Diacylglycerol kinase alpha mediates HGF-induced Rac activation and membrane ruffling by regulating atypical PKC and RhoGDI

Aberrant MET activation impairs perinuclear actin cap organization with YAP1 cytosolic relocation

Little is known about the signaling network responsible for the organization of the perinuclear actin cap, a recently identified structure holding unique roles in the regulation of nuclear shape and cell directionality. In cancer cells expressing a constitutively active MET, we show a rearrangement of the actin cap filaments, which crash into perinuclear patches associated with spherical nuclei, meandering cell motility and inactivation of the mechano-transducer YAP1. MET ablation is sufficient to reactivate YAP1 and restore the cap, leading to enhanced directionality and flattened nuclei. Consistently, the introduction of a hyperactive MET in normal epithelial cells, enhances nuclear height and alters the cap organization, as also confirmed by TEM analysis. Finally, the constitutively active YAP1 mutant YAP5SA is able to overcome the effects of oncogenic MET. Overall, our work describes a signaling axis empowering MET-mediated YAP1 dampening and actin cap misalignment, with implications for nuclear shape and cell motility.

Crosstalk between diacylglycerol kinase and protein kinase A in the regulation of airway smooth muscle cell proliferation

BACKGROUND

Diacylglycerol kinase (DGK) regulates intracellular signaling and functions by converting diacylglycerol (DAG) into phosphatidic acid. We previously demonstrated that DGK inhibition attenuates airway smooth muscle (ASM) cell proliferation, however, the mechanisms mediating this effect are not well established. Given the capacity of protein kinase A (PKA) to effect inhibition of ASM cells growth in response to mitogens, we employed multiple molecular and pharmacological approaches to examine the putative role of PKA in the inhibition of mitogen-induced ASM cell proliferation by the small molecular DGK inhibitor I (DGK I).

METHODS

We assayed cell proliferation using CyQUANT™ NF assay, protein expression and phosphorylation using immunoblotting, and prostaglandin E2 (PGE2) secretion by ELISA. ASM cells stably expressing GFP or PKI-GFP (PKA inhibitory peptide-GFP chimera) were stimulated with platelet-derived growth factor (PDGF), or PDGF + DGK I, and cell proliferation was assessed.

RESULTS

DGK inhibition reduced ASM cell proliferation in cells expressing GFP, but not in cells expressing PKI-GFP. DGK inhibition increased cyclooxygenase II (COXII) expression and PGE2 secretion over time to promote PKA activation as demonstrated by increased phosphorylation of (PKA substrates) VASP and CREB. COXII expression and PKA activation were significantly decreased in cells pre-treated with pan-PKC (Bis I), MEK (U0126), or ERK2 (Vx11e) inhibitors suggesting a role for PKC and ERK in the COXII-PGE2-mediated activation of PKA signaling by DGK inhibition.

CONCLUSIONS

Our study provides insight into the molecular pathway (DAG-PKC/ERK-COXII-PGE2-PKA) regulated by DGK in ASM cells and identifies DGK as a potential therapeutic target for mitigating ASM cell proliferation that contributes to airway remodeling in asthma.

Underappreciated roles for Rho GDP dissociation inhibitors (RhoGDIs) in cell function: Lessons learned from the pancreatic islet β-cell

Rho subfamily of G proteins (e.g., Rac1) have been implicated in glucose-stimulated insulin secretion from the pancreatic β-cell. Interestingly, metabolic stress (e.g., chronic exposure to high glucose) results in sustained activation of Rac1 leading to increased oxidative stress, impaired insulin secretion and β-cell dysfunction. Activation-deactivation of Rho G proteins is mediated by three classes of regulatory proteins, namely the guanine nucleotide exchange factors (GEFs), which facilitate the conversion of inactive G proteins to their active conformations; the GTPase-activating proteins (GAPs), which convert the active G proteins to their inactive forms); and the GDP-dissociation inhibitors (GDIs), which prevent the dissociation of GDP from G proteins. Contrary to a large number of GEFs (82 members) and GAPs (69 members), only three members of RhoGDIs (RhoGDIα, RhoGDIβ and RhoGDIγ) are expressed in mammalian cells.Even though relatively smaller in number, the GDIs appear to play essential roles in G protein function (e.g., subcellular targeting) for effector activation and cell regulation. Emerging evidence also suggests that the GDIs are functionally regulated via post-translational modification (e.g., phosphorylation) and by lipid second messengers, lipid kinases and lipid phosphatases. We highlight the underappreciated regulatory roles of RhoGDI-Rho G protein signalome in islet β-cell function in health and metabolic stress. Potential knowledge gaps in the field, and directions for future research for the identification of novel therapeutic targets to loss of functional β-cell mass under the duress of metabolic stress are highlighted.

Autocrine regulation of airway smooth muscle contraction by diacylglycerol kinase

Diacylglycerol kinase (DGK), a lipid kinase, catalyzes the conversion of diacylglycerol (DAG) to phosphatidic acid, thereby terminating DAG-mediated signaling by Gq-coupled receptors that regulate contraction of airway smooth muscle (ASM). A previous study from our laboratory demonstrated that DGK inhibition or genetic ablation leads to reduced ASM contraction and provides protection for allergen-induced airway hyperresponsiveness. However, the mechanism by which DGK regulates contractile signaling in ASM is not well established. Herein, we investigated the role of prorelaxant cAMP-protein kinase A (PKA) signaling in DGK-mediated regulation of ASM contraction. Pretreatment of human ASM cells with DGK inhibitor I activated PKA as demonstrated by the phosphorylation of PKA substrates, VASP, Hsp20, and CREB, which was abrogated when PKA was inhibited pharmacologically or molecularly using overexpression of the PKA inhibitor peptide, PKI. Furthermore, inhibition of DGK resulted in induction of cyclooxygenase (COX) and generation of prostaglandin E₂ (PGE₂ ) with concomitant activation of Gs-cAMP-PKA signaling in ASM cells in an autocrine/paracrine fashion. Inhibition of protein kinase C (PKC) or extracellular-signal-regulated kinase (ERK) attenuated DGK-mediated production of PGE₂ and activation of cAMP-PKA signaling in human ASM cells, suggesting that inhibition of DGK activates the COX-PGE₂ pathway in a PKC-ERK-dependent manner. Finally, DGK inhibition-mediated attenuation of contractile agonist-induced phosphorylation of myosin light chain 20 (MLC-20), a marker of ASM contraction, involves COX-mediated cAMP production and PKA activation in ASM cells. Collectively these findings establish a novel mechanism by which DGK regulates ASM contraction and further advances DGK as a potential therapeutic target to provide effective bronchoprotection in asthma.

Diacylglycerol Kinase η Activity in Cells Using Protein Myristoylation and Cellular Phosphatidic Acid Sensor

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol to produce phosphatidic acid (PtdOH) and regulates the balance between two lipid second messengers: diacylglycerol and PtdOH. Several lines of evidence suggest that the η isozyme of DGK is involved in the pathogenesis of bipolar disorder. However, the detailed molecular mechanisms regulating the pathophysiological functions remain unclear. One reason is that it is difficult to detect the cellular activity of DGKη. To overcome this difficulty, we utilized protein myristoylation and a cellular PtdOH sensor, the N-terminal region of α-synuclein (α-Syn-N). Although DGKη expressed in COS-7 cells was broadly distributed in the cytoplasm, myristoylated (Myr)-AcGFP-DGKη and Myr-AcGFP-DGKη-KD (inactive (kinase-dead) mutant) were substantially localized in the plasma membrane. Moreover, DsRed monomer-α-Syn-N significantly colocalized with Myr-AcGFP-DGKη but not Myr-AcGFP-DGKη-KD at the plasma membrane. When COS-7 cells were osmotically shocked, all DGKη constructs were exclusively translocated to osmotic shock-responsive granules (OSRG). DsRed monomer-α-Syn-N markedly colocalized with only Myr-AcGFP-DGKη at OSRG and exhibited a higher signal/background ratio (3.4) than Myr-AcGFP-DGKη at the plasma membrane in unstimulated COS-7 cells (2.5), indicating that α-Syn-N more effectively detects Myr-AcGFP-DGKη activity in OSRG. Therefore, these results demonstrated that the combination of myristoylation and the PtdOH sensor effectively detects DGKη activity in cells and that this method is convenient to examine the molecular functions of DGKη. Moreover, this method will be useful for the development of drugs targeting DGKη. Furthermore, the combination of myristoylation (intensive accumulation in membranes) and α-Syn-N can be applicable to assays for various cytosolic PtdOH-generating enzymes.

Therapeutic Targeting of DGKA-Mediated Macropinocytosis Leads to Phospholipid Reprogramming in Tuberous Sclerosis Complex

Lymphangioleiomyomatosis is a rare destructive lung disease affecting primarily women and is the primary lung manifestation of tuberous sclerosis complex (TSC). In lymphangioleiomyomatosis, biallelic loss of TSC1/2 leads to hyperactivation of mTORC1 and inhibition of autophagy. To determine how the metabolic vulnerabilities of TSC2-deficient cells can be targeted, we performed a high-throughput screen utilizing the "Repurposing" library at the Broad Institute of MIT and Harvard (Cambridge, MA), with or without the autophagy inhibitor chloroquine. Ritanserin, an inhibitor of diacylglycerol kinase alpha (DGKA), was identified as a selective inhibitor of proliferation of Tsc2^-/- mouse embryonic fibroblasts (MEF), with no impact on Tsc2^+/+ MEFs. DGKA is a lipid kinase that metabolizes diacylglycerol to phosphatidic acid, a key component of plasma membranes. Phosphatidic acid levels were increased 5-fold in Tsc2^-/- MEFs compared with Tsc2^+/+ MEFs, and treatment of Tsc2^-/- MEFs with ritanserin led to depletion of phosphatidic acid as well as rewiring of phospholipid metabolism. Macropinocytosis is known to be upregulated in TSC2-deficient cells. Ritanserin decreased macropinocytic uptake of albumin, limited the number of lysosomes, and reduced lysosomal activity in Tsc2^-/- MEFs. In a mouse model of TSC, ritanserin treatment decreased cyst frequency and volume, and in a mouse model of lymphangioleiomyomatosis, genetic downregulation of DGKA prevented alveolar destruction and airspace enlargement. Collectively, these data indicate that DGKA supports macropinocytosis in TSC2-deficient cells to maintain phospholipid homeostasis and promote proliferation. Targeting macropinocytosis with ritanserin may represent a novel therapeutic approach for the treatment of TSC and lymphangioleiomyomatosis. SIGNIFICANCE: This study identifies macropinocytosis and phospholipid metabolism as novel mechanisms of metabolic homeostasis in mTORC1-hyperactive cells and suggest ritanserin as a novel therapeutic strategy for use in mTORC1-hyperactive tumors, including pancreatic cancer. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/8/2086/F1.large.jpg.

PKCδ mediates mitochondrial ROS generation and oxidation of HSP60 to relieve RKIP inhibition on MAPK pathway for HCC progression

Both protein kinase C (PKC) and reactive oxygen species (ROS) are well-known signaling messengers cross-talking with each other to activate mitogen-activated protein kinases (MAPKs) for progression of hepatocellular carcinoma (HCC). However, the underlying mechanisms are not well elucidated. Especially, whether mitochondrial ROS (mtROS) is involved and how it triggers MAPK signaling are intriguing. In this study, we found mtROS generation and phosphorylation of MAPKs were mediated by PKCδ in HCCs treated with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Heat shock protein 60 (HSP60), one of the chaperones in mitochondria was the major protein oxidized in TPA-treated HCCs. Moreover, depletion of HSP60 or expression of HSP60 cysteine mutant prevented TPA-induced phosphorylation of MAPKs. To delineate how HSP60 mediated MAPK activation, the role of Raf kinase inhibitor protein (RKIP), a negative regulator of MAPK, was investigated. TPA dissociated RKIP from HSP60 in both mitochondria and cytosol, concurrently with translocation of HSP60 and MAPK from mitochondria to cytosol, which was associated with robust phosphorylation of MAPKs in the cytosol. Moreover, TPA induced opposite phenotypical changes of HCCs, G1 cell cycle arrest, and cell migration, which were prevented by mtROS scavengers and depletion of PKCδ and HSP60. Consistently, TPA increased the migration-related genes, hydrogen peroxide inducible clone5, matrix metalloproteinase-1/3, lamininγ2, and suppressed the cell cycle regulator cyclin E1 (CCNE1) via PKCδ/mtROS/HSP60/MAPK-axis. Finally, c-jun and c-fos were required for TPA-induced expression of the migration-related genes and a novel microRNA, miR-6134, was responsible for TPA-induced suppression of CCNE1. In conclusion, PKCδ cross-talked with mtROS to trigger HSP60 oxidation for release of RKIP to activate MAPK, regulating gene expression for migration, and G1 cell cycle arrest in HCC. Targeted therapy aiming at key players like PKCδ, RKIP, and HSP60 is promising for preventing HCC progression.

Identification of Key Phospholipids That Bind and Activate Atypical PKCs

PKCζ and PKCι/λ form the atypical protein kinase C subgroup, characterised by a lack of regulation by calcium and the neutral lipid diacylglycerol. To better understand the regulation of these kinases, we systematically explored their interactions with various purified phospholipids using the lipid overlay assays, followed by kinase activity assays to evaluate the lipid effects on their enzymatic activity. We observed that both PKCζ and PKCι interact with phosphatidic acid and phosphatidylserine. Conversely, PKCι is unique in binding also to phosphatidylinositol-monophosphates (e.g., phosphatidylinositol 3-phosphate, 4-phosphate, and 5-phosphate). Moreover, we observed that phosphatidylinositol 4-phosphate specifically activates PKCι, while both isoforms are responsive to phosphatidic acid and phosphatidylserine. Overall, our results suggest that atypical Protein kinase C (PKC) localisation and activity are regulated by membrane lipids distinct from those involved in conventional PKCs and unveil a specific regulation of PKCι by phosphatidylinositol-monophosphates.

Interplay Between SNX27 and DAG Metabolism in the Control of Trafficking and Signaling at the IS

Recognition of antigens displayed on the surface of an antigen-presenting cell (APC) by T-cell receptors (TCR) of a T lymphocyte leads to the formation of a specialized contact between both cells named the immune synapse (IS). This highly organized structure ensures cell–cell communication and sustained T-cell activation. An essential lipid regulating T-cell activation is diacylglycerol (DAG), which accumulates at the cell–cell interface and mediates recruitment and activation of proteins involved in signaling and polarization. Formation of the IS requires rearrangement of the cytoskeleton, translocation of the microtubule-organizing center (MTOC) and vesicular compartments, and reorganization of signaling and adhesion molecules within the cell–cell junction. Among the multiple players involved in this polarized intracellular trafficking, we find sorting nexin 27 (SNX27). This protein translocates to the T cell–APC interface upon TCR activation, and it is suggested to facilitate the transport of cargoes toward this structure. Furthermore, its interaction with diacylglycerol kinase ζ (DGKζ), a negative regulator of DAG, sustains the precise modulation of this lipid and, thus, facilitates IS organization and signaling. Here, we review the role of SNX27, DAG metabolism, and their interplay in the control of T-cell activation and establishment of the IS.

Diacylglycerol Kinase Alpha in Radiation-Induced Fibrosis: Potential as a Predictive Marker or Therapeutic Target

Radiotherapy is an efficient tool in cancer treatment, but it brings along the risk of side effects such as fibrosis in the irradiated healthy tissue thus limiting tumor control and impairing quality of life of cancer survivors. Knowledge on radiation-related fibrosis risk and therapeutic options is still limited and requires further research. Recent studies demonstrated that epigenetic regulation of diacylglycerol kinase alpha (DGKA) is associated with radiation-induced fibrosis. However, the specific mechanisms are still unknown. In this review, we scrutinized the role of DGKA in the radiation response and in further cellular functions to show the potential of DGKA as a predictive marker or a novel target in fibrosis treatment. DGKA was reported to participate in immune response, lipid signaling, exosome production, and migration as well as cell proliferation, all processes which are suggested to be critical steps in fibrogenesis. Most of these functions are based on the conversion of diacylglycerol (DAG) to phosphatidic acid (PA) at plasma membranes, but DGKA might have also other, yet not well-known functions in the nucleus. Current evidence summarized here underlines that DGKA activation may play a central role in fibrosis formation post-irradiation and shows a potential of direct DGKA inhibitors or epigenetic modulators to attenuate pro-fibrotic reactions, thus providing novel therapeutic choices.

Diacylglycerol kinase ζ promotes actin cytoskeleton remodeling and mechanical forces at the B cell immune synapse

Diacylglycerol kinases (DGKs) limit antigen receptor signaling in immune cells by consuming the second messenger diacylglycerol (DAG) to generate phosphatidic acid (PA). Here, we showed that DGKζ promotes lymphocyte function-associated antigen 1 (LFA-1)-mediated adhesion and F-actin generation at the immune synapse of B cells with antigen-presenting cells (APCs), mostly in a PA-dependent manner. Measurement of single-cell mechanical force generation indicated that DGKζ-deficient B cells exerted lower forces at the immune synapse than did wild-type B cells. Nonmuscle myosin activation and translocation of the microtubule-organizing center (MTOC) to the immune synapse were also impaired in DGKζ-deficient B cells. These functional defects correlated with the decreased ability of B cells to present antigen and activate T cells in vitro. The in vivo germinal center response of DGKζ-deficient B cells was also reduced compared with that of wild-type B cells, indicating that loss of DGKζ in B cells impaired T cell help. Together, our data suggest that DGKζ shapes B cell responses by regulating actin remodeling, force generation, and antigen uptake-related events at the immune synapse. Hence, an appropriate balance in the amounts of DAG and PA is required for optimal B cell function.

Biological regulation of diacylglycerol kinases in normal and neoplastic tissues: New opportunities for cancer immunotherapy

In the recent years, the arsenal of anti-cancer therapies has evolved to target T lymphocytes and restore their capacity to destroy tumor cells. However, the clinical success is limited, with a large number of patients that never responds and others that ultimately develop resistances. Overcoming the hypofunctional state imposed by solid tumors to T cells has revealed critical but challenging due to the complex strategies that tumors employ to evade the immune system. The Diacylglycerol kinases (DGK) limit DAG-dependent functions in T lymphocytes and their upregulation in tumor-infiltrating T lymphocytes contribute to limit T cell cytotoxic potential. DGK blockade could reinstate T cell attack on tumors, limiting at the same time tumor cell growth, thanks to the DGK positive input into several oncogenic pathways. In this review we summarize the latest findings regarding the regulation of specific DGK isoforms in healthy and anergic T lymphocytes, as well as their contribution to oncogenic phenotypes. We will also revise the latest advances in the search for pharmacological inhibitors and their potential as anti-cancer agents, either alone or in combination with immunomodulatory agents.

Characterization of α-synuclein N-terminal domain as a novel cellular phosphatidic acid sensor

Tracking the localization and dynamics of the intracellular bioactive lipid phosphatidic acid (PA) is important for understanding diverse biological phenomena. Although several PA sensors have been developed, better ones are still needed for comprehensive PA detection in cells. We recently found that α-synuclein (α-Syn) selectively and strongly bound to PA in vitro. Here, we revealed that the N-terminal region of α-Syn (α-Syn-N) specifically bound to PA, with a dissociation constant of 6.6 μm. α-Syn-N colocalized with PA-producing enzymes, diacylglycerol kinase (DGK) β at the plasma membrane (PM), myristoylated DGKζ at the Golgi apparatus, phorbol ester-stimulated DGKγ at the PM, and phospholipase D2 at the PM and Golgi but not with the phosphatidylinositol-4,5-bisphosphate-producing enzyme in COS-7 cells. However, α-Syn-N failed to colocalize with them in the presence of their inhibitors and/or their inactive mutants. These results indicate that α-Syn-N specifically binds to cellular PA and can be applied as an excellent PA sensor.

Analytical Method for Diacylglycerol Kinase ζ Activity in Cells Using Protein Myristoylation and Liquid Chromatography-Tandem Mass Spectrometry

Specific inhibitors of diacylglycerol kinase (DGK) ζ can be promising anticancer medications via the activation of cancer immunity. Although the detection of cellular activities of target enzymes is essential for drug screening in addition to in vitro assays, it is difficult to detect the activity of DGKζ in cells. In the present study, we generated AcGFP-DGKζ cDNA with a consensus N-myristoylation sequence at the 5' end (Myr-AcGFP-DGKζ) to target DGKζ to membranes. Using liquid chromatography (LC)-tandem mass spectrometry (MS/MS) (LC-MS/MS), we showed that Myr-AcGFP-DGKζ, but not AcGFP-DGKζ without the myristoylation sequence, substantially augmented the levels of several phosphatidic acid (PtdOH) species. In contrast to Myr-AcGFP-DGKζ, its inactive mutant did not exhibit an increase in PtdOH production, indicating that the increase in PtdOH production was DGK activity-dependent. This method will be useful in chemical compound selection for the development of drugs targeting DGKζ and can be applicable to various soluble (nonmembrane bound) lipid-metabolizing enzymes, including other DGK isozymes.

Regulation of Rho GTPases by RhoGDIs in Human Cancers

Rho GDP dissociation inhibitors (RhoGDIs) play important roles in various cellular processes, including cell migration, adhesion, and proliferation, by regulating the functions of the Rho GTPase family. Dissociation of Rho GTPases from RhoGDIs is necessary for their spatiotemporal activation and is dynamically regulated by several mechanisms, such as phosphorylation, sumoylation, and protein interaction. The expression of RhoGDIs has changed in many human cancers and become associated with the malignant phenotype, including migration, invasion, metastasis, and resistance to anticancer agents. Here, we review how RhoGDIs control the function of Rho GTPases by regulating their spatiotemporal activity and describe the regulatory mechanisms of the dissociation of Rho GTPases from RhoGDIs. We also discuss the role of RhoGDIs in cancer progression and their potential uses for therapeutic intervention.

The Volume-Regulated Anion Channel in Glioblastoma

Malignancy of glioblastoma multiforme (GBM), the most common and aggressive form of human brain tumor, strongly depends on its enhanced cell invasion and death evasion which make surgery and accompanying therapies highly ineffective. Several ion channels that regulate membrane potential, cytosolic Ca²⁺ concentration and cell volume in GBM cells play significant roles in sustaining these processes. Among them, the volume-regulated anion channel (VRAC), which mediates the swelling-activated chloride current (IClswell) and is highly expressed in GBM cells, arguably plays a major role. VRAC is primarily involved in reestablishing the original cell volume that may be lost under several physiopathological conditions, but also in sustaining the shape and cell volume changes needed for cell migration and proliferation. While experimentally VRAC is activated by exposing cells to hypotonic solutions that cause the increase of cell volume, in vivo it is thought to be controlled by several different stimuli and modulators. In this review we focus on our recent work showing that two conditions normally occurring in pathological GBM tissues, namely high serum levels and severe hypoxia, were both able to activate VRAC, and their activation was found to promote cell migration and resistance to cell death, both features enhancing GBM malignancy. Also, the fact that the signal transduction pathway leading to VRAC activation appears to involve GBM specific intracellular components, such as diacylglicerol kinase and phosphatidic acid, reportedly not involved in the activation of VRAC in healthy tissues, is a relevant finding. Based on these observations and the impact of VRAC in the physiopathology of GBM, targeting this channel or its intracellular regulators may represent an effective strategy to contrast this lethal tumor.

Of local translation control and lipid signaling in neurons

Fine-tuned regulation of new proteins synthesis is key to the fast adaptation of cells to their changing environment and their response to external cues. Protein synthesis regulation is particularly refined and important in the case of highly polarized cells like neurons where translation occurs in the subcellular dendritic compartment to produce long-lasting changes that enable the formation, strengthening and weakening of inter-neuronal connection, constituting synaptic plasticity. The changes in local synaptic proteome of neurons underlie several aspects of synaptic plasticity and new protein synthesis is necessary for long-term memory formation. Details of how neuronal translation is locally controlled only start to be unraveled. A generally accepted view is that mRNAs are transported in a repressed state and are translated locally upon externally cued triggering signaling cascades that derepress or activate translation machinery at specific sites. Some important yet poorly considered intermediates in these cascades of events are signaling lipids such as diacylglycerol and its balancing partner phosphatidic acid. A link between these signaling lipids and the most common inherited cause of intellectual disability, Fragile X syndrome, is emphasizing the important role of these secondary messages in synaptically controlled translation.

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

The reactive oxygen species, generally labeled toxic due to high reactivity without target specificity, are gradually uncovered as signaling molecules involved in a myriad of biological processes. But one important feature of ROS roles in macromolecule movement has not caught attention until recent studies with technique advance and design elegance have shed lights on ROS signaling for intercellular and interorganelle communication. This review begins with the discussions of genetic and chemical studies on the regulation of symplastic dye movement through intercellular tunnels in plants (plasmodesmata), and focuses on the ROS regulatory mechanisms concerning macromolecule movement including small RNA-mediated gene silencing movement and protein shuttling between cells. Given the premise that intercellular tunnels (bridges) in mammalian cells are the key physical structures to sustain intercellular communication, movement of macromolecules and signals is efficiently facilitated by ROS-induced membrane protrusions formation, which is analogously applied to the interorganelle communication in plant cells. Although ROS regulatory differences between plant and mammalian cells exist, the basis for ROS-triggered conduit formation underlies a unifying conservative theme in multicellular organisms. These mechanisms may represent the evolutionary advances that have enabled multicellularity to gain the ability to generate and utilize ROS to govern material exchanges between individual cells in oxygenated environment.

HGF/Met Signaling in Cancer Invasion: The Impact on Cytoskeleton Remodeling

The invasion of cancer cells into surrounding tissue and the vasculature is essential for tumor metastasis. Increasing evidence indicates that hepatocyte growth factor (HGF) induces cancer cell migration and invasion. A broad spectrum of mechanisms underlies cancer cell migration and invasion. Cytoskeletal reorganization is of central importance in the development of the phenotype of cancer cells with invasive behavior. Through their roles in cell mechanics, intracellular trafficking, and signaling, cytoskeleton proteins participate in all essential events leading to cell migration. HGF has been involved in cytoskeleton assembly and reorganization, and its role in regulating cytoskeleton dynamics is still expanding. This review summarizes our current understanding of the role of HGF in regulating cytoskeleton remodeling, distribution, and interactions.

[The importance of marker proteins of the nervous tissue for morphological diagnostics of the craniocerebral injury]

The present review of the literature involves 50 publications concerning various substrates of importance as the biological markers of axonal damages with special reference to the secondary molecular and cellular mechanisms on which to base in vitro and in vivo modeling of the craniocerebral injury. The results of the investigations with the application of mass-spectrometry for the identification of the proteins specifically synthesized in response to the injury are presented; their biological functions are described. The use of the sequential microscopic imaging technique and the immunohistochemical methods made it possible to determine that the majority of the marker proteins are involved in the specific intracellular processes that are triggered in response to the traumatic impact including apoptosis, proliferation, formation of lamellipodia, axon regeneration, actin remodeling, cell migration and inflammation. In addition, a rise in the amount of intracellular actin-associated proteins has been observed. It is concluded that the investigations into the properties and the physiological role of beta-amyloid precursor protein (beta-APP) are of special value for the characteristic of nervous tissue damages and morphological diagnostics of the craniocerebral injury.

Diacylglycerol Kinases: Shaping Diacylglycerol and Phosphatidic Acid Gradients to Control Cell Polarity

Diacylglycerol kinases (DGKs) terminate diacylglycerol (DAG) signaling and promote phosphatidic acid (PA) production. Isoform specific regulation of DGKs activity and localization allows DGKs to shape the DAG and PA gradients. The capacity of DGKs to constrain the areas of DAG signaling is exemplified by their role in defining the contact interface between T cells and antigen presenting cells: the immune synapse. Upon T cell receptor engagement, both DGK α and ζ metabolize DAG at the immune synapse thus constraining DAG signaling. Interestingly, their activity and localization are not fully redundant because DGKζ activity metabolizes the bulk of DAG in the cell, whereas DGKα limits the DAG signaling area localizing specifically at the periphery of the immune synapse. When DGKs terminate DAG signaling, the local PA production defines a new signaling domain, where PA recruits and activates a second wave of effector proteins. The best-characterized example is the role of DGKs in protrusion elongation and cell migration. Indeed, upon growth factor stimulation, several DGK isoforms, such as α, ζ, and γ, are recruited and activated at the plasma membrane. Here, local PA production controls cell migration by finely modulating cytoskeletal remodeling and integrin recycling. Interestingly, DGK-produced PA also controls the localization and activity of key players in cell polarity such as aPKC, Par3, and integrin β1. Thus, T cell polarization and directional migration may be just two instances of the general contribution of DGKs to the definition of cell polarity by local specification of membrane identity signaling.

Characterization of Novel Molecular Mechanisms Favoring Rac1 Membrane Translocation

The Rac1 GTPase plays key roles in cytoskeletal organization, cell motility and a variety of physiological and disease-linked responses. Wild type Rac1 signaling entails dissociation of the GTPase from cytosolic Rac1-Rho GDP dissociation inhibitor (GDI) complexes, translocation to membranes, activation by exchange factors, effector binding, and activation of downstream signaling cascades. Out of those steps, membrane translocation is the less understood. Using transfections of a expression cDNA library in cells expressing a Rac1 bioreporter, we previously identified a cytoskeletal feedback loop nucleated by the F-actin binding protein coronin 1A (Coro1A) that promotes Rac1 translocation to the plasma membrane by facilitating the Pak-dependent dissociation of Rac1-Rho GDI complexes. This screening identified other potential regulators of this process, including WDR26, basigin, and TMEM8A. Here, we show that WDR26 promotes Rac1 translocation following a Coro1A-like and Coro1A-dependent mechanism. By contrast, basigin and TMEM8A stabilize Rac1 at the plasma membrane by inhibiting the internalization of caveolin-rich membrane subdomains. This latter pathway is F-actin-dependent but Coro1A-, Pak- and Rho GDI-independent.

Epigenetic regulation of diacylglycerol kinase alpha promotes radiation-induced fibrosis

Radiotherapy is a fundamental part of cancer treatment but its use is limited by the onset of late adverse effects in the normal tissue, especially radiation-induced fibrosis. Since the molecular causes for fibrosis are largely unknown, we analyse if epigenetic regulation might explain inter-individual differences in fibrosis risk. DNA methylation profiling of dermal fibroblasts obtained from breast cancer patients prior to irradiation identifies differences associated with fibrosis. One region is characterized as a differentially methylated enhancer of diacylglycerol kinase alpha (DGKA). Decreased DNA methylation at this enhancer enables recruitment of the profibrotic transcription factor early growth response 1 (EGR1) and facilitates radiation-induced DGKA transcription in cells from patients later developing fibrosis. Conversely, inhibition of DGKA has pronounced effects on diacylglycerol-mediated lipid homeostasis and reduces profibrotic fibroblast activation. Collectively, DGKA is an epigenetically deregulated kinase involved in radiation response and may serve as a marker and therapeutic target for personalized radiotherapy.

Radiotherapy can induce fibrosis in cancer patients, limiting its use in clinical settings. Here, the authors identify a differentially methylated enhancer of the lipid kinase DGKA in fibroblasts from breast cancer patients developing fibrosis after radiotherapy and they show that DGKA inhibition affects lipid homeostasis and reduces pro-fibrotic fibroblast activation.

Regulation of Macropinocytosis by Diacylglycerol Kinase ζ

Macropinosomes arise from the closure of plasma membrane ruffles to bring about the non-selective uptake of nutrients and solutes into cells. The morphological changes underlying ruffle formation and macropinosome biogenesis are driven by actin cytoskeleton rearrangements under the control of the Rho GTPase Rac1. We showed previously that Rac1 is activated by diacylglycerol kinase ζ (DGKζ), which phosphorylates diacylglycerol to yield phosphatidic acid. Here, we show DGKζ is required for optimal macropinocytosis induced by growth factor stimulation of mouse embryonic fibroblasts. Time-lapse imaging of live cells and quantitative analysis revealed DGKζ was associated with membrane ruffles and nascent macropinosomes. Macropinocytosis was attenuated in DGKζ-null cells, as determined by live imaging and vaccinia virus uptake experiments. Moreover, macropinosomes that did form in DGKζ-null cells were smaller than those found in wild type cells. Rescue of this defect required DGKζ catalytic activity, consistent with it also being required for Rac1 activation. A constitutively membrane bound DGKζ mutant substantially increased the size of macropinosomes and potentiated the effect of a constitutively active Rac1 mutant on macropinocytosis. Collectively, our results suggest DGKζ functions in concert with Rac1 to regulate macropinocytosis.

Endosomal Na+/H+ exchanger NHE5 influences MET recycling and cell migration

The neuron-enriched Na⁺/H⁺ exchanger NHE5 is expressed in C6 glioma cells, acidifies recycling endosomes, and modulates cell surface abundance of receptor tyrosine kinases MET and EGFR. NHE5 depletion impairs MET recycling and facilitates degradation, thereby impairing cell migration and polarity.

Increased recycling and elevated cell surface expression of receptors serve as a mechanism for persistent receptor-mediated signaling. We show that the neuron-enriched Na⁺/H⁺ exchanger NHE5 is abundantly expressed in C6 glioma cells and plays an important part in regulating cell surface expression of the receptor tyrosine kinases MET and EGF receptor. NHE5 is associated with transferrin receptor (TfR)- and Rab11-positive recycling endosomal membranes, and NHE5 knockdown by short hairpin RNA significantly elevates pH of TfR-positive recycling endosomes. We present evidence that NHE5 facilitates MET recycling to the plasma membrane, protects MET from degradation, and modulates HGF-induced phosphatidylinositol-3-kinase and mitogen-activated protein kinase signaling. Moreover, NHE5 depletion abrogates Rac1 and Cdc42 signaling and actin cytoskeletal remodeling. We further show that NHE5 knockdown impairs directed cell migration and causes loss of cell polarity. Our study highlights a possible role of recycling endosomal pH in regulating receptor-mediated signaling through vesicular trafficking.

Diacylglycerol kinases in membrane trafficking

Diacylglycerol kinases (DGKs) belong to a family of cytosolic kinases that regulate the phosphorylation of diacylglycerol (DAG), converting it into phosphatidic acid (PA). There are 10 known mammalian DGK isoforms, each with a different tissue distribution and substrate specificity. These differences allow regulation of cellular responses by fine-tuning the delicate balance of cellular DAG and PA. DGK isoforms are best characterized as mediators of signal transduction and immune function. However, since recent studies reveal that DAG and PA are also involved in the regulation of endocytic trafficking, it is therefore anticipated that DGKs also plays an important role in membrane trafficking. In this review, we summarize the literature discussing the role of DGK isoforms at different stages of endocytic trafficking, including endocytosis, exocytosis, endocytic recycling, and transport from/to the Golgi apparatus. Overall, these studies contribute to our understanding of the involvement of PA and DAG in endocytic trafficking, an area of research that is drawing increasing attention in recent years.

Physiological Signaling and Structure of the HGF Receptor MET

The "hepatocyte growth factor" also known as "scatter factor", is a multifunctional cytokine with the peculiar ability of simultaneously triggering epithelial cell proliferation, movement and survival. The combination of those proprieties results in the induction of an epithelial to mesenchymal transition in target cells, fundamental for embryogenesis but also exploited by tumor cells during metastatization. The hepatocyte growth factor receptor, MET, is a proto-oncogene and a prototypical transmembrane tyrosine kinase receptor. Inhere we discuss the MET molecular structure and the hepatocyte growth factor driven physiological signaling which coordinates epithelial proliferation, motility and morphogenesis.

A mitochondrial switch promotes tumor metastasis

Metastatic progression of cancer is associated with poor outcome, and here we examine metabolic changes underlying this process. Although aerobic glycolysis is known to promote metastasis, we have now identified a different switch primarily affecting mitochondria. The switch involves overload of the electron transport chain (ETC) with preserved mitochondrial functions but increased mitochondrial superoxide production. It provides a metastatic advantage phenocopied by partial ETC inhibition, another situation associated with enhanced superoxide production. Both cases involved protein tyrosine kinases Src and Pyk2 as downstream effectors. Thus, two different events, ETC overload and partial ETC inhibition, promote superoxide-dependent tumor cell migration, invasion, clonogenicity, and metastasis. Consequently, specific scavenging of mitochondrial superoxide with mitoTEMPO blocked tumor cell migration and prevented spontaneous tumor metastasis in murine and human tumor models.

The diacylglycerol kinase α/atypical PKC/β1 integrin pathway in SDF-1α mammary carcinoma invasiveness

Diacylglycerol kinase α (DGKα), by phosphorylating diacylglycerol into phosphatidic acid, provides a key signal driving cell migration and matrix invasion. We previously demonstrated that in epithelial cells activation of DGKα activity promotes cytoskeletal remodeling and matrix invasion by recruiting atypical PKC at ruffling sites and by promoting RCP-mediated recycling of α5β1 integrin to the tip of pseudopods. In here we investigate the signaling pathway by which DGKα mediates SDF-1α-induced matrix invasion of MDA-MB-231 invasive breast carcinoma cells. Indeed we showed that, following SDF-1α stimulation, DGKα is activated and localized at cell protrusion, thus promoting their elongation and mediating SDF-1α induced MMP-9 metalloproteinase secretion and matrix invasion. Phosphatidic acid generated by DGKα promotes localization at cell protrusions of atypical PKCs which play an essential role downstream of DGKα by promoting Rac-mediated protrusion elongation and localized recruitment of β1 integrin and MMP-9. We finally demonstrate that activation of DGKα, atypical PKCs signaling and β1 integrin are all essential for MDA-MB-231 invasiveness. These data indicates the existence of a SDF-1α induced DGKα - atypical PKC - β1 integrin signaling pathway, which is essential for matrix invasion of carcinoma cells.

Receptor tyrosine kinase c-Met controls the cytoskeleton from different endosomes via different pathways

Receptor tyrosine kinases (RTKs) are increasingly recognized as having the capacity to signal post-internalization. Signalling outputs and/or duration, and subsequent cellular outcome, are thought to be distinct when emanating from endosomes compared with those from the plasma membrane. Here we show, in invasive, basal-like human breast cell models, that different mechanisms are engaged by the RTK c-Met in two different endosomes to control the actin cytoskeleton via the key migratory signal output Rac1. Despite an acute activation of Rac1 from peripheral endosomes (PEs), c-Met needs to traffic to a perinuclear endosome (PNE) to sustain Rac1 signalling, trigger optimal membrane ruffling, cell migration and invasion. Unexpectedly, in the PNE but not in the PE, PI3K and the Rac-GEF Vav2 are required. Thus we describe a novel endosomal signalling mechanism whereby one signal output, Rac1, is stimulated through distinct pathways by the same RTK depending on which endosome it is localized to in the cell.

Increased diacylglycerol kinase ζ expression in human metastatic colon cancer cells augments Rho GTPase activity and contributes to enhanced invasion

BACKGROUND

Unraveling the signaling pathways responsible for the establishment of a metastatic phenotype in carcinoma cells is critically important for understanding the pathology of cancer. The acquisition of cell motility is a key property of metastatic tumor cells and is a prerequisite for invasion. Rho GTPases regulate actin cytoskeleton reorganization and the cellular responses required for cell motility and invasion. Diacylglycerol kinase ζ (DGKζ), an enzyme that phosphorylates diacylglycerol to yield phosphatidic acid, regulates the activity of the Rho GTPases Rac1 and RhoA. DGKζ mRNA is highly expressed in several different colon cancer cell lines, as well as in colon cancer tissue relative to normal colonic epithelium, and thus may contribute to the metastatic process.

METHODS

To investigate potential roles of DGKζ in cancer metastasis, a cellular, isogenic model of human colorectal cancer metastatic transition was used. DGKζ protein levels, Rac1 and RhoA activity, and PAK phosphorylation were measured in the non-metastatic SW480 adenocarcinoma cell line and its highly metastatic variant, the SW620 line. The effect of DGKζ silencing on Rho GTPase activity and invasion through Matrigel-coated Transwell inserts was studied in SW620 cells. Invasiveness was also measured in PC-3 prostate cancer and MDA-MB-231 breast cancer cells depleted of DGKζ.

RESULTS

DGKζ protein levels were elevated approximately 3-fold in SW620 cells compared to SW480 cells. There was a concomitant increase in active Rac1 in SW620 cells, as well as substantial increases in the expression and phosphorylation of the Rac1 effector PAK1. Similarly, RhoA activity and expression were increased in SW620 cells. Knockdown of DGKζ expression in SW620 cells by shRNA-mediated silencing significantly reduced Rac1 and RhoA activity and attenuated the invasiveness of SW620 cells in vitro. DGKζ silencing in highly metastatic MDA-MB-231 breast cancer cells and PC-3 prostate cancer cells also significantly attenuated their invasiveness.

CONCLUSION

Elevated DGKζ expression contributes to increased Rho GTPase activation and the enhanced motility of metastatic cancer cells. These findings warrant further investigation of the clinical relevance of DGKζ upregulation in colon and other cancers. Interfering with DGKζ function could provide a means of inhibiting invasion and metastasis.

Identification of key signaling molecules involved in the activation of the swelling-activated chloride current in human glioblastoma cells

The swelling-activated chloride current (I Cl,Vol) is abundantly expressed in glioblastoma (GBM) cells, where it controls cell volume and invasive migration. The transduction pathway mediating I Cl,Vol activation in GBM cells is, however, poorly understood. By means of pharmacological and electrophysiological approaches, on GL-15 human GBM cells we found that I Cl,Vol activation by hypotonic swelling required the activity of a U73122-sensitive phospholipase C (PLC). I Cl,Vol activation could also be induced by the membrane-permeable diacylglycerol (DAG) analog OAG. In contrast, neither calcium (Ca(2+)) chelation by BAPTA-AM nor changes in PKC activity were able to affect I Cl,Vol activation by hypotonic swelling. We further found that R59022, an inhibitor of diacylglycerol kinase (DGK), reverted I Cl,Vol activation, suggesting the involvement of phosphatidic acid. In addition, I Cl,Vol activation required the activity of a EHT1864-sensitive Rac1 small GTPase and the resulting actin polymerization, as I Cl,Vol activation was prevented by cytochalasin B. We finally show that I Cl,Vol can be activated by the promigratory fetal calf serum in a PLC- and DGK-dependent manner. This observation is potentially relevant because blood serum can likely come in contact with glioblastoma cells in vivo as a result of the tumor-related partial breakdown of the blood-brain barrier. Given the relevance of I Cl,Vol in GBM cell volume regulation and invasiveness, the several key signaling molecules found in this study to be involved in the activation of the I Cl,Vol may represent potential therapeutic targets against this lethal cancer.

Depression of type I diacylglycerol kinases in pancreatic β-cells from male mice results in impaired insulin secretion

Diacylglycerol kinase (DGK) catalyzes the conversion of diacylglycerol (DAG) to phosphatidic acid. This study investigated the expression and function of DGK in pancreatic β-cells. mRNA expression of type I DGK isoforms (α, β, γ) was detected in mouse pancreatic islets and the β-cell line MIN6. Protein expression of DGKα and DGKγ was also detected in mouse β-cells and MIN6 cells. The type I DGK inhibitor R59949 inhibited high K(+)- and glucose-induced insulin secretion in MIN6 cells. Moreover, single knockdown of DGKα or DGKγ by small interfering RNA slightly but significantly decreased glucose- and high K(+)-induced insulin secretions, and the double knockdown further decreased them to the levels comparable with those induced by R59949. R59949 and DiC8, a membrane permeable DAG analog, decreased intracellular Ca(2+) concentration elevated by glucose and high K(+) in MIN6 cells. Real-time imaging in MIN6 cells expressing green fluorescent protein-tagged DGKα or DGKγ showed that the DGK activator phorbol 12-myristate 13-acetate rapidly induced translocation of DGKγ to the plasma membrane, whereas high K(+) slowly translocated DGKα and DGKγ to the plasma membrane. R59949 increased the DAG content in MIN6 cells when stimulated with high KCl, whereas it did not increase the DAG content but decreased the phosphatidic acid content when stimulated with high glucose. Finally, R59949 was confirmed to inhibit high K(+)-induced insulin secretion from mouse islets and glucose-induced insulin secretion from rat islets. These results suggest that DGKα and DGKγ are present in β-cells and that the depression of these DGKs causes a decrease in intracellular Ca(2+) concentration, thereby reducing insulin secretion.

Negative charges in the flexible N-terminal domain of Rho GDP-dissociation inhibitors (RhoGDIs) regulate the targeting of the RhoGDI-Rac1 complex to membranes

In its resting state, Rho GDP-dissociation inhibitor (RhoGDI) α forms a soluble cytoplasmic heterodimer with the GDP-bound form of Rac. Upon stimulation, the dissociation of RhoGDIα from the RhoGDIα-Rac complex is a mandatory step for Rac activation; however, this mechanism is poorly understood. In this study, we examined how the cytoplasm/membrane cycles of the RhoGDI-Rac complex are regulated, as well as where RhoGDI dissociates from the RhoGDI-Rac complex, during FcγR-mediated phagocytosis. The negatively charged and flexible N terminus (25 residues) of RhoGDIα, particularly its second negative amino acid cluster possessing five negatively charged amino acids, was a pivotal regulator in the cytoplasm/membrane cycles of the RhoGDI-Rac complex. We also found that RhoGDIα translocated to the phagosomes as a RhoGDIα-Rac1 complex, and this translocation was mediated by an interaction between the polybasic motif in the C terminus of Rac1 and anionic phospholipids produced on phagosomes, such as phosphatidic acid, that is, by a phagosome-targeting mechanism of Rac1. Thus, we demonstrated that the targeting/accumulation of the RhoGDIα-Rac1 complex to phagosomes is regulated by a balance between three factors: 1) the negatively charged and flexible N-terminal of RhoGDIα, 2) the binding affinity of RhoGDIα for Rac1, and 3) anionic phospholipids produced on phagosomes. Moreover, we demonstrated that the mechanism of targeting/accumulation of the RhoGDIα-Rac1 complex is also applicable for the RhoGDIβ-Rac1 complex.

Cholecystokinin-mediated RhoGDI phosphorylation via PKCα promotes both RhoA and Rac1 signaling

RhoA and Rac1 have been implicated in the mechanism of CCK-induced amylase secretion from pancreatic acini. In all cell types studied to date, inactive Rho GTPases are present in the cytosol bound to the guanine nucleotide dissociation inhibitor RhoGDI. Here, we identified the switch mechanism regulating RhoGDI1-Rho GTPase dissociation and RhoA translocation upon CCK stimulation in pancreatic acini. We found that both Gα13 and PKC, independently, regulate CCK-induced RhoA translocation and that the PKC isoform involved is PKCα. Both RhoGDI1 and RhoGDI3, but not RhoGDI2, are expressed in pancreatic acini. Cytosolic RhoA and Rac1 are associated with RhoGDI1, and CCK-stimulated PKCα activation releases the complex. Overexpression of RhoGDI1, by binding RhoA, inhibits its activation, and thereby, CCK-induced apical amylase secretion. RhoA translocation is also inhibited by RhoGDI1. Inactive Rac1 influences CCK-induced RhoA activation by preventing RhoGDI1 from binding RhoA. By mutational analysis we found that CCK-induced PKCα phosphorylation on RhoGDI1 at Ser96 releases RhoA and Rac1 from RhoGDI1 to facilitate Rho GTPases signaling.

PKC mediates fluctuant ERK-paxillin signaling for hepatocyte growth factor-induced migration of hepatoma cell HepG2

Hepatocyte growth factor (HGF) is critical for triggering metastasis of hepatocellular carcinoma cell (HCC). Extracellular signal-regulated kinase (ERK) mediates HGF-induced cell migration via focal adhesion signaling. Protein kinase C (PKC) is a negative regulator of ERK activation, however, both PKC and ERK were required for HGF-induced cell migration. To address this intriguing issue, the signal mechanisms for HGF-induced HepG2 cell migration were investigated in a long-term fashion. HGF-induced phosphorylations of ERK, Src (at Tyr 416) and paxillin (at Ser178 and Tyr31) were up and down for 3 times within 24h. HGF also induced fluctuant PKC activation and Rac degradation. Consistently, HGF induced intermittent actin polarization within 24h, which can be blocked by the inhibitors of PKC (Bisindolymaleimide) and ERK. Inhibitor studies revealed that ERK was required for HGF-induced paxillin phosphorylation at Ser178, whereas PKC and Rac-1 may suppress HGF-induced phosphorylation of ERK and paxillin (at Ser178) and upregulate phosphorylation of paxillin at Tyr31. Based on shRNA technique, PKCα and δ were responsible for suppressing HGF-induced phosphorylation of ERK and paxillin (at Ser178), whereas PKC ε and ζ were required for phosphorylation of paxillin at Tyr31. The HGF-induced fluctuant signaling is reminiscent of c-Met endocytosis. Using Concanavalin A, an inhibitor of endocytosis, we found that c-Met endocytosis was required for PKC to suppress ERK phosphorylation. Moreover, HGF-induced c-Met degradation was also fluctuant, which can be prevented by Bisindolymaleimide. In conclusion, PKC is critical for mediating HGF-induced fluctuant ERK-paxillin signaling during cell migration, probably via triggering endosomal degradation of c-Met.

Regulation of small GTPases by GEFs, GAPs, and GDIs

Small GTPases use GDP/GTP alternation to actuate a variety of functional switches that are pivotal for cell dynamics. The GTPase switch is turned on by GEFs, which stimulate dissociation of the tightly bound GDP, and turned off by GAPs, which accelerate the intrinsically sluggish hydrolysis of GTP. For Ras, Rho, and Rab GTPases, this switch incorporates a membrane/cytosol alternation regulated by GDIs and GDI-like proteins. The structures and core mechanisms of representative members of small GTPase regulators from most families have now been elucidated, illuminating their general traits combined with scores of unique features. Recent studies reveal that small GTPase regulators have themselves unexpectedly sophisticated regulatory mechanisms, by which they process cellular signals and build up specific cell responses. These mechanisms include multilayered autoinhibition with stepwise release, feedback loops mediated by the activated GTPase, feed-forward signaling flow between regulators and effectors, and a phosphorylation code for RhoGDIs. The flipside of these highly integrated functions is that they make small GTPase regulators susceptible to biochemical abnormalities that are directly correlated with diseases, notably a striking number of missense mutations in congenital diseases, and susceptible to bacterial mimics of GEFs, GAPs, and GDIs that take command of small GTPases in infections. This review presents an overview of the current knowledge of these many facets of small GTPase regulation.

Identification of injury specific proteins in a cell culture model of traumatic brain injury

The complicated secondary molecular and cellular mechanisms following traumatic brain injury (TBI) are still not fully understood. In the present study, we have used mass spectrometry to identify injury specific proteins in an in vitro model of TBI. A standardized injury was induced by scalpel cuts through a mixed cell culture of astrocytes, oligodendrocytes and neurons. Twenty-four hours after the injury, cell culture medium and whole-cell fractions were collected for analysis. We found 53 medium proteins and 46 cell fraction proteins that were specifically expressed after injury and the known function of these proteins was elucidated by an extensive literature survey. By using time-lapse microscopy and immunostainings we could link a large proportion of the proteins to specific cellular processes that occur in response to trauma; including cell death, proliferation, lamellipodia formation, axonal regeneration, actin remodeling, migration and inflammation. A high percentage of the proteins uniquely expressed in the medium after injury were actin-related proteins, which normally are situated intracellularly. We show that two of these, ezrin and moesin, are expressed by astrocytes both in the cell culture model and in mouse brain subjected to experimental TBI. Interestingly, we found many inflammation-related proteins, despite the fact that cells were present in the culture. This study contributes with important knowledge about the cellular responses after trauma and identifies several potential cell-specific biomarkers.

ErbB2-dependent chemotaxis requires microtubule capture and stabilization coordinated by distinct signaling pathways

Activation of the ErbB2 receptor tyrosine kinase stimulates breast cancer cell migration. Cell migration is a complex process that requires the synchronized reorganization of numerous subcellular structures including cell-to-matrix adhesions, the actin cytoskeleton and microtubules. How the multiple signaling pathways triggered by ErbB2 coordinate, in time and space, the various processes involved in cell motility, is poorly defined. We investigated the mechanism whereby ErbB2 controls microtubules and chemotaxis. We report that activation of ErbB2 increased both cell velocity and directed migration. Impairment of the Cdc42 and RhoA GTPases, but not of Rac1, prevented the chemotactic response. RhoA is a key component of the Memo/ACF7 pathway whereby ErbB2 controls microtubule capture at the leading edge. Upon Memo or ACF7 depletion, microtubules failed to reach the leading edge and cells lost their ability to follow the chemotactic gradient. Constitutive ACF7 targeting to the membrane in Memo-depleted cells reestablished directed migration. ErbB2-mediated activation of phospholipase C gamma (PLCγ) also contributed to cell guidance. We further showed that PLCγ signaling, via classical protein kinases C, and Memo signaling converged towards a single pathway controlling the microtubule capture complex. Finally, inhibiting the PI3K/Akt pathway did not affect microtubule capture, but disturbed microtubule stability, which also resulted in defective chemotaxis. PI3K/Akt-dependent stabilization of microtubules involved repression of GSK3 activity on the one hand and inhibition of the microtubule destabilizing protein, Stathmin, on the other hand. Thus, ErbB2 triggers distinct and complementary pathways that tightly coordinate microtubule capture and microtubule stability to control chemotaxis.

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

Diacylglycerol kinase δ modulates Akt phosphorylation through pleckstrin homology domain leucine-rich repeat protein phosphatase 2 (PHLPP2)

Discovering proteins that modulate Akt signaling has become a critical task, given the oncogenic role of Akt in a wide variety of cancers. We have discovered a novel diacylglycerol signaling pathway that promotes dephosphorylation of Akt. This pathway is regulated by diacylglycerol kinase δ (DGKδ). In DGKδ-deficient cells, we found reduced Akt phosphorylation downstream of three receptor tyrosine kinases. Phosphorylation upstream of Akt was not affected. Our data indicate that PKCα, which is excessively active in DGKδ-deficient cells, promotes dephosphorylation of Akt through pleckstrin homology domain leucine-rich repeats protein phosphatase (PHLPP) 2. Depletion of either PKCα or PHLPP2 rescued Akt phosphorylation in DGKδ-deficient cells. In contrast, depletion of PHLPP1, another Akt phosphatase, failed to rescue Akt phosphorylation. Other PHLPP substrates were not affected by DGKδ deficiency, suggesting mechanisms allowing specific modulation of Akt dephosphorylation. We found that β-arrestin 1 acted as a scaffold for PHLPP2 and Akt1, providing a mechanism for specificity. Because of its ability to reduce Akt phosphorylation, we tested whether depletion of DGKδ could attenuate tumorigenic properties of cultured cells and found that DGKδ deficiency reduced cell proliferation and migration and enhanced apoptosis. We have, thus, discovered a novel pathway in which diacylglycerol signaling negatively regulates Akt activity. Our collective data indicate that DGKδ is a pertinent cancer target, and our studies could lay the groundwork for development of novel cancer therapeutics.

Cell motility: The necessity of Rac1 GDP/GTP flux

The Ras proto-oncogenic proteins, prototypes of the small GTPases, work as molecular switches: they are active when bound to GTP and inactive when bound to GDP. A variety of evidence suggested that the Ras paradigm is not fully valid for the Rho-family of small GTPases. Indeed, permanent activation is not sufficient but it is rather the continuous oscillation between the GDP-bound and GTP-bound conformations (namely the GDP/GTP cycling or GTPase flux), that is required for Rho-GTPases to perform their biological functions and properly coordinate actin cytoskeleton reorganization. In our recent study, we show that Rac1 needs to cycle between the GDP and GTP states in order to efficiently control cell motility. Similarly, it was previously reported that GDP/GTP cycling is required by RhoA for cytokinesis and by Cdc42 for cell polarization. The future challenge is to understand why the GTPase flux is so important for the biological actions of Rho GTPases.

A potential role of progestin-induced laminin-5/α6-integrin signaling in the formation of side branches in the mammary gland

Mammary organoids from adult mice produce tubules, analogous to mammary ducts in vivo, in response to hepatocyte growth factor (HGF) when cultured in collagen gels. The combination of HGF plus progestin (R5020) causes reduced tubule number and length. We hypothesized that the inhibitory effect on tubulogenesis was due to progestin-mediated alteration of HGF/c-Met signaling. Using molecular inhibitors and short hairpin RNA, it was determined that HGF activation of Ras-related C3 botulinum toxin substrate (Rac1) was required for the formation of cytoplasmic extensions, the first step of tubulogenesis, and that Rac1 activity was Src kinase (Src) and focal adhesion kinase (FAK) dependent. The highly novel finding was that R5020 reduced tubulogenesis by up-regulating and increasing extracellular laminin and α6-integrin ligation to reduce activation of the Src, focal adhesion kinase, and Rac1 pathway. Receptor activator of nuclear factor-κB ligand, another progesterone-induced paracrine factor, did not replicate this effect of R5020. The inhibitory effect of R5020 on tubulogenesis was likely mediated through progesterone receptor (PR) isoform A (PRA), because PRA is the predominant PR isoform expressed in the organoids, and the progestin-induced effect was prevented by the PR antagonist RU486. These results provide a plausible mechanism that explains progestin/PRA-mediated blunting of HGF-induced tubulogenesis in vitro and is proposed to be relevant to progesterone/PRA-induced side-branching in vivo during pregnancy.

Interactions between the PDZ domains of Bazooka (Par-3) and phosphatidic acid: in vitro characterization and role in epithelial development

The polarity regulator Bazooka (Par-3) is shown to directly bind the signaling lipid phosphatidic acid via its PDZ domains in vitro. In vivo, the interaction appears to support Bazooka functions important for Drosophila embryonic epithelial structure. Thus Bazooka has a closer connection to membrane lipids than previously recognized.

Bazooka (Par-3) is a conserved polarity regulator that organizes molecular networks in a wide range of cell types. In epithelia, it functions as a plasma membrane landmark to organize the apical domain. Bazooka is a scaffold protein that interacts with proteins through its three PDZ (postsynaptic density 95, discs large, zonula occludens-1) domains and other regions. In addition, Bazooka has been shown to interact with phosphoinositides. Here we show that the Bazooka PDZ domains interact with the negatively charged phospholipid phosphatidic acid immobilized on solid substrates or in liposomes. The interaction requires multiple PDZ domains, and conserved patches of positively charged amino acid residues appear to mediate the interaction. Increasing or decreasing levels of diacylglycerol kinase or phospholipase D—enzymes that produce phosphatidic acid—reveal a role for phosphatidic acid in Bazooka embryonic epithelial activity but not its localization. Mutating residues implicated in phosphatidic acid binding revealed a possible role in Bazooka localization and function. These data implicate a closer connection between Bazooka and membrane lipids than previously recognized. Bazooka polarity landmarks may be conglomerates of proteins and plasma membrane lipids that modify each other's activities for an integrated effect on cell polarity.

Diacylglycerol kinase α controls RCP-dependent integrin trafficking to promote invasive migration

Phosphatidic acid generation by DGK-α is essential for the localization of Rab11-coupling protein to invasive pseudopods and subsequent invasive migration by tumor cells.

Inhibition of αvβ3 integrin or expression of oncogenic mutants of p53 promote invasive cell migration by enhancing endosomal recycling of α5β1 integrin under control of the Rab11 effector Rab-coupling protein (RCP). In this paper, we show that diacylglycerol kinase α (DGK-α), which phosphorylates diacylglycerol to phosphatidic acid (PA), was required for RCP to be mobilized to and tethered at the tips of invasive pseudopods and to allow RCP-dependent α5β1 recycling and the resulting invasiveness of tumor cells. Expression of a constitutive-active mutant of DGK-α drove RCP-dependent invasion in the absence of mutant p53 expression or αvβ3 inhibition, and conversely, an RCP mutant lacking the PA-binding C2 domain was not capable of being tethered at pseudopod tips. These data demonstrate that generation of PA downstream of DGK-α is essential to connect expression of mutant p53s or inhibition of αvβ3 to RCP and for this Rab11 effector to drive the trafficking of α5β1 that is required for tumor cell invasion through three-dimensional matrices.

Nuclear envelope phosphatase 1-regulatory subunit 1 (formerly TMEM188) is the metazoan Spo7p ortholog and functions in the lipin activation pathway

Lipin-1 catalyzes the formation of diacylglycerol from phosphatidic acid. Lipin-1 mutations cause lipodystrophy in mice and acute myopathy in humans. It is heavily phosphorylated, and the yeast ortholog Pah1p becomes membrane-associated and active upon dephosphorylation by the Nem1p-Spo7p membrane complex. A mammalian ortholog of Nem1p is the C-terminal domain nuclear envelope phosphatase 1 (CTDNEP1, formerly "dullard"), but its Spo7p-like partner is unknown, and the need for its existence is debated. Here, we identify the metazoan ortholog of Spo7p, TMEM188, renamed nuclear envelope phosphatase 1-regulatory subunit 1 (NEP1-R1). CTDNEP1 and NEP1-R1 together complement a nem1Δspo7Δ strain to block endoplasmic reticulum proliferation and restore triacylglycerol levels and lipid droplet number. The two human orthologs are in a complex in cells, and the amount of CTDNEP1 is increased in the presence of NEP1-R1. In the Caenorhabditis elegans embryo, expression of nematode CTDNEP1 and NEP1-R1, as well as lipin-1, is required for normal nuclear membrane breakdown after zygote formation. The expression pattern of NEP1-R1 and CTDNEP1 in human and mouse tissues closely mirrors that of lipin-1. CTDNEP1 can dephosphorylate lipins-1a, -1b, and -2 in human cells only in the presence of NEP1-R1. The nuclear fraction of lipin-1b is increased when CTDNEP1 and NEP1-R1 are co-expressed. Therefore, NEP1-R1 is functionally conserved from yeast to humans and functions in the lipin activation pathway.

HGF-induced PKCζ activation increases functional CXCR4 expression in human breast cancer cells

The chemokine receptor CXCR4 and its ligand CXCL12 have been shown to mediate the metastasis of many malignant tumors including breast carcinoma. Interaction between hepatocyte growth factor (HGF) and the Met receptor tyrosine kinase mediates development and progression of cancers. HGF is able to induce CXCR4 expression and contributes to tumor cell invasiveness in breast carcinoma. However, the mechanism of the CXCR4 expression modulated by c-Met-HGF axis to enhance the metastatic behavior of breast cancer cells is still unclear. In this study, we found that HGF induced functional CXCR4 receptor expression in breast cancer cells. The effect of HGF was specifically mediated by PKCζ activity. After transfection with PKCζ-siRNA, the phosphorylation of PKCζ and CXCR4 was abrogated in breast cancer cells. Interference with the activation of Rac1, a downstream target of HGF, prevented the HGF-induced increase in PKCζ activity and CXCR4 levels. The HGF-induced, LY294002-sensitive translocation of PKCζ from cytosol to plasma membrane indicated that HGF was capable of activating PKCζ, probably via phosphoinositide (PI) 3-kinases. HGF treatment also increased MT1-MMP secretion. Inhibition of PKCζ, Rac-1 and phosphatidylinositol 3-kinase may attenuate MT1-MMP expression in cells exposed to HGF. Functional manifestation of the effects of HGF revealed an increased ability for migration, chemotaxis and metastasis in MDA-MB-436 cells in vitro and in vivo. Our findings thus provided evidence that the process of HGF-induced functional CXCR4 expression may involve PI 3-kinase and atypical PKCζ. Moreover, HGF may promote the invasiveness and metastasis of breast tumor xenografts in BALB/c-nu mice via the PKCζ-mediated pathway, while suppression of PKCζ by RNA interference may abrogate cancer cell spreading.

SAP-mediated inhibition of diacylglycerol kinase α regulates TCR-induced diacylglycerol signaling

Diacylglycerol kinases (DGKs) metabolize diacylglycerol to phosphatidic acid. In T lymphocytes, DGKα acts as a negative regulator of TCR signaling by decreasing diacylglycerol levels and inducing anergy. In this study, we show that upon costimulation of the TCR with CD28 or signaling lymphocyte activation molecule (SLAM), DGKα, but not DGKζ, exits from the nucleus and undergoes rapid negative regulation of its enzymatic activity. Inhibition of DGKα is dependent on the expression of SAP, an adaptor protein mutated in X-linked lymphoproliferative disease, which is essential for SLAM-mediated signaling and contributes to TCR/CD28-induced signaling and T cell activation. Accordingly, overexpression of SAP is sufficient to inhibit DGKα, whereas SAP mutants unable to bind either phospho-tyrosine residues or SH3 domain are ineffective. Moreover, phospholipase C activity and calcium, but not Src-family tyrosine kinases, are also required for negative regulation of DGKα. Finally, inhibition of DGKα in SAP-deficient cells partially rescues defective TCR/CD28 signaling, including Ras and ERK1/2 activation, protein kinase C membrane recruitment, induction of NF-AT transcriptional activity, and IL-2 production. Thus SAP-mediated inhibition of DGKα sustains diacylglycerol signaling, thereby regulating T cell activation, and it may represent a novel pharmacological strategy for X-linked lymphoproliferative disease treatment.