DNA microarray analysis reveals a role for lysophosphatidic acid in the regulation of anti-inflammatory genes in MC3T3-E1 cells

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.

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.

Thrombin induced by the extrinsic pathway and PAR-1 regulated inflammation at the site of fracture repair

Thrombin (coagulation factor IIa) is a serine protease encoded by the F2 gene. Pro-thrombin (coagulation factor II) is cut to generate thrombin in the coagulation cascade that results in a reduction of blood loss. Procoagulant states that lead to activation of thrombin are common in bone fracture sites. However, its physiological roles and relationship with osteoblasts in bone fractures are largely unknown. We herein report various effects of thrombin on mouse osteoblastic MC3T3-E1 cells. MC3T3-E1 cells expressed proteinase-activated receptor 1 (PAR1), also known as the coagulation factor II receptor. They also produced monocyte chemoattractant protein (MCP-1), tissue factor (TF), MCSF and IL-6 upon thrombin stimulation through the PI3K-Akt and MEK-Erk1/2 pathways. Furthermore, MCP-1 obtained from thrombin-stimulated MC3T3-E1 cells induced migration by macrophage RAW264 cells. All these effects of thrombin on MC3T3-E1 cells were abolished by the selective non-peptide thrombin receptor inhibitor SCH79797. We also found that thrombin, PAR-1, MCP-1, TF as well as phosphorylated AKT and p42/44 were significantly expressed at the fracture site of mouse femoral bone. Collectively, thrombin/PAR-1 interaction regulated MCP-1, TF, MCSF and IL-6 production by MC3T3-E1 cells. Furthermore, MCP-1 induced RAW264 cell migration. Thrombin may thus be a novel cytokine that regulates several aspects of osteoblast function and fracture healing.

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.

Heat shock protein 60 protects skeletal tissue against glucocorticoid-induced bone mass loss by regulating osteoblast survival

Excessive glucocorticoid administration accelerates osteoblast apoptosis and skeletal deterioration. Heat shock proteins (HSPs) regulate metabolic activities in osteoblastic cells. This study characterized the biological significance of HSP60 in glucocorticoid-induced bone loss. Rats were treated with glucocorticoid, HSP60 antisense oligonucleotides, or adenovirus-mediated HSP60 gene transfer. Bone mineral density, metaphyseal trabecular micro-architecture, and fragility were analyzed by dual X-ray absorptiometry, micro-computed tomography, and material testing, respectively. Differential proteomic profiles of bone tissue extracts were detected by bi-dimensional electrophoresis and mass spectrometry. Survival and proapoptotic signal transduction were quantified by immunoblotting. Glucocorticoid-treated rats had low bone mineral density and metaphyseal trabecular microstructure in association with downregulation of collagen 1α1 and HSP60 expressions in bone tissue. Gain of HSP60 function by adenovirus-mediated HSP60 gene transfer abrogated the deleterious effects of glucocorticoid treatment on bone mass, trabecular microstructure, and mechanical strength. Enhancement of HSP60 signaling attenuated the glucocorticoid-induced loss of trabecular bone volume, mineral acquisition reactions and osteoblast surface. HSP60 gene transfer activated ERK and Akt and reduced Bax and cytochrome c release, as well as caspase-3 cleavage, which attenuated the inhibitory effects of glucocorticoid treatment on osteoblast survival. Loss of HSP60 function by HSP60 antisense oligonucleotides accelerated mitochondrial apoptotic programs and osteoblast apoptosis. Knockdown of HSP60 induced loss of bone mass, micro-architecture integrity, and mechanical property. Taken together, loss of HSP60 signaling contributes to the glucocorticoid-induced enhancement of pro-apoptotic reactions, thereby accelerating osteoblast apoptosis and bone mass loss. Enhancement of HSP60 function is beneficial for protecting bone tissue against the glucocorticoid-induced inhibition of bone cell viability and bone formation.

Regulation of gene expression and subcellular protein distribution in MLO-Y4 osteocytic cells by lysophosphatidic acid: Relevance to dendrite outgrowth

Osteoblastic and osteocytic cells are highly responsive to the lipid growth factor lysophosphatidic acid (LPA) but the mechanisms by which LPA alters bone cell functions are largely unknown. A major effect of LPA on osteocytic cells is the stimulation of dendrite membrane outgrowth, a process that we predicted to require changes in gene expression and protein distribution. We employed DNA microarrays for global transcriptional profiling of MLO-Y4 osteocytic cells grown for 6 and 24h in the presence or absence of LPA. We identified 932 transcripts that displayed statistically significant changes in abundance of at least 1.25-fold in response to LPA treatment. Gene ontology (GO) analysis revealed that the regulated gene products were linked to diverse cellular processes, including DNA repair, response to unfolded protein, ossification, protein-RNA complex assembly, and amine biosynthesis. Gene products associated with the regulation of actin microfilament dynamics displayed the most robust expression changes, and LPA-induced dendritogenesis in vitro was blocked by the stress fiber inhibitor cytochalasin D. Mass spectrometry-based proteomic analysis of MLO-Y4 cells revealed significant LPA-induced changes in the abundance of 284 proteins at 6h and 844 proteins at 24h. GO analysis of the proteomic data linked the effects of LPA to cell processes that control of protein distribution and membrane outgrowth, including protein localization, protein complex assembly, Golgi vesicle transport, cytoskeleton-dependent transport, and membrane invagination/endocytosis. Dendrites were isolated from LPA-treated MLO-Y4 cells and subjected to proteomic analysis to quantitatively assess the subcellular distribution of proteins. Sets of 129 and 36 proteins were enriched in the dendrite fraction as compared to whole cells after 6h and 24h of LPA exposure, respectively. Protein markers indicated that membranous organelles were largely excluded from the dendrites. Highly represented among the proteins with elevated abundances in dendrites were molecules that regulate cytoskeletal function, cell motility and membrane adhesion. Our combined transcriptomic/proteomic analysis of the response of MLO-Y4 osteocytic cells to LPA indicates that dendritogenesis is a membrane- and cytoskeleton-driven process with actin dynamics playing a particularly critical role.

Purinergic signaling is required for fluid shear stress-induced NF-κB translocation in osteoblasts

Fluid shear stress regulates gene expression in osteoblasts, in part by activation of the transcription factor NF-κB. We examined whether this process was under the control of purinoceptor activation. MC3T3-E1 osteoblasts under static conditions expressed the NF-κB inhibitory protein IκBα and exhibited cytosolic localization of NF-κB. Under fluid shear stress, IκBα levels decreased, and concomitant nuclear localization of NF-κB was observed. Cells exposed to fluid shear stress in ATP-depleted medium exhibited no significant reduction in IκBα, and NF-κB remained within the cytosol. Similar results were found using oxidized ATP or Brilliant Blue G, P2X(7) receptor antagonists, indicating that the P2X(7) receptor is responsible for fluid shear-stress-induced IκBα degradation and nuclear accumulation of NF-κB. Pharmacologic blockage of the P2Y6 receptor also prevented shear-induced IκBα degradation. These phenomena involved neither ERK1/2 signaling nor autocrine activation by P2X(7)-generated lysophosphatidic acid. Our results suggest that fluid shear stress regulates NF-κB activity through the P2Y(6) and P2X(7) receptor.

Lysophosphatidic acid-induced ERK activation and chemotaxis in MC3T3-E1 preosteoblasts are independent of EGF receptor transactivation

Bone-forming osteoblasts and their progenitors are target cells for the lipid growth factor lysophosphatidic acid (LPA) which is produced by degranulating platelets at sites of tissue injury. LPA is a potent inducer of bone cell chemotaxis, proliferation and survival in vitro, and this lipid factor is an attractive candidate to facilitate preosteoblast migration during skeletal regeneration in vivo. In this study we sought to more clearly define the intracellular signaling pathways mediating the effects of LPA on bone cells. LPA-treated MC3T3-E1 preosteoblastic cells exhibited a bimodal activation of extracellular signal-related kinase (ERK1/2) with maximal phosphorylation at 5 and 60 min. MEK1/2 activation was detected within 2.5 min of LPA exposure and remained elevated for at least an hour. ERK1/2 phosphorylation was not coupled to Ras activation or to LPA-induced elevations in cytosolic Ca(2+). While LPA exposure transactivates the EGF receptor in many cell types, LPA-stimulated ERK1/2 activation in MC3T3-E1 cells was unaffected by the inhibition of EGF receptor function. ERK isoforms can function as transcription factors and ERK1/2 rapidly accumulated in the nuclei of LPA-treated cells, a process that was blocked if ERK1/2 phosphorylation was prevented. Blocking ERK1/2 phosphorylation also led to significant decreases in LPA-induced MC3T3-E1 cell chemotaxis, while the inhibition of EGF receptor function had no effect on the stimulation of preosteoblast motility by LPA. Our results identify ERK1/2 activation as a mediator of LPA-stimulated MC3T3-E1 cell migration that may be relevant to preosteoblast motility and gene expression during bone repair in vivo. J. Cell. Physiol. 219: 716-723, 2009. (c) 2009 Wiley-Liss, Inc.

Multiple actions of lysophosphatidic acid on fibroblasts revealed by transcriptional profiling

Background

Lysophosphatidic acid (LPA) is a lipid mediator that acts through specific G protein-coupled receptors to stimulate the proliferation, migration and survival of many cell types. LPA signaling has been implicated in development, wound healing and cancer. While LPA signaling pathways have been studied extensively, it remains unknown how LPA affects global gene expression in its target cells.

Results

We have examined the temporal program of global gene expression in quiescent mouse embryonic fibroblasts stimulated with LPA using 32 k oligonucleotide microarrays. In addition to genes involved in growth stimulation and cytoskeletal reorganization, LPA induced many genes that encode secreted factors, including chemokines, growth factors, cytokines, pro-angiogenic and pro-fibrotic factors, components of the plasminogen activator system and metalloproteases. Strikingly, epidermal growth factor induced a broadly overlapping expression pattern, but some 7% of the genes (105 out of 1508 transcripts) showed differential regulation by LPA. The subset of LPA-specific genes was enriched for those associated with cytoskeletal remodeling, in keeping with LPA's ability to regulate cell shape and motility.

Conclusion

This study highlights the importance of LPA in programming fibroblasts not only to proliferate and migrate but also to produce many paracrine mediators of tissue remodeling, angiogenesis, inflammation and tumor progression. Furthermore, our results show that G protein-coupled receptors and receptor tyrosine kinases can signal independently to regulate broadly overlapping sets of genes in the same cell type.

Molecular genetic studies of gene identification for osteoporosis

This review comprehensively summarizes the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of September 2007. It is intended to constitute a sequential update of our previously published reviews covering the available data up to the end of 2004. Evidence from candidate gene-association studies, genome-wide linkage and association studies, as well as functional genomic studies (including gene-expression microarray and proteomics) on osteogenesis and osteoporosis, are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. The major results of all studies are tabulated for comparison and ease of reference. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.