoptosis. The signaling actions of extracellular LPA are terminated by its dephosphorylation by a family of lipid phosphate phosphatases (LPP) that act as ecto-enzymes. In addition, lipid phosphate phoshatase-1 attenuates signaling downstream of the activation of both LPA receptors and receptor tyrosine kinases. This makes many cancer cells hypersensitive to the action of various growth factors since they often express low LPP1/3 activity. Increasing our understanding of the complicated signaling pathways that are used by LPA to stimulate cell survival should identify new therapeutic targets that can be exploited to increase the efficacy of chemo- and radio-therapy. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

The polybasic insertion in autotaxin α confers specific binding to heparin and cell surface heparan sulfate proteoglycans

Autotaxin (ATX) is a secreted lysophospholipase D that generates the lipid mediator lysophosphatidic acid (LPA), playing a key role in diverse physiological and pathological processes. ATX exists in distinct splice variants, but isoform-specific functions remain elusive. Here we characterize the ATXα isoform, which differs from the canonical form (ATXβ) in having a 52-residue polybasic insertion of unknown function in the catalytic domain. We find that the ATXα insertion is susceptible to cleavage by extracellular furin-like endoproteases, but cleaved ATXα remains structurally and functionally intact due to strong interactions within the catalytic domain. Through ELISA and surface plasmon resonance assays, we show that ATXα binds specifically to heparin with high affinity (K(d) ~10(-8) M), whereas ATXβ does not; furthermore, heparin moderately enhanced the lysophospholipase D activity of ATXα. We further show that ATXα, but not ATXβ, binds abundantly to SKOV3 carcinoma cells. ATXα binding was abolished after treating the cells with heparinase III, but not after chondroitinase treatment. Thus, the ATXα insertion constitutes a cleavable heparin-binding domain that mediates interaction with heparan sulfate proteoglycans, thereby targeting LPA production to the plasma membrane.

Structure-function relationships of autotaxin, a secreted lysophospholipase D

Autotaxin (ATX or ENPP2) is an ectonucleotide pyrophosphatase/phosphodiesterase (ENPP) that functions as a secreted lysophospholipase D to produce the multifunctional lipid mediator lysophosphatidic acid (LPA) from more complex lysophospholipids. LPA acts on distinct G protein-coupled receptors thereby activating multiple signaling cascades and cellular responses. The ATX-LPA signaling axis is implicated in a remarkably wide variety of physiological and pathological processes, ranging from vascular and neural development to lymphocyte homing, fibrosis and cancer. Despite much progress in understanding LPA receptor signaling, the precise mode of action of ATX has long remained elusive due to the lack of structural data. In particular, it has been unclear what makes ATX a unique lysophospholipase D and how the enzyme is targeted to LPA-responsive cells. Recent structural studies have begun to clarify these issues. Here we discuss new insights and inferences from the ATX structure.

Galectin-3 contributes to melanoma growth and metastasis via regulation of NFAT1 and autotaxin

Melanoma is the deadliest form of skin cancer in which patients with metastatic disease have a 5-year survival rate of less than 10%. Recently, the overexpression of a β-galactoside binding protein, galectin-3 (LGALS3), has been correlated with metastatic melanoma in patients. We have previously shown that silencing galectin-3 in metastatic melanoma cells reduces tumor growth and metastasis. Gene expression profiling identified the protumorigenic gene autotaxin (ENPP2) to be downregulated after silencing galectin-3. Here we report that galectin-3 regulates autotaxin expression at the transcriptional level by modulating the expression of the transcription factor NFAT1 (NFATC2). Silencing galectin-3 reduced NFAT1 protein expression, which resulted in decreased autotaxin expression and activity. Reexpression of autotaxin in galectin-3 silenced melanoma cells rescues angiogenesis, tumor growth, and metastasis in vivo. Silencing NFAT1 expression in metastatic melanoma cells inhibited tumor growth and metastatic capabilities in vivo. Our data elucidate a previously unidentified mechanism by which galectin-3 regulates autotaxin and assign a novel role for NFAT1 during melanoma progression.

Constitutive autotaxin transcription by Nmyc-amplified and non-amplified neuroblastoma cells is regulated by a novel AP-1 and SP-mediated mechanism and abrogated by curcumin

The motility, angiogenesis and metastasis-stimulating factor Autotaxin (Atx), over expressed by human neuroblastomas (NB), is constitutively expressed by human Nmyc-amplified SK-N-BE and non-Nmyc-amplified SH-SY5Y NB cells. Here, we characterise a novel Atx transcriptional mechanism, utilised by both cell lines, that is restricted to the first 285bp of the Atx promoter and involves AP-1 and SP transcription factors, acting through a CRE/AP-1-like element at position -142 to -149 and a GAbox at position -227 to -235 relative to the Atx translational start site. This novel transcriptional mechanism can be inhibited by internally initiated SP-3 and the natural phenol curcumin.

Lysophosphatidic acid: a potential mediator of osteoblast-osteoclast signaling in bone

Osteoclasts (bone resorbing cells) and osteoblasts (bone forming cells) play essential roles in skeletal development, mineral homeostasis and bone remodeling. The actions of these two cell types are tightly coordinated, and imbalances in bone formation and resorption can result in disease states, such as osteoporosis. Lysophosphatidic acid (LPA) is a potent bioactive phospholipid that influences a number of cellular processes, including proliferation, survival and migration. LPA is also involved in wound healing and pathological conditions, such as tumor metastasis and autoimmune disorders. During trauma, activated platelets are likely a source of LPA in bone. Physiologically, osteoblasts themselves can also produce LPA, which in turn promotes osteogenesis. The capacity for local production of LPA, coupled with the proximity of osteoblasts and osteoclasts, leads to the intriguing possibility that LPA acts as a paracrine mediator of osteoblast-osteoclast signaling. Here we summarize emerging evidence that LPA enhances the differentiation of osteoclast precursors, and regulates the morphology, resorptive activity and survival of mature osteoclasts. These actions arise through stimulation of multiple LPA receptors and intracellular signaling pathways. Moreover, LPA is a potent mitogen implicated in promoting the metastasis of breast and ovarian tumors to bone. Thus, LPA released from osteoblasts is potentially an important autocrine and paracrine mediator - physiologically regulating skeletal development and remodeling, while contributing pathologically to metastatic bone disease. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Bone defects in LPA receptor genetically modified mice

LPA and LPA(1) have been shown to increase osteoblastic proliferation and differentiation as well as activation of osteoclasts. Cell and animal model studies have suggested that LPA is produced by bone cells and bone tissues. We obtained data from invalidated mice which support the hypothesis that LPA(1) is involved in bone development by promoting osteogenesis. LPA(1)-invalidated mice demonstrate growth and sternal and costal abnormalities, which highlights the specific roles of LPA(1) during bone development. Microcomputed tomography and histological analysis demonstrate osteoporosis in the trabecular and cortical bone of LPA(1)-invalidated mice. Moreover, bone marrow mesenchymal progenitors from these mice displayed decreased osteoblastic differentiation. Infrared analysis did not indicate osteomalacia in the bone tissue of LPA(1)-invalidated mice. LPA(1) displays opposite effects to LPA(4) on the related G proteins G(i) and G(s), responsible for decrease and increase of the cAMP level respectively, which itself is essential to the control of osteoblastic differentiation. The opposite effects of LPA(1) and LPA(4) during osteoblastic differentiation support the possibility that new pharmacological agents derived from the LPA pathways could be found and used in clinical practice to positively influence bone formation and treat osteoporosis. The paracrine effect of LPA is potentially modulated by its concentration in bone tissues, which may result from various intracellular and extracellular pathways. The relevance of LPA(1) in bone remodeling, as a receptor able to influence both osteoblast and osteoclast activity, still deserves further clarification. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Controlling cancer through the autotaxin-lysophosphatidic acid receptor axis

LPA (lysophosphatidic acid, 1-acyl-2-hydroxy-sn-glycero-3-phosphate), is a growth factor-like lipid mediator that regulates many cellular functions, many of which are unique to malignantly transformed cells. The simple chemical structure of LPA and its profound effects in cancer cells has attracted the attention of the cancer therapeutics field and drives the development of therapeutics based on the LPA scaffold. In biological fluids, LPA is generated by ATX (autotaxin), a lysophospholipase D that cleaves the choline/serine headgroup from lysophosphatidylcholine and lysophosphatidylserine to generate LPA. In the present article, we review some of the key findings that make the ATX-LPA signalling axis an emerging target for cancer therapy.

Autotaxin and LPA receptor signaling in cancer

Lysophosphatidic acid (LPA; monoacyl-glycerol-3-phosphate) is a lipid mediator that functions as a mitogen and motility factor for many cell types. LPA signals through six specific G protein-coupled receptors, named LPA(1-6), which trigger both overlapping and distinct signaling pathways. LPA is produced from extracellular lysophosphatidylcholine by a secreted lysophospholipase D, named autotaxin (ATX), originally identified as an "autocrine motility factor" for tumor cells. ATX-LPA signaling is vital for embryonic development and promotes tumor formation, angiogenesis, and experimental metastasis in mice. Elevated expression of ATX and/or aberrant expression of LPA receptors are found in several human malignancies, while loss of LPA(6) function has been implicated in bladder cancer. In this review, we summarize our present understanding of ATX and LPA receptor signaling in cancer.

The emerging role of lysophosphatidic acid (LPA) in skeletal biology

Lysophosphatidic acid (LPA) is the simplest signalling lipid eliciting pleiotropic actions upon most mammalian cell types. Although LPA has an established role in many biological processes, particularly wound healing and cancer, the participation of LPA in skeletal biology is just beginning to emerge. Early studies, identified in this review, gave a solid indication that LPA, via binding to one of several cell surface receptors, activated multiple intracellular systems culminating in altered cell morphology, growth, motility and survival. More recently the ablation of murine LPA1 and 4 receptors implies that this lipid has a role in skeletal development and post natal bone accrual. Greater understanding of the ability of LPA to influence, for example, osteoblast growth, maturation and survival could be advantageous in developing novel strategies aimed at improving skeletal tissue repair and regeneration. Herein this review provides an insight into the diversity of studies exploring the actions of a small lipid on those major cell types key to skeletal tissue health and homeostasis.

Targeting lysophosphatidic acid receptor type 1 with Debio 0719 inhibits spontaneous metastasis dissemination of breast cancer cells independently of cell proliferation and angiogenesis

Metastasis is the main cause of death for cancer patients. Targeting factors that control metastasis formation is a major challenge for clinicians. Lysophosphatidic acid (LPA) is a bioactive phospholipid involved in cancer. LPA activates at least six independent G protein-coupled receptors (LPA_1–6). Tumor cells frequently co-express multiple LPA receptors, puzzling the contribution of each one to cancer progression. All three receptors, LPA₁, LPA₂ and LPA₃, act as oncogenes and prometastatic factors in the mouse mammary gland. The competitive inhibitor of LPA₁ and LPA₃ receptors, Ki16425, inhibits efficiently breast cancer bone metastases in animal models. We showed here that Debio 0719, which corresponds to the R-stereoisomer of Ki16425 exhibited highest antagonist activities at LPA₁ (IC₅₀=60 nM) and LPA₃ (IC₅₀=660 nM) than Ki16425 [IC₅₀=130 nM (LPA₁); IC₅₀=2.3 μM (LPA₃)]. In vitro, Debio 0719, inhibited LPA-dependent invasion of the 4T1 mouse mammary cancer cells. In vivo, early but not late administration of Debio 0719 (50 mg/kg p.o. twice daily) to BALB/c mice during the course of orthotopic 4T1 primary tumor growth reduced the number of spontaneously disseminated tumor cells to bone and lungs without affecting the growth of primary tumors and tumor-induced angiogenesis. We found that increased LPA₁ mRNA expression in primary tumors of breast cancer patients correlated significantly with their positive lymph node status (p<0.001). Altogether, our results suggest that LPA₁ controls early events of metastasis independently of cell proliferation and angiogenesis. Therefore, targeting this receptor with Debio 0719 has a high therapeutic potential against metastasis formation for breast cancer patients.

LPA is a chemorepellent for B16 melanoma cells: action through the cAMP-elevating LPA5 receptor

Lysophosphatidic acid (LPA), a lipid mediator enriched in serum, stimulates cell migration, proliferation and other functions in many cell types. LPA acts on six known G protein-coupled receptors, termed LPA_1–6, showing both overlapping and distinct signaling properties. Here we show that, unexpectedly, LPA and serum almost completely inhibit the transwell migration of B16 melanoma cells, with alkyl-LPA(18∶1) being 10-fold more potent than acyl-LPA(18∶1). The anti-migratory response to LPA is highly polarized and dependent on protein kinase A (PKA) but not Rho kinase activity; it is associated with a rapid increase in intracellular cAMP levels and PIP3 depletion from the plasma membrane. B16 cells express LPA₂, LPA₅ and LPA₆ receptors. We show that LPA-induced chemorepulsion is mediated specifically by the alkyl-LPA-preferring LPA₅ receptor (GPR92), which raises intracellular cAMP via a noncanonical pathway. Our results define LPA₅ as an anti-migratory receptor and they implicate the cAMP-PKA pathway, along with reduced PIP3 signaling, as an effector of chemorepulsion in B16 melanoma cells.

Stat3 mediates expression of autotaxin in breast cancer

We determined that signal transducer and activator of transcription 3 (Stat3) is tyrosine phosphorylated in 37% of primary breast tumors and 63% of paired metastatic axillary lymph nodes. Examination of the distribution of tyrosine phosphorylated (pStat3) in primary tumors revealed heterogenous expression within the tumor with the highest levels found in cells on the edge of tumors with relatively lower levels in the central portion of tumors. In order to determine Stat3 target genes that may be involved in migration and metastasis, we identified those genes that were differentially expressed in primary breast cancer samples as a function of pStat3 levels. In addition to known Stat3 transcriptional targets (Twist, Snail, Tenascin-C and IL-8), we identified ENPP2 as a novel Stat3 regulated gene, which encodes autotaxin (ATX), a secreted lysophospholipase which mediates mammary tumorigenesis and cancer cell migration. A positive correlation between nuclear pStat3 and ATX was determined by immunohistochemical analysis of primary breast cancer samples and matched axillary lymph nodes and in several breast cancer derived cell lines. Inhibition of pStat3 or reducing Stat3 expression led to a decrease in ATX levels and cell migration. An association between Stat3 and the ATX promoter, which contains a number of putative Stat3 binding sites, was determined by chromatin immunoprecipitation. These observations suggest that activated Stat3 may regulate the migration of breast cancer cells through the regulation of ATX.

Differential roles for the p101 and p84 regulatory subunits of PI3Kγ in tumor growth and metastasis

Phosphoinositide 3-kinase γ (PI3Kγ) consists of a catalytic subunit p110γ, which forms mutually exclusive dimers with one of the regulatory subunits called p101 and p84/p87(PIKAP). Recently, PI3Kγ emerged as being a potential oncogene because overexpression of the catalytic subunit p110γ or the regulatory subunit p101 leads to oncogenic cellular transformation and malignancy. However, the contribution of the individual subunits to tumor growth and metastasis and the mechanisms involved are not understood. We therefore individually knocked down the PI3Kγ subunits (p84, p101 and p110γ) in MDA-MB-231 cells, which reduced in vitro migration of the cell lines. Knockdown of p110γ or p101 inhibited apoptosis, Akt phosphorylation and lung colonization in SCID mice. Similarly, the knockdown of p110γ and p101 in murine epithelial carcinoma 4T1.2 cells inhibited primary tumor growth and spontaneous metastasis, as well as lung colonization. In contrast, knockdown of p84 in MDA-MB-231 cells enhanced Akt phosphorylation and lung colonization. These findings are the first to implicate differential functions of the two PI3Kγ regulatory subunits in the process of oncogenesis, and indicate that loss of p101 is sufficient to reduce in vivo tumor growth and metastasis to the same extent as that of p110γ.

Ligand-based autotaxin pharmacophore models reflect structure-based docking results

The autotaxin (ATX) enzyme exhibits lysophospholipase D activity responsible for the conversion of lysophosphatidyl choline to lysophosphatidic acid (LPA). ATX and LPA have been linked to the initiation of atherosclerosis, cancer invasiveness, and neuropathic pain. ATX inhibition therefore offers currently unexploited therapeutic potential, and substantial interest in the development of ATX inhibitors is evident in the recent literature. Here we report the performance-based comparison of ligand-based pharmacophores developed on the basis of different combinations of ATX inhibitors in the training sets against an extensive database of compounds tested for ATX inhibitory activity, as well as with docking results of the actives against a recently reported ATX crystal structure. In general, pharmacophore models show better ability to select active ATX inhibitors binding in a common location when the ligand-based superposition shows a good match to the superposition of actives based on docking results. Two pharmacophore models developed on the basis of competitive inhibitors in combination with the single inhibitor crystallized to date in the active site of ATX were able to identify actives at rates over 40%, a substantial improvement over the <10% representation of active site-directed actives in the test set database.

Insights into autotaxin: how to produce and present a lipid mediator

Autotaxin (ATX) is a secreted phosphodiesterase that produces the lipid mediator lysophosphatidic acid (LPA). LPA acts through specific guanine-nucleotide-binding protein (G protein)-coupled receptors to stimulate migration, proliferation, survival and other functions in many cell types. ATX is important in vivo for processes as diverse as vasculogenesis, lymphocyte trafficking and tumour progression. However, the inner workings of ATX have long been elusive, in terms of both its substrate specificity and how localized LPA signalling is achieved. Structural studies have shown how ATX recognizes its substrates and may interact with the cell surface to promote specificity in LPA signalling.

Dual function of ERRα in breast cancer and bone metastasis formation: implication of VEGF and osteoprotegerin

Bone metastasis is a complication occurring in up to 70% of advanced breast cancer patients. The estrogen receptor-related receptor alpha (ERRα) has been implicated in breast cancer and bone development, prompting us to examine whether ERRα may function in promoting the osteolytic growth of breast cancer cells in bone. In a mouse xenograft model of metastatic human breast cancer, overexpression of wild-type ERRα reduced metastasis, whereas overexpression of a dominant negative mutant promoted metastasis. Osteoclasts were directly affected and ERRα upregulated the osteoclastogenesis inhibitor, osteoprotegerin (OPG), providing a direct mechanistic basis for understanding how ERRα reduced breast cancer cell growth in bone. In contrast, ERRα overexpression increased breast cancer cell growth in the mammary gland. ERRα-overexpressing primary tumors were highly vascularized, consistent with an observed upregulation of angiogenic growth factor, the VEGF. In support of these findings, we documented that elevated expression of ERRα mRNA in breast carcinomas was associated with high expression of OPG and VEGF and with disease progression. In conclusion, our results show that ERRα plays a dual role in breast cancer progression in promoting the local growth of tumor cells, but decreasing metastatic growth of osteolytic lesions in bone.

Characterization of a sphingosine 1-phosphate receptor antagonist prodrug

Sphingosine 1-phosphate (S1P) is a phospholipid that binds to a set of G protein-coupled receptors (S1P(1)-S1P(5)) to initiate an array of signaling cascades that affect cell survival, differentiation, proliferation, and migration. On a larger physiological scale, the effects of S1P on immune cell trafficking, vascular barrier integrity, angiogenesis, and heart rate have also been observed. An impetus for the characterization of S1P-initiated signaling effects came with the discovery that FTY720 [fingolimod; 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol] modulates the immune system by acting as an agonist at S1P(1). In the course of structure-activity relationship studies to better understand the functional chemical space around FTY720, we discovered conformationally constrained FTY720 analogs that behave as S1P receptor type-selective antagonists. Here, we present a pharmacological profile of a lead S1P(1/3) antagonist prodrug, 1-(hydroxymethyl)-3-(3-octylphenyl)cyclobutane (VPC03090). VPC03090 is phosphorylated by sphingosine kinase 2 to form the competitive antagonist species 3-(3-octylphenyl)-1-(phosphonooxymethyl)cyclobutane (VPC03090-P) as observed in guanosine 5'-O-(3-[(35)S]thio)triphosphate binding assays, with effects on downstream S1P receptor signaling confirmed by Western blot and calcium mobilization assays. Oral dosing of VPC03090 results in an approximate 1:1 phosphorylated/alcohol species ratio with a half-life of 30 h in mice. Because aberrant S1P signaling has been implicated in carcinogenesis, we applied VPC03090 in an immunocompetent mouse mammary cancer model to assess its antineoplastic potential. Treatment with VPC03090 significantly inhibited the growth of 4T1 primary tumors in mice. This result calls to attention the value of S1P receptor antagonists as not only research tools but also potential therapeutic agents.

Development of an activity-based probe for autotaxin

Autotaxin (ATX), or ecto-nucleotide pyrophosphatase/phosphodiesterase 2 (ENPP2), is a secreted lysophospholipase D that hydrolyses lysophosphatidylcholine into the lipid mediator lysophosphatidic acid (LPA), a mitogen and chemoattractant for many cell types. ATX has been implicated in tumour progression and inflammation, and might serve as a biomarker. Here we describe the development of a fluorescent activity-based probe that covalently binds to the active site of ATX. The probe consists of a lysophospholipid-based backbone linked to a trapping moiety that becomes reactive after phosphate ester hydrolysis, and a Cy5 fluorescent dye to allow visualisation of active ATX. The probe reacts specifically with the three known isoforms of ATX, it competes with small-molecule inhibitors for binding to ATX and allows ATX activity in plasma to be determined. Our activity-based reporter will be useful for monitoring ATX activity in biological fluids and for inhibitor screening.

Structural basis of substrate discrimination and integrin binding by autotaxin

Autotaxin (ATX, also known as ectonucleotide pyrophosphatase/phosphodiesterase-2, ENPP2) is a secreted lysophospholipase D that generates the lipid mediator lysophosphatidic acid (LPA), a mitogen and chemoattractant for many cell types. ATX-LPA signaling is involved in various pathologies including tumor progression and inflammation. However, the molecular basis of substrate recognition and catalysis by ATX and the mechanism by which it interacts with target cells are unclear. Here, we present the crystal structure of ATX, alone and in complex with a small-molecule inhibitor. We have identified a hydrophobic lipid-binding pocket and mapped key residues for catalysis and selection between nucleotide and phospholipid substrates. We have shown that ATX interacts with cell-surface integrins through its N-terminal somatomedin B-like domains, using an atypical mechanism. Our results define determinants of substrate discrimination by the ENPP family, suggest how ATX promotes localized LPA signaling and suggest new approaches for targeting ATX with small-molecule therapeutic agents.

Contribution of platelets to tumour metastasis

Extensive experimental evidence shows that platelets support tumour metastasis. The activation of platelets and the coagulation system have a crucial role in the progression of cancer. Within the circulatory system, platelets guard tumour cells from immune elimination and promote their arrest at the endothelium, supporting the establishment of secondary lesions. These contributions of platelets to tumour cell survival and spread suggest platelets as a new avenue for therapy.

CARMA3: A novel scaffold protein in regulation of NF-κB activation and diseases

CARD recruited membrane associated protein 3 (CARMA3) is a novel scaffold protein. It belongs to the CARMA protein family, and is known to activate nuclear factor (NF)-κB. However, it is still unknown which receptor functions upstream of CARMA3 to trigger NF-κB activation. Recently, several studies have demonstrated that CARMA3 serves as an indispensable adaptor protein in NF-κB signaling under some G protein-coupled receptors (GPCRs), such as lysophosphatidic acid (LPA) receptor and angiotensin (Ang) II receptor. Mechanistically, CARMA3 recruits its essential downstream molecules Bcl10 and MALT1 to form the CBM (CARMA3-Bcl10-MALT1) signalosome whereby it triggers NF-κB activation. GPCRs and NF-κB play pivotal roles in the regulation of various cellular functions, therefore, aberrant regulation of the GPCR/NF-κB signaling axis leads to the development of many types of diseases, such as cancer and atherogenesis. Recently, the GPCR/CARMA3/NF-κB signaling axis has been confirmed in these specific diseases and it plays crucial roles in the pathogenesis of disease progression. In ovarian cancer cell lines, knockdown of CARMA3 abolishes LPA receptor-induced NF-κB activation, and reduces LPA-induced ovarian cancer invasion. In vascular smooth cells, downregulation of CARMA3 substantially impairs Ang-II-receptor-induced NF-κB activation, and in vivo studies have confirmed that Bcl10-deficient mice are protected from developing Ang-II-receptor-induced atherosclerosis and aortic aneurysms. In this review, we summarize the biology of CARMA3, describe the role of the GPCR/CARMA3/NF-κB signaling axis in ovarian cancer and atherogenesis, and speculate about the potential roles of this signaling axis in other types of cancer and diseases. With a significant increase in the identification of LPA- and Ang-II-like ligands, such as endothelin-1, which also activates NF-κB via CARMA3 and contributes to the development of many diseases, CARMA3 is emerging as a novel therapeutic target for various types of cancer and other diseases.

Autotaxin inhibitors: a perspective on initial medicinal chemistry efforts

The lysophospholipase D enzyme, autotaxin (ATX), has been linked to numerous human diseases including cancer, neurophatic pain, obesity and Alzheimer's disease. Although the ATX protein was initially purified and characterized in 1992, a link to bioactive lipid metabolism was not made until 2002. In the past decade, metal chelators, lysophospholipid product analogs, and more recently, small non-lipid inhibitors of the enzyme were successfully identified. The majority of these inhibitors have been characterized using recombinant purified ATX in vitro, with very few examples studied in more complex systems. Translation of ATX inhibitors from the hands of medicinal chemists to clinical use will require substantially expanded characterization of ATX inhibitors in vivo.

Lysophosphatidic acid signals through multiple receptors in osteoclasts to elevate cytosolic calcium concentration, evoke retraction, and promote cell survival

Lysophosphatidic acid (LPA) is a bioactive phospholipid whose functions are mediated by multiple G protein-coupled receptors. We have shown that osteoblasts produce LPA, raising the possibility that it mediates intercellular signaling among osteoblasts and osteoclasts. Here we investigated the expression, signaling and function of LPA receptors in osteoclasts. Focal application of LPA elicited transient increases in cytosolic calcium concentration ([Ca(2+)](i)), with 50% of osteoclasts responding at approximately 400 nm LPA. LPA-induced elevation of [Ca(2+)](i) was blocked by pertussis toxin or the LPA(1/3) receptor antagonist VPC-32183. LPA caused sustained retraction of osteoclast lamellipodia and disrupted peripheral actin belts. Retraction was insensitive to VPC-32183 or pertussis toxin, indicating involvement of a distinct signaling pathway. In this regard, inhibition of Rho-associated kinase stimulated respreading after LPA-induced retraction. Real-time reverse transcription-PCR revealed transcripts encoding LPA(1) and to a lesser extent LPA(2), LPA(4), and LPA(5) receptor subtypes. LPA induced nuclear translocation of NFATc1 and enhanced osteoclast survival, effects that were blocked by VPC-32183 or by a specific peptide inhibitor of NFAT activation. LPA slightly reduced the resorptive activity of osteoclasts in vitro. Thus, LPA binds to at least two receptor subtypes on osteoclasts: LPA(1), which couples through G(i/o) to elevate [Ca(2+)](i), activate NFATc1, and promote survival, and a second receptor that likely couples through G(12/13) and Rho to evoke and maintain retraction through reorganization of the actin cytoskeleton. These findings reveal a signaling axis in bone through which LPA, produced by osteoblasts, acts on multiple receptor subtypes to induce pleiotropic effects on osteoclast activity and function.

c-Src-mediated phosphorylation of thyroid hormone receptor-interacting protein 6 (TRIP6) promotes osteoclast sealing zone formation

Osteoclasts resorb bone through the formation of a unique attachment structure called the sealing zone. In this study, a role for thyroid hormone receptor-interacting protein 6 (TRIP6) in sealing zone formation and osteoclast activity was examined. TRIP6 was shown to reside in the sealing zone through its association with tropomyosin 4, an actin-binding protein that regulates sealing dimensions and bone resorptive capacity. Suppression of TRIP6 in mature osteoclasts by RNA interference altered sealing zone dimensions and inhibited bone resorption, whereas overexpression of TRIP6 increased the sealing zone perimeter and enhanced bone resorption. Treatment of osteoclasts with lysophosphatidic acid (LPA), which phosphorylates TRIP6 at tyrosine 55 through a c-Src-dependent mechanism, caused increased association of TRIP6 with the sealing zone, as did overexpression of a TRIP6 cDNA bearing a phosphomimetic mutation at tyrosine 55. Further, LPA treatment caused increases in osteoclast fusion, sealing zone perimeter, and bone resorptive capacity. In contrast, overexpression of TRIP6 containing a nonphosphorylatable amino acid residue at position 55 severely diminished sealing zone formation and bone resorption and suppressed the effects of LPA on the cytoskeleton. LPA effects were mediated through its receptor isoform LPA(2), as indicated by treatments with receptor-specific agonists and antagonists. Thus, these studies suggest that TRIP6 is a critical downstream regulator of c-Src signaling and that its phosphorylation is permissive for its presence in the sealing zone where it plays a positive role in osteoclast bone resorptive capacity.