Cancer cells are defined by their ability to invade through the basement membrane, a critical step during metastasis. While increased secretion of proteases, which facilitates degradation of the basement membrane, and alterations in the cytoskeletal architecture of cancer cells have been previously studied, the contribution of the mechanical properties of cells in invasion is unclear. Here we apply a magnetic tweezer system to establish that stiffness of patient tumor cells and cancer cell lines inversely correlates with migration and invasion through three-dimensional basement membranes, a correlation known as a power law. We found that cancer cells with the highest migratory and invasive potential are five times less stiff than cells with the lowest migration and invasion potential. Moreover, decreasing cell stiffness by pharmacological inhibition of myosin II increases invasiveness, while increasing cell stiffness by restoring expression of the metastasis suppressor TβRIII/betaglycan decreases invasiveness. These findings are the first demonstration of the power law relation between the stiffness and the invasiveness of cancer cells and show that mechanical phenotypes can be used to grade the metastatic potential of cell populations with the potential for single cell grading. The measurement of a mechanical phenotype, taking minutes rather than hours needed for invasion assays, is promising as a quantitative diagnostic method and as a discovery tool for therapeutics. By demonstrating that altering stiffness predictably alters invasiveness, our results indicate that pathways regulating these mechanical phenotypes are novel targets for molecular therapy of cancer.

Loss of expression of the TGF-␤ superfamily coreceptor, the type III TGF-␤ receptor (T␤RIII or betaglycan), occurs in a broad spectrum of human cancers including breast, lung, ovarian, pancreatic, prostate, and renal cell cancer. T␤RIII suppresses cancer progression in vivo, at least in part, by reducing cancer cell motility. However, the mechanism by which T␤RIII regulates migration is unknown. Here, we demonstrate an unexpected TGF-␤ signaling independent role for T␤RIII in activating Cdc42, altering the actin cytoskeleton and reducing directional persistence to inhibit random migration of both cancer and normal epithelial cells. Functionally, T␤RIII through its interaction with the scaffolding protein ␤-arrestin2, activates Cdc42 and inhibits migration. These studies identify a TGF-␤ independent homeostatic function for T␤RIII in regulating cell migration.C ell migration is a complex process required for physiological functions including embryonic development and wound healing. Alterations in cell migration are also critical to disease pathogenesis, including inflammatory and vascular diseases, tumor cell invasion, and metastases (1, 2). The ability of cells to migrate is regulated both by the speed and the directionality of migration that can be triggered by external cues (i.e., chemotaxis) or because of the intrinsic property of cells to migrate (i.e., intrinsic persistence) that are in turn regulated by the Rho family of GTPases, integrins, the actin cytoskelton, and microtubules (3, 4). Several migratory processes in development and tissue remodeling occur without any evidence of extrinsic chemotactic signaling, relying instead on intrinsic cell migratory properties (5).The type III TGF-␤ receptor (T␤RIII/betaglycan), an 849 aa heparan sulfate proteoglycan, is the most abundant and ubiquitously expressed TGF-␤ superfamily coreceptor. T␤RIII has the potential to increase or decrease TGF-␤ signaling through mechanisms yet to be fully defined (6-8). T␤RIII is classically thought to function as a coreceptor, presenting TGF-␤ superfamily ligands to their respective signaling receptors (8). Recent studies have suggested essential, nonredundant roles for T␤RIII in regulating signaling through T␤RII and T␤RI as well as independently of T␤RII and T␤RI (9). T␤RIII null embryos die on embryonic day 13.5, exhibiting hepatic and cardiovascular defects (10). An essential role for T␤RIII has also been demonstrated in mesenchymal transformation during chick embryonic heart development (11, 12) and in mediating TGF-␤ resistance in intestinal goblet cells (13). We have defined essential roles for the cytoplasmic domain of T␤RIII in mediating TGF-␤ signaling independent of the ligand presentation role (14), along with regulating cell-surface levels of T␤RIII and T␤RII through interactions with G␣-interacting protein-interacting protein, C terminus (GIPC) (15) and ␤-arrestin2 (16).Loss of T␤RIII expression has been reported in multiple cancers, with loss of expression correlating with disease progression, advanced stage, or ...

Signaling co-receptors are diverse, multifunctional components of most major signaling pathways, with roles in mediating and regulating signaling in both physiological and pathophysiological circumstances. Many of these signaling co-receptors, including CD44, glypicans, neuropilins, syndecans and TβRIII/betaglycan are also proteoglycans. Like other co-receptors, these proteoglycan signaling co–receptors can bind multiple ligands, promoting the formation of receptor signaling complexes and regulating signaling at the cell surface. The proteoglycan signaling co-receptors can also function as structural molecules to regulate adhesion, cell migration, morphogenesis and differentiation. Through a balance of these signaling and structural roles, proteoglycan signaling co-receptors can have either tumor promoting or tumor suppressing functions. Defining the role and mechanism of action of these proteoglycan signaling co-receptors should enable more effective targeting of these co-receptors and their respective pathways for the treatment of human disease.

In budding yeast, the mitotic spindle is comprised of 32 kinetochore microtubules (kMTs) and ∼8 interpolar MTs (ipMTs). Upon anaphase onset, kMTs shorten to the pole, whereas ipMTs increase in length. Overlapping MTs are responsible for the maintenance of spindle integrity during anaphase. To dissect the requirements for anaphase spindle stability, we introduced a conditionally functional dicentric chromosome into yeast. When centromeres from the same sister chromatid attach to opposite poles, anaphase spindle elongation is delayed and a DNA breakage-fusion-bridge cycle ensues that is dependent on DNA repair proteins. We find that cell survival after dicentric chromosome activation requires the MT-binding proteins Kar3p, Bim1p, and Ase1p. In their absence, anaphase spindles are prone to collapse and buckle in the presence of a dicentric chromosome. Our analysis reveals the importance of Bim1p in maintaining a stable ipMT overlap zone by promoting polymerization of ipMTs during anaphase, whereas Kar3p contributes to spindle stability by cross-linking spindle MTs.

Reactive oxygen species (ROS) are acknowledged generally to be multi-faceted regulators of cellular functions that trigger various pathological states when present chronically or transiently at non-physiologically high levels. Here we focus on the physiological involvement of ROS in cellular motility, with special emphasis on endothelial cells (EC). An important source of ROS within EC is the non-phagocytic NAD(P)H oxidase, and the small GTPase Rac1 plays a central role in activating this complex. Rac1 is one of the three Rho-family molecules (Rac, Rho and Cdc42) involved in the control of the actin cytoskeleton in response to various signals. In this review we examine the evidence linking ROS production, Rac1 activation and actin organization to EC motility, considering mechanisms for direct interaction of ROS and actin and the effects of ROS on proteins that regulate the actin cytoskeleton. The accumulated evidence suggests that ROS are important regulators of the actin cytoskeletal dynamics and cellular motility, and more in-depth studies are needed to understand the underlying mechanisms.

Both the transforming growth factor b (TGF-b) and integrin signalling pathways have well-established roles in angiogenesis. However, how these pathways integrate to regulate angiogenesis is unknown. Here, we show that the extracellular matrix component, fibronectin, and its cellular receptor, a5b1 integrin, specifically increase TGF-b1-and BMP-9-induced Smad1/5/8 phosphorylation via the TGF-b superfamily receptors endoglin and activin-like kinase-1 (ALK1). Fibronectin and a5b1 integrin increase Smad1/5/8 signalling by promoting endoglin/ ALK1 cell surface complex formation. In a reciprocal manner, TGF-b1 activates a5b1 integrin and downstream signalling to focal adhesion kinase (FAK) in an endoglindependent manner. a5b1 integrin and endoglin form a complex on the cell surface and co-internalize, with their internalization regulating a5b1 integrin activation and signalling. Functionally, endoglin-mediated fibronectin/ a5b1 integrin and TGF-b pathway crosstalk alter the responses of endothelial cells to TGF-b1, switching TGF-b1 from a promoter to a suppressor of migration, inhibiting TGF-b1-mediated apoptosis to promote capillary stability, and partially mediating developmental angiogenesis in vivo. These studies provide a novel mechanism for the regulation of TGF-b superfamily signalling and endothelial function through crosstalk with integrin signalling pathways.

While loss of antioxidant expression and the resultant oxidant-dependent damage to cellular macromolecules is key to tumorigenesis, it has become evident that effective oxidant scavenging is conversely necessary for successful metastatic spread. This dichotomous role of antioxidant enzymes in cancer highlights their context-dependent regulation during different stages of tumor development. A prominent example of an antioxidant enzyme with such a dichotomous role and regulation is the mitochondria-localized manganese superoxide dismutase SOD2 (MnSOD). SOD2 has both tumor suppressive and promoting functions, which are primarily related to its role as a mitochondrial superoxide scavenger and H2O2 regulator. However, unlike true tumor suppressor- or onco-genes, the SOD2 gene is not frequently lost, or rarely mutated or amplified in cancer. This allows SOD2 to be either repressed or activated contingent on context-dependent stimuli, leading to its dichotomous function in cancer. Here, we describe some of the mechanisms that underlie SOD2 regulation in tumor cells. While much is known about the transcriptional regulation of the SOD2 gene, including downregulation by epigenetics and activation by stress response transcription factors, further research is required to understand the post-translational modifications that regulate SOD2 activity in cancer cells. Moreover, future work examining the spatio-temporal nature of SOD2 regulation in the context of changing tumor microenvironments is necessary to allows us to better design oxidant- or antioxidant-based therapeutic strategies that target the adaptable antioxidant repertoire of tumor cells.

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