Proteomic identification and functional characterization of a novel ARF6 GTPase-activating protein, ACAP4

PLK1 phosphorylation of ZW10 guides accurate chromosome segregation in mitosis

Stable transmission of genetic information during cell division requires faithful chromosome segregation. Mounting evidence has demonstrated that polo-like kinase 1 (PLK1) dynamics at kinetochores control correct kinetochore-microtubule attachments and subsequent silencing of the spindle assembly checkpoint. However, the mechanisms underlying PLK1-mediated silencing of the spindle checkpoint remain elusive. Here, we identified a regulatory mechanism by which PLK1-elicited zeste white 10 (ZW10) phosphorylation regulates spindle checkpoint silencing in mitosis. ZW10 is a cognate substrate of PLK1, and the phosphorylation of ZW10 at Ser12 enables dynamic ZW10-Zwint1 interactions. Inhibition of ZW10 phosphorylation resulted in misaligned chromosomes, while persistent expression of phospho-mimicking ZW10 mutant caused premature anaphase, in which sister chromatids entangled as cells entered anaphase. These findings reveal the previously uncharacterized PLK1-ZW10 interaction through which dynamic phosphorylation of ZW10 fine-tunes accurate chromosome segregation in mitosis.

Ez-Metastasizing: The Crucial Roles of Ezrin in Metastasis

Ezrin is the cytoskeletal organizer and functions in the modulation of membrane-cytoskeleton interaction, maintenance of cell shape and structure, and regulation of cell-cell adhesion and movement, as well as cell survival. Ezrin plays a critical role in regulating tumor metastasis through interaction with other binding proteins. Notably, Ezrin has been reported to interact with immune cells, allowing tumor cells to escape immune attack in metastasis. Here, we review the main functions of Ezrin, the mechanisms through which it acts, its role in tumor metastasis, and its potential as a therapeutic target.

Ras superfamily GTPase activating proteins in cancer: Potential therapeutic targets?

To search more therapeutic strategies for Ras-mutant tumors, regulators of the Ras superfamily involved in the GTP/GDP (guanosine triphosphate/guanosine diphosphate) cycle have been well concerned for their anti-tumor potentials. GTPase activating proteins (GAPs) provide the catalytic group necessary for the hydrolysis of GTPs, which accelerate the switch by cycling between GTP-bound active and GDP-bound inactive forms. Inactivated GAPs lose their function in activating GTPase, leading to the continuous activation of downstream signaling pathways, uncontrolled cell proliferation, and eventually carcinogenesis. A growing number of evidence has shown the close link between GAPs and human tumors, and as a result, GAPs are believed as potential anti-tumor targets. The present review mainly summarizes the critically important role of GAPs in human tumors by introducing the classification, function and regulatory mechanism. Moreover, we comprehensively describe the relationship between dysregulated GAPs and the certain type of tumor. Finally, the current status, research progress, and clinical value of GAPs as therapeutic targets are also discussed, as well as the challenges and future direction in the cancer therapy.

The expression of ASAP3 and NOTCH3 and the clinicopathological characteristics of adult glioma patients

ASAP3 is involved in a variety of biological activities, including cancer progression in humans. In adult glioma, we explore the effects of ASAP3 and NOTCH3 and their relationships on prognosis. The Oncomine, TIMER, and Gene Expression Profiling Interactive Analysis databases were used to investigate ASAP3 expression. Immunohistochemistry was used to assess the levels of ASAP3 and NOTCH3 expressions. The effects of ASAP3 and NOTCH3 on prognosis were assessed using survival analysis. The results revealed that the amount of ASAP3 mRNA in gliomas was much higher than in normal tissue (P < 0.01). Glioma patients with high ASAP3 mRNA expression had a worse overall survival and progression-free survival. ASAP3 overexpression is directly associated with the NOTCH signaling system. Immunohistochemistry revealed that ASAP3 and NOTCH3 were overexpressed in glioblastomas (GBMs). ASAP3 expression was associated with age, recurrence, tumor resection, postoperative chemoradiotherapy, World Health Organization (WHO) grade, and Ki-67 expression. ASAP3 expression was related to the isocitrate dehydrogenase-1 mutation in low-grade glioma. Gender, local recurrence, tumor resection, postoperative radio-chemotherapy, WHO grade, recurrence, and ATRX expression were all associated with NOTCH3 expression. ASAP3 was shown to be positively associated with NOTCH3 (r = 0.337, P = 0.000). Therefore, ASAP3 and NOTCH3 as oncogene factors have the potential to be prognostic biomarkers and therapeutic targets in adult glioma.

Phosphorylation of CENP-R by Aurora B regulates kinetochore-microtubule attachment for accurate chromosome segregation

Error-free mitosis depends on accurate chromosome attachment to spindle microtubules via a fine structure called the centromere that is epigenetically specified by the enrichment of CENP-A nucleosomes. Centromere maintenance during mitosis requires CENP-A-mediated deposition of constitutive centromere-associated network that establishes the inner kinetochore and connects centromeric chromatin to spindle microtubules during mitosis. Although previously proposed to be an adaptor of retinoic acid receptor, here, we show that CENP-R synergizes with CENP-OPQU to regulate kinetochore-microtubule attachment stability and ensure accurate chromosome segregation in mitosis. We found that a phospho-mimicking mutation of CENP-R weakened its localization to the kinetochore, suggesting that phosphorylation may regulate its localization. Perturbation of CENP-R phosphorylation is shown to prevent proper kinetochore-microtubule attachment at metaphase. Mechanistically, CENP-R phosphorylation disrupts its binding with CENP-U. Thus, we speculate that Aurora B-mediated CENP-R phosphorylation promotes the correction of improper kinetochore-microtubule attachment in mitosis. As CENP-R is absent from yeast, we reasoned that metazoan evolved an elaborate chromosome stability control machinery to ensure faithful chromosome segregation in mitosis.

Elevated expression of ADAP2 is associated with aggressive behavior of human clear-cell renal cell carcinoma and poor patient survival

BACKGROUND

Clear cell renal cell carcinoma (ccRCC) is the most common and lethal cancer of the adult kidney. ADAP2 is a GTPase-activating protein was upregulated in clear cell renal cell carcinoma. The role of ADAP2 in ccRCC progression is unknown.

METHODS

ADAP2 expression in ccRCC cell lines and tissues was examined via real-time PCR, Western blot and IHC. MTS, colony formation and transwell assay to explore the role of ADAP2 in ccRCC. ADAP2 in growth and metastasis of ccRCC were evaluated in vivo through ccRCC xenograft tumor growth, lung metastatic mice model. The prognostic role of ADAP2 was evaluated by survival analysis.

RESULTS

ADAP2 mRNA was expressed at significantly higher levels in 23 pairs of ccRCC tissues than in normal kidney tissues (P < 0.01). Immunohistochemical analysis of 298 ccRCC tissues revealed elevated ADAP2 expression as an independent unfavorable prognostic factor for the overall survival (P = 0.0042) and progression-free survival (P = 0.0232) of patients. The KaplanMeier survival curve showed that patients with a higher expression of ADAP2 showed a significantly lower overall survival rate and disease-free survival rate. Moreover, high expression of ADAP2 at the mRNA level was associated with a worse prognosis for overall survival (P = 0.0083) in The Cancer Genome Atlas (TCGA) cohort. In vivo and in vitro functional study showed that overexpression of ADAP2 promotes ccRCC cell proliferation and metastasis ability, whereas knockdown of ADAP2 inhibited cell proliferation, colony formation, migration and invasion.

CONCLUSION

ADAP2 is a novel prognostic marker and could promotes tumor progression in ccRCC.

Acetylation of ezrin regulates membrane-cytoskeleton interaction underlying CCL18-elicited cell migration

Ezrin, a membrane–cytoskeleton linker protein, plays an essential role in cell polarity establishment, cell migration, and division. Recent studies show that ezrin phosphorylation regulates breast cancer metastasis by promoting cancer cell survivor and promotes intrahepatic metastasis via cell migration. However, it was less characterized whether there are additional post-translational modifications and/or post-translational crosstalks on ezrin underlying context-dependent breast cancer cell migration and invasion. Here we show that ezrin is acetylated by p300/CBP-associated factor (PCAF) in breast cancer cells in response to CCL18 stimulation. Ezrin physically interacts with PCAF and is a cognate substrate of PCAF. The acetylation site of ezrin was mapped by mass spectrometric analyses, and dynamic acetylation of ezrin is essential for CCL18-induced breast cancer cell migration and invasion. Mechanistically, the acetylation reduced the lipid-binding activity of ezrin to ensure a robust and dynamic cycling between the plasma membrane and cytosol in response to CCL18 stimulation. Biochemical analyses show that ezrin acetylation prevents the phosphorylation of Thr567. Using atomic force microscopic measurements, our study revealed that acetylation of ezrin induced its unfolding into a dominant structure, which prevents ezrin phosphorylation at Thr567. Thus, these results present a previously undefined mechanism by which CCL18-elicited crosstalks between the acetylation and phosphorylation on ezrin control breast cancer cell migration and invasion. This suggests that targeting PCAF signaling could be a potential therapeutic strategy for combating hyperactive ezrin-driven cancer progression.

Pals1 prevents Rac1-dependent colorectal cancer cell metastasis by inhibiting Arf6

Loss of apical-basal polarity and downregulation of cell-cell contacts is a critical step during the pathogenesis of cancer. Both processes are regulated by the scaffolding protein Pals1, however, it is unclear whether the expression of Pals1 is affected in cancer cells and whether Pals1 is implicated in the pathogenesis of the disease.

Using mRNA expression data and immunostainings of cancer specimen, we show that Pals1 is frequently downregulated in colorectal cancer, correlating with poorer survival of patients. We further found that Pals1 prevents cancer cell metastasis by controlling Rac1-dependent cell migration through inhibition of Arf6, which is independent of the canonical binding partners of Pals1. Loss of Pals1 in colorectal cancer cells results in increased Arf6 and Rac1 activity, enhanced cell migration and invasion in vitro and increased metastasis of transplanted tumor cells in mice. Thus, our data reveal a new function of Pals1 as a key inhibitor of cell migration and metastasis of colorectal cancer cells. Notably, this new function is independent of the known role of Pals1 in tight junction formation and apical-basal polarity.

Identification of miR-145 as a regulator of the cardiomyocyte inflammatory response and oxidative stress under hyperglycemia

The current study aimed to explore the effects of microRNA (miR)-145 on the inflammatory response and oxidative stress (OS) in high glucose (HG)-induced cardiomyocytes, as well as the specific mechanism underlying this action. H9c2 cells were treated with 33 mmol/l glucose (HG group) or cotreated with 24.5 mmol/l mannitol and 5.5 mmol/l glucose (hypertonic group), and the expression levels of miR-145 and ADP ribosylation factor 6 (ARF6) were detected. The cells were transfected with pcDNA3.1-ARF6, miR-145 mimics or corresponding negative controls prior to the assessment of cell survival rate. Levels of lactate dehydrogenase (LDH), reactive oxygen species (ROS) and malondialdehyde (MDA), as well as the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), and the levels of IL-6, TNF-α and monocyte chemoattractant protein-1 (MCP-1) were subsequently determined. The apoptotic rate of H9c2 cells was examined by flow cytometry. The interaction between miR-145-ARF6 was predicted and confirmed by luciferase reporter assays. In the HG group, miR-145 expression was significantly decreased and ARF6 expression significantly increased compared with controls. Furthermore, the levels of inflammatory factors (IL-6, TNF-α and MCP-1), LDH, ROS and MDA were significantly elevated in the HG group compared with controls. Significantly decreased SOD, CAT and GPx activities and significantly increased numbers of apoptotic cells were observed in the HG group compared with controls. The cells transfected with miR-145 mimics exhibited significantly decreased LDH, ROS and MDA levels, significantly increased antioxidant enzyme activities and significantly decreased apoptotic rates compared with controls, while the opposite results were observed in cells transfected with pcDNA3.1-ARF6. Moreover, co-transfection with miR-145 mimics and pcDNA3.1-ARF6 exacerbated the inflammatory response and OS injury in HG-induced cardiomyocytes compared with cells transfected with miR-145 mimics alone. Furthermore, miR-145 negatively targeted ARF6. miR-145 attenuated the HG-induced inflammatory response and OS injury in cardiomyocytes by negatively regulating ARF6, which may contribute to providing a theoretical basis for the treatment of diabetic cardiomyopathy.

Paternal contributors in recurrent pregnancy loss: Cues from comparative proteome profiling of seminal extracellular vesicles

Recent evidence entail paternal factors as plausible contributors in spontaneous recurrent pregnancy loss (RPL). Seminal extracellular vesicles secreted from cells of male reproductive tract carry regulatory proteins and RNAs. They are proposed to regulate sperm maturation and function while their fusion to endometrial stromal cells helps in decidualization. Nevertheless, the mechanism(s) involved in these processes are poorly understood. This study aims at elucidating the molecular basis of paternal contribution by comparative proteomics (label-free LC-MS/MS) of isolated seminal extracellular vesicles from fertile men and partners of patients with RPL (n = 21 per group). Bioinformatics analysis revealed the identified differentially expressed proteins to be involved in DNA replication, recombination and repair, gene expression, cellular assembly and organization, cell death, and survival. Major disease pathways affected were identified as developmental, hereditary, and immunological disorders. Of the three identified hub genes regulating the above disease pathways, two (HNRNPC and HNRNPU) are overexpressed while RUVBL1 is underexpressed along with over expression of HIST1H1C, DDX1, surmising defective chromatin packaging, and histone removal in spermatozoa resulting in improper expression in paternal genes thereby leading to abnormal embryo development. Besides, alteration in GSTP1 expression points oxidative predominance in RPL group. Differential expression of C3, C4a/C4b, CFB, and GDF 15 may be involved in altered maternal immune response to paternal antigens resulting in impaired decidualization.

RNA-seq analysis identifies cytoskeletal structural genes and pathways for meat quality in beef

RNA sequencing (RNA-seq) has allowed for transcriptional profiling of biological systems through the identification of differentially expressed (DE) genes and pathways. A total of 80 steers with extreme phenotypes were selected from the University of Florida multibreed Angus-Brahman herd. The average slaughter age was 12.91±8.69 months. Tenderness, juiciness and connective tissue assessed by sensory panel, along with marbling, Warner-Bratzler Shear Force (WBSF) and cooking loss, were measured in longissimus dorsi muscle. Total RNA was extracted from muscle and one RNA-seq library per sample was constructed, multiplexed, and sequenced based on protocols by Illumina HiSeq-3000 platform to generate 2×101 bp paired-end reads. The overall read mapping rate using the Btau_4.6.1 reference genome was 63%. A total of 8,799 genes were analyzed using two different methodologies, an expression association and a DE analysis. A gene and exon expression association analysis was carried out using a meat quality index on all 80 samples as a continuous response variable. The expression of 208 genes and 3,280 exons from 1,565 genes was associated with the meat quality index (p-value ≤ 0.05). A gene and isoform DE evaluation was performed analyzing two groups with extreme WBSF, tenderness and marbling. A total of 676 (adjusted p-value≤0.05), 70 (adjusted p-value≤0.1) and 198 (adjusted p-value≤0.1) genes were DE for WBSF, tenderness and marbling, respectively. A total of 106 isoforms from 98 genes for WBSF, 13 isoforms from 13 genes for tenderness and 43 isoforms from 42 genes for marbling (FDR≤0.1) were DE. Cytoskeletal and transmembrane anchoring genes and pathways were identified in the expression association, DE and the gene enrichment analyses; these proteins can have a direct effect on meat quality. Cytoskeletal proteins and transmembrane anchoring molecules can influence meat quality by allowing cytoskeletal interaction with myocyte and organelle membranes, contributing to cytoskeletal structure and architecture maintenance postmortem.

ACAP4 interacts with CrkII to promote the recycling of integrin β1

ACAP4, a GTPase-activating protein (GAP) for the ADP-ribosylation factor 6 (ARF6), plays import roles in cell migration, cell polarity, vesicle trafficking and tumorigenesis. Similarly, the ubiquitously expressed adaptor protein CrkII functions in a wide range of cellular activities, including cell proliferation, T cell adhesion and activation, tumorigenesis, and bacterial pathogenesis. Here, we demonstrate that ACAP4 physically interacts with CrkII. Biochemical experiments revealed that ACAP4^550-660 and the SH3N domain of CrkII are responsible for the interaction. Functional characterization showed that the interaction is required for the recruitment of ACAP4 to the plasma membrane where ACAP4 functions to regulate the recycling of the signal transducer integrin β1. Thus, we suggest that the CrkII-ACAP4 complex may be involved in regulation of cell adhesion.

Inhibition of the aberrant A1CF-FAM224A-miR-590-3p-ZNF143 positive feedback loop attenuated malignant biological behaviors of glioma cells

Background

Glioma is the most common and lethal type of malignant brain tumor. Accumulating evidence has highlighted that RNA binding protein APOBEC1 complementation factor (A1CF) is involved in various cellular processes by modulating RNA expression, and acts as an oncogene in breast cancer. However, the function of A1CF in glioma remained unclear.

Methods

Quantitative RT-PCR and western blot analysis were employed to detect the expression levels of A1CF, lncRNA family with sequence similarity 224 member A (FAM224A), miR-590-3p, zinc finger protein 143 (ZNF143) and ArfGAP with SH3 domain, ankyrin repeat and PH domain 3 (ASAP3) in glioma tissues and cell lines. The Cell Counting Kit-8 assay, migration and invasion assays, and flow cytometry analysis were conducted to evaluate the function of A1CF, FAM224A, miR-590-3p, ZNF143 and ASAP3 in the malignant biological behaviors of glioma cells. Moreover, luciferase reporter, RIP and ChIP assays were used to investigate the interactions among A1CF, FAM224A, miR-590-3p, ZNF143, ASAP3 and MYB. Finally, the xenograft tumor growth assay further ascertained the biological roles of A1CF, FAM224A and miR-590-3p in glioma cells.

Results

A1CF was upregulated and functioned as an oncogene via stabilizing and increasing FAM224A expression; moreover, high A1CF and FAM224A expression levels indicated a poorer prognosis for glioma patients. Conversely, miR-590-3p was downregulated and exerted a tumor-suppressive function in glioma cells. Inhibition of A1CF significantly restrained cell proliferation, migration and invasion, and promoted apoptosis by upregulating miR-590-3p in a FAM224A-dependent manner. FAM224A was a molecular sponge of miR-590-3p and they were in an RNA-induced silencing complex. ZNF143 was upregulated in glioma tissues and cell lines. MiR-590-3p could negatively modulate the expression of ZNF143 via binding to the ZNF143 3′ UTR. Moreover, ZNF143 participated in miR-590-3p-induced tumor-suppressive activity on glioma cells. ASAP3 and MYB were transcriptionally activated by ZNF143, and importantly, ZNF143 could directly target the promoter of FAM224A and stimulate its expression, collectively forming a positive feedback loop.

Conclusions

The present study clarifies that the A1CF-FAM224A-miR-590-3p-ZNF143 positive feedback loop conducts critical regulatory effects on the malignant progression of glioma cells, which provides a novel molecular target for glioma therapy.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1200-5) contains supplementary material, which is available to authorized users.

Gastric Parietal Cell Physiology and Helicobacter pylori-Induced Disease

Acidification of the gastric lumen poses a barrier to transit of potentially pathogenic bacteria and enables activation of pepsin to complement nutrient proteolysis initiated by salivary proteases. Histamine-induced activation of the PKA signaling pathway in gastric corpus parietal cells causes insertion of proton pumps into their apical plasma membranes. Parietal cell secretion and homeostasis are regulated by signaling pathways that control cytoskeletal changes required for apical membrane remodeling and organelle and proton pump activities. Helicobacter pylori colonization of human gastric mucosa affects gastric epithelial cell plasticity and homeostasis, promoting epithelial progression to neoplasia. By intervening in proton pump expression, H pylori regulates the abundance and diversity of microbiota that populate the intestinal lumen. We review stimulation-secretion coupling and renewal mechanisms in parietal cells and the mechanisms by which H pylori toxins and effectors alter cell secretory pathways (constitutive and regulated) and organelles to establish and maintain their inter- and intracellular niches. Studies of bacterial toxins and their effector proteins have provided insights into parietal cell physiology and the mechanisms by which pathogens gain control of cell activities, increasing our understanding of gastrointestinal physiology, microbial infectious disease, and immunology.

LRIF1 interacts with HP1α to coordinate accurate chromosome segregation during mitosis

Heterochromatin protein 1α (HP1α) regulates chromatin specification and plasticity during cell fate decision. Different structural determinants account for HP1α localization and function during cell division cycle. Our earlier study showed that centromeric localization of HP1α depends on the epigenetic mark H3K9me3 in interphase, while its centromeric location in mitosis relies on uncharacterized PXVXL-containing factors. Here, we identified a PXVXL-containing protein, ligand-dependent nuclear receptor-interacting factor 1 (LRIF1), which recruits HP1α to the centromere of mitotic chromosomes and its interaction with HP1α is essential for accurate chromosome segregation during mitosis. LRIF1 interacts directly with HP1α chromoshadow domain via an evolutionarily conserved PXVXL motif within its C-terminus. Importantly, the LRIF1-HP1α interaction is critical for Aurora B activity in the inner centromere. Mutation of PXVXL motif of LRIF1 leads to defects in HP1α centromere targeting and aberrant chromosome segregation. These findings reveal a previously unrecognized direct link between LRIF1 and HP1α in centromere plasticity control and illustrate the critical role of LRIF1-HP1α interaction in orchestrating accurate cell division.

Acetylation of ACAP4 regulates CCL18-elicited breast cancer cell migration and invasion

Tumor metastasis represents the main causes of cancer-related death. Our recent study showed that chemokine CCL18 secreted from tumor-associated macrophages regulates breast tumor metastasis, but the underlying mechanisms remain less clear. Here, we show that ARF6 GTPase-activating protein ACAP4 regulates CCL18-elicited breast cancer cell migration via the acetyltransferase PCAF-mediated acetylation. CCL18 stimulation elicited breast cancer cell migration and invasion via PCAF-dependent acetylation. ACAP4 physically interacts with PCAF and is a cognate substrate of PCAF during CCL18 stimulation. The acetylation site of ACAP4 by PCAF was mapped to Lys311 by mass spectrometric analyses. Importantly, dynamic acetylation of ACAP4 is essential for CCL18-induced breast cancer cell migration and invasion, as overexpression of the persistent acetylation-mimicking or non-acetylatable ACAP4 mutant blocked CCL18-elicited cell migration and invasion. Mechanistically, the acetylation of ACAP4 at Lys311 reduced the lipid-binding activity of ACAP4 to ensure a robust and dynamic cycling of ARF6-ACAP4 complex with plasma membrane in response to CCL18 stimulation. Thus, these results present a previously undefined mechanism by which CCL18-elicited acetylation of the PH domain controls dynamic interaction between ACAP4 and plasma membrane during breast cancer cell migration and invasion.

DDEFL1 correlated with Rho GTPases activity in breast cancer

DDEFL1 is related to maintaining a limiting amount of ARF6 in GTP-loaded form by accelerating its GTP hydrolysis activity, which has been implicated in hepatocellular cancer pathogenesis and lung cancer development. We investigated DDEFL1 expression in breast cancer and paired normal breast tissues by immunohistochemistry and found that DDEFL1 expression was significantly associated with tumor size, lymph node metastasis, high content of elastosis and TNM stage but not with menopausal status or age. We detected the mRNA and protein expression of DDEFL1 in breast cancer cell lines by Western blotting and quantitative real-time PCR (qRT-PCR). DDEFL1 was obvious in MDA-MB-435s and MDA-MB-231 but very weak in ZR-75-1. Further experiments were conducted to evaluate the effect of DDEFL1 small interfering RNA (siRNA) transfection on the biological behavior of MDA-MB-231. After transfection, the effects of DDEFL1 inhibition on expression of mRNA and protein were also analyzed by Western blotting and qRT-PCR. Increased apoptosis and invasive ability, decreased cellular proliferation was found in MDA-MB-231 with successful DDEFL1 siRNA transient transfection (p < 0.05). Western blotting and qRT-PCR results showed that the DDEFL1 inhibition up-regulated Caspase-3, Apaf-1, cytochrome c, and Bax expression and down-regulated Bcl-2 expression. The DDEFL1 inhibition also down-regulated the mRNA and protein expression of Rho, CDC42 and Rac1. Our study provided a functional linkage through DDEFL1 with breast cancer biological behaviours by Rho GTPases. Possible implication of our main finding for the DDEFL1 role in breast cancer and the downstream signaling pathways for the treatment of breast cancer.

MST4 kinase phosphorylates ACAP4 protein to orchestrate apical membrane remodeling during gastric acid secretion

Digestion in the stomach depends on acidification of the lumen. Histamine-elicited acid secretion is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. Our recent study revealed the functional role of PKA-MST4-ezrin signaling axis in histamine-elicited acid secretion. However, it remains uncharacterized how the PKA-MST4-ezrin signaling axis operates the insertion of H,K-ATPases into the apical plasma membranes of gastric parietal cells. Here we show that MST4 phosphorylates ACAP4, an ARF6 GTPase-activating protein, at Thr⁵⁴⁵ Histamine stimulation activates MST4 and promotes MST4 interaction with ACAP4. ACAP4 physically interacts with MST4 and is a cognate substrate of MST4 during parietal cell activation. The phosphorylation site of ACAP4 by MST4 was mapped to Thr⁵⁴⁵ by mass spectrometric analyses. Importantly, phosphorylation of Thr⁵⁴⁵ is essential for acid secretion in parietal cells because either suppression of ACAP4 or overexpression of non-phosphorylatable ACAP4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, persistent overexpression of MST4 phosphorylation-deficient ACAP4 results in inhibition of gastric acid secretion and blockage of tubulovesicle fusion to the apical membranes. Significantly, phosphorylation of Thr⁵⁴⁵ enables ACAP4 to interact with ezrin. Given the location of Thr⁵⁴⁵ between the GTPase-activating protein domain and the first ankyrin repeat, we reason that MST4 phosphorylation elicits a conformational change that enables ezrin-ACAP4 interaction. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ACAP4 signaling cascade to polarized acid secretion in gastric parietal cells.

Upregulation of ASAP3 contributes to colorectal carcinogenesis and indicates poor survival outcome

The function and clinical implication of ArfGAP with SH3 domain, ankyrin repeat, and PH domain 3 (ASAP3) in colorectal cancer (CRC) remains undefined. In the present study, we showed that the expression level of ASAP3 was dramatically increased in CRC and its upregulation was associated with American Joint Committee on Cancer stage (P < 0.001) and poor prognosis (P = 0.0022). The combination of stage and ASAP3 expression improved the prediction of survival in CRC patients. Suppression of ASAP3 inhibited cell proliferation by inducing G1 phase arrest without influencing apoptosis. ASAP3 promoted growth of colon tumors in mice with colitis, and accelerated cell invasion and migration in vitro. Increased ASAP3 was associated with activation of the nuclear factor-κB (NF-κB) canonical pathway in CRC. Upregulation of ASAP3 increased the phosphorylation and nuclear translocation of the p65 NF-κB subunit. Mechanistically, ASAP3 interacts with NF-κB essential modulator (NEMO) and could reduce the polyubiquitinylation of NEMO. Overall, ASAP3 might regulate NF-κB via binding to NEMO. ASAP3 acts as an oncogene in colonic cancer and could be a potential biomarker of colon carcinogenesis.

The Microtubule Plus End Tracking Protein TIP150 Interacts with Cortactin to Steer Directional Cell Migration

Cell migration is orchestrated by dynamic interactions of microtubules with the plasma membrane cortex. How these interactions facilitate these dynamic processes is still being actively investigated. TIP150 is a newly characterized microtubule plus end tracking protein essential for mitosis and entosis (Ward, T., Wang, M., Liu, X., Wang, Z., Xia, P., Chu, Y., Wang, X., Liu, L., Jiang, K., Yu, H., Yan, M., Wang, J., Hill, D. L., Huang, Y., Zhu, T., and Yao, X. (2013) Regulation of a dynamic interaction between two microtubule-binding proteins, EB1 and TIP150, by the mitotic p300/CBP-associated factor (PCAF) orchestrates kinetochore microtubule plasticity and chromosome stability during mitosis. J. Biol. Chem. 288, 15771-15785; Xia, P., Zhou, J., Song, X., Wu, B., Liu, X., Li, D., Zhang, S., Wang, Z., Yu, H., Ward, T., Zhang, J., Li, Y., Wang, X., Chen, Y., Guo, Z., and Yao, X. (2014) Aurora A orchestrates entosis by regulating a dynamic MCAK-TIP150 interaction. J. Mol. Cell Biol. 6, 240-254). Here we show that TIP150 links dynamic microtubules to steer cell migration by interacting with cortactin. Mechanistically, TIP150 binds to cortactin via its C-terminal tail. Interestingly, the C-terminal TIP150 proline-rich region (CT150) binds to the Src homology 3 domain of cortactin specifically, and such an interaction is negatively regulated by EGF-elicited tyrosine phosphorylation of cortactin. Importantly, suppression of TIP150 or overexpression of phospho-mimicking cortactin inhibits polarized cell migration. In addition, CT150 disrupts the biochemical interaction between TIP150 and cortactin in vitro, and perturbation of the TIP150-cortactin interaction in vivo using a membrane-permeable TAT-CT150 peptide results in an inhibition of directional cell migration. We reason that a dynamic TIP150-cortactin interaction orchestrates directional cell migration via coupling dynamic microtubule plus ends to the cortical cytoskeleton.

IRE1-RACK1 axis orchestrates ER stress preconditioning-elicited cytoprotection from ischemia/reperfusion injury in liver

Endoplasmic reticulum (ER) stress is involved in ischemic preconditioning that protects various organs from ischemia/reperfusion (I/R) injury. We established an in vivo ER stress preconditioning model in which tunicamycin was injected into rats before hepatic I/R. The hepatic I/R injury, demonstrated by serum aminotransferase level and the ultra-structure of the liver, was alleviated by administration of tunicamycin, which induced ER stress in rat liver by activating inositol-requiring enzyme 1 (IRE1) and upregulating 78 kDa glucose-regulated protein (GRP78). The proteomic identification for IRE1 binders revealed interaction and cooperation among receptor for activated C kinase 1 (RACK1), phosphorylated AMPK, and IRE1 under ER stress conditions in a spatiotemporal manner. Furthermore, in vitro ER stress preconditioning was induced by thapsigargin and tunicamycin in L02 and HepG2 cells. Surprisingly, BCL2 was found to be phosphorylated by IRE1 under ER stress conditions to prevent apoptotic process by activation of autophagy. In conclusion, ER stress preconditioning protects against hepatic I/R injury, which is orchestrated by IRE1-RACK1 axis through the activation of BCL2. Our findings provide novel insights into the molecular pathways underlying ER stress preconditioning-elicited cytoprotective effect against hepatic I/R injury.

Signaling Scaffold Protein IQGAP1 Interacts with Microtubule Plus-end Tracking Protein SKAP and Links Dynamic Microtubule Plus-end to Steer Cell Migration

Cell migration is orchestrated by dynamic interaction of microtubules with the plasma membrane cortex. However, the regulatory mechanisms underlying the cortical actin cytoskeleton and microtubule dynamics are less characterized. Our earlier study showed that small GTPase-activating proteins, IQGAPs, regulate polarized secretion in epithelial cells (1). Here, we show that IQGAP1 links dynamic microtubules to steer cell migration via interacting with the plus-end tracking protein, SKAP. Biochemical characterizations revealed that IQGAP1 and SKAP form a cognate complex and that their binding interfaces map to the WWIQ motif and the C-terminal of SKAP, respectively. The WWIQ peptide disrupts the biochemical interaction between IQGAP1 and SKAP in vitro, and perturbation of the IQGAP1-SKAP interaction in vivo using a membrane-permeable TAT-WWIQ peptide results in inhibition of directional cell migration elicited by EGF. Mechanistically, the N-terminal of SKAP binds to EB1, and its C terminus binds to IQGAP1 in migrating cells. Thus, we reason that a novel IQGAP1 complex orchestrates directional cell migration via coupling dynamic microtubule plus-ends to the cell cortex.

Dynamic autophosphorylation of mps1 kinase is required for faithful mitotic progression

The spindle assembly checkpoint (SAC) is a surveillance mechanism monitoring cell cycle progression, thus ensuring accurate chromosome segregation. The conserved mitotic kinase Mps1 is a key component of the SAC. The human Mps1 exhibits comprehensive phosphorylation during mitosis. However, the related biological relevance is largely unknown. Here, we demonstrate that 8 autophosphorylation sites within the N-terminus of Mps1, outside of the catalytic domain, are involved in regulating Mps1 kinetochore localization. The phospho-mimicking mutant of the 8 autophosphorylation sites impairs Mps1 localization to kinetochore and also affects the kinetochore recruitment of BubR1 and Mad2, two key SAC effectors, subsequently leading to chromosome segregation errors. Interestingly, the non-phosphorylatable mutant of the 8 autophosphorylation sites enhances Mps1 kinetochore localization and delays anaphase onset. We further show that the Mps1 phospho-mimicking and non-phosphorylatable mutants do not affect metaphase chromosome congression. Thus, our results highlight the importance of dynamic autophosphorylation of Mps1 in regulating accurate chromosome segregation and ensuring proper mitotic progression.

NEDD9/Arf6-dependent endocytic trafficking of matrix metalloproteinase 14: a novel mechanism for blocking mesenchymal cell invasion and metastasis of breast cancer

NEDD9 is an established marker of invasive and metastatic cancers. NEDD9 downregulation has been shown to dramatically reduce cell invasion and metastasis in multiple tumors. The mechanisms by which NEDD9 regulates invasion are largely unknown. In the current study, we have found that NEDD9 is required for MMP14 enzymatic recovery/recycling through the late endosomes to enable disengagement of tissue inhibitor of matrix metalloproteinase 2 (TIMP2) and tumor invasion. Depletion of NEDD9 decreases targeting of the MMP14/TIMP2 complex to late endosomes and increases trafficking of MMP14 from early/sorting endosomes back to the surface in a small GTPase Arf6-dependent manner. NEDD9 directly binds to Arf6-GAP, ARAP3, and Arf6 effector GGA3 thereby facilitating the Arf6 inactivation required for MMP14/TIMP2 targeting to late endosomes. Re-expression of NEDD9 or a decrease in Arf6 activity is sufficient to restore MMP14 activity and the invasive properties of tumor cells. Importantly, NEDD9 inhibition by Vivo-Morpholinos, an antisense therapy, decreases primary tumor growth and metastasis in xenograft models of breast cancer. Collectively, our findings uncover a novel mechanism to control tumor cells dissemination through NEDD9/Arf6-dependent regulation of MMP14/TIMP2 trafficking, and validates NEDD9 as a clinically relevant therapeutic target to treat metastatic cancer.

The 68-kDa telomeric repeat binding factor 1 (TRF1)-associated protein (TAP68) interacts with and recruits TRF1 to the spindle pole during mitosis

The telomere capping protein TRF1 is a component of the multiprotein complex "shelterin," which organizes the telomere into a high order structure. Besides telomere maintenance, telomere-associated proteins also have nontelomeric functions. For example, tankyrase 1 and TRF1 are required for the maintenance of faithful mitotic progression. However, the functional relevance of their centrosomal localization has not been established. Here, we report the identification of a TRF1-binding protein, TAP68, that interacts with TRF1 in mitotic cells. TAP68 contains two coiled-coil domains and a structural maintenance of chromosome motifs and co-localizes with TRF1 to telomeres during interphase. Immediately after nuclear envelope breakdown, TAP68 translocates toward the spindle poles followed by TRF1. Dissociation of TAP68 from the telomere is concurrent with the Nek2A-dependent phosphorylation at Thr-221. Biochemical characterization demonstrated that the first coiled-coil domain of TAP68 binds and recruits TRF1 to the centrosome. Inhibition of TAP68 expression by siRNA blocked the localization of TRF1 and tankyrase 1 to the centrosome. Furthermore, siRNA-mediated depletion of TAP68 perturbed faithful chromosome segregation and genomic stability. These findings suggest that TAP68 functions in mediating TRF1-tankyrase 1 localization to the centrosome and in mitotic regulation.

Loss of ASAP3 destabilizes cytoskeletal protein ACTG1 to suppress cancer cell migration

ArfGAP with SH3 domain, ankyrin repeat and PH domain 3 (ASAP3), previously known as ACAP4, DDEFL1 and UPLC1, is considered to be an important regulator in cancer cell migration/invasion and actin-based cytoskeletal remodeling. However, the underlying mechanisms through which ASAP3 mediates these processes are not well-elucidated. This study reported that in certain types of cancer cells, loss of ASAP3 suppressed cell migration/invasion, in part by destabilizing γ-actin-1 (ACTG1), a cytoskeletal protein considered to be an integral component of the cell migratory machinery, essential for the rearrangement of the dynamic cytoskeletal networks and important in diseases, such as brain malformation, hearing loss and cancer development. The data, for the first time, link ASAP3 with ACTG1 in the regulation of cytoskeletal maintenance and cell motility.

Phosphorylation of the Bin, Amphiphysin, and RSV161/167 (BAR) domain of ACAP4 regulates membrane tubulation

ArfGAP With Coiled-Coil, Ankyrin Repeat And PH Domains 4 (ACAP4) is an ADP-ribosylation factor 6 (ARF6) GTPase-activating protein essential for EGF-elicited cell migration. However, how ACAP4 regulates membrane dynamics and curvature in response to EGF stimulation is unknown. Here, we show that phosphorylation of the N-terminal region of ACAP4, named the Bin, Amphiphysin, and RSV161/167 (BAR) domain, at Tyr34 is necessary for EGF-elicited membrane remodeling. Domain structure analysis demonstrates that the BAR domain regulates membrane curvature. EGF stimulation of cells causes phosphorylation of ACAP4 at Tyr34, which subsequently promotes ACAP4 homodimer curvature. The phospho-mimicking mutant of ACAP4 demonstrates lipid-binding activity and tubulation in vitro, and ARF6 enrichment at the membrane is associated with ruffles of EGF-stimulated cells. Expression of the phospho-mimicking ACAP4 mutant promotes ARF6-dependent cell migration. Thus, the results present a previously undefined mechanism by which EGF-elicited phosphorylation of the BAR domain controls ACAP4 molecular plasticity and plasma membrane dynamics during cell migration.

CENP-E kinesin interacts with SKAP protein to orchestrate accurate chromosome segregation in mitosis

Mitotic chromosome segregation is orchestrated by the dynamic interaction of spindle microtubules with the kinetochore. Although previous studies show that the mitotic kinesin CENP-E forms a link between attachment of the spindle microtubule to the kinetochore and the mitotic checkpoint signaling cascade, the molecular mechanism underlying dynamic kinetochore-microtubule interactions in mammalian cells remains elusive. Here, we identify a novel interaction between CENP-E and SKAP that functions synergistically in governing dynamic kinetochore-microtubule interactions. SKAP binds to the C-terminal tail of CENP-E in vitro and is essential for an accurate kinetochore-microtubule attachment in vivo. Immunoelectron microscopic analysis indicates that SKAP is a constituent of the kinetochore corona fibers of mammalian centromeres. Depletion of SKAP or CENP-E by RNA interference results in a dramatic reduction of inter-kinetochore tension, which causes chromosome mis-segregation with a prolonged delay in achieving metaphase alignment. Importantly, SKAP binds to microtubules in vitro, and this interaction is synergized by CENP-E. Based on these findings, we propose that SKAP cooperates with CENP-E to orchestrate dynamic kinetochore-microtubule interaction for faithful chromosome segregation.

ACAP4 protein cooperates with Grb2 protein to orchestrate epidermal growth factor-stimulated integrin β1 recycling in cell migration

ARF6 GTPase is an important regulator of membrane trafficking and actin-based cytoskeleton dynamics active at the leading edge of migrating cells. The integrin family heterodimeric transmembrane proteins serve as major receptors for extracellular matrix proteins, which play essential roles in cell adhesion and migration. Our recent proteomic analyses of ARF6 effectors have identified a novel ARF6 GTPase-activating protein, ACAP4, essential for EGF-induced cell migration. However, molecular mechanisms underlying ACAP4-mediated cell migration have remained elusive. Here, we show that ACAP4 regulates integrin β1 dynamics during EGF-stimulated cell migration by interaction with Grb2. Our biochemical study shows that EGF stimulation induces phosphorylation of tyrosine 733, which enables ACAP4 to bind Grb2. This interaction of ACAP4 with Grb2 regulates integrin β1 recycling to the plasma membrane. Importantly, knockdown of ACAP4 by siRNA or overexpression of ACAP4 decreased recycling of integrin β1 to the plasma membrane and reduced integrin-mediated cell migration. Taken together, these results suggest a novel function for ACAP4 in the regulation of cell migration through controlling integrin β1 dynamics.

Rho kinase phosphorylation promotes ezrin-mediated metastasis in hepatocellular carcinoma

During progression of hepatocellular carcinoma, multiple genetic and epigenetic alterations act to posttranslationally modulate the function of proteins that promote cancer invasion and metastasis. To define such abnormalities that contribute to liver cancer metastasis, we carried out a proteomic comparison of primary hepatocellular carcinoma and samples of intravascular thrombi from the same patient. Mass spectrometric analyses of the liver cancer samples revealed a series of acidic phospho-isotypes associated with the intravascular thrombi samples. In particular, we found that Thr567 hyperphosphorylation of the cytoskeletal protein ezrin was tightly correlated to an invasive phenotype of clinical hepatocellular carcinomas and to poor outcomes in tumor xenograft assays. Using phospho-mimicking mutants, we showed that ezrin phosphorylation at Thr567 promoted in vitro invasion by hepatocarcinoma cells. Phospho-mimicking mutant ezrinT567D, but not the nonphosphorylatable mutant ezrinT567A, stimulated formation of membrane ruffles, suggesting that Thr567 phosphorylation promotes cytoskeletal-membrane remodeling. Importantly, inhibition of Rho kinase, either by Y27632 or RNA interference, resulted in inhibition of Thr567 phosphorylation and a blockade to cell invasion, implicating Rho kinase-ezrin signaling in hepatocellular carcinoma cell invasion. Our findings suggest a strategy to reduce liver tumor metastasis by blocking Rho kinase-mediated phosphorylation of ezrin.

The structure of an Arf-ArfGAP complex reveals a Ca2+ regulatory mechanism

Arfs are small G proteins that have a key role in vesicle trafficking and cytoskeletal remodeling. ArfGAP proteins stimulate Arf intrinsic GTP hydrolysis by a mechanism that is still unresolved. Using a fusion construct we solved the structure of the ArfGAP ASAP3 in complex with Arf6 in the transition state. This structure clarifies the ArfGAP catalytic mechanism and shows a glutamine((Arf6)) and an arginine finger((ASAP3)) as the important catalytic residues. Unexpectedly the structure shows a calcium ion, liganded by both proteins in the complex interface, stabilizing the interaction and orienting the catalytic machinery. Calcium stimulates the GAP activity of ASAPs, but not other members of the ArfGAP family. This type of regulation is unique for GAPs and any other calcium-regulated processes and hints at a crosstalk between Ca(2+) and Arf signaling.

Phospho-regulated ACAP4-Ezrin interaction is essential for histamine-stimulated parietal cell secretion

The ezrin-radixin-moesin proteins provide a regulated linkage between membrane proteins and the cortical cytoskeleton and also participate in signal transduction pathways. Ezrin is localized to the apical membrane of parietal cells and couples the protein kinase A activation cascade to the regulated HCl secretion. Our recent proteomic study revealed a protein complex of ezrin-ACAP4-ARF6 essential for volatile membrane remodeling (Fang, Z., Miao, Y., Ding, X., Deng, H., Liu, S., Wang, F., Zhou, R., Watson, C., Fu, C., Hu, Q., Lillard, J. W., Jr., Powell, M., Chen, Y., Forte, J. G., and Yao, X. (2006) Mol. Cell Proteomics 5, 1437-1449). However, knowledge of whether ACAP4 physically interacts with ezrin and how their interaction is integrated into membrane-cytoskeletal remodeling has remained elusive. Here we provide the first evidence that ezrin interacts with ACAP4 in a protein kinase A-mediated phosphorylation-dependent manner through the N-terminal 400 amino acids of ACAP4. ACAP4 locates in the cytoplasmic membrane in resting parietal cells but translocates to the apical plasma membrane upon histamine stimulation. ACAP4 was precipitated with ezrin from secreting but not resting parietal cell lysates, suggesting a phospho-regulated interaction. Indeed, this interaction is abolished by phosphatase treatment and validated by an in vitro reconstitution assay using phospho-mimicking ezrin(S66D). Importantly, ezrin specifies the apical distribution of ACAP4 in secreting parietal cells because either suppression of ezrin or overexpression of non-phosphorylatable ezrin prevents the apical localization of ACAP4. In addition, overexpressing GTPase-activating protein-deficient ACAP4 results in an inhibition of apical membrane-cytoskeletal remodeling and gastric acid secretion. Taken together, these results define a novel molecular mechanism linking ACAP4-ezrin interaction to polarized epithelial secretion.

PinX1 is recruited to the mitotic chromosome periphery by Nucleolin and facilitates chromosome congression

Mitotic chromosome movements are orchestrated by interactions between spindle microtubules and chromosomes. It is well known that kinetochore is the major site where microtubule-chromosome attachment occurs. However, the functions of other domains of chromosome such as chromosome periphery have remained elusive. Our previous studies show that PinX1 distributes to chromosome periphery and kinetochore during mitosis, and harbors the microtubule binding activity. Here we report that PinX1 interacts with Nucleolin, a chromosome periphery protein, through its C-termini. Deconvolution microscopic analyses show PinX1 mainly co-localizes with Nucleolin at chromosome periphery in prometaphase. Moreover, depletion of Nucleolin abolishes chromosome periphery localizations of PinX1, suggesting a functional interrelationship between PinX1 and Nucleolin. Importantly, repression of PinX1 and Nucleolin abrogates chromosome segregation in real-time mitosis, validating the functional importance of PinX1-Nucleolin interaction. We propose PinX1 is recruited to chromosome periphery by Nucleolin and a complex of PinX1 and Nucleolin is essential for faithful chromosome congression.

Centaurin-like protein Cnt5 contributes to arsenic and cadmium resistance in fission yeast

Arsenic (As) and cadmium (Cd) are two of the most hazardous substances in the environment and have been implicated in a number of human diseases including cancer. Their mechanisms of toxicity and subsequent carcinogenesis are not understood. To identify the genes involved in As/Cd detoxification, we screened a random insertional mutagenesis library of Schizosaccharomyces pombe for mutants that are hypersensitive to As/Cd. Mutations were mapped to spc1(+) (sty1(+)) and SPBC17G9.08c. Spc1 is a stress-activated protein kinase orthologous to human p38. A fragment of SPBC17G9.08c was previously identified as csx2, a high-copy suppressor of cut6 that encodes an acetyl-CoA carboxylase involved in fatty acid biosynthesis. SPBC17G9.08c is a member of the centaurin ADP ribosylation factor GTPase activating protein family found in a variety of fungi, plants and metazoans, but not in Saccharomyces cerevisiae. Cnt5, so named because its closest human homolog is centaurin beta-5, binds to phosphatidic acid and phosphatidyl serine in vitro. Microscopic localization of Cnt5-GFP indicates significant redistribution of Cnt5 from the cytoplasm to the cell membranes in response to As stress. These data suggest a model in which Cnt5 contributes to As/Cd resistance by maintaining membrane integrity or by modulating membrane trafficking.

Helicobacter pylori VacA disrupts apical membrane-cytoskeletal interactions in gastric parietal cells

Helicobacter pylori persistently colonize the human stomach and have been linked to atrophic gastritis and gastric carcinoma. Although it is well known that H. pylori infection can result in hypochlorhydria, the molecular mechanisms underlying this phenomenon remain poorly understood. Here we show that VacA permeabilizes the apical membrane of gastric parietal cells and induces hypochlorhydria. The functional consequences of VacA infection on parietal cell physiology were studied using freshly isolated rabbit gastric glands and cultured parietal cells. Secretory activity of parietal cells was judged by an aminopyrine uptake assay and confocal microscopic examination. VacA permeabilization induces an influx of extracellular calcium, followed by activation of calpain and subsequent proteolysis of ezrin at Met(469)-Thr(470), which results in the liberation of ezrin from the apical membrane of the parietal cells. VacA treatment inhibits acid secretion by preventing the recruitment of H,K-ATPase-containing tubulovesicles to the apical membrane of gastric parietal cells. Electron microscopic examination revealed that VacA treatment disrupts the radial arrangement of actin filaments in apical microvilli due to the loss of ezrin integrity in parietal cells. Significantly, expression of calpain-resistant ezrin restored the functional activity of parietal cells in the presence of VacA. Proteolysis of ezrin in VacA-infected parietal cells is a novel mechanism underlying H. pylori-induced inhibition of acid secretion. Our results indicate that VacA disrupts the apical membrane-cytoskeletal interactions in gastric parietal cells and thereby causes hypochlorhydria.

ASAP3 is a focal adhesion-associated Arf GAP that functions in cell migration and invasion

ASAP3, an Arf GTPase-activating protein previously called DDEFL1 and ACAP4, has been implicated in the pathogenesis of hepatocellular carcinoma. We have examined in vitro and in vivo functions of ASAP3 and compared it to the related Arf GAP ASAP1 that has also been implicated in oncogenesis. ASAP3 was biochemically similar to ASAP1: the pleckstrin homology domain affected function of the catalytic domain by more than 100-fold; catalysis was stimulated by phosphatidylinositol 4,5-bisphosphate; and Arf1, Arf5, and Arf6 were used as substrates in vitro. Like ASAP1, ASAP3 associated with focal adhesions and circular dorsal ruffles. Different than ASAP1, ASAP3 did not localize to invadopodia or podosomes. Cells, derived from a mammary carcinoma and from a glioblastoma, with reduced ASAP3 expression had fewer actin stress fiber, reduced levels of phosphomyosin, and migrated more slowly than control cells. Reducing ASAP3 expression also slowed invasion of mammary carcinoma cells. In contrast, reduction of ASAP1 expression had no effect on migration or invasion. We propose that ASAP3 functions nonredundantly with ASAP1 to control cell movement and may have a role in cancer cell invasion. In comparing ASAP1 and ASAP3, we also found that invadopodia are dispensable for the invasive behavior of cells derived from a mammary carcinoma.

Arf GAPs as regulators of the actin cytoskeleton

The Arf (ADP-ribosylation factor) GAPs (GTPase-activating proteins) are a family of proteins with a common catalytic domain that induces hydrolysis of GTP bound to Arf GTP-binding proteins. At least three groups of multidomain Arf GAPs affect the actin cytoskeleton and cellular activities, such as migration and movement, that depend on the cytoskeleton. One role of the Arf GAPs is to regulate membrane remodelling that accompanies actin polymerization. Regulation of membrane remodelling is mediated in part by the regulation of Arf proteins. However, Arf GAPs also regulate actin independently of effects on membranes or Arf. These functions include acting as upstream regulators of Rho family proteins and providing a scaffold for Rho effectors and exchange factors. With multiple functional elements, the Arf GAPs could integrate signals and biochemical activities that result in co-ordinated changes in actin and membranes necessary for a wide range of cellular functions.

Contribution of AZAP-Type Arf GAPs to cancer cell migration and invasion

Arf GAPs are a family of proteins with a common catalytic domain that induces hydrolysis of GTP bound to the small GTP-binding protein Arf. The proteins are otherwise structurally diverse. Several subtypes of Arf GAPs have been found to be targets of oncogenes and to control cell proliferation and cell migration. The latter effects are thought to be mediated by coordinating changes in actin remodeling and membrane traffic. In this chapter, we discuss Arf GAPs that have been linked to oncogenesis and the molecular mechanisms underlying the effects of these proteins in cancer cells. We also discuss the enzymology of the Arf GAPs related to possible targeted inhibition of specific subtypes of Arf GAPs.

Phosphoinositides: regulators of membrane traffic and protein function

Phosphoinositides serve as important spatio-temporal regulators of intracellular trafficking and cell signalling events. In addition to their recognition by specific phosphoinositide binding domains present within cytoplasmic adaptor proteins or membrane integral channels and transporters phosphoinositides may affect membrane transport by eliciting conformational changes within proteins or by regulating enzymatic activities. During adaptor-mediated membrane traffic phosphoinositides form part of coincidence detection systems that aid in targeting pools of specific phosphoinositides to select intracellular transport pathways. In this review, we discuss potential mechanisms for conferring selectivity onto the phosphoinositide code as well as possible avenues for future research.

Endogenous ARF6 interacts with Rac1 upon angiotensin II stimulation to regulate membrane ruffling and cell migration

ARF6 and Rac1 are small GTPases known to regulate remodelling of the actin cytoskeleton. Here, we demonstrate that these monomeric G proteins are sequentially activated when HEK 293 cells expressing the angiotensin type 1 receptor (AT(1)R) are stimulated with angiotensin II (Ang II). After receptor activation, ARF6 and Rac1 transiently form a complex. Their association is, at least in part, direct and dependent on the nature of the nucleotide bound to both small G proteins. ARF6-GTP preferentially interacts with Rac1-GDP. AT(1)R expressing HEK293 cells ruffle, form membrane protrusions, and migrate in response to agonist treatment. ARF6, but not ARF1, depletion using small interfering RNAs recapitulates the ruffling and migratory phenotype observed after Ang II treatment. These results suggest that ARF6 depletion or Ang II treatment are functionally equivalent and point to a role for endogenous ARF6 as an inhibitor of Rac1 activity. Taken together, our findings reveal a novel function of endogenously expressed ARF6 and demonstrate that by interacting with Rac1, this small GTPase is a central regulator of the signaling pathways leading to actin remodeling.