Platelet-derived growth factor stimulates synthesis of PtdIns(3,4,5)P3 by activating a PtdIns(4,5)P2 3-OH kinase

TLR4/MyD88/PI3K interactions regulate TLR4 signaling

TLRs activate immune responses by sensing microbial structures such as bacterial LPS, viral RNA, and endogenous "danger" molecules released by damaged host cells. MyD88 is an adapter protein that mediates signal transduction for most TLRs and leads to activation of NF-kappaB and MAPKs and production of proinflammatory cytokines. TLR4-mediated signaling also leads to rapid activation of PI3K, one of a family of kinases involved in regulation of cell growth, apoptosis, and motility. LPS stimulates phosphorylation of Akt, a downstream target of PI3K, in wild-type (WT) mouse macrophages. LPS-induced phosphorylation of Akt serine 473 was blunted in MyD88(-/-) macrophages and was completely TLR4-dependent. MyD88 and p85 were shown previously to co-immunoprecipitate, and a YXXM motif within the Toll-IL-1 resistance (TIR) domain of MyD88 was suggested to be important for this interaction. To test this hypothesis, we compared expressed MyD88 variants with mutations within the YXXM motif or lacking the TIR domain or death domain and measured their capacities to bind PI3K p85, MyD88, and TLR4 by co-immunoprecipitation analyses. The YXXM --> YXXA mutant MyD88 bound more strongly to p85, TLR4, and WT MyD88 than the other variants, yet was significantly less active than WT MyD88, suggesting that sustained interaction of MyD88/PI3K with the TLR4 intracellular "signaling platform" negatively regulates signaling. We propose a hypothetical model in which sustained PI3K activity at the membrane limits the availability of the PI3K substrate, thereby negatively regulating signaling.

Single-molecule fluorescence studies of a PH domain: new insights into the membrane docking reaction

Proteins containing membrane targeting domains play essential roles in many cellular signaling pathways. However, important features of the membrane-bound state are invisible to bulk methods, thereby hindering mechanistic analysis of membrane targeting reactions. Here we use total internal reflection fluorescence microscopy (TIRFM), combined with single particle tracking, to probe the membrane docking mechanism of a representative pleckstrin homology (PH) domain isolated from the general receptor for phosphoinositides, isoform 1 (GRP1). The findings show three previously undescribed features of GRP1 PH domain docking to membranes containing its rare target lipid, phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P(3)]. First, analysis of surface diffusion kinetics on supported lipid bilayers shows that in the absence of other anionic lipids, the PI(3,4,5)P(3)-bound protein exhibits the same diffusion constant as a single lipid molecule. Second, the binding of the anionic lipid phosphatidylserine to a previously unidentified secondary binding site slows both diffusion and dissociation kinetics. Third, TIRFM enables direct observation of rare events in which dissociation from the membrane surface is followed by transient diffusion through solution and rapid rebinding to a nearby, membrane-associated target lipid. Overall, this study shows that in vitro single-molecule TIRFM provides a new window into the molecular mechanisms of membrane docking reactions.

Measurement of polyphosphoinositides in cultured mammalian cells

The seven phosphorylated derivatives of phosphatidylinositol (PtdIns), often collectively referred to as polyphosphoinositides (PPIn), are a minor component of eukaryotic cell membranes. Nevertheless, their synthesis is needed for an ever-increasing spectrum of cellular processes, including regulation of the actin cytoskeleton, chemotaxis, membrane trafficking, glucose uptake, and organelle acidification. PPIn metabolism is regulated dynamically by a network of kinases and phosphatases. Furthermore, synthesis of PPIn can be provoked by external stimuli; for example, the second messenger phosphatidylinositol 3,4,5-trisphosphate rapidly and transiently accumulates in cells challenged with agonists such as PDGF that activate receptor tyrosine kinases. The measurement of PPIn levels in in vivo cultured cells has been vital to our understanding of the metabolism and function of these important signaling molecules; methods are described herein that allow measurement of PPIn levels in culture cells in vivo.

Regulation of T-cell responses by PTEN

The phosphoinositide 3-kinase (PI3K) signaling pathway plays a critical role in the development, activation, and homeostasis of T cells by modulating the expression of survival and mitogenic factors in response to a variety of stimuli. Ligation of the antigen receptor, costimulatory molecules, and cytokine receptors activate PI3K, resulting in the production of the lipid second messenger phosphatidylinositol-3,4,5-triphosphate (PIP(3)). A number of molecules help to regulate the activity of this pathway, including the lipid phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10). By limiting the amount of PIP(3) available within the cell, PTEN directly opposes PI3K activity and influences the selection of developing thymocytes as well as the activation requirements of mature T cells. T cells with unchecked PI3K activity, as a result of PTEN deficiency, contribute to the development of both autoimmune disease and lymphoma. This review dissects our current understanding of PI3K and PTEN and discusses why appropriate balance of these molecules is necessary to maintain normal T-cell responses.

Lower phosphoinositide 3-kinase (PI 3-kinase) activity and differential expression levels of selective catalytic and regulatory PI 3-kinase subunit isoforms in prefrontal cortex and hippocampus of suicide subjects

Phosphoinositide 3 (PI 3)-kinase is one of the key signaling enzymes that participates in a myriad of physiological functions in brain and is utilized by neurotrophins to mediate neuronal plasticity, cell survival, and inhibition of apoptosis for several neuronal subtypes. Our recent demonstration that expression of neurotrophic factors and activation of the receptor tyrosine kinase B are significantly altered in postmortem brain of suicide subjects led us to examine whether suicide brain is associated with alterations in PI 3-kinase signaling. In prefrontal cortex (PFC), hippocampus, and cerebellum of suicide (n=28) and nonpsychiatric control (n=21) subjects we examined catalytic activation of PI 3-kinase, and mRNA and protein levels of regulatory (p85alpha, p85beta) and catalytic (p110alpha, p110beta) subunits of PI 3-kinase. It was observed that the catalytic activity of PI 3-kinase was significantly reduced in PFC and hippocampus of suicide subjects compared with nonpsychiatric control subjects. Competitive PCR analysis revealed significantly reduced mRNA expression of p85beta and p110alpha and increased expression of p85alpha subunit isoforms in PFC and hippocampus of suicide subjects. Alterations in these catalytic and regulatory subunits were accompanied by changes in their respective protein levels. These changes were not present in cerebellum of suicide subjects. Also, these changes were present in all suicide subjects irrespective of psychiatric diagnosis. Our findings of reduced activation and altered expression of specific PI 3-kinase regulatory and catalytic subunit isoforms demonstrate abnormalities in this signaling pathway in postmortem brain of suicide subjects and suggest possible involvement of aberrant PI 3-kinase signaling in the pathogenic mechanisms of suicide.

Loss of Vac14, a regulator of the signaling lipid phosphatidylinositol 3,5-bisphosphate, results in neurodegeneration in mice

The signaling lipid, phosphatidylinositol 3,5-bisphosphate (PI(3,5)P(2)), likely functions in multiple signaling pathways. Here, we report the characterization of a mouse mutant lacking Vac14, a regulator of PI(3,5)P(2) synthesis. The mutant mice exhibit massive neurodegeneration, particularly in the midbrain and in peripheral sensory neurons. Cell bodies of affected neurons are vacuolated, and apparently empty spaces are present in areas where neurons should be present. Similar vacuoles are found in cultured neurons and fibroblasts. Selective membrane trafficking pathways, especially endosome-to-TGN retrograde trafficking, are defective. This report, along with a recent report on a mouse with a null mutation in Fig4, presents the unexpected finding that the housekeeping lipid, PI(3,5)P(2), is critical for the survival of neural cells.

Differential association of phosphatidylinositol 3-kinase, SHIP-1, and PTEN with forming phagosomes

In macrophages, enzymes that synthesize or hydrolyze phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P(3)] regulate Fcgamma receptor-mediated phagocytosis. Inhibition of phosphatidylinositol 3-kinase (PI3K) or overexpression of the lipid phosphatases phosphatase and tensin homologue (PTEN) and Src homology 2 domain-containing inositol phosphatase (SHIP-1), which hydrolyze PI(3,4,5)P(3) to phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4-bisphosphate [PI(3,4)P(2)], respectively, inhibit phagocytosis in macrophages. To examine how these enzymes regulate phagosome formation, the distributions of yellow fluorescent protein (YFP) chimeras of enzymes and pleckstrin homology (PH) domains specific for their substrates and products were analyzed quantitatively. PTEN-YFP did not localize to phagosomes, suggesting that PTEN regulates phagocytosis globally within the macrophage. SHIP1-YFP and p85-YFP were recruited to forming phagosomes. SHIP1-YFP sequestered to the leading edge and dissociated from phagocytic cups earlier than did p85-cyan fluorescent protein, indicating that SHIP-1 inhibitory activities are restricted to the early stages of phagocytosis. PH domain chimeras indicated that early during phagocytosis, PI(3,4,5)P(3) was slightly more abundant than PI(3,4)P(2) at the leading edge of the forming cup. These results support a model in which phagosomal PI3K generates PI(3,4,5)P(3) necessary for later stages of phagocytosis, PTEN determines whether those late stages can occur, and SHIP-1 regulates when and where they occur by transiently suppressing PI(3,4,5)P(3)-dependent activities necessary for completion of phagocytosis.

Biology-enabling inositol phosphates, phosphatidylinositol phosphates and derivatives

This review highlights inositol polyphosphate- and phosphatidylinositol-based small molecule probes that have advanced our understanding of intracellular signalling.

Non-invasive visualization of the lipid product of class I PI3K in transgenic mouse models

PI3Ks (phosphoinositide 3-kinases) regulate many critical cellular responses by producing PI(3,4,5)P3 (phosphatidylinositol 3,4,5-trisphosphate). To facilitate the spatio-temporal characterization of PI(3,4,5)P3 in living primary cells, we generated a novel strain of transgenic mice [AktPH (Akt pleckstrin homology domain)–GFP (green fluorescent protein) Tg (transgenic) mice] that express a fluorescent bioprobe for PI(3,4,5)P3/PI(3,4)P2 (phosphatidylinositol 3,4-bisphosphate). By crossing AktPH–GFP Tg mice with strains of gene-targeted ‘knockout’ mice lacking a particular phosphoinositide-metabolizing enzyme, we have been able to evaluate the contribution of each enzyme to PI(3,4,5)P3 localization in migrating neutrophils. Our results indicate that PI3Kγ and the PI(3,4,5)P3 phosphatase SHIP1 [SH2 (Src homology 2)-containing inositol phosphatase-1] are the key regulators of PI(3,4,5)P3 dynamics during fMet-Leu-Phe (N-formylmethionyl-leucylphenylalanine; ‘chemotactic peptide’)-stimulated neutrophil migration. Our study has also validated the fluorescent transgenic strategy for studying PI(3,4,5)P3 metabolism in physiological and pathological situations.

Monocyte p110alpha phosphatidylinositol 3-kinase regulates phagocytosis, the phagocyte oxidase, and cytokine production

Mononuclear phagocytes are critical modulators and effectors of innate and adaptive immune responses, and PI-3Ks have been shown to be multifunctional monocyte regulators. The PI-3K family includes eight catalytic isoforms, and only limited information is available about how these contribute to fine specificity in monocyte cell regulation. We examined the regulation of phagocytosis, the phagocyte oxidative burst, and LPS-induced cytokine production by human monocytic cells deficient in p110alpha PI-3K. We observed that p110alpha PI-3K was required for phagocytosis of IgG-opsonized and nonopsonized zymosan in differentiated THP-1 cells, and the latter was inhibitable by mannose. In contrast, p110alpha PI-3K was not required for ingestion serum-opsonized zymosan. Taken together, these results suggest that FcgammaR- and mannose receptor-mediated phagocytosis are p110alpha-dependent, whereas CR3-mediated phagocytosis involves a distinct isoform. It is notable that the phagocyte oxidative burst induced in response to PMA or opsonized zymosan was also found to be dependent on p110alpha in THP-1 cells. Furthermore, p110alpha was observed to exert selective and bidirectional effects on the secretion of pivotal cytokines. Incubation of p110alpha-deficient THP-1 cells with LPS showed that p110alpha was required for IL-12p40 and IL-6 production, whereas it negatively regulated the production of TNF-alpha and IL-10. Cells deficient in p110alpha also exhibited enhanced p38 MAPK, JNK, and NF-kappaB phosphorylation. Thus, p110alpha PI-3K appears to uniquely regulate important monocyte functions, where other PI-3K isoforms are uninvolved or unable to fully compensate.

c-Src control of chloride channel support for osteoclast HCl transport and bone resorption

Bone degradation by osteoclasts depends upon active transport of hydrogen ions to solubilize bone mineral. This transport is supported by the parallel actions of a proton ATPase and a chloride channel located in the osteoclast ruffled membrane. We have previously identified a novel chloride channel, p62, which appears to be the avian counterpart to CLIC-5b and is expressed coincident with the appearance of acid secretion as avian osteoclasts differentiate in culture. In this article, we show that suppression of CLIC-5b in differentiating avian osteoclasts results in decreased acidification by vesicles derived from these cells and decreased ability of the cells to resorb bone. Acidification is rescued by the presence of valinomycin, consistent with a selective loss of chloride channel but not proton pump activity. Osteoclast bone resorption is known to be dependent on the expression of the tyrosine kinase, c-Src. We show that CLIC-5b from osteoclasts has affinity for both Src SH2 and SH3 domains. We find that suppression of expression of Src in developing osteoclasts results in decreased vesicular acidification, which is rescued by valinomycin, consistent with the loss of chloride conductance in the proton pump-containing vesicles. Suppression of c-Src causes no change in the steady state level of CLIC-5b expression, but does result in failure of proton pump and CLIC-5b to colocalize in cultured osteoclast precursors. We conclude that suppression of c-Src interferes with osteoclast bone resorption by disrupting functional co-localization of proton pump and CLIC-5b.

Quantitative model of Ras-phosphoinositide 3-kinase signalling cross-talk based on co-operative molecular assembly

In growth-factor-stimulated signal transduction, cell-surface receptors recruit PI3Ks (phosphoinositide 3-kinases) and Ras-specific GEFs (guanine nucleotide-exchange factors) to the plasma membrane, where they produce 3'-phosphorylated phosphoinositide lipids and Ras-GTP respectively. As a direct example of pathway networking, Ras-GTP also recruits and activates PI3Ks. To refine the mechanism of Ras-PI3K cross-talk and analyse its quantitative implications, we offer a theoretical model describing the assembly of complexes involving receptors, PI3K and Ras-GTP. While the model poses the possibility that a ternary receptor-PI3K-Ras complex forms in two steps, it also encompasses the possibility that receptor-PI3K and Ras-PI3K interactions are competitive. In support of this analysis, experiments with platelet-derived growth factor-stimulated fibroblasts revealed that Ras apparently enhances the affinity of PI3K for receptors; in the context of the model, this suggests that a ternary complex does indeed form, with the second step greatly enhanced through membrane localization and possibly allosteric effects. The apparent contribution of Ras to PI3K activation depends strongly on the quantities and binding affinities of the interacting molecules, which vary across different cell types and stimuli, and thus the model could be used to predict conditions under which PI3K signalling is sensitive to interventions targeting Ras.

Deterministic model of dermal wound invasion incorporating receptor-mediated signal transduction and spatial gradient sensing

During dermal wound healing, platelet-derived growth factor (PDGF) serves as both a chemoattractant and mitogen for fibroblasts, potently stimulating their invasion of the fibrin clot over a period of several days. A mathematical model of this process is presented, which accurately accounts for the sensitivity of PDGF gradient sensing through PDGF receptor/phosphoinositide 3-kinase-mediated signal transduction. Analysis of the model suggests that PDGF receptor-mediated endocytosis and degradation of PDGF allows a constant PDGF concentration profile to be maintained at the leading front of the fibroblast density profile as it propagates, at a constant rate, into the clot. Thus, the constant PDGF gradient can span the optimal concentration range for asymmetric phosphoinositide 3-kinase signaling and fibroblast chemotaxis, with near-maximal invasion rates elicited over a relatively broad range of PDGF secretion rates. A somewhat surprising finding was that extremely sharp PDGF gradients do not necessarily stimulate faster progression through the clot, because maintaining such a gradient through PDGF consumption is a potentially rate-limiting process.

Quantitative elucidation of a distinct spatial gradient-sensing mechanism in fibroblasts

Migration of eukaryotic cells toward a chemoattractant often relies on their ability to distinguish receptor-mediated signaling at different subcellular locations, a phenomenon known as spatial sensing. A prominent example that is seen during wound healing is fibroblast migration in platelet-derived growth factor (PDGF) gradients. As in the well-characterized chemotactic cells Dictyostelium discoideum and neutrophils, signaling to the cytoskeleton via the phosphoinositide 3-kinase pathway in fibroblasts is spatially polarized by a PDGF gradient; however, the sensitivity of this process and how it is regulated are unknown. Through a quantitative analysis of mathematical models and live cell total internal reflection fluorescence microscopy experiments, we demonstrate that PDGF detection is governed by mechanisms that are fundamentally different from those in D. discoideum and neutrophils. Robust PDGF sensing requires steeper gradients and a much narrower range of absolute chemoattractant concentration, which is consistent with a simpler system lacking the feedback loops that yield signal amplification and adaptation in amoeboid cells.

Somatostatin Increases Phospholipase D Activity and Phosphatidylinositol 4,5-bisphosphate Synthesis in Clonal β Cells HIT-T15

In the presence of arginine vasopressin (AVP), somatostatin increases [Ca(2+)](i), leading to a transient increase in insulin release from clonal beta cells HIT-T15 via G(i/o) and phospholipase C (PLC) pathway (Cheng et al., 2002a). The present study was to elucidate the mechanisms underlying somatostatin-induced [Ca(2+)](i) increase in the presence of AVP. We found that the effect of somatostatin was mediated by betagamma subunits but not by the alpha subunit of G(i/o). Because somatostatin alone failed to increase [Ca(2+)](i), we hypothesized that somatostatin increases phosphatidylinositol 4,5-bisphosphate (PIP(2)) synthesis, providing extra substrate for preactivated PLC-beta to generate inositol 1,4,5-trisphosphate (IP(3)). Somatostatin alone did not increase IP(3) levels, but AVP + somatostatin did. Somatostatin increased PIP(2) levels but decreased phosphatidylinositol 4-phosphate levels. We further hypothesized that PLD mediates somatostatin-induced changes in PIP(2) levels. Both the phospholipase D (PLD) inhibitors and antibody versus PLD1 antagonized AVP-somatostatin-induced increases in [Ca(2+)](i). PLD inhibitor also antagonized somatostatin-induced increase in PIP(2) levels. In addition, somatostatin increased PLD activity. These results suggest that activation of somatostatin receptors that are coupled to the betagamma dimer of G(i/o) led to PLD1 activation, thus promoting the synthesis of phosphatidic acid. Phosphatidic acid activates PIP-5 kinase, which evokes an increase in PIP(2) synthesis. The PIP(2) generated by somatostatin administration increases substrate for preactivated phospholipase C-beta, which hydrolyzes PIP(2) to form IP(3), leading to an increase in [Ca(2+)](i). The regulation of PIP(2) synthesis by G(i/o)-coupled receptors via PLD activation represents a novel signaling mechanism for somatostatin and a novel concept in the cross-talk between G(q)- and G(i/o)-coupled receptors in beta cells.

GRP1 pleckstrin homology domain: activation parameters and novel search mechanism for rare target lipid

Pleckstrin homology (PH) domains play a central role in a wide array of signaling pathways by binding second messenger lipids of the phosphatidylinositol phosphate (PIP) lipid family. A given type of PIP lipid is formed in a specific cellular membrane where it is generally a minor component of the bulk lipid mixture. For example, the signaling lipid PI(3,4,5)P(3) (or PIP(3)) is generated primarily in the inner leaflet of the plasma membrane where it is believed to never exceed 0.02% of the bulk lipid. The present study focuses on the PH domain of the general receptor for phosphoinositides, isoform 1 (GRP1), which regulates the actin cytoskeleton in response to PIP(3) signals at the plasma membrane surface. The study systematically analyzes both the equilibrium and kinetic features of GRP1-PH domain binding to its PIP lipid target on a bilayer surface. Equilibrium binding measurements utilizing protein-to-membrane fluorescence resonance energy transfer (FRET) to detect GRP1-PH domain docking to membrane-bound PIP lipids confirm specific binding to PIP(3). A novel FRET competitive binding measurement developed to quantitate docking affinity yields a K(D) of 50 +/- 10 nM for GRP1-PH domain binding to membrane-bound PIP(3) in a physiological lipid mixture approximating the composition of the plasma membrane inner leaflet. This observed K(D) lies in a suitable range for regulation by physiological PIP(3) signals. Interestingly, the affinity of the interaction decreases at least 12-fold when the background anionic lipids phosphatidylserine (PS) and phosphatidylinositol (PI) are removed from the lipid mixture. Stopped-flow kinetic studies using protein-to-membrane FRET to monitor association and dissociation time courses reveal that this affinity decrease arises from a corresponding decrease in the on-rate for GRP1-PH domain docking with little or no change in the off-rate for domain dissociation from membrane-bound PIP(3). Overall, these findings indicate that the PH domain interacts not only with its target lipid, but also with other features of the membrane surface. The results are consistent with a previously undescribed type of two-step search mechanism for lipid binding domains in which weak, nonspecific electrostatic interactions between the PH domain and background anionic lipids facilitate searching of the membrane surface for PIP(3) headgroups, thereby speeding the high-affinity, specific docking of the domain to its rare target lipid.

Sorting nexin homologues are targets of phosphatidylinositol 3-phosphate in sporulation of Schizosaccharomyces pombe

Schizosaccharomyces pombe defective in phosphatidylinositol (PtdIns) 3-kinase shows various defects in forespore membrane formation, including onset, growth orientation, and closure. Downstream factors of PtdIns 3-kinase in this system were explored. Among various phox homology (PX) domain-containing proteins, Vps5p and Vps17p, homologues of sorting nexins, were found to be required for efficient sporulation. Cells defective in these proteins showed a disordered growth orientation of the forespore membrane, as is the case with Deltapik3 cells. Vps5p and Vps17p with mutations in the PX domains failed to suppress the defects of their relevant disruptants. Vps5p and Vps17p migrated toward the the forespore membrane in a pik3+-dependent manner, suggesting that these proteins may interact with PtdIns(3)P. Electron-microscopic analysis revealed that the forespore membrane fails to engulf the nucleus in some of these cells, accumulating vesicle-like bodies similar to those seen in Deltaspo3 cells. These results suggest that Vps5p and Vps17p are the targets of PtdIns(3)P in vesicle transport required for onset of the forespore membrane formation.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Spatial analysis of 3' phosphoinositide signaling in living fibroblasts: II. Parameter estimates for individual cells from experiments

Fibroblast migration is directed by gradients of platelet-derived growth factor (PDGF) during wound healing. As in other chemotactic systems, it has been shown recently that localized stimulation of intracellular phosphoinositide (PI) 3-kinase activity and production of 3' PI lipids in the plasma membrane are important events in the signaling of spatially biased motility processes. In turn, 3' PI localization depends on the effective diffusion coefficient, D, and turnover rate constant, k, of these lipids. Here we present a systematic and direct comparison of mathematical model calculations and experimental measurements to estimate the values of the effective 3' PI diffusion coefficient, D, turnover rate constant, k, and other parameters in individual fibroblasts stimulated uniformly with PDGF. In the context of our uniform stimulation model, the values of D and k in each cell were typically estimated within 10-20% or less, and the mean values across all of the cells analyzed were D = 0.37 +/- 0.25 microm2/s and k = 1.18 +/- 0.54 min(-1). In addition, we report that 3' PI turnover is not affected by PDGF receptor signaling in our cells, allowing us to focus our attention on the regulation of 3' PI production as this system is studied further.

Hippocampal neuronal polarity specified by spatially localized mPar3/mPar6 and PI 3-kinase activity

How a neuron becomes polarized remains an outstanding question. Here, we report that selection of the future axon among neurites of a cultured hippocampal neuron requires the activity of growth factor receptor tyrosine kinase, phosphatidylinositol 3-kinase (PI 3-kinase), as well as atypical protein kinase C (aPKC). The PI 3-kinase activity, highly localized to the tip of the newly specified axon of stage 3 neurons, is essential for the proper subcellular localization of mPar3, the mammalian homolog of C. elegans polarity protein Par3. Polarized distribution of not only mPar3 but also mPar6 is important for axon formation; ectopic expression of mPar6 or mPar3, or just the N terminus of mPar3, leaves neurons with no axon specified. Thus, neuronal polarity is likely to be controlled by the mPar3/mPar6/aPKC complex and the PI 3-kinase signaling pathway, both serving evolutionarily conserved roles in specifying cell polarity.

Phosphatidylinositol 3-kinase is a key regulator of early phase differentiation in keratinocytes

The survival and growth of epithelial cells depend on adhesion to the extracellular matrix. Because epidermal keratinocytes differentiate as they leave the basement membrane, an adhesion signal may regulate the initiation of differentiation. Phosphatidylinositol 3-kinase (PI3K) is a fundamental signaling molecule that regulates the adhesion signal. Transfection of a dominant negative form of PI3K into keratinocytes using an adenovirus vector resulted in significant morphological changes comparable to differentiation and the induction of differentiation markers, keratin (K) 1 and K10. In turn, transfection with the constitutively active form of PI3K almost completely abolished the induction of K1 and K10 by differentiation in suspension cultures using polyhydroxyethylmethacrylate-coated dishes. PI3K activity was lost in suspension culture, except by cells bearing the constitutively active form of PI3K. These data demonstrate that blockade of PI3K results in differentiation and that activation of PI3K prevents differentiation. Furthermore, expression of the dominant negative form of PI3K significantly inhibited keratinocyte adhesion to the extracellular matrix and reduced the surface expression of alpha(6) and beta(1) integrins in suspension culture. Moreover, expression of the active form of PI3K restored the mRNA levels of adhesion molecules that were reduced in suspension culture, including alpha(3), alpha(6), and beta(1) integrins, BP180, and BP230. In conclusion, loss of PI3K activity results in keratinocytes leaving the basement membrane and the initiation of a "default" differentiation mechanism.

Distinct role of phosphatidylinositol 3-kinase and Rho family GTPases in Vav3-induced cell transformation, cell motility, and morphological changes

Vav3 is a member of the Vav family of guanine nucleotide exchange factors (GEFs) for the Rho family GTPases. Deleting the N-terminal calponin homology (CH) domain to generate Vav3-(5-10) or deleting both the CH and the acidic domain to generate Vav3-(6-10) results in activating the transforming potential of Vav3. Expression of either the full-length Vav3 or its truncation mutants led to activation of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), focal adhesion kinase (FAK), and Stat3. We investigated the requirement of these signaling molecules for Vav3-induced focus formation and found that PI3K and its downstream signaling molecules, Akt and p70 S6 kinase, are required, albeit to varying degrees. Inhibition of PI3K had a more dramatic effect than inhibition of MAPK on Vav3-(6-10)-induced focus formation. Activated PI3K enhanced the focus-forming activity of Vav3-(6-10). Wild type FAK but not Y397F mutant FAK enhanced Vav3-(6-10)-induced focus formation. Dominant negative (dn) mutant of Stat3 resulted in a 60% inhibition of the focus-forming activity of Vav3-(6-10). Moreover, Rac1, RhoA, and to a lesser extent, Cdc42, are important for Vav3-(6-10)-induced focus formation. Constitutively activated (ca) Rac synergizes with Vav3-(6-10) in focus formation. This synergy requires signaling via Rho-associated kinase (ROK) and p21-activated kinase (PAK), downstream effectors of Rac. Consistently, a ca PAK mutant enhanced, whereas a dn PAK mutant inhibited the focus-forming ability of Vav3-(6-10). Despite having potent focus-forming ability, Vav3-(6-10) has very weak colony-forming ability. This colony-forming ability of Vav3-(6-10) can be enhanced dramatically by co-expressing an activated PI3K and to some extent by co-expressing an activated PAK mutant or c-Myc. Interestingly, inhibition of PI3K and MAPK had no effect on the ability of either wild type or Vav3-(6-10) to induce cytoskeletal changes including formation of lamellipodia and filopodia in NIH 3T3 cells. Over expression of Vav3 or Vav3-(6-10) resulted in an enhancement of cell motility. This enhancement was dependent on PI3K, Rac1, and Cdc42 but not on Rho. Overall, our results show that signaling pathways of PI3K, MAPK, and Rho family GTPases are differentially required for Vav3-induced focus formation, colony formation, morphological changes, and cell motility.

3-phosphoinositides modulate cyclic nucleotide signaling in olfactory receptor neurons

Phosphatidylinositol 3-kinase (PI3K)-dependent phosphoinositide signaling has been implicated in diverse cellular systems coupled to receptors for many different ligands, but the extent to which it functions in sensory transduction is yet to be determined. We now report that blocking PI3K activity increases odorant-evoked, cyclic nucleotide-dependent elevation of [Ca(2+)](i) in acutely dissociated rat olfactory receptor neurons and does so in an odorant-specific manner. These findings imply that 3-phosphoinositide signaling acts in vertebrate olfactory transduction to inhibit cyclic nucleotide-dependent excitation of the cells and that the interaction of the two signaling pathways is important in odorant coding, indicating that 3-phosphoinositide signaling can play a role in sensory transduction.

The p85 subunit of phosphoinositide 3-kinase is associated with beta-catenin in the cadherin-based adhesion complex

Cell adhesion is fundamental to establishing and maintaining the discrete tissues in multicellular organisms. Adhesion must be sufficiently strong to preserve tissue architecture, whilst having the capacity to readily dissociate to permit fundamental processes, such as wound repair, to occur. However, very little is known about the signalling mechanisms involved in temporary down-regulation of cell adhesion to facilitate such processes. Cadherins are the principal mediators of cell-cell adhesion in a wide variety of tissues and species and form multi-protein complexes with cytosolic and cytoskeletal proteins to express their full adhesive capacity. In the present study we report that the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase) is associated with the cadherin-based adhesion complex in human epithelial cells. The interaction of p85 with the complex is via beta-catenin. We also show that the interaction of p85 and beta-catenin is direct, involves the N-terminal Src homology domain 2 of p85 and is regulated by tyrosine phosphorylation. These data suggest that PI 3-kinase may play a role in the functional regulation of the cadherin-based adhesion complex.

Phosphoinositide 3-kinase isoforms selectively couple receptors to vascular L-type Ca(2+) channels

Heterodimeric class I phosphoinositide 3-kinase (PI3K) has been shown to be involved in the stimulation of voltage-gated Ca(2+) channels by various mediators. In this study, we bring evidences that vascular L-type Ca(2+) channels can be modulated by both tyrosine kinase-regulated class Ia and G protein-regulated class Ib PI3Ks. Purified recombinant PI3Ks increased the peak Ca(2+) channel current density when applied intracellularly. Furthermore, PI3Kalpha-, beta-, and delta-mediated stimulations of Ca(2+) channel currents were increased by preactivation by a phosphotyrosyl peptide, whereas PI3Kgamma- and beta-mediated effects were increased by Gbetagamma. In freshly isolated and cultured vascular myocytes, angiotensin II and Gbetagamma stimulated L-type Ca(2+) channel current. In contrast, platelet-derived growth factor (PDGF)-BB and the phosphotyrosyl peptide did not stimulate Ca(2+) channel current in freshly isolated cells despite the presence of endogenous PDGF receptors and PI3Kalpha and PI3Kgamma. Interestingly, when endogenous PI3Kbeta expression arose in cultured myocytes, both PDGF and phosphotyrosyl peptide stimulated Ca(2+) channels through PI3Kbeta, as revealed by the inhibitory effect of an anti-PI3Kbeta antibody. These results suggest that endogenous PI3Kbeta but not PI3Kalpha is specifically involved in PDGF receptor-induced stimulation of Ca(2+) channels and that different isoforms of PI3K regulate physiological increases of Ca(2+) influx in vascular myocytes stimulated by vasoconstrictor or growth factor.

Endothelial cell apoptosis: biochemical characteristics and potential implications for atherosclerosis

The high turnover of endothelial cells (EC) in atherosclerosis suggests that an increase in the frequency of both cell proliferation and cell death is important in the pathogenesis of this common disorder. Further, increased apoptosis of EC, smooth muscle cells (SMC) and immune cells has been observed in atheromatous plaques. Many pro-atherogenic factors, including oxidized low-density lipoproteins, angiotensin II and oxidative stress, can induce EC apoptosis. Such damage to the endothelium may be an initiating event in atherogenesis since EC apoptosis may compromise vasoregulation, increase SMC proliferation, SMC migration and blood coagulation. In addition, EC overlying vascular lesions have been shown to increase their expression of pro-apoptotic proteins, such as Fas and Bax, while decreasing levels of anti-apoptotic factors. Therefore, understanding EC apoptotic pathways that are altered in atherosclerosis may enable a greater understanding of disease pathogenesis and foster the development of new therapies. The present discussion outlines the biochemical characteristics of EC apoptosis and the role that altered regulation of apoptosis plays in vasculopathy.

The EphA8 receptor regulates integrin activity through p110gamma phosphatidylinositol-3 kinase in a tyrosine kinase activity-independent manner

Recent genetic studies suggest that ephrins may function in a kinase-independent Eph receptor pathway. Here we report that expression of EphA8 in either NIH 3T3 or HEK293 cells enhanced cell adhesion to fibronectin via alpha(5)beta(1)- or beta(3) integrins. Interestingly, a kinase-inactive EphA8 mutant also markedly promoted cell attachment to fibronectin in these cell lines. Using a panel of EphA8 point mutants, we have demonstrated that EphA8 kinase activity does not correlate with its ability to promote cell attachment to fibronectin. Analysis using EphA8 extracellular and intracellular domain mutants has revealed that enhanced cell adhesion is dependent on ephrin A binding to the extracellular domain and the juxtamembrane segment of the cytoplasmic domain of the receptor. EphA8-promoted adhesion was efficiently inhibited by wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor. Additionally, we found that EphA8 had associated PI 3-kinase activity and that the p110gamma isoform of PI 3-kinase is associated with EphA8. In vitro binding experiments revealed that the EphA8 juxtamembrane segment was sufficient for the formation of a stable complex with p110gamma. Similar results were obtained in assay using cells stripped of endogenous ephrin A ligands by treatment with preclustered ephrin A5-Fc proteins. In addition, a membrane-targeted lipid kinase-inactive p110gamma mutant was demonstrated to stably associate with EphA8 and suppress EphA8-promoted cell adhesion to fibronectin. Taken together, these results suggest the presence of a novel mechanism by which the EphA8 receptor localizes p110gamma PI 3-kinase to the plasma membrane in a tyrosine kinase-independent fashion, thereby allowing access to lipid substrates to enable the signals required for integrin-mediated cell adhesion.

Odor-stimulated phosphatidylinositol 3-kinase in lobster olfactory receptor cells

Two antagonists of phosphoinositide 3-OH kinases (PI3Ks), LY294002 and Wortmannin, reduced the magnitude of the receptor potential in lobster olfactory receptor neurons (ORNs) recorded by patch clamping the cells in vivo. An antibody directed against the c-terminus of human PI3K-P110 beta detected a molecule of predicted size in the outer dendrites of the ORNs. Two 3-phosphoinositides, PI(3,4)P(2) (1--4 microM) and PI(3,4,5)P(3) (1--4 microM) applied to the cytoplasmic side of inside-out patches taken from cultured lobster ORNs, reversibly activated a Na(+)-gated channel previously implicated in the transduction cascade in these cells. 3-Phosphoinositides were the most effective phosphoinositide (1 microM) in enhancing the open probability of the channel. Collectively, these results implicate 3-phosphoinositides in lobster olfactory transduction and raise the need to consider the 3-phosphoinositide pathway in olfactory transduction.

Phosphoinositide 3-kinases in T lymphocyte activation

Biochemical experiments have established that the metabolism of inositol phospholipids by phosphoinositide 3-kinases (PI3Ks) and lipid-phosphatases is triggered by many receptors that control T lymphocyte function, including antigen-receptors, costimulatory molecules, cytokines and chemokines. Novel effectors of PI3K have been identified in the immune system and shown to be important in the control of lymphocyte activation. Moreover, key lipid-phosphatases have been identified that act to terminate or modulate PI3K signalling in cells of the immune system.

An Arabidopsis inositol phospholipid kinase strongly expressed in procambial cells: synthesis of PtdIns(4,5)P2 and PtdIns(3,4,5)P3 in insect cells by 5-phosphorylation of precursors

We have cloned a phosphatidylinositol-4-phosphate 5-kinase (PIP5K) cDNA (AtP5K1) from Arabidopsis thaliana. By the application of cell permeabilization and short-term nonequilibrium labelling we show that expression of AtP5K1 in Baculovirus-infected insect (Spodoptera frugiperda) cells directs synthesis of PtdIns(4,5)P2 and PtdIns(3,4,5)P3. The same phosphoinositides were produced by isolated whole-cell membrane fractions of AtP5K1-expressing insect cells. Their synthesis was not affected by adding defined precursor lipids, that is PtdIns(3)P, PtdIns(4)P, PtdIns(3,4)P2, or PtdIns(4,5)P2, in excess, indicating that substrates for the plant enzyme were not limiting in vivo. Enzymatic dissection of lipid headgroups revealed that AtP5K1-directed synthesis of PtdIns(4,5)P2 and PtdIns(3,4,5)P3 proceeds via 5-phosphorylation of precursors. Analysis of promoter-reporter gene (beta-glucuronidase) fusions in transgenic plants revealed that expression of the AtP5K1 gene is strongest in vascular tissues of leaves, flowers, and roots, namely in cells of the lateral meristem, that is the procambium. Single-cell sampling of sap from flower stem meristem tissue and neighbouring phloem cells, when coupled to reverse transcriptase--polymerase chain reaction, confirmed preferential expression of AtP5K1 in procambial tissue. We hypothesize that AtP5K1, like animal and yeast PIP5K, may be involved in the control of cell proliferation.

Cellular signalling: Stressing the importance of PIP3

The lipid second messenger PIP3 was previously thought to be generated exclusively by type I PI 3-kinases. Now, a novel route of PIP3 synthesis, controlled by an unrelated enzyme family, has been discovered, increasing our understanding of the versatility of PIP3 in cellular signalling.

Mammalian inositol polyphosphate 5-phosphatase II can compensate for the absence of all three yeast Sac1-like-domain-containing 5-phosphatases

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] plays a complex role in generating intracellular signalling molecules, and also in regulating actin-binding proteins, vesicular trafficking and vacuolar fusion. Four inositol polyphosphate 5-phosphatases (hereafter called 5-phosphatases) have been identified in Saccharomyces cerevisiae: Inp51p, Inp52p, Inp53p and Inp54p. Each enzyme contains a 5-phosphatase domain which hydrolyses PtdIns(4,5)P(2), forming PtdIns4P, while Inp52p and Inp53p also express a polyphosphoinositide phosphatase domain within the Sac1-like domain. Disruption of any two yeast 5-phosphatases containing a Sac1-like domain results in abnormalities in actin polymerization, plasma membrane, vacuolar morphology and bud-site selection. Triple null mutant 5-phosphatase strains are non-viable. To investigate the role of PtdIns(4,5)P(2) in mediating the phenotype of double and triple 5-phosphatase null mutant yeast, we determined whether a mammalian PtdIns(4,5)P(2) 5-phosphatase, 5-phosphatase II, which lacks polyphosphoinositide phosphatase activity, could correct the phenotype of triple 5-phosphatase null mutant yeast and restore cellular PtdIns(4,5)P(2) levels to near basal values. Mammalian 5-phosphatase II expressed under an inducible promoter corrected the growth, cell wall, vacuolar and actin polymerization defects of the triple 5-phosphatase null mutant yeast strains. Cellular PtdIns(4,5)P(2) levels in various 5-phosphatase double null mutant strains demonstrated significant accumulation (4.5-, 3- and 2-fold for Deltainp51Deltainp53, Deltainp51Deltainp52 and Deltainp52Deltainp53 double null mutants respectively), which was corrected significantly following 5-phosphatase II expression. Collectively, these studies demonstrate the functional and cellular consequences of PtdIns(4,5)P(2) accumulation and the evolutionary conservation of function between mammalian and yeast PtdIns(4,5)P(2) 5-phosphatases.

Phosphoinositide 3-kinase signalling pathways

Phosphoinositide 3-kinases (PI3Ks) phosphorylate the 3′-OH position of the inositol ring of inositol phospholipids, producing three lipid products: PtdIns(3)P, PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3). These lipids bind to the pleckstrin homology (PH) domains of proteins and control the activity and subcellular localisation of a diverse array of signal transduction molecules. Three major classes of signalling molecule are regulated by binding of D-3 phosphoinositides to PH domains: guanine-nucleotide-exchange proteins for Ρ family GTPases, the TEC family tyrosine kinases such as BTK and ITK in B and T lymphocytes, respectively, and the AGC superfamily of serine/threonine protein kinases. These molecules are activated by a variety of extracellular stimuli and have been implicated in a wide range of cellular processes, including cell cycle progression, cell growth, cell motility, cell adhesion and cell survival.

A novel pathway of cellular phosphatidylinositol(3,4,5)-trisphosphate synthesis is regulated by oxidative stress

BACKGROUND

Phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P(3)] is a key second messenger found ubiquitously in higher eukaryotic cells. The activation of Class I phosphoinositide 3-kinases and the subsequent production of PtdIns(3,4,5)P(3) is an important cell signaling event that has been causally linked to the activation of a variety of downstream cellular processes, such as cell migration and proliferation. Although numerous proteins regulating a variety of biological pathways have been shown to bind PtdIns(3,4,5)P(3), there are no data to demonstrate multiple mechanisms for PtdIns(3,4,5)P(3) synthesis in vivo.

RESULTS

In this study, we demonstrate an alternative pathway for the in vivo production of PtdIns(3,4,5)P(3) mediated by the action of murine Type Ialpha phosphatidylinositol 4-phosphate 5-kinase (Type Ialpha PIPkinase), an enzyme best characterized as regulating cellular PtdIns(4,5)P(2) levels. Analysis of this novel pathway of PtdIns(3,4,5)P(3) synthesis in cellular membranes leads us to conclude that in vivo, Type Ialpha PIPkinase also acts as a PtdIns(3,4)P(2) 5-kinase. We demonstrate for the first time that cells actually contain an endogenous PtdIns(3,4)P(2) 5-kinase, and that during oxidative stress, this enzyme is responsible for PtdIns(3,4,5)P(3) synthesis. Furthermore, we demonstrate that by upregulating the H(2)O(2)-induced PtdIns(3,4,5)P(3) levels using overexpression studies, the endogenous PtdIns(3,4)P(2) 5-kinase is likely to be Type Ialpha PIPkinase.

CONCLUSIONS

We describe for the first time a novel in vivo activity for Type Ialpha PIPkinase, and a novel pathway for the in vivo synthesis of functional PtdIns(3,4,5)P(3), a key lipid second messenger regulating a number of diverse cellular processes.

Growth factor-dependent signaling and cell cycle progression

There are three central ideas contained within this review. Firstly, growth factor-stimulated signaling is not restricted to a 30-60 min window, but occurs at a much later time as well. Secondly, the second wave of signaling overlaps temporally with the cell cycle program and may be directly responsible for engaging it. Thirdly, the G1 to S interval appears to encompass two distinct phases of the cell cycle, during which the coordinated activation of distinct sets of signaling enzymes drives cell cycle progression. Each of these concepts is likely to initiate new investigation and hence provide additional insight into the fundamental question of how growth factors drive cell proliferation.

SH2-containing inositol 5'-phosphatase SHIP2 associates with the p130(Cas) adapter protein and regulates cellular adhesion and spreading

In a previous study, we found that the SHIP2 protein became tyrosine phosphorylated and associated with the Shc adapter protein in response to the treatment of cells with growth factors and insulin (T. Habib, J. A. Hejna, R. E. Moses, and S. J. Decker, J. Biol. Chem. 273:18605-18609, 1998). We describe here a novel interaction between SHIP2 and the p130(Cas) adapter protein, a mediator of actin cytoskeleton organization. SHIP2 and p130(Cas) association was detected in anti-SHIP2 immunoprecipitates from several cell types. Reattachment of trypsinized cells stimulated tyrosine phosphorylation of SHIP2 and increased the formation of a complex containing SHIP2 and a faster-migrating tyrosine-phosphorylated form of p130(Cas). The faster-migrating form of p130(Cas) was no longer recognized by antibodies to the amino terminus of p130(Cas) and appeared to be generated through proteolysis. Interaction of the SHIP2 protein with the various forms of p130(Cas) was mediated primarily through the SH2 domain of SHIP2. Immunofluorescence studies indicated that SHIP2 localized to focal contacts and to lamellipodia. Increased adhesion was observed in HeLa cells transiently expressing exogenous WT-SHIP2. These effects were not seen with SHIP2 possessing a mutation in the SH2 domain (R47G). Transfection of a catalytic domain deletion mutant of SHIP2 (DeltaRV) inhibited cell spreading. Taken together, our studies suggest an important role for SHIP2 in adhesion and spreading.

Connecting signaling and cell cycle progression in growth factor-stimulated cells

A widely used model system to investigate cell proliferation is stimulation of serum-arrested cells with growth factors. Recent data suggest that there are two waves of growth factor-dependent signaling events required for a proliferative response. One is an acute burst of signaling, which occurs immediately after growth factor stimulation and lasts for 30 - 60 min. The other occurs in a different time frame (8 - 12 h post stimulation), and involves activation of cyclin dependent kinases (Cdks). In addition to a general overview of growth factor-dependent signaling, we present our 'two wave' hypothesis for how signaling and cell cycle progression are linked.

Glut-1 translocation in FRTL-5 thyroid cells: role of phosphatidylinositol 3-kinase and N-glycosylation

It was previously demonstrated that insulin or TSH treatment of FRTL-5 cells resulted in an elevation of glucose transport and in an increase of cell surface expression of the glucose transporter Glut-1. However, the signaling mechanisms leading to the insulin or TSH-induced increase in the cell surface expression of Glut-1 were not investigated. In the present study, we demonstrated that wortmannin and LY294002, two specific inhibitors of phosphatidylinositol 3-kinase (PI3-kinase), interfere both in the signaling pathways of insulin and TSH leading to glucose consumption enhancement and Glut-1 translocation. Two hours after insulin treatment, TSH or cAMP analog (Bu)2cAMP stimulation, glucose transport was increased and most of the intracellular Glut-1 pool was translocated to plasma membranes. Wortmannin or LY294002 blocked the insulin, (Bu)2cAMP, and the TSH-induced translocation of Glut-1. Wortmannin or LY294002 alone did not alter the basal ratio between intracellular and cell surface Glut-1 molecules. These results suggest that in FRTL-5 cells wortmannin and LY294002 inhibited the insulin, (Bu)2cAMP and TSH events leading to Glut-1 translocation from an intracellular compartment to the plasma membrane. Likewise, (Bu)2cAMP effects on glucose transport and Glut-1 translocation to plasma membrane were repressed by PI3-kinase inhibitors but not by the protein kinase A (PKA) inhibitor H89. We suggest that (Bu)2cAMP stimulates Glut-1 translocation to plasma membrane through PI3-kinase-dependent and PKA-independent signaling pathways. To further elucidate mechanisms that regulate the translocation of Glut-1 to cell membrane, we extended this study to the role played by the N-glycosylation in the translocation and in the biological activity of Glut-1 in FRTL-5 cells. For this purpose we used tunicamycin, an inhibitor of the N-glycosylation. Our experiments with tunicamycin clearly showed that both the glycosylated and unglycosylated forms of the transporter reached the cell surface. Likewise, a decrease in glucose consumption (-50%) after treatment of cells with tunicamycin was accompanied by a decrease (-70% vs. control) in the membrane expression of a 50-kDa form of Glut-1 and an increase in its unglycosylated 41-kDa form. These results suggest that carbohydrate moiety is essential for the biological activity of glucose transport but is not required for the translocation of Glut-1 from the intracellular membrane pool to the plasma membrane.

Phosphatidylinositol 3-kinase signaling is important for smooth muscle cell replication after arterial injury

The phosphoinositide 3-kinase [PI(3)K] pathway is a key signaling pathway important for replication of mammalian cells. In this study, we examined the role of PI(3)K in smooth muscle cell (SMC) replication after balloon catheter injury of rat carotid arteries. Protein kinase B (PKB), a downstream target of PI(3)K, was phosphorylated at 30 and 60 minutes after injury and to a lesser degree after 6 hours and 1 and 2 days but not after 7 days. Wortmannin (10 microgram per rat), a PI(3)K inhibitor, given to rats 60 and 5 minutes before and 11 hours after balloon injury, reduced the levels of phosphorylated PKB. SMC replication quantified between 24 to 48 hours was significantly reduced compared with control replication, as were the levels of cyclin D(1). Wortmannin was also administered to rats between days 7 and 8 and between days 7 and 9 after balloon catheter injury. A reduction in levels of phosphorylated PKB was detected, but no decrease in the replication of intimal SMCs was observed in either experiment. These data demonstrate that the PI(3)K signal transduction pathway plays an important role in medial but not intimal SMC replication.

Interaction of GRASP, a protein encoded by a novel retinoic acid-induced gene, with members of the cytohesin family of guanine nucleotide exchange factors

A novel, retinoic acid-induced gene, GRP1-associated scaffold protein (GRASP), was isolated from P19 embryonal carcinoma cells using a subtractive screening strategy. GRASP was found to be highly expressed in brain and exhibited lower levels of expression in lung, heart, embryo, kidney, and ovary. The predicted amino acid sequence of GRASP is characterized by several putative protein-protein interaction motifs, suggesting that GRASP may be a component of a larger protein complex in the cell. Although GRASP does not harbor a predicted membrane spanning domain(s), the protein was observed to be associated with the plasma membrane of transiently transfected mammalian cells. Yeast two-hybrid screening revealed that GRASP interacted strongly with the General Receptor for Phosphoinositides 1 (GRP1), a brefeldin A-insensitive guanine nucleotide exchange factor for the ADP-ribosylation factor family of proteins. GRASP. GRP1 interactions were also demonstrated in vitro and in mammalian cells in which GRASP was shown to enhance GRP1 association with the plasma membrane. Furthermore, GRASP colocalized with endogenous ADP-ribosylation factors at the plasma membrane in transfected cells, suggesting that GRASP may modulate signaling by this family of small GTPases.

Class II phosphoinositide 3-kinases are downstream targets of activated polypeptide growth factor receptors

The class II phosphoinositide 3-kinases (PI3K) PI3K-C2alpha and PI3K-C2beta are two recently identified members of the large PI3K family. Both enzymes are characterized by the presence of a C2 domain at the carboxy terminus and, in vitro, preferentially utilize phosphatidylinositol and phosphatidylinositol 4-monophosphate as lipid substrates. Little is understood about how the catalytic activity of either enzyme is regulated in vivo. In this study, we demonstrate that PI3K-C2alpha and PI3K-C2beta represent two downstream targets of the activated epidermal growth factor (EGF) receptor in human carcinoma-derived A431 cells. Stimulation of quiescent cultures with EGF resulted in the rapid recruitment of both enzymes to a phosphotyrosine signaling complex that contained the EGF receptor and Erb-B2. Ligand addition also induced the appearance of a second, more slowly migrating band of PI3K-C2alpha and PI3K-C2beta immunoreactivity on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since both PI3K enzymes can utilize Ca(2+) as an essential divalent cation in lipid kinase assays and since the catalytic activity of PI3K-C2alpha is refractory to the inhibitor wortmannin, these properties were used to confirm the recruitment of each PI3K isozyme to the activated EGF receptor complex. To examine this interaction in greater detail, PI3K-C2beta was chosen for further investigation. EGF and platelet-derived growth factor also stimulated the association of PI3K-C2beta with their respective receptors in other cells, including epithelial cells and fibroblasts. The use of EGF receptor mutants and phosphopeptides derived from the EGF receptor and Erb-B2 demonstrated that the interaction with recombinant PI3K-C2beta occurs through E(p)YL/I phosphotyrosine motifs. The N-terminal region of PI3K-C2beta was found to selectively interact with the EGF receptor in vitro, suggesting that it mediates the association of this PI3K with the receptor. However, the mechanism of this interaction remains unclear. We conclude that class II PI3K enzymes may contribute to the generation of 3' phosphoinositides following the activation of polypeptide growth factor receptors in vivo and thus mediate certain aspects of their biological activity.

Roles of non-catalytic subunits in gbetagamma-induced activation of class I phosphoinositide 3-kinase isoforms beta and gamma

By using purified preparations we show that nanomolar concentrations of Gbetagamma significantly stimulated lipid kinase activity of phosphatidylinositol 3-kinase (PI3K) beta and PI3Kgamma in the presence as well as in the absence of non-catalytic subunits such as p85alpha or p101. Concomitantly, Gbetagamma stimulated autophosphorylation of the catalytic subunit of PI3Kgamma (EC(50), 30 nM; stoichiometry >/=0.6 mol of P(i)/mol of p110gamma), which also occurred in the absence of p101. Surprisingly, we found that p101 affected the lipid substrate preference of PI3Kgamma in its Gbetagamma-stimulated state. With phosphatidylinositol as substrate, p110gamma but not p101/p110gamma was significantly stimulated by Gbetagamma to form PI-3-phosphate (EC(50), 20 nM). The opposite situation was found when PI-4,5-bisphosphate served as substrate. Gbetagamma efficiently and potently (EC(50), 5 nM) activated the p101/p110gamma heterodimer but negligibly stimulated the p110gamma monomer to form PI-3,4,5-trisphosphate. However, this weak stimulatory effect on p110gamma was overcome by excess concentrations of Gbetagamma (EC(50), 100 nM). This finding is in accordance with the in vivo situation, where activated PI3K catalyzes the formation of PI-3,4,5-trisphosphate but not PI-3-phosphate. We conclude that p101 is responsible for PI-4, 5-bisphosphate substrate selectivity of PI3Kgamma by sensitizing p110gamma toward Gbetagamma in the presence of PI-4,5-P(2).

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Mouse phosphoinositide 3-kinase p110alpha gene: cloning, structural organization, and localization to chromosome 3 band B

Phosphoinositide 3-Kinases (PI3-Kinases) are a family of dual specificity enzymes with a unique lipid kinase activity toward the D-3 position of the inositol ring of phosphoinositides and a less well characterized serine/threonine protein kinase activity. Class IA PI3-Kinases comprise a 110-120 kDa catalytic subunit (usually termed p110) and an 85 kDa or 50 to 55 kDa regulatory subunit (often called p85). cDNAs for three mammalian Class IA PI3-Kinase catalytic subunits designated p110alpha, p110beta, and p110delta have been cloned from several species. A YAC clone for the human p110alpha gene has also been cloned and mapped to chromosome 3q26.3. However, structural organization for any of the PI3-Kinase p110alpha genes has not been reported. Here, we report the cloning, structural organization, and chromosomal localization of the mouse PI3-Kinase p110alpha gene. The translated portion of the mouse p110alpha gene is encoded by 19 exons that span at least 24 kb. Dual color fluorescence in situ hybridization (FISH) was performed to determine the chromosomal localization of the mouse PI3-Kinase p110alpha gene. FISH results and DAPI banding demonstrated localization of the p110alpha gene to band B on mouse chromosome 3, a region syntenic with human chromosome 3q26.3.

Localization of a class II phosphatidylinositol 3-kinase, PI3KC2alpha, to clathrin-coated vesicles

We have analysed phosphatidylinositol 3-kinase activity associated with subcellular fractions prepared from rat brains. Phosphatidylinositol 3-kinase activity is not markedly enriched with synaptic vesicle purification; whilst the activity associated with the most pure fractions is inhibited at low concentrations of wortmannin (IC50 approximately 4-5 nM). In contrast, clathrin-coated vesicle (CCV) fractions showed increased enzyme activity compared to light membrane fractions from which they are purified. In addition to a wortmannin-sensitive activity, we also detected an activity that could only be inhibited at higher concentrations of wortmannin (IC50 approximately 400 nM), characteristic of certain class II enzymes (including phosphatidylinositol 3-kinase C2alpha) to be highly enriched in CCV fractions. Immunoblotting with an antibody raised against phosphatidylinositol 3-kinase C2alpha, confirmed that this enzyme is highly enriched in CCVs and displays an enrichment profile during the purification that mirrors enrichment of the low nanomolar wortmannin-insensitive activity. If the CCV purification protocol is adapted to favour nerve terminally derived vesicles, we find reduced levels of the C2alpha enzyme in the CCV fractions, suggesting that the enzyme may principally reside on vesicles associated with the cell body.