Abstract
BackgroundThe expression of the drug-metabolizing enzyme cytochrome P450 3A4 (CYP3A4) is regulated by the pregnane × receptor (PXR), which is modulated by numerous signaling pathways, including the cyclin-dependent kinase (Cdk) pathway. Flavonoids, commonly consumed by humans as dietary constituents, have been shown to modulate various signaling pathways (e.g., inhibiting Cdks). Flavonoids have also been shown to induce CYPs expression, but the underlying mechanism of action is unknown. Here, we report the mechanism responsible for flavonoid-mediated PXR activation and CYP expression.ResultsIn a cell-based screen designed to identify compounds that activate PXR-mediated CYP3A4 gene expression in HepG2 human carcinoma cells, we identified several flavonoids, such as luteolin and apigenin, as PXR activators. The flavonoids did not directly bind to PXR, suggesting that an alternative mechanism may be responsible for flavonoid-mediated PXR activation. Consistent with the Cdk5-inhibitory effect of flavonoids, Cdk5 and p35 (a non-cyclin regulatory subunit required to activate Cdk5) were expressed in HepG2. The activation of Cdk5 attenuated PXR-mediated CYP3A4 expression whereas its downregulation enhanced it. The Cdk5-mediated downregulation of CYP3A4 promoter activity was restored by flavonoids, suggesting that flavonoids activate PXR by inactivating Cdk5. In vitro kinase assays showed that Cdk5 directly phosphorylates PXR. The Cdk kinase profiling assay showed that apigenin inhibits multiple Cdks, suggesting that several Cdks may be involved in activation of PXR by flavonoids.ConclusionsOur results for the first time link the stimulatory effect of flavonoids on CYP expression to their inhibitory effect on Cdks, through a PXR-mediated mechanism. These results may have important implications on the pharmacokinetics of drugs co-administered with herbal remedy and herbal-drug interactions.
Highlights
The expression of the drug-metabolizing enzyme cytochrome P450 3A4 (CYP3A4) is regulated by the pregnane × receptor (PXR), which is modulated by numerous signaling pathways, including the cyclin-dependent kinase (Cdk) pathway
The activity of PXR is regulated by direct ligand binding [4] and by various cell signaling pathways [5], such as those mediated by protein kinase C (PKC) [6], protein kinase A (PKA) [7,8], cyclin-dependent kinase 2 (Cdk2) [9], 70kDa form of ribosomal protein S6 kinase (p70 S6K) [10], forkhead in rhabdomyosarcoma (FKHR) [11], and nuclear factor κB (NF-κB) [12,13,14]
Since these flavonoids did not directly bind to PXR, and flavonoids might inhibit Cdk5, we studied the effect of flavonoids on the activity of Cdk5/p35 and the regulation of PXR by Cdk5 in order to determine the possible role of flavonoids in regulating PXR-mediated gene expression of CYP3A4
Summary
The expression of the drug-metabolizing enzyme cytochrome P450 3A4 (CYP3A4) is regulated by the pregnane × receptor (PXR), which is modulated by numerous signaling pathways, including the cyclin-dependent kinase (Cdk) pathway. Flavonoids, commonly consumed by humans as dietary constituents, have been shown to modulate various signaling pathways (e.g., inhibiting Cdks). We report the mechanism responsible for flavonoid-mediated PXR activation and CYP expression. The activity of PXR is regulated by direct ligand binding [4] and by various cell signaling pathways [5], such as those mediated by protein kinase C (PKC) [6], protein kinase A (PKA) [7,8], cyclin-dependent kinase 2 (Cdk2) [9], 70kDa form of ribosomal protein S6 kinase (p70 S6K) [10], forkhead in rhabdomyosarcoma (FKHR) [11], and nuclear factor κB (NF-κB) [12,13,14]. The widespread use of flavonoids, coupled with their potentially beneficial effects, has triggered studies on the mechanism by which they modulate signaling pathways
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