Lysophosphatidic acid inhibits Ca2+ signaling in response to epidermal growth factor receptor stimulation in human astrocytoma cells by a mechanism involving phospholipase C(gamma) and a G(alphai) protein

Secreted PLA2 induces proliferation in astrocytoma through the EGF receptor: another inflammation-cancer link

We have investigated mechanisms that contribute to reinforce the relationship between inflammation and cancer. Secreted phospholipase A(2) group IIA (sPLA(2)-IIA) is a molecule relevant in inflammatory events and has been proposed as a marker for some of these. Previously, we reported the mitogenic properties of this sPLA(2) in the human astrocytoma cell line 1321N1. Here, we go deeper into the mechanisms that link this inflammatory protein with proliferation in one of the most aggressive types of tumors. We found that phosphorylation of the extracellular regulated kinase (ERK) was preceded by the activation of the small GTPase Ras, and both failed to be activated by inhibiting protein kinase C (PKC). Fractionation and immunofluorescence studies revealed translocation of PKC alpha, delta, and epsilon to the membrane fraction upon stimulation with sPLA(2)-IIA. Immunoprecipitation analysis showed that sPLA(2)-IIA induces phosphorylation of the epidermal growth factor receptor (EGFR) through a PKC-dependent pathway. We found that phosphorylation of this receptor contributed to Ras and ERK activation and that inhibition of ERK, PKC, and EGFR blocked the mitogenic response induced by sPLA(2)-IIA. This study showed that sPLA(2)-IIA is able to bring into play EGFR to trigger its signaling and that PKC leads the distribution of resources. Interestingly, we found that this is not a cell-specific response, because sPLA(2)-IIA was also able to transactivate EGFR in MCF7 human breast cancer cells. Therefore, this mechanism could contribute to worsen the prognosis of a tumor in an inflammatory microenvironment. We also present more links of the tumor chain possibly susceptible to targeting.

Role of Ca2+-independent phospholipase A2 in cell growth and signaling

Phospholipase A(2) (PLA(2)) are esterases that cleave glycerophospholipids to release fatty acids and lysophospholipids. Several studies demonstrate that PLA(2) regulate growth and signaling in several cell types. However, few of these studies have focused on Ca2+-independent phospholipase A(2) (iPLA(2) or Group VI PLA(2)). This class of PLA(2) was originally suggested to mediate phospholipid remodeling in several cell types including macrophages. As such, it was labeled as a housekeeping protein and thought not to play as significant of roles in cell growth as its older counterparts cytosolic PLA(2) (cPLA(2) or Group IV PLA(2)) and secretory PLA(2) (sPLA(2) or Groups I-III, V and IX-XIV PLA(2)). However, several recent studies demonstrate that iPLA(2) mediate cell growth, and do so by participating in signal transduction pathways that include epidermal growth factor receptors (EGFR), mitogen activated protein kinases (MAPK), mdm2, and even the tumor suppressor protein p53 and the cell cycle regulator p21. The exact mechanism by which iPLA(2) mediates these pathways are not known, but likely involve the generation of lipid signals such as arachidonic acid, lysophosphatidic acid (LPA) and lysophosphocholines (LPC). This review discusses the role of iPLA(2) in cell growth with special emphasis placed on their role in cell signaling. The putative lipid signals involved are also discussed.

Rho-family GTPases modulate Ca(2+) -dependent ATP release from astrocytes

Previously, we reported that activation of G protein-coupled receptors (GPCR) in 1321N1 human astrocytoma cells elicits a rapid release of ATP that is partially dependent on a G(q)/phophospholipase C (PLC)/Ca(2+) mobilization signaling cascade. In this study we assessed the role of Rho-family GTPase signaling as an additional pathway for the regulation of ATP release in response to activation of protease-activated receptor-1 (PAR1), lysophosphatidic acid receptor (LPAR), and M3-muscarinic (M3R) GPCRs. Thrombin (or other PAR1 peptide agonists), LPA, and carbachol triggered quantitatively similar Ca(2+) mobilization responses, but only thrombin and LPA caused rapid accumulation of active GTP-bound Rho. The ability to elicit Rho activation correlated with the markedly higher efficacy of thrombin and LPA, relative to carbachol, as ATP secretagogues. Clostridium difficile toxin B and Clostridium botulinum C3 exoenzyme, which inhibit Rho-GTPases, attenuated the thrombin- and LPA-stimulated ATP release but did not decrease carbachol-stimulated release. Thus the ability of certain G(q)-coupled receptors to additionally stimulate Rho-GTPases acts to strongly potentiate a Ca(2+)-activated ATP release pathway. However, pharmacological inhibition of Rho kinase I/II or myosin light chain kinase did not attenuate ATP release. PAR1-induced ATP release was also reduced twofold by brefeldin treatment suggesting the possible mobilization of Golgi-derived, ATP-containing secretory vesicles. ATP release was also markedly repressed by the gap junction channel inhibitor carbenoxolone in the absence of any obvious thrombin-induced change in membrane permeability indicative of hemichannel gating.

Selective attenuation of Toll-like receptor 2 signalling may explain the atheroprotective effect of sphingosine 1-phosphate

AIMS

Vascular inflammation is a major atherogenic factor and Toll-like receptor (TLR) 2 ligands, including bacterial and serum lipoproteins, seem to be involved in atherogenesis. On this basis, we analysed the effect of lipoproteins and different lipid components on TLR2-dependent signalling.

METHODS AND RESULTS

In TLR2-transfected human embryonic kidney 293 cells and human monocytes, oxidized low-density lipoproteins inhibited nuclear factor (NF)-kappaB-driven transcriptional activity and chemokine gene expression in response to TLR2 ligands. Sphingosine 1-phosphate (S1P) and oxidized palmitoyl-arachidonoyl-phosphatidylcholine, but not lipoprotein-carried lysophospholipids, inhibited TLR2 activation. Silencing experiments in TLR2-transfected 293 cells showed that the S1P-mediated attenuation effect is mediated by S1P receptors type 1 and type 2. To address the physiological significance of these findings, additional experiments were performed in human peripheral blood monocytes and monocyte-derived macrophages. In both cell types, S1P selectively attenuated TLR2 signalling, as NF-kappaB and extracellular signal-regulated kinase activation, but not c-Jun amino terminal kinase phosphorylation, were inhibited by physiologically relevant concentrations of S1P. Moreover, the attenuation of TLR2 signalling was partially reverted by pharmacological inhibition of phosphoinositide 3-kinase (PI3K) and Ras pathways. In addition, S1P inhibited the chemokine gene expression elicited by TLR2, but not by TLR4 ligands.

CONCLUSION

These findings disclose a cross-talk mechanism between lipoprotein components and TLR in which engagement of S1P receptors exert selective attenuation of TLR2-dependent activation via PI3K and Ras signalling. A corollary to these data is that the negative cross-talk of S1P receptors and TLR2 signalling might be involved in the atheroprotective effects of S1P.

Blood-brain barrier transport of non-viral gene and RNAi therapeutics

The development of gene- and RNA interference (RNAi)-based therapeutics represents a challenge for the drug delivery field. The global brain distribution of DNA genes, as well as the targeting of specific regions of the brain, is even more complicated because conventional delivery systems, i.e. viruses, have poor diffusion in brain when injected in situ and do not cross the blood-brain barrier (BBB), which is only permeable to lipophilic molecules of less than 400 Da. Recent advances in the "Trojan Horse Liposome" (THL) technology applied to the transvascular non-viral gene therapy of brain disorders presents a promising solution to the DNA/RNAi delivery obstacle. The THL is comprised of immunoliposomes carrying either a gene for protein replacement or small hairpin RNA (shRNA) expression plasmids for RNAi effect, respectively. The THL is engineered with known lipids containing polyethyleneglycol (PEG), which stabilizes its structure in vivo in circulation. The tissue target specificity of THL is given by conjugation of approximately 1% of the PEG residues to peptidomimetic monoclonal antibodies (MAb) that bind to specific endogenous receptors (i.e. insulin and transferrin receptors) located on both the BBB and the brain cellular membranes, respectively. These MAbs mediate (a) receptor-mediated transcytosis of the THL complex through the BBB, (b) endocytosis into brain cells and (c) transport to the brain cell nuclear compartment. The present review presents an overview of the THL technology and its current application to gene therapy and RNAi, including experimental models of Parkinson's disease and brain tumors.

RNA interference and nonviral targeted gene therapy of experimental brain cancer

The human epidermal growth factor receptor (EGFR) plays an oncogenic role in solid cancer, including brain primary and metastatic cancers. Transvascular nonviral gene therapy in combination with EGFR-RNA interference (RNAi) represents a new therapeutic approach to silencing oncogenic genes in solid cancers. This is achieved with pegylated immunoliposomes (PIL) carrying short hairpin RNA expression plasmids driven by the U6 RNA polymerase promoter and directed to target EGFR expression by RNAi. The PIL is comprised of a mixture of known lipids containing polyethyleneglycol (PEG), which stabilizes the PIL structure in vivo in circulation. The tissue target specificity of PILs is given by conjugation of approximately 1% of the PEG residues to monoclonal antibodies (mAbs) that bind to specific endogenous receptors (i.e., insulin and transferrin receptors) located in the brain vascular endothelium, which forms the blood brain barrier (BBB), and brain cellular membranes, respectively. These mAbs are known to induce 1) receptor-mediated transcytosis of the PIL complex through the BBB and 2) transport to the brain cell nuclear compartment. Treatment of an experimental human brain tumor model in scid mice is possible with weekly intravenous RNAi gene therapy causing reduced tumor expression of EGFR and 88% increase in survival time of these mice with advanced intracranial brain cancer. The availability of additional RNAi tumor targets may improve the therapeutic efficacy of this new anticancer drug. The accessibility to chimeric and/or humanized mAbs directed to human BBB and brain cell specific-receptors may accelerate the application of this technology to the treatment of human tumors.

Intravenous RNA interference gene therapy targeting the human epidermal growth factor receptor prolongs survival in intracranial brain cancer

PURPOSE

The human epidermal growth factor receptor (EGFR) plays an oncogenic role in solid cancer, including brain cancer. The present study was designed to prolong survival in mice with intracranial human brain cancer with the weekly i.v. injection of nonviral gene therapy causing RNA interference (RNAi) of EGFR gene expression.

EXPERIMENTAL DESIGN

Human U87 gliomas were implanted in the brain of adult scid mice, and weekly i.v. gene therapy was started at day 5 after implantation of 500000 cells. An expression plasmid encoding a short hairpin RNA directed at nucleotides 2529-2557 within the human EGFR mRNA was encapsulated in pegylated immunoliposomes. The pegylated immunoliposome was targeted to brain cancer with 2 receptor-specific monoclonal antibodies (MAb), the murine 83-14 MAb to the human insulin receptor and the rat 8D3 MAb to the mouse transferrin receptor.

RESULTS

In cultured glioma cells, the delivery of the RNAi expression plasmid resulted in a 95% suppression of EGFR function, based on measurement of thymidine incorporation or intracellular calcium signaling. Weekly i.v. RNAi gene therapy caused reduced tumor expression of immunoreactive EGFR and an 88% increase in survival time of mice with advanced intracranial brain cancer.

CONCLUSIONS

Weekly i.v. nonviral RNAi gene therapy directed against the human EGFR is a new therapeutic approach to silencing oncogenic genes in solid cancers. This is enabled with a nonviral gene transfer technology that delivers liposome-encapsulated plasmid DNA across cellular barriers with receptor-specific targeting ligands.

Lysophosphatidic acid (LPA) protects primary chronic lymphocytic leukemia cells from apoptosis through LPA receptor activation of the anti-apoptotic protein AKT/PKB

Lysophosphatidic acid (LPA) protects epithelial and fibroblast cell lines from apoptosis. In B-cells, LPA acts as a growth factor promoting cell proliferation. Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of CD19+/CD5+ B-lymphocytes primarily through a block in apoptosis. The mechanisms underlying this defect are not fully understood. We investigated whether LPA could be a survival factor in CLL cells. Herein, we demonstrate that LPA protects B-cell lines BJAB and I-83 and primary CLL cells but not normal B-cells from fludarabine- and etoposide-induced apoptosis. Furthermore, LPA prevented spontaneous apoptosis in primary CLL cells. The LPA1 expression was found to be increased in primary CLL cells compared with normal B-cells correlating with LPA prevention of apoptosis. Treatment of primary CLL cells with the LPA receptor antagonist, diacylglycerol pyrophosphate, reverses the protective effect of LPA against apoptosis, and down-regulation of the LPA1 by siRNA blocked LPA-mediated protection against spontaneous apoptosis in primary CLL cells. The protective effect of LPA was inhibited by blocking activation of the phosphatidylinositol 3-kinase/AKT signaling pathway. These results indicate that LPA is a survival factor in B-cell lines and primary CLL cells but not normal B-cells. Thus, drugs targeting the LPA receptors might be an effective therapy against B-cell-derived malignancies such as CLL.

Lysophosphatidic acid induces chemotaxis, oxygen radical production, CD11b up-regulation, Ca2+ mobilization, and actin reorganization in human eosinophils via pertussis toxin-sensitive G proteins

Lysophosphatidic acid (LPA) is a bioactive lipid mediator, which is generated by secretory type II phospholipase A(2) and is thought to play a major role in the pathogenesis of atopic diseases. In this study, the biological activity of LPA on human eosinophils was characterized. We showed by reverse transcription and PCR that human eosinophils express the mRNA of the LPA receptors endothelial differentiation gene (EDG)-2 and EDG-7. Experiments revealed that LPA has chemotactic activity toward eosinophils, stimulates the production of reactive oxygen metabolites, and induces up-regulation of the integrin CD11b. Signal pathway measurements indicated Ca(2+)-mobilization from intracellular stores and transient actin polymerization upon stimulation with LPA. Cell responses elicited by LPA were inhibited by pertussis toxin indicating that in eosinophils the LPA receptor(s), presumably EDG-2 and/or EDG-7, are coupled to G(i/o) proteins. Moreover, LPA-induced activation of eosinophils could be completely blocked by the EDG-2/EDG-7 antagonist diacylglycerol pyrophosphate. In addition, at optimal doses the changes induced by LPA were comparable to those obtained by the other well-characterized chemotaxins. These results indicate that LPA is a strong chemotaxin and activator of eosinophils. These findings point to a novel role of LPA in the pathogenesis of diseases with eosinophilic inflammation such as atopic diseases as chemotaxin as well as activator of proinflammatory effector functions.

Native and Recombinant Human Edg4 Receptor-Mediated Ca2+ Signalling

We have developed an assay system suitable for assessment of compound action on the Edg4 subtype of the widely expressed lysophosphatidic acid (LPA)-responsive Edg receptor family. Edg4 was stably overexpressed in the rat hepatoma cell line Rh 7777, and a Ca(2+)-based FLIPR assay developed for measurement of functional responses. In order to investigate the mechanisms linking Edg4 activation to cytosolic Ca(2+) elevation, we have also studied LPA signalling in a human neuroblastoma cell line that endogenously expresses Edg4. LPA responses displayed similar kinetics and potency in the two cell lines. The Ca(2+) signal generated by activation of LPA-sensitive receptors in these cells is mediated primarily by endoplasmic reticulum. However, there is a substantial inhibition of the LPA response by FCCP, indicating that mitochondria also play a key role in the LPA response. Partial inhibition of the response by cyclosporin A could indicate an active Ca(2+) release role for mitochondria in the LPA response. The inositol 1,4,5-triphosphate receptor antagonist 2-aminoethyl diphenyl borate markedly inhibits, but does not abolish, the Ca(2+) response to LPA, suggesting further complexity to the signalling pathways activated by Edg receptors. In comparing Edg signalling in recombinant and native cells, there is a striking overall similarity in receptor expression pattern, agonist potency, and the effect of modulators on the Ca(2+) response. This indicates that the Edg4-overexpressing Rh7777 cell line is a very useful model system for studying receptor pharmacology and signalling mechanisms, and for investigating the Edg4 receptor's downstream effects.

The influence of lysophosphatidic acid on the functions of human dendritic cells

Lysophosphatidic acid (LPA) is a bioactive lipid mediator which is generated by secretory phospholipase A(2). In this study, we studied the biological activity of LPA on human dendritic cells (DCs), which are specialized APCs characterized by their ability to migrate into target sites and secondary lymphoid organs to process Ags and activate naive T cells. We show that immature and mature DCs express the mRNA for different LPA receptors such as endothelial differentiation gene (EDG)-2, EDG-4, and EDG-7. In immature DCs, LPA stimulated pertussis toxin-sensitive Ca(2+) increase, actin polymerization, and chemotaxis. During the maturation process, DCs lost their ability to respond toward LPA with Ca(2+) transients, actin polymerization, and chemotaxis. However, LPA inhibited in a pertussis toxin-insensitive manner the secretion of IL-12 and TNFalpha as well as enhanced secretion of IL-10 from mature DCs. Moreover, LPA did not affect the endocytic or phagocytic capacities and the surface phenotype of DCs, although it increased the allostimulatory function of mature DC and inhibited their capacity to induce Th1 differentiation. In summary, our study implicates that LPA might regulate the trafficking, cytokine production, and T cell-activating functions of DCs.

Cloning and characterization of glioma BK, a novel BK channel isoform highly expressed in human glioma cells

Voltage-dependent large-conductance Ca2+-activated K+ channels (BK channels) are widely expressed in excitable and nonexcitable cells. BK channels exhibit diverse electrophysiological properties, which are attributable in part to alternative splicing of their alpha-subunits. BK currents have been implicated in the growth control of glial cells, and BK channels with novel biophysical properties have recently been characterized in human glioma cells. Here we report the isolation, cloning, and functional characterization of glioma BK (gBK), a novel splice isoform of hSlo, the gene that encodes the alpha-subunits of human BK channels. The primary sequence of gBK is 97% identical to its closest homolog hbr5, but it contains an additional 34-amino-acid exon at splice site 2 in the C-terminal tail of BK channels. hSlo transcripts containing this novel exon are expressed ubiquitously in various normal tissues as well as in neoplasmic samples, suggesting that the novel exon may modulate important physiological functions of BK channels. Expression of gBK in Xenopus oocytes gives rise to iberiotoxin-sensitive (IbTX) currents, with an IC(50) for IbTX of 5.7 nm and a Hill coefficient of 0.76. Single gBK channels have a unitary conductance of similar250 pS, and the currents show significantly slower activation and higher Ca2+ sensitivity than hbr5. Ca2+ sensitivity was enhanced specifically at physiologically relevant [Ca2+]i (100-500 nm). Examination of biopsies from patients with malignant gliomas has revealed specific overexpression of BK channels in gliomas compared with nonmalignant human cortical tissues. Importantly, tumor malignancy grades have correlated positively with BK channel expression, suggesting an important role for the gBK channel in glioma biology.

Effective cytotoxicity against human leukemias and chemotherapy-resistant leukemia cell lines by N-N-dimethylsphingosine

We evaluated the cytotoxicity of dimethylsphingosine (DMS) against four human leukemia cell lines: two acute (HL60 and a multi-drug resistance MDR-positive derivative HL60-dox) and two blast crisis chronic myelogenous leukemias (JFP1, from a treatment refractory patient and K562), and against blasts isolated from 11 leukemia patients. Cell line viability decreased proportionally to DMS concentration and treatment time (P<0.001). HL60-dox and JFP1 were the most sensitive, indicating DMS efficacy against human leukemia MDR. Importantly, leukemia samples showed a similar sensitivity to DMS as that of the cell lines, firstly demonstrating PKC-independent sphingolipid activity against fresh human tumor specimens. DMS-based chemotherapy may improve leukemia treatment.

Antiangiogenic potential of 10-hydroxycamptothecin

To investigate the antiangiogenic potential of 10-hydroxycamptothecin (HCPT), the proliferation of human microvascular endothelial cells (HMEC) and seven human tumor cell lines were detected by SRB assay, and the endothelial cell migration and tube formation were assessed using two in vitro model systems. Also, inhibition of angiogenesis was determined with a modification of the chick embryo chorioallantoic membrane (CAM) assay in vivo. Morphological assessment of apoptosis was performed by fluorescence microscope. HCPT 0.313-5 micromol x L(-1) treatment resulted in a dose-dependent inhibition of proliferation, migration and tube formation in HMEC cells, and HCPT 6.25-25 nmol x egg(-1) inhibited angiogenesis in CAM assay. HCPT 1.25-5 micromol x L(-1) elicited typical morphological changes of apoptosis including condensed chromatin, nuclear fragmentation, and reduction in volume in HMEC cells. HCPT significantly inhibited angiogenesis both in vitro and in vivo at relatively low concentrations, and this effect was related with induction of apoptosis in HMEC cells. These results taken collectively suggest that HCPT may be a potent antiangiogenetic and cytotoxic drug and further investigation is warranted.