PI 3-kinase gamma and protein kinase C-zeta mediate RAS-independent activation of MAP kinase by a Gi protein-coupled receptor

An updated patent review of autotaxin inhibitors (2017-present)

INTRODUCTION

The ATX-LPA axis is an attractive target for therapeutic intervention in a variety of diseases, such as tumor metastasis, fibrosis, pruritus, multiple sclerosis, inflammation, autoimmune conditions, metabolic syndrome, and so on. Accordingly, considerable efforts have been devoted to the development of new chemical entities capable of modulating the ATX-LPA axis.

AREAS COVERED

This review aims to provide an overview of novel ATX inhibitors reported in patents from September 2016 to August 2020, discussing their structural characteristics and inhibitory potency in vitro and in vivo.

EXPERT OPINION

In the past four years, the classification of ATX inhibitors based on binding modes has brought great benefits to the discovery of more efficacious inhibitors. In addition to GLPG1690 currently in phase III clinical studies for IPF, BBT-877, and BLD-0409 as potent ATX inhibitors have been enrolled in phase I clinical evaluation; meanwhile, many effective molecules were also reported successively. However, most emerging ATX inhibitors in the last four years are closely analogs of previous entities, such as GLPG1690 and PF-8380, which translate into the urgently identification of ATX inhibitors with diverse structural features and promising properties in the near future.

Ovarian Cancer Dissemination-A Cell Biologist's Perspective

Epithelial ovarian cancer (EOC) comprises multiple disease states representing a variety of distinct tumors that, irrespective of tissue of origin, genetic aberrations and pathological features, share common patterns of dissemination to the peritoneal cavity. EOC peritoneal dissemination is a stepwise process that includes the formation of malignant outgrowths that detach and establish widespread peritoneal metastases through adhesion to serosal membranes. The cell biology associated with outgrowth formation, detachment, and de novo adhesion is at the nexus of diverse genetic backgrounds that characterize the disease. Development of treatment for metastatic disease will require detailed characterization of cellular processes involved in each step of EOC peritoneal dissemination. This article offers a review of the literature that relates to the current stage of knowledge about distinct steps of EOC peritoneal dissemination, with emphasis on the cell biology aspects of the process.

Noncanonical regulation of insulin-mediated ERK activation by phosphoinositide 3-kinase γ

Classically, Class IB phosphoinositide 3-kinase (PI3Kγ) plays a role in ERK activation following G-protein–coupled receptor (GPCR) activation. Here we show that PI3Kγ noncanonically regulates ERK phosphorylation in a kinase-independent mechanism, irrespective of the upstream signals. PI3Kγ sequesters PP2A, allowing sustained ERK function.

Classically Class IB phosphoinositide 3-kinase (PI3Kγ) plays a role in extracellular signal–regulated kinase (ERK) activation following G-protein coupled receptor (GPCR) activation. Knock-down of PI3Kγ unexpectedly resulted in loss of ERK activation to receptor tyrosine kinase agonists such as epidermal growth factor or insulin. Mouse embryonic fibroblasts (MEFs) or primary adult cardiac fibroblasts isolated from PI3Kγ knock-out mice (PI3KγKO) showed decreased insulin-stimulated ERK activation. However, expression of kinase-dead PI3Kγ resulted in rescue of insulin-stimulated ERK activation. Mechanistically, PI3Kγ sequesters protein phosphatase 2A (PP2A), disrupting ERK–PP2A interaction, as evidenced by increased ERK–PP2A interaction and associated PP2A activity in PI3KγKO MEFs, resulting in decreased ERK activation. Furthermore, β-blocker carvedilol-mediated β-arrestin-dependent ERK activation is significantly reduced in PI3KγKO MEF, suggesting accelerated dephosphorylation. Thus, instead of classically mediating the kinase arm, PI3Kγ inhibits PP2A by scaffolding and sequestering, playing a key parallel synergistic step in sustaining the function of ERK, a nodal enzyme in multiple cellular processes.

Autocrine Regulation of UVA-Induced IL-6 Production via Release of ATP and Activation of P2Y Receptors

Extracellular nucleotides, such as ATP, are released from cells in response to various stimuli and act as intercellular signaling molecules through activation of P2 receptors. Exposure to the ultraviolet radiation A (UVA) component of sunlight causes molecular and cellular damage, and in this study, we investigated the involvement of extracellular nucleotides and P2 receptors in the UVA-induced cellular response. Human keratinocyte-derived HaCaT cells were irradiated with a single dose of UVA (2.5 J/cm²), and ATP release and interleukin (IL)-6 production were measured. ATP was released from cells in response to UVA irradiation, and the release was blocked by pretreatment with inhibitors of gap junction hemichannels or P2X7 receptor antagonist. IL-6 production was increased after UVA irradiation, and this increase was inhibited by ecto-nucleotidase or by antagonists of P2Y11 or P2Y13 receptor. These results suggest that UVA-induced IL-6 production is mediated by release of ATP through hemichannels and P2X7 receptor, followed by activation of P2Y11 and P2Y13 receptors. Interestingly, P2Y11 and P2Y13 were associated with the same pattern of IL-6 production, though they trigger different intracellular signaling cascades: Ca²⁺-dependent and PI3K-dependent, respectively. Thus, IL-6 production in response to UVA-induced ATP release involves at least two distinct pathways, mediated by activation of P2Y11 and P2Y13 receptors.

Combined inhibition of PI3Kβ and PI3Kγ reduces fat mass by enhancing α-MSH-dependent sympathetic drive

Obesity is defined as an abnormal increase in white adipose tissue and has become a major medical burden worldwide. Signals from the brain control not only appetite but also energy expenditure, both of which contribute to body weight. We showed that genetic or pharmacological inhibition of two phosphatidylinositol 3-kinases (PI3Kβ and PI3Kγ) in mice reduced fat mass by promoting increased energy expenditure. This effect was accompanied by stimulation of lipolysis and the acquisition of the energy-burning characteristics of brown adipocytes by white adipocytes, a process referred to as "browning." The browning of the white adipocytes involved increased norepinephrine release from the sympathetic nervous system. We found that PI3Kβ and PI3Kγ together promoted a negative feedback loop downstream of the melanocortin 4 receptor in the central nervous system, which controls appetite and energy expenditure in the periphery. Analysis of mice with drug-induced sympathetic denervation suggested that these kinases controlled the sympathetic drive in the brain. Administration of inhibitors of both PI3Kβ and PI3Kγ to mice by intracerebroventricular delivery induced a 10% reduction in fat mass as quickly as 10 days. These results suggest that combined inhibition of PI3Kβ and PI3Kγ might represent a promising treatment for obesity.

Mechanisms linking obesity and cancer

The incidence of obesity in US adults has been steadily increasing over the past few decades. Many comorbidities associated with obesity have been well-established such as type 2 diabetes and cardiovascular diseases. However, more recently an epidemiological relationship between obesity and the prevalence of a variety of cancers has also been uncovered. The shift of the paradigm surrounding white adipose tissue function from purely an energy storage tissue, to one that has both endocrine and metabolic relevance, has led to several mechanisms implicated in how obesity drives cancer prevalence and cancer deaths. Currently, there are four categories into which these mechanisms fall - increased lipids and lipid signaling, inflammatory responses, insulin resistance, and adipokines. In this review, we examine each of these categories and the mechanisms through which they drive cancer pathogenesis. Understanding the relationship(s) between obesity and cancer and especially the nodal points of control in these cascades will be essential in developing effective therapeutics or interventions for combating this deadly combination. This article is part of a Special Issue entitled Lipid Metabolism in Cancer.

Functional interactions between the oxytocin receptor and the β2-adrenergic receptor: implications for ERK1/2 activation in human myometrial cells

The Gq-coupled oxytocin receptor (OTR) and the Gs-coupled β(2)-adrenergic receptor (β(2)AR) are both expressed in myometrial cells and mediate uterine contraction and relaxation, respectively. The two receptors represent important pharmacological targets as OTR antagonists and β(2)AR agonists are used to control pre-term uterine contractions. Despite their physiologically antagonistic effects, both receptors activate the MAP kinases ERK1/2, which has been implicated in uterine contraction and the onset of labor. To determine the signalling pathways involved in mediating the ERK1/2 response, we assessed the effect of blockers of specific G protein-associated pathways. In human myometrial hTERT-C3 cells, inhibition of Gαi as well as inhibition of the Gαq/PKC pathway led to a reduction of both OTR- and β(2)AR-mediated ERK1/2 activation. The involvement of Gαq/PKC in β(2)AR-mediated ERK1/2 induction was unexpected. To test whether the emergence of this novel signalling mechanism was dependent on OTR expression in the same cell, we conducted experiments in HEK 293 cells that were transfected with the β(2)AR alone or co-transfected with the OTR. Using this approach, we found that β(2)AR-mediated ERK1/2 responses became sensitive to PKC inhibition only in cells co-transfected with the OTR. Inhibitor studies indicated the involvement of an atypical PKC isoform in this process. We confirmed the specific involvement of PKCζ in this pathway by assessing PKCζ translocation to the cell membrane. Consistent with our inhibitor studies, we found that β(2)AR-mediated PKCζ translocation was dependent on co-expression of OTR. The present demonstration of a novel β(2)AR-coupled signalling pathway that is dependent on OTR co-expression is suggestive of a molecular interaction between the two receptors.

Lysophosphatidic acid-induced p21Waf1 expression mediates the cytostatic response of breast and ovarian cancer cells to TGFβ

Lysophosphatidic acid (LPA) is a multifunctional intercellular phospholipid mediator present in blood and other biological fluids. In cancer cells, LPA stimulates expression or activity of inflammatory cytokines, angiogenic factors, matrix metalloproteinases, and other oncogenic proteins. In this study, we showed that LPA upregulated expression of the cyclin-dependent kinase inhibitor p21(Waf1) in TGFβ-sensitive breast and ovarian cancer cells, but not in TGFβ-resistant ones. We examined the possibility that LPA-induced p21 might contribute to the cytostatic response to TGFβ. In serum-free conditions, TGFβ alone induced p21 expression weakly in TGFβ-sensitive cells. Serum or serum-borne LPA cooperated with TGFβ to elicit the maximal p21 induction. LPA stimulated p21 via LPA(1) and LPA(2) receptors and Erk-dependent activation of the CCAAT/enhancer binding protein beta transcription factor independent of p53. Loss or gain of p21 expression led to a shift between TGFβ-sensitive and -resistant phenotypes in breast and ovarian cancer cells, indicating that p21 is a key determinant of the growth inhibitory activity of TGFβ. Our results reveal a novel cross-talk between LPA and TGFβ that underlies TGFβ-sensitive and -resistant phenotypes of breast and ovarian cancer cells.

Differential regulation of protrusion and polarity by PI3K during neutrophil motility in live zebrafish

Cell polarity is crucial for directed migration. Here we show that phosphoinositide 3-kinase (PI(3)K) mediates neutrophil migration in vivo by differentially regulating cell protrusion and polarity. The dynamics of PI(3)K products PI(3,4,5)P(3)-PI(3,4)P(2) during neutrophil migration were visualized in living zebrafish, revealing that PI(3)K activation at the leading edge is critical for neutrophil motility in intact tissues. A genetically encoded photoactivatable Rac was used to demonstrate that localized activation of Rac is sufficient to direct migration with precise temporal and spatial control in vivo. Similar stimulation of PI(3)K-inhibited cells did not direct migration. Localized Rac activation rescued membrane protrusion but not anteroposterior polarization of F-actin dynamics of PI(3)K-inhibited cells. Uncoupling Rac-mediated protrusion and polarization suggests a paradigm of two-tiered PI(3)K-mediated regulation of cell motility. This work provides new insight into how cell signaling at the front and back of the cell is coordinated during polarized cell migration in intact tissues within a multicellular organism.

Phosphoinositide 3-kinase Akt signaling pathway interacts with protein kinase Cbeta2 in the regulation of physiologic developmental hypertrophy and heart function

The phosphoinositide 3-kinase (PI3-kinase)-protein kinase B (Akt) signaling pathway is essential in the induction of physiological cardiac hypertrophy. In contrast, protein kinase C beta2 (PKCbeta2) is implicated in the development of pathological cardiac hypertrophy and heart failure. Thus far, no clear association has been demonstrated between these two pathways. In this study, we examined the potential interaction between the PI3-kinase and PKCbeta2 pathways by crossing transgenic mice with cardiac specific expression of PKCbeta2, constitutively active (ca) PI3-kinase, and dominant-negative (dn) PI3-kinase. In caPI3-kinase/PKCbeta2 and dnPI3-kinase/PKCbeta2 double-transgenic mice, the heart weight-to-body weight ratios and cardiomyocyte sizes were similar to those observed in caPI3-kinase and dnPI3-kinase transgenic mice, respectively, suggesting that the regulation of physiological developmental hypertrophy via modulation of cardiomyocyte size proceeds through the PI3-kinase pathway. In addition, we observed that caPI3-kinase/PKCbeta2 mice showed improved cardiac function while the function of dnPI3-kinase/PKCbeta2 mice was similar to that of the PKCbeta2 group. PKCbeta2 protein levels in both dnPI3-kinase/PKCbeta2 and PKCbeta2 mice were significantly upregulated. Interestingly, however, PKCbeta2 protein expression was significantly attenuated in caPI3-kinase/PKCbeta2 mice. PI3-kinase activity measured by Akt phosphorylation was not affected by PKCbeta2 overexpression. These data suggest a potential interaction between these two pathways in the heart, where PI3-kinase is predominantly responsible for the regulation of physiological developmental hypertrophy and may act as an upstream modulator of PKCbeta2 with the potential for rescuing the pathological cardiac dysfunction induced by overexpression of PKCbeta.

PSM/SH2-B distributes selected mitogenic receptor signals to distinct components in the PI3-kinase and MAP kinase signaling pathways

The Pro-rich, PH, and SH2 domain containing mitogenic signaling adapter PSM/SH2-B has been implicated as a cellular partner of various mitogenic receptor tyrosine kinases and related signaling mechanisms. Here, we report in a direct comparison of three peptide hormones, that PSM participates in the assembly of distinct mitogenic signaling complexes in response to insulin or IGF-I when compared to PDGF in cultured normal fibroblasts. The complex formed in response to insulin or IGF-I involves the respective peptide hormone receptor and presumably the established components leading to MAP kinase activation. However, our data suggest an alternative link from the PDGF receptor via PSM directly to MEK1/2 and consequently also to p44/42 activation, possibly through a scaffold protein. At least two PSM domains participate, the SH2 domain anticipated to link PSM to the respective receptor and the Pro-rich region in an association with an unidentified downstream component resulting in direct MEK1/2 and p44/42 regulation. The PDGF receptor signaling complex formed in response to PDGF involves PI 3-kinase in addition to the same components and interactions as described for insulin or IGF-I. PSM associates with PI 3-kinase via p85 and in addition the PSM PH domain participates in the regulation of PI 3-kinase activity, presumably through membrane interaction. In contrast, the PSM Pro-rich region appears to participate only in the MAP kinase signal. Both pathways contribute to the mitogenic response as shown by cell proliferation, survival, and focus formation. PSM regulates p38 MAP kinase activity in a pathway unrelated to the mitogenic response.

Connexin hemichannels and gap junction channels are differentially influenced by lipopolysaccharide and basic fibroblast growth factor

Gap junction (GJ) channels are formed by two hemichannels (connexons), each contributed by the cells taking part in this direct cell-cell communication conduit. Hemichannels that do not interact with their counterparts on neighboring cells feature as a release pathway for small paracrine messengers such as nucleotides, glutamate, and prostaglandins. Connexins are phosphorylated by various kinases, and we compared the effect of various kinase-activating stimuli on GJ channels and hemichannels. Using peptides identical to a short connexin (Cx) amino acid sequence to specifically block hemichannels, we found that protein kinase C, Src, and lysophosphatidic acid (LPA) inhibited GJs and hemichannel-mediated ATP release in Cx43-expressing C6 glioma cells (C6-Cx43). Lipopolysaccharide (LPS) and basic fibroblast growth factor (bFGF) inhibited GJs, but they stimulated ATP release via hemichannels in C6-Cx43. LPS and bFGF inhibited hemichannel-mediated ATP release in HeLa-Cx43 cells, but they stimulated it in HeLa-Cx43 with a truncated carboxy-terminal (CT) domain or in HeLa-Cx26, which has a very short CT. Hemichannel potentiation by LPS was inhibited by blockers of the arachidonic acid metabolism, and arachidonic acid had a potentiating effect like LPS and bFGF. We conclude that GJ channels and hemichannels display similar or oppositely directed responses to modulatory influences, depending on the balance between kinase activity and the activity of the arachidonic acid pathway. Distinctive hemichannel responses to pathological stimulation with LPS or bFGF may serve to optimize the cell response, directed at strictly controlling cellular ATP release, switching from direct GJ communication to indirect paracrine signaling, or maximizing cell-protective strategies.

How important is protein kinase C in μ-opioid receptor desensitization and morphine tolerance?

The repeated administration of opiate drugs such as morphine results in the development of tolerance to their analgesic, rewarding (euphoric) and respiratory-depressant effects; thus, to obtain the same level of response with subsequent administrations, a greater dose must be used. Tolerance can limit the clinical efficacy of opiate drugs and enhance the social problems that are inherent in recreational opioid abuse. Surprisingly, the mechanism (or mechanisms) underlying the development of morphine tolerance remains controversial. Here, we propose that protein kinase C could have a crucial role in the desensitization of mu-opioid receptors by morphine and that this cellular process could contribute to the development and maintenance of morphine tolerance in vivo.

Mechanisms Linking Adenosine A1 Receptors and Extracellular Signal-Regulated Kinase 1/2 Activation in Human Trabecular Meshwork Cells

This study was designed to evaluate the signaling pathways coupling adenosine A1 receptors and extracellular signal-regulated kinase (ERK) 1 and 2 in human trabecular meshwork (HTM) cells. Studies were conducted using cultures of primary HTM cells and the HTM-3 cell line. Activation of ERK1/2, location of protein kinase C (PKC) isoforms, and matrix metalloproteinase (MMP) secretion were determined by Western blotting. In primary HTM cells and the HTM-3 cell line, administration of the A1 agonist N6-cyclohexyladenosine (CHA) produced a concentration-dependent increase in ERK1/2 activation. This CHA-induced ERK activation was blocked by pretreatment with the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine or pertussis toxin. Transfection with dominant negative N17 Ras produced only a small (31%) decline in CHA-induced ERK activation, and the response was not altered by pretreatment with the Src tyrosine kinase inhibitor, PP2 [3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-D] pyrimidin-4-amine], the phosphoinositide kinase-3 inhibitor, LY-294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one], or the A3 receptor antagonist, MRS-1191 [3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate]. Administration of CHA also induced the translocation of PKCalpha from the cytosol to the membrane, and pretreatment with the phospholipase C (PLC) inhibitor, U73122 [1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]-hexyl]-1H-pyrrole-2,5-dione], blocked ERK1/2 activation induced by CHA. Transfection of short interfering RNA targeting PKCalpha blocked the CHA-induced ERK1/2 activation and the secretion of MMP-2. These results confirm the existence of functional adenosine A1 receptors in the trabecular meshwork cells. These receptors are coupled to the activation of ERK1/2 through G(i/o) proteins and dependent upon the upstream activation of PLC and PKCalpha. These studies provide evidence that adenosine A1 receptor agonists increase outflow facility through sequential activation of G(i/o) > PLC > PKCalpha > c-Raf > mitogen-activated protein kinase kinase > ERK1/2, leading to secretion of MMP-2.

RhoA-GTPase facilitates entry of Kaposi's sarcoma-associated herpesvirus into adherent target cells in a Src-dependent manner

Kaposi's sarcoma-associated herpesvirus (KSHV) (human herpesvirus 8) binds to adherent target cell surface heparan sulfate molecules via its envelope glycoproteins gB and gpK8.1A, to integrins via gB, to the transporter CD98/xCT complex, and possibly to another molecule(s). This is followed by virus entry overlapping with the induction of preexisting host cell signal pathways, such as focal adhesion kinase, Src, phosphatidylinositol 3-kinase (PI3-K), Rho-GTPases, protein kinase C-zeta, and extracellular signal-regulated kinase 1/2. Here, using hemagglutinin-tagged plasmids expressing wild-type, dominant-positive, and dominant-negative forms of RhoA in HEK (human embryonic kidney) 293 cells, we investigated the role of RhoA-GTPase in virus entry. The dominant-negative form of RhoA GTPase and treatment of target cells with Clostridium difficile toxin B (CdTxB), a specific inactivator of Rho-GTPases, significantly blocked KSHV entry. KSHV infection induced closely similar levels of FAK and PI3-K in all three cell types. In contrast, very strong Src activation was observed in KSHV-infected dominant-positive RhoA cells compared to wild-type cells, and only moderate Src activation was seen in dominant-negative cells. Inhibition of Src activation by CdTxB and reduction of RhoA activation by Src inhibitors suggest that KSHV-induced Src is involved in RhoA activation, which in turn is involved in a feedback-sustained activation of Src. Since the decreased entry in RhoA dominant-negative cells may be due to inefficient signaling downstream of RhoA, we examined the induction of RhoA-activated Dia-2, which is also known to induce Src. Dia-2 coimmunoprecipitated with activated Src, which was inhibited by Src inhibitors, in the infected cells. Together with the reduced virus entry in RhoA dominant-negative cells, these results suggest that activated RhoA-dependent Dia-2 probably functions as a link between RhoA and Src in KSHV-infected cells, mediating the sustained Src activation, and that KSHV-induced Src and RhoA play roles in facilitating entry into adherent target cells.

Requirement of androgen-dependent activation of protein kinase Czeta for androgen-dependent cell proliferation in LNCaP Cells and its roles in transition to androgen-independent cells

A cell line that we designed, AILNCaP, proliferated in androgen-depleted medium after emerging from long-term androgen-depleted cultures of an androgen-sensitive prostate cancer cell line, LNCaP. Using this cell line as a model of progression to androgen independence, we demonstrated that the activity of the mammalian target of rapamycin/p70 S6 kinase transduction pathway is down-regulated after androgen depletion in LNCaP, whereas its activation is related to transition of this cell line to androgen-independent proliferation. Kinase activity of protein kinase Czeta is regulated by androgen stimulation in LNCaP cells, whereas it is activated constitutively in AILNCaP cells under androgen-depleted conditions. Treatment with a protein kinase Czeta pseudosubstrate inhibitor reduced p70 S6 kinase activity and cell proliferation in both cell lines. We identified that both protein kinase Czeta and p70 S6 kinase were associated in LNCaP cells and this association was enhanced by the androgen stimulation. We examined the expression of phospho-protein kinase Czeta and phospho-p70 S6 kinase in hormone-naive prostate cancer specimens and found that the expression of both kinases was correlated with each other in those specimens. Significant correlation was observed between the expression of both kinases and Ki67 expression. Most of the prostate cancer cells that survived after prior hormonal treatment also expressed both kinases. This is the first report that shows the significance of this pathway for both androgen-dependent and -independent cell proliferation in prostate cancer. Our data suggest that protein kinase Czeta/mammalian target of rapamycin/S6 kinase pathway plays an important role for the transition of androgen-dependent to androgen-independent prostate cancer cells.

Protein kinase C zeta plays an essential role for Mycobacterium tuberculosis-induced extracellular signal-regulated kinase 1/2 activation in monocytes/macrophages via Toll-like receptor 2

This study characterized the upstream signalling molecules involved in extracellular signal-regulated kinase (ERK) 1/2 activation and determined their effects on differential tumour necrosis factor (TNF)-alpha expression by monocytes/macrophages infected with virulent or avirulent mycobacteria. The avirulent Mycobacterium tuberculosis (MTB) strain H37Ra (MTBRa) induced higher levels of activation of ERK 1/2 and the upstream MAPK kinase (MEK)1 and, subsequently, higher levels of TNF-alpha expression in human primary monocytes and monocyte-derived macrophages, as compared with MTB strain H37Rv (MTBRv). The MTB-induced activation of ERK 1/2 was not dependent on Ras or Raf. However, inhibition of the activity of atypical protein kinase C (PKC) zeta decreased the in vitro phosphorylation of MEK, ERK 1/2 activation and subsequent TNF-alpha induction caused by MTBRv or MTBRa. Toll-like receptor (TLR) 2 was found to play a major role in MTB-induced TNF-alpha expression and PKCzeta phosphorylation. Co-immunoprecipitation experiments showed that PKCzeta interacts physically with TLR2 after MTB stimulation. Moreover, PKCzeta phosphorylation was increased more in macrophages following MTBRa, versus MTBRv, infection. This is the first demonstration that PKCzeta interacts with TLR2 to play an essential role in MTB-induced ERK 1/2 activation and subsequent TNF-alpha expression in monocytes/macrophages.

Mechanism of polarized protrusion formation on neuronal precursors migrating in the developing chicken cerebellum

Directed cell migration results from the polarization of the cellular motile apparatus by integration of extracellular signals, which are presented in a three-dimensional, spatiotemporal manner in living organisms. To investigate the mechanism underlying the highly polarized and directional nature of migration in vivo, we have developed an imaging system for observing rhombic lip cell migration in the developing chicken cerebellum. First, we show that Cdc42 is the central regulator of the overall polarity, morphology and protrusion formation in these cells. However, perturbation of canonical polarity effectors of Cdc42, e.g. the Par6-Par3-aPKC complex, does not disrupt the cell asymmetry, whereas it affects orientation of the tip of the leading process. In contrast to Cdc42, Rac is required for the generation of protrusions but not the overall polarity. Function interference of class IA phosphoinositide 3-kinase abrogates both directional extension and maintenance of the long leading process, whereas PTEN modulates the size of the protrusion. Actomyosin contractility is important for coordinated spreading of the tip of the leading process in situ. Finally, ErbB4 functions in the generation of protrusions on the rhombic lip cells. These results suggest that polarized protrusion formation on neuronal precursors may occur by a more divergent and complex mechanism than that seen in studies of other cell types growing on planar substrates.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

Atypical PKCzeta is involved in RhoA-dependent mitogenic signaling by the P2Y12 receptor in C6 cells

When nucleotide hydrolysis is prevented, agonists of the P2Y(12) receptor enhance the proliferation of C6 glioma cells by RhoA-dependent, protein kinase C (PKC)-dependent activation of the extracellular signal-regulated kinase (ERK) pathway [Claes P, Grobben B, Van Kolen K, Roymans D & Slegers H (2001) Br J Pharmacol134, 402-408; Grobben B, Claes P, Van Kolen K, Roymans D, Fransen P, Sys SU & Slegers H (2001) J Neurochem78, 1325-1338]. In this study, we show that ERK1/2 phosphorylation was not affected by transfection of the cells with the Gbetagamma-subunit-scavenging adrenergic receptor kinase peptide [betaARK1-(495-689)] or with Rap1GAPII, indicating that P2Y(12) receptor stimulation enhances ERK1/2 phosphorylation by G(i)alpha subunit-mediated signaling independently of Rap1 activation. Inhibition of the RhoA downstream effector Rho-associated coiled-coil-containing kinase (ROCK) with Y-27632 did not affect the P2Y(12) receptor-induced increase in ERK1/2 phosphorylation but abrogated the mitogenic response. Involvement of growth factor receptor transactivation in the signaling towards ERK phosphorylation could be ruled out by the lack of an effect of PP2, AG1024, AG1296 or SU1498, inhibitors of Src, insulin-like growth factor receptor, platelet-derived growth factor receptor and vascular endothelial growth factor receptor kinase activity, respectively. Experiments with bisindolylmaleimide I and IX indicated the requirement of PKC activity. Classical and novel PKC isoforms could be excluded by treatment of the cells with Gö6976 and calphostin C, whereas addition of a myristoylated PKCzeta pseudosubstrate inhibitor completely abolished P2Y(12) receptor-induced ERK1/2 activation. Moreover, coimmunoprecipitation experiments revealed PKCzeta/Raf1 and PKCzeta/ERK association, indicating the involvement of PKCzeta. From the data presented, we can conclude that the P2Y(12) receptor enhances cell proliferation by a G(i)alpha-dependent, RhoA-dependent PKCzeta/Raf1/MEK/ERK pathway that requires activation of ROCK, which is not involved in ERK1/2 signaling.

The anti-apoptotic effect of leukotriene B4 in neutrophils: A role for phosphatidylinositol 3-kinase, extracellular signal-regulated kinase and Mcl-1

The constitutive commitment of neutrophils to apoptosis is a key process for the control and resolution of inflammation and it can be delayed by various inflammatory mediators including leukotriene B4 (LTB4). The mechanisms by which LTB4 contributes to neutrophil survival are still unclear and the present work aims at identifying intracellular pathways underlying this effect. Inhibition of human neutrophil apoptosis by LTB4 was abrogated by the phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin and by the specific MEK inhibitor PD98059. In contrast, inhibitors of p38 MAPK, Jak2/3 and Src did not hinder the anti-apoptotic effect of LTB4. We also investigated the effects of members of the Bcl-2 family as they play a crucial role in the regulation of programmed cell death. When neutrophils were incubated with LTB4 for 1 to 6 h, the mRNA levels of the anti-apoptotic protein Mcl-1 were upregulated approximately 2-fold, while those of the pro-apoptotic protein Bax were downregulated 3- to 4-fold, as determined by real-time PCR. Accordingly, Western blot analysis revealed that the expression of Mcl-1 was upregulated in presence of LTB4, while flow cytometric analysis revealed that Bax protein was downregulated. Furthermore, the modulatory effects of LTB4 on Mcl-1 and Bax proteins were abolished in the presence of either wortmannin or PD98059. Taken together, these results demonstrate the participation of PI3-K and MEK/ERK kinases, as well as regulatory apoptotic proteins such as Mcl-1 and Bax, in the anti-apoptotic effects of LTB4 in human neutrophils.

Reelin induces the detachment of postnatal subventricular zone cells and the expression of the Egr-1 through Erk1/2 activation

Reelin binds to very low-density lipoprotein receptor and apolipoprotein E receptor 2, thereby inducing mDab1 phosphorylation and activation of the phosphatidylinositide 3 kinase (PI3K) pathway. Here we demonstrate that Reelin activates the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) pathway, which leads to the phosphorylation of Erk1/2 proteins. The inhibition of Src family kinases (SFK) blocked Reelin-dependent Erk1/2 activation. This was also shown in neuronal cultures from mDab1-deficient mice. Although rat sarcoma viral oncogene was weakly activated upon Reelin treatment, pharmacological inhibition of the PI3K pathway blocked Reelin-dependent ERK activation, which indicates cross talk between the ERK and PI3K pathways. We show that blockade of the ERK pathway does not prevent the chain migration of neurons from the subventricular zone (SVZ) but does inhibit the Reelin-dependent detachment of migrating neurons. We also show that Reelin induces the transcription of the early growth response 1 transcription factor. Our findings demonstrate that Reelin triggers ERK signaling in an SFK/mDab1- and PI3K-dependent manner and that ERK activation is required for Reelin-dependent transcriptional activation and the detachment of neurons migrating from the SVZ.

Phosphorylation of adaptor protein-2 mu2 is essential for Na+,K+-ATPase endocytosis in response to either G protein-coupled receptor or reactive oxygen species

Activation of G protein-coupled receptor by dopamine and hypoxia-generated reactive oxygen species promote Na+,K+-ATPase endocytosis. This effect is clathrin dependent and involves the activation of protein kinase C (PKC)-zeta and phosphorylation of the Na+,K+-ATPase alpha-subunit. Because the incorporation of cargo into clathrin vesicles requires association with adaptor proteins, we studied whether phosphorylation of adaptor protein (AP)-2 plays a role in its binding to the Na+,K+-ATPase alpha-subunit and thereby in its endocytosis. Dopamine induces a time-dependent phosphorylation of the AP-2 mu2 subunit. Using specific inhibitors and dominant-negative mutants, we establish that this effect was mediated by activation of the adaptor associated kinase 1/PKC-zeta isoform. Expression of the AP-2 mu2 bearing a mutation in its phosphorylation site (T156A) prevented Na+,K+-ATPase endocytosis and changes in activity induced by dopamine. Similarly, in lung alveolar epithelial cells, hypoxia-induced endocytosis of Na+,K+-ATPase requires the binding of AP-2 to the tyrosine-based motif (Tyr-537) located in the Na+,K+-ATPase alpha-subunit, and this effect requires phosphorylation of the AP-2 mu2 subunit. We conclude that phosphorylation of AP-2 mu2 subunit is essential for Na+,K+-ATPase endocytosis in response to a variety of signals, such as dopamine or reactive oxygen species.

Tailoring Ras-pathway--inhibitor combinations for cancer therapy

Constitutive activation of Ras pathways plays a critical role in cancer development and maintenance. Inhibitors of such pathways are already in use for cancer therapy, with significant but as yet only partial success in the most deadly types of human cancers, against which even combinations of Ras-pathway inhibitors with classic cytotoxic drugs or irradiation are insufficient. Combinations of farnesyl transferase inhibitors (FTI's), inhibitors of Ras pathways, are now in use in clinical trials. In this review we analyze possible reasons for the limited efficacy--including the diverse and sometimes even contradictory effects of active Ras pathways in tumor cells--and propose possible alternative methods of tailoring Ras-pathway inhibitor combinations for cancer therapy. Such tailoring is now possible thanks to increased knowledge of the complexity of Ras pathways, their cooperation with other oncogenic pathways, and their "addictive" nature. We provide examples demonstrating that this knowledge can be translated into useful drug combinations that disrupt multiple oncogenic pathways and hit a weak point of a given tumor cell. One such example is combination treatment with a Ras inhibitor and a glycolysis blocker for pancreatic tumor cells. The future design of such potential drug combination therapies and the follow-up of their outcome will undoubtedly be facilitated by gene-expression profiling and proteomic methods.

Class II phosphoinositide 3-kinase defines a novel signaling pathway in cell migration

The lipid products of phosphoinositide 3-kinase (PI3K) are involved in many cellular responses such as proliferation, migration, and survival. Disregulation of PI3K-activated pathways is implicated in different diseases including cancer and diabetes. Among the three classes of PI3Ks, class I is the best characterized, whereas class II has received increasing attention only recently and the precise role of these isoforms is unclear. Similarly, the role of phosphatidylinositol-3-phosphate (PtdIns-3-P) as an intracellular second messenger is only just beginning to be appreciated. Here, we show that lysophosphatidic acid (LPA) stimulates the production of PtdIns-3-P through activation of a class II PI3K (PI3K-C2β). Both PtdIns-3-P and PI3K-C2β are involved in LPA-mediated cell migration. This study is the first identification of PtdIns-3-P and PI3K-C2β as downstream effectors in LPA signaling and demonstration of an intracellular role for a class II PI3K. Defining this novel PI3K-C2β–PtdIns-3-P signaling pathway may help clarify the process of cell migration and may shed new light on PI3K-mediated intracellular events.

Role of Ras/PKCζ/MEK/ERK1/2 signaling pathway in angiotensin II-induced vascular smooth muscle cell proliferation

The role of protein kinase C (PKC) and its cross talk with extracellular signal-regulated kinase (ERK) cascade in angiotensin II (AngII)-elicited vascular smooth muscle cell (VSMC) proliferation are still unclear. In this study, the PKC pathway of AngII to activate ERK1/2 and induce cell proliferation was investigated in rat aortic smooth muscle cells. The proliferation of VSMCs was tested by [3H]-thymidine incorporation assay. Phosphorylated and non-phosphorylated PKCzeta, ERK1/2, Elk-1, and mitogen-activated ERK-activating kinase (MEK) were estimated by Western blot analysis. The interactions of signal molecules were examined by immunoprecipitation. AngII-induced VSMC proliferation and activation of ERK1/2 and nuclear transcription factor Elk-1 were all down-regulated by PKC non-specific inhibitor (staurosporine) and PKCzeta pseudosubstrate inhibitor (PS-PKCzeta). Dominant negative Ras transfection into VSMCs decreased AngII-induced PKCzeta and ERK1/2 phosphorylation. AngII stimulated the association of PKCzeta with Ras. AngII-induced MEK phosphorylation was inhibited by PKCzeta pseudosubstrate inhibitor and the PKCzeta-MEK complex was detected by immunoprecipitation. These results suggest that PKCzeta isoform is involved in VSMC proliferation and Elk-1 activation. AngII can activate ERK1/2 by Ras/PKCzeta/MEK pathway, which may be one of the important signal transduction pathways in AngII-induced VSMC proliferation.

Functional analysis of PTPN11/SHP-2 mutants identified in Noonan syndrome and childhood leukemia

Noonan syndrome (NS) is characterized by short stature, characteristic facial features, and heart defects. Recently, missense mutations of PTPN11, the gene encoding protein tyrosine phosphatase (PTP) SHP-2, were identified in patients with NS. Further, somatic mutations in PTPN11 were detected in childhood leukemia. Recent studies showed that the phosphatase activities of five mutations identified in NS and juvenile myelomonocytic leukemia (JMML) were increased. However, the functional properties of the other mutations remain unidentified. In this study, in order to clarify the differences between the mutations identified in NS and leukemia, we examined the phosphatase activity of 14 mutants of SHP-2. We identified nine mutations, including a novel F71I mutation, in 16 of 41 NS patients and two mutations, including a novel G503V mutation, in three of 29 patients with leukemia. Immune complex phosphatase assays of individual mutants transfected in COS7 cells showed that ten mutants identified in NS and four mutants in leukemia showed 1.4-fold to 12.7-fold increased activation compared with wild-type SHP-2. These results suggest that the pathogenesis of NS and leukemia is associated with enhanced phosphatase activity of mutant SHP-2. A comparison of the phosphatase activity in each mutant and a review of previously reported cases showed that high phosphatase activity observed in mutations at codons 61, 71, 72, and 76 was significantly associated with leukemogenesis.

Elastin Peptides Activate Extracellular Signal-Regulated Kinase 1/2 via a Ras-Independent Mechanism Requiring Both p110γ/Raf-1 and Protein Kinase A/B-Raf Signaling in Human Skin Fibroblasts

Elastin peptides (EPs) produced during cancer progression bind to the elastin binding protein (EBP) found at the surface of dermal fibroblasts, leading to the expression of collagenase-1 gene. The production of this enzyme involved in stromal reaction is caused by the sustained activation of the extracellular signal-regulated kinases 1/2 (ERK1/2) pathway via cAMP/protein kinase A (PKA) and phosphatidylinositol 3-kinase (PI3K). However, the mechanism of these signaling events remains unknown. We show that kappa-elastin (kappaE), a commonly used EP, induces maximum phosphorylation of mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)1/2 and ERK1/2 after 30 min. The simultaneous inhibition of PKA and PI3K, by N-(2-(p-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide (H89) and 2-(4-morpholynil)-8-phenyl-4H-1-bemzopyran-4-one (LY294002), respectively, blocked MEK1/2 and ERK1/2 phosphorylation, as did lactose, an EBP antagonist. kappaE induced Raf-1 phosphorylation and activation in a PI3K-dependent manner. In our system, the PI3K p110gamma is expressed and activated by betagamma-derived subunits from a pertussis toxin-sensitive G protein after fibroblast stimulation. Pertussis toxin also blocks the Raf-1/MEK1/2/ERK1/2 phosphorylation cascade. In addition, we found that B-Raf is expressed in dermal fibroblasts and activated in a PKA-dependent manner after kappaE treatment, thereby integrating PKA signals to MEK1/2. It is noteworthy that Ras involvement was excluded because ERK1/2 activation by kappaE was not blocked in RasN17-transfected fibroblasts. Together, our results identify a novel Ras-independent ERK1/2 activation system in which p110gamma/Raf-1/MEK1/2 and PKA/B-Raf/MEK1/2 cooperate to activate ERK1/2. Thus, p110gamma and B-Raf seem to be important modulators of dermal fibroblasts physiology and should now qualify as therapeutic targets in strategies aiming at limiting elastin degradation contribution to cancer progression.

Erythropoietin Just Before Reperfusion Reduces Both Lethal Arrhythmias and Infarct Size via the Phosphatidylinositol-3 Kinase-Dependent Pathway in Canine Hearts

Although recent studies suggest that erythropoietin (EPO) may reduce multiple features of the myocardial ischemia/reperfusion injury, the cellular mechanisms and the clinical implications of EPO-induced cardioprotection are still unclear. Thus, in this study, we clarified dose-dependent effects of EPO administered just before reperfusion on infarct size and the incidence of ventricular fibrillation and evaluated the involvement of the phosphatidylinositol-3 (PI3) kinase in the in vivo canine model. The canine left anterior descending coronary artery was occluded for 90 min followed by 6 h of reperfusion. A single intravenous administration of EPO just before reperfusion significantly reduced infarct size (high dose (1,000 IU/kg): 7.7 +/- 1.6%, low dose (100 IU/kg): 22.1 +/- 2.4%, control: 40.0 +/- 3.6%) in a dose-dependent manner. Furthermore, the high, but not low, dose of EPO administered as a single injection significantly reduced the incidence of ventricular fibrillation during reperfusion (high dose: 0%, low dose: 40.0%, control: 50.0%). An intracoronary administration of a PI3 kinase inhibitor, wortmannin, blunted the infarct size-limiting and anti-arrhythmic effects of EPO. Low and high doses of EPO equally induced Akt phosphorylation and decreased the equivalent number of TUNEL-positive cells in the ischemic myocardium of dogs. These effects of EPO were abolished by the treatment with wortmannin. In conclusion, EPO administered just before reperfusion reduced infarct size and the incidence of ventricular fibrillation via the PI3 kinase-dependent pathway in canine hearts. EPO administration can be a realistic strategy for the treatment of acute myocardial infarction.

PKC isotypes in post-activated and fertilized mouse eggs: association with the meiotic spindle

Several isotypes of protein kinase C (PKC) have been reported to be expressed in mammalian eggs, but it is unknown whether these isotypes have a common function in the egg during or within the first few hours of fertilization. Here we show that the isotypes of PKC exhibit distinct patterns of enrichment immediately after mouse egg activation. PKCalpha and gamma accumulate in the egg cortex 25 min post-activation, while only PKCalpha accumulates at the contractile ring of the forming second polar body about 1.5 h post-activation. PKCzeta exhibits some unique features that resulted in it being the focus of more extensive analysis. PKCzeta is tightly associated with the meiotic spindle as determined by detergent extraction and is closely associated with alpha-tubulin as determined by FRET analysis in the metaphase II (MII) egg. In addition, after egg activation, PKCzeta remains associated with the spindle as it transits into anaphase II and later telophase II, becoming associated with the midzone microtubules. Antibodies to the active form of PKCzeta are enriched on the spindle poles and later in development on the midzone microtubules. Active PKCzeta also is enriched in both pronuclei in the 6-h post-fertilization and in the 14-h post-fertilization embryo as well as in the nuclei of the two-cell embryo. Inhibition of PKCzeta, but not inhibition of other isotypes of PKC, results in rapid disruption of the meiotic spindle. This study suggests that PKCzeta has a role in spindle stability, while other PKC isotypes have different roles in the conversion of the egg to the zygote.

Role of atypical protein kinase C in estradiol-triggered G1/S progression of MCF-7 cells

Expression of a dominant negative atypical protein kinase C (aPKC), PKCzeta, prevents nuclear translocation of extracellular regulated kinase 2 (ERK-2), p27 nuclear reduction, and DNA synthesis induced by estradiol in human mammary cancer-derived MCF-7 cells. aPKC action upstream of these events has been analyzed. In hormone-stimulated NIH 3T3 and Cos cells ectopically expressing human estrogen receptor alpha (hERalpha), aPKC is activated by phosphatidylinositol 3-kinase (PI 3-kinase) and, in turn, controls the Ras/MEK-1/ERK cascade. In MCF-7 and Cos cells stimulated by hormone, PI 3-kinase activates PKCzeta by Thr410 phosphorylation. Serine phosphorylation of PKCzeta is simultaneously induced. PKCzeta activation leads to recruitment of Ras to a multimolecular complex that also includes hERalpha, Src, PI 3-kinase, and aPKC. We propose that PKCzeta pushes Ras and the signaling complex close together in such a way that it facilitates the Src-dependent Ras activation. This activation is crucial for the interplay between estradiol-triggered signaling and cell cycle machinery.

Activation of insulin receptors by lagerstroemin

Lagerstroemin, an ellagitannin isolated from the leaves of Lagerstroemia speciosa (L.) Pers. (Lythraceae), was examined for its biological activities. In rat adipocytes, the compound increased the rate of glucose uptake and decreased the isoproterenol-induced glycerol release. In Chinese hamster ovary cells expressing human insulin receptors, it increased the Erk activity. These insulin-like actions were accompanied by the increased tyrosine-phosphorylation of the beta-subunit of the insulin receptors. Tryptic digestion of the extracellular sites of the insulin receptors markedly increased the effective concentrations of insulin without changing those of lagerstroemin. Thus lagerstroemin was considered to cause its insulin-like actions by a mechanism different from that employed by insulin.

Comparative Studies of Molecular Mechanisms of Dopamine D2 and D3 Receptors for the Activation of Extracellular Signal-regulated Kinase

Dopamine D(2) and D(3) receptors (D(2)R/D(3)R), which have similar structural architecture as well as functional similarities, are expressed in the same brain dopaminergic neurons. It is intriguing that two receptor proteins with virtually the same functional roles are expressed in the same neuron. Recently we have shown that D(2)R and D(3)R possess different regulatory processes including intracellular trafficking properties, which implies that they might employ different signaling mechanisms for regulation of the same cellular processes. Here we studied the signaling pathways of ERK activation mediated by D(2)R and D(3)R in HEK-293 cells and corroborated them with concomitant studies in COS-7 cells and C6 cells. Our results show that Src, phosphatidylinositol 3-kinase, and atypical protein kinase C were commonly involved in D(2)R-/D(3)R-mediated ERK activation. However, beta-arrestin and sequestration of D(2)R/D(3)R were found not to be involved. ERK activations mediated by D(3)R, but not D(2)R, were blocked by betaARK-CT, AG1478 epidermal growth factor receptor (EGFR) inhibitor, and by dominant negative mutants of Ras and Raf, suggesting the involvement of the Gbetagamma(i) pathway. The alpha-subunit of G(o) (Galpha(o)) was able to couple with D(3)R to mediate ERK activation. We conclude that D(3)R mainly utilizes the betagamma pathway of G(i) protein, which involves the transactivation of EGFR in HEK-293 cells. In contrast, the alpha-subunit of the G(i) protein plays a main role in D(2)R-mediated ERK activation. Our study suggests one example of intricate cellular regulations in the brain, that is, dopaminergic neurons could regulate ERK activity more flexibly through alternative usage of either the D(2)R or D(3)R pathway depending on the cellular situation.

Novel p21-activated kinase-dependent protrusions characteristically formed at the edge of transformed cells

During long-term culture, certain lines become neoplastic while accumulating changes in cell shape. Early and late cell populations have characteristic shape phenotypes that have been quantified by computerized assay. Phenotypes are determined from variables describing three-dimensional aspects of the subcellular distribution of mass. The features of cells can be recognized by use of latent factors, which are theoretical variables based on the covariance of the primary variables. Factor #7 represented a cell edge feature different from filopodia. We studied the morphological characteristics and morphogenesis of the feature. Brief exposure of cells from rat tracheal epithelium to phorbol 12-myristate 13-acetate (PMA) enhanced #7 values. The time to reach maximal #7 values was prolonged if PMA was administered with calcium ionophore or lysophosphatidic acid (LPA). Factor #7 was elevated during periods of ruffling suppression and stress fiber reorganization. Cells showing high #7 values were examined by scanning electron microscopy (SEM) and found to exhibit strap-shaped and cupola-shaped projections. Because RhoA regulates stress fiber formation, we sought to perturb #7 features by introducing dominant-acting negative and positive constructs of RhoA, RhoA-N19, and RhoA-V14. Neither affected #7 values. Although overexpression of the kinase inhibitory domain of p21-activated kinase 1 (PAK) had no effect on #7 values, they were affected by overexpression of a domain binding PAK-interacting guanine nucleotide exchange factor (PIX). Because a PAK-PIX complex is implicated in the remodeling of focal complexes (FCs) and recycling of PAK to the cytoplasm, the results implicate a component of FCs in the formation of #7 features. The data suggested that feature formation is driven by activated Cdc42-binding kinase (ACK) and Rac. Moreover, they suggested that the #7 protrusions are neurite-like structures and that their development involves FC regulation.

Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 envelope glycoprotein gB induces the integrin-dependent focal adhesion kinase-Src-phosphatidylinositol 3-kinase-rho GTPase signal pathways and cytoskeletal rearrangements

Human herpesvirus 8 (HHV-8; Kaposi's sarcoma-associated herpesvirus) envelope glycoprotein gB possesses an RGD motif, interacts with alpha 3 beta 1 integrin, and uses it as one of the entry receptors. HHV-8 induces the integrin-dependent focal adhesion kinase (FAK), a critical step in the outside-in signaling pathways necessary for the subsequent phosphorylation of other cellular kinases, cytoskeletal rearrangements, and other functions. As an initial step toward deciphering the role of HHV-8 gB-integrin interaction in infection, signal pathways induced by gB were examined. A truncated form of gB without the transmembrane and carboxyl domains (gB Delta TM), a gB Delta TM mutant form (gB Delta TM-RGA) with an RGD-to-RGA mutation, and inhibitors of cellular kinases were used. HHV-8 gB Delta TM, but not gB Delta TM-RGA, induced FAK phosphorylation in target cells, which was in part dependent on the presence of alpha 3 beta 1 integrin. FAK was critical for the subsequent phosphorylation of Src by gB Delta TM, and Src induction was essential for the phosphorylation of phosphatidylinositol 3-kinase (PI-3K). HHV-8 gB Delta TM-induced PI-3K was essential for the induction of RhoA and Cdc42 Rho GTPases that was accompanied by the cytoskeletal rearrangements. These gB-induced morphological changes were inhibited by the PI-3K inhibitors. Ezrin, one of the essential elements required to cross-link the actin cytoskeleton with the plasma membrane and to induce the morphological changes, was induced by the Rho GTPases. Inhibition of cellular tyrosine kinases by the brief treatment of cells with 4',5,7-trihydroxyisoflavone (genistein) blocked the entry of HHV-8 into target cells. These findings suggest that, independently of other viral glycoproteins and via its RGD motif, HHV-8 gB induces integrin-dependent pre-existing FAK-Src-PI-3K-Rho GTPase kinases. Since these signal pathways play vital roles in host cell endocytosis and movement of particulate materials in the cytoplasm, the early stages of HHV-8 gB interaction with host cells may provide a very conducive environment for the successful infection of target cells.

Contribution of PKC-dependent and -independent processes in temporal ERK regulation by ET-1, PDGF, and EGF in rat myometrial cells

Endothelin-1 (ET-1), platelet-derived growth factor (PDGF), and epidermal growth factor (EGF) stimulated thymidine incorporation with different efficiency (PDGF ≫ EGF = ET-1) in rat myometrial cells. They also stimulated ERK activation, which culminated at 5 min and then declined to reach a plateau (at 45 min: EGF > 90%, PDGF = 50%, and ET-1 < 10% of maximum). Inhibition and downregulation of PKC demonstrated that ERK activation at 5 min involved PKCδ and -ζ for ET-1 and PKCα plus another PKC isoform for PDGF. By contrast, the EGF response did not involve PKC. Stimulation of Ras was more important with EGF than with PDGF, with ET-1 being the weakest activator. The simultaneous incubation of the cells with EGF and ET-1 potentiated the ERK activation at 5 min and mimicked the plateau phase obtained with PDGF. Under these conditions thymidine incorporation was comparable to that induced by PDGF. Taken together, our results indicated that the kinetic profile of ERK activation and its impact on cell proliferation can be modulated by the differential involvement of PKC isoforms and the amplitude of Ras activation.

Characterization of ERK1/2 signalling pathways induced by adenosine receptor subtypes in newborn rat cardiomyocytes

1. Adenosine A(1), A(2A), and A(3) receptors (ARs) and extracellular signal-regulated kinase 1/2 (ERK1/2) play a major role in myocardium protection from ischaemic injury. In this study, we have characterized the adenosine receptor subtypes involved in ERK1/2 activation in newborn rat cardiomyocytes. 2. Adenosine (nonselective agonist), CPA (A(1)), CGS 21680 (A(2A)) or Cl-IB-MECA (A(3)), all increased ERK1/2 phosphorylation in a time- and dose-dependent manner. The combined maximal response of the selective agonists was similar to adenosine alone. Theophylline (nonselective antagonist) inhibited completely adenosine-mediated ERK1/2 activation, whereas a partial inhibition was obtained with DPCPX (A(1)), ZM 241385 (A(2A)), and MRS 1220 (A(3)). 3. PD 98059 (MEK1; ERK kinase inhibitor) abolished all agonist-mediated ERK1/2 phosphorylation. Pertussis toxin (PTX, G(i/o) blocker) inhibited completely CPA- and partially adenosine- and Cl-IB-MECA-induced ERK1/2 activation. Genistein (tyrosine kinase inhibitor) and Ro 318220 (protein kinase C, PKC inhibitor) partially reduced adenosine, CPA and Cl-IB-MECA responses, without any effect on CGS 21680-induced ERK1/2 phosphorylation. H89 (protein kinase A, PKA inhibitor) abolished completely CGS 21680 and partially adenosine and Cl-IB-MECA responses, without any effect on CPA response. 4. Cl-IB-MECA-mediated increases in cAMP accumulation suggest that A(3)AR-induced ERK1/2 phosphorylation involves adenylyl cyclase activation via phospholipase C (PLC) and PKC stimulation. 5. In summary, we have shown that ERK1/2 activation by adenosine in cardiomyocytes results from an additive stimulation of A(1), A(2A), and A(3)ARs, which involves G(i/o) proteins, PKC, and tyrosine kinase for A(1) and A(3)ARs, and Gs and PKA for A(2A)ARs. Moreover, the A(3)AR response also involves a cAMP/PKA pathway via PKC activation.

Norepinephrine activates extracellular-regulated kinase in cortical neurons

BACKGROUND

Previous studies demonstrate that indirect activation of monoamine receptors by antidepressant treatment increases neurotrophic factors that activate the mitogen-activated protein kinase cascade; however, it is also possible that these monoamine receptors influence the mitogen-activated protein kinase pathway independent of neurotrophic factors. The influence of norepinephrine on the phosphorylation of extracellular-regulated protein kinase is characterized.

METHODS

Primary cerebral cortical cultures were prepared from embryonic day 18 rat brains and were subsequently incubated with norepinephrine in the absence or presence of agents acting as noradrenergic receptors or as intracellular signaling proteins. Levels of phosphorylated extracellular-regulated protein kinase were determined by immunoblot.

RESULTS

The results demonstrate that incubation with norepinephrine produces a time- and dose-dependent activation of phosphorylated extracellular-regulated protein kinase and that this increase is dependent on activation of alpha(2)- and beta-adrenergic receptor subtypes. In addition, the results demonstrate that norepinephrine activation of phosphorylated extracellular-regulated protein kinase is dependent on a pertussis toxin-sensitive G protein, a receptor tyrosine kinase, and activation of phosphatidylinositol 3-kinase.

CONCLUSIONS

The findings suggest that activation of the mitogen-activated protein kinase cascade by norepinephrine can occur via a tyrosine kinase-dependent signaling pathway but independent of classical second-messenger or Src-dependent kinases.

PI3-kinase inhibitors abolish the enhanced chronotropic effects of angiotensin II in spontaneously hypertensive rat brain neurons

Angiotensin II (Ang II), acting at Ang II type 1 receptors (AT1Rs), increases the firing rate of neurons from Wistar-Kyoto (WKY) rat brain via protein kinase C (PKC)- and calcium-calmodulin kinase II (CaMKII)-dependent mechanisms. The objectives of this study were twofold; first, to compare the Ang-II-stimulated increase in firing of neurons from WKY and spontaneous hypertensive rats (SHR) and second, to elucidate the signaling mechanisms involved. Action potentials were measured in neurons cultured from SHR and WKY rat brains using the whole cell configuration of the patch-clamp technique in the current-clamp mode. Ang II (100 nM) caused three- and sixfold increases in neuronal firing rate in WKY rat and SHR neurons, respectively; effects that were abolished by the AT1R antagonist Losartan (1 microM). Co-administration of calphostin C (10 microM, a PKC inhibitor) and KN-93 (10 microM, a CaMKII inhibitor) completely blocked this Ang II action in WKY rat neurons, while they caused only a approximately 50% attenuation in SHR neurons. The residual increase in firing rate produced by Ang II in SHR neurons was blocked by inhibitors of phosphatidylinositol 3 kinase (PI3-kinase), either LY 294002 (10 microM) or wortmannin (100 nM). These observations suggest that a PI3-kinase signaling pathway may be responsible for the enhanced chronotropic effect produced by Ang II in SHR neurons.

Signalling from adenosine receptors to mitogen-activated protein kinases

The purine nucleoside adenosine acts via four distinct adenosine receptor subtypes: the adenosine A(1), A(2A), A(2B), and A(3) receptor. They are all G protein-coupled receptors (GPCR) coupling to classical second messenger pathways such as modulation of cAMP production or the phospholipase C (PLC) pathway. In addition, they couple to mitogen-activated protein kinases (MAPK), which could give them a role in cell growth, survival, death and differentiation. Although each of the adenosine receptors can activate one or more of the MAPKs, the mechanisms appear to differ substantially, both between receptor subtypes in the same cell type and between the same receptor in different cell types.

The emerging role of lysophosphatidic acid in cancer

The bioactive phospholipid lysophosphatidic acid (LPA) stimulates cell proliferation, migration and survival by acting on its cognate G-protein-coupled receptors. Aberrant LPA production, receptor expression and signalling probably contribute to cancer initiation, progression and metastasis. The recent identification of ecto-enzymes that mediate the production and degradation of LPA, as well as the development of receptor-selective analogues, indicate mechanisms by which LPA production or action could be modulated for cancer therapy.

Phosphatidylinositol 3-kinase regulates the CD4/CD8 T cell differentiation ratio

The signaling pathways that control T cell differentiation have only begun to be elucidated. Using T cell lines, it has been shown that class IA phosphatidylinositol 3-kinase (PI3K), a heterodimer composed of a p85 regulatory and a p110 catalytic subunit, is activated after TCR stimulation. Nonetheless, the contribution of p85/p110 PI3K isoforms in T cell development has not been described. Mice deficient in the other family of class I PI3K, p110gamma, which is regulated by G protein-coupled receptors, exhibit reduced thymus size. Here we examine T cell development in p110gamma-deficient mice and in mice expressing an activating mutation of the p85 regulatory subunit, p65(PI3K), in T cells. We show that p110gamma-deficient mice have a partial defect in pre-TCR-dependent differentiation, which is restored after expression of the p65(PI3K) activating mutation. Genetic alteration of both PI3K isoforms also affects positive selection; p110gamma deletion decreased and p65(PI3K) expression augmented the CD4(+)/CD8(+) differentiation ratio. Finally, data are presented showing that both PI3K isoforms influenced mature thymocyte migration to the periphery. These observations underscore the contribution of PI3K in T cell development, as well as its implication in determining the CD4(+)/CD8(+) T cell differentiation ratio in vivo.

Permeabilization in a cerebral endothelial barrier model by pertussis toxin involves the PKC effector pathway and is abolished by elevated levels of cAMP

Respiratory tract infections caused by Bordetella pertussis are occasionally accompanied by severe neurologic disorders and encephalopathies. For these sequelae to occur the integrity of cerebral barriers needs to be compromised. The influence of pertussis toxin, a decisive virulence factor in the pathogenesis of pertussis disease, on barrier integrity was investigated in model systems for blood-liquor (epithelial) and blood-brain (endothelial) barriers. While pertussis toxin did not influence the barrier function in Plexus chorioideus model systems, the integrity of cerebral endothelial monolayers was severely compromised. Cellular intoxication by pertussis toxin proceeds via ADP-ribosylation of alpha-G(i) proteins, which not only interferes with the homeostatic inhibitory regulation of adenylate cyclase stimulation but also results in a modulation of the membrane receptor coupling. Increasing intra-endothelial cAMP levels by employing cholera toxin or forskolin even inhibited the pertussis toxin-induced permeabilization of endothelial barriers. Therefore, pertussis-toxin-induced permeabilization has to be mediated via a cAMP-independent pathway. To investigate potential signalling pathways we employed several well established cellular drugs activating or inhibiting central effectors of signal transduction pathways, such as phosphatidylinositol 3-kinase, adenylate cyclase, phospholipase C, myosin light chain kinase and protein kinase C. Only inhibitors and activators of protein kinase C and phosphatidylinositol 3-kinase affected the pertussis toxin-induced permeability. In summary, we conclude that permeabilization of cerebral endothelial monolayers by pertussis toxin does not depend on elevated cAMP levels and proceeds via the phosphokinase C pathway.

Optimal chemotactic responses of leukemic T cells to stromal cell-derived factor-1 requires the activation of both class IA and IB phosphoinositide 3-kinases

Stromal cell-derived factor-1 (SDF-1) and its receptor CXCR4 are a multifunctional chemokine/receptor system with essential roles in the development of the immune system and other aspects of embryogenesis, including vascularization and organ development. SDF-1 is also a potent chemoattractant for T cells and has roles in both inflammation and immune homeostasis. Our group has previously demonstrated that phosphoinositide 3-kinase (PI 3-kinase) is activated in SDF-1-stimulated T cells and is indeed required for SDF-1-mediated chemotaxis. In this study Jurkat clones were established, stably expressing dominant negative constructs of class IA and class IB PI 3-kinases under the control of the tetracycline off inducible gene system, to determine the relative roles of these PI 3-kinases in SDF-1 signaling. Our results show that expression of either kinase-dead PI3Kgamma (KD-PI3Kgamma) or Deltap85 (a construct unable to bind class I(A) p110alpha, -beta, or -delta) leads to a partial inhibition of SDF-1-stimulated protein kinase B phosphorylation, but had no effect on SDF-1-induced phosphorylation of the mitogen-activated protein kinase ERK1/2. Functional studies demonstrated that expression of KD-PI3Kgamma markedly inhibited SDF-1-mediated chemotaxis, typically eliciting 40-60% inhibition. Interestingly, the expression of Deltap85 also leads to inhibition of the SDF-1-mediated chemotactic response, albeit to a much lesser extent than achieved with the KD-PI3Kgamma mutant, typically in the range of 20-40% inhibition. Furthermore, the inhibition of chemotaxis by the expression of dominant negative class IA or class IB PI 3-kinases could be enhanced by the presence of the PI 3-kinase inhibitor LY294002. Together, these results demonstrate that optimal chemotactic response of leukemic T cells to SDF-1 requires the activation of both class IA and class IB PI 3-kinases.

Activation of the mitogen-activated protein kinases Erk1/2 by erythropoietin receptor via a G(i )protein beta gamma-subunit-initiated pathway

We have recently shown that a heterotrimeric G(i) protein is coupled to the erythropoietin (Epo) receptor. The G(i) protein constitutively associates in its heterotrimeric form with the intracellular domain of Epo receptor (EpoR). After Epo stimulation G(i) is released from the receptor and activated. In the present study we have investigated the functional role of the heterotrimeric G(i) protein bound to EpoR. In Chinese hamster ovary cells expressing EpoR, the G(i) inhibitor pertussis toxin blocked mitogen-activated protein kinase (MAPK) Erk1/2 activation induced by Epo. Epo-dependent MAPK activation was also sensitive to the G beta gamma competitive inhibitor beta ARK1-ct (C-terminal fragment of the beta-adrenergic receptor kinase), to the Ras dominant negative mutant RasN17, and to the phosphoinositide 3-kinase (PI3K) inhibitor LY 294002. A region of 7 amino acids (469-475) in the C-terminal end of EpoR was shown to be required for G(i) binding to EpoR in vivo. Deletion of this region in EpoR abolished both MAPK and PI3K activation in response to Epo. We conclude that in Chinese hamster ovary cells, Epo activates MAPK via a novel pathway dependent on G(i) association to EpoR, G beta gamma subunit, Ras, and PI3K. The tyrosine kinase Jak2 also contributes to this new pathway, more likely downstream of beta gamma and upstream of Ras and PI3K. This pathway is similar to the best characterized pathway used by seven transmembrane receptors coupled to G(i) to activate MAPK and may cooperate with other described Epo-dependent MAPK activation pathways in hematopoietic cells.

Thrombin stimulation of vascular adhesion molecule-1 in endothelial cells is mediated by protein kinase C (PKC)-delta-NF-kappa B and PKC-zeta-GATA signaling pathways

We recently demonstrated that thrombin induces the expression of vascular adhesion molecule-1 (VCAM-1) in endothelial cells by an NF-kappaB- and GATA-dependent mechanism. In the present study, we describe the signaling pathways that mediate this response. Thrombin stimulation of the VCAM-1 gene and promoter in human umbilical vein endothelial cells was inhibited by preincubation with the phosphatidylinositol 3-kinase inhibitor, LY294002, the protein kinase C (PKC)-delta inhibitor, rottlerin, a PKC-zeta peptide inhibitor, or by overexpression of dominant negative (DN)-PKC-zeta. In electrophoretic mobility shift assays, thrombin-mediated induction of NF-kappaB p65 binding to two NF-kappaB motifs in the upstream promoter region of VCAM-1 was blocked by LY294002 and rottlerin, whereas the inducible binding of GATA-2 to a tandem GATA motif was inhibited by LY294002 and the PKC-zeta peptide inhibitor. In co-transfection assays, thrombin stimulation of a minimal promoter containing multimerized VCAM-1 NF-kappaB sites was inhibited by DN-PKC-delta but not DN-PKC-zeta. In contrast, thrombin-mediated transactivation of a minimal promoter containing tandem VCAM-1 GATA motifs was inhibited by DN-PKC-zeta but not DN-PKC-delta. Finally, thrombin failed to induce VCAM-1 expression in vascular smooth muscle cells. Taken together, these data suggest that the endothelial cell-specific effect of thrombin on VCAM-1 expression involves the coordinate activity of PKC-delta-NF-kappaB and PKC-zeta-GATA signaling pathways.