BACTERIAL LIPOPOLYSACCHARDIE PLUS INTERFERON-GAMMA ELICIT A VERY FASTINHIBITION OF A CA2-DEPENDENT NITRIC-OXIDE SYNTHASE ACTIVITY IN HUMANASTROCYTOMA-CELLS()

Previous results indicate that induction of inducible nitric-oxide syn thase (iNOS) expression may be kept suppressed by the endogenous NO le vel as produced by a constitutive NOS (cNOS) enzyme. In cell types pos sessing both cNOS and iNOS, this may represent an evident paradox. Her e, we report that lipopolysaccharide and interferon-gamma, which are a ble to strongly induce iNOS in astrocytoma cells, can rapidly inhibit the NO production generated by the constitutive NOS isoform, thus obta ining the best conditions for iNOS induction and resolving the apparen t paradox. In fact, a 30-min treatment of T67 cells with the combinati on of lipopolysaccharide plus interferon-gamma (MM) strongly inhibits the cNOS activity, as determined by measuring [H-3]citrulline producti on. In addition, the effect of MM is also observed by measuring nitrit e, the stable breakdown product of NO: a 30-min pretreatment of T67 ce lls with MIX is able to reduce significantly the N-methyl-D-aspartate- induced nitrite production. Finally, using reverse transcriptase-polym erase chain reaction, we have observed that a 30-min treatment of T67 cells with MM does not affect expression of mRNA coding for the neuron al NOS-I isoform. These results suggest the novel concept of a possibl e role of a cNOS isoform in astrocytes as a control function on iNOS i nduction.