CHAPTER 103 Invasion in Malignant Glioma
There is little question that the infiltrative behavior of malignant gliomas in the brain is one of the primary confounders in modern brain tumor management. A profound paradox exists, however, in that the destructive and infiltrative growth of gliomas in the brain is rarely accompanied by far-flung systemic metastases.1 This is so despite expression of the same cast of proteases, motility factors, and in vitro behavior seen in metastatic tumors from other sources.2 This chapter describes the clinical phenotype of glioma infiltration, its radiologic characteristics, and its significance with regard to clinical outcome. A discussion of the cell biology of glioma cells and their extracellular matrix (ECM)3 and cytoskeletal elements4 follows, as well as a survey of factors associated with invasive behavior5,6 and growth control.7 Finally, these molecules are reviewed as potential therapeutic targets.
Clinical Patterns of Spread
Contiguous Spread
At autopsy, 45% of cases of glioblastoma multiforme (GBM) extend beyond one lobe, 25% involve an entire hemisphere, and 25% to 30% cross over to the opposite hemisphere.8,9 Almost 60% of laterally located supratentorial gliomas spread in an anteroposterior direction, and approximately 20% invade deep supratentorial structures and infratentorial areas by invading along fiber tracts in a vertical direction.9 Frontal lobe gliomas tend to invade the frontal lobe by crossing through the corpus callosum (CC), whereas temporal lobe gliomas are prone to invade the midbrain and pons. These tendencies are observed in high-grade as well as low-grade gliomas. Gliomas arising in regions below the CC are largely confined and tend to invade basal structures, such as the thalamus and peduncles along the corticospinal tracts.10 Bilateral extension occurs through the thalamus, hypothalamus, and anterior commissure, especially in the region of the basal ganglia.8,11 In fact, Scherer found that all thalamic and hypothalamic gliomas extended bilaterally.8 Pontine tumors frequently spread cephalad and invade the midbrain and thalamic areas. Occasionally, caudal spread directly to the upper cervical cord occurs. In untreated lesions, neoplastic cells were observed within 3 cm of the necrotic tissue, whereas in recurrences, the tumor cells extended far beyond the primary tumor and were found in the opposite hemisphere in 80% of cases.12
Distant Recurrence
All treatment strategies for managing high-grade gliomas eventually fail. The pattern of tumor recurrence after treatment has been studied extensively since Kramer advocated whole-brain radiation therapy with a focal boost in 1959.13 The tumor can recur at the same site or close to it (local recurrence), or it can recur at a distant site (distant recurrence), arbitrarily defined as more than 2 cm from the original tumor site. Autopsy studies have shown that radiotherapy somewhat controls local disease (no recurrence at the primary site in 50% of cases), but the incidence of distant recurrence increased from 3% without radiotherapy to 19% to 22% with it.14,15 Clinically, 5% to 7% of high-grade glioma patients may have additional lesions away from the original site when they are seen for the first time.16,17 With respect to the field of radiation, recurrences totally outside the boost field can occur in 2% to 25% of cases, and recurrences partially outside the boost field can occur in 23% to 48% of cases.18,19
Brachytherapy was thought to be more effective than conventional treatment in improving survival, but it also results in an increased incidence of distant recurrence in patients with malignant glioma.20–22 Recurrences 2 to 5 cm away from the original tumor developed in 36% to 50% of patients, and recurrences more than 5 cm from the original tumor developed in 20% to 45% of patients. In another study, the recurrence after brachytherapy was extensive; about 39% of 23 patients experienced multicentric and subependymal spread.23 Other studies, however, do not report such a high incidence of distant metastasis. One analysis of 50 cases of recurrent tumor after brachytherapy found that only 16% of the recurrences extended beyond the 2-cm margin.
The more effectively the disease is controlled locally and the longer the survival, the greater the chance of recurrence at a distant site. Massey and Wallner found that the incidence of distant metastasis is higher in the second recurrence than in the first and observed that once surgery was performed for an initial recurrence, a second, distant recurrence (>4 cm from the original tumor) developed in as many as 25% of cases.24 Choucair and coworkers17 and Sneed and colleagues,25 in contrast, did not find any significant increase in the incidence of distant recurrence; local tumor progression was the predominant pattern of failure after brachytherapy. Late follow-up, though, showed distinctly separate lesions or subependymal or systemic spread in 40% of cases, thus reaffirming that with malignant glioma, increased survival means an increase in the number of distant recurrences. The role of brachytherapy in both newly diagnosed and recurrent glioma remains unclear, however. Reirradiation of recurrent malignant gliomas with GliaSite, in which an aqueous solution of 125I is delivered through an expandable balloon catheter, resulted in only a modest increase in survival.26 Despite initial enthusiasm for the role of 125I brachytherapy, phase III trials have failed to demonstrate a survival benefit.27,28 Since these trials, the use of brachytherapy has decreased greatly.
Radiosurgery has also been evaluated as a mode of delivering local radiation therapy. In a comparison study, distant recurrences were found in approximately 20% of patients treated by radiosurgery or brachytherapy; the rate of distant recurrence was significantly higher, however, in those not treated with either of these modalities.22 Other innovative and aggressive treatments of local glioma have resulted in a similar increase in distant recurrence, with rates as high as 75% after intra-arterial chemotherapy and 57% after radioimmunotherapy.29,30 As with brachytherapy, there was some enthusiasm for the use of radiosurgery to treat glioblastoma after positive results in phase I and II trials, but a randomized prospective trial failed to demonstrate a survival benefit when compared with conventional radiotherapy alone.31
Low-grade gliomas (Kernohan grades I and II) can also recur at distant sites after many years.32 In 20 cases of low-grade recurrence, nearly a fourth were situated outside the radiotherapy field.18
Multifocal Glioma and Gliomatosis Cerebri
Most gliomas occur as single masses. However, wide dissemination of glioma cells may lead to the appearance on magnetic resonance imaging (MRI) of multiple masses separated by intervening brain tissue. These multiple gliomas can be classified as “multifocal” if there is an apparent route of dissemination (i.e., a white matter tract). In contrast, if there is no obvious pathway for spread, it is termed multicentric.33 This is not to be confused with gliomatosis cerebri, which is characterized as a diffuse infiltration of glial tumor cells with preservation of the underlying cytoarchitecture and sparing of neurons. A rare type of neoplasm representing approximately 1% of total glial neoplasms, these tumors involve two or more lobes, often extending bilaterally or to infratentorial structures.34
Surgery, radiotherapy, and other adjuvant treatments, along with increased life expectancy and delayed recurrence, have increased the incidence of multifocal glioma.14,15,17,35,36 The average incidence of multifocal glioma is 1.5% at diagnosis but 7.5% after treatment. In an autopsy series of 209 gliomas, multifocal glioma was found in 27.8% of cases, and the ratio of multiple to multicentric glioma was 10.6:1.37 Tertiary centers may see an unusually high number of multifocal gliomas because of referral bias. In one study, the percentage of multifocal glioma was 30% in 47 malignant gliomas on initial examination and 56% in 25 malignant gliomas on follow-up.38
Multiple lesions occur as a result of tumor invasion from a monoclonal origin. The more time available for glioma cells to migrate, the higher the incidence of multifocal glioma. Anaplastic astrocytoma exhibits more infiltrative growth than GBM does.39,40 Thus, multifocal glioma should occur more frequently in anaplastic astrocytoma than in GBM. In fact, in one study the incidence of multifocal glioma at initial evaluation was higher in moderately anaplastic astrocytoma (“low grade,” 4.3%) than in GBM (0.98%).17 After treatment, only 18 of 405 GBMs (4.4%) recurred with multifocal lesions as compared with 54 of 630 anaplastic gliomas (8.6%) (Fig. 103-1).
Gliomatosis cerebri continues to carry a poor prognosis, although survival is variable. At diagnosis, gliomatosis cerebri is widely migratory and invasive. Despite its often lower grade appearance on histology, a majority of patients with gliomatosis cerebri will have a prognosis similar to that of GBM. Because of the diffusely infiltrative nature of this lesion, resective surgery is often not possible, which leaves radiotherapy and chemotherapy as the mainstays of treatment. Genetic abnormalities are shared with other diffuse gliomas, thus suggesting that gliomatosis may not be a separate entity from gliomas, as has been postulated, but rather a particularly migratory and invasive, but less proliferative subtype of diffuse glioma.41
Computed Tomography and Magnetic Resonance Imaging
Computed tomography (CT) and MRI have aided tremendously in understanding the pathobiology of glioma. Autopsy studies have confirmed the reliability of CT in defining the gross and microscopic extent of the tumor within a 2-cm margin, including the presence of multicentric lesions and recurrences. MRI is more sensitive and accurate than CT in studying gliomas.42–47 The superiority of MRI in assessing the presence of tumor extends to the immediate postoperative period as well. In one study, MRI detected residual tumor postoperatively in 77% of patients, whereas CT detected residual tumor in only 40.5%.45 Contrast-enhanced MRI performed within 72 hours (preferably 24 hours) postoperatively predates the appearance of normal postoperative enhancement and is therefore optimal for evaluating residual macroscopic tumor.48 The persistence of enhancing areas on MRI within 72 hours after tumor resection correlates with a high tendency for tumor recurrence in the early postoperative period.45,49
High-grade gliomas usually consist of a core of macroscopic growth surrounded by tumor cells. These tumor cells infiltrate the parenchyma around the core.12,37,38,44,50 The enhancement seen on MRI and CT correlates with the central mass, which has the highest ratio of abnormal vessels. An area of ring enhancement with surrounding hypodensity is the typical picture of malignant glioma on CT. The solid component of the tumor correlates with the contrast enhancement on CT, the hypodensity within the contrast-enhancing region is usually the central necrotic portion, and the surrounding hypodensity is a mixture of edema and infiltrated glioma cells.12,14,37,51 In contrast to the high-grade variety, low-grade gliomas generally consist of masses of tumor cells that infiltrate functioning parenchyma and frequently appear as hypodensity on CT and as more extensive T2 abnormalities on MRI.38,49
Both autopsy and stereotactic biopsy studies have confirmed the presence of tumor cells in the hypodense areas.30,42,43 In stereotactic biopsy studies by Kelly and associates in 39 patients with low- or high-grade glioma, CT showed tumor cells infiltrating the hypodense regions in 74 of 98 biopsy samples (76%).40 The hypodensity, however, does not delineate tumor-infiltrated brain parenchyma; both stereotactic biopsy and autopsy studies have shown isodense areas beyond the tumor-related hypodensity permeated by tumor cells in as many as 80% of biopsies.12,52
The infiltrating tumor cells in the periphery are better approximated by the hyperintensity on T2-weighted images than by any signal abnormality on T1-weighted MRI or CT with or without contrast enhancement.38,43,49,53,54 It should be emphasized that the T2-weighted abnormality reflects the result of edema, demyelination, and other degenerative changes, not the cellularity or atypia of the tumor cells.44 As with the hypodensity on CT, tumor cells are found infiltrating the brain beyond the hyperintensity signal changes on T2-weighted images when stereotactic biopsies are compared with MRI scans.38,44,49,51 Kelly and associates studied a group of patients with untreated glial tumors, 28% of whom had grade III or IV astrocytoma.40,51 Isolated tumor cell infiltration was found extending at least as far as the T2 weighting–defined abnormality, with tumor cells detected outside this region in approximately half the patients. In high-grade glioma, Watanabe and coworkers found tumor cells in 94% of biopsy specimens within areas of T2-weighted hyperintensity and in 22% of samples outside the areas of T2-weighted hyperintensity.46 Postmortem studies have suggested that T2-weighted hyperintensity may be a sensitive indicator of tumor cell invasion in untreated high-grade glioma patients but may be overrepresented (24%) or underrepresented (28%) in cases of recurrence.49 Heavily T2-weighted fluid attenuation inversion recovery (FLAIR) sequences improve the definition of tumor dissemination. FLAIR may also be a useful imaging modality for assessing response to treatment. Patients treated with bevacizumab, an antiangiogenic agent, have displayed a discordance between enhancement on T1 weighting and FLAIR. This may represent increased tumor dissemination with decreased vascularity.55
Newer MRI techniques have further elucidated the extent of spread in gliomas. Magnetic resonance spectroscopy (MRS) is a technique used to measure metabolites in tissue. Some metabolites commonly detected with MRS include choline, creatine, lactate, lipid, and N-acetylaspartate. Choline is thought to be an indicator of cell membrane turnover, thus being increased in proliferating tissues such as gliomas. In addition, neuronal markers such as N-acetylaspartate and creatine are decreased in glioma tissue. Using these parameters, tumor spread has been found beyond the volume defined by contrast enhancement on T1 imaging or beyond that defined by T2 hyperintensity. In addition, an increase in choline concentration is not necessarily correlated with the area of gadolinium enhancement.56
Diffusion-weighted MRI and diffusion tensor imaging have also been used to define the extent of tumor invasion. Diffusion tensor imaging describes the movement of water in tissue by using two measurements, mean diffusivity and fractional anisotropy, which represent the magnitude and direction of water molecule movement, respectively. Abnormalities in these values are due to a combination of increased water content and tumor infiltration leading to more disorganized diffusion. These alterations in value also extend beyond the border of gadolinium enhancement and T2 hyperintensity, thus suggesting that tumor invasion is more diffuse than was previously believed.57
Mechanism of Glioma Spread
Invasion of tumor cells into peripheral normal tissue is thought to be a multifactorial process requiring the interaction of tumor cells with the ECM and surrounding normal healthy tissue. Unlike other neoplasms, both benign and malignant gliomas infiltrate brain parenchyma early during inception, and they continuously infiltrate and spread from the focus of origin. Scherer defined and categorized the mechanism of glioma spread according to the presence of certain secondary structures that are formed during this process.58 These growths are perineural, surface, perivascular, perifascicular, intrafascicular, interfibrillary, white and gray matter, or a combination of these types. In one analysis, the pattern of spread in malignant glioma was studied by MRI in 47 patients.36,59 Spread along the myelin tract was detected in 34% of invading tumors (28% involving the CC and 6% involving other white matter tracts). T2-weighted studies suggested that a larger proportion of tumors located close to the CC invaded this white matter tract. Among tumors located near the CC (within 2 cm), spread throughout the CC was observed in 16 of 27 patients (59%) at diagnosis and in 14 of 17 patients (82%) at follow-up. Invasion through the adjacent ECM was noted in 38% of patients, along the basement membranes (predominantly subependymal) in 16%, and through cerebrospinal fluid (CSF) in 6%. The track was unclassifiable in 6%. Presumably, the perivascular, perineural, or leptomeningeal growth56 or subependymal spread36 is related to ECM molecules organized as a basement membrane or otherwise. Thus, the most frequent mechanism of spread for gliomas is invasion through the white matter or the ECM.
Spread of Glioma through White Matter
Spread of glioma through white matter was originally observed by Strobe60 and later described in detail by Scherer.58 GBM originates predominantly and spreads diffusely in the subcortical white matter.8,61 Oligodendrogliomas display a similar pattern of growth in white matter.59 Spread of malignant gliomas through all the white matter tracts (i.e., CC, uncinate fasciculus, fasciculus longitudinalis and occipitofrontalis, auditory and visual bundles, and corona radiata) has been documented.11 The CC is the main white matter tract that leads to bilateral growth.8,9 Maxwell’s autopsy study of GBM revealed that in 28 patients with GBM, as many as 75% had interhemispheric extension via the CC visible to the naked eye.10 In MRI studies of living patients, malignant glioma spread across the CC in 34% of cases, and in gliomas that originated within 2 cm of the CC, more than 80% invaded along that pathway.
Spread of Glioma along the Basement Membrane or Extracellular Matrix
Although observed rarely by Scherer,58 subependymal growth was noted in 16% of high-grade gliomas in our series (Fig. 103-2). This disparity might be a reflection of today’s improved survival. Of 48 recurrent cases of GBM studied by Sato and colleagues, 83% extended along the periventricular region.62 The most frequent pathway for the spread of malignant glioma in the study by Rosenblum and coworkers was the adjacent brain parenchyma (38%).38
Cerebrospinal Fluid Dissemination
The spread of malignant glioma through the CSF usually involves the basal leptomeninges and occurs more frequently than is generally realized.59 The reported incidence varies from 6.7% to 21%.35,36,60,63 Because of microscopic seeding that is not apparent clinically or radiographically, CSF dissemination is detected more commonly at autopsy. Children have the highest rate of CSF dissemination at diagnosis; it is either already present or develops during follow-up in a third of children.64,65 Spinal cord seeding can occur in 1.2% to 4.7% of malignant gliomas, and the rate may be higher in those with low-grade glioma.17,18
Molecular Basis of Glioma Invasion
In vivo and in vitro investigations have shown the predilection of human glioma cells or glioma cell lines to invade white matter tracts and along ECM substrates.48,66–72 The CC in particular is a major pathway for the migration of implanted neoplastic cells. Glioma invasion along blood vessels and perivascular spaces, between the ependyma and subependyma, and along the glia limitans can be explained by the migration of glioma cells along the basal lamina. The preference of implanted tumor cells to migrate to perivascular spaces and myelinated fiber tracts has also been demonstrated by Pedersen and associates.69 Prominent invasion of the CC was observed, with tumor cells found in the contralateral hemisphere. Chicoine and Silbergeld found tumor cells in the contralateral hemisphere as well, with cells predominantly in the white matter and lining the CSF pathways after implantation.50 Using cell adhesion and monolayer migration assays, Giese and colleagues demonstrated that myelin and the ECM protein merosin are the most permissive substrates for tumor cell attachment and migration.68 Other investigations have suggested that either fibronectin or laminin produced by mesenchymal cells in the brain (blood vessels, leptomeninges) plays a pivotal role in the non–white matter–dependent infiltration of glioma cells into surrounding parenchyma.67,73
Gliomas have been found to have two discrete populations of cells: a core of proliferating cells surrounded by cells invading the brain parenchyma. The gene expression profile between these two populations has also been found to differ. Several genes currently under investigation, including those for P311,74 death-associated protein 3,75 Fn14,76 and phosphatidylinositol-3′-kinase,77 have been found to have increased expression in invasive-phenotype cells.
Adhesion Molecules in Glioma
ECM molecules, cell surface adhesion molecules (CAMs), and molecules that act as receptors for ECM components or CAMs are collectively called adhesion molecules. They are secreted by cells and accumulate on the cell surface and in the extracellular space around the cells.78–80 It should be noted that these are not distinct classes of molecules. Some ECM molecules may act as CAMs or receptors on the cell surface, and some CAMs may be released and incorporated in the ECM. In vitro, almost all these molecules modify adhesion to the substrates and are therefore called adhesion molecules. Some individual molecules or their fragments may actually have a negative effect on adhesion of cells to the substratum. Besides adhesion, they have a more complex and integral function in modifying the cellular response to external stimuli during development and in the mature state. Such functions include signaling for trophic effect, triggering or suppressing apoptosis, and binding growth factors, proteases, and protease inhibitors; adhesion may or may not be the primary role of these adhesion molecules.
Extracellular Matrix Molecules
The “ground substance” initially recognized by Golgi as a network of fibrillar and amorphous material surrounding the neurons81 was discredited for several decades. This changed in the 1970s and 1980s, when careful examination of the nervous ultrastructure and the use of sensitive immunohistochemistry confirmed the presence of an ECM in the nervous system. The ECM in the brain is scant and unorganized except for the basement membrane around the blood vessels, yet by some estimates about 17% to 20% of total brain volume consists of ECM.82 Some CAMs and certain trophic factors can also be incorporated into the ECM.
The ECM has an important role in the invasion of gliomas into surrounding tissue. Nearly all of the ECM proteins are found in the perivascular space, which is a preferred mode of invasion of glioma cells.83 In addition, various studies have shown that gliomas cause alterations in the composition of ECM and that glioma cells require the presence of ECM proteins to convert to a migratory phenotype.84 Glioma cells grown in isolation in culture do not migrate, whereas exposure to certain ECM components such as laminin, collagen IV, tenascin-C, and vitronectin stimulates radial migration from glioma spheroids (Table 103-1).85
| ECM COMPONENT | CHARACTERISTICS |
|---|---|
| Collagen | Restricted to the mesenchymal component of glioma, except type IV |
| Fibronectin | Expression restricted to glioma blood vessels |
| Laminin | Associates with the mesenchymal component of glioma |
| Tenascin-C |
ECM, extracellular matrix; SPARC, secreted protein, acidic, and rich in cysteine.
Collagen
Collagen is the major glycoprotein in extraneural tissue. At least 19 different forms of collagen and 35 genes that encode for them have been found.77 The mesenchymal tissue (blood vessels and meninges) of normal brain and glioma expresses fibril-forming collagen (types I, III, V, VI, and VII). Collagen type IV, which is classified as a sheet-forming collagen, is extensively expressed in the developing nervous system but is restricted to the synaptic basal lamina in the developed brain. Although glioma cells can deposit different types of collagen in vitro, collagen in situ is restricted to the mesenchymal component of the glioma, with the exception of type IV, which may surround individual glioma cells.77
Fibronectin
Expressed abundantly by fetal neurons and glia, fibronectin is restricted to the mesenchymal structures in normal brain. Fibronectin can be expressed in vitro by several glioma cell lines, whereas in situ expression of fibronectin by glioma cells is scant and somewhat restricted to glioma blood vessels. Fibronectin can also be detected weakly in brain tissue infiltrated by glioma cells.71,86–88
Laminin
Laminin is a cruciform molecule formed by disulfide bonding of three chains. Each chain has a few isoforms that in turn give rise to at least 18 isoforms of laminin. Expression of laminin-1 in the normal human brain and in gliomas is codistributed with collagen type IV; laminin-2, also known as merosin, is the predominant form expressed by reactive astrocytes and occasional glioma cells in situ and by primary astrocytes and glioma cell lines in vitro.71,77,89 Expression of laminin parallels that of fibronectin in low- or high-grade glioma (i.e., it associates with the mesenchymal tissue of the tumor, with only occasional variants of glioma expressing it around individual neoplastic cells).85,86
Tenascin-C
Tenascin-C was initially identified as an antigen that not only is present in the ECM of gliomas but also is expressed in embryonic brain and other fetal tissue.90,91 Its expression generally declines as the brain matures, but in contrast to other ECM proteins such as fibronectin and laminin, it is found in vertebrate brain tissue throughout life.92,93
Each arm has two recurrent motifs: centrifugally, there is a repeat of approximately 13 epidermal growth factor (EGF)-like domains, and centripetally, there is a string of 8 to 15 fibronectin (FN) type III domains.76,78 Besides binding to cell surface integrin receptors through RGD sequences, tenascin also binds fibronectin and chondroitin sulfate proteoglycans such as phosphocan and neurocan in the ECM.94–96
The biologic role of tenascin seems to be more complex than that of any other ECM protein. Tenascin exhibits both adhesive and repulsive properties ascribable to separate molecular domains, mediates neuron-glia interaction, and also creates inhibitory boundaries within the brain and glial scars.90,94 It is upregulated extensively in granulation tissue and astroglial scars, as well as in a variety of mesenchymal tumors and cancers such as glioma, fibrosarcoma, osteosarcoma, melanoma, mammary carcinoma, and squamous cell carcinoma (see Fig. 103-2).86,87 In gliomas, its expression correlates with the degree of anaplasia, although the expression may be more heterogeneous in anaplastic astrocytomas and GBMs.88,92,97
Tenascin-specific antibodies labeled with 131I were used in an attempt to treat gliomas by directing local radiation therapy. Only a slight increase in survival times was noted, although this may have been complicated by study design. In addition, tenascin-C may play a role in angiogenesis. By inhibiting angiogenesis, invasiveness of the tumor may have actually been increased, thus confounding any therapeutic response from local irradiation.98
Thrombospondin
Thrombospondins are a family of at least four related trimeric proteins consisting of three approximately 180-kD subunits and are the product of four related genes. They are expressed in both embryonic and adult brain, and their expression correlates with mitotic and migratory events in certain embryonic nervous tissues.76,77 Thrombospondin-1 and thrombospondin-2 are closely related proteins within the larger group of thrombospondin proteins. Both have been found to have an antiangiogenic effect generated through CD36 receptor signaling.99 In addition, both have been found to have increased expression in glioblastomas.100 This may represent a host antitumor response. Expression levels, however, have not been found to have any prognostic value in patients.101 Given that angiogenesis is found in most glioma cells, it is probable that any antiangiogenic response was overwhelmed by proangiogenic factors.
Other Glycoproteins
Only a few other ECM glycoproteins have been found to play a significant role in brain tumor biology in that they are upregulated in glioma. Such glycoproteins include vitronectin, osteopontin, and SPARC (secreted protein, acidic, and rich in cysteine). SPARC, also known as osteonectin, is developmentally regulated and plays an important role in cell migration, matrix mineralization, and angiogenesis. SPARC is strongly upregulated in malignant tumors, including malignant glioma. Although its precise mode of action remains unclear, SPARC leads to increased invasiveness in confrontation assays.102 In contrast, increased expression of SPARC is associated with decreased proliferation.103 Thus, SPARC may induce an invasive phenotype in which proliferation is deferred for migration.
Osteopontin, another acidic glycoprotein associated with bones and present at very low levels in normal brain, is upregulated in gliomas in proportion to the degree of malignancy and can be produced by glioma cells themselves.104 Vitronectin, expressed mainly by hepatocytes, is expressed at high levels by mesenchymal tissues in the brain.105 Vitronectin is also expressed in high-grade gliomas but is undetectable in low-grade gliomas. In xenograft models, vitronectin was preferentially detected at invading tumor borders, although it was not clear whether it was expressed by the tumor cells or by the invaded normal brain tissue.106
Matrix Metalloproteinases
Matrix metalloproteinases (MMPs) are a group of Zn2+-dependent enzymes that are integral for normal ECM turnover. They have also been found to be elevated in glioma cells and may contribute to the invasiveness of these tumors. In particular, MMP-2, MMP-9, and the membrane-type MMPs (MT-MMPs) are thought to have a crucial role in the invasiveness of gliomas. MMP-2, MMP-9, and MT-MMPs can degrade all components of the ECM and are activated in tumor tissues. In addition, activated MMPs have been associated with a poorer prognosis, but small interfering RNA (siRNA) directed toward these enzymes decreases the invasiveness of tumor cells.107
Glycosaminoglycans and Proteoglycans
A proteoglycan consists of a core protein to which glycosaminoglycan chains are attached. Thus far, chondroitin sulfate proteoglycan and heparan sulfate proteoglycan seem to be important for the central nervous system. Some dermatan sulfate proteoglycans such as decorin and biglycan, which may also contain chondroitin sulfate glycosaminoglycans, are expressed in the nervous system and upregulated in neural injury and degenerative processes.76,77 Most of the proteoglycans are present in the ECM, but some may be cell surface proteoglycans and might serve as receptors for growth factors, as well as for ECM components. These proteoglycans include syndecans (which have both heparan sulfate and chondroitin sulfate chains), betaglycan, and NG2.76
