Sane Rucha S, Buckley Donna J, Buckley Arthur R, Nallani Srikanth C, Desai Pankaj B
Division of Pharmaceutical Sciences, College of Pharmacy, University of Cincinnati Medical Center, 3225 Eden Avenue, Cincinnati, OH 45267-0004, USA.
Drug Metab Dispos. 2008 May;36(5):946-54. doi: 10.1124/dmd.107.018598. Epub 2008 Feb 25.
Previously we observed that the antiestrogens tamoxifen and 4-hydroxytamoxifen (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. First, we observed that transfection of the primary cultures of human hepatocytes with PXR-specific small interfering RNA reduced the PXR mRNA expression and the extent of CYP3A4 induction by tamoxifen and 4OHT by 50%. Second, in LS174T colon carcinoma cells, which were observed to have significantly lower PXR expression relative to human hepatocytes, neither tamoxifen nor 4OHT induced CYP3A4. Third, N-desmethyltamoxifen, which did not induce CYP3A4 in human hepatocytes, also did not activate PXR in LS174T cells. We then used cell-based reporter assay to evaluate the effects of other receptors such as glucocorticoid receptor GR alpha and estrogen receptor ER alpha on the transcriptional activation of PXR. The cotransfection of GR alpha in LS174T cells augmented PXR activation by tamoxifen and 4OHT. On the other hand, the presence of ER alpha inhibited PXR-mediated basal activation of CYP3A4 promoter, possibly via competing for common cofactors such as steroid receptor coactivator 1 and glucocorticoid receptor interacting protein 1. 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 contributes to the extent of the inductive effect.
此前我们观察到，抗雌激素药物他莫昔芬和4-羟基他莫昔芬（4OHT）可在原代人肝细胞中诱导CYP3A4，并在基于细胞的报告基因检测中激活人孕烷X受体（PXR）。鉴于核受体之间复杂的相互作用、CYP3A4的组织特异性表达，以及他莫昔芬和4OHT与众多受体相互作用的可能性，开展了本研究以深入了解他莫昔芬和4OHT的诱导作用机制。首先，我们观察到用PXR特异性小干扰RNA转染原代人肝细胞培养物可使PXR mRNA表达以及他莫昔芬和4OHT诱导的CYP3A4程度降低50%。其次，在相对于人肝细胞而言PXR表达显著较低的LS174T结肠癌细胞中，他莫昔芬和4OHT均未诱导CYP3A4。第三，在人肝细胞中不诱导CYP3A4的N-去甲基他莫昔芬在LS174T细胞中也未激活PXR。然后我们使用基于细胞的报告基因检测来评估其他受体如糖皮质激素受体GRα和雌激素受体ERα对PXR转录激活的影响。在LS174T细胞中共转染GRα可增强他莫昔芬和4OHT对PXR的激活作用。另一方面，ERα的存在抑制了PXR介导的CYP3A4启动子基础激活，可能是通过竞争共同辅因子如类固醇受体辅激活因子1和糖皮质激素受体相互作用蛋白1。总体而言，我们的研究结果表明，他莫昔芬和4OHT诱导CYP3A4主要由PXR介导，但其他核受体和转录辅因子的整体化学计量也有助于诱导作用的程度。
