Prolonged hypoxia alleviates prolyl hydroxylation-mediated suppression of RIPK1 to promote necroptosis and inflammation

The prolyl hydroxylation of hypoxia-inducible factor 1α (HIF-1α) mediated by the EGLN-pVHL pathway represents a classic signalling mechanism that mediates cellular adaptation under hypoxia. Here we identify RIPK1, a known regulator of cell death mediated by tumour necrosis factor receptor 1 (TNFR1), as a target of EGLN1-pVHL. Prolyl hydroxylation of RIPK1 mediated by EGLN1 promotes the binding of RIPK1 with pVHL to suppress its activation under normoxic conditions. Prolonged hypoxia promotes the activation of RIPK1 kinase by modulating its proline hydroxylation, independent of the TNFα-TNFR1 pathway. As such, inhibiting proline hydroxylation of RIPK1 promotes RIPK1 activation to trigger cell death and inflammation. Hepatocyte-specific Vhl deficiency promoted RIPK1-dependent apoptosis to mediate liver pathology. Our findings illustrate a key role of the EGLN-pVHL pathway in suppressing RIPK1 activation under normoxic conditions to promote cell survival and a model by which hypoxia promotes RIPK1 activation through modulating its proline hydroxylation to mediate cell death and inflammation in human diseases, independent of TNFR1.

Mesenchymal stem/stromal cells in breast cancer development and management

Mesenchymal stem/stromal cells (MSCs) encompass a heterogeneous population of fibroblastic progenitor cells that reside in multiple tissues around the body. They are endowed with capacities to differentiate into multiple connective tissue lineages, including chondrocytes, adipocytes, and osteoblasts, and are thought to function as trophic cells recruited to sites of injury and inflammation where they contribute to tissue regeneration. In keeping with these roles, MSCs also to home to sites of breast tumorigenesis, akin to their migration to wounds, and participate in tumor stroma formation. Mounting evidence over the past two decades has described the critical regulatory roles for tumor-associated MSCs in various aspects of breast tumor pathogenesis, be it tumor initiation, growth, angiogenesis, tumor microenvironment formation, immune evasion, cancer cell migration, invasion, survival, therapeutic resistance, dissemination, and metastatic colonization. In this review, we present a brief summary of the role of MSCs in breast tumor development and progression, highlight some of the molecular frameworks underlying their pro-malignant contributions, and present evidence of their promising utility in breast cancer therapy.

In vivo gain-of-function cDNA library screening for colonization genes in a mouse model of pulmonary metastasis

We provide a protocol for gain-of-function (GOF) cDNA screen of genes that foster cancer cell colonization of secondary tissues, the last and most lethal step of the metastasis cascade. We present techniques for cDNA viral library preparation and delivery leading up to the recovery of colonization-promoting sequences in a proof-of-concept DU145-based mouse model of pulmonary metastasis. Adapted to other cDNA libraries and cancer models, this approach would prove widely useful in enumerating intrinsic genetic determinants underlying metastatic colonization.

For complete details on the use and execution of this protocol, please refer to Tu et al. (2021).

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Colonization is the last and most lethal bottleneck step of the metastasis cascade

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Gain-of-function (GOF) cDNA screen was adapted to probe cancer colonization of lung

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DU145 cells harboring GOF prostate cDNA library generated pulmonary macro-metastases

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Genomic PCR of macro-colonies exposed inserted cDNAs of potential colonization genes

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Endocrine regulation of cancer stem cell compartments in breast tumors

Cancer cells within breast tumors exist within a hierarchy in which only a small and rare subset of cells is able to regenerate growths with the heterogeneity of the original tumor. These highly malignant cancer cells, which behave like stem cells for new cancers and are called "cancer stem cells" or CSCs, have also been shown to possess increased resistance to therapeutics, and represent the root cause underlying therapy failures, persistence of residual disease, and relapse. As >90% of cancer deaths are due to refractory tumors, identification of critical molecular drivers of the CSC-state would reveal vulnerabilities that can be leveraged in designing therapeutics that eradicate advanced disease and improve patient survival outcomes. An expanding and complex body of work has now described the exquisite susceptibility of CSC pools to the regulatory influences of local and systemic hormones. Indeed, breast CSCs express a plethora of hormonal receptors, which funnel hormonal influences over every aspect of breast neoplasia - be it tumor onset, growth, survival, invasion, metastasis, or therapy resistance - via directly impacting CSC behavior. This article is intended to shed light on this active area of investigation by attempting to provide a systematic and comprehensive overview of the available evidence directly linking hormones to breast CSC biology.

The LINC01119-SOCS5 axis as a critical theranostic in triple-negative breast cancer

The development of triple-negative breast cancer (TNBC) is critically regulated by certain tumor-microenvironment-associated cells called mesenchymal stem/stromal cells (MSCs), which we and others have shown promote TNBC progression by activating pro-malignant signaling in neighboring cancer cells. Characterization of these cascades would better our understanding of TNBC biology and bring about therapeutics that eliminate the morbidity and mortality associated with advanced disease. Here, we focused on the emerging class of RNAs called long non-coding RNAs or lncRNAs and utilized a MSC-supported TNBC progression model to identify specific family members of functional relevance to TNBC pathogenesis. Indeed, although some have been described to play functional roles in TNBC, activities of lncRNAs as mediators of tumor-microenvironment-driven TNBC development remain to be fully explored. We report that MSCs stimulate robust expression of LINC01119 in TNBC cells, which in turn induces suppressor of cytokine signaling 5 (SOCS5), leading to accelerated cancer cell growth and tumorigenesis. We show that LINC01119 and SOCS5 exhibit tight correlation across multiple breast cancer gene sets and that they are highly enriched in TNBC patient cohorts. Importantly, we present evidence that the LINC01119-SOCS5 axis represents a powerful prognostic indicator of adverse outcomes in TNBC patients, and demonstrate that its repression severely impairs cancer cell growth. Altogether, our findings identify LINC01119 as a major driver of TNBC development and delineate critical non-coding RNA theranostics of potential translational utility in the management of advanced TNBC, a class of tumors in most need of effective and targeted therapy.

In vivo library screening identifies the metabolic enzyme aldolase A as a promoter of metastatic lung colonization

Elucidations of the factors that promote the growth of disseminated tumor cells (DTCs) into life-threatening lesions stand to provide much needed prognostic and therapeutic targets of translational utility for patients with metastatic cancer. To identify such regulators, we conducted gain-of-function cDNA library screening to discover genes that foster prostate cancer cell colonization of mouse lungs as an experimental model. Our efforts identified the metabolic enzyme aldolase A (ALDOA) as a driver of cancer cell motility, anchorage-independent growth, and metastatic colonization, and as a prognosticator of adverse patient outcome across many malignancies, including prostate, breast, pancreatic, and liver cancers. Metabolomics coupled with biochemical and functional analyses revealed that ALDOA triggered the activation of adenosine-5'-monophosphate (AMP)-activated protein kinase (AMPK), which we demonstrate played essential promalignant activities in ALDOA-expressing cells. Collectively, these findings unveiled vivo approaches to identify metastatic colonization regulators and uncovered previously undescribed roles for ALDOA-AMPK pathway in tumor progression.

Microenvironmental Regulation of Long Noncoding RNA LINC01133 Promotes Cancer Stem Cell-Like Phenotypic Traits in Triple-Negative Breast Cancers

The fibrotic tumor microenvironment is a critical player in the pathogenesis of triple-negative breast cancers (TNBCs), with the presence of fibroblastic infiltrates particularly correlating with tumors that are clinically advanced. On this front, we previously demonstrated that TNBCs are highly enriched in fibroblastic stromal progenitor cells called mesenchymal stem/stromal cells (MSCs) and that such cells play critical roles in promoting TNBC initiation and progression. How TNBC cells respond to MSC stimulation, however, is not fully understood, and stands to reveal contextual signals used by TNBC cells during tumor development and provide biomarkers and therapeutic targets of pertinence to TNBC management. Here, we report that MSCs strongly induced the long noncoding RNA (lncRNA) LINC01133 in neighboring TNBC cells. Indeed, although lncRNAs have been tightly associated with cancer development, their contributions to breast cancer in general, and to TNBC pathogenesis in particular, have not been fully elucidated, and we set out to determine if LINC01133 regulated malignant traits in TNBC cells. We establish that LINC01133 is sufficient, on its own, in promoting phenotypic and growth characteristics of cancer stem cell-like cells, and that it is a direct mediator of the MSC-triggered miR-199a-FOXP2 pathway in TNBC models. Furthermore, we show that LINC01133 is a critical regulator of the pluripotency-determining gene Kruppel-Like Factor 4 (KLF4), and that it represents a biomarker and prognosticator of disease outcome in the clinic. Collectively, our findings introduce LINC01133 as a novel functional driver of malignancy and a potential theranostic in TNBC. Stem Cells 2019;37:1281-1292.

Targeting Cancer Stem Cells-A Renewed Therapeutic Paradigm

Metastasis is often accompanied by radio- and chemotherapeutic resistance to anticancer treatments and is the major cause of death in cancer patients. Better understanding of how cancer cells circumvent therapeutic insults and how disseminated cancer clones generate life-threatening metastases would therefore be paramount to the development of effective therapeutic approaches for clinical management of malignant disease. Mounting reports over the past two decades have provided evidence for the existence of a minor population of highly malignant cells within liquid and solid tumors, which are capable of self-renewing and of regenerating secondary growths with the heterogeneity of the primary tumors from which they derive. These cells, called tumor-initiating cells or cancer stem cells (CSCs) exhibit increased resistance to standard radio- and chemotherapies and appear to have mechanisms that enable them to evade immune surveillance. CSCs are therefore considered to be responsible for systemic residual disease after cancer therapy, as well as for disease relapse. How CSCs develop, the nature of the interactions they establish with their microenvironment, their phenotypic and functional characteristics, as well as their molecular dependencies have all taken center stage in cancer therapy. Indeed, improved understanding of CSC biology is critical to the development of important CSC-based anti-neoplastic approaches that have the potential to radically improve cancer management. Here, we summarize some of the most pertinent elements regarding CSC development and properties, and highlight some of the clinical modalities in current development as anti-CSC therapeutics.

Science Signaling Podcast for 21 February 2017: Pentraxin-3 in basal-like breast cancer

This Podcast features an interview with Antoine Karnoub, senior author of a Research Article that appears in the 21 February 2017 issue of Science Signaling, about how pentraxin-3 (PTX3) links increased phosphoinositide 3-kinase (PI3K) signaling to stem cell properties in basal-like breast cancer (BLBC). BLBC is an aggressive type of breast cancer in which the tumor cells exhibit increased PI3K signaling and have stem cell-like properties. Thomas et al found that aberrant PI3K signaling in BLBCs stimulated the expression of PTX3, which encodes a protein that functions in innate immunity. PTX3 promoted stem cell-like traits in mammary epithelial cells and stimulated the growth of breast cancer cells. Conversely, decreasing the abundance of PTX3 in breast cancer cells reduced both the growth of these cells and their expression of stem cell markers. The abundance of PTX3 transcripts in breast tumors negatively correlated with patient survival, suggesting that PTX3 may be a useful biomarker for stratifying BLBC patients and that targeting PTX3 may suppress tumor growth in some BLBC patients.Listen to Podcast.

Pentraxin-3 is a PI3K signaling target that promotes stem cell-like traits in basal-like breast cancers

Basal-like breast cancers (BLBCs) exhibit hyperactivation of the phosphoinositide 3-kinase (PI3K) signaling pathway because of the frequent mutational activation of the PIK3CA catalytic subunit and the genetic loss of its negative regulators PTEN (phosphatase and tensin homolog) and INPP4B (inositol polyphosphate-4-phosphatase type II). However, PI3K inhibitors have had limited clinical efficacy in BLBC management because of compensatory amplification of PI3K downstream signaling loops. Therefore, identification of critical PI3K mediators is paramount to the development of effective BLBC therapeutics. Using transcriptomic analysis of activated PIK3CA-expressing BLBC cells, we identified the gene encoding the humoral pattern recognition molecule pentraxin-3 (PTX3) as a critical target of oncogenic PI3K signaling. We found that PTX3 abundance is stimulated, in part, through AKT- and nuclear factor κB (NF-κB)-dependent pathways and that presence of PTX3 is necessary for PI3K-induced stem cell-like traits. We further showed that PTX3 expression is greater in tumor samples from patients with BLBC and that it is prognostic of poor patient survival. Our results thus reveal PTX3 as a newly identified PI3K-regulated biomarker and a potential therapeutic target in BLBC.

Lysyl Oxidase Is a Strong Determinant of Tumor Cell Colonization in Bone

Lysyl oxidase (LOX) is a secreted copper-dependent amine oxidase whose primary function is to drive collagen crosslinking and extracellular matrix stiffness. LOX in colorectal cancer synergizes with hypoxia-inducible factor-1 (HIF-1) to promote tumor progression. Here we investigated whether LOX/HIF1 endows colorectal cancer cells with full competence for aggressive colonization in bone. We show that a high LOX expression in primary tumors from patients with colorectal cancer was associated with poor clinical outcome, irrespective of HIF-1 In addition, LOX was expressed by tumor cells in the bone marrow from colorectal cancer patients with bone metastases. In vivo experimental studies show that LOX overexpression in colorectal cancer cells or systemic delivery of the conditioned medium from LOX-overexpressing colorectal cancer cells promoted tumor cell dissemination in the bone marrow and enhanced osteolytic lesion formation, irrespective of HIF-1 Conversely, silencing or pharmacologic inhibition of LOX activity blocked dissemination of colorectal cancer cells in the bone marrow and tumor-driven osteolytic lesion formation. In vitro, tumor-secreted LOX supported the attachment and survival of colorectal cancer cells to and in the bone matrix, and inhibited osteoblast differentiation. LOX overexpression in colorectal cancer cells also induced a robust production of IL6. In turn, both LOX and IL6 were acting in concert to promote RANKL-dependent osteoclast differentiation, thereby creating an imbalance between bone resorption and bone formation. Collectively, our findings show that LOX supports colorectal cancer cell dissemination in the bone marrow and they reveal a novel mechanism through which LOX-driven IL6 production by colorectal cancer cells impairs bone homeostasis. Cancer Res; 77(2); 268-78. ©2016 AACR.

Silencing FOXP2 in breast cancer cells promotes cancer stem cell traits and metastasis

In a recent article in Cell Stem Cell, we showed that mesenchymal stem cells (MSCs), progenitor cells that populate the breast tumor stroma, induce microRNA-mediated FOXP2 repression in breast cancer cells (BCCs), thus promoting cancer stem cell (CSC) and metastatic traits. Here, we discuss the implications of these findings for understanding metastatic CSC genesis.

Abstract P1-07-02: Mesenchymal stem cell regulated microRNAs converge on the speech gene FOXP2 and regulate breast cancer metastasis

About 90% of breast cancer mortalities are due to the spread of breast cancer cells (BCCs) from a primary tumor to distant organs, a process known as metastasis. However, the molecular mechanisms underlying metastasis remain poorly understood. Substantial evidence now supports a major role for the tumor microenvironment (TME) in catalyzing breast cancer metastasis. Indeed, observations indicate that proximal interactions between BCCs and cells of the TME induce altered gene expression programs in BCCs, allowing for the navigation of the various steps of the metastatic cascade. Our group and others observed that breast tumors recruit mesenchymal stem cells (MSCs): multipotent fibroblasts that normally exert tissue maintenance functions. We and others have observed that physical interactions of MSCs with BCCs are sufficient to drive their metastatic dissemination in murine xenograft models, via the induction of epithelial-mesenchymal transition (EMT) and dedifferentiation into stem cell-like states (cancer stem cells, or CSCs), states tightly associated with the capacity to seed new tumors (for example in foreign tissues) and with chemotherapeutic resistance. However, the TME-induced molecular pathways regulating such mechanisms remain poorly understood.

MicroRNAs (miRNAs, miRs) are small noncoding RNAs that regulate gene expression via base-pair interactions with messenger RNAs (mRNAs), resulting in mRNA degradation or translational inhibition. Due to their ability to interact with large numbers of target mRNAs simultaneously, miRNAs are major regulators of cell identity, and thereby serve critical roles in metastasis.

We performed miRnome-wide screening of MSC-stimulated BCCs to determine if TME interactions might contribute to BCC metastasis via the deregulation of miRNAs. We observed that proximal MSCs induce aberrant expression of a specific set of miRNAs in BCCs, which had not been previously implicated in breast cancer pathogenesis. These miRNAs, led by the transcriptionally co-regulated miR-199a-3p and miR-214, were sufficient to actuate the metastasis of weakly metastatic human BCCs in xenograft models. We observed that exogenous expression of the miRNAs provided BCCs with phenotypes and gene markers characteristic of CSCs, including enhanced tumor initiation capacities. Interestingly, we found that the MSC-induced miRNAs function as an interrelated network, and converge upon a common novel target: the speech associated gene FOXP2. Knockdown of FOXP2 phenocopied the metastatic phenotypes observed in MSC-induced miRNA expressing BCCs. Importantly, elevated levels of the MSC-induced miRNAs or depressed levels of FOXP2 could predict patient prognosis in the clinic. Altogether, our results incriminate FOXP2 and it’s MSC-induced miRNA regulatory network as novel determinants of breast cancer metastasis.

Citation Format: Benjamin G Cuiffo, Antoine Campagne, George W Bell, Antonio Lembo, Francesca Orso, Evan Lien, Manoj K Bhasin, Monica Raimo, Summer E Hanson, Andriy Marusyk, Peiman Hematti, Kornelia Polyak, Odette Mariani, Stefano Volinia, Anne Vincent-Salomon, Daniela Taverna, Antoine E Karnoub. Mesenchymal stem cell regulated microRNAs converge on the speech gene FOXP2 and regulate breast cancer metastasis [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-07-02.

MSC-regulated microRNAs converge on the transcription factor FOXP2 and promote breast cancer metastasis

Mesenchymal stem/stromal cells (MSCs) are progenitor cells shown to participate in breast tumor stroma formation and to promote metastasis. Despite expanding knowledge of their contributions to breast malignancy, the underlying molecular responses of breast cancer cells (BCCs) to MSC influences remain incompletely understood. Here, we show that MSCs cause aberrant expression of microRNAs, which, led by microRNA-199a, provide BCCs with enhanced cancer stem cell (CSC) properties. We demonstrate that such MSC-deregulated microRNAs constitute a network that converges on and represses the expression of FOXP2, a forkhead transcription factor tightly associated with speech and language development. FOXP2 knockdown in BCCs was sufficient in promoting CSC propagation, tumor initiation, and metastasis. Importantly, elevated microRNA-199a and depressed FOXP2 expression levels are prominent features of malignant clinical breast cancer and are associated significantly with poor survival. Our results identify molecular determinants of cancer progression of potential utility in the prognosis and therapy of breast cancer.

Mesenchymal stem cells in tumor development: emerging roles and concepts

Mesenchymal stem cells (MSCs) are multipotent progenitor cells that participate in the structural and functional maintenance of connective tissues under normal homeostasis. They also act as trophic mediators during tissue repair, generating bioactive molecules that help in tissue regeneration following injury. MSCs serve comparable roles in cases of malignancy and are becoming increasingly appreciated as critical components of the tumor microenvironment. MSCs home to developing tumors with great affinity, where they exacerbate cancer cell proliferation, motility, invasion and metastasis, foster angiogenesis, promote tumor desmoplasia and suppress anti-tumor immune responses. These multifaceted roles emerge as a product of reciprocal interactions occurring between MSCs and cancer cells and serve to alter the tumor milieu, setting into motion a dynamic co-evolution of both tumor and stromal tissues that favors tumor progression. Here, we summarize our current knowledge about the involvement of MSCs in cancer pathogenesis and review accumulating evidence that have placed them at the center of the pro-malignant tumor stroma.

Mesenchymal stem cells in the pathogenesis and therapy of breast cancer

Mesenchymal stem cells (MSCs) are a heterogeneous mix of stromal stem cells that can give rise to cells of mesodermal lineages, namely adipocytes, osteocytes and chondrocytes. They can home to sites of injury where they promote the repair and regeneration of damaged tissues. MSCs also home to sites of tumorigenesis, and as such, are utilized as efficient cellular vehicles for the delivery of anti-neoplastic therapeutics. Recently, MSCs within the tumor microenvironment have been shown to contribute to the desmoplastic reaction and to facilitate tumor formation and progression, sparking renewed interest in their pro-tumorigenic attributes and their roles as tumor stromal cells. Here, we describe the evidence linking MSCs to inflammatory processes and breast cancer development, and discuss their newly discovered physiological roles in the context of the tumor microenvironment.

Ras oncogenes: split personalities

Extensive research on the Ras proteins and their functions in cell physiology over the past 30 years has led to numerous insights that have revealed the involvement of Ras not only in tumorigenesis but also in many developmental disorders. Despite great strides in our understanding of the molecular and cellular mechanisms of action of the Ras proteins, the expanding roster of their downstream effectors and the complexity of the signalling cascades that they regulate indicate that much remains to be learnt.

Mesenchymal stem cells within tumour stroma promote breast cancer metastasis

Mesenchymal stem cells have been recently described to localize to breast carcinomas, where they integrate into the tumour-associated stroma. However, the involvement of mesenchymal stem cells (or their derivatives) in tumour pathophysiology has not been addressed. Here, we demonstrate that bone-marrow-derived human mesenchymal stem cells, when mixed with otherwise weakly metastatic human breast carcinoma cells, cause the cancer cells to increase their metastatic potency greatly when this cell mixture is introduced into a subcutaneous site and allowed to form a tumour xenograft. The breast cancer cells stimulate de novo secretion of the chemokine CCL5 (also called RANTES) from mesenchymal stem cells, which then acts in a paracrine fashion on the cancer cells to enhance their motility, invasion and metastasis. This enhanced metastatic ability is reversible and is dependent on CCL5 signalling through the chemokine receptor CCR5. Collectively, these data demonstrate that the tumour microenvironment facilitates metastatic spread by eliciting reversible changes in the phenotype of cancer cells.

Transformation of different human breast epithelial cell types leads to distinct tumor phenotypes

We investigated the influence of normal cell phenotype on the neoplastic phenotype by comparing tumors derived from two different normal human mammary epithelial cell populations, one of which was isolated using a new culture medium. Transformation of these two cell populations with the same set of genetic elements yielded cells that formed tumor xenografts exhibiting major differences in histopathology, tumorigenicity, and metastatic behavior. While one cell type (HMECs) yielded squamous cell carcinomas, the other cell type (BPECs) yielded tumors closely resembling human breast adenocarcinomas. Transformed BPECs gave rise to lung metastases and were up to 10(4)-fold more tumorigenic than transformed HMECs, which are nonmetastatic. Hence, the pre-existing differences between BPECs and HMECs strongly influence the phenotypes of their transformed derivatives.

Chemokine networks and breast cancer metastasis

Chemokines have been initially characterized as chemoattractants for leukocytes infiltrating into inflamed tissues. However, over the past few years, accumulating evidence has described the critical involvement of this superfamily of intercellular signaling proteins in a variety of biological processes, which include embryogenesis, organogenesis, and tissue homeostasis. Moreover, recent work has demonstrated novel roles for chemokines and their receptors in regulating various aspects of the transformed phenotype, such as tumor growth, angiogenesis, invasion, and metastasis. Here, we review the current knowledge regarding chemokine/chemokine receptor involvement in breast cancer pathogenesis with primary emphases on their role in the metastatic spread of cancer cells and in tumor-stroma interactions.

Auto-inhibition of the Dbl family protein Tim by an N-terminal helical motif

Dbl-related oncoproteins are guanine nucleotide exchange factors specific for Rho-family GTPases and typically possess tandem Dbl homology (DH) and pleckstrin homology domains that act in concert to catalyze exchange. Because the ability of many Dbl-family proteins to catalyze exchange is constitutively activated by truncations N-terminal to their DH domains, it has been proposed that the activity of Dbl-family proteins is regulated by auto-inhibition. However, the exact mechanisms of regulation of Dbl-family proteins remain poorly understood. Here we show that the Dbl-family protein, Tim, is auto-inhibited by a short, helical motif immediately N-terminal to its DH domain, which directly occludes the catalytic surface of the DH domain to prevent GTPase activation. Similar to the distantly related Vav isozymes, auto-inhibition of Tim is relieved by truncation, mutation, or phosphorylation of the auto-inhibitory helix. A peptide comprising the helical motif inhibits the exchange activity of Tim in vitro. Furthermore, substitutions within the most highly conserved surface of the DH domain designed to disrupt interactions with the auto-inhibitory helix also activate the exchange process.

Vav transformation requires activation of multiple GTPases and regulation of gene expression

Although Vav can act as a guanine nucleotide exchange factor for RhoA, Rac1, and Cdc42, its transforming activity has been ascribed primarily to its ability to activate Rac1. However, because activated Vav, but not Rac-specific guanine nucleotide exchange factors, exhibits very potent focus-forming transforming activity when assayed in NIH 3T3 cells, Vav transforming activity must also involve activation of Rac-independent pathways. In this study, we determined the involvement of other Rho family proteins and their signaling pathways in Vav transformation. We found that RhoA, Rac1, and Cdc42 functions are all required for Vav transforming activity. Furthermore, we determined that Vav activation of nuclear factor-kappaB and the Jun NH2-terminal kinase mitogen-activated protein kinase (MAPK) is necessary for full transformation by Vav, whereas p38 MAPK does not seem to play an important role. We also determined that Vav is a weak activator of Elk-1 via a Ras- and MAPK/extracellular signal-regulated kinase kinase-dependent pathway, and this activity was essential for Vav transformation. Thus, we conclude that full Vav transforming activation is mediated by the activation of multiple small GTPases and their subsequent activation of signaling pathways that regulate changes in gene expression. Because Vav is activated by the epidermal growth factor receptor and other tyrosine kinases involved in cancer development, defining the role of aberrant Vav signaling may identify activities of receptor tyrosine kinases important for human oncogenesis.

Vav Transformation Requires Activation of Multiple GTPases and Regulation of Gene Expression

Although Vav can act as a guanine nucleotide exchange factor for RhoA, Rac1, and Cdc42, its transforming activity has been ascribed primarily to its ability to activate Rac1. However, because activated Vav, but not Rac-specific guanine nucleotide exchange factors, exhibits very potent focus-forming transforming activity when assayed in NIH 3T3 cells, Vav transforming activity must also involve activation of Rac-independent pathways. In this study, we determined the involvement of other Rho family proteins and their signaling pathways in Vav transformation. We found that RhoA, Rac1, and Cdc42 functions are all required for Vav transforming activity. Furthermore, we determined that Vav activation of nuclear factor-κB and the Jun NH2-terminal kinase mitogen-activated protein kinase (MAPK) is necessary for full transformation by Vav, whereas p38 MAPK does not seem to play an important role. We also determined that Vav is a weak activator of Elk-1 via a Ras- and MAPK/extracellular signal-regulated kinase kinase–dependent pathway, and this activity was essential for Vav transformation. Thus, we conclude that full Vav transforming activation is mediated by the activation of multiple small GTPases and their subsequent activation of signaling pathways that regulate changes in gene expression. Because Vav is activated by the epidermal growth factor receptor and other tyrosine kinases involved in cancer development, defining the role of aberrant Vav signaling may identify activities of receptor tyrosine kinases important for human oncogenesis.

Rac1b, a tumor associated, constitutively active Rac1 splice variant, promotes cellular transformation

A novel splice variant of Rac1, designated Rac1b, is expressed in human breast and colon carcinoma cells. Rac1b contains an additional 19 amino-acid insert immediately behind the switch II domain, a region important for Rac1 interaction with regulators and effectors. Recent studies showed that Rac1b exhibited the biochemical properties of a constitutively activated GTPase, yet it showed impaired interaction with downstream effectors, suggesting that Rac1b may be defective in biological activity. Whether Rac1b is a biologically active protein was not addressed. Therefore, we evaluated the biochemical, signaling and growth-promoting properties of authentic Rac1b. Similar to previous observations, we found that Rac1b showed enhanced intrinsic guanine nucleotide exchange activity, impaired intrinsic GTPase activity, and failed to interact with RhoGDI. Surprisingly, we found that Rac1b, like the constitutively-activated and transforming Rac1(Q61L) mutant, promoted growth transformation of NIH3T3 cells. Rac1b-expressing cells also showed a loss of density-dependent and anchorage-dependent growth. Surprisingly, unlike activated Rac1(61L), Rac1b did not show enhanced activation of the nuclear factor kappaB (NF-kappaB) transcription factor or stimulate cyclin D1 expression, the signaling activities that best correlate with Rac1 transforming activity. However, Rac1b did promote activation of the AKT serine/threonine kinase. Therefore, we suggest that Rac1b selectively activates a subset of Rac1 downstream signaling pathways to facilitate cellular transformation.

Molecular Basis for Rho GTPase Signaling Specificity

There is now considerable evidence for the involvement of aberrant Rho GTPase activation in breast cancer development. Like Ras, Rho GTPases function as signaling nodes regulated by diverse extracellular stimuli. Rho GTPase activation is facilitated by multiple regulatory proteins, in particular guanine nucleotide exchange factors (GEFs) such as Dbl family proteins. Activated Rho GTPases in turn interact with and regulate a spectrum of functionally diverse downstream effectors, initiating a network of cytoplasmic and nuclear signaling cascades. Thus, Rho GTPases represent points of signaling convergence as well as relay switches that disseminate signaling divergence. In this review, we highlight issues relating to the structural basis by which Dbl family GEFs facilitate signaling convergence and Rho GTPase activation, and how Rho GTPases promote signal dissemination through downstream effectors.

Critical role of the pleckstrin homology domain in Dbs signaling and growth regulation

Dbl family proteins act as guanine nucleotide exchange factors and positive regulators of Rho GTPase function by stimulating formation of the active, GTP-bound state. All Dbl family Rho guanine nucleotide exchange factors possess an invariant tandem domain structure consisting of a Dbl homology (DH) catalytic domain followed by a pleckstrin homology (PH) regulatory domain. We determined previously that the PH domain of Dbs was critical for the intrinsic catalytic activity of the DH domain in vitro and for Dbs transformation in vivo. In this study, we evaluated the role of phosphoinositide binding to the PH domain in regulating the DH domain function of Dbs in vitro and in vivo. We determined that mutation of basic amino acids located within the beta1-beta2 and beta3-beta4 loops of the PH domain resulted in impaired phospholipid binding in vitro, yet full guanine nucleotide exchange activity in vitro was retained for RhoA and Cdc42. Surprisingly, these mutants were compromised in their ability to activate Rho GTPases in vivo and to cause transformation of NIH 3T3 cells. However, Dbs subcellular localization was impaired by these PH domain mutations, supporting a role for phospholipid interactions in facilitating membrane association. Despite the importance of phospholipid binding for Dbs function in vivo, we found that Dbs signaling and transforming activity was not stimulated by phosphatidylinositol 3-kinase activation. We suggest that the PH domain of Dbs facilitates two distinct roles in the regulation of DH domain function, one critical for GTPase association and activation in vitro and one critical for phosphoinositide binding and GTPase interaction in vivo, that together promote Dbs association with membranes.

Role of the pleckstrin homology domain in intersectin-L Dbl homology domain activation of Cdc42 and signaling

Intersectin-long (ITSN-L) contains the invariant Dbl homology (DH) and pleckstrin homology (PH) domain structure characteristic of the majority of Dbl family proteins. This strict domain topography suggests that the PH domain serves an essential, conserved function in the regulation of the intrinsic guanine nucleotide exchange activity of the DH domain. We evaluated the role of the PH domain in regulating the DH domain function of ITSN-L. Surprisingly, we found that the PH domain was dispensable for guanine nucleotide exchange activity on Cdc42 in vitro, yet the PH domain enhanced the ability of the DH domain to activate Cdc42 signaling in vivo. PH domains can interact with phosphoinositide substrates and products of phosphatidylinositol 3-kinase (PI3K). However, PI3K activation did not modulate ITSN-L DH domain function in vivo.

RhoG signals in parallel with Rac1 and Cdc42

RhoG is a member of the Rho family of small GTPases and shares high sequence identity with Rac1 and Cdc42. Previous studies suggested that RhoG mediates its effects through activation of Rac1 and Cdc42. To further understand the mechanism of RhoG signaling, we studied its potential activation pathways, downstream signaling properties, and functional relationship to Rac1 and Cdc42 in vivo. First, we determined that RhoG was regulated by guanine nucleotide exchange factors that also activate Rac and/or Cdc42. Vav2 (which activates RhoA, Rac1, and Cdc42) and to a lesser degree Dbs (which activates RhoA and Cdc42) activated RhoG in vitro. Thus, RhoG may be activated concurrently with Rac1 and Cdc42. Second, some effectors of Rac/Cdc42 (IQGAP2, MLK-3, PLD1), but not others (e.g. PAKs, POSH, WASP, Par-6, IRSp53), interacted with RhoG in a GTP-dependent manner. Third, consistent with this differential interaction with effectors, activated RhoG stimulated some (JNK and Akt) but not other (SRF and NF-kappaB) downstream signaling targets of activated Rac1 and Cdc42. Finally, transient transduction of a tat-tagged Rac1(17N) dominant-negative fusion protein inhibited the induction of lamellipodia by the Rac-specific activator, Tiam1, but not by activated RhoG. Together, these data argue that RhoG function is mediated by signals independent of Rac1 and Cdc42 activation and instead by direct utilization of a subset of common effectors.

Distinct requirements for Ras oncogenesis in human versus mouse cells

The spectrum of tumors associated with oncogenic Ras in humans often differs from those in mice either treated with carcinogens or engineered to sporadically express oncogenic Ras, suggesting that the mechanism of Ras transformation may be different in humans. Ras stimulates primarily three main classes of effector proteins, Rafs, PI3-kinase, and RalGEFs, with Raf generally being the most potent at transforming murine cells. Using oncogenic Ras mutants that activate single effectors as well as constitutively active effectors, we find that the RalGEF, and not the Raf or PI3-kinase pathway, is sufficient for Ras transformation in human cells. Thus, oncogenic Ras may transform murine and human cells by distinct mechanisms, and the RalGEF pathway--previously deemed to play a secondary role in Ras transformation--could represent a new target for anti-cancer therapy.

Structural and biophysical insights into the role of the insert region in Rac1 function

A 13 amino acid insertion that forms a short 3(10) helix between beta-strand 5 and alpha-helix 4 is a distinguishing feature among most members of the Rho family of GTPases, yet the precise role of this region in signal transduction is poorly understood. Previous in vivo functional studies have implicated the insert region of RhoA, Rac1, and Cdc42 to be important for cell transformation, regulation of the actin cytoskeleton, controlling DNA synthesis, and in the activation of downstream targets. In Rac1, our recent biological studies have suggested that the insert is important for SRF activation and the formation of lamellipodia but is dispensable for all other cellular functions of this protein. In the studies reported herein, we have characterized the effect of the insert deletion on Rac1 structure, thermodynamic stability, and the kinetics of nucleotide association. These in vitro studies help clarify biological data and provide further insights as to the role of this 13 amino acid region in modulating Rac1 function. The studies reveal that deletion of the insert has no effect on Rac1 structure and causes only a marginal (approximately 0.8 kcal/mol) decrease in the DeltaG(fold) of Rac1*GDP*Mg2+. The intrinsic rate of nucleotide dissociation of Rac1*Delta(ins) is decreased by about 1.5-fold compared to that of wild type, and a 3-fold increase in the GEF (Vav2)-mediated exchange rate is observed. In addition, deletion of the insert does not change the K(D) for the interaction of Rac1 with GDI, and similar to that previously observed for Cdc42, no inhibition of GDP dissociation is observed for the deletion mutant relative to that for the native protein. Taken together, the structural and biochemical studies reported here are consistent with our biological data reported previously and suggest that the most likely role of the insert region must be to serve as a binding interface for downstream effectors, particularly those important for actin regulation.

Role of MLK3-mediated activation of p70 S6 kinase in Rac1 transformation

The signaling pathways that mediate the transforming activity of the Rac1 GTPase remain to be determined. In the present study, we used effector domain mutants of the constitutively activated Rac(61L) mutant that display differential transforming activities and differential activation of downstream effector pathways to investigate the contribution of p70 S6 kinase (p70(S6K)) to Rac1 transformation and to decipher the signaling pathways leading from Rac1 to p70(S6K). First, we found that Rac1 transforming activity could be dissociated from Rac1 activation of p70(S6K). A weakly transforming Rac1 mutant retained the ability to activate p70(S6K), whereas some potently transforming effector mutants were impaired in their ability to activate p70(S6K). These data suggest that p70(S6K) is not necessary to promote full Rac1 transforming activity. We also found a strong correlation between the ability of the Rac(61L) effector mutants to activate p70(S6K) and their ability to activate the JNK mitogen-activated protein kinase. We found that the MLK3 serine/threonine kinase activated JNK and p70(S6K), whereas activation of p70(S6K) by Rac(61L) was significantly inhibited by dominant-negative MLK3. Additionally, the ability of the Rac(61L) effector mutants to activate MLK3 correlated well with their ability to activate p70(S6K) and JNK. Taken together, these results provide evidence that Rac1 coordinately activates p70(S6K) and JNK via MLK3 activation. Finally, we found that co-expression of wild type, but not kinase-dead, MLK3 significantly inhibited Rac1 transforming activity. These results suggest that MLK3 may be a negative regulator of the growth-promoting and transforming properties of Rac1.

Molecular basis for Rac1 recognition by guanine nucleotide exchange factors

Rho GTPases are activated by a family of guanine nucleotide exchange factors (GEFs) known as Dbl family proteins. The structural basis for how GEFs recognize and activate Rho GTPases is presently ill defined. Here, we utilized the crystal structure of the DH/PH domains of the Rac-specific GEF Tiam1 in complex with Rac1 to determine the structural elements of Rac1 that regulate the specificity of this interaction. We show that residues in the Rac1 beta2-beta3 region are critical for Tiam1 recognition. Additionally, we determined that a single Rac1-to-Cdc42 mutation (W56F) was sufficient to abolish Rac1 sensitivity to Tiam1 and allow recognition by the Cdc42-specific DH/PH domains of Intersectin while not impairing Rac1 downstream activities. Our findings identified unique GEF specificity determinants in Rac1 and provide important insights into the mechanism of DH/PH selection of GTPase targets.

The insert region of Rac1 is essential for membrane ruffling but not cellular transformation

The Rho family of Ras-related proteins, which includes Rac1, RhoA, and Cdc42, is distinguished from other members of the Ras superfamily of small GTPases in that its members possess additional sequences positioned between beta-strand 5 and alpha-helix 4, designated the insert region. Previous studies have established the importance of an intact insert region for the transforming, but not actin cytoskeletal reorganization, activities of Cdc42 and RhoA. Similarly, the insert region was determined to be essential for Rac1-mediated mitogenesis. Additionally, an intact insert region was also determined to be required for the antiapoptotic activity of Rac1 as well as for Rac1 activation of reactive oxygen species and the NF-kappaB transcription factor. However, it has not been determined whether the insert region is important for Rac1-mediated growth transformation. In this study, we assessed the requirement for the insert region in Rac1 transformation and signaling in NIH 3T3 cells. Unexpectedly, we found that a mutant of constitutively activated Rac1 that lacked the insert region retained potent transforming activity. The insert region of Rac1 was dispensable for Rac1 stimulation of transcription from the cyclin D1 promoter and for activation of the c-Jun, NF-kappaB, and E2F-1 transcription factors but was essential for Rac1 induction of serum response factor activity. While an intact insert region was dispensable for inducing reactive oxygen species production in vivo, it was required for Rac1 induction of lamellipodia. When taken together, these results show that the insert region of Rac1 serves roles in regulating actin organization and cell growth that are distinct from those of the analogous regions of Cdc42 and RhoA and support its involvement in regulating specific downstream effector interactions.