Nitric oxide downregulates tumour necrosis factor in mRNA in RAW 264.7 cells

Nitric oxide (NO) and tumour necrosis factor (TNF) are essential mediators in a number of biological processes, including the immune response. TNF stimulates NO production via expression of inducible NO synthase (iNOS), with L-arginine being the only substrate. Previously, we demonstrated that, inversely, NO inhibits lipopolysaccharide (LPS)-induced TNF synthesis. We have now investigated whether this reduction of TNF bioactivity is also reflected at the level of TNF mRNA in the murine macrophage cell line RAW 264.7. TNF mRNA was quantified by Northern analysis using an alpha[33P]dCTP-labelled probe. Cells stimulated with 10 microg/ml LPS in the absence of L-arginine, in order to prevent endogenous NO formation, contained more TNF mRNA than cells supplied with 1 mM L-arginine at 14 h and 20 h after stimulation. By contrast, no difference was observed at 4 h. This time course is compatible with the involvement of iNOS. The half-life of TNF mRNA in the presence of NO was roughly half that observed under L-arginine-free conditions (41 min versus 77 min, respectively). L-citrulline (1 mM), which has been shown to be recycled in RAW 264.7 cells to L-arginine, completely restored attenuation of TNF bioactivity and TNF message to control levels obtained with 1 mM L-arginine. Together, these findings suggest that endogenous NO regulates TNF mRNA, mainly by reducing its half-life. In addition, a distinct additional band (approximately 1.4 kb) hybridizing with the TNF probe was consistently observed in non-stimulated cells. This may correspond to TNF mRNA specifically hydrolysed at the AU-rich region, possibly reflecting another control point for TNF expression.