Alternative splicing of the actin binding domain of human cortactin affects cell migration

Stiffness promotes cell migration, invasion, and invadopodia in nasopharyngeal carcinoma by regulating the WT-CTTN level

Matrix stiffness potently promotes the malignant phenotype in various biological contexts. Therefore, identification of gene expression to participate in mechanical force signals transduced into downstream biochemical signaling will contribute substantially to the advances in nasopharyngeal carcinoma (NPC) treatment. In the present study, we detected that cortactin (CTTN) played an indispensable role in matrix stiffness-induced cell migration, invasion, and invadopodia formation. Advances in cancer research have highlighted that dysregulated alternative splicing contributes to cancer progression as an oncogenic driver. However, whether WT-CTTN or splice variants (SV1-CTTN or SV2-CTTN) regulate matrix stiffness-induced malignant phenotype is largely unknown. We proved that alteration of WT-CTTN expression modulated matrix stiffness-induced cell migration, invasion, and invadopodia formation. Considering that splicing factors might drive cancer progression through positive feedback loops, we analyzed and showed how the splicing factor PTBP2 and TIA1 modulated the production of WT-CTTN. Moreover, we determined that high stiffness activated PTBP2 expression. Taken together, our findings showed that the PTBP2-WT-CTTN level increases upon stiffening and then promotes cell migration, invasion, and invadopodia formation in NPC.

Targeted splicing therapy: new strategies for colorectal cancer

RNA splicing is the process of forming mature mRNA, which is an essential phase necessary for gene expression and controls many aspects of cell proliferation, survival, and differentiation. Abnormal gene-splicing events are closely related to the development of tumors, and the generation of oncogenic isoform in splicing can promote tumor progression. As a main process of tumor-specific splicing variants, alternative splicing (AS) can promote tumor progression by increasing the production of oncogenic splicing isoforms and/or reducing the production of normal splicing isoforms. This is the focus of current research on the regulation of aberrant tumor splicing. So far, AS has been found to be associated with various aspects of tumor biology, including cell proliferation and invasion, resistance to apoptosis, and sensitivity to different chemotherapeutic drugs. This article will review the abnormal splicing events in colorectal cancer (CRC), especially the tumor-associated splicing variants arising from AS, aiming to offer an insight into CRC-targeted splicing therapy.

Wiskott-Aldrich syndrome protein forms nuclear condensates and regulates alternative splicing

The diverse functions of WASP, the deficiency of which causes Wiskott-Aldrich syndrome (WAS), remain poorly defined. We generated three isogenic WAS models using patient induced pluripotent stem cells and genome editing. These models recapitulated WAS phenotypes and revealed that WASP deficiency causes an upregulation of numerous RNA splicing factors and widespread altered splicing. Loss of WASP binding to splicing factor gene promoters frequently leads to aberrant epigenetic activation. WASP interacts with dozens of nuclear speckle constituents and constrains SRSF2 mobility. Using an optogenetic system, we showed that WASP forms phase-separated condensates that encompasses SRSF2, nascent RNA and active Pol II. The role of WASP in gene body condensates is corroborated by ChIPseq and RIPseq. Together our data reveal that WASP is a nexus regulator of RNA splicing that controls the transcription of splicing factors epigenetically and the dynamics of the splicing machinery through liquid-liquid phase separation.

TGF-β-mediated epithelial-mesenchymal transition and tumor-promoting effects in CMT64 cells are reflected in the transcriptomic signature of human lung adenocarcinoma

Epithelial-mesenchymal transition (EMT) is a cellular process during which epithelial cells acquire mesenchymal phenotypes. Cancer cells undergo EMT to acquire malignant features and TGF-β is a key regulator of EMT. Here, we demonstrate for the first time that TGF-β could elicit EMT in a mouse lung adenocarcinoma cell line. TGF-β signaling activation led to cell morphological changes corresponding to EMT and enhanced the expression of mesenchymal markers and EMT-associated transcription factors in CMT64 lung cancer cells. RNA-sequencing analyses revealed that TGF-β increases expression of Tead transcription factors and an array of Tead2 target genes. TGF-β stimulation also resulted in alternative splicing of several genes including Cd44, tight junction protein 1 (Tjp1), and Cortactin (Cttn). In parallel with EMT, TGF-β enhanced cell growth of CMT64 cells and promoted tumor formation in a syngeneic transplantation model. Of clinical importance, the expression of TGF-β-induced genes identified in CMT64 cells correlated with EMT gene signatures in human lung adenocarcinoma tissue samples. Furthermore, TGF-β-induced gene enrichment was related to poor prognosis, underscoring the tumor-promoting role of TGF-β signaling in lung adenocarcinoma. Our cellular and syngeneic transplantation model would provide a simple and useful experimental tool to study the significance of TGF-β signaling and EMT.

Antisense Oligonucleotide-Mediated Reduction of HDAC6 Does Not Reduce Tau Pathology in P301S Tau Transgenic Mice

Acetylation of tau protein is dysregulated in Alzheimer's Disease (AD). It has been proposed that acetylation of specific sites in the KXGS motif of tau can regulate phosphorylation of nearby residues and reduce the propensity of tau to aggregate. Histone deacetylase 6 (HDAC6) is a cytoplasmic enzyme involved in deacetylation of multiple targets, including tau, and it has been suggested that inhibition of HDAC6 would increase tau acetylation at the KXGS motifs and thus may present a viable therapeutic approach to treat AD. To directly test the contribution of HDAC6 to tau pathology, we intracerebroventricularly injected an antisense oligonucleotide (ASO) directed against HDAC6 mRNA into brains of P301S tau mice (PS19 model), which resulted in a 70% knockdown of HDAC6 protein in the brain. Despite a robust decrease in levels of HDAC6, no increase in tau acetylation was observed. Additionally, no change of tau phosphorylation or tau aggregation was detected upon the knockdown of HDAC6. We conclude that HDAC6 does not impact tau pathology in PS19 mice.

Cortactin in Epithelial-Mesenchymal Transition

Cortactin, a member of the actin-binding protein family, plays an important role in cell movement involving the cytoskeleton, as cell movement mediated by cortactin may induce the epithelial–mesenchymal transition. Cortactin participates in tumor proliferation, migration, and invasion and other related disease processes by binding to different proteins and participating in different pathways and mechanisms that induce the occurrence of these disease processes. Therefore, this article reviews the correlations between cortactin, the actin cytoskeleton, and the epithelial–mesenchymal transition and discusses its clinical importance in tumor therapy.

Hypoxia-induced alternative splicing: the 11th Hallmark of Cancer

Hypoxia-induced alternative splicing is a potent driving force in tumour pathogenesis and progression. In this review, we update currents concepts of hypoxia-induced alternative splicing and how it influences tumour biology. Following brief descriptions of tumour-associated hypoxia and the pre-mRNA splicing process, we review the many ways hypoxia regulates alternative splicing and how hypoxia-induced alternative splicing impacts each individual hallmark of cancer. Hypoxia-induced alternative splicing integrates chemical and cellular tumour microenvironments, underpins continuous adaptation of the tumour cellular microenvironment responsible for metastatic progression and plays clear roles in oncogene activation and autonomous tumour growth, tumor suppressor inactivation, tumour cell immortalization, angiogenesis, tumour cell evasion of programmed cell death and the anti-tumour immune response, a tumour-promoting inflammatory response, adaptive metabolic re-programming, epithelial to mesenchymal transition, invasion and genetic instability, all of which combine to promote metastatic disease. The impressive number of hypoxia-induced alternative spliced protein isoforms that characterize tumour progression, classifies hypoxia-induced alternative splicing as the 11th hallmark of cancer, and offers a fertile source of potential diagnostic/prognostic markers and therapeutic targets.

Splicing Busts a Move: Isoform Switching Regulates Migration

Cell migration is essential for normal development, neural patterning, pathogen eradication, and cancer metastasis. Pre-mRNA processing events such as alternative splicing and alternative polyadenylation result in greater transcript and protein diversity as well as function and activity. A critical role for alternative pre-mRNA processing in cell migration has emerged in axon outgrowth during neuronal development, immune cell migration, and cancer metastasis. These findings suggest that migratory signals result in expression changes of post-translational modifications of splicing or polyadenylation factors, leading to splicing events that generate promigratory isoforms. We summarize this recent progress and suggest emerging technologies that may facilitate a deeper understanding of the role of alternative splicing and polyadenylation in cell migration.

High cortactin expression in B-cell acute lymphoblastic leukemia is associated with increased transendothelial migration and bone marrow relapse

Cancer is a major cause of death in children worldwide, with B-lineage cell acute lymphoblastic leukemia (B-ALL) being the most frequent childhood malignancy. Relapse, treatment failure and organ infiltration worsen the prognosis, warranting a better understanding of the implicated mechanisms. Cortactin is an actin-binding protein involved in cell adhesion and migration that is overexpressed in many solid tumors and in adult B-cell chronic lymphocytic leukemia. Here, we investigated cortactin expression and potential impact on infiltration and disease prognosis in childhood B-ALL. B-ALL cell lines and precursor cells from bone marrow (BM) and cerebrospinal fluid (CSF) of B-ALL patients indeed overexpressed cortactin. In CXCL12-induced transendothelial migration assays, transmigrated B-ALL cells had highest cortactin expression. In xenotransplantation models, only cortactinhigh-leukemic cells infiltrated lungs, brain, and testis; and they colonized more easily hypoxic BM organoids. Importantly, cortactin-depleted B-ALL cells were significantly less efficient in transendothelial migration, organ infiltration and BM colonization. Clinical data highlighted a significant correlation between high cortactin levels and BM relapse in drug-resistant high-risk B-ALL patients. Our results emphasize the importance of cortactin in B-ALL organ infiltration and BM relapse and its potential as diagnostic tool to identify high-risk patients and optimize their treatments.

Distinct Functions for Mammalian CLASP1 and -2 During Neurite and Axon Elongation

Mammalian cytoplasmic linker associated protein 1 and -2 (CLASP1 and -2) are microtubule (MT) plus-end tracking proteins that selectively stabilize MTs at the edge of cells and that promote MT nucleation and growth at the Golgi, thereby sustaining cell polarity. In vitro analysis has shown that CLASPs are MT growth promoting factors. To date, a single CLASP1 isoform (called CLASP1α) has been described, whereas three CLASP2 isoforms are known (CLASP2α, -β, and -γ). Although CLASP2β/γ are enriched in neurons, suggesting isoform-specific functions, it has been proposed that during neurite outgrowth CLASP1 and -2 act in a redundant fashion by modulating MT dynamics downstream of glycogen synthase kinase 3 (GSK3). Here, we show that in differentiating N1E-115 neuroblastoma cells CLASP1 and CLASP2 differ in their accumulation at MT plus-ends and display different sensitivity to GSK3-mediated phosphorylation, and hence regulation. More specifically, western blot (WB) analysis suggests that pharmacological inhibition of GSK3 affects CLASP2 but not CLASP1 phosphorylation and fluorescence-based microscopy data show that GSK3 inhibition leads to an increase in the number of CLASP2-decorated MT ends, as well as to increased CLASP2 staining of individual MT ends, whereas a reduction in the number of CLASP1-decorated ends is observed. Thus, in N1E-115 cells CLASP2 appears to be a prominent target of GSK3 while CLASP1 is less sensitive. Surprisingly, knockdown of either CLASP causes phosphorylation of GSK3, pointing to the existence of feedback loops between CLASPs and GSK3. In addition, CLASP2 depletion also leads to the activation of protein kinase C (PKC). We found that these differences correlate with opposite functions of CLASP1 and CLASP2 during neuronal differentiation, i.e., CLASP1 stimulates neurite extension, whereas CLASP2 inhibits it. Consistent with knockdown results in N1E-115 cells, primary Clasp2 knockout (KO) neurons exhibit early accelerated neurite and axon outgrowth, showing longer axons than control neurons. We propose a model in which neurite outgrowth is fine-tuned by differentially posttranslationally modified isoforms of CLASPs acting at distinct intracellular locations, thereby targeting MT stabilizing activities of the CLASPs and controlling feedback signaling towards upstream kinases. In summary, our findings provide new insight into the roles of neuronal CLASPs, which emerge as regulators acting in different signaling pathways and locally modulating MT behavior during neurite/axon outgrowth.

An update of knowledge on cortactin as a metastatic driver and potential therapeutic target in oral squamous cell carcinoma

Cortactin is a protein encoded by the CTTN gene, localized on chromosome band 11q13. As a result of the amplification of this band, an important event in oral carcinogenesis, CTTN is also usually amplified, promoting the frequent overexpression of cortactin. Cortactin enhances cell migration in oral cancer, playing a key role in the regulation of filamentous actin and of protrusive structures (invadopodia and lamellipodia) on the cell membrane that are necessary for the acquisition of a migratory phenotype. We also analyze a series of emerging functions that cortactin may exert in oral cancer (cell proliferation, angiogenesis, regulation of exosomes, and interactions with the tumor microenvironment). We review its molecular structure, its most important interactions (with Src, Arp2/3 complex, and SH3-binding partners), the regulation of its functions, and its specific oncogenic role in oral cancer. We explore the mechanisms of its overexpression in cancer, mainly related to genetic amplification. We analyze the prognostic implications of the oncogenic activation of cortactin in potentially malignant disorders and in head and neck cancer, where it appears to be relevant in the development of lymph node metastasis. Finally, we discuss its usefulness as a therapeutic target and suggest future research lines.

Cortactin stabilization of actin requires actin-binding repeats and linker, is disrupted by specific substitutions, and is independent of nucleotide state

The actin-binding protein cortactin promotes the formation and maintenance of actin-rich structures, including lamellipodial protrusions in fibroblasts and neuronal dendritic spines. Cortactin cellular functions have been attributed to its activation of the Arp2/3 complex, which stimulates actin branch nucleation, and to its recruitment of Rho family GTPase regulators. Cortactin also binds actin filaments and significantly slows filament depolymerization, but the mechanism by which it does so and the relationship between actin binding and stabilization are unclear. Here we elucidated the cortactin regions that are necessary and sufficient for actin filament binding and stabilization. Using actin cosedimentation assays, we found that the cortactin repeat region binds actin but that the adjacent linker region is required for binding with the same affinity as full-length cortactin. Using total internal reflection fluorescence microscopy to measure the rates of single filament actin depolymerization, we observed that cortactin-actin interactions are sufficient to stabilize actin filaments. Moreover, conserved charged residues in repeat 4 were necessary for high-affinity actin binding, and substitution of these residues significantly impaired cortactin-mediated actin stabilization. Cortactin bound actin with higher affinity than did its paralog, hematopoietic cell-specific Lyn substrate 1 (HS1), and the effects on actin stability were specific to cortactin. Finally, cortactin stabilized ADP-actin filaments, indicating that the stabilization mechanism does not depend on the actin nucleotide state. Together, these results indicate that cortactin binding to actin is necessary and sufficient to stabilize filaments in a concentration-dependent manner, specific to conserved residues in the cortactin repeats, and independent of the actin nucleotide state.

High expression of PTBP1 promote invasion of colorectal cancer by alternative splicing of cortactin

Polypyrimidine tract-binding protein 1 (PTBP1) involving in almost all steps of mRNA regulation including alternative splicing metabolism during tumorigenesis due to its RNA-binding activity. Initially, we found that high expressed PTBP1 and poor prognosis was interrelated in colorectal cancer (CRC) patients with stages II and III CRC, which widely different in prognosis and treatment, by immunohistochemistry. PTBP1 was also upregulated in colon cancer cell lines. In our study, knockdown of PTBP1 by siRNA transfection decreased cell proliferation and invasion in vitro. Denovirus shRNA knockdown of PTBP1 inhibited colorectal cancer growth in vivo. Furthermore, PTBP1 regulates alternative splicing of many target genes involving in tumorgenesis in colon cancer cells. We confirmed that the splicing of cortactin exon 11 which was only contained in cortactin isoform-a, as a PTBP1 target. Knockdown of PTBP1 decreased the expression of cortactin isoform-a by exclusion of exon 11. Also the mRNA levels of PTBP1 and cortactin isoform-a were cooperatively expressed in colorectal cancer tissues. Knocking down cortactin isoform-a significantly decreased cell migration and invasion in colorectal cancer cells. Overexpression of cortactin isoform-a could rescue PTBP1-knockdown effect of cell motility. In summary the study revealed that PTBP1 facilitates colorectal cancer migration and invasion activities by inclusion of cortactin exon 11.

Cortactin function in invadopodia

Actin remodeling plays an essential role in diverse cellular processes such as cell motility, vesicle trafficking or cytokinesis. The scaffold protein and actin nucleation promoting factor Cortactin is present in virtually all actin-based structures, participating in the formation of branched actin networks. It has been involved in the control of endocytosis, and vesicle trafficking, axon guidance and organization, as well as adhesion, migration and invasion. To migrate and invade through three-dimensional environments, cells have developed specialized actin-based structures called invadosomes, a generic term to designate invadopodia and podosomes. Cortactin has emerged as a critical regulator of invadosome formation, function and disassembly. Underscoring this role, Cortactin is frequently overexpressed in several types of invasive cancers. Herein we will review the roles played by Cortactin in these specific invasive structures.

Decreased expression of cortactin in the schizophrenia brain

Schizophrenia is a severe psychiatric disorder that is characterized by a wide array of symptoms and a complex neuropathology. A well-characterized neurobiological feature of schizophrenia is abnormal synaptic plasticity, although the mechanisms underlying this are not fully understood. Numerous studies have demonstrated a link between proper functioning of the cytoskeleton and synaptic plasticity. The actin-related protein-2/3 (Arp2/3) complex is responsible for the nucleation of new actin filaments and elongation of existing actin filaments and is thus crucial to cytoskeletal dynamics, especially actin polymerization and organization. To determine whether the Arp2/3 complex is abnormally expressed in schizophrenia, we measured the protein expression of Arp2 and Arp3, as well as Arp2/3 complex binding partners and associated proteins including cortactin, neuronal-Wiskott-Aldrich syndrome protein (WASP), WASP-family verprolin homologous protein 1 (WAVE1), and Abelson interactor 1 (Abi1) in the superior temporal gyrus of paired schizophrenia and comparison participants. No changes were found in Arp2, Arp3, neuronal-WASP, WAVE1, or Abi1. However, all three isoforms of cortactin were decreased in schizophrenia. Specifically, the 62 kDa isoform was decreased by 43%; the 71 kDa isoform was decreased by 32%; and the 58 kDa isoform was decreased by 35%. Cortactin regulates branching of filamentous actin through its binding and activation of the Arp2/3 complex, and it is thus critical to the formation of stable actin networks. These findings contribute to a growing body of evidence implicating altered cytoskeletal dynamics in schizophrenia.

A novel splice variant of supervillin, SV5, promotes carcinoma cell proliferation and cell migration

Supervillin is an actin-associated protein that regulates actin dynamics by interacting with Myosin II, F-actin, and Cortactin to promote cell contractility and cell motility. Two splicing variants of human Supervillin (SV1 and SV4) have been reported in non-muscle cells; SV1 lacks 3 exons present in the larger isoform SV4. SV2, also called archvillin, is present in striated muscle; SV3, also called smooth muscle archvillin or SmAV, was cloned from smooth muscle. In the present study, we identify a novel splicing variant of Supervillin (SV5). SV5 contains a new splicing pattern. In the mouse tissues and cell lines examined, SV5 was predominantly expressed in skeletal and cardiac muscles and in proliferating cells, but was virtually undetectable in most normal tissues. Using RNAi and rescue experiments, we show here that SV5 displays altered functional properties in cancer cells, and regulates cell proliferation and cell migration.

Tuning cell migration: contractility as an integrator of intracellular signals from multiple cues

There has been immense progress in our understanding of the factors driving cell migration in both two-dimensional and three-dimensional microenvironments over the years. However, it is becoming increasingly evident that even though most cells share many of the same signaling molecules, they rarely respond in the same way to migration cues. To add to the complexity, cells are generally exposed to multiple cues simultaneously, in the form of growth factors and/or physical cues from the matrix. Understanding the mechanisms that modulate the intracellular signals triggered by multiple cues remains a challenge. Here, we will focus on the molecular mechanism involved in modulating cell migration, with a specific focus on how cell contractility can mediate the crosstalk between signaling initiated at cell-matrix adhesions and growth factor receptors.

Alternative Splicing in Adhesion- and Motility-Related Genes in Breast Cancer

Breast cancer is the most common tumor and the second leading cause of cancer death among woman, mainly caused by the metastatic spread. Tumor invasiveness is due to an altered expression of adhesion molecules. Among them, semaphorins are of peculiar interest. Cancer cells can manipulate alternative splicing patterns to modulate the expression of adhesion- and motility-related molecules, also at the isoform level. In this study, combining RNA-Sequencing on MCF-7 to targeted experimental validations—in human breast cell lines and breast tumor biopsies—we identified 12 new alternative splicing transcripts in genes encoding adhesion- and motility-related molecules, including semaphorins, their receptors and co-receptors. Among them, a new SEMA3F transcript is expressed in all breast cell lines and breast cancer biopsies, and is translated into a new semaphorin 3F isoform. In silico analysis predicted that most of the new putative proteins lack functional domains, potentially missing some functions and acquiring new ones. Our findings better describe the extent of alternative splicing in breast cancer and highlight the need to further investigate adhesion- and motility-related molecules to gain insights into breast cancer progression.

PI(3,5)P2 controls endosomal branched actin dynamics by regulating cortactin-actin interactions

The late endosomal lipid PI(3,5)P₂ binds to cortactin through the filamentous actin (F-actin) binding domain of cortactin, leading to removal of cortactin from endosomal actin networks and inhibition of cortactin-mediated branched actin nucleation and stabilization.

Branched actin critically contributes to membrane trafficking by regulating membrane curvature, dynamics, fission, and transport. However, how actin dynamics are controlled at membranes is poorly understood. Here, we identify the branched actin regulator cortactin as a direct binding partner of phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) and demonstrate that their interaction promotes turnover of late endosomal actin. In vitro biochemical studies indicated that cortactin binds PI(3,5)P₂ via its actin filament-binding region. Furthermore, PI(3,5)P₂ competed with actin filaments for binding to cortactin, thereby antagonizing cortactin activity. These findings suggest that PI(3,5)P₂ formation on endosomes may remove cortactin from endosome-associated branched actin. Indeed, inhibition of PI(3,5)P₂ production led to cortactin accumulation and actin stabilization on Rab7⁺ endosomes. Conversely, inhibition of Arp2/3 complex activity greatly reduced cortactin localization to late endosomes. Knockdown of cortactin reversed PI(3,5)P₂-inhibitor–induced actin accumulation and stabilization on endosomes. These data suggest a model in which PI(3,5)P₂ binding removes cortactin from late endosomal branched actin networks and thereby promotes net actin turnover.

Abl2/Abl-related gene stabilizes actin filaments, stimulates actin branching by actin-related protein 2/3 complex, and promotes actin filament severing by cofilin

Both Arp2/3 complex and the Abl2/Arg nonreceptor tyrosine kinase are essential to form and maintain diverse actin-based structures in cells, including cell edge protrusions in fibroblasts and cancer cells and dendritic spines in neurons. The ability of Arg to promote cell edge protrusions in fibroblasts does not absolutely require kinase activity, raising the question of how Arg might modulate actin assembly and turnover in the absence of kinase function. Arg has two distinct actin-binding domains and interacts physically and functionally with cortactin, an activator of the Arp2/3 complex. However, it was not known whether and how Arg influences actin filament stability, actin branch formation, or cofilin-mediated actin severing or how cortactin influences these reactions of Arg with actin. Arg or cortactin bound to actin filaments stabilizes them from depolymerization. Low concentrations of Arg and cortactin cooperate to stabilize filaments by slowing depolymerization. Arg stimulates formation of actin filament branches by Arp2/3 complex and cortactin. An Arg mutant lacking the C-terminal calponin homology actin-binding domain stimulates actin branch formation by the Arp2/3 complex, indicative of autoinhibition. ArgΔCH can stimulate the Arp2/3 complex even in the absence of cortactin. Arg greatly potentiates cofilin severing of actin filaments, and cortactin attenuates this enhanced severing. The ability of Arg to stabilize filaments, promote branching, and increase severing requires the internal (I/L)WEQ actin-binding domain. These activities likely underlie important roles that Arg plays in the formation, dynamics, and stability of actin-based cellular structures.

Cortactin, another player in the Lyn signaling pathway, is over-expressed and alternatively spliced in leukemic cells from patients with B-cell chronic lymphocytic leukemia

Cortactin, an actin binding protein and Lyn substrate, is up-regulated in several cancers and its level is associated with increased cell migration, metastasis and poor prognosis. The identification that the Src kinase Lyn and its substrate HS1 are over-expressed in B-cell chronic lymphocytic leukemia and involved in resistance to chemotherapy and poor prognosis, prompted us to investigate the role of cortactin, an HS1 homolog, in the pathogenesis and progression of this disorder. In this study, we observed that cortactin is over-expressed in leukemic cells of patients (1.10 ± 0.12) with respect to normal B lymphocytes (0.19 ± 0.06; P=0.0065). Fifty-three percent of our patients expressed the WT mRNA and p80/85 protein isoforms, usually lacking in normal B lymphocytes which express the SV1 variant and the p70/75 protein isoforms. Moreover, we found an association of the cortactin overexpression and negative prognostic factors, including ZAP-70 (P<0.01), CD38 (P<0.01) and somatic hypermutations in the immunoglobulin heavy-chain variable region (P<0.01). Our results show that patients with B-cell chronic lymphocytic leukemia express high levels of cortactin with a particular overexpression of the WT isoform that is lacking in normal B cells, and a correlation to poor prognosis, suggesting that this protein could be relevant in the pathogenesis and aggressiveness of the disease.

Oncogenic alternative splicing switches: role in cancer progression and prospects for therapy

Alterations in the abundance or activities of alternative splicing regulators generate alternatively spliced variants that contribute to multiple aspects of tumor establishment, progression and resistance to therapeutic treatments. Notably, many cancer-associated genes are regulated through alternative splicing suggesting a significant role of this post-transcriptional regulatory mechanism in the production of oncogenes and tumor suppressors. Thus, the study of alternative splicing in cancer might provide a better understanding of the malignant transformation and identify novel pathways that are uniquely relevant to tumorigenesis. Understanding the molecular underpinnings of cancer-associated alternative splicing isoforms will not only help to explain many fundamental hallmarks of cancer, but will also offer unprecedented opportunities to improve the efficacy of anti-cancer treatments.

Dynamin rings: not just for fission

The GTPase dynamin has captivated researchers for over two decades, even managing to establish its own research field. Dynamin's allure is partly due to its unusual biochemical properties as well as its essential role in multiple cellular processes, which include the regulation of clathrin-mediated endocytosis and of actin cytoskeleton. On the basis of the classic model, dynamin oligomerization into higher order oligomers such as rings and helices directly executes the final fission reaction in endocytosis, which results in the generation of clathrin-coated vesicles. Dynamin's role in the regulation of actin cytoskeleton is mostly explained by its interactions with a number of actin-binding and -regulating proteins; however, the molecular mechanism of dynamin's action continues to elude us. Recent insights into the mechanism and role of dynamin oligomerization in the regulation of actin polymerization point to a novel role for dynamin oligomerization in the cell.

Phosphorylation of the alternative mRNA splicing factor 45 (SPF45) by Clk1 regulates its splice site utilization, cell migration and invasion

Alternative mRNA splicing is a mechanism to regulate protein isoform expression and is regulated by alternative splicing factors. The alternative splicing factor 45 (SPF45) is overexpressed in cancer, although few biological effects of SPF45 are known, and few splicing targets have been identified. We previously showed that Extracellular Regulated Kinase 2 (ERK2) phosphorylation of SPF45 regulates cell proliferation and adhesion to fibronectin. In this work, we show that Cdc2-like kinase 1 (Clk1) phosphorylates SPF45 on eight serine residues. Clk1 expression enhanced, whereas Clk1 inhibition reduced, SPF45-induced exon 6 exclusion from Fas mRNA. Mutational analysis of the Clk1 phosphorylation sites on SPF45 showed both positive and negative regulation of splicing, with a net effect of inhibiting SPF45-induced exon 6 exclusion, correlating with reduced Fas mRNA binding. However, Clk1 enhanced SPF45 protein expression, but not mRNA expression, whereas inhibition of Clk1 increased SPF45 degradation through a proteasome-dependent pathway. Overexpression of SPF45 or a phospho-mimetic mutant, but not a phospho-inhibitory mutant, stimulated ovarian cancer cell migration and invasion, correlating with increased fibronectin expression, ERK activation and enhanced splicing and phosphorylation of full-length cortactin. Our results demonstrate for the first time that SPF45 overexpression enhances cell migration and invasion, dependent on biochemical regulation by Clk1.

Differential protein expression profiles in estrogen receptor-positive and -negative breast cancer tissues using label-free quantitative proteomics

Identification of the proteins that are associated with estrogen receptor (ER) status is a first step towards better understanding of the hormone-dependent nature of breast carcinogenesis. Although a number of gene expression analyses have been conducted, protein complement has not been systematically investigated to date. Because proteins are primary targets of therapeutic drugs, in this study, we have attempted to identify proteomic signatures that demarcate ER-positive and -negative breast cancers. Using highly enriched breast tumor cells, replicate analyses from 3 ERα+ and 3 ERα- human breast tumors resulted in the identification of 2,995 unique proteins with ≥2 peptides. Among these, a number of receptor tyrosine kinases and intracellular kinases that are abundantly expressed in ERα+ and ERα- breast cancer tissues were identified. Further, label-free quantitative proteome analysis revealed that 236 proteins were differentially expressed in ERα+ and ERα- breast tumors. Among these, 141 proteins were selectively up-regulated in ERα+, and 95 proteins were selectively up-regulated in ERα- breast tumors. Comparison of differentially expressed proteins with a breast cancer database revealed 98 among these have been previously reported to be involved in breast cancer. By Gene Ontology molecular function, dehydrogenase, reductase, cytoskeletal proteins, extracellular matrix, hydrolase, and lyase categories were significantly enriched in ERα+, whereas selected calcium-binding protein, membrane traffic protein, and cytoskeletal protein were enriched in ERα- breast tumors. Biological process and pathway analysis revealed that up-regulated proteins of ERα+ were overrepresented by proteins involved in amino acid metabolism, proteasome, and fatty acid metabolism, while up-regulated proteins of ERα- were overrepresented by proteins involved in glycolysis pathway. The presence and relative abundance of 4 selected differentially abundant proteins (liprin-α1, fascin, DAP5, and β-arrestin-1) were quantified and validated by immunohistochemistry. In conclusion, unlike in vitro cell culture models, the in vivo signaling proteins and pathways that we have identified directly from human breast cancer tissues may serve as relevant therapeutic targets for the pharmacological intervention of breast cancer.

Comprehensive exon array data processing method for quantitative analysis of alternative spliced variants

Alternative splicing of pre-mRNA generates protein diversity. Dysfunction of splicing machinery and expression of specific transcripts has been linked to cancer progression and drug response. Exon microarray technology enables genome-wide quantification of expression levels of the majority of exons and facilitates the discovery of alternative splicing events. Analysis of exon array data is more challenging than the analysis of gene expression data and there is a need for reliable quantification of exons and alternatively spliced variants. We introduce a novel, computationally efficient methodology, Multiple Exon Array Preprocessing (MEAP), for exon array data pre-processing, analysis and visualization. We compared MEAP with existing pre-processing methods, and validation of six exons and two alternatively spliced variants with qPCR corroborated MEAP expression estimates. Analysis of exon array data from head and neck squamous cell carcinoma (HNSCC) cell lines revealed several transcripts associated with 11q13 amplification, which is related with decreased survival and metastasis in HNSCC patients. Our results demonstrate that MEAP produces reliable expression values at exon, alternatively spliced variant and gene levels, which allows generating novel experimentally testable predictions.

Cortactin is associated with perineural invasion in the deep invasive front area of laryngeal carcinomas

The cortactin gene, mapped at 11q13, has been associated with an aggressive clinical course in many cancers because of its function of invasiveness. This study evaluated CTTN protein and its prognostic value in the deep invasive front and superficial areas of laryngeal squamous cell carcinomas. The transcript expression levels were evaluated in a subset of cases. Overexpression of CTTN cytoplasmatic protein (80% of cases in both the deep invasive front and superficial areas) and transcript (30% of samples) was detected in a significant number of cases. In more than 20% of cases, observation verified membrane immunostaining in the deep invasive front and superficial areas. Perineural invasion was significantly associated with N stage and recurrence (P = .0058 and P = .0037, respectively). Higher protein expression levels were correlated with perineural invasion (P = .004) in deep invasive front cells, suggesting that this area should be considered a prognostic tool in laryngeal carcinomas. Although most cases had moderate to strong CTTN expression on the tumor surface, 2 sets of cases revealed a differential expression pattern in the deep invasive front. A group of cases with absent to weak expression of CTTN in the deep invasive front showed good prognosis parameters, and a second group with moderate to strong expression of CTTN were associated with an unfavorable prognosis, suggesting an association with worse outcome. Taken together, these results suggest that the deep invasive front might be considered a grading system in laryngeal carcinomas and that cortactin is a putative marker of worse outcome in the deep invasive front of laryngeal carcinomas.

Cortactin: a multifunctional regulator of cellular invasiveness

Branched actin assembly is critical for a variety of cellular processes that underlie cell motility and invasion, including cellular protrusion formation and membrane trafficking. Activation of branched actin assembly occurs at various subcellular locations via site-specific activation of distinct WASp family proteins and the Arp2/3 complex. A key branched actin regulator that promotes cell motility and links signaling, cytoskeletal and membrane trafficking proteins is the Src kinase substrate and Arp2/3 binding protein cortactin. Due to its frequent overexpression in advanced, invasive cancers and its general role in regulating branched actin assembly at multiple cellular locations, cortactin has been the subject of intense study. Recent studies suggest that cortactin has a complex role in cellular migration and invasion, promoting both on-site actin polymerization and modulation of autocrine secretion. Diverse cellular activities may derive from the interaction of cortactin with site-specific binding partners.

Cortactin phosphorylated by ERK1/2 localizes to sites of dynamic actin regulation and is required for carcinoma lamellipodia persistence

Background

Tumor cell motility and invasion is governed by dynamic regulation of the cortical actin cytoskeleton. The actin-binding protein cortactin is commonly upregulated in multiple cancer types and is associated with increased cell migration. Cortactin regulates actin nucleation through the actin related protein (Arp)2/3 complex and stabilizes the cortical actin cytoskeleton. Cortactin is regulated by multiple phosphorylation events, including phosphorylation of S405 and S418 by extracellular regulated kinases (ERK)1/2. ERK1/2 phosphorylation of cortactin has emerged as an important positive regulatory modification, enabling cortactin to bind and activate the Arp2/3 regulator neuronal Wiskott-Aldrich syndrome protein (N-WASp), promoting actin polymerization and enhancing tumor cell movement.

Methodology/Principal Findings

In this report we have developed phosphorylation-specific antibodies against phosphorylated cortactin S405 and S418 to analyze the subcellular localization of this cortactin form in tumor cells and patient samples by microscopy. We evaluated the interplay between cortactin S405 and S418 phosphorylation with cortactin tyrosine phosphorylation in regulating cortactin conformational forms by Western blotting. Cortactin is simultaneously phosphorylated at S405/418 and Y421 in tumor cells, and through the use of point mutant constructs we determined that serine and tyrosine phosphorylation events lack any co-dependency. Expression of S405/418 phosphorylation-null constructs impaired carcinoma motility and adhesion, and also inhibited lamellipodia persistence monitored by live cell imaging.

Conclusions/Significance

Cortactin phosphorylated at S405/418 is localized to sites of dynamic actin assembly in tumor cells. Concurrent phosphorylation of cortactin by ERK1/2 and tyrosine kinases enables cells with the ability to regulate actin dynamics through N-WASp and other effector proteins by synchronizing upstream regulatory pathways, confirming cortactin as an important integration point in actin-based signal transduction. Reduced lamellipodia persistence in cells with S405/418A expression identifies an essential motility-based process reliant on ERK1/2 signaling, providing additional understanding as to how this pathway impacts tumor cell migration.

Direct dynamin-actin interactions regulate the actin cytoskeleton

The large GTPase dynamin assembles into higher order structures that are thought to promote endocytosis. Dynamin also regulates the actin cytoskeleton through an unknown, GTPase-dependent mechanism. Here, we identify a highly conserved site in dynamin that binds directly to actin filaments and aligns them into bundles. Point mutations in the actin-binding domain cause aberrant membrane ruffling and defective actin stress fibre formation in cells. Short actin filaments promote dynamin assembly into higher order structures, which in turn efficiently release the actin-capping protein (CP) gelsolin from barbed actin ends in vitro, allowing for elongation of actin filaments. Together, our results support a model in which assembled dynamin, generated through interactions with short actin filaments, promotes actin polymerization via displacement of actin-CPs.

Invadopodia: specialized tumor cell structures for the focal degradation of the extracellular matrix

Invasive tumor-derived or transformed cells, cultured on a flat extracellular matrix substratum, extend specialized proteolytically active plasma membrane protrusions. These structures, termed invadopodia, are responsible for the focal degradation of the underlying substrate. Considerable progress has been made in recent years towards understanding the basic molecular components and regulatory circuits and the ultrastructural features of invadopodia. This has generated substantial interest in invadopodia as a paradigm to study the complex interactions between the intracellular trafficking, signal transduction and cytoskeleton regulation machineries; hopes are high that they may also represent valid biological targets to help advance the anti-cancer drug discovery process. Current knowledge will be reviewed here with an emphasis on the many open questions in invadopodia biology.

Cortactin colocalizes with filopodial actin and accumulates at IgCAM adhesion sites in Aplysia growth cones

Both IgCAMs and the actin cytoskeleton play critical roles in neuronal growth cone motility and guidance. However, it is unclear how IgCAM receptors transduce signals from the plasma membrane to induce actin remodeling. Previous studies have shown that local clustering and immobilization of apCAM, the Aplysia homolog of NCAM, induces Src kinase activity and F-actin polymerization in the peripheral domain of cultured Aplysia bag cell growth cones. Therefore, we wanted to test whether the Src kinase substrate and actin regulator cortactin could be a molecular link between Src activity and actin assembly during apCAM-mediated growth cone guidance. Here, we cloned Aplysia cortactin and showed that it is abundant in the nervous system. Immunostaining of growth cones revealed a strong colocalization of cortactin with F-actin in filopodial bundles and at the leading edge of lamellipodia. Perturbation of the cytoskeleton indicated that cortactin distribution largely depends on actin filaments. Furthermore, active Src colocalized with cortactin in regions of actin assembly, including leading edge and filopodia tips. Finally, we observed that cortactin, like F-actin, localizes to apCAM adhesion sites mediating growth cone guidance. Altogether, these data suggest that cortactin is a mediator of IgCAM-triggered actin assembly involved in growth cone motility and guidance.

Cortactin promotes migration and platelet-derived growth factor-induced actin reorganization by signaling to Rho-GTPases

Dynamic actin rearrangements are initiated and maintained by actin filament nucleators, including the Arp2/3-complex. This protein assembly is activated in vitro by distinct nucleation-promoting factors such as Wiskott-Aldrich syndrome protein/Scar family proteins or cortactin, but the relative in vivo functions of each of them remain controversial. Here, we report the conditional genetic disruption of murine cortactin, implicated previously in dynamic actin reorganizations driving lamellipodium protrusion and endocytosis. Unexpectedly, cortactin-deficient cells showed little changes in overall cell morphology and growth. Ultrastructural analyses and live-cell imaging studies revealed unimpaired lamellipodial architecture, Rac-induced protrusion, and actin network turnover, although actin assembly rates in the lamellipodium were modestly increased. In contrast, platelet-derived growth factor-induced actin reorganization and Rac activation were impaired in cortactin null cells. In addition, cortactin deficiency caused reduction of Cdc42 activity and defects in random and directed cell migration. Reduced migration of cortactin null cells could be restored, at least in part, by active Rac and Cdc42 variants. Finally, cortactin removal did not affect the efficiency of receptor-mediated endocytosis. Together, we conclude that cortactin is fully dispensable for Arp2/3-complex activation during lamellipodia protrusion or clathrin pit endocytosis. Furthermore, we propose that cortactin promotes cell migration indirectly, through contributing to activation of selected Rho-GTPases.

Cell and molecular biology of invadopodia

The controlled degradation of the extracellular matrix is crucial in physiological and pathological cell invasion alike. In vitro, degradation occurs at specific sites where invasive cells make contact with the extracellular matrix via specialized plasma membrane protrusions termed invadopodia. Considerable progress has been made in recent years toward understanding the basic molecular components and their ultrastructural features; generating substantial interest in invadopodia as a paradigm to study the complex interactions between the intracellular trafficking, signal transduction, and cytoskeleton regulation machineries. The next level will be to understand whether they may also represent valid biological targets to help advance the anticancer drug discovery process. Current knowledge will be reviewed here together with some of the most important open questions in invadopodia biology.

Overexpression of cortactin in head and neck squamous cell carcinomas can be uncoupled from augmented EGF receptor expression

BACKGROUND

The gene encoding cortactin, CTTN (locus 11q13), an actin-binding substrate of Src kinases, is frequently amplified in breast and head and neck squamous cell carcinomas (HNSCC) and cortactin overexpression is thought to contribute in a significant way to the invasive phenotype of these tumors. Elevated Epidermal Growth Factor receptor (EGFR) expression is also commonly observed in HNSCC and has been associated with poor prognosis and resistance to cytotoxic agents, including ionizing radiation. It has been suggested that cortactin overexpression may increase EGFR levels in these tumors by affecting receptor downregulation, however we recently found by multivariate analysis, that cortactin expression status remained an independent prognostic factor for local recurrence, disease-free survival, and overall survival.

MATERIAL AND METHODS

To examine the potential link between cortactin overexpression and EGFR status, we compared cortactin and EGFR levels in a series of tumor lines derived from HNSCC. RNAi-mediated silencing was performed in cortactin overexpressing cells and in vivo tumoral potential with respect to cortactin and EGFR status was analyzed.

RESULTS AND DISCUSSION

Cortactin and EGFR levels were not strictly coupled in these lines and cortactin depletion did not decrease steady state receptor levels, although it did affect the epithelial to mesenchymal phenotypic conversion of cells. These results, together with clinical findings point to the existence of an EGFR-independent role of cortactin in HNSCC that may have important implications regarding the design of targeted therapies to combat tumor spread.

Cortactin branches out: roles in regulating protrusive actin dynamics

Since its discovery in the early 1990's, cortactin has emerged as a key signaling protein in many cellular processes, including cell adhesion, migration, endocytosis, and tumor invasion. While the list of cellular functions influenced by cortactin grows, the ability of cortactin to interact with and alter the cortical actin network is central to its role in regulating these processes. Recently, several advances have been made in our understanding of the interaction between actin and cortactin, providing insight into how these two proteins work together to provide a framework for normal and altered cellular function. This review examines how regulation of cortactin through post-translational modifications and interactions with multiple binding partners elicits changes in cortical actin cytoskeletal organization, impacting the regulation and formation of actin-rich motility structures.

PTP1B regulates cortactin tyrosine phosphorylation by targeting Tyr446

The emergence of protein-tyrosine phosphatase 1B (PTP1B) as a potential drug target for treatment of diabetes, obesity, and cancer underlies the importance of understanding its full range of cellular functions. Here, we have identified cortactin, a central regulator of actin cytoskeletal dynamics, as a substrate of PTP1B. A trapping mutant of PTP1B binds cortactin at the phosphorylation site Tyr(446), the regulation and function of which have not previously been characterized. We show that phosphorylation of cortactin Tyr(446) is induced by hyperosmolarity and potentiates apoptotic signaling during prolonged hyperosmotic stress. This study advances the importance of Tyr(446) in the regulation of cortactin and provides a potential mechanism to explain the effects of PTP1B on processes including cell adhesion, migration, and tumorigenesis.

Cortactin adopts a globular conformation and bundles actin into sheets

Cortactin is a filamentous actin-binding protein that plays a pivotal role in translating environmental signals into coordinated rearrangement of the cytoskeleton. The dynamic reorganization of actin in the cytoskeleton drives processes including changes in cell morphology, cell migration, and phagocytosis. In general, structural proteins of the cytoskeleton bind in the N-terminal region of cortactin and regulatory proteins in the C-terminal region. Previous structural studies have reported an extended conformation for cortactin. It is therefore unclear how cortactin facilitates cross-talk between structural proteins and their regulators. In the study presented here, circular dichroism, chemical cross-linking, and small angle x-ray scattering are used to demonstrate that cortactin adopts a globular conformation, thereby bringing distant parts of the molecule into close proximity. In addition, the actin bundling activity of cortactin is characterized, showing that fully polymerized actin filaments are bundled into sheet-like structures. We present a low resolution structure that suggests how the various domains of cortactin interact to coordinate its array of binding partners at sites of actin branching.

Cortactin expression predicts poor survival in laryngeal carcinoma

Amplification of the 11q13 region is one of the most frequent aberrations in squamous cell carcinomas of the head and neck region (HNSCC). Amplification of 11q13 has been shown to correlate with the presence of lymph node metastases and decreased survival. The 11q13.3 amplicon carries numerous genes including cyclin D1 and cortactin. Recently, we reported that FADD becomes overexpressed upon amplification and that FADD protein expression predicts for lymph node positivity and disease-specific mortality. However, the gene within the 11q13.3 amplicon responsible for this correlation is yet to be identified. In this paper, we compared, using immunohistochemical analysis for cyclin D1, FADD and cortactin in a series of 106 laryngeal carcinomas which gene correlates best with lymph node metastases and increased disease-specific mortality. Univariate Cox regression analysis revealed that high expression of cyclin D1 (P=0.016), FADD (P=0.003) and cortactin (P=0.0006) predict for increased risk to disease-specific mortality. Multivariate Cox analysis revealed that only high cortactin expression correlates with disease-specific mortality independent of cyclin D1 and/or FADD. Of genes located in the 11q13 amplicon, cortactin expression is the best predictor for shorter disease-specific survival in late stage laryngeal carcinomas.

Amplicon mapping and expression profiling identify the Fas-associated death domain gene as a new driver in the 11q13.3 amplicon in laryngeal/pharyngeal cancer

PURPOSE

Amplification of the 11q13 region is a frequent event in human cancer. The highest incidence (36%) is found in head and neck squamous cell carcinomas. Recently, we reported that the amplicon size in 30 laryngeal and pharyngeal carcinomas with 11q13 amplification is determined by unique genomic structures, resulting in the amplification of a set of genes rather than a single gene.

EXPERIMENTAL DESIGN

To investigate which gene(s) drive the 11q13 amplicon, we determined the smallest region of overlap with amplification and the expression levels of all genes within this amplicon.

RESULTS

Using array-based comparative genomic hybridization analysis, we detected a region of approximately 1.7 Mb containing 13 amplified genes in more than 25 of the 29 carcinomas. Quantitative reverse transcription-PCR revealed that overexpression of 8 potential driver genes including, cyclin D1, cortactin, and Fas-associated death domain (FADD), correlated significantly with DNA amplification. FADD protein levels correlated well with DNA amplification, implicating that FADD is also a candidate driver gene in the 11q13 amplicon. Analysis of 167 laryngeal carcinomas showed that increased expression of FADD (P = 0.007) and Ser(194) phosphorylated FADD (P = 0.011) were associated with a worse disease-specific survival. FADD was recently reported to be involved in cell cycle regulation, and cancer cells expressing high levels of the Ser(194) phosphorylated isoform of FADD proved to be more sensitive to Taxol-induced cell cycle arrest.

CONCLUSION

Because of the frequent amplification of the 11q13 region and concomitant overexpression of FADD in head and neck squamous cell carcinomas, we hypothesize that FADD is a marker to select patients that might benefit from Taxol-based chemoradiotherapy.

Multiple regulatory inputs converge on cortactin to control invadopodia biogenesis and extracellular matrix degradation

Invadopodia are proteolytically active protrusions formed by invasive tumoral cells when grown on an extracellular matrix (ECM) substratum. Although many molecular components have been defined, less is known of the formation and regulation of invadopodia. The multidomain protein cortactin, which is involved in the regulation of actin polymerisation, is one such component, but how cortactin is modulated to control the formation of invadopodia has not been elucidated. Here, a new invadopodia synchronization protocol is used to show that the cortactin N-terminal acidic and SH3 domains, involved in Arp2/3 complex and N-WASP binding and activation, respectively, are both required for invadopodia biogenesis. In addition, through a combination of RNA interference and a wide array of cortactin phosphorylation mutants, we were able to show that three convergent regulatory inputs based on the regulation of cortactin phosphorylation by Src-family kinases, Erk1/Erk2 and PAK are necessary for invadopodia formation and extracellular matrix degradation. These findings suggest that cortactin is a scaffold protein bringing together the different components necessary for the formation of the invadopodia, and that a fine balance between different phosphorylation events induces subtle changes in structure to calibrate cortactin function.

Alternative splicing and differential gene expression in colon cancer detected by a whole genome exon array

Background

Alternative splicing is a mechanism for increasing protein diversity by excluding or including exons during post-transcriptional processing. Alternatively spliced proteins are particularly relevant in oncology since they may contribute to the etiology of cancer, provide selective drug targets, or serve as a marker set for cancer diagnosis. While conventional identification of splice variants generally targets individual genes, we present here a new exon-centric array (GeneChip Human Exon 1.0 ST) that allows genome-wide identification of differential splice variation, and concurrently provides a flexible and inclusive analysis of gene expression.

Results

We analyzed 20 paired tumor-normal colon cancer samples using a microarray designed to detect over one million putative exons that can be virtually assembled into potential gene-level transcripts according to various levels of prior supporting evidence. Analysis of high confidence (empirically supported) transcripts identified 160 differentially expressed genes, with 42 genes occupying a network impacting cell proliferation and another twenty nine genes with unknown functions. A more speculative analysis, including transcripts based solely on computational prediction, produced another 160 differentially expressed genes, three-fourths of which have no previous annotation. We also present a comparison of gene signal estimations from the Exon 1.0 ST and the U133 Plus 2.0 arrays.

Novel splicing events were predicted by experimental algorithms that compare the relative contribution of each exon to the cognate transcript intensity in each tissue. The resulting candidate splice variants were validated with RT-PCR. We found nine genes that were differentially spliced between colon tumors and normal colon tissues, several of which have not been previously implicated in cancer. Top scoring candidates from our analysis were also found to substantially overlap with EST-based bioinformatic predictions of alternative splicing in cancer.

Conclusion

Differential expression of high confidence transcripts correlated extremely well with known cancer genes and pathways, suggesting that the more speculative transcripts, largely based solely on computational prediction and mostly with no previous annotation, might be novel targets in colon cancer. Five of the identified splicing events affect mediators of cytoskeletal organization (ACTN1, VCL, CALD1, CTTN, TPM1), two affect extracellular matrix proteins (FN1, COL6A3) and another participates in integrin signaling (SLC3A2). Altogether they form a pattern of colon-cancer specific alterations that may particularly impact cell motility.

Transgenic mice with mammary gland targeted expression of human cortactin do not develop (pre-malignant) breast tumors: studies in MMTV-cortactin and MMTV-cortactin/-cyclin D1 bitransgenic mice

Background

In human breast cancers, amplification of chromosome 11q13 correlates with lymph node metastasis and increased mortality. To date, two genes located within this amplicon, CCND1 and EMS1, were considered to act as oncogenes, because overexpression of both proteins, respectively cyclin D1 and cortactin, correlated well with 11q13 amplification. Cyclin D1 is involved in cell cycle regulation and the F-actin-binding protein cortactin in cytoskeletal dynamics and cell migration. To study the role of cortactin in mammary gland tumorigenesis, we examined mouse mammary tumor virus (MMTV)-cortactin transgenic mice and MMTV-cortactin/-MMTV-cyclin D1 bitransgenic mice.

Methods

MMTV-cortactin transgenic mice were generated and intercrossed with previously described MMTV-cyclin D1 transgenic mice. Immunohistochemical, Northern and Western blot analyses were performed to study the expression of human transgene cortactin during mammary gland development and in mammary tumors. For tumor incidence studies, forced-bred, multiparous mice were used to enhance transgene expression in the mammary gland. Microscopical examination was performed using haematoxylin and eosin staining.

Results

Mammary gland tumors arose stochastically (incidence 21%) with a mean age of onset at 100 weeks. This incidence, however, did not exceed that of aged-matched control FVB/N mice (38%), which unexpectedly, also developed spontaneous mammary gland tumors. We mimicked 11q13 amplification by generating MMTV-cortactin/-MMTV-cyclin D1 bitransgenic mice but did not observe any synergistic effect of cortactin on cyclin D1-induced mammary hyperplasias or carcinomas, nor development of distant metastasis.

Conclusion

From this study, we conclude that development of (pre-malignant) breast tumors in either wild type or MMTV-cyclin D1 mice was not augmented due to mammary gland targeted overexpression of human cortactin.

Cortactin affects cell migration by regulating intercellular adhesion and cell spreading

Cortactin is an F-actin binding protein that stabilizes F-actin networks and promotes actin polymerization by activating the Arp2/3 complex. Overexpression of cortactin, as observed in several human cancers, stimulates cell migration, invasion, and experimental metastasis; however, the underlying mechanism is not understood. To investigate the importance of cortactin in cell migration, we downregulated its expression using RNA interference (RNAi). Stable downregulation of cortactin in HBL100 breast epithelial cells resulted in (i) decreased cell migration and invasion, (ii) enhanced cell-cell adhesion, and (iii) accelerated cell spreading. These phenotypic changes were reversed by expression of RNAi-resistant mouse cortactin. Cortactin colocalized with cadherin and beta-catenin in adherens junctions, consistent with its role in intercellular adhesion. Remarkably, cortactin deficiency did not affect lamellipodia formation. Instead, downregulation of cortactin in human squamous carcinoma cells that overexpress cortactin changed the cytoskeletal organization. We conclude that increased levels of cortactin, as found in human carcinomas, promote cell migration and invasion by reducing cell spreading and intercellular adhesive strength.

Cortactin overexpression results in sustained epidermal growth factor receptor signaling by preventing ligand-induced receptor degradation in human carcinoma cells

The chromosome 11q13 region is frequently amplified in human carcinomas and results in an increased expression of various genes including cortactin, and is also associated with an increased invasive potential. Cortactin acts as an important regulator of the actin cytoskeleton. It is therefore very tempting to speculate that cortactin is the crucial gene within the 11q13 amplicon that mediates the invasive potential of these carcinomas. Cortactin also participates in receptor-mediated endocytosis, and recent findings have shown that, during receptor internalization, cortactin overexpression inhibits the ubiquitylation-mediated degradation of the epidermal growth factor receptor, resulting in a sustained ligand-induced epidermal growth factor receptor activity.

Cortactin phosphorylation as a switch for actin cytoskeletal network and cell dynamics control

Cortactin is an important molecular scaffold for actin assembly and organization. Novel mechanistic functions of cortactin have emerged with more interacting partners identified, revealing its multifaceted roles in regulating actin cytoskeletal networks that are necessary for endocytosis, cell migration and invasion, adhesion, synaptic organization and cell morphogenesis. These processes are mediated by its multi-domains binding to F-actin and Arp2/3 complex and various SH3 targets. Furthermore, its role in actin remodeling is subjected to regulation by tyrosine and serine/threonine kinases. Elucidating the mechanisms underlying cortactin phosphorylation and its functional consequences would provide new insights to various aspects of cell dynamics control.

Cortactin signalling and dynamic actin networks

Cortactin was first identified over a decade ago, and its initial characterization as both an F-actin binding protein and v-Src substrate suggested that it was likely to be a key regulator of actin rearrangements in response to tyrosine kinase signalling. The recent discovery that cortactin binds and activates the actin related protein (Arp)2/3 complex, and thus regulates the formation of branched actin networks, together with the identification of multiple protein targets of the cortactin SH3 domain, have revealed diverse cellular roles for this protein. This article reviews current knowledge regarding the role of cortactin in signalling to the actin cytoskeleton in the context of these developments.