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

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

MSC-Regulated MicroRNAs Converge on the Transcription Factor FOXP2 and Promote Breast Cancer Metastasis

SummaryMesenchymal 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

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

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

Lysyl oxidase is a strong determinant of tumor cell colonization in bone. Short Title: LOX triggers tumor cell colonization in bone

International audienceLysyl 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 (CRC) synergizes with hypoxia-inducible factor-1 (HIF-1 to promote tumor progression. Here we investigated whether LOX/HIF1 endows CRC cells with full competence for aggressive colonization in bone. We show that a high LOX expression in primary tumors from CRC patients was associated with poor clinical outcome, irrespective of HIF-1. Additionally, LOX was expressed by tumor cells in the bone marrow from CRC patients with bone metastases. In vivo experimental studies show that LOX overexpression in CRC cells or systemic delivery of the conditioned medium from LOX-overexpressing CRC cells promoted tumor cell dissemination in the bone marrow and enhanced osteolytic lesion formation, irrespective of HIF-1. Conversely, silencing or pharmacological inhibition of LOX activity blocked dissemination of CRC cells in the bone marrow and tumor-driven osteolytic lesion formation. In vitro, tumor-secreted LOX supported the attachment and survival of CRC cells to and in the bone matrix, and inhibited osteoblast differentiation. LOX overexpression in CRC cells also induced a robust production of IL-6. In turn, both LOX and IL-6 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 CRC cell dissemination in the bone marrow and they reveal a novel mechanism through which LOX-driven IL-6 production by CRC cells impairs bone homeostasis

Lysyl oxidase is a strong determinant of tumor cell colonization in bone. Short Title: LOX triggers tumor cell colonization in bone

International audienceLysyl 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 (CRC) synergizes with hypoxia-inducible factor-1 (HIF-1 to promote tumor progression. Here we investigated whether LOX/HIF1 endows CRC cells with full competence for aggressive colonization in bone. We show that a high LOX expression in primary tumors from CRC patients was associated with poor clinical outcome, irrespective of HIF-1. Additionally, LOX was expressed by tumor cells in the bone marrow from CRC patients with bone metastases. In vivo experimental studies show that LOX overexpression in CRC cells or systemic delivery of the conditioned medium from LOX-overexpressing CRC cells promoted tumor cell dissemination in the bone marrow and enhanced osteolytic lesion formation, irrespective of HIF-1. Conversely, silencing or pharmacological inhibition of LOX activity blocked dissemination of CRC cells in the bone marrow and tumor-driven osteolytic lesion formation. In vitro, tumor-secreted LOX supported the attachment and survival of CRC cells to and in the bone matrix, and inhibited osteoblast differentiation. LOX overexpression in CRC cells also induced a robust production of IL-6. In turn, both LOX and IL-6 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 CRC cell dissemination in the bone marrow and they reveal a novel mechanism through which LOX-driven IL-6 production by CRC cells impairs bone homeostasis

Critical role for lysyl oxidase in mesenchymal stem cell-driven breast cancer malignancy

Mesenchymal stem cells (MSCs) are multipotent progenitor cells with the ability to differentiate into multiple mesoderm lineages in the course of normal tissue homeostasis or during injury. We have previously shown that MSCs migrate to sites of tumorigenesis, where they become activated by cancer cells to promote metastasis. However, the molecular and phenotypic attributes of the MSC-induced metastatic state of the cancer cells remained undetermined. Here, we show that bone marrow-derived human MSCs promote de novo production of lysyl oxidase (LOX) from human breast carcinoma cells, which is sufficient to enhance the metastasis of otherwise weakly metastatic cancer cells to the lungs and bones. We also show that LOX is an essential component of the CD44-Twist signaling axis, in which extracellular hyaluronan causes nuclear translocation of CD44 in the cancer cells, thus triggering LOX transcription by associating with its promoter. Processed and enzymatically active LOX, in turn, stimulates Twist transcription, which mediates the MSC-triggered epithelial-to-mesenchymal transition (EMT) of carcinoma cells. Surprisingly, although induction of EMT in breast cancer cells has been tightly associated with the generation of cancer stem cells, we find that LOX, despite being critical for EMT, does not contribute to the ability of MSCs to promote the formation of cancer stem cells in the carcinoma cell populations. Collectively, our studies highlight a critical role for LOX in cancer metastasis and indicate that the signaling pathways controlling stroma-induced EMT are distinct from pathways regulating the development of cancer stem cells.David & Lucile Packard Foundatio