Coupled inositide phosphorylation and phospholipase D activation initiates clathrin-coat assembly on lysosomes

Mechanism of the Regulation of Plasma Cholesterol Levels by PI(4,5)P2

Elevated low-density lipoprotein (LDL) cholesterol (LDLc) is one of the most well-established risk factors for cardiovascular disease, while high levels of high-density lipoprotein (HDL) cholesterol (HDLc) have been associated with protection from cardiovascular disease. Cardiovascular disease remains one of the leading causes of death worldwide; thus it is important to understand mechanisms that impact LDLc and HDLc metabolism. In this chapter, we will discuss molecular processes by which phosphatidylinositol-(4,5)-bisphosphate, PI(4,5)P₂, is thought to modulate LDLc or HDLc. Section 1 will provide an overview of cholesterol in the circulation, discussing processes that modulate the various forms of lipoproteins (LDL and HDL) carrying cholesterol. Section 2 will describe how a PI(4,5)P₂ phosphatase, transmembrane protein 55B (TMEM55B), impacts circulating LDLc levels through its ability to regulate lysosomal decay of the low-density lipoprotein receptor (LDLR), the primary receptor for hepatic LDL uptake. Section 3 will discuss how PI(4,5)P₂ interacts with apolipoprotein A-I (apoA1), the key apolipoprotein on HDL. In addition to direct mechanisms of PI(4,5)P₂ action on circulating cholesterol, Sect. 4 will review how PI(4,5)P₂ may indirectly impact LDLc and HDLc by affecting insulin action. Last, as cholesterol is controlled through intricate negative feedback loops, Sect. 5 will describe how PI(4,5)P₂ is regulated by cholesterol.

Phagosome resolution regenerates lysosomes and maintains the degradative capacity in phagocytes

Phagocytes engulf unwanted particles into phagosomes that then fuse with lysosomes to degrade the enclosed particles. Ultimately, phagosomes must be recycled to help recover membrane resources that were consumed during phagocytosis and phagosome maturation, a process referred to as “phagosome resolution.” Little is known about phagosome resolution, which may proceed through exocytosis or membrane fission. Here, we show that bacteria-containing phagolysosomes in macrophages undergo fragmentation through vesicle budding, tubulation, and constriction. Phagosome fragmentation requires cargo degradation, the actin and microtubule cytoskeletons, and clathrin. We provide evidence that lysosome reformation occurs during phagosome resolution since the majority of phagosome-derived vesicles displayed lysosomal properties. Importantly, we show that clathrin-dependent phagosome resolution is important to maintain the degradative capacity of macrophages challenged with two waves of phagocytosis. Overall, our work suggests that phagosome resolution contributes to lysosome recovery and to maintaining the degradative power of macrophages to handle multiple waves of phagocytosis.

Emerging roles of PtdIns(4,5)P2--beyond the plasma membrane

Phosphoinositides are a collection of lipid messengers that regulate most subcellular processes. Amongst the seven phosphoinositide species, the roles for phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] at the plasma membrane, such as in endocytosis, exocytosis, actin polymerization and focal adhesion assembly, have been extensively studied. Recent studies have argued for the existence of PtdIns(4,5)P2 at multiple intracellular compartments, including the nucleus, endosomes, lysosomes, autolysosomes, autophagic precursor membranes, ER, mitochondria and the Golgi complex. Although the generation, regulation and functions of PtdIns(4,5)P2 are less well-defined in most other intracellular compartments, accumulating evidence demonstrates crucial roles for PtdIns(4,5)P2 in endolysosomal trafficking, endosomal recycling, as well as autophagosomal pathways, which are the focus of this Commentary. We summarize and discuss how phosphatidylinositol phosphate kinases, PtdIns(4,5)P2 and PtdIns(4,5)P2-effectors regulate these intracellular protein and membrane trafficking events.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Phosphatidylinositol 4-phosphate and phosphatidylinositol 3-phosphate regulate phagolysosome biogenesis

Professional phagocytic cells ingest microbial intruders by engulfing them into phagosomes, which subsequently mature into microbicidal phagolysosomes. Phagosome maturation requires sequential fusion of the phagosome with early endosomes, late endosomes, and lysosomes. Although various phosphoinositides (PIPs) have been detected on phagosomes, it remained unclear which PIPs actually govern phagosome maturation. Here, we analyzed the involvement of PIPs in fusion of phagosomes with various endocytic compartments and identified phosphatidylinositol 4-phosphate [PI(4)P], phosphatidylinositol 3-phosphate [PI(3)P], and the lipid kinases that generate these PIPs, as mediators of phagosome-lysosome fusion. Phagosome-early endosome fusion required PI(3)P, yet did not depend on PI(4)P. Thus, PI(3)P regulates phagosome maturation at early and late stages, whereas PI(4)P is selectively required late in the pathway.

The AP2 clathrin adaptor protein complex regulates the abundance of GLR-1 glutamate receptors in the ventral nerve cord of Caenorhabditis elegans

Regulation of glutamate receptor trafficking controls synaptic strength and plasticity. This study takes advantage of viable, null mutations in subunits of the clathrin adaptor protein 2 (AP2) complex in Caenorhabditis elegans to reveal a novel and unexpected AP2-dependent trafficking step for glutamate receptors early in the secretory pathway.

Regulation of glutamate receptor (GluR) abundance at synapses by clathrin-mediated endocytosis can control synaptic strength and plasticity. We take advantage of viable, null mutations in subunits of the clathrin adaptor protein 2 (AP2) complex in Caenorhabditis elegans to characterize the in vivo role of AP2 in GluR trafficking. In contrast to our predictions for an endocytic adaptor, we found that levels of the GluR GLR-1 are decreased at synapses in the ventral nerve cord (VNC) of animals with mutations in the AP2 subunits APM-2/μ2, APA-2/α, or APS-2/σ2. Rescue experiments indicate that APM-2/μ2 functions in glr-1–expressing interneurons and the mature nervous system to promote GLR-1 levels in the VNC. Genetic analyses suggest that APM-2/μ2 acts upstream of GLR-1 endocytosis in the VNC. Consistent with this, GLR-1 accumulates in cell bodies of apm-2 mutants. However, GLR-1 does not appear to accumulate at the plasma membrane of the cell body as expected, but instead accumulates in intracellular compartments including Syntaxin-13– and RAB-14–labeled endosomes. This study reveals a novel role for the AP2 clathrin adaptor in promoting the abundance of GluRs at synapses in vivo, and implicates AP2 in the regulation of GluR trafficking at an early step in the secretory pathway.

LAPTM4B is a PtdIns(4,5)P2 effector that regulates EGFR signaling, lysosomal sorting, and degradation

Lysosomal degradation is essential for the termination of EGF-stimulated EGF receptor (EGFR) signaling. This requires EGFR sorting to the intraluminal vesicles (ILVs) of multi-vesicular endosomes (MVEs). Cytosolic proteins including the ESCRT machineries are key regulators of EGFR intraluminal sorting, but roles for endosomal transmembrane proteins in receptor sorting are poorly defined. Here, we show that LAPTM4B, an endosomal transmembrane oncoprotein, inhibits EGF-induced EGFR intraluminal sorting and lysosomal degradation, leading to enhanced and prolonged EGFR signaling. LAPTM4B blocks EGFR sorting by promoting ubiquitination of Hrs (an ESCRT-0 subunit), which inhibits the Hrs association with ubiquitinated EGFR. This is counteracted by the endosomal PIP kinase, PIPKIγi5, which directly binds LAPTM4B and neutralizes the inhibitory function of LAPTM4B in EGFR sorting by generating PtdIns(4,5)P2 and recruiting SNX5. PtdIns(4,5)P2 and SNX5 function together to protect Hrs from ubiquitination, thereby promoting EGFR intraluminal sorting. These results reveal an essential layer of EGFR trafficking regulated by LAPTM4B, PtdIns(4,5)P2 signaling, and the ESCRT complex and define a mechanism by which the oncoprotein LAPTM4B can transform cells and promote tumor progression.

Role of phospholipase d in g-protein coupled receptor function

Prolonged agonist exposure of many G-protein coupled receptors induces a rapid receptor phosphorylation and uncoupling from G-proteins. Resensitization of these desensitized receptors requires endocytosis and subsequent dephosphorylation. Numerous studies show the involvement of phospholipid-specific phosphodiesterase phospholipase D (PLD) in the receptor endocytosis and recycling of many G-protein coupled receptors e.g., opioid, formyl or dopamine receptors. The PLD hydrolyzes the headgroup of a phospholipid, generally phosphatidylcholine (PC), to phosphatidic acid (PA) and choline and is assumed to play an important function in cell regulation and receptor trafficking. Protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families regulate the two mammalian PLD isoforms 1 and 2. Mammalian and yeast PLD are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. The PA product is an intracellular lipid messenger. PLD and PA activities are implicated in a wide range of physiological processes and diseases including inflammation, diabetes, oncogenesis or neurodegeneration. This review discusses the characterization, structure, and regulation of PLD in the context of membrane located G-protein coupled receptor function.

Phospholipase D and the maintenance of phosphatidic acid levels for regulation of mammalian target of rapamycin (mTOR)

Phosphatidic acid (PA) is a critical metabolite at the heart of membrane phospholipid biosynthesis. However, PA also serves as a critical lipid second messenger that regulates several proteins implicated in the control of cell cycle progression and cell growth. Three major metabolic pathways generate PA: phospholipase D (PLD), diacylglycerol kinase (DGK), and lysophosphatidic acid acyltransferase (LPAAT). The LPAAT pathway is integral to de novo membrane phospholipid biosynthesis, whereas the PLD and DGK pathways are activated in response to growth factors and stress. The PLD pathway is also responsive to nutrients. A key target for the lipid second messenger function of PA is mTOR, the mammalian/mechanistic target of rapamycin, which integrates both nutrient and growth factor signals to control cell growth and proliferation. Although PLD has been widely implicated in the generation of PA needed for mTOR activation, it is becoming clear that PA generated via the LPAAT and DGK pathways is also involved in the regulation of mTOR. In this minireview, we highlight the coordinated maintenance of intracellular PA levels that regulate mTOR signals stimulated by growth factors and nutrients, including amino acids, lipids, glucose, and Gln. Emerging evidence indicates compensatory increases in one source of PA when another source is compromised, highlighting the importance of being able to adapt to stressful conditions that interfere with PA production. The regulation of PA levels has important implications for cancer cells that depend on PA and mTOR activity for survival.

Isoform 5 of PIPKIγ regulates the endosomal trafficking and degradation of E-cadherin

Phosphatidylinositol phosphate kinases (PIPKs) have distinct cellular targeting, allowing for site-specific synthesis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] to activate specific signaling cascades required for cellular processes. Several C-terminal splice variants of PIPKIγ (also known as PIP5K1C) exist, and have been implicated in a multitude of cellular roles. PI(4,5)P2 serves as a fundamental regulator of E-cadherin transport, and PI(4,5)P2-generating enzymes are important signaling relays in these pathways. We present evidence that the isoform 5 splice variant of PIPKIγ (PIPKIγi5) associates with E-cadherin and promotes its lysosomal degradation. Additionally, we show that the endosomal trafficking proteins SNX5 and SNX6 associate with PIPKIγi5 and inhibit PIPKIγi5-mediated E-cadherin degradation. Following HGF stimulation, activated Src directly phosphorylates PIPKIγi5. Phosphorylation of the PIPKIγi5 C-terminus regulates its association with SNX5 and, consequently, E-cadherin degradation. Additionally, this PIPKIγi5-mediated pathway requires Rab7 to promote degradation of internalized E-cadherin. Taken together, the data indicate that PIPKIγi5 and SNX5 are crucial regulators of E-cadherin sorting and degradation. PIPKIγi5, SNX and phosphoinositide regulation of lysosomal sorting represent a novel area of PI(4,5)P2 signaling and research. PIPKIγi5 regulation of E-cadherin sorting for degradation might have broad implications in development and tissue maintenance, and enhanced PIPKIγi5 function might have pathogenic consequences due to downregulation of E-cadherin.

Hepatic gluconeogenesis is enhanced by phosphatidic acid which remains uninhibited by insulin in lipodystrophic Agpat2-/- mice

In this study we examined the role of phosphatidic acid (PA) in hepatic glucose production (HGP) and development of hepatic insulin resistance in mice that lack 1-acylglycerol-3-phosphate O-acyltransferase 2 (AGPAT2). Liver lysophosphatidic acid and PA levels were increased ∼2- and ∼5-fold, respectively, in male Agpat2(-/-) mice compared with wild type mice. In the absence of AGPAT2, the liver can synthesize PAs by activating diacylglycerol kinase or phospholipase D, both of which were elevated in the livers of Agpat2(-/-) mice. We found that PAs C16:0/18:1 and C18:1/20:4 enhanced HGP in primary WT hepatocytes, an effect that was further enhanced in primary hepatocytes from Agpat2(-/-) mice. Lysophosphatidic acids C16:0 and C18:1 failed to increase HGP in primary hepatocytes. The activation of HGP was accompanied by an up-regulation of the key gluconeogenic enzymes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. This activation was suppressed by insulin in the WT primary hepatocytes but not in the Agpat2(-/-) primary hepatocytes. Thus, the lack of normal insulin signaling in Agpat2(-/-) livers allows unrestricted PA-induced gluconeogenesis significantly contributing to the development of hyperglycemia in these mice.

Endosomal type Iγ PIP 5-kinase controls EGF receptor lysosomal sorting

Endosomal trafficking and degradation of epidermal growth factor receptor (EGFR) play an essential role in the control of its signaling. Phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P(2)) is an established regulator of endocytosis, whereas PtdIns3P modulates endosomal trafficking. However, we demonstrate here that type I gamma phosphatidylinositol phosphate 5-kinase i5 (PIPKIγi5), an enzyme that synthesizes PtdIns4,5P(2), controls endosome-to-lysosome sorting of EGFR. In this pathway, PIPKIγi5 interacts with sorting nexin 5 (SNX5), a protein that binds PtdIns4,5P(2) and other phosphoinositides. PIPKIγi5 and SNX5 localize to endosomes, and loss of either protein blocks EGFR sorting into intraluminal vesicles (ILVs) of the multivesicular body. Loss of ILV sorting greatly enhances and prolongs EGFR signaling. PIPKIγi5 and SNX5 prevent Hrs ubiquitination, and this facilitates the Hrs association with EGFR that is required for ILV sorting. These findings reveal that PIPKIγi5 and SNX5 form a signaling nexus that controls EGFR endosomal sorting, degradation, and signaling.

PLD$ is involved in phagocytosis of microglia: expression and localization changes of PLD4 are correlated with activation state of microglia

Phospholipase D4 (PLD4) is a recently identified protein that is mainly expressed in the ionized calcium binding adapter molecule 1 (Iba1)-positive microglia in the early postnatal mouse cerebellar white matter. Unlike PLD1 and PLD2, PLD4 exhibits no enzymatic activity for conversion of phosphatidylcholine into choline and phosphatidic acid, and its function is completely unknown. In the present study, we examined the distribution of PLD4 in mouse cerebellar white matter during development and under pathological conditions. Immunohistochemical analysis revealed that PLD4 expression was associated with microglial activation under such two different circumstances. A primary cultured microglia and microglial cell line (MG6) showed that PLD4 was mainly present in the nucleus, except the nucleolus, and expression of PLD4 was upregulated by lipopolysaccharide (LPS) stimulation. In the analysis of phagocytosis of LPS-stimulated microglia, PLD4 was co-localized with phagosomes that contained BioParticles. Inhibition of PLD4 expression using PLD4 specific small interfering RNA (siRNA) in MG6 cells significantly reduced the ratio of phagocytotic cell numbers. These results suggest that the increased PLD4 in the activation process is involved in phagocytosis of activated microglia in the developmental stages and pathological conditions of white matter.

The dual effect of Rac2 on phospholipase D2 regulation that explains both the onset and termination of chemotaxis

We document a biphasic effect of Rac2 on the activation and inhibition of PLD2. Cells overexpressing Rac2 and PLD2 simultaneously show a robust initial (<10 min) response toward a chemoattractant that is later (>30 min) greatly diminished over PLD2-only controls. The first phase is due to the presence of a Rac2-PLD2 positive-feedback loop. To explain the mechanism for the Rac2-led PLD2 inhibition (the second phase), we used leukocytes from wild-type (WT) and Rac2(-/-) knockout mice. Rac2(-/-) cells displayed an enhanced PLD2 (but not PLD1) enzymatic activity, confirming the inhibitory role of Rac2. Late inhibitory responses on PLD2 due to Rac2 were reversed in the presence of phosphatidylinositol 4,5-bisphosphate (PIP(2)) both in vitro (purified GST-PH-PLD2, where GST is glutathione S-transferase and PH is pleckstrin homology) and in vivo. Coimmunoprecipitation and immunofluorescence microscopy indicated that PLD2 and Rac2 remain together. The presence of an "arc" of Rac2 at the leading edge of leukocyte pseudopodia and PLD2 physically posterior to this wave of Rac2 was observed in late chemotaxis. We propose Rac-led inhibition of PLD2 function is due to sterical interference of Rac with PLD2's PH binding site to the membrane and deprivation of the PIP(2). This work supports the importance of functional interactions between PLD and Rac in the biological response of cell migration.

FRMD4A regulates epithelial polarity by connecting Arf6 activation with the PAR complex

The Par-3/Par-6/aPKC/Cdc42 complex regulates the conversion of primordial adherens junctions (AJs) into belt-like AJs and the formation of linear actin cables during epithelial polarization. However, the mechanisms by which this complex functions are not well elucidated. In the present study, we found that activation of Arf6 is spatiotemporally regulated as a downstream signaling pathway of the Par protein complex. When primordial AJs are formed, Par-3 recruits a scaffolding protein, termed the FERM domain containing 4A (FRMD4A). FRMD4A connects Par-3 and the Arf6 guanine-nucleotide exchange factor (GEF), cytohesin-1. We propose that the Par-3/FRMD4A/cytohesin-1 complex ensures accurate activation of Arf6, a central player in actin cytoskeleton dynamics and membrane trafficking, during junctional remodeling and epithelial polarization.

PIP5K-driven PtdIns(4,5)P2 synthesis: regulation and cellular functions

It has long been known that phosphoinositides are present in cellular membranes, but only in the past four decades has our understanding of their importance for proper cell function advanced significantly. Key to determining the biological roles of phosphoinositides is understanding the enzymes involved in their metabolism. Although many such enzymes have now been identified, there is still much to learn about their cellular functions. Phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) are a group of kinases that catalyse the production of phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2]. As well as being a substrate for the enzymes phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI3K), PtdIns(4,5)P2 acts as a second messenger in its own right, influencing a variety of cellular processes. In this Commentary, we review how PIP5Ks are modulated to achieve regulated PtdIns(4,5)P2 production, and discuss the role of these proteins in different cellular processes.

Role of phospholipase D in parathyroid hormone type 1 receptor signaling and trafficking

The role of phospholipase D (PLD) in the regulation of the traffic of the PTH type 1 receptor (PTH1R) was studied in Chinese hamster ovary cells stably transfected with a human PTH1R (CHO-R3) and in rat osteosarcoma 17/2.8 (ROS) cells. PTH(1-34) increased total PLD activity by 3-fold in CHO-R3 cells and by 2-fold in ROS cells. Overexpression of wild-type (WT) PLD1 and WT-PLD2 increased basal PLD activity in CHO-R3 but not in ROS cells. Ligand-stimulated PLD activity greatly increased in CHO-R3 cells transfected with WT-PLD1 and WT-PLD2. However, only WT-PLD2 expression increased PTH-dependent PLD activity in ROS cells. Expression of the catalytically inactive mutants R898K-PLD1 (DN-PLD1) and R758K-PLD2 (DN-PLD2) inhibited ligand-dependent PLD activity in both cell lines. PTH(1-34) induced internalization of the PTH1R with a concomitant increase in the colocalization of the receptor with PLD1 in intracellular vesicles and in a perinuclear, ADP ribosylation factor-1-positive compartment. The distribution of PLD1 and PLD2 remained unaltered after PTH treatment. Expression of DN-PLD1 had a small effect on endocytosis of the PTH1R; however, DN-PLD1 prevented accumulation of the PTH1R in the perinuclear compartment. Expression of DN-PLD2 significantly retarded ligand-induced PTH1R internalization in both cell lines. The differential effects of PLD1 and PLD2 on receptor traffic were confirmed using isoform-specific short hairpin RNA constructs. We conclude that PLD1 and PLD2 play distinct roles in regulating PTH1R traffic; PLD2 primarily regulates endocytosis, whereas PLD1 regulates receptor internalization and intracellular receptor traffic.

Clathrin regulates the association of PIPKIgamma661 with the AP-2 adaptor beta2 appendage

The AP-2 clathrin adaptor differs fundamentally from the related AP-1, AP-3, and AP-4 sorting complexes because membrane deposition does not depend directly on an Arf family GTPase. Instead phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) appears to act as the principal compartmental cue for AP-2 placement at the plasma membrane as well as for the docking of numerous other important clathrin coat components at the nascent bud site. This PtdIns(4,5)P(2) dependence makes type I phosphatidylinositol 4-phosphate 5-kinases (PIPKIs) lynchpin enzymes in the assembly of clathrin-coated structures at the cell surface. PIPKIgamma is the chief 5-kinase at nerve terminals, and here we show that the 26-amino acid, alternatively spliced C terminus of PIPKIgamma661 is an intrinsically unstructured polypeptide that binds directly to the sandwich subdomain of the AP-2 beta2 subunit appendage. An aromatic side chain-based, extended interaction motif that also includes the two bulky C-terminal residues of the short PIPKIgamma635 variant is necessary for beta2 appendage engagement. The clathrin heavy chain accesses the same contact surface on the AP-2 beta2 appendage, but because of additional clathrin binding sites located within the unstructured hinge segment of the beta2 subunit, clathrin binds the beta2 chain with a higher apparent affinity than PIPKIgamma661. A clathrin-regulated interaction with AP-2 could allow PIPKIgamma661 to be strategically positioned for regional PtdIns(4,5)P(2) generation during clathrin-coated vesicle assembly at the synapse.

Role of phosphatidic acid in the coupling of the ERK cascade

The production of phosphatidic acid plays a crucial role in the activation of the ERK cascade. This role was linked to the binding of phosphatidate to a specific polybasic site within the kinase domain of Raf-1. Here we show that phosphatidate promotes ERK phosphorylation in intact cells but does not activate Raf in vitro. The kinase suppressor of Ras (KSR) contains a sequence homologous to the phosphatidate binding site of Raf-1. Direct binding of phosphatidate to synthetic peptides derived from the sequences of the binding domains of Raf-1 and KSR was demonstrated by spectroscopic techniques. The specificity of these interactions was confirmed using synthetic lipids and mutated peptides in which the core of the phosphatidic acid binding domain was disrupted. Insulin and exogenous dioleoyl phosphatidate induced a rapid translocation of a mouse KSR1-EGFP construct to the plasma membrane of HIRcB cells. Mutation of two arginines located in the core of the putative phosphatidate binding site abolished dioleoyl phosphatidate- and insulin-induced translocation of KSR1. Overexpression of the mutant KSR1 in HIRcB cells inhibited insulin-dependent MEK and ERK phosphorylation. The addition of dioleoyl phosphatidate or insulin increased the co-localization of KSR1 and H-Ras and promoted the formation of plasma membrane patches enriched in both proteins and phosphatidic acid. These results, in conjunction with our previous work, suggest the formation of phosphatidate-enriched membrane microdomains that contain all components of the ERK cascade. We propose that these domains act as molecular scaffolds in the coupling of signaling events.

Integral and associated lysosomal membrane proteins

We searched for novel proteins in lysosomal membranes, tentatively participating in molecular transport across the membrane and/or in interactions with other compartments. In membranes purified from placental lysosomes, we identified 58 proteins, known to reside at least partially in the lysosomal membrane. These included 17 polypeptides comprising or associated with the vacuolar adenosine triphosphatase. We report on additional 86 proteins that were significantly enriched in the lysosomal membrane fraction. Among these, 12 novel proteins of unknown functions were found. Three were orthologues of rat proteins that have been identified in tritosomes by Bagshaw RD et al. (A proteomic analysis of lysosomal integral membrane proteins reveals the diverse composition of the organelle. Mol Cell Proteomics 2005;4:133-143). Here, the proteins encoded by LOC201931 (FLJ38482) and LOC51622 (C7orf28A) were expressed with an appended fluorescent tag in HeLa cells and found to be present in lysosomal organelles. Among the lysosomally enriched proteins, also 16 enzymes and transporters were detected that had not been assigned to lysosomal membranes previously. Finally, our results identified a particular set of proteins with known functions in signaling and targeting to be at least partially associated with lysosomes.

Phosphatidic acid regulates the affinity of the murine phosphatidylinositol 4-phosphate 5-kinase-Ibeta for phosphatidylinositol-4-phosphate

Type I phosphatidylinositol 4-phosphate 5-kinase (PI4P5K) catalyzes the phosphorylation of phosphatidylinositol 4 phosphate [PI(4)P] at carbon 5, producing phosphatidylinositol 4,5 bisphosphate [PI(4,5)P2]. Phosphatidic acid (PA) activates PI4P5K in vitro and plays a central role in the activation of PIP5K pathways in vivo. This report demonstrates that actin fiber formation in murine fibroblasts involves PA activation of PIP5Ks and defines biochemical interactions between PA and the PIP5Ks. Inhibition of phospholipase D production of PA results in the loss of actin fibers. Overexpression of the beta isoform of the type I murine phosphatidylinositol 4-phosphate 5-kinase (mPIP5K-Ibeta) maintains actin fiber structure in the face of phospholipase D inhibition. PA activates mPIP5K-Ibeta by direct binding to mPIP5K-Ibeta through both electrostatic and hydrophobic interactions, with the fatty acid acyl chain length and degree of saturation acting as critical determinants of binding and activation. Furthermore, kinetic analysis suggests that phosphorylation of the PI(4)P substrate does not follow classical Michaelis-Menten kinetics. Instead, the kinetic data are consistent with a model in which mPIP5K-Ibeta initially binds to the lipid micelle and subsequently binds the PI(4)P substrate. In addition, the kinetics indicate substrate inhibition, suggesting that mPIP5K-Ibeta contains an inhibitory PI(4)P-binding site. These results suggest a model in which mPIP5K-Ibeta is surrounded by PI(4)P, but is unable to catalyze its conversion to PI(4,5)P2 unless PA is bound.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

A mutation in Epstein-Barr virus LMP2A reveals a role for phospholipase D in B-Cell antigen receptor trafficking

Epstein-Barr virus (EBV) latent infection of B cells blocks the interrelated signaling and antigen-trafficking functions of the BCR through the activity of its latent membrane protein 2A (LMP2A). At present, the molecular mechanisms by which LMP2A exerts its control of BCR functions are only poorly understood. Earlier studies showed that in B cells expressing LMP2A containing a tyrosine mutation at position 112 in its cytoplasmic domain (Y112-LMP2A), the BCR could initiate signaling but could not properly traffic antigen for processing. Here, we show that BCR signaling in Y112-LMP2A-expressing cells is attenuated with a reduction in both the degree and duration of phosphorylation of key components of the BCR signaling cascade including Syk, BLNK, PI3K, and Btk. Notably, Y112-LMP2A expression completely blocked the BCR-induced activation of phospholipase D (PLD), a lipase implicated in the intracellular trafficking of a variety of surface receptors. We show that blocking PLD activity, by expressing Y112-LMP2A, treating cells with the PLD inhibitor 1-butanol or reducing PLD expression by siRNA, blocked BCR trafficking to class II-containing compartments. Moreover, Y112-LMP2A expression blocked the recruitment of phosphorylated forms of the downstream BCR signaling components, Erk and JNK, through both PLD-dependent and PLD-independent mechanisms. Thus, the investigation of the mechanism by which Y112-LMP2A blocks BCR function revealed an essential role for PLD in BCR trafficking for antigen processing.

Role of lipids and actin in the formation of clathrin-coated pits

Assembly of clathrin-coated pits and their maturation into coated vesicles requires coordinated interactions between specific lipids and several structural and regulatory proteins. In the presence of primary alcohols, phospholipase D generates phosphatidylalcohols instead of PA, reducing stimulation of phosphatidyl inositol 5-kinase (PI5K) and hence decreasing formation of phosphoinositide-4,5-biphosphate (PIP(2)). Using live-cell imaging, we have shown that acute treatment of cells with 1-butanol or other small primary alcohols induces rapid disassembly of coated pits at the plasma membrane and blocks appearance of new ones. Addition of exogenous PIP(2) reverses this effect. Coated pits and vesicles reappear synchronously upon removal of 1-butanol; we have used this synchrony to assess the role of actin in coated vesicle assembly. Prolonged inhibition of actin polymerization by latrunculin A or cytochalasin D reduced by approximately 50% the frequency of coated pit formation without affecting maturation into coated vesicles. As in control cells, removal of 1-butanol in the continued presence of an actin depolymerizer led to synchronous appearance of new pits, which matured normally. Thus, remodeling of the actin cytoskeleton is not essential for clathrin-coated vesicle assembly but may indirectly affect the nucleation of clathrin-coated pits.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

MORN motifs in plant PIPKs are involved in the regulation of subcellular localization and phospholipid binding

Multiple repeats of membrane occupation and recognition nexus (MORN) motifs were detected in plant phosphatidylinositl monophosphate kinase (PIPK), a key enzyme in PI-signaling pathway. Structural analysis indicates that all the MORN motifs (with varied numbers at ranges of 7-9), which shared high homologies to those of animal ones, were located at N-terminus and sequentially arranged, except those of OsPIPK1 and AtPIPK7, in which the last MORN motif was separated others by an approximately 100 amino-acid "island" region, revealing the presence of two kinds of MORN arrangements in plant PIPKs. Through employing a yeast-based SMET (sequence of membrane-targeting) system, the MORN motifs were shown being able to target the fusion proteins to cell plasma membrane, which were further confirmed by expression of fused MORN-GFP proteins. Further detailed analysis via deletion studies indicated the MORN motifs in OsPIPK1, together with the 104 amino-acid "island" region are involved in the regulation of differential subcellular localization, i.e. plasma membrane or nucleus, of the fused proteins. Fat Western blot analysis of the recombinant MORN polypeptide, expressed in Escherichia coli, showed that MORN motifs could strongly bind to PA and relatively slightly to PI4P and PI(4,5)P2. These results provide informative hints on mechanisms of subcellular localization, as well as regulation of substrate binding, of plant PIPKs.

Stimulation of phosphatidylinositol kinase type I-mediated phosphatidylinositol (4,5)-bisphosphate synthesis by AP-2mu-cargo complexes

Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P(2)] is an important factor for a variety of cellular functions ranging from cell signaling to actin cytoskeletal dynamics and endocytic membrane traffic. Here, we have identified the clathrin adaptor complex AP-2 as a regulator of phosphatidylinositol 4-phosphate 5-kinase (PIPK)-mediated PI(4,5)P(2) synthesis. AP-2 directly interacts with the kinase core domain of type I PIPK isozymes via its mu2-subunit in vitro and in native protein extracts. Endocytic cargo protein binding to mu2 leads to a potent stimulation of PIPK activity. These data thus identify a positive feedback loop consisting of endocytic cargo proteins, AP-2mu, and PIPK type I which may provide a specific pool of PI(4,5)P(2) dedicated to clathrin/AP-2-dependent receptor internalization.

Dynamics and function of phospholipase D and phosphatidic acid during phagocytosis

Phospholipase D (PLD) produces phosphatidic acid (PA), an established intracellular signalling lipid that has been also implicated in vesicular trafficking, and as such, PLD could play multiple roles during phagocytosis. Using an RNA interference strategy, we show that endogenous PLD1 and PLD2 are necessary for efficient phagocytosis in murine macrophages, in line with results obtained with wild-type constructs and catalytically inactive PLD mutants which, respectively, enhance and inhibit phagocytosis. Furthermore, we found that PA is transiently produced at sites of phagosome formation. Macrophage PLD1 and PLD2 differ in their subcellular distributions. PLD1 is associated with cytoplasmic vesicles, identified as a late endosomal/lysosomal compartment, whereas PLD2 localizes at the plasma membrane. In living cells undergoing phagocytosis, PLD1 vesicles are recruited to nascent and internalized phagosomes, whereas PLD2 is only observed on nascent phagosomes. These results provide evidence that both PLD isoforms are required for phagosome formation, but only PLD1 seems to be implicated in later stages of phagocytosis occurring after phagosomal internalization.

Role of phospholipase D2 in the agonist-induced and constitutive endocytosis of G-protein coupled receptors

We have recently shown that the mu-opioid receptor [MOR1, also termed mu-opioid peptide (MOP) receptor] is associated with the phospholipase D2 (PLD2), a phospholipid-specific phosphodiesterase located in the plasma membrane. We further demonstrated that, in human embryonic kidney (HEK) 293 cells co-expressing MOR1 and PLD2, treatment with (D-Ala2, Me Phe4, Glyol5)enkephalin (DAMGO) led to an increase in PLD2 activity and an induction of receptor endocytosis, whereas morphine, which does not induce opioid receptor endocytosis, failed to activate PLD2. In contrast, a C-terminal splice variant of the mu-opioid receptor (MOR1D, also termed MOP(1D)) exhibited robust endocytosis in response to both DAMGO and morphine treatment. We report here that MOR1D also mediates an agonist-independent (constitutive) PLD2-activation facilitating agonist-induced and constitutive receptor endocytosis. Inhibition of PLD2 activity by over-expression of a dominant negative PLD2 (nPLD2) blocked the constitutive PLD2 activation and impaired the endocytosis of MOR1D receptors. Moreover, we provide evidence that the endocytotic trafficking of the delta-opioid receptor [DOR, also termed delta-opioid peptide (DOP) receptor] and cannabinoid receptor isoform 1 (CB1) is also mediated by a PLD2-dependent pathway. These data indicate the generally important role for PLD2 in the regulation of agonist-dependent and agonist-independent G protein-coupled receptor (GPCR) endocytosis.

Clathrin-coated vesicle formation from isolated plasma membranes

Endocytic clathrin-coated vesicle (CCV) formation is a complex process involving a large number of proteins and lipids. The minimum machinery and the hierarchy of the events involved in CCV formation have yet to be defined. Here we describe an in vitro assay for CCV formation from highly purified rat liver plasma membranes. This rapid and easy assay can be used to quantitatively evaluate the different protein requirements for different endocytic receptors.

PldB, a putative phospholipase D homologue in Dictyostelium discoideum mediates quorum sensing during development

Quorum sensing, also known as cell-density sensing in the unicellular eukaryote Dictyostelium discoideum, is required for efficient entry into the differentiation and development segment of its life cycle. Quorum sensing is accomplished by simultaneously secreting and sensing the glycoprotein Conditioned Medium Factor, or CMF. When the density of starving cells is high, CMF levels are high, which leads to aggregation followed by development. Here, we describe the role of pldB, a gene coding for a putative phospholipase D (PLD) homologue, in quorum sensing. We find that in submerged culture, adding butanol, an inhibitor of PLD-catalyzed phosphatidic acid production, allows cells to bypass the requirement for CMF mediated quorum sensing and aggregate at low cell density. Deletion of pldB mimics the presence of butanol, allowing cells to aggregate at low cell density. pldB- cells also initiate and finish aggregation rapidly. Analysis of early developmental gene expression in pldB- cells reveals that the cyclic AMP receptor cAR1 is expressed at higher levels earlier than in wild-type cells, which could explain the rapid aggregation phenotype. As would be predicted, cells overexpressing pldB are unable to aggregate even at high cell density. Adding CMF to these pldB- overexpressing cells does not rescue aggregation. Both of these phenotypes are cell autonomous, as mixing a small number of pldB- cells with wild-type cells does not cause the wild-type cells to behave like pldB- cells.

Diacylglycerol kinase zeta regulates phosphatidylinositol 4-phosphate 5-kinase Ialpha by a novel mechanism

Phosphatidylinositol 4,5-bisphosphate (PIP2) plays an important role during actin polymerization and is produced by the type I phosphatidylinositol 4-phosphate 5-kinases (PIP5KI), which are activated by phosphatidic acid (PA). As diacylglycerol kinases (DGKs) generate PA by phosphorylating diacylglycerol (DAG), we investigated whether DGKs were involved in controlling PIP2 levels by regulating PIP5KI activity. Here we show that expression of DGKzeta significantly enhances PIP5KIalpha activity in thrombin-stimulated HEK293 cells, and DGK activity is required for this stimulation. We also observed that DGKzeta co-immunoprecipitated and co-localized with PIP5KIalpha, suggesting that they reside in a regulated signaling complex. To explore the role of DGKzeta in actin polymerization, we examined the subcellular distribution of DGKzeta, PIP5KIalpha and actin, and found that these proteins co-localized with actin in lamellipodial protrusions. Supporting that PIP5KIalpha regulation occurs at the sites of actin polymerization, we found that PIP2 also accumulated in the actin-rich regions of lamellipodia. Significantly, in wounding assays, DGKzeta, PIP5KIalpha and PIP2 accumulated at the leading edge of migrating A172 cells, where massive actin polymerization is known to occur. Combined, these data support a novel function for DGKzeta: by generating PA, it stimulates PIP5KIalpha activity to increase local PIP2, which regulates actin polymerization.

Ral and phospholipase D2-dependent pathway for constitutive metabotropic glutamate receptor endocytosis

G-protein-coupled receptors play a central role in the regulation of neuronal cell communication. Class 1 metabotropic glutamate receptors (mGluRs) mGluR1a and mGluR5a, which are coupled with the hydrolysis of phosphoinositides, are essential for modulating excitatory neurotransmission at glutamatergic synapses. These receptors are constitutively internalized in heterologous cell cultures, neuronal cultures, and intact neuronal tissues. We show here that the small GTP-binding protein Ral, its guanine nucleotide exchange factor RalGDS (Ral GDP dissociation stimulator), and phospholipase D2 (PLD2) are constitutively associated with class 1 mGluRs and regulate constitutive mGluR endocytosis. Moreover, both Ral and PLD2 are colocalized with mGluRs in endocytic vesicles in both human embryonic kidney 293 (HEK 293) cells and neurons. Ral and PLD2 activity is required for the internalization of class 1 mGluRs but is not required for the internalization of the beta2-adrenergic receptor. Constitutive class 1 mGluR internalization is not dependent on the downstream Ral effector proteins Ral-binding protein 1 and PLD1 or either ADP-ribosylation factors ARF1 or ARF6. The treatment of HEK 293 cells and neurons with small interfering RNA both downregulates PLD2 expression and blocks mGluR1a and mGluR5a endocytosis. The constitutive internalization of mGluR1a and mGluR5a is also attenuated by the treatment of cells with 1-butanol to prevent PLD2-mediated phosphatidic acid formation. We propose that the formation of a mGluR-scaffolded RalGDS/Ral/PLD2 protein complex provides a novel alternative mechanism to beta-arrestins for the constitutive endocytosis of class 1 mGluRs.

Lipid regulation of the synaptic vesicle cycle

Membrane vesicle cycling is orchestrated through the combined actions of proteins and lipids. At neuronal synapses, this orchestration must meet the stringent demands of speed, fidelity and sustainability of the synaptic vesicle cycle that mediates neurotransmission. Historically, the lion's share of the attention has been focused on the proteins that are involved in this cycle; but, in recent years, it has become clear that the previously unheralded plasma membrane and vesicle lipids are also key regulators of this cycle. This article reviews recent insights into the roles of lipid-modifying enzymes and lipids in the acute modulation of neurotransmission.

Regulation and cellular roles of phosphoinositide 5-kinases

The membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP(2)), plays a critical role in various, apparently very different cellular processes. As precursor for second messengers generated by phospholipase C isoforms and class I phosphoinositide 3-kinases, PIP(2) is indispensable for cellular signaling by membrane receptors. In addition, PIP(2) directly affects the localization and activity of many cellular proteins via specific interaction with unique phosphoinositide-binding domains and thereby regulates actin cytoskeletal dynamics, vesicle trafficking, ion channel activity, gene expression and cell survival. The activity and subcellular localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) isoforms, which catalyze the formation of PIP(2), are actively regulated by membrane receptors, by phosphorylation and by small GTPases of the Rho and ARF families. Spatially and temporally organized regulation of PIP(2) synthesis by PIP5K enables dynamic and versatile PIP(2) signaling and represents an important link in the execution of cellular tasks by Rho and ARF GTPases.

Phospholipases D1 and D2 Coordinately Regulate Macrophage Phagocytosis

Phagocytosis is a fundamental feature of the innate immune system, required for antimicrobial defense, resolution of inflammation, and tissue remodeling. Furthermore, phagocytosis is coupled to a diverse range of cytotoxic effector mechanisms, including the respiratory burst, secretion of inflammatory mediators and Ag presentation. Phospholipase D (PLD) has been linked to the regulation of phagocytosis and subsequent effector responses, but the identity of the PLD isoform(s) involved and the molecular mechanisms of activation are unknown. We used primary human macrophages and human THP-1 promonocytes to characterize the role of PLD in phagocytosis. Macrophages, THP-1 cells, and other human myelomonocytic cells expressed both PLD1 and PLD2 proteins. Phagocytosis of complement-opsonized zymosan was associated with stimulation of the activity of both PLD1 and PLD2, as demonstrated by a novel immunoprecipitation-in vitro PLD assay. Transfection of dominant-negative PLD1 or PLD2 each inhibited the extent of phagocytosis (by 55-65%), and their combined effects were additive (reduction of 91%). PLD1 and PLD2 exhibited distinct localizations in resting macrophages and those undergoing phagocytosis, and only PLD1 localized to the phagosome membrane. The COS-7 monkey fibroblast cell line, which has been used as a heterologous system for the analysis of receptor-mediated phagocytosis, expressed PLD2 but not PLD1. These data support a model in which macrophage phagocytosis is coordinately regulated by both PLD1 and PLD2, with isoform-specific localization. Human myelomonocytic cell lines accurately model PLD-dependent signal transduction events required for phagocytosis, but the heterologous COS cell system does not.

Phosphoinositides in Constitutive Membrane Traffic

Proteins that make, consume, and bind to phosphoinositides are important for constitutive membrane traffic. Different phosphoinositides are concentrated in different parts of the central vacuolar pathway, with phosphatidylinositol 4-phosphate predominate on Golgi, phosphatidylinositol 4,5-bisphosphate predominate at the plasma membrane, phosphatidylinositol 3-phosphate the major phosphoinositide on early endosomes, and phosphatidylinositol 3,5-bisphosphate found on late endocytic organelles. This spatial segregation may be the mechanism by which the direction of membrane traffic is controlled. Phosphoinositides increase the affinity of membranes for peripheral membrane proteins that function for sorting protein cargo or for the docking and fusion of transport vesicles. This implies that constitutive membrane traffic may be regulated by the mechanisms that control the activity of the enzymes that produce and consume phosphoinositides. Although the lipid kinases and phosphatases that function in constitutive membrane traffic are beginning to be identified, their regulation is poorly understood.

Peptidoglycan and lipoteichoic acid in gram-positive bacterial sepsis: receptors, signal transduction, biological effects, and synergism

In sepsis and multiple organ dysfunction syndrome (MODS) caused by gram-negative bacteria, lipopolysaccharide (LPS) initiates the early signaling events leading to the deleterious inflammatory response. However, it has become clear that LPS can not reproduce all of the clinical features of sepsis, which emphasize the roles of other contributing factors. Gram-positive bacteria, which lack LPS, are today responsible for a substantial part of the incidents of sepsis with MODS. The major wall components of gram-positive bacteria, peptidoglycan and lipoteichoic acid, are thought to contribute to the development of sepsis and MODS. In this review, the literature underlying our current understanding of how peptidoglycan and lipoteichoic acid activate inflammatory responses will be presented, with a focus on recent advances in this field.

Protein kinase Calpha translocates to the perinuclear region to activate phospholipase D1

The inhibition of phorbol ester activation of phospholipase D1 (PLD1) by protein kinase C (PKC) inhibitors has been considered proof of phosphorylation-dependent activation of PLD1 by PKCalpha. We studied the effect of the PKC inhibitors Ro-31-8220 and bisindolylmaleimide I on PLD1 activation and found that they inhibited the activation by interfering with PKCalpha binding to PLD1. Further studies showed that only unphosphorylated PKCalpha could bind to and activate PLD1 and that both inhibitors induced phosphorylation of PKCalpha. The phosphorylation status of either PLD1 or PKCalpha per se did not affect PLD1 activation in vitro. Immunofluorescence studies showed that PLD1 remained in the perinuclear region after phorbol ester treatment, whereas PKCalpha translocated from cytosol to both plasma membrane and perinuclear regions. Both Ro-31-8220 and bisindolylmaleimide I blocked the translocation of PKCalpha to the perinuclear region but not to the plasma membrane. Studies with okadaic acid suggested that phosphorylation regulated the relocation of PKCalpha from the plasma membrane to the perinuclear region. It is proposed that localization and interaction of PKCalpha with PLD1 in the perinuclear region is required for PLD1 activation and that PKC inhibitors inhibit this through phosphorylation of PKCalpha, which blocks its translocation.

Overexpression of the amplified Pip4k2β gene from 17q11–12 in breast cancer cells confers proliferation advantage

Gene amplification is common in solid tumors and is associated with adverse prognosis, disease progression, and development of drug resistance. A small segment from chromosome 17q11-12 containing the HER-2/Neu gene is amplified in about 25% of breast cancer. HER-2/Neu amplification is associated with adverse prognosis and may predict response to chemotherapy and hormonal manipulation. Moreover, HER-2/Neu amplification may select patients for anti-HER-2/Neu-based therapy with Herceptin. We and others recently described a common sequence element from the HER-2/Neu region that was amplified in breast cancer cells. In addition, most, if not all, of the amplified genes from this region display overexpression. This raises the intriguing possibility that genes immediately adjacent to HER-2/Neu may influence the biological behavior of breast cancer carrying HER-2/Neu amplification and serve as rational targets for therapy. By extracting sequence information from public databases, we have constructed a contig in bacterial artificial chromosomes (BACs) that extends from HER-2/Neu to a phosphotidylinositol phosphate kinase (PIPK), Pip4k2beta from 17q11-12. Although a role of PI-3-kinase and AKT in cancer biology has been previously described, PIPK has not been previously implicated. We show that Pip4k2beta, initially known as Pip5k2beta, is amplified in a subset of breast cancer cell lines and primary breast cancer samples that carry HER-2/Neu amplification. Out of eight breast cancer cell lines with HER-2/Neu amplification, three have concomitant amplification of the Pip4k2beta gene--UACC-812, BT-474 and ZR-75-30. Similarly, two out of four primary breast tumors with HER-2/Neu amplification carry Pip4k2beta gene amplification. Intriguingly, one tumor displays an increase in the gene copy number of Pip4k2beta that is significantly more than that of HER-2/Neu. Moreover, dual color FISH reveals that amplified Pip4k2beta gene may exist in a distinct structure from that of HER-2/Neu in ZR-75-30 cell line. These studies suggest that Pip4k2beta may reside on an amplification maximum distinct from that of HER-2/Neu and serve as an independent target for amplification and selective retention. Pip4k2beta amplification is associated with overexpression at the RNA and protein level in breast cancer cell lines. Stable expression of Pip4k2beta in breast cancer cell lines with and without HER-2/Neu amplification increases cell proliferation and anchorage-independent growth. The above observations implicate Pip4k2beta in the development and/or progression of breast cancer. Our study suggests that Pip4k2beta may be a distinct target for gene amplification and selective retention from 17q11-12.

A novel AP-2 adaptor interaction motif initially identified in the long-splice isoform of synaptojanin 1, SJ170

Phosphoinositides play a fundamental role in clathrin-coat assembly at the cell surface. Several endocytic components and accessory factors contain independently folded phosphoinositide-binding modules that facilitate, in part, membrane placement at the bud site. As the clathrin-coat assembly process progresses toward deeply invaginated buds, focally synthesized phosphoinositides are dephosphorylated, principally through the action of the phosphoinositide polyphosphatase synaptojanin 1. Failure to catabolize polyphosphoinositides retards the fission process and endocytic activity. The long-splice isoform of synaptojanin 1, termed SJ170, contains a carboxyl-terminal extension that harbors interaction motifs for engaging several components of the endocytic machinery. Here, we demonstrate that in addition to DPF and FXDXF sequences, the SJ170 carboxyl terminus contains a novel AP-2 binding sequence, the WXXF motif. The WXXF sequence engages the independently folded alpha-subunit appendage that projects off the heterotetrameric AP-2 adaptor core. The endocytic protein kinases AAK1 and GAK also contain functional WXX(FW) motifs in addition to two DPF repeats, whereas stonin 2 harbors three tandem WXXF repeats. Each of the discrete SJ170 adaptor-interaction motifs bind to appendages relatively weakly but, as tandemly arrayed within the SJ170 extension, can cooperate to bind bivalent AP-2 with good apparent affinity. These interactions likely contribute to the appropriate targeting of certain endocytic components to clathrin bud sites assembling at the cell surface.

ARF6 stimulates clathrin/AP-2 recruitment to synaptic membranes by activating phosphatidylinositol phosphate kinase type Igamma

Clathrin-mediated endocytosis of synaptic vesicle membranes involves the recruitment of clathrin and AP-2 adaptor complexes to the presynaptic plasma membrane. Phosphoinositides have been implicated in nucleating coat assembly by directly binding to several endocytotic proteins including AP-2 and AP180. Here, we show that the stimulatory effect of ATP and GTPgammaS on clathrin coat recruitment is mediated at least in part by increased levels of PIP2. We also provide evidence for a role of ADP-ribosylation factor 6 (ARF6) via direct stimulation of a synaptically enriched phosphatidylinositol 4-phosphate 5-kinase type Igamma (PIPKIgamma), in this effect. These data suggest a model according to which activation of PIPKIgamma by ARF6-GTP facilitates clathrin-coated pit assembly at the synapse.

Phosphoinositide regulation of the actin cytoskeleton

Phosphoinositides [PPIs, which collectively refer to phosphorylated derivatives of phosphatidylinositol (PI)] have a pivotal role as precursors to important second messengers and as bona fide signaling and scaffold targeting molecules. This review focuses on recent advances that elucidate how PPIs, particularly PI(4,5)P2 (PIP2), directly regulate the actin cytoskeleton in vivo by modulating the activity and targeting of actin regulatory proteins. The role of PIP2 in stimulating actin polymerization and in establishing cytoskeleton-plasma membrane linkages is emphasized. In addition, the review presents tantalizing evidence that suggests how binding of selected cytoskeletal proteins to membrane PPIs may promote PPI clustering into raft lipid microdomains, alter their accessibility to other proteins, and even distort the bilayer conformation. These actions have profound implications for many other PPI-regulated membrane functions that are beginning to be uncovered, and they suggest how PPIs can mediate crosstalk between the actin cytoskeleton and an expanding spectrum of essential cellular functions.

Mastoparan selectively activates phospholipase D2 in cell membranes

Both known isoforms of phospholipase (PL) D, PLD1 and PLD2, require phosphatidylinositol 4,5-bisphosphate for activity. However, PLD2 is fully active in the presence of this phospholipid, whereas PLD1 activation is dependent on additional factors such as ADP-ribosylation factor-1 (ARF-1) and protein kinase Calpha. We find that mastoparan, an activator of G(i) and mast cells, stimulates an intrinsic PLD activity, most likely PLD2, in fractions enriched in plasma membranes from rat basophilic leukemia 2H3 mast cells. Overexpression of PLD2, but not of PLD1, results in a large increase in the mastoparan-inducible PLD activity in membrane fractions, particularly those enriched in plasma membranes. As in previous studies, expressed PLD2 is localized primarily in the plasma membrane and PLD1 in granule membranes. Studies with pertussis toxin and other agents indicate that mastoparan stimulates PLD2 independently of G(i), ARF-1, protein kinase C, and calcium. Kinetic studies indicate that mastoparan interacts synergistically with phosphatidylinositol 4,5-bisphosphate and that oleate, itself a weak stimulant of PLD2 at low concentrations, is a competitive inhibitor of mastoparan stimulation of PLD2. Therefore, mastoparan may be useful for investigating the regulation of PLD2, particularly in view of the well studied molecular interactions of mastoparan with certain other strategic signaling proteins.

Proteomic analysis of early melanosomes: identification of novel melanosomal proteins

Melanin is a heterogeneous biopolymer produced only by specific cells termed melanocytes, which synthesize and deposit the pigment in specialized membrane-bound organelles known as melanosomes. Although melanosomes have been suspected of being closely related to lysosomes and platelets, the total number of melanosomal proteins is still unknown. Thus far, six melanosome-specific proteins have been identified, and the challenge is to characterize the complete proteome of the melanosome to further understand its mechanism of biogenesis. In this report, we used mass spectrometry and subcellular fractionation to identify protein components of early melanosomes. Using this approach, we have identified all 6 of the known melanosome-specific proteins, 56 proteins that are shared with other organelles, and confirmed the presence of 6 novel melanosomal proteins using western blotting and by immunohistochemistry.

The role of phosphatidic acid in the regulation of the Ras/MEK/Erk signaling cascade

Phosphatidic acid (PA) is an important second messenger produced by the activation of numerous cell surface receptors. Recent data have suggested that PA regulates multiple cellular processes. This review addresses primarily the role of PA in the regulation of the Erk1/2 cascade pathway. A model for the regulation of Erk1/2 phosphorylation by cell surface receptors is presented. According to this model, agonists stimulate the binding of GTP to Ras and the activation of phospholipase D to generate phosphatidic acid. PA promotes the binding of cRaf-1 kinase to the membrane, where it interacts with Ras.GTP and other regulatory components of the pathway. Ras-Raf complexes remain bound to the surface of endosomes, where scaffolding complexes involving Ras, cRaf-1, MEK and Erk are formed. Complete activation and coupling of the cascade requires endocytosis, a process that is also modulated by PA.