Mechanisms of lysophosphatidic acid-induced DNA synthesis in vascular smooth muscle cells

Signalling by lysophosphatidate and its health implications

Extracellular lysophosphatidate (LPA) signalling is regulated by the balance of LPA formation by autotaxin (ATX) versus LPA degradation by lipid phosphate phosphatases (LPP) and by the relative expressions of six G-protein-coupled LPA receptors. These receptors increase cell proliferation, migration, survival and angiogenesis. Acute inflammation produced by tissue damage stimulates ATX production and LPA signalling as a component of wound healing. If inflammation does not resolve, LPA signalling becomes maladaptive in conditions including arthritis, neurologic pain, obesity and cancers. Furthermore, LPA signalling through LPA1 receptors promotes fibrosis in skin, liver, kidneys and lungs. LPA also promotes the spread of tumours to other organs (metastasis) and the pro-survival properties of LPA explain why LPA counteracts the effects of chemotherapeutic agents and radiotherapy. ATX is secreted in response to radiation-induced DNA damage during cancer treatments and this together with increased LPA1 receptor expression leads to radiation-induced fibrosis. The anti-inflammatory agent, dexamethasone, decreases levels of inflammatory cytokines/chemokines. This is linked to a coordinated decrease in the production of ATX and LPA1/2 receptors and increased LPA degradation through LPP1. These effects explain why dexamethasone attenuates radiation-induced fibrosis. Increased LPA signalling is also associated with cardiovascular disease including atherosclerosis and deranged LPA signalling is associated with pregnancy complications including preeclampsia and intrahepatic cholestasis of pregnancy. LPA contributes to chronic inflammation because it stimulates the secretion of inflammatory cytokines/chemokines, which increase further ATX production and LPA signalling. Attenuating maladaptive LPA signalling provides a novel means of treating inflammatory diseases that underlie so many important medical conditions.

Role of lysophosphatidic acid in vascular smooth muscle cell proliferation

Lysophosphatidic acid (LPA) is an important lipid molecule for signal transduction in cell proliferation. Although the effects of LPA on vascular smooth muscle (VSM) cell growth have been reported previously, the underlying mechanisms of its action are not fully understood. The present study was undertaken to investigate the effects of some inhibitors of different protein kinases and other molecular targets on LPA-induced DNA synthesis as well as gene expression in the aortic VSM cells. The DNA synthesis was studied by the [³H]thymidine incorporation method and the gene expression was investigated by the real-time PCR technique. It was observed that the LPA-induced DNA synthesis was attenuated by inhibitors of protein kinase C (PKC) (staurosporine, calphostin C, and bisindolylmaleimide), phosphoinositide 3-kinase (PI3K) (wortmannin and LY294002), and ribosomal p70S6 kinase (p70S6K) (rapamycin). The inhibitors of guanine protein coupled receptors (GPCR) (pertussis toxin), phospholipase C (PLC) (U73122 and D609), and sodium-hydrogen exchanger (NHE) (amiloride and dimethyl amiloride) were also shown to depress the LPA-induced DNA synthesis. Furthermore, gene expressions for PLC β1 isoform, PKC δ and ε isoforms, casein kinase II β isoform, and endothelin-1A receptors were elevated by LPA. These results suggest that the LPA-induced proliferation of VSM cells is mediated through the activation of GPCR and multiple protein kinases as well as gene expressions of some of their specific isoforms.

Lysophosphatidic acid enhances neointimal hyperplasia following vascular injury through modulating proliferation, autophagy, inflammation and oxidative stress

Lysophosphatidic acid (LPA), which is one of the intermediate products of membrane phospholipid metabolism, is a bioactive phospholipid that possesses diverse activities. In the present study, the effects of LPA on neointimal formation following vascular injury were investigated. A carotid artery balloon injury model was employed in the present study, and following vascular injury, rats received an intraperitoneal injection of 1 mg/kg LPA. Subsequently, histopathological alterations were assessed by hematoxylin and eosin staining, the expression levels of proliferating cell nuclear antigen (PCNA) were detected by immunohistochemistry, apoptosis was assessed via a terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay, and the expression levels of apoptosis-associated and autophagy-associated proteins were detected by western blotting. In addition, inflammatory and oxidative stress-associated factors were assessed by reverse transcription-quantitative polymerase chain reaction or corresponding kits. The results of the present study demonstrated that LPA enhanced vascular injury-induced neointimal hyperplasia. LPA further elevated the expression levels of PCNA in the injured carotid artery tissues. LPA exhibited no effect on apoptosis in carotid artery tissues, whereas it modulated autophagy in the injured carotid artery tissues. Furthermore, LPA enhanced vascular injury-induced inflammation and oxidative stress. The present study demonstrated that LPA may enhance neointimal hyperplasia following vascular injury by modulating proliferation, autophagy, inflammation and oxidative stress, but not apoptosis. Furthermore LPA may contribute to the pathology of atherosclerosis and may be considered a promising therapeutic target for the treatment of atherosclerosis.

Protein kinase CK2 inhibition suppresses neointima formation via a proline-rich homeodomain-dependent mechanism

Neointimal hyperplasia is a product of VSMC replication and consequent accumulation within the blood vessel wall. In this study, we determined whether inhibition of protein kinase CK2 and the resultant stabilisation of proline-rich homeodomain (PRH) could suppress VSMC proliferation. Both silencing and pharmacological inhibition of CK2 with K66 antagonised replication of isolated VSMCs. SiRNA-induced knockdown as well as ectopic overexpression of proline-rich homeodomain indicated that PRH disrupts cell cycle progression. Mutation of CK2 phosphorylation sites Ser¹⁶³ and Ser¹⁷⁷ within the PRH homeodomain enabled prolonged cell cycle arrest by PRH. Concomitant knockdown of PRH and inhibition of CK2 with K66 indicated that the anti-proliferative action of K66 required the presence of PRH. Both K66 and adenovirus-mediated gene transfer of S163C:S177C PRH impaired neointima formation in human saphenous vein organ cultures. Importantly, neither intervention had notable effects on cell cycle progression, cell survival or migration in cultured endothelial cells.

Effect of lysophosphatidylglycerol on intracellular free Ca2+ concentration in A10 vascular smooth muscle cells

Although plasma levels of lysophosphatidylglycerol (LPG) are increased in hypertension, its role in the pathogenesis of vascular defects is not clear. In view of the importance of Ca²⁺ overload in causing vascular smooth muscle (VSM) dysfunction, the action of LPG on [Ca²⁺]_i in cultured A10 VSM cell line was examined by using Fura 2-AM acetoxymethyl ester technique. LPG was found to induce a concentration-dependent increase in [Ca²⁺]_i in VSM cells. This change was dependent both on the extracellular and intracellular Ca²⁺ sources, as it was reduced by 30% by EGTA, an extracellular Ca²⁺ chelator, and 70% by thapsigargin, a sarcoplasmic reticulum (SR) Ca²⁺-pump inhibitor. However the increase in [Ca²⁺]_i due to LPG was not altered by caffeine or ryanodine, which affect Ca²⁺-release through the ryanodine receptors in the SR. On the other hand, LPG-induced change in [Ca²⁺]_i was suppressed by 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate, a phospholipase C (PLC) inhibitor, as well as by xestospongin and 2-aminoethoxydiphenyl borate, two inositol trisphosphate (IP3) receptor inhibitors in the SR. These observations support the view that LPG-induced increase in [Ca²⁺]_i in VSM cells is mainly a result of Ca²⁺ release from Ca²⁺ pool in the SR through PLC/IP3-sensitive signal transduction mechanism. Furthermore, it is suggested that the elevated level of LPG may induce intracellular Ca²⁺ overload and thus play a critical role in the development of vascular abnormalities.

Dental Pulp Fibroblasts Contain Target Cells for Lysophosphatidic Acid

Lysophosphatidic acid (LPA) is a locally produced bioactive phospholipid which is involved in tissue repair. The objective of this study was to determine whether dental pulp tissue also responds to the phospholipid. Effects of LPA on proliferation, differentiation, and mitogen-activated protein kinase (MAPK) signaling of dental pulp fibroblasts (DPF) were examined in vitro. We report that DPF express LPA receptors LPA1, LPA2, and LPA3 and respond to the ligand with increased mitogenic activity. Involvement of extracellular signal-regulated kinase, p38 MAPK, and c-Jun NH2-terminal kinase in LPA signaling could be demonstrated by use of specific inhibitors and detection of the phosphorylation status of the kinases. An increased mitogenic activity paralleled a decreased number of alkaline-phosphatase-positive cells and expression levels of dentin sialophosphoprotein and osteocalcin. Together, these results suggest that dental pulp fibroblasts can respond to LPA, a process that may play a role in pulp tissue repair.

Reduction of blood pressure by store-operated calcium channel blockers

The voltage-operated Ca(2+) channels (VOCC), which allow Ca(2+) influx from the extracellular space, are inhibited by anti-hypertensive agents such as verapamil and nifedipine. The Ca(2+) entering from outside into the cell triggers Ca(2+) release from the sarcoplasmic reticulum (SR) stores. To refill the depleted Ca(2+) stores in the SR, another type of Ca(2+) channels in the cell membrane, known as store-operated Ca(2+) channels (SOCC), are activated. These SOCCs are verapamil and nifedipine resistant, but are SKF 96465 (SK) and gadolinium (Gd(3+) ) sensitive. Both SK and Gd(3+) have been shown to reduce [Ca(2+) ]i in the smooth muscle, but their effects on blood pressure have not been reported. Our results demonstrated that both SK and Gd(3+) produced a dose-dependent reduction in blood pressure in rat. The combination of SK and verapamil produced an additive action in lowering the blood pressure. Furthermore, SK, but not Gd(3+) suppressed proliferation of vascular smooth muscle cells in the absence or presence of lysophosphatidic acid (LPA). SK decreased the elevation of [Ca(2+) ]i induced by LPA, endothelin-1 (ET-1) and angiotensin II (Ang II), but did not affect the norepinephrine (NE)-evoked increase in [Ca(2+) ]i . On the other hand, Gd(3+) inhibited the LPA and Ang II induced change in [Ca(2+) ]i , but had no effect on the ET-1 and NE induced increase in [Ca(2+) ]i . The combination of verapamil and SK abolished the LPA- or adenosine-5'-triphosphate (ATP)-induced [Ca(2+) ]i augmentation. These results suggest that SOCC inhibitors, like VOCC blocker, may serve as promising drugs for the treatment of hypertension.

Different effects of lysophosphatidic acid on L-type calcium current in neonatal rat ventricular myocytes with and without H2O2 treatment

L-type calcium current (I(Ca-L)) alterations are implicated in various cardiac diseases, and the lysophosphatidic acid (LPA) level increases in several ischemic heart diseases. We investigated the effects of LPA on I(Ca-L) in normal and H2O2-treated neonatal rat ventricular myocytes. LPA treatment (24h) increased the action potential duration (APD) and I(Ca-L) in normal ventricular myocytes, but it decreased these parameters in H2O2-treated myocytes. LPA increased the single-channel open probability of L-type calcium channels in both normal and H2O2-treated myocytes. LPA activated calcineurin (CaN) and induced the cytoplasm-to-nucleus translocation of nuclear factor of activated T-cells (NFAT) in H2O2-treated cardiomyocytes. In H2O2-treated cardiomyocytes, LPA decreased Ca(v)1.2 mRNA and protein expression levels at 4 and 8h, respectively. A CaN inhibitor (FK-506) prevented LPA-induced APD, I(Ca-L), and Ca(v)1.2 mRNA and protein down-regulation. The LPA-induced I(Ca-L) increase in normal cardiomyocytes was CaN-NFAT signaling-independent, and LPA did not affect Ca(v)1.2 mRNA or protein expression. In conclusion, LPA increases the I(Ca-L) in normal ventricular myocytes by increasing the single-channel open probability of L-type calcium channels, and LPA decreases I(Ca-L) in H2O2-treated cardiomyocytes via the CaN-NFAT pathway.

Suppression of high lipid diet induced by atherosclerosis sarpogrelate

Sarpogrelate (SP), a serotonin (5-HT2A) receptor antagonist, is used as an anti-platelet agent for the treatment of some vascular diseases. SP has been reported to inhibit 5-HT induced coronary artery spasm, increase in intracellular calcium and smooth muscle cells proliferation. This study was undertaken to test that SP suppresses the development of atherosclerosis due to high cholesterol diet (HCD) by decreasing blood viscosity and oxidative stress. For this purpose, 29 rabbits were divided into four groups: control group (normal diet); normal diet group with SP at the dose of 5 mg/kg/day; HCD group fed 1% cholesterol; and HCD group with SP at the dose of 5 mg/kg/day. After 90 days of the experiment, blood samples were collected and the animals were killed; the thoracic aorta was stained by the Oil Red O staining method. The results indicate that plasma levels of cholesterol, triglycerides and malondialdehyde were increased in rabbits fed HCD. Plasma viscosity and whole blood viscosity were also higher in the HCD group than that in normal diet group. Treatment with SP prevented these alterations induced by HCD whereas this agent had no significant effect in rabbits fed normal diet. Morphological examination of the aorta revealed that SP treatment prevented the formation of foam cells and atherosclerotic plaque. It is suggested that the beneficial effects of SP in atherosclerosis may be due to actions on blood viscosity, lipid levels and oxidative stress.

Lysophosphatidic acid in atherosclerotic diseases

Lysophosphatidic acid (LPA) is a potent bioactive phospholipid. As many other biological active lipids, LPA is an autacoid: it is formed locally on demand, and it acts locally near its site of synthesis. LPA has a plethora of biological activities on blood cells (platelets, monocytes) and cells of the vessel wall (endothelial cells, smooth muscle cells, macrophages) that are all key players in atherosclerotic and atherothrombotic processes. The specific cellular actions of LPA are determined by its multifaceted molecular structures, the expression of multiple G-protein coupled LPA receptors at the cell surface and their diverse coupling to intracellular signalling pathways. Numerous studies have now shown that LPA has thrombogenic and atherogenic actions. Here, we aim to provide a comprehensive, yet concise, thoughtful and critical review of this exciting research area and to pinpoint potential pharmacological targets for inhibiting thrombogenic and atherogenic activities of LPA. We hope that the review will serve to accelerate knowledge of basic and clinical science, and to foster drug development in the field of LPA and atherosclerotic/atherothrombotic diseases.

Signaling pathways involved in DNA synthesis and migration in response to lysophosphatidic acid and low-density lipoprotein in coronary artery smooth muscle cells

Low-density lipoprotein (LDL) and lysophosphatidic acid (LPA), one of the lipid components of lipoprotein, induced the DNA synthesis of coronary artery smooth muscle cells (CASMCs). The LDL- and LPA-induced DNA synthesis was markedly inhibited by the LPA receptor antagonist Ki16425, pertussis toxin, small interfering RNAs targeted for LPA1 receptors, and a potent calcineurin inhibitor cyclosporine A. It has been reported that LDL and LPA induced a migration response in a manner sensitive to Ki16425, pertussis toxin, and a LPA1 receptor-specific small interfering RNA. However, cyclosporine A was ineffective in inhibiting the migration response. Instead, an epidermal growth factor (EGF) receptor tyrosine kinase inhibitor markedly suppressed the migration response to LDL and LPA without having any significant effect on DNA synthesis. Thus, the LDL-induced stimulation of DNA synthesis and migration in CASMCs is mediated by its component LPA through LPA1 receptors and G(i/o)-proteins. Ca2+/calcineurin pathways and transactivation of EGF receptors mediate LPA1-receptor-induced DNA synthesis and migration, respectively.

Lysophosphatidic acid signaling during embryo development in sheep: involvement in prostaglandin synthesis

We investigated the lysophosphatidic acid (LPA) pathway during early pregnancy in sheep. LPA was detected in the uteri of early-stage pregnant ewes. Using quantitative RT-PCR, the expression of autotaxin, the LPA-generating enzyme, was found in the endometrium and conceptus. In the latter autotaxin, transcript levels were low on d 12-14 and increased on d 15-16, in parallel with the level of LPA. Autotaxin was localized in the luminal epithelium and superficial glands of the endometrium and in trophectoderm cells of the conceptus. The expression of G protein-coupled receptors for LPA was also examined in the ovine conceptus. LPA receptor LPAR1 and LPAR3 transcripts were expressed during early pregnancy and displayed a peak on d 14, whereas the highest level of protein for both receptors was observed at d 17. LPAR1 was localized in cellular membranes and nuclear compartments of the trophectoderm cells, whereas LPAR3 was revealed only in membranes. LPA activated phosphorylation of the MAPK ERK1/2 in ovine trophectoderm-derived cells. Moreover, the bioactive lipid increased the proliferation of trophectoderm cells in culture, as shown by thymidine and bromodeoxyuridine incorporation. Furthermore, LPA induced changes to the organization of beta-actin and alpha-tubulin, suggesting a role for it in rearrangement of trophectoderm cells cytoskeleton. Because a link had previously been established between prostaglandin and LPA pathways, we analyzed the effect of LPA on prostaglandin synthesis. LPA induced an increase in the release of prostaglandin F2alpha and prostaglandin E2, with no significant modifications to cytosolic phospholipase A2alpha and prostaglandin synthase-2 expression. Taken together, our results suggest a new role for LPA-mediated signaling in the ovine conceptus at the time of implantation.

Therapeutic potential of autotaxin/lysophospholipase d inhibitors

Lysophosphatidic acids (LPAs) are structurally simple lipid phosphate esters with a widely appreciated role as extracellular signaling molecules. LPA binds to selective cell surface receptors to promote cell growth, survival, motility and differentiation. Studies using LPA receptor knockout mice and experimental therapeutics targeting these receptors identify roles for LPA signaling in processes that include cardiovascular disease and function, angiogenesis, reproduction, cancer progression and neuropathic pain. These studies identify considerable functional redundancy between these receptors and raise the possibility that additional lysophosphatidic acid receptors remain to be identified. LPA is present in the blood and other biological fluids at physiologically relevant concentrations and can likely be rapidly generated and degraded in different locations, for example at sites of inflammation, vascular injury and thrombosis or in the tumor micro environment. Recent work identifies a secreted enzyme, autotaxin (ATX), as the key component of an extracellular pathway for generation of lysophosphatidic acid by lysophospholipase D catalyzed hydrolysis of lysophospholipid substrates. In contrast to the apparently redundant functions of LPA receptors, studies using ATX knock out and transgenic mice indicate that this enzyme is uniquely required for LPA signaling during early development and serves as the primary determinant of circulating LPA levels in adult animals. Accordingly, pharmacological inhibition of ATX may be a viable and potentially effective way to interfere with LPA signaling in the cardiovascular system and possibly other settings such as tumor metastasis for therapeutic benefit. In this review we provide an update on recent advances in defining roles for LPA signaling in major disease processes and discuss recent progress in understanding the regulation and function of autotaxin focusing on strategies for the identification and initial evaluation of small molecule autotaxin inhibitors.

Mechanisms of the lysophosphatidic acid-induced increase in [Ca(2+)](i) in skeletal muscle cells

Although lysophosphatidic acid (LPA) is known to increase intracellularfree calcium concentration ([Ca²⁺]_i) in different cell types, the effect of LPA on the skeletal muscle cells is not known. The present study was therefore undertaken to examine the effect of LPA on the [Ca²⁺]_i in C2C12 cells. LPA induced a concentration and time dependent increase in [Ca²⁺]_i, which was inhibited by VPC12249, VPC 32183 and dioctanoyl glycerol pyrophosphate, LPA1/3 receptor antagonists. Pertussis toxin, a Gⁱ protein inhibitor, also inhibited the LPA-induced increase in [Ca²⁺]_i. Inhibition of tyrosine kinase activities with tyrphostin A9 and genistein also prevented the increase in [Ca²⁺]_i due to LPA. Likewise, wortmannin and LY 294002, phosphatidylinositol 3-kinase (PI3-K) inhibitors, inhibited [Ca²⁺]_i response to LPA. The LPA effect was also attenuated by ethylene glycolbis(β-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), an extracellular Ca²⁺ chelator, Ni²⁺ and KB-R7943, inhibitors of the Na⁺-Ca²⁺ exchanger; the receptor operated Ca²⁺ channel (ROC) blockers, 2-aminoethoxydiphenyl borate and SK&F 96365. However, the L-type Ca²⁺ channel blockers, verapamil and diltiazem; the store operated Ca²⁺ channel blockers, La³⁺ and Gd³⁺; a sarcoplasmic reticulum calcium pump inhibitor, thapsigargin; an inositol trisphosphate receptor antagonist, xestospongin and a phospholipase C inhibitor, U73122, did not prevent the increase [Ca²⁺]_i due to LPA. Our data suggest that the LPA-induced increase in [Ca²⁺]_i might occur through Gⁱ-protein coupled LPA_1/3 receptors that may be linked to tyrosine kinase and PI3-K, and may also involve the Na⁺-Ca²⁺ exchanger as well as the ROC. In addition, LPA stimulated C2C12 cell proliferation via PI3-K. Thus, LPA may be an important phospholipid in the regulation of [Ca²⁺]_i and growth of skeletal muscle cells.

Targeting platelets for prevention and treatment of cardiovascular disease

Platelets play an important role in the development of thrombosis, atherosclerosis, hypertension, heart attack and stroke. As a result, pharmacologic interventions that influence platelet functions, such as adhesion, aggregation and the release of different factors, are considered useful for the prevention and treatment of cardiovascular disease. Although classical anti-platelet agents have proven beneficial effects for the treatment of some specific cardiovascular diseases, there are limitations for their use as these drugs target platelet function directly. In contrast, newly developed anti-platelet agents have broad applications for the treatment of cardiovascular disease as they not only influence platelet function but are also considered to affect cardiac and vascular smooth muscle cell functions. Natural food products and nutraceutical agents also appear to modify cardiovascular abnormalities by affecting various platelet functions; however, the mechanisms of their actions remain to be investigated. Accordingly, this article is focused to discuss emerging pharmacologic, nutritional and nutraceutical interventions that may influence the prevention or progression of a broad range of cardiovascular diseases.

Pharmacological basis of different targets for the treatment of atherosclerosis

The development of atherosclerotic plaque is a highly regulated and complex process which occurs as a result of structural and functional alterations in endothelial cells, smooth muscle cells (SMCs), monocytes/macrophages, T-lymphocytes and platelets. The plaque formation in the coronary arteries or rupture of the plaque in the peripheral vasculature in latter stages of atherosclerosis triggers the onset of acute ischemic events involving myocardium. Although lipid lowering with statins has been established as an important therapy for the treatment of atherosclerosis, partially beneficial effects of statins beyond decreasing lipid levels has shifted the focus to develop newer drugs that can affect directly the process of atherosclerosis. Blockade of renin angiotensin system, augmentation of nitric oxide availability, reduction of Ca(2+) influx, prevention of oxidative stress as well as attenuation of inflammation, platelet activation and SMC proliferation have been recognized as targets for drug treatment to control the development, progression and management of atherosclerosis. A major challenge for future drug development is to formulate a combination therapy affecting different targets to improve the treatment of atherosclerosis.

Lysophosphatidic acid mediates pleiotropic responses in skeletal muscle cells

Lysophosphatidic acid (LPA) is a potent modulator of growth, cell survival, and apoptosis. Although all four LPA receptors are expressed in skeletal muscle, very little is known regarding the role they play in this tissue. We used RT-PCR to demonstrate that cultured skeletal muscle C2C12 cells endogenously express multiple LPA receptor subtypes. The demonstration that LPA mediates the activation of ERK1/2 MAP kinase and Akt/PKB in C2C12 cells is consistent with the widely observed mitogenic properties of LPA. In spite of these observations, LPA did not induce proliferation in C2C12 cells. Paradoxically, we found that prolonged treatment of C2C12 cells with LPA led to caspase 3 and PARP cleavage as well as the activation of stress-associated MAP kinases JNK and p38. In spite of these typically pro-apoptotic responses, LPA did not induce cell death. Blocking ERK1/2 and Akt/PKB activation with specific pharmacological inhibitors, nevertheless, stimulated LPA-mediated apoptosis. Taken together, these results suggest that both mitogenic and apoptotic responses serve to counterbalance the effects of LPA in cultured C2C12 cells.

Role of atrial natriuretic peptide in the suppression of lysophosphatydic acid-induced rat aortic smooth muscle (RASM) cell growth

Lysophosphatidic acid (LPA) is a lipid mediator with multiple biological functions. In the present study we investigated the possible role of atrial natriuretic peptide (ANP), a hormone affecting cardiovascular homeostasis and inducing antimitogenic effects in different cell types, on LPA-induced cell growth and reactive oxygen species (ROS) production in rat aortic smooth muscle (RASM) cells. Both LPA effects on cell growth and levels of ROS were totally abrogated by physiological concentrations of ANP, without modifying the overexpression of LPA-receptors. These effects were also affected by cell pretreatment with wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K). Moreover, the LPA-induced activation of Akt, a downstream target of PI3K, was completely inhibited by physiological concentrations of ANP, which were also able to inhibit p42/p44 phosphorylation. Taken together, our data suggest that PI3K may represent an important step in the LPA signal transduction pathway responsible for ROS generation and DNA synthesis in RASM cells. At same time, the enzyme could also represent an essential target for the antiproliferative effects of ANP.

Mechanisms of lysophosphatidic acid-induced increase in intracellular calcium in vascular smooth muscle cells

Although lysophosphatidic acid (LPA) is known to cause an increase in intracellular Ca2+ concentration ([Ca2+]i) in vascular smooth muscle cells (VSMCs), the mechanisms of [Ca2+]i mobilization by LPA are not fully understood. In the present study, the effect of LPA on [Ca2+]i mobilization in cultured A10 VSMCs was examined by Fura-2 fluorescence technique. The expression of LPA receptors was studied by immunostaining. LPA was observed to increase [Ca2+]i in a concentration-dependent manner; this increase was dependent on the concentration of extracellular Ca2+. Both sarcolemmal (SL) Na(+)-Ca2+ exchange inhibitors (amiloride, Ni2+ and KB-R7943) and Na(+)-H+ exchange inhibitor (MIA) as well as SL store-operated Ca2+ channel (SOC) antagonists (SK&F 96365, tyrphostin A9 and gadolinium), unlike SL Ca2+ channel antagonists (verapamil and diltiazem), inhibited the LPA-induced increase in [Ca2+]i. In addition, sarcoplasmic reticulum (SR) Ca2+ channel blocker (ryanodine), SR Ca2+ channel opener (caffeine), SR Ca2+ pump ATPase inhibitor (thapsigargin) and inositol 1,4,5-trisphosphate (InsP3) receptor antagonists (xestospongin and 2-aminoethoxydiphenyl borate) were found to inhibit the LPA-induced Ca2+ mobilization. Furthermore, phospholipase C (PLC) inhibitor (U 73122) and protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate) attenuated the LPA-induced increase in [Ca2+]i. These results indicate that Ca2+ mobilization by LPA involves extracellular Ca2+ entry through SL Na(+)-Ca2+ exchanger, Na(+)-H+ exchanger and SL SOCs. In addition, ryanodine-sensitive and InsP(3)-sensitive intracellular Ca2+ pools may be associated with the LPA-induced increase in [Ca2+]i. Furthermore, the LPA-induced [Ca2+]i mobilization in VSMCs seems to be due to the activation of both PLC and PKC.

Protein Kinase Cδ Mediates Lysophosphatidic Acid-induced NF-κB Activation and Interleukin-8 Secretion in Human Bronchial Epithelial Cells

Lysophosphatidic acid (LPA), a potent bioactive lipid, elicits many of its biological actions via the specific G-protein-coupled receptors LPA1, LPA2, LPA3, and LPA4. Recently, we have shown that LPA-induced transactivation of platelet-derived growth factor receptor-beta is regulated by phospholipase D2 in human bronchial epithelial cells (HBEpCs) (Wang, L., Cummings, R. J., Zhao, Y., Kazlauskas, A., Sham, J., Morris, A., Brindley, D. N., Georas, S., and Natarajan, V. (2003) J. Biol. Chem. 278, 39931-39940). Here, we report that protein kinase Cdelta (PKCdelta) mediates LPA-induced NF-kappaB transcription and interleukin-8 (IL-8) secretion in HBEpCs. Treatment of HBEpCs with LPA increased both IL-8 gene and protein expression, which was coupled to Gi and G(12/13) proteins. LPA caused a marked activation of NF-kappaB in HBEpCs as determined by IkappaB phosphorylation and of NF-kappaB nuclear translocation and a strong induction of NF-kappaB promoter-mediated luciferase activity. Furthermore, LPA-activated PKCdelta and the LPA-mediated activation of NF-kappaB and IL-8 production were attenuated by overexpression of dominant-negative PKCdelta and rottlerin. Intratracheal administration of LPA in mice resulted in elevated levels of macrophage inflammatory protein-2, a murine homolog of IL-8, and an influx of neutrophils in the bronchoalveolar lavage fluid. These results demonstrate for the first time that LPA is a potent stimulator of IL-8 production in HBEpCs, which involves PKCdelta/NF-kappaB signaling pathways.

Differential gene expression in infarct scar and viable myocardium from rat heart following coronary ligation

Post-myocardial infarction (MI) remodeling of cardiac myocytes and the myocardial interstitium results in alteration of gross ventricular geometry and ventricular dysfunction. To investigate the mechanisms of the remodeling process of the heart after large MI, the expression of various genes in viable left ventricle and infarct scar tissue were examined at 16 weeks post-MI. Steady-state expression of Na(+)-K+ ATPase alpha-1 and -2, phospholamban (PLB), alpha-myosin heavy chain (alpha-MHC), ryanodine receptor (Rya) and Ca2+ ATPase (Serca2) mRNAs were decreased in the infarct scar vs noninfarcted sham-operated controls (P < 0.05). On the other hand, Gialpha2 and beta-MHC mRNAs were upregulated (P < 0.05, respectively) in the infarct scar whereas Na(+)-K+ ATPase-beta, Na(+)-Ca2+ exchanger and Gs mRNAs were not altered vs control values. In viable left ventricle, the alpha-1 subunit of Na(+)-K+ ATPase, alpha-3, beta-isoforms, Rya, beta-MHC, Gialpha2, Gs and Na(+)-Ca2+ exchanger were significantly elevated while expression of the alpha-2 subunit of Na(+)-K+ ATPase, PLB and Serca2 were significantly decreased compared to controls. Expression of CK2alpha mRNA was elevated in noninfarcted heart (145 +/- 15%) and diminished in the infarct scar (66 +/- 13%) vs controls. Expression of beta-MHC mRNA was elevated in both viable and infarct scar tissues of experimental hearts (140 +/- 31% and 183 +/- 30% vs. controls, respectively). These results suggest that cardiac genes in the infarcted tissue and viable left ventricle following MI are differentially regulated.

Attenuation of the serotonin-induced increase in intracellular calcium in rat aortic smooth muscle cells by sarpogrelate

Although serotonin (5-HT) induced proliferation of vascular smooth muscle cells is considered to involve changes in intracellular Ca2+([Ca2+]i), the mechanism of Ca2+mobilization by 5-HT is not well defined. In this study, we examined the effect of 5-HT on rat aortic smooth muscle cells (RASMCs) by Fura-2 microfluorometry for [Ca2+]imeasurements. 5-HT was observed to increase the [Ca2+]iin a concentration- and time-dependent manner. This action of 5-HT was dependent upon the extracellular concentration of Ca2+([Ca2+]e) and was inhibited by both Ca2+channel antagonists (verapamil and diltiazem) and inhibitors of sarcoplasmic reticular Ca2+pumps (thapsigargin and cyclopia zonic acid). The 5-HT-induced increase in [Ca2+]iwas blocked by sarpogrelate, a 5-HT2A-receptor antagonist, but not by different agents known to block other receptor sites. 5-HT-receptor antagonists such as ketanserin, cinanserin, and mianserin, unlike methysergide, were also found to inhibit the 5-HT-induced Ca2+mobilization, but these agents were less effective in comparison to sarpogrelate. On the other hand, the increase in [Ca2+]iin RASMCs by ATP, angiotensin II, endothelin-1, or phorbol ester was not affected by sarpogrelate. These results indicate that Ca2+mobilization in RASMCs by 5-HT is mediated through the activation of 5-HT2Areceptors and support the view that the 5-HT-induced increase in [Ca2+]iinvolves both the extracellular and intracellular sources of Ca2+.Key words: sarpogrelate, serotonin, vascular smooth muscle cells, intracellular Ca2+.