A role for microtubules in endothelial cell protrusion in three-dimensional matrices

Increased Microtubule Growth Triggered by Microvesicle-mediated Paracrine Signaling is Required for Melanoma Cancer Cell Invasion

The acquisition of cell invasiveness is the key transition from benign melanocyte hyperplasia to aggressive melanoma. Recent work has provided an intriguing new link between the presence of supernumerary centrosomes and increased cell invasion. Moreover, supernumerary centrosomes were shown to drive non-cell-autonomous invasion of cancer cells. Although centrosomes are the principal microtubule organizing centers, the role of dynamic microtubules for non-cell-autonomous invasion remains unexplored, in particular, in melanoma. We investigated the role of supernumerary centrosomes and dynamic microtubules in melanoma cell invasion and found that highly invasive melanoma cells are characterized by the presence of supernumerary centrosomes and by increased microtubule growth rates, both of which are functionally interlinked. We demonstrate that enhanced microtubule growth is required for increased three-dimensional melanoma cell invasion. Moreover, we show that the activity to enhance microtubule growth can be transferred onto adjacent noninvasive cells through microvesicles involving HER2. Hence, our study suggests that suppressing microtubule growth, either directly using anti-microtubule drugs or through HER2 inhibitors might be therapeutically beneficial to inhibit cell invasiveness and thus, metastasis of malignant melanoma.

Significance

This study shows that increased microtubule growth is required for melanoma cell invasion and can be transferred onto adjacent cells in a non-cell-autonomous manner through microvesicles involving HER2.

Microtubules tune mechanosensitive cell responses

Mechanotransduction is a process by which cells sense the mechanical properties of their surrounding environment and adapt accordingly to perform cellular functions such as adhesion, migration and differentiation. Integrin-mediated focal adhesions are major sites of mechanotransduction and their connection with the actomyosin network is crucial for mechanosensing as well as for the generation and transmission of forces onto the substrate. Despite having emerged as major regulators of cell adhesion and migration, the contribution of microtubules to mechanotransduction still remains elusive. Here, we show that talin- and actomyosin-dependent mechanosensing of substrate rigidity controls microtubule acetylation (a tubulin post-translational modification) by promoting the recruitment of α-tubulin acetyltransferase 1 (αTAT1) to focal adhesions. Microtubule acetylation tunes the mechanosensitivity of focal adhesions and Yes-associated protein (YAP) translocation. Microtubule acetylation, in turn, promotes the release of the guanine nucleotide exchange factor GEF-H1 from microtubules to activate RhoA, actomyosin contractility and traction forces. Our results reveal a fundamental crosstalk between microtubules and actin in mechanotransduction that contributes to mechanosensitive cell adhesion and migration.

Microtubules tune mechanosensitive cell responses

Mechanotransduction is a process by which cells sense the mechanical properties of their surrounding environment and adapt accordingly to perform cellular functions such as adhesion, migration and differentiation. Integrin-mediated focal adhesions are major sites of mechanotransduction and their connection with the actomyosin network is crucial for mechanosensing as well as for the generation and transmission of forces onto the substrate. Despite having emerged as major regulators of cell adhesion and migration, the contribution of microtubules to mechanotransduction still remains elusive. Here, we show that talin- and actomyosin-dependent mechanosensing of substrate rigidity controls microtubule acetylation (a tubulin post-translational modification) by promoting the recruitment of α-tubulin acetyltransferase 1 (αTAT1) to focal adhesions. Microtubule acetylation tunes the mechanosensitivity of focal adhesions and Yes-associated protein (YAP) translocation. Microtubule acetylation, in turn, promotes the release of the guanine nucleotide exchange factor GEF-H1 from microtubules to activate RhoA, actomyosin contractility and traction forces. Our results reveal a fundamental crosstalk between microtubules and actin in mechanotransduction that contributes to mechanosensitive cell adhesion and migration.

Defining the role of cytoskeletal components in the formation of apoptopodia and apoptotic bodies during apoptosis

During apoptosis, dying cells undergo dynamic morphological changes that ultimately lead to their disassembly into fragments called apoptotic bodies (ApoBDs). Reorganisation of the cytoskeletal structures is key in driving various apoptotic morphologies, including the loss of cell adhesion and membrane bleb formation. However, whether cytoskeletal components are also involved in morphological changes that occur later during apoptosis, such as the recently described generation of thin apoptotic membrane protrusions called apoptopodia and subsequent ApoBD formation, is not well defined. Through monitoring the progression of apoptosis by confocal microscopy, specifically focusing on the apoptopodia formation step, we characterised the presence of F-actin and microtubules in a subset of apoptopodia generated by T cells and monocytes. Interestingly, targeting actin polymerisation and microtubule assembly pharmacologically had no major effect on apoptopodia formation. These data demonstrate apoptopodia as a novel type of membrane protrusion that could be formed in the absence of actin polymerisation and microtubule assembly.

Microtubule-targeting agents and their impact on cancer treatment

Microtubule-targeting agents (MTAs) constitute a diverse group of chemical compounds that bind to microtubules and affect their properties and function. Disruption of microtubules induces various cellular responses often leading to cell cycle arrest or cell death, the most common effect of MTAs. MTAs have found a plethora of practical applications in weed control, as fungicides and antiparasitics, and particularly in cancer treatment. Here we summarize the current knowledge of MTAs, the mechanisms of action and their role in cancer treatment. We further outline the potential use of MTAs in anti-metastatic therapy based on inhibition of cancer cell migration and invasiveness. The two main problems associated with cancer therapy by MTAs are high systemic toxicity and development of resistance. Toxic side effects of MTAs can be, at least partly, eliminated by conjugation of the drugs with various carriers. Moreover, some of the novel MTAs overcome the resistance mediated by both multidrug resistance transporters as well as overexpression of specific β-tubulin types. In anti-metastatic therapy, MTAs should be combined with other drugs to target all modes of cancer cell invasion.

Flubendazole and mebendazole impair migration and epithelial to mesenchymal transition in oral cell lines

Benzimidazole anthelmintics flubendazole and mebendazole are microtubule-targeting drugs that showed considerable anti-cancer activity in different preclinical models. In this study, the effects of flubendazole and mebendazole on proliferation, migration and cadherin switching were studied in a panel of oral cell lines in vitro. Both compounds reduced the viability of the PE/CA-PJ15 and H376 oral squamous carcinoma cells and of the premalignant oral keratinocytes DOK with the IC₅₀ values in the range of 0.19-0.26 μM. Normal oral keratinocytes and normal gingival fibroblasts were less sensitive to the treatment. Flubendazole and mebendazole also reduced the migration of the PE/CA-PJ15 cell in concentrations that had no anti-migratory effects on the normal gingival fibroblasts. Levels of the focal adhesion kinase FAK, Rho-A and Rac1 GTPases and the Rho guanine nucleotide exchange factor GEF-H1 were decreased in both PE/CA-PJ15 cells and gingival fibroblasts following treatment. Both drugs also interfered with cadherin switching in the model of TGF-β-induced epithelial to mesenchymal transition (EMT) in the DOK cell line. Levels of N-cadherin were reduced in the TGF-β induced cells co-treated with flubendazol and mebendazole in very low concentration (50 nM). These results suggest direct effects of both benzimidazoles on selected processes of EMT in oral cell lines such as cadherin switching as well as cellular migration.

A cell surface display fluorescent biosensor for measuring MMP14 activity in real-time

Despite numerous recent advances in imaging technologies, one continuing challenge for cell biologists and microscopists is the visualization and measurement of endogenous proteins as they function within living cells. Achieving this goal will provide a tool that investigators can use to associate cellular outcomes with the behavior and activity of many well-studied target proteins. Here, we describe the development of a plasmid-based fluorescent biosensor engineered to measure the location and activity of matrix metalloprotease-14 (MMP14). The biosensor design uses fluorogen-activating protein technology coupled with a MMP14-selective protease sequence to generate a binary, “switch-on” fluorescence reporter capable of measuring MMP14 location, activity, and temporal dynamics. The MMP14-fluorogen activating protein biosensor approach is applicable to both short and long-term imaging modalities and contains an adaptable module that can be used to study many membrane-bound proteases. This MMP14 biosensor promises to serve as a tool for the advancement of a broad range of investigations targeting MMP14 activity during cell migration in health and disease.

The RhoGAP protein ARHGAP18/SENEX localizes to microtubules and regulates their stability in endothelial cells

Localization of a regulator of RhoGTPases (ARHGAP18) is important for microtubule stability and endothelial cell function. The localization is demonstrated by advanced imaging and biochemical techniques.

RhoGTPases are important regulators of the cell cytoskeleton, controlling cell shape, migration and proliferation. Previously we showed that ARHGAP18 in endothelial cells is important in cell junctions. Here we show, using structured illumination microscopy (SIM), ground-state depletion (GSD), and total internal reflection fluorescence microscopy (TIRF) that a proportion of ARHGAP18 localizes to microtubules in endothelial cells, as well as in nonendothelial cells, an association confirmed biochemically. In endothelial cells, some ARHGAP18 puncta also colocalized to Weibel–Palade bodies on the microtubules. Depletion of ARHGAP18 by small interfering RNA or analysis of endothelial cells isolated from ARHGAP18-knockout mice showed microtubule destabilization, as evidenced by altered morphology and decreased acetylated α-tubulin and glu-tubulin. The destabilization was rescued by inhibition of ROCK and histone deacetylase 6 but not by a GAP-mutant form of ARHGAP18. Depletion of ARHGAP18 resulted in a failure to secrete endothelin-1 and a reduction in neutrophil transmigration, both known to be microtubule dependent. Thrombin, a critical regulator of the Rho-mediated barrier function of endothelial cells through microtubule destabilization, enhanced the plasma membrane–bound fraction of ARHGAP18. Thus, in endothelial cells, ARHGAP18 may act as a significant regulator of vascular homeostasis.

Microtubules in 3D cell motility

Three-dimensional (3D) cell motility underlies essential processes, such as embryonic development, tissue repair and immune surveillance, and is involved in cancer progression. Although the cytoskeleton is a well-studied regulator of cell migration, most of what we know about its functions originates from studies conducted in two-dimensional (2D) cultures. This research established that the microtubule network mediates polarized trafficking and signaling that are crucial for cell shape and movement in 2D. In parallel, developments in light microscopy and 3D cell culture systems progressively allowed to investigate cytoskeletal functions in more physiologically relevant settings. Interestingly, several studies have demonstrated that microtubule involvement in cell morphogenesis and motility can differ in 2D and 3D environments. In this Commentary, we discuss these differences and their relevance for the understanding the role of microtubules in cell migration in vivo. We also provide an overview of microtubule functions that were shown to control cell shape and motility in 3D matrices and discuss how they can be investigated further by using physiologically relevant models.

Mesenchymal Cell Invasion Requires Cooperative Regulation of Persistent Microtubule Growth by SLAIN2 and CLASP1

Microtubules regulate signaling, trafficking, and cell mechanics, but the respective contribution of these functions to cell morphogenesis and migration in 3D matrices is unclear. Here, we report that the microtubule plus-end tracking protein (+TIP) SLAIN2, which suppresses catastrophes, is not required for 2D cell migration but is essential for mesenchymal cell invasion in 3D culture and in a mouse cancer model. We show that SLAIN2 inactivation does not affect Rho GTPase activity, trafficking, and focal adhesion formation. However, SLAIN2-dependent catastrophe inhibition determines microtubule resistance to compression and pseudopod elongation. Another +TIP, CLASP1, is also needed to form invasive pseudopods because it prevents catastrophes specifically at their tips. When microtubule growth persistence is reduced, inhibition of depolymerization is sufficient for pseudopod maintenance but not remodeling. We propose that catastrophe inhibition by SLAIN2 and CLASP1 supports mesenchymal cell shape in soft 3D matrices by enabling microtubules to perform a load-bearing function.

Movers and shakers: cell cytoskeleton in cancer metastasis

Metastasis is responsible for the greatest number of cancer deaths. Metastatic disease, or the movement of cancer cells from one site to another, is a complex process requiring dramatic remodelling of the cell cytoskeleton. The various components of the cytoskeleton, actin (microfilaments), microtubules (MTs) and intermediate filaments, are highly integrated and their functions are well orchestrated in normal cells. In contrast, mutations and abnormal expression of cytoskeletal and cytoskeletal-associated proteins play an important role in the ability of cancer cells to resist chemotherapy and metastasize. Studies on the role of actin and its interacting partners have highlighted key signalling pathways, such as the Rho GTPases, and downstream effector proteins that, through the cytoskeleton, mediate tumour cell migration, invasion and metastasis. An emerging role for MTs in tumour cell metastasis is being unravelled and there is increasing interest in the crosstalk between key MT interacting proteins and the actin cytoskeleton, which may provide novel treatment avenues for metastatic disease. Improved understanding of how the cytoskeleton and its interacting partners influence tumour cell migration and metastasis has led to the development of novel therapeutics against aggressive and metastatic disease.

LINKED ARTICLES

This article is part of a themed section on Cytoskeleton, Extracellular Matrix, Cell Migration, Wound Healing and Related Topics. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-24.