A mechanism for tamoxifen-mediated inhibition of acidification

Tamoxifen has been reported to inhibit acidification of cytoplasmic organel les in mammalian cells. Here, the mechanism of this inhibition is investiga ted using in vitro assays on isolated organelles and liposomes. Tamoxifen i nhibited ATP-dependent acidification in organelles from a variety of source s, including isolated microsomes from mammalian cells, vacuoles from Saccha romyces cerevisiae, and inverted membrane vesicles from Escherichia coil. T amoxifen increased the ATPase activity of the vacuolar proton ATPase but de creased the membrane potential (V-m) generated by this proton pump, suggest ing that tamoxifen may act by increasing proton permeability. In liposomes, tamoxifen increased the rate of pH dissipation. Studies comparing the effe ct of tamoxifen on pH gradients using different salt conditions and with ot her known ionophores suggest that tamoxifen affects transmembrane pH throug h two independent mechanisms. First, as a lipophilic weak base, it partitio ns into acidic vesicles, resulting in rapid neutralization. Second, it medi ates coupled, electroneutral transport of proton or hydroxide with chloride . An understanding of the biochemical mechanism(s) for the effects of tamox ifen that are independent of the estrogen receptor could contribute to pred icting side effects of tamoxifen and in designing screens to select for est rogen-receptor antagonists without these side effects.