Fbx8 makes Arf6 refractory to function via ubiquitination

The ARF GTPase regulatory network in collective invasion and metastasis

The ability to remodel and move cellular membranes, and the cargoes regulated by these membranes, allows for specialised functions to occur in distinct regions of the cell in a process known as cellular polarisation. The ability to collectively co-ordinate such polarisation between cells allows for the genesis of multicellularity, such as the formation of organs. During tumourigenesis, the rules for such tissue polarisation become dysregulated, allowing for collective polarity rearrangements that can drive metastasis. In this review, we focus on how membrane trafficking underpins collective cell invasion and metastasis in cancer. We examine this through the lens of the ADP-ribosylation factor (ARF) subfamily of small GTPases, focusing on how the ARF regulatory network - ARF activators, inactivators, effectors, and modifications - controls ARF GTPase function.

FBX8 promotes metastatic dormancy of colorectal cancer in liver

Patients with colorectal cancer (CRC) often develop malignant regrowth of metastatic dormant tumor cells in liver years after primary treatment. FBX8 is involved in suppressing tumor metastasis. Short-term chemotherapy experiments and liver metastasis mice model of orthotopic injection into the cecum were performed to construct the dormant models. GST-pull-down assay, Co-IP and immunofluorescence were used to confirm the bindings among FBX8 and its substrates. FBX8 upregulated the expression of epithelial and stemness markers, while downregulated the expression of mesenchymal and proliferative markers associated with tumor cell dormancy. FBX8 promoted the maintenance of metastatic dormancy of CRC cells. Mechanistically, FBX8 directly bound to HIF-1α, CDK4 and C-myc through its Sec7 domain and led to the ubiquitin degradation of these proteins, thereby inhibiting cell cycle progression, proliferation, angiogenesis, and metastasis. Clinically, FBX8 expression was negatively correlated with the HIF-1α, CDK4, and c-Myc in CRC tissues. Our study reveals a novel mechanism of FBX8 in regulating tumor metastatic dormancy in liver and provides new strategies for the treatment of CRC metastasis.

Ancient complement and lineage-specific evolution of the Sec7 ARF GEF proteins in eukaryotes

Guanine nucleotide exchange factors (GEFs) are the initiators of signaling by every regulatory GTPase, which in turn act to regulate a wide array of essential cellular processes. To date, each family of GTPases is activated by distinct families of GEFs. Bidirectional membrane trafficking is regulated by ADP-ribosylation factor (ARF) GTPases and the development throughout eukaryotic evolution of increasingly complex systems of such traffic required the acquisition of a functionally diverse cohort of ARF GEFs to control it. We performed phylogenetic analyses of ARF GEFs in eukaryotes, defined by the presence of the Sec7 domain, and found three subfamilies (BIG, GBF1, and cytohesins) to have been present in the ancestor of all eukaryotes. The four other subfamilies (EFA6/PSD, IQSEC7/BRAG, FBX8, and TBS) are opisthokont, holozoan, metazoan, and alveolate/haptophyte specific, respectively, and each is derived from cytohesins. We also identified a cytohesin-derived subfamily, termed ankyrin repeat-containing cytohesin, that independently evolved in amoebozoans and members of the SAR and haptophyte clades. Building on evolutionary data for the ARF family GTPases and their GTPase--activating proteins allowed the generation of hypotheses about ARF GEF protein function(s) as well as a better understanding of the origins and evolution of cellular complexity in eukaryotes.

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.

FBX8 degrades GSTP1 through ubiquitination to suppress colorectal cancer progression

F-box only protein 8 (FBX8), as a critical component of the SKP1-CUL1-F-box (SCF) E3 ubiquitin ligases, has been associated with several malignancies through interacting with a member of proteins. However, the substrates of FBX8 for destruction in the progression of colorectal carcinoma (CRC) need to be explored. Here, we show that loss of FBX8 accelerates chemical-induced colon tumorigenesis. FBX8 directly targets GSTP1 for ubiquitin-mediated proteasome degradation in CRC. GSTP1 promotes the proliferation, invasion, and metastasis of CRC cells. Furthermore, GSTP1 is upregulated in CRC tissue samples and predicts poor prognosis of CRC patients. The inactivation of FBX8 negatively correlated with increased levels and stability of GSTP1 in clinical CRC tissues and FBX8 knockout transgenic mice. These findings identify a novel ubiquitination pathway as FBX8-GSTP1 axis that regulates the progression of CRC, which might be a potential prognostic biomarker for CRC patients.

Elevated Na+/H+ exchanger-1 expression enhances the metastatic collective migration of head and neck squamous cell carcinoma cells

Cancer cells can migrate as collectives during invasion and/or metastasis; however, the precise molecular mechanisms of this form of migration are less clear compared with single cell migration following epithelial-mesenchymal transition. Elevated Na⁺/H⁺ exchanger1 (NHE1) expression has been suggested to have malignant roles in a number of cancer cell lines and in vivo tumor models. Furthermore, a metastatic human head and neck squamous cell carcinoma (HNSCC) cell line (SASL1m) that was isolated based on its increased metastatic potential also exhibited higher NHE1 expression than its parental line SAS. Time-lapse video recordings indicated that both cell lines migrate as collectives, although with different features, e.g., SASL1m was much more active and changed the direction of migration more frequently than SAS cells, whereas locomotive activities were comparable. SASL1m cells also exhibited higher invasive activity than SAS in Matrigel invasion assays. shRNA-mediated NHE1 knockdown in SASL1m led to reduced locomotive and invasive activities, suggesting a critical role for NHE1 in the collective migration of SASL1m cells. SASL1m cells also exhibited a higher metastatic rate than SAS cells in a mouse lymph node metastasis model, while NHE1 knockdown suppressed in vivo SASL1m metastasis. Finally, elevated NHE1 expression was observed in human HNSCC tissue, and Cariporide, a specific NHE1 inhibitor, reduced the invasive activity of SASL1m cells, implying NHE1 could be a target for anti-invasion/metastasis therapy.

FBX8 is a metastasis suppressor downstream of miR-223 and targeting mTOR for degradation in colorectal carcinoma

F-box proteins are critical components of the SKP1-CUL1-F-box (SCF) E3 ubiquitin ligases and involved in the ubiquitin-dependent proteolytic pathway. Dysregulation of F-box protein-mediated proteolysis often leads to human malignancies. F-box only protein 8 (FBX8), a novel component of F-box proteins, is down-regulated in several tumors and closely correlates with tumor progression. However, little is known about its function, regulatory mechanisms and substrates in the progression of colorectal carcinoma (CRC). Combining microRNA (miRNA) assay, functional characterization, mechanistic studies with clinical validation, we identify FBX8 as a CRC metastasis suppressor downstream of miR-223, a metastasis promoting miRNA that is transcriptionally regulated by Myocyte enhancer factor (MEF2A). mTOR is a substrate of FBX8 for ubiquitin-mediated degradation and is required for FBX8 induced cell proliferation and invasion in CRC cells. FBX8 is down-regulated in human CRC tissues and correlates with MEF2A, miR-223 and mTOR expression levels. Notably, low FBX8 expression status in CRC tissues was a significant prognostic factor for poor overall survival of patients. These findings illustrate FBX8 as a metastasis suppressor that functions through mTOR signaling pathway and has significant prognostic power.

Regulators and Effectors of Arf GTPases in Neutrophils

Polymorphonuclear neutrophils (PMNs) are key innate immune cells that represent the first line of defence against infection. They are the first leukocytes to migrate from the blood to injured or infected sites. This process involves molecular mechanisms that coordinate cell polarization, delivery of receptors, and activation of integrins at the leading edge of migrating PMNs. These phagocytes actively engulf microorganisms or form neutrophil extracellular traps (NETs) to trap and kill pathogens with bactericidal compounds. Association of the NADPH oxidase complex at the phagosomal membrane for production of reactive oxygen species (ROS) and delivery of proteolytic enzymes into the phagosome initiate pathogen killing and removal. G protein-dependent signalling pathways tightly control PMN functions. In this review, we will focus on the small monomeric GTPases of the Arf family and their guanine exchange factors (GEFs) and GTPase activating proteins (GAPs) as components of signalling cascades regulating PMN responses. GEFs and GAPs are multidomain proteins that control cellular events in time and space through interaction with other proteins and lipids inside the cells. The number of Arf GAPs identified in PMNs is expanding, and dissecting their functions will provide important insights into the role of these proteins in PMN physiology.

The role of cullin proteins in gastric cancer

The cullin proteins are a family of scaffolding proteins that associate with RING proteins and ubiquitin E3 ligases and mediate substrate-receptor bindings. Thus, cullin proteins regulate the specificity of ubiquitin targeting in the regulation of proteins involved in various cellular processes, including proliferation, differentiation, and apoptosis. There are seven cullin proteins that have been identified in eukaryotes: CUL1, CUL2, CUL3, CUL4A, CUL4B, CUL5, and CUL7/p53-associated parkin-like cytoplasmic protein. All of these proteins contain a conserved cullin homology domain that binds to RING box proteins. Cullin-RING ubiquitin ligase complexes are activated upon post-translational modification by neural precursor cell-expressed, developmentally downregulated protein 8. The aberrant expression of several cullin proteins has been implicated in many cancers though the significance in gastric cancer has been less well investigated. This review provides the first systematic discussion of the associations between all members of the cullin protein family and gastric cancer. Functional and regulatory mechanisms of cullin proteins in gastric carcinoma progression are also summarized along with a discussion concerning future research areas. Accumulating evidence suggests a critical role of cullin proteins in tumorigenesis, and a better understanding of the function of these individual cullin proteins and their targets will help identify potential biomarkers and therapeutic targets.

F-box protein interactions with the hallmark pathways in cancer

F-box proteins (FBP) are the substrate specifying subunit of Skp1-Cul1-FBP (SCF)-type E3 ubiquitin ligases and are responsible for directing the ubiquitination of numerous proteins essential for cellular function. Due to their ability to regulate the expression and activity of oncogenes and tumour suppressor genes, FBPs themselves play important roles in cancer development and progression. In this review, we provide a comprehensive overview of FBPs and their targets in relation to their interaction with the hallmarks of cancer cell biology, including the regulation of proliferation, epigenetics, migration and invasion, metabolism, angiogenesis, cell death and DNA damage responses. Each cancer hallmark is revealed to have multiple FBPs which converge on common signalling hubs or response pathways. We also highlight the complex regulatory interplay between SCF-type ligases and other ubiquitin ligases. We suggest six highly interconnected FBPs affecting multiple cancer hallmarks, which may prove sensible candidates for therapeutic intervention.

Significance of FBX8 in progression of gastric cancer

F-box only protein 8 (FBX8), a novel component of F-box proteins, has recently been observed in several malignancies. However, its clinical implication in the progression of gastric cancer still remains unclear. The aim of this study was to explore the role of FBX8 in gastric cancer (GC) and analyze its correlation with tumor progression and prognosis. The expression of FBX8 in GC cell lines and matched pairs of fresh gastric cancer tissues were detected by real-time RT-PCR and Western blotting. Immunohistochemistry was used to analyze clinicopathological patterns of FBX8 in 136 cases of clinical paraffin-embedded GC tissues. A series of functional assays were conducted to evaluate the effect of FBX8 on proliferation and invasion in vitro and metastasis in vivo. FBX8 was markedly down-regulated in GC tissues compared to adjacent normal tissues. Patients with low FBX8 had shorter overall survival time and poor prognosis. Knocking down FBX8 obviously promoted proliferation and invasion in BGC823 cells, while over-expression of FBX8 in SGC7901 and AGS cells had the opposite effects. Moreover, FBX8 was sufficient to suppress metastasis in nude mice. Down-regulation of FBX8 significantly correlates with invasion, metastasis and poor survival time in GC patients. FBX8 may serve as a promising therapeutic target for inhibition of GC metastasis.

Down-expression of F box only protein 8 correlates with tumor grade and poor prognosis in human glioma

F box only protein 8 (FBX8) is a novel component of F-box proteins which involved in the ubiquitin-dependent proteolytic pathway. Recent studies have revealed that FBX8 was unregulated in tumor cells and was closely associated with tumor progression and metastasis of other cancer, but little research has been done yet to test its usefulness as a prognostic marker in human glioma. In the present study, we investigated the expression of FBX8 in glioma tissues using immunohistochemical analysis and evaluated its prognostic significance in glioma. We found that 44/77 (57.14%) gilomas had positive expression of FBX8, while 65/77 (84.42%) normal brain tissue had positive expression of FBX8. The expression level of FBX8 was remarkably down-regulated in glioma tissues compared with normal brain tissues (P < 0.001). The down-expression of FBX8 in tumor cells was strongly correlated with tumor grade of patients with glioma (P < 0.05). Patients with lower expression of FBX8 protein had shorter overall survival time than those with higher level expression of FBX8 (P < 0.05). Multivariate analysis showed that FBX8 down-expression was an independent prognostic indicator for glioma patient's survival. Our results suggest that a potential application of FBX8 in prognosis prediction and therapeutic application in glioma.

A CULLINary ride across the secretory pathway: more than just secretion

Mulitmeric cullin-RING ubiquitin ligases (CRLs) represent the largest class of ubiquitin ligases in eukaryotes. However, most CRL ubiquitylation pathways remain uncharacterized. CRLs control a myriad of functions by catalyzing mono- or poly-ubiquitylation of target proteins. Recently, novel CRLs have been identified along the secretory pathway where they modify substrates involved in diverse cellular processes such as vesicle coat assembly and cell cycle progression. This review discusses our current understanding of CRL ubiquitylation within the secretory pathway, with special emphasis on the emerging role of the Golgi as a ubiquitylation platform. CRLs are also implicated in endosome function, where their specific roles are less well understood.

Deubiquitinating enzymes in oocyte maturation, fertilization and preimplantation embryo development

Post-translational modifications of cellular proteins by ubiquitin and ubiquitin-like protein modifiers are important regulatory events involved in diverse aspects of gamete and embryo physiology including oocyte maturation, fertilization and development of embryos to term. Deubiquitinating enzymes (DUBs) regulate proteolysis by reversing ubiquitination, which targets proteins to the 26S proteasome. The ubiquitin C-terminal hydrolases (UCHs) comprise are DUBs that play a role in the removal of multi-ubiquitin chains. We review here the roles of UCHs in oocytes maturation, fertilization and development in mouse, bovine, porcine and rhesus monkeys. Oocyte UCHs contributes to fertilization and embryogenesis by regulating the physiology of the oocyte and blastomere cortex as well as oocyte spindle. Lack of UCHs in embryos reduces fertilization, while mutant embryos fail to undergo compaction and blastocyst formation. In addition to advancing our understanding of reproductive process, research on the role of deubiquitinating enzymes will allow us to better understand and treat human infertility, and to optimize reproductive performance in agriculturally important livestock species.

FBX8 Acts as an Invasion and Metastasis Suppressor and Correlates with Poor Survival in Hepatocellular Carcinoma

Background

F-box only protein 8 (FBX8), a novel component of F-box proteins, is lost in several cancers and has been associated with invasiveness of cancer cells. However, its expression pattern and role in the progression of hepatocellular carcinoma remain unclear. This study investigated the prognostic significance of FBX8 in hepatocellular carcinoma samples and analyzed FBX8 function in hepatocellular carcinoma cells by gene manipulation.

Methodology

The expression of FBX8 was detected in 120 cases of clinical paraffin-embedded hepatocellular carcinoma tissues, 20 matched pairs of fresh tissues and five hepatocellular carcinoma cell lines by immunohistochemistry with clinicopathological analyses, real-time RT-PCR or Western blot. The correlation of FBX8 expression with cell proliferation and invasion in five HCC cell lines was analyzed. Moreover, loss of function and gain of function assays were performed to evaluate the effect of FBX8 on cell proliferation, motility, invasion in vitro and metastasis in vivo.

Conclusions

We found that FBX8 was obviously down-regulated in HCC tissues and cell lines (P<0.05). The FBX8 down-regulation correlated significantly with poor prognosis, and FBX8 status was identified as an independent significant prognostic factor. Over-expression of FBX8 decreased proliferation, migration and invasion in HepG2 and 97H cells, while knock-down of FBX8 in 7721 cells showed the opposite effect. FBX8 negatively correlated with cell proliferation and invasion in 7701, M3, HepG2 and 97H cell lines. In vivo functional assays showed FBX8 suppressed tumor growth and pulmonary metastatic potential in mice. Our results indicate that down-regulation of FBX8 significantly correlates with invasion, metastasis and poor survival in hepatocellular carcinoma patients. It may be a useful biomarker for therapeutic strategy and control in hepatocellular carcinoma treatment.

GBF1 bears a novel phosphatidylinositol-phosphate binding module, BP3K, to link PI3Kγ activity with Arf1 activation involved in GPCR-mediated neutrophil chemotaxis and superoxide production

In neutrophils, Arf1 is activated upon GPCR stimulation. GBF1, a GEF for Arf, is primarily responsible for Arf1 activation upon GPCR stimulation and is important for chemotaxis and superoxide production. GBF1 also binds to products of PI3Kγ . The results indicate a novel mechanism that links PI3Kγ with chemotaxis and superoxide production.

Most chemoattractants for neutrophils bind to the Gα_i family of heterotrimeric G protein–coupled receptors (GPCRs) and release Gβγ subunits to activate chemotaxis and superoxide production. GIT2, a GTPase-activating protein for Arf1, forms a complex with Gβγ and is integral for directional sensing and suppression of superoxide production. Here we show that GBF1, a guanine nucleotide exchanging factor for Arf-GTPases, is primarily responsible for Arf1 activation upon GPCR stimulation and is important for neutrophil chemotaxis and superoxide production. We find that GBF1 bears a novel module, namely binding to products of phosphatidyl inositol 3-kinase (PI3K), which binds to products of PI3Kγ. Through this binding, GBF1 is translocated from the Golgi to the leading edge upon GPCR stimulation to activate Arf1 and recruit p22phox and GIT2 to the leading edge. Moreover, GBF1-mediated Arf1 activation is necessary to unify cell polarity during chemotaxis. Our results identify a novel mechanism that links PI3Kγ activity with chemotaxis and superoxide production in GPCR signaling.

Essential role of ubiquitin C-terminal hydrolases UCHL1 and UCHL3 in mammalian oocyte maturation

Ubiquitin C-terminal hydrolases (UCHs) comprise a family of deubiquitinating enzymes that play a role in the removal of multi-ubiquitin chains from proteins that are posttranslationally modified by ubiquitination to be targeted for proteolysis by the 26S proteasome. The UCH-enzymes also generate free monomeric ubiquitin from precursor multi-ubiquitin chains and, in some instances, may rescue ubiquitinated proteins from degradation. This study examined the roles of two oocyte-expressed UCHs, UCHL1, and UCHL3 in murine and rhesus monkey oocyte maturation. The Uchl1 and Uchl3 mRNAs were highly expressed in GV and MII oocytes, and were associated with the oocyte cortex (UCHL1) and meiotic spindle (UCHL3). Microinjection of the UCH-family enzyme inhibitor, ubiquitin-aldehyde (UBAL) to GV oocytes prevented oocyte meiotic progression beyond metaphase I in a majority of treated oocytes and caused spindle and first polar body anomalies. Injection of antibodies against UCHL3 disrupted oocyte maturation and caused meiotic anomalies, including abnormally long meiotic spindles. A selective, cell permeant inhibitor of UCHL3, 4, 5, 6, 7-tetrachloroidan-1, 3-dione also caused meiotic defects and chromosome misalignment. Cortical granule localization in the oocyte cortex was disrupted by UBAL injected after oocyte maturation. We conclude that the activity of oocyte UCHs contributes to oocyte maturation by regulating the oocyte cortex and meiotic spindle.

ARF family G proteins and their regulators: roles in membrane transport, development and disease

The ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF proteins, ARF-like (ARL) proteins and SAR1, regulates membrane traffic and organelle structure, and each family member is regulated through a cycle of GTP binding and GTP hydrolysis, which activate and inactivate, respectively, the G protein.

Traditionally, ARFs have been characterized for their immediate effects in the recruitment of coat proteins to drive cargo sorting, the recruitment of enzymes that can alter membrane lipid composition and the regulation of cytoskeletal factors. Now, new roles for ARFs have been discovered at the Golgi complex, for example in driving lipid transport. ARL proteins are also being increasingly linked to coordination of trafficking with cytoskeletal processes, for example during ciliogenesis.

There is particular interest in the mechanisms that control recruitment of the ARF guanine nucleotide exchange factors (GEFs) that mediate GTP binding to ARFs and, in the case of the cytohesin (also known as ARNO) GEF, membrane recruitment is coupled to relief of autoinhibition. GEFs such as cytohesin may also participate in a cascade of activation between particular pairs of ARFs.

Traditionally, G protein signalling has been viewed as a linear pathway, with the GDP-bound form of an ARF protein being inactive; however, more recent studies have highlighted novel roles for these GDP-bound forms and have also shown that GEFs and GTPase-activating proteins (GAPs) themselves can engage in distinct signalling responses through scaffolding functions.

The ADP-ribosylation factor (ARF) and ARF-like (ARL) family of G proteins, which are known to regulate membrane traffic and organelle structure, are emerging as regulators of diverse processes, including lipid and cytoskeletal transport. Although traditionally viewed as part of a linear signalling pathway, ARFs and their regulators must now be considered to exist within functional networks, in which both the 'inactive' ARF and the regulators themselves can mediate distinct effects.

Members of the ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF-like (ARL) proteins and SAR1, regulate membrane traffic and organelle structure by recruiting cargo-sorting coat proteins, modulating membrane lipid composition, and interacting with regulators of other G proteins. New roles of ARF and ARL proteins are emerging, including novel functions at the Golgi complex and in cilia formation. Their function is under tight spatial control, which is mediated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that catalyse GTP exchange and hydrolysis, respectively. Important advances are being gained in our understanding of the functional networks that are formed not only by the GEFs and GAPs themselves but also by the inactive forms of the ARF proteins.

Ras superfamily GEFs and GAPs: validated and tractable targets for cancer therapy?

There is now considerable and increasing evidence for a causal role for aberrant activity of the Ras superfamily of small GTPases in human cancers. These GTPases function as GDP-GTP-regulated binary switches that control many fundamental cellular processes. A common mechanism of GTPase deregulation in cancer is the deregulated expression and/or activity of their regulatory proteins, guanine nucleotide exchange factors (GEFs) that promote formation of the active GTP-bound state and GTPase-activating proteins (GAPs) that return the GTPase to its GDP-bound inactive state. In this Review, we assess the association of GEFs and GAPs with cancer and their druggability for cancer therapeutics.

c-Myc stimulates cell invasion by inhibiting FBX8 function

c-Myc is a cellular onco-protein and a transcriptional activator important for cell growth, cell division, and tumorigenesis. Despite all that is known of its function, the mechanism of how c-Myc contributes to tumorigenesis is unclear. To gain insight into the mechanism through which c-Myc protein exerts its oncogenic activity, we performed large-scale, tandem repeat affinity purification and identified the F box only protein 8 (FBX8), an F-box- and Sec7 domain-containing protein, as a novel Myc-binding protein. The c-Myc/FBX8 interaction was mediated by the c-Myc box II (MBII) region. We also confirmed that Myc protein overexpression in 293T cells affected FBX8 cellular translocation and led to recovery from FBX8-mediated inhibition of ADP-ribosylation factor 6 (ARF6) function during cell invasion. Together, these results suggest that FBX8 is a novel c-Myc binding protein and that c-Myc induces cell invasive activity through the inhibition of FBX8 effects on ARF6 function during cell invasion.

Cytohesin-2/ARNO, through its interaction with focal adhesion adaptor protein paxillin, regulates preadipocyte migration via the downstream activation of Arf6

The formation of primitive adipose tissue is the initial process in adipose tissue development followed by the migration of preadipocytes into adipocyte clusters. Comparatively little is known about the molecular mechanism controlling preadipocyte migration. Here, we show that cytohesin-2, the guanine-nucleotide exchange factor for the Arf family GTP-binding proteins, regulates migration of mouse preadipocyte 3T3-L1 cells through Arf6. SecinH3, a specific inhibitor of the cytohesin family, markedly inhibits migration of 3T3-L1 cells. 3T3-L1 cells express cytohesin-2 and cytohesin-3, and knockdown of cytohesin-2 with its small interfering RNA effectively decreases cell migration. Cytohesin-2 preferentially acts upstream of Arf6 in this signaling pathway. Furthermore, we find that the focal adhesion protein paxillin forms a complex with cytohesin-2. Paxillin colocalizes with cytohesin-2 at the leading edges of migrating cells. This interaction is mediated by the LIM2 domain of paxillin and the isolated polybasic region of cytohesin-2. Importantly, migration is inhibited by expression of the constructs containing these regions. These results suggest that cytohesin-2, through a previously unexplored complex formation with paxillin, regulates preadipocyte migration and that paxillin plays a previously unknown role as a scaffold protein of Arf guanine-nucleotide exchange factor.

Molecular characterization and association analysis of FBXO40 with partial hematological indexes in pig

F-box proteins are quite significant ubiquitin-proteasome pathway regulators in eukaryotic cells. FBXO40, a member of this large family, alters its expression pattern in muscle atrophy. Here we isolated most of the verified porcine FBXO40 coding sequence (CDS) (2258 bp) and assigned it to the porcine chromosome 13q4.1-4.6 by using the INRA-Minnesota porcine radiation hybrid panel, and we also explored the tissue expression distributions, which is relatively high in longissimus dorsi muscle, heart, low in kidney, small intestine, brain, hypophysis, lymphonode, thymus, spleen, large intestine, ovary, stomach, and undetectable in testis, liver, uterus and thyroid gland. Inferring phylogenetic tree was constructed to study the evolutionary implications. Moreover, a HindII (HincII)-RFLP (A/C) polymorphism in 3'-untranslated region (3'-UTR) of porcine FBXO40 gene was demonstrated by sequencing and PCR-restriction fragment length polymorphism (PCR-RFLP) analysis. Statistical analysis result of this polymorphism showed that the allele A was predominant in all detected indigenous breeds, but C in western introduced commercial breeds. The SNP was further analyzed in our experimental pig population including Tongcheng, Landrace, Large White, and crossbreds of Large White × (Landrace × Tongcheng) and Landrace × (Large White × Tongcheng). The association analysis results indicated that the A/C base substitution was associate with some hematological indexes, the hemoglobin concentration (P < 0.0001), mean corpuscular volume hemoglobin concentration (P = 0.0002) and mean corpuscular volume (P = 0.0138).

The EGFR-GEP100-Arf6-AMAP1 signaling pathway specific to breast cancer invasion and metastasis

Tumors are tissue-specific diseases, and their mechanisms of invasion and metastasis are highly diverse. In breast cancer, biomarkers that specifically correlate with the invasive phenotypes have not been clearly identified. A small GTPase Arf6 primarily regulates recycling of plasma membrane components. We have shown that Arf6 and its effector AMAP1 (DDEF1, DEF1, ASAP1 and centaurin β4) are abnormally overexpressed in some breast cancers and used for their invasion and metastasis. Overexpression of these proteins is independent of the transcriptional upregulation of their genes, and occurs only in highly malignant breast cancer cells. We recently identified GEP100 (BRAG2) to be responsible for the Arf6 activation to induce invasion and metastasis, by directly binding to ligand-activated epidermal growth factor receptor (EGFR). A series of our studies revealed that for activation of the invasion pathway of EGFR, it is prerequisite that Arf6 and AMAP1 both are highly overexpressed, and that EGFR is activated by ligands. Pathological analyses indicate that a significant large population of human ductal cancers may utilize the EGFR-GEP100-Arf6-AMAP1 pathway for their malignancy. Microenvironments have been highly implicated in the malignancy of mammary tumors. Our results reveal an aspect of the precise molecular mechanisms of some breast cancers, in which full invasiveness is not acquired just by intracellular alterations of cancer cells, but extracellular factors from microenvironments may also be necessary. Possible translation of our knowledge to cancer therapeutics will also be discussed.