Osmotic regulation of a novel flavin-containing monooxygenase in primary cultured cells from rainbow trout (Oncorhynchus mykiss).

A cDNA encoding a hepatic isoform of flavin-containing monooxygenase (hFMO) (EF063736) containing an open reading frame of 1792 base pairs (bp) and encoding 554 amino acids was cloned and sequenced from liver mRNA of rainbow trout (Oncorhynchus mykiss). The genomic sequence of hFMO was also characterized and was 4.379 kilobases, possessing 10 exons and 9 introns (EU519462). Structural analysis of the promoter region showed several cis-acting elements including putative glucocorticoid and osmoregulatory response elements, which have been reported to be functionally related to induction of flavin-containing monooxygenase (FMO) proteins in vertebrates. The amino acid sequence showed 74% identity to a putative FMO gene from fugu (Takifugu rubripes; Q6ZZY9), 52 to 55% to zebrafish (Brachydanio rerio; Q5RGM6, Q5RGM3, Q6TLD2, Q7T1D7) FMO5, and 54 and 50% to human FMO1 (Q01740), FMO3 (P49326), and FMO5 (P49326). Southern blot analysis using a 180-bp fragment of the hFMO cDNA indicated at least seven potential genes. Treatment of primary trout hepatocytes with cortisol and sodium chloride for 24 h enhanced hFMO expression. Expression of hFMO was not detected in untreated or solute-treated primary cultures of gill epithelial cells, suggesting tissue-specific expression of hFMO. Induction of hFMO is consistent with the occurrence of cis-osmoregulatory and glucocorticoid response elements identified in the 5'-upstream sequence, indicating regulation of hFMO in response to hypersaline conditions and the osmoregulatory hormone cortisol.