Metabolic detoxication pathways for sterigmatocystin in primary tracheal epithelial cells

Abstract

Human health effects of inhaled mycotoxins remain poorly documented, despite the large amounts present in bioaerosols. Among these mycotoxins, sterigmatocystin is one of the most prevalent. Our aim was to study the metabolism and cellular consequences of sterigmatocystin once it is in contact with the airway epithelium. Metabolites were analyzed first in vitro, using recombinant P450 1A1, 1A2, 2A6, 2A13, and 3A4 enzymes, and subsequently in porcine tracheal epithelial cell (PTEC) primary cultures at an air-liquid interface. Expressed enzymes and PTECs were exposed to sterigmatocystin, uniformly enriched with (13)C to confirm the relationship between sterigmatocystin and metabolites. Induction of the expression of xenobiotic-metabolizing enzymes upon sterigmatocystin exposure was examined by real-time quantitative real-time polymerase chain reaction. Incubation of 50 μM sterigmatocystin with recombinant P450 1A1 led to the formation of three metabolites: monohydroxy-sterigmatocystin (M1), dihydroxy-sterigmatocystin (M2), and one glutathione adduct (M3), the latter after the formation of a transient epoxide. Recombinant P450 1A2 also led to M1 and M3. P450 3A4 led to only M3. In PTEC, 1 μM sterigmatocystin metabolism resulted in a glucuro conjugate (M4) mainly excreted at the basal side of cells. If PTEC were treated with β-naphthoflavone prior to sterigmatocystin incubation, two other products were detected, i.e., a sulfo conjugate (M5) and a glucoro conjugate (M6) of hydroxy-sterigmatocystin. Exposure of PTEC for 24 h to 1 μM sterigmatocystin induced an 18-fold increase in the mRNA levels of P450 1A1, without significantly induced 7-ethoxyresorufin O-deethylation activity. These data suggest that sterigmatocystin is mainly detoxified and is unable to produce significant amounts of reactive epoxide metabolites in respiratory cells. However, sterigmatocystin increases the P450 1A1 mRNA levels with unknown long-term consequences. These in vitro results obtained in the porcine pulmonary tract need to be confirmed in human epithelial cells.