Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

Role of the steroid receptor coactivator SRC-3 in cell growth

Steroid receptor coactivator 3 (SRC-3/p/CIP/AIB1/ACTR/RAC3/TRAM-1) is a member of the p160 family of nuclear receptor coactivators, which includes SRC-1 (NCoA-1) and SRC-2 (TIF2/GRIP1/NCoA2). Previous studies indicate that SRC-3 is required for normal animal growth and is often amplified or overexpressed in many cancers, including breast and prostate cancers. However, the mechanisms of SRC-3-mediated growth regulation remain unclear. In this study, we show that overexpression of SRC-3 stimulates cell growth to increase cell size in prostate cancer cell lines. Furthermore, our results indicate that overexpression of SRC-3 can modulate the AKT signaling pathway in a steroid-independent manner, which results in the activation of AKT/mTOR signaling concomitant with an increase in cell size. In contrast, down-regulation of SRC-3 expression in cells by small interfering RNA decreases cell growth, leading to a smaller cell size. Similarly, in SRC-3 null mutant mice, AKT signaling is down-regulated in normally SRC-3-expressing tissues. Taken together, these results suggest that SRC-3 is an important modulator for mammalian cell growth.

Early stage-specific inhibitions of cardiomyocyte differentiation and expression of Csx/Nkx-2.5 and GATA-4 by phosphatidylinositol 3-kinase inhibitor LY294002

Inhibition of phosphatidylinositol 3-kinase (PI3-kinase) has been reported to block cardiomyocyte differentiation. However, at which stage PI3-kinase plays this important role and what its molecular targets are remain unknown. To answer these questions, we induced cardiomyocyte differentiation of P19CL6 mouse embryonal carcinoma cells and investigated the activation of PI3-kinase by analyzing phospho-Akt. We also treated P19CL6 cells with the PI3-kinase-specific inhibitor LY294002 either continuously or at various time points and monitored the expression of cardiac contractile proteins and transcription factors. Most cells differentiated into sarcomeric myosin heavy chain (MHC)-positive cardiomyocytes on day 16 after induction. An increase in phospho-Akt was observed after induction and was maintained throughout the differentiation. LY294002 treatment restricted to the phase from days 0 to 4 was sufficient to inhibit cardiomyocyte differentiation in a dose-dependent manner. In contrast, LY294002 treatment either from days 4 to 8 or from days 8 to 12 did not cause significant changes in sarcomeric MHC expression. LY294002 treatment from days 0 to 4 also suppressed Csx/Nkx-2.5 and GATA-4 expression. These results demonstrate that PI3-kinase becomes activated and plays a pivotal role at a very early stage of cardiomyocyte differentiation, possibly by modulating the expression of the cardiac transcription factors.

Sphingomyelin in the suppression of colon tumors: prevention versus intervention

Intestinal cells are regularly exposed to sphingolipid metabolites, i.e., ceramide and sphingoid bases, after hydrolysis of complex sphingolipids from the diet. These metabolites are known regulators of cell growth, differentiation, and death. Non-pharmacological amounts in the diet have been shown to inhibit early stages of chemically induced colon cancer in mice. To distinguish between chemopreventive and chemotherapeutic effects of sphingomyelin supplements, mice were fed sphingomyelin before and after tumor initiation. Both applications drastically reduced tumor formation, without a significant difference among the groups, indicating that sphingolipids are as effective in the chemoprevention of tumors as in early intervention. The normalization of cell proliferation and rate of apoptosis, but not the induction of differentiation, seem to be key players in the suppression of tumor formation by dietary sphingomyelin. This may have implications for the development of a cancer prevention or treatment strategy with sphingolipids as an alternative to conventional drugs.

The phosphoinositide 3-kinase/Akt pathway regulates cell cycle progression of HL60 human leukemia cells through cytoplasmic relocalization of the cyclin-dependent kinase inhibitor p27(Kip1) and control of cyclin D1 expression

The serine/threonine protein kinase Akt, a downstream effector of phosphoinositide 3-kinase (PI3K), plays a pivotal role in tumorigenesis because it affects the growth and survival of cancer cells. Several laboratories have demonstrated that Akt inhibits transcriptional activation of a number of related forkhead transcription factors now referred to as FoxO1, FoxO3, and FoxO4. Akt-regulated forkhead transcription factors are involved in the control of the expression of both the cyclin-dependent kinase (cdk) inhibitor p27(Kip1) and proapoptotic Bim protein. Very little information is available concerning the importance of the PI3K/Akt pathway in HL60 human leukemia cells. Here, we present our findings showing that the PI3K/Akt axis regulates cell cycle progression of HL60 cells through multiple mechanisms also involving the control of FoxO1 and FoxO3. To this end, we took advantage of a HL60 cell clone (HL60AR cells) with a constitutively activated PI3K/Akt axis. When compared with parental (PT) HL60 cells, HL60AR cells displayed higher levels of phosphorylated FoxO1 and FoxO3. In AR cells forkhead factors localized predominantly in the cytoplasm, whereas in PT cells they were mostly nuclear. AR cells proliferated faster than PT cells and showed a lower amount of the cdk inhibitor p27(Kip1), which was mainly found in the cytoplasm and was hyperphosphorylated on threonine residues. AR cells also displayed higher levels of cyclin D1 and phosphorylated p110 Retinoblastoma protein. The protein levels of cdk2, cdk4, and cdk6 were not altered in HL60AR cells, whereas the activities of both ckd2 and cdk6 were higher in AR than in PT cells. These results show that in HL60 cells the PI3K/Akt signaling pathway may be involved in the control of the cell cycle progression most likely through mechanisms involving the activation of forkhead transcription factors.

Nuclear magnetic resonance structure of the P395S mutant of the N-SH2 domain of the p85 subunit of PI3 kinase: an SH2 domain with altered specificity

Understanding the specificity of Src homology 2 (SH2) domains is important because of their critical role in cell signaling. Previous genetic analysis has characterized mutants of the N-terminal src homology 2 (SH2) domain of the p85 subunit of phosphoinositide 3-kinase (PI3K). The P395S mutant exhibits a specificity for phosphopeptide binding different from that of the wild-type SH2. The P395S mutant has an increased affinity for the platelet-derived growth factor receptor (PDGFr) compared to polyomavirus middle T antigen (MT). Solution structures of the P395S mutant of the p85 N-SH2 alone and complexed to a PDGFr phosphopeptide were determined to explain the change in specificity. Chemical shift perturbations caused by different peptides were compared for mutant and wild-type structures. The results show that the single P395S mutation has broad effects on the structure. Furthermore, they provide a rationale for the observed changes in binding preference.

Topographical expression of class IA and class II phosphoinositide 3-kinase enzymes in normal human tissues is consistent with a role in differentiation

Background

Growth factor, cytokine and chemokine-induced activation of PI3K enzymes constitutes the start of a complex signalling cascade, which ultimately mediates cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. The PI3K enzyme family is divided into 3 classes; class I (subdivided into IA and IB), class II (PI3K-C2α, PI3K-C2β and PI3K-C2γ) and class III PI3K. Expression of these enzymes in human tissue has not been clearly defined.

Methods

In this study, we analysed the immunohistochemical topographical expression profile of class IA (anti-p85 adaptor) and class II PI3K (PI3K-C2α and PI3K-C2β) enzymes in 104 formalin-fixed, paraffin embedded normal adult human (age 33–71 years, median 44 years) tissue specimens including those from the gastrointestinal, genitourinary, hepatobiliary, endocrine, integument and lymphoid systems. Antibody specificity was verified by Western blotting of cell lysates and peptide blocking studies. Immunohistochemistry intensity was scored from undetectable to strong.

Results

PI3K enzymes were expressed in selected cell populations of epithelial or mesenchymal origin. Columnar epithelium and transitional epithelia were reactive but mucous secreting and stratified squamous epithelia were not. Mesenchymal elements (smooth muscle and endothelial cells) and glomerular epithelium were only expressed PI3K-C2α while ganglion cells expressed p85 and PI3K-C2β. All three enzymes were detected in macrophages, which served as an internal positive control. None of the three PI3K isozymes was detected in the stem cell/progenitor compartments or in B lymphocyte aggregates.

Conclusions

Taken together, these data suggest that PI3K enzyme distribution is not ubiquitous but expressed selectively in fully differentiated, non-proliferating cells. Identification of the normal in vivo expression pattern of class IA and class II PI3K paves the way for further analyses which will clarify the role played by these enzymes in inflammatory, neoplastic and other human disease conditions.

Phosphatidylinositol 3'-kinase-dependent activation of renal mesangial cell Ki-Ras and ERK by advanced glycation end products

Advanced glycation end products (AGEs) are produced by the non-enzymatic glycation of proteins and lipids. AGE levels are pathologically elevated in a number of inflammatory diseases and in diabetes mellitus. There is evidence that AGEs, acting through the receptor for AGEs, contribute to diabetic complications. Nephropathy is a major complication of diabetes mellitus. However, the initiating molecular events that trigger diabetic renal disease are unknown. Renal mesangial cells produce excess extracellular matrix in response to treatment with transforming growth factor-beta, and excess mesangial cell matrix production, by impairing glomerular filtration, contributes to diabetic nephropathy. AGEs are known to trigger the autocrine production and release of transforming growth factor-beta. However, it is unclear how AGEs signal in mesangial cells. Here we show that treatment of mesangial cells with AGEs and with the receptor for AGEs agonist S100 triggers activation of the extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3'-kinase (PI3K) pathways. AGEs trigger the GTP loading of mesangial cell Ras, and AGE activation of ERK requires Ras. We observe that Ki-Ras, but not Ha-Ras, is the target of AGE action. Surprisingly, inhibition of PI3K blocks both ERK and Ki-Ras activation. We also observe that activation of ERK and the PI3K target kinase protein kinase-B is blocked with free radical scavengers, indicating a role for reactive oxygen species in AGE recruitment of PI3K. Thus, AGEs signal to Ki-Ras and ERK through reactive oxygen species-dependent activation of PI3K.

Beta-arrestin1 mediates insulin-like growth factor 1 (IGF-1) activation of phosphatidylinositol 3-kinase (PI3K) and anti-apoptosis

beta-arrestins (1 and 2) are widely expressed cytosolic proteins that play central roles in G protein-coupled receptor signaling. beta-arrestin1 is also recruited to the insulin-like growth factor 1 (IGF-1) receptor, a receptor tyrosine kinase, upon agonist binding. Here we report that, in response to IGF-1 stimulation, beta-arrestin1 mediates activation of phosphatidylinositol 3-kinase in a pathway that leads to the subsequent activation of Akt and anti-apoptosis. This process is independent of both Gi and ERK activity. The pathway fails in mouse embryo fibroblasts lacking both beta-arrestins and is restored by stable transfection of beta-arrestin1. Remarkably, this pathway is insensitive to chemical inhibition of IGF-1 receptor tyrosine kinase activity. These results suggest that, in addition to their roles in G protein-coupled receptor signaling, beta-arrestins couple the IGF-1 receptor tyrosine kinase to the phosphatidylinositol 3-kinase system and suggest that this mechanism is operative independently of the tyrosine kinase activity of the receptor.

SHIP-2 and PTEN are expressed and active in vascular smooth muscle cell nuclei, but only SHIP-2 is associated with nuclear speckles

Recently, the control of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3)-dependant signaling by phosphatases has emerged, but there is a shortage of information on intranuclear PtdIns(3,4,5)P3 phosphatases. Therefore, we investigated the dephosphorylation of [32P]PtdIns(3,4,5)P3 specifically labeled on the D-3 position of the inositol ring in membrane-free nuclei isolated from pig aorta vascular smooth muscle cells (VSMCs). In vitro PtdIns(3,4,5)P3 phosphatase assays revealed the production of both [32P]PtdIns(3,4)P2 and inorganic phosphate, demonstrating the presence of PtdIns(3,4,5)P3 5- and 3-phosphatase activities inside the VSMC nucleus, respectively. Both activities presented the same potency in cellular lysates, whereas the nuclear PtdIns(3,4,5)P3 5-phosphatase activity appeared to be the most efficient. Immunoblot experiments showed for the first time the expression of the 5-phosphatase SHIP-2 (src homology 2 domain-containing inositol phosphatase) as well as the 3-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10) in VSMC nuclei. In addition, immunoprecipitations from nuclear fractions indicated a [32P]PtdIns(3,4,5)P3 dephosphorylation by both SHIP-2 and PTEN. Moreover, confocal microscopy analyses demonstrated that SHIP-2 but not PTEN colocalized with a speckle-specific component, the SC35 splicing factor. These results suggest that SHIP-2 may be the primary enzyme for metabolizing PtdIns(3,4,5)P3 into PtdIns(3,4)P2 within the nucleus, thus producing another second messenger, whereas PTEN could down-regulate nuclear phosphoinositide 3-kinase signaling. Finally, intranuclear PtdIns(3,4,5)P3 phosphatases might be involved in the control of VSMC proliferation and the pathogenesis of vascular proliferative disorders.

Class I phosphoinositide 3-kinase p110beta is required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by macrophages

Phosphoinositide 3-kinases (PI3Ks) play an important role in a variety of cellular functions, including phagocytosis. PI3Ks are activated during phagocytosis induced by several receptors and have been shown to be required for phagocytosis through the use of inhibitors such as wortmannin and LY294002. Mammalian cells have multiple isoforms of PI3K, and the role of the individual isoforms during phagocytosis has not been addressed. The class I PI3Ks consist of a catalytic p110 isoform associated with a regulatory subunit. Mammals have three genes for the class IA p110 subunits encoding p110alpha, p110beta, and p110delta and one gene for the class IB p110 subunit encoding p110gamma. Here we report a specific recruitment of p110beta and p110delta (but not p110alpha) isoforms to the nascent phagosome during apoptotic cell phagocytosis by fibroblasts. By microinjecting inhibitory antibodies specific to class IA p110 subunits, we have shown that p110beta is the major isoform required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by primary mouse macrophages. Macrophages from mice expressing a catalytically inactive form of p110delta showed no defect in the phagocytosis of apoptotic cells and IgG-opsonized particles, confirming the lack of a major role for p110delta in this process. Similarly, p110gamma-deficient macrophages phagocytosed apoptotic cells normally. Our findings demonstrate that p110beta is the major class I catalytic isoform required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by primary macrophages.

Phosphatidylinositol 3-kinase interacts with the adaptor protein Dab1 in response to Reelin signaling and is required for normal cortical lamination

Reelin is a large secreted signaling protein that binds to two members of the low density lipoprotein receptor family, the apolipoprotein E receptor 2 and the very low density lipoprotein receptor, and regulates neuronal positioning during brain development. Reelin signaling requires activation of Src family kinases as well as tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This results in activation of phosphatidylinositol 3-kinase (PI3K), the serine/threonine kinase Akt, and the inhibition of glycogen synthase kinase 3beta, a protein that is implicated in the regulation of axonal transport. Here we demonstrate that PI3K activation by Reelin requires Src family kinase activity and depends on the Reelin-triggered interaction of Dab1 with the PI3K regulatory subunit p85alpha. Because the Dab1 phosphotyrosine binding domain can interact simultaneously with membrane lipids and with the intracellular domains of apolipoprotein E receptor 2 and very low density lipoprotein receptor, Dab1 is preferentially recruited to the neuronal plasma membrane, where it is phosphorylated. Efficient Dab1 phosphorylation and activation of the Reelin signaling cascade is impaired by cholesterol depletion of the plasma membrane. Using a neuronal migration assay, we also show that PI3K signaling is required for the formation of a normal cortical plate, a step that is dependent upon Reelin signaling.

Analysis of gene expression following norepinephrine stimulation in the rat pineal gland using DNA microarray technique

Several studies have demonstrated that norepinephrine (NE) is the critical neurotransmitter for the regulation of gene expression in the pineal gland. We studied the acute effect of NE stimulation in cultured rat pineal glands using Affymetrix rat genome microarray GeneChip probe arrays. Our data demonstrate that NE stimulation affects regulation of several genes; 44 and 29 genes were up- or down-regulated more than 2.5-fold, respectively. As shown in previous studies, arylalkylamine N-acetyltransferase, cyclic AMP responsive element modulator, jun-B and c-fos mRNA levels were increased by NE stimulation. Genes that were not previously reported and increased by NE stimulation in the pineal gland were protein tyrosine phosphatase, nuclear receptors, and activity and neurotransmitter-induced early genes. Unlike up-regulated genes, most of the down-regulated genes were not reported previously. Genes encoding enzymes involved in metabolism and structural proteins were decreased following NE stimulation. Identification of genes affected by NE stimulation would provide valuable information to understanding pineal biology fully.

Genetic analysis of the myotubularin family of phosphatases in Caenorhabditis elegans

Myotubularins (MTMs) constitute a large family of lipid phosphatases that specifically dephosphorylate phosphatidylinositol (3)P. MTM1 and MTM2 are mutated in X-linked myotubular myopathy and Charcot-Marie-Tooth disease (type 4B), respectively, although the mechanisms whereby MTM dysfunction leads to these diseases is unknown. To gain insight into MTM function, we undertook the study of MTMs in the nematode Caenorhabditis elegans, which possesses representative homologues of the four major subgroups of MTMs identified in mammals. As in mammals, we found that C. elegans MTMs mediate distinct functions. let-512 (vps34) encodes the C. elegans homologue of the yeast and mammalian homologue of the phosphatidylinositol 3-kinase Vps34. We found that reduction of mtm-6 (F53A2.8) function by RNA inhibition rescued the larval lethality of let-512 (vps34) mutants and that the reduction of mtm-1 (Y110A7A.5) activity by RNA inhibition rescued the endocytosis defect of let-512 animals. Together, these observations provide genetic evidence that MTMs negatively regulate phosphatidylinositol (3)P levels. Analysis of MTM expression patterns using transcriptional green fluorescence protein reporters demonstrated that these two MTMs exhibit mostly non-overlapping expression patterns and that MTM-green fluorescence protein fusion proteins are localized to different subcellular locations. These observations suggest that some of the different functions of MTMs might, in part, be a consequence of unique expression and localization patterns. However, our finding that at least three C. elegans MTMs play essential roles in coelomocyte endocytosis, a process that also requires VPS34, indicates that MTMs do not simply turn off VPS34 but unexpectedly also function as positive regulators of biological processes.

Role of phosphoinositide 3-kinase signaling in mast cells: new insights from knockout mouse studies

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases essential for diverse physiological reactions. In recent years a series of gene-targeted mice lacking different types of PI3Ks and related molecules have been generated which enable us to understand the role of PI3K pathways, particularly class I members, in vivo. Analyses of such gene-targeted mice have led to major discoveries in the physiological roles of PI3K signaling in mast cell biology. In particular the role of PI3Ks has been extensively studied in signaling through the high-affinity IgE receptor (FcepsilonRI), since mast cells are the main effector cells in type I allergic reaction associated with IgE-dependent mechanisms. Furthermore, the knockout mice have provided significant information concerning the role of PI3K signals in mast cell differentiation. This review presents several new insights into mast cell biology, which have been elucidated by the analyses of these knockout mice.

A new selective AKT pharmacological inhibitor reduces resistance to chemotherapeutic drugs, TRAIL, all-trans-retinoic acid, and ionizing radiation of human leukemia cells

It is now well established that the reduced capacity of tumor cells of undergoing cell death through apoptosis plays a key role both in the pathogenesis of cancer and in therapeutic treatment failure. Indeed, tumor cells frequently display multiple alterations in signal transduction pathways leading to either cell survival or apoptosis. In mammals, the pathway based on phosphoinositide 3-kinase (PI3K)/Akt conveys survival signals of extreme importance and its downregulation, by means of pharmacological inhibitors of PI3K, considerably lowers resistance to various types of therapy in solid tumors. We recently described an HL60 leukemia cell clone (HL60AR cells) with a constitutively active PI3K/Akt pathway. These cells were resistant to multiple chemotherapeutic drugs, all-trans-retinoic acid (ATRA), and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Treatment with two pharmacological inhibitors of PI3K, wortmannin and Ly294002, restored sensitivity of HL60AR cells to the aforementioned treatments. However, these inhibitors have some drawbacks that may severely limit or impede their clinical use. Here, we have tested whether or not a new selective Akt inhibitor, 1L-6-hydroxymethyl-chiro-inositol 2(R)-2-O-methyl-3-O-octadecylcarbonate (Akt inhibitor), was as effective as Ly294002 in lowering the sensitivity threshold of HL60 cells to chemotherapeutic drugs, TRAIL, ATRA, and ionizing radiation. Our findings demonstrate that, at a concentration which does not affect PI3K activity, the Akt inhibitor markedly reduced resistance of HL60AR cells to etoposide, cytarabine, TRAIL, ATRA, and ionizing radiation. This effect was likely achieved through downregulation of expression of antiapoptotic proteins such as c-IAP1, c-IAP2, cFLIP(L), and of Bad phosphorylation on Ser 136. The Akt inhibitor did not influence PTEN activity. At variance with Ly294002, the Akt inhibitor did not negatively affect phosphorylation of protein kinase C-zeta and it was less effective in downregulating p70S6 kinase (p70S6K) activity. The Akt inhibitor increased sensitivity to apoptotic inducers of K562 and U937, but not of MOLT-4, leukemia cells. Overall, our results indicate that selective Akt pharmacological inhibitors might be used in the future for enhancing the sensitivity of leukemia cells to therapeutic treatments that induce apoptosis or for overcoming resistance to these treatments.

FOXO transcription factors as regulators of immune homeostasis: molecules to die for?

Regulation of phosphatidylinositol 3-kinase (PI3K) activity has been demonstrated to be critical for correct lymphocyte function. The molecular targets of this lipid kinase have been the subject of extensive research, and many functional effects of PI3K activation are thought to be mediated by the serine-threonine kinase protein kinase B (PKB/c-akt). Genetic analyses in the nematode worm Caenorhabditis elegans have identified a novel PI3K-regulated signaling pathway that regulates organism lifespan through inhibition of a Forkhead (FOX) transcription factor, DAF-16. Recent studies have subsequently revealed an evolutionarily conserved signaling module in higher eukaryotes in which PKB can directly phosphorylate and inactive a family of Forkhead box class O (FOXO) transcription factors. Phosphorylation results in nuclear exclusion and inhibition of transcription. FOXO transcription factors have been found to play critical roles in regulation of proliferation, apoptosis and control of oxidative stress. This occurs through both activation and repression of target gene expression by multiple mechanisms. Here the regulation and function of these transcription factors is discussed with specific relevance to immune homeostasis. A greater understanding of the regulation and function of this signaling pathway in lymphocytes may provide novel therapeutic opportunities for immune diseases.

Hyaluronan-mediated CD44 interaction with RhoGEF and Rho kinase promotes Grb2-associated binder-1 phosphorylation and phosphatidylinositol 3-kinase signaling leading to cytokine (macrophage-colony stimulating factor) production and breast tumor progression

In this study we have examined CD44 (a hyaluronan (HA) receptor) interaction with a RhoA-specific guanine nucleotide exchange factor (p115RhoGEF) in human metastatic breast tumor cells (MDA-MB-231 cell line). Immunoprecipitation and immunoblot analyses indicate that both CD44 and p115RhoGEF are expressed in MDA-MB-231 cells and that these two proteins are physically associated as a complex in vivo. The binding of HA to MDA-MB-231 cells stimulates p115RhoGEF-mediated RhoA signaling and Rho kinase (ROK) activity, which, in turn, increases serine/threonine phosphorylation of the adaptor protein, Gab-1 (Grb2-associated binder-1). Phosphorylated Gab-1 promotes PI 3-kinase recruitment to CD44v3. Subsequently, PI 3-kinase is activated (in particular, alpha, beta, gamma forms but not the delta form of the p110 catalytic subunit), AKT signaling occurs, the cytokine (macrophage-colony stimulating factor (M-CSF)) is produced, and tumor cell-specific phenotypes (e.g. tumor cell growth, survival and invasion) are up-regulated. Our results also demonstrate that HA/CD44-mediated oncogenic events (e.g. AKT activation, M-CSF production and breast tumor cell-specific phenotypes) can be effectively blocked by a PI 3-kinase inhibitor (LY294002). Finally, we have found that overexpression of a dominant-negative form of ROK (by transfection of MBA-MD-231 cells with the Rho-binding domain cDNA of ROK) not only inhibits HA/CD44-mediated RhoA-ROK activation and Gab-1 phosphorylation but also down-regulates oncogenic signaling events (e.g. Gab-1.PI 3-kinase-CD44v3 association, PI 3-kinase-mediated AKT activation, and M-CSF production) and tumor cell behaviors (e.g. cell growth, survival, and invasion). Taken together, these findings strongly suggest that CD44 interaction with p115RhoGEF and ROK plays a pivotal role in promoting Gab-1 phosphorylation leading to Gab-1.PI 3-kinase membrane localization, AKT signaling, and cytokine (M-CSF) production during HA-mediated breast cancer progression.

Phosphoinositide 3-kinase is required for process outgrowth and cell polarization of gastrulating mesendodermal cells

BACKGROUND

During vertebrate gastrulation, cell polarization and migration are core components in the cellular rearrangements that lead to the formation of the three germ layers, ectoderm, mesoderm, and endoderm. Previous studies have implicated the Wnt/planar cell polarity (PCP) signaling pathway in controlling cell morphology and movement during gastrulation. However, cell polarization and directed cell migration are reduced but not completely abolished in the absence of Wnt/PCP signals; this observation indicates that other signaling pathways must be involved.

RESULTS

We show that Phosphoinositide 3-Kinases (PI3Ks) are required at the onset of zebrafish gastrulation in mesendodermal cells for process formation and cell polarization. Platelet Derived Growth Factor (PDGF) functions upstream of PI3K, while Protein Kinase B (PKB), a downstream effector of PI3K activity, localizes to the leading edge of migrating mesendodermal cells. In the absence of PI3K activity, PKB localization and cell polarization are strongly reduced in mesendodermal cells and are followed by slower but still highly coordinated and directed movements of these cells.

CONCLUSIONS

We have identified a novel role of a signaling pathway comprised of PDGF, PI3K, and PKB in the control of morphogenetic cell movements during gastrulation. Furthermore, our findings provide insight into the relationship between cell polarization and directed cell migration at the onset of zebrafish gastrulation.

MDA-7 negatively regulates the beta-catenin and PI3K signaling pathways in breast and lung tumor cells

mda-7 is a novel tumor suppressor with cytokine properties. Adenoviral mda-7 (Ad-mda7) induces apoptosis and cell death selectively in tumor cells. The molecular mechanisms underlying the anti-tumor activity of Ad-mda7 in breast and lung cancer lines were investigated. Microarray analyses implicated both the beta-catenin and the PI3K signaling pathways. Ad-mda7 treatment increased protein expression from tumor suppressor genes, including E-cadherin, APC, GSK-3beta, and PTEN, and decreased expression of proto-oncogenes involved in beta-catenin and PI3K signaling. Ad-mda7 caused a redistribution of cellular beta-catenin from the nucleus to the plasma membrane, resulting in reduced TCF/LEF transcriptional activity, and upregulated the E-cadherin-beta-catenin adhesion complex in a tumor cell-specific manner. Expression of the PI3K pathway members (p85 PI3K, FAK, ILK-1, Akt, and PLC-gamma) was downregulated and expression of the PI3K antagonist PTEN was increased. Consistent with this result, pharmacological inhibition of PI3K by wortmannin did not abrogate killing by Ad-mda7. Killing of breast cancer cells by Ad-mda7 required both MAPK and MEK1/2 signaling pathways, whereas these pathways were not essential for MDA-7-mediated killing in lung cancer cells. Thus, in breast and lung tumor cells MDA-7 protein expression modulates cell-cell adhesion and intracellular signaling via coordinate regulation of the beta-catenin and PI3K pathways.

Phosphoinositide signaling disorders in human diseases

Phosphoinositides (PIs) play an essential role in diverse cellular functions. Their intracellular level is strictly regulated by specific PI kinases, phosphatases and phospholipases. Recent discoveries indicate that dysfunctions in the control of their level often lead to pathologies. This review will focus on some human diseases whose etiologies involve PI-metabolizing enzymes. The role of PTEN (phosphatase and tensin homolog deleted on chromosome ten) in cancer, the impact of the Src homology 2-containing inositol-5-phosphatase phosphatases in acute myeloid leukemia or diabetes, the involvement of myotubularin family members in genetic diseases and the implication of OCRL1 in Lowe syndrome will be emphasized. We will also review how some bacterial pathogens have evolved strategies to specifically manipulate the host cell PI metabolism to efficiently infect them.

Class I phosphoinositide 3-kinases

Phosphoinositide 3-kinases (PI3-kinases) are a family of enzymes that 3'-phosphorylate the inositol head group of membrane phosphoinositides. They are subdivided into three major classes (I, II and III) based on their structural homology, regulation and substrate specificity. It is now becoming clear that PI3-kinase isoforms are subject to differential regulation and may play distinct roles within the cell. PI3-kinases and their second messenger lipid products have been implicated in a plethora of cellular responses with increasing evidence for involvement in the pathogenesis of human diseases. The future development of specific PI3-kinase isoform inhibitors may offer therapeutic benefit in a broad range of clinical settings, related to cancer, inflammatory and immunological diseases.

Activated G alpha q inhibits p110 alpha phosphatidylinositol 3-kinase and Akt

Some Gq-coupled receptors have been shown to antagonize growth factor activation of phosphatidylinositol 3-kinase (PI3K) and its downstream effector, Akt. We used a constitutively active Galphaq(Q209L) mutant to explore the effects of Galphaq activation on signaling through the PI3K/Akt pathway. Transient expression of Galphaq(Q209L) in Rat-1 fibroblasts inhibited Akt activation induced by platelet-derived growth factor or insulin treatment. Expression of Galphaq(Q209L) also attenuated Akt activation promoted by coexpression of constitutively active PI3K in human embryonic kidney 293 cells. Galphaq(Q209L) had no effect on the activity of an Akt mutant in which the two regulatory phosphorylation sites were changed to acidic amino acids. Inducible expression of Galphaq(Q209L) in a stably transfected 293 cell line caused a decrease in PI3K activity in p110alpha (but not p110beta) immunoprecipitates. Receptor activation of Galphaq also selectively inhibited PI3K activity in p110alpha immunoprecipitates. Active Galphaq still inhibited PI3K/Akt in cells pretreated with the phospholipase C inhibitor U73122. Finally, Galphaq(Q209L) co-immunoprecipitated with the p110alpha-p85alpha PI3K heterodimer from lysates of COS-7 cells expressing these proteins, and incubation of immunoprecipitated Galphaq(Q209L) with purified recombinant p110alpha-p85alpha in vitro led to a decrease in PI3K activity. These results suggest that agonist binding to Gq-coupled receptors blocks Akt activation via the release of active Galphaq subunits that inhibit PI3K. The inhibitory mechanism seems to be independent of phospholipase C activation and might involve an inhibitory interaction between Galphaq and p110alpha PI3K.

Gene-targeting reveals physiological roles and complex regulation of the phosphoinositide 3-kinases

Phosphoinositide 3-kinases (PI3Ks) are represented by a family of eight distinct enzymes that can be divided into three classes based on their structure and function. The class I PI3Ks are heterodimeric enzymes that are regulated by recruitment to plasma membrane following receptor activation and which control numerous cellular functions, including growth, differentiation, migration, survival, and metabolism. New light has been shed on the biological role of individual members of the class I PI3Ks and their regulatory subunits through gene-targeting experiments. In addition, these experiments have brought the complexity of how PI3K activation is regulated into focus.

FIZZ1/RELMalpha, a novel hypoxia-induced mitogenic factor in lung with vasoconstrictive and angiogenic properties

In a mouse chronic hypoxia model of pulmonary hypertension, we discovered a novel hypoxia-inducible gene in lung, FIZZ1/RELMalpha, first through a cDNA array analysis and then confirmed by RT-PCR. Western blot and immunohistochemistry revealed that its expression was induced by hypoxia only in lung. The hypoxia-upregulated gene expression was located in the pulmonary vasculature, bronchial epithelial cells, and type II pneumocytes. 3H-thymidine incorporation demonstrated that the recombinant protein stimulated rat pulmonary microvascular smooth muscle cell (RPSM) proliferation dose-dependently ranging from 3.3x10(-9) to 3.3x10(-8) mol/L. Therefore, we renamed this gene as hypoxia-induced mitogenic factor (HIMF). HIMF strongly activated Akt phosphorylation. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (10 micromol/L) inhibited HIMF-activated Akt phosphorylation. It also inhibited HIMF-stimulated RPSM proliferation. Thus, the PI3K/Akt pathway, at least in part, mediates the proliferative effect of HIMF. Further studies showed that HIMF had angiogenic and vasoconstrictive properties. HIMF increased pulmonary arterial pressure and vascular resistance more potently than either endothelin-1 or angiotensin II.

CCAAT/enhancer-binding protein and activator protein-1 transcription factors regulate the expression of interleukin-8 through the mitogen-activated protein kinase pathways in response to mechanical stretch of human airway smooth muscle cells

Here we investigated the mechanisms by which mechanical stretch regulates the production of IL-8 in primary human airway smooth muscle cells (HASMC). Bronchial HASMC were subjected to cyclic mechanical stretch (12%, 1 Hz) using the computer-controlled Flexcell Strain system. Mechanical stretch increased IL-8 mRNA expression and protein production. Cyclic stretch of HASMC also increased the kinase activities of ERK1/2, JNK1, p38, and the DNA binding activities of AP-1 and C/EBP transcription factors with little effect on NF-kappa B. The inhibition of AP-1 and C/EBP transcriptional activities blocked the production of IL-8 in culture supernatants. Furthermore, the inhibition of ERK1/2 and p38 but not JNK1 caused a significant down-regulation in the expression and production of IL-8 in response to cyclic stretch. Although protein tyrosine kinases were required for the activation of both ERK1/2 and p38 kinase, stretch-activated channels, small GTPase proteins, and extracellular Ca2+ influx were required only for the activation of p38 kinase whereas phosphoinositide 3-kinase was needed for ERK1/2 activation. In addition, the phosphorylation of ERK1/2 was essential for the activation of AP-1 whereas p38 MAP kinase was needed for the activation of C/EBP. Our data demonstrate that the cyclic stretch of HASMC causes the increased production of IL-8 by activating the AP-1 and C/EBP transcription factors through the activation of ERK1/2 and p38 kinase signaling pathways.

Stimulation of the alpha1A adrenergic receptor inhibits PDGF-induced PDGF beta receptor Tyr751 phosphorylation and PI 3-kinase activation

Several reports indicate that some G(alphaq)-coupled receptors antagonize the activation of phosphatidylinositol (PI) 3-kinase by receptor tyrosine kinases. We used Rat-1 fibroblasts expressing the alpha(1A) adrenergic receptor to study how this G(alphaq)-coupled receptor inhibits platelet-derived growth factor (PDGF) activation of PI 3-kinase. Phenylephrine (PE) stimulation of the alpha(1A) adrenergic receptor inhibited PDGF-induced binding of PI 3-kinase to the PDGF receptor (PDGFR) and phosphorylation of the PDGFR at Tyr751, which forms a docking site for PI 3-kinase. By contrast, activation of phospholipase C gamma by PDGF and phosphorylation of the PDGFR at Tyr716 and Tyr771 were not inhibited by PE. The protein tyrosine phosphatase SHP-2, which dephosphorylates Tyr751 on the PDGFR, was more active in cells treated with PDGF plus PE than in cells treated with either agent alone. PDGF-induced PI 3-kinase signaling was also inhibited by treatment of cells with Pasteurella multocida toxin to activate G(alphaq). These results suggest that the alpha(1A) adrenergic receptor, and perhaps other G(alphaq)-coupled receptors, uses tyrosine dephosphorylation to block PI 3-kinase activation by PDGF.

Akt is a major downstream target of PI3-kinase involved in angiotensin II-induced proliferation

Different signal transduction cascades have been implicated in angiotensin II (Ang II)-mediated cell growth, such as the extracellular signal-regulated kinase 1/2 (ERK1/2) and the phosphatidylinositol 3-kinase (PI3K) pathways. To identify the downstream targets of PI3K involved in Ang II-induced proliferation, we used both rat aortic smooth muscle (RASM) cells and a CHO cell line stably expressing the rat AT1A receptor. The ERK1/2 and PI3K pathways are independently activated and implicated in Ang II-mediated DNA synthesis and cell number increase in these 2 cell lines. In addition, a specific inhibitor of Akt inhibited Ang II-induced Akt phosphorylation, DNA synthesis and proliferation in CHO-AT1A or RASM cells. A dominant-negative mutant of Akt was also found to selectively block Ang II-induced proliferation of CHO-AT1A cells. To further elucidate the signaling events leading to Akt activation, we used an AT1 receptor mutant (AT1AD74E), deficient for Gq protein coupling, and the intracellular calcium chelator BAPTA-AM. Although altered Akt and ERK1/2 activation was observed in the CHO-AT1AD74E cell line, blockade of intracellular calcium elevation did not affect phosphorylation of these kinases. These results provide the first evidence of a specific and necessary role of Akt in Ang II-induced proliferation through a Gq protein-dependent calcium-independent pathway.

The role of PI3K in immune cells

Members of the phosphoinositide-3 kinase (PI3K) family control several cellular responses including cell growth, survival, cytoskeletal remodeling and the trafficking of intracellular organelles in many different types of cell. In particular PI3K has important functions in the immune system. It has been difficult to evaluate the roles of distinct PI3Ks in cellular immune responses because no PI3K inhibitors are specific for individual family members and because most stimuli activate several PI3K enzymes. The development of gene-targeted mice now enables us to examine the physiological functions of individual PI3K enzymes in the immune system in vivo.

Identification and characterization of the autophosphorylation sites of phosphoinositide 3-kinase isoforms beta and gamma

Class I phosphoinositide 3-kinases (PI3Ks) are bifunctional enzymes possessing lipid kinase activity and the capacity to phosphorylate their catalytic and/or regulatory subunits. In this study, in vitro autophosphorylation of the G protein-sensitive p85-coupled class I(A) PI3K beta and p101-coupled class I(B) PI3K gamma was examined. Autophosphorylation sites of both PI3K isoforms were mapped to C-terminal serine residues of the catalytic p110 subunit (i.e. serine 1070 of p110 beta and serine 1101 of p110 gamma). Like other class I(A) PI3K isoforms, autophosphorylation of p110 beta resulted in down-regulated PI3K beta lipid kinase activity. However, no inhibitory effect of p110 gamma autophosphorylation on PI3K gamma lipid kinase activity was observed. Moreover, PI3K beta and PI3K gamma differed in the regulation of their autophosphorylation. Whereas p110 beta autophosphorylation was stimulated neither by G beta gamma complexes nor by a phosphotyrosyl peptide derived from the platelet-derived growth factor receptor, autophosphorylation of p110 gamma was significantly enhanced by G beta gamma in a time- and concentration-dependent manner. In summary, we show that autophosphorylation of both PI3K beta and PI3K gamma occurs in a C-terminal region of the catalytic p110 subunit but differs in its regulation and possible functional consequences, suggesting distinct roles of autophosphorylation of PI3K beta and PI3K gamma.

Autophagy: a barrier or an adaptive response to cancer

Macroautophagy or autophagy is a degradative pathway terminating in the lysosomal compartment after the formation of a cytoplasmic vacuole that engulfs macromolecules and organelles. The recent discovery of the molecular controls of autophagy that are common to eukaryotic cells from yeast to human suggests that the role of autophagy in cell functioning is far beyond its nonselective degradative capacity. The involvement of proteins with properties of tumor suppressor and oncogenic properties at different steps of the pathway implies that autophagy must be considered in tumor progression. Autophagy as a stress response mechanism protects cancer cells from low nutrient supply or therapeutic insults. Autophagy is also involved in the elimination of cancer cells by triggering a non-apoptotic cell death program, suggesting a negative role in tumor development. These two aspects of autophagy will be discussed in this review.

Immunocytochemical detection of phosphatidylinositol 3-kinase activation by insulin and leptin

Intracellular signaling mediated by phosphatidylinositol 3-kinase (PI3K) is important for a number of cellular processes and is stimulated by a variety of hormones, including insulin and leptin. A histochemical method for assessment of PI3K signaling would be an important advance in identifying specific cells in histologically complex organs that are regulated by growth factors and peptide hormones. However, current methods for detecting PI3K activity require either homogenization of the tissue or cells or the ability to transfect probes that bind to phosphatidylinositol 3,4,5 trisphosphate (PIP3), the reaction product of PI3K catalysis. Here we report the validation of an immunocytochemical method to detect changes in PI3K activity, using a recently developed monoclonal antibody to PIP3, in paraformaldehyde-fixed bovine aortic endothelial cells (BAECs) in culture and in hepatocytes of intact rat liver. Treatment with either insulin or leptin increased BAEC PIP3 immunoreactivity, and these effects were blocked by pretreatment with PI3K inhibitors. Furthermore, infusion of insulin into the hepatic portal vein of fasted rats caused an increase of PIP3 immunostaining in hepatocytes that was associated with increased serine phosphorylation of the downstream signaling molecule protein kinase B/Akt (PKB/Akt). We conclude that immunocytochemical PIP3 staining can detect changes in PI3K activation induced by insulin and leptin in cell culture and intact liver.

Regulation of cardiac myocyte cell death

Cardiac myocyte death, whether through necrotic or apoptotic mechanisms, is a contributing factor to many cardiac pathologies. Although necrosis and apoptosis are the widely accepted forms of cell death, they may utilize the same cell death machinery. The environment within the cell probably dictates the final outcome, producing a spectrum of response between the two extremes. This review examines the probable mechanisms involved in myocyte death. Caspases, the generally accepted executioners of apoptosis, are significant in executing cardiac myocyte death, but other proteases (e.g., calpains, cathepsins) also promote cell death, and these are discussed. The two principal cell death pathways (death receptor- and mitochondrial-mediated) are described in relation to the emerging structural information for the principal proteins, and they are discussed relative to current understanding of myocyte cell death mechanisms. Whereas the mitochondrial pathway is probably a significant factor in myocyte death in both acute and chronic phases of myocardial diseases, the death receptor pathway may prove significant in the longer term. The Bcl-2 family of proteins are key regulators of the mitochondrial death pathway. These proteins are described and their possible functions are discussed. The commitment to cell death is also influenced by protein kinase cascades that are activated in the cell. Whereas certain pathways are cytoprotective (e.g., phosphatidylinositol 3'-kinase), the roles of other kinases are less clear. Since myocyte death is implicated in a number of cardiac pathologies, attenuation of the death pathways may prove important in ameliorating such disease states, and possible therapeutic strategies are explored.

High affinity molecules disrupting GRB2 protein complexes as a therapeutic strategy for chronic myelogenous leukaemia

Chronic myelogenous leukaemia (CML) is one of the most intensively studied human malignancies. It has been the focus of major efforts to develop potent drugs for several decades, but until recently cure rates remained low. A breakthrough in CML therapy was very likely accomplished with the clinical introduction of STI-571 [imatinib mesylate; Gleevec (USA); Glivec (other countries)] in 2000/2001. Despite the hope that STI-571 has generated for many CML patients, development of resistance to this drug is already apparent in some cases, especially if the CML is diagnosed in its later stages. Therefore, novel drugs which can be used alone or in combination with STI-571 are highly desirable. This review briefly summarises the current understanding and therapy of CML and then discusses in more detail basic laboratory research that attempts to target Grb2, an adaptor protein known to directly interact with the Bcr portion of the Bcr-Abl fusion protein. Blocking the binding of Grb2 to the GDP-releasing protein SoS is well known to abrogate the activation of the GTPase Ras, a major driving force of the central mitogenic (MAP kinase) pathway. Additional Grb2 effector proteins may also contribute to the proliferation-inhibiting effects observed upon uncoupling Grb2 from its downstream signalling system. Since Grb2 is a known signal transducer for several major human oncogenes, this approach may have applications for a wider range of human cancers.

PI3K and Btk differentially regulate B cell antigen receptor-mediated signal transduction

Phosphoinositide-3 kinase (PI3K) is thought to activate the tyrosine kinase Btk. However, through analysis of PI3K-/- and Btk-/- mice, B cell antigen receptor (BCR)-induced activation of Btk in mouse B cells was found to be unaffected by PI3K inhibitors or by a lack of PI3K. Consistent with this observation, PI3K-/- Btk-/- double-deficient mice had more severe defects than either single-mutant mouse. NF-kappaB activation along with Bcl-xL and cyclin D2 induction were severely blocked in both PI3K-/- and Btk-/- single-deficient B cells. Transgenic expression of Bcl-xL restored the development and BCR-induced proliferation of B cells in PI3K-/- mice. Our results indicate that PI3K and Btk have unique roles in proximal BCR signaling and that they have a common target further downstream in the activation of NF-kappaB.

Altered signaling and cell cycle regulation in embryonal stem cells with a disruption of the gene for phosphoinositide 3-kinase regulatory subunit p85alpha

The p85alpha regulatory subunit of class I(A) phosphoinositide 3-kinases (PI3K) is derived from the Pik3r1 gene, which also yields alternatively spliced variants p50alpha and p55alpha. It has been proposed that excess monomeric p85 competes with functional PI3K p85-p110 heterodimers. We examined embryonic stem (ES) cells with heterozygous and homozygous disruptions in the Pik3r gene and found that wild type ES cells express virtually no monomeric p85alpha. Although, IGF-1-stimulated PI3K activity associated with insulin receptor substrates was unaltered in all cell lines, p85alpha-null ES cells showed diminished protein kinase B activation despite increased PI3K activity associated with the p85beta subunit. Furthermore, p85alpha-null cells demonstrated growth retardation, increased frequency of apoptosis, and altered cell cycle regulation with a G(0)/G(1) cell cycle arrest and up-regulation of p27(KIP), whereas signaling through CREB and MAPK was enhanced. These phenotypes were reversed by re-expression of p85alpha via adenoviral gene transfer. Surprisingly, all ES cell lines could be differentiated into adipocytes. In these differentiated ES cells, however, compensatory p85beta signaling was lost in p85alpha-null cells while increased signaling by CREB and MAPK was still observed. Thus, loss of p85alpha in ES cells induced alterations in IGF-1 signaling and regulation of apoptosis and cell cycle but no defects in differentiation. However, differentiated ES cells partially lost their ability for compensatory signaling at the level of PI3K, which may explain some of the defects observed in mice with homozygous deletion of the Pik3r1 gene.

The c-Cbl-associated protein and c-Cbl are two new partners of the SH2-containing inositol polyphosphate 5-phosphatase SHIP2

SHIP2 is a phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) 5-phosphatase which contains motifs susceptible to mediate protein-protein interaction. Using yeast two-hybrid, GST-pulldown, and coimmunoprecipitation studies, we isolated the CAP cDNA as a specific partner of SHIP2 proline-rich domain and showed by GST-pulldown experiments that the interaction took place with the SH3C of CAP. The interaction was not modulated in COS-7 cells stimulated by EGF neither in CHO cells overexpressing the insulin receptor in the presence or absence of insulin stimulation. We also showed that SHIP2 was able to coimmunoprecipitate with endogenous c-Cbl protein in the absence of CAP and with the insulin receptor in CHO-IR cell extracts. The presence of SHIP2 in a complex around the insulin receptor could account for the very specific increase in insulin sensitivity of SHIP2 knock-out mice.

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

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

Roles of G beta gamma in membrane recruitment and activation of p110 gamma/p101 phosphoinositide 3-kinase gamma

Receptor-regulated class I phosphoinositide 3-kinases (PI3K) phosphorylate the membrane lipid phosphatidylinositol (PtdIns)-4,5-P2 to PtdIns-3,4,5-P3. This, in turn, recruits and activates cytosolic effectors with PtdIns-3,4,5-P3-binding pleckstrin homology (PH) domains, thereby controlling important cellular functions such as proliferation, survival, or chemotaxis. The class IB p110 gamma/p101 PI3K gamma is activated by G beta gamma on stimulation of G protein-coupled receptors. It is currently unknown whether in living cells G beta gamma acts as a membrane anchor or an allosteric activator of PI3K gamma, and which role its noncatalytic p101 subunit plays in its activation by G beta gamma. Using GFP-tagged PI3K gamma subunits expressed in HEK cells, we show that G beta gamma recruits the enzyme from the cytosol to the membrane by interaction with its p101 subunit. Accordingly, p101 was found to be required for G protein-mediated activation of PI3K gamma in living cells, as assessed by use of GFP-tagged PtdIns-3,4,5-P3-binding PH domains. Furthermore, membrane-targeted p110 gamma displayed basal enzymatic activity, but was further stimulated by G beta gamma, even in the absence of p101. Therefore, we conclude that in vivo, G beta gamma activates PI3K gamma by a mechanism assigning specific roles for both PI3K gamma subunits, i.e., membrane recruitment is mediated via the noncatalytic p101 subunit, and direct stimulation of G beta gamma with p110 gamma contributes to activation of PI3K gamma.

Estrogen receptor phosphorylation

Estrogen receptor alpha (ERalpha) is phosphorylated on multiple amino acid residues. For example, in response to estradiol binding, human ERalpha is predominately phosphorylated on Ser-118 and to a lesser extent on Ser-104 and Ser-106. In response to activation of the mitogen-activated protein kinase pathway, phosphorylation occurs on Ser-118 and Ser-167. These serine residues are all located within the activation function 1 region of the N-terminal domain of ERalpha. In contrast, activation of protein kinase A increases the phosphorylation of Ser-236, which is located in the DNA-binding domain. The in vivo phosphorylation status of Tyr-537, located in the ligand-binding domain, remains controversial. In this review, I present evidence that these phosphorylations occur, and identify the kinases thought to be responsible. Additionally, the functional importance of ERalpha phosphorylation is discussed.

Novel agents for the prevention of breast cancer: targeting transcription factors and signal transduction pathways

Transformation of breast cells occurs through loss or mutation of tumor suppressor genes, or activation or amplification of oncogenes, leading to deregulation of signal transduction pathways, abnormal amplification of growth signals, and aberrant expression of genes that ultimately transform the cells into invasive cancer. The goal of cancer preventive therapy, or "chemoprevention," is to eliminate premalignant cells or to block the progression of normal cells into cancer. Multiple alterations in signal pathways and transcription factors are observed in mammary gland tumorigenesis. In particular, estrogen receptor (ER) deregulation plays a critical role in breast cancer development and progress, and targeting ER with selective ER modulators (SERMs) has achieved significant reduction of breast cancer incidence in women at high risk for breast cancer. However, not all breast cancer is prevented by SERMs, because 30-40% of the tumors are ER-negative. Other receptors for retinoids, vitamin D analogs and peroxisome proliferator-activiator, along with transcription factors such as AP-1, NF-kappaB, and STATs (signal transducers and activators of transcription) affect breast tumorigenesis. This is also true for the signal transduction pathways, for example cyclooxygenase 2 (Cox-2), HER2/neu, mitogen-activated protein kinase (MAPK), and PI3K/Akt. Therefore, proteins in pathways that are altered during the process of mammary tumorigenesis may be promising targets of future chemopreventive drugs. Many newly-developed synthetic or natural compounds/agents are now under testing in preclinical studies and clinical trials. Receptor selective retinoids, receptor tyrosine kinase inhibitors (TKIs), SERMs, Cox-2 inhibitors, and others are some of the promising novel agents for the prevention of breast cancer. The chemopreventive activity of these agents and other novel signal transduction inhibitors are discussed in this chapter.

Retinal neuroprotection by growth factors: a mechanistic perspective

For more than a decade it has been known that certain growth factors inhibit apoptosis in genetically determined and experimental models of inner and outer retinal degeneration. The molecular mechanisms underlying these protective effects and the signaling that supports the survival of photoreceptors and retinal ganglion cells in these models have recently come under more in depth investigation. This paper reviews our current understanding of the balance of pro- and antiapoptotic signals that determine cell fate in the retina and how the activation of key signal transduction pathways by specific classes of neurotrophins protects retinal neurons.

Reelin-mediated signaling locally regulates protein kinase B/Akt and glycogen synthase kinase 3beta

Reelin is a large secreted protein that controls cortical layering by signaling through the very low density lipoprotein receptor and apolipoprotein E receptor 2, thereby inducing tyrosine phosphorylation of the adaptor protein Disabled-1 (Dab1) and suppressing tau phosphorylation in vivo. Here we show that binding of Reelin to these receptors stimulates phosphatidylinositol 3-kinase, resulting in activation of protein kinase B and inhibition of glycogen synthase kinase 3beta. We present genetic evidence that this cascade is dependent on apolipoprotein E receptor 2, very low density lipoprotein receptor, and Dab1. Reelin-signaling components are enriched in axonal growth cones, where tyrosine phosphorylation of Dab1 is increased in response to Reelin. These findings suggest that Reelin-mediated phosphatidylinositol 3-kinase signaling in neuronal growth cones contributes to final neuron positioning in the mammalian brain by local modulation of protein kinase B and glycogen synthase kinase 3beta kinase activities.

UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,6 N-acetylglucosaminyltransferase V (Mgat5) deficient mice

Targeted gene mutations in mice that cause deficiencies in protein glycosylation have revealed functions for specific glycans structures in embryogenesis, immune cell regulation, fertility and cancer progression. UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,6 N-acetylglucosaminyltransferase V (GlcNAc-TV or Mgat5) produces N-glycan intermediates that are elongated with poly N-acetyllactosamine to create ligands for the galectin family of mammalian lectins. We generated Mgat5-deficient mice by gene targeting methods in embryonic stem cells, and observed a complex phenotype in adult mice including susceptibility to autoimmune disease, reduced cancer progression and a behavioral defect. We found that Mgat5-modified N-glycans on the T cell receptor (TCR) complex bind to galectin-3, sequestering TCR within a multivalent galectin-glycoprotein lattice that impedes antigen-dependent receptor clustering and signal transduction. Integrin receptor clustering and cell motility are also sensitive to changes in Mgat5-dependent N-glycosylation. These studies demonstrate that low affinity but high avidity interactions between N-glycans and galectins can regulate the distribution of cell surface receptors and their responsiveness to agonists.

Activation of the PI3K/Akt pathway and chemotherapeutic resistance

The resistance of many types of cancer to conventional chemotherapies is a major factor undermining successful cancer treatment. In this review, the role of a signal transduction pathway comprised of the lipid kinase, phosphatidylinositol 3-kinase (PI3K), and the serine/threonine kinase, Akt (or PKB), in chemotherapeutic resistance will be explored. Activation of this pathway plays a pivotal role in essential cellular functions such as survival, proliferation, migration and differentiation that underlie the biology of human cancer. Akt activation also contributes to tumorigenesis and tumor metastasis, and as shown most recently, resistance to chemotherapy. Modulating Akt activity is now a commonly observed endpoint of chemotherapy administration or administration of chemopreventive agents. Studies performed in vitro and in vivo combining small molecule inhibitors of the PI3K/Akt pathway with standard chemotherapy have been successful in attenuating chemotherapeutic resistance. As a result, small molecules designed to specifically target Akt and other components of the pathway are now being developed for clinical use as single agents and in combination with chemotherapy to overcome therapeutic resistance. Specifically inhibiting Akt activity may be a valid approach to treat cancer and increase the efficacy of chemotherapy.

Armed therapeutic viruses: strategies and challenges to arming oncolytic viruses with therapeutic genes

Oncolytic viruses are attractive therapeutics for cancer because they selectively amplify, through replication and spread, the input dose of virus in the target tumor. To date, clinical trials have demonstrated marked safety but have not realized their theoretical efficacy potential. In this review, we consider the potential of armed therapeutic viruses, whose lytic potential is enhanced by genetically engineered therapeutic transgene expression from the virus, as potential vehicles to increase the potency of these agents. Several classes of therapeutic genes are outlined, and potential synergies and hurdles to their delivery from replicating viruses are discussed.

Embryonic testis cord formation and mesonephric cell migration requires the phosphotidylinositol 3-kinase signaling pathway

Mesonephric cell migration and seminiferous cord formation are critical processes in embryonic testis development at the time of male sex determination. Extracellular growth factors shown to influence seminiferous cord formation such as neurotropin-3 utilize in part the phosphotidylinositol 3-kinase (PI3K) signal transduction pathway. The current study investigates the hypothesis that the PI3K pathway is critical in seminiferous cord formation and testis development. The role of the PI3K signaling pathway in testicular cord formation was examined using an Embryonic Day 13 organ culture system and a PI3K-specific inhibitor LY294002. The actions of a mitogen-activated protein (MAP) kinase-specific inhibitor PD98059 was also examined. The PI3K inhibitor blocked cord formation or reduced the number of cords in a concentration-dependent manner. The actions of LY294002 were found to have a developmental stage specificity in that cord inhibition was observed in organs from embryos with 16-17 tail somites, while organs from embryos with 19 or more tail somites had no block in cord formation and only a small reduction in cord number. In contrast, the MAP kinase inhibitor PD98059 did not block cord formation and only caused a slight reduction in cord number. Neither PI3K or MAP kinase inhibitor altered apoptotic cell number, suggesting apoptosis was not the reason for the inhibition of cord formation. Embryonic testis cell migration assays showed that the PI3K inhibitor LY294002 blocked mesonephros cell migration into the testis, while the MAP kinase inhibitor had no effect. Observations suggest the interference of cell migration is the cause for the inhibition of cord formation. Western blot analysis confirmed that LY294002 and PD98509 inhibited phosphorylation of Akt and ERK1/ERK2, respectively. Combined observations demonstrate that the PI3K signaling pathway is involved in embryonic testis cord formation and mesonephros cell migration.

The significance of LMP1 expression in nasopharyngeal carcinoma

The Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) is a key effector of EBV-mediated B cell transformation. LMP1 displays potent oncogenic properties in rodent fibroblasts, and induces a wide range of effects in B cells and epithelial cells. LMP1 functions as a constitutively active tumor necrosis factor receptor (TNFR) engaging a multitude of signaling pathways that include NF-kappaB, the mitogen-activated protein kinases (MAPKs), JNK, p38, the JAK/STAT pathway and, more recently, the small Rho GTPases. The constitutive activation of these signaling cascades explains LMP1's ability to induce such a diverse array of morphological and phenotypic effects in cells and provides an insight into how LMP1 may induce cell transformation. The frequent expression of LMP1 in undifferentiated nasopharyngeal carcinoma (NPC) points to a role for this viral oncoprotein as a key effector molecule in NPC pathogenesis.