INDUCIBLE NITRIC-OXIDE-SYNTHASE MESSENGER-RNA IS TRANSIENTLY EXPRESSED AND DESTROYED BY A CYCLOHEXIMIDE-SENSITIVE PROCESS

Nitric oxide is a mediator of a wide range of physiological processes. It is produced by an enzyme family, the nitric-oxide synthases, one f orm of which is induced in many cells following stimulation with cytok ines and lipopolysaccharide. The aim of the experiments reported in th is study was to investigate the regulation of mRNA expression for this inducible nitric-oxide synthase in smooth muscle cells and macrophage s. Stimulation of these cells with cytokines and lipopolysaccharide re sults in a marked elevation of nitric-oxide-synthase mRNA levels, whic h however do not remain elevated, but reach a maximum at 3-6h after st imulation before returning to baseline levels over the next 20 h. Enzy me activity, however, remained virtually constant for 48 h following s timulation. Inspection of the 3' untranslated segment of both murine a nd human inducible nitric-oxide-synthase mRNAs showed the presence of a conserved AU-rich octanucleotide sequence, previously identified in cytokine and oncogene mRNAs and shown to mediate mRNA instability. A p articular feature of the breakdown of mRNAs bearing this sequence is t hat degradation is prevented by protein-synthesis inhibition. We show in this study that the half-life of inducible nitric-oxide-synthase mR NA is 6 h and that in the presence of an inhibitor of protein synthesi s this breakdown is prevented. Thus, the mRNA for inducible nitric-oxi de synthase shares some features in common with cytokines such as the transient expression and decay of its mRNA which can be prevented by p rotein-synthesis inhibition.