Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Development of a transendothelial shuttle by macrophage modification

One of the limiting factors in tissue regeneration, particularly in the context of chronic disease such as myodystrophy, motor neuron disease, sarcopenia, and cardiovascular disease, is limited availability of stem cells. We propose employing autologous macrophages to deliver stem cells, thereby facilitating tissue regeneration, by a novel and relatively non-invasive therapeutic intervention. Circulatory monocytic cells of M1 phenotype have capacity for transendothelial migration to infiltrate damaged tissue, making them ideal delivery vehicles. However, in order to deliver viable stem cells, these macrophages must undergo phagosome maturation arrest. Our aim was to induce phagosome maturation arrest in prepolarised M1 macrophages, whilst maintaining capacity for phagocytic engulfment (including phagosome formation) and transendothelial migration. Primary human M1 macrophages were treated with a wortmannin-concanamycin A-chloroquine cocktail to induce arrest. Modified cells were allowed to ingest 4.5 μm protein-coated fluorescent latex beads (simulated stem cells), before migratory capacity in response to MCP-1 was assessed over a 2-hr period in a Transwell co-culture system. Data indicate that phagosome acidification (as indicated by pHrodo®) was prevented in treated cells, effectively limiting digestion of ingested "cargo" (1.23 ± 0.26% vs. 7.52 ± 0.98% in controls; p < .0001). Neither phagocytic engulfment capacity (68.67 ± 3.51% vs. 61.19 ± 4.68%) nor migratory capacity (70.14 ± 12.6 vs. 72.86 ± 16.0 migrated cells per well) was compromised. We conclude that macrophages were successfully modified into transendothelial delivery vehicles, without compromising required functionality. This delivery system can be exploited to develop a novel method for focussed stem cell and/or drug delivery.

Prevention of the wortmannin-induced inhibition of phosphoinositide 3-kinase by sulfhydryl reducing agents

The effects of the sulfhydryl reducing agents 2-mercaptoethanol and dithiothreitol on wortmannin-induced inhibition of phosphoinositide 3-kinase (PI3K) were studied in order to examine whether the sulfhydryl reducing agents directly affect the wortmannin inhibition of PI3K. These reducing agents are commonly used to stabilize enzyme structures by maintaining protein sulfhydryl groups in the reduced state. Preincubation of wortmannin with millimolar levels of 2-mercaptoethanol, a sulfhydryl derivative of ethanol, markedly prevented subsequent wortmannin-induced inhibition of PI3K. In contrast, ethanol, 2-mercaptoethanol lacking sulfhydryl group, and 2-(methylthio)ethanol, a methyl derivative of the sulfhydryl group of 2-mercaptoethanol, had little effect on the wortmannin-induced inhibition of PI3K, which suggests that the prevention of wortmannin-induced inhibition by 2-mercaptoethanol occurs through the sulfhydryl group of this agent. Moreover, dithiothreitol, a second sulfhydryl reducing agent, also markedly prevented wortmannin-induced inhibition of PI3K. These results indicate that the wortmannin-induced inhibition of PI3K is markedly prevented by millimolar concentrations of sulfhydryl reducing agents such as 2-mercaptoethanol and dithiothreitol in the medium, presumably by the binding of wortmannin to the agents.

Targeting the undruggable proteome: the small molecules of my dreams

Biologically active small molecules have long proven useful in the exploration of cell biology. Although many early compounds were by-products of drug development efforts, recent increased small molecule screening efforts in academia have expanded the repertoire of biological processes investigated to include areas of biology that are not of immediate pharmaceutical interest. Many of these new bioassays score for small molecule-induced phenotypic changes at the cellular or even organismal level and thus have been described as "chemical genetic" screens. However, this analogy with traditional genetic screens is misleading; although each gene has roughly an equivalent chance of being mutated in a traditional genetic screen, the amount of "proteomic space" that a chemical genetics approach can reach using current small molecule libraries is considerably smaller. Thus, new chemical biology methodologies are needed to target the remaining "undruggable proteome" with small druglike molecules.

Effect of pycnogenol on glucose transport in mature 3T3-L1 adipocytes

Pycnogenol, a procyanidins-enriched extract of Pinus maritima bark, possesses antidiabetic properties, which improves the altered parameters of glucose metabolism that are associated with type 2 diabetes mellitus (T2DM). Since the insulin-stimulated antidiabetic activities of natural bioactive compounds are mediated by GLUT4 via the phosphatidylinositol-3-kinase (PI3K) and/or p38 mitogen activated protein kinase (p38-MAPK) pathway, the effects of pycnogenol were examined on the molecular mechanism of glucose uptake by the glucose transport system. 3T3-L1 adipocytes were treated with various concentrations of pycnogenol, and glucose uptake was examined using a non-radioisotope enzymatic assay and by molecular events associated with the glucose transport system using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). The results show that pycnogenol increased glucose uptake in fully differentiated 3T3-L1 adipocytes and increased the relative abundance of both GLUT4 and Akt mRNAs through the PI3K pathway in a dose dependent manner. Furthermore, pycnogenol restored the PI3K antagonist-induced inhibition of glucose uptake in the presence of wartmannin, an inhibitor of the PI3K. Overall, these results indicate that pycnogenol may stimulate glucose uptake via the PI3K dependent tyrosine kinase pathways involving Akt. Further the results suggest that pycnogenol might be useful in maintaining blood glucose control.

Electrophysiological identification of glucose-sensing neurons in rat nodose ganglia

The vagal afferent system is strategically positioned to mediate rapid changes in motility and satiety in response to systemic glucose levels. In the present study we aimed to identify glucose-excited and glucose-inhibited neurons in nodose ganglia and characterize their glucose-sensing properties. Whole-cell patch-clamp recordings in vagal afferent neurons isolated from rat nodose ganglia demonstrated that 31/118 (26%) neurons were depolarized after increasing extracellular glucose from 5 to 15 mm; 19/118 (16%) were hyperpolarized, and 68/118 were non-responsive. A higher incidence of excitatory response to glucose occurred in gastric- than in portal vein-projecting neurons, the latter having a higher incidence of inhibitory response. In glucose-excited neurons, elevated glucose evoked membrane depolarization (11 mV) and an increase in membrane input resistance (361 to 437 M). Current reversed at 99 mV. In glucose-inhibited neurons, membrane hyperpolarization (13 mV) was associated with decreased membrane input resistance (383 to 293 M). Current reversed at 97 mV. Superfusion of tolbutamide, a K(ATP) channel sulfonylurea receptor blocker, elicited identical glucose-excitatory but not glucose-inhibitory responses. Kir6.2 shRNA transfection abolished glucose-excited but not glucose-inhibited responses. Phosphatidylinositol bisphosphate (PIP(2)) depletion using wortmannin increased the fraction of glucose-excited neurons from 26% to 80%. These results show that rat nodose ganglia have glucose-excited and glucose-inhibited neurons, differentially distributed among gastric- and portal vein-projecting nodose neurons. In glucose-excited neurons, glucose metabolism leads to K(ATP) channel closure, triggering membrane depolarization, whereas in glucose-inhibited neurons, the inhibitory effect of elevated glucose is mediated by an ATP-independent K(+) channel. The results also show that PIP(2) can determine the excitability of glucose-excited neurons.

Covalent reactions of wortmannin under physiological conditions

Wortmannin (Wm), a steroid-like molecule of 428.4 Da, appears to be unstable in biological fluids (apparent chemical instability), yet it exhibits an antiproliferative activity in assays employing a 48 hr incubation period (prolonged bioactivity), a situation we refer to as the "wortmannin paradox." Under physiological conditions, Wm covalently reacts with nucleophiles such as the side chains of cysteine, N-methyl hexanoic acid, lysine, or proline at the C20 position on the furan ring. Like Wm, WmC20 amino acid derivatives had significant antiproliferative activities. Three Wm derivatives, WmC20-proline, WmC20-cysteine, and a WmC20-N-methyl hexanoic acid, generated Wm that then reacted with lysine in an exchange-type reaction. This unusual, reversible, covalent reaction of Wm with nucleophiles under physiological conditions provides an explanation for the wortmannin paradox.

Bruton's tyrosine kinase together with PI 3-kinase are part of Toll-like receptor 2 multiprotein complex and mediate LTA induced Toll-like receptor 2 responses in macrophages

Lipoteichoic acid (LTA) of Gram-positive bacteria initiates innate immune responses via Toll-like receptor-2 (TLR2), resulting in the activation of intracellular signaling and production of inflammatory cytokines in macrophages. Although Bruton's tyrosine kinase (Btk) is biologically important molecule implicated in immune regulation and recently in TLR signaling its importance for LTA-TLR2 mediated responses has not been evaluated. In this study, we detected Btk in the LTA signaling complex with TLR2 and PI 3-kinase (PI3K). The constitutive interaction of these proteins was mediated via PI3K Src homology (SH3) -domain. Both Btk and PI3K were activated by LTA stimulation and the LTA induced cytokine expression was differentially modulated by these kinases. LTA induced the activation of nuclear factor kappaB (NFkappaB), however, only Btk inhibition affected the LTA induced Ser536 phosphorylation and DNA-binding of NFkappaB. In conclusion, our results demonstrate that Btk and PI3K occupy important roles in TLR2-induced activation of macrophages, resulting in selective regulation of cytokines.

Wortmannin-C20 conjugates generate wortmannin

We report on C20-6-(N-methylamino)hexanoic conjugates of wortmannin featuring a tertiary enamine attached to the C20 that inhibit phosphoinositol-3-OH kinase (PI3K) by producing wortmannin (Wm) through an intramolecular attack. The generation of Wm by these conjugates permits the design of Wm based PI3K inhibitors that need not fit into the ATP pocket of PI3K, including Wm conjugates of BSA, IgG, or beads. Wm generating WmC20-N(Me)-hexanoate conjugates offer an approach to the design of targeted or slow release forms of Wm which may inhibit PI3K in tissues more selectively than the parent Wm, a compound which has desirable anti-inflammatory and anti-proliferative activities but which also has a variety of toxic effects.

Scaffolding adapter Grb2-associated binder 2 requires Syk to transmit signals from FcepsilonRI

Scaffolding adapter Grb2-associated binder 2 (Gab2) is a key component of FcepsilonRI signaling in mast cells, required for the activation of PI3K. To understand how Gab2 is activated in FcepsilonRI signaling, we asked which protein tyrosine kinase is required for Gab2 phosphorylation. We found that Gab2 tyrosyl phosphorylation requires Lyn and Syk. In agreement with published results, we found that Fyn also contributes to Gab2 tyrosyl phosphorylation. However, Syk activation is defective in Fyn(-/-) mast cells, suggesting that Syk is the proximal kinase responsible for Gab2 tyrosyl phosphorylation. Then, we asked which domains in Gab2 are required for Gab2 tyrosyl phosphorylation. We found that the Grb2-Src homology 3 (SH3) binding sites are required for, whereas the pleckstrin homology (PH) domain contributes to, Gab2 tyrosyl phosphorylation. Using a protein/lipid overlay assay, we determined that the Gab2 PH domain preferentially binds the PI3K lipid products, PI3, 4,5P3 and PI3, 4P2. Furthermore, the Grb2-SH3 binding sites and PH domain binding to PI3K lipid products are required for Gab2 function in FcepsilonRI-evoked degranulation and Akt activation. Our data strongly suggest a model for Gab2 action in FcepsilonRI signaling. The Grb2 SH3 binding sites play a critical role in bringing Gab2 to FcepsilonRI, whereupon Gab2 becomes tyrosyl-phosphorylated in a Syk-dependent fashion. Phosphorylated Gab2 results in recruitment and activation of PI3K, whose lipid products bind the PH domain of Gab2 and acts in positive feedback loop for sustained PI3K recruitment and phosphatidylinositol-3,4,5-trisphosphate production, required for FcepsilonRI-evoked degranulation of mast cells.

IN VITRO METABOLISM OF THE PHOSPHATIDYLINOSITOL 3-KINASE INHIBITOR, WORTMANNIN, BY CARBONYL REDUCTASE

The phosphatidylinositol 3-kinase inhibitor, wortmannin, is extensively used in molecular signaling studies and has been proposed as a potential antineoplastic agent. The failure to detect wortmannin in mouse plasma after i.v. administration prompted in vitro studies of wortmannin metabolism. Wortmannin was incubated with mouse tissue homogenates, homogenate fractions, or purified, recombinant human carbonyl reductase in the presence of specified cofactors and inhibitors. Reaction products were characterized and quantified with liquid chromatography (LC)/mass spectrometry. Reaction rates were characterized using Michaelis-Menten kinetics. Wortmannin was metabolized to a material 2 atomic mass units greater than wortmannin. Liver homogenate had the highest metabolic activity. Some metabolism occurred in kidney and lung homogenates. Very little metabolism occurred in brain or red blood cell homogenates. Liver S9 fraction and cytosol metabolized wortmannin in the presence of NADPH and, to a much lesser extent, in the presence of NADH. Microsomal metabolism of wortmannin was minimal. Purified, recombinant human carbonyl reductase metabolized wortmannin. Quercetin, a carbonyl reductase inhibitor, greatly decreased wortmannin metabolism by S9, cytosol, and carbonyl reductase. The K(M) for wortmannin metabolism by purified, recombinant human carbonyl reductase was 119 +/- 9 microM, and the V(max) was 58 +/- 9 nmol/min/mg of protein. LC-tandem mass spectrometry spectra indicated that carbonyl reductase metabolized wortmannin to 17-OH-wortmannin. Wortmannin reduction by carbonyl reductase may partly explain why wortmannin is not detected in plasma after being administered to mice. Metabolism of wortmannin to 17-OH-wortmannin has mechanistic, and possibly toxicologic, implications because 17-OH-wortmannin is 10-fold more potent an inhibitor of phosphatidylinositol 3-kinase than is wortmannin.

Use of high-performance liquid chromatography to characterize the rapid decomposition of wortmannin in tissue culture media

Although wortmannin is extensively used in molecular signaling studies, its stability in tissue culture medium has not been assessed precisely. Therefore, we used high-performance liquid chromatography (HPLC) and mass spectrometry (MS) to characterize the decomposition of wortmannin in five commonly used media. Wortmannin was added to medium alone or to medium supplemented with 10% unheated or heat-inactivated fetal bovine serum and incubated at 37 degrees C. After 0, 5, 10, 20, 35, and 60 min, wortmannin remaining in the medium was quantified, and its decay constant and half-life were calculated. In all media, wortmannin decomposed monoexponentially, with half-lives between 8 and 13 min. HPLC/MS indicated that wortmannin decomposed to materials with m/z 447, 433, 373, and 313. Acidification of material produced by incubation of wortmannin in tissue culture medium or 1 microM NaOH converted the material with m/z 447 back to one that cochromatographed with and had an m/z (429) identical to that of wortmannin. Therefore wortmannin is much less stable in tissue culture medium than previously thought although some apparent loss of wortmannin reflects reversible, pH-dependent opening of the lactone ring of wortmannin. This rapid and complex decomposition of wortmannin argues for care being taken in how it is used in in vitro studies.

Phosphoinositide 3-kinase activity leads to silica-induced NF-kappaB activation through interacting with tyrosine-phosphorylated I(kappa)B-alpha and contributing to tyrosine phosphorylation of p65 NF-kappaB

The role of the subunits of phosphoinositide (PI) 3-kinase in NF-kappaB activation in silica-stimulated RAW 264.7 cells was investigated. Results indicate that PI3-kinase activity was increased in response to silica. The p85alpha subunit of PI3-kinase interacted with tyrosine-phosphorylated I(kappa)B-alpha in silica-stimulated cells. PI3-kinase specific inhibitors, such as wortmannin and LY294003, substantially blocked both silica-induced PI3-kinase and NF-kappaB activation. The inhibition of NF-KB activation by PI3-kinase inhibitors was also observed in pervanadate-stimulated but not in LPS-stimulated cells. Furthermore, tyrosine phosphorylation of NF-kappaB p65 was enhanced in cells stimulated with silica, pervanadate or LPS, and wortmannin substantially inhibited the phosphorylation event induced by the first two stimulants but not LPS. Antioxidants, such as superoxide dismutase (SOD), N-acetylcysteine (NAC) and pyrrolidine dithiocarbamate (PDTC), blocked silica-induced PI3-kinase activation, suggesting that reactive oxygen species may be important regulatory molecules in NF-kappaB activation by mediating PI3-kinase activation. Our data suggest that p85 and p110 subunits of PI3-kinase play a role in NF-kappaB activation through interaction with tyrosine-phosphorylated I(kappa)B-alpha and contributing to tyrosine phosphorylation of p65 NF-kappaB.

Vascular endothelial growth factor causes pulmonary vasodilation through activation of the phosphatidylinositol-3-kinase-nitric oxide pathway in the late-gestation ovine fetus

Vascular endothelial growth factor (VEGF) causes vasodilation in adult models of peripheral vascular disease and myocardial ischemia through the acute release of nitric oxide (NO). However, the hemodynamic effects of VEGF and its effects on NO production have not been studied in the developing lung circulation. We hypothesized that VEGF causes fetal pulmonary vasodilation, and that its actions are mediated through the release of endogenous NO. We performed surgery in 16 fetal lambs (125-135 d gestation; term = 147 d), and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic flow transducer was placed on the left pulmonary artery (LPA) to measure blood flow, and a catheter was placed in the LPA for local drug infusion. Pulmonary vascular resistance in the left lung was calculated as pulmonary artery pressure minus left atrial pressure divided by LPA flow. Fetal lambs were treated with brief infusions of recombinant human VEGF (dose, 0.5-2.0 micro g) into the LPA. Recombinant human VEGF infusions acutely increased LPA flow by up to 3-fold (p < 0.02) and decreased pulmonary vascular resistance by 65% (p < 0.05) in a dose-related fashion, without affecting aortic pressure or heart rate. To determine the mechanism of VEGF-induced vasodilation, we studied the effects of nitro-L-arginine, an NO synthase inhibitor, and LY294002, a phosphatidylinositol-3-kinase inhibitor, on the response to VEGF. We found that pretreatment with either nitro-L-arginine or LY294002 completely inhibited the vasodilator response to recombinant human VEGF (p < 0.005). These findings suggest that recombinant human VEGF causes fetal pulmonary vasodilation, and that this response is likely mediated by the release of NO through activation of phosphatidylinositol-3-kinase.

TSH signaling and cell survival in 3T3-L1 preadipocytes

Thyroid-stimulating hormone (TSH) action in adipose tissue remains largely unknown. Our previous work indicates that human preadipocytes express functional TSH receptor (TSHR) protein, demonstrated by TSH activation of p70 S6 kinase (p70 S6K). We have now studied murine 3T3-L1 preadipocytes to further characterize TSH signaling and cellular action. Western blot analysis of 3T3-L1 preadipocyte lysate revealed the 100-kDa mature processed form of TSHR. TSH activated p70 S6K and protein kinase B (PKB/Akt), as measured by immunoblot analysis. Preincubation with wortmannin or LY-294002 completely blocked TSH activation of p70 S6K and PKB/Akt, implicating phosphoinositide 3-kinase (PI3K) in their regulation. TSH increased phosphotyrosine protein(s) in the 125-kDa region and augmented the associated PI3K activity fourfold. TSH had no effect on cAMP levels in 3T3-L1 preadipocytes, suggesting that adenylyl cyclase is not involved in TSH activation of the PI3K-PKB/Akt-p70 S6K pathway. 3T3-L1 preadipocyte cell death was reduced by 29-76% in serum-deprived (6 h) preadipocytes treated with 1-20 microM TSH. In the presence of 20 microM TSH, an 88% reduction in terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL)-positive cells was observed in serum-starved (3 h) 3T3-L1 preadipocytes as well as a 93% reduction in the level of cleaved activated caspase 3. In summary, TSH acts as a survival factor in 3T3-L1 preadipocytes. TSH does not stimulate cAMP accumulation in these cells but instead activates a PI3K-PKB/Akt-p70 S6K pathway.

Coordinate regulation of catecholamine uptake by rab3 and phosphoinositide 3-kinase

Previously we observed that rab3 GTPases modulate both the secretion of catecholamines from PC12 neuroendocrine cells and the steady-state accumulation of exogenous norepinephrine (NE) into these cells (Weber, E., Jilling, T., and Kirk, K. L. (1996) J. Biol. Chem. 271, 6963-6971). Here we addressed the mechanisms by which these monomeric GTPases stimulate NE uptake by PC12 cells including their effects on uptake kinetics, their sites of action (secretory granule membrane versus plasma membrane), and the involvement of rab3-interacting proteins in this process. We observed that rab3B stimulated the rate and maximal accumulation of radiolabeled NE into large dense core vesicles within intact PC12 cells. rab3A and rab3B also increased NE uptake into large dense core vesicles in digitonin-permeabilized PC12 cells, which indicates that these GTPases stimulate catecholamine uptake at the level of the secretory granule membrane. In an attempt to identify rab3B targets that may mediate this effect on NE uptake, we found that rab3B interacts directly with phosphoinositide 3-kinase (PI3K) in a GTP-dependent fashion and that PI3K activity was elevated in PC12 cells overexpressing rab3B. Furthermore, two structurally distinct inhibitors of PI3K (wortmannin and LY294002) inhibited NE uptake in intact as well as digitonin-permeabilized PC12 cells, but had no effect on calcium-evoked NE secretion. Our results indicate that rab3 and PI3K positively and coordinately regulate NE uptake in PC12 neuroendocrine cells at least in part by stimulating the secretory vesicle uptake step.

Thrombin-stimulated cell proliferation mediated through activation of Ras/Raf/MEK/MAPK pathway in canine cultured tracheal smooth muscle cells

The elevated level of thrombin has been detected in the airway fluids of asthmatic patients and shown to stimulate cell proliferation in tracheal smooth muscle cells (TSMCs). However, the implication of thrombin in the cell proliferation was not completely understood. In this study, thrombin stimulated [3H]thymidine incorporation and p42/p44 mitogen-activated protein kinase (MAPK) phosphorylation in a time- and concentration-dependent manner in TSMCs. Pretreatment of TSMCs with pertussis toxin (PTX) significantly inhibited [3H]thymidine incorporation and phosphorylation of MAPK induced by thrombin. These responses were attenuated by tyrosine kinase inhibitors genistein and herbimycin A, phosphatidyl inositide (PI)-phospholipase C (PLC) inhibitor U73122, protein kinase C inhibitor GF109203X, removal of Ca2+ by addition of BAPTA/AM plus EGTA, PI 3-kinase inhibitors wortmannin and LY294002, and inhibitor of MEK1/2 PD98059. Furthermore, overexpression of dominant negative mutants, H-Ras-15A and Raf-N4, significantly suppressed p42/p44 MAPK activation induced by thrombin and PDGF-BB, indicating that Ras and Raf may be required for activation of these kinases. These results conclude that the mitogenic effect of thrombin was mediated through the activation of Ras/Raf/MEK/MAPK pathway. Thrombin-mediated MAPK activation was modulated by PI-PLC, Ca2+, PKC, tyrosine kinase, and PI 3-kinase associated with cell proliferation in canine cultured TSMCs.

Phosphatidylinositol 3-kinase and NF-kappaB regulate motility of invasive MDA-MB-231 human breast cancer cells by the secretion of urokinase-type plasminogen activator

Cell migration is a fundamental aspect of the neoplastic cell metastasis. Here, we show that phosphatidylinositol (PI) 3-kinase is constitutively active and controls cell motility of highly invasive breast cancer cells by the activation of transcription factor, NF-kappaB. The urokinase-type plasminogen activator (uPA) promoter contains an NF-kappaB binding site, and uPA expression in MDA-MB-231 cells is induced by the constitutively active NF-kappaB. Thus, motility was inhibited by overexpression of a dominant negative p85alpha regulatory subunit of PI 3-kinase (p85DN), as well as by pretreatment of cells with specific inhibitors of the p110 catalytic subunit of PI 3-kinase, wortmannin and LY294002. The involvement of gene transcription in cell motility was suggested because treatment with actinomycin D and cycloheximide, which inhibit transcription and new protein synthesis, respectively, abolished endogenous migration of MDA-MB-231 cells. Although wortmannin, Ly294002, or overexpression of p85DN did not significantly reduce DNA binding activity of NF-kappaB in nuclear extracts, wortmannin, Ly294002, and the overexpression of p85DN or IkappaBalpha inhibited constitutive activation of NF-kappaB in a reporter gene assay. Highly invasive MDA-MB-231 cells constitutively secreted uPA in amounts significantly higher than poorly invasive MCF-7 cells. Furthermore, inhibition of NF-kappaB markedly attenuated endogenous migration, and inhibition of PI 3-kinase and NF-kappaB reduced secretion of uPA. Our data suggest a link between constitutively active PI 3-kinase, NF-kappaB, and secretion of uPA, which is responsible for the migration of highly invasive breast cancer cells. Thus, constitutively active PI 3-kinase controls cell motility by the regulation of expression of uPA through the activation of NF-kappaB.

Chemoattractant-stimulated NF-kappaB activation is dependent on the low molecular weight GTPase RhoA

Chemoattractants bind to seven transmembrane-spanning, G-protein-coupled receptors on monocytes and neutrophils and induce a variety of functional responses, including activation of the transcription factor NF-kappaB. The signaling mechanisms utilized by chemoattractants to activate NF-kappaB in human peripheral blood monocytes are poorly defined. We previously demonstrated that fMet-Leu-Phe (fMLP) stimulates NF-kappaB activation, and this function of fMLP requires phosphatidylinositol 3-kinase (PI3K). Here we present evidence that fMLP activates RhoA and that fMLP-induced NF-kappaB activation requires this small GTPase. Stimulation of monocytes with fMLP rapidly activated RhoA as well as NF-kappaB, and their activation was markedly reduced by pertussis toxin treatment. Pretreatment of monocyte with a RhoA inhibitor, C3 transferase from Clostridium botulinum, effectively blocked fMLP-induced NF-kappaB activation as well as interleukin-1beta gene expression. A dominant negative form of RhoA (T19N) also inhibited fMLP-stimulated reporter gene expression in a kappaB-dependent manner. Cotransfection of the monocytic THP1 cells with a constitutively active form of RhoA (Q63L) with the promoter reporter plasmid results in a marked increase in NF-kappaB-mediated reporter gene expression. Furthermore, the PI3K inhibitors wortmannin and LY294002 block RhoA activation induced by fMLP. These results demonstrate that low molecular weight GTPase RhoA is a novel signal transducer for fMLP-induced NF-kappaB activation and Galpha(i) or Galpha(o) class of heterotrimeric G proteins likely mediate RhoA activation via PI3K in human peripheral blood monocytes.

Rapamycin-sensitive phase of 3T3-L1 preadipocyte differentiation after clonal expansion

Inhibition of insulin-induced 3T3-L1 preadipocyte differentiation by rapamycin has been attributed to a blockade of the early critical clonal expansion phase of the adipogenic program. Rapamycin binds to, and inhibits, mTOR (mammalian target of rapamycin), leading to diminution of p70 S6 kinase activity and eukaryotic initiation factor 4E binding protein 1 (eIF4E-BP1) function. Our objective was to determine if rapamycin-sensitive pathways exist subsequent to the clonal expansion phase. We determined that the mitotic clonal expansion was complete by day 4 of the differentiation protocol, based on the response to Ara-C (cytosine beta-D-arabinofuranoside), which only inhibits differentiation when administered during this phase. Treatment of differentiating 3T3-L1 cells with rapamycin, starting on day 4, exerted potent negative effects on glycerol phosphate dehydrogenase activity, and triacylglycerol accumulation, as well as on the protein expression of adipogenic transcription factors, C/EBPalpha and PPARgamma. Insulin-stimulated p70 S6 kinase activity, and its inhibition by rapamycin, were comparable in preadipocytes at day 0 vs. day 4 post-differentiation. We conclude that a component of the adipogenic program, operating after the completion of clonal expansion, is inhibited by rapamycin, suggesting an ongoing need for mTOR function in this process.

Essential role for Gab2 in the allergic response

Dos/Gab family scaffolding adapters (Dos, Gab1, Gab2) bind several signal relay molecules, including the protein-tyrosine phosphatase Shp-2 and phosphatidylinositol-3-OH kinase (PI(3)K); they are also implicated in growth factor, cytokine and antigen receptor signal transduction. Mice lacking Gab1 die during embryogenesis and show defective responses to several stimuli. Here we report that Gab2-/- mice are viable and generally healthy; however, the response (for example, degranulation and cytokine gene expression) of Gab2-/- mast cells to stimulation of the high affinity immunoglobulin-epsilon (IgE) receptor Fc(epsilon)RI is defective. Accordingly, allergic reactions such as passive cutaneous and systemic anaphylaxis are markedly impaired in Gab2-/- mice. Biochemical analyses reveal that signalling pathways dependent on PI(3)K, a critical component of Fc(epsilon)RI signalling, are defective in Gab2-/- mast cells. Our data identify Gab2 as the principal activator of PI(3)K in response to Fc(epsilon)RI activation, thereby providing genetic evidence that Dos/Gab family scaffolds regulate the PI(3)K pathway in vivo. Gab2 and/or its associated signalling molecules may be new targets for developing drugs to treat allergy.

Mechanisms of thrombin-induced MAPK activation associated with cell proliferation in human cultured tracheal smooth muscle cells

The elevated level of thrombin has been detected in the airway fluids of asthmatic patients. However, the implication of thrombin in the pathogenesis of bronchial hyperreactivity was not completely understood. Therefore, in this study we investigated the effect of thrombin on cell proliferation and p42/p44 mitogen-activated protein kinase (MAPK) activation in human tracheal smooth muscle cells (TSMCs). Thrombin stimulated [3H]thymidine incorporation and p42/p44 MAPK phosphorylation in a time- and concentration-dependent manner in TSMCs. Pretreatment of TSMCs with pertussis toxin (PTX) significantly inhibited [3H]thymidine incorporation and phosphorylation of MAPK induced by thrombin. These responses were attenuated by tyrosine kinase inhibitors genistein and herbimycin A, phosphatidyl inositide (PI)-phospholipase C (PLC) inhibitor U73122, protein kinase C (PKC) inhibitor GF109203X, removal of Ca(2+) by addition of BAPTA/AM plus EGTA, and PI 3-kinase inhibitors wortmannin and LY294002. In addition, thrombin-induced [3H]-thymidine incorporation and p42/p44 MAPK phosphorylation was completely inhibited by PD98059 (an inhibitor of MEK1/2), indicating that activation of MEK1/2 was required for these responses. Furthermore, overexpression of dominant negative mutants, RasN17 and Raf-301, significantly suppressed p42/p44 MAPK activation induced by thrombin and PDGF-BB, indicating that Ras and Raf may be required for activation of these kinases. These results conclude that the mitogenic effect of thrombin was mediated through the activation of Ras/Raf/MEK/MAPK pathway. Thrombin-mediated MAPK activation was modulated by PI-PLC, Ca(2+), PKC, tyrosine kinase, and PI 3-kinase associated with cell proliferation in cultured human TSMCs.

Phosphatidylinositol-3,4,5-trisphosphate is required for insulin-like growth factor 1-mediated survival of 3T3-L1 preadipocytes

Adipocyte number, a determinant of adipose tissue mass, reflects the balance between the rates of proliferation/differentiation vs. apoptosis of preadipocytes. The percentage of 3T3-L1 preadipocytes undergoing cell death following serum deprivation was reduced by 10 nM insulin-like growth factor (IGF)-1 (from 50.0 +/- 0.7% for control starved cells to 27.5 +/- 3.1%). TUNEL staining confirmed the apoptotic nature of the cell death. The protective effect of IGF-1 was blocked by phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin, and LY294002, but was unaffected by rapamycin, PD98059, or SB203580, which inhibit mammalian target of rapamycin (mTOR), ERK kinase (MEK1), and p38 MAPK respectively. Exogenous PI(3,4,5)P3 (10 microM), the principal product of IGF-1-stimulated PI3K in 3T3-L1 preadipocytes, had a modest survival effect on its own, reducing cell death from 47.9 +/- 3.4% to 35.6 +/- 3.5%. When added to the combination of IGF-1 and LY294002, PI(3,4,5)P3 reversed most of the inhibitory effect of LY294002 on IGF-1-dependent cell survival, protein kinase B/Akt phosphorylation, and caspase-3 activity. Taken together, these results implicate PI(3,4,5)P3 as a necessary signal for the anti-apoptotic action of IGF-1 on 3T3-L1 preadipocytes.

Functional TSH receptor in human abdominal preadipocytes and orbital fibroblasts

Controversy continues about whether, and to what levels of abundance, thyroid-stimulating hormone receptors (TSHR) are found in human tissues other than the thyroid gland. Restricted expression to the thyroid and orbit would suggest that TSHR represents the target autoantigen in thyroid-associated ophthalmopathy. A more generalized pattern of tissue expression would be inconsistent with TSHR acting as the autoantigen that is solely responsible for selectively targeting the immune system to the orbit. We have detected TSHR mRNA in human abdominal adipose tissue by Northern blot analysis. TSHR protein was also detected, by immunoblotting with two different antibodies, in preadipocytes isolated from human abdominal subcutaneous and omental adipose tissue and in derivative adipocytes differentiated in primary culture. Preadipocytes treated with thyroid-stimulating hormone (TSH) exhibited a sevenfold increase in the activity of p70 S6 kinase, a serine/threonine kinase recently recognized as a downstream target of TSHR in thyroid cells. Activation of p70 S6 kinase by TSH was also observed in orbital fibroblasts. Thus TSHR protein expression is found in fibroblasts from several anatomic locations, suggesting that factors other than site-limited TSHR expression must be involved in restricting the distribution of Graves' disease manifestations. Furthermore, the presence of functional TSHR in preadipocytes raises the possibility of a novel role for TSHR signaling in adipose tissue development.

Action of rolipram on specific PDE4 cAMP phosphodiesterase isoforms and on the phosphorylation of cAMP-response-element-binding protein (CREB) and p38 mitogen-activated protein (MAP) kinase in U937 monocytic cells

U937 monocytic cells are shown here to express a range of PDE4, cAMP-specific phosphodiesterase (PDE) isoenzymes: the long isoenzymes, PDE4A4, PDE4D5 and PDE4D3, plus the short isoenzyme, PDE4B2. These isoenzymes provide around 76% of the total cAMP PDE activity of U937 cells. The specific activities of the total PDE4A, PDE4B and PDE4D activities were 0.63+/-0.09, 8.8+/-0.2 and 34.4+/-2.9 pmol/min per mg of protein respectively. The PDE4 selective inhibitor, rolipram, inhibited immunopurified PDE4B and PDE4D activities similarly, with IC(50) values of approx. 130 nM and 240 nM respectively. In contrast, rolipram inhibited immunopurified PDE4A activity with a dramatically lower IC(50) value of around 3 nM. Rolipram increased phosphorylation of cAMP-response-element-binding protein (CREB) in U937 cells in a dose-dependent fashion, which implied the presence of both high affinity (IC(50) value approx. 1 nM) and low affinity (IC(50) value approx. 120 nM) components. Rolipram dose-dependently inhibited the interferon-gamma (IFN-gamma)-stimulated phosphorylation of p38 mitogen-activated protein (MAP) kinase in a simple monotonic fashion with an IC(50) value of approx. 290 nM. On this basis, it is suggested that rolipram inhibition of PDE4A4 is involved in regulating CREB phosphorylation but not IFN-gamma-stimulated p38 MAP kinase phosphorylation. PDE4A4 was also selectively activated by challenge of U937 cells with either bacterial lipopolysaccharide (LPS) or IFN-gamma through a process which was attenuated by both wortmannin and rapamycin. It is proposed that the PDE4A4 isoform is involved in compartmentalized cAMP signalling responses in U937 monocytes.

Aminopeptidase N/CD13 is directly linked to signal transduction pathways in monocytes

In the present study, we characterized in monocytes the rise in [Ca(2+)](i) evoked by monoclonal antibodies (mAbs) to aminopeptidase N (APN)/CD13, showing a two-phase calcium increase with a small-belled [Ca(2+)](i) rise due to the release of calcium from intracellular stores and a more sustained plateau due to the influx of calcium from the extracellular environment. Tyrosine kinase inhibitors were able to inhibit the rise in [Ca(2+)](i) induced by ligation APN/CD13, as were inhibitors of the phosphatidylinositol 3-kinase. For the first time we can show that mAbs to APN/CD13 provoke phosphorylation of the mitogen-activated protein kinases ERK1/2, JNK, and p38. Furthermore, we show that mRNA of the chemotactic cytokine IL-8 is upregulated under the influence of APN/CD13 ligation. Although the in vivo ligand as well as possible cooperating membrane molecules remains to be identified, our results suggest that the membrane ectoenzyme APN/CD13 is a novel signal transduction molecule in monocytes.

Platelet-derived growth factor rapidly increases activity and cell surface expression of the EAAC1 subtype of glutamate transporter through activation of phosphatidylinositol 3-kinase

Na(+)-dependent glutamate transporters are the primary mechanism for removal of excitatory amino acids (EAAs) from the extracellular space of the central nervous system and influence both physiologic and pathologic effects of these compounds. Recent evidence suggests that the activity and cell surface expression of a neuronal subtype of glutamate transporter, EAAC1, are rapidly increased by direct activation of protein kinase C and are decreased by wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K). We hypothesized that this regulation could be analogous to insulin-induced stimulation of the GLUT4 subtype of glucose transporter, which is dependent upon activation of PI3-K. Using C6 glioma, a cell line that endogenously and selectively expresses EAAC1, we report that platelet-derived growth factor (PDGF) increased Na(+)-dependent L-[(3)H]-glutamate transport activity within 30 min. This effect of PDGF was not due to a change in total cellular EAAC1 immunoreactivity but was instead correlated with an increase cell surface expression of EAAC1, as measured using a membrane impermeant biotinylation reagent combined with Western blotting. A decrease in nonbiotinylated intracellular EAAC1 was also observed. These studies suggest that PDGF causes a redistribution of EAAC1 from an intracellular compartment to the cell surface. These effects of PDGF were accompanied by a 35-fold increase in PI3-K activity and were blocked by the PI3-K inhibitors, wortmannin and LY 294002, but not by an inhibitor of protein kinase C. Other growth factors, including insulin, nerve growth factor, and epidermal growth factor had no effect on glutamate transport nor did they increase PI3-K activity. These studies suggest that, as is observed for insulin-mediated translocation of GLUT4, EAAC1 cell surface expression can be rapidly increased by PDGF through activation of PI3-K. It is possible that this PDGF-mediated increase in EAAC1 activity may contribute to the previously demonstrated neuroprotective effects of PDGF.

fMet-Leu-Phe stimulates proinflammatory cytokine gene expression in human peripheral blood monocytes: the role of phosphatidylinositol 3-kinase

The fMLP-stimulated release of proinflammatory cytokines such as IL-1 by human peripheral blood monocytes is an important component of the inflammatory process. The signaling mechanisms used by fMLP to stimulate the release of cytokines are still incompletely understood. We previously demonstrated that fMLP-stimulated NF-kappaB activation in PBMC and now we present evidence that the lipid products of phosphatidylinositol 3-kinase (PI 3-kinase) are required for fMLP-stimulated activation of NF-kappaB. Pretreatment with the PI 3-kinase inhibitors, wortmannin and LY294002, effectively blocked fMLP-induced IL-1beta gene expression as well as NF-kappaB activation. Transient transfection of THP1 cells with a dominant-negative mutant of the PI 3-kinase p85 subunit also abrogated fMLP-induced kappaB activity. These results suggest a potential role of fMLP in the transcription of proinflammatory cytokines and provide the first evidence that such regulation may occur through PI 3-kinase activity.

Phospholipase C-linked receptors regulate the ATP-sensitive potassium channel by means of phosphatidylinositol 4,5-bisphosphate metabolism

In the COS7 cells transfected with cDNAs of the Kir6.2, SUR2A, and M(1) muscarinic receptors, we activated the ATP-sensitive potassium (K(ATP)) channel with a K(+) channel opener and recorded the whole-cell K(ATP) current. The K(ATP) current was reversibly inhibited by the stimulation of the M(1) receptor, which is linked to phospholipase C (PLC) by the G(q) protein. The receptor-mediated inhibition was observed even when protein kinase C (PKC) was inhibited by H-7 or by chelating intracellular Ca(2+) with 10 mM 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) included in the pipette solution. However, the receptor-mediated inhibition was blocked by U-73122, a PLC inhibitor. M(1)-receptor stimulation failed to inhibit the K(ATP) current activated by the injection of exogenous phosphatidylinositol 4,5-bisphosphate (PIP(2)) through the whole-cell patch pipette. The receptor-mediated inhibition became irreversible when the replenishment of PIP(2) was blocked by wortmannin (an inhibitor of phosphatidylinositol kinases), or by including adenosine 5'-[beta,gamma-imido]triphosphate (AMPPNP, a nonhydrolyzable ATP analogue) in the pipette solution. In inside-out patch experiments, the ATP sensitivity of the K(ATP) channel was significantly higher when the M(1) receptor in the patch membrane was stimulated by acetylcholine. The stimulatory effect of pinacidil was also attenuated under this condition. We postulate that stimulation of PLC-linked receptors inhibited the K(ATP) channel by increasing the ATP sensitivity, not through PKC activation, but most probably through changing PIP(2) levels.

A role for phosphatidylinositol 3-kinase in platelet aggregation in response to low, but not high, concentrations of PAF or thrombin

In this study we show that platelet activating factor (PAF) activates PI 3-kinase over a rapid time course that correlates closely with the aggregation response. Tyrosine kinases are involved in this response, since there is increased PI 3-kinase activity associated with tyrosine-phosphorylated proteins. PI 3-kinase inhibitors were used to probe the dependence of PAF-induced aggregation on PI 3-kinase. Both wortmannin and LY-294002 inhibited PAF-induced aggregation that correlated with PI 3-kinase inhibition only when using lower concentrations of PAF giving reversible aggregation (primary phase). Similar results were obtained with human platelets using thrombin or thrombin receptor activating peptide. The same pattern of response was observed when activation of GPIIbIIIa was assessed by flow cytometry, i.e., PI 3-kinase inhibitors blocked integrin activation only when lower concentrations of agonist were used. We suggest that PI 3-kinase is important for reversible (primary) aggregation of platelets in response to PAF or thrombin, perhaps by contributing to the 'inside-out' activation of the platelet integrin GPIIbIIIa, only when submaximal concentrations of agonists are used. The lack of effect of PI 3-kinase inhibitors, when high concentrations of agonist are used, suggests that PI 3-kinase-independent pathways contribute to aggregation under these conditions.

Agonists cause nuclear translocation of phosphatidylinositol 3-kinase gamma. A Gbetagamma-dependent pathway that requires the p110gamma amino terminus

In hematopoietic cells, the signals initiated by activation of the phosphoinositide 3-kinase (PI3K) family have been implicated in cell proliferation and survival, membrane and cytoskeletal reorganization, chemotaxis, and the neutrophil respiratory burst. Of the four isoforms of human PI3K that phosphorylate phosphatidylinositol 4, 5-bisphosphate, only p110gamma (or PI3Kgamma) is associated with the regulatory subunit, p101, and is stimulated by G protein betagamma heterodimers. We performed immunolocalization of transfected p110gamma in HepG2 cells and found that, under resting conditions, p110gamma was present in a diffuse cytoplasmic pattern, but translocated to the cell nucleus after serum stimulation. Serum-stimulated p110gamma translocation was inhibited by pertussis toxin and could also be induced by overexpression of Gbetagamma in the absence of serum. In addition, we found that deletion of the amino-terminal 33 residues of p110gamma had no effect on association with p101 or on its agonist-regulated translocation, but truncation of the amino-terminal 82 residues yielded a p110gamma variant that did not associate with p101 and was constitutively localized in the nucleus. This finding implies that the intracellular localization of p110gamma is regulated by p101 as well as Gbetagamma. The effect of PI3Kgamma in the nucleus is an area of active investigation.

Light-evoked recovery from wortmannin-induced inhibition of catecholamine secretion and synaptic transmission

Wortmannin (WT) is known to inhibit catecholamine (CA) secretion in chromaffin cells. This effect was found to be sensitive to UV light in experiments designed to perform simultaneous monitoring of changes in [Ca2+]i and CA secretion in perfused rat adrenal medullas. When the change in [Ca2+]i was measured using calcium green-1 (490 nm excitation), a 35-min treatment with 10 microM WT caused a 69% inhibition of CA secretion evoked by excess (30 mM) extracellular K+ and a moderate inhibition of the [Ca2+]i response. In contrast, the same treatment of fura-2-loaded cells with WT caused only an 11% inhibition of the high-K+-evoked secretion and no significant attenuation of the [Ca2+]i response. However, during interruption of fluorometry with fura-2, the inhibitory effect of WT developed at a rate similar to that exhibited in calcium green-1-loaded cells. The WT-induced inhibition of high-K+- or bradykinin-evoked secretory responses, which was otherwise irreversible, was reversed by exposing WT-treated chromaffin cells to 380-nm light. When WT was reapplied to the cells of which the secretory ability had been restored by light irradiation, the secretory response was inhibited with a time course similar to that shown during the initial treatment with WT. The photosensitive effect of WT was also demonstrated using bullfrog sympathetic ganglia in which WT-induced inhibition of synaptic transmission was reversed by irradiation with 380-nm light. These results suggest that UV light removes the inhibitory effects of WT by disrupting the covalent bond formed between WT and a target molecule which remains to be determined, although myosin light chain kinase has been reported as the target molecule in both cases examined in this study.

Participation of protein kinases in staurosporine-induced interleukin-6 production by rat peritoneal macrophages

The incubation of rat peritoneal macrophages in the presence of staurosporine, a non-specific protein kinase inhibitor, induced interleukin-6 (IL-6) production in a time- and concentration-dependent manner at 6.3-63 nM, but at 210 nM, the stimulant effect on IL-6 production was reduced. The levels of IL-6 mRNA as determined by a reverse transcription-polymerase chain reaction were also increased by staurosporine in parallel with the ability to induce IL-6 production. Compounds structurally related to staurosporine including K-252a (non-specific protein kinase inhibitor) and KT-5720 (inhibitor of cyclic AMP-dependent protein kinase, PKA), did not increase IL-6 production by peritoneal macrophages. Staurosporine-induced increases in IL-6 production and expression of IL-6 mRNA were decreased by the PKC inhibitors, H-7 (2.7-27 microM), Ro 31-8425 (1-10 microM) and calphostin C (0.3-3 microM) and by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor LY294002 (30-100 microM), but were further increased by the protein tyrosine kinase (PTK) inhibitor, genistein (12-37 microM). The staurosporine-induced increase in IL-6 production was not affected by the PKA inhibitor, H-89 (0.1-3 microM). These findings suggest that the induction of IL-6 production by staurosporine is secondary to elevation of IL-6 mRNA level, which, in turn, is positively regulated by the activation of PKC and PI 3-kinase and negatively regulated by the activation of PTK. PKA does not appear to play a significant role.

Requirement of phosphatidylinositol 3-kinase activity for bradykinin stimulation of NF-kappaB activation in cultured human epithelial cells

The signaling mechanisms utilized by bradykinin (BK) to activate the transcription factor nuclear factor kappaB (NF-kappaB) are poorly defined. We previously demonstrated that BK-stimulated NF-kappaB activation requires the small GTPase RhoA. We present evidence that BK-induced NF-kappaB activation both activates and requires phosphatidylinositol 3-kinase (PI 3-kinase) in A549 human epithelial cells. Pre-treatment with the PI 3-kinase-specific inhibitors, wortmannin, and LY294002 effectively blocked BK-induced PI 3-kinase activity. Wortmannin and LY294002 also abolished BK-induced NF-kappaB activation, as did transient transfection with a dominant negative mutant of the p85 subunit. BK-stimulated PI 3-kinase activity and NF-kappaB activation were sensitive to pertussis but not cholera toxin, suggesting that the B2 BK receptors transducing the response were coupled to Galphai or Galphao heterotrimeric G proteins. Tumor necrosis factor alpha (TNFalpha) also stimulated increased PI 3-kinase activity, however TNFalpha-stimulated NF-kappaB activation was not affected by the PI 3-kinase inhibitors or the p85 dominant negative mutant. These findings provide evidence that BK-induced NF-kappaB activation utilizes a signaling pathway that requires activity of both RhoA and PI 3-kinase and is distinct from the signaling pathway utilized by TNFalpha. Furthermore, we show that the p85 regulatory subunit is required for activation of PI 3-kinase activity by this G protein-coupled receptor.

The inhibitory guanine nucleotide-binding protein Gi2alpha induces and potentiates adipocyte differentiation

The present study further elucidates the involvement of the alpha-subunit of the GTP-binding protein Gi2 in the differentiation of murine 3T3-L1 cells. Control and vector-transfected cells attained a fully differentiated adipocyte phenotype showing ample lipid droplets. Cells expressing wild type (WT)-Gi2alpha or the constitutively active R179E-Gi2alpha, however, became enlarged, less confluent, and produced large amounts of lipids. Differentiation consistently increased the triglyceride (TAG) content in control cells. In both WT-Gi2alpha and R179E-Gi2alpha clones, a marked increase in TAG could be detected even prior to insulin/dexamethasone/isobutyl methylxanthine exposure. The activity of palmitoyl-CoA synthetase (PCS) and glycerophosphate acyltransferase (GPAT) also increased upon differentiation. WT-Gi2alpha and R179E-Gi2alpha overexpression also enhanced PCS and GPAT activities even before differentiation medium was added. The total amount of phospholipids (PL) generally increased upon differentiation; however, pre- and postdifferentiation values were insignificantly different in cells expressing WT-Gi2alpha and R179E-Gi2alpha. Differentiation altered the PL profile with a relative shift from phosphatidylcholine and phosphatidylethanolamine to phosphatidylinositol (PI) in differentiated cells. Finally, differentiation yielded a general increase in the activity of basal PI-phospholipase-C activity. Again, cells expressing WT-Gi2alpha and R179E-Gi2alpha demonstrated elevated enzyme activity and enhanced second messenger accumulation subsequent to differentiation. In summary, cells with the R179E-mutants of Gi2alpha exhibited stimulated lipid turnover and accumulation in both undifferentiated and differentiated cells.

Inhibition of Ca2+-dependent contraction in swine carotid artery by myosin kinase inhibitors

Experiments were designed to examine the efficacy of the MLCK inhibitors wortmannin and ML-9 in intact smooth muscle to determine whether contractile agonists can induce a Ca(2+) and myosin light chain phosphorylation-independent contraction. Both wortmannin and ML-9 reduced active stress in a dose-dependent manner. Both inhibitors interfered with Ca2+ mobilization in either the K(+)-depolarized or agonist activated swine carotid media at concentrations greater than 10 microM. Wortmannin reduced MRLC phosphorylation and stress to resting levels in stimulated tissues while Ca2+ remained above resting levels. There was no evidence for Ca2+ and MRLC phosphorylation-independent stress generation in swine arterial smooth muscle.

Wortmannin, an inhibitor of phosphatidylinositol kinases, blocks the MgATP-dependent recovery of Kir6.2/SUR2A channels

1. In order to investigate the mechanism underlying MgATP-dependent recovery of ATP-sensitive potassium (KATP) channels, we expressed Kir6.2/SUR2A (inwardly rectifying K+ channel subunit/sulfonylurea receptor) or C-terminal-truncated Kir6.2 (Kir6.2DeltaC26) in COS7 cells (Green monkey kidney cells), and carried out inside-out patch clamp experiments. 2. After patch excision in ATP-free internal solution, the activity of Kir6.2/SUR2A channels could be maximally recovered by the application of 5 mM MgATP. Subsequent application of 100 microM Ca2+ induced a rapid decay of Kir6.2/SUR2A activity to 11.6 +/- 1.1 % (mean +/- s.e.m.) of the control level (Ca2+-induced run-down; n = 64). 3. MgATP (5 mM) recovered 99.4 +/- 4.2 % (n = 13) of the Ca2+-induced run-down. Protein kinase inhibitors such as W-7, H-7, H-8 and genistein did not inhibit this reaction. However, wortmannin, an inhibitor of phosphatidylinositol 3- and 4-kinases, blocked the MgATP-dependent recovery in a concentration-dependent manner; the magnitudes of recovery were 35.7 +/- 7.2 % (10 microM) and 4.3 +/- 2.5 % (100 microM) of the Ca2+-induced run-down. 4. MgUDP (10 mM) reversed the Ca2+-induced run-down of Kir6.2/SUR2A channels by 60.4 +/- 7.6 % (n = 5). Wortmannin failed to modify this reaction. 5. Kir6.2DeltaC26 channels, which opened in the absence of SUR2A, were less sensitive to Ca2+; Kir6.2DeltaC26 channels were inactivated to 44.8 +/- 4.4 % (n = 14) by 100 microM Ca2+. MgATP recovered the Ca2+-induced run-down of Kir6.2DeltaC26 by 89.8 +/- 7. 7 % (n = 9), and 100 microM wortmannin inhibited this reaction (1.8 +/- 2 %, n = 7). 6. Application of 10 microM phosphatidylinositol-4, 5-bisphosphate (PI-4,5-P2) recovered the activity of Kir6.2/SUR2A channels after Ca2+-induced run-down (104.3 +/- 6.4 %, n = 10). Even after the MgATP-dependent recovery was blocked by 100 microM wortmannin, PI-4,5-P2 reactivated the channels (102.3 +/- 8.6 %, n = 5). Similar results were obtained with Kir6.2DeltaC26. 7. These results suggest that the entity of MgATP-dependent recovery may be membrane lipid phosphorylation rather than protein phosphorylation, and that synthesis of PI-4,5-P2 or phosphatidylinositol-3,4, 5-trisphosphate may upregulate Kir6.2 channels.

Basic secretagogues activate protein tyrosine phosphorylation and release of arachidonic acid in mast cells via a novel protein kinase C and phosphatidylinositol 3-kinase-dependent mechanism

Mast cells play a central role in inflammatory and immediate-type allergic reactions. These granulated cells release by a process of regulated exocytosis a variety of biologically active substances which are either preformed (e.g. histamine), or synthesized de novo following activation [e. g. metabolites of arachidonic acid (AA) and multifunctional cytokines]. Exocytosis in mast cells is activated either in response to aggregation of the receptors for immunoglobulin E (FcepsilonRI) or by the direct activation of pertussis toxin-sensitive G-proteins by a class of receptor mimetic agents, collectively known as basic secretagogues of mast cells. In the present study we show that compound 48/80 (c48/80), a synthetic member of the class of basic secretagogues, stimulates protein tyrosine phosphorylation of a number of as yet unidentified cellular substrates. These phosphorylations were inhibited by the tyrphostin AG-18, by the phosphatidylinositol 3-kinase inhibitor wortmannin and by the protein kinase C inhibitors K252a and GF1 09203X. These inhibitors also inhibited the release of AA induced by c48/80 but had no effect on exocytosis. Taken together, our findings suggest that basic secretagogues induce protein tyrosine phosphorylation as part of their parallel multiple signaling pathways which are presumably mediated by more than one G-protein. Both protein kinase C and phosphatidylinositol 3-kinase serve as intermediates in this signaling pathway. The protein tyrosine kinase signaling pathway, which mediates the activation of AA release, does not contribute to secretion of the preformed mediators such as histamine, but it might largely contribute to the de novo production of inflammatory mediators like leukotrienes and prostaglandins.

Nerve growth factor- and epidermal growth factor-stimulated translocation of the ADP-ribosylation factor-exchange factor GRP1 to the plasma membrane of PC12 cells requires activation of phosphatidylinositol 3-kinase and the GRP1 pleckstrin homology domain

ADP-ribosylation factors (ARFs) are small GTP-binding proteins that are regulators of vesicle trafficking in eukaryotic cells. GRP1 is a member of a family of ARF guanine-nucleotide-exchange factors that binds in vitro the lipid second messenger phosphatidylinositol 3,4, 5-trisphosphate [PtdIns(3,4,5)P3]. In order to study the effects of PtdIns(3,4,5)P3 on the function of GRP1, we have cloned the human homologue of GRP1, encoding for a protein which is 98.8% identical to mouse brain GRP1. Human GRP1 binds, via its pleckstrin homology (PH) domain, the inositol head group of PtdIns(3,4,5)P3, inositol 1, 3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4], with high affinity (Kd 32. 2+/-5.2 nM) and inositol phosphate specificity [Kd values for Ins(1, 3,4,5,6)P5, InsP6, Ins(1,3,4)P3 and Ins(1,4,5)P3: 283+/-32, >10000, >10000 and >10000 nM, respectively). Furthermore, GRP1 can accommodate addition of glycerol or diacetylglycerol to the 1-phosphate of Ins(1,3,4,5)P4, data that are consistent with its proposed role as a putative PtdIns(3,4,5)P3 receptor. To address whether GRP1 binds PtdIns(3,4,5)P3 in vivo, we have expressed a chimaera of green fluorescent protein (GFP) fused to the N-terminus of GRP1 in PC12 cells and, using confocal microscopy, examined its resultant localization in live cells. Stimulation with either nerve growth factor or epidermal growth factor (both at 100 ng/ml) results in a rapid, PH-domain dependent, translocation of GFP-GRP1 from the cytosol to the plasma membrane, which occurs with a time course that parallels the production of PtdIns(3,4,5)P3. This translocation is dependent on the activation of phosphatidylinositol 3-kinase, since it is inhibited by wortmannin (100 nM), LY294002 (50 microM) and by the co-expression with dominant negative p85. Taken together these data strongly suggest that GRP1 interacts in vivo with plasma membrane-located PtdIns(3,4,5)P3 and hence constitutes a true PtdIns(3,4,5)P3 receptor.

Multiple signaling pathways regulate cell surface expression and activity of the excitatory amino acid carrier 1 subtype of Glu transporter in C6 glioma

Neuronal and glial sodium-dependent transporters are crucial for the control of extracellular glutamate levels in the CNS. The regulation of these transporters is relatively unexplored, but the activity of other transporters is regulated by protein kinase C (PKC)- and phosphatidylinositol 3-kinase (PI3K)-mediated trafficking to and from the cell surface. In the present study the C6 glioma cell line was used as a model system that endogenously expresses the excitatory amino acid carrier 1 (EAAC1) subtype of neuronal glutamate transporter. As previously observed, phorbol 12-myristate 13-acetate (PMA) caused an 80% increase in transporter activity within minutes that cannot be attributed to the synthesis of new transporters. This increase in activity correlated with an increase in cell surface expression of EAAC1 as measured by using a membrane-impermeant biotinylation reagent. Both effects of PMA were blocked by the PKC inhibitor bisindolylmaleimide II (Bis II). The putative PI3K inhibitor, wortmannin, decreased L-[3H]-glutamate uptake activity by >50% within minutes. Wortmannin decreased the Vmax of L-[3H]-glutamate and D-[3H]-aspartate transport, but it did not affect Na+-dependent [3H]-glycine transport. Wortmannin also decreased cell surface expression of EAAC1. Although wortmannin did not block the effects of PMA on activity, it prevented the PMA-induced increase in cell surface expression. This trafficking of EAAC1 also was examined with immunofluorescent confocal microscopy, which supported the biotinylation studies and also revealed a clustering of EAAC1 at cell surface after treatment with PMA. These studies suggest that the trafficking of the neuronal glutamate transporter EAAC1 is regulated by two independent signaling pathways and also may suggest a novel endogenous protective mechanism to limit glutamate-induced excitotoxicity.

Wortmannin inhibits spreading and chemotaxis of rat osteoclasts in vitro

Wortmannin (WT) and 17beta-hydroxywortmannin (HWT), which are inhibitors of phosphatidylinositol-3(OH)-kinase (PI3K), have been shown previously to inhibit bone resorption in vitro and in vivo, possibly by interfering with formation of the osteoclast ruffled border. Since migration of osteoclasts also plays an important role in the process of bone resorption, we investigated the effects of these inhibitors on osteoclast morphology and motility. Both HWT and WT caused a sustained decrease in the planar area of osteoclasts in vitro (half maximal effect at 25 and 165 nM, respectively), with the effect of HWT on cell area more readily reversible than WT. These agents also caused accumulation of intracellular vesicles. Time-lapse video microscopy was used to record the migration of osteoclasts in response to macrophage colony-stimulating factor (M-CSF) or vehicle, flowing passively from a micropipette positioned 200-400 microm from the cell. M-CSF caused directed migration of osteoclasts, indicating chemotaxis (over 3 h osteoclasts migrated 96 +/- 14 microm in response to M-CSF vs. 11 +/- 2 microm in control experiments). Both WT (100 or 500 nM) and LY294002 (100 microM), a specific PI3K inhibitor structurally unrelated to WT, significantly inhibited osteoclast chemotaxis in response to M-CSF. Taken together, these effects of WT, HWT, and LY294002 are consistent with an important role for PI3K in regulating cytoskeletal function in osteoclasts. The inhibitory effects of WT and HWT on bone resorption may be due, in part, to impairment of osteoclast motility.

Downstream signalling events regulated by phosphatidylinositol 3-kinase activity

The phosphatidylinositol (PI) 3-kinase family of enzymes is now known to be regulated by several different upstream pathways in response to virtually all growth factors and cytokines. In the past few years, the phosphoinositides phosphorylated at the 3-OH position of the inositol ring have been shown to be lipid second messengers that may directly or indirectly regulate the activity of several different serine/threonine kinases. Consistent with the many different cellular events in which PI 3-kinase plays an important role, a diverse group of serine/threonine kinases are regulated downstream of PI 3-kinases, including protein kinase C (PKC) isoforms, p70 S6 kinase, and PKB/Akt. This review summarises studies done primarily in the past few years that have begun to unravel these targets of PI 3-kinase activity.

Stimulation of p70S6 kinase via a growth hormone-controlled phosphatidylinositol 3-kinase pathway leads to the activation of a PDE4A cyclic AMP-specific phosphodiesterase in 3T3-F442A preadipocytes

The challenge of 3T3-F442A fibroblasts with growth hormone led to both a decrease in the mobility on SDS/PAGE and activation of the PDE4A cyclic AMP-specific phosphodiesterase isoform PDE4A5. Activation was mediated by a JAK-2-dependent pathway coupled to the activation of phosphatidylinositol 3-kinase and p70S6 kinase. Activation was not dependent on the ability of growth hormone to stimulate ERK2 or protein kinase C or any effect on transcription. Blockade of activation of murine PDE4A5 ablated the ability of growth hormone to decrease intracellular cAMP levels. Antisense depletion of murine PDE4A5 mimicked the ability of rolipram to enhance the growth hormone-stimulated differentiation of 3T3-F442A cells to adipocytes. It is suggested that activation of PDE4A5 by growth hormone serves as a brake on the differentiation processes.

Phosphatidylinositol 3-kinase inhibitors block aortic smooth muscle cell proliferation in mid-late G1 phase: effect on cyclin-dependent kinase 2 and the inhibitory protein p27KIP1

In the present study, we investigated the involvement of phosphatidylinositol 3-kinase (PI 3-kinase) activity in the progression of vascular smooth muscle cells (VSMCs) throughout the G1 phase of cell cycle. Addition of two selective inhibitors of PI 3-kinase, LY 294002 or wortmannin, to quiescent VSMCs prevented serum-induced DNA synthesis in a dose-dependent manner with IC50 of 8.7 +/- 2.0 microM and 53.9 +/- 8.5 nM, respectively. Time course studies revealed that the two PI 3-kinase inhibitors blocked VSMC proliferation in mid-late G1 phase, about 6 h before the G1/S transition. This G1 growth arrest was due, at least in part, to the reduction of the CDK2 associated kinase activity resulting mainly from the upregulation of the inhibitory protein p27KIP1.

Action of insulin on the surface morphology of hepatocytes: role of phosphatidylinositol 3-kinase in insulin-induced shape change of microvilli

In previous studies we have shown that the insulin-responding glucose transporter isoform of 3T3-L1 adipocytes, GluT4, is almost completely located on microvilli. Furthermore, insulin caused the integration of these microvilli into the plasma membrane, suggesting that insulin-induced stimulation of glucose uptake may be due to the destruction of the cytoskeletal diffusion barrier formed by the actin filament bundle of the microvillar shaft regions [Lange et al. (1990) FEBS Lett. 261, 459-463; Lange et al. (1990) FEBS Lett. 276, 39-41]. Similar shape changes in microvilli were observed when the transport rates of adipocytes were modulated by glucose feeding or starvation. Here we demonstrate that the action of insulin on the surface morphology of hepatocytes is identical to that on 3T3L1 adipocytes; small and narrow microvilli on the surface of unstimulated hepatocytes were rapidly shortened and dilated on top of large domed surface areas. The aspect and mechanism of this effect are closely related to "membrane ruffling" induced by insulin and other growth factors. Pretreatment of hepatocytes with the PI 3-kinase inhibitor wortmannin (100 nM), which completely prevents transport stimulation by insulin in adipocytes and other cell types, also inhibited insulin-induced shape changes in microvilli on the hepatocyte surface. In contrast, vasopressin-induced microvillar shape changes in hepatocytes [Lange et al. (1997) Exp. Cell Res. 234, 486-497] were insensitive to wortmannin pretreatment. These findings indicate that PI 3-kinase products are necessary for stimulation of submembrane microfilament dynamics and that cytoskeletal reorganization is critically involved in insulin stimulation of transport processes. The mechanism of the insulin-induced cytoskeletal reorganization can be explained on the basis of the recent finding of Lu et al. [Biochemistry 35(1996) 14027-14034] that PI 3-kinase products exhibit much higher affinity for the profilin-actin complex than the primary products, PIP and PIP2. Thus, activated PI 3-kinase may direct a flux of profilin-actin complexes to the membrane locations of activated insulin receptors, where, due to the release of actin monomers after binding of profilactin to PI(3,4)P2 and PI(3,4,5)P3, massive actin polymerization is initiated. As a consequence, PI 3-kinase activation initiates a vectorial reorganization of the cellular actin system to membrane sites neighboring activated insulin receptors, giving rise to local membrane stress as visualized by extensive surface deformations and shortening of microvilli. In addition, extensive high-affinity binding of F-actin-barbed endcapping proteins enhances the cytoplasmic concentration of rapidly polymerizing filament ends. Consequently, the actin monomer concentration is lowered and the (cytoplasmic) pointed ends of the microvillar shaft bundle depolymerize and become shorter. The observations presented strengthen the previously postulated diffusion-barrier concept of glucose- and ion-uptake regulation and provide a mechanistic basis for explaining the action of insulin and other growth factors on transport processes across the plasma membrane.

Biology of chronic myelogenous leukemia

This article reviews the biology of chronic myelogenous leukemia (CML) and its effect on the process of hematopoiesis. The relevance of the BCR-ABL fusion protein as well as murine models are also discussed. CML has been studied more extensively than any other malignancy, yet the correlation between the clinical symptoms of chronic phase CML and the BCR-ABL oncoprotein is poorly understood. Insights from recent efforts both to develop a good in vivo animal model and to characterize the effect of the BCR-ABL oncoprotein on relevant signal molecules may lead to a better understanding of the pathophysiology of chronic phase CML and, thereby, to the development of targeted therapeutic approaches.

Regulation of GRP1-catalyzed ADP ribosylation factor guanine nucleotide exchange by phosphatidylinositol 3,4,5-trisphosphate

Cellular levels of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) are rapidly elevated in response to activation of growth factor receptor tyrosine kinases. This polyphosphoinositide binds the pleckstrin homology (PH) domain of GRP1, a protein that also contains 200 residues with high sequence similarity to a segment of the yeast Sec7 protein that functions as an ADP ribosylation exchange factor (ARF) (Klarlund, J., Guilherme, A., Holik, J. J., Virbasius, J. V., Chawla, A., and Czech, M. P. (1997) Science 275, 1927-1930). Here we show that dioctanoyl PtdIns(3,4,5)P3 binds the PH domain of GRP1 with a Kd = 0.5 microM, an affinity 2 orders of magnitude greater than dioctanoyl-PtdIns(4,5)P2. Further, the Sec7 domain of GRP1 is found to catalyze guanine nucleotide exchange of ARF1 and -5 but not ARF6. Importantly, PtdIns(3,4,5)P3, but not PtdIns(4,5)P2, markedly enhances the ARF exchange activity of GRP1 in a reaction mixture containing dimyristoylphosphatidylcholine micelles, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid, and a low concentration of sodium cholate. PtdIns(3,4,5)P3-mediated ARF nucleotide exchange through GRP1 is selectively blocked by 100 microM inositol 1,3,4,5-tetrakisphosphate, which also binds the PH domain of GRP1. Taken together, these data are consistent with the hypothesis that selective recruitment of GRP1 to PtdIns(3,4,5)P3 in membranes activates ARF1 and -5, known regulators of intracellular membrane trafficking.

Thrombospondin signaling of focal adhesion disassembly requires activation of phosphoinositide 3-kinase

Thrombospondin is an extracellular matrix protein involved in modulating cell adhesion. Thrombospondin stimulates a rapid loss of focal adhesion plaques and reorganization of the actin cytoskeleton in cultured bovine aortic endothelial cells. The focal adhesion labilizing activity of thrombospondin is localized to the amino-terminal domain, specifically amino acids 17-35. Use of a synthetic peptide (hep I), containing amino acids 17-35 of thrombospondin, enables us to examine the signaling mechanisms specifically involved in thrombospondin-induced disassembly of focal adhesions. We tested the hypothesis that activation of phosphoinositide 3-kinase is a necessary step in the thrombospondin-induced signaling pathway regulating focal adhesion disassembly. Both wortmannin and LY294002, membrane permeable inhibitors of phosphoinositide 3-kinase activity, blocked hep I-induced disassembly of focal adhesions. Similarly, wortmannin inhibited hep I-mediated actin microfilament reorganization and the hep I-induced translocation of alpha-actinin from focal adhesion plaques. Hep I also stimulated phosphoinositide 3-kinase activity approximately 2-3-fold as measured in anti-phosphoinositide 3-kinase and anti-phosphotyrosine immunoprecipitates. Increased immunoreactivity for the 85-kDa regulatory subunit in anti-phosphotyrosine immunoprecipitates suggests that the p85/p110 form of phosphoinositide 3-kinase is involved in this pathway. In 32Pi-labeled cells, hep I increased levels of phosphatidylinositol (3,4,5)-trisphosphate, the major product of phosphoinositide 3-kinase phosphorylation. These results suggest that thrombospondin signals the disassembly of focal adhesions and reorganization of the actin cytoskeleton by a pathway involving stimulation of phosphoinositide 3-kinase activity.