Products of cells from gliomas: IX. Evidence that two fundamentally different mechanisms change extracellular matrix expression by gliomas

Gliosarcoma stem cells undergo glial and mesenchymal differentiation in vivo

Cancer stem cells (CSCs) are characterized by their self-renewing potential and by their ability to differentiate and phenocopy the original tumor in orthotopic xenografts. Long-term propagation of glioblastoma (GBM) cells in serum-containing medium results in loss of the CSCs and outgrowth of cells genetically and biologically divergent from the parental tumors. In contrast, the use of a neurosphere assay, a serum-free culture for selection, and propagation of central nervous system-derived stem cells allows the selection of a subpopulation containing CSCs. Gliosarcoma (GS), a morphological variant comprising approximately 2% of GBMs, present a biphasic growth pattern, composed of glial and metaplastic mesenchymal components. To assess whether the neurosphere assay would allow the amplification of a subpopulation of cells with "gliosarcoma stem cell" properties, capable of propagating both components of this malignancy, we have generated neurospheres and serum cultures from primary GS and GBM surgical specimens. Neurosphere cultures from GBM and GS samples expressed neural stem cell markers Sox2, Musashi1, and Nestin. In contrast to the GBM neurosphere lines, the GS neurospheres were negative for the stem cell marker CD133. All neurosphere lines generated high-grade invasive orthotopic tumor xenografts, with histological features strikingly similar to the parental tumors, demonstrating that these cultures indeed are enriched in CSCs. Remarkably, low-passage GS serum cultures retained the expression of stem cell markers, the ability to form neurospheres, and tumorigenicity. The GS experimental tumors phenocopied the parental tumor, exhibiting biphasic glial and mesenchymal components, constituting a clinically relevant model to investigate mesenchymal differentiation in GBMs.

Endogenous tenascin-C enhances glioblastoma invasion with reactive change of surrounding brain tissue

Tenascin-C is an extracellular matrix glycoprotein implicated in embryogenesis, wound healing and tumor progression. We previously revealed that tenascin-C expression is correlated with the prognosis of patients with glioblastoma. However, the exact role of endogenous tenascin-C in regulation of glioblastoma proliferation and invasion remains to be established. We show here that endogenous tenascin-C facilitates glioblastoma invasion, followed by reactive change of the surrounding brain tissue. Although shRNA-mediated knockdown of endogenous tenascin-C does not affect proliferation of glioblastoma cells, it abolishes cell migration on a two-dimensional substrate and tumor invasion with brain tissue changes in a xenograft model. The tyrosine phosphorylation of focal adhesion kinase, a cytoplasmic tyrosine kinase that associates with integrins, was decreased in tenascin-C-knockdown cells. In the analysis of clinical samples, tenascin-C expression correlates with the volume of peritumoral reactive change detected by magnetic resonance imaging. Interestingly, glioblastoma cells with high tenascin-C expression infiltrate brain tissue in an autocrine manner. Our results suggest that endogenous tenascin-C contributes the invasive nature of glioblastoma and the compositional change of brain tissue, which renders tenascin-C as a prime candidate for anti-invasion therapy for glioblastoma.

Primary glioblastomas express mesenchymal stem-like properties

Glioblastoma is the most common and aggressive primary brain cancer. Recent isolation and characterization of brain tumor-initiating cells supports the concept that transformed neural stem cells may seed glioblastoma. We previously identified a wide array of mesenchymal tissue transcripts overexpressed in a broad set of primary glioblastoma (de novo) tumors but not in secondary glioblastoma (derived from lower-grade) tumors, low-grade astrocytomas, or normal brain tissues. Here, we extend this observation and show that a subset of primary glioblastoma tumors and their derived tumor lines express cellular and molecular markers that are associated with mesenchymal stem cells (MSC) and that glioblastoma cell cultures can be induced to differentiate into multiple mesenchymal lineage-like cell types. These findings suggest either that a subset of primary glioblastomas derive from transformed stem cells containing MSC-like properties and retain partial phenotypic aspects of a MSC nature in tumors or that glioblastomas activate a series of genes that result in mesenchymal properties of the cancer cells to effect sustained tumor growth and malignant progression.

Mechanisms of glioma cell invasion

Invasive growth is one of the characteristics of gliomas--local infiltration into the surrounding nerve tissue decisively restricts all treatment strategies. Particularly the merit of all local treatment modalities is queried. The question whether a glioma represents a diffuse disease of the CNS or a local disturbance with unrestrained expansion tendency is still at issue. Understanding of the invasion mechanisms is of importance inasmuch as biologically reasonable and effective strategies of limiting and suppressing glioma invasion can only hence be derived. The affinity of glioma cells towards certain structures of the extracellular matrix as well as taking advantage of tumour vascularisation with regard to extension play a decisive role. Still not fully understood are tumour host interactions. Future thinking will have to take into account these interactions as well as evidence to be derived from development neurobiology and regeneration capacity of the CNS. The present review is meant to give a short overview and disclose many questions.

Expression of different extracellular matrix components in human brain tumor and melanoma cells in respect to variant culture conditions

Local tumor invasion into the surrounding brain tissue is a major characteristic of malignant gliomas. These processes critically depend on the interaction of tumor cells with various extracellular matrix (ECM) components. Because only little quantitative information about expression of ECM gene products in general and expression in response to alterations of the surrounding environment is available, the present study was designed. Four human glioblastoma cell lines (U373MG, U138MG, U251MG, GaMG) as well as four human melanoma cell lines (MV3, BLM, 530, IF6) were tested with semiquantitative RT-PCR for their ability to express mRNA of different human ECM components (fibronectin, decorin, tenascin, collagen I, collagen IV, versican). In addition, two human medulloblastoma (MHH-Med 1, MHH-Med 4) and two fibrosarcoma (HT1080, U2OS) cell lines were analyzed. Cells which were grown in DMEM medium containing 10% FCS expressed most of the analyzed protein components. When the same medium, but depleted of ECM proteins by filtrating through a membrane with cut-off at > 100 kD was used, basal mRNA expression of the ECM proteins was changed in most of the examined cell lines. Using serum free conditions, most of the cell lines again showed a variation in the expression pattern of mRNA encoding for the different ECM proteins compared to the other medium conditions. Comparing different cell lines from one tumor entity or different tumor groups, ECM expression was heterogeneous with regard to the different tumor entities as well as within the entities themselves. Migration assays revealed heterogeneous responses between the different cell lines, ECM components and culture conditions, making it difficult to correlate ECM expression patterns and migratory behavior. Our results revealed that all examined cell lines are able to produce ECM proteins in vitro. This suggests that tumor cells can modulate their microenvironment in vitro which has to be taken into consideration for studies related to migration and invasion.

Astrocytoma cell interaction with elastin substrates: implications for astrocytoma invasive potential

Elastin has been identified within the meninges and the microvasculature of the normal human brain. However, the role that elastin plays in either facilitating astrocytoma cell attachment to these structures or modulating astrocytoma invasion has not been previously characterized. We have recently shown that astrocytoma cell lines and specimens produce tropoelastin, and express the 67 kDa elastin binding protein (EBP). In the present report, we have established that astrocytoma cells attach to elastin as a substrate in vitro. The U87 MG astrocytoma cell line demonstrated the greatest degree of adhesion. In addition, all astrocytoma cell lines examined were capable of penetrating and migrating through an intact elastin membrane, and of degrading tritiated-elastin, a process that could be prevented by the pre-incubation of astrocytoma cells with EDTA, but not with alpha1-antitrypsin. Astrocytoma cells were also capable of penetrating 1 mm sections of human brain tissue maintained as organotypic cultures. Interestingly, the invasive potential of cultured astrocytoma cells plated on organotypic cultures of human brain was significantly increased after exposure to elastin degradation products (kappa-elastin), which interact with astrocytoma cell surface EBP. Our data show that astrocytoma cells express a functional 67 kDa EBP, enabling them to potentially recognize and attach to elastin as a substrate. These data also suggest that this elastin receptor may be involved in processes which regulate regional astrocytoma invasion.

ECM-mediated glioma cell invasion

Cell adhesion receptors of the integrin superfamily, CD44, and adhesion receptors of the immunoglobulin superfamily are expressed by high-grade astrocytic tumors of the central nervous system. These receptors are critical for the invasion of these tumors in the nervous system. Glioma cells utilize these receptors to adhere to and migrate along the components of the extracellular matrix, which is uniquely distributed and regulated within the brain and the spinal cord. For this reason, glioma cell invasion into the adjacent brain tissue is dependent on the interaction of glioma cells with the extracellular matrix. The receptor-ECM component interaction is discussed, focusing on the role of cell adhesion molecules of the integrin family and CD44 in glioma cell adhesion and invasion.

Insights about brain tumors gained through immunohistochemistry and in situ hybridization of nuclear and phenotypic markers

Immunohistochemistry (IHC) has provided major insights about the classification of brain tumors by identifying cellular markers of phenotype and about tumor growth potential with nuclear markers of proliferation. In situ hybridization (ISH) research shows promise for diagnostic applications in tumor classification. The avidin-biotin conjugate IHC procedure is highlighted for diagnostic use on routinely processed clinical specimens. The immunophenotypes of brain tumors are tabulated in reference to their common IHC markers. Tumors that have been correctly classified by their IHC phenotypes include the giant-cell glioblastoma, primary brain lymphoma, and central neurocytoma. Phenotypes that may be more definitively detected by ISH, such as pituitary hormone, immunoglobulin light chain, and collagen messages are described. IHC of nuclear proliferation markers correlates with grade of malignancy, predicts tumor growth potential, and is prognostic for patient survival. The incorporation of bromodeoxyuridine, the expression of proliferating cell nuclear antigen, and the expression of Ki-67 antigen detected by MIB-1 antibody are compared in regard to their cell cycle activity and labeling index determinations. Fluorescence in situ hybridization (FISH) of brain tumor interphase nuclei and chromosomes is described. Abnormal FISH signals of specific chromosomes are associated with different types of brain tumors, with different grades of malignancy, and with mesenchymal drift of glioma cells in culture.

Electrophysiological properties of human astrocytic tumor cells In situ: enigma of spiking glial cells

To better understand physiological changes that accompany the neoplastic transition of astrocytes to become astrocytoma cells, we studied biopsies of low-grade, pilocytic astrocytomas. This group of tumors is most prevalent in children and the tumor cells maintain most antigenic features typical of astrocytes. Astrocytoma cells were studied with the use of whole cell patch-clamp recordings in acute biopsy slices from 4-mo- to 14-yr-old pediatric patients. Recordings from 53 cells in six cases of low-grade astrocytomas were compared to either noncancerous peritumoral astrocytes or astrocytes obtained from other surgeries. Astrocytoma cells almost exclusively displayed slowly activating, sustained, tetraethylammonium (TEA)-sensitive outward potassium currents (delayed rectifying potassium currents; IDR) and transient, tetrodotoxin (TTX)-sensitive sodium currents (INa). By contrast, comparison glial cells from peritumoral regions or other surgeries showed IDR and INa, but in addition these cells also expressed transient "A"-type K+ currents and inwardly rectifying K+ currents (IIR), both of which were absent in astrocytoma cells. IIR constituted the predominant conductance in comparison astrocytes and was responsible for a high-resting K+ conductance in these cells. Voltage-activated Na+ currents were observed in 37 of 53 astrocytoma cells. Na+ current densities in astrocytoma cells, on average, were three- to fivefold larger than in comparison astrocytes. Astrocytoma cells expressing INa could be induced to generate slow action potential-like responses (spikes) by current injections. The threshold for generating such spikes was -34 mV (from a holding potential of -70 mV). The spike amplitude and time width were 52.5 mV and 12 ms, respectively. No spikes could be elicited in comparison astrocytes, although some of them expressed Na+ currents of similar size. Comparison of astrocytes to astrocytoma cells suggests that the apparent lack of IIR, which leads to high-input resistance (>500 MOmega), allows glioma cells to be sufficiently depolarized to generate Na+ spikes, whereas the high resting K+ conductance in astrocytes prevents their depolarization and thus generation of spikes. Consistent with this notion, Na+ spikes could be induced in spinal cord astrocytes in culture when IIR was experimentally blocked by 10 microM Ba2+, suggesting that the absence of IIR in astrocytoma cells is primarily responsible for the unusual spiking behavior seen in these glial tumor cells. It is unlikely that such glial spikes ever occur in vivo.

Stimulation of extracellular matrix components in the normal brain by invading glioma cells

Malignant gliomas are characterized by an extensive invasion of tumor cells into the normal brain parenchyma. A substantial amount of data indicates that cell movement in general is regulated by specific interactions between extracellular matrix components and specific cell-surface receptors. In the present work, multicellular spheroids from 4 human glioma cell lines (U-373Mg, A-172Mg, U-251Mg and HF-66) were confronted with normal rat brain cell aggregates in vitro, which resulted in a progressive invasion of tumor cells into the brain aggregates. The co-cultures were then sectioned and immuno-stained for specific extracellular matrix components (laminin, fibronectin and collagen type IV) and for specific cell-surface receptors which bind to these components (integrins beta1, beta4, alpha3, alpha6). In addition, flow-cytometric measurements and Northern blot analyses showed expression of several different integrins within the cell lines. The alpha3 subunit was expressed strongly in all cell lines. Whereas the beta1 subunit was expressed weakly in exponentially growing monolayer cultures, it showed a pronounced expression in multicellular spheroids, indicating that the integrin expression may vary depending on the micro-environment within a tumor. Furthermore, normal brain tissue was able to produce laminin when confronted with the glioma cells, which also was observed for fibronectin and collagen type IV. The relevance of our observations to the in vivo situation was investigated further by immuno-staining 5 human glioma biopsy samples for laminin. In some areas of the tumors, specific deposits of laminin were observed. In conclusion, we have shown that normal brain tissue has the ability to produce extracellular matrix components, such as laminin, collagen type IV and fibronectin, when confronted with invading glioma cells. Our results show that the glioma cells express specific integrins which can interact with these extracellular matrix components. Such interactions may facilitate tumor cell migration and invasion.