ARF6 activation by Galpha q signaling: Galpha q forms molecular complexes with ARNO and ARF6

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.

Regulation of smooth muscle contraction by monomeric non-RhoA GTPases

Smooth muscle contraction in the cardiovascular system, airways, prostate and lower urinary tract is involved in the pathophysiology of many diseases, including cardiovascular and obstructive lung disease plus lower urinary tract symptoms, which are associated with high prevalence of morbidity and mortality. This prominent clinical role of smooth muscle tone has led to the molecular mechanisms involved being subjected to extensive research. In general smooth muscle contraction is promoted by three major signalling pathways, including the monomeric GTPase RhoA pathway. However, emerging evidence suggests that monomeric GTPases other than RhoA may be involved in signal transduction in smooth muscle contraction, including Rac GTPases, cell division control protein 42 homologue, adenosine ribosylation factor 6, Ras, Rap1b and Rab GTPases. Here, we review these emerging functions of non-RhoA GTPases in smooth muscle contraction, which has now become increasingly more evident and constitutes an emerging and innovative research area of high clinical relevance.

A NAV2729-sensitive mechanism promotes adrenergic smooth muscle contraction and growth of stromal cells in the human prostate

Voiding symptoms in benign prostatic hyperplasia (BPH) are driven by prostate smooth muscle contraction and prostate growth. Smooth muscle contraction in the prostate and other organs critically depends on activation of the small monomeric GTPase RhoA and probably Rac1. A role of another GTPase, ADP-ribosylation factor 6 (ARF6), for smooth muscle contraction has been recently suggested by indirect evidence but remains to be proven for any organ. Here, we report effects of NAV2729, an inhibitor with assumed specificity for ARF6, in human prostate tissues and cultured prostate stromal cells (WPMY-1). NAV2729 (5 μm) inhibited neurogenic and α₁-adrenergic contractions of human prostate tissues. Contractions induced by endothelin-1, by the thromboxane A₂ agonist U46619, or by high molar KCl were not inhibited. Correlation analyses suggested up-regulation of prostatic ARF6 expression with increasing degree of BPH, as ARF6 expression increased with the content of prostate-specific antigen (PSA) of prostate tissues. NAV2729 inhibited ARF6 activity but not other GTPases (ARF1, RhoA, Rac1) in prostate tissues and in WPMY-1 cells. Proliferation of WPMY-1 cells was inhibited concentration-dependently by NAV2726, as reflected by decreased viability, 5-ethynyl-2'-deoxyuridine (EdU) assay, colony formation assay, and expression of Ki-67. Silencing of ARF6 expression mimicked effects of NAV2729 on viability and in the EdU assay. Effects of NAV2729 on viability and proliferation were attenuated in cells with silenced ARF6 expression. Our findings suggest that a NAV2729-sensitive mechanism promotes adrenergic contraction and stromal cell growth in the human prostate, which is probably ARF6-mediated. Similar actions in other organs and urodynamic effects of NAV2729 appear possible.

Pallidin is a novel interacting protein for cytohesin-2 and regulates the early endosomal pathway and dendritic formation in neurons

Cytohesin-2 is a member of the guanine nucleotide exchange factors for ADP ribosylation factor 1 (Arf1) and Arf6, which are small GTPases that regulate membrane traffic and actin dynamics. In this study, we first demonstrated that cytohesin-2 localized to the plasma membrane and vesicles in various subcellular compartment in hippocampal neurons by immunoelectron microscopy. Next, to understand the molecular network of cytohesin-2 in neurons, we conducted yeast two-hybrid screening of brain cDNA libraries using cytohesin-2 as bait and isolated pallidin, a component of the biogenesis of lysosome-related organelles complex 1 (BLOC-1) involved in endosomal trafficking. Pallidin interacted specifically with cytohesin-2 among cytohesin family members. Glutathione S-transferase pull-down and immunoprecipitation assays further confirmed the formation of a protein complex between cytohesin-2 and pallidin. Immunofluorescence demonstrated that cytohesin-2 and pallidin partially colocalized in various subsets of endosomes immunopositive for EEA1, syntaxin 12, and LAMP2 in hippocampal neurons. Knockdown of pallidin or cytohesin-2 reduced cytoplasmic EEA1-positive early endosomes. Furthermore, knockdown of pallidin increased the total dendritic length of cultured hippocampal neurons, which was rescued by co-expression of wild-type pallidin but not a mutant lacking the ability to interact with cytohesin-2. In contrast, knockdown of cytohesin-2 had the opposite effect on total dendritic length. The present results suggested that the interaction between pallidin and cytohesin-2 may participate in various neuronal functions such as endosomal trafficking and dendritic formation in hippocampal neurons. Cover Image for this issue: doi: 10.1111/jnc.14197.

Inhibition of smooth muscle contraction and ARF6 activity by the inhibitor for cytohesin GEFs, secinH3, in the human prostate

Prostate smooth muscle contraction is critical for etiology and treatment of male lower urinary tract symptoms (LUTS) and is promoted by small monomeric GTPases (RhoA and Rac). GTPases may be activated by guanosine nucleotide exchange factors (GEFs). GEFs of the cytohesin family may indirectly activate Rac, or ADP ribosylation factor (ARF) GTPases directly. Here we investigated the expression of cytohesin family GEFs and effects of the cytohesin inhibitor Sec7 inhibitor H3 (secinH3) on smooth muscle contraction and GTPase activities in human prostate tissues. Of all four cytohesin isoforms, cytohesin-1 and -2 showed the highest expression in real-time PCR. Western blot and fluorescence staining suggested that cytohesin-2 may be the predominant isoform in prostate smooth muscle cells. Contractions induced by norepinephrine, the α₁-adrenoceptor agonist phenylephrine, the thromboxane A₂ analog U-46619 , and endothelin-1 and -3, as well as neurogenic contractions induced by electric field stimulation (EFS), were reduced by secinH3 (30 µM). Inhibition of EFS-induced contractions appeared to have efficacy similar to that of inhibition by the α₁-adrenoceptor antagonist tamsulosin (300 nM). Combined application of secinH3 plus tamsulosin caused larger inhibition of EFS-induced contractions than tamsulosin alone. Pull-down assays demonstrated inhibition of the small monomeric GTPase ARF6 by secinH3, but no inhibition of RhoA or Rac1. In conclusion, we suggest that a cytohesin-ARF6 pathway takes part in smooth muscle contraction. This may open attractive new possibilities in medical treatment of male LUTS.

Computational analysis for GNAQ mutations: New insights on the molecular etiology of Sturge-Weber syndrome

Somatic activating mutations in the GNAQ have been recently associated with several congenital genetic disorders and tumors; however, the molecular mechanism/etiology that leads to GNAQ somatic mosaic mutation are unknown. Here, we reported a case of Sturge-Weber Syndrome (SWS) manifesting cutaneous vascular malformations (hemifacial Port-wine stain), cerebral and ocular vascular abnormalities (including epilepsy and glaucoma) and harboring a c.548G>A (p.R183Q) somatic mosaic mutation in GNAQ. Computational modeling studies were performed to assistant with the comprehension of the functional impact of p.R183Q and p.Q209L mutations in GNAQ, which encodes a G protein subunit alpha q (Gαq). The p.R183Q mutation was predicted to abolish hydrogen bonds between R183 residue and GDP molecule, destabilizing the inactive GDP-bound conformation of the Gαq mutants. Furthermore, replacement of R183 by Q183 residue was predicted to promote conformation changes in protein surface features affecting the switch I region, a key region that undergoes conformational changes triggered by receptor binding during signal transduction. In addition, replacement of Q209 by L209 residue was predicted to affect the molecular interaction between Gαq and Gβ subunit, impairing formation of the inactive heterotrimeric complex. These findings, in association with PPI network analysis, indicate that p.R183Q and p.Q209L mutations result in the over-activation of different downstream effectors, which in turn will determine the distinct cell responses and phenotype. These findings bring new insights on molecular etiology of vascular malformations associated to SWS and on different mechanisms underlying hyperactivation of downstream pathways to Gαq.

The small GTPase ARF6 regulates protein trafficking to control cellular function during development and in disease

The activation of the small GTPase ARF6 has been implicated in promoting several pathological processes related to vascular instability and tumor formation, growth, and metastasis. ARF6 also plays a vital role during embryonic development. Recent studies have suggested that ARF6 carries out these disparate functions primarily by controlling protein trafficking within the cell. ARF6 helps direct proteins to intracellular or extracellular locations where they function in normal cellular responses during development and in pathological processes later in life. This transport of proteins is accomplished through a variety of mechanisms, including endocytosis and recycling, microvesicle release, and as yet uncharacterized processes. This Commentary will explore the functions of ARF6, while focusing on the role of this small GTPase in development and postnatal physiology, regulating barrier function and diseases associated with its loss, and tumor formation, growth, and metastasis.

The BRAG/IQSec family of Arf GEFs

The IQSec/BRAG proteins are a subfamily of Arf-nucleotide exchange factors. Since their discovery almost 15 y ago, the BRAGs have been reported to be involved in diverse physiological processes from myoblast fusion, neuronal pathfinding and angiogenesis, to pathophysiological processes including X-linked intellectual disability and tumor metastasis. In this review we will address how, in each of these situations, the BRAGs are thought to regulate the surface levels of adhesive and signaling receptors. While in most cases BRAGs are thought to enhance the endocytosis of these receptors, how they achieve this remains unclear. Similarly, while all 3 BRAG proteins contain calmodulin-binding IQ motifs, little is known about how their activities might be regulated by calcium. These are some of the questions that are likely to form the basis of future research.

ARF6 Is an Actionable Node that Orchestrates Oncogenic GNAQ Signaling in Uveal Melanoma

Activating mutations in Gαq proteins, which form the α subunit of certain heterotrimeric G proteins, drive uveal melanoma oncogenesis by triggering multiple downstream signaling pathways, including PLC/PKC, Rho/Rac, and YAP. Here we show that the small GTPase ARF6 acts as a proximal node of oncogenic Gαq signaling to induce all of these downstream pathways as well as β-catenin signaling. ARF6 activates these diverse pathways through a common mechanism: the trafficking of GNAQ and β-catenin from the plasma membrane to cytoplasmic vesicles and the nucleus, respectively. Blocking ARF6 with a small-molecule inhibitor reduces uveal melanoma cell proliferation and tumorigenesis in a mouse model, confirming the functional relevance of this pathway and suggesting a therapeutic strategy for Gα-mediated diseases.

β-Arrestin-mediated Angiotensin II Signaling Controls the Activation of ARF6 Protein and Endocytosis in Migration of Vascular Smooth Muscle Cells

Angiotensin II (Ang II) is a vasopressive hormone but is also a potent activator of cellular migration. We have previously shown that it can promote the activation of the GTPase ARF6 in a heterologous overexpressing system. The molecular mechanisms by which receptors control the activation of this small G protein remain, however, largely unknown. Furthermore, how ARF6 coordinates the activation of complex cellular responses needs to be further elucidated. In this study, we demonstrate that Ang II receptors engage β-arrestin, but not Gq, to mediate ARF6 activation in HEK 293 cells. To further confirm the key role of β-arrestin proteins, we overexpressed β-arrestin2-(1-320), a dominant negative mutant known to block receptor endocytosis. We show that expression of this truncated construct does not support the activation of the GTPase nor cell migration. Interestingly, β-arrestin2 can interact with the ARF guanine nucleotide exchange factor ARNO, although the C-terminally lacking mutant does not. We finally examined whether receptor endocytosis controlled ARF6 activation and cell migration. Although the clathrin inhibitor PitStop2 did not impact the ability of Ang II to activate ARF6, cell migration was markedly impaired. To further show that ARF activation regulates key signaling events leading to migration, we also examined MAPK activation. We demonstrate that this signaling axis is relevant in smooth muscle cells of the vasculature. Altogether, our findings show for the first time that Ang II receptor signaling to β-arrestin regulates ARF6 activation. These proteins together control receptor endocytosis and ultimately cell migration.

Association of activated Gαq to the tumor suppressor Fhit is enhanced by phospholipase Cβ

BACKGROUND

G proteins are known to modulate various growth signals and are implicated in the regulation of tumorigenesis. The tumor suppressor Fhit is a newly identified interaction partner of Gq proteins that typically stimulate the phospholipase C pathway. Activated Gαq subunits have been shown to interact directly with Fhit, up-regulate Fhit expression and enhance its suppressive effect on cell growth and migration. Other signaling molecules may be involved in modulating Gαq/Fhit interaction.

METHODS

To test the relationship of PLCβ with the interaction between Gαq and Fhit, co-immunoprecipication assay was performed on HEK293 cells co-transfected with different combinations of Flag-Fhit, Gα16, Gα16QL, pcDNA3 vector, and PLCβ isoforms. Possible associations of Fhit with other effectors of Gαq were also demonstrated by co-immunoprecipitation. The regions of Gαq for Fhit interaction and PLCβ stimulation were further evaluated by inositol phosphates accumulation assay using a series of Gα16/z chimeras with discrete regions of Gα16 replaced by those of Gαz.

RESULTS

PLCβ1, 2 and 3 interacted with Fhit regardless of the expression of Gαq. Expression of PLCβ increased the affinities of Fhit for both wild-type and activated Gαq. Swapping of the Fhit-interacting α2-β4 region of Gαq with Gαi eliminated the association of Gαq with Fhit without affecting the ability of the mutant to stimulate PLCβ. Other effectors of Gαq including RGS2 and p63RhoGEF were unable to interact with Fhit.

CONCLUSIONS

PLCβ may participate in the regulation of Fhit by Gq in a unique way. PLCβ interacts with Fhit and increases the interaction between Gαq and Fhit. The Gαq/PLCβ/Fhit complex formation points to a novel signaling pathway that may negatively regulate tumor cell growth.

The small GTPase ADP-Ribosylation Factor 1 mediates the sensitivity of triple negative breast cancer cells to EGFR tyrosine kinase inhibitors

The clinical use of EGFR-targeted therapy, in triple negative breast cancer patients, has been limited by the development of resistance to these drugs. Although activated signaling molecules contribute to this process, the molecular mechanisms remain relatively unknown. We have previously reported that the small GTPase ADP-Ribosylation Factor 1 (ARF1) is highly expressed in invasive breast cancer cells and acts as a molecular switch to activate EGF-mediated responses. In this study, we aimed at defining whether the high expression of ARF1 limits sensitivity of these tumor cells to EGFR inhibitors, such as gefitinib. Here, we show that the knock down of ARF1 expression or activity decreased the dose and latency time required by tyrosine kinase inhibitors to induce cell death. This may be explained by the observation that the depletion of ARF1 suppressed gefitinib-mediated activation of key mediators of survival such as ERK1/2, AKT and Src, while enhancing cascades leading to apoptosis such as the p38MAPK and JNK pathways, modifying the Bax/Bcl2 ratio and cytochrome c release. In addition, inhibiting ARF1 expression and activation also results in an increase in gefitinib-mediated EGFR internalization and degradation further limiting the ability of this receptor to promote its effects. Interestingly, we observed that gefitinib treatment resulted in the enhanced activation of ARF1 by promoting its recruitment to the receptor AXL, an important mediator of EGFR inhibition suggesting that ARF1 may promote its pro-survival effects by coupling to alternative mitogenic receptors in conditions where the EGFR is inhibited. Together our results uncover a new role for ARF1 in mediating the sensitivity to EGFR inhibition and thus suggest that limiting the activation of this GTPase could improve the therapeutic efficacy of EGFR inhibitors.

The adaptor proteins p66Shc and Grb2 regulate the activation of the GTPases ARF1 and ARF6 in invasive breast cancer cells

Signals downstream of growth factor receptors play an important role in mammary carcinogenesis. Recently, we demonstrated that the small GTPases ARF1 and ARF6 were shown to be activated downstream of the epidermal growth factor receptor (EGFR) and act as a key regulator of growth, migration, and invasion of breast cancer cells. However, the mechanism via which the EGFR recruits and activates ARF1 and ARF6 to transmit signals has yet to be fully elucidated. Here, we identify adaptor proteins Grb2 and p66Shc as important regulators mediating ARF activation. We demonstrate that ARF1 can be found in complex with Grb2 and p66Shc upon EGF stimulation of the basal-like breast cancer MDA-MB-231 cell line. However, we report that these two adaptors regulate ARF1 activation differently, with Grb2 promoting ARF1 activation and p66Shc blocking this response. Furthermore, we show that Grb2 is essential for the recruitment of ARF1 to the EGFR, whereas p66Shc hindered ARF1 receptor recruitment. We demonstrate that the negative regulatory role of p66Shc stemmed from its ability to block the recruitment of Grb2/ARF1 to the EGFR. Conversely, p66Shc potentiates ARF6 activation as well as the recruitment of this ARF isoform to the EGFR. Interestingly, we demonstrate that Grb2 is also required for the activation and receptor recruitment of ARF6. Additionally, we show an important role for p66Shc in modulating ARF activation, cell growth, and migration in HER2-positive breast cancer cells. Together, our results highlight a central role for adaptor proteins p66Shc and Grb2 in the regulation of ARF1 and ARF6 activation in invasive breast cancer cells.

Rab family proteins regulate the endosomal trafficking and function of RGS4

RGS4, a heterotrimeric G-protein inhibitor, localizes to plasma membrane (PM) and endosomal compartments. Here, we examined Rab-mediated control of RGS4 internalization and recycling. Wild type and constitutively active Rab5 decreased RGS4 PM levels while increasing its endosomal targeting. Rab5, however, did not appreciably affect the PM localization or function of the M1 muscarinic receptor (M1R)/Gq signaling cascade. RGS4-containing endosomes co-localized with subsets of Rab5-, transferrin receptor-, and Lamp1/Lysotracker-marked compartments suggesting RGS4 traffics through PM recycling or acidified endosome pathways. Rab7 activity promoted TGN association, whereas Rab7(dominant negative) trapped RGS4 in late endosomes. Furthermore, RGS4 was found to co-localize with an endosomal pool marked by Rab11, the protein that mediates recycling/sorting of proteins to the PM. The Cys-12 residue in RGS4 appeared important for its Rab11-mediated trafficking to the PM. Rab11(dominant negative) decreased RGS4 PM levels and increased the number of RGS4-containing endosomes. Inhibition of Rab11 activity decreased RGS4 function as an inhibitor of M1R activity without affecting localization and function of the M1R/Gq signaling complex. Thus, both Rab5 activation and Rab11 inhibition decreased RGS4 function in a manner that is independent from their effects on the localization and function of the M1R/Gq signaling complex. This is the first study to implicate Rab GTPases in the intracellular trafficking of an RGS protein. Thus, Rab GTPases may be novel molecular targets for the selective regulation of M1R-mediated signaling via their specific effects on RGS4 trafficking and function.

ADP-ribosylation factors 1 and 6 regulate Wnt/β-catenin signaling via control of LRP6 phosphorylation

It has been shown that inhibition of GTPase-activating protein of ADP-ribosylation factor (Arf), ArfGAP, with a small molecule (QS11) results in synergistic activation of Wnt/β-catenin signaling. However, the role of Arf in Wnt/β-catenin signaling has not yet been elucidated. Here, we show that activation of Arf is essential for Wnt/β-catenin signaling. The level of the active form of Arf (Arf-GTP) transiently increased in the presence of Wnt, and this induction event was abrogated by blocking the interaction between Wnt and Frizzled (Fzd). In addition, knockdown of Fzds, Dvls or LRP6 blocked the Wnt-mediated activation of Arf. Consistently, depletion of Arf led to inhibition of Wnt-mediated membrane PtdIns (4,5)P2 (phosphatidylinositol 4, 5-bisphosphate) synthesis and LRP6 phosphorylation. Overall, our data suggest that transient activation of Arf modulates LRP6 phosphorylation for the transduction of Wnt/β-catenin signaling.

ARF6-dependent regulation of P2Y receptor traffic and function in human platelets

Adenosine diphosphate (ADP) is a critical regulator of platelet activation, mediating its actions through two G protein-coupled receptors, the P2Y₁ and P2Y₁₂ purinoceptors. Recently, we demonstrated that P2Y₁ and P2Y₁₂ purinoceptor activities are rapidly and reversibly modulated in human platelets, revealing that the underlying mechanism requires receptor internalization and subsequent trafficking as an essential part of this process. In this study we investigated the role of the small GTP-binding protein ADP ribosylation factor 6 (ARF6) in the internalization and function of P2Y₁ and P2Y₁₂ purinoceptors in human platelets. ARF6 has been implicated in the internalization of a number of GPCRs, although its precise molecular mechanism in this process remains unclear. In this study we show that activation of either P2Y₁ or P2Y₁₂ purinoceptors can stimulate ARF6 activity. Further blockade of ARF6 function either in cell lines or human platelets blocks P2Y purinoceptor internalization. This blockade of receptor internalization attenuates receptor resensitization. Furthermore, we demonstrate that Nm23-H1, a nucleoside diphosphate (NDP) kinase regulated by ARF6 which facilitates dynamin-dependent fission of coated vesicles during endocytosis, is also required for P2Y purinoceptor internalization. These data describe a novel function of ARF6 in the internalization of P2Y purinoceptors and demonstrate the integral importance of this small GTPase upon platelet ADP receptor function.

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.

ADP-ribosylation factor 6 expression and activation are reduced in myometrium in complicated pregnancies

Background

ARF6 (ADP-ribosylation factor 6) small GTP binding protein plays critical roles in actin cytoskeleton rearrangements and membrane trafficking, including internalisation of G protein coupled receptors (GPCR). ARF6 operates by cycling between GDP-bound (inactive) and GTP-bound (active) forms and is a potential regulator of GPCR-mediated uterine activity during pregnancy and labour. ARF6 contains very low intrinsic GTP binding activity and depends on GEFs (guanine nucleotide exchange factors) such as CYTH3 (cytohesin 3) to bind GTP. ARF6 and CYTH3 were originally cloned from human placenta, but there is no information on their expression in other reproductive tissues.

Methods

The expression of ARF6, ARF1, and CYTH1-4 was investigated by measuring mRNA (using RT-PCR) and protein levels (using immunoblotting) in samples of myometrium obtained from non-pregnant women, and women with normal pregnancies, before or after the spontaneous onset of labour. We also analysed myometrial samples from women with spontaneous preterm labour and from women with complicated pregnancies requiring emergency preterm delivery. The GST)-effector pull down assay was used to study the presence of active ARF6 and ARF1 in all myometrial extracts.

Results

ARF6, ARF1 and CYTH3 but not CYTH1, CYTH2 and CYTH4 were expressed in all samples and the levels did not change with pregnancy or labour. However, ARF6 and CYTH3 but not ARF1 levels were significantly reduced in complicated pregnancies. The alterations in the expression of ARF6 and its GEF in human myometrium indicate a potential involvement of this signalling system in modulating the response of myometrial smooth muscle in complicated pregnancies. The levels of ARF6-GTP or ARF1-GTP did not change with pregnancy or labour but ARF6-GTP levels were significantly decreased in women with severe complications of pregnancy.

Conclusions

We have demonstrated a functional ARF6 system in human myometrium and a correlation between ARF6 level and activity in uterine and abnormal pregnancy.

Mono-ADP-ribosylation of the G protein betagamma dimer is modulated by hormones and inhibited by Arf6

Mono-ADP-ribosylation is a reversible post-translational modification that can modulate the functions of target proteins. We have previously demonstrated that the β subunit of heterotrimeric G proteins is endogenously mono-ADP-ribosylated, and once modified, the βγ dimer is inactive toward its effector enzymes. To better understand the physiological relevance of this post-translational modification, we have studied its hormonal regulation. Here, we report that Gβ subunit mono-ADP-ribosylation is differentially modulated by G protein-coupled receptors. In intact cells, hormone stimulation of the thrombin receptor induces Gβ subunit mono-ADP-ribosylation, which can affect G protein signaling. Conversely, hormone stimulation of the gonadotropin-releasing hormone receptor (GnRHR) inhibits Gβ subunit mono-ADP-ribosylation. We also provide the first demonstration that activation of the GnRHR can activate the ADP-ribosylation factor Arf6, which in turn inhibits Gβ subunit mono-ADP-ribosylation. Indeed, removal of Arf6 from purified plasma membranes results in loss of GnRHR-mediated inhibition of Gβ subunit mono-ADP-ribosylation, which is fully restored by re-addition of purified, myristoylated Arf6. We show that Arf6 acts as a competitive inhibitor of the endogenous ADP-ribosyltransferase and is itself modified by this enzyme. These data provide further understanding of the mechanisms that regulate endogenous ADP-ribosylation of the Gβ subunit, and they demonstrate a novel role for Arf6 in hormone regulation of Gβ subunit mono-ADP-ribosylation.

The CNK1 scaffold binds cytohesins and promotes insulin pathway signaling

Protein scaffolds play an important role in signal transduction, regulating the localization of signaling components and mediating key protein interactions. Here, we report that the major binding partners of the Connector Enhancer of KSR 1 (CNK1) scaffold are members of the cytohesin family of Arf guanine nucleotide exchange factors, and that the CNK1/cytohesin interaction is critical for activation of the PI3K/AKT cascade downstream from insulin and insulin-like growth factor 1 (IGF-1) receptors. We identified a domain located in the C-terminal region of CNK1 that interacts constitutively with the coiled-coil domain of the cytohesins, and found that CNK1 facilitates the membrane recruitment of cytohesin-2 following insulin stimulation. Moreover, through protein depletion and rescue experiments, we found that the CNK1/cytohesin interaction promotes signaling from plasma membrane-bound Arf GTPases to the phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) to generate a PIP(2)-rich microenvironment that is critical for the membrane recruitment of insulin receptor substrate 1 (IRS1) and signal transmission to the PI3K/AKT cascade. These findings identify CNK1 as a new positive regulator of insulin signaling.

Mutation of a TADR protein leads to rhodopsin and Gq-dependent retinal degeneration in Drosophila

The Drosophila photoreceptor is a model system for genetic study of retinal degeneration. Many gene mutations cause fly photoreceptor degeneration, either because of excessive stimulation of the visual transduction (phototransduction) cascade, or through apoptotic pathways that in many cases involve a visual arrestin Arr2. Here we report a gene named tadr (for torn and diminished rhabdomeres), which, when mutated, leads to photoreceptor degeneration through a different mechanism. Degeneration in the tadr mutant is characterized by shrunk and disrupted rhabdomeres, the light sensory organelles of photoreceptor. The TADR protein interacted in vitro with the major light receptor Rh1 rhodopsin, and genetic reduction of the Rh1 level suppressed the tadr mutation-caused degeneration, suggesting the degeneration is Rh1-dependent. Nonetheless, removal of phospholipase C (PLC), a key enzyme in phototransduction, and that of Arr2 failed to inhibit rhabdomeral degeneration in the tadr mutant background. Biochemical analyses revealed that, in the tadr mutant, the G(q) protein of Rh1 is defective in dissociation from the membrane during light stimulation. Importantly, reduction of G(q) level by introducing a hypomorphic allele of G(alphaq) gene greatly inhibited the tadr degeneration phenotype. These results may suggest that loss of a potential TADR-Rh1 interaction leads to an abnormality in the G(q) signaling, which in turn triggers rhabdomeral degeneration independent of the PLC phototransduction cascade. We propose that TADR-like proteins may also protect photoreceptors from degeneration in mammals including humans.

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.

Endothelin-1 promotes migration of endothelial cells through the activation of ARF6 and the regulation of FAK activity

Several proteins act in concert to promote remodeling of the actin cytoskeleton during migration. This process is highly regulated by small GTP-binding proteins of the ADP-ribosylation factor (ARF) family of proteins. Here, we show that endothelin-1 (ET-1) can promote the activation of ARF6 and migration of endothelial cells through the activation of ET(B) receptors. Inhibition of ARF6 expression using RNA interference markedly impairs basal and ET-1 stimulated cell migration. In contrast, depletion of ARF1 has no significant effect. In order to delineate the underlying mechanism, we examined the signaling events activated in endothelial cells following ET-1 stimulation. Here, we show that this hormone promotes the phosphorylation of focal adhesion kinase (FAK), Erk1/2, and the association of FAK to Src, as well as of FAK to GIT1. These have been shown to be important for the formation and turnover of focal adhesions. In non-stimulated cells, depletion of ARF6 leads to increased FAK and Erk1/2 phosphorylation, similar to what is observed in ET-1 treated cells. In these conditions, FAK is found constitutively associated with the soluble tyrosine kinase, Src. In contrast, depletion of ARF6 impairs the ability of GIT1 to form an agonist promoted complex with FAK, thereby preventing disassembly of focal adhesions. As a consequence, ARF6 depleted endothelial cells are impaired in their ability to form capillary tubes. Taken together, our data suggest that ARF6 is central in regulating focal adhesion turnover in endothelial cells. Our study provides a molecular mechanism by which, this small GTPase regulates cell motility, and ultimately angiogenesis.

Prolonged G(q) activity triggers fly rhodopsin endocytosis and degradation, and reduces photoreceptor sensitivity

Rapid deactivation of the Drosophila light receptor rhodopsin, through a visual arrestin Arr2 and a pathway that involves a transcription factor dCAMTA, is required for timely termination of light responses in the photoreceptor neuron. Here we report that this process is also critical for maintenance of the photoreceptor sensitivity. In both dCAMTA- and arr2-mutant flies, the endocytosis of the major rhodopsin Rh1 was dramatically increased, which was mediated by a G(q) protein that signals downstream of rhodopsin in the visual transduction pathway. Consequently, the Rh1 level was downregulated and the photoreceptor became less sensitive to light. Remarkably, the G(q)-stimulated Rh1 endocytosis does not require phospholipase C, a known effector of G(q), but depends on a tetraspanin protein. Our work has identified an arrestin-independent endocytic pathway of G protein-coupled receptor in the fly. This pathway may also function in mammals and mediate an early feedback regulation of receptor signaling.

The N-terminal coiled-coil domain of the cytohesin/ARNO family of guanine nucleotide exchange factors interacts with Galphaq

Cytohesins are guanine-nucleotide exchange factors (GEF) for the Arf family of GTPases. One member of the Arf family, ARF6, plays an active role in the intracellular trafficking of G protein-coupled receptors. We have previously reported that Galphaq signaling leads to the activation of ARF6, possibly through a direct interaction with cytohesin-2/ARNO. Here, we report that Galphaq can directly interact with cytohesin-1, another Arf-GEF of the ARNO/cytohesin family. Cytohesin-1 preferentially associated with a constitutively active mutant of Galphaq (Galphaq-Q209L) compared to wild-type Galphaq in HEK293 cells. Stimulation of TPbeta, a Galphaq-coupled receptor, to activate Galphaq resulted in the promotion of a protein complex between Galphaq and cytohesin-1. Confocal immunofluorescence microscopy revealed that wild-type Galphaq and cytohesin-1 co-localized in intracellular compartments and at or near the plasma membrane. In contrast, expression of Galphaq-Q209L induced a drastic increase in the localization of cytohesin-1 at the plasma membrane. Expression of a dominant-negative mutant of cytohesin-1 reduced by 40% the agonist-induced internalization of TPbeta, a process that we previously demonstrated to be dependent on Galphaq-mediated signaling and Arf6 activation. Using deletion mutants, we show that cytohesin-1 interacts with Galphaq through its N-terminal coiled-coil domain. Cytohesin-1 and cytohesin-2/ARNO mutants lacking the coiled-coil domain were unable to relay Galphaq-mediated activation of Arf6. This is the first report of an interaction between the coiled-coil domain of the cytohesin/ARNO family of Arf-GEFs and a member of the heterotrimeric G proteins.