H-Ras-specific activation of Rac-MKK3/6-p38 pathway: its critical role in invasion and migration of breast epithelial cells

Human tumors frequently exhibit constitutively activated Ras signaling, which contributes to the malignant phenotype. Mounting evidence suggests unique roles of the Ras family members, H-Ras, N-Ras and K-Ras, in normal and pathological conditions. In an effort to dissect distinct Ras isoform-specific functions in malignant phenotypic changes, we previously established H-Ras- and N-Ras-activated MCF10A human breast epithelial cell lines. Using these, we showed that p38 kinase is a key signaling molecule differentially regulated between H-Ras and N-Ras, leading to H-Ras-specific induction of invasive and migrative phenotypes. The present study is to further investigate H-Ras- and N-Ras-mediated signaling pathways and to unveil how these pathways are integrated for regulation of invasive/migrative phenotypic conversion of human breast epithelial cells. Here we report that the Rac-MAPK kinase (MKK)3/6-p38 pathway is a unique signaling pathway activated by H-Ras, leading to the invasive/migrative phenotype. In contrast, Raf-MEK-ERK and phosphatidylinositol 3-kinase-Akt pathways, which are fundamental to proliferation and differentiation, are activated by both H-Ras and N-Ras. A significant role for p38 in cell invasion is further supported by the observation that p38 activation by MKK6 transfection is sufficient to induce invasive and migrative phenotypes in MCF10A cells. Activation of the MKK6-p38 pathway results in a marked induction of matrix metalloproteinase (MMP)-2, whereas it had little effect on MMP-9, suggesting MMP-2 up-regulation by MKK6-p38 pathway as a key step for H-Ras-induced invasion and migration. We also provide evidence for cross-talk among the Rac, Raf, and phosphatidylinositol 3-kinase pathways critical for regulation of MMP-2 and MMP-9 expression and invasive phenotype. Taken together, the present study elucidated the role of the Rac-MKK3/6-p38 pathway leading to H-Ras-specific induction of malignant progression in breast epithelial cells, providing implications for developing therapeutic strategies for mammary carcinoma to target Ras downstream signaling molecules required for malignant cancer cell behavior but less critical for normal cell functions.     

Activation of H-Ras and Rac1 correlates with epidermal growth factor-induced invasion in Hs578T and MDA-MB-231 breast carcinoma cells

There is considerable experimental evidence that hyperactive Ras proteins promote breast cancer growth and development including invasiveness, despite the low frequency of mutated forms of Ras in breast cancer. We have previously shown that H-Ras, but not N-Ras, induces an invasive phenotype mediated by small GTPase Rac1 in MCF10A human breast epithelial cells. Epidermal growth factor (EGF) plays an important role in aberrant growth and metastasis formation of many tumor types including breast cancer. The present study aims to investigate the correlation between EGF-induced invasiveness and Ras activation in four widely used breast cancer cell lines. Upon EGF stimulation, invasive abilities and H-Ras activation were significantly increased in Hs578T and MDA-MB-231 cell lines, but not in MDA-MB-453 and T47D cell lines. Using small interfering RNA (siRNA) to target H-Ras, we showed a crucial role of H-Ras in the invasive phenotype induced by EGF in Hs578T and MDA-MB-231 cells. Moreover, siRNA-knockdown of Rac1 significantly inhibited the EGF-induced invasiveness in these cells. Taken together, this study characterized human breast cancer cell lines with regard to the relationship between H-Ras activation and the invasive phenotype induced by EGF. Our data demonstrate that the activation of H-Ras and the downstream molecule Rac1 correlates with EGF-induced breast cancer cell invasion, providing important information on the regulation of malignant progression in mammary carcinoma cells.     

Global gene expression profiling unveils S100A8/A9 as candidate markers in H-ras-mediated human breast epithelial cell invasion

The goal of the present study is to unveil the gene expression profile specific to the biological processes of human breast epithelial cell invasion and migration using an MCF10A model genetically engineered to constitutively activate the H-ras or N-ras signaling pathway. We previously showed that H-Ras, but not N-Ras, induces MCF10A cell invasion/migration, whereas both H-Ras and N-Ras induce cell proliferation and phenotypic transformation. Thus, these cell lines provide an experimental system to separate the gene expression profile associated with cell invasion apart from cell proliferation/transformation. Analysis of whole human genome microarray revealed that 412 genes were differentially expressed among MCF10A, N-Ras MCF10A, and H-Ras MCF10A cells and hierarchical clustering separated 412 genes into four clusters. We then tested whether S100A8 and S100A9, two of the genes which are most highly up-regulated in an H-Ras-specific manner, play a causative role for H-Ras-mediated MCF10A cell invasion and migration. Importantly, small interfering RNA-mediated knockdown of S100A8/A9 expression significantly reduced H-Ras-induced invasion/migration. Conversely, the induction of S100A8/A9 expression conferred the invasive/migratory phenotype to parental MCF10A cells. Furthermore, we provided evidence of signaling cross-talk between S100A8/A9 and the mitogen-activated protein kinase signaling pathways essential for H-Ras-mediated cell invasion and migration. Taken together, this study revealed S100A8/A9 genes as candidate markers for metastatic potential of breast epithelial cells. Our gene profile data provide useful information which may lead to the identification of additional potential targets for the prognosis and/or therapy of metastatic breast cancer.     

The Function of Vacuolar ATPase (V-ATPase) a Subunit Isoforms in Invasiveness of MCF10a and MCF10CA1a Human Breast Cancer Cells

The vacuolar H⁺ ATPases (V-ATPases) are ATP-driven proton pumps that transport protons across both intracellular and plasma membranes. Previous studies have implicated V-ATPases in the invasiveness of various cancer cell lines. In this study, we evaluated the role of V-ATPases in the invasiveness of two closely matched human breast cancer lines. MCF10a cells are a non-invasive, immortalized breast epithelial cell line, and MCF10CA1a cells are a highly invasive, H-Ras-transformed derivative of MCF10a cells selected for their metastatic potential. Using an in vitro Matrigel assay, MCF10CA1a cells showed a much higher invasion than the parental MCF10a cells. Moreover, this increased invasion was completely sensitive to the specific V-ATPase inhibitor concanamycin. MCF10CA1a cells expressed much higher levels of both a1 and a3 subunit isoforms relative to the parental line. Isoforms of subunit a are responsible for subcellular localization of V-ATPases, with a3 and a4 targeting V-ATPases to the plasma membrane of specialized cells. Knockdown of either a3 alone or a3 and a4 together using isoform-specific siRNAs inhibited invasion by MCF10CA1a cells. Importantly, overexpression of a3 but not the other a subunit isoforms greatly increased the invasiveness of the parental MCF10a cells. Similarly, overexpression of a3 significantly increased expression of V-ATPases at the plasma membrane. These studies suggest that breast tumor cells employ particular a subunit isoforms to target V-ATPases to the plasma membrane, where they function in tumor cell invasion.     

Invasiveness of transformed human breast epithelial cell lines is related to cathepsin B and inhibited by cysteine proteinase inhibitors

The activities'of the lysosomal cysteine proteinases cathepsin B and L are regulated by their endogenous inhibitors, stefins A and B, and cystatin C, and their imbalance may be associated with increased invasiveness and development of the malignant cell phenotype. The aim of this study was to investigate mRNA, protein and activity levels of the above proteins in relation to in vitro invasiveness and to the reported in vivo tumorigenicity of four human breast tumor cell lines: the spontaneously immortalized cell line MCF10A, its c-Ha-ras transfectant MCF10AT, and two tumorigenic derivative cell lines, MCF10AT-Ca1a and MCF10AT-Ca1d. Invasiveness did not correlate with tumorigenicity, since the MCF10AT cell was the most invasive and the remaining three were at about half of its level. Cathepsin B expression paralleled the in vitro invasiveness through matrigel at all levels of expression, but cathepsin L did not. Stefin levels were elevated several-fold in the tumorigenic cell lines, but not in MCF10AT. The hypothesis that cathepsin B plays an active role in the invasion of breast cancer cell lines was confirmed by the fact that synthetic cysteine proteinase inhibitors, particularly those selective for cathepsin B, significantly reduced the invasion of the MCF10AT cells.     

Identification of annexin II as a novel secretory biomarker for breast cancer

Early prediction of metastatic breast cancer is important for improvement of prognosis and survival rate. The present study aimed to identify secreted protein biomarkers for detection of invasive breast cancer. To this end, we performed a comparative proteomic analysis by a combination of 2DE and MALDI‐TOF MS analysis of conditioned media from invasive H‐Ras MCF10A human breast epithelial cells and noninvasive MCF10A and N‐Ras MCF10A cells. We identified a list of 25 proteins that were strongly detected in media of H‐Ras MCF10A and focused on annexin II, which was shown to be involved in cell motility. Invasive triple‐negative human breast carcinoma cells, Hs578T, and MDA‐MB‐231, showed increased levels of annexin II in media, demonstrating that secretion of annexin II correlated well with the invasive phenotype of cells. We demonstrated a crucial role of annexin II in breast cell invasion/migration and actin cytoskeleton reorganization required for filopodia formation. Annexin II levels in the plasma samples and breast cancer tissues of breast cancer patients were significantly higher than those of normal groups, providing a clinical relevance to our in vitro findings. Taken together, we identified annexin II as a novel secretory biomarker candidate for invasive breast cancer, especially estrogen receptor‐negative breast cancer.     

Expression of endothelins and their receptors promotes an invasive phenotype of breast tumor cells but is insufficient to induce invasion in benign cells

There is increased staining of endothelins (ET-1, -2, and -3) and receptors (ET-RA and -RB) in invasive breast tumors compared to nonneoplastic tissue, and ETs stimulate MCF-7 cell invasion in vitro. We analyzed ETstimulation of benign and transformed mammary epithelial cells, and whether expression of ETs is sufficient to induce invasiveness. In breast cancer patient serum, ET-1 was increased in those patients with lymph node metastases compared to those with no lymph node involvement; ETs, however, had no mitogenic effect on breast tumor cell lines in vitro. The benign mammary epithelial cell line, hTERT-HME1, and the poorly invasive breast tumor cell line MCF-7 secreted low levels of ET-1, while the invasive cell lines SKBR3 and MDAMB231 secreted high levels. Expression of the ETs and receptors by the cell lines broadly correlated with their in vitro invasiveness; overexpression of ETs in MCF-7 cells increased basal invasion. ET-mediated invasion involved both receptors and a calcium influx to induce a pertussis toxin-sensitive MAPK pathway. MMP-14 activity was induced via ET-RA in an autocrine manner. In contrast to transformed cells, ET stimulation or overexpression did not induce an invasive phenotype in benign cells. Benign cells do not respond to ETs, and ET expression is not sufficient to induce invasion; however, the level of ET production by tumor cells correlates with their invasiveness, and increasing expression of the ET axis promotes breast tumor cell invasion via both receptors, while MMP-14 is induced via ET-RA.     

Linoleic acid induces an EMT-like process in mammary epithelial cells MCF10A

Epidemiological studies and animal models suggest an association between high levels of dietary fat intake and an increased risk of developing breast cancer. Epithelial-mesenchymal-transition (EMT) is a process, by which epithelial cells are transdifferentiated to a mesenchymal state, and it has been implicated in cancer progression, including invasion and metastasis. Linoleic acid (LA) induces proliferation and invasion in breast cancer cells. However, the role of LA on the EMT process in human mammary epithelial cells remains to be studied. In the present study, we demonstrate that LA induces a transient down-regulation of E-cadherin expression, accompanied with an increase of Snail1, Snail2, Twist1, Twist2 and Sip1 expressions. Furthermore, LA induces FAK and NFκB activation, MMP-2 and -9 secretions, migration and invasion. In summary, our findings demonstrate, for the first time, that LA promotes an EMT-like process in MCF10A human mammary epithelial cells.     

β-Arrestin2 Regulates Lysophosphatidic Acid-Induced Human Breast Tumor Cell Migration and Invasion via Rap1 and IQGAP1

β-arrestins play critical roles in chemotaxis and cytoskeletal reorganization downstream of several receptor types, including G protein-coupled receptors (GPCRs), which are targets for greater than 50% of all pharmaceuticals. Among them, receptors for lysophosphatidic acid (LPA), namely LPA₁ are overexpressed in breast cancer and promote metastatic spread. We have recently reported that β-arrestin2 regulates LPA₁-mediated breast cancer cell migration and invasion, although the underlying molecular mechanisms are not clearly understood. We show here that LPA induces activity of the small G protein, Rap1 in breast cancer cells in a β-arrestin2-dependent manner, but fails to activate Rap1 in non-malignant mammary epithelial cells. We found that Rap1A mRNA levels are higher in human breast tumors compared to healthy patient samples and Rap1A is robustly expressed in human ductal carcinoma in situ and invasive tumors, in contrast to the normal mammary ducts. Rap1A protein expression is also higher in aggressive breast cancer cells (MDA-MB-231 and Hs578t) relative to the weakly invasive MCF-7 cells or non-malignant MCF10A mammary cells. Depletion of Rap1A expression significantly impaired LPA-stimulated migration of breast cancer cells and invasiveness in three-dimensional Matrigel cultures. Furthermore, we found that β-arrestin2 associates with the actin binding protein IQGAP1 in breast cancer cells, and is necessary for the recruitment of IQGAP1 to the leading edge of migratory cells. Depletion of IQGAP1 blocked LPA-stimulated breast cancer cell invasion. Finally, we have identified that LPA enhances the binding of endogenous Rap1A to β-arrestin2, and also stimulates Rap1A and IQGAP1 to associate with LPA₁. Thus our data establish novel roles for Rap1A and IQGAP1 as critical regulators of LPA-induced breast cancer cell migration and invasion.     

Tetraspanin CD9 Promotes the Invasive Phenotype of Human Fibrosarcoma Cells via Upregulation of Matrix Metalloproteinase-9

Tumor cell metastasis, a process which increases the morbidity and mortality of cancer patients, is highly dependent upon matrix metalloproteinase (MMP) production. Small molecule inhibitors of MMPs have proven unsuccessful at reducing tumor cell invasion in vivo. Therefore, finding an alternative approach to regulate MMP is an important endeavor. Tetraspanins, a family of cell surface organizers, play a major role in cell signaling events and have been implicated in regulating metastasis in numerous cancer cell lines. We stably expressed tetraspanin CD9 in an invasive and metastatic human fibrosarcoma cell line (CD9-HT1080) to investigate its role in regulating tumor cell invasiveness. CD9-HT1080 cells displayed a highly invasive phenotype as demonstrated by matrigel invasion assays. Statistically significant increases in MMP-9 production and activity were attributed to CD9 expression and were not due to any changes in other key tetraspanin complex members or MMP regulators. Increased invasion of CD9-HT1080 cells was reversed upon silencing of MMP-9 using a MMP-9 specific siRNA. Furthermore, we determined that the second extracellular loop of CD9 was responsible for the upregulation of MMP-9 production and subsequent cell invasion. We demonstrated for the first time that tetraspanin CD9 controls HT1080 cell invasion via upregulation of an integral member of the MMP family, MMP-9. Collectively, our studies provide mounting evidence that altered expression of CD9 may be a novel approach to regulate tumor cell progression.     

Type I collagen-induced pro-MMP-2 activation is differentially regulated by H-Ras and N-Ras in human breast epithelial cells

Tumor cell invasion and metastasis are often associated with matrix metalloproteinases (MMPs), among which MMP-2 and MMP-9 are of central importance. We previously showed that H-Ras, but not N-Ras, induced invasion of MCF10A human breast epithelial cells in which the enhanced expression of MMP-2 was involved. MMP-2 is produced as a latent pro-MMP-2 (72 kDa) to be activated resulting the 62 kDa active MMP-2. The present study investigated if H-Ras and/or N-Ras induces pro-MMP-2 activation of MCF10A cells when cultured in two-dimensional gel of type I collagen. Type I collagen induced activation of pro-MMP-2 only in H-Ras MCF10A cells but not in N-Ras MCF10A cells. Induction of active MMP-2 by type I collagen was suppressed by blocking integrin alpha2, indicating the involvement of integrin signaling in pro-MMP-2 activation. Membrane-type (MT)1-MMP and tissue inhibitor of metalloproteinase (TIMP)-2 were up-regulated by H-Ras but not by N-Ras in the type I collagen-coated gel, suggesting that H-Ras-specific up-regulation of MT1-MMP and TIMP-2 may lead to the activation of pro-MMP-2. Since acquisition of pro-MMP-2 activation can be associated with increased malignant progression, these results may help understanding the mechanisms for the cell surface matrix-degrading potential which will be crucial to the prognosis and therapy of breast cancer metastasis.     

Actopaxin (α-Parvin) Phosphorylation Is Required for Matrix Degradation and Cancer Cell Invasion

Dysregulation of cell adhesion and motility is known to be an important factor in the development of tumor malignancy. Actopaxin (α-parvin) is a paxillin, integrin-linked kinase, and F-actin binding focal adhesion protein with several serine phosphorylation sites in the amino terminus that contribute to the regulation of cell spreading and migration. Here, phosphorylation of actopaxin is shown to contribute to the regulation of matrix degradation and cell invasion. Osteosarcoma cells stably expressing wild type (WT), nonphosphorylatable (Quint), and phosphomimetic (S4D/S8D) actopaxin demonstrate that actopaxin phosphorylation is necessary for efficient Src and matrix metalloproteinase-driven degradation of extracellular matrix. Rac1 was found to be required for actopaxin-induced matrix degradation whereas inhibition of myosin contractility promoted degradation in the phosphomutant-expressing Quint cells, indicating that a balance of Rho GTPase signaling and regulation of cellular tension are important for the process. Furthermore, actopaxin forms a complex with the Rac1/Cdc42 GEF β-PIX and Rac1/Cdc42 effector PAK1, to regulate actopaxin-dependent matrix degradation. Actopaxin phosphorylation is elevated in the invasive breast cancer cell line MDA-MB-231 compared with normal breast epithelial MCF10A cells. Expression of the nonphosphorylatable Quint actopaxin in MDA-MB-231 cells inhibits cell invasion whereas overexpression of WT actopaxin promotes invasion in MCF10A cells. Taken together, this study demonstrates a new role for actopaxin phosphorylation in matrix degradation and cell invasion via regulation of Rho GTPase signaling.     

PHLDA1 (pleckstrin homology-like domain,family A,member 1) knockdown promotes migration and invasion of MCF10A breast epithelial cells

PHLDA1 (pleckstrin homology-like domain, family A, member 1) is a multifunctional protein that plays distinct roles in several biological processes including cell death and therefore its altered expression has been identified in different types of cancer. Progressively loss of PHLDA1 was found in primary and metastatic melanoma while its overexpression was reported in intestinal and pancreatic tumors. Previous work from our group showed that negative expression of PHLDA1 protein was a strong predictor of poor prognosis for breast cancer disease. However, the function of PHLDA1 in mammary epithelial cells and the tumorigenic process of the breast is unclear. To dissect PHLDA1 role in human breast epithelial cells, we generated a clone of MCF10A cells with stable knockdown of PHLDA1 and performed functional studies. To achieve reduced PHLDA1 expression we used shRNA plasmid transfection and then changes in cell morphology and biological behavior were assessed. We found that PHLDA1 downregulation induced marked morphological alterations in MCF10A cells, such as changes in cell-to-cell adhesion pattern and cytoskeleton reorganization. Regarding cell behavior, MCF10A cells with reduced expression of PHLDA1 showed higher proliferative rate and migration ability in comparison with control cells. We also found that MCF10A cells with PHLDA1 knockdown acquired invasive properties, as evaluated by transwell Matrigel invasion assay and showed enhanced colony-forming ability and irregular growth in low attachment condition. Altogether, our results indicate that PHLDA1 downregulation in MCF10A cells leads to morphological changes and a more aggressive behavior.     

Id-1 induces cell invasiveness in immortalized epithelial cells by regulating cadherin switching and Rho GTPases

Epithelial-mesenchymal transition (EMT), characterized by cadherin switching, contributes to cancer metastasis. Our recent study showed that Id-1 (inhibitor of differentiation-1) promotes metastasis in esophageal cancer cells, but whether the invasive and metastatic dynamics can be induced early in the carcinogenesis process is still unclear. Immortalization is regarded as the initial stage in the malignant transformation of normal cells. In this study, we investigated the role and mechanisms of Id-1 in inducing EMT and cell invasiveness in immortalized esophageal epithelial cells. We found that immortalized epithelial cells expressed higher endogenous levels of Id-1 compared with normal cells. Ectopic Id-1 expression inhibited the differentiation of immortalized esophageal epithelial cells and promoted cadherin switching, which was accompanied by increased adhesiveness to extracellular matrix, cell motility, migratory potential and matrix metalloproteinase-dependent invasiveness. GTPase activity assays showed that over-expression or short-hairpin RNA knockdown of Id-1 led to corresponding changes in Rac1 activity, whereas RhoA activity was significantly decreased with Id-1 depletion. Inhibitors targeting Rac1, RhoA, and Rho kinase suppressed the invasiveness of Id-1-expressing NE2-hTERT cells. Knockdown of N-cadherin in Id-1-over-expressing cells inhibited cell invasiveness and down-regulated RhoA activity. These data suggest that the Id-1-induced invasive potential may be regulated through the N-cadherin-RhoA axis and Rac1 activation.     

Characterization of Spontaneous and TGF-β-Induced Cell Motility of Primary Human Normal and Neoplastic Mammary Cells In Vitro Using Novel Real-Time Technology

The clinical complications derived from metastatic disease are responsible for the majority of all breast cancer related deaths. Since cell migration and invasion are a prerequisite for metastasis their assessment in patient cancer cells in vitro may have prognostic value for the tumor''s metastatic capacity. We employed real-time cell analysis (RTCA) on the xCELLigence DP system to determine in vitro motility of patient-derived primary human breast cancer epithelial cells (HBCEC). Initially, the RTCA assay was validated using established human breast cancer cell lines with either an invasive (MDA-MB-231, MDA-MB-435s) or a non-invasive phenotype (MCF-7, MDA-MB-468), and primary NSCLC cells (Tu459). Previous standard assays of cell migration/invasion revealed that only MDA-MB-231, −435s, and Tu459 cells exhibited spontaneous and TGF-β1-stimulated migration and invasion through a Matrigel barrier. In the present study, the TGF-β1-stimulated activities could be blocked by SB431542, a potent kinase inhibitor of the TGF-β type I receptor ALK5. Application of the RTCA assay to patient-derived tumor cells showed that 4/4 primary HBCEC and primary NSCLC cells, but not normal human mammary epithelial cells (HMEC), displayed high spontaneous migratory and invasive activity which correlated with higher MMP-2 expression and uPA protein levels in HBCEC compared to HMEC. Upon treatment with TGF-β1, HBCEC exhibited morphologic and gene regulatory alterations indicative of epithelial-to-mesenchymal transition. However, exclusively the invasive but not the migratory activity of HBCEC was further enhanced by TGF-β1. This indicates the requirement for molecular, e.g. integrin interactions with Matrigel components in HBCEC in order to become responsive to pro-invasive TGF-β effects. Together, these results show for the first time that tumorigenic HBCEC but not normal HMEC possess a strong basal migratory as well as a basal and TGF-β1-inducible invasive potential. These findings qualify the RTCA assay as an in vitro migration/invasion testing system for patient-specific primary breast cancer cells.