Short Periods of Hypoxia Upregulate Sphingosine Kinase 1 and Increase Vasodilation of Arteries to Sphingosine 1-Phosphate (S1P) via S1P3.

Sphingosine kinase [(SK), isoforms SK1 and SK2] catalyzes the formation of the bioactive lipid, sphingosine 1-phosphate (S1P). This can be exported from cells and bind to S1P receptors to modulate vascular function. We investigated the effect of short-term hypoxia on SK1 expression and the response of arteries to S1P. SK1 expression in rat aortic and coronary artery endothelial cells was studied using immunofluorescence and confocal microscopy. Responses of rat aortic rings were studied using wire myography and reversible hypoxia induced by bubbling myography chambers with 95% N2:5% CO2 Inhibitors were added 30 minutes before induction of hypoxia. S1P induced endothelium-dependent vasodilation via activation of S1P3 receptors and generation of nitric oxide. Hypoxia significantly increased relaxation to S1P and this was attenuated by (2R)-1-[[(4-[[3-methyl-5-[(phenylsulfonyl)methyl] phenoxy]methyl]phenyl]methyl]-2-pyrrolidinemethanol [(PF-543), SK1 inhibitor] but not (R)-FTY720 methyl ether [(ROMe), SK2 inhibitor]. Hypoxia also increased vessel contractility to the thromboxane mimetic, 9,11-dideoxy-11α,9α-epoxymethanoprostaglandin F2α, which was further increased by PF-543 and ROMe. Hypoxia upregulated SK1 expression in aortic and coronary artery endothelial cells and this was blocked by PF-543 and 2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole [(SKi), SK1/2 inhibitor]. The effects of PF-543 and SKi were associated with increased proteasomal/lysosomal degradation of SK1. A short period of hypoxia increases the expression of SK1, which may generate S1P to oppose vessel contraction. Under hypoxic conditions, upregulation of SK1 is likely to lead to increased export of S1P from the cell and vasodilation via activation of endothelial S1P3 receptors. These data have significance for perfusion of tissue during episodes of ischemia.

Therapeutic potential of targeting sphingosine kinases and sphingosine 1-phosphate in hematological malignancies.

Sphingolipids, such as ceramide, sphingosine and sphingosine 1-phosphate (S1P) are bioactive molecules that have important functions in a variety of cellular processes, which include proliferation, survival, differentiation and cellular responses to stress. Sphingolipids have a major impact on the determination of cell fate by contributing to either cell survival or death. Although ceramide and sphingosine are usually considered to induce cell death, S1P promotes survival of cells. Sphingosine kinases (SPHKs) are the enzymes that catalyze the conversion of sphingosine to S1P. There are two isoforms, SPHK1 and SPHK2, which are encoded by different genes. SPHK1 has recently been implicated in contributing to cell transformation, tumor angiogenesis and metastatic spread, as well as cancer cell multidrug-resistance. More recent findings suggest that SPHK2 also has a role in cancer progression. This review is an overview of our understanding of the role of SPHKs and S1P in hematopoietic malignancies and provides information on the current status of SPHK inhibitors with respect to their therapeutic potential in the treatment of hematological cancers.

Sphingosine kinase 1 enables communication between melanoma cells and fibroblasts that provides a new link to metastasis.

In this issue of Oncogene, Albinet et al. have demonstrated a critical role of melanoma sphingosine kinase 1, which catalyses formation of sphingosine 1-phosphate (S1P), in promoting the differentiation of fibroblasts into myofibroblasts. The myofibroblast sphingosine kinase 1 then promotes the S1P-dependent dissemination (metastasis) of melanoma cells via a S1P receptor 3-mediated mechanism. These findings are of major significance because they provide a novel mechanism of interaction between melanoma and the microenvironment niche in promoting metastasis. These studies therefore identify S1P derived from myofibroblasts and melanoma cells as a novel target for therapeutic intervention.

Regulation of cell survival by sphingosine-1-phosphate receptor S1P1 via reciprocal ERK-dependent suppression of Bim and PI-3-kinase/protein kinase C-mediated upregulation of Mcl-1.

Although the ability of bioactive lipid sphingosine-1-phosphate (S1P) to positively regulate anti-apoptotic/pro-survival responses by binding to S1P1 is well known, the molecular mechanisms remain unclear. Here we demonstrate that expression of S1P1 renders CCL39 lung fibroblasts resistant to apoptosis following growth factor withdrawal. Resistance to apoptosis was associated with attenuated accumulation of pro-apoptotic BH3-only protein Bim. However, although blockade of extracellular signal-regulated kinase (ERK) activation could reverse S1P1-mediated suppression of Bim accumulation, inhibition of caspase-3 cleavage was unaffected. Instead S1P1-mediated inhibition of caspase-3 cleavage was reversed by inhibition of phosphatidylinositol-3-kinase (PI3K) and protein kinase C (PKC), which had no effect on S1P1 regulation of Bim. However, S1P1 suppression of caspase-3 was associated with increased expression of anti-apoptotic protein Mcl-1, the expression of which was also reduced by inhibition of PI3K and PKC. A role for the induction of Mcl-1 in regulating endogenous S1P receptor-dependent pro-survival responses in human umbilical vein endothelial cells was confirmed using S1P receptor agonist FTY720-phosphate (FTY720P). FTY720P induced a transient accumulation of Mcl-1 that was associated with a delayed onset of caspase-3 cleavage following growth factor withdrawal, whereas Mcl-1 knockdown was sufficient to enhance caspase-3 cleavage even in the presence of FTY720P. Consistent with a pro-survival role of S1P1 in disease, analysis of tissue microarrays from ER(+) breast cancer patients revealed a significant correlation between S1P1 expression and tumour cell survival. In these tumours, S1P1 expression and cancer cell survival were correlated with increased activation of ERK, but not the PI3K/PKB pathway. In summary, pro-survival/anti-apoptotic signalling from S1P1 is intimately linked to its ability to promote the accumulation of pro-survival protein Mcl-1 and downregulation of pro-apoptotic BH3-only protein Bim via distinct signalling pathways. However, the functional importance of each pathway is dependent on the specific cellular context.

Expression of sphingosine 1-phosphate receptor 4 and sphingosine kinase 1 is associated with outcome in oestrogen receptor-negative breast cancer.

BACKGROUND: We previously reported that sphingosine 1-phosphate receptor 4 (S1P(4)) is expressed and stimulates the ERK-1/2 pathway via a human epidermal growth factor receptor 2 (HER2)-dependent mechanism in oestrogen receptor-negative (ER(-)) MDA-MB-453 breast cancer cells. METHODS: Clinical relevance of S1P(4) and sphingosine kinase 1 (SK1, which catalyses the formation of S1P) was assessed in a cohort of 140 ER(-) breast tumours by immunohistochemistry (IHC) and the weighted histoscore method. Additional evidence for a functional interaction between S1P(4) and SK1 and between HER2 and SK1 was obtained using MDA-MB-453 cells. RESULTS: High S1P(4) expression is associated with shorter disease-free (P=0.014) and disease-specific survival (P=0.004), and was independent on multivariate analysis. In addition, patients with tumours that contain high and low levels of SK1 and S1P(4), respectively, have a significantly shorter disease-free survival (P=0.043) and disease-specific survival (P=0.033) compared with patients whose tumours contain both low S1P(4) and SK1 levels. In addition, high tumour expression of SK1 was significantly associated with shorter disease-specific survival (P=0.0001) in patients with HER2-positive tumours. Treatment of MDA-MB-453 cells with the SK1 inhibitor, SKi (2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole) reduced the basal and S1P/S1P(4)-induced activation of ERK-1/2 and altered HER2 trafficking in these cells. CONCLUSION: These findings highlight an important role for S1P(4) and SK1 in ER(-) breast cancer progression.

Interaction between anandamide and sphingosine-1-phosphate in mediating vasorelaxation in rat coronary artery.

BACKGROUND AND PURPOSE: Anandamide and sphingosine-1-phosphate (S1P) both regulate vascular tone in a variety of vessels. This study aimed to examine the mechanisms involved in the regulation of coronary vascular tone by anandamide and S1P, and to determine whether any functional interaction occurs between these receptor systems. EXPERIMENTAL APPROACH: Mechanisms used by anandamide and S1P to regulate rat coronary artery (CA) reactivity were investigated using wire myography. Interactions between S1P and the cannabinoid (CB)(2) receptor were determined using human embryonic kidney 293 (HEK293) cells that stably over-express recombinant CB(2) receptor. KEY RESULTS: Anandamide and S1P induced relaxation of the rat CA. CB(2) receptor antagonists attenuated anandamide-induced relaxation, while S1P-mediated relaxation was dependent on the vascular endothelium and S1P(3). Anandamide treatment resulted in an increase in the phosphorylation of sphingosine kinase-1 within the CA. Conversely, anandamide-mediated relaxation was attenuated by inhibition of sphingosine kinase. Moreover, S1P(3), specifically within the vascular endothelium, was required for anandamide-mediated vasorelaxation. In addition to this, S1P-mediated relaxation was also reduced by CB(2) receptor antagonists and sphingosine kinase inhibition. Further evidence that S1P functionally interacts with the CB(2) receptor was also observed in HEK293 cells over-expressing the CB(2) receptor. CONCLUSIONS AND IMPLICATIONS: In the vascular endothelium of rat CA, anandamide induces relaxation via a mechanism requiring sphingosine kinase-1 and S1P/S1P(3). In addition, we report that S1P may exert some of its effects via a CB(2) receptor- and sphingosine kinase-dependent mechanism, where subsequently formed S1P may have privileged access to S1P(3) to induce vascular relaxation.

The role of the PDE4D cAMP phosphodiesterase in the regulation of glucagon-like peptide-1 release.

BACKGROUND AND PURPOSE: Increases in intracellular cyclic AMP (cAMP) augment the release/secretion of glucagon-like peptide-1 (GLP-1). As cAMP is hydrolysed by cAMP phosphodiesterases (PDEs), we determined the role of PDEs and particularly PDE4 in regulating GLP-1 release. EXPERIMENTAL APPROACH: GLP-1 release, PDE expression and activity were investigated using rats and GLUTag cells, a GLP-1-releasing cell line. The effects of rolipram, a selective PDE4 inhibitor both in vivo and in vitro and stably overexpressed catalytically inactive PDE4D5 (D556A-PDE4D5) mutant in vitro on GLP-1 release were investigated. KEY RESULTS: Rolipram (1.5 mg x kg(-1) i.v.) increased plasma GLP-1 concentrations approximately twofold above controls in anaesthetized rats and enhanced glucose-induced GLP-1 release in GLUTag cells (EC(50) approximately 1.2 nmol x L(-1)). PDE4D mRNA transcript and protein were detected in GLUTag cells using RT-PCR with gene-specific primers and Western blotting with a specific PDE4D antibody respectively. Moreover, significant PDE activity was inhibited by rolipram in GLUTag cells. A GLUTag cell clone (C1) stably overexpressing the D556A-PDE4D5 mutant, exhibited elevated intracellular cAMP levels and increased basal and glucose-induced GLP-1 release compared with vector-transfected control cells. A role for intracellular cAMP/PKA in enhancing GLP-1 release in response to overexpression of D556A-PDE4D5 mutant was demonstrated by the finding that the PKA inhibitor H89 reduced both basal and glucose-induced GLP-1 release by 37% and 39%, respectively, from C1 GLUTag cells. CONCLUSIONS AND IMPLICATIONS: PDE4D may play an important role in regulating intracellular cAMP linked to the regulation of GLP-1 release.

Lipid phosphate phosphatases and lipid phosphate signalling.

Mammalian LPPs (lipid phosphate phosphatases) are integral membrane proteins that belong to a superfamily of lipid phosphatases/phosphotransferases. They have broad substrate specificity in vitro, dephosphorylating PA (phosphatidic acid), S1P (sphingosine 1-phosphate), LPA (lysophosphatidic acid) etc. Their physiological role may include the attenuation of S1P- and LPA-stimulated signalling by virtue of an ecto-activity (i.e. dephosphorylation of extracellular S1P and LPA), thereby limiting the activation of LPA- and S1P-specific G-protein-coupled receptors at the cell surface. However, our recent work suggests that an intracellular action of LPP2 and LPP3 may account for the reduced agonist-stimulated p42/p44 mitogen-activated protein kinase activation of HEK-293 (human embryonic kidney 293) cells. This may involve a reduction in the basal levels of PA and S1P respectively and the presence of an early apoptotic phenotype under conditions of stress (serum deprivation). Additionally, we describe a model whereby LPP2, but not LPP3, may be functionally linked to the phospholipase D1-derived PA-dependent recruitment of sphingosine kinase 1 to the perinuclear compartment. We also consider the potential regulatory mechanisms for LPPs, which may involve oligomerization. Lastly, we highlight many aspects of the LPP biology that remain to be fully defined.

Receptor tyrosine kinase-GPCR signal complexes.

The formation of complexes between growth factor receptors and members of a family of G-protein-coupled receptors whose natural ligands are S1P (sphingosine 1-phosphate) and LPA (lysophosphatidic acid) represents a new signalling entity. This receptor complex allows for integrated signalling in response to growth factor and/or S1P/LPA and provides a mechanism for more efficient activation (due to integrated close-proximity signalling from both receptor classes) of the p42/p44 MAPK (mitogen-activated protein kinase) pathway. This article provides information on the molecular events at the interface between receptor tyrosine kinases and S1P/LPA receptors. Examples include the PDGF (platelet-derived growth factor)-induced tyrosine phosphorylation of G(i)alpha, released upon S1P(1) receptor activation, which is required for initiation of the p42/p44 MAPK pathway. Critical to this event is the formation of endocytic vesicles containing functionally active PDGFbeta receptor-S1P(1) receptor complexes, which are internalized and relocated with components of the p42/p44 MAPK pathway. We also report examples of cross-talk signal integration between the Trk A (tropomyosin receptor kinase A) receptor and the LPA(1) receptor in terms of the NGF (nerve growth factor)-dependent regulation of the p42/p44 MAPK pathway. NGF induces recruitment of the LPA(1) receptor to the nucleus (delivery might be Trk A-dependent), whereupon the LPA(1) receptor may govern gene expression via novel nuclear signalling processes.

Cyclic nucleotide phosphodiesterases in pancreatic islets.

Cyclic nucleotide phosphodiesterases (PDEs) comprise a family of enzymes (PDE1-PDE11) which hydrolyse cyclic AMP and cyclic GMP to their biologically inactive 5' derivatives. Cyclic AMP is an important physiological amplifier of glucose-induced insulin secretion. As PDEs are the only known mechanism for inactivating cyclic nucleotides, it is important to characterise the PDEs present in the pancreatic islet beta cells. Several studies have shown pancreatic islets or beta cells to contain PDE1C, PDE3B and PDE4, with some evidence for PDE10A. Most evidence suggests that PDE3B is the most important in relation to the regulation of insulin release, although PDE1C could have a role. PDE3-selective inhibitors augment glucose-induced insulin secretion. In contrast, activation of beta-cell PDE3B could mediate the inhibitory effect of IGF-1 and leptin on insulin secretion. In vivo, although PDE3 inhibitors augment glucose-induced insulin secretion, concomitant inhibition of PDE3B in liver and adipose tissue induce insulin resistance and PDE3 inhibitors do not induce hypoglycaemia. The development of PDE3 inhibitors as anti-diabetic agents would require differentiation between PDE3B in the beta cell and that in hepatocytes and adipocytes. Through their effects in regulating beta-cell cyclic nucleotide concentrations, PDEs could modulate beta-cell growth, differentiation and survival; some work has shown that selective inhibition of PDE4 prevents diabetes in NOD mice and that selective PDE3 inhibition blocks cytokine-induced nitric oxide production in islet cells. Further work is required to understand the mechanism of regulation and role of the various PDEs in islet-cell function and to validate them as targets for drugs to treat and prevent diabetes.

The gamma subunit of the rod photoreceptor cGMP phosphodiesterase can modulate the proteolysis of two cGMP binding cGMP-specific phosphodiesterases (PDE6 and PDE5) by caspase-3.

We have investigated whether the proteolysis of members of the cGMP binding phosphodiesterases (PDE6, PDE5A1, and PDE10A2) by caspase-3 is modulated by the gamma inhibitor subunit of PDE6. We show here that purified caspase-3 proteolyses PDE6, an enzyme composed of two nonidentical catalytic subunits (termed alpha and beta) with molecular mass of 88 and 84 kDa. The proteolysis of PDE6 produced a single fragment with a molecular mass of 78 kDa. This corresponds to the possible cleavage of the caspase-3 consensus DFVD site (amino acids: 164-168) in the alpha subunit and leads to a 50% decrease in the cGMP hydrolysing activity of the enzyme. The addition of rod PDEgamma to the incubation completely blocked the cleavage of PDE6 by caspase-3. In contrast, rod PDEgamma converted PDE5A1 (molecular mass of 98 kDa) to a better substrate for caspase-3. This resulted in the formation of four major fragments with molecular mass of 82-83, 67, 43, and 34 kDa. In addition, caspase-3 induced an approximately 80% reduction in the activity of a partially purified preparation of PDE5A1 in the presence of rod PDEgamma. Caspase-3 also cleaved PDE10A2 (molecular mass of 95 kDa) to a single 48-kDa fragment. This was consistent with cleavage of the DLFD site (amino acids: 312-315) in PDE10A2. In contrast with both PDE6 and PDE5A1, rod PDEgamma was without effect on this enzyme. These data show that rod PDEgamma interacts with at least two members of the cGMP binding PDE family (PDE5A1 and PDE6) and can exert differential effects on the cleavage of these enzymes by caspase-3.

Short-term local delivery of an inhibitor of Ras farnesyltransferase prevents neointima formation in vivo after porcine coronary balloon angioplasty.

BACKGROUND: Mitogenic stimuli present at the site of coronary arterial balloon injury contribute to the progression and development of a restenotic lesion, many signaling through a common pathway involving the small G protein p21(ras). Our aim was to demonstrate in biochemical studies that farnesyl protein transferase inhibitor III (FPTIII) is an inhibitor of p21(ras) processing and that when it is given locally in vivo at the site of coronary balloon injury in a porcine model, it can inhibit neointima formation. METHODS AND RESULTS: FPTIII (1 to 25 micromol/L) concentration-dependently reduced p21(ras) levels in porcine coronary artery smooth muscle cell membranes. FPTIII also prevented p42/p44 MAPK activation and DNA synthesis in response to platelet-derived growth factor in these cells at a concentration of 25 micromol/L. Application of 25 micromol/L FPTIII locally for 15 minutes to balloon-injured porcine coronary arteries in vivo prevented neointima formation assessed at 4 weeks, reduced proteoglycan deposition, and inhibited adventitial hypertrophy. Coronary arteries from FPTIII-treated pigs had no deterioration in contraction or in endothelium-dependent relaxation. CONCLUSIONS: The study demonstrates in the pig that short-term local delivery of inhibitors of p21(ras)-dependent mitogenic signal transduction prevents restenosis after balloon angioplasty.

The inhibitory gamma subunit of the type 6 retinal cyclic guanosine monophosphate phosphodiesterase is a novel intermediate regulating p42/p44 mitogen-activated protein kinase signaling in human embryonic kidney 293 cells.

The inhibitory gamma subunits of the retinal rod and cone photoreceptor type 6 retinal cyclic guanosine monophosphate phosphodiesterase (PDEgamma) are expressed in non-retinal tissues. Here, we show that PDEgamma interacts with the G-protein-coupled receptor kinase 2 signaling system to regulate the epidermal growth factor- and thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase in human embryonic kidney 293 cells. This is based upon several lines of evidence. First, the transfection of cells with an antisense rod PDEgamma plasmid construct, which reduced endogenous rod PDEgamma expression, ablated the epidermal growth factor- and thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase. Second, the transfection of cells with recombinant rod or cone PDEgamma and/or G-protein-coupled receptor kinase 2 increased the stimulation of p42/p44 mitogen-activated protein kinase by epidermal growth factor or thrombin. In contrast, a G-protein-coupled receptor kinase 2 phosphorylation-resistant rod PDEgamma mutant failed to increase the epidermal growth factor- or thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase and, in fact, functioned as a dominant negative. Thrombin also stimulated the association of endogenous rod PDEgamma with dynamin II, which was increased in cells transfected with rod PDEgamma or G-protein-coupled receptor kinase 2. Dynamin II plays a critical role in regulating endocytosis of receptor signal complexes required for activation of p42/p44 mitogen-activated protein kinase. Therefore, PDEgamma may have an important role in promoting endocytosis of receptor signal complexes leading to the activation of p42/p44 mitogen-activated protein kinase. We conclude that PDEgamma is an entirely novel intermediate regulating mitogenic signaling from both receptor tyrosine kinase and G-protein-coupled receptors in human embryonic kidney 293 cells.

Assessment of agonism at G-protein coupled receptors by phosphatidic acid and lysophosphatidic acid in human embryonic kidney 293 cells.

Several different molecular species of phosphatidic acid (PA) bind to a G-protein coupled receptor (GPCR) to induce activation of the p42/p44 mitogen-activated protein kinase (p42/p44 MAPK) pathway in HEK 293 cells. PA is active at low nanomolar concentrations and the response is sensitive to pertussis toxin (which uncouples GPCRs from G(i/o)). The de-acylated product of PA, lysophosphatidic acid (LPA), which binds to members of the endothelial differentiation gene (EDG) family of receptors also stimulated p42/p44 MAPK in a pertussis toxin sensitive manner, but with an approximately 100 - 1000 fold lower potency compared with the different molecular species of PA. RT - PCR using gene-specific primers showed that HEK 293 cells express EDG2 and PSP24, the latter being a lipid binding GPCR out with the EDG cluster. We conclude that PA is a novel high potency GPCR agonist.

Nerve growth factor stimulation of p42/p44 mitogen-activated protein kinase in PC12 cells: role of G(i/o), G protein-coupled receptor kinase 2, beta-arrestin I, and endocytic processing.

In this study, we have shown that nerve growth factor (NGF)-dependent activation of the p42/p44 mitogen-activated protein kinase (p42/p44 MAPK) pathway in PC12 cells can be partially blocked by pertussis toxin (which inactivates the G proteins G(i/o)). This suggests that the Trk A receptor may use a G protein-coupled receptor pathway to signal to p42/p44 MAPK. This was supported by data showing that the NGF-dependent activation of p42/p44 MAPK is potentiated in cells transfected with G protein-coupled receptor kinase 2 (GRK2) or beta-arrestin I. Moreover, GRK2 is constitutively bound with the Trk A receptor, whereas NGF stimulates the pertussis toxin-sensitive binding of beta-arrestin I to the TrkA receptor-GRK2 complex. Both GRK2 and beta-arrestin I are involved in clathrin-mediated endocytic signaling to p42/p44 MAPK. Indeed, inhibitors of clathrin-mediated endocytosis (e.g., monodansylcadaverine, concanavalin A, and hyperosmolar sucrose) reduced the NGF-dependent activation of p42/p44 MAPK. Finally, we have found that the G protein-coupled receptor-dependent component regulating p42/p44 MAPK is required for NGF-induced differentiation of PC12 cells. Thus, NGF-dependent inhibition of DNA synthesis was partially blocked by PD098059 (inhibitor of MAPK kinase-1 activation) and pertussis toxin. Our findings are the first to show that the Trk A receptor uses a classic G protein-coupled receptor-signaling pathway to promote differentiation of PC12 cells.

Tethering of the platelet-derived growth factor beta receptor to G-protein-coupled receptors. A novel platform for integrative signaling by these receptor classes in mammalian cells.

Here we provide evidence to show that the platelet-derived growth factor beta receptor is tethered to endogenous G-protein-coupled receptor(s) in human embryonic kidney 293 cells. The tethered receptor complex provides a platform on which receptor tyrosine kinase and G-protein-coupled receptor signals can be integrated to produce more efficient stimulation of the p42/p44 mitogen-activated protein kinase pathway. This was based on several lines of evidence. First, we have shown that pertussis toxin (which uncouples G-protein-coupled receptors from inhibitory G-proteins) reduced the platelet-derived growth factor stimulation of p42/p44 mitogen-activated protein kinase. Second, transfection of cells with inhibitory G-protein alpha subunit increased the activation of p42/p44 mitogen-activated protein kinase by platelet-derived growth factor. Third, platelet-derived growth factor stimulated the tyrosine phosphorylation of the inhibitory G-protein alpha subunit, which was blocked by the platelet-derived growth factor kinase inhibitor, tyrphostin AG 1296. We have also shown that the platelet-derived growth factor beta receptor forms a tethered complex with Myc-tagged endothelial differentiation gene 1 (a G-protein-coupled receptor whose agonist is sphingosine 1-phosphate) in cells co-transfected with these receptors. This facilitates platelet-derived growth factor-stimulated tyrosine phosphorylation of the inhibitory G-protein alpha subunit and increases p42/p44 mitogen-activated protein kinase activation. In addition, we found that G-protein-coupled receptor kinase 2 and beta-arrestin I can associate with the platelet-derived growth factor beta receptor. These proteins play an important role in regulating endocytosis of G-protein-coupled receptor signal complexes, which is required for activation of p42/p44 mitogen-activated protein kinase. Thus, platelet-derived growth factor beta receptor signaling may be initiated by G-protein-coupled receptor kinase 2/beta-arrestin I that has been recruited to the platelet-derived growth factor beta receptor by its tethering to a G-protein-coupled receptor(s). These results provide a model that may account for the co-mitogenic effect of certain G-protein-coupled receptor agonists with platelet-derived growth factor on DNA synthesis.

The role of the cyclic GMP-inhibited cyclic AMP-specific phosphodiesterase (PDE3) in regulating clonal BRIN-BD11 insulin secreting cell survival.

We report here that the cyclic GMP-inhibited cyclic AMP specific phosphodiesterase (PDE3B) is expressed as a membrane-bound protein in clonal insulin-secreting BRIN-BD11 cells. This was shown using SKF94836 (PDE3 inhibitor) which maximally inhibited membrane-bound cyclic AMP PDE activity by approximately 25-30% and by RT-PCR. We also demonstrated that insulin growth factor-1 (IGF-1) activates PDE3B in BRIN-BD11 cells. We therefore evaluated the role of phosphoinositide 3-kinase (PI3K) and p42/p44 mitogen-activated protein kinase (p42/p44 MAPK) pathways in regulating this enzyme. We report here that the PI3K inhibitor, wortmannin, prevented the IGF-1-dependent stimulation of PDE3B activity. In contrast, the inhibitor of MEK-1 activation, PD098059 (which reduced IGF-1-stimulated p42/p44 MAPK phosphorylation), had no effect on PDE3B activation. Furthermore, IGF-1-dependent stimulation of p42/p44 MAPK and PDE3B was abolished in serum-deprived cells and this was associated with apoptosis. We propose that the deregulation of the PI3K/PDE3B pathway might result in increased intracellular cyclic AMP accumulation, which promotes apoptosis. This was supported by the finding that the adenylyl cyclase activator, forskolin, also induced apoptosis. Finally, we found that orthovanadate (a phosphotyrosine phosphatase inhibitor) fully restored the activation of p42/p44 MAPK in serum-deprived cells, but had only a small effect on PDE activity. This confirmed that p42/p44 MAPK is on a separate pathway to PDE3B. Therefore, IGF-1-dependent regulation of PDE3B may be linked to cell survival through PI3K and not p42/p44 MAPK.

The platelet-derived growth factor receptor stimulation of p42/p44 mitogen-activated protein kinase in airway smooth muscle involves a G-protein-mediated tyrosine phosphorylation of Gab1.

Using cultured airway smooth muscle cells, we showed previously that the platelet-derived growth factor (PDGF) receptor uses the G-protein, G(i), to stimulate Grb-2-associated phosphoinositide 3-kinase (PI3K) activity. We also showed that this was an intermediate step in the activation of p42/p44 mitogen-activated protein kinase (p42/p44 MAPK) by PDGF. We now present two lines of evidence that provide further support for this model. First, we report that PDGF stimulates the G(i)-mediated tyrosine phosphorylation of the Grb-2 adaptor protein, Gab1. This phosphorylation appears to be necessary for association of PI3K1a with the Gab1-Grb-2 complex. Second, PI3K appears to promote the subsequent association of dynamin II (which is involved in clathrin-mediated endocytic processing) with the complex. Furthermore, inhibitors of PI3K and clathrin-mediated endocytosis reduced the PDGF-dependent activation of p42/p44 MAPK, suggesting a role for PI3K in the endocytic signaling process leading to stimulation of p42/p44 MAPK. Together, these results begin to define a common signaling model for certain growth factor receptors (e.g., PDGF, insulin, insulin-like growth factor-1, and fibroblast growth factor) which use G(i) to transmit signals to p42/p44 MAPK.

Sphingosine 1-phosphate signalling in mammalian cells.

Sphingosine 1-phosphate is formed in cells in response to diverse stimuli, including growth factors, cytokines, G-protein-coupled receptor agonists, antigen, etc. Its production is catalysed by sphingosine kinase, while degradation is either via cleavage to produce palmitaldehyde and phosphoethanolamine or by dephosphorylation. In this review we discuss the most recent advances in our understanding of the role of the enzymes involved in metabolism of this lysolipid. Sphingosine 1-phosphate can also bind to members of the endothelial differentiation gene (EDG) G-protein-coupled receptor family [namely EDG1, EDG3, EDG5 (also known as H218 or AGR16), EDG6 and EDG8] to elicit biological responses. These receptors are coupled differentially via G(i), G(q), G(12/13) and Rho to multiple effector systems, including adenylate cyclase, phospholipases C and D, extracellular-signal-regulated kinase, c-Jun N-terminal kinase, p38 mitogen-activated protein kinase and non-receptor tyrosine kinases. These signalling pathways are linked to transcription factor activation, cytoskeletal proteins, adhesion molecule expression, caspase activities, etc. Therefore sphingosine 1-phosphate can affect diverse biological responses, including mitogenesis, differentiation, migration and apoptosis, via receptor-dependent mechanisms. Additionally, sphingosine 1-phosphate has been proposed to play an intracellular role, for example in Ca(2+) mobilization, activation of non-receptor tyrosine kinases, inhibition of caspases, etc. We review the evidence for both intracellular and extracellular actions, and extensively discuss future approaches that will ultimately resolve the question of dual action. Certainly, sphingosine 1-phosphate will prove to be unique if it elicits both extra- and intra-cellular actions. Finally, we review the evidence that implicates sphingosine 1-phosphate in pathophysiological disease states, such as cancer, angiogenesis and inflammation. Thus there is a need for the development of new therapeutic compounds, such as receptor antagonists. However, identification of the most suitable targets for drug intervention requires a full understanding of the signalling and action profile of this lysosphingolipid. This article describes where the research field is in relation to achieving this aim.

The cAMP-specific phosphodiesterase PDE4A5 is cleaved downstream of its SH3 interaction domain by caspase-3. Consequences for altered intracellular distribution.

The unique N-terminal region of the cAMP-specific phosphodiesterase PDE4A5, which confers an ability to bind to certain protein SH3 domains, is cleaved during apoptosis in both Rat-1 fibroblasts and PC12 cells. Cleavage was abolished by the caspase-3-selective inhibitor, z-DEVD-CHO but not the caspase-1 selective inhibitor, z-YVAD-CHO. Caspase-3 treatment of PDE4A5, expressed either transiently in COS cells or generated in vitro by coupled transcription translation, generated a similar cleavage product of 100 kDa compared with the native 110-kDa PDE4A5. This product could be detected immunochemically with an antibody raised to a C-terminal PDE4A5 peptide but not an antibody raised to the N terminus of PDE4A5, indicating that caspase-3 caused N-terminal cleavage of PDE4A5. Deletion of the putative caspase-3 cleavage site, (69)DAVD(72), in PDE4A5, or generation of either the D72A or the D69A mutants, ablated the ability of caspase-3 to cause cleavage. The N-terminal truncate PDE4A5-DeltaP3 was engineered to mimic the caspase-cleaved product of PDE4A5. This showed altered catalytic activity and, unlike PDE4A5, was unable to interact with the SH3 domain of the tyrosyl kinase, LYN. Although both PDE4A5 and PDE4A5-DeltaP3 were localized at cell cortical regions (ruffles), the distinct perinuclear association noted for both PDE4A5 and LYN was not seen for PDE4A5-DeltaP3. Staurosporine-induced apoptosis caused a marked redistribution of PDE4A5 but not PDE4A8 in stably transfected Rat-1 cells. The PDE4-selective inhibitor, rolipram together with the adenylyl cyclase activator forskolin, caused a synergistic increase in the apoptosis of Rat-1 cells. Overexpression of PDE4A5 in Rat-1 cells protected against staurosporine-induced apoptosis in contrast to overexpression of PDE4A8, which potentiated apoptosis. PDE4A5 may be the sole PDE4 family member to provide a substrate for caspase-3 cleavage and this action serves to remove the SH3 binding domain that is unique to this isoform within the PDE4A family and to alter its intracellular targeting.