Regulation of cytochrome P4501A1 expression in rat small intestine

The predominant inducible cytochrome P450 (CYP) in rat small intestine is CYP1A1, which, when induced to elevated levels by xenobiotics or dietary constituents, has the potential to metabolize and consequently reduce the systemic uptake of low concentrations of orally ingested, bioactivatable polycyclic aromatic hydrocarbons and heterocyclic aromatic amines. We investigated the regulation of small intestinal CYP1A1 in an effort to develop its anticancer potential. The time courses of hepatic and intestinal CYP1A1 induction by beta-naphthoflavone (BNF) were compared quantitatively at the protein and mRNA levels by immunoblot and competitive RNA-polymerase chain reaction analyses. CYP1A1 mRNA levels in both organs increased sharply and were maximal at approximately 6 hr and returned to near basal levels by 12 hr after BNF treatment. In contrast, hepatic CYP1A2 mRNA levels increased much more gradually. Small intestinal CYP1A2 mRNA concentrations were insufficient to support translation of detectable protein. Maximal levels of intestinal and hepatic CYP1A1 protein occurred between 12 and 24 hr, and 24 and 48 hr, respectively, after BNF. Intestinal CYP1A1 protein was detectable earlier and for a shorter duration than hepatic CYP1A1. CYP1A1 induction was first detected in crypt cells 3 hr before the appearance of activity in villous cells, and maximal levels of activity were reached in crypt cells 12 to 18 hr before maximal and 1.5-fold (per mg protein) higher responses in villous cells-induction thus occurs in both villous and crypt cells. Previously detected decreases in CYP1A1 inducibility from duodenum to ileum correlated with decreases in immunoblot determined-Ah receptor levels. Intestinal CYP1A1 induction does not involve the glucocorticoid receptor in contrast to hepatic induction. These studies have revealed several novel features of small intestinal CYP1A1 regulation.

Characterization of rat small intestinal cytochrome P450 composition and inducibility

The composition and inducibility of cytochrome P450 (P450) in rat small intestinal epithelial cells were investigated with the use of RNA-polymerase chain reaction and immunoblot techniques. The complement of intestinal P450s is more restricted than hepatic forms. P450s 1A1, 2B1, and 3A1 were detected in enterocytes of untreated rats and were inducible by beta-naphthoflavone (BNF), phenobarbital, and pregnenolone-16alpha-carbonitrile or dexamethasone, respectively. In addition, P450s 2C6 and 2C11 were both constitutively expressed at low levels. In contrast, several P450 forms, which are found in the liver, were not detected in enterocytes of untreated or induced rats, including P450s 2A1, 2B2, 2E1, 3A2, and 4A1. P4501A2 mRNA was detected only in BNF-induced rat small intestine and at levels that did not result in its detectable translation. The most prominent inducible form in rat small intestine is P4501A1. Its inducibility diminishes markedly along the length of the small intestine from the duodenum to the ileum. Furthermore, the induction of P4501A1 in enterocytes was affected by the route of administration of the inducing agent. Thus, intestinal P4501A1 was more sensitive to orally administered BNF, whereas induction of hepatic P4501A1 was more sensitive to intraperitoneal administered BNF. Overall, the results demonstrate the differential regulation of P450s between the liver and small intestine, and provide a basis for further studies in assessing the potential of intestinal P450s to protect against orally ingested polycyclic aromatic hydrocarbon carcinogens.