Anti-NR1I3 (full length) polyclonal antibody

Xenobiotics induce the transcription of cytochromes P450 (CYPs) 2B and 3A through the constitutive androstane receptor (CAR; NR1I3) and pregnane X receptor (PXR; NR1I2), respectively, In this report, we have systematically compared a series of xenobiotics and natural steroids for their effects on mouse and human CAR and PXR, Our results demonstrate dual regulation of PXR and CAR by a subset of compounds that affect CYP expression, Moreover, there are marked pharmacological differences between the mouse (m) and human (h) orthologs of both CAR and PXR, For example, the planar hydrocarbon 1,4-bis[2-(3,5-dichloropyridyl-oxy)]benzene activates mCAR and hPXR but has little or no activity on hCAR and mPXR, In contrast, the CAR deactivator androstanol activates both mouse and human PXR, Similarly, the PXR activator clotrimazole is a potent deactivator of hCAR, Using radioligand binding and fluorescence resonance energy transfer assays, we demonstrate that several of the compounds that regulate mouse and human CAR, including natural steroids, bind directly to the receptors, Our results suggest that CAR, like PXR, is a steroid receptor that is capable of recognizing structurally diverse compounds, Moreover, our findings underscore the complexity in the physiologic response to xenobiotics.

Cyclophosphamide (CPA) is one of the most widely used chemotherapeutic prodrugs that undergoes hepatic bioactivation mediated predominantly by cytochrome P450 (CYP) 2B6. Given that the CYP2B6 gene is primarily regulated by the constitutive androstane receptor (CAR, NR1I3), we hypothesize that selective activation of CAR can enhance systemic exposure of the pharmacologically active 4-hydroxycyclophosamide (4-OH-CPA), with improved efficacy of CPA-based chemotherapy. In this study, we have developed a unique human primary hepatocyte (HPH)-leukemia cell coculture model; the chemotherapeutic effects of CPA on leukemia cells can be directly investigated in vitro in a cellular environment where hepatic metabolism was well maintained. Our results demonstrated that activation of CAR preferentially induces the expression of CYP2B6 over CYP3A4 in HPHs, although endogenous expression of these enzymes in leukemia cells remains negligible. Importantly, coadministration of CPA with a human CAR activator led to significantly enhanced cytotoxicity in leukemia cells by inducing the apoptosis pathways, without concomitant increase in the off-target hepatotoxicity. Associated with the enhanced antitumor activity, a time and concentration-dependent increase in 4-OH-CPA formation was observed in the coculture system. Together, our findings offer proof of concept that CAR as a novel molecular target can facilitate CPA-based chemotherapy by selectively promoting its bioactivation. (Blood. 2013;121(2):329-338)