Coinduction of nitric-oxide synthase and arginase I in cultured rat peritoneal macrophages and rat tissues in vivo by lipopolysaccharide

Nitric oxide is synthesized by nitric-oxide synthase from arginine, a common substrate of arginase. Rat peritoneal macrophages were cultured in the presence of bacterial lipopolysaccharide (LPS), and expression of the inducible isoform of nitric oxide synthase (iNOS) and liver-type arginase (arginase I) was analyzed, mRNAs for iNOS and arginase I were induced by LPS in a dose-dependent manner, iNOS mRNA appeared 2 h after LPS treatment and increased to a near maximum at 8-12 h. On the other hand, arginase I mRNA that was undetectable prior to the treatment began to increase after 4 h with a lag time and reached a maximum at 12 h. Immunoblot analysis showed that iNOS and arginase I proteins were also induced, mRNA for arginase II, an arginase isozyme, was not detected in the LPS-activated peritoneal cells, mRNA for CCAAT/enhancer-binding protein beta (C/EBP beta), a transactivator of the arginase I gene, was also induced, and the induction was more rapid than that of arginase I mRNA. Changes in iNOS and arginase I mRNAs were also examined in LPS-injected rats in vivo. iNOS mRNA increased rapidly in the lung and spleen, reached a maximum 2-6 h after the LPS treatment, and decreased thereafter. Arginase I mRNA was induced markedly and more slowly in both tissues, reaching a maximum in 12 h. Thus, arginase I appears to have an important role in down regulating nitric oxide synthesis in murine macrophages by decreasing the availability of arginine, and the induction of arginase I is mediated by C/EBP beta.