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

New insights into pathogenesis of congenital pseudarthrosis of tibia in children using periosteum proteomics analysis

RATIONALE

The exact etiology and pathogenesis of congenital pseudarthrosis of tibia (CPT) are not clear. Quantitative proteomics analysis plays a vital role in disease pathology research. Tandem mass tag (TMT)-based proteomics techniques were employed to identify and analyze the differentially expressed proteins (DEP) in the tibia periosteum tissues of CPT patients.

METHODS

The samples were divided into three groups: CPT with NF1 group, CPT without NF1 group (non-NF1-CPT), and control group (patients with open tibial fracture). A fold change ≥1.5 or ≤0.66 and P-value <0.05 were used as the thresholds to screen DEPs. Subsequently, bioinformatics resources such as online tools DAVID and String were used to generate gene ontology (GO) annotation, KEGG pathways enrichment, and protein-protein interaction (PPI) network for these DEPs.

RESULTS

The results show that a total of 347 proteins were differentially expressed in NF1-CPT groups, 212 of which were upregulated and 135 were downregulated. There were more DEPs in non-NF1-CPT groups; we identified 467 DEPs, including 281 upregulated and 186 downregulated. Among them, NF1-CPT groups and non-NF1-CPT groups shared 231 DEPs, and the remaining 230 DEPs showed the same expression trend in the two disease groups, with 117 upregulated and 113 downregulated. In particular, 116 proteins were altered only in NF1-CPT groups (94 were upregulated and 22 were downregulated), whereas 236 proteins were altered only in non-NF1-CPT groups (164 were upregulated and 72 were downregulated). Finally, compared with non-NF1-CPT groups, 47 proteins changed 1.5-fold and P-value < 0.05 in NF1-CPT groups.

CONCLUSIONS

To sum up, we found that common DEPS in periosteum of NF1-CPT and non-NF1-CPT groups are mainly involved in cell matrix assembly, cell adhesion, AKT-PI3K signal pathway activation, and vascular agglutination, which indicate that these are the pathological characteristics of CPT. The osteogenic ability is weak, the osteoclastic ability is strong, the vascular lumen is narrow, the invasive growth and the proliferation of fibroblasts are enhanced in CPT patients.

Autotaxin/Lysophosphatidic Acid Axis: From Bone Biology to Bone Disorders

Lysophosphatidic acid (LPA) is a natural bioactive phospholipid with pleiotropic activities affecting multiple tissues, including bone. LPA exerts its biological functions by binding to G-protein coupled LPA receptors (LPA_1-6) to stimulate cell migration, proliferation, and survival. It is largely produced by autotaxin (ATX), a secreted enzyme with lysophospholipase D activity that converts lysophosphatidylcholine (LPC) into active LPA. Beyond its enzymatic activity, ATX serves as a docking molecule facilitating the efficient delivery of LPA to its specific cell surface receptors. Thus, LPA effects are the result of local production by ATX in a given tissue or cell type. As a consequence, the ATX/LPA axis should be considered as an entity to better understand their roles in physiology and pathophysiology and to propose novel therapeutic strategies. Herein, we provide not only an extensive overview of the relevance of the ATX/LPA axis in bone cell commitment and differentiation, skeletal development, and bone disorders, but also discuss new working hypotheses emerging from the interplay of ATX/LPA with well-established signaling pathways regulating bone mass.

Pain-like behavior in the collagen antibody-induced arthritis model is regulated by lysophosphatidic acid and activation of satellite glia cells

Inflammatory and neuropathic-like components underlie rheumatoid arthritis (RA)-associated pain, and lysophosphatidic acid (LPA) is linked to both joint inflammation in RA patients and to neuropathic pain. Thus, we investigated a role for LPA signalling using the collagen antibody-induced arthritis (CAIA) model. Pain-like behavior during the inflammatory phase and the late, neuropathic-like phase of CAIA was reversed by a neutralizing antibody generated against LPA and by an LPA_1/3 receptor inhibitor, but joint inflammation was not affected. Autotaxin, an LPA synthesizing enzyme was upregulated in dorsal root ganglia (DRG) neurons during both CAIA phases, but not in joints or spinal cord. Late-phase pronociceptive neurochemical changes in the DRG were blocked in Lpar1 receptor deficient mice and reversed by LPA neutralization. In vitro and in vivo studies indicated that LPA regulates pain-like behavior via the LPA₁ receptor on satellite glia cells (SGCs), which is expressed by both human and mouse SGCs in the DRG. Furthermore, CAIA-induced SGC activity is reversed by phospholipid neutralization and blocked in Lpar1 deficient mice. Our findings suggest that the regulation of CAIA-induced pain-like behavior by LPA signalling is a peripheral event, associated with the DRGs and involving increased pronociceptive activity of SGCs, which in turn act on sensory neurons.

Metabolomic biomarkers of low BMD: a systematic review

Due to the metabolic nature of osteoporosis, this study was conducted to identify metabolomic studies investigating the metabolic profile of low bone mineral density (BMD) and osteoporosis. A comprehensive systematic literature search was conducted through PubMed, Web of Science, Scopus, and Embase databases up to April 08, 2020, to identify observational studies with cross-sectional or case-control designs investigating the metabolic profile of low BMD in adults using biofluid specimen via metabolomic platform. The quality assessment panel specified for the "omics"-based diagnostic research (QUADOMICS) tool was used to estimate the methodologic quality of the included studies. Ten untargeted and one targeted approach metabolomic studies investigating biomarkers in different biofluids through mass spectrometry or nuclear magnetic resonance platforms were included in the systematic review. Some metabolite panels, rather than individual metabolites, showed promising results in differentiating low BMD from normal. Candidate metabolites were of different categories including amino acids, followed by lipids and carbohydrates. Besides, certain pathways were suggested by some of the studies to be involved. This systematic review suggested that metabolic profiling could improve the diagnosis of low BMD. Despite valuable findings attained from each of these studies, there was great heterogeneity regarding the ethnicity and age of participants, samples, and the metabolomic platform. Further longitudinal studies are needed to validate the results and confirm the predictive role of metabolic profile on low BMD and fracture. It is also mandatory to address and minimize the heterogeneity in future studies by using reliable quantitative methods. Summary: Due to the metabolic nature of osteoporosis, researchers have considered metabolomic studies recently. This systematic review showed that metabolic profiling including different categories of metabolites could improve the diagnosis of low BMD. However, great heterogeneity was observed and it is mandatory to address and minimize the heterogeneity in future studies.

Rheumatoid Arthritis in the View of Osteoimmunology

Rheumatoid arthritis is characterized by synovial inflammation and irreversible bone erosions, both highlighting the immense reciprocal relationship between the immune and bone systems, designed osteoimmunology two decades ago. Osteoclast-mediated resorption at the interface between synovium and bone is responsible for the articular bone erosions. The main triggers of this local bone resorption are autoantibodies directed against citrullinated proteins, as well as pro-inflammatory cytokines and the receptor activator of nuclear factor-κB ligand, that regulate both the formation and activity of the osteoclast, as well as immune cell functions. In addition, local bone loss is due to the suppression of osteoblast-mediated bone formation and repair by inflammatory cytokines. Similarly, inflammation affects systemic bone remodeling in rheumatoid arthritis with the net increase in bone resorption, leading to systemic osteoporosis. This review summarizes the substantial progress that has been made in understanding the pathophysiology of systemic and local bone loss in rheumatoid arthritis.

Lysophosphatidic acid receptors 2 and 3 regulate erythropoiesis at different hematopoietic stages

Hematopoiesis, the complex developmental process that forms blood components and replenishes the blood system, involves multiple intracellular and extracellular mechanisms. We previously demonstrated that lysophosphatidic acid (LPA), a lipid growth factor, has opposing regulatory effects on erythrocyte differentiation through activation of LPA receptors 2 and 3; yet the mechanisms underlying this process remain unclear. In this study, LPA₂ is observed that highly expressed in common myeloid progenitors (CMP) in murine myeloid cells, whereas the expression of LPA₃ displaces in megakaryocyte-erythroid progenitors (MEP) of later stage of myeloid differentiation. Therefore, we hypothesized that the switching expression of LPA₂ and LPA₃ determine the hematic homeostasis of mammalian megakaryocytic-erythroid lineage. In vitro colony-forming unit assays of murine progenitors reveal that LPA₂ agonist GRI reduces the erythroblast differentiation potential of CMP. In contrast, LPA₃ agonist OMPT increases the production of erythrocytes from megakaryocyte-erythrocyte progenitor cells (MEP). In addition, treatment with GRI reduces the erythroid, CMP, and MEP populations in mice, indicating that LPA₂ predominantly inhibits myeloid differentiation at an early stage. In contrast, activation of LPA₃ increases the production of terminally differentiated erythroid cells through activation of erythropoietic transcriptional factor. We also demonstrate that the LPA₃ signaling is essential for restoration of phenylhydrazine (PHZ)-induced acute hemolytic anemia in mice and correlates to erythropoiesis impairment of Hutchinson-Gilford progeria Symptom (HGPS) premature aging expressed K562 model. Our results reveal the distinct roles of LPA₂ and LPA₃ at different stages of hematopoiesis in vivo, providing potentiated therapeutic strategies of anemia treatment.

[Research progress on the biological regulatory function of lysophosphatidic acid in bone tissue cells]

Lysophosphatidic acid (LPA) is a small phospholipid that is present in all eukaryotic tissues and blood plasma. As an extracellular signaling molecule, LPA mediates many cellular functions by binding to six known G protein-coupled receptors and activating their downstream signaling pathways. These functions indicate that LPA may play important roles in many biological processes that include organismal development, wound healing, and carcinogenesis. Recently, many studies have found that LPA has various biological effects in different kinds of bone cells. These findings suggest that LPA is a potent regulator of bone development and remodeling and holds promising application potential in bone tissue engineering. Here, we review the recent progress on the biological regulatory function of LPA in bone tissue cells.

A Membranome-Centered Approach Defines Novel Biomarkers for Cellular Subtypes in the Intervertebral Disc

OBJECTIVE

Lack of specific marker-sets prohibits definition and functional distinction of cellular subtypes in the intervertebral disc (IVD), such as those from the annulus fibrosus (AF) and the nucleus pulposus (NP).

DESIGN

We recently generated immortalized cell lines from human NP and AF tissues; these comprise a set of functionally distinct clonal subtypes. Whole transcriptome analyses were performed of 12 phenotypically distinct clonal cell lines (4× NP-Responder, 4× NP-nonResponder, 2× AF-Sheet forming, and 2× AF-nonSheet forming). Data sets were filtered for membrane-associated marker genes and compared to literature.

RESULTS

Comparison of our immortal cell lines to published primary NP, AF, and articular chondrocytes (AC) transcriptome datasets revealed preservation of AF and NP phenotypes. NP-specific membrane-associated genes were defined by comparison to AF cells in both the primary dataset (46 genes) and immortal cell-lines (161 genes). Definition of AF-specific membrane-associated genes yielded 125 primary AF cell and 92 immortal cell-line markers. Overlap between primary and immortal NP cells yielded high-confidence NP-specific marker genes for NP-R (CLDN11, TMEFF2, CA12, ANXA2, CD44) and NP-nR (EFNA1, NETO2, SLC2A1). Overlap between AF and immortal AF subtypes yielded specific markers for AF-S (COLEC12, LPAR1) and AF-nS (CHIC1).

CONCLUSIONS

The current study provides a reference platform for preclinical evaluation of novel membrane-associated cell type-specific markers in the IVD. Future research will focus on their biological relevance for IVD function in development, homeostasis, and degenerate conditions.

Adjuvant drug-assisted bone healing: Part II - Modulation of angiogenesis

 The treatment of critical-size bone defects following complicated fractures, infections or tumor resections is a major challenge. The same applies to fractures in patients with impaired bone healing due to systemic inflammatory and metabolic diseases. Despite considerable progress in development and establishment of new surgical techniques, design of bone graft substitutes and imaging techniques, these scenarios still represent unresolved clinical problems. However, the development of new active substances offers novel potential solutions for these issues. This work discusses therapeutic approaches that influence angiogenesis or hypoxic situations in healing bone and surrounding tissue. In particular, literature on sphingosine-1-phosphate receptor modulators and nitric oxide (NO•) donors, including bi-functional (hybrid) compounds like NO•-releasing cyclooxygenase-2 inhibitors, was critically reviewed with regard to their local and systemic mode of action.

Lysophosphatidic Acid and Hematopoiesis: From Microenvironmental Effects to Intracellular Signaling

Vertebrate hematopoiesis is a complex physiological process that is tightly regulated by intracellular signaling and extracellular microenvironment. In recent decades, breakthroughs in lineage-tracing technologies and lipidomics have revealed the existence of numerous lipid molecules in hematopoietic microenvironment. Lysophosphatidic acid (LPA), a bioactive phospholipid molecule, is one of the identified lipids that participates in hematopoiesis. LPA exhibits various physiological functions through activation of G-protein-coupled receptors. The functions of these LPARs have been widely studied in stem cells, while the roles of LPARs in hematopoietic stem cells have rarely been examined. Nonetheless, mounting evidence supports the importance of the LPA-LPAR axis in hematopoiesis. In this article, we have reviewed regulation of hematopoiesis in general and focused on the microenvironmental and intracellular effects of the LPA in hematopoiesis. Discoveries in these areas may be beneficial to our understanding of blood-related disorders, especially in the context of prevention and therapy for anemia.

The roles of autotaxin/lysophosphatidic acid in immune regulation and asthma

Lysophosphatidic acid (LPA) species are present in almost all organ systems and play diverse roles through its receptors. Asthma is an airway disease characterized by chronic allergic inflammation where various innate and adaptive immune cells participate in establishing Th2 immune response. Here, we will review the contribution of LPA and its receptors to the functions of immune cells that play a key role in establishing allergic airway inflammation and aggravation of allergic asthma.

Autotaxin Implication in Cancer Metastasis and Autoimunne Disorders: Functional Implication of Binding Autotaxin to the Cell Surface

Autotaxin (ATX) is an exoenzyme which, due to its unique lysophospholipase D activity, is responsible for the synthesis of lysophosphatidic acid (LPA). ATX activity is responsible for the concentration of LPA in the blood. ATX expression is increased in various types of cancers, including breast cancer, where it promotes metastasis. The expression of ATX is also remarkably increased under inflammatory conditions, particularly in the osteoarticular compartment, where it controls bone erosion. Biological actions of ATX are mediated by LPA. However, the phosphate head group of LPA is highly sensitive to degradation by the action of lipid phosphate phosphatases, resulting in LPA inactivation. This suggests that for efficient action, LPA requires protection, which is potentially achieved through docking to a carrier protein. Interestingly, recent reports suggest that ATX might act as a docking molecule for LPA and also support the concept that binding of ATX to the cell surface through its interaction with adhesive molecules (integrins, heparan sulfate proteoglycans) could facilitate a rapid route of delivering active LPA to its cell surface receptors. This new mechanism offers a new vision of how ATX/LPA works in cancer metastasis and inflammatory bone diseases, paving the way for new therapeutic developments.

Role of CX3CL1/CX3CR1 Signaling Axis Activity in Osteoporosis

Osteoporosis is a civilization disease which is still challenging for contemporary medicine in terms of treatment and prophylaxis. It results from excessive activation of the osteoclastic cell line and immune cells like macrophages and lymphocytes. Cell-to-cell inflammatory information transfer occurs via factors including cytokines which form a complex network of cell humoral correlation, called cytokine network. Recently conducted studies revealed the participation of CX3CL1 chemokine in the pathogenesis of osteoporosis. CX3CL1 and its receptor CX3CR1 present unique properties among over 50 described chemokines. Apart from its chemotactic activity, CX3CL1 is the only chemokine which may function as an adhesion molecule which facilitates easier penetration of immune system cells through the vascular endothelium to the area of inflammation. The present study, based on world literature review, sums and describes convincing evidences of a significant role of the CX3CL1/CX3CR1 axis in processes leading to bone mineral density (BMD) reduction. The CX3CL1/CX3CR1 axis plays a principal role in osteoclast maturation and binding them with immune cells to the surface of the bone tissue. It promotes the development of inflammation and production of many inflammatory cytokines near the bone surface (i.e., TNF-α, IL-1β, and IL-6). High concentrations of CX3CL1 in serum are directly proportional to increased concentrations of bone turnover and inflammatory factors in human blood serum (TRACP-5b, NTx, IL-1β, and IL-6). Regarding the fact that acting against the CX3CL1/CX3CR1 axis is a potential target of immune treatment in osteoporosis, the number of available papers tackling the topic is certainly insufficient. Therefore, it seems justified to continue research which would precisely determine its role in the metabolism of the bone tissue as one of the most promising targets in osteoporosis therapy.

microRNA-329 reduces bone cancer pain through the LPAR1-dependent LPAR1/ERK signal transduction pathway in mice

Background:

Bone cancer pain (BCP) is a common symptom occurring among patients with cancer and has a detrimental effect on their quality of life. Growing evidence has implicated microRNA-329 (miR-329) in the progression of bone diseases. In the present study, we aimed to elucidate the potential effects of miR-329 on BCP in a BCP mouse model via binding to lysophosphatidic acid receptor 1 (LPAR1) through the LPAR1/extracellular signal-regulated kinase (ERK) signaling pathway.

Methods:

Initially, a BCP mouse model was established via injection of 4 × 10⁴ murine breast tumor (4T1 cell) cells (4 μl). The interaction between miR-329 and LPAR1 was identified using a bioinformatics website and dual luciferase reporter gene assay. The modeled mice were subsequently treated with miR-329 mimic, LPAR1 shRNA, or both, in order to examine the effect of miR-329 on the paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) of mice, the expression of LPAR1/ERK signaling pathway-related genes.

Results:

The positive expression rate of LPAR1 protein and extent of ERK1/2 phosphorylation were increased in BCP mouse models. LPAR1 is a target gene of miR-329, which can inhibit the expression of LPAR1. In response to miR-329 overexpression and LPAR1 silencing, BCP mice showed increased PWT and PWL, along with decreased LPAR1 expression and ratio of p-ERK/ERK.

Conclusions:

Altogether, the results obtained indicated that miR-329 can potentially alleviate BCP in mice via the inhibition of LPAR1 and blockade of the LPAR1/ERK signaling pathway, highlighting that upregulation of miR-329 could serve as a therapeutic target for BCP treatment.

Osteoclast-Derived Autotaxin, a Distinguishing Factor for Inflammatory Bone Loss

OBJECTIVE

The severity of rheumatoid arthritis (RA) correlates directly with bone erosions arising from osteoclast (OC) hyperactivity. Despite the fact that inflammation may be controlled in patients with RA, those in a state of sustained clinical remission or low disease activity may continue to accrue erosions, which supports the need for treatments that would be suitable for long-lasting inhibition of OC activity without altering the physiologic function of OCs in bone remodeling. Autotaxin (ATX) contributes to inflammation, but its role in bone erosion is unknown.

METHODS

ATX was targeted by inhibitory treatment with pharmacologic drugs and also by conditional inactivation of the ATX gene Ennp2 in murine OCs (ΔATX^{C tsk} ). Arthritic and erosive diseases were studied in human tumor necrosis factor-transgenic (hTNF^+/- ) mice and mice with K/BxN serum transfer-induced arthritis. Systemic bone loss was also analyzed in mice with lipopolysaccharide (LPS)-induced inflammation and estrogen deprivation. Joint inflammation and bone erosion were assessed by histology and micro-computed tomography. The role of ATX in RA was also examined in OC differentiation and activity assays.

RESULTS

OCs present at sites of inflammation overexpressed ATX. Pharmacologic inhibition of ATX in hTNF^+/- mice, as compared to vehicle-treated controls, significantly mitigated focal bone erosion (36% decrease; P < 0.05) and systemic bone loss (43% decrease; P < 0.05), without affecting synovial inflammation. OC-derived ATX was revealed to be instrumental in OC bone resorptive activity and was up-regulated by the inflammation elicited in the presence of TNF or LPS. Specific loss of ATX in OCs from mice subjected to ovariectomy significantly protected against the systemic bone loss and erosion that had been induced with LPS and K/BxN serum treatments (30% reversal of systemic bone loss [P < 0.01]; 55% reversal of erosion [P < 0.001]), without conferring bone-protective properties.

CONCLUSION

Our results identify ATX as a novel OC factor that specifically controls inflammation-induced bone erosions and systemic bone loss. Therefore, ATX inhibition offers a novel therapeutic approach for potentially preventing bone erosion in patients with RA.

Regulation of osteoclast function via Rho-Pkn3-c-Src pathways

BACKGROUND

Wnt signaling pathways are largely divided into the β-catenin-dependent canonical pathway and β-catenin-independent non-canonical pathways. The roles of Wnt signaling in bone metabolism have been extensively investigated. We previously attempted to clarify the roles of Wnt-non-canonical signaling in bone resorption and demonstrated that Wnt5a-receptor tyrosine kinase-like orphan receptor 2 (Ror2) signaling promoted osteoclast differentiation by enhancing RANK expression in osteoclast precursor cells. However, the roles of Wnt5a-Ror2 signaling in osteoclast function remain unclear.

HIGHLIGHT

Trabecular bone mass was significantly greater in osteoclast-specific Ror2-deficient (Ror2^ΔOCL/ΔOCL) mice than in control mice due to the decreased bone-resorbing activity of osteoclasts. Wnt5a-Ror2 signaling activated Rho in osteoclasts via dishevelled-associated activator of morphogenesis 2 (Daam2). The expression of protein kinase N3 (Pkn3), a Rho effector, increased during osteoclast differentiation. Trabecular bone mass was significantly greater in Pkn3-deficient mice than in wild-type mice due to the decreased bone-resorbing activity of osteoclasts. Pkn3 bound to c-Src and Pyk2 in a Wnt5a-Ror2 signaling-dependent manner, thereby enhancing the kinase activity of c-Src in osteoclasts. The binding of Pkn3 to c-Src was essential for the bone-resorbing activity of osteoclasts.

CONCLUSION

Wnt5a-Ror2 signaling promotes the bone-resorbing activity of osteoclasts by activating the Daam2-Rho-Pkn3-c-Src pathways. Pkn3 inhibitors, therefore, have potential as therapeutic agents for osteoporosis and bone destruction in inflammatory diseases.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Lysophosphatidic acid: Its role in bone cell biology and potential for use in bone regeneration

Lysophosphatidic acid (LPA) is a simple phospholipid that exerts pleiotropic effects on numerous cell types by activating its family of cognate G protein-coupled receptors (GPCRs) and participates in many biological processes, including organismal development, wound healing, and carcinogenesis. Bone cells, such as bone marrow mesenchymal stromal (stem) cells (BMSCs), osteoblasts, osteocytes and osteoclasts play essential roles in bone homeostasis and repair. Previous studies have identified the presence of specific LPA receptors in these bone cells. In recent years, an increasing number of cellular effects of LPA, such as the induction of cell proliferation, survival, migration, differentiation and cytokine secretion, have been found in different bone cells. Moreover, some biomaterials containing LPA have shown the ability to enhance osteogenesis. This review will focus on findings associated with LPA functions in these bone cells and present current studies related to the application of LPA in bone regenerative medicine. Further understanding this information will help us develop better strategies for bone healing.

Untargeted Metabolomics Reveals the Protective Effect of Fufang Zhenshu Tiaozhi (FTZ) on Aging-Induced Osteoporosis in Mice

Fufang Zhenzhu Tiaozhi (FTZ), as an effective traditional Chinese medicine, has been prescribed for more than 20 years. It has proven clinical efficacy as a prescription for patients with dyslipidemia, glucocorticoid- and high-fat-induced osteoporosis, but its effect on osteoporosis induced by aging is still unclear. The aim of this study was to investigate the anti-osteoporosis effect of FTZ in aging mice and revealed its biochemical action mechanism using metabolomics. Model of primary osteoporosis induced by aging was established. The mice in treatment group received a therapeutic dose of oral FTZ extract once daily during the experiment. The model and control groups received the corresponding volume of oral normal saline solution. Plasma samples of all three groups were collected after 12 weeks. Clinical biochemical parameters and biomechanics were determined in the osteoporosis model induced by normal aging to evaluate anti-osteoporosis effect of FTZ. Ultra performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-QTOF/MS) was used to analyze metabolic changes. The changes of histomorphometric and biomechanic parameters of femurs, as well as osteoblast and osteoclast activity indicated that FTZ administration reduced the risk of osteoporosis. Partial least squares discriminant analysis (PLS-DA) score plot revealed a clear separation trend between model and controls. Moreover, PLS-DA score plot indicated the anti-osteoporosis effect of FTZ with sphingosine 1-phosphate, LPA (16:0) and arachidonic acid (AA) among key biomarkers. The pivotal pathways revealed by pathway analysis including sphingolipid metabolism, glycerophospholipid metabolism, and AA metabolism. The mechanism by which FTZ reduces the risk of primary age-related osteoporosis in mice might be related to disorders of the above-mentioned pathways. FTZ has a protective effect against osteoporosis induced by aging, which may be mediated via interference with sphingolipid, glycerophospholipid, and AA metabolisms in mice.

Regulation of tumor cell - Microenvironment interaction by the autotaxin-lysophosphatidic acid receptor axis

The lipid mediator lysophosphatidic acid (LPA) in biological fluids is primarily produced by cleavage of lysophospholipids by the lysophospholipase D enzyme Autotaxin (ATX). LPA has been identified and abundantly detected in the culture medium of various cancer cell types, tumor effusates, and ascites fluid of cancer patients. Our current understanding of the physiological role of LPA established its role in fundamental biological responses that include cell proliferation, metabolism, neuronal differentiation, angiogenesis, cell migration, hematopoiesis, inflammation, immunity, wound healing, regulation of cell excitability, and the promotion of cell survival by protecting against apoptotic death. These essential biological responses elicited by LPA are seemingly hijacked by cancer cells in many ways; transcriptional upregulation of ATX leading to increased LPA levels, enhanced expression of multiple LPA GPCR subtypes, and the downregulation of its metabolic breakdown. Recent studies have shown that overexpression of ATX and LPA GPCR can lead to malignant transformation, enhanced proliferation of cancer stem cells, increased invasion and metastasis, reprogramming of the tumor microenvironment and the metastatic niche, and development of resistance to chemo-, immuno-, and radiation-therapy of cancer. The fundamental role of LPA in cancer progression and the therapeutic inhibition of the ATX-LPA axis, although highly appealing, remains unexploited as drug development to these targets has not reached into the clinic yet. The purpose of this brief review is to highlight some unique signaling mechanisms engaged by the ATX-LPA axis and emphasize the therapeutic potential that lies in blocking the molecular targets of the LPA system.

Metabolomic profiles associated with bone mineral density in US Caucasian women

Background

Individuals’ peak bone mineral density (BMD) achieved and maintained at ages 20–40 years is the most powerful predictor of low bone mass and osteoporotic fractures later in life. The aim of this study was to identify metabolomic factors associated with peak BMD variation in US Caucasian women.

Methods

A total of 136 women aged 20–40 years, including 65 subjects with low and 71 with high hip BMD, were enrolled. The serum metabolites were assessed using a liquid chromatography-mass spectrometry (LC-MS) method. The partial least-squares discriminant analysis (PLS-DA) method and logistic regression models were used, respectively, to examine the associations of metabolomic profiles and individual metabolites with BMD.

Results

The low and high BMD groups could be differentiated by the detected serum metabolites using PLS-DA (P_permutation = 0.008). A total of 14 metabolites, including seven amino acids and amino acid derivatives, five lipids (including three bile acids), and two organic acids, were significantly associated with the risk for low BMD. Most of these metabolites are novel in that they have never been linked with BMD in humans earlier. The prediction model including the newly identified metabolites significantly improved the classification of the groups with low and high BMD. The area under the receiver operating characteristic curve without and with metabolites were 0.88 (95% CI: 0.83–0.94) and 0.97 (95% CI: 0.94–0.99), respectively (P for the difference = 0.0004).

Conclusion

Metabolomic profiling may improve the risk prediction of osteoporosis among Caucasian women. Our findings also suggest the potential importance of the metabolism of amino acids and bile acids in bone health.

Platelets, autotaxin and lysophosphatidic acid signalling: win-win factors for cancer metastasis

Platelets play a crucial role in the survival of metastatic cells in the blood circulation. The interaction of tumour cells with platelets leads to the production of plethoric factors among which our review will focus on lysophosphatidic acid (LPA), because platelets are the highest producers of this bioactive lysophospholipid in the organism. LPA promotes platelet aggregation, and blocking platelet function decreases LPA signalling and leads to inhibition of breast cancer cell metastasis. Autotaxin (ATX), a lysophospholipase D responsible for the basal concentration of LPA in blood, was detected in platelet α-granules. Functionally, active ATX is eventually released following tumour cell-induced platelet aggregation, thereby promoting metastasis. Megakaryocytes do not express ATX but respond to LPA stimulation. Whether LPA-primed megakaryocytes contribute to the recently reported negative action of megakaryocytes on cancer metastasis is not yet known. However, an understanding of the ATX/LPA signalling pathways in platelets, cancer cells and megakaryocytes opens up new approaches for fighting cancer metastasis.

An Ectopic Imaging Window for Intravital Imaging of Engineered Bone Tissue

Tissue engineering is a promising branch of regenerative medicine, but its clinical application remains limited because thorough knowledge of the in vivo repair processes in these engineered implants is limited. Common techniques to study the different phases of bone repair in mice are destructive and thus not optimal to gain insight into the dynamics of this process. Instead, multiphoton‐intravital microscopy (MP‐IVM) allows visualization of (sub)cellular processes at high resolution and frequency over extended periods of time when combined with an imaging window that permits optical access to implants in vivo. In this study, we have developed and validated an ectopic imaging window that can be placed over a tissue‐engineered construct implanted in mice. This approach did not interfere with the biological processes of bone regeneration taking place in these implants, as evidenced by histological and micro–computed tomography (μCT)‐based comparison to control ectopic implants. The ectopic imaging window permitted tracking of individual cells over several days in vivo. Furthermore, the use of fluorescent reporters allowed visualization of the onset of angiogenesis and osteogenesis in these constructs. Taken together, this novel imaging window will facilitate further analysis of the spatiotemporal regulation of cellular processes in bone tissue–engineered implants and provides a powerful tool to enhance the therapeutic potential of bone tissue engineering. © 2017 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Lysophosphatidic acid protects against acetaminophen-induced acute liver injury

We investigated the effect of lysophosphatidic acid (LPA) in experimental acetaminophen (APAP)-induced acute liver injury. LPA administration significantly reduced APAP-challenged acute liver injury, showing attenuated liver damage, liver cell death and aspartate aminotransferase and alanine aminotransferase levels. APAP overdose-induced mortality was also significantly decreased by LPA administration. Regarding the mechanism involved in LPA-induced protection against acute liver injury, LPA administration significantly increased the glutathione level, which was markedly decreased in APAP challenge-induced acute liver injury. LPA administration also strongly blocked the APAP challenge-elicited phosphorylation of JNK, ERK and GSK3β, which are involved in the pathogenesis of acute liver injury. Furthermore, LPA administration decreased the production of TNF-α and IL-1β in an experimental drug-induced liver injury animal model. Mouse primary hepatocytes express LPA_1,3–6, and injection of the LPA receptor antagonist KI16425 (an LPA_1,3-selective inhibitor) or H2L 5765834 (an LPA_1,3,5-selective inhibitor) did not reverse the LPA-induced protective effects against acute liver injury. The therapeutic administration of LPA also blocked APAP-induced liver damage, leading to an increased survival rate. Collectively, these results indicate that the well-known bioactive lipid LPA can block the pathogenesis of APAP-induced acute liver injury by increasing the glutathione level but decreasing inflammatory cytokines in an LPA_1,3,5-independent manner. Our results suggest that LPA might be an important therapeutic agent for drug-induced liver injury.

Contradictory Role of CD97 in Basal and Tumor Necrosis Factor-Induced Osteoclastogenesis In Vivo

OBJECTIVE

CD97, a member of the 7-transmembrane epidermal growth factor family of adhesion G protein-coupled receptors, is expressed on various cell types. This study was undertaken to elucidate the functions of CD97 in bone and inflammation in an experimental mouse model, by examining the effect of CD97 on osteoclastogenesis in vitro, characterizing the skeletal phenotype of CD97-deficient (CD97-knockout [KO]) mice, and assessing the responses to tumor necrosis factor (TNF) treatment.

METHODS

Femoral tissue and bone marrow (BM)-derived cells from CD97-KO and wild-type (WT) mice were assessed using histomorphometric analyses, in vitro cultures, and reverse transcription-polymerase chain reaction. Serum cytokine and chemokine levels in the presence or absence of TNF challenge were analyzed by multiplex assay.

RESULTS

In cultures of mouse BM-derived macrophages in vitro, RANKL induced the expression of CD97. In vivo, the trabecular bone volume of the femurs of female CD97-KO mice was increased, and this was associated with a decrease in the number of osteoclasts. Compared to WT mice, CD97-KO mice had a reduced potential to form osteoclast-like cells in vitro. Furthermore, TNF treatment augmented the formation of osteoclasts in the calvaria of CD97-KO mice in vivo, by increasing the production of RANKL and other cytokines and chemokines and by reducing the production of osteoprotegerin by calvarial cells.

CONCLUSION

These findings demonstrate that CD97 is a positive regulator of osteoclast-like cell differentiation, a mechanism that influences bone homeostasis. However, the presence of CD97 may be essential to suppress the initial osteoclastogenesis that occurs in response to acute and local inflammatory stimuli.

The critical role of the ZNF217 oncogene in promoting breast cancer metastasis to the bone

Bone metastasis affects >70% of patients with advanced breast cancer. However, the molecular mechanisms underlying this process remain unclear. On the basis of analysis of clinical datasets, and in vitro and in vivo experiments, we report that the ZNF217 oncogene is a crucial mediator and indicator of bone metastasis. Patients with high ZNF217 mRNA expression levels in primary breast tumours had a higher risk of developing bone metastases. MDA-MB-231 breast cancer cells stably transfected with ZNF217 (MDA-MB-231-ZNF217) showed the dysregulated expression of a set of genes with bone-homing and metastasis characteristics, which overlapped with two previously described 'osteolytic bone metastasis' gene signatures, while also highlighting the bone morphogenetic protein (BMP) pathway. The latter was activated in MDA-MB-231-ZNF217 cells, and its silencing by inhibitors (Noggin and LDN-193189) was sufficient to rescue ZNF217-dependent cell migration, invasion or chemotaxis towards the bone environment. Finally, by using non-invasive multimodal in vivo imaging, we found that ZNF217 increases the metastatic growth rate in the bone and accelerates the development of severe osteolytic lesions. Altogether, the findings of this study highlight ZNF217 as an indicator of the emergence of breast cancer bone metastasis; future therapies targeting ZNF217 and/or the BMP signalling pathway may be beneficial by preventing the development of bone metastases. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Metastasis: new functional implications of platelets and megakaryocytes

Platelets are essential components of hemostasis. Due to a plethora of factors released on activation, platelet functions are also connected to tumor growth, notably by acting on angiogenesis. It is now well recognized that major roles of platelets in the poor outcome of cancer patients occurs during hematogenous dissemination of cancer cells. In this review, we describe recent insights into the molecular mechanisms supporting the prometastatic activity of platelets. Platelets have been shown to promote survival of circulating tumor cells (CTCs) in the bloodstream by conferring resistance to the shear stress and attack from natural killer cells. Recently, platelets were found to promote and/or maintain the state of epithelial to mesenchymal transition on CTCs through platelet secretion of transforming growth factor β in response to CTC activation. At a later stage in the metastatic process, platelets promote extravasation and establishment of metastatic cells in distant organs as observed in bone. This particular environment is also the site of hematopoiesis, megakaryocytopoiesis, and platelet production. Increasing the number of megakaryocytes (MKs) in the bone marrow results in a high bone mass phenotype and inhibits skeletal metastasis formation of prostate cancer cells. As a result of their specific location in vascular niches in the bone marrow, MK activity might contribute to the "seed and soil" suitability between CTCs and bone. In conclusion, recent findings have made a great advance in our knowledge on how platelets contribute to the metastatic dissemination of cancer cells and that may support the development of new antimetastasis therapies.

ATX-LPA1 axis contributes to proliferation of chondrocytes by regulating fibronectin assembly leading to proper cartilage formation

The lipid mediator lysophosphatidic acid (LPA) signals via six distinct G protein-coupled receptors to mediate both unique and overlapping biological effects, including cell migration, proliferation and survival. LPA is produced extracellularly by autotaxin (ATX), a secreted lysophospholipase D, from lysophosphatidylcholine. ATX-LPA receptor signaling is essential for normal development and implicated in various (patho)physiological processes, but underlying mechanisms remain incompletely understood. Through gene targeting approaches in zebrafish and mice, we show here that loss of ATX-LPA₁ signaling leads to disorganization of chondrocytes, causing severe defects in cartilage formation. Mechanistically, ATX-LPA₁ signaling acts by promoting S-phase entry and cell proliferation of chondrocytes both in vitro and in vivo, at least in part through β1-integrin translocation leading to fibronectin assembly and further extracellular matrix deposition; this in turn promotes chondrocyte-matrix adhesion and cell proliferation. Thus, the ATX-LPA₁ axis is a key regulator of cartilage formation.

Understanding the local actions of lipids in bone physiology

The adult skeleton is a metabolically active organ system that undergoes continuous remodeling to remove old and/or stressed bone (resorption) and replace it with new bone (formation) in order to maintain a constant bone mass and preserve bone strength from micro-damage accumulation. In that remodeling process, cellular balances--adipocytogenesis/osteoblastogenesis and osteoblastogenesis/osteoclastogenesis--are critical and tightly controlled by many factors, including lipids as discussed in the present review. Interest in the bone lipid area has increased as a result of in vivo evidences indicating a reciprocal relationship between bone mass and marrow adiposity. Lipids in bones are usually assumed to be present only in the bone marrow. However, the mineralized bone tissue itself also contains small amounts of lipids which might play an important role in bone physiology. Fatty acids, cholesterol, phospholipids and several endogenous metabolites (i.e., prostaglandins, oxysterols) have been purported to act on bone cell survival and functions, the bone mineralization process, and critical signaling pathways. Thus, they can be regarded as regulatory molecules important in bone health. Recently, several specific lipids derived from membrane phospholipids (i.e., sphingosine-1-phosphate, lysophosphatidic acid and different fatty acid amides) have emerged as important mediators in bone physiology and the number of such molecules will probably increase in the near future. The present paper reviews the current knowledge about: (1°) bone lipid composition in both bone marrow and mineralized tissue compartments, and (2°) local actions of lipids on bone physiology in relation to their metabolism. Understanding the roles of lipids in bone is essential to knowing how an imbalance in their signaling pathways might contribute to bone pathologies, such as osteoporosis.

CXCL8 and CCL20 Enhance Osteoclastogenesis via Modulation of Cytokine Production by Human Primary Osteoblasts

Generalized osteoporosis is common in patients with inflammatory diseases, possibly because of circulating inflammatory factors that affect osteoblast and osteoclast formation and activity. Serum levels of the inflammatory factors CXCL8 and CCL20 are elevated in rheumatoid arthritis, but whether these factors affect bone metabolism is unknown. We hypothesized that CXCL8 and CCL20 decrease osteoblast proliferation and differentiation, and enhance osteoblast-mediated osteoclast formation and activity. Human primary osteoblasts were cultured with or without CXCL8 (2–200 pg/ml) or CCL20 (5–500 pg/ml) for 14 days. Osteoblast proliferation and gene expression of matrix proteins and cytokines were analyzed. Osteoclast precursors were cultured with CXCL8 (200 pg/ml) and CCL20 (500 pg/ml), or with conditioned medium (CM) from CXCL8 and CCL20-treated osteoblasts with or without IL-6 inhibitor. After 3 weeks osteoclast formation and activity were determined. CXCL8 (200 pg/ml) and CCL20 (500 pg/ml) enhanced mRNA expression of KI67 (2.5–2.7-fold), ALP (1.6–1.7-fold), and IL-6 protein production (1.3–1.6-fold) by osteoblasts. CXCL8-CM enhanced the number of osteoclasts with 3–5 nuclei (1.7-fold), and with >5 nuclei (3-fold). CCL20-CM enhanced the number of osteoclasts with 3–5 nuclei (1.3-fold), and with >5 nuclei (2.8-fold). IL-6 inhibition reduced the stimulatory effect of CXCL8-CM and CCL20-CM on formation of osteoclasts. In conclusion, CXCL8 and CCL20 did not decrease osteoblast proliferation or gene expression of matrix proteins. CXCL8 and CCL20 did not directly affect osteoclastogenesis. However, CXCL8 and CCL20 enhanced osteoblast-mediated osteoclastogenesis, partly via IL-6 production, suggesting that CXCL8 and CCL20 may contribute to osteoporosis in rheumatoid arthritis by affecting bone cell communication.

Plasma and synovial fluid autotaxin correlate with severity in knee osteoarthritis

BACKGROUND

This study aimed to investigate the relationships between plasma and synovial autotaxin and the severity in knee osteoarthritis (OA) patients.

METHODS

A total of 90 participants (70 knee OA patients and 20 controls) were recruited. Autotaxin and high-sensitivity C-reactive protein (hs-CRP) levels were determined. The symptomatic and radiographic severity of OA was assessed using the Western Ontario McMaster University Osteoarthritis Index (WOMAC) scores and the Kellgren-Lawrence grades.

RESULTS

OA patients had significantly higher circulating autotaxin and hs-CRP than controls. Plasma autotaxin was directly correlated with synovial fluid autotaxin (r=0.639, P<0.001). Additionally, plasma and synovial fluid autotaxin were associated with radiographic severity (P<0.001). Furthermore, plasma and synovial fluid autotaxin levels were positively correlated with WOMAC scores (r=0.558, P<0.001 and r=0.371, P=0.002, respectively).

CONCLUSION

Plasma and synovial fluid autotaxin levels were positively correlated with the severity of OA. Thus, autotaxin has potential as a biomarker reflecting the severity of knee OA.

Identification of heparin-binding EGF-like growth factor (HB-EGF) as a biomarker for lysophosphatidic acid receptor type 1 (LPA1) activation in human breast and prostate cancers

Lysophosphatidic acid (LPA) is a natural bioactive lipid with growth factor-like functions due to activation of a series of six G protein-coupled receptors (LPA_1–6). LPA receptor type 1 (LPA₁) signaling influences the pathophysiology of many diseases including cancer, obesity, rheumatoid arthritis, as well as lung, liver and kidney fibrosis. Therefore, LPA₁ is an attractive therapeutic target. However, most mammalian cells co-express multiple LPA receptors whose co-activation impairs the validation of target inhibition in patients because of missing LPA receptor-specific biomarkers. LPA₁ is known to induce IL-6 and IL-8 secretion, as also do LPA₂ and LPA₃. In this work, we first determined the LPA induced early-gene expression profile in three unrelated human cancer cell lines expressing different patterns of LPA receptors (PC3: LPA_1,2,3,6; MDA-MB-231: LPA_1,2; MCF-7: LPA_2,6). Among the set of genes upregulated by LPA only in LPA₁-expressing cells, we validated by QPCR and ELISA that upregulation of heparin-binding EGF-like growth factor (HB-EGF) was inhibited by LPA_1–3 antagonists (Ki16425, Debio0719). Upregulation and downregulation of HB-EGF mRNA was confirmed in vitro in human MDA-B02 breast cancer cells stably overexpressing LPA₁ (MDA-B02/LPA₁) and downregulated for LPA₁ (MDA-B02/shLPA₁), respectively. At a clinical level, we quantified the expression of LPA₁ and HB-EGF by QPCR in primary tumors of a cohort of 234 breast cancer patients and found a significantly higher expression of HB-EGF in breast tumors expressing high levels of LPA₁. We also generated human xenograph prostate tumors in mice injected with PC3 cells and found that a five-day treatment with Ki16425 significantly decreased both HB-EGF mRNA expression at the primary tumor site and circulating human HB-EGF concentrations in serum. All together our results demonstrate that HB-EGF is a new and relevant biomarker with potentially high value in quantifying LPA₁ activation state in patients receiving anti-LPA₁ therapies.