Involvement of CAR and PXR in the transcriptional regulation of CYP2B6 gene expression by ingredients from herbal medicines.

1. Induction of hepatic drug-metabolizing enzymes can affect drug efficacy and cause toxicity. However, so far, limited information is available regarding the molecular mechanism how herbal medicines induce human CYP2B6, which metabolizes many of the clinically used therapeutics and activates several pro-carcinogens or toxicants. Accumulated evidence suggests that the human constitutive androstane receptor (hCAR) and the human pregnane X receptor (hPXR) play important roles in trans-activation of CYP2B6. In this study, we investigated the effects of 68 Chinese herbal ingredients on the receptor specificity of hPXR/hCAR-mediated CYP2B6 induction by luciferase reporter gene assays in transiently transfected HepG2 cells and on the expression of CYP2B6 in LS174T cells. 2. The HepG2 cells were transiently transfected with human CYP2B6 luciferase promoter reporter plasmids along with hPXR or hCAR3. The results indicated that apigenin (Api), curcumol (Cur) and praeruptorin A (Pra A) were identified as potent activators of hPXR, and Pra A was also a ligand of hCAR. 3. Furthermore, CYP2B6 mRNA expression in LS174T cells treated with the three herbal ingredients was determined by real-time polymerase chain reaction. By combining western blot and LC-MS/MS, CYP2B6 protein expression and catalytic activity induced by the three herbal ingredients were measured. 4. Our observation showed Api and Cur up-regulated CYP2B6 expression by transactivation of hPXR, and Pra A acted as the ligand of both hPXR and hCAR to induce CYP2B6 expression.