Induction of nitric oxide and nitric oxide synthase mRNA by silica and lipopolysaccharide in PMA-primed THP-1 cells

Nitric oxide (NO), a nitrogen-free radical, plays an important role in mediating inflammatory reaction and cytotoxicity of tissue. To determine whether NO was involved in silica-induced pulmonary tissue damage, we studied the effects of silica on nitric oxide (NO) production and inducible NO synthase (iNOS) mRNA expression by THP-I cells, a monocyte-like cell line with properties of the pulmonary alveolar macrophage. Experimental results showed that silica elicited a marked stimulation of nitric oxide production in a time-dependent manner by THP-I cells in vitro following the priming of these cells with the phorbol ester PMA. Both nitric oxide synthase inhibitor N-monomethyl-L-arginine (NMMA) and xanthine oxidase inhibitor allopurinol can partially suppress silica-induced NO production in PMA-primed THP-I cells. Northern blot analysis indicated that, after 2 h of silica exposure, PMA-primed THP-I cells began to express iNOS mRNA, which reached peak expression at 8 h. Endotoxin treatment of these cells produced a similar effect. These results indicated that silica is a potent inducer of NO production in macrophages and its ability to induce tissue damage may partially be attributed to its ability to initiate excessive production of nitric oxide from macrophages.