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Interaction of tamoxifen with the multidrug resistance P-glycoprotein
1Imperial Cancer Research Fund Laboratories, Institute for Molecular Medicine, Oxford, UK.
Abstract:
Tamoxifen is an anti-oestrogen which is currently being assessed as a prophylactic for women at high risk of breast cancer. Taxoxifen has also been shown to reverse multidrug resistance in P-glycoprotein (P-gp)-expressing cells, although the mechanism of action is unknown. In this study we demonstrate that tamoxifen interacts directly with P-gp. Plasma membranes from P-gp-expressing cells bound [3H]tamoxifen in a specific and saturable fashion. A 180 kDa membrane protein in these membranes, labelled by the affinity analogue tamoxifen aziridine and azidopine, was shown to be P-gp. Tamoxifen reduced the binding of vinblastine and azidopine to P-gp, and tamoxifen increased [3H]vinblastine accumulation in P-gp-expressing cells to levels approaching those in non-P-gp-expressing cells. However, the cellular accumulation of [3H]tamoxifen itself was not influenced by the presence of P-gp. Thus, tamoxifen appears to reverse multidrug resistance by binding to P-gp and inhibiting the transport of cytotoxic drugs, but does not itself appear to be transported by the protein.
Insights
Tamoxifen directly binds to P-glycoprotein (P-gp), reversing multidrug resistance by blocking drug transport. This interaction inhibits cytotoxic drug efflux but tamoxifen itself is not transported by P-gp.
Area of Science:
- Biochemistry
- Molecular Biology
- Pharmacology
Background:
- Tamoxifen, an anti-oestrogen, is investigated for breast cancer prophylaxis.
- Tamoxifen is known to reverse multidrug resistance in P-glycoprotein (P-gp)-expressing cells.
- The precise mechanism by which tamoxifen reverses multidrug resistance is not fully understood.
Purpose of the Study:
- To elucidate the mechanism of tamoxifen's action in reversing multidrug resistance.
- To investigate the direct interaction between tamoxifen and P-glycoprotein.
Main Methods:
- Utilized radiolabeled [3H]tamoxifen to assess binding to P-gp expressing cell membranes.
- Employed affinity labeling with tamoxifen aziridine and azidopine to identify P-gp.
- Measured the effect of tamoxifen on the binding of vinblastine and azidopine to P-gp.
- Quantified the accumulation of [3H]vinblastine in P-gp-expressing cells.
Main Results:
- Demonstrated specific and saturable binding of [3H]tamoxifen to plasma membranes of P-gp-expressing cells.
- Identified P-gp as a 180 kDa membrane protein that binds tamoxifen.
- Showed that tamoxifen reduces the binding of vinblastine and azidopine to P-gp.
- Observed an increase in [3H]vinblastine accumulation in P-gp-expressing cells upon tamoxifen treatment.
- Found no significant influence of P-gp on the cellular accumulation of [3H]tamoxifen itself.
Conclusions:
- Tamoxifen directly interacts with and binds to P-glycoprotein.
- Tamoxifen reverses multidrug resistance by inhibiting P-gp-mediated transport of cytotoxic drugs.
- Tamoxifen is not a substrate for P-gp, meaning it is not transported by the protein.