Activation of the Steroid and Xenobiotic Receptor (Human Pregnane X Receptor) by Nontaxane Microtubule-Stabilizing Agents

• Published in: Clinical cancer research Vol. 11; no. 17; pp. 6359 - 6369
• Main Authors: Mani, Sridhar, Huang, Haiyan, Sundarababu, Sumathy, Liu, Wenjing, Kalpana, Ganjam, Smith, Amos B, Horwitz, Susan B
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA American Association for Cancer Research 01-09-2005
• Subjects: Alkanes - pharmacology; Animals; Antineoplastic agents; Antineoplastic Agents - pharmacology; Antineoplastic Agents, Phytogenic - pharmacology; Aryl Hydrocarbon Hydroxylases - metabolism; Biological and medical sciences; Blotting, Northern; Carbamates - pharmacology; CYP450; Cytochrome P-450 CYP3A; Cytochrome P-450 Enzyme System - metabolism; discodermolide; DNA-Binding Proteins - metabolism; drug metabolism; epothilones; Epothilones - pharmacology; Glutathione Transferase - metabolism; Histone Acetyltransferases; Humans; Immunoblotting; Lactones - pharmacology; Medical sciences; Mice; Mice, Inbred C57BL; Nuclear Proteins - metabolism; Nuclear Receptor Co-Repressor 1; Nuclear Receptor Co-Repressor 2; Nuclear Receptor Coactivator 1; Oxidoreductases, N-Demethylating - metabolism; Paclitaxel - pharmacology; Pharmacology. Drug treatments; Plasmids; Pregnane X Receptor; Pyrones; Receptors, Cytoplasmic and Nuclear - genetics; Receptors, Cytoplasmic and Nuclear - metabolism; Receptors, Steroid - genetics; Receptors, Steroid - metabolism; Reflex, Abnormal; Repressor Proteins - metabolism; taxanes; Transcription Factors - metabolism; Transcription, Genetic - drug effects; Transcriptional Activation; Two-Hybrid System Techniques; Xenobiotics; Human; Steroid; Stability; Pregnane X receptor; Cytoskeleton; Activation; Microtubule; Xenobiotic; Biological receptor
• ISSN: 1078-0432; 1557-3265
• DOI: 10.1158/1078-0432.CCR-05-0252

Purpose: Because induction of drug efflux transporters is one of the major underlying mechanisms of drug resistance in cancer chemotherapy,
and human pregnane X receptor (hPXR) is one of the principal “xenobiotic” receptors whose activation induces transporter and
drug-metabolizing enzyme gene transcription, it would be ideal to develop chemotherapy drugs that do not activate hPXR. This
report describes studies undertaken to explore the characteristics of hPXR stimulation and mechanisms of drug-receptor interactions
in vitro with new anti-tubulin drugs.
Experimental Design: In vitro transient transcription, glutathione S -transferase pull-down assays, and mammalian one-hybrid and two-hybrid systems were used to explore drug-receptor interactions.
Loss of righting reflex was used to assess effects of drugs on PXR activity in vivo .
Results: The current study showed that paclitaxel, discodermolide, and an analogue of epothilone B, BMS-247550, induced CYP3A4 protein
expression in HepG2 hepatoma cells. Transient transcription assays of a luciferase reporter in the presence and absence of
a GAL4-steroid and xenobiotic receptor (SXR) plasmid in HepG2 cells showed that these drugs activate hPXR. This was not true
for the inactive analogue of paclitaxel, baccatin III, or for an analogue of epothilone A, analogue 5, none of which stabilizes
microtubules. To determine the mechanisms by which paclitaxel, discodermolide, and BMS-247550 activate hPXR, a mammalian two-hybrid
assay was done using VP16SRC-1 (coactivator) and GAL4-SXR. SRC-1 preferentially augmented the effects of these drugs on hPXR.
Expression of SMRT (corepressor) but not NCoR suppressed the drug-induced activation of SXR by ∼50%, indicating a selectivity
in corepressor interaction with hPXR. These drugs resulted in shortened duration of loss of righting reflex in vivo , indicating drug-induced activation of PXR in mice.
Conclusion: These findings suggest that activation of hPXR with selective displacement of corepressors is an important mechanism by which
microtubule-stabilizing drugs induce drug-metabolizing enzymes both in vitro and in vivo .