Introduction: Expression of iNOS in human tumor was first described in colorectal adenocardinoma cell lines (Radomski et al., 1991). Because metastatic activity, host defense mechanisms, and level of differentiation seem to be correlated to iNOS expression, a role for iNOS has been proposed in the onset and progression of malignant diseases. Under physiological condition, NO acts as an intracellular secondary messenger and provides an efficient system for cellular regulation, interaction, and defense. The expression of 3 isoforms of NOS in human gliomas and in peritumoral areas have been analyzed by several groups. Although the induction of iNOS has been noted in human meningioma (Bakshi et al., 1998; Broholm et al., 2003; Ellie et al., 1995; Hara et al., 1996), a clear correlation between meningiomas and iNOS expression has not been established. Methods: Human meningiomas of benign, atypical, and anaplastic/malignant grade (n=10) were excised and primary explant cultures obtained. Histopathological confirmation of grade was obtained. Using western blot , expression of inducible nitric oxide synthase (iNOS) was demonstrated for cultured cells and intact tissue fragments. Integrity of downstream NO signaling pathways was tested by exposing cultured cells at early passage to NO donor compounds. The downstream actions of NO take two forms: (1) cGMP-dependent; and (2) cGMP-independent, which are mediated by reactive nitrogen species produced by the interaction of NO with oxygen (O2) or with superoxide radicals (O2.-). The first was tested by measuring levels of guanyl cyclase and of cGMP, and the second by assay of nitrotyrosine formation, in NO-donor stimulated cells of each grade. Results: A clear correlation was shown between iNOS expression and the degree of tumor malignancy, with lowest expression seen in benign tumors and highest seen in those of anaplastic grade. In malignant meningioma cells both membrane-bound and soluble guanylyl cyclase (sGC) were un-regulatable and the cGMP levels were very low despite significantly high levels of iNOS expression. In contrast, in benign meningioma cells sGC responded to NO normally and a marked increase of cGMP was observed upon NO donor stimulation. Thus, the downstream pathways of NO signaling in malignant meningioma appear to be shut down. In additional experiments a significant increase in nitrotyrosine formation (a biomarker for cGMP-independent NO action) was detected in malignant cells. Conclusion: iNOS expression correlates with degree of malignancy in meningiomas. Although , in anaplastic meningiomas cGMP-dependent signaling pathways are inactive but cGMP-independent pathways are not. These data suggest that in malignant meningioma cells, iNOS overexpression may contribute to a survival advantage mediated by protein tyrosine nitration.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]