The identification of novel therapeutic targets for the treatment of malignant brain tumors.

A two-step strategy was developed consisting of differential display reverse transcriptase polymerase chain reaction (DDRT-PCR) with cultured normal human fetal astrocytes and U-373MG glioma cells followed by reverse Northern analysis of normal brain and primary tumor tissues. hu-dek, alpha-NAC, ribosomal proteins L7a and L35a, and five novel genes were identified. Since none of these genes has been previously shown to be associated with malignant brain tumor formation, this approach may be useful to identify novel targets for the diagnosis and treatment of brain tumors.

Gene transfection-mediated overexpression of beta1,4-N-acetylglucosamine bisecting oligosaccharides in glioma cell line U373 MG inhibits epidermal growth factor receptor function.

N-linked oligosaccharides appear to be important for the function of the epidermal growth factor (EGF) receptor. In a previous study (Rebbaa, A., Yamamoto, H., Moskal, J. R., and Bremer, E. G. (1996) J. Neurochem. 67, 2265-2272), we showed that binding of the erythroagglutinating phytohemagglutin lectin from Phaseolus vulgaris to the bisecting structures on the EGF receptor from U373 MG glioma cells blocked EGF binding and receptor autophosphorylation. In this study we examined the consequences of overexpression of the bisecting structure on the EGF receptor by gene transfection of U373 MG cells with the N-acetylglucosaminyltransferase III (GnT-III). This modification leads to a significant decrease in EGF binding and EGF receptor autophosphorylation. In addition, the cellular response to EGF was found to be altered. Proliferation of U373 MG cells in serum-free medium is inhibited by EGF. In contrast, proliferation of the GnT-III-transfected cells was stimulated by EGF. These data demonstrate that changes in EGF receptor glycosylation by GnT-III transfection reduces the number of the active receptors in U373 MG cells and that this change results in change in the cellular response to EGF.

Ganglioside GM3 inhibition of EGF receptor mediated signal transduction.

Ganglioside GM3 is a membrane component that has been described to modulate cell growth through inhibition of EGF receptor associated tyrosine kinase. In order to determine if the inhibition of cell growth by this ganglioside is specifically mediated through EGF receptor signaling, the effects of GM3 on key enzymes implicated in EGF signaling were determined and compared to another inhibitor of the EGF receptor kinase. Treatment of A1S cells in culture by GM3 or a tyrosine kinase inhibitor, leflunomide, led to the inhibition of MAP kinase and PI3 kinase activities. There was no detectable effect on phosphotyrosine phosphatases. In a cell free system, however, GM3 had no effect on the activity of these signaling intermediates. Leflunomide was able to directly inhibit MAP kinase activity. GM3 and leflunomide were also found to act differently on the expression of the early immediate genes. The expression of c-fos and c-jun was inhibited by both GM3 and leflunomide. The expression of c-myc, however, was only inhibited by leflunomide. These findings suggest that the action of GM3 on cell growth and signaling is specifically mediated by EGF receptor and that this ganglioside does not act directly on the intracellular intermediates of EGF receptor signaling. In addition, soluble small molecule tyrosine kinase inhibitors such as leflunomide can directly affect the activity of MAP kinases and possibly other signaling intermediates. The direct effects of leflunomide on signaling intermediates may explain the differential effects of leflunomide and GM3 on gene expression and cell growth.

The expression of Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase and alpha 2,6-linked sialoglycoconjugates in human brain tumors.

CMP-NeuAc: Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase (alpha 2,6-ST) [EC 2.4.99.1] is developmentally regulated, shows a high degree of tissue specificity, and appears to play a role in oncogenic transformation and metastasis. In the present study, we have performed the first detailed analysis of the expression of alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugates in human brain tumors. We used a polyclonal, monospecific anti-rat alpha 2,6-ST antibody and the alpha 2,6-linked sialic acid-specific lectin, Sambucus nigra agglutinin (SNA) for histochemical studies, and a human alpha 2,6-ST-specific cDNA probe for Northern analysis. Meningiomas, chordomas and craniopharyngiomas frequently expressed alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugates. Among the different meningioma subtypes, meningothelial meningiomas stained more strongly with both anti-alpha 2,6-ST antibody and SNA than the fibroblastic and anaplastic meningiomas. On the other hand, all tumors of glial origin and medulloblastomas were virtually devoid of either alpha 2,6-ST or alpha 2,6-linked sialoglycoconjugate expression. Moreover, very weak to negligible expression of both alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugates was observed in brain metastases. In conclusion, alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugate expression is associated with non-neuroectodermal epithelial-like tumors.

Multidrug resistance phenotype associated with selection of an aminopterin resistant dog kidney cell line.

A determination of the mechanisms of drug resistance in tumour cells is important for developing strategies to combat such resistance in persons receiving chemotherapy. This report describes a combined cellular, biochemical, and molecular analysis of a dog kidney cell line selected for resistance to increasing levels of the hydrophilic antifolate, aminopterin. Three distinct drug resistance phenotypes were observed in cells exhibiting high levels of aminopterin resistance. Two of these phenotypes were decreased aminopterin accumulation and increased levels of dihydrofolate reductase specific activity. The third drug resistance phenotype was noted initially as cross resistance to a variety of hydrophobic drugs indicating multidrug resistance. Biochemical assays demonstrated reduced accumulation of the hydrophobic fluorescent drug daunorubicin and of 3H-colchicine in the aminopterin resistant cells. These results were then correlated with increased levels of the multidrug resistance (mdr) gene product, P-glycoprotein, and mdr mRNA levels in the aminopterin resistant cells. However, experiments designed to prove a role for expression of the mdr gene in providing a degree of aminopterin resistance were unsuccessful. It is concluded that aminopterin selection in these dog kidney cells resulted in expression of at least three distinct drug resistance phenotypes and that one of these phenotypes, multidrug resistance, represented a secondary response to the aminopterin selection.