ROR2/Wnt5a Signaling Regulates Directional Cell Migration and Early Tumor Cell Invasion in Ovarian Cancer

Adhesion to and clearance of the mesothelial monolayer are key early events in metastatic seeding of ovarian cancer. ROR2 is a receptor tyrosine kinase that interacts with Wnt5a ligand to activate noncanonical Wnt signaling and has been previously shown to be upregulated in ovarian cancer tissue. However, no prior study has evaluated the mechanistic role of ROR2 in ovarian cancer. Through a cellular high-throughput genetic screen, we independently identified ROR2 as a driver of ovarian tumor cell adhesion and invasion. ROR2 expression in ovarian tumor cells serves to drive directed cell migration preferentially toward areas of high Wnt5a ligand, such as the mesothelial lined omentum. In addition, ROR2 promotes ovarian tumor cell adhesion and clearance of a mesothelial monolayer. Depletion of ROR2, in tumor cells, reduces metastatic tumor burden in a syngeneic model of ovarian cancer. These findings support the role of ROR2 in ovarian tumor cells as a critical factor contributing to the early steps of metastasis. Therapeutic targeting of the ROR2/Wnt5a signaling axis could provide a means of improving treatment for patients with advanced ovarian cancer.

IMPLICATIONS

This study demonstrates that ROR2 in ovarian cancer cells is important for directed migration to the metastatic niche and provides a potential signaling axis of interest for therapeutic targeting in ovarian cancer.

Senescent CAFs Mediate Immunosuppression and Drive Breast Cancer Progression

The tumor microenvironment (TME) profoundly influences tumorigenesis, with gene expression in the breast TME capable of predicting clinical outcomes. The TME is complex and includes distinct cancer-associated fibroblast (CAF) subtypes whose contribution to tumorigenesis remains unclear. Here, we identify a subset of myofibroblast CAFs (myCAF) that are senescent (senCAF) in mouse and human breast tumors. Utilizing the MMTV-PyMT;INK-ATTAC (INK) mouse model, we found that senCAF-secreted extracellular matrix specifically limits natural killer (NK) cell cytotoxicity to promote tumor growth. Genetic or pharmacologic senCAF elimination unleashes NK cell killing, restricting tumor growth. Finally, we show that senCAFs are present in HER2+, ER+, and triple-negative breast cancer and in ductal carcinoma in situ (DCIS) where they predict tumor recurrence. Together, these findings demonstrate that senCAFs are potently tumor promoting and raise the possibility that targeting them by senolytic therapy could restrain breast cancer development.

SIGNIFICANCE

senCAFs limit NK cell-mediated killing, thereby contributing to breast cancer progression. Thus, targeting senCAFs could be a clinically viable approach to limit tumor progression.

DDR2-regulated arginase activity in ovarian cancer-associated fibroblasts promotes collagen production and tumor progression

Ovarian cancer has poor survival outcomes particularly for advanced stage, metastatic disease. Metastasis is promoted by interactions of stromal cells, such as cancer-associated fibroblasts (CAFs) in the tumor microenvironment (TME), with tumor cells. CAFs play a key role in tumor progression by remodeling the TME and extracellular matrix (ECM) to result in a more permissive environment for tumor progression. It has been shown that fibroblasts, in particular myofibroblasts, utilize metabolism to support ECM remodeling. However, the intricate mechanisms by which CAFs support collagen production and tumor progression are poorly understood. In this study, we show that the fibrillar collagen receptor, Discoidin Domain Receptor 2 (DDR2), promotes collagen production in human and mouse omental CAFs through arginase activity. CAFs with high DDR2 or arginase promote tumor colonization in the omentum. In addition, DDR2-depleted CAFs had decreased ornithine levels leading to decreased collagen production and polyamine levels compared to WT control CAFs. Tumor cell invasion was decreased in the presence CAF conditioned media (CM) depleted of DDR2 or arginase-1, and this invasion defect was rescued in the presence of CM from DDR2-depleted CAFs that constitutively overexpressed arginase-1. Similarly, the addition of exogenous polyamines to CM from DDR2-depleted CAFs led to increased tumor cell invasion. We detected SNAI1 protein at the promoter region of the arginase-1 gene, and DDR2-depleted CAFs had decreased levels of SNAI1 protein at the arginase-1 promoter region. Furthermore, high stromal arginase-1 expression correlated with poor survival in ovarian cancer patients. These findings highlight how DDR2 regulates collagen production by CAFs in the tumor microenvironment by controlling the transcription of arginase-1, and CAFs are a major source of arginase activity and L-arginine metabolites in ovarian cancer models.

Functional analysis of recurrent CDC20 promoter variants in human melanoma

Small nucleotide variants in non-coding regions of the genome can alter transcriptional regulation, leading to changes in gene expression which can activate oncogenic gene regulatory networks. Melanoma is heavily burdened by non-coding variants, representing over 99% of total genetic variation, including the well-characterized TERT promoter mutation. However, the compendium of regulatory non-coding variants is likely still functionally under-characterized. We developed a pipeline to identify hotspots, i.e. recurrently mutated regions, in melanoma containing putatively functional non-coding somatic variants that are located within predicted melanoma-specific regulatory regions. We identified hundreds of statistically significant hotspots, including the hotspot containing the TERT promoter variants, and focused on a hotspot in the promoter of CDC20. We found that variants in the promoter of CDC20, which putatively disrupt an ETS motif, lead to lower transcriptional activity in reporter assays. Using CRISPR/Cas9, we generated an indel in the CDC20 promoter in human A375 melanoma cell lines and observed decreased expression of CDC20, changes in migration capabilities, increased growth of xenografts, and an altered transcriptional state previously associated with a more proliferative and less migratory state. Overall, our analysis prioritized several recurrent functional non-coding variants that, through downregulation of CDC20, led to perturbation of key melanoma phenotypes.

Stromal DDR2 Promotes Ovarian Cancer Metastasis through Regulation of Metabolism and Secretion of Extracellular Matrix Proteins

Ovarian cancer is the leading cause of gynecologic cancer-related deaths. The propensity for metastasis within the peritoneal cavity is a driving factor for the poor outcomes associated with this disease, but there is currently no effective therapy targeting metastasis. In this study, we investigate the contribution of stromal cells to ovarian cancer metastasis and identify normal stromal cell expression of the collagen receptor, discoidin domain receptor 2 (DDR2), that acts to facilitate ovarian cancer metastasis. In vivo, global genetic inactivation of Ddr2 impairs the ability of Ddr2-expressing syngeneic ovarian cancer cells to spread throughout the peritoneal cavity. Specifically, DDR2 expression in mesothelial cells lining the peritoneal cavity facilitates tumor cell attachment and clearance. Subsequently, omentum fibroblast expression of DDR2 promotes tumor cell invasion. Mechanistically, we find DDR2-expressing fibroblasts are more energetically active, such that DDR2 regulates glycolysis through AKT/SNAI1 leading to suppressed fructose-1,6-bisphosphatase and increased hexokinase activity, a key glycolytic enzyme. Upon inhibition of DDR2, we find decreased protein synthesis and secretion. Consequently, when DDR2 is inhibited, there is reduction in secreted extracellular matrix proteins important for metastasis. Specifically, we find that fibroblast DDR2 inhibition leads to decreased secretion of the collagen crosslinker, LOXL2. Adding back LOXL2 to DDR2 deficient fibroblasts rescues the ability of tumor cells to invade. Overall, our results suggest that stromal cell expression of DDR2 is an important mediator of ovarian cancer metastasis.

IMPLICATIONS

DDR2 is highly expressed by stromal cells in ovarian cancer that can mediate metastasis and is a potential therapeutic target in ovarian cancer.

Genetic characterization of primary and metastatic high-grade serous ovarian cancer tumors reveals distinct features associated with survival

High-grade serous ovarian cancer (HGSC) is the most lethal histotype of ovarian cancer and the majority of cases present with metastasis and late-stage disease. Over the last few decades, the overall survival for patients has not significantly improved, and there are limited targeted treatment options. We aimed to better characterize the distinctions between primary and metastatic tumors based on short- or long-term survival. We characterized 39 matched primary and metastatic tumors by whole exome and RNA sequencing. Of these, 23 were short-term (ST) survivors (overall survival (OS) < 3.5 years) and 16 were long-term (LT) survivors (OS > 5 years). We compared somatic mutations, copy number alterations, mutational burden, differential gene expression, immune cell infiltration, and gene fusion predictions between the primary and metastatic tumors and between ST and LT survivor cohorts. There were few differences in RNA expression between paired primary and metastatic tumors, but significant differences between the transcriptomes of LT and ST survivors in both their primary and metastatic tumors. These findings will improve the understanding of the genetic variation in HGSC that exist between patients with different prognoses and better inform treatments by identifying new targets for drug development.

A Cdh3-β-catenin-laminin signaling axis in a subset of breast tumor leader cells control leader cell polarization and directional collective migration

Carcinoma dissemination can occur when heterogeneous tumor and tumor-stromal cell clusters migrate together via collective migration. Cells at the front lead and direct collective migration, yet how these leader cells form and direct migration are not fully appreciated. From live videos of primary mouse and human breast tumor organoids in a 3D microfluidic system mimicking native breast tumor microenvironment, we developed 3D computational models, which hypothesize that leader cells need to generate high protrusive forces and overcome extracellular matrix (ECM) resistance at the leading edge. From single-cell sequencing analyses, we find that leader cells are heterogeneous and identify and isolate a keratin 14- and cadherin-3-positive subpopulation sufficient to lead collective migration. Cdh3 controls leader cell protrusion dynamics through local production of laminin, which is required for integrin/focal adhesion function. Our findings highlight how a subset of leader cells interact with the microenvironment to direct collective migration.

DDR2 Expression in Cancer-Associated Fibroblasts Promotes Ovarian Cancer Tumor Invasion and Metastasis through Periostin-ITGB1

Ovarian cancer has the highest mortality of all gynecologic malignancies. As such, there is a need to identify molecular mechanisms that underlie tumor metastasis in ovarian cancer. Increased expression of receptor tyrosine kinase, DDR2, has been associated with worse patient survival. Identifying downstream targets of DDR2 may allow specific modulation of ovarian cancer metastatic pathways. Additionally, stromal cells play a critical role in metastasis. The crosstalk between tumor and stromal cells can lead to tumor progression. We first identified that tumor cells co-cultured with DDR2-expressing fibroblasts had lower periostin expression when compared to tumor cells co-cultured with DDR2-depleted fibroblasts. We confirmed that DDR2 regulates POSTN expression in ovarian cancer-associated fibroblasts (CAFs). We found that mesothelial cell clearance and invasion by tumor cells were enhanced three-fold when DDR2 and POSTN-expressing CAFs were present compared to DDR2 and POSTN-depleted CAFs. Furthermore, DDR2-depleted and POSTN-overexpressing CAFs co-injected with ovarian tumor cells had increased tumor burden compared to mice injected with tumor cells and DDR2 and POSTN-depleted CAFs. Furthermore, we demonstrated that DDR2 regulates periostin expression through integrin B1 (ITGB1). Stromal DDR2 is highly correlated with stromal POSTN expression in ovarian cancer patient tumors. Thus, DDR2 expression in CAFs regulates the steps of ovarian cancer metastasis through periostin.

Abstract 2423: A Cdh3 - Lam332 signaling axis by a unique subset of leader cells controls breast tumor organoid protrusion dynamics and directed collective migration

Carcinoma dissemination can occur when heterogeneous tumor and tumor stromal cells clusters migrate together via collective migration. Cells at the front lead and direct collective migration, yet how these leader cells form and interact with the microenvironment to direct migration are not fully appreciated. From live videos of primary mouse and human breast tumor organoids in a 3D microfluidic system that mimics the native breast tumor microenvironment, we developed 3D computational models. These hypothesize that for leader cells to polarize to the leading edge and lead directed collective migration they need to generate high protrusive forces and overcome extracellular matrix (ECM) resistance. From scRNAseq of migrating primary tumor organoids, we identify significant K14+ “leader” cell heterogeneity that differs depending upon the environmental signal. We isolate a unique Cadherin-3 (Cdh3) positive leader cell subpopulation that is necessary and sufficient to lead collective migration. Cdh3 controls leader cell protrusion dynamics through controlling the local production of Laminin-332 which is required for integrin/focal adhesion function. Loss of Cdh3 expression in mouse and human primary breast tumor organoids and invasive breast tumor cell lines significantly impairs directed collective migration. In the absence of leader cell Cdh-Cdh3 intercellular adhesions there is reduced cellular β-catenin levels and Laminin 332 production. In syngeneic, orthotopic transplant mouse models, Cdh3 depletion in tumor cells resulted in decreased laminin 332 production at the invasive edge of primary tumors and decreased lung metastasis. Chromatin Immunoprecipitation experiments revealed that in contacted cells β-catenin and TCF4 are present on Lam α3, β3, and γ2 promoter regions. Laminin 332 production by leader cells was required for optimal Integrin/FA function and cellular protrusion stability. These results indicated that in leader cells local Cdh3-Cdh3/β-catenin regulated Laminin 332 production controls protrusion dynamics and overcomes resistance to ECM to lead directed collective tumor migration. Our findings highlight how a unique subset of leader cells in breast tumors interact with the microenvironment to direct collective migration.

Citation Format: Yanyang Cao, Priscilla Y. Hwang, Maria Clarke, Jose Almeida, Amit Pathak, Gregory D. Longmore. A Cdh3 - Lam332 signaling axis by a unique subset of leader cells controls breast tumor organoid protrusion dynamics and directed collective migration [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2423.

Collagen Linearization within Tumors

It is now well appreciated that the tumor microenvironment (TME) surrounding primary tumors impacts tumor growth, progression (invasion and migration), and response to therapy. Broadly speaking, the TME is composed of cells (immune cells, activated fibroblasts, adipocytes, endothelial cells), acellular extracellular matrix (ECM), and cytokines or growth factors, some of which are bound or tethered to the ECM proteins. All these compartments undergo significant changes during tumor development and progression. Changes to the ECM, in particular, can dramatically influence cancer biology. This has stimulated the development of therapies that directly reverse or prevent the structural changes in the TME ECM that facilitate cancer progression. But to do so, in a rational manner, we need to understand how structural changes to tumor ECM arise, are remodeled, and function to facilitate tumor cell invasion and migration that give rise to metastatic disease, which is the main cause of cancer-related deaths. In this issue of Cancer Research, Janjanam and colleagues show that the ratio of WISP1/WISP2 in tumors is critical for ECM collagen fiber linearization and important for metastasis. WISP2 binds ECM collagen directly and can inhibit WISP1-mediated collagen linearization. These new results offer a new approach for targeting the altered collagen ECM in tumors by preventing or reversing collagen linearization.See related article by Janjanam et al., p. 5666.

Tyrosine kinase-independent actions of DDR2 in tumor cells and cancer-associated fibroblasts influence tumor invasion, migration and metastasis

Both tumor cell-intrinsic signals and tumor cell-extrinsic signals from cells within the tumor microenvironment influence tumor cell dissemination and metastasis. The fibrillar collagen receptor tyrosine kinase (RTK) discoidin domain receptor 2 (DDR2) is essential for breast cancer metastasis in mouse models, and high expression of DDR2 in tumor and tumor stromal cells is strongly associated with poorer clinical outcomes. DDR2 tyrosine kinase activity has been hypothesized to be required for the metastatic activity of DDR2; however, inhibition of DDR2 tyrosine kinase activity, along with that of other RTKs, has failed to provide clinically relevant responses in metastatic patients. Here, we show that tyrosine kinase activity-independent action of DDR2 in tumor cells can support Matrigel invasion and in vivo metastasis. Paracrine actions of DDR2 in tumor cells and cancer-associated fibroblasts (CAFs) also support tumor invasion, migration and lung colonization in vivo. These data suggest that tyrosine kinase-independent functions of DDR2 could explain failures of tyrosine kinase inhibitor treatment in metastatic breast cancer patients and highlight the need for alternative therapeutic strategies that inhibit both tyrosine kinase-dependent and -independent actions of RTKs in the treatment of breast cancer. This article has an associated First Person interview with the first author of the paper.

Contextual cues from cancer cells govern cancer-associated fibroblast heterogeneity

Cancer cells function as primary architects of the tumor microenvironment. However, the molecular features of cancer cells that govern stromal cell phenotypes remain unclear. Here, we show that cancer-associated fibroblast (CAF) heterogeneity is driven by lung adenocarcinoma (LUAD) cells at either end of the epithelial-to-mesenchymal transition (EMT) spectrum. LUAD cells that have high expression of the EMT-activating transcription factor ZEB1 reprogram CAFs through a ZEB1-dependent secretory program and direct CAFs to the tips of invasive projections through a ZEB1-driven CAF repulsion process. The EMT, in turn, sensitizes LUAD cells to pro-metastatic signals from CAFs. Thus, CAFs respond to contextual cues from LUAD cells to promote metastasis.

Bota-Rabassedas et al. show that EMT in lung adenocarcinoma cells activates a secretory process that governs CAF heterogeneity and, in turn, sensitizes lung adenocarcinoma cells to pro-metastatic signals from CAFs. Thus, EMT positions lung adenocarcinoma cells at the apex of a signaling hierarchy in the tumor microenvironment.

Micro-strains in the extracellular matrix induce angiogenesis

An improved understanding of biomechanical factors that control tumor development, including angiogenesis, could explain why few of the promising treatment strategies discovered via in vitro models translate well into in vivo or clinical studies. The ability to manipulate and in real-time study the multiple independent biomechanical properties on cellular activity has been limited, primarily due to limitations in traditional in vitro platforms or the inability to manipulate such factors in vivo. We present a novel microfluidic platform that mimics the vascularized tumor microenvironment with independent control of interstitial flow and mechanical strain. The microtissue platform design isolates mechanically-stimulated angiogenesis in the tumor microenvironment, by manipulating interstitial flow to eliminate soluble factors that could drive blood vessel growth. Our studies demonstrate that enhanced mechanical strain induced by cancer-associated fibroblasts (CAFs) promotes angiogenesis in microvasculature models, even when preventing diffusion of soluble factors to the growing vasculature. Moreover, small but significant decreases in micro-strains induced by inhibited CAFs were sufficient to reduce angiogenesis. Ultimately, we believe this platform represents a significant advancement in the ability to investigate biomechanical signals while controlling for biochemical signals, with a potential to be utilized in fields beyond cancer research.

Guidelines and definitions for research on epithelial-mesenchymal transition

Epithelial–mesenchymal transition (EMT) encompasses dynamic changes in cellular organization from epithelial to mesenchymal phenotypes, which leads to functional changes in cell migration and invasion. EMT occurs in a diverse range of physiological and pathological conditions and is driven by a conserved set of inducing signals, transcriptional regulators and downstream effectors. With over 5,700 publications indexed by Web of Science in 2019 alone, research on EMT is expanding rapidly. This growing interest warrants the need for a consensus among researchers when referring to and undertaking research on EMT. This Consensus Statement, mediated by ‘the EMT International Association’ (TEMTIA), is the outcome of a 2-year-long discussion among EMT researchers and aims to both clarify the nomenclature and provide definitions and guidelines for EMT research in future publications. We trust that these guidelines will help to reduce misunderstanding and misinterpretation of research data generated in various experimental models and to promote cross-disciplinary collaboration to identify and address key open questions in this research field. While recognizing the importance of maintaining diversity in experimental approaches and conceptual frameworks, we emphasize that lasting contributions of EMT research to increasing our understanding of developmental processes and combatting cancer and other diseases depend on the adoption of a unified terminology to describe EMT.

In this Consensus Statement, the authors (on behalf of the EMT International Association) propose guidelines to define epithelial–mesenchymal transition, its phenotypic plasticity and the associated multiple intermediate epithelial–mesenchymal cell states. Clarification of nomenclature and definitions will help reduce misinterpretation of research data generated in different experimental model systems and promote cross-disciplinary collaboration.

DDR2 controls breast tumor stiffness and metastasis by regulating integrin mediated mechanotransduction in CAFs

Biomechanical changes in the tumor microenvironment influence tumor progression and metastases. Collagen content and fiber organization within the tumor stroma are major contributors to biomechanical changes (e., tumor stiffness) and correlated with tumor aggressiveness and outcome. What signals and in what cells control collagen organization within the tumors, and how, is not fully understood. We show in mouse breast tumors that the action of the collagen receptor DDR2 in CAFs controls tumor stiffness by reorganizing collagen fibers specifically at the tumor-stromal boundary. These changes were associated with lung metastases. The action of DDR2 in mouse and human CAFs, and tumors in vivo, was found to influence mechanotransduction by controlling full collagen-binding integrin activation via Rap1-mediated Talin1 and Kindlin2 recruitment. The action of DDR2 in tumor CAFs is thus critical for remodeling collagen fibers at the tumor-stromal boundary to generate a physically permissive tumor microenvironment for tumor cell invasion and metastases.

Randomly Distributed K14+ Breast Tumor Cells Polarize to the Leading Edge and Guide Collective Migration in Response to Chemical and Mechanical Environmental Cues

Collective cell migration is an adaptive, coordinated interactive process involving cell-cell and cell-extracellular matrix (ECM) microenvironmental interactions. A critical aspect of collective migration is the sensing and establishment of directional movement. It has been proposed that a subgroup of cells known as leader cells localize at the front edge of a collectively migrating cluster and are responsible for directing migration. However, it is unknown how and when leader cells arrive at the front edge and what environmental cues dictate leader cell development and behavior. Here, we addressed these questions by combining a microfluidic device design that mimics multiple tumor microenvironmental cues concurrently with biologically relevant primary, heterogeneous tumor cell organoids. Prior to migration, breast tumor leader cells (K14⁺) were present throughout a tumor organoid and migrated (polarized) to the leading edge in response to biochemical and biomechanical cues. Impairment of either CXCR4 (biochemical responsive) or the collagen receptor DDR2 (biomechanical responsive) abrogated polarization of leader cells and directed collective migration. This work demonstrates that K14⁺ leader cells utilize both chemical and mechanical cues from the microenvironment to polarize to the leading edge of collectively migrating tumors. SIGNIFICANCE: These findings demonstrate that pre-existing, randomly distributed leader cells within primary tumor organoids use CXCR4 and DDR2 to polarize to the leading edge and direct migration.

LATS1/2 kinases trigger self-renewal of cancer stem cells in aggressive oral cancer

Cancer stem cells (CSCs), which play important roles in tumor initiation and progression, are resistant to many types of therapies. However, the regulatory mechanisms underlying CSC-specific properties, including self-renewal, are poorly understood. Here, we found that LATS1/2, the core Hippo pathway-kinases, were highly expressed in the oral squamous cell carcinoma line SAS, which exhibits high capacity of CSCs, and that depletion of these kinases prevented SAS cells from forming spheres under serum-free conditions. Detailed examination of the expression and activation of LATS kinases and related proteins over a time course of sphere formation revealed that LATS1/2 were more highly expressed and markedly activated before initiation of self-renewal. Moreover, TAZ, SNAIL, CHK1/2, and Aurora-A were expressed in hierarchical, oscillating patterns during sphere formation, suggesting that the process consists of four sequential steps. Our results indicate that LATS1/2 trigger self-renewal of CSCs by regulating the Hippo pathway, the EMT, and cell division.

Cancer-associated fibroblasts support vascular growth through mechanical force

The role of cancer-associated fibroblasts (CAFs) as regulators of tumor progression, specifically vascular growth, has only recently been described. CAFs are thought to be more mechanically active but how this trait may alter the tumor microenvironment is poorly understood. We hypothesized that enhanced mechanical activity of CAFs, as regulated by the Rho/ROCK pathway, contributes to increased blood vessel growth. Using a 3D in vitro tissue model of vasculogenesis, we observed increased vascularization in the presence of breast cancer CAFs compared to normal breast fibroblasts. Further studies indicated this phenomenon was not simply a result of enhanced soluble signaling factors, including vascular endothelial growth factor (VEGF), and that CAFs generated significantly larger deformations in 3D gels compared to normal fibroblasts. Inhibition of the mechanotransductive pathways abrogated the ability of CAFs to deform the matrix and suppressed vascularization. Finally, utilizing magnetic microbeads to mechanically stimulate mechanically-inhibited CAFs showed partial rescue of vascularization. Our studies demonstrate enhanced mechanical activity of CAFs may play a crucial and previously unappreciated role in the formation of tumor-associated vasculature which could possibly offer potential novel targets in future anti-cancer therapies.

Past matrix stiffness primes epithelial cells and regulates their future collective migration through a mechanical memory

During morphogenesis and cancer metastasis, grouped cells migrate through tissues of dissimilar stiffness. Although the influence of matrix stiffness on cellular mechanosensitivity and motility are well-recognized, it remains unknown whether these matrix-dependent cellular features persist after cells move to a new microenvironment. Here, we interrogate whether priming of epithelial cells by a given matrix stiffness influences their future collective migration on a different matrix - a property we refer to as the 'mechanical memory' of migratory cells. To prime cells on a defined matrix and track their collective migration onto an adjoining secondary matrix of dissimilar stiffness, we develop a modular polyacrylamide substrate through step-by-step polymerization of different PA compositions. We report that epithelial cells primed on a stiff matrix migrate faster, display higher actomyosin expression, form larger focal adhesions, and retain nuclear YAP even after arriving onto a soft secondary matrix, as compared to their control behavior on a homogeneously soft matrix. Priming on a soft ECM causes a reverse effect. The depletion of YAP dramatically reduces this memory-dependent migration. Our results present a previously unidentified regulation of mechanosensitive collective cell migration by past matrix stiffness, in which mechanical memory depends on YAP activity.

Action of SNAIL1 in Cardiac Myofibroblasts Is Important for Cardiac Fibrosis following Hypoxic Injury

Hypoxic injury to the heart results in cardiac fibrosis that leads to cardiac dysfunction and heart failure. SNAIL1 is a zinc finger transcription factor implicated in fibrosis following organ injury and cancer. To determine if the action of SNAIL1 contributed to cardiac fibrosis following hypoxic injury, we used an endogenous SNAIL1 bioluminescence reporter mice, and SNAIL1 knockout mouse models. Here we report that SNAIL1 expression is upregulated in the infarcted heart, especially in the myofibroblasts. Utilizing primary cardiac fibroblasts in ex vivo cultures we find that pro-fibrotic factors and collagen I increase SNAIL1 protein level. SNAIL1 is required in cardiac fibroblasts for the adoption of myofibroblast fate, collagen I expression and expression of fibrosis-related genes. Taken together this data suggests that SNAIL1 expression is induced in the cardiac fibroblasts after hypoxic injury and contributes to myofibroblast phenotype and a fibrotic scar formation. Resultant collagen deposition in the scar can maintain elevated SNAIL1 expression in the myofibroblasts and help propagate fibrosis.

Cell density and actomyosin contractility control the organization of migrating collectives within an epithelium

The mechanisms underlying collective migration are important for understanding development, wound healing, and tumor invasion. Here we focus on cell density to determine its role in collective migration. Our findings show that increasing cell density, as might be seen in cancer, transforms groups from broad collectives to small, narrow streams. Conversely, diminishing cell density, as might occur at a wound front, leads to large, broad collectives with a distinct leader-follower structure. Simulations identify force-sensitive contractility as a mediator of how density affects collectives, and guided by this prediction, we find that the baseline state of contractility can enhance or reduce organization. Finally, we test predictions from these data in an in vivo epithelium by using genetic manipulations to drive collective motion between predicted migratory phases. This work demonstrates how commonly altered cellular properties can prime groups of cells to adopt migration patterns that may be harnessed in health or exploited in disease.

Abstract PR01: Disruption of DDR2-collagen interactions in tumor and stroma with a novel small molecule inhibitor blunts cancer metastasis

Discoidin Domain Receptor 2 (DDR2) is a receptor tyrosine kinase that utilizes the extracellular matrix protein collagen as its ligand. Recently, DDR2 was shown to be critical in facilitating breast cancer metastasis. In tumor cells DDR2 expression, which is absent in normal breast epithelium, is induced during Epithelial Mesenchymal transition (EMT), and serves to stabilize protein levels of the EMT inducing transcription factor, Snail1. DDR2 expression is present in a majority of human invasive ductal breast carcinomas, and expression is predominately localized to the tumor-stroma boundary. In these tumors DDR2 is acting in a positive feedback manner to maintain Snail1 levels and activity at the leading, invasive edge of the tumors, where cells have undergone EMT and come into contact with extracellular matrix (ECM) collagen I. This allows continued invasion through the ECM and contributes to breast cancer metastasis. Interestingly, in genetic models of breast cancer in mice, selectively eliminating DDR2 in either the tumor (MMTV-PyMT; DDR2 fl/fl; K14-Cre) or the fibroblasts within the stroma (MMTV-PyMT; DDR2 fl/fl; FSP-Cre) leads to a dramatic inhibition of tumor metastasis. This indicates that in addition to maintaining EMT in the tumor cells, DDR2 functions within the stromal compartment to facilitate metastasis as well. In culture, the matrix synthesized and remodeled by cancer associated fibroblasts (CAFs) is organized very differently than that of normal fibroblasts. When DDR2 is depleted from CAFS, the matrix more closely resembles that produced by normal fibroblasts, with less organization of matrix proteins. Therefore, it appears that DDR2 engagement by collagen provides a means for both tumor and stromal cells to enhance metastatic potential. As such, DDR2 is a novel RTK target for the treatment of breast cancer metastasis, and we have developed potent and selective small molecule inhibitors of DDR2. These compounds provide a means of blunting breast cancer metastasis by targeting the tumor and stromal cells, thereby disrupting DDR2 signaling in both compartments. The developed inhibitors have nanomolar cellular potency at blocking DDR2 activation. Unlike traditional tyrosine kinase inhibitors (TKIs) that target the intracellular kinase domain, the inhibitors that we have developed act on the extracellular domain of DDR2 and disrupt DDR2 signaling by accelerating the disassembly of the DDR2-collagen complex. Therefore these inhibitors have a unique mode of action that is distinct from the canonical classes of TKIs. These compounds have the ability to attenuate DDR2 signaling both in cell culture, as well as in vivo as demonstrated by using a Snail1-clic beetle green reporter in a syngeneic transplant model. Treating CAFs with the developed inhibitors in culture reverts the organizational phenotype of the resulting matrix to that resembling the matrix produced by normal fibroblasts. Furthermore, using mouse models of late stage breast tumor metastasis, we have shown that these inhibitors reduce metastatic burden in the lungs of mice, to a level comparable to that of genetic knockdown of DDR2. Together these data support further investigation of this novel class of DDR2 inhibitors as anti-metastasis agents, potentially for use in combination with standard of care therapy to halt cancer progression and prevent relapse. Further, it will be important to study the role of these inhibitors in other cancers where DDR2 expression has been shown to promote metastasis, including, but not limited to, ovarian, head and neck, and nonsmall cell lung carcinoma.

This abstract is also presented as Poster B13.

Citation Format: Whitney R. Grither, Gregory D. Longmore. Disruption of DDR2-collagen interactions in tumor and stroma with a novel small molecule inhibitor blunts cancer metastasis. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr PR01.

Disruption of the Cdc42/Par6/aPKC or Dlg/Scrib/Lgl Polarity Complex Promotes Epithelial Proliferation via Overlapping Mechanisms

The establishment and maintenance of apical-basal polarity is a defining characteristic and essential feature of functioning epithelia. Apical-basal polarity (ABP) proteins are also tumor suppressors that are targeted for disruption by oncogenic viruses and are commonly mutated in human carcinomas. Disruption of these ABP proteins is an early event in cancer development that results in increased proliferation and epithelial disorganization through means not fully characterized. Using the proliferating Drosophila melanogaster wing disc epithelium, we demonstrate that disruption of the junctional vs. basal polarity complexes results in increased epithelial proliferation via distinct downstream signaling pathways. Disruption of the basal polarity complex results in JNK-dependent proliferation, while disruption of the junctional complex primarily results in p38-dependent proliferation. Surprisingly, the Rho-Rok-Myosin contractility apparatus appears to play opposite roles in the regulation of the proliferative phenotype based on which polarity complex is disrupted. In contrast, non-autonomous Tumor Necrosis Factor (TNF) signaling appears to suppress the proliferation that results from apical-basal polarity disruption, regardless of which complex is disrupted. Finally we demonstrate that disruption of the junctional polarity complex activates JNK via the Rho-Rok-Myosin contractility apparatus independent of the cortical actin regulator, Moesin.

Mechanical signals regulate and activate SNAIL1 protein to control the fibrogenic response of cancer-associated fibroblasts

Increased deposition of collagen in extracellular matrix (ECM) leads to increased tissue stiffness and occurs in breast tumors. When present, this increases tumor invasion and metastasis. Precisely how this deposition is regulated and maintained in tumors is unclear. Much has been learnt about mechanical signal transduction in cells, but transcriptional responses and the pathophysiological consequences are just becoming appreciated. Here, we show that the SNAIL1 (also known as SNAI1) protein level increases and accumulates in nuclei of breast tumor cells and cancer-associated fibroblasts (CAFs) following exposure to stiff ECM in culture and in vivo SNAIL1 is required for the fibrogenic response of CAFs when exposed to a stiff matrix. ECM stiffness induces ROCK activity, which stabilizes SNAIL1 protein indirectly by increasing intracellular tension, integrin clustering and integrin signaling to ERK2 (also known as MAPK1). Increased ERK2 activity leads to nuclear accumulation of SNAIL1, and, thus, avoidance of cytosolic proteasome degradation. SNAIL1 also influences the level and activity of YAP1 in CAFs exposed to a stiff matrix. This work describes a mechanism whereby increased tumor fibrosis can perpetuate activation of CAFs to sustain tumor fibrosis and promote tumor metastasis through regulation of SNAIL1 protein level and activity.

Ajuba proteins

Schimizzi and Longmore summarise what we know about Ajuba LIM-domain proteins and their various subcellular roles.

Stromal-Initiated Changes in the Bone Promote Metastatic Niche Development

More than 85% of advanced breast cancer patients suffer from metastatic bone lesions, yet the mechanisms that facilitate these metastases remain poorly understood. Recent studies suggest that tumor-derived factors initiate changes within the tumor microenvironment to facilitate metastasis. However, whether stromal-initiated changes are sufficient to drive increased metastasis in the bone remains an open question. Thus, we developed a model to induce reactive senescent osteoblasts and found that they increased breast cancer colonization of the bone. Analysis of senescent osteoblasts revealed that they failed to mineralize bone matrix and increased local osteoclastogenesis, the latter process being driven by the senescence-associated secretory phenotype factor, IL-6. Neutralization of IL-6 was sufficient to limit senescence-induced osteoclastogenesis and tumor cell localization to bone, thereby reducing tumor burden. Together, these data suggest that a reactive stromal compartment can condition the niche, in the absence of tumor-derived signals, to facilitate metastatic tumor growth in the bone.

Three-dimensional matrix fiber alignment modulates cell migration and MT1-MMP utility by spatially and temporally directing protrusions

Multiple attributes of the three-dimensional (3D) extracellular matrix (ECM) have been independently implicated as regulators of cell motility, including pore size, crosslink density, structural organization, and stiffness. However, these parameters cannot be independently varied within a complex 3D ECM protein network. We present an integrated, quantitative study of these parameters across a broad range of complex matrix configurations using self-assembling 3D collagen and show how each parameter relates to the others and to cell motility. Increasing collagen density resulted in a decrease and then an increase in both pore size and fiber alignment, which both correlated significantly with cell motility but not bulk matrix stiffness within the range tested. However, using the crosslinking enzyme Transglutaminase II to alter microstructure independently of density revealed that motility is most significantly predicted by fiber alignment. Cellular protrusion rate, protrusion orientation, speed of migration, and invasion distance showed coupled biphasic responses to increasing collagen density not predicted by 2D models or by stiffness, but instead by fiber alignment. The requirement of matrix metalloproteinase (MMP) activity was also observed to depend on microstructure, and a threshold of MMP utility was identified. Our results suggest that fiber topography guides protrusions and thereby MMP activity and motility.

Dachsous1b cadherin regulates actin and microtubule cytoskeleton during early zebrafish embryogenesis

Dachsous (Dchs), an atypical cadherin, is an evolutionarily conserved regulator of planar cell polarity, tissue size and cell adhesion. In humans, DCHS1 mutations cause pleiotropic Van Maldergem syndrome. Here, we report that mutations in zebrafish dchs1b and dchs2 disrupt several aspects of embryogenesis, including gastrulation. Unexpectedly, maternal zygotic (MZ) dchs1b mutants show defects in the earliest developmental stage, egg activation, including abnormal cortical granule exocytosis (CGE), cytoplasmic segregation, cleavages and maternal mRNA translocation, in transcriptionally quiescent embryos. Later, MZdchs1b mutants exhibit altered dorsal organizer and mesendodermal gene expression, due to impaired dorsal determinant transport and Nodal signaling. Mechanistically, MZdchs1b phenotypes can be explained in part by defective actin or microtubule networks, which appear bundled in mutants. Accordingly, disruption of actin cytoskeleton in wild-type embryos phenocopied MZdchs1b mutant defects in cytoplasmic segregation and CGE, whereas interfering with microtubules in wild-type embryos impaired dorsal organizer and mesodermal gene expression without perceptible earlier phenotypes. Moreover, the bundled microtubule phenotype was partially rescued by expressing either full-length Dchs1b or its intracellular domain, suggesting that Dchs1b affects microtubules and some developmental processes independent of its known ligand Fat. Our results indicate novel roles for vertebrate Dchs in actin and microtubule cytoskeleton regulation in the unanticipated context of the single-celled embryo.

Transient SNAIL1 expression is necessary for metastatic competence in breast cancer

SNAIL1 has been suggested to regulate breast cancer metastasis based on analyses of human breast tumor transcriptomes and experiments using cancer cell lines and xenografts. However, in vivo genetic experimental support for a role for SNAIL1 in breast cancer metastasis that develops in an immunocompetent tumor microenvironment has not been determined. To address this question, we created a genetic SNAIL1 model by coupling an endogenous SNAIL1 reporter with an inducible SNAIL1 transgene. Using multiple genetic models of breast cancer, we demonstrated that endogenous SNAIL1 expression was restricted to primary tumors that ultimately disseminate. SNAIL1 gene deletion either during the premalignant phase or after primary tumors have reached a palpable size blunted metastasis, indicating that late metastasis was the main driver of metastasis and that this was dependent on SNAIL1. Importantly, SNAIL1 expression during breast cancer metastasis was transient and forced transient, but not continuous. SNAIL1 expression in breast tumors was sufficient to increase metastasis.

Rho1 regulates adherens junction remodeling by promoting recycling endosome formation through activation of myosin II

Once adherens junctions (AJs) are formed between polarized epithelial cells they must be maintained because AJs are constantly remodeled in dynamic epithelia. AJ maintenance involves endocytosis and subsequent recycling of E-cadherin to a precise location along the basolateral membrane. In the Drosophila pupal eye epithelium, Rho1 GTPase regulates AJ remodeling through Drosophila E-cadherin (DE-cadherin) endocytosis by limiting Cdc42/Par6/aPKC complex activity. We demonstrate that Rho1 also influences AJ remodeling by regulating the formation of DE-cadherin-containing, Rab11-positive recycling endosomes in Drosophila postmitotic pupal eye epithelia. This effect of Rho1 is mediated through Rok-dependent, but not MLCK-dependent, stimulation of myosin II activity yet independent of its effects upon actin remodeling. Both Rho1 and pMLC localize on endosomal vesicles, suggesting that Rho1 might regulate the formation of recycling endosomes through localized myosin II activation. This work identifies spatially distinct functions for Rho1 in the regulation of DE-cadherin-containing vesicular trafficking during AJ remodeling in live epithelia.

β-Catenin serves as a clutch between low and high intercellular E-cadherin bond strengths

A wide range of invasive pathological outcomes originate from the loss of epithelial phenotype and involve either loss of function or downregulation of transmembrane adhesive receptor complexes, including Ecadherin (Ecad) and binding partners β-catenin and α-catenin at adherens junctions. Cellular pathways regulating wild-type β-catenin level, or direct mutations in β-catenin that affect the turnover of the protein have been shown to contribute to cancer development, through induction of uncontrolled proliferation of transformed tumor cells, particularly in colon cancer. Using single-molecule force spectroscopy, we show that depletion of β-catenin or the prominent cancer-related S45 deletion mutation in β-catenin present in human colon cancers both weaken tumor intercellular Ecad/Ecad bond strength and diminishes the capacity of specific extracellular matrix proteins-including collagen I, collagen IV, and laminin V-to modulate intercellular Ecad/Ecad bond strength through α-catenin and the kinase activity of glycogen synthase kinase 3 (GSK-3β). Thus, in addition to regulating tumor cell proliferation, cancer-related mutations in β-catenin can influence tumor progression by weakening the adhesion of tumor cells to one another through reduced individual Ecad/Ecad bond strength and cellular adhesion to specific components of the extracellular matrix and the basement membrane.

Nuclear ubiquitination by FBXL5 modulates Snail1 DNA binding and stability

The zinc finger transcription factor Snail1 regulates epithelial to mesenchymal transition, repressing epithelial markers and activating mesenchymal genes. Snail1 is an extremely labile protein degraded by the cytoplasmic ubiquitin-ligases β-TrCP1/FBXW1 and Ppa/FBXL14. Using a short hairpin RNA screening, we have identified FBXL5 as a novel Snail1 ubiquitin ligase. FBXL5 is located in the nucleus where it interacts with Snail1 promoting its polyubiquitination and affecting Snail1 protein stability and function by impairing DNA binding. Snail1 downregulation by FBXL5 is prevented by Lats2, a protein kinase that phosphorylates Snail1 precluding its nuclear export but not its polyubiquitination. Actually, although polyubiquitination by FBXL5 takes place in the nucleus, Snail1 is degraded in the cytosol. Finally, FBXL5 is highly sensitive to stress conditions and is downregulated by iron depletion and γ-irradiation, explaining Snail1 stabilization in these conditions. These results characterize a novel nuclear ubiquitin ligase controlling Snail1 protein stability and provide the molecular basis for understanding how radiotherapy upregulates the epithelial to mesenchymal transition-inducer Snail1.

The Arp2/3 complex mediates multigeneration dendritic protrusions for efficient 3-dimensional cancer cell migration

Arp2/3 is a protein complex that nucleates actin filament assembly in the lamellipodium in adherent cells crawling on planar 2-dimensional (2D) substrates. However, in physiopathological situations, cell migration typically occurs within a 3-dimensional (3D) environment, and little is known about the role of Arp2/3 and associated proteins in 3D cell migration. Using time resolved live-cell imaging and HT1080, a fibrosarcoma cell line commonly used to study cell migration, we find that the Arp2/3 complex and associated proteins N-WASP, WAVE1, cortactin, and Cdc42 regulate 3D cell migration. We report that this regulation is caused by formation of multigeneration dendritic protrusions, which mediate traction forces on the surrounding matrix and effective cell migration. The primary protrusions emanating directly from the cell body and prolonging the nucleus forms independent of Arp2/3 and dependent on focal adhesion proteins FAK, talin, and p130Cas. The Arp2/3 complex, N-WASP, WAVE1, cortactin, and Cdc42 regulate the secondary protrusions branching off from the primary protrusions. In 3D matrices, fibrosarcoma cells as well as migrating breast, pancreatic, and prostate cancer cells do not display lamellipodial structures. This study characterizes the unique topology of protrusions made by cells in a 3D matrix and show that these dendritic protrusions play a critical role in 3D cell motility and matrix deformation. The relative contribution of these proteins to 3D migration is significantly different from their role in 2D migration.

The collagen receptor discoidin domain receptor 2 stabilizes SNAIL1 to facilitate breast cancer metastasis

Increased stromal collagen deposition in human breast tumours correlates with metastases. We show that activation of the collagen I receptor DDR2 (discoidin domain receptor 2) regulates SNAIL1 stability by stimulating ERK2 activity, in a Src-dependent manner. Activated ERK2 directly phosphorylates SNAIL1, leading to SNAIL1 nuclear accumulation, reduced ubiquitylation and increased protein half-life. DDR2-mediated stabilization of SNAIL1 promotes breast cancer cell invasion and migration in vitro, and metastasis in vivo. DDR2 expression was observed in most human invasive ductal breast carcinomas studied, and was associated with nuclear SNAIL1 and absence of E-cadherin expression. We propose that DDR2 maintains SNAIL1 level and activity in tumour cells that have undergone epithelial-mesenchymal transition (EMT), thereby facilitating continued tumour cell invasion through collagen-I-rich extracellular matrices by sustaining the EMT phenotype. As such, DDR2 could be an RTK (receptor tyrosine kinase) target for the treatment of breast cancer metastasis.

Abstract 3939: The collagen receptor discoidin domain receptor 2 facilitates breast cancer metastasis by stabilizing Snail1 protein

Mammographic density is a leading risk factor for the development of breast cancer and these cancers are more invasive and metastatic. The increase in breast density is, in part, due to increased fibrillar collagen deposition. In an RNAi human kinome screen to identify post-transcriptional regulators of Snail1 protein stability we found the collagen receptor DDR2, but not related DDR1, to stabilize Snail1 protein as a result of collagen I activated DDR2 stimulation of ERK2 activity. This can occur independent of β1-integrin and TGFβ signals. DDR2 is not required for TGFβ-induced Epithelial Mesenchymal transition (EMT) or EMT-induced Snail1 transcription. Rather DDR2 expression, which is absent in normal breast epithelia, is induced during EMT as breast tumor cells assume invasive, mesenchymal features. DDR2-induced ERK2 activity stabilizes Snail1 protein level in two ways. First, ERK2 interacts with and phosphorylates Snail1 and second ERK2 inhibits GSK3b.

DDR2 was found to be critical for breast cancer cell invasion and migration in vitro, in 3D collagen gels, and breast cancer metastasis in vivo, in syngeneic xenograft models. A majority of human invasive ductal breast carcinomas express DDR2 protein and there is a strong association between nuclear Snail1 expression, DDR2 expression, and the absence of E-cadherin expression. The small group of breast cancer patients with tumors harboring increased DDR2 gene copy number has diminished survival. We propose that DDR2 acts in a positive feedback loop to maintain Snail1 protein level and activity in breast tumor cells at the leading, invasive edge where tumor cells that have undergone EMT and invaded through the basement membrane to encounter extracellular matrix collagen I. This facilitates the continued tumor cell invasion and migration through the collagen I-rich ECM, and thereby, contributes to breast cancer metastasis.

That the collagen I receptor DDR2 maintains the EMT-inducing Snail1 protein level and activity and contributes to breast cancer metastasis could explain the increase in metastasis of breast cancers in women with dense breasts. As such, DDR2 could be considered a novel RTK target for the treatment of breast cancer metastasis.

Citation Format: Kun Zhang, Callie Corsa, Julie Prior, David Piwnica-Worms, Gregory D. Longmore. The collagen receptor discoidin domain receptor 2 facilitates breast cancer metastasis by stabilizing Snail1 protein. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3939. doi:10.1158/1538-7445.AM2013-3939

α-actinin1 and 4 tyrosine phosphorylation is critical for stress fiber establishment, maintenance and focal adhesion maturation

In polarized, migrating cells, stress fibers are a highly dynamic network of contractile acto-myosin structures composed of bundles of actin filaments held together by actin cross-linking proteins such as α-actinins. As such, α-actinins influence actin cytoskeleton organization and dynamics and focal adhesion maturation. In response to environmental signals, α-actinins are tyrosine phosphorylated and this affects their binding to actin stress fibers; however, the cellular role of α-actinin tyrosine phosphorylation remains largely unknown. We found that non-muscle α-actinin1/4 are critical for the establishment of dorsal stress fibers and maintenance of transverse arc stress fibers. Analysis of cells genetically depleted of α-actinin1 and 4 reveals two distinct modes for focal adhesion maturation. An α-actinin1 or 4 dependent mode that uses dorsal stress fiber precursors as a template for establishing focal adhesions and their maturation, and an α-actinin-independent manner that uses transverse arc precursors to establish focal adhesions at both ends. Focal adhesions formed in the absence of α-actinins are delayed in their maturation, exhibit altered morphology, have decreased amounts of Zyxin and VASP, and reduced adhesiveness to extracellular matrix. Further rescue experiments demonstrate that the tyrosine phosphorylation of α-actinin1 at Y12 and α-actinin4 at Y265 is critical for dorsal stress fiber establishment, transverse arc maintenance and focal adhesion maturation.

Actin cap associated focal adhesions and their distinct role in cellular mechanosensing

The ability for cells to sense and adapt to different physical microenvironments plays a critical role in development, immune responses, and cancer metastasis. Here we identify a small subset of focal adhesions that terminate fibers in the actin cap, a highly ordered filamentous actin structure that is anchored to the top of the nucleus by the LINC complexes; these differ from conventional focal adhesions in morphology, subcellular organization, movements, turnover dynamics, and response to biochemical stimuli. Actin cap associated focal adhesions (ACAFAs) dominate cell mechanosensing over a wide range of matrix stiffness, an ACAFA-specific function regulated by actomyosin contractility in the actin cap, while conventional focal adhesions are restrictively involved in mechanosensing for extremely soft substrates. These results establish the perinuclear actin cap and associated ACAFAs as major mediators of cellular mechanosensing and a critical element of the physical pathway that transduce mechanical cues all the way to the nucleus.

Mismatch in mechanical and adhesive properties induces pulsating cancer cell migration in epithelial monolayer

The mechanical and adhesive properties of cancer cells significantly change during tumor progression. Here we assess the functional consequences of mismatched stiffness and adhesive properties between neighboring normal cells on cancer cell migration in an epithelial-like cell monolayer. Using an in vitro coculture system and live-cell imaging, we find that the speed of single, mechanically soft breast carcinoma cells is dramatically enhanced by surrounding stiff nontransformed cells compared with single cells or a monolayer of carcinoma cells. Soft tumor cells undergo a mode of pulsating migration that is distinct from conventional mesenchymal and amoeboid migration, whereby long-lived episodes of slow, random migration are interlaced with short-lived episodes of extremely fast, directed migration, whereas the surrounding stiff cells show little net migration. This bursty migration is induced by the intermittent, myosin II-mediated deformation of the soft nucleus of the cancer cell, which is induced by the transient crowding of the stiff nuclei of the surrounding nontransformed cells, whose movements depend directly on the cadherin-mediated mismatched adhesion between normal and cancer cells as well as α-catenin-based intercellular adhesion of the normal cells. These results suggest that a mechanical and adhesive mismatch between transformed and nontransformed cells in a cell monolayer can trigger enhanced pulsating migration. These results shed light on the role of stiff epithelial cells that neighbor individual cancer cells in early steps of cancer dissemination.

The LIMD1 protein bridges an association between the prolyl hydroxylases and VHL to repress HIF-1 activity

There are three prolyl hydroxylases (PHD1, 2 and 3) that regulate the hypoxia-inducible factors (HIFs), the master transcriptional regulators that respond to changes in intracellular O(2) tension. In high O(2) tension (normoxia) the PHDs hydroxylate two conserved proline residues on HIF-1α, which leads to binding of the von Hippel-Lindau (VHL) tumour suppressor, the recognition component of a ubiquitin-ligase complex, initiating HIF-1α ubiquitylation and degradation. However, it is not known whether PHDs and VHL act separately to exert their enzymatic activities on HIF-1α or as a multiprotein complex. Here we show that the tumour suppressor protein LIMD1 (LIM domain-containing protein) acts as a molecular scaffold, simultaneously binding the PHDs and VHL, thereby assembling a PHD-LIMD1-VHL protein complex and creating an enzymatic niche that enables efficient degradation of HIF-1α. Depletion of endogenous LIMD1 increases HIF-1α levels and transcriptional activity in both normoxia and hypoxia. Conversely, LIMD1 expression downregulates HIF-1 transcriptional activity in a manner depending on PHD and 26S proteasome activities. LIMD1 family member proteins Ajuba and WTIP also bind to VHL and PHDs 1 and 3, indicating that these LIM domain-containing proteins represent a previously unrecognized group of hypoxic regulators.

Temporal and spatial cooperation of Snail1 and Twist1 during epithelial-mesenchymal transition predicts for human breast cancer recurrence

Epithelial-mesenchymal transition (EMT) is a normal developmental program that is considered to also play an important role in cancer metastasis. Ultimate inducers of EMT are transcriptional repressors that individually can induce experimental EMT, yet in many cells, particularly cancer cells, multiple inducers are expressed simultaneously. Why, and if, and how they interact to regulate EMT is unanswered. Using RNA interference technology to affect protein knockdown and avoid potential overexpression artifact coupled with transient TGFβ treatment to better mimic in vivo conditions we show, in both nontumorigenic and tumorigenic epithelial cancer cells, that Snail1 is uniquely required for EMT initiation, whereas Twist1 is required to maintain late EMT. Twist1, present in resting epithelial cells, is dispensable for EMT initiation. Mechanistically, in response to transient TGFβ treatment, transient Snail1 expression represses Twist1 transcription directly, which is subsequently upregulated, as Snail1 levels decrease, to sustain E-cadherin downregulation and growth arrest of EMT. Persistent Twist1 expression is associated with a p38 and extracellular signal-regulated kinase signal feedback loop that sustains growth-inhibitory signals characteristic of quiescent micrometastatic tumors. This Snail1-Twist1 temporal and spatial cooperation was also observed in vivo during human breast cancer progression to metastasis. Twist1 level, but not Snail1 level, and Twist1:Snail1 ratio in disseminated micrometastatic bone marrow tumor cells was found to correlate with survival and treatment resistance and is highly predictive of metastatic or recurrent disease.

Lats2 kinase potentiates Snail1 activity by promoting nuclear retention upon phosphorylation

Snail1 is a central regulator of epithelial cell adhesion and movement in epithelial-to-mesenchymal transitions (EMTs) during embryo development; a process reactivated during cancer metastasis. While induction of Snail1 transcription precedes EMT induction, post-translational regulation of Snail1 is also critical for determining Snail1's protein level, subcellular localization, and capacity to induce EMT. To identify novel post-translational regulators of Snail1, we developed a live cell, bioluminescence-based screen. From a human kinome RNAi screen, we have identified Lats2 kinase as a novel regulator of Snail1 protein level, subcellular localization, and thus, activity. We show that Lats2 interacts with Snail1 and directly phosphorylates Snail1 at residue T203. This occurs in the nucleus and serves to retain Snail1 in the nucleus thereby enhancing its stability. Lats2 was found to positively influence cellular EMT and tumour cell invasion, in a Snail1-dependent manner. Indeed during TGFβ-induced EMT Lats2 is activated and Snail1 phosphorylated at T203. Analysis in mouse and zebrafish embryo development confirms that Lats2 acts as a positive modulator of Snail1 protein level and potentiates its in vivo EMT activity.

A distinctive role for focal adhesion proteins in three-dimensional cell motility

Focal adhesions are large multi-protein assemblies that form at the basal surface of cells on planar dishes, and that mediate cell signalling, force transduction and adhesion to the substratum. Although much is known about focal adhesion components in two-dimensional (2D) systems, their role in migrating cells in a more physiological three-dimensional (3D) matrix is largely unknown. Live-cell microscopy shows that for cells fully embedded in a 3D matrix, focal adhesion proteins, including vinculin, paxillin, talin, alpha-actinin, zyxin, VASP, FAK and p130Cas, do not form aggregates but are diffusely distributed throughout the cytoplasm. Despite the absence of detectable focal adhesions, focal adhesion proteins still modulate cell motility, but in a manner distinct from cells on planar substrates. Rather, focal adhesion proteins in matrix-embedded cells regulate cell speed and persistence by affecting protrusion activity and matrix deformation, two processes that have no direct role in controlling 2D cell speed. This study shows that membrane protrusions constitute a critical motility/matrix-traction module that drives cell motility in a 3D matrix.

The Cdc42/Par6/aPKC polarity complex regulates apoptosis-induced compensatory proliferation in epithelia

BACKGROUND

In response to stress- or tissue-damage-induced apoptosis, unaffected epithelial cells undergo compensatory proliferation to maintain the integrity of the epithelium. Proximal signals regulating this response are not fully understood, but c-Jun N-terminal kinase (JNK) activity appears to be critical for both apoptosis and compensatory proliferation. Disruption of epithelial cell apical-basal polarity occurs in early cancer development and is often correlated with increased proliferation by means not fully characterized. We considered whether disruption of the various polarity complexes could provide signals identifying damaged epithelial cells and thus lead to apoptosis-induced compensatory proliferation.

RESULTS

We identify the Cdc42/Par6/atypical protein kinase C (aPKC) Par polarity complex as uniquely and specifically regulating apoptosis-induced compensatory proliferation in Drosophila epithelia. Genetic depletion of individual components or disruption of formation and localization of this complex, but not other polarity complexes, induces JNK-dependent apoptosis and JNK-dependent compensatory proliferation following radiation injury. When apoptosis execution is blocked, by p35 expression, Cdc42/Par6/aPKC-depleted tissues uniquely hyperproliferate, leading to tissue and organ overgrowth. Disruption of Cdc42/Par6/aPKC leads to activation of JNK through increased Rho1 and Rok activity and Rok's capacity to activate myosin but not F-actin.

CONCLUSIONS

We show that the Cdc42/Par6/aPKC polarity complex influences both a physiologic compensatory proliferation response after irradiation injury and a contrived compensatory non-cell-autonomous hyperproliferation response when cell-autonomous apoptosis, resulting from Cdc42/Par6/aPKC disruption, is inhibited. These results suggest the possibility that in cancer where apoptotic regulation is disrupted, loss of Cdc42/Par6/aPKC polarity complex organization or localization could contribute to tumor hyperproliferation and explain how polarity disruption contributes to tumor development.

Ajuba LIM proteins are negative regulators of the Hippo signaling pathway

The mammalian Ajuba LIM proteins (Ajuba, LIMD1, and WTIP) are adaptor proteins that exhibit the potential to communicate cell adhesive events with nuclear responses to remodel epithelia. Determining their role in vivo, however, has been challenging due to overlapping tissue expression and functional redundancy. Thus, we turned to Drosophila, where a single gene, CG11063 or djub, exists. Drosophila lacking the djub gene or depleted of dJub by RNA interference identify djub as an essential gene for development and a novel regulator of epithelial organ size as a component of the conserved Hippo (Hpo) pathway, which has been implicated in both tissue size control and cancer development. djub-deficient tissues were small and had decreased cell numbers as a result of increased apoptosis and decreased proliferation, due to downregulation of DIAP1 and cyclin E. This phenocopies tissues deficient for Yorkie (Yki), the downstream target of the Hippo pathway. djub genetically interacts with the Hippo pathway, and epistasis suggests that djub lies downstream of hpo. In mammalian and Drosophila cells, Ajuba LIM proteins/dJub interact with LATS/Warts (Wts) and WW45/Sav to inhibit phosphorylation of YAP/Yki. This work describes a novel role for the Ajuba LIM proteins as negative regulators of the Hippo signaling pathway.