Synthesis of nitric oxide and nitrosamine by immortalized woodchuck hepatocytes.
Woodchuck (Marmota monax) hepatic cells, which were immortalized by the simian virus 40 large T antigen (SV40 Tag) produced nitric oxide (NO; measured as nitrite) in vitro from L-arginine (L-Arg) after lipopolysaccharide (LPS) treatment. NO synthesis was related to L-Arg and LPS concentration and plateaued at 1.0 mM L-Arg and 1.0 microgram/ml LPS. LPS-stimulated cells nitrosated morpholine to form N-nitrosomorpholine (NMOR) in the presence of L-Arg at pH 7.4. NMOR production increased 7-fold in LPS stimulated cells compared to unstimulated hepatocytes. N-nitrosodimethylamine (NDMA) was detected in the cell culture medium in the presence of LPS and L-Arg but without added dimethylamine. NG-monomethyl-L-arginine, a selective inhibitor of nitric oxide synthase, inhibited formation of NO and NMOR, indicating that NO and nitrosating agents were formed via the L-Arg-nitric oxide pathway. These data are the first to report NO and N-nitrosamine production by immortalized hepatocytes and confirm earlier work showing that primary hepatocytes form NO in culture. This suggests that hepatic formation of N-nitroso compounds and/or NO could be an etiologic factor in hepatocellular carcinoma. Immortalized woodchuck hepatic cells may be useful as in vitro models to study the L-Arg-nitric oxide pathway and its possible role in liver carcinogenesis.