Metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in primary cultures of rat alveolar type II cells

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-buta none (NNK) induces primarily lung tumors, which are assumed to derive from malignant transformation of alveolar type II (AII) cells within the lung. T o elicit its carcinogenic effects, NNK requires metabolic activation by cyt ochrome P-450 (CYP)-mediated alpha-hydroxylation. Therefore, in this study the metabolism of NNK and expression of the NNK-activating CYP isoform CYP2 B1 were investigated in primary cultures of rat AII cells. Although basal e xpression of CYP2B1 decreased in a time-dependent manner during culture of AII cells, substantial CYP2B1 protein expression was observed in AII cell c ultures after the first 24 h. When AII cells were incubated with 0.05 mu M [5-H-3]NNK, N-oxidation of NNK, which is thought to represent a detoxificat ion pathway, was predominant (42%). alpha-Hydroxylated metabolites resultin g from metabolic activation of NNK amounted to 35% of all detected metaboli tes. However, the proportion of alpha-hydroxylated metabolites decreased to 17% of all detected metabolites when AII cells were incubated with a 100-f old higher concentration of NNK (5 mu M). In summary, this study indicates a remarkable activity of cultured AII cells to metabolize NNK, leading to s ubstantial metabolic activation of NNK, which was more pronounced in incuba tions at low NNK concentration. Because exposure to NNK via cigarette smoki ng is thought to lead to very low plasma NNK concentrations (1-15 pM), thes e data suggest that metabolic activation of NNK in cigarette smokers might occur to a larger extent than would be expected according to previous metab olic studies performed with high (micromolar) NNK concentrations.