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The multicopper oxidase ceruloplasmin plays a key role in iron homeostasis, and its ferroxidase activity is required to stabilize cell surface ferroportin, the only known mammalian iron exporter. Missense mutations causing the rare autosomal neurodegenerative disease aceruloplasminemia were investigated by testing their ability to prevent ferroportin degradation in rat glioma C6 cells silenced for endogenous ceruloplasmin. Most of the mutants did not complement (i.e. did not stabilize ferroportin) because of the irreversible loss of copper binding ability. Mutant R701W, which was found in a heterozygous very young patient with severe neurological problems, was unable to complement per se but did so in the presence of copper-glutathione or when the yeast copper ATPase Ccc2p was co-expressed, indicating that the protein was structurally able to bind copper but that metal loading involving the mammalian copper ATPase ATP7B was impaired. Notably, R701W exerted a dominant negative effect on wild type, and it induced the subcellular relocalization of ATP7B. Our results constitute the first evidence of "functional silencing" of ATP7B as a novel molecular defect in aceruloplasminemia. The possibility to reverse the deleterious effects of some aceruloplasminemia mutations may disclose new possible therapeutic strategies.

Previous results have indicated that lipopolysaccharide (LPS) plus interferon-gamma (IFNgamma) inhibits nitric-oxide synthase (NOS)-I activity in glial cells. We report here that arachidonic acid (AA) plays a pivotal role in this response, which was consistently reproduced in different glial cell lines and in primary rat astrocytes. This notion was established using pharmacological inhibitors of phospholipase A2 (PLA2), cytosolic PLA2 (cPLA2) antisense oligonucleotides, and AA add-back experiments. This approach not only allowed the demonstration that AA promotes inhibition of NOS-I activity but also produced novel experimental evidence that LPS/IFNgamma itself is a potential stimulus for NOS-I. Indeed, LPS/IFNgamma fails to generate nitric oxide (NO) via NOS-I activation simply because it activates the AA-dependent signal that impedes NOS-I activity. Otherwise, LPS/IFNgamma promotes NO formation, sensitive to exogenous AA, in cells in which cPLA2 is pharmacologically inhibited or genetically depleted. Because NO suppresses the NFkappaB-dependent NOS-II expression, inactivation of NOS-I by the LPS/IFNgamma-induced AA pathway provides optimal conditions for NFkappaB activation and subsequent NOS-II expression. Inhibition of cPLA2 activity, while reducing the availability of AA, consistently inhibited NFkappaB activation and NOS-II mRNA induction and delayed NO formation. These responses were promptly reestablished by addition of exogenous AA. Finally, we have demonstrated that the LPS/IFNgamma-dependent tyrosine phosphorylation of NOS-I and inhibition of its activity are mediated by endogenous AA.

Human astrocytoma T67 cells constitutively express a neuronal NO synthase (NOS-I) and, following administration of lipopolysaccharide (LPS) plus interferon-gamma (IFNgamma), an inducible NOS isoform (NOS-II). Previous results indicated that a treatment of T67 cells with the combination of LPS plus IFNgamma, by affecting NOS-I activity, also inhibited NO production in a very short time. Here, we report that under basal conditions, a NOS-I protein of about 150 kDa was weakly and partially tyrosine-phosphorylated, as verified by immunoprecipitation and Western blotting. Furthermore, LPS plus IFNgamma increased the tyrosine phosphorylation of NOS-I, with a concomitant inhibition of its enzyme activity. The same effect was observed in the presence of vanadate, an inhibitor of phosphotyrosine-specific phosphatases. On the contrary, genistein, an inhibitor of protein-tyrosine kinases, reduced tyrosine phosphorylation of NOS-I, enhancing its enzyme activity. Finally, using reverse transcriptase-polymerase chain reaction, we have observed that a suboptimal induction of NOS-II mRNA expression in T67 cells was enhanced by vanadate (or L-NAME) and inhibited by genistein. Because exogenous NO has been found to suppress NOS-II expression, the decrease of NO production that we have obtained from the inactivation of NOS-I by LPS/IFNgamma-induced tyrosine phosphorylation provides the best conditions for NOS-II expression in human astrocytoma T67 cells.

Previous results indicate that induction of inducible nitric-oxide synthase (iNOS) expression may be kept suppressed by the endogenous NO level as produced by a constitutive NOS (cNOS) enzyme. In cell types possessing both cNOS and iNOS, this may represent an evident paradox. Here, we report that lipopolysaccharide and interferon-gamma, which are able to strongly induce iNOS in astrocytoma cells, can rapidly inhibit the NO production generated by the constitutive NOS isoform, thus obtaining the best conditions for iNOS induction and resolving the apparent paradox. In fact, a 30-min treatment of T67 cells with the combination of lipopolysaccharide plus interferon-gamma (MIX) strongly inhibits the cNOS activity, as determined by measuring [3H]citrulline production. In addition, the effect of MIX is also observed by measuring nitrite, the stable breakdown product of NO: a 30-min pretreatment of T67 cells with MIX is able to reduce significantly the N-methyl-D-aspartate-induced nitrite production. Finally, using reverse transcriptase-polymerase chain reaction, we have observed that a 30-min treatment of T67 cells with MIX does not affect expression of mRNA coding for the neuronal NOS-I isoform. These results suggest the novel concept of a possible role of a cNOS isoform in astrocytes as a control function on iNOS induction.

The reactive nitrogen species, nitric oxide (NO), plays an important role in the pathogenesis of neurodegenerative diseases. The suppression of NO production may be fundamental for survival of neurons. Here, we report that pretreatment of human ramified microglial cells with nearly physiological levels of exogenous NO prevents lipopolysaccharide (LPS)/tumor necrosis factor alpha (TNF alpha)-inducible NO synthesis, because by affecting NF-kappa B activation it inhibits inducible Ca(2+)-independent NO synthase isoform (iNOS) mRNA expression. Using reverse transcriptase polymerase chain reaction, we have found that both NO donor sodium nitroprusside (SNP) and authentic NO solution are able to inhibit LPS/TNF alpha-inducible iNOS gene expression; this effect was reversed by reduced hemoglobin, a trapping agent for NO. The early presence of SNP during LPS/TNF alpha induction is essential for inhibition of iNOS mRNA expression. Furthermore, SNP is capable of inhibiting LPS/TNF alpha-inducible nitrite release, as determined by Griess reaction. Finally, using electrophoretic mobility shift assay, we have shown that SNP inhibits LPS/TNF alpha-elicited NF-kappa B activation. This suggests that inhibition of iNOS gene expression by exogenous NO may be ascribed to a decreased NF-kappa B availability.