Significance of experimental models in neurooncology

Angiogenesis and cell proliferation in human craniopharyngioma xenografts in nude mice

OBJECT

Craniopharyngioma is one of the most common congenital tumors of the sellar and suprasellar regions and accounts for between 4 and 6% of all intracranial tumors. Its oncogenesis and biological behavior have not been well studied, and neither a cell line nor an animal model have been established. To better understand the tumor and improve its clinical management, the authors investigated the angiogenesis and cellular proliferation in subcutaneous craniopharyngioma xenografts obtained by implanting human tumor cells into athymic nude mice.

METHODS

Human craniopharyngioma cells obtained from surgical specimens were subcutaneously implanted into BALB/c-nu/nu nude mice to establish a preliminary animal model of a transplanted tumor. Immunohistochemical staining with streptavidin-peroxidase complex was used to identify the cell phenotype and to evaluate the angiogenesis and proliferation in the xenografts. Expression of cytokeratin, minichromosome maintenance deficient 6 (MCM6) protein, and endothelial cell marker CD34 on the xenograft sections were assayed quantitatively by computer-assisted microscopy. Twenty-seven surviving subcutaneous xenografts were obtained in 15 nude mice. The total implantation success rate was 28.12% (adamantine epithelioma [AE], 37.50%; squamous papillary tumor [SPT], 18.75%). Formation of capillaries and cell proliferation were observed in all of these xenografts. Microvessel density and degree of MCM6 immunostaining were positively correlated in the surviving grafts (r = 0.410, p < 0.05), but there was no significant difference in these variables between the AE and SPT groups (p > 0.05).

CONCLUSIONS

A preliminary animal model of human craniopharyngioma was established in the nude mouse by heterotopic implantation. Surviving xenografts maintained their vascularization and proliferation activities until harvesting at 12 weeks.

Molecular genetic characterisation of intracerebrally transplanted brain tumours

The aim of the present study was the characterisation of genetic alterations in two different experimental gliomas, induced in rats from the inbred strain BDIX by transplacental ethylnitrosourea with subsequent serial transplantation. The genes investigated have been shown previously to be altered during human glial tumour progression and include the gene for the epidermal growth factor receptor (EGFR), the genes for the cell cycle regulators cyclin dependent kinase 4 (CDK4), cyclinD1 (cycD1), the p16 gene (MTS1/INK4) and the retinoblastoma gene (RB). Using a semi-quantitative PCR-based screening method no gross alterations could be detected in these genes, demonstrating that nitrosourea-induced glial tumours of rats do not harbour those genetic changes which typically arise in human malignant gliomas. Thus, the use of this tumour model for gene therapy trials is questionable.

AgNOR content and PCNA expression in transplanted malignant neurinoma in rats

Malignant neurinomas can be induced in BD IX rats by transplacental application of ethylnitroso-urea during pregnancy. Tumors develop in the offspring in trigeminal and spinal nerves and can be easily transplanted upon rats of the same strain. During the first passages a considerable shortening of subsequent induction periods takes place. Concurrently, silver stained Nucleolar Organizer Regions (AgNORs) increase in number and other measured AgNOR parameters change in a similar way. The number of cells that express the proliferation marker PCNA equally becomes more frequent during the first subcutaneously transplanted generations. There is high correlation between AgNOR parameters, number of PCNA expressing cells, induction times and passage. It is concluded that the first generations of the transplantation model of these tumors can be used to test the validity of proliferation indicators. Our results show further that AgNORs in fact belong to the group of markers of proliferation.

Immunohistochemical investigations in experimentally induced tumors of the nervous system

35 tumors of brain, spinal cord and cranial and peripheral nerves were induced with ENU (ethyl-nitrosourea) in the offspring of treated BD-IX pregnant rats. 36 tumors--35 of the nervous system, one nephroblastoma--were observed in 14 rats. With these results, the number of experimental nervous system tumors of the own collection induced in BD-IX rats and classified next to the rules of human neurooncology, amounts to 2,216. All 35 tumors of the nervous system were treated by a panel of immunohistochemical reactions comprising antibodies against cytoskeleton intermediary filaments such as GFAP (glial fibrillary acid protein), neurofilament proteins, vimentin and cytokeratins and some nervous system antigens such as NSE (neuron specific enolase), MBP (myelin basic protein) and S-100 protein. In central tumors, considered to be malignant gliomas, focal reactivity against vimentin and GFAP was found. Expression of other tested markers was weak or absent. In neurinoma of trigeminal and peripheral nerves, reactivity to S-100 antigen was lacking, whilst there was strong reaction to the vimentin antigen.

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.

Chemotherapy in intracerebrally transplanted tumors induced with transplacental administration of ethylnitrosourea in rats

Four tumors of the spinal cord were induced with ethylnitrosourea in rats by transplacental administration and transplanted into the brains of animals of the same strain. One of these intracerebrally grafted tumor lines (G-XIII) was followed up over the first 10 passages and treated with CCNU and other alkylating drugs. The results were compared with findings in an earlier established line (G-XII) in passage 12 and 59, which in the first instance was sensitive to CCNU. The CCNU application prolonged survival in treated animals in various treatment schedules in the first 10 intracerebrally grafted generations of the tumor up to 59%. Induction times of tumors became increasingly shorter. The susceptibility of early passages was similar in both lines. Its loss in late passages went together with diffuse growth of the tumor and reticulin fiber production. In addition, glial fibrillary acid protein expression and formation of intermediate filaments in perivascular tumor cells was lost.

Morphology of early stages of ENU-induced brain tumors in rats

Tumors were induced in experimental animals in order to investigate early tumor growth with conventional histology and gliofibrillary acid protein (GFAP) demonstration with the peroxidase-antiperoxidase (PAP) method. In one experiment, the animals were killed after transplacental ENU administration, when microtumors were suspected; in the second experiment, the animals were followed up to their natural death and microtumors found at random were used for analysis. Conventional histology revealed 3 types of microtumors: growth restricted to the subventricular matrix, growth in the neighbourhood of the ventricles with obvious or probable connection to the ventricular zones and small tumors observed exclusively in the white matter. The latter tumors by conventional staining were composed of small round cells, considered to be oligodendrocytes. They did not contain GFA-protein positive cells within the tumor. The tumor in presumable and visible connection with the ventricular lining did contain GFAP-positive astrocytes. In the very small subventricular tumors, the small round cells (oligodendrocytes) were in continuous contact with the identical interfascicular glial cells, while GFAP positive astrocytes seemed to stem from the subventricular astrocytes (tanycytes, ependymoglia). The ependyma itself was always preserved. A twofold origin of these experimental tumors with probable development into one common cytological glial type is assumed.