A phospholipase D and protein kinase C inhibitor blocks the spreading of murine mammary adenocarcinoma cells altering f-actin and beta1-integrin point contact distribution

Emerging role of lncRNAs as mechanical signaling molecules in mechanotransduction and their association with Hippo-YAP signaling: a review

Cells within tissues are subject to various mechanical forces, including hydrostatic pressure, shear stress, compression, and tension. These mechanical stimuli can be converted into biochemical signals through mechanoreceptors or cytoskeleton-dependent response processes, shaping the microenvironment and maintaining cellular physiological balance. Several studies have demonstrated the roles of Yes-associated protein (YAP) and its homolog transcriptional coactivator with PDZ-binding motif (TAZ) as mechanotransducers, exerting dynamic influence on cellular phenotypes including differentiation and disease pathogenesis. This regulatory function entails the involvement of the cytoskeleton, nucleoskeleton, integrin, focal adhesions (FAs), and the integration of multiple signaling pathways, including extracellular signal-regulated kinase (ERK), wingless/integrated (WNT), and Hippo signaling. Furthermore, emerging evidence substantiates the implication of long non-coding RNAs (lncRNAs) as mechanosensitive molecules in cellular mechanotransduction. In this review, we discuss the mechanisms through which YAP/TAZ and lncRNAs serve as effectors in responding to mechanical stimuli. Additionally, we summarize and elaborate on the crucial signal molecules involved in mechanotransduction.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

A comprehensive model that explains the regulation of phospholipase D2 activity by phosphorylation-dephosphorylation

We report here that the enzymatic activity of phospholipase D2 (PLD2) is regulated by phosphorylation-dephosphorylation. Phosphatase treatment of PLD2-overexpressing cells showed a biphasic nature of changes in activity that indicated the existence of "activator" and "inhibitory" sites. We identified three kinases capable of phosphorylating PLD2 in vitro-epidermal growth factor receptor (EGFR), JAK3, and Src (with JAK3 reported for the first time in this study)-that phosphorylate an inhibitory, an activator, and an ambivalent (one that can yield either effect) site, respectively. Mass spectrometry analyses indicated the target of each of these kinases as Y(296) for EGFR, Y(415) for JAK3, and Y(511) for Src. The extent to which each site is activated or inhibited depends on the cell type considered. In COS-7, cells that show the highest level of PLD2 activity, the Y(415) is a prominent site, and JAK3 compensates the negative modulation by EGFR on Y(296). In MCF-7, cells that show the lowest level of PLD2 activity, the converse is the case, with Y(296) unable to compensate the positive modulation by Y(415). MTLn3, with medium to low levels of lipase activity, show an intermediate pattern of regulation but closer to MCF-7 than to COS-7 cells. The negative effect of EGFR on the two cancer cell lines MTLn3 and MCF-7 is further proven by RNA silencing experiments that yield COS-7 showing lower PLD2 activity, and MTLn3 and MCF-7 cells showing an elevated activity. MCF-7 is a cancer cell line derived from a low-aggressive/invasive form of breast cancer that has relatively low levels of PLD activity. We propose that PLD2 activity is low in the breast cancer cell line MCF-7 because it is kept downregulated by tyrosyl phosphorylation of Y(296) by EGFR kinase. Thus, phosphorylation of PLD2-Y(296) could be the signal for lowering the level of PLD2 activity in transformed cells with low invasive capabilities.

Design of isoform-selective phospholipase D inhibitors that modulate cancer cell invasiveness

Phospholipase D (PLD) is an essential enzyme responsible for the production of the lipid second messenger phosphatidic acid. Phosphatidic acid participates in both G protein-coupled receptor and receptor tyrosine kinase signal transduction networks. The lack of potent and isoform-selective inhibitors has limited progress in defining the cellular roles of PLD. We used a diversity-oriented synthetic approach and developed a library of PLD inhibitors with considerable pharmacological characterization. Here we report the rigorous evaluation of that library, which contains highly potent inhibitors, including the first isoform-selective PLD inhibitors. Specific members of this series inhibit isoforms with >100-fold selectivity both in vitro and in cells. A subset of inhibitors was shown to block invasiveness in metastatic breast cancer models. These findings demonstrate the power of diversity-oriented synthesis combined with biochemical assays and mass spectrometric lipid profiling of cellular responses to develop the first isoform-selective PLD inhibitors--a new class of antimetastatic agents.

Phospholipase D1 regulates cell migration in a lipase activity-independent manner

Cell migration, a complex biological process, requires dynamic cytoskeletal remodeling. Phospholipase D (PLD) generates phosphatidic acid, a lipid second messenger. Although PLD activity has been proposed to play a role in cytoskeletal rearrangement, the manner in which PLD participates in the rearrangement process remains obscure. In this study, by silencing endogenous PLD isozymes using small interfering RNA in HeLa cells, we demonstrate that endogenous PLD1 is required for the normal organization of the actin cytoskeleton, and, more importantly, for cell motility. PLD1 silencing in HeLa cells resulted in dramatic changes in cellular morphology, including the accumulation of stress fibers, as well as cell elongation and flattening, which appeared to be caused by an increased number of focal adhesions, which ultimately culminated in enhanced cell-substratum interactions. Accordingly, serum-induced cell migration was profoundly inhibited by PLD1-silencing. Moreover, the augmented cell substratum interaction and retarded cell migration induced by PLD1-silencing could be restored by the adding back not only of wild type, but also of lipase-inactive PLD1 into knockdown cells. Taken together, our results strongly suggest that endogenous PLD1 is a critical factor in the organization of the actin-based cytoskeleton, with regard to cell adhesion and migration. These effects of PLD1 appear to operate in a lipase activity-independent manner. We also discuss the regulation of Src family kinases by PLD1, as related to the modulation of Pyk2 and cell migration.

Atypical protein kinase C-zeta modulates clonogenicity, motility, and secretion of proteolytic enzymes in murine mammary cells

In this paper, we investigated whether protein kinase C-zeta (PKC zeta), a member of the atypical PKC family, induces phenotypic alterations associated with malignant transformation and tumor progression in mammary cells. The stable overexpression of PKC zeta in immortalized mammary epithelial cells (NMuMG), activates the mitogenic extracellular signal-regulated kinase (ERK) pathway, enhanced clonal cell growth and exerts profound effects on proteases secretion. The effect on proteases expression seems to be specific for urokinase-type plasminogen activator and metalloproteinase-9 (MMP-9) because no modulation in MMP-2 and MMP-3 production could be detected. In addition, our experiments demonstrated that PKC zeta overexpression markedly altered the adhesive, spreading, and migratory abilities of NMuMG cells. The overexpression of this enzyme was not sufficient to confer an anchorage-independent growth capacity. An extensive mutational analysis of PKC zeta revealed that the effects observed in NMuMG cells were strictly dependent on the kinase (catalytic) domain of the enzyme. Taken together, these results suggest that in mammary cells PKC zeta modulates several of the critical events involved in tumor development and dissemination through the activation of mitogen activated protein kinase (MAPK)/ERK pathway.

Herpes simplex virus thymidine kinase/ganciclovir system in multicellular tumor spheroids

We have developed multicellular spheroids (MCS) established from LM05e and LM3 spontaneous Balb/c-murine mammary adenocarcinoma and B16 C57-murine melanoma derived cell lines as an in vitro model to study the efficacy of the herpes simplex virus thymidine kinase/ganciclovir (HSVtk/GCV) suicide system. We demonstrated for the first time that HSVtk-expressing cells assembled as MCS manifested a GCV resistance phenotype compared to the same cells grown as sparse monolayers. HSVtk-expressing LM05e, LM3 and B16 spheroids were 16-, three- and nine-fold less sensitive to GCV than their respective monolayers, even though they could express transgenes 10-, eight- and five-fold more efficiently. Mixed populations of HSVtk- and their respective beta gal-expressing cells displayed a cell-type specific bystander effect that was higher in monolayers than in MCS. However, HSVtk-expressing cells in two- or three-dimensional cultures were always significantly more sensitive to GCV than the beta gal-expressing counterparts, supporting the feasibility of this suicide approach in vivo. We present evidence showing that HSVtk-expressing tumor cells, when transferred from monolayers to MCS, displayed: (i) lower GCV cytotoxic activity and bystander effect; (ii) higher and efficient expression of genes transferred as lipoplexes; (iii) lower cell proliferation rates; and (iv) changes in intracellular Bax/Bcl-xL rheostat of mitochondria-mediated apoptosis.

Inhibition of invasion and metastasis by glypican-3 in a syngeneic breast cancer model

Glypican-3 (GPC3), a proteoglycan bound to the cell membrane through a GPI anchor, is widely expressed in the embryo but down regulated in most adult tissues, with some exceptions as mammary cells. GPC3 is involved in the regulation of cell proliferation and survival in specific cell types. LM3, a murine mammary tumor cell line unable to express GPC3, was stably transfected with the rat GPC3 gene to analyze its role in tumor progression. Upon injection into syngeneic BALB/c mice LM3-GPC3 clones showed less local invasiveness and developed fewer spontaneous and experimental lung metastasis than controls. GPC3-expressing cells were more sensitive to apoptosis induced by serum depletion, exhibited a delay in the first steps of spreading and were less motile than controls. On the other hand, LM3-GPC3 cells were significantly more adherent to FN than control ones. We observed that GPC3 transfectants presented a higher expression of E-cadherin and beta-catenin, molecules whose down regulation has been associated with tumor progression. Exogenous TGF-beta increased MMP-9 activity in both control and GPC3-expressing cells, but did not modulate MMP-2. Contrarily, GPC3 expression prevented the increase of MMP-2 activity induced by IGF-II. Our results suggest that GPC3 has a protective role against mammary cancer progression.

Effect of bradykinin and PMA on the synthesis of proteoglycans during the cell cycle of endothelial cells in culture

Bradykinin (BK) and phorbol 12-myristate-13-acetate (PMA) were used in the present work to study the biosynthesis of proteoglycans (PG) during the cell cycle of endothelial cells. PMA, an activator of PKC, stimulated the synthesis of heparan sulfate proteoglycan (HSPG) secreted to the medium of endothelial cells mainly during the G(1) phase of the cell cycle [J. Cell. Biochem. 70 (1998) 563]. BK is a vasoactive peptide that increases calcium levels inside the cells indirectly stimulating PKC. Treatment of the endothelial cells with BK, as well as PMA, stimulated the synthesis of HSPG secreted to the medium and produced an antimitogenic effect on the cell cycle. These results led to the conclusion that PKC is directly involved in the synthesis of HSPG secreted to the medium. Also, comparing the effect showed by BK with PMA, one may suggest that different PKC isoforms are involved in these two processes and that their isoforms are mainly Ca(2+) dependent.

Phospholipase D2 stimulates cell protrusion in v-Src-transformed cells

Phospholipase D (PLD) activity has been implicated in several aspects of cell physiology including vesicle transport, signal transduction, cell proliferation, cytoskeletal structure, and oncogenic transformation. Two PLD isoforms (PLD1 and PLD2) have been identified and characterized. We have expressed both wild-type and catalytically inactive forms of PLD1 and PLD2 in 3Y1 rat fibroblasts and in 3Y1 cells transformed by v-Src, a tyrosine kinase that elevates PLD activity. The v-Src-transformed 3Y1 cells have small, but distinct cell protrusions, implicated in cell migration and metastasis. We report here that elevated expression of PLD2 substantially increased the length of the cell protrusions and that a catalytically inactive PLD2 mutant abolished the cell protrusions. The extended protrusions in the PLD2-overexpressing cells were dependent upon microtubule assembly. These data suggest a role for PLD2 in the v-Src-mediated formation of cell protrusions that may be critical for the invasive properties of v-Src-transformed cells.

Involvement of phospholipase D in oxidative stress-induced necrosis of vascular smooth muscle cells

Phospholipase D (PLD) has been associated with necrosis. However, it is not clear whether PLD plays a causative role in this cellular process. We investigated the role of PLD in oxidative stress-induced necrosis of vascular smooth muscle cells (VSMCs). Pervanadate (hydrogen peroxide plus orthovanadate) but not hydrogen peroxide alone activated PLD in a dose- and time-dependent manner. Exposure of VSMCs to pervanadate resulted in necrosis. Pretreatment with butan-1-ol, a PLD inhibitor, attenuated both pervanadate-induced necrosis and increase of intracellular Ca(2+). Removal of extracellular Ca(2+) inhibited pervanadate-induced necrosis by 50%. These results suggest that PLD activation mediates pervanadate-induced necrosis of VSMCs, which is at least partly due to Ca(2+) toxicity.

Modulation of urokinase-type plasminogen activator and metalloproteinase activities in cultured mouse mammary-carcinoma cells: enhancement by paclitaxel and inhibition by nocodazole

Paclitaxel is a potent anti-tumor drug used in the treatment of breast cancer. It induces de-centralization of the microtubular system in tumor cells, blocking cell division. In the search for dissemination to a secondary site, cancer cells are capable of degrading most components of the extracellular matrix via an extracellular proteolytic cascade, including urokinase-type plasminogen activator (uPA) and the matrix metalloproteinases (MMPs). In the present study, the effects of paclitaxel and nocodazole, 2 drugs known to affect microtubules with opposite mechanisms of action, have been tested for their effect on the secretion of uPA and MMPs in cultures of F3II mouse mammary-tumor cells. Tumor-derived uPA activity significantly increased after pre-treatment of tumor cells for 24 hr with micromolar concentrations of paclitaxel (4 microM), while decreasing after pre-treatment with nocodazole (1 microM). A similar modulation was found for MMP-9 by zymographic analysis. Immunofluorescence and Western-blot analysis confirmed the formation of parallel microtubule fragments in paclitaxel-treated cells and almost complete de-polymerization of microtubules in nocodazole-treated ones. Our data suggest that, through opposite actions on microtubule organization and dynamics, paclitaxel and nocodazole exert inverse modulation of tumor-derived proteolytic activity in mammary tumor cells.

Involvement of fibronectin in the regulation of urokinase production and binding in murine mammary tumor cells

Tumor invasion and metastasis development is a multistep process involving adhesion molecules as well as tumor proteases. It has been reported that tumor cells lacking fibronectin (FN) expression and engineered to re-express FN showed a marked reduction in metastatic ability. Besides its effects on cell adhesion and migration, FN could be modulating other cellular events associated with the metastatic cascade. To test this hypothesis, we analyzed the production of urokinase-type plasminogen activator (uPA), and its receptor (uPAR), 2 molecules involved in the invasive phenotype, in cells over-expressing RGD wild-type FN (FNwt clones) or RGD-mutated FN (FN RGD-minus clones). Secreted uPA activity and antigen were significantly up-regulated in FN-expressing clones, although RGD-minus cells secreted approximately 50% less uPA than the FNwt ones. Interestingly, while control and FN RGD-minus clones were able to readily bind uPA to their surface, FNwt clones exhibited impaired uPA binding. Furthermore, treatment of the parental cell line as well as the control and FN-expressing clones with exogenous purified FN or RGD peptides induced up-regulation of uPA production and the reduction of uPA membrane binding, which was associated with lower expression of uPAR. This modulation by FN was found to be dependent on RGD sequence and beta1 integrin. These results strongly suggest a novel activity for the multifunctional glycoprotein FN regarding the regulation of uPA production as well as the capacity of tumor cells to bind uPA.

Cytoplasmic elongation and rupture in megakaryoblastic leukemia cells via activation of adhesion and motility by staurosporine on fibronectin-bound substratum

Human megakaryoblastic leukemia Meg-01 cells were attached to fibronectin (FN)-coated substratum, on which remarkable spreading and cytoplasmic elongation was induced by treatment with a protein kinase inhibitor, staurosporine (stp). This effect was inhibited by RGDS and was also not seen on FN-lacking substratum. The extended cytoplasm had swollen terminals and nodes, which contained GpIIb and beta-thromboglobulin, occasionally included alpha granules, and tended to form particles (2-5 microm) after rupture of the narrowed cytoplasm. Among other protein kinase modulators tested, only K252a promoted the elongation, while calphostin, herbimycin, TPA, and calyculin suppressed it. The cells began to migrate soon after addition of stp, with attachment to the substratum held at some sites during the migration. This tethered movement seemed to cause the cytoplasmic elongation and the rupture into particles. The elongation was retarded by pretreating the cells with cytochalasin A and Clostridium C3 toxin but not with demecolcine. Actin microfilaments in the stp-treated Meg-01 cells accumulated in the filopodia and periphery of the extended cytoplasm, in which vinculin was colocalized as adhesion plaques. The microtubules were longitudinally oriented through the cytoplasmic extension and showed no ring profile in the nodes and particles. Thus, stp in the presence of FN appears to stimulate reorganization of actin-based cytoskeleton and formation of focal contacts in Meg-01 cells. This leads to the activation of cell adhesion and motility, and then cytoplasmic elongation and rupture into particles.

Function and expression of CD44 during spreading, migration, and invasion of murine carcinoma cells

The cell surface glycoprotein CD44 is proposed as a main participant in cell adhesion and migration. We studied the function, expression, and distribution of CD44 in the invasive and metastatic F3II murine carcinoma cell line during adhesion, spreading, migration, and invasion. A mAb anti-CD44 (KM 201) dramatically blocked F3II cell adhesion on both plastic and hyaluronic acid coatings, as well as spreading on uncoated plastic surfaces (P < 0.01). KM201 mAb significantly inhibited F3II cell migration and invasion in Transwell chambers. Immunocytochemistry of spreading cells revealed that CD44 distributed in bands on the cell surface, particularly in the tip of leading edges and in the perinuclear zones of the cell membrane. CD44 antigen was never detected in filopodia or lamellipodia nor in focal adhesion-like structures, but was also detectable as strong interlamellar bands. Fully spread cells showed a decreased CD44 signal compared to cells in early stages of spreading. This decrease correlated with a reduced expression of CD44 as detected by Western blot. We also investigated the signals that may regulate CD44 expression in F3II cells. Treatment of F3II cells, with phorbol myristate acetate (PMA) or phosphatidic acid (PA, the product of PLD-dependent hydrolysis of phosphatidylcholine), significantly enhanced CD44 expression. Conversely, the treatment of F3II cells with H7, a specific PKC inhibitor, or propranolol, which blocks PA conversion to DAG, significantly decreased CD44 expression levels. These results suggest the involvement of PKC and PLD pathways in CD44 expression. These results demonstrate that CD44 plays an important role during F3II cells adhesion, spreading, migration, and invasion. In addition we provide information linking the PLD- and PKC-dependent pathways with the regulation of CD44 expression.

Reduction of mouse mammary tumor formation and metastasis by lovastatin, an inhibitor of the mevalonate pathway of cholesterol synthesis

Lovastatin, a fungal antibiotic used in the treatment of hypercholesterolemia, is an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the key regulatory enzyme in the mevalonate pathway of cholesterol synthesis. We examined the antitumor properties of lovastatin on the F3II sarcomatoid mammary carcinoma, a highly invasive and metastatic murine tumor model. Female BALB/c inbred mice were inoculated subcutaneously with F3II tumor cells and injected i.p. daily with 10 mg/kg body weight of lovastatin or administered p.o. at a level corresponding to the human dosage of 1-2 mg/kg/day. Treatment significantly prolonged tumor latency and reduced tumor formation and metastatic dissemination to the lungs from established mammary tumors. In vitro, antitumor properties of lovastatin were strongly associated with inhibition of tumor cell attachment and migration. These actions were prevented by addition of mevalonate but not by equivalent concentrations of farnesyl pyrophosphate. In accordance, Western blot assays showed that lovastatin effects did not appear to be related to modifications in Ras oncoproteins in our model. The present data indicate that lovastatin could be an antitumor agent with potentially useful clinical applications in breast cancer.