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Bioimaging real-time PXR-dependent mdr1a gene regulation in mdr1a.fLUC reporter mice
Long Gu1, Jasmine Chen, Timothy W Synold
1Department of Cancer Biology, Beckman Research Institute at City of Hope, Duarte, California, USA.
Abstract:
The MDR1 gene encodes P-glycoprotein, a transmembrane drug efflux transporter that confers multidrug resistance in cancer cells and affects drug pharmacokinetics by virtue of its expression in the liver, kidney, and colon. Nuclear receptors human steroid and xenobiotic receptor (SXR) and constitutive androstane receptor (CAR) are possible master regulators of xenobiotic-inducible MDR1 expression in drug processing organs, but the mechanism of MDR1 regulation has yet to be directly demonstrated in vivo. Moreover, it has previously been impossible to determine the sustained or cumulative effect of repeated doses of xenobiotics on in vivo MDR1 expression. We previously reported a mouse model containing firefly luciferase (fLUC) knocked into the mdr1a genomic locus, allowing noninvasive bioimaging of intestinal mdr1a gene expression in live animals. In the current study, we crossed mdr1a.fLUC mice into the pxr knockout (pxr(-/-)) genetic background and injected mice with pregnenolone-16α-carbonitrile (PCN), a strong mouse pregnane X receptor (PXR) ligand, and two therapeutically relevant taxanes, paclitaxel and docetaxel. All three agents induced mdr1a.fLUC expression (bioluminescence), but only PCN and docetaxel appeared to act primarily via PXR. Luminescence returned to baseline by 24-48 hours after drug injection and was reinducible over two additional rounds of drug dosing in pxr(+/+) mice. TCPOBOP, a CAR ligand, modestly induced mdr1a.fLUC in pxr(+/+) and pxr(-/-) strains, consistent with CAR's minor role in mdr1a regulation. Collectively, these results demonstrate that the mdr1a.fLUC bioimaging model can capture changes in mdr1 gene expression under conditions of repeated xenobiotic treatment in vivo and that it can be used to probe the mechanism of gene regulation in response to different xenobiotic agents.
Insights
This study shows that a novel mouse model can track changes in the MDR1 gene, which affects drug resistance. The model successfully demonstrated how different drugs regulate this gene in live animals, even with repeated dosing.
Area of Science:
- Pharmacology
- Molecular Biology
- Genetics
Background:
- The MDR1 gene encodes P-glycoprotein, a transporter influencing drug resistance and pharmacokinetics.
- Nuclear receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are implicated in regulating MDR1 expression, but in vivo mechanisms remain unclear.
- Assessing sustained or cumulative effects of xenobiotics on in vivo MDR1 expression has been challenging.
Purpose of the Study:
- To investigate the in vivo regulation of the MDR1 gene by xenobiotics using a novel bioimaging mouse model.
- To elucidate the roles of PXR and CAR in mediating MDR1 induction by specific ligands and taxanes.
- To evaluate the model's capability in capturing dynamic changes in MDR1 expression under repeated drug exposure.
Main Methods:
- Generation of a mouse model with firefly luciferase (fLUC) knocked into the mdr1a locus (mdr1a.fLUC) for noninvasive bioimaging.
- Crossing mdr1a.fLUC mice with PXR knockout (pxr(-/-)) mice to assess PXR-dependent regulation.
- Administration of PXR ligand (PCN) and taxanes (paclitaxel, docetaxel), and CAR ligand (TCPOBOP) to evaluate their effects on mdr1a.fLUC expression.
Main Results:
- PCN, docetaxel, and paclitaxel induced mdr1a.fLUC expression; PCN and docetaxel primarily acted via PXR.
- MDR1a expression returned to baseline within 24-48 hours post-injection and was reinducible with repeated dosing in wild-type mice.
- TCPOBOP induced mdr1a.fLUC expression modestly in both PXR-sufficient and PXR-knockout mice, indicating a minor role for CAR.
Conclusions:
- The mdr1a.fLUC bioimaging model effectively captures dynamic changes in MDR1 gene expression in vivo.
- The model allows for probing the mechanisms of MDR1 gene regulation by various xenobiotics, including under repeated exposure conditions.
- Results confirm PXR as a major regulator of MDR1 induction by certain xenobiotics, with CAR playing a lesser role.
