The guanine nucleotide exchange factor ARNO mediates the activation of ARF and phospholipase D by insulin

Golgi Complex: A Signaling Hub in Cancer

The Golgi Complex is the central hub in the endomembrane system and serves not only as a biosynthetic and processing center but also as a trafficking and sorting station for glycoproteins and lipids. In addition, it is an active signaling hub involved in the regulation of multiple cellular processes, including cell polarity, motility, growth, autophagy, apoptosis, inflammation, DNA repair and stress responses. As such, the dysregulation of the Golgi Complex-centered signaling cascades contributes to the onset of several pathological conditions, including cancer. This review summarizes the current knowledge on the signaling pathways regulated by the Golgi Complex and implicated in promoting cancer hallmarks and tumor progression.

Physiological and Pathological Roles of the Cytohesin Family in Neurons

The cytohesin proteins, consisting of four closely related members (cytohesins-1, -2, -3, and -4), are a subfamily of the Sec7 domain-containing guanine nucleotide exchange factors for ADP ribosylation factors (Arfs), which are critical regulators of membrane trafficking and actin cytoskeleton remodeling. Recent advances in molecular biological techniques and the development of a specific pharmacological inhibitor for cytohesins, SecinH3, have revealed the functional involvement of the cytohesin-Arf pathway in diverse neuronal functions from the formation of axons and dendrites, axonal pathfinding, and synaptic vesicle recycling, to pathophysiological processes including chronic pain and neurotoxicity induced by proteins related to neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer's disease. Here, we review the physiological and pathological roles of the cytohesin-Arf pathway in neurons and discuss the future directions of this research field.

The GEF Cytohesin-2/ARNO Mediates Resistin induced Phenotypic Switching in Vascular Smooth Muscle Cells

The pro-inflammatory adipokine resistin induces a phenotypic switch of vascular smooth muscle cells (VSMC), a process decisive for atherosclerosis, including morphological changes, increased synthetic activity, proliferation and migration. The guanine-exchange factor ARNO (Cytohesin-2) has been shown to be important for morphological changes and migration of other cell types. In this study we dissected the role of ARNO in resistin induced VSMC phenotypic switching and signalling. Firstly, treatment with the cytohesin inhibitor Secin H3 prevented the resistin mediated induction of morphological changes in VSMC. Secondly, Secin H3 treatment as well as expression of an inactive ARNO (EK) reduced resistin induced VSMC synthetic activity, as assessed by matrix metalloproteinase 2 (MMP-2) expression, as well as the migration into a wound in vitro compared to ARNO WT expression. Thirdly, we found ARNO to influence MMP-2 expression and migration via activation of p38 MAPK and the JNK/AP-1 pathway. Interestingly, these processes were shown to be dependent on the binding of PIP₃, as mutation of the ARNO PH-domain inhibited VSMC migration, MMP-2 expression as well as p38 MAPK and JNK signalling. Thus, we demonstrate that ARNO is an important link in resistin dependent cell signalling leading to morphological changes, MMP-2 production and migration of VSMC.

Insulin activates intracellular transport of lipid droplets to release triglycerides from the liver

Kumar et al. describe a detailed pathway for channeling fat from the liver into blood across fed/fasted cycles. Insulin, phosphatidic acid, and kinesin collaborate in hepatocytes to deliver lipid droplets to the smooth ER, where they are catabolized to supply fat for lipoprotein production and secretion.

Triglyceride-rich lipid droplets (LDs) are catabolized with high efficiency in hepatocytes to supply fatty acids for producing lipoprotein particles. Fasting causes a massive influx of adipose-derived fatty acids into the liver. The liver in the fasted state is therefore bloated with LDs but, remarkably, still continues to secrete triglycerides at a constant rate. Here we show that insulin signaling elevates phosphatidic acid (PA) dramatically on LDs in the fed state. PA then signals to recruit kinesin-1 motors, which transport LDs to the peripherally located smooth ER inside hepatocytes, where LDs are catabolized to produce lipoproteins. This pathway is down-regulated homeostatically when fasting causes insulin levels to drop, thus preventing dangerous elevation of triglycerides in the blood. Further, we show that a specific peptide against kinesin-1 blocks triglyceride secretion without any apparent deleterious effects on cells. Our work therefore reveals fundamental mechanisms that maintain lipid homeostasis across metabolic states and leverages this knowledge to propose a molecular target against hyperlipidemia.

ARF GTPases and their GEFs and GAPs: concepts and challenges

Detailed structural, biochemical, cell biological, and genetic studies of any gene/protein are required to develop models of its actions in cells. Studying a protein family in the aggregate yields additional information, as one can include analyses of their coevolution, acquisition or loss of functionalities, structural pliability, and the emergence of shared or variations in molecular mechanisms. An even richer understanding of cell biology can be achieved through evaluating functionally linked protein families. In this review, we summarize current knowledge of three protein families: the ARF GTPases, the guanine nucleotide exchange factors (ARF GEFs) that activate them, and the GTPase-activating proteins (ARF GAPs) that have the ability to both propagate and terminate signaling. However, despite decades of scrutiny, our understanding of how these essential proteins function in cells remains fragmentary. We believe that the inherent complexity of ARF signaling and its regulation by GEFs and GAPs will require the concerted effort of many laboratories working together, ideally within a consortium to optimally pool information and resources. The collaborative study of these three functionally connected families (≥70 mammalian genes) will yield transformative insights into regulation of cell signaling.

Promiscuity of the catalytic Sec7 domain within the guanine nucleotide exchange factor GBF1 in ARF activation, Golgi homeostasis, and effector recruitment

The integrity of the Golgi and trans-Golgi network (TGN) is disrupted by brefeldin A (BFA), which inhibits the Golgi-localized BFA-sensitive factor (GBF1) and brefeldin A-inhibited guanine nucleotide-exchange factors (BIG1 and BIG2). Using a cellular replacement assay to assess GBF1 functionality without interference from the BIGs, we show that GBF1 alone maintains Golgi architecture; facilitates secretion; activates ADP-ribosylation factor (ARF)1, 3, 4, and 5; and recruits ARF effectors to Golgi membranes. Unexpectedly, GBF1 also supports TGN integrity and recruits numerous TGN-localized ARF effectors. The impact of the catalytic Sec7 domain (Sec7d) on GBF1 functionality was assessed by swapping it with the Sec7d from ARF nucleotide-binding site opener (ARNO)/cytohesin-2, a plasma membrane GEF reported to activate all ARFs. The resulting chimera (GBF1-ARNO-GBF1 [GARG]) targets like GBF1, supports Golgi/TGN architecture, and facilitates secretion. However, unlike GBF1, GARG activates all ARFs (including ARF6) at the Golgi/TGN and recruits additional ARF effectors to the Golgi/TGN. Our results have general implications: 1) GEF's targeting is independent of Sec7d, but Sec7d influence the GEF substrate specificity and downstream effector events; 2) all ARFs have access to all membranes, but are restricted in their distribution by the localization of their activating GEFs; and 3) effector association with membranes requires the coincidental presence of activated ARFs and specific membrane identifiers.

Role of catechins on ET-1-induced stimulation of PLD and NADPH oxidase activities in pulmonary smooth muscle cells: determination of the probable mechanism by molecular docking studies

The treatment of human pulmonary artery smooth muscle cells with ET-1 stimulates the activity of PLD and NADPH oxidase, but this stimulation is inhibited by pretreatment with bosentan (ET-1 receptor antagonist), FIPI (PLD inhibitor), apocynin (NADPH oxidase inhibitor), and EGCG and ECG (catechins having a galloyl group), but not EGC and EC (catechins devoid of a galloyl group). Herein, using molecular docking analyses based on our biochemical studies, we determined the probable mechanism by which the catechins containing a galloyl group inhibit the stimulation of PLD activity induced by ET-1. The ET-1-induced stimulation of PLD activity was inhibited by SecinH3 (inhibitor of cytohesin). Arf6 and cytohesin-1 are associated in the cell membrane, which is not inhibited by the catechins during ET-1 treatment of the cells. However, EGCG and ECG inhibited the binding of GTPγS with Arf6, even in the presence of cytohesin-1. The molecular docking analyses revealed that the catechins containing a galloyl group (EGCG and ECG) with cytohesin-1–Arf6GDP, but not the catechins without a galloyl group (EGC and EC), prevent GDP–GTP exchange in Arf6, which seems to be an important mechanism for inhibiting the activation of PLD induced by ET-1, and subsequently increases the activity of NADPH oxidase.

Kinesin-dependent mechanism for controlling triglyceride secretion from the liver

The liver secretes lipids in a controlled manner despite vast changes in its internal lipid content. This buffering function of the liver is essential for lipid/energy homeostasis, but its molecular and cellular mechanism is unknown. We show that motor protein kinesin transports lipid droplets (LDs) to the endoplasmic reticulum (ER) in liver cells, engineering ER−droplet contacts and supplying lipids to the ER for secretion as lipoprotein. However, when fasting induces massive lipid accumulation in liver, kinesin is removed from LDs, inhibiting lipid supply to the ER and homeostatically tempering lipid secretion from liver in a fasted state. Interestingly, reducing kinesin also blocks propagation of hepatitis-C virus inside liver cells, possibly because viral proteins cannot transfer from the ER to LDs.

Despite massive fluctuations in its internal triglyceride content, the liver secretes triglyceride under tight homeostatic control. This buffering function is most visible after fasting, when liver triglyceride increases manyfold but circulating serum triglyceride barely fluctuates. How the liver controls triglyceride secretion is unknown, but is fundamentally important for lipid and energy homeostasis in animals. Here we find an unexpected cellular and molecular mechanism behind such control. We show that kinesin motors are recruited to triglyceride-rich lipid droplets (LDs) in the liver by the GTPase ARF1, which is a key activator of lipolysis. This recruitment is activated by an insulin-dependent pathway and therefore responds to fed/fasted states of the animal. In fed state, ARF1 and kinesin appear on LDs, consequently transporting LDs to the periphery of hepatocytes where the smooth endoplasmic reticulum (sER) is present. Because the lipases that catabolize LDs in hepatocytes reside on the sER, LDs can now be catabolized efficiently to provide triglyceride for lipoprotein assembly and secretion from the sER. Upon fasting, insulin is lowered to remove ARF1 and kinesin from LDs, thus down-regulating LD transport and sER–LD contacts. This tempers triglyceride availabiity for very low density lipoprotein assembly and allows homeostatic control of serum triglyceride in a fasted state. We further show that kinesin knockdown inhibits hepatitis-C virus replication in hepatocytes, likely because translated viral proteins are unable to transfer from the ER to LDs.

Role of ADP ribosylation factor6- Cytohesin1-PhospholipaseD signaling axis in U46619 induced activation of NADPH oxidase in pulmonary artery smooth muscle cell membrane

Treatment of human pulmonary artery smooth muscle cells (HPASMCs) with the thromboxane A2 receptor antagonist, SQ29548 inhibited U46619 stimulation of phospholipase D (PLD) and NADPH oxidase activities in the cell membrane. Pretreatment with apocynin inhibited U46619 induced increase in NADPH oxidase activity. The cell membrane contains predominantly PLD2 along with PLD1 isoforms of PLD. Pretreatment with pharmacological and genetic inhibitors of PLD2, but not PLD1, attenuated U46619 stimulation of NADPH oxidase activity. U46619 stimulation of PLD and NADPH oxidase activities were insensitive to BFA and Clostridium botulinum C3 toxin; however, pretreatment with secinH3 inhibited U46619 induced increase in PLD and NADPH oxidase activities suggesting a major role of cytohesin in U46619-induced increase in PLD and NADPH oxidase activities. Arf-1, Arf-6, cytohesin-1 and cytohesin-2 were observed in the cytosolic fraction, but only Arf-6 and cytohesin-1 were translocated to the cell membrane upon treatment with U46619. Coimmunoprecipitation study showed association of Arf-6 with cytohesin-1 in the cell membrane fraction. In vitro binding of GTPγS with Arf-6 required the presence of cytohesin-1 and that occurs in BFA insensitive manner. Overall, BFA insensitive Arf6-cytohesin1 signaling axis plays a pivotal role in U46619-mediated activation of PLD leading to stimulation of NADPH oxidase activity in HPASMCs.

Role of curcumin in PLD activation by Arf6-cytohesin1 signaling axis in U46619-stimulated pulmonary artery smooth muscle cells

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine to produce phosphatidic acid (PA) which in some cell types play a pivotal role in agonist-induced increase in NADPH oxidase-derived [Formula: see text]production. Involvement of ADP ribosylation factor (Arf) in agonist-induced activation of PLD is known for smooth muscle cells of systemic arteries, but not in pulmonary artery smooth muscle cells (PASMCs). Additionally, role of cytohesin in this scenario is unknown in PASMCs. We, therefore, determined the involvement of Arf and cytohesin in U46619-induced stimulation of PLD in PASMCs, and the probable mechanism by which curcumin, a natural phenolic compound, inhibits the U46619 response. Treatment of PASMCs with U46619 stimulated PLD activity in the cell membrane, which was inhibited upon pretreatment with SQ29548 (Tp receptor antagonist), FIPI (PLD inhibitor), SecinH3 (inhibitor of cytohesins), and curcumin. Transfection of the cells with Tp, Arf-6, and cytohesin-1 siRNA inhibited U46619-induced activation of PLD. Upon treatment of the cells with U46619, Arf-6 and cytohesin-1 were translocated and associated in the cell membrane, which were not inhibited upon pretreatment of the cells with curcumin. Cytohesin-1 appeared to be necessary for in vitro binding of GTPγS with Arf-6; however, addition of curcumin inhibited binding of GTPγS with Arf-6 even in the presence of cytohesin-1. Our computational study suggests that although curcumin to some extent binds with Tp receptor, yet the inhibition of Arf6_GDP to Arf6_GTP conversion appeared to be an important mechanism by which curcumin inhibits U46619-induced increase in PLD activity in PASMCs.

Distinct regulations of ARF1 by two Aplysia Sec7 isoforms

Sec7 protein is a guanine nucleotide exchange factor in the ADP-ribosylation factor (ARF) family of small GTP-binding proteins. Aplysia Sec7 proteins (ApSec7s) play many roles in neurite outgrowth and synaptic facilitation in Aplysia neurons. However, the binding property of Aplysia ARF1 by ApSec7 isoforms has not been examined. In this study, we found that the cloned Aplysia ARF1 (ApARF1) protein only localized to the Golgi complex when it was expressed alone in HEK293T cells; however, if ApARF1 was co-expressed with plasma membrane-targeted ApSec7, it localized to both the plasma membrane and the Golgi complex via association with the Sec7 domain of ApSec7. Moreover, in HEK293T cells expressing both ApARF1 and another Sec7 isoform, ApSec7(VPKIS), the pleckstrin homology domain of ApSec7(VPKIS) associated with ApARF1, resulting in its localization to the Golgi complex. Overall, we propose a model in which ApSec7(VPKIS) activates ApARF1 in the Golgi complex, while ApSec7 recruits ApARF1 to the plasma membrane where it activates ApARF1/6 downstream signaling.

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.

Modulation of Insulin Sensitivity of Hepatocytes by the Pharmacological Downregulation of Phospholipase D

Background. The role of phospholipase D (PLD) as a positive modulator of glucose uptake activation by insulin in muscle and adipose cells has been demonstrated. The role of PLD in the regulation of glucose metabolism by insulin in the primary hepatocytes has been determined in this study. Methods. For this purpose, we studied effects of inhibitors of PLD on glucose uptake and glycogen synthesis stimulation by insulin. To determine the PLD activity, the method based on determination of products of transphosphatidylation reaction, phosphatidylethanol or phosphatidylbutanol, was used. Results. Inhibition of PLD by a general antagonist (1-butanol) or specific inhibitor, halopemide, or N-hexanoylsphingosine, or by cellular ceramides accumulated in doxorubicin-treated hepatocytes decreased insulin-stimulated glucose metabolism. Doxorubicin-induced hepatocytes resistance to insulin action could be abolished by inhibition of ceramide production. Halopemide could nullify this effect. Addition of propranolol, as well as inhibitors of phosphatidylinositol 3-kinase (PI3-kinase) (wortmannin, LY294002) or suppressors of Akt phosphorylation/activity, luteolin-7-O-glucoside or apigenin-7-O-glucoside, to the culture media could block cell response to insulin action. Conclusion. PLD plays an important role in the insulin signaling in the hepatocytes. PLD is activated downstream of PI3-kinase and Akt and is highly sensitive to ceramide content in the liver cells.

Lysophosphatidic acid acyltransferase beta regulates mTOR signaling

Lysophosphatidic acid acyltransferase (LPAAT-β) is a phosphatidic acid (PA) generating enzyme that plays an essential role in triglyceride synthesis. However, LPAAT-β is now being studied as an important regulator of cell growth and differentiation and as a potential therapeutic target in cancer since PA is necessary for the activity of key proteins such as Raf, PKC-ζ and mTOR. In this report we determine the effect of LPAAT-β silencing with siRNA in pancreatic adenocarcinoma cell lines. We show for the first time that LPAAT-β knockdown inhibits proliferation and anchorage-independent growth of pancreatic cancer cells. This is associated with inhibition of signaling by mTOR as determined by levels of mTORC1- and mTORC2-specific phosphorylation sites on 4E-BP1, S6K and Akt. Since PA regulates the activity of mTOR by modulating its binding to FKBP38, we explored the possibility that LPAAT-β might regulate mTOR by affecting its association with FKBP38. Coimmunoprecipitation studies of FKBP38 with mTOR show increased levels of FKBP38 associated with mTOR when LPAAT-β protein levels are knocked down. Furthermore, depletion of LPAAT-β results in increased Lipin 1 nuclear localization which is associated with increased nuclear eccentricity, a nuclear shape change that is dependent on mTOR, further confirming the ability of LPAAT-β to regulate mTOR function. Our results provide support for the hypothesis that PA generated by LPAAT-β regulates mTOR signaling. We discuss the implications of these findings for using LPAAT-β as a therapeutic target.

Activation machinery of the small GTPase Arf6

The small GTPase ADP-ribosylation factor 6 (Arf6) plays pivotal roles in a wide variety of cellular events, including exocytosis, endocytosis, actin cytoskeleton reorganization and phosphoinositide metabolism, in various types of cells. To control such a wide variety of actions of Arf6, activation of Arf6 could be precisely controlled by its activators, guanine nucleotide exchange factors (GEFs), in spatial and temporal manners. In this manuscript, we summarize and discuss the characteristics of previously identified GEFs specific to Arf6 and activation machineries of Arf6.

Small inhibitors of ADP-ribosylation factor activation and function in mammalian cells

Small GTP-binding proteins of the ADP-ribosylation factor (Arf) family control various cell functional responses including protein transport and recycling between different cellular compartments, phagocytosis, proliferation, cytoskeletal remodelling, and migration. The activity of Arfs is tightly regulated. GTPase-activating proteins (GAPs) inactivate Arfs by stimulating GTP hydrolysis, and guanine nucleotide exchange factors (GEFs) stimulate the conversion of inactive GDP-bound Arf to the active GTP-bound conformation. There is increasing evidence that Arf small GTPases contribute to cancer growth and invasion. Increased expression of Arf6 and of Arf-GEPs, or deregulation Arf-GAP functions have been correlated with enhanced invasive capacity of tumor cells and metastasis. The spatiotemporal specificity of Arf activation is dictated by their GEFs that integrate various signals in stimulated cells. Brefeldin A (BFA), which inactivates a subset of Arf-GEFs, has been very useful for assessing the function of Golgi-localized Arfs. However, specific inhibitors to investigate the individual function of BFA-sensitive and insensitive Arf-GEFs are lacking. In recent years, specific screens have been developed, and new inhibitors with improved selectivity and potency to study cell functional responses regulated by BFA-sensitive and BFA-insensitive Arf pathways have been identified. These inhibitors have been instrumental for our understanding of the spatiotemporal activation of Arf proteins in cells and demonstrate the feasibility of developing small molecules interfering with Arf activation to prevent tumor invasion and metastasis.

Nucleotide exchange factors: Kinetic analyses and the rationale for studying kinetics of GEFs

Exchange factors are enzymes that catalyze the exchange of GTP for GDP on guanine nucleotide binding proteins. Progress in understanding the molecular basis of action and the cellular functions of these enzymes has largely come from structural determinations (e.g., crystal structures) and studying effects on cells when expression levels of the exchange factors are perturbed or mutated exchange factors are expressed. Proportionally little effort has been expended on studying the kinetics of exchange; however, reaction rates are central to understanding enzymes. Here, we discuss the importance of kinetic analysis of exchange factors for guanine nucleotide binding proteins, with a focus on ADP-ribosylation factor (Arf) and heterotrimeric G proteins, for providing unique insights into molecular mechanisms and regulation as well as how kinetic analyses are used to complement other approaches.

cAMP antagonizes ERK-dependent antiapoptotic action of insulin

Insulin has antiapoptotic activity in various cell types. However, the signaling pathways underlying the antiapoptotic activity of insulin is not yet known. This study was conducted to determine if cAMP affects the antiapoptotic activity of insulin and the activity of PI3K and ERK in CHO cells expressing human insulin receptors (CHO-IR). Insulin-stimulated ERK activity was completely suppressed by cAMP-elevating agents like as pertussis toxin (Ptx) and cholera toxin (Ctx) after 4 h treatment. Insulin-stimulated PKB/Akt activity was not affected at all. Ptx treatment together with insulin increased the number of apoptotic cells and the degree of DNA fragmentation. Ctx or 8-brcAMP treatment also increased the number of apoptotic cells and stimulated the cleavage of caspase-3 and the hydrolysis of PARP. Taken together, cAMP antagonizes the antiapoptotic activity of insulin and the main target molecule of cAMP in this process is likely ERK, not PI3K-dependent PKB/Akt.

Kinetic studies of the Arf activator Arno on model membranes in the presence of Arf effectors suggest control by a positive feedback loop

Proteins of the cytohesin/Arno/Grp1 family of Arf activators are positive regulators of the insulin-signaling pathway and control various remodeling events at the plasma membrane. Arno has a catalytic Sec7 domain, which promotes GDP to GTP exchange on Arf, followed by a pleckstrin homology (PH) domain. Previous studies have revealed two functions of the PH domain: inhibition of the Sec7 domain and membrane targeting. Interestingly, the Arno PH domain interacts not only with a phosphoinositide (phosphatidylinositol 4,5-bisphosphate or phosphatidylinositol 3,4,5-trisphosphate) but also with an activating Arf family member, such as Arf6 or Arl4. Using the full-length membrane-bound forms of Arf1 and Arf6 instead of soluble forms, we show here that the membrane environment dramatically affects the mechanism of Arno activation. First, Arf6-GTP stimulates Arno at nanomolar concentrations on liposomes compared with micromolar concentrations in solution. Second, mutations in the PH domain that abolish interaction with Arf6-GTP render Arno completely inactive when exchange reactions are reconstituted on liposomes but have no effect on Arno activity in solution. Third, Arno is activated by its own product Arf1-GTP in addition to a distinct activating Arf isoform. Consequently, Arno activity is strongly modulated by competition with Arf effectors. These results show that Arno behaves as a bistable switch, having an absolute requirement for activation by an Arf protein but, once triggered, becoming highly active through the positive feedback effect of Arf1-GTP. This property of Arno might provide an explanation for its function in signaling pathways that, once triggered, must move forward decisively.

Vascular endothelial growth factor receptor-2 activates ADP-ribosylation factor 1 to promote endothelial nitric-oxide synthase activation and nitric oxide release from endothelial cells

Vascular endothelial growth factor (VEGF) induces angiogenesis and regulates endothelial function via production and release of nitric oxide (NO), an important signaling molecule. The molecular basis leading to NO production involves phosphatidylinositiol-3 kinase (PI3K), Akt, and endothelial nitric-oxide synthase (eNOS) activation. In this study, we have examined whether small GTP-binding proteins of the ADP-ribosylation factor (ARF) family act as molecular switches to regulate signaling cascades activated by VEGF in endothelial cells. Our results show that this growth factor can promote the rapid and transient activation of ARF1. In endothelial cells, this GTPase is present on dynamic plasma membrane ruffles. Inhibition of ARF1 expression, using RNA interference, markedly impaired VEGF-dependent eNOS phosphorylation and NO production by preventing the activation of the PI3K/Akt signaling axis. Furthermore, our data indicate that phosphorylation of Tyr(801), on VEGF receptor 2, is essential for activating Src- and ARF1-dependent signaling events leading to NO release from endothelial cells. Lastly, this mediator is known to regulate a broad variety of endothelial cell functions. Depletion of ARF1 markedly inhibits VEGF-dependent increase of vascular permeability as well as capillary tubule formation, a process important for angiogenesis. Taken together, our data indicate that ARF1 is a novel modulator of VEGF-stimulated NO release and signaling in endothelial cells.

Cytohesin-1 regulates the Arf6-phospholipase D signaling axis in human neutrophils: impact on superoxide anion production and secretion

Polymorphonuclear neutrophil (PMN) stimulation with fMLP stimulates small G proteins such as ADP-ribosylation factors (Arfs) Arf1 and Arf6, leading to phospholipase D (PLD) activation and functions such as degranulation and the oxidative burst. However, the molecular links between fMLF receptors and PLD remain unclear. PMNs express cytohesin-1, an Arf-guanine exchange factor that activates Arfs, and its expression is strongly induced during the acquisition of the neutrophilic phenotype by neutrophil-like cells. The role of cytohesin-1 in the activation of the fMLF-Arf-PLD signaling axis, and the accomplishment of superoxide anion production, and degranulation was investigated in PMNs using the selective inhibitor of cytohesin, Sec 7 inhibitor H3 (secinH3). Cytohesin-1 inhibition with secinH3 leads to Arf6 but not Arf1 inhibition, demonstrating the specificity for Arf6, and fMLF-mediated activation of PLD and of the oxidative burst as well. We observed a decrease in fMLF-mediated protein secretion and expression of cell surface markers corresponding to primary (CD63/myeloperoxidase), secondary (CD66/lactoferrin), and tertiary (matrix metalloproteinase-9) granules in PMNs incubated with secinH3. Similarly, silencing cytohesin-1 or Arf6 in PLB-985 cells negatively affected fMLF-induced activation of PLD, superoxide production, and expression of granule markers on the cell surface. In contrast, stable overexpression of cytohesin-1 in PLB-985 cells enhanced fMLF-induced activation of Arf6, PLD, and NADPH oxidase. The results of this study provide evidence for an involvement of cytohesin-1 in the regulation of the functional responses of human PMNs and link these events, in part at least, to the activation of Arf6.

ADP-ribosylation factor 1 controls the activation of the phosphatidylinositol 3-kinase pathway to regulate epidermal growth factor-dependent growth and migration of breast cancer cells

Activation of intracellular signaling pathways by growth factors is one of the major causes of cancer development and progression. Recent studies have demonstrated that monomeric G proteins of the Ras family are key regulators of cell proliferation, migration, and invasion. Using an invasive breast cancer cell lines, we demonstrate that the ADP-ribosylation factor 1 (ARF1), a small GTPase classically associated with the Golgi, is an important regulator of the biological effects induced by epidermal growth factor. Here, we show that this ARF isoform is activated following epidermal growth factor stimulation and that, in MDA-MB-231 cells, ARF1 is found in dynamic plasma membrane ruffles. Inhibition of endogenous ARF1 expression results in the inhibition of breast cancer cell migration and proliferation. The underlying mechanism involves the activation of the phosphatidylinositol 3-kinase pathway. Our data demonstrate that depletion of ARF1 markedly impairs the recruitment of the phosphatidylinositol 3-kinase catalytic subunit (p110alpha) to the plasma membrane, and the association of the regulatory subunit (p85alpha) to the activated receptor. These results uncover a novel molecular mechanism by which ARF1 regulates breast cancer cell growth and invasion during cancer progression.

ARAP1 regulates endocytosis of EGFR

Signaling through the EGF receptor is regulated by endocytosis. ARAP1 is a protein with Arf guanosine triphosphatase-activating protein (GAP) and Rho GAP domains. We investigated the role of ARAP1 in EGF receptor endocytic trafficking. Following EGF treatment of cells, ARAP1 rapidly and transiently associated with the edge of the cell and punctate structures containing Rab5, rabaptin 5 and EGFR but not early embryonic antigen 1 (EEA1). EGF associated with the ARAP1-positive punctate structures prior to EEA1-positive early endosomes. Recruitment of ARAP1 to the punctate structures required active Rab5 and an additional signal from EGFR. Decreasing ARAP1 levels with small interfering RNA accelerated association of EGF with EEA1 endosomes and degradation of EGFR. Phosphorylation of extracellular-signal-regulated kinase (ERK) and c-Jun-amino-terminal kinase (JNK) was diminished and more transient in cells with reduced levels of ARAP1 than in controls. Based on these findings, we propose that ARAP1 regulates the endocytic traffic of EGFR and, consequently, the rate of EGFR signal attenuation.

Active Arf6 recruits ARNO/cytohesin GEFs to the PM by binding their PH domains

ARNO is a soluble guanine nucleotide exchange factor (GEF) for the Arf family of GTPases. Although in biochemical assays ARNO prefers Arf1 over Arf6 as a substrate, its localization in cells at the plasma membrane (PM) suggests an interaction with Arf6. In this study, we found that ARNO activated Arf1 in HeLa and COS-7 cells resulting in the recruitment of Arf1 on to dynamic PM ruffles. By contrast, Arf6 was activated less by ARNO than EFA6, a canonical Arf6 GEF. Remarkably, Arf6 in its GTP-bound form recruited ARNO to the PM and the two proteins could be immunoprecipitated. ARNO binding to Arf6 was not mediated through the catalytic Sec7 domain, but via the pleckstrin homology (PH) domain. Active Arf6 also bound the PH domain of Grp1, another ARNO family member. This interaction was direct and required both inositol phospholipids and GTP. We propose a model of sequential Arf activation at the PM whereby Arf6-GTP recruits ARNO family GEFs for further activation of other Arf isoforms.

Phosphatidic acid and diacylglycerol generation is regulated by insulin in cerebral cortex synaptosomes from adult and aged rats

Insulin receptor associated with the cerebral cortex (CC) has been shown to be involved in brain cognitive functions. Furthermore, deterioration of insulin signaling has been associated with age-related brain degeneration. We have reported previously that aging stimulates phospholipase D/phosphatidate phosphohydrolase 2 (PLD/PAP2) pathway in CC synaptosomes from aged rats, generating a differential availability of their reaction products: diacylglycerol (DAG) and phosphatidic acid (PA). The aim of this work was to determine the effect of aging on DAG kinase (DAGK), as an alternative pathway for PA generation, and to evaluate the effect of insulin on PLD/PAP2 pathway and DAGK. PLD, PAP2, and DAGK activities were measured using specific radiolabeled substrates in CC synaptosomes from adult (4 months old) and aged rats (28 months old). In adult animals, in the presence of the tyrosine phosphatase inhibitor (sodium o-vanadate), insulin stimulated PLD activity at 5 min incubation. DAGK activity was also increased at the same time of incubation and PAP2 was inhibited. In aged animals, PLD activity was not modified by the presence of insulin plus vanadate, PAP2 was inhibited, and DAGK was stimulated by the hormone. Insulin, vanadate, and the combination of both induced protein tyrosine phosphorylation in adult CC synaptosomes. Aged rats showed a lower level of protein phosphorylation with respect to adult rats. Our results show that insulin modulates PA and DAG availability through the regulation of PLD/PAP2 and DAGK pathways in adult rat CC synaptosomes. Additionally, we demonstrated that PA and DAG generation is regulated differentially by insulin during aging.

Insulin-stimulated plasma membrane fusion of Glut4 glucose transporter-containing vesicles is regulated by phospholipase D1

Insulin stimulates glucose uptake in fat and muscle by mobilizing Glut4 glucose transporters from intracellular membrane storage sites to the plasma membrane. This process requires the trafficking of Glut4-containing vesicles toward the cell periphery, docking at exocytic sites, and plasma membrane fusion. We show here that phospholipase D (PLD) production of the lipid phosphatidic acid (PA) is a key event in the fusion process. PLD1 is found on Glut4-containing vesicles, is activated by insulin signaling, and traffics with Glut4 to exocytic sites. Increasing PLD1 activity facilitates glucose uptake, whereas decreasing PLD1 activity is inhibitory. Diminished PA production does not substantially hinder trafficking of the vesicles or their docking at the plasma membrane, but it does impede fusion-mediated extracellular exposure of the transporter. The fusion block caused by RNA interference-mediated PLD1 deficiency is rescued by exogenous provision of a lipid that promotes fusion pore formation and expansion, suggesting that the step regulated by PA is late in the process of vesicle fusion.