Initiation and transduction of stretch-induced RhoA and Rac1 activation through caveolae: cytoskeletal regulation of ERK translocation

The Rho family small GTPases play a crucial role in mediating cellular responses to stretch. However, it remains unclear how force is transduced to Rho signaling pathways. We investigated the effect of stretch on the activation and caveolar localization of RhoA and Rac1 in neonatal rat cardiomyocytes. In unstretched cardiomyocytes, RhoA and Rac1 were detected in both caveolar and non-caveolar fractions as assessed using detergent-free floatation analysis. Stretching myocytes for 4 min activated RhoA and Rac1. By 15 min of stretch, RhoA and Rac1 had dissociated from caveolae, and there was decreased coprecipitation of RhoA and Rac1 with caveolin-3. To determine whether compartmentation of RhoA and Rac1 within caveolae was necessary for stretch signaling, we disrupted caveolae with methyl beta-cyclodextrin (MbetaCD). Treatment with 5 mm MbetaCD for 1 h dissociated both RhoA and Rac1 from caveolae. Under this condition, stretch failed to activate RhoA or Rac1. Stretch-induced actin cytoskeletal organization was concomitantly impaired. Interestingly the ability of stretch to activate extracellular signal-regulated kinase (ERK) was unaffected by MbetaCD treatment, but ERK translocation to the nucleus was impaired. Stretch-induced hypertrophy was also inhibited. Actin cytoskeletal disruption with cytochalasin-D also prevented stretch from increasing nuclear ERK, whereas actin polymerization with jasplakinolide restored nuclear translocation of activated ERK in the presence of MbetaCD. We suggest that activation of RhoA or Rac1, localized in a caveolar compartment, is essential for sensing externally applied force and transducing this signal to the actin cytoskeleton and ERK translocation.     

Signals from type 1 sphingosine 1-phosphate receptors enhance adult mouse cardiac myocyte survival during hypoxia

Sphingosine 1-phosphate (S1P) is a biologically active lysophospholipid that serves as a key regulator of cellular differentiation and survival. Immune stimuli increase S1P synthesis and secretion by mast cells and platelets, implicating this molecule in tissue responses to injury and inflammation. Binding of S1P to G(i) protein-coupled receptors activates phosphatidylinositol 3-kinase and Akt in a variety of tissues. To elucidate the mechanisms by which S1P enhances adult cardiac myocyte survival during hypoxia, we used a mouse cell culture system in which S1P(1) receptors were observed to transduce signals from exogenous S1P, an S1P(1) receptor antibody with agonist properties, and the pharmacological agents FTY720 and SEW2871. S1P(1) receptor mRNA and protein were abundantly expressed by adult mouse cardiac myocytes. S1P-S1P(1) receptor axis enhancement of myocyte survival during hypoxia was abolished by phosphatidylinositol 3-kinase inhibition. S1P(1) receptor function was closely associated with activation of Akt, inactivation of GSK-3beta, and reduction of cytochrome c release from heart mitochondria. These observations highlight the importance of S1P(1) receptors on ventricular myocytes as mediators of inducible resistance against cellular injury during severe hypoxic stress.     

Mapping pathways downstream of sphingosine 1-phosphate subtype 1 by differential chemical perturbation and proteomics

Sphingosine 1-phosphate subtype 1 (S1P(1)) receptor agonists alter lymphocyte trafficking and endothelial barrier integrity in vivo. Among these is the potent, non-selective agonist, FTY720-P, whose mechanism of action has been suggested to correlate with S1P(1) down-regulation. Discovery of the in vivo active S1P(1)-selective agonist, SEW2871, has broadened our understanding of minimal requirements for S1P(1) function while highlighting differences regarding agonist effect on S1P(1) fate, because SEW2871 does not degrade S1P(1). To further understand the mechanism of agonist-induced S1P(1) down-regulation, we compared signaling and fate of human S1P(1)-green fluorescent protein (GFP) in stable 293 cells, using AFD-R, a chiral analog of FTY720-P, SEW2871, and S1P. Although all agonists acutely internalized S1P(1) to late endosomal vesicles and activated GTPgammaS(35) binding and pERK to similar maxima, only AFD-R led to significant S1P(1) down-regulation, as shown by GFP immunoprecipitation studies. Down-regulation was time- and concentration-dependent, was partially blocked by proteasomal inhibition and reversed by chloroquine and an antagonist to S1P(1). All agonists induced a receptor-associated increase in ubiquitination, with AFD-R inducing 3-fold more accumulation than S1P and being 3-4 logs more potent than SEW2871. The formation of AFD-R-receptor ubiquitin complex was inhibited by antagonist and chloroquine and was enhanced by proteasomal inhibition. Identification of proteins by PAGE liquid chromatography-tandem mass spectrometry in cells treated with AFD-R confirmed the co-migration of ubiquitin peptides with those of S1P(1) and GFP, relative to vehicle alone. These data suggest that the hierarchy of ubiquitin recruitment to S1P(1) (AFD-R > S1P > SEW2871) correlates with the efficiency of lysosomal receptor degradation and reflects intrinsic differences between agonists.     

Sphingosine 1-phosphate S1P2 and S1P3 receptor-mediated Akt activation protects against in vivo myocardial ischemia-reperfusion injury

Sphingosine 1-phosphate (S1P) is released at sites of tissue injury and effects cellular responses through activation of G protein-coupled receptors. The role of S1P in regulating cardiomyocyte survival following in vivo myocardial ischemia-reperfusion (I/R) injury was examined by using mice in which specific S1P receptor subtypes were deleted. Mice lacking either S1P(2) or S1P(3) receptors and subjected to 1-h coronary occlusion followed by 2 h of reperfusion developed infarcts equivalent to those of wild-type (WT) mice. However, in S1P(2,3) receptor double-knockout mice, infarct size following I/R was increased by >50%. I/R leads to activation of ERK, JNK, and p38 MAP kinases; however, these responses were not diminished in S1P(2,3) receptor knockout compared with WT mice. In contrast, activation of Akt in response to I/R was markedly attenuated in S1P(2,3) receptor knockout mouse hearts. Neither S1P(2) nor S1P(3) receptor deletion alone impaired I/R-induced Akt activation, which suggests redundant signaling through these receptors and is consistent with the finding that deletion of either receptor alone did not increase I/R injury. The involvement of cardiomyocytes in S1P(2) and S1P(3) receptor mediated activation of Akt was tested by using cells from WT and S1P receptor knockout hearts. Akt was activated by S1P, and this was modestly diminished in cardiomyocytes from S1P(2) or S1P(3) receptor knockout mice and completely abolished in the S1P(2,3) receptor double-knockout myocytes. Our data demonstrate that activation of S1P(2) and S1P(3) receptors plays a significant role in protecting cardiomyocytes from I/R damage in vivo and implicate the release of S1P and receptor-mediated Akt activation in this process.     

Sphingosine-1-phosphate receptors mediate neuromodulatory functions in the CNS

Sphingosine-1-phosphate (S1P) is a ubiquitous, lipophilic cellular mediator that acts in part by activation of G-protein-coupled receptor. Modulation of S1P signaling is an emerging pharmacotherapeutic target for immunomodulatory drugs. Although multiple S1P receptor types exist in the CNS, little is known about their function. Here, we report that S1P stimulated G-protein activity in the CNS, and results from [³⁵S]GTPγS autoradiography using the S1P₁-selective agonist SEW2871 and the S1P_1/3-selective antagonist VPC44116 show that in several regions a majority of this activity is mediated by S1P₁ receptors. S1P receptor activation inhibited glutamatergic neurotransmission as determined by electrophysiological recordings in cortical neurons in vitro , and this effect was mimicked by SEW2871 and inhibited by VPC44116. Moreover, central administration of S1P produced in vivo effects resembling the actions of cannabinoids, including thermal antinociception, hypothermia, catalepsy and hypolocomotion, but these actions were independent of CB₁ receptors. At least one of the central effects of S1P, thermal antinociception, is also at least partly S1P₁ receptor mediated because it was produced by SEW2871 and attenuated by VPC44116. These results indicate that CNS S1P receptors are part of a physiologically relevant and widespread neuromodulatory system, and that the S1P₁ receptor contributes to S1P-mediated antinociception.     

Sphingosine 1-phosphate triggers both apoptotic and survival signals for human hepatic myofibroblasts

Hepatic myofibroblasts (hMFs) are central in the development of liver fibrosis during chronic liver diseases, and their removal by apoptosis contributes to the resolution of liver fibrosis. We previously identified Edg receptors for sphingosine 1-phosphate (S1P) in human hMFs. Here, we investigated the effects of S1P on hMF apoptosis. S1P reduced viability of serum-deprived hMFs by an apoptotic process that was unrelated to the conversion of S1P into sphingosine and ceramide. The apoptotic effects of S1P were receptor-independent because dihydro-S1P, an Edg agonist, had no effect. S1P also stimulated a receptor-dependent survival pathway, revealed by enhanced activation of caspase-3 by S1P in the presence of pertussis toxin. Cell survival relied on two pertussis toxin-sensitive events, activation of ERK and activation of phosphatidylinositol 3-kinase (PI3K)/Akt by S1P. Both pathways were also activated by dihydro-S1P. Blunting either ERK or PI3K enhanced caspase-3 stimulation by S1P, and simultaneous inhibition of both pathways resulted in additive effects on caspase-3 activation. In conclusion, S1P induces apoptosis of human hMFs via a receptor-independent mechanism and stimulates a survival pathway following activation of Edg receptors. The survival pathway arises from the sequential activation of G(i)/G(o) proteins and independent stimulations of ERK and PI3K/Akt. Therefore, blocking Edg receptors may sensitize hepatic myofibroblasts to apoptosis by S1P.     

Protection of adult rat cardiac myocytes from ischemic cell death: role of caveolar microdomains and delta-opioid receptors

The role of caveolae, membrane microenvironments enriched in signaling molecules, in myocardial ischemia is poorly defined. In the current study, we used cardiac myocytes prepared from adult rats to test the hypothesis that opioid receptors (OR), which are capable of producing cardiac protection in vivo, promote cardiac protection in cardiac myocytes in a caveolae-dependent manner. We determined protein expression and localization of delta-OR (DOR) using coimmunohistochemistry, caveolar fractionation, and immunoprecipitations. DOR colocalized in fractions with caveolin-3 (Cav-3), a structural component of caveolae in muscle cells, and could be immunoprecipitated by a Cav-3 antibody. Immunohistochemistry confirmed plasma membrane colocalization of DOR with Cav-3. Cardiac myocytes were subjected to simulated ischemia (2 h) or an ischemic preconditioning (IPC) protocol (10 min ischemia, 30 min recovery, 2 h ischemia) in the presence and absence of methyl-beta-cyclodextrin (MbetaCD, 2 mM), which binds cholesterol and disrupts caveolae. We also assessed the cardiac protective effects of SNC-121 (SNC), a selective DOR agonist, on cardiac myocytes with or without MbetaCD and MbetaCD preloaded with cholesterol. Ischemia, simulated by mineral oil layering to inhibit gas exchange, promoted cardiac myocyte cell death (trypan blue staining), a response blunted by SNC (37 +/- 3 vs. 59 +/- 3% dead cells in the presence and absence of 1 muM SNC, respectively, P < 0.01) or by use of the IPC protocol (35 +/- 4 vs. 62 +/- 3% dead cells, P < 0.01). MbetaCD treatment, which disrupted caveolae (as detected by electron microscopy), fully attenuated the protective effects of IPC or SNC, resulting in cell death comparable to that of the ischemic group. By contrast, SNC-induced protection was not abrogated in cells incubated with cholesterol-saturated MbetaCD, which maintained caveolae structure and function. These findings suggest a key role for caveolae, perhaps through enrichment of signaling molecules, in contributing to protection of cardiac myocytes from ischemic damage.     

Sphingosine 1-Phosphate (S1P) Receptor Agonists Mediate Pro-fibrotic Responses in Normal Human Lung Fibroblasts via S1P2 and S1P3 Receptors and Smad-independent Signaling

Synthetic sphingosine 1-phosphate receptor 1 modulators constitute a new class of drugs for the treatment of autoimmune diseases. Sphingosine 1-phosphate (S1P) signaling, however, is also involved in the development of fibrosis. Using normal human lung fibroblasts, we investigated the induction of fibrotic responses by the S1P receptor (S1PR) agonists S1P, FTY720-P, ponesimod, and SEW2871 and compared them with the responses induced by the known fibrotic mediator TGF-β1. In contrast to TGF-β1, S1PR agonists did not induce expression of the myofibroblast marker α-smooth muscle actin. However, TGF-β1, S1P, and FTY720-P caused robust stimulation of extracellular matrix (ECM) synthesis and increased pro-fibrotic marker gene expression including connective tissue growth factor. Ponesimod showed limited and SEW2871 showed no pro-fibrotic potential in these readouts. Analysis of pro-fibrotic signaling pathways showed that in contrast to TGF-β1, S1PR agonists did not activate Smad2/3 signaling but rather activated PI3K/Akt and ERK1/2 signaling to induce ECM synthesis. The strong induction of ECM synthesis by the nonselective agonists S1P and FTY720-P was due to the stimulation of S1P₂ and S1P₃ receptors, whereas the weaker induction of ECM synthesis at high concentrations of ponesimod was due to a low potency activation of S1P₃ receptors. Finally, in normal human lung fibroblast-derived myofibroblasts that were generated by TGF-β1 pretreatment, S1P and FTY720-P were effective stimulators of ECM synthesis, whereas ponesimod was inactive, because of the down-regulation of S1P₃R expression in myofibroblasts. These data demonstrate that S1PR agonists are pro-fibrotic via S1P₂R and S1P₃R stimulation using Smad-independent pathways.     

Localization of VEGFR-2 and PLD2 in endothelial caveolae is involved in VEGF-induced phosphorylation of MEK and ERK

To clarify the role of caveolae in VEGF/VEGF receptor-2 (VEGFR-2)-mediated signaling cascades, primary cultured human umbilical vein endothelial cells (HUVECs) were fractionated to isolate caveolae-enriched cell membranes. Interestingly, VEGFR-2, phospholipase D2 (PLD2), and Ras were enriched in caveolae-enriched fractions. Moreover, VEGF increased PLD activity in a time- and dose-dependent manner in HUVECs, whereas a ligand specific for VEGFR-1 placental growth factor did not change PLD activity. A PLD inhibitor, 1-butanol, almost completely suppressed VEGF-induced ERK phosphorylation and cellular proliferation, whereas the negative control for 1-butanol, 3-butanol, did not produce significant changes. Addition of phosphatidic acid negated the 1-butanol-induced suppression. Pharmacological analyses using several inhibitors indicated that PKC-delta regulates the VEGF-induced activation of PLD/ERK. Thus PLD2 could be involved in MEK/ERK signaling cascades that are induced by the VEGF/VEGFR-2/PKC-delta pathway in endothelial cells. Pretreatment with the cholesterol depletion agent methyl-beta-cyclodextrin (MbetaCD) almost completely disassembled caveolar structures, whereas the addition of cholesterol to MbetaCD-treated cells restored caveolar structures. Pretreatment with MbetaCD largely abolished phosphorylation of MEK/ERK by VEGF, whereas the addition of cholesterol restored VEGF-induced MEK/ERK phosphorylations. These results indicate that intact caveolae are required for the VEGF/VEGFR-2-mediated MEK/ERK signaling cascade.     

Nrg-1 belongs to the endothelial differentiation gene family of G protein-coupled sphingosine-1-phosphate receptors

The previously cloned rat nerve growth factor-regulated G protein-coupled receptor NRG-1 (Glickman, M., Malek, R. L., Kwitek-Black, A. E., Jacob, H. J., and Lee N. H. (1999) Mol. Cell. Neurosci. 14, 141-52), also known as EDG-8, binds sphingosine-1-phosphate (S1P) with high affinity and specificity. In this paper we examined the signal transduction pathways regulated by the binding of S1P to EDG-8. In Chinese hamster ovary cells heterologously expressing EDG-8, S1P inhibited forskolin-induced cAMP accumulation and activated c-Jun NH2-terminal kinase. Surprisingly, S1P inhibited serum-induced activation of extracellular regulated protein kinase 1 and 2 (ERK1/2). Treatment with pertussis toxin, which ADP-ribosylates and inactivates G(i), blocked S1P-mediated inhibition of cAMP accumulation, but had no effect on c-Jun NH2-terminal kinase activation or inhibition of ERK1/2. The inhibitory effect of S1P on ERK1/2 activity was abolished by treatment with orthovanadate, suggesting the involvement of a tyrosine phosphatase. A subunit selective [35S] guanosine 5'-3-O-(thio)triphosphate binding assay demonstrates that EDG-8 activated G(i/o) and G12 but not Gs and G(q/11) in response to S1P. In agreement, EDG-8 did not stimulate phosphoinositide turnover or cAMP accumulation. The ability of S1P to induce mitogenesis in cells expressing the EDG-1 subfamily of G protein-coupled receptors is well characterized. In contrast, S1P inhibited proliferation in Chinese hamster ovary cells expressing EDG-8 but not empty vector. The antiproliferative effect, like S1P-mediated ERK1/2 inhibition, was orthovanadate-sensitive and pertussis toxin-insensitive. Our results indicate that EDG-8, a member of the EDG-1 subfamily, couples to unique signaling pathways.     

High Density Lipoprotein Stimulated Migration of Macrophages Depends on the Scavenger Receptor Class B,Type I,PDZK1 and Akt1 and Is Blocked by Sphingosine 1 Phosphate Receptor Antagonists

HDL carries biologically active lipids such as sphingosine-1-phosphate (S1P) and stimulates a variety of cell signaling pathways in diverse cell types, which may contribute to its ability to protect against atherosclerosis. HDL and sphingosine-1-phosphate receptor agonists, FTY720 and SEW2871 triggered macrophage migration. HDL-, but not FTY720-stimulated migration was inhibited by an antibody against the HDL receptor, SR-BI, and an inhibitor of SR-BI mediated lipid transfer. HDL and FTY720-stimulated migration was also inhibited in macrophages lacking either SR-BI or PDZK1, an adaptor protein that binds to SR-BI''s C-terminal cytoplasmic tail. Migration in response to HDL and S1P receptor agonists was inhibited by treatment of macrophages with sphingosine-1-phosphate receptor type 1 (S1PR1) antagonists and by pertussis toxin. S1PR1 activates signaling pathways including PI3K-Akt, PKC, p38 MAPK, ERK1/2 and Rho kinases. Using selective inhibitors or macrophages from gene targeted mice, we demonstrated the involvement of each of these pathways in HDL-dependent macrophage migration. These data suggest that HDL stimulates the migration of macrophages in a manner that requires the activities of the HDL receptor SR-BI as well as S1PR1 activity.     

Mapping structural determinants within third intracellular loop that direct signaling specificity of type 1 corticotropin-releasing hormone receptor

The type 1 corticotropin-releasing hormone receptor (CRH-R1) influences biological responses important for adaptation to stressful stimuli, through activation of multiple downstream effectors. The structural motifs within CRH-R1 that mediate G protein activation and signaling selectivity are unknown. The aim of this study was to gain insights about important structural determinants within the third intracellular loop (IC3) of the human CRH-R1α important for cAMP and ERK1/2 pathways activation and selectivity. We investigated the role of the juxtamembrane regions of IC3 by mutating amino acid cassettes or specific residues to alanine. Although simultaneous tandem alanine mutations of both juxtamembrane regions Arg(292)-Met(295) and Lys(311)-Lys(314) reduced ligand binding and impaired signaling, all other mutant receptors retained high affinity binding, indistinguishable from wild-type receptor. Agonist-activated receptors with tandem mutations at the proximal or distal terminal segments enhanced activation of adenylyl cyclase by 50-75% and diminished activation of inositol trisphosphate and ERK1/2 by 60-80%. Single Ala mutations identified Arg(292), Lys(297), Arg(310), Lys(311), and Lys(314) as important residues for the enhanced activation of adenylyl cyclase, partly due to reduced inhibition of adenylyl cyclase activity by pertussis toxin-sensitive G proteins. In contrast, mutation of Arg(299) reduced receptor signaling activity and cAMP response. Basic as well as aliphatic amino acids within both juxtamembrane regions were identified as important for ERK1/2 phosphorylation through activation of pertussis toxin-sensitive G proteins as well as G(q) proteins. These data uncovered unexpected roles for key amino acids within the highly conserved hydrophobic N- and C-terminal microdomains of IC3 in the coordination of CRH-R1 signaling activity.     

bFGF induces S1P1 receptor expression and functionality in human pulmonary artery smooth muscle cells

Sphingosine-1-phosphate (S1P), acting through five closely related G-protein coupled receptors termed S1P1-5, has recently emerged as a possible regulator of smooth muscle cell (SMC) physiology with the potential to induce contraction, proliferation and stress fiber formation. In the present study, real-time quantitative PCR was used to determine the expression patterns of S1P receptor subtypes in human primary pulmonary artery smooth muscle cells (PASMC). We report here that subconfluent PASMC express predominantly S1P2 and S1P3 receptors and we show that S1P1 receptor mRNA levels are significantly up-regulated following basic fibroblast growth factor (bFGF) treatment. As a consequence, increased responsiveness, as measured by impedance and ERK1/2 phosphorylation, was observed upon stimulation with a specific S1P1 receptor agonist SEW2871. We therefore demonstrate, for the first time, that a growth factor that was previously shown to be involved in physiological and pathological changes of SMC function induced S1P1 receptor expression and we propose that S1P1 receptor up-regulation could contribute to vascular remodeling.     

Post-Transplant Immunosuppression: Regulation of the Efflux of Allospecific Effector T Cells from Lymphoid Tissues

A functional sphingosine-1-phosphate (S1P) receptor antagonist specifically inhibited the egress of activated allospecific T cells from draining popliteal lymph nodes in alloantigen-sensitised mice. The level of S1P receptor 1 (S1PR1) mRNA was similarly reduced 1 and 3 days after mitogenic activation of T cells. However, the response of these cells to the S1PR1-specific agonist SEW2871 was only reduced on the first day after T cell activation with normal receptor-mediated Akt-phosphorylation restored by day 3. Longitudinal analysis of CD69 expression showed that almost all T cells expressed this antigen on days 1 and 3 after activation. However, the absolute level of cell-surface expression of CD69 peaked on undivided T cells and was then halved by each of the first 3 cycles of mitosis. CD69-specific small interfering RNA (siRNA) reduced the maximal level of CD69 expression by undivided, mitogen-stimulated T cells. These cells retained their capacity to phosphorylate Akt in response to stimulation with SEW2871. These data show that S1P receptors are involved in controlling the egress of activated T cells from lymph nodes, and that S1PR1 function is regulated by the level of T cell surface CD69. They suggest a potential for augmentation of this process to deplete alloreactive effector cells after organ transplantation.     

Adaptive increase in adenylyl cyclase activity in NG108-15 and S49 cells induced by chronic treatment with inhibitory drugs is not due to a decrease in cyclic AMP concentrations.

NG108-15 neuroblastoma x glioma hybrid cells and S49 lymphoma cells exhibit an enhancement in adenylyl cyclase activity after chronic treatment with receptor agonists that acutely inhibit the enzyme. Using agonists that activate five distinct inhibitory receptors in NG108-15 cells, we have found that there is a correlation between the extent of acute inhibition of prostaglandin E1 (PGE1)-stimulated cAMP accumulation and efficacy for induction of enhanced PGE1 stimulation of cAMP accumulation after chronic treatment and withdrawal. Chronic treatment with dideoxyadenosine, which acutely inhibits adenylyl cyclase activity by a mechanism independent or cell surface receptors or pertussis toxin-sensitive G proteins, did not induce enhanced PGE1 stimulation of cAMP accumulation in NG108-15 cells or forskolin stimulation of cAMP accumulation in S49 cells. While control basal cAMP concentrations were acutely decreased by carbachol in NG108-15 cells and by somatostatin in S49 cells, when the cAMP concentrations were maintained above the control basal values with a phosphodiesterase inhibitor, chronic treatment with these inhibitory drugs nonetheless resulted in enhanced cAMP responses in both NG108-15 and S49 cells. These results provide evidence that the initial decrement in cAMP concentrations caused by inhibitory drug is not the requisite signal for inducing the subsequent sensitization of adenylyl cyclase in NG108-15 and S49 cells but that activation of a pertussis toxin-sensitive G protein is involved in the development of this important adaptation.     

Localisation of endothelin B receptor variants to plasma membrane microdomains and its effects on downstream signalling

The endothelin B (ET_B) receptor can undergo a proteolytic cleavage resulting in an unglycosylated N-terminally truncated receptor. We investigated whether ET_B receptor processing affects caveolar localisation and mitogenic signalling. Distinct subcellular localisations of ET_B receptor constructs and epidermal growth factor (EGF) receptor ligands were analysed performing detergent-free caveolae preparations and total internal reflection fluorescence microscopy. ET_B receptor-induced transactivation of the EGF receptor and its downstream signalling was investigated performing shedding assays and ERK1/2 phosphorylation analyses. In COS7 cells, the N-terminally truncated but not the full-length or glycosylation-deficient ET_B receptor localised to caveolae. In caveolae-free HEK293 cells, only ET_B receptor constructs fused to caveolin-2 localised to membrane microdomains. A caveolar accumulation of the ET_B receptor disfavoured EGF receptor ligand shedding. Nonetheless, the activation of ERK1/2 was efficient and long-lasting. In HEK293 cells, the shedding activity was also impaired by N-terminal truncation. The subsequent ERK1/2 phosphorylation was long-lasting only for the full-length ET_B receptor. We conclude that the ET_B receptor localisation might depend on the presence of caveolae within the cell investigated. The data further suggest that caveolar enrichment of ET_B receptors does not facilitate the release of EGF receptor ligands. However, independent of their localisation, ET_B receptors are able to induce an ERK1/2 phosphorylation.     

The functional PDGFbeta receptor-S1P1 receptor signaling complex is involved in regulating migration of mouse embryonic fibroblasts in response to platelet derived growth factor

We report here that mouse embryonic fibroblasts (MEF) express a functional PDGFbeta receptor-S1P(1) receptor complex. The S1P(1) receptor is constitutively active and functions to enhance PDGF-stimulated migration of MEF. This was based on three pieces of evidence. Firstly, the S1P(1) receptor and PDGFbeta receptor are co-immunoprecipitated from cell lysates using anti-PDGFbeta receptor antibody. These findings suggest that the receptors form a complex in MEF. Secondly, inverse agonism of the S1P(1) receptor with SB649146 to eliminate the constitutive activity of the S1P(1) receptor reduced the PDGF-induced activation of p42/p44 MAPK in MEF. Thirdly, SB649146 inhibited the migration of MEF in response to the selective S1P(1) receptor agonist, SEW2871 or PDGF. In contrast, S1P inhibited PDGF-stimulated MEF migration, possibly mediated by the inhibitory S1P(2) receptor. These findings resolve an important issue regarding the functional role of the S1P(1) receptor in regulating MEF migration and suggest an important role within the context of PDGFbeta receptor-S1P(1) receptor complex signaling.     

Migration of CD4 T cells and dendritic cells toward sphingosine 1-phosphate (S1P) is mediated by different receptor subtypes: S1P regulates the functions of murine mature dendritic cells via S1P receptor type 3

Dendritic cells (DCs) and lymphocytes are known to show a migratory response to the phospholipid mediator, sphingosine 1-phosphate (S1P). However, it is unclear whether the same S1P receptor subtype mediates the migration of lymphocytes and DCs toward S1P. In this study, we investigated the involvement of S1P receptor subtypes in S1P-induced migration of CD4 T cells and bone marrow-derived DCs in mice. A potent S1P receptor agonist, the (S)-enantiomer of FTY720-phosphate [(S)-FTY720-P], at 0.1 nM or higher and a selective S1P receptor type 1 (S1P(1)) agonist, SEW2871, at 0.1 muM or higher induced a dose-dependent down-regulation of S1P(1). The pretreatment with these compounds resulted in a significant inhibition of mouse CD4 T cell migration toward S1P. Thus, it is revealed that CD4 T cell migration toward S1P is highly dependent on S1P(1). Mature DCs, when compared with CD4 T cells or immature DCs, expressed a relatively higher level of S1P(3) mRNA. S1P at 10-1000 nM induced a marked migration and significantly enhanced the endocytosis of FITC-dextran in mature but not immature DCs. Pretreatment with (S)-FTY720-P at 0.1 microM or higher resulted in a significant inhibition of S1P-induced migration and endocytosis in mature DCs, whereas SEW2871 up to 100 microM did not show any clear effect. Moreover, we found that S1P-induced migration and endocytosis were at an extremely low level in mature DCs prepared from S1P(3)-knockout mice. These results indicate that S1P regulates migration and endocytosis of murine mature DCs via S1P(3) but not S1P(1).     

Compartmentation of cAMP signaling in cardiac myocytes: a computational study

Receptor-mediated changes in cAMP production play an essential role in sympathetic and parasympathetic regulation of the electrical, mechanical, and metabolic activity of cardiac myocytes. However, responses to receptor activation cannot be easily ascribed to a uniform increase or decrease in cAMP activity throughout the entire cell. In this study, we used a computational approach to test the hypothesis that in cardiac ventricular myocytes the effects of beta(1)-adrenergic receptor (beta(1)AR) and M(2) muscarinic receptor (M(2)R) activation involve compartmentation of cAMP. A model consisting of two submembrane (caveolar and extracaveolar) microdomains and one bulk cytosolic domain was created using published information on the location of beta(1)ARs and M(2)Rs, as well as the location of stimulatory (G(s)) and inhibitory (G(i)) G-proteins, adenylyl cyclase isoforms inhibited (AC5/6) and stimulated (AC4/7) by G(i), and multiple phosphodiesterase isoforms (PDE2, PDE3, and PDE4). Results obtained with the model indicate that: 1), bulk basal cAMP can be high ( approximately 1 microM) and only modestly stimulated by beta(1)AR activation ( approximately 2 microM), but caveolar cAMP varies in a range more appropriate for regulation of protein kinase A ( approximately 100 nM to approximately 2 microM); 2), M(2)R activation strongly reduces the beta(1)AR-induced increases in caveolar cAMP, with less effect on bulk cAMP; and 3), during weak beta(1)AR stimulation, M(2)R activation not only reduces caveolar cAMP, but also produces a rebound increase in caveolar cAMP following termination of M(2)R activity. We conclude that compartmentation of cAMP can provide a quantitative explanation for several aspects of cardiac signaling.     

Lysophosphatidic acid and receptor-mediated activation of endothelial nitric-oxide synthase

Both lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are platelet-derived phospholipids that elicit diverse biological responses. In endothelial cells, S1P stimulates the EDG-1 receptor-mediated activation of the endothelial isoform of nitric oxide synthase (eNOS), but the role of LPA in eNOS regulation is less well understood. We now report that LPA treatment of bovine aortic endothelial cells (BAEC) activates eNOS enzyme activity in a pathway that involves phosphorylation of eNOS on serine 1179 by protein kinase Akt. In contrast to the cellular responses elicited by S1P in COS-7 cells, LPA can stimulate the activation of eNOS and Akt independently of EDG-1 receptor transfection. LPA-stimulated enzyme activation was significantly attenuated in an eNOS mutant lacking the site that is phosphorylated by kinase Akt (eNOS S1179A). In BAEC, activation of eNOS by LPA is completely blocked by pertussis toxin, by the intracellular calcium chelator BAPTA (1,2-bis(aminophenoxy) ethane-N,N,N',N'-tetraacetic acid), and by the phosphoinositide 3-kinase (PI3-K) inhibitor wortmannin, but is unaffected by U0126, an inhibitor of mitogen-activated protein (MAP) kinase pathways. Analysis of the LPA dose response for eNOS activation reveals an EC(50) of approximately 40 nM, a concentration well below the potency of LPA at the EDG-1 receptor. Taken together, these results indicate that LPA potently activates eNOS in BAEC in a pathway distinct from the EDG-1 receptor, but mediated by a similar receptor-mediated pathway dependent on pertussis toxin-sensitive G proteins and involving activation of the PI3-K/Akt pathway. These studies have identified a role for the phospholipid LPA in eNOS activation, and point out the complementary role of distinct platelet-derived lipids in endothelial signaling pathways.