Lysophosphatidic acid and sphingosine 1-phosphate stimulate endothelial cell wound healing

Lysophosphatidic acid (LPA) and angiogenesis

Lysophosphatidic acid (LPA) is a simple lipid with many important biological functions such as the regulation of cellular proliferation, cellular migration, differentiation, and suppression of apoptosis. Although a direct angiogenic effect of LPA has not been reported to date, there are indications that LPA promotes angiogenesis. In addition, LPA is a chemoattractant for cultured endothelial cells and promotes barrier function in such cultures. To test the hypothesis that LPA is angiogenic, we used the chicken chorio-allantoic membrane (CAM) assay. Sequence analysis of the cloned, full-length chicken LPA receptor cDNAs revealed three receptor types that are orthologous to the mammalian LPA(1), LPA(2), and LPA(3) receptors. We document herein that LPA is angiogenic in the CAM system and further that synthetic LPA receptor agonists and antagonists mimic or block this response, respectively. Our results predict that LPA receptor antagonists are a possible therapeutic route to interdicting angiogenesis.

Sphingosine 1-phosphate induces platelet/endothelial cell adhesion molecule-1 phosphorylation in human endothelial cells through cSrc and Fyn

Sphingosine 1-phosphate (S1P) is a multifunctional phospholipid which acts through a specific family of G protein-coupled receptors. Platelet/endothelial cell adhesion molecule-1 (PECAM-1) form trans-homophilic binding at lateral cell border. Upon stimulation, its cytoplasmic tyrosine residues could be phosphorylated and interact with various downstream signaling molecules. In this study, we demonstrated that S1P induced PECAM-1 tyrosine phosphorylation in human umbilical cord vein cells (HUVECs). By pharmacological inhibitors, it was suggested that G(i) and Src family kinases were involved in PECAM-1 phosphorylation. Moreover, cSrc and Fyn siRNA significantly suppressed S1P-induced PECAM-1 phosphorylation. These results suggested that S1P-induced PECAM-1 phosphorylation through G(i) and subsequent cSrc and Fyn. Our findings provide further understanding of S1P and PECAM-1 signaling as well as their functions in endothelial cells.

Sustained release of sphingosine 1-phosphate for therapeutic arteriogenesis and bone tissue engineering

Sphingosine 1-phosphate (S1P) is a bioactive phospholipid that impacts migration, proliferation, and survival in diverse cell types, including endothelial cells, smooth muscle cells, and osteoblast-like cells. In this study, we investigated the effects of sustained release of S1P on microvascular remodeling and associated bone defect healing in vivo. The murine dorsal skinfold window chamber model was used to evaluate the structural remodeling response of the microvasculature. Our results demonstrated that 1:400 (w/w) loading and subsequent sustained release of S1P from poly(lactic-co-glycolic acid) (PLAGA) significantly enhanced lumenal diameter expansion of arterioles and venules after 3 and 7 days. Incorporation of 5-bromo-2-deoxyuridine (BrdU) at day 7 revealed significant increases in mural cell proliferation in response to S1P delivery. Additionally, three-dimensional (3D) scaffolds loaded with S1P (1:400) were implanted into critical-size rat calvarial defects, and healing of bony defects was assessed by radiograph X-ray, microcomputed tomography (muCT), and histology. Sustained release of S1P significantly increased the formation of new bone after 2 and 6 weeks of healing and histological results suggest increased numbers of blood vessels in the defect site. Taken together, these experiments support the use of S1P delivery for promoting microvessel diameter expansion and improving the healing outcomes of tissue-engineered therapies.

Dehydroepiandrosterone stimulates endothelial proliferation and angiogenesis through extracellular signal-regulated kinase 1/2-mediated mechanisms

Dehydroepiandrosterone (DHEA) activates a plasma membrane receptor on vascular endothelial cells and phosphorylates ERK 1/2. We hypothesize that ERK1/2-dependent vascular endothelial proliferation underlies part of the beneficial vascular effect of DHEA. DHEA (0.1-10 nm) activated ERK1/2 in bovine aortic endothelial cells (BAECs) by 15 min, causing nuclear translocation of phosphorylated ERK1/2 and phosphorylation of nuclear p90 ribosomal S6 kinase. ERK1/2 phosphorylation was dependent on plasma membrane-initiated activation of Gi/o proteins and the upstream MAPK kinase because the effect was seen with albumin-conjugated DHEA and was blocked by pertussis toxin or PD098059. A 15-min incubation of BAECs with 1 nm DHEA (or albumin-conjugated DHEA) increased endothelial proliferation by 30% at 24 h. This effect was not altered by inhibition of estrogen or androgen receptors or nitric oxide production. There was a similar effect of DHEA to increase endothelial migration. DHEA also increased the formation of primitive capillary tubes of BAECs in vitro in solubilized basement membrane. These rapid DHEA-induced effects were reversed by the inhibition of either Gi/o-proteins or ERK1/2. Additionally, DHEA enhanced angiogenesis in vivo in a chick embryo chorioallantoic membrane assay. These findings indicate that exposure to DHEA, at concentrations found in human blood, causes vascular endothelial proliferation by a plasma membrane-initiated activity that is Gi/o and ERK1/2 dependent. These data, along with previous findings, define an important vascular endothelial cell signaling pathway that is activated by DHEA and suggest that this steroid may play a role in vascular function.

Protein kinase C-epsilon regulates sphingosine 1-phosphate-mediated migration of human lung endothelial cells through activation of phospholipase D2, protein kinase C-zeta, and Rac1

The signaling pathways by which sphingosine 1-phosphate (S1P) potently stimulates endothelial cell migration and angiogenesis are not yet fully defined. We, therefore, investigated the role of protein kinase C (PKC) isoforms, phospholipase D (PLD), and Rac in S1P-induced migration of human pulmonary artery endothelial cells (HPAECs). S1P-induced migration was sensitive to S1P(1) small interfering RNA (siRNA) and pertussis toxin, demonstrating coupling of S1P(1) to G(i). Overexpression of dominant negative (dn) PKC-epsilon or -zeta, but not PKC-alpha or -delta, blocked S1P-induced migration. Although S1P activated both PLD1 and PLD2, S1P-induced migration was attenuated by knocking down PLD2 or expressing dnPLD2 but not PLD1. Blocking PKC-epsilon, but not PKC-zeta, activity attenuated S1P-mediated PLD stimulation, demonstrating that PKC-epsilon, but not PKC-zeta, was upstream of PLD. Transfection of HPAECs with dnRac1 or Rac1 siRNA attenuated S1P-induced migration. Furthermore, transfection with PLD2 siRNA, infection of HPAECs with dnPKC-zeta, or treatment with myristoylated PKC-zeta peptide inhibitor abrogated S1P-induced Rac1 activation. These results establish that S1P signals through S1P(1) and G(i) to activate PKC-epsilon and, subsequently, a PLD2-PKC-zeta-Rac1 cascade. Activation of this pathway is necessary to stimulate the migration of lung endothelial cells, a key component of the angiogenic process.

Sphingosine-1-phosphate signaling and the skin

Sphingolipids have long been viewed as rather passive structural components of cellular membranes. More recently, it has become evident that metabolism of sphingomyelin yields several lipid mediators that evoke diverse and specific responses in different cell types. One sphingomyelin derivate, sphingosine-1-phosphate (S1P), has attracted particular attention for its effect on epidermal cells, which differs from those on most other cell types. S1P inhibits keratinocyte proliferation and induces keratinocyte differentiation and migration, suggesting a role for S1P in the re-epithelialization of wounds. The migratory response involves the phosphorylation and activation of Smad3. In epithelial tumors, S1P signaling has been linked with potential oncogenic effects, but has also been found to inhibit metastasis in a mouse melanoma model. S1P promotes endothelial cell survival, acts as a chemoattractant for vascular cells, and exerts a protective effect on the endothelial barrier. Conversely, S1P receptor knockout leads to embryonic lethality mainly due to impaired vascular maturation. S1P presumably modulates peripheral T-lymphocyte levels by stimulating their egress from lymphoid organs rather than by promoting T-cell proliferation. The S1P analog FTY720 (fingolimod) acts as a functional antagonist by inhibiting lymphocyte egress, and thus holds great promise as an immunosuppressant drug for the prevention of allograft rejection and treatment of T-lymphocyte-driven inflammatory skin diseases, such as lupus erythematosus, psoriasis, and atopic dermatitis. Topical use of S1P and other sphingosine compounds is also under investigation, particularly for the treatment of acne vulgaris.

Sphingosine kinase 1 gene transfer reduces postoperative peritoneal adhesion in an experimental model

BACKGROUND

Recovery of the surgically damaged mesothelial cell layer is a major process in reducing postoperative peritoneal adhesions. Sphingosine kinase (SPK) 1 is a signalling molecule involved in the regulation of proliferation and migration of various cell types. This study determined the effect of SPK-1 gene transfer on the recovery of damaged mesothelial cells and on peritoneal adhesion formation after surgery.

METHODS

Rat mesothelial cells were isolated and characterized by their expression of cytokeratin and vimentin. Their migration was determined by scratch wound motility assay. Cellular SPK-1 activity was measured by [gamma-32P]adenosine 5'-triphosphate incorporation. Wistar rats underwent laparotomy with subsequent caecum or uterine horn abrasion. Rats were randomized to either SPK-1 gene (Ad-SPK-1) transfer or control groups. The animals were killed 14 days after operation and peritoneal adhesions were graded.

RESULTS

Adenovirus-mediated SPK-1 gene transfer increased the cellular SPK-1 activity of mesothelial cells, leading to enhanced migration. Median adhesion scores were significantly lower in the Ad-SPK-1 group than in controls in both rat caecum (0.98 versus 2.60; P < 0.001) and rat uterine horn (0.28 versus 1.83; P < 0.001) models.

CONCLUSION

Adenovirus-mediated SPK-1 gene transfer promotes recovery of the surgically damaged mesothelial cell layer and prevents postoperative peritoneal adhesion formation.

Lysophosphatidic acid decreases the nuclear localization and cellular abundance of the p53 tumor suppressor in A549 lung carcinoma cells

Lysophosphatidic acid (LPA) is a bioactive lipid that promotes cancer cell proliferation and motility through activation of cell surface G protein-coupled receptors. Here, we provide the first evidence that LPA reduces the cellular abundance of the tumor suppressor p53 in A549 lung carcinoma cells, which express endogenous LPA receptors. The LPA effect depends on increased proteasomal degradation of p53 and it results in a corresponding decrease in p53-mediated transcription. Inhibition of phosphatidylinositol 3-kinase protected cells from the LPA-induced reduction of p53, which implicates this signaling pathway in the mechanism of LPA-induced loss of p53. LPA partially protected A549 cells from actinomycin D induction of both apoptosis and increased p53 abundance. Expression of LPA(1), LPA(2), and LPA(3) receptors in HepG2 hepatoma cells, which normally do not respond to LPA, also decreased p53 expression and p53-dependent transcription. In contrast, neither inactive LPA(1) (R124A) nor another G(i)-coupled receptor, the M(2) muscarinic acetylcholine receptor, reduced p53-dependent transcription in HepG2 cells. These results identify p53 as a target of LPA action and provide a new dimension for understanding how LPA stimulates cancer cell division, protects against apoptosis, and thereby promotes tumor progression.

Lysophosphatidic acid stimulates thrombomodulin lectin-like domain shedding in human endothelial cells

Thrombomodulin (TM) is an anticoagulant glycoprotein highly expressed on endothelial cell surfaces. Increased levels of soluble TM in circulation have been widely accepted as an indicator of endothelial damage or dysfunction. Previous studies indicated that various proinflammatory factors stimulate TM shedding in various cell types such as smooth muscle cells and epithelial cells. Lysophosphatidic acid (LPA) is a bioactive lipid mediator present in biological fluids during endothelial damage or injury. In the present study, we first observed that LPA triggered TM shedding in human umbilical vein endothelial cells (HUVECs). By Cyflow analysis, we showed that the LPA-induced accessibility of antibodies to the endothelial growth factor (EGF)-like domain of TM is independent of matrix metalloproteinases (MMPs), while LPA-induced TM lectin-like domain shedding is MMP-dependent. Furthermore, a stable cell line expressing TM without its lectin-like domain exhibited a higher cell proliferation rate than a stable cell line expressing full-length TM. These results imply that LPA induces TM lectin-like domain shedding, which might contribute to the exposure of its EGF-like domain for EGF receptor (EGFR) binding, thereby stimulating subsequent cell proliferation. Based on our findings, we propose a novel mechanism for the exposure of TM EGF-like domain, which possibly mediates LPA-induced EGFR transactivation.

F2L, a peptide derived from heme-binding protein, inhibits LL-37-induced cell proliferation and tube formation in human umbilical vein endothelial cells

F2L, a peptide derived from heme-binding protein, was originally identified as an endogenous ligand for formyl peptide receptor-like (FPRL)2. Previously, we reported that F2L inhibits FPR and FPRL1-mediated signaling in neutrophils. Since endothelial cells express functional FPRL1, we examined the effect of F2L on LL-37 (an FPRL1 agonist)-induced signaling in human umbilical vein endothelial cells (HUVECs). F2L stimulated the chemotactic migration in HUVECs. However, F2L inhibited FPRL1 activity, resulting in the inhibition of cell proliferation and tube formation induced by LL-37 in HUVECs. We suggest that F2L will potentially be useful in the study of FPRL1 signaling and the development of drugs to treat diseases involving the FPRL1 in the vascular system.

Lysophosphatidic acid-induced interleukin-1 beta expression is mediated through Gi/Rho and the generation of reactive oxygen species in macrophages

Lysophosphatidic acid (LPA), a low-molecular-weight lysophospholipid enriched in platelets and mildly oxidized low-density lipoproteins, is known to regulate inflammation and atherosclerosis by binding to its cognate receptors. In this study, we reported that LPA upregulated interleukin-1 beta (IL-1 beta) expression in mouse J774A.1 macrophages. By using pharmacological inhibitors, it was suggested that G(i)/Rho activation and subsequent reactive oxygen species (ROS) production were involved in IL-1 beta induction. In addition, IL-1 beta induction by LPA was also observed in human primary macrophages. In summary, LPA is involved in the processes of inflammation by affecting macrophage behavior.

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

Lysophosphatidic acid (LPA) is a bioactive lipid with functional properties that overlap those of growth factors and cytokines. LPA production in vivo is linked to platelet degranulation and the biological activities of this lipid are associated with wound healing. Osteoblasts and their progenitor cells are exposed to high levels of this lipid factor in regions adjacent to bone fractures and we postulate a role for LPA in skeletal healing. The regeneration of bone injuries requires a complex array of changes in gene expression, but the effects of LPA on mRNA levels in bone cells have not been investigated. We performed a genome-wide expression analysis in LPA-treated MC3T3-E1 pre-osteoblastic cells using Affymetrix GeneChip arrays. Cells exposed to LPA for 6 h exhibited 513 regulated genes, whereas changes in the levels of 54 transcripts were detected after a 24-h LPA treatment. Gene ontology analysis linked LPA-regulated gene products to biological processes that are known to govern bone healing, including cell proliferation, response to stress, organ development, chemotaxis/motility, and response to stimuli. Among the gene products most highly up-regulated by LPA were transcripts encoding the anti-inflammatory proteins sST2, ST2L, and heat-shock protein 25 (HSP25). RT-PCR analysis confirmed that these mRNAs were increased significantly in MC3T3-E1 cells and primary osteoblasts exposed to LPA. The response of cells to LPA is mediated by G-protein-coupled receptors, and the stimulation of anti-inflammatory gene expression in MC3T3-E1 cells was blocked by Ki16425, an inhibitor of LPA(1) and LPA(3) receptor forms. Pertussis toxin impaired only the LPA-induced expression of sST2. LPA-stimulated levels of sST2, ST2L and HSP25 mRNAs persisted if the cytosolic Ca(2+) elevations elicited by this lipid were blocked with BAPTA. In contrast to the stimulatory effect of LPA, exposure of MC3T3-E1 cells to fluid shear reduced the transcript levels of all three anti-inflammatory genes. The induction of sST2, ST2L and HSP25 expression by LPA suggests a role for this lipid factor in the regulation of osteoblastic cell function during periods of inflammation.

Targeting the lipids LPA and S1P and their signalling pathways to inhibit tumour progression

The bioactive lipids lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), the enzymes that generate and degrade them, and the receptors that receive their signals are all potential therapeutic targets in cancer. LPA and S1P signalling pathways can modulate a range of cellular processes that contribute to tumourigenesis, such as proliferation and motility, and components of the signalling pathways often show aberrant expression and altered activity upon malignant transformation. This article reviews LPA- and S1P-mediated activities that might contribute to the aetiology of cancer, and examines the potential of the many antagonists that have been developed to inhibit LPA and S1P signalling pathways. In addition, the outcomes of various clinical trials using LPA- and S1P-associated targets in cancer and other diseases are described, and future directions are discussed.

Lysophosphatidic acid regulates inflammation-related genes in human endothelial cells through LPA1 and LPA3

Lysophosphatidic acid (LPA) is a low-molecular-weight lysophospholipid (LPL), which regulates endothelial cells participating in inflammation processes via interactions with endothelial differentiation gene (Edg) family G protein-coupled receptors. In this study, we attempted to determine which LPA receptors mediate the inflammatory response in human endothelial cells. Introduction of siRNA against LPA1 significantly suppressed LPA-induced ICAM-1 mRNA, total protein, and cell surface expressions, and subsequent U937 monocyte adhesion to LPA-treated human umbilical endothelial cells (HUVECs). By knock down of LPA1 and LPA3 in HUVECs, LPA-enhanced IL-1beta mRNA expression was significantly attenuated. Moreover, LPA1 and LPA3 siRNA also inhibited LPA-enhanced IL-1-dependent long-term IL-8 and MCP-1 mRNA expression, and subsequent THP-1 cell chemotaxis toward LPA-treated HUVEC-conditioned media. These results suggest that the expression of LPA-induced inflammatory response genes is mediated by LPA1 and LPA3. Our findings suggest the possible utilization of LPA1 or LPA3 as drug targets to treat severe inflammation.

Lysophosphatidic acid is detectable in human bronchoalveolar lavage fluids at baseline and increased after segmental allergen challenge

BACKGROUND

Lysophosphatidic acid (LPA) is a biologically active lysophospholipid and a component of normal plasma. LPA binds to receptors expressed on circulating and structural lung cells and affects cell growth and activation. Whether LPA is present in the lung has not been previously reported.

OBJECTIVE

To develop an assay to measure LPA in bronchoalveolar lavage (BAL) fluids, and to study the association between LPA and allergic airway inflammation.

METHODS

Seventeen allergic subjects underwent bronchoscopy and segmental allergen challenge, followed 18 h later by BAL. Supernatants were analysed for LPA content using liquid chromatography and mass spectroscopy. Expression of LPA receptors on primary bronchial epithelial cells was analysed by immunolabelling, and the effects of LPA on epithelial cell barrier function was investigated by measuring transepithelial resistance.

RESULTS

LPA was detectable in BAL from control lung segments, and significantly increased 18 h after allergen challenge. Polyunsaturated species of LPA were especially increased following segmental allergen challenge. LPA levels did not strongly correlate with the number or percentages of eosinophils, neutrophils of lymphocytes, whereas MIP-3alpha (CCL20) levels correlated significantly with the allergen-driven influx of lymphocytes. The levels of LPA from control sites correlated inversely with BAL protein content, suggesting that LPA promoted epithelial barrier integrity at baseline. Experiments using primary human bronchial epithelial cells confirmed that LPA tightened the epithelial cell barrier.

CONCLUSION

Lysophosphatidic acid is detectable in human BAL fluids at baseline and its expression increases during allergic inflammation. LPA does not appear to be a dominant chemoattractant for eosinophils or lymphocytes during allergic airway inflammation. In the absence of ongoing inflammation, LPA may promote epithelial barrier integrity.

Endothelial cell migration on RGD-peptide-containing PEG hydrogels in the presence of sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) is a potent chemokinetic agent for endothelial cells that is released by activated platelets. We previously developed Arg-Gly-Asp (RGD)-containing polyethylene glycol biomaterials for the controlled delivery of S1P to promote endothelialization. Here, we studied the effects of cell adhesion strength on S1P-stimulated endothelial cell migration in the presence of arterial levels of fluid shear stress, since an upward shift in optimal cell adhesion strengths may be beneficial for promoting long-term cell adhesion to materials. Two RGD peptides with different integrin-binding specificities were added to the polyethylene glycol hydrogels. A linear RGD bound primarily to beta(3) integrins, whereas a cyclic RGD bound through both beta(1) and beta(3) integrins. We observed increased focal adhesion formation and better long-term adhesion in flow with endothelial cells on linear RGD peptide, versus cyclic RGD, even though initial adhesion strengths were higher for cells on cyclic RGD. Addition of 100 nM S1P increased cell speed and random motility coefficients on both RGD peptides, with the largest increases found on cyclic RGD. For both peptides, much of the increase in cell migration speed was found for smaller cells (<1522 microm(2) projected area), although the large increases on cyclic RGD were also due to medium-sized cells (2288-3519 microm(2)). Overall, a compromise between high cell migration rates and long-term adhesion will be important in the design of materials that endothelialize after implantation.

Sphingosine 1-phosphate (S1P) inhibits monocyte-endothelial cell interaction by regulating of RhoA activity

Recent studies suggest that sphingosine 1-phosphate (S1P) protects against atherosclerosis. We assessed the effects of S1P on monocyte-endothelial interaction in the presence of inflammatory mediators. Pretreatment of THP-1 cells with S1P abolished Phorbol 12 myristate 13-acetate (PMA)-induced THP-1 cell adhesion to human umbilical vein endothelial cells (HUVECs). S1P inhibited PMA-induced activation of RhoA, but not PKCs. S1P activated p190Rho GTPase activation protein (GAP) only in the presence of PMA, suggesting an inhibitory effect of S1P and PMA to suppress RhoA. In conclusion, S1P inhibited monocyte-endothelial interactions by inhibiting RhoA activity which may explain its anti-atherogenic effects.

Sphingosine 1-phosphate accelerates wound healing in diabetic mice

BACKGROUND

Blood platelets store sphingosine 1-phosphate (S1P) abundantly and release this bioactive lipid extracellularly. S1P acts as an intercellular mediator through interaction with the endothelial differentiation gene (EDG)/S1P family of G protein-coupled receptors. Of the EDG family S1P receptors, EDG-5 (S1P2) is inhibited in migration induced by S1P. Diabetes impairs numerous aspects of tissue repair. Failure of wound angiogenesis is known to delay diabetic wound healing.

OBJECTIVES

We examined whether S1P subcutaneous injection could improve the healing of full-thickness skin wounds in healthy and diabetic mice. We further determine if the combined S1P and EDG-5 (S1P2) antagonist injection in diabetic mice could affect wound healing. Finally, we examined the histopathological findings of the wound following S1P injection in diabetic mice.

METHODS

Eight- to 10-week-old BALA/c mice, diabetic db/db mice and Wister rats were used for the studies. A full-thickness wound was made on the dorsal skin of the healthy and diabetic mice. Either 10 microM or 100 microM of S1P or vehicle control (BSA/PBS) was injected into the wound bed every day. We calculated the wound area after each injection. EDG-5 (S1P2) antagonist (JTE-013) or vehicle (DMSO) was then injected in addition to the S1P around the dorsal wound of diabetic mice and the wound diameter was measured. Wound tissue samples were excised following injection for histopathological examination.

RESULTS

Wound area in normal BALA/c mice did not significantly decrease upon S1P injection compared to S1P-untreated controls. S1P injection alone showed significant promotion of wound healing in diabetic mice compared to no S1P treatment. The combination of S1P and EDG-5 (S1P2) receptor antagonist administration induced maximal wound healing in diabetic mice. Histopathological examination revealed that S1P induces neo-vascularization potential in rats and diabetic mice wound.

CONCLUSIONS

S1P injection in diabetic mice significantly accelerated cutaneous wound healing in the neo-vascularization process. The results demonstrate that S1P affects and sustains all key cellular processes responsible for wound repair and point to a unique potential for this molecule in the therapy of diabetic wounds, particularly as an angiogenic agent in treatment of diabetic wounds.

Lysophosphatidic acid decreases epidermal growth factor receptor binding in airway epithelial cells

We showed previously that treatment of human airway smooth muscle cells and lung fibroblasts with lysophosphatidic acid (LPA) increases the binding of epidermal growth factor (EGF) to EGF receptors (EGFRs). The purpose of this study was to determine whether LPA also regulates EGFR binding in airway epithelial cells. Airway epithelial cells were incubated in the absence or presence of 10 microM LPA for increasing times, and binding of 125I-EGF to intact cells on ice was measured. Exposure to LPA for only 15 min caused a 30 to 70% decrease in EGFR binding in a dose-dependent manner, depending on the cell line. This decrease in binding was sustained to at least 18 h in BEAS-2B and primary human bronchial epithelial cells. In contrast, the LPA-induced decrease in binding reversed rapidly in two lung cancer epithelial cell lines, H292 and A549, returning to control levels within 3 h. LPA increased phosphorylation of the EGFR in BEAS-2B cells, and this phosphorylation was inhibited by both 4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline (AG1478; EGFR tyrosine kinase inhibitor) and N-[(2R)-2-(hydroxamidocarbonylmethyl)-4-methylpentanoyl]-l-tryptophan methylamide (GM6001; matrix metalloproteinase inhibitor) but not by CRM197 (heparin-binding EGF inhibitor). AG-1478 and GM6001 also inhibited the LPA-induced decrease in EGFR binding but only by 50%, suggesting only partial involvement of EGFR transactivation in the decrease in EGFR binding. In summary, LPA stimulates a decrease in EGFR binding in airway epithelial cells that is sustained in normal cells but that rapidly reverses in cancer cells. LPA-induced transactivation of EGFRs occurs and contributes to the decrease in EGFR binding, but additional pathway(s) may also be involved.

Hic-5 promotes endothelial cell migration to lysophosphatidic acid

Endothelial cell migration is critical for proper blood vessel development. Signals from growth factors and matrix proteins are integrated through focal adhesion proteins to alter cell migration. Hydrogen peroxide-inducible clone 5 (Hic-5), a paxillin family member, is enriched in the focal adhesions in bovine pulmonary artery endothelial (BPAE) cells, which migrate to lysophosphatidic acid (LPA) on denatured collagen. In this study, we investigate the role of Hic-5 in LPA-stimulated endothelial cell migration. LPA recruits Hic-5 to the focal adhesions and to the pseudopodia in BPAE cells plated on collagen, suggesting that recruitment of Hic-5 to focal adhesions is associated with endothelial cell migration. Knockdown of endogenous Hic-5 significantly decreases migration toward LPA, confirming involvement of Hic-5 in migration. To address the role of Hic-5 in endothelial cell migration, we exogenously expressed wild-type (WT) Hic-5 and green fluorescent protein Hic-5 C369A/C372A (LIM3 mutant) constructs in BPAE cells. WT Hic-5 expression increases chemotaxis of BPAE cells to LPA, whereas migration toward LPA of the green fluorescent protein Hic-5 C369A/C372A-expressing cells is similar to that shown in vector control cells. Additionally, ERK phosphorylation is enhanced in the presence of LPA in WT Hic-5 cells. A pharmacological inhibitor of MEK activity inhibits LPA-stimulated WT Hic-5 cell migration and ERK phosphorylation, suggesting Hic-5 enhances migration via MEK activation of ERK. Together, these studies indicate that Hic-5, a focal adhesion protein in endothelial cells, is recruited to the pseudopodia in the presence of LPA and enhances migration.

Shape change controls supporting cell proliferation in lesioned mammalian balance epithelium

Mature mammals are uniquely vulnerable to permanent auditory and vestibular deficits, because the cell proliferation that produces replacement hair cells in other vertebrates is limited in mammals. To investigate the cellular mechanisms responsible for that difference, we created excision lesions in the sensory epithelium of embryonic and 2-week-old mouse utricles. Lesions in embryonic utricles closed in <24 h via localized expansion of supporting cells, which then reentered the cell cycle. Pharmacological treatments combined with time-lapse microscopy demonstrated that the healing depended on Rho-mediated contraction of an actin ring at the leading edge of the lesion. In contrast, lesions in utricles from 2-week-old and older mice remained open even after 48 h. Supporting cells in those utricles remained compact and columnar and had significantly stouter cortical actin belts than those in embryonic sensory epithelia. This suggests that cytoskeletal changes may underlie the age-related loss of proliferation in mammalian ears by limiting the capacity for mature supporting cells to change shape. In mature utricles, exogenous stimulation with lysophosphatidic acid overcame this maturational block and induced closure of lesions, promoting supporting cell expansion and subsequent proliferation. After lysophosphatidic acid treatment, 85% of the mature supporting cells that had spread to a planar area >300 microm2 entered S-phase, whereas only 10% of those cells that had a planar area <100 microm2 entered S-phase. Together, these results indicate that cellular shape change can overcome the normal postnatal cessation of supporting cell proliferation that appears to limit regeneration in mammalian vestibular epithelia.

Lysophosphatidic acid modulates the activation of human monocyte-derived dendritic cells

Lysophosphatidic acid (LPA) is a biologically active lysophospholipid that can regulate immune activation. LPA can activate T cells and dendritic cells (DCs), but the effects of LPA on the ability of DCs to influence T cell polarization are not well understood. Human monocyte-derived DCs were differentiated in vitro in the presence of interleukin-4 (IL-4) and granulocyte-macrophage colonystimulating factor (GM-CSF), and matured with LPA and lipopolysaccharide (LPS) alone or in combination. DC activation was monitored by analyzing cell-surface expression of co-stimulatory receptors and cytokine production. The ability of DCs to influence T cell activation was determined using two models of DC:T cell co-culture. Maturation with LPS induced dose-dependent DC activation characterized by enhanced expression of co-stimulatory molecules (e.g., CD86) and production of cytokines including IL-6 and IL-10. Co-incubation with LPA attenuated the LPS-induced production of IL-6, without significantly affecting IL-10 secretion or the ability of DC to promote T cell proliferation. DCs matured in the presence of both LPA and LPS enhanced the production of interferon-gamma (IFN-gamma) when co-cultured with allogeneic T cells, compared with DC matured by LPS alone. Similar results were found using a model of allogeneic naïve T cell differentiation, where LPA- plus LPS-matured DC enhanced IFN-gamma as well as IL-4 secretion after restimulation. Lysophosphatidic acid fine-tunes the effects of LPS on human myeloid DC maturation, but does not exert a dominant effect on the ability of DC to influence Th cell polarization.

Erythropoietin, a hypoxia-regulated factor, elicits a pro-angiogenic program in human mesenchymal stem cells

OBJECTIVE

The ability of erythropoietin (EPO) to elicit a pro-angiogenic effect on human mesenchymal stem cells (hMSC) was tested. hMSC are currently under study as therapeutic delivery agents that target tumor vessels. Hypoxia favors the differentiation of hMSC towards a pro-angiogenic program. However, the classical angiogenic factors, vascular endothelial growth factor and basic fibroblast growth factor, are not fully capable of restoring this effect. The hypoxia-regulated factor, EPO, induces angiogenesis in endothelial cells. Here, EPO's pro-angiogenic effect on hMSC was analyzed.

METHODS

hMSC were tested for EPO receptor expression by western blot, immunofluorescence, and flow cytometry assays. Downstream receptor signaling components JAK and STAT were measured by standard assays. Pro-angiogenesis effects mediated by EPO treatment of hMSC were measured by proliferation, cytokine, or pro-angiogenesis factor secretion, metalloprotease activation, migration, invasion, wound healing, and tubule formation assays.

RESULTS

hMSC express the cognate EPO receptor and are capable of promoting angiogenesis following EPO treatment in all the angiogenesis assays tested. EPO-treated hMSC proliferate and secrete pro-angiogenesis factors more readily than untreated hMSC. EPO leads to increased hMSC chemotaxis, migration, and activation of matrix metalloprotease-2. This treatment causes greater recruitment of vessels as measured in an in vivo angiogenesis assay.

CONCLUSION

EPO is capable of eliciting a pro-angiogenesis program in hMSC that instigates secretion of angiogenic factors and the subsequent recruitment of endothelium. This study defines a novel mechanism for tumor cell recruitment of blood vessels that is important to consider in the design of stem cell-based therapies.

Sphingosine 1-phosphate regulates inflammation-related genes in human endothelial cells through S1P1 and S1P3

Sphingosine 1-phosphate (S1P) is a bioactive lysophospholipid (LPL) ligand that binds endothelial differentiation gene (Edg) family G-protein-coupled receptors and has been implicated as an important regulator in endothelial cells during inflammation processes. In this study, we attempt to determine which S1P receptors mediating the inflammatory response in human endothelial cells. Our results indicated that introduction of siRNA against S1P(1) significantly suppressed S1P-induced ICAM-1 mRNA, total protein, and cell surface expressions in human umbilical vein endothelial cells (HUVECs). Moreover, U937 cells adhesion to S1P-treated HUVECs was profoundly reduced by knock-down of S1P(1) in HUVECs. By knock-down of S1P(1) or S1P(3) in HUVECs, S1P-enhanced IL-8, MCP-1 mRNA expression, and THP-1 cell chemotaxis toward S1P-treated HUVEC-conditioned media was profoundly reduced. These results suggested that S1P-induced inflammatory response genes expression is mediated through S1P(1) and S1P(3). Our findings suggest the possible utilization of S1P(1) or S1P(3) as drug targets to treat severe inflammation.

Malignancy-related 67kDa laminin receptor in adenoid cystic carcinoma. Effect on migration and beta-catenin expression

Adenoid cystic carcinoma is a malignant salivary gland neoplasm with recurrence and metastasis. We studied the expression of a malignancy-related non-integrin laminin receptor, the 67LR, in this neoplasm. Immunohistochemistry showed 67LR in adenoid cystic carcinoma. This receptor binds a sequence of laminin beta1 chain, the YIGSR peptide. We studied the effect of 67LR and YIGSR in cells (CAC2) from adenoid cystic carcinoma. Three-dimensional cultures of cells embedded into either laminin-111 gel (controls) or YIGSR-enriched laminin-111 (treated) were prepared and studied by light microscopy. CAC2 cells treated with YIGSR appeared fibroblast-like, while control cells were epithelioid. Blockage of 67LR by antibody abolished YIGSR effect in three-dimensional cultures. We analysed the relevance of 67LR and YIGSR on beta-catenin expression in CAC2 cells. Immunofluorescence and immunoblot showed that YIGSR decreased beta-catenin, while blockage of 67LR restored the presence of this molecule. The 67LR and YIGSR induced fibroblast-like morphology in CAC2 cells, with disruption of cell-cell contacts and decrease of beta-catenin. These features resemble epithelial-mesenchymal transition (EMT). EMT also increases cell migration. In monolayer assays YIGSR increased migration of CAC2 cells. We conclude that 67LR and YIGSR are involved in epithelial-mesenchymal transition, modulation of beta-catenin expression, and migratory activity of CAC2 cells.

Statins induce S1P1 receptors and enhance endothelial nitric oxide production in response to high-density lipoproteins

BACKGROUND AND PURPOSE

Sphingosine 1-phosphate (S1P) is a serum-borne naturally occurring sphingolipid, specifically enriched in high-density lipoprotein (HDL) fractions. S1P binds to G-protein-coupled S1P1 receptors to activate endothelial NO synthase (eNOS) in vascular endothelial cells. We explored whether and how statins, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, modulate expression of S1P1 receptors and endothelial responses for subsequent stimulation with S1P or with HDL.

EXPERIMENTAL APPROACH

Protein expression and phosphorylation and mRNA expression in cultured bovine aortic endothelial cells (BAEC) were determined using immunoblots and reverse transcription PCR analyses, respectively. NO synthesis was assessed as nitrite production.

KEY RESULTS

Stimulation of BAEC with pitavastatin or atorvastatin led to significant increases in S1P1-receptors, at levels of protein and mRNA, in a dose-dependent manner. When BAEC were treated with pitavastatin prior to stimulation with S1P or with normal human HDL, phosphorylation and activation of eNOS evoked by S1P or by HDL was enhanced. These effects of statins were counteracted by L-mevalonate and were mimicked by an inhibitor of geranylgeranyl transferase I, suggesting that inhibition of HMG-CoA reductase activity and subsequent decreases in protein geranylgeranylation may contribute to these actions of statins. Specific knock down of S1P1 receptors by small interfering RNA led to attenuation of eNOS responses to HDL.

CONCLUSIONS AND IMPLICATIONS

Statins induce S1P1 receptors and potentiate responses of endothelial cells to HDL-associated sphingolipids, identifying a novel aspect of the pleiotropic actions of statins through which they may exert NO-dependent vascular protective effects.

Lysophospholipids increase IL-8 and MCP-1 expressions in human umbilical cord vein endothelial cells through an IL-1-dependent mechanism

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are both low-molecular-weight lysophospholipid (LPL) ligands which are recognized by the Edg family of G protein-coupled receptors (GPCRs). In endothelial cells, these two ligands activate Edg receptors resulting in cell proliferation and cell migration. Interleukin-8 (IL-8) is a C-X-C chemokine and acts as a chemoattractant of neutrophils, whereas monocyte chemoattractant protein-1 (MCP-1) is a C-C chemokine and functions mainly as a chemoattractant of monocytes/macrophages. Both factors are secreted from endothelial cells and have been implicated in the processes leading to atherosclerosis. We examined the effects of LPLs on the expression of IL-8 and MCP-1, key regulators of leukocyte recruitment in human umbilical cord vein endothelial cells (HUVECs). Work illustrated in this article showed that LPA and S1P enhanced IL-8 and MCP-1 mRNA expressions, and protein secretions in dose- and time-dependent fashions. Maximal mRNA expression appeared at 16 hr post-ligand treatment. Using prior treatments with chemical inhibitors, LPLs enhanced IL-8 and MCP-1 expressions through a Gi-, Rho-, and NFkappaB-dependent mechanism. In a chemotaxis assay system, LPL treatments of endothelial cells enhanced monocyte recruitment through upregulating IL-8 and MCP-1 protein secretions. Pre-incubation with AF12198, an IL-1 receptor antagonist or IL-1 functional blocking antibody both suppressed the enhanced effects elicited by LPLs of IL-8 and MCP-1 mRNA expressions in HUVECs. These results suggest that LPLs released by activated platelets might enhance the IL-8- and MCP-1-dependent chemoattraction of monocytes toward the endothelium through an IL-1-dependent mechanism, which may play an important role in facilitating wound-healing and inflammation processes.

Tumor cell invasion of collagen matrices requires coordinate lipid agonist-induced G-protein and membrane-type matrix metalloproteinase-1-dependent signaling

Background

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are bioactive lipid signaling molecules implicated in tumor dissemination. Membrane-type matrix metalloproteinase 1 (MT1-MMP) is a membrane-tethered collagenase thought to be involved in tumor invasion via extracellular matrix degradation. In this study, we investigated the molecular requirements for LPA- and S1P-regulated tumor cell migration in two dimensions (2D) and invasion of three-dimensional (3D) collagen matrices and, in particular, evaluated the role of MT1-MMP in this process.

Results

LPA stimulated while S1P inhibited migration of most tumor lines in Boyden chamber assays. Conversely, HT1080 fibrosarcoma cells migrated in response to both lipids. HT1080 cells also markedly invaded 3D collagen matrices (~700 μm over 48 hours) in response to either lipid. siRNA targeting of LPA₁ and Rac1, or S1P₁, Rac1, and Cdc42 specifically inhibited LPA- or S1P-induced HT1080 invasion, respectively. Analysis of LPA-induced HT1080 motility on 2D substrates vs. 3D matrices revealed that synthetic MMP inhibitors markedly reduced the distance (~125 μm vs. ~45 μm) and velocity of invasion (~0.09 μm/min vs. ~0.03 μm/min) only when cells navigated 3D matrices signifying a role for MMPs exclusively in invasion. Additionally, tissue inhibitors of metalloproteinases (TIMPs)-2, -3, and -4, but not TIMP-1, blocked lipid agonist-induced invasion indicating a role for membrane-type (MT)-MMPs. Furthermore, MT1-MMP expression in several tumor lines directly correlated with LPA-induced invasion. HEK293s, which neither express MT1-MMP nor invade in the presence of LPA, were transfected with MT1-MMP cDNA, and subsequently invaded in response to LPA. When HT1080 cells were seeded on top of or within collagen matrices, siRNA targeting of MT1-MMP, but not other MMPs, inhibited lipid agonist-induced invasion establishing a requisite role for MT1-MMP in this process.

Conclusion

LPA is a fundamental regulator of MT1-MMP-dependent tumor cell invasion of 3D collagen matrices. In contrast, S1P appears to act as an inhibitory stimulus in most cases, while stimulating only select tumor lines. MT1-MMP is required only when tumor cells navigate 3D barriers and not when cells migrate on 2D substrata. We demonstrate that tumor cells require coordinate regulation of LPA/S1P receptors and Rho GTPases to migrate, and additionally, require MT1-MMP in order to invade collagen matrices during neoplastic progression.

Lysophosphatidic acid induces endothelial cell death by modulating the redox environment

Oxidant stress plays a significant role in hypoxic-ischemic injury to the susceptible microvascular endothelial cells. During oxidant stress, lysophosphatidic acid (LPA) concentrations increase. We explored whether LPA caused cytotoxicity to neuromicrovascular cells and the potential mechanisms thereof. LPA caused a dose-dependent death of porcine cerebral microvascular as well as human umbilical vein endothelial cells; cell death appeared oncotic rather than apoptotic. LPA-induced cell death was mediated via LPA(1) receptor, because the specific LPA(1) receptor antagonist THG1603 fully abrogated LPA's effects. LPA decreased intracellular GSH levels and induced a p38 MAPK/JNK-dependent inducible nitric oxide synthase (NOS) expression. Pretreatment with the antioxidant GSH precursor N-acetyl-cysteine (NAC), as well as with inhibitors of NOS [N(omega)-nitro-l-arginine (l-NNA); 1400W], significantly prevented LPA-induced endothelial cell death (in vitro) to comparable extents; as expected, p38 MAPK (SB203580) and JNK (SP-600125) inhibitors also diminished cell death. LPA did not increase indexes of oxidation (isoprostanes, hydroperoxides, and protein nitration) but did augment protein nitrosylation. Endothelial cytotoxicity by LPA in vitro was reproduced ex vivo in brain and in vivo in retina; THG1603, NAC, l-NNA, and combined SB-203580 and SP600125 prevented the microvascular rarefaction. Data implicate novel properties for LPA as a modulator of the cell redox environment, which partakes in endothelial cell death and ensued neuromicrovascular rarefaction.

Of Spiders and Crabs: The Emergence of Lysophospholipids and Their Metabolic Pathways as Targets for Therapy in Cancer: Fig. 1

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), two small lysophospholipids, are potent inducers of many of the hallmarks of cancer including cell proliferation, survival, migration, invasion, and neovascularization in in vitro and in vivo tumor models. Furthermore, the enzymes metabolizing LPA and S1P and their receptors are aberrant in multiple cancer lineages and exhibit transforming activity altering patterns and targets for metastasis. Several recent studies show the remarkable activity of new chemical genomics and/or potential novel drugs in preclinical models. Combined with the physiologic and pathophysiologic activities of LPA and S1P, these studies suggest the implementation of preclinical and clinical evaluation of LPA and S1P as therapeutic targets.

Sphingosine 1-phosphate protects rat liver sinusoidal endothelial cells from ethanol-induced apoptosis: Role of intracellular calcium and nitric oxide

In alcoholic liver disease, ethanol-induced damage to sinusoidal endothelial cells (SECs) appears to be important in the progression of liver damage. However, little is known about the mechanisms responsible for protection of SECs against ethanol-induced injury. To elucidate the role of sphingosine 1-phosphate (S1P), which is stored in platelets and may be released from them on their activation, we investigated the effect of S1P on rat liver SECs in primary culture. Pretreatment of cells with 1 mumol/L S1P attenuated ethanol-induced apoptosis. Electron microscopy confirmed this protective effect of S1P on damaged SECs in liver tissues after perfusion of ethanol. In the absence of ethanol, S1P increased DNA synthesis as determined via incorporation of bromodeoxyuridine. S1P also ameliorated the decreased DNA synthesis of cells induced by ethanol. Addition of S1P to cells induced an increase in intracellular calcium concentrations and NO production in cells. Western blotting revealed that S1P significantly induced the activation of endothelial NO synthase (eNOS), but not Akt, and that S1P-induced activation of eNOS was blocked by trifluoperazine, a calmodulin inhibitor. Furthermore, N(G)-nitro-L-arginine methyl ester, a NO synthase inhibitor, cancelled the effect of S1P on DNA synthesis, apoptosis, and NO production in vitro as well as the protective effect of S1P on cell damage in situ. In conclusion, the biological effect of S1P is at least partially mediated by Ca(2+)-sensitive eNOS activation and subsequent NO formation; extracellular S1P could contribute to sinusoidal protection and remodeling in alcoholic liver injury.

Hydrogen peroxide induces S1P1 receptors and sensitizes vascular endothelial cells to sphingosine 1-phosphate, a platelet-derived lipid mediator

Sphingosine 1-phosphate (S1P) is a platelet-derived angiogenic lipid growth factor, modulating G-protein-coupled S1P(1) receptors (S1P(1)-R) to activate endothelial nitric oxide synthase (eNOS), as well as MAPK pathways in endothelial cells. We explored whether and how hydrogen peroxide (H(2)O(2)), a representative reactive oxygen species, alters S1P(1)-R expression and influences S1P signaling in cultured bovine aortic endothelial cells (BAECs). When BAECs are treated with pathophysiologically relevant concentrations of H(2)O(2) (150 microM for 30 min), S1P(1)-R protein expression levels are acutely augmented by approximately 30-fold in a dose-dependent fashion. When BAECs have been pretreated with H(2)O(2), subsequent S1P stimulation (100 nM) leads to a higher degree of eNOS enzyme activation (assessed as intracellular cGMP content, 1.7 +/- 0.2-fold vs. no H(2)O(2) pretreatment groups, P < 0.05), associated with a higher magnitude of phosphorylation responses of eNOS and MAPK ERK1/2. PP2, an inhibitor of Src-family tyrosine kinase, abolished the effects of H(2)O(2) on both S1P(1)-R protein upregulation and enhanced BAEC responses to S1P. H(2)O(2) does not augment S1P(1) mRNA expression, whereas VEGF under identical cultures leads to increases in S1P(1) mRNA signals. Whereas H(2)O(2) attenuates proliferation of BAECs, addition of S1P restores growth responses of these cells. These results demonstrate that extracellularly administered H(2)O(2) increases S1P(1)-R expression and promotes endothelial responses for subsequent S1P treatment. These results may identify potentially important points of cross-talk between reactive oxygen species and sphingolipid pathways in vascular responses.

Lysophosphatidic acid induces chemotaxis in MC3T3-E1 osteoblastic cells

Lysophosphatidic acid (LPA) is a bioactive lipid that has pleiotropic effects on a variety of cell types and enhances the migration of endothelial and cancer cells, but it is not known if this lipid can alter osteoblast motility. We performed transwell migration assays using MC3T3-E1 osteoblastic cells and found LPA to be a potent chemotactic agent. Quantitative time-lapse video analysis of osteoblast migration after wounds were introduced into cell monolayers indicated that LPA stimulated both migration velocity and the average migration distance per cell. LPA also elicited substantial changes in cell shape and actin cytoskeletal structure; lipid-treated cells contained fewer stress fibers and displayed long membrane processes that were enriched in F-actin. Quantitative RT-PCR analysis showed that MC3T3-E1 cells express all four known LPA-specific G-protein-coupled receptors (LPA1-LPA4) with a relative mRNA abundance of LPA1>LPA4>LPA2>>LPA3. LPA-induced changes in osteoblast motility and morphology were antagonized by both pertussis toxin and Ki16425, a subtype-specific blocker of LPA1 and LPA3 receptor function. Cell migration in many cell types is linked to changes in intracellular Ca2+. Ki16425 also inhibited LPA-induced Ca2+ signaling in a dose-dependent manner, suggesting a link between LPA-induced Ca2+ transients and osteoblast chemotaxis. Our data show that LPA stimulates MC3T3-E1 osteoblast motility via a mechanism that is linked primarily to the G-protein-coupled receptor LPA1.

Delivery of sphingosine 1-phosphate from poly(ethylene glycol) hydrogels

While protein growth factors promote therapeutic angiogenesis, delivery of lipid factors such as sphingosine 1-phosphate (S1P) may provide better stabilization of newly formed vessels. We developed a biomaterial for the controlled delivery of S1P, a bioactive lipid released from activated platelets. Multiarm poly(ethylene glycol)-vinyl sulfone was cross-linked with albumin, a lipid-transporting protein, to form hydrogels. The rate of S1P release from the materials followed Fickian kinetics and was dependent upon the presence of lipid carriers in the release solution. Delivery of S1P from RGD-modified hydrogels increased the cell migration speed of endothelial cells growing on the materials. The materials also induced angiogenesis in the chorioallantoic membrane assay. Our data demonstrate that the storage and release of lipid factors provides a new route for the induction of angiogenesis by artificial materials.

De novo biosynthesis of dihydrosphingosine-1-phosphate by sphingosine kinase 1 in mammalian cells

Sphingosine kinase 1 (SK1) is one of the two known kinases, which generates sphingosine-1-phosphate (S1P), a potent endogenous lipid mediator involved in cell survival, proliferation, and cell-cell interactions. Activation of SK1 and intracellular generation of S1P were suggested to be part of the growth and survival factor-induced signaling, and overexpression of SK1 provoked cell tumorigenic transformation. Using a highly selective and sensitive LC-MS/MS approach, here we show that SK1 overexpression, but not SK2, in different primary cells and cultured cell lines results in predominant upregulation of the synthesis of dihydrosphingosine-1-phosphate (DHS1P) compared to S1P. Stable isotope pulse-labeling experiments in conjunction with LC-MS/MS quantitation of different sphingolipids demonstrated strong interference of overexpressed SK1 with the de novo sphingolipid biosynthesis by deviating metabolic flow of newly formed sphingoid bases from ceramide formation toward the synthesis of DHS1P. On the contrary, S1P biosynthesis was not directly linked to the de novo sphingoid bases transformations and was dependent on catabolic generation of sphingosine from complex sphingolipids. As a result of SK1 overexpression, migration and Ca2+-response of human pulmonary artery endothelial cells (HPAEC) to stimulation with external S1P, but not thrombin, was strongly impaired. In contrast, selective increase in intracellular content of DHS1P or S1P through the uptake and phosphorylation of corresponding sphingoid bases had no effect on S1P-induced signaling or facilitation of wound healing. Furthermore, infection of human bronchial epithelial cells (HBEpC) with RSV A-2 virus increased SK1-mediated synthesis of DHS1P and S1P, whereas TNF-alpha enhanced only S1P production in HPAEC. These findings uncover a new functional role for SK1, which can control survival/death (DHS1P-S1P/ceramides) balance by targeting sphingolipid de novo biosynthesis and selectively generating DHS1P at a metabolic step preceding ceramide formation.

Lysophosphatidic Acid (LPA)-Induced Vascular Endothelial Growth Factor (VEGF) by Mesothelial Cells and Quantification of Host-Derived VEGF in Malignant Ascites

BACKGROUND

Lysophosphatidic acid (LPA) is a lipid mediator with multiple biological activities that may affect the progression of various cancers. Malignant ascites contains high levels of LPA as well as vascular endothelial growth factor (VEGF). Although LPA receptors are widely expressed in normal as well as cancer cells, little is known about the effect of LPA on host cells. Therefore, we evaluated the effect of LPA specifically on peritoneal mesothelial cells (PMC), and assessed another aspect of LPA in tumor biology mediated through the host cells.

MATERIALS AND METHODS

The effect of LPA on the production of VEGF was evaluated by ELISA and northern blotting. Next, we quantified human- and mouse-VEGF separately in ascitic fluid of nude mice inoculated intraperitoneally with a human gastric cancer, MKN45, and thus evaluated the ratio of host-derived VEGF in malignant ascites.

RESULTS

Addition of 10 to 80 mum LPA enhanced VEGF production by PMC through gene activation. The effect was strongly inhibited by pre-treatment with PTX or Ki16425, indicating that the effect was mainly dependent on the LPA1 signal. Of the VEGF in ascitic fluid at 3 weeks after tumor inoculation, 12.8% was derived from mouse cells. At 6 weeks, however, the ratio of host-derived VEGF was reduced to 5.0%, suggesting that the ratio of host-derived VEGF may be higher in the earlier phase.

CONCLUSION

Because tumor growth is often associated with an increase of LPA concentration in ascites, stimulation of VEGF production in PMC might have an important role in the growth of cancer cells disseminated in the peritoneal cavity.

Development and application of automatic high-resolution light microscopy for cell-based screens

Large-scale microscopy-based screens offer compelling advantages for assessing the effects of genetic and pharmacological modulations on a wide variety of cellular features. However, development of such assays is often confronted by an apparent conflict between the need for high throughput, which usually provides limited information on a large number of samples, and a high-content approach, providing detailed information on each sample. This chapter describes a novel high-resolution screening (HRS) platform that is able to acquire large sets of data at a high rate and light microscope resolution using specific "reporter cells," cultured in multiwell plates. To harvest extensive morphological and molecular information in these automated screens, we have constructed a general analysis pipeline that is capable of assigning scores to multiparameter-based comparisons between treated cells and controls. This chapter demonstrates the structure of this system and its application for several research projects, including screening of chemical compound libraries for their effect on cell adhesion, discovery of novel cytoskeletal genes, discovery of cell migration-related genes, and a siRNA screen for perturbation of cell adhesion.

Rac1 modulates sphingosine 1-phosphate-mediated activation of phosphoinositide 3-kinase/Akt signaling pathways in vascular endothelial cells

Sphingosine 1-phosphate (S1P) is a platelet-derived sphingolipid that activates G protein-coupled S1P receptors and initiates a broad range of responses in vascular endothelial cells. The small GTPase Rac1 is implicated in diverse S1P-modulated cellular responses in endothelial cells, yet the molecular mechanisms involved in S1P-mediated Rac1 activation are incompletely understood. We studied the pathways involved in S1P-mediated Rac1 activation in bovine aortic endothelial cells (BAEC) and found that S1P-induced Rac1 activation is impaired following chelation of G protein betagamma subunits by transfection of betaARKct. Treatment with the Src tyrosine kinase inhibitor PP2 completely attenuated S1P-mediated Rac1 activation; however, pretreatment of BAEC with wortmannin, an inhibitor of phosphoinositide (PI) 3-kinase, had no effect on Rac1 activation while completely blocking S1P-induced Akt phosphorylation. We used Rac1-specific small interfering RNA (siRNA) duplexes to "knock down" endogenous Rac1 expression and found that siRNA-mediated Rac1 knockdown significantly impaired basal as well as S1P-induced phosphorylation of protein kinase Akt, as well as several downstream targets of Akt including endothelial nitric-oxide synthase and glycogen synthase kinase 3beta. By contrast, S1P-induced phosphorylation of the mitogen-activated protein kinases ERK1/2 was unperturbed by siRNA-mediated Rac1 knockdown. We found that overexpression of the Rac1 guanine nucleotide exchange factor (GEF) Tiam1 markedly enhanced Rac1 activity, whereas a dominant negative Tiam1 mutant significantly attenuated S1P-mediated Rac1 activation. Taken together, these studies identify G protein betagamma subunits, Src kinase and the GEF Tiam1 as upstream modulators of S1P-mediated Rac1 activation, and establish a central role for Rac1 in S1P-mediated activation of PI 3-kinase/Akt/endothelial nitric-oxide synthase signaling in vascular endothelial cells.

Fluid shear stress modulates cell migration induced by sphingosine 1-phosphate and vascular endothelial growth factor

The rational design of drug delivery systems requires the ability to predict the environment-specific responses of target cells to the delivered drug. Here we describe the in vitro effects of fluid shear stress, vascular endothelial growth factor (VEGF), and sphingosine 1-phosphate (S1P) on the migration of human umbilical vein endothelial cells (HUVEC). Endothelial cell migration into a scrape wound was enhanced in S1P- or VEGF-stimulated HUVEC by the addition of fluid shear stress. In both cases, scrape wound closure rates were near a maximal value that was not exceeded when cells were exposed to all three factors. We also found that cell migration into a scrape wound due to S1P stimulation was correlated with the S1P1 mRNA concentration, in systems where cell migration was not already near maximal. The present work represents our initial steps toward predicting cell migration based upon the activation state of the receptors and enzymes involved in the chemokinetic response. These results also illustrate the importance of context-dependent analysis of cell signaling cascades.

Lysophospholipids enhance matrix metalloproteinase-2 expression in human endothelial cells

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are both low-molecular-weight lysophospholipids, which promote cell proliferation, migration, and invasion via interaction with a family of specific G protein-coupled receptors. Matrix metalloproteinases (MMPs) are zinc-dependent proteolytic enzymes, which are involved in degradation of the extracellular matrix and play critical roles in endothelial cell migration and matrix remodeling during angiogenesis. Among these MMPs, MMP-2 is known to trigger cell migration. In our present study, we examined the effects of LPA and S1P on MMP-2 expression in human endothelial cells. We showed that LPA and S1P enhanced MMP-2 expression in mRNA, protein levels, and also enzymatic activity of cells of the EAhy926 human endothelial cell line. The enhancement effects occurred in concentration- and time-dependent manners. Results from real-time PCR, Western blots, and substrate gels indicated that these enhancement effects were mediated through MAPK kinase/ERK-, nuclear factor-kappaB-, and calcium influx-dependent pathways. Furthermore, we show that endothelial cell invasion of the gel was enhanced by lysophospholipids, and the induction could be prevented by an MMP inhibitor, GM6001. These observations suggest that LPA and S1P may play important roles in endothelial cell invasion by regulating the expression of MMP-2.

Impairment of angiogenesis and cell migration by targeted aquaporin-1 gene disruption

Aquaporin-1 (AQP1) is a water channel protein expressed widely in vascular endothelia, where it increases cell membrane water permeability. The role of AQP1 in endothelial cell function is unknown. Here we show remarkably impaired tumour growth in AQP1-null mice after subcutaneous or intracranial tumour cell implantation, with reduced tumour vascularity and extensive necrosis. A new mechanism for the impaired angiogenesis was established from cell culture studies. Although adhesion and proliferation were similar in primary cultures of aortic endothelia from wild-type and from AQP1-null mice, cell migration was greatly impaired in AQP1-deficient cells, with abnormal vessel formation in vitro. Stable transfection of non-endothelial cells with AQP1 or with a structurally different water-selective transporter (AQP4) accelerated cell migration and wound healing in vitro. Motile AQP1-expressing cells had prominent membrane ruffles at the leading edge with polarization of AQP1 protein to lamellipodia, where rapid water fluxes occur. Our findings support a fundamental role of water channels in cell migration, which is central to diverse biological phenomena including angiogenesis, wound healing, tumour spread and organ regeneration.

Ischemic preconditioning in deceased donor liver transplantation: a prospective randomized clinical trial of safety and efficacy

Ischemic preconditioning (IPC) has the potential to decrease graft injury and morbidity after liver transplantation. We prospectively investigated the safety and efficacy of 5 minutes of IPC induced by hilar clamping in local deceased donor livers randomized 1:1 to standard (STD) recovery (N = 28) or IPC (N = 34). Safety was assessed by measurement of heart rate, blood pressure, and visual inspection of abdominal organs during recovery, and efficacy by recipient aminotransferases (aspartate aminotransferase [AST] and alanine aminotransferase [ALT], both measured in U/L), total bilirubin, and international normalized ratio of prothrombin time (INR) after transplantation. IPC performed soon after laparotomy did not cause hemodynamic instability or visceral congestion. Recipient median AST, median ALT, and mean INR, in STD vs. IPC were as follows: day 1 AST 696 vs. 841 U/L; day 3 AST 183 vs. 183 U/L; day 1 ALT 444 vs. 764 U/L; day 3 ALT 421 vs. 463 U/L; day 1 INR 1.7 +/- .4 vs. 2.0 +/- .8; and day 3 INR 1.3 +/- .2 vs. 1.4 +/- .3; all P > .05. No instances of nonfunction occurred. The 6-month graft and patient survival STD vs. IPC were 82 vs. 91% and median hospital stay was 10 vs. 8 days; both P > .05. In conclusion, deceased donor livers tolerated 5 minutes of hilar clamping well, but IPC did not decrease graft injury. Further trials with longer periods of preconditioning such as 10 minutes are needed.

Sphingosine-1-phosphate signaling promotes critical migratory events in vasculogenesis

Here we have investigated the role of sphingosine-1-phosphate (S1P) signaling in the process of vasculogenesis in the mouse embryo. At stages preceding the formation of blood vessels (7.5-8 dpc) in the embryo proper, yolk sac, and allantois, the S1P receptor S1P(2) is expressed in conjunction with S1P(1) and/or S1P(3). Additionally, sphingosine kinase-2 (SK2), an enzyme that catalyzes the formation of S1P, is expressed in these tissues throughout periods of vasculogenesis. Using the cultured mouse allantois explant model of blood vessel formation, we found that vasculogenesis was dependent on S1P signaling. We showed that S1P could replace the ability of serum to promote vasculogenesis in cultured allantois explants. Instead of small poorly reticulated clusters of rounded endothelial cells that formed under serum-free conditions, S1P promoted the formation of elongated endothelial cells that arranged into expansive branched networks of capillary-like vessels. These effects could not be reproduced by vascular endothelial growth factor or basic fibroblast growth factor administration. The ability of S1P to promote blood vessel formation was not due to effects on cell survival or on changes in numbers of endothelial cells (Flk1(+)/PECAM(+)), angioblasts (Flk1(+)/PECAM(-)), or undifferentiated mesodermal cells (Flk1(-)/PECAM(-)). The S1P effect on blood vessel formation was attributed to it promoting migratory activities of angioblasts and early endothelial cells required for the expansion of vascular networks. Together, our findings suggest that migratory events critical to the de novo formation of blood vessels are under the influence of S1P, possibly synthesized via the action of SK2, with signaling mediated by S1P receptors that include S1P(1), S1P(2), and S1P(3).

Lysophosphatidic Acid Interacts with Transforming Growth Factor-β Signaling to Mediate Keratinocyte Growth Arrest and Chemotaxis

Lysophosphatidic acid (LPA, 1-acyl-glycerol-3-phosphate) plays an important role in diverse biological responses including cell proliferation, differentiation, survival, migration, and tumor cell invasion. The most prominent source of LPA is platelets from which it is released after thrombin activation and is assumed to be an essential function of this lysophospholipid in cutaneous wound closure. Therefore, we examined the role of LPA on biological responses of keratinocytes. Although LPA potently enhances keratinocyte migration, it strongly induces growth arrest of proliferating epidermal cells. Thus, LPA possesses analogous actions to transforming growth factor-beta (TGF-beta), which is also released from degranulating platelets at wounded sites. In contrast to LPA, the intracellular signaling events of TGF-beta have been clearly identified and indicate that Smad3 is involved in chemotaxis and cell growth arrest of keratinocytes induced by this cytokine. Here we show that LPA, although it does not alter TGF-beta release is capable to activate Smad3 and results in a heteromerization with Smad4 and binding of the complex to its specific DNA-promoter elements. LPA completely fails to induce chemotaxis in Smad3-deficient cells, whereas growth inhibition is at least in part reduced. These findings indicate an essential role of Smad3 in diverse biological properties of LPA-stimulated keratinocytes.

Alkyl lysophosphatidic acid and fluoromethylene phosphonate analogs as metabolically-stabilized agonists for LPA receptors

We describe an efficient method for the synthesis of alkyl lysophosphatidic acid (LPA) analogs as well as alkyl LPA mono- and difluoromethylene phosphonate analogs. Each alkyl LPA analog was evaluated for subtype-specific LPA receptor agonist activity using a cell migration assay for LPA(1) activation in cancer cells and an intracellular calcium mobilization assay for LPA(2) and LPA(3) activation. Alkyl LPAs induced pronounced cell migration activity with equivalent or higher potency than sn-1-oleoyl LPA, while the alkyl LPA fluoromethylene phosphonates proved to be less potent agonists in this assay. However, each alkyl LPA analog activated Ca(2+) release by activation of LPA(2) and LPA(3) receptors. Interestingly, the absolute configuration of the sn-2 hydroxyl group of the alkyl LPA analogs was not recognized by any of the three LPA receptors. The use of alkyl LPA analogs further expands the scope of structure-activity studies, which will better define LPA-LPA receptor interactions.

Lysophospholipid receptors: signaling and biology

Lysophospholipids (LPs), such as lysophosphatidic acid and sphingosine 1-phosphate, are membrane-derived bioactive lipid mediators. LPs can affect fundamental cellular functions, which include proliferation, differentiation, survival, migration, adhesion, invasion, and morphogenesis. These functions influence many biological processes that include neurogenesis, angiogenesis, wound healing, immunity, and carcinogenesis. In recent years, identification of multiple cognate G protein-coupled receptors has provided a mechanistic framework for understanding how LPs play such diverse roles. Generation of LP receptor-null animals has allowed rigorous examination of receptor-mediated physiological functions in vivo and has identified new functions for LP receptor signaling. Efforts to develop LP receptor subtype-specific agonists/antagonists are in progress and raise expectations for a growing collection of chemical tools and potential therapeutic compounds. The rapidly expanding literature on the LP receptors is herein reviewed.

Lysophosphatidic acid inhibits TGF-β-mediated stimulation of type I collagen mRNA stability via an ERK-dependent pathway in dermal fibroblasts

Lysophosphatidic acid (LPA) is a serum-derived pleiotropic mediator with a potential role in wound repair. Since extracellular matrix (ECM) deposition is a critical part of wound healing, this study was designed to examine whether LPA is involved in ECM regulation. Using human dermal fibroblasts, we demonstrate that LPA counteracts transforming growth factor-beta (TGF-beta) stimulation of type I collagen mRNA and protein. This factor elicits its inhibitory effects at the posttranscriptional level via destabilization of type I collagen mRNA. Furthermore, using the mitogen-activated protein kinase kinase (MEK) inhibitor PD98059, we show that the extracellular signal-regulated kinase (ERK) pathway is a negative regulator of the TGF-beta-induced stabilization of type I collagen mRNA, and that the activation of the ERK pathway by LPA mediates their inhibitory effects on collagen production. In conclusion, this study describes a novel function for LPA as an antagonist of TGF-beta induced ECM deposition. These findings may be relevant to physiologic wound repair and may be useful in designing therapeutic agents to prevent excessive scarring.