title: Tamoxifen inhibits TRPV6 activity via estrogen receptor-independent pathways in TRPV6-expressing MCF-7 breast cancer cells.
abstract: The epithelial calcium channel TRPV6 is upregulated in breast carcinoma compared with normal mammary gland tissue. The selective estrogen receptor modulator tamoxifen is widely used in breast cancer therapy. Previously, we showed that tamoxifen inhibits calcium uptake in TRPV6-transfected Xenopus oocytes. In this study, we examined the effect of tamoxifen on TRPV6 function and intracellular calcium homeostasis in MCF-7 breast cancer cells transiently transfected with EYFP-C1-TRPV6. TRPV6 activity was measured with fluorescence microscopy using Fura-2. The basal calcium level was higher in transfected cells compared with nontransfected cells in calcium-containing solution but not in nominally calcium-free buffer. Basal influxes of calcium and barium were also increased. In transfected cells, 10 mumol/L tamoxifen reduced the basal intracellular calcium concentration to the basal calcium level of nontransfected cells. Tamoxifen decreased the transport rates of calcium and barium in transfected cells by 50%. This inhibitory effect was not blocked by the estrogen receptor antagonist, ICI 182,720. Similarly, a tamoxifen-induced inhibitory effect was also observed in MDA-MB-231 estrogen receptor-negative cells. The effect of tamoxifen was completely blocked by activation of protein kinase C. Inhibiting protein kinase C with calphostin C decreased TRPV6 activity but did not alter the effect of tamoxifen. These findings illustrate how tamoxifen might be effective in estrogen receptor-negative breast carcinomas and suggest that the therapeutic effect of tamoxifen and protein kinase C inhibitors used in breast cancer therapy might involve TRPV6-mediated calcium entry. This study highlights a possible role of TRPV6 as therapeutic target in breast cancer therapy.
authors.affiliation: Institute of Biochemistry and Molecular Medicine, University of Bern, Bern, Switzerland CH-3012.
authors.forename: Katrin A
authors.initials: KA
authors.lastname: Bolanz
authors.forename: Gergely Gy
authors.initials: GG
authors.lastname: Kovacs
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Landowski
authors.forename: Matthias A
authors.initials: MA
authors.lastname: Hediger
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D001464:Barium / Q000378:metabolism; D001943:Breast Neoplasms / Q000201:enzymology / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000235:genetics / Q000378:metabolism; D045744:Cell Line, Tumor; D005260:Female; D005455:Fluorescent Antibody Technique; D016257:Fura-2 / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D008164:Luminescent Proteins / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011960:Receptors, Estrogen / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D050916:TRPV Cation Channels / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D013629:Tamoxifen / Q000494:pharmacology*; D014162:Transfection
chemical_list: D001426:Bacterial Proteins; D015220:Calcium Channels; D008164:Luminescent Proteins; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D050916:TRPV Cation Channels; C422827:TRPV6 protein, human; C054562:yellow fluorescent protein, Bacteria; D013629:Tamoxifen; D001464:Barium; D011493:Protein Kinase C; D002118:Calcium; D016257:Fura-2