N-WASP-mediated invadopodium formation is involved in intravasation and lung metastasis of mammary tumors

WASPs and WAVEs: from molecular function to physiology in hematopoietic cells

The actin cytoskeleton is critically involved in a variety of cell functions. The Arp2/3 complex mediates branching of filamentous actin. The members of the Wiskott-Aldrich syndrome protein (WASP) family are major regulators of the complex. As such, the family proteins are also involved in numerous aspects of cell biology. In this short review, we first define the expanding WASP family. Next, we compare the domain structure of the members, and explain the known or proposed functions of each domain or region. Finally, we demonstrate the well-characterized roles of the proteins in specific cellular functions.

Regulatory networks defining EMT during cancer initiation and progression

Epithelial to mesenchymal transition (EMT) is essential for driving plasticity during development, but is an unintentional behaviour of cells during cancer progression. The EMT-associated reprogramming of cells not only suggests that fundamental changes may occur to several regulatory networks but also that an intimate interplay exists between them. Disturbance of a controlled epithelial balance is triggered by altering several layers of regulation, including the transcriptional and translational machinery, expression of non-coding RNAs, alternative splicing and protein stability.

Actin binding proteins: their ups and downs in metastatic life

In order to metastasize away from the primary tumor site and migrate into adjacent tissues, cancer cells will stimulate cellular motility through the regulation of their cytoskeletal structures. Through the coordinated polymerization of actin filaments, these cells will control the geometry of distinct structures, namely lamella, lamellipodia and filopodia, as well as the more recently characterized invadopodia. Because actin binding proteins play fundamental functions in regulating the dynamics of actin polymerization, they have been at the forefront of cancer research. This review focuses on a subset of actin binding proteins involved in the regulation of these cellular structures and protrusions, and presents some general principles summarizing how these proteins may remodel the structure of actin. The main body of this review aims to provide new insights into how the expression of these actin binding proteins is regulated during carcinogenesis and highlights new mechanisms that may be initiated by the metastatic cells to induce aberrant expression of such proteins.

Breaking away: matrix remodeling from the leading edge

Substantial progress has been made in recent years toward understanding the molecular mechanisms by which tumor cells, and the supporting stroma, degrade confining matrix during migration. Significant attention has been focused on understanding the biology of several dynamic and distinct, but remarkably related, cell structures that include lamellipodia, focal adhesions (FAs), filopodia, podosomes, and invadopodia. How these invasive organelles assemble and function is a topic of intense study. Most exciting has been the recent progress made by combining advanced microscope technologies with a wide variety of different 3D matrices, tissue explants, or even living model organisms. From these approaches, it has become increasingly evident that the conventional definitions of these invasive structures may be less clear than was previously thought.

Markers of circulating breast cancer cells

The detection of circulating tumor cells (CTC) aids in diagnosis of disease, prognosis, disease recurrence, and therapeutic response. The molecular aspects of metastasis are reviewed including its relevance in the identification and characterization of putative markers that may be useful in the detection thereof. Also discussed are methods for CTC enrichment using molecular strategies. The clinical application of CTC in the metastatic disease process is also summarized.

Crk and ABI1: binary molecular switches that regulate abl tyrosine kinase and signaling to the cytoskeleton

The nonreceptor tyrosine kinases Abl and Arg are among the most well-characterized tyrosine kinases in the human genome. The activation of Abl by N-terminal fusions with Bcr (Bcr-Abl) or Gag (v-Abl) is responsible for chronic myeloid leukemia or Ph+ acute lymphoblastic leukemia and mouse leukemia virus, respectively. In addition, aberrant Abl and Arg activation downstream of several oncogenic growth factor receptors contributes to the development and progression of a variety of human cancers, often associated with poor clinical outcome, drug resistance, and tumor invasion and metastasis. Abl activation can occur by a variety of mechanisms that include domain interactions involving structural remodeling of autoinhibited conformations as well as direct phosphorylation by upstream kinases and phosphatases. Constitutive activation of Abl plays a significant role in regulating the actin cytoskeleton by modulating cell adhesion, motility, and invadopodia. This review addresses the role of Abl and Arg in tumor progression with particular emphasis on the roles of Crk and Abi1 adapter proteins as distinct molecular switches for Abl transactivation. These insights, combined with new insights into the structure of these kinases, provide the rationale to envision that Crk and Abi1 fine-tune Abl regulation to control signaling to the cytoskeleton.

Diverse roles for the paxillin family of proteins in cancer

The paxillin family of intracellular scaffold proteins includes paxillin, Hic-5, and leupaxin, and all have been identified as key regulators of the cellular migration machinery in both 2- and 3-dimensional microenvironments. Herein, we provide insight into the roles of these proteins during tumorigenesis and metastasis, highlighting their functions in cancer initiation as well as tumor cell dissemination and survival. Furthermore, we speculate on the potential of paxillin family proteins as both future prognostic and therapeutic targets.

WIP: WASP-interacting proteins at invadopodia and podosomes

Regulated cell invasion resulting from migratory and matrix-degrading events is an essential step in physiological processes such as the inflammatory response and tissue repair. Cell invasion is also thought to be a critical parameter in pathological conditions such as cancer metastasis. The migration of normal and cancer cells is largely driven by the actin cytoskeleton, which controls cell shape, adhesion and contractility. Podosomes and invadopodia are actin-rich protrusions that drive invasion in normal and cancer cells. These structures protrude from the basal region of the cell facing the extracellular matrix, where they adhere to and degrade the matrix, thus facilitating invasive migration. WASP (Wiskott-Aldrich syndrome protein) and WIP (WASP-interacting protein) localise to the actin rich core of podosomes and play a critical role in their formation. More recently, studies performed on microarray data sets from cancer patients of several tumour categories show a strong correlation between reduced WIP expression and improved prognosis. In this article, we identify endogenous WIP at the distal tips of cancer cell invasive protrusions and we summarise recent advances in the study of the roles of WIP- and WASP-protein families during migration and invasion of normal and cancer cells related to podosome and invadopodium generation.

N-WASP coordinates the delivery and F-actin-mediated capture of MT1-MMP at invasive pseudopods

Metastasizing tumor cells use matrix metalloproteases, such as the transmembrane collagenase MT1-MMP, together with actin-based protrusions, to break through extracellular matrix barriers and migrate in dense matrix. Here we show that the actin nucleation-promoting protein N-WASP (Neural Wiskott-Aldrich syndrome protein) is up-regulated in breast cancer, and has a pivotal role in mediating the assembly of elongated pseudopodia that are instrumental in matrix degradation. Although a role for N-WASP in invadopodia was known, we now show how N-WASP regulates invasive protrusion in 3D matrices. In actively invading cells, N-WASP promoted trafficking of MT1-MMP into invasive pseudopodia, primarily from late endosomes, from which it was delivered to the plasma membrane. Upon MT1-MMP's arrival at the plasma membrane in pseudopodia, N-WASP stabilized MT1-MMP via direct tethering of its cytoplasmic tail to F-actin. Thus, N-WASP is crucial for extension of invasive pseudopods into which MT1-MMP traffics and for providing the correct cytoskeletal framework to couple matrix remodeling with protrusive invasion.

Adhesion rings surround invadopodia and promote maturation

Invasion and metastasis are aggressive cancer phenotypes that are highly related to the ability of cancer cells to degrade extracellular matrix (ECM). At the cellular level, specialized actin-rich structures called invadopodia mediate focal matrix degradation by serving as exocytic sites for ECM-degrading proteinases. Adhesion signaling is likely to be a critical regulatory input to invadopodia, but the mechanism and location of such adhesion signaling events are poorly understood. Here, we report that adhesion rings surround invadopodia shortly after formation and correlate strongly with invadopodium activity on a cell-by-cell basis. By contrast, there was little correlation of focal adhesion number or size with cellular invadopodium activity. Prevention of adhesion ring formation by inhibition of RGD-binding integrins or knockdown (KD) of integrin-linked kinase (ILK) reduced the number of ECM-degrading invadopodia and reduced recruitment of IQGAP to invadopodium actin puncta. Furthermore, live cell imaging revealed that the rate of extracellular MT1-MMP accumulation at invadopodia was greatly reduced in both integrin-inhibited and ILK-KD cells. Conversely, KD of MT1-MMP reduced invadopodium activity and dynamics but not the number of adhesion-ringed invadopodia. These results suggest a model in which adhesion rings are recruited to invadopodia shortly after formation and promote invadopodium maturation by enhancing proteinase secretion. Since adhesion rings are a defining characteristic of podosomes, similar structures formed by normal cells, our data also suggest further similarities between invadopodia and podosomes.

Pathological roles of invadopodia in cancer invasion and metastasis

Invadopodia are actin-rich membrane protrusions formed by invasive cancer cells. Invadopodia mediate the focal degradation of pericellular extracellular matrix (ECM) by the localized proteolytic activity of matrix metalloproteinases (MMPs). Over the last 2 decades, much progress has been made in identifying the molecular components of invadopodia and understanding the molecular mechanisms underlying their formation. Although the physiological and pathological roles of invadopodia have long been elusive, emerging evidence has begun to reveal their importance in local invasion during cancer metastasis. This review highlights recent findings on the roles of invadopodia in cancer invasion and metastasis and discusses the possibility of and strategies for targeting invadopodia formation for the development of novel anticancer therapeutics.

Directed cell invasion and migration during metastasis

Metastasis requires tumor cell dissemination to different organs from the primary tumor. Dissemination is a complex cell motility phenomenon that requires the molecular coordination of the protrusion, chemotaxis, invasion and contractility activities of tumor cells to achieve directed cell migration. Recent studies of the spatial and temporal activities of the small GTPases have begun to elucidate how this coordination is achieved. The direct visualization of the pathways involved in actin polymerization, invasion and directed migration in dissemination competent tumor cells will help identify the molecular basis of dissemination and allow the design and testing of more specific and selective drugs to block metastasis.