Comprehensive understanding of multiple actions of anticancer drug tamoxifen in isolated mitochondria
Yufu Unten1, Masatoshi Murai1, Tomoki Koshitaka1
1Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan.
Abstract:
Tamoxifen has been widely used in the treatment of estrogen receptor (ER)-positive breast cancer, whereas it also exhibits ER-independent anticancer effects in various cancer cell types. As one of the convincing mechanisms underlying the ER-independent effects, induction of apoptosis through mitochondrial dysfunction has been advocated. However, the mechanism of action of tamoxifen even at the isolated mitochondrial level is not fully understood and remains controversial. Here, we attempted to comprehensively understand tamoxifen's multiple actions in isolated rat liver mitochondria through not only revisiting the actions hitherto reported but also conducting originally designed experiments. Using submitochondrial particles, we found that tamoxifen has potential as an inhibitor of both respiratory complex I and ATP synthase. However, these inhibitory effects were not elicited in intact mitochondria, likely because penetration of tamoxifen across the inner mitochondrial membrane is highly restricted owing to its localized positive charge (-N+H(CH3)2). This restricted penetration may also explain why tamoxifen is unable to function as a protonophore-type uncoupler in mitochondria. Moreover, tamoxifen suppressed opening of the mitochondrial permeability transition pore induced by Ca2+ overload through enhancing phosphate uptake into the matrix. The photoaffinity labeling experiments using a photolabile tamoxifen derivative (pTAM1) indicated that pTAM1 specifically binds to voltage-dependent anion channels (VDACs) 1 and 3, which regulate transport of various substances into mitochondria. The binding of tamoxifen to VDAC1 and/or VDAC3 could be responsible for the enhancement of phosphate uptake. Taking all the results together, we consider the principal impairment of mitochondrial functions caused by tamoxifen.
Insights
Tamoxifen, used for breast cancer, may induce apoptosis via mitochondrial dysfunction. This study reveals tamoxifen
Area of Science:
- Biochemistry
- Mitochondrial Biology
- Cancer Pharmacology
Background:
- Tamoxifen is a standard breast cancer therapy targeting estrogen receptors (ER).
- Tamoxifen also shows ER-independent anticancer effects, potentially through mitochondrial dysfunction and apoptosis.
- The precise mitochondrial mechanisms of tamoxifen remain incompletely understood and debated.
Purpose of the Study:
- To comprehensively investigate tamoxifen's actions on isolated rat liver mitochondria.
- To elucidate the ER-independent mechanisms of tamoxifen's anticancer effects at the mitochondrial level.
Main Methods:
- Experiments using submitochondrial particles and intact mitochondria.
- Assessment of respiratory complex I and ATP synthase inhibition.
- Investigation of protonophore-uncoupler activity.
- Analysis of mitochondrial permeability transition pore opening.
- Photoaffinity labeling with a tamoxifen derivative (pTAM1).
Main Results:
- Tamoxifen showed potential inhibition of respiratory complex I and ATP synthase in submitochondrial particles, but not intact mitochondria, due to restricted inner membrane penetration.
- Tamoxifen suppressed Ca2+-induced mitochondrial permeability transition pore opening by enhancing matrix phosphate uptake.
- Photoaffinity labeling identified voltage-dependent anion channels (VDACs) 1 and 3 as specific binding sites for tamoxifen.
- Tamoxifen binding to VDACs likely mediates the observed enhancement of phosphate uptake.
Conclusions:
- Tamoxifen's restricted inner mitochondrial membrane penetration limits its direct inhibitory effects on respiratory complexes and ATP synthase in intact mitochondria.
- Tamoxifen modulates mitochondrial function by enhancing phosphate uptake via VDACs, suppressing the permeability transition pore.
- These VDAC-mediated effects on mitochondrial function represent a key mechanism for tamoxifen's ER-independent anticancer activity.
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