St. John's wort induces hepatic drug metabolism through activation of the pregnane X receptor

Abstract

St. John's wort (Hypericum perforatum) is an herbal remedy used widely for the treatment of depression. Recent clinical studies demonstrate that hypericum extracts increase the metabolism of various drugs, including combined oral contraceptives, cyclosporin, and indinavir. In this report, we show that hyperforin, a constituent of St. John's wort with antidepressant activity, is a potent ligand (K(i) = 27 nM) for the pregnane X receptor, an orphan nuclear receptor that regulates expression of the cytochrome P450 (CYP) 3A4 monooxygenase. Treatment of primary human hepatocytes with hypericum extracts or hyperforin results in a marked induction of CYP3A4 expression. Because CYP3A4 is involved in the oxidative metabolism of >50% of all drugs, our findings provide a molecular mechanism for the interaction of St. John's wort with drugs and suggest that hypericum extracts are likely to interact with many more drugs than previously had been realized.

Figure 1

Hyperforin is a potent activator of PXR. (A) CV-1 cells were transfected with expression plasmids for human PXR and the (CYP3A1)₂-tk-CAT reporter. Cells were treated with extracts prepared from three different commercial preparations of St. John's wort [extract 1, Nature's Way (9 μg/ml); extract 2, Nature's Plus (75 μg/ml); extract 3, Solaray (7 μg/ml)] or with 10 μM of the indicated pure compounds, except for hyperforin, which was tested at 1 μM. Hyperforin was toxic to cells at concentrations >1 μM. Cell extracts subsequently were assayed for CAT activity. Data represent the mean of assays performed in quadruplicate ± SE and are plotted as -fold activation relative to transfected cells treated with vehicle alone. (B) Chemical structure of hyperforin. (C) CV-1 cells were transfected as in A and treated with increasing concentrations of hyperforin (squares) or rifampicin (circles). Data points represent the mean of assays performed in triplicate.

Figure 2

Hyperforin binds to PXR with high affinity. Competition binding assays were performed with purified human PXR LBD and 10 nM of the high-affinity PXR radioligand [³H]SR12813 in the presence of the indicated concentrations of hyperforin (squares), SR12813 (circles), or umbelliferone (triangles). Each data point represents the mean of assays performed in triplicate. The K_i value of hyperforin was calculated to be 27 nM by nonlinear regression analysis.

Figure 3

St. John's wort extracts and hyperforin induce CYP3A4 expression in human hepatocytes. Northern blot analysis was performed with total RNA (10 μg) prepared from primary cultures of human hepatocytes treated for 30 h with extracts prepared from three different commercial preparations of St. John's wort [extract 1, Nature's Way (9 μg/ml); extract 2, Nature's Plus (75 μg/ml); extract 3, Solaray (7 μg/ml)], 1 μM hyperforin, or vehicle alone (0.1% ethanol). The blot was probed sequentially with ³²P-labeled fragments of CYP3A4 and β-actin.