Integrin engagement differentially modulates epithelial cell motility by RhoA/ROCK and PAK1

Biomechanical properties of laminins and their impact on cancer progression

Laminins (LMs) constitute a family of heterotrimeric glycoproteins essential for the formation of basement membranes (BM). They act as molecular bridges between cells and the extracellular matrix (ECM), thereby transmitting signals influencing cell behavior and tissue organization. In the realm of cancer pathobiology, LMs regulate key processes such as migration, differentiation, or fibrosis. This review critically examines the multifaceted impact of LMs on tumor progression, with a particular focus on the isoform-specific structure-function relationships, and how this structural diversity contributes to the biomechanical properties of BMs. LM interactions with integrin and non-integrin cell surface receptors, as well as with other ECM proteins, modify the response of cancer cells to the ECM stiffness, ultimately influencing the capacity of malignant cells to breach the BM, a limiting step in metastatic dissemination. Comprehension of the mechanisms underlying LM-driven tumor biomechanics holds potential for better understand cancer pathobiology and design new targeted therapeutic strategies.

Long range juxtacrine signalling through cadherin for collective cell orientation

Many life phenomena, such as development, morphogenesis, tissue remodelling, and wound healing, are often driven by orderly and directional migration of collective cells. However, when cells are randomly oriented or localized disorder exists in orderly oriented collective cells, cell migration cannot occur in an orderly manner although various motion modes such as global rotation and local swirling and/or various motion patterns such as radial pattern and chiral pattern often occur. Therefore, it is important to control cell orientation to ensure the orderly migration of collective cells. Here, we show that it is not force transmission, but juxtacrine signalling through cadherin that plays a critical role in the orientation of collective cells. Surprisingly, juxtacrine signalling for cell orientation reached cells on a plastic dish that were not directly subjected to mechanical stimulation, up to 7 mm away from the actuator. The present study suggests that even weak mechanical stimulation is transmitted in a long range without force transmission through juxtacrine signalling. The long range juxtacrine signalling might play an important role in various life phenomena. STATEMENT OF SIGNIFICANCE: Juxtacrine signalling is direct cell-cell contact-dependent signalling, which plays a crucial role in cell behaviors such as mechanosensing, mechanotransduction and collective cell behaviors, however, there is not enough understanding about juxtacrine signalling. The present study has demonstrated that juxtacrine signalling for collective cell orientation is transmitted over a long range through cadherin. To the best of our knowledge, this is the first report of long range juxtacrine signalling. This finding may lead to the elucidation of various life phenomena such as development, morphogenesis, tissue remodelling, and wound healing.

Mitochondrial bioenergetics, metabolism, and beyond in pancreatic β-cells and diabetes

In Type 1 and Type 2 diabetes, pancreatic β-cell survival and function are impaired. Additional etiologies of diabetes include dysfunction in insulin-sensing hepatic, muscle, and adipose tissues as well as immune cells. An important determinant of metabolic health across these various tissues is mitochondria function and structure. This review focuses on the role of mitochondria in diabetes pathogenesis, with a specific emphasis on pancreatic β-cells. These dynamic organelles are obligate for β-cell survival, function, replication, insulin production, and control over insulin release. Therefore, it is not surprising that mitochondria are severely defective in diabetic contexts. Mitochondrial dysfunction poses challenges to assess in cause-effect studies, prompting us to assemble and deliberate the evidence for mitochondria dysfunction as a cause or consequence of diabetes. Understanding the precise molecular mechanisms underlying mitochondrial dysfunction in diabetes and identifying therapeutic strategies to restore mitochondrial homeostasis and enhance β-cell function are active and expanding areas of research. In summary, this review examines the multidimensional role of mitochondria in diabetes, focusing on pancreatic β-cells and highlighting the significance of mitochondrial metabolism, bioenergetics, calcium, dynamics, and mitophagy in the pathophysiology of diabetes. We describe the effects of diabetes-related gluco/lipotoxic, oxidative and inflammation stress on β-cell mitochondria, as well as the role played by mitochondria on the pathologic outcomes of these stress paradigms. By examining these aspects, we provide updated insights and highlight areas where further research is required for a deeper molecular understanding of the role of mitochondria in β-cells and diabetes.

Sci-Seq of Human Fetal Salivary Tissue Introduces Human Transcriptional Paradigms and a Novel Cell Population

Multiple pathologies and non-pathological factors can disrupt the function of the non-regenerative human salivary gland including cancer and cancer therapeutics, autoimmune diseases, infections, pharmaceutical side effects, and traumatic injury. Despite the wide range of pathologies, no therapeutic or regenerative approaches exist to address salivary gland loss, likely due to significant gaps in our understanding of salivary gland development. Moreover, identifying the tissue of origin when diagnosing salivary carcinomas requires an understanding of human fetal development. Using computational tools, we identify developmental branchpoints, a novel stem cell-like population, and key signaling pathways in the human developing salivary glands by analyzing our human fetal single-cell sequencing data. Trajectory and transcriptional analysis suggest that the earliest progenitors yield excretory duct and myoepithelial cells and a transitional population that will yield later ductal cell types. Importantly, this single-cell analysis revealed a previously undescribed population of stem cell-like cells that are derived from SD and expresses high levels of genes associated with stem cell-like function. We have observed these rare cells, not in a single niche location but dispersed within the developing duct at later developmental stages. Our studies introduce new human-specific developmental paradigms for the salivary gland and lay the groundwork for the development of translational human therapeutics.

Generation of ductal organoids from normal mammary luminal cells reveals invasive potential

Here we present an experimental model for human luminal progenitor cells that enables single, primary cells isolated from normal tissue to generate complex branched structures resembling the ductal morphology of low-grade carcinoma of no special type. Thereby, we find that ductal structures are generated through invasive branching morphogenesis via matrix remodeling and identify reduced actomyosin contractility as a prerequisite for invasion. In addition, we show that knockout of E-cadherin causes a dissolution of duct formation as observed in invasive lobular carcinoma, a subtype of invasive carcinomas where E-cadherin function is frequently lost. Thus, our model shows that invasive capacity can be elicited from normal luminal cells in specific environments, which results in low-grade no special type morphology. This assay offers a platform to investigate the dynamics of luminal cell invasion and unravel the impact of genetic and non-genetic aberrations on invasive morphology. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

RSV attenuates epithelial cell restitution by inhibiting actin cytoskeleton-dependent cell migration

The airway epithelium's ability to repair itself after injury, known as epithelial restitution, is an essential mechanism enabling the respiratory tract's normal functions. Respiratory Syncytial Virus (RSV) is the leading cause of lower respiratory tract infections worldwide. We sought to determine whether RSV delays the airway epithelium wound repair process both in vitro and in vivo. We found that RSV infection attenuated epithelial cell migration, a step in wound repair, promoted stress fiber formation, and mediated assembly of large focal adhesions (FA). Inhibition of Rho kinase (ROCK), a master regulator of actin function, reversed these effects. There was increased RhoA and phospho-myosin light chain (pMLC2) following RSV infection. In vivo, mice were intraperitoneally inoculated with naphthalene to induce lung injury, followed by RSV infection. RSV infection delayed re-epithelialization. There were increased concentrations of pMLC2 in day 7 naphthalene plus RSV animals which normalized by day 14. This study suggests a key mechanism by which RSV infection delays wound healing.

Screening of integrins localized on the surface of human epidermal melanocytes

In vivo, melanocytes occupy three-dimensional (3D) space. Nevertheless, most experiments involving melanocytes are performed in a two-dimensional microenvironment, resulting in difficulty obtaining accurate results. Therefore, it is necessary to construct an artificial in vivo-like 3D microenvironment. Here, as a step towards engineering a precisely defined acellular 3D microenvironment supporting the maintenance of human epidermal melanocytes (HEMs), we examined the types of integrin heterodimers that are expressed transcriptionally, translationally, and functionally in HEMs. Real-time PCR and fluorescent immunoassay analyses were used to elucidate the expression of integrin α and β subunit genes at the transcriptional and translational levels, respectively. The functionality of the presumed integrin heterodimers was confirmed using attachment and antibody-inhibition assays. Among the genes encoding 12 integrin subunits (α₁, α₂, α₃, α₄, α₅, α₆, α₇, α_V, β₁, β₃, β₅, and β₈) showing significantly higher transcription levels, proteins translated from the integrin α₂, α₄, α₅, β₁, β₃, and β₅ subunit genes were detected on the surface of HEMs. These HEMs showed significantly increased adhesion to collagen I, fibronectin, laminin, and vitronectin, and functional blockade of the integrin α₂ subunits significantly inhibited adhesion to collagen I, fibronectin, and laminin. In addition, there was no significant inhibition of the adhesion to fibronectin or vitronectin in HEMs with functional blockade of the integrin α₄, α₅, or α_V subunits. These results indicate that the active integrin α₂β₁ heterodimer and the inactive integrin α₄, α₅, α_V, β₃, and β₅ subunits are all localized on the surface of HEMs.

Laminin 332 in cancer: When the extracellular matrix turns signals from cell anchorage to cell movement

Laminin 332 is crucial in the biology of epithelia. This large extracellular matrix protein consists of the heterotrimeric assembly of three subunits - α3, β3, and γ2 - and its multifunctionality relies on a number of extracellular proteolytic processing events. Laminin 332 is central to normal epithelium homeostasis by sustaining cell adhesion, polarity, proliferation, and differentiation. It also supports a major function in epithelial tissue formation, repair, and regeneration by buttressing cell migration and survival and basement membrane assembly. Interest in this protein increased after the discovery that its expression is perturbed in tumor cells, cancer-associated fibroblasts, and the tumor microenvironment. This review summarizes current knowledge regarding the established involvement of the laminin 332 γ2 chain in tumor invasiveness and discusses the role of its α3 and β3 subunits.

Rebalancing of actomyosin contractility enables mammary tumor formation upon loss of E-cadherin

E-cadherin (CDH1) is a master regulator of epithelial cell adherence junctions and a well-established tumor suppressor in Invasive Lobular Carcinoma (ILC). Intriguingly, somatic inactivation of E-cadherin alone in mouse mammary epithelial cells (MMECs) is insufficient to induce tumor formation. Here we show that E-cadherin loss induces extrusion of luminal MMECs to the basal lamina. Remarkably, E-cadherin-deficient MMECs can breach the basal lamina but do not disseminate into the surrounding fat pad. Basal lamina components laminin and collagen IV supported adhesion and survival of E-cadherin-deficient MMECs while collagen I, the principle component of the mammary stromal micro-environment did not. We uncovered that relaxation of actomyosin contractility mediates adhesion and survival of E-cadherin-deficient MMECs on collagen I, thereby allowing ILC development. Together, these findings unmask the direct consequences of E-cadherin inactivation in the mammary gland and identify aberrant actomyosin contractility as a critical barrier to ILC formation.

Deciphering the Mammary Stem Cell Niche: A Role for Laminin-Binding Integrins

Integrins, which bind laminin, a major component of the mammary basement membrane, are strongly expressed in basal stem cell-enriched populations, but their role in controlling mammary stem cell function remains unclear. We found that stem cell activity, as evaluated in transplantation and mammosphere assays, was reduced in mammary basal cells depleted of laminin receptors containing α3- and α6-integrin subunits. This was accompanied by low MDM2 levels, p53 stabilization, and diminished proliferative capacity. Importantly, disruption of p53 function restored the clonogenicity of α3/α6-integrin-depleted mammary basal stem cells, while inhibition of RHO or myosin II, leading to decreased p53 activity, rescued the mammosphere formation. These data suggest that α3/α6-integrin-mediated adhesion plays an essential role in controlling the proliferative potential of mammary basal stem/progenitor cells through myosin II-mediated regulation of p53 and indicate that laminins might be important components of the mammary stem cell niche.

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α3- and α6-integrins are required for mammary basal stem cell function

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p53 is activated in mammary basal cells depleted of α3- and α6-integrins

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RHO and myosin II mediate p53 activation in α3- and α6-integrin-depleted cells

Co-targeting PI3K, mTOR, and IGF1R with small molecule inhibitors for treating undifferentiated pleomorphic sarcoma

Undifferentiated pleomorphic sarcomas (UPSs) are aggressive mesenchymal malignancies with no definitive cell of origin or specific recurrent genetic hallmarks. These tumors are largely chemoresistant; thus, identification of potential therapeutic targets is necessary to improve patient outcome. Previous studies demonstrated that high expression of activated protein kinase B (AKT) in patients with UPS corresponds to poor disease-specific survival. Here, we demonstrate that inhibiting phosphatidylinositol-3-kinase/mammalian target of rapamycin (PI3K/mTOR) signaling using a small molecule inhibitor reduced UPS cell proliferation and motility and xenograft growth; however, increased phosphorylation of insulin-like growth factor 1 receptor (IGF1R) indicated the potential for adaptive resistance following treatment through compensatory receptor activation. Co-treatment with a dual PI3K/mTOR inhibitor and an anti-IGF1R kinase inhibitor reduced in vivo tumor growth rates despite a lack of antiproliferative effects in vitro. Moreover, this combination treatment significantly decreased UPS cell migration and invasion, which is linked to changes in p27 subcellular localization. Our results demonstrate that targeted inhibition of multiple components of the IGF1R/PI3K/mTOR pathway was more efficacious than single-agent therapy and suggest that co-targeting this pathway could be a beneficial therapeutic strategy for patients with UPS.

Ganglioside, disialosyl globopentaosylceramide (DSGb5), enhances the migration of renal cell carcinoma cells

About one third of renal cell carcinoma (RCC) patients exhibit metastasis upon initial presentation. However, the molecular basis for RCC metastasis is not fully understood. A ganglioside, disialosyl globopentaosylceramide (DSGb5), was originally isolated from RCC tissue extracts, and its expression is correlated with RCC metastatic potential. DSGb5 is synthesized by GalNAc α2,6-sialyltransferase VI (ST6GalNAcVI) and is expressed on the surface of RCC cells. Importantly, DSGb5 binds to sialic acid-binding Ig-like lectin-7 (Siglec-7) expressed on natural killer (NK) cells, thereby inhibiting NK-cell cytotoxicity. However, the role of DSGb5 in RCC progression remains obscure. To address this issue, we used ACHN cells derived from malignant pleural effusion of a patient with metastatic RCC. Using the limiting dilution method, we isolated three independent clones with different DSGb5 expression levels. Comparison of these clones indicated that the cloned cells with high DSGb5 expression levels exhibited greater migration potential, compared to the clone with low DSGb5 expression levels. In contrast, DSGb5 expression levels exerted no significant effect on cell proliferation. We then established the ACHN-derived cell lines that stably expressed siRNA against ST6GalNAcVI mRNA or control siRNA. Importantly, the ST6GalNAcVI-knockdown cells expressed low levels of DSGb5. We thus demonstrated the significantly decreased migration potential of the ST6GalNAcVI-knockdown cells with low DSGb5 expression levels, compared to the control siRNA-transfected cells expressing high DSGb5 levels, but no significant difference in the cell proliferation. Thus, DSGb5 expression may ensure the migration of RCC cells. We propose that DSGb5 expressed on RCC cells may determine their metastatic capability.

Molecular Basis of Laminin-Integrin Interactions

Laminins are composed of three polypeptide chains, designated as α, β, and γ. The C-terminal region of laminin heterotrimers, containing coiled-coil regions, short tails, and laminin globular (LG) domains, is necessary and sufficient for binding to integrins, which are the major laminin receptor class. Laminin recognition by integrins critically requires the α chain LG domains and a glutamic acid residue of the γ chain at the third position from the C-terminus. Furthermore, the C-terminal region of the β chain contains a short amino acid sequence that modulates laminin affinity for integrins. Thus, all three of the laminin chains act cooperatively to facilitate integrin binding. Mammals possess 5 α (α1-5), 3 β (β1-3), and 3 γ (γ1-3) chains, combinations of which give rise to 16 distinct laminin isoforms. Each isoform is expressed in a tissue-specific and developmental stage-specific manner, exerting its functions through binding of integrins. In this review, we detail the current knowledge surrounding the molecular basis and physiological relevance of specific interactions between laminins and integrins, and describe the mechanisms underlying laminin action through integrins.

The effect of ROCK on TNF-α-induced CXCL8 secretion by intestinal epithelial cell lines is mediated through MKK4 and JNK signaling

Intestinal epithelial cells (IEC) play a role in mucosal inflammatory responses by producing important chemokines like CXCL8 when stimulated by TNF-α. Previously, we found that IEC cell lines required the Rho-associated kinase, ROCK, for CXCL8 responses after IL-1 stimulation. This study extends these findings by showing that inhibiting ROCK suppressed TNF-α-induced CXCL8 secretion by Caco-2 and DLD1 colonic epithelial cell lines and CXCL8 mRNA levels in Caco-2 cells. RNAi knockdown experiments indicated that the inhibitory effect was mediated by ROCK2, and not ROCK1. Inhibiting ROCK had no effect on TNF-stimulated IκBα phosphorylation and degradation or p38 MAPK phosphorylation indicating that ROCK plays no role in these signaling pathways. However, inhibiting ROCK suppressed TNF-induced phosphorylation of the p54 JNK isoform and phosphorylation of the upstream MKK4 kinase. These results suggest that ROCK is required for CXCL8 responses by TNF-stimulated IEC by affecting intracellular signaling through MKK4 and JNK.

α integrin cytoplasmic tails can rescue the loss of Rho-family GTPase signaling in the C. elegans somatic gonad

Integrin signaling relies on multiple, distinct pathways to impact a diverse set of cell behaviors. The Rho family of GTPases are well-established downstream signaling partners of integrins that regulate cell shape, polarity, and migration. The nematode C. elegans provides a simple in vivo system for studying both integrins and the Rho family. Our previous work showed that the C. elegans α integrin cytoplasmic tails have tissue-specific functions during development. Here, we use chimeric α integrins to show that the cytoplasmic tails can rescue the loss of the Rho family of GTPases in three cell types in the somatic gonad. Knockdown of rho-1 by RNAi causes defects in sheath cell actin organization, ovulation, and vulva morphology. Chimeric α integrin ina-1 with the pat-2 cytoplasmic tail can rescue both actin organization and ovulation after rho-1 RNAi, yet cannot restore vulva morphology. Knockdown of cdc-42 by RNAi causes defects in sheath cell actin organization, ovulation, vulva morphology, and distal tip cell migration. Chimeric α integrin pat-2 with the ina-1 cytoplasmic tail can rescue vulva morphology defects and distal tip cell migration after cdc-42 RNAi, yet cannot restore sheath cell actin organization or ovulation. Disruption of Rac yields the same phenotype in distal tip cells regardless of α integrin cytoplasmic tail composition. Taken together, the cytoplasmic tails of α integrins can bypass signaling from members of the Rho family of GTPases during development.

The three-way switch operation of Rac1/RhoA GTPase-based circuit controlling amoeboid-hybrid-mesenchymal transition

Metastatic carcinoma cells exhibit at least two different phenotypes of motility and invasion - amoeboid and mesenchymal. This plasticity poses a major clinical challenge for treating metastasis, while its underlying mechanisms remain enigmatic. Transitions between these phenotypes are mediated by the Rac1/RhoA circuit that responds to external signals such as HGF/SF via c-MET pathway. Using detailed modeling of GTPase-based regulation to study the Rac1/RhoA circuit's dynamics, we found that it can operate as a three-way switch. We propose to associate the circuit's three possible states to the amoeboid, mesenchymal and amoeboid/mesenchymal hybrid phenotype. In particular, we investigated the range of existence of, and the transition between, the three states (phenotypes) in response to Grb2 and Gab1 - two downstream adaptors of c-MET. The results help to explain the regulation of metastatic cells by c-MET pathway and hence can contribute to the assessment of possible clinical interventions.

Hypoxia-induced changes to integrin α 3 glycosylation facilitate invasion in epidermoid carcinoma cell line A431

Hypoxia is a critical microenvironmental factor that drives cancer progression through angiogenesis and metastasis. Glycoproteins, especially those on the plasma membrane, orchestrate this process; however, questions remain regarding hypoxia-perturbed protein glycosylation in cancer cells. We focused on the effects of hypoxia on the integrin family of glycoproteins, which are central to the cellular processes of attachment and migration and have been linked with cancer in humans. We employed electrostatic repulsion hydrophilic interaction chromatography coupled with iTRAQ labeling and LC-MS/MS to identify and quantify glycoproteins expressed in A431. The results revealed that independent of the protein-level change, N-glycosylation modifications of integrin α 3 (ITGA3) were inhibited by hypoxia, unlike in other integrin subunits. A combination of Western blot, flow cytometry, and cell staining assays showed that hypoxia-induced alterations to the glycosylation of ITGA3 prevented its efficient translocation to the plasma membrane. Mutagenesis studies demonstrated that simultaneous mutation of glycosites 6 and 7 of ITGA3 prevented its accumulation at the K562 cell surface, which blocked integrin α 3 and β 1 heterodimer formation and thus abolished ITGA3's interaction with extracellular ligands. By generating A431 cells stably expressing ITGA3 mutated at glycosites 6 and 7, we showed that lower levels of ITGA3 on the cell surface, as induced by hypoxia, conferred an increased invasive ability to cancer cells in vitro under hypoxic conditions. Taken together, these results revealed that ITGA3 translocation to the plasma membrane suppressed by hypoxia through inhibition of glycosylation facilitated cell invasion in A431.

Cellular signaling by collagen-binding integrins

The four collagen-binding αI domain integrins form their own subgroup among cell adhesion receptors. The signaling functions of α1β1 and α2β1 integrins have been analyzed in many experimental models, whereas less studies are available about the more recently found α10β1 and α11β1 heterodimers. Interestingly, collagen binding by α1β1 and α2β1 often generates opposite cellular responses. For example α1β1 has often been reported to promote cell proliferation and to suppress collagen synthesis, whereas α2β1 can in many model systems inhibit growth and promote collagen synthesis. There are obviously cell type dependent factors modifying the signaling. Additionally the structure and the organization of collagenous matrix play a critic role. Many recent studies have also stressed the importance of the crosstalk between the integrins and other cell surface receptors.

Matrikine and matricellular regulators of EGF receptor signaling on cancer cell migration and invasion

Cancer invasion is a complex process requiring, among other events, extensive remodeling of the extracellular matrix including deposition of pro-migratory and pro-proliferative moieties. In recent years, it has been described that while invading through matrices cancer cells can change shape and adapt their migration strategies depending on the microenvironmental context. Although intracellular signaling pathways governing the mesenchymal to amoeboid migration shift and vice versa have been mostly elucidated, the extracellular signals promoting these shifts are largely unknown. In this review, we summarize findings that point to matrikines that bind specifically to the EGF receptor as matricellular molecules that enable cancer cell migrational plasticity and promote invasion.

Integrins regulate centrosome integrity and astrocyte polarization following a wound

In response to a wound, astrocytes in culture extend microtubule-rich processes and polarize, orienting their centrosomes and Golgi apparatus woundside. β1 Integrin null astrocytes fail to extend processes toward the wound, and are disoriented, and often migrate away orthogonal, to the wound. The centrosome is unusually fragmented in β1 integrin null astrocytes. Expression of a β1 integrin cDNA in the null background yields cells with intact centrosomes that polarize and extend processes normally. Fragmented centrosomes rapidly assemble following integrin ligation and cell attachment. However, several experiments indicated that cell adhesion is not necessary. For example, astrocytes in suspension expressing a chimeric β1 subunit that can be activated by an antibody assemble centrosomes suggesting that β1 activation is sufficient to cause centrosome assembly in the absence of cell adhesion. siRNA knockdown of PCM1, a major centrosomal protein, inhibits cell polarization, consistent with the notion that centrosomes are necessary for polarity and that integrins regulate polarity via centrosome integrity. Screening inhibitors of molecules downstream of integrins indicate that neither FAK nor ILK is involved in regulation of centrosome integrity. In contrast, blebbistatin, a specific inhibitor of non-muscle myosin II (NMII), mimics the response of β1 integrin null astrocytes by disrupting centrosome integrity and cell polarization. Blebbistatin also inhibits integrin-mediated centrosome assembly in astrocytes attaching to fibronectin, consistent with the hypothesis that NMII functions downstream of integrins in regulating centrosome integrity.

Lipid rafts control human melanoma cell migration by regulating focal adhesion disassembly

Tumor cell migration is a crucial step in the metastatic cascade, and interruption of this step is considered to be logically effective in preventing tumor metastasis. Lipid rafts, distinct liquid ordered plasma membrane microdomains, have been shown to influence cancer cell migration, but the underlying mechanisms are still not well understood. Here, we report that lipid rafts regulate the dynamics of actin cytoskeleton and focal adhesion in human melanoma cell migration. Disrupting the integrity of lipid rafts with methyl-β cyclodextrin enhances actin stress fiber formation and inhibits focal adhesion disassembly, accompanied with alterations in cell morphology. Furthermore, actin cytoskeleton, rather than microtubules, mediates the lipid raft-dependent focal adhesion disassembly by regulating the dephosphorylation of focal adhesion proteins and the internalization of β3 integrin. We also show that Src-RhoA-Rho kinase signaling pathway is responsible for lipid raft disruption-induced stress fiber formation. Taken together, these observations provide a new mechanism to further explain how lipid rafts regulate the migration of melanoma cell and suggest that lipid rafts may be novel and attractive targets for cancer therapy.

The CD9/CD81 tetraspanin complex and tetraspanin CD151 regulate α3β1 integrin-dependent tumor cell behaviors by overlapping but distinct mechanisms

Integrin α3β1 potently promotes cell motility on its ligands, laminin-332 and laminin-511, and this may help to explain why α3β1 has repeatedly been linked to breast carcinoma progression and metastasis. The pro-migratory functions of α3β1 depend strongly on lateral interactions with cell surface tetraspanin proteins. Tetraspanin CD151 interacts directly with the α3 integrin subunit and links α3β1 integrin to other tetraspanins, including CD9 and CD81. Loss of CD151 disrupts α3β1 association with other tetraspanins and impairs α3β1-dependent motility. However, the extent to which tetraspanins other than CD151 are required for specific α3β1 functions is unclear. To begin to clarify which aspects of α3β1 function require which tetraspanins, we created breast carcinoma cells depleted of both CD9 and CD81 by RNA interference. Silencing both of these closely related tetraspanins was required to uncover their contributions to α3β1 function. We then directly compared our CD9/CD81-silenced cells to CD151-silenced cells. Both CD9/CD81-silenced cells and CD151-silenced cells showed delayed α3β1-dependent cell spreading on laminin-332. Surprisingly, however, once fully spread, CD9/CD81-silenced cells, but not CD151-silenced cells, displayed impaired α3β1-dependent directed motility and altered front-rear cell morphology. Also unexpectedly, the CD9/CD81 complex, but not CD151, was required to promote α3β1 association with PKCα in breast carcinoma cells, and a PKC inhibitor mimicked aspects of the CD9/CD81-silenced cell motility defect. Our data reveal overlapping, but surprisingly distinct contributions of specific tetraspanins to α3β1 integrin function. Importantly, some of CD9/CD81's α3β1 regulatory functions may not require CD9/CD81 to be physically linked to α3β1 by CD151.

The neural cell adhesion molecule (NCAM) associates with and signals through p21-activated kinase 1 (Pak1)

The Neural cell adhesion molecule (NCAM) plays an important role in regulation of nervous system development. To expand our understanding of the molecular mechanisms via which NCAM influences differentiation of neurons, we used a yeast two-hybrid screening to search for new binding partners of NCAM and identified p21-activated kinase 1 (Pak1). We show that NCAM interacts with Pak1 in growth cones of neurons. The autophosphorylation and activity of Pak1 were enhanced when isolated growth cones were incubated with NCAM function triggering antibodies, which mimic the interaction between NCAM and its extracellular ligands. The association of Pak1 with cell membranes, the efficiency of Pak1 binding to its activators, and Pak1 activity were inhibited in brains of NCAM-deficient mice. NCAM-dependent Pak1 activation was abolished after lipid raft disruption, suggesting that NCAM promotes Pak1 activation in the lipid raft environment. Phosphorylation of the downstream Pak1 effectors LIMK1 and cofilin was reduced in growth cones from NCAM-deficient neurons, which was accompanied by decreased levels of filamentous actin and inhibited filopodium mobility in the growth cones. Dominant-negative Pak1 inhibited and constitutively active Pak1 enhanced the ability of neurons to increase neurite outgrowth in response to the extracellular ligands of NCAM. Our combined observations thus indicate that NCAM activates Pak1 to drive actin polymerization to promote neuronal differentiation.

RHOC: a key gene for endometriosis

Considerable efforts have been invested in elucidating the potential mechanisms involved in the physiopathology of endometriosis. The aims of our study were to investigate whether RHOC expression is differentially altered in the endometrium and in endometriotic lesions. A total of 40 patients diagnosed with endometriosis and 15 healthy fertile women were selected for the study. Paired biopsies of endometrial tissue (eutopic endometrium) and endometriotic lesions (ectopic endometrium) were obtained from the patients with endometriosis. Endometrium from women without endometriosis was used as a control. Expression of the RHOC gene was analyzed by real-time polymerase chain reaction in autologous endometrial tissues of women with endometriosis and in the endometrium of control women. Increased RHOC expression was detected in endometriotic lesions compared to the eutopic endometrium of women with endometriosis and control women. RHOC changes may be among the key elements involved in the origin and the maintenance of endometriosis.

A novel role for the Rho-associated kinase, ROCK, in IL-1-stimulated intestinal epithelial cell responses

Intestinal epithelial cells (IEC) play a role in mucosal inflammation by producing pro-inflammatory chemokines that may initiate or amplify local responses. IL-1 is a potent activator of IEC and its receptor localizes to focal adhesions. Since the Rho-associated kinase, ROCK, also localizes to focal adhesions, we examined the role of ROCK in IL-1-induced chemokine responses in IEC cell lines. Suppressing ROCK with the Y27632 inhibitor suppressed IL-1-stimulated Caco-2 cell CXCL8/IL-8 and IEC-6 cell CCL2/MCP-1 secretion and mRNA levels. ROCK inhibition also suppressed IL-1-induced JNK phosphorylation in both cell lines, but high levels of the inhibitor had no significant effect on IL-1-stimulated Caco-2 IκBα phosphorylation and degradation or IKK phosphorylation and kinase activity. Therefore, ROCK may exert an effect on IL-1-stimulated JNK signaling to AP-1 activation, with little effect on IKK/IκBα signaling, defining a potentially important mechanism for regulating IL-1 signaling in IEC that may be essential for optimal cytokine responses.

Integrin α3β1 regulates tumor cell responses to stromal cells and can function to suppress prostate cancer metastatic colonization

Integrin α3β1 promotes tumor cell adhesion, migration, and invasion on laminin isoforms, and several clinical studies have indicated a correlation between increased tumoral α3β1 integrin expression and tumor progression, metastasis, and poor patient outcomes. However, several other clinical and experimental studies have suggested that α3β1 can possess anti-metastatic activity in certain settings. To help define the range of α3β1 functions in tumor cells in vivo, we used RNAi to silence the α3 integrin subunit in an aggressive, in vivo-passaged subline of PC-3 prostate carcinoma cells. Loss of α3 integrin impaired adhesion and proliferation on the α3β1 integrin ligand, laminin-332 in vitro. Despite these deficits in vitro, the α3-silenced cells were significantly more aggressive in a lung colonization model in vivo, with a substantially increased rate of tumor growth that significantly reduced survival. In contrast, silencing the related α6 integrin subunit delayed metastatic growth in vivo. The increased colonization of α3-silenced tumor cells in vivo was recapitulated in 3D collagen co-cultures with lung fibroblasts or pre-osteoblast-like cells, where α3-silenced cells showed dramatically enhanced growth. The increased response of α3-silenced tumor cells to stromal cells in co-culture could be reproduced by fibroblast conditioned medium, which contains one or more heparin-binding factors that selectively favor the growth of α3-silenced cells. Our new data suggest a scenario in which α3β1 regulates tumor-host interactions within the metastatic tumor microenvironment to limit growth, providing some of the first direct evidence that specific loss of α3 function in tumor cells can have pro-metastatic consequences in vivo.

p21-Activated kinase 6 (PAK6) inhibits prostate cancer growth via phosphorylation of androgen receptor and tumorigenic E3 ligase murine double minute-2 (Mdm2)

The androgen receptor (AR) signaling pathway plays a crucial role in the development and growth of prostate malignancies. Regulation of AR homeostasis in prostate tumorigenesis has not yet been fully characterized. In this study, we demonstrate that p21-activated kinase 6 (PAK6) inhibits prostate tumorigenesis by regulating AR homeostasis. First, we demonstrated that in normal prostate epithelium, AR co-localizes with PAK6 in the cytoplasm and translocates into the nucleus in malignant prostate. Furthermore, AR phosphorylation at Ser-578 by PAK6 promotes AR-E3 ligase murine double minute-2 (Mdm2) association, causing AR degradation upon androgen stimuli. We also showed that PAK6 phosphorylates Mdm2 on Thr-158 and Ser-186, which is critical for AR ubiquitin-mediated degradation. Moreover, we found that Thr-158 collaborates with Ser-186 for AR-Mdm2 association and AR ubiquitin-mediated degradation as it facilitates PAK6-mediated AR homeostasis. PAK6 knockdown promotes prostate tumor growth in vivo. Interestingly, we found a strong inverse correlation between PAK6 and AR expression in the cytoplasm of prostate cancer cells. These observations indicate that PAK6 may be important for the maintenance of androgen-induced AR signaling homeostasis and in prostate malignancy, as well as being a possible new therapeutic target for AR-positive and hormone-sensitive prostate cancer.

Rotavirus infection of cells in culture induces activation of RhoA and changes in the actin and tubulin cytoskeleton

Rotavirus infection induces an increase in [Ca²⁺]_cyto, which in turn may affect the distribution of the cytoskeleton proteins in the infected cell. Changes in microfilaments, including the formation of stress fibers, were observed starting at 0.5 h.p.i. using fluorescent phalloidin. Western blot analysis indicated that RhoA is activated between 0.5 and 1 h.p.i. Neither the phosphorylation of RhoA nor the formation of stress fibers were observed in cells infected with virions pre-treated with an anti-VP5* non-neutralizing mAb, suggesting that RhoA activation is stimulated by the interaction of the virus with integrins forming the cell receptor complex. In addition, the structure of the tubulin cytoskeleton was also studied. Alterations of the microtubules were evident starting at 3 h.p.i. and by 7 h.p.i. when microtubules were markedly displaced toward the periphery of the cell cytoplasm. Loading of rotavirus-infected cells with either a Ca²⁺ chelator (BAPTA) or transfection with siRNAs to silence NSP4, reversed the changes observed in both the microfilaments and microtubules distribution, but not the appearance of stress fibers. These results indicate that alterations in the distribution of actin microfilaments are initiated early during infection by the activation of RhoA, and that latter changes in the Ca²⁺ homeostasis promoted by NSP4 during infection may be responsible for other alterations in the actin and tubulin cytoskeleton.

α3β1 integrins regulate CD151 complex assembly and membrane dynamics in carcinoma cells within 3D environments

Integrins are extracellular matrix (ECM) receptors that are key players in the regulation of tumour cell invasion. The laminin-binding integrin α3β1 has previously been shown to regulate adhesion and migration of carcinoma cells in part through co-operative signalling with the tetraspanin family of transmembrane proteins. However, the spatial and temporal regulation of crosstalk between these families of transmembrane proteins in intact cells remains poorly understood. Here we have used fluorescence resonance energy transfer (FRET) to demonstrate for the first time that α3β1 and the tetraspanin CD151 directly associate at the front and retracting rear of polarised migrating breast carcinoma cells in both two-dimentional (2D) and three-dimentional (3D)matrices. Furthermore, localised α3β1-CD151 binding correlates with lower CD151 homodimerisation in cells migrating on laminin or within matrigel. Loss of α3β1 integrin leads to increased CD151 homodimer formation, increased activation of Rho GTPase, loss of cell polarity and decreased invasion in 3D ECM. As a result, α3-silenced cells show decreased actin-based membrane protrusion and retraction in both 2D and 3D environments. These data demonstrate that associations between α3β1 and CD151 occur dynamically within discrete subcellular compartments and act to establish local GTPase signalling to promote tumour cell invasion. These novel findings shed light on the complex crosstalk and switching between receptor complexes in response to different extracellular cues during cell invasion in 3D environments.

TGF-β autocrine pathway and MAPK signaling promote cell invasiveness and in vivo mammary adenocarcinoma tumor progression

Breast cancer progression and metastasis have been linked to abnormal signaling by transforming growth factor-β (TGF-β) cytokines. In early-stage breast cancers, TGF-β exhibits tumor suppressor activity by repressing cell proliferation and inducing cell death, whereas in advanced-stage tumors, TGF-β promotes invasion and metastatic dissemination. The molecular mechanisms underlying pro-oncogenic activities of TGF-β are not fully understood. The present study validates the role of TGF-β signaling in cancer progression and explores mediators of pro-oncogenic TGF-β activities using the LM3 mammary adenocarcinoma cell line, derived from a spontaneous murine mammary adenocarcinoma. Expression of kinase-inactive TGF-β receptors decreased both basal and TGF-β-induced invasion. Analysis of signal transduction mediators showed that p38MAPK and MEK contribute to TGF-β stimulation of cell motility and invasion. TGF-β disrupted the epithelial actin structures supporting cell-cell adhesions, and increased linear actin filaments. Moreover, MEK and p38MAPK pathways showed opposite effects on actin remodeling in response to TGF-β. Blockade of Raf-MEK signaling enhanced TGF-β induction of actin stress-fibers whereas p38MAPK inhibitors blocked this effect. A novel observation was made that TGF-β rapidly activates the actin nucleation Arp2/3 complex. In addition, TGF-β stimulated matrix metalloproteinase MMP-9 secretion via a MAPK-independent pathway. Experiments using syngeneic mice showed that kinase-inactive TGF-β receptors inhibit the first stages of LM3 tumor growth in vivo. Our studies demonstrate that autocrine TGF-β signaling contributes to the invasive behavior of mammary carcinoma cells. Moreover, we show that both MAPK-dependent and -independent pathways are necessary for TGF-β-induced effects. Therefore, MEK-ERK and p38 MAPK pathways are potential venues for therapeutic intervention in pro-oncogenic TGF-β signaling.

Parenchymal-stromal switching for extracellular matrix production on invasion of oral squamous cell carcinoma

It is poorly understood which cell type, tumor cells, or stromal cells are responsible for the production of extracellular matrix molecules in the neoplastic stroma. We studied the expression of 4 extracellular matrix molecules at the protein and messenger RNA levels in monocellular and 2 kinds of coculture systems between human squamous cell carcinoma (ZK-1) and fibroblast (OF-1) cell lines, which may correspond to carcinoma in situ and squamous cell carcinoma, respectively. Squamous cell carcinoma and carcinoma in situ tissue sections were also investigated by immunohistochemistry and in situ hybridization for extracellular matrix. Immunohistochemically, perlecan and tenascin C were localized in carcinoma cells in carcinoma in situ, whereas they were in the stromal space in squamous cell carcinoma. In monocellular culture conditions, expression levels for perlecan, tenascin C, and laminin were more predominant in ZK-1 than in OF-1, although those for fibronectin were more enhanced in OF-1. However, these extracellular matrix expression levels of OF-1 were elevated, whereas those of ZK-1 dropped when they were in coculture conditions. The differences between ZK-1 and OF-1 were significantly more evident in direct contact (ZK-1/OF-1, 56%-22%) than in indirect contact (63%-39%). These results indicate that oral squamous cell carcinoma cells produce extracellular matrix in the absence of stromal fibroblasts (or in carcinoma in situ) and that they stop producing extracellular matrix in the presence of fibroblasts (or in squamous cell carcinoma). It is hence suggested that stromal fibroblasts after direct contact with invading squamous cell carcinoma cells are more responsible than squamous cell carcinoma cells for the formation of neoplastic stroma, whereas carcinoma in situ cells have to produce and deposit extracellular matrix by themselves to form intraepithelial microstromal spaces.

Interplay between β1-integrin and Rho signaling regulates differential scattering and motility of pancreatic cancer cells by snail and Slug proteins

The Snail family of transcription factors has been implicated in pancreatic cancer progression. We recently showed that Snail (Snai1) promotes membrane-type 1 matrix metalloproteinase (MT1-MMP)- and ERK1/2-dependent scattering of pancreatic cancer cells in three-dimensional type I collagen. In this study, we examine the role of Slug (Snai2) in regulating pancreatic cancer cell scattering in three-dimensional type I collagen. Although Slug increased MT1-MMP expression and ERK1/2 activity, Slug-expressing cells failed to scatter in three-dimensional collagen. Moreover, in contrast to Snail-expressing cells, Slug-expressing cells did not demonstrate increased collagen I binding, collagen I-driven motility, or α2β1-integrin expression. Significantly, inhibiting β1-integrin function decreased migration and scattering of Snail-expressing cells in three-dimensional collagen. As Rho GTPases have been implicated in invasion and migration, we also analyzed the contribution of Rac1 and Rho signaling to the differential migration and scattering of pancreatic cancer cells. Snail-induced migration and scattering were attenuated by Rac1 inhibition. In contrast, inhibiting Rho-associated kinase ROCK1/2 increased migration and scattering of Slug-expressing cells in three-dimensional collagen and thus phenocopied the effects of Snail in pancreatic cancer cells. Additionally, the increased migration and scattering in three-dimensional collagen of Slug-expressing cells following ROCK1/2 inhibition was dependent on β1-integrin function. Overall, these results demonstrate differential effects of Snail and Slug in pancreatic cancer and identify the interplay between Rho signaling and β1-integrin that functions to regulate the differential scattering and migration of Snail- and Slug-expressing pancreatic cancer cells.

Activation of EGFR promotes squamous carcinoma SCC10A cell migration and invasion via inducing EMT-like phenotype change and MMP-9-mediated degradation of E-cadherin

EGFR is a potent stimulator of invasion and metastasis in head and neck squamous cell carcinomas (HNSCC). However, the mechanism by which EGFR may stimulate tumor cell invasion and metastasis still need to be elucidated. In this study, we showed that activation of EGFR by EGF in HNSCC cell line SCC10A enhanced cell migration and invasion, and induced loss of epitheloid phenotype in parallel with downregulation of E-cadherin and upregulation of N-cadherin and vimentin, indicating that EGFR promoted SCC10A cell migration and invasion possibly by an epithelial to mesenchymal transition (EMT)-like phenotype change. Interestingly, activation of EGFR by EGF induced production of matrix metalloproteinase-9 (MMP-9) and soluble E-cadherin (sE-cad), and knockdown of MMP-9 by siRNA inhibited sE-cad production induced by EGF in SCC10A. Moreover, both MMP-9 knockdown and E-cadherin overexpression inhibited cell migration and invasion induced by EGF in SCC10A. The results indicate that EGFR activation promoted cell migration and invasion through inducing MMP-9-mediated degradation of E-cadherin into sE-cad. Pharmacologic inhibition of EGFR, MEK, and PI3K kinase activity in SCC10A reduced phosphorylated levels of ERK-1/2 and AKT, production of MMP-9 and sE-cad, cell migration and invasion, and expressional changes of EMT markers (E-cadherin and N-cadherin) induced by EGF, indicating that EGFR activation promotes cell migration and invasion via ERK-1/2 and PI3K-regulated MMP-9/E-cadherin signaling pathways. Taken together, the data suggest that EGFR activation promotes HNSCC SCC10A cell migration and invasion by inducing EMT-like phenotype change and MMP-9-mediated degradation of E-cadherin into sE-cad related to activation of ERK-1/2 and PI3K signaling pathways.

Two distinct integrin-mediated mechanisms contribute to apical lumen formation in epithelial cells

BACKGROUND

Formation of apical compartments underlies the morphogenesis of most epithelial organs during development. The extracellular matrix (ECM), particularly the basement membrane (BM), plays an important role in orienting the apico-basal polarity and thereby the positioning of apical lumens. Integrins have been recognized as essential mediators of matrix-derived polarity signals. The importance of β1-integrins in epithelial polarization is well established but the significance of the accompanying α-subunits have not been analyzed in detail.

PRINCIPAL FINDINGS

Here we demonstrate that two distinct integrin-dependent pathways regulate formation of apical lumens to ensure robust apical membrane biogenesis under different microenvironmental conditions; 1) α2β1- and α6β4-integrins were required to establish a basal cue that depends on Rac1-activity and guides apico-basal cell polarization. 2) α3β1-integrins were implicated in positioning of mitotic spindles in cysts, a process that is essential for Cdc42-driven epithelial hollowing.

SIGNIFICANCE

Identification of the separate processes driven by particular integrin receptors clarifies the functional hierarchies between the different integrins co-expressed in epithelial cells and provides valuable insight into the complexity of cell-ECM interactions thereby guiding future studies addressing the molecular basis of epithelial morphogenesis during development and disease.

Hemidesmosomes and focal contact proteins: functions and cross-talk in keratinocytes, bullous diseases and wound healing

The outer most layer of the skin, the epidermis, is attached to the dermis via a sheet of extracellular matrix proteins termed the basement membrane zone (BMZ). In the intact skin, adhesion of the keratinocytes in the basal layer of the epidermis to the BMZ is facilitated primarily by hemidesmosomes which associate with the keratin cytoskeleton. Cultured keratinocytes do not assemble bona fide hemidesmosomes although hemidesmosome protein clusters (stable anchoring contacts) are found along the substrate-attached surface of the cells and towards the leading edge of keratinocytes repopulating scratch wounds. Actin cytoskeleton-associated matrix adhesion devices termed focal contacts are not thought to play an important role in the adhesion of keratinocytes to the BMZ in intact skin but are prominent in cultured keratinocytes where they are believed to regulate cell migration. We review the molecular components, functions, dynamics and cross-talk of hemidesmosomes and focal contacts in keratinocytes. In addition, we briefly describe what is known about their role in autoimmune and genetic blistering diseases of the skin. We also discuss recent publications which indicate, contrary to expectation, that certain focal contact proteins retard keratinocyte migration while hemidesmosomal proteins regulate directed keratinocyte motility during wound healing.

Spatial organization of adhesion: force-dependent regulation and function in tissue morphogenesis

Integrin- and cadherin-mediated adhesion is central for cell and tissue morphogenesis, allowing cells and tissues to change shape without loosing integrity. Studies predominantly in cell culture showed that mechanosensation through adhesion structures is achieved by force-mediated modulation of their molecular composition. The specific molecular composition of adhesion sites in turn determines their signalling activity and dynamic reorganization. Here, we will review how adhesion sites respond to mecanical stimuli, and how spatially and temporally regulated signalling from different adhesion sites controls cell migration and tissue morphogenesis.

Characterization of the roles of RHOC and RHOA GTPases in invasion, motility, and matrix adhesion in inflammatory and aggressive breast cancers

BACKGROUND

The 2 closely related small GTPases, RHOC and RHOA, are involved in mammary gland carcinogenesis; however, their specific roles in determining cancer cell adhesion and invasion have not been elucidated.

METHODS

RHOA and RHOC are highly homologous, thereby posing a major challenge to study their individual functions in cancer cells. By selectively knocking down these proteins, we have been able to alternatively inhibit RHOC and RHOA, while preserving expression of the other rho protein. Quantitative analyses of the growth patterns and invasion in the aggressive estrogen receptor negative cell lines MDA-231 and SUM149 were carried out on collagen I and Matrigel substrates.

RESULTS

RHOC, and not RHOA, modulates surface expression and colocalization of alpha2 and beta1 integrins in MDA-MB-231 on collagen I. Neither RHOC or RHOA affected integrin expression in the inflammatory breast cancer cell line SUM149, further highlighting the different regulation of adhesion and motility in inflammatory breast cancer.

CONCLUSIONS

This work shows that RHOC and RHOA play different roles in cell-matrix adhesion, motility, and invasion of MDA-MB-231 and reaffirms the crucial role of RHOC-GTPase in inflammatory breast cancer cell invasion.

Nitrogen-containing bisphosphonate, YM529/ONO-5920, inhibits tumor metastasis in mouse melanoma through suppression of the Rho/ROCK pathway

The small GTPases of the Ras and Rho families are widely involved in tumorigenesis and metastasis. We recently showed that YM529/ONO-5920, a new developed bisphosphonate, inhibits the mevalonate pathway, is required for the prenylation of the small GTPases. In this study, we investigated whether YM529/ONO-5920 inhibits tumor cell migration, invasion, adhesion, and metastasis in B16BL6 cells, a mouse melanoma cell line. It was found that YM529/ONO-5920 significantly inhibited lung metastasis, cell migration, invasion, and adhesion at concentrations that did not show anti-proliferative effects on B16BL6 cells. YM529/ONO-5920 also inhibited the expression of matrix metalloproteinases (MMPs) and very late antigens (VLAs). Furthermore, YM529/ONO-5920 suppressed Rho activation, but not activation of Ras. The results indicate that YM529/ONO-5920 suppresses the Rho/ROCK pathways, thereby inhibiting B16BL6 cell migration, invasion, adhesion and metastasis. These findings suggest that YM529/ONO-5920 has potential clinical applications for the treatment of tumour cell metastasis.

Extracellular matrix effect on RhoA signaling modulation in vascular smooth muscle cells

Morphological adaptations of vascular smooth muscle cells (VSMC) to the mechanically active environment in which they reside, are mediated by direct interactions with the extracellular matrix (ECM) which induces physiological changes at the intracellular level. This study aimed to analyze the effects of the ECM on RhoA-induced mechanical signaling that controls actin organization and focal adhesion formation. VSMC were transfected with RhoA constructs (wild type, dominant negative or constitutively active) and plated on different ECM proteins used as substrate (fibronectin, collagen IV, collagen I, and laminin) or poly-l-lysine as control. Morphological changes of the VSMC were detected by fluorescence confocal microscopy and total internal reflection fluorescence (TIRF) microscopy, and were independently verified using adhesion assays and Western blot analysis. Our results showed that the ECM has an important role in cell spreading, adhesion and morphology with a direct effect on modulating RhoA signaling. RhoA activity significantly affected the stress fibers and focal adhesions reorganization, but in a context imposed by the ECM. Thus, RhoA activity modulation in VSMC induced an increased activation of stress fibers and FA formation at 5h, while a significant inhibition was recorded at 24h after plating on the different ECM. Our findings provide biophysical evidence that ECM modulates VSMC response to mechanical stimuli inducing intracellular biochemical signaling involved in cellular adaptation to the local microenvironment.

Integrin alpha3 subunit regulates events linked to epithelial repair, including keratinocyte migration and protein expression

Two integrins, alpha3beta1 and alpha6beta4, are high-affinity receptors for laminin 332, the major laminin isoform of the dermal-epidermal junction, although they are thought to have different functions. Biological and genetic studies have firmly established that the alpha6beta4 integrin is indispensable for the stable anchorage of the epidermis to the underlying dermis. In contrast, the alpha3beta1 integrin is thought to be important for cell migration, although the issue is controversial, and both positive and negative effects have been reported. To address the function of alpha3beta1 integrin, we used small interfering RNA to down-regulate the alpha3 subunit in human keratinocytes. The resulting phenotype indicates that lack of alpha3beta1 integrin compromises intercellular adhesion and collective migration, while it enhances single cell migration with a concomitant increase of both focal adhesion kinase and extracellular signal-regulated kinase. In addition, down-regulation of integrin alpha3 subunit results in an increased expression of fibronectin and precursor laminin 332, two extracellular matrix proteins known to be up-regulated during wound healing. Thus, down-regulation of alpha3beta1 integrin recapitulates crucial events governing keratinocyte migration associated with wound healing and tissue repair.

Pak1 regulates branching morphogenesis in 3D MDCK cell culture by a PIX and beta1-integrin-dependent mechanism

Branching morphogenesis is a fundamental process in the development of the kidney. This process gives rise to a network of ducts, which form the collecting system. Defective branching can lead to a multitude of kidney disorders including agenesis and reduced nephron number. The formation of branching tubules involves changes in cell shape, cell motility, and reorganization of the cytoskeleton. However, the exact intracellular mechanisms involved are far from understood. We have used the three-dimensional (3D) Madin-Darby canine kidney (MDCK) cell culture system to study how p21-activated kinase 1 (Pak1), which is an important regulator of the cytoskeleton, modulates branching. Our data reveal that Pak1 plays a crucial role in regulating branching morphogenesis. Expression of a dominant-negative Pak1 mutant (DN-Pak1) in MDCK cysts resulted in the spontaneous formation of extensions and branching tubules. Cellular contractility and levels of phosphorylated myosin light chain (pMLC) were increased in DN-Pak1 cells in collagen. Expression of a DN-Pak1 mutant that does not bind to PIX (DN-Pak1-DeltaPIX) failed to form extensions in collagen and did not have increased contractility. This shows that the DN-Pak1 mutant requires PIX binding to generate extensions and increased contractility in 3D culture. Furthermore, a beta1-integrin function-blocking antibody (AIIB2) inhibited the formation of branches and blocked the increased contractility in DN-Pak1 cysts. Taken together, our work shows that DN-Pak1-induced branching morphogenesis requires PIX binding and beta1-integrin signaling.

Laminin-binding integrins and their tetraspanin partners as potential antimetastatic targets

Within the integrin family of cell adhesion receptors, integrins alpha3beta1, alpha6beta1, alpha6beta4 and alpha7beta1 make up a laminin-binding subfamily. The literature is divided on the role of these laminin-binding integrins in metastasis, with different studies indicating either pro- or antimetastatic functions. The opposing roles of the laminin-binding integrins in different settings might derive in part from their unusually robust associations with tetraspanin proteins. Tetraspanins organise integrins into multiprotein complexes within discrete plasma membrane domains termed tetraspanin-enriched microdomains (TEMs). TEM association is crucial to the strikingly rapid cell migration mediated by some of the laminin-binding integrins. However, emerging data suggest that laminin-binding integrins also promote the stability of E-cadherin-based cell-cell junctions, and that tetraspanins are essential for this function as well. Thus, TEM association endows the laminin-binding integrins with both pro-invasive functions (rapid migration) and anti-invasive functions (stable cell junctions), and the composition of TEMs in different cell types might help determine the balance between these opposing activities. Unravelling the tetraspanin control mechanisms that regulate laminin-binding integrins will help to define the settings where inhibiting the function of these integrins would be helpful rather than harmful, and may create opportunities to modulate integrin activity in more sophisticated ways than simple functional blockade.

Sdc1 negatively modulates carcinoma cell motility and invasion

During cancer progression, tumor cells eventually invade the surrounding collagen-rich extracellular matrix. Here we show that squamous cell carcinoma cells strongly adhere to Type I collagen substrates but display limited motility and invasion on collagen barriers. Further analysis revealed that in addition to the alpha2beta1 integrin, a second collagen receptor was identified as Syndecan-1 (Sdc1), a cell surface heparan sulfate proteoglycan. We demonstrate that siRNA-mediated depletion of Sdc1 reduced adhesion efficiency to collagen I, whereas knockdown of Sdc4 was without effect. Importantly, silencing Sdc1 expression caused reduced focal adhesion plaque formation and enhanced cell spreading and motility on collagen I substrates, but did not alter cell motility on other ECM substrates. Sdc1 depletion ablated adhesion-induced RhoA activation. In contrast, Rac1 was strongly activated following Sdc1 knockdown, suggesting that Sdc1 may mediate the link between integrin-induced actin remodeling and motility. Taken together, these data substantiate the existence of a co-adhesion receptor system in tumor cells, whereby Sdc1 functions as a key regulator of cell motility and cell invasion by modulating RhoA and Rac activity. Downregulation of Sdc1 expression during carcinoma progression may represent a mechanism by which tumor cells become more invasive and metastatic.