SYNTHESIS AND BIOLOGICAL EFFECTS OF NO IN MALIGNANT GLIOMA-CELLS - MODULATION BY CYTOKINES INCLUDING CD95L AND TGF-BETA, DEXAMETHASONE, ANDP53 GENE-TRANSFER

Nitric oxide (NO) is thought to play an important role in neurotransmi ssion, inflammation, and regulation of cell death in the mammalian bra in. Here, we examined the synthesis and biological effects of NO in hu man malignant glioma cells. Exposure to cytokines such as interferon ( IFN)-gamma, tumor necrosis factor (TNF)-alpha or interleukin (IL)-1 be ta and lipopolysaccharide (LPS) induced NO synthesis in rat C6 and A17 2 human glioma cells, but not in LN-229, T98G or LN-18 human malignant glioma cells. Induced release of NO involved enhanced expression of i nducible NO synthase (iNOS). Failure to detect NO release in the latte r cell lines was not overcome by neutralization of endogenous TGF-beta or by coexposure to cytokines, LPS, and antioxidants. Apoptosis induc ed by CD95 ligand (CD95L) did not involve NO formation. Neither NOS in hibitors nor NO donators modulated CD95L-induced apoptosis. Dexamethas one (DEX)-mediated protection of glioma cells from CD95L-induced apopt osis was also independent of DEX effects on NO metabolism. DEX inhibit ed not only cytokine/ LPS-evoked NO release but also attenuated the to xicity of NO in three of five cell lines. Forced expression of tempera ture-sensitive p53 val(135) in C6 cells in either mutant or wild-type conformation inhibited cytokine/LPS-induced NO synthesis. Further, acc umulation of p53 in both mutant or wild-type conformation protected gl ioma cells from the toxicity of exogenous NO, consistent with a gain o f p53 function associated with p53 accumulation. We conclude that resi stance to NO-dependent immune defense mechanisms may contribute to the malignant progression of human cancers with p53 alterations, notably those associated with the accumulation of mutant p53 protein.