Abstract
The human colon carcinoma cell line, Caco-2, is widely used as a model for oral absorption of xenobiotics. The usefulness of Caco-2 cells has been limited, however, because they do not express appreciable quantities of CYP3A4, the principle cytochrome P450 present in human small bowel epithelial cells. We report that treatment of Caco-2 cells with 1α,25-dihydroxyvitamin D3, beginning at confluence, results in a dose- and duration-dependent increase in CYP3A4 mRNA and protein, with little apparent effect on the expression of CYP3A5 or CYP3A7. This treatment also results in increases in NADPH cytochrome P450 reductase and P-glycoprotein (the MDR1 gene product) but has no detectable effect on expression of CYP1A1, CYP2D6, cytochrome b 5, liver or intestinal fatty acid binding proteins, or villin. Maximal expression of CYP3A4 requires an extracellular matrix on a permeable support and the presence of serum. In the treated cells, the intrinsic formation clearance of 1′-hydroxymidazolam (a reaction characteristically catalyzed by CYP3A enzymes) was estimated to be somewhat lower than that of human jejunal mucosa (1.14 and 3.67 ml/min/g of cells, respectively). The 1′-OH-midazolam/4-OH-midazolam product ratio produced by the cells (∼5.3) is comparable to, but somewhat lower than, that observed in human jejunal microsomes (7.4–15.4), which may reflect the presence of CYP3A7 in the Caco-2 cells. 25-Hydroxyvitamin D3 is less efficacious but reproduces the effects of the dihydroxy compound, whereas unhydroxylated vitamin D is without appreciable effect. These observations, together with the time course of response, suggest that the vitamin D receptor may be involved in CYP3A4 regulation. The culture model we describe should prove useful in defining the role of CYP3A4 in limiting the oral bioavailability of many xenobiotics.
Footnotes
- Received November 6, 1996.
- Accepted January 24, 1997.
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Send reprint requests to: Paul B. Watkins, M.D., University of Michigan Medical Center, Room A7119 University Hospital, Box 0108, 1500 East Medical Center Drive, Ann Arbor, MI 48109. E-mail:pwatkins{at}umich.edu
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This work was supported by National Institutes of Health Grants GM38149 (P.B.W.) and GM48349 (K.E.T.).
- The American Society for Pharmacology and Experimental Therapeutics
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