ADP Ribosylation Factor 6 Promotes Contraction and Proliferation, Suppresses Apoptosis and Is Specifically Inhibited by NAV2729 in Prostate Stromal Cells.

The presumed ADP ribosylation factor (ARF) 6 inhibitor NAV2729 inhibits human prostate smooth muscle contraction and proliferation of stromal cells, which are driving factors of voiding symptoms in benign prostatic hyperplasia (BPH). However, its specificity and a confirmed role of ARF6 for smooth muscle contraction are still pending. Here, we generated monoclonal ARF6 knockouts in human prostate stromal cells (WPMY-1), and characterized phenotypes of contractility, growth-related functions, and susceptibility to NAV2729 in knockout and control clones. ARF6 knockout was verified by Western blot. Knockout clones showed impaired contraction and actin organization, reduced proliferation and viability, and increased apoptosis and cell death. In ARF6-expressing control clones, NAV2729 (5 µM) strongly inhibited contraction (67% inhibition across all three control clones), actin organization (72%), proliferation (97%), and viability (up to 82%), and increased apoptosis (5-fold) and cell death (6-fold). In ARF6 knockouts, effects of NAV2729 (5 µM) were widely reduced, including lacking or minor effects on contractions (0% inhibition across all three knockout clones), actin (18%) and proliferation (13%), and lacking increases of apoptosis and cell death. Viability was reduced by NAV2729 with an IC50 of 3.3 µM across all three ARF6 control clones, but of 4.5-8.2 µM in ARF6 knockouts. In conclusion, ARF6 promotes prostate smooth muscle contraction and proliferation of stromal cells. Both are inhibited by NAV2729, which showed high specificity for ARF6 up to 5 µM and represents an attractive compound in the context of BPH. Considering the relevance of smooth muscle-based diseases, shared roles of ARF6 in other smooth muscle types merit further investigation. SIGNIFICANCE STATEMENT: By knockout of ARF6 in prostate stromal cells, this study demonstrates the involvement of ARF6 in promotion of prostate smooth muscle contraction and stromal growth, and defines concentration ranges for their ARF6-specific inhibition by NAV2729. Besides the context of benign prostatic hyperplasia and lower urinary tract symptoms, analog ARF6 functions in contraction and growth appear possible in other smooth muscle-rich organs, which merits further attention considering the high clinical relevance of smooth muscle-based diseases.

CLIC4/Arf6 Pathway.

RATIONALE: Increased expression of CLIC4 (chloride intracellular channel 4) is a feature of endothelial dysfunction in pulmonary arterial hypertension, but its role in disease pathology is not fully understood. OBJECTIVE: To identify CLIC4 effectors and evaluate strategies targeting CLIC4 signaling in pulmonary hypertension. METHODS AND RESULTS: Proteomic analysis of CLIC4-interacting proteins in human pulmonary artery endothelial cells identified regulators of endosomal trafficking, including Arf6 (ADP ribosylation factor 6) GTPase activating proteins and clathrin, while CLIC4 overexpression affected protein regulators of vesicular trafficking, lysosomal function, and inflammation. CLIC4 reduced BMPRII (bone morphogenetic protein receptor II) expression and signaling as a result of Arf6-mediated reduction in gyrating clathrin and increased lysosomal targeting of the receptor. BMPRII expression was restored by Arf6 siRNA, Arf inhibitor Sec7 inhibitor H3 (SecinH3), and inhibitors of clathrin-mediated endocytosis but was unaffected by chloride channel inhibitor, indanyloxyacetic acid 94 or Arf1 siRNA. The effects of CLIC4 on NF-κB (nuclear factor-kappa B), HIF (hypoxia-inducible factor), and angiogenic response were prevented by Arf6 siRNA and SecinH3. Sugen/hypoxia mice and monocrotaline rats showed elevated expression of CLIC4, activation of Arf6 and NF-κB, and reduced expression of BMPRII in the lung. These changes were established early during disease development. Lung endothelium-targeted delivery of CLIC4 siRNA or treatment with SecinH3 attenuated the disease, reduced CLIC4/Arf activation, and restored BMPRII expression in the lung. Endothelial colony-forming cells from idiopathic pulmonary hypertensive patients showed upregulation of CLIC4 expression and Arf6 activity, suggesting potential importance of this pathway in the human condition. CONCLUSIONS: Arf6 is a novel effector of CLIC4 and a new therapeutic target in pulmonary hypertension.

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.

Sphingolipids inhibit endosomal recycling of nutrient transporters by inactivating ARF6.

Endogenous sphingolipids (ceramide) and related synthetic molecules (FTY720, SH-BC-893) reduce nutrient access by decreasing cell surface expression of a subset of nutrient transporter proteins. Here, we report that these sphingolipids disrupt endocytic recycling by inactivating the small GTPase ARF6. Consistent with reported roles for ARF6 in maintaining the tubular recycling endosome, MICAL-L1-positive tubules were lost from sphingolipid-treated cells. We propose that ARF6 inactivation may occur downstream of PP2A activation since: (1) sphingolipids that fail to activate PP2A did not reduce ARF6-GTP levels; (2) a structurally unrelated PP2A activator disrupted tubular recycling endosome morphology and transporter localization; and (3) overexpression of a phosphomimetic mutant of the ARF6 GEF GRP1 prevented nutrient transporter loss. ARF6 inhibition alone was not toxic; however, the ARF6 inhibitors SecinH3 and NAV2729 dramatically enhanced the killing of cancer cells by SH-BC-893 without increasing toxicity to peripheral blood mononuclear cells, suggesting that ARF6 inactivation contributes to the anti-neoplastic actions of sphingolipids. Taken together, these studies provide mechanistic insight into how ceramide and sphingolipid-like molecules limit nutrient access and suppress tumor cell growth and survival.

Dissecting the localization of lipopolysaccharide-responsive and beige-like anchor protein (LRBA) in the endomembrane system.

INTRODUCTION AND OBJECTIVES: LRBA deficiency is caused by loss of LRBA protein expression, due to either homozygous or compounds heterozygous mutations in LRBA. LRBA deficiency has been shown to affect vesicular trafficking and autophagy. To date, LRBA has been observed in the cytosol, Golgi apparatus and some lysosomes in LPS-stimulated murine macrophages. The objectives of the present study were to study the LRBA localization in organelles involved in vesicular traffic, phagocytosis, and autophagy in mononuclear phagocytes (MP). MATERIALS AND METHODS: We analyzed LRBA colocalization with different endosomes markets using confocal microscopy in MP. We used the autophagy inhibitors to determine the role of LRBA in formation, maturation or degradation of the autophagosome. RESULTS: LRBA intracellular trafficking depends on the activity of the GTPase ADP ribosylation factor-1 (ARF) in MP. LRBA was identified in early, late endosomes but did not colocalize strongly with lysosomal markers. Although LRBA appears not to be recruited during the phagocytic cargo uptake, it greatly colocalized with the microtubule-associated protein 1A/1B-light chain 3 (LC3) under a steady state and this decreased after the induction of autophagy flux. Although the use of inhibitors of lysosome fusion did not restore the LRBA/LC3 colocalization, inhibitors of either early to late endosomes trafficking or PI3K pathway did. CONCLUSIONS: Taken together, our results show that LRBA is located in endomembrane system vesicles, mainly in the early and late endosomes. Although LRBA appears not to be involved in the phagocytic uptake, it is recruited in the early steps of the autophagy flux.

Commensal microbiota-induced microRNA modulates intestinal epithelial permeability through the small GTPase ARF4.

The intestinal tract contains many commensal bacteria that modulate various physiological host functions. Dysbiosis of commensal bacteria triggers dysfunction of the intestinal epithelial barrier, leading to the induction or aggravation of intestinal inflammation. To elucidate whether microRNA plays a role in commensal microbiome-dependent intestinal epithelial barrier regulation, we compared transcripts in intestinal epithelial cells (IECs) from conventional and germ-free mice and found that commensal bacteria induced the expression of miR-21-5p in IECs. miR-21-5p increased intestinal epithelial permeability and up-regulated ADP ribosylation factor 4 (ARF4), a small GTPase, in the IEC line Caco-2. We also found that ARF4 expression was up-regulated upon suppression of phosphatase and tensin homolog (PTEN) and programmed cell death 4 (PDCD4), which are known miR-21-5p targets, by RNAi. Furthermore, ARF4 expression in epithelial cells of the large intestine was higher in conventional mice than in germ-free mice. ARF4 suppression in the IEC line increased the expression of tight junction proteins and decreased intestinal epithelial permeability. These results indicate that commensal microbiome-dependent miR-21-5p expression in IECs regulates intestinal epithelial permeability via ARF4, which may therefore represent a target for preventing or managing dysfunction of the intestinal epithelial barrier.

Rab35 protein regulates evoked exocytosis of endothelial Weibel-Palade bodies.

Weibel-Palade bodies (WPB) are secretory organelles of endothelial cells that undergo evoked exocytosis following intracellular Ca2+ or cAMP elevation, thereby supplying the vasculature with factors controlling hemostasis. Several cytosolic and membrane-associated proteins, including the Rab family members Rab3, Rab15, and Rab27a, have been implicated in regulating the acute exocytosis of WPB. Here, we carried out a genome-wide screen to identify Rab pathways affecting WPB exocytosis. Overexpression of a specific subset of Rab GTPase-activating proteins (RabGAPs) inhibited histamine-evoked, Ca2+-dependent WPB exocytosis, presumably by inactivating the target Rab GTPases. Among these RabGAPs, we concentrated on TBC1D10A and showed that the inhibitory effect depends on its GAP activity. We confirmed that Rab35 was a target Rab of TBC1D10A in human endothelial cells; Rab35 interacted with TBC1D10A, and expression of the GAP-insensitive Rab35(Q67A) mutant rescued the inhibitory effect of TBC1D10A overexpression on WPB exocytosis. Furthermore, knockdown of Rab35 and expression of a dominant-negative Rab35 mutant both inhibited histamine-evoked secretion of the WPB cargos von Willebrand factor and P-selectin. Pulldown and co-immunoprecipitation experiments identified the ArfGAP with coiled-coil, Ank repeat, and pleckstrin homology domain-containing protein ACAP2 as an Rab35 effector in endothelial cells, and depletion as well as overexpression approaches revealed that ACAP2 acts as a negative regulator of WPB exocytosis. Interestingly, a known ACAP2 target, the small GTPase Arf6, supported histamine-evoked WPB exocytosis, as shown by knockdown and overexpression of a dominant-negative Arf6 mutant. Our data identify Rab35 as a novel regulator of WPB exocytosis, most likely acting through the downstream effectors ACAP2 and Arf6.

MiR-221-3p targets ARF4 and inhibits the proliferation and migration of epithelial ovarian cancer cells.

Epithelial ovarian cancer (EOC) is the most lethal gynecologic cancer. Although molecular diagnostic tools and targeted therapies have been developed over the past few decades, the survival rate is still rather low. Numerous researches suggest that some microRNAs (miRNAs) are key regulators of tumor progression. Among those miRNAs that has attracted much attention for their multiple roles in human cancers, the function of miR-221-3p in EOC has not been elucidated. Herein, we examined the expression of miR-221-3p in EOC patients and cell lines. Our data revealed that higher expression of miR-221-3p was linked to better overall survival in EOC patients. In-vitro experiments indicated that miR-221-3p inhibited EOC cell proliferation and migration. By performing subsequent systematic molecular biological and bioinformatic analyses, we found ADP-ribosylation factor (ARF) 4 is one of the putative target genes, the direct binding relationship was further confirmed by dual-luciferase reporter assay. Finally, a distinct gene expression between miR-221-3p and ARF4 in EOC group and normal group was identified, and the negative correlation between their expression levels in EOC specimens was further confirmed. Taken together, our research uncovered the tumor suppressive role of miR-221-3p in EOC and directly targeted ARF4, suggesting that miR-221-3p might be a novel potential candidate for clinical prognosis and therapeutics of EOC.

Deficiency of primary cilia in kidney epithelial cells induces epithelial to mesenchymal transition.

Primary cilium is a microtubule-based non-motile organelle that plays critical roles in kidney pathophysiology. Our previous studies revealed that the lengths of primary cilia decreased upon renal ischemia/reperfusion injury and oxidative stress, and restored with recovery. Here, we tested the hypothesis that lack of primary cilium causes epithelial to mesenchymal transition (EMT) of kidney tubule cells. We investigated the alteration of length of primary cilia in TGF-ß-induced EMT via visualization of primary cilia by fluorescence staining against acetylated α-tubulin. EMT was determined by measuring mesenchymal protein expression using quantitative PCR and indirect fluorescence staining. As a result, TGF-ß treatment decreased ciliary length along with EMT. To test whether defect of primary cilia trigger onset of EMT, cilia formation was disturbed by knock down of ciliary protein using siRNA along with/without TGF-ß treatment. Knock down of Arl13b and Ift20 reduced cilia elongation and increased expression of EMT markers such as fibronectin, α-SMA, and collagen III. TGF-ß-induced EMT was greater as well in Arl13b and Ift20-knock down cells compared to control cells. Taken together, deficiency of primary cilia trigger EMT and exacerbates it under pro-fibrotic signals.

Identification of ARL4C as a Peritoneal Dissemination-Associated Gene and Its Clinical Significance in Gastric Cancer.

BACKGROUND: In gastric cancer (GC), peritoneal dissemination (PD) occurs frequently and is incurable. In this study, we aimed to identify PD-associated genes in GC. METHODS: We identified a PD-associated gene using three GC datasets: highly disseminated peritoneal GC cell lines, the Singapore dataset and The Cancer Genome Atlas (TCGA) dataset. We assessed the clinicopathological significance of the gene expression using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and performed immunohistochemical analysis for the gene in our patient cohort. We also performed survival analyses of the gene in our patient cohort, the Singapore dataset and the GSE62254 datasets. Moreover, gene set enrichment analysis (GSEA) was performed using the Singapore and TCGA datasets. Finally, in vitro experiments such as invasion/migration assays, immunofluorescence staining of actin filaments, epidermal growth factor (EGF) treatment analysis, and gene expression analysis were conducted using three gene-knockdown GC cell lines (AGS, 58As9, MKN45). RESULTS: ADP-ribosylation factor-like 4c (ARL4C) was identified as a PD-associated gene, and immunohistochemical analysis showed that ARL4C was overexpressed in GC cells. High ARL4C expression was associated with the depth of invasion (p < 0.01) and PD (p < 0.05) and was a poor prognostic factor (p < 0.05) in our patient cohort, the Singapore dataset and the GSE62254 dataset. ARL4C expression positively correlated with the epithelial-mesenchymal transition (EMT) gene set in GSEA. Moreover, ARL4C knockdown reduced invasion/migration capacity, SLUG expression, and the formation of lamellipodia or filopodia in AGS and 58As9 cells. Finally, EGF treatment increased ARL4C expression in MKN45 cells. CONCLUSIONS: ARL4C was associated with PD and was a poor prognostic factor in GC, possibly through promoting invasive capacity by activation of both EMT and motility.

Frequent overexpression of AMAP1, an Arf6 effector in cell invasion, is characteristic of the MMTV-PyMT rather than the MMTV-Neu human breast cancer model.

BACKGROUND: The small GTPase Arf6 and its downstream effector AMAP1 (also called ASAP1/DDEF1) constitute a signaling pathway promoting cell invasion, in which AMAP1 interacts with several different proteins, including PRKD2, EPB41L5, paxillin, and cortactin. Components of this pathway are often overexpressed in human breast cancer cells, to be correlated with poor prognosis of the patients, whereas overexpression of the Arf6 pathway did not correlate with the four main molecular classes of human breast tumors. In this pathway, receptor tyrosine kinases, including EGFR and Her2, activate Arf6 via GEP100. MMTV-PyMT mice and MMTV-Neu mice are well-established models of human breast cancer, and exhibit the early dissemination and the lung metastasis, by utilizing protein tyrosine phosphorylation for oncogenesis. PyMT-tumors and Neu-tumors are known to have overlapping gene expression profiles, which primarily correspond to the luminal B-type of human mammary tumors, although they differ in the time necessary for tumor onset and metastasis. Given the common usage of protein tyrosine phosphorylation, as well as the frequent use of these animal models for studying breast cancer at the molecular level, we here investigated whether mammary tumors in these mouse models utilize the Arf6-based pathway for invasion. METHODS: Expression levels of Arf6, AMAP1, and GEP100 were analyzed in PyMT-tumors and Neu-tumors by western blotting. Expression of Arf6 and AMAP1 was also analyzed by immunohistochemistry. The involvement of AMAP1 in invasion, and the possible correlation of its high expression levels with cancer mesenchymal properties were also investigated. RESULTS: We found that PyMT-tumors, but not Neu-tumors, frequently overexpress AMAP1 and use it for invasion, whereas both types of tumors expressed Arf6 and GEP100 at different levels. High levels of the AMAP1 expression among PyMT-tumor cells were frequently correlated with loss of the epithelial marker CK8 and also with expression of the mesenchymal marker vimentin both at the primary sites and at sites of the lung metastases. CONCLUSIONS: PyMT-tumors appear to frequently utilize the Arf6-based invasive machinery, whereas Neu-tumors do not. Our results suggest that MMTV-PyMT mice, rather than MMTV-Neu mice, are useful to study the Arf6-based mammary tumor malignancies, as a representative model of human breast cancer.

Family-wide Analysis of the Inhibition of Arf Guanine Nucleotide Exchange Factors with Small Molecules: Evidence of Unique Inhibitory Profiles.

Arf GTPases and their guanine nucleotide exchange factors (ArfGEFs) are major regulators of membrane traffic and organelle structure in cells. They are associated with a variety of diseases and are thus attractive therapeutic targets for inhibition by small molecules. Several inhibitors of unrelated chemical structures have been discovered, which have shown their potential in dissecting molecular pathways and blocking disease-related functions. However, their specificity across the ArfGEF family has remained elusive. Importantly, inhibitory responses in the context of membranes, which are critical determinants of Arf and ArfGEF cellular functions, have not been investigated. Here, we compare the efficiency and specificity of four structurally distinct ArfGEF inhibitors, Brefeldin A, SecinH3, M-COPA, and NAV-2729, toward six ArfGEFs (human ARNO, EFA6, BIG1, and BRAG2 and Legionella and Rickettsia RalF). Inhibition was assessed by fluorescence kinetics using pure proteins, and its modulation by membranes was determined with lipidated GTPases in the presence of liposomes. Our analysis shows that despite the intra-ArfGEF family resemblance, each inhibitor has a specific inhibitory profile. Notably, M-COPA is a potent pan-ArfGEF inhibitor, and NAV-2729 inhibits all GEFs, the strongest effects being against BRAG2 and Arf1. Furthermore, the presence of the membrane-binding domain in Legionella RalF reveals a strong inhibitory effect of BFA that is not measured on its GEF domain alone. This study demonstrates the value of family-wide assays with incorporation of membranes, and it should enable accurate dissection of Arf pathways by these inhibitors to best guide their use and development as therapeutic agents.

MHC class II presentation is controlled by the lysosomal small GTPase, Arl8b.

Dendritic cells (DCs) are specialized APCs with the ability to prime naive T cells. DCs first sample Ags from the environment and then orchestrate their processing and loading onto MHC class II (MHC II) Ag-presenting molecules in lysosomes. Once MHC II molecules have bound a peptide, the MHC II-peptide complex is delivered to the cell surface for presentation to CD4(+) T cells. Regulation of Ag uptake via macropinocytosis and phagocytosis has been extensively studied, as well as trafficking in early endocytic vesicles notably regulated by the small GTPase Rab5 and its effectors. However, little is known about the regulators of Ag delivery from early endosomes to lysosomal compartments where the proper pH, proteases, MHC II, invariant chain, and HLA-DM reside, awaiting exogenous Ags for loading. In this article, we report the crucial role of the small GTPase ADP-ribosylation factor-like 8b (Arl8b) in MHC II presentation in DCs. We show for the first time, to our knowledge, that Arl8b localizes to MHC II compartments in DCs and regulates formation of MHC II-peptide complexes. Arl8b-silenced DCs display a defect in MHC II-Ag complex formation and its delivery to the cell surface during infection resulting in a defect in T cell recognition. Our results highlight the role of Arl8b as a trafficking regulator of the late stage of complex formation and MHC II presentation in DCs.

GGA and Arf proteins modulate retrovirus assembly and release.

The Gag protein is the major structural determinant of retrovirus assembly. Although a number of cellular factors have been reported to facilitate retrovirus release, little is known about the cellular machinery that directs Gag to the site of virus assembly. Here, we report roles for the Golgi-localized gamma-ear containing Arf-binding (GGA) and ADP ribosylation factor (Arf) proteins in retrovirus particle assembly and release. Whereas siRNA-mediated depletion of GGA2 and GGA3 led to a significant increase in particle release in a late domain-dependent manner, GGA overexpression severely reduced retrovirus particle production by impairing Gag trafficking to the membrane. GGA overexpression inhibited retroviral assembly and release by disrupting Arf protein activity. Furthermore, disruption of endogenous Arf activity inhibited particle production by decreasing Gag-membrane binding. These findings identify the GGA proteins as modulators of HIV-1 release and the Arf proteins as critical cellular cofactors in retroviral Gag trafficking to the plasma membrane.

A comparative structure-function analysis of active-site inhibitors of Vibrio cholerae cholix toxin.

Cholix toxin from Vibrio cholerae is a novel mono-ADP-ribosyltransferase (mART) toxin that shares structural and functional properties with Pseudomonas aeruginosa exotoxin A and Corynebacterium diphtheriae diphtheria toxin. Herein, we have used the high-resolution X-ray structure of full-length cholix toxin in the apo form, NAD(+) bound, and 10 structures of the cholix catalytic domain (C-domain) complexed with several strong inhibitors of toxin enzyme activity (NAP, PJ34, and the P-series) to study the binding mode of the ligands. A pharmacophore model based on the active pose of NAD(+) was compared with the active conformation of the inhibitors, which revealed a cationic feature in the side chain of the inhibitors that may determine the active pose. Moreover, a conformational search was conducted for the missing coordinates of one of the main active-site loops (R-loop). The resulting structural models were used to evaluate the interaction energies and for 3D-QSAR modeling. Implications for a rational drug design approach for mART toxins were derived.

Nm23-H1 regulates glucose-stimulated insulin secretion in pancreatic ß-cells via Arf6-Rac1 signaling axis.

BACKGROUND: A growing body of evidence implicates novel roles for nm23-like proteins in the regulation of cellular functions. However, roles of these proteins in islet function and glucose-stimulated insulin secretion (GSIS) remain largely unknown. METHODS: siRNA-nm23-H1 and nucleoside diphosphate kinase and histidine kinase-deficient mutants of nm23-H1 (K12Q and H118F) were used to assess roles of nm23-H1 in GSIS. RESULTS: siRNA-mediated knockdown of the expression of nm23-H1 markedly inhibited GSIS in INS-1 832/13 cells. Nm23-H1 knockdown also resulted in significant inhibition of glucose-mediated activation of Arf6, a small G-protein, which has been implicated in GSIS. Expression of K12Q and H118F mutants of nm23-H1 in INS-1 832/13 cells led to inhibition of glucose-induced translocation and membrane association of Rac1, another small G-protein, which is downstream to Arf6 in the signaling events leading to GSIS. A significant inhibition of GSIS was also seen in these cells expressing K12Q and H118F. CONCLUSIONS: We conclude that the nm23-H1 activation step is upstream of Arf6 activation in signaling events leading to GSIS. NDP kinase and histidine kinase functions of nm23-H1 are necessary for glucose-induced membrane association of Rac1 and ensuing insulin secretion. We present the first evidence for regulation of GSIS by nm23-H1 in pancreatic ß-cells.

ADP-ribosylation factor-like 3, a manchette-associated protein, is essential for mouse spermiogenesis.

During spermiogenesis, the manchette is an important structure for sperm head and tail formation. However, the mechanisms responsible for this process are poorly understood. In a previous study, we established a comparative proteome profile for mouse testis during the first wave of spermatogenesis, and provided lists of proteins of potential importance in the regulation of male fertility and infertility. Here we have selected Arl3, one of these interesting proteins, and investigated its expression and function during spermiogenesis. Western blotting was used to determine Arl3 expression levels in mice at different time points from birth to adulthood. The results show Arl3 was expressed from birth, and the expression level increased significantly from Week 4, when mouse spermiogenesis begins. Immunohistochemistry and indirect immunofluorescence were used to investigate the Arl3 expression during sperm development, and the intracellular localization of Arl3 in more detail. In elongating spermatids from steps 8 to 15, Arl3 was localized to the posterior section of the head, in a similar pattern to the manchette. The Arl3 signal was colocalized during spermiogenesis with α-tubulin, a marker for the manchette. To investigate the possible functional role of Arl3, mouse testes were injected with small interfering RNA (siRNA) against Arl3, or control siRNA. Western blotting showed a 60% reduction in the Arl3 expression after 72 h, and a significant increase in sperm abnormalities after 3 weeks compared with the negative control. In conclusion, we propose that Arl3 is a novel manchette-related protein with an important role in spermiogenesis.

ADP ribosylation factor 1 activity is required to recruit AP-1 to the major histocompatibility complex class I (MHC-I) cytoplasmic tail and disrupt MHC-I trafficking in HIV-1-infected primary T cells.

HIV-1-infected cells are partially resistant to anti-HIV cytotoxic T lymphocytes (CTLs) due to the effects of the HIV Nef protein on antigen presentation by major histocompatibility complex class I (MHC-I), and evidence has been accumulating that this function of Nef is important in vivo. HIV Nef disrupts the normal expression of MHC-I by stabilizing a protein-protein interaction between the clathrin adaptor protein AP-1 and the MHC-I cytoplasmic tail. There is also evidence that Nef activates a phosphatidylinositol 3 kinase (PI3K)-dependent GTPase, ADP ribosylation factor 6 (ARF-6), to stimulate MHC-I internalization. However, the relative importance of these two pathways is unclear. Here we report that a GTPase required for AP-1 activity (ARF-1) was needed for Nef to disrupt MHC-I surface levels, whereas no significant requirement for ARF-6 was observed in Nef-expressing T cell lines and in HIV-infected primary T cells. An ARF-1 inhibitor blocked the ability of Nef to recruit AP-1 to the MHC-I cytoplasmic tail, and a dominant active ARF-1 mutant stabilized the Nef-MHC-I-AP-1 complex. These data support a model in which Nef and ARF-1 stabilize an interaction between MHC-I and AP-1 to disrupt the presentation of HIV-1 epitopes to CTLs.

ARFGAP2 and ARFGAP3 are essential for COPI coat assembly on the Golgi membrane of living cells.

Coat protein complex I (COPI) vesicles play a central role in the recycling of proteins in the early secretory pathway and transport of proteins within the Golgi stack. Vesicle formation is initiated by the exchange of GDP for GTP on ARF1 (ADP-ribosylation factor 1), which, in turn, recruits the coat protein coatomer to the membrane for selection of cargo and membrane deformation. ARFGAP1 (ARF1 GTPase-activating protein 1) regulates the dynamic cycling of ARF1 on the membrane that results in both cargo concentration and uncoating for the generation of a fusion-competent vesicle. Two human orthologues of the yeast ARFGAP Glo3p, termed ARFGAP2 and ARFGAP3, have been demonstrated to be present on COPI vesicles generated in vitro in the presence of guanosine 5'-3-O-(thio)triphosphate. Here, we investigate the function of these two proteins in living cells and compare it with that of ARFGAP1. We find that ARFGAP2 and ARFGAP3 follow the dynamic behavior of coatomer upon stimulation of vesicle budding in vivo more closely than does ARFGAP1. Electron microscopy of ARFGAP2 and ARFGAP3 knockdowns indicated Golgi unstacking and cisternal shortening similarly to conditions where vesicle uncoating was blocked. Furthermore, the knockdown of both ARFGAP2 and ARFGAP3 prevents proper assembly of the COPI coat lattice for which ARFGAP1 does not seem to play a major role. This suggests that ARFGAP2 and ARFGAP3 are key components of the COPI coat lattice and are necessary for proper vesicle formation.

Newly discovered and characterized antivirulence compounds inhibit bacterial mono-ADP-ribosyltransferase toxins.

The mono-ADP-ribosyltransferase toxins are bacterial virulence factors that contribute to many disease states in plants, animals, and humans. These toxins function as enzymes that target various host proteins and covalently attach an ADP-ribose moiety that alters target protein function. We tested compounds from a virtual screen of commercially available compounds combined with a directed poly(ADP-ribose) polymerase (PARP) inhibitor library and found several compounds that bind tightly and inhibit toxins from Pseudomonas aeruginosa and Vibrio cholerae. The most efficacious compounds completely protected human lung epithelial cells against the cytotoxicity of these bacterial virulence factors. Moreover, we determined high-resolution crystal structures of the best inhibitors in complex with cholix toxin to reveal important criteria for inhibitor binding and mechanism of action. These results provide new insight into development of antivirulence compounds for treating many bacterial diseases.