Tightly controlled MRTF-A activity regulates epithelial differentiation during formation of mammary acini.

BACKGROUND: Myocardin-related transcription factors (MRTF) A and B link actin dynamics and mechanotransduction to gene expression. In mice, MRTF-A is involved in mammary gland differentiation, but its role in human mammary epithelial cells remains unclear. METHODS: Three-dimensional cultures of human mammary epithelial MCF10A cells were used to model acinar morphogenesis. Stable MRTF-A knockdown, MRTF-A/B rescue and MRTF-A/B overexpression was established to characterize the functional role during morphogenesis using confocal microscopy and expression analysis. Breast cancer patient databases were analyzed for MRTF-A expression. RESULTS: We showed that a precise temporal control of MRTFs is required for normal morphogenesis of MCF10A mammary acini. MRTF transcriptional activity, but not their protein amounts, is transiently induced during 3D acini formation. MRTF-A knockdown dramatically reduces acini size and prevents lumen formation. These effects are rescued by re-expression of MRTF-A, and partially by MRTF-B. Conversely, overexpression of MRTF-A and MRTF-B increases acini size, resulting in irregular spheroids without lumen and defective apico-basal polarity. These phenotypes correlate with deregulated expression of cell cycle inhibitors p21/Waf1, p27/Kip1 and altered phosphorylation of retinoblastoma protein. In MRTF overexpressing spheroids, proliferation and apoptosis are simultaneously increased at late stages, whilst neither occurs in control acini. MRTFs interfere with anoikis of the inner cells and cause an integrin switch from α6 to α5, repression of E-cadherin and induction of mesenchymal markers vimentin, Snai2 and Zeb1. Moreover, MRTF-overexpressing spheroids are insensitive to alteration in matrix stiffness. In two breast cancer cohorts, high expression of MRTF-A and known target genes was associated with decreased patient survival. CONCLUSION: MRTF-A is required for proliferation and formation of mammary acini from luminal epithelial cells. Conversely, elevated MRTF activity results in pre-malignant spheroid formation due to defective proliferation, polarity loss and epithelial-mesenchymal transition.

Polysialylation of the neural cell adhesion molecule: interfering with polysialylation and migration in neuroblastoma cells.

Polysialic acid represents a unique posttranslational modification of the neural cell adhesion molecule (NCAM). It is built as a homopolymer of up to 150 molecules of alpha 2-8-linked sialic acids on N-glycans of the fifth immunoglobulin-like domain of NCAM. Besides its role in cell migration and axonal growth during development, polysialic acids are closely related to tumor malignancy as they are linked to the malignant potential of several tumors, such as undifferentiated neuroblastoma. Polysialic acid expression is significantly more frequent in high-grade tumors than in low-grade tumors. It is synthesized in the Golgi apparatus by the activity of two closely related enzymes, the polysialyltransferases ST8SiaII and ST8SiaIV. Interestingly, polysialylation of tumors is not equally synthesized by both polysialyltransferases. It has been shown that especially the ST8SiaII gene is not expressed in some normal tissue, but is strongly expressed in tumor tissue. Here we summarize some knowledge on the role of polysialic acid in cell migration and tumor progression and present novel evidence that interfering with polysialylation using unnatural sialic acid precursors decreases the migration of neuroblastoma cells.

Role of phospholipase D2/phosphatidic acid signal transduction in micro- and delta-opioid receptor endocytosis.

We demonstrated recently that opioid-induced activation of phospholipase D2 (PLD2) enhances mu- (MOPr) and delta-opioid receptor endocytosis/recycling and thus reduces the development of opioid receptor desensitization and tolerance. However, the mechanistic basis for the PLD2-mediated induction of opioid receptor endocytosis is currently unknown. Here we show that PLD2-generated phosphatidic acid (PA) might play a key role in facilitating the endocytosis of opioid receptors. However, PLD2-derived PA is known to be further converted to diacylglycerol (DAG) by PA phosphohydrolase (PPAP2). In fact, blocking of PA phosphohydrolase activity by propranolol or PPAP2-short interfering RNA (siRNA) transfection significantly attenuated agonist-induced opioid receptor endocytosis. The primary importance of PA-derived DAG in the induction of opioid receptor endocytosis was further supported by the finding that increasing the DAG level by inhibiting the reconversion of DAG into PA with the DAG kinase inhibitor 3-[2-(4-[bis-(4-fluorophenyl)methylene]-1-piperidinyl)ethyl]-2,3-dihydro-2-thioxo-4(1H)quinazolinone (R59949) or the addition of the synthetic cell-permeable DAG analog 1,2-dioctanoyl-sn-glycerol (DOG), further increased the agonist-induced opioid receptor endocytosis. Moreover, the addition of DOG bypasses the PLD2-siRNA- or PPAP2-siRNA-mediated impairment of DAG synthesis and resulted in a restoration of agonist-induced opioid receptor internalization. Further studies established a functional link between PA-derived DAG and the activation of p38 mitogen-activated protein kinase (MAPK) and the subsequent phosphorylation of the Rab5 effector early endosome antigen 1, which has been demonstrated recently to be required for the induction of MOPr endocytosis. Taken together, our results revealed that the regulation of opioid receptor endocytosis by PLD2 involves the conversion of its product PA to DAG resulting in an activation of the p38 MAPK pathway.

mu-opioid receptor-stimulated synthesis of reactive oxygen species is mediated via phospholipase D2.

We have recently shown that the activation of the rat mu-opioid receptor (MOPr, also termed MOR1) by the mu-agonist [D-Ala(2), Me Phe(4), Glyol(5)]enkephalin (DAMGO) leads to an increase in phospholipase D2 (PLD2) activity and an induction of receptor endocytosis, whereas the agonist morphine which does not induce opioid receptor endocytosis fails to activate PLD2. We report here that MOPr-mediated activation of PLD2 stimulates production of reactive oxygen molecules via NADH/NADPH oxidase. Oxidative stress was measured with the fluorescent probe dichlorodihydrofluorescein diacetate and the role of PLD2 was assessed by the PLD inhibitor D-erythro-sphingosine (sphinganine) and by PLD2-small interfering RNA transfection. To determine whether NADH/NADPH oxidase contributes to opioid-induced production of reactive oxygen species, mu-agonist-stimulated cells were pre-treated with the flavoprotein inhibitor, diphenylene iodonium, or the specific NADPH oxidase inhibitor, apocynin. Our results demonstrate that receptor-internalizing agonists (like DAMGO, beta-endorphin, methadone, piritramide, fentanyl, sufentanil, and etonitazene) strongly induce NADH/NADPH-mediated ROS synthesis via PLD-dependent signaling pathways, whereas agonists that do not induce MOPr endocytosis and PLD2 activation (like morphine, buprenorphine, hydromorphone, and oxycodone) failed to activate ROS synthesis in transfected human embryonic kidney 293 cells. These findings indicate that the agonist-selective PLD2 activation plays a key role in the regulation of NADH/NADPH-mediated ROS formation by opioids.

TCDD mediates inhibition of p53 and activation of ERalpha signaling in MCF-7 cells at moderate hypoxic conditions.

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) is known to promote cancer initiation and progression and accumulates in mammary fat tissue. Effects of TCDD are mediated by the aryl hydrocarbon receptor (AhR). Physiological conditions of moderate hypoxia in breast cancer also activate another transcription factor, hypoxia-inducible factor-1 alpha (HIF-1alpha). In addition, the transcription factors p53 and the estrogen receptor alpha (ERalpha) are important key players in breast cancer progression. Here, human breast cancer cells cultured under mild hypoxic conditions were exposed to TCDD and analyzed for regulation of p53 signaling and ERalpha transactivation. Simultaneous exposure to TCDD and hypoxia resulted in a moderate but reproducible inhibition of p53 expression. Both the direct activation of the ERalpha and the transcriptional regulation of Hdm2 mediated this inhibition. As consequence the p53-mediated target gene expression (Dusp5) was reduced. Silencing of Dusp5 by simultaneous exposure of TCDD and hypoxia or by RNAi led to increased phosphorylation of ERK1/2. This increase resulted in transactivation of ERalpha and induction of ERalpha-mediated transcription of Hdm2 and SOCS3. Specificity of ERalpha-transactivation by ERK1/2 was confirmed by treatment with MAPKK-inhibitor PD98059. The combination of inhibition of functional p53 protein and induction of ERalpha signaling could serve as a model for the operational sequence of TCDD effects to prevent cell death and promote breast tumor progression.

TCDD induces cell migration via NFATc1/ATX-signaling in MCF-7 cells.

Breast cancer is characterized, among others, by the concurrence of lipophilic xenobiotica such as 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD) with hypoxic tissue conditions. This condition activates the transcription factors hypoxia inducible factor-1alpha (HIF-1alpha) and aryl hydrocarbon receptor (AhR) that are known to promote tumor progression. An interrelation between these transcription factors and nuclear factor of activated T-cells (NFAT) was implied by gene array analysis. In the present study, the interplay of the three transcription factors was studied and correlated with the migration of MCF-7 cells in response to TCDD and/or hypoxia. An AhR-activation by 10nM TCDD and HIF-1alpha activation by 5% oxygen induced activation of NFATc1. The effects were inhibited by cyclosporine A (CsA), suggesting that the activation of NFAT by AhR or HIF-1alpha signaling is calcineurin-dependent. The expression/activity of the NFAT target gene autotaxin (ATX) was increased. ATX is known to stimulate migration of tumor cells. The hydrolysis product of ATX, lysophosphatidic acid (LPA), increased the migration of MCF-7 cells under normoxia but not under hypoxia. This effect correlated with increased migration observed after TCDD treatment. Hypoxia did not promote migration of MCF-7 cells, suggesting that ATX down-stream signaling was inhibited by hypoxia. In conclusion, the TCDD-mediated activation of NFATc1 is suggested to promote cell migration via ATX/LPA-signaling.

Adult stem cells and their trans-differentiation potential--perspectives and therapeutic applications.

Stem cells are self-renewing multipotent progenitors with the broadest developmental potential in a given tissue at a given time. Normal stem cells in the adult organism are responsible for renewal and repair of aged or damaged tissue. Adult stem cells are present in virtually all tissues and during most stages of development. In this review, we introduce the reader to the basic information about the field. We describe selected stem cell isolation techniques and stem cell markers for various stem cell populations. These include makers for endothelial progenitor cells (CD146/MCAM/MUC18/S-endo-1, CD34, CD133/prominin, Tie-2, Flk1/KD/VEGFR2), hematopoietic stem cells (CD34, CD117/c-Kit, Sca1), mesenchymal stem cells (CD146/MCAM/MUC18/S-endo-1, STRO-1, Thy-1), neural stem cells (CD133/prominin, nestin, NCAM), mammary stem cells (CD24, CD29, Sca1), and intestinal stem cells (NCAM, CD34, Thy-1, CD117/c-Kit, Flt-3). Separate section provides a concise summary of recent clinical trials involving stem cells directed towards improvement of a damaged myocardium. In the last part of the review, we reflect on the field and on future developments.

The cellular localization of autotaxin impacts on its biological functions in human thyroid carcinoma cells.

Autotaxin (ATX/NPP2) shows a nucleotide pyrophosphatase/phosphodiesterase and lysophospholipase D (lysoPLD) activity and is a member of a family of structurally-related mammalian ecto-nucleotide pyrophosphate/phosphodiesterases (E-NPP1-3). ATX is unique among E-NPP as it is secreted and not membrane-bound as are NPP1 and -3. The ATX gene activity is significantly higher in undifferentiated anaplastic (UTC) as compared to follicular (FTC) and papillary thyroid carcinomas (PTC) or goiter tissues. ATX also enhances the motility of thyroid tumor cells. We bio-engineered stable transfectants of the human thyroid carcinoma cell line FTC-238 expressing either bioactively-secreted (sATX) or membrane-anchored ATX (mATX) to identify the biological functions of ATX which critically depend on the E-NPP member being secreted and provide insight into the effects of high local ATX concentrations and cellular responses. An increased cell motility was exclusively observed with FTC-238 sATX transfectants, whereas membrane-anchored ATX appeared to impair motility. We identified IL-1beta as an upstream suppressor of ATX expression in FTC-238, ATX-mediated motility in FTC-238 and stable transfectants, with IL-1beta having the strongest motility-suppressive effect on FTC-238 sATX clones. sATX and mATX strongly increased the anchorage-independent colony formation of FTC-238 but the size and number of colonies formed in the soft agar were significantly smaller in FTC-238 mATX versus the FTC-238 sATX clones. The cancer-testis antigen BAGE was identified as a novel target gene of ATX in FTC-238. Transcript levels for BAGE were 6-fold higher in FTC-238 mATX versus sATX clones. Increased BAGE transcript levels were also detected in tissues of patients with UTC versus FTC, PTC or goiter tissues. In summary, enhanced tumor cell motility and tumorigenic capacity critically depended on sATX in thyroid carcinoma cells. Irrespective of its compartmentalization, the cancer-testis antigen BAGE was identified as a novel target gene of ATX in FTC-238 and a potential new tissue marker in UTC tissues, which we had previously shown to express high levels of ATX.

Cancer stem cells as targets for cancer therapy: selected cancers as examples.

It is becoming increasingly evident that cancer constitutes a group of diseases involving altered stem-cell maturation/differentiation and the disturbance of regenerative processes. The observed malignant transformation is merely a symptom of normal differentiation processes gone astray rather than the primary event. This review focuses on the role of cancer stem cells (CSCs) in three common but also relatively under-investigated cancers: head and neck, ovarian, and testicular cancer. For didactic purpose, the physiology of stem cells is first introduced using hematopoietic and mesenchymal stem cells as examples. This is followed by a discussion of the (possible) role of CSCs in head and neck, ovarian, and testicular cancer. Aside from basic information about the pathophysiology of these cancers, current research results focused on the discovery of molecular markers specific to these cancers are also discussed. The last part of the review is largely dedicated to signaling pathways active within various normal and CSC types (e.g. Nanog, Nestin, Notch1, Notch2, Oct3 and 4, Wnt). Different elements of these pathways are also discussed in the context of therapeutic opportunities for the development of targeted therapies aimed at CSCs. Finally, alternative targeted anticancer therapies arising from recently identified molecules with cancer-(semi-)selective capabilities (e.g. apoptin, Brevinin-2R) are considered.

Significance of prolyl hydroxylase 2 in the interference of aryl hydrocarbon receptor and hypoxia-inducible factor-1 alpha signaling.

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) and the aryl hydrocarbon receptor (AhR) work as environmental sensors in human tissues. These proteins are members of the helix-loop-helix/Per-ARNT-SIM transcription factor family and form heterodimers with the aryl hydrocarbon receptor nuclear translocator. HIF-1 alpha can be activated by low oxygen concentrations and hypoxia-inducing agents. The AhR is activated by xenobiotica such as dioxins. Here, we analyze the interference between the AhR signaling, activated by 10 nM 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD), and the HIF-1 alpha pathway, induced by hypoxia (5% O2), in two human cell lines, the breast carcinoma cell line MCF-7 and the hepatocyte cell line HepG2. In both cell lines, treatment with TCDD and hypoxia clearly reduced the stabilization of HIF-1 alpha and HRE-mediated promoter activity when compared to the induction under hypoxia alone. Because these effects were not observed after alpha-naphthoflavone treatment and HIF-1 alpha mRNA was not down-regulated, HIF-1 alpha stabilization was revealed to be the target by TCDD in an AhR-depended mechanism. Under exposure to TCDD or hypoxia, the main regulator of HIF-1 alpha stability, the prolyl hydroxylase domain containing protein 2 (PHD2) showed an increase in promoter activity, transcript numbers, and protein amount. Therefore, PHD2 expression is regulated in an AhR-dependent manner under normoxia. The AhR-dependent regulation of PHD2 under normoxia, however, is overwritten by the TCDD-mediated destabilization of HIF-1 alpha. The destabilization of HIF-1 alpha is the dominant effect causing the reduced PHD2 expression after simultaneous exposure to TCDD and hypoxia. We conclude that PHD2 does not mediate the TCDD-mediated HIF-1 alpha destabilization and does not control the interference of AhR and HIF-1 alpha pathways.

Expression, regulation and function of autotaxin in thyroid carcinomas.

Autotaxin (ATX/NPP2) is a tumor cell motility-stimulating factor that displays both a nucleotide pyrophosphatase/phosphodiesterase activity and a recently described lysophospholipase D (lysoPLD) activity. The precise function of ATX in tumor cells and the role of ATX in thyroid carcinoma remains unclear. We have quantified ATX mRNA expression in thyroid carcinoma cell lines and in tissues of patients with thyroid carcinomas. ATX gene activity was significantly higher in undifferentiated anaplastic thyroid carcinoma cell lines (UTC) and tumor tissues as compared to follicular thyroid carcinoma (FTC) cell lines, FTC tissues or goiter tissues that were used as a control. In the thyroid carcinoma cell line 1736, EGF and bFGF stimulated ATX mRNA expression, whereas the cytokines IL-4, IL-1beta and TGF-beta reduced ATX transcriptional levels. FTC-133 cells, stably transfected with an expression vector for ATX, showed a higher lysoPLD activity, a higher proliferation rate and an increased migratory behavior. In addition, ATX also displayed a paracrine stimulatory effect on the motility of different thyroid carcinoma cell lines. Overexpression of ATX in the stably transfected FTC-133 resulted in down-regulation of CD54/ intercellular adhesion molecule-1 (ICAM-1) gene expression and augmented gene activity of the pro-angiogenic chemokine IL-8. We conclude that ATX may be regarded as a new tissue marker for undifferentiated human thyroid carcinoma cells. ATX increases the proliferation and migration of thyroid carcinoma cell lines and may also affect the angiogenic potential of thyroid carcinoma cells. Further studies are needed to provide insight into the role of ATX in the normal and neoplastic thyroid gland.