The guanine nucleotide exchange factor (GEF) Ect2 is an oncogene in human cancer

Ect2 is an oncogene in multiple human cancers. Ect2 is aberrantly overexpressed in multiple human tumor types, often as a result of targeted amplification of the ECT2 gene as part of the 3q26 amplicon. Ect2 is important for proliferation, migration and invasion of various types of cancer cells in vitro, and for NSCLC tumorigenicity in vivo. The role of Ect2 in cellular transformation is distinct from its physiologic role in cytokinesis, and many tumor cells appear to have evolved Ect2-independent cytokinesis mechanisms. In NSCLC cells, the ability of Ect2 to support transformation is linked to its mislocalization to the cytoplasm and activation of a Rac1-Pak-Mek1,2-Erk1,2 signaling axis that is regulated through its binding to the oncogenic PKCι/Par6α complex (Fig. 4). Therefore, Ect2 and PKCι are genetically linked due to their frequent co-amplification as part of the 3q26 amplicon, and functionally and biochemically linked through formation of an oncogenic PKCι-Par6-Ect2 complex that drives transformation. Further experiments will be required to determine if Ect2 and PKCι are similarly linked in other tumors, particularly those harboring 3q26 amplification. In addition, further work is needed to elucidate the molecular mechanisms that regulate the formation, dynamics and activity of the oncogenic PKCι-Par6α-Ect2 complex. These studies hold the promise of identifying novel therapeutic approaches to cancer treatment based on inhibiting Ect2 function in cancer cells.     

Matrix metalloproteinase-10 promotes Kras-mediated bronchio-alveolar stem cell expansion and lung cancer formation

Matrix metalloproteinase 10 (MMP-10; stromelysin 2) is a member of a large family of structurally related matrix metalloproteinases, many of which have been implicated in tumor progression, invasion and metastasis. We recently identified Mmp10 as a gene that is highly induced in tumor-initiating lung bronchioalveolar stem cells (BASCs) upon activation of oncogenic Kras in a mouse model of lung adenocarcinoma. However, the potential role of Mmp10 in lung tumorigenesis has not been addressed. Here, we demonstrate that Mmp10 is overexpressed in lung tumors induced by either the smoke carcinogen urethane or oncogenic Kras. In addition, we report a significant reduction in lung tumor number and size after urethane exposure or genetic activation of oncogenic Kras in Mmp10 null (Mmp10(-/-)) mice. This inhibitory effect is reflected in a defect in the ability of Mmp10-deficient BASCs to expand and undergo transformation in response to urethane or oncogenic Kras in vivo and in vitro, demonstrating a role for Mmp10 in the tumor-initiating activity of Kras-transformed lung stem cells. To determine the potential relevance of MMP10 in human cancer we analyzed Mmp10 expression in publicly-available gene expression profiles of human cancers. Our analysis reveals that MMP10 is highly overexpressed in human lung tumors. Gene set enhancement analysis (GSEA) demonstrates that elevated MMP10 expression correlates with both cancer stem cell and tumor metastasis genomic signatures in human lung cancer. Finally, Mmp10 is elevated in many human tumor types suggesting a widespread role for Mmp10 in human malignancy. We conclude that Mmp10 plays an important role in lung tumor initiation via maintenance of a highly tumorigenic, cancer-initiating, stem-like cell population, and that Mmp10 expression is associated with stem-like, highly metastatic genotypes in human lung cancers. These results indicate that Mmp10 may represent a novel therapeutic approach to target lung cancer stem cells.     

Oncogenic activity of Ect2 is regulated through protein kinase C iota-mediated phosphorylation

The Rho GTPase guanine nucleotide exchange factor Ect2 is genetically and biochemically linked to the PKCι oncogene in non-small cell lung cancer (NSCLC). Ect2 is overexpressed and mislocalized to the cytoplasm of NSCLC cells where it binds the oncogenic PKCι-Par6 complex, leading to activation of the Rac1 small GTPase. Here, we identify a previously uncharacterized phosphorylation site on Ect2, threonine 328, that serves to regulate the oncogenic activity of Ect2 in NSCLC cells. PKCι directly phosphorylates Ect2 at Thr-328 in vitro, and RNAi-mediated knockdown of either PKCι or Par6 leads to a decrease in phospho-Thr-328 Ect2, indicating that PKCι regulates Thr-328 Ect2 phosphorylation in NSCLC cells. Both wild-type Ect2 and a phosphomimetic T328D Ect2 mutant bind the PKCι-Par6 complex, activate Rac1, and restore transformed growth and invasion when expressed in NSCLC cells made deficient in endogenous Ect2 by RNAi-mediated knockdown. In contrast, a phosphorylation-deficient T328A Ect2 mutant fails to bind the PKCι-Par6 complex, activate Rac1, or restore transformation. Our data support a model in which PKCι-mediated phosphorylation regulates Ect2 binding to the oncogenic PKCι-Par6 complex thereby activating Rac1 activity and driving transformed growth and invasion.     

Matrix metalloproteinase-10 is required for lung cancer stem cell maintenance, tumor initiation and metastatic potential

Matrix metalloproteinases (Mmps) stimulate tumor invasion and metastasis by degrading the extracellular matrix. Here we reveal an unexpected role for Mmp10 (stromelysin 2) in the maintenance and tumorigenicity of mouse lung cancer stem-like cells (CSC). Mmp10 is highly expressed in oncosphere cultures enriched in CSCs and RNAi-mediated knockdown of Mmp10 leads to a loss of stem cell marker gene expression and inhibition of oncosphere growth, clonal expansion, and transformed growth in vitro. Interestingly, clonal expansion of Mmp10 deficient oncospheres can be restored by addition of exogenous Mmp10 protein to the culture medium, demonstrating a direct role for Mmp10 in the proliferation of these cells. Oncospheres exhibit enhanced tumor-initiating and metastatic activity when injected orthotopically into syngeneic mice, whereas Mmp10-deficient cultures show a severe defect in tumor initiation. Conversely, oncospheres implanted into syngeneic non-transgenic or Mmp10(-/-) mice show no significant difference in tumor initiation, growth or metastasis, demonstrating the importance of Mmp10 produced by cancer cells rather than the tumor microenvironment in lung tumor initiation and maintenance. Analysis of gene expression data from human cancers reveals a strong positive correlation between tumor Mmp10 expression and metastatic behavior in many human tumor types. Thus, Mmp10 is required for maintenance of a highly tumorigenic, cancer-initiating, metastatic stem-like cell population in lung cancer. Our data demonstrate for the first time that Mmp10 is a critical lung cancer stem cell gene and novel therapeutic target for lung cancer stem cells.     

Protein kinase Cα promotes cell migration through a PDZ-dependent interaction with its novel substrate discs large homolog 1 (DLG1)

Protein scaffolds maintain precision in kinase signaling by coordinating kinases with components of specific signaling pathways. Such spatial segregation is particularly important in allowing specificity of signaling mediated by the 10-member family of protein kinase C (PKC) isozymes. Here we identified a novel interaction between PKCα and the Discs large homolog (DLG) family of scaffolds that is mediated by a class I C-terminal PDZ (PSD-95, disheveled, and ZO1) ligand unique to this PKC isozyme. Specifically, use of a proteomic array containing 96 purified PDZ domains identified the third PDZ domains of DLG1/SAP97 and DLG4/PSD95 as interaction partners for the PDZ binding motif of PKCα. Co-immunoprecipitation experiments verified that PKCα and DLG1 interact in cells by a mechanism dependent on an intact PDZ ligand. Functional assays revealed that the interaction of PKCα with DLG1 promotes wound healing; scratch assays using cells depleted of PKCα and/or DLG1 have impaired cellular migration that is no longer sensitive to PKC inhibition, and the ability of exogenous PKCα to rescue cellular migration is dependent on the presence of its PDZ ligand. Furthermore, we identified Thr-656 as a novel phosphorylation site in the SH3-Hook region of DLG1 that acts as a marker for PKCα activity at this scaffold. Increased phosphorylation of Thr-656 is correlated with increased invasiveness in non-small cell lung cancer lines from the NCI-60, consistent with this phosphorylation site serving as a marker of PKCα-mediated invasion. Taken together, these data establish the requirement of scaffolding to DLG1 for PKCα to promote cellular migration.