Activation of the steroid and xenobiotic receptor (human pregnane X receptor) by nontaxane microtubule-stabilizing agents.
OBJECTIVE
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.


METHODS
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 approximately 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.


CONCLUSIONS
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.
Mani
Sridhar
S
Albert Einstein Cancer Center and Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461, USA. smani@montefiore.org
Huang
Haiyan
H
Sundarababu
Sumathy
S
Liu
Wenjing
W
Kalpana
Ganjam
G
Smith
Amos B
AB
Horwitz
Susan B
SB
D000473:Alkanes / Q000494:pharmacology; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology; D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology; D001189:Aryl Hydrocarbon Hydroxylases / Q000378:metabolism; D015152:Blotting, Northern; D002219:Carbamates / Q000494:pharmacology; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D034261:Epothilones / Q000494:pharmacology; D005982:Glutathione Transferase / Q000378:metabolism; D051548:Histone Acetyltransferases; D006801:Humans; D015151:Immunoblotting; D007783:Lactones / Q000494:pharmacology; D051379:Mice; D008810:Mice, Inbred C57BL; D009687:Nuclear Proteins / Q000378:metabolism; D056971:Nuclear Receptor Co-Repressor 1; D056985:Nuclear Receptor Co-Repressor 2; D056920:Nuclear Receptor Coactivator 1; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism; D017239:Paclitaxel / Q000494:pharmacology; D010957:Plasmids; D000077297:Pregnane X Receptor; D011753:Pyrones; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D011987:Receptors, Steroid / Q000235:genetics / Q000378:metabolism*; D012021:Reflex, Abnormal; D012097:Repressor Proteins / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects*; D015533:Transcriptional Activation; D020798:Two-Hybrid System Techniques; D015262:Xenobiotics
D000473:Alkanes; D000970:Antineoplastic Agents; D000972:Antineoplastic Agents, Phytogenic; D002219:Carbamates; D004268:DNA-Binding Proteins; D034261:Epothilones; D007783:Lactones; C539127:NCOR1 protein, human; C539130:NCOR2 protein, human; C539128:Ncor1 protein, mouse; C539131:Ncor2 protein, mouse; D009687:Nuclear Proteins; D056971:Nuclear Receptor Co-Repressor 1; D056985:Nuclear Receptor Co-Repressor 2; D000077297:Pregnane X Receptor; D011753:Pyrones; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; D012097:Repressor Proteins; D014157:Transcription Factors; D015262:Xenobiotics; D003577:Cytochrome P-450 Enzyme System; C071229:discodermolide; D001189:Aryl Hydrocarbon Hydroxylases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D010089:Oxidoreductases, N-Demethylating; D051548:Histone Acetyltransferases; C538859:NCOA1 protein, human; C538861:Ncoa1 protein, mouse; D056920:Nuclear Receptor Coactivator 1; D005982:Glutathione Transferase; C430592:ixabepilone; D017239:Paclitaxel