Received for publication September 5, 2007.
Revised February 14, 2008.
Accepted for publication February 15, 2008.

Role of human Pregnane X Receptor in tamoxifen and 4-hydroxytamoxifen mediated CYP3A4 induction in primary human hepatocytes and LS174T cells

Abstract

Previously we observed that the anti-estrogens tamoxifen (tamoxifen) and 4-hyrdoxytamoxifen (4OHT) induce CYP3A4 in primary human hepatocytes and activate human pregnane X receptor (PXR) in cell based reporter assays. Given the complex cross-talk between nuclear receptors, tissue-specific expression of CYP3A4 and the potential for tamoxifen and 4OHT to interact with a myriad of receptors, this study was undertaken to gain mechanistic insights into the inductive effects of tamoxifen and 4OHT. Firstly, we observed that transfection of the primary cultures of human hepatocytes with PXR-specific siRNA reduced the PXR mRNA expression and the extent of CYP3A4 induction by tamoxifen and 4OHT by 50%. Secondly, in LS174T colon carcinoma cells, which were observed to have significantly lower PXR expression relative to human hepatocytes, neither tamoxifen nor 4OHT induced CYP3A4. Thirdly, N-desmethyltamoxifen (NDMT), which did not induce CYP3A4 in human hepatocytes, also did not activate PXR in LS174T cells. We then employed cell based reporter assay to evaluate the effects of other receptors such as GR and ER on the transcriptional activation of PXR. The co-transfection of GR in LS174T cells augmented PXR activation by tamoxifen and 4OHT. The presence of ER on the other hand, inhibited PXR mediated basal activation of CYP3A4 promoter, possibly via competing for common co-factors such as SRC1 and GRIP1. Collectively, our findings suggest that the CYP3A4 induction by tamoxifen and 4OHT is primarily mediated by PXR but the overall stoichiometry of other nuclear receptors and transcription cofactors also contribute to the extent of the inductive effect.

Key words: anticancer agents, CYP3A, enzyme induction, extrahepatic cytochrome P450, hepatocytes, nuclear receptors, PXR, regulation of gene expression