MYCN oncogene and angiogenesis: down-regulation of endothelial growth inhibitors in human neuroblastoma cells. Purification, structural, and functional characterization

Angiogenesis, the formation of new blood vessels, is seen during embryonic development and tumor progression, but the mechanisms have remained unclear. Recent data indicate that tumor angiogenesis can be induced by cellular oncogenes, leading to the enhanced activity of molecules stimulating angiogenesis. However, activated oncogenes might also facilitate angiogenesis by down-regulating endogenous inhibitors of angiogenesis. We report here that enhanced expression of the N-myc oncogene in human neuroblastoma cells down-regulates three inhibitors of endothelial cell proliferation. One of them was identified by amino acid sequencing as being identical with activin A, a developmentally-regulated protein. Down-regulation involves interaction of the N-myc protein with the activin A promoter. Work is ongoing to characterize the other two endothelial cell inhibitors. We suggest that the N-myc induced down-regulation of angiogenesis inhibitors could contribute to tumor angiogenesis.

A comparative cell-based high throughput screening strategy for the discovery of selective tyrosine kinase inhibitors with anticancer activity

Growth factor receptor tyrosine kinases (RTK) have been implicated in tumor growth, metastasis and angiogenesis, and are thus considered promising targets for therapeutic intervention in malignant diseases. We present a novel drug discovery strategy to find inhibitors of RTKs based on comparative screening of compound libraries employing functional cellular assays. Cell lines stably expressing HER2 and the receptors for hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), insulin-like growth factor-I (IGF-I) and epidermal growth factor (EGF) have been established. All cell lines are based on FDC-P1, a murine myeloid progenitor cell line which allows a direct comparison of results obtained in primary screens. In addition, the same cell lines are suitable for compound optimization and for animal studies. Using this strategy we report the identification of promising lead candidates for further drug development which are highly selective, non-cytotoxic and cell permeable with potencies in the low micromolar range.

Antigen presentation by astrocytes primes rat T lymphocytes for apoptotic cell death. A model for T-cell apoptosis in vivo

In this study we have characterized apoptotic cell death of autoreactive T cells resulting from their interaction with astrocytes and the modulatory effect of steroid hormones. Time kinetics of T-cell activation by interferon (IFN)-gamma-treated astrocytes from neonatal Lewis rats and by professional antigen presenting cells (APCs) from bulk suspensions of thymus or spleen were performed. [3H]Thymidine incorporation of neuritogenic P2- and encephalitogenic myelin basic protein (MBP)-specific T-cell lines declined after 48 h in culture with astrocytes. A similar suppressive effect was observed when T cells were cocultured with thymic APCs and astrocytes. This effect disappeared when astrocytes were separated by a transwell system. After 72 h of culture with astrocytes a mean of 17.5 +/- 12.4% T cells exhibited morphological signs of apoptosis. Apoptosis was identified by light microscopy, and confirmed by electron microscopy, by in situ tailing reaction and by agarose gel electrophoresis. Glucocorticosteroids and oestrogen specifically enhanced T-cell apoptosis within 8 h (69.8 +/- 23.1% and 69 +/- 17.1%, respectively) when added after 72 h to the astrocyte system, but not at earlier time points of T-cell activation or when thymic APCs were used. Glucocorticoid-mediated T-cell apoptosis was inhibited by the steroid-receptor antagonist RU 486. Pregnenolone, lipocortin-1, indomethacin and transforming growth factor-beta did not induce apoptosis in this system. The steroid effect was not associated with CD28, IL-2 receptor, or transferrin-receptor expression, which were equally upregulated on T cells activated by astrocytes or thymic APC as shown by fluorescence activated cell sorting (FACS) analysis. We conclude that astrocytes as CNS-specific APC may render T cells susceptible for induction of apoptotic cell death. Some steroid hormones can markedly enhance this process in vitro and may augment an additional effect of a systemic corticosteroid response in vivo during recovery from autoimmune encephalomyelitis.

Transplantation of an oligodendrocyte cell line leading to extensive myelination

Oligodendrocytes, the myelin-forming cells of the central nervous system, can be generated from progenitor cell lines and assayed for their myelinating properties after transplantation. A growth-factor-dependent cell line of rat oligodendrocyte progenitors (CG4) was carried through 31-48 passages before being transplanted into normal newborn rat brain or the spinal cord of newborn myelin-deficient (md) rats. In md rat spinal cord, CG4 oligodendrocyte progenitors migrated up to 7 mm along the dorsal columns, where they divided and myelinated numerous axons 2 weeks after grafting. CG4 cells were transfected with the bacterial lacZ gene and selected for high beta-galactosidase expression. The cell migration and fate of these LacZ+ cells were analyzed after transplantation. In normal newborn brain, LacZ+ oligodendrocyte progenitors migrated along axonal tracts from the site of injection and integrated in the forming white matter. In md rats, extensive migration (up to 12 mm) was revealed by staining for beta-galactosidase activity of the intact spinal cord where many grafted cells had moved into the posterior columns. Similar migration and integration of grafted cells occurred in the spinal cord of normal myelinated rats and after a noninvasive grafting procedure. Thus, oligodendrocyte progenitors can maintain their ability to migrate and myelinate axons in vivo after multiple passages in vitro. Such progenitor cell lines can be used to study the molecular mechanisms underlying oligodendrocyte development and the repair of myelin in dysmyelinating diseases.

Intercellular regulation of major histocompatibility complex class I expression in neural cells

We have studied the effect of rat central nervous system (CNS) neurons on the inducibility of major histocompatibility complex (MHC) class I molecules on syngeneic astrocytes. In a co-culture system composed of embryonic rat cortical neurons and neonatal astrocytes, intact neurons decreased constitutive expression of MHC class I determinants and inhibited the induction of class I products on astrocytes. Viability of the neurons and direct contact with astrocytes was critical for this effect. Soluble factors released by neurons were inefficient. Our data indicate that the lack of MHC class I expression on astrocytes in situ might be the result of an active suppression mechanism rather than merely due to the absence of activating factors in the CNS.

Interleukin-6 production in "normal" and HTLV-1 tax-expressing brain-specific endothelial cells

Abnormal cytokine production can contribute in many instances to the development of pathology. Our study focuses on the regulation of interleukin (IL)-6 production in vitro in brain-specific endothelial cells (BEC) under physiological conditions and in a model of human T leukemia virus-1 (HTLV-1) infection. IL-6 production was strongly up-regulated in a dose-dependent mode upon exposure to recombinant IL-1 beta, although nearly not detectable in unstimulated BEC. This induction of IL-6 production could be achieved by reagents known to increase intracellular levels of cAMP, such as forskolin, prostaglandin E or pentoxifylline. Furthermore, transcription and production of IL-6 was inducible by addition of dibutyryl cAMP, but not by addition of calcium ionophores or diacylglycerol. To assess a potential role of HTLV-1-infected BEC in the pathogenesis of tropical spastic paraparesis (TSP), the HTLV-1 tax gene was expressed in BEC. Tax gene-expressing BEC produced constitutively very high amounts of IL-6, which were not longer hyperinducible by IL-1 beta or cAMP derivatives. Our results indicate that HTLV-1 tax induces hyperproduction of IL-6 in brain-specific endothelial cells directly by an intracellular mechanism which subsequently renders IL-6 production independent of exogenous stimuli or activators of (cAMP-dependent) second messenger levels. On the basis of these findings we suggest that tax-mediated hyperactivation of IL-6 production in BEC contributes to elevated IL-6 levels found in serum and cerebrospinal fluid of patients with TSP and might have a significance in the immune pathogenesis of the disease.

Cortical neurons selectively inhibit MHC class II induction in astrocytes but not in microglial cells

Astrocytes have been shown to act as potent accessory cells for MHC class II-restricted T cell responses in vitro after treatment with interferon-gamma. In contrast, even under conditions of severe central nervous system (CNS) inflammation, they seem to express little, if any, class II molecules in vivo. Thus the role of astroglial cells as accessory cells in immune responses in the CNS remains to be determined. We have studied neuron--glia interactions with respect to induction of MHC class II molecules. Surprisingly, in a co-culture system, viable neurons inhibited the induction of class II restriction elements on astrocytes. This effect was only observed when neurons had contact to astrocytes; neuron derived soluble factors alone were insufficient. Most interestingly, the suppressive effect of neurons on class II inducibility operated specifically on astrocytes, while microglial cells were left unaffected.

Dissociation of antigen-presenting capacity of astrocytes for peptide-antigens versus superantigens

Although superantigens and their molecular interactions with MHC class II molecules have been well characterized recently, little is known concerning the physiological function of different types of APC in inducing superantigen-mediated T cell activation. To evaluate the potential of nonhematopoetic cells to present superantigens to T cells, we have tested astrocytes as a typical "nonprofessional" APC. Although astrocytes can express appropriate levels of MHC class II products and adhesion molecules, they turned out to be unable to mediate superantigen-driven activation of normal T lymphocytes, even in the presence of rather high concentrations of toxins. In contrast, they could properly present equimolar amounts of nominal Ag to various Ag-specific T cell lines under the same experimental conditions. Inability of astrocytes to support T cell responses to superantigens could not be overcome by addition of cytokines IL-1 and IL-6. Binding studies with class II-expressing astrocytes revealed that T cell unresponsiveness was not due to a general failure of astrocytes to bind the superantigen. Moreover, the resulting SA-class II complex was recognizable by TCR, as demonstrated by the capacity to activate IL-2 secretion in T cell hybridomas. Our results extend previous studies demonstrating marked differences of various types of APC to trigger T cell responses to superantigens and describe for the first time a dissociation of the Ag-presenting capacity for peptide-Ag vs superantigen on an accessory cell.

Dissociation of antigen-presenting capacity of astrocytes for peptide-antigens versus superantigens

Although superantigens and their molecular interactions with MHC class II molecules have been well characterized recently, little is known concerning the physiological function of different types of APC in inducing superantigen-mediated T cell activation. To evaluate the potential of nonhematopoetic cells to present superantigens to T cells, we have tested astrocytes as a typical "nonprofessional" APC. Although astrocytes can express appropriate levels of MHC class II products and adhesion molecules, they turned out to be unable to mediate superantigen-driven activation of normal T lymphocytes, even in the presence of rather high concentrations of toxins. In contrast, they could properly present equimolar amounts of nominal Ag to various Ag-specific T cell lines under the same experimental conditions. Inability of astrocytes to support T cell responses to superantigens could not be overcome by addition of cytokines IL-1 and IL-6. Binding studies with class II-expressing astrocytes revealed that T cell unresponsiveness was not due to a general failure of astrocytes to bind the superantigen. Moreover, the resulting SA-class II complex was recognizable by TCR, as demonstrated by the capacity to activate IL-2 secretion in T cell hybridomas. Our results extend previous studies demonstrating marked differences of various types of APC to trigger T cell responses to superantigens and describe for the first time a dissociation of the Ag-presenting capacity for peptide-Ag vs superantigen on an accessory cell.

Two cloned cerebral endothelial cell phenotypes: an in vitro model for angiogenesis?

Expression of smooth muscle alpha-actin and migratory behaviour of cloned cerebral endothelial cells (cEC) which exhibited two distinct phenotypes (type I, type II) were studied. Removal of mitogenic factors (alpha ECGF, ECGS) and heparin from the culture medium resulted in a smooth muscle-like appearance (type II) of the cells, expression of smooth muscle alpha-actin protein and smooth muscle actin mRNA and in an increased migratory activity. In contrast, addition of growth factors and heparin led to a cobblestone-like phenotype (type I) which lacked the expression of smooth muscle alpha-actin but expressed other proteins as determined by 2-D-gel electrophoresis.

Reversible expression of sm alpha-actin protein and sm alpha-actin mRNA in cloned cerebral endothelial cells

The expression of smooth muscle (sm) alpha-actin was studied in cloned capillary cerebral endothelial cells of two phenotypes. Type I cells were cultured in medium containing 10% FCS, heparin and ECGS (or alpha-ECGF) and stained positive for a specific endothelial cell marker (Bandeiraea simplicifolia). Depletion of heparin and ECGS resulted in a smooth muscle-like appearance after 2-3 days. Cells of this phenotype, (type II) stained positive for the endothelial cell marker and for sm alpha-actin. In contrast to type I cells, type II cells expressed sm alpha-actin protein and mRNA as evidenced by Immunoblots and Northern blots. This phenotypic switch was shown to be reversible and so was the expression of sm alpha-actin.

Glial cells and neurons induce blood-brain barrier related enzymes in cultured cerebral endothelial cells

The blood-brain barrier (BBB) in mammals is created and maintained by cerebral endothelial cells (cEC) that express specialized functional properties, including intercellular tight junctions, absence of fenestrae and specific membrane transport systems. It has been proposed that the differentiation of these characteristics, acquired during brain development, is controlled by the neural environment. Co-culture experiments of cloned cEC with astroglial cells, C6 glioma cells and cortical neurons, with plasma membranes or conditioned media of these cells, were used to study induction of some BBB characteristics in vitro. Activities of Na+,K(+)-ATPase and gamma-glutamyl transpeptidase (GGTP), an enzyme responsible for amino acid transport across the BBB, were taken as parameters for BBB function. Co-culture of cEC with C6 glioma cells caused a two-fold increase in GGTP activity and this activity was likewise amplified by incubation with plasma membrane fractions derived from C6 glioma cells, embryonic brain cells and cortical neurons; conditioned media (soluble factors) had no effect. Na+,K(+)-ATPase activity, estimated from the ouabain inhibitable fraction of 86Rb uptake, was increased by about 90% in cEC incubated with C6 glioma plasma membranes. We propose from these data that both neurons and glial cells confer BBB characteristics on cEC via cell-cell contact.

Gamma-glutamyl-transpeptidase (GGTP) and NA+K(+)-ATPase activities in different subpopulations of cloned cerebral endothelial cells: responses to glial stimulation

Glial stimulation of Na+K(+)-ATPase and gamma-glutamyl-transpeptidase was taken as parameter for blood brain barrier function in cloned cerebral endothelial cells of different phenotypes. In type I cells ("cobblestone" phenotypus) gamma-glutamyl-transpeptidase activity increased 10-12 fold and Na+K(+)-ATPase activity was 2-fold increased after glial stimulation. In type II cells ("spindle-form" phenotype) gamma-glutamyl-transpeptidase was only 2-fold increased, whereas Na+K(+)-ATPase was even depressed. K(+)-(86Rb) uptake was twice as high in type I cells. These data indicate that type I cells are involved in blood brain barrier function.

Glial-conditioned medium and attachment to ConA are essential for long-term culture of cortical neurons

Cortical brain cells from 14-day-old mouse embryos were seeded on various substrates and cultivated in serum-free medium with or without conditioned medium from astrocytes or C6 glioma cells. Poly-L-lysine was shown to be the best substrate for cell attachment followed by Concanavalin A (ConA) and adhesion particles derived from glia cells. Cells grown on ConA sprouted rapidly and formed large networks. Survival of neurons was greatly prolonged when glia-conditioned medium (GCM) was present in the culture medium. Cells grown on ConA were then viable for more than 4 weeks. Without GCM, neurons survived in culture for about 2 weeks, regardless of the substrate. Endothelial cell growth supplement or acidic fibroblast growth factor increased survival of neurons but also stimulated proliferation of astrocytes.

Isolation, characterization, and long-term cultivation of porcine and murine cerebral capillary endothelial cells

We present a simple method for isolation and long-term cultivation of porcine and murine cerebral capillary endothelial cells (cEC). Two major points are made. First, that the "characteristic" morphology of the endothelial cells depends mainly on the presence of endothelial cell growth factors in the culture medium and second, that the identification of the cells as endothelial cells requires a special lectin instead of criteria used for large vessel endothelial cells, such as factor VIII staining or LDL uptake. Pure cerebral capillaries were isolated by means of a series of centrifugation steps; endothelial cells were released by collagenase treatment and cultivated on plastic petri dishes, which proved to be better for cell attachment than collagen or gelatin coating. The microvascular cells were cultivated in either the presence or absence of growth factors. Medium 199 + 10% FCS produced mainly spindle-shaped cells, growing in the "hills and valleys" pattern, which, if not passaged for weeks, showed three dimensional tubular structures. Cells of the "cobblestone" phenotype were promoted in medium 199 + 10% FCS, enriched with endothelial cell growth supplement (ECGS) and heparin (referred to as complete medium). These cells retained their phenotype for months and could be passaged up to 35 times till now. If ECGS and heparin were omitted from these cultures, the cells became elongated and resembled smooth muscle cells. This effect was reversible when the cells were transferred to complete medium. With cEC, cloned by limiting dilution, we noticed this reversal phenomenon as well. We used several markers to characterize the microvascular cells and could show that the lectin of Bandeiraea simplicifolia is a highly reliable marker for endothelial cells and that the monoclonal antibody alpha-sm-1 (anti-smooth muscle cell actin) is excellent for determining smooth muscle cells.