Imaging glioma cell invasion in vivo reveals mechanisms of dissemination and peritumoral angiogenesis

Infiltration of cancer cells into normal tissue is a hallmark of malignant gliomas and compromises treatment options. A lack of appropriate models limits the study of this invasion in vivo, which makes it difficult to fully understand its anatomy and the role of dynamic interactions with structures of the normal brain. We developed a novel methodology by utilizing multiphoton laser scanning microscopy (MPLSM) to image the movement of glioma cells deep within the normal brain of live mice in real time. This allowed us to track the invasion of individual RFP-expressing GL261 cells in relation to perfused vasculature or GFP-labeled endothelial cells repetitively over days, up to a depth of 0.5 mm. Glioma cells moved faster and more efficiently when the abluminal site of a blood vessel was utilized for invasion. Cells that invaded perivascularly were frequently found next to (a) multiple capillary structures where microvessels run parallel to each other, (b) capillary loops or glomeruloid-like bodies, and (c) dilated capillaries. Dynamic MPLSM for more than 48 h revealed that single invasive glioma cells induced intussusceptive microvascular growth and capillary loop formation, specifically at the microvascular site with which they had contact. As the main tumor grew by cooption of existing brain vessels, these peritumoral vascular changes may create a beneficial environment for glioma growth. In conclusion, our study revealed new mechanisms of peritumoral angiogenesis and invasion in gliomas, providing an explanation for their interdependence.

Induction of glioma cell migration by vitronectin in human serum and cerebrospinal fluid

OBJECT

Malignant gliomas are often highly invasive and can migrate along blood vessels. The purpose of the current study was to identify the substance in human serum and/or cerebrospinal fluid (CSF) that promotes glioma cell migration.

METHODS

The authors used a Boyden chamber cell migration assay to study the effect of serum from patients with glioma and healthy volunteers on chemotaxis of A172 human glioma cells. Heat inactivation, trypsinization, and ultrafiltration of serum were used to establish the nature of the active factor. Vitronectin and fibronectin were chosen for further investigations; chemotactic effects were studied in both serum and CSF.

RESULTS

Serum from both patients with glioma and healthy volunteers was found to promote chemotaxis of human glioma cells. This activity was greatly reduced by heat inactivation or trypsinization. Fractionation of the serum by ultrafiltration through membranes with various pore sizes showed that the active molecule was larger than 50 kD. Antibodies against integrin alphav or alphavbeta5 or arginine-glycine-aspartic acid-containing peptides, both of which block the vitronectin-glioma cell interactions, significantly reduced serum-induced cell migration, whereas blocking the interaction of glioma cells with fibronectin had no effect. Furthermore, the ability of serum to promote the migration of A 172 or T98G glioma cells was suppressed by immunodepletion of vitronectin and restored by the addition of exogenous vitronectin. The migration of glioma cells induced by CSF collected from the postoperative cavity of a malignant glioma patient was also reduced by blocking the interaction of glioma cells with vitronectin.

CONCLUSIONS

These results suggest that vitronectin is one of the major factors in serum- and CSF-induced glioma cell migration.

Tumor growth instability and the onset of invasion

Motivated by experimental observations, we develop a mathematical model of chemotactically directed tumor growth. We present an analytical study of the model as well as a numerical one. The mathematical analysis shows that: (i) tumor cell proliferation by itself cannot generate the invasive branching behavior observed experimentally, (ii) heterotype chemotaxis provides an instability mechanism that leads to the onset of tumor invasion, and (iii) homotype chemotaxis does not provide such an instability mechanism but enhances the mean speed of the tumor surface. The numerical results not only support the assumptions needed to perform the mathematical analysis but they also provide evidence of (i), (ii), and (iii). Finally, both the analytical study and the numerical work agree with the experimental phenomena.

The impact of "search precision" in an agent-based tumor model

Several cell surface receptors are overexpressed in malignant brain tumors and reportedly involved in tumor progression and invasion. It is unclear, however, whether such an improvement of cellular signal reception leads to a monotonic increase in the tumor system's average velocity during invasion or whether there is a maximum threshold beyond which the average velocity starts to decelerate. To gain more insight into the systemic effects of such cellular search precision modulations, this study proposes a two-dimensional agent-based model in which the spatio-temporal expansion of malignant brain tumor cells is guided by environmental heterogeneities in mechanical confinement, toxic metabolites and nutrient sources. Here, the spatial field of action is represented by an adaptive grid lattice, which corresponds to the experimental finding that tumor cells are more likely to follow each other along preformed pathways. Another prominent feature is the dual threshold concept for both nutrient level and toxicity, which determine whether cells proliferate, migrate, remain quiescent or die in the next period. The numerical results from varying the key parameters encoding the capability of tumor cells to invade and their ability to proliferate indicate an emergent behavior. Specifically, increasing invasiveness not only leads to an increase in maximum expansion velocity, but also requires a more precise spatial search process, corresponding to an improved cell signal reception, in order to obtain maximum velocity. To increase cellular invasiveness beyond the maximum that can be achieved by exclusively tuning the motility parameter, it requires an additional reduction in the cells' proliferation rate and prompts an even more biased search process. Most interestingly, however, a prominent phase transition suggests that tumor cells do not employ a 100 percent search precision to attain maximum spatial velocity. These findings argue for a selection advantage conferred by limited randomness in processing spatial search and indicate that our computational platform may prove valuable in investigating emergent, multicellular tumor patterns caused by alterations on the molecular level.

Analysis of the antitumoral mechanisms of lipopolysaccharide against glioblastoma multiforme

Our objective was to analyze the lipopolysaccharide (LPS) antitumoral effect upon glioblastoma, including whether the lipid A subunit alone can elicit glioblastoma regression, whether dexamethasone suppresses this response to LPS, whether B and T lymphocytes factor in this response, and whether this antitumoral effect of LPS provides resistance against subsequent challenge with glioblastoma. Mice (BALB/c, nude or SCID) implanted with s.c. DBT glioblastomas were treated with LPS (with or without dexamethasone) or with lipid A. A subset of BALB/c mice in which s.c. DBT glioblastomas had previously been eradicated using LPS were re-implanted with s.c. or intracranial (i.c.) DBT cells. For mice with s.c. tumors, mean tumor masses (MTM) were compared between groups. Survival was compared for mice with i.c. tumors. Lipid A caused near complete tumor regression of DBT glioblastomas in BALB/c mice (p<0.0001). Dexamethasone did not alter the antitumoral effect of LPS (p=0.48). LPS reduced the MTM of s.c. glioblastomas in T lymphocyte-deficient nude mice, but not as effectively as in immunocompetent mice. The antitumoral response to LPS for T and B lymphocyte-deficient SCID mice bearing DBT glioblastomas was similar to that for nude mice. Eradication of s.c. DBT glioblastoma in BALB/c provided partial resistance to subsequent challenge with s.c. or i.c. glioblastoma. We conclude that the LPS-mediated antitumoral response against glioblastoma is dependent upon the lipid A subunit of LPS, partially dependent upon T lymphocytes, independent of B lymphocytes, unaffected by dexamethasone and provides partial protection against subsequent challenges with glioblastoma.

Glioblastoma and cerebral microvascular endothelial cell migration in response to tumor-associated growth factors

OBJECTIVE

Glioma cell migration is determined by a complex interplay between soluble motogens and extracellular matrix components. Several growth factors are thought to be involved in glioma cell migration; however, little is known about their motogenic potency relative to one another.

METHODS

Using modified Boyden chamber assays, we compared the chemotactic effects of scatter factor/hepatocyte growth factor (SF/HGF), transforming growth factor (TGF)-alpha, TGF-beta1, TGF-beta2, epidermal growth factor (EGF), fibroblast growth factor (FGF)-1, FGF-2, insulin-like growth factor (IGF)-1, IGF-2, platelet-derived growth factor (PDGF)-AA, PDGF-BB, vascular endothelial growth factor (VEGF), pleiotrophin (PTN), and midkine (MK) in concentrations ranging from 1 pmol/L to 50 nmol/L on three different human glioblastoma cell lines. Checkerboard analyses distinguished between chemotaxis and chemokinesis. We further investigated the motogenic effects on human cerebral microvascular endothelial cells and analyzed receptor expression profiles.

RESULTS

SF/HGF was the most potent chemotactic factor for all three glioblastoma cell lines, inducing up to 33-fold stimulation of migration. TGF-alpha showed the second strongest effect (up to 17-fold stimulation), and FGF-1 was also chemotactic for all three glioblastoma cell lines analyzed (maximal 4-fold effect). EGF, FGF-2, IGF-1, IGF-2, TGF-beta1, and TGF-beta2 were chemotactic for one or two of the cell lines (2- to 4-fold effects), whereas PDGF-AA, PDGF-BB, VEGF, PTN, and MK had no effect. In contrast, the most potent stimulators of cerebral microvascular endothelial cell migration were PDGF-AA (4-fold) and PDGF-BB (6-fold).

CONCLUSION

The expression levels of SF/HGF and TGF-alpha as well as their respective receptors, MET and EGFR, are known to correlate with glioma malignancy grade. The particularly strong motogenic effects of these two growth factors suggest that they could be promising targets for an antimigratory component of glioma therapy, at least in comparison with the 12 other factors that were analyzed.

Intratumoral injection of lipopolysaccharide causes regression of subcutaneously implanted mouse glioblastoma multiforme

OBJECTIVE

Anecdotal reports documented extended survival times for patients who developed infections at the site of resection of malignant gliomas. Hypothesized mechanisms for this phenomenon include immune responses triggered by lipopolysaccharide (LPS). This investigation assessed whether LPS could produce tumor regression in an in vivo model of malignant glioma.

METHODS

Delayed brain tumor cells (2 x 10(6)) were injected subcutaneously into female BALB/c mice. LPS (300-500 microg) was injected intratumorally or subcutaneously at a contralateral site on Days 10, 17, and 24. Control animals received phosphate-buffered saline intratumorally or subcutaneously. Mice were killed on Day 28, and tumors were removed. Mean tumor masses for control animals and the two LPS-treated groups (intratumoral or contralateral subcutaneous treatment) were compared. Histological assessments of treated and control tumors were performed.

RESULTS

Complete or nearly total tumor regression was achieved in all 20 mice with subcutaneous delayed brain tumor cell tumors treated intratumorally with 400 microg of LPS (mean tumor mass of 0.09 +/- 0.38 g versus 2.42 +/- 2.46 g for control animals, P < 0.0001). Intratumoral administration of 300 microg of LPS or subcutaneous injection of 300 or 400 microg of LPS at a contralateral site resulted in less consistent regression of subcutaneous tumors. Administration of 500 microg of LPS resulted in tumor regression similar to that observed with lower doses but was limited by treatment-related deaths in 40% of animals. Histological assessment revealed lymphocytic infiltration of LPS-treated tumors.

CONCLUSION

Intratumoral injections of LPS caused dramatic regression of subcutaneously implanted delayed brain tumor cell mouse gliomas. Investigation of this antitumoral effect may improve treatment responses for patients with malignant gliomas.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

Roles of transforming growth factor-alpha and related molecules in the nervous system

The epidermal growth factor (EGF) family of polypeptides is regulators for tissue development and repair, and is characterized by the fact that their mature forms are proteolytically derived from their integral membrane precursors. This article reviews roles of the prominent members of the EGF family (EGF, transforming growth factor-alpha [TGF-alpha] and heparin-binding EGF [HB-EGF]) and the related neuregulin family in the nerve system. These polypeptides, produced by neurons and glial cells, play an important role in the development of the nervous system, stimulating proliferation, migration, and differentiation of neuronal, glial, and Schwann precursor cells. These peptides are also neurotrophic, enhancing survival and inhibiting apoptosis of post-mitotic neurons, probably acting directly through receptors on neurons, or indirectly via stimulating glial proliferation and glial synthesis of other molecules such as neurotrophic factors. TGF-alpha, EGF, and neuregulins are involved in mediating glial-neuronal and axonal-glial interactions, regulating nerve injury responses, and participating in injury-associated astrocytic gliosis, brain tumors, and other disorders of the nerve system. Although the collective roles of the EGF family (as well as those of the neuregulins) are shown to be essential for the nervous system, redundancy may exist among members of the EGF family.

Glioma cell migration is associated with glioma-induced angiogenesis in vivo

To simultaneously assess glioma cell invasion and glioma angiogenesis in vivo by non-invasive and quantitative means, DiI-labeled C6 glioma spheroids were implanted into the dorsal skinfold chamber preparation of nude mice (n = 6). Heat-inactivated spheroids served as controls to distinguish between active and passive cell spread. Using multi-fluorescent intravital videomicroscopy, glioma cell migration was analyzed on days 1-4, 6, and 10 and spheroid vascularization was analyzed on days 3, 6, and 10 after implantation. Additionally, C6 glioma spheroids were implanted into the chronic cranial window of nude mice as an orthotopic implantation site (n = 4). In the dorsal skinfold chamber, spheroids were vascularized within 10 days and revealed a tumor-specific microvasculature. In parallel, individual glioma cells detached from the spheroid edge and migrated centrifugally demonstrating an affinity to tumor and host vessels. Glioma cells demonstrated a heterogeneous pattern of their regional migratory activity (0.2-9.6 microm/h) which correlated well with regional glioma angiogenesis (r = 0.733). Using the cranial window, glioma cells spread similarly demonstrating an affinity to the perivascular space of pial/subpial vessels with preference to the arteriolar segments. Intravital fluorescence microscopy represents a versatile technique to assess the complex relationship between glioma-driven angiogenesis and glioma cell invasion.

Extracellular matrix degradation by metalloproteinases and central nervous system diseases

Matrix metalloproteinases (MMPs) are a gene family of neutral proteases involved in normal and pathological processes in the central nervous system (CNS). Normally released into the extracellular space, MMPs break down the extracellular matrix (ECM) to allow cell growth and to facilitate remodeling. Proteolysis becomes pathological when the normal balance between the proteases and their inhibitors, tissue inhibitors to metalloproteinases (TIMPs), is lost. Cancer cells secrete neutral proteases to facilitate spread through the ECM. MMPs increase capillary permeability, and they have been implicated in demyelination. Neurological diseases, such as brain tumors, multiple sclerosis, Guillain-Barré, ischemia, Alzheimer's disease, and infections, lead to an increase in the matrix-degrading proteases. Two classes of neutral proteases have been extensively studied, namely the MMPs and the plasminogen activators (PAs), which act in concert to attack the ECM. After proteolytic injury occurs, the process of ECM remodeling begins, which can lead to fibrosis of blood vessels and gliosis. TIMPs are increased after the acute injury and may add to the fibrotic buildup of ECM components. Thus, an imbalance in proteolytic activity either during the acute injury or in recovery may aggravate the underlying disease process. Agents that affect the proteolytic process at any of the regulating sites are potentially useful in therapy.

The hydroxyurea-induced loss of double-minute chromosomes containing amplified epidermal growth factor receptor genes reduces the tumorigenicity and growth of human glioblastoma multiforme

OBJECTIVE

We investigated whether the hydroxyurea-induced loss of double-minute chromosomes containing amplified epidermal growth factor receptor (EGFR) genes would lead to a loss of tumorigenicity of a glioblastoma multiforme cell line.

METHODS

Glioblastoma multiforme cells were treated in vitro with 0 (HU0) or 100 micromol/L (HU100) hydroxyurea and then injected into the flanks of nude mice. Survival and tumor volumes were evaluated. Pulsed-field gel electrophoresis, Southern blot hybridization, and slot-blot analysis were used to determine EGFR amplification levels. Flow cytometry and immunofluorescent staining were used for cell-cycle analysis and EGFR protein expression.

RESULTS

Prior to injection, HU100 cells lost 95% of their amplified EGFR genes and developed into tumors 6 weeks after injection versus 3 weeks for HU0 cells. Mice with HU100 tumors had a median survival of 62 days versus 43 days for control mice with HU0 tumors. Pulse-field gel electrophoresis analysis showed that HU100 tumors had reamplified the EGFR gene as double-minute chromosomes of the same size as those originally present before hydroxyurea treatment. When HU100 cells were cultured in the absence of hydroxyurea, the EGFR gene also reamplified. HU100 cells grew at less than half the rate of untreated HU0 control cells in culture and showed a decreased number of cells entering the cell cycle. Immunofluorescent staining of HU150 (150 micromol/L) cells showed decreased EGFR protein expression.

CONCLUSION

The EGFR gene is important for tumorigenicity in mice and growth in culture. Hydroxyurea induces the loss of double-minute chromosome-amplified EGFR genes against a selection gradient and significantly delays the onset of tumors. These results support the potential use of low-dose hydroxyurea for the treatment of human glioblastoma multiforme.

Basic fibroblast growth factor induces cell migration and proliferation after glia-specific gene transfer in mice

Basic fibroblast growth factor (bFGF) is overexpressed in most high-grade human gliomas, implying that it is involved in the pathogenesis of these tumors. To assess the biological effect of inappropriate production of bFGF in normal astrocytes, we developed a system for glia-specific gene transfer in transgenic mice. A transgene encoding the receptor for subgroup A avian leukosis virus and controlled by the astrocyte-specific glial fibrillary acidic protein promoter permits efficient glia-specific transfer of genes carried by subgroup A avian leukosis virus vectors. With this system, we have demonstrated that bFGF induces proliferation and migration of glial cells in vivo, without the induction of tumors.

Implantation of C6 astrocytoma spheroid into collagen type I gels: invasive, proliferative, and enzymatic characterizations

A three-dimensional model has been developed in which C6 astrocytoma spheroids of defined sizes are embedded into collagen type I gels. The authors have monitored cell invasive behavior; obtained quantitative data on cell invasion, proliferation, and enzymatic activity; assessed cell-cell interactions by altering the spheroid size used; and studied cell-matrix interactions by modifying the matrix components. Their results show that C6 astrocytoma cells detach from the spheroid surface and invade the gel as single cells by means of a system that appears to be dependent on metalloprotease function. These invasive cells have a low proliferative index. Larger spheroids with central hypoxic microregions possess cells that invade the gel at faster rates; this could be correlated with the release of increased collagen type I degrading activity. Extracellular matrix proteins, such as laminin, fibronectin, and collagen type IV have no significant influence on invasive activity, whereas hyaluronic acid decreases and human central nervous system myelin increases invasion. New strategies directed at the treatment of malignant gliomas must take into account the subpopulation of malignant cells located long distances from the major tumor mass. The spheroid invasion model may provide specific insights into the behavior of these invasive cells.

Influence of epidermal growth factor and gastrin on the cell proliferation of human meningiomas versus astrocytic tumors maintained as ex vivo tissue cultures

The hormone sensitivity of a tumor is traditionally based on the presence of steroid receptors. Other factors should be taken into consideration. Here, we studied the influence of 10 nM epidermal growth factor (EGF) or gastrin on the proliferation of human ex vivo tumor cultures by means of [3H]thymidine autoradiography. The immunohistochemical EGF-receptor expression was also quantified by means of computer-assisted microscopy. The results demonstrated that the proliferation of 6/11 astrocytic tumors and 3/16 meningiomas was sensitive to at least one factor tested, i.e. EGF or gastrin (P < 0.01), and 5 of these 9 'hormone-sensitive' tumors were sensitive to both factors. The immunohistochemical labeling index for the EGF receptor was higher than 80% in 15/16 meningiomas, but only in 6/11 gliomas (P < 0.01). These results suggest that EGF and gastrin are important for astrocytic tumor proliferation and significantly (P < 0.01) less important for meningiomas. Thus, astrocytic tumors may be steroid insensitive in term of cell growth, but are certainly not hormone insensitive.

Isolation and characterization of human malignant glioma cells from histologically normal brain

Brain invasion prevents complete surgical extirpation of malignant gliomas; however, invasive cells from distant, histologically normal brain previously have not been isolated, cultured, and characterized. To evaluate invasive human malignant glioma cells, the authors established cultures from gross tumor and histologically normal brain. Three men and one woman, with a mean age of 67 years, underwent two frontal and two temporal lobectomies for tumors, which yielded specimens of both gross tumor and histologically normal brain. Each specimen was acquired a minimum of 4 cm from the gross tumor. The specimens were split: a portion was sent for neuropathological evaluation (three glioblastomas multiforme and one oligodendroglioma) and a portion was used to establish cell lines. Morphologically, the specimens of gross tumor and histologically normal brain were identical in three of the four cell culture pairs. Histochemical staining characteristics were consistent both within each pair and when compared with the specimens sent for neuropathological evaluation. Cultures demonstrated anchorage-independent growth in soft agarose and neoplastic karyotypes. Growth rates in culture were greater for histologically normal brain than for gross tumor in three of the four culture pairs. Although the observed increases in growth rates of histologically normal brain cultures do not correlate with in vivo behavior, these findings corroborate the previously reported stem cell potential of invasive glioma cells. Using the radial dish assay, no significant differences in motility between cultures of gross tumor and histologically normal brain were found. In summary, tumor cells were cultured from histologically normal brain acquired from a distance greater than 4 cm from the gross tumor, indicating the relative insensitivity of standard histopathological identification of invasive glioma cells (and hence the inadequacy of frozen-section evaluation of resection margins). Cell lines derived from gross tumor and histologically normal brain were usually histologically identical and demonstrated equivalent motility, but had different growth rates.

Mechanisms of glioma invasion: role of matrix-metalloproteinases

One of the most lethal properties of high grade gliomas is their ability to invade the surrounding normal brain tissue, as infiltrated cells often escape surgical resection and inevitably lead to tumour recurrence. The consequent poor prognosis and survival rate underscore the need to further understand and target the cellular mechanisms that underly tumour invasiveness. Proteases which degrade the surrounding stromal cells and extracellular matrix proteins have been demonstrated to be critical effectors of invasion for tumours of both central and peripheral origin. Within the nervous system, the role of metalloproteinases as well as other classes of proteases in mediating the invasive phenotype of high grade gliomas has been an intense area of research. We present in this article a review of this literature and address the possibility that these proteases and the biochemical pathways that regulate their expression, such as protein kinase C, may represent potential targets in the therapy of high grade gliomas.

Migration of human glioma cells in response to tumour cyst fluids

Glial tumours often show high degrees of local invasion that lead to local recurrence of the disease. Extracellular matrix components as well as soluble factors may play a critical role in this poorly understood process. Cyst fluid from human brain tumours may accumulate such autocrine produced factors and may represent a source were those factors may be easily obtained and studied. We have studied the effect of cyst fluids harvested from 17 glial tumours, 3 meningiomas, and three metastases on the motility of established human glioma cell lines. Both cyst fluids of high grade and low grade gliomas contained varying degrees of motility enhancing activity. No such activity was identified in cyst fluids obtained from meningiomas. The relation of mitogenic and motogenic activity in three selected cyst fluids was analysed using a quantitative monolayer migration assay. Quantitative analysis of cyst fluid effects on both proliferation and migration indicate that tumour cyst fluids contain factors that strongly stimulate cell migration and that maximum stimulation of migration did not occur at concentrations optimal for cell proliferation. Our findings indicate that glial tumours in fact produce and secrete soluble factors that may contribute to their dissemination in brain tissue.