Abstract
Cytochrome P450 (CYP) 3A4, which is abundant in human liver and small intestine and participates in the metabolism of various drugs and xenochemicals, is known to be induced by 1,25-dihydroxyvitamin D 3 (1,25(OH) 2D 3) in the colon carcinoma cell line Caco-2 cells. Nitric oxide (NO) is able to inhibit CYP3A4 expression and catalytic activity. In this study, we investigated the mechanism of suppression by NO of 1,25(OH) 2D 3-induced CYP3A4 expression in Caco-2 cells. Caco-2 cells were exposed for 36 h to 400 nM 1,25(OH) 2D 3, and the induction of CYP3A4 mRNA expression was detected by real-time PCR. Because c-Myc regulates the expression of several genes, we examined its effect on the CYP3A4 expression induced by 1,25(OH) 2D 3. The expression of c-myc mRNA was increased in the early stage but decreased 36 h after the treatment of Caco-2 cells with 1,25(OH) 2D 3. The NO donor NOR-4 suppressed CYP3A4 expression induced by 1,25(OH) 2D 3 in Caco-2 cells in contrast, it significantly induced c-myc gene expression. Treatment of Caco-2 cells with the c-myc antisense oligonucleotide reversed the inhibitory effect of NOR-4 on CYP3A4 expression induced by 1,25(OH) 2D 3. These results suggest that the suppression of 1,25(OH) 2D 3-induced CYP3A4 expression by NO is due to c-myc expression.
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