Regulation of phosphatidylinositol 3-kinase activity and phosphatidylinositol 3,4,5-trisphosphate accumulation by neutrophil priming agents

Neutrophil priming by agents such as TNF-alpha and GM-CSF causes a dramatic increase in the response of these cells to secretagogue agonists and affects the capacity of neutrophils to induce tissue injury. In view of the central role of phosphatidylinositol 3-kinase (PI3-kinase) in regulating NADPH oxidase activity we examined the influence of priming agents on agonist-stimulated phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) accumulation in human neutrophils. Pretreatment of neutrophils with TNF-alpha or GM-CSF, while not influencing fMLP-stimulated PtdIns(3,4,5)P3 accumulation at 5 s, caused a major increase in PtdIns(3,4,5)P3 at later times (10-60 s), which paralleled the augmented superoxide anion (O2-) response. The intimate relationship between PtdIns(3,4,5)P3 accumulation and O2- release was confirmed using platelet-activating factor, which caused full but transient priming of both responses. Likewise, LY294002, a PI3-kinase inhibitor, and genistein, a tyrosine kinase inhibitor, caused parallel inhibition of O2- generation and PtdIns(3,4,5)P3 accumulation; in contrast, radicicol, which inhibits receptor-mediated activation of p85 PI3-kinase, had no effect on either response. Despite major increases in PI3-kinase activity observed in p85 and anti-phosphotyrosine immunoprecipitates in growth factor-stimulated smooth muscle cells, no such increase was observed in primed/stimulated neutrophils. In contrast, both fMLP and TNF-alpha alone caused a 3-fold increase in PI3-kinase activity in p110gamma PI3-kinase immunoprecipitates. p21(ras) activation (an upstream regulator of PI3-kinase) was unaffected by priming. These data demonstrate that timing and magnitude of PtdIns(3,4,5)P3 accumulation in neutrophils correlate closely with O2- generation, that PI3-kinase-gamma is responsible for the enhanced PtdIns(3,4,5)P3 production seen in primed cells, and that factors other than activation of p21(ras) underlie this response.     

Direct activation of the epithelial Na(+) channel by phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate produced by phosphoinositide 3-OH kinase

The phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) is accepted to be a direct modulator of ion channel activity. The products of phosphoinositide 3-OH kinase (PI3K), PtdIns(3,4)P(2) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)), in contrast, are not. We report here activation of the epithelial Na(+) channel (ENaC) reconstituted in Chinese hamster ovary cells by PI3K. Insulin-like growth factor-I also activated reconstituted ENaC and increased Na(+) reabsorption across renal A6 epithelial cell monolayers via PI3K. Neither IGF-I nor PI3K affected the levels of ENaC in the plasma membrane. The effects of PI3K and IGF-I on ENaC activity paralleled changes in the plasma membrane levels of the PI3K product phospholipids, PtdIns(3,4)P(2)/PtdIns(3,4,5)P(3), as measured by evanescent field fluorescence microscopy. Both PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3) activated ENaC in excised patches. Activation of ENaC by PI3K and its phospholipid products corresponded to changes in channel open probability. We conclude that PI3K directly modulates ENaC activity via PtdIns(3,4)P(2) and PtdIns(3,4,5)P(3). This represents a novel transduction pathway whereby growth factors, such as IGF-I, rapidly modulate target proteins independent of signaling elicited by kinases downstream of PI3K.     

Regulation of phosphoinositide kinases in T cells. Evidence that phosphatidylinositol 3-kinase is not a substrate for T cell antigen receptor-regulated tyrosine kinases.

A phosphoinositide kinase that can phosphorylate phosphatidylinositol (PtdIns) is present in 4G10 monoclonal antibody (mAb) phosphotyrosine immunoprecipitates isolated from T cells activated via the T cell antigen receptor (TCR).CD3 complex. This PtdIns kinase is not the PtdIns 3-kinase that associates with activated protein tyrosine kinases in fibroblasts, since Western blotting and immunoprecipitation experiments with antibodies specific for the p85 alpha subunit of the PtdIns 3-kinase indicate that this polypeptide is not immunoprecipitated by the 4G10 mAb from TCR.CD3-activated Jurkat cells. Moreover, immunoprecipitated PtdIns 3-kinase isolated from T cells with p85 antibodies is inhibited when PtdIns is presented in Nonidet P-40, whereas the PtdIns kinase activity present in 4G10 mAb phosphotyrosine immunoprecipitates is enhanced in the presence of Nonidet P-40. In vitro kinase assays of PtdIns 3-kinase immunoprecipitated with p85 antibodies from T cells indicate that it associates with a serine kinase that can phosphorylate a p85 polypeptide. However, no protein tyrosine kinase activity capable of tyrosine phosphorylating p85 in vitro associates with p85 alpha immunoprecipitates in quiescent or TCR.CD3-activated T cells. These data suggest that the TCR.CD3 complex does not regulate PtdIns 3-kinase activity by a mechanism that involves protein tyrosine kinases.     

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.     

Nuclear phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-kinase, Akt, and PTen: emerging key regulators of anti-apoptotic signaling and carcinogenesis

Inositol lipid-derived second messengers have long been known to have an important regulatory role in cell physiology. Phosphatidylinositol 3-kinase (PI3K) synthesizes the second messenger 3,4,5'-phosphatidylinositol trisphosphate (Ptdlns 3,4,5P3) which controls a multitude of cell functions. Down-stream of PI3K/PtdIns 3,4,5P3 is the serine/threonine protein kinase Akt (protein kinase B, PKB). Since the PI3K/ PtdIns 3,4,5P3 /Akt pathway stimulates cell proliferation and suppresses apoptosis, it has been implicated in carcinogenesis. The lipid phosphatase PTEN is a negative regulator of this signaling network. Until recently, it was thought that this signal transduction cascade would promote its anti-apoptotic effects when activated in the cytoplasm. Several lines of evidence gathered over the past 20 years, have highlighted the existence of an autonomous nuclear inositol lipid cycle, strongly suggesting that lipids are important components of signaling pathways operating at the nuclear level. PI3K, PtdIns(3,4,5)P3, Akt, and PTEN have been identified within the nucleus and recent findings suggest that they are involved in cell survival also by operating in this organelle, through a block of caspase-activated DNase and inhibition of chromatin condensation. Here, we shall summarize the most updated and intriguing findings about nuclear PI3K/ PtdIns(3,4,5)P3/Akt/PTEN in relationship with carcinogenesis and suppression of apoptosis.     

Novel function of phosphoinositide 3-kinase in T cell Ca2+ signaling. A phosphatidylinositol 3,4,5-trisphosphate-mediated Ca2+ entry mechanism

This study presents evidence that phosphoinositide (PI) 3-kinase is involved in T cell Ca(2+) signaling via a phosphatidylinositol 3,4, 5-trisphosphate PI(3,4,5)P(3)-sensitive Ca(2+) entry pathway. First, exogenous PI(3,4,5)P(3) at concentrations close to its physiological levels induces Ca(2+) influx in T cells, whereas PI(3,4)P(2), PI(4, 5)P(2), and PI(3)P have no effect on [Ca(2+)](i). This Ca(2+) entry mechanism is cell type-specific as B cells and a number of cell lines examined do not respond to PI(3,4,5)P(3) stimulation. Second, inhibition of PI 3-kinase by wortmannin and by overexpression of the dominant negative inhibitor Deltap85 suppresses anti-CD3-induced Ca(2+) response, which could be reversed by subsequent exposure to PI(3,4,5)P(3). Third, PI(3,4,5)P(3) is capable of stimulating Ca(2+) efflux from Ca(2+)-loaded plasma membrane vesicles prepared from Jurkat T cells, suggesting that PI(3,4,5)P(3) interacts with a Ca(2+) entry system directly or via a membrane-bound protein. Fourth, although D-myo-inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4, 5)P(4)) mimics PI(3,4,5)P(3) in many aspects of biochemical functions such as membrane binding and Ca(2+) transport, we raise evidence that Ins(1,3,4,5)P(4) does not play a role in anti-CD3- or PI(3,4,5)P(3)-mediated Ca(2+) entry. This PI(3,4,5)P(3)-stimulated Ca(2+) influx connotes physiological significance, considering the pivotal role of PI 3-kinase in the regulation of T cell function. Given that PI 3-kinase and phospholipase C-gamma form multifunctional complexes downstream of many receptor signaling pathways, we hypothesize that PI(3,4,5)P(3)-induced Ca(2+) entry acts concertedly with Ins(1,4,5)P(3)-induced Ca(2+) release in initiating T cell Ca(2+) signaling. By using a biotinylated analog of PI(3,4,5)P(3) as the affinity probe, we have detected several putative PI(3,4,5)P(3)-binding proteins in T cell plasma membranes.     

Specific binding of the C-terminal Src homology 2 domain of the p85alpha subunit of phosphoinositide 3-kinase to phosphatidylinositol 3,4,5-trisphosphate. Localization and engineering of the phosphoinositide-binding motif

Phosphoinositide second messengers, generated from the action of phosphoinositide 3-kinase (PI3K), mediate an array of signaling pathways through the membrane recruitment and activation of downstream effector proteins. Although pleckstrin domains of many target proteins have been shown to bind phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) and/or phosphatidylinositol 3,4-bisphosphate (PI(3,4)P(2)) with high affinity, published data concerning the phosphoinositide binding specificity of Src homology 2 (SH2) domains remain conflicting. Using three independent assays, we demonstrated that the C-terminal (CT-)SH2 domain, but not the N-terminal SH2 domain, on the PI3K p85alpha subunit displayed discriminative affinity for PIP(3). However, the binding affinity diminished precipitously when the acyl chain of PIP(3) was shortened. In addition, evidence suggests that the charge density on the phosphoinositol ring represents a key factor in determining the phosphoinositide binding specificity of the CT-SH2 domain. In light of the largely shared structural features between PIP(3) and PI(4,5)P(2), we hypothesized that the PIP(3)-binding site on the CT-SH2 domain encompassed a sequence that recognized PI(4,5)P(2). Based on a consensus PI(4,5)P(2)-binding sequence (KXXXXXKXKK; K denotes Arg, Lys, and His), we proposed the sequence (18)RNKAENLLRGKR(29) as the PIP(3)-binding site. This binding motif was verified by using a synthetic peptide and site-directed mutagenesis. More importantly, neutral substitution of flanking Arg(18) and Arg(29) resulted in a switch of ligand specificity of the CT-SH2 domain to PI(4,5)P(2) and PI(3,4)P(2), respectively. Together with computer modeling, these mutagenesis data suggest a pseudosymmetrical relationship in the recognition of the phosphoinositol head group at the binding motif.     

Evidence that the lipid phosphatase SHIP-1 regulates T lymphocyte morphology and motility

SHIP-1 negatively regulates the PI3K pathway in hematopoietic cells and has an emerging role in T lymphocyte biology. PI3K and SHIP can regulate cell migration in leukocytes, particularly in neutrophils, although their role in T cell migration has been less clear. Therefore, we sought to explore the role of SHIP-1 in human CD4(+) T lymphocyte cell migration responses to chemoattractants using a lentiviral-mediated expression system and a short hairpin RNA approach. Silencing of SHIP-1 leads to increased basal phosphorylation of protein kinase B/Akt and its substrate GSK3β, as well as an increase in basal levels of polymerized actin, suggesting that SHIP-1 might regulate changes in the cytoskeleton. Accordingly, silencing of SHIP-1 led to loss of microvilli and ezrin/radixin/moesin phosphorylation, which could not be rescued by the PI3K inhibitor Ly294002. There were striking morphological changes, including a loss of microvilli projections, which mirrored changes in wild type cells after stimulation with the chemokine CXCL11. There was no defect in directional T cell migration toward CXCL11 in the SHIP-1-silenced cells but, importantly, there was a defect in the overall basal motility of SHIP-1 knockdown cells. Taken together, these results implicate SHIP-1 as a key regulator of basal PI3K signaling in human CD4(+) T lymphocytes with important phosphatase-independent actions, which together are key for maintaining normal morphology and basal motility.     

Phosphoinositide lipid phosphatases: natural regulators of phosphoinositide 3-kinase signaling in T lymphocytes

The phosphoinositide 3-kinase signaling pathway has been implicated in a range of T lymphocyte cellular functions, particularly growth, proliferation, cytokine secretion, and survival. Dysregulation of phosphoinositide 3-kinase-dependent signaling and function in leukocytes, including B and T lymphocytes, has been implicated in many inflammatory and autoimmune diseases. As befits a pivotal signaling cascade, several mechanisms exist to ensure that the pathway is tightly regulated. This minireview focuses on two lipid phosphatases, viz. the 3'-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10) and SHIP (Src homology 2 domain-containing inositol-5-phosphatase). We discuss their role in regulating T lymphocyte signaling as well their potential as future therapeutic targets.     

The CXC chemokine stromal cell-derived factor activates a Gi-coupled phosphoinositide 3-kinase in T lymphocytes

The cellular effects of stromal cell-derived factor-1 (SDF-1) are mediated primarily by binding to the CXC chemokine receptor-4. We report in this study that SDF-1 and its peptide analogues induce a concentration- and time-dependent accumulation of phosphatidylinositol-(3,4,5)-trisphosphate (PtdIns(3,4,5)P3) in Jurkat cells. This SDF-1-stimulated generation of D-3 phosphoinositide lipids was inhibited by pretreatment of the cells with an SDF-1 peptide antagonist or an anti-CXCR4 Ab. In addition, the phosphoinositide 3 (PI 3)-kinase inhibitors wortmannin and LY294002, as well as the Gi protein inhibitor pertussis toxin, also inhibited the SDF-1-stimulated accumulation of PtdIns(3,4,5)P3. The effects of SDF-1 on D-3 phosphoinositide lipid accumulation correlated well with activation of the known PI 3-kinase effector protein kinase B, which was also inhibited by wortmannin and pertussis toxin. Concentrations of PI 3-kinase inhibitors, sufficient to inhibit PtdIns(3,4,5)P3 accumulation, also inhibited chemotaxis of Jurkat and peripheral blood-derived T lymphocytes in response to SDF-1. In contrast, SDF-1-stimulated actin polymerization was only partially inhibited by PI 3-kinase inhibitors, suggesting that while chemotaxis is fully dependent on PI 3-kinase activation, actin polymerization requires additional biochemical inputs. Finally, SDF-1-stimulated extracellular signal-related kinase (ERK)-1/2 mitogen-activated protein kinase activation was inhibited by PI 3-kinase inhibitors. In addition, the mitogen-activated protein/ERK kinase inhibitor PD098059 partially attenuated chemotaxis in response to SDF-1. Hence, it appears that ERK1/2 activation is dependent on PI 3-kinase activation, and both biochemical events are involved in the regulation of SDF-1-stimulated chemotaxis.     

Overexpression of NBS1 contributes to transformation through the activation of phosphatidylinositol 3-kinase/Akt

Nijmegen breakage syndrome (NBS) is a chromosomal instability syndrome associated with cancer predisposition, radiosensitivity, microcephaly, and growth retardation. The NBS gene product, NBS1 (p95) or nibrin, is a part of the hMre11 complex, a central player associated with double strand break repair. We previously demonstrated that c-Myc directly activates NBS1 expression. Here we have shown that constitutive expression of NBS1 in Rat1a and HeLa cells induces/enhances their transformation. Repression of endogenous NBS1 levels using short interference RNA reduces the transformation activity of two tumor cell lines. Increased NBS1 expression is observed in 40-52% of non-small cell lung carcinoma, hepatoma, and esophageal cancer samples. NBS1 overexpression stimulates phosphatidylinositol (PI) 3-kinase activity, leading to increased phosphorylation levels of Akt and its downstream targets such as glycogen synthase kinase 3beta and mammalian target of rapamycin in different cell lines and tumor samples. Transformation induced by NBS1 overexpression can be inhibited by a PI3-kinase inhibitor (LY294002). Repression of endogenous Akt expression by short interference RNA decreases the transformation activity of Rat1a cells overexpressing NBS1. These results indicate that overexpression of NBS1 is an oncogenic event that contributes to transformation through the activation of PI3-kinase/Akt.     

EFR3 and phosphatidylinositol 4-kinase IIIα regulate insulin-stimulated glucose transport and GLUT4 dispersal in 3T3-L1 adipocytes

Insulin stimulates glucose transport in muscle and adipocytes. This is achieved by regulated delivery of intracellular glucose transporter (GLUT4)-containing vesicles to the plasma membrane where they dock and fuse, resulting in increased cell surface GLUT4 levels. Recent work identified a potential further regulatory step, in which insulin increases the dispersal of GLUT4 in the plasma membrane away from the sites of vesicle fusion. EFR3 is a scaffold protein that facilitates localization of phosphatidylinositol 4-kinase type IIIα to the cell surface. Here we show that knockdown of EFR3 or phosphatidylinositol 4-kinase type IIIα impairs insulin-stimulated glucose transport in adipocytes. Using direct stochastic reconstruction microscopy, we also show that EFR3 knockdown impairs insulin stimulated GLUT4 dispersal in the plasma membrane. We propose that EFR3 plays a previously unidentified role in controlling insulin-stimulated glucose transport by facilitating dispersal of GLUT4 within the plasma membrane.     

Naringenin inhibits phosphoinositide 3-kinase activity and glucose uptake in 3T3-L1 adipocytes

Previous studies have shown that flavonoids inhibit glucose uptake in cultured cells. In this report, we show that the grapefruit flavanone naringenin inhibited insulin-stimulated glucose uptake in 3T3-L1 adipocytes in a dose-dependent manner. Naringenin acts by inhibiting the activity of phosphoinositide 3-kinase (PI3K), a key regulator of insulin-induced GLUT4 translocation. Although naringenin did not alter the phosphotyrosine status of the insulin receptor, insulin receptor substrate proteins, or PI3K, it did inhibit the phosphorylation of the downstream signaling molecule Akt. In an in vitro kinase assay, naringenin inhibited PI3K activity. A physiologically attainable dose of 6 microM naringenin reduced insulin-stimulated glucose uptake by approximately 20%. This inhibitory effect remained 24h after the removal of naringenin from the culture medium. Collectively, our findings suggest that the regular consumption of naringenin in grapefruit may exacerbate insulin resistance in susceptible individuals via impaired glucose uptake in adipose tissue.     

Interleukin 2- and polyomavirus middle T antigen-induced modification of phosphatidylinositol 3-kinase activity in activated T lymphocytes.

Stimulation of activated T lymphocytes with interleukin 2 (IL-2) results in rapid increases in intracellular protein tyrosine phosphorylation. Both the identity of the protein tyrosine kinase (PTK) activated by IL-2 receptor ligation and the identities of the critical target proteins for this PTK remain largely undefined. In this article, we demonstrate that stimulation of activated murine or human T cells with IL-2 for 10 to 30 min induces two- to threefold increases in the level of phosphatidylinositol (PtdIns) 3-kinase activity present in antiphosphotyrosine (p-Tyr) antibody immunoprecipitates from these cells. Furthermore, substantial levels of PtdIns 3-kinase activity were coprecipitated from IL-2-deprived T cells by antibodies to the src-related PTK p59fyn. Cellular stimulation with IL-2 induced a two- to threefold increase in the level of p59fyn-associated PtdIns 3-kinase activity. To examine the effect of a constitutive increase in PtdIns 3-kinase activity on the growth factor responsiveness of activated T cells, murine CTLL-2 cells were transfected with a polyomavirus middle T antigen (MTAg) expression vector. Anti-p-Tyr and anti-p59fyn immunoprecipitates from MTAg-transfected CTLL-2 cells contained three- to sixfold higher levels of PtdIns 3-kinase activity than wild-type cells. Immune complex kinase assays revealed that MTAg expression concomitantly induced a constitutive threefold increase in the PTK activity of p59fyn in these cells. However, stable MTAg expression did not abrogate the dependence of CTLL-2 cells on exogenous IL-2 for continued growth and proliferation.     

Evidence that phosphatidylinositol 3-kinase is involved in sperm-induced tyrosine kinase signaling in Xenopus egg fertilization

Background  

Studies have examined the function of PI 3-kinase in the early developmental processes that operate in oocytes or early embryos of various species. However, the roles of egg-associated PI 3-kinase and Akt, especially in signal transduction at fertilization, are not well understood.     

Localization and insulin-regulated relocation of phosphoinositide 5-kinase PIKfyve in 3T3-L1 adipocytes

The mammalian phosphoinositide kinase PIKfyve catalyzes the synthesis of phosphatidylinositol 5-P and phosphatidylinositol 3,5-P(2), thought essential in cellular functions, including membrane trafficking. To discern the intracellular loci of PIKfyve products' formation, we have examined the localization of PIKfyve protein versus enzymatic activity and a possible acutely regulated redistribution in 3T3-L1 adipocytes. Subcellular fractions of resting cells that were positive for immunoreactive PIKfyve, such as cytosol ( approximately 76%), internal structures (low density microsomal fraction (LDM), composed of recycling endosomes, GLUT4 storage compartment, Golgi, and cytoskeletal elements) ( approximately 20%), and plasma membrane (( approximately )4%), expressed enzymatically active PIKfyve. While the presence of a FYVE finger in PIKfyve predicts early endosome targeting, density gradient sedimentation, immunoadsorption, and fluorescence microscopy analyses segregated the LDM-associated PIKfyve from the membranes of the recycling endosomes and GLUT4. PIKfyve fluorescence staining largely coincided with trans-Golgi network/multivesicular body markers, indicating PIKfyve's role in the late endocytic/biosynthetic pathways. A subfraction of particulate PIKfyve resisted nonionic detergent treatment, implying association with cytoskeletal structures, previously found positive for key members of the insulin signaling cascade. Upon acute stimulation of 3T3-L1 adipocytes with insulin or pervanadate, a portion of the cytosolic PIKfyve was recruited onto LDM, which was coupled with a commensurate increase of PIKfyve lipid kinase activity and an electrophoretic mobility shift. We suggest the recruited PIKfyve specifies the site and timing of phosphoinositide signals that are relevant to the acute insulin action.     

Increased phosphoinositide 3-kinase activity induces a lymphoproliferative disorder and contributes to tumor generation in vivo.

Alterations in the cell division:cell death ratio induce multiple autoimmune and transformation processes. Phosphoinositide 3-kinase (PI3K) controls cell division and cell death in vitro, but its effect on the function of the cellular immune system and on tumor formation in mammals is poorly characterized. Here we show that transgenic mice expressing in T lymphocytes an active form of PI3K derived from a thymic lymphoma, p65(PI3K), developed an infiltrating lymphoproliferative disorder and autoimmune renal disease with an increased number of T lymphocytes exhibiting a memory phenotype and reduced apoptosis. This pathology was strikingly similar to that described in mice exhibiting heterozygous loss of the tumor suppressor PTEN, a lipid and protein phosphatase. We show that overexpression of PTEN selectively blocks p65(PI3K)-induced 3T3 fibroblast transformation. Moreover, the early development of T cell lymphomas in p65(PI3K) Tg p53(-/-) mice indicated that PI3K contributes to tumor development. These observations demonstrate that constitutive activation of PI3K extends T cell survival in vivo, affects T cell homeostasis, and contributes to tumor generation, supporting a model in which selective increases in one type of PTEN substrate, the PI3K-derived lipid products, induce tumorigenesis. PI3K thus emerges as a potential target in autoimmune disease and cancer therapy.     

Calmodulin antagonists inhibit insulin-stimulated GLUT4 (glucose transporter 4) translocation by preventing the formation of phosphatidylinositol 3,4,5-trisphosphate in 3T3L1 adipocytes

It has been previously reported that calmodulin plays a regulatory role in the insulin stimulation of glucose transport. To examine the basis for this observation, we examined the effect of a panel of calmodulin antagonists that demonstrated a specific inhibition of insulin-stimulated glucose transporter 4 (GLUT4) but not insulin- or platelet-derived growth factor (PDGF)-stimulated GLUT1 translocation in 3T3L1 adipocytes. These treatments had no effect on insulin receptor autophosphorylation or tyrosine phosphorylation of insulin receptor substrate 1 (IRS1). Furthermore, IRS1 or phosphotyrosine antibody immunoprecipitation of phosphatidylinositol (PI) 3-kinase activity was not affected. Despite the marked insulin and PDGF stimulation of PI 3-kinase activity, there was a near complete inhibition of protein kinase B activation. Using a fusion protein of the Grp1 pleckstrin homology (PH) domain with the enhanced green fluorescent protein, we found that the calmodulin antagonists prevented the insulin stimulation of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] formation in vivo. Similarly, although PDGF stimulation increased PI 3-kinase activity in in vitro immunoprecipitation assays, there was also no significant formation of PI(3,4,5)P3 in vivo. These data demonstrate that calmodulin antagonists prevent insulin-stimulated GLUT4 translocation by inhibiting the in vivo production of PI(3,4,5)P3 without directly affecting IRS1- or phosphotyrosine-associated PI 3-kinase activity. This phenomenon is similar to that observed for the PDGF stimulation of 3T3L1 adipocytes.