Immunohistochemical studies on cellular character of microtumors induced by ethylnitrosourea in the rat brain utilizing anti-Leu 7 and anti-glial fibrillary acidic protein antibodies

Alterations in cerebrospinal fluid proteins in a presymptomatic primary glioma model

Background

Understanding the early relationship between brain tumor cells and their environment could lead to more sensitive biomarkers and new therapeutic strategies. We have been using a rodent model of neurocarcinogenesis in which all animals develop brain tumors by six months of age to establish two early landmarks in glioma development: the appearance of a nestin⁺ cell at thirty days of age and the appearance of cellular hyperplasia between 60 and 120 days of age. We now report an assessment of the CSF proteome to determine the changes in protein composition that occur during this period.

Materials and Methods

Nestin⁺ cell clusters and microtumors were assessed in 63 ethylnitrosourea-exposed rats on 30, 60, and 90 days of age. CSF was obtained from the cisterna magna from 101 exposed and control rats at 30, 60, and 90 days and then analyzed using mass spectrometry. Differentially expressed peaks were isolated and identified.

Results

Nestin⁺ cells were noted in all ethylnitrosourea-exposed rats assessed pathologically. Small microtumors were noted in 0%, 18%, and 67% of 30-, 60-, and 90-day old rats, respectively (p<0.05, Chi square). False Discovery Rate analysis of peak intensities showed that the number of true discoveries with p<0.05 increased markedly with increasing age. Isolation and identification of highly differentially detected proteins at 90 days of age revealed increases in albumin and a fragment of α1 macroglobulin and alterations in glutathionylated transthyretin.

Conclusions

The presence of increased albumin, fragments of cerebrospinal fluid proteins, and glutathione breakdown in temporal association with the development of cellular hyperplasia, suggests that, similar to many other systemic cancers, inflammation and oxidative stress is playing an important early role in the host’s response to brain tumor development and may be involved in affecting the early growth of brain tumor.

Neurogenic tumors in rats induced by ethylnitrosourea

Ethylnitrosourea (ENU) was injected intravenously into Sprague-Dawley rats on day 15 of gestation at doses of 0, 2.50, 6.25 and 10.00 mg/kg. The resulting 1980 progeny were observed for up to 24 months in a life-time study (900 rats) or for periods of 171-325 days in a serial sacrifice study (1080 rats). The rats in both studies were randomized into three groups, one exposed to a radiofrequency, one sham-exposed and one cage control. Since no effects of the radiofrequency were observed on the ENU-induced tumors, the exposure groups were combined to facilitate study of the tumors by dose rate over time. All rats were necropsied and major organs were examined histologically including the brain, entire spinal cord, trigeminal nerves and all tumors. A total of 48 spinal cord tumors (SCT), 251 spinal nerve tumors, 264 cranial nerve tumors and 1058 brain tumors were studied. The tumors were characterized by incidence, histologic type, volume, malignancy and multiplicity. Ethylnitrosouria, as given in this study, was determined to be an effective carcinogen reliably inducing (in order of frequency) brain, cranial nerve, spinal nerve and SCT. Dose of ENU correlated positively with the frequency, multiplicity, volume, malignancy, and negatively with latency of brain tumors and to a lesser extent with nerve tumors.

Isolation of immortalized, INK4a/ARF-deficient cells from the subventricular zone after in utero N-ethyl-N-nitrosourea exposure

OBJECT

Brain tumors, including gliomas, develop several months after rats are exposed in utero to N-ethyl-N-nitroso-urea (ENU). Although pathological changes cannot be detected until these animals are several weeks old, the process that eventually leads to glioma formation must begin soon after exposure given the rapid clearance of the carcinogen and the observation that transformation of brain cells isolated soon after exposure occasionally occurs. This model can therefore potentially provide useful insights about the early events that precede overt glioma formation. The authors hypothesized that future glioma cells arise from stem/progenitor cells residing in or near the subventricular zone (SVZ) of the brain.

METHODS

Cells obtained from the SVZ or corpus striatum in ENU-exposed and control rats were cultured in an epidermal growth factor (EGF)-containing, chemically defined medium. Usually, rat SVZ cells cultured in this manner (neurospheres) are nestin-positive, undifferentiated, and EGF-dependent and undergo cell senescence. Consistent with these prior observations, control SVZ cells undergo senescence by the 12th to 15th doubling (20 of 20 cultures). In contrast, three of 15 cultures of cells derived from the SVZs of individual ENU-treated rats continue to proliferate for more than 60 cell passages. Each of these nestin-expressing immortalized cell lines harbored a common homozygous deletion spanning the INK4a/ARF locus and was unable to differentiate into neural lineages after exposure to specific in vitro stimuli. Nevertheless, unlike the rat C6 glioma cell line, these immortalized cell lines demonstrate EGF dependence and low clonogenicity in soft agar and did not form tumors after intracranial transplantation.

CONCLUSIONS

Data in this study indicated that immortalized cells may represent glioma precursors that reside in the area of the SVZ after ENU exposure that may serve as a reservoir for further genetic and epigenetic hits that could eventually result in a full glioma phenotype.

Aberrant nestin expression during ethylnitrosourea-(ENU)-induced neurocarcinogenesis

Nestin is a unique intermediate filament protein. While it is robustly expressed in developing brain, postnatal expression is limited to the brain's subventricular zone (SVZ) and endothelial cells. Reexpression occurs, however, under several pathological conditions, including injury and neoplasia. We hypothesized that nestin would be a sensitive marker of early neoplasia after transplacental exposure of rats to ethylnitrosourea (ENU). Rats of various ages were administered bromodeoxyuridine (BudR) before sacrifice, and brain sections were examined for proliferative cells and several immunohistochemical markers, including nestin. Additional rats were examined after a stab wound injury to assess the expression of two of these markers, GFAP and nestin, in reactive astrocytes. All ENU-induced brain tumors (n = 9) were classified as gliomas (astrocytomas or oligoastrocytomas) based on their histology and immunophenotype. Nestin expression was noted in all tumors examined and was present in tumor cells as well as endothelial cells. During tumor development, we consistently noted nestin-expressing cells bearing multiple processes distributed throughout brain parenchyma. Both single cells and multiple cell clusters were observed as early as postnatal day 30 in all ENU-exposed brains examined (n = 11). Such distinctive nestin-expressing cells were not seen in nestin-stained control brains or ENU-exposed brains stained for GFAP or vimentin, nor was such a cell seen in a stab wound model used to assess reactive astrocytosis. While the number of these clusters was highly variable among rats, their size increased between 30 and 90 days. The data suggest that these nestin-expressing cells represent an early stage of the neoplastic process. It remains to be determined whether these cells become apparent at 30 days of age due to "dedifferentiation" of a local resident astrocyte or astrocyte precursor cell or migration of a relatively undifferentiated precursor/stem cell from the SVZ.

Neural stem cells and neuro-oncology: quo vadis?

Conventionally, gliomas are assumed to arise via transformation of an intraparenchymal glial cell that forms a mass that then expands centrifugally, eventually invading surrounding tissues. We propose an alternative model in which gliomas arise via initiation and promotion of cells within the brain's subependymal layer or subventricular zone, the source of a recently characterized pool of neural cells with the properties of self-renewal and multipotentiality (i.e., stem cells) that persists into adulthood. In this model, the particular histological subtype of glioma would represent the effects of temporal and spatial environmental influences rather than the particular cell of origin and the disease's centrifugal point would be the subependymal layer. The implications of such a model are discussed.

Magnetic resonance imaging of ethyl-nitrosourea-induced rat gliomas: a model for experimental therapeutics of low-grade gliomas

Human low-grade gliomas represent a population of brain tumors that remain a therapeutic challenge. Preclinical evaluation of agents, to test their preventive or therapeutic efficacy in these tumors, requires the use of animal models. Spontaneous gliomas develop in models of chemically induced carcinogenesis, such as in the transplacental N-ethyl-N-nitrosourea (ENU) rat model. However, without the ability to detect initial tumor formation, multiplicity or to measure growth rates, it is difficult to test compounds for their interventional or preventional capabilities. In this study Fisher-334 rats, treated transplacentally with ENU, underwent magnetic resonance imaging (MRI) examination in order to evaluate this approach for detection of tumor formation and growth. ENU-induced intracranial cerebral tumors were first observable in T2-weighted images beginning at 4 months of age and grew with a mean doubling time of 0.487 +/- 0.112 months. These tumors were found histologically to be predominately mixed gliomas. Two therapeutic interventions were evaluated using MRI, vitamin A (all-trans retinol palmitate, RP), as a chemopreventative agent and the anti-angiogenic drug SU-5416. RP was found to significantly delay the time to first tumor observation by one month (P = 0.05). No differences in rates of tumor formation or growth rates were observed between control and RP-treated groups. MRI studies of rats treated with SU-5416 resulted in reduction in tumor growth rates compared to matched controls. These results show that MRI can be used to provide novel information relating to the therapeutic efficacy of agents against the ENU-induced tumor model.

Evaluation of ENU-induced gliomas in rats: nomenclature, immunochemistry, and malignancy

Rats developed mixed gliomas, oligodendrogliomas, and a few astrocytomas in response to transplacental ethylnitrosourea. The neoplastic cell composition of mixed gliomas must be defined; this study required a 20-80% admixture of neoplastic astrocytes and oligodendroglia for the diagnosis of mixed glioma. A battery of immunoantibodies, including Leu-7, S-100, and vimentin, were helpful in classifying rat gliomas, and the histologic features of each tumor type are described. Other brain tumor characteristics that may decide the outcome of carcinogenicity studies include incidence, multiplicity, latency, fatality, size, and malignancy. The size of tumors was determined by measuring their 3-dimensional volumes. Brain tumor volume was found to be highly correlated with malignancy and fatality. Systematic evaluation of the malignancy of brain tumors is an important but often overlooked adjunct method of measuring the effectiveness of a carcinogen. A system to estimate malignancy, one that grades 9 tumor characteristics and weights, each according to clinical outcome, was developed. It was found that mixed gliomas grew larger, had a shorter latency, and were significantly more malignant than were other gliomas.

Immunohistochemical localization and biological significance of the phylogenically conserved thymus-brain antigen (UB-13 antigen) in skate, rat and human

A monoclonal antibody (UB-13) originally raised against the brain of the skate (Raja kenojei, a cartilaginous-fish) was found to react with lymphoid and brain tissues from many species when examined immunohistochemically. In rat and human thymus, UB-13 antigen was observed to be closely associated with reticular tissue in the medulla and cortex. Interestingly, a few or several thymocytes were encircled by the UB-13-reactive reticular tissue. At 14 days gestation, rat thymus consisted mainly of reticular epithelial tissue, after which strong thymocyte production started. At this stage, some of the reticular tissue was heavily stained with UB-13. In the thymus tissues of the irradiated and recovering rats, where reduction and massive reproduction of thymocytes were observed, extensive UB-13 antigen expression localized on the reticular epithelial tissue, an observation which may support the thymocyte re-population. These findings suggest that the antigen recognized by UB-13 may be important for thymocyte proliferation and maturation. UB-13 antigen was found in the fibrous structure of the molecular and granular layer of the human cerebellum. Some glial cells were also stained strongly with UB-13 in the human cerebellar or cerebral grey and white matter. In rat, glial cells, especially astroglias, and the endothelial structure of blood vessels were stained strongly with UB-13. These findings suggest that UB-13 may be a useful monoclonal antibody for analysis of brain-lymphoid antigen in many species.

S-100 protein and glial fibrillary acidic protein (GFAP) in experimentally induced and spontaneous tumours of peripheral nerves in BDVI rats. Light microscopic and immunohistochemical study

Twenty-five ethylnitrosourea (ENU) induced and 24 spontaneous tumours of peripheral nerves as well as 28 spontaneous mesenchymal tumours in BDVI rats were studied by light microscopy and immunohistochemically for the presence of S-100 protein (S-100). Early ENU-induced schwannomas*) representing the thickenings of nerves showed weak or negative S-100 immunoreactivity. S-100 positivity (both in the cytoplasm and nucleus) was observed in all large ENU-induced tumours found in animals dying or killed at advanced age. Immunostaining was present in both cystic and solid areas of schwannomas. S-100 positivity was found in 20 of 24 spontaneous schwannomas: 14 of 20 positive tumours contained cysts. Twenty-seven schwannomas (12 ENU-induced and 15 spontaneous) were studied for the presence of glial fibrillar acid protein (GFAP) and 13 were positive (7 had cystic areas). GFAP-positivity was relatively high in 4 tumours (3 ENU-induced and 1 spontaneous); these tumours also showed intense S-100 reactivity. Immunoreactivity for S-100 occurred more frequently and was much more intense than that for GFAP. The incidence of spontaneous peripheral nerve tumours in BDVI males reached 4%, cystic schwannomas being the most frequent type. All spontaneous mesenchymal tumours except lipoma (S-100 positive) were negative for S-100 protein and for GFAP.

Relationship between glial reaction to a stab wound and tumor development after receiving transplacental ethylnitrosourea in the rat

Fisher 344 rats born from mothers treated with ethylnitrosourea (ENU) 50 mg/kg intravenously were injured at the 1st and 2nd month of extrauterine life by a transcranial stab. The wound affected cerebral cortex, white matter and basal ganglia. The animals were killed 15 and 45 days and 5 months after injury and cell reaction was studied histologically and immunohistochemically. Bromodeoxyuridine (BrdUrd) was administered 1 h before sacrifice and the labeled cells were evaluated. In ENU-treated rats injured at 1 month of age only minor differences were found in comparison with injured controls. In ENU rats injured at 2 months of age and killed 15 days later, a higher number of BrdUrd-labeled cells was found in comparison with controls; 45 days after injury the cell reaction acquired the aspect of a microtumor, however, no microtumor unrelated with the needle track was present. In ENU rats killed 5 months after the injury, there was no difference between injured and not injured ENU-treated rats, as far as the aspect and the number of tumors were concerned. The tumor phenotype was, thus, anticipated by the cell response to trauma in ENU rats. The interpretation is that the additional cell division, in response to trauma, anticipate not only the phenotypic, but also the cell kinetics changes, as indicated by BrdUrd labeling.

Immunohistochemical recognition of ethylnitrosourea induced rat brain microtumors by anti-Leu 7 monoclonal antibody

This immunocytochemical study was undertaken to clarify the histogenesis of ethylnitrosourea-induced rat brain tumors. The tumors induced in offspring of Sprague-Dawley rats injected with ethylnitrosourea on day 18 of gestation were used in these experiments. Controls consisted of pregnant Sprague-Dawley rats similarly injected with saline alone. Both microtumors (less than 1 mm) and macrotumors were examined immunocytochemically. The cells present in both macro- and microtumors were reactive with anti-Leu 7, an antibody which recognizes oligodendrocytes. Intermixed with, but distinct from the tumor cells were glial fibrillary acidic protein positive cells morphologically identical to astrocytes found in other areas distant to tumors in the treated animals, and in controls. These data suggest that both early and late tumors are oligodendrogliomas, not astrocytomas or mixed gliomas, and that the cell of origin of the tumor is the oligodendrocyte rather than an uncommitted stem cell as previously suggested.

Cellular and molecular aspects of neurocarcinogenesis

Although the morphology of neural tumors induced in rats by N-ethyl-N-nitrosourea (NEU) and related alkylating agents has been extensively investigated, their histogenesis and the molecular basis of malignant transformation are still largely unknown. This review gives an account of the interaction of neurocarcinogenic agents with cellular DNA, the possible role of promutagenic O6-alkyldeoxyguanine and their deficient repair by the cerebral O6-alkylguanine-DNA alkyltransferase. A new experimental model is described in which neural tumors are induced in fetal brain transplants. Pregnant rats received a single iv dose of NEU (50 mg/kg) on the 14th day of gestation. One day later, suspensions were prepared from the fetal forebrain and stereotactically injected into the caudoputamen of adult rats. After additional exposure to NEU of the host animals 8 days and 9 weeks post transplantation, all rats developed brain tumors within the neural graft. Histopathologically, all neoplasms were classified as olidogdendrogliomas. Other neoplasms typically induced by NEU transplacentally (astrocytomas, mixed gliomas, ependymomas) were absent. The selective induction of oligodendrogliomas indicates that neoplastic transformation in the nervous system can occur in a differentiated glial cell or a precursor cell committed to oligodendrocytic differentiation, and that transformation of a pluripotential stem cell is not necessary. Transplacental exposure of the donor fetuses to NEU alone, i.e., without additional postgrafting exposure, did not produce brain tumors in any of the experimental animals indicating that in the microenvironment of fetal brain transplants the multistep development of gliomas requires additional mutational events. Malignant schwanomas perinatally induced by NEU carry a point mutation in the transmembrane domain of the neu gene. The mode of oncogene activation in NEU-induced CNS gliomas has not yet been elucidated. We have used cerebral grafting techniques to study the effects of known oncogenes on the developing nervous system, taking advantage of efficient gene transfer by replication-defective retroviral vectors and of the extraordinary capacity of fetal CNS to differentiate in and fully integrate with the host brain. Rats carrying transplants exposed in vitro to the polyoma medium T-antigen developed endothelial hemangiomas in the graft which often led to fatal cerebral hemorrhage within 13-50 days after transplantation. Introduction of the viral src gene caused astrocytic and mesenchymal tumors after latency periods of 2-6 months. Following infection of fetal donor cells with a vector encoding the v-myc oncogene, only a single embryonal CNS tumor was observed whereas exposure to v-H-ras produced a low incidence of gliomas.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of vimentin and glial fibrillary acidic protein in ethylnitrosourea-induced rat gliomas and glioma cell lines

The expression of glial fibrillary acidic protein (GFAP) and vimentin was investigated immunohistochemically in 104 experimental gliomas induced by transplancental application of ethylnitrosourea (ENU) in CDF rats. Immunoreactivity for vimentin was prominent in many astrocytic tumor cells and especially in small glioma cells forming anaplastic medulloblastoma-like foci in many tumors. The majority of tumor cells in oligodendroglial tumors were vimentin negative, except for some of the large polymorphous oligodendrogliomas which contained intermingled vimentin positive glioma cells. GFAP immunoreactivity was detectable only in a low fraction of tumor astrocytes and in a few exceptional cases some oligodendroglial tumor cells stained positive. Immunohistochemistry with antibodies against neurofilaments and cytokeratins revealed no staining in tumor cells of ENU-induced gliomas, while all oligodendrogliomatous tumors stained positive for HNK-1. Immunocytological and immunoblot investigations of the two rat glioma cell clones RG2 and F98, which are both derived from ENU-induced gliomas, showed a prominent expression of vimentin in monolayer cultures and in syngeneic intracerebral transplantation tumors. F98 additionally demonstrated a fraction of GFAP positive cells especially in confluent cultures and in intracerebral tumors. RG2, on the other hand, exhibited virtually no GFAP immunoreactivity in culture but showed individual GFAP positive tumor cells in intracerebral tumors. Our results revealed a more precise picture of the cellular differentiation in ENU-induced rat gliomas and in two widely used glioma cell lines. They underline the heterogeneity of experimental rat gliomas which may comprise cells at different stages of differentiation towards oligodendroglial or astroglial phenotype.

Ultrastructural findings in transplanted experimental brain tumors and their significance for the cytogenesis of such tumors

Tumors induced by transplacental action in the spinal cord of rats were transplanted into the brains of the same rat strain. They were followed up by electron microscopy during the first ten passages. Three architectural features were detected: First pure tumor parts, second myelin breakdown and phagocytosis, and third the resulting accumulation of resting macrophages. Architecture two and three were interpreted as result of considerable phagocytotic activity of tumor cells localized within the white substance of the brain and spinal cord. Only architecture one was considered to represent proper tumor. Since this was low differentiated and partial astrocytic differentiation only occurred around vessels to remarkable extent, the thesis is put forward that these transplacentally induced tumors correspond to human primitive neuroectodermal tumors.

An approach to the determination of the relative potencies of chemical agents during the stages of initiation and promotion in multistage hepatocarcinogenesis in the rat

The potency of carcinogenic agents in eliciting neoplastic lesions has long been a concern of investigators in the field of oncology. This paper describes a method, based on quantitative stereologic calculations, to estimate the relative potency of chemicals as initiating and/or promoting agents. The parameters defined in this paper are: (a) Initiation index = no. foci induced X liver-1 X [mmole/kg body weight]-1; and (b) Promotion index = Vf/Vc X mmol-1 X wk-1. These parameters have been calculated for a number of chemical agents, based both on data from this laboratory and others published in the literature. Neither parameter varied significantly with the dose of two different initiating agents used in this study. The range of promotion indices extended over more than eight orders of magnitude, whereas that of the initiation indices was much less variable. Such parameters may be useful as quantitative estimates of the potency of hepatocarcinogenic agents not only in rodents, but potentially in quantitative risk estimations in the human.

Immunohistochemical recognition of human neuroepithelial tumors by anti-Leu 7 (HNK-1) monoclonal antibody

The immunoreactivity of the anti-Leu 7 (HNK-1) monoclonal antibody, a marker for natural killer cells, was evaluated with the peroxidase-anti-peroxidase (PAP) method on sections of human paraffin-embedded tissues from 135 tumors of the central nervous system and five esthesioneuroblastomas. As shown independently by others, the antibody was found to react with most types of neoplastic neuroepithelial cells. Our findings indicate that the reaction is most often localized on the cytoplasmic membranes. The immunoreactive cell membranes were generally those of well-differentiated tumor cells and of neoplastic cells found in tumors that usually were not embryonal in nature. Parallel immunostaining either of the same or of successive sections with an anti-glial fibrillary acidic protein serum was of considerable assistance in discriminating between different immunoreactive cells, e.g., between astrocytes and cells presumed to be oligodendrocytes. Despite its cross-recognition of cells of various histogenesis, the anti-Leu 7 monoclonal antibody can, in well-defined circumstances, elucidate specific differential diagnostic problems involving neurogenic neoplasms that cannot be resolved with routine staining techniques.