Reduced bactericidal activity and nitric oxide production in metallothionein-deficient macrophages in response to lipopolysaccharide stimulation

This study was designed to investigate bactericidal activity of and nitric oxide (NO) production in lipopolysaccharide (LPS)-stimulated peritoneal exudate macrophages (Mϕ) from metallothionein (MT)-null mice. Control Mϕ had a bactericidal effect on Staphylococcus aureus, but MT-null Mϕ had significantly lower activity. NO is an important factor in the bactericidal function of Mϕ. LPS-stimulated MT-null Mϕ produced less NO than those of control mice. LPS-stimulated Mϕ produce cytokines such as tumor necrosis factor (TNF)-alpha. TNF-alpha activate Mϕ and stimulates NO production. We evaluated NO production by TNF-alpha-stimulated Mϕ. MT-null Mϕ produced less NO in response to TNF-alpha stimulation. Levels of expression of inducible NO synthase (iNOS) mRNA and production of iNOS protein in response to LPS stimulation were similar in MT-null and control cells, as were levels of expression of arginase, which competes in arginine metabolism with iNOS. No notable changes were found in arginine uptake or in expression of cationic amino acid transporter 2 (a major arginine transporter in Mϕ) between control and MT-null Mϕ. The rate of conversion of [14C]-l-arginine to citrulline, which is formed with NO by the action of iNOS, was much lower in MT-null Mϕ than in control cells. These results indicate that the reduced production of NO in MT-deficient Mϕ is due mainly to reduced activity of iNOS. Thus, MT plays important roles in bactericidal activity, NO production, and arginine metabolism in activated Mϕ.