Src can regulate carboxy terminal interactions with AFAP-110, which influence self-association, cell localization and actin filament integrity

CSNK2B modulates IRF1 binding to functional DNA elements and promotes basal and agonist-induced antiviral signaling

Interferon regulatory factor 1 (IRF1) is a critical component of cell-intrinsic innate immunity that regulates both constitutive and induced antiviral defenses. Due to its short half-life, IRF1 function is generally considered to be regulated by its synthesis. However, how IRF1 activity is controlled post-translationally has remained poorly characterized. Here, we employed a proteomics approach to identify proteins interacting with IRF1, and found that CSNK2B, a regulatory subunit of casein kinase 2, interacts directly with IRF1 and constitutively modulates its transcriptional activity. Genome-wide CUT&RUN analysis of IRF1 binding loci revealed that CSNK2B acts generally to enhance the binding of IRF1 to chromatin, thereby enhancing transcription of key antiviral genes, such as PLAAT4 (also known as RARRES3/RIG1/TIG3). On the other hand, depleting CSNK2B triggered abnormal accumulation of IRF1 at AFAP1 loci, thereby down-regulating transcription of AFAP1, revealing contrary effects of CSNK2B on IRF1 binding at different loci. AFAP1 encodes an actin crosslinking factor that mediates Src activation. Importantly, CSNK2B was also found to mediate phosphorylation-dependent activation of AFAP1-Src signaling and exert suppressive effects against flaviviruses, including dengue virus. These findings reveal a previously unappreciated mode of IRF1 regulation and identify important effector genes mediating multiple cellular functions governed by CSNK2B and IRF1.

LncRNA AFAP1-AS1 Modulates the Proliferation and Invasion of Gastric Cancer Cells by Regulating AFAP1 via miR-205-5p

Purpose

The present study investigated the expression and function of the long noncoding RNA (lncRNA) actin filament associated protein 1 antisense RNA1 (AFAP1-AS1) related to gastric cancer (GC), based on previous results from a microarray analysis.

Methods

Real-time quantitative polymerase chain reaction (qPCR) was used to verify the expression of AFAP1-AS1 in 97 fresh GC tissues and paired non-GC tissues, as well as in six different GC cell lines (BGC-823, SGC-7901, MGC-803, AGS, MKN-45, and MKN-28). The expression levels were subsequently correlated with the clinicopathological features of patients. siRNA against AFAP1-AS1 was transfected into GC cell lines, and cell proliferation, migration, and invasion were detected before and after silencing of AFAP1-AS1 expression. Luciferase reporter gene analysis was used to confirm the target gene of microRNA-205-5p (miR-205-5p) in 293T cells. The potential mechanism was subsequently investigated.

Results

qPCR results showed that AFAP1-AS1 was significantly overexpressed in GC tumor tissues and also GC cell lines, comparing to their paired non-GC tissues. Furthermore, statistical analysis revealed that the overexpression of AFAP1-AS1 was significantly correlated with tumor size (p=0.018) and grade of differentiation (p=0.042). Subsequently, artificially decreasing the expression of AFAP1-AS1 with its specific siRNA dramatically inhibited the proliferation, migration and invasion of GC cell lines (SGC-7901 and BGC-823 cells). Mechanical analysis suggested that AFAP1-AS1 is involved in regulation of its maternal gene, AFAP1, at both mRNA level and protein level. Luciferase reporter gene assay indicated that lncRNA AFAP1-AS1, as a ceRNA, is able to sponge miR-205-5p. Moreover, miR-205-5p has been well demonstrated to participate in the regulation of AFAP1 expression and the phenotypes of GC cells, including proliferation, migration and invasion.

Conclusion

AFAP1-AS1, as a novel biomarker of GC, promotes the proliferation migration and invasion of GC cells and function as ceRNA to target AFAP1 by sponging miR-205-5p.

Long non-coding RNA AFAP1-AS1 plays an oncogenic role in promoting cell migration in non-small cell lung cancer

Long non-coding RNA (lncRNA) plays an important role in tumor progression and metastasis. Emerging evidence indicates that lncRNA actin filament-associated protein 1-antisense RNA 1 (AFAP1-AS1) is dysregulated in certain tumors. However, the function of AFAP1-AS1 in non-small cell lung cancer (NSCLC) remains elusive. In this study, we conducted global lncRNA profiling and identified that AFAP1-AS1 is significantly upregulated in NSCLC, suggesting that AFAP1-AS1 may be important for lung cancer development. For the first time, the transcription initiation and termination sites of AFAP1-AS1 were identified by rapid amplification of cDNA ends technology, and the sequencing data indicated that AFAP1-AS1 in lung cancer cells is a novel transcript variant. Through gain- and loss-of-function studies, AFAP1-AS1 was demonstrated to promote cell migration and invasion. Mechanistically, AFAP1-AS1 functions through positively regulating the expression of AFAP1 protein. On the other hand, the expression of lncRNA AFAP1-AS1 negatively correlates with CpG methylation status of its gene promoter, identified in both lung cancer cells and patient tissues, and treatment with DNA methyltransferase inhibitor decitabine significantly activates AFAP1-AS1 expression, strongly supporting that AFAP1-AS1 expression is tightly regulated by DNA methylation. Taken together, this study demonstrates that AFAP1-AS1 acts as an oncogene in NSCLC to promote cell migration partly by upregulating AFAP1 expression, while its own expression is controlled by DNA methylation, and highlights its diagnostic and therapeutic values for NSCLC patients.

PARP1 Is Up-Regulated in Non-small Cell Lung Cancer Tissues in the Presence of the Cyanobacterial Toxin Microcystin

Non-small cell lung cancer (NSCLC) is the major form of lung cancer, with adenocarcinoma (LUAD) and squamous cell carcinoma (LUSC) being its major subtypes. Smoking alone cannot completely explain the lung cancer etiology. We hypothesize that altered lung microbiome and chronic inflammatory insults in lung tissues contribute to carcinogenesis. Here we explore the microbiome composition of LUAD samples, compared to LUSC and normal samples. Extraction of microbiome DNA in formalin-fixed, paraffin-embedded (FFPE) lung tumor and normal adjacent tissues was meticulously performed. The 16S rRNA product from extracted microbiota was subjected to microbiome amplicon sequencing. To assess the contribution of the host genome, CD36 expression levels were analyzed then integrated with altered NSCLC subtype-specific microbe sequence data. Surprisingly phylum Cyanobacteria was consistently observed in LUAD samples. Across the NSCLC subtypes, differential abundance across four phyla (Proteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes) was identified based on the univariate analysis (p-value < 6.4e-4 to 3.2e-2). In silico metagenomic and pathway analyses show that presence of microcystin correlates with reduced CD36 and increased PARP1 levels. This was confirmed in microcystin challenged NSCLC (A427) cell lines and Cyanobacteria positive LUAD tissues. Controlling the influx of Cyanobacteria-like particles or microcystin and the inhibition of PARP1 can provide a potential targeted therapy and prevention of inflammation-associated lung carcinogenesis.

Phosphatidylinositol 3-Kinase-Associated Protein (PI3KAP)/XB130 Crosslinks Actin Filaments through Its Actin Binding and Multimerization Properties In Vitro and Enhances Endocytosis in HEK293 Cells

Actin-crosslinking proteins control actin filament networks and bundles and contribute to various cellular functions including regulation of cell migration, cell morphology, and endocytosis. Phosphatidylinositol 3-kinase-associated protein (PI3KAP)/XB130 has been reported to be localized to actin filaments (F-actin) and required for cell migration in thyroid carcinoma cells. Here, we show a role for PI3KAP/XB130 as an actin-crosslinking protein. First, we found that the carboxyl terminal region of PI3KAP/XB130 containing amino acid residues 830-840 was required and sufficient for localization to F-actin in NIH3T3 cells, and this region is directly bound to F-actin in vitro. Moreover, actin-crosslinking assay revealed that recombinant PI3KAP/XB130 crosslinked F-actin. In general, actin-crosslinking proteins often multimerize to assemble multiple actin-binding sites. We then investigated whether PI3KAP/XB130 could form a multimer. Blue native-PAGE analysis showed that recombinant PI3KAP/XB130 was detected at 250-1200 kDa although the molecular mass was approximately 125 kDa, suggesting that PI3KAP/XB130 formed multimers. Furthermore, we found that the amino terminal 40 amino acids were required for this multimerization by co-immunoprecipitation assay in HEK293T cells. Deletion mutants of PI3KAP/XB130 lacking the actin-binding region or the multimerizing region did not crosslink actin filaments, indicating that actin binding and multimerization of PI3KAP/XB130 were necessary to crosslink F-actin. Finally, we examined roles of PI3KAP/XB130 on endocytosis, an actin-related biological process. Overexpression of PI3KAP/XB130 enhanced dextran uptake in HEK 293 cells. However, most of the cells transfected with the deletion mutant lacking the actin-binding region incorporated dextran to a similar extent as control cells. Taken together, these results demonstrate that PI3KAP/XB130 crosslinks F-actin through both its actin-binding region and multimerizing region and plays an important role in endocytosis.

Upregulated long non-coding RNA AFAP1-AS1 expression is associated with progression and poor prognosis of nasopharyngeal carcinoma

Altered expression of long noncoding RNAs (lncRNAs) associated with human carcinogenesis. We performed a cDNA microarray analysis of lncRNA expression in 12 cases of nasopharyngeal carcinoma (NPC) and 4 non-tumor nasopharyngeal epitheliums. One lncRNA, actin filament associated protein 1 antisense RNA1 (AFAP1-AS1), was identified and selected for further study. AFAP1-AS1 expression was upregulated in NPC and associated with NPC metastasis and poor prognosis. In vitro experiments demonstrated that AFAP1-AS1 knockdown significantly inhibited the NPC cell migration and invasive capability. AFAP1-AS1 knockdown also increased AFAP1 protein expression. Proteomic and bioinformatics analyses suggested that AFAP1-AS1 affected the expression of several small GTPase family members and molecules in the actin cytokeratin signaling pathway. AFAP1-AS1 promoted cancer cell metastasis via regulation of actin filament integrity. AFAP1-AS1 might be a potential novel marker that can predict cancer patient prognosis and as a potential therapeutic target for NPC.

Actin filament-associated protein 1 is required for cSrc activity and secretory activation in the lactating mammary gland

Actin filament-associated protein 1 (AFAP1) is an adaptor protein of cSrc that binds to filamentous actin and regulates the activity of this tyrosine kinase to affect changes to the organization of the actin cytoskeleton. In breast and prostate cancer cells, AFAP1 has been shown to regulate cellular responses requiring actin cytoskeletal changes such as adhesion, invadopodia formation and invasion. However, a normal physiologic role for AFAP1 has remained elusive. In this study, we generated an AFAP1 knockout mouse model that establishes a novel physiologic role for AFAP1 in lactation. Specifically, these animals displayed a defect in lactation that resulted in an inability to nurse efficiently. Histologically, the mammary glands of the lactating knockout mice were distinguished by the accumulation of large cytoplasmic lipid droplets in the alveolar epithelial cells. There was a reduction in lipid synthesis and the expression of lipogenic genes without a corresponding reduction in the production of β-casein, a milk protein. Furthermore, these defects were associated with histologic and biochemical signs of precocious involution. This study also demonstrated that AFAP1 responds to prolactin, a lactogenic hormone, by forming a complex with cSrc and becoming tyrosine phosphorylated. Taken together, these observations pointed to a defect in secretory activation. Certain characteristics of this phenotype mirrored the defect in secretory activation in the cSrc knockout mouse, but most importantly, the activity of cSrc in the mammary gland was reduced during early lactation in the AFAP1-null mouse and the localization of active cSrc at the apical surface of luminal epithelial cells during lactation was selectively lost in the absence of AFAP1. These data define, for the first time, the requirement of AFAP1 for the spatial and temporal regulation of cSrc activity in the normal breast, specifically for milk production.

AFAP1L1, a novel associating partner with vinculin, modulates cellular morphology and motility, and promotes the progression of colorectal cancers

We have previously identified actin filament-associated protein 1-like 1 (AFAP1L1) as a metastasis-predicting marker for spindle cell sarcomas by gene expression profiling, and demonstrated that AFAP1L1 is involved in the cell invasion process by in vitro analyses. However, its precise molecular function has not been fully elucidated, and it remains unknown whether AFAP1L1 could be a prognostic marker and/or therapeutic target of other malignancies. In this study, we found a marked elevation of AFAP1L1 gene expression in colorectal cancer (CRC) tissues as compared to the adjacent normal mucosa. Multivariate analysis revealed that AFAP1L1 was an independent and significant factor for the recurrence of rectal cancers. Moreover, the addition of the AFAP1L1 expression level to the lymph node metastasis status provided more predictive information regarding postoperative recurrence in rectal cancers. AFAP1L1-transduced CRC cells exhibited a rounded shape, increased cell motility on planar substrates, and resistance to anoikis in vitro. AFAP1L1 localized to the ringed structure of the invadopodia, together with vinculin, and AFAP1L1 was identified as a novel associating partner of vinculin by immunoprecipitation assay. AFAP1L1-transduced cells showed accelerated tumor growth in vivo, presumably reflecting the anoikis resistance of these AFAP1L1-expressing cells. Furthermore, the local administration of a siRNA against AFAP1L1 significantly suppressed the in vivo tumor growth of xenografts, suggesting that AFAP1L1 might be a candidate therapeutic target for CRCs. These results suggest that AFAP1L1 plays a role in the progression of CRCs by modulating cell shape and motility and by inhibiting anoikis, presumably through interactions with vinculin-including protein complexes.

Low molecular weight hyaluronan induces lymphangiogenesis through LYVE-1-mediated signaling pathways

Hyaluronan (HA), a large nonsulfated glycosaminogycan in the extracellular matrix, whose degraded fragments termed as low molecular weight hyaluronan (LMW-HA), has been reported as an important regulator of angiogenesis. However, little is known about the influence of LMW-HA on lymphangiogenesis. In this study, we try to explore the in vitro effects of LMW-HA on lymphangiogenesis and identify the underlying molecular mechanisms. Our results showed that LMW-HA stimulation significantly increased lymphatic endothelial cells (LECs) proliferation, migration and tube formation. Further experiments demonstrated that LMW-HA altered actin cytoskeleton rearrangement and increased the formation of intense stress fibers, lamellipodia and filopodia. Mechanistically, LMW-HA stimulation resulted in rapid tyrosine phosphorylation of protein kinase C α/βII (PKCα/βII) and extracellular-regulated kinase 1/2 (ERK1/2). Lymphalic vessel endotheilial hyaluronan receptor 1 (LYVE-1), a homologue of CD44, is the main cell surface receptor for HA in LECs. Blocking the binding interaction of LMW-HA with LYVE-1 using neutralizing anti-LYVE-1 antibodies significantly inhibited LECs proliferation, migration, tube formation and signal transduction induced by LMW-HA, suggesting that LMW-HA may play a critical role in the processes required for lymphangiogenesis through interactions with its receptor LYVE-1 and triggering intracellular signal cascades.

The actin-binding domain of actin filament-associated protein (AFAP) is involved in the regulation of cytoskeletal structure

Actin filament-associated protein (AFAP) plays a critical role in the regulation of actin filament integrity, formation and maintenance of the actin network, function of focal contacts, and cell migration. Here, we show that endogenous AFAP was present not only in the cytoskeletal but also in the cytosolic fraction. Depolymerization of actin filaments with cytochalasin D or latrunculin A increased AFAP in the cytosolic fraction. AFAP harbors an actin-binding domain (ABD) in its C-terminus. AFAPΔABD, an AFAP mutant with selective ABD deletion, was mainly in the cytosolic fraction when overexpressed in the cells, which was associated with a disorganized cytoskeleton with reduced stress fibers, accumulation of F-actin on cellular membrane, and formation of actin-rich small dots. Cortactin, a well-known podosome marker, was colocalized with AFAPΔABD in these small dots at the ventral surface of the cell, indicating that these small dots fulfill certain criteria of podosomes. However, these podosome-like small dots did not digest gelatin matrix. This may be due to the reduced interaction between AFAPΔABD and c-Src. When AFAPΔABD-transfected cells were stimulated with phorbol ester, they formed podosome-like structures with larger sizes, less numerous and longer life span, in comparison with wild-type AFAP-transfected cells. These results indicate that the association of AFAP with F-actin through ABD is crucial for AFAP to regulate cytoskeletal structures. The AFAPΔABD, as cytosolic proteins, may be more accessible to the cellular membrane, podosome-like structures, and thus be more interactive for the regulation of cellular functions.

Interaction of low molecular weight hyaluronan with CD44 and toll-like receptors promotes the actin filament-associated protein 110-actin binding and MyD88-NFκB signaling leading to proinflammatory cytokine/chemokine production and breast tumor invasion

Both high and low molecular weight hyaluronan (HMW-HA vs. LMW-HA) exist in various tissues and cells. In this study, we investigated LMW-HA-mediated CD44 interaction with Toll-like receptors (TLRs), the actin filament-associated protein (AFAP-110), and a myeloid differentiation factor (MyD88) in breast tumor cells (MDA-MB-231 cells). Our data indicate that LMW-HA (but not HMW-HA) preferentially stimulates a physical association between CD44 and TLRs followed by a concomitant recruitment of AFAP-110 and MyD88 into receptor-containing complexes in breast tumor cells. LMW-HA-activated AFAP-110 then binds to filamentous actin (F-actin) resulting in MyD88/nuclear factor-κB (NF-κB) nuclear translocation, NF-κB-specific transcription, and target gene [interleukine 1β and interleukine-8 (IL-1β and IL-8)] expression. These signaling events lead to proinflammatory cytokine/chemokine production in the breast tumor cells. AFAP-110-F-actin (activated by LMW-HA) also promotes tumor cell invasion. Downregulation of AFAP-110 or MyD88 by transfecting breast tumor cells with AFAP-110 siRNA or MyD88 siRNA, respectively not only blocks the ability of LMW-HA to stimulate AFAP-110-actin function, but also impairs MyD88-NF-κB nuclear translocation and NF-κB transcriptional activation. Consequently, both IL-1β/IL-8 production and tumor cell invasion are impaired. Taken together, these findings suggest that LMW-HA plays an important role in CD44-TLR-associated AFAP-110-actin interaction and MyD88-NF-κB signaling required for tumor cell behaviors, which may contribute to the progression of breast cancer.

AFAP1L1 is a novel adaptor protein of the AFAP family that interacts with cortactin and localizes to invadosomes

The actin-filament associated protein (AFAP) family of adaptor proteins consists of three members: AFAP1, AFAP1L1, and AFAP1L2/XB130 with AFAP1 being the best described as a cSrc binding partner and actin cross-linking protein. A homology search of AFAP1 recently identified AFAP1L1 which has a similar sequence, domain structure and cellular localization; however, based upon sequence variations, AFAP1L1 is hypothesized to have unique functions that are distinct from AFAP1. While AFAP1 has the ability to bind to the SH3 domain of the nonreceptor tyrosine kinase cSrc via an N-terminal SH3 binding motif, it was unable to bind cortactin. However, the SH3 binding motif of AFAP1L1 was more efficient at interacting with the SH3 domain of cortactin and not cSrc. AFAP1L1 was shown by fluorescence microscopy to decorate actin filaments and move to punctate actin structures and colocalize with cortactin, consistent with localization to invadosomes. Upon overexpression in A7r5 cells, AFAP1L1 had the ability to induce podosome formation and move to podosomes without stimulation. Immunohistochemical analysis of AFAP1L1 in human tissues shows differential expression when contrasted with AFAP1 with localization of AFAP1L1 to unique sites in muscle and the dentate nucleus of the brain where AFAP1 was not detectable. We hypothesize AFAP1L1 may play a similar role to AFAP1 in affecting changes in actin filaments and bridging interactions with binding partners, but we hypothesize that AFAP1L1 may forge unique protein interactions in which AFAP1 is less efficient, and these interactions may allow AFAP1L1 to affect invadosome formation.

Tks5 recruits AFAP-110, p190RhoGAP, and cortactin for podosome formation

Podosome formation in vascular smooth muscle cells is characterized by the recruitment of AFAP-110, p190RhoGAP, and cortactin, which have specific roles in Src activation, local down-regulation of RhoA activity, and actin polymerization, respectively. However, the molecular mechanism that underlies their specific recruitment to podosomes remains unknown. The scaffold protein Tks5 is localized to podosomes in Src-transformed fibroblasts and in smooth muscle cells, and may serve as a specific recruiting adapter for various components during podosome formation. We show here that induced mislocalization of Tks5 to the surface of mitochondria leads to a major subcellular redistribution of AFAP-110, p190RhoGAP, and cortactin, and to inhibition of podosome formation. Analysis of a series of similarly mistargeted deletion mutants of Tks5 indicates that the fifth SH3 domain is essential for this recruitment. A Tks5 mutant lacking the PX domain also inhibits podosome formation and induces the redistribution of AFAP-110, p190RhoGAP, and cortactin to the perinuclear area. By expressing a catalytically inactive point mutant and by siRNA-mediated expression knock-down we also provide evidence that p190RhoGAP is required for podosome formation. Together our findings demonstrate that Tks5 plays a central role in the recruitment of AFAP-110, p190RhoGAP, and cortactin to drive podosome formation.

Tiam1 and Rac1 are required for platelet-activating factor-induced endothelial junctional disassembly and increase in vascular permeability

It is known that platelet-activating factor (PAF) induces severe endothelial barrier leakiness, but the signaling mechanisms remain unclear. Here, using a wide range of biochemical and morphological approaches applied in both mouse models and cultured endothelial cells, we addressed the mechanisms of PAF-induced disruption of interendothelial junctions (IEJs) and of increased endothelial permeability. The formation of interendothelial gaps filled with filopodia and lamellipodia is the cellular event responsible for the disruption of endothelial barrier. We observed that PAF ligation of its receptor induced the activation of the Rho GTPase Rac1. Following PAF exposure, both Rac1 and its guanine nucleotide exchange factor Tiam1 were found associated with a membrane fraction from which they co-immunoprecipitated with PAF receptor. In the same time frame with Tiam1-Rac1 translocation, the junctional proteins ZO-1 and VE-cadherin were relocated from the IEJs, and formation of numerous interendothelial gaps was recorded. Notably, the response was independent of myosin light chain phosphorylation and thus distinct from other mediators, such as histamine and thrombin. The changes in actin status are driven by the PAF-induced localized actin polymerization as a consequence of Rac1 translocation and activation. Tiam1 was required for the activation of Rac1, actin polymerization, relocation of junctional associated proteins, and disruption of IEJs. Thus, PAF-induced IEJ disruption and increased endothelial permeability requires the activation of a Tiam1-Rac1 signaling module, suggesting a novel therapeutic target against increased vascular permeability associated with inflammatory diseases.

Iron oxide nanoparticles induce human microvascular endothelial cell permeability through reactive oxygen species production and microtubule remodeling

Background

Engineered iron nanoparticles are being explored for the development of biomedical applications and many other industry purposes. However, to date little is known concerning the precise mechanisms of translocation of iron nanoparticles into targeted tissues and organs from blood circulation, as well as the underlying implications of potential harmful health effects in human.

Results

The confocal microscopy imaging analysis demonstrates that exposure to engineered iron nanoparticles induces an increase in cell permeability in human microvascular endothelial cells. Our studies further reveal iron nanoparticles enhance the permeability through the production of reactive oxygen species (ROS) and the stabilization of microtubules. We also showed Akt/GSK-3β signaling pathways are involved in iron nanoparticle-induced cell permeability. The inhibition of ROS demonstrate ROS play a major role in regulating Akt/GSK-3β – mediated cell permeability upon iron nanoparticle exposure. These results provide new insights into the bioreactivity of engineered iron nanoparticles which can inform potential applications in medical imaging or drug delivery.

Conclusion

Our results indicate that exposure to iron nanoparticles induces an increase in endothelial cell permeability through ROS oxidative stress-modulated microtubule remodeling. The findings from this study provide new understandings on the effects of nanoparticles on vascular transport of macromolecules and drugs.

AFAP120 regulates actin organization during neuronal differentiation

During development, dynamic changes in the actin cytoskeleton determine both cell motility and morphological differentiation. In most mature tissues, cells are generally minimally motile and have morphologies specialized to their functions. In metastatic cancer, cells generally lose their specialized morphology and become motile. Therefore, proteins that regulate the transition between the motile and morphologically differentiated states can play important roles in determining cancer outcomes. AFAP120 is a neuronal-specific protein that binds Src kinase and protein kinase C (PKC) and cross-links actin filaments. Here we report that expression and tyrosine phosphorylation of AFAP120 are developmentally regulated in the cerebellum. In cerebellar cultures, PKC activation induces Src kinase-dependent phosphorylation of AFAP120, indicating that AFAP120 may be a downstream effector of Src. In neuroblastoma cells induced to differentiate by treatment with a PKC activator, tyrosine phosphorylation of AFAP120 appears to regulate the formation of the lamellar actin structures and subsequent neurite initiation. Together, these results indicate that AFAP120 plays a role in organizing dynamic actin structures during neuronal differentiation and suggest that AFAP120 may help regulate the transition from motile precursor to morphologically differentiated neurons.

The actin cross-linking protein AFAP120 regulates axon elongation in a tyrosine phosphorylation-dependent manner

Growth cone guidance and axon elongation require the dynamic coordinated regulation of the actin cytoskeleton. As the growth cone moves, actin-dependent forces generate tension that enables protrusive activity in the periphery and drives growth cone translocation. This dynamic remodeling of the actin cytoskeleton in response to membrane tension requires activation of Src kinase. Although it has been proposed that these actin-dependent forces vary with the extent of actin cross-linking, the identity of the cross-linking protein(s) remains unknown. AFAP120 is a nervous system specific actin cross-linking protein that is regulated by Src kinase phosphorylation. Here, we report that AFAP120 is expressed and tyrosine phosphorylated in differentiating cerebellar granule cells, where it is enriched in the axon and growth cone. Over-expression of AFAP120 enhances neurite elongation in a tyrosine phosphorylation-dependent manner. These findings suggest that AFAP120 may coordinate Src signaling with the dynamic changes in the actin cytoskeleton that drive growth cone motility and axon elongation.

Phosphorylation of AFAP-110 affects podosome lifespan in A7r5 cells

AFAP-110 is an actin-binding and -crosslinking protein that is enriched in Src and phorbol ester (PE)-induced podosomes. In vascular smooth muscle cells endogenous AFAP-110 localized to actin stress fibers and, in response to treatment with phorbol-12,13-dibutyrate (PDBu), to actin-rich podosomes. Since PEs can activate PKCalpha, AFAP-110 is a substrate of PKCalpha and PKCalpha-AFAP-110 interactions direct podosome formation, we sought to identify a PE-induced phosphorylation site in AFAP-110 and determine whether phosphorylation is linked to the formation of podosomes. Mutational analysis revealed Ser277 of AFAP-110 to be phosphorylated in PE-treated cells. The use of a newly generated, phospho-specific antibody directed against phosphorylated Ser277 revealed that PKCalpha activation is associated with PE-induced AFAP-110 phosphorylation. In PDBu-treated A7r5 rat vascular smooth muscle cells, immunolabeling using the phospho-specific antibody showed that phospho-AFAP-110 is primarily associated with actin in podosomes. Although mutation of Ser at position 277 to Ala (AFAP-110(S277A)) did not alter the ability of AFAP-110 to localize to podosomes, overexpression of AFAP-110(S277A) in treated and untreated A7r5 cells resulted in an increased number of cells that display podosomes. Video microscopy demonstrated that AFAP-110(S277A) expression correlates with an increased number of long-lived podosomes. Therefore, we hypothesize that AFAP-110 phosphorylation and/or dephosphorylation is involved in the regulation of podosome stability and lifespan.

AFAP-110 is required for actin stress fiber formation and cell adhesion in MDA-MB-231 breast cancer cells

Regulation of actin organization and dynamics is a highly complex process that involves a number of actin-binding proteins, including capping, branching, severing, sequestering, and cross-linking proteins. The actin-binding and cross-linking protein AFAP-110 is expressed in normal myoepithelial cells. Screening of different breast epithelial cell lines revealed high expression levels of AFAP-110 in the human breast cancer cell lines MDA-MB-231 and MDA-MB-435. Knockdown of AFAP-110 expression in MDA-MB-231 cells does not result in any changes in cell proliferation but did result in a loss of actin stress fiber cross-linking and decreased adhesion to fibronectin. An inducible knockdown approach confirms that MDA-MB-231 breast cancer cells require AFAP-110 expression for stress fiber formation and adhesion. Thus, AFAP-110 may provide cytoskeletal tension through stress fiber formation, which is required for focal adhesion formation. Indeed, we could not detect any focal contacts or focal adhesions in AFAP-110 knockdown cells after adhesion to fibronectin. Although expression levels of crucial focal adhesion components were not influenced by AFAP-110 expression levels, treatment of AFAP-110 knockdown cells with LPA did not result in induction of actin stress fibers and focal adhesions. In summary, AFAP-110 plays an important role in MDA-MB-231 breast cancer cell adhesion possibly by regulating stress filament cross-linking which would promote focal adhesion formation.

An active Src kinase-β-actin association is linked to actin dynamics at the periphery of colon cancer cells

Src controls the dynamic actin cytoskeleton in fibroblasts and in cancer cells, although it is not known how direct its effects are. Using FRET/FLIM imaging, we found that wild type Src associates directly, or indirectly, with peripheral beta-actin at integrin adhesions after serum stimulation, and that an active Src kinase domain is essential. Beta-actin can be directly tyrosine-phosphorylated by Src in vitro, and in a Src-dependent manner in cells. Moreover, beta-actin dynamics are suppressed when Src is rendered kinase-inactive. Surprisingly, debilitating mutations in the Src SH2 or SH3 domains do not suppress association of Src with beta-actin. This may therefore be an example of a spatially regulated Src kinase/substrate interaction that is controlling peripheral actin dynamics. Interestingly, there is no FRET between Src and beta-actin at cadherin-mediated cell-cell contacts, despite apparent co-localization there, demonstrating precise spatial specificity of Src/beta-actin complexes.

XB130, a Novel Adaptor Protein for Signal Transduction

Adaptor proteins are important mediators in signal transduction. In the present study, we report the cloning and characterization of a novel adaptor protein, XB130. This gene is located on human chromosome 10q25.3 and encodes a protein of 818 amino acids. It contains several Src homology (SH)2- and SH3-binding motifs, two pleckstrin homology domains, a coiled-coil region, and a number of potential tyrosine or serine/threonine phosphorylation sites. Endogenous XB130 interacts with c-Src tyrosine kinase. Their co-expression in COS-7 cells resulted in activation of c-Src and elevated tyrosine phosphorylation of multiple proteins, including XB130 itself. XB130 expression in HEK293 cells enhanced serum response element- and AP-1-dependent transcriptional activation mediated by c-Src. XB130DeltaN, an N-terminal deletion mutant lacking a putative SH3-binding motif and several putative SH2-binding sites, reduced its ability to mediate Src signal transduction. Down-regulation of endogenous XB130 with siRNA reduced c-Src activity, IL-8 production, EGF-induced phosphorylation of Akt and GSK3beta, and altered cell cycles in human lung epithelial cells. These data suggest that XB130 as an adaptor may play an important role in the regulation of signal transduction and cellular functions.

PI3K activation is required for PMA-directed activation of cSrc by AFAP-110

Activation of PKCalpha will induce the cSrc binding partner AFAP-110 to colocalize with and activate cSrc. The ability of AFAP-110 to colocalize with cSrc is contingent on the integrity of the amino-terminal pleckstrin homology (PH1) domain, while the ability to activate cSrc is dependent on the integrity of its SH3 binding motif, which engages the cSrc SH3 domain. The outcome of AFAP-110-directed cSrc activation is a change in actin filament integrity and the formation of podosomes. Here, we address what cellular signals promote AFAP-110 to colocalize with and activate cSrc, in response to PKCalpha activation or PMA treatment. Because PH domain integrity in AFAP-110 is required for colocalization, and PH domains are known to interact with both protein and lipid binding partners, we sought to determine whether phosphatidylinositol 3-kinase (PI3K) activation played a role in PMA-induced colocalization between AFAP-110 and cSrc. We show that PMA treatment is able to direct activation of PI3K. Treatment of mouse embryo fibroblast with PI3K inhibitors blocked PMA-directed colocalization between AFAP-110 and cSrc and subsequent cSrc activation. PMA also was unable to induce colocalization or cSrc activation in cells that lacked the p85alpha and -beta regulatory subunits of PI3K. This signaling pathway was required for migration in a wound healing assay. Cells that were null for cSrc or the p85 regulatory subunits or expressed a dominant-negative AFAP-110 also displayed a reduction in migration. Thus PI3K activity is required for PMA-induced colocalization between AFAP-110 and cSrc and subsequent cSrc activation, and this signaling pathway promotes cell migration.

ILK mediates actin filament rearrangements and cell migration and invasion through PI3K/Akt/Rac1 signaling

One of the hallmarks of integrin signaling is an increase in cell migration and invasion, both of which are associated with actin filament rearrangements. Integrin-linked kinase (ILK) is a cytoplasmic effector of integrin receptors. ILK is known to be involved in multiple cellular functions. However, the signaling pathways involved in ILK-mediated cellular structure and motility remain to be elucidated. Here, we have demonstrated that overexpression of ILK was sufficient to induce actin filament rearrangements, to form cell motility structures, and to increase cell migration and invasion in a phosphatidylinositol 3-kinase (PI3K)-dependent manner. This corresponds with the activation of both Akt and p70 ribosomal protein S6 kinase (p70S6K1). Overexpression of dominant-negative mutants of Akt inhibited ILK-dependent activation of p70S6K1, indicating that Akt is upstream of p70S6K1 in response to ILK signaling. Overexpression of ILK was sufficient to induce Rac1 activation, which was abolish by a PI3K inhibitor, indicating that Rac1 activity is involved in ILK signaling in a PI3K dependent manner. Inhibition of Akt, Rac1, or p70S6K1 inhibited the effects of ILK on actin filaments and cell migration, suggesting a regulatory role of the PI3K/Akt/p70S6K1/Rac1 signaling pathway in response to ILK signaling. We have shown that overexpression of a dominant-negative ILK was sufficient to abolish fibronectin peptide (PHSRN)-induced rearrangements of actin filaments and cell migration and invasion. Taken together, our results identify a mechanism through which ILK can regulate both integrin-associated rearrangements of actin filaments and cell migration and invasion at the integrin receptor-proximal region.

Cdc42 Regulates Arsenic-induced NADPH Oxidase Activation and Cell Migration through Actin Filament Reorganization

Although arsenic is a human carcinogen, the molecular mechanisms of its action remain to be understood. The present study reports that exposure to arsenic induced actin filament reorganization, resulting in lamellipodia and filopodia structures through the activation of Cdc42 in SVEC4-10 endothelial cells. It was also found that arsenic induced the formation of the superoxide anion (O2*) in SVEC4-10 cells. Immunoprecipitation and Western blotting analysis demonstrated that arsenic stimulation induced serine phosphorylation of p47phox, a key component of NADPH oxidase, indicating that arsenic induces O2* formation through NADPH oxidase activation. Inhibition of arsenic-induced actin filament reorganization by either overexpression of a dominant negative Cdc42 or pretreatment of an actin filament stabilizing regent, jasplakinolide, abrogated arsenic-induced NADPH oxidase activation, showing that the activation of NADPH oxidase was regulated by Cdc42-mediated actin filament reorganization. This study also showed that overexpression of a dominant negative Rac1 was sufficient to abolish arsenic-induced O2*- production, implying that Rac1 activities are required for Cdc42-mediated NADPH oxidase activation in response to arsenic stimulation. Furthermore, arsenic stimulation induced cell migration, which can be inhibited by the inactivation of either Cdc42 or NADPH oxidase. Taken together, the results indicate that arsenic is able to activate NADPH oxidase through Cdc42-mediated actin filament reorganization, leading to the induction of an increase in cell migration in SVEC4-10 endothelial cells.

Conversion of mechanical force into biochemical signaling

Physical forces play important roles in regulating cell proliferation, differentiation, and death by activating intracellular signal transduction pathways. How cells sense mechanical stimulation, however, is largely unknown. Most studies focus on cellular membrane proteins such as ion channels, integrins, and receptors for growth factors as mechanosensory units. Here we show that mechanical stretch-induced c-Src protein tyrosine kinase activation is mediated through the actin filament-associated protein (AFAP). Distributed along the actin filaments, AFAP can directly active c-Src through binding to its Src homology 3 and/or 2 domains. Mutations at these specific binding sites on AFAP blocked mechanical stretch-induced c-Src activation. Therefore, mechanical force can be transmitted along the cytoskeleton, and interaction between cytoskeletal associated proteins and enzymes related to signal transduction may convert physical forces into biochemical reactions. Cytoskeleton deformation-induced protein-protein interaction via specific binding sites may represent a novel intracellular mechanism for cells to sense mechanical stimulation.

Protein kinase Calpha activates c-Src and induces podosome formation via AFAP-110

We report that the actin filament-associated protein AFAP-110 is required to mediate protein kinase Calpha (PKCalpha) activation of the nonreceptor tyrosine kinase c-Src and the subsequent formation of podosomes. Immunofluorescence analysis demonstrated that activation of PKCalpha by phorbol 12-myristate 13-acetate (PMA), or ectopic expression of constitutively activated PKCalpha, directs AFAP-110 to colocalize with and bind to the c-Src SH3 domain, resulting in activation of the tyrosine kinase. Activation of c-Src then directs the formation of podosomes, which contain cortactin, AFAP-110, actin, and c-Src. In a cell line (CaOV3) that has very little or no detectable AFAP-110, PMA treatment was unable to activate c-Src or effect podosome formation. Ectopic expression of AFAP-110 in CaOV3 cells rescued PKCalpha-mediated activation of c-Src and elevated tyrosine phosphorylation levels and subsequent formation of podosomes. Neither expression of activated PKCalpha nor treatment with PMA was able to induce these changes in CAOV3 cells expressing mutant forms of AFAP-110 that are unable to bind to, or colocalize with, c-Src. We hypothesize that one major function of AFAP-110 is to relay signals from PKCalpha that direct the activation of c-Src and the formation of podosomes.

Analysis of the role of the leucine zipper motif in regulating the ability of AFAP-110 to alter actin filament integrity

AFAP-110 has an intrinsic ability to alter actin filament integrity as an actin filament crosslinking protein. This capability is regulated by a carboxy terminal leucine zipper (Lzip) motif. The Lzip motif facilitates self-association stabilizing the AFAP-110 multimers. Deletion of the Lzip motif (AFAP-110(Deltalzip)) reduces the stability of the AFAP-110 multimer and concomitantly increases its ability to crosslink actin filaments, in vitro, and to activate cSrc and alter actin filament integrity, in vivo. We sought to determine how the Lzip motif regulates AFAP-110 function. Substitution of the c-Fos Lzip motif in place of the AFAP-110 Lzip motif (AFAP-110(fos)) was predicted to preserve the alpha-helical structure while changing the sequence. To alter the structure of the alpha-helix, a leucine to proline mutation was generated in the AFAP-110 alpha-helical Lzip motif (AFAP-110(581P)), which largely preserved the sequence. The helix mutants, AFAP-110(Deltalzip), AFAP-110(fos), and AFAP-110(581P), demonstrated reduced multimer stability with an increased capacity to crosslink actin filaments, in vitro, relative to AFAP-110. An analysis of opposing binding sites indicated that the carboxy terminus/Lzip motif can contact sequences within the amino terminal pleckstrin homology (PH1) domain indicating an auto-inhibitory mechanism for regulating multimer stability and actin filament crosslinking. In vivo, only AFAP-110(Deltalzip) and AFAP-110(581P) were to activate cSrc and to alter cellular actin filament integrity. These data indicate that the intrinsic ability of AFAP-110 to crosslink actin filaments is dependent upon both the sequence and structure of the Lzip motif, while the ability of the Lzip motif to regulate AFAP-110-directed activation of cSrc and changes in actin filament integrity in vivo is dependent upon the structure or presence of the Lzip motif. We hypothesize that the intrinsic ability of AFAP-110 to crosslink actin filaments or activate cSrc are distinct functions.

Actin cytoskeleton remodelling via local inhibition of contractility at discrete microdomains

Activation of conventional protein kinase C by phorbol ester triggers the Src-dependent remodelling of the actin cytoskeleton and the formation of podosomes in vascular smooth muscle cells. Rearrangement of actin cytoskeleton in response to phorbol-12,13-dibutyrate is characterised by the simultaneous disassembly of peripheral actin stress fibres and focal adhesions, focal de novo actin polymerisation and actomyosin contraction in the cell center, indicating a spatially and temporally segregated, differential modulation of actin-cytoskeleton stability and turnover. Taking advantage of the prominent actin cytoskeleton in A7r5 cells we show here, that the molecular basis for the local inhibition of contractility is the specific recruitment of p190RhoGAP to specialised microdomains at the focal adhesion/stress fibre interface, which are constitutively enriched in cortactin. The microdomains contain structurally altered actin filaments inaccessible to phalloidin. However, the filaments remain decorated with high molecular weight tropomyosins. Clustering of cortactin during podosome formation causes the rapid, local dispersion of myosin and tropomyosin, and interferes with the F-actin binding of h1calponin, consistent with a RhoGAP-mediated reduction of contractility. Phorbol ester-induced podosome formation is efficiently blocked by expression of constitutively active Dia1, which leads to the dispersion of cortactin. The results provide direct evidence for the spatially restricted inhibition of contractility via the recruitment and accumulation of cortactin and p190RhoGAP.

PI3K induced actin filament remodeling through Akt and p70S6K1: implication of essential role in cell migration

This study was designed to identify the molecular mechanisms of phosphatidylinositol 3-kinase (PI3K)-induced actin filament remodeling and cell migration. Expression of active forms of PI3K, v-P3k or Myr-P3k, was sufficient to induce actin filament remodeling to lead to an increase in cell migration, as well as the activation of Akt in chicken embryo fibroblast (CEF) cells. Either the inhibition of PI3K activity using a PI3K-specific inhibitor, LY-294002, or the disruption of Akt activity restored the integrity of actin filaments in CEF cells and inhibited PI3K-induced cell migration. We also found that expression of an activated form of Akt (Myr-Akt) was sufficient to remodel actin filaments to lead to an increase in cell migration, which was unable to be inhibited by the presence of LY-294002. Furthermore, we found that p70S6K1 kinase was a downstream molecule that can mediate the effects of both PI3K and Akt on actin filaments and cell migration. Overexpression of an active form of p70S6K1 was sufficient to induce actin filament remodeling and cell migration in CEF cells, which requires Rac activity. These results demonstrate that activation of PI3K activity alone is sufficient to remodel actin filaments to increase cell migration through the activation of Akt and p70S6K1 in CEF cells.

The SH4-Unique-SH3-SH2 domains dictate specificity in signaling that differentiate c-Yes from c-Src

c-Src and c-Yes are highly homologous members of the Src family of non-receptor tyrosine kinases. The overall sequence similarity between c-Src and c-Yes allows them to perform many overlapping functions. However, the phenotypes of the c-src and c-yes knockout mice, and cells derived from them, are quite different, indicating functional specificity between the two proteins. Specifically, c-src-/- cells are deficient in several processes that require dynamic regulation of the actin cytoskeleton. In order to begin to understand why c-Yes is unable to compensate for c-Src signaling, we used a series of Src/Yes chimeras in which the non-catalytic functional domains of Src527F were replaced by those of c-Yes. Using chicken embryo fibroblasts as a model system, our results indicate that the c-Yes N-terminal SH4-Unique domains are sufficient to inhibit the ability of Src527F to alter cell morphology, induce actin filament rearrangements or stimulate motility or invasive potential. The data also indicate that the SH4-Unique-SH3-SH2 domains of c-Yes work cooperatively and prevent activation of signaling proteins associated with Src527F transformation, including activation of phosphatidylinositol 3-kinase, phosphorylation of c-Raf and Akt and downregulation of RhoA-GTP. These data indicate that c-Yes may not modulate signals associated with c-Src-induced changes in actin filament integrity and may explain why c-Yes fails to compensate for c-Src signaling in src-/- cells.

Hydrogen peroxide formation and actin filament reorganization by Cdc42 are essential for ethanol-induced in vitro angiogenesis

This report focuses on the identification of the molecular mechanisms of ethanol-induced in vitro angiogenesis. The manipulation of angiogenesis is an important therapeutic approach for the treatment of cancer, cardiovascular diseases, and chronic inflammation. Our results showed that ethanol stimulation altered the integrity of actin filaments and increased the formation of lamellipodia and filopodia in SVEC4-10 cells. Further experiments demonstrated that ethanol stimulation increased cell migration and invasion and induced in vitro angiogenesis in SVEC4-10 cells. Mechanistically, ethanol stimulation activated Cdc42 and produced H(2)O(2) a reactive oxygen species intermediate in SVEC4-10 cells. Measuring the time course of Cdc42 activation and H(2)O(2) production upon ethanol stimulation revealed that the Cdc42 activation and the increase of H(2)O(2) lasted more than 3 h, which indicates the mechanisms of the long duration effects of ethanol on the cells. Furthermore, either overexpression of a constitutive dominant negative Cdc42 or inhibition of H(2)O(2) production abrogated the effects of ethanol on SVEC4-10 cells, indicating that both the activation of Cdc42 and the production of H(2)O(2) are essential for the actions of ethanol. Interestingly, we also found that overexpression of a constitutive dominant positive Cdc42 itself was sufficient to produce H(2)O(2) and to induce in vitro angiogenesis. Taken together, our results suggest that ethanol stimulation can induce H(2)O(2) production through the activation of Cdc42, which results in reorganizing actin filaments and increasing cell motility and in vitro angiogenesis.

Protein expression levels of the Src activating protein AFAP are developmentally regulated in brain

The Src family of nonreceptor tyrosine kinases plays an important role in modulating signals that affect growth cone extension, neuronal differentiation, and brain development. Recent reports indicate that the Src SH2/SH3 binding partner AFAP-110 has the capacity to modulate actin filament integrity as a cSrc activating protein and as an actin filament bundling protein. Both AFAP-110 and a brain specific isoform called AFAP-120 (collectively referred to as AFAP) exist at high levels in chick embryo brain. We sought to identify the localization of AFAP in mouse brain in order to identify its expression pattern and potential role as a cellular modulator of Src family kinase activity and actin filament integrity in the brain. In E16 mouse embryos, AFAP expression levels were very high and concentrated in the olfactory bulb, cortex, forebrain, cerebellum, and various peripheral sensory structures. In P3 mouse pups, overall expression was reduced compared to E16 embryos, and AFAP was found primarily in olfactory bulb, cortex, and cerebellum. AFAP expression levels were significantly reduced in adult mice, with high expression levels only detected in the olfactory bulb. Western blot analysis indicated that concentrated expression of AFAP correlates well with the AFAP-120 isoform, which appears to be a splice variant of AFAP-110. As the expression pattern of AFAP overlaps with the reported expression patterns of cSrc and Fyn, we hypothesize that AFAP is positioned to modulate signal transduction cascades that direct activation of these nonreceptor tyrosine kinases and concomitant cellular changes that occur in actin filaments during brain development.

Molecular cloning of actin filament-associated protein: a putative adaptor in stretch-induced Src activation

Mechanical stretch-induced activation of c-Src is an important step for signal transduction of stretch-induced fetal rat lung cell proliferation. This process appears to be mediated through actin filament-associated protein (AFAP), encoded by a gene originally cloned from the chicken. In the present study, we cloned the rat AFAP gene from fetal rat lungs. Its mRNA and protein are differentially expressed among various tissues. The protein is colocalized with actin filaments in fetal rat lung epithelial cells and fibroblasts. Mechanical stretch increased tyrosine phosphorylation of rat AFAP and its binding to c-Src within the initial several minutes. Src SH2 and SH3 binding motifs are highly conserved in the AFAP proteins (from chicken, rat to human). On the basis of the molecular structure of AFAP protein, we speculate that it is an adaptor in mechanical stretch-induced activation of c-Src. A novel model of mechanoreception is proposed.

Src protein kinase pp60c-src influences adhesion stabilization of HT-29 colon carcinoma cells to extracellular matrix components under dynamic conditions of laminar flow

Tumor cell adhesion to extracellular matrix (ECM) and its stabilization are important determinants in metastasis formation, and they are mediated, in part, by integrins and regulated by a variety of protein kinases. Protein tyrosine kinase pp60c-src is found in adhesion-dependent focal adhesion plaques where it may regulate different integrin-mediated signaling cascades. Using human HT-29 colon carcinoma cells stably transfected with pp60c-src--specific antisense oligonucleotides (HT-29AS15), we investigated the role of pp60c-src in integrin-mediated adhesion and its stabilization to ECM components collagen I or IV under static and laminar fluid flow conditions. Under static adhesion conditions transfection of pp60c-src antisense oligonucleotides did not modify adhesive properties. Phosphorylation of focal adhesion kinase (FAK) and paxillin induced by static adhesion to collagen I or IV were similar in HT-29P and HT-29AS15 cells. However, using hydrodynamic conditions in a laminar flow chamber we found a slight reduction in early adhesion events and an even greater difference in adhesion stabilization rates (ASRs). The transfected cells showed a significant reduction in their ability to withstand shear forces and stabilize adhesive bounds. These changes correlated with the cellular expression levels of pp60c-src. Our results suggest that pp60c-src may be involved in stabilization of dynamic HT-29 cell adhesion to ECM components, and this kinase appears to be part of a mechanosensory protein complex during integrin-mediated cell adhesion.

PC phosphorylation increases the ability of AFAP-110 to cross-link actin filaments

The actin filament-associated protein and Src-binding partner, AFAP-110, is an adaptor protein that links signaling molecules to actin filaments. AFAP-110 binds actin filaments directly and multimerizes through a leucine zipper motif. Cellular signals downstream of Src(527F) can regulate multimerization. Here, we determined recombinant AFAP-110 (rAFAP-110)-bound actin filaments cooperatively, through a lateral association. We demonstrate rAFAP-110 has the capability to cross-link actin filaments, and this ability is dependent on the integrity of the carboxy terminal actin binding domain. Deletion of the leucine zipper motif or PKC phosphorylation affected AFAP-110's conformation, which correlated with changes in multimerization and increased the capability of rAFAP-110 to cross-link actin filaments. AFAP-110 is both a substrate and binding partner of PKC. On PKC activation, stress filament organization is lost, motility structures form, and AFAP-110 colocalizes strongly with motility structures. Expression of a deletion mutant of AFAP-110 that is unable to bind PKC blocked the effect of PMA on actin filaments. We hypothesize that upon PKC activation, AFAP-110 can be cooperatively recruited to newly forming actin filaments, like those that exist in cell motility structures, and that PKC phosphorylation effects a conformational change that may enable AFAP-110 to promote actin filament cross-linking at the cell membrane.

The intrinsic ability of AFAP-110 to alter actin filament integrity is linked with its ability to also activate cellular tyrosine kinases

The actin filament-associated protein of 110 kDa (AFAP-110) is a Src binding partner that represents a potential modulator of actin filament integrity in response to cellular signals. Previous reports have demonstrated that AFAP-110 is capable of directly binding and altering actin filaments. Deletion of the leucine zipper motif of AFAP-110 (AFAP-110(Deltalzip)) has been shown to induce a phenotype which resembles Src-transformed cells, by repositioning actin filaments into rosettes. This deletion also mimics a conformational change in AFAP-110 that is detected in Src-transformed cells. The results presented here indicate that unlike AFAP-110, AFAP-110(Deltalzip) is capable of activating cellular tyrosine kinases, including Src family members, and that AFAP-110(Deltalzip) itself is hyperphosphorylated. The newly tyrosine phosphorylated proteins and activated Src-family members appear to be associated with actin-rich lamellipodia. A point mutation that alters the SH3-binding motif of AFAP-110(Deltalzip) prevents it from activating tyrosine kinases and altering actin filament integrity. In addition, a deletion within a pleckstrin homology (PH) domain of AFAP-110(Deltalzip) will also revert its effects upon actin filaments. Lastly, dominant-positive RhoA(V14) will block the ability of AFAP-110(Deltalzip) from inducing actin filament rosettes, but does not inhibit Src activation. Thus, conformational changes in AFAP-110 enable it to activate cellular kinases in a mechanism requiring SH3 and/or PH domain interactions. We hypothesize that cellular signals which alter AFAP-110 conformation, enable it to activate cellular kinases such as cSrc, which then direct changes in actin filament integrity in a Rho-dependent fashion.

The actin filament-associated protein AFAP-110 is an adaptor protein that modulates changes in actin filament integrity

The actin filament-associated protein of 110 kDa (AFAP-110) was first identified as an SH3/SH2 binding partner for the nonreceptor tyrosine kinase, Src. Subsequent data have demonstrated that AFAP-110 can interact with other Src family members. AFAP-110 contains additional protein binding modules including two pleckstrin homology domains, a leucine zipper motif and a target sequence for serine/threonine phosphorylation. AFAP-110 interacts with actin filaments directly via a carboxy terminal actin-binding domain. Thus AFAP-110 may function as an adaptor protein by linking Src family members and/or other signaling proteins to actin filaments. AFAP-110 also has an intrinsic capability to alter actin filament integrity that can be revealed upon conformational changes associated with phosphorylation or mutagenesis. Recent data has indicated that AFAP-110 may also serve to activate cSrc in response to this conformational change as well. Thus, AFAP-110 may function in several ways by (1) acting as an adaptor protein that links signaling molecules to actin filaments, (2) serving as a platform for the construction of larger signaling complexes, (3) serving as an activator of Src family kinases in response to cellular signals that alter its conformation and (4) directly effecting actin filament organization as an actin filament cross-linking protein. Here, we will review the structure and function of AFAP-110 as well as potential binding partners and effectors of AFAP-110's ability to alter actin filament integrity.

The SH3 domain directs acto-myosin-dependent targeting of v-Src to focal adhesions via phosphatidylinositol 3-kinase

The v-Src oncoprotein is translocated to integrin-linked focal adhesions, where its tyrosine kinase activity induces adhesion disruption and cell transformation. We previously demonstrated that the intracellular targeting of Src is dependent on the actin cytoskeleton, under the control of the Rho family of small G proteins. However, the assembly of v-Src into focal adhesions does not require its catalytic activity or myristylation-dependent membrane association. Here, we report that the SH3 domain is essential for the assembly of focal adhesions containing the oncoprotein by mediating a switch from a microtubule-dependent, perinuclear localization to actin-associated focal adhesions; furthermore, v-Src translocation to focal adhesions requires myosin activity, at least under normal conditions when the actin cytoskeleton is being dynamically regulated. Although the SH3 domain of v-Src is also necessary for its association with focal adhesion kinase (FAK), which is often considered a likely candidate mediator of focal adhesion targeting via its carboxy-terminal targeting sequence, we show here that binding to FAK is not essential for the targeting of v-Src to focal adhesions. The p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase also associates with v-Src in an SH3-dependent manner, but in this case inhibition of PI 3-kinase activity suppressed assembly of focal adhesions containing the oncoprotein. Thus, the Src SH3 domain, which binds PI 3-kinase and which is necessary for activation of Akt downstream, is required for the actin-dependent targeting of v-Src to focal adhesions.

Ischemia reperfusion-induced dynamic changes of protein tyrosine phosphorylation during human lung transplantation

BACKGROUND

We have recently demonstrated that more than 20% of lung cells undergo apoptosis within the first 2 hr of graft reperfusion after human lung transplantation. It has been found that changes of protein tyrosine phosphorylation are involved in the regulation of apoptosis in various cell types.

METHODS

To determine the protein tyrosine phosphorylation status and related biochemistry changes, lung tissue biopsies were collected from six human lung transplant procedures after cold ischemic preservation (2-5 hr at 4 degrees C), after completing the implantation procedure (approximately 1 hr), and 1 or 2 hr after graft reperfusion. Western blotting was performed to determine protein tyrosine phosphorylation and several signal transduction proteins. Protein tyrosine kinase (PTK) and protein tyrosine phosphatase (PTP) activities were also measured.

RESULTS

Protein tyrosine phosphorylation was significantly increased after lung implantation and before reperfusion, and significantly decreased during the first 2 hr of graft reperfusion. The activity of Src PTKs was reduced by 50% during graft reperfusion, which was associated with a decrease of Src proteins and human actin filament associated protein, a cofactor for Src activation. PTP activity significantly decreased after lung implantation and remained at a low level 1 hr after reperfusion. After 2 hr of reperfusion, however, PTP activity returned to the basal level.

CONCLUSION

These dynamic changes of PTK and PTP likely explain the observed alterations of protein tyrosine phosphorylation. The significant decrease in protein tyrosine phosphorylation may be related to the observed apoptotic cell death during human lung transplantation.

The carboxy terminus of AFAP-110 modulates direct interactions with actin filaments and regulates its ability to alter actin filament integrity and induce lamellipodia formation

The actin filament-associated protein AFAP-110 is an SH2/SH3 binding partner for Src. AFAP-110 contains several protein-binding motifs in its amino terminus and has been hypothesized to function as an adaptor molecule that could link signaling proteins to actin filaments. Recent studies using deletional mutagenesis demonstrated that AFAP-110 can alter actin filament integrity in SV40 transformed Cos-1 cells. Thus, AFAP-110 may be positioned to modulate the effects of Src upon actin filaments. In this report, we sought to determine whether (a) AFAP-110 could interact with actin filaments directly and (b) deletion mutants could affect actin filament integrity and cell shape in untransformed fibroblast cells. The data demonstrate that the carboxy terminus of AFAP-110 is both necessary and sufficient for actin filament association, in vivo and in vitro. Analysis of the carboxy terminus revealed a mean 40% similarity with other known actin-binding motifs, indicating a mechanism for binding to actin filaments. AFAP-110 can also induce lamellipodia formation. Contiguous with the alpha-helical, actin-binding motif is an alpha-helical, leucine zipper motif. Deletion of the leucine zipper motif (AFAP(Deltalzip)) followed by cellular expression enabled AFAP(Deltalzip) to alter actin filament integrity and cell shape in untransformed cells as evidenced by the induction of lamellipodia formation. We hypothesize that AFAP-110 may be an important signaling protein that can directly modulate changes in actin filament integrity and induce lamellipodia formation.

Leucine zipper-mediated homodimerization of the adaptor protein c-Cbl. A role in c-Cbl's tyrosine phosphorylation and its association with epidermal growth factor receptor

The 120-kDa proto-oncogenic protein c-Cbl is a multidomain adaptor protein that is phosphorylated in response to the stimulation of a broad range of cell surface receptors and participates in the assembly of signaling complexes that are formed as a result of the activation of various signal transduction pathways. Several structural features of c-Cbl, including the phosphotyrosine-binding domain, proline-rich domain, and motifs containing phosphotyrosine and phosphoserine residues, mediate the association of c-Cbl with other components of these complexes. In addition to those domains that have been demonstrated to play a role in the binding of c-Cbl to other signaling molecules, c-Cbl also contains a RING finger motif and a putative leucine zipper. In this study, we demonstrate that the previously identified putative leucine zipper mediates the formation of Cbl homodimers. Using the yeast two-hybrid system, we show that deletion of the leucine zipper domain is sufficient to abolish Cbl homodimerization, while Cbl mutants carrying extensive N-terminal truncations retain the ability to dimerize with the full-length Cbl. The requirement of the leucine zipper for the homodimerization of Cbl was confirmed by in vitro binding assays, using deletion variants of the C-terminal half of Cbl with and without the leucine zipper domain, and in cells using Myc and green fluorescent protein (GFP) N-terminal-tagged Cbl variants. In cells, the deletion of the leucine zipper caused a decrease in both the tyrosine phosphorylation of Cbl and its association with the epidermal growth factor receptor following stimulation with epidermal growth factor, thus demonstrating a role for the leucine zipper in c-Cbl's signaling functions. Thus, the leucine zipper domain enables c-Cbl to homodimerize, and homodimerization influences Cbl's signaling function, modulating the activity of Cbl itself and/or affecting Cbl's associations with other signaling proteins in the cell.

Monoclonal antibodies directed against AFAP-110 recognize species-specific and conserved epitopes

The actin filament-associated protein, AFAP-110, is a Src SH2/SH3 binding partner that can modulate changes in actin filament structure. AFAP-110 contains a carboxy terminal motif that facilitates actin filament interactions, as well as amino terminal protein binding motifs, including an SH3 binding motif, two SH2 binding motifs, and two Pleckstrin homology domains. Two monoclonal antibodies (MAbs) were developed that recognized epitopes in either the amino terminus (MAb 4C3) or the carboxy terminus (anti-AFAP-110) of AFAP-110. Site-directed mutations that change key proline residues to alanine in the SH3 binding motif and an adjacent proline-rich motif abrogated MAb 4C3 binding. These same mutations have been shown to prevent SH3 interactions between AFAP-110 and Src527F. These data indicate that MAb 4C3 recognizes an epitope that is part of the SH3 binding motif. Interestingly, MAb 4C3 is not efficiently reactive with mammalian homologs of AFAP-110. Sequence analysis of a putative cDNA clone that encodes the amino terminus of the human AFAP-110 isoform predicted a one amino acid difference within this epitope, indicating a mechanism for species-specific binding by MAb 4C3. A second, MAb anti-AFAP-110, recognizes AFAP-110 across species and binds to an epitope within the carboxy terminus. This epitope includes the 5th heptad repeat of the carboxy terminal, leucine zipper motif (amino acids 592-598)--a motif that facilitates self-associations and may regulate the function of AFAP-110. These MAbs will be useful for analyzing the effects of AFAP-110 upon cell morphology and actin filament integrity. In addition, the avian-specific MAb 4C3 may be useful for studying the effects of avian AFAP-110 constructs expressed in mammalian cells, by providing an internal epitope tag.